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Cite this: Chem. Soc. Rev., 2016,
45,6327
Applications of N-heterocyclic imines in main
group chemistry
Tatsumi Ochiai,
a
Daniel Franz
b
and Shigeyoshi Inoue*
ab
The imidazolin-2-imino group is an N-heterocyclic imino functionality that derives from the class of
compounds known as guanidines. The exocyclic nitrogen atom preferably bonds to electrophiles and
its electron-donating character is markedly enhanced by efficient delocalization of cationic charge
density into the five-membered imidazoline ring. Thus, this imino group is an excellent choice for
thermodynamic stabilization of electron-deficient species. Due to the variety of available imidazoline-
based precursors to this ligand, its steric demand can be tailored to meet the requirements for kinetic
stabilization of otherwise highly reactive species. Consequently, it does not come as a surprise that the
imidazolin-2-iminato ligand has found widespread applications in transition-metal chemistry to furnish
pincer complexes or ‘‘pogo stick’’ type compounds. In comparison, the field of main-group metal
compounds of this ligand is still in its infancy; however, it has received growing attention in recent years.
A considerable number of electron-poor main-group element species have been described today which
are stabilized by N-heterocyclic iminato ligands. These include low-valent metal cations and species that
are marked by formerly unknown bonding modes. In this article we provide an overview on the present
chemistry of main-group element compounds of the imidazolin-2-iminato ligand, as well as selected
examples for the related imidazolidin- and benzimidazolin-2-imino system.
Introduction
The imidazolin-2-imino group is a potent electron pair donor
In coordination chemistry nitrogen is particularly recognized
for its role as a strong electron-donor atom in ligand systems.
Seemingly, this contradicts the fact that this element belongs
to the highly electronegative members of the periodic table.
a
Institut fu
¨r Chemie, Technische Universita
¨t Berlin, Straße des 17. Juni 135,
b
Department of Chemistry, Institute of Silicon Chemistry and Catalysis Research
Center, Technische Universita
¨tMu
¨nchen, Lichtenbergstraße 4, 85748 Garching,
Germany
Tatsumi Ochiai
Tatsumi Ochiai was born in
Shizuoka, Japan, in 1987. In 2012
he received his MSc degree from
the University of Tsukuba under
the supervision of Prof. Akira
Sekiguchi, for which he received
the Dean’s Prize. He worked
on heteroatom-substituted tetra-
hedranes. He then moved to
Berlin in 2012 starting doctoral
work in the research group of
Prof. Shigeyoshi Inoue. His
research interest mainly focuses
on the synthesis of low-coordinate
heavier group 14 elements,
metallylenes and metallyliumyl-
idene ions.
Daniel Franz
Daniel Franz studied chemistry at
the Goethe University Frankfurt
am Main, Germany, where he
received his PhD degree under
the supervision of Prof. Matthias
Wagner in 2012. Following this
he moved to Berlin to carry out
postdoctoral studies in the group of
Prof. Shigeyoshi Inoue. Today he is
pursuing his research interests in
coordination chemistry of group 13
and 14 elements at the Technische
Universita
¨tMu
¨nchen under the
supervision of Prof. Inoue.
Received 29th February 2016
DOI: 10.1039/c6cs00163g
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However, the trivalent nitrogen atom in amines, as well as in
imines has high electron density in the form of a lone pair that
it readily shares with various types of hard and soft Lewis acids.
Tertiary amines and secondary ketimines resemble in their
nucleophilic properties but, in sharp contrast, the unsaturated
carbon atom of the imino functionality is prone to the reaction
with nucleophiles or reducing agents whereas the amino carbon
atom is inert (Fig. 1). This reactivity results from the p-interaction
with the more electronegative nitrogen atom which provides
the higher bond order but also drains electron density from the
carbon centre in the s-, as well as the p-scaffold. Due to the
orthogonal orientation of the nitrogen lone pair this is not
compensated by pback donation. Interestingly, the electronic
properties of the imino-nitrogen atom are, vice versa, stronger
affected by the characteristics of the carbon atom than it may
be the case for the amino-nitrogen centre. In this regard, the
electron-rich p-system of an imidazoline ring not only mitigates
the electrophilicity of an imino-carbon atom incorporated at
the 2-position of the cycle but also pushes electron density
to the exocyclic imino-nitrogen atom (Fig. 1). Notably, phosphoran-
imines (R
3
PQNR) resemble the imidazolin-2-imines in the
electron-donating character of the imino nitrogen atom (Fig. 1).
The resemblance of these two ligand classes is reasoned by the
similarities in the electronic properties of the parent phosphine
and imidazolin-2-ylidene, respectively. Furthermore, one must
recognize the isoelectronic relation between imidazolin-2-imines
and N-heterocyclic olefins, which function as strong Lewis
bases due to the ylide-like nature of the exocyclic alkene
bond (Fig. 1).
1
The allocation of electron density from the five-membered
imidazoline ring to the exocyclic nitrogen atom is illustrated by
conceivable resonance structures of the anionic imidazolin-2-
iminato ligand A(Fig. 2).
2,3
The canonical form Bin which the
exocyclic nitrogen atom bears two formal anionic charges
suggests a significant boost of its electron-donating properties
as compared to ketimines (Fig. 1 and 2). Form Crepresents the
partial N-heterocyclic carbene (NHC) character of the imid-
azoline moiety (Fig. 2). As apparent from the canonical forms
(A–C) the imidazolin-2-iminato ligand represents a 2selectron
donor with potential to contribute an additional two or even
four p-electrons. Consequently, its metal complexes (A
M
) may
exhibit significant metalla-2-aza-allene (B
M
) or metalimide (C
M
)
character (Fig. 2). This manifests in an expansion of the imino
group’s CN distance and shortening of the N–M bond length.
Concomitantly, the C–N–M bond angle is widened to approach
the angle of 1801in the ideal CCC allene structure motive. As a
result of its electron-donating properties, the imidazolin-2-iminato
ligand is an efficient tool for the thermodynamic stabilization of
electron-poor species. Moreover, the bulkiness of the imidazoline
ring can be conveniently modified to meet individual require-
ments for kinetic stabilization of otherwise elusive compounds.
The scope of this review
In this article we focus on the coordination chemistry of the
imidazolin-2-iminato ligand, as well as the strongly related
imidazolidin-2-imino group and the benzimidazolin-2-imino
group with regard to main-group elements. For the latter two
Fig. 1 Overview of selected N-donor ligands and the related N-heterocyclic
olefin (R = organyl or H).
Fig. 2 Selected resonance structures for the anionic imidazolin-2-iminato
ligand, as well as a model complex with M
+
(R = organyl).
Shigeyoshi Inoue
Shigeyoshi Inoue studied chem-
istry at the University of Tsukuba
where he completed his PhD
degree under the supervision of
Prof. Akira Sekiguchi in 2008.
After being a Humboldt Post-
doctoral fellow, as well as a JSPS
postdoctoral fellow for research
abroad with Prof. Matthias Drieß
at the Technische Universita
¨t
Berlin, he began his independent
research career as a Sofia
Kovalevskaja Professor in 2010
at the same university. Since 2015
he has been a W2-Tenure-Track Professor of Silicon Chemistry at the
Technische Universita
¨tMu
¨nchen. His research interests are focused on
the synthesis and reactivity investigation of low-valent main group
compounds with the goal of finding novel applications in synthesis
and catalysis.
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only relevant examples will be given. An earlier review of Kuhn,
Frenking and coworkers on imidazolin-2-imines includes main-
group metal complexes but dates back about 13 years.
2
The
broad spectrum of transition metal complexes that comprise
this ligand class and the methods for the synthesis of the ligand
have recently been reviewed by Tamm and coworkers and will
be discussed only in part.
3
Moreover, only selected examples will be discussed for
compounds of this iminato ligand with the non-metals carbon
and nitrogen because this belongs to the field of organic
chemistry rather than coordination chemistry.
Group 1 and group 2 element
complexes
Background
About 20 years ago Kuhn and coworkers started their pioneering
studies on the chemistry of imidazolin-2-imines.
4
Afewalkaline
5,6
and alkaline earth
7
compounds of the imino group were reported
but not investigated thoroughly probably because of the pro-
nounced polar nature of the N–M bond (M = alkaline or alkaline
earth metal). This puts them in the role of a reactive intermediate
for ligand transfer via salt metathesis rather than a species with its
own follow-up reactivity with sustainment of the N–M bond.
Accordingly, the chemistry of group 1 and group 2 imidazolin-2-
iminato complexes is only explored to a minor degree to date.
Lithium and potassium complexes
The reaction of L
Me
2
NH (L
Me
2
= 1,3-dimethyl-imidazolin-2-
ylidene) with MeLi in Et
2
O produces L
Me
2
NLi (1) which is the
N-lithiated derivative of the imidazolin-2-imine.
5
The species
was characterized by
1
H NMR analysis and according to the
reported CHN elemental analysis no solvent was present in the
isolated material. If the conversion was carried out in THF/Et
2
O
with MeLi that was prepared from H
3
CCl and elemental lithium
without prior separation of lithium chloride, crystals of the
unexpected composition [Li
12
O
2
Cl
2
(L
Me
2
N)
8
(thf)
4
]8THF (2)were
retrieved in low yield (Scheme 1).
The solid state structure of 2is marked by a Li
12
N
8
O
2
Cl
2
cage
that comprises a peroxo moiety in its core (Fig. 3). The authors
reasoned that the O
22
group resulted from contamination of the
solvent with traces of peroxide. Crystals of dimeric [L
Dip
NLi]
2
toluene
(3toluene, L
Dip
= 1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene)
were isolated in good yield after the reaction of L
Dip
NH with
nBuLi in toluene/hexane.
8
Apparently, the formation of higher
aggregates is hampered by the bulkier Dip groups (Dip = 2,6-
diisopropylphenyl). Bringing into contact L
Me
2
NH and freshly
prepared MeK in Et
2
O afforded the heavier alkaline derivative
L
Me
2
NK (4).
6
The compound was characterized by elemental
analysis and its existence was verified by the synthesis of
the dithiocarbiminate L
Me
2
NCS
2
K(5, Scheme 2). Interestingly,
the latter shows structural characteristics that account for a
bonding situation as represented by resonance structure 5
B
(Scheme 2) with the C–N
imino
bond length significantly
increased (a range from 1.369(16) Å to 1.379(18) Å is observed
in the solid state structure; cf. 2: C–N
imino
= 1.260(4)–1.263(4) Å;
3: C–N
imino
= 1.241(3) Å, 1.242(4) Å). Accordingly, the C–S
distances in 5(1.733(13)–1.755(13) Å) resemble typical CS single
bond lengths.
Magnesium complexes
As rare examples for N-heterocyclic iminato complexes of group
2 metals the magnesium compounds (L
Me
2
NH)
4
MgI
2
(6[I]
2
), as
well as {L
Me
2
NMgI}
n
(7) and {(L
Me
2
N)
2
Mg}
n
(8), were reported
by Kuhn and coworkers (nZ1).
7
They are accessed through
L
Me
2
NH via conversion with 0.25 MgI
2
, MeMgI and (nBu)
2
Mg,
respectively (Scheme 3). Single crystal XRD (X-ray diffraction)
data were obtained for 6[I]
2
(Fig. 4) while the degree of aggrega-
tion (n)of7and 8was not elucidated by structural analysis.
Scheme 1 Conversion of the imino lithium species 1into the imino-
stabilized LiOCl aggregate 2(the lithium chloride derives from the methyl-
lithium synthesis and peroxide from contaminated solvent). Formation of
the bulky imino lithium dimer 3(Dip = 2,6-diisopropylphenyl).
Fig. 3 Ellipsoid plot (30% level) of the Li
12
N
8
O
2
Cl
2
cage in 2with adjacent
imino-carbon atoms and oxygen atoms of coordinated THF.
Scheme 2 Conversion of potassium imide 4with carbon disulfide to the
thiocarbiminate 5(represented by resonance structures 5
A
and 5
B
).
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Notably, 6[I]
2
also formed if less than four equiv. of the imine
were reacted with MgI
2
. The authors attributed this observation
to the high basicity of the ligand.
Group 13 element complexes
Background
Of group 13 elements only a few aluminium complexes with an
N-heterocyclic iminato ligand had been reported until respec-
tive research was resumed by our group.
6
Reports
8–10
in the
year 2014 were the first to describe imidazolin-2-imino com-
plexes of boron. In contrast, the coordination chemistry of
related phosphoranimines of boron
11
and aluminium
11a,c,d,12
is thoroughly studied.
Boron complexes
The Lewis acid base adducts LNH(BH
3
)(9, Scheme 4) between
L
Dip
NH, as well as L
Mes
NH (L
Mes
= 1,3-dimesityl-imidazolin-2-
ylidene), and the parent borane were isolated after conversion
of the imine with Me
2
SBH
3
in toluene.
9
When treated with
imidazolin-2-ylidenes (L) dihydrogen is abstracted from the
HN–BH moiety of these imine–borane compounds and along
with L(H
2
) (hydrogenated at the formerly carbenic centre) the
NHC-adducts of respective imino boron dihydrides (LN(BH
2
)L)
are formed (10, Scheme 4).
9
These NHC-adducts undergo
hydride-mediated ring-expansion reaction, that is, the boron
atom transfers its two hydrides to the adjacent carbon atom and
inserts into the C
carbenoid
NbondoftheNHC(11,Scheme4).
9
Presumably, the interaction between the lone pair at the imino
nitrogen atom and the unoccupied p-orbital at the boron centre
supports the trigonalization of the metalloid atom. Interestingly,
this insertion occurs at higher temperatures with more sterically
hindered substituents at the boron atom. Moreover, H
2
L
Mes
(1,3-dimesityl-imidazolidin-2-ylidene, NHC saturated at the ligand
backbone) is subject to ring-expansion reaction at significantly
lower temperatures than its congener of very similar sterical
encumbrance L
Mes
(NHC unsaturated at the ligand backbone).
It was reasoned that the conjugated ring system in L
Mes
is more
efficient for the delocalization of positive charge density and,
thus, stabilizes the boron dihydride form (LNBH
2
L).
Similarly, ring-activation and expansion reaction of L
Dip
took place by heating the amido-substituted hydridoborane
L
Dip
(BH
2
)HNDip reported by Rivard and coworkers.
13
Conversion of the bis(iminiumtosylate) 12[OTs]
2
with Li[BH
4
]
furnishes the boronium salt 13[OTs] (Scheme 5).
10
The com-
pound reacts with yellow sulfur to give a rare example of a
cationic thioxoborane 14[OTs] that was structurally characterized
(Scheme 5).
10
The B–S bond length (1.710(5) Å) in 14
+
is the
shortest that has been reported to date for a molecular complex.
Notably, the B–N bond lengths significantly decrease upon
transformation of tetrahedral 13
+
into trigonal-planar 14
+
(13
+
: 1.573(5) Å, 1.577(5) Å; 14
+
: 1.483(5) Å, 1.493(5) Å) and,
consequently, a partial double bond character can be attributed
to the boron–nitrogen interactions in 14
+
. Concomitantly, the
C–N distances of the imino groups increase (13
+
: 1.317(5) Å,
1.318(4) Å; 14
+
: 1.359(4) Å, 1.363(4) Å) which is in accordance
with the formulation of resonance structure 14
A+
(Scheme 5)
that represents the delocalization of positive charge density
into the N-heterocycles and the polarization of the BS bond
towards the sulfur atom (NBO charge at S = 0.58; NBO =
Natural Bond Orbital). DFT (density functional theory) calcula-
tions supported the interpretation of the remarkably short B–S
distance in terms of a boron sulfur double bond. For example,
the HOMO (highest occupied molecular orbital) shows mainly
Scheme 3 Synthesis of the imino-magnesium compounds 6[I]
2
,7and 8.
Fig. 4 Ball and stick representation of (L
Me
2
NH)
4
Mg
2+
(6
2+
) as derived
from XRD analysis (non-N-bonded hydrogen atoms have been omitted).
Scheme 4 Conversion of the imine–borane adduct 9with NHC to
iminoboron dihydride NHC complex 10 and the formation of 11 via ring
expansion reaction. R
1
= Mes or Dip; R
2
= Me (for R
3
= Me), Mes or Dip
(both for R
3
= H); R
2
=MesandR
3
= H for unsaturated backbone; not all
combinations of imine and NHC are viable for 10 and 11.
9
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the sulfur lone pair and the HOMO1 reveals the BQS
p-bonding orbital. The NBO charge at the boron centre of 14
+
was calculated to be +0.63 which accounts for the boron cation
character of the complex as illustrated by the canonical form
14
B+
(Scheme 5).
Very recently, Rivard and coworkers described the conver-
sion of the imidazolin-2-imino trimethylsilane 15 to the imino
boron dichloride 16 and its organyl derivative 17 by the reaction
of 15 with BCl
3
and PhBCl
2
, respectively (Scheme 6).
14
The
bisimino boron monochloride 18 was furnished in a reaction
between 15 and 16 (Scheme 6). Moreover, the synthesis of the
diphenyl congener 19 was accomplished by conversion of 17
with phenylmagnesium bromide (Scheme 6). The solid state
structure of the dihalide 16 hints toward the significant bora-2-
aza-allene properties of the CNB moiety (type B
M
, Fig. 2) as
concluded from the C–N–B bond angle of 1801and the short
B–N bond length (1.302(6) Å) which implies high boron–nitrogen
double bond character. Remarkably, 16,17 and 19 react
with amine–boranes (R
n
H
3n
NBH
3
; R = H, Me; n=1,2)to
produce the respective dihydrogenated imino boron com-
pounds L
Dip
NH(B(H)R0R00 )20–22 (R0=R
00 = Cl for 20;R
0= Cl,
R00 = Ph for 21;R
0=R
00 = Ph for 22) and a mixture of the amine–
borane dehydrogenation products (Scheme 6).
14
The authors
conclude that this imino boron compound acts as an intra-
molecular frustrated Lewis acid base pair. It should be noted that
compound 17 displays catalytic activity in the dehydrocoupling
of MeNH
2
BH
3
to yield [MeNBH]
3
along with oligomeric amino-
boranes, which shows its great potential with respect to further
application in metal-free catalysis for the dehydrocoupling of
amine–boranes and related species.
Aluminium complexes
The bisimino aluminium complexes (L
Me
2
NSiMe
3
)
2
AlMe
2
[Cl] (23)
and (L
Me
2
NH)
2
AlMe
2
[Cl] (24) were synthesized by conversion of
L
Me
2
NSiMe
3
(25) and L
Me
2
NH, respectively, with 0.5 equiv. of
AlMe
2
Cl.
7
The ion-separated forms were postulated on the
basis of NMR spectroscopic data. The related Lewis acid base
adduct L
Me
2
NSiMe
3
AlCl
3
(26) releases Me
3
SiCl upon heating the
neat compound to 180 1C and is converted into {L
Me
2
NAlCl
2
}
3
(27,Scheme7).
7
An X-ray crystallographic analysis of compound
27 verified its trimeric structure with a six-membered Al
3
N
3
cycle.
The imino aluminium dihydride {L
Dip
NAlH
2
}
2
(28) results
from the reaction of L
Dip
NH with Me
3
NAlH
3
(Scheme 7).
8
From
the dihydride one can derive the dihalides {L
Dip
NAlX
2
}
2
(29–31,
X = Cl, Br, I) by conversion with BX
3
(two equiv.) which were
described to form dimers in the solid state, as well as in solution
(Scheme 8).
7
Obviously, the bulkier iminato ligand in 29 leads
to the formation of a four-membered Al
2
N
2
ring with smaller
N–Al–N angles (87.8(1)1, 92.3(1)1) in comparison to the six-
membered ring in 27 with larger angles (108–1101). Interestingly,
the sterically hindered phosphoranimino aluminium dihydride
and -dichloride form dimers with four-membered Al
2
N
2
rings
Scheme 5 Reaction of the bis(iminiumtosylate) 12[OTs]
2
with lithium
borohydride to the boronium salt 13[OTs] (Ts = tosyl) and its conversion
to the thioxoborane salt 14[OTs].
Scheme 6 Conversion of the imino trimethylsilane 15 to the imino
chloroboranes 16–18 and the imino diphenylborane 19. Formation of
the imine–borane adducts via abstraction of dihydrogen from amine–
borane adducts.
Scheme 7 Conversion of the trimethylsilylimine 25 to the aluminium
trichloride imine complex 26 and its transformation into trimeric 27.
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({R
3
PNAlX
2
}
2
; R = iPr, tBu; X = H, Cl), as well.
11c,12a
Further-
more, the dihydride 28 reacts with the electrophiles Me
2
SBH
3
(four equiv.) and Me
3
SiOTf (two equiv., Tf = triflyl) to yield the
aluminum borohydride 32 and the aluminium monohydride
triflate 33, respectively (Scheme 8).
8
Notably, the substitution of
both aluminium bonded hydrides in 33 for triflate substituents
could not be accomplished by the use of a larger excess of
Me
3
SiOTf, even at elevated temperature. In contrast, the con-
version of 28 with only two equiv. of Me
2
SBH
3
does not afford
the expected aluminium monohydride borohydride as a pro-
duct but yields mixtures of 28 and 32. Obviously, the electron
withdrawing triflyl groups in 33 mitigate the hydride-donor
strength of the remaining AlH functionality. Accordingly, only
aluminium monohydride triflates of the related phosphoran-
iminato or the 1,3-diketiminato ligand have been reported.
11c,15
The conversion of the aluminium dihydride 28 with yellow
sulfur affords a rare example of an aluminium hydride
hydrogensulfide complex (34) by insertion of a sulfur atom into
the AlH bond (Scheme 8).
16
Similar to 33 the remaining hydride
functionalities at the aluminium centres in 34 are less reactive
than in the parent compound. However, the transformation
with S
8
to form the bis(hydrogensulfide) 35 can be forced onto
the system by heating (90 1C for four days, Scheme 8).
16
As apparent from the XRD study the Al–S distances in 35
(2.231(1)–2.240(1) Å) are slightly shorter than the respective
distances in the monohydrogensulfide 34 (2.250(1) Å and
2.252(1) Å).
In order to furnish a heavier aluminium chalcogenide of
the imidazolin-2-iminato ligand, 28 was converted with the
tellurium atom transfer reagent nBu
3
PTe (two equiv.).
17
This
conversion yields ditopic aluminium ditelluride 36 as a rare
example of an electron-precise aluminium complex with the
chalcogen in the oxidation state 1 (Scheme 8). The hydrides
left at the aluminium centres in 36 do not react further with
excess nBu
3
PTe. However, the compound converts with NHC
(L
Et
,5equiv.,L
Et
= 1,3-diethyl-4,5-dimethyl-imidazolin-2-ylidene)
in a dehydrogenative redox process to form the monotopic
aluminium telluride 37 (Fig.5,Scheme8)withthechalcogenin
the oxidation state 2 along with dihydrogenated NHC (L
Et
(H
2
)).
17
Scheme 8 Overview on syntheses of imino aluminium compounds derived from the aluminium dihydride 28: reaction of 28 to the dihalides 29–31, the
borohydride 32 and the triflate 33 (X = Cl, Br, I). Synthesis of the aluminium mono- and bis(hydrogensulfides) 34, as well as 35. Conversion of 28 to the
ditelluride 36 and its monotopic aluminium telluride offspring 37 (L
Et
= 1,3-diethyl-4,5-dimethyl-imidazolin-2-ylidene). Transformation of 37 to ditopic
39 via the presumed intermediate 38.
Fig. 5 Ellipsoid plot (30% level) of the aluminium telluride 37 (hydrogen atoms,
isopropyl groups and non-N-bonded methyl groups have been omitted).
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The structural study of 37 revealed a remarkably short Al–Te
distance of 2.5130(14) Å and DFT calculations determined an
enhanced aluminium–tellurium interaction (WBI
AlTe
= 1.20;
NPA charges: Al = +1.24, Te = 0.95; WBI = Wiberg bond index,
NPA = natural population analysis). It has to be pointed out that
the terminal position of the tellurium atom is a very scant
structural motif as group 16 atoms commonly assume bridging
positions in aluminium chalcogenides. Upon heating a benzene
solution of 37 to 80 1C one of the two L
Et
ligands is released and
the putative intermediate L
Dip
N(AlTe)L
Et
(38) undergoes aggrega-
tion to form 39 (Scheme 8).
The reaction pathway via 38 was suggested by DFT calcula-
tions; however, the isolation of a bulkier congener of this elusive
species was not accomplished by the use of more sterically
hindered NHC. The structural investigation of 39 revealed signifi-
cantly increased Al–Te distances (2.6143(14) Å, 2.6211(15) Å) and
a decreased bond order for the AlTe interaction (WBI
AlTe
= 0.75;
NPA charges: Al = +1.21, Te = 0.79) with respect to 37.
17
It should be noted that in ditopic 39 the aluminium centres are
bridged via the tellurium atoms. Notably, 37 and 39 contrast
the other given examples for aluminium complexes of the
imidazolin-2-iminato ligand in that the aluminium centres
are not connected via the nitrogen atoms of the imino groups.
Taking into account the marked changes in the Al–Te distances
and the values for the WBI
AlTe
upon transformation of 37 into
39 the nature of the AlTe interaction in 37 was presumed to
possess high AlQTe double bond character.
Group 14 element complexes
Background
In initial reports on the chemistry of N-heterocyclic iminato
ligands Kuhn and coworkers described the imino trimethyl-
silane 25 (Scheme 7) which was used as an alternative trans-
metallation reagent to the alkaline metal salts mentioned
above.
4,7
Presumably, the bulkier L
Mes
NSiMe
3
is formed as an
intermediate in the synthesis of L
Mes
NH via a Staudinger-type
reaction described by Cameron, Jenkins, Clyburne and coworkers in
2001.
18
Tamm and coworkers established the general method for
the preparation of trimethylsilyl-functionalized bulkier imidazolin-2-
iminato ligands such as L
Mes
NSiMe
3
and L
Dip
NSiMe
3
in 2004.
19
This
method has tremendous advantages for the convenient and high-
yield synthesis of various imidazolin-2-imines. Moreover, a silicon
atom was incorporated into the spacer group between the imino-
and the arene moiety in oligodentate ligands reported by Tamm
and coworkers.
20
However, it played a rather passive role in the
chemistry of the transition metal complexes derived from this
ligand system. As outlined in the following section a considerable
time elapsed from Kuhn’s initial report until the coordination
chemistry of the imidazolin-2-iminato ligand with tetrel atoms
was thoroughly investigated.
Silicon complexes
Our group commenced work on main group element complexes
of the imidazolin-2-iminato ligand a few years ago and described
its complex with a silicon(II) centre in 2012.
21
High interest for
molecular low-valent silicon compounds originates from their
various applications in catalysis and bond activation.
22,23
Conversion of L
Dip
NLi with Cp*Si[B(C
6
F
5
)
4
] (Cp* = pentamethyl-
cyclopentadienyl) as a source of silicon(II) afforded the penta-
methylcyclopentadienyl imino silylene 40 with Z
2
coordination
of the silicon centre by the organyl ligand (Scheme 9). An
alternative synthetic route by which 40 can be accessed is via
reaction of L
Dip
NLi with Cp*SiBr
3
followed by reductive dehalo-
genation of L
Dip
NSi(Br
2
)Cp* (41). Unfortunately, this method
affords only very poor yields of the silylene. DFT calculations
on 40 show some pbonding interaction between the imino
nitrogen lone pair and the unoccupied p-orbital at the silicon
centre. The WBI
SiN
of 0.80 and the Si–N bond length (1.691(5) Å)
imply single bond character. Thus, multiple bond interaction
as illustrated by the general canonical structures B
M
and C
M
(Fig. 2) cannot be concluded for 40. A key motivation of the
study was to explore potential silylene–nitrene character of com-
plexes between a low-valent silicon atom and the imidazolin-2-
iminato ligand as represented by the canonical structure 40
A
(Scheme 9). However, structural and theoretical investigation
verified the imino-substituted silylene formulation 40 with no
relevant silylene–nitrene character (40
A
). Conversion of 40 with
tris(pentafluorophenyl)borane furnished the silylene–borane
adduct 42. It is interesting to note that the Cp* ligand is
coordinated in a Z
1
-mode with one sbond to the silicon atom
in sharp contrast to the precursor 40, in which Z
2
-mode Si–Cp*
bonding is observed. As compared to 40 the Si–N bond length is
considerably reduced to 1.605(3) Å and its WBI
SiN
is increased
to 0.90 which account for partial SiN double bond character.
The C–N–Si angle of 158.7(3)1in 42 is wider than in 40
(136.6(4)1). Accordingly, the relevant 1-sila-2-aza-allene nature
(cf. B
M
, Fig. 2) can be attributed to 42 as represented by
resonance structure 42
A
(Scheme 9).
In order to exploit the strongly electron-donating properties
of an N-heterocyclic iminato ligand for tuning the reactivity of
low-valent silicon species Rivard and coworkers attempted the
synthesis of a hypothetical bisiminosilylene. Access to the
bisiminodibromosilane precursor 43 is granted by conversion
of L
Dip
NSiMe
3
(15) with SiBr
4
in appropriate stoichiometry
(Scheme 10).
24
The monoimino derivative L
Dip
NSiBr
3
(44)is
Scheme 9 Synthesis of Cp*-substituted iminosilylene 40 and its borane
adduct 42, as well as the dibromide precursor 41. Silylene–nitrene formu-
lation 40
A
and sila-2-aza-allene canonical structure 42
A
.
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synthesized in a similar fashion (Scheme 10).
24
The reductive
dehalogenation of 43 with KC
8
(excess) yielded the potassium salt
45 instead of the desired silylene (L
Dip
N)
2
Si (46, Scheme 10).
24
This product (45) was presumed to result from an intermediate
potassium silanide via migration of a Dip group. The formation
of minor amounts of the siloxane 47 was reasoned by the pre-
sence of silicon grease in the reaction mixture (Scheme 10).
Germanium complexes
The reductive dehalogenation of the bulky bisiminodichloro-
germane 48 with sodium naphthalenide affords the bisimino-
germylene 49 as reported by Rivard and coworkers (Scheme 11).
24
Notably, the related monoiminotrichlorogermane 50 was also
described (Scheme 11). In the solid state the germanium(II)
compound (49) exhibits longer Ge–N distances (both: 1.8194(15) Å)
and a decreased N–Ge–N bond angle (99.48(10)1) with respect to
its halogenated precursor 48 (Ge–N = 1.7528(14) Å, 1.7582(14) Å;
N–Ge–N = 106.33(7)1; Fig. 6). These structural features were
interpreted by the authors in terms of a higher p-character of the
Ge–N bond in 49 as compared to 48. Theoretical calculations
indicated a low singlet–triplet gap of 45.8 kcal mol
1
for the
bisiminogermylene 49, a value which is similar to that of the
elusive bisiminosilylene 46 (44.5 kcal mol
1
). This computational
study suggests high inclination for the sterically hindered metal
centre to insert into element–element bonds of small substrate
molecules. However, upon conversion of 49 with dihydrogen
Rivard and coworkers observed the formation of L
Dip
NH as
the only soluble species instead of the expected (L
Dip
N)
2
GeH
2
.
24
This may account for the pronounced proton affinity of the
imidazolin-2-imino group. Interestingly, the bisiminogermane
is also not formed in the reaction of 48 with hydride transfer
reagents such as K[BHsBu
3
] or potassium hydride.
24
Another synthetic approach to germanium(II) complexes of
the imidazolin-2-iminato ligand uses the Lappert’s germylene
((Me
3
Si)
2
N)
2
Ge as a low-valent metal source. Its conversion with one
equiv. of L
Dip
NH at 50 1C furnishes the amino(imino)germylene 51
in the form of a viscous liquid (Scheme 12).
25
It acts as a ligand
towards iron carbonyls as demonstrated by the formation of the
germylene complex 52 after reaction of 51 with diironnonacarbonyl
(Scheme 12).
25a
The XRD analysis of 52 reveals a Ge–N
imino
distance
of 1.755(2) Å which is significantly shorter than the Ge–N
amino
bond
length of 1.839(2) Å and also with respect to the free bisimino-
germylene 49 (vide supra). Considering the WBIs of the Ge–N
bonds in 52 (Ge–N
imino
= 0.86, Ge–N
amino
= 0.60) it is reasonable
to assume that the bulky imidazolin-2-iminato ligand bonds
stronger to the germanium(II) centre than the bis(trimethylsilyl)-
amino group, presumably as a result of the iminato ligand’s
higher electron-donating character.
If treated with tris(pentafluorophenyl)borane compound 51
undergoes a methyl-abstraction and ring-closing reaction to
form the cyclic germyliumylidene 53[MeB(C
6
F
5
)
3
] as an example
for a cationic complex of germanium(II) (Scheme 13, Fig. 7).
25a
The bonding situation in 53
+
is found to be suitably described
as an amino-bonded cationic germanium(II) atom that is stabi-
lized via dative bond type interaction with an intramolecularly
tethered imino group. This is indicated by a weaker interaction
Scheme 10 Reduction of dibromosilane 43 with KC
8
to form the unexpected
anionic compound 45 instead of intended 46. The siloxane 47, as well as
the tribromide 44.
Scheme 11 Reductive dehalogenation of the dichlorogermane 48 to the
bisiminogermylene 49. The trichlorogermane 50.
Fig. 6 Ellipsoid plot (30% level) of the bisiminogermylene 49 (hydrogen
atoms and isopropyl groups have been omitted).
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between the Ge(II) centre and the N
imino
atom (Ge–N
imino
=
1.9694(14) Å, WBI
GeN
= 0.48) and a stronger bond between the
Ge(II) centre and the N
amino
atom (Ge–N
amino
= 1.8437(15) Å,
WBI
GeN
= 0.73). This bonding situation between the metal
centre and the N atoms appears to be in contrast to the
situation in the uncharged congeners 51 and 52 (vide supra).
Moreover, the C–N
imino
distance of 1.335(2) Å in 53
+
is greater
in comparison to 52 (C–N
imino
= 1.296(3) Å) and hints towards
delocalization of cationic charge into the imidazoline ring
system similar to the observations reported for 5, as well as
14
+
. This accounts for the pronounced ability of the imidazolin-
2-iminato ligand to stabilize cationic species which was verified
yet again by a very recent report on the isolation of bifunctional
germylene–germyliumylidenes.
25b
Tin complexes
In 2015 the formation of the amino(imino)stannylene 54
was reported that proceeds in a similar fashion to the lighter
congener 51 via reaction of ((Me
3
Si)
2
N)
2
Sn with L
Dip
NH at
60 1C (Scheme 12).
26
Notably, the
119
Sn NMR chemical shift
of 208 ppm for 54 (C
6
D
6
) is considerably shifted to higher
field with respect to the precursor (767 ppm, C
6
D
6
) which was
accredited to an aggregated species in solution with a higher
coordinate tin(II) centre. The compound (54) was obtained as a
pale red powder and reacted with 4-dimethylamino-pyridine
(dmap) to give the solid tin(II) adduct 55 that exhibited a
resonance at 3 ppm in the
119
Sn NMR spectroscopic analysis
(Scheme 12).
26
The XRD study of 55 shows a shorter Sn–N
imino
contact (2.0588(13) Å) and a longer Sn–N
amino
distance
(2.1647(12) Å). This was interpreted in terms of a stronger bond
of the metal centre to the iminato ligand and a weaker inter-
action with the amino group as described for the germanium
congener 52, as well (vide supra).
25,26
The reaction of 54 with
tris(pentafluorophenyl)borane affords the stannyliumylidene
salt 56[MeB(C
6
F
5
)
3
] in a methyl abstraction and ring closing
reaction similar to the process that afforded the germanium
analogue 53[MeB(C
6
F
5
)
3
] (Scheme 13, vide supra). The bonding
situations in 56
+
and 53
+
resemble, that is, an amino bonded
metallyliumylidene cation which is stabilized by a dative bond
to the imino group. Accordingly, the Sn–N
imino
distance of
2.197(2) Å in 56
+
is longer than the Sn–N
amino
bond length of
2.062(2) Å which is an observation that is in contrast to that
reported for the uncharged congener 55 that possesses a
shorter Sn–N
imino
contact. The isolation of 56[MeB(C
6
F
5
)
3
]is
another example for the high potential of N-heterocyclic imino
systems to stabilize cationic species. Interestingly, the amino-
(imino)stannylene 54 converts with azido trimethylsilane to the
dimeric iminostannylene azide 57 (Scheme 12).
26
Apparently, an
expected stannaimine of the type (L
Dip
N)((Me
3
Si)
2
N)Sn(NSiMe
3
)
is not formed but ligand exchange results in the liberation of
(Me
3
Si)
3
N from the system. Interestingly, in solution (THF-d
8
)
dimeric 57 (d(
119
Sn) = 285 ppm) exists in equilibrium with a
monomeric species (d(
119
Sn) = 39 ppm; 570, Scheme 12).
Very recently, our group described the bisiminochlorostannate
58 which forms by the reaction of L
Dip
NLiwithhalfanequivalent
of SnCl
2
1,4-dioxane (Scheme 14).
27
The
119
Sn NMR spectrum
of 58 (thf-d
8
) shows a resonance at 18 ppm that is shifted to a
lower field in comparison to common monomeric trigonal
pyramidal-coordinate 1,3-diketiminato tin(II) chlorides (118 ppm
to 337 ppm).
28
The XRD analysis of 58 reveals a butterfly-shaped
four-membered SnN
2
Li stannacycle with no bonding interaction
Scheme 12 Preparation of amino(imino)metallylenes 51 and 54 and the
iron carbonyl 52, as well as the dmap adduct 55 (dmap = 4-dimethylamino-
pyridine). Conversion of 54 to dimeric stannylene azide 57.
Scheme 13 Synthesis of four-membered metallyliumylidenes 53
+
and
56
+
by methyl-abstraction from the amino(imino)metallylenes 51 and 54.
Fig. 7 Ellipsoid plot (30% level) of the germyliumylidene 53
+
(hydrogen
atoms and isopropyl groups have been omitted).
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between the Sn atom and the Li atom (Fig. 8). The Sn–N
imino
distances of 2.143(5) Å and 2.179(4) Å are longer compared with
that of the dmap adduct 55. The Li–N
imino
bond lengths of
1.946(9) Å and 2.004(9) Å are comparable to those reported for
the imino lithium dimer [L
Dip
NLi]
2
toluene (3toluene).
8
Compound 58 reacts with electrophiles such as I
2
and MeI to
form the oxidative addition products 59 and 60 which demon-
strates its stannylene character. For the bulky substrate Me
3
SiCl
the analogous formation of the stannane (Me
3
Si)ClSn(L
Dip
)
2
is
suppressed and L
Dip
SiMe
3
is formed along with the dimeric
chlorostannylene [L
Dip
SnCl]
2
(61). The bisiminochlorostannate
(58) may also act as an electrophile. Its conversion with MeLi
leads to the formation of Me-substituted stannate 62 that
exhibited a planar four-membered LiN
2
Sn ring in the XRD
analysis. Theoretical calculations on 58 revealed the high single-
bond character of the Sn–N
imino
interactions as concluded from
the comparison of the WBI values of 58 (0.43 and 0.44) with the
ones in 61 (0.24 and 0.24) which mark considerable dative-bond
character for the latter. Moreover, the computational study of the
natural population analysis (NPA) charge distribution in 58 and 61
shows that the Sn atom in 58 is less positively polarized (+1.22)
than that in 61 (+1.42). These theoretical results account for the
stannyl anion character of 58 as illustrated by the resonance
structure 58
A
(Scheme 14). However, the ambiphilic reactivity of
the tin(II)centrein58, that is, it functions as a nucleophile in the
synthesis of 59 and 60 andasanelectrophileintheconversion
to 62, has to be pointed out. It allows for the conclusion that the
compound (58) possesses high stannylenoid character and thus
represents a heavier congener of carbenoids.
Miscellaneous: survey of carbon chemistry
In the field of coordination chemistry the tethering of the
exocyclic imino-nitrogen atom of an N-heterocyclic imino group
to a carbon atom mostly serves the creation of tailor-made ligand
systems. These synthetic methods have been reviewed elsewhere.
2,3
They can be complemented by the report of Tamm and coworkers
in 2014 on the modified synthesis of the bisimine 1,2-(L
iPr
2
Me
2
N)
2
-
C
2
H
4
(L
iPr
2
Me
2
= 1,3-diisopropyl-4,5-dimethyl-imidazolin-2-ylidene,
Scheme 15), a chelate-fashioned ligand system which had been
described before in the year 2007.
29,30
Group 15 element complexes
Background
For the pnictogen family compounds of phosphorus with the
imidazolidin-2-imino group (saturated in the ligand backbone)
dominate the field. As outlined in the following section this
ligand system is often implemented for the stabilization of
phosphorus-centred radicals and suits the requirements for
the isolation of cationic species similar to the strongly related
imidazolin-2-imino group (unsaturated in the ligand backbone).
Scheme 14 Synthesis of bisiminostannylenoid 58 and reactivity with I
2
,
MeI, ClSiMe
3
and MeLi to products 59–62 (diox = 1,4-dioxane).
Fig. 8 Ellipsoid plot (30% level) of the stannylenoid 58 (hydrogen atoms
and isopropyl groups have been omitted).
Scheme 15 Synthesis of the bisimine compound 1,2-(L
iPr
2
Me
2
N)
2
-C
2
H
4
(L
iPr
2
Me
2
= 1,3-diisopropyl-4,5-dimethyl-imidazolin-2-ylidene) from an
imidazolium salt precursor.
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Most notably, pioneering work on imidazolin-2-imino-substituted
phosphanes was reported by Kuhnandcoworkersin1996and
1998.
31
Also, compounds with the imidazolin-2-imino structural
motifadjacenttoanitrogenatomare abundant. Examples include
common types of organic compounds such as azines of cyclic
ureas, cyclic bisguanidines, as well as triazenes and diazotates with
the corresponding C
3
N
2
five-membered ring backbone. These will
be discussed in the miscellaneous section of this review. Interest-
ingly, the respective chemistry of the heavier pnictogens remains
largely unexplored to date.
Phosphorus compounds
Phosphorus mononitrides and phosphinonitrenes have developed
into an established subgenre of the iminato ligand-stabilized
phosphorus chemistry and respective research was sparked
by Bertrand and coworkers in 2010. They reported the use of
the imidazolidin-2-imino lithium reagent (H
2
)L
Dip
NLi for the
synthesis of the phosphorus dichloride 63 which undergoes
reductive dehalogenation with magnesium in the presence of a
cyclic alkyl(amino) carbene (CAAC) to afford 64 (Scheme 16,
Fig. 9).
32
The authors demonstrated that this compound (64)can
be regarded as a molecular congener of phosphorus mononitride
stabilized by a CAAC as a ligand to the phosphorus atom and an
NHC at the P-bonded nitrogen atom (64
A
,Scheme16).This
resonance structure (64
A
) is reminiscent of the diphosphorus
compounds 65 and 66 that bear two NHC ligands or two CAACs,
respectively (Scheme 16).
33,34
The formulation 64 represents the phosphazabutadiene
character of the compound. In the
31
P NMR spectroscopic
analysis the chemical shift of 64 is observed at 134 ppm which
is shifted to a lower field with respect to the heavier congeners
65 and 66 (range: from 59 ppm to 74 ppm). As derived from
XRD analysis the geometry of 64 (Fig. 9) was described as trans-
bent with a short P–C
CAAC
bond (1.719(2) Å), as well as an N–C
NHC
distance (1.282(3) Å) that is in the range of C–N bond lengths of
imino groups (vide supra). The P–N distance of 1.7085(16) Å is
similar to that of typical P–N single bonds. Oxidation of 64 with
Ph
3
C[B(C
6
F
5
)
4
] (trityl tetrakis(pentafluorophenyl)borate) afforded
the radical cation 64
+
(Scheme 16).
32
This process is reversible as
was shown by the regeneration of uncharged 64 via reduction
with potassium graphite (KC
8
, Scheme 16). In the theoretical
analysis of 64 and 64
+
the shapes of the HOMO and the SOMO
(singly occupied molecular orbital), respectively, are very similar.
They majorly comprise a p* orbital of the PN group that shows
bonding interaction with a p-type orbital at the carbenic atom of
the NHC, as well as the CAAC ligand.
32
The EPR study of 64
+
in
frozen fluorobenzene at 100 K revealed g-tensors of g
x
= 2.0052,
g
y
=2.0087andg
z
=2.0028whicharecomparabletotherespective
values in 65
+
and 66
+
.
32–34
The scope of applications of N-heterocyclic imines in phos-
phorus chemistry was extended in 2011 when Bertrand and
coworkers reported the reduction of the bisimino compound
67[OTf] (Tf = triflyl) with KC
8
to the uncharged phosphinyl
radical 67(Scheme 17, Fig. 10).
35
Notably, the synthesis of
67[OTf] proceeds via the chloride salt 67[Cl] that could not be
isolated in analytically pure form at that time but is purified in
Scheme 16 Synthesis of phosphazabutadiene 64 and its conversion to the
radical cation 64
+
. Phosphorus mononitride formulation 64
A
. NHC- and
CAAC-stabilized diphosphorus complexes 65 and 66.
Fig. 9 Ball and stick representation of the phosphazabutadiene 64 as
derived from XRD analysis (hydrogen atoms and isopropyl groups have
been omitted).
Scheme 17 Conversion of the bisiminophosphonium salt 67[OTf] to the
phosphinyl radical 67. Synthesis of the imino(vanadyl)phosphinyl radical 69
via chlorophosphine 68. Vanadium-centre formulation 69
A
(Np = neopentyl).
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the course of the anion exchange (chloride vs. triflate,
Scheme 17).
35
The paramagnetic nature of 67was verified by
its EPR study at 100 K in frozen THF from which the g-tensors
g
x
= 2.0074, g
y
= 2.0062 and g
z
= 2.0024 were derived. A com-
parison with hyperfine coupling constants of atomic phos-
phorus revealed that an unpaired electron is primarily localized
on the 3p(P) orbital (62%) with a small contribution of the 3s(P)
orbital (2%).
Bertrand and coworkers extended their investigation of imino-
substituted phosphinyl radicals: the phosphorus dichloride 63
served as a precursor to the nitridovanadium-functionalized
phosphorus monochloride 68 (Scheme 17).
35
In an analogous
fashion as for 67the reduction of 68 with KC
8
furnished the
phosphinyl radical 69(Scheme 17).
35
From the EPR study of
69g-tensors of g
x
= 1.9726, g
y
= 2.0048 and g
z
= 1.9583 were
determined. Taking into account the hyperfine coupling con-
stants to
51
V, as well as
31
P it was concluded that the spin density
in 69mainly resides on the vanadium centre (67%) and is only
localized to a minor degree on the phosphorus atom and the
NHC moiety. In contrast, the spin density of the bisimino
derivative 67was found to reside with 62% in the 3p(P) orbital
and with 2% in the 3s(P) orbital. In line with the structural and
theoretical analysis the authors concluded that the bisimino
radical 67is a phosphorus centred radical with little spin
delocalization over the iminato ligands. The nitridovanadium
congener 69, however, is best represented by the canonical
structure 69
A
, that is a vanadium(IV) complex with a phosphin-
imide ligand.
In 2012 Betrand and coworkers reported the remarkable
transformation of the azido bisiminophosphane 70 to the
phosphinonitrene species 71 via irradiation at 254 nm
(Scheme 18).
36
As a starting material the bisiminophosphenium
salt 67[Cl] was used that had also been implemented in the
synthesis of the phosphinyl radical 67(Scheme 17).
35,36
The
theoretical analysis of the nitrene (71) suggested that the back
donation of a nitrogen lone-pair into accessible s*orbitalsatthe
phosphorus atom significantly contributes to the thermodynamic
stability of the compound. The phosphorus atom in 71 is the
centre of a trigonal-plane and the sum of the angles around the
P atom amounts to 3601(Fig. 11). Notably, the P–N
nitrene
bond
length of 1.457(8) Å in 71 is significantly shorter than the
P–N
imino
distances (1.618(8) Å, 1.629(8) Å), as well as the P–N
azido
distance of 1.895(11) Å in the precursor (70). This is in good
agreement with the upfield shift in the
31
P NMR spectrum of 71
(8 ppm) in comparison to 70 (111 ppm) which indicates the
multiple-bond character of the PN functionality. The phosphino-
nitrene (71) reacts with isopropyl isonitrile (iPrNC) to yield
the carbodiimide 72 that was not structurally characterized
(Scheme 18).
36
In consequence, the created NCNiPr group can
be abstracted from the phosphorus atom by implementing
isopropyltriflate as an alkylating agent. In the outcome the
starting material 67
+
is generated in the form of the triflate salt
(67[OTf], Scheme 18).
Bertrand and coworkers described the transformation of the
phosphinonitrene 71 to iminophosphonium triflates in 2013.
37
The methylation or protonation of 71 using methyltriflate or triflic
acid, respectively, furnished 73[OTf] or 74[OTf] (Scheme 19).
The PQN bond length of the phosphoranimine functionality in
74
+
amounts to 1.526(2) Å which is longer than the distance of
these atoms in the precursor 71 (vide supra, note that the struc-
tural parameters of 73
+
are not discussed due to poor data quality).
Fig. 10 Ellipsoid plot (30% level) of the bisiminophosphinyl radical 67
(hydrogen atoms and isopropyl groups have been omitted).
Scheme 18 Synthesis of phosphinonitrene 71 by photoirradiation of
azidophosphane 70 and conversion to carbodiimide 72.
Fig. 11 Ball and stick representation of the phosphinonitrene 71 as derived
from XRD analysis (hydrogen atoms and isopropyl groups have been omitted).
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On the other hand, the P–N
imino
distances in 74
+
are decreased
to 1.553(2) Å and 1.559(2) Å in comparison to 71 (vide supra).
This suggests a stronger interaction between the P-centre and
the imino nitrogen atoms and accounts for the potential of
N-heterocyclic imino systems in stabilizing cationic species.
The addition of water to 73[OTf] or 74[OTf] yielded the cationic
phosphine oxides 75[OTf] or 76[OTf], respectively (Scheme 19).
37
The expected electrophilic properties of 74
+
were verified by its
conversion with dmap that generated the Lewis acid base adduct
77[OTf] (Scheme 19).
37
Notably, the
31
P NMR chemical shift
of 77
+
was observed at a significantly higher field (1 ppm)
compared to the precursor 74
+
(73 ppm).
In 2014 the literature on the phosphinonitrene species (71)was
enriched by Bertrand and coworkers with their investigation of
coinage metal–nitrene compounds.
38
By conversion of 71 with a
corresponding equivalent of copper-orsilvertriflate(MOTf)the
respective complexes with bridging or terminal phosphinonitrene
ligands are generated (78–83,Scheme20).Thereactionof71 with
two equiv. of MOTf furnished the bimetallic complexes 78 or 79
with a bridging nitrenic atom. These showed similar structural
features in the solid state, that is, a planar coordination environ-
ment of the phosphorus atom and significantly increased
P–N
nitrenic
bond lengths (1.510(5) Å for 78 and 1.528(3) Å for 79)
with respect to 71.Furthermore,theP–N
imino
bond lengths are
shortened (1.573(3) Å for 78 and 1.561(3) Å for 79)and,vice versa,
the C–N
imino
distances are lengthened (range of 1.31–1.36 Å for 78
and 79) which indicates the stronger allocation of electron density
from the imidazolidin-2-imino system to the phosphorus atom
than in the precursor. After conversion of two equiv. of 71 with
MOTf the linear complexes 82 and 83 with terminal bis-
(phosphinonitrene) ligands were obtained.
38
Notably, the M–N
nitrenic
bond lengths (1.801(2) Å, 1.807(3) Å for 82 and 2.017(3) Å, 2.029(4) Å
for 83) in the linear complexes are decreased in comparison to the
bimetallic systems (1.817(3) Å for 78 and 2.080(3) Å, 2.086(3) Å
for 79). Interestingly, the conversion of the phosphinonitrene and
MOTf in a one to one ratio afforded 80 and 81, respectively, as
confirmed by NMR spectroscopic analysis. However, these com-
pounds were found in a dynamic equilibrium with their bridging
and terminal congeners (78,82 for 80 and 79,83 for 81).
38
A thorough study on the reactivity of the phosphinonitrene
71 was published in 2015.
39
The authors described its thermal
transformation to the iminophosphorane 84, as well as several
conversions with typical small molecule substrates (Scheme 21).
At elevated temperature quantitative rearrangement of 71 was
observed. The nitrenic atom inserts into a tertiary carbon CH
bond of an isopropyl side chain followed by migration of the Dip
moiety to the phosphorus centre to create the five membered
PNC
3
ring in 84. The addition of an excess amount of acetonitrile
to the phosphinonitrene (71) afforded a mixture (16 :1) of
the ketenimine 85 and the diazaphosphete 86 (Scheme 21).
39
Notably, the ketenimine is transformed into the diazaphosphete
at elevated temperature (90 1C). This process was reasoned by the
Scheme 19 Methylation and protonation of 71 to 73[OTf] and 74[OTf]
and their reactivity towards H
2
O and dmap to produce (75–77)[OTf].
Scheme 20 The formations of metal–nitrene complexes 78–83 from
phosphinonitrene 71.
Scheme 21 Thermal conversion of phosphinonitrene 71 to 84 and reactivity
with small molecules to imino complexes 85–91.
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initial deprotonation of acetonitrile by the nitrenic centre and
nucleophilic attack of the generated cyanomethylanion at the
phosphorus atom to afford 85. Subsequent cyclization and
proton migration leads to the formation of 86. Reaction of
the phosphinonitrene (71) with carbon dioxide or carbon
disulfide yields the isocyanate 87 or the isothiocyanate 88,
respectively (Scheme 21).
39
One should point out the cleavage
of the thermodynamically stable CQE double bond (E = O, S) in
this process. Compound 71 activates elemental sulfur (S
8
), as
well as white phosphorus (P
4
).
39
The reaction with S
8
furnishes
the phosphine sulfide 89 that bears a thiosulfinylamino group
at the phosphorus atom (Scheme 21). The conversion of 71 with
P
4
affords phosphorus enriched 90 with a unique P
5
N moiety
via insertion of the PN
nitrene
fragment into a P–P single bond of
the P
4
cluster (Scheme 21). With a slight excess of water the
phosphinonitrene (71) reacted to yield the aminophosphine
oxide 91 as the product of the addition of H
2
O to the PN
nitrene
bond (Scheme 21).
39
By implementing the imidazolin-2-imino trimethylsilane
92 Vidovic
´and coworkers synthesized the imino phosphorus
dichloride 93 that was reacted with carbodiphosphorane to
yield the phosphenium salt 94[Cl] (Scheme 22).
40
The latter was
subjected to chloride abstraction with two equiv. of silver
hexafluoroantimonate and in the outcome the dicationic phos-
phinimine 95[SbF
6
]
2
was formed (Scheme 22).
40
The dication
assumes a trans-bent structure motif and the P–N distance of
1.594(6) Å is significantly shorter than the respective bond
lengths in the CAAC congener 64 (1.7085(16) Å) and its radical
cation 64
+
(1.645(4) Å). This suggests relevant double bond
character for the PN fragment in 95
2+
. As concluded from the
theoretical analysis of the dication the authors attributed
the increased PN interaction to the removal of electrons from the
HOMO which majorly comprises the PN p* antibonding orbital.
Taking into account structural parameters such as the comparably
long C–N
imino
bond (1.367(8) Å) and bond polarizations derived
from the NBO analysis it was presumed that 95
2+
possesses
dicationic phosphorus mononitride character to a minor degree
(95
A2+
, Scheme 22). Regardless of the dominant resonance
structure of 95
2+
its isolation confirms the potential of the
imidazolin-2-imino ligand for stabilizing cationic species.
The application of (benz)imidazolin-2-imino substituents as
supporting groups for P-based ligands has recently emerged
as a subgenre of the field of phosphorus compounds of the
iminato ligand. The chemistry relies on the pioneering work
of Kuhn and coworkers, who converted L
Me
2
SiMe
3
(25) to the
imino dichlorophosphane 96 (Schemes 7 and 23).
31a
If treated
with AlCl
3
this compound reacts to yield the ditopic phosphenium
salt 97[AlCl
4
]
2
, the structural formulation of which was based on
NMR spectroscopic characteristics (Scheme 23).
31a
The authors
described that in solution 97
2+
is in equilibrium with 96
depending on the nucleophilic properties of the solvent. Moreover,
Kuhn and coworkers described the iminophosphanes 98 and 99,
as well as the conversion of 99 to the iminophosphorane 100
(Scheme 23).
31b
In 2015 Mallik, Panda and coworkers reported the imino
diphenylphosphine 101 that was converted to the borane
adduct 102, as well as the phosphorus chalcogenides 103–106
(Scheme 24).
41
Also in 2015 Dielmann and coworkers established the use
of imidazolin-2-imino-substituted phosphines as electron-rich
ligands to transition metals.
42
By lithiation of the benzimidazolin-
2-imine BL
iPr
NH (107,BL
iPr
= 1,3-diisopropylbenzimidazolin-2-
ylidene) and reaction with the corresponding chlorophosphines
the synthesis of the iminophosphines 108–113 was accomplished
(Scheme 25).
42
In addition, the conversion of the bulkier
L
Me
2
Mes
2
NSiMe
3
(114,L
Me
2
Mes
2
= 1,3-dimesityl-4,5-dimethyl-imid-
azolin-2-ylidene) with the respective chlorophosphines led to the
Scheme 22 Reaction of imino phosphorus dichloride to the phosphenium
salt 94[Cl] and its conversion to the dicationic phosphinimine 95
2+
. Phosphorus
mononitride formulation 95
A2+
.
Scheme 23 Preparation of the imino phosphorus dichloride 96 from the
imino trimethylsilane 25. Reaction to the dimeric phosphenium cation 97
2+
.
The iminophosphanes 98 and 99, as well as the iminophosphorane 100.
Scheme 24 Reaction of the imino diphenylphosphine 101 to the borane-
adduct 102, as well as the phosphorus chalcogenides 103–106.
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iminophosphines 115 and 116 in the outcome (Scheme 25).
42
To assess the electron-donor strength of these phosphorus-based
ligands the Tolman electronic parameters (TEP) of their nickel
tricarbonyl complexes were determined.
43
Moreover, they were
evaluated according to the Huynh’s method. The
13
C NMR-
spectroscopic shift of the carbene carbon of the BL
iPr
group in
the trans-{PdBr
2
(BL
iPr
)ligand} is sensitive to the donor strength of
the ligand, in which the carbene resonance of the BL
iPr
is down-
field shifted with increasing donor strength of the ligand trans
to the BL
iPr
group.
44
The Huynh’s parameters of the imino-
phosphines show the same qualitative trend as the TEP analysis.
As a result, many of the iminophosphines were found to be more
potent electron-pair donors than most electron-rich trialkyl-
phosphines. Remarkably, the authors concluded that the imino-
phosphines 111 and 112,aswellas115 and 116, are stronger
donor ligands than classical NHCs. In addition, the bisimine 116
was presumed to be a more potent donor ligand than the very
strongly electron donating abnormal NHCs exceeding the cap-
ability of monoimine 115, as well as the bis- and the trisimine
111 and 112. Consequently, it was reasoned that the imidazolin-
2-iminato ligand is a stronger p-electron donor than the related
benzimidazolin-2-iminato ligand.
Uncharged organosuperbases that comprise the imidazolidin-
2-imino fragment as a chiral bis(guanidine)iminophosphorane
were described by Takeda and Terada in 2013.
45
The respective
iminophosphonium salts 117HCl, 118HCl, 119HCl and 120HBr
were synthesized by conversion of respective aminoguanidinium
halides with phosphorus pentachloride in the presence of base
followed by acidic work-up (Scheme 26).
45
The stability of the
iminophosphonium hydrohalide salt, and thus the high
Brønsted basicity of the uncharged compounds, relies on the
properties of the iminophosphorane as an electron-rich oligo-
dentate ligand. The free base was not characterized but generated
by reaction with potassium tertbutoxide and used in situ for the
assessment of catalytic activity in the electrophilic amination of
tetralones with azodicarboxylate.
45
Notably, no particular reason
for the use of the imidazolidin-2-imino group instead of acyclic
guanidino functionalities was pointed out by the authors. We
assume that its implementation rather follows synthetic applic-
ability for furnishing the chiral bis(guanidine)iminophosphorane
species. Notably, the scope of catalytic applications of this
compound as a chiral uncharged organosuperbase was expanded
in recent years.
46
Miscellaneous: survey of related nitrogen compounds
Compounds in which the exocyclic nitrogen atom of an
N-heterocyclic imino fragment bonds to another nitrogen atom
are abundant in the literature but may be accounted for in the
field of classical organic chemistry rather than inorganic or
organometallic coordination chemistry which is the focus of
this review. They can be categorized into triazenes
47
(represen-
tative example: 121, Scheme 27), azines
18,48
(122, subcategory:
bisguanidines, Scheme 27), as well as diazotates
49
(123,
Scheme 28) and their azoimidazolium
50
derivatives (124[BPh
4
],
Scheme 28).
Scheme 25 Synthesis of benzimidazolin-2-imino-substituted phosphines
108–113, as well as the imidazolin-2-imino-substituted phosphines 115 and
116. (a) (1) nBuLi, THF, 78 1C; (2) chlorophosphine, room temperature.
(b) (1) nBuLi, THF, 78 1C; (2) [Fe(C
5
H
4
PCl
2
)
2
]. (c) 115:iPr
2
PCl, THF; 116:(1)PCl
3
,
THF, 78 1C; (2) iPrMgCl, THF, 78 1C.
Scheme 26 Preparation of the chiral imino phosphonium halides
(M)-117HCl–(M)-120HBr and the stereoisomer (P)-117HCl.
Scheme 27 Synthesis of representative examples for the compound classes:
triazenes (121)andazines(122).
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Imidazolyl triazenes (121) release dinitrogen under thermal
conditions which leads to the formation of imino organyls
whereas their exposure to acidic conditions generates a diazonium
species along with the imine. The latter accounts for the pro-
nounced proton affinity of the imidazolin-2-imino group. Apart
from applications in organic chemistry azines (122) and diazotates
(123), as well as their azoimidazolium spin-offs (124[BPh
4
]), are
found to be employed as ligands to main group elements or
transition metals in rare instances. The mechanism for the
formation of 124[BPh
4
] is proposed to involve AlCl
3
-mediated
oxygen abstraction to afford the dicationic diazonium compound,
followed by its azo coupling with mesitylene. As an interesting
difference in their bonding modes the C–N
imino
distance in azines
is generally shorter than in the reported diazotates. This suggests
high CN double bond character for the former and considerable
single bond character along with delocalization of positive charge
into the imidazoline ring for the latter. Similarly, the azoimid-
azolium cation 124
+
exhibits a comparably long C–N
imino
bond
length of 1.386(2) Å which indicates that the positive charge is
majorly distributed among the atoms of the five-membered ring
(Scheme 28, Fig. 12).
Conclusions
This survey of the coordination chemistry of main group element
complexes with N-heterocyclic imines shows that these ligands
are suitable for the isolation of otherwise elusive species (e.g.
AlQTe double bond, stannylenoid, phosphinonitrene). In particular,
they have proven valuable for the thermodynamic stabilization of
electron-deficient central atoms, and thus enabled the isolation
of rare types of low-coordinate cationic metal complexes (e.g.
cationic thioxoborane, germyliumylidene). The strongly electron-
donating character of the imidazolin-2-iminato ligand derives
from the efficient delocalization of cationic charge density into
the five-membered ring system. The lengthening of the C–N
imino
distance is an indicator for the allocation of electron density
by the ligand as it is often observed upon transformation of an
uncharged species into a cationic offspring. The exploration of
the phosphorus chemistry of this imino ligand demonstrates its
applicability for the stabilization of charged, as well as uncharged
phosphorus-centred radicals. Moreover, the electron-rich nature
of the imidazolin-2-imino group has resulted in a new class of
phosphines that bear supporting imino groups and act as highly
electron donating phosphorus-centred ligands.
The chemistry of N-heterocyclic iminato complexes of main
group elements is still in its infancy as compared to the wide-
spread field of metal amides.
51
However, the growing interest in
N-heterocyclic imines in recent years underlines their usefulness
as ancillary ligands and distinguishes them from other classes of
nitrogen-based ligand systems. Future work should study yet
unexplored complexes of the imidazoli(di)n-2-imino group with
heavier main group metals and focus on catalytic applications of
the respective systems.
Acknowledgements
Financial support of the WACKER Chemie AG, as well as
the European Research Council (SILION 637394) is gratefully
acknowledged.
Notes and references
1(a) N. Kuhn, H. Bohnen, J. Kreutzberg, D. Bla
¨ser and R. Boese,
J. Chem. Soc., Chem. Commun., 1993, 1136; (b)S.M.
Ibrahim Al-Rafia, A. C. Malcolm, S. K. Liew, M. J. Ferguson,
R. McDonald and E. Rivard, Chem. Commun., 2011, 47, 6987;
(c)S.M.IbrahimAl-Rafia,M.J.FergusonandE.Rivard,Inorg.
Chem., 2011, 50, 10543; (d) S. Kronig, P. G. Jones and M. Tamm,
Eur. J. Inorg. Chem., 2013, 2301; (e) Y. Wang, M. Y. Abraham,
R.J.Gilliard,Jr.,D.R.Sexton,P.WeiandG.H.Robinson,
Organometallics, 2013, 32, 6639.
2 N. Kuhn, M. Go
¨hner, M. Grathwohl, J. Wiethoff, G. Frenking
and Y. Chen, Z. Anorg. Allg. Chem., 2003, 629, 793.
3(a) A. G. Trambitas, T. K. Panda and M. Tamm, Z. Anorg.
Allg. Chem., 2010, 636, 2156; (b) Y. Wu and M. Tamm, Coord.
Chem. Rev., 2014, 260, 116.
4 N. Kuhn, R. Fawzi, M. Steimann, J. Wiethoff, D. Bla
¨ser and
R. Boese, Z. Naturforsch., 1995, 50b, 1779.
Scheme 28 Synthesis of representative examples for the compound classes:
diazotates (123) and azoimidazolium salts (124[BPh
4
]).
Fig. 12 Ellipsoid plot (30% level) of the azoimidazolium cation 124
+
(hydrogen
atoms have been omitted).
Review Article Chem Soc Rev
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This article is licensed under a
Creative Commons Attribution 3.0 Unported Licence.
View Article Online
This journal is ©The Royal Society of Chemistry 2016 Chem. Soc. Rev., 2016, 45,6327--6344 | 6343
5 N. Kuhn, U. Abram, C. Maichle-Mo
¨ßmer and J. Wiethoff,
Z. Anorg. Allg. Chem., 1997, 623, 1121.
6 N. Kuhn, R. Fawzi, M. Steimann, J. Wiethoff and G. Henkel,
Z. Anorg. Allg. Chem., 1997, 623, 1577.
7 N. Kuhn, R. Fawzi, M. Steimann and J. Wiethoff, Z. Anorg.
Allg. Chem., 1997, 623, 554.
8 D. Franz, E. Irran and S. Inoue, Dalton Trans., 2014,
43, 4451.
9 D. Franz and S. Inoue, Chem. – Asian J., 2014, 9, 2083.
10 D. Franz, E. Irran and S. Inoue, Angew. Chem., Int. Ed., 2014,
53, 14264.
11 (a) K. Dehnicke and F. Weller, Coord. Chem. Rev., 1997,
158, 103; (b) S. Courtenay, J. Y. Mutus, R. W. Schurko and
D. W. Stephan, Angew. Chem., Int. Ed., 2002, 41, 498;
(c) S. Courtenay, D. Walsh, S. Hawkeswood, P. Wei,
A. K. Das and D. W. Stephan, Inorg. Chem., 2007, 46, 3623;
(d) O. Alhomaidan, E. Hollink and D. W. Stephan, Organo-
metallics, 2007, 26, 3041; (e) M. H. Holthausen, I. Mallov and
D. W. Stephan, Dalton Trans., 2014, 43,15201;(f) K. Spannhoff,
R. Rojas, R. Fro
¨hlich, G. Kehr and E. Erker, Organometallics,
2011, 30, 2377; (g)K.Jaiswal,B.Prashanth,S.Ravi,K.R.
Shamasundar and S. Singh, Dalton Trans., 2015, 44, 15779.
12 (a) C. M. Ong, P. McKarns and D. W. Stephan, Organo-
metallics, 1999, 18, 4197; (b) K. Aparna, R. McDonald,
M. Ferguson and R. G. Cavell, Organometallics, 1999,
18, 4241; (c) Z.-X. Wang and Y.-X. Li, Organometallics,
2003, 22, 4900; (d) J. Guo, J.-S. Lee, M.-C. Foo, K.-C. Lau,
H.-W. Xi, K. H. Lim and C.-W. So, Organometallics, 2010,
29, 939; (e)C.V.Ca
´rdenas, M. A
´. M. Herna
´ndez and J.-M.
Gre
´vy, Dalton Trans., 2010, 39, 6441.
13 S. M. I. Al-Rafia, R. McDonald, M. J. Ferguson and E. Rivard,
Chem. – Eur. J., 2012, 18, 13810.
14 M. W. Lui, N. R. Paisley, R. McDonald, M. J. Ferguson and
E. Rivard, Chem. – Eur. J., 2016, 22, 2134.
15 Z.Yang,M.Zhong,X.Ma,S.De,C.Anusha,P.Parameswaran
and H. W. Roesky, Angew. Chem., Int. Ed., 2015, 54, 10225.
16 D. Franz and S. Inoue, Chem. – Eur. J., 2014, 20, 10645.
17 (a) D. Franz, T. Szilva
´si, E. Irran and S. Inoue, Nat. Commun.,
2015, 6, 10037; (b) D. Franz and S. Inoue, Dalton Trans.,
2016, 45, 9385.
18 J. M. Hopkins, M. Bowdridge, K. N. Robertson, T. S. Cameron,
H. A. Jenkins and J. A. C. Clyburne, J. Org. Chem., 2001,
66, 5713.
19 (a) M. Tamm, S. Randoll, T. Bannenberg and E. Herdtweck,
Chem. Commun., 2004, 876; (b) M. Tamm, D. Petrovic,
S. Randoll, S. Beer, T. Bannenberg, P. G. Jones and
J. Grunenberg, Org. Biomol. Chem., 2007, 5, 523.
20 S. Randoll, P. G. Jones and M. Tamm, Organometallics, 2008,
27, 3232.
21 S. Inoue and K. Leszczyn
´ska, Angew. Chem., Int. Ed., 2012,
51, 8589.
22 M. Asay, C. Jones and M. Driess, Chem. Rev., 2011, 111, 354.
23 G.-H. Lee, R. West and T. Mu
¨ller, J. Am. Chem. Soc., 2003,
125, 8114.
24 M. W. Lui, C. Merten, M. J. Ferguson, R. McDonald, Y. Xu
and E. Rivard, Inorg. Chem., 2015, 54, 2040.
25 (a) T. Ochiai, D. Franz, X.-N. Wu and S. Inoue, Dalton Trans.,
2015, 44, 10952; (b) T. Ochiai, T. Szilva
´si, D. Franz, E. Irran
and S. Inoue, Angew. Chem., Int. Ed., 2016, DOI: 10.1002/
anie.201605636.
26 (a) T. Ochiai, D. Franz, E. Irran and S. Inoue, Chem. – Eur. J.,
2015, 21, 6704; (b) T. Ochiai and S. Inoue, Phosphorus, Sulfur
Silicon Relat. Elem., 2016, 191, 624.
27 T. Ochiai, D. Franz, X.-N. Wu, E. Irran and S. Inoue, Angew.
Chem., Int. Ed., 2016, 55, 6983.
28 (a) A. Akkari, J. J. Byrne, I. Saur, G. Rima, H. Gornitzka and
J. Barrau, J. Organomet. Chem., 2001, 622, 190; (b) Y. Ding,
H. W. Roesky, M. Noltemeyer, H.-G. Schmidt and P. P.
Power, Organometallics, 2001, 20, 1190; (c) P. B. Hitchcock,
J. Hu, M. F. Lappert and J. R. Severn, Dalton Trans., 2004,
4193; (d) A. P. Dove, V. C. Gibson, E. L. Marshall,
H. S. Rzepa, A. J. P. White and D. J. Williams, J. Am. Chem.
Soc., 2006, 128, 9834; (e) S. L. Choong, C. Schenk, A. Stasch,
D. Dange and C. Jones, Chem. Commun., 2012, 48, 2504;
(f) R. Olejnı
´k, Z. Pade
ˇlkova
´, R. Mundil, J. Merna and
A. Ru
˚zˇic
ˇka, Appl. Organomet. Chem., 2014, 28, 405.
29 J. Volbeda, P. G. Jones and M. Tamm, Inorg. Chim. Acta,
2014, 422, 158.
30 D. Petrovic, T. Glo
¨ge, T. Bannenberg, C. G. Hrib, S. Randoll,
P. G. Jones and M. Tamm, Eur. J. Inorg. Chem., 2007, 3472.
31 (a) N. Kuhn, R. Fawzi, M. Steimann and J. Wiethoff, Chem.
Ber., 1996, 129, 479; (b) N. Kuhn, H. Kotowski and J. Wiethoff,
Phosphorus, Sulfur Silicon Relat. Elem., 1998, 133, 237.
32 R. Kinjo, B. Donnadieu and G. Bertrand, Angew. Chem.,
Int. Ed., 2010, 49, 5930.
33 Y. Wang, Y. Xie, P. Wie, R. B. King, H. F. Schaefer, III,
P. v. R. Schleyer and G. H. Robinson, J. Am. Chem. Soc., 2008,
130, 14970.
34 O. Back, B. Donnadieu, P. Parameswaran, G. Frenking and
G. Bertrand, Nat. Chem., 2010, 2, 369.
35 O. Back, B. Donnadieu, M. v. Hopffgarten, S. Klein,
R. Tonner, G. Frenking and G. Bertrand, Chem. Sci., 2011,
2, 858.
36 F. Dielmann, O. Back, M. Henry-Ellinger, P. Jerabek,
G. Frenking and G. Bertrand, Science, 2012, 337, 1526.
37 F. Dielmann, C. E. Moore, A. Rheingold and G. Bertrand,
J. Am. Chem. Soc., 2013, 135, 14071.
38 F. Dielmann, D. M. Andrada, G. Frenking and G. Bertrand,
J. Am. Chem. Soc., 2014, 136, 3800.
39 F. Dielmann and G. Bertrand, Chem. – Eur. J., 2015, 21, 191.
40 Y. K. Loh, C. Gurnani, R. Ganguly and D. Vidovic
´,Inorg.
Chem., 2015, 54, 3087.
41 K. Naktode, S. D. Gupta, A. Kundu, S. K. Jana, H. P. Nayek,
B. S. Mallik and T. K. Panda, Aust. J. Chem., 2015, 68, 127.
42 M. A. Wu
¨nsche, P. Mehlmann, T. Witteler, F. Buß,
P. Rathmann and F. Dielmann, Angew. Chem., Int. Ed.,
2015, 54, 11857.
43 C. A. Tolman, Chem. Rev., 1977, 77, 313.
44 H. V. Huynh, Y. Han, R. Jothibasu and J. A. Yang, Organo-
metallics, 2009, 28, 5395.
45 T. Takeda and M. Terada, J. Am. Chem. Soc., 2013,
135, 15306.
Chem Soc Rev Review Article
Open Access Article. Published on 08 August 2016. Downloaded on 15/05/2017 16:01:58.
This article is licensed under a
Creative Commons Attribution 3.0 Unported Licence.
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6344 |Chem. Soc. Rev., 2016, 45,6327--6344 This journal is ©The Royal Society of Chemistry 2016
46 (a) T. Takeda and M. Terada, Aust. J. Chem., 2014, 67, 1124;
(b) A. Kondoh, M. Oishi, T. Takeda and M. Terada, Angew.
Chem., Int. Ed., 2015, 54, 15836; (c) T. Takeda, A. Kondoh
and M. Terada, Angew. Chem., Int. Ed., 2016, 55, 4734.
47 (a) D. M. Khramov and C. W. Bielawski, Chem. Commun.,
2005, 4958; (b) D. M. Khramov and C. W. Bielawski, J. Org.
Chem., 2007, 72, 9407; (c) D. J. Coady, D. M. Khramov,
B. C. Norris, A. G. Tennyson and C. W. Bielawsky, Angew.
Chem., Int. Ed., 2009, 48, 5187; (d) A. G. Tennyson, D. M.
Khramov, C. D. Varnado Jr., P. T. Creswell, J. W. Kamplain,
V. M. Lynch and C. W. Bielawski, Organometallics, 2009,
28, 5142; (e) A. G. Tennyson, E. J. Moorhead, B. L. Madison,
J. A. V. Er, V. M. Lynch and C. W. Bielawski, Eur. J. Org.
Chem., 2010, 6277; ( f) R. J. Ono, Y. Suzuki, D. M. Khramov,
M. Ueda, J. L. Sessler and C. W. Bielawski, J. Org. Chem.,
2011, 76, 3239; (g) D. Jishkariani, C. D. Hall, A. Demircan,
B. J. Tomlin, P. J. Steel and A. R. Katritzky, J. Org. Chem.,
2013, 78, 3349; (h) S. Patil, K. White and A. Bugarin,
Tetrahedron Lett., 2014, 55, 4826; (i) F. W. Kimani and
J. C. Jewett, Angew. Chem., Int. Ed., 2015, 54, 4051.
48 (a) B. Bildstein, M. Malaun, H. Kopacka, K.-H. Ongania and
K. Wurst, J. Organomet. Chem., 1999, 572, 177; (b) M. Reinmuth,
C. Neuha
¨user,P.Walter,M.Enders,E.KaiferandH.-J.Himmel,
Eur. J. Inorg. Chem.,2011,83;(c) J. Tauchman, K. Hladı
´kova
´,
F. Uhlı
´k, I. Cı
´sar
ˇova
´and P. S
ˇte
ˇpnic
ˇka, New J. Chem., 2013,
37, 2019; (d) H. Herrmann, M. Reinmuth, S. Wiesner,
O. Hu
¨bner, E. Kaifer, H. Wadepohl and H.-J. Himmel, Eur.
J. Inorg. Chem., 2015, 2345.
49 (a) A. G. Tskhovrebov, E. Solari, M. D. Wodrich, R. Scopelliti
and K. Severin, Angew. Chem., Int. Ed., 2012, 51, 232;
(b) A. G. Tskhovrebov, E. Solari, M. D. Wodrich,
R. Scopelliti and K. Severin, J. Am. Chem. Soc., 2012,
134, 1471; (c) A. G. Tskhovrebov, B. Vuichoud, E. Solari,
R. Scopelliti and K. Severin, J. Am. Chem. Soc., 2013,
135, 9486.
50 A. G. Tskhovrebov, L. C. E. Naested, E. Solari, R. Scopelliti
and K. Severin, Angew. Chem., Int. Ed., 2015, 54, 1289.
51 (a) M. F. Lappert, P. P. Power, A. V. Protchenko and
A. L. Seeber, Metal Amide Chemistry, Wiley-VCH, Weinheim,
2009; (b) D. L. Kays, Chem. Soc. Rev., 2016, 45, 1004.
Review Article Chem Soc Rev
Open Access Article. Published on 08 August 2016. Downloaded on 15/05/2017 16:01:58.
This article is licensed under a
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