molecules
Article
Direct Base-Assisted C-H Cyclonickelation of
6-Phenyl-2,20-bipyridine
Nicolas Vogt 1, Vasily Sivchik 2,3,* , Aaron Sandleben 1, Gerald Hörner 4,5
and Axel Klein 1,*
1Department für Chemie, Institut für Anorganische Chemie, Universität zu Köln, Greinstraße 6,
2Department of Chemistry, University of Eastern Finland, 80101 Joensuu, Finland
3School of Chemistry, University of East Anglia, Earlham Road, Norwich NR4 7TJ, UK
4Institut für Chemie, Anorganische Chemie IV, Universität Bayreuth, Universitätsstraße 30,
D-95440 Bayreuth, Germany; gerald.hoerner@uni-bayreuth.de
5Theoretische Chemie, Technische Universität Berlin, Straße des 17. Juni 135, D-10623 Berlin, Germany
*Correspondence: v[email protected] (V.S.); [email protected] (A.K.); Tel.: +49-221-470-4006 (A.K.)
Academic Editor: Burgert Blom
Received: 4 February 2020; Accepted: 20 February 2020; Published: 24 February 2020
Abstract:
The organonickel complexes [Ni(Phbpy)X] (X =Br, OAc, CN) were obtained for the first
time in a direct base-assisted arene C(sp
2
)–H cyclometalation reaction from the rather unreactive
precursor materials NiX
2
and HPhbpy (6-phenyl-2,2
0
-bipyridine) or from the versatile precursor
[Ni(HPhbpy)Br
2
]
2
. Different from previously necessary C-Br oxidative addition at Ni(0), an extended
scan of reaction conditions allowed quantitative access to the title compound from Ni(II) on
synthetically useful timescales through base-assisted C-H activation in nonpolar media at elevated
temperature. Optimisation of the reaction conditions (various bases, solvents, methods) identified 1:2
mixtures of acetate and carbonate as unrivalled synergetic base pairs in the optimum protocol that
holds promise as a readily usable and easily tuneable access to a wide range of direct nickelation
products. While for the base-assisted C-H metalation of the noble metals Ru, Ir, Rh, or Pd, this
acetate/carbonate method has been established for a few years, our study represents the leap into the
world of the base metals of the 3d series.
Keywords: cyclometalation; cyclonickelation; C–H activation; organonickel; base-assisted
1. Introduction
Metal-driven C(sp
2
)-H activation and turnover of the resulting metalated species in catalytic
processes has for a long time been the domain of heavy elements from the 4d and 5d rows,
such as ruthenium, iridium and, especially, palladium. Platinum-promoted C-H activation of
hydrocarbons was one of the first milestones in this endeavour [
1
]. The intramolecular variant coined
as cyclometalation initiated a period of rapid knowledge growth, owing to favourable metalation
kinetics and thermodynamics [
2
,
3
]. Von Zelewsky et al. prepared the first biscycloplatinated derivatives
of 2-phenylpyridine by transmetalation of the carbolithiated precursor back in the 1980s as well as
discovered the intriguing luminescence of the obtained cyclometalated compounds [
4
]. Later, Constable
etal. introducedcycloruthenation,cycloplatination, andcyclopalladationthroughC-Hactivation(direct
cyclometalation) of aryl/N-donor moieties [
5
,
6
]. This straightforward synthetic approach coupled with
the potential diversity of
π
-conjugated derivatives of CˆN ligands to tailor photophysical properties led
to intensive development in the field with more than 1000 congeners published today [
7
–
19
]. For Pd(II)
salts (e.g., chloride or acetate) the direct cyclometalation of 2-phenylpyridine proceeds already at
room temperature [
6
,
14
,
20
]. Not surprisingly, a considerable number of
π
-conjugated CˆN-derivatives
Molecules 2020,25, 997; doi:10.3390/molecules25040997 www.mdpi.com/journal/molecules
Molecules 2020,25, 997 2 of 13
are reported with interesting photophysical and biomedical properties [
20
,
21
]. Moreover, the ease
of cyclopalladation resulted in the development of ground-breaking catalytic technologies based on
Pd-catalysed directed C−H functionalisation [22–25].
In sharp contrast to the literally thousands of examples available for Pd(II) and Pt(II),
cyclometalated Ni(II) derivatives of nitrogen-donating
π
-conjugated moieties have remained rare
species, although the first example of a cyclometalated Ni(II) complex, [Ni(Cp)(o-phenylazo)-phenyl)],
was prepared from [Ni(Cp)
2
] (Cp
−
=cyclopentadienide) in 1963 [
26
]. Van Koten et al. introduced
two approaches for the synthesis of Ni(NˆCˆN)-pincer complexes: (a) transmetalation of a Au(I)
precursor; or (b) oxidative addition of zero-valent nickel to the bromoaryl precursor [
27
]. Much later,
Wolczanski et al. directly metalated 2-phenylpyridine under harsh heating conditions (Scheme 1) [
28
],
whereas Klein et al. performed a two-step cyclometalation via Pd-directed ortho-bromination of
6-phenyl-2,2
0
-bipyridine followed by the oxidative addition of zero-valent nickel, akin to van Koten’s
approach [
29
,
30
]. Direct, base-assisted metalation with NiCl
2
was demonstrated by Gong and Song
in a pincer topology [
31
] and, in a similar approach by Zargarian et al. [
32
], it was found to be also
feasible in macrocyclic NˆCˆNˆC or NˆCˆNˆN setups [
33
]. These reactions were considered to be
electrophilic substitutions (H
+
versus Ni(II)) that are archetypal for Pt(II) and Pd(II) complexes [
1
,
25
]
but quite unique for Ni(II). Mechanistic insights were provided by van der Vlugt et al. for the formation
of a [Ni(CNP)Br] pincer complex from the tetrahedral [Ni(HCˆNˆP)Br
2
] precursor using NEt
3
as a
base (Scheme 2) [
34
]. The isolation of a tri-coordinated T-shaped Ni complex with weak (an)agostic
Ni
. . .
(H-C) interaction invokes C-H bond coordination to Ni(II), forming a sigma-complex as a key
step of the process with base neutralising the proton outside of the coordination sphere.
Molecules 2020, 25, x FOR PEER REVIEW 2 of 13
biomedical properties [20,21]. Moreover, the ease of cyclopalladation resulted in the development of
ground-breaking catalytic technologies based on Pd-catalysed directed C−H functionalisation [22–
25].
In sharp contrast to the literally thousands of examples available for Pd(II) and Pt(II),
cyclometalated Ni(II) derivatives of nitrogen-donating π-conjugated moieties have remained rare
species, although the first example of a cyclometalated Ni(II) complex, [Ni(Cp)(o-phenylazo)-
phenyl)], was prepared from [Ni(Cp)2] (Cp− = cyclopentadienide) in 1963 [26]. Van Koten et al.
introduced two approaches for the synthesis of Ni(N^C^N)-pincer complexes: (a) transmetalation of
a Au(I) precursor; or (b) oxidative addition of zero-valent nickel to the bromoaryl precursor [27].
Much later, Wolczanski et al. directly metalated 2-phenylpyridine under harsh heating conditions
(Scheme 1) [28], whereas Klein et al. performed a two-step cyclometalation via Pd-directed ortho-
bromination of 6-phenyl-2,2′-bipyridine followed by the oxidative addition of zero-valent nickel, akin
to van Koten’s approach [29,30]. Direct, base-assisted metalation with NiCl2 was demonstrated by
Gong and Song in a pincer topology [31] and, in a similar approach by Zargarian et al. [32], it was
found to be also feasible in macrocyclic N^C^N^C or N^C^N^N setups [33]. These reactions were
considered to be electrophilic substitutions (H+ versus Ni(II)) that are archetypal for Pt(II) and Pd(II)
complexes [1,25] but quite unique for Ni(II). Mechanistic insights were provided by van der Vlugt et
al. for the formation of a [Ni(CNP)Br] pincer complex from the tetrahedral [Ni(HC^N^P)Br2]
precursor using NEt3 as a base (Scheme 2) [34]. The isolation of a tri-coordinated T-shaped Ni complex
with weak (an)agostic Ni…(H‒C) interaction invokes C‒H bond coordination to Ni(II), forming a
sigma-complex as a key step of the process with base neutralising the proton outside of the
coordination sphere.
Scheme 1. Cyclonickelation of C(sp2)‒H bonds with monodentate and bidentate N-heterocyclic
directing groups.
Evidence for an inner-sphere attack of coordinated base was provided by Punji et al., who
proposed the anion exchange of chloride or acetate by bis(trimethylsilyl)amide as a step precluding
N-directed C‒H bond activation under solvent-free conditions [35] (Scheme 2). In keeping with this,
Musaev et al. suggested a cesium/carboxylate cluster as the active basic principle based on
computational studies [36]. The latter treatment appears to be the most comprehensive one to explain
the “carboxylate effect”, which is a recurrent motif in metalation at d8 metal centres, as reviewed a
while ago by Ackermann [37] and recently discussed in deep mechanistic detail by Macgregor et al.
for Ru, Ir, Rh, or Pd systems [38–40]. Schafer, Love et al. elegantly adopted this motif for the C‒H
activation of H3C‒N(Cy)C(O)N(H)(quinolin-8-yl), forming [Ni(‒CH2‒N(Cy)(O)N(quinolinyl)(PEt3)]
[41].
Scheme 1.
Cyclonickelation of C(sp
2
)-H bonds with monodentate and bidentate N-heterocyclic
directing groups.
Evidence for an inner-sphere attack of coordinated base was provided by Punji et al., who proposed
the anion exchange of chloride or acetate by bis(trimethylsilyl)amide as a step precluding N-directed
C-H bond activation under solvent-free conditions [
35
] (Scheme 2). In keeping with this, Musaev
et al. suggested a cesium/carboxylate cluster as the active basic principle based on computational
studies [
36
]. The latter treatment appears to be the most comprehensive one to explain the “carboxylate
effect”, which is a recurrent motif in metalation at d
8
metal centres, as reviewed a while ago by
Ackermann [
37
] and recently discussed in deep mechanistic detail by Macgregor et al. for Ru, Ir, Rh,
or Pd systems [
38
–
40
]. Schafer, Love et al. elegantly adopted this motif for the C-H activation of
H3C-N(Cy)C(O)N(H)(quinolin-8-yl), forming [Ni(-CH2-N(Cy)(O)N(quinolinyl)(PEt3)] [41].
Molecules 2020,25, 997 3 of 13
Herein, we describe the development of a user-friendly protocol for a quantitative direct,
base-assisted C(sp
2
)-H cyclonickelation procedure, relying on a simple ortho-phenyl pyridine system
and NiBr
2
as the starting material. As a generic prototype of (di)nitrogen-directing moieties, we
have selected 6-phenyl-2,20-bipyridine (HPhbpy), rendering the protocol suitable for a wide range of
bidentate and tridentate ligands. Bidentate directional group assisted C–H activations have recently
gained high interest in organometallic catalysis [
36
,
37
,
42
–
45
]. In a thorough and comprehensive
scan of the reaction conditions (solvent; counter ion; temperature; external base(s); Schemes 2
and 3) we identified the combination of high-boiling nonpolar solvents and acetate/carbonate as a
homogeneous/heterogeneous base couple to enable rather rapid and very efficient nickelation.
Molecules 2020, 25, x FOR PEER REVIEW 3 of 13
Herein, we describe the development of a user-friendly protocol for a quantitative direct, base-
assisted C(sp2)‒H cyclonickelation procedure, relying on a simple ortho-phenyl pyridine system and
NiBr2 as the starting material. As a generic prototype of (di)nitrogen-directing moieties, we have
selected 6-phenyl-2,2′-bipyridine (HPhbpy), rendering the protocol suitable for a wide range of
bidentate and tridentate ligands. Bidentate directional group assisted C–H activations have recently
gained high interest in organometallic catalysis [36,37,42–45]. In a thorough and comprehensive scan
of the reaction conditions (solvent; counter ion; temperature; external base(s); Schemes 2 and 3) we
identified the combination of high-boiling nonpolar solvents and acetate/carbonate as a
homogeneous/heterogeneous base couple to enable rather rapid and very efficient nickelation.
Scheme 2. Examples of base-assisted cyclonickelations and scope of this work.
Scheme 3. Cyclonickelation methods A–C and prepared compounds.
2. Results
We worked out different procedures for the direct C‒H cyclonickelation of 6-phenyl-2,2′-
bipyridine (HPhbpy) to yield the previously reported red complex [Ni(Phbpy)Br] (Scheme 3) [29,30].
The first synthesis attempts using NiBr2 and HPhbpy in boiling ethanol or methanol were
unsuccessful. After the initial dissolution of the precursor materials, the typical red colour was
observed. However, further reaction and isolation yielded grey-green materials. This is in line with
Scheme 2. Examples of base-assisted cyclonickelations and scope of this work.
Molecules 2020, 25, x FOR PEER REVIEW 3 of 13
Herein, we describe the development of a user-friendly protocol for a quantitative direct, base-
assisted C(sp2)‒H cyclonickelation procedure, relying on a simple ortho-phenyl pyridine system and
NiBr2 as the starting material. As a generic prototype of (di)nitrogen-directing moieties, we have
selected 6-phenyl-2,2′-bipyridine (HPhbpy), rendering the protocol suitable for a wide range of
bidentate and tridentate ligands. Bidentate directional group assisted C–H activations have recently
gained high interest in organometallic catalysis [36,37,42–45]. In a thorough and comprehensive scan
of the reaction conditions (solvent; counter ion; temperature; external base(s); Schemes 2 and 3) we
identified the combination of high-boiling nonpolar solvents and acetate/carbonate as a
homogeneous/heterogeneous base couple to enable rather rapid and very efficient nickelation.
Scheme 2. Examples of base-assisted cyclonickelations and scope of this work.
Scheme 3. Cyclonickelation methods A–C and prepared compounds.
2. Results
We worked out different procedures for the direct C‒H cyclonickelation of 6-phenyl-2,2′-
bipyridine (HPhbpy) to yield the previously reported red complex [Ni(Phbpy)Br] (Scheme 3) [29,30].
The first synthesis attempts using NiBr2 and HPhbpy in boiling ethanol or methanol were
unsuccessful. After the initial dissolution of the precursor materials, the typical red colour was
observed. However, further reaction and isolation yielded grey-green materials. This is in line with
Scheme 3. Cyclonickelation methods A–C and prepared compounds.
2. Results
We worked out different procedures for the direct C-H cyclonickelation of 6-phenyl-2,2
0
-bipyridine
(HPhbpy) to yield the previously reported red complex [Ni(Phbpy)Br] (Scheme 3) [29,30].
Molecules 2020,25, 997 4 of 13
The first synthesis attempts using NiBr
2
and HPhbpy in boiling ethanol or methanol were
unsuccessful. After the initial dissolution of the precursor materials, the typical red colour was
observed. However, further reaction and isolation yielded grey-green materials. This is in line
with earlier reports on the title complex [
29
,
30
] and related work on the complex [Ni(Mes)(bpy)Br]
(Mes =2,4,6-trimethylphenyl; bpy =2,2
0
-bipyridine), which decomposes in protic solvents yielding
greenish octahedral Ni(II) species [
46
]. Avoiding protic solvents and working in vigorously dried
solvents, we heated HPhbpy with NiBr
2
in toluene, but no yield was observed after 60 h at 130
◦
C
(Table 1, entry 1). In keeping with the few literature reports, we ascribe the lack of reactivity to the
lower basicity of the coordinated bromide ligand; similar observations have been reported by Campeau
et al. [
47
] for a Pd catalysed bimolecular arylation and by Maseras and Echavarren et al. [
48
] for an
intramolecular Pd-dependent arylation. In the following, we added external bases [
49
] such as NEt
3
,
NaHCO
3
, Na
2
CO
3
, or Cs
2
CO
3
to the reaction mixture (Entries 2–6). While for most reaction conditions
the yields of [Ni(Phbpy)Br] were very poor (up to 2%), for Na
2
CO
3
/90 h/reflux, a crude yield of 19%
was obtained (Table 1, Entry 6). In one of these experiments, MeOH was added to dissolve the starting
materials, and the reaction time could be tremendously shortened (Entry 3). However, yield was poor
due to decomposition, which was in line with the initial experiments. A further experiment in the melt
(Entry 4) showed no yield.
Table 1.
Essential parameters for the optimisation of the cyclonickelation of 6-phenyl-2,2
0
-bipyridine
(HPhbpy).
aNiX2Base (eq.) Solvent Time (h) T (◦C) Yield (%) Analysis
1ANiBr2no base toluene 60 111 g0visual b
2ANiBr2NEt3(11.5) toluene 48 111 g1.5 1H NMR c
3ANiBr2NaHCO3(2) toluene/MeOH 3 111 g2.5 1H NMR
4ANiBr2Na2CO3(4) solid 1 170 0 visual b
5BNiBr2Cs2CO3(2) toluene 66 111 gtrace 1H NMR
6BNiBr2Na2CO3(3) toluene 90 111 g19 1H NMR
7C[Ni(HPhbpy)Br2] NaOtBu (2) diethyl ether 16 23 0 1H NMR
8C[Ni(HPhbpy)Br2]KOAc/K2CO3
(2/4) toluene 16 111 g61H NMR
9C[Ni(HPhbpy)Br2]KOAc/K2CO3
(2/4) toluene 62 111 g28 1H NMR
10
ANi(OAc)2.4H2O -
toluene/EtOH//DME
15 111 5dXRD
11
ANi(OAc)2- solid 1 250 0 evisual b
12
ANi(OAc)2- solid 1 180 4fXRD
a
Method A: Strictly direct cyclonickelation using NiBr
2
or Ni(OAc)
2
.Method B: Cyclonickelation via in situ
formation of [Ni(HPhbpy)Br
2
]
2
.Method C: Starting from isolated [Ni(HPhbpy)Br
2
]
2
.
b
The target complex has a
characteristic red colour and can be easily traced visually in the reaction solution by the naked eye.
c
see Figures
S1–S4 in the Supplementary Materials.
d
[Ni(Phbpy)(OAc)] was obtained.
e
Red material recrystallised from HOAc
gave green-grey decomposed material.
f
Solid mixture heated while evaporated, trapped by NaCN in THF, and
recrystallised from CH
2
Cl
2
, yielding [Ni(Phbpy)(CN)].
g
The boiling temperature (reflux) is reported. Further details
in Table S1 in the Supplementary Materials.
During this series of experiments, we noticed the quantitative formation of the non-cyclometalated
1:1 adduct [Ni(HPhbpy)Br
2
]
2
upon heating NiBr
2
and HPhbpy in THF [
50
]. We were able to grow single
crystals of this compound. Single-crystal XRD revealed a binuclear structure with an almost isotropic
Ni
2
(
µ
-Br)
2
diamond core with two triplet Ni(II) centres. Two other bromides are terminal ligands
completing a five-coordinated trigonal bipyramidal geometry around nickel. (Figure 1; pertinent
metrical data in Table S2 in the Supplementary Materials). In the following work, [Ni(HPhbpy)Br
2
]
2
proved to be beneficial as a convenient, well-defined, and long-term storable form and was used as
precursor (Table 1, Entries 7–9).
Molecules 2020,25, 997 5 of 13
Molecules 2020, 25, x FOR PEER REVIEW 5 of 13
Figure 1. Ellipsoid view (two asymmetric units resulting in a binuclear structure) of [Ni(HPhbpy)Br
2
]
2
(left) and crystal structure of [Ni(HPhbpy)Br
2
]
2
viewed along the a axis (right). Ellipsoids are drawn
at the 50% probability level.
Starting from Ni(OAc)
2.
4H
2
O, the acetato complex [Ni(Phbpy)(OAc)] was obtained in traces
(Entry 10), and the crystal and molecular structure could be solved (Figure 2, crystal and molecular
data in Table S3 in the Supplementary Materials). The coordination geometry around Ni(II) is
distorted square-planar.
Figure 2. Ellipsoid view of [Ni(Phbpy)(OAc)] (left) and crystal structure viewed along the c axis
(right). Ellipsoids are drawn at the 50% probability level.
In a further experiment, anhydrous nickel acetate was used in a solid-state reaction to produce
the acetato complex, but the product could not be isolated due to the degradation by acetic acid (Entry
11). Quenching the same reaction mixture with cyanide yielded some crystals of [Ni(Phbpy)(CN)]
(Entry 12, Figure S6, Table S4 and further details in the Supplementary Materials). Unfortunately, the
crystal quality was too low for a satisfying crystal structure solution and refinement. However, the
atom connectivity is unequivocal. Thus, in contrast to Br
−
, acetate can obviously act as base.
Based on the encouraging 19% yield obtained from the reaction starting from the in situ
formation of [Ni(HPhbpy)Br
2
]
2
(Table 1, Entry 6) we started a series of reactions using isolated
[Ni(HPhbpy)Br
2
]
2
. The reaction progress was monitored using UV-vis absorption spectroscopy with
the spectrum of the previously reported [Ni(Phbpy)Br] [29] as reference. Using a 1:2 ratio and
equimolar amounts (regarding the precursor) of KOAc and K
2
CO
3
, we achieved a yield of 43% within
three days of heating in toluene (Table 2). The yields further increased to 91% upon prolonged heating
in chlorobenzene, reaching 98% in 1,2-dichlorobenzene and quantitative yield in p-xylene within 25
h (Figure 3). The use of the high-boiling benzonitrile did not yield the product. After 20 h, no yield
was observed and further heating at 200 °C for more than 60 h yielded a brown precipitated material
that does not contain the targeted complex. We assume that the coordinating abilities of benzonitrile
Figure 1.
Ellipsoid view (two asymmetric units resulting in a binuclear structure) of [Ni(HPhbpy)Br
2
]
2
(
left
) and crystal structure of [Ni(HPhbpy)Br
2
]
2
viewed along the aaxis (
right
). Ellipsoids are drawn at
the 50% probability level.
Starting from Ni(OAc)
2.
4H
2
O, the acetato complex [Ni(Phbpy)(OAc)] was obtained in traces
(Entry 10), and the crystal and molecular structure could be solved (Figure 2, crystal and molecular
data in Table S3 in the Supplementary Materials). The coordination geometry around Ni(II) is
distorted square-planar.
Molecules 2020, 25, x FOR PEER REVIEW 5 of 13
Figure 1. Ellipsoid view (two asymmetric units resulting in a binuclear structure) of [Ni(HPhbpy)Br
2
]
2
(left) and crystal structure of [Ni(HPhbpy)Br
2
]
2
viewed along the a axis (right). Ellipsoids are drawn
at the 50% probability level.
Starting from Ni(OAc)
2.
4H
2
O, the acetato complex [Ni(Phbpy)(OAc)] was obtained in traces
(Entry 10), and the crystal and molecular structure could be solved (Figure 2, crystal and molecular
data in Table S3 in the Supplementary Materials). The coordination geometry around Ni(II) is
distorted square-planar.
Figure 2. Ellipsoid view of [Ni(Phbpy)(OAc)] (left) and crystal structure viewed along the c axis
(right). Ellipsoids are drawn at the 50% probability level.
In a further experiment, anhydrous nickel acetate was used in a solid-state reaction to produce
the acetato complex, but the product could not be isolated due to the degradation by acetic acid (Entry
11). Quenching the same reaction mixture with cyanide yielded some crystals of [Ni(Phbpy)(CN)]
(Entry 12, Figure S6, Table S4 and further details in the Supplementary Materials). Unfortunately, the
crystal quality was too low for a satisfying crystal structure solution and refinement. However, the
atom connectivity is unequivocal. Thus, in contrast to Br
−
, acetate can obviously act as base.
Based on the encouraging 19% yield obtained from the reaction starting from the in situ
formation of [Ni(HPhbpy)Br
2
]
2
(Table 1, Entry 6) we started a series of reactions using isolated
[Ni(HPhbpy)Br
2
]
2
. The reaction progress was monitored using UV-vis absorption spectroscopy with
the spectrum of the previously reported [Ni(Phbpy)Br] [29] as reference. Using a 1:2 ratio and
equimolar amounts (regarding the precursor) of KOAc and K
2
CO
3
, we achieved a yield of 43% within
three days of heating in toluene (Table 2). The yields further increased to 91% upon prolonged heating
in chlorobenzene, reaching 98% in 1,2-dichlorobenzene and quantitative yield in p-xylene within 25
h (Figure 3). The use of the high-boiling benzonitrile did not yield the product. After 20 h, no yield
was observed and further heating at 200 °C for more than 60 h yielded a brown precipitated material
that does not contain the targeted complex. We assume that the coordinating abilities of benzonitrile
Figure 2.
Ellipsoid view of [Ni(Phbpy)(OAc)] (
left
) and crystal structure viewed along the caxis (
right
).
Ellipsoids are drawn at the 50% probability level.
In a further experiment, anhydrous nickel acetate was used in a solid-state reaction to produce
the acetato complex, but the product could not be isolated due to the degradation by acetic acid (Entry
11). Quenching the same reaction mixture with cyanide yielded some crystals of [Ni(Phbpy)(CN)]
(Entry 12, Figure S6, Table S4 and further details in the Supplementary Materials). Unfortunately, the
crystal quality was too low for a satisfying crystal structure solution and refinement. However, the
atom connectivity is unequivocal. Thus, in contrast to Br−, acetate can obviously act as base.
Based on the encouraging 19% yield obtained from the reaction starting from the in situ formation
of [Ni(HPhbpy)Br
2
]
2
(Table 1, Entry 6) we started a series of reactions using isolated [Ni(HPhbpy)Br
2
]
2
.
The reaction progress was monitored using UV-vis absorption spectroscopy with the spectrum of
the previously reported [Ni(Phbpy)Br] [
29
] as reference. Using a 1:2 ratio and equimolar amounts
(regarding the precursor) of KOAc and K
2
CO
3
, we achieved a yield of 43% within three days
of heating in toluene (Table 2). The yields further increased to 91% upon prolonged heating in
chlorobenzene, reaching 98% in 1,2-dichlorobenzene and quantitative yield in p-xylene within 25 h
(Figure 3). The use of the high-boiling benzonitrile did not yield the product. After 20 h, no yield
Molecules 2020,25, 997 6 of 13
was observed and further heating at 200
◦
C for more than 60 h yielded a brown precipitated material
that does not contain the targeted complex. We assume that the coordinating abilities of benzonitrile
hamper the reaction by forming nitrile complexes [
46
], thus interfering with the formation of the
intermediate [Ni(HPhbpy)Br
2
]
2
. This is in line with low yields for Ni-catalysed C(sp
2
)
−
H and C(sp
3
)
−
H
functionalisation of aminoquinoline in polar solvents such as DMSO, dioxane, and MeCN [51].
Table 2. Solvent influence on the optimised cyclonickelation reaction of HPhbpy a.
Entry Solvent Time (h) Tbat (◦C) Tb.p. (◦C) EN
T
bYield (%)
13 toluene 72 120 111
0.099
43
14 chlorobenzene 64 160 131
0.188
91
15 1,2-dichlorobenzene 25 190 180
0.225
98
16 p-xylene 25 160 138
0.074
100
17 benzonitrile 20 200 190
0.333
0
18 p-xylene c(from NiBr2) 64 160 138
0.074
93
19 1,2-dichlorobenzene d(no base) 64 190 180
0.225
15
a
Using [Ni(HPhbpy)Br
2
]
2
(65 mg, 0.14 mmol), KOAc (28 mg, 0.28 mmol), and K
2
CO
3
(39 mg, 0.54 mmol), Dean–Stark
apparatus, or activated molecular sieve. Yields as observed from UV-vis absorption measurements of the reaction
solutions were based on the reported molar extinction coefficients of the long-wavelength maximum around 510
nm of the target complex [Ni(Phbpy)Br] [
29
,
30
].
bEN
T
=normalised empirical parameter of solvent polarity [
52
].
c
Reaction starting from anhydrous NiBr2.dReaction without added base.
Molecules 2020, 25, x FOR PEER REVIEW 6 of 13
hamper the reaction by forming nitrile complexes [46], thus interfering with the formation of the
intermediate [Ni(HPhbpy)Br2]2. This is in line with low yields for Ni-catalysed C(sp2)−H and
C(sp3)−H functionalisation of aminoquinoline in polar solvents such as DMSO, dioxane, and MeCN
[51].
Table 2. Solvent influence on the optimised cyclonickelation reaction of HPhbpy a.
Entry Solvent Time (h) Tbat (°C) Tb.p. (°C) 𝐸
b Yield (%)
13 toluene 72 120 111 0.099 43
14 chlorobenzene 64 160 131 0.188 91
15 1,2-dichlorobenzene 25 190 180 0.225 98
16 p-xylene 25 160 138 0.074 100
17 benzonitrile 20 200 190 0.333 0
18 p-xylene c (from NiBr2) 64 160 138 0.074 93
19 1,2-dichlorobenzene d (no base) 64 190 180 0.225 15
a Using [Ni(HPhbpy)Br2]2 (65 mg, 0.14 mmol), KOAc (28 mg, 0.28 mmol), and K2CO3 (39 mg, 0.54
mmol), Dean–Stark apparatus, or activated molecular sieve. Yields as observed from UV-vis
absorption measurements of the reaction solutions were based on the reported molar extinction
coefficients of the long-wavelength maximum around 510 nm of the target complex [Ni(Phbpy)Br]
[29,30]. b 𝐸
= normalised empirical parameter of solvent polarity [52]. c Reaction starting from
anhydrous NiBr2. d Reaction without added base.
0 102030405060
0
20
40
60
80
100
1,2-dichlorobenzene
p-xylene
p-xylene, from NiBr
2
chlorobenzene
1,2-dichlorobenzene, no base
yield / %
time / h
Figure 3. Yields of the cyclometalation reaction in various solvents, as monitored using UV-vis
absorption spectroscopy. The traces represent the reaction entries 14, 15, 16, 18 and 19 in Table 2.
A detailed look on the time-dependence of the reaction for selected examples (Figure 3) shows
that p-xylene and 1,2-dichlorobenzene reach high yields already after about 20 h, while
chlorobenzene requires at least 40 h. Using NiBr2 instead of [Ni(HPhbpy)Br2]2 for the reaction in p-
xylene under the same conditions afforded only 18% yield after 20 h but could be brought to 93%
yield when heating for 64 h. This observation is in line with the assumed pre-formation of the p-
xylene-soluble [Ni(HPhbpy)Br2]2 from insoluble NiBr2. Finally, a blank experiment in 1,2-
dichlorobenzene (no external base) delivered a mediocre 15% yield after 64 h.
3. Discussion
Summarising the experimental results shows that (i) protic solvents and water-containing Ni
salts must be avoided. Although they speed up reaction times due to perfect dissolution of the
precursor compounds especially simple Ni(II) salts, the final product is quenched, presumably
forming non-cyclometalated hexacoordinate Ni(II) species. (ii) The acetate/carbonate mixture is
Figure 3.
Yields of the cyclometalation reaction in various solvents, as monitored using UV-vis
absorption spectroscopy. The traces represent the reaction entries 14, 15, 16, 18 and 19 in Table 2.
A detailed look on the time-dependence of the reaction for selected examples (Figure 3) shows
that p-xylene and 1,2-dichlorobenzene reach high yields already after about 20 h, while chlorobenzene
requires at least 40 h. Using NiBr
2
instead of [Ni(HPhbpy)Br
2
]
2
for the reaction in p-xylene under
the same conditions afforded only 18% yield after 20 h but could be brought to 93% yield when
heating for 64 h. This observation is in line with the assumed pre-formation of the p-xylene-soluble
[Ni(HPhbpy)Br
2
]
2
frominsolubleNiBr
2
. Finally, a blankexperiment in 1,2-dichlorobenzene(no external
base) delivered a mediocre 15% yield after 64 h.
3. Discussion
Summarising the experimental results shows that (i) protic solvents and water-containing Ni
salts must be avoided. Although they speed up reaction times due to perfect dissolution of the
precursor compounds especially simple Ni(II) salts, the final product is quenched, presumably forming
non-cyclometalated hexacoordinate Ni(II) species. (ii) The acetate/carbonate mixture is capable of
driving the C-H metalation to completion, whereas single-component bases such as CO
32−
, HCO
3−
,
Molecules 2020,25, 997 7 of 13
−
OtBu, and NEt
3
promote the reaction on a trace level. (iii) The reaction proceeds preferably via the
formation of the NˆN-coordinated intermediate [Ni(HPhbpy)Br
2
]
2
. (iv) This precursor allows the use
of high boiling nonpolar solvents, while protic solvents and water-containing polar solvents, which are
necessary to dissolve the NiBr2precursor, can be avoided.
The observation of high reaction temperatures suggests a high activation barrier for the direct
cyclonickelation of N-donor/aryl systems. Similarly, cyclonickelation to form CˆN or NˆCˆN chelates
required harsh heating [
28
,
31
,
32
]. High reaction temperatures seem to be the price for starting
from rather unreactive Ni(II) precursors and a not very reactive C(sp
2
)–H bond. In addition, the
base-assisted N–H
2
C(sp
3
)–H nickelation yielding [Ni(-CH
2
-N(Cy)(O)N(quinolinyl)(PEt
3
)] required
temperatures above 70
◦
[
41
]. In contrast to this, the CˆNˆP nickelation in the recently reported
complex [Ni(HPh-Py-O-PR
2
)Br
2
] (R =tBu) proceeds at 50
◦
C in an hour [
34
]. The significantly lower
activation barrier can be tentatively explained by the following factors: (i) the P-donor brings the
Ni(II) centre closer to the C–H activation site to enable (an)agnostic interactions, since phosphorus
has a bigger covalent radius versus nitrogen (107 pm versus 71 pm), resulting in a Ni–P bond length
of approximately 2.29 Å compared with the Ni–N bond of approximately 2.04 Å; (ii) this P-donor
(decorated by two tBu groups) is a much stronger
σ
-donor and perhaps a better
π
-acceptor than the
N-site in our study. The Ni
. . .
C distance in our precursor is 2.97(1) Å, which is quite short compared
to the 3.039(3) Å of the phosphine complex. However, this distance is largely depending on the
angle of the freely rotating phenyl group toward the binding plane, and the values represent only
the solid state and not the molecules in solution. Moreover, the Ni–C bond in the cyclonickelated
species [Ni(Ph-Py-O-PR
2
)Br] [
34
], [Ni(Phbpy)Br] [
29
], and [Ni(Phbpy)OAc] (Table S3) is of the same
magnitude (approximately 1.9 Å). Therefore, the crystallographic data suggest that the difference in
the electronic properties of the P- and N-donors rather than steric effects are responsible for the much
lower activation barrier.
It is tempting to correlate the enhanced reaction efficiency with the high boiling points of
these solvents, indicating simple Arrhenius behaviour. However, the complete lack of reactivity in
benzonitrile as a solvent with a similarly high boiling point indicates that aspects of solvent polarity
and coordination ability also affect the reaction progress. This generally agrees with the recent work of
Sandford et al., who reported low cyclonickelation yields in polar-coordinating solvents [
51
]. In our
study, alongside the series of solvents, an increase in polarity (
EN
Tvalues)
[
52
] leads to a decrease
in activity. This inverse relationship points to a nonpolar transition state of the cyclometalation
reaction, in stark contrast to the findings of Davies and Macgregor et al. [
39
], who inferred from
substituent Hammett plots a substantial accumulation of positive charge in the transition state of a
cycloruthenation reaction. Thus, in future work, we will model this reaction using density functional
theory (DFT) methods.
4. Materials and Methods
4.1. Materials
Commercially available chemicals were purchased from Sigma-Aldrich, Acros, ABCR, or
Fisher-Scientific and were used without further purification. Dry THF was obtained from distillation
over sodium/potassium alloy. The preparation of N-(1-(2-pyridyl)-1-oxo-2-ethyl)pyridinium-iodide
(Kroehnke salt) and 6-Phenyl-2,20-bipyridine (HPhbpy) is described in the Supplementary Materials.
Preparation of [Ni(HPhbpy)Br2]2
(adopted from Yang et al.) [
50
]: NiBr
2
(328 mg, 1.50 mmol),
HPhbpy (348 mg, 1.50 mmol) in THF (125 mL) were heated under reflux for 15 h. After cooling to
room temperature, a yellow solid was collected by filtration on a glass frit, washed using diethyl
ether (3
×
10 mL), and dried in vacuo for 15 h. Yield: 72% (487 mg). Elemental analysis found (calc.
for C16H12Br2N2Ni) C 42.62 (42.63); H 2.86 (2.68); N 6.23 (6.21)%.
Molecules 2020,25, 997 8 of 13
4.2. Cyclonickelation Experiments
General considerations:
In comparison with Pt or Pd congeners, cyclometalated Ni(II) complexes
are less stable due to weaker Ni-C and Ni-N bonds [
38
,
53
]. The Ni–N bond, being also weaker than
Ni–P bonds, is susceptible to facile ligand substitution by oxygen donors (e.g., alcohols) even in the
case of chelating NˆN-donors. [
46
]. The Ni–C bond is also chemically unstable. In addition to facile
hydrolysis [
46
], oxygen insertion can occur in the presence of dioxygen with the formation of C-O-Ni
bonds [
54
,
55
]. Thus, the cyclonickelation in this study was conducted under N
2
atmosphere in solvents
purified by distillation over Na-benzophenone ketyl (toluene or THF) or rigorously dried molecular
sieves (chlorobenzene, 1,2-dichlorobenzene, and benzonitrile).
Preparation of [Ni(Phbpy)Br] via C-H-activation:
After a number of optimisation experiments
(Tables 1and 2), the reaction was carried out as follows. Under inert atmosphere [Ni(HPhbpy)Br
2
]
2
(112.7 mg, 0.25 mmol, 1 eq.) was suspended in p-xylene (140 mL). After adding KOAc (25.5 mg,
0.25 mmol, 1 eq.) and K
2
CO
3
(34.6 mg, 0.25 mmol, 1 eq.), the mixture was heated to reflux. For the
separation of forming water, the reaction flask was interconnected by a glass frit filled with activated
molecular sieveto therefluxcondenser. After25h, the mixture wasallowed tocool to roomtemperature,
and the solvent was evaporated under reduced pressure. The deep red residue was dissolved in dry
CH
2
Cl
2
(100 mL) and filtered through a plug of Na
2
SO
4
/Celite. After evaporating the solvent, the
product was received as a red powder. The yield was determined from the UV-vis absorption band of
the product [Ni(Phbpy)Br] in THF (see Figure S5). The isolated material (183 mg, 0.495 mmol, 99%)
was identified through its 1H NMR spectrum [29,30].
From previous work, it was known that the [Ni(Phbpy)Br] is somewhat stable and soluble in
CH
2
Cl
2
(
1
H NMR) and THF (reaction medium, crystallisation solvent). The complex has a characteristic
red colour [
29
,
30
]. In the course of our investigation, we found that [Ni(Phbpy)Br] is insoluble in
diethyl ether. Profiting from its moderate solubility and stability in acetone,
1
H and
1
H-
1
H COSY NMR
spectra was measured in acetone-d
6
. Additionally, the compound can be recrystallised by the vapour
diffusion of diethyl ether to CH
2
Cl
2
or acetone at room temperature without special precautions. At the
final stage, the red crystals can be quickly washed using methanol to eliminate the contaminant (yellow
powder, presumably NiBr
2
). Yet, [Ni(Phbpy)Br] degrades in methanol solution, as can be seen from the
colour change from red to yellow and green, and
1
H NMR (CD
3
OD) gave very broad signals indicating
the formation of paramagnetic species. Details on the cyclometalation reactions yielding [Ni(Phbpy)X]
(X =Br, OAc, CN) through base-assisted C-H activation (direct cyclometalation) are available in the
Supplementary Materials.
Instrumentation: 1
H,
13
C and
19
F NMR spectra were recorded on a Bruker Avance II 300 MHz
(
1
H: 300 MHz,
13
C: 75 MHz,
19
F: 282 MHz, Ettlingen, Germany) equipped with a double resonance
(BBFO) 5 mm observe probehead with z-gradient coil or on Bruker Avance 400 spectrometer (
1
H:
400 MHz,
13
C: 100 MHz, Ettlingen, Germany). Chemical shifts were relative to TMS. UV–vis absorption
spectra were recorded on a Varian Cary 05E spectrophotometer (Troisdorf, Germany). Elemental
analyses were obtained using a HEKAtech CHNS EuroEA 3000 analyzer (Wegberg, Germany).
EI-MS spectra were measured using a Finnigan MAT 95 mass spectrometer (Bremen, Germany).
Simulations were performed using ISOPRO 3.0 (Sunnyvale, CA, USA). Single crystal structure analysis
(XRD): Measurement of [Ni(HPhbpy)Br
2
]
2
was performed at 170(2) K using an IPDS IIT (STOE
and Cie., Darmstadt, Germany) diffractometer, with Mo-K
α
radiation (
λ
=0.71073 Å) employing
ω
-
ϕ
-2
θ
scan technique. The structure was solved by direct methods using SIR 2014 [
56
], and
refinement was carried out with SHELXL 2018 employing full-matrix least-squares methods on F
02
≥
2
σ
(F
02
) [
57
]. The numerical absorption corrections (X-RED V1.22; Stoe & Cie, 2001, Darmstadt,
Germany) were performed after optimising the crystal shapes using X-SHAPE V1.06 (Stoe & Cie, 1999,
Darmstadt, Germany) [
58
]. The non-hydrogen atoms were refined with anisotropic displacement
parameters without any constraints. The hydrogen atoms were included by using appropriate riding
models. Measurement of [Ni(Phbpy)(OAc)] was conducted at 150 K. The X-ray diffraction data were
collected with Bruker Kappa Apex-II (Ettlingen, Germany) diffractometer by using Mo-K
α
radiation
Molecules 2020,25, 997 9 of 13
(
λ
=0.71073 Å). The APEX2 [
59
] program package was used for cell refinements and data reduction.
Using OLEX2 [
60
], the structure was solved with theSHELXS program structure solution program using
direct methods and refined with the SHELXL refinement package using least squares minimisation [
61
].
A numerical absorption correction (SADABS) [
62
] was applied to all data. The non-hydrogen atoms
were refined with anisotropic displacement parameters without any constraints. All H atoms were
positioned geometrically and constrained to ride on their parent atoms, with C
−
H=0.95
−
0.99 Å and
U
iso
=1.2
−
1.5 U
eq
(parent atom). Data of both structure solutions and refinements can be obtained
free of charge at https://summary.ccdc.cam.ac.uk/structure-summary-form or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-336-033 or
2
]
2
) and 1956739 ([Ni(Phbpy)(OAc)]).
5. Conclusions
Herein, we have addressed the central problem of a direct metalation of arene C(sp
2
)-H functions
using rather unreactive Ni(II) precursors such as NiBr
2
or Ni(OAc)
2
, which lifts the limitation of
user-friendlymetalationroutinesto the 4dand5d rows. Thetargetorganonickelcomplex [Ni(Phbpy)Br]
(HPhbpy =6-phenyl-2,2
0
-bipyridine), which previously was accessible only from C-Br activation
through a Ni(0) precursor, has now been obtained for the first time through C-H activation in a direct,
base-assisted cyclonickelation. Optimal reaction conditions were identified to require (i) anhydrous
precursor materials, ii) elevated temperature, (iii) nonpolar media (p-xylene, (di)chlorobenzene),
and (iv) the presence of carbonate and acetate as a synergetic base pair. On the other hand, the
di-nitrogen coordination step of NiX
2
(X =Br, OAc) proceeds slowly in nonpolar media. Therefore,
[Ni(HPhbpy)Br
2
]
2
obtained by Ni(II) coordination in THF proved to be a convenient precursor for the
direct, base-assisted metallation. The concerted fulfilment of all four criteria allows quantitative direct
C-H activation on a synthetically reasonable timescale.
Although the reaction requires high temperatures >100
◦
C, overall, the described quantitative
direct nickelation appears superior to previous protocols with respect to a potential diversification.
Variation of the substitution pattern of the metalated arene including deuteration and DFT modelling
is subject of ongoing work. The high efficiency motivates a future extension to the more challenging
cyclometalation of C(sp
3
)-H protoligands, which was elegantly demonstrated recently by Schafer, Love
et al. for the formation of [Ni(-CH
2
-N(Cy)C(O)N(quinoline-8-yl)(PEt
3
)] [
41
]. While for the noble metals
Ru, Ir, Rh, or Pd, the base-assisted C(sp
2
)-H deprotonation and metalation using acetate/carbonate has
been established for a few years [
38
–
40
] our study represents an important step into the world of the
base metals of the 3d series.
Supplementary Materials:
Details on the C-H activation reactions and synthesis of the new complexes, NMR
spectra, pictures of the crystal and molecular structure of [Ni(Phbpy)(CN)] and UV-vis absorption spectra with
tables of important structural parameters of [Ni(HPhbpy)Br2]2, [Ni(Phbpy)(OAc)], and [Ni(Phbpy)(CN)] are
available (see Supplementary Materials). The following are available online: Figure S1. 600 MHz
1
H NMR
spectrum of [Ni(Phbpy)Br] in CD
2
Cl
2
. Figure S2. 400 MHz
1
H NMR spectrum of [Ni(Phbpy)Br] in CD
2
Cl
2
.
Figure S3. 400 MHz
1
H NMR spectrum of [Ni(Phbpy)Br] in acetone-d
6
. Figure S4. 400 MHz
1
H-
1
H COSY
spectrum of [Ni(Phbpy)Br] in acetone-d
6
. Figure S5. UV-vis absorption spectra recorded during the reaction of
[Ni(HPhbpy)Br
2
]
2
in p-xylene in the presence of KOAc/K
2
CO
3
. Figure S6. Molecular structure of [Ni(Phbpy)(CN)]
(left) and crystal structure viewed along the caxis (right). Note that the structure could not be solved satisfactorily.
These plots show only preliminary results. Table S1. Parameters for the optimisation of the cyclonickelation
of HPhbpy. Table S2. Crystal data, refinement parameters, and important bond parameters of the structure of
the binuclear complex [Ni(HPhbpy)Br
2
]
2
. Table S3. Crystal data, structure refinement, and important bond
parameters for [Ni(Phbpy)(OAc)]. Table S4. Crystal data for [Ni(Phbpy)(CN)].
Author Contributions:
Conceptualisation, A.K. and V.S.; methodology, A.K. and G.H.; syntheses and analysis,
V.S., N.V. and A.S.; crystal structures, A.S.; writing, review, and editing, V.S., A.K., and G.H.; visualisation, N.V.;
funding acquisition, A.K. All authors have read and agreed to the published version of the manuscript.
Funding: The Deutsche Forschungsgemeinschaft DFG KL1194/15-1 is acknowledged for funding of this project.
Acknowledgments:
G.H. thanks the DFG for financial support within the cluster of excellence UNICAT and SFB
840 (Von partikulären Nanosystemen zur Mesotechnologie) and Martin Kaupp (TU Berlin) for generous support.
Molecules 2020,25, 997 10 of 13
We also grateful to Igor Koshevoy, Department of Chemistry, University of Eastern Finland, Joensuu for support
and helpful discussions.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability:
Samples of the compounds [Ni(Phbpy)Br], [Ni(HPhbpy)Br2]2, and [Ni(Phbpy)(OAc)] are
available from the authors.
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