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Cite this: Chem. Commun., 2014,
50,13469
Synthesis and vibrational spectroscopy
of
57
Fe-labeled models of [NiFe] hydrogenase: first
direct observation of a nickel–iron interaction†
David Schilter,*
a
Vladimir Pelmenschikov,
b
Hongxin Wang,
cd
Florian Meier,
b
Leland B. Gee,
c
Yoshitaka Yoda,
e
Martin Kaupp,
b
Thomas B. Rauchfuss
a
and
Stephen P. Cramer*
cd
A new route to iron carbonyls has enabled synthesis of
57
Fe-labeled
[NiFe] hydrogenase mimic (OC)
357
Fe(pdt)Ni(dppe). Its study by nuclear
resonance vibrational spectroscopy revealed Ni–
57
Fe vibrations, as
confirmed by calculations. The modes are absent for [(OC)
357
Fe(pdt)-
Ni(dppe)]
+
, which lacks Ni–
57
Fe bonding, underscoring the utility of
the analyses in identifying metal–metal interactions.
Despite our extremely low atmospheric concentration of dihydrogen
(B1 ppm), this substrate is a key metabolite of many anaerobic
bacteria.
1
In such living systems can be found the most prevalent
enzymes for hydrogen processing, the nickel–iron hydrogenases
([NiFe]–H
2
ases).
1,2
These electrocatalysts specifically mediate the
redox reaction H
2
"2H
+
+2e
at several hundred turnovers
per second.
3
Their heterobimetallic active sites exist in several states,
some of which are summarized below (Fig. 1, left and centre).
The active sites feature Ni bound to four cysteinato residues,
two of which bridge to an Fe(CO)(CN)
2
fragment. In the Ni–C
state, Ni(III)Fe(II) centres bind a bridging hydride (H
), reductive
elimination of which affords Ni–L.
4
Thus, H
+
is abstracted by a
terminal cys ligand (Fig. 1, centre top), leaving a Ni(I)Fe(II) core
with a 2e
bond between the metals.
4
The use of vibrational spectroscopy to study [NiFe]–H
2
ase is
convenient in that its active site features chromophores easily
identifiable by such techniques. Spectral analyses are often aided
by comparison to data from synthetic models
5
whose structures are
well understood. Specificity for
57
Fe-coupled modes is afforded
by nuclear resonance vibrational spectroscopy (NRVS, vide infra).
This has recently enabled observation of characteristic Fe–CN/
Fe–CO bending and stretching modes in [NiFe]–H
2
ase (Ni–A and
Ni–R) and the Fe subsite model [Fe(benzenedithiolato)(CN)
2
CO]
2
.
6
However, no NRVS studies have reported on metal–metal bonding,
which is expected for low-valent clusters like Ni–L and its models.
4,7
A near-complete [NiFe]–H
2
ase mimic is the Ni(II)Fe(I) species
[(OC)
3
Fe(pdt)Ni(dppe)]
+
([1]
+
,pdt
2
=
S(CH
2
)
3
S
; dppe = 1,2-bis-
(diphenylphosphino)ethane), a model for Ni–L, albeit with metal
oxidation states reversed (Fig. 1, right).
8
This S= 1/2 model is
prepared from (OC)
3
Fe(pdt)Ni(dppe) (1),
9
itself the subject of density
functional theory (DFT) and resonance Raman investigations.
10
Disclosed here is methodology for
57
Fe-labeled prototypes
[(OC)
357
Fe(pdt)Ni(dppe)]
0/+
([1]
0/+
), enabling the study of metal–
metal bonding with NRVS.
The Ni(I)Fe(I) complex 1is usually accessed by interaction of
(pdt)Ni(dppe) with an Fe carbonyl such as Fe
2
(CO)
9
or Fe(CO)
4
I
2
.
11–14
The precursor to these, Fe(CO)
5
, is not conveniently prepared from
elemental Fe, a factor that necessitated a new route adaptable to
57
Fe incorporation. Thus, metallic Fe was converted to the organo-
soluble FeI
2
source Fe
2
I
4
(
i
PrOH)
4
,
15
which, upon combination with
(pdt)Ni(dppe), gave the known diiodide I
2
Fe(pdt)Ni(dppe).
14
While
the diiodide does not bind CO in CH
2
Cl
2
,whentreatedwith
AgBF
4
it converts to the putative electrophile ‘[IFe(pdt)Ni(dppe)]
+
’
(or perhaps its dimer), which undergoes carbonylation to afford
Fig. 1 Key [NiFe]–H
2
ase states (left and centre) and two model complexes
(right).
a
Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana,
b
Institut fu
¨r Chemie, Technische Universita
¨t Berlin, 10623 Berlin, Germany
c
Department of Chemistry, University of California, Davis, CA 95616, USA.
E-mail: spjcramer@ucdavis.edu
d
Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley,
CA 94720, USA
e
JASRI, SPring-8, Sayo-gun, Hyogo 679-5198, Japan
†Electronic supplementary information (ESI) available: Experimental proce-
dures, spectral data, computational chemistry details, animated vibrational
modes as GIFs. See DOI: 10.1039/c4cc04572f
Received 16th June 2014,
Accepted 9th September 2014
DOI: 10.1039/c4cc04572f
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[(OC)
3
FeI(pdt)Ni(dppe)]
+
([1I]
+
).
14
Reduction with CoCp
2
gave 1
in yields comparable to the (pdt)Ni(dppe)/Fe
2
(CO)
9
route.
11
In adapting the synthesis to the Ni
57
Fe target, elemental
57
Fe
was oxidized to
57
Fe
2
I
4
(
i
PrOH)
4
,
12
which was converted into violet
[(OC)
357
FeI(pdt)Ni(dppe)]
+
(ESI-MS: m/z829.5 [10I]
+
vs. 828.9 [1I]
+
),
with reduction affording green (OC)
357
Fe(pdt)Ni(dppe) (10)in35%
yield (Scheme 1).
While n
CO
energies and analytical and
31
P{
1
H} NMR data for 10
are virtually identical to those of natural abundance 1, LI-FDI-MS
(liquid introduction field desorption ionization mass spectrometry)
analyses are telling. Soft ionization of non-polar 10and 1allowed
for the detection of parent cations at m/z702.9 and 701.9, respec-
tively (Fig. S4, ESI†). Oxidation of 10with FcBF
4
afforded mixed-
valent salt [10]BF
4
, whose EPR signal is broadened relative to that of
[1]BF
4
due to hyperfine interactions.
Investigations into Ni–Fe bonding in the new
57
Fe-labeled
variants of the reduced and oxidized complexes (respectively 10and
[10]
+
) were undertaken using NRVS. This technique, enabled by
the development of third generation synchrotron sources, insertion
devices and advanced X-ray optics,
16–18
involves scanning an
extremely monochromatic (e.g. 0.8 meV) X-ray beam through a
nuclear resonance of a Mo
¨ssbauer-active isotope (e.g. 14.4 keV for
57
Fe). Subsequent relaxation causes the generation/annihilation of
phonons, the detection of which reveals all modes in which the
57
Fe
nucleus moves along the direction of the incident X-ray. NRVS has
several advantages over traditional IR and resonance Raman spectro-
scopies,
19
not least in terms of element and isotope specificity
and absence of the optical selection rules, which have allowed for
the resolution of n
Fe–X
(X=S,P,Cl,CO,CN,NO)vibrations
in complicated systems.
6,13,19,20
This relatively new but powerful
technique in inorganic and biological iron chemistry is applied here
to [10]
0/+
. The analysed spectra in terms of
57
Fe partial vibrational
density of states (PVDOS) are given in Fig. 2.
Intense bands assigned to n
Fe–CO
and d
Fe–CO
modes were observed
at 440–630 cm
1
, with full-range NRVS spectra and Fe–C(O) kinetic
energy distribution (KED) diagrams presented in Fig. S9 (ESI†).
Similar signals were found for [NiFe]–H
2
ase.
6
It was expected that
n
Fe–S
,d
Fe–S
and n
Fe–Ni
bands, if observable, would lie at low energies
(r400 cm
1
). Upon comparing data for 10and [10]
+
(Fig.2a),asharp
and prominent NRVS peak at 158 cm
1
for 10was noticed. This
band, absent from the spectra of [10]
+
, was tentatively ascribed to
vibration of the Ni–Fe bond, such interactions not being significant
in [10]
+
. Differences in NRVS data of 10and [10]
+
were less marked in
other spectral regions, although peaks for [10]
+
were broader.
The assignment of vibrational bands was elaborated using
DFT calculations on [10]
0/+
as detailed in the ESI;†simulated NRVS
(
57
Fe PVDOS) and Fe–Ni KED diagrams for [10]
+
and 10are also
presented in Fig. 2b and c, respectively. Band positions and inten-
sities in the calculated and observed spectra are largely in agreement
(particularly in Fig. 2c; see also Fig. S9, ESI†).Inthecaseof[10]
+
,
some differences are assigned to impurities (10and/or [10H]
+
). The
band for 10at 158 cm
1
, calculated by DFT at 157 cm
1
, indeed
involves stretching of the Fe–Ni bond symmetrical to a Ni–P1 stretch
(see Fig. 3, and the ESI†for animations). Such a vibrational coupling
in the Fe–Ni–P1 triad implicates a strong Fe–Ni interaction, and it
has no complement in the normal modes pool calculated for [1]
+
.
While very prominent in NRVS (owing to the large
57
Fe displace-
ment), the 157 cm
1
band is weaker (B5%) in the Ni–Fe KED
diagram, which reflects relative motion of Fe and Ni. Other Ni–Fe
stretches, such as those calculated at 266, 311, and 386 cm
1
(see in
Fig. 2c), are considerably stronger (representing 13%, 9%, and 11%
total Ni–Fe KED, respectively). Yet only the first two modes can be
associated with bands observed at 262 and 303 cm
1
,whilethelast
onehasvanishingNRVSintensity(Fig.2a).Themodesat266,311,
and 386 cm
1
have lower
57
Fe PVDOS intensity as they involve
displacement mostly of Ni (rather than Fe), this movement being
evident from the DFT results (Fig. S10, ESI†). Analysis of Fe–Ni
vibrations is further complicated by vibrational coupling to C/P/S
atoms, in particular the bridging S donors. Notably, mixed n
Fe–Ni
modes in the 220–360 cm
1
region involving up to 14% contribution
from the Fe–Ni stretch were also reported for 1.
10
Structural (X-ray diffraction) and DFT studies on unlabeled
[1]
0/+
have suggested that the reduced Ni(I)Fe(I) species may
feature Ni–Fe bonding,
9
while the oxidized Ni(II)Fe(I)doesnot.
8
This was confirmed by re-analysing bonding in [1]
0/+
using ELF,
21
Scheme 1
Fig. 2 Observed NRVS spectra for [10]
+
(thick red lines, (a) and (b)) and 10
(thick blue lines, (a) and (c)) vs. DFT calculated
57
Fe PVDOS spectra for [10]
+
(thin red line, (b)) and 10(thin blue line, (c)). Calculated Fe–Ni KED (green) is
given in (b) for [10]
+
, and in (c) for 10. Key bands observed for 10are labelled
in (a), with DFT counterparts in (c) indicated by vertical lines. Modes giving
rise to bands with significant Fe–Ni character are marked (*) and shown in
Fig. 3 and Fig. S10 (ESI†). For 0–650 cm
1
spectra, see Fig. S9 (ESI†).
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ELI-D,
22
and QTAIM
23
electron density-based methods (Fig. 4;
Fig. S11 and S12, ESI†), all of which indicated a Ni–Fe bond in 1.
Notably, the ELF/ELI-D bond attractor and the bond critical point
found by QTAIM are shifted from the Ni–Fe vector away from the
bridgingSatoms,suchthatFe(
I) is (pseudo)octahedral. Bonding
involves overlap of two singly-occupied d(z
2
) orbitals and is absent
when Ni is oxidized, with [1]
+
featuring a Fe-localized d(z
2
) SOMO. In
contrast, Ni–L has a SOMO with Ni d(z
2
)andd(x
2
y
2
)character,
4
and a 2e
Ni-Fe dative bond (optimized Fe–Ni distance: 2.47 Å).
4
While [1]
+
and Ni–L have the same electron count for the [NiFe]
3+
site, only one has Ni–Fe bonding. A key distinction is Ni geometry,
which is planar in [1]
+
but SF
4
-like in Ni–L.
Since the Fe(I) centre in [1]
+
is electron poor (average n
CO
=
2010 cm
1
)
8
it cannot supply 2e
for a Fe-Ni coordinate bond.
Another possibility, a covalent Ni–Fe bond, is unlikely due to the
low donicity of Ni(II). The metal centres in [1]
+
are distant (our
DFT result: 2.80 Å), while those in electron-rich 1(average n
CO
=
1977 cm
1
) are sufficiently proximal (experimental: 2.47 Å,
9
DFT:
2.46 Å) for covalent Fe–Ni bonding (see Table S2 and Fig. S3, ESI†
for structural and IR details).
Obtaining direct evidence of metal–metal bonding in molecular
systems is nontrivial, and distances do not guarantee presence/
absence of bonding. For example, EXAFS studies reveal similar
Fe–Ni distances for Ni–L and Ni–C,
24
despite Ni not being bonded
to Fe in the latter. This highlights that (i) [NiFe]–H
2
aseactivesite
ligation is inflexible (relative to [1]
0/+
) and that (ii) EXAFS/XRD
studies typically only afford nuclear positions via core electron
density, with bonding electron density between metals being poorly
resolved.
25
In rare cases when very high quality single crystal data
are obtained, multipole analysis, followed by topological analysis
of static electron density (versus inspection of difference maps)
can give insights into metal–metal bonding, as exemplified by
(re)investigation of the archetypal Mn
2
(CO)
10
.
26
Metal–metal bonding is common in organoiron chemistry, but
it also plays a role in the reduced states of some metalloenzymes,
stabilizing low-valent metal centres poised for substrate activation.
7
In addition to [NiFe]–H
2
ase, Ni–Fe interactions are also proposed
for carbon monoxide dehydrogenase (CODH),
7,27
with Ni–Ni
and Fe–Fe bonds being present in acetyl-CoA synthetase (ACS)
7
and [FeFe]–H
2
ase,
28
respectively. NRVS is demonstrably effec-
tive in unambiguous identification of low energy
57
Fe-coupled
modes.
13,20,29
Compared to IR and resonance Raman, it avoids
interference from solvent and ‘fingerprint’ bands, enabling
identification of Fe-coupled vibrations, such as the 158 cm
1
n
Fe–Ni
mode here. The use of NRVS to probe iron–metal modes
in everything from small molecules to iron enzymes is antici-
pated to provide a wealth of information on these catalysts.
While primordial routes to iron carbonyls have been reported
(e.g. the one-pot preparation of (CO)
3
Fe(pdt)Fe(CO)
3
from
FeCl
2
),
30
they are typically limited in scope and reproducibility.
In contrast, the Fe
2
I
4
(
i
PrOH)
4
/CO/CoCp
2
strategy will likely be
generalizable and afford iron carbonyls of relevance to H
2
ases
and organoiron chemistry in general. With the isolation of
57
Fe-labeled [NiFe]–H
2
ase mimics [10]
0/+
, the element and isotope
selectivity of NRVS was exploited to obtain the first vibrational
spectroscopic evidence of Fe–Ni interactions. These results serve
as an important benchmark, opening the door to work probing
metal–metal bonding in redox-active H
2
ase enzymes as well as
related enzymatic and model systems.
Thanks are given to Drs Mark J. Nilges and Haijun Yao for
assistance with EPR and LI-FDI-MS, respectively. Financial
support was provided by the National Institutes of Health
(GM061153-10 to T.B.R. and GM-65440 to S.P.C.), U.S. Depart-
ment of Energy Office of Biological and Environmental Research
(DOE OBER) (S.P.C.), and the ‘Unifying Concepts in Catalysis’
initiative of the German Research Council (V.P., F.M., and M.K.).
NRVS experiments performed at SPring-8 BL09XU were funded
by JASRI (beamtime proposal 2013A0032).
Fig. 3 Scaled arrow depiction of nuclear displacements for the normal
mode calculated for 10at 157 cm
1
(a symmetric Fe–Ni–P1 stretch, see
corresponding
57
Fe PVDOS band in Fig. 2c). Key [10]
0/+
modes are animated
in the ESI.†
Fig. 4 Electron localization function (ELF) analysis of the Ni–Fe bonding
in [1]
0/+
(top/bottom). Ni–Fe bond attractor position for [1]
0
is indicated by
the localized area in green (center-top), absent for [1]
+
. See Fig. S11 and
S12 (ESI†) for alternative bonding representations.
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