Mixed-valence nickel-iron dithiolate models of the [NiFe]-hydrogenase active site

Article abstract of DOI:10.1021/ic202329y

A series of mixed-valence nickel-iron dithiolates is described. Oxidation of (diphosphine)Ni(dithiolate)Fe(CO)₃ complexes 1, 2, and 3 with ferrocenium salts affords the corresponding tricarbonyl cations [(dppe)Ni(pdt)Fe-(CO)₃]⁺ ([1]⁺), [(dppe)Ni(edt)Fe(CO)₃]⁺ ([2]⁺)and [(dcpe)Ni(pdt)Fe(CO)₃]⁺ ([3]⁺), respectively, where dppe = PH₂PCH₂CH₂PPH₂, dcpe = Cy₂PCH₂CH₂PCy₂, (Cy = cyclohexyl), pdtH₂ = HSCH₂CH₂CH₂SH, and edtH ₂ = HSCH₂CH₂SH. The cation [2]⁺ proved unstable, but the propanedithiolates are robust. IR and EPR spectroscopic measurements indicate that these species exist as Cs-symmetric species. Crystallographic characterization of [3]BF₄ shows that Ni is square planar. Interaction of [1]BF₄ with P-donor ligands (L)afforded a series of substituted derivatives of type [(dppe)Ni(pdt)Fe(CO) ₂L]BF₄ for L = P(OPh)₃ ([4a]BF₄), P(p-C₆H₄Cl)₃ ([4b]BF₄), PPH ₂(2-py)([4c]BF₄), PPH₂(OEt)([4d]BF ₄), PPh₃ ([4e]BF₄), PPH₂-(o-C ₆H₄OMe)([4f]BF₄), PPH₂(o-C ₆H₄OCH₂OMe)([4g]BF₄), P(p-tol) ₃ ([4h]BF₄), P(p-C₆H₄OMe) ₃ ([4i]BF₄), and PMePH₂ ([4j]BF₄). EPR analysis indicates that ethanedithiolate [2]⁺ exists as a single species at 110 K, whereas the propanedithiolate cations exist as a mixture of two conformers, which are proposed to be related through a flip of the chelate ring. Moessbauer spectra of 1 and oxidized S = 1/2 [4e]BF₄ are both consistent with a low-spin Fe(I)state. The hyperfine coupling tensor of [4e]BF₄ has a small isotropic component and significant anisotropy. DFT calculations using the BP86, B3LYP, and PBE0 exchange-correlation functionals agree with the structural and spectroscopic data, suggesting that the SOMOs in complexes of the present type are localized in an Fe(I)-centered d(z2)orbital. The DFT calculations allow an assignment of oxidation states of the metals and rationalization of the conformers detected by EPR spectroscopy. Treatment of [1]⁺ with CN-and compact basic phosphines results in complex reactions. With dppe, [1]⁺ undergoes quasi-disproportionation to give 1 and the diamagnetic complex [(dppe)Ni(pdt)Fe(CO)₂(dppe)] ²⁺ ([5] ²⁺), which features square-planar Ni linked to an octahedral Fe center.

Full text of DOI:10.1021/ic202329y

Article
pubs.acs.org/IC
Mixed-Valence Nickel−Iron Dithiolate Models of the
[NiFe]-Hydrogenase Active Site
David Schilter,† Mark J. Nilges,† Mrinmoy Chakrabarti,‡ Paul A. Lindahl,*,‡ Thomas B. Rauchfuss,*,†
and Matthias Stein*,§
†Department of Chemistry, University of Illinois, 600 South Goodwin Avenue, Urbana, Illinois 61801, United States
‡Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012, United States
§Max Planck Institute for Dynamics of Complex Technical Systems, Sandtorstrasse 1, 39106 Magdeburg, Germany
S
* Supporting Information
ABSTRACT: A series of mixed-valence nickel−iron dithiolates is described.
Oxidation of (diphosphine)Ni(dithiolate)Fe(CO)3 complexes 1, 2, and 3 with
ferrocenium salts affords the corresponding tricarbonyl cations [(dppe)Ni(pdt)Fe-
(CO)3]+ ([1]+), [(dppe)Ni(edt)Fe(CO)3]+ ([2]+) and [(dcpe)Ni(pdt)Fe(CO)3]+
([3]+), respectively, where dppe = Ph2PCH2CH2PPh2, dcpe = Cy2PCH2CH2PCy2,
(Cy = cyclohexyl), pdtH2 = HSCH2CH2CH2SH, and edtH2 = HSCH2CH2SH. The
cation [2]+ proved unstable, but the propanedithiolates are robust. IR and EPR
spectroscopic measurements indicate that these species exist as Cs-symmetric species.
Crystallographic characterization of [3]BF4 shows that Ni is square planar. Interaction
of [1]BF4 with P-donor ligands (L) afforded a series of substituted derivatives of type
[(dppe)Ni(pdt)Fe(CO)2L]BF4 for L = P(OPh)3 ([4a]BF4), P(p-C6H4Cl)3
([4b]BF4), PPh2(2-py) ([4c]BF4), PPh2(OEt) ([4d]BF4), PPh3 ([4e]BF4), PPh2-
(o-C6H4OMe) ([4f]BF4), PPh2(o-C6H4OCH2OMe) ([4g]BF4), P(p-tol)3 ([4h]BF4),
P(p-C6H4OMe)3 ([4i]BF4), and PMePh2 ([4j]BF4). EPR analysis indicates that ethanedithiolate [2]+ exists as a single species at
110 K, whereas the propanedithiolate cations exist as a mixture of two conformers, which are proposed to be related through a
flip of the chelate ring. Mossbauer spectra of 1 and oxidized S = 1/2 [4e]BF are both consistent with a low-spin Fe(I) state. The
̈
4
hyperfine coupling tensor of [4e]BF4 has a small isotropic component and significant anisotropy. DFT calculations using the
BP86, B3LYP, and PBE0 exchange−correlation functionals agree with the structural and spectroscopic data, suggesting that the
SOMOs in complexes of the present type are localized in an Fe(I)-centered d(z2) orbital. The DFT calculations allow an
assignment of oxidation states of the metals and rationalization of the conformers detected by EPR spectroscopy. Treatment of
[1]+ with CN− and compact basic phosphines results in complex reactions. With dppe, [1]+ undergoes quasi-disproportionation
to give 1 and the diamagnetic complex [(dppe)Ni(pdt)Fe(CO)2(dppe)]2+ ([5]2+), which features square-planar Ni linked to an
octahedral Fe center.
Scheme 1
INTRODUCTION
■
The modeling of the active sites of the hydrogenases (H2ases)
is a fertile area for research because the inorganic chemistry
at play is unusual. Most distinctive is the prevalence of para-
magnetic states; this contrasts the chemistry of metal carbonyls
in which open shell species are uncommon. Why have H2ases
adopted S = 1/2 states, especially given that the H2/H+ redox
process involves 2e−? While good progress has been made
on modeling the paramagnetic states of the [FeFe]-H2ases,1−3
synthetic analogs for the corresponding odd-electron states of
the [NiFe]-H2ases have not been characterized as well.
The active sites of [NiFe]-H2ases exist in several S = 1/2
states, but only the Ni-C state (Scheme 1)4 is implicated in the
catalytic mechanism.5 Spectroscopic analysis of the Ni-C state
indicates a Ni(III)Fe(II) center bridged by a hydrido ligand.6,7
Upon irradiation at low-temperatures, Ni-C converts to Ni-L
(Scheme 2), another S = 1/2 state first observed in enzymes
from Allochromatium vinosum.8 Spectroscopic analysis and DFT
calculations indicate a Ni(I)Fe(II) core in which the hydrido
bridge is absent. Thus, Ni-C undergoes a photoreduction in
Received: October 27, 2011
Published: February 3, 2012
© 2012 American Chemical Society
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although an additional CO replaces the thioether at the Fe(II)
center and the chelating diphosphine dppe is used in place
of the dithiocarbamate. Dppe has also been employed as a
surrogate for terminal thiolates in [(dppe)Ni(pdt)Fe(CO)3]
Scheme 2
(1), a model described by Schroder and co-workers.19 In con-
̈
trast to the models discussed above, the Fe center is bound to
only three ligands besides the bridging thiolates. This derivative
features metal−metal bonding absent in more oxidized NiFe
complexes such as those described by Tatsumi, Jiang, and their
co-workers.
which the two-electron more reduced Ni-L is reached. Photo-
conversion appears to involve the deprotonation of Ni-C by a
proximal base that has not been identified.9 EPR and ENDOR
experiments and DFT calculations on the Ni-L state have
shown that the majority of the unpaired spin density is localized
at the Ni center, consistent with a Ni(I)Fe(II) oxidation state
assignment.
Ni-A (unready) and Ni-B (ready) are more oxidized S = 1/2
states of H2ases, both with Ni(III)Fe(II) cores. High-resolution
crystallographic studies have been reported for the enzymes
from Desulfovibrio gigas (85% Ni-A/15% Ni-B),10 Desulfovibrio
fructosovorans (Ni-B),11 Desulfovibrio vulgaris Miyazaki F
(Ni-B,12 Ni-SI/Ni-R),13 and Desulfovibrio desulfuricans (Ni-B).14
Regardless of the state of the enzyme, the active site consists
of Ni coordinated to four thiolato (or thiol) ligands, two of
which bridge to an “organometallic” Fe(CN)2(CO) fragment.15
Although the first crystal structure of [NiFe]-H2ase appeared in
1995, synthetic modeling of this family of hydrogenases has
only recently afforded complexes with structures approaching
this active site.
The formally Ni(I)Fe(I) species 1 readily protonates to
furnish the conjugate acid [(dppe)Ni(pdt)(μ-H)Fe(CO)3]BF4
([1H]BF4), which features a hydrido ligand bridging the Ni(II)
and Fe(II) centers (Scheme 3).20 This complex was the first
hydride of a NiFe thiolate, and the significance of this stoich-
iometric resemblance is enhanced by this complex’s ability to
catalyze the reduction of protons to H2. More recently, the Fe
coordination sphere has been varied to afford more electron-
rich derivatives of type [(dppe)Ni(pdt)(μ-H)Fe(CO)2(PR3)]+.21
This family of hydrido complexes represent promising models
for the Ni−R state of [NiFe]-H2ase, which is characterized by
an S = 0 Ni(II)Fe(II) core with a proposed bridging hydrido
ligand.13
In a preliminary communication, we reported that oxidation
of the neutral complex 1 using a ferrocenium salt (FcBF4) al-
lowed for the in situ preparation of [(dppe)Ni(pdt)Fe(CO)3]-
BF4 ([1]BF4), which was characterized by IR and EPR spec-
troscopy.20 This salt remains the only potential mimic of a
paramagnetic state of the enzyme. In this paper, we describe the
synthesis of a series of S = 1/2 NiFe carbonyls, the first models
for Ni-L. Such compounds have spin distributions inverse to
that of Ni-L, and their characterization sheds new light on
the role of the ligands in fine-tuning electronic structure of bi-
metallic species. Additionally, our studies revealed the tendency
of some members of this series to undergo disproportionation,
thereby affording potential models for Ni-SCO state.
Synthetic models featuring a NiFe core in which Ni(II) is
bound by four sulfur ligands have been reported by the Tatsumi
group (Figure 1, left).16 This dithiocarbamato complex, in
Figure 1. Structural mimics of the active site of the [NiFe]-H2ases.
RESULTS
addition to reproducing the Ni coordination environment,
also features the Fe(CN)2(CO) fragment, albeit bound to a
thioether. The CN and CO bands in the IR spectrum match
those of the enzyme in the unready Ni-A state, suggesting that
the Fe(II) centers in the model complex and [NiFe]-H2ase
have similar electron densities. Such 34e- complexes may
be considered models for Ni-SCO, an EPR-silent, CO-bound,
H−-free state of [NiFe]-H2ase in which the Ni and Fe centers
are in the +2 oxidation state.17 A key difference, however, can
be found in the Ni and Fe coordination numbers: in such
models these are 4 and 6, respectively, whereas in Ni-SCO the
extrinsic CO binds Ni, causing both centers to be 5-coordinate.
Jiang and co-workers have prepared a related Ni(II)Fe(II)
structural model (Figure 1, right) that incorporates a Fe(CN)2-
(CO)2 fragment bridged to Ni by a pair of thiolate ligands.18
This complex is closely related to the dithiocarbamate example,
■
[(Diphosphine)Ni(pdt)Fe(CO)3]+. Preliminary studies re-
vealed that (dppe)Ni(pdt)Fe(CO)3 (1) and related complexes
undergo oxidation at approximately −0.5 V vs Fc+/0. For this
reason, the new cations were prepared from the neutral Ni(I)-
Fe(I) complexes and FcBF4. The first example of the series is
[1]BF4, which could be isolated as a brown powder. This salt
dissolves in CH2Cl2 to give moderately stable solutions, which
are unaffected by ambient light but are sensitive to O2 and
H2O. Solutions in THF and MeCN decompose to unidenti-
fied products. Using the appropriate ferrocenium salts, we also
prepared solutions of [1]BArF (BArF − = tetrakis(3,5-bis-
4
4
(trifluoromethyl)phenyl)borate) and [1]PF6. The latter could
be isolated as a brown powder. Although the solubility prop-
erties of these salts varied, their IR and EPR spectra (see below)
were very similar.
Scheme 3
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Figure 2. X-band EPR spectra (CH2Cl2/PhMe, 110 K) of [1]BF4 (exp.) and [1′]BF4 (exp.′). In each case the experimental spectrum could be
simulated as sum of two components, denoted A+B (for [1]BF4), and A′+B′ (for [1′]BF4).
Under 13CO (1 atm), 1 in CH2Cl2 solution converted to
[(dppe)Ni(pdt)Fe(13CO)3] (1′), for which the νCO bands are
shifted to lower energies (1982 (m), 1913 (s) cm−1 vs 2028 (m),
1932 (s) cm−1). Similarly, [1]BF4 also exchanges with 13CO,
allowing for the generation of [(dppe)Ni(pdt)Fe(13CO)3]BF4
([1′]BF4), which has νCO bands consistent with complete labeling
(2009 (m), 1941 (s) cm−1 vs 2057 (m), 1986 (s) cm−1). Analo-
gously to the parent complex, the labeled salt can also be prepared
by the oxidation of [(dppe)Ni(pdt)Fe(13CO)3] (1′) with FcBF4.
The X-band EPR spectrum of a frozen CH2Cl2/PhMe solu-
tion of [1]BF4 (Figure 2, exp.) exhibits two comparably intense
signals, including an axial signal (A) and rhombic signal (B).
These signals are consistent with the presence of two isomers.
The absence of any resolved 31P hyperfine coupling indicates
that negligible spin density resides on the Ni center, suggesting
a Fe(I)Ni(II) oxidation state. This assignment was further
confirmed by comparing this spectrum to that of the 13CO-
labeled compound [1′]BF4 (exp.′). The EPR spectrum of the
latter again suggests the presence of two closely related isomers
with signals split by a single I = 1/2 nucleus (A = 62, 53, 72
MHz), proposed to be the apical 13C atom. One signal was
further split/broadened (A = 13, 30, 17 MHz) owing to weak
interactions with the two equivalent basal 13CO ligands. For
comparison, the coupling observed for the Fe(I)-13CO spin
system in the Hox state of [FeFe]-H2ase is 21 MHz.22 EPR
[3]BF4, which is similar to [1]BF4 in terms of its spectroscopy
(Figures S14, S15, Supporting Information) and stability.
Structure of [(dcpe)Ni(pdt)Fe(CO)3]BF4. X-ray crystallo-
graphy allowed for structural confirmation of [3]BF4, the
results of which are presented in Figure 3.
spectra for [1]BF4, [1]PF6, and [1]BArF were similar.
Figure 3. ORTEP of [3]BF4 with ellipsoids drawn at the 50%
probability level. The H atoms, BF4− anion, and two CH2Cl2 solvate
molecules are omitted for clarity. Selected distances (Å, distances calcu-
lated using BP/TZVP are given in parentheses): Ni1−Fe1, 2.818 (2.76);
Ni1−P1, 2.191 (2.23); Ni1−P2, 2.188 (2.22); Ni1−S1, 2.235 (2.25);
Ni1−S2, 2.227 (2.24); Fe1−S1, 2.296 (2.31); Fe1−S2, 2.288 (2.34);
Fe1−C30, 1.833 (1.80); Fe1−C31, 1.799 (1.79); Fe1−C32, 1.790 (1.79).
4
Two analogs of [1]+ were examined involving replacement
of the dithiolate and the diphosphine. Oxidation of ethan-
edithiolate [(dppe)Ni(edt)Fe(CO)3] (2)23 with ferrocenium
salts gave unstable products regardless of the counteranion.
In situ IR analysis of these solutions confirmed that the cation
[2]+ (νCO = 2059 (m), 1988 (s) cm−1) closely resembles [1]+,
although these solutions also contained significant amounts of 2
and [2H]+. The instability of [2]+ contrasts with the good
stability of the corresponding hydrido complex [2H]+, whose
structure is expected to closely resemble that of the mixed-
valence derivative. The EPR analyses of [2]+ are discussed later.
Lastly, oxidation of the more electron-rich complex [(dcpe)-
Ni(pdt)Fe(CO)3] (3) with FcBF4 in CH2Cl2 solution afforded
The Ni−Fe distance in [3]BF4 (2.818 Å) is longer than that
in 1 (2.467 Å), as well as the Ni(II)(μ-H)Fe(II) complex
[3H]BF4 (2.684 Å), in which a hydrido ligand is present.
Whereas the atom connectivity in 3 and [3]+ is identical, a
dramatic conformational change occurs upon oxidation. The Ni
center in [3]BF4 exists in an almost square planar coordination
environment, as indicated by the twist angle of 3.55° between
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the NiP2 and NiS2 planes. In contrast, the Ni center in 3 is thought
to be tetrahedral, the relevant twist angle in the closely related dppe
analog 1 being 84.91°.19 By comparison, the (dcpe)Ni(pdt) frag-
ment in [3H]BF4 is also almost square planar, and the average
Ni−P distance for this complex (2.189 Å)20 is identical to that
in [3]BF4.21 The solid-state data are consistent with [3]BF4 fea-
turing a Ni(II) center. The Fe center is situated in an approxi-
mately square-pyramidal environment, with C30 occupying the
apical position. Such a geometry is often adopted by mono-
nuclear Fe(I)24 and dinuclear Fe(I)Fe(II) compounds.25,26 The
Fe−Cbasal distances (1.799, 1.790 Å) are somewhat shorter than
the Fe−Capical distance (1.833 Å), as might be expected for a
complex with CO ligands trans to strong σ-donors.
Furthermore, the average Fe−C distance in [3]BF4 (1.807 Å)
is similar to that for 1 (1.798 Å), consistent with the Fe centers
in both complexes being in the same +1 oxidation state.
Substituted Derivatives of [1]+. In an attempt to intro-
duce CN− ligands, a solution of [1]BArF was treated with NBu4CN.
However, this reaction afforded 1 as the predominant species
according to IR analysis. The redox process was rapid and
accompanied by effervescence, believed to result from evolution
of NCCN. The formation of 1 was also observed when [1]BF4
was treated with either the N-heterocyclic carbene bis(2,6-
diisopropylphenyl)imidazolin-2-ylidene or the isocyanide MeNC.
Given the propensity of C-donors to reduce [1]+, less basic
ligands were investigated. Addition of [1]BF4 to an excess of
PPh3 in CH2Cl2 solution resulted in evolution of CO and
formation of a dark red-brown solution. The reaction was
instantaneous, and a red-brown powder could be preci-
pitated with pentane. The IR spectrum of this product fea-
tures a pair of comparably intense νCO bands at 1988 and
1929 cm−1, consistent with the formation of the monosubsti-
tuted cis-dicarbonyl species [(dppe)Ni(pdt)Fe(CO)2PPh3]BF4
([4e]BF4) according to Scheme 4. The mean shift in νCO upon
The νCO values for the compounds prepared are given in
Table 1. These data correlate inversely with the basicity of the
Table 1. IR Data for Compounds of Type
[(dppe)Ni(pdt)Fe(CO)2L]BF4 in CH2Cl2 Solution, Ordered
a
in Decreasing νCO
−1
−1
L
compound
νCO, cm
TEP, cm
[4a]BF4
[4a′]BF4
[4b]BF4
[4c]BF4
[4d]BF4
[4e]BF4
[4e′]BF4
[4f]BF4
[4g]BF4
2007, 1952
1961, 1907
1991, 1933
1990, 1932
1989, 1929
1988, 1929
1941, 1885
1987, 1929
1987, 1928
P(OPh)3
2085.3
2072.8
P(p-C6H4Cl)3
PPh2(2-py)
PPh2(OEt)
PPh3
2068.9
2066.1
PPh2(o-C6H4OMe)
PPh2(o-
C6H4OCH2OMe)
P(p-tol)3
[4h]BF4
[4i]BF4
[4j]BF4
1985, 1927
1985, 1926
2066.7
2066.1
2067.1
P(p-C6H4OMe)3
PMePh2
1985, 1977, 1928,
1907
a
Data for the 13CO-labeled analogs, [(dppe)Ni(pdt)Fe(13CO)2L]BF4,
are included, these compounds being denoted with prime symbols.
monodentate P-donor ligands employed. As expected, the
trend in νCO values roughly mirrors the trend in the Tolman
Electronic Parameter27 (TEP) for each P-donor ligand. The
similarity of the IR data for complexes of PPh3, PPh2(2-py)
([4c]BF4), PPh2(o-C6H4OMe) ([4f]BF4), and PPh2(o-C6H4-
OCH2OMe) ([4g]BF4) indicates that ether and pyridyl sub-
stituents do not interact significantly with the metal centers in
the monocations.
In many cases, the two νCO bands have discernible shoulders
at lower energy, suggesting that the complexes are present as two
species in solution. This effect is most pronounced in the case of
the PMePh2 complex ([4j]BF4), where the absorptions can be
somewhat resolved (Figure S48, Supporting Information). The
isomerism involved is addressed in the following section.
Scheme 4
EPR Spectroscopy of [(dppe)Ni(pdt)Fe(CO)2L]+. All sub-
stituted mixed-valence derivatives were characterized by X-band
EPR spectroscopy; selected spectra were simulated in order to
extract g and A(31P) values, which are presented in Table 2. Spectra
not presented here can be found in the Supporting Information.
The EPR spectrum of the PPh3-containing compound
[4e]BF4 (Figure 4) features a pattern of two overlapping rhom-
bic signals, consistent with the lower symmetry of [4e]+ relative
to [1]+. The hyperfine splitting of the signals is attributed to the
coupling to the PPh3 ligand, the associated A values being in
the range 117−217 MHz. Spectra for [4a−j]BF4 are similar.
BP/TZP calculations predict a large isotropic hyperfine coup-
ling of +202 MHz for 31PPh3 in [4e]+ and a dipolar contri-
bution of (−17, −16, +33) MHz . The hyperfine coupling to
the dppe P nuclei is significantly weaker (Aiso = 4 MHz).
The 13CO-labeled derivative [(dppe)Ni(pdt)Fe-
(13CO)2PPh3]BF4 ([4e′]BF4, prepared from [1′]), was studied
in order to derive further structural information about com-
plexes of the present type. In their EPR spectra, the g-values for
[4e]BF4 and [4e′]BF4 are almost identical (Table 2). In addition
to the 31P coupling arising from Fe-bound PPh3, further signal
broadening is observed in the case of [4e′]BF4, a result of weak
interactions with the two 13CO ligands (Figure S35, Supporting
Information). The data suggest that in such arrangements a
substitution is −63 cm−1 relative to the tricarbonyl parent
compound [1]BF4 (vCO = 2057, 1986 cm−1). An identical shift
in vCO was observed upon replacement of one CO for PPh3
in [1H]BF4 to give [(dppe)Ni(pdt)(μ-H)Fe(CO)2PPh3]BF4.21
Further information regarding the identity of [4e]BF4 was
gleaned from its positive-ion ESI mass spectrum, which fea-
tured a base peak at m/z 936.3 for the parent ion [4e]+.
Analogous to the preparation of [4e]BF4, a range of other
monosubstituted derivatives were prepared ([4a−j]BF4), each
of which was characterized according to analytical and ESI-MS
data, as well as by IR and EPR spectroscopy. ESI-MS analyses
supported the formulation of the new complexes as [(dppe)-
Ni(pdt)Fe(CO)2L]+. Ionization by loss of BF4− from the salts
allowed for the detection of the parent ions, with the isotopic
distributions being consistent with their structures. In some
cases significant fragmentation was also observed, in which the
cations lose a terminal ligand to afford species of the type
[(diphosphine)Ni(dithiolate)Fe(CO)2]+ and [(dppe)Ni(pdt)-
Fe(CO)L]+.
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a
Table 2. EPR Simulation Parameters for Salts of Mixed-Valence Complexes
compound
g-factor
A(31P) (MHz)
line width (G)
relative abundance
2.052, 2.050, 2.005
2.055, 2.038, 2.009
2.053, 2.051, 2.005
2.054, 2.037, 2.008
13, 18, 12
14, 17, 13
11, 11, 7
16, 11, 10
5, 11, 4
0.57
0.43
0.62
0.38
[1]BF4
b
c
[1′]BF4
[2]BF4
[4a]BF4
2.054, 2.053, 2.010
2.058, 2.031, 2.007
2.053, 2.051, 2.004
2.058, 2.032, 2.007
18, 15, 16
345, 345, 364
351, 311, 372
345, 345, 364
351, 311, 372
167, 165, 211
177, 147, 211
168, 183, 195
152, 117, 227
170, 182, 197
147, 122, 228
8, 10, 6
0.68
0.32
0.68
0.32
0.77
0.23
0.66
0.34
0.66
0.34
14, 20, 6
8, 10, 6
d
[4a′]BF4
[4e]BF4
[4e′]BF4
[4h]BF4
e
2.053, 2.051, 2.004
2.066, 2.036, 2.006
2.066, 2.045, 2.004
2.066, 2.038, 2.006
14, 20, 6
6, 13, 10
22, 5, 11
10, 13, 8
12, 20, 6
11, 13, 8
13, 20, 6
f
g
2.070, 2.042, 2.007
2.064, 2.036, 2.004
2.069, 2.037, 2.004
a
Where two entries exist for a given compound, two isomers are represented, the relative abundances of which are given in the last column.
b
c
Parameters for [4a]BF4 were derived from both X- and Q-band data. A(13C) = 62, 53, 72 and 13, 30, 17. A(13C) = 57, 80, 50 (coupling for basal
d
e
f
g
13C unresolved). A(13C) = 16, 19, 20. A(13C) = 12, 12, 20. A(13C) = 18, 17, 23. A(13C) = 20, 14, 22.
of 13CO ligands results in only slight signal broadening, sug-
gesting that the P(OPh)3 ligand is apically bound to Fe(I), with
the basal 13CO ligands interacting only weakly with the SOMO.
EPR spectra of the Ni(II)Fe(I) compounds [1]BF4, [3]BF4,
and [4a−j]BF4 (and the 13CO-labeled analogs prepared) indi-
cate, in each case, the presence of two species. For [4a−j]BF4,
these isomers have almost identical EPR parameters. Indeed,
satisfactory simulations could only be obtained when two spec-
ies were considered. It is suggested that these species are very
similar, and the data are consistent with the isomers being
related by a ring flip of the bridging pdt2− ligand (Scheme 5).
Figure 4. X-band EPR spectra (CH2Cl2/PhMe, 110 K) of [4e]BF4.
The experimental spectrum (exp.) could be simulated as the sum of
two components, denoted A+B.
Scheme 5
ligand must be in the apical position to interact strongly with
the Fe-centered SOMO.
The compound [4a]BF4 is noteworthy owing to its high AP
values, which average 354 and 345 MHz for the axial and rhom-
bic components (2:1), respectively (Figure 5). These values
The presence of two isomers is more pronounced when the
room temperature spectra are considered (selected spectra pres-
ented in Supporting Information), suggesting a slow interconver-
sion between conformers (“flipamers”). Additionally, the dis-
persion afforded by Q-band EPR was exploited in the analysis of
one example, [4e]BF4. In this case, the high-field (gz) resonance
has obvious shoulders, which are evident in one of the two
modeled signals (Figure S32, Supporting Information).
The existence of flipamers is found in pdt-bridged dinuclear
species, the ring inversion/flipping barriers typically being 8−
11 kcal/mol.28,29 For example, EPR spectroscopy was used
to detect the two flipamers of the [FeFe]-H2ase model com-
plex [(dppv)(CO)Fe(pdt)Fe(CO)2(PMe3)]+ (dppv = cis-1,2-
bis(diphenylphosphino)ethene).25
This isomerism is not possible for the ethanedithiolate cation
[2]+, and although it was too unstable to be isolated as a salt,
EPR analysis was conducted on a sample prepared in situ. The
spectrum recorded at 110 K (Figure 6) revealed an almost
purely axial signal, corresponding to a single spin system. The
signal at higher field is split into a triplet, whereas the hyperfine
splitting of the low-field resonance is unresolved.
Figure 5. X-band EPR spectra (CH2Cl2/PhMe, 110 K) of [4a]BF4
(exp.) and [4a′]BF4 (exp.′). Simulated spectra (sim. and sim.′,
respectively) are also presented.
suggest significant spin delocalization onto the P(OPh)3 ligand,
which is the best π-acceptor of the P-donor ligands employed.
The 13CO-labeled compound [4a′]BF4 was prepared to clarify
the assignment. As with the PPh3 congener [4e′]BF4, the presence
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component from ge (2.0023), are indicative of the presence of
Ni(I) in this state of the enzyme. Calculations using the BP86
(Becke exchange, Perdew correlation),30,31 B3LYP (Becke ex-
change, Lee−Yang−Parr correlation),32 and PBE0 (Perdew,
Burke and Ernzerhof)33,34 hybrid functionals give g-tensors for
the compounds [1]+, [2]+, [3]+, [4a]+, and [4e]+ (Table 1 of the
Supporting Information) in which the smallest g-component, g3,
is close to ge, effectively ruling out Ni(I) species.
In general BP86 tends to underestimate g-shifts by about 19
ppt for g1, whereas PBE0 tends to slightly overestimate g-shifts.
Best agreement with experiment is obtained with the B3LYP
hybrid functional. The self-consistent consideration of spin−
orbit coupling in spin-restricted calculations with Slater basis
functions gives larger g-shifts compared with the effective poten-
tial approach through the coupled-perturbed SCF equation.
The calculations are accurate enough to assign the two confor-
mers (see Scheme 5). For example, the experimental g-tensors
for [1+] (2.052, 2.050, 2.005 and 2.055, 2.038, 2.009) can be
compared with those derived computationally (ZORA, B3LYP).
Calculations suggest that axial g-values (2.052, 2.051, 2.006)
can be expected when the central CH2 is oriented toward Ni
(flipamer a). Slightly lower g-values can be observed (2.051,
2.047, 2.005) when this group is instead closer to Fe (as it is in
flipamer b). Whereas g1 remains almost unchanged, the largest
decrease occurs for g2. This effect can be rationalized by the g2-
axis pointing toward the thiolate bridging ligand and being thus
sensitive to the orientation of the pdt2− ligand. In two cases,
the relative energies between flipamers and their barrier to
interconversion were computed. For the tricarbonyl complex
[1]+, the Gibbs free energy of flipamer ‘a’ is lower by 1 kcal/mol,
the activation barrier for conversion to flipamer ‘b’ being
9 kcal/mol (see Figure 7). The flipamers of the PPh3-substituted
derivative [4e]+ are almost isoenergetic: flipamer ‘b’ is favored by
0.2 kcal/mol, the barrier being 10.5 kcal/mol. The calculated
barriers are comparable to those reported for diiron propane-
dithiolates (vide supra) and are consistent with the observation of
two species by EPR spectroscopy. Of course, such a treatment is
not applicable for the ethanedithiolate complex [2]+, the EPR
spectrum of which can be modeled using a single conformer.
The calculated atomic spin densities show that the major-
ity of the unpaired spin (0.7−0.9 e− ) resides on the Fe atom,
with the Ni center only carrying ∼0.1 e− (Table 3). According
to these calculations, the conformation of the pdt ring has little
influence on the spin density distribution and thus on the
Figure 6. X-band EPR spectra (CH2Cl2/PhMe) of [2]BF4 collected at
110 K (110 K exp.) and room temperature (rt exp.). Simulated spectra
are also presented.
Coupling (A = 14 MHz) was also observed in the room
temperature isotropic spectrum, again implying the presence of
a single species in which an unpaired electron is coupled to two
equivalent I = 1/2 nuclei. Such a signal arises either from the
coupling of an Fe(I) center with two protons on the edt2−
ligand or a Ni(I)−31P interaction; this question is addressed in
the following section. The fact that this complex is present as a
single species in solution is consistent with the two EPR signals
from [1]BF4 arising from the presence of flipamers.
DFT Calculations. Density functional theoretical calcula-
tions were performed in an effort to rationalize EPR data and
assign the oxidation states. The calculations support the assign-
ments of Ni(I)Fe(I) for the neutral complexes and Ni(II)Fe(I)
for the singly oxidized derivatives. The calculated structural
parameters agree with those determined experimentally to
within 0.01 Å for bond lengths and 1−2° for bond angles. The
structure calculated for [1]+ (Figure 7) is very similar to that
observed crystallographically for [3]+, with the Ni−Fe distances
being 2.818 and 2.80 Å, respectively.
With the observed structure, the Mulliken population of the
unpaired spin on Fe is 0.72 and on Ni is 0.12 (see Figure 7).
Upon ring inversion of pdt2− chelate ring, the distribution of
the unpaired spin density remains nearly unchanged, but small
differences in g-values are detectable (vide infra).
The calculated g-tensor principal values from the spin-
unrestricted CP-SCF calculations and spin-restricted ZORA
calculations give a consistent picture of the electronic structure
of the mixed-valence NiFe dithiolates. The g-values reported for
Ni-L (2.30, 2.12, 2.05), in particular the deviation of the gz
Figure 7. Isocontour plots of the unpaired spin density distribution (violet) at 0.005 e−/a03 for the two conformers of [1]+. The central methylene of the
pdt2− ligand can be oriented toward Ni (left, conformer a) or Fe (right, conformer b). Unpaired atomic spin densities are given for selected nuclei.
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It was reasoned that the apparent inconsistency could be due
to intermolecular spin−spin interactions. Accordingly, [4e]BF4
was studied at 4.3 K in a strong applied magnetic field (6 T),
where these interactions would be less apparent. Indeed, the
high-field data indicated the presence of magnetic hyperfine
interactions, which prompted us to examine a frozen solution
of [4e]BF4 (5 mM, CH2Cl2/PhMe, 1:2). At 6 K and 0.05 T
Table 3. Calculated Atomic Spin Populations (BP/TZVP)
for Complexes [1]+, [2]+, [3]+, [4a]+, and [4e]+. The
superscripts and refer to the conformations of the pdt-
chelate ring (Figure 7)
a
b
BP86/TZVP
ρ(Fe)
ρ(Ni)
ρ(Pdppe
)
ρ(apical) C/P
[1]+a
[1]+b
[2]+
[3]+a
[3]+b
[4a]+a
[4a]+b
0.72
0.70
0.72
0.71
0.70
0.71
0.74
0.12
0.12
0.12
0.15
0.13
0.13
0.11
0.01, 0.01
0.01, 0.01
0.01, 0.01
0.01, 0.01
0.01, 0.01
0.01, 0.02
0.01, 0.01
0.05
0.05
0.05
0.05
0.06
0.05
0.05
applied field, the Mossbauer spectrum of the solution indicated
̈
the presence of magnetic hyperfine interactions. Such low con-
centrations circumvented spin−spin interactions but resulted in
noisy spectra. Analysis of a more concentrated sample (40 mM)
afforded a similar low-field, low-temperature spectrum but with
higher S/N (Figure S53, Supporting Information). Indeed, data
collected at applied fields of 4 and 6 T were, apart from the
poorer S/N, indistinguishable from those of solid [4e]BF4.
The low field spectrum of this frozen solution and the high-
temperature, high-field data of the solid sample could be simu-
lated with an S = 1/2 Hamiltonian (Figure 9, parameters in
hyperfine interactions. The 31P nuclei of the bidentate dppe and
dcpe ligands give rise to only small hyperfine interactions with
isotropic components each between −4 and −15 MHz.
In complexes of type [(diphosphine)Ni(dithiolate)Fe(CO)2L]+,
the apical ligand on the Fe atom can acquire about 5% of the un-
paired spin density. For the tricarbonyl cations, this delocalization
results in an almost isotropic 13C hyperfine interaction of +69 to
+76 MHz for [1]+ and [2]+, respectively, with a smaller dipolar
contribution (−5, −3, +8 MHz). The hyperfine couplings to the
other two 13CO ligands are significantly smaller (−16, −17 MHz)
and cannot be resolved in the experimental EPR spectra. In [4e]+,
there exists a large isotropic hyperfine interaction (+178 and +204
MHz for the two conformers) with the 31PPh3 ligand in addition
to a small dipolar component (−18, −16, +34 MHz).
̈
Mossbauer Spectra. The neutral complex 1 and the mixed-
valence species [4e]BF4, [4h]BF4, and [4i]BF4 were studied by
Mossbauer spectroscopy. At 6 K and 0.05 T applied field, 1 exhi-
̈
bited a quadrupole doublet (δ = 0.04 mm/s and ΔEQ = 0.68 mm/s),
consistent with a diamagnetic complex containing low-spin Fe(I).
Under similar conditions, analysis of the diiron(I) species (CO)3Fe-
(pdt)Fe(CO)3 yielded the parameters δ = 0.04 mm/s and ΔEQ =
0.77 mm/s (Figure S52, Supporting Information). Similar values
have been reported for the related complexes (CO)3Fe(SCH2O-
Figure 9. Mossbauer spectra of [4e]BF recorded at the applied fields
̈
4
35
CH2S)(CO)3 and (CO)3Fe(SCH2Si(CH3)2CH2S)(CO)3.36
and temperatures indicated. Spectra A, C, and D were obtained on a
solid sample, while spectrum B was obtained on a 40 mM frozen
solution. The magnetic field was parallel to the γ beam for spectra A
and B and transverse to the γ beam for spectra C and D. The solid line
through the data is a simulation using an S = 1/2 Hamiltonian with the
parameters δ = 0.18 mm/s, ΔEQ = 0.79 mm/s, η = 0.7, A/(gnβn) =
(+6.2, −5.5, −28.1) KG, βefg = 45°, and γefg = 90°.
These data further highlight the parallels between these two
low-spin Fe(I) metal−metal bonded derivatives.
Surprisingly, the one-electron oxidized S = 1/2 complexes
[4e]BF4, [4h]BF4, and [4i]BF4 in the solid state also exhibited
quadrupole doublets at 6 K and 0.05 T. The spectra appeared
to be inconsistent with the EPR and DFT studies (vide supra),
which indicated unpaired spin density on these complexes,
primarily on the Fe (Figure 8). In such cases, it was expected
caption). The DFT calculated Mossbauer parameters of η = 0.8
̈
and a NQCC of 11 MHz agree well with those measured. The
57Fe hyperfine tensor for [4e]BF4 has a small isotopic component,
Aiso = (Ax+ Ay+ Az)/3 = −9.1 MHz and significant anisotropy.
The calculated 57Fe isotropic hyperfine coupling parameters
(Aiso = −4 MHz for SR UKS ZORA B3LYP/TZP and Aiso
=
−1.5 MHz for SR UKS ZORA BP/TZP) and larger anisotropic
hyperfine tensors (SR UKS B3LYP/TZP (−22, −10, +32)
MHz and SR UKS BP/TZP (−18, −9, +27) MHz) are in good
agreement with those measured.
Disproportionation Reactions. Interaction of [1]+ with
small basic phosphine ligands results in complicated mixtures.
For example, addition of [1]+ to excess PBu3 in CH2Cl2 induces
a “disproportionation” reaction affording 1 and a product for-
mulated as [(dppe)Ni(pdt)Fe(CO)2(PBu3)2](BF4)2. Precipita-
tion of the latter compound allowed for its identification ac-
cording to ESI−MS (m/z 1166.1 [M − BF4−]+, 539.5 [M −
2BF4−]2+) and 31P NMR (60 (dppe), 28 (PBu3) ppm) data.
A strong νCO band could be observed at 1965 cm−1, with a
Figure 8. Isocontour plots of the unpaired spin density distribution
3
(violet) at 0.005 e−/a0 for the two conformers of [4a]+. The central
methylene of the pdt2− ligand can be oriented toward Ni (left,
conformer a) or Fe (right, conformer b).
that paramagnetic hyperfine structure would be observed under
the experimental conditions employed.
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Scheme 6
weaker one at 2037 cm−1. Along with the NMR data, these
data indicate the presence of a Fe(II)(CO)2(PBu3)2 fragment
in which the CO ligands are mutually trans, although other
isomers could be present. Similar products were observed when
PMe3 (νCO 1971 cm−1, m/z 913.5 [M − BF4−]+, 413.6 [M −
2BF4−]2+) and PMe2Ph (νCO 1971 cm−1, m/z 1037.5 [M −
BF4−]+, 475.3 [M − 2BF4−]2+) were employed in place of PBu3.
Overall, these data suggest that a disporportionation-type reac-
tion is triggered when a sufficiently small pair of ligands bind Fe.
More tractable products of disproportionation were obtained
when the diphosphine dppe was used. At room temperature, a
rapid reaction was indicated by IR spectroscopy; the products
are 1 and the new salt formulated as [(dppe)Ni(pdt)Fe(CO)2-
(dppe)](BF4)2 ([5](BF4)2 (Scheme 6).
of a Ni(II)Fe(II) complex lacking a metal−metal bond. The
Fe−C distances (1.800, 1.817 Å) are similar, and these solid-
state data are corroborated by the IR solution spectrum, which
features a single CO band (1975 cm−1), consistent with the pre-
sence of two trans CO ligands. Indeed, the complex is roughly
symmetric, as evidenced by the two Fe−S (2.325, 2.334 Å),
Ni−P (2.180, 2.186 Å), and Ni−S (2.219, 2.253 Å) bonds of
similar pairwise lengths. These are comparable to the average
Ni−P (2.176 Å) and Ni−S (2.222 Å) distances in the related
dinickel(II) complex {(pdt)[Ni(dppe)]2}(BF4)2.37 The product
resembles [(dppe)Ni(pdt)Fe(CO)2(CN)2]18 and might be
compared with the 34e− Ni(II)Fe(II) core present in the Ni-
SCO state of [NiFe]-H2ase.
DISCUSSION
This compound can be prepared in good yield by treatment
of a CH2Cl2 solution of 1 and FcBF4 (2 equiv) with dppe. On
■
Oxidation of Ni(I)Fe(I) dithiolates affords mixed-valence
derivatives, which are described as Ni(II)Fe(I) species. Several
lines of evidence support this assignment: (i) crystallographic
analysis shows that Fe−C bond lengths are unaffected by oxi-
dation of the Ni(I)Fe(I) precursor whereas the Ni coordination
geometry changes from being tetrahedral to square planar,
(ii) in the EPR spectra of the cations, hyperfine coupling is ob-
served with 13CO (on Fe) but not the diphosphine (on Ni),
1
the basis of its H and 31P NMR spectra, [5]2+ is diamagnetic
and symmetrical in CD2Cl2 solution. Further evidence sup-
porting the proposed structure could be obtained by ESI−MS,
which allowed for the detection of ions at m/z 1158.7 and
536.1, assigned to {[5]BF4}+ and [5]2+, respectively.
Structure of [(dppe)Ni(pdt)Fe(CO)2(dppe)](BF4)2. The
solid state structure of [5]2+ was established by X-ray crystal-
lography (Figure 10).
and (iii) the Mossbauer parameters observed are similar to
̈
other Fe(I) sites. Additionally, as has recently been described,
the potentials for the oxidation of Ni(dppe) derivative 1 vs, the
Ni(dcpe) derivative 3 differ strongly (E1/2 = −0.54 and −0.84 V
vs Fc/Fc+, respectively).23
In view of the electron-rich Fe(CN)2(CO) fragment present
in the [NiFe]-H2ase active site, it was of interest to further
modify the tricarbonyl cations by introducing donor ligands. It
was anticipated that the displacement of one or more of the CO
ligands in [1]+ with stronger σ-donors would stabilize oxida-
tion states matching those of the Ni-L state of the enzyme
(Ni(I)Fe(II)). For example, ENDOR measurements on the Ni-
A state revealed a small A(57Fe) of 1 MHz, and no hyperfine
could be resolved for the enzyme in the Ni-B and Ni-C states.38
However, the substituted complexes [4a−j]+ also appear to be
Ni(II)Fe(I) derivatives in which the monodentate P-donor
ligands occupy the apical Fe coordination site. Donor atoms at
this site are proposed to interact strongly with the Fe-centered
SOMO, as indicated by the observation of strong hyperfine
coupling to only a single 13CO center in [1′]+. This trend is
supported by considering the minimal effect that 13CO labeling
has on the EPR spectrum of [4e]+, suggesting that the (basal)
13CO ligands in this complex are only weakly coupled to the
SOMO.
Figure 10. ORTEP of [5](BF4)2·4CH2Cl2 with ellipsoids drawn at the
50% probability level. The H atoms, disordered BF4− anions, and four
CH2Cl2 solvate molecules are omitted for clarity. Selected distances
(Å): Ni1−Fe1, 2.818; Fe1−P1, 2.254; Fe1−P2, 2.262; Fe1−C27,
1.800; Fe1−C28, 1.817; Fe1−S1, 2.325; Fe1−S2, 2.334; Ni1−S1,
2.219; Ni1−S2, 2.253; Ni1−P3, 2.186; Ni1−P4, 2.180.
The substituted derivatives [4a−j]+ are expected to be ap-
proximately isostructural to one another, given the correlation
between νCO and TEP values for the complexes. The apical
location of the P-donor ligands in these salts contrasts with the
structure of the hydride [(dppe)Ni(pdt)(μ-H)Fe(CO)2PPh3]+,
in which PPh3 occupies a basal coordination site.21 The latter
conformation is undoubtedly stabilized by the mutually trans
arrangement of the H− (strong σ-donor) and CO (π-acceptor)
As expected, the complex dication features a pdt2− ligand
bridging the Ni and Fe centers, which exist in square planar and
octahedral ligand environments, respectively. These coordina-
tion geometries, and a Ni−Fe distance of 3.203 Å, are reflective
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ligands. The stereochemistry at Fe in the new complexes also
differs from the enzyme, in which the two strongly σ-donating
CN− ligands occupy the basal sites.
degree of oxidation, but other low-spin Fe(I) centers with similar δ
(0.12 mm/s) have been reported.47 The low-spin Fe(I)Fe(II)
cluster in Hox from C. pasterurianum has 0.1 < δ < 0.3 mm/s;
in the Hox-CO state from D. vulgaris, it has δ1,2 = 0.13,
0.17 mm/s,45,48 similar to the δ value associated with the Fe(I) in
[4e]BF4.
Using 13CO labeling, we confirmed that both 1 and [1]BF4
rapidly exchange with CO, which may be relevant to the well-
known observation that exogenous CO inhibits [NiFe]-H2ase.
Binding of CO to Ni-SIa affords the EPR-silent Ni-SCO and
Ni-SCOred states,39 while Ni-L reacts with CO to give the Ni-
CO state.40 The latter state is paramagnetic (S = 1/2) and
features CO coordinated to Ni(I).41 Although no analogous
CO adduct of [1]BF4 was observed in our work, short-lived CO
adducts would explain the facility by which these cations
exchange with CO. It appears likely that the exogenous CO
ligand would bind the Fe center; a similar adduct with tertiary
phosphine ligands would be an intermediate in the conversion
of [1]BF4 to [4a−j]BF4.
SUMMARY
■
The first examples of mixed-valence (S = 1/2) nickel−iron
dithiolates have been prepared. Complexes of the type [(dxpe)-
Ni(xdt)Fe(CO)2L]+ are characterized as Ni(II)Fe(I) mixed-
valence species on the basis of structural and spectroscopic
data. The Ni(II)Fe(I) assignment contrasts the Ni(I)Fe(II)
core present in the Ni-L state of [NiFe]-H2ase and is perhaps
more related to the Fe(I)Fe(II) fragment in the Hox state of
[FeFe]-H2ase. The crystallographic results highlight the large
geometric changes upon oxidation of the Ni(I)Fe(I) precursor,
suggesting that stabilization of Ni(I)Fe(II) complexes may
require not just changes in the terminal ligands, but also greater
control of the nickel coordination sphere to better match the
seesaw geometry observed for the enzyme.
There are notable differences between the spectroscopy of
the mixed-valence complexes discussed here and data reported
for the Ni-L state of [NiFe]-H2ase.38 Solution IR data for the
mixed-valence complexes suggest that the Fe center in each of
these derivatives is still electron-poor relative to the enzyme
(νCO = 1911 cm−1 for Ni-L state of D. vulgaris Miyazaki F).42
More significantly, the spin in this state is predominantly local-
ized in the Ni d(x2 − y2) and d(z2) orbitals, with the g-values
obtained being 2.298, 2.116, and 2.043.43 These g-values are
considerably larger than those found for [2]BF4 (2.053, 2.054,
2.010), in which the spin likely resides on the Fe center. It
appears that the incorporation of monophosphines into [1]+ is
insufficient to reverse the Ni(II)Fe(I) oxidation state assign-
ment of [1]+. In fact, instead of resembling Ni-L, the data
for the new complexes are closer to those for the [2Fe]H
component in the Hox state of [FeFe]-H2ase. In the enzyme
isolated from C. pasteurianum, this Fe(I)Fe(II) cluster exhibited
an EPR signal with g = 2.097, 2.039, 1.999.44
EXPERIMENTAL SECTION
■
Unless otherwise stated, all chemicals were purchased from com-
mercial sources and used as received. PMe3 was distilled from CaH2.
The compounds 1,22 2, 3,23 and PPh2(o-C6H4OCH2OCH3)49 were
prepared according to the literature methods.
All reactions were conducted in an MBraun glovebox equipped with
a solvent purification system; the concentrations of O2 and H2O in the
N2 atmosphere were less than 1 ppm. The mixed-valence complexes
were stored at −28 °C. IR spectra of complexes (in CH2Cl2) were
recorded on a Perkin-Elmer Spectrum 100 FTIR spectrometer. EPR
spectra of complexes (∼1 mM in CH2Cl2/PhMe, 1:1) were recorded
on a Varian E-line 12″ Century Series X-band CW spectrometer.
ESI−MS data were acquired using a Waters Micromass Quattro II
spectrometer. Analytical data were acquired using an Exeter Analytical
CE-440 elemental analyzer. UV−vis data were acquired on a Varian
Cary 50 Bio spectrophotometer. NMR spectra were recorded at room
temperature on a Varian Unity 500 spectrometer. 31P{1H} spectra
were collected at 500 MHz and chemical shifts are referenced to
external 85% H3PO4. Crystallographic data were collected using a
Siemens SMART diffractometer equipped with a Mo Kα source (λ =
Mossbauer spectra of the diamagnetic Ni(I)Fe(I) complex 1
̈
and the closely related one-electron-oxidized S = 1/2 species
were collected to give evidence for the description of the
latter as Ni(II)Fe(I) species, given that a more delocalized de-
scription might have been possible. Most useful in this analysis
were the Fe isomer shifts δ, as well as the magnitude and aniso-
tropy of the A-tensors. The δ value was found to be slightly
greater for [4e]BF4 relative to 1, a fact that could suggest the Fe
center in the former to be oxidized relative to the Fe(I) center
in the latter. However, the isomer shift could also reflect dif-
ferences in coordination environment, given that a CO ligand is
substituted for PPh3 in [4e]BF4. Low-spin Fe(II) can be ruled
out given that a very low Fe-centered spin density and A-values
would be expected in such a case. The A-tensor of [4e]BF4
is modest but large enough to support an Fe(I) assignment.
Popescu and co-workers also required a relatively small
0.71073 Å) and an Apex II detector. Mossbauer spectra were collected
̈
for samples either suspended in mineral oil or dissolved in CH2Cl2/
PhMe (1:2). Of the instruments used (MS4 WRC and 12CNDT-6T
spectrometers, SEE Co., Edina MN), the former allowed data collec-
tion at 6 K with 0.05 T field applied parallel to the γ rays, while
the latter gave data at 4.3 K with perpendicular fields as high as 6 T.
Spectra were analyzed with WMOSS software. Isomer shifts are
quoted relative to α-iron at 298 K.
[(dppe)Ni(pdt)Fe(13CO)3] (1′). Compound 1 (21.1 mg, 30.0 μmol)
was dissolved in CH2Cl2 (5 mL), and the solution was frozen with
liquid N2. The reaction vessel was evacuated; the solution was placed
under 13CO (1 atm) and warmed to room temperature. The mixture
was briefly agitated, allowed to stand for 2 h, and evaporated to
dryness to afford the product as a green powder (quant.).
FT-IR: νCO = 1982, 1913 cm−1. 13C NMR (126 MHz) 209.3 ppm.
31P NMR (202 MHz) 63.5 ppm. Anal. Calcd for C2913C3H30FeNiO3-
P2S2·0.5CH2Cl2: C, 52.14; H, 4.17; N, 0.00. Found: C, 52.41; H,
4.36; N, 0.00.
Aiso value (9.5 MHz) to simulate Mossbauer spectra of the
̈
[2Fe]H component in the Hox-CO state of [FeFe]-H2ase from
Clostridium pasterurianum,45 although the magnetic hyperfine
tensors used were isotropic unlike those employed to simulate
spectra of [4e]BF4. From DFT studies using the ADF program,
Brunold and co-workers calculated highly anisotropic A values,
including +11, −1, and −27.5 MHz, for the Fe(I) center in
Hox.46 These values are similar in both sign and magnitude to
those used in our simulation, indicating that the 57Fe hyperfine
tensor for [4e]BF4 is best explained by a low-spin Fe(I) center.
The assignment is also corroborated by our DFT calculations,
which gave an unpaired spin density of 0.85e− at the Fe atom of
[4e]BF4. The increase of δ, relative to that of 1, suggests some
[(dppe)Ni(pdt)Fe(CO)3]BF4 ([1]BF4). Compound 1 (14.1 mg,
20.0 μmol) and FcBF4 (5.5 mg, 20.0 μmol) were dissolved in CH2Cl2
(2 mL) with rapid stirring. After 1 min, pentane (−28 °C, 15 mL) was
added, and the mixture was allowed to stand at −28 °C for 10 min.
A solid was isolated by filtration, washed with pentane (−28 °C, 2 ×
2 mL), and dried briefly to afford the product as a brown powder
(12.0 mg, 15.2 μmol, 76%).
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Article
FT-IR: νCO = 2057, 1986 cm−1. ESI-MS: m/z 702.1 [M − BF4−]+.
Anal. Calcd for C32H30BF4FeNiO3P2S2·0.5CH2Cl2: C, 46.89; H, 3.75;
N, 0.00. Found: C, 46.73; H, 3.85; N, 0.28.
PPh2(OEt) − BF4−]2+. Anal. Calcd for C45H45BF4FeNiO3P3S2·
0.25CH2Cl2: C, 53.63; H, 4.53; N, 0.00. Found: C, 53.50; H, 4.74;
N, 0.06.
[(dppe)Ni(pdt)Fe(CO)3]PF6 ([1]PF6). This salt was prepared
analogously to [1]BF4, using FcPF6 in place of FcBF4. Yield: 78%,
brown powder.
[(dppe)Ni(pdt)Fe(CO)2PPh3]BF4 ([4e]BF4). Yield: 93%, yellow-
brown powder. ESI-MS: m/z 936.3 [M − BF4−]+. Anal. Calcd for
C49H45BF4FeNiO2P3S2·0.75CH2Cl2: C, 54.92; H, 4.31; N, 0.00. Found:
C, 54.70; H, 4.32; N, 0.34.
FT-IR: νCO = 2057, 1986 cm−1. Anal. Calcd for C32H30F6FeNiO3-
P3S2·0.25CH2Cl2: C, 44.55; H, 3.54; N, 0.00. Found: C, 44.51; H,
3.61; N, 0.32.
[(dppe)Ni(pdt)Fe(13CO)2PPh3]BF4 ([4e′]BF4). Yield: 73%, yellow-
brown powder. ESI-MS: m/z 938.0 [M − BF4−]+. Anal. Calcd for
C4713C2H45BF4FeNiO2P3S2·0.75CH2Cl2: C, 54.82; H, 4.30; N, 0.00.
Found: C, 54.70; H, 4.29; N, 0.00.
[(dppe)Ni(pdt)Fe(13CO)3]BF4 ([1′]BF4). This salt was prepared
analogously to [1]BF4, using 1′ as the precursor. Yield: 78%, yellow-
brown powder.
[(dppe)Ni(pdt)Fe(CO)2PPh2(o-C6H4OMe)]BF4 ([4f]BF4). Yield: 67%,
brown powder. ESI-MS: m/z 966.2 [M − BF4−]+. Anal. Calcd for
C50H47BF4FeNiO3P3S2·0.5CH2Cl2: C, 55.30; H, 4.41; N, 0.00. Found:
C, 55.09; H, 4.25; N, 0.38.
[(dppe)Ni(pdt)Fe(CO)2PPh2(o-C6H4OCH2OMe)]BF4 ([4g]BF4).
Yield: 63%, brown powder. ESI-MS: m/z 996.4 [M − BF4−]+. Anal.
Calcd for C51H49BF4FeNiO4P3S2·CH2Cl2: C, 53.41; H, 4.40; N, 0.00.
Found: C, 53.44; H, 4.35; N, 0.34.
FT-IR: νCO = 2009, 1941 cm−1. Anal. Calcd for C3213C3H30BF4-
FeNiO3P2S2·0.75CH2Cl2: C, 45.91; H, 3.71; N, 0.00. Found: C, 45.96;
H, 3.80; N, 0.00.
[1]BArF . Compound 1 (7.0 mg, 10 μmol) and FcBArF (10.5 mg,
4
4
10 μmol) were dissolved in CH2Cl2 (1 mL) with rapid stirring. The
deep brown solution was used in situ for reactivity studies.
FT-IR: νCO = 2057, 1986 cm−1.
[(dppe)Ni(pdt)Fe(CO)2P(p-tol)3]BF4 ([4h]BF4). Yield: 89%, yellow-
brown powder. ESI-MS: m/z 978.3 [M − BF4−]+. Anal. Calcd for
C52H51BF4FeNiO2P3S2·0.5CH2Cl2: C, 56.87; H, 4.73; N, 0.00. Found:
C, 56.58; H, 4.63; N, 0.32.
[(dppe)Ni(pdt)Fe(CO)2P(p-C6H4OMe)3]BF4 ([4i]BF4). Yield: 96%,
yellow-brown powder. ESI-MS: m/z 1026.3 [M − BF4−]+. Anal.
Calcd for C52H51BF4FeNiO5P3S2·0.25CH2Cl2: C, 55.26; H, 4.57; N,
0.00. Found: C, 55.26; H, 4.75; N, 0.25.
[(dppe)Ni(pdt)Fe(CO)2PMePh2]BF4 ([4j]BF4). Yield: 76%, brown
powder. ESI-MS: m/z 874.3 [M − BF4−]+. Anal. Calcd for C44H43BF4-
FeNiO2P3S2·CH2Cl2: C, 51.62; H, 4.33; N, 0.00. Found: C, 51.90; H,
4.35; N, 0.23.
[(dppe)Ni(pdt)Fe(CO)2dppe](BF4)2 ([5](BF4)2). Compound 1
(14.1 mg, 20 μmol) and FcBF4 (10.9 mg, 40 μmol) were partially
dissolved in CH2Cl2 (2 mL) with rapid stirring. After 1 min, the
solution was treated dropwise with dppe (8.0 mg, 20 μmol) in CH2Cl2
(0.5 mL). The solution was stirred for another 0.5 min, pentane
(−28 °C, 15 mL) was added, and the mixture was allowed to stand
at −28 °C for 10 min. The solids were isolated by filtration, washed
with pentane (−28 °C, 2 × 2 mL), and dried briefly to afford the title
compound (16.4 mg, 13.2 μmol, 66%) as an orange powder.
FT-IR: νCO = 1967 cm−1. 31P{1H} NMR (CD2Cl2, 202 MHz) 59.5,
58.3 ppm. Anal. Calcd for C57H54B2F8FeNiO2P4S2: C, 54.89; H, 4.36;
N, 0.00. Found: C, 54.77; H, 4.41; N, 0.00.
[(dppe)Ni(edt)Fe(CO)3]BF4 ([2]BF4). Compound 2 (6.9 mg,
10.0 μmol) and FcBF4 (2.7 mg, 10.0 μmol) were dissolved in
CH2Cl2 (1 mL) with rapid stirring. The deep brown solution was used
for in situ EPR and IR analyses, the latter indicating that significant
amounts of the diamagnetic species 2 and [2H]BF4 were present in
the crude mixture. The solution was diluted with PhMe for the EPR
analysis.
FT-IR: νCO = 2059, 1988 cm−1. ESI-MS: m/z 688.1 [M − BF4−]+.
[(dcpe)Ni(pdt)Fe(CO)3]BF4 ([3]BF4). This salt was prepared ana-
logously to [1]BF4, using 3 as the precursor. Yield: 78%, brown
powder.
FT-IR: νCO = 2054, 1982 cm−1. ESI-MS: m/z 726.3 [M − BF4−]+.
Anal. Calcd for C32H54BF4FeNiO3P2S2·2.25CH2Cl2: C, 40.92; H, 5.87;
N, 0.00. Found: C, 40.90; H, 5.82; N, 0.00.
Brown plate-like single crystals of [3]BF4·2CH2Cl2 were grown by
layering a concentrated CH2Cl2 solution with pentane and allowing
the mixture to stand at −28 °C. One crystal (0.544 × 0.295 ×
0.062 mm3) was subjected to X-ray diffraction at 193 K. Its space
group was determined to be monoclinic P21/c with cell parameters:
a = 16.680 Å, b = 15.715 Å, c = 17.193 Å, α = 90.00°, β = 92.84°, γ =
90.00°. Integration of 3591 reflections and solution by direct methods
using SHELXTL V6.1250,51 afforded a model with R1 = 0.0527 and
wR2 = 0.1228.
Phosphine-Substituted Derivatives ([4a−j]BF4). Compound 1
(14.1 mg, 20 μmol) and FcBF4 (5.5 mg, 20 μmol) were dissolved in
CH2Cl2 (2 mL) with rapid stirring. After 1 min, the solution was
added dropwise to the appropriate phosphine (200 μmol) in CH2Cl2
(0.5 mL). The solution was stirred for another 0.5 min, pentane
(−28 °C, 15 mL) was added, and the mixture was allowed to stand
at −28 °C for 1 h. The solids were isolated by filtration, washed with
pentane (−28 °C, 2 × 2 mL), and dried briefly to afford the respective
phosphine complexes. The 13CO derivatives were prepared analog-
ously using 1′ as the precursor.
Orange prismatic single crystals of [5](BF4)2·4CH2Cl2 were grown
by layering a concentrated CH2Cl2 solution with pentane and allowing
the mixture to stand at −28 °C. One crystal (0.261 × 0.237 × 0.108
mm3) was subjected to X-ray diffraction at 193 K. Its space group was
determined to be triclinic P1 with cell parameters: a = 16.680 Å, b =
̅
14.195 Å, c = 21.288 Å, α = 78.98°, β = 75.07°, γ = 64.26°. Integration
of 9944 reflections and solution by direct methods using SHELXTL
V6.1250,51 afforded a model with R1 = 0.0465 and wR2 = 0.1126.
[(dppe)Ni(pdt)Fe(CO)2P(OPh)3]BF4 ([4a]BF4). Yield: 78%, yellow-
brown powder. ESI-MS: m/z 983.9 [M − BF4−]+. Anal. Calcd for
C49H45BF4FeNiO5P3S2·0.5CH2Cl2: C, 53.33; H, 4.16; N, 0.00. Found:
C, 53.41; H, 4.11; N, 0.31.
CALCULATIONS
■
Calculations of structural parameters and the electronic
structure were performed using ORCA.52 Geometry optimiza-
tions were performed using the BP86 exchange−correlation
functional30,31 and a triple-ζ basis set with polarization func-
tions that were obtained from the TURBOMOLE53 library.
This combination of exchange−correlation functional and
basis set was shown to give accurate structural parameters. In
addition, single-point calculations using the hybrid B3LYP32
and PBE033,34 functionals on the BP86/TZVP geometry
optimized structures were carried out. This combination of
exchange−correlation functional and basis set was shown to
give accurate structural parameters. IR spectra were generated
by numerically calculating second derivatives; calculations of
g-tensors were performed using an effective mean-field spin−
orbit coupling operator, with the center-of-mass as the origin
[(dppe)Ni(pdt)Fe(13CO)2P(OPh)3]BF4 ([4a′]BF4). Yield: 70%, yellow-
brown powder. ESI-MS: m/z 986.0 [M − BF4−]+. Anal. Calcd for
C4713C2H45BF4FeNiO5P3S2·0.5CH2Cl2: C, 53.24; H, 4.15; N, 0.00.
Found: C, 52.97; H, 4.15; N, 0.00.
[(dppe)Ni(pdt)Fe(CO)2P(p-C6H4Cl)3]BF4 ([4b]BF4). Yield: 87%,
yellow-brown powder. ESI-MS: m/z 1040.0 [M − BF4−]+. Anal.
Calcd for C49H42BF4FeNiO2P3S2Cl3·0.67CH2Cl2: C, 50.36; H, 3.69;
N, 0.00. Found: C, 50.49; H, 3.77; N, 0.01.
[(dppe)Ni(pdt)Fe(CO)2PPh2(2-py)]BF4 ([4c]BF4). Yield: 80%, yellow-
brown powder. ESI-MS: m/z 937.1 [M − BF4−]+, 908.0 [M − CO −
BF4−]+. Anal. Calcd for C48H44BF4FeNiNO2P3S2·0.25CH2Cl2: C,
55.38; H, 4.29; N, 1.34. Found: C, 55.30; H, 4.28; N, 1.58.
[(dppe)Ni(pdt)Fe(CO)2PPh2(OEt)]BF4 ([4d]BF4). Yield: 81%, yellow-
brown powder. ESI-MS: m/z 904.0 [M − BF4−]+, 567.2 [M +
2347
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Article
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basis set was used for spin-restricted g-tensor calculations,58 and
a TZP basis set was used for spin-unrestricted scalar relativistic
hyperfine and quadrupole coupling tensor calculations.59,60
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ASSOCIATED CONTENT
■
S
* Supporting Information
Spectroscopy results and calculated g-tensor values for the
studied compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENTS
■
The authors wish to thank Bryan Barton for helpful discussions
and Drs. Danielle L. Gray and Amy L. Fuller for X-ray
crystallography. This work was supported by the National
Institutes of Health (Grants GM061153 to T.R., GM46441 and
GM084266 to P.L.).
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