Mechanism of H2 Production by Models for the [NiFe]-Hydrogenases: Role of Reduced Hydrides

Article abstract of DOI:10.1021/jacs.6b04579

The intermediacy of a reduced nickel-iron hydride in hydrogen evolution catalyzed by Ni-Fe complexes was verified experimentally and computationally. In addition to catalyzing hydrogen evolution, the highly basic and bulky (dppv)Ni(μ-pdt)Fe(CO)(dppv) ([1]⁰ dppv = cis-C₂H₂(PPh₂)₂) and its hydride derivatives have yielded to detailed characterization in terms of spectroscopy, bonding, and reactivity. The protonation of [1]⁰ initially produces unsym-[H1]⁺, which converts by a first-order pathway to sym-[H1]⁺. These species have C₁ (unsym) and C_s (sym) symmetries, respectively, depending on the stereochemistry of the octahedral Fe site. Both experimental and computational studies show that [H1]⁺ protonates at sulfur. The S = 1/2 hydride [H1]⁰ was generated by reduction of [H1]⁺ with Cp?₂Co. Density functional theory (DFT) calculations indicate that [H1]⁰ is best described as a Ni(I)-Fe(II) derivative with significant spin density on Ni and some delocalization on S and Fe. EPR spectroscopy reveals both kinetic and thermodynamic isomers of [H1]⁰. Whereas [H1]⁺ does not evolve H₂ upon protonation, treatment of [H1]⁰ with acids gives H₂. The redox state of the "remote" metal (Ni) modulates the hydridic character of the Fe(II)-H center. As supported by DFT calculations, H₂ evolution proceeds either directly from [H1]⁰ and external acid or from protonation of the Fe-H bond in [H1]⁰ to give a labile dihydrogen complex. Stoichiometric tests indicate that protonation-induced hydrogen evolution from [H1]⁰ initially produces [1]⁺, which is reduced by [H1]⁰. Our results reconcile the required reductive activation of a metal hydride and the resistance of metal hydrides toward reduction. This dichotomy is resolved by reduction of the remote (non-hydride) metal of the bimetallic unit.

Full text of DOI:10.1021/jacs.6b04579

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Article
2
Mechanism of H Production by Models for the
[NiFe]-Hydrogenases: Role of Reduced Hydrides
Olbelina A. Ulloa, Mioy T. Huynh, Casseday P. Richers, Jeffery A. Bertke,
Mark J. Nilges, Sharon Hammes-Schiffer, and Thomas B. Rauchfuss
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b04579 • Publication Date (Web): 21 Jun 2016
Downloaded from http://pubs.acs.org on June 26, 2016
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Mechanism of H2 Production by Models for the [NiFe]-
Hydrogenases: Role of Reduced Hydrides
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Olbelina A. Ulloa,§ Mioy T. Huynh,§ Casseday P. Richers,§ Jeffery A. Bertke, Mark J. Nilges,
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Sharon HammesꢀSchiffer,* Thomas B. Rauchfuss*
Department of Chemistry, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
Abstract
The intermediacy of a reduced nickelꢀiron hydride in hydrogen evolution catalyzed by
NiꢀFe complexes was verified experimentally and computationally. In addition to catalyzing
hydrogen evolution, the highly basic and bulky (dppv)Ni(ꢀꢀpdt)Fe(dppv)(CO) ([1]0, dppv = cisꢀ
C2H2(PPh2)2) and its hydride derivatives have yielded to detailed characterization in terms of
spectroscopy, bonding, and reactivity. The protonation of [1]0 initially produces unsymꢀ[H1]+,
which converts by a first order pathway to symꢀ[H1]+. These species have C1 (unsym) and Cs
(sym) symmetry, respectively, depending on the stereochemistry of the octahedral Fe site. Both
experimental and computational studies show that [H1]+ protonates at sulfur. The S = ½ hydride
[H1]0 was generated by reduction of [H1]+ with Cp*2Co. DFT calculations indicate that [H1]0 is
best described as a Ni(I)ꢀFe(II) derivative with significant spin density on Ni and some
delocalization on S and Fe. EPR spectroscopy reveals both kinetic and thermodynamic isomers
of [H1]0. Whereas [H1]+ does not evolve H2 upon protonation, treatment of [H1]0 with acids
gives H2. The redox state of the "remote" metal (Ni) modulates the hydridic character of the
Fe(II)ꢀH center. As supported by DFT calculations, H2 evolution proceeds either directly from
[H1]0 and external acid, or from protonation of the FeꢀH bond in [H1]0 to give a labile
dihydrogen complex. Stoichiometric tests indicate that protonationꢀinduced hydrogen evolution
from [H1]0 initially produces [1]+, which is reduced by [H1]0. Our results reconcile the required
reductive activation of a metal hydride and the resistance of metal hydrides toward reduction.
This dichotomy is resolved by reduction of the remote (nonꢀhydride) metal of the bimetallic unit.
§These authors contributed equally to this work.
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Introduction
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The crystallographic characterization of the active sites of [NiFe]ꢀ and [FeFe]ꢀ
hydrogenase (H2ases) sparked the synthesis and study of many low molecular weight models.
These models replicate some aspects of the biological reactions,1 shedding light on how these
enzymes operate and perhaps opening avenues for bioinspired if not biomimetic reactivity of
interest in the energy sector.
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Scheme 1. Active site states of [NiFe] hydrogenases. Only unidirectional conversions are shown for
simplicity.
Mechanistic attention is particularly focused on hydrideꢀcontaining models since hydride
is the substrate both in H2 oxidation and proton reduction. Only a small number of hydrideꢀ
containing containing models for the [NiFe]ꢀenzymes have been designed.2ꢀ4 A versatile family
2–
of models are of the type [(diphos)Ni(ꢀꢀpdt)(ꢀꢀH)Fe(PR3)x(CO)3–x]+ (pdt2– = S2C3H6 ; diphos =
dppe = 1,2ꢀC2H4(PPh2)2, dppbz = 1,2ꢀC6H4(PPh2)2, dppv = cisꢀ1,2ꢀC2H2(PPh2)2). In the case of
the Fe(CO)3 and Fe(CO)2(PR3) derivatives, the NiꢀFeꢀhydrides are notable because they catalyze
proton reduction,2,3,5,6 as well as, in one case,7 hydrogen oxidation.8
Hydrogen evolution catalyzed both by the enzymes and by the models operates by the
intermediacy of Ni(II)ꢀHꢀFe(II) species.9ꢀ11 In the enzyme, this species, the NiꢀR state, releases
H2 via protonolysis to give an unsaturated Ni(II)ꢀFe(II) species (NiꢀSIa) (Scheme 1). The
intimate details of H2 release in the enzyme remain an area of active investigation,12 one
complicating aspect being the protonation state of the terminal thiolate cofactors. The Ni(II)ꢀHꢀ
Fe(II) models do not release H2 upon treatment with proton donors. Instead, hydrogen evolution
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occurs only upon reduction.3,5,13,14 A similar pattern is often observed with Fe(II)ꢀHꢀFe(II)
(bridging hydride14) and Fe(II)ꢀFe(II)ꢀH (terminal hydride15) complexes, which also release H2
only upon reduction.16 For the [FeFe]ꢀhydrogenase models, the ETꢀinduced protonation of an
Fe(II)ꢀFe(II)ꢀH species is explained by the involvement of a transient Hsred state featuring an
appended [4Feꢀ4S]+ subunit.9
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Since hydrogen evolution from NiꢀHꢀFe complexes commences upon reduction, reduced
NiꢀHꢀFe hydrides represent important targets for characterization. Electrochemical reduction of
the [(diphos)Ni(ꢀꢀpdt)(ꢀꢀH)Fe(PR3)x(CO)3–x]+species leads to unstable products, however,
probably owing to bimolecular degradation pathways.3 Such bimolecular processes should be
suppressed in sterically bulky NiꢀFe hydrides. For these reasons, our attention turned to
[(dppv)Ni(ꢀꢀpdt)(ꢀꢀH)Fe(CO)(dppv)]z, a new family of [NiFe]ꢀhydrogenase models related to
the [(dppv)(CO)Fe(ꢀꢀpdt)(ꢀꢀH)Fe(CO)(dppv)] platform that has illuminated aspects of the
[FeFe]ꢀhydrogenase chemistry.14,17 The dppv ligand confers stability to these models and
minimizes side reactions that can plague more flexible chelating diphosphines.18,19
Starting with the preparation and protonation of [(dppv)Ni(ꢀꢀpdt)(ꢀꢀH)Fe(CO)(dppv)]0,
this report describes previously unseen insights related to hydrogen evolution catalysis. Of
particular interest is the observation of the reduced hydride, which is the species that is
protonated, thereby liberating H2. Our results reconcile the required reductive activation of a
metal hydride and the resistance of metal hydrides toward reduction.20 This dichotomy is shown
to be resolved when ET occurs at the remote (nonꢀhydride) metal of the bimetallic unit.
Results and Discussion
Following precedent for related reactions,2 the synthesis of [1]0 began with the reaction of
Fe(pdt)(dppv)(CO)2, NiCl2(dppv), and NaBF4 to give the Ni(II)ꢀFe(II) species [(dppv)Ni(ꢀꢀ
pdt)(ꢀꢀCl)Fe(CO)(dppv)]BF4 ([Cl1]BF4). Although Fe(pdt)(dppv)(CO)2 consists of a 1:4
mixture of allꢀcis and trans isomers, the initially formed [Cl1]+ consists only of the
unsymmetrical (unsym) isomer wherein the dppv on Fe spans apical and basal coordination sites.
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Reduction of a THF solution of unsymꢀ[Cl1]BF4 with KFe(CO)2Cp gave [1]0 (
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+
S
P
S
S
P
S
CO
CO
S
2 KFp
S
P
NiCl2(dppv)
NaBF4
Fe(II)
P
Ni(II)
P
P
Fe(II)
Ni(I)
Fe(I)
P
P
Cl
P
9
OC
P
OC
[Cl1]+
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unsymꢀ[1]0
H+
+
+
S
Fe(II)
P
OC
CO
P
Fe(II)
S
S
S
S
S
Ni(II)
P
Fe(II)
Ni(II)
P
P
P
P
OC
P
P
H
P
H
OC
symꢀ[H1]+
unsymꢀ[H1]+
eꢀ
eꢀ
OC
Fe(II)
P
P
Fe(II)
P
P
S
S
S
S
S
S
P
P
Ni(I)
Ni(I)
Ni(I)
Fe(I)
P
P
P
P
H
P
P
H
OC
OC
unsymꢀ[1]0
symꢀ[H1]0
unsymꢀ[H1]0
P
S
S
Ni(II)
P
P
Fe(0)
P
OC
Scheme 2).
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At room temperature, the 31P{1H} NMR spectrum of [1]0 in C6D6 solution is simple:
signals assigned to the Fe(dppv) center appear as a triplet (δ 89.2), and those assigned to the
Ni(dppv) center appear as two doublets (δ 74.2 and 46.2). These assignments are consistent with
the 31P{1H} NMR spectrum of the 57Fe derivative, which revealed broadening of the signal
assigned to the Fe(dppv)(CO) center (Figure S19). The chemical shifts of the signals assigned to
the Ni(dppv) group are similar to those for [(dppe)Ni(ꢀꢀpdt)Fe(CO)3]0 (δ 81 and 46, at –68 °C).3
The Ni(dppe) center of [(dppe)Ni(ꢀꢀpdt)Fe(CO)3]0 is dynamic on the NMR timescale, such that
only a single 31P signal (δ 63.6) appears at 20 °C.6 In contrast, two 31P signals assigned to the
Ni(dppv) site in [1]0 are observed even up to 90 ºC, indicating a barrier to rotation at Ni of >14
kcal/mol (Figure S23). Upon cooling a toluene solution of [1]0 below –65 °C, four signals
appear in the 31P{1H} NMR spectrum. This decoalescence reveals the full complexity of this
chiral species.
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Scheme 2. Synthesis of [(dppv)Ni(µꢀpdt)Fe(CO)(dppv)]0 ([1]0) and its hydride derivatives (PhꢀP
substituents omitted for clarity).
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Crystallographic analysis of [1]0 confirms the unsymmetrical structure suggested by the
NMR data (Figure 1). The FeꢀNi bond of 2.5261(4) Å is 0.06 Å longer than that for
[(dppe)Ni(ꢀꢀpdt)Fe(CO)3]0 (2.466(6) Å),19 an elongation attributed to steric interactions between
the two bulky M(dppv) centers. The iron center of [1]0 adopts a distorted octahedral geometry,
viewing the FeꢀNi bond as a sixth ligand. The nickel center adopts a distorted trigonal
bipyramidal geometry, viewing the FeꢀNi bond as a fifth ligand, in addition to two phosphines
and two thiolates. The Fe1ꢀNi1ꢀP2 angle is 162.2° and the dihedral angle is 80.94° for the S1ꢀNiꢀ
S2 and P1ꢀNiꢀP2 planes. For comparison, the corresponding values for [(dppe)Ni(ꢀꢀ
pdt)Fe(CO)3]0 are 169.8° and 84.9°.19 The NiꢀP distances are comparable to those in Ni(dppv)2
(2.1414(6) ꢀ 2.1586(6) Å) as well as [(dppe)Ni(ꢀꢀpdt)Fe(CO)3]0 (2.159 ꢀ 2.146 Å).19
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Figure 1. Molecular structure of the nonꢀhydrogen atoms of [1]0 showing 50% probability ellipsoids.
DFT-Optimized Structure of [1]0. The DFTꢀoptimized structure of [1]0 is in good
agreement with the crystal structure. Similar to related dithiolate compounds,6,19 [1]0 also
contains a NiꢀFe σꢀbond that can be localized using either the Natural Bond Orbital (NBO)21 or
the PipekꢀMezey criterion (Figure S61).22
The 31P NMR resonance at δ89 is assigned to the Fe(dppv)(CO) center. This signal
decoalesces at low temperatures in toluene, indicating a rotational barrier of ~9 kcal/mol, in good
agreement with the DFTꢀcalculated free energy barrier of ꢁG‡ = 9.55 kcal/mol in toluene.
Interchange of the PFe sites could conceivably occur via an intermediate wherein the dppv
occupies dibasal sites (i.e., sites that are trans to the dithiolate) and the CO ligand occupies the
apical site trans to nickel. According to our calculations, the formation of this dibasal
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intermediate is thermodynamically accessible (ꢁG° = 5.26 kcal/mol) but is subject to a high free
energy barrier of 27 kcal/mol in toluene. A lower free energy pathway is shown in Scheme 3
with a transition state (TS) in which the dppv ligand occupies sites cis to the dithiolate. In the TS
structure, the NiꢀFe bond remains ~2.52 Å. A similar pathway has been invoked for Fe2(ꢀꢀ
pdt)(CO)4(diphos) and related complexes.23,24
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Pb
Fe
Pa
Fe
S
S
Pb
S
S
S
P
S
P
P
Ni
Ni
Ni
Fe
Pa
OC
Pb
OC
OC
P
P
P
Pa
Scheme 3. DFTꢀcalculated lowꢀenergy pathway for degenerate isomerization of [1]0. Notice that the
dibasal isomer is not an intermediate in this process (PhꢀP groups omitted for clarity)
The calculations do not reveal a minimum with a square planar Ni center. In contrast,
previous calculations on [(dppe)Ni(ꢀꢀpdt)Fe(CO)3]0 showed that the valence isomers, with either
square planar Ni(II) or tetrahedral Ni(I) centers, are nearly isoergic. Formation of square planar
Ni(II) requires an intramolecular electron transfer to convert Fe(I) to Fe(0). The [(dppe)Ni(ꢀꢀ
pdt)Fe(CO)3]+/0 couple (–0.59 V) is ~300 mV milder than the [1]+/0 couple, indicating the greater
reluctance for the (RS)2Fe(dppv)(CO) vs (RS)2Fe(CO)3 site to undergo reduction from Fe(I) to
Fe(0) in [1]0.6
The IR spectrum of [1]0 in THF solution shows a dominant νCO band at 1897 cm–1 with a
shoulder at ~1912 cm–1. These signals probably arise from two conformers ("flippamers"),
resulting from the relative orientation of the propanedithiolate backbone. DFT predicts 1905 and
1908 cm–1 for the two flippamers of the unsym isomer.
Redox of [1]0. Cyclic voltammetry (CV) studies on a 1,2ꢀC6H4F2 solution of [1]0
revealed a single redox couple assigned to [1]+/0 at –1.13 V vs Fc0/+. The couple is quasiꢀ
reversible on the basis of the current ratio, ipa/ipc ~ 1.16, and a fairly linear dependence of i vs
(scan rate)1/2 (Figures S37). Reflecting the highly reactive nature of [1]+, the CV of a PhCN
solution was very different, involving two oxidation and two reductive processes. The latter
processes are almost certainly the result of solvent coordination.
DFT Characterization of [1]+. Using DFT, two valence isomers were characterized for
[1]+. These two isomers differ in oxidation states for the two metals: Ni(I)Fe(II) vs Ni(II)Fe(I).
The calculations indicate that these two species also differ with respect to the geometry at Ni
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(Figure 2). Moreover, the FeꢀNi bond is disrupted (2.91 Å) in Ni(II)Fe(I) compared to the
Ni(I)Fe(II) valence isomer (2.52 Å). The sym isomer of [1]+ was also characterized using DFT,
and is qualitatively similar to the unsym isomer presented here.
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Figure 2. Spin density plots (isovalue = 0.005) for the Ni(I)Fe(II) (left) and Ni(II)Fe(I) (right) valence
isomers of unsymꢀ[1]+.
The spin densities for these two valence isomers are given in
Table 1. The calculated reaction free energy is small (ꢁG° = −1.36 kcal/mol) for the
isomerization from Ni(II)Fe(I) to Ni(I)Fe(II), and the calculated free energy barrier for this
isomerization is also relatively low (ꢁG‡ = 2.75 kcal/mol). Thus, the calculations indicate that
both isomers could be thermally accessible at room temperature but do not allow for the
identification of a kinetic or thermodynamic preference. The calculate νCO values are 1948 and
1913 cm–1 for the Ni(I)Fe(II) and Ni(II)Fe(I) valence isomers, respectively. Solutions of [1]+
were generated oxidation of [1]0 with [Fc]PF6 followed by immediate analysis by FTꢀIR and
EPR spectroscopy.
Table 1. Calculated Spin Densities for the Valence Isomers of [1]+.a
Valence
Isomer
ρ(Ni)
ρ(Fe)
ρ(2S) ρ(2PNi)b ρ(2PFe)c
0.05
0.05
0.59
0.64
0.87
0.90
0.04
–0.05
–0.02 0.01
–0.02 0.01
0.05
0.05
–0.02
0.00
Ni(II)Fe(I)
0.33
0.32
0.03
0.05
Ni(I)Fe(II)
aFor each entry, the first and second entries corresponds to the unsym and sym isomers, respectively.
bThese values refer to the P atoms of the dppv ligand on the Ni center.
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bThese values refer to the P atoms of the dppv ligand on the Fe center.
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Protonation of [1]0. Protonation of [1]0 with HOTf or H(OEt2)2BArF afforded a single
unsymmetrical isomer of [H1]+ (
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+
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S
P
S
S
P
S
CO
CO
S
2 KFp
S
P
NiCl2(dppv)
NaBF4
Fe(II)
P
Ni(II)
P
P
Fe(II)
Ni(I)
Fe(I)
P
P
Cl
P
OC
P
OC
[Cl1]+
unsymꢀ[1]0
H+
+
+
S
Fe(II)
P
OC
CO
P
Fe(II)
S
S
S
S
S
Ni(II)
P
Fe(II)
Ni(II)
P
P
P
OC
P
P
P
H
P
H
OC
symꢀ[H1]+
unsymꢀ[H1]+
eꢀ
eꢀ
OC
Fe(II)
P
P
Fe(II)
P
S
P
S
S
S
S
S
P
P
Ni(I)
Ni(I)
Ni(I)
Fe(I)
P
P
P
P
H
P
P
H
OC
OC
unsymꢀ[1]0
symꢀ[H1]0
unsymꢀ[H1]0
P
S
S
Ni(II)
P
P
Fe(0)
P
OC
Scheme 2). Three 31P{1H} NMR signals (intensities: 1:1:2) for unsymꢀ[H1]+ appear at δ
93.4, 79.4, and 73.5, respectively. The signals at δ 93.4 and 79.4 are assigned to the Fe(dppv)
group, and the signal at δ 73.5 is assigned to the Ni(dppv) group. Upon cooling the toluene
solution of [H1]+ to –40 °C, the Ni(dppv) signal decoalesces into two broad singlets (Figure
S33). These observations are consistent with the Fe center being more rigid than the Ni site
(Scheme 4).
+
+
+
Pb
Fe
Pa
Fe
S
S
S
S
Pb
Fe
S
S
slow
fast
P
P
Ni
Ni
P'
Ni
OC
Pb
OC
P'
P
P
H
H
OC
Pa
H
Pa
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Scheme 4. Variable temperature NMR measurements establish that Ni(II) site is nonrigid in unsymꢀ
[H1]+. PhꢀP groups were omitted for clarity.
8
9
Over the course of a few hours in MeCN solution at 25 ºC, unsymꢀ[H1]+ partially
converts to symꢀ[H1]+ (
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+
S
P
S
S
P
S
CO
CO
S
2 KFp
S
P
NiCl2(dppv)
NaBF4
Fe(II)
P
Ni(II)
P
P
Fe(II)
Ni(I)
Fe(I)
P
P
Cl
P
OC
P
OC
[Cl1]+
unsymꢀ[1]0
H+
+
+
S
Fe(II)
P
OC
CO
P
Fe(II)
S
S
S
S
S
Ni(II)
P
Fe(II)
Ni(II)
P
P
P
P
OC
P
P
H
P
H
OC
symꢀ[H1]+
unsymꢀ[H1]+
eꢀ
eꢀ
OC
Fe(II)
P
P
Fe(II)
P
P
S
S
S
S
S
S
P
P
Ni(I)
Ni(I)
Ni(I)
Fe(I)
P
P
P
P
H
P
P
H
OC
OC
unsymꢀ[1]0
symꢀ[H1]0
unsymꢀ[H1]0
P
S
S
Ni(II)
P
P
Fe(0)
P
OC
Scheme 2). The conversion follows firstꢀorder kinetics with a rate constant of 5.2 x 10–5
s–1. Based on transition state theory, this rate for conversion of unsymꢀ[H1]+ to symꢀ[H1]+
corresponds to ꢁG‡ ~ 23 kcal/mol. The DFTꢀcalculated free energy barrier for this isomerization
is 24 kcal/mol. The symmetrical isomer gives rise to two singlets in the 31P{1H} NMR spectrum.
Consistent with its high symmetry, symꢀ[H1]+ exhibits a 31P{1H} NMR spectrum that does not
change over the range +35 to –40 ºC. At equilibrium, the sym:unsym ratio is 1:5 and 1:8 in
MeCN and CH2Cl2 solutions, respectively.
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Crystallographic and DFT Analysis of the Ni(II)-H-Fe(II) Complex. The salt unsymꢀ
[H1]BArF was characterized crystallographically. It features an octahedral iron center and an
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approximately square pyramidal nickel center (Figure 3). The Fe center adopts the unsym
geometry, which is the predominant isomer in solution.
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Figure 3. Molecular structure of the cation in [H1]BArF with 50% probability thermal ellipsoids. This
structure corresponds to the unsym isomer. Hydrogen atoms that were not refined, the counter ion, and
solvent molecules have been omitted for clarity.
4
The structures of both the sym and unsym isomers of [H1]+ were optimized using DFT,
and the unsym isomer was found to be 2.35 kcal/mol lower in free energy than the sym isomer.
The optimized unsym structure agrees well with the crystallographic result, as shown by the
selected bond lengths and angles provided in Table 2. Consistent with the crystal structure of
unsymꢀ[H1]+, the optimized structures of symꢀ[H1]+ and unsymꢀ[H1]+ both feature
asymmetrically bound hydrides, with the hydride closer to Fe than to Ni. The DFTꢀoptimized
structure of symꢀ[H1]+ is provided in the SI (Table S13). Moreover, consistent with previous
DFT calculations on NiFe models,24 the CO stretching frequencies (νCO) for the optimized
structures agree well with the experimental IR data (Table 3). The values 1946 and 1957 cm–1
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were calculated for the sym and unsym isomers, respectively. These values shift by 1ꢀ4 cm–1,
depending on the conformation of the pdt backbone (Table S15).
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–
Table 2. Selected Structural Metrics (Å, °) for [1]0, unsymꢀ[H1]+ (as BArF salt) and symꢀ[H1]0.
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8
unsym-[1]0
Expt
unsym-[H1]+
Expt
sym-[H1]0
Calca
Calca
2.523
ꢀ
Calca
2.686
1.609
1.812
2.187,
2.210
2.277,
2.257
17.56
2.5261(4)
ꢀ
2.6461(5)
1.56(3)
2.691
9
Fe-Ni
Fe-H
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1.631
ꢀ
ꢀ
1.80(2)
1.808
Ni-H
2.1557(8),
2.1640(6)
2.2174(5),
2.1923(8)
80.94
2.186
2.184
2.203
2.256
79.19
2.1379(8)
2.1557(8)
2.2021(8)
2.2250(8)
15.97
2.254,
2.200
Ni-P1,
Ni-P2
2.278,
2.241
Fe-P3,
Fe-P4
104.78
P1-Ni-P2/S1-Ni-
S2
dihedral angle
(°)b
89.54
86.27
90.09
87.08
87.50
86.43
88.67
87.21
87.26
87.17
P1-Ni-P2
bite angle (°)
P3-Fe-P4
bite angle (°)
aThe omission of counterions in the calculations may introduce discrepancies between the crystal structure and DFTꢀ
optimized structure.
bDihedral refers to the angle between the planes formed by the groups of three atoms listed.
Table 3. Experimental and Calculated Values for νCO (cm–1).
νCO Expa,b
1946e
vCO DFTc
n/a
[Cl1]BF4
[1]0
[H1]+
1912 (sh)d, 1897
1905
1953
1946 (sym)
1957 (unsym)
1980 (unsym)
1922
1960 (sh)d
1994 (unsym)
1920
[H1H]2+
[H1]0
aExperiments performed in THF solution unless otherwise noted.
bCounterions for salts are BArF – except in the cases [1]PF6 and [Cl1]BF4.
4
cCentral CH2 of the pdt is oriented toward the Ni center in all cases. The νCO values for the
structures with the central CH2 of the pdt oriented toward the Fe center is provided in the SI
(Table S15). These values shift by 1ꢀ4 cm–1 depending on the pdt conformation.
dsh = shoulder.
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Acid-Base Properties of [H1]+. The pKa of [H1]+ was determined by treating [1]0 with
varying amounts of acetic acid (pKaMeCN = 21.56) to give mixtures of [1]0 and unsymꢀ[H1]+. The
31P{1H} NMR signals for [1]0 and unsymꢀ[H1]+ are distinct, consistent with slow proton
exchange between the metal hydride and its conjugate base.25,26 For these experiments, PhCN
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was used as the solvent under the assumption that pKaMeCN ~ pKaPhCN 27
In MeCN solution,
.
HOAc and OAc− tend to associate, as indicated by the homoconjugation constant of 3.21.26 Our
calculations corrected for this effect (Table S7). The pKa of [H1]+ is estimated to be 16.6 ±
0.04.25 A pKa of 17.7 for [H1]+ was calculated by DFT relative to the reference pKa for
[(dppe)Ni(ꢀꢀpdt)(ꢀꢀH)Fe(CO)2(PPh3)]+. As expected for a more electronꢀrich metalꢀhydride, the
pKa of [H1]+ is higher than that for related complexes substituted by only two or three
phosphines. For example, pKaPhCN = 10.7 and 14.9 for [(dppe)Ni(ꢀꢀpdt)(ꢀꢀH)Fe(CO)3]+ and
[(dppe)Ni(ꢀꢀpdt)(ꢀꢀH)Fe(CO)2(PPh3)]+, respectively.3 The DFT calculated relative pKa’s for
these species are also in good agreement with the experimental values and are provided in the
Supporting Information (Table S18).
In CH2Cl2 solution, [H1]+ is protonated by [H(OEt2)2]BArF to give a species denoted
4
[H1H]2+. The reaction is indicated by a color change from orange to redꢀbrown. In the IR
spectrum, νCO shifts from ~1956 cmꢀ1 to 1994 cmꢀ1 (Figure 4). The same protonation cannot be
effected in THF, and addition of THF (100 ꢂL) to a ~9 mM solution of [H1H]2+ in CH2Cl2 gave
back [H1]+, attesting to the highly acidic nature of the dication. The 31P NMR spectrum of
[H1H]2+ shows four multiplets, indicating that the four phosphorus centers are nonequivalent,
which indicates that (i) is relatively rigid and (ii) protonation is regiospecific. In contrast, symꢀ
[H1]+ is dynamic on the 31P NMR timescale owing to rotation of the square planar Ni center.
Solutions of [H1H]2+ in CD2Cl2 are stable for hours. The 1H NMR spectrum of [H1H]2+ shows a
single hydride signal at δ −13.1. This hydride signal is shifted upfield of [H1]+ by 7.2 ppm and
integrates to 1 relative to the pdt2– ligand.
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Figure 4. Spectroscopic changes induced by protonation of [H1]+ at room temperature. Top: FTIR
spectra of [H1]+ (dashed) and [H1H]2+ (solid) in CH2Cl2. Bottom: 31P NMR spectra of a thermally
equilibrated spectrum of [H1]+ (top) and a freshly prepared solution of [H1H]2+ (bottom) in CH2Cl2 (weak
signals at δ 88 and 68 in the upper spectrum correspond to the sym isomer).
The structure of [H1H]2+ and its interaction with OEt2 were examined by DFT. The
inclusion of neutral OEt2 in these calculations is relevant because the acid was [H(OEt2)2]+. The
calculations show that H+ and H(OEt2)+ regiospecifically protonate one of the two diastereotopic
thiolate sulfur centers. Protonation by H+ breaks the NiꢀS bond trans to the basal FeꢀP, as
indicated by elongation of the Ni−S distance from 2.25 to 3.66 Å. However, the NiꢀS bond is
not disrupted when the ether is included in the optimization of [H1H]2+ (Figure 5). A possible
explanation for the observation that ether stabilizes the bidentate over the monodentate structure
is that the hydrogen bonding of ether to the SH group increases the basicity of the sulfur,
allowing it to donate electrons more easily to the positively charged Ni center. The DFTꢀ
calculated values for νCO for the protonated species are 1980 and 1981 cmꢀ1 for the structures
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with and without OEt2, respectively. The Sꢀprotonation of bridging dithiolates in diiron
complexes by HBF4 and HOTf, as well as the disruption of the metalꢀsulfur bond, has been
observed previously.28
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Figure 5. DFTꢀcalculated structure of [H1H]2+ with (top) and without (bottom) the interaction of the
acidic proton with neutral OEt2. Phenyl groups and most hydrogens were omitted for clarity. Another
minimum energy structure was obtained by starting with the lower monodentate structure and adding an
ether within hydrogenꢀbonding distance of S2H; this optimized structure retained the monodentate
character and was ~8 kcal/mol higher in free energy than the upper bidentate structure.
Redox Experiments on Hydrides. Cyclic voltammetry (CV) studies on [H1]+ revealed
that the hydride [H1]+ reversibly reduces to [H1]0 (Figure 6) at −1.48 V vs Fc0/+ in MeCN (Table
4). In fact, all complexes of the type [(diphosphine)NiII(ꢀꢀpdt)(ꢀꢀH)FeII(CO)3ꢀx(PR3)x]+ reduce at
similar potentials regardless of the substitution at Fe, consistent with a Niꢀcentered reductions.3
The main difference among their redox behavior is the reversibility, which improves with steric
bulk such that the current ratio, ipa/ipc, for the [H1]+/0 couple is 0.94 at 100 mV/s in MeCN (Table
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S3). The DFTꢀcalculated potential for the [H1]+/0 couple agrees with the experimental value
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(Table 4).
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Figure 6. Cyclic voltammogram of [H1]+/0 at the scan rates 50ꢀ500 mV/s. Conditions: 0.1M [Bu4N]PF6
in MeCN, 1 mM [H1]OTf, glassy carbon working electrode, Ag/AgCl reference electrode, and Pt wire
counter electrode.
Table 4. Current Ratios (ipa/ipc) and Observed and Calculated Redox Potentials (V vs
Fc0/+) for NiꢀHꢀFe Complexes.a
Redox Couple
E expt ipa/ipc E DFT
½
½
–1.443
0.933 –1.44b
[(dppe)Ni(ꢀꢀpdt)(ꢂꢀH)Fe(CO)2(P(OPh)3)]+/0
–1.493
–1.293
–1.48
0.063 –1.53
0.263 –1.37
0.94 –1.45
[(dppe)Ni(ꢀꢀpdt)(ꢂꢀH)Fe(CO)2(PPh3)]+/0
[(dppe)Ni(ꢀꢀpdt)(ꢂꢀH)Fe(CO)3]+/0
[(dppv)Ni(ꢀꢀpdt)(ꢀꢀH)Fe (CO)(dppv)]+/0 ([H1]+/0)
a 1 mM complex in MeCN, 0.1 M [NBu4]BF4 electrolyte, glassy carbon electrode, 100 mV/s scanꢀrate.
b This couple was used as the reference for all other calculated potentials so the experimental and
calculated values agree by construction. Because the experimental reference is E1/2, the calculated
potentials are also assumed to be E1/2.
Hydrogen Evolution Catalysis. The hydrides are catalysts for hydrogen evolution. In
the presence of added TsOH (pKaMeCN = 8.7), CF3CO2H (pKaMeCN = 12.7),29 and ClCH2CO2H
(pKaMeCN = 15.3), [H1]+ in MeCN solution shows increased currents at potentials corresponding
to the reductive wave near −1.4 V, clear evidence for catalysis. For example, the rate of
hydrogen evolution approaches "plateau catalytic current" at high [H+] and scan rates. The data
were instead interpreted by the footꢀofꢀtheꢀwave method, focusing on the slope of the iꢀV
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response at the onset of a catalytic current.30 The latter yields a TOF of 480 s–1 for TsOH (Figure
S42). The production of hydrogen was confirmed by gas chromatographic analysis of the
headspace gas after bulk electrolysis. Electrolysis for 1 h at −1.8 V vs Fc0/+ (100 mV more
negative than the onset potential at 50 mM of CF3CO2H in MeCN with [Bu4N]BF4 electrolyte)
afforded a Faradaic yield of 93% H2 (Figure S43). Electrocatalytic production of H2 was
monitored for 2 h. Rapid catalyst decomposition was observed at [CF3CO2H] > 100 mM.
Reduced Hydride [(dppv)Ni(ꢀ-pdt)(ꢀ-H)Fe(CO)(dppv)]0, [H1]0. Characterization of
the reduced hydride was pursued to gain insight into the mechanism for hydrogen evolution. As
indicated above, the [H1]+/0 couple proceeds with good reversibility. Treatment of THF
solutions of [H1]+ with one equiv of CoCp*2 afforded a dark brown species with a single new
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ν
CO band, shifted by 25 cm–1 toward lower energy (Figure ).
Figure 7. FTꢀIR spectra of THF solutions of [H1]BArF (dashed line) and [H1]0 (solid line).
4
The stability of THF solutions of [H1]0 ranges from hours to days at room temperature, the rates
apparently depending on the sample quality. Analysis of the headspace gas from a 50 mM
solution of [H1]0 that had been standing at 26 ºC for 3.6 h revealed that approximately 0.35 equiv
of H2 (70% of theory, eq 1) was produced for every equiv of [H1]0 consumed. IR spectra in the
ν
CO region showed isosbestic behavior (Figures S46 and S47). When the decay of [H1]0 was
followed by 31P{1H} NMR spectroscopy, [1]0 was the exclusive product.
2 [H1]0 ꢀ 2 [1]0 + H2
(1)
The EPR spectrum of freshly prepared [H1]0 were simulated as three overlapping sets of
signals with giso = 2.059, 2.056, and 2.030 (Figure 8). The ratio changes over the course of
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several minutes after the reduction (Figure S51), a process attributed to isomerization. The timeꢀ
dependence of the EPR spectrum is mirrored by IR measurements: a freshly reduced sample
showed νCO at 1914 cm–1, which shifted to 1920 cm–1 after several minutes (Figure S48). The
rate of isomerization, proceeding with a half life of 2 minutes, is about 1000x faster than the
rates for interconversion of symꢀ and unsymꢀ[H1]0. The ratio of the three species is initially
1:1.2:1, respectively, reaching 1:0.1:0.1 upon equilibration (Figure 9). Each of these three
species gives
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Figure 8. EPR spectrum of a THF solution of [H1]0 freshly reduced at –77 °C (black), together with a
simulated spectra of the three components and their sum.
rise to a similar splitting pattern arising from A ~ 360 and 160 MHz and two smaller hyperfine
splittings around 25 MHz (Table 5). The large hyperfine coupling constants are assigned to
interactions of the unpaired electron with the two 31P(Ni) centers, while the two smaller coupling
constants are assigned to interactions of the unpaired electron with the two 31P(Fe) centers. Two
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of the three species are attributed to two flippamers of unsymꢀ[H1]0, while the third species is
assigned as symꢀ[H1]0. The observation of only one flippamer of symꢀ[H1]0 is consistent with
the unsymmetrical steric environment around the central CH2 of the pdt2– ligand. The EPR
spectra of [H1]0 and [H1ꢀ57Fe]0 were indistinguishable, consistent with the spin primarily
localized on Ni (Figure S52). Coupling assignable to the hydride was not observed as indicated
by the small difference between the experimental and simulated spectra.16
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Figure 9. EPR spectrum of a THF solution of [H1]0 equilibrated at 25 °C in red, together with a simulated
spectrum in black.
Table 5. EPR Simulation Parameters for [H1]0.
Species
g-factor
A (31P) (MHz)
unsymꢀ[H1]0
(flippamer a)
unsymꢀ[H1]0
(flippamer b)
symꢀ[H1]0
2.059
355, 166, 34, 30
2.030
2.056
348, 164, 33, 28
387, 199, 22, 14
The changes in the EPR spectrum suggest that the kinetic product, presumably enriched
in the unsym isomer as dominates in [H1]+, isomerizes to a second species, this thermodynamic
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product presumably being the sym isomer. To independently verify this analysis, a thermally
equilibrated MeCN solution of [H1]0 was treated with one equiv of FcBF4 to "freeze" the isomer
ratio. The result, measured at 20 min at room temperature, was a 53:47 ratio of unsymꢀ
[H1]+:symꢀ[H1]+. This isomer ratio is significantly enriched in the sym isomer relative to the
thermodynamic ratio (85:15 favoring unsym).
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DFT Characterization of [H1]0. DFT calculations were also carried out on the reduced
mixedꢀvalence species to obtain further structural, energetic, and mechanistic insights. The
calculations indicate that the Ni center changes from square planar to tetrahedral in the
conversion of [H1]+ to [H1]0 (
+
S
P
S
S
P
S
CO
CO
S
2 KFp
S
P
NiCl2(dppv)
NaBF4
Fe(II)
P
Ni(II)
P
P
Fe(II)
Ni(I)
Fe(I)
P
P
Cl
P
OC
P
OC
[Cl1]+
unsymꢀ[1]0
H+
+
+
S
Fe(II)
P
OC
CO
P
Fe(II)
S
S
S
S
S
Ni(II)
P
Fe(II)
Ni(II)
P
P
P
P
OC
P
P
H
P
H
OC
symꢀ[H1]+
unsymꢀ[H1]+
eꢀ
eꢀ
OC
Fe(II)
P
P
Fe(II)
P
P
S
S
S
S
S
S
P
P
Ni(I)
Ni(I)
Ni(I)
Fe(I)
P
P
P
P
H
P
P
H
OC
OC
unsymꢀ[1]0
symꢀ[H1]0
unsymꢀ[H1]0
P
S
S
Ni(II)
P
P
Fe(0)
P
OC
Scheme 2). This change in geometry is consistent with the description of the reduced
complex [H1]0 as Ni(I)Fe(II).31 Although the geometry at Ni changes dramatically, the bond
distances (NiꢀFe, NiꢀH, and FeꢀH) change by less than 5% (Table 2).
Spin densities were analyzed to determine the localization of the unpaired electron in the
mixedꢀvalence reduced bridging hydride species. In this species, the spin resides primarily (41ꢀ
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47%) on the Ni center, although spin density is also localized near the Fe, P, and S centers (Table
6). As observed for related NiFe dithiolates,6,31,32 the spin densities are relatively insensitive to
the orientation of the pdt ring (Table S16). The unpaired spin density plots for both the unsym
and sym isomers of [H1]0 are depicted in Figure 10. Overall, the reduction of the bridging
hydride species is consistently predicted to occur primarily at the Ni center.
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Table 6. Spin Densities Calculated for the Reduced Hydride Complex [H1]0.
Complex
ρ(Ni) ρ(Fe) ρ(2S) ρ(2PNi)a
0.47 0.19 0.14 0.12
0.41 0.28 0.14 0.12
symꢀ[H1]0
unsymꢀ[H1]0
aThese values refer to the P atoms of the dppv ligand on the Ni center.
Similar to the parent Ni(II)Fe(II) derivatives, the hydride ligand for the reduced mixedꢀ
valence hydrides is also asymmetrically bound, with a shorter distance to Fe than to Ni (Table 2).
This asymmetry is consistent with the greater spin density on Ni, which indicates that the species
is best described as a Ni(I) paramagnet ligated to an Fe(II)ꢀhydride.
Figure 10. Topꢀdown view of the isocontour plot (isovalue = 0.005) of the unpaired spin density of
[H1]0 for the unsym (apicalꢀbasalꢀdppv on Fe, left) and sym (dibasalꢀdppv on Fe, right) isomers
showing predominant localization around the Ni coordination sphere, with some localization near Fe
as well. This difference is clarified by the numerical atomic spin densities provided in Table 6.
Protonation of [H1]0. The aim of preparing [H1]0 was to test its role in the hydrogen
evolution but in the absence of an electrode. Indeed, when treated with one equiv HBF4·Et2O,
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solutions of [H1]0 immediately gave H2 concomitant with a color change from brown to orange.
The 31P{1H} NMR spectrum recorded 20 min after the addition of acid showed that a 72:28
mixture of unsymꢀ[H1]+ and symꢀ[H1]+ is produced in 96% yield. Based on the rates of the
relevant isomerizations, a ratio of ~75:25 is predicted. Analysis of the headspace by gas
chromatography revealed that 0.32 equiv of H2 is formed. This yield of H2 corresponds to 64%
of the theoretically predicted value according to the stoichiometry in eq 2.
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[H1]0 + H+ ꢀ [H1]+ + 0.5 H2
(2)
In terms of the mechanism for hydrogen evolution, the steps in eq 3ꢀ4 are invoked.
Protonation of [H1]0 affords [1]+ and H2 (vide infra):
[H1]0 + H+ ꢀ [1]+ + H2
Electron transfer from [H1]0 to [1]+:
(3)
(4)
(5)
[H1]0 + [1]+ ꢀ [H1]+ + [1]0
Protonation of [1]0:
[1]0 + H+ ꢀ [H1]+
The redox process in eq 4 is favorable since our electrochemical measurements show that [H1]0
is about 0.5 V more reducing than [1]0.
Three predictions arising from the proposed mechanism were tested. First, protonation of
[H1]0 with D+ sources should give HD. When the reaction of [H1]0 and DOTf was monitored by
1H NMR spectroscopy, the HD/H2 ratio was 5.3:1. Second, treatment of [H1]0 with a deficiency
of H+ should give some [1]0. Indeed, addition of 0.5 equiv of H(OEt2)2BArF to a solution of
4
[H1]0 gave substantial but variable amounts of [1]0. The variability of these yields is attributed
to the sensitivity of [H1]0 toward impurities, which accelerate hydrogen evolution according to
eq 1. Finally, the ratio unsymꢀ[H1]+/symꢀ[H1]+ should reflect on the mechanism since oxidation
of [H1]0 predominantly gives significant sym, and protonation of [1]0 generates exclusively
unsym (see above). Indeed, treatment of a MeCN solution of [H1]0 with 1 equiv of HBF4·Et2O
led to a 72:28 ratio of unsymꢀ[H1]+ to symꢀ[H1]+ 20 min after treatment. This ratio is consistent
with a mechanism in which half of the [H1]0 undergoes a sequence of protonation, reduction, and
protonation to form almost exclusively unsymꢀ[H1]+, and the other half is oxidized to a mixture
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of unsymꢀ[H1]+ and symꢀ[H1]+, which was shown independently to reach a 53:47 ratio 20 min
after oxidation in MeCN (vide supra).
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Several scenarios for the protonolysis of [H1]0 were examined with DFT to gain insight
into the pathway for H2 evolution. The free energy diagram for a proposed mechanism is shown
in Scheme 5. The free energies were calculated with CF3CO2H as the acid (pKaMeCN = 12.7) and
at an applied potential of –1.80 V vs Fc0/+ in MeCN, consistent with the bulk electrolysis
experiments described above. Under these conditions, DFT calculations indicate that the
production of H2 via interaction of [H1]0 with CF3CO2H acid to give [1]+ is thermodynamically
favorable (ꢁG° = –7.0 kcal/mol, indicated by the red dashed line in Scheme 4). This process
could occur as a concerted but most likely asynchronous protonation of the hydride and
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Scheme 5. Experimentally and computationallyꢀsupported pathway for hydrogen evolution
catalyzed by 1. The free energies are calculated with CF3CO2H acid (pKaMeCN = 12.7) and at an
applied potential of –1.8 V vs. Fc0/+ in MeCN to coincide with the bulk electrolysis experiments.
H2 evolution is proposed to occur directly from interaction of [H1]0 and CF3CO2H acid under
these conditions (red dashed line), but stronger acids may also evolve H2 from an intermediate
dihydrogen complex, [H21]+ (gray dashed lines). In the last step, an alternative mechanism for
reduction of [1]+ by [H1]0 could also occur under certain conditions. The relative free energies
for half reactions corresponding to electron transfer are calculated with respect to the applied
potential, and the free energy difference between the initial and final states in the pathway
corresponds to the potential associated with the double reduction of 2HA to produce 2A– and H2
relative to this applied potential. An analogous scheme calculated relative to the HA/H2 couple
is presented in Figure S5. PhꢀP groups omitted for clarity.
release of H2, where ‘concerted’ indicates the absence of a thermally stable intermediate. A
pathway involving protonation of the FeꢀH bond in [H1]0 to form an intermediate dihydrogen
complex, [H21]+, is also possible but is thermodynamically unfavorable (gray dashed lines in
Scheme 5) under these specific conditions. However, this protonation step is more
thermodynamically favorable when stronger acids, such as HBArF ·2Et2O, are used. Under such
4
strong acid conditions, H2 is evolved directly after the H2 adduct is formed upon protonation of
[H1]0. Thus, both mechanisms may be involved in H2 evolution. Regardless, evolution of H2
produces [1]+, which is then reduced to form [1]0 in the bulk electrolysis experiments. However,
note that in the protonation experiments discussed above, an alternative mechanism, in which
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[1]+ is reduced by [H1]0 (eq 4), occurs in the absence of an electrode and could in principle also
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occur in the electrochemical experiments.
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Conclusions
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The pathway has been elucidated for hydrogen evolution catalyzed by a synthetic
analogue of the [NiFe]ꢀhydrogenase active site. The model system is [(dppv)Ni(ꢀꢀpdt)( ꢀꢀ
H)Fe(CO)(dppv)]+, which does not release H2 even upon protonation. Instead, reduction is
required to initiate hydrogen evolution. Of particular interest is the spectroscopic and
computational characterization of the reduced hydride complex, which is stabilized by the steric
protection afforded by the two dppv ligands. The reduced hydride complex is assigned as a
Ni(I)Fe(II) species on the basis of FTꢀIR and EPR measurements and supported by DFT
calculations. These studies revealed that reduction of the Ni center enhances the hydridic
character of the FeꢀH center. Hydrogen evolution occurs by protonation of the FeꢀH bond
leading to release of dihydrogen. It is intriguing to contrast the hydrogen evolving properties of
[H1]0 with that the related mixed valence homobimetallic complex [HFe2(pdt)(CO)2(dppv)2],
where the hydride is a spectator (it is not involved in H2 release).14
Comparisons of these NiFe models with the [NiFe]ꢀhydrogenases are instructive.
Overall, the biological system operates at one oxidation state higher than these models (Table 7).
This difference reflects the effects of charge, since the model complexes are neutral or cationic,
whereas the active site is highly anionic, which stabilizes higher oxidation states. With seven
anionic ligands (4×RS−, 2×CN−, H−), the active site in the NiꢀR state is 3− or 2−, depending on
the degree of protonation of a terminal thiolate ligand.9
Table 7. Comparison of Redox States of NiꢀHꢀFe Centers in the Enzyme and Models.
formula
[(RS)2Ni(ꢀ-SR)2
(ꢀ-H)Fe(CO)(CN)2]3-
NiꢀR
(PR3)2Ni(ꢀ-SR)2
(ꢀ-H)Fe(CO)(PR3)2
reduced hydride
[(RS)2Ni(ꢀ-SR)2
(ꢀ-H)Fe(CO)(CN)2]2-
NiꢀC
[(PR3)2Ni(ꢀ-SR)2
(ꢀ-H)Fe(CO)(PR3)2]+
oxidized hydride
label
Ni(II), Fe(II)
Ni(I), Fe(II)
Ni(III), Fe(II)
Ni(II), Fe(II)
oxidation
states
0
1+
net charge
.reactivity
3−
2−
protic
hydridic
(H+ gives H2)
hydridic
(H+ gives H2)
protic
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Crystallographic and DFT analyses for the NiꢀR and NiꢀC states in the enzyme reveal that the
NiꢀH and FeꢀH distances are similar, with dNiꢀH ~ 1.6 and dFeꢀH = 1.7 Å.11 In synthetic models,
the FeꢀH distances are always shorter than the NiꢀH distances, sometimes by 0.6 Å.4,33 Xꢀray
spectroscopic studies indicate that the FeꢀH site only weakly interacts with the Ni center in these
models.33 Studies on both the enzyme and models indicate, however, that these distances are not
strongly affected by the metal oxidation state.9,11 The most prominent difference between the
enzyme and the active site models is the coordination sphere of the nickel center.34 In the
models, the Ni(SR)2(PR3)2 site alternates between square planar and tetrahedral, whereas in the
protein, the Ni(SR)4 center maintains a distorted SF4ꢀlike geometry.9,35 Future studies must
address these three gaps: the rigidity of the Ni site, the anionic character of the site, and the fact
that redox is confined to Ni. These daunting challenges can only, we believe, be addressed by
radical changes in the coordination sphere of the Ni center.
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Experimental Section
General Considerations. Unless otherwise stated, reactions were conducted using
standard Schlenk techniques, reagents were purchased from conventional sources, and solvents
were HPLC grade and purified using an alumina filtration system (Glass Contour, Irvine, CA).
ESIꢀMS data were acquired using a Waters Micromass Quattro II or ZMD spectrometer.
Analytical data were acquired using an Exeter Analytical CEꢀ440 elemental analyzer. NMR
spectra were acquired with Varian U500 and VXR500 spectrometers. Chemical shifts (in ppm)
were referenced to residual solvent peaks (for 1H, 13C) or external 85% H3PO4 (for 31P). Solution
IR spectra were recorded on a PerkinꢀElmer Spectrum 100 FTIR spectrometer. Crystallographic
data were collected using a Siemens SMART diffractometer equipped with a Mo Kα source (λ =
0.71073 Å) and an Apex II detector. Photolysis employed a Spectroline MBꢀ100 instrument
(365 nm). Syntheses of Fe(CO)2(pdt)(dppv)36 and K[FeCp(CO)2]37 have been reported. CV's
were simulated with the CHI630D package. EPR spectra were recorded on a Varian Eꢀline Eꢀ12
Century series Xꢀband CW spectrometer. Spectra were collected at room temperature with a
microwave frequency of 9.30 GHz, 1 gauss modulation, and 20 mW microwave power. EPR
spectra were simulated using the program SIMPOW6.38 Samples of [1]0 and [H1]0 were handled
in a glovebox with the exclusion of chlorocarbons.
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[(dppv)Ni(ꢀ-Cl)(ꢀ-pdt)Fe(dppv)(CO)]BF4, [3Cl]BF4. To a solution of
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Fe(pdt)(CO)2(dppv) (1.00 g, 1.63 mmol) in 150 mL of acetone was added solid NiCl2(dppv)
(0.854 g, 1.63 mmol) and NaBF4 (0.196 g, 1.78 mmol). The resulting heterogeneous mixture
was heated at reflux for 4 h at which point no solid was visible, and the red color had deepened.
An FTꢀIR spectrum confirmed that Fe(CO)2(pdt)(dppv) was fully consumed. After cooling in an
ice bath, the reaction solution was filtered through a pad of Celite. The solvent was removed
from the filtrate, and the solid was extracted into 50 mL of CH2Cl2. The solution was diluted
with 150 mL of hexanes, and the mixture was cooled to 0 °C for several days. Crystals of the
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1
product were isolated and dried under vacuum. Yield: 1.76 g (93%). H NMR (500 MHz,
CD2Cl2, 20 °C): δ 8.05 – 7.87 (m, 2H, P(CH)2P), 7.78 – 7.22 (m, 40H, Ph), 7.14 – 6.95 (m, 2H,
P(CH)2P), 2.05 (s, 1H, (SCH2)2CH2), 2.02 (s, 2H, (SCH2)2CH2), 2.00 – 1.93 (m, 2H,
(SCH2)2CH2), 1.94 – 1.85 (m, 1H, (SCH2)2CH2), 1.04 – 0.91 (m, 2H, (SCH2)2CH2). 31P{1H}
NMR (202 MHz, CD2Cl2): δ 73.3 (d, JPP = 16.6 Hz, 2 P, FeP), 57.6 (d, JPP = 16.5 Hz, 2 P, NiP).
IR (acetone): νCO = 1946 (s) cmꢀ1. Although not obtained in analytical purity (%C varied by at
least 3%) crude samples were converted to the reduced complex.
(dppv)Ni(ꢀ-pdt)Fe(dppv)(CO), [1]0. In a 500ꢀmL Schlenk flask was dissolved
[Cl1]BF
4
(1.25 g, 1.07 mmol) in 200 mL of acetone and stirred at room temperature. This
] (0.464, 2.14 mmol) in 60 mL of
solution was treated dropwise with a solution of K[FeCp(CO)
2
distilled acetone. The reaction mixture was allowed to stir for ~50 min. and was then
concentrated to ~100 mL. The solution was diluted with 100 mL of pentane and was cooled in
an ice bath for 4 h to yield a brown precipitate. The precipitate was isolated by cannula filtration
while cold and rinsed with pentane. The precipitate was then extracted into 2 x 30 mL of cold
toluene, and these extracts were filtered through a pad of Celite The toluene was removed under
.
vacuum, and the remaining brown solid was recrystallized from 50 mL of THF and 150 mL of
1
pentane at –40 °C. Yield: 0.972 g (87 %). H NMR (500 MHz, C6D6): δ 8.08 ꢀ 6.93 (44H, Ph
and P(CH)2P), 2.32 (m, 2H, (SCH2)CH2), 1.79 (m, 1H, (SCH2)CH2), 1.56 (m, 1H, (SCH2)CH2);
2H, (SCH2)CH2). 31P{1H} NMR (202 MHz, C6D6): δ 89.2 (t, JPP = 13.6 Hz, 2P, Fe2P), 74.2 (dt,
JPP = 30.1, 15.5 Hz, 1P, NiꢀP), 46.2 (dt, JPP = 28.3, 12.1 Hz, 1P, NiꢀP). IR (THF): νCO = 1911
(sh), 1897 (s) cmꢀ1. Anal. Calcd for C56H50FeNiOP4S2: C, 64.58; H, 4.84. Found: C, 64.17; H,
4.70. Single crystals of [1]0 were grown from THF solutions layered with pentane.
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[(dppv)Ni(ꢀ-H)(ꢀ-pdt)Fe(dppv)(CO)]OTf, [H1]OTf. A solution of [1]0 (0.10 g, 0.10
mmol) in 20 mL of THF was treated with HOTf (10 ꢀL, 0.11 mmol), causing a color change
from brown to orange solution. An FTꢀIR spectrum confirmed that the starting material was
fully consumed. The solution was concentrated to ~10 mL. The concentrated solution was
layered with 10 mL of pentane and cooled to −40 °C for four days. The resulting orange crystals
were collected, washed with pentane, and dried under vacuum. Yield: 0.114 g (99%). unsym
isomer: 1H NMR (500 MHz, d3ꢀMeCN, 20 °C): δ –4.27 (t, 2JPH = 35.3 Hz, 1H, Fe(ꢀꢀH)Ni).
31P{1H} NMR (202 MHz, d3ꢀMeCN, 20 °C): δ 87.8 (s, 2P, Fe2P), 69.1 (s, 2P, sym.Niꢀ2P).
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1
Unsym isomer: H NMR (500 MHz, d3ꢀMeCN, 20 °C): δ – 5.80 (t, 2JPH = 42.3 Hz, 1H, Fe(ꢀꢀ
31
H)Ni). P{1H} NMR (202 MHz, d3ꢀMeCN, 20 °C): δ 93.4 (s, 1P, FeP), 79.4 (s, 1P, FeP), 73.5
(s, 2P, Niꢀ2P). IR (THF): νCO = 1962 (m), 1952 (s) cmꢀ1. Anal. Calcd for C57H51F3FeNiO4P4S3:
C, 57.45; H, 4.31. Found: C, 57.22; H, 4.41.
[(dppv)Ni(ꢀ-H)(ꢀ-pdt)Fe(dppv)(CO)]BArF , [H1]BArF . A solution of [H1]OTf
4
4
(0.071 g, 0.06 mmol) in 10 mL of THF was treated with NaBArF (0.058 g, 0.065 mmol), and the
4
reaction mixture was heated to 35 °C for 5 min. After cooling to room temperature and filtering,
the reaction solution was evaporated, leaving an orange solid residue. Sym isomer: 1H NMR
(500 MHz, C6D6, 20 °C): δ ꢀ4.37 (t, JPH = 34.0 Hz, 1H, Fe(ꢀꢀH)Ni). 31P{1H} NMR (202 MHz,
C6D6, 20 °C): δ 88.2 (s, 2P, Fe2P), 68.4 (s, 2P, Ni2P). Unsym isomer: δ 8.45 – 6.84 (m, 52 H,
Ar), 6.42ꢀ 6.21 (m, 4 H, P(CH)2P, 1.71 – 1.60 (m, JHH = 11 Hz, 1H, (SCH2)CH2), 1.49 (d, JHH
14.1 Hz, 1H, (SCH2)CH2), 1.45 – 1.35 (m, 1H, (SCH2)CH2), 1.03 – 0.95 (m, 2H, (SCH2)CH2),
0.10 (t, JHH = 13.5 Hz, 1H, (SCH2)CH2), ꢀ5.77 (t, 2JPH = 42.5 Hz, 1H, Fe(ꢀꢀH)Ni). 31P{1H}
NMR (202 MHz, C6D6, 20 °C): δ 87.82 (s, 1P, FeꢀP), 80.60 (s, 1P, FeꢀP), 72.82 (s, 2P, Niꢀ2P).
IR (THF): νCO = 1953 (s), 1960 (sh).
=
[(dppv)Ni(ꢀ-H)(ꢀ-pdt)Fe(dppv)(CO)]BF4, [H1]BF4. A solution of [1]0 (0.500 g, 0.48
mmol) in 75 mL of THF was treated with HBF4·Et2O (0.1 M, 0.5 mL, 0.05 mmol) to give an
orange solution. After concentrating to half volume, the solution was layered with 100 mL of
pentane at 0 ºC. Small orange crystals were collected by filtration and dried under vacuum.
1
Yield: 0.485 g (90%). The unsymmetrical isomer crystallized exclusively. H NMR (500 MHz,
CD2Cl2, 20 °C): δ 8.10 (t, 2JPH = 8.3 Hz, 2H, P(CH)2P)), 7.71ꢀ7.12 (m, 40H, Ph), 6.91 – 6.77 (m,
2H, P(CH)2P), 1.54 (t, JHH = 14.4 Hz, 1H, (SCH2)CH2), 1.36 – 1.23 (m, 1H, (SCH2)CH2), 1.09
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