Biomimetics of [NiFe]-Hydrogenase: Nickel- or Iron-Centered Proton

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Biomimetics of [NiFe]-Hydrogenase: Nickelor Iron-Centered Proton Reduction Catalysis? Hao Tang, and Michael B. Hall J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10425 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

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Journal of the American Chemical Society

Biomimetics of [NiFe]-Hydrogenase: Nickel- or Iron-Centered Proton Reduction Catalysis? Hao Tang and Michael B. Hall* Department of Chemistry, Texas A&M University, College Station, Texas 77845, USA KEYWORDS. Hydrogenase, H2 evolution, Density functional theory, Nickel, Iron. ABSTRACT: The [NiFe] hydrogenase (H2ase) has been characterized in the Ni-R state with a hydride bridging between Fe and Ni but displaced toward the Ni. In nearly all of the synthetic Ni-R models reported so far, the hydride ligand is either displaced toward Fe, or terminally bound to Fe. Recently, a structural and functional [NiFe]-H2ase mimic (Nat. Chem. 2016, 8, 1054-1060) was reported to produce H2 catalytically via EECC mechanism through a Ni-centered hydride intermediate like the enzyme. Here, a comprehensive DFT study shows a much lower energy route via an E[ECEC] mechanism through an Fe-centered hydride intermediate. Although catalytic H2 production occurs at the potential corresponding to complex’s second reduction, a third electron is needed to induce the second proton addition from the weak acid. The first two-electron reductions and a proton addition produce a semi-bridging hydride with a short Fe-H bond like other structured [NiFe]-biomimetics, but this species is not basic enough to add another proton from the weak acid without the third electron. The calculated mechanism provides insight into the origin of this structure in the enzyme.

1. Introduction Hydrogenases (H2ase) have attracted increasing attention because of their efficient production and oxidation of molecular hydrogen,1 a carbon-neutral energy carrier. In contrast to the more common biomimetics for [Fe]H2ases1,2 and [FeFe]-H2ases,1,3 biomimetics for [NiFe]H2ases have proven challenging.1,4 The active site of the [NiFe]-H2ases features a nickel tetrathiolate (four cysteines) with two S bridges to an Fe(CN)2(CO) center.5 Four key states in the catalytic cycle are: Ni-SIa (NiIIFeII), Ni–L (NiIFeII), Ni–C (NiIIIµ(H)FeII), and Ni–R (NiIIµ(H)FeII).6-8 Despite intensive design efforts experimentally and theoretically,4 the metal-hydrides in nearly all of the synthetic NiFe models feature a short Fe-H bond and a long Ni-H bond, including the only structurally characterized ones by the Ogo4f and Rauchfuss4d-e,i,k groups. Although some mononuclear Ni species have been described as hydrogen production catalysts,9-11 only a few heterodinuclear models show Ni-centered reactivity.10-13 For example, using DFT calculations, Greco predicted that ferrocene modified derivative of a structural and non-functioning NiFe model4a,b would bind H2 if Ni was oxidized to NiIII.12 Gan et al.10 and Das et al.11 showed a functional but non-structural model, also having a ferrocene attached, produced H2 at a low overpotential. In a very recent report, the reaction between the dithiolate NiIIL species (L2- = 2,2′-(2,2′-bipyridine-6,6′diyl)bis(1,1-diphenylethanethiolate)) and the FeII species ([CpFe(CO)(MeCN)2]+) produced the heterodinuclear

Scheme 1. Proposed catalytic cycles for H2 production: high-energy (outer) path13 and low-energy (inner) path

[LNiIIFeIICp(CO)]+ complex (denoted as [NiIIFeIIL]+ in Scheme 1), which was structured and characterized spectroscopically.13 Upon electrochemical reduction in weak acid, [NiIIFeIIL]+ produced H2 at high rates near its second reduction potential.13 Scheme 1 depicts proposed mechanisms for H2 evolution mediated by this biomimetic system. Beginning with [NiIIFeIIL]+, a mimic for Ni-SIa, one-electron reduction affords [NiIFeIIL], a mimic of the Ni-L state, which was characterized spectroscopically. Further reduction produced a species that was believed to be the Ni-bipyridine diradical [NiIFeIIL•]–. Near the potential of this second reduction, molecular hydrogen was generated catalytically in the presence of the weak acid, [HNEt3]+. A doubly reduced, protonated intermediate was proposed as a Ni-hydride, [(NiIIH)FeIIL], a model of Ni-R. The proposed species was spectroscopically characterized from a reaction of [NiIIFeIIL]+ with NaBH4. Molecular hydrogen appeared to be generated from [(NiIIH)FeIIL] by addition of a

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proton, which would be an EECC catalytic mechanism (outer in Scheme 1). Knowledge of several mechanistic details is still missing, including the other possible sites for electron and proton uptake, the alternative sequences of the various electron and proton transfer steps, the other roles of the

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redox-active, non-innocent bipyridine ligand, and the possibility of M-S bond cleavage to afford a proton relay as reported in the theoretical studies of other [NiFe]-14 and [FeFe]-H2ases models.15 Here, alternative detailed H2 production mechanisms catalyzed by this [NiFe] mimic are predicted by DFT (Scheme 1).

Scheme 2. Calculated electrocatalytic cycles for H2 production at B3P86 levela

[1]+

+ eEexp = -1.29

[1] N N

OC

N N

Fe NiII S S

1CS,3∆G

+ eEexp = -1.90 OC

N

FeI NiII S S

N

[1b] 2∆G = 2.1; E cal = -1.01 O C N N I FeII Ni S S [1a] 2∆G = 0.0; E cal = -0.92

II

= 0.0/12.0

+ H+

[1b]-

1CS,3∆G

= +3.4/+4.5

∆pKa = -9.7/-10.5 [1H2]+

= 15.6; -5.1

N

+

FeII

TS2

1.77 1.83 0.80 2.94

S1

H2 H1 Fe

Fe-H1 Fe-H2 H1-H2 S1-H2

H+

+ H+

= +11.0/+11.8

+ eEcal = -1.43

N

CO

TS1

H

N

N

H

FeII NiII CO S S [Ni(FeH)L] 2∆G = 0.0

[H1H] H

[1H2]

H H N FeII FeII N NiI CO NiI CO S S H S S ‡ = 11.0 ∆G 2 1 [Ni(Fe−η H2)L] [Ni(FeH)(SH)] 2∆G = 0.0 2∆G = 9.8 N

FeII

Fe

+ H+ ∆pKa = +3.3

∆pKa = -9.8/-10.6

H

Ni

H2 H1

[1H]-

[H1H]+ N

1.55 2.05 1.00 1.71

S1 a

NiII CO S S H S S 1CS∆G ‡ = 9.0 3 [Ni(FeH)(SH)]+ [Ni(Fe−η 2H2)L]+ 1CS,3∆G = 3.2/12.3 1CS,3∆G = 3.3/0.0 NiI

E[ECEC]

Ni

FeII N NiII CO S S [Ni(FeH)L] 1CS,3∆G = 1.1/0.0

H H

N 1CS∆G ‡ 4 1CS∆G =

N

H S S + [NiFe(SH)L] 2∆G = 0.0 + H+

[EECC]

1,3∆pK

[1H]

FeII

NiI

Fe0 NiII S S

+ H+

OC N

Fe-H1 Fe-H2 H1-H2 S1-H2

OC

= 6.7; Ecal = -2.02 O C N N I FeII Ni S S [1a]- 1OS,3∆G = 1.0/0.0 1OS,3E cal = -1.77/-1.73

[1H]+

H2

∆G2‡ = 8.6; ∆G = -2.4

1CS∆G

∆pKa = -11.1

N

H2

[1]-

N

a

The relative Gibbs free energies (∆G) and barriers (∆G‡) are given in kcal/mol. The reduction potentials (E0 vs Fc+/0) are given in V. The relative acidities (∆pKa = pKa(CatH) - pKa(Et3NH+)) are reported with reference to Et3NH+ in MeCN. The more positive ∆pKa values indicated that the protonation reaction is thermodynamically favorable, while the more negative ∆pKa values indicate that the protonation reaction is thermodynamically unfavorable. DFT-optimized structures of TS1 and TS2 correspond to the intramolecular proton transfer and H2 release, respectively. The Gibbs free energy barrier of TS2 (∆G2‡ = 8.6) is lower than that of [1H2] (∆G = 9.8), which is caused by the harmonic frequencies overestimating the thermal corrections for [1H2] more than that for TS2; [1H2] is more stable than TS2 in the enthal– py. The most stable species of [1H]+, [1H], [1H] , [H1H]+, and [H1H] are displayed in Scheme 2, while the other isomers are displayed in SI.

2. Computational methods All the geometry optimizations were performed in the gas-phase using Gaussian 0916 at the B3P8617/def2TZVP level of theory, because the B3P86 functional reproduces the experimental geometries very well (see benchmarking with various DFT functionals in the Supporting Information, SI). Single point energies were cal-

culated at these optimized structures with the B3P86/def2-TZVPP level of theory. Solvent effects using acetonitrile as the solvent were described with continuum solvation model SMD.18 Stable intermediates and transition states (TSs) were established by their harmonic frequencies with no imaginary frequencies for minima and only one imaginary frequency for TSs. TSs


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were confirmed to lead to the corresponding intermediates through intrinsic reaction coordinate (IRC) calculations.19 The stabilities of the DFT wavefunctions were tested. The 〈S〉2 values for the open shell cases were examined and have little spin contamination. The openshell singlet states were searched for the cases that have the similar energies for the closed-shell singlet and triplet states. The υCO frequencies are obtained from the harmonic frequencies calculations. All the reported Gibbs free energies (∆G) and barriers (∆G‡) include: the electronic energy in solution, gas-phase thermal correction at 298.15 K, and solvation free energy. All the reduction potentials (E) are calculated with respect to Fc+/0 as in the experiment. The relative acidities (∆pKa) are calculated relative to the pKa of Et3NH+ as in the experiment. 3. Results and discussion Scheme 2 displays our DFT-predicted mechanism for H2 production by the lower-spin states. Consistent with the electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) measurements,13 DFT predicts that [1]+ has a singlet ground state with low-spin NiII and FeII. The triplet state is predicted to be more than 12 kcal/mol higher in energy (Scheme 2). The DFT(B3P86)-optimized structure of singlet [1]+ has a square planar NiII center, two thiolates bridging the NiII and FeII ions (both Fe-S1 and Fe-S2 distances at 2.32 Å (exp. 2.305 and 2.298 Å), both Ni-S1 and Ni-S2 distances at 2.18 Å (exp. 2.175 and 2.179 Å), respectively), and a Ni-Fe distance of 2.92 Å (exp. 2.918 Å) (Figure S1). This DFT(B3P86)-optimized structure is in closer accord (less than 0.03 Å errors) with the experimental one than those with other tested DFT functionals including the DFT(BP86)-optimized structure (Table S2).13 Reduction of [1]+ produces [1], which has two energetically close valence isomers. The most stable, [1a], is a NiIFeII complex (see spin density in Figure S2), in which NiI is five-coordinate such that the CO ligand bridges the Fe and Ni ions (Ni-C1 distance of 2.15 Å), and two thiolates are bridging the Ni and Fe ions (both Fe-S1 and Fe-S2 distances at 2.35 Å). The less stable isomer, [1b], is a NiIIFeI complex (Figure S2), in which NiII is four-coordinate like [1]+ and the CO ligand is terminally bound to the Fe ion, but one Fe-S bond is broken (Fe-S1 and Fe-S2 distances of 3.67 and 2.27 Å). Moreover, the Ni-Fe bond is calculated to be 2.56 Å in [1a], close enough for some metal-metal bonding, while this Fe-Ni bond is disrupted (2.96 Å) in [1b]. In addition to being 2.1 kcal/mol lower in free energy, [1a] corresponds to the spectroscopically determined structure.13 Knowledge of the super-reduced state [1]– remains scant from experiment, while DFT predicts two low energy isomers (Scheme 2). The isomer [1a]– has a triplet ground state with a close-lying open-shell singlet state (1.0 kcal/mol less stable). The spin density results (Figure S3) for [1]– reveal that these two states have single

electrons residing primarily on Ni and on the bipyridine ligand, ferromagnetically coupled and antiferromagnetically coupled, respectively, which is in accord with a [NiIFeIIL•]– assignment.13 The optimized geometries in both states feature a bridging CO ligand (Ni-C distance of 2.02 Å) and two thiolates bridging the Ni and Fe ions (both Fe-S1 and Fe-S2 distances at 2.36 Å). The isomer [1b]–, in which one Fe-S bond is broken and the CO ligand is not bridging, is calculated 6.7 kcal/mol higher in free energy. In the absence of added acid, the CV scans show two reversible and diffusion-controlled one-electron reduction waves at -1.29 V and -1.90 V.13 Consistent with experiment, DFT calculations (Scheme 2) show reduction of [1]+ to [1a] at Ecal = -0.92 V (Ni reduction), and reduction of [1a] to [1a]– at Ecal = -1.73 V (bipyridine reduction). In the presence of acid, protonation of [1] is predicted to be thermodynamically unlikely based on the negative ∆pKa values. As shown in Scheme 2, the most stable isomer among eleven calculated isomers of [1H]+ (Scheme S1) involves protonation of [1a] on S with one S-Fe bond broken. The ∆pKa (vs Et3NH+) value for the sulfur-protonated species is calculated to be -11.1, indicating of unfavorable thermodynamic process. Thus, protonation only occurs after the next reduction to [1]–.

O

N1

N1

S1

H

H

C1 Ni

Ni N2

S1

Fe C1

S2

N2

Fe S2

O

(a) [Ni(FeH)L] (b) [(NiH)FeL] 1CS,3∆G = 36.0/21.3 = 1.1/0.0 1CS,3∆pK = +11.0/+11.8 1CS,3∆pK = -14.6/-3.8 a a Fe-H 3.41 / 3.49 Fe-H 1.54 / 1.58 Ni-H 1.56 / 1.56 Ni-H 1.93 / 1.72 Ni-Fe 2.89 / 2.73 Ni-Fe 3.30 / 3.18 Fe-S1 3.97 / 4.03 Fe-S1 2.32 / 2.35 Fe-S2 2.25 / 2.27 Fe-S2 2.29 / 2.35 Figure 1. DFT(B3P86)-optimized singlet geometries (Å, see the triplet geometries in Figure S4), relative free energies (∆G kcal/mol), and ∆pKa values of (a) this work Fe-protonated and (b) Ni-protonated species13 in the singlet and triplet states for [1H]. 1CS,3∆G

Protonation of [1]– is predicted to be thermodynamically favorable, producing [1H] (Scheme 2). Among ten calculated isomers of [1H] (Scheme S2), the most stable isomer involves protonation of [1a]– on Fe to afford nearly degenerate closed-shell singlet and triplet states with one S-Fe bond broken, as indicated by the elongation of the Fe-S1 distance from 2.36 Å in [1a]– to 3.97 Å in [1H] (Figures 1a and S3). The DFT-optimized struc-



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ture (Figure 1a) shows a short Fe-H bond semi-bridging to the Ni, which is similar to the three crystallographically characterized species [(diphos)Ni(pdt)(µH)Fe(CO)2L]+,4d,e [(amine)2Ni(SR)2(µ-H)FeL3]+,4f and [(dppv)Ni(µ-pdt)Fe(CO)(dppv)].4k In contrast to the proposed Ni-based hydride,13 our DFT calculations show that protonation of [1a]– at the Ni center to yield singlet and triplet Ni-hydrides is thermodynamically unfavorable by 36.0 and 21.3 kcal/mol (and very negative ∆pKa values) relative to the Fe-protonated triplet species (Figure 1). Previous work reported the mononuclear NiII species, [NiL], alone can produce H2 at the NiII/NiI reduction potential of -1.82 V,9,13 attaching the cationic moiety [CpFeII(CO)]+ to produce [1]+ causes an anodic shift of the NiII/NiI reduction potential by 0.53 V, and changes the Ni-centered proton reduction exhibited in the mononuclear [NiL] species to the Fe center in the heterodinuclear NiFe catalyst, which illustrates how the catalytic activity can be greatly modified by such changes. Protonation of [1]– at Fe rather than at the low oxidation state Ni can be rationalized by S-Fe bond cleavage and intramolecular charge transfer. Specifically, upon protonation of [1a]–, one S-Fe bond of [1a]– cleaves and the two electrons previously residing on the nickel’s highly destabilized dx2-y2 antibonding orbital and on the bipyridine ligand are simultaneously transferred to the unsaturated Fe (16e-), leading to the final oxidation states of NiII (16e-) and FeII (18e-). This is the same species that would arise from protonation of the Fe0 in [1b]–. This case is further confirmation of the necessity of Fe-S bond cleavage for proton reduction at electronically saturated metals as proposed by our recent work.14 Subsequently, a second protonation of singlet or triplet [1H] is predicted to be thermodynamically unfavorable, as indicated by the negative 1CS,3∆pKa values of -9.8/10.6 for the most stable species with thiolate S as a proton relay among eight calculated isomers of [H1H]+ (Scheme S3). Furthermore, direct protonation of the FeH bond in [1H] is also thermodynamically unfavorable (1CS,3∆pKa = -9.7/-10.5 kcal/mol, Scheme 2). On the basis of the slightly unfavorable overall thermodynamic data (1CS,3∆G = 3.4/4.5 kcal/mol, Scheme 2) for direct reaction of [1H] with acid to regenerate [1]+ and H2, the EECC mechanism as previously proposed13 could only serve as a very minor pathway for H2 evolution. Thus, a third electron is needed before the weak acid can add a second proton: the reduction of [1H] to [1H]– is predicted to occur at Ecal = -1.43 V, which is less negative than that of the [1a]0/– couple by 0.3 V (Scheme 2). The spin density (Figure 2) indicates that the bipyridine moiety again acts as the electron reservoir in [1H]–, so that this species is best described as [NiIIHFeIIL•]–. Reduction results in the significant elongation of the Ni-H distance from 1.72 to 2.69 Å and of the Ni-Fe distance from 2.73 to 3.47 Å. Hence, [1H]– is more accurately

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described as a terminal FeII hydride than a semi-bridging hydride. Note that the Ni-protonated isomer of [1H]– is calculated to be 25.9 kcal/mol higher in free energy with respect to the Fe-protonated one (Figure 2).

N1

H Ni

S1

Ni

O C1 Fe

Fe N2

S2

N2

H

N1

S1

S2

C1 O

(a) [Ni(FeH)L]2∆G

Fe-H Ni-H Ni-Fe Fe-S1 Fe-S2

= 0.0 1.49 2.69 3.47 4.05 2.27

ρNi 0.04 ρFe 0.01 ρS1 -0.00 ρS2 -0.00 ρ2Py 0.97 ρH 0.00

(b) [(NiH)FeL]2∆G

= 25.9

Fe-H Ni-H Ni-Fe Fe-S1 Fe-S2

3.41 1.57 3.27 2.35 2.35

ρNi ρFe ρS1 ρS2 ρ2Py ρH

1.32 -0.02 0.08 0.08 -0.72 0.22

Figure 2. DFT(B3P86)-optimized geometries (Å), relative free energies (∆G kcal/mol), and spin densities (ρ) of (a) Fe-protonated and (b) Ni-protonated species in the doublet state for [1H]–. For the addition of the second proton to [1H]–, direct proton delivery from [Et3NH]+ to the hydride in [1H]- to give [1H2] is predicted to be kinetically and thermodynamically unfavorable compared to proton delivery to the sulfur open site (by ∆∆G‡ = 7.8 and ∆∆G = 4.2 kcal/mol, Figure S6). Thus, the most likely pathway involves S serving as the proton relay rendering the formation of the thiol-hydride [H1H] (∆pKa = +3.3, Scheme S4), followed by the H+/H- coupling (∆G1‡ = 11.0 kcal/mol) to afford the side–on Fe-η2-H2-adduct intermediate, [1H2], which is stabilized by the H2 σelectron donation into the vacant Fe dx2-y2 orbital and back-donation from the doubly occupied Fe dxz orbital into the unoccupied H2 σ*-orbital (Figures S7 and S8). Finally, [1H2] releases H2 and regenerates the reduced species [1b] (∆G2‡ = 8.6 and ∆G = -0.3 kcal/mol) and then [1b] relaxes to [1a] through a very small barrier releasing additional free energy (∆G = -2.4 kcal/mol). Thus, this catalytic cycle reflects an E[ECEC] mechanism, wherein the first reduction serves as an activation step, while the second and third reduction form the catalytic cycle with the third one occurring in the same window as the second, a prediction consistent with the experiments.13 The resulting [H1H] has a protonated S and a bridging hydride, analogous to the proton-hydride pair recently characterized in the Ni-R state of the [NiFe]-H2ase.7b The difference between them is that the hydride is displaced towards the Fe site in [H1H] (Fe-H and Ni-H distances of 1.57 and 1.75 Å, respectively, Figure S9),


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while it is displaced towards the Ni site in Ni-R (Ni-H and Fe-H distances of 1.58 and 1.78 Å, respectively).7b Note that the distance of H+…H– in [H1H] is 2.74 Å, similar to the H+…H– coupling distance of 2.45 Å observed in Ni-R.7b Table 1. IR data for υCO (cm-1) of the species

be protonated at the ‘free’ thiolate to form the thiolhydride [NiI(HFeII)(SH)]. H2 can be evolved by first coupling H+/H- to yield the Fe-η2-H2-adduct. This predicted H2 production mechanism might be helpful for the rational design and understanding of [NiFe]-H2ases biomimetics.

Species

Exp.13

L/H

L/H

ASSOCIATED CONTENT

[1]+

1929

1937

1929

Supporting Information

[1a]

1770

1812

1804

1946/1936

1924/1920

The Supporting Information is available free of charge on the ACS Publications website.

1904/1913

1867/1882

[(NiH)FeL] [Ni(HFe)L]

1838

BP86a

B3P86a

a

The absolute values are given because BP86 functional provides good agreement with the experimental values without the application of scaling factors. The values calculated using B3P86, which gives the good geometries are scaled with the application of scaling factors of 0.93. L/H indicates of the low and high spin states. All the absolute values were given in Table S4.

The calculated CO frequencies are in reasonably good accord with the experimental values (Table 1); the BP86 frequencies are unscaled from BP86 geometries, while the B3P86 frequencies at the B3P86-optimized geometries are scaled. Upon reduction of [1]+ (1929 cm-1) a band appears at 1770 cm-1, which from DFT calculations is assigned to isomer [1a] with a semi-bridging CO.13 The calculated υCO values for the semi-bridging Fehydride [Ni(HFe)L] and Ni-hydride [(NiH)FeL] reveal that the semi-bridging hydride’s υCO value is in closer accord with experiment.13 The NMR predicted shift of 3.92 ppm for the semi-bridging Fe-hydride [Ni(HFe)L] is in better agreement with the experimental value of 6.80 ppm, compared to the shift of 10.35 ppm for Nihydride [(NiH)FeL] (Tables S7 and S8). When the reaction of [1]+ and NaBH4 was followed by 1H NMR spectroscopy, [1a] was the main product and the metalhydride was produced in up to 15% yield.13 This ratio may be rationalized by a thermoneutral mechanism in which the semi-bridging hydrides [Ni(HFe)L] react with itself to form [1a] and H2 (2[1H] → 2[1a] + H2, ∆G = 1.3 kcal/mol). 4. Conclusion In summary, our DFT calculations reveal that heterodinuclear NiFe complex [LN2S2NiIIFeIICp(CO)]+, [NiIIFeIIL]+, undergoes Fe-centered proton reduction catalysis, rather Ni-centered reactivity.13 The crystallographically characterized [NiIIFeIIL]+ undergoes a oneelectron reduction at the Ni center to afford [NiIFeIIL], which is further reduced to produce the Ni-bipyridine diradical [NiIFeIIL•]-. Protonation of [NiIIFeIIL•]- occurs at the Fe site and forms the stable semi-bridging hydride [NiII(HFeII)L], in which one bridging Fe-S bond breaks to afford a terminal thiolate. The bipyridine moiety accepts a third electron, rendering the formation of the Fehydride [NiII(HFeII)L•]-. This species is basic enough to

Benchmarking with various DFT functionals; Cartesian coordinates and absolute energies; Tables S1-S8, Figures S1-S11, and Schemes S1-S4, which include energies, spin densities, geometries, ∆pKa, υCO values and NMR from other functionals. (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge financial support from the National Science Foundation under Grant No. CHE-1664866 and the Welch Foundation under Grant No. A-0648. Computer time was provided by the TAMU High Performance Research Computing Facility.

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Fe-centered Proton Reduction [(NiH)FeL]

[NiFeL] N

S H

H2 Fe

[Ni(FeH2)L]

+

e-

[NiFeL]

+ e-

[NiFeL]

Ni O

C

+ H+

IPT [Ni(FeH)(SH)]

+ H+

[Ni(FeH)L]


+ e-

[Ni(FeH)L]

7

Biomimetics of [NiFe]-Hydrogenase: Nickel- or Iron-Centered Proton

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