Hemilabile Bridging Thiolates as Proton Shuttles in Bioinspired H2 Production Electrocatalysts

Article abstract of DOI:10.1021/jacs.6b06461

Synthetic analogues and computationally assisted structure-function analyses have been used to explore the features that control proton-electron and proton-hydride coupling in electrocatalysts inspired by the [NiFe]-hydrogenase active site. Of the bimetallic complexes derived from aggregation of the dithiolato complexes MN₂S₂ (N₂S₂ = bismercaptoethane diazacycloheptane; M = Ni or Fe(NO)) with (η⁵-C₅H₅)Fe(CO)⁺ (the Fe′ component) or (η⁵-C₅H₅)Fe(CO)₂⁺, Fe″, which yielded Ni-Fe′⁺, Fe-Fe′⁺, Ni-Fe″⁺, and Fe-Fe″⁺, respectively, both Ni-Fe′⁺ and Fe-Fe′⁺ were determined to be active electrocatalysts for H₂ production in the presence of trifluoroacetic acid. Correlations of electrochemical potentials and H₂ generation are consistent with calculated parameters in a predicted mechanism that delineates the order of addition of electrons and protons, the role of the redox-active, noninnocent NO ligand in electron uptake, the necessity for Fe′-S bond breaking (or the hemilability of the metallodithiolate ligand), and hydride-proton coupling routes. Although the redox active {Fe(NO)}⁷ moiety can accept and store an electron and subsequently a proton (forming the relatively unstable Fe-bound HNO), it cannot form a hydride as the NO shields the Fe from protonation. Successful coupling occurs from a hydride on Fe′ with a proton on thiolate S and requires a propitious orientation of the H-S bond that places H⁺ and H^- within coupling distance. This orientation and coupling barrier are redox-level dependent. While the Ni-Fe′ derivative has vacant sites on both metals for hydride formation, the uptake of the required electron is more energy intensive than that in Fe-Fe′ featuring the noninnocent NO ligand. The Fe′-S bond cleavage facilitated by the hemilability of thiolate to produce a terminal thiolate as a proton shuttle is a key feature in both mechanisms. The analogous Fe″-S bond cleavage on Ni-Fe″ leads to degradation.

Full text of DOI:10.1021/jacs.6b06461

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Article
Hemi-labile Bridging Thiolates as Proton Shuttles
2
in Bio-inspired H Production Electrocatalysts
Shengda Ding, Pokhraj Ghosh, Allen M. Lunsford, Ning Wang,
Nattamai Bhuvanesh, Michael B. Hall, and Marcetta Y. Darensbourg
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b06461 • Publication Date (Web): 19 Aug 2016
Downloaded from http://pubs.acs.org on August 19, 2016
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Hemi-labile Bridging Thiolates as Proton Shuttles in Bio-inspired H2 Production Electrocatalysts
Shengda Ding†§, Pokhraj Ghosh†§, Allen M. Lunsford†§, Ning Wang†#, Nattamai Bhuvanesh†,
Michael B. Hall*† and Marcetta Y. Darensbourg*†
†Department of Chemistry, Texas A & M University, College Station, TX 77843, United States
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Abstract
Synthetic analogues and computationally assisted structureꢀfunction analyses have been used to
explore the features that control protonꢀelectron and protonꢀhydride coupling in electrocatalysts inspired
by the [NiFe]ꢀhydrogenase active site. Of the bimetallic complexes derived from aggregation of the
dithiolato complexes MN2S2 (N2S2 = bismercaptoethane diazacycloheptane; M = Ni or Fe(NO)) with (η5ꢀ
C5H5)Fe(CO)+ (the Fe’ component) or (η5ꢀC5H5)Fe(CO)2 , Fe”, which yielded Ni-Fe’+, Fe-Fe’+, Ni-Fe”+,
+
and Fe-Fe”+, respectively, both Ni-Fe’+ and Fe-Fe’+ were determined to be active electrocatalysts for H2
production in the presence of trifluoroacetic acid. Correlations of electrochemical potentials and H2
generation are consistent with calculated parameters in a predicted mechanism that delineates the order of
addition of electrons and protons, the role of the redoxꢀactive, nonꢀinnocent NO ligand in electron uptake,
the necessity for Fe’ꢀS bond breaking (or the hemiꢀlability of the metallodithiolate ligand), and hydrideꢀ
proton coupling routes. Although the redox active {Fe(NO)}7 moiety can accept and store an electron and
subsequently a proton (forming the relatively unstable Feꢀbound HNO), it cannot form a hydride as the
NO shields the Fe from protonation. Successful coupling occurs from a hydride on Fe’ with a proton on
thiolate S and requires a propitious orientation of the HꢀS bond that places H+ and Hꢀ within coupling
distance. This orientation and coupling barrier are redoxꢀlevel dependent. While the Ni-Fe’ derivative
has vacant sites on both metals for hydride formation, the uptake of the required electron is more energy
intensive than that in Fe-Fe’ featuring the nonꢀinnocent NO ligand. The Fe’ꢀS bond cleavage facilitated
by the hemiꢀlability of thiolate to produce a terminal thiolate as a proton shuttle is a key feature in both
mechanisms. The analogous Fe”ꢀS bond cleavage on Ni-Fe” leads to degradation.
Introduction
Heterobimetallic molecular compositions utilizing thiolateꢀsulfurs as bridges are widespread in
biology, especially in the active sites of metalloenzymes such as the [FeFe]ꢀ and [NiFe]ꢀH2ase and Acetyl
CoA Synthase.1,2 That these biocatalysts facilitate organometallicꢀlike transformations, using firstꢀ
row/abundant transition metals, has inspired chemists to address the features that control their
mechanisms of action through the syntheticꢀanalogue approach. Synergy between synthesis and theory
has developed by linking the mechanistic interpretation of assays, such as electrocatalytic proton
reduction or hydrogen oxidation in the active sites of the hydrogenases, with those of the model
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complexes.3 While the structures of individual components of the
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biocatalysts that are siteꢀisolated by the protein are clear, functional
reproductions in small molecule models have not been entirely successful.
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The role of a pendant amine base nearby an open site on iron was
determined to be critical to the remarkable rates of hydrogen production in
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the [FeFe]ꢀH2ase2 and has been successfully used to design H+ reduction
and H2 oxidation electrocatalysts in nickelꢀbased complexes outfitted with
the PNPꢀ and P2N2ꢀtype ligands of Dubois, et al.3ꢀ8 Their team has also provided dramatic, bona fide
examples of heterolytic H2 cleavage products in (η5ꢀC5H4R)FeII(P2N2)+ complexes, suggesting that the
P2N2 ligand in structure I, and its pendant base capabilities, might be considered as a surrogate for the
Ni(SR)4 metalloligand in the [NiFe]ꢀH2ase active site.9ꢀ11 Thus, while the catalytic center of [NiFe]ꢀH2ase
does not have a pendant amine as operative base, there is structural support from high resolution protein
crystallography that a terminal cysteinyl thiolate on the nickel might serve in that capacity.12,13 Such a
suggestion was made earlier in the mechanistic study of Niu and Hall.14 Other persistent questions
regarding the requirement of two metals in such active sites are as follows: Do they assist each other by
dual electron storage? Does one tune the electronic character and redox potential of the other? Is a
metallodithiolate biology’s ultimate redoxꢀactive, nonꢀinnocent ligand?
There is an extensive class of biꢀ and polymetallic complexes derived from transition metals,
largely NiII, in tetradentate E2S22ꢀ (E = N, P, S) binding sites that use excess lone pairs on the cis thiolate
sulfurs for binding in a bidentate manner to an additional metal, M’.15,16
Analogous to the (η5ꢀC5H4R)FeII(P2N2)+ complexes described above,
myriad heterobimetallics have been reported in a developing area that
uses η5ꢀcyclopentadienide (η5ꢀC5H5 or η5ꢀC5Me5 i.e., Cp and Cp*,
respectively) or η6ꢀarenes bound to d6 FeII or RuII, as M’, which in
,
combination with the bridging dithiolates from the NiN2S2 may offer a
single open site for reactivity at M’, structure II.15,17ꢀ22 The tunability at the piꢀligand offers some control
for oxidative addition in stoichiometric reactions, including both H2 and O2 activation.23ꢀ26 Reports of
proton reduction under electrochemical conditions by such CpFeII or CpRuII entities are scarce in the
literature; however, there are examples of an S’2NiS2 (S’ = thioether sulfur; S = thiolate sulfur)
metalloligand bound to CpFe’ and Cp*Fe’ that demonstrated modest electrocatalysis and H2
production.18,20 The MN2S2 platform offers opportunity to modify a metallodithiolate ligand by changing
only the M, retaining consistency in steric features such that the Sꢀdonor and M’ꢀacceptor effects might
be deconvoluted. Thus, we have designed experimental and computational protocols to analyze the
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proton reduction possibilities of the heterobimetallics represented in Figure 1, with focus on the potential
sites for electron and proton uptake, the order of their addition, and the requirements for hemiꢀlability and
Sꢀprotonation of the MN2S2 metallodithiolate ligands at various redox levels.
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Results
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Scheme 1. The synthesis of Fe-Fe’+ and Ni-Fe’+ complexes as
BF4 salts. The IR frequencies of CO and NO are in red and
Synthesis
and
Characterization.
ꢀ
Scheme 1 displays the synthetic protocol used
blue respectively.
to prepare the bimetallic complexes,
MN2S2•CpFe(CO)+BF4 (M = Fe(NO), Ni, the
Fe in CpFe(CO) is Fe’), Fe-Fe’+ and Ni-Fe’+,
in this work. The reaction of MN2S2 and
[CpFe(CO)2(Solv)]+, prepared in situ from
CpFe(CO)2I and AgBF4 in CH2Cl2, at 22°C,
ꢀ
formed
an
intermediate
ꢀ
species
MN2S2•CpFe(CO)2 BF4 , Fe-Fe”+ and Ni-Fe”+
(the Fe in CpFe(CO)2 is Fe”). Subsequent
photolysis released CO and permitted bidentate
binding of the metallodithiolate ligands. While
the intermediate species, Fe-Fe”+ and Ni-Fe”+,
+
are light and air sensitive, the Fe-Fe’+ and Ni-Fe’+ complexes are isolated as intensely colored crystalline
ꢀ
BF4 salts that are thermally and air stable in the solid form. Stringent conditions (CO pressure of 11 bar
and 50°C) partially return the MFe’+ to the MFe”+, see Figure S9.
Xꢀray diffraction analysis of crystalline Ni-Fe’+, Fe-Fe’+, and Ni-Fe”+ revealed molecular
structures with typical pianoꢀstool geometry about the CpFe’(CO)+ unit and butterflyꢀlike [M(ꢁꢀSR)2Fe’]
cores in the Ni-Fe’+ and Fe-Fe’+ derivatives, Figure 1. Specifically, the bridging thiolate sulfur lone pairs
impose a hinge angle (the intersection of the best N2S2 plane with the S2Fe’ plane) of ca. 125°. The
mesocyclic diazacycloheptane framework in the MN2S2 portion of each provides similar NꢀꢀꢀN and SꢀꢀꢀS
distances, and ∠
of ca. 82o. In the Fe-Fe’+ complex the NO is transoid to the CO on the CpFe’
S–Fe’–S
unit; the ∠
angle is 163.8°. The M…Fe’ distance in Fe-Fe’+ and Ni-Fe’+ are 3.203(1) and 3.016(1)
FeꢀNꢀO
Å, respectively. In contrast, the Ni-Fe”+ dicarbonyl complex finds the NiN2S2 plane is shifted away from
where it was in the Ni-Fe’+, opening the Ni–SꢀFe” bond angle to ca. 121.4(1)° from ca. 85.44(3)° in the
Ni-Fe’+, and yielding a NiꢀFe” distance some 0.7 to 0.9 Å greater than in the bidentate MN2S2ꢀFe’
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complex. The Fe”ꢀS dative bond distance in Ni-Fe”+ is 2.285(3) Å and the nonꢀbonded thiolate S is at
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3.999(3)Å from the Fe”.
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ꢀ
Figure 1. Molecular structures of Ni-Fe”+, Fe-Fe’+, and Ni-Fe’+ complexes. The BF4 ions are
omitted for clarity. aBonded sulfur. b Nonꢀbonded sulfur. c Average MꢀS distance
While the Ni-Fe’+ complex is diamagnetic, the Fe-Fe’+ has S = 1/2, consistent with the wellꢀ
known {Fe(NO)}7 electronic configuration.27,28 The 298 K, Xꢀband EPR spectrum, shows an isotropic
triplet of g value = 2.04 with hyperfine coupling constant of 15.3 G, and only minor differences to the
free metalloligand.29 Details of the lowꢀ and variable field Mössbauer spectra of the M-Fe’+ and M-Fe”+
complexes will be presented and discussed in a separate study.
ꢀ
Electrochemistry: Cyclic voltammograms (CV) of BF4 salts of Fe-Fe’+, Figure S30, and Ni-
Fe’+, Figure S34, were recorded at 22° C under Ar. All scans are referenced to internal Fc0/+ at E1/2 = 0.0
V. Full scans of both complexes initiated in the positive direction as well as peak isolation and scan rate
dependence can be found in the SI, Figures S30ꢀS37. On initiating the electrochemical scan in the
cathodic direction, two reduction events, and, upon reversal, two oxidation events were observed for both
complexes within the MeCN solvent window. The initial reductive event, at ꢀ1.64 V in the case of the Ni-
Fe’+, is assigned to the NiII/I couple; its irreversibility is addressed in the computational section below. In
contrast, for the Fe-Fe’+ complex, the first reduction is quasiꢀreversible and at a more positive position, ꢀ
1.19 V; it is assigned to the {Fe(NO)7/8 redox couple. In both cases, the first observed or more positive
reduction event is anodically shifted compared to the MN2S2 (free metalloligand) precursors, thus
illustrating the electronꢀwithdrawing nature of the [CpFe’(CO)]+ unit and its ability to modulate redox
events on the MN2S2 unit.28,30 The second, more negative, irreversible reduction event in the Fe-Fe’+
complex is assigned to the Fe’II/I couple in the [CpFe’(CO)]+ unit. For the Ni-Fe’+ complex, assignment
of the more negative event is not straightꢀforward due to the irreversibility of the previous redox event;
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computational studies, vide infra, indicate an intramolecular NiI to FeII electron transfer concomitant with
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structural rearrangement accounts for this irreversible behavior.
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Addition of trifluoroacetic
acid (TFA) to the electrochemical
cell containing Ni-Fe’+ or Fe-Fe’+
increases the current of the initial
reduction events described above.
[N.B. Methanesulfonic acid gave
similar results as TFA, see figure S38
ꢀ S39, however considerable fouling
of the electrode surface discouraged
extensive studies with this acid.] For
the Ni-Fe’+ complex, this current
continues to increase with additional
equivalents of TFA, Figure 2A, while
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Figure 2. CV of 2 mM A) Ni-Fe’+ and B) Fe-Fe’+ under Ar in
for the Fe-Fe’+ complex the initial
CH3CN solutions containing 0.1 M [tBu4N][PF6] as supporting
reduction event’s current is saturated
after addition of 12 equiv. of TFA,
Figure 2B. With greater than 6
equiv. of TFA, a new peak at ꢀ1.66 V
electrolyte with addition of equivalents of trifluoroacetic acid. C) An
overlay of Ni-Fe’+ and Fe-Fe’+ in the presence of 50 equivalents of
TFA as well as 50 equivalents of TFA in the absence of either
catalyst. The dotted line denotes the potential applied during bulk
electrolysis, ꢀ1.56 V.
appears for the Fe-Fe’+ complex and its intensity increases with additional equiv. of TFA. An overlay of
both complexes after addition of 50 equiv. of TFA as well as TFA in the absence of either catalyst is
displayed in Figure 2C. The large current enhancement was attributed to the catalytic production of H2,
which was quantified by bulk electrolysis studies described below. From the CV experiments, turnover
frequencies (TOFs) of 69 sꢀ1 and 52 sꢀ1 (experimental barriers: 14.9 and 15.1 kcal/mol at 298.15 K by
Eyring equation) and overpotentials of 938 mV and 942 mV for the Fe-Fe’+ and Ni-Fe’+ complexes
respectively, were obtained.31ꢀ33 The calculation of TOFs and overpotentials follows the approach
described by Helm, Appel, and Wiese, see the SI for specifics.33,34 It is noteworthy to mention the
overserved barrier is a comprehensive parameter reflecting the activation of electron transfer, proton
transfer and intraꢀ/interꢀmolecular processes throughout the catalytic cycle. It is often higher than the
calculated barriers of intramolecular processes, vide infra. The H/D kinetic isotope effects on Fe-Fe’+
and Ni-Fe’+ turnover frequencies (kH/kD) were determined to be 1.46 and 1.56, respectively. While kH/kD
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isotope effects are known to vary widely, these relatively low ratios are consistent with the likely
involvement of metalꢀhydride species in the catalytic cycles.35 36
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Electrocatalytic H2 production: The headspace of the bulk electrolysis setup was analyzed for H2
using gas chromatography after applying a constant potential at ꢀ1.56 V (dotted line in Figure 2) in the
presence of catalyst and 50 equivalents of TFA. Due to the overlap of the background TFA peak and the
catalytic peaks, the H2 evolving from the acid itself must be deducted, Table S3. All values obtained are
an average of three separate bulk electrolysis experiments. After 30 min of electrolysis with the Ni-Fe’+
catalyst, 0.98 ± 0.04 Coulombs (after acid subtraction) was passed through the solution resulting in a turnꢀ
overꢀnumber (TON) of 0.26 ± 0.01 with a Faradaic efficiency of 96.0 ± 2.9 % for H2 production, Table
S4. Similarly in the presence of the Fe-Fe’+ catalyst, passage of 1.29 ± 0.06 Coulombs through the
solution gave a TON of 0.33 ± 0.02 with a Faradaic efficiency of 77.2 ± 7.9 % for H2, Table S5. These
results confirm that the current enhancement in the cyclic voltammogram is in fact due to the reduction of
protons to H2 by the Ni-Fe’+ and Fe-Fe’+ catalysts in the presence of TFA.
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Computational investigation: assignment of redox events and mechanistic studies
The complexities of the cyclic voltammograms of the Ni-Fe’+ or Fe-Fe’+ complexes in the
presence of added acid, which indicate the existence of protonated and/or rearranged species, stimulated
computational studies as complements to electrocatalytic proton reduction studies. A minimum of two
chemical steps (C steps, i.e. protonation) and two electrochemical steps (E steps, i.e. reduction) is
required to produce H2 from protons and electrons. The exact order of C and E steps depends on the pKa
of the acid vs. catalyst and the redox potential of the catalyst, respectively; they often take place in an
alternating order to prevent the accumulation of charges.37 To computationally construct the E and C steps
in catalytic cycles, structures of the precursor complexes from xꢀray diffraction were compared to the
calculated structures as validity checks, Table S11; the redox potentials (E0 vs. Fc+/0) and relative acidities
ꢂ
ꢃ
ꢂ
ꢃ
(ꢂpKa = pꢀꢁ CatH − pꢀꢁ CFꢄCOOH ) of components were predicted by calculations. Alternative sites
for location of the added protons were carefully examined to determine which sites were lowest in energy.
Detailed methodology information and optimized geometries (xyz files) are deposited in the SI.
Computational approaches to electrocatalytic proton reduction mechanisms have become fairly
standard,37ꢀ39 especially for biomimetics of the hydrogenase active sites. From protein crystallography the
features of the protein ensconced molecular catalysts and second coordination spheres are readily
apparent but their roles are just beginning to be firmly established.1 Hence, our starting points for the
predicted mechanisms lie in paths deemed reasonable for the biocatalysts and for previous studies of
biomimics; structures are accepted or rejected according to comparative energies (E0 and pKa) and
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activation barriers between structures. The bimetallic constitution of our complexes, Fe-Fe’+ and Ni-Fe’+
enables them to buffer electrons, with additional stabilization from the nonꢀinnocent ligands, particularly
NO in the case of Fe-Fe’+.29 At some point, typically after reduction(s), a complex must be able to accept
a proton, convert it into a hydride on the metal, be poised to react with an additional proton, located on
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some basic site, to yield H2. Our model complexes, however, lack an obvious builtꢀin pendant base to
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1,38,40ꢀ42
serve as a proton reservoir, a role played by the bridgehead amine in [FeFe]ꢀH2ase,
or a terminal
thiolate in the [NiFe]ꢀH2ase active site.1,12,13 Instead, the hemiꢀlabile bridging thiolates on Fe-Fe’+ and
Ni-Fe’+ may dissociate one of two Fe’ꢀS bonds; the veracity of such a monoꢀdentate Sꢀbridging species is
supported by the isolated Ni-Fe”+ shown in Scheme 1. Such dissociation creates reactive sites both on S
and Fe’; i.e., a Lewis acidꢀbase pair that can be used as proton and hydride storage depots is generated.
Interestingly, the possibility of conversion of a bridging thiolate into an available proton base was
inspired by the early theoretical studies of the [FeFe]ꢀH2ase.38,43 The advent of semiꢀsynthetic approaches
to biohybrids in recent years
that unambiguously identified
a bridgehead amine in the S to
S linker of the diiron unit in
[FeFe]ꢀH2ase has established
the pivotal role of this pendant
base in proton transfer, thus
negating the requirement for
FeꢀS bond cleavage in such
functionalized dithiolates. 44ꢀ47
Figures
the
3
and
4
display
calculated
electrocatalytic cycles for H2
production with Fe-Fe’+ and
Ni-Fe’+,
electrocatalysts.
respectively,
as
A
Figure 3. The calculated electrocatalytic cycles for H2 production
on Fe-Fe’+ in the presence of TFA. The relative Gibbs free energies
are provided in kcal/mol and the reference point (G = 0) resets after
every reduction or protonation. The redox potentials (E) are
reported in V with reference to the standard redox couple Fc+/0 and
the relative acidities (ꢂpKa) are reported with reference to TFA.
Note: superscripts DN and UP on S refer to the positioning of the
proton in Sꢀprotonated species.
description of the former is as
follows. In the absence of
added acid, the CV scans of
Fe-Fe’+ show two reduction
events; the first quasiꢀ
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reversible one was calculated to be ꢀ1.11 V (exp. ꢀ1.19V) and is assigned to the Fe(NO) unit, i.e., the
redox couple {Fe(NO)}7/8ꢀFe’II. Such an assignment was confirmed by the IR shifts of the diatomic
ligands (exp.: ꢀ57 and ꢀ23 cmꢀ1; calc’d ꢀ84 and ꢀ31 for NO and CO respectively, Figure S11, Table S12).
The resulting neutral Fe-Fe’ has a linear triplet {Fe(NO)}8 moiety, formed by high spin FeII
ꢀ
29,48
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antiferromagnetically coupled to highꢀspin NO
.
It may be further reduced irreversibly, calculated at ꢀ
1.99 V (exp. ꢀ 2.07 V), to Fe-Fe’–, in which one SꢀFe’ bond dissociates to accommodate the added
electron on Fe’ with a final redox level of {Fe(NO)}8ꢀFe’I.
In the presence of TFA the first reduction event at ꢀ1.19 V in the cyclic voltammogram was
observed to increase in current without shifting position. This behavior is explained by the reaction of
TFA with the reduced Fe-Fe’ state and its depletion, thus enhancing diffusion of Fe-Fe’+ into the double
layer at the electrode. By calculations, the thiolate S was determined to be the optimal protonation site.
Other possibilities (Table S9) were considered, including the ironꢀbound NO which would produce the
HNO ligand. It was found however to be thermodynamically less likely and also nonꢀproductive for
subsequent H2 formation as a metalꢀhydride is needed for the H+/H– coupling. Upon protonation on sulfur
the bond cleavage at Fe’ꢀS immediately follows, stabilizing the system by 3.7 kcal/mol. The ꢂpKa (vs.
TFA) values for ringꢀclosed (Fe-Fe’-S*H+ ) and ringꢀopened (Fe-Fe’-SUPH+) sulfurꢀprotonated species
are ꢀ5.6 and ꢀ2.7, respectively, indicating slightly unfavorable thermodynamic processes. Thus, excess
acid is needed to drive the protonation of Fe-Fe’, explaining why the observed saturation of current
enhancement requires multiple equivalents (> 12 equiv.) of added acid and rules out the possibility of an
immediate second protonation on Fe-Fe’-SUPH+ (to Fe-Fe’H-SDNH2+, ꢂpKa = ꢀ14.3). Despite the
increase in current response, the electrochemical event at ꢀ1.11 V (ꢀ1.19 V exp.) is not catalytic as this
reduction potential is insufficient (vide infra) to pass a second electron and close the catalytic cycle.
A second current enhancement, which appears in CV scans with added acid at ꢀ1.66 V (shifted by
0.41 V from ꢀ 2.07 V in the absence of acid), suggests reactions of new species, Fe-Fe’-SUPH+, generated
by protonation. One should be reminded that the production of Fe-Fe’-SUPH+ is energetically unfavorable
such that the reduction event of Fe-Fe’-SUPH+ observed at ꢀ1.66 V becomes dominant only with the
presence of more than 6 equiv. of TFA. The reduction of Fe-Fe’-SUPH+ has a calculated potential of ꢀ1.32
V, changing the FeII of Fe’ to FeI , a redox state capable of converting a proton into a hydride. The direct
product of reduction, Fe-Fe’-SUPH (G =1.4 kcal/mol) may transform into a hydrideꢀbearing species Fe-
Fe’H (G =1.7 kcal/mol) via the SꢀH inversion species Fe-Fe’-SDNH (G = 0 kcal/mol) traversing two lowꢀ
lying transition states (G = 4.2 and 7.6 kcal/mol). The Fe-Fe’H species is at the {Fe(NO)}8ꢀFe’III redox
level as the electrons forming the ironꢀhydride are donated by FeI of the reduced Fe’.
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There are two pathways shown in Figure 3 for addition of the second proton. Although Fe-Fe’-
SDNH is the dominant species, the next protonation step, either on S of Fe-Fe’H or on Fe’ of Fe-Fe’-
SDNH, produces the same thiolꢀhydride, Fe-Fe’H-SDNH+ and both protonations are thermodynamically
favored, with ꢂpKa values of 6.6 or 5.3 kcal/mol, respectively. The spatial positioning of the hydride and
the proton on Fe-Fe’H-SDNH+ allows the coupling reaction over a barrier of G = 11.6 kcal/mol. The
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+
resulting H2 σꢀcomplex Fe-Fe’H2 then overcomes another barrier at G = 12.0 kcal/mol to dissociate H2
and to regenerate the catalyst Fe-Fe’+. This catalyst cycle thus closes with an [ECEC] mechanism. This
mechanism uses the thiolate sulfur as a proton relay. One may argue TFA may directly deliver the proton
+
to the hydride of Fe-FeH’ to accomplish an intermolecular coupling to form Fe-FeH2 , skipping the
intermediate Fe-FeH-SDNH’+. The relatively high barrier at 16.2 kcal/mol (Figure S43) renders this
possibility less likely.
In
contrast the delivery of proton
into the sulfur open site only
incurs
a
negligible barrier
(Figure S43).
Alternatively, Fe-Fe’H-
SDNH+ may accept
third
a
electron at a redox potential of ꢀ
1.27 V and the highest reaction
barrier
dramatically drops to 4.9
kcal/mol. In this case the
for
H2
formation
reduced Fe-Fe’ is regenerated
instead of Fe-Fe’+ and closes an
E[CECE] working catalytic
cycle, in which the first
reduction
event
essentially
serves as an activation step.
According to the calculations,
Figure 4. The calculated electrocatalytic cycles for H2 production
on Ni-Fe’+ in the presence of TFA; see caption of Figure 3 for
additional description. The Gibbs free energy of the barrier
between Ni-Fe’H2 and Ni-Fe’, G = ꢀ4.6 kcal/mol, as marked with an
asterisk, is lower than that of Ni-Fe’H2, G = 1.2 kcal/mol. This is
caused by the preference of solvation correction over the transition
state. This transition may be accepted as barrierless.
the
current
enhancement
associated with the second
reduction event at ꢀ1.32 V
(calc’d; observed at ꢀ1.66 V) is
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considered to be catalytic and productive in either the slow or fast catalytic cycle as subsequent reduction
events are all calculated to be less negative than ꢀ1.32 V.
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The nickel species Ni-Fe’+ has mechanisms similar to those of Fe-Fe’+ with a few exceptions,
Figure 4. The first reduction of Ni-Fe’+ is initially localized on the NiN2S2 moiety with its fourꢀmembered
Ni(µꢀSR)2Fe’ unit intact as was that of Fe-Fe’. However, the fourꢀcoordinate nickel lacks the electronic
flexibility of Fe(NO) in Fe-Fe’ and can only accommodate the added electron on nickel’s highly
destabilized antibonding dx2-y2 orbital, achieving an oxidation state of NiIꢀFe’II in Ni-Fe’*. As a result the
calculated redox potential rises significantly to ꢀ2.00 V (exp. ꢀ1.64 V). Following the reduction, one SꢀFe
bond of the Ni(µꢀSR)2Fe’ core breaks to open the NiꢀS2ꢀFe’ ring. The electron previously added to the
nickel is concomitantly transferred to the unsaturated (16ꢀeꢀ) Fe’ with bond cleavage, bringing the
electron counts back to a 16ꢀeꢀ NiII and a 17ꢀeꢀ FeI. This arrangement stabilizes the ringꢀopened species
Ni-Fe’ by 1.0 kcal/mol, accounting for observed irreversibility of the CV event. The experimental IR
shift, ꢀ157 cmꢀ1 (Figure S10), upon the reduction of Ni-Fe’+, confirms Fe-Fe’ (calc’d shift: ꢀ127 cmꢀ1,
Table S12) is the reduced product, rather than Fe-Fe’* (calc’d shift: ꢀ43 cmꢀ1).
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In the absence of acid, following the ringꢀopening process and intramolecular charge transfer, the
successive reduction on Ni-Fe’ puts the second electron again within the NiII/I couple. The calculations
also affirm that the first redox potential is more negative than that of any subsequent steps in the catalytic
cycles in the presence of TFA (Figure 4), so that the CV current enhancement at ꢀ1.64 V is acknowledged
as catalytic. The followꢀup protonation on Ni-Fe’ goes directly to the reduced Fe’ rather than S as the FeI
has sufficient electron density to convert the proton into a FeIIIꢀhydride. The next steps are similar to
those of Fe-Fe’+ in Figure 3. The Ni-Fe’+ may also have two working catalytic cycles, either [ECEC] or
E[CECE] depending on the occurrence of a nonꢀmandatory, third reduction event.
The homoconjugation of TFA,31,49 i.e. the stabilization of the conjugate base TFAꢀ by another
molecule of HꢀTFA, was evaluated by calculations to enhance the the acidity by ꢀ 5.6 pKa units (exp. ꢀ
3.9)31 on standard conditions. The acidity increase, though less significant when the acid concentration is
low, may further facilitate these protonation processes outlined in Figure 3 and 4 at the cost of faster
depletion of the available acid on the electrode surface. However, it may not be able to activate another
route. An immediate second protonation requires a much stronger acid, vide supra.
By proceeding along the predicted mechanistic pathway, the monoꢀdentate species, Ni-Fe”+,
breaks its single FeꢀS bond upon reduction and the complex decomposes, as experimentally observed.
The cleaved fragment, the •FeCp(CO)2 radical, is also catalytically active for H2 production before its fast
deactivation by dimerization.50
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This work provides a paradigm for deconvoluting electrocatalytic protonꢀreduction mechanisms
in dithiolate bridged bimetallics. Salient points to be made regarding the mechanistic features of the two
[MN2S2·CpFe(CO)]+ electrocatalysts are as follows:
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• The initial electron uptake is at the M in the N2S2 pocket, rather than the CpFe’(CO)+, for both
M = NiII and {[Fe(NO)}7 ; the latter however presents a softer, delocalized landing for the electron,
without permitting subsequent FeꢀH formation, as the iron is not adequately basic (Table S9). Another
key difference lies in the fact that the added electron is stored on the {[Fe(NO)}8 unit (within the
Fe(NO)N2S2 metalloꢀligand) throughout the catalytic cycle rendering that unit a “redoxꢀactive, spectator
ligand”51 to the reactive center, the CpFe(CO) unit, in the preferred E[CECE] path. In contrast, the firstꢀ
formed NiIN2S2 readily transfers its electron to Fe’, with NiIIꢀ(µꢀSR)2ꢀFeI’ ring opening in advance of
protonation. Thus, the NiII in the monoꢀdentate NiN2S2 metalloligand cannot accept a proton to form a
NiꢀH bond resembling the recent NMR characterized Niꢀbound hydride in a NiꢀR model, which contains
a nonꢀinnocent ligand with Ni to buffer the electron.52 Besides, Fe is also protected from the proton by
open sites on S and on reduced Fe’.
• The hemiꢀlability of
the MN2S2 metalloꢀligand,
necessary for producing an
open site on the active iron of
the CpFe’ unit (a site that is
occupied by CO in the Ni-Fe”+
congener or procatalyst), as
well as an available Sꢀbase site,
is facilitated by reduction of the
dithiolate bridged bimetallic.
Figure 5. Species featuring proximate protonꢀhydride pairs and the
comparisons of H+ꢀHꢀ distances. The τ value, a measure of square
pyramid (τ = 0) vs. trigonal bipyramid (τ = 1) geometry in the
A further role for this hemiꢀlability is displayed in the monoꢀdentate bridging thiolate bound to the Fe’IIIꢀ
hydride in Fe-Fe’H-SDNH+. The Fe’III with a formal electron count of 17 is able to accept partial donation
from an available πꢀdonor pair on S, serving as a σ+π ligand, while Fe’II in Fe-Fe’H-SDNH+ is completely
saturated and the S is merely a σꢀdonating ligand. (See Table S10 for Fe’ꢀS bonding analysis.) This
additional π bonding in the oxidized Fe-Fe’H-SDNH+ species is exemplified by its short Fe’ꢀS bond
distance at 2.230 Å that elongates to 2.342 Å upon reduction to the Fe-Fe’H-SDNH species.
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• The H2 evolution from the diꢀprotonated, doubly or triply reduced species requires optimally
oriented protonated thiol and iron hydride. In this regard it is instructive to compare H+ꢀꢀꢀHꢀ distances in
our calculated intermediate thiolꢀhydrides with experimental data from the doubly protonated
P2N2FeCpR(CO) complex of Liu, et al.,10 Figure 5, finding concurrence in the reduced Fe-Fe’H-SDNH
form (1.486Å) with that found in the amine pendant base complex (1.489 Å). Note that reduction of Fe-
Fe’H-SDNH+ shortens the H+ꢀꢀꢀHꢀ distance from 2.634Å to 1.486Å via structural shifts in the
Fe(NO)N2S(SH) metalloligand, involving both a rotation around the Fe’ꢀS bond as well as a small change
in the τ parameter53 that defines the extent of square pyramid vs. trigonal bipyramid character in the
Fe(NO)N2S(SH) unit. These changes push the protonꢀhydride pair into a close position, creating an early
transition state according to Hammond's postulate,54 amenable for H2 elimination via the E[CECE], low
barrier path. In contrast at 2.634Å the H+/Hꢀ coupling following the [ECEC] mechanistic path must
surmount a much higher barrier. Note that the H+ꢀꢀꢀHꢀ coupling distance in the Fan and Hall calculated
mechanism for proton reduction in the [FeFe]ꢀH2ase active site is 1.472Å, remarkably consistent with the
experimental value from structure I, and the calculated value (1.486Å) for our reduced diprotonated
intermediate Fe-Fe’H-SDNH in Figure 5.36 Notably, the proton/hydride pair recently characterized in the
NiꢀR state of the [NiFe]ꢀH2ase active site is at 2.45Å,12 a distance related to the intermediate in our slow
route for H2 production, and perhaps consistent with the [NiFe]ꢀH2ase enzyme’s bias towards H2 uptake
and oxidation rather than production.
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In conclusion, the wellꢀstudied P2N2 ligand of Dubois, et al.4 has control of optimal proton
placement via the chair/boat interconversion of the sixꢀmembered FeP2C2N cyclohexaneꢀlike ring
described in Figure 5,10 a feature that was exploited in the design and development of further generations
of the Ni(P2N2)2 catalyst(s) and presaged by Nature's azadithiolate bidentate bridging ligand in the [FeFe]ꢀ
H2ase active site.1 The heterobimetallics explored herein demonstrate the possibility for very stable
bidentate ligands based on metallodithiolates (a metalꢀtamed Sꢀdonor or Nature’s version of a phosphine
Pꢀdonor) that respond to an electrochemical event by switching a coordinate covalent bond into a Lewis
acidꢀbase pair and concomitantly placing a proton and hydride within an optimal coupling
distance. Easily accessible molecular motions and coordination sphere distortions are available to render
the tethered thiolate into a pendant base of greater activity for proton delivery to the metalꢀhydride. The
opportunities for tuning catalysts according to this approach lie both on the metal responsible for the
hydride activity and, as we have also shown, the metal that holds and orients the pendant base. Our future
plans are to optimize the catalysts via the bidentate SꢀMꢀS angle and to pursue experimental evidence for
the thiolꢀhydride pair.
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Supporting Information
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Experimental, additional spectroscopic, electrochemical and computational details, Xꢀray crystallographic
data (CIF) from the structure for complexes Fe-Fe’+, Ni-Fe’+, Ni-Fe”+, and computational xyz files. This
material is available free of charge via the Internet at http://pubs.acs.org.
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Author Information
Corresponding author emails: marcetta@chem.tamu.edu
#Currently at School of Chemistry and Chemical Engineering, Henan University of Technology,
Zhengzhou 450001, China.
§Equal contributors to this paper; listed in alphabetical order.
Acknowledgements
We are grateful for financial support from the National Science Foundation (CHEꢀ1266097 to M.Y.D.,
and CHEꢀ1300787 to M.B.H.), the Robert A. Welch Foundation (Aꢀ0924 to M.Y.D., and Aꢀ0648 to
M.B.H.). Part of the experimental work (salary for Allen Lunsford and Pokhraj Ghosh) was made
possible by an NPRP award (NPRP 6ꢀ1184ꢀ1ꢀ224) from the Qatar National Research Fund (a member of
Qatar Foundation). The authors acknowledge the Laboratory for Molecular Simulation at Texas A&M
University for providing computing resources. Appreciation is expressed to Drs. Eric Wiedner and Aaron
Appel for helpful advice dealing with interpretation of the cyclic voltammetry, bulk electrolysis and gas
quantification.
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