Complexes of MN2S2·Fe(η5-C5R5)(CO) as platform for exploring cooperative heterobimetallic effects in HER electrocatalysis

Article abstract of DOI:10.1039/c6dt04666e

The control of aggregation at sulfur by metallodithiolates (MN₂S₂) has made them prime candidates as building blocks for the synthesis of biomimetics of various bimetallic enzyme active sites, with reactivity consequences implicating redox control by both metal centers. Recent studies of MN₂S₂ (M = Ni²⁺, Fe(NO)²⁺) bound to [(η⁵-C₅H₅)Fe(CO)]⁺ as electrocatalysts for proton reduction, the hydrogen evolution reaction, demonstrated reduction-induced hemi-lability of the bridging cis-dithiolates as a key step in the electrochemical proton reduction process (Ding, et al., J. Am. Chem. Soc., 2016, 138, 12920-12927). The MN₂S₂·Fe(η⁵-C₅R₅)(CO) platform offers numerous possibilities for tuning the electronic character of the M(μ-S₂)Fe core. As well as modifying M within the metallodithiolate ligand, replacing H by CH₃ at the η⁵-C₅R₅ moiety increases the electron density at the Fe center, which might facilitate the reductive Fe-S bond cleavage. Although release of a free thiolate in these hemi-labile ligands creates a needed internal pendant base, this benefit might be countered by the increase in over-potential for addition of the first electron. Herein we report the preparation and characterization of four bimetallic aggregates with the (η⁵-C₅R₅)Fe(CO) (R = H, CH₃; Fe′ or Fe?′, respectively) or the dicarbonyl (η⁵-C₅R₅)Fe(CO)₂ scaffold (R = H, CH₃; Fe′′ or Fe?′′, respectively) bound to redox active MN₂S₂ ligands (M = Ni²⁺, Co(NO)²⁺; N₂S₂ = bismercaptoethane diazacycloheptane) Co-Fe?′, Ni-Fe?′, Co-Fe′ and Co-Fe?′′ complexes. The bidentate complexes were found to be electrocatalysts for proton reduction, although at high over-potential, especially for the derivatives of the electron-rich (η⁵-C₅(CH₃)₅)Fe(CO)⁺. The turnover (TON) and turnover frequencies (TOF) were determined and found to be comparable to the previously reported MN₂S₂·Fe(η⁵-C₅H₅)(CO)⁺ analogues.

Full text of DOI:10.1039/c6dt04666e

Dalton
Transactions
View Article Online
View Journal
PAPER
Complexes of MN2S2·Fe(η5-C5R5)(CO) as platform
for exploring cooperative heterobimetallic effects
in HER electrocatalysis†
Cite this: DOI: 10.1039/c6dt04666e
Pokhraj Ghosh, Manuel Quiroz, Ning Wang,‡ Nattamai Bhuvanesh and
Marcetta Y. Darensbourg*
The control of aggregation at sulfur by metallodithiolates (MN2S2) has made them prime candidates as
building blocks for the synthesis of biomimetics of various bimetallic enzyme active sites, with reactivity
consequences implicating redox control by both metal centers. Recent studies of MN2S2 (M = Ni2+
,
Fe(NO)2+) bound to [(η5-C5H5)Fe(CO)]+ as electrocatalysts for proton reduction, the hydrogen evolution
reaction, demonstrated reduction-induced hemi-lability of the bridging cis-dithiolates as a key step in the
electrochemical proton reduction process (Ding, et al., J. Am. Chem. Soc., 2016, 138, 12920–12927). The
MN2S2·Fe(η5-C5R5)(CO) platform offers numerous possibilities for tuning the electronic character of the
M(µ-S2)Fe core. As well as modifying M within the metallodithiolate ligand, replacing H by CH3 at the
η5-C5R5 moiety increases the electron density at the Fe center, which might facilitate the reductive Fe–S
bond cleavage. Although release of a free thiolate in these hemi-labile ligands creates a needed internal
pendant base, this benefit might be countered by the increase in over-potential for addition of the first
electron. Herein we report the preparation and characterization of four bimetallic aggregates with the
(η5-C5R5)Fe(CO) (R = H, CH3; Fe’ or Fe*’, respectively) or the dicarbonyl (η5-C5R5)Fe(CO)2 scaffold (R = H,
CH3; Fe’’ or Fe*’’, respectively) bound to redox active MN2S2 ligands (M = Ni2+, Co(NO)2+; N2S2 = bismer-
captoethane diazacycloheptane) Co-Fe*’, Ni-Fe*’, Co-Fe’ and Co-Fe*’’ complexes. The bidentate com-
plexes were found to be electrocatalysts for proton reduction, although at high over-potential, especially
for the derivatives of the electron-rich (η5-C5(CH3)5)Fe(CO)+. The turnover (TON) and turnover frequen-
cies (TOF) were determined and found to be comparable to the previously reported MN2S2·Fe(η5-C5H5)
(CO)+ analogues.
Received 9th December 2016,
Accepted 1st February 2017
DOI: 10.1039/c6dt04666e
rsc.li/dalton
metal aggregation via monodentate or bidentate binding.9,10
The ability of the N2S2 tetradentate ligand to securely bind a
Introduction
Synthetic approaches to small molecule models of the active variety of metals, including the redox active {Fe(NO)}7/8 and
sites [FeFe]-, [NiFe]-hydrogenases1 and Acetyl CoA synthase {Co(NO)}8/9 units (Enemark Feltham notation11), encourages
(ACS)2,3 and potential heterobimetallic catalysts have exploited explorations of redox cooperativity between MN2S2 donors and
the mono/bidentate binding possibilities of metallodithiolates, receivers. Various receiver units such as {Fe(NO)2}9/10, as well as
particularly MN2S2, in development of a number of structural FeII or RuII bound to η5-C5R5 (where R = H, CH3) or η6-arenes
and functional analogues.4–8 In particular, the π* interaction have been used in this rapidly growing area of bioinspired
resulting from filled Nid–Sp orbital contacts in square planar heterobimetallics.12–24 Especially notable are the NiN2S2 deriva-
NiN2S2 or NiS′2S2 (S′ = thioether) complexes, promote the tives of (η6-arene)RuII or [P(OR)3]3FeII from the Ogo group which
nucleophilic reactivity of cis-dithiolate sulfurs including specific demonstrated impressive heterolytic splitting of H2 in aqueous
or methanol media, generating
a
bridging hydride.25,26
Replacement of the neutral methyl-substituted arenes on such
Ni–Ru/Fe complexes with anionic η5-C5Me5, Cp*, showed oxi-
Department of Chemistry, Texas A & M University, College Station, Texas 77843,
USA. E-mail: marcetta@chem.tamu.edu
2−
dative addition of O2, resulting in η2-O2
derivatives of
RuIV/FeIV complexes that were isolated and character-
ized.14,19,27,28 Thus subtle modulations in the electronic features
of the model complexes lead to unique organometallic chem-
istry that exploits a wide range of electrochemical potentials.
†Electronic supplementary information (ESI) available. CCDC 1521062–1521064
1521078. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c6dt04666e
‡Present address: School of Chemistry and Chemical Engineering,
Henan University of Technology, Zhengzhou 450001, China.
This journal is © The Royal Society of Chemistry 2017
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Recent studies from our laboratory of MN2S2 (M = Ni2+, and electronic properties for assessing features of significance
[Fe(NO)]2+) bound to [(η5-C5H5)Fe′(CO)]+, demonstrated electro- to electrocatalysis of the hydrogen evolution reaction.
catalytic HER (h̲ydrogen e̲volution r̲eactivity) in the presence of
trifluoroacetic acid (TFA). A computation-based approach for
assignment of the electrochemical events suggested that
reduction-induced, iron–sulfur bond cleavage, i.e., the hemi-
lability of the N2S2 metalloligand,29 should be a key step in the
HER mechanism.30 That is, in such complexes that do not
have an open site on the metal envisioned as an electron/
proton landing site for conversion to a hydride, nor a pendant
base for positioning a proton, the hemi-lability of a metallo-
dithiolate may be invoked to provide both needed components
for the heterolytic H−/H+ coupling to make dihydrogen, Fig. 1.
Results and discussion
Synthesis
Similar to published reports,8,14,17,18,23,30 the preparation of
heterobimetallic complexes, [MN2S2·FeCp(CO)]+[BF4]− and
[MN2S2·FeCp*(CO)]+[PF6]−, (M = Ni, [Co(NO)]) is readily accom-
plished by thermal and photochemical displacement of CO as
shown in Scheme 1. The receiver units for the bidentate pro-
ducts, FeCp(CO) and FeCp*(CO), are hereafter abbreviated as
Fe′ and Fe*′, respectively; while the monodentate products,
FeCp(CO)2 and FeCp*(CO)2 are abbreviated as Fe″ and Fe*″,
respectively. Overall, the room temperature reaction of [CpFe
(CO)2(Solv)]+[BF4]− or [Cp*Fe(CO)3]+[PF6]− with MN2S2 in
CH2Cl2 forms the monodentate complexes, [κ1-MN2S2·FeCp
(CO)2]+[BF4]− (Co-Fe″) and [κ1-MN2S2·FeCp*(CO)2]+[PF6]−
(Ni-Fe*″ and Co-Fe*″). Although Co-Fe″ can be synthesized and
isolated as a pure purple solid, the Ni-Fe*″ and Co-Fe*″ are
present as a mixture with the starting material. Positive-ion
ESI mass spectrum of Ni-Fe*″ and Co-Fe*″ displayed promi-
nent signals at the molecular ion masses, m/z 523.05 and m/z
554.05, respectively, with characteristic isotopic distribution
patterns which closely matched the calculated bundle, Fig. S25
and S26.†
Photolysis of the monodentate (M-Fe″ or M-Fe*″) complexes
resulted in loss of one CO to form air-stable, bidentate com-
−
−
plexes, Co-Fe′ as the BF4 salt, and Ni-Fe*′and Co-Fe*′ as PF6
salts. Additional photolysis of the bidentate complexes led to
Fig. 1 Salient features in the computationally proposed mechanistic
path for electrocatalytic H2 production catalyzed by MN2S2·[Fe(η5-C5H5)
(CO)]+ as catalyst.30
The consequence of exchange of η5-C5H5 with the electron rich
η5-C5Me5 unit, generating the (η5-C5Me5)Fe(CO)+ receiver
group for the bidentate metallodithiolate ligand, might be
anticipated as follows: expected for hemi-lability, the
M(µ-SR)2Fe′ ring opening should be enhanced by the
additional electron richness, thus more easily generating the
open sites on sulfur and on Fe′. The proton affinity of the avail-
able thiolate sulfur could be enhanced, although likely not by
much. Most debatable is the reduction potential of the Fe′,
which in the (η5-C5Me5) derivative is expected to be more nega-
tive and hence increase the overpotential for the overall reac-
tion. However the enhanced proton affinity at the Fe′, coupled
with the proximal thiolate S–H orientation that yields the
optimal H+/H− coupling distance might compensate for the
overpotential. These are questions to be addressed, beginning
with the synthesis of complexes with MN2S2 metallodithiolate
ligands bound to [(η5-C5R5)Fe(CO)]+ (R = H, CH3; Cp and Cp*,
respectively). Herein we report the synthesis and characteriz-
−
Scheme 1 Synthesis of Co-Fe’’ and Co-Fe’ as BF4 salts and Co-Fe*’
and Ni-Fe*’ as PF6− salts. The IR frequencies of the complexes in CH2Cl2
ation of a set of such bimetallics, emphasizing their structural are shown in the table.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Dalton Transactions
Paper
Table 1 Selected metric parameters of Co-Fe’’, Co-Fe’, Co-Fe*’ and
Ni-Fe*’
decomposition. The ν(CO) stretching frequency is an impor-
tant reporter unit for the electron density at the Fe′ or Fe*′
centers in accordance with the conventional σ-donor/π-back-
bonding arguments. Due to the greater electron donor ability
of the η5-C5Me5, the ν(CO) stretching frequency of Ni-Fe*′
and Co-Fe*′, (1905 and 1907 cm−1 respectively), showed a
bathochromic shift of ca. 25 cm−1 compared to the
[NiN2S2·FeCp(CO)]+[BF4]− and Co-Fe′ congeners.30 The effect on
the ν(NO) stretching frequency was less prominent with an
overall lowering of ν(NO) by 3 cm−1 for Co-Fe*′. Slightly lower
ν(CO) stretching frequencies of Ni-Fe*′, compared to Co-Fe*′, has
precedence from previous studies with (η5-C5H5)Fe(CO) or
W(CO)4 reporter units, showcasing the higher electron donor
ability of NiN2S2 over (NO)CoN2S2 metalloligands.30,31 The
monodentate complex, Co-Fe″, showed two ν(CO) stretching fre-
quencies at 2045 and 2002 cm−1 and a ν(NO) at 1630 cm−1. All
the newly synthesized complexes are diamagnetic and were
characterized by 1H and 13C NMR spectroscopies, Fig. S28–S35.†
Co-Fe″
Co-Fe′
Co-Fe*′
Ni-Fe*′
M⋯Fe′/Å
3.913 (1)
4.559 (2)a
3.321 (1)
2.545 (3)
124.7 (9)
—
95.9 (3)
80.4 (9)
118.3 (3)c
0.361
3.153 (1)
3.386 (4)
2.994 (2)
2.598 (5)
131.4 (4)
81.9 (2)
85.8 (5)
81.7 (1)
126.2
3.136 (2)
3.370 (4)
2.977 (1)
2.585 (4)
126.6 (3)
80.8 (3)
85.2 (4)
81.7 (1)
121.9
3.005 (1)
3.096 (5)
2.954 (2)
2.571 (9)
—
79.6 (6)
87.1 (7)
83.7 (3)
121.5
M⋯CCO/Å
S⋯S/Å
N⋯N/Å
∠M–N–O/°
∠S–Fe–S/°
∠S–M–S/°
∠N–M–N/°
Hinge angleb/°
M–N2S2 disp.d/Å
0.432
0.429
0.205
a Average value. b The angle between N2S2 and S2Fe′ best planes. c The
∠Co–S–Fe is given in lieu of the hinge angle. d Displacement of M from
N2S2 best plane.
straining the ∠S–M–S by ca. 7–9° compared to the free metallo-
dithiolate ligands.10 The hinge angle (the intersection of the
N2S2 best plane with the S2Fe′ or S2Fe*′ best plane) is ca. 126°
for Co-Fe′ which constricts by ca. 4° in Ni-Fe*′ and Co-Fe*′.
This might be related to the added electron density and steric
encumbrance in the Ni-Fe*′ and Co-Fe*′ due to the (η5-C5Me5)
compared to the (η5-C5H5). The Co-Fe′ and Co-Fe*′ derivatives
find the NO positioned transoid to the CO on the FeCp and
FeCp* units. Furthermore, in the bidentate complexes, Co-Fe′
and Co-Fe*′, the NO is on the side of the N2S2 ligand that has
the 3-carbon chain between the two N atoms, whereas in the
monodentate complex, Co-Fe″, the NO resides on the side of
the 2-carbon chain. In all three bidentate complexes, the N to
S linkers are eclipsed and the bridging dithiolate hinge posi-
tions the downward oriented methylene groups, α to S, so as to
X-ray diffraction analysis
X-ray quality crystals were obtained by layering a concentrated
solution of the complexes in CH2Cl2 with hexanes at −28 °C.
While the monodentate Co-Fe″ was obtained as dark needles,
the Co-Fe′ and Co-Fe*′ presented as dark violet blocks, and the
Ni-Fe*′ complex crystallized as brown blocks. The molecular
structures of the complexes Co-Fe″, Co-Fe′, Ni-Fe*′ and Co-Fe*′
are shown in Fig. 2 as ball and stick renditions and selected
metric parameters are tabulated in Table 1. Full structural
reports are deposited in the Cambridge Crystallographic Data
Centre. The structures feature typical piano-stool coordination
geometries about the iron in FeCp(CO)+ and FeCp*(CO)+ units.
In Co-Fe′, Ni-Fe*′ and Co-Fe*′ the convergent lone pairs on the
bridging thiolates enforce bidentate binding at the iron (Fe′ or
Fe″) center leading to a butterfly-like [M(μ-SR)2Fe] core, con-
cradle the Fe–CO moiety, resulting in CCO to Hα-CH distances
2
of ca. 2.7–2.8 Å. Space-filling models highlight the close
encounter of the hydrocarbon linkers on the CO, Fig. 3, and
the minimal space taken up by the folded structure.
Nevertheless, the M⋯Fe′ and M⋯Fe*′ distances of 3.0–3.2 Å in
our complexes are too long to implicate a bonding interaction.
The report of Kure, Tanase, et al., of analogous MP2S2·M′Cp*
structures (M = NiII, PdII, PtII; M′ = RhIII, IrIII) that find a µ-H
between M and M′ describe this pocket as a nest, and the
bridging hydride as a “nesting” hydride.24
The bridging thiolate (μ-SR) of the monodentate complex,
+
uses one of its divergent lone pairs to bind FeCp(CO)2 , allow-
ing the latter to be oriented away from the free thiolate.
The ∠Co–S–Fe′ opens up by ca. 30° compared to the bidentate
complexes.
A
similar phenomenon was observed for
[κ1-NiN2S2·FeCp(CO)2]+[BF4]−.30
Electrochemistry
Cyclic voltammetry
−
Cyclic voltammograms (CV) of Co-Fe′ as BF4 salt and Co-Fe*′
Fig. 2 Molecular structures of Co-Fe’’, Co-Fe’, Co-Fe*’ and Ni-Fe*’. The
BF4 counter anions of Co-Fe’’ and Co-Fe’ and the PF6 counter anions
of Co-Fe*’ and Ni-Fe*’ are omitted for clarity.
−
−
−
and Ni-Fe*′ as PF6 salts were recorded at room temperature
under argon in CH3CN solutions containing 0.1 M [tBu4N][PF6]
This journal is © The Royal Society of Chemistry 2017
Dalton Trans.

View Article Online
Paper
Dalton Transactions
The complexes in general showed two distinct redox events.
The quasi-reversible event at slightly positive potential was
assigned to the Fe′II/III or Fe*II/III redox couple, and the one at
negative potential was assigned to the MII/IN2S2 reduction. The
oxidation event at 0.32 V assigned to the Fe′II/III couple for the
Co-Fe′ complex showed a positive shift of ca. 200 mV with
respect to the Fe*′II/III couple of Co-Fe*′ implying a greater ease
of oxidation at the Fe center electron-enriched by the η5-C5Me5
unit. Consistently, the {Co(NO)}8/9 reduction event of Co-Fe*′ at
−1.19 V differs by 70 mV as compared to the Co-Fe′ indicating
the greater stabilization of the oxidized {Co(NO)}8 state in the
Co-Fe*′ complex. The Fe*′II/III couple of the Ni-Fe*′ complex
showed a quasi-reversible oxidation event at 0.07 V, which is
ca. 220 mV less positive than the Fe′II/III couple of the
[NiN2S2·FeCp(CO)]+[BF4]− complex, for similar reasons.30
Likewise a negative shift of the NiII/I couple by 280 mV was
observed for the Ni-Fe*′ complex relative to the [NiN2S2·FeCp
(CO)]+[BF4]− complex.30 All complexes showed an irreversible
oxidation event at greater than 1.00 V, possibly due to dithio-
late-based oxidation. Overall, the effect of greater electron
donor ability of η5-C5Me5 over the η5-C5H5 unit was manifested
in a more energetically demanding reduction couple and, con-
sistently, more facile oxidation events.
Fig. 3 Space filling models of, Co-Fe*’ and Co-Fe’ shown in two orien-
tations; (B) is rotated by 90° from (A).
Electrocatalysis
Electrochemical studies of Co-Fe′, Co-Fe*′ and Ni-Fe*′ were
carried out in the presence of trifluoroacetic acid (TFA) and are
shown in Fig. 5. The increase in the cathodic current that was
observed at the {Co(NO)}8/9 reduction event for Co-Fe*′ (−1.19
V) and Co-Fe′ (−1.12 V), was saturated after addition of 8
equivalents of TFA. An additional event at −1.75 V appeared
for the Co-Fe*′ on addition of first equivalent of acid and
further acid additions showed a steady increase in this current
response. A new major current event appeared at −1.56 V, for
complex Co-Fe′, with TFA > 6 equivalents. A similar pre-peak
phenomenon was observed for [(NO)FeN2S2·FeCp(CO)]+[BF4]−
complex at −1.66 V.30 Similarly, addition of TFA to the Ni-Fe*′
complex showed the formation of a pre-peak at −1.74 V that
increased in current intensity along with the major NiII/I
reduction event at −1.91 V.
as supporting electrolyte using a glassy carbon working elec-
trode. The scans at 200 mV s−1 are shown in Fig. 4 with assign-
ments listed therein. Scan rate overlays to determine reversibil-
ity or quasi-reversibility are given in Fig. S15–S19.†
The attribution of the reduction-induced cathodic current
enhancement upon addition of TFA to electrocatalytic H2 pro-
duction was verified by gas chromatography and quantified by
an average of two consistent bulk electrolysis experiments. A
constant potential at −1.80 V (dotted line in Fig. 5D), for
30 min, was applied in a CH3CN solution containing 2 mM of
catalyst and 50 equivalents of TFA. Since the background H2
evolved from TFA itself at this potential overlaps the response
from the electrocatalyst, the former was subtracted to obtain
corrected values of the catalytic response, Fig. S1.† The turn-
over numbers (TON) for Ni-Fe*′, Co-Fe*′ and Co-Fe′ were found
to be 0.32 0.05, 0.15 0.01 and 0.35 0.05, respectively, with
Faradaic efficiencies of 88
4%, 99
0.2% and 92
1%,
respectively, for H2 production. Following the approach of
Wiese32 and Helm and Appel,33 turnover frequencies (TOF),
derived from the cyclic voltammetry studies, were determined
Fig. 4 Cyclic voltammograms of Co-Fe’, Co-Fe*’ and Ni-Fe*’ as 2 mM
CH3CN solutions containing 0.1 M [tBu4N][PF6].
Dalton Trans.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Dalton Transactions
Paper
Fig. 5 Cyclic voltammograms of (A) Co-Fe’, (B) Co-Fe*’ and (C) Ni-Fe*’ as 2 mM CH3CN solutions containing 0.1 M [tBu4N][PF6] as supporting elec-
trolyte with added equivalents of TFA. (D) Overlay of catalytic current responses of Co-Fe’, Co-Fe*’ and Ni-Fe*’ in presence of 50 equivalents of TFA
and 50 equivalents of TFA in absence of catalyst. The dotted line indicates the constant potential applied for bulk electrolysis, −1.80 V.
to be 32 s−1, 65 s−1, 79 s−1 for Ni-Fe*′, Co-Fe*′ and Co-Fe′,
respectively. The high overpotentials for Ni-Fe*′ and Co-Fe*′
Conclusions
complexes, 1.31 V and 1.27 V, respectively, are consistent with In summary, the proficiencies of three MN2S2·Fe(η5-C5R5)(CO)+
the electron rich η5-C5Me5 unit present in these complexes complexes (R = H or CH3) for catalysis of the HER were deter-
while the moderately lower overpotential for Co-Fe′, 1.06 V, mined by standard electrochemical analyses and structure/
with the η5-C5H5 unit, is comparable to those reported for the function comparisons are made. The Co-Fe′ complex relates to
[(NO)FeN2S2·FeCp(CO)]+[BF4]− and [NiN2S2·FeCp(CO)]+[BF4]− M-Fe′ complexes, M = Ni2+ and [Fe(NO)]2+ reported earlier.30
complexes.30 Table 2 shows a comparative listing of overpoten- Its properties are by and large precise analogues of the Fe(NO)
tial, turnover number (TON) and turnover frequency (TOF) of derivative, Fe-Fe′, and can be used as a reference to connect
the different complexes used in this study. The data obtained the two studies.30 The CO and NO ligands that report via
for calculating such values are given in Tables S1–S7.† The vibrational spectroscopy on electronic results from aggregating
complexes, [(NO)FeN2S2·FeCp(CO)]+[BF4]− and [NiN2S2·FeCp the individual components of the bimetallics are entirely con-
(CO)]+[BF4]−, showed H/D kinetic isotope effects on TOF and sistent with the shifts in redox potentials of the Fe′ and Fe*′
the (kH/kD) were determined to be ca. 1.5.30 The consistency in units and the metallodithiolate ligands. The overall conclusion
the TOF values for M-Fe*′ and M-Fe′ complexes leads to the must be that the Cp* enriches the electron density mainly at
overall assumption that the reaction path for the catalytic the Fe center, and has an effect, however less prominent, at
process is similar and involves a metal-hydride species as an the metallodithiolate ligand. At this point, we have not
intermediate in the catalytic cycle.
attempted an extensive computational study of the mechanism
as was done for the prior study.30 We assume that the reaction
path is similar (or identical) and the energy requirement for
the ring opening process that creates the needed Lewis pair for
H+⋯H− coupling is a dominant factor for determining over-
potential but has less significance for other electrochemical
features such as turnover number and turnover frequency. Our
working hypothesis continues to focus on the bimetallic mech-
anism, assuming that the bimetallic remains intact via a
single thiolate bridge. Clearly it would be reassuring to obtain
experimental evidence for the presumed iron-hydride/proto-
nated thiolate sulfur intermediate.30 At this point we still
Table 2 Overpotential, turnover number (TON) and turnover frequency
(TOF) of Ni-Fe*’, Co-Fe*’, Ni-Fe’,30 Fe-Fe’30 and Co-Fe’
Overpotential
(V)
Turnover number
(TON)
Turnover
freq. (TOF) (s−1
)
Ni-Fe*′
Co-Fe*′
Ni-Fe′30
Fe-Fe′30
Co-Fe′
1.31
1.27
0.94
0.94
1.06
0.32 0.05
0.15 0.01
0.26 0.01
0.33 0.02
0.35 0.05
32
65
52
69
79
This journal is © The Royal Society of Chemistry 2017
Dalton Trans.

View Article Online
Paper
Dalton Transactions
search for these illusive species, and other mechanistic possi- and structure refinement parameters are summarized in
bilities cannot be ruled out. Concomitant with the more Tables S8–S11.†
negative reduction event is a more accessible, reversible oxi-
dation event. Hence the catalytic propensities of these Cp*
Electrochemistry
complexes might be more advantageously applied to oxidation A three-electrode cell was use to record Cyclic voltammograms
catalysis.19,27
(CVs) on the CHI600E electrochemical analyzer (HCH instru-
ments, Inc.). The counter electrode was a platinum wire, the
reference electrode was a CH3CN solution of Ag/AgNO3 in a
Vycor-tipped glass tube, and the working electrode was a
0.071 cm2 glassy carbon dish electrode. Diamond paste (3 µm)
was used to polish the glassy carbon working electrode as
needed throughout the CV experiment. A blanket of Ar was
kept over the solution during the experiments following an
Ar purge for 5 min. All solutions were in CH3CN and the
measurements were done at room temperature using 0.1 M
[n-Bu4N][PF6] as the electrolyte and 2.0 mM in analyte. All
potentials were shown relative to the Fc/Fc+ couple at 0.00 V.
A custom made three-necked truncated conical shaped
flask with an outlet port/gas inlet was the apparatus used for
bulk electrolysis experiments. A Ni–Cr-coiled wire counter elec-
trode, a Ag/AgNO3 reference electrode, and a 3 mm glassy
carbon working electrode were placed in the necks of the cell.
The Ni–Cr-coiled wire placed in a glass tube with a medium
glass frit served as the counter electrode. A glass tube contain-
ing a Ag wire immersed in a 1 mM solutions of AgNO3 in
MeCN separated from the main solution by a Vycor frit was the
make-up of the reference electrode. To the electrochemical cell
10 mL of 0.1 M [n-Bu4N][PF6] in CH3CN was added and then
purged with Ar to deplete O2. To the cell 2 × 10−5 mol of the
appropriate catalyst and 50 equivalents of trifluoroacetic acid
were added. To ensure that the experiment was under catalytic
Experimental section
Methods and materials
The MBraun Manual Solvent Purification System with Alcoa
F200 activated alumina desiccant was used to purify aceto-
nitrile (CH3CN), benzene, dichloromethane (CH2Cl2), hexane
and diethylether. Reactions were carried out using standard
Schlenk conditions under N2 atmosphere. All reagents were
used as received from standard vendors unless otherwise
stated. Published procedures were followed to synthesize the
known complexes (NO)Co(bme-dach) (bme-dach = N,N-bis
(2-mercaptoethyl)-1,5-diazacycloheptane),34
Ni(bme-dach),35
(η5-C5H5)Fe(CO)2I,36 and [(η5-C5Me5)Fe(CO)3][PF6].37 HPLC-
grade acetonitrile, [n-Bu4N][PF6], and AgBF4 were reagent grade
and purchased from Sigma-Aldrich.
Physical measurements
The Bruker Tensor 37 Fourier transform IR (FTIR) spectro-
meter was used to record solution infrared spectra, using CaF2
cell with 0.2 mm path length. The laboratory for Biological
Mass Spectrometry at Texas A&M University performed and
provided all mass spectrometry (ESI-MS). The Atlantic
Microlab Inc., located in Norcross, Ga, performed all elemental
1
analyses. H NMR and 13C NMR spectra were recorded on the
conditions,
a cyclic voltammogram was recorded. After
30 minutes of bulk electrolysis performed at −1.80 V vs. Fc/Fc+,
1 mL of methane was added as the internal standard.
Detection of H2 gas using GC is discussed in the ESI.†
Inova 500 MHz superconducting NMR instrument.
X-ray diffraction analyses
The crystal structures of Co-Fe″, Co-Fe′ were measured at low
temperatures (150 and 110 K, respectively) by BRUKER APEX 2
X-ray (three-circle) diffractometer with Mo sealed X-ray tube
Synthesis
[(η5-C5H5)Fe(CO)2(bme-dach)Co(NO)][BF4], Co-Fe″. A solu-
(Kα = 0.70173 Å). The single crystals of Co-Fe*′ and Ni-Fe*′ were tion of CpFe(CO)2I (256 mg, 1.00 mmol) and AgBF4 (195 mg
measured using BRUKER Quest X-ray (fixed-Chi geometry) 1.00 mmol) in CH2Cl2 (20 mL) was stirred for 30 min in the
diffractometer with Mo-Iμs X-ray tube (Kα = 0.71073 Å) and dark at room temperature and then filtered through celite. The
BRUKER Venture X-ray (kappa geometry) diffractometer with filtrate was added to a solution of (NO)Co(bme-dach) (307 mg,
Cu-Iμs X-ray tube (Kα = 1.5418 Å), respectively, at 100 K. The 1.00 mmol) in CH2Cl2 (20 mL) and stirred in the dark for 3 h.
structures were refined by weighted least squares refinement The resulting solution was then redissolved in CH2Cl2 after it
on F2. At idealized positions, hydrogen atoms were placed and was dried under vacuum and washed with 20 mL (×3) diethyl
fixed isotropic displacement parameters were used to refine ether. The solution was filtered through celite to filter any
them. For all non-hydrogen atoms, anisotropic displacement unreacted starting material and X-ray quality crystals for
parameters were employed. The following programs were used: complex Co-Fe″ were obtained by layering a CH2Cl2 solution of
data collection, APEX3;38 data reduction, SAINT;39 absorption the product with hexanes. IR (CH2Cl2, cm−1): ν(CO) 2045,
correction, SADABS;40 cell refinement and structure solutions, 2001; ν(NO) 1632; ESI-MS+: m/z 483.95 (Calc. for [M]+, 483.96).
SHELXS/XT;40 The final data presentation and structure plots
[(η5-C5H5)Fe(CO)(bme-dach)Co(NO)][BF4], Co-Fe′. A solution
were generated in Olex2.41 Crystallographic data for the com- of CpFe(CO)2I (256 mg, 1.00 mmol) and AgBF4 (195 mg
plexes Co-Fe″, Co-Fe′, Co-Fe*′ and Ni-Fe*′ are deposited in the 1.00 mmol) in CH2Cl2 (20 mL) was stirred for 30 min in the
Cambridge Crystallographic Data Centre; their numbers are: dark at room temperature and then filtered through celite. The
CCDC 1521064 (Co-Fe″), 1521078 (Co-Fe′), 1521063 (Co-Fe*′) filtrate was added to a solution of (NO)Co(bme-dach) (307 mg,
and 1521062 (Ni-Fe*′). For convenience pertinent crystal data 1.00 mmol) in CH2Cl2 (20 mL) and the solution was stirred
Dalton Trans.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Dalton Transactions
Paper
under UV lamp irradiation. IR spectrum was used to monitor
the reaction. Upon completion of the reaction, the product was
purified through a silica column with CH2Cl2/CH3OH (20 : 1,
v/v) as the eluent. X-ray quality crystals for complex Co-Fe′ were
also obtained by layering a CH2Cl2 solution of the product
with hexanes. IR (CH2Cl2, cm−1): ν(CO) 1934; ν(NO) 1650;
ESI-MS+: m/z 455.99 (Calc. for [M]+, 455.99). Elem. Anal. Calc’d
(found) for C15H23BCoFeF4N3O2S2 (MW = 543 g mol−1): C,
33.17 (33.25); H, 4.27 (4.29); N, 7.74 (7.65).
4 D. Schilter, J. M. Camara, M. T. Huynh, S. Hammes-Schiffer
and T. B. Rauchfuss, Chem. Rev., 2016, 116, 8693–8749.
5 T. R. Simmons and V. Artero, Angew. Chem., Int. Ed., 2013,
52, 6143–6145.
6 S. Ogo, Chem. Commun., 2009, 3317–3325.
7 F. Gloaguen and T. B. Rauchfuss, Chem. Soc. Rev., 2009, 38,
100–108.
8 D. Brazzolotto, M. Gennari, N. Queyriaux, T. R. Simmons,
J. Pécaut, S. Demeshko, F. Meyer, M. Orio, V. Artero and
C. Duboc, Nat. Chem., 2016, 8, 1054–1060.
9 C. A. Grapperhaus and M. Y. Darensbourg, Acc. Chem. Res.,
1998, 31, 451–459.
[(η5-C5Me5)Fe(CO)(bme-dach)Co(NO)][PF6], Co-Fe*′. A solu-
tion of (NO)Co(bme-dach) (307 mg, 1.00 mmol) and [Fe(Cp*)
(CO)3]PF6 (419 mg, 1.00 mmol) in CH2Cl2 (25 mL) were stirred
under UV lamp irradiation. The dark purple reaction mixture 10 J. A. Denny and M. Y. Darensbourg, Chem. Rev., 2015, 115,
was monitored by IR spectroscopy. Once the reaction was com- 5248–5273.
pleted, the dark purple product was purified by passing 11 J. H. Enemark and R. D. Feltham, Coord. Chem. Rev., 1974,
through a silica gel column with CH2Cl2/CH3OH (10 : 1, v/v) as 13, 339–406.
the eluent. X-ray quality crystals for complex Co-Fe*′ were 12 C.-H. Hsieh, S. Ding, O. F. Erdem, D. J. Crouthers, T. Liu,
obtained by layering a CH2Cl2 solution of the product with
hexanes. IR (CH2Cl2, cm−1): ν(CO) 1907; ν(NO) 1647; ESI-MS+:
m/z 526.13 (Calc. for [M]+, 526.07). Anal. Calc’d (found) for
C. C. L. McCrory, W. Lubitz, C. V. Popescu,
J. H. Reibenspies, M. B. Hall and M. Y. Darensbourg, Nat.
Commun., 2014, 5, 3684–3688.
C20H33CoF6FeN3O2PS2 (MW = 671 g mol−1): C, 35.78 (36.31); 13 N. T. Nguyen, Y. Mori, T. Matsumoto, T. Yatabe, R. Kabe,
H, 4.95 (5.16); N, 6.26 (5.83).
H. Nakai, K.-S. Yoon and S. Ogo, Chem. Commun., 2014, 50,
13385–13387.
[(η5-C5Me5)Fe(CO)(bme-dach)Ni][PF6], Ni-Fe*′. In a similar
manner to that of complex Co-Fe*′, complex Ni-Fe*′ was pre- 14 M. A. Reynolds, T. B. Rauchfuss and S. R. Wilson,
pared by using Ni(bme-dach) (276 mg, 1.00 mmol) and Organometallics, 2003, 22, 1619–1625.
[Fe(Cp*)(CO)3]PF6 (419 mg, 1.00 mmol) in CH2Cl2 (25 mL) 15 C. Mealli and T. B. Rauchfuss, Angew. Chem., Int. Ed., 2007,
which was stirred under UV lamp irradiation. The tan reaction 46, 8942–8944.
mixture was monitored by changes in the IR spectrum and 16 W. Zhu, A. C. Marr, Q. Wang, F. Neese, D. J. E. Spencer,
X-ray quality crystals were obtained by layering a CH2Cl2 solu-
A. J. Blake, P. A. Cooke, C. Wilson and M. Schröder, Proc.
Natl. Acad. Sci. U. S. A., 2005, 102, 18280–18285.
tion of the product with hexanes. IR (CH2Cl2, cm−1): ν(CO)
1905; ESI-MS+: m/z 495.06 (Calc. for [M]+, 495.07). Anal. Calc’d 17 K. Weber, Ö. F. Erdem, E. Bill, T. Weyhermüller and
(found) for C20H33F6FeN2NiOPS2 (MW = 640 g mol−1): C, 37.47
(36.99); H, 5.19 (5.38); N, 4.37 (4.31).
W. Lubitz, Inorg. Chem., 2014, 53, 6329–6337.
18 S. Canaguier, M. Field, Y. Oudart, J. Pecaut, M. Fontecave
and V. Artero, Chem. Commun., 2010, 46, 5876–5878.
19 T. Kishima, T. Matsumoto, H. Nakai, S. Hayami, T. Ohta
and S. Ogo, Angew. Chem., Int. Ed., 2016, 55, 724–727.
20 T. Matsumoto, R. Kabe, K. Nonaka, T. Ando, K.-S. Yoon,
H. Nakai and S. Ogo, Inorg. Chem., 2011, 50, 8902–8906.
Acknowledgements
We sincerely thank the National Science Foundation
(CHE-1266097 to M. Y. D.) and the Robert A. Welch 21 K. Ichikawa, T. Matsumoto and S. Ogo, Dalton Trans., 2009,
Foundation (A-0924 to M. Y. D.). Part of the experimental work 4304–4309.
(salary for Pokhraj Ghosh) was allowed by a NPRP award 22 T. Matsumoto, B. Kure and S. Ogo, Chem. Lett., 2008, 37,
(NPRP 6-1184-1-224) from the Qatar National Research Fund 970–971.
(a member of Qatar Foundation). The authors appreciate 23 D. Yang, Y. Li, L. Su, B. Wang and J. Qu, Eur. J. Inorg.
Dr Allen M. Lunsford and Shengda Ding for helpful
discussions.
Chem., 2015, 2015, 2965–2973.
24 B. Kure, M. Sano, T. Nakajima and T. Tanase,
Organometallics, 2014, 33, 3950–3965.
25 S. Ogo, R. Kabe, K. Uehara, B. Kure, T. Nishimura,
S. C. Menon, R. Harada, S. Fukuzumi, Y. Higuchi,
T. Ohhara, T. Tamada and R. Kuroki, Science, 2007, 316,
585–587.
26 S. Ogo, K. Ichikawa, T. Kishima, T. Matsumoto, H. Nakai,
K. Kusaka and T. Ohhara, Science, 2013, 339, 682–684.
27 K. Kim, T. Kishima, T. Matsumoto, H. Nakai and S. Ogo,
Organometallics, 2013, 32, 79–87.
Notes and references
1 W. Lubitz, H. Ogata, O. Rüdiger and E. Reijerse, Chem.
Rev., 2014, 114, 4081–4148.
2 M. Can, F. A. Armstrong and S. W. Ragsdale, Chem. Rev.,
2014, 114, 4149–4174.
3 C. Darnault, A. Volbeda, E. J. Kim, P. Legrand, X. Vernède,
P. A. Lindahl and J. C. Fontecilla-Camps, Nat. Struct. Biol., 28 K. Kim, T. Matsumoto, A. Robertson, H. Nakai and S. Ogo,
2003, 10, 271–279.
Chem. – Asian J., 2012, 7, 1394–1400.
This journal is © The Royal Society of Chemistry 2017
Dalton Trans.

View Article Online
Paper
Dalton Transactions
29 J. C. Jeffrey and T. B. Rauchfuss, Inorg. Chem., 1979, 18, 35 J. J. Smee, M. L. Miller, C. A. Grapperhaus,
2658–2666.
J. H. Reibenspies and M. Y. Darensbourg, Inorg. Chem.,
2001, 40, 3601–3605.
30 S. Ding, P. Ghosh, A. M. Lunsford, N. Wang, N. Bhuvanesh,
M. B. Hall and M. Y. Darensbourg, J. Am. Chem. Soc., 2016, 36 S. El-Tarhuni, M. Ho, M. H. Kawser, S. Shi and
138, 12920–12927. M. W. Whiteley, Organomet. Chem., 2014, 752, 30–36.
31 J. L. Hess, H. L. Conder, K. N. Green and 37 D. Catheline and D. Astruc, Organometallics, 1984, 3, 1094–
M. Y. Darensbourg, Inorg. Chem., 2008, 47, 2056–2063. 1100.
32 S. Wiese, U. J. Kilgore, M.-H. Ho, S. Raugei, D. L. DuBois, 38 APEX2, Bruker AXS Inc., Madison, WI, 2007.
R. M. Bullock and M. L. Helm, ACS Catal., 2013, 3, 2527–2535. 39 SAINT, Bruker AXS Inc., Madison, WI, 2007.
33 A. M. Appel and M. L. Helm, ACS Catal., 2014, 4, 630–633.
34 C.-Y. Chiang, J. Lee, C. Dalrymple, M. C. Sarahan,
40 G. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystallogr.,
2008, 64, 112–122.
J. H. Reibenspies and M. Y. Darensbourg, Inorg. Chem., 41 O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard
2005, 44, 9007–9016.
and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339–341.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2017