A matrix of heterobimetallic complexes for interrogation of hydrogen evolution reaction electrocatalysts

Article abstract of DOI:10.1039/c7sc03378h

Experimental and computational studies address key questions in a structure-function analysis of bioinspired electrocatalysts for the HER. Combinations of NiN₂S₂ or [(NO)Fe]N₂S₂ as donors to (η⁵-C₅H₅)Fe(CO)⁺ or [Fe(NO)₂]^+/0 generate a series of four bimetallics, gradually softened by increasing nitrosylation, from 0 to 3, by the non-innocent NO ligands. The nitrosylated NiFe complexes are isolated and structurally characterized in two redox levels, demonstrating required features of electrocatalysis. Computational modeling of experimental structures and likely transient intermediates that connect the electrochemical events find roles for electron delocalization by NO, as well as Fe-S bond dissociation that produce a terminal thiolate as pendant base well positioned to facilitate proton uptake and transfer. Dihydrogen formation is via proton/hydride coupling by internal S-H⁺?^-H-Fe units of the harder bimetallic arrangements with more localized electron density, while softer units convert H^-?H^-via reductive elimination from two Fe-H deriving from the highly delocalized, doubly reduced [Fe₂(NO)₃]^- derivative. Computational studies also account for the inactivity of a Ni₂Fe complex resulting from entanglement of added H⁺ in a pinched -S^δ-?H⁺?^δ-S- arrangement.

Full text of DOI:10.1039/c7sc03378h

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A Matrix of Heterobimetallic Complexes for Interrogation
of Hydrogen Evolution Reaction Electrocatalysts
†
†
†
†
‡
Pokhraj Ghosh, Shengda Ding, Rachel B. Chupik, Manuel Quiroz, ChungꢀHung Hsieh,
†
†
†
Nattami Bhuvanesh, Michael B. Hall and Marcetta Y. Darensbourg *
†
Department of Chemistry, Texas A & M University, College Station, TX 77843, United States
‡
Department of Chemistry, Tamkang University, New Taipei City, Taiwan 25157
Abstract
Experimental and computational studies address key questions in a structureꢀfunction
analysis of bioinspired electrocatalysts for the HER. Combinations of NiN S or [(NO)Fe]N S
2 2
2
2
5
+
+/0
as donors to (η ꢀC H )Fe(CO) or [Fe(NO) ] generate a series of four bimetallics, gradually
5
5
2
“softened” by increasing nitrosylation, from 0 to 3, by the nonꢀinnocent NO ligands. The
nitrosylated NiFe complexes are isolated and structurally characterized in two redox levels,
demonstrating required features of electrocatalysis. Computational modeling of experimental
structures and likely transient intermediates that connect the electrochemical events find roles for
electron delocalization by NO, as well as FeꢀS bond dissociation that produce a terminal thiolate
as pendant base well positioned to facilitate proton uptake and transfer. Dihydrogen formation
+
−
via proton/hydride coupling by internal SꢀH ••• HꢀFe units of the “harder” bimetallic
−
−
arrangements with more localized electron density, while softer units convert H •••H via
reductive elimination from two FeꢀH deriving from the highly delocalized, doubly reduced
−
[
Fe (NO) ] derivative. Computational studies also account for the inactivity of a Ni Fe complex
2
3
2
+
δ−
+
δ−
resulting from entanglement of added H in a pinched –S •••H ••• S−arrangement.
Introduction
From protein crystallography the bimetallic active site structures in enzymes such as
[
NiFe]ꢀ, [FeFe]ꢀhydrogenases (H ase), COꢀdehydrogenases and acetyl coA synthase (ACS) have
2
1

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been convincingly interpreted in terms of characteristics needed for their organometallicꢀlike
1
, 2
functions. Whereas most major homogeneous catalytic applications involving redox processes
use precious metals that can operate as single site catalysts, the intricate molecular arrangements
in nature’s biocatalysts harness combinations of at least two first row transition metals,
3
ꢀ5
connected by sulfides or thiolates, along with Lewis acid/base sites. Over the past two decades
a rich area in synthetic chemistry inspired by such natural products has developed, yielding
biomimetics for insight into enzyme mechanisms. In addition the link between the [NiFe]ꢀ and
[
FeFe]ꢀH ase active sites and base metal, sustainable catalysts for the Hydrogen Evolution
2
Reaction (HER) holds promise for the production of H from “solar” (photovoltaic) electrons via
2
6
electrocatalysis. Specific efforts have
been directed towards the use of
metallodithiolates
complexes as bidentate donor ligands
readily deduced from the structure of
from
MN2S2
(
the ACS enzyme active site), that bind to
receiver metal units via bridging
Figure 1.Structure and redox activity of
+
+
[
(NO)FeN2S2
•
Fe(NO) ] ,[FeꢀFe] ; protonation of the
2
8
7
ꢀ25
oneꢀelectron reduced diiron complex yields H2.
dithiolates.
The
electronic
requirements of the thiolate sulfurs have a steric consequence in the butterfly M(µꢀSR) M’ cores
2
2
6
that are seen in the H ase active sites, placing M and M’ within close proximity.
2
The advancement of chemistry via structure/function analysis of sets of compounds with
wellꢀknown differences in composition and structure is a challenge in the complicated area of
HER electrocatalysis. Nonetheless the metallodithiolateꢀasꢀsynthon approach, inspired from the
ACS active site, permits modular design that includes some features of the bimetallic [NiFe]ꢀ
2

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and [FeFe]ꢀH ase active sites beyond the obvious dithiolate core structures. An initial foray
2
8
, 27
explored the properties of the diiron, trinitrosyl complex shown in Figure 1.
With it we
7
/8
intended to exploit the redoxꢀactivity of {Fe(NO)} in the (NO)FeN S metalloligand bound to a
2
2
9/10
redoxꢀactive
{Fe(NO)2}
,
iron
dinitrosyl
unit.
Electrochemical
studies
of
+
+
[
(NO)FeN S •Fe(NO) ] , [FeꢀFe] , Figure 1, found two singleꢀelectron, reversible reduction
2
2
2
9
/10
7/8
8
events, ꢀ0.78 V and ꢀ1.33 V, assigned to {Fe(NO)2}
and {Fe(NO)} couples, respectively.
The Fe of the (NO)FeN S metalloligand is herein distinguished as Fe; the electron count of the
2
2
2
8
iron nitrosyl units uses the Enemark–Feltham approach. Consistent with the stoichiometric
9
/10
reaction shown in Figure 1, the {Fe(NO)2}
couple, at ꢀ0.78 V, was the catalytically active
center for electrochemical proton reduction in the presence of strong acid, HBF •Et O. Although
4
2
modest in overpotential and TOF, electrocatalysis for H production was observed at this
2
8
potential; preliminary computational studies indicated that a hydrideꢀbound {Fe(NO) } could
2
likely be a transient intermediate, however the overall H releasing mechanism was at that stage
2
8
incomplete. Interestingly, the second reduction process, related to the more negative
7
/8
{
Fe(NO)} couple, showed a current response to added weak acid, however H was not
2
detected. Computational study attributed this to a nonꢀproductive reduction event with
8
protonation on the nitrosyl, which terminates the catalytic cycle.
We have made analogues of the diirion trinitrosyl complex making use of NiN S and
2
2
5
9,10
II
(
NO)FeN S metallodithiolates in combination with η ꢀC R derivatives (R = H, CH ), of Fe
2 2 5 5 3
8
shown in Figure 2.The large differences in reduction potential of the MN S ligands, with the d ꢀ
2
2
II
Ni being more negative because of a more rigid, less polarizable electronic structure as
7
compared to the delocalized {Fe(NO)} unit, of greater electronic flexibility, inspired the labels
of “hard” for the former MN S unit, and “soft” Sꢀdonor unit for the latter. The Feꢀreceivers also
2
2
3

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differ in electronic flexibility and their ease of electron uptake, the term “soft” describing the
9
highly delocalized {Fe(NO) } unit, and the indefinite oxidation state of the iron, as compared to
2
II
5
8,9
5
the definite Fe of the η ꢀC R , “hard” receiver derivatives. The hard receiver unit, (η ꢀ
5
5
+
C H )Fe(CO) , is herein distinguished from the soft Fe(NO) unit by Fe’ and Fe, respectively.
5
5
2
Notable from the computational approach that guided the interpretation of
electrochemical events of the
+
+
[NiꢀFe’]
and
[FeꢀFe’]
complexes in the presence of
acid was the indication of a
reductive ironꢀsulfur bond
cleavage (shown in red, Figure
2,) that converted the bidentate
dithiolate into a monodentate
Figure 2.Abbreviated computational mechanisms for
+
Sꢀdonor, thus creating
a
electrocatalysis of H production by the [FeꢀFe’] and [Niꢀ
2
9
+
Fe’] electrocatalysts. Shown in red is the Fe – S bond that
undergoes reductive bond cleavage.
transient frustrated Lewis pair,
i.e., yielding reactive sites for proton and electron uptake on the free thiolate and the open site on
9
iron, respectively, see Figure 2. In this way, complexes, that do not have an amine pendant base
for proton uptake and storage, or open sites on iron for a hydride, as in the [FeFe]ꢀH ase active
2
2
9ꢀ33
site,
adjust their structures to accommodate coupled electron/proton uptake. While the mild
+
potential for the first EC process for the [FeꢀFe’] complex required both proton/electron uptake
+
for genesis of the pendant base, the more negative potential that reduces the [NiꢀFe’] labilizes
III
8,9
the sulfur and creates an Fe ꢀH at the first reduction, Figure 2.
4

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In this report we provide a more complete matrix of MN S ꢀFe complexes of
2
2
electrocatalytic potential for experimental and computational analysis. Specifically a redox
innocent (“hard”) metalloligand, NiN S , of more negative reduction potential, is incorporated in
2
2
II
0/+
place of “soft” (NO)FeN S . The thus generated [Ni N S •Fe(NO) ] , a “hard”/“soft” complex
2
2
2
2
2
may be compared to the other members of the matrix. The solid state structures of
II
0/+
[
Ni N S •Fe(NO) ] in two redox levels and characteristics as an electrocatalyst (robustness
2 2 2
0
0
Scheme 1.A) The syntheses of [NiꢀFe] and [NiFe(CO)] as
and turnover frequency), for proton
2
+
+
ꢀ
neutral complexes, and [Ni ꢀFe ] and B) [Ni ꢀFe] as BF
4
reduction are also described. salt. The IR values (in cm ) of CO and NO are in red and blue,
2
2
2
ꢀ1
respectively.
Computational study, addresses the
A)
S
S
O
N
O
Ni
N
N
Fe
diversity of geometries of diꢀ and
polyꢀmetallic compounds containing
N S metalloligands by inspecting the
N
C
O
[
Ni-Fe(CO)]
1
734 (m), 1690 (s)
2006 (m)
S
S
S
S
O
N
CO
CO
Ni
Ni
Fe
N
N
2
2
N
N
N
O
[Ni-Fe]0
681 (m), 1630 (s)
Ni(bme-daco)
+
versatile bonding orbitals of the
metalloligands. The computational
mechanisms contrast the working
electrocatalysts against a nonꢀworking
analogue by exploring possible
intermediates in the proposed catalytic
cycles. Here important roles for hemiꢀ
labile and redox active ligands are
revealed.
1
S
S
O
N
Ni
Fe
N
N
N
O
+
[
Ni-Fe]
O
N
2+
O
O
N
N
S
Fe
Fe
S
O
N
Ni
N
S
N
N
N
Ni
S
2
+
[
Ni2-Fe2]
1
1
805 (m), 1794 (s)
749 (m), 1732 (s)
+
B)
O
N
O
N
S
S
Ni
[Fe(CO)3(NO)]-
S
Fe
2
N
S
N
2
[NO]BF4
THF, 5 h
S
Ni
Ni
S
N
n
N
N
N
n = 0 (bme-dach)
n = 1 (bme-daco)
n
n
+
[
Ni -Fe]
2
n = 0: 1790 (m), 1736 (s)
n = 1: 1789 (m), 1736 (s)
5

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Results and Discussion
Synthesis and Characterization
0
Shown in Scheme 1A are the synthetic routes to NiN S •Fe(NO) , [NiꢀFe] , and its oneꢀ
2
2
2
+
electron oxidized analogue,[NiꢀFe] , isolated and crystallographically characterized as a dimer,
2
+
2+
[
NiN S •Fe(NO) ] or [Ni ꢀFe ] , (N S = N,Nꢀbis(2ꢀmercaptoethyl)ꢀ1,5ꢀdiazacyclooctane or
2 2 2 2 2 2 2 2
bmeꢀdaco). Infrared values listed for the diatomic ligands were recorded in CH Cl or THF
2
2
o
solution. Freshly prepared Fe(CO) (NO) in THF readily reacts with NiN S at 22 C, with
2
2
2 2
o
replacement of one CO, releasing the second CO under photolysis, or within 20 min at 40 C,
3
4ꢀ 37
thus converting the NiN S from monoꢀ to bidentate ligand.
2
2
0
From this approach the [NiꢀFe] complex was isolated as a brown solid that is stable at
0
+
ꢀ
o
ambient temperature under Ar. Oxidation of [NiꢀFe] by Fc PF at 0 C resulted in a color
6
change of the THF solution from brown to dark purple with concomitant shifts of the ν(NO)
ꢀ1
values by an average of ca. 110 cm to higher wave numbers. The reversibility of this oxidation
0
was confirmed by reaction with cobaltocene and return to the reduced [NiꢀFe] . The ν(NO) bands
2
+
listed under [Ni ꢀFe ] , Scheme 1A, reflect the presence of overlapping components which were
2
2
resolved into two sets of absorbances, interpreted as a likely mixture of monomeric cation and
dicationic dimer, with the set at lower values slightly less in intensity (Figure S24). As other
experimental data, vide infra, as well as computational studies, indicate the predominance of
+
monomeric [NiꢀFe] , we postulate that the set of absorbances at slightly lower wavenumbers (as
2
+
shoulders on the major bands) are due to the dimeric [Ni ꢀFe ] .We note that the electronꢀspray
2
2
+
ionization mass spectrum of [NiꢀFe] has a parent ion with isotopic bundle distribution at m/z
6

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+
2+
that is consistent with a monomeric [NiꢀFe] rather than a dimeric [Ni ꢀFe ] , Figure S28. The
2
2
difference between two consecutive isotopic mass units is ~1, rather than 0.5, which indicates the
+
predominance of the monomer, [NiꢀFe] , in the polar solvents in which they are soluble.
+
+
The magnetic moments of [NiꢀFe] and [Ni ꢀFe] are 1.54 B.M. and 1.76 B.M.,
2
respectively, measured by Evans’ method at room temperature in CD Cl . This is consistent with
2
2
the presence of a single unpaired electron, Figures S1 and S2. The EPR spectra for both
9
complexes display the isotropic g = 2.03 signal that is prototypical of the {Fe(NO) } unit, Figure
2
+
S21, S22, respectively. The 77 K EPR spectrum of the [NiꢀFe] displayed fine structure
requiring two signals for simulation: A major isotropic signal of g = 2.035 showed coupling
1
4
with nitrogen of A( N) = 32.74 MHz and a minor anisotropic signal had parameters of g
=
xyz
2.183, 2.012, 1.908 and no observable hyperfine coupling, Figure S21.
Xꢀray diffraction quality crystals of the oxidized NiꢀFe compound were obtained from the
+
oneꢀpot reaction of equimolar NiN S and (putative) [Fe(CO) (NO) ] (prepared in situ by
2
2
2
2
−
38
reacting [Fe(CO) (NO)] with two equivalents of [NO]BF ) in CH Cl at ambient temperature,
3
4
2
2
+
Scheme 1. A third NiꢀFe complex, [Ni ꢀFe] , was obtained on combining NiN S with
2
2 2
+
[
Fe(CO) (NO) ] in 2:1 ratio in THF solution, Scheme 1B. Xꢀray quality crystals of this
2 2
ꢀ
trimetallic as its BF salt were obtained from hexane/THF layering at ꢀ28°C. Its v(NO) IR
4
9
spectral features are typical of monomeric DNICs in the {Fe(NO) } redox level.
2
Xꢀray Diffraction Studies
0
2+
The molecular structures of the heterometallic complexes [NiꢀFe] , [Ni ꢀFe ] and
2
2
+
[
Ni ꢀFe] are shown in Figure 3.
2
7

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0
2+
+
Figure 3.Molecular structures of a) [NiꢀFe] , b)[Ni ꢀFe ] and c)[Ni ꢀFe] from Xꢀray diffraction.
2
2
2
ꢀ
2+
+
The BF counter ions of [Ni ꢀFe ] and [Ni ꢀFe] are omitted for clarity.
4
2
2
2
0
The bimetallic complex [NiꢀFe] , exhibits an overall butterflyꢀlike Ni(µꢀSR) Fe core,
2
II 36
analogous to the report of Pohl et al., where an open chain N S ligand chelated the Ni . The
2
2
converging lone pairs (see below) on the cisꢀdithiolates engage in bidentate binding and impose a
hinge angle (the intersection of the best N S plane with the S Feplane) of ca. 117 °, visꢀàꢀvis
2
2
2
o
39
constricting the S−Ni−S angle by ca. 4 compared to the free metalloligand. The two nitrosyl
units bound to the pseudo tetrahedral iron center are slightly bent towards each other, in an
4
0
“
attracto” orientation; the average Fe−N−O angle is ~ 163.8 °. The Ni•••Fe distance of 3.001
3
6
(2) Å, is slightly longer (by 0.022 Å) than that obtained in the Pohl, et al. structure.
2
+
ꢀ
The oxidized NiFe complex crystallizes as dimeric [Ni ꢀFe ] with two BF anions; two
2
2
4
dinitrosyl iron units are bridged by two NiN S metalloligands.The tetrahedral geometry about
2
2
each Fe(NO) unit is thus completed by two thiolates from different NiN S metalloligands, thus
2
2 2
generating an abbreviated paddlewheel, or propeller type, complex seen in examples of nickelꢀ
41
gold
tetrametallic
complexes.
Likewise,
the
molecular
structure
of
+
[
Ni ꢀFe] demonstrates that one Fe(NO) unit bridges two NiN S metalloligands, each acting as a
2
2
2 2
+
monodentate ligand. As shown in the [Ni ꢀFe] structure, Figure 3, the free unbound thiolates of
2
8

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+
two NiN S units are transoid to each other. The addition of a second Fe(NO) unit to generate
2
2
2
2
+
the dication, [Ni ꢀFe ] , would require rotation about one FeꢀS bond in order to align the two
2
2
metalloligands.
2
+
+
The average Ni•••Fe distances in [Ni ꢀFe ] and [Ni ꢀFe] , are 3.680 (2) Å and 3.521 (2)
2
2
2
0
II
Å, respectively, and are longer than in the [NiꢀFe] reduced complex by ca. 0.5 Å. The Ni
maintains a square planar geometry in the reduced and oxidized complexes with a displacement
of no more than 0.1 Å from the N S best plane. Overall these structures demonstrate the
2
2
impressive adaptability of the NiN S metalloligands, and their potential to template clusters
2
2
7
through Sꢀbased aggregation.
Computational Structural Study
This computational section uses density functional theory (DFT) analysis to address the
structural question in particular that was raised by the xꢀray diffraction report: Is there an
electronic factor that governs the different µ ꢀSR binding modes found in the three forms of NiFe
2
heterometallic aggregates? The functional/basis set combination, TPSS/6ꢀ311++G(d,p), and
natural bond orbital (NBO) analysis were applied to the computational structural modeling of the
0
2+
free metalloligand NiN S and its derivatives [NiꢀFe] , [Ni ꢀFe ] ; more details of the
2
2
2
2
computational methodology is available in SI.
The divergent or convergent orientation of S lone pairs of NiN S metalloligand and
2
2
influences on structures of NiN S •M’ heterobimetallics. Traditional bidentate ligands such
2
2
as diphosphines, diamines and bipyridyls have a single lone pair on each donor site. These lone
x
42
pairs are positioned on orbitals originating from sp ꢀhybridization and are highly directional.
They provide fixed binding orientations that match the coordination preference of traditional
9

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metal receivers. In contrast, the binding between the sulfurs of the metallothiolate NiN S and an
2
2
exogeneous metal are more geometrically flexible because of the multiple S lone pairs. From
NBO bonding analysis, sulfur in the NiN S metalloligand is found to use mainly p orbitals for
2
2
4
3, 44
bonding to Ni and C.
the
SꢀC bonds (C and C refer to the C H linker connecting S and N where C is directly bound to
For example, in a free NiN S , p character makes up 83% and 86% of
2 2
S
contributions
in
the
SꢀNi
bonds
and
α
α
β
2
4
α
S, Figure 6A), which leaves one lone pair in a p orbital and another in an sꢀdominated orbital on
each S. Because a receiver group, a Fe(NO) unit in our case, may bind to either lobe of the p
2
lone pair(s), whose orientation is determined by the NiꢀSꢀC torsion angle, a diversity of
α
7
, 26
structures results.
The orientation of this remaining p lone pair in the NiN S metalloligand is determined
2
2
by the NiN S metalloligand’s NiꢀSꢀC ꢀC ꢀN fiveꢀmembered rings that adopt a nonꢀplanar
2
2
α
β
envelope conformation like cyclopentane. The C carbon, the “flap” of the envelope
α
conformation, puckers towards one side or the other of the N S plane, Figure 3. Figure 4 shows
2
2
how this puckering tilts the remaining 3pꢀlone pair on each sulfur from the normal to the N2S2
plane. This tilt causes two pꢀorbital lobes (green lobes) to converge on the side to which the flap
puckers, while the red lobes diverge on the opposite side. The orientation of the added Fe(NO)2
receiver unit(s), will be thus determined by such directional property of the donor p lone pairs.
0
The structure of the reduced monomer [NiꢀFe] shows the Fe(NO) fragment is on the same side
2
2
+
as the flap; while in the oxidized dimer [Ni ꢀFe ] the flap and the Fe(NO) fragment(s) appear
2
2
2
on different sides of each N S plane, thus, binding to the other end of the p lone pair. Based on
2
2
the analysis above, the converging lobes of the p donor lone pairs maximize contact to the
0
Fe(NO) unit in the monomer [NiꢀFe] , while the diverging lobes are preferred by two bridging
2
1
0

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2
+
Fe(NO) units between two metalloligands in the dimer [Ni ꢀFe ] . The utilization of the
2
2
2
divergent lobes apparently lessens the steric repulsion between Fe(NO) units. In summary, the
2
binding position of the Fe(NO) unit with respect to the flap in the NiꢀSꢀC ꢀC ꢀN fiveꢀmembered
2
α
β
rings are correlated by the competition between chemical bond directionality of the binding
sulfurs and steric repulsion of the receiver units.
A
B
Cβ
Cα
Figure 4.A) The geometry of a free metalloligand NiN S and B) its two 3p lone pairs
2
2
presented one on each sulfur (contour plots at isovalue = 0.05 a.u. by NBO analysis). Note
the relative positions of the SꢀC / SꢀNi bonds and the 3pꢀlone pair.
α
Electrochemistry
The cyclic voltammograms of [NiꢀFe] (in CH Cl ), [Ni ꢀFe] (in CH CN), as BF salts,
+
+
ꢀ
2
2
2
3
4
0
o
0/+
and [NiꢀFe] (in CH Cl ), were recorded under Ar at 22 C, and referenced to Fc (E1/2 = 0.0 V)
2
2
0
as an internal standard. Both the neutral complex [NiꢀFe] and the cationic analogue,
+
ꢀ
[
NiꢀFe] , used in the CV studies as its BF salt, displayed reversible reduction events at ca. ꢀ0.73
4
9
/10
V (CH Cl ), assigned to the {Fe(NO) }
couple; see Figure S13ꢀS15 for CV scans. The NiFe
2
2
2
complexes also present two irreversible oxidation events at ca. ꢀ0.10 V and ca. +0.45 V, with
minor differences in intensities according to the neutral or cationic sources. Both of these events
are assumed to be Sꢀbased.
1
1

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+
In CH CN, the trimetallic complex [Ni ꢀFe] , showed a reversible event at, E = ꢀ0.75
3
2
1/2
9
/10
V, assigned to the {Fe(NO)2} couple and an irreversible oxidation event at E = ꢀ0.05 V, see
1/2
9
/10
Figure S16. The E value for the {Fe(NO) }
couple, is anodically shifted by ca. 30 mV
1
/2
2
relative to the 1:1, NiFe complexes, resulting from the greater electron donation of two NiN2S2
9
/10
centers to the Fe(NO) redox marker. The scan rate dependences of the {Fe(NO) } couple for
2
2
all three complexes support the assignments to reversible or quasiꢀreversible as described above,
see Figures S17 – S19.
+
Cyclic voltammetry and response to added acid. Electrochemical studies of [NiꢀFe] and [Ni ꢀ
2
+
Fe] were carried out in presence of HBF •Et O under a N /Ar atmosphere. For comparison the
4
2
2
+
[
FeꢀFe] complex was examined under similar experimental conditions. Sequential addition of
+
2+
HBF •Et O to a CH Cl solution of [NiꢀFe] (2 mM[Ni ꢀFe ] ) showed an increase in the
4
2
2
2
2
2
9
/10
cathodic current at the {Fe(NO)2}
redox event at ꢀ0.73 V. The initial cathodic current
response at ꢀ0.8 V saturates with ~20 equivalents of the acid, Figure 5 (inset). A second rise in
cathodic current at ꢀ1.10 V, commences upon addition of > 12 equivalents of the acid, which
continues to rise as the catalytic current response, Figure 5. The first response is attributed to the
+
reduction of [NiꢀFe] followed by a protonation. The second response is assigned to the upꢀtake
+
of another electron by the reduced and protonated counterpart of [NiꢀFe] . The mechanism
below connects the successive protonation to the production of H , thus closing the catalytic
2
cycle. Overlays of this response of the NiFe complex in presence of 50 equivalents of
HBF •Et O (0.1 M), as well as the CV of the free acid, are shown in Figure 5. The catalytic H
2
4
2
produced was confirmed by applying a constant potential at ꢀ1.12 V for 60 min (black bold line
in Figure 5), and analysis of the headspace by gas chromatography. The H was quantified by an
2
average of two consistent constant potential coulometry experiments with subtraction of the H2
1
2

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9
,10
+
+
produced from the free acid. The nitrosylated compounds [NiꢀFe] and [FeꢀFe] were found to
have low turnover numbers and Faradaic efficiencies, 68 ± 2 % and 58 ± 1%, respectively, for H2
+
production. In contrast the [NiꢀFe’] gave a Faradeic efficiency of ca. 96%.
We assume that
the former involves alternate protonation pathways, particularly at NO, that lead to degradation
and hence low F.E. In addition the TON for the robust NiꢀFe’ complex in 50 equiv of TFA,
measured at ꢀ1.73 V and over a period of 8 h was found to be 6.7, assuring catalytic proton
0
reduction. The electrocatalytic response of the reduced complex, [NiꢀFe] in the presence of
+
HBF •Et O, is, as expected, the same as [NiꢀFe] and is shown in Figure S20ꢀB.”
4
2
2
+
Figure 5. Cyclic voltammograms of 2.0 mM CH Cl solutions of [Ni ꢀFe ] (or
[
2
2
2
2
+
NiꢀFe] (blue)); with 50 equiv. (0.1 M) of added HBF •Et O (red); and, for reference, 50
4 2
equiv. (0.1 M) of HBF •Et O (green). The black line indicates the constant potential applied
4
2
2
+
+
for bulk electrolysis, ꢀ1.12 V. Inset: Cyclic voltammograms of [Ni ꢀFe ] (or [NiꢀFe] ) with 2
2
2
1
3

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2
+
ꢀ
to 20 equiv. aliquots of HBF •Et O. Crystalline [Ni ꢀFe ] was dissolved as its BF salt, in
4
2
2
2
4
t
0
.1 M Bu NPF as supporting electrolyte, with a glassy carbon electrode at scan rate of 200
4 6
mV/s. Note: equivalents of HBF •Et O was calculated with respect to the dimeric [Ni ꢀFe ] .
2+
4
2
2
2
Following the approach
4
5
of Helm and Appel,
and
4
6
Wiese,
the
turnover
frequency (TOF) as calculated
from the CV experiment for
+
ꢀ1
[
NiꢀFe] , was 39.7 s , which is
+
slightly better than the [FeꢀFe]
ꢀ
1
complex, 26.7 s , calculated
under similar
conditions. The [NiꢀFe] shows a
saturation of the more negative catalytic current upon addition of 80 equivalents of HBF •Et O,
Figure 6.Cyclic voltammograms of 2.0 mM CH
3
CN solution of
+
[
Ni ꢀFe] (blue), with 1 and 2 equiv. of added HBF •Et O (red
2
4
2
experimental and green, respectively). Note: the N S ligand used in this
2
2
compound is bmeꢀdach.
+
4
2
+
i.e., ~0.16 M CH Cl solution. Notably, the behavior of the [FeꢀFe] complex is similar, and
2
2
further addition of acid leads to decomposition of both catalysts. The precise calculation of
overpotential is indeterminable as the thermodynamic potential (EHBF4/H2,BF4ꢀ) of 0.1 M
4
7
HBF •Et O in CH Cl is unavailable. Using the thermodynamic potential of HBF •Et O in
4
2
2
2
4
2
4
8, 49
+
+
acetonitrile(ꢀ0.26 V),
an estimate of the overpotential of [NiꢀFe] and [FeꢀFe] were 711 mV
+
+
and 660 mV, respectively, which are lower than those of the [NiꢀFe’]
and [FeꢀFe’]
electrocatalysts by over 220 mV.
In contrast to the NiFe complexes, addition of HBF •Et O to a 2.0 mM CH CN solution
4
2
3
+
of [Ni ꢀFe] , (the N S ligand used in this electrochemical study is bmeꢀdach) did not show an
2
2 2
1
4

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9
/10
increase in the cathodic current at ꢀ0.75 V, the reversible {Fe(NO)2}
redox event. Instead, a
new reversible redox event at E1/2 = ꢀ0.52 V, appeared upon addition of two equivalents of
HBF •Et O with a concomitant disappearance of the original redox process, Figure 6. Further
4
2
addition of acid resulted in electrode fouling, Figure S20ꢀA. A possible explanation, from
+
computational chemistry, vide infra, for the positive 230 mV shift is that [Ni ꢀFe] can be
2
protonated on its exposed thiolate sulfur by HBF •Et O, vide infra. Such would account for the
4
2
9
/10
+
greater ease of reduction for the {Fe(NO)2}
couple, compared to the [Ni ꢀFe] complex.
2
Supporting this conclusion is that addition of 1 equivalent of HBF •Et O to a CH CN solution of
4
2
3
+
[
Ni ꢀFe] produced a small but definite shift of the ν(NO) in the IR spectrum from 1787 and
2
ꢀ1
ꢀ1
1
734cm to 1793 and 1737 cm , Figure S27.In addition, the irreversible oxidation event at 0.07
V, which is assumed to besulfurꢀbased oxidation, shows a decrease in the anodic current upon
addition of acid, indicating disulfide formation is inhibited under acidic conditions.
Computational Mechanistic Study
The electrochemical study points to additional questions for computational study: A)
How do the calculated electrocatalytic mechanisms compare for the hardꢀsoft vs. softꢀsoft
donor/receiver adducts? B) Can computational analysis clarify those cases of nonꢀcatalytic
+
electrochemical responses to added protons? Modeling is extended to [Ni ꢀFe] ,along with
2
0
+
[
NiꢀFe] , [NiꢀFe] , in various oxidation states and with multiple added protons to answer these
questions.
+
0
Mechanistic perspectives of the proton reduction electrocatalysis by [NiꢀFe] /[NiꢀFe] and
+
comparison to [FeꢀFe] . An earlier computational analysis of a HER electrocatalysis
+
mechanism proposed for the [FeꢀFe] complex, Figure 1, focused on the first reduction event
8
with a strong acid proton source. The successive reduction event ultimately allowed for double
1
5

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8
, 50
proton addition to the Fe(NO) unit and formation of a dihydride, panel A of Figure 7.
2
8
Making use of electron transfer from the reduced {Fe(NO)} , reductive elimination from the
6
8
2
7
9
{
Fe(NO)} ꢀ{Fe(NO) } morphed into an η ꢀH –Fe(NO) , restoring {Fe(NO)} ꢀ{Fe(NO)} , with
2 2 2
9
H formation and loss. Note that no hemiꢀlability of the metallodithiolate ligand is necessary
2
here as the mechanism does not entail hydride/proton coupling to H , but rather reductive
2
7
elimination from two hydrides.
A Soft donor unit – Soft receiver unit
B Hard donor unit – Soft receiver unit
+
Figure 7. Comparative catalytic cycles for H production catalyzed by [FeꢀFe] and
2
+
[
NiꢀFe] . All pKa, thermodynamic, and metric data for the two mechanisms are available
50
in a separate report.
+
0
The [NiꢀFe] and its reduced counterpart [NiꢀFe] are determined to be electrocatalysts at
ꢀ
0.73 V for H production with HBF •Et O, Figure 5. [Note: The computational study finds that
2
4
2
2
+
the [Ni ꢀFe ] , whose dimeric structure was established in the solid state by crystallography,
2
2
+
finds greater stability in solution as the monomeric form, [NiꢀFe] . Experimental evidences
including ESIꢀMS and determination of ꢁ support this thesis, vide supra.]The catalytic cycle is
eff
1
6

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+
thus initiated with the monomer [NiꢀFe] (Figure 7B). As indicated in panel B of Figure 7, the
9
+
{
Fe(NO) } in the [NiꢀFe] unit accepts the first incoming electron, followed by the first proton,
2
8
to create a hydride on the now {Fe(NO) } unit. Addition of a second electron activates the
2
hemiꢀlability of the bridging thiolate to break one SꢀFe bond, while the terminal hydride
becomes bridging between Fe and Ni. The cleavage of the SꢀFe dative bond essentially releases
one p lone pair of the thiolate so that S can act as a pendant base to accept the second proton and
guide it to a coupling position with the hydride and produce H . Details of the full catalytic cycle
2
+
+
with energetics and analysis of electronic structure evolution for both [FeꢀFe] and [NiꢀFe] are
5
0
presented in a separate report.
+
Explanation for the Absence of Ctalytic Activity of [Ni ꢀFe] . While one might have
2
+
expected the dangling thiolates in the 2:1 complex [Ni ꢀFe] to act as a pendant base, in fact this
2
complex does not show any catalytic activity in the presence of strong acid, HBF •Et O, within
4
2
the solvent potential window. A computational study, summarized in Figure 8, reveals that while
reduction still occurs on the Fe(NO) unit, the protonation process is diverted from the Fe(NO)2
2
unit. In this 2:1 complex, the computations show that only one thiolate from each NiN S binds
2
2
to Fe(NO) , while the other thiolate, is “free” to interact with other electron acceptors; thus it
2
9
may also be protonated, even before the reduction of the {Fe(NO) } unit occurs, see Table S9,
2
and shifts the its potential, which is supported by experiment, vide supra.
0
According to the computations, in the reduced [Ni ꢀFe] the “free” thiolate competes with
2
1
0
the reduced {Fe(NO)2} unit for the incoming proton (Figure 8A); in addition, by rotation
around an FeꢀS bond, the two NiN S ligands may orient their “free” thiolate sulfurs to pinch the
2
2
proton, i.e., consequently forming a strong hydrogen bond. (Figure 8 A/B) Spectroscopic
+
evidence supports protonation on S even before reduction, i.e., in [Ni ꢀFe] , Figure S27. Two
2
1
7

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+
+
geometries of the pinched proton by two “free” thiolates, [Ni ꢀFeꢀSHSꢀ1] and [Ni ꢀFeꢀSHSꢀ2]
2
2
can be achieved by either translating or rotating one NiN S unit of [Ni ꢀFe], respectively.
2
2
2
5
1
Precedent in Dubois’ Ni(P N ) catalysts, a proton pinched between two amine N bases is
2
2 2
relatively stable; in our case, the pinched proton is even more stable than a hydride on Fe(NO)2
5
0
(
Figure 8A). However, the mechanistic clue from the [NiꢀFe] complex indicates the
1
0
requirement for a proton to be reduced into a hydride, by {Fe(NO) } , before the H can be
2
2
produced by the protonꢀhydride coupling mechanism. Therefore, the formation of a stable
pinched proton likely prevents the generation of the hydride and cuts off the catalytic cycle. The
thiolate already bound to Fe(NO) also helps stabilize the proton on a “free” thiolate, to a smaller
2
+
extent, with the example of [Ni ꢀFeꢀSHSꢀ3] (Figure 8A).
2
A
B
+
+
[
Ni ꢀFeꢀSHSꢀ1]
[Ni ꢀFeꢀSHSꢀ2]
2
2
1
8

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+
Figure 8.The protonation of [Ni ꢀFe], the reduced form of [Ni ꢀFe] and a) possible
2
2
protonated products with b) 3D geometric presentations of selected species featuring the
pinched proton. The computationally derived structures are rendered so as to show the
NiN S metalloligand without altering the rigidity of the N S planar structure. All
2
2
2 2
hydrogens except the one pinched between two sulfurs are omitted.
Conclusions
Our collection of hydrogen evolution reaction catalysts is summarized in Figure 9. While
the small differences in donor units and acceptor units do not influence the overall structures of
the Sꢀbridged bimetallics; all have butterflyꢀlike [M(ꢁꢀSR) Fe] core and the potential for opening
2
up sites for proton addition via the hemiꢀlability of the metallothiolate donors. Nevertheless,
demonstrable and explicable differences are seen in their catalytic performances as indicated by
catalytic potential, required acid strength, and TOF.
Figure 9. A comparative schematic for a matrix of bimetallic electrocatalysts containing
hard/soft donor/acceptor units.
1
9

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Analogous to the HSAB (Hard and Soft (Lewis) Acids and Bases) concept, we offer an
electronic parallel, “soft vs. hard donor/receiver units”, in this case directed towards the number
of NO ligands in the bimetallics ranging from 0 to 3, with increasing flexibility (i.e., soft) of
electronic structure within each unit. The nonꢀinnocence of the NO ligand confers electron
uptake at milder potentials, which we have seen in both the donor units and acceptor units. Thus
the incorporation of NO ligands on the acceptor units, the ‘hardꢀsoft’ and ‘softꢀsoft’
electrocatalysts lead to energetically more accessible catalytic current, however, at the cost of a
stronger acid and a lower TOF in comparison to the bimetallics with hard acceptor units.
While these electrocatalysts are only moderately efficient for H production, they are
2
wellꢀbehaved and have demonstrated reproducibility. Two of the catalysts, c and d, with soft
receivers, are isolated and crystallized in both oxidized and reduced forms at ambient conditions
lending confidence to the presumed catalytic cycle.
Features in the electrochemical scans may be reasonably ascribed to protonation products
whose identities are further described by computational chemistry. The resulting computational
mechanisms identify key features that may guide future synthetic targets. For example, the hemiꢀ
lability of the Sꢀdonors may be optimized by steric constraints; the usefulness of the Fe(NO)2
unit as electron depot and protonation site with low redox potential, should encourage
explorations with other redoxꢀactive, soft acceptors. The computations also suggest a
ꢀ
+
mechanistic paradigm of heterolytic H /H , hydrideꢀproton, coupling for bimetallics a,b and c
from the chart, and reductive elimination from d arising in the softꢀsoft construct. Such a
+
supposition derives from extreme electron delocalization in the trinitrosylated [FeꢀFe] complex
2
0

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and argues that suitably constructed first row, bimetallic complexes may take on twoꢀelectron
processes that emulate noble metals.
Supporting Information
Experimental, spectroscopic, additional electrochemical and computational details, Xꢀray
0
2+
+
crystallographic data (CIF) from the structure of the complexes [NiꢀFe] , [Ni ꢀFe ] , [Ni ꢀFe] ,
2
2
2
and computational coordinates are available in the Supporting Information. Crystallographic data
0
2+
+
for the complexes [NiꢀFe] , [Ni ꢀFe ] and [Ni ꢀFe] were deposited in the Cambridge
2
2
2
0
Crystallographic Data Centre. The following CCDC numbers were assigned to them: [NiꢀFe]
2
+
+
(
CCDC 1045461), [Ni ꢀFe ] (CCDC 1045460) and [Ni ꢀFe] (CCDC 1565539).
2 2 2
Author Information
Corresponding author emails: marcetta@chem.tamu.edu
Conflicts of Interest
There are no conflicts of interest to declare.
Acknowledgements
We are thankful for financial support from the National Science Foundation (CHEꢀ1266097,
CHEꢀ1665258 to M.Y.D., and CHEꢀ1300787, CHEꢀ1664866 to M.B.H.), the Robert A. Welch
Foundation (Aꢀ0924 to M.Y.D., and Aꢀ0648 to M.B.H.). A portion of the salary of 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 Dr. David Russell and Dr. Doyong Kim for interpretation of the
high resolution ESIꢀMS. The authors would also like to acknowledge Dr. Allen M. Lunsford for
helpful discussions.
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Chemical Science
Page 24 of 24
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DOI: 10.1039/C7SC03378H
Nitrosyls as electron reservoirs guide protons to favorable sites in bimetallic HER catalysts.