Oxygen-Tolerant H2 Production by [FeFe]-H2ase Active Site Mimics Aided by Second Sphere Proton Shuttle

Article abstract of DOI:10.1021/jacs.8b05983

The instability of [FeFe]-H₂ases and their biomimetics toward O₂ renders them inefficient to implement in practical H₂ generation (HER). Previous investigations on synthetic models as well as natural enzymes proved that re

Full text of DOI:10.1021/jacs.8b05983

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Article
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Oxygen-Tolerant H Production by [FeFe]-Hase
nd
Active Site Mimics Aided by 2 Sphere Proton Shuttle
Md Estak Ahmed, Subal Dey, Marcetta Y. Darensbourg, and Abhishek Dey
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05983 • Publication Date (Web): 05 Sep 2018
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Oxygen-Tolerant H2 Production by [FeFe]-H2ase Active
Site Mimics Aided by 2nd Sphere Proton Shuttle
Md Estak Ahmed, Subal Dey, Marcetta Y. Darensbourg*#, Abhishek Dey*
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Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Kolkata, India 700032
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# Department of Chemistry, Texas A&M University, College Station, TX 77843-3255
ABSTRACT: The instability of [FeFe]-H2ases and their biomimetics towards O2 renders them
inefficient to implement in practical H2 generation (HER). Previous investigations on synthetic
models as well as natural enzymes proved that reactive oxygen species (ROS) generated on O2
exposure oxidatively degrades the 2Fe sub-cluster within the H-cluster, active site. Recent
electrochemical studies, coupled with theoretical investigations on [FeFe]-H2ase suggested that
selective O2 reduction to H2O could eliminate the ROS and hence tolerance against oxidative
degradation could be achieved (Nat. Chem., 2017, 9, 88-95). We have prepared a series of 2Fe
subsite mimics with substituted arenes attached to bridgehead N-atoms in the S to S linker, (µ-
S2(CH2)2NAr)[Fe(CO)3]2. Structural analyses find the nature of the substituent on the arene
offers steric control of the orientation of bridgehead N-atoms, affecting their proton uptake and
translocation ability. The heterogeneous electrochemical studies of these complexes
physiadsorbed on edge plane graphite (EPG) electrode show the onset of HER activity at ~180
mV overpotential in pH 5.5 water. In addition, bridgehead N-protonation, and subsequent H-
bonding capability, is established to facilitate the O-O bond cleavage resulting in selective O2
reduction to H2O. This allows a synthetic [FeFe]-H2ase model to reduce protons to H2 unabated
in the presence of dissolved O2 in water at nearly neutral pH (pH 5.5); i.e., O2-tolerant, stable
HER activity is achieved.
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Introduction
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The diiron [FeFe]-hydrogenase (H2ase) is one of the two naturally occurring H2ases that can
catalyze the reduction of H+ to H2 reversibly.1-3 The active site of the [FeFe]-H2ase (H-cluster)
contains a Fe4S4 cluster that is cysteine sulfur-linked to one of the iron centers (Fep) of a Fe2S2
sub-cluster (the 2Fe subsite), the redox active catalytic centre (Scheme 1, centre). An
azadithiolato (ADT) bridge holds the two irons together in the 2Fe subsite; the geometry about
the irons is completed with additional two cyanides and three carbonyl ligands. The consensus
mechanism finds that the iron of the 2Fe subsite that is distal to the Fe4S4 cluster (Fed) directly
interacts with H+ or molecular H2.4,5 Recent spectroelectrochemical investigations under
anaerobic conditions suggest that the Fe(I)Fe(I) state of the enzyme is responsible for binding
the first added proton at the bridgehead nitrogen base.6,7 Successive reduction and proton
rearrangement results in a hydride-bound Fe(II)Fe(II)-H- species (Scheme 1).6 Further
protonation occurs at the bridgehead amine, well positioned to couple with the hydride, to
release H2. The opposite direction is evoked for H2 oxidation. The bridgehead nitrogen atom
shuttles proton to and from the diiron active site8,9, ergo the orientation of the nitrogen lone
pair towards Fed is key to H2ase activity.10-13 The 1st reduction of the 2Fe subsite [Fe(I)Fe(I)
state] is poised at - 420 to - 450 mV13,14 at pH 7.0 where the enzyme operates with an
overpotential (the additional energy needed above the thermodynamic potential of H+ + e- = ½
H2) of only 20-50 mV.3 The turnover rates of [FeFe] H2ase vary between 100-10000 s-1.3
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Scheme 1: Proposed mechanistic cycle of [FeFe]-H2ase enzyme adapted from J. Am. Chem.
Soc. 2017, 139, 15122−15134. The active site of the H-cluster is shown at the centre. The
formal oxidation states of the irons in the 2Fe subsite are indicated.
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As the [FeFe]-H2ases and their synthetic analogues are intolerant towards the presence of
oxygen during catalytic turnovers, their utility as a practical water splitting agent is greatly
compromised.15-17 Hence, investigation of O2 reactivities of H2ases have emerged as an intense
field of research. Initial EXAFS analysis with the enzyme suggested that reactive oxygen
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-
species (ROS), e.g., H2O2 and O2 , generated on O2 exposure destroys the F4S4-cluster,
rescinding the e--shuttle and permanently damages the HER activity (scheme 2a).18,19 Later, a
theoretical investigation suggested that ROS may oxidize either the Fe4S4, the 2Fe subsite, or
any of the S-atoms present in the H-cluster.20-22 Studies of the electrocatalytic O2 reduction
with synthetic functional 2Fe subsite mimics have indicated that a large amount of ROS
degrades the 2Fe sub-cluster models through S-oxygenation.23 The major ROS responsible was
proposed to be H2O2 resulting from the hydrolysis of a Fe(1.5)Fe(1.5)-OOH intermediate
species involved in O2 reduction by the Fe(I)Fe(0) state of these synthetic analogues (Scheme
2b).23 The use of ROS scavenging additives like catalase, in the electrolytic solution, enhanced
the catalytic turnovers for 2Fe subsite mimics as well as other H2 evolution catalysts.23,24
Subsequent crystallographic investigations on enzymes demonstrated that indeed the 2Fe sub-
cluster degrades first on O2 exposure keeping the Fe4S4 cluster almost intact; activity could be
reactivated through reinsertion of an intact 2Fe moiety.25
Initial experimental data on site-directed mutants show that the nearby Cys299 residue in the
[FeFe]-H2ase from Chlamydomonas pastaurianum (Cp) and Cys169 from Chlamydomonas
reinhardtii (Cr) is oxidized to sulfenic acid (S-OH) by the ROS generated by the 2Fe sub-
cluster in the presence of O2.26 Molecular dynamics calculations on the pathways leading to
the generation of deleterious ROS in the enzyme active site suggested that reaction of
molecular O2 at the Fed site of Fe(II)-Fe(I) state and further reduction and protonation gives an
Fe(II)Fe(II)-OOH species that could either easily be released as H2O2 by taking a proton from
adjacent Cys-SH or other reactive radical species.26 Similar types of oxidative inactivation
mechanisms were also established for the [NiFe]-H2ase enzyme. In this case, a Ni(III)-(OOH)-
Fe(II) species (scheme 2c), bearing resemblance to the Fe(II)Fe(II)-OOH species in [FeFe]-
H2ase, is formed on O2 exposure which immediately oxidizes the bridging thiolate ligand of
the active site to sulfonate and generates the Ni-A (unready) state; the latter could be easily
regenerated by reducing the sulfonates to thiol.27 However, for [FeFe]-H2ase, H-bonding
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between the cysteine thiol and the peroxide species was proposed to be important for stabilizing
the Fe(II)Fe(II)-OOH intermediate and activating its O-O bond for cleavage, reducing the
amount of ROS produced in the reaction of O2 with the reduced active site.26
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Similar hydrogen-bonding stabilization of such peroxide intermediates by an N-protonated
ADT ligand was also suggested in synthetic analogues which showed a two-fold reduction of
ROS produced from O2 reduction at acidic, relative to neutral, pH’s.23 The protonation of the
pendant aniline group, only achieved at very low pHs, helped cleave the O-O bond and resulted
in “partial” 4e-/4H+ reduction of O2 to benign H2O offering the 2Fe cluster partial protection
from ROS.23 Recently, partial reversibility of the reaction of the reduced cluster with O2 could
be achieved and was ascribed to the rapid delivery of protons and electrons to the 2Fe subsite
affecting 4e-/4H+ reduction of O2.15,26,28,29 In [NiFe]-H2ases oxygen tolerance is inherited from
4e-/4H+ reduction of O2 to benign H2O.27,30,31 Thus, results obtained from both H2ase enzymes
and synthetic analogues have hinted at the importance of a local proton source in affecting 4e-
/4H+ reduction of O2 at the 2Fe active site, precluding the formation of ROS which are
responsible for irreversible damage to the 2Fe active site.
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a
c
b
Scheme 2: Representative schemes summarizing O2 reactivities of H2ases and synthetic
models. In all schemes the M-OOH is established as key reactive species for degradation. (a)
The [FeFe]-H2ase enzyme undergoes incomplete reduction to generate ROS and loses its
activity; (b) in the case of [FeFe]-H2ase models, the bridgehead N-protonation plays a key role
in lowering the amount of ROS; and (c) the [NiFe]-H2ase enzyme active site undergoes O2
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reduction in the Ni-C state to produce the Ni-(OOH)-Fe intermediate that oxidizes one of the
Cys-S bridges to sulfenate ligand (Ni-B).
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Through a biohybrid approach, i.e., the combination of synthetic analogues and apo-enzyme
from [FeFe]-H2ase, the requirement for the bridgehead nitrogen in the ADT moiety has been
unambiguously established.10,32,33 The orientation of the nitrogen lone pair (N-lp) is also key
for efficient heterolytic H-H bond formation or cleavage.11,12 In fact, crystallographic evidence
of a dihydrogen-bound intermediate has only been achieved for synthetic complexes where the
orientation of the N-lp is directed towards the metal-hydride formation site.12,34,35 While
synthetic mimics of the Fe2S2 site are well explored,36,37 only recently have promising
electrocatalytic activities of these models been reported in aqueous environments.38-40
Typically, these catalysts require substantial overpotentials and strongly acidic conditions to
be functional. In comparison, of the synthetic catalysts, only those of Dubois works as low as
50-100 mV overpotentials albeit in organic solvents: their superior activity have been attributed
to advantageous orientation of the basic moiety which offers both thermodynamic and kinetic
advantage to the HER process.34,41-44 The importance of the bridging atom on the ADT in the
HER reactivity of a 2Fe sub-cluster in physiologically relevant aqueous medium is yet to be
demonstrated.
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Our current study, with complexes physiadsorbed on EPG, demonstrates the remarkable effect
of o-substitution on the geometry (through fixing the orientation of the N-lp) and reactivity of
N-Arylated, ADT-bridged synthetic mimics of [FeFe]-H2ase Fe2S2 subsite (Fig. 1: 1, 2, 3). As
1, 2, and 3 complexes show HER activity in near neutral pH (pH 5.5), whereas complex 4,
having the N-lp delocalized into the aromatic ring, only works under very acidic conditions,
the advantage of directional N-lp in reducing the overpotential in electrochemical H-H bond
formation is thus established. In addition, the basic and directed N-lp affects 4e-/4H+ reduction
of O2 to H2O and achieves the long sought after O2-tolerant aerobic proton reduction in aqueous
medium.
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a
1
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R1 = Et, R2 = H;
R1 = Me, R2 = H;
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R1 = Me, R2 = CO2Et; 3
R1 = H, R2 = Br;
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R1 = H, R2 = NO2;
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Figure 1: (a) Representative structures of the 2Fe subsite mimics of [FeFe]-H2ases investigated
in this report. 1, 2, 3, 4 & 5 are the XRD-determined structures of 2-ethyl, 2-methyl, 2-methyl-
4-ethylethanoate, 4-bromo and 4-nitro substituted phenyl amine containing ADT bridged 2Fe
subsite mimics, respectively. 6 is the XRD-determined structure of N-Me substituted amine
containing ADT-bridged 2Fe subsite mimic.
Results & Analysis
Synthesis & Characterization: Complexes 1, 2 and 3 were synthesized by following a slightly
modified literature procedure.38,45 Generally, mixing of 2 equivalents of p-formaldehyde with
aromatic amines in CH2Cl2 for 3-4 hours, followed by SOCl2 addition, gives the bis-
chloromethylated aromatic amine; condensation with the Li2(µ-S)2[Fe(CO)3]2 salt in dry THF
at low temperature leads to the target complex. The same protocol is followed for the p-
bromoaniline and 4-nitroaniline, yielding 4 and 5, respectively.38,45-48 To obtain 1, 2 and 3, (1
= o-Et aniline derivative; 2 = o-Methyl; 3 = o-Methyl-p-ethylcarboxylato) the reaction mixture
must be stirred for 48 h (Figure 2, top). Although the bischloromethylated ligand was isolated
in analytically pure form as evident from the 1H-NMR data, the yields (37-48 %) of 1, 2 and 3
are quite low compared to 4 and 5 (72-84 %). The additional steric effect of o-substitution in
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the aromatic ring is the likely source of the lower yield. Nevertheless, 1, 2 & 3 show
characteristic CO vibrations in the 2074 to 1997 cm-1 range in CH3CN similarly to 4 and 5
(Table 1).45,48
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The H NMR spectrum of 1 in CDCl3 shows very sharp signals (Fig. 2, bottom, sky blue)
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suggesting that there is very little, if any, conformational flexibility in solution at RT.
Furthermore, the CH2 resonances are shifted up-field in complex 1 relative to complex 4
(Figure 2, bottom, red) directly implicating an increase in electron density on the -CH2- group
of complex 1 suggestive of the diminishing conjugation of the adjacent bridgehead N-lp with
1
the aromatic ring. Similar H NMR spectra was also obtained for 2 & 3 (Figure S1 & S2).
Hence, the o-substitution provides a framework which renders the N-lp conjugation with the
aromatic ring.
Figure 2: (top) General scheme for synthesis of 2-Et-Hyd (1) and 2-Me-Hyd (2) and 3-methyl-
1
4-aminobenzeneethanoate (3). (bottom) Overlay of H NMR data of complex 1 (in sky blue)
and complex 4 (in red). (inset) Overlay of FTIR data of the same complexes in acetonitrile
solution.
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Table 1: Geometric & spectroscopic comparison of Complex 1, 2, 3, 4, 5 and 6 as
4
5
determined from X-ray crystal structure
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Complex
ν(CO) IR
(cm-1)
Bonds
Average Bond
Length (Å)
Dihedral Angle
(C-C-N-C) (deg)
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2-Et-Hyd (1)
2074, 2037,
1997
Fe-Fe
Fe-S
2.510 (± 0.001)
2.250 (± 0.001)
1.801 (± 0.001)
2.505 (± 0.001)
2.246 (± 0.001)
1.801 (± 0.001)
2.489 (± 0.001)
2.252 (± 0.001)
1.794 (± 0.001)
2.497 (± 0.001)
2.257 (± 0.001)
1.795 (± 0.001)
2.507 (± 0.001)
2.266 (± 0.001)
1.801 (± 0.001)
80.90
80.90
Fe-CO
Fe-Fe
Fe-S
2-Me-Hyd (2)
2073, 2036,
1999
Fe-CO
Fe-Fe
Fe-S
2-Me-4-
Carboxyethanoate-
Hyd (3)
2074, 2038,
1998
78.50
6.01
2.89
Fe-CO
Fe-Fe
Fe-S
4-Br-Hyd (4)
2076, 2040,
2000
Fe-CO
Fe-Fe
Fe-S
4-NO2-Hyd (5)
N-Me-Hyd (6)
2075, 2039,
2000
Fe-CO
2074, 2037,
1996
Fe-Fe
Fe-S
2.492 (± 0.001)
2.259 (± 0.001)
1.791 (± 0.001)
NA
Fe-CO
Figure 3: Structures of complexes 2 and 4 showing on left the orientation of phenyl (in green)
and the -H2C-N-CH2- planes of the ADT (in red), 80.09° and 6.01°, respectively; (right) the
chemical structures of 2 and 4.
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Deep red needle-like x-ray quality single crystals could be obtained by slow evaporation of a
CH2Cl2 solution of these complexes and the X-ray crystallographic structures are displayed in
Figure 1. The Fe-Fe, Fe-S and Fe-CO bond lengths of complex 1, 2 & 3 are in the range reported
for similar complexes in the literature, e.g., complex 4, given in Table 1. Importantly, the
crystal structures show that the dihedral angles defined by the C2C1 carbons of the Aryl ring
and the NC of the ADT ligand in complex 1, 2 and 3 are ~ 80.5  1.5° (Table 1, C-C-N-C) are
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relatively large compared to only 6° in 4 and 2.89° in 5. In sharp contrast to all known structures
of N-aryl substituted ADT where the phenyl plane is almost co-planar with the C-N-C plane
(Figure 3, right), the planes of the aromatic rings in complexes 1, 2, and 3 are rotated (Figure
3, green) substantially away from the C-N-C plane (Figure 3, red) of the ADT. The three C-N-
C angles are 112°, 112° and 113°. Thus, the N centres in complexes 1, 2 and 3 are tetrahedral
suggesting a sp3 hybridization at the N relative to a more planar geometry in other N-Aryl
substituted complexes. A greater basicity is thus expected for the nitrogens of 1, 2 and 3 since
resonance delocalization of the nitrogen lone pair into the aromatic ring is operative in 4 and
5. Hence, the high field shift in  (ppm) values of 1 relative to 4 could easily be due to the loss
of the delocalization of the N-lp in the aromatic ring. The increased electron density on the N-
atom in complexes 1, 2 and 3 is delocalized into the Fe2S2 cluster and results in minor, but
perceptible, shifts in C-O vibrational frequencies in the FTIR in complex 1 (Figure 2 inset, sky
blue) relative to those of complex 4 (Figure 2 inset, red) indicating increased electron density
into the diiron unit resulting in enhanced backbonding in 1. It is also important to note that the
direction of the N-lp of complexes 1, 2 and 3 towards one of the Fe’s is considered key for
efficient HER activity.10,34
Electrochemical Investigations: The cyclic voltammetry (CV) of complex 1 in CH3CN (1
mM solution (scan rate of 20 mV/s, a glassy carbon (GC) working electrode, and 0.1 M
nBu4NClO4 as supporting electrolyte) shows a quasi-reversible redox event at -1.55 V vs Fc+/0,
characteristic of these clusters (Figure 4, blue). 38,49,50,51 Complexes 2 and 3 also show similar
behavior in CH3CN solution (Figure 4, green and red, respectively). These complexes can be
physiadsorbed on edge-plane graphite (EPG) electrodes allowing investigation of these
modified electrodes in aqueous medium – an approach that was successfully applied for
complex 4.38 X-ray photo-electron spectroscopy (XPS) of the EPG electrode bearing complex
1 show ionization energies 710.9 and 724.8 eV corresponding to the ionization of the 2p1/2 and
2p3/2 levels of the Fe(I) canters, respectively. The peak centered at 399.8 eV represents the
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ionization of the tertiary amine nitrogen atoms and the peak at 162.3 eV corresponds to the
ionization of 2p of thiolate sulphur characteristic of these type of complexes (Figure S3).52,53
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In an unbuffered aqueous solution containing 0.1 M KPF6 (measured pH 5.5), a cathodic
process at -50 mV and anodic process at 290 mV vs NHE (Figure 5a, blue) is assigned to the
1/1- redox event (i. e. Fe(I)Fe(I) + e- ⇌ Fe(0)Fe(I) redox process) where the reduced Fe(I)Fe(0)
state has been reported to reduce O2.23 On sweeping the electrode potential further negative,
another redox process associated with a faradaic current is observed at -500 mV vs NHE
(Figure 5b, blue). Very similar observations were also observed for 2 (Figure 5b, green) and 3
(Figure 5a & b, red) as well. These catalytic responses represent proton reduction by the
Fe(0)Fe(0) redox state at pH 5.5 with an onset potential of -500 mV. The onset potential is
defined here by the substantial enhancement of the current with the complex-modified
electrode compared to the bare EPG electrode under the same condition. The large “(+)ve”
shift in potential for these complexes was previously attributed to the greater aqueous solvation
of the anionic species formed on reduction.38 Note that the reduction potentials of complexes
1, 2, 3, and 4 are very similar in both organic and aqueous solutions. Thus, the HER observed
at low overpotentials in the ortho substituted derivatives strongly implicates a role for the N-
lp in these.
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Figure 4: Homogeneous cyclic voltammetry of complex 1-4 in ACN solution (1 mM solution,
scan rate: 20 mV/s, GC working electrode (diameter: 3 mm) and Pt counter electrode, 0.1M
nBu4NClO4 supporting electrolyte).
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Product analysis and catalytic parameters: To establish the nature of the Faradaic process
at -500 mV vs NHE, rotating ring disc electrochemistry (RRDE) was performed to enable in
situ product analysis.54 All the potentials for the heterogeneous electrochemistry is reported vs
NHE as they are recorded using water as electrolytic medium. In an RRDE experiment any H2
produced at the working electrode bearing these complexes diffuse as a result of the
hydrodynamic force created on rotation of the shaft holding the EPG working electrode; the H2
produced is concomitantly detected at a Pt ring (secondary electrode, poised at a constant
potential of 0.5 V) encircling the working EPG electrode, where the H2 is re-oxidized to H+
(Figure 5c).36 This technique is very widely used for various catalytic process associated in
energy transduction processes. Indeed, a RRDE experiment using a EPG electrode modified
with 1 shows detection of H2 at the Pt ring (Figure 5c, purple) with an onset potential of -500
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mV under anaerobic condition. The overpotentials () for HER (determined by E0 = -

0.059xpH – Eonset) is ~180 mV for complex 1 (Figure S4, 2 and 3 are given in the SI) at pH 5.5;
this value is considerably lower than other [FeFe]-H2ase mimics investigated in aqueous
medium. The corresponding Tafel plot of the electrochemical polarography also shows a
similar overpotential (Figure S5). Bulk electrolysis (BE) at -700 mV with glassy carbon counter
electrode results in a steady current (Figure 5b and a comparison with Pt counter in Figure S6
left) indicating that 1 is stable under these conditions and the gas collected is confirmed as H2
using gas chromatography (GC) fitted with a thermal conductivity detector (TCD) (Figure S7).
The overall faradaic yield (which is a direct measure of the charge converted to product)
obtained as a ratio of H2 gas collected to the charge dispensed during BE is > 95 ± 2%. The
TOFmax of the catalysis is determined from the Icat/Ip ratio at room temperature which is a
convenient way of describing the catalytic efficiency for heterogeneous electrocatalysts55,56; it
is calculated to be 13.3 s-1 and 40.1 s-1 at -600 mV and – 800 mV vs NHE, respectively. The
TOFmax was also determined for complexes 2 and 3 and lies within 11.2-15.4 s-1 at -600 mV
and 32.2-49.4 s-1 at -800 mV. The TON of complex 1 for HER is determined from bulk
electrolysis data obtained at -700 mV for 10 h to be ≫ 106 (SI, page S9). Similar results were
also obtained for 2 and 3.
It has been earlier reported that the complexes within this series are quite stable as catalysts for
electrocatalytic HER.38 Previous reports on a [Fe-Fe]-H2ase mimic-SDS composite shows
Icat/Ip value < 3 s-1 at near neutral pH (pH 5.5)40 while the rest of the H2ase mimics show HER
activity at much lower pHs.38,39 Post electrolysis surface analysis of the modified EPG by XPS
show that the characteristic ionization energies remain almost unaltered during bulk
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electrolysis (Figure S8). Hence the HER activity originates from the molecular complex 1.
Complex 1 shows substantial higher activity (Figure 5b, blue line) compared to complex 4
(Figure 5b, purple line) at near neutral pH’s. Note that complex 4 is known to be a facile HER
catalyst in acidic solution with 500 mV overpotentials; however it has almost no activity at pH
5.5 (Figure 5b) where complex 1 is very active.38 A very similar trend is observed for
complexes 2 and 3 for HER (Figure 5b, green and red). The results suggest that the high activity
of 1, 2 and 3 are due to the presence of the well-oriented N-lp with higher pKa adjacent to the
proton binding site. The N-lp may either assist protonation at higher pHs or may facilitate the
formation of the H-H bond by retaining a proton in the vicinity of a Fe-H species formed after
the initial protonation and electron transfer from the cluster.
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a
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Figure 5: (a) Overlay of Cyclic Voltammetry (CV) data of complexes 1, 3, 4 and 6
physiadsorbed on EPG surface in KPF6 solution (pH 5.5) on different potential range (scan rate
50 mV/s). (b) Overlay of linear sweep voltammetry data in KPF6 solution (scan rate: 50 mV/s).
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(c) RRDE data of 1 on EPG electrode in KPF6 solution; red line indicates H2 generation and
the violet line represents the H2 oxidation at the Pt ring, the ring held at a constant potential of
0.5 V. The Pt current is enhanced 25x. (d) Controlled potential bulk electrolysis at − 0.7 V in
aqueous KPF6 solution of complex 1 (blue) with GC counter electrode and blank EPG (red).
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Oxygen Reduction & Aerobic H2 Production: In a previous report, we suggested that
bridgehead N-protonation may lead to the facile O-O bond cleavage resulting in 4e-/4H+
reduction of O2 to benign water, ergo, lower amounts of ROS which is also known as PROS
(partially reduced oxygen species as generated under electrolytic condition through
insufficient transfer of electron to molecular oxygen), produced during competitive O2
reduction by [FeFe]-H2ase models under aerobic conditions. In the N-Aryl bridged models,
where the pKa of the nitrogen is quite low, a smaller amount of ROS from O2 reduction could
only be observed at pH <1.23 The increased basicity of complexes 1, 2 and 3, as evident from
HER at near neutral pH, may allow facile 4e-/4H+ reduction of O2 offering air stable HER
reactivity. The RRDE scan with a complex 1 modified EPG electrode (Figure 6a, blue line)
showed two catalytic responses with -150 mV and – 550 mV onset potential, respectively, in
aerobic electrolytic solution (pH 5.5, KPF6 solution). The 1st catalytic process overlays with
the Fe(I)Fe(I)/Fe(I)Fe(0) process whereas the 2nd catalytic process overlays with the HER
response of 1, 2 and 3 and corresponds to the Fe(I)Fe(0)/Fe(0)Fe(0) process. Previous work by
this group showed that electrocatalytic O2 reduction by synthetic mimics of [FeFe]-H2ase
develops at the Fe(I)Fe(I)/Fe(I)Fe(0) state.23 Accordingly, the 1st catalytic process is the
electrocatalytic oxygen reduction and the 2nd catalytic process will have contributions from
HER and, possibly, ORR catalyzed by the Fe(0)Fe(0) state.
Rotating ring disc electrochemistry (RRDE) is a convenient technique to analyze the individual
products of these two catalytic processes, H+ and/or O2 reduction (Figure 6a, b). If the Pt ring
is held at a fixed potential of 0.5 V, it can oxidize H2, the product of HER, whereas holding it
above 0.7 V permits detection of both H2 and any ROS produced during O2 reduction. Holding
the Pt ring at 0.5 V showed no current for the 1st catalytic process whereas a H2 oxidation
current was observed during the 2nd catalytic process at -0.52 V for 1, 2 and 3 due to H2 formed
from HER (Figure 6b). Holding the potential of the ring at 0.7 V shows no current
corresponding to the ORR at -0.52 V from complexes 1, 2 and 3 indicating no ROS production
during O2 reduction (Figure 6a). On the contrary complexes 4 and 5, without the ortho
substituents, shows a large Pt ring current corresponding to the O2 reduction process at -0.5 V
indicating generation of large amount (~ 50%) of ROS during O2 reduction (Figure 6a).
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Similarly, complex 6 also shows a large amount of ROS (~ 50%) at pH 5 phosphate buffer
when the Pt ring is hold at a potential of 0.7 V (Figure 6c). These results unambiguously
demonstrate that complexes 1, 2 and 3 efficiently reduce O2 by 4e-/4H+ to H2O with high
selectivity, however complexes 4, 5 and 6 produces ~50% H2O2 under the same conditions.
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The ROS produced by complexes 4, 5 and 6 on reaction with O2 in their reduced states results
in the oxidation of the bridging thiolate ligands leading to degradation of the 2Fe cluster.23 The
lack of ROS during O2 reduction by complexes 1, 2 and 3, due to 4e-/4H+ reduction of O2 to
water, raises the possibility of stable HER by these complexes in the presence of O2. To check
the stability and aerobic HER activity, bulk electrolysis using 1 was performed at -700 mV,
under aerobic conditions for hours with GC counter as well Pt counter electrode. The results
do not show any loss in the HER activity at least over few hours (Figure S9). The % of faradaic
yield (FY) of aerobic HER is estimated to be ~ 70 % for complex 1-3. Post electrolysis analysis
of the electrode by XPS shows that the complexes does not degrades much even under aerobic
electrolysis time courses (Fig. S10). We presented the % of FY and % of PROS for complexes
1-5 as a bar diagram in figure 6d. The results suggest that diminishing the amount of PROS for
complex 1-3 enhances the stability as well as FY for HER in the presence of O2. On the other
hand, complexes 4 & 5 show competitive 2e-/2H+ ORR resulting in PROS, leading to faster
degradation of these complexes. The average TOFaerobic can be calculated from aerobic bulk
electrolysis data (Figure 5d) and the values are determined to be 16.2 – 23.1 s-1 for complex 1-
3 in the presence of oxygen. Spectrometric quantification of the ROS generated, by Xylenol
orange assay, was done with the electrolytic solution after BE and the results suggest that only
a small amount of ROS is generated over 10 hours of BE (SI, page S13). Post aerobic bulk
electrolysis XPS analysis of the modified EPG electrode show the retention of the characteristic
ionization peaks of complex 1 on the electrode surface (Figure S10). The ATR-IR spectra of
the modified EPG electrode before and after aerobic bulk electrolysis of complex 1 show no
changes of the CO vibrations (Figure S11). Thus, rotation of the phenyl ring off resonance due
to the ortho substitution not only allows HER at near neutral pH’s with low overpotential but
also affects 4e-/4H+ reduction of O2 allowing oxygen-tolerant HER. These results strongly
corroborate the findings in both [FeFe]- and [NiFe]-H2ases where a sufficient or excess supply
of electrons and a local proton relay allows selective 4e-/4H+ reduction of O2 to benign H2O,
precluding the formation of harmful reactive oxygen species (ROS), affecting oxygen tolerant
HER.
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Figure 6: (a) Overlay of aerobic RRDE responses of complexes 1-5, holding the Pt ring at 0.70
V. Scan rate 50 mV/s and at pH 5.5 (KPF6 solution and 300 rpm). The ‘dashed line’ represents
the corresponding Pt-ring current. The ‘dotted line’ is the LSV of complexes 1 and 2 under
anaerobic condition and with the electrodes held static. (b) Overlay of aerobic RRDE data for
complexes 1-4, holding the Pt ring at 0.5 V. Scan rate 50 mV/s and at pH 5.5 (KPF6 solution).
The Pt ring current is multiplied by a factor of 15 for all complexes in b. On the other hand, in
a, the multiplication factor is 15 for 1, 2 and 3 and 10 for 4 and 5. (c) Overlay of aerobic RRDE
responses of complex 6, holding the Pt ring at 0.70 V. Scan rate 50 mV/s and at pH 5 phosphate
buffer. The Pt ring current is multiplied by a factor of 2 in c. (d) A bar diagram of % PROS
and % FY of HER for complexes 1 to 5.
To corroborate the O2 reduction ability of a working proton-reduction catalyst, the experiment
described in Figure 7 was carried out. Here, the HER activity of complex 1 at pH 5.5 in a
degassed electrolyte solution (Figure 7, cyan) is overwhelmed by ORR current when the
electrolytic solution is saturated with O2 (Figure 7, red). The retained HER activity under these
circumstances is evidenced by the H2 detection current at the Pt ring (Figure 7, dashed cyan
and dashed red for degassed and aerobic, respectively). On removing the dissolved O2 by
purging with Ar gas, the ORR electrocatalytic activity is lost and the original HER activity is
reinstated (Figure 7, yellow and H2 detection dashed yellow) thereby emphasizing the oxygen
tolerant HER catalyzed by these complexes.
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Figure 7: Overlay of RRDE of complex 1 in different conditions. The cyan is RRDE in
anaerobic condition, red one is the RRDE in O2 purge condition and yellow one is the RRDE
in Ar purge condition. During anaerobic (sky blue) and Ar purge (yellow) conditions the Pt
ring was held at 0.5 V. In the case of O2 purge condition (red) Pt ring was held at 0.7 V.
‘Dashed line’ indicates the corresponding Pt current scaled by 25.
Figure 8: Proposed mechanistic scheme for O2 reduction by ortho substituted ADT bridged
[Fe-Fe]-H2ase mimics.23
DFT Calculations
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The experimental results indicate that complexes 1 and 2 show selective 4e-/4H+ reduction of
O2 to H2O at near neutral pH’s whereas complexes 4 and 6 show substantial ROS at these pH’s.
Previous calculations of the reaction of the Fe(I)Fe(0) state with O2 had identified that a
putative Fe(1.5)Fe(1.5)-OOH species is responsible for the hydrolytic release of H2O2.23 The
cleavage of the O-O bond in this Fe(1.5)Fe(1.5)-OOH species results in 4e-/4H+ reduction of
O2 (Figure 8). Thus, the experimental data suggests that the presence of the pendant protonated
amines in 1 and 2 lowers hydrolysis and activates the O-O bond for cleavage. Recent
investigations using iron porphyrins have revealed that the selectivity for 4e-/4H+ reduction of
O2 can be enhanced by a) activation of the O-O bond of a peroxide intermediate and b)
stabilization of the a bound peroxide against hydrolysis via H-bonding.57,58 In the absence of
direct spectroscopic data on such species, geometry-optimized density functional theory
calculations are utilized to provide insight.
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Table 2: Fe-O and O-O Stretching frequency and bond length of different complexes
determined from the DFT optimized structures.
Fe-O (cm-1) O-O (cm-1)
G (Kcal/mol)
PA
Complexes
Fe-O
(Å)
H2O2 Release (Kcal/mmol)
Complex 1
Complex 2
Complex 4
Complex 6
428
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839
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847
1.996
1.997
2.014
2.005
4.67
4.66
0
-21
-20
0
9.03
-16
The optimized structures of a putative Fe(1.5)Fe(1.5)-OOH intermediate for complexes 1, 2, 4
and 6 (Fig. 9) show that the protonated amine is hydrogen-bonded to the proximal oxygen of
the terminal hydroperoxide ligand. The gas phase proton affinities (PA, Fe(I)Fe(I) state) for
complexes 1, 2 and 6 relative to complex 4 are calculated to be -21, -20 and -16 Kcal/mol,
respectively, suggesting that the protonation of the amine in complexes 1 and 2 are likely to
occur at higher pH’s relative to complexes 4 and 6. The calculated Fe-O stretching frequencies
(Fe-O, Table 2, Fig. 9) in the optimized Fe(1.5)Fe(1.5)-OOH structures are higher for
complexes 1 and 2 relative to 4 and 6 (Table 2). The stronger Fe-O bonds in the hydroperoxide
adducts of complexes 1 and 2 are complemented by weaker O-O bonds (O-O, Table 2).
The ΔG of hydrolysis of H2O2 from the Fe(1.5)Fe(1.5)-OOH species is uphill in complexes 1,
2 and 6 relative to complex 4 (Table 2). Thus, the DFT calculations suggest stabilization of the
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labile hydroperoxide (higher ΔG hydrolysis) group and activation of its O-O (weaker O-O
)
bond by hydrogen bonding by the pendant basic group consistent with selective 4e-/4H+
reduction of O2 by complexes 1 and 2 at higher pH (higher PA). While complex 6 shows
stabilization of the bound hydroperoxide by hydrogen bonding, the hydroperoxide ligand is not
activated for O-O cleavage (strong O-O) relative to complex 4. As a result, the activation barrier
for O-O bond cleavage is likely higher in complex 6 which produces substantial H2O2 (2e-/2H+
product) relative to complex 1 and 2.
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Figure 9: DFT optimized hydroperoxide intermediate structures of (a) complex 1, (b) complex
2, (c) complex 4 and (d) complex 6.
Conclusion
In summary, ortho substitution of the Aryl groups in N-Aryl substituted ADT bridging
dithiolate in synthetic mimics of the 2Fe subsite of H2ase induces pyramidality at the
bridgehead nitrogen that appears to foster the ability of these complexes to reduce protons in
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the presence of oxygen at near neutral pH’s with low overpotential. The ortho substitution does
not affect the formal reduction potentials of the complexes (active Fe(0)Fe(0) state at ~ -550
mV vs. NHE), rather it results in rotation of the phenyl ring rescinding resonance delocalization
of the N-lp into the ring. The increased electron density at the well-oriented, pyramidal nitrogen
allows protonation at nearly neutral pH in contrast to un-substituted N-Aryl derivatives which
show HER only at pH < 1. Thus, despite having very similar formal potentials, the presence of
an active local proton relay at higher pH’s (pH 5.5) allows HER at -500 mV vs NHE, much
closer to the thermodynamic H+ + e- ↔ ½ H2 potential at that pH resulting in very low
overpotentials for HER. The same proton relay allows the unprecedented selective 4e-/4H+
reduction of O2 to benign H2O by these synthetic 2Fe models at neutral pH’s when their
predecessor, without the ortho substituent, produce >50% detrimental H2O2. Thus, emulation
of the 2nd sphere interactions, deemed important for O2 tolerance in H2ases, in synthetic
complexes, proffers unprecedented O2 tolerant, bioinspired HER catalysts that work efficiently
at near neutral pH’s. The highly versatile diiron complex, with its manifold of subtle tuning
capabilities has shown in this study that the electrophilicity of O2 is similar to that of the proton,
and the reactive site for proton binding and reduction equally well competes for O2 binding and
reduction.59
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Experimental Section
General procedure: Syntheses of all the compounds were performed either under anaerobic
atmosphere or in an Ar Glove Box from MBRAUN. Complexes 1 and 2 were synthesized using
standard Schlenk techniques. 2-methylaniline, 2-Ethylaniline and the solvents were used fresh
after distillation. Thionyl chloride (SOCl2) was purchased from Spectrochem Pvt. Ltd. (India),
p-formaldehyde and superhydride (Li[HBEt3], 1 M in THF) were from Sigma-Aldrich
chemicals Pvt. Ltd., and anhydrous Na2SO4 and sulfuric acid (98%) were purchased from
Merck Specialities, Pvt. Ltd. (India). Silica (60-120 mesh), for column chromatography, was
purchased from SISCO Pvt. Ltd. (India). Edge Plane Graphite (EPG) discs were purchased
from Pine Instruments. The FT-IR data are measured on the Perkin-Elmer FTIR Frontier
instrument. All the NMR spectra were recorded on Bruker DPX-400 or DPX-500
spectrometers at room temperature. The mass spectra are recorded on an QTOF Micro YA263
instrument. The H2 gas from controlled potentiometry proton reduction was detected on a GC
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instrument of model no. 7890B (G3440B), serial no. CN14333203 fitted with a Thermal
Conductivity Detector (TCD).
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General procedures for synthesis
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Complex 1, [(µ-SCH2)2N(2-Ethylphenyl)CH2S)[Fe(CO)3]2: Complex 1 was synthesized
using standard Schlenk techniques by slightly modifying the synthetic protocol described in
the literature.45 To a solution of 2-Ethylaniline (1.23 mL, 10 mmol), paraformaldehyde (780
mg, 26 mmol) was added; the solution was stirred for 2 days at room temperature. Then SOCl2
was slowly added at 0 °C and the mixture was brought to RT with stirring. After half an hour
the solvent was evaporated completely to dryness to remove all the acid vapour. A 440 mg
sample of (µ-S2)[Fe(CO)3] 2 (1 mmol) was dissolved in 20 mL of dry tetrahydrofuran (THF).
Two mL of LiBEt3H (1 M in THF) was added dropwise to the red solution at −78 °C, with
immediate color change to deep green. Following cooling to -15°C, a THF solution of the
ligand 2-EtC6H4N(CH2)2Cl2 (4 equiv.) was added to the reaction mixture. The colour
immediately changed to red. The reaction mixture was further stirred for 1 h at RT. Vacuum
removal of the solvent produced a dark brown precipitate. A crystalline red solid product was
obtained after column chromatography with a hexane-toluene (9:1) mixture. Single crystals
1
were grown by slow evaporation of a CH2Cl2 solution of the complex. Overall yield: 41%. H
NMR (CDCl3, 400 MHz): δ 7.22 (J = 7.2, 1H, d), 7.12 (2H, m), 6.94 (J = 7.6, 1H, d), 3.72 (s,
4H), 2.61 (J = 7.466, 2H, quar), 1.23 (J = 7.6, 3H, t); IR (CH3CN): 2074 cm−1(s), 2037 cm−1(s)
and 1997 cm−1(s). ESI-MS data (M+H)+ = 497.9379 (m/z); Elemental Analysis (found): C,
38.28; H, 3.81; N, 2.65; (calculated): C, 38.65; H, 3.85; N, 2.82.
Complex 2, [(µ-SCH2)2N(2-Methylphenyl)CH2S)[Fe(CO)3]2 : Complex 2 was synthesized
following exact same procedure described for 1, using 2-Methylaniline instead of 2-
Ethylaniline. The column chromatography was performed using hexane-toluene (9:1) mixture.
Suitable single crystals were obtained by slow evaporation of a CH2Cl2 solution of the complex.
1
Overall yield is 35%. H NMR (CDCl3): δ 7.25 (1H, d), 7.14 (2H, m), 6.94 (1H, d), 3.71 (s,
4H), 2.60 (3H, s); IR (CH3CN): 2073 cm−1 (s), 2038 cm−1 (s), 1999 cm−1 (s). ESI-MS data
(M+H)+ = 483.9214 (m/z); Elemental Analysis (found): C, 37.34; H, 3.79; N, 2.42;
(calculated): C, 37.29; H, 3.55; N, 2.90.
Complex
3,
[(µ-SCH2)2N(2-Methyl-4-carboxyethanoate-phenyl)CH2S)[Fe(CO)3]2:
Complex 3 was synthesized exactly following the same procedures described for 1 and 2, using
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3-methyl-4-aminobenzeneethanoate (details of ligand synthesis are in the SI) . The column
chromatography was performed with hexane-CH2Cl2 (4:1) mixture. Suitable single crystals
were obtained by slow evaporation of a CH2Cl2 solution of the complex. The overall yield is
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49%. H NMR (CDCl3): δ 7.83 (1H, s), 7.81 (1H, s), 6.94 (1H, d), 4.34 (2H, quart), 3.94 (s,
4H), 2.28 (3H, s), 1.38 (3H, t); IR (CH3CN): 2075 cm−1(s), 2037 cm−1(s), 1999 cm−1 (s). ESI-
MS data (M+H)+ = 555.957 (m/z); Elemental Analysis (found): C, 38.67; H, 3.96; N, 2.49;
(calculated): C, 38.94; H, 3.81; N, 2.52.
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Electrochemical Measurements: All the electrochemical experiments were performed using
a CH Instruments (model CHI 710D) Electrochemical Analyzer. Reference electrodes,
Teflon® plate material evaluating cell (ALS Japan) were purchased from CH Instruments. The
rotating ring disk electrochemical (RRDE) set up from Pine Research Instrumentation (E6
series Change Disk tips with AFE6M rotor) was used to obtain the RRDE and LSV data. A Pt
wire was used as a counter electrode. All the measurements were performed against Ag/AgCl
aqueous leak-proof reference electrode (sat’d. KCl). In case of aqueous measurements the
potentials were converted to NHE by adding 200 mV with the value obtained by using Ag/AgCl
(sat’d. KCl). Anaerobic CV experiments were performed in a Glove Box filled with N2 or
within a 5-neck S3 water jacketed electrochemical cell (PINE Instruments) by thoroughly
degassing the whole set up with Ar gas. Particulars of various electrochemical techniques are
given in successive sections.
Homogeneous CV: A glassy carbon electrode and a Pt wire were the working and the counter
electrode, respectively. The measurements were done against aqueous Ag/AgCl reference
electrode (sat’d. KCl) and corrected to Fc+/0. Homogeneous CV experiments were performed
with 1 mM solution of any complex and 0.1M [n-Bu4N][ClO4] in CH3CN in a Glove Box at
RT. The working glassy carbon electrode was polished and washed well before each single
experiment.
Construction of Graphite Electrodes: The edge plane graphite (EPG) electrode was freshly
polished and cleaned with polishing kit before each single experimental use. A 200 μL portion
of a dilute solution (∼1 mM, in CH2Cl2) was uniformly drop casted on the cleaned EPG disc
(5 mm outer diameter) fitted within a shaft. After the CH2Cl2 was evaporated, the surface was
washed with CHCl3 thoroughly and sonicated in ethanol to remove any loosely bound catalyst
on the surface and washed with triple distilled water.
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Heterogeneous CV: The aqueous cyclic voltamogram was recorded with the same instrument
and same software in 0.1 M KPF6 water solution with all of the complexes adsorbed onto the
EPG disc (5 mm outer diameter) held within a shaft (Pine Instruments, AFE6MB). An
anaerobic environment was maintained within a water jacketed electrochemical cell (Pine
Instrument, RRPG138) by purging with Ar gas for 1 h. The aqueous Ag/AgCl reference (Pine
instruments, RREF0021) and Pt counter (AFCTR5) electrodes were attached to the cell through
airtight joints.
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Rotating Disc Electrochemistry: The RDE/RRDE measurements were performed in a CHI
710D or 720D bipotentiostat. An EPG disc was used as the working electrode which was
mounted inside of a Pt ring assembly (Pine Instrument, AFE6RIP), itself mounted at the tip of
a shaft (Pine Instruments, AFE6MB) which in turn was fitted to a MRS rotator. A water
jacketed electrochemical cell (Pine Instrument, RRPG138) is used where the rotor was inserted
through a taper plug assembly (AC01TPA6M) which allows free rotation of the vertical rotor
while maintaining an airtight seal. An aqueous Ag/AgCl reference (Pine instruments,
RREF0021) and Pt counter (AFCTR5) electrodes are attached to the cell through airtight joints.
The EPG fitted shaft is then tethered just before the experiment is begun.
Electrochemical Hydrogen Production: The shaft bearing the EPG electrode and the Pt ring
was vertically inserted into the water jacketed cell, which was then purged with Ar for 50-60
mins. The anaerobic HER experiments were carried out in thoroughly degassed solutions (free-
pump-thaw) to eliminate any competing electrocatalytic O2 reduction. Background scans were
performed under the same conditions with bare EPG electrodes. Linear sweep voltammetry
(LSV) was performed at 300 rpm (rotations per minute) to investigate the electrocatalytic H2
generation. For aerobic HER, the electrolytic solutions were used without any freeze-pump-
thaw manipulations.
Measurement of ROS: The ROS represent the ratio of the catalytic O2 reduction current in
the disc and the H2O2 oxidation current observed in the Pt ring. The H2O2 produced due to
incomplete O2 reduction is radially defused out to the Pt ring where it is oxidized back to O2
producing a oxidation current (i.e. current having an opposite sign of the O2 reduction current).
The collection efficiency (~20-22%) is estimated before every new set of experiments using
the ferrocyanide/ferricyanide redox system.
Controlled Potential Coulometry and Faradaic Yield: The controlled potential bulk
electrolysis experiments were performed in the same water jacketed electrochemical cell that
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is used for the LSV. These experiments were performed in unbuffered degassed as well as
aerated KPF6 solutions with pH 5.5 at −0.7 V vs NHE for a particular time. The shaft bearing
the working electrode was rotated at 600 rpm to disperse H2 bubbles formed on electrolysis.
During electrolysis the H2 generated was collected through an outlet of the cell and collected
using an inverted buret setup. The ratio of the moles of H2 produced in the process (estimated
from the volume of H2 collected) and the charge dispersed is reported as the FY.
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Detection of H2: H2 is generally detected by head space gas analysis by a GC fitted with TCD.
Artero et al. reported an electrochemical method for detecting H2 using rotating ring disc
electrochemistry (RRDE). In an RRDE experiment a Pt ring encircling the working EPG
electrode is held at a constant potential of 0.5 V. At this potential Pt electrode oxidizes H2,
generated at the working electrode and radially diffused outward to the Pt electrode because of
the hydrodynamic current produced by the rotating shaft back to H+ generating an oxidation
current. Thus, a reduction current is observed at the working electrode and an oxidation current
is observed at the Pt electrode. The viability of this method is previously established by this
group by purging pure H2 gas in the vicinity of a working Pt-wire electrode held at constant
potential of 0.5 V; details are described in prior reports.38 The gas evolved during BE was
detected by using GC instrument brought from Agilent Technologies with model no. 7890B
(G3440B), serial no. CN14333203 fitted with TCD. Approximately 200 L gas was collected
by a gas tight syringe and was injected into the inlet of the GC.
X-ray Photoelectron Spectroscopy: The EPG electrode was freshly polished with silicon
carbide paper to get rid of all the contaminations out for each single use. The complexes were
dissolved in DCM (1 mM) and 200 µl solution was drop-casted prior to heterogeneous
electrochemical investigations. Additional complex was removed by repeated washing with
chloroform, ethanol and water subsequently. Then long time anaerobic BE was done in
unbuffered KPF6 solution. After long time BE the EPG surface was washed with deionised
water and ethanol and dried under Ar atmosphere. XPS (XPS, Omicron, model: 1712-62-11)
measurement was done using a high resolution monochromatic Mg−Kα radiation source at
1253.6 eV under 15 kV voltage and 5 mA current condition.
X-ray Structure Determinations: X-ray data were collected either at 150 K on a Bruker
SMART APEX-II diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.710
73 Å) controlled by the APEX 2 (v. 2010.1−2) software package (Complex 1 & 2) or at 100 K
on a Bruker D8VENTURE Microfocus diffractometer equipped with PHOTON II Detector,
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with Mo Kα radiation (λ = 0.710 73 Å), controlled by the APEX3 (v2017.3−0) software
package (Complex 3). Raw data were integrated and corrected for Lorentz and polarization
effects using the Bruker APEX II95/APEX III program suite. Absorption corrections were
performed using SADABS. All the structures were solved by direct methods and were refined
against all data in the reported 2θ ranges by full-matrix least-squares on F2 with the SHELXL
program suite98 using the OLEX 299 interface. Hydrogen atoms at idealized positions were
included during the final refinements of each structure. The OLEX 2 interface was used for
structure visualization, analysis of bond distances and angles, and drawing ORTEP100,101
plots
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ASSOCIATED CONTENT
Corresponding Author
icad@iacs.res.in
Marcetta@chem.tamu.edu
Notes
The authors declare no competing financial interest. All the potentials mentioned in this
manuscript are recorded with Ag/AgCl (sat. KCl) references and presented referenced against
Fc+/0 for nonaqueous and against NHE for aqueous electrochemistry by addition of 520 mV
and 200 mV, respectively.
ACKNOWLEDGMENT
The research was funded by MNRE 103/180/2010-NT and CEFIPRA project 5405-1. E.A.
acknowledges CSIR-SRF and S.D. acknowledges IACS Institute Fellow Scheme. The XPS
facility in IACS is funded by DST Nano Mission. MYD acknowledges the USA National
Science Foundation for financial support (CHE-1266097 and CHE-1665258), and the Robert
A. Welch Foundation (A-0924).
Supporting Information
Additional experimental details, spectroscopic data, electrochemical data and the crystal
structures are included as supporting information. This material is available free of charge via the
Internet at http://pubs.acs.org.
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