Assessment of two cobalt(II) complexes with pincer ligands for the electrocatalytic hydrogen evolution reaction. A comparison of the SNS vs ONS coordination

Article abstract of DOI:10.1016/j.ica.2020.119497

Bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide (pdcta) and N-(2,5- dimethoxyphenyl)-6-[(2,5-dimethoxyphenyl)carbamothioyl]pyridine-2-carboxamide (pcta) were synthesized from a one pot mixture as pincer ligands. Crystals of pcta, grown from CDCl<

Full text of DOI:10.1016/j.ica.2020.119497

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Research paper
Assessment of two cobalt(II) complexes with pincer ligands for the electroca-
talytic hydrogen evolution reaction. A comparison of the SNS vs ONS coordi-
nation
Mark A.W. Lawrence, Willem Mulder, Michael J. Celestine, Colin D.
McMillen, Alvin A. Holder
PII:
S0020-1693(19)31588-9
https://doi.org/10.1016/j.ica.2020.119497
ICA 119497
DOI:
Reference:
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Inorganica Chimica Acta
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Revised Date:
Accepted Date:
17 October 2019
28 January 2020
1 February 2020
Please cite this article as: M.A.W. Lawrence, W. Mulder, M.J. Celestine, C.D. McMillen, A.A. Holder, Assessment
of two cobalt(II) complexes with pincer ligands for the electrocatalytic hydrogen evolution reaction. A comparison
of the SNS vs ONS coordination, Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.119497
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1
Assessment of two cobalt(II) complexes with pincer ligands for the electrocatalytic
hydrogen evolution reaction. A comparison of the SNS vs ONS coordination
1
*
1
2
3
Mark A.W. Lawrence, Willem Mulder, Michael J. Celestine, Colin D. McMillen, and Alvin
A. Holder2
1
Department of Chemistry, The University of the West Indies Mona, Kingston 7, Jamaica
Department of Chemistry and Biochemistry, Old Dominion University, 4541 Hampton Boulevard,
2
Norfolk, VA 23529, U.S.A.
3
Department of Chemistry, Clemson University, 379 Hunter Laboratories Clemson, SC 29634, U.S.A.
E-mail: mark.lawrence02@uwimona.edu.jm
Abstract
Bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide
(pdcta)
and
N-(2,5-
dimethoxyphenyl)-6-[(2,5- dimethoxyphenyl)carbamothioyl]pyridine-2-carboxamide (pcta) were
synthesized from a one pot mixture as pincer ligands. Crystals of pcta, grown from CDCl were
3
structurally characterized, and found to crystallize in the space group P2 /c. The Co(II) complexes
1
3
3
[
Co(II)( -SNS-pdcta)(CH CO )(H O)] (1) and [Co(II)( -ONS-pcta)(CH CO )(H O)]•H O (2)
3
2
2
3
2
2
2
were prepared from the reaction of pdcta and pcta and Co(CH CO ) •4H O in ethanol at reflux
3
2 2
2
under argon atmosphere. The identities of 1 and 2 were confirmed from their elemental analyses,
ESI MS and a series of spectroscopic measurements. Both complexes displayed electrocatalytic
hydrogen evolution with p-toluene sulfonic acid monohydrate in the presence and absence of bpy
or PPh co-ligands. The hydrogen evolution occurred at moderate overpotentials (605-780) mV,
3
with 1 giving better Faradaic efficiencies than 2 under all the conditions explored herein.
Thermodynamic estimates, based on the reduction potentials, suggests that the homolytic pathway
III
for the production of hydrogen from the Co -hydride species generated in the presence of the
proton source, is more favourable than the heterolytic pathway in CH CN.
3
Keywords: cobalt(II); SNS/SNO pincer ligand; electrochemistry; hydrogen; co-ligands
1
Introduction
An ever-increasing consumption of fossil fuels has led to the global energy crisis and
environmental deterioration, through the effects of global warming [1, 2]. Batteries and electrical
energy is considered as one of the most promising alternatives for replacing fossil fuels energy,
but current (popular) methods of electricity generation accounts for ca 25% of global greenhouse

2
gas emissions [3]. Efficient solar‐powered conversion systems exploiting inexpensive and robust
catalytic materials for the photo‐ and photo‐electro‐catalytic water splitting, photovoltaic cells,
fuel cells, and usage of waste products (such as CO ) as chemical fuels are appealing solutions [4].
2
Thus there is a growing need for the direct generation of molecular hydrogen from water as a result
of it being a convenient and clean energy vector, while utilising renewable resources, for example,
water and sun light [5-10].
The hydrogen evolution reaction (HER) has been actively studied as the globe seeks viable long-
term alternatives to the use of fossil fuels. Hydrogen [6, 7, 9, 10] is a clean, renewable and high-
energy-density source, and its production through the reduction of water appears to be a convenient
solution for long-term storage and accessibility. Hydrogenases are known to catalyze the
(reversible) reduction of protons into molecular hydrogen efficiently in nature. There is a search
for catalysts that can facilitate the reduction of protons to hydrogen, while using cheap and Earth
abundant metals such as Fe, Ni, Co, and Zn, and this represents an area of current interest [11-29].
Such catalysts include cobaloximes [30, 31], cobalt diimine–dioximes [17, 24], and cobalt
dithiolenes [19] to name a few (Figure 1).
Fihri et al. [32] reported the first cobaloxime-based photocatalytic system for hydrogen
2
+
evolution. The complex [(bpy) Ru(L-pyr)Co(dmgBF ) (OH )]
(where L-pyr = (4-
2
2 2
2
pyridine)oxazolo[4,5-f]phenanthroline and dmgBF = difluoroboryldimethylglyoximato) harvests
2
2
+
photons from the [Ru(bpy) ] moiety and drives electrons to the [Co(dmgBF ) ] moiety, which is
3
2 2
a hydrogen evolving catalyst. This complex was able to perform around 105 turnovers over 15
hours [32]. The issue with this complex is that it requires UV irradiation and is relatively inactive
under visible irradiation (λ > 380 nm). Fihri et al. [14] also synthesized two more complexes to
get past the issue of the first complex. The two complexes, [(dmphen) Ru(L-
2
pyr)Co(dmgBF ) (OH )](PF ) and [(ppy) Ir(L-pyr)Co(dmgBF ) (OH )]PF , were comparable or
2
2
2
6 2
2
2 2
2
6
better than platinum-based systems [14]. The complexes were able to produce hydrogen under
visible irradiation with a higher number of turnovers, up to 273 turnovers.
Cropek et al. [33] reported the effects of ligand structure on the photocatalytic production
of hydrogen in acidic media. The complexes used were three mixed metal complexes with a Ru(II)
moiety as a photosensitizer and a Co(II) moiety as a catalyst. Photochemical studies were done
with the complexes in acetonitrile with p-cyanoanilinium tetrafluoroborate as the proton source

3
and either triethylamine or triethanolamine as the sacrificial electron donor. From the
photocatalytic studies, they concluded that the terminal ligand affects the catalyst and its
performance, and the sacrificial electron donor affects the amount of hydrogen produced, and the
lifespan of the catalyst. One of the complexes, [Ru(pbt) (L-pyr)Co(dmgBF ) (OH )](PF ) (where
2
2 2
2
6 2
pbt = 2-(2′-pyridyl)benzothiazole), performed better than the other complexes, as this complex
was able to produce hydrogen over a 42 hour period with triethylamine as the sacrificial electron
donor [33].
Others have reported on hydrogen production from acidic media with cobalt(III) complexes
as electrocatalysts. Peng et al. [34] reported on a water soluble complex, [(phen) Co(CN) ]NO ,
2
2
3
producing hydrogen from both acetic acid and an aqueous buffer. In acetic acid, the complex could
produce hydrogen through electro-catalysis with a turnover frequency of 0.0154 mol of H per
2
mole of catalyst. In a neutral aqueous buffer, the turnover frequency was 0.25 mol of H per mole
2
of catalyst. The most plausible mechanism for the production of hydrogen during the catalysis
involves the formation of a Co(I) species which reacts with a proton source to form a Co(III)-
hydride via oxidative addition, from which hydrogen may be produced via a homolytic or
heterolytic pathway [35-37]. The formation and stability of the Co(I) intermediate is thus an
important consideration in the development of a suitable (pre-)catalyst for the hydrogen evolution
reaction (HER). To this end, various researchers have studied the spectral characteristics of Co(I)
species generated via photochemical [38], chemical [39-42], and electrochemical [42-45]
reduction methods. Stubbert et al. [22] reported that the ligand may also directly aid in the
electrocatalytic process of a cobalt bis(iminopyridine) complex, and it was also noted that certain
ligands, such as 1,5-diphenylcarbazone, may actually inhibit electrocatalytic behavior, if they can
be easily protonated and cause solvolysis [46]. Hickey et al. [45] through a series of N,N-bidentate
ligands, also demonstrated that if the ancillary ligand(s) is (are) not bulky enough to minimize
disproportionation, the electrocatalytic process can be suppressed and the formation of cobalt
metal on the electrode surface prevails.

4
Cl
H O
2
O N N O
Co H
F
O N N O
Co
F
F
H
B
B
O N N O
F
O N N O
N
OH2
cobaloximes
2+
+
Br
N
NH
N
N
N
N
Co
Co
NH
N
Br
[
14]diene-N4
Br
[14]tetraene-N4
Ph P
3
F
F
O N
O N
N
N
O N
O N
N
N
B
Co
H
Co
Br
imine/oxime
phosphine coordinated
-
S
S
S
Co
S
dithiolene
Figure 1. Structural examples of cobalt complexes used in HER.
For many years, pincer complexes of various designs have been applied with varying success in
catalytic and electrocatalytic HER [47-49]. Like pincer ligands, thioamide moieties have attracted
attention due to their ability to tautomerize upon deprotonation, and coordinate to a variety of
transition metals [50-56]. Palladium(II) complexes bearing thioamide pincer ligands of the type
2
,6-bis-thioamidophenyl and 2,6-bis-thioamidopyridine have been reported as active catalysts for
the Mizoroki-Heck cross-coupling reaction [52], and bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-
dicarbothioamide (pdcta) and 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-
pyridinecarbothioamide (pbcta) have shown potential in HER [57]. Our interest in the electron
transfer processes involving transition metal complexes [51, 58-64], as well as the electrocatalytic
reduction of protons and small molecules, such CO [41, 42, 65, 66], has led to the development
2
of pincer ligands, bearing sulfur (from thioamides) and nitrogen coordination sites of the types 3-
SNS and SNN [51, 57]. In this report, we examine the electrocatalytic HER and the effect of
3
substituting a sulfur with and oxygen atom on the same ligand back bone, via a  -SNS and ONS

5
pincer ligand system; and also the effect of co-ligands 2,2’-bipyridine (bpy) and
triphenylphosphine (PPh ) on the voltammetric response of the cobalt complexes in situ, on the
3
path to efficient molecular systems for HER.
2
Experimental
All reagents were purchased from commercial sources (BDH and Sigma-Aldrich) and solvents
were purchased as HPLC grade and used without further purification. Absorbance measurements
were performed on a HP 8453A diode array spectrophotometer. IR spectra were recorded as neat
samples using an ATIR accessory on a Bruker Vector 22 FTIR spectrophotometer. NMR spectra
were recorded on a Bruker ACE 500 MHz FT spectrometer and the resonances are reported in 
units downfield from TMS as an internal standard or to the residual protons in the incompletely
deuterated solvent.
All high resolution ESI MS spectra were acquired via positive electrospray ionization on a Bruker
1
2 Tesla APEX –Qe FTICR-MS with and Apollo II ion source at the College of Sciences Major
Instrument Cluster (COSMIC), Old Dominion University, 143 Oceanography & Physical Sciences
Building Norfolk, VA 23529, U.S.A. Samples were dissolved in acetonitrile or 1:1
dichloromethane/acetonitrile, followed by direct injection using a syringe pump with a flow rate
–
1
of 2 L s . The data was processed using Bruker Daltonics Data Analysis Version 3.4.
All electrochemical experiments were performed on a DigiIvy DY2312 potentiostat, under an
argon atmosphere at room temperature. A standard three electrode cell setup was employed, using
a glassy carbon working electrode (diameter = 3 mm), a silver wire quasi-reference electrode and
a platinum wire as an auxiliary electrode. Ferrocene, which was used as an internal reference
showed a reversible wave at +0.48 V in CH CN. The ionic strength was maintained at 0.1 M
3
[nBu N]PF . The solvents used in the electrochemical experiments were dried using standard
4
6
procedures [67].
Controlled-potential electrolysis (CPE) measurements for the production of hydrogen were
conducted at –1.10 and –1.25 V (vs Ag), on unstirred solutions for 20 min in a sealed two-
chambered H-cell separated by a fine frit, where one chamber held the working and reference

6
n
electrodes in 10 mL of 0.3 mM of complex in 0.1 M [ BuN ]PF (supporting electrolyte) with 11
4
6
mM p-toluenesulfonic acid monohydrate (p-TSOH) (as a proton source) and the second chamber
held the auxiliary electrode in 5 mL of the solvent with the supporting electrolyte. A glassy carbon
plate (contact area ~2 cm × 1 cm × 3 mm) and Pt wire were used as the working and auxiliary
electrodes, respectively, with Ag wire as the reference electrode. The solution was purged with Ar
for 20 minutes and then sealed under an Ar atmosphere before the start of each electrolysis
experiment. The amount of H evolved was calculated based on the charge accumulated from the
2
catalyst solution during CPE minus the charge from the same solution without any catalyst.[68]
After electrolysis, 50 L of the headspace was injected into the gas chromatograph (GC). GC
experiments were performed on an HP (Agilent) 5890 series II instrument with an ECD detector
o
-1
(
40 C isothermal; 1.0 mL min flow rate; He carrier gas) [69]. The remainder of the gas in the
headspace was analyzed with a PGas-22 H gas detector.
2
2
.1 Semi-Empirical and Density Functional Theory calculations
Density functional theory and semi-empirical calculations were carried out using the GAMESS
†
software package [70, 71]. The structures were optimized in the gas phase as indicated by the
absence of imaginary frequencies in the Hessian, using PM3, and B3LYP methods. The Pople
double-eta with the common polarization and diffuse options basis set, 6-31G(d,p) was employed
in all DFT calculations. The GAMESS input files were generated using Avogadro [72] version
‡
1
.2.0 , and the output file viewed using the same.
2
.2 Preparation of bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide (pdcta)
and N-(2,5- dimethoxyphenyl)-6-[(2,5- dimethoxyphenyl)carbamothioyl]pyridine-2-
carboxamide (pcta)
High yield of bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide (pdcta) was prepared
3
following the reported procedure [51, 57]. In a 100 cm round bottom flask, bis-N-(2,5-
†
GAMESS: an open-source general ab initio quantum chemistry package.
https://www.msg.chem.iastate.edu/gamess/index.html
‡
Avogadro: an open-source molecular builder and visualization tool. Version 1.2.0. http://avogadro.cc/

7
dimethoxyphenyl) pyridine-2,6-dicarboxamide (0.25 g, 0.57 mmol) and 2,4-bis(4-
methoxyphenyl)-1,3,2,4-dithiadiphosphetane 2,4-disulfide (Lawesson’s reagent (LR); 0.30 g, 0.74
3
mmol) were suspended in toluene (35 cm ) and refluxed under argon for 7 h. This mixture was
3
transferred to a 100 cm beaker and evaporated to dryness under a strong stream of air. This crude
material was purified by column chromatography (silica) using dichloromethane/hexanes (3:2),
followed by ethyl acetate. The dichloromethane/hexanes solvent mixture gave a yellow–orange
crystalline product, bis-N-(2,5-dimethoxyphenyl)pyridine-2,6-dicarbothioamide, pdcta (R = 0.24
F
o
in dichloromethane/hexanes (3:2); yield = 74 mg (28%), m.p. = 163–165 C, and the back fraction
(
from ethyl acetate) gave an orange-yellow crystalline product N-(2,5- dimethoxyphenyl)-6-[(2,5-
dimethoxyphenyl)carbamothioyl]pyridine-2-carboxamide, pcta (RF 0.05 in
dichloromethane/hexanes (3:2); 119 mg, 46%), mp 140–142 C. Elemental analysis: Found: C,
0.85; H, 5.10; N, 9.31. Calc. for C H N O S: C, 60.91; H, 5.11; N, 9.27%.  (CDCl ) 3.79
=
o
6
2
3
23
3
5
H
3
(3H, s), 3.83 (3H, s), 3.85 (6H, s), 6.67 (1H, d), 6.79 (1H, d), 6.86 (1H, d), 6.94 (1H, d), 8.09 (1H,
t), 8.30 (1H, s), 8.44 (1H, d), 8.95 (1H, d), 9.26 (1H, s), 10.27 (1H, br s, NH), 12.30 (1H br s, NH).
C (CDCl ) 55.9, 56.0, 56.1, 56.5, 106.5, 107.1, 109.5, 110.9, 111.1, 111.8, 124.6, 127.6, 127.8,
3
1
3
28.9, 139.1, 143.1, 144.5, 147.9, 151.7, 153.3, 154.0, 161.3, 185.5. Selected max (neat)/cm–1
366 (NH), 3280 (NH), 1689 (amide I), 1599, 1530, 1491, 1443, (pyridyl and aryl), 1220.
2
.3 Preparation of [Co(pdcta)(CH CO )(H O)] (1)
3
2
2
In a pressure tube, pdcta (30.1 mg, 64 mol), and Co(CH CO ) •4H O (16.1 mg, 65 mol) were
3
2 2
2
suspended in EtOH (5 mL). The mixture was sparged with Ar and refluxed for 2.5 h, following
which it was concentrated under a strong stream of air. The reaction mixture was filtered; then the
residue was washed with cold Et O and air dried. Yield = 31 mg (80%). High resolution ESI MS
2
III
+
(
positive mode) m/z = 604.60336 for the [Co (pdcta)(CH CO )(H O)] and 605.12019 for the
3 2 2
II
+
[
Co (pdcta)(CH CO )(H O)]-H species. Elemental analysis: Found: C, 49.27; H, 4.14; N, 6.72.
3
2
2
–
1
–1
Calc. for C H N O S : C, 49.67; H, 4.50; N, 6.95%.  (DMSO)/nm (/M cm ) 283 (19480),
2
5
27
3
7
2
max
–
1
–1
3
4
89 (8100) and 493 sh (3500); (CH CN)/nm (/M cm ) 198 (14250), 281 (4800), 392 (2300),
3
–
1
88 sh (1000). Selected max (KBr)/cm 3549, 3475 (OH), 3239 (NH), 1619 (C=N).

8
2
.4 Preparation of [Co(pcta)(CH CO )(H O)]•H O (2)
3 2 2 2
In a pressure tube, pcta (30.0 mg, 66 mol), and Co(CH CO ) •4H O (16.5 mg, 66 mol) were
3
2 2
2
suspended in EtOH (5 mL). The mixture was sparged with Ar and refluxed for 2.5 h, following
which it was concentrated under a strong stream of air. The reaction mixture was filtered; then the
residue was washed with cold Et O and air dried. Yield = 38 mg) (95%). High-resolution ESI MS
2
II
+
(
positive mode) m/z = 589.10577 for the [Co (pcta)(CH CO )(H O)]-H and 588.04567 for the
3 2 2
III
+
[
Co (pcta)(CH CO )(H O)] species. Elemental analysis: Found: C, 49.20; H, 4.35; N, 6.92. Calc.
3
2
2
–
1
–1
for C H N O S: C, 49.51; H, 4.82; N, 6.93%.  (DMSO)/nm (/M cm ) 270 (12700), 400
2
5
29
3
9
max
–
1
–1
and 500 sh (3500 and 1400); (CH CN)/nm (/M cm ) 201 (17100), 264 (4490), 319 and 478 sh
3
–
1
(
2514 and 479). Selected max (KBr)/cm 3443 br (OH), 3380 (NH), 1685 (amide I), 1619 (C=N).
2
.5 Voltammograms in the presence of co-ligands PPh and bpy
3
Stoichiometric amounts of triphenyl phosphine or 2,2’-bipyridine were added to 1.4 mM
acetonitrile solutions of 1 or 2 in the supporting electrolyte. The solution was stirred and sparged
with Ar for 10-15 minutes following which the voltammograms were acquired.
2
.6 X-ray crystallography
A crystal of pcta grown from CDCl , of approximate dimensions 0.06 × 0.16 × 0.21 mm, was used
3
for the X-ray crystallographic analysis. A Bruker D8 Venture diffractometer with Mo Kα radiation
(IµS, Incoatec, λ = 0.71073 Å) and a Photon 100 detector were used for the data collection. The
structure was solved (intrinsic phasing, SHELXT) and refined (full-matrix least squares on F2,
SHELXL) using the Bruker SHELXTL [73] Software Package. The space group selection of P2 /c
1
was made based on the systematic absences. All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms attached to carbon atoms were refined in geometrically optimized positions using
riding models. The hydrogen atoms attached to nitrogen atoms were identified from the difference
electron density map and fully refined. Details of data collection and structural solution for pcta
are provided in the supporting crystallographic files (CCDC deposition number 1957918), and
Table 1.

9
3
3
3
Results and Discussion
.1 Synthesis and characterization
.1.1 Ligands pcta and pdcta
The synthesis of the asymmetric pincer ligand pcta was affected by modifying the reaction time
and ratio of Lawessons’s reagent in the protocol used to prepare pdcta (Scheme 1). Both ligands
can be prepared from a one-pot mixture (see experimental section) as they are each other’s by-
product in the thionation employing Lawessons’s reagent. Consequently, the yield of pdcta is
1
13
determined by the ratio of Lawessons’s reagent and reaction time. In the H and C NMR spectra
of pcta, the amide and thioamide protons and carbonyl groups are clearly resolved (Fig. S1), and
1
this can be used to assess the quality of the purification process. In the H NMR spectrum measured
in CDCl , the amide proton of pcta is observed as a singlet at 10.27 ppm, whereas the thioamide
3
proton in this molecule is observed at 12.30 ppm. In pdcta the thioamide protons are observed as
1
3
a singlet (integrating for 2 H) at 12.84 ppm. In the C NMR spectra, the amide and thioamide of
pcta are observed at 161.3 and 185.5, respectively, whereas in pdcta, the thioamide is observed at
1
84.3 ppm. The asymmetry of the molecule also resulted in the splitting of the methoxy protons.
The formulation of pcta was further confirmed from the elemental analysis and X-ray
crystallography (Table 1). Crystals of pcta grown from CDCl were characterized by single crystal
3
X-ray diffraction, resulting in a well-refined structure in space group P2 /c (Fig. 2). The bond
1
lengths and angles are within the normal ranges, and are very similar to the analogous pdcta [51].
The two aryl rings are non-coplanar with one another, as the ring containing the thioamide moiety
is twisted out of the plane of the pyridyl and aryl ring with amide moiety by 31.83°. Weak C-H···O
intermolecular interactions from the C14 and C22 methyl groups (C14-H14B···O3 = 3.446(3) Å,
1
75.9° and C22-H22B···O1 = 3.317(3) Å, 158.4°) may contribute to the conformation of the
molecule, and provide packing interactions along the a- and b-axes. Stacking interactions between
the pi systems of neighbouring molecules (shortest C···C distance = 3.305 Å) provide additional
stability. Intramolecular N-H···O and N-H···N interactions (D-H···A = 2.575(2)-2.705(2) Å) may
also contribute to the conformation of the molecule.

1
0
O
O
X
S
N
N
O
NH
O
HN
O
O
O
NH
O
HN
O
O
(i)
X = S : pdcta
X = O : pcta
Scheme 1. Preparation of pyridylcarbo(thio)amides. Reagents and conditions: (i) Lawesson’s
reagent, toluene, reflux.
Table 1. Crystal data and structure refinement for pcta.
Identification code
Chemical formula
Formula weight
Temperature
pcta
C H N O S
2
3
23
3
5
453.50 g/mol
173(2) K
Wavelength
0.71073 Å
Crystal size
0.06 × 0.16 × 0.21 mm
Crystal system, space group
Unit cell dimensions
Monoclinic, P2 /c
1
a = 17.0733(17) Å
b = 8.5106(8) Å
c = 16.8262(15) Å
β = 117.271(3)°
2173.2(4)
4, 1.386
0.190
952
2.68 to 26.39°
–21/21, –10/10, –21/20
50314/4448 [R(int) = 0.0716]
1.0000 and 0.9277
4448/2/301
Volume (Å3)
Z, Calculated density (g cm )
-
3
-
1
Absorption coefficient (mm )
F(000)

range for data collection (o)
Range of h, k, l
Reflections collected/unique
Max. and min. transmission
Data/restraints/parameters
2
Goodness-of-fit on F
1.048
Final R indices [3484 data; I>2σ(I)]
R indices (all data)
Largest diff. peak and hole (eÅ )
CCDC no.
R = 0.0471, wR = 0.1107
1
2
R = 0.0661, wR = 0.1199
1
2
-3
0.386 and -0.430
1957918

1
1
Figure 2. Structure of pcta, shown as 50 % probability ellipsoids. Selected bond lengths (Å):
C6-S1 = 1.659(2), C6-N2 = 1.343(3), C7-N2 = 1.408(3), C1-N1 = 1.343(3), C5-N1 = 1.344(2),
C15-N3 = 1.352(3), C16-N3 = 1.402(3), C15-O3 = 1.236(2).
3
.1.2 Complexes 1 and 2
The synthesis of cobalt(II) complexes 1 and 2, was affected following the general procedure
illustrated in Scheme 2, and the species were isolated as dark brown solids in yields of 80 and
9
5%, respectively. The spectroscopic and elemental analyses, along with the ESI-MS are
consistent with the proposed formulations. In the IR spectra, a broad band in the region associated
with water was observed in the spectra of 1 and 2, indicating the presence of water in the structure
of the solid. In 2, the reduction in the wavenumber of the amide I band is suggestive of a pseudo-
coordination between the metal centre and the oxygen of the amide moiety. In the paramagnetic
1H NMR spectra of both 1 and 2 (Fig. S2 and S3), the pyridyl ring is deshielded by the
paramagnetic envelope of the Co(II), however, the aryl rings which are further away from the
influence of the Co(II) are less affected and are observed in the diamagnetic region. In both
complexes NH proton is observed above 10 ppm, which is consistent with the formulation
proposed in scheme 2.
The presence of exchangeable solvent in the coordination sphere suggests there is most likely an
exchange of the water ligand with the solvent upon dissolution. This can also be inferred from the
effect of the solvent on the molar absorptivity values as well as its effect on the profile of the
electronic spectra of the complexes (Figs. S5 and S6). The UV-visible spectra are dominated by

1
2
low energy transitions in the visible region corresponding to Co(II) d-d transitions which merges
into MLCT and -* transitions of the ligands in the UV regions in the spectra of both species.
The features of the electronic spectra are consistent with a low spin Co(II) complex [33].
O
O
H
O
NH
NH
N
O
N
O
N
N
Co(OAc)
2
.
2
4H O
X
S
X
S
EtOH,

Co
O
O
O
O
O
OH2
X = O : pcta
X = S : pdcta
X = S :
X = O :
1
2
80%
95%
Scheme 2. Preparation of the Co pincer complexes.
.2 Voltammetric properties of pcta and complexes 1 and 2
3
Density functional theory calculations analysing the molecular orbitals of pcta (Fig. S7) revealed
that the HOMO and LUMO+1 are centred on the amidic moiety whereas the LUMO and HOMO-1
are centred on the thioamide moiety. Further assessment of the iminothiol tautomerized molecule
of pcta (which is the protonated form of the coordinated molecule) indicated that the HOMO also
resided on the amide portion of the molecule, the LUMO on the iminothiol and the LUMO+1
switches also back to the amide moiety. From the calculations, the features of the redox chemistry
of either the thioamide or iminothiol tautomer is thus expected to be similar, with differences
occurring in the redox potentials. Similar assessment of pdcta revealed that the thioamide moiety
is dominant in its redox chemistry [57]. In the cyclic and square wave voltammograms of pdcta,
the characteristic feature was indeed the reduction and oxidation of the thioamide moiety [57]. The
reduction followed the pattern of: (i) an irreversible reduction to yield the thienyl radical anion,
followed by (ii) a second irreversible reduction to produce the thienyl dianion; and with (iii)
subsequent reductions occurring on the aromatic rings [57]. Similarly, the initial phase of the
voltammograms of pcta (Fig. S8) is characterized by the reduction of the thiomaide moiety which
is more accessible than the amide moiety however, the on the anodic side the amide oxidation
partially overlaps that of the thioamide. Similar to another pincer ligand pbcta (where pbcta = 6-
(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-pyridinecarbothioamide) [57], the
asymmetry of pcta renders the first reduction of the thioamide (at E = –1.16 V vs Ag) reversible,
½
and the second reduction quasi-reversible (at E = –1.57 V), followed by two closely spaced
½
reductions between E = –1.90 to –2.05 V assigned to the amide moiety. The oxidations are
pc

1
3
observed at E = +1.62 (ca 4 electrons) and +1.8 V. The multi-electron oxidation was also
pa
observed for the pdcta molecule.
The voltammetric response of 1 and 2 is summarized in Scheme 3. In the voltammograms of the
cobalt(II) complexes, an adsorption wave is observed at ca –0.5 V (which disappears on repetitive
cycling, Fig. S9). The coordination of pcta to Co(II) to produce 2 removes the reduction wave (and
also the oxidation wave) associated with the thioamide moiety, and a new quasi-reversible (Ipa/Ipc
=
0.61) wave is observed at E = –1.22 V which is ca 60 mV more negative than the first reduction
½
2
+
+
wave of the ligand (Fig. 3a and b). This new reduction is assigned to the Co → Co process in
+
0
complex 2. In the cyclic and square wave voltammograms, the Co → Co reduction is observed
as a shoulder at –1.50 V (vs Ag) on the ligand reduction occurring at E = –1.68 V. The oxidation
pc
2
+
3+
of Co → Co is observed at E = +1.30 V as a shoulder on the ligand oxidation at E = +1.47
pa
pa
V (Fig. 3 and Fig. S10). DFT calculations suggests that the HOMO and LUMO and LUMO+1 of
are largely localized the metal centre, with some delocalization across the ligand (Fig. 4), and
2
are thus consistent with the assignments. The delocalization of the orbitals may also be a
contributing factor in the quasi-reversibility of the metal reduction observed in the
voltammograms. In acetonitrile it is expected that the aqua ligand will be readily substituted by
the solvent, and it is also known that the coordination number of Co(I) changes to five, from
coordination number six of the Co(II) precursor, which naturally results in the exclusion of a ligand
[44]. Furthermore the Co(I) metal centre has a greater affinity for nitrogen based ligands compared
to aqua and halogen ligands [16, 44, 74], and it is thus likely that the acetate ligand is excluded.
This might be the driving force behind the quasi-reversibility.
The reaction of pdcta with Co(II) to produce 1, resulted in the suppression the thioamide reduction
and produced a new quasi-reversible (I /I = 0.75) reduction wave at E = –1.08 V. This reduction
pa pc
½
2
+
+
potential has been ascribed to the Co → Co reduction in complex 1. The second reduction at E
½
+
0
=
–1.74 V is tentatively assigned to the Co → Co based on its quasi-reversible nature as well as
the 1:1 cathodic current when compared to the first reduction wave (Fig. S8). On the oxidation
2
+
3+
side, the Co → Co is observed at E = +1.05 V, also as a shoulder on a second oxidation at E
pa
pa
=
+1.33 V, which is ligand-based. Like 2, DFT calculations suggests that the HOMO, LUMO, and
LUMO+1 may also involve the ligand orbitals. The voltammetric response of the ligands when
coordinated to the cobalt metal is similar in nature to those reported for the Pd(II) systems

1
4
containing pbcta and pdcta [51, 57], and consequently these effects induced by the metal ion on
the reduction potentials of the ligands are in accord with the proposed structure in which the ligand
–
exists in its anionic (tautomerized, N=C–S ) form. In both complexes 1 and 2, scan rate variations
around Epc,1 and Epc,2 revealed that the peak currents increase linearly with the square root of the
scan rate, which is consistent with electrochemical processes that are diffusion controlled.
Reduction
Oxidation
II
–
I
II
–
III
Co L + e → Co L
Co L – e → Co L
I
–
0
III
–
III 2+
Co L + e → Co L
Co L – 2e → Co L
0
–
0 2–
Co L + 2e → Co L
Scheme 3. Summary of the voltammetric responses of 1 and 2 in CH CN.
3
§
Figure 3. Left: Cyclic voltammograms of (a) 3.3 mM pcta in DMF , (b) = 2.6 mM 2, (c) 3.0 mM
pdcta, (d) 2.4 mM 1 in CH CN on a glassy carbon working electrode. Supporting electrolyte = 0.1
3
§
3
The molecule pcta is sparingly soluble in CH CN.

1
5
n
–1
M [ Bu N]PF , scan rate = 200 mV s . Right: Normalized (I/conc) squarewave voltammograms
4
6
of the solutions given on the left hand side of the figure (squarewave frequency = 10 Hz).
LUMO +1
LUMO
HOMO
1
2
Figure 4. MOs of 1 and 2 calculated at the B3LYP/6-31G(d,p) level optimized in the gas phase.
3
.2.1 Effect of co-ligands 2,2’-bipyridine and triphenyl phosphine on the voltammetric
behaviour of 1 and 2
The presence of an exchangeable solvent molecule in the coordination sphere, along with the fairly
labile solvent and acetate ligands on the Co(II) metal centre, led us to investigate the effect of a
strongly coordinating ligands 2,2’-bipyridine (bpy) and triphenylphosphine (PPh ) on the
3
voltammetric response of complexes 1 and 2. The ligands bpy and PPh are known to impart
3
favourable electrochemical properties on various metal centres, such as improving the reversibility
of various oxidation states, altering the redox potentials, and providing steric bulk to improve the
stability of reduced states [24, 38, 42, 55, 64, 75, 76]. The effects of the concentrations of PPh3
and bpy on the E values and the i of the Co(II/I) redox couples of 1 and 2 are demonstrated in
1
/2
p,c
the Electronic Supporting Information (Section B). Voltammograms on CH CN solutions of 1 and
3
2
containing one equivalent of bpy showed shifts in the reduction potentials assigned to the metal

1
6
centres (Fig. 5, Table 2), as well as the removal of the adsorption wave occurred in its absence or
its presence with complex and PPh mixtures. The addition of one equivalent of bpy to 1 resulted
3
2
+
+
+
0
in the Co → Co reduction shifting from E = –1.08 to –1.03 V, the Co → Co shifting to –1.74
½
2
+
3+
to –1.67 V, and the Co → Co oxidation shifted from E = +1.05 to to E = +0.48 V (see Fig.
pa
½
2
+
+
4
). In complex 2, the addition of one equivalent of bpy resulted in the Co → Co reduction
+
0
shifting from E = –1.25 to –1.01 V. The Co → Co is resolved from the ligand reduction wave,
½
2
+
3+
and is observed at E = –1.32 V. The Co → Co oxidation of 2 is also clearly resolved from
pc
ligand oxidation and is observed at E = +0.44 V. The addition of bpy resulted in the Co(II/I)
½
reduction potentials of both 1 and 2 becoming more positive by an average of 190 mV. The bpy
co-ligand also shifted the CoIII/I waves closer to zero by about 500 mV on average. The addition
II/I
of PPh resulted in the intermediate shift (relative to bpy) in the Co redox process for both 1 and
3
2
. In the case of 1, the reduction also become more Nernstian in behaviour (see Table 1), via the
I/0
decrease in E , and an increase in I /I . The Co reduction is observed at –1.72 V in 1, and
p
pa pc
appears at –1.49 V as a shoulder on the ligand reduction in 2. The PPh co-ligand had a marginal
3
2
+
3+
effect on the Co → Co oxidation potential which occurred at E = +1.32 and +1.24 V for 1 and
pa
2
, respectively. Although our primary interest is the Co(I) oxidation state, the dramatic shift
induced by the bpy co-ligand on the CoII/III redox potentials was surprising. Also, the large and
positive values of the CoII/III redox potential confirmed the remarkable stability of 1 and 2 towards
aerobic oxidation.
2
+
+
Table 2. Effect of co-ligand on the reduction properties of the Co → Co in CH CN (0.1 M
3
[nBu N]PF ).
4
6
Complex
E / V
I /I
pa pc
E / mV
1
/2
p
1
/bpy
/PPh3
2
/bpy
/PPh3
–1.08
–1.03
–1.00
–1.23
–1.01
–1.17
0.75
0.80
0.83
0.61
0.70
0.60
260
150
130
150
250
170
1
1
2
2

1
7
Figure 5. The effect of bpy and PPh on the voltammograms (cyclic (left) at scan rate = 200 mV
3
–
1
s and (normalized as I/conc) square wave (right) at SW frequency = 10 Hz) of 1 and 2 in
n
CH CN on a glassy carbon working electrode. Supporting electrolyte = 0.1 M [ Bu N]PF .
3
4
6
The stability of the Co(I) species is of interest in the application of Co-containing complexes as
2
+
+
electro- or photo-catalytic species. In both complexes, the Co → Co reduction has a quasi-
reversible nature. The formation of Co(I) was examined by spectroelectrochemical methods. The
spectral changes of 1 upon reduction at a constant potential of –1.1 V vs Ag is shown in Fig. S11.
Upon reduction, the band at ca 300 nm is transformed into a shoulder on the MLCT band at 200
nm, and simultaneously, the intensity of the charge transfer band increases. Tetra-n-butyl
n
borohydride ([ Bu N]BH ) reduction of transition metals is also ubiquitous, and coordination
4
4
complexes of Co(II) can be conveniently reduced to Co(I) state using the borohydride anion [41,
n
4
2]. The reduction of 1 and 2 with [ Bu N]BH resulted in similar spectral transformations as those
4
4
obtained via spectroelectrochemical methods, further suggesting that the spectral transformation
was indeed that of the corresponding Co(I) species, and hence supports the CoII/I assignments in
the voltammograms.

1
8
3
.3 Electrochemical behaviour of 1 and 2 in the presence of p-toluene sulfonic acid
monohydrate
Numerous cobalt-containing species have been associated with the electrocatalytic reduction of
protons [11, 12, 41, 42, 77, 78]. The electrochemical behaviour of both complexes was investigated
for electrocatalytic activity in the presence of p-toluene sulfonic acid (p-TSOH) as a proton source.
In the presence of the proton source, both species showed a new cathodic wave within the vicinity
of the Co(II/I) reduction wave (e.g., Fig. 6). This new wave was scan rate dependent (Fig. S12)
and also increased with the concentration of p-TSOH. The electrocatalytic behaviour was also
investigated with the bpy and PPh co-ligands. In the absence of the co-ligands, both 1 and 2
3
exhibited a pre-wave in the vicinity of the adsorption wave observed in the absence of the proton
source, and also an unexpected symmetrical peak on the return scan. This feature was only
observed in the voltammograms with p-TSOH, at its variation with scan rate and [p-TSOH], and
0
is suggestive of a stripping wave of the Co from the electrode. In the presence of bpy and p-TSOH,
complex 1 produced irreproducible voltammograms on consecutive cycling, and thus required
cleaning of the electrode between acquisitions of each voltammogram. This is suggestive of
modification or passivation of the electrode during electro-catalysis by a non-catalytic species [79-
8
1], as rinsing the electrode and repeating the experiment resulted in diminished currents on each
cycle. In the 1/PPh /p-TSOH mixture, normal catalytic response was observed with successive
3
additions of p-TSOH. On the other hand, the addition of p-TSOH to complex 2 resulted in cyclic
voltammograms which displayed a pre-wave as well as an unexpected diffusional wave on the
return scan, following the electrocatalytic wave. However, in the 2/bpy/p-TSOH mixture, normal
catalytic responses were observed (Fig. S13). Interestingly, in the 2/PPh /p-TSOH mixture, normal
3
catalytic response was observed at low [p-TSOH], and hysteresis was observed in the higher [p-
TSOH]. Thus, the apparent catalytic response of 1 is improved with PPh , and 2 is improved with
3
bpy.
Further studies on the electrocatalytic activity of the complexes were carried out by
controlled potential electrolysis (CPE) of CH CN solutions of the complexes (with and without
3
co-ligands) in the presence of p-TSOH. A CPE experiment on complex 1, without co-ligands, at –
1
.1 V versus Ag, when corrected for a catalyst-free solution under the same potential, produced a
net charge of 1.237 C after 1000 s of electrolysis, with accompanying evolution H gas. A similar
2
experiment on complex 2 at –1.25 V vs Ag gave a net charge of 0.828 C. The net volume of H2

1
9
evolved correspond to 88 and 44 L, representing 56 and 41% Faradaic efficiency for complexes
1
and 2, respectively. In the presence of the co-ligands, CPE experiments at –0.90 V with bpy and
1.0 V with PPh , netted remarkably improved Faradaic efficiencies with 1/PPh compared to the
–
3
3
marginal gain obtained in the 2/bpy mixture (Table 3). The overpotential associated with the
*
*
catalysed proton reductions (Eq. 2 ) [82, 83] varied between the complexes and amongst the co-
ligands. The results suggest that the bpy ligand has a positive effect on the Faradaic efficiency of
the electrocatalytic response of 2, however it has a negative effect on 1 in CH CN, whereas PPh
3
3
had a positive effect on 1 but an adverse effect on 2 (Table 3). The magnitude of the Gibbs free
energy for the homolytic and heterolytic H evolution from p-toluenesulfonic acid monohydrate
2
(
p-TSOH) in CH CN can be determined using the methodology suggested by Kellett and Spiro
3
[
43], on the basis of the values for the potentials of the cobalt and p-TSOH/H couples versus
2
+
Fc /Fc. In the analyses, it is evident that the thermodynamics of the hydrogen evolution tends to
favour the homolytic pathway [35, 37], except in the 1/bpy and 1/PPh systems. In the 1/PPh
3
3
system, where both pathways are differentiated by ca. 14 kJ/mol, the highest Faradaic efficiency
was obtained. Similarly, the 2/bpy system, which possessed the smallest separation in energy for
the competing pathways with pre-catalyst 2 (ca 27 kJ/mol), yielded the greatest Faradaic efficiency
for pre-catalyst 2. The turnover frequency (TOF) followed the trend bpy > no coligand > PPh in
3
both complexes. It is clear that the steric bulk retarded the hydrogen evolution, which supports a
general preference for a homolytic pathway suggested by the thermodynamic analysis. The
Faradaic efficiencies obtained are comparable to some Pd(II)-hydrazonic derivatives reported
recently [84]. These results seem favourable when compared to cobalt complexes with [14]-
tetraene-N4 (Tim) ligands of the type [Co(TimPh/Me)X ] (X = Br or CH CN; n = 1 with X = Br
n+
2
3
and n = 3 with X = CH CN) which yielded 20-25% Faradaic efficiency [16]. However the 1/PPh
3
3
system with its almost quantitative efficiency is comparable to some cobalt diglyoxime complexes
in CH CN [16].
3
*
*
+
where ηcat/2 is overpotential at the catalytic half-wave potential, E°HA (–0.65 V vs Fc /Fc [82] G.A.N. Felton, R.S.
Glass, D.L. Lichtenberger, D.H. Evans, Inorg. Chem., 45 (2006) 9181-9184.doi:10.1021/ic060984e) is the reduction
3 a 3
potential of p-TSOH in CH CN, pK of p-TSOH (8.7 [82] ibid.) in CH
CN and Ecat/2 is determined from CoII/I in the
+
presence p-TSOH vs Fc /Fc using square wave voltammetry.

2
0
ηcat/2 = E°HA – Ecat/2
(2)
Figure 6. Cyclic voltammograms of 1 (left) and 1/bpy (right) in the presence of varying quantities
p-toluenesulfonic acid monohydrate (p-TSOH), at a glassy carbon working electrode in CH CN.
3
n
[
1] = 1.2 mM, [1/bpy] = 1.4 mM, supporting electrolyte = 0.1 M [ Bu N]PF , scan rate = 200 mV
4
6
–
1
s .
Table 3. Effect of the co-ligands bpy and PPh on the overpotential, Faradaic efficiency, turn over
3
frequency, and free energy of the catalysed proton reduction of p-TSOH in CH CN.
3
G / kJ mol–1 G / kJ mol–1
TOF /
Faradaic
efficiency / %
흁풎풐풍 푯ퟐ
Complex  cat/2/ mV
풉풓 ―ퟏ
(heterolytic
pathway)
–15.0
(homolytic
pathway)
–87.8
(
)
흁풎풐풍 풄풂풕
1
/bpy
/PPh3
2
/bpy
/PPh3
750
650
680
780
605
780
56
51
97
41
44
40
5.5
6.2
2.2
2.1
3.0
0.93
1
1
–101
–83.0
–93.6
–80.0
–20.3
–101
2
2
–53.1
–81.0
–16.9
–102

2
1
Scheme 4. Hydrogen evolution pathways from Co(I) intermediate[35, 43]. Ligands are excluded
for clarity.
4
Conclusions
3
Two pincer Co(II) complexes of the type  -SNS and ONS were synthesized. Both complexes
displayed electrocatalytic hydrogen evolution in the presence and absence of bpy or PPh co-
3
3
ligands. The hydrogen evolution occurred at moderate overpotentials with the  -SNS complex
3
giving better Faradaic efficiencies than the  -ONS under all the conditions explored. Further
studies are in progress to isolate these and other mixed ligand systems in order to assess their
potency in the hydrogen evolution reaction.
Acknowledgements
The authors are grateful for the kind assistance of the Department of Chemistry at The University
of the West Indies, Mona and The University of Technology Jamaica for financial support. AAH
would like to thank the National Science Foundation (NSF) for the NSF CAREER Award, as this
material is based upon work supported by the NSF under CHE-1431172 (Formerly CHE-
1
151832). AAH would also like to thank Old Dominion University’s Faculty Proposal Preparation
Program (FP3). The authors are also grateful for the suggestions made by the reviewers.
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5
Highlights:
3
3


Two Co(II)-pincer ligands based on pyridyl(di)thiocarboamide of  -SNS and  -ONS
are reported
The crystal structure of N-(2,5- dimethoxyphenyl)-6-[(2,5-
dimethoxyphenyl)carbamothioyl]pyridine-2-carboxamide (pcta) is reported
Both complexes of Co(II) displayed electro-catalytic hydrogen evolution (HER)


The addition of co-ligands bpy or PPh had varying effects on the HER
3

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Author Statement
Mark A.W. Lawrence: Conceptualization, Methodology, Experimentation and Data processing,
Initial draft preparation, Graphical Preparation, Reviewing, Editing.
Willem Mulder: Methodology, Data checking, Writing, Reviewing, Editing.
Michael J. Celestine: Experimentation, Data processing, Editing
Colin D. McMillen: Experimentation, X-ray structure solution, Writing, Reviewing, Editing
Alvin A. Holder: Writing, Reviewing, Editing