Electrochemical response of a Ru(II) benzothiazolyl-2-pyridinecarbothioamide pincer towards carbon dioxide and transfer hydrogenation of aryl ketones in air

Article abstract of DOI:10.1016/j.molstruc.2020.128829

A ruthenium(II) complex of 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-pyridinecarbothioamide (pbcta), of the formula [Ru(pbcta)Cl₂(dmf)] (1, where DMF = dimethyl formamide) was prepared from RuCl₃?xH₂O and pbcta in DMF at reflux under argon atmosphere. The identity of 1 was confirmed from its elemental analysis, ESI MS, and a series of spectroscopic measurements. Voltammetric measurements on 1 in DMF and DFT studies on the structure optimized in the gas phase revealed predominantly ligand based electron transfer processes under argon. In the presence of a proton source, proton coupled electron transfer to the ligand occurs. Under a carbon dioxide atmosphere, voltammetric studies revealed that 1 is inactive for CO₂ reduction, and the redox responses observed in the presence of the proton source and/or CO₂ are ligand based leading to reactions with the coordinated pbcta. Transfer hydrogenation (TH) of aryl ketones was efficiently carried out in 2-propanol using 1 at reflux. TH of the aryl ketone substrates proceeded in air with almost quantitative conversions at 0.2–1.0 molpercent catalyst.

Full text of DOI:10.1016/j.molstruc.2020.128829

Journal Pre-proof
Electrochemical response of a Ru(II) benzothiazolyl-2-pyridinecarbothioamide pincer
towards carbon dioxide and transfer hydrogenation of aryl ketones in air
Shannen C. Lorraine, Mark A.W. Lawrence, Michael Celestine, Alvin A. Holder
PII:
S0022-2860(20)31154-6
DOI:
https://doi.org/10.1016/j.molstruc.2020.128829
MOLSTR 128829
Reference:
To appear in: Journal of Molecular Structure
Received Date: 30 April 2020
Revised Date: 25 June 2020
Accepted Date: 3 July 2020
Please cite this article as: S.C. Lorraine, M.A.W. Lawrence, M. Celestine, A.A. Holder, Electrochemical
response of a Ru(II) benzothiazolyl-2-pyridinecarbothioamide pincer towards carbon dioxide and transfer
hydrogenation of aryl ketones in air, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/
j.molstruc.2020.128829.
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition
of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of
record. This version will undergo additional copyediting, typesetting and review before it is published
in its final form, but we are providing this version to give early visibility of the article. Please note that,
during the production process, errors may be discovered which could affect the content, and all legal
disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.

Electrochemical response of a Ru(II) benzothiazolyl-2-pyridinecarbothioamide pincer
towards carbon dioxide and transfer hydrogenation of aryl ketones in air
Shannen C. Lorraine,a Mark A.W. Lawrence,a*Michael Celestineb, and Alvin A. Holderb
aDepartment of Chemistry, The University of the West Indies Mona, Kingston 7, Jamaica, W.I.
bDepartment of Chemistry and Biochemistry, Old Dominion University, Norfolk, VA 23529
Email: mark.lawrence02@uwimona.edu.jm
Abstract
A ruthenium(II) complex of 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-
pyridinecarbothioamide (pbcta), of the formula [Ru(pbcta)Cl2(dmf)] (1, where DMF = dimethyl
formamide) was prepared from RuCl3•xH2O and pbcta in DMF at reflux under argon
atmosphere. The identity of 1 was confirmed from its elemental analysis, ESI MS, and a series of
spectroscopic measurements. Voltammetric measurements on 1 in DMF and DFT studies on the
structure optimized in the gas phase revealed predominantly ligand based electron transfer
processes under argon. In the presence of a proton source, proton coupled electron transfer to the
ligand occurs. Under a carbon dioxide atmosphere, voltammetric studies revealed that 1 is
inactive for CO2 reduction, and the redox responses observed in the presence of the proton
source and/or CO2 are ligand based leading to reactions with the coordinated pbcta. Transfer
hydrogenation (TH) of aryl ketones was efficiently carried out in 2-propanol using 1 at reflux.
TH of the aryl ketone substrates proceeded in air with almost quantitative conversions at 0.2–1.0
mol% catalyst.
Keywords: Ruthenium(II); NNS pincer ligand; Transfer hydrogenation; Aryl ketones
1
Introduction
The use of transition metal complexes as catalysts presents tremendous scope for
performance optimization by tuning the ligands about the metal centre [1]. The metal often act as
the catalytic site, and the ligands help to stabilize the metal in its various oxidation states
throughout the catalytic cycle. In some cases, the ligand has been suggested to play a role in the
catalytic process by acting as a proton or electron sink [2-4]. Consequently, non-innocent ligands
i.e. ligands that can participate in acid base reactions whilst coordinated, and are also redox
active at accessible potentials, may be useful in improving catalytic processes [5-10]. Although

many different types of complexes have shown significant activity, ruthenium polypyridyl
complexes are particularly attractive due to their wide scope of applications [11-16]. For
example, transfer hydrogenation (TH) is a useful protocol for the reduction of ketones and
aldehydes to their corresponding alcohols [17]. Ruthenium(II) and iridium(III) complexes are
usually applied as the most useful catalysts for the transfer hydrogenation of ketones [18-20].
Bifunctional catalysts containing ruthenium(II) complexes and (monotosylated) 1,2-diamines or
aminoalcohols can offer high catalytic activity and selectivity due to the presence of a N–H
functionality [21]. A great variety of related ligands and transition metal complex catalysts have
been developed to mitigate the need for bifunctional systems. These include aminophosphines
[22-25], N-heterocyclic carbenes (NHCs) [26, 27], pincer ligands of the κ3-NNN type [28-30], to
name a few. Catalytic TH involving transition metals are often performed in 2-propanol as the
hydrogen donor, and under inert atmosphere to prevent oxidation of the catalyst, thus affording
high conversion of the ketone to the desired alcohol [21]. Other systems employed include
HCO2H–Et3N [31], or aqueous solution of HCO2Na [32]. Nevertheless, the operational
simplicity offered by the use of an inexpensive and non-toxic solvent such as 2-propanol,
provides the additional advantage of the acetone by-product which can be easily removed. The
reductive conditions enabled by the presence of a strong base such as KOH or KtBuO, readily
facilitates the reduction of C=O to C-OH bonds with good functional group tolerance. Despite
the success of various systems, there is still a need to develop cheap, simple and effective
systems, to improve the accessibility and scope of TH reaction.
Ruthenium complexes have also been implicated in the electro-catalytic reduction of carbon
dioxide on various electrode surfaces and solvents in the presence of proton sources such as
water and alcohols [8, 15, 33]. Electro-catalytic proton reduction is often coupled to carbon
dioxide reduction, however Ru(II) is not associated with proton reduction, despite the
accessibility of the Ru(IV/III/II) reduction couples in many ruthenium containing complexes [34,
35]. However, Sponholz et al [36] illustrated the homogenous catalytic hydrogen generation
from ethanol. The homogeneously catalyzed hydrogenation of carbon dioxide (CO2) to formic
acid was first reported in 1976 [37]. In recent times, various authors [15, 38-40] have continued
to illustrate the ability of ruthenium (and other metals) to directly hydrogenate CO2 either with
hydrogen gas (hydrogenation), or via transfer hydrogenation to produce species such as formate.
Transfer hydrogenation is less explored than hydrogenation, but it is a safer and more

economical route as stated above. The direct reduction of CO2 is not only of environmental
importance [41], but it is also of industrial significance [42, 43] as species such as formate are
useful synthons, and potential hydrogen store for the hydrogen economy. Other studies have also
investigated the conversion of CO2 to methanol, and an excellent review is given by Li et al [44].
In previous studies we reported on the development of pyridyl carbothioamide derivatives as
pincer ligands [45, 46], as well as various coordination complexes for electro-catalytic proton
reduction [46-49]. In those studies, it was shown the ligand framework possess accessible redox
potentials, which may present an opportunity for them to act as electron shuttles. In this report,
the synthesis of the ruthenium(II) complex of 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-
dimethoxyphenyl)-2-pyridinecarbothioamide (pbcta), its electrochemical behavior towards CO2,
and application to the transfer hydrogenation of some aryl ketones are outlined.
Figure 1. Examples of complexes employed in catalytic (transfer) hydrogenation.
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 ATR accessory on a Bruker Vector 22 FTIR

spectrophotometer. High-resolution ESI MS spectra were acquired via positive electrospray
ionization on a Bruker 12 Tesla APEX –Qe FTICR-MS with an Apollo II ion source. Samples
were dissolved in acetonitrile or 1:1 dichloromethane/acetonitrile, followed by direct injection
using a syringe pump with a flow rate of 2 µL s–1. The data was processed using Bruker
1
Daltonics Data Analysis Version 3.4. Solution H NMR spectra were measured using a Bruker
ACE 500-MHz Fourier transform spectrometer and were referenced internally to the residual
protons of the incompletely deuterated solvent.
All electrochemical experiments were performed on a DigiIvy DY2312 potentiostat, under
an argon atmosphere (unless otherwise stated) 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 +0.65 V in DMF. The ionic
strength was maintained at 0.1 M [nBu4N]PF6. The solvents used in the electrochemical
experiments were dried using standard procedures [50].
Controlled-potential electrolysis (CPE) measurements for the production of hydrogen were
conducted at –1.10 V (vs Ag), on stirred solutions for 20 min in a sealed two-chambered H-cell
separated by a fine frit, where one chamber held the working and reference electrodes in 10 mL
of 0.36 mM of complex in 0.1 M [nBuN4]PF6 (supporting electrolyte) with 12 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.
Transfer hydrogenation experiments were performed under an inert atmosphere of dry argon
or ambient air. The solvents were purified using standard procedures [51] and, where necessary,
were distilled under argon or nitrogen atmosphere using the appropriate drying agent. The ketone
substrates were obtained from commercial suppliers and used without further purification. GC
analysis was carried out on a Hewlett-Packard 6890 gas chromatograph equipped with a 5973
MSD detector and β-DEX 120 chiral capillary column (30 m × 0.25 mm; Supelco, USA).

.
2.1 Semi-Empirical and Density Functional Theory calculations
Density functional theory calculations were carried out using the GAMESS software
package1 [52, 53]. The structures were optimized (see supporting info) in the gas phase as
indicated by the absence of imaginary frequencies in the Hessian, using PW91X/SBKJC [54, 55]
with the common polarization and spherical coordinates. Solvent optimization in DMF using the
SMD solvation method [56]. The GAMESS input file was generated using MacMolPlt 7.72 [57],
and the output file viewed using the same. The SBKJC basis set results was initially compared to
those obtained using the Sapporo core/valence relativistic basis sets (SPK) double zeta potentials
(PW91X/SPKr-DZP) [58-60]. This basis set provided identical FMO, to the SBKJC basis set,
albeit with different energies (as was expected, see supporting information). However, the family
of SPK basis sets were more computationally demanding while not providing more useful
information. Consequently only the calculations based SBKJC basis set are discussed.
2.2 Preparation of [Ru(pdcta)Cl2(DMF)] (1)
The
6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-
pyridinecarbothioamide ligand (pbcta) was prepared following literature protocol [45]. In a
pressure tube, RuCl3•xH2O (23 mg, 0.11 mmol) and pbcta (49 mg, 0.10 mmol) were dissolved in
DMF (20 mL) and Et3N (0.5 mL). The mixture was then thoroughly sparged with Ar and then
refluxed for 6 h, following which, the solution was almost completely distilled under reduced
pressure. The residue was dissolved and transferred to a beaker using DCM (20 mL) and dried
under a strong stream of air. The resulting solid was washed with copious amounts of water and
diethyl ether, then air dried. A brown solid was isolated with yield 83% (59.4 mg). High-
resolution ESI MS (positive mode) m/z = of 640.04567 for the [Ru(pdcta)Cl2]-H+ species (see
supporting information). Elemental analysis: Found: C, 44.16; H, 3.22; N, 7.86. Calc. for
C23H21Cl2N3O4RuS2•C3H7NO: C, 43.82; H, 3.96; N, 7.86%. Selected IR (ATR) / cm–1: 3423
1 GAMESS: an open-source general ab initio quantum chemistry package.
https://www.msg.chem.iastate.edu/gamess/index.html
2
MacMolPlt:
an
open-source
molecular
builder
and
visualization
tool
for
GAMESS.
http://brettbode.github.io/wxmacmolplt/

ν(NH), 2834 ν(CHDMF), 1639 ν(C=ODMF) 1598–1500 ν(C=Caryl). λmax (DMSO) / nm (ε ± 200 /
M–1 cm–1) 274 (18600), 320 (16100), broad shoulder from 460–1000. δH(dmso-d6): 3.77 (3H, s),
3.99 (6H), 4.04 (3H) overlapping singlets, 6.56–6.74 (1H), 7.07 (3H), 7.96 (1 H), 8.14 (1H,
CHDMF), 8.26-8.54 (3H, m), 10.79 (1H, NH).
2.3 General procedure for transfer hydrogenation of ketones
In a typical catalytic run, a mixture of 1 (2.1 mg, 0.003 mmol, 1 equiv) and the base (0.03
mmol, 10 equiv.) were stirred in 2-propanol (3 mL) in a pressure tube (Radley tubeTM) under air
or inert atmosphere for 15-20 min. To this mixture, the ketone (1.5 mmol, 500 equiv.) was added
and the resulting mixture stirred at the desired temperature. At the end of the reaction, the
mixture was filtered through a short pad of silica gel, transferred and made up in a volumetric
flask prior to dilution and injection on the GC column. This was followed by vacuum distillation
and the product isolated as a pale yellow oil after purification by silica gel chromatography with
hexane: ethyl acetate (5:1) as the eluent.
3
Results and discussion
3.1 Synthesis and characterization
The
6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-
pyridinecarbothioamide ligand (pbcta) was prepared according to a protocol reported in an
earlier study [45]. The preparation of the ruthenium(II) complex [Ru(pbcta)Cl2(DMF)] (1) from
RuCl3•xH2O followed a similar protocol to that of [Ru(pbt)2Cl2]•0.25CH3COCH3 (where pbt =
2-(2’-pyridyl)benzothiazole) [61]. In the IR and NMR spectra of the [RuCl2(pdcta)(DMF)]
complex, the amide NH is retained, suggesting that the ligand is in a neutral form [62, 63], unlike
its Pd(II) analogue [45, 46]. The elemental analysis, the mass spectrum and other spectroscopic
data (see electronic supporting information) are consistent with the proposed formulation.
Scheme 1. Preparation of [Ru(pbcta)Cl2(DMF)] (1).

3.2 DFT and Electrochemical studies
In the gas phase optimized structure, the ν(C=ODMF) is calculated at 1637 cm–1 and
observed at 1639 cm–1. The bond length of the C=S in 1 is predicted to be 1.787 Å compared to
1.666 Å in the crystal structure of pbcta [45], and the C-S bonds in the benzothiazole ring are
predicted at 1.810 and 1.840 Å compared to 1.723 and 1.747 Å observed. On average, bond
lengths of the C-S are predicted at ca 5% greater than those observed in the free ligand, whereas,
the C=S bond in 1 is predicted to be ca 7% longer than the free C=S bond. This extra bond
lengthening is suggesting a strong interaction between the Ru and coordinated sulfur, and the
LUMO and LUMO+1 are mixed π*/σ*/d-π orbitals consistent with the π-back-bonding
characteristics of sulfur of the thioamide and the nitrogen of the benzothiazole [64]. However,
the retention of N-H stretching frequency in the experimental IR spectrum is clearly indicating
that the thioamide functionality is retained, and thus the pbcta ligand remains neutral. Density
functional theory calculations in the gas phase (and in DMF using the SMD solvation method
[56]) analyzing the molecular orbitals of 1 (Fig. 2), revealed that the LUMO and LUMO+1
largely involves the coordinated pbcta moiety. Specifically the LUMO heavily populated by the
thioamide group and LUMO+1 is delocalized across the pyridothiazole portion of the
coordinated pbcta. On the other hand, the HOMO is centered on Ru-Cl moiety and HOMO-1 and
-2 are fairly mixed with metal center and the thioamide moieties.

Figure 2. DFT optimized structures in the gas phase (PW91X/SBKJC) and in DMF
(PW91X/SBKJC/SMD) for 1.
Cyclic and square wave voltammograms of 1 compared to pbcta, revealed a series of
predominantly ligand based redox processes. In the voltammograms of pbcta, there are two
reversible reductions, one electron each, between –1.1 → –1.7 V versus Ag [46] (Fig. 3 and Fig.
S4). In 1, the first of these reduction waves observed in pbcta is absent; however the first
reduction wave of 1 which is also a reversible wave observed at E = –1.48 V (see Fig. S5),
½
coincides with the second reduction wave of pbcta. When compared to pbcta this first wave in 1
appears to be a two electron reduction (Fig. 3). There is second reduction which was observed at
Epc = –1.93 V (two electrons) and a third at Epc = –2.10 V versus Ag, both of which are nearly
identical to those observed in pbcta. The coordination of pbcta to the Ru(II) metal centre
removes the initial reduction of the thioamide that leads to a thioamide radical anion in
uncoordinated pbcta. This behavior is suggesting a significant change about the sulfur of the
C=S moiety upon coordination to the Ru metal centre, and is also consistent with the intense p-
π/d-π mixing and the lowering of the energies of its orbitals due to bonding. The second

reduction wave in 1 is a two electron wave resulting reduction of the benzothiazole ring of the
pbcta ligand. At anodic potentials, the Ru(II/III) oxidation was observed at +1.20 V followed by
ligand oxidation (thioamide) at +1.52 V. The redox potentials are comparable to similar Ru(II)
containing species [8, 63, 65], and are consistent with the locations of the MOs as discussed
above.
Figure 3. Normalized cyclic voltammograms of 1 and pbcta in DMF on a glassy carbon
electrode. [1] = 0.84 mM, pbcta = 1.63 mM, supporting electrolyte = 0.1 M (nBu4N]PF6).
Ruthenium(II) systems are well known for their ability to reduce carbonyl compounds in
a variety of reactions such as hydrogenation, transfer hydrogenation, and as electrocatalysis to
name a few, whereas Ru(III) polypyridyls are well known for the water oxidation reaction. To
this end, complex 1 was investigated in the electrocatalytic reduction of CO2 in the presence and
absence of proton sources, by voltammetric techniques. Other systems based on the ligand
framework demonstrated electro-catalytic response on the presence of a proton source [46, 66].
Ruthenium is not known for this reaction, and it was confirmed by controlled potential
electrolysis experiments of 1 in the presence and absence of p-toluene sulfonic acid (see Fig. S6).
•–
The proton coupled reduction of CO2 is more energetically favorable than generating the CO2

[67, 68]. In the presence of weak proton donors such as 2-propanol (iPrOH) or ethanol, the
current at the first reduction wave approximately doubles (Fig. 4). A similar behavior is observed
in pbcta in the potential range at which 1 is reduced. Further studies indicate that under a CO2
atmosphere, the current at the first reduction potential is doubled and the voltammogram of 1
was restored upon sparging with Ar, suggesting that the interaction with CO2 was a non-covalent
interaction. Voltammograms in a CO2 atmosphere with the proton source resulted in negligible
i
increase in the peak current compared to CO2 or PrOH only (Fig. 4). Several radical anions are
known to reduce carbon dioxide to oxalate [69]. However, the nature of reactions with CO2 is
suggesting that the thiolate generated from the reduction of the thioamide most likely reacts with
CO2 to generate a thiocarbonate species. Though not desired for this reaction, thiocarbonates can
be used as a carrier for CO2, as these are readily oxidized to release CO2 and produce a disulfide
[70]. These data indicate that 1 is inactive for the direct CO2 reduction, and the redox responses
observed in the presence of the proton source and/or CO2 are ligand based leading to reactions
with the coordinated pbcta (Scheme 2). In this scheme, the coordination complex is suggested to
undergo a net 4e–, 4H+, however it is expected to proceed via two sequential 2e–, 2H+ reductions.
In the presence of CO2, the thiolate anion from the initial 2e– process is a potent nucleophile that
readily attacks the electrophilic carbon of CO2 to generate the thiocarbonate species. Though not
shown in the scheme, the free ligand is also expected to first form a thienyl radical which will be
readily protonated in the presence of a proton source, followed by additional reductions and
protonation.

i
Figure 4. Cyclic voltammograms of 1 (left) and pbcta (right) with and without PrOH under Ar
and CO2 atmospheres on a glassy carbon electrode. Conditions: [1] = 0.84 mM, [pbcta] = 1.63
mM, CE = Pt wire, supporting electrolyte = 0.1 M (nBu4N]PF6).
Scheme 2. Proposed mechanism for reduction of 1 in DMF, where L = ligand framework and S
= sulfur of the thioamide.

3.3 Transfer hydrogenation of aryl ketones
Whilst there have been a few reports of SNS [71] and NNS [72-74] ligand systems in
(asymmetric) hydrogenation of C=X bonds, reports of these systems in transfer hydrogenation is
sparse. Transfer hydrogenation (TH) of carbonyl compounds under mild conditions such as room
temperature and/or in air, require stable yet highly active transition metal catalysts. To this end, a
number of pincer ligands of the NNN type has been reported to generate efficient Ru(II) (pre-
)catalyst for the TH of carbonyl compounds in refluxing 2-propanol [1, 27-29]. Many of the
systems explored with (asymmetric) TH require inert atmosphere at fairly low (0.05-1.0%)
catalyst loading. The excellent catalytic performance of several planar tridentate ligands and
their ruthenium(II) complexes in TH prompted us to investigate the catalytic activity of a
substituted pyridylbenzothiazole (pbcta) NNS pincer ligand system and its ruthenium(II)
complex in these transformations.
In our initial TH studies, acetophenone (2a) was selected as the model substrate, with
KtBuO as base, to screen the reaction conditions (Scheme 3). With 0.4 mol % of 1 as (pre-
)catalyst, >99% conversion to 1-phenylethanol (3a) was observed under inert atmosphere, and
89% conversion in air (Table 1, entry 1). GC-MS analysis indicated that a racemic mixture of 1-
phenylethanol was isolated as the only product. Complex 1 displayed both air and moisture-
stability which allowed for the TH reactions to be conducted under inert atmosphere or in the
presence of air. Further attempts to improve the enantioselectivity and activity of the system
were investigated using a 5:2 mixture of trimethylamine/formic acid or sodium formate.
However, 1 appeared to be incompatible with the TEAF mixture or sodium formate, since little
to no conversion was observed (Table 1, entries 2 and 3).
Lowering the catalyst loading to 0.2 mol%, gave conversions of 88% and 89% within 24 h
under inert atmosphere and aerobic conditions, respectively (entry 4). After 48 hours, the
conversions had improved to 96% and >99% under inert atmosphere and aerobic conditions,
respectively (entry 5). Evidently, TH could only be effected at reflux since no conversion was
observed under ambient conditions (entry 6). Under similar conditions, the relatively cheaper
base KOH, gave similar conversions to KtBuO (Table 1, entry 7). With either base, the
reductions under aerobic conditions gave slightly better conversions after 48 h.

Scheme 3. Transfer hydrogenation of aryl ketones.
Table 1. Transfer hydrogenation of acetophenone catalysed by 1.
Yielda (%)
inert Air
S/C
Temp Time
(h)
reflux 24
Entry Base
ratio (oC)
1
KtBuO
250
250
250
500
500
500
500
500
500
500
500
500
>99
6
89
--
2
3
4
5
6
7
TEAF
NaHCO2
KtBuO
KtBuO
KtBuO
KOH
50
50
24
24
0
--
reflux 24
reflux 48
88
96
0
89
>99
--
r.t.
48
reflux 48
reflux 24
reflux 48
95
0
97
--
8
none
KOH
KOH
KOH
9b
10c
11c
12d
--
0
r.t.
48
--
0
reflux 48
reflux 48
--
--
61
54
KOH
a By GC-MS analysis; b cat = RuCl3•xH2O/pbcta (1:1); c cat = 1/PPh3 (1:1); d cat = 1/bpy (1:1)
The literature has illustrated the importance of the base, even though its true role remains
unclear. To this end, TH reactions with 1 was investigated in the absence of base, and no
conversion to the desired 1-phenylethanol was obtained (Table 1, entry 8). Evidently, for
effective TH, a base was required for the generation of the active catalytic species. The TH of
acetophenone was also examined in situ with the (pre-)catalyst precursors, RuCl3•xH2O/pbcta
(1:1) (Table 1, entry 9). There was no conversion to the desired alcohol. This suggests that
Ru(III) is not the active (pre-)catalyst nor is Ru(II) likely generated from Ru(III) under the
reaction conditions. Even though the activity of 1 under aerobic conditions was quite pleasing,
the absence of chiral induction remained a major concern. In a previous study, it was
demonstrated that co-ligands, triphenylphoshine (PPh3) or 2,2’-bipyridine (bpy) when added to
complexes of with a similar ligand frame work to pbcta resulted in enhanced Faradaic
efficiencies of the Co(II) compounds in electrocatalytic proton reduction [66]. To this end, the
addition of PPh3 (1 equiv.) as a co-ligand to the reaction mixtures was investigated. PPh3 is

expected to add steric bulk at the metal centre and also potentially offer additional stability to the
transition state(s), thereby enhancing the enantioselectivity [75]. The addition of PPh3 resulted in
the decreased activity of 1 (entries 10-11), and the chiral induction remained unimproved.
Evidently, the bulkiness of the 1/PPh3 system hindered the reaction progress rather than
accelerate it. A similar conclusion can be made of co-ligand bpy (entry 12), which is suggesting
that the co-ligands are reducing the sites available for substrate binding.
The catalytic reductions with complex 1 as a (pre-)catalyst were extended to other aryl
ketone substrates 2b-e. Increasing the steric congestion at the ketone group as in the case of
propiophenone, 2b (Table 2, entries 1-3) required an increased loading of 1 mol% to achieve
97% conversions after 48 h (entry 3). The p-halosubstituted substrates, 4-chloroacetophenone
(2c), and 4-bromoacetophenone (2d), gave 82 and 14% conversions, respectively, at 0.2 mol%
catalyst loading after 48 h (entries 4 and 5). Interestingly, 4-bromoacetophenone required 1
mol% of the (pre-)catalyst to achieve quantitative conversion after 48 h (>99%, entry 7). The
reason for this discrepancy is not immediately understood. Increasing the catalyst loading from
0.2 to 0.5 mol% (Table 2, entry 9) resulted in 98% conversion of 3-methoxyacetophenone (2e) to
1-(3-methoxyphenyl)ethanol, which is an important synthon for the synthesis of 3-methoxy-2,6-
dimethylphenethyl alcohol [76], and one of its enantiomer for rivastigmine hydrogen tartrate [77-
79].
It is plausible that the present transfer hydrogenation may follow an inner-sphere
mechanism wherein generation of a Ru(II) hydride is the catalytically active species [28, 80].
The TH of the ketone is presumably initiated from the in situ generation of a ruthenium(II)
alkoxide species, by abstracting a proton from the alcohol to extrude 1 equivalent of hydrogen
chloride. The resulting alkoxide species undergoes β-hydride elimination to give the true
catalytic species, a Ru–H intermediate, and the concomitant release of acetone. Coordination of
the ketone substrate to the Ru–H species followed by hydride transfer from the metal to the
coordinated substrate carbonyl gives another Ru(II)-alkoxide. The hydrogenated product is
released upon the transfer of hydrogen from 2-propanol to the substrate and regeneration of the
ruthenium(II) alkoxide species.
In summary, an active Ru(II) complex was developed on the NNS pincer ligand as (pre-
)catalyst for the TH of ketones to the corresponding alcohols in air. The system appears to

tolerate a variety of functional groups, however it does not facilitate enantio-differentiation.
Efforts are underway to optimize the reaction times, and to enhance the enantioselectivity of the
ligand system.
Table 2. Transfer hydrogenation of aryl ketones catalysed by 1 in air.
Yielda
(%)
19
69
97
Entry
Substrate
S:C
O
1
2
3
500
200
100
2b
O
4
500
82
Cl
2c
5
6
7
500
200
100
14
56
>99
O
Br
2d
8
9
500
200
87
98
O
O
2e
a By GC-MS analysis; reaction conditions: base = KOH, reaction time = 48 h, temp. = reflux.
Conclusions
A novel Ru(II) pincer compound was synthesized and voltammetric studies indicated that
it redox properties are ligand centred. The compound was inactive towards the electrocatalytic
reduction of CO2, however it showed good activity towards the transfer hydrogenation of aryl
ketones under aerobic conditions.
Acknowledgments
SCL, AAH and MAWL would like to thank Professor Tara P. Dasgupta (sunrise:
Wednesday, January 29, 1941 and sunset: Monday, April 20, 2020) for his beloved guidance as a

father, a Ph.D. research advisor and a mentor to make us what we are today. R.I.P. AAH would
also like to thank the National Science Foundation, 2415 Eisenhower Avenue, Alexandria, VA
22314, U.S.A., for a National Science Foundation CAREER Award, as this material is based
upon work supported by the National Science Foundation under CHE-14321172 (formerly CHE-
1151832). AAH would also like to thank the Old Dominion University startup package that
allowed for the successful completion of this work. The authors are also grateful for the
suggestions made by the reviewers.
References
[1] M.A.W. Lawrence, K.-A. Green, P.N. Nelson, S.C. Lorraine, Polyhedron, 143 (2018) 11-
27.doi:10.1016/j.poly.2017.08.017
[2] P. Wang, G. Liang, C.L. Boyd, C.E. Webster, X. Zhao, Eur. J. Inorg. Chem., 2019 (2019)
2134-2139.doi:10.1002/ejic.201801418
[3] P. Wang, G. Liang, M.R. Reddy, M. Long, K. Driskill, C. Lyons, B. Donnadieu, J.C.
Bollinger, C.E. Webster, X. Zhao, J. Am. Chem. Soc., 140 (2018) 9219-
9229.doi:10.1021/jacs.8b05108
[4] T. Straistari, R. Hardré, J. Fize, S. Shova, M. Giorgi, M. Réglier, V. Artero, M. Orio, Chem.
Eur. J., 24 (2018) 8779-8786.doi:10.1002/chem.201801155
[5] M.A.W. Lawrence, S.C. Lorraine, K.-A. Wilson, K. Wilson, Polyhedron, 173 (2019)
114111.doi:10.1016/j.poly.2019.114111
[6] S. Angelova, V. Paskaleva, N. Kochev, L. Antonov, Mol. Phys., (2018) 1-
9.doi:10.1080/00268976.2018.1548717
[7] T. Straistari, R. Hardré, J. Massin, M. Attolini, B. Faure, M. Giorgi, M. Réglier, M. Orio,
Eur. J. Inorg. Chem., 2018 (2018) 2259-2266.doi:10.1002/ejic.201800120
[8]
A.
Begum,
P.G.
Pickup,
Electrochem.
Comm.,
9
(2007)
2525-
2528.doi:10.1016/j.elecom.2007.07.034
[9] E.J. Thompson, L.A. Berben, Angew. Chem. Int. Ed., 54 (2015) 11642-
11646.doi:10.1002/anie.201503935
[10] J. Jiang, K.L. Materna, S. Hedström, K.R. Yang, R.H. Crabtree, V.S. Batista, G.W. Brudvig,
Angew. Chem. Int. Ed., 56 (2017) 9111-9115.doi:10.1002/anie.201704700
[11] S.E. Clapham, A. Hadzovic, R.H. Morris, Coord. Chem. Rev., 248 (2004) 2201-
2237.doi:10.1016/j.ccr.2004.04.007
[12] M.J. Clarke, Coord. Chem. Rev., 232 (2002)
[13] B.J. Coe, Coord. Chem. Rev., 257 (2013) 1438-1458.doi:10.1016/j.ccr.2012.06.025
[14] P. Crochet, V. Cadierno, Dalton Trans., 43 (2014) 12447-12462.doi:10.1039/C4DT01494D
[15] G.A. Filonenko, R. van Putten, E.N. Schulpen, E.J.M. Hensen, E.A. Pidko, ChemCatChem,
6 (2014) 1526-1530.doi:10.1002/cctc.201402119
[16] A.A. Holder, S. Swavey, K.J. Brewer, Inorg. Chem., 43 (2004) 303-
308.doi:10.1021/ic035029t
[17] D. Wang, D. Astruc, Chem. Rev., 115 (2015) 6621-6686.doi:10.1021/acs.chemrev.5b00203

[18] F. Foubelo, C. Nájera, M. Yus, Tetrahedron: Asymmetry, 26 (2015) 769-
790.doi:10.1016/j.tetasy.2015.06.016
[19] B. Štefane, F. Požgan, Top. Curr. Chem., 374 (2016) 18.doi:10.1007/s41061-016-0015-5
[20] X. Wu, X. Li, A. Zanotti-Gerosa, A. Pettman, J. Liu, A.J. Mills, J. Xiao, Eur. J. Chem., 14
(2008) 2209-2222.doi:10.1002/chem.200701258
[21] R. Noyori, S. Hashiguchi, Acc. Chem. Res., 30 (1997) 97-102.doi:10.1021/ar9502341
[22] M.S. Rahman, M. Oliana, K.K. Hii, Tetrahedron: Asymmetry, 15 (2004) 1835-
1840.doi:10.1016/j.tetasy.2004.05.001
[23] M. Aydemir, A. Baysal, S. Özkar, L.T. Yıldırım, Polyhedron, 30 (2011) 796-
804.doi:10.1016/j.poly.2010.12.011
[24] P. Crochet, J. Gimeno, J. Borge, S. García-Granda, New J. Chem., 27 (2003) 414-
420.doi:10.1039/B206119H
[25] C. Kayan, N. Meriç, M. Aydemir, Y.S. Ocak, A. Baysal, Appl. Organomet. Chem., 28
(2014) 127-133.doi:10.1002/aoc.3096
[26] S. Yaşar, E.Ö. Karaca, Ç. Şahin, İ. Özdemir, O. Şahin, O. Büyükgüngör, J. Organomet.
Chem., 789-790 (2015) 1-7.doi:10.1016/j.jorganchem.2015.04.012
[27] X.-H. Zhu, L.-H. Cai, C.-X. Wang, Y.-N. Wang, X.-Q. Guo, X.-F. Hou, J. Mol. Catal. A:
Chem., 393 (2014) 134-141.doi:10.1016/j.molcata.2014.06.004
[28] M. Zhao, Z. Yu, S. Yan, Y. Li, Tetrahedron Lett., 50 (2009) 4624-
4628.doi:10.1016/j.tetlet.2009.05.100
[29] W. Du, Q. Wang, Z. Yu, Chin. J. Catal., 34 (2013) 1373-1377.doi:10.1016/S1872-
2067(12)60583-X
[30] S. Enthaler, B. Hagemann, S. Bhor, G. Anilkumar, M.K. Tse, B. Bitterlich, K. Junge, G.
Erre, M. Beller, Adv. Synth. Catal., 349 (2007) 853-860.doi:10.1002/adsc.200600475
[31] A. Fujii, S. Hashiguchi, N. Uematsu, T. Ikariya, R. Noyori, J. Am. Chem. Soc., 118 (1996)
2521-2522.doi:10.1021/ja954126l
[32] X. Wu, X. Li, W. Hems, F. King, J. Xiao, Org. Biomol. Chem., 2 (2004) 1818-
1821.doi:10.1039/B403627A
[33] Z. Chen, J.J. Concepcion, M.K. Brennaman, P. Kang, M.R. Norris, P.G. Hoertz, T.J. Meyer,
Proc. Natl. Acad. Sci. U.S.A., 109 (2012) 15606–15611.doi:10.1073/pnas.1203122109/-
/DCSupplemental
[34] M. Abe, Y. Sasaki, Y. Yamada, K. Tsukahara, S. Yano, T. Yamaguchi, M. Tominaga, I.
Taniguchi, T. Ito, Inorg. Chem., 35 (1996) 6724-6734.doi:10.1021/ic960430j
[35] Q.G. Mulazzani, S. Emmi, P.G. Fuochi, M.Z. Hoffman, M. Venturi, J. Am. Chem. Soc., 100
(1978) 981-983.doi:10.1021/ja00471a059
[36] P. Sponholz, D. Mellmann, C. Cordes, P.G. Alsabeh, B. Li, Y. Li, M. Nielsen, H. Junge, P.
Dixneuf, M. Beller, ChemSusChem, 7 (2014) 2419-2422.doi:10.1002/cssc.201402426
[37] Y. Inoue, H. Izumida, Y. Sasaki, H. Hashimoto, Chem. Lett., 5 (1976) 863-
864.doi:10.1246/cl.1976.863
[38] N. Makuve, G. Mehlana, R. Tia, J. Darkwa, B.C.E. Makhubela, J. Organomet. Chem., 899
(2019) 120892.doi:10.1016/j.jorganchem.2019.120892
[39] P. Munshi, A.D. Main, J.C. Linehan, C.-C. Tai, P.G. Jessop, J. Am. Chem. Soc., 124 (2002)
7963-7971.doi:10.1021/ja0167856
[40] S. Sanz, A. Azua, E. Peris, Dalton Trans., 39 (2010) 6339-6343.doi:10.1039/C003220D
[41] J.H. Butler, M. Battle, M.L. Bender, S.A. Montzka, A.D. Clarke, E.S. Saltzman, C.M.
Sucher, J.P. Severinghaus, J.W. Elkins, Nature, 399 (1999) 749-755.doi:10.1038/21586

[42] A.-H. Liu, R. Ma, C. Song, Z.-Z. Yang, A. Yu, Y. Cai, L.-N. He, Y.-N. Zhao, B. Yu, Q.-W.
Song, Angew. Chem. Int. Ed., 51 (2012) 11306-11310.doi:10.1002/anie.201205362
[43] M. Tamura, M. Honda, Y. Nakagawa, K. Tomishige, Journal of Chemical Technology &
Biotechnology, 89 (2014) 19-33.doi:10.1002/jctb.4209
[44] Y.-N. Li, R. Ma, L.-N. He, Z.-F. Diao, Catal. Sci. Technol., 4 (2014) 1498-
1512.doi:10.1039/C3CY00564J
[45] M.A.W. Lawrence, Y.A. Jackson, W.H. Mulder, P.M. Björemark, M. Håkansson, Aust. J.
Chem., 68 (2015) 731-741.doi:10.1071/CH14380
[46]
M.A.W.
Lawrence,
W.H.
Mulder,
ChemistrySelect,
3
(2018)
8387-
8394.doi:10.1002/slct.201802065
[47] M. Bakir, M.A.W. Lawrence, M. Ferhat, R.R. Conry, J. Coord. Chem., 70 (2017) 3048-
3064.doi:10.1080/00958972.2017.1374379
[48] M. Bakir, M.W. Lawrence, P. Nelson, M.B. Yamin, J. Coord. Chem., 72 (2019) 2261-
2278.doi:10.1080/00958972.2019.1645329
[49] M.A.W. Lawrence, A.A. Holder, Inorg. Chim. Acta, 441 (2016) 157-
168.doi:10.1016/j.ica.2015.11.016
[50] W.L.F. Armarego, C.L.L. Chai, Chapter 4 - Purification of Organic Chemicals, Purification
of Laboratory Chemicals (Fifth Edition), Butterworth-Heinemann, Burlington, 2003, pp. 80-388.
[51] W.L.F. Armarego, C.L.L. Chai, Purification of Laboratory Chemicals 6th ed., Butterworth-
Heinemann, Oxford, 2009.
[52] M.S. Gordon, M.W. Schmidt, Chapter 41 - Advances in electronic structure theory:
GAMESS a decade later, in: C.E. Dykstra, G. Frenking, K.S. Kim, G.E. Scuseria (Eds.) Theory
and Applications of Computational Chemistry, Elsevier, Amsterdam, 2005, pp. 1167-1189.
[53] M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S.T. Elbert, M.S. Gordon, J.H. Jensen, S.
Koseki, N. Matsunaga, K.A. Nguyen, S. Su, T.L. Windus, M. Dupuis, J.A. Montgomery Jr, J.
Comput. Chem., 14 (1993) 1347-1363.doi:10.1002/jcc.540141112
[54] W.J. Stevens, H. Basch, M. Krauss, J. Chem. Phys., 81 (1984) 6026-
6033.doi:10.1063/1.447604
[55] W.J. Stevens, M. Krauss, H. Basch, P.G. Jasien, Can. J. Chem., 70 (1992) 612-
630.doi:10.1139/v92-085
[56] A.V. Marenich, C.J. Cramer, D.G. Truhlar, J. Phys. Chem. B, 113 (2009) 6378-
6396.doi:10.1021/jp810292n
[57] B.M. Bode, M.S. Gordon, J. Mol. Graphics and Modeling, 16 (1999) 133-
138.doi:10.1016/S1093-3263(99)00002-9
[58] T. Koga, H. Tatewaki, H. Matsuyama, Y. Satoh, Theoret. Chem. Acc., 102 (1999) 105-
111.doi:10.1007/s002140050479
[59] H. Tatewaki, T. Koga, J. Chem. Phys., 104 (1996) 8493-8499.doi:10.1063/1.471599
[60] T. Noro, M. Sekiya, T. Koga, S.L. Saito, Chem. Phys. Lett., 481 (2009) 229-
233.doi:10.1016/j.cplett.2009.09.044
[61] D.M. Cropek, A. Metz, A.M. Muller, H.B. Gray, T. Horne, D.C. Horton, O. Poluektov,
D.M. Tiede, R.T. Weber, W.L. Jarrett, J.D. Phillips, A.A. Holder, Dalton Trans., 41 (2012)
13060-13073.doi:10.1039/c2dt30309d
[62] Y. Komiyama, J. Kuwabara, T. Kanbara, Organometallics, 33 (2014) 885-
891.doi:10.1021/om400969p
[63] T. Teratani, T.-a. Koizumi, T. Yamamoto, K. Tanakab, T. Kanbara, Dalton Trans., 40
(2011) 8879-8886.doi:10.1039/c0dt01283a

[64] V. Artero, M. Fontecave, Coord. Chem. Rev., 249 (2005) 1518-
1535.doi:10.1016/j.ccr.2005.01.014
[65] N.A. Samy, V. Alexander, Dalton Trans., 40 (2011) 8630-8642.doi:10.1039/C1DT10811E
[66] M.A.W. Lawrence, W.H. Mulder, M.J. Celestine, C.D. McMillen, A.A. Holder, Inorg.
Chim. Acta, 506 (2020) 119497.doi:10.1016/j.ica.2020.119497
[67] A. Goeppert, M. Czaun, J.-P. Jones, G.K. Surya Prakash, G.A. Olah, Chem. Soc. Rev., 43
(2014) 7995-8048.doi:10.1039/C4CS00122B
[68] J. Schneider, H. Jia, J.T. Muckerman, E. Fujita, Chem. Soc. Rev., 41 (2012) 2036-
2051.doi:10.1039/C1CS15278E
[69] A. Gennaro, A.A. Isse, J.-M. Savéant, M.-G. Severin, E. Vianello, J. Am. Chem. Soc., 118
(1996) 7190-7196.doi:10.1021/ja960605o
[70] P. Singh, J.H. Rheinhardt, J.Z. Olson, P. Tarakeshwar, V. Mujica, D.A. Buttry, J. Am.
Chem. Soc., 139 (2017) 1033-1036.doi:10.1021/jacs.6b10806
[71] D. Spasyuk, S. Smith, D.G. Gusev, Angew. Chem. Int. Ed., 52 (2013) 2538-
2542.doi:10.1002/anie.201209218
[72] B.M. Stadler, P. Puylaert, J. Diekamp, R. van Heck, Y. Fan, A. Spannenberg, S. Hinze, J.G.
de Vries, Adv. Synth. Catal., 360 (2018) 1151-1158.doi:10.1002/adsc.201701607
[73] P.A. Dub, B.L. Scott, J.C. Gordon, Organometallics, 34 (2015) 4464-
4479.doi:10.1021/acs.organomet.5b00432
[74] P. Puylaert, A. Dell'Acqua, F. El Ouahabi, A. Spannenberg, T. Roisnel, L. Lefort, S. Hinze,
S. Tin, J.G. de Vries, Catal. Sci. Technol., 9 (2019) 61-64.doi:10.1039/C8CY02218F
[75] W. Ye, M. Zhao, Z. Yu, Chem. Eur. J., 18 (2012) 10843-
10846.doi:10.1002/chem.201201703
[76] A.L. Meyer, R.B. Turner, Tetrahedron, 27 (1971) 2609-2615.doi:10.1016/S0040-
4020(01)98049-4
[77] P.-C. Yan, G.-L. Zhu, J.-H. Xie, X.-D. Zhang, Q.-L. Zhou, Y.-Q. Li, W.-H. Shen, D.-Q.
Che, Org. Process Res. Dev., 17 (2013) 307-312.doi:10.1021/op3003147
[78] M. Hu, F.-L. Zhang, M.-H. Xie, Synth. Commun., 39 (2009) 1527-
1533.doi:10.1080/00397910802531948
[79] J. Mangas-Sánchez, M. Rodríguez-Mata, E. Busto, V. Gotor-Fernández, V. Gotor, J. Org.
Chem., 74 (2009) 5304-5310.doi:10.1021/jo900784g
[80] M.U. Raja, N. Raja, R. Ramesh, The Open Catalysis Journal, 3 (2010) 30-
33.doi:10.2174/1876214X01003010030

Highlights:
•
•
•
•
Ru(II) κ3-SNN pincer ligands is reported for transfer hydrogenation in air
The frontier molecular orbitals are assessed using DFT methods
Voltammetric studies revealed that compound is inactive for CO2 reduction
TH of the aryl ketones proceeded in air with almost quantitative conversions

Author Statement
Shannen C. Lorraine: Conceptualization, Methodology, Experimentation and Data processing,
Reviewing, Editing.
Mark A.W. Lawrence: Conceptualization, Methodology, Experimentation and Data processing,
Initial draft preparation, Graphical Preparation, Reviewing, Editing.
Michael J. Celestine: Experimentation, Data processing, Editing.
Alvin A. Holder: Writing, Reviewing, Editing.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: