Iridium and Palladium CO2Hydrogenation in Water by Catalyst Precursors with Electron-Rich Tetrazole Ligands

Article abstract of DOI:10.1021/acs.organomet.0c00276

Hydrogenation of CO2 to formate serves as a means of H2 storage. By utilizing robust, electron-rich and bulky N∧N bidentate tetrazolylpyridyl ligands, we have prepared CO2 hydrogenation iridium(III) and palladium(II) catalyst precursors (3-7) that operate in water as a solvent. In situ 1H NMR spectroscopic data obtained when the hydrogenation catalyst is 5 detect that a hydrido-bridged binuclear iridium intermediate compound is formed prior to the formation of the catalytically active iridium hydride species. This hydrido-bridged iridium intermediate is observable by in situ 1H NMR spectroscopy even after extended periods of storage. In addition, we demonstrate that product formation in the CO2 hydrogenation, catalyzed by these tetrazolylpyridyl iridium catalysts, proceeds via either CO2 hydrogenation or NaHCO3 reduction.

Full text of DOI:10.1021/acs.organomet.0c00276

pubs.acs.org/Organometallics
Article
Iridium and Palladium CO2 Hydrogenation in Water by Catalyst
Precursors with Electron-Rich Tetrazole Ligands
Edward Ocansey, James Darkwa, and Banothile C. E. Makhubela*
Cite This: https://dx.doi.org/10.1021/acs.organomet.0c00276
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Hydrogenation of CO2 to formate serves as a means
of H2 storage. By utilizing robust, electron-rich and bulky N∧N
bidentate tetrazolylpyridyl ligands, we have prepared CO2
hydrogenation iridium(III) and palladium(II) catalyst precursors
(3−7) that operate in water as a solvent. In situ 1H NMR
spectroscopic data obtained when the hydrogenation catalyst is 5
detect that a hydrido-bridged binuclear iridium intermediate
compound is formed prior to the formation of the catalytically
active iridium hydride species. This hydrido-bridged iridium
1
intermediate is observable by in situ H NMR spectroscopy even
after extended periods of storage. In addition, we demonstrate that product formation in the CO2 hydrogenation, catalyzed by these
tetrazolylpyridyl iridium catalysts, proceeds via either CO2 hydrogenation or NaHCO3 reduction.
make use of the synergistic effect of imidazolium-based ionic
liquids with their associated acetate anions, which act as an
acid buffer and stabilizer for the catalytically active Ru−H
species.
INTRODUCTION
■
Currently, the use of CO2 as a carbon source in a variety of
synthetic processes is receiving attention as a result of its
abundance, nontoxicity, and low cost.1 Homogeneous catalytic
hydrogenation of CO2 to various products such as formates,
methanol, and dimethyl ether provides one approach to
transform CO2 into valuable chemicals. CO2 is considered an
environmental nuisance and a pollutant; as such, hydro-
genating it serves as a means of storing hydrogen for other
catalytic processes (such as in transfer hydrogenation) and
molecular hydrogenation. Indeed, transfer hydrogenation
reactions are of industrial importance, especially in the
pharmaceutical, perfume, and agrochemical industries, where
it has been shown to be an easy and safer way to work with
hydrogen carriers (such as methanol, formic acid, and
isopropyl alcohol) as opposed to molecular hydrogen.2−4
Several metals, such as iridium, ruthenium, palladium, and
nickel, have been reported as catalysts with varying degrees of
success for the hydrogenation of CO2. The pioneering research
by Inoue and co-workers with Ru, Rh, and Ir triphenylphos-
phine complexes as catalysts gave some of the first examples of
CO2 hydrogenation to formic acid.5 They hydrogenated CO2
to formic acid using a mixture of CO2 and H2 (2.5 MPa/2.5
MPa), the catalyst precursor, and a base at room temperature.
Since then, other metals such as Pd and Ni, as either
homogeneous or heterogeneous catalysts, have been used in
CO2 hydrogenation. In addition, relatively cheaper metals,
such as Mn and Fe, can catalyze CO2 hydrogenation.6,7
However, it is not only the metal in the catalysts that is
effective in the hydrogenation; the nature of the ligand also has
an effect on the catalysis. For example, Weilhard et al.8
reported highly active Ru catalysts for CO2 hydrogenation that
Generally homogeneous catalysts with phosphine ligands are
effective for a variety of catalytic reactions.9−11 For example,
Milstein and co-workers have reported a P∧N∧P iron(II)
dihydride pincer complex that was utilized as a catalyst for CO2
hydrogenation. This catalyst is able to catalyze the hydro-
genation of both CO2 and NaHCO3 to formate with TONs
ranging between 300 and 800, on par with those of other
noble-metal catalysts, even with a low catalyst loading (0.1 mol
%) and short reaction times (ca. 5−16 h).12 Similarly, the
N o z a k i g r o u p h a s r e p o r t e d
a
( 2 , 6 - b i s -
(diisopropylphosphinomethyl)pyridine)iridium(III) trihydride
complex that performs remarkably in hydrogenating CO2
under basic aqueous conditions.13 Even with a 1/1 CO2/H2
mixture and 5 MPa pressure at 200 °C, the Nozaki catalyst
shows remarkable activity (i.e., TOF = 150000 h−1 in 2 h).13
Another (P∧N)iridium complex that has excellent catalytic
activity (TON = 348000) is ((diisopropylphoshinoethyl)-
amine)iridium(III) hydride, reported by Hazari and co-
workers.14 The ligand in this catalyst has an N−H group in
the secondary coordination sphere of the iridium complex.
Received: April 22, 2020
https://dx.doi.org/10.1021/acs.organomet.0c00276
© XXXX American Chemical Society
Organometallics XXXX, XXX, XXX−XXX
A

Organometallics
pubs.acs.org/Organometallics
Article
Scheme 1. Synthesis of Ligands and Metal Complexes
Thus, the excellent catalytic activity is attributed to H bonding
between the N−H group in the ligand and CO2, which
thereafter undergoes reductive elimination to produce the
catalytically active iridium hydride species. Computational
evaluation of the catalytic system indicated that CO2 insertion
could happen more readily in the catalytic system, since it is
easily stabilized by bridging via hydrogen bonding with the
outer-sphere N−H group.14
In addition to the effect of ligands on catalytic activity, the
nature of the solvent used for this reaction has also been shown
to affect the catalytic activity. This is because catalytic
intermediates can be stabilized by solvents or such solvents
may influence the entropy differences between the reactants
and products via solvation.5 For instance, we recently utilized
ruthenium(II) and iridium(III) half-sandwich complexes as
catalysts for the CO2 hydrogenation using a variety of solvents
which suggested a solvent-dependent hydrogenation of CO2,
with THF/H2O being an ideal solvent mixture.15,16
Generally, phosphine ligands are mostly air and moisture
sensitive in catalytic systems for CO2 hydrogenation, and so
nonphosphine ligands, such as N∧N and N∧C ligands, have
also been studied to good effect. For example, the Himeda
group has observed the generation of CO2/H2 in the course of
transfer hydrogenation of ketones with the half-sandwich
complex [Cp*Rh(bpy)Cl]Cl in aqueous solutions of formic
acid. They then later noticed that the Rh complex could also
catalyze CO2 hydrogenation in water.17 This observation then
led to the synthesis of the series of half-sandwich complexes
[(CnMen)M(4,4′-R2-bpy)Cl]+ (n = 5, 6; M = Ir, Rh, Ru; R =
OH, OMe, Me, H) for the hydrogenation of CO2. In this
study, complexes containing hydroxyl groups were found to be
very responsive to protonation and deprotonation of the
hydroxyl substituents at different pHs. This led to a class of
compounds referred to as “proton-responsive ligands”, which
have tunable electronic properties at different pHs and which
greatly improve their ability to catalyze CO2 hydrogenation.
Indeed, the Himeda water-soluble Ir(III) complex [Cp*Ir-
(6,6′-R2-bpy)(OH2)]SO4 (bpy = 2,2′-bipyridine, R= OH) is
able to demonstrate the role “proton-responsive ligands” play
in activating CO2 via electronic and second-sphere coordina-
tion effects.18 As a result of this, they were able achieve high
catalytic activity (TON = 24000) in CO2 hydrogenation at
ambient temperatures and pressures. A strong correlation
between hydrogen (H−), methyl, methoxy, and hydroxy
substituents on the bipyridine ring in the catalysts and the
catalytic activities, as a result of electronic effects, was also
demonstrated. They thus proposed a mechanism that involves
the generation of active Ir−H species, assisted by the pendant
base O− in the catalyst. These oxyanion groups are electron-
rich and contribute to H2 cleavage, while they play stablilizing
roles in various intermediates. The role of the oxyanion was
proven experimentally by NMR experiments and by DFT
calculations.18 A similar observation of O−-assisted H2 cleavage
leading to efficient CO2 hydrogenation by bidentate
(pentamethylcylopentadienyl)iridium(III) pyridine-diazole
catalysts (TON = 11000)19 under mild reaction conditions
of temperature (50 °C) and pressure (1 MPa CO2/H2 1/1)
was reported. Here, the catalyst precursors contain a hydroxyl
and uncoordinated NH functional groups that are responsible
for the “proton-responsive” behavior.19
Tetrazole-containing compounds have recently found
applications in CO2 capture, having been used as linkers in
B
https://dx.doi.org/10.1021/acs.organomet.0c00276
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
metal−organic frameworks (MOFs).20 In addition, tetrazole-
stabilized catalyst precursors have served as sources of
electron-rich N atoms, which contribute toward improved
catalytic activity at the metal center in hydrogenation
reactions.18 It is the ability of tetrazole-containing compounds
to capture and possibly activate CO2 that led to the present
study. We designed new electron-rich half-sandwich
(tetrazolylpyridyl)iridium(III) and (tetrazolylpyridyl)-
palladium(II) chloride complexes, with the potential to exhibit
“proton responsiveness” at the N2 position of the tetrazole
ring, and investigated their ability as catalysts for CO2
hydrogenation.
undertaken with the iridium precursor, and when reactions
were performed with compound 2 and the iridium precursor,
caution was taken by connecting the Schlenk tube to a vacuum line
in order to release any built-up pressure and also minimize
agitation in the initial stages of the reaction. Synthesis of the
iridium complex 4 (Scheme 1) was achieved by reacting
compound 2 with [Ir(C5Me5)Cl2]2 in a 2/1 ratio in
dichloromethane. Compound 4 was obtained in pure form as
a yellow solid in quantitative yield after the solvent was
removed in vacuo. The 1H NMR spectrum of 4 indicated more
significant shifts downfield in the proton signals of about 0.4−
0.8 ppm, especially for the pyridinyl protons, in comparison to
the 1H NMR peaks of 2. In contrast to compound 2, there was
no indication from the 1H NMR spectrum that 4 had
rotameric forms, since the spectrum of N-k-tert-butyl-
substituted iridium complex 4 (i.e., the 2-anti form) is
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the Tetrazole-
Based Ligands. The synthesis of compound 1 was achieved
by first reacting 2-cyanopyridine with a slight excess of NaN3 in
the presence of excess Et3N·HCl (Scheme 1). Compound 1
was then isolated after adding a solution of HCl to precipitate
the product as an air- and moisture-stable brown solid in
excellent yield. Compound 1 is soluble in most organic
solvents and was characterized by NMR spectroscopy and
elemental analysis. The 1H NMR spectrum of 1 showed only a
slight downfield shift of about 0.1−0.2 ppm in comparison to
1
expected to be different from the H NMR spectrum of N-l-
tert-butyl-substituted iridium complex 4 (i.e., the 2-syn form).
1
Indeed, H NMR data show the product to be the sterically
hindered N-k-tert-butyl-substituted iridium complex 4.
Reaction of 4 with Ag2SO4 as a halide abstractor in water
yielded a sulfato complex, 5, instead of the expected iridium
aqua complex, 6. This is in contrast to various reports of halide
abstraction from [(N∧N)Ir(Cl)] complexes with Ag2SO4 in
water that produce the corresponding aqua complexes
[(N∧N)Ir(OH2)]+.24 For most iridium chloro complexes,
halide abstraction in water leads to cationic iridium aquo
species. However, a (N∧N)iridium sulfato complex formed
following chloride abstraction from the (N∧N)iridium chloro
starting material, using Ag2SO4 as reported in the literature
supports the formation of 5 from 4.25 Formation of the iridium
aqua complex 6 was eventually achieved by reaction of the
iridium sulfato complex 5 with KPF6 in water. The formation
of both iridium complexes 5 and 6 was confirmed by 1H NMR
spectroscopy. In addition, the 13P{1H} NMR of 6 with peaks
around −143 ppm confirmed the presence of the PF6
counterion. Further support for the formation of 5 and 6
was provided by mass spectrometry, for which m/z [M + H]+
628.1513 and [M]+ 550.1962 were observed for 5 and 6,
respectively. The reaction of complex 5 with HCOONa,
followed by a counterion exchange with NaPF6, resulted in the
formation of the iridium hydride species 7. The presence of the
hydride ligand was confirmed by a 1H NMR signal peak
around −11 ppm that integrates for one proton (Figure SI-1).
Similarly, the 13P{1H} NMR spectrum (Figure SI-1) with
peaks at around 140 ppm confirmed the presence of the PF6
counterion in complex 7. Complexes 5−7 are all soluble in
DMSO; however, only 5 is soluble in water.
Single Crystal X-ray Structure of Complex 4. Yellow
single crystals of 4 were grown by slow evaporation of the
complex in dichloromethane. Crystallographic data and the
molecular structure of this complex are shown in Table 1 and
Figure 1. Complex 4 crystallizes in the monoclinic space group
C2/c with a piano-stool geometry (Figure 1). As expected, the
iridium−N(pyridine) bond distance (2.146(7) Å) is longer
than that of the iridium−N(tetrazole) bond (2.078(7) Å).
This suggests a stronger iridium−N(tetrazole) bond in
comparison to the iridium−N(pyridine) bond. This is possibly
as a result of delocalization of the several lone pairs of the
nitrogens in the tetrazole ring, resulting in enhanced electron
donation to the iridium center and hence a shorter Ir−N bond
length. The bond lengths between iridium and the donating
ligands as well as the bond angles are in conformity with
1
the H NMR spectrum of 2-cyanopyridine.
Alkylation of 1 with tert-butyl alcohol (Scheme 1) resulted
in the formation of 2-tert-butyl-5-(2-pyridyl)-2H-tetrazole (2).
Compound 2 was obtained as a white solid in good yield
(77%) and is soluble in organic solvents but insoluble in water.
The key 1H NMR peak of 2 is at 1.8 ppm, integrating for nine
protons, confirming that the alkylation of 1 had occurred. In
fact, selective alkylation of the two nitrogen atoms b and c in 1
is possible in the presence of a strong acid, because on
protonation of the nitrogen atoms of the pyridine the d
nitrogen of 1 should result in the delocalization of charge
among nitrogens a, e, and d (Scheme 1). This would lead to
two equivalent nitrogens (i.e., b and c) as available sites for the
alkylating agent to attack.21 It is also interesting to note that
due to rotation along the C−C bond between the pyridine and
tetrazole rings, compound 2 can exist as rotamers. Indeed,
quantum mechanical calculations have shown that in DMSO
and DMF the total energy of 2-syn is only about 1 kJ mol−1
lower than that of 2-anti.22
Synthesis of Tetrazolyl Palladium and Iridium
Complexes. The synthesis of the new palladium complex 3
(Scheme 1) was achieved by reacting a dichloromethane
solution of compound 1 with [PdCl2(MeCN)2] in a 1/1 ratio.
Complex 3 precipitated out of solution as a yellowish solid in
good yield and is partially soluble in chloroform but
1
completely soluble in DMSO. The H NMR spectrum of 3
showed only slight shifts in proton signals of 0.1−0.2 ppm in
1
comparison to the H NMR spectrum of 1. For example,
proton f (Scheme 1) shifted downfield by about 0.1 ppm while
proton h shifted downfield by about 0.2 ppm and proton i
shifted upfield by about 0.2 ppm, and there were no observed
shifts in the signal of g.
Interestingly, the reaction of compound 1 with [Ir(C5Me5)-
Cl2]2 in a 2/1 ratio resulted in an immediate loud sound
accompanied by the violent ejection of the septum that was
used to seal the Schlenk tube in which the reaction was run.
We suspect this was the result of the detonation of the
tetrazole to give off molecular nitrogen,23 possibly from stirring
of the reaction mixture. There were no further reactions of 1
C
https://dx.doi.org/10.1021/acs.organomet.0c00276
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
Table 1. Crystal Data and Structure Refinement Details for
Complex 4
empirical formula
formula wt
temp/K
cryst syst
space group
a/Å
b/Å
c/Å
α/deg
C20H28N5Cl2Ir
601.14
100
monoclinic
C2/c
33.847(7)
9.694(2)
16.767(4)
90
β/deg
γ/deg
V/Å3
119.082(4)
90
4808.1(17)
48
Figure 2. Electrostatic potential map of 4.
Z
ρcalcd/g cm−3
μ/mm−1
F(000)
1.6504
5.796
2217.2
0.3 × 0.2 × 0.2
Mo Kα (λ = 0.71073)
2.76−56.76
(Table SI-T1). This indicated that water plays a crucial role in
the process during the CO2 hydrogenation reaction. Water
may serve as a labile ligand which stabilizes various reaction
intermediates during the catalytic reaction. In addition, water
could contribute toward increasing reaction rates by facilitating
hydrogen bonding between the generally accepted catalytically
active iridium hydride species and oxygen atoms present in
CO2.19
cryst size/mm3
radiation
2θ range for data collection/
deg
index ranges
no. of rflns collected
no. of indep rflns
no. of data/restraints/params
goodness of fit on F2
final R indexes (I ≥ 2σ(I))
final R indexes (all data)
largest diff peak, hole/e Å−3
−45 ≤ h ≤ 44, −12 ≤ k ≤ 12, −22 ≤ l ≤ 22
77162
6017 (Rint = 0.2239, Rσ = 0.1067)
6017/0/267
This observation was further noticed when a water/THF
mixture (4/1) was used, which resulted in a drop in TONs. As
expected, increasing the temperature from 120 to 160 °C also
resulted in an increase in formate production and the
corresponding TONs. Changing the base from KOH to
DBU did not result in a significant change in TON; however,
using the weak base triethylamine resulted in a drop in TONs.
Using an equimolar amount of K2CO3 resulted in an increase
in TON, possibly as a result of an increase in the amount of
potassium ions present in solution. A higher concentration of
K+ readily stabilizes any formate present, resulting in increased
formate generation. Reducing the amount of CO2 with respect
to H2 resulted in an increase in TON, with the optimum ratio
being determined as a 1/2 CO2/H2 mixture. Reducing the
reaction time from 24 to 18 h resulted in a drop in both the
amount of formate and TON; however, lowering the catalyst
loading to 1 μmol resulted in a significant drop in the amount
1.004
R1 = 0.0563, wR2 = 0.1293
R1 = 0.0942, wR2 = 0.1513
3.94, −2.27
1
of formate formed but an increased TON. Representative H
and 13C{1H} NMR spectra after CO2 hydrogenation reaction
using 4 are shown in Figure 4.
Figure 1. Molecular structure of complex 4. Hydrogen atoms, the Cl
counterion, and residual solvents are omitted for clarity. Selected
bond distances (Å) and bond angles (deg); Ir(1)−Cl(1) 2.394(2),
Ir(1)−N(1) 2.078(7), Ir(1)−N(2) 2.146(7); N(1)−Ir(1)−Cl(1)
88.3(2), N(2)−Ir(1)−N(1) 75.0(3).
Precatalysts 3 and 5−7 were also used for the CO2
hydrogenation under reaction conditions identical with those
used for 4. This was done to evaluate the effect of different
metals as well as the various coligands (hydride, aqua, sulfato)
(Table 2). All five precatalysts were used to run the
hydrogenation reaction in the presence of mercury and in
each instance gave slightly lower TONs (Table 2) in
comparison to the reactions without mercury, signifying that
the active catalysts in all cases were largely homogeneous.
Precatalyst 3 is the least active (TON = 91; Table 2, entry 1),
while complex 7 is the most active (TON = 561; Table 2, entry
5). This is expected, as the other iridium precatalysts (4−6)
would have to generate the iridium hydride species in situ
before catalyzing the CO2 hydrogenation reaction. However, it
is interesting to note that the aqua complex 6 was slightly more
active than the sulfato complex 5. This suggests that the
formation of an iridium aqua species may be an important
intermediate in the catalytic process. The reaction mixtures at
the end of the hydrogenation showed a peak at 160 ppm in the
13C{1H} NMR spectra of these mixtures, assignable to a
several iridium N∧N bidentate complexes that have been
reported in the literature.26−28
A space-filling model of 4 showing the electrostatic potential
map indicates the highly electron rich site is found at the
iridium center due to electron donation from the Cp* and the
pyridyl-tetrazole donor sites in 4. In addition, N2 in the
tetrazole ring is fairly electron rich (Figure 2), which suggests
that it is possibly available to act as a “proton-responsive” site
during the catalytic cycle of CO2 hydrogenation.
Optimization of CO2 Hydrogenation Reaction Con-
ditions Using 4. When CO2 hydrogenation was carried out
using 4 as a catalyst precursor in either ethanol or DMSO as
the solvent, there was only a marginal amount of formate
produced with a TON of around 40 (Figure 3). When water
was used as the solvent, TONs almost doubled to around 80
D
https://dx.doi.org/10.1021/acs.organomet.0c00276
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
Figure 3. Optimization of reaction conditions using 4. The reactions were performed in 10 mL of the appropriate solvent (or mixture) and 3.55
mmol of base at the appropriate temperature and total pressure (50 bar) with stirring (1500 rpm) in the presence of complex 4 (μmol) for 24 h
−
using DMF as a standard. TON = (calculated mmol of [HCO2 ])/(mmol of catalyst).
Table 2. Evaluating Precatalysts 3−7 for CO2
a
Hydrogenation
product
TON
d
c
entry
catalyst
(product )/mmol [HCO2−]/mM
(TON )
1
2
3
4
5
6
7
3
4
5
6
7
4
0.45 (0.39)
1.42 (1.32)
1.97 (1.80)
2.16 (1.88)
2.80 (2.02)
45
142
197
216
280
58
91 (78)
283 (264)
396 (360)
433 (376)
561 (403)
116
b
0.58
[Cp*Ir(L1)
0.26
0.91
26
1300
e
(Cl)]Cl
8
[Cp*Ir(L2)
91
4600
e
(Cl)]Cl
a
1
Figure 4. Typical (A) H NMR spectrum and (B) 13C{1H} NMR
Reaction conditions unless indicated otherwise: performed in 10 mL
of H2O and 3.55 mmol of K2CO3 at 160 °C under 50 bar of CO2/H2
(1/2) with stirring (1500 rpm) in the presence of a complex (5 μmol)
spectrum of the contents of the reaction mixture after the CO2
hydrogenation reaction using DMF as an internal standard.
b
for 24 h. Reaction carried out in the presence of only 33 bar of H2.
c
−
Calculated amount of [HCO2 ] in the presence of excess mercury.
d
−
Calculated TON of [HCO2 ] when the reaction was carried out in
bicarbonate ion (Figure SI-3). This suggested that the actual
species undergoing hydrogenation are bicarbonate ions formed
in solution from a combination of both CO2 and K2CO3. Such
a hydrogenation of bicarbonate ions has been reported in the
literature.29 In addition, CO2 has been shown to exist in a
variety of forms in aqueous solution in the presence of
carbonate ions.30 In order to confirm this, the hydrogenation
reaction was carried out with K2CO3 but without CO2 (Table
2, entry 6). The product from this reaction was formate but at
reduced TON, confirming the participation of the carbonate
ion in the CO2 hydrogenation when K2CO3 is used as the base.
The catalytic activities of 4−7 in comparison to those of
similar N∧N bidentate azole iridium Cp* precatalysts19 are
inferior. However, the amount of product formed for these
N∧N bidentate azole iridium Cp* precatalysts19 was lower in
comparison to 4−7. Thus, higher TONs at a lower catalyst
e
the presence of excess mercury. Reference 19: L1 = 2-(1H-imidazol-
2-yl)pyridine, L2 = 2-(1H-pyrazol-3-yl)pyridine. The reactions were
performed in 10 mL of a 2.0 M KHCO3 aqueous solution (pH 8.5) at
50 °C under 1.0 MPa of H2/CO2 (1/1) with stirring (1500 rpm) in
the presence of a complex (20 μm) for 24 h.
loading do not necessarily translate to higher TONs at
increased catalyst loading. It has to be noted that, for these
N∧N bidentate azole iridium Cp* precatalysts,19 the increased
activity is attributed to the presence of proton-responsive
groups on the ligands. As such, it could be reasonably expected
that the activity of 4−7 may be improved by introducing
proton-responsive groups.
Proposed Mechanism for the Iridium-Catalyzed CO2
Hydrogenation. When 5 was reacted with H2 in DMSO-d6,
E
https://dx.doi.org/10.1021/acs.organomet.0c00276
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
the new iridium species B was observed, as evidenced by its 1H
NMR spectrum, which displayed a peak at around −14 ppm
(Figure SI-4). If B were the molecular hydrogen complex,
doubling of the tetrazole aromatic peaks would be seen, as was
reported for a similar N∧N bidentate Cp* iridium dihydrogen
complex (where the N∧N bidentate ligand was an
centers). Similar observations of such a monohydrido-bridged
iridium dinuclear system have been reported in the literature.33
Intermediate B was found to be stable for ∼13 months when it
was left under ambient conditions.
A mercury drop test confirmed that CO2 was hydrogenated
by a molecular pathway (Table 2), since addition of metallic
mercury to the reaction contents did not inhibit the
transformation of CO2. As such, a plausible reaction
mechanism involving 4 as a catalyst precursor is indicated in
Scheme 3. In the presence of a base and water, 4 undergoes an
aquation reaction. This results in the chloro ligand being
replaced with a water ligand to give 6 (Scheme 2).
imidazole).31 However, the H NMR spectrum of B showed
1
no such doubling of the tetrazole aromatic peak signals. In
addition, since the dihydrogen iridium complex is expected to
be very acidic, it should undergo facile deprotonation in water
in order to yield 7.32 However, this was not observed when B
was kept in a water solution containing DMSO-d6 (Figure 5).
In the presence of H2, 6 (which potentially exists in
equilibrium with intermediate A) is converted to the hydrido-
bridged iridium dinuclear species B. This step is probably
followed by addition of a second mole of H2 in the presence of
a base, leading to intermediate C (7). However, with the
presence of carbonate ions in aqueous solution, formation of
the hydrido complex C might also be achieved by a
bicarbonate- or water-assisted H2 heterolysis.19 It appears
that although N-2 is electron-rich (Figure 2), this is not
sufficient for it to act as a proton-responsive group that results
in high TON in the range that was observed by Himeda and
co-workers.26 This is evidenced from optimizations of the
solvent and base, for which higher TONs were obtained when
a water/carbonate mixture was used in comparison to other
solvent/base mixtures.
It is interesting to note that, for most of these half-sandwich
iridium(III) catalytic systems, it has been proven by either
experiments or computational calculations that CO2 is the
actual substrate undergoing reduction.18,19 However, for the
tetrazolylpyridyl catalytic systems, an alternative route toward
formate formation is proposed. This route involves insertion of
a bicarbonate ion into the iridium-hydride bond of 7, yielding
intermediate D. This can then be followed by loss of a
hydroxyl group from D to afford intermediate E, which then
subsequently reductively eliminates, resulting in expulsion of
formate. Reactions of 7 with NaHCO3, under CO2 pressure
and separately with CO2 in the presence of NaOH, provided
1
Figure 5. H NMR spectra of (a) 7 and (b) intermediate B.
We have proposed the structure of B to be a hydrido-bridged
iridium dinuclear species containing an Ir···H···Ir bond (Figure
5). In addition, the hydride signal of B is upfield in comparison
to that of 7 (−11 ppm). This implies that the hydride in B is
more hydridic (in this case being bonded to two iridium
Scheme 2. Formation of Catalytically Active Iridium Hydride Species
F
https://dx.doi.org/10.1021/acs.organomet.0c00276
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
1
Figure 6. Investigation of the route of formation of iridium formato species by H NMR spectroscopy. The reaction was performed in a high-
pressure NMR tube; to 5 mg of 7 was added 10 mg of NaHCO3 in 0.5 mL dmso-d6 and the reaction monitored over time.
Scheme 3. Plausible Reaction Mechanism of N∧N Bidentate Iridium Complexes for the CO2 Hydrogenation Reaction
valuable insights into the actual hydrogenation step leading to
the formation of the iridium formato species E (Figure 6).
The reaction of 7 with NaHCO3 over a period of 3 h
showed a gradual disappearance of the hydride peak of
complex 7 (at −11 ppm) with the formation of a new iridium
species with peaks at around −15 and −16 ppm. On
consideration of the very low chemical shifts at which these
peaks appear, they are likely to be due to an iridium hydride
bicarbonate species that is bridged by a hydride or possibly
species D, as indicated in Scheme 3.
Similar peaks were observed when a combination of CO2
and NaOH was reacted with 7 (Figure 7). This is possibly as a
result of the in situ formation of bicarbonate species from a
basic aqueous CO2 mixture. However, in the absence of a base,
there seems to be no CO2 insertion into the metal−hydride
bond. This is in contrast to reported cases of various N∧N
bidentate iridium hydride systems.5,19 However, such direct
CO2 insertion in the metal−hydride bond cannot be
completely ruled out, as hydrogenating CO2 in the presence
of Et3N resulted in formate production. As observed from the
in situ H NMR experiments, there is no reaction between B
1
and NaHCO3, CO2, or a mixture of CO2 + NaOH, since the
peak at ∼−14 ppm remains over the course of the reaction.
This indicates that 7 is the catalytically active species for the
CO2 hydrogenation reaction. Finally, in the last step of the
reaction, the formate is expelled from the iridium formato E
and further stabilized by K+ ions while A is regenerated. In
attempts to isolate the formato species E, iridium hydride 7
was reacted with excess NaHCO3 at 70 °C for 16 h. Over the
course of the reaction, 7, which is not soluble in water,
gradually dissolves in water, resulting in a yellow solution. The
1H NMR spectrum of this reaction solution (Figure SI-6)
showed the disappearance of the hydride peak accompanied by
the appearance of a new formate peak at ∼8.4 ppm. However,
G
https://dx.doi.org/10.1021/acs.organomet.0c00276
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
Figure 7. Investigation of the direct CO2 insertion route for the formation of iridium formato species by 1H NMR spectroscopy. (blue spectrum)
The reaction was performed in a high-pressure NMR tube; to 5 mg of 7 in 0.5 mL of dmso-d6 was added 50 bar of CO2 and the reaction monitored
over time. (red spectrum) The reaction was performed in a high-pressure NMR tube; to 5 mg of 7 in 0.5 mL of dmso-d6 were added 5 mg of
NaOH and 50 bar of CO2 and the reaction was monitored over time.
1
a Thermos Scientific FLASH 2000 CHNS-O Analyzer. Mass
spectrometry was performed using a Waters Synapt G2 mass
spectrometer with both ESI positive and cone voltage 15 V. XRD
spectra were obtained with a Bruker APEX-II CCD diffractometer.
Synthesis of 2-(2H-Tetrazol-5-yl)pyridine (1). Into a round-
bottom flask containing 100 mL of toluene were placed 5.20 g (50
mmol) of 2-cyanopyridine, 4.2 g (65 mmol) of NaN3, and 20.6 g (150
mmol) of Et3N·HCl, and the mixture was stirred at 110 °C for 24 h.
After the reaction mixture was cooled to room temperature, 36% HCl
was added until the mixture was acidic (pH 2) to precipitate the crude
product. The mixture was then filtered, and the residue was washed
with 36% HCl. The solid residue was then oven-dried for 20 min at
110 °C. The crude product was purified by dissolving in methanol
followed by filtration and drying of the filtrate. Appearance: brown
solid (yield 6.1 g, 83%). Solubility: soluble in water, methanol, and
a variety of H NMR signals were observed, and this could be
as a result of the formation of iridium aqua/hydroxyl species.
CONCLUSION
■
Two tetrazole-containing ligands and five tetrazole-containing
iridium(III) and palladium(II) complexes have been synthe-
sized and characterized using NMR spectroscopy, mass
spectrometry, and infrared spectroscopy and in addition
using single-crystal X-ray diffraction for complex 4. The
complexes were evaluated for their catalytic activity in CO2
hydrogenation, for which there were varying degrees of
success. The highest TON achieved was 560, which is
significantly lower than those of other iridium(III) half-
sandwich catalytic systems previously reported, therefore
implying that the N2 position of the tetrazolyl ligand in the
current systems is poorly proton responsive. Nonetheless, the
iridium precatalysts were found to be more active in
comparison to the palladium variant. The complexes were
found to hydrogenate CO2 in the presence of water via a
homogeneous catalytic approach. The iridium precatalysts
were observed to first undergo aquation in water. This was
followed by formation of a bridging hydrido and a subsequent
iridium hydride species. This iridium hydride species then goes
on to hydrogenate bicarbonate species or CO2 that was present
in solution, yielding formate.
1
3
DMSO. H NMR (400 MHz, DMSO, 30 °C) (ppm): 8.80 (d, J =
4.4 Hz, 1H, Harom), 8.23 (d, 3J = 8.0 Hz, 1H, Harom), 8.09 (t, 3J = 6.4
3
Hz, 1H, Harom), 7.66 (t, J = 5.2 Hz, 1H, Harom). 13C{1H} NMR
(100.6 MHz, DMSO-d6): 155.26; 150.53; 144.14; 138.71; 126.58;
123.08. Anal. Calcd for C6H5N5: C, 48.98; H, 3.43; N, 47.60. Found:
C, 48.78; H, 3.30; N, 47.44.
Synthesis of 2-(2-(tert-Butyl)-2H-tetrazol-5-yl)pyridine (2).
In a round-bottom flask, tert-butyl alcohol (0.32 mL, 3.5 mmol) was
slowly added to a suspension of 0.515 g of 2-(2H-tetrazol-5-
yl)pyridine (1; 3.5 mmol) in 1.25 mL of 65% perchloric acid with
stirring. The mixture was then stirred for 2 h at room temperature,
after which 10 mL of 10 M NaOH was added. The aqueous mixture
was then extracted with 3 × 25 mL portions of chloroform and dried
in vacuo to yield 2. Appearance: white solid (yield 0.55 g, 77%).
EXPERIMENTAL SECTION
■
1
Solubility: soluble in methanol, chloroform, and DMSO. H NMR
All reactions were carried out in air unless otherwise stated. All
solvents used were reagent grade, purchased from Sigma-Aldrich, and
dried under nitrogen before use. 2-Cyanopyridine, sodium azide,
triethylamine hydrochloride, tert-butyl alcohol, and 65% perchloric
acid were purchased from Sigma-Aldrich and used without further
purification. PdCl2 and iridium(III) chloride were purchased from
Heraeus (South Africa) and used as received. [PdCl2(MeCN)2]34 and
3
(400 MHz, CDCl3, 30 °C) (ppm): 8.77 (d, J = 4 Hz, 1H, Harom),
8.25 (d, 3J = 7.6 Hz, 1H, Harom), 7.82 (t, 3J = 1.6 Hz, 1H, Harom), 7.36
(t, 3J = 4.8 Hz, 1H, Harom), 1.80 (s, 9H, HtBu). 13C{1H} NMR (100.6
MHz, DMSO): 164.18; 150.56; 146.91; 138.03; 125.61; 122.81;
64.69; 29.31. Anal. Calcd for C10H13N5: C, 59.10; H, 6.45; N, 34.46.
Found: C, 59.22; H, 6.30; N, 34.44.
35
Synthesis of [PdCl2(1)] (3). Into a Schlenk tube containing 10
mL of CH2Cl2 were placed 100 mg (0.67 mmol) of 2-(2H-tetrazol-5-
yl)pyridine (1) and 176 mg (0.67 mmol) of [PdCl2(MeCN)2]. The
reaction mixture was then stirred for 24 h at room temperature. After
the reaction time had elapsed, the solvent was reduced in vacuo to
yield the product. Appearance: brown solid (yield 176 mg, 81%).
Solubility: soluble in DMSO. 1H NMR (400 MHz, DMSO-d6, 30 °C)
[Ir(C5Me5)Cl2]2 were synthesized according to literature proce-
dures. NMR spectra were recorded on a Bruker 400 MHz NMR
spectrometer (1H at 400 MHz, 13C{1H} at 100 MHz, 13P{1H} NMR
at 161.99 MHz). Spectrometer chemical shifts were reported relative
to the internal standard tetramethylsilane (δ 0.00 ppm) and
referenced to the residual proton and carbon signals at 7.24 and
77.0 ppm, respectively, of CDCl3. Infrared spectra were obtained neat
using a PerkinElmer Spectrum BX II spectrometer fitted with an ATR
probe. Melting points were obtained using a Gallenkamp 5A 6797
digital melting-point apparatus. Elemental analysis was performed on
3
3
(ppm): 8.98 (d, J = 5.6 Hz, 1H, Harom), 8.22 (t, J = 7.6 Hz, 1H,
3
3
Harom), 8.11 (d, J = 6.8 Hz, 1H, Harom), 7.67 (t, J = 6.8 Hz, 1H,
Harom). HR-ESI-MS: m/z [M − N2]+ 293.0515. Anal. Calcd for
H
https://dx.doi.org/10.1021/acs.organomet.0c00276
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
C6H5N5PdCl2: C, 22.21; H, 1.55; N, 21.59. Found: C, 22.51; H, 1.11;
N, 21.77.
90 °C under vacuum to remove the solvent. Appearance: yellowish
brown solid (yield 15 mg, 75%). Solubility: soluble in DMSO and
1
Synthesis of [IrCp*Cl(2)]Cl− (4). Into a Schlenk tube containing
5 mL of CH2Cl2 were placed 200 mg (0.98 mmol) of 2-(2-(tert-
butyl)-2H-tetrazol-5-yl)pyridine (2) and 388 mg (0.49 mmol) of
[Ir(C5Me5)Cl2]2. The reaction mixture was then stirred for 24 h at
room temperature. After the reaction time had elapsed, the solvent
was reduced in vacuo to yield the product. Appearance: yellow solid
insoluble in chloroform, diethyl ether, and hexane. H NMR (400
3
MHz, DMSO, 30 °C) (ppm): 8.76 (d, J = 6.2 Hz, 2H, Harom), 8.15
(d, 3J = 7.6 Hz, 2H, Harom), 8.02 (t, 3J = 7.2 Hz, 2H, Harom), 7.57 (t, 3J
t
= 6.2 Hz, 2H, Harom), 1.98 (s, 30H, HCp*), 1.77 (s, 18H, HBu),
−14.34 (s, 1H, HIr−H−Ir). 13C{1H} NMR (100.6 MHz, DMSO-d6):
165.32; 154.66; 145.06; 142.40; 130.55; 124.64; 94.90; 69.85; 29.00;
8.55. HR-ESI-MS: [M − 2CH3]+ m/z 1032.27, [M − NtBu]+ m/z
993.26, [M − NtBu − tBu]+ m/z 934.16. Anal. Calcd for
C40H58Ir2N10O8S2: C, 38.27; H, 4.66; N, 11.16. Found: C, 38.62;
H, 4.91; N, 11.44.
1
(yield 545 mg, 93%). Solubility: soluble in DCM and CHCl3. H
3
NMR (400 MHz, CDCl3, 30 °C) (ppm): 9.10 (d, J = 5.2 Hz, 1H,
Harom), 8.35 (m, 2H, Harom), 8.11 (t, 3J = 5.6 Hz, 1H, Harom), 1.86 (s,
9H, HBu), 1.77 (s, 15H, HCp*). 13C{1H} NMR (100.6 MHz,
t
General CO2 Hydrogenation Reaction. In a stainless steel
reactor, an appropriate amount of the appropriate catalyst was
dissolved in 10 mL of the appropriate solvent. A 3.55 mmol amount
of the appropriate base was added and the resulting solution
pressurized with the required CO2/H2 mixtures. The reaction mixture
was then stirred at an appropriate temperature for the required time at
1500 rpm. After the appropriate reaction time had elapsed, the reactor
was cooled and the gases were slowly vented. A 0.4 mL portion of the
contents of the reactor was sampled and added to 0.2 mL of D2O and
5 μL of DMF (as a standard). The amount of product (formate)
obtained was calculated by integral relations between the singlet
formate peak at 8.3 ppm and the singlet DMF formamide peak at 7.9
DMSO): 165.81; 153.41; 142.74; 141.23; 131.30; 124.27; 89.90;
69.46; 29.33; 9.31. HR-ESI-MS: [M]+ m/z 566.1660. Anal. Calcd for
C20H28Cl2IrN5: C, 39.93; H, 4.69; N, 11.64. Found: C, 40.48; H,
4.30; N, 11.44.
Synthesis of [IrCp*SO4(2)] (5). Into a Schlenk tube containing 5
mL of water was placed 20 mg (0.033 mmol) of 4 and the mixture
stirred for 10 min followed by the addition of 10.37 mg (0.033 mmol)
of Ag2SO4. The reaction mixture was then stirred for 16 h at 40 °C.
After the reaction time had elapsed, the reaction mixture was filtered
and the filtrate dried in vacuo to yield the product. Appearance: yellow
1
solid (yield 18 mg, 87%). Solubility: soluble in water and CHCl3. H
3
NMR (400 MHz, D2O, 30 °C) (ppm): 9.06 (d, J = 5.2 Hz, 1H,
1
3
3
ppm in the H NMR spectrum of the sample.
Harom), 8.38 (d, J = 7.6 Hz, 1H, Harom), 8.29 (t, J = 7.6 Hz, 1H,
3
t
Experimental Description of Generating an Electrostatic
Potential Map. Calculation was carried out with the Spartan 14
Molecular Modeling program (Wavefunction, 2014) at the semi-
empirical PM6 level of theory. The starting geometry of the molecular
system was constructed using the Spartan graphical model builder and
minimized interactively using the sybyl force field. The geometry was
fully optimized without any symmetry constraints. The optimized
geometry was subjected to full frequency calculations to verify the
nature of the stationary points. The equilibrium geometry was
characterized by the absence of imaginary frequencies.
Harom), 7.84 (t, J = 6.0 Hz, 1H, Harom), 1.77 (s, 9H, HBu), 1.63 (s,
15H, HCp*). 13C{1H} NMR (100.6 MHz, D2O): 166.71; 152.76;
143.76; 142.41; 130.21; 124.34; 89.75; 69.92; 28.16; 8.86. HR-ESI-
MS: [M + H]+ m/z 628.1513. Anal. Calcd for C20H28IrN5O4S: C,
38.33; H, 4.50; N, 11.17. Found: C, 38.48; H, 4.77; N, 11.44.
Synthesis of [IrCp*H2O(2)](PF6)2 (6). Into a Schlenk tube
containing 5 mL of water was placed 27 mg (0.043 mmol) of 5 and
the mixture stirred for 10 min followed by the addition of 14.47 mg
(0.086 mmol) of NaPF6. The reaction mixture was then stirred for 30
min at room temperature. After the reaction time had elapsed, the
reaction mixture was filtered. To the residue was added 15 mL of
methanol, and the resulting suspension was filtered. Drying this
filtrate in vacuo for 6 h yielded the desired product. Appearance:
yellow solid (yield 31 mg, 86%). Solubility: soluble in methanol and
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00276.
■
sı
1
insoluble in water and CHCl3. H NMR (400 MHz, MeOD, 30 °C)
3
3
(ppm): 9.30 (d, J = 5.6 Hz, 1H, Harom), 8.61 (d, J = 7.6 Hz, 1H,
1H NMR and mass spectra of selected complexes and
reactions as well as data for the optimization of reaction
conditions (PDF)
Cartesian coordinates of the calculated structure (XYZ)
Spartan file for complex 4 (ZIP)
3
3
Harom), 8.52 (t, J = 8.0 Hz, 1H, Harom), 8.09 (t, J = 6.0 Hz, 1H,
Harom), 1.97 (s, 9H, HBu), 1.82 (s, 15H, HCp*). 13C{1H} NMR (100.6
t
MHz, D2O): 152.95; 142.61; 141.27; 130.39; 124.87; 124.11; 90.40;
90.22; 27.87; 7.62. HR-ESI-MS: [M]+ m/z 550.1962. Anal. Calcd for
C20H30F12IrN5OP2: C, 28.64; H, 3.61; N, 8.35. Found C, 28.48; H,
3.77; N, 8.44.
Synthesis of [IrCp*H(2)](PF6) (7). Into a Schlenk tube
containing 5 mL of water was placed 20 mg (0.0319 mmol) of 5
and the mixture stirred for 10 min. This was followed by the addition
of 867.76 mg (12.76 mmol) of HCOONa, and the mixture was
heated at 70 °C for 30 min. KPF6 (0.0587g, 0.0319 mmol) was then
added to the reaction mixture, which was then stirred for 30 min at
room temperature. After the reaction time had elapsed, the reaction
mixture was filtered and the residue dried in an oven at 80 °C for 20
min to yield the desired product. Appearance: yellow solid (yield 18
Accession Codes
CCDC 1935810 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
AUTHOR INFORMATION
Corresponding Author
mg, 83%). Solubility: soluble in DMSO and insoluble in water. H
■
3
NMR (400 MHz, DMSO, 30 °C) (ppm): 8.99 (d, J = 5.6 Hz, 1H,
3
3
Harom), 8.50 (d, J = 7.6 Hz, 1H, Harom), 8.26 (t, J = 7.2 Hz, 1H,
Banothile C. E. Makhubela − Research Centre for Synthesis
and Catalysis, Department of Chemical Sciences, University of
Johannesburg, Auckland Park 2006, South Africa;
orcid.org/0000-0002-2292-7400; Email: bmakhubela@
uj.ac.za
Harom), 7.79 (t, 3J = 6.0 Hz, 1H, Harom), 1.84 (s, 15H, HCp*), 1.77 (s,
9H, HBu), −11.12 (s, 1H, HIrH). 13C{1H} NMR (100.6 MHz,
t
DMSO): 155.88; 141.82; 141.22; 131.32; 125.27; 124.10; 90.80;
90.20; 28.07; 7.92. HR-ESI-MS: [M]+ m/z 532.2062. Anal. Calcd for
C20H29F6IrN5P: C, 35.50; H, 4.32; N, 10.35. Found: C, 35.88; H,
3.97; N, 10.44.
Isolation of Intermediate B. In a stainless steel reactor, 20 mg of
5 (0.03 mmol) was dissolved in 0.5 mL of DMSO-d6. The reactor was
pressurized with 5 bar of H2 and the solution subsequently stirred at
100 °C for 1 h. After the reaction time had elapsed, the reactor was
cooled and the gas vented out. The crude mixture was then dried at
Authors
Edward Ocansey − Research Centre for Synthesis and Catalysis,
Department of Chemical Sciences, University of Johannesburg,
Auckland Park 2006, South Africa; orcid.org/0000-0002-
6847-2915
I
https://dx.doi.org/10.1021/acs.organomet.0c00276
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
(13) Tanaka, R.; Yamashita, M.; Nozaki, K. Catalytic Hydrogenation
of Carbon Dioxide Using Ir(III)-Pincer Complexes. J. Am. Chem. Soc.
2009, 131 (40), 14168−14169.
(14) Schmeier, T. J.; Dobereiner, G. E.; Crabtree, R. H.; Hazari, N.
Secondary Coordination Sphere Interactions Facilitate the Insertion
Step in an Iridium(III) CO2 Reduction Catalyst. J. Am. Chem. Soc.
2011, 133 (24), 9274−9277.
(15) Malaza, S. S. P.; Makhubela, B. C. E. Direct and Indirect CO2
Hydrogenation Catalyzed by Ir(III), Rh(III), Ru(II), and Os(II) Half-
Sandwich Complexes to Generate Formates and N,N-Diethylforma-
mide. J. CO2 Util. 2020, 39, 101149.
(16) Makuve, N.; Mehlana, G.; Tia, R.; Darkwa, J.; Makhubela, B. C.
E. Hydrogenation of Carbon Dioxide to Formate by α-Diimine RuII,
RhIII, IrIII Complexes as Catalyst Precursors. J. Organomet. Chem.
2019, 899, 120892.
(17) Himeda, Y.; Onozawa-Komatsuzaki, N.; Sugihara, H.; Arakawa,
H.; Kasuga, K. Transfer Hydrogenation of a Variety of Ketones
Catalyzed by Rhodium Complexes in Aqueous Solution and Their
Application to Asymmetric Reduction Using Chiral Schiff Base
Ligands. J. Mol. Catal. A: Chem. 2003, 195, 95−100.
(18) Wang, W.-H.; Hull, J. F.; Muckerman, J. T.; Fujita, E.; Himeda,
Y. Second-Coordination-Sphere and Electronic Effects Enhance
Iridium(III)-Catalyzed Homogeneous Hydrogenation of Carbon
Dioxide in Water near Ambient Temperature and Pressure. Energy
Environ. Sci. 2012, 5 (7), 7923−7926.
(19) Suna, Y.; Himeda, Y.; Fujita, E.; Muckerman, J. T.; Ertem, M.
Z. Iridium Complexes with Proton-Responsive Azole-Type Ligands as
Effective Catalysts for CO2 Hydrogenation. ChemSusChem 2017, 10
(22), 4535−4543.
James Darkwa − Research Centre for Synthesis and Catalysis,
Department of Chemical Sciences, University of Johannesburg,
Auckland Park 2006, South Africa
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00276
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the National Research Foundation of South
Africa (NRF) for financial assistance (Grant Nos. 117989 and
112943). We thank Sasol SA, the University of Johannesburg
Centre for Synthesis and Catalysis, and the UJ Faculty of
Science Spectrum for use of facilities. The authors also thank
Dr. Gift Mehlana (Midlands State University, Zimbabwe) for
his assistance in the solution of the crystal structure.
REFERENCES
■
̈
̈
(1) Krocher, O.; Koppel, R. A.; Baiker, A. Highly Active Ruthenium
Complexes with Bidentate Phosphine Ligands for the Solvent-Free
Catalytic Synthesis of N,N-Dimethylformamide and Methyl Formate.
Chem. Commun. 1997, 2 (5), 453−454.
(2) Dayan, O.; Dayan, S.; Kani, I.; Çetinkaya, B. Ruthenium(II)
Complexes Bearing Pyridine-Based Tridentate and Bidentate Ligands:
Catalytic Activity for Transfer Hydrogenation of Aryl Ketones. Appl.
Organomet. Chem. 2012, 26 (12), 663−670.
(3) Sharma, A. K.; Joshi, H.; Sharma, K. N.; Gupta, P. L.; Singh, A.
K. 2 - Propanol vs Glycerol as Hydrogen Source in Catalytic
Activation of Transfer Hydrogenation with (η6-Benzene)Ruthenium-
(II) Complexes of Unsymmetrical Bidentate Chalcogen Ligands.
Organometallics 2014, 33, 3629−3639.
(20) Li, X. Y.; Yin, Z.; Ma, W. M.; Wang, C.; Yu, Y. N.; Cheng, Y.
Aperiodic Chemical Sequence in a Rod-Spacer Metal-Organic
Framework from Linear Tetrazole-Benzene-Carboxylate Linker.
Inorg. Chem. Commun. 2020, 116, 107925.
(21) Koren, P. N.; Gaponik, V. A.; Ostrovskii, O. Reactions of
Azolium Cations. I. Kinetics and Mechanism of Alkylation of 5-
phenyltetrazole with Isopropyl Alcohol in Aqueous Sulfuric Acid
Media. Int. J. Chem. Kinet. 1993, 25, 1043−1051.
(22) Mosalkova, A. P.; Voitekhovich, S. V.; Lyakhov, A. S.;
Ivashkevich, L. S.; Lach, J.; Kersting, B.; Gaponik, P. N.;
Ivashkevich, O. A. 2-Tert-Butyl-5-(2-Pyridyl)-2H-Tetrazole as a
Chelating Ligand in the Direct Synthesis of Novel Cu(II) and
Heterobimetallic Cu(II)/Mn(II) Complexes. Dalt. Trans. 2013, 42
(8), 2985−2997.
(23) Downard, A. J.; Peter, J.; Steenwijka, J. Syntheses of Chelating
Tetrazole-Containing Ligands and Studies of Their Palladium(II) and
Ruthenium(II) Complexes. Aust. J. Chem. 1995, 48, 1625−1642.
(24) Himeda, Y.; Onozawa-Komatsuzaki, N.; Miyazawa, S.;
Sugihara, H.; Hirose, T.; Kasuga, K. PH-Dependent Catalytic Activity
and Chemoselectivity in Transfer Hydrogenation Catalyzed by
Iridium Complex with 4,4’-Dihydroxy-2,2’- Bipyridine. Chem. - Eur.
J. 2008, 14 (35), 11076−11081.
(25) Wang, S.; Huang, H.; Dorcet, V.; Roisnel, T.; Bruneau, C.;
Cedric, F. Efficient Iridium Catalysts for Base-Free Hydrogenation of
Levulinic Acid Table of Contents. Organometallics 2017, 36 (16),
3152−3162.
(26) Kanega, R.; Onishi, N.; Szalda, D. J.; Ertem, M. Z.; Muckerman,
J. T.; Fujita, E.; Himeda, Y. CO2 Hydrogenation Catalysts with
Deprotonated Picolinamide Ligands. ACS Catal. 2017, 7 (10), 6426−
6429.
(27) Siek, S.; Burks, D. B.; Gerlach, D. L.; Liang, G.; Tesh, J. M.;
Thompson, C. R.; Qu, F.; Shankwitz, J. E.; Vasquez, R. M.; Chambers,
N.; Szulczewski, G. J.; Grotjahn, D. B.; Webster, C. E.; Papish, E. T.
Iridium and Ruthenium Complexes of N-Heterocyclic Carbene- and
Pyridinol-Derived Chelates as Catalysts for Aqueous Carbon Dioxide
Hydrogenation and Formic Acid Dehydrogenation: The Role of the
Alkali Metal. Organometallics 2017, 36 (6), 1091−1106.
(28) Gunasekar, G. H.; Yoon, Y.; Baek, I. H.; Yoon, S. Catalytic
Reactivity of an Iridium Complex with a Proton Responsive N-Donor
(4) Zeng, L.; Wu, F.; Li, Y. Y.; Dong, Z. R.; Gao, J. X. Synthesis and
Characterization of Novel Chiral Bidentate P,N-Containing Ligands
and Ruthenium(II) Complex. The Application in Asymmetric
Transfer Hydrogenation of Ketones. J. Organomet. Chem. 2014, 762
(II), 34−39.
(5) Inoue, Y.; Izumida, H.; Sasaki, Y.; Hashimoto, H. Catalytic
fixation of carbon dioxide to formic acid by transition-metal
complexes under mild conditions. Chem. Lett. 1976, 5, 863−864.
̈
(6) Bertini, F.; Glatz, M.; Gorgas, N.; Stoger, B.; Peruzzini, M.;
Veiros, L. F.; Kirchner, K. A.; Gonsalvi, L. Carbon Dioxide
Hydrogenation Catalysed by Well- Defined Mn(I) PNP Pincer
Hydride Complexes. Chem. Sci. 2017, 8, 5024−5029.
̈
(7) Bertini, F.; Gorgas, N.; Stoger, B.; Peruzzini, M.; Veiros, L. F.;
Kirchner, K. A.; Gonsalvi, L. Efficient and Mild Carbon Dioxide
Hydrogenation to Formate Catalyzed by Fe(II)-Hydridocarbonyl
Complexes Bearing 2, 6- (Diaminopyridyl) Diphosphine Pincer
Ligands. ACS Catal. 2016, 6 (5), 2889−2893.
(8) Weilhard, A.; Qadir, M. I.; Sans, V.; Dupont, J. Selective CO2
Hydrogenation to Formic Acid with Multifunctional Ionic Liquids.
ACS Catal. 2018, 8 (3), 1628−1634.
(9) Grossman, O.; Azerraf, C.; Gelman, D. Palladium Complexes
Bearing Novel Strongly Bent Trans-Spanning Diphosphine Ligands:
Synthesis, Characterization, and Catalytic Activity. Organometallics
2006, 25 (2), 375−381.
(10) Burgess, S. A.; Kendall, A. J.; Tyler, D. R.; Linehan, J. C.; Appel,
A. M. Hydrogenation of CO2 in Water Using a Bis(Diphosphine) Ni-
H Complex. ACS Catal. 2017, 7 (4), 3089−3096.
(11) Bays, J. T.; Priyadarshani, N.; Jeletic, M. S.; Hulley, E. B.;
Miller, D. L.; Linehan, J. C.; Shaw, W. J. The Influence of the Second
and Outer Coordination Spheres on Rh(Diphosphine)2 CO2
Hydrogenation Catalysts. ACS Catal. 2014, 4, 3663−3670.
(12) Langer, R.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J. W.; Ben-
David, Y.; Milstein, D. Low-Pressure Hydrogenation of Carbon
Dioxide Catalyzed by an Iron Pincer Complex Exhibiting Noble Metal
Activity. Angew. Chem., Int. Ed. 2011, 50, 9948−9952.
J
https://dx.doi.org/10.1021/acs.organomet.0c00276
Organometallics XXXX, XXX, XXX−XXX

Organometallics
pubs.acs.org/Organometallics
Article
Ligand in CO2 Hydrogenation to Formate. RSC Adv. 2018, 8 (3),
1346−1350.
(29) Himeda, Y. Highly Efficient Hydrogen Evolution by
Decomposition of Formic Acid Using an Iridium Catalyst with 4,4’-
Dihydroxy-2,2’-Bipyridine. Green Chem. 2009, 11 (12), 2018−2022.
(30) Tenno, T.; Rikmann, E.; Zekker, I.; Tenno, T.; Daija, L.;
Mashirin, A. Modelling Equilibrium Distribution of Carbonaceous
Ions and Molecules in a Heterogeneous System of CaCO3-Water-Gas.
Proc. Est. Acad. Sci. 2016, 65 (1), 68.
(31) Brewster, T. P.; Miller, A. J. M.; Heinekey, D. M.; Goldberg, K.
I. Hydrogenation of Carboxylic Acids Catalyzed by Half-Sandwich
Complexes of Iridium and Rhodium. J. Am. Chem. Soc. 2013, 135,
16022−16025.
(32) Vogt, M.; Pons, V.; Heinekey, D. M. Synthesis and
Characterization of a Dicationic Dihydrogen Complex of Iridium
with a Bis-Carbene Ligand Set. Organometallics 2005, 24 (8), 1832−
1836.
(33) Kashiwame, Y.; Kuwata, S.; Ikariya, T. Metal - Pyrazole
Bifunction in Half-Sandwich C-N Chelate Iridium Complexes:
Pyrazole - Pyrazolato Interconversion and Application to Catalytic
Intramolecular Hydroamination of Aminoalkene. Chem. - Eur. J. 2010,
16, 766−770.
(34) Anderson, G. K.; Lin, M.; Sen, A.; Gretz, E. Bis(Benzonitrile)
Dichloro Complexes of Palladium and Platinum; Wiley: Hoboken, NJ,
USA, 2007. DOI: DOI: 10.1002/9780470132593.ch13.
( 3 5 ) W h i t e , A . ; Y a t e s , P . M . ; M a i t l i s , C .
(Pentamethylcyclopentadienyl)Rhodium and Iridium Compounds.
In Inorganic Syntheses; Wiley Online Library: 1992; p 228.
K
https://dx.doi.org/10.1021/acs.organomet.0c00276
Organometallics XXXX, XXX, XXX−XXX