Selective reduction of alkynes to alkenes with hydrogen or formic acid catalyzed by cis,mer-[IrH2Cl(mtppms)3]

Article abstract of DOI:10.1016/j.ica.2021.120359

In this work we have found, that the water-soluble cis,mer-[IrH₂Cl(mtppms)₃] (mtppms = monosulfonated triphenylphosphine Na-salt) was an excellent catalyst for reduction of terminal alkynes by hydrogen transfer from aqueous HCOOH/HCOONa mixtures. The conversions strongly depended on the pH of the reaction mixtures, and the highest rate of phenylacetylene transfer hydrogenation was observed at pH 3. The same dihydrido-Ir(III) complex actively catalyzed also the hydrogenation of terminal alkynes under mild conditions (T = 50 °C; P(H₂) = 2–10 bar). Importantly, both the hydrogenation and hydrogen transfer reductions afforded exclusively the corresponding alkenes as products. Phenylacetylene and its substituted derivatives reacted smoothly, while benzylic and aliphatic alkynes were less reactive or did not react at all. It was also found, that an excess of the mtppms ligand inhibited the reaction. This was rationalized by formation of cisz-[IrH₂(mtppms)₄]⁺ which was also confirmed with multinuclear NMR spectroscopy. On the basis of the experimental results, a joint mechanism was suggested for both the hydrogenation and transfer hydrogenation pathways. The mechanism of hydrogenation and transfer hydrogenation of phenylacetylene was also studied by DFT calculations, which revealed several possibilities for protonation of a vinyl intermediate as the crucial step in formation of the styrene product.

Full text of DOI:10.1016/j.ica.2021.120359

Inorganica Chimica Acta 522 (2021) 120359
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Selective reduction of alkynes to alkenes with hydrogen or formic acid
catalyzed by cis,mer-[IrH2Cl(mtppms)3]☆
a
a
a
a
a
´
´
¨
´
´
´
´
Gyorgy Hanko , Richard Marton , Antal Udvardy , Mihaly Purgel , Agnes Katho ,
a,
b
a,*
´
´
Ferenc Joo , Gabor Papp
a Department of Physical Chemistry, University of Debrecen, P.O. Box 400, H-4002 Debrecen, Hungary
b MTA-DE Redox and Homogeneous Catalytic Reaction Mechanisms Research Group, P.O.Box 400, H-4002 Debrecen, Hungary
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this work we have found, that the water-soluble cis,mer-[IrH2Cl(mtppms)3] (mtppms = monosulfonated tri-
phenylphosphine Na-salt) was an excellent catalyst for reduction of terminal alkynes by hydrogen transfer from
aqueous HCOOH/HCOONa mixtures. The conversions strongly depended on the pH of the reaction mixtures, and
the highest rate of phenylacetylene transfer hydrogenation was observed at pH 3. The same dihydrido-Ir(III)
complex actively catalyzed also the hydrogenation of terminal alkynes under mild conditions (T = 50 ◦C; P
(H2) = 2–10 bar). Importantly, both the hydrogenation and hydrogen transfer reductions afforded exclusively the
corresponding alkenes as products. Phenylacetylene and its substituted derivatives reacted smoothly, while
benzylic and aliphatic alkynes were less reactive or did not react at all. It was also found, that an excess of the
mtppms ligand inhibited the reaction. This was rationalized by formation of cisz-[IrH2(mtppms)4]+ which was
also confirmed with multinuclear NMR spectroscopy. On the basis of the experimental results, a joint mechanism
was suggested for both the hydrogenation and transfer hydrogenation pathways. The mechanism of hydroge-
nation and transfer hydrogenation of phenylacetylene was also studied by DFT calculations, which revealed
several possibilities for protonation of a vinyl intermediate as the crucial step in formation of the styrene product.
Alkynes
Aqueous
Homogeneous catalysis
Hydrogenation
Iridium
Transfer hydrogenation
1. Introduction
by the problem of separating the catalysts from the products, and
catalyst recycling is also difficult [9]. One potential method for capi-
The selective hydrogenation of unsaturated organic compounds is a
highly significant research area from a practical point of view, because
the resulting partially or fully hydrogenated derivatives are important
materials of the fine chemical industry. From the pharmaceutical or
perfume industries to the manufacturing of pesticides, it is increasingly
important to develop more efficient and cost-effective processes. Thus,
the heterogeneous or homogeneous catalytic selective hydrogenation of
polyunsaturated compounds is a key aspect of catalysis research.
Heterogeneous catalysts suitable for the semi-hydrogenation of al-
kynes are widely known in the literature [1–3] and numerous transition
metal complexes proved to be highly active in these reactions in ho-
mogeneous phase as well [4–8]. Homogeneous catalysis is accompanied
talizing on the efficiency and selectivity characteristic of homogeneous
catalysts is performing aqueous-organic biphasic catalysis. In this case,
the water-soluble catalyst (obtained –for example– by incorporating
water-soluble ligands, such as e.g. sulfonated triphenylphosphines into
the metal complex catalysts) is easily separable from the organic prod-
ucts (or reagents) and is recyclable [6,10].
A safer alternative for hydrogenation is the method of hydrogen
transfer from suitable hydrogen donor substances. In these cases, the
hydrogen source is not gaseous hydrogen (often under high pressure),
but amine-borane [11], alcohol/base (EtOH [12,13], MeOH [14], 2-
PrOH [15]), hypophosphorous acid [16], or in several cases HCOOH/
HCOONa [17–21], to name a few.
Abbreviations: mtppms, monosulfonated triphenylphosphine Na-salt (3-diphenylphopshinobenzenesulfonic acid Na-salt); mtppts, trisulfonated triphenylphosphine
trisodium salt (trisodium 3,3′,3′′-phosphinetriylbenzenesulfonate); TOF, turnover frequency = (mol reacted substrate)×(mol catalyst×time)–1 = TON×time–1; TON,
turnover number = (mol reacted substrate)×(mol catalyst)–1; PMe3, trimethylphosphine; PPh3, triphenylphosphine.
☆
Dedicated to Professor Maurizio Peruzzini on the occasion of his 65th birthday in recognition of his numerous achievements in coordination chemistry and
homogeneous catalysis.
* Corresponding author.
E-mail address: papp.gabor@science.unideb.hu (G. Papp).
https://doi.org/10.1016/j.ica.2021.120359
Received 13 December 2020; Received in revised form 25 February 2021; Accepted 18 March 2021
Available online 23 March 2021
0020-1693/© 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

´
G. Hanko et al.
Inorganica Chimica Acta 522 (2021) 120359
[IrH2Cl(PPh3)3] has long been known as a hydrogenation catalyst
placed into a home-made medium pressure glass reactor fitted with a
silicon rubber septum (total volume 80 ± 2 mL) which –after several
vacuum/argon cycles– was finally filled with atmospheric Ar. The sol-
vents were added through the septum with a syringe/hypodermic needle
and the solids were dissolved with magnetic stirring. After the addition
of the substrate solution, the reactor was filled with H2-gas up to the
desired pressure, placed into a thermostated bath and the reaction
mixture was stirred vigorously. At the desired reaction time, the rector
was cooled down in crushed ice, vented slowly and opened to air. The
liquid phases were separated, a sample of the organic phase was dried by
passing through a plug of anhydrous MgSO4, and was analyzed by gas
chromatography.
[22]. Seminal papers on water-soluble Ir(I)-phosphine complexes were
published by Atwood et al, whose work mostly dealt with carbonyl
complexes including cis,mer-[IrH2(CO)L3]+ (L = mtppms, mtppts), too
[23–28]. Merola and co-workers made extensive studies on the synthesis
and aqueous organometallic chemistry of mer-[IrH2X(PMe3)3] com-
plexes (PMe3 = trimethylphosphine, X = various halides or other an-
ions) [29,30]. However, it was only in 2016, that our research group first
published the synthesis of the closest water-soluble analog of [IrH2Cl
(PPh3)3], cis,mer-[IrH2Cl(mtppms)3], and its first application in selective
HCOOH decomposition [31]. It was established that this catalyst per-
formed the fastest hydrogen evolution from aqueous formic acid/
formate solutions (max. TOF = 298 000 hꢀ 1; max. TON = 67 650).
Furthermore, it was successfully demonstrated that the catalyst was
recyclable without significant loss of activity – HCOOH decomposition
was carried out with the same catalyst in five consecutive cycles.
The same catalytic system was used in the high-resolution analytical
assay of glycans, in which the reaction is technically a homogeneous
phase reductive amination [32]. HCOOH acted as a hydrogen source in
our method, substituting the highly toxic, but still widely used cyano-
borohydride. The cis,mer-[IrH2Cl(mtppms)3] catalyst was investigated in
the para-hydrogenation of various water-soluble alkyne derivatives, too,
where the semi-hydrogenated alkene products maintained the enhanced
magnetically labeled signals. Accordingly, these catalytic systems have
the potential to be applied in medical diagnostics [33].
For transfer hydrogenation of alkynes, the H2 atmosphere in the
reactor was replaced with Ar, and aqueous solutions of HCOOH and
HCOONa were used instead of water as the aqueous phase.
2.3. Computational details
DFT calculations [35] were carried out using Gaussian09 software
package [36]. Geometry optimizations were performed with M06
functional.[37] Def2TZVP ECP/basis set was employed on iridium [38],
together with the TZVP basis set for non-metal atoms [39,40]. Fre-
quency calculations were done at the level of the theory of geometry
optimization. Relative free energies (ΔG) are reported at 298.15 K and
atmospheric pressure. The solvent (water) effect was accounted for with
the use of the Polarized Continuum Model (IEF-PCM) [41]. Stationary
points on the Potential Energy Surface (PES) are reactants (R), in-
termediates (I), transition states (TS), and products (P) as first-order
saddle points of PES. All stationary points were confirmed by fre-
quency analysis where minima had all positive frequencies while TSs
had one imaginary frequency related to the actual movement of the
reaction coordinate. For the calculations, the sulfonated phosphine li-
gands in cis,mer-[IrH2Cl(mtppms)3] and its derivatives were replaced by
PPh3.
In this publication, we describe the application of cis,mer-[IrH2Cl
(mtppms)3] dihydride in the selective hydrogenation of various alkyne
derivatives (Scheme 1). Both gaseous hydrogen and aqueous HCOOH/
HCOONa mixtures were used as hydrogen sources in aqueous-organic
biphasic reactions.
2. Experimental section
2.1. Materials and methods
cis,mer-[IrH2Cl(mtppms)3] [31] and mtppms [34] were synthesized
as described earlier and their purities were checked by NMR spectros-
copy. Phenylacetylene and other alkynes were purchased from Sigma-
Aldrich and used as received. IrCl3⋅3H2O was obtained from Pressure
Chemical Co. and Alfa Aesar. HCOOH, HCOONa, and other inorganic
salts were supplied by Sigma-Aldrich and VWR. Argon, nitrogen and
hydrogen (99.99% purity) were provided by Linde. Deuterated solvents
(99.9%) were purchased from Cambridge Isotope Laboratories Inc. and
Sigma-Aldrich. NMR spectra were recorded on BRUKER AVANCE DRX
360 and BRUKER AVANCE I 400 MHz spectrometers and evaluated
using the Bruker TopSpin program. 1H, 13C and 31P NMR spectra were
referenced to tetramethylsilane (TMS), 2,2-dimethyl-2-silapentane-5-
sulfonate (DSS), 85% H3PO4 and residual solvent peaks, respectively.
Gas chromatographic measurements were performed with the use of an
3. Results and discussion
3.1. Hydrogenation of alkynes catalyzed by cis,mer-[IrH2Cl(mtppms)3]
The catalytic properties of cis,mer-[IrH2Cl(mtppms)3] was studied in
Agilent Technologies 7890 A instrument (Optima®-1, 0.25 μm, 30 m ×
0.32 mm, FID 300 ◦C, carrier gas: Nitrogen 1.9 mL minꢀ 1).
2.2. General procedure for the hydrogenation and transfer hydrogenation
of alkynes
Fig. 1. Hydrogenation of phenylacetylene in aqueous-organic biphasic system.
Conditions: nphenylacetylene = 5x10-4 mol; VH2O = 5.0 mL; Vtoluene = 3.0 mL; T =
50 ◦C; P(H2) = 10 bar; 1 mol% catalyst.
All manipulations were performed under oxygen-free conditions
using standard Schlenk-techniques. The solid catalyst, excess of phos-
phine ligand (if required), and naphthalene (internal standard) were
Scheme 1. Hydrogenation of phenylacetylene.
2

´
G. Hanko et al.
Inorganica Chimica Acta 522 (2021) 120359
detail in hydrogenation of phenylacetylene in aqueous-organic biphasic
reactions. Time course of the reaction is shown by Fig. 1.
attempted with cis,mer-[IrH2Cl(mtppms)3] as the catalyst. Importantly,
these two substrates also afforded the corresponding alkenes as exclu-
sive products. However, their reaction rates were largely different:
under the conditions of Fig. 1, but at 80 ◦C, the required reaction time
for 100% conversion was 90 min with 4-ethynylbromobenzene and 240
min with benzylacetylene, compared to less than 10 min with phenyl-
acetylene. Aliphatic alkynes (4-phenyl-1-butyne and 1-hexyne) were not
hydrogenated and diphenylacetylene proved unreactive, too.
As shown by Fig. 1, cis,mer-[IrH2Cl(mtppms)3] efficiently catalyzed
the hydrogenation of phenylacetylene even under mild reaction condi-
tions (50 ◦C, 10 bar H2, 1 mol% catalyst). The turnover frequency, TOF,
calculated from the initial linear part of the conversion vs. reaction time
graph, reached 180 hꢀ 1 (TOF = (mol reacted substrate)×(mol catalyst ×
time)-1). This TOF compares favorably to those in the literature [4,5,8]
given the biphasic nature of the reaction mixture and the low solubility
(456 mg/L at 25 ◦C [42]) of phenylacetylene in water.
3.2. Transfer hydrogenation of phenylacetylene
The rate of hydrogenation increased with rising temperature, and
under the conditions of Fig. 1 but at 80 ◦C temperature, the reaction time
needed for 100% conversion of phenylacetylene to styrene was less than
10 min (which implies TOF = 600 hꢀ 1 as a lower limit). An increase of
hydrogen pressure from 2 to 10 bar led to an approximately threefold
increase in the reaction rate (Table S1).
Reductions with hydrogen transfer from suitable donor molecules
instead of molecular H2 have several advantages over conventional
hydrogenations [45]. In addition to alcohols (most often 2-propanol),
applied together with appropriate bases, formic acid, formates or their
mixtures are often used as sources of hydrogen for the most diverse
reactions [8,45].
An important feature of the reaction is its selectivity to the alkene
product. In no experiments in this study, was formation of products
other than styrene detected in hydrogenation of phenylacetylene with
cis,mer-[IrH2Cl(mtppms)3] as the catalyst. For example, even when a
reaction was performed at 80 ◦C for 2 h (other conditions as at Fig. 1),
only semi-hydrogenation to styrene was observed, despite a 100%
conversion of phenylacetylene to styrene within the first 10 min. Under
our conditions, the products of alkyne hydration/hydrogenation (i.e.
ketones and alcohols) –as described by Luo et al. [43] and Xiao et al.
[44]– were not detected.
We have described earlier, that cis,mer-[IrH2Cl(mtppms)3] was an
outstandingly active catalyst for the decomposition of formic acid in the
presence of Na-formate [31]. The reaction rate strongly depended on the
solution pH, i.e. on the [HCOOH]/[HCOONa] ratio in the reaction
mixture (actually, the aqueous formic acid/formate mixture served as a
pH buffer, too). In addition, cis,mer-[IrH2Cl(mtppms)3] was also used as
the catalyst in a reductive amination-based analytical method [32], in
which the reduction of an imine was achieved by catalytic H-transfer
from HCOOH.
The study of phenylacetylene conversion as a function of the sub-
strate to catalyst concentration ratio ([S]/[C]) revealed an important
feature of the reaction. As shown in Fig. 2, the conversion decreased only
slightly in the [S]/[C] = 100–200 interval, however, this was followed
by a sharp drop, so much that at [S]/[C] = 1000, only a 3% conversion
was determined. In the [S]/[C] = 100–1000 range, the TOF values
varied according to a maximum curve. It is noteworthy that even at the
highest alkyne concentration the specific reaction rate (expressed as
TOF) still was about 20% of its highest value. Furthermore, the selec-
tivity to styrene was retained at all [S]/[C] conditions.
Based on the above findings, we attempted transfer hydrogenation of
phenylacetylene and other alkynes using cis,mer-[IrH2Cl(mtppms)3] as
the catalyst. Indeed, the reaction proceeded with reasonable rates,
leading to 67% conversion with 1 mol% catalyst at 80 ◦C at pH 2.9
(Fig. 3). This conversion corresponds to TOF = 34 hꢀ 1, which is com-
parable to the TOF observed in hydrogenations of the same substrate
with the same catalyst at T = 50 ◦C and 10 bar H2 pressure.
As can be seen on Fig. 4, the rate of transfer hydrogenation of phe-
nylacetylene as the function of the pH of the formate buffer showed a
sharp maximum at pH ~ 3 when [HCOOH]/[HCOO–] is 7/3.
The data of Fig. 4 may explain the slowing down of the transfer
hydrogenation after the first 0.5–1 h reaction time, as appears on Fig. 3.
Since the reaction is slow in neutral or basic solutions, it may be sup-
posed, that the actual hydride donor is HCOOH. With increasing reac-
tion times, not only the concentration of one of the reactants (HCOOH) is
decreased, but the consumption of formic acid leads to a gradual in-
crease of the pH of the mixture, too. Consequently, these two effects
result in a steady decrease of the rate of further hydrogen transfer to
phenylacetylene.
The likely reason for this phenomenon may be in substrate inhibition
occurring at higher concentrations. Since the minimum substrate con-
centration is 100 times higher than that of the catalyst, formation of a
presumed unreactive bis(alkyne)Ir(III) species would not decrease
significantly the total available substrate concentration but could
decrease the concentration of the catalytically active Ir-species sub-
stantially (see also the suggested mechanism in Section 3.3).
In addition to phenylacetylene, hydrogenation of 4-ethynylbromo-
benzene and 3-phenyl-1-propyne (benzylacetylene) were also
The rate of the reaction showed strong dependence on the temper-
ature (Figure S2). An Arrhenius-type plot of the TOF values as the
Fig. 2. Conversion of the substrate (a) and turnover frequency of the catalyst
(b) in hydrogenation of phenylacetylene catalyzed by cis,mer-[IrH2Cl
(mtppms)3] as functions of the substrate to catalyst concentration ratio ([S]/
[C]). Conditions: ncatalyst = 5x10-6 mol; VH2O = 5.0 mL; Vtoluene = 3.0 mL; T =
50 ◦C; P(H2) = 10 bar, t = 30 min.
Fig. 3. Time course of the transfer hydrogenation of phenylacetylene in
aqueous-organic biphasic system catalyzed by cis,mer-[IrH2Cl(mtppms)3].
Conditions: ncatalyst = 5x10-6 mol; nsubstrate = 5x10-4 mol VH2O = 6.0 mL; Vtoluene
= 3.0 mL; T = 80 ◦C; (nHCOOH + nHCOONa 0
)
= 4.0x10-3 mol; pH = 2.9;
3

´
G. Hanko et al.
Inorganica Chimica Acta 522 (2021) 120359
hydrogenation of phenylacetylene resulted in 43% conversion, and 4-
ethynylanisole (37% conversion) and 4-ethynyltoluene (45% conver-
sion) also reacted smoothly, then 4-ethynyl-bromobenzene showed only
18% conversion. In contrast, diphenylacetylene, 3-phenyl-1-propyne, 4-
phenyl-1-butyne, and 1-hexyne did not react at all (conditions: ncatalyst
= 5 × 10-6 mol; nsubstrate = 5 × 10-4 mol; VH2O = 6.0 mL; Vtoluene = 3.0
mL; pH = 3.90; nHCOOH = nHCOONa = 7.5x10–3 mol T = 80 ◦C; t = 1 h).
With the aim to get some insight into the reaction mechanism, we
studied the effect of added phosphine ligand on the rate of phenyl-
acetylene reduction by hydrogen transfer from formic acid/formate
mixtures with cis,mer-[IrH2Cl(mtppms)3] as the catalyst. The results are
presented in Table 1.
As can be seen from the data of Table 1, the conversion of phenyl-
acetlyne gradually decreased with increased mtppms concentration,
approaching only half of the original conversion at a 5-fold ligand excess
over cis,mer-[IrH2Cl(mtppms)3]. In addition, 1H and 31P NMR mea-
surements revealed, that upon the addition of mtppms, the following
reaction took place:
Fig. 4. Conversion of phenylacetylene as a function of pH in transfer hydro-
genation from HCOOH/HCOONa catalyzed by cis,mer-[IrH2Cl(mtppms)3].
Conditions: ncatalyst = 5x10-6 mol; nsubstrate = 5x10-4 mol; VH2O = 6.0 mL; Vto-
= 3.0 mL; T = 80 ◦C; (nHCOOH + nHCOONa 0
)
= 2.0x10–3 mol; t = 2 h.
luene
cis,mer-[IrH2Cl(mtppms)3] + mtppms ⇌ cis-[IrH2(mtppms)4]+ (1)
Note, that in this reaction a coordinated halide is replaced relatively
easily by a phosphine ligand giving rise to a cationic complex. This
behaviour is not without precedent in aqueous solutions, where both the
halide anion and the complex cation become stabilized by strong hy-
dration [29,30,46]. Merola and co-workers synthesized a similar com-
pound cis,mer-[IrH2Cl(PMe3)3], which, in aqueous solution, underwent
replacement of chloride by PMe3 in a process analogous to Equation 1
[29,30].
function of the inverse absolute temperature yielded a temperature co-
efficient of 21.3 kJ/mol (Figure S3). However, in lack of more detailed
kinetic experiments and data, this number can be regarded only as a
composite value representing also the temperature dependence of the
partition equilibrium of phenylacetylene between the aqueous and the
toluene phase.
It is important to note here, that similar to the hydrogenations with
H2, transfer hydrogenation of phenylacetylene from formic acid/
formate also resulted in formation of styrene with 100% selectivity.
The effect of the total amount of the H-donor (i.e. the concentration
of the formate buffer) was also established. In these experiments the pH
was set to a constant 3.9, and only the [H-donor]/[substrate] ratio was
varied – the results are shown on Fig. 5. As expected, at the beginning
the conversion increased with the increased amount of the H-donor,
however, after reaching a sharp maximum at nHCOOH + nHCOONa = 7.5 ×
10-3 mol, it showed a gradual drop with further increase of the total
formic acid/formate concentration. In the specific case of Fig. 5, the
maximum conversion was observed at the [H-donor]:[substrate]:[cata-
lyst] = 1500:100:1 ratio.
The 1H and 31P NMR parameters of cis-[IrH2(mtppms)4]+ formed by
the exchange of halide to mtppms are displayed in Fig. 6.
Since the NMR measurements were performed at 25 ◦C in MeOH/
H2O mixed solvent, the results cannot be strictly parallelled with the
catalytic results determined at 80 ◦C. Nevertheless, it is safe to say, that
the catalytic activity of cis-[IrH2(mtppms)4]+ is lower than that of the
initial cis,mer-[IrH2Cl(mtppms)3] complex.
3.3. Suggested mechanism of hydrogenation or hydrogen transfer
reduction of phenylacetylene catalyzed by cis,mer-[IrH2Cl(mtppms)3]
At the moment we do not have a clear explanation of the large drop
in phenylacetylene conversion at large formic acid/formate concentra-
tions. In more concentrated electrolyte solutions, some salting out of
phenylacetylene from the aqueous phase may happen. Alternatively, the
increased HCOO– concentration should facilitate the coordination of this
ion to the cationic Ir(III) complexes which –presumably– act as the
active species in the catalytic cycle (see below).
Based on the experimental results described in Sections 3.2 and 3.3,
we suggest the following mechanism for the reduction of phenyl-
acetylene both by hydrogenation (involving H2) and by transfer hy-
drogenation (involving formic acid/formate mixtures as H-donor)
(Scheme 2).
According to this mechanism, in the first step of the reaction, a water
molecule coordinates in place of the halide of cis,mer-[IrH2Cl(mtppms)3]
(A), yielding cis,mer-[IrH2(H2O)(mtppms)3]+ (B), the entry point to the
catalytic cycle. Formation of this aquo species was confirmed by
determination of chloride with the use of a chloride selective electrode:
in a 1.00 mM aqueous solution of cis,mer-[IrH2Cl(mtppms)3], [Cl–] =
0.96 mM was found experimentally (for details see Supporting Infor-
mation). The labile water ligand in this complex is later replaced by the
substrate phenylacetylene yielding C. Hydride migration to the β-carbon
Concerning other alkynes it was found, that while the transfer
Table 1
Effect of added mtppms on transfer hydrogenation of phenylacetylene catalyzed
by cis,mer-[IrH2Cl(mtppms)3] and on formation of cis-[IrH2(mtppms)4]+.
[mtppms]/[IrH2Cl(mtppms)3]
Conversion (%)
cis-[IrH2(mtppms)4]+ (%) *
0
1
3
5
53
33
29
30
0
20
33
57
Fig. 5. Transfer hydrogenation of phenylacetylene from formic acid/formate
mixtures catalyzed by cis,mer-[IrH2Cl(mtppms)3]: conversion of the alkyne as a
function of the total amount of the H-donor (nHCOOH + nHCOONa). Conditions:
ncatalyst = 5 × 10-6 mol; nsubstrate = 5 × 10-4 mol; VH2O = 6.0 mL; Vtoluene = 3.0
mL; pH = 3.90; T = 80 ◦C; t = 2 h.
Conditions: nsubstrate = 5x10-4 mol; VH2O = 6.0 mL; Vtoluene = 3.0 mL; T = 80 ◦C; t
= 2 h; nHCOOH = nHCOONa = 7.5x10-3 mol; pH = 3.90; 1 mol% catalyst.
*
Molar ratio of cis-[IrH2(mtppms)4]+ in the cis,mer-[IrH2Cl(mtppms)3] + n
mtppms mixtures, determined by 31P NMR spectroscopy (T = 25 ◦C, solvent:
MeOH-H2O).
4

´
G. Hanko et al.
Inorganica Chimica Acta 522 (2021) 120359
Fig. 6. Structure and the measured NMR parameters of cis-[IrH2(mtppms)4]+.
mtppms, which –via formation of cis-[IrH2(mtppms)4]+– removes (part
of) the catalytically important cis,mer-[IrH2(H2O)(mtppms)3]+ from the
catalytic cycle.
Another important question should be considered. Since cis,mer-
[IrH2Cl(mtppms)3] is an extremely good catalyst of formic acid
decomposition to H2 + CO2, there is a chance for the reduction of
phenylacetylene to proceed simply by hydrogenation using the H2
supplied by prior formic acid decomposition. To check this possibility,
phenylacetylene was injected into the reaction mixture of a fast running
formate decomposition reaction which was followed by measuring the
volume of the evolved gas in a thermostated gas burette. Following the
addition of phenylacetylene, gas evolution slowed down immediately
and completely ceased in a few minutes. Upon further stirring no change
of the gas volume (neither gas evolution, nor uptake) was observed.
However, after 2 h reaction time, 40% conversion of phenylacetylene to
styrene was determined. This shows, that in the presence of HCOOH/
HCOONa mixture, only transfer hydrogenation is operative. However,
transfer hydrogenation of phenylacetylene –although comparable in
rate– is slower than its hydrogenation with H2. In agreement with the
results from the gas burette measurements, we did not observe devel-
opment of pressure in the reactors during transfer hydrogenation re-
actions. These observations support the existence of two different
pathways for hydrogenation and hydrogen transfer.
3.4. DFT calculations on phenylactylene hydrogenation and transfer
hydrogenation
We performed DFT calculations to find out which of the proposed
mechanisms can be reasonable and comparable to the experimental
results. We found that the coordinated phenylacetylene in the axial
Scheme 2. Suggested mechanism of phenylacetylene hydrogenation and
transfer hydrogenation from formic acid/formate mixtures, catalyzed by cis,
mer-[IrH2Cl(mtppms)3].
position is favorable (I1; ꢀ 21.9 kJ molꢀ 1
) referred to cis,mer-
[IrH2(PPh3)3] and outer-sphere phenylacetylene system (R). From this,
the energy barrier of the hydride transfer’s elementary step is 56.1 kJ
of the coordinated phenylacetylene results in a vinyl species (D) with a
molꢀ 1 (TS1; 34.9 kJ molꢀ 1) resulting in the
σ-styryl intermediate (I2;
vacant coordination site on Ir. Release of styrene requires protonation
ꢀ 17.0 kJ molꢀ 1). To the vacant equatorial side, the coordination of the
hydrogen molecule was the most preferred (I3a; ꢀ 25.6 kJ molꢀ 1) while
the coordination of a water molecule was slightly favored (I3b; ꢀ 8.9 kJ
molꢀ 1) compared to the coordination of formic acid (I3c; ꢀ 6.3 kJ
molꢀ 1). The proton transfer was a two-step process from I3a: in the first
on α-carbon and there are several possibilities for this step:
a) The vacant site in D is filled by a H2 molecule. Protonation of the
vinyl intermediate happens from the coordinated dihydrogen and leads
to release of styrene (replaced by H2O) and yields directly the catalyti-
cally active cis,mer-[IrH2(H2O)(mtppms)3]+ species (B).
step, an H H cleavage occurred (TS2a; ꢀ 25.3 kJ molꢀ 1) resulting in a
–
b) The vacant site in D is filled by a water molecule. The coordinated
H2O protonates the α-carbon of the neighbouring vinyl ligand leading to
species in which one of the hydrogen atoms stayed in the equatorial
position while the other one went to the axial position forming a
hydrogen molecule in axial position (I4a; ꢀ 66.3 kJ molꢀ 1). From this, an
interesting transition state was found (TS3a; ꢀ 27.8 kJ molꢀ 1) in which
both hydrogen atoms had a rearrangement and therefore was similar to
tetrahydride species in which one of the hydrides was replaced by the
the release of styrene and formation of mer-[IrH(OH)(H2O)(P)3]+ com-
plex. Further reaction of this complex with H2 leads to regeneration of B.
c) The vacant site in D is occupied by a formic acid molecule with
monodentate coordination through its carbonyl oxygen. The coordi-
nated formic acid protonates the α-carbon of the neighbouring vinyl
moiety resulting in the release of styrene and formation of [IrH(HCOO)
(P)3]+. Internal redox reaction of this complex with CO2 loss regenerates
cis,mer-[IrH2(H2O)(mtppms)3]+ (B).
carbon atom of
σ
-styryl. The hydrogen from the axial position went back
to the equatorial position while from the equatorial side the hydrogen
transferred to the
σ-styryl resulting in the final product having a disso-
d) The vinyl intermediate D is protonated by H+ from the bulk of the
aqueous solution. This step is more probable in acidic solutions but
becomes less favoured under basic conditions.
ciated styrene and the cis,mer-[IrH2(PPh3)3] catalyst (P; ꢀ 152.9 kJ
molꢀ 1). The coordinated water molecule had less preferred steric con-
dition than hydrogen, therefore, a significantly higher energy barrier
occurred (TS2b; 36.5 kJ molꢀ 1). We found, however; that the κ1-O state
This mechanism is in agreement with the inhibition by excess
5

´
G. Hanko et al.
Inorganica Chimica Acta 522 (2021) 120359
of the coordinated formic acid had an excellent orientation which
resulted in a very low energy barrier of the one-step proton transfer
(TS2c; ꢀ 4.4 kJ molꢀ 1). Both proton transfers resulted in very favored
intermediates (I4b; ꢀ 83.8 kJ molꢀ 1 and I4c; ꢀ 119.0 kJ molꢀ 1) which
needed other steps to get back the cis,mer-[IrH2(PPh3)3] catalyst, see
Fig. S6–S8 (Fig. 7).
(mtppts)2] and [IrH2(CO)(mtppts)3]+, however, no mechanism was
suggested for the reactions [26]. In our case, cis,mer-[IrH2(H2O)
(mtppms)3]+, too, underwent H/D exchange, shown by the loss of
couplings in the otherwise well-resolved 1H NMR (hydride region), as
well as by the increase of the water signal intensity (see Supporting
Information). This leads us to believe, that the vinyl intermediate in
phenylacetylene hydrogenation may be efficiently protonated by a H2O
ligand in cis-position.
3.5. The relative contributions of intermediates I3a-I3c to styrene release
via proton transfer
Our DFT calculations showed that protonation by a coordinated
dihydrogen of the vinyl-ligand in D is much preferred to protonation by
coordinated H2O. However, there is precedent in the literature (other
than the mentioned H/D exchange) where such an interaction was
considered [47]. We have also found that coordinated water could be
involved in such a protonation process in hydrogenation of cinna-
maldehyde catalyzed by [{RuCl2(mtppms)2}2] [48]. It should also be
mentioned, that taking into account the large excess of water molecule
(55.5 M) relative to the concentration of hydrogen and formic acid, the
energy of I3b, TS2b and I4b can be corrected by ꢀ 18.0 kJ/mol. This
value is calculated by the -RTln(cbulk/cideal gas) equation, where R is
8.314 J/molK, T = 298 K, cbulk = 55.5 M and cideal gas is 1/24.5 M (1 atm
ideal gas solute state). In this case the protonation from the coordinated
water molecule would be more relevant due to the stabilization of I3b.
As can be seen from Section 3.4, a crucial step of the mechanism is the
way of protonation of the vinyl intermediate whether it is mer-[IrH(H2)
(C6H5C2H2)(P3)]+ (I3a) or mer-[IrH(H2O)(C6H5C2H2)(P3)]+ (I3b) in
hydrogenation or mer-[IrH(HCOOH)(C6H5C2H2)(P3)] (I3c) in hydrogen
transfer. In neutral and basic solutions, proton concentration in the bulk
aqueous reaction mixture is probably too low to favour direct proton-
ation, although Paterniti and Atwood observed instantaneous proton-
ation of the methyl ligand in trans-[Ir(CO)(Me)L2] (L = mtppms-Na or
mtppms-K) upon dissolution in water [24]. Conversely, while direct
involvement of undissociated HCOOH agrees with the rate increase from
pH 7 to pH 3, it seems incompatible with the decrease of the reaction
rate below pH 3. The solubility of H2 in water is 0.81 mM (20 ◦C), while
the catalyst concentration in our case is 1 mM. In contrast, in such
aqueous solutions, water is in a very large excess over the catalyst, and
fast conversion of the starting chloro-complex (A) to cis,mer-[IrH2(H2O)
(mtppms)3]+ (B) was demonstrated by determination of the concentra-
tion of free chloride. Merola et al have observed the same replacement of
chloride by H2O in aqueous solutions of cis,mer-[IrH2Cl(PMe)3] yielding
cis,mer-[IrH2(H2O)(PMe)3]+. They have suggested, that the mechanism
of H/D exchange in cis,mer-[IrH2(H2O)(PMe)3]+ dissolved in D2O pro-
ceeded via protonation of a hydride ligand in the complex by the
neighbouring D2O (yielded by a H2O/D2O exchange) followed by
4. Conclusion
The easily available, water-soluble cis,mer-[IrH2Cl(mtppms)3] was
found highly active in the selective hydrogenation of phenylacetylene to
styrene in aqueous-organic biphasic reaction mixtures. Reduction of
phenylacetylene could be effected by using either molecular H2, or an
aqueous formic acid/Na-formate mixture, although transfer hydroge-
nations proceeded with somewhat lower reaction rates. Hydrogenations
(with H2) were retarded in the presence of a large excess of the alkyne
substrate, while the rate of transfer hydrogenations strongly depended
on the pH of the aqueous phase of the reaction mixture. Nevertheless, in
deprotonation of the resulting (
η
2-HD)Ir species [29]. Similar H/D ex-
change was observed by Atwood et al. in D2O solutions of [IrClH2(CO)
Fig. 7. The Gibbs free energy profile of the mechanism; energies are in kJ molꢀ 1
.
6

´
G. Hanko et al.
Inorganica Chimica Acta 522 (2021) 120359
´
[7] F. Joo, Aqueous Organometallic Catalysis, Kluwer, The Netherlands (2001),
all cases, the reaction yielded exclusively styrene as product. DFT cal-
culations were performed to identify the key species of the reaction
mechanism. On the basis of both the experimental findings and the re-
sults of calculations, an unified reaction mechanism was proposed for
both hydrogenation and transfer hydrogenation of alkynes with cis,mer-
[IrH2Cl(mtppms)3] as the catalyst. It is revealed, that the aqueous phase
significantly contributes to the formation of the catalytically active
species, cis-mer-[IrH2(H2O)(mtppms)3]+, presumably by strong solva-
tion of Cl– and the complex cation resulting from chloride dissociation
from cis,mer-[IrH2Cl(mtppms)3].
https://doi.org/10.1007/0-306-47510-3.
[8] K.C.K. Swamy, A.S. Reddy, K. Sandeep, A. Kalyani, Advances in chemoselective
and/or stereoselective semihydrogenation of alkynes, Tetrahedron Lett. 59 (5)
(2018) 419–429, https://doi.org/10.1016/j.tetlet.2017.12.057.
´
´
´
[9] H.H. Horvath, G. Papp, C. Csajagi, F. Joo, Selective catalytic hydrogenations in a
microfluidics-based high throughput flow reactor on ion-exchange supported
transition metal complexes: A modular approach to the heterogenization of soluble
complex catalysts, Catal. Commun. 8 (3) (2007) 442–446, https://doi.org/
10.1016/j.catcom.2006.07.016.
´
´
[10] H.H. Horvath, F. Joo, Stereoselective homogeneous catalytic hydrogenation of
disubstituted alkynes in aqueous-organic biphasic media, React Kinet Catal Lett 85
(2) (2005) 355–360, https://doi.org/10.1007/s11144-005-0281-7.
[11] S. Fu, N.-Y. Chen, X. Liu, Z. Shao, S.-P. Luo, Q. Liu, Ligand-Controlled Cobalt-
Catalyzed Transfer Hydrogenation of Alkynes: Stereodivergent Synthesis of Z- and
E-Alkenes, J. Am. Chem. Soc. 138 (27) (2016) 8588–8594, https://doi.org/
10.1021/jacs.6b0427110.1021/jacs.6b04271.s00110.1021/jacs.6b04271.s002.
[12] Y. Wang, Z. Huang, Z. Huang, Catalyst as colour indicator for endpoint detection to
enable selective alkyne trans-hydrogenation with ethanol, Nat Catal 2 (6) (2019)
529–536, https://doi.org/10.1038/s41929-019-0299-2.
cis,mer-[IrH2Cl(mtppms)3] proved to be a practical catalyst, too. It is
an easily synthesized, air-stable solid, well-soluble in water. It efficiently
catalyses the reduction of alkynes either by conventional hydrogenation
or by hydrogen transfer from aqueous HCOOH/HCOONa, and can be
simply recovered in the aqueous phase of aqueous-organic biphasic re-
action mixtures.
[13] J. Yang, C. Wang, Y. Sun, X. Man, J. Li, F. Sun, Ligand-controlled iridium-catalyzed
semihydrogenation of alkynes with ethanol: highly stereoselective synthesis of E-
and Z-alkenes, Chem. Commun. 55 (13) (2019) 1903–1906, https://doi.org/
10.1039/C8CC09714C.
CRediT authorship contribution statement
[14] K. Tani, A. Iseki, T. Yamagata, Efficient transfer hydrogenation of alkynes and
alkenes with methanol catalysed by hydrido(methoxo)iridium(III) complexes,
Chem. Commun. (1999) 1821–1822, https://doi.org/10.1039/A905765J.
[15] T. Kaicharla, B.M. Zimmermann, M. Oestreich, J.F. Teichert, Using alcohols as
simple H2-equivalents for copper-catalysed transfer semihydrogenations of
alkynes, Chem. Commun. 55 (89) (2019) 13410–13413, https://doi.org/10.1039/
C9CC06637C.
¨
´
´
´
Gyorgy Hanko: Investigation. Richard Marton: Investigation.
´
Antal Udvardy: Investigation, Methodology. Mihaly Purgel: Formal
´
´
analysis, Methodology, Writing - review & editing. Agnes Katho:
´
Conceptualization, Writing - review & editing. Ferenc Joo: Writing -
´
review & editing. Gabor Papp: Investigation, Methodology, Writing -
[16] H. Cao, T. Chen, Y. Zhou, D. Han, S.-F. Yin, L.-B. Han, Copper-Catalyzed Selective
Semihydrogenation of Terminal Alkynes with Hypophosphorous Acid, Adv. Synth.
Catal. 356 (4) (2014) 765–769, https://doi.org/10.1002/adsc.v356.410.1002/
adsc.201300916.
original draft, Writing - review & editing.
Declaration of Competing Interest
[17] A.M. Kalsin , T.y.A. Peganova, V.V. Novikov, A.I. Zhamoytina, L. Gonsalvi, M.
Peruzzini, Transfer Hydrogenation of Ketones Catalyzed by Surface-Active
Ruthenium and Rhodium Complexes in Water, Chem. Eur. J. 20 (2014) 846-854.
https://doi.org/10.1002/chem.201301679.
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.
[18] A. Guerriero, M. Peruzzini, L. Gonsalvi, Ruthenium(II)-Arene Complexes of the
Water-Soluble Ligand CAP as Catalysts for Homogeneous Transfer Hydrogenations
in Aqueous Phase, Catalysts 8 (2018) 88, https://doi.org/10.3390/catal8020088.
[19] S. Musa, A. Ghosh, L. Vaccaro, L. Ackermann, D. Gelman, Efficient E-Selective
Transfer Semihydrogenation of Alkynes by Means of Ligand-Metal Cooperating
Ruthenium Catalyst, Adv. Synth. Catal. 357 (10) (2015) 2351–2357, https://doi.
org/10.1002/adsc.v357.1010.1002/adsc.201500372.
Acknowledgements
The research was supported by the EU and co-financed by the Eu-
ropean Regional Development Fund (under the projects GINOP-2.3.2-
15-2016-00008 and GINOP-2.3.3-15-2016-00004), and by the The-
matic Excellence Programme of the Ministry for Innovation and Tech-
nology of Hungary (ED_18-1-2019-0028), within the framework of the
Vehicle Industry thematic programme of the University of Debrecen.
The financial support of the Hungarian National Research, Development
and Innovation Office (FK-128333) is greatly acknowledged. This work
was partially supported by the European Union and the European Social
Fund through project Supercomputer, the National Virtual Lab, grant
´
´
´
´
[20] I. Szatmari, G. Papp, F. Joo, A. Katho, Unexpectedly fast catalytic transfer
hydrogenation of aldehydes by formate in 2-propanol–water mixtures under mild
conditions, Catal. Tod. 247 (2015) 14–19, https://doi.org/10.1016/j.
cattod.2014.06.023.
´
´
´
´
[21] A. Katho, I. Szatmari, G. Papp, F. Joo, Effect of 2-Propanol on the Transfer
Hydrogenation of Aldehydes by Aqueous Sodium Formate using a Rhodium(I)-
sulfonated Triphenylphosphine Catalyst, CHIMIA 69 (6) (2015) 339–344, https://
doi.org/10.2533/chimia.2015.339.
[22] R.S. Coffey, The decomposition of formic acid catalysed by soluble metal
complexes, Chem. Commun. (18) (1967) 923b, https://doi.org/10.1039/
c1967000923b.
[23] D.P. Paterniti, P.J. Roman, J.D. Atwood, Solvent effect for oxidative addition of
hydrogen in solvents from toluene to water: reaction of H2 with trans-[Ir(CO)Cl
{PPh2(C6H4SO3K-m)}2], Chem. Commun. (1996) 2659–2660, https://doi.org/
10.1039/CC9960002659.
´
no.: TAMOP-4.2.2.C-11/1/KONV-2012-0010.
[24] D.P. Paterniti, J.D. Atwood, Hydrolysis of an iridium–methyl bond, formation of
trans-[Ir(CO)(OH)L2] from trans-[Ir(CO)MeL2] [L = PPh2(m-C6H4SO3Na), PPh2
(m-C6H4SO3K) or P(C6H4SO3Na)3] upon dissolving in water, Chem. Commun.
(1997) 1665–1666, https://doi.org/10.1039/A702197F.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2021.120359.
[25] P.J. Roman, D.P. Paterniti, R.F. See, M.R. Churchill, J.D. Atwood, Synthesis,
Properties, and Reactions of Monosulfonated Triphenylphosphine (PPh2(m-
C6H4SO3K) = TPPMS) Complexes of Iridium(I). Crystal and Molecular Structure of
[N(CH2C6H5)(C2H5)3+][PPh2(m-C6H4SO3)-]⋅H2O, Organometallics 16 (7)
(1997) 1484–1490, https://doi.org/10.1021/om960911a.
References
[26] D.P. Paterniti, P.J. Roman, J.D. Atwood, Activation of Hydrogen on Iridium(I)
Centers in Water: Solvent Effects, pH Effects, and Iridiumꢀ Hydride Exchange with
D2O, Organometallics 16 (15) (1997) 3371–3376, https://doi.org/10.1021/
om9702690.
[1] H. Lindlar, R. Dubuis, Palladium Catalyst for Partial Reduction of Acetylenes,
Organic Syntheses 89 (1966), https://doi.org/10.15227/orgsyn.046.0089.
[2] T.N. Gieshoff, A. Welther, M.T. Kessler, M.H.G. Prechtl, A. Jacobi von Wangelin,
Stereoselective iron-catalyzed alkyne hydrogenation in ionic liquids, Chem.
Commun. 50 (18) (2014) 2261–2264, https://doi.org/10.1039/C3CC49679A.
[3] T. Mitsudome, M. Yamamoto, Z. Maeno, T. Mizugaki, K. Jitsukawa, K. Kaneda,
One-step Synthesis of Core-Gold/Shell-Ceria Nanomaterial and Its Catalysis for
Highly Selective Semihydrogenation of Alkynes, J. Am. Chem. Soc. 137 (42) (2015)
13452–13455, https://doi.org/10.1021/jacs.5b07521.
[27] P.J. Roman, J.D. Atwood, Carbonylation of trans-Ir(CO)Cl(TPPTS)2 and Reactivity
of [Ir(CO)2(TPPTS)3]Cl (TPPTS = P(m-C6H4SO3Na)3) in DMSO and Water,
Organometallics 16 (25) (1997) 5536–5540, https://doi.org/10.1021/
om970351v.
[28] D.P. Paterniti, L.W. Francisco, J.D. Atwood, Synthesis of Cationic Iridium(I)
Complexes of Water-Soluble Phosphine Ligands, [Ir(CO)(TPPMS)3]CF3SO3, [Ir
(CO)(H2O)(TPPTS)2]CF3SO3, and [Ir(CO)2(TPPMS)3]ClO4 (TPPMS = PPh2(m-
C6H4SO3K), TPPTS = P(m-C6H4SO3Na)3), Organometallics 18 (2) (1999)
123–127, https://doi.org/10.1021/om9806139.
[4] A.M. Kluwer, C. J., in: Elsevier, in Handbook of Homogeneous Hydrogenation,
Wiley-VCH, Weinheim, 2007, pp. 375–411, https://doi.org/10.1002/
9783527619382.ch14.
´
´
´
[5] F. Joo, A. Katho, in: in Handbook of Homogeneous Hydrogenation, Wiley-VCH,
[29] J. S. Merola, T. L. Husebo, K. E. Matthews Aqueous Organometallic Chemistry of
mer-Ir(H)2(PMe3)3X Complexes, Organometallics 31, 10, (2012) 3920–3929.
https://doi.org/10.1021/om300140f.
Weinheim, 2007, pp. 1327–1359.
´
[6] F. Joo, in: Water in Organic Synthesis, Georg Thieme Verlag, Stuttgart, 2012,
pp. 95–119.
7

´
G. Hanko et al.
Inorganica Chimica Acta 522 (2021) 120359
[30] T.X. Le, J.S. Merola, Synthesis and reaction chemistry of water-soluble mer-(Me3P)
3Ir(H)(H)Cl: activation by water of alkyne insertion into an iridium-hydrogen
bond, Organometallics 12 (10) (1993) 3798–3799, https://doi.org/10.1021/
om00034a006.
[38] D. Andrae, U. Hӓußermann, M. Dolg, H. Stoll, H. Preuß, Energy-adjustedab initio
pseudopotentials for the second and third row transition elements, Theoret. Chim.
Acta 77 (2) (1990) 123–141, https://doi.org/10.1007/BF01114537.
¨
[39] A. Schafer, H. Horn, R. Ahlrichs, Fully optimized contracted Gaussian basis sets for
¨
´
´
´
´
[31] G. Papp, G. Olveti, H. Horvath, A. Katho, F. Joo, Highly efficient dehydrogenation
of formic acid in aqueous solution catalysed by an easily available water-soluble
iridium(iii) dihydride, Dalton Trans. 45 (2016) 14516–14519, https://doi.org/
10.1039/C6DT01695B.
atoms Li to Kr, J. Chem. Phys. 97 (4) (1992) 2571–2577, https://doi.org/10.1063/
1.463096.
¨
[40] A. Schafer, C. Huber, R. Ahlrichs, Fully optimized contracted Gaussian basis sets of
triple zeta valence quality for atoms Li to Kr, J. Chem. Phys. 100 (8) (1994)
´
´
´
[32] Z. Kovacs, G. Papp, H. Horvath, F. Joo, A. Guttman, A novel carbohydrate labeling
method utilizing transfer hydrogenation-mediated reductive amination,
J. Pharmaceut. Biomed. 142 (2017) 324–327, https://doi.org/10.1016/j.
jpba.2017.05.017.
5829–5835, https://doi.org/10.1063/1.467146.
ˇ
[41] S. Miertus, E. Scrocco, J. Tomasi, Electrostatic interaction of a solute with a
continuum. A direct utilizaion of AB initio molecular potentials for the prevision of
solvent effects, Chem. Phys. 55 (1) (1981) 117–129, https://doi.org/10.1016/
0301-0104(81)85090-2.
´
´
[33] G. Papp, H. Horvath, F. Joo, A Simple and Efficient Procedure for Rh(I)- and Ir(I)-
complex Catalyzed Para-hydrogenation of Alkynes and Alkenes in Aqueous Media
Resulting in Strong PHIP Effects, ChemCatChem 11 (13) (2019) 3000–3003,
https://doi.org/10.1002/cctc.v11.1310.1002/cctc.201900602.
[42] S.C. Valvanix, S.H. Yalkowsky, T.J. Roseman, Solubility and Partitioning IV:
Aqueous Solubility and Octanol-Water Partition Coefficients of Liquid
Nonelectrolytes, J. Pharm. Sci. 70 (5) (1981) 502–507, https://doi.org/10.1002/
jps.2600700510.
´
´
´
´
´
[34] F. Joo, J. Kovacs, A. Katho, A.C. Benyei, T. Decuir, D.J. Darensbourg, A. Miedaner,
D.L. Dubois, Meta- Sulfonatophenyl) Diphenylphosphine, Sodium Salt and its
Complexes with Rhodium(I), Ruthenium(II), Iridium(I, Inorg. Synth. (1998) 1–8,
https://doi.org/10.1002/9780470132630.ch1.
[43] N. Luo, Y. Zhong, J.-T. Liu, L. Ouyang, R. Luo, An Efficient Hydration and Tandem
Transfer Hydrogenation of Alkynes for the Synthesis of Alcohol in Water, Synthesis
52 52 (22) (2020) 3439–3445, https://doi.org/10.1055/s-0040-1707233.
[44] J. Li, C. Wang, D. Xue, Y. Wei, J. Xiao, One-pot transformation of alkynes into
alcohols and amines with formic acid, Green Chem. 15 (2013) 2685–2689, https://
doi.org/10.1039/C3GC41133H.
[35] R.G. Parr, W. Yang, Density Functional Theory of Atoms and Molecules, Oxford
University Press, New York, USA, 1989.
[36] Frisch G.W.T. M. J. , Schlegel H. B., Scuseria G. E., Robb M. A., Cheeseman J. R.,
Scalmani G., Barone V., Mennucci B., Petersson G. A., Nakatsuji H., Caricato M., Li
X., Hratchian H. P., Izmaylov A. F., Bloino J., Zheng G., Sonnenberg J. L., Hada M.,
Ehara M., Toyota K., Fukuda R., Hasegawa J., Ishida M., Nakajima T., Honda Y.,
Kitao O., Nakai H., Vreven T., Montgomery J. A., Peralta Jr., J. E., Ogliaro F.,
Bearpark M., Heyd J. J., Brothers E., Kudin K. N., Staroverov V. N., Kobayashi R.,
Normand J., Raghavachari K., Rendell A., Burant J. C., Iyengar S. S., Tomasi J.,
Cossi M., Rega N., Millam J. M., Klene M., Knox J. E., Cross J. B., Bakken V., Adamo
C., Jaramillo J., Gomperts R., Stratmann R. E., Yazyev O., Austin A. J., Cammi R.,
Pomelli C., Ochterski J. W., Martin R. L., Morokuma K., Zakrzewski V. G., Voth G.
[45] D. Wang, D. Astruc, The Golden Age of Transfer Hydrogenation, Chem. Rev. 115
(13) (2015) 6621–6686, https://doi.org/10.1021/acs.chemrev.5b00203.
´
´
´
´
[46] G. Papp, H. Horvath, A. Katho, F. Joo, Aqueous Organometallic Chemistry.
Synthesis and Solution Equilibria of Trisodium Carbonylchlorotris[3-
(diphenylphosphino-κP)benzenesulfonato]hydridoruthenate(3ꢀ ) ([RuH(Cl)(CO)
{m-(Ph2P) C6H4 SO3Na}3]) and Trisodium Aquacarbonyltris[3-
(diphenylphosphino-κP)benzenesulfonato] hydridoruthenate(2ꢀ )
Tetrafluoroborate(1ꢀ ) ([RuH(CO)(H2O){m-(Ph2P) C6H4 SO3Na}3] [BF4]), Helv.
Chim. Acta 88 (2005) 566–573, https://doi.org/10.1002/hlca.200590039.
¨
´
´
A., Salvador P., Dannenberg J. J., Dapprich S., Daniels A. D., Farkas O., Foresman
[47] P.P. Feher, F. Joo, G. Papp, M. Purgel, Hydrogenation of Cinnamaldehyde by
Water-Soluble Ruthenium(II) Phosphine Complexes: A DFT Study on the
Selectivity and Viability of trans-Dihydride Pathways, Eur. J. Inorg. Chem 2021 (3)
(2021) 236–242, https://doi.org/10.1002/ejic.v2021.310.1002/ejic.202000933.
J. B., Ortiz J. V., Cioslowski J., and Fox D. J., Gaussian 09, Revision E.01,
Wallingford CT, (2009).
[37] Y. Zhao, D.G. Truhlar, The M06 suite of density functionals for main group
thermochemistry, thermochemical kinetics, noncovalent interactions, excited
states, and transition elements: two new functionals and systematic testing of four
M06-class functionals and 12 other functionals, Theor. Chem. Account. 120 (1-3)
(2008) 215–241, https://doi.org/10.1007/s00214-007-0310-x.
´
´
´
[48] G. Kovacs, G. Ujaque, A. Lledos, F. Joo, The Role of Water in the Stereoselective
Hydrogenation of 1,2-Diphenylacetylene Catalyzed by the Water-Soluble [{RuCl2
(mtppms)2}2], Eur. J. Inorg. Chem. 2007 (18) (2007) 2879–2889, https://doi.org/
10.1002/(ISSN)1099-068210.1002/ejic.v2007:1810.1002/ejic.200601200.
8