Promoter effect of bicarbonate in hydrogenation of cinnamaldehyde catalyzed by a water-soluble Ru(II)-phosphine complex

Article abstract of DOI:10.1016/j.ica.2017.06.061

The highly selective formation of cinnamalcohol in hydrogenation of trans-cinnamaldehyde with [{RuCl₂(mtppms)₂}₂] + mtppms as catalyst (mtppms = monosulfonated triphenylphosphine) in aqueous solution was substantially accelerated by NaHCO₃ (in 20–50 mol% quantities relative to cinnamaldehyde). More than double conversion compared to bicarbonate-free systems was observed at n(NaHCO₃)/n(Ru) = 20. Prehydrogenation of the reaction mixture before the addition of cinnamaldehyde resulted in further rate increase (45.3% conversion vs. 13.1% in water). ¹H, ¹³C and ³¹P NMR studies revealed that formate produced in hydrogenation of bicarbonate facilitated formation of trans-[Ru(H)₂(H₂O)(mtppms)₃], a better catalyst than cis-fac-[Ru(H)₂(H₂O)(mtppms)₃] which is the product of the reaction of [{RuCl₂(mtppms)₂}₂] + mtppms with H₂ in the absence of formate (or bicarbonate). Accordingly, NaHCO₂ produced even higher rate increase than the same amount of NaHCO₃.

Full text of DOI:10.1016/j.ica.2017.06.061

Accepted Manuscript
Promoter effect of bicarbonate in hydrogenation of cinnamaldehyde catalyzed
by a water-soluble Ru(II)-phosphine complex
Imre SZATMÁRI, Gábor PAPP, Ferenc JOÓ, Ágnes KATHÓ
PII:
S0020-1693(17)30582-0
http://dx.doi.org/10.1016/j.ica.2017.06.061
ICA 17710
DOI:
Reference:
Inorganica Chimica Acta
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15 April 2017
21 June 2017
27 June 2017
Please cite this article as: I. SZATMÁRI, G. PAPP, F. JOÓ, A. KATHÓ, Promoter effect of bicarbonate in
hydrogenation of cinnamaldehyde catalyzed by a water-soluble Ru(II)-phosphine complex, Inorganica Chimica
Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.06.061
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1
Promoter effect of bicarbonate in hydrogenation of cinnamaldehyde
catalyzed by a water-soluble Ru(II)-phosphine complex¶
Imre SZATMÁRI1, Gábor PAPP1,2, Ferenc JOÓ1,2, Ágnes KATHÓ1*
1Department of Physical Chemistry, University of Debrecen, P.O.Box 400, H-4002 Hungary
2MTA-DE Homogeneous Catalysis and Reaction Mechanisms Research Group, P.O.Box 400,
H-4002 Hungary
* Corresponding author:
Ágnes Kathó
Phone: +3652512900, ext. 22383
E-mail: katho.agnes@science.unideb.hu
¶Dedicated to Professor Imre Sóvágó, our excellent teacher, esteemed colleague and longtime
friend in recognition of his fundamental contributions to biocoordination chemistry, on the
occasion of his 70th birthday.

2
Bullet points:
- NaHCO3 strongly accelerates cinnamaldehyde hydrogenation with a water-soluble Ru(II)-
phosphine catalyst
- The reaction is highly selective towards formation of the unsaturated alcohol
- A plausible reaction mechanism is suggested on basis of kinetic and NMR measurements
- Formate (from hydrogenation of bicarbonate) plays active role in catalysis
- trans-[Ru(H)2(H2O)(mtppms)3] was identified as key catalytic species

3
Abstract
The highly selective formation of cinnamalcohol in hydrogenation of trans-cinnamaldehyde
with [{RuCl2(mtppms)2}2] + mtppms as catalyst (mtppms = monosulfonated
triphenylphosphine) in aqueous solution was substantially accelerated by NaHCO3 (in 20-50
mol% quantities relative to cinnamaldehyde). More than double conversion compared to
bicarbonate-free systems was observed at n(NaHCO3)/n(Ru) = 20. Prehydrogenation of the
reaction mixture before the addition of cinnamaldehyde resulted in further rate increase
(45.3% conversion vs. 13.1% in water). 1H-, 13C- and 31P-NMR studies revealed that formate
produced in hydrogenation of bicarbonate facilitated formation of trans-
[Ru(H)2(H2O)(mtppms)3], a better catalyst than cis-fac-[Ru(H)2(H2O)(mtppms)3] which is the
product of the reaction of [{RuCl2(mtppms)2}2] + mtppms with H2 in the absence of formate
(or bicarbonate). Accordingly, NaHCO2 produced even higher rate increase than the same
amount of NaHCO3.
Keywords
Hydrogenation; Isomerization; Monosulfonated triphenylphosphine (mtppms); Ruthenium;
Water-soluble.

4
1. Introduction
Selective reduction of unsaturated aldehydes (such as trans-cinnamaldehyde, CAL, Scheme
1) is a synthetically useful reaction and much research effort has been directed to selectively
obtain from such substrates either saturated aldehydes or -and more importantly- unsaturated
alcohols by catalytic hydrogenation or by hydrogen transfer from suitable H-donor
compounds.[1-13] During our studies on hydrogenations in homogeneous aqueous solutions
or in aqueous-organic biphasic systems we observed that [{RuCl2(mtppms)2}2] (1; mtppms
=diphenylphosphinobenzene-m-sulfonic acid Na-salt) in the presence of phosphine excess
catalyzed exclusive formation of cinnamalcohol by catalytic hydrogen transfer from aqueous
formate. [4, 14]
Scheme 1. Hydrogenation of trans-cinnamaldehyde.
Formation of unsaturated alcohols was also detected when unsaturated aldehydes were
hydrogenated in an H2 atmosphere (in the absence of formate) with the same Ru(II)-mtppms
catalyst. In this case, a closer scrutiny of the effects of reaction parameters revealed that
selectivity towards formation of either cinnamalcohol (COL) or 3-phenylpropanal (HCAL)
critically depended on the pH of the aqueous phase (as well as on H2 pressure[15]). At
pH ≥ 8 hydrogenation yielded unsaturated alcohol while at pH ≤ 5 the exclusive product was
saturated aldehyde – the pH even could be used as a selectivity switch.[16, 17] A detailed
study later showed that depending on the pH and the hydrogen pressure in solutions of
[{RuCl2(mtppms)2}2] + n mtppms, altogether five hydrido-Ru(II) species could be formed, of
which trans-[RuH2(mtppms)4], cis-fac-[RuH2(H2O)(mtppms)3] and [RuH2(η2-H2)(mtppms)3]
catalyzed the selective hydrogenation of the C=O function of aldehydes while
[{RuHCl(mtppms)2}2] and [RuHCl(mtppms)3] were active and selective in the saturation of
the C=C bond.[18] It should be added here, that similar pH dependence of the rate and

5
selectivity could not be studied in case of catalytic hydrogen transfer to unsaturated aldehydes
from sodium formate since 1 is an active catalyst for decomposition of HCO2H formed from
˗
HCO2 in acidic solutions.
It has been discovered recently that in aqueous solutions [{RuCl2(mtppms)2}2] -in the
presence of excess mtppms- actively catalyzed hydrogenation of bicarbonate to formate [19,
20] as well as the reverse reaction i.e. dehydrogenation of aqueous formate to
bicarbonate.[21]
˗
˗
HCO3 + H2
HCO2 + H2O
Considering the good H-donor properties of aqueous formate together with the high activity
of [{RuCl2(mtppms)2}2] + n mtppms in hydrogenation of bicarbonate to formate we reasoned
that aqueous hydrogenation of aldehydes (with H2) catalyzed by 1 + n mtppms, might be
facilitated by addition of bicarbonate in sub-stoichiometric amounts relative the aldehyde.
This possibility is shown schematically on Scheme 2.
Scheme 2. Selective catalytic hydrogenation of aldehydes in presence of bicarbonate.
˗
Indeed, we have found a substantial promoter effect of HCO3 on hydrogenation of
cinnamaldehyde catalyzed by 1. Here we report the details of the reaction together with NMR
studies on the possible Ru(II)-phosphine species participating in the catalytic cycles. A
plausible reaction mechanism is also suggested.
2. Experimental Section
2.1. Materials and instruments
[{RuCl2(mtppms)2}2] and mtppms were synthesized as described earlier [22] and their purities
were checked by NMR spectroscopy. Cinnamaldehyde was purchased from Sigma-Aldrich

6
and used as received. RuCl3×3H2O was obtained from Pressure Chemical Co., NaHCO3 and
NaHCO2 were supplied by Sigma-Aldrich and Spectrum 3D. Argon and nitrogen (99.99%
purity) were provided by Linde and used directly from cylinders. The pH of the solutions was
adjusted to desired values using 0.2 M phosphate buffer solutions prepared from analytical
grade Na2HPO4×12H2O and NaH2PO4×2H2O. Deuterated solvents (99.9%) were purchased
from Cambridge Isotope Laboratories Inc. and Sigma Aldrich.
NMR spectra were recorded on a BRUKER DRX 360 spectrometer and evaluated
using the WinNMR 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 made on
HP5890 Series II; Chrompack WCOT Fused Silica 30m*32mm CP WAX52CB; FID; carrier
gas: argon; isothermal; temperatures: injector 250°C, oven 130°C, detector 250°C.
2.2. General experimental procedure for hydrogenation of cinnamaldehyde in the presence of
NaHCO3
In a Schlenk-flask, NaHCO3 (16.8 mg, 0.2 mmol), [{RuCl2(mtppms)2}2] (10.0 mg, 0.005
mmol) and mtppms (32.0 mg, 0.08 mmol) were dissolved in water (7.5 mL) under an argon
atmosphere followed by addition of cinnamaldehyde (50 µL, 0.4 mmol). Argon was replaced
by hydrogen and the solution was stirred at 50 °C for the desired reaction time. Samples (0.2
mL) were withdrawn periodically from the vigorously stirred solution and were diluted with
1.0 mL of water before extraction with 1.0 mL of chlorobenzene. The organic layer was dried
(MgSO4) and analyzed by gas chromatography. In experiments with prehydrogenation, the
above solution of NaHCO3, [{RuCl2(mtppms)2}2] and mtppms was stirred in an H2
atmosphere for 10 min at 50 °C before addition of cinnamaldehyde. Reactions with P(H2) ˃ 1
bar were carried out in home-made glass pressure tubes equipped with a needle valve, a
pressure gauge and a septum inlet.
3. Results and discussion
When 0.4 mmol cinnamaldehyde was heated at 50°C in 7.5 mL 0.2M phosphate buffer of pH
8.3 under 1 bar H2 for 1 hour in the presence of 1 and an excess of mtppms, a conversion of
13.1% was determined and the sole product of the reaction was cinnamalcohol. Addition of
increasing amounts of NaHCO3 resulted in a substantial increase of conversion which reached
a maximum value of 29.9% with 0.2 mmol NaHCO3 corresponding to n(NaHCO3)/n(Ru) =
20. Note that this amount of NaHCO3 is only 50 mol% of that of cinnamaldehyde (as can be

7
seen later the effect can be conveniently studided in the 20-50 mol% range). Larger amounts
of NaHCO3 did not further increase the reaction rate, instead a small decrease of conversion
was observed (Figure 1). In separate series of experiments the given amounts of NaHCO3
were prehydrogenated under the same reaction conditions for 10 min at which time the
substrate cinnamaldehyde was injected. In this experimental arrangement even larger increase
of cinnamaldehyde conversion was observed (relative to bicarbonate-free conditions) with a
maximum value of 45.3%. This conversion is more than three times higher than in the
˗
absence a HCO3 demonstrating a strong promoter effect of bicarbonate (Figure 1).
Figure 1. Conversions in hydrogenation of cinnamaldehyde catalyzed by 1 in the presence of
NaHCO3 and the effect of prehydrogenation of NaHCO3. n(CAL) = 0.4 mmol, n(1) = 0.005
mmol, n(mtppms) = 0.08 mmol, V(H2O) = 7.5 mL, P(H2) = 1 bar, T = 50°C, t = 1 h. Symbols:
● no prehydrogenation; ▲ 10 min prehydrogenation of NaHCO3; ♦ in the absence of
NaHCO3 the reaction was done in 0.2 M phosphate buffer, pH 8.30, V= 7.5 mL.
The major product at all NaHCO3 concentrations, with or without prehydrogenation was the
unsaturated alcohol (COL) and the maximum conversion to the saturated aldehyde (HCAL)
never exceeded 2.5%.
Similar experiments were also conducted at 10 bar H2 pressure (other conditions as on
Figure 1). Again, NaHCO3 had a pronounced positive effect on the hydrogenation rate of
cinnamaldehyde resulting in a maximum turnover frequency of TOF =299 h-1 at
n(NaHCO3)/n(Ru) = 40 as opposed to TOF = 102 h-1 in the absence of NaHCO3 (TOF = mol
converted substrate × (mol catalyst × time)-1. In accordance with our earlier observation [15]

8
at this H2 pressure the reaction was completely selective to cinnamalcohol. Interestingly, the
turnover frequencies of cinnamaldehyde hydrogenation as a function of hydrogen pressure
showed a much steeper increase in the presence than in the absence of NaHCO3 (Figure 2).
For comparison, hydrogenations in the absence of NaHCO3 were conducted in 0.2 M
phosphate buffer solutions of pH=8.30 (corresponding to that of the NaHCO3 solutions used
in this comparative study).
Figure 2. Turnover frequencies of cinnamaldehyde hydrogenation catalyzed by 1 as a
function of H2 pressure in the absence and presence of NaHCO3. n(CAL) = 1.0 mmol, n(1) =
0.0025 mmol, n(mtppms) = 0.04 mmol, V(aqueous phase) = 7.5 mL, T = 50°C, t = 1 h.
Aqueous phase: ● 0.2 M phosphate buffer, pH = 8.30, ▲ 13.3 mM NaHCO3.
Table 1 shows the effect of added mtppms (ligand excess) on the conversion obtained in
cinnamaldehyde hydrogenation at 10 bar H2 pressure with [{RuCl2(mtppms)2}2] catalyst as a
function of the [mtppms]/[1] ratio (please note the dimeric nature of the complex). Increasing
concentrations of mtppms result in substantially increased conversions which may refer to
coordination of more than 2 phosphine ligands in the actual catalytically active Ru(II)-species.
Table 1. Effect of added phosphine on hydrogenation of cinnamaldehyde catalyzed by
[{RuCl2(mtppms)2}2] (1)
Conversiona
[mtppms]/[1]
TOF (h1)
(%)

9
0
2
68
8.5b
16.4
23.8
36.6
49.7
131
190
293
398
6
12
16
Conditions: n(CAL) = 1.0 mmol, n(1) = 0.0025 mmol, n(NaHCO3) = 0.2 mmol, V(H2O) =
7.5 mL, P(H2) = 10 bar, T = 50°C, t = 15 min. aProduct is cinnamalcohol, except b5.4%
cinnamalcohol + 3.1% 3-phenylpropanal.
Effect of temperature
The promoter effect of NaHCO3 can be observed also in the temperature dependence of
cinnamaldehyde hydrogenation (Figure 3). Under conditions of Figure 3, in the presence of
NaHCO3 conversion of cinnamaldehyde to hydrogenated products was as high as 65% in
comparison to 30% determined in the bicarbonate-free system. Yield of cinnamalcohol was
always predominant (maximum yield 55%) while those of 3-phenylpropanal and 3-
phenylpropanol did not exceed 5.5 %, each (see Table 2 for details).
Figure 3. Conversions of cinnamaldehyde hydrogenation catalyzed by 1 in the absence and
presence of NaHCO3 as a function of the temperature. n(CAL) = 0.4 mmol, n(1) = 0.005

10
mmol, n(mtppms) = 0.08 mmol, V(aqueous phase) = 7.5 mL, P(H2) = 1 bar, T = 50°C, t = 1 h.
Aqueous phase: ● 0.2 M phosphate buffer, pH = 8.30, ▲ 26.7 mM NaHCO3.
Similarly, at 10 bar H2 pressures hydrogenation rates of cinnamaldehyde were consistently
and substantially higher in presence of NaHCO3 than in phosphate buffer solutions of the
same pH (Figure 4).
Figure 4. Effect of NaHCO3 on the rate of cinnamaldehyde hydrogenation at 10 bar H2
pressure. n(CAL) = 1.0 mmol, n(1) = 0.0025 mmol, n(mtppms) = 0.04 mmol, V(aqueous
phase) = 7.5 mL, T = 50°C, t = 1 h. Aqueous phase: ● 0.2 M phosphate buffer, pH = 8.30, ▲
26.7 mM NaHCO3.
In addition to cinnamaldehyde hydrogenations, multinuclear NMR measurements were also
made in order to follow the reactions of [{RuCl2(mtppms)2}2] (1) under hydrogenation
conditions (composition of the NMR sample: n(1) = 0.01 mmol, n(mtppms) = 0.042 mmol,
n(NaHCO3) = 0.4 mmol, V(H2O) = 0.5 mL, V(MeOD) = 0.1 mL, p(H2) = 1 bar; room
temperature). On the basis of kinetic and NMR data we suggest the following reaction
˗
mechanism (Scheme 3). In the first step [{RuCl2(mtppms)2}2] (1) reacts with HCO3 yielding
the known [20] monomeric [Ru(H2O)2(HCO3)2(mtppms)2] (2). In this work, formation of 2
was investigated with the use of NaH13CO3 and was evidenced by the broad triplet 13C-NMR
˗
signal of coordinated bicarbonate at δ=167.3 ppm (non-coordinated HCO3 resonates at
δ=161.1 ppm). In addition, 31P-NMR also showed the broad singlet signal of 2 at δ=51.0 ppm.
Next, reaction of 2 and H2 yields free formate (13C-NMR: δ=170.7 ppm (s)) and trans-

11
[Ru(H)2(H2O)(mtppms)3] (3). The trans-dihydrido-Ru(II) complex 3 has previously been
obtained in reaction of [{RuCl2(mtppms)2}2] + mtppms with NaHCO2 in dilute aqueous
solutions and was extensively characterized by its 1H- and 31P-NMR spectra [18].
Characteristic signals of 3, such as 1H-NMR: δ= -17.6 ppm (dt, 2JHP=25 and 27 Hz), and 31P-
NMR: δ=44.0 ppm (br, d) and 77.3 ppm (br, t) were, indeed, observed in our hydrogenation
system with added NaHCO3. Addition of cinnamaldehyde to aqueous solution of 3 leads to a
shift of the NMR signals with no change in the pattern and signal multiplicities, 1H-NMR:
δ= -18.7 ppm (br), and 31P-NMR: δ=46.4 ppm (br, d) and 79.0 ppm (br, t). We suppose that
cinnamaldehyde (CAL) replaces the H2O ligand in 3 with formation of trans-
[Ru(H)2(CAL)(mtppms)3] (4), however, this complex was not studied in more detail. Hydride
migration in 4 and subsequent reactions with H2 and H2O yield cinnamalcohol (COL) as
product and regenerate the catalytically active species, trans-[Ru(H)2(H2O)(mtppms)3] (3).
Scheme 3. Suggested mechanism of cinnamaldehyde hydrogenation catalyzed by
[{RuCl2(mtppms)2}2] + mtppms in water in the presence of catalytic amounts of NaHCO3.
It is important to mention here, that reaction of [{RuCl2(mtppms)2}2] and mtppms with H2 in
˗
˗
the absence of HCO2 (or HCO3 ) results in formation of a different hydride species, namely
cis-fac-[Ru(H)2(H2O)(mtppms)3] (5) with clearly distinct NMR spectra (1H-NMR: δ= -10.4

12
ppm (dt, 2JHP=39 and 34 Hz), and 31P-NMR: δ=42.0 ppm (br) and 58.0 ppm (br) [18] –
however, these signals were not detected in the presence of NaHCO3.
The mechanism suggested above makes possible the rationalization of the
experimental findings on cinnamaldehyde hydrogenation with added bicarbonate. NaHCO3 is
catalytically reduced to formate which directs the reaction of [{RuCl2(mtppms)2}2] and
mtppms with H2 to yield trans-[Ru(H)2(H2O)(mtppms)3] (3) rather than cis-fac-
[Ru(H)2(H2O)(mtppms)3] (5). 3 may be a better catalyst for aldehyde hydrogenation than 5
due to the strong strong trans-labilizing effect of a trans-hydride ligand which may facilitate
hydride migration to coordinated CAL. Furthermore, it can also be conceived that in addition
to H2, formate also plays an important role in regeneration of 3 in the product forming step.
This would be in accordance with the known easy formation of 3 in dilute formate solutions
[18]. The advantageous effect of prehydrogenation of [{RuCl2(mtppms)2}2] (1), mtppms and
NaHCO3 in the absence of cinnamaldehyde is also explained by the suggested mechanism, by
that the reaction yields 3 and formate. Finally, the need for a ligand excess over 1 is
rationalized by the fact that the species in the suggested catalytic cycle, 3 and 4 contain three
mtppms ligands.
To obtain further evidence on the role of formate in speeding up the hydrogenation of
cinnamaldehyde catalyzed by 1 + mtppms, we carried out reactions with added NaHCO2
instead of NaHCO3. The results are shown in Table 2.
Table 2. Effect of Na-formate in comparison to that of Na-bicarbonate on hydrogenation of
cinnamaldehyde catalyzed by [{RuCl2(mtppms)2}2] + mtppms.
Promoter
T (°C)
Products
HCAL (%) HCOL (%)
COL (%)
NaHCO2
NaHCO3
none*
60
60
60
70.0
49.0
20.9
2.6
3.3
0.0
2.6
0.0
0.0
NaHCO2
NaHCO3
none*
70
70
70
80.5
54.5
30.3
1.7
5.4
2.4
7.1
0.0
1.9

13
Conditions: n(CAL) = 1.0 mmol, n(1) = 0.005 mmol, n(mtppms) = 8 mmol, n(NaHCO2) =
n(NaHCO3) = 0.2 mmol, V(H2O or *0.2 M phosphate buffer) = 7.5 mL, P(H2) = 1 bar, t = 1
h, 10 min prehydrogenation before addition of CAL.
As can be seen, at both temperatures NaHCO2 is more efficient in speeding up hydrogenation
of cinnamaldehyde catalyzed with [{RuCl2(mtppms)2}2] + mtppms (although the higher
conversions are accompanied by a small loss in selectivity). This shows that in agreement
with the suggested mechanism the most important species in the promoter effect of NaHCO3
is in fact the formate, produced by hydrogenation of bicarbonate.
4. Conclusion
NaHCO3 acts as promoter in hydrogenation of cinnamaldehyde to cinnamalcohol in aqueous
systems with [{RuCl2(mtppms)2}2] + mtppms catalyst. Study of the effects of reaction
parameters together with multinuclear NMR measurements revealed that the origin of this
promoter effect is in the formation of trans-[Ru(H)2(H2O)(mtppms)3] (3) rather than cis-fac-
[Ru(H)2(H2O)(mtppms)3] (5) mediated by formate arising from hydrogenation of bicarbonate
under reaction conditions. Formate can also accelerate regeneration of the real catalytic
species, i.e. 3. Accordingly, NaHCO2 produces even higher rate increase than the same
amount of NaHCO3.
Acknowledgements
This work was supported by the EU and co-financed by the European Regional Development
Fund under the project GINOP-2.3.2-15-2016-00008. Financial support of the Hungarian
National Research Fund (NKFI-1PD115535 to G.P. and OTKA-NKFIH K101372 to F.J.) is
gratefully acknowledged.
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15
Highlights:
- NaHCO3 strongly accelerates cinnamaldehyde hydrogenation with a water-soluble Ru(II)-
phosphine catalyst
- The reaction is highly selective towards formation of the unsaturated alcohol
- A plausible reaction mechanism is suggested on basis of kinetic and NMR measurements
- Formate (from hydrogenation of bicarbonate) plays active role in catalysis
- trans-[Ru(H)2(H2O)(mtppms)3] was identified as key catalytic species