Dehalogenation of organic halides in aqueous media by hydrogen transfer from formate catalyzed by water-soluble Ru(II)-N-heterocyclic carbene complexes

Article abstract of DOI:10.1016/j.molcata.2016.09.036

Water-soluble [RuCl(NHC)(L)(η⁶-arene)] complexes (NHC?=?bmim?=?1-butyl-3-methyl-imidazole-2-ylidene;?L?=?tertiary phosphine, such as mtppms, mtppts, pta, pta-Me and pta-Bn; η⁶-arene?=?η⁶-p-cymene) were succesfully applied for the first time as catalysts in hydrodehalogenation of organic halides by hydrogen transfer from aqueous Na-formate with turnover frequencies up to TOF?=?112?h^?1 at 80?°C. Simultaneous to hydrodehalogenation, aqueous formate was also decomposed to H₂ and HCO₃^?. In case of [RuCl(bmim)(pta)(η⁶-p-cymene)]Cl (pta?=?1,3,5-triaza-7-phosphaadamantane) a reaction mechanism is suggested on basis of kinetic and NMR measurements which accounts for both hydrodehalogenation and formate dehydrogenation and involves [RuH(bmim)(pta)(η⁶-p-cymene)]⁺ as the key catalytic species for both cycles.

Full text of DOI:10.1016/j.molcata.2016.09.036

Accepted Manuscript
Title: Dehalogenation of organic halides in aqueous media by
hydrogen transfer from formate catalyzed by water-soluble
Ru(II)-N-heterocyclic carbene complexes
Author: Nat a´ lia Marozs a´ n Henrietta Horv a´ th Anik o´ Erdei
Ferenc Joo´
PII:
S1381-1169(16)30419-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.09.036
MOLCAA 10060
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
10-8-2016
27-9-2016
29-9-2016
Please cite this article as: Nat a´ lia Marozs a´ n, Henrietta Horv a´ th, Anik o´ Erdei, Ferenc
Jo o´ , Dehalogenation of organic halides in aqueous media by hydrogen transfer from
formate catalyzed by water-soluble Ru(II)-N-heterocyclic carbene complexes, Journal
of Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.09.036
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Dehalogenation of organic halides in aqueous media by hydrogen transfer
from formate catalyzed by water-soluble Ru(II)-N-heterocyclic carbene
complexes
a
b
a
a,b
Natália Marozsán *, Henrietta Horváth , Anikó Erdei , Ferenc Joó *
aDepartment of Physical Chemistry, University of Debrecen, Debrecen, P.O. Box 400,
H-4002 Hungary
bMTA-DE Homogeneous Catalysis and Reaction Mechanisms Research Group, Debrecen,
P.O. Box 400, H-4002 Hungary
*
Corresponding authors:
Natália Marozsán
Phone: +3652512900, ext. 22387
E-mail: marozsan.natalia@science.unideb.hu
Ferenc Joó
Phone: +3652512900, ext. 22382
E-mail: joo.ferenc@science.unideb.hu

2
Graphical abstract
Highlights

Organic halides were hydrodehalogenated by homogeneous catalysis in aqueous
system



Dehalogenations proceeded by hydrogen transfer from aqueous sodium formate
Water-soluble, mixed-ligand NHC-posphine-Ru(II) catalysts were employed
A catalytic cycle is proposed on basis of kinetic and NMR measurements
Abstract:
6
Water-soluble [RuCl(NHC)(L)(η -arene)] complexes (NHC = bmim = 1-butyl-3-methyl-
imidazole-2-ylidene; L = tertiary phosphine, such as mtppms, mtppts, pta, pta-Me and pta-Bn;
6
6
η -arene = η -p-cymene) were succesfully applied for the first time as catalysts in
hydrodehalogenation of organic halides by hydrogen transfer from aqueous Na-formate with

3
-1
turnover frequencies up to TOF=112 h at 80°C. Simultaneous to hydrodehalogenation,
-
6
aqueous formate was also decomposed to H
2
and HCO . In case of [RuCl(bmim)(pta)(η -p-
3
cymene)]Cl (pta =1,3,5-triaza-7-phosphaadamantane) a reaction mechanism is suggested on
basis of kinetic and NMR measurements which accounts for both hydrodehalogenation and
6
+
formate dehydrogenation and involves [RuH(bmim)(pta)(η -p-cymene] as the key catalytic
species for both cycles.
Keywords: hydrodehalogenation; formate dehydrogenation; N-heterocyclic carbene
complexes; ruthenium; water-soluble
1
.
Introduction
Reductive dehalogenation of organic halides plays important role in synthetic chemistry as
well as in chemical destruction of halogen-containing organic pollutants [1-8]. Homogeneous
catalytic methods apply metal complexes based mostly on palladium [8-16], however,
complexes of ruthenium [17-22], rhodium [20, 22-27], cobalt and nickel [28] are also found
among the most active catalysts. It is of interest in this regard, that the extensively studied
various C-C coupling reactions with organic halides as one of the reactants also imply
catalytic rupture of the C-Halogen bond; it comes as no surprise that palladium complexes are
most often featured as catalysts of choice in C-C couplings, too. In addition to H , other
2
hydrogen donors, such as silanes [4, 17, 29], and formates [14, 28] have also been used. For
example, Na-formate dissolved in 2-propanol was found to be a very efficient hydrogen donor
in dehalogenations of hexachlorocyclohexane and hexachlorobenzene catalyzed by Rh(I)-
complexes [25].
Traditionally, tertiary phosphine complexes play important role in
hydrodehalogenations. Nevertheless, in the last two decades N-heterocyclic carbene (NHC)

4
complexes were introduced into many areas of organometallic catalysis [30, 31]. NHC ligands
(such as e.g. imidazole-2-ylidenes) form stable bonds to transition metal ions and the resulting
complexes become suitable for application as catalysts under harsh conditions (at high
temperatures, in the presence of strong bases, etc), too. Typical reactions which include
transition metal NHC complexes are C-C couplings, olefin metathesis, cycloadditions,
hydroformylation, hydrogenation, transfer hydrogenation, redox isomerization,
hydrosilylation, hydration, etc. [32]. N-heterocyclic carbene complexes were used in
hydrodehalogenations [10-13], as well, and were found promising in catalysis of the usually
difficult reductive defluorination reactions, too [7, 33].
Application of transition metal NHC complexes in aqueous catalysis has expanded
enormously in the last 10 years, so much that it was covered by three extensive reviews [34-
3
6]. Water as solvent has many attractive features in organic and organometallic chemistry
[37, 38]. Certainly, the main impetus comes from the possible elimination of hazardous
organic solvents (the green aspect) although treatment of polluted wastewater also needs
serious attention. Hydrodehalogenation in aqueous systems was rarely reported, one example
-1
-1
being the slow (TOF<6.3 h ; TOF=mol reacted substrate×(mol catalyst×time) )
dehalogenation of water-soluble chloro- and bromopropanoic acids by aqueous Na- and NH
4
-
formates catalyzed by various Cp*Ir complexes [39]. In another study, Bényei et al [19]
reported a very efficient system for hydrodehalogenation of alkyl, benzyl, and aryl halides
using water-soluble Ru(II)-phosphine complexes as catalysts for hydrogen transfer from
-1
aqueous Na-formate; the initial TOF of CHCl dehalogenation reached 1000 h . However,
3
hydrodehalogenation in aqueous media catalyzed by water-soluble transition metal NHC
complexes has not been reported so far.

5
For several years, we have been interested in syntheses of water-soluble
organometallic complexes and their catalytic applications in aqueous solutions or aqueous-
organic biphasic systems [37, 40-44]. Here we report the first successful application of water-
soluble Ru(II)-NHC-phosphine complexes as catalysts for hydrodehalogenation of organic
halides in aqueous systems with hydrogen transfer from Na-formate. The catalysts were
6
obtained in reactions of [RuCl
2
(bmim)(η -p-cymene)] [40] and various water-soluble
phosphine ligands shown on Scheme 1. Reactions catalysed by one of these complexes,
6
[
RuCl(bmim)(pta)(η -p-cymene)]Cl (pta=1,3,5-triaza-7-phosphaadamantane=1,3,5-triaza-7-
3
,7
phosphatricyclo[3.3.1.1] decane) were studied in detail. It was found, that in addition to
hydrodehalogenation this complex also catalyzed the decomposition of aqueous formate to H
and HCO -
2
3
, too. On basis of kinetic studies, a possible mechanism for the simultaneous
hydrodehalogenation and formate dehydrogenation is also suggested.
2
.
Experimental
2
.1
Materials, general instrumentation and procedures
All commercial reagents were high purity products of Sigma Aldrich and were used as
6
received. [{RuCl
mtppts-Na
according to known methods. [RuCl
cymene)}
2
(η -p-cymene)}
2
] [45], and the water-soluble phosphines, mtppms-Na [46],
3
[47], pta [48], [pta-Me](OSO
2
CF
3
) [49], and [pta-Bn]Cl [50] were prepared
6
6
2
(bmim)(η -p-cymene)] was prepared from [{RuCl
2
(η -p-
2
] and 1-butyl-3-methylimidazolium chloride, [bmim]Cl by the silver-carbene
1
13
transfer methodology as described earlier [40]; H and C NMR spectra of the product were
identical to those in the literature.
1
13
31
H, C, and P NMR spectra were recorded by Bruker 400 MHz and Bruker 360
MHz NMR spectrometers and referenced to DSS (4,4-dimethyl-4-silapentane-1-sulfonic acid
sodium salt), TMS (tetramethylsilane), 85% phosphoric acid, and residual solvent peaks,

6
respectively. For elemental analyses an ElementarVario Micro (CHNS) equipment was used.
ESI-MS data were collected on a BRUKER BioTOF II ESI-TOF spectrometer in positive ion
mode.
Gas chromatographic analysis of hydrodehalogenation reaction mixtures were done by
®
using an Agilent 7890A equipment (automatic injector; column: MN Optima -1, 0.25 µm x
3
0 m x 0.32 mm; carrier gas: N ; injector at 280°C; FID at 300°C). Sensitivity of the detector
2
for all substrates and their dehalogenation products was calibrated using naphthalene as
internal standard. The Agilent Data System program was used for data collection and analysis.
2
.2
General procedure of the dehalogenation
Hydrodehalogenation reactions were run with catalysts prepared in situ from
6
[RuCl
2
(bmim)(η -p-cymene)] and the appropriate phosphine ligand. The reactions were run
under argon at constant temperature either in Schlenk tubes (atmospheric pressure) or in home
made thick-wall glass reactors (reactions under pressure). A representative example of the
catalytic hydrodehalogenation of chloroform is as follows. 4.8 mg (0.01 mmol) of
6
[RuCl
2
(bmim)(η -p-cymene)] and 1.6 mg (0.01 mmol) pta was placed into a Schlenk tube
closed by a rubber septum. The tube was evacuated then filled with argon gas several times. 1
mL of aqueous solution of HCOONa (136 mg, 2 mmol) was injected through the septum with
needle, followed by 80 µL (1 mmol) of CHCl containing 10 mg naphthalene (internal
3
standard). The tube was immersed into a thermostated bath (80.0±0.1 °C) and the mixture was
vigorously stirred for 120 min. After this time the reaction mixture was cooled to room
temperature then kept in the freezer for 10 minutes. Then 1 mL of toluene (1 mL hexane in
case of benzyl halides) was injected through the septum and the product mixture was
thoroughly extracted at room temperature. The separated organic phase was filtered through a

7
short column of anhydrous MgSO
4
and analyzed by gas chromatography. Catalytic
dehalogenation data disclosed in Tables and on Figures are means of 3-5 experiments.
6
2
.3
Synthesis of the [RuCl(bmim)(pta)(η -p-cymene)]Cl catalyst
6
[
RuCl(bmim)(pta)(η -p-cymene)]Cl was synthetized by reacting under argon 150 mg (0.34
6
mmol) [RuCl (bmim)(η -p-cymene)] and 49.4 mg (0.34 mmol) pta in 10 mL dry methanol for
2
2
h at room temperature. The red reaction mixture was filtered through a HyfloSuperCell pad
and evaporated to dryness to yield a sticky dark solid. This was triturated with several
portions of diethyl ether after which an orange solid product was formed (164 mg, 81%).
Elemental analysis (%): Found C, 46.27; H, 6.22; N, 10.66; Calculated for C24
H
40
N
5
PCl Ru
2
C, 47.92; H, 6.70; N, 11.64. Despite all our efforts we were unable to obtain samples with
elemental analysis closer to the calculated and the composition of the product was established
mainly on basis of NMR and MS data.
Samples for ESI-MS were prepared in methanol and showed peaks corresponding to
6
+
[
RuCl(bmim)(pta)(η -p-cymene)]Cl at m/z 566.174 ([M-Cl] , calc. 566.175) and 409.095
+
(
[M-Cl-pta] , calc. 409.098) with correct isotopic distribution patterns.
1
13
31
1
Characterization by H, C, and P NMR-spectroscopy: H NMR (400 MHz, 298 K,
DMSO-d ) δ/ppm: 0.84 (t, 3H, N-CH CH CH CH ), 1.05 (d, 6H, CH -CH-CH ), 1.19 (sextet,
H, N-CH CH CH CH ), 1.68 (quintet, 2H, N-CH CH CH CH ), 1.83 (s, 3H, C-CH ), 2.45
heptet, 1H, CH -CH-CH ), 3.71 (s, 3H, N-CH ), 4.02 (t, 2H, N-CH CH CH CH ), 4.12 (s,
6
2
2
2
3
3
3
2
2
2
2
3
2
2
2
3
3
(
3
3
3
2
2
2
3
6
7
H, PCH
2
N), 4.37 (s, 6H, NCH N), 5.67-5.70 (m, 4H, -CH-), 7.36 (d, 1H, N-CH=CH-N),
2
.42 (d, 1H, N-CH=CH-N). 13C NMR (100 MHz, 298 K, DMSO-d
) δ/ppm: 13.93 (-
6
CH CH ), 18.33 (C-CH ), 19.51 (N-CH CH CH ), 22.19 (CH-CH ), 30.56 (CH-CH ), 33.43
2
3
3
2
2
2
3
3
1
N), 72.66 (d, 3JP-
(
N-CH CH ), 38.59 (N-CH ), 50.97 (N-CH ), 52.03 (d, JP-C= 16 Hz, PCH
2
2
2
3
2
C
= 6 Hz, NCH N) 85.16, 88.37 (CH-CH), 95.44 (C-CH ), 104.48 (CH-CH(CH ) ), 122.16,
2
3 3 2

8
3
1
1
1
23.36 (N-CH=CH-N), 178.51 (NCN). P{ H} NMR (162 MHz, 298 K, DMSO-d ) δ/ppm:
6
6
-
35.54, -37.55, -38.50. Addition of [RuCl
2
(pta)(η -p-cymene)] increased the intensity of the
resonance at -35.54 ppm what shows that during the synthesis part of the bmim ligand in
6
[RuCl
2
(bmim)(η -p-cymene)] was replaced by pta.
After column chromatography on silica gel (eluent methanol:triethylamine=1:1) a
3
1
brown sticky solid was obtained. This product showed two singlets in the P NMR spectrum
(
solvent: 0.1 M KCl in D
the product mixture.
In contrast to the isolated product, mixing of aqueous solutions of [RuCl
2
O, 298 K) at δ= -36.21 ppm and -36.93 ppm (in a 8/2 intensity ratio)
6
2
(bmim)(η -p-
cymene)] and pta followed with heating for 30 min at 60°C yielded a solution displaying two
3
1
singlet resonances in the P NMR spectrum at -36.93 ppm and -37.53 ppm (in H O) with a
2
6
8
/2 intensity ratio, and the reaction mixture contained only 4% of [RuCl
2
(pta)(η -p-cymene)]
impurity. This led us to investigate catalytic hydrodehalogenation of organic halides with
6
catalysts prepared in situ from [RuCl
phosphine ligands.
2
(bmim)(η -p-cymene)] and the appropriate tertiary
3
.
Results and discussion
3
.1
Dehalogenation of organic halides
6
[RuCl
2
(bmim)(η -p-cymene)], 1 (Scheme 2) was found to efficiently catalyze
hydrodehalogenation of aliphatic and aromatic halides. As yet, no similar half-sandwich
Ru(II)-NHC complexes have been applied for catalytic hydrogenolysis of C-X (X = halogen)
bonds in water.
In aqueous systems, 1 is known to dissociate chloride to yield [RuCl(H
O)(bmim)(η6-
2
+
p-cymene)] in which the aqua ligand can be easily displaced by phosphines [40]. The various
6
[
RuCl(bmim)(L)(η -p-cymene)] complexes formed in situ in the reaction of 1 with

9
L = mtppms, mtppts, pta, pta-Me or pta-Bn were also active in catalysis of
hydrodehalogenation of various halides (Scheme 2). Data for dehalogenation of chloroform
are given in Table 1.
As shown by the data in Table 1, 1 itself is an active catalyst of chloroform dehalogenation in
the biphasic CHCl
3
/H O mixtures leading to 17.2% conversion in 2 hr. The catalytic activity
2
can be increased by addition of pta or mtppms; in fact the highest conversion (25.8% in 2 h)
was achieved with the latter ligand. Monosulphonated triphenylphosphine (mtppms) is a
known surfactant [37] and this may facilitate the mass transport between the catalyst-
containing aqueous phase and the organic phase of the neat substrate. The charged
6
2-
[RuCl(bmim)(pta)(η -p-cymene)]+
6
[
RuCl(bmim)(mtppts)(η -p-cymene)] ,
and
6
2+
[
RuCl(bmim)(L)(η -p-cymene)] complexes (L = pta-Bn or pta-Me) showed lower catalytic
6
activity than [RuCl(bmim)(mtppms)(η -p-cymene)]. In general, the catalytic activities of the
complexes in Table 1 compare favorably to literature data for homogeneous Ru(II) catalysts.
For example, with [RuCl
2
(PPh
3
)
3
] catalyst Xie et al [18] observed 19 turnovers in 36 h
in xylene, while in the dehalogenation of
-1
(
TOF = 0.53 h ) in hydrodehalogenation of CHCl
3
-1
benzyl chloride, Marčec et al [20] obtained a TOF=4.4 h using HCOOLi as hydrogen source
and refluxing dioxane as solvent. On the other hand, Bényei et al reported an outstandingly
efficient system for hydrodehalogenation of CCl
{RuCl (mtppms) ] + mtppms as catalyst and aqueous HCOONa as hydrogen donor. At
0°C, dehalogenation started with an initial TOF as high as 1000 h , however, the reactions
stopped abruptly at about 60% conversion of CCl (although the average TOF for the total 3
hours reaction time was still a respectable 160 h ) [19].
4
and other aliphatic halides composed of
[
2
}
2 2
-1
8
4
-1

1
0
In most of our further investigations on the details of catalytic hydrodehalogenation
6
we used the catalyst prepared in situ from [RuCl
2
(bmim)(η -p-cymene)] and pta. The effect of
the [pta]/[Ru] ratio on the activity of the catalyst is shown on Figure 1. The highest activity
was observed with a [pta]/[Ru] = 1/1 ratio, and this refers to the involvement of the known
6
+
[
RuCl(bmim)(pta)(η -p-cymene)] mixed NHC-phosphine complex [40] among the most
important catalytic species. Hydrodehalogenation of CHCl was also carried out with the
3
catalyst synthetized as described in the Experimental; essentially the same catalytic activity
was observed (20.2% conversion in 2 h) as with corresponding in situ mixtures of 1 and pta
(19.6 %, Table 1, Entry 3).
In the absence of HCOONa no reaction occured. However, with an increase of the formate
concentration a steep increase of the reaction rate was observed followed by a plateau and
even with a slight decrease; the highest conversion (21.6%) was determined at
[
HCOONa] = 2 M (Figure 2). The most probable explanation of the rate decrease above this
formate concentration may be in the decreased solubility of CHCl in such aqueous formate
solutions. Nevertheless, the conversion could be increased with a simultaneous increase of
both the formate concentration and the reaction time. Accordingly, CHCl was dehalogenated
3
3
with 36.4% conversion with 5 M HCOONa in 6 h (other conditions as in Figure 2).
On increasing the temperature the reaction rate increased exponentially. While hardly any
conversion (1.2%) was detected at 40°C, 34.3% of the substrate CHCl
CH Cl at 90°C in 2 h (Figure 3). Conversions in the first 2 h represent well the initial rates
of CHCl
3
was converted to
2
2
2
3
dehalogenations (Figure 4) and the data gave a good linear Arrhenius plot (R =

1
1
0
.999) allowing calculation of a formal activation energy of E =64.0 kJ/mol. Note that in
a
addition to changes in the rate of the various steps of the chemical reaction with changing
temperatures, in a liquid biphasic system the rate of the overall reaction may be influenced
also by changes in the solubility of the substrate(s) and product(s) in the two liquid phases
(themselves temperature dependent).
Table 2 summarizes the results of the hydrodehalogenation of various organic halides with the
6
+
[
RuCl(bmim)(pta)(η -p-cymene)] catalyst in aqueous formate solutions. In accord with the
reactivity of CHCl , the reaction of carbon tetrachloride yielded both chloroform and
3
dichloromethane (entries 1, 2). Concerning the reactivities of various substrates, our findings
follow the expected trends. Accordingly, aliphatic halides reacted faster than halobenzenes, so
much that in the case of bromo- and iodobenzene the reaction times had to be extended from 2
to 19 h to achieve meaningful conversions (chlorobenzene still gave low yield of benzene).
The reactivity order was R-Cl < R-Br < R-I; these results are in agreement with the general
reactivity of the C-Halogen bonds. Dehalogenation of benzyl halides was accompanied by
hydrolysis to benzyl alcohol and for that reason it was not investigated in more detail.
While dehalogenation proceeded effectively under argon with HCOONa as the hydrogen
source, H
In the presence of HCOONa, admisson of 10 bar H
conversions of the most reactive substrates (CCl and CHBr
was only slightly influenced by H
2
alone proved insufficient, and formate, too, had to be given to initiate the reactions.
2
caused noticeable changes in the
4
3
, entries 2 and 6). Conversely,
and this effect was not further
dehalogenation of CHCl
scrutinized.
3
2
Under the standard reaction conditions, hydrodehalogenation of CHCl
smoothly and no other product than CH Cl was observed. However, the time course of the
reaction (Figure 4) revealed that the reactions stopped well before 100% conversion of the
3
proceeded
2
2

1
2
-1
substrate (31.0% conv. in 4 h). With the more reactive CHBr (initial TOF = 112 h ) higher
3
conversions were observed (86.5% conv. in 4 h), although there was no complete conversion
in this case either.
6
or CHCl
3
; 0.01 mmol [RuCl
2
(bmim)(η -p-cymene)] (1); 0.01 mmol pta; 2.0 mmol HCOONa;
solvent = 1 mL H
2
O; T = 80°C; argon atmosphere (1 bar).
3
.2
Catalysis of formate dehydrogenation concomitant to hydrodehalogenation
A possible explanation of incomplete conversions of organic halides is in the catalytic
-
decomposition of formate to H
2
and HCO
3
(Eq. 1) concomitant to dehalogenation. Formate
decomposition is a reaction of great interest since –when coupled to catalytic hydrogenation
of bicarbonate– it can serve as basis for reversible hydrogen storage/delivery systems. As yet
only a few Ru(II)- and Ir(I)-based homogeneous catalysts have been identified capable for
catalysis of the reversible reaction (1) in both directions, influenced only by the hydrogen
pressure [42, 43, 51-53].
-
-
HCO
2
+ H
2
O
H
2
+ HCO
3
(1)
Indeed, we have found that under the reaction conditions of dehalogenation but in the absence
6
+
of an organic halide, [RuCl(bmim)(pta)(η -p-cymene)] catalyzed the formation of H from
2
aqueous formate - the reaction could be followed by using an atmospheric gas burette. The
results show (Figure 5) that 13% of the initial formate decomposed in 2 h at 80°C. This is in
good agreement with the 77.6% dehalogenation of CHBr
3
during the same time (Figure 4).
Note that dehalogenation of CHBr
apparently due to lack of formate.
3
stopped completely after 4 h (maximum conversion 86%),

1
3
With the aim to assess the role of formate decomposition in hydrodehalogenation of
haloorganics, we studied the kinetics of H evolution. The initial rates of H evolution at
2
2
temperatures at which the reaction could be followed by gas volumetry were determined from
the slopes of the linear parts of the graphs in the 15-40 min intervals (Figure 5). An
Arrhenius-plot of these data yielded an activation energy of E
a
= 119 kJ/mol. There are only a
few data in the literature on the activation energy of catalytic decomposition of HCOONa in
aqueous solution yielding H
2
and bicarbonate. The closest to our system is the
6
dehydrogenation of aqueous Na-formate by the half-sandwich η -arene complex catalyst
obtained in situ from [{RuCl
studied by Beller and co-workers [52]. They reported initial rates of formate decomposition at
0°C and at 40°C (in DMF/H O solvent mixtures); from the data an activation energy of 83
kJ/mol can be calculated. This is significantly less than the value determined by us for
2
(benzene)}
2
] + 1,2-bis(diphenylphosphino)methane (dppm)
6
2
6
+
[
RuCl(bmim)(pta)(η -p-cymene)] for formate dehydrogenation (119 kJ/mol) but still
substantially higher than the apparent activation energy of hydrodehalogention of CHCl by
3
the same complex (64.0 kJ/mol, see 3.1).
6
+
-
3
.3
Reactions of [RuCl(bmim)(pta)(η -p-cymene)] with aqueous HCOO
6
Addition of 1 equivalent of pta to a solution of [RuCl
2
(bmim)(η -p-cymene)] in H
2
O resulted
in an immediate color change from orange to yellow. After heating the reaction mixture for 30
3
1
min at 60°C two singlet resonances were observed in the P NMR spectrum at -36.93 and
-
37.53 ppm (in H
2
O) in a 8/2 ratio. The main component in this solution is
6
+
(bmim)(η6-
O)(bmim)(η -p-cymene)] we assign the less intensive
[
RuCl(bmim)(pta)(η -p-cymene)] (δ= -36.93 ppm) [40]. Since aquation of [RuCl
2
6
+
p-cymene)] is known to yield [RuCl(H
2
31P resonance at δ= -37.53 ppm to [Ru(H
O)(bmim)(pta)(η -p-cymene)] . Addition of 50
6
2+
2
equivalents of HCOONa at room temperature caused a shift of these singlets to -36.99 ppm

1
4
1
and -37.60 ppm. Upon heating this solution at 80°C for 30 min, H-NMR showed two
doublets in the hydride region centered at δ = -10.92 ppm (2JH-P=52.9 Hz) and at -10.95 ppm
(2 H-P=52.9 Hz). Similarly, in the 31P NMR spectrum two doublets were observed at
J
δ = -21.55 ppm (2JP-H=51.5 Hz) and δ = -21.70 ppm (2JP-H=52.5 Hz). Proton decoupling led to
collapse of these doublets to singlets, similarly, the doublets in the hydride region collapsed to
singlets upon phosphorus decoupling. These spectral data are consistent with the presence of
6
+
half-sandwich monohydrido complexes [RuH(bmim)(pta)(η -p-cymene)] (major product,
3
1
1
1
31
6
P{ H} δ = -21.55 ppm, H{ P} δ = -10.95 ppm) and probably [RuH(HCOO)(pta)(η -p-
3
1
1
1
31
cymene)] (minor product, P{ H} δ = -21.70 ppm, H{ P} δ = -10.92 ppm). Presence of
6
n+
-
[
RuHX(pta)(η -p-cymene)] (X=Cl , n=0; X=H O, n=1) complexes can be excluded since
2
1
1
31
they show doublets in the H NMR spectra at δ = -10.35 and -10.43 ppm (singlets in H{ P}
NMR, in both cases 2JP-H = 40 Hz) [54]. Under no conditions were triplet hydride resonances
1
observed in the H NMR spectra, so despite the lability of the complexes and possible
redistribution of pta among various ruthenium species (as observed in case of similar Ru(II)-
arene-pta complexes [54]) formation of hydrido-bisphosphine-Ru(II) species (such as e.g.
6
+
[
RuH(pta) (η -p-cymene)] [54]) during catalysis need not be considered.
2
3
.4
Suggested reaction mechanism of simultaneous hydrodehalogenation and formate
6
+
dehydrogenation catalyzed by [RuCl(bmim)(pta)(η -p-cymene)]
On the basis of the kinetic and structural findings described in 3.1-3.3, the following
mechanism is suggested for the simultaneous hydrodehalogenation and formate
dehydrogenation (Scheme 3).
6
As shown on Scheme 3, in a fast reaction [RuCl
2
(bmim)(η -p-cymene)] and pta yield
6
+
[
RuCl(bmim)(pta)(η -p-cymene)] in which chloride is replaced easily by formate. Heating

1
5
-
the aqueous solution to 80°C induces decomposition of coordinated HCOO with release of
-
6
+
CO
hydride ligand of this complex attacks the positively polarized carbon in the substrate (e.g.
CHCl ) which also results in the split of the C-Cl bond to yield the dehalogenated product
CH Cl ). The resulting coordinatively unsaturated Ru(II)-moiety may pick up a chloride or
formate and enters again the catalytic cycle of hydrodehalogenation. Alternatively,
2
(as HCO
3
) and concomitant formation of [RuH(bmim)(pta)(η -p-cymene)] . The
3
(
2
2
6
+
[
RuH(bmim)(pta)(η -p-cymene)] is protonated to yield H
2
and in the presence of excess
HCOO provides [Ru(HCOO)(bmim)(pta)(η -p-cymene)] (hydrogen generation cycle). With
this catalyst, formate decomposition (E = 119 kJ/mol) cannot be rate determining in
hydrodehalogenation of CHCl for which an activation energy of 64.0 kJ/mol was determined.
-
6
+
a
3
Obviously, the two activation energy values belong to different steps in the relevant catalytic
cycles. It seems reasonable to assume that in hydrodehalogenation the hydride transfer from
δ+
δ-
the metal to carbon in the C -Cl bond is the rate determining step, while in case of formate
from the [RuH(bmim)(pta)(η -p-cymene)]+
6
dehydrogenation it is the release of H
2
intermediate which limits the rate of H generation.
2
Summary
In summary, we have succesfully used for the first time water-soluble mixed-ligand Ru(II)-
NHC-phosphine complex catalysts such as [RuCl(bmim)(pta)(η -p-cymene)]+ for
hydrodehalogenation of organic halides by hydrogen transfer from sodium formate in aqueous
6
-1
media with turnover frequencies up to TOF=112 h at 80°C. The reactions proceed in purely
6
aqueous media and require no organic solvents, bases or other additives. [RuCl
2
(bmim)(η -p-
6
cymene)] (1) and the water-soluble [RuCl(bmim)(L)(η -p-cymene)] complexes (L=mtppms,
mtppts, pta-Me and pta-Bn) derived from 1 in reaction with L were also active in

1
6
hydrodehalogenation but –except the mtppms derivative– showed lower activity than
6
+
[
RuCl(bmim)(pta)(η -p-cymene)] . Aliphatic as well as aromatic halides could equally be
dehalogenated with decreasing reactivity in this order. It was also found that in addition to
6
+
hydrodehalogenation [RuCl(bmim)(pta)(η -p-cymene)] catalyzed also the decomposition of
aqueous formate to H and bicarbonate. Kinetics of both hydrodehalogenation and formate
decomposition were studied and a possible reaction mechanism was suggested involving
2
6
+
[
RuH(bmim)(pta)(η -p-cymene)] as the key catalytic species in both reactions.
Acknowledgement
The research was supported by the EU and co-financed by the European Social Fund under
the project ENVIKUT (TÁMOP-4.2.2.A-11/1/KONV-2012-0043). Thanks are also due to the
Hungarian Research, Development and Innovation Office - Hungarian Research Fund
(NKFIH-OTKA K101372) for financial support.
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[
[
[

2
0
25
20
15
10
5
0
0
1
2
3
4
[
pta]/[Ru]
Figure 1. Hydrodehalogenation of CHCl
3
by aqueous Na-formate; effect of the [pta]/[Ru]
6
ratio on the activity of the catalyst formed in situ from [RuCl
Conditions: 1.0 mmol CHCl
1); solvent = 1 mL H O; argon atmosphere (1 bar); T = 80°C; t = 2 h.
2
(bmim)(η -p-cymene)] and pta.
6
3
; 2.0 mmol HCOONa; 0.01 mmol [RuCl
2
(bmim)(η -p-cymene)]
(
2

2
1
30
25
20
15
10
5
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Cformate (M)
Figure 2. Hydrodehalogenation of CHCl
3
with aqueous Na-formate catalyzed by
6
[RuCl
2
(bmim)(η -p-cymene)] + pta; conversion as a function of formate concentration.
6
Conditions: 1.0 mmol CHCl
solvent = 1 mL H
3
; 0.01 mmol [RuCl
2
(bmim)(η -p-cymene)] (1); 0.01 mmol pta;
2
O; argon atmosphere (1 bar); T = 80°C; t = 2 h.

2
2
40
35
30
25
20
15
10
5
0
3
0
40
50
60
70
80
90
100
Temperature (°C)
Figure 3. Hydrodehalogenation of CHCl
3
with aqueous Na-formate catalyzed by
6
[RuCl
2
(bmim)(η -p-cymene)] + pta; conversion as a function of the temperature. Conditions:
6
1
.0 mmol CHCl
3
; 0.01 mmol [RuCl
2
(bmim)(η -p-cymene)] (1); 0.01 mmol pta; 2.0 mmol
HCOONa; solvent = 1 mL H
2
O; argon atmosphere (1 bar); t = 2 h.

2
3
100
90
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
Time (h)
Figure 4. Time course of the hydrodehalogenation of CHBr
3
(●) and CHCl
3
(■) with aqueous
6
Na-formate catalyzed by [RuCl
2
(bmim)(η -p-cymene)] + pta. Conditions: 1.0 mmol CHBr
3

2
4
1
1
1
4
2
0
8
6
4
2
0
0
20
40
60
80
100
120
140
Time (min)
-
Figure 5. Time-course of H
2
evolution in decomposition of aqueous HCO
2
catalyzed by
6
[
RuCl
RuCl
2
2
(bmim)(η -p-cymene)] + pta at various temperatures. Conditions: 0.01 mmol
6
[
(bmim)(η -p-cymene)] (1); 0.01 mmol pta; 2.0 mmol HCOONa; solvent = 1 mL H
2
O;
T = 70°C (■), 75°C (▲) and 80°C (♦) ; initial atmosphere: argon (1 bar).

2
5
Scheme 1. Water-soluble phosphines used in this study

2
6
Scheme 2. General scheme of hydrodehalogenation of organic halides

2
7
Scheme 3. Suggested mechanism of hydrodehalogenation of organic halides and concomitant
6
dehydrogenation of aqueous formate catalyzed by the [RuCl
catalyst
2
(bmim)(η -p-cymene)] + pta

2
8
Table 1. Hydrodehalogenation of CHCl with aqueous Na-formate catalyzed by 1 and by the
3
6
[
RuCl(bmim)(L)(η -p-cymene)] complexes formed in situ in the reaction of 1 with L.
-1
Entry
1a, b
2
L
Conversion (%) TOF (h )
pta
0
0
-
17.2
19.6
13.9
7.6
8.5
9.8
6.9
3.8
3a
pta
4a
pta-Bn
pta-Me
mtppms
mtppts
5
6
25.8
11.7
12.9
5.8
7
6
Conditions: 1.0 mmol CHCl
solvent = 1 mL H O; P(H
3
; 2.0 mmol HCOONa; 0.01 mmol [RuCl
2
(bmim)(η -p-cymene)] (1); 0.01 mmol L;
a
b
2
2
) = 10 bar; T = 80°C; t = 2 h; argon atmosphere (1 bar); without 1.

2
9
Table 2. Hydrodehalogenation of organic halides with aqueous Na-formate catalyzed by the
6
mixed-ligand NHC-phosphine complex formed in situ from [RuCl
2
(bmim)(η -p-cymene)] and
pta.
Entry Substrate Time Atmosphere Conversion
Products
(%)
(h)
(%)
1
2
3
4
5
6
7
8
9
1
CCl
CCl
4
4
2
Ar
29.7
41.9
19.6
21.6
77.6
66.4
0.7
CHCl
CHCl
3
3
(18.0) + CH
2
Cl
2
(11.7)
(12.9)
2
H
2
(29.0) + CH
2
Cl
2
CHCl
CHCl
3
3
2
Ar
CH
CH
CH
CH
2
2
2
2
Cl
Cl
2
2
H
2
2
CHBr
CHBr
PhBr
PhCl
PhBr
PhI
3
3
2
Ar
Br
Br
2
2
2
H
2
2
Ar
Ar
Ar
Ar
Benzene
Benzene
Benzene
Benzene
19
19
19
1.7
7.4
0
26.5
6
Conditions: 1.0 mmol substrate; 2.0 mmol HCOONa; 0.01 mmol [RuCl
2
(bmim)(η -p-cymene)] (1); 0.01 mmol
pta; solvent = 1 mL H O; T = 80°C; P(Ar) = 1 bar; P(H ) = 10 bar.
2
2