Ruthenium(II)-Catalyzed Hydrogen Generation from Formic Acid using Cationic, Ammoniomethyl-Substituted Triarylphosphine Ligands

Article abstract of DOI:10.1002/cctc.201200782

The selective catalytic decomposition of formic acid into hydrogen and carbon dioxide has been achieved in water under mild conditions. For the first time, a ruthenium ion in combination with a series of oligocationic, ammoniomethyl-substituted triarylphosphines was used for this reaction, as opposed to previously used anionic and neutral ligands. These cationic phosphines vary in size and charge and therefore have different hydrophilic, steric, and electronic properties. Excellent catalytic activities were achieved in the formic acid dehydrogenation reaction and correlations between the activity and the ligand structure were made. High turnover frequencies (TOFs) of 1950h^-1 and turnover numbers (TONs) over 10000 were obtained through optimization of the catalytic system.

Full text of DOI:10.1002/cctc.201200782

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DOI: 10.1002/cctc.201200782
Ruthenium(II)-Catalyzed Hydrogen Generation from
Formic Acid using Cationic, Ammoniomethyl-Substituted
Triarylphosphine Ligands
Weijia Gan, Dennis J. M. Snelders, Paul J. Dyson, and Gꢀbor Laurenczy*[a]
The selective catalytic decomposition of formic acid into hy-
drogen and carbon dioxide has been achieved in water under
mild conditions. For the first time, a ruthenium ion in combina-
tion with a series of oligocationic, ammoniomethyl-substituted
triarylphosphines was used for this reaction, as opposed to
previously used anionic and neutral ligands. These cationic
phosphines vary in size and charge and therefore have differ-
ent hydrophilic, steric, and electronic properties. Excellent cata-
lytic activities were achieved in the formic acid dehydrogena-
tion reaction and correlations between the activity and the
ligand structure were made. High turnover frequencies (TOFs)
of 1950 hÀ1 and turnover numbers (TONs) over 10000 were ob-
tained through optimization of the catalytic system.
Introduction
With the continuing increase in worldwide energy demand, re-
newable energy technologies are required to reduce the de-
pendence on fossil fuels and emission of greenhouse gases
and other pollutants. As an energy carrier, hydrogen is proba-
bly the most promising alternative solution for a new, clean,
and sustainable energy system for transport/mobile applica-
tions. Moreover, efficient energy conversion can be achieved
when hydrogen is utilized in fuel cells.[1] The technical chal-
lenges of using hydrogen as a power source for transport and
small- to medium-scale electricity generation include real-time
production and convenient storage. Current hydrogen storage
and release technology includes high-pressure gas cylinders
and cryogenic tanks,[2] as well as physical storage materials
(carbon materials,[1,3] metal–organic frameworks[4]) and chemi-
cal storage materials (metal hydrides,[5,6] complex hydrides,[7,8]
amine boranes,[9,10] alcohols,[11–13] biomass[14–16]). At present,
none of these systems are entirely satisfactory because of cost,
weight, and/or safety issues. The use of formic acid (HCOOH),
which contains 53 g hydrogen per liter (or 4.4 wt%) and salts
of its conjugate base, formate (HCOOÀ), are currently being
considered as hydrogen sources and potential hydrogen-stor-
age materials.[17] The appeal of formic acid as a vector to store
hydrogen lies in its simplicity. The dehydrogenation of formic
acid and its salts under catalytic conditions to yield hydrogen
and carbon dioxide is an important process for mobile applica-
tions; and at a later stage, the carbon dioxide can be recycled
back into formic acid by reduction. This presents a reversible
hydrogen storage–release system with potentially zero net CO2
emissions, thereby contributing to sustainable energy storage
in the future.
The formic acid decomposition reaction that produces H2
and CO2 has been extensively studied with heterogeneous[18–20]
and homogeneous[21–24] catalytic systems. These catalysts have
various disadvantages, which include poor stability, limited cat-
alyst lifetimes, and low selectivity.[25] In 2006, promising homo-
geneous catalytic systems were developed in our group, which
were based on a ruthenium(II)/(III) precatalyst with water-solu-
ble meta-trisulfonated triphenylphosphine (mTPPTS)[26–28] and
several other hydrophilic phosphines.[29] By using two equiva-
lents of these water-soluble phosphines per ruthenium ion,
significant turnover frequencies (TOF) were observed for the
selective decomposition of formic acid into carbon monoxide-
free hydrogen and carbon dioxide over a wide pressure
range.[30] For mobile applications, we attempted to immobilize
the water-soluble ruthenium(II) catalysts based on the princi-
ples of physical absorption, ion exchange, and coordination to
polymers.[31] Beller et al. have also found that a number of
ruthenium based precursors with arene and/or phosphine li-
gands catalyze the selective decomposition of formic acid into
H2 and CO2.[32–38] In particular, the influence of different amine
adducts and irradiation with light have been studied in detail.
Very recently, a highly effective and robust iron catalyst,[39,40]
which is a finely-tuned complex formed from an iron(II) cation
bound by the four phosphorus centers of a tetradentate phos-
phine
ligand,
tris[(2-diphenyl-phosphino)ethyl]phosphine
[P(CH2CH2PPh2)3 (PP3)],[41] has proven very active in this reac-
tion. These homogeneous catalytic systems for formic acid de-
composition reactions have the potential for practical applica-
tions.[42,43] Wills et al. described the decomposition of formic
acid in a HCOOH/Et3N azeotrope by using a Rh/TsDPEN
(TsDPEN=N-(4-toluenesulfonyl)-1,2-diphenylethylenediamine)
tethered catalyst and suggested that the reaction mechanism
[a] W. Gan, Dr. D. J. M. Snelders, Prof. P. J. Dyson, Prof. G. Laurenczy
Institut des Sciences et Ingꢀnierie Chimiques
ꢁcole Polytechnique Fꢀdꢀrale de Lausanne (EPFL)
CH-1015 Lausanne (Switzerland)
Fax: (+41)21-693-9780
E-mail: gabor.laurenczy@epfl.ch
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201200782.
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is closely related to that of the transfer hydrogenation of ke-
tones.[44] Later, a continuous-flow method for hydrogen gener-
ation from formic acid was described based on a system con-
sisting of RuCl2(DMSO)4 or RuCl3 in the presence of tertiary
amine base.[45] Himeda et al. used an iridium-based catalyst
with 4,4’-dihydroxy-2,2’-bipyridine as the ligand in this reaction
and obtained a high initial turnover frequency (TOF).[46] They
investigated the formic acid decomposition activity of half-
sandwich rhodium and ruthenium catalysts with 4,4’-dihy-
droxy-2,2’-bipyridine (DHBP)[47] as well as a binuclear Cp*Ir
(Cp*=pentamethylcyclopentadienyl) catalyst with 4,4’,6,6’-tet-
rahydroxy-2,2’-bipyrimidine (thbpym) as a bridging ligand.[48]
Fukuzumi et al. have shown that formic acid can be dehydro-
genated selectively in aqueous solution by using [Rh(Cp*)-
(bpy)(H2O)]2+ (bpy=2,2’-bipyridine) and other related com-
plexes.[49] They also studied a heteronuclear iridium(III)–ruthe-
nium(II) complex [Ir(Cp*)(H2O)(bpm)Ru(bpy)2][SO4]2 (bpm=2,2’-
bipyrimidine), which is active in acidic aqueous solutions under
ambient conditions.[50] Some other catalysts, such as [RuCl2-
cant for the catalytic activity of these complexes. Furthermore,
the presence of positively charged groups in the backbone of
the triarylphosphine is expected to enhance hydrophilicity and
increase the interligand Coulombic repulsion between neigh-
boring ligands, and a variety of Tolman cone angles will be
present.[56] These effects could consequently increase the pref-
erence for the formation of transition metal complexes of coor-
dinatively unsaturated phosphines, which may in turn increase
the interaction between the catalysts and substrate molecules.
In particular, the presence of anionic substrates in the reaction
medium would enhance this effect as the positive charges on
the complexes can attract the negatively charged ions to pro-
vide a superior catalytic activity. Some of these types of struc-
ture–activity relationships were reported in 2011.[61]
We have recently explored the Ru-catalyzed H2 generation
from formic acid by using, for the first time, different oligoca-
tionic, ammoniomethyl-substituted triarylphosphines as preca-
talysts in aqueous solution. In this paper, we describe the use
of these phosphines in the catalytic formic acid dehydrogena-
tion reaction and compare the oligocationic phosphines to the
other hydrophilic ligands.
[51]
[52]
(mTPPMS)2]2
and RhCl(mTPPMS)3 (mTPPMS=meta-mono-
sulfonated triphenylphosphine), for the reverse reaction were
investigated by Joꢂ et al. Activity was observed in Ru-catalyzed
formic acid and sodium formate decomposition, as well as Rh-
catalyzed calcium formate decomposition, followed by calcium
carbonate precipitation. In particular, [RuCl2(mTPPMS)2]2 was
shown to catalyze both sodium bicarbonate hydrogenation
and sodium formate decomposition reactions in an aqueous
phase.[53] This system acts as a charge–discharge device for the
storage and delivery of hydrogen without the need to isolate
either formate or bicarbonate. A similar system has very re-
cently been published based on ruthenium(II) benzene/phos-
phine catalysts.[54]
Results and Discussion
In this study, a series of water-soluble, oligocationic, ammoni-
um-functionalized triarylphosphine ligands that differ in size
and charge were prepared and applied as precatalysts. These
ligands are based on a triphenylphosphine structure in which
a certain number of the aryl rings are substituted by one or
two cationic ammoniomethyl groups with charges balanced
À
by BF4 or IÀ counterions (Figure 1). In addition to the previ-
ously reported para-substituted tricationic (3a, 3b), meta-sub-
stituted dicationic (4), meta-substituted tetracationic (5), and
meta-substituted hexacationic (6) triarylphosphines, two new
phosphines have been prepared following similar synthetic
routes with some modifications (see Supporting Information
for the synthesis of 1 and 2). The para-substituted monocat-
ionic (1) and para-substituted dicationic (2) ligands differ in
their solubility in water as a result of their different formal
charges. The two classes of phosphines are either substituted
at the 4- or both the 3- and 5-positions and possess the fol-
lowing order of increasing steric demand: 1<2<3; 4<5<6.
If water-solubilizing substituents are attached at the para posi-
tion of the triarylphosphine, it would be expected that any dif-
ference in steric properties would be small. To evaluate the
electronic properties of these phosphine ligands, the corre-
sponding phosphine selenides were prepared from elemental
In many of the cases mentioned above, water was used as
a solvent/reaction medium in the context of green and sustain-
able chemistry. One of the challenges for catalysis in water is
the design of soluble, hydrophilic ligands with defined elec-
tronic and steric characteristics to ensure the adequate stability
and activity of the catalysts. Most of the ligands developed for
aqueous formic acid/H2 +CO2 interconversion are anionic phos-
phines that contain mainly sulfonate, or to a less extent, car-
boxyl or hydroxyl moieties, as well as neutral phosphines, such
as 1,3,5-triaza-7-phosphaadamantane (PTA). Recently, Klein
Gebbink et al. reported a series of new oligocationic, ammo-
niomethyl-substituted phosphine ligands.[55,56] These com-
pounds vary in size from small molecules to dendritic ligands
through the variation of the number and position of the qua-
ternary ammonium substituents. The homogeneous catalysts
formed from these phosphines in combination with suitable
metal precursors have shown promising results in Pd-catalyzed
Suzuki–Miyaura reactions[56–58] as well as Rh-catalyzed hydrofor-
mylations.[59] Shaughnessy recently stated that the presence of
different ionic substituents attached to triaryl phosphines can
have a significant influence on their coordination behavior and
structural features.[60] Thus, the ionic character of the cationic
phosphines substituted with quaternary ammonium moieties
is not only important for the solubility of the catalysts in water,
but the electronic and steric properties can be equally signifi-
1
Se and analyzed by 31P NMR. The coupling constants JP,Se de-
termined from the 31P NMR spectra correlate to the s-donation
1
strength of the corresponding phosphine ligand. A small JP,Se
coupling constant indicates strong s-donation and the strong
basicity of the phosphine.[62, 63]
1
Table 1 summarizes the JP,Se coupling constants for the pre-
pared phosphine selenides. These data show that the para-
1
substituted cationic compounds have small JP,Se coupling con-
stants, and thus the P center is more electron donating. For
both the para- and meta-substituted cationic phosphines, an
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was encountered in the first
cycle of each system that con-
tained phosphine ligands and
a RuIII precursor and it was also
observed previously for the sys-
tems that contained anionic
phosphine ligands. NMR and UV/
Vis spectroscopy indicate a re-
duction of the RuIII species to
RuII during this time, which ex-
plains why an activation period
is observed.[28] In subsequent
cycles, the catalysts exhibit near-
identical activities, indicative of
the formation of stable catalyti-
cally active species (Figure 2).
The cationic ligands in combi-
nation with RuII were compared
Figure 1. Structures of ammoniomethyl-substituted cationic phosphine ligands 1–6.
1
Table 1. JP,Se coupling constants of phosphine selenides in the 31P NMR
spectra in D2O.
Ligand
1JP,Se [Hz]
Ref.
1
2
3a
3b
4
5
6
678
689
703
701
697
717
735
707
698
[56]
[71]
mTPPTS
mTPPDS
Figure 2. Kinetic curves for HCOOH decomposition with 1. Conditions:
RuCl3·xH2O (28 mm), HCOOH/HCOONa (9:1, 10m), two equivalents of phos-
phine to Ru, 908C; addition of HCOOH (0.38 mL) for recycling.
increased number of ammoniomethyl groups leads to a step-
1
wise increase in the coupling constant JP,Se of the phosphine
selenides, which indicates that the phosphine becomes
a weaker s-donor ligand. The para-tricationic ligand 3, which
has three para-ammoniomethyl substituents, displays a cou-
pling constant similar to that of meta-substituted dicationic
phosphine 4. Interestingly, the magnitude of the coupling con-
stant for 3 is also comparable to that of mTPPTS despite their
opposite charges. Additionally, comparison of the data for 3a
and 3b shows that the anions of the para-substituted trica-
with the highly water-soluble anionic [meta-trisulfonated tri-
phenylphosphine (mTPPTS) and meta-disulfonated triphenyl-
phosphine (mTPPDS)] and neutral (PTA) phosphine ligands typ-
ically used in formic acid decomposition. The catalysts were
prepared in situ by simply mixing the precursor RuCl3·xH2O
with the phosphine ligand in an aqueous HCOOH/HCOONa so-
lution. These manipulations were performed without any inert-
atmosphere protection, as the catalysts are air-stable, especial-
ly after the activation period. In general, when combined with
two equivalents of these ligands, RuII formed efficient catalysts
for the decomposition reaction with high HCOOH conversion
(>95%). The resulting gases were analyzed by FTIR spectros-
copy (with a detection limit of 3 ppm for CO, see Experimental
Section). No CO was detected in any of the reactions, conse-
quently the H2 produced by using these catalysts is suitable
for direct use in fuel cells, which typically require less than
50 ppm CO.[64]
1
tionic molecules have no significant effect on the JP,Se value.
Formation of RuII phosphine catalysts and the catalytic de-
composition of formic acid to generate H2
Cationic phosphine-coordinated Ru catalysts were formed
in situ by mixing RuCl3·xH2O with two equivalents of the ap-
propriate ligand in a formic acid/sodium formate aqueous so-
lution. After heating the homogeneous mixture to 908C, very
rapid formic acid decomposition occurs, which is observed as
a pressure increase that corresponds to the formation of H2
and CO2. This represented a conversion of 30–70% based on
the initial concentration of formic acid. This reaction profile
In Figure 3 the TOF of the Ru catalysts with different cationic
(1–6), anionic (mTPPTS and mTPPDS), and neutral (PTA) hydro-
philic phosphine ligands are shown. TOF values were calculat-
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(IÀ) impart different solubility properties to the phosphines.
However, no significant anion effect was detected in the cata-
lytic HCOOH decomposition. Notably, the small (Tolman cone
angle of 1038),[65] strongly basic, and highly hydrophilic neutral
phosphine PTA exhibits a relatively low activity. As no obvious
catalyst deactivation or aggregation was observed after the
first five cycles, this behavior is possibly related to the protona-
tion of PTA at low pH values (PTA has a pKa of approximately
6.5), which would significantly reduce the donating strength of
the phosphine.[66,67] The high TOF of 1430 hÀ1 obtained for 4 is
probably a result of a good balance between the suitable elec-
tron donation strength, ideal steric properties, its high hydro-
philicity, and the positively charged structure.
To obtain more active catalytic systems, further studies were
performed to optimize the reaction conditions for the Ru com-
plex with 3a (Ru-3a). The effects of initial pH in the solution,
catalyst concentration, ligand-to-Ru ratio, and reaction temper-
ature were investigated and are summarized in Table 2.
Figure 3. Comparison of the activities of Ru-catalyzed HCOOH decomposi-
tion in the presence of cationic (1–6), anionic (mTPPTS and mTPPDS), and
neutral (PTA) phosphine ligands. Catalysts formed in situ with RuCl3·xH2O
(28 mm; 56 mm for the PTA system), HCOOH/HCOONa (9:1, 10m), and two
equivalents of phosphine to Ru at 908C; TOF was calculated for the fifth
cycle of each catalyst.
Table 2. Optimization of the catalyst and reaction conditions of formic
acid decomposition catalyzed by Ru-3a. The reactions were performed in
aqueous solution (H2O/D2O 1:1, 1 mL); TOF was calculated for the fifth
cycle of each catalyst; a known amount of HCOOH was added after each
cycle to reach a total formate concentration of 10m for recycling.
ed after the fifth cycle of each catalyst according to the
HCOOH decomposition rate until 95% conversion was reached
(determined by 13C NMR spectroscopy). From the results, it is
[b]
1
Entry
T [8C] [Ru] [mm] P/Ru [HCOOH]/[HCOONa][a] TOF [hÀ1
]
noticeable that the ligands that possess smaller JP,Se coupling
1
2
3
4
5
6
7
8
90
90
90
90
90
90
90
90
90
90
35
60
120
25
60
90
120
90
28
28
28
28
28
28
14
44
70
140
28
28
28
28
28
28
28
28
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
3:1
3:1
3:1
3:1
1:1
9:1
7:1
5:1
3:1
1:1
1:2
9:1
9:1
9:1
9:1
9:1
9:1
9:1
9:1
9:1
9:1
9:1
9:1
1030
860
940
850
1070
550
580
810
780
430
<4
90
1950
<4
70
860
1950
23
constants, for example, 1, 2, 3, 4, mTPPTS, and mTPPDS, cata-
lyze the decomposition at a higher rate. This indicates that the
stronger s-donating ligands could promote the HCOOH de-
composition, although no linear correlation can be made be-
tween the electronic parameters of the ligands and the catalyt-
ic activity data. Notably, the systems that contain ligands with
charges of 1+, 2+, and 3+ (1–4) afford excellent catalytic ac-
tivities, and these catalysts are stable to at least ten reaction
cycles without any deactivation observed. These new cationic
phosphines promote faster reactions than the previously re-
ported negatively charged ligands mTPPTS and mTPPDS under
the same conditions. The observed high catalytic activity of
these ligands is probably because of the positive charge,
which renders a cationic environment around the catalytic
sites and, therefore, faster coordination of the negatively
charged species, for example, HCOOÀ, HCO3À, and HÀ. In addi-
tion, steric effects may also play a significant role. The steric
bulk of 5 and 6 potentially inhibits access of the substrate mol-
ecules to the available metal coordination site, more than the
smaller phosphines with lesser steric demands. The reason for
this is that the ligands 5 and 6 do not contain dendritic shells,
in contrast to previously published ligands in this series of cat-
ionic ligands.[55,56] The Coulombic repulsion effect was not
studied in this work and we cannot conclude whether or not it
occurs in the present system. Hydrophobic effects are also ex-
pected to perturb the catalytic activity as the hydrophobic
groups may reduce the interaction between the catalyst and
hydrophilic substrates. However, at low catalyst concentrations
(28 mm in our case) all the ligands dissolved completely in
water to form homogeneous solutions without any aggrega-
tion or precipitation observed. The anions in 3a (BF4À) and 3b
9
10
11
12
13
14
15
16
17
18[c]
[a] Initial ratio. [b] TOF was calculated until 95% HCOOH conversion.
[c] TOF was calculated for the second cycle as Ru precipitate was ob-
served after two cycles.
The dependence of the HCOOH decomposition rate on the
initial pH was studied by varying the acid-to-base ratio
([HCOOH]/[HCOONa]), keeping the total substrate concentra-
tion fixed (10m), and recycling the catalyst with the addition
of pure formic acid (Table 2, entries 1–6). High TOF (850–
1070 hÀ1) were observed for reactions in the presence of for-
mate between acid-to-base ratios of 9:1 and 1:1, and the de-
composition was retarded in a more basic solution (HCOOH/
HCOOÀ =1:2). The moderate influence of the initial acid-to-
base ratio on the reaction rate may be because of the relative-
ly small pH change in the solution (2.8–4.1). Similarly, slower
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reaction rates were observed in each catalytic cycle when
formic acid conversions approached 100%, which also repre-
sented solutions with higher pH values. Upon varying the cata-
lyst concentration, the TOF reached a maximum at 28 mm [Ru]
(Table 2, entries 1, 7–10) and decreased gradually at higher
concentrations. This phenomenon was observed with other
catalysts that used RuCl3 as the metal source for which it was
rationalized that with increasing chloride concentrations, the
formation of the catalytically less active chloro-bridged dimers
was much more probable. The number of molar equivalents of
phosphine ligand with respect to RuII was also investigated (P/
Ru ratio, Table 2, entries 1, 16, and 18). Similar to previous stud-
ies, in the Ru-3a system, the required minimal P/Ru ratio is 2:1.
Using lower ratios resulted in the precipitation of elemental Ru
in the reactor. Conversely, increasing the P/Ru ratio above 2:1
would enhance the stability of the catalytic system but with
a slight decrease in the reaction rate as a result of stepwise
phosphine coordination and a crowded first coordination
sphere. From the data on P/Ru variation, we could see that
both two and three equivalents of phosphine per metal center
were sufficient to afford stable catalysts with high activity. The
recycling experiments of these two catalytic systems were per-
formed at different temperatures from 25–1208C (Table 2, en-
tries 1, 11–17). The rate of decomposition increased with tem-
perature, as expected. At all temperatures, including room
temperature, conversions over 95% could be achieved. At
608C, a moderate TOF of 90 hÀ1 for the 2:1 system and 70 hÀ1
for the 3:1 system was observed. The highest TOF (approxi-
mately 2000 hÀ1) were obtained by using either two or three
equivalents of 3a per Ru center at 1208C. In the following
cycles at 908C (after the cycle at 1208C), no deactivation was
observed. In a noteworthy run, the catalyst with two equiva-
lents of 3a was further recycled at 908C by the addition of
pure HCOOH for more than 30 cycles without any loss in activi-
ty (Figure 4). The total turnover number (TON) exceeded
10000 after 10 h of use at 908C over a pressure range of
1–100 bar.
Mechanistic studies on hydride formation
In the mixture of precatalyst RuCl3 and 3a, a new hydride spe-
cies H1 was detected upon the addition of 1m HCOOH. H1 ex-
1
hibits a doublet of triplets at À7.1 ppm in the H NMR spec-
trum, which collapses to a singlet in the corresponding
1H{31P} NMR spectrum (Figure 5). This indicates that the hydride
in H1 couples with nonequivalent P atoms with a structure of
Figure 5. 1H NMR spectrum of H1 with coupling constants.
H(Pa)2Pb (Figure 6). The small mag-
nitude of the coupling constant
2JH,P =23.8 Hz for the triplets indi-
cis
cates that two identical P nuclei in
cis positions couple with the hy-
dride, and are disposed mutually
trans to each other. The large cou-
Figure 6. Proposed structure
of H1 based on NMR spec-
2
pling constant JH,P =101.7 Hz
trans
that corresponds to the doublet
suggests that the hydride is trans
to an additional phosphine ligand.
The remaining RuII coordination
troscopy (P= 1 or 3a;
X=H2O or Cl).
sites are presumably occupied by H2O or ClÀ, as no corre-
sponding signals were observed in 13C NMR experiments. In
the system that contained RuCl3 and 1, a similar hydride struc-
ture was also detected if the solution was under H2 pressure.
In both cases, the hydride species disappear upon standing
the solution overnight at room temperature or heating it to
508C as determined by NMR spectroscopy. The poor stability
of this structure may be because of the steric hindrance of the
three phosphine ligands.
Conclusions
We have shown that RuII in conjunction with a series of posi-
tively charged phosphine ligands affords highly active and
stable catalysts for the dehydrogenation of formic acid. The hy-
drophilic, electronic, and steric properties of the phosphines
were analyzed and consequently related to the catalytic behav-
ior of the corresponding complexes. On optimization of the re-
action conditions with 3a as the ligand, a high TOF of
1950 hÀ1 was achieved with well-defined P/Ru ratios of 2:1 and
3:1. The 3a/Ru (2:1) system is very stable over 30 experimental
cycles to give a total TON of over 10000. With the help of mul-
Figure 4. Kinetic curves for recycling HCOOH decomposition with 3a. Condi-
tions: RuCl3·xH2O (28 mm), HCOOH/HCOONa (9:1,10m), two equivalents of
phosphine to Ru, 908C (12th cycle at 1208C); addition of HCOOH (0.38 mL)
for recycling.
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129.10 (d, 3JC,P =8.3 Hz, m-Ph), 129.06 (s, p-Ar), 67.41 (s, NCH2),
51.86 ppm (s, N(CH3)3); elemental analysis calcd (%) for
C26H35B2F8N2P (580.26): C 53.77, H 6.08, N 4.83; found: C 53.54, H
6.12, N 4.79.
tinuclear NMR spectroscopy, an important hydride intermedi-
ate was detected, which provides useful information for further
mechanistic studies. The cationic phosphine ligands demon-
strate great potential in a HCOOH–H2 (CO2) storage system
through the formation of efficient and flexible aqueous-phase
catalysis.
Synthesis of phosphine selenides 1(Se)–6(Se)
The phosphine selenides were prepared by stirring a solution of
the phosphine (20–30 mg) in MeOH (2 mL) in the presence of 10
equivalents of elemental Se at 508C for 2 h. The mixtures were fil-
tered through Celite and dried in vacuum. The resulting white
powders were dissolved in deuterated water and analyzed by NMR
spectroscopy.
Experimental Section
General manipulations
Manipulations of air-sensitive compounds were carried out under
a N2 atmosphere by using standard Schlenck techniques. Cationic
phosphines 3–6 were synthesized according to literature proce-
dures.[56,59] The two new cationic ligands 1 and 2 were synthesized
according to procedures given in the Supporting Information and
their characterization data are detailed below. mTPPTS was pre-
pared according to a literature procedure[68] with a modified purifi-
cation process[69] and contained <1% of mTPPDS and their phos-
phine oxides. All organic solvents for ligand synthesis were dried
and deoxygenated before use. All other chemicals were purchased
and used as received.
Kinetic studies
Kinetic measurements were performed in 10 mm external diameter
medium-pressure sapphire NMR tubes.[70] In a typical formic acid
decomposition reaction, RuCl3·xH2O (0.028 mmol, 7.4 mg) was dis-
solved in a formic acid/sodium formate (9:1, 10m) aqueous solu-
tion (1 mL H2O/D2O 1:1) that contained two equivalents of the ap-
propriate water-soluble phosphine ligand (0.056 mmol). The cata-
lytically active species were formed in situ by heating the sapphire
tube at 908C in an electric heating jacket in which the decomposi-
tion reaction was monitored by the pressure increase or in the
NMR instrument in which the reaction was monitored by recording
1H and 13C NMR spectra. Conversions were calculated by integra-
tion of the formic acid/formate 13C NMR peak with a proper relaxa-
tion time (T1) setting, before, during, and after the reaction. After
the tube was cooled to room temperature and depressurized to
the air, recycling experiments were performed by the addition of
further formic acid (10 mmol, 0.38 mL).
NMR spectroscopy
1H, 13C, 13C{1H}, 31P, and 31P{1H} NMR spectra were recorded by using
a Bruker DRX-400 NMR spectrometer (5 mm tubes) or a Bruker
Avance DRX-400 NMR spectrometer (10 mm sapphire tubes). 3-(Tri-
methylsilyl)-1-propanesulfonate and phosphoric acid (external
standards) were used as references for 1H and 31P NMR experi-
ments, respectively. The spectra were fitted by using TOPSPIN,
1DWINNMR, and NMRICMA/MATLAB programs (nonlinear least-
square fit to determine the spectral parameters).
FTIR analysis of the gas phase
Characterization data for 1
Analysis of the product gas streams was performed by FTIR spec-
troscopy by using a PerkinElmer IR gas cell (path length 10 cm)
with KBr windows filled with gas products from the sapphire tube
reactor. The spectrometer was operated under a N2 atmosphere.
For the detection of CO, a mixture of H2/CO2 (1:1) was used as
background and samples that contained 10, 5, 3, and 2 ppm of CO
were prepared for calibration. For all spectra, 32 scans were mea-
sured from 4000–400 cmÀ1 (CO band area 2200–2100 cmÀ1). This
procedure allowed the CO concentration to be determined with
a lower limit of 3 ppm.
3
1H NMR (400 MHz, [D6]DMSO): d=7.54 (d, JH,H =7.3 Hz, 2H, m-Ar),
3
7.46–7.40 (m, 2H, p-Ph), 7.43 (d, JH,H =2.3 Hz, 4H, m-Ph), 7.33 (dd,
3JH,H =7.4 Hz, JH,P =7.4 Hz, 2H, o-Ar), 7.30–7.25 (m, 4H, o-Ph), 4.51
3
(s, 2H, NCH2), 3.02 ppm (s, 9H, N(CH3)3); 31P{1H} NMR (162 MHz,
[D6]DMSO): d=À6.91 ppm; 13C{1H} NMR (101 MHz, [D6]DMSO): d=
1
1
139.61 (d, JC,P =13.4 Hz, i-Ar), 135.94 (d, JC,P =11.1 Hz, i-Ph), 133.49
(d, 2JC,P =20.0 Hz, o-Ph), 133.29 (d, 2JC,P =20.9 Hz, o-Ar), 132.98 (d,
3JC,P =6.4 Hz, m-Ar), 129.30 (s, p-Ph), 128.91 (d, JC,P =7.0 Hz, m-Ph),
3
128.75 (s, p-Ar), 67.40 (s, NCH2), 51.83 ppm (s, N(CH3)3); elemental
analysis calcd (%) for C22H25BF4NP (421.17): C 62.68, H 5.98, N 3.32;
found: C 61.55, H 5.66, N 3.20.
Acknowledgements
We thank the Swiss National Science Foundation (SNSF) and the
EPFL for their financial support. The students Yanouk Cudrꢀ, Ma-
thieu Marmier, Arnauld Thevenon, Ewan Frost-Penninton, and Ka-
trina Kendall are thanked for their involvement during their
master studies.
Characterization data for 2
1H NMR (400 MHz, [D6]DMSO): d=7.56 (d, JH,H =7.4 Hz, 4H, m-Ar),
3
3
7.45 (d, JH,H =4.5 Hz, 2H, m-Ph), 7.51–7.43 (m, 1H, p-Ph), 7.37 (dd,
3
3
3
3JH,P =7.6 Hz, JH,H =7.6 Hz, 4H, o-Ar), 7.33 (dd, JH,H =6.9 Hz, JH,P
=
4.4 Hz, 2H, o-Ph), 4.51 (s, 4H, NCH2), 3.02 ppm (s, 18H, N(CH3)3).
31P{1H} NMR (162 MHz, [D6]DMSO): d=À7.13 ppm; 13C{1H} NMR
Keywords: homogeneous catalysis
effects · P ligands · ruthenium
·
hydrogen
· ligand
1
(101 MHz, [D6]DMSO): d=138.92 (d, JC,P =13.2 Hz, i-Ar), 135.34 (d,
2
2
1JC,P =10.7 Hz, i-Ph), 133.83 (d, JC,P =20.4 Hz, o-Ph), 133.58 (d, JC,P
=
19.3 Hz, o-Ar), 133.14 (d, 3JC,P =6.7 Hz, m-Ar), 129.66 (s, p-Ph),
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 1126 – 1132 1131

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Received: October 31, 2012
Published online on March 13, 2013
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ChemCatChem 2013, 5, 1126 – 1132 1132