(para-Diphenylphosphino)benzenesulfonic acid and its ruthenium(II) complexes: An old water soluble phosphine ligand in a new perspective

Article abstract of DOI:10.1139/v01-077

Chloro- and hydrido-complexes of ruthenium(II) with potassium (4-diphenylphosphino)benzenesulfonate (pTPPMS) were prepared and their properties were compared with those of the related complexes of (3-diphenylphosphino)benzenesulfonates. An X-ray crystal structure determination revealed that the para-sulfonated ligand has a Tolman cone angle slightly smaller than that of PPh₃. It was established by pH-potentiometric measurements and by ¹H and ³¹P NMR studies that the distribution of the hydride derivatives [RuClH(pTPPMS)₃] and [RuH₂(pTPPMS)_3,4] is strongly pH-dependent. Hydrogenation of trans-cinnamaldehyde in aqueous-organic mixtures is catalyzed by [RuCl₂(pTPPMS)₄] and is also strongly influenced by the pH of the aqueous phase due to the pH-dependent formation of the above hydrides. In acidic solutions (pH < 2) an exclusive C=C reduction takes place, while in alkaline solutions (pH > 6) a selective C=O hydrogenation was observed.

Full text of DOI:10.1139/v01-077

635
(para-Diphenylphosphino)benzenesulfonic acid
and its ruthenium(II) complexes: an old water
soluble phosphine ligand in a new perspective
Gábor Papp, József Kovács, Attila Cs. Bényei, Gábor Laurenczy,
Levente Nádasdi, and Ferenc Joó
Abstract: Chloro- and hydrido-complexes of ruthenium(II) with potassium (4-diphenylphosphino)benzenesulfonate
(pTPPMS) were prepared and their properties were compared with those of the related complexes of (3-
diphenylphosphino)benzenesulfonates. An X-ray crystal structure determination revealed that the para-sulfonated ligand
has a Tolman cone angle slightly smaller than that of PPh3. It was established by pH-potentiometric measurements and
1
by H and 31P NMR studies that the distribution of the hydride derivatives [RuClH(pTPPMS)3] and [RuH2(pTPPMS)3,4
is strongly pH-dependent. Hydrogenation of trans-cinnamaldehyde in aqueous–organic mixtures is catalyzed by
[RuCl2(pTPPMS)4] and is also strongly influenced by the pH of the aqueous phase due to the pH-dependent formation
of the above hydrides. In acidic solutions (pH < 2) an exclusive C=C reduction takes place, while in alkaline solutions
(pH > 6) a selective C=O hydrogenation was observed.
]
Key words: ruthenium, water soluble, hydrogenation, hydrides, sulfonated phosphines, biphasic.
Résumé : On a préparé les complexes chloro- et hydrido de ruthénium(II) avec le (4-diphénylphosphino)benzènesulfo-
nate de potassium (pTPPMS) et on a comparé leurs propriétés avec celles des complexes apparentés des (3-
diphénylphosphino)benzènesulfonates. Une détermination de structure par diffraction des rayons X a permis de mettre
en évidence que le ligand para-sulfoné présente un cône de Tolman qui est légèrement inférieur à celui du PPh3. Des
1
mesures potentiométriques de pH et des études de RMN du H et du 31P ont permit de déterminer que la distribution
des dérivés hydrures [RuClH(pTPPMS)3] et [RuH2(pTPPMS)3,4] dépend beaucoup du pH. L’hydrogénation du trans-cin-
namaldéhyde, dans des mélanges eau/solvant organique, est catalysée par le [RuCl2(pTPPMS)4] et qu’elle est fortement
influencée par le pH de la phase aqueuse en raison de la formation des hydrures mentionnés plus haut qui est in-
fluencée par le pH. En solutions acides (pH < 2), on observe exclusivement la réduction de la liaison C=C alors qu’en
solutions alcalines (pH > 6), on observe une hydrogénation sélective de C=O.
Mots clés : ruthénium, soluble dans l’eau, hydrogénation, hydrures, phosphines sulfonées, à deux phases.
[Traduit par la Rédaction] Papp et al. 641
Introduction
triarylphosphine by fuming sulfuric acid. In fact, the first
water soluble phosphine ligand (3-diphenylphosphino)ben-
zenesulfonic acid (mTPPMS) (as a Na+ or K+ salt), intro-
duced into coordination chemistry by Ahrland et al. (6), was
prepared this way as early as 1958. As a result of the
protonation of phosphorus in fuming sulfuric acid these
compounds contain the -SO3 moiety in the meta position to
the P donor atom. Based on a large body of experimental
findings in coordination chemistry (7) and in organometallic
catalysis (1–3, 8, 9) it could be concluded that such a modi-
fication of triarylphosphines did not lead to spectacular
changes in complexation properties of these ligands despite
Aqueous organometallic chemistry and catalysis has at-
tracted a great deal of interest recently mostly as a conse-
quence of the relatively easy and cheap regeneration of the
catalysts and isolation of products in aqueous–organic biphasic
systems (1–3). Although by far not exclusive, water soluble
tertiary phosphines are the most important ligands in this
field; the best known example being 3,3′,3′′-phosphinetriyl-
benzenesulfonic acid (Na+ or K+ salts) (TPPTS) or more pre-
cisely, mTPPTS (4, 5). The simplest way of preparation of
sulfonated arylphosphines is the sulfonation of the parent
Received September 5, 2000. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on June 30, 2001.
Dedicated to Professor Brian R. James on the occasion of his 65th birthday.
G. Papp, J. Kovács, and F. Joó.1,2 Research Group of Homogeneous Catalysis, Hungarian Academy of Sciences, University of
Debrecen, 4010 Debrecen, P.O. Box 7, Hungary.
A.Cs. Bényei and L. Nádasdi. Institute of Physical Chemistry, University of Debrecen, 4010 Debrecen, P.O. Box 7, Hungary.
G. Laurenczy. Institute of Inorganic and Analytical Chemistry, University of Lausanne, 1015 Lausanne-Dorigny, Switzerland.
1Corresponding author (telephone: (36-52)-512900; fax: (36-52)-512915; e-mail: jooferenc@tigris.klte.hu).
2Present address: Institute of Physical Chemistry, University of Debrecen, 4010 Debrecen, P.O. Box 7, Hungary.
Can. J. Chem. 79: 635–641 (2001)
© 2001 NRC Canada
DOI: 10.1139/cjc-79-5/6-635

636
Can. J. Chem. Vol. 79, 2001
the somewhat diminished electron density on phosphorus
and the obvious increase in size. Nevertheless, a few distinct
changes have been noticed. For example, although
[RuCl2(PPh3)3], [RuCl2(PPh3)4], and [{RuCl2(PPh3)2}2] are
all known in solid form (10, 11), with the meta-
monosulfonated triphenylphosphine only [{RuCl2(mTPPMS)2}2]
could be isolated (12, 13). Furthermore, the water soluble
analog of the easily isolable [RhH(PPh3)4], i.e.,
[RhH(mTPPMS)4] could not be prepared at all (14). There
are only a few X-ray crystal structures of complexes with
sulfonated arylphosphines available (15–17) that would al-
low comparison of ligands having the sulfonate substituent
in different positions; no systematic study was devoted to
this question. Both (4-diphenylphosphino) benzenesulfonates
(pTPPMS) (18) and 4,4′,4′′-phosphinetriylbenzenesulfonates
(pTPPTS) (19, 20) have already been described in the literature.
The para-monosulfonated triphenylphosphine, pTPPMS was
first prepared by Schindlbauer (18) in 1965, but since then
only one study (21) concerned its use in catalysis.
Ruthenium(II)–phosphine complexes play an important role
in catalysis (22) and the dissociation equilibria and reactivity
of [RuCl2(PPh3)3] was thoroughly studied in this context
(23, 24). Therefore, it seemed of interest to us to prepare ru-
thenium(II) complexes of pTPPMS and compare them to the
well known Ru(II) complexes of mTPPMS. Herein, we re-
port the results of our structural and catalytic studies.
salt could be obtained by recrystallization of 0.1 mmol of
the pTPPMS–K+ salt from 12 mL of a 0.4 M CsCl solution.
[RuCl2(pTPPMS)4] was prepared by an adaptation of the
method of Borowski et al. (28) for the preparation of various
mTPPMS complexes. To a solution of [RuCl2(PPh3)3] (288 mg,
0.3 mmol) in tetrahydrofuran (THF) (50 mL) was added
pTPPMS (570 mg, 1.5 mmol). The resulting clear solution
was stirred at room temperature for 24 h during which the
THF-insoluble [RuCl2(pTPPMS)4] (as a stable tetrahydrate)
precipitated as a light brown powder. For several batches the
yields were consistently higher than 90%. The ruthenium
and phosphorus content was determined on a Spectroflame
ICP-AES (
λRu
λP
= 267.876 nm, = 178.287 nm). Anal. calcd.
for C72H64Cl2K4O16P4S4Ru: Ru 5.72, P 7.02, [P]/[Ru] = 4;
found: Ru 5.58, P 7.02, [P]/[Ru] = 4.10.
Catalytic hydrogenation of trans-cinnamaldehyde (50 µL,
0.4 mmol) was carried out in a thoroughly stirred two-phase
liquid mixture comprised of an aqueous phosphate buffer
(3.00 mL) of known pH, containing KCl (45 mg), and
chlorobenzene (5.00 mL) at 80°C. The reaction was started
by the addition of [RuCl2(pTPPMS)4] (13.0 mg, 7.4 µmol)
together with pTPPMS (12.0 mg, 30 µmol), and the organic
layer was analyzed by gas chromatography after 240 min
reaction time. GC conditions: Chrom 5; packed column,
Carbowax 20M on Chromosorb, 240 cm; injector 220°C,
column 200°C, detector (FID) 200°C; carrier Ar.
X-ray crystal structure determination of Ph2PC6H4-4-
SO3Cs·H2O
Experimental
A colourless prism of dimensions of 0.35 × 0.20 ×
0.15 mm was mounted on a glass fiber with epoxy cement.
Data collection was performed on an Enraf–Nonius
MACH3/PC diffractometer in non-profiled ω/2θ scans with
θmax = 23.01°. A total of 7775 independent reflections were
collected. The structure was solved with the use of direct
methods (29) and refined by using SHELXL 97 (30). Hydro-
gen atoms were treated by a mixture of independent and
constrained refinement. Scattering factors for all atoms and
anomalous dispersion corrections for the non-hydrogen atoms
were taken from ref. 31. Pertinent crystal data are shown in
Table 1.
All manipulations were done under an argon or nitrogen
atmosphere. All solvents were purified by distillation and
carefully deaerated before use. Doubly distilled water was
used throughout.
Chlorodiphenylphosphine, triphenylphosphine, chlorosulfonic
acid, and fluorobenzene were obtained from Aldrich. Fuming
sulfuric acid (30% SO3) was supplied by Merck. RuCl3 (aq)
was a loan from Johnson Matthey. D2O (99.9%) was pur-
chased from Cambridge Isotope Laboratories. H2, N2, and
Ar were acquired from Carbagas-CH or from Messer (Hun-
gary). trans-Cinnamaldehyde (Aldrich) was distilled at re-
duced pressure before use. All other reagents were obtained
from Aldrich and used without further purification.
Infrared spectra were recorded on a PerkinElmer Paragon
Results and discussion
1
1000 PC FT-IR spectrometer on KBr discs. H and proton
decoupled 31P NMR measurements were run on Bruker AC
200, AM 360, DRX 400, and DRX 600 NMR spectrometers
and are referenced to 3-(trimethylsilyl)-1-propanesulfonic acid
sodium salt (TSPSA, Fluka) and 85% phosphoric acid, re-
spectively. The spectra were fitted with WIN-NMR, GNMR
4.0, and NMRICMA/MATLAB programs using a PC. Con-
ditions of the pH-potentiometric measurements are described
in detail in refs. 25 and 26, and those of HPLC analysis of
the sulfonated phosphines in ref. 13.
Preparation and properties of pTPPMS
Potassium (4-diphenylphosphino)benzenesulfonate (pTPPMS)
was prepared by the reaction of potassium 4-fluoro-
benzenesulfonate and KPPh2 in THF as described by Novak
and co-workers (21). In some cases we have observed for-
mation of small amounts of the water insoluble 1,4-
bis(diphenylphosphino)benzene. In fact, the latter compound
was prepared in high yield by Schindlbauer (18) by heating
at reflux (160–180°C) 1,4-ClC6H4SO3Na and KPPh2 in
β,β′-diethoxy-diethyl ether. Similar to mTPPMS, in aqueous
solutions pTPPMS behaves as a surfactant, which may result
in solubilization of unsulfonated phosphines (13). Infrared
stretching frequencies of the -SO3 group in pTPPMS are
very close to those in mTPPMS (13): 1038 and 1042 cm–1
(A1) and 1200 and 1196 cm–1 (E), respectively.
The sulfonated phosphines pTPPMS (K+ salt, monohydrate)
(21) and mTPPMS (Na+ salt) (13), as well as the complexes
[RuCl2(PPh3)3] (10), [{RuCl2(mTPPMS)2}2] (13), and
[{Ru(H2O)6}(tosylate)2] (27) were prepared as described in
the literature. The purity of pTPPMS was checked by 31P
NMR (D2O, δ = –5.87 ppm, s) and by HPLC (13); both
methods revealed a uniform compound with no phosphine
oxide present. X-ray quality crystals of the pTPPMS–Cs+
We could obtain X-ray quality crystals of the cesium salt
of pTPPMS by recrystallization of the K-salt from a CsCl
© 2001 NRC Canada

Papp et al.
637
Table 1. Crystallographic parameters for Ph2PC6H4-4-SO3Cs·H2O.
Fig. 1. ORTEP representation of Ph2PC6H4-4-SO3Cs·H2O. Proba-
bility displacement ellipsoids are drawn to 50%.
Formula
MW
Crystal dimensions (mm)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C36H32Cs2O8P2S2
984.5
0.35 × 0.2 × 0.15
Triclinic (colourless prism)
O1W
C16
C17
C18
C15
P1
O3
C13
C2
9.271(2)
10.561(2)
21.30(4)
92.948(10)
90.136(10)
108.314(10)
1.997(4)
2
C14
C3
C5
C4
O1
S1
C1
P1
C7
C6
C8
O2
Cs1
C9
C12
C11
γ (°)
V (× 109) (pm3)
Z
C10
Dx (Mg m–3)
µ (mm–1)
1.65
2.078
Table 2. Selected X-ray crystallographic parameters for
Ph2PC6H4-4-SO3Cs·H2O.
Data collection
Radiation (λ (Å))
T (K)
T min/max
Measured reflections
Independent reflections
Index ranges
(∆/σ)max
Extinction correction
Data/restraints/parameters
R(F)
wR(F2)
Mo Kα (0.71073)
293(2)
0.2031/0.7128
7775
Interatomic
distances (pm)
Interatomic
angles (°)
C1—P1
C7—P1
C13—P1
C1′—P1′
C7′—P1′
C13′—P1′
182.9(8)
183.7(7)
184.5(7)
183.9(8)
182.6(8)
183.8(8)
C1-P1-C7
101.4(3)
101.7(3)
101.5(3)
101.4(4)
103.1(4)
102.2(0)
C1-P1-C13
C7-P1-C13
C1′-P1′-C7′
C1′-P1′-C13′
C7′-P1′-C13′
7775
h = –3–11, k = –12–12, l = –20–20
0.076
SHELXL
7775/0/464
0.0667
0.224
Note: Primed values refer to a second phosphine molecule in the unit
cell.
GoF (S)
∆ρ min/max (e Å–3)
1.063
1.796/–1.429 (around Cs atoms)
sentially as described by Darensbourg et al. (16, 17). To this
end, a virtual H atom was placed at a distance of 228 pm
from the phosphorus atom and the cone angle of the smallest
cone which originates from the coordinates of this hydrogen
and circumscribes the whole molecule (without the cation)
was computed. For comparison, similar calculations were
made using X-ray crystal structural data of other tertiary
arylphosphines taken from the Cambridge Structural Data-
base. In the case of para-subsituted arylphosphines it is also
important to consider the space requirement of the hydrogen
atoms in ortho-positions of the aryl rings; this was accounted
for by using the Bohr radius of H (52.9 pm). Cone angles of
several substituted triphenylphosphines and that of PPh3 are
collected in Table 3.
Our calculations resulted in a cone angle of 141.5° for
PPh3, which is in a reasonable agreement with the value of
145° reported by Tolman (35). More interestingly, the cone
angles for pTPPMS and pTPPTS are slightly smaller (by 4°
and 2°, respectively), while for mTPPMS and mTPPTS are
considerably larger (approximately by 35°) than that of PPh3.
It can be concluded, therefore, that the space requirement of
para-sulfonated triphenylphosphines is practically the same
as that of PPh3 itself, meta-sulfonated triphenylphosphines,
however, are much more space-demanding. Furthermore, it
is the position and not the number of the -SO3 substituents
solution. The structure of the pTPPMS–Cs+ salt was deter-
mined by X-ray diffraction and this gives an opportunity to
compare it to a recently published structure (32) of the
benzyltriethylammonium salt of mTPPMS. An ORTEP rep-
resentation is shown in Fig. 1. Each unit cell contains two
phosphine molecules and one H2O molecule. The two mole-
cules consistently have slightly different geometric parame-
ters, however, the difference is within the error of structure
determination. Selected interatomic distances and angles are
given in Table 2.3
As expected, the geometry around the phosphorus atom is
trigonal pyramidal, and the average value (for both phos-
phine moieties in the asymmetric unit) of the three P—C
bonds is 183.6 pm and that of the C-P-C angles is 101.9°.
These values are insignificantly different from those deter-
mined for PPh3 (182.8 pm and 103.0° (33) and 183.1 pm
and 102.8° (34)), and from those obtained in the case of
the mono- and tri-sulfonated triphenylphosphines
+
–
[N(CH2C6H5)(C2H5)3 ](Ph2PC6H4-3-SO3 ·H2O] (182.6 pm and
102.5° (32)) and [P(C6H4-4-SO3K)3·KCl·0.5H2O] (184.3 pm
and 103.5° (20)), respectively. The cone angle, as defined by
Tolman (35), was determined from these structural data es-
3 Supplementary material may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research Council
Canada, Ottawa, ON K1A 0S2, Canada. For information on obtaining material electronically go to http://www.nrc.ca/cisti/irm/unpub_e.shtml.
Crysallographic information has been deposited with the Cambridge Crystallographic Data Centre. Copies of the data can be obtained (de-
position number 157 706), free of charge, on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (Fax: 44-1223-
336033 or e-mail: deposit@ccdc.cam.ac.uk).
© 2001 NRC Canada

638
Can. J. Chem. Vol. 79, 2001
Table 3. Cone angles of tertiary arylphosphines cal-
culated from crystallographic data.
free phosphine (–5.8 ppm, br s). Upon addition of pTPPMS
in excess (up to [P]/[Ru] = 15) the signal at 53.7 ppm be-
came dominant at the expense of the signal at 54.5 ppm,
which shows the existence of at least two ruthenium complexes
in the solution. However, the signals were still unresolved,
and therefore, unsuitable for structural characterization. To
obtain more information we wanted to study the 19F NMR
spectra of related fluoro complexes. Addition of KF to solu-
tions of [RuCl2(pTPPMS)4] ([F–]/[Ru] = 2–10) produced
complex spectra, therefore, chloride-free samples were pre-
pared in the reaction of [Ru(H2O)6]2+ + 2F– + 8(pTPPMS).
The only major 31P NMR signal was observed at 53.2 ppm
(broad but symmetric) accompanied by several small peaks
at 54.0, 52.0, 45.1, and 39.2 ppm (in addition to free pTPPMS
Cone angle
(°)
Ref. for
crystallographic data
Phosphine
pTPPMS
pTPPTS
PPh3
mTPPMS
mTPPTS
137.7
139.2
141.5
177.6
170.0
This work
20
34
32
16, 17
that defines the size of these ligands (at least in terms of
their cone angles).
1
and some of its oxide). H or 19F decoupling did not reveal
any structure of the resonances either, as it did not cause any
noticable line sharpening (for technical reasons simultaneous
decoupling was not possible). The 19F NMR spectrum of the
same solution was more informative. Remarkably, it showed
no signs of free F– indicating that the dominant peak in the
31P NMR spectrum should belong to a difluoro species.
Preparation and reactions of [RuCl2(pTPPMS)4] and its
catalytic activity in hydrogenation of trans-
cinnamaldehyde
When a THF solution of [RuCl2(PPh3)3] was reacted with
5 equiv of pTPPMS, metathesis of the phosphine ligands
produced the title compound, which is only sparingly solu-
ble in THF, in high yield.
Furthermore, a well resolved triplet of triplets centered at
2
–108.70 ppm was observed with coupling constants JP
=
trans ,F
The overall composition of the complex in solid form as
[RuCl2(pTPPMS)4] was confirmed by determination of its
ruthenium–phosphorus content (1:4). This is an interesting
difference in comparison with mTPPMS. Ruthenium(II)
complexes of (3-diphenylphosphino)benzenesulfonic acid
(Na salt) can be prepared either by heating at reflux
ethanolic solutions of aq RuCl3 with an excess of mTPPMS,
or by phosphine metathesis in THF (13), similar to the pro-
cedure described above with pTPPMS. However, in both re-
actions the product is [{RuCl2(mTPPMS)2}2]. While one can
expect a larger extent of phosphine dissociation, and hence,
an increased probability of dimerization at reflux tempera-
ture, the exclusive formation of the dimeric mTPPMS deriv-
ative in room temperature metathesis reactions is in contrast
to the formation of [RuCl2(PPh3)3] in refluxing ethanol and
to that of [RuCl2(PPh3)4] in cold methanol (10). On the other
hand, the relative stability of the various monomeric and
dimeric compounds correlates well with the space require-
ment of PPh3, pTPPMS, and mTPPMS as represented by the
respective Tolman cone angles. Such a lowering of the num-
ber of coordinated phosphine ligands was also observed in
the case of [Pd(mTPPMS)3] (15) and in the failed attempts
to prepare [RhH(mTPPMS)4] (14) (Na+ or K+ salts).
Characterization of the new water soluble phosphine com-
plexes by NMR spectroscopy presented considerable diffi-
culties. Unfortunately, in all measurements described below
the signals were found to be very broad (50–150 Hz) and
8.5 Hz and 2 JP
= 5.2 Hz. 31P decoupling resulted in par-
cis ,F
tial collapse of the structure of the 19F signal, although with
our technical capabilities full collapse to a singlet could not
be observed. These data are consistent with the presence of a
cis-[RuF2(pTPPMS)4] complex, and the small values of the
coupling constants explain the insensitivity of the 31P NMR
signals for fluorine decoupling. Such a species should show
two 31P resonances of equal intensity, which we do not ob-
serve separately as they may overlap or be hidden under-
neath the broad envelope at 53.2 ppm. Although one may
not extrapolate with certainty the conclusions regarding the
fluoro-complex(es) to the case of the chloride-containing
species, the similarity of the 31P NMR spectra at high pTPPMS
excess allows us to believe that at least some of the ruthe-
nium is present as cis-[RuCl2(pTPPMS)4]. Stabilization of
higher phosphine species by hydrogen bonding as in case of
sulfonated phosphine complexes relative to the analogous
PPh3 compounds has already been observed and quantita-
tively established for the phosphine dissociation equilibria of
[Rh(CO)H(mTPPTS)3] vs. [Rh(CO)H(PPh3)3] (36).
We have described earlier that [{RuCl2(mTPPMS)2}2] is
prone to hydrolysis in water (25, 26). This complex activates
H2 via heterolytic fission, and therefore, formation of both
[RuClH(mTPPMS)3] and [RuH2(mTPPMS)4] is accompanied
by proton production. All these reactions were followed by
pH-potentiometry and the resulting ruthenium complexes
1
were identified by H and 31P NMR spectroscopy (25, 26).
1
were basically uneffected by H decoupling, changes in the
Similarly, in aqueous solutions at slightly elevated tempera-
temperature between 0–50°C, or by addition of small amounts
of MeOD, which were found to be useful in other cases (14).
Although the line width of the signals refer to intra- and (or)
intermolecular dynamic processes or fast relaxation, insuffi-
cient solubility of the compounds in the ususal NMR sol-
vents other than D2O (alcohols, THF, acetone etc.) precluded
measurements in a wide temperature range.
tures [RuCl2(pTPPMS)4] readily reacts with molecular hy
-
drogen. A combined potentiometric–NMR study of this
reaction was undertaken. In short, [RuCl2(pTPPMS)4] was
dissolved under an inert gas atmosphere (Ar) in a solution of
a supporting electrolyte (KCl) and excess pTPPMS at a pre-
set constant pH. In case of any hydrolysis the pH was kept
constant by delivering KOH solution of known concentration
by an autoburette. The volume of the KOH solution con-
sumed is proportional to the extent of hydrolysis at that par-
ticular pH. After attaining equilibrium the solution was
In D2O solutions of [RuCl2(pTPPMS)4] the 31P NMR
spectra showed two overlapping unresolved, symmetric reso-
nances at 54.5 and 53.7 ppm in addition to resonances repre-
senting some phosphine oxide (36.0 ppm, br s) and some
© 2001 NRC Canada

Papp et al.
639
Fig. 3. Distribution of ruthenium into [RuClH(pTPPMS)3] (᭡)
and [RuH2(pTPPMS)3,4] (᭿) in aqueous solution as a function of
pH (220 mg [RuCl2(pTPPMS)4], 200 mg pTPPMS, 10 mL
0.2 M KCl, H2O:D2O = 9:1, 60°C, 1 bar of Ar or H2).
Fig. 2. Proton production upon dissolution (middle set of bars)
and further hydrogenation (front set) of solutions of
[RuCl2(pTPPMS)4] as a function of pH. Total proton production
is shown by the back set of bars (220 mg [RuCl2(pTPPMS)4],
200 mg pTPPMS, 10 mL 0.2 M KCl, H2O:D2O = 9:1, 60°C,
1 bar of Ar or H2).
100
80
60
40
20
0
2
1
0
total
3
4
5
Ar
6
7
8
9
H2
10
pH
2
4
6
8
10
pH
placed under hydrogen gas. Formation of any ruthenium hy-
drides with heterolytic splitting of H2 was, again, indicated
by further KOH consumption until the reaction attained a
new equilibrium position at the particular (preset) pH. This
way the whole pH-range, amenable to measurements with a
glass electrode, was investigated for hydrolysis of and hy-
dride formation from [RuCl2(pTPPMS)4] under H2. The re-
sults are shown in Figs. 2 and 3.
(intensity ratio 1:2) is assigned to trisphosphine species, i.e.,
we suggest that part of the ruthenium(II) is present as the
trigonal bipyramidal cis-[RuH2(pTPPMS)3] and (or) the struc-
turally close octahedral trans-[RuH2(H2O)(pTPPMS)3] (anal-
ogous to [RuH2(PPh3)3(THF)] (40)). Such complexes have
been identified as the catalytically active species in transfer
hydrogenation experiments (42, 44).
It is seen from Fig. 2 that below pH 5 there is no or negligible
proton production upon dissolution of [RuCl2(pTPPMS)4].
However, hydrogenation of the same solutions results in the
formation of 1 mol of H+ for 1 mol of Ru. During this reac-
Based on literature reports (40–43), exclusive formation of
trisphosphine complexes is unlikely in the presence of ex-
cess phosphine such as pTPPMS used in our experiments
([P]total/[Ru] = 8). In agreement with the above, the two other
31P NMR signals at around 41.5 and 54.6 ppm (intensity ratio
1:1) may belong to cis-[RuH2(pTPPMS)4]. In two earlier
studies the analogous cis-[RuH2(TPPTS)4] was found to have
31P chemical shifts at δ = 42.4 (br s) and 55.0 (br s) (38) and
at δ = 41.9 and 53.9 ppm (39). The same authors observed the
hydride signal of cis-[RuH2(TPPTS)4] at δ = –10.6 ppm
1
tion the colour of the solution turns deep cherry red. The H
and 31P NMR spectra of these solutions were recorded (90%
H2O – 10% D2O, 50°C). 1H NMR δ: –17.9 (q, 2JP,H = 24.9 Hz).
31P NMR δ: 58.7 (br s). These data are consistent with the
formation of [RuHCl(pTPPMS)3]; similar [RuHCl(P)3] com-
plexes have already been reported with PPh3 (37), mTPPMS
(25, 26), and mTPPTS ligands (38, 39).
2
In basic solutions significant proton production is ob-
served upon dissolution of [RuCl2(pTPPMS)4] with excess
pTPPMS under argon indicating extensive hydrolysis. Bub-
bling hydrogen through these solutions resulted in further pro-
ton production (Fig. 2) giving [H+]total/[Ru] = 2 at pH 10.
Above pH 9 the hydrogenated solutions are bright yellow. 31P
NMR spectra of these yellow solutions recorded at 81.01 and
161.94 MHz (90% H2O – 10% D2O, 50°C) showed broad
singlets at δ = 41.5, 52.0, and 54.6 ppm in a ratio of 2:2:1, re-
spectively. However, at 242.94 MHz the signal at 41.5 ppm
split into two resonances of equal intensity at 41.39 and
41.57 ppm. The only 1H NMR signal was observed at
−10.3 ppm (br td or pseudo q) with the coupling constants
(“pseudo-quartet,” JP,H = 33 Hz) (38) and at δ = –10.5 ppm
2
(“four intense merging lines,” JP,H = 35 Hz) (39). In our case
the putative cis-[RuH2(pTPPMS)4] species is also expected to
1
show a pseudo-quartet in H NMR around δ = –10.3 ppm
which may coincide with the signal(s) of the above trans-
[RuH2(H2O)(pTPPMS)3] or cis-[RuH2(pTPPMS)3] complexes.
Selective 31P decoupling of either Pa or Pb of cis-
[RuH2(pTPPMS)4] should result in collapse of the pseudo-
quartet to a triplet, however, this may be obscured by the
1
changes in the H NMR spectra of the trisphosphine hydride
complexes. The presence of both kinds of dihydrides is fur-
ther supported by the simulation of the hydride resonance.
The experimental spectrum could not be fitted properly with
either trans-[RuH2(H2O)P3] or cis-[RuH2P3] alone, however,
an acceptable fit was obtained by considering a mixture of the
trisphosphine complexes with cis-[RuH2(pTPPMS)4] around
1:1 ratio. These simulations gave coupling constants for the td
2 JP ,H = 34.4 Hz and JP ,H = 35.0 Hz. Selective 31P decoup-
2
ling at 41.5 ppm caused bthis signal to collapse into a triplet
a
while irradiation at 52.0 ppm resulted in a doublet in the hy-
dride region. According to the results of the pH-static hydro-
genation (2 mol H+ is produced for 1 mol Ru) the hydride
species present should be dihydride(s), so any monohydride
which otherwise would fit the NMR data can be disregarded.
Consequently, one pair of the 31P NMR signals, one of those
at 41.39 and 41.57 ppm together with the resonance at 52.0
2
2
part of the spectrum as JP ,H = 33 Hz and JP ,H = 39 Hz
a
b
(compared to 34.4 and 35.0 Hz, respectively, from the
31P NMR spectrum) and for the supposed pseudo-quartet as
2JP,H = 44 Hz, its center being shifted by 11 Hz from the cen-
ter of the td signal. Based on these considerations we conclude
© 2001 NRC Canada

640
Can. J. Chem. Vol. 79, 2001
Fig. 4. Hydrogenation of trans-cinnamaldehyde.
Fig. 5. Selectivity of the hydrogenation of trans-cinnamaldehyde
to saturated aldehyde (᭜) and unsaturated alcohol (᭿) catalyzed
by [RuCl2(pTPPMS)4] as a function of pH. For conditions see
Experimental.
phine ligands seem to play no important role. However, the
higher stability of the corresponding pTPPMS complexes
manifests itself in the onset of the formation of
[RuH2(pTPPMS)3,4] at somewhat lower pH, than for
[RuH2(mTPPMS)4]. Under strictly comparable conditions
there is only a negligible fraction of Ru(II) in the form of
[RuH2(mTPPMS)4] at pH 6 (25, 26), while [RuH2(pTPPMS)3,4]
represents about 20% of all ruthenium at this pH and
[RuH2(pTPPMS)3,4] can be detected in a small relative con-
centration already at pH ≈ 2 (Fig. 3). This has important con-
sequences in catalysis.
100
80
60
40
20
0
Selective reduction of an aldehyde or ketone function in
the presence of an olefinic C=C bond is a challenge in catal-
ysis and ruthenium phosphine complexes are among the best
catalysts of such processes (25, 38, 44, 48). Hydrogenation
of trans-cinnamaldehyde (Fig. 4) is actively catalyzed by the
hydrido-complexes formed from [RuCl2(pTPPMS)4]. De-
pending on the pH, overall turnover frequencies up to 13 h–1
were determined under the conditions specified in Experi-
mental (complete conversion of cinnamaldehyde). However,
the selectivity of the reaction was a striking function of the
pH (Fig. 5). In acidic solutions, around and below pH 2 slow
hydrogenation of trans-cinnamaldehyde yields the saturated
aldehyde, 3-phenylpropanal (C=C reduction). An increase in
the pH of the aqueous phase results in a sharp increase in the
overall rate and more importantly in the selectivity of hydro-
genation in favour of the unsaturated alcohol, 3-phenyl-2-
propenol (C=O reduction), which represents about 80% of
all products already at pH 4. Although this change in selec-
tivity does not strictly parallel the concentration distribution
of [RuClH(pTPPMS)3] and [RuH2(pTPPMS)3,4] as a func-
tion of the pH, it can be easily rationalized by assuming a
higher specific rate of C=O reduction catalyzed by
[RuH2(pTPPMS)3,4] than that of C=C reduction catalyzed by
[RuClH(pTPPMS)3]. In comparison to catalysis of hydroge-
nation of trans-cinnamaldehyde by [RuHCl(mTPPMS)3] and
[RuH2(mTPPMS)4], the basic characteristics are very similar
except that the change in the selectivity happens in a lower
pH range with complexes of pTPPMS (pH 2–4 compared to
4.5–6.5 with mTPPMS). This may be a useful feature of ca-
talysis by [RuCl2(pTPPMS)3,4] in cases where lower pH is
beneficial, such as the hydrogenation of bicarbonates (49,
50) under CO2 pressure. The mechanism of the reaction is
thought to be similar to that of the hydrogenation of unsatu-
rated aldehydes with [{RuCl2(mTPPMS)2}2] + mTPPMS
(25, 26), however, elucidation of a detailed picture requires
further measurements.
1
3
5
7
9
11 13
pH
that in our solutions prepared from [RuCl2(pTPPMS)4] with ex-
cess pTPPMS under hydrogen at high pH (>9) ruthenium is
present in form of a mixture of tris- and tetrakisphosphine
dihydrides, [RuH2X(pTPPMS)3] (X = nothing or H2O) and
cis-[RuH2(pTPPMS)4]; for a general description of the ruthe-
nium hydrides in such solutions in the following we use
[RuH2(pTPPMS)3,4].
In the intermediate pH range (5 < pH < 9) both mono- and
dihydrides are present and their concentration distribution
was calculated from the integrated intensities of the respec-
tive hydride 1H NMR signals. Under these conditions a
small amount of a third complex was also detected (90%
2
H2O – 10% D2O, 50°C). 1H NMR δ: –8.7 (td, JP,H
=
37.7 Hz, JP,H = 7.9 Hz). 31P NMR δ: 51.4 (br s). These data
are very close to those of [{RuClH(mTPPMS)2}2] identified
earlier by Sánchez-Delgado et al. (45) (1H NMR δ: –8.7 (td,
2
2JP,H = 38 Hz, JH,H = 9 Hz). 31P NMR δ: 52.11 (AB q, JA,B
=
38 Hz)) and suggest the presence of [{RuClH(pTPPMS)2}2].
No signs of other hydrides, such as e.g., the analogs of
[RuClH(PPh3)3]2
(46)
and
[(PPh3)2(H)Ru(µ-H)(µ-
Cl)RuH(PPh3)2]– (47) were seen by NMR spectroscopy in
solutions of any pH.
As expected, both the H and 31P NMR data of the Ru-
1
pTPPMS complexes are very close to the corresponding pa-
rameters of [RuHCl(mTPPMS)3] and [RuH2(mTPPMS)4],
since in these hydrides the steric requirements of the phos-
© 2001 NRC Canada

Papp et al.
641
17. D.J. Darensbourg, C.J. Bischoff, and J.H. Reibenspies. Inorg.
Chem. 30, 1144 (1991).
Conclusions
In this study we have shown that the (4-diphenyl-
phosphino)benzenesulfonate anion pTPPMS has approxi-
mately the same Tolman cone angle (determined by X-ray
diffraction as its Cs+ salt) as PPh3. Consequently, the steric
requirements for its coordination are considerably smaller
than those of the closest analog, the (3-diphenyl-
phosphino)benzenesulfonate anion, mTPPMS. NMR data of
the two phosphines and their Ru(II)-complexes showed the
ligand electronic properties to be very similar. Due to its
smaller size (cone angle) pTPPMS forms more stable
tetrakisphosphino–Ru(II) complexes, such as [RuCl2P4] and
[RuH2P4] than mTPPMS; this is manifested in the distribu-
tion of the hydride complexes and the resulting selectivity of
hydrogenation of trans-cinnamaldehyde as a function of pH.
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Acknowledgments
This work was supported by the Hungarian National Re-
search Foundation (OTKA T29934 and D25136) and by the
Office Fédéral de l’Education et de la Science, Suisse
(OFES 98.0011). The generous loan of aq RuCl3 by Johnson
Matthey is gratefully acknowledged. This research is part of
the collaboration within the COST Action D10–Working
Group 0001.
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© 2001 NRC Canada