Synthesis, characterization and reactivity of heterobimetallic complexes (η5-C5R5)Ru(CO)(μ-dppm)M(CO)2 (η5-C5H5) (R = H, CH3; M = Mo, W). Interconversion of hydrogen/carbon dioxide and formic acid by these complexes

Article abstract of DOI:10.1039/b306835h

The heterobimetallic complexes [(η⁵-C₅R₅)Ru(CO)(μ-dppm) M(CO)₂(η⁵-C₅H₅)] (M = Mo, W; R = H, CH₃) (1-4) are prepared by metathetical reactions between (η⁵-C₅R₅)Ru(dppm)Cl and Na⁺[(η⁵-C₅H₅) M(CO)₃]^-. IR spectroscopic and X-ray structural studies show that each of these complexes contains a semi-bridging carbonyl ligand. The low activity of these complexes in catalytic CO₂ hydrogenation to formic acid might be attributed to their non-facile reactions with H₂ to yield the active dihydride species. The metal-metal bonds can be protonated to form the cationic complexes, which contain strong Ru-H-M bridges. The bridging hydrogen atom is weakly acidic since it can be removed by a strong base, and it protonates BPh₄^- to give BPh₃ and benzene. It also reacts with the hydridic hydrogen of Et₃SiH to yield H₂. The bridging hydrogen, however, cannot be removed by hydride scavengers such as Ph₃C⁺OTf^- and Me₃Si⁺OTf^-. The sluggishness of the catalytic formic acid decomposition by 1-4 is attributable to the stability of the protonated bimetallic intermediate [(η⁵-C₅R₅)Ru(CO)(μ-dppm)(μ-H) M(CO)₂(η⁵-C₅H₅)]⁺ HCOO^- formed during the catalysis.

Full text of DOI:10.1039/b306835h

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Synthesis, characterization and reactivity of heterobimetallic
complexes (ꢀ5-C5R5)Ru(CO)(ꢁ-dppm)M(CO)2(ꢀ5-C5H5) (R ؍ H,
CH3; M ؍ Mo, W). Interconversion of hydrogen/carbon dioxide
and formic acid by these complexes†
Man Lok Man, Zhongyuan Zhou, Siu Man Ng and Chak Po Lau*
Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic
University, Hung Hom, Kowloon, Hong Kong, China.
E-mail: bccplau@polyu.edu.hk; Fax: 852-23649932; Tel: 852-27665619
Received 16th June 2003, Accepted 14th August 2003
First published as an Advance Article on the web 26th August 2003
The heterobimetallic complexes [(η5-C5R5)Ru(CO)(µ-dppm)M(CO)2(η5-C5H5)] (M = Mo, W; R = H, CH3) (1–4) are
prepared by metathetical reactions between (η5-C5R5)Ru(dppm)Cl and Naϩ[(η5-C5H5)M(CO)3]Ϫ. IR spectroscopic
and X-ray structural studies show that each of these complexes contains a semi-bridging carbonyl ligand. The low
activity of these complexes in catalytic CO2 hydrogenation to formic acid might be attributed to their non-facile
reactions with H2 to yield the active dihydride species. The metal–metal bonds can be protonated to form the cationic
complexes, which contain strong Ru–H–M bridges. The bridging hydrogen atom is weakly acidic since it can be
removed by a strong base, and it protonates BPh4Ϫ to give BPh3 and benzene. It also reacts with the hydridic hydrogen
of Et3SiH to yield H2. The bridging hydrogen, however, cannot be removed by hydride scavengers such as Ph3CϩOTfϪ
and Me3SiϩOTfϪ. The sluggishness of the catalytic formic acid decomposition by 1–4 is attributable to the stability of
the protonated bimetallic intermediate [(η5-C5R5)Ru(CO)(µ-dppm)(µ-H)M(CO)2(η5-C5H5)]ϩHCOOϪ formed during
the catalysis.
formato complexes Cp2Zr(OC(O)H)(µ-H)(µ-NtBu)IrCp*,
Introduction
which could be converted stoichiometrically to formic acid.6
The unconventional hydrogen bond (dihydrogen bond) between
More recently, Puddephatt and co-workers have demonstrated
a transition metal hydride and a common hydrogen bond donor
that the bimetallic complex [Ru2(µ-CO)(CO)4(µ-dppm)2] has
containing an O–H or an N–H group is a hydride–proton inter-
action. Complexes containing this H ؒ ؒ ؒ H interaction of the
considerably higher activity in catalytic reversible reaction of
formic acid to CO2 and H2 than the comparable mononuclear
ruthenium complex catalysts. The relatively high activity might
be related to facile generation of coordination unsaturation in
type M–H ؒ ؒ ؒ H–O or M–H ؒ ؒ ؒ H–N might be regarded as
intermediates in hydride protonation to generate a η2-H2 ligand
and the reverse reaction, i.e., heterolytic splitting of dihydrogen.
the binuclear system. This is the first binuclear homogeneous
The dihydrogen bond is also recognized for activating the M–H
catalyst that is reported to directly involve in either the decom-
bond for reactions such as H/D exchanges, proton transfer, and
position of formic acid or the hydrogenation of CO2 to formic
substitution.1 It is also well-demonstrated that the dihydrogen
acid.7
bond plays an important role in controlling the reactivity and
In our attempt to construct the dihydrogen-bonded bi-
metallic systems, we synthesized, characterized, and studied the
reactivity of some Ru–M (M = Mo, W) bimetallic com-
stereochemistry of organometallic hydride complexes.2
We have synthesized and characterized intramolecular Ru–
H ؒ ؒ ؒ H–N dihydrogen-bonding in aminocyclopentadienyl
plexes; the catalysis of the reversible reaction of formic acid to
ruthenium hydride complexes and have also demonstrated that
carbon dioxide and hydrogen with these complexes was also
the Ru–H ؒ ؒ ؒ H–N dihydrogen bond is a key feature in the
investigated.
catalytic hydrogenation of CO2 to formic acid.3 We envision
that replacing the N–H function with a metal fragment M–H,
Results and discussion
which is relatively less hydridic, might generate bimetallic
systems containing a Ru–H ؒ ؒ ؒ H–M dihydrogen bond, these
systems might exhibit interesting reactivity.
Syntheses and characterizations of bimetallic complexes 1–4
Bimetallic complexes have attracted considerable interest
because they have the potential of giving unique chemistry,
and are promising candidates for new chemical reactions and
catalysis.4 The proximity of the metal centers offers the possi-
bility of cooperative reactivity – the metal centers operating in
concert, with reactivity of the individual metal atoms comple-
menting each other.5 It was reported that the unsymmetrical
metal–metal bond in the early–late heterobimetallic complex
Cp2Zr(µ-NtBu)IrCp* was cleaved by H2 to form the hydride
complex Cp2Zr(H)(µ-H)(µ-NtBu)IrCp*; CO2 insertion into the
terminal hydride gave two diastereomeric heterobimetallic
The Ru–Mo and Ru–W bimetallic complexes were prepared by
reacting the ruthenium chloro complex with the sodium salt of
the cyclopentadienyl molybdenum or tungsten carbonyl anion
(eqn. (1)).
(1)
† Electronic supplementary information (ESI) available: Selected bond
lengths and angles and ORTEP diagrams for complexes 2–4 and 6ؒCl
and discussion on the structure of 6ؒCl. See http://www.rsc.org/supp-
data/dt/b3/b306835h/
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 3 7 2 7 – 3 7 3 5
3727

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Similar metathetical reactions have been employed to prepare
bimetallic complexes containing metal–metal bonds.8 It
was reported that a similar heterobimetallic complex A formed
Ϫ
by reaction of Liϩ[η5-C5H4P(C6H4-p-CH3)2]Mo(CO)3 with
{[(C6H4-p-CH3)PCH2]2RhCl}2 contains
a highly polarized
δϩRh–MoδϪ bond; changing the donating phosphine ligands on
Rh to carbonyls led to reduction of the metal–metal bond
polarity.9
Fig. 1 Molecular structure of CpRu(CO)(µ-dppm)Mo(CO)2Cp (1).
The 31P{1H} NMR spectrum of each of the complexes 1–4
shows a pair of doublets for the dppm ligand. For 1 at δ 49.9
and 53.7 ppm (2J(PP) = 75.6 Hz); for 2 at δ 46.9 and 58.9 ppm
(2J(PP) = 76.1 Hz); for 3 at δ 21.1 and 56.1 ppm (2J(PP) =
74.3 Hz); for 4 at δ 21.3 and 48.2 ppm (2J(PP) = 72.9 Hz). The
methylene hydrogen atoms of the dppm ligand of each complex
are diastereotopic, they appear as a pair of multiplets. For 1 at
δ 3.85 and 4.50 ppm; for 2 at δ 2.87 and 3.67 ppm; for 3 at δ 4.21
and 4.55 ppm; for 4 at δ 2.88 and 3.66 ppm. Each complex
shows three carbonyl peaks in the 13C NMR spectrum: for 1, at
molecular structures, crystal data and refinement details,
selected bond distances and angles of complexes 2–4 can be
found in ESI.†) The bimetallic complexes are isostructural. In
each complex, the ruthenium fragment can be considered as a
three-legged piano stool, and the other metal fragment exhibits
a four-legged piano-stool geometry. A carbonyl ligand is co-
ordinated to the ruthenium center, while two CO groups are
bonded to the other metal center. The metal–metal distances of
3.0277(6) and 3.0693(4) Å, respectively, in 1 and 2, are quite
long but do not rule out the presence of metal–metal bonds.
A Ru–Mo bond distance of 3.058(1) Å was reported for the
heterobimetallic complex [RuMo(CO)6(dppm)2].10 In a recently
reported structure of a cluster containing metal–metal bonds
linking high-valent molybdenum and low-valent ruthenium
centers, one of the ruthenium–molybdenum bonds in the clus-
ter, which is dative in origin (Ru Mo), has a Ru–Mo bond
distance of 3.1094(8) Å.13 We therefore infer the existence of
ruthenium–molybdenum metal–metal bonds in 1 and 2. Simi-
larly, the metal–metal bonds are believed to be present in the
isostructural ruthenium–tungsten complexes. The metal–metal
distances in 3 and 4 are respectively, 3.0246(11) and 3.0695(4)
Å. It is noticeable that in each of the complexes 1–4, the M–C–
O (M = Mo, W) angle of the carbonyl ligand, which is closer to
the ruthenium center [in 1, Mo(1)–C(36)–O(1) 168.9(3)Њ], devi-
ates more from linearity than the M–C–O angle of the other
CO ligand on M. Moreover, the distances of the carbon atoms
of the less linear carbonyl ligands on M from the ruthenium
centers in 1–4 fall in the range 2.744–2.906 Å, indicative of
weak interaction between these CO ligands and the ruthenium
centers. In a Zr–Ru bimetallic complex, the zirconium-bound
CO is bent slightly away from the Ru (Zr–C–O 167Њ) indicating
weak interaction with the ruthenium center, which is 2.70 Å
from the carbon atom of the carbonyl ligand.14 One of the
iridium bound carbonyls in the bimetallic complex [RhIr-
(CO)3(dppm)2] is semi-bridging; the Ir–C–O (171.9(6)Њ) deviates
significantly from linearity, and the Rh–C distance is 2.644(7)
Å.15
2
δ 249.7 (d, J(PC) = 23.2 Hz), 238.4 (s), and 214.5 ppm (d,
2J(PC) = 21.0 Hz); for 2, at δ 252.3 (d, 2J(PC) = 24.1 Hz), 251.3
2
(s), and 229.6 ppm (d, J(PC) = 15.5 Hz); for 3, at δ 238.0 (d,
2J(PC) = 16.8 Hz), 226.5 (d, 2J(PC) = 7.4 Hz), and 212.7 ppm (d,
2J(PC) = 21.2 Hz); for 4, at δ 239.7 (d, 2J(PC) = 15.6 Hz), 227.4
(d, 2J(PC) = 7.4 Hz), and 218.5 ppm (d, 2J(PC) = 19.9 Hz). The
most downfield signal in each case is probably due to the semi-
bridging carbonyl (vide infra). The carbonyl signals of all four
complexes are sharp at room temperature, indicating that there
is no scrambling of the carbonyl ligands. IR spectroscopy pro-
vides more information about the structures of the complexes.
It is observed that in the KBr IR spectrum of each of the com-
plexes 1–4, in addition to the bands due to terminal carbonyl
groups, a relatively low energy CO stretching frequency is also
present (1: 1791 cmϪ1; 2: 1777 cmϪ1; 3: 1792 cmϪ1; 4: 1784 cmϪ1).
The presence of the lower CO stretching frequencies indicates
that bridging or semi-bridging carbonyl groups are present in
the bimetallic complexes. Moreover, the solution spectra of the
complexes in CH2Cl2 and THF are basically identical to the
KBr spectra, indicative of the perseverance of the bridging or
semi-bridging carbonyl groups of 1–4 in solution. Semi-bridg-
ing carbonyls are not uncommon in bimetallic complexes. For
example, the X-ray structures and solid-state IR spectroscopy
10
show that the bimetallic complexes MoRu(CO)6(dppm)2 and
[RhIr(CH3)(CO)2(dppm)2]ϩ 11 contain semi-bridging carbonyl
groups. However, in solutions, the structures of these complexes
are somewhat different than in the solid state; the solution IR
spectra of these complexes show no CO stretches assignable to
semi-bridging carbonyl groups. On the other hand, a semi-
bridging carbonyl group was shown by IR spectroscopy to be
present in [MoRh(CO)4(dppm)(CNtBu)]Cl, both in the solid
and in solution.12
The carbonyl ligand on M, which interacts with the
ruthenium center, can be regarded as semi-bridging CO group.
In this bonding mode, the CO accepts electron in its π* orbital
from the ruthenium center, which in turn draws electron from
M (Chart 1). This semi-bridging bonding mode was studied
theoretically by Sargent and Hall.16 The presence of semi-
bridging CO group accounts for the low wavenumber CO
stretches in 1–4.
Structures of complexes 1–4
The structures of 1–4 have been determined by X-ray crystallo-
graphy in order to establish whether the carbonyl groups that
correspond to the low stretches in IR spectroscopy have bridg-
ing or semi-bridging geometry. The molecular structure of the
complex 1 is shown in Fig. 1.
Crystal data and refinement details are given in Table 1.
Selected bond distances and angles are given in Table 2. (The
Chart 1
D a l t o n T r a n s . , 2 0 0 3 , 3 7 2 7 – 3 7 3 5
3728

Table 1 Crystal data and refinement details for 1–4, 5ؒBF4 and 6ؒCl (Bruker Smart 1000CCD diffractometer, refinement method full-matrix least-squares on F 2)
1
2
3
4
5ؒBF4
6ؒCl
Empirical forumla
Mr
T/K
RuMoC38H32O3P2
795.59
294(2)
RuMoC43H37O3P2.C2H5OC2H5
939.84
294(2)
RuWC38H32O3P2
883.50
294(2)
RuWC43H37O3P2.C2H5OC2H5
1027.75
294(2)
C38H33BF4O3P2RuMo
883.40
294(2)
C40H37Cl5O3P2RuW
1089.81
294(2)
λ/Å
0.71073
Monoclinic
C2/c
0.71073
Monoclinic
P21/n
0.71073
Monoclinic
C2/c
0.71073
Monoclinic
P21/n
0.71073
Monoclinic
Cc
0.71073
Monoclinic
P21/c
Crystal system
Space group
Unit cell dimensions
a/Å
b/Å
c/Å
15.198(3)
17.865(4)
24.564(5)
98.92(3)
6589(2), 8
1.604
0.973
13.2833(19)
15.640(2)
21.157(3)
105.055(3)
4244.5(11), 4
1.471
15.181(8)
13.246(2)
15.526(2)
21.175(3)
104.618(4)
4214.1(11), 4
1.620
20.923(3)
10.3501(14)
17.962(3)
111.211(3)
3626.2(9), 4
1.618
9.1011(19)
23.816(5)
19.937(4)
98.592(4)
4272.8(16), 4
1.694
17.843(10)
24.543(13)
98.901(11)
6568(6), 8
1.787
β/Њ
V/Å3, Z
Dc/g cmϪ3
µ/mmϪ1
0.769
4.094
3.20
0.908
3.467
F(000)
3200
1928
3456
2056
1768
2136
Crystal size/mm
θ Range/Њ
Limiting indices, hkl
0.20 × 0.16 × 0.14
1.68–27.50
Ϫ19 to 13, Ϫ15 to 23,
Ϫ31 to 31
20989
0.20 × 0.18 × 0.12
1.64–27.55
Ϫ17 to 12, Ϫ20 to 14,
Ϫ27 to 26
28358
0.16 × 0.12 × 0.10
1.77–27.60
Ϫ16 to 19, Ϫ21 to 23,
Ϫ31 to 20
21041
0.20 × 0.16 × 0.14
1.65–27.56
Ϫ17 to 11, Ϫ19 to 20,
Ϫ25 to 27
27228
0.30 × 0.28 × 0.10
2.09–27.54
Ϫ26 to 27, Ϫ13 to 13,
Ϫ23 to 18
12074
0.20 × 0.16 × 0.14
2.07–27.60
Ϫ11 to 11, Ϫ30 to 28,
Ϫ25 to 18
28667
Reflections collected
Independent reflections (Rint
)
7532 (0.0515)
99.4
9772 (0.0298)
99.7
7411 (0.2278)
97.2
9704 (0.0383)
99.6
6262 (0.0274)
99.7
9832 (0.1499)
99.4
.
Completeness (%) to θ = 27.50Њ
Absorption correction
SADABS
0.8758/0.8292
7532/0/406
1.028
SADABS
SADABS
0.6849/0.5603
7411/20/400
1.036
SADABS
0.6631/0.5673
9704/4/472
0.695
SADABS
SADABS
0.6425/0.5440
9832/84/468
0.986
Max./min. transmission
Data/restraints/parameters
Goodness-of-fit on F 2
0.9133/0.8613
9772/0/477
0.791
0.9147/0.7724
6262/23/447
1.061
Final R indices [I > 2σ(I )]
R1 = 0.0383,
wR2 = 0.0790
R1 = 0.0574,
wR2 = 0.0841
R1 = 0.0335,
wR2 = 0.0978
R1 = 0.0509,
wR2 = 0.1159
R1 = 0.0801,
wR2 = 0.2009
R1 = 0.1240,
wR2 = 0.2189
R1 = 0.0332,
wR2 = 0.0913
R1 = 0.0572,
wR2 = 0.1110
R1 = 0.0353,
wR2 = 0.0849
R1 = 0.0410,
wR2 = 0.0878
R1 = 0.0686,
wR2 = 0.1373
R1 = 0.2343,
wR2 = 0.1681
R indices (all data)

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Table 2 Selected bond distances (Å) and angles (Њ) for CpRu(CO)-
(µ-dppm)Mo(CO)2Cp (1)
Table 4 Times for complete decomposition of formic acid with 1–4a
Entry
Complex/solvent
Reaction time/h
Ru(1)–Mo(1)
Ru(1)–P(2)
Mo(1)–P(1)
O(1)–C(36)
O(3)–C(38)
3.0277(6)
2.2641(8)
2.4310(8)
1.173(3)
1.158(3)
Mo(1)–C(36)
Mo(1)–C(37)
Ru(1)–C(38)
O(2)–C(37)
1.949(3)
1.934(3)
1.845(3)
1.157(4)
1
2
3
4
5
6
7
8
1/THF-d8
1/C6D6
2/THF-d8
2/C6D6
3/THF-d8
3/C6D6
4/THF-d8
4/C6D6
6
15
16
24
24
40
24
40
P(2)–Ru(1)–Mo(1)
P(1)–Mo(1)–Ru(1)
P(2)–C(11)–P(1)
C(18)–P(1)–Mo(1) 120.74(7)
C(24)–P(2)–Ru(1) 122.60(8)
93.18(2)
84.80(2)
107.30(11)
O(1)–C(36)–Mo(1) 168.9(3)
O(2)–C(37)–Mo(1) 174.2(2)
O(3)–C(38)–Ru(1)
C(30)–P(2)–Ru(1)
C(12)–P(1)–Mo(1)
172.8(3)
110.56(8)
112.48(8)
a Typical experimental conditions: complex 2.51 µmol, solvent 0.4 mL,
formic acid 25.1 µmol, temperature 80 ЊC.
Table 3 Catalytic hydrogenation of CO2 with complexes 1–4a
2
doublet of doublet of doublets (2J(HP) = 28.2 Hz, J(HP) =
Entry
Complex/solvent
Turnover no.b (TON)
21.0 Hz, J(HH) = 5.4 Hz), it is coupled to the phosphorus
atoms and one of the methylene protons of dppm. Coupling of
the hydride ligand with one of the methylene protons of dppm
is not uncommon.17 In a double resonance study, irradiation of
the dppm methylene signal at δ 4.46 ppm reduced the hydride
signal to a doublet of doublets, and conversely irradiating the
hydride signal simplified the methylene signal from a multiplet
to a pseudo-quartet. The proton signal of the formate cannot
be unequivocally identified because it overlaps with the signal
of the free formic acid. It is noteworthy that the concentration
of 5ؒHCOO decreased with the decomposition of formic acid,
and it competely reverted to 1 upon the conclusion of the cata-
1
2
3
4
5
6
7
8
1/THF
1/benzene
2/THF
2/benzene
3/THF
3/benzene
4/THF
4/benzene
10
43
Trace
Trace
5
28
Trace
Trace
a Typical reaction conditions: catalyst = 0.009 mmol, solvent: THF or
benzene (8 mL), triethylamine (2 mL), H2 = 30 atm, CO2 = 30 atm,
reaction time = 45 h, temperature = 120 ЊC. b TON = mol of product/
mol of complex.
1
lytic reaction, as illustrated by H and 31P{1H} NMR spectro-
1
scopy. Taking the H and 31P{1H} NMR data together, it is
possible to deduce that the intermediate 5ؒHCOO is a dppm-
spanned bimetallic complex containing a hydrogen bridge.
Reactions of dinuclear metal complexes with acids to form
hydride-bridged bimetallic species are well documented.10b,15,18
For instance, treatment of the dirhodium species [Rh2(CO)3-
(dppm)2] with HPF6 or p-CH3C6H4SO3H has been shown
to lead to the formation of the hydride-bridged bimetallic
complex [Rh2(µ-H)(µ-CO)(CO)2(dppm)2]ϩ.18 The generation of
5ؒHCOO was much slower using benzene-d6 in place of
THF-d8 as the solvent; 1 was only partially converted to
5ؒHCOO after heating the solution at 80 ЊC for 11 h.
CO2 hydrogenation to formic acid with 1–4
We monitored the reactions of 1–4 with pressurized H2 (∼45
1
atm) in C6D6 at 120 ЊC for 60 h with high-pressure H and
31P{1H} NMR spectroscopy, but we found in each case that the
bimetallic complex remained unchanged in the experiment, it
was the only NMR observable organometallic species through-
out. The sharp singlet signal of dissolved H2 was observed at
1
δ 4.60 ppm in the H NMR spectrum. Although 1–4 do not
seem to react with H2 to yield detectable metal hydride species,
the bimetallic complexes are able to effect the hydrogenation of
CO2 to formic acid in benzene and THF solutions, albeit in very
low yields (Table 3).
Reactions of 1 and 3 with acids
We have monitored a 1-catalyzed CO2 hydrogenation reac-
tion by high-pressure 1H and 31P{1H} NMR spectroscopy, and
have found that 1 was the only observable organometallic
species throughout the experiment. Furthermore, we have also
learned in separate experiments that the mononuclear systems
CpRu(PPh3)(CO)H, CpM(PPh3)(CO)2H, CpRu(dppm)(CO)ϩ,
At this point we digress somewhat to describe the acidification
reactions of 1–4. In view of the fact that 1 was not completely
converted to 5ؒHCOO during the catalytic formic acid decom-
position reaction, we reacted 1 with other acids to see if its
acidification would go to completion. Stirring a THF solution
of 1 with an excess of concentrated hydrochloric acid at room
temperature for 30 min led to the formation of the complex
5ؒCl, whose 31P{1H} NMR spectrum is identical to that of
Ϫ
CpRu(dppm)Cl/Agϩ and CpM(CO)3 are all inactive in CO2
hydrogenation reaction. Therefore, it is probably true that the
CO2 hydrogenation reactions with 1–4 are heterobimetallic
catalytic processes in which fragmentation of the bimetallic
systems do not seem to occur.
1
5ؒHCOO. The H NMR spectrum of 5ؒCl is also identical to
that of 5ؒHCOO if we ignore the formate and formic acid sig-
nals of the latter. Attempts to ascertain the structure of 5ؒCl by
X-ray crystallography was frustrated by failure to obtain good
single crystals for this purpose. We then reacted 1 with HBF4ؒ
Formic acid decomposition with 1–4
1
It is generally true that metal complexes, which are active in
catalytic hydrogenation of CO2 to formic acid, can effect the
reverse reaction as well. Table 4 shows the results of formic acid
decomposition to CO2 and H2 catalyzed by 1–4. Although the
catalytic processes are slow, monitoring of the reaction with
NMR spectroscopy reveals some interesting results. It was
found that after heating a THF-d8 solution containing 2.51
µmol of 1 and 25.1 µmol of formic acid in a sealed 5 mm NMR
tube at 80 ЊC for 2.5 h, the former was partially converted to the
hydride complex 5ؒHCOO, which was characterized by 1H and
31P{1H} NMR spectroscopy. The 31P{1H} NMR spectrum
showed two doublets at δ 39.7 and 48.5 ppm (J(PP) = 56 Hz) for
the two phosphorus atoms of dppm. In the 1H NMR spectrum
of the hydride complex, the hydride signal at δ Ϫ17.16 ppm is a
Et2O in THF to obtain the hydride complex 5ؒBF4, whose H
and 31P{1H} NMR spectra are identical to those of 5ؒCl. In
view of the identical NMR spectroscopic data of 5ؒHCOO,
Ϫ
5ؒCl, and 5ؒBF4 and the fact that BF4 is generally non-
coordinating, it is very likely that the three complexes are ionic
species with identical cationic portions, but different anions.
Unlike 5ؒCl, which resists single crystal growing, 5ؒBF4 affords
single crystals suitable for X-ray diffraction study readily. Fig. 2
shows the molecular structure of the cation [CpRu(CO)-
(µ-dppm)(µ-H)Mo(CO)2Cp]ϩ (5؉) of 5ؒBF4. The structure-
refinement information for 5ؒBF4 can be found in Table 1.
Selected bond distances and angles are given in Table 5. The
hydride ligand in 5؉ was located and refined to give Mo–H and
Ru–H distances of 1.74(6) and 1.87(6) Å, respectively. The
D a l t o n T r a n s . , 2 0 0 3 , 3 7 2 7 – 3 7 3 5
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ively.20 The molybdenum dihydride [CpMo(CO)2(PR3)H2]ϩ has
Table 5 Selected bond distances (Å) and angles (Њ) for [CpRu(CO)-
(µ-dppm)(µ-H)Mo(CO)2Cp][BF4] (5ؒBF4)
been implicated as an intermediate in catalytic ionic hydro-
genation, but has not been directly observed.21 It has also been
reported that reaction of diphosphazane-bridged derivatives of
nonacarbonyldiruthenium, [Ru2(µ-CO)(CO)4{µ-(RO)2PN(Et)-
P(OR)2}2] (R = Me or Pri) with HCl and HBr first gives the
hydride species [Ru2H(CO)5{µ-(RO)2PN(Et)P(OR)2}2]ϩ, which
further reacts with the acid to produce the halogeno-bridged
complexes [Ru2(µ-X)(CO)5{µ-(RO)2PN(Et)P(OR)2}2]ϩ (X = Cl,
Br). In these reactions, the acids, in a formal sense, exhibit a
type of umpolung behavior.22
Ru(1)–H
1.87(6)
Mo(1)–H
1.74(6)
Ru(1)–P(1)
Ru(1)–C(11)
Mo(1)–C(12)
Mo(1)–C(13)
2.3053(8)
1.880(3)
1.982(3)
1.953(3)
Mo(1)–P(2)
O(1)–C(11)
O(2)–C(12)
O(3)–C(13)
2.4703(8)
1.127(4)
1.150(4)
1.155(4)
P(1)–C(26)–P(2)
C(20)–P(1)–Ru(1)
C(33)–P(2)–Mo(1) 109.87(10)
O(2)–C(12)–Mo(1) 173.3(3)
115.07(14)
118.90(10)
C(14)–P(1)–Ru(1)
115.58(10)
C(27)–P(2)–Mo(1) 119.63(10)
O(1)–C(11)–Ru(1) 172.0(3)
O(3)–C(13)–Mo(1) 175.0(2)
Reactions of 5ؒOTf with Ph3C؉OTf؊ and Me3Si؉OTf؊
In their study of catalytic ionic hydrogenation of ketones with
molybdenum and tungsten catalysts, Bullock and Voges gener-
ated the catalyst precursors [CpM(PR )(CO) (O᎐CEt )]ϩ (M =
᎐
3
2
2
Mo, W) by abstracting the hydrogen of the M–H bond of
CpM(PR3)(CO)2H as a hydride with Ph3Cϩ in the presence of
3-pentanone.21 The bridging hydrogen atom in 5ؒOTf, however,
could not be removed by the action of Ph3CϩOTfϪ. Similarly,
5ؒOTf failed to react with Me3SiϩOTfϪ.
Reactions of 5ؒBF4 with bases
After our attempts to remove the bridging hydrogen atom of
5ؒBF4 as a hydride ligand failed, we tested the reactivity of the
complex towards the weak base Et3N. It was found that 5ؒBF4
remained intact after refluxing a THF solution of the complex
in the presence of an excess of Et3N for 45 h. However, 1 was
regenerated when 5ؒBF4 was allowed to react with the strong
base sodium methoxide (eqn. (2)).
Fig. 2 Molecular structure of [CpRu(CO)(µ-dppm)(µ-H)Mo(CO)2-
Cp][BF4] (5ؒBF4).
(2)
slightly asymmetric position of the bridging hydride in 5؉ is
manifested by the unequal H–P coupling constants exhibited by
the hydride signal in the H NMR spectrum. It was reported
1
that the hydride signals which showed unequal H–P coupling
constants in the complexes [Ru(CO)2(µ-H)(µ-dppm)2Mo(CF3-
COO)(CO)3]10b and [Ir2(µ-H)(µ-Pz)2H3(NCCH3)(PiPr3)2],19 are
consistent with the bridging hydride ligands being not equi-
distant from the metal centers. All carbonyl groups in 5؉ are
terminal, in consonance with the IR spectrum of 5ؒBF4 or 5ؒCl,
which shows no carbonyl stretch below 1880 cmϪ1. The metal
centers in 5؉ are separated by a distance of 3.277(6) Å,
although it is longer than that of the parent complex 1, it can-
not exclude the existence of metal–metal bond in the complex.
The presence of a three-center–two-electron bond is highly
probable. In congruence with the large metal–metal separation,
the P–C–P angle of the dppm ligand in 5؉ is much larger than
that in the parent complex 1 [115.07(14) vs. 107.30(11)Њ].
Reactions of 5ؒBF4 and 5ؒCl with Et3SiH
We anticipate that reaction of 5؉ with silane would provide a
silyl complex, which may demonstrate some interesting charac-
teristics. Complex 5ؒBF4 was reacted with triethylsilane in THF;
however, the complex remained unchanged after refluxing the
solution for 24 h. On the other hand, 5ؒCl behaved quite differ-
ently, its reaction with Et3SiH yielded 1 and chlorotriethyl-
silane; the latter was detected by 29Si{1H} NMR spectroscopy.
Reaction of 5ؒCl with Na؉BPh4؊
Since it was difficult to obtain single crystals of 5ؒCl for X-ray
diffraction study, we had tried to change the chloride anion
to tetraphenylborate by reacting 5ؒCl with a stoichiometric
amount of NaϩBPh4Ϫ. However, instead of yielding the
expected complex, the reaction regenerated 1 quantitatively,
along with the formation of BPh3 and benzene, which were
Reaction of 3 with an excess of concentrated hydrochloric
acid in THF at room temperature yielded the protonated com-
plex 6ؒCl, which is analogous to 5ؒCl. The structure of 6؉ is
very similar to that of 5؉ (For the characterization and X-ray
structure determination of 6ؒCl, see ESI†).
1
detected by 11B{1H} and H NMR spectroscopy, respectively
It is noteworthy that the chloride anions in 5ؒCl and 6ؒCl are
non-coordinating, probably for steric reasons. Although the
protonation reactions of 1 and 3 to give 5ؒCl and 6ؒCl, respect-
ively, were carried out in the presence of an excess of concen-
trated hydrochloric acid, further protonation of 5ؒCl and 6ؒCl
did not occur. Similarly, reaction of 5 and 6 with an excess of
HBF4ؒEt2O or triflic acid only gave the mono-protonated prod-
ucts 5ؒBF4 (or 5ؒOTf ) and 6ؒBF4 (or 6ؒOTf ), respectively, again
no further protonation was observed. It has been reported that
CpW(CO)2(PR3)H (R = Me, Cy, Ph) can be protonated by
(eqn. (3)). The reaction depicted in eqn. (3) results from
(3)
Ϫ
strong acids such as HOTf and [H(Et2O)2]ϩBArЈ4 (ArЈ = 3,5-
bis(trifluoromethyl)phenyl) to give the dihydrides [CpW(CO)2-
Ϫ
(PR3)H2]ϩOTfϪ and [CpW(CO)2(PR3)H2]ϩBArЈ4
, respect-
D a l t o n T r a n s . , 2 0 0 3 , 3 7 2 7 – 3 7 3 5
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acidification of BPh4Ϫ, suggestive of the acidic nature of the
bridging hydrogen of 5ؒCl. Proton-induced cleavage of an aryl
group from tetraarylborates is well-documented.23
hydridicity of the Ru–H and the higher acidity of the M–H
(M = Mo or W), we would be able to generate interesting
bimetallic systems containing the dihydrogen-bonded moiety
Ru–H ؒ ؒ ؒ H–M. Unfortunately, we have not been able to
observe these dihydrogen-bonded dihydride species in the
in-situ high-pressure NMR studies of the reactions of 1–4 with
H2. However, 1–4 show activity (albeit very low) in the catalysis
of CO2 hydrogenation to formic acid and the reverse reac-
tion, i.e., the decomposition of formic acid to H2 and CO2. A
reaction mechanism, involving the transient intermediacy of
the dihydrogen-bonded dihydride species, is proposed to
account for the catalytic reactions. We attribute the low
activity of 1–4 in CO2 hydrogenation to their non-facile reac-
tions with H2 to generate the active dihydride species; on the
other hand, the sluggishness of the catalytic formic acid
decomposition by these bimetallic complexes is probably due to
the formation of a stable intermediate, which contains a strong
Ru–H–M bridge.
Possible mechanism for CO2 hydrogenation and formic acid
decomposition catalyzed by 1–4
In a metal complex-catalyzed CO2 hydrogenation reaction,
activation of the H2 molecule by the metal center is an import-
ant step. Although no metal hydride species were detectable by
1H and 31P{1H} NMR spectroscopy in the 1–4 catalyzed CO2
hydrogenation reactions, we propose that a metal hydride
intermediate is generated as a transient species during the
catalysis (species B in Scheme 1). It is present in very low con-
centration or is very short-lived, so that it evades observation by
NMR spectroscopy. Since the hydride on ruthenium is more
hydridic24 and the one on M is more acidic,26 the Ru–H ؒ ؒ ؒ
H–M dihydrogen bond is likely to be present in B. Reaction of
B with CO2 yields the intermediate C, and it does not accumu-
late to such a concentration that it is detectable by NMR
spectroscopy in the hydrogenation reaction. The low catalytic
activities of 1–4 in CO2 hydrogenation can probably be attri-
buted to the non-facile generation of the hydride intermediate
B.
Experimental
All reactions were carried out under a dry N2 atmosphere using
standard Schlenk techniques. All solvents were distilled and
degassed prior to use. Dichloromethane was distilled from
calcium hydride. Tetrahydrofuran, diethyl ether, toluene and
hexane were distilled from sodium–benzophenone ketyl. Meth-
anol and ethanol were distilled from magnesium and iodine.
CpRu(dppm)Cl,28 CpMo(CO)3ϪNaϩ,29 and CpW(CO)3ϪNaϩ 29
were synthesized according to published procedures. Formic
acid and sodium formate were purchased from BDH and used
as received. Trifluoromethanesulfonic acid and HBF4.Et2O
were purchased from Aldrich.
Infrared spectra were obtained from a Bruker Vector 22 FT-
IR spectrophotometer. 1H NMR spectra were obtained from a
Bruker DPX 400 spectrometer. Chemical shifts (δ, ppm) were
measured relative to the proton residue of the deuterated sol-
vent (THF-d8 δ 1.85 ppm, CD3COCD3 δ 2.20 ppm, C6D6 δ 7.40
ppm.) 31P{1H} NMR spectra were taken on a Bruker DPX-400
spectrometer at 161.98 MHz. 31P{1H} NMR chemical shifts
were externally referenced to 85% H3PO4 in D2O (δ 0.00 ppm).
29Si{1H} NMR spectra were taken on a Bruker DPX-400
spectrometer at 79.5 MHz. 11B{1H} NMR spectra were taken
on a Bruker DPX-400 spectrometer at 128.3 MHz. High-
pressure NMR studies were carried out in a sapphire HP NMR
tube; the 10 mm sapphire NMR tube was purchased from
Saphikon, Milford, NH, and the titanium high-pressure valve
was constructed at the ISSECC-CNR, Firenze, Italy. Electro-
spray Ionization Mass Spectrometry was carried out with a
Finnigan MAT 95S mass spectrometer by first dissolving the
sample in CH2Cl2–MeOH. Elemental analyses were performed
by M–H–W Laboratories, Phoenix, AZ, USA.
Scheme
1
Proposed mechanism of CO2 hydrogenation and
decomposition of formic acid catalysed by 1–4.
The catalysis of formic acid decomposition is proposed to
proceed by the same catalytic cycle, but in a reversed manner.
That the intermediate 5 or 6 can be detected by NMR spectro-
scopy in the early stage of the decomposition reaction is prob-
ably due to the presence of relatively high concentration of
HCOOH, therefore shifting the equilibrium towards 5 or 6.
Further support of the proposed mechanism for formic acid
decomposition in Scheme 1 could be obtained from the reac-
tions of 5ؒOTf and 6ؒOTf with sodium formate. After heating a
THF-d8 solution of 5ؒOTf (or 6ؒOTf ) with one equiv. of
sodium formate in a sealed NMR tube at 80 ЊC for 6 h, 31P{1H}
NMR spectroscopy showed that the metal–metal bonded com-
CpRu(CO)(ꢁ-dppm)Mo(CO)2Cp (1)
Degassed THF (8 mL) was added to a solid mixture of
CpMo(CO)3Na (0.10 g, 0.38 mmol) and CpRu(dppm)Cl
(0.22 g, 0.38 mmol), the reaction mixture was heated at 120 ЊC
for 48 h, during which, the color of the solution changed from
orange to dark red. The resulting solution was then filtered to
remove the salt formed. Upon removal of the solvent of the
filtrate under vacuum, 5 mL of hexane was added to the resi-
dual paste with vigorous stirring to produce a red solid, which
was collected by filtration. The solid was washed with methanol
(2 × 2 mL) and diethyl ether (2 × 2 mL) and then dried in vacuo.
Yield: 0.22 g (73%). Anal. Calc. for RuMoC38H32O3P2: C 57.37,
1
plex 1 (or 3) was regenerated. H NMR spectroscopy showed
the disappearance of the signal of the formate ion and the
appearance of the signal of free H2 (δ 4.60 ppm). One of the
reasons for the sluggishness of the formic acid decomposition
by 1–4 is probably the difficulty encountered by the formate
anion in opening up the hydrogen bridge, direct hydride deliv-
ery from the formate to the M–H might also be non-facile.
H 4.05. Found: C 56.98, H 4.02%. IR (KBr, cmϪ1): ν(C᎐O) 1791
᎐
᎐
Conclusion
1
(m), 1873 (s) and 1894 (sh). H NMR (400.13 MHz, C6D6, 25
The main thrust of this work is to synthesize Ru–Mo and
Ru–W bimetallic complexes (1–4) and study the activation of
H2 with these complexes. We hope, by virtue of the greater
ЊC): δ 3.85 (m, 1H, PCHHP), 4.50 (m, 1H, PCHHP), 5.06 (s,
5H, CpRu), 5.08 (s, 5H, CpMo), 7.02–7.76 (m, 20H of dppm).
2
31P{1H} NMR (161.99 MHz, C6D6, 25 ЊC): δ 49.9 [d, J(PP) =
D a l t o n T r a n s . , 2 0 0 3 , 3 7 2 7 – 3 7 3 5
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2
75.6 Hz], 53.7 [d, J(PP) = 75.6 Hz]. ESI-MS (CH2Cl2–MeOH
(80%). Anal. Calc. for RuMoC38H33ClO3P2: C 54.85, H 4.00.
as solvent) m/z: 795 [M]ϩ.
Found: 54.79, H 3.94%. IR (KBr, cmϪ1): ν(C᎐O). 1880 (s), 1949
᎐
᎐
1
(sh) and 1972 (s). H NMR (400.13 MHz, THF-d8, 25 ЊC):
2
2
Cp*Ru(CO)(ꢁ-dppm)Mo(CO)2Cp (2)
δ Ϫ17.16 [ddd, J(HP) = 28.0 Hz, J(HP) = 20.8 Hz, J(HH) =
5.6 Hz, 1H, Mo–H–Ru], 3.90 (m, 1H, PCHHP), 4.42 (m, 1H,
PCHHP), 5.20 (s, 5H, CpRu), 5.56 (s, 5H, CpMo), 7.10–
7.77 (m, 20H of dppm). 31P{1H} NMR (161.99 MHz, THF-d8,
25 ЊC): δ 39.7 [dd, 2J(PP) = 56.4 Hz, J(HH) = 13.1 Hz], 48.4 [dd,
2J(PP) = 56.4 Hz, J(HH) = 13.1 Hz]. ESI-MS (CH2Cl2–MeOH
as solvent) m/z: 832 [M]ϩ.
The procedure for 1 was followed exactly, except that Cp*Ru-
(dppm)Cl was used in place of CpRu(dppm)Cl. Yield: 0.21 g
(70%). Anal. Calc. for RuMoC43H37O3P2: C 60.00, H 4.33.
Found: C 59.74, H 4.52%. IR (KBr, cmϪ1): ν(C᎐O) 1777 (m),
᎐
᎐
1
1854 (s) and 1874 (sh). H NMR (400.13 MHz, C6D6, 25 ЊC):
δ 1.50 (s, 15H, methyls of Cp*) δ 2.87 (m, 1H, PCHHP), 3.67
(m, 1H, PCHHP), 5.06 (s, 5H, CpMo), 7.00–7.50 (m, 20H of
dppm). 31P{1H} NMR (161.99 MHz, C6D6, 25 ЊC): δ 46.9 [d,
2J(PP) = 76.1 Hz], 58.9 [d, 2J(PP) = 76.1 Hz]. ESI-MS (CH2Cl2–
MeOH as solvent) m/z: 865 [M]ϩ.
Synthesis of [CpRu(CO)(ꢁ-dppm)(ꢁ-H)Mo(CO)2Cp]؉[BF4]؊
(5ؒBF4)
Complex 1 (0.020 g, 0.025 mmol) was dissolved in 10 mL of
THF in a Schlenk flask. HBF4ؒEt2O (54%, 6.9 µL, 0.050 mmol)
was added and the resulting solution was heated at 80 ЊC for
2 h. Solvent was removed under vacuum. Hexane (5 mL) was
added to the residue, with stirring, to produce an orange solid.
This was collected and washed with water (2 × 1 mL) and di-
ethyl ether (2 × 2 mL), and dried under vacuum. Yield: 0.017 g
CpRu(CO)(ꢁ-dppm)W(CO)2Cp (3)
This complex was prepared by using the same procedure as for
the preparation of 1, except that CpW(CO)3Na was used
instead of CpMo(CO)3Na. Yield: 0.21 g (78%). Anal. Calc. for
RuWC38H32O3P2: C 51.66, H 3.65. Found: C 51.30, H 3.58%. IR
1
(77%). The product was identified by H and 31P{1H} NMR
(KBr, cmϪ1): ν(C᎐O) 1792 (s), 1872 (s) and 1892 (sh). H NMR
1
᎐
᎐
spectroscopy. 1H NMR (400.13 MHz, THF-d8, 25 ЊC):
(400.13 MHz, C6D6, 25 ЊC): δ 4.21 (m, 1H, PCHHP), 4.55 (m,
2
2
δ Ϫ17.16 [ddd, J(HP) = 28.2 Hz, J(HP) = 21.0 Hz, J(HH) =
5.6 Hz, 1H, Mo–H–Ru], 3.98 (m, 1H, PCHHP), 4.46 (m, 1H,
PCHHP), 5.16 (s, 5H, CpRu), 5.61 (s, 5H, CpMo), 7.05–7.72
(m, 20H of dppm). 31P{1H} NMR (161.99 MHz, THF-d8, 25
ЊC): δ 39.7 [d, 2J(PP) = 56.3 Hz], 48.5 [d, 2J(PP) = 56.3 Hz].
1H, PCHHP), 5.03 (s, 5H, CpRu), 5.04 (s, 5H, CpMo), 6.90–
7.79 (m, 20H of dppm). 31P{1H} NMR (161.99 MHz, C6D6,
2
2
25 ЊC): δ 21.1 [d, J(PP) = 74.3 Hz], 56.1 [d, J(PP) = 74.3 Hz].
ESI-MS (CH2Cl2–MeOH as solvent) m/z: 881 [M]ϩ.
Cp*Ru(CO)(ꢁ-dppm)W(CO)2Cp (4)
Synthesis of [CpRu(CO)(ꢁ-dppm)(ꢁ-H)Mo(CO)2Cp]؉[OTf]؊
(5ؒOTf)
This complex was prepared by using the same procedure as for
the preparation of 3 except that Cp*Ru(dppm)Cl was used in
place of CpRu(dppm)Cl. Yield: 0.23 g (76%). Anal. Calc. for
The procedure for preparation of 5ؒOTf is similar to that of
5ؒBF4, except that trifluoromethanesulfonic acid (4.40 µL, 0.050
mmol) was used instead of HBF4ؒEt2O. Yield: 0.019 g (80%) 1H
NMR (400.13 MHz, THF-d8, 25 ЊC): δ Ϫ17.17 [ddd, 2J(HP) =
RuWC43H37O3P2: C 54.44, H 3.93. Found: C 54.32, H 4.02%. IR
(KBr, cmϪ1): ν(C᎐O) 1784 (med), 1854 (s) and 1873 (sh). H
1
᎐
᎐
NMR (400.13 MHz, C6D6, 25 ЊC): δ 1.55 (s, 15H, methyls of
2
28.8 Hz, J(HP) = 21.0 Hz, J(HH) = 5.6 Hz, 1H, Mo–H–Ru],
Cp*) δ 2.88 (m, 1H, PCHHP), 3.66 (m, 1H, PCHHP), 4.84 (s,
5H, CpMo), 6.76–7.29 (m, 20H of dppm). 31P{1H} NMR
(161.99 MHz, C6D6, 25 ЊC): δ 21.3 [d, 2J(PP) = 72.9 Hz], 48.2[d,
2J(PP) = 72.9 Hz]. ESI-MS (CH2Cl2–MeOH as solvent) m/z:
951 [M]ϩ.
3.98 (m, 1H, PCHHP), 4.46 (m, 1H, PCHHP), 5.20 (s, 5H,
CpRu), 5.65 (s, 5H, CpMo), 7.09–7.61 (m, 20H of dppm).
31P{1H} NMR (161.99 MHz, THF-d8, 25 ЊC): δ 39.7 [d, 2J(PP)
= 56.3 Hz], 48.5 [d, 2J(PP) = 56.3 Hz].
Synthesis of [CpRu(CO)(ꢁ-dppm)(ꢁ-H)W(CO)2Cp]؉[Cl]؊
(6ؒCl)
Hydrogenation of CO2 by 1–4
The reactions were carried out in a 250 mL stainless steel auto-
clave. In a typical run, 0.009 mmol of the complex was dissol-
ved in 8 mL of THF or benzene and 2 mL of triethylamine was
added. The solution was heated under 60 atm of CO2/H2 (30
atm/30 atm) at 120 ЊC for 45 h. The reactor was cooled rapidly
Complex 3 (0.020 g, 0.023 mmol) was dissolved in 10 mL of
THF, and an excess of concentrated HCl (37%, aq.) (0.5 mL)
was added. The solution was stirred for 30 min, and the solvent
was removed under vacuum. Hexane (5 mL) was added to the
residue, with stirring, to produce an orange solid. The solid was
collected and washed with water (2 × 1 mL) and diethyl ether
(2 × 2 mL), and then dried under vacuum. Yield: 0.016 g (78%).
Anal. Calc. for RuWC38H33ClO3P2: C 49.61, H 3.62. Found: C
1
and carefully vented. Formic acid formed was analyzed by H
NMR spectroscopy, with DMF (0.5 µL) as internal standard.
Decomposition of formic acid by 1–4
49.64, H 3.81%. IR (KBr, cmϪ1): ν(C᎐O). 1867 (m), 1947 (sh)
᎐
᎐
The reactions were performed in a 5 mm NMR tube and were
monitored by 1H and 31P{1H} NMR spectroscopy. In a typical
run, 2.51 µmol of the complex was dissolved in 0.4 mL of C6D6
or THF-d8 in a 5 mm NMR tube. 10 equiv of formic acid (0.964
µL, 0.0251 mmol) was then added and the mixture was heated
at 80 ЊC. The progress of the decomposition reaction was moni-
tored by 1H NMR spectroscopy in 2 h intervals.
and 1972 (s). 1H NMR (400.13 MHz, THF-d8, 25 ЊC): δ Ϫ17.72
[ddd, 2J(HP) = 27.6 Hz, 2J(HP) = 19.2 Hz, J(HH) = 5.2 Hz, 1H,
1
W–H–Ru; in the phosphorus-decoupled H NMR spectrum,
the W satellites of the bridging hydride signal were observed,
J(HW) = 47 Hz], 3.91 (m, 1H, PCHHP), 4.89 (m, 1H, PCHHP),
5.20 (s, 5H, CpRu), 5.66 (s, 5H, CpW), 7.10–7.61 (m, 20H of
dppm). 31P{1H} NMR (161.99 MHz, THF-d8, 25 ЊC): δ 14.3
[dd, 2J(PP) = 53.5 Hz, J(HH) = 10.7 Hz], 40.5 [dd, 2J(PP) = 53.5
Hz, J(HH) = 10.7 Hz]. ESI-MS (CH2Cl2–MeOH as solvent)
m/z: 884 [M Ϫ Cl]ϩ.
Preparation of [CpRu(CO)(ꢁ-dppm)(ꢁ-H)Mo(CO)2Cp]؉[Cl]؊
(5ؒCl)
Complex 1 (0.020 g, 0.025 mmol) was dissolved in 10 mL of
THF. An excess of concentrated HCl (37%, aq) (0.5 mL) was
added to the solution and the mixture was stirred for 30 min.
Solvent was removed under vacuum. Hexane (5 mL) was added
to the residue, with stirring, to produce an orange solid. This
was collected and washed with water (2 × 1 mL) and diethyl
ether (2 × 2 mL), and dried under vacuum. Yield: 0.016 g
Synthesis of [CpRu(CO)(ꢁ-dppm)(ꢁ-H)W(CO)2Cp]؉[BF4]؊
(6ؒBF4)
Complex 3 (0.020 g, 0.023 mmol) was dissolved in 10 mL of
THF. HBF4ؒEt2O (54%, 6.3 µL, 0.046 mmol) was added and the
solution was heated at 80 ЊC for 2 h. Solvent was removed under
vacuum. Hexane (5 mL) was added to the residue, with stirring,
D a l t o n T r a n s . , 2 0 0 3 , 3 7 2 7 – 3 7 3 5
3733

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to produce an orange solid. This was collected and washed with
water (2 × 1 mL) and diethyl ether (2 × 2 mL), and dried under
vacuum. Yield: 0.019 g (78%). The product was then identified
by 1H and 31P{1H} NMR spectroscopy. 1H NMR (400.13 MHz,
formate (0.9 mg, 10.5 µmol) were dissolved in THF-d8 (0.5 mL)
in a NMR tube. The mixture was then allowed to heat at 80 ЊC
for 6 h. The reaction products were identified by 1H and
31P{1H} NMR spectroscopy.
2
2
THF-d8, 25 ЊC): δ Ϫ17.72 [ddd, J(HP) = 27.6 Hz, J(HP) =
Reaction of [CpRu(CO)(ꢁ-dppm)(ꢁ-H)W(CO)2Cp]؉OTf؊
(6ؒOTf) with sodium formate
19.2 Hz, J(HH) = 5.2 Hz, 1H, W–H–Ru; in the phosphorus-
1
decoupled H NMR spectrum, the W satellites of the bridging
hydride signal were observed, J(HW) = 47 Hz], 3.91 (m, 1H,
PCHHP), 4.89 (m, 1H, PCHHP), 5.20 (s, 5H, CpRu), 5.66 (s,
5H, CpW), 7.10–7.61 (m, 20H of dppm). 31P{1H} NMR
The procedure of this reaction is the same as that of
[CpRu(CO)(µ-dppm)(µ-H)Mo(CO)2Cp]ϩOTfϪ (5ؒOTf ), except
that [CpRu(CO)(µ-dppm)(µ-H)W(CO)2Cp]ϩOTfϪ (6ؒOTf ) was
used in place of [CpRu(CO)(µ-dppm)(µ-H)Mo(CO)2Cp]ϩOTfϪ
(5ؒOTf ).
2
(161.99 MHz, THF-d8, 25 ЊC): δ 14.3 [dd, J(PP) = 53.5 Hz,
J(HH) = 10.7 Hz], 40.5 [dd, 2J(PP) = 53.5 Hz, J(HH) = 10.7 Hz].
Synthesis of [CpRu(CO)(ꢁ-dppm)(ꢁ-H)W(CO)2Cp]؉[OTf]؊
(6ؒOTf)
Crystallographic analysis for CpRu(CO)(ꢁ-dppm)Mo(CO)2Cp
(1), Cp*Ru(CO)(ꢁ-dppm)Mo(CO)2Cp (2), CpRu(CO)(ꢁ-dppm)-
W(CO)2Cp(3),Cp*Ru(CO)(ꢁ-dppm)W(CO)2Cp(4),[CpRu(CO)-
(ꢁ-dppm)(ꢁ-H)Mo(CO)2Cp]؉[BF4]؊ (5ؒBF4) and [CpRu(CO)-
(ꢁ-dppm)(ꢁ-H)W(CO)2Cp]؉[Cl]؊ (6ؒCl)
The procedure for preparation of 6ؒOTf is similar to that of
6ؒBF4, except that trifluoromethanesulfonic acid (4.10 µL, 0.046
mmol) was used instead of HBF4ؒEt2O. Yield: 0.020 g (77%).
1H NMR (400.13 MHz, THF-d8, 25 ЊC): δ Ϫ17.72 [ddd, 2J(HP)
= 27.6 Hz, 2J(HP) = 19.2 Hz, J(HH) = 5.2 Hz, 1H, W–H–Ru; in
the phosphorus-decoupled 1H NMR spectrum, the W satellites
of the bridging hydride signal were observed, J(HW) = 47 Hz],
3.91 (m, 1H, PCHHP), 4.89 (m, 1H, PCHHP), 5.20 (s, 5H,
CpRu), 5.66 (s, 5H, CpW), 7.10–7.61 (m, 20H of dppm).
31P{1H} NMR (161.99 MHz, THF-d8, 25 ЊC): δ 14.3 [dd, 2J(PP)
= 53.5 Hz, J(HH) = 10.7 Hz], 40.5 [dd, 2J(PP) = 53.5 Hz, J(HH)
= 10.7 Hz].
Crystals suitable for an X-ray diffraction study for 1,3, 5ؒBF4,
and 6ؒCl were obtained by layering of hexane on CH2Cl2 solu-
tions of these complexes, while crystals of 2 and 4 were
obtained by layering of diethyl ether on their CH2Cl2 solutions.
A suitable crystal of each of the complexes was mounted on a
Bruker CCD area detector diffractometer using Mo-Kα radi-
ation (λ = 0.71073 Å) from a generator operating at 50 kV and
30 mA. The intensity data of 1–4, 5ؒBF4 and 6ؒCl were collected
in the 2θ range 3–55Њ, with oscillation frames of ꢀ and ω in the
range 0–180Њ. 1321 Frames were taken in four shells. An empir-
ical absorption correction of the SADABS (G. M. Sheldrick,
1996) program based on Fourier coefficient fitting was applied.
The crystal structures were determined by the direct method,
which yielded the positions of part of the non-hydrogen atoms,
and subsequent difference Fourier syntheses were employed to
locate all of the remaining non-hydrogen atoms which did not
show up in the initial structure. Hydrogen atoms were located
based on difference Fourier syntheses connecting geometrical
analysis. All non-hydrogen atoms were refined anisotropically
Reaction of 5ؒCl with sodium methoxide
Sodium methoxide was prepared by reacting sodium metal
(0.5 g) with methanol (10 mL). After all the sodium metal
was reacted, the sodium methoxide solution was transferred
to another Schlenk flask loaded with complex 5ؒCl (0.18 g,
0.22 mmol). The mixture was refluxed for 16 h after which the
solvent was removed under vacuum. Hexane (10 mL) was
added to the residue with stirring to obtain a red solid, which
was found to be 1 by 1H and 31P{1H} NMR spectroscopy.
2
with weight function w = 1/[σ2(Fo ) ϩ 0.1000p]2 ϩ 0.0000p],
2
where p = (Fo ϩ 2Fc2)/3 were refined. Hydrogen atoms were
Reaction of 5ؒCl with Et3SiH
refined with fixed individual displacement parameters. All
experiment and computation were performed on a Bruker CCD
Area Detector Diffractometer and PC computer with program
of Bruker Smart and Bruker SHELXTL packages.
CCDC reference numbers 211626–211631.
See http://www.rsc.org/suppdata/dt/b3/b306835h/ for crystal-
lographic data in CIF or other electronic format.
A sample of 5ؒCl (0.0013 g, 1.56 µmol) was dissolved in THF-d8
(0.5 mL) in a 5 mm NMR tube and triethylsilane (0.25 µL, 1.56
µmol) was added. The mixture was then refluxed for 24 h. The
resulting mixture was analyzed at room-temperature by 1H,
1
31P{1H} and 29Si{1H} NMR spectroscopy. H NMR (400.13
MHz, THF-d8, 25 ЊC): δ 0.63 (q, J(HH) = 7.8 Hz, 2H, SiCH2),
1.07 (t, J(HH) = 7.8 Hz, 3H, SiCH2CH3), 3.85 (m, 1H,
PCHHP), 4.50 (m, 1H, PCHHP), 5.06 (s, 5H, CpRu), 5.08 (s,
5H, CpMo), 7.02–7.76 (m, 20H of dppm). 31P{1H} NMR
(161.99 MHz, THF-d8, 25 ЊC): δ 49.9 [d, 2J(PP) = 75.6 Hz], 53.7
Acknowledgements
We thank the Hong Kong Polytechnic University for financial
support.
2
[d, J(PP) = 75.6 Hz]. 29Si{1H} NMR (79.5 MHz, THF-d8, 25
ЊC): δ 20.8.
References and notes
Reaction of 5ؒCl with sodium tetraphenylborate
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1
The resulting mixture was analyzed by room-temperature H,
11B{1H} and 31P{1H} NMR spectroscopy. The 1H and 31P{1H}
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regenerated. The formation of triphenylborane was confirmed
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δ 68.8.
Reaction of [CpRu(CO)(ꢁ-dppm)(ꢁ-H)Mo(CO)2Cp]؉OTf؊
(5ؒOTf) with sodium formate
The complex [CpRu(CO)(µ-dppm)(µ-H)Mo(CO)2Cp]ϩOTfϪ
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(5ؒOTf ) (0.0020 g, 2.1 µmol) and a five-fold excess of sodium
D a l t o n T r a n s . , 2 0 0 3 , 3 7 2 7 – 3 7 3 5
3734

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