Complexes with platinum-ruthenium bonds: The stepwise formation of complexes containing PtRu and PtRu2 units

Article abstract of DOI:10.1021/om990032z

The reaction of [Pt(dppm)₂]Cl₂, dppm = Ph₂PCH₂PPh₂, with [HRu(CO)₄]⁺ in a 1:2 ratio leads in a multistep reaction to the new heteronuclear cluster [PtRu₂(CO)₅

Full text of DOI:10.1021/om990032z

2162
Organometallics 1999, 18, 2162-2167
Com p lexes w ith P la tin u m -Ru th en iu m Bon d s: Th e
Step w ise F or m a tion of Com p lexes Con ta in in g P tRu a n d
P tRu 2 Un its
Brian T. Sterenberg, Michael C. J ennings, and Richard J . Puddephatt*
Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 5B7
Received J anuary 20, 1999
The reaction of [Pt(dppm)2]Cl2, dppm ) Ph2PCH2PPh2, with [HRu(CO)4]- in a 1:2 ratio
leads in a multistep reaction to the new heteronuclear cluster [PtRu2(CO)5(µ-CO)(µ-dppm)2],
which contains a triangle of metal atoms having both Pt-Ru edges bridged by dppm ligands.
The initial step in the reaction is the formation of the heterobimetallic complex [PtRuH-
(CO)3(µ-dppm)2]+, which then reacts with additional [HRu(CO)4]- to form the PtRu2 cluster.
In the absence of excess [HRu(CO)4]-, the complex [PtRuH(CO)3(µ-dppm)2]+ undergoes
chloride for carbonyl substitution at ruthenium to give [PtRuHCl(CO)(µ-CO)(µ-dppm)2]. This
complex then reacts further with chlorinated solvent to give [PtRu(Cl)2(CO)2(µ-dppm)2], which
exists in two isomeric forms, one of which contains a semibridging carbonyl ligand. The
structures of [PtRu2(CO)5(µ-CO)(µ-dppm)2] and one isomer of [PtRu(Cl)2(CO)2(µ-dppm)2] have
been determined crystallographically.
Sch em e 1
In tr od u ction
Mixed metal cluster complexes of platinum have
received considerable attention due to their potential
to act as models for mixed metal catalysts.1 Platinum-
ruthenium bonded clusters are of particular interest
because supported Pt/Ru/alumina catalysts are useful
in petroleum refining and platinum-ruthenium elec-
trodes are used to catalyze methanol oxidation in fuel
cells.2 Platinum-ruthenium clusters themselves have
also been shown to be catalysts for hydrogenation and
hydrosilylation of alkynes.3
The ability of many different metals, including main
group metals such as tin, mid-transition metals such
as rhenium, and several group 8-10 metals such as
ruthenium or iridium to enhance the utility of platinum
catalysts is remarkable and prompted a research pro-
gram on the planned synthesis of complexes containing
Pt-M bonds and studies of their reactivity.4 It was
shown recently that complexes containing PtIr (1 and
2) and PtIr2 (3a and 3b) units could be formed by the
stepwise addition of [Ir(CO)4]- to [Pt(dppm)2]2+ as
shown in Scheme 1.5 This paper reports the analogous
formation of PtRu and PtRu2 bonded clusters by reac-
tion of [HRu(CO)4]- with [Pt(dppm)2]2+. Although sev-
eral mixed metal clusters of the form PtRu2 are known,
(1) (a) Farrugia, L. J . Adv. Organomet. Chem. 1990, 31, 301. (b) The
Chemistry of Heteronuclear Clusters and Multimetallic Catalysts;
Adams, R. D., Herrmann, W. A., Eds.; Polyhedron 1988, 7, 2251-2462.
(c) Comprehensive Organometallic Chemistry II; Abel, E. W., Stone,
F. G. A., Wilkinson, G., Eds.; Heteronuclear Metal-Metal Bonds, Vol.
10; Adams, R. D., Ed.; Elsevier: Oxford, 1995.
(2) (a) Miura, H.; Taguchi, H.; Sugiyama, K.; Matsuda, T.; Gonzalez,
R. D. J . Catal. 1990, 124, 194. (b) Lee, C. E.; Bergens, S. H. J . Phys.
Chem. B 1998, 102, 193.
simple routes to them are not common. Previously
characterized examples include [PtRu2(CO)8(dppe)],6
[PtRu2(CO)6(dppe)2],7 [Ru2Pt(CO)5(CNBut)(PPh3)4],8 and
[PtRu2(µ-PPh2)(µ-H)(CO)7(PCy3)].9 These clusters were
(3) (a) Adams, R. D.; Barnard, T. S.; Li, Z. Y.; Wu, W. G.; Yamamoto,
J . H. J . Am. Chem. Soc. 1994, 116, 9103. (b) Adams, R. D.; Barnard,
T. S. Organometallics 1998, 17, 2567.
(4) Xiao, J .; Puddephatt, R. J . Coord. Chem. Rev. 1995, 143, 457.
(5) Sterenberg, B. T.; J enkins, H. A.; Puddephatt, R. J . Organome-
tallics, in press.
(6) (a) Adams, R. D.; Chen, G.; Wang, J .; Wu, W. Organometallics
1990, 9, 1339. (b) Bruce, M. I.; Shaw, G.; Stone, F. G. A. J . Chem.
Soc., Dalton Trans. 1972, 1781.
(7) Adams, R. D.; Chen, G.; Wu, W. J . Cluster Sci. 1993, 4, 119.
(8) Bruce, M. I.; Matisons, J . G.; Skelton, B. W.; White, A. H. Aust.
J . Chem. 1982, 35, 687.
10.1021/om990032z CCC: $18.00 © 1999 American Chemical Society
Publication on Web 05/04/1999

Complexes with Pt-Ru Bonds
Organometallics, Vol. 18, No. 11, 1999 2163
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
Ch a r t 1. Kn ow n P tRu 2 a n d P tRu Bon d ed
Com p lexes
4
7a
formula
fw
temp (K)
wavelength (Å)
space group
a (Å)
C
56H44O6P4PtRu2
C55.5H52Cl4O2P4PtRu
1312.81
292(2)
0.710 73
Cc
19.788(4)
16.897(3)
17.302(4)
106.37(3)
5550.8(19)
4
334.02
100(2)
0.710 73
P21/n
12.444(3)
16.842(3)
23.757(5)
96.19
b (Å)
c (Å)
â (deg)
volume (Å3)
Z
4950(2)
4
F(calc) (Mg/m3)
abs coeff (mm-1
no. of ind reflns
no. of data/
restraints/
params
1.790
3.603
1.571
3.137
)
10 042 (Rint ) 0.0460) 5627 [Rint ) 0.0940]
10042/0/622
5627/9/608
G-o-F on F2
1.059
1.047
R[I > 2σ(I)] R1
0.0288
wR2 0.0621
0.0497
0.1384
0.0526
0.1426
R(all data) R1
0.0429
wR2 0.0667
Sch em e 2
as an orange solid and characterized both spectroscopi-
cally and by an X-ray structure determination.
The 31P NMR spectrum of complex 4 contains two
2
resonances at δ ) 16.8 [d, J (PP) ) 48 Hz, RuP] and
8.3 [d, 2J (PP) ) 48 Hz, 1J (PtP) ) 3130 Hz, PtP] in a 1:1
ratio. The presence of only two resonances in the 31P
spectrum indicates that the molecule 4 has an effective
mirror plane which contains the Pt atom and bisects
the Ru-Ru bond. Hence both dppm ligands bridge Pt-
Ru edges of the PtRu2 triangle and the Ru-Ru edge is
not bridged. The structure thus differs from those of the
analogous PtIr2 clusters 3a and 3b, in which the dppm
ligands bridge one Pt-Ir edge and the Ir-Ir edge, with
1
one Pt-Ir edge unbridged. The H NMR spectrum of 4
contains a single resonance at δ ) 4.48 for the dppm
methylene groups, indicating that there is an effective
mirror plane containing the PtRu2(PCP)2 atoms. The
spectrum contains no hydride resonance, and the stoi-
chiometry of formation of 4 requires loss of the hydride
ligands from the 2 equiv of the reagent [HRu(CO)4]- as
dihydrogen. When the reaction was carried out in a
sealed NMR tube, dihydrogen was positively identified
either formed by degradation of a higher nuclearity
cluster or isolated in low yield from mixtures containing
other cluster complexes.6-9 Heterobimetallic complexes
containing Pt-Ru bonds are surprisingly rare,10 and
dppm-bridged PtRu complexes are unknown. Some of
the known complexes containing PtRu or PtRu2 units
are shown in Chart 1.
1
by its H NMR spectrum (δ ) 4.6), thus supporting the
proposed stoichiometry. The 13C NMR spectrum of 4
shows two carbonyl resonances in a 2:1 ratio for the four
axial carbonyls and the two equatorial carbonyls, re-
spectively. The resonance for the equatorial carbonyls
shows a long-range coupling 3J (PPtRuC) ) 21 Hz, which
is consistent with the phosphorus and carbonyl being
approximately collinear with the PtRu bond. The reso-
nance for the axial carbonyls shows a coupling J (PtC)
) 34 Hz, perhaps indicating a weak bridging of these
carbonyls to platinum. However, the lowest carbonyl
stretching frequency in the IR spectrum occurs at 1878
cm-1, which suggests a weak semibridging interaction
only.
Resu lts a n d Discu ssion
Syn th esis of th e P tRu 2 Clu ster [P tRu 2(CO)5(µ-
CO)(µ-d p p m )2], 4. By analogy with the known reac-
tions of [Pt(dppm)2]Cl2 with [Ir(CO)4]- to give PtIr2
clusters 3 (Scheme 1),5 the synthesis of PtRu2 clusters
by the reaction of [Pt(dppm)2]2+ with [PPN][HRu(CO)4]
was studied. The reaction led to the complex [PtRu2-
(CO)5(µ-CO)(µ-dppm)2], 4, Scheme 2, which was isolated
The basic features of the structure of 4 deduced from
the spectra were confirmed by the structure determi-
nation (Table 1). A view of the structure is shown in
Figure 1, and selected bond lengths and angles are given
in Table 2. The three metal atoms form a triangle, and
the two Pt-Ru edges of the triangle are each bridged
by a dppm ligand. The three metals and four phosphorus
(9) (a) Powell, J .; Brewer, J . C.; Gulia, G.; Sawyer, J . F. Inorg. Chem.
1989, 28, 4470. 1650. (b) Kuwata, S.; Mizobe, Y.; Hidai, M. J . Am.
Chem. Soc. 1993, 115, 8499.
(10) (a) Mague, J . T.; Balakrishna, M. S. J . Organomet. Chem. 1996,
23, 4259. (b) Anderson, S.; Mullica, D. F.; Sappenfield, E. L.; Stone, F.
G. A. Organometallics 1995, 14, 3516. (c) Akabori, S.; Kumagai, T.;
Shirahige, T. Organometallics 1987, 6, 526. (d) Powell, J .; Fuchs, E.;
Gregg, M. R.; Phillips, J .; Stainer, M. V. R. Organometallics 1990, 9,
387.

2164 Organometallics, Vol. 18, No. 11, 1999
Sterenberg et al.
Sch em e 3
F igu r e 1. View of the structure of [PtRu2(CO)5(µ-CO)-
(µ-dppm)2], 4. Thermal ellipsoids are drawn at 30% prob-
ability. Hydrogen atoms have been omitted for clarity.
weakly semibridging. The semibridging interaction of
the carbonyl on Ru(2) results in a deformation of the
other carbonyl ligands, with the result that the carbonyl
ligands on the two ruthenium atoms are in a staggered
rather than eclipsed conformation (Figure 1).
Ta ble 2. Selected Bon d s Len gth s (Å) a n d An gles
(d eg) for [P tRu 2(CO)5(µ-CO)(µ-d p p m )2], 4
Pt-P(1)
2.255(1)
2.268(1)
2.534(4)
2.6757(6)
2.6793(9)
1.906(5)
1.915(4)
1.928(4)
2.318(1)
2.8214(7)
1.913(4)
Ru(2)-C(1)
Ru(2)-C(3)
Ru(2)-P(4)
C(1)-O(1)
C(2)-O(2)
C(3)-O(3)
C(4)-O(4)
C(5)-O(5)
C(6)-O(6)
1.926(4)
1.940(5)
2.304(1)
1.149(5)
1.140(5)
1.144(5)
1.160(5)
1.147(5)
1.144(5)
Pt-P(3)
The presence of a single semibridging carbonyl in the
solid-state structure leads to overall Cs symmetry for
the complex. This is clearly inconsistent with the C2v
symmetry in solution deduced from the NMR spectra.
The apparent symmetry in solution results from a
fluxional process in which the four axial carbonyl
ligands exchange between terminal and weakly semi-
bridging bonding modes, as shown in Scheme 3. At-
tempts were made to freeze out the fluxional process
using variable-temperature NMR, but it is still fast on
the NMR time scale even at -90 °C. From examination
of the structure in Figure 1, it is clear that only a very
small movement of the carbonyls is required for this
equilibration.
Pt-C(1)
Pt-Ru(1)
Pt-Ru(2)
Ru(1)-C(6)
Ru(1)-C(4)
Ru(1)-C(5)
Ru(1)-P(2)
Ru(1)-Ru(2)
Ru(2)-C(2)
P(1)-Pt-P(3)
105.79(4) C(6)-Ru(1)-C(5)
128.5(1) C(4)-Ru(1)-C(5)
95.2(2)
163.0(2)
106.8(1)
88.5(1)
96.3(1)
154.7(1)
68.5(1)
94.7(1)
95.18(4)
P(1)-Pt-C(1)
P(3)-Pt-C(1)
92.76(9) C(6)-Ru(1)-P(2)
94.63(4) C(4)-Ru(1)-P(2)
158.19(3) C(5)-Ru(1)-P(2)
80.03(9) C(6)-Ru(1)-Pt
156.51(3) C(4)-Ru(1)-Pt
97.11(3) C(5)-Ru(1)-Pt
P(1)-Pt-Ru(1)
P(3)-Pt-Ru(1)
C(1)-Pt-Ru(1)
P(1)-Pt-Ru(2)
P(3)-Pt-Ru(2)
C(1)-Pt-Ru(2)
Ru(1)-Pt-Ru(2)
C(6)-Ru(1)-C(4)
C(5)-Ru(1)-Ru(2)
Th e Mech a n ism of Clu ster F or m a tion . The mech-
anism of formation of 4 clearly requires the addition of
2 equiv of [HRu(CO)4]- to [Pt(dppm)2]2+, with loss of
H2 + 2CO. Attempts were made to observe intermediate
complexes in this overall reaction by monitoring the
reaction by NMR under varying conditions. Only the
bimetallic intermediate complex [HPtRu(CO)3(µ-dppm)2]+,
5, Scheme 2, was detected in this way. Compound 5 can
be synthesized by reaction of [Pt(dppm)2]Cl2 with 1
equiv of PPN[HRu(CO)4] at low temperature, but it is
difficult to purify, as discussed below, and so was
characterized by its spectroscopic data.
43.3(1)
P(2)-Ru(1)-Pt
63.59(2) C(6)-Ru(1)-Ru(2) 100.5(1)
99.1(2)
81.2(1)
C(4)-Ru(1)-Ru(2)
C(2)-Ru(2)-Ru(1) 104.5(1)
87.0(1)
P(2)-Ru(1)-Ru(2) 152.74(3) C(1)-Ru(2)-Ru(1)
87.7(1)
82.8(1)
Pt-Ru(1)-Ru(2)
C(2)-Ru(2)-C(1)
C(2)-Ru(2)-C(3)
C(1)-Ru(2)-C(3)
C(2)-Ru(2)-P(4)
C(1)-Ru(2)-P(4)
C(3)-Ru(2)-P(4)
C(2)-Ru(2)-Pt
C(1)-Ru(2)-Pt
C(3)-Ru(2)-Pt
P(4)-Ru(2)-Pt
58.27(3) C(3)-Ru(2)-Ru(1)
99.7(2)
94.9(2)
164.2(2)
104.8(1)
88.9(1)
93.2(1)
155.1(1)
64.3(1)
99.9(1)
P(4)-Ru(2)-Ru(1) 150.63(3)
Pt-Ru(2)-Ru(1)
O(1)-C(1)-Ru(2)
O(1)-C(1)-Pt
58.14(2)
173.8(3)
112.6(3)
72.4(1)
176.8(4)
176.4(4)
175.7(4)
177.1(3)
176.0(4)
Ru(2)-C(1)-Pt
O(2)-C(2)-Ru(2)
O(3)-C(3)-Ru(2)
O(4)-C(4)-Ru(1)
O(5)-C(5)-Ru(1)
1
The H NMR spectrum of 5 shows a hydride peak at
δ ) -3.95 [t, 2J (PH) ) 10 Hz, 1J (PtH) ) 1270 Hz],
clearly indicating the presence of a terminal Pt-H
group. The dppm methylene hydrogens appear as a
single resonance at δ ) 4.34, indicating that there is a
mirror plane containing the two metal and four phos-
phorus atoms. The 31P NMR spectrum of 5 shows two
multiplet resonances characteristic of an AA′XX′ spin
system at δ ) 36.8 [RuP] and 24.0 [1J (PtP) ) 3260 Hz,
PtP], clearly showing that the dppm ligands bridge the
PtRu unit. In the 13C NMR spectrum of 5, two reso-
nances are observed at δ ) 195.5 [equatorial CO] and
207.8 [axial CO] in a 1:2 ratio, neither showing resolved
J (PtC) coupling. The IR spectrum shows three terminal
bands at 2042, 1988, and 1970 cm-1. The structure can
thus be unambiguously assigned as having a terminal
94.33(3) O(6)-C(6)-Ru(1)
atoms are approximately coplanar. The two Pt-Ru
distances are very similar at 2.6757(6) and 2.6793(9)
Å, while the unbridged Ru-Ru distance is significantly
longer at 2.8214(7) Å. There are three carbonyl ligands
on each ruthenium atom, which naturally gives a 16-
electron configuration at platinum and an 18-electron
configuration at each ruthenium. On Ru(1), all three
carbonyls are terminal but one of the carbonyl ligands
on Ru(2) forms a weak semibridging interaction to
platinum. The Pt-C(1) distance is 2.534(4) Å, while the
Ru(2)-C(1) distance is 1.926(4) Å, and the angles Pt-
C(1)-O(1) (112.6(3)°) and Ru(2)-C(1)-O(1) (173.8(3)°)
clearly show that the carbonyl is best considered as

Complexes with Pt-Ru Bonds
Sch em e 4
Organometallics, Vol. 18, No. 11, 1999 2165
formation from 6 requires the overall replacement of the
hydride by chloride (Scheme 4). Complex 6 is detected
as an intermediate, and complete conversion to 7 takes
about 3 days.
The 1H NMR spectrum of 6 shows a hydride reso-
2
3
nance at δ ) -5.45 [tt, J (PPtH) ) 16 Hz, J (PRuPtH)
) 3 Hz, 1J (PtH) ) 1050 Hz], and the large value of
1J (PtH) proves the presence of a terminal PtH group.
Two resonances are seen for the dppm methylene
hydrogen atoms, indicating that there is no plane of
symmetry containing the PtRu(PCP)2 atoms and hence
that the chloride is axial rather than equatorial on
ruthenium. The 13C NMR spectrum shows two reso-
nances at δ ) 207.7 [terminal CO] and 225.1 [bridging
CO]. Although the chemical shift at δ ) 225.1 suggests
a bridging carbonyl, no 1J (PtC) coupling is observed, and
it is possible that the chemical shift reflects a strongly
semibridging carbonyl. The IR spectrum shows two
bands at 1880 and 1770 cm-1 and so lends support to
the proposal that a bridging carbonyl is present.
In the 1H NMR spectrum of 7 there is no hydride
resonance, but the spectra are otherwise similar to those
of 6. Thus, there are two resonances in the 1H NMR
spectrum due to the CHaHb protons of the dppm ligands,
and the 31P NMR spectrum shows two multiplets due
to the RuP and PtP atoms. The 13C NMR spectrum gives
two resonances at δ ) 208.0 and 192.4, both in the usual
range for terminal RuCO groups and which show small
couplings J (PtC) ) 117 and 180 Hz, respectively.
hydride on platinum, three terminal carbonyls on
ruthenium, and a chloride counterion, as shown in
Scheme 2. This structure therefore contains a 16-
electron platinum center and an 18-electron ruthenium
center. If the Pt-Ru bond is considered as a single
covalent bond, the formal oxidation states are Pt(I)Ru-
(I), but it is also possible to consider this as a ruthenium
to platinum donor-acceptor bond, and then the formal
oxidation states are Pt(II)Ru(0).
The overall reaction leading to formation of cluster 4
is then suggested to occur as follows. The initial step is
-
The IR spectrum of 7 shows unusual features. In
either the solid state or solution, an analytically pure
sample of 7 showed four carbonyl bands at 2025, 1945,
1905, and 1792 cm-1, though 7 contains only two
carbonyl ligands. The relative intensities of the bands
changed between the solid and solution state, indicating
that the extra bands result from a second isomer of 7.
The bands at 1905 and 1792 cm-1 are favored in the
solid state and are assigned to isomer 7a , and the low-
frequency stretch at 1792 cm-1 indicates the presence
of a bridging carbonyl in 7a . The bands at 2025 and
1945 cm-1 are more prominent in the solution state and
indicate that the second isomer, 7b, has only terminal
carbonyl ligands. Note that both carbonyl bands for 7b
are at higher energy than those of 7a . This is reasonable
since the semibridging carbonyl in 7a is involved in
back-bonding with platinum so the remaining terminal
carbonyl on ruthenium also back-bonds more strongly
than the carbonyls in 7a . Although the IR spectra
clearly indicate that two isomers are present in solution,
the NMR spectra contain only one set of peaks even at
-90 °C, indicating that the interconversion of the two
isomers is very fast on the NMR time scale but slow on
the IR time scale. The resonance in the 13C NMR
spectrum at δ ) 208.0 can now be assigned to the
carbonyl that is semibridging in 7a but terminal in 7b.
Thus, it has a significantly higher chemical shift than
the resonance at δ ) 192, which is assigned to the
carbonyl which is terminally bonded in both isomers.
combination of the first equivalent of HRu(CO)4 and
[Pt(dppm)2]2+ to form [PtRuH(CO)3(µ-dppm)2]Cl, 5. This
reaction occurs by nucleophilic attack of [HRu(CO)4]-
on 1, with rearrangement of the dppm ligands from
chelating to bridging coordination, migration of hydride
from ruthenium to platinum, and with loss of 1 equiv
of CO. The next step is attack on the bimetallic complex
5 by the second equivalent of [HRu(CO)4]- with loss of
H2 and CO, to form the trimetallic cluster 4. The initial
attack by the second equivalent of [HRu(CO)4]- on 5
might be expected to occur at the 16-electron platinum
center, but that should lead to insertion of ruthenium
into a Pt-P bond and hence to formation of a Ru2(µ-
dppm) edge, whereas the structure of 4 clearly requires
insertion into a Ru-P bond and suggests attack by
[HRu(CO)4]- at the ruthenium center of 5 with loss of
CO. Migration of hydride from ruthenium to platinum
followed by reductive elimination of H2 would then lead
to formation of 4.
Fu r th er Ch em istr y of th e Heter obim etallic Com -
p lex 5. Attempts to prepare complex 5 in pure form led
instead to the isolation of two new PtRu bonded
complexes, [PtRuHCl(CO)(µ-CO)(µ-dppm)2], 6, and
[PtRuCl2(CO)2(µ-dppm)2], 7. The compound [PtRuHCl-
(CO)(µ-CO)(µ-dppm)2], 6, is formed from 5 by displace-
ment of one carbonyl ligand by the chloride counterion
as shown in Scheme 4. It is formed as the main product
if the reaction of PPN[HRu(CO)4] and [Pt(dppm)2][Cl]2
is carried out in tetrahydrofuran solvent at room tem-
perature and if the PPN[HRu(CO)4] is added in a
dropwise fashion. Complex 6 could not be isolated in
pure form (it could not be separated from some impurity
of cluster 4), but it was completely characterized in
solution. Complex 7 is formed when the above reaction
is carried out in solution in dichloromethane, and its
One isomer of complex 7 has been structurally char-
acterized. A diagram of the structure is shown in Figure
2, and selected bond lengths and angles are shown in
Table 3. Clearly, the structurally characterized isomer
is 7a , since it contains a bridging carbonyl ligand. Recall
that 7a is the isomer favored in the solid state. The

2166 Organometallics, Vol. 18, No. 11, 1999
Sterenberg et al.
the metal-metal bond such that the angle Pt-Ru-C(1)
is 156.4(6)°, substantially less than 180°.
In the absence of structural characterization, the
precise structure of the isomer 7b must be deduced from
the structure of 7a and from the spectroscopic data.
Clearly, the interconversion of the isomers is very facile
so the difference in structures is probably minor. We
suggest that formation of 7b from 7a occurs simply by
breaking the semibridging Pt‚‚‚CO interaction, thus
allowing the ruthenium to adopt a more regular octa-
hedral structure. In this way, the loss of the Pt‚‚‚CO
bonding component is balanced by the relief of angle
strain at ruthenium, such that the isomers have similar
energies. Interconversion between the isomers probably
requires a change in conformation of the bridging dppm
ligands, and this provides a small steric barrier since
the ruthenium center is sterically crowded.
The structural characterization of 7a clearly lends
more support to the proposed structure of 6. It is,
however, interesting that the hydride-chloride complex
6 exists only in the bridging carbonyl form analogous
to 7a , with no evidence of a second isomer analogous to
7b. The role of the semibridging carbonyl in 7a is clearly
to remove excess charge from platinum by back-bonding.
Complex 6 contains the stronger electron-donating
hydride ligand, which makes the platinum more elec-
tron rich and increases the strength of the semibridging
carbonyl interaction. Hence, there is no equilibrium with
the terminal carbonyl isomer analogous to 7b.
F igu r e 2. View of the structure of [PtRu(Cl)2(CO)(µ-CO)-
(µ-dppm)2], 7a . Thermal ellipsoids are drawn at 30%
probability. Hydrogen atoms have been omitted for clarity.
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for [P tRu (Cl)2(CO)(µ-CO)(µ-d p p m )2] (7a )
Pt-C(2)
Pt-P(4)
Pt-P(1)
Pt-Cl(1)
Pt-Ru
2.20(1)
Ru-C(2)
Ru-P(2)
Ru-P(3)
Ru-Cl(2)
C(1)-O(1)
C(2)-O(2)
1.92(1)
2.359(3)
2.360(3)
2.498(3)
1.16(2)
1.15(2)
2.295(4)
2.319(3)
2.406(4)
2.823(1)
1.81(1)
Ru-C(1)
C(2)-Pt-P(4)
C(2)-Pt-P(1)
P(4)-Pt-P(1)
C(2)-Pt-Cl(1)
P(4)-Pt-Cl(1)
P(1)-Pt-Cl(1)
P(4)-Pt-Ru
94.1(3)
97.6(3)
164.4(1)
139.4(4)
88.8(1)
89.0(1)
91.5(1)
90.11(8)
177.8(2)
105.3(7)
88.3(5)
97.0(4)
C(1)-Ru-P(3)
C(2)-Ru-P(3)
P(2)-Ru-P(3)
C(1)-Ru-Cl(2)
C(2)-Ru-Cl(2)
P(2)-Ru-Cl(2)
P(3)-Ru-Cl(2)
C(1)-Ru-Pt
88.3(5)
93.1(4)
169.9(1)
113.3(6)
141.4(4)
85.5(1)
Con clu sion s
The reaction of [RuH(CO)4]- with [Pt(dppm)2]2+ oc-
curs in a stepwise manner to give [PtRuH(CO)3(µ-
dppm)2]+, 5, and then [PtRu2(CO)6(µ-dppm)2], 4. Com-
plex 5 acts as a precursor to the further Pt-Ru bonded
complexes [PtRuHCl(CO)2(µ-dppm)2], 6, and [PtRuCl2-
(CO)3(µ-dppm)2], 7. Reliable synthetic routes to these
heterobimetallic Pt-Ru bonded complexes are particu-
larly useful because there are few other examples of
such complexes, which are the simplest models for the
Pt/Ru surface chemistry that is important in bimetallic
catalysis. A feature of both the binuclear Pt-Ru com-
plexes, 6 and 7a , and the PtRu2 cluster complex, 4, is
the presence of semibridging carbonyl ligands, whose
function appears to be to equalize the charge densities
at the platinum and ruthenium centers by back-bonding
of electron density away from the electron-rich platinum
center.
87.0(1)
P(1)-Pt-Ru
156.4(6)
93.64(8)
93.24(8)
90.3(1)
Cl(1)-Pt-Ru
C(1)-Ru-C(2)
C(1)-Ru-P(2)
C(2)-Ru-P(2)
P(2)-Ru-Pt
P(3)-Ru-Pt
Cl(2)-Ru-Pt
structure of 7a contains a trans,trans-PtRu(µ-dppm)2
group with a metal-metal separation Pt-Ru ) 2.823-
(1) Å, which is consistent with a single metal-metal
bond, though the distance is longer than in most related
complexes.6-10 For example, it is considerably longer
than the Pt-Ru distances in cluster 4 (2.6757(6) and
2.6793(9) Å) and somewhat longer than in the binuclear
complexes [PtRuCp(PPh3)(Me2PCH2PMe2)2] (2.769(1) Å)
and [PtRuH(µ-σ,η5-7,8-C2B9H10)(CO)2(PEt3)2] (2.802(1)
Å).10a,b In addition to the phosphine ligands, the ruthe-
nium atom is bound to two carbonyl ligands and one
chloride ligand, while the platinum atom is bound to
one chloride and also forms a semibridging interaction
with one of the ruthenium carbonyl ligands. This
configuration results in an 18-electron Ru(I) center and
a 16-electron Pt(I) center. The geometry at ruthenium
is best considered as a highly distorted octahedron. The
angles within the PtClP2 unit are all close to 90°, so
the stereochemistry at platinum is roughly square
planar ignoring the semibridging carbonyl. The carbonyl
ligand C(2)O(2) is bent toward the Pt atom to form the
semibridging interaction. The angles Ru-C(2)-O(2) )
157(1)°, Pt-C(2)-O(2) ) 116(1)° and distances Ru-C(2)
) 1.92(1), Pt-C(2) ) 2.20(1) Å clearly indicate a
semibridging carbonyl. The terminal carbonyl C(1)O(1)
is also bent from its ideal octahedral position trans to
Exp er im en ta l Section
Infrared spectra were recorded as Nujol mulls using a
1
Perkin-Elmer 2000 FTIR spectrometer. The H, 31P{1H}, and
13C{1H} NMR spectra were recorded by using a Varian Gemini
300 NMR spectrometer. The compounds [PPN][Ir(CO)4]11 and
5
[Pt(dppm)2]Cl2 were prepared by literature methods. All
manipulations were carried out using standard Schlenk
techniques under an atmosphere of prepurified argon except
as indicated. Once formed, the pure Pt-Ru bonded complexes
were only slowly decomposed by air.
[P tRu 2(CO)5(µ-CO)(µ-d p p m )2] (4). [Pt(dppm)2]Cl2 (40 mg,
0.038 mmol) and excess PPN[HRu(CO)4] (150 mg, 0.199 mmol)
were cooled to -80 °C and dissolved in 30 mL of CH2Cl2 which
(11) Walker, H. W.; Ford, P. C. J . Organomet. Chem. 1981, 214, C43.

Complexes with Pt-Ru Bonds
Organometallics, Vol. 18, No. 11, 1999 2167
had been degassed with three freeze-pump-thaw cycles. The
resulting red-orange solution was allowed to warm to room
temperature and then stirred for 4 days. The solvent was
removed in vacuo, and the residue was chromatographed on
a silica gel column using CH2Cl2 as the eluent. The orange
band was isolated, and the solvent was removed in vacuo. The
residue was recrystalized from CH2Cl2/pentane and dried in
vacuo. Yield: 32 mg, 62%. Anal. Calcd for C56H44O6P4PtRu2:
C, 50.41; H, 3.32. Found: C, 50.29; H, 3.32. IR (Nujol): ν(CO)
2007 (s), 1960 (s), 1940 (sh), 1924 (s), 1893 (s), 1878 (b). NMR
the residue was separated by chromatography (silica gel, CH2-
Cl2/acetone 50:50), and the yellow band was isolated. The
solvent was removed in vacuo, and the residue was recrystal-
ized from CH2Cl2/Et2O. Yield: 85 mg, 74%. Anal. Calcd for
C
52H44Cl2O2P4PtRu: C, 52.40; H, 3.72; Cl, 5.95. Found: C,
52.35; H, 3.71; Cl, 5.81. IR: ν(CO) ) 2025 ss, 1945 ss,1905 sb,
1792 mb. NMR in CD2Cl2: δ(1H) ) 4.70 [m, 2H, PCHHP]; 4.09
1
[m, 2H, PCHHP]. δ(31P) ) 24.3 [m, RuP]; 11.6 [m, J (PtP) )
3060 Hz, PtP]. δ(13C) ) 208.0 [t, 2J (PC) ) 11 Hz, 1J (PtC) )
2
117 Hz]; 192.4 [m, J (PtC) ) 180 Hz].
2
in CD2Cl2: δ(1H) ) 4.48 [PCH2P, m]. δ(31P) ) 16.8 [d, J (PP)
X-r a y Str u ctu r e Deter m in a tion s. [P tRu 2(CO)5(µ-CO)-
(µ-d p p m )2] (4). Crystals of 4 were grown by slow diffusion of
pentane into a methylene chloride solution. A red crystal was
mounted on a glass fiber. Data were collected at 100 K on a
Nonius Kappa-CCD diffractometer using COLLECT (Nonius,
1998) software. The unit cell parameters were calculated and
refined from the full data set. Crystal cell refinement and data
reduction was carried out using the Nonius DENZO package.
The data were scaled using SCALEPACK (Nonius, 1998), and
no other absorption corrections were applied. The reflection
data and systematic absences were consistent with the mono-
clinic space group P21/n. The SHELXTL 5.03 (Sheldrick, G.
M., Madison, WI) program package was used to solve the
structure by direct methods and successive difference Fouriers.
All non-hydrogen atoms were refined with anisotropic thermal
parameters. The hydrogen atoms were calculated geometri-
cally and rode on their respective carbon atoms. The largest
residual electron density peak (1.523 e/Å3) was associated with
the platinum atom.
) 48 Hz, RuP], 8.3 [d, 2J (PP) ) 48 Hz, 1J (PtP) ) 3130 Hz,
3
PtP]. δ(13C) ) 204 [2C, dm, J (PC) ) 21 Hz, equatorial CO];
222 [4C, m, 1J (PtC) ) 34 Hz, axial CO]. If the reaction to form
complex 4 is carried out in situ in CD2Cl2, compound 5 is
observed by 31P NMR as a reaction intermediate.
[P tRu H(CO)3(µ-d p p m )2][Cl] (5). [Pt(dppm)2]Cl2 (30 mg,
0.028 mmol) and PPN[HRu(CO)4] (22 mg, 0.029 mmol) were
placed in an NMR tube and cooled to -80 °C. Deuterated
methylene chloride (0.5 mL) was then added to the tube,
resulting in the formation of an orange solution, which was
allowed to slowly warm to room temperature. Compound 5 is
stable in solution for several hours, but over longer periods it
is transformed into 6 and 7. Attempts to isolate solid samples
of 5 resulted in mixtures of 6 and 7. IR (CH2Cl2 solution): 2042
(m), 1988 (s), 1970 (sh). NMR in CD2Cl2: δ(1H) ) -3.95 [t,
2
1
1H, J (PH) ) 10 Hz, J (PtH) ) 1270 Hz, PtH]; 4.34 [m, 4H,
3J (PtH) ) 30 Hz, PCH2P]. δ(31P) ) 36.8 [m, RuP]; 24.0 [m,
1J (PtP) ) 3260, PtP]. δ(13C) ) 195.5 [t, 1C, J (PC) ) 14 Hz,
2
2J (PtC) ) 28 Hz, CO anti to Pt]; 207.8 [t, 2C, J (PC) ) 13 Hz,
[P t R u Cl2(CO)(µ-CO)(µ-d p p m )2] (7a ). Crystals of 7a ‚
CH2Cl2‚1/2pentane were grown by slow diffusion of pentane
into a methylene chloride solution. A crystal was mounted in
a capillary tube. Data were collected and treated as above.
Systematic absences were consistent with either Cc or C2/c,
but the noncentrosymmetric choice (Cc) had a better combined-
figure-of-merit. The choice was borne out by a successful
solution of the structure. Of the two solvent molecules, the
methylene chloride was well behaved, while the pentane was
disordered. It was modeled by fixing the R and â C-C bond
lengths. An extinction correction was applied, and the absolute
structure parameter was refined to a value of 0.334. The
largest residual electron density peak (1.299 e/Å3) was associ-
ated with the pentane of solvation. The crystal data and
refinement parameters are listed in Table 1. Selected inter-
atomic distances and angles are listed in Table 3.
2
CO syn to Pt].
[P tRu HCl(CO)(µ-CO)(µ-d p p m )2] (6). [Pt(dppm)2]Cl2 (100
mg, 0.096 mmol) was suspended in THF (50 mL). [PPN][HRu-
(CO)4] (72 mg, 0.096 mmol) was dissolved in THF (20 mL) and
added dropwise to the suspension over 30 min, resulting in a
bright yellow solution and a white precipitate. The solvent was
removed in vacuo, and the residue was extracted into THF (2
mL) and filtered. The solvent was removed from the filtrate
in vacuo, and the residue was recrystallized from THF/Et2O/
pentane and dried in vacuo. Yield: 111 mg. The sample
contained small amounts of a second compound, which was
shown to be [PtRu(Cl)2(CO)2(µ-dppm)2] (7). IR: ν(CO) ) 1880
sb, 1770 mb. NMR in CD2Cl2: δ(1H) ) -5.45 [tt, 1H, 2J (HPtP)
3
1
) 16 Hz, J (HPtRuP) ) 3 Hz, J (HPt) ) 1050 Hz, PtH]; 4.23
(m, 1H, CH2); 3.88 [m, 1H, 3J (PtH) ) 70 Hz, CH2]. δ(31P) )
38.0 [m, RuP]; 17.8 [m, 1J (PtP) ) 2940 Hz, PtP]. δ(13C) ) 207.7
[m, 1C]; 225.1 [m, 1C].
Ack n ow led gm en t. We thank the NSERC (Canada)
[P tRu Cl2(CO)2(d p p m )2] (7). PPN[HRu(CO)4] (72 mg, 0.096
mmol) was dissolved in CH2Cl2 (15 mL), and the solution was
cooled to -80 °C. This solution was then added to [Pt(dppm)2]-
[Cl2] (100 mg, 0.096 mmol) in CH2Cl2 (15 mL) also at -80 °C,
resulting in the formation of an orange solution. The solution
was allowed to gradually warm to room temperature and then
stirred for 4 days. The solvent was then removed in vacuo,
and the PRF for financial support.
Su p p or tin g In for m a tion Ava ila ble: Table of X-ray data
in CIF format are available on the web only. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM990032Z