Carbon-carbon bond formation by cumulene insertion into "Rh(μ-CH2)M" moieties (M = Ru, Os): Roles of the cumulenes and the metals in product formation

Article abstract of DOI:10.1021/om040074

The reactions of allene, methylene, and 1,1-dimethylene with [RhM(CO) ₄(μ-CH₂)-(dppm)][X] complex was studied. The goal of this study was to obtain models for C₃-bridged species, proposed as intermediates in a methylene-coupling sequence. Propanediyl-bridged complexes were observed and were also proposed as unstable intermediates in the coupling of ethylene with methylene-bridged units to give propene. The results show that it is significant that although reactivity is initiated at the Rh center in all cases, the identity of the final product can depend on the nature of the adjacent metal.

Full text of DOI:10.1021/om040074

Organometallics 2004, 23, 4759-4770
4759
Ca r bon -Ca r bon Bon d F or m a tion by Cu m u len e In ser tion
in to “Rh (µ-CH2)M” Moieties (M ) Ru , Os): Roles of th e
Cu m u len es a n d th e Meta ls in P r od u ct F or m a tion
Amala Chokshi, Bryan D. Rowsell, Steven J . Trepanier,
Michael J . Ferguson,† and Martin Cowie*
Department of Chemistry, University of Alberta Edmonton, AB, Canada T6G 2G2
Received May 26, 2004
The reactions of allene, methylallene, and 1,1-dimethylallene with [RhM(CO)4(µ-CH2)-
(dppm)2][X] (M ) Os, X ) BF4 (1); M ) Ru, X ) CF3SO3 (3); dppm ) µ-Ph2PCH2PPh2) in the
presence of 1 equiv of trimethylamine oxide yield a variety of products. Under these conditions
compounds 1 and 3 react with allene to yield the trimethylenemethane-bridged products
[RhM(CO)2(µ-η3:η1-C(CH2)3)(dppm)2][X] (M ) Os (5), Ru (6)), in which the trimethylene-
methane fragment is η1-bound to the group 8 metal by one methylene group and η3-bound
to Rh by the rest of the fragment. These compounds react with CO and PMe3 to yield [RhM-
(CO)2L(µ-η3:η1-C(CH2)3)(dppm)2][X] (L ) CO, PMe3), in which L is coordinated to Rh and
the η3:η1-binding mode of trimethylenemethane is retained. Although attack of L occurs at
Rh for the Rh/Ru species, carbonyl attack in the Rh/Os compound initially occurs at Os
with subsequent scrambling throughout the three carbonyl sites; attack of PMe3 on the Rh/
Os compound occurs at Rh. Methylallene reacts with 1 and Me3NO to give a species analogous
to 5 and 6, having the methyl substituent on one of the terminal carbons of the allylic
functionality. This methyl group is involved in an agostic interaction with Rh, which can be
displaced by CO. The tricarbonyl complex [RhOs(CO)3(µ-CH2)(dppm)2][CF3SO3] (2) reacts
with 1,1-dimethylallene at -10°C to give [RhOs(CO)3(µ-η1:η1-(CH3)2CCCH2CH2)(dppm)2]-
[CF3SO3], in which the dimethylvinyl group is adjacent to Rh, exo to the RhCCH2CH2Os
dimetallacycle. At ambient temperature this species decomposes to give 1,1-dimethyl-1,3-
butadiene. Compound 3 reacts with 1,1-dimethylallene in the presence of Me3NO to yield
[RhRu(CO)2((CH3)2CCCH2CH2CO)(dppm)2][CF3SO3] (14), in which [2+1+1] coupling of the
allene, the bridging methylene group, and a coordinated carbonyl has occurred. The dative
bond from the bridging acyl moiety of 14 to Ru is displaced in reactions with H2 and CO to
give [RhRu(CO)2(C(O)CH2CH2C(dC(CH3)2))(µ-H)2(dppm)2][CF3SO3] and [RhRu(CO)3(C(O)CH2-
CH2C(dC(CH3)2))(dppm)2][CF3SO3], respectively. A rationalization of this chemistry is
presented.
In tr od u ction
surface-bound alkenyl group, in the proposal by Basset2a
and later by Dry2b it is proposed to be a surface-bound
olefin, whereas others have suggested the involvement
of surface-bound vinylidenes.3 A recent study from our
laboratory, in which diazomethane-generated methylene
groups couple to yield either allyl or butanediyl frag-
ments at a Rh/Os core,4 appears consistent with the
proposal of Basset and Dry, at least in the early stages
of methylene coupling. In our study4 the stepwise
transformations of a bridging methylene group to a
bridging ethylene to a bridging propanediyl group were
proposed before divergence to the allyl or butanediyl
products occurred, either by â-hydrogen elimination
from the C3H6-bridged intermediate or insertion of a
fourth methylene group, respectively. The propanediyl-
bridged intermediate is pivotal to this scheme of meth-
ylene coupling, observed in the Rh/Os system,4 as well
as to the Basset/Dry scheme, so we set out to learn more
The coupling of unsaturated C2 fragments with surface-
bound methylene groups has been suggested as an
important part of the chain-propagating steps of the
Fischer-Tropsch reaction.1-3 In the proposal by Maitlis
and co-workers1 this C2 fragment is suggested to be a
* Corresponding author. E-mail: martin.cowie@ualberta.ca.
† X-ray Crystallography Laboratory.
(1) (a) Turner, M. L.; Byers, P. K.; Long, H. C.; Maitlis, P. M. J .
Am. Chem. Soc. 1993, 115, 4417. (b) Turner, M. L.; Long, H. C.;
Stenton, A.; Byers, P. K.; Maitlis, P. M. Chem. Eur. J . 1995, 1, 549. (c)
Maitlis, P. M.; Long, H. C.; Quyoum, R.; Turner, M. L.; Wang, Z.-Q.
Chem. Commun. 1996, 1. (d) Long, H. C.; Turner, M. L.; Fornasiero,
P.; Kaspar, J .; Graziani, M.; Maitlis, P. M. J . Catal. 1997, 167, 172.
(e) Quyoum, R.; Berdini, V.; Turner, M. L.; Long, H. C.; Maitlis, P. M.
J . Catal. 1998, 173, 355. (f) Maitlis, P. M.; Quyoum, R.; Long, H. C.;
Turner, M. L. Appl. Catal. A: Gen. 1999, 186, 363. (g) Turner, M. L.;
Marsih, N.; Mann, B. E.; Quyoum, R.; Long, H. C.; Maitlis, P. M. J .
Am. Chem. Soc. 2002, 124, 10456.
(2) (a) Hugues, R.; Besson, B.; Bussie`re, P.; Dalmon, J . A.; Basset,
J . M.; Olivier, D. Nouv. J . Chim. 1981, 5, 207. (b) Dry, M. E. Appl.
Catal. A: Gen. 1996, 138, 319.
(3) See for example: (a) McCandlish, L. E. J . Catal. 1983, 83, 362.
(b) Gibson, V. C.; Parkin, G.; Bercaw, J . E. Organometallics 1991, 10,
220.
(4) (a) Trepanier, S. J .; Sterenberg, B. T.; McDonald, R.; Cowie, M.
J . Am. Chem. Soc. 1999, 121, 2613. (b) Trepanier, S. J .; Dennett, J . N.
L.; Sterenberg, B. T.; McDonald, R.; Cowie, M. J . Am. Chem. Soc. 2004,
126, 8046.
10.1021/om040074+ CCC: $27.50 © 2004 American Chemical Society
Publication on Web 09/01/2004

4760 Organometallics, Vol. 23, No. 20, 2004
Chokshi et al.
Sch em e 1
bridged complexes of Rh/Os and Rh/Ru in attempts to
generate models for the above-mentioned propanediyl-
bridged intermediates. By investigating this series of
cumulenes and these two metal combinations we hoped
to gain information about the functions of the cumulene
substituents and the different metals in the carbon-
carbon bond formation leading to C3-bridged complexes.
Exp er im en ta l Section
Gen er a l Com m en ts. All solvents were dried using the
appropriate desiccants, distilled before use, and stored under
argon. Reactions were carried out under an argon atmosphere
using standard Schlenk techniques. Allene gas was purchased
from Praxair, while methylallene and dimethylallene were
purchased from Fluka Chemicals. The trimethylphosphine was
purchased from Aldrich as a 1.0 M solution in toluene. The
13CO (99% enrichment) was purchased from Isotech, Inc. The
complexes [RhOs(CO)x(µ-CH2)(dppm)2][BF4] (x ) 4 (1), 3 (2))4
and [RhRu(CO)x(µ-CH2)(dppm)2][CF3SO3] (x ) 4 (3), 3 (4))10
(dppm ) µ-Ph2PCH2PPh2) were prepared as previously re-
ported.
The H, 13C{1H}, H-1H COSY, and 31P{1H} NMR spectra
were recorded on a Varian iNova-400 spectrometer operating
at 399.8 MHz for 1H, 161.8 MHz for 31P, and 100.6 MHz for
13C{1H}. Infrared spectra were obtained on a Nicolet Magna
750 FTIR spectrometer with a NIC-Plan IR microscope either
in the solid state or as a solution. The elemental analyses were
performed by the microanalytical service within the depart-
ment. Electrospray ionization mass spectra were run on a
Micromass ZabSpec spectrometer by the staff in the mass
spectrometry service laboratory. In all cases, the distribution
of isotope peaks for the appropriate parent ion matched that
calculated for the formula given. Spectroscopic data for all
compounds are given in Table 1.
P r ep a r a tion of Com p ou n d s. (a ) [Rh Os(CO)2(µ-η3:η1-
C(CH2)3)(d p p m )2][BF 4] (5). [RhOs(CO)4(µ-CH2)(dppm)2][BF4]
(1) (50 mg, 0.039 mmol) was dissolved in 15 mL of CH2Cl2.
Allene was passed through this solution at a rate of 2 mL/
min for 2 min. At this stage, no reaction was observed.
Addition of 5 mL of a CH2Cl2 solution of Me3NO (3.5 mg, 1.2
equiv) via cannula to the solution of 1 and allene resulted in
a color change from yellow to dark red. The solution was
stirred for 30 min and filtered, and a red-orange solid was
isolated by the slow addition of 10 mL of ether followed by 20
mL of pentane. The isolated solid was washed with ether and
dried under a stream of argon (yield 87%). Anal. Calcd for
about such species. Propanediyl-bridged complexes have
been observed5 and are also proposed as unstable
intermediates in the coupling of ethylene with methyl-
ene-bridged units to give propene.6 The probable mech-
anism for propene formation in the latter example is
via a sequence of â-hydrogen-elimination and reductive-
elimination steps, the facility of which also explains the
paucity of propanediyl-bridged species.
Attempts by us to generate propanediyl-bridged prod-
ucts through ethylene insertion into a series of “Rh(µ-
CH2)M”-containing species (M ) Ru, Os) were unsuc-
cessful, leading either to no reaction or to propene
formation.4,7 We therefore sought to model these C3-
bridged species by insertions of other unsaturated
substrates such as alkynes or cumulenes. Although
alkyne insertions into the Rh(µ-CH2)M-based complexes
worked well to give model C3-bridged products,8,9 these
products differed from the propanediyl-bridged targets
in having unsaturation within the C3 framework. We
reasoned that cumulenes could yield insertion products
as diagrammed in Scheme 1, in which the unsaturation
was exo to the C3 fragment, possibly leading to better
models of the propanediyl-bridged targets.
For cumulenes having substitution at only one end,
coordination to Rh (the site of unsaturation) should be
favored at the unsubstituted end, leading to two product
types, in which insertion into the Rh-CH2 bond has
occurred to give a vinylic group adjacent to Rh (structure
B) or having an allylic group in a symmetric position
between the two methylene groups (structure D). Allene,
having no substituents, can give rise to two analogous
products. In product D the absence of â-hydrogens
eliminates the decomposition route proposed for pro-
panediyl-bridged species.6
1
1
C
56H50F4O2P4RhOsB: C, 51.78; H, 3.89. Found: C, 52.20; H,
3.75 MS: m/z 1173 (M+ - BF4).
(b) [Rh Ru (CO)2(µ-η3:η1-C(CH2)3)(d p p m )2][CF 3SO3] (6).
A Schlenk flask was charged with the compound [RhRu(CO)4-
(µ-CH2)(dppm)2][CF3SO3] (3) (85 mg, 0.068 mmol) and Me3-
NO (8.70 mg, 0.116 mmol, 1.7 equiv). The flask was cooled to
-8 °C using a saltwater/ice bath, evacuated, then back-filled
to ca. 1 atm with allene. Then 5 mL of acetone was added to
the flask, causing the solution color to change immediately
from red to yellow-orange. The ice bath was removed, and the
solution was stirred for 1 h, after which it was filtered over
Celite. The filtrate was concentrated to ca. 2 mL, and subse-
quent dropwise addition of Et2O caused the formation of an
orange solid. The supernatant was removed, and the solid was
washed with 2 × 15 mL of Et2O and dried in vacuo (yield: 54
mg, 65%). Anal. Calcd for C57H50F3O5P4RhRuS: C, 55.52; H,
4.09. Found: C, 55.69; H, 4.09.
In this paper we report the reactions of allene,
methylallene, and 1,1-dimethylallene with methylene-
(5) (a) Theopold, K. H.; Bergman, R. G. J . Am. Chem. Soc. 1980,
102, 5964. (b) Howard, T. R.; Lee, J . B.; Grubbs, R. H. J . Am. Chem.
Soc. 1980, 102, 6876. (c) Lee, J . B.; Gajola, G. J .; Schaefer, W. P.;
Howard, T. R.; Ikariya, T.; Straus, D. A.; Grubbs, R. H. J . Am. Chem.
Soc. 1981, 103, 7358. (d) Motyl, K. M.; Norton, J . R.; Schauer, C. K.;
Anderson, O. P. J . Am. Chem. Soc. 1982, 2, 914.
(6) (a) Sumner, C. E., J r.; Riley, P. E.; Davis, R. E.; Pettit, R. J .
Am. Chem. Soc. 1980, 102, 1752. (b) Theopold, K. H.; Bergman, R. G.
J . Am. Chem. Soc. 1981, 103, 2489. (c) Kao, S. C.; Thiel, C. H.; Pettit,
R. Organometallics 1983, 2, 914.
(c) [Rh Os(CO)3(µ-η3:η1-C(CH2)3)(d p p m )2][BF 4] (7). Com-
pound 5 (30 mg, 0.026 mmol) was dissolved in 10 mL of CH2-
Cl2, and carbon monoxide was passed over the solution for 5
(7) Rowsell, B. D.; Cowie, M. Unpublished results.
(8) Rowsell, B. D.; McDonald, R.; Ferguson, M. J .; Cowie, M.
Organometallics 2003, 22, 2944.
(9) (a) Chokshi, A.; Graham, T. W.; Wigginton, J . R.; McDonald, R.;
Ferguson, M. J .; Cowie, M. Manuscript in preparation. (b) Chokshi,
A. MSc Thesis, University of Alberta, 2004.
(10) Rowsell, B. D.; Trepanier, S. J .; Lam, R.; McDonald, R.; Cowie,
M. Organometallics 2002. 21, 3228.

C-C Bond Formation by Cumulene Insertion
Organometallics, Vol. 23, No. 20, 2004 4761
Ta ble 1. Sp ectr oscop ic Da ta for Com p ou n d s
NMRd,e
a-c
compound
IR (cm-1
)
31P{1H} (ppm)f
1H (ppm)g,h
13C{1H} (ppm)h,i
2
[RhOs(CO)2(µ-η3:η1-C(CH2)3)-
(dppm)2][BF4] (5)
2003 (s), 1929 (s)
40.6 (dm,
4.39 (m, 2H, dppm), 4.10
181.6 (t, J PC ) 8 Hz,
2
1J RhP ) 152
Hz), -1.4 (m)
(s, 2H), 4.01 (m, 2H, dppm),
1C), 172.4 (t, J PC
)
3
2
1.95 (d, J PH ) 8 Hz, 2H),
7 Hz, 1C), 73.5 (t, J PC
) 7 Hz), 7.27 (s, br)
194.5 (m, 1C), 191.7
(m, 1C), 13.8 (s, 1C)
3
1.79 (t, J PH ) 12 Hz, 2H)
[RhRu(CO)2(µ-η3:η1-C(CH2)3)-
(dppm)2][CF3SO3] (6)
2021 (m), 1951 (s)
30.9 (om)
4.30 (m, 2H, dppm), 3.90
(m, 2H, dppm), 4.02 (s, 2H),
3
1.98 (d, J PH ) 8 Hz, 2H),
3
1.49 (t, J PH ) 8 Hz, 2H)
1
[RhOs(CO)3(µ-η3:η1-C(CH2)3)-
(dppm)2] [BF4] (7)
13.8 (dm,
4.74 (m, 2H, dppm), 4.31
194.7(dt, J RhC ) 60
1J RhP ) 137
Hz), -8.1 (m)
(m, 2H, dppm), 2.63 (d, J PH
Hz, 1C), 185.9 (m,
1C), 177.3 (m, 1C)
3
) 9 Hz, 2H), 2.03 (s, 2H), 1.69
3
(t, J PH ) 9 Hz, 2H)
[RhRu(CO)3(µ-η3:η1-C(CH2)3)-
(dppm)2][CF3SO3] (8)
2010 (m), 1980 (m),
1935 (m)
30.4 (m),
13.2 (dm)
4.16 (m, 2H, dppm), 4.06
205.2 (m, 1C), 198.7
(m, 1C), 184.2 (dm,
1J RhC ) 57 Hz, 1C)
3
(m, 2H, dppm), 2.64 (d, J PH
) 9 Hz, 2H), 1.79 (s, 2H),
3
1.52 (t, J PH ) 9 Hz, 2H)
[RhOs(CO)2(PMe3)(µ-η3:η1-
C(CH2)3)(dppm)2][BF4] (9)
14.3 (dm,
4.87 (m, 2H, dppm), 4.55
(m, 2H, dppm), 3.21 (s,
1J RhP ) 140
Hz), -13.7
(m), -49.8 (d,
1J RhP ) 104
Hz)
3
2H), 2.07 (t, J PH ) 11 Hz,
3
2H), 1.87 (d, J PH ) 8 Hz,
2
2H), 1.76 (d, J PH ) 13 Hz,
9H)
[RhRu(CO)2(PMe3)(µ-η3:η1-
C(CH2)3)(dppm)2][CF3SO3] (10)
26.3 (m), 13.3
4.32 (m, 2H, dppm), 4.12
(m, 2H, dppm), 1.86 (s, br,
2H), 1.73 (m, br, 2H), 1.50
(dm), -34.6 (dm)j
2
(s, br, 2H), 0.39 (d, J PH
) 7 Hz, 9H)j
[RhOs(CO)2(µ-η3:η1-CH(CH3)-
C(CH2)2)(dppm)2][BF4] (11)
2001 (s), 1923 (s)
42.8 (m), 36.9
(m), -0.1 (m),
-7.2 (m)
4.41 (om, 3H, dppm; 1H,
CHCH3), 3.9 (m, 1H, dppm),
3.61 (s, 1H, C(CHH)CHCH3),
2.12 (d, 1H, J PH ) 8 Hz
CHHCHCH3), 2.01 (m, 1H,
1J CH ) 130 Hz, Os-CH2),
181.2 (m, 1C), 172.7
(m, 1C), 81.9 (dd, 1C,
1J RhC ) 29 Hz, J PC
)
6 Hz, CHCH3), 64.6
1
(dd, 1C, J RhC ) 32
Hz, J PC ) 7 Hz,
C(CH2)CHCH3),
15.6 (s, 1C, CH3),
9.52 (s, 1C,
1
1.90 (m, 1H, J CH ) 130 Hz,
Os-CH2), -0.22 (dd, 3H,
4J PH ) 6 Hz, CHCH3,
3J HH ) 6 Hz)
Os-CH2)
184.9 (m), 183.9 (m),
177.1(m), -4.1 (s),
[RhOs(CO)3(µ-η3:η1-CH(CH3)-
C(CH2)2)(dppm)2][BF4] (12)
11.3 (om),
-3.2 (m),
-16.2 (m)
5.35 (m, 1H, dppm), 4.82 (m,
1H, dppm), 4.59 (m, 1H,
dppm), 4.21 (m, 1H,
CHCH3), 4.10 (m, 1H,
dppm), 1.35 (m, 3H, CHCH3)
4.90 (m, dppm, 2H), 4.07 (m,
dppm, 2H), 2.41 (s, br, CH2,
2H), 1.63 (t, br, Os-CH2,
2H), 0.78 (t, J ) 6 Hz, CH3,
3H), 0.10 (s, br, CH3, 3H)
1
[RhOs(CO)3(µ-η1:η1-(CH3)2C-
CH2CH2)(dppm)2][CF3SO3] (13)
24.3 (dm,
191.4 (dt, J RhC ) 48
2
1J RhP ) 121
Hz), -3.2 (m)
Hz, J PC ) 11 Hz),
182.5 (s, br, Os(CO)),
172.8 (t, J PC ) 8
Hz, Os(CO)),
2
2
2.69 (t, J PC ) 8 Hz)
1
[RhRu(CO)2((CH3)2CCH2-
CH2CO)(dppm)2][CF3SO3] (14)
1975 (m), 1910 (s)
35.5 (m),
20.4 (dm)
3.58 (m, 2H, dppm), 3.41
(m, 2H, dppm), 2.73 (m, 2H),
1.31 (s, 3H), 0.96 (s, 3H),
0.37 (m, 2H)
3.94 (m, 2H, dppm), 3.60
(m, 2H, dppm), 1.99 (s, 3H),
1.73 (m, 2H), 1.16 (s, 3H),
0.93 (m, 2H), -7.89
250.7 (dt, J RhC ) 33
2
Hz, J PC ) 8 Hz, 1C)
220.2 (m, 1C), 202.4 (t,
2J PC ) 23 Hz, 1C)
1
[RhRu(CO)2(µ-H)2(η1:η1-
C(O)CH2CH2C(dC(CH3)2)-
(dppm)2][CF3SO3] (15)
2053 (m), 2001 (s),
1683 (w)
31.9 (m),
22.3 (dm)
236.3 (dt, J RhC ) 28
2
Hz, J PC ) 7 Hz, 1C),
196.8 (m, 1C), 190.7
2
(t, J PC ) 11 Hz, 1C)
(m, 1H), -10.01 (m, 1H)
4.40 (m, 2H, dppm), 3.99
(m, 2H, dppm), 1.98 (t,
2H), 1.69 (s, 3H), 1.17
(s, 3H), 1.08 (br, 2H)
1
[RhRu(CO)3(η1:η1-C(O)-
CH2CH2C(dC(CH3)2)-
(dppm)2][CF3SO3] (16)
2025 (m), 1975 (m),
1951 (m), 1687 (w)
31.9 (m),
17.9 (dm)
231.9 (dt, J RhC ) 30
Hz,2J PC ) 8 Hz, 1C),
207.6 (m, 1C), 206.4
(m, 1C), 195.6 (t, J PC
2
) 12 Hz, 1C)
a
b
IR abbreviations: s ) strong, m ) medium, w ) weak, sh ) shoulder. CH2Cl2 solutions unless otherwise stated, in units of cm-1
.
c Carbonyl stretches unless otherwise noted. NMR abbreviations: s ) singlet, d ) doublet, t ) triplet, m ) multiplet, dm ) doublet of
d
multiplets, om ) overlapping multiplets, br ) broad, dt ) doublet of triplets. e NMR data at 25 °C for Rh/Ru compounds and at -80 °C
for Rh/Os compounds in CD2Cl2 unless otherwise stated. f 31P chemical shifts referenced to external 85% H3PO4. Chemical shifts for the
g
phenyl hydrogens are not given. 1H and 13C chemical shifts referenced to TMS. i 13C{1H} NMR performed with 13C enrichment of both
h
j
carbonyls and methylene groups. NMR data at -60 °C.
min at a rate of 1 mL/min. The solution slowly turned from
red-orange to yellow. The solution was stirred for 30 min and
filtered, and then a yellow solid was precipitated by the slow
addition of 10 mL of ether and 20 mL of pentane. The solid
was washed with ether and dried under a stream of argon
(yield 78%). Satisfactory elemental analyses for this compound
could not be obtained due to facile CO loss. Characterization
was based on spectroscopic studies.

4762 Organometallics, Vol. 23, No. 20, 2004
Chokshi et al.
(d ) [Rh Ru (CO)3(µ-η3:η1-C(CH2)3)(d p p m )2][CF 3SO3] (8).
Compound 6 (100 mg, 0.081 mmol) was dissolved in 4 mL of
CH2Cl2, and carbon monoxide gas was passed over the solution
for ca. 5 min. The solution slowly turned from yellow-orange
to orange. After 0.5 h of stirring, the solution was concentrated
to ca. 3 mL using a stream of Ar gas. Slow addition of 15 mL
of Et2O resulted in the precipitation of orange microcrystals.
The supernatant was removed, and the crystals were dried in
vacuo (yield: 80 mg, 78%). Anal. Calcd for C58H50F3O6P4-
RhRuS: C, 55.24; H, 4.00. Found: C, 55.23; H, 3.87.
(e) [Rh Os(CO)2(P Me3)(µ-η3:η1-C(CH2)3)(dppm )2][BF4] (9).
Compound 5 (30 mg, 0.026 mmol) was dissolved in 0.7 mL of
CD2Cl2 and cooled to -78 °C. An excess of 1.0 M trimeth-
ylphosphine in toluene (40 µL, 0.040 mmol, 1.6 equiv) was
added at this temperature. Although no color change was
observed, NMR characterization at -80 °C revealed complete
conversion to a new product. This product was stable up to
-20 °C. However at higher temperatures, facile PMe3 loss
occurred, necessitating its characterization at low temperature
using NMR spectroscopy.
(f) [Rh Ru (CO)2(P Me3)(µ-η3:η1-C(CH2)3)(dppm )2][CF3SO3]
(10). Compound 6 (10 mg, 0.008 mmol) was dissolved in 0.7
mL of CD2Cl2 and cooled to -78 °C. To this was added 9 µL of
a 1.0 M solution of PMe3 in THF (0.009 mmol, 1.1 equiv),
causing no noticeable color change. However, monitoring the
solution at -80 °C by NMR spectroscopy showed essentially
quantitative conversion into a new compound, 10. This was
the only product formed between -80 and -40 °C; however,
at temperatures higher than -40 °C this product began to
decompose into several unidentified products. As a result,
characterization of 10 was achieved by 31P{1H} and 1H NMR
spectroscopy at the lower temperatures.
spectrum of this solution showed only compound 11 and
unreacted PMe3
ent.
, between temperatures of -80 °C and ambi-
(k ) Rea ction of 1 w ith (CH3)2CdCdCH2. To an NMR tube
containing [RhOs(CO)4(µ-CH2)(dppm)2][BF4] (1) (10 mg, 0.008
mmol) dissolved in 0.5 mL of CD2Cl2, was added Me2CdCd
CH2 (4 µL, 0.04 mmol). After approximately 12 h, a small
amount of the compound [RhOs(CO)4(dppm)2][BF4]11 was
observed by NMR spectroscopy. After 3 days all of 1 had
disappeared, leaving [RhOs(CO)4(dppm)2][BF4] as the only
phosphorus-containing product observed. 1,1-Dimethyl-1,3-
butadiene was also identified on the basis of its 1H NMR
spectrum.
(l) [Rh Os(CO)3(µ-η1:η1-((CH3)2CCCH2CH2)(d p p m )2][CF 3-
SO3] (13). The tricarbonyl complex [RhOs(CO)3(µ-CH2)(dppm)2]-
[CF3SO3] (2) (20 mg, 0.015 mmol) was dissolved in 0.7 mL of
CD2Cl2 and cooled to -10 °C. To the cooled solution, 1.8 µL
(1.2 equiv) of dimethylallene was added and stirred for 5 min.
The resultant dark red solution could be characterized only
by NMR spectroscopy, as the compound rapidly decomposed
at ambient temperature. Compound 13 could also be generated
-
as the BF4 salt from the tetracarbonyl (1) in the presence of
Me3NO; however the route described above gave a sample
having a cleaner 1H NMR spectrum without the presence of
N(CH3)3 and Me3NO.
(m ) [Rh Ru (CO)2((CH3)2CCCH2CH2CO)(dppm )2][CF3SO3]‚
0.5CH2Cl2 (14). A Schlenk flask was charged with 115 mg
(0.092 mmol) of [RhRu(CO)4(µ-CH2)(dppm)2][CF3SO3] (3), 9.5
µL of dimethylallene (0.096 mmol, 1.04 equiv), and 6 mL of
acetone. The solution then was cooled to -10 °C, and 6 mL of
a 0.025 M solution of Me3NO in acetone (0.150 mmol, 1.7 equiv)
was added dropwise, causing the immediate color change from
yellow to dark red to orange. The solution was stirred for 1 h,
after which it was warmed to ambient temperature and then
filtered over Celite. The solvent was removed in vacuo, and
the orange residue was dissolved in 2 mL of CH2Cl2. Orange
microcrystals precipitated after dropwise addition of 17 mL
of Et2O. The supernatant was removed, and the crystals were
(g) Attem p ted Rea ction s of 5 a n d 6 w ith CH2N2. In
separate experiments compounds 5 and 6 (20 mg) were each
dissolved in 0.7 mL of CD2Cl2 and cooled to -78 °C. An excess
of diazomethane was passed through the solution at a rate of
10 mL/min for 2 min. The reaction was stirred for 20 min at
low temperature. In both cases the NMR spectra of the
solutions at -80°C showed a mixture of unidentifiable prod-
ucts. The same results were obtained if compounds 5 and 6
were reacted with diazomethane at ambient temperature.
dried in vacuo (yield: 70 mg, 57%). Anal. Calcd for C60.5H55
-
ClF3O6P4RhRuS: C, 54.62; H, 4.17; Cl, 2.66. Found: C, 54.80;
H, 4.16; Cl, 2.16.
(n )
[R h R u (CO)2(C(O)CH 2CH 2C(dC(CH 3)2))(µ-H )2-
(h ) [Rh Os(CO)2(µ-η3:η1-CH(CH3)C(CH2)2)(d p p m )2][BF 4]
(11). Compound 1 (30 mg, 0.024 mmol) was dissolved in 10
mL of CH2Cl2, and methylallene was slowly passed through
the solution at a rate of 0.5 mL/min. No reaction was observed.
A 2 mL solution of Me3NO (2.1 mg, 1.2 equiv) was added via
cannula to the solution containing both compound 1 and
methylallene. The resultant orange solution was stirred for
30 min and then filtered. An orange solid was precipitated by
the slow addition of 15 mL of pentane, and the isolated solid
was washed with 20 mL of ether and dried under a stream of
argon (yield 86%). Anal. Calcd for C57H50F4O2P4RhOsB: C,
52.17; H, 3.93. Found: C, 51.88, H, 3.90. MS: m/z 1187 (M+
- BF4).
(i) [Rh Os(CO)3(µ-η3:η1-CH(CH3)C(CH2)2)(d p p m )2][BF 4]
(12). Compound 11 (30 mg, 0.025 mmol) was dissolved in 10
mL of CH2Cl2, and carbon monoxide was passed through the
solution at a rate of 2 mL/min for 2 min. This caused an
immediate color change from orange to yellow. The solution
was stirred for 20 min and precipitated by the slow addition
of 20 mL of ether followed by 10 mL of pentane, filtered, and
dried under a stream of argon. This compound was prone to
facile CO loss, and thus it was characterized using NMR
techniques.
(d p p m )2][CF 3SO3] (15). Compound 14 (67 mg, 0.052 mmol)
was dissolved in 5 mL of CH2Cl2. Hydrogen gas was passed
over the solution for ca. 5 min, after which no noticeable color
change occurred. The solution was stirred under H2 for 1 h,
the solvent was removed in vacuo, and the orange residue was
dissolved in 2 mL of CH2Cl2. Slow addition of 10 mL of Et2O
caused formation of a dark orange solid. The supernatant was
removed, and the solid was washed with 2 × 10 mL of Et2O
and dried in vacuo (yield: 52 mg, 77%). Anal. Calcd for
C
60H56F3O6P4RhRuS: C, 55.86; H, 4.38. Found: C, 55.23; H,
4.29.
(o) [Rh Ru (CO)3(C(O)CH2CH2C(dC(CH3)2))(dppm )2][CF3-
SO3] (16). A Schlenk flask was charged with 15 mg (0.011
mmol) of [RhRu(CO)2(C(O)CH2CH2C(dC(CH3)2))(dppm)2][CF3-
SO3] (14) and ca. 1 atm of CO. To this, 10 mL of CH2Cl2 was
added, causing the formation of a yellow solution. The mixture
was stirred for 30 min, after which it was concentrated via an
Ar stream to 2 mL. Dropwise addition of 25 mL of Et2O
afforded a dark yellow solid, which was washed with 2 × 10
mL of Et2O after the supernatant was removed. Satisfactory
elemental analysis could not be obtained for 16, perhaps owing
to the air-sensitive nature of this compound.
X-r a y Da ta Collection . Orange crystals of [RhRu(CO)2-
((CH3)2CCCH2CH2CO)(dppm)2][CF3SO3]‚(CH3)2CO (14) were
obtained by slow diffusion of diethyl ether into an acetone
solution of the compound. Data were collected on a Bruker
(j) Attem p ted Rea ction of 11 w ith P Me3. An NMR tube
charged with compound 11 (15 mg, 0.013 mmol) was dissolved
in 0.7 mL of CD2Cl2 and cooled to -78 °C. A 25 µL portion of
PMe3 (0.025 mmol, 1.25 equiv) was added to the solution via
syringe. No color change was observed, and the 31P {1H} NMR
(11) Hilts, R. W.; Franchuk, R. A.; Cowie, M. Organometallics 1991,
10, 304.

C-C Bond Formation by Cumulene Insertion
Organometallics, Vol. 23, No. 20, 2004 4763
Ta ble 2. Cr ysta llogr a p h ic Da ta for Com p ou n d 14
Sch em e 2
[RhRu(CO)2((CH3)2CCCH2CH2CO)-
(dppm)2][CF3SO3]‚(CH3)2CO (14)
C63H60F3O7P4RhRuS
formula
fw
1346.03
cryst dimens, mm
0.17 × 0.16 × 0.14
cryst syst
space group
a, Å
triclinic
P1h (No. 2)
13.228(1)a
15.403(2)
b, Å
c, Å
15.544(2)
R deg
â, deg
γ, deg
V, Å3
75.761(2)
74.776(2)
72.884(2)
2871.1(5)
Z
d
2
calcd, g cm-3
1.557
0.762
µ, mm-1
radiation (λ, Å)
graphite-monochromated
Mo KR (0.71073)
-80
ω scans (0.2°)
(25 s exposures)
52.70
T, °C
scan type
completed using the HKLF 5 reflection file (containing all the
reflection data: exactly overlapped, partially overlapped, and
nonoverlapped) and the program SHELXL-93.15 Hydrogen
atoms were assigned positions on the basis of the geometries
of their attached carbon atoms and were given thermal
parameters 20% greater than those of the attached carbons.
The final model for 14 refined to values of R1(F) ) 0.0560 (for
2θ (max), deg
no. of unique reflns
no of observns
11 377 (Rint ) 0.053)
11 148 [Fo g 2σ( Fo2)]
2
range of transmn factors
no. of data/restraints/
params
0.9008-0.8813
18 118 [Fo g -3σ( Fo2)]/0/724
2
11 148 data with Fo g 2σ(Fo2)) and wR2(F2) ) 0.1377 (for all
2
residual density, e/Å3
1.104 to -1.353
0.0560
R1(Fo > 2σ (Fo2))b
2
18 118 data).
wR2 [Fo g -3σ(Fo2)]b
0.1377
2
2
GOF (S)c
0.953 [Fo g -3σ(Fo2)]
Resu lts a n d Com p ou n d Ch a r a cter iza tion
a
Cell parameters obtained from least-squares refinement of
3998 centered reflections. b R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑w(Fo
2
Attempts to react a series of cumulenes with the
methylene-bridged tetracarbonyl complexes [RhM(CO)4-
(µ-CH2)(dppm)2][X] (M ) Os, X ) BF4 (1); M ) Ru, X )
CF3SO3 (3)) failed except in the case of 1 and dimethyl-
allene, which react over a period of several days at
ambient temperature, as described later. All other
combinations of compounds 1 and 3 with allene, meth-
ylallene, and 1,1-dimethylallene gave no reaction after
several days under the same conditions. However, all
of these cumulenes react instantly with the above
complexes in the presence of 1 equiv of trimethylamine
oxide. Previous studies have shown that the reactions
of 1 and 3 with Me3NO yield the respective tricarbonyl
complexes [RhM(CO)3(µ-CH2)(dppm)2][X] (M ) Os (2),
Ru (4)),4,10 which are presumably the species reacting
with the cumulenes. However it usually proved most
convenient and reproducible to carry out these reactions
by generating the tricarbonyl compounds in situ from
their tetracarbonyl precursors in the presence of the
cumulene of interest by the addition of Me3NO. In all
schemes involving carbonyl removal from 1 and 3 the
respective tricarbonyls 2 and 4 are shown as the
precursors.
- Fc2)2/∑w(Fo4)]1/2. Refinement on Fo for all reflections (having
2
2
Fo g -3σ(Fo2). wR2 and S based on Fo2; R1 based on Fo, with Fo
set to zero for negative Fo2. The observed criterion of Fo2 > 2σ(Fo
)
2
is used only for calculating R1 and is not relevant to the choice of
reflections for refinement. c S ) [∑w(Fo - Fc2)2/(n - p)]1/2 (n )
2
number of data, p ) number of parameters varied; w ) [σ2(Fo2) +
(0.0638P)2]-1, where P ) [max(Fo2, 0) + 2Fc2]/3.
PLATFORM/SMART 1000 CCD diffractometer12 using Mo KR
radiation at -80 °C. Preliminary crystallographic experiments
indicated that the crystal used for data collection displayed
nonmerohedral twinning. Both components of the twin were
indexed with the program GEMINI13 on reflections extracted
from four sets of 200 frames. The first twin component can be
related to the second component by 180° rotation about the
[141] axis in reciprocal space and the [010] axis in direct space.
The integrated intensities for the reflections for the two
components were extracted from the frames data using the
program SAINT,12 corrected for absorption using the Gaussian
integration (face-indexed) method, and written into a single
SHELXL-93 HKLF 5 reflection file with the program GEMINI
using all reflection data (exactly overlapped, partially over-
lapped, and nonoverlapped). Unit cell parameters were ob-
tained from a least-squares refinement of the setting angles
of 3998 reflections from the data collection. The space group
was determined to be P1h [No. 2]. Data were corrected for
absorption through use of the SADABS procedure. See Table
2 for a summary of crystal data and X-ray data collection
information.
(a ) Allen e Rea ction s. Under the above conditions,
compounds 2 and 4 react with allene at subambient
temperatures to yield the trimethylenemethane-bridged
products [RhM(CO)2(µ-η3:η1-C(CH2)3)(dppm)2]+ (M ) Os
(5), Ru (6)), as shown in Scheme 2. The spectral
parameters for both species are closely comparable and
in agreement with the formulation shown, in which one
methylene group of the trimethylenemethane unit is η1-
Str u ctu r e Solu tion a n d Refin em en t. The structure for
compound 14 was solved by automated Patterson location of
the heavy metal atoms and structure expansion via the
DIRDIF-9914 program system, using a partial HKLF reflection
file containing only the nonoverlapping reflections from the
major component of the twinned crystal. Refinement was
(14) Beurskens, P. T.; Beurskens, G.; de Gelder, R.; Garcia-Granda,
S.; Israel, R.; Gould, R. O.; Smits, J . M. M. DIRDIF-99 program system;
Crystallography Laboratory, University of Nijmegen: The Nether-
lands, 1999.
(15) Sheldrick, G. M. SHELXL-93: Program for crystal structure
determination; University of Go¨ttingen: Go¨ttingen, Germany, 1993.
(12) Programs for diffractometer operation, data reduction, and
absorption correction were those supplied by Bruker.
(13) Gemini Twinning Solution Program Suite, Version 1.0; Bruker-
AXS: Madison, WI, 1999.

4764 Organometallics, Vol. 23, No. 20, 2004
Chokshi et al.
Sch em e 3
bound to the group 8 metal while the rest of this unit is
η3-bound to Rh. For compound 5, as an example, the
31P{1H} NMR spectrum shows two resonances, typical
for such species in which the low-field signal displays
152 Hz coupling to Rh. In the 1H NMR spectrum, in
addition to the phenyl resonances, the dppm methylenes
appear at δ 4.39 and 4.01 and three additional signals
corresponding to the trimethylenemethane moiety ap-
pear as a triplet at δ 1.79, a doublet at δ 1.95, and a
broad singlet at δ 4.10. The high-field triplet corre-
sponds to the methylene group that is σ-bound to Os,
displaying 12 Hz coupling to the phosphorus nuclei on
this metal. The mid-field doublet corresponds to a pair
of protons on the η3-allyl portion of the trimethylene-
methane unit that are anti to the Os-bound CH2 group;
this pair displays 8 Hz coupling to one of the Rh-bound
phosphines. Finally, the broad singlet corresponds to the
syn protons of the allyl moiety. The two resonances for
the allyl functionality are very similar to those reported
for the 2-methylallyl ligand in a series of compounds,
[Rh(η3-CH2C(CH3)CH2)L2] (L2 ) disphosphine),16 in
which the syn protons appear as broad singlets at lower
field (between δ 3.20 and 4.00) and the anti protons
appear as doublets at somewhat higher field (between
δ 2.36 and 2.71), showing coupling to one of the 31P
nuclei of the diphosphine ligands.
Rh, an interesting difference in the site of ligand attack
is observed in the Rh/Os complex. The addition of PMe3
to 5 at -80 °C gives rise to a doublet for the coordinated
PMe3 group at δ -49.8 in the 31P{1H} NMR spectrum,
displaying 104 Hz coupling to Rh. The magnitude of the
Rh-P coupling clearly identifies that the added phos-
phine is bound to Rh, and the appearance of this product
as the sole species at -80 °C suggests initial attack at
this metal. Warming to ambient temperature does not
lead to rearrangement. In contrast, addition of 13CO to
5 at -80 °C shows that initial carbonyl attack occurs
at Os with the labeled 13CO appearing at δ 185.9 in the
13C{1H} NMR spectrum and displaying no coupling to
Rh. Upon warming to ambient temperature, scrambling
of the label over all carbonyl sites (δ 194.7, 185.9, and
177.3) is observed, with the low-field signal displaying
typical Rh coupling of 60 Hz.
In the case of 13CO and PMe3 addition to the Rh/Ru
analogue 6, low-temperature addition at Rh occurs in
both cases with no scrambling detected upon warming
the sample to ambient temperature. Another curious
difference in the reactivity of the Rh/Os (5) and the Rh/
Ru (6) compounds is that whereas reaction of the former
with CO and PMe3 is reversible, with the added ligand
being labile, the CO ligand is bound irreversibly in 8
while the PMe3 adduct (10) is unstable, decomposing
to unidentified products.
A sample of 5 in which the carbonyls are 13C enriched
displays two carbonyl resonances at δ 181.6 and 172.4;
selective phosphorus-decoupling experiments and the
absence of Rh coupling establishes that both carbonyls
are bound to Os. The IR spectrum of an unenriched
sample shows the carbonyl stretches at 2003 and 1929
cm-1, which together with the high-field chemical shifts
of these groups in the NMR spectrum, establishes that
both carbonyls are terminally bound.
When compound 5 is generated using a 13CH2-
enriched sample of 2, the label is found to be uniquely
situated in the site that is σ-bound to Os (at δ 7.27 in
the 13C{1H} NMR spectrum); no scrambling of this label
is observed over the other two sites of the trimethyl-
enemethane unit. The unique placement of the labeled
CH2 group adjacent to Os indicates that allene insertion
into the Rh-CH2 bond of the precursor occurs. Similarly
no scrambling was observed for the 13CH2-labeled Rh/
Ru analogue 6. Although not initially observed in the
13C NMR experiment of 5, the methylene carbons of the
allylic fragment in this species were identified at δ 73.5
in 2-D HMBC and HMQC experiments.17 Attempts to
assign the central carbon of the trimethylenemethane
unit in an HMBC experiment failed, as it did not show
the appropriate correlation.
Attempts to incorporate additional methylene groups
into the trimethylenemethane fragments by reactions
of compounds 5 and 6 with diazomethane did not yield
isolable products. Although these reactions occurred
readily, even at -80 °C, a complex mixture of products
was obtained in each case, from which none of the
species could be identified.
The addition of CO or PMe3 to compounds 5 and 6
yields the respective adducts [RhM(CO)3(µ-η3:η1-C(CH2)3)-
(dppm)2]+ (M ) Os (7), Ru (8)) and [RhM(PMe3)(CO)2-
(µ-η3:η1-C(CH2)3)(dppm)2]+ (M ) Os (9), Ru (10)). In all
(b) Meth yla llen e Rea ction s. The reaction of meth-
ylallene with 1 in the presence of Me3NO yields [RhOs-
(CO)2(µ-η3:η1-CH(CH3)C(CH2)2)(dppm)2][BF4] (11), shown
in Scheme 3. This product appears to be analogous to
the allene-insertion products 5 and 6 except having a
methyl substituent at one end of the allyl moiety.
Methyl substitution at this end lowers the symmetry
of the product, leading to the inequivalence of all four
phosphorus nuclei. As a result, four signals are seen in
the 31P{1H} NMR spectrum; the low-field pair of reso-
nances at δ 42.8 and 36.9 correspond to the 31P nuclei
bound to Rh, and those at δ -0.1 and -7.2 correspond
to the Os-bound pair.
1
cases, the H NMR spectra display the singlet, doublet,
triplet pattern for the trimethylenemethane unit, as
observed for compounds 5 and 6, suggesting that this
moiety has remained η1-bound to the group 8 metal and
η3-bound to Rh. Although all products appear analogous,
now having a terminal ligand (CO or PMe3) bound to
(16) Fryzuk, M. D. Inorg. Chem. 1982, 21, 2134.
(17) Braun, S.; Kalinowski, H.-O.; Berger, S. In 100 and More Basic
NMR Experiments: A Practical Course; VCH Publ.: New York, 1996;
Chapter 10.

C-C Bond Formation by Cumulene Insertion
Organometallics, Vol. 23, No. 20, 2004 4765
1
Sch em e 4
In the H NMR spectrum the syn protons (at δ 3.61
and 4.41) appear at low field, while the sole anti proton
(δ 2.12) is at higher field, as was observed for compounds
5-10 and in the similarly substituted [Rh(η3-allyl)L2]
series of compounds.16 Furthermore, as is also typical
of this substitution pattern, the observed syn proton
signal appears as a broad singlet, while the anti proton
is a doublet displaying typical 8 Hz coupling to one of
the Rh-bound phosphines. The signal for the syn proton
that is adjacent to the methyl substituent was found
buried beneath one of the dppm methylene signals at δ
1
4.41 through H COSY experiments. The methyl protons
appear as a doublet of doublets (4J PH ) 3J HH ) 6 Hz) at
δ -0.22. Formulation of this signal as resulting from
identical coupling to a Rh-bound phosphine and to the
adjacent proton was confirmed by selective 31P decou-
pling resulting in its collapse to a doublet with only
H-H coupling remaining. The above information indi-
cates that the methyl substituent occupies one of the
sites on the allyl moiety anti to the Os-bound methylene
group.
that reacts with one of the tetracarbonyl precursors. In
this reaction involving compound 1 the products ob-
tained after several days are [RhOs(CO)4(dppm)2][BF4]11
and 1,1-dimethyl-1,3-butadiene. No other species is
observed at intermediate times. However, the addition
of 1,1-dimethylallene to [RhOs(CO)3(µ-CH2)(dppm)2][CF3-
SO3] (2-CF 3SO3) results in an instantaneous reaction
yielding [RhOs(CO)3(µ-η1:η1-(CH3)2CCCH2CH2)(dppm)2]-
[CF3SO3] (13) as shown in Scheme 4. This product is
unstable upon workup, decomposing within hours at
ambient temperature, yielding 1,1-dimethyl-1,3-buta-
diene, [RhOs(CO)4(dppm)2][CF3SO3], and unidentified
decomposition products, which presumably are the
source of the additional carbonyl in the binuclear
product. Owing to its instability, characterization of 13
was carried out by NMR at low temperatures. In
addition to the dppm resonances in the 1H NMR
spectrum, two other methylene resonances are observed;
a broad triplet at δ 1.63, having 8 Hz coupling to the
osmium-bound phosphines, is consistent with an Os-
bound methylene group, while the other appears as a
broad singlet at δ 2.41. 31P decoupling experiments
support these assignments, with only the former signal
displaying coupling to phosphorus. 2-D proton COSY
experiments show coupling between the two adjacent
methylene groups. The methyl signals appear at δ 0.78
and 0.10; the lower-field signal is a triplet with 6 Hz
coupling to the Rh-bound phosphines, while the other
is a broad unresolved singlet. In the 13C{1H} NMR
spectrum of a 13CO-enriched sample three resonances
are observed (δ 191.4, 182.5, 172.8) with only the low-
field signal displaying Rh coupling. It is significant that
whereas the initial η1:η3-trimethylenemethane products
are dicarbonyls, having no Rh-bound carbonyl, complex
13, having the η1:η1-bound ligand, has a Rh-bound
carbonyl ligand. This additional carbonyl ligand is
required in order to give Rh a 16e configuration.
Again, the use of 13CH2-enriched compound 1 confirms
that the original metal-bound methylene group remains
bound to Os with no scrambling to the other methylene
site. In the 13C{1H} NMR spectrum this group in 11
appears as a broad unresolved signal at δ 9.52. Also
analogous to compounds 5 and 6, the two carbonyls in
11 are established as being bound to Os on the basis
that no Rh coupling is observed.
The unusually high-field chemical shift for the methyl
1
resonance in the H NMR spectrum of 11 leads us to
propose that this group is involved in an agostic interac-
tion with Rh, in which rapid exchange between the
agostic hydrogen and the other two is occurring. Facile
exchange of the three protons of a methyl group when
one of the C-H bonds is involved in an agostic interac-
tion is common.18 In this compound the agostic interac-
tion proposed would give Rh an 18e configuration. We
anticipated that this agostic interaction might be dis-
placed by another ligand. Accordingly, CO addition to
11 yields the carbonyl adduct [RhOs(CO)3(µ-η3:η1-CH-
(CH3)C(CH2)2)(dppm)2][BF4] (12), which displays one
Rh-bound carbonyl and two on Os. Consistent with the
suggestion that CO addition has displaced the agostic
1
interaction involving the methyl group, the H signal
for this group has shifted substantially downfield to δ
1.35, a value that is more characteristic of a normal
methyl group. Attempts to determine the initial site of
CO attack were unsuccessful since no reaction is seen
at low temperatures. At temperatures at which reaction
occurs, the labeled carbonyl is observed scrambled over
all three sites in the complex.
No reaction of 11 with PMe3 was observed over a
range of temperatures. Presumably, coordination of the
larger phosphine ligand at Rh is inhibited by the methyl
substituent on the allyl moiety. It is also surprising that
no product analogous to 11 was observed for the Rh/Ru
system. Reaction of 4 with methylallene over a range
of temperatures gave only mixtures of unidentified
products.
Reaction of dimethylallene with the Rh/Ru precursor
(3) in the presence of Me3NO (generating 4 in situ) gives
a very different result, yielding [RhRu(CO)2((CH3)2-
CCCH2CH2CO)(dppm)2][CF3SO3] (14), as diagrammed
in Scheme 5. In the 1H NMR spectrum two methyl
resonances (singlets) are observed at δ 1.31 and 0.96,
while four multiplets for the methylene groups are
observed at δ 3.58, 3.41, 2.73, and 0.37. The former two
are identified as dppm methylene groups by their
characteristic appearance as an AB quartet upon broad-
band 31P decoupling experiments. In the 13C{1H} NMR
spectrum of a 13CO-enriched sample one resonance
appears at δ 250.7 as a doublet of triplets with coupling
to Rh (33 Hz) and to the Rh-bound phosphorus nuclei.
(c) 1,1-Dim eth yla llen e Rea ction s. As noted earlier,
1,1-dimethylallene is the only cumulene investigated
(18) (a) Brookhart, M.; Green, M. L. H.; Wong, L. Prog. Inorg. Chem.
1988, 36, 1. (b) Brookhart, M.; Green, M. L. H. J . Organomet. Chem.
1983, 250, 395.

4766 Organometallics, Vol. 23, No. 20, 2004
Chokshi et al.
Sch em e 5
F igu r e 1. Perspective view of the complex cation of [RhRu-
(CO)2((CH3)2CCCH2CH2CO)(dppm)2][CF3SO3] (14) showing
the atom-labeling scheme. Non-hydrogen atoms are rep-
resented by Gaussian ellipsoids at the 20% probability
level. Hydrogen atoms are shown with arbitrarily small
thermal parameters. Only the ipso carbons of the dppm
phenyl rings are shown.
The low-field chemical shift of this species suggests an
acyl carbonyl group.19 Two other carbonyl resonances,
at δ 220.2 and 202.4, are assigned as a semibridging
carbonyl, having no resolved Rh coupling, and a termi-
nal Ru-bound carbonyl, respectively. The IR spectrum
shows carbonyl bands at 1975 and 1910 cm-1 consistent
with this assignment. No stretch for the acyl carbonyl
is observed. Failure to observe the carbonyl stretch for
a µ-η1:η1 acyl group, of the type diagrammed in Scheme
5, has been noted previously19 and presumably results
from some contribution by an oxycarbene resonance
structure, which shifts this stretch to lower frequency,
where it can become obscured by other bands. In one
acetyl-bridged complex, Cp2Zr(µ-CH3CO)Mo(CO)2Cp,
spectrum is also at slightly higher field than previous
examples.19
The acyl carbonyl is clearly oriented toward Ru in
such a way as to give rise to a bonding interaction with
this metal, although this distance (2.449(4) Å) is long,23
suggesting that this interaction is weak. We suggest
that the weakness of the O(3)-Ru bond arises because
of strain within the bridging acyl moiety; the Ru-O(3)-
C(3) angle (107.5(3)°) is significantly more acute than
the 120° predicted on the basis of the sp2 hybridization
of an acyl group, again being more consistent with the
oxy-carbene formulation. It is suggested that the opti-
mum overlap between the acyl oxygen and Ru cannot
be achieved without further elongation of the Rh-Ru
bond, which at 2.9833(6) Å is already unusually long;
the parameters observed are a compromise between the
conflicting needs for the Rh-Ru and Ru-O(3) bonds.
Reaction of 14 with H2 yields the dihydride product
[RhRu(η1:η1-C(O)CH2CH2CdC(CH3)2)(CO)2(µ-H)2(dppm)2]-
[CF3SO3] (15) as shown in Scheme 5. Both hydride
resonances are complex multiplets, consistent with their
bridging environments in which they couple to all four
phosphorus nuclei, Rh, and each other. Selective 31P
decoupling experiments confirm coupling of these hy-
dride signals to both sets of chemically inequivalent
phosphorus nuclei, and broadband 31P decoupling allows
observation of 10 Hz coupling of the low-field hydride
and 18 Hz coupling of the high-field hydride to Rh and
additional 8 Hz mutual coupling. 13C{1H} NMR spectra
clearly demonstrate that both terminal carbonyls are
bound to Ru, while the low-field acyl resonance (δ 236.3)
displays 28 Hz coupling to Rh, confirming that it
remains bound to this metal. Location of the hydrides,
bridging the metals, indicates that the acyl fragment
must have moved from the bridging site it occupied in
the precursor 14, so is shown in Scheme 5 bound solely
the acyl carbonyl stretch was observed at 1339 cm-1 20
.
An unambiguous structural assignment is based on
the X-ray structure determination of 14, and a repre-
sentation of the complex cation is shown in Figure 1,
with relevant bond lengths and angles appearing in
Table 3. The X-ray structure confirms that a [2+1+1]
condensation of the dimethylallene, methylene, and
carbonyl groups has occurred, yielding the metallacycle
proposed in Scheme 5. Within the five-membered me-
tallacycle the bond lengths and angles appear normal
for the formulation proposed, and the unit is quite
planar, with the largest out-of-plane deviation (0.051(7)
Å) being that of C(4). The bonding parameters involving
the acyl group are intermediate between acyl and
oxycarbene formulations. Therefore, the Rh-C(3) dis-
tance (1.915(5) Å) is short, suggesting some carbene
character in this bond, and can be compared with the
Rh-C(6) bond (2.077(5) Å) involving the vinylic group.
Despite the short “carbene-like” Rh-C(3) distance, the
C(3)-O(3) distance of 1.234(6) Å is normal for a double
bond21 and is somewhat shorter than the range observed
(1.24-1.29 Å) in related species.19a,d,e,20,22 Furthermore,
the 13C chemical shift for this acyl carbon in the NMR
(19) (a) J effrey, J . C.; Orpen, A. G.; Stone, F. G. A.; Went, M. J . J .
Chem. Soc., Dalton Trans. 1986, 173. (b) J ohnson, K. A.; Gladfelter,
W. L. Organometallics 1990, 9, 2101. (c) Gao, Y.; J ennings, M. C.;
Puddephatt, R. J . Organometallics 2001, 20, 1882. (d) Trepanier, S.
J .; McDonald, R.; Cowie, M. Organometallics 2003, 22, 2638. (e)
Rowsell, B. D.; McDonald, R.; Cowie, M. Organometallics 2004, 23,
3873.
1
to Rh. The H NMR resonances for the acyl ligand are
(22) (a) Lindley, P. F.; Mills, O. S. J . Chem. Soc., A 1969, 1279. (b)
Blickensderfer, J . R.; Knobler, C. B.; Kaesz, H. D. J . Am. Chem. Soc.
1975, 97, 2686. (c) Hendrick, K.; Iggo, J . A.; Mays, M. J .; Raithby, P.
R. J . Chem. Soc., Chem. Commun. 1984, 209. (d) Shafiq, F.; Kramarz,
K. W.; Eisenberg, R. Inorg. Chim. Acta 1993, 213, 111.
(20) Longato, B.; Norton, J . R.; Huffman, J . C.; Marsella, J . A.;
Caulton, K. G. J . Am. Chem. Soc. 1981, 103, 209.
(21) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J . Chem. Soc., Perkin Trans 2 1987, 51.
(23) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson,
D. G.; Taylor, R. J . Chem. Soc., Dalton Trans. 1989, 51.

C-C Bond Formation by Cumulene Insertion
Organometallics, Vol. 23, No. 20, 2004 4767
Ta ble 3. Selected Dista n ces (Å) a n d An gles (d eg) for Com p ou n d 14
atom 1
atom 2
distance
atom 1
atom 2
distance
Rh
Rh
Rh
Rh
Rh
Rh
Ru
Ru
Ru
Ru
Ru
Ru
P1
P3
C2
C3
C6
P2
P4
O3
C1
C2
2.9833(6)
2.368(2)
2.363(2)
2.238(5)
1.915(5)
2.077(5)
2.361(2)
2.341(2)
2.449(4)
1.837(5)
1.919(5)
P1
P3
O1
O2
O3
C3
C4
C5
C6
C7
C7
P2
P4
C1
C2
C3
C4
C5
C6
C7
C8
C9
3.089(2)a
3.090(2)a
1.160(6)
1.162(5)
1.234(6)
1.497(7)
1.536(7)
1.510(7)
1.329(7)
1.488(7)
1.520(7)
atom 1
atom 2
atom 3
angle
atom 1
atom 2
atom 3
angle
Ru
Ru
Ru
Ru
Ru
P1
C2
C2
C3
Rh
Rh
Rh
Rh
Rh
P2
O3
Rh
Rh
Rh
Rh
Rh
Rh
Rh
Rh
Rh
Ru
Ru
Ru
Ru
Ru
Ru
Ru
P1
P3
C2
C3
C6
P3
C3
C6
C6
P2
P4
O3
C1
C2
P4
C1
90.23(4)
89.56(4)
40.0(1)
O3
C1
Ru
C6
C6
C8
C5
Rh
Rh
C3
C4
Rh
Rh
C4
Rh
Ru
Ru
Ru
O3
C7
C7
C7
C6
C6
C6
C4
C5
C3
C3
C3
C2
C2
C2
C2
C3
C8
C9
C9
C7
C5
C7
C5
C6
O3
C4
O3
O2
O2
108.2(2)
104.0(2)
107.5(3)
122.8(5)
121.5(5)
115.7(4)
125.8(5)
112.3(3)
122.0(4)
112.3(4)
112.6(4)
119.3(4)
118.0(4)
122.7(4)
120.5(4)
148.0(4)
73.6(2)
158.1(1)
175.71(5)
113.6(2)
161.8(2)
84.6(2)
92.31(4)
93.01(4)
59.59(8)
152.6(2)
48.6(2)
169.51(5)
147.8(2)
a
Nonbonded distance.
also consistent with this ligand fragment remaining
intact. The IR spectrum of 15 shows two terminal
carbonyl stretches in addition to a weak stretch due to
with the cumulenes studied (only 1 reacted very slowly
with dimethylallene), facile reactions were observed in
all cases by the removal of a carbonyl ligand using Me3-
NO. As part of our strategy to generate C3-bridged
ligands, we had anticipated that cumulene insertion
could occur in one of two ways, as shown in Scheme 1.
Of all reactions studied, only the reaction of 2 and 1,1-
dimethylallene gives one of the anticipated products.
However, we suggest in what follows that in all cases
initial insertion of the cumulene molecule into the Rh-
CH2 bond occurs to give either structure B or D, but
that in all but one example these intermediates are not
observed, instead rearranging to the products observed.
It appears that whether intermediate B or D is obtained
is determined by the steric demands of the cumulene
molecule; in the cases of allene and methylallene, initial
cumulene coordination at Rh occurs as shown in C,
subsequently leading to D, whereas for 1,1-dimethyl-
allene, coordination as shown for A occurs, which upon
insertion leads to B. As noted, in most cases both B and
D undergo further transformations to give the observed
products. We assume that the orientation of the cumu-
lene molecule upon coordination to Rh and the subse-
quent nature of the insertion product are dictated by
the nonbonded contacts between the cumulene substit-
uents and the phenyl groups of the dppm ligands.
the acyl group at 1683 cm-1
.
Reaction of 14 with CO yields the tricarbonyl complex
[RhRu(η1:η1-C(O)CH2CH2CdC(CH3)2)(CO)3(dppm)2][CF3-
SO3] (16). Again, all carbonyl ligands are shown, by 13C-
{1H} NMR studies, to be terminally bound to Ru, while
the acyl carbonyl is bound to Rh (1J RhC ) 30 Hz). Three
stretches in the IR spectrum correspond to the terminal
carbonyl groups, while the acyl stretch is observed at
1687 cm-1. The 1H resonances of the acyl ligand are very
similar to those of 15, indicating that this ligand
fragment has remained intact and is presumably bound
to Rh as suggested for 15. Both 15 and 16 can be
thought of as resulting from displacement of the oxygen-
ruthenium dative bond from the bridging acyl group in
14 by H2 or CO, respectively.
Discu ssion
As noted earlier, one goal of this study was to effect
cumulene insertion into the Rh-CH2 bond of the meth-
ylene-bridged precursors [RhM(CO)4(µ-CH2)(dppm)2][X]
(M ) Os (1), Ru (3)) in attempts to generate models for
the C3H6-bridged species that was proposed to be a
pivotal intermediate in the coupling of diazomethane-
generated methylene groups, promoted by a Rh/Os
complex.4 Such C3-containing products could also model
surface-bound species that had been suggested2 in
Fischer-Tropsch chemistry. Furthermore, by comparing
the chemistries of both the Rh/Os and Rh/Ru combina-
tions of metals a better understanding of the roles of
the different metals in this chemistry was anticipated.
Although both compounds were found to be unreactive
The reactions of compounds 2 and 4 (generated in situ
from the reactions of 1 and 3 with Me3NO) with allene
give the analogous trimethylenemethane-bridged prod-
ucts [RhM(CO)2(µ-η3:η1-C(CH2)3)(dppm)2]+ (M ) Os (5),
Ru (6)), in which the trimethylenemethane unit is
σ-bound to the group 8 metal through one methylene
group while binding as an η3-allylic fragment to Rh
through the remainder of the ligand. The retention of
the labeled methylene group from the precursors (2, 4)

4768 Organometallics, Vol. 23, No. 20, 2004
Chokshi et al.
Although the η3:η1 coordination mode for the tri-
methylenemethane unit is not that of our original
target, we assume that it derives readily from the
targeted structure D, in which the exocyclic allyl moiety
that is â to both Rh and the group 8 metal rearranges
to an η3-allylic moiety with concomitant CO loss from
Rh. The stability of the η3-allyl binding mode over the
η1-mode is well documented.27a Clearly, the reverse
binding mode, having the allylic fragment bound to the
group 8 metal, is also possible. However, as already
noted, it is assumed that the greater steric crowding in
this case is destabilizing, favoring instead the observed
species. In the case of the Os complexes (5 and 11),
retention of the strong Os-CH2 σ-bond would also favor
the structures observed.
Sch em e 6
Since the rearrangement of D to the η3-allyl product
was assumed to occur with concomitant CO loss, it
seemed likely that ligand (CO or PMe3) addition to 5 or
6 would reverse this rearrangement, allowing us to
observe the targeted η1:η1-binding mode. However in all
cases ligand attack leading to species 7-10 and having
either CO or PMe3 bound to Rh resulted in retention of
the η3-binding mode of the allylic fragment. In the
precursors 5 and 6 the Rh center is coordinatively
unsaturated, so additional ligand coordination can occur
without transformation to an η1-allylic binding mode.
Surprisingly, the initial site of attack on the Rh/Os
compound 5 differs for CO and PMe3. Whereas at -80
°C PMe3 attacks, as expected, at the coordinatively
unsaturated Rh, initial carbonyl attack at this temper-
ature occurs at Os, with scrambling occurring at higher
temperatures. Although the Os center in 5 is saturated,
transfer of a carbonyl to Rh can give rise to unsaturation
at Os. Presumably, the larger PMe3 group cannot
readily gain access at the more crowded Os, so attacks
instead at Rh. In the Rh/Ru analogue, there is no
evidence of either attack at Ru or carbonyl scrambling.
With the methylallene analogue 11, the coordinative
unsaturation at Rh is alleviated by an agostic interac-
tion involving a C-H bond of the methyl substituent,
but this interaction is readily displaced by a carbonyl
generating the tricarbonyl analogue of 7.
1,1-Dimethylallene appears to coordinate to the Rh
centers in both the Rh/Os and Rh/Ru analogues, as
shown in structure A, which upon insertion into the
Rh-CH2 bond yields B. In each case, however, an
additional transformation occurs. In the reaction of 1
with dimethylallene, no complex containing this sub-
strate is observed, but 1 is quantitatively consumed over
several days, yielding 1,1-dimethyl-1,3-butadiene and
[RhOs(CO)4(dppm)2]+. A species analogous to the pre-
sumed intermediate in this transformation, [RhOs(CO)3-
(µ-η1:η1-(CH3)2CCCH2CH2)(dppm)2]+ (13), is observed at
low temperature when the reaction is carried out with
compound 2. This product is unstable at ambient
temperature, decomposing to the 1,1-dimethylbutadiene
and the above tetracarbonyl products, together with
other decomposition products, which presumably give
rise to the additional carbonyl group. We assume that
this transformation results as outlined in Scheme 7
(dppm ligands omitted). â-Hydrogen elimination from
as the one that is σ-bound to Os or Ru in the products
(5, 6) confirms that cumulene insertion into the Rh-
CH2 bond has occurred. In the case of the reaction of 2
with methylallene, the nonbonded contacts noted above
favor a product, [RhOs(CO)2(µ-η3:η1-CH(CH3)C(CH2)2)-
(dppm)2]+ (11), that is exactly analogous to compounds
5 and 6, except having a methyl substituent on one of
the terminal allylic carbons.
Trimethylenemethane complexes of mononuclear24
and binuclear25 complexes are known, and in the latter
cases the µ-η3:η1-binding mode proposed herein has also
been proposed or observed. Although some mononuclear
trimethylenemethane complexes have been shown to be
fluxional by rotation about the ligand-metal axis,24a
this fluxionality has been reported with only one bridged
complex.25d Certainly in compounds 5 and 6, prepared
from 13CH2-labeled compounds 2 and 4, no scrambling
of the label over the three methylene sites of the ligand
is observed; the label remains bound to Os or Ru. For
the Os species (5) we might argue that this results from
the stronger Os-CH2 bond;26 however, this argument
is less compelling for the Ru analogue given the
presumed similarity in Rh-C and Ru-C bond
strengths.26 If scrambling of the trimethylenemethane
moiety were to occur, this would presumably take place
as diagrammed in Scheme 6 (dppm ligands above and
below the plane of the drawing are omitted), through
an intermediate in which the η3-allylic fragment is
bound to the group 8 metal. We propose that such an
intermediate would be unstable owing to the greater
steric crowding at this metal by virtue of its higher
coordination number.
(24) (a) J ones, M. D.; Kemmitt, R. D. W. Adv. Organomet. Chem.
1987, 27, 279. (b) Fryzuk, M. D.; J oshi, K.; Rettig, S. J . Organometallics
1991, 10, 1642. (c) Herberich, G. E.; Spaniol, T. P. J . Chem. Soc., Chem.
Commun. 1991, 1457. (d) Herberich, G. E.; Spaniol, T. P. J . Chem.
Soc., Dalton Trans. 1993, 2471. (e) Donaldson, W. A.; Cushie, C. D.;
Guo, S.; Kramer, M. J .; Bennette, D. W. Transition Met. Chem. 1997,
22, 592. (f) Moor, T.; Kielty, C.; Reeves, P. C. J . Organomet. Chem.
2001, 620, 308.
(25) (a) Wolfe, S.; Hason, S. K.; Campbell, J . R. Chem. Commun.
1070, 1420. (b) Whitesides, T. H.; Slaven, R. W. J . Organomet. Chem.
1974, 67, 99. (c) Fildes, M. J .; Knox, S. A. R.; Orpen, A. G.; Turner, M.
L.; Yates, M. I. J . Chem. Soc., Chem. Commun. 1989, 1680. (d)
Chetcuti, M. J .; Fanwick. P. E.; Grant, B. E. Organometallics 1991,
10, 3003.
(26) (a) Ziegler, T.; Tschinke, V. In Bonding Energetics in Organo-
metallic Compounds; Marks, T. J ., Ed.; American Chemical Society:
Washington, DC, 1990; Chapter 19. (b) Ziegler, T.; Tschinke, V.;
Ursenbach, B. J . Am. Chem. Soc. 1987, 109, 4825. (c) Armentrout, P.
B. In Bonding Energetics in Organometallic Compounds; Marks, T.
J ., Ed.; American Chemical Society: Washington, DC, 1990; Chapter
2.
(27) (a) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books; Mill Valley, CA, 1987; Chapter 3. (b) Chapter
6.

C-C Bond Formation by Cumulene Insertion
Organometallics, Vol. 23, No. 20, 2004 4769
Sch em e 7
aldehyde. However, reductive elimination from the
dihydride product 15 is not observed even upon mild
heating.
Con clu sion s
The products obtained in the reactions of methylene-
bridged complexes of Rh/Os and Rh/Ru with a series of
cumulenes are consistent with insertion of cumulenes
into the Rh-CH2 bond and depend both on the cumu-
lene substituents and on the combinations of metals.
Although the initial products of cumulene insertion are
not all observed, the observed final products are con-
sistent with insertion occurring such that the uncoor-
dinated “CdCRR’” moiety is exo to the resulting five-
membered dimetallacycle, either adjacent to the original
metal-bound methylene group or adjacent to Rh. The
nature of the initial product depends on the degree of
substitution of the cumulene. Therefore, the unsubsti-
tuted allene initially yields an η1:η1-trimethylenemethane
unit having the allylic “CdCH2” group in a symmetrical
site between the two metal-bound methylene groups,
while the bulkier 1,1-dimethylallene yields an initial
species in which the dimethylvinyl group is adjacent to
Rh. In the first case, both metal combinations (Rh/Os
and Rh/Ru) behave identically and the presumed η1:η1-
trimethylenemethane intermediate rearranges to an η3:
η1-binding mode having one methylene group σ-bound
to the group 8 metal and the remainder of the fragment
η3-bound to Rh. This transformation reflects the well-
documented preference for η3- over η1-allyls.27a
Sch em e 8
In the second case the differing reactivity of the two
metal combinations becomes obvious. Both metal com-
binations give similar initial products in which the
metals are bridged by an η1:η1-(CH3)2CdCCH2CH2
fragment. However, with the Rh/Os metal combination,
â-hydrogen elimination occurs followed by reductive
elimination of 1,1-dimethyl-1,3-butadiene. With the Rh/
Ru combination, migration of the Ru-bound end of the
hydrocarbyl fragment to a Ru-bound carbonyl occurs,
yielding an acyl group. The difference in these latter
two products is consistent with the greater tendency of
the second-row metal (Ru) to undergo migratory inser-
tion compared to the third-row Os27b and with the
greater propensity of the third-row metal for C-H bond
activation.28
The stated goal of this study was to obtain models
for C3-bridged species, proposed as intermediates in a
methylene-coupling sequence.4 In this regard, the study
was only partially successful; only 1,1-dimethylallene
with the Rh/Os complex yielded the anticipated product
as an observable species. The subsequent transforma-
tion of this product was consistent with transformations
of related C3 products containing â-hydrogens.4,6 In all
other cases, products different from those anticipated
were obtained. Under these conditions, these products
are certainly not useful models for Fischer-Tropsch
chemistry. Nevertheless, the different products obtained
demonstrate how small modifications to the system can
result in significant reactivity differences. In particular,
it is significant that although reactivity is initiated at
the central carbon of the C3-bridged unit and reductive
elimination, in which hydrogen transfer to the vinylic
Rh-bound carbon occurs, would yield the butadiene
molecule. We had anticipated that allene insertion into
the Rh(µ-CH2)M units might give rise to better models
of propanediyl-bridged species than the products of
alkyne insertion. The above proposed sequence, leading
to dimethylbutadiene formation, supports this conten-
tion since it parallels the conversion of the bridging
propanediyl fragment to propene.6
In the case of the Rh/Ru complex the only product
observed in its reaction with 1,1-dimethylallene is the
acyl-containing 14, which has resulted from 2+1+1
coupling of dimethylallene, the bridging methylene unit,
and a carbonyl. We propose that this also proceeds via
an intermediate like B. In this case however, substitu-
tion of Os by Ru leads to a greater tendency for carbonyl
migratory insertion,27b so migration of the Ru-bound
methylene group to an adjacent carbonyl, as dia-
grammed in Scheme 8 (dppm groups omitted), yields a
Ru-bound acyl, and subsequent migration of the acyl
to Rh to give a favorable five-membered metallacycle,
accompanied by coordination of the acyl oxygen at Ru
and carbonyl migration from Rh to Ru, yields the
product 14.
The observation that the Ru-O distance of the
bridging acyl unit in 14 was long, presumably indicating
a weak interaction, suggested that displacement of this
Ru-acyl interaction could be induced by ligand addition.
This has been confirmed by the addition of either H2 or
CO, as shown earlier in Scheme 5. We had anticipated
that, in the case of H2 addition, hydrogenolysis of the
Rh-C bonds might occur, yielding the appropriate
(28) (a) Ziegler, T.; Tschinke, V.; Fan, L. Y.; Becke, A. D. J . Am.
Chem. Soc. 1989, 111, 9177. (b) Lees, A. J . Organomet. Chem. 1998,
554, 1. (c) Su, M. D.; Chu, S. Y. J . Am. Chem. Soc. 1997, 119, 5373. (d)
Su, M. D.; Chu, S. Y. Int. J . Quantum Chem. 1999, 72, 405.

4770 Organometallics, Vol. 23, No. 20, 2004
Chokshi et al.
the Rh center in all cases, the identity of the final
product can depend on the nature of the adjacent metal.
It is important to gain an understanding of such
reactivity trends in heterobinuclear complexes in order
to fully appreciate the roles of different metals in
bimetallic catalysts.
this research and NSERC for funding the Bruker
PLATFORM/SMART 1000-CCD diffractometer.
Su p p or tin g In for m a tion Ava ila ble: Tables of X-ray
experimental details, atomic coordinates, interatomic distances
and angles, anisotropic thermal parameters, and hydrogen
parameters for compound 14. This material is available free
of charge via the Internet at http://pubs.acs.org.
Ack n ow led gm en t. We thank the Natural Sciences
and Engineering Research Council of Canada (NSERC)
and the University of Alberta for financial support of
OM040074+