Oxidative additions of coordinated ligands at unsaturated molybdenum and tungsten diphosphine-bridged carbonyl dimers. 3. Decarbonylation reactions of [MoW(η5-C5H5)2(CO) 4(μ-Ph2PCH2PPh2)]

Article abstract of DOI:10.1021/om960920j

Decarbonylation of the heterometallic title complex [MoWCp₂(CO)₄(μ-dppm)] (Cp = η⁵-C₅H₅; dppm = Ph₂PCH₂PPh₂) in refluxing tetrahydrofuran leads to the phosphido complex [MoWCp₂(μ-CH₂PPh₂)(μ-PPh ₂)(CO)₂], presumably via the tricarbonylic species [MoWCp₂(μ-CH₂PPh₂)(μ-PPh ₂)(μ-CO)(CO)₂]. The latter is an unstable compound which can be generated upon reaction of the former with CO and also contains the phosphinomethyl ligand C-bonded to the tungsten atom. In contrast, photolytic decarbonylation of the title complex at -10 °C leads reversibly to the hydrido compound [MoW(μ-η¹:η⁵-C₅H ₄)Cp(μ-H)(CO)₃(μ-dppm)], in which the cyclopentadienylidene ligand is specifically η⁵-bonded to tungsten and η¹-bonded to molybdenum. Further photolysis of this complex at 10 °C leads to the triply bonded dimer [MoWCp₂(CO)₂(μ-dppm)] with concomitant regeneration of a H-C (cyclopentadienyl) bond. The latter species reacts with ^tBuCN to give [MoWCp₂(μ-η¹:η²-CN ^tBu)(CO)₂(μ-dppm)], in which the isocyanide ligand is specifically η¹-bonded to tungsten and η²-bonded to molybdenum. In order to account for the metal selectivity observed in the above species, two different intermediates, having either μ-η¹-CO or μ-η¹:η²-CO ligands, are thought to be involved in the decarbonylation reactions of the title compound.

Full text of DOI:10.1021/om960920j

1378
Organometallics 1997, 16, 1378-1383
Oxid a tive Ad d ition s of Coor d in a ted Liga n d s a t
Un sa tu r a ted Molybd en u m a n d Tu n gsten
Dip h osp h in e-Br id ged Ca r bon yl Dim er s. 3.
Deca r bon yla tion Rea ction s of
[MoW(η5-C5H5)2(CO)4(µ-P h 2P CH2P P h 2)]
Celedonio Alvarez, M. Esther Garc´ıa, V´ıctor Riera, and Miguel A. Ruiz*
Departamento de Qu´ımica Orga´nica e Inorga´nica/ IUQOEM, Universidad de Oviedo,
E-33071 Oviedo, Spain
Received October 31, 1996X
Decarbonylation of the heterometallic title complex [MoWCp2(CO)4(µ-dppm)] (Cp ) η5-
C5H5; dppm ) Ph2PCH2PPh2) in refluxing tetrahydrofuran leads to the phosphido complex
[MoWCp2(µ-CH2PPh2)(µ-PPh2)(CO)2], presumably via the tricarbonylic species [MoWCp2(µ-
CH2PPh2)(µ-PPh2)(µ-CO)(CO)2]. The latter is an unstable compound which can be generated
upon reaction of the former with CO and also contains the phosphinomethyl ligand C-bonded
to the tungsten atom. In contrast, photolytic decarbonylation of the title complex at -10 °C
leads reversibly to the hydrido compound [MoW(µ-η1:η5-C5H4)Cp(µ-H)(CO)3(µ-dppm)], in
which the cyclopentadienylidene ligand is specifically η5-bonded to tungsten and η1-bonded
to molybdenum. Further photolysis of this complex at 10 °C leads to the triply bonded dimer
[MoWCp2(CO)2(µ-dppm)] with concomitant regeneration of a H-C (cyclopentadienyl) bond.
t
The latter species reacts with BuCN to give [MoWCp2(µ-η1:η2-CNtBu)(CO)2(µ-dppm)], in
which the isocyanide ligand is specifically η1-bonded to tungsten and η2-bonded to
molybdenum. In order to account for the metal selectivity observed in the above species,
two different intermediates, having either µ-η1-CO or µ-η1:η2-CO ligands, are thought to be
involved in the decarbonylation reactions of the title compound.
Ch a r t 1
In tr od u ction
In the previous parts of this series1,2 we have shown
that the unsaturated species generated through ejection
of carbon monoxide from the single-metal-metal-
bonded dimers [M2Cp2(CO)4(µ-dppm)] (Cp ) η5-C5H5;
dppm ) Ph2PCH2PPh2; M ) W,1 Mo2) are able to
activate C-H or P-C (sp3) bonds in the coordinated
cyclopentadienyl or diphosphine ligands, respectively.
The results are strongly dependent on the metal. Thus,
when M ) Mo, only P-C bond activation is observed,
which leads irreversibly to the phosphido-phosphinom-
ethyl complex [Mo2Cp2(µ-CH2PPh2)(µ-PPh2)(CO)2]. In
contrast, when M ) W a reversible C-H bond activation
occurs, which gives the hydrido cyclopentadienylidene
compound [W2(µ-η1:η5-C5H4)Cp(µ-H)(CO)3(µ-dppm)]. From
the above studies we could not give a fully satisfactory
explanation for this striking difference in the chemical
behavior of the dimolybdenum and ditungsten systems.
Thus, while the behavior of the dimolybdenum system
could be in part due to the low thermodynamic stability
of the cyclopentadienylidene ligand when bonded to this
metal,3 it was not clear why the P-C cleavage of the
backbone of the dppm ligand occurs readily in the
dimolybdenum complex but is almost absent in the
ditungsten analogue under all decarbonylation condi-
tions examined (thermal or photochemical). Because of
the fact that both P-C4 and C-H5 bond cleavage
reactions are relevant processes in the chemistry of
organometallic compounds, we were interested in gain-
ing more insight about the influence of the metal on
these activation processes. We therefore decided to
study the decarbonylation reactions of the mixed-metal
dimer [MoWCp2(CO)4(µ-dppm)] (1).
Resu lts a n d Discu ssion
Syn th esis a n d Str u ctu r e of [MoWCp 2(CO)4(µ-
d p p m )] (1). As found for its homonuclear analogues,
* To whom correspondence should be addressed. E-mail: mara@
sauron.quimica.uniovi.es.
(3) Alvarez, M. A.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.; Bois, C.;
J eannin, Y. J . Am. Chem. Soc. 1995, 117, 1324.
(4) (a) Chaudret, B.; Delavaux, B.; Poilblanc, R. Coord. Chem. Rev.
1988, 86, 191. (b) Garrou, P. E. Chem. Rev. 1985, 85, 171.
(5) (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; p 295. (b) Crabtree,
R. H. Angew. Chem., Int. Ed. Engl. 1993, 32, 789. (c) Ryabov, A. D.
Chem. Rev. 1990, 90, 403. (d) J ones, W. D.; Feher, F. J . Acc. Chem.
Res. 1989, 22, 91. (e) Crabtree, R. H. Chem. Rev. 1985, 85, 245. (f)
Brookhart, M.; Green, M. L. H. J . Organomet. Chem. 1983, 250, 395.
X Abstract published in Advance ACS Abstracts, March 1, 1997.
(1) (a) Part 1: Alvarez, M. A.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.;
Falvello, L. R.; Bois, C. Organometallics 1997, 16, 354. (b) Alvarez, M.
A.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.; Bois, C.; J eannin, Y. J . Am.
Chem. Soc. 1993, 115, 3786.
(2) (a) Part 2: Garc´ıa, G.; Garc´ıa, M. E.; Melo´n, S.; Riera, V.; Ruiz,
M. A.; Villafan˜e, F. Organometallics 1997, 16, 624. (b) Riera, V.; Ruiz,
M. A.; Villafan˜e, F.; Bois, C.; J eannin, Y. J . Organomet. Chem. 1989,
375, C23.
S0276-7333(96)00920-X CCC: $14.00 © 1997 American Chemical Society

Mo and W Diphosphine-Bridged CO Dimers
Organometallics, Vol. 16, No. 7, 1997 1379
Ta ble 1. IR a n d 31P {1H} NMR Da ta for New Com p ou n d s
b
compd
νst(CO)a/cm-1
δ(P) (assignt, J PW
)
J PP
[MoWCp2(CO)4(µ-dppm)] (1)
1913 (m), 1877 (vs)
1840 (w), 1819 (m)
1828 (m, sh), 1790 (s)d
1776 (s)d,f
47.7 (MoP), 10.7 (WP) {isomer A}
34.9 (MoP), -1.7 (WP) {isomer B}
71.7 (µ-P, 227), 39.6 (MoP, 28)
176.3 (µ-P, 275), 25.7 (MoP, 66)
51.6 (µ-P, 200), 53.8 (MoP)
108c
75c
14e
4e
[MoWCp2(µ-CH2PPh2)(µ-PPh2)(CO)2] (2)
[MoWCp2(µ-CH2PPh2)(O)(µ-PPh2)(CO)] (3)
[MoWCp2(µ-CH2PPh2)(µ-PPh2)(µ-CO)(CO)2] (4)
[MoW(µ-η1:η5-C5H4)Cp(µ-H)(CO)3(µ-dppm)] (5)
[MoWCp2(CO)2(µ-dppm)] (6)
1938 (vs), 1878 (w), 1656 (m)
1930 (vs), 1852 (s), 1793 (m)
1735 (vs)
7g
72.5 (MoP), 22.2 (WP, 294)
70h
74g
83
54.9 (MoP), 28.7 (WP, 425)
51.7 (MoP), 8.12 (WP, 394)
[MoWCp2(µ-η1:η2-CNtBu)(CO)2(µ-dppm)] (7)
1824 (s), 1787 (vs)i
a
b
Recorded in THF solution, unless otherwise stated. Recorded at 161.97 MHz and 291 K in toluene-d8 solution, unless otherwise
stated. δ is given in ppm relative to external 85% aqueous H3PO4 and J in hertz. c In acetone-d6 solution at 248 K. Dichloromethane
d
solution. e C6D6 solution. νst(W-O) 931 cm-1 in Nujol mull. CD2Cl2 solution. 253 K. νst(CN) 1629 (w) cm-1
.
f
g
h
i
Ch a r t 2
Ch a r t 3
later) indicate that no rearrangement of the phosphorus
ligands has occurred upon carbonylation, so that the
phosphinomethyl group binds the tungsten atom also
through its carbon atom (Chart 3).
We must note that the dimolybdenum analogue of
complex 2 failed to give a CO adduct stable enough to
be detected spectroscopically under the usual experi-
mental conditions.2 Thus, it seems that complexes of
the type 4 give up CO more easily as we replace
tungsten for molybdenum atoms, as is usually observed
in the chemistry of carbonylic complexes of these ele-
ments.
compound 1 can be synthesized in good yield by addition
of dppm to the triply bonded dimer [MoWCp2(CO)4].
Spectroscopic data for 1 (Table 1 and Experimental
Section) are strongly related to those of the homonuclear
dimers, thus indicating a close structural relationship.
As expected,1,2 two isomers are observed in solution,
their ratio (ca. 1:1 in Me2CO-d6 at 248 K) being midway
between those observed for the dimolybdenum and
ditungsten complexes. The structures of these isomers
have been previously discussed1,2 and need no further
comments.
Th er m a l Deca r bon yla tion of Com p ou n d 1. Com-
plex 1 is readily decarbonylated in refluxing tetrahy-
drofuran to give the unsaturated, air-sensitive complex
[MoWCp2(µ-CH2PPh2)(µ-PPh2)(CO)2] (2) in good yield,
along with trace amounts of the oxo compound [MoWCp2-
(µ-CH2PPh2)(O)(µ-PPh2)(CO)] (3). The latter is formed
due to the presence of trace amouts of atmospheric
oxygen in the reaction medium and during manipulation
of the reaction mixture. Thus, complex 3 is the major
species formed when solutions of 2 are exposed to air,
although extensive decomposition also occurs.
Spectroscopic data for 2 and 3, which will be discussed
later on, clearly show that these complexes are obtained
as single isomers in each case, with the carbon atom of
the phosphinomethyl ligand specifically bonded to the
tungsten atom (Chart 2). The oxo ligand in 3 is also
bonded to the tungsten atom, as expected in view of the
structures of its homonuclear analogues [M2Cp2(µ-CH2-
PPh2)(O)(µ-PPh2)(CO)] (M ) Mo,6 W1).
Complex 2 reacts readily with CO (1 atm) to give the
electron-precise species [MoWCp2(µ-CH2PPh2)(µ-PPh2)-
(µ-CO)(CO)2] (4). The latter compound, however, could
not be isolated as a pure solid because it starts to
decarbonylate when the CO atmosphere is removed so
as to give back the parent dicarbonyl 2. Thus, com-
pound 4 is assumed to be an intermediate species in
the thermal formation of 2. In fact, IR monitoring of
the thermolytic reaction leading to dicarbonyl 2 shows
the presence of weak νst(CO) bands that can be assigned
to complex 4. On the other hand, NMR data for 4 (see
P h otoch em ica l Deca r bon yla tion of Com p ou n d
1. Complex 1 easily loses a single CO molecule when
irradiated with visible-UV light in THF solution at -10
°C to give cleanly the hydrido cyclopentadienylidene
complex [MoW(µ-η1:η5-C5H4)Cp(µ-H)(CO)3(µ-dppm)] (5).
Complex 5 is thermally unstable and decomposes readily
above 0 °C. When further irradiated at 10 °C, it rapidly
gives the triply metal-metal bonded dicarbonyl [MoW-
Cp2(CO)2(µ-dppm)] (6) in good yield. As expected, the
latter can be obtained through direct irradiation of 1 at
10 °C, a process that can be reversed by just bubbling
CO through a solution of compound 6.
Complex 6 is isostructural with its ditungsten1 and
dimolybdenum2 analogues and also displays fluxional
behavior in solution, so that the linear semibridging
carbonyls exchange their positions between molybde-
num and tungsten atoms. This is clearly indicated in
the 13C NMR spectrum by the presence of a single
carbonyl resonance at 248.4 ppm (to be compared with
254.2 and 240.7 ppm for the corresponding dimolybde-
num and ditungsten complexes, respectively). This
dynamic behavior has been also observed in the triply
bonded heterometallic complex [MoWCp2(CO)4]7 and
requires only a modest energy of activation.
As found for 2, the cyclopentadienylidene complex 5
is obtained as a single isomer. NMR data (see later)
allow us to establish clearly that the C5H4 group is
specifically η5-bonded to tungsten and η1-bonded to
molybdenum (Chart 4). This is just opposite to what
we would have expected on the basis of our previous
(6) Riera, V.; Ruiz, M. A.; Villafan˜e, F.; Bois, C.; J eannin, Y.
Organometallics 1993, 12, 124.
(7) Curtis, M. D.; Fotinos, M. A.; Messerle, L.; Sattelberger, A. P.
Inorg. Chem. 1983, 22, 1559.

1380 Organometallics, Vol. 16, No. 7, 1997
Alvarez et al.
Ch a r t 4
Sch em e 1. Exp ected Sp ecificity in th e C-H Bon d
Clea va ge P r ocess (Not Obser ved ) of Com p lex 7a
Ch a r t 5
studies on the structure of heterometallic species [M2M′-
(µ-η1:η5-C5H4)Cp2(CO)6] (M, M′ ) Mo, W), which re-
vealed a clear preference of the C5H4 ligand to form σ
bonds to tungsten rather than to molybdenum atoms.3
The above data suggested to us that the structure
exhibited by compound 5 might be not only dictated by
the strength of the σ bond between the C5H4 ligand and
the Mo or W atoms but also influenced by the nature of
its precursors.
a
P-C-P ) Ph2PCH2PPh2.
tungsten and π to molybdenum) of a bridging carbonyl
ligand in the tricarbonylic precursor of 5.
Str u ctu r a l Ch a r a cter iza tion of New Com p lexes.
With the exception of 4, all heterometallic complexes
1-7 have dimolybdenum and/or ditungsten counter-
parts. Therefore, the structural characterization of
these heterometallic species is straightforward by com-
parison of their IR and NMR (1H, 31P, 13C) data with
those of the corresponding homonuclear compounds1,2
and needs then no detailed comments. Typically,
νst(CO) frequencies in the heterometallic complexes
(Table 1) are intermediate between those observed in
the corresponding homometallic species. As for NMR
chemical shifts, they are mostly dictated by the identity
of the metal to which the nucleus under observation is
bonded. For example, δP values for compound 6 mea-
In order to trace back the origin of the stereospecific
formation of 5, we examined the reaction of the dicar-
t
bonyl species 6 with BuCN. In the case of the ditung-
sten analogue of 6, this has been shown1 to proceed
through a µ-η1:η2-tBuCN complex, which then experi-
ences C-H bond oxidative addition to yield a hydrido
cyclopentadienylidene complex analogous to 5. Indeed,
tBuCN readily reacts with 6 to yield the isocyanide-
bridged complex [MoWCp2(µ-η1:η2-CNtBu)(CO)2(µ-dppm)]
(7). This product, however, does not experience any
detectable C-H activation process at room temperature,
thus resembling the behavior of its dimolybdenum
analogue.2
sured in CD2Cl2 at 248 K are 54.9 and 28.7 ppm (J PP
)
Once more, NMR data indicate that complex 7 is
formed as a single isomer, out of the two possible ones.
From these data we conclude that the isocyanide ligand
is σ-bonded to tungsten and π-bonded to molybdenum.
This is not surprising itself, as W-C σ bonds are usually
stronger than Mo-C ones.8 However, this structural
feature might be at the origin of the failure of 7 to
experience C-H (cyclopentadienyl) bond oxidative ad-
dition. Were such a process to occur for 7, we would
expect this to render the cyclopentadienylidene ligand
in its less favorable disposition: that is, σ-bonded to
molybdenum instead of tungsten (Scheme 1).
Complex 7 can be taken as a model for the tricarbo-
nylic intermediate presumably formed after ejection of
the first CO molecule in compound 1 (see later).1,2 Thus,
the anomalous presence of the cyclopentadienylidene
ligand specifically σ-bonded to molybdenum in complex
5 would be the result of the specific coordination (σ to
108 Hz), very close to the values found respectively for
the corresponding dimolybdenum (57.9 ppm) and di-
tungsten (27.2 ppm, J PP ) 112 Hz) analogues. Thus,
the assignments of the NMR resonances in the hetero-
metallic species 1-7 are quite obvious in general. Of
relevance in the present context are only the data which
allow us to identify the coordination position of ligands
when two isomers are possible.
For compounds 2 and 3, the disposition of the phos-
phinomethyl ligand P-bonded to molybdenum is denoted
by the similarity of their 31P chemical shifts (Table 1)
to those of the dimolybdenum analogues2,6 and the
reduced value of the corresponding P-W couplings (30-
60 Hz), almost an order of magnitude lower than one-
bond couplings measured in related ditungsten com-
plexes.1 The oxo ligand in 3 can be safely placed on
tungsten by exclusion, because the single carbonyl of
the molecule exhibits chemical shifts and P-C couplings
(to both phosphorus atoms) almost identical with those
of the dimolybdenum analogue.6
(8) 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; p 100.

Mo and W Diphosphine-Bridged CO Dimers
Organometallics, Vol. 16, No. 7, 1997 1381
Sch em e 2. P r op osed Rea ction P a th w a ys in th e
Deca r bon yla tion Rea ction s of Com p ou n d 1a
As was said before, compound 4 has no homometallic
counterpart. In spite of this, the spectroscopic data for
4 allow a precise definition of its structure. First, the
phosphorus atoms of the phosphinomethyl and phos-
phido groups are weakly coupled (J PP ) 7 Hz), as was
found for 2 and 3, which seems characteristic of a
transoid disposition of these bridging ligands, so as to
define an almost flat five-atom ring.1,2,6 The IR and 13C
NMR spectra denote the presence of two terminal and
one bridging carbonyl (Table 1 and Experimental Sec-
tion). The relative intensity of the high-energy νst(CO)
bands (strong and weak, in order of decreasing fre-
quency) is indicative of the presence of a cisoid M2(CO)2
oscillator,9 as found in the isoelectronic complexes [W2-
Cp2(µ-X)(µ-CO)(CO)2(µ-dppm)] PF6 (X ) halogen).10
Therefore, the bridging CO must be located in a transoid
disposition with respect to the terminal carbonyls (Chart
3).
The dppm ligand in 5 displays two 31P NMR reso-
nances at 72.5 and 22.2 ppm (J PW ) 294 Hz), so that
the latter corresponds to the phosphorus atom bonded
to tungsten. The unique cyclopentadienyl ligand in the
1
molecule gives rise to a doublet (J HP ) 1 Hz) in the H
NMR spectrum. On selective 31P decoupling of the 72.5
ppm signal, this cyclopentadienyl resonance becomes a
singlet, which proves that this ligand is bonded to the
molybdenum atom. Then, the cyclopentadienylidene
ligand must be η5-bonded to tungsten and η1-bonded to
molybdenum (Chart 4).
a
P-C-P ) Ph2PCH2PPh2.
during high-temperature (thermolytic) decarbonylation.
First, we have been able to detect and characterize
complex 4, which is most likely the immediate precursor
of the dicarbonylic product 2. We have also shown that
the phosphinomethyl ligand in 4 is C-bonded to the
tungsten atom. As a result, it is unlikely that complex
4 can be derived directly from intermediate A, because
this should expectedly render, after P-C bond oxidative
addition, a phosphinomethyl complex having this ligand
C-bonded to molybdenum rather than to tungsten, as
observed in the C-H oxidative addition leading to 5.
As an alternative explanation, it could be thought that,
at high temperatures, intermediate A might be able to
experience at some extent an isomerization process so
that the bridging CO group would be σ-bonded to
molybdenum and π-bonded to tungsten, which then
could allow the formation of 4. However, we consider
this possibility as rather unlikely in light of the dynamic
information available on fluxional homometallic com-
plexes having four-electron CO groups. Thus, in the
case of [Mn2(µ-η1:η2-CO)(CO)4(µ-dppm)2]11 and [Mn2(µ-
η1:η2-CO)(CO)5L(µ-dppm)] (L ) CO,12a PR312b), NMR
experiments have shown that the bridging CO ligand
exchanges its position only with terminal carbonyls in
the metal atom to which it is σ-bonded.
In the case of the isocyanide compound 7, the coor-
dination of the bridging isocyanide as η1-bonded to
tungsten and η2-bonded to molybdenum is based on the
NMR chemical shift of its carbon atom (214.2 ppm), very
similar to that measured for the corresponding ditung-
sten complex (212.3)1 and substantially lower than that
for the dimolybdenum analogue (223.1 ppm).2
Rea ction P a th w a ys in th e Deca r bon yla tion of
Com p lex 1. The results discussed above indicate that
decarbonylation of the heterometallic complex 1 can lead
to either P-C cleavage (complex 2) or C-H cleavage
(complex 5) products, in addition to the triply bonded
dicarbonyl 6. Therefore, this behavior has common
characteristics with both the molybdenum and tungsten
homometallic systems and can be understood by con-
sidering similar reaction pathways,1,2 slightly modified
as shown in Scheme 2. The transformations 1/6 and
1/5, which are the dominant processes at low temper-
atures (photochemical decarbonylations), have been
discussed in detail for the homometallic systems. Here
it should only be noted that the bridging carbonyl in
intermediate A is assumed to be σ-bonded to tungsten
and π-bonded to molybdenum, as found for the isocya-
nide-bridged complex 7. In this way, the C-H bond
oxidative addition of the cyclopentadienyl ligand is
expected to render, via the agostic intermediate B, a
cyclopentadienylidene ligand σ-bonded to molybdenum,
as found for 5. Finally, because of the reversibility of
these low-temperature transformations, the nature of
the photochemical products (5 or 6) is just governed by
the extent of decarbonylation induced in the system.
The heterometallic system, however, offers two new
pieces of information concerning the processes occurring
In order to explain the structure shown by the P-C
cleavage products 4 and 2, we then propose that, at high
temperatures, the unsaturated tricarbonylic intermedi-
ate C is formed, having a two-electron bridging CO
group. P-C bond oxidative addition for this species
could now equally occur on tungsten or molybdenum
centers, so that the process would then occur so as to
give the most stable product.
While we do not have direct evidence for the formation
of intermediate C, there is at least a precedent of this
sort of species as products of decarbonylation in metal-
(9) Braterman, P. S. Metal Carbonyl Spectra; Academic Press:
London, U.K., 1975.
(10) Alvarez, M. A.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.; Bois, C.;
J eannin, Y. Angew. Chem., Int. Ed. Engl. 1993, 32, 1156.
(11) Marsella, J . A.; Caulton, K. G. Organometallics 1982, 1, 274.
(12) (a) Liu, X. Y.; Riera, V.; Ruiz, M. A. Organometallics 1994, 13,
2925. (b) Liu, X. Y.; Riera, V.; Ruiz, M. A. Unpublished results.

1382 Organometallics, Vol. 16, No. 7, 1997
Alvarez et al.
metal-bonded carbonyl dimers. Thus, different studies
have shown that photochemical decarbonylation of [Fe2-
Cp2(CO)4] may yield two independent isomeric products,
the unsaturated [Fe2Cp2(µ-CO)3]13 and the electron-
precise [Fe2Cp2(µ-η1:η2-CO)(CO)2].14,15 The latter is the
most stable isomer and can be isolated as a solid,15 but
the unsaturated isomer is the most stable in the case
of the pentamethylcyclopentadienyl system.16 The above
information suggests that, in the case of our heterome-
tallic system, intermediates A and C might not differ
much in energy, although the electron-precise A is
expected to be more stable. However, even after ac-
cepting the involvement of intermediate C in the forma-
tion of compounds 4 and 2, we cannot yet make a
reasonable proposal about the question of whether A
and C are independent primary products of the decar-
bonylation of 1 or, alternatively, one precedes the other.
It is interesting to compare the relative efficiencies
of the pathways leading to P-C cleavage products in
compound 1 and its homometallic analogues. For the
ditungsten system, P-C bond cleavage of the backbone
of the dppm ligand is only observed at high tempera-
tures and as a very low-efficiency process.1 In contrast,
the dimolybdenum system experiences such a process
with very high efficiency at 70 °C and remains some-
what active even at 0 °C (photochemical decarbonyla-
tion).2 The heterometallic system behaves in an inter-
mediate way, so that the P-C cleavage process is
dominant at high temperatures but is fully suppressed
during low-temperature (photochemical) decarbonyla-
tions. Clearly, the above facts do not have a thermo-
dynamic origin (relative reluctance of the P-C bond to
oxidatively add to tungsten atoms) as, in fact, the P-C
bond oxidative addition in the heterometallic complex
occurs on the tungsten atom. Thus, we conclude that
the relative efficiency of the P-C cleavage process
occurring in these dimetallic complexes has a kinetic
origin. Replacement of tungsten for molybdenum atoms
seems to decrease the relevant energy barriers so as to
increase the rate of oxidative addition of this bond.
ligand. On the basis of the stereochemistry of the
heterometallic products derived from 1, it is proposed
that two different intermediates, having a bridging CO
ligand in either a µ-η1 or µ-η1:η2 coordination mode, are
possibly involved in these decarbonylation reactions.
Exp er im en ta l Section
Gen er a l Com m en ts. The general experimental techniques
and manipulation procedures are described in ref 1a. [MoWCp2-
(CO)4]7 and dppm17 were prepared by literature methods.
NMR spectra were recorded at 291 K and 300.13 (1H), 121.5
(31P{1H}), or 75.47 MHz (13C{1H}) unless otherwise stated.
Chemical shifts are given in ppm, relative to internal TMS
(1H, 13C) or external 85% H3PO4 aqueous solution (31P).
Coupling constants (J ) are given in hertz. Compound 2 is
highly air-sensitive, whereas 4 and 5 are thermally unstable,
so that satisfactory elemental analyses could not be obtained
in these cases.
P r ep a r a tion of [MoWCp 2(CO)4(µ-d p p m )] (1). A dichlo-
romethane solution (20 mL) of [MoWCp2(CO)4] (0.521 g, 1
mmol) was treated with dppm (0.384 g, 1 mmol) and the
mixture was stirred at room temperature for 10 min. Solvent
was then removed under vacuum; the residue was dissolved
in a minimum of toluene, and this solution was chromato-
graphed on a tap-water-refrigerated alumina column (activity
III, 30 × 2.5 cm) prepared in petroleum ether. Elution with
petroleum ether gave a minor fraction containing some
[MoWCp2(CO)6]. Elution with toluene gave a red-orange
fraction, which was taken to dryness under vacuum. Crystal-
lization of the residue from toluene/petroleum ether at -20
°C yielded complex 1 as dark brown crystals (0.679 g, 75%).
Anal. Calcd for C39H32O4P2MoW (1): C, 51.54; H, 3.56.
Found: C, 51.87; H, 3.78. 1H NMR (400.13 MHz, Me2CO-d6,
248 K): δ 8.18-7.04 (Ph), 5.82 (ddd, J HH ) 17, J HP ) 12, 4,
1H, CH2, isomer A), 5.56 (ddd, J HH ) 17, J HP ) 16, 9, 1H, CH2,
isomer A), 5.48 (s, 5H, Cp, isomer B), 5.33 (s, 5H, Cp, isomer
B), 5.17 (m, 1H, CH2, isomer B), 4.70 (m, 1H, CH2, isomer B),
4.60 (s, 5H, Cp, isomer A), 4.54 (s, 5H, Cp, isomer A). Ratio
A:B ca. 1:1.
P r ep a r a t ion of [MoWCp 2(µ-CH2P P h 2)(µ-P P h 2)(CO)2]
(2). A tetrahydrofuran (THF) solution (15 mL) of compound
1 (0.045 g, 0.05 mmol) was refluxed for 4 h to give a dark brown
solution. The solvent was then removed under vacuum; the
residue was dissolved in toluene, and this solution was
chromatographed at 10 °C on an alumina column (activity III,
20 × 2.5 cm) prepared in petroleum ether. Elution with the
latter solvent gave a fraction containing a small amount of
[MoWCp2(CO)6]. Elution with toluene gave a greenish-brown
fraction followed by a minor violet fraction. Removal of solvent
under vacuum from the main fraction yielded complex 2 as
an air-sensitive brown powder (0.027 g, 65%). 1H NMR (200.13
MHz, toluene-d8): δ 7.80-6.80 (20H, Ph), 4.81 (d, J HP ) 0.5,
5H, WCp), 4.77 (s, 5H, MoCp), 3.23 (dd, J HH ) 11, J HP ) 6,
1H, CH2), 2.06 (ddd, J HH ) 11, J HP ) 17, 2, 1H, CH2). 13C-
{1H} NMR (100.61 MHz, C6D6): δ 229.7 (dd, J PC ) 14, 10,
MoCO), 224.2 (dd, J PC ) 7, 5, J WC ) 210, WCO), 148.7-128.0
(Ph), 89.9, 89.7 (2 × s, Cp), -5.74 (d, J CP ) 8, J CW ) 50, WCH2).
Removal of solvent under vacuum from the violet fraction
yielded the oxo compound 3 as a dark gray powder in variable
amounts (usually less than 5%). Anal. Calcd for C36H32O2P2-
MoW (3): C, 51.43; H, 3.34. Found: C, 51.05; H, 3.15. 1H
NMR (C6D6): δ 7.90-6.90 (20H, Ph), 5.27 (s, 5H, WCp), 4.71
(s, 5H, MoCp), 3.33 (td, J HH ) 12, J HP ) 12, 5, 1H, CH2), 2.43
(dd, J HH ) 12, J HP ) 8, 1H, CH2). 13C{1H} NMR (100.61 MHz,
C6D6): δ 234.2 (dd, J PC ) 11, 5, MoCO), 147.0-128.0 (Ph),
106.9 (s, WCp), 87.9 (s, MoCp), -2.20 (d, J CP ) 10, WCH2).
P r ep a r a tion of [MoWCp 2(µ-CH2P P h 2)(µ-P P h 2)(µ-CO)-
(CO)2] (4). Essentially pure complex 4 (by IR and 31P NMR
spectroscopy) is rapidly formed when CO (1 atm) is gently
Con clu d in g Rem a r k s
Depending on reaction conditions (thermolytic or
photolytic), decarbonylation of the heterometallic dimer
1 can result in the oxidative addition of C-H (cyclo-
pentadienyl) or P-C (dppm backbone) bonds to the
dimetal center. In contrast to what might have been
deduced only on the basis of the behavior of the
dimolybdenum or tungsten analogues of 1, both pro-
cesses can occur at either molybdenum or tungsten
centers. The presence of tungsten atoms stabilizes the
products arising from the C-H cleavage, in agreement
with previous findings,3 while the presence of molyb-
denum atoms seems to reduce the temperature thresh-
old for the P-C cleavage of the backbone of the dppm
(13) (a) Caspar, J . V.; Meyer, T. J . J . Am. Chem. Soc. 1980, 102,
7794. (b) Bloyce, P. E.; Campen, A. K.; Hooker, R. H.; Rest, A. J .;
Thomas, N. R.; Bitterwolf, T. E.; Shade, J . E. J . Chem. Soc., Dalton
Trans. 1990, 2833. (c) Dixon, A. J .; George, M. W.; Hughes, C.;
Poliakoff, M.; Turner, J . J . J . Am. Chem. Soc. 1992, 114, 1719.
(14) (a) Zhang, S.; Brown, T. L. J . Am. Chem. Soc. 1992, 114, 2723.
(b) Zhang, S.; Brown, T. L. J . Am. Chem. Soc. 1993, 115, 1779.
(15) Alvarez, M. A.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A. Unpublished
results.
(16) Blaha, J . P.; Burnsten, B. E.; Dewan, J . C.; Frankel, R. B.;
Randolph, C. L.; Wilson, B. A.; Wrighton, M. S. J . Am. Chem. Soc.
1985, 107, 4561.
(17) Agiar, A. M.; Beisler, J . J . Org. Chem. 1964, 29, 1660.

Mo and W Diphosphine-Bridged CO Dimers
Organometallics, Vol. 16, No. 7, 1997 1383
bubbled at room temperature through THF or CH2Cl2 solutions
of complex 2 for about 30 s. Removal of solvent under vacuum
from these solutions gave almost quantitative back-conversion
of 4 into 2. NMR data for 4 were then obtained by carrying
out the above carbonylation process in the deuterated solvent,
inside the NMR tube. 1H NMR (400.13 MHz, CD2Cl2): δ 7.7-
the same solvent mixture gave a very minor orange fraction
containing trace amounts of [MoWCp2(CO)6], followed by a
green fraction. Removal of solvents under vacuum from the
latter yielded complex 6 as a black-green air-sensitive solid
(0.026 g, 62%). Anal. Calcd for C37H32O2P2MoW (6): C, 52.11;
H, 3.79. Found: C, 51.75; H, 3.66. 1H NMR (CD2Cl2): δ 7.60-
7.20 (20H, Ph), 5.73 (t, J HP ) 10, 2H, CH2), 4.57 (d, J HP ) 1,
5H, Cp), 4.54 (d, J HP ) 1, 5H, Cp), 13C{1H} NMR (100.61 MHz,
CD2Cl2): δ 248.4 (d, J PC ) 5, CO), 139.6-125.5 (Ph), 89.4, 87.9
(2 × s, Cp), 67.4 (dd, J CP ) 22, 5, CH2).
7.0 (20H, Ph), 4.95 (s, 5H, Cp), 4.57 (s, 5H, Cp), 1.74 (t, J HH
J HP ) 12, 1H, CH2), 1.43 (dd, J HH ) 12, J HP ) 14, 1H, CH2).
13C{1H} NMR (100.61 MHz, CD2Cl2, 213 K): δ 308.3 (d, J PC
)
)
24, µ-CO), 250.1 (t, J PC ) 13, MoCO), 237.4 (s, WCO), 149.0-
127.1 (Ph), 94.1, 91.0 (2 × s, Cp), -20.0 (d, J CP ) 7, CH2).
P r ep a r a t ion of [MoW(µ-η1:η5-C5H 4)Cp (µ-H )(CO)3(µ-
d p p m )] (5). A THF solution (15 mL) of compound 1 (0.045 g)
was irradiated with visible-UV light at -10 °C for 1 h while
nitrogen was bubbled through the solution gently. This
solution was then filtered at the same temperature, the solvent
was removed under vacuum, and the residue washed with
petroleum ether to give almost pure compound 5 as a brown
solid. This species is unstable at room temperature in solution
or even in the solid state. For NMR studies, the solid
compound was dissolved at -10 °C in toluene-d8. 1H NMR
(400.13 MHz, toluene-d8, 253 K): δ 6.16 (br, 1H, C5H4), 4.99
(br, 1H, C5H4), 4.85 (d, J HP ) 1, 5H, MoCp), 4.63 (m, 1H, C5H4),
4.30 (m, 1H, C5H4), 4.14 (dt, J HH ) 13, J HP ) 10, 1H, CH2),
3.94 (q, br, J HH ≈ J HP ≈ 11, 1H, CH2), -14.36 (ddd, J HP ) 38,
29, J HH ) 1, 1H, µ-H).
P r epar ation of [MoWCp2(CO)2(µ-η1:η2-CNtBu )(µ-dppm )]
(7). tBuCN (6 µL, 0.05 mmol) was added to a THF solution
(15 mL) of compound 6 (0.043 g, 0.05 mmol), and the mixture
was stirred at room temperature for 15 min to give a red-brown
solution. Solvent was then removed under vacuum and the
residue washed with petroleum ether (2 × 5 mL) and extracted
with THF (15 mL). The filtered solution was then concen-
trated under vacuum; petroleum ether was added, and this
mixture was stored overnight at -20 °C. In this way, red-
brown crystals of 7 were obtained (0.037 g, 78%). Anal. Calcd
for C42H41O2NP2MoW (7): C, 53.90; H, 4.42; N, 1.50. Found:
C, 53.61; H, 4.27; N, 1.40. 1H NMR (toluene-d8): δ 8.45-6.86
(20H, Ph), 5.96 (m, 1H, CH2), 5.64 (m, 1H, CH2), 4.58 (d, J HP
) 1, 5H, Cp), 4.22 (s, 5H, Cp), 1.39 (s, 9H, Me). 13C{1H} NMR
(toluene-d8): δ 260.2 (br, MoCO), 227.8 (d, J PC ) 9, WCO),
214.2 (br, WCN), 145.0-120.0 (Ph), 93.6, 88.8 (2 × s, Cp), 79.1
(br, CH2), 56.4 (s, CMe3), 31.5 (s, Me).
P r ep a r a t ion of [MoWCp 2(CO)2(µ-d p p m )] (6). Com-
pound 1 (0.045 g, 0.05 mmol) was photolyzed as described for
5, but at 10 °C for 2 h, to yield a greenish mixture. Solvent
was then removed under vacuum; the residue was dissolved
in dichloromethane/petroleum ether (1:1), and this solution
was chromatographed at 10 °C on an alumina column (activity
III, 20 × 2.5 cm) prepared in petroleum ether. Elution with
Ack n ow led gm en t. We thank the DGICYT of Spain
for financial support (Project PB91-0678).
OM960920J