Formation and cleavage of C-C, C-O, and O-H bonds involving methoxycarbyne and hydroxycarbyne ligands at unsaturated dimolybdenum complexes

Article abstract of DOI:10.1021/om050550b

Reversible carbyne-carbyne coupling occurs upon addition of simple ligands L (CO, ^tBuNC) to the bis(methoxycarbyne) complex [Mo ₂Cp₂(μ-COMe)₂-(μ-PCy₂)]BF ₄ to give the dimethoxyacetylene-br

Full text of DOI:10.1021/om050550b

4122
Organometallics 2005, 24, 4122-4124
Formation and Cleavage of C-C, C-O, and O-H Bonds
Involving Methoxycarbyne and Hydroxycarbyne Ligands
at Unsaturated Dimolybdenum Complexes
Celedonio M. Alvarez, M. Angeles Alvarez, M. Esther Garc´ıa,
Daniel Garc´ıa-Vivo´, and Miguel A. Ruiz*
Departamento de Quı´mica Orga´nica e Inorga´nica/IUQOEM, Universidad de Oviedo,
E-33071 Oviedo, Spain
Received June 30, 2005
Scheme 1
Summary: Reversible carbyne-carbyne coupling occurs
t
upon addition of simple ligands L (CO, BuNC) to
the bis(methoxycarbyne) complex [Mo2Cp2(µ-COMe)2-
(µ-PCy2)]BF4 to give the dimethoxyacetylene-bridged
derivatives [Mo2Cp2{µ-η2:η2-C2(OMe)2}(µ-PCy2)L2]BF4.
Upon demethylation, the dicarbonyl cation experiences
a 1,2-methoxyl shift to give the carboxycarbyne derivative
[Mo2Cp2{µ-C(CO2Me)}(µ-PCy2)(CO)2].
Recently we reported the synthesis of the unsaturated
complex [Mo2Cp2(µ-COMe)(µ-PCy2)(µ-CO)] (1), which
contains a methoxycarbyne ligand bridging a triple
intermetallic bond.1 The presence of a bridging carbonyl
in this relatively electron-rich molecule makes the
corresponding oxygen atom a nucleophilic site, and this
prompted us to examine the reactions of 1 with some
electrophiles as a rational route to compounds contain-
ing two alkoxycarbyne ligands bridging a multiple
metal-metal bond, no examples of which appear to have
been described previously. In the context of the widely
accepted analogy between metal surfaces and metal
clusters,2 a binuclear complex having a multiple metal-
metal bond can be considered the simplest (if crude)
model for a metal surface, as the former shares some of
the principal features of the latter: that is, coordinative
and electronic unsaturation at the metal centers. The
study of the reactivity of such complexes, therefore, can
yield results of interest not only from the point of view
of the stoichiometric and homogeneous reactions but
also as models for elemental steps possibly occurring
during chemical reactions heterogeneously catalyzed by
metal surfaces or nanoparticles.
In this paper we report the synthesis of the bis-
(methoxycarbyne) compound [Mo2Cp2(µ-COMe)2(µ-PCy2)]-
BF4 (2) and the hydroxy- and methoxycarbyne analogue
[Mo2Cp2(µ-COH)(µ-COMe)(µ-PCy2)]BF4 (3), as well as
some unusual transformations experienced by these
highly unsaturated cations, which involve the formation
and cleavage of C-C, C-O, or O-H bonds under very
mild conditions (Scheme 1). Among the latter transfor-
mations we should note the observation of a reversible
coupling between methoxycarbyne ligands induced on
2 by carbonylation. While there are several reports of
carbyne-carbyne coupling yielding coordinated alkynes
or the reverse,3 rarely have these processes been found
as involving alkoxycarbyne ligands4 or being reversible.
In fact, we can quote only two previous examples of
reversible coupling between carbyne ligands of any type,
these occurring at Fe3 or Fe4 clusters.5 Our results thus
show for the first time that two metal centers are
enough to sustain a reversible coupling between carbyne
ligands. This finding might be of interest in the context
of the metal-catalyzed hydrogenation of CO under either
homogeneous or heterogeneous conditions, a subject of
substantial academic and industrial interest, especially
in the current (and future) scene of high prices for crude
oil.6 As for the heterogeneous processes (Fischer-
(3) (a) Shapley, J. R.; Comstock, M. C. Coord. Chem. Rev. 1995, 143,
501. (b) Adams, R. D.; Horvath, I. T. Prog. Inorg. Chem. 1985, 33, 127.
(c) Stone, F. G. A. Angew. Chem., Int. Ed. Engl. 1984, 23, 89.
(4) (a) Bronk, B. S.; Protasiewicz, J. D.; Pence, L. E.; Lippard, S. J.
Organometallics 1995, 14, 2177. (b) Wen-Yann, Y.; Shapley, J. R. J.
Organomet. Chem. 1986, 315, C29. (c) Nuel, D.; Daham, F.; Mathieu,
R. J. Am. Chem. Soc. 1985, 107, 1658.
(5) (a) Nuel, D.; Daham, F.; Mathieu, R. Organometallics 1985, 4,
1436. (b) Okazaki, M.; Ohtani, T.; Ogino, H. J. Am. Chem. Soc. 2004,
126, 4104.
* To whom correspondence should be addressed. E-mail: mara@
fq.uniovi.es.
(1) Garc´ıa, M. E.; Melo´n, S.; Ramos, A.; Riera, V.; Ruiz, M. A.;
Belletti, D.; Graiff, C.; Tiripicchio, A. Organometallics 2003, 22, 1983.
(2) Muetterties, E. L.; Rhodin, T. N.; Band, E.; Brucker, C. F.;
Pretzer, W. R. Chem. Rev. 1979, 79, 91.
(6) For some recent reviews on the subject see, for example: (a)
Maitlis, P. M. J. Organomet. Chem. 2004, 689, 4366. (b) Maitlis, P. M.
J. Mol. Catal. A 2003, 204-205, 55. (c) Dry. M. E. Catal. Today 2002,
71, 227.
10.1021/om050550b CCC: $30.25 © 2005 American Chemical Society
Publication on Web 07/16/2005

Communications
Organometallics, Vol. 24, No. 17, 2005 4123
K, thus giving the hydroxycarbyne derivative 3.11
Spectroscopic data for 3 are similar to those for 2, and
thus both cations are assumed to have the same
structure. Hydroxycarbyne complexes are extremely
rare, generally unstable species,12 and this is also the
case for 3, as it rearranges above 253 K in dichlo-
romethane solution to give the corresponding hydride-
carbonyl isomer [Mo2Cp2(µ-COMe)(H)(µ-PCy2)(CO)]BF4
(4).13 The latter complex is unique in having its hydride
ligand terminally bonded to a metal engaged in a triple
intermetallic bond (bridging hydrides are usually found
for this type of organometallic molecule),14 but this
leaves the dimetal center relatively unprotected, and
further reactions occur at room temperature, currently
under study.
In contrast to the hydroxycarbyne 3, the bis(methoxy-
carbyne) complex 2 is quite stable thermally, and it can
be heated in refluxing 1,2-dichloroethane or acetonitrile
for several hours without significant change. However,
compound 2 reacts readily with simple electron donors
such as CO (293 K, 60 bar) and CNtBu (above 243 K) to
give the corresponding alkyne-bridged derivatives
[Mo2Cp2{µ-η2:η2-C2(OMe)2}(µ-PCy2)L2]BF4 (L ) CO (5),
CNtBu (6)),15,16 as a result of a carbyne-carbyne coup-
ling forced by the incorporation of two new ligands to
the dimetal site. No intermediates were detected in
these reactions. The cation in 6 contains a dimethoxy-
acetylene molecule almost perpendicularly bridging
(twist angle ca. 10°) the dimolybdenum center (Figure
2),17 and the terminal isocyanide groups are arranged
in a trans disposition. Incidentally, we note that com-
pound 6 is the first complex containing the unstable
dimethoxyacetylene molecule to be structurally char-
acterized. The C(1)-C(2) distance, 1.325(3) Å, is quite
short, perhaps indicative of some multiplicity in that
bond, which would imply that the alkyne ligand in 6
could be acting as a donor of less than 4 electrons, thus
yielding a total electron count for the complex below 32.
As a result, the formal order for the intermetallic bond
should be somewhat higher than 2, in agreement with
the Mo(1)-Mo(2) distance of 2.6550(5) Å, a figure
shorter than that expected for a double bond (cf. 2.71 Å
for the 32-electron complex [Mo2Cp2(µ-PPh2)2(CO)2]).18
Figure 1. ORTEP diagram for the cation in compound 2.
Cyclohexyl groups (except the C1 atoms) and H atoms are
omitted for clarity. Selected bond lengths (Å) and angles
(deg): Mo(1)-Mo(2) ) 2.4742(14), Mo(1)-C(1) ) 2.00(1),
Mo(1)-C(2) ) 1.99(1), Mo(1)-P(1) ) 2.405(3), Mo(2)-C(1)
) 2.00(1), Mo(2)-C(2) ) 2.01(1), Mo(2)-P(1) ) 2.409(3),
C(1)-O(1) ) 1.31(1), O(1)-C(3) ) 1.47(2), C(2)-O(2) )
1.33(1), O(2)-C(4) ) 1.48(1); Mo(1)-P(1)-Mo(2) ) 61.9-
(1), Mo(1)-C(1)-Mo(2) ) 76.4(4), Mo(1)-C(2)-Mo(2) )
76.6(4), C(1)-O(1)-C(3) ) 115.8(10), C(2)-O(2)-C(4) )
116.6(9).
Tropsch synthesis), our results suggest that C-C coup-
ling in FT reactions might also occur (even if as a side
pathway) through hydroxycarbyne intermediates, with-
out involvement of fully hydrogenated (CHx) surface
species. As for the homogeneous hydrogenations, our
results suggest that a hypothetical binuclear catalyst
for the formation of C2 oxygenates might not require
extensive hydrogenation up to hydroxymethyl interme-
diates prior to C-C coupling but might promote this
coupling through hydroxycarbyne intermediates as
well. We note that hydroxycarbyne ligands have
been suggested previously as possible intermediates in
carbon monoxide hydrogenation,7 and we have even
accomplished a model COH to CH reduction at the
unsaturated ditungsten cation [W2Cp2(µ-COH)(CO)2-
(µ-Ph2PCH2PPh2)]+.8 However, no experimental evi-
dence for the involvement of hydroxycarbyne species in
real catalytic systems has yet been found.
Compound 1 experiences easy O-alkylation with
(Me3O)BF4 to give the bis(methoxycarbyne) derivative
2.9 The symmetrical cation in this product (Figure 1)10
displays a very short intermetallic length of 2.4742(14)
Å, this being almost identical with that measured at
the neutral precursor 1 and consistent with the triple
Mo-Mo bond formulated for these 30-electron mol-
ecules.1 The bridging carbyne atoms define a Mo2(µ-C)2
dihedral angle of ca. 130° and are not far away from
each other (C(1)‚‚‚C(2) ) 2.836 Å), a circumstance that
surely facilitates their easy coupling (see below).
Compound 1 can be also protonated at the oxygen
position by using HBF4‚OEt2 in dichloromethane at 233
(11) Selected spectroscopic data for 3: 1H NMR (CD2Cl2, 233 K) δ
13.16 (br, 1H, COH), 6.18 (s, 10H, Cp), 3.94 (s, 3H, OMe) ppm; 31P-
{1H} NMR (CD2Cl2, 233 K) δ 261.5 ppm; 13C{1H} NMR (CD2Cl2, 233
K) δ 365.8 (d, br, JCP ) 13 Hz, µ-COH), 363.0 (d, JCP ) 13 Hz, µ-COMe)
ppm.
(12) (a) Garc´ıa, M. E.; Riera, V.; Rueda, M. T.; Ruiz, M. A. J. Am.
Chem. Soc. 1999, 121, 1960. (b) Alvarez, M. A.; Garc´ıa, M. E.; Riera,
V.; Ruiz, M. A. Organometallics 1999, 18, 634 and references therein.
(13) Selected spectroscopic data for 4: νCO (CH2Cl2) 1873 cm-1 1H
;
NMR (CD2Cl2, 243 K) δ 6.00, 5.91 (2 × s, 2 × 5H, Cp), 4.16 (s, 3H,
OMe), -1.40 (d, JHP ) 32 Hz, Mo-H) ppm; 31P{1H} NMR (CD2Cl2,
243 K) δ 294.5 ppm.
(14) Alvarez, C. M.; Alvarez, M. A.; Garc´ıa, M. E.; Ramos, A.; Ruiz,
M. A.; Lanfranchi, M.; Tiripicchio, A. Organometallics 2005, 24, 7 and
references therein.
(15) Selected spectroscopic data for 5: νCO (CH2Cl2) 1954 (vs) cm-1
;
1H NMR (CD2Cl2) δ 5.93 (s, 10H, Cp), 3.95 (s, 6H, OMe) ppm; 31P{1H}
NMR (CD2Cl2) δ 156.7 ppm; 13C{1H} NMR (CD2Cl2) δ 217.1 (d, JCP
12 Hz, CO), 151.3 (d, JCP ) 3 Hz, µ-CC) ppm.
)
(7) (a) Nicholas, K. M. Organometallics 1982, 1, 1713. (b) Muetter-
ties, E. L.; Stein, J. Chem. Rev. 1979, 79, 470.
(16) Selected spectroscopic data for 6: νCN (CH2Cl2) 2113 (vs) cm-1
;
(8) Alvarez, M. A.; Bois, C.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.
Angew. Chem., Int. Ed. Engl. 1996, 35, 102.
1H NMR (CD2Cl2) δ 5.35 (s, 10H, Cp), 3.92 (s, 6H, OMe), 1.16 (s, 18H,
tBu) ppm; 31P{1H} NMR (CD2Cl2) δ 167.0 ppm; 13C{1H} NMR (CD2-
Cl2) δ 160.3 (d, JCP ) 10 Hz, Mo-CN), 147.4 (s, µ-CC) ppm.
(17) X-ray data for 6‚CH2Cl2: red crystals, monoclinic (P21/n), a )
9.2905(19) Å, b ) 17.630(4) Å, c ) 25.898(5) Å, â ) 94.555(4)°, V )
4228.5(15) Å3, T ) 293 K, Z ) 4, R ) 3.04 (observed data with I >
2σ(I)), GOF ) 1.054.
(9) Selected spectroscopic data for 2: 1H NMR (CD2Cl2) δ 6.22 (s,
10H, Cp), 4.00 (s, 6H, OMe) ppm; 31P{1H} NMR (CD2Cl2); δ 264.2 ppm;
13C{1H} NMR (CD2Cl2) δ 366.0 (d, JCP ) 13 Hz, µ-COMe) ppm.
(10) X-ray data for 2‚CH2Cl2: brown crystals, monoclinic (C2/c), a
) 33.404(6) Å, b ) 13.912(2) Å, c ) 13.422(2) Å, â ) 95.131(3)°, V )
6212.4(17) Å3, T ) 293 K, Z ) 8, R ) 7.37 (observed data with I >
2σ(I)), GOF ) 1.028.
(18) Adatia, T.; McPartlin, M.; Mays, M. J.; Morris, M. J.; Raithby,
P. R. J. Chem. Soc., Dalton Trans. 1989, 1555.

4124 Organometallics, Vol. 24, No. 17, 2005
Communications
toward C-O cleavage. Demethylation is cleanly ac-
complished by treatment of 5 with DBU (1,8-diazabicyclo-
[5.4.0]undec-7-ene) but gives unexpectedly the neutral
carboxycarbyne complex [Mo2Cp2{µ-C(CO2Me)}(µ-PCy2)-
(CO)2] (7).20 This seems to result from a 1,2-shift of the
methoxyl group remaining at the ketenyl intermediate
(not detected), which is presumably formed after initial
demethylation in 5, a rearrangement for which we have
found no precedent in the literature.
In summary, we have shown that the presence of
multiple bonds at dimolybdenum centers induces or
allows several unusual transformations involving meth-
oxycarbyne or hydroxycarbyne bridging ligands, these
including hydroxycarbyne/hydride-carbonyl isomeriza-
tion, reversible coupling between methoxycarbyne
ligands, and 1,2-methoxyl shifts at ketenyl intermedi-
ates. Apart from their novelty, these transformations
might be used as models for some of the complex
reactions taking place during metal-catalyzed reduc-
tions of carbon monoxide, under either homogeneous or
heterogeneous conditions. Further work is now under
way to extend the scope of the above transformations.
Figure 2. ORTEP diagram for the cation in compound 6.
Cy (except the C1 atoms) and tBu groups and H atoms are
omitted for clarity. Selected bond lengths (Å) and angles
(deg): Mo(1)-Mo(2) ) 2.6550(5), Mo(1)-C(1) ) 2.103(2),
Mo(1)-C(2) ) 2.262(2), Mo(1)-P(1) ) 2.3796(8), Mo(2)-
C(1) ) 2.275(2), Mo(2)-C(2) ) 2.107(2), Mo(2)-P(1) )
2.3801(7), Mo(1)-C(5) ) 2.055(2), Mo(2)-C(6) ) 2.058(2),
C(1)-O(1) ) 1.374(3), O(1)-C(3) ) 1.427(3), C(2)-O(2) )
1.371(3), O(2)-C(4) ) 1.435(3); C(5)-Mo(1)-Mo(2) )
90.2(1), C(6)-Mo(2)-Mo(1) ) 92.8(1), Mo(1)-P(1)-Mo(2)
) 67.81(2), Mo(1)-C(1)-Mo(2) ) 74.6(1), Mo(1)-C(2)-
Mo(2) ) 74.7(1), C(1)-O(1)-C(3) ) 112.7(2), C(2)-O(2)-
C(4) ) 112.6(2).
Acknowledgment. We thank the MCYT/MECD of
Spain for a grant (to D.G.-V.) and financial support
(Grant No. BQU2003-05471).
The coupling reaction leading to compound 5 is fully
reversible, and the starting carbyne complex 2 can be
rapidly (ca. 5 min) and quantitatively (by NMR) regen-
erated through photochemical decarbonylation of tet-
rahydrofuran solutions of 5 at 285 K. No intermediates
were detected in this double-decarbonylation process.
As stated above, only two precedents for reversible
coupling between carbyne ligands have been reported
previously,5 and the closest precedent involving bi-
nuclear complexes is found in the (irreversible) cleavage
of alkynes by triply bonded ditungsten alkoxide or
silyloxide complexes.19
Supporting Information Available: Text giving experi-
mental procedures and spectroscopic data for new compounds
and a CIF file giving crystallographic data for compounds 2
and 6. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM050550B
(19) (a) Listemann, M. L.; Schrock, R. R. Organometallics 1985, 4,
74. (b) Chamberlin, R. M. L.; Rosenfeld, D. C.; Wolczanski, P. T.;
Lobkovsky, E. B. Organometallics 2002, 21, 2724.
(20) Selected spectroscopic data for 7: νCO (CH2Cl2) 1900 (vs, Mo-
CO), 1652 (w, CdO) cm-1; 1H NMR (C6D6) δ 5.27 (s, 10H, Cp), 4.02 (s,
3H, OMe) ppm; 31P{1H} NMR (C6D6) δ 128.2 ppm; 13C{1H} NMR (C6D6)
δ 402.8 (s, µ-C), 221.4 (d, JCP ) 11 Hz, Mo-CO), 191.8 (s, CdO) ppm.
The dimethoxyacetylene ligand in the cation 5 is
activated not only toward C-C bond cleavage but also