Chemistry of unsaturated group 6 metal complexes with bridging hydroxy and methoxycarbyne ligands. 3. Formation and cleavage of C-C and C-O bonds in the reactions of the complexes [Mo2Cp2(μ-COMe)(μ-COR) (μ-PCy2)]BF4 (R = Me, Et)

Article abstract of DOI:10.1021/om7010358

The reactions of the 30-electron complexes [Mo₂Cp ₂(μ-COMe)(μ-COR)(μ-PCy₂)JBF₄ (Cp = η⁵-C₅H₅; R = Me, Et) with CO, CN ^tBu, or tetraethylpyrophosphite (tedip) lead to the new 32-electron complexes [Mo₂Cp₂{μ-η²: η²-C(OMe)C(OR)}(μ-PCy₂)(CO)₂]BF ₄, [Mo₂Cp₂(μ-η²: η²-C(OMe)C(OMe)}(μ-PCy₂)(CN^tBu) ₂]BF₄, and [Mo₂Cp₂{μ- η²:η²-C(OMe)C(OMe)}(μ-PCy₂)(μ- tedip)]BF₄, respectively. These products contain a bonded dialkoxyacetylene molecule resulting from the coupling of two alkoxycarbyne ligands, induced by the addition of the donor molecules to the unsaturated dimetal center of the starting material, and this coupling can be fully reversed in the dicarbonyl product upon photochemical treatment. The structure of the diisocyanide and tedip complexes, both displaying quite short intermetallic lengths (ca. 2.66 A), was confirmed crystallographically. The structure of the dicarbonyl derivative, which in solution exhibits cis and trans isomers, was optimized using DFT methods, which also led to the prediction of short intermetallic distances (2.75 A). Demethylation of the dicarbonyl complexes leads to the new carboxycarbyne derivatives [Mo₂Cp ₂(μ-C(CO₂R)} (μ-PCy₂)(CO)₂] (R = Me, Et) as a result of the unexpected 1,2-shift of the methoxyl group occurring in the ketenyl intermediate (not detected) presumably formed first. This proposal is supported by DFT calculations, these predicting a moderate activation barrier (ΔG?₂₉₈ = 78 kJmol^-1) for the spontaneous transformation (ΔG₂₉₈ = -51 kJ mol ^-1) of the ketenyl intermediate into the carbyne complex finally isolated. The bis(methoxycarbyne) complex is also reactive toward the 16-electron fragment Ru(CO)₄, to give the 46-electron cluster [Mo₂RuCp₂(μ-COMe)₂(μ-PCy ₂)(CO)₄]BF₄, which concentrates most of its electronic unsaturation at the dimolybdenum center (Mo-Mo = 2.686(1) A), in spite of the rearrangement of the carbyne ligands, both of them now bridging one of the Mo-Ru edges, according to an X-ray diffraction study.

Full text of DOI:10.1021/om7010358

Organometallics 2008, 27, 543–554
543
Chemistry of Unsaturated Group 6 Metal Complexes with Bridging
Hydroxy and Methoxycarbyne Ligands. 3. Formation and Cleavage
of C-C and C-O Bonds in the Reactions of the Complexes
[Mo2Cp2(µ-COMe)(µ-COR)(µ-PCy2)]BF4 (R ) Me, Et)
M. Esther García, Daniel García-Vivó, and Miguel A. Ruiz*
Departamento de Química Orgánica e Inorgánica/IUQOEM, UniVersidad de OViedo,
E-33071 OViedo, Spain
ReceiVed October 16, 2007
The reactions of the 30-electron complexes [Mo2Cp2(µ-COMe)(µ-COR)(µ-PCy2)]BF4 (Cp ) η5-C5H5;
R ) Me, Et) with CO, CNtBu, or tetraethylpyrophosphite (tedip) lead to the new 32-electron complexes
[Mo2Cp2{µ-η2:η2-C(OMe)C(OR)}(µ-PCy2)(CO)2]BF4,
[Mo2Cp2{µ-η2:η2-C(OMe)C(OMe)}(µ-
PCy2)(CNtBu)2]BF4, and [Mo2Cp2{µ-η2:η2-C(OMe)C(OMe)}(µ-PCy2)(µ-tedip)]BF4, respectively. These
products contain a bonded dialkoxyacetylene molecule resulting from the coupling of two alkoxycarbyne
ligands, induced by the addition of the donor molecules to the unsaturated dimetal center of the starting
material, and this coupling can be fully reversed in the dicarbonyl product upon photochemical treatment.
The structure of the diisocyanide and tedip complexes, both displaying quite short intermetallic lengths
(ca. 2.66 Å), was confirmed crystallographically. The structure of the dicarbonyl derivative, which in
solution exhibits cis and trans isomers, was optimized using DFT methods, which also led to the prediction
of short intermetallic distances (2.75 Å). Demethylation of the dicarbonyl complexes leads to the new
carboxycarbyne derivatives [Mo2Cp2{µ-C(CO2R)}(µ-PCy2)(CO)2] (R ) Me, Et) as a result of the
unexpected 1,2-shift of the methoxyl group occurring in the ketenyl intermediate (not detected) presumably
formed first. This proposal is supported by DFT calculations, these predicting a moderate activation
barrier (∆Gq298 ) 78 kJmol-1) for the spontaneous transformation (∆G298 ) -51 kJ mol-1) of the ketenyl
intermediate into the carbyne complex finally isolated. The bis(methoxycarbyne) complex is also reactive
toward the 16-electron fragment Ru(CO)4, to give the 46-electron cluster [Mo2RuCp2(µ-COMe)2(µ-
PCy2)(CO)4]BF4, which concentrates most of its electronic unsaturation at the dimolybdenum center
(Mo-Mo ) 2.686(1) Å), in spite of the rearrangement of the carbyne ligands, both of them now bridging
one of the Mo-Ru edges, according to an X-ray diffraction study.
quote only two previous examples of reversible coupling
between carbyne ligands of any type, these occurring at the Fe3
or Fe4 metal centers of the complexes [Fe3(µ-CCH3)(µ-CO-
Et)(CO)9]5 and [Fe4(η5-C5H4Me)4(µ-CH)2(µ3-CO)3],6 these be-
ing triggered through carbonylation and two-electron reduction,
respectively. Moreover, the behavior of the dimethoxyacetylene
complex 3a was also unusual in yielding the carboxycarbyne
complex [Mo2Cp2{µ-C(CO2Me)}(µ-PCy2)(CO)2] upon dem-
ethylation, this implying the occurrence of an unexpected 1,2-
shift of the OMe group.2 We thus decided to study these unusual
transformations in more detail. We have noted previously that
the study of the reactivity of hydroxy- and alkoxycarbyne ligands
at unsaturated binuclear centers might also be of interest in the
context of metal-catalyzed hydrogenation of CO under either
homogeneous or heterogeneous conditions.7 In this context, our
preliminary results on the reactivity of complex 1a were relevant
since for the first time they suggested that the C-C coupling
Introduction
Recently we have described full details of the synthesis and
electronic structure of the unsaturated bis(alkoxycarbyne) cat-
ionic complexes [Mo2Cp2(µ-COMe)(µ-COR)(µ-PCy2)]BF4 [R
) Me (1a), Et (1b)].1 A preliminary study on the bis(methoxy-
carbyne) complex 1a had revealed that this unsaturated cation
might experience unusual transformations involving the forma-
tion and cleavage of C-C or C-O bonds under very mild
conditions.2 In particular, a rare coupling between methoxy-
carbyne ligands could be induced on 1a by the double addition
of two-electron donor molecules, to give the alkyne-bridged
complexes [Mo2Cp2{µ-η2:η2-C2(OMe)2}(µ-PCy2)(L)2]BF4 [L )
CNtBu (2), L ) CO (3a)]. Interestingly, the coupling process
leading to compound 3a could be fully reversed through
photochemical decarbonylation, to regenerate compound 1a
quantitatively.2 While there are several reports of carbyne-
carbyne coupling reactions yielding coordinated alkynes or the
reverse,3 rarely have these processes been found involving
alkoxycarbyne ligands,4 or being reversible. In fact, we can
(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.
* To whom correspondence should be addressed. E-mail: mara@
uniovi.es.
(1) García, M. E.; García-Vivó, D.; Ruiz, M. A.; Aullón, G.; Alvarez,
S. Organometallics 2007, 26, 4930.
(5) Nuel, D.; Daham, F.; Mathieu, R. Organometallics 1985, 4, 1436.
(6) Okazaki, M.; Ohtani, T.; Ogino, H. J. Am. Chem. Soc. 2004, 126,
4104.
(2) Alvarez, C. M.; Alvarez, M. A.; García, M. E.; García-Vivó, D.;
Ruiz, M. A. Organometallics 2005, 24, 4122.
10.1021/om7010358 CCC: $40.75
2008 American Chemical Society
Publication on Web 02/02/2008

544 Organometallics, Vol. 27, No. 4, 2008
García et al.
Chart 1
reactions in the Fischer–Tropsch synthesis might occur (even
if as a side pathway) at only two metal centers and without the
involvement of fully hydrogenated (CHx) surface species, via
hydroxycarbyne derivatives. However, although hydroxycarbyne
ligands have been suggested as possible intermediates in carbon
monoxide hydrogenation,8 we still note that no experimental
evidence of their formation in the real catalytic systems has
yet been found.
In this paper we report full details of our study on the
reactions of the unsaturated cations 1a,b toward simple donor
molecules to give alkoxyalkyne-bridged derivatives. This has
been complemented with density functional theory (DFT)
calculations on the carbonyl derivatives, to gain a better
understanding of the reversibility of the C-C coupling between
alkoxycarbyne ligands so induced. Full details of the demethy-
lation reactions of the alkoxyalkyne-bridged complexes are also
included, these being in turn complemented by DFT calculations
of the corresponding reaction profile, here involving the
unexpected 1,2-shift of a methoxyl group at an unstable ketenyl
intermediate.
Results and Discussion
Reactions of Complexes 1a,b toward Donor Molecules.
As expected for 30-electron species,1 the complexes 1a,b react
readily with simple electron-donor molecules to give the
corresponding products derived from the addition of the donors
to their unsaturated Mo2 centers. However, this is accompanied
by a C-C coupling between the alkoxycarbyne ligands present
in the starting substrate, thus yielding the corresponding alkyne-
bridged derivatives.
reversing the synthetic route leading to 1a.1 However, the
reaction of compound 1a with the diphosphite (EtO)2POP(OEt)2
(tedip) proceeds slowly (24 h) to give in quantitative yield the
corresponding addition product [Mo2Cp2{µ-η2:η2-C2(OMe)2}(µ-
PCy2)(µ-tedip)]BF4 (4), in which the P atoms are forced to
occupy cisoid positions due to the geometrical constraints
imposed by the diphosphite backbone.
Solid-State and Solution Structure of Compound 2. The
structure of compound 2 was confirmed through a single-crystal
X-ray diffraction study (Figure 1).2 The molecule displays two
MoCp(CNtBu) fragments in a transoid arrangement and bridged
by dicyclohexylphosphide and dimethoxyacetylene ligands, so
that the P and Mo atoms and the centroid of the C-C bond of
the alkyne are all placed roughly in the same plane. The alkyne
ligand is slightly twisted (ca. 12°) away from the ideal
perpendicular positioning of the C-C bond relative to the
intermetallic vector, which results in the presence of two shorter
(ca. 2.10 Å) and two longer (ca. 2.27 Å) Mo-C bonds. Although
we are not aware of any previous structural determination of a
complex containing the unstable dimethoxyacetylene molecule,
we note that these lengths only slightly exceed the range of
Compound 1a reacts with CNtBu (above 243 K) or CO (60
bar, 293 K) to give quantitatively the alkyne-bridged complexes
[Mo2Cp2{µ-η2η2-C2(OMe)2}(µ-PCy2)(L)2]BF4 [L ) CNtBu (2),
CO (3a)]. In solution, compound 2 is present only as the
corresponding trans isomer, whereas the dicarbonyl complex
3a is obtained as a mixture of the isomers trans-3a and cis-3a
(Chart 1), which in solution are in equilibrium (trans-3a/cis-3a
) 8 in CD2Cl2 at 293 K). In a similar way, compound 1b reacts
under the same conditions with CO to give the new alkyne-
bridged complexes [Mo2Cp2{µ-η2:η2-C2(OEt)(OMe)}(µ-PCy2)-
(CO)2]BF4 (3b). This complex also displays cis/trans isomerism
in solution, but here the presence of an asymmetric alkyne ligand
renders two possible cis isomers, depending on the relative
orientation of the alkoxy and carbonyl groups (Chart 1), both
of which are present in similar (even if small) amounts (see
Experimental Section). The absence of any cis isomer for the
diisocyanide complex 2 surely is a steric effect, since the almost
parallel arrangement of the isocyanide ligands thus implied
would lead to severe repulsions between the corresponding tBu
groups.
As for the bidentate P-donor molecules, we found that
compound 1a does not react with the diphosphine Ph2PCH2PPh2
even in refluxing tetrahydrofuran solutions, perhaps due to
unfavorable steric effects. Not surprisingly, the use of a more
basic diphosphine such as Me2PCH2PMe2 leads to demethylation
of one of the methoxycarbyne ligands, to give almost cleanly
the neutral complex [Mo2Cp2(µ-COMe)(µ-PCy2)(µ-CO)], thus
(7) (a) Alvarez, M. A.; García, M. E.; Riera, V.; Ruiz, M. A.; Robert,
F. Organometallics 2002, 21, 11777. (b) Alvarez, M. A.; García, M. E.;
Riera, V.; Ruiz, M. A. Organometallics 1999, 18, 634. (c) Alvarez, M. A.;
Bois, C.; García, M. E.; Riera, V.; Ruiz, M. A. Angew. Chem., Int. Ed.
Engl. 1996, 35, 102.
Figure 1. ORTEP diagram (30% probability) of the cation in
t
compound 2.2 Cy rings (except the C1 atoms), Bu groups, and H
(8) (a) Nicholas, K. M. Organometallics 1982, 1, 1713. (b) Muetterties,
E. L.; Stein, J. Chem. ReV. 1979, 79, 470.
atoms are omitted for clarity.

Unsaturated Group 6 Metal Complexes
Organometallics, Vol. 27, No. 4, 2008 545
Table 1. Selected IRa and NMRb Data for New Compounds
compound
ν(CO)
δ(P)
δ(µ-C)/(JCP)
[Mo2Cp2{µ-η2:η2-C2(OMe)2}(µ-PCy2)(CNtBu)2]BF4 (2)
[Mo2Cp2{µ-η2:η2-C2(OMe)2}(µ-PCy2)(CO)2]BF4 (trans-3a)
(cis-3a)
2113 (vs)c, 2063 (w, sh)c, 1617 (w)d
1954 (vs), 1602 (w)d
1992 (s)e
167.0
156.7
187.7
156.2
147.4
151.3 (3)
[Mo2Cp2{µ-η2:η-C2(OEt)(OMe)}(µ-PCy2)(CO)2]BF4 (trans-3b) 1953 (vs), 1602 (w)d
150.7 (3), 149.8 (3)
(cis-3b), (cis-3b′)
1994 (s)e
187.7, 181.1
[Mo2Cp2{µ-η2:η2-C2(OMe)2}(µ-PCy2)(µ-tedip)]BF4 (4)
[Mo2Cp2{µ-C(CO2Me)}(µ-PCy2)(CO)2] (5a)
[Mo2Cp2{µ-C(CO2Et)}(µ-PCy2)(CO)2] (5b)
[Mo2Cp2{µ-C(CO2Me)}(µ-PCy2)(µ-CO)] (6)
[Mo2RuCp2(µ-COMe)2(µ-PCy2)(CO)4]BF4 (7)
164.9 (t), 159.7 (d)f 158.1 (12, 2), 117.0 (28, 3)
1929 (w, sh), 1900 (s), 1652 (w)g
1929 (w, sh), 1900 (s), 1652 (w)g
1702 (s), 1665 (s)g
128.2h
128.1h
237.1
402.8h
403.3h
361.1 (15)
385.2 (4), 375.0 (4)
2071 (vs), 2026 (s), 1996 (s), 1956 (w) 200.5
a Recorded in dichloromethane solution, ν in cm-1
.
b Recorded in CD2Cl2 solutions at 290 K and 121.50 (31P) or 75.48 (13C) MHz, δ in ppm relative
31P); J in Hz. c ν(CN). d ν (CC). e The expected antisymmetric stretch for these isomers is
to internal TMS (13C) or external 85% aqueous H3PO4
(
masked by the strong band of the major trans isomer. JPP ) 22. g Stretching band of the CO2R group. h Recorded in C6D6 solution.
f
values found for alkyne-bridged compounds of the type [M2Cp2-
(CO)4(µ-η2:η2-C2R2)] (M ) Mo, R ) H, Et, Ph;9a M ) W, R
) H)9b (M-C lengths in the range 2.22–2.12 Å). In the latter
complexes the alkyne ligand is best described as a four-electron
donor, displaying C-C bond lengths of ca. 1.33 Å, a figure
quite similar to that found for the dimethoxyacetylene ligand
in 2 (1.325(3) Å). Thus we can conclude that the bonding
situation in compound 2 is similar, although we have to be aware
that carbon-carbon distances are not a very reliable probe for
analyzing the metal-alkyne bonding.10 With this caution, we
then should formulate a double M-M bond for this 32-electron
complex. In agreement with this, the Mo-Mo distance is quite
short, 2.6550(5) Å, consistent with a double metal-metal bond
in the presence of small donor atoms at the bridging positions.
For instance, the intermetallic bond length in the isoelectronic
carbyne-bridged complex [Mo2Cp2{µ-C(CO2Me)}(µ-PCy2)-
(CO)2] is just slightly longer (2.686(1) Å, see below) and that
in the azavinylidene complex trans-[Mo2Cp2(µ-NdCHPh)(µ-
PCy2)(CO)2] is slightly shorter (2.632(1) Å),11 with the differ-
ence here being mainly attributed to the lower covalent radius
of nitrogen, relative to that of carbon.
The spectroscopic data for compound 2 (Table 1 and
Experimental Section) are fully consistent with its solid-state
structure. In particular, the coupling process leading to the
alkyne ligand is clearly denoted by the disappearance of the
characteristic highly deshielded 13C NMR resonances of the meth-
oxycarbyne groups (366.0 ppm in 1a), to render a quite shielded
resonance instead (147.4 ppm). The chemical shifts of these C
atoms are similar to those previously reported by us for the
isoelectronic alkyne-bridged cations [Mo2Cp2{µ-η2:η2-HC2(p-
tol)}(L)2(µ-L′2)]+2 (L ) CO, CNtBu; L′2 ) dppm, dmpm);12
however, we have to point out that all these values are slightly
higher than those found for the neutral and electron-precise
complexes [M2Cp2(CO)4(µ-η2:η2-C2R2)] mentioned above.9 This
possibly reflects not only the higher charge and oxidation state
of the molybdenum atoms in our complexes, but also the
difference in the total electron count for these molecules (32-
electron vs 34-electron species). On the other hand, the 31P NMR
spectrum of 2 displays a relatively deshielded resonance (167.0
ppm) when compared to other related 32-electron complexes
such as [Mo2Cp2(µ-η1:η2-CMedCH2)(µ-PCy2)(CO)2] (131.2
ppm).13 This might possibly be attributed to a less efficient donor
ability of the dimethoxyacetylene bridge in the cationic complex,
when compared with that of an alkenyl ligand, since such
electronic property seems to correlate with the 31P chemical
shifts in molecules of the type trans-[Mo2Cp2(µ-PCy2)(µ-
X)(CO)2].14 We finally note that the IR spectrum of 2 shows,
in addition to the main C-N stretching band at 2113 cm-1, a
weak additional C-N band at 2063 cm-1. This is not unusual
for terminal isocyanide ligands, which can adopt slightly bent
conformations in solution, along with the more stable (linear)
conformations.15
Solution Structure of the Dicarbonyls 3a,b. As stated
above, these complexes are obtained as a mixture of isomers
differing in the relative positioning of the MoCp(CO) fragments,
with the isomer trans being dominant in both cases.
For compound 3a two sharp resonances are present in its 31
P
NMR spectrum. The resonance for the major isomer (trans-3a)
is only slightly more shielded than that of the trans-diisocyanide
analogue 2 (156.7 vs 167.0 ppm), while the minor isomer (cis-
3a) exhibits a resonance some 30 ppm more deshielded (187.7
ppm). This seems to be a spectroscopic feature of the cis/trans
isomerism in these 32-electron complexes, and it has been
previously detected in the isoelectronic bis(phosphide) pairs cis-
and trans-[Mo2Cp2(µ-PR2)(µ-PR′2)(CO)2] (R ) R′ ) Ph; R )
t
Ph, R′ ) Bu), with the resonances corresponding to the cis
isomers appearing some 45 ppm above those of the trans
isomers.16 The IR spectrum also reflects the presence of
two isomers: for the major isomer (trans-3a) we can clearly
identify one strong band in the region of terminal carbonyls
(1954 cm-1), consistent with the presence of two CO ligands
arranged trans, almost antiparallel to each other;17 in addition,
there is a weak band at higher frequencies (1992 cm-1), which
can be assigned to the symmetric C-O stretch of its cis-
dicarbonyl isomer cis-3a. Similar trends in the C-O stretching
frequencies have been found for the cis and trans isomers of
the mentioned bis(phosphide) complexes.16 The existence of a
C2 symmetry axis in the major isomer trans-3a is clearly denoted
1
by the simplicity of the H and 13C NMR spectra, displaying
single resonances for the pairs of equivalent Cp, OMe, and CO
(9) (a) Bailey, W. I.; Chisholm, M. H.; Cotton, F. A.; Rankel, L. A.
J. Am. Chem. Soc. 1978, 100, 5764. (b) Ginley, D. S.; Bock, C. R.;
Wrighton, M. S.; Fischer, B. E.; Tipton, D. L.; Bau, R. J. Organomet. Chem.
1978, 157, 41.
(13) García, M. E.; Melón, S.; Ramos, A.; Riera, V.; Ruiz, M. A.;
Belletti, D.; Graiff, C.; Tiripicchio, A. Organometallics 2003, 22, 1983.
(14) García; , M. E.; Ramos, A.; Ruiz, M. A.; Lanfranchi, M.; Marchio,
L. Organometallics 2007, 26, 6197.
(10) (a) Templeton, J. L. AdV. Organomet. Chem. 1989, 29, 1. (b) Ricard,
L.; Weiss, R.; Newton, W. E.; Chen, G. J.; McDonald, J. W. J. Am. Chem.
Soc. 1978, 100, 1318.
(15) Singleton, E.; Oosthuizen, H. E. AdV. Organomet. Chem. 1983,
22, 209.
(16) (a) García, M. E.; Riera, V.; Ruiz, M. A.; Rueda, M. T.; Sáez, D.
Organometallics 2002, 21, 5515. (b) Alvarez, M. A.; García, M. E.;
Martinez, M. E.; Ramos, A.; Ruiz, M. A.; Sáez, D.; Vaissermann, J. Inorg.
Chem. 2006, 45, 6965.
(11) Alvarez, M. A.; García, M. E.; Ramos, A.; Ruiz, M. A. Organo-
metallics 2007, 26, 1461.
(12) Alvarez, M. A.; Anaya, Y.; García, M. E.; Ruiz, M. A. J.
Organomet. Chem. 2007, 692, 983.
(17) Braterman, P. S. Metal Carbonyl; Academic Press: London, 1975.

546 Organometallics, Vol. 27, No. 4, 2008
García et al.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Compound 4
dimethoxyacetylene, diphosphite, and dicyclohexylphosphide,
with the Mo and P atoms, and the centroid of the C-C bond of
the alkyne placed almost in the same plane, while the diphos-
phite ligand is placed perpendicular to that plane (P-Mo-P
angles ca. 94°). The intermetallic distance, 2.655(1) Å, is
identical to that found for the diisocyanide complex 2 and,
therefore, consistent with the double metal-metal bond for-
mulated according to the EAN rule. The cisoid arrangement of
MoCp moieties in 4 has little effect (compared to 2) on the
Mo-C and C-C bond lengths of the alkyne either, although
the alkyne ligand is now placed almost perfectly perpendicular
to the metal-metal bond (twist angle of ca. 4°), probably as a
consequence of the steric pressure induced by the proximity of
the ethoxy groups of the tedip ligand. We note also that both
the phosphide and diphosphite ligands display similar M-P
bond lengths, a feature previously found by us for related
compounds such as the electron-precise hydride [Mo2Cp2(µ-
H)(µ-PCy2)(CO)2(µ-tedip)].11
Spectroscopic data in solution for compound 4 are fully
consistent with the structure found in the crystal. The 31P NMR
spectrum displays a doublet at 159.7 ppm, in the same region
as other tedip-bridged dimolybdenum complexes,11,18 and a
triplet at 164.9 ppm, which is therefore assigned to the phosphide
ligand, with a chemical shift similar to that in 2. The coupling
between phosphite and phosphide P atoms (JPP ) 22 Hz) falls
in the lower part of the range usually found for a cis arrangement
in this type of cyclopentadienyl complexes.19 On the other hand,
the presence of a symmetry plane (rather than a C2 symmetry
axis) relating both metal centers in 4 causes the appearance in
the corresponding 1H and 13C NMR spectra of just one
resonance for the equivalent Cp ligands, but two separated ones
for the methoxyl groups of the bridging alkyne. The metal-bound
C atoms also give rise to separated resonances (δC ) 158.1
ppm, triplet of doublets(td) with JCP ) 12, 2 Hz; δC ) 117.0
ppm, td with JCP ) 28, 3 Hz). By considering the general trends
for 2JXY in complexes of the type CpMXYL2,20,21 we can justify
that the lower couplings (with values of 2 and 3 Hz, respectively)
correspond to those with the phosphide ligand, since the
corresponding C-Mo-P angles are all similar (ca. 103° for
C(1b) and 108° for C(2b), Figure 2). In addition, the values of
the tedip-alkyne P-C couplings allow us to assign the more
strongly coupled resonance (117 ppm) to that of the carbon atom
placed closer to the tedip bridge (C(2b) in Figure 2), since this
implies the smaller P-Mo-C angle (ca. 78° in the crystal).
In contrast, the carbon atom closer to the Cp ligands (C(1b) in
Figure 2) displays much larger C-Mo-P angles (ca. 113° in
the crystal), and therefore it is expected to display a smaller
Mo(1b)-Mo(2b)
Mo(1b)-C(1b)
Mo(2b)-C(1b)
Mo(1b)-C(2b)
Mo(2b)-C(2b)
Mo(1b)-P(1b)
Mo(2b)-P(1b)
Mo(1b)-P(2b)
Mo(2b)-P(3b)
C(1b)-C(2b)
C(1b)-O(1b)
O(1b)-C(3b)
C(2b)-O(2b)
O(2b)-C(4b)
2.655(1)
2.131(7)
2.207(6)
2.273(7)
2.166(6)
2.401(2)
2.394(2)
2.390(2)
2.373(2)
1.345(9)
1.369(7)
1.428(8)
1.356(8)
1.417(10)
Mo(1b)-Mo(2b)-P(3b)
Mo(2b)-Mo(1b)-P(2b)
Mo(1b)-C(1b)-Mo(2b)
Mo(1b)-C(2b)-Mo(2b)
Mo(1b)-P(1b)-Mo(2b)
P(1b)-Mo(1b)-P(2b)
P(1b)-Mo(2b)-P(3b)
C(1b)-Mo(1b)-P(2b)
C(1b)-Mo(2b)-P(3b)
C(2b)-Mo(1b)-P(2b)
C(2b)-Mo(2b)-P(3b)
C(1b)-O(1b)-C(3b)
C(2b)-O(2b)-C(4b)
P(2b)-O(5b)-P(3b)
94.6(1)
90.1(1)
75.5(2)
73.4(2)
67.3(1)
93.3(1)
93.5(1)
111.7(2)
115.9(2)
76.3(2)
80.4(2)
114.6(5)
113.6(5)
121.9(3)
groups, as well as six 13C resonances for the equivalent
cyclohexyl groups. Due to its low relative proportion, not all
resonances of the isomer cis-3a could be identified in these
spectra, but the data available are informative enough to support
1
the proposed structure. In particular, the H and 13C spectra
exhibit one Cp but two OMe resonances, in agreement with
the presence of a symmetry plane, rather than a C2 axis, relating
the metal fragments of the molecule.
The spectroscopic data for the ethoxy(methoxy)acetylene
complex 3b (Table 1 and Experimental Section) are very similar
to those just discussed for the complex 3a, and then a detailed
analysis is not needed, although there are some minor differ-
ences. First, its IR spectrum lacks a high-frequency band,
indicative of the inexistence or very low proportion of any cis
isomers. In agreement with this, the 31P NMR spectrum reveals
the presence of two very minor resonances in the region
expected for these cis-dicarbonyls (ca. 187 ppm), along with
the resonance of the major trans isomer (156.2 ppm). The
asymmetry of the alkyne ligand implies for the major isomer
trans-3b the absence of any symmetry elements, this explaining
the appearance of independent resonances for each of the Cp
and CO groups and the appearance of 12 13C cyclohexyl
resonances. In contrast, both cis isomers (differing in the relative
orientation of the alkoxy groups with respect to the CO ligands,
Chart 1) display equivalent cyclopentadienyl ligands.
Solid-State and Solution Structure of Compound 4. The
structure of compound 4 was confirmed through a single-crystal
X-ray diffraction study (Table 2 and Figure 2). The asymmetric
unit contains two independent molecules of the complex with
no major differences between them. As it can be seen, the cation
in 4 displays two cisoid MoCp moieties bridged by three ligands,
P-C coupling, as found for the 158.1 ppm resonance (JCP
)
12 Hz). Note the large chemical shift difference (δ ) 41.1 ppm)
between these two carbon resonances, derived just from their
different positioning with respect to the tedip ligand, an effect
perhaps related to their distinct average Mo-C bond lengths
[2.17 Å for C(1b) and 2.22 Å for C(2b)].
Reversibility of the Carbyne-Carbyne Coupling. The
C-C bond formation process leading to the dimethoxyacetylene
complexes 2 to 4 was found to be fully reversible in the case
(18) Riera, V.; Ruiz, M. A.; Villafañe, F. Organometallics 1992, 11,
2854.
(19) Alvarez, M. A.; Anaya, Y.; García, M. E.; Riera, V.; Ruiz, M. A.
Organometallics 2004, 23, 433.
(20) Jameson, C. H. In Phosphorous-31 NMR Spectroscopy in Stereo-
chemical Analysis; Verkade, J. G., Quin, L. D., Eds.; VCH: New York,
1987; Chapter 6.
(21) Wrackmeyer, B.; Alt, H. G.; Maisel, H. E. J. Organomet. Chem.
1990, 399, 125.
Figure 2. ORTEP diagram (30% probability) of the cation in
t
compound 4. Cy rings (except the C1 atoms), Bu and Et groups,
and H atoms are omitted for clarity. Only one of the two
independent molecules present in the asymmetric unit is shown.

Unsaturated Group 6 Metal Complexes
Organometallics, Vol. 27, No. 4, 2008 547
Table 3. Selected Data of the DFT-Optimized Structures of
Compounds cis-3a and trans-3aa
parameter
trans-3a
cis-3a
Mo(1)-Mo(2)
Mo(1)-C(1)
Mo(2)-C(2)
Mo(1)-C(3)
Mo(1)-C(4)
Mo(2)-C(3)
Mo(2)-C(4)
Mo(1)-P
2.750
1.988
1.985
2.108
2.475
2.473
2.108
2.450
2.450
1.358
1.338
1.338
84.7
2.759
1.995
2.014
2.086
2.527
2.397
2.102
2.446
2.446
1.368
1.349
1.329
87.5
Mo(2)-P
C(3)-C(4)
C(3)-OMe
C(4)-OMe
C(1)-Mo(1)-Mo(2)
C(2)-Mo(2)-Mo(1)
Mo(1)-P-Mo(2)
85.9
68.3
99.4
68.7
Figure 3. Optimized geometries for complexes cis-3a and trans-
3a, with hydrogen atoms omitted for clarity.
a Bond lengths and angles in Å and deg, respectively, according to
the labeling shown in the figure below.
of the dicarbonyl products 3a,b. In fact, the latter experience
easily the loss of both CO ligands under visible-UV irradiation
in THF solutions, to regenerate rapidly (ca. 5 min) and
quantitatively (by NMR) the corresponding parent bis(alkoxy-
carbyne) complexes 1a,b. No intermediates were detected in
this double-decarbonylation process. As stated above, only two
precedents for a reversible coupling between carbyne ligands
have been reported previously,5,6 and the closest precedent
involving binuclear complexes is the irreversible cleavage of
alkynes induced by triply bonded ditungsten alkoxide or
silyloxide complexes.22
Chart 2
In contrast to the above results, neither of the complexes 2
or 4 undergoes a related transformation under thermal or
photochemical activation. Since the geometric parameters
calculated for the dicarbonyls cis- and trans-3a show no special
weakness in the C-C bond of the dimethoxyacetylene ligand
(see below), then we have to conclude that the reversibility of
the C-C coupling reaction involved in the 1/3 transformation
is due to no special labilization effect on that C-C bond, but
just to the experimental facilities to add and remove CO
molecules from the dimetal center in these complexes.
DFT Calculations on the Dicarbonyl Compounds 3a. The
great computational efficiency and chemical accuracy of the
DFT methods currently under use allows carrying out quantum
mechanical studies of real (rather than model) systems of the
size of the binuclear compounds here discussed.23 We have thus
carried out DFT calculations on both isomers of compound 3a
(cis and trans, see Experimental Section for further details), to
gain a better understanding of the bonding and easy C-C bond
cleavage in these cations.
previously that the B3LYP-optimized bond distances involving
transition metal atoms tend to be slightly longer than those
determined crystallographically, the difference found here (ca.
0.1 Å) is noticeably higher. Thus, we conclude that the
intermetallic length in the dicarbonyls 2a surely is genuinely
higher than those in the complexes 2 and 4. It is thus tempting
to associate this weakening of the intermetallic interaction to
the better π-acceptor properties of the carbonyl ligands (when
compared to those of the CNtBu or tedip groups), which implies
a lower electron density at the metal centers. We also note that
the molecular orbitals (MOs) calculated for both cis-3a and
trans-3a are in good agreement with the double M-M bond
proposed for these cations on the basis of the EAN rules, since
the expected σMM and πMM bonding MOs can be found in both
cases (see Supporting Information).
The most relevant parameters derived from the geometry
optimization of these cations (Figure 3) can be found in Table
3. The optimized geometries for both isomers exhibit similar
bond lengths, so it can be concluded that the relative arrange-
ment of the MoCp(CO) moieties has little effect on the bonding
within the central core of the molecule. The intermetallic
distances found in both isomers (ca. 2.755 Å) are somewhat
longer than those experimentally determined for the complexes
2 and 4 (ca. 2.66 Å). Although we,1 and others,23 have found
A relevant feature in the structure of these isomers is the
strong twist angle of the alkyne ligand with respect to the
perpendicular of the intermetallic vector (26-28°, to be
compared with 4° and 12° for compounds 4 and 2, respectively).
This causes large differences (ca. 0.4 Å) in the Mo-C bond
lengths involving the alkyne, with the short distances (ca. 2.10
Å) approaching the values calculated for the Mo-carbyne bonds
in 1a (ca. 2.02 Å)1 and the long distances (ca. 2.50 Å) being
much longer than expected for single Mo-C bonds. In contrast,
the C-C bond displays a normal length of ca. 1.36 Å. All this
suggests a significant distortion in the coordination mode of
the alkyne, away from the perpendicular mode, toward the
parallel bis-carbene mode, which also implies a four-electron
contribution to the dimetal center (A and B in Chart 2). Indeed,
the environment around the Mo-bound alkyne atoms is close
to trigonal rather than tetrahedral (sum of angles around these
(22) (a) Chamberlain, R. M. L.; Rosenfeld, D. C.; Wolczanski, P. T.;
Lobkovsky, E. B. Organometallics 2002, 21, 2724. (b) Listemann, M. L.;
Schrock, R. R. Organometallics 1985, 4, 74.
(23) (a) Koch, W.; Holthausen, M. C. A. Chemist′s Guide to Density
Functional Theory, 2nd ed.; Wiley-VCH: Weinheim, 2002. (b) Ziegler, T.
Chem. ReV. 1991, 91, 651. (c) Foresman, J. B., Frisch, A. E. Exploring
Chemistry with Electronic Structure Methods, 2nd ed.; Gaussian, Inc.:
Pittsburgh, 1996.

548 Organometallics, Vol. 27, No. 4, 2008
García et al.
Table 4. Topological Properties of the Electron Density at the Bond
Critical Pointsa
Chart 3
cis-3a
trans-3a
bond
F
2F
F
2F
Mo(1)-Mo(2)
Mo(1)-C(1)
Mo(2)-C(2)
Mo(1)-C(3)
Mo(2)-C(4)
C(3)-C(4)
0.397
0.849
0.810
0.790
0.750
2.178
1.968
2.052
0.869
9.554
9.570
5.089
5.706
0.403
0.867
0.860
0.760
0.760
2.242
2.024
2.026
0.933
9.760
9.767
4.728
4.721
-20.836
-5.060
-3.223
-22.301
-4.895
-4.825
C(3)-OMe
C(4)-OMe
a Labeling scheme according to the figure in Table 3. Values of the
electron density (F) and its Laplacian ( 2F) at these points are given in
e Å-3 and e Å-5, respectively.
Scheme 1. Proposed Reaction Pathway for the Formation of
Compound 5a [Mo ) MoCp(CO); the bridging PCy2 ligand
is omitted for clarity]
atoms being ca. 359° for cis-3a and 358° for trans-3a). We
feel that further studies, these including different alkyne ligands,
are needed to conclude whether the strong twist of the alkyne
bridge in these dicarbonyl cations has an electronic or steric
origin (or a combination of both), and experimental work in
that direction is being undertaken in our laboratory.
To obtain complementary information on the coordination
of the alkyne in the dicarbonyls 3a, we have also carried out a
topological analysis of the electron density on these systems
by means of the quantum theory of atoms in molecules (AIM),24
with the resulting values of the electron density (F) and its
Laplacian ( 2F) at several bond critical points (bcp) of both
cis- and trans-3a being collected in Table 4. The first point of
interest is the location of the corresponding M-M bcp, a feature
not granted for metal-metal bonded complexes having bridging
ligands.25 The values of the electron density at these bcp’s (ca.
0.40 e Å-3) are similar to those recently computed by us for
other molecules with formal metal-metal double bonds such
as the methoxycarbyne complex [Mo2Cp2(µ-COMe)(µ-
PCy2)(CO)2] (0.42 e Å-3).26 As for the Mo2C2 tetrahedron, only
the bcp’s corresponding to the shorter Mo-C bonds are located.
The corresponding electron densities (0.76–0.79 e Å-3) are
intermediate between the values computed for the Mo-C single
bonds of bridging carbonyls (ca. 0.71 e Å-3) and the Mo-C
multiple bonds of bridging methoxycarbynes (formal bond order
1.5; 0.83–0.93 e Å-3),1,26 and thus suggest the presence of
substantial multiplicity in these bonds, which is in agreement
with the trigonal environment of the C atoms and with an
incipient carbene-like character of these Mo-C bonds (Chart
2). Finally, the values of the electron density at the C-C bcp
of the alkyne ligand (ca. 2.21 e Å-3) are lower than those
computed for free alkynes (2.44–2.56 e Å-3),27 in agreement
with the expected decrease of the electron density upon
coordination,10a but we have no data on coordinated alkynes to
be used for comparative purposes. We note, however, that the
calculated densities at the alkyne C-C bcp’s in cis-3a and trans-
3a are yet considerably higher than that computed for the
C(sp2)-C(sp2) single bond in [Mo2Cp2{µ-C(CO2Me)}(µ-
PCy2)(CO)2] (1.93 e Å-3).26
triggered by two-electron redox processes.6 We thus decided
to check whether redox reactions could be used to modify the
C-C bonding of the alkyne ligand in the cations 2 to 4.
Unfortunately, these complexes turned to be quite resistant to
oxidation, surely due to the positive charge already present in
these molecules. As for the reduction reactions, we found that
complexes 2 and 3a,b react readily with strong reducing agents
such as Na(Hg) or sodium naphthalenide, but complex mixtures
were obtained in both cases. Demethylation of the coordinated
alkyne was suspected to be one of the undergoing processes.
Indeed, when the dicarbonyl complex 3a is treated with the
strong base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), de-
methylation of the bonded alkyne is cleanly accomplished, but
givesunexpectedlytheneutralcomplex[Mo2Cp2{µ-C(CO2Me)}(µ-
PCy2)(CO)2] (5a) (Chart 3), bearing a 2-methoxy-2-oxo-
ethylidyne bridging ligand (also referred to as carboxycarbyne).
This seems to be the result of a 1,2-shift of the methoxyl group
remaining in the ketenyl intermediate (not detected), which is
presumably formed after the initial demethylation in 3a (Scheme
1, see below). However, the treatment of the ethoxy(methoxy)
acetylene complex 3b with DBU has a poor selectivity, and
both demethylation and deethylation occur in similar propor-
tions, to give a roughly equimolar mixture of the complexes 5a
and its ethoxyl-substituted analogue [Mo2Cp2{µ-C(CO2Et)}(µ-
PCy2)(CO)2] (5b). Unfortunately, these very similar compounds
could not be fully separated by either chromatography or
crystallization, and therefore the data for compound 5b had to
be extracted from the spectroscopic analysis of these mixtures.
Reaction of the Alkyne-Bridged Complexes 3a,b with
DBU. As stated above, the formation and cleavage of the C-C
bond in the cluster [Fe4(η5-C5H4Me)4(µ-CH)2(µ3-CO)3] can be
(24) (a) Bader, R. F. W. Atoms in Molecules-A Quantum Theory; Oxford
University Press: Oxford, 1990. (b) Bader, R. F. W. Chem. ReV. 1991, 91,
893.
(25) Macchi, P.; Sironi, A. Coord. Chem. ReV. 2003, 383, 238–239.
(26) García, M. E.; García-Vivó, D.; Ruiz, M. A.; Aullón, G.; Alvarez,
S. Organometallics 2007, 26, 5912.
Solid-State and Solution Structure of Compounds 5a,b.
The structure of compound 5a was confirmed through a single-
crystal X-ray diffraction study (Table 5 and Figure 4). The
(27) Grabowski, S. J.; Krygowski, T. M. Chem. Phys. Lett. 2004, 389,
51.

Unsaturated Group 6 Metal Complexes
Organometallics, Vol. 27, No. 4, 2008 549
Table 5. Selected Bond Lengths (Å) and Angles (deg) for
Compound 5a
Mo(1)-Mo(2)
Mo(1)-C(1)
Mo(2)-C(1)
Mo(1)-C(4)
Mo(2)-C(5)
Mo(1)-P
2.656(1)
1.993(5)
1.977(6)
1.993(8)
2.015(7)
2.409(2)
2.394(2)
1.447(8)
1.215(7)
1.353(7)
1.448(7)
Mo(1)-Mo(2)-C(5)
Mo(2)-Mo(1)-C(4)
Mo(1)-C(1)-Mo(2)
Mo(1)-P-Mo(2)
Mo(1)-C(1)-C(2)
Mo(2)-C(1)-C(2)
C(1)-C(2)-O(2)
O(2)-C(2)-O(3)
C(1)-C(2)-O(3)
C(4)-Mo(1)-P
82.7(2)
85.5(3)
84.0(2)
67.2(1)
139.8(4)
135.5(4)
124.7(5)
122.3(5)
113.1(5)
84.0(3)
85.3(2)
Mo(2)-P
C(1)-C(2)
C(2)-O(2)
C(2)-O(3)
O(3)-C(3)
C(5)-Mo(2)-P
Figure 4. ORTEP diagram (30% probability) of compound 5a. Cy
molecule displays two MoCp(CO) fragments in a transoid
arrangement bridged by dicyclohexylphosphide and carboxy-
carbyne ligands, with the latter group twisted ca. 29° from the
perpendicular to the intermetallic vector and the carbonyl ligands
slightly bent over that vector (C-Mo-Mo ca. 84°). The latter
is a characteristic feature of electron-deficient molecules of the
type [Mo2Cp2(µ-PCy2)(µ-X)(CO)2] (X ) one- to three-electron
donor group).11,14 The Mo-Mo separation is quite short,
2.656(1) Å, consistent with a double metal-metal bond in the
presence of a bridging atom of small covalent radius, as
discussed above (see discussion of the structures of complexes
2 and 4). On the other hand, the Mo-C(carbyne) distances (ca.
1.98 Å) are almost identical to those previously measured for
alkoxycarbyne complexes such as 1a1 or [Mo2Cp2(µ-COEt)(µ-
PCy2)(µ-CO)],13 an observation that in turn is consistent with
our recent calculations pointing to a substantial similarity
between these two types of carbyne ligands.26 Finally, we note
that the twisted conformation of the CO2Me group with respect
to the intermetallic vector rules out the possibility of a π
interaction with the carbyne C atom. Accordingly, the bonding
distances within this fragment are similar to those found in
organic molecules. For instance, the C(1)-C(2) bond length of
1.447(8) Å is fully consistent with the presence of a single C-C
bond between these sp2-hybridized atoms.28
The spectroscopic data in solution for compound 5a (Table
1 and Experimental Section) are fully consistent with its solid-
state structure, and those for the ethoxyl compound 5b are
almost identical and therefore need no specific comments. The
carbonyl ligands in these complexes give rise to two C-O
stretching bands with the typical pattern of trans-dicarbonyl
complexes (weak and strong, in order of decreasing frequency)
defining angles between CO groups close to 180°.17 We note,
however, that the average C-O stretching frequencies (1915
cm-1) are substantially higher than that figure in the isostructural
methoxycarbyne complex [Mo2Cp2(µ-COMe)(µ-PCy2)(CO)2]
(1878 cm-1).26 This suggests a considerable electron-acceptor
character of the carboxycarbyne ligand, as far as the metal
centers are concerned. On the other hand, the 31P NMR spectrum
of 5a exhibits a resonance at 128.2 ppm that falls in the range
found for related 32-electron compounds of the type trans-
[Mo2Cp2(µ-PCy2)(µ-X)(CO)2] (X ) 3e-donor ligand),13,14,16
while the presence of the bridging carbyne is denoted by the
appearance of a strongly deshielded resonance in the 13C NMR
spectrum (402.8 ppm), with a shift similar to that of the
isoelectronic methoxycarbyne complex [Mo2Cp2(µ-COMe)(µ-
PCy2)(CO)2] (404.0 ppm).26 As expected, the 13C NMR
spectrum of 5a displays just one resonance for the CO and Cp
rings (except the C1 atoms) and H atoms are omitted for clarity.
ligands, in agreement with the presence of a C2 symmetry axis
(by assuming free rotation of the CO2Me around the single C-C
bond) and a six-resonances pattern for the equivalent cyclohexyl
rings.
Decarbonylation of Compound 5a. The methoxycarbyne
complex [Mo2Cp2(µ-COMe)(µ-PCy2)(CO)2] undergoes photo-
chemical decarbonylation to give the triply bonded complex
[Mo2Cp2(µ-COMe)(µ-PCy2)(µ-CO)].26 It was thus expected that
the carboxycarbyne complex 5a might exhibit a similar behavior.
Indeed, the photolysis of toluene solutions of 5a with visible-UV
light gives with good yield the monocarbonyl derivative
[Mo2Cp2{µ-C(CO2Me)}(µ-PCy2)(µ-CO)] (6) (Chart 3).
Spectroscopic data for compound 6 (Table 1 and Experimental
Section) are similar to those of the related alkoxycarbyne
complexes [Mo2Cp2(µ-COR)(µ-PCy2)(µ-CO)] (R ) Me, Et)13
and firmly support the geometry proposed, made up of two
MoCp moieties bridged by CO, carboxycarbyne, and dicyclo-
hexylphosphide ligands (Chart 3). The bridging carbonyl gives
rise to a low-frequency C-O stretching band (1665 cm-1) and
to a characteristic strongly deshielded 13C NMR resonance
(299.5 ppm), to be compared with the values of 1674 cm-1
and 305.0 ppm for the carbonyl ligand in [Mo2Cp2(µ-COMe)(µ-
PCy2)(µ-CO)].13 As expected, the bridgehead C(carbyne) atom
displays an even more deshielded 13C NMR resonance (361.1
ppm, to be compared with a value of 352.0 ppm for the
mentioned methoxycarbyne complex). Finally, the presence of
only one resonance for the Cp ligands in the 1H and 13C NMR
spectra, and the appearance of eight 13C resonances for the
inequivalent cyclohexyl rings, gives further support to the overall
symmetry proposed for this molecule.
DFT Study of the 1,2-Methoxyl Shift Leading to the
Carbyne Complex 5a. DFT methods currently under use have
proved to be useful for the prediction of geometries and energies
not only of ground states but also of the transition states of
metal complexes.29 In the case of the reaction transforming the
dimethoxyacetylene complex 3a into the carboxycarbyne 5a we
could detect no intermediates by IR or NMR monitoring of the
reaction mixture. Yet we can rationalize the results by assuming
that a demethylation of the dimethoxyacetylene ligand of 3a
would occur first, to initially generate a µ-η2:η2-bonded ketenyl
ligand (Scheme 1), which was taken as the starting point of
our calculations. The latter show that this structure is unstable
with respect to its µ-η1:η1-ketenyl isomer (I-1), so we suppose
that this change in the coordination mode of the ketenyl ligand
occurs very rapidly. The undetected intermediate I-1 is calcu-
lated to undergo a 1,2-shift of the OMe group through the
(28) (a) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry:
Principles of Structure and ReactiVity, 4th ed.; HarperCollins College
Publications: New York, 1993. (b) Allen, F. H.; Kennard, O.; Watson, D. G.;
Brammer, L.; Orpen, A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987,
S1.
(29) (a) Ziegler, T.; Autschbach, J. Chem. ReV. 2005, 105, 2695. (b)
Niu, S.; Hall, M. B. 2000, 100, 353.

550 Organometallics, Vol. 27, No. 4, 2008
García et al.
Chart 4
computed for the phenyl complex [Mo2Cp2(µ-Ph)(µ-PCy2)-
(CO)2] (2.333 Å),14 taken here as a rough model for a one-
electron C-donor ligand, while the C-C length (1.35 Å) is
somewhat higher than that expected for a C(sp2)-C(sp) double
bond (ca. 1.31 Å).28
In the transition state TS-1 the Mo-C distances become
shorter, reflecting the progressive transformation of the hydro-
carbyl bridge into a carbyne ligand, which displays even shorter
Mo-C bond distances (2.00 Å), then corresponding to a formal
Mo-C bond order of 1.5. As expected, the methoxyl group is
located in the middle of the way from one carbon to the other,
and the corresponding C-O(1) lengths are comparable to each
other (ca. 1.78 Å) and both of them are longer than the reference
single C-O bond lengths (ca. 1.4 Å). The computed activation
Figure 5. Energy profile (kJ/mol) for the 1,2-shift of the OMe group
leading to compound 5a, showing the DFT-optimized structures
for I-1 and TS-1 (see text), with Cy rings (except the C1 atoms)
and Cp ligands omitted for clarity.
Table 6. Selected Data of the DFT-Optimized Structures of
Complexes I-1, TS-1, and 5aa
parameter
I-1
TS-1
5ab
Mo(1)-Mo(2)
Mo(1)-C(1)
Mo(2)-C(1)
Mo(1)-CO
Mo(2)-CO
C(1)-C(2)
2.630
2.236
2.202
1.968
1.953
1.351
1.412
2.377
1.175
1.422
2.447
2.451
2.657
2.116
2.120
1.963
1.958
1.353
1.775
1.797
1.173
1.424
2.452
2.450
68.9
2.699
2.000
2.006
1.981
1.978
1.455
2.441
1.364
1.220
1.430
2.454
2.460
barrier corresponding to this movement is modest (∆Gq
)
298
78 kJ mol-1), and the overall process from I-1 to 5a is
thermodynamically favorable, as expected (∆G298 )78 kJ
mol-1). We should note that, to the best of our knowledge, no
ketenyl complex having an alkoxyl substituent seems to have
been ever detected, a circumstance perhaps related to the facile
migration of this group between adjacent carbon atoms here
analyzed. This behavior might be specific of alkoxyl substitu-
ents, as opposed to alkyl or aryl groups. In fact, we have recently
prepared in our laboratory an isoelectronic phenylketenyl
analogue of complex I-1, [Mo2Cp2{µ-η1:η1-C(CO)Ph}(µ-
PCy2)(CO)2], a molecule with a structure comparable to that of
I-1, according to an X-ray diffraction study, but thermally stable,
with no tendency for aryl migration.31
C(1)-O(1)
C(2)-O(1)
C(2)-O(2)
O(1)-Me
Mo(1)-P
Mo(2)-P
O(1)-C(1)-C(2)
C(1)-C(2)-O(2)
118.6
177.0
118.5
121.6
168.5
a Bond lengths (Å) and angles (deg) according to the labeling shown
in the figure (see text). b Data taken from ref 26.
Reaction of Compound 1a with [Ru3(CO)12]. In the
previous sections we have shown that the reaction of compound
1a with different donor molecules induces C-C coupling
between the methoxycarbyne ligands there present. We then
decided to check whether the incorporation of 16-electron metal
fragments of the type M(CO)x or MCp(CO)x, isolobally related
to the methylene fragment, might also induce related coupling
reactions. Unfortunately, perhaps due to its cationic nature,
complex 1a has a very low basicity and does not react with
most of the classical precursors of 16-electron metal fragments
such as [MnCp′(THF)(CO)2] (Cp′ ) C5H4Me), [Fe2(CO)9], or
[Mo(CO)6] under either thermal or photochemical activation.
However, reaction with [Ru3(CO)12] does occur under visible-UV
light irradiation to give almost quantitatively the new 46-electron
trinuclear cluster [Mo2RuCp2(µ-COMe)2(µ-PCy2)(CO)4]BF4 (7)
(Chart 3). Indeed the formation of this cluster is accompanied
by a rearrangement of the methoxycarbyne ligands, but no C-C
coupling is induced in the process.
The structure of compound 7 was confirmed through a single-
crystal X-ray diffraction study (Figure 6 and Table 7). The cation
can be viewed as the result of the addition of a Ru(CO)4
fragment to the unsaturated dimetal center of 1a, followed by
a reorganization of the carbyne and carbonyl ligands so that
the two methoxycarbyne groups are now placed over one of
the Mo-Ru edges in an almost perfect transoid arrangement
[torsion angle C(2)-Mo(2)-Ru-C(4) ) 167°] while one of
transition state TS-1 to finally give the carboxycarbyne com-
pound 5a. We note that the DFT-optimized geometry and
electronic structure of 5a was reported recently by us in the
context of an analysis of the metal-alkoxycarbyne bonding.26
Figure 5 shows a diagram of the reaction profile for the I-1/
5a transformation including the views of the optimized geom-
etries for I-1 and TS-1. The most relevant parameters derived
from the geometry optimization of these structures can be found
in Table 6. As for the ketenyl complex I-1, its intermetallic
distance (2.630 Å) is intermediate between those computed for
the doubly bonded carbyne 5a (2.699 Å)26 and for the triply
bonded hydride [Mo2Cp2(µ-H)(µ-PCy2)(CO)2] (2.535 Å),14 and
therefore suggests that the ketenyl ligand in this intermediate
is effectively providing the dimetal center with more than one
electron. This effect has been previously found in other ketenyl
complexes, such as the heterometallic complex [FeWCp{µ-
C(SiPh3)CO}(CO)5], and can be rationalized by assuming the
contribution of ketenyl (one-electron donor) and acylium (three-
electron donor) resonant forms to the electronic structure of the
bridging ligand (Chart 4).30 Accordingly, the Mo-C bond
distances in I-1 (ca. 2.22 Å) are significantly shorter than those
(30) Jeffery, J. C.; Ruiz, M. A. ; Stone, F. G. A. J. Organomet. Chem.
1988, 355, 231, and references therein.
(31) Alvarez, M. A.; García, M. E.; Martínez, M. E.; Ruiz, M. A.
Unpublished results.

Unsaturated Group 6 Metal Complexes
Organometallics, Vol. 27, No. 4, 2008 551
indicative of the presence of a further, weak semibridging
interaction with the third metal atom.
The spectroscopic data in solution for compound 7 (Table 1
and Experimental Section) are essentially consistent with its
solid-state structure. Thus, the IR spectrum exhibits four C-O
stretching bands, three of them being characteristic of a
pyramidal Ru(CO)3 fragment [2071 (vs), 2026 (s), and 1996
(s) cm-1], while the fourth band, at lower frequency [1956 (w)
cm-1], can be associated to the Mo-bound carbonyl ligand. The
31P spectrum exhibits a quite deshielded resonance (200.5 ppm),
with a shift in the range of those previously reported by us for
related 46-electron heterometallic Mo2M clusters exhibiting
1
Mo2(µ-PCy2) moieties (ca. 160–210 ppm).32 The H and 13C
NMR spectra reflect the great asymmetry of the cation by
exhibiting independent resonances for the inequivalent Cp and
COMe ligands. The latter give rise to characteristic highly
deshielded 13C NMR resonances (385.2 and 375.0 ppm), with
shifts somewhat higher than those previously reported for the
isoelectronic alkylcarbyne cluster [Mo2RuCp2(µ3-CR)(µ-
PPh2)(CO)5] [R ) Me (350.8 ppm), Et (363.5 ppm)].34c This
can be due both to the different electron-withdrawing properties
of the alkyl and alkoxyl substituents and to the different
coordination modes (µ2 vs µ3) of the carbyne ligands.
Figure 6. ORTEP diagram (30% probability) of compound 7. Cy
rings (except the C1 atoms) and H atoms are omitted for clarity.
Table 7. Selected Bond Lengths (Å) and Angles (deg) for compound 7
Mo(1)-Mo(2)
Mo(1)-Ru
Mo(2)-Ru
Mo(1)-C(1)
Mo(2)-C(2)
Mo(2)-C(4)
Ru-C(2)
2.686(1)
2.813(1)
2.846(1)
2.004(5)
1.954(5)
2.005(5)
2.091(5)
2.045(5)
1.934(5)
1.966(5)
Ru-C(8)
1.952(5)
1.333(6)
1.306(6)
61.0(1)
56.7(1)
67.7(1)
C(2)-O(2)
C(4)-O(4)
Mo(1)-Mo(1)-Ru
Mo(1)-Ru-Mo(2)
Mo(1)-P-Mo(2)
C(1)-Mo(1)-P
C(2)-Mo(2)-P
C(4)-Mo(2)-P
C(7)-Ru-C(2)
Concluding Remarks. The high unsaturation of the 30-
electron bis(alkoxycarbyne) complexes 1a,b allows them to
readily react with simple electron-donor molecules (L) at room
temperature to give the corresponding double-addition products.
This induces the C-C coupling between both alkoxycarbyne
ligands to generate quantitatively the new dialkoxyacetylene
complexes of the type [Mo2Cp2{µ-η2:η2-C2(OMe)(OR)}(µ-
PCy2)(L)2]+ (R ) Me, Et). In the case of the dicarbonyl
complex, this coupling process can be fully and quantitatively
reversed upon photochemical decarbonylation, even when there
is no special weakening effect on the corresponding C-C bond
of the coordinated alkyne, according to DFT calculations on
both cis and trans isomers of the alkyne complex. The addition
of the 16-electron fragment Ru(CO)4 to the unsaturated com-
pound 1a can be accomplished though the reaction with
[Ru3(CO)12] under photochemical activation, although no C-C
coupling between the carbyne ligands is thus induced, but just
their migration from Mo-Mo to Mo-Ru bonds. Removal of a
methyl cation from the dicarbonyl alkyne-bridged complexes
renders initially an unstable µ-η2:η2-ketenyl complex, which,
according to our DFT calculations, would rapidly rearrange into
a µ-η1:η1-ketenyl intermediate, which in turn undergoes a fast
1,2-shift of the methoxyl group to generate a new 2-methoxy-
2-oxo-ethylidyne bridging ligand. The computed activation
barrier for the methoxyl shift is only 78 kJ mol-1, which is in
agreement with our inability to detect spectroscopically any
intermediates in the experimental reaction. This facile migration
process might be a characteristic property of the alkoxyl (as
opposed to alkyl or aryl) groups.
93.3(2)
Ru-C(4)
101.1(2)
104.9(2)
82.2(2)
Ru-C(6)
Ru-C(7)
the carbonyls of the entering Ru(CO)4 fragment has been
transferred to the Mo(1) center. The Mo(1)-Mo(2) bond
distance, 2.686(1) Å, is significantly shorter than those previ-
ously found for single Mo-Mo bonds in related trinuclear
clusters, such as the trimolybdenum complex [Mo3Cp3(µ-
PCy2)(µ3-CO)(CO)4] (ca. 3.1 Å).32 In fact, this distance is even
shorter than those measured for doubly bonded binuclear species
such as the bis(diphenylphosphide) complex [Mo2Cp2(µ-
PPh2)2(CO)2] (2.713(1) Å)33 and comparable to those measured
in our 32-electron complexes 2, 4, and 5a. All this suggests
that the electronic unsaturation of the trinuclear cluster is
essentially localized on this pair of metal atoms. Yet, the
Mo-Ru separations, 2.813(1) and 2.846(1) Å, are also some-
what shorter than those measured in related compounds having
Mo-Ru single bonds (2.90–3.1 Å),34 perhaps as an effect of
the positive charge of cluster. The coordination sphere of the
molybdenum centers is completed by one Cp ligand and the
dicyclohexylphosphide ligand bridging the Mo-Mo edge. This
latter ligand exhibits a slightly asymmetric coordination mode,
being placed ca. 0.05 Å closer to Mo(1), doubtless as a
consequence of the different coordination numbers around the
Mo atoms. On the other hand, the methoxycarbyne ligands
display slightly different coordination modes; thus, the bridge-
head carbon atom C(4) exhibits an almost perfect and rather
symmetrical µ2-coordination mode (Mo(2)-C(4) 2.005(5) Å and
Ru-C(4) 2.045(5) Å), whereas the C(2) atom exhibits a more
asymmetric coordination (Mo(2)-C(2) 1.954(5) Å and Ru-C(2)
2.091(5) Å), with the Mo(1)-C(2) distance of 2.426(5) Å being
Experimental Section
General Procedures and Starting Materials. All manipulations
and reactions were carried out under a nitrogen (99.995%)
atmosphere using standard Schlenk techniques. Solvents were
purified according to literature procedures and distilled prior to
use.35 Petroleum ether refers to that fraction distilling in the range
338–343 K. The compounds [Mo2Cp2(µ-COMe)2(µ-PCy2)]BF4 (1a)
and [Mo2Cp2(µ-COEt)(µ-COMe)(µ-PCy2)]BF4 (1b) were prepared
(32) Alvarez, C. M.; Alvarez, M. A.; García, M. E.; Ramos, A.; Ruiz,
M. A.; Graiff, C.; Tiripicchio, A. Organometallics 2007, 26, 321.
(33) Adatia, T.; McPartlin, M.; Mays, M. J.; Morris, M. J.; Raithby,
P. R. J. Chem. Soc., Dalton Trans. 1989, 1555.
(34) (a) Khorasani-Motlagh, M.; Safari, N.; Pamplin, C. B.; Patrick,
B. O.; James, B. R. Can. J. Chem. 2006, 84, 330. (b) Bridgeman, A. J.;
Mays, M. J.; Woods, A. D. Organometallics 2001, 20, 2076. (c) Adams,
H.; Bailey, N. A.; Gill, L. J.; Morris, M. J.; Sadler, N. D. J. Chem. Soc.,
Dalton Trans. 1997, 3041.
(35) Armarego, W. L. F.; Chai, C. Purification of Laboratory Chemicals,
5th ed.; Butterworth-Heinemann: Oxford, UK, 2003.

552 Organometallics, Vol. 27, No. 4, 2008
García et al.
as described previously.1 Chromatographic separations were carried
out using jacketed columns cooled by tap water (ca. 288 K) or by
a closed 2-propanol circuit, kept at the desired temperature with a
cryostat. Commercial aluminum oxide (Aldrich, activity I, 150
mesh) was degassed under vacuum prior to use. The latter was
mixed under nitrogen with the appropriate amount of water to reach
the activity desired. All other reagents were obtained from the usual
commercial suppliers and used as received. IR stretching frequencies
were measured in solution or Nujol mulls and are referred to as ν
(solvent) or ν (Nujol) respectively. Nuclear magnetic resonance
(NMR) spectra were routinely recorded at 300.13 (1H), 121.50
(31P{1H}), or 75.47 MHz (13C{1H}) at 290 K in CD2Cl2 solutions
unless otherwise stated. Chemical shifts (δ) are given in ppm,
relative to internal tetramethylsilane (1H, 13C) or external 85%
aqueous H3PO4 (31P). Coupling constants (J) are given in Hz.
JCP ) 3, 2 × µ-CC), 94.1, 94.0 (2 × s, 2 × Cp), 74.1 (s, OCH2),
64.1 (s, OMe), 47.4, 47.0 [2 × d, JCP )20, 2 × C1(Cy)], 34.7 [s,
C2,6(Cy)], 32.7 [d, JCP ) 5, C6,2(Cy)], 27.7 [d, JCP ) 11, C3,5(Cy)],
27.6 [d, JCP ) 11, C5,3(Cy)], 25.8 [s, 2 × C4(Cy)], 15.5 (s, Me).
Data for cis-3b and cis-3b′: 1H NMR: δ 5.85 (s, 10H, Cp). 13C{1H}
NMR: δ 92.5, 92.4 (2 × s, 2 × Cp).
Preparation of [Mo2Cp2{µ-η2:η2-C2(OMe)2}(µ-PCy2)(µ-
tedip)]BF4 (4). Compound 1a (0.050 g, 0.072 mmol) and tedip
(30 µL, 0.123 mmol) were stirred in dichloromethane (10 mL) for
24 h in a bulb equipped with a Young’s valve. After removal of
solvents under vacuum, dichloromethane (10 mL) and petroleum
ether (5 mL) were added. Removal of the solvents from the latter
mixture under vacuum and washing of the residue with petroleum
ether gave compound 4 as a brown powder (0.064 g, 93%). The
crystals used in the X-ray study were grown by slow diffusion of
petroleum ether into a tetrahydrofuran solution of the complex at
253 K. Anal. Calcd for C34H58BF4Mo2O7P3: C, 42.97; H, 6.15.
Preparation of [Mo2Cp2{µ-η2:η2-C2(OMe)2}(µ-PCy2)(CNtBu)2]-
BF4 (2). A solution of compound 1a (0.100 g, 0.144 mmol) in
t
1
dichloromethane (10 mL) was stirred with BuNC (40 µL, 0.353
Found: C, 42.85; H, 5.97. H NMR: δ 5.45 (s, 10H, Cp), 4.18 (s,
mmol) for 5 min to give a red solution. The solvent was then
removed under vacuum, and the residue was washed with petroleum
ether to give compound 2 as a red powder (0.116 g, 94%). The
crystals used in the X-ray study were grown by slow diffusion of
petroleum ether into a dichloromethane solution of the complex at
253 K. Anal. Calcd for C36H56BF4Mo2N2PO2 · CH2Cl2: C, 47.10;
N, 2.96; H, 6.19. Found: C, 47.16; N, 2.99; H, 6.23. 1H NMR
(300.13 MHz): δ 5.35 (s, 10H, Cp), 3.92 (s, 6H, OMe), 1.16 (s,
3H, OMe), 4.09 (s, 4H, OCH2), 3.94 (m, 4H, OCH2), 2.75 (s, 3H,
OMe), 2.5–0.4 (m, 22H, Cy), 1.25 (t, JHH ) 7, 6H, Me), 1.22 (t,
JHH ) 7, 6H, Me). 31P{1H} NMR: δ 164.9 (t, JPP ) 22, µ-PCy2),
159.7 (d, JPP ) 22, µ-tedip). 13C{1H} NMR: δ 158.1 (td, JCP
)
12, 2, µ-CC), 117.0 (td, JCP ) 28, 3, µ-CC), 90.9 (s, Cp), 64.9 (ft,
AXX′, JHP + JHP′ ) 9, OCH2), 62.7 (ft, AXX′, JHP + JHP′ ) 12,
OCH2), 60.2, 59.9 (2 × s, 2 × OMe), 53.8 [d, JCP ) 23, C1(Cy)],
46.1 [d, JCP ) 14, C1(Cy)], 34.8, 33.8 [2 × s, 2 × C2(Cy)], 28.3,
27.8 [2 × d, JCP ) 11, 2 × C3(Cy)], 26.3, 26.0 [2 × s, 2 × C4(Cy)],
16.3, 16.1 (2 × s, 2 × Me).
t
18H, Bu), 2.0–1.2 (m, 22H, Cy). 13C{1H} NMR: δ 160.3 (d, JCP
) 10, CN), 147.4 (s, µ-CC), 90.9 (s, Cp), 62.4 (s, OMe), 58.7 [s,
C1(tBu)], 50.0 [d, JCP ) 18, C1(Cy)], 34.7 [d, JCP ) 4, C2,6(Cy)],
33.4 [s, C6,2(Cy)], 30.3 [s, C2(tBu)], 28.4 [d, JCP ) 12, C3,5(Cy)],
28.3 [d, JCP ) 10, C5,3(Cy)], 26.4 [s, 2 × C4(Cy)].
Preparation of [Mo2Cp2{µ-C(CO2Me)}(µ-PCy2)(CO)2] (5a).
Neat 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, 20 µL, 0.134
mmol) was added to a dichloromethane solution (10 mL) of
compound 3a (0.090 g, 0.120 mmol), and the mixture was stirred
for 10 min to give an orange solution. The solvent was then removed
under vacuum, the residue was extracted with CH2Cl2-petroleum
ether (1:2), and the extracts were chromatographed on alumina
(activity IV, 20 × 3 cm) at 253 K. Elution with dichloromethane-
petroleum ether (3:1) gave an orange fraction. Removal of solvents
from this fraction gave compound 5a as an orange powder (0.049
g, 63%). The crystals used in the X-ray study were grown by
cooling at 253 K a saturated petroleum ether solution of the
complex. Anal. Calcd for C27H35Mo2O4P: C, 50.16; H, 5.46. Found:
Preparation of [Mo2Cp2{µ-η2:η2-C2(OMe)2}(µ-PCy2)(CO)2]-
BF4 (3a). A solution of compound 1a (0.100 g, 0.144 mmol) in
dichloromethane (10 mL) was stirred at room temperature in a high-
pressure reactor under a CO atmosphere (60 bar) for 12 h. After
depressurization of the reactor, the red resulting solution was
transferred into a Schlenk flask. Petroleum ether was then added,
the solvents were removed under vacuum, and the residue was then
washed with petroleum ether to yield compound 3a as a red
microcrystalline solid (0.101 g, 94%). Upon dissolution of this solid
in dichloromethane, the presence of the major isomer trans-3a along
with a small amount of the isomer cis-3a can be detected by 1H or
31P NMR spectroscopy. After ca. 3 h at room temperature, an
equilibrium ratio trans-3a/cis-3a of ca. 8 is reached. Anal. Calcd
for C28H38BF4Mo2O4P · CH2Cl2: C, 41.80; H, 4.83. Found: C, 41.68;
H, 5.25. Spectroscopic data for trans-3a: 1H NMR: δ 5.93 (s, 10H,
Cp), 3.95 (s, 6H, OMe), 2.4–1.1 (m, 22H, Cy). 13C{1H} NMR: δ
217.1 (d, JCP ) 12, CO), 151.3 (d, JCP ) 3, µ-CC), 94.2 (s, Cp),
64.3 (s, OMe), 47.5 [d, JCP )20, C1(Cy)], 34.9 [d, JCP ) 4,
C2,6(Cy)], 33.0 [s, C6,2(Cy)], 27.9 [d, JCP ) 12, C3,5(Cy)], 27.8 [d,
JCP ) 20, C5,3(Cy)], 25.9 [s, C4(Cy)]. Data for cis-3a: 1H NMR: δ
5.71 (s, 10H, Cp), 4.00 (s, 3H, OMe), 3.99 (s, 3H, OMe). 13C{1H}
NMR: δ 92.5 (s, Cp), 63.0, 62.9 (2 × s, 2 × OMe); other resonances
of this minor isomer were masked by those of the major isomer.
1
C, 50.22; H, 5.40. H NMR (C6D6): δ 5.27 (s, 10H, Cp), 4.02 (s,
3H, OMe), 2.5–1.1 (m, 22H, Cy). 13C{1H} NMR (C6D6): δ 402.8
(br, s, µ-C), 221.4 (d, JCP ) 11, CO), 191.8 (s, C)O), 91.5 (s,
Cp), 50.9 (s, OMe), 45.4 [d, JCP ) 18, C1(Cy)], 36.1 [s, C2,6(Cy)],
34.3 [s, C6,2(Cy)], 28.4 [d, JCP ) 12, C3,5(Cy)], 28.2 [d, JCP ) 12,
C5,3(Cy)], 26.5 [s, C4(Cy)].
Reaction of 3b with DBU. Neat 1,8-diazabicyclo[5.4.0]undec-
7-ene (DBU, 20 µL, 0.134 mmol) was added to a dichloromethane
solution (10 mL) of compound 3b (0.050 g, 0.066 mmol), and the
mixture was stirred for 10 min to give an orange solution. The
solvent was then removed under vacuum, the residue was extracted
with CH2Cl2-petroleum ether (1:2), and the extracts were chro-
matographed on alumina (activity IV, 20 × 3 cm) at 253 K. Elution
with dichloromethane-petroleum ether (3:1) gave an orange
fraction, this containing a 5:4 mixture of compounds 5a and
[Mo2Cp2{µ-C(CO2Et)}(µ-PCy2)(CO)2] (5b), respectively. Spectro-
Preparation of [Mo2Cp2{µ-η2:η2-C2(OEt)(OMe)}(µ-PCy2)-
(CO)2]BF4 (3b). The procedure is identical to that described for
3a, but using compound 1b (0.100 g, 0.142 mmol) instead. This
gives compound 3b (0.100 g, 93%) as a red microcrystalline solid.
Upon dissolution of this solid in dichloromethane, the presence of
small amounts of the isomers cis-3b and cis-3b′ can be detected
by 1H or 31P NMR spectroscopy. After ca. 3 h at room temperature,
an equilibrium ratio trans-3b/cis-3b/cis-3b′ of ca. 20/1/1 is reached.
Anal. Calcd for C29H40BF4Mo2PO4: C, 45.69; H, 5.29. Found: C,
1
scopic data for compound 5b: H NMR (C6D6): δ 5.30 (s, 10H,
Cp), 4.73, 4.55 (ABX3, JAB ) 10, JAX ) JBX ) 7, 2H, OCH2),
1.37 (t, JHH ) 7, 3H, Me), 2.5–1.2 (m, 22H, Cy). 13C{1H} NMR
(C6D6): δ 403.3 (s, µ-C), 221.5 (d, JCP ) 7, CO), 191.3 (s, CdO),
91.5 (s, Cp), 59.6 (s, OCH2), 45.4 [d, JCP ) 18, C1(Cy)], 36.1 [s,
C2,6(Cy)], 34.3 [s, C6,2(Cy)], 28.4 [d, JCP ) 12, C3,5(Cy)], 28.2 [d,
JCP ) 12, C5,3(Cy)], 26.5 [s, C4(Cy)], 15.8 (s, CH3).
1
45.77; H, 5.29. Spectroscopic data for trans-3b: H NMR: δ 5.92
(s, 10H, Cp), 4.30 (q, JHH ) 7, 2H, OCH2), 3.92 (s, 3H, OMe),
2.5–1.1 (m, 22H, Cy), 1.40 (t, JHH ) 7, 3H, Me). 13C{1H} NMR:
δ 217.2, 217.1 (2 × d, JCP ) 12, 2 × CO), 150.7, 149.8 (2 × d,
Preparation of [Mo2Cp2{µ-C(CO2Me)}(µ-PCy2)(µ-CO)] (6).
A toluene solution (10 mL) of compound 5a (0.050 g, 0.077 mmol)
was irradiated with visible—UV light for 30 min in a Pyrex-jacketed

Unsaturated Group 6 Metal Complexes
Organometallics, Vol. 27, No. 4, 2008 553
Schlenk tube refrigerated by water, while keeping a gentle N2 purge.
Solvent was removed under vacuum from the brown-reddish
resulting solution, the residue was then extracted with dichloro-
methane-petroleum ether (1:1), and the extracts were filtered
through diatomaceous earth. Removal of the solvents from the
filtrate yielded compound 6 as a red powder (0.037 g, 78%). Anal.
Calcd for C26H35Mo2O3P: C, 50.50; H, 5.70. Found: C, 50.44; H,
leading to compound 5a, the Synchronous Transit-Guided Quasi-
Newton (STQN) was used, as implemented in the GAUSSIAN03
suite, employing a tight cutoff on forces and step size to determine
convergence (keyword Opt)QST2,tight). In all cases, negative
frequencies were absent, except for TS-1, for which one, and only
one, imaginary frequency (466i) was found, as expected for a
transition state. Molecular orbitals and frequencies were visualized
using the Molekel42 program. The topological analysis of the
electron density was carried out using the Xaim routine.43
1
5.65. H NMR: δ 5.96 (s, 10H, Cp), 3.57 (s, 3H, OMe), 1.9–0.3
(m, 22H, Cy). 13C{1H} NMR: δ 361.1 (d, JCP ) 15, µ-C), 299.5
(d, JCP ) 11, µ-CO), 180.2 (s, CdO), 96.1 (s, Cp), 51.4 (s, OMe),
41.7, 40.2 [2 × d, JCP ) 18, 2 × C1(Cy)], 33.3, 32.6 [2 × s, 2 ×
C2(Cy)], 27.3 [d, JCP ) 13, C3(Cy)], 27.1 [d, JCP ) 12, C3(Cy)],
25.9 [s, 2 × C4(Cy)].
X-ray Structure Determination of Compounds 4 and 5a. Data
were collected on a Smart-CCD-1000 Bruker diffractometer using
graphite-monochromated Mo-KR radiation at 100 K (for 4) and
120 K (for 5a). Cell dimensions and orientation matrixes were
initially determined from least-squares refinements on reflections
measured in three sets of 30 exposures collected in three different
ω regions and eventually refined against all reflections. The software
SMART44 was used for collecting frames of data, indexing
reflections, and determining lattice parameters. The collected frames
were then processed for integration by the software SAINT,44 and
a multiscan absorption correction was applied with SADABS.45
Using the program suite WinGX,46 the structure was solved by
Patterson interpretation and phase expansion and refined with full-
matrix least-squares on F2 with SHELXL97.47 For compound 4,
there are two independent molecules in the asymmetric unit, both
displaying similar structural parameters; one of the cyclohexyl rings
and two ethoxyl groups (of the tedip ligand) were found to be
disordered in two sites. All the nonhydrogen atoms were refined
anisotropically, whereas the hydrogen atoms were constrained to
ideal geometries and refined with fixed isotropic displacement
parameters. The cyclohexyl ligand could be modeled in two
positions with stably refined populations of ratio 40:60, which were
fixed for final refinement. Similarity restraints were used for the
two disordered components, and fixed positional parameters were
imposed for the minor component. For the ethoxyl groups a similar
scheme was employed, and during final refinements a possible
molecule of a solvent (most probably THF) was found to be present
in the asymmetric unit. Therefore, the SQUEEZE48 program, as
implemented in PLATON,49 was used to model the corresponding
electron densities. After convergence, the strongest residual peaks
(1.17–0.96 e Å-3) were located around the disordered cyclohexyl
group. The refinement converged with the residuals shown in Table
8. For compound 5a all non-hydrogen atoms were refined aniso-
tropically, and all hydrogen atoms were fixed at calculated
geometric positions in the last least-squares refinements, with a
given isotropic thermal factor. One of the Cp ligands and one of
the cyclohexyl groups were disordered over two positions. The
populations of the two congeners refined stably to a 37:63 (for the
Cp) and 40:60 (for the cyclohexyl) ratios. Rigid-bond restraints50
and isotropic restraints were placed on the anisotropic displacement
Preparation of [Mo2RuCp2(µ-COMe)2(µ-PCy2)(CO)4]BF4
(7). A tetrahydrofuran solution (10 mL) of compound 1a (0.040 g,
0.058 mmol) and [Ru3(CO)12] (0.017 g, 0.027 mmol) was irradiated
with visible-UV light for 2.5 h in a Pyrex-jacketed Schlenk tube
refrigerated by water, while keeping a gentle N2 purge. Solvent
was removed under vacuum from the green-brownish resulting
solution, and the residue was washed with petroleum ether (5 ×
20 mL). This solid was then dissolved in dichloromethane (10 mL)
and filtered through diatomaceous earth. Petroleum ether (10 mL)
was then added to the filtrate, and the solvents were removed under
vacuum to give compound 7 as a green solid (0.038 g, 72%). The
crystals used in the X-ray diffraction study were grown by slow
diffusion of a layer of diethyl ether into a dichloromethane solution
of the complex at 253 K. Anal. Calcd for C30H38BF4Mo2O6PRu:
1
C, 39.80; H, 4.23. Found: C, 39.87; H, 4.32. H NMR: δ 5.60,
5.58 (2 × s, 2 × 5H, 2 × Cp), 4.52, 3.99 (2 × s, 2 × 3H, 2 ×
OMe), 2.1–1.0 (m, 22H, Cy). 13C{1H} NMR: δ 385.2, 375.0 (2 ×
d, 2 × JCP ) 4, 2 × µ-COMe), 228.9 (s, MoCO), 199.7 (s, RuCO),
197.1 (s, 2 × RuCO), 94.7, 92.3 (2 × s, 2 × Cp), 76.5, 70.5 (2 ×
s, 2 × OMe), 49.0 [d, JCP ) 22, C1(Cy)], 43.5 [d, JCP ) 15, C1(Cy)],
35.8, 34.3 [2 × s, 2 × C2(Cy)], 32.0 [s, br, 2 × C2(Cy)], 28.0–27.0
[m, 4 × C3(Cy)], 26.20, 26.15 [2 × s, 2 × C4(Cy)].
Computational Details. All computations were carried out using
the GAUSSIAN03 package,36 in which the hybrid method B3LYP
was applied with the Becke three-parameter exchange functional37
and the Lee–Yang–Parr correlation functional.38 Effective core
potentials (ECP) and their associated double-ꢀ LANL2DZ basis
set were used for the molybdenum and phosphorus atoms,39
supplemented by an extra d-polarization function in the case of
P.40 The light elements (O, C, and H) were described with
6-31G*.41 Geometry optimizations were performed under no
symmetry restrictions using initial coordinates derived from X-ray
data, when possible, and frequency analyses were performed to
ensure that minimum structures were achieved with no imaginary
frequencies. For the localization of the transition state (TS-1)
(36) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R. ,Montgomery,J. A., Jr.; Vreven, T.; Kudin, K. N.
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T. ; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
ReVision B.02; Gaussian, Inc.: Wallingford, CT, 2004.
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(b) Petersson, G. A.; Al-Laham, M. A. J. Chem. Phys. 1991, 94, 6081. (c)
Petersson, G. A.; Bennett, A.; Tensfeldt, T. G.; Al-Laham, M. A.; Shirley,
W. A.; Mantzaris, J. J. Chem. Phys. 1988, 89, 2193.
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Graphics Tool. CHIMIA 2000, 54, 766.
(43) Ortiz J. C.; Bo, C. Xaim; Departament de Química Física i
Inorgànica (Universidad Rovira i Virgili): Tarragona, Spain, 1998.
(44) SMART & SAINT Software Reference Manuals, Version 5.051
(Windows NT Version); Bruker Analytical X-ray Instruments: Madison,
WI, 1998.
(45) Sheldrick, G. M., SADABS, Program for Empirical Absortion
Correction; University of Göttingen: Germany, 1996.
(46) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(47) Sheldrick, G. M., SHELXL97: Program for the Refinement of
Crystal Structures; University of Göttingen: Göttingen, Germany, 1997.
(48) Van der Sluis, P.; Spek, A. L. Acta Crystallogr., Sect. A 1990,
A46, 194.
(49) (a) Spek, A. L. Acta Crystallogr., Sect. A 1990, A46, C34. (b) Spek,
A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht University:
Utrecht, The Netherlands, 1998.
(37) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(38) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
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A.; Jonas, V.; Köhler, K. F.; Stegman, R.; Veldkamp, A.; Frenking, G.
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554 Organometallics, Vol. 27, No. 4, 2008
García et al.
Table 8. Crystal Data for Compounds 4, 5a, and 7
4
5a
7
mol formula
mol wt
C34H58BF4Mo2O7P3
950.43
C27H35Mo2O4P
646.40
C31H40BCl2F4Mo2O6PRu
990.26
cryst syst
triclinic
P1
0.71073
triclinic
P1
0.71073
9.819(2)
11.145(2)
12.932(3)
73.307(4)
76.161(3)
86.211(3)
1316.2(5)
2
monoclinic
P21/c
0.71073
17.9342(9)
14.7605(5)
13.5710(7)
90
99.491(2)
90
j
j
space group
radiation (λ, Å)
a, Å
b, Å
c, Å
11.967(2)
19.929(4)
20.267(4)
62.708(4)
76.801(4)
75.315(4)
4119.6(13)
4
R, deg
, deg
γ, deg
V, Å3
3543.3(3)
4
Z
calcd density, g cm-3
absorp coeff, mm-1
temperature, K
θ range (deg)
index ranges (h, k, l)
no. of reflns collected
no. of indep reflns (Rint
1.532
0.786
120(2)
1.631
1.044
100.0(1)
1.856
1.377
100.1(2)
1.14–26.43
-14, 14; -18, 24; 0, 25
22564
15289 (0.0411)
9369
R1 ) 0.0496
wR2 ) 0.1075b
R1 ) 0.1001 wR2 ) 0.1225b
0.923
1.69–26.41
-11, 12; -13, 13; 0, 16
14549
5357 (0.0344)
4145
R1 ) 0.0506
wR2 ) 0.119c
R1 ) 0.0671 wR2 ) 0.1262c
1.064
0.41–25.68
0, 21; -17, 0; -16, 16
19960
6369 (0.0453)
4621
R1 ) 0.0361
wR2 ) 0.0807d
R1 ) 0.0599 wR2 ) 0.0952d
1.061
)
no. of reflns with I > 2θ(I)
R indexes
[data with I > 2σ(I)]a
R indexes (all data)a
GOF
no. of restraints/params
39/990
0.914, -1.38
46/395
2.293, -1.178
0/435
1.049, -1.072
∆F (max., min.), e Å-3
a R ) ∑||Fo| - |Fc||/∑|Fo|. wR ) [∑w(|Fo|2 - |Fc|2)2/∑w|Fo|2]1/2. w ) 1/[σ2(Fo ) + (aP)2 + bP] where P ) (Fo + 2Fc2)/3. b a ) 0.0597, b ) 0. c a )
2
2
0.0500, b ) 5.6363. d a ) 0.0481, b ) 0.
parameters of the minor components. The refinement converged
with the residuals shown in Table 8.
with full-matrix least-squares on F2 with SHELXL97.47 All non-
hydrogen atoms were refined anisotropically. All hydrogen atoms
were fixed at calculated positions and were given an overall
isotropic thermal parameter. Crystallographic data and structure
refinement details for compound 7 are collected in Table 8.
X-ray Structure Determination of Compound 7. Data collec-
tion for compound 7 was performed at 100(2) K on a Nonius Kappa
CCD single diffractometer, using graphite-monochromated Mo KR
radiation. Images were collected at a 29 mm fixed crystal-detector
distance, using the oscillation method, with 1° oscillation and 50 s
exposure time per image. Data collection strategy was calculated
with the program Collect.51 Data reduction and cell refinement were
Acknowledgment. We thank the MEC of Spain for a grant
(to D.G.) and financial support (Projects BQU2003-05471
and CTQ2006-01207).
performed with the programs HKL Denzo and Scalepack.52
A
semiempirical absorption correction was applied using the program
SORTAV.53 Using the program suite WinGX,46 the structure was
solved by Patterson interpretation and phase expansion and refined
Supporting Information Available: Tables of selected molec-
ular orbitals for complexes trans-3a and cis-3a (pdf) and a CIF
file containing the crystallographic data for the structural analysis
of compounds 4, 5a, and 7. This material is available free of charge
via the Internet at http://pubs.acs.org.
(51) Collect, data collection software; Nonius B. V.: The Netherlands,
1997–2004.
(52) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
(53) Blessing, R. H. Acta Crystallogr. 1995, A51, 33.
OM7010358