Reactivity of the α-agostic methyl bridge in the unsaturated complex [Mo2(η5-C5H5) 2(μ-η1:η2-CH3)(μ- PCy2)(CO)2]: Migratory behavior and methylid

Article abstract of DOI:10.1021/om800061z

The bridging methyl ligand present in the title compound can migrate up to the cyclopentadienyl (Cp) site upon reaction with CO to achieve an overall exchange between the methyl and hydrogen (Cp) positions. In the presence of different metal-carbonyl comp

Full text of DOI:10.1021/om800061z

Organometallics 2008, 27, 1973–1975
1973
Reactivity of the r-Agostic Methyl Bridge in the Unsaturated
Complex [Mo2(η5-C5H5)2(µ-η1:η2-CH3)(µ-PCy2)(CO)2]: Migratory
Behavior and Methylidyne Derivatives
M. Angeles Alvarez, Daniel Garc´ıa-Vivo´, M. Esther Garc´ıa, M. Eugenia Mart´ınez,
Alberto Ramos, and Miguel A. Ruiz*
Departamento de Qu´ımica Orga´nica e Inorga´nica/IUQOEM, UniVersidad de OViedo, E-33071 OViedo, Spain
ReceiVed January 23, 2008
Chart 1
Summary: The bridging methyl ligand present in the title
compound can migrate up to the cyclopentadienyl (Cp) site upon
reaction with CO to achieVe an oVerall exchange between the
methyl and hydrogen (Cp) positions. In the presence of different
metal-carbonyl complexes, easy dehydrogenation of the methyl
group takes place under photochemical conditions to giVe
methylidyne-bridged heterometallic clusters.
few of them display multiple intermetallic bonding,8 and their
reactivity has not been explored. In this context, our recent
preparation of the unsaturated methyl complex [Mo2Cp2(µ-η1:
η2-CH3)(µ-PCy2)(CO)2] (1;9 Cp ) η5-C5H5) gave us the op-
portunity to study the chemical behavior of an R-agostic methyl
ligand bridging a multiple metal-metal bond. In this paper we
report our preliminary results on the reactivity of compound 1,
which reveal several unusual features such as the facile
migration of the methyl ligand up to the coordinated cyclopen-
tadienyl groups and its easy dehydrogenation in the presence
of metal-carbonyl fragments, then providing a rational synthetic
route to novel heterometallic clusters having methylidyne
bridges (Scheme 1).
Methyl- and other alkyl-bridged complexes are species of
interest for several reasons, including the fact that they serve
as models both for intermediates in alkyl-transfer processes and
for adsorbates in several heterogeneously catalyzed reactions
such as the Fischer-Tropsch synthesis and also because they
are implied as catalysts or precursors of the homogeneous
catalysts used in the polymerization of olefins.1,2 Although a
large number of such binuclear complexes have been reported
so far, only some of them display metal-metal bonds, these
generally exhibiting an asymmetric coordination of the methyl
(or alkyl) bridge in which a C-H bond is involved in an
R-agostic interaction with one of the metal atoms, the ligand
then behaving formally as a three-electron donor (Chart 1). The
reactivity reported so far for the methyl bridges in the latter
complexes includes the oxidative addition of the agostic C-H
bond at trimetal centers,3 rearrangement of the ligand to a
terminal coordination mode,4 deprotonation,5 reductive elimina-
tion with other ligands,6 and insertion of CO.4b,c,7 Among all
the alkyl-bridged complexes described so far, however, only a
According to recent DFT calculations,9a the agostic interaction
in 1 is rather weak; therefore, the view of the methyl ligand as
a 3-electron donor to yield a 32-electron species is somewhat
exaggerated yet is a useful formalism to interpret its reactivity.
In any case, the presence of this weak agostic interaction along
with the multiple intermetallic bond makes this compound quite
reactive toward simple donors such as carbon monoxide,
isocyanides, and diphosphines at room temperature, to give a
variety of products, many of which involve the migration of
the methyl ligand. The most remarkable migratory behavior is
that observed in the reaction with CO, this giving as major
products the acetyl-bridged complex [Mo2Cp2{µ-η1:κ1-C(Me)O}-
(µ-PCy2)(CO)3] (2) and the hydrides [Mo2Cp(η5-C5H4Me)(µ-
H)(µ-PCy2)(CO)4] (3) and [Mo2Cp{η5-C5H4C(O)Me}(µ-H)(µ-
PCy2)(CO)4] (4), the last two complexes having methyl- and
acetyl-substituted cyclopentadienyl ligands, respectively. The
relative amounts of these products are dependent on the
experimental conditions, and separate experiments indicate that
toluene solutions of the acetyl complex 2 decompose progres-
sively at room temperature to give mainly the methylcyclopen-
tadienyl hydride complex 3, along with small amounts of 4 and
other products.
* To whom correspondence should be addressed. E-mail:
mara@uniovi.es.
(1) (a) Braunstein, P.; Boag, N. M. Angew. Chem., Int. Ed. 2001, 40,
2427. (b) Marks, T. J. Acc. Chem. Res. 1992, 25, 57.
(2) For some recent work on alkyl-bridged complexes see, for example: (a)
Bolton, P. D.; Clot, E.; Cowley, A. R.; Mountford, P. J. Am. Chem. Soc.
2006, 128, 15005. (b) Dietrich, H. M.; Grove, H.; To¨rnroos, K. W.;
Anwander, R. J. Am. Chem. Soc. 2006, 128, 1458. (c) Weng, Z.; Teo, S.;
Koh, L. L.; Hor, T. S. A. Chem. Commun. 2006, 1319.
(3) (a) Calvert, R. B.; Shapley, J. R. J. Am. Chem. Soc. 1977, 99, 5225.
(b) Dutta, T. K.; Vites, J. C.; Jacobsen, G. B.; Fehlner, T. P. Organometallics
1987, 6, 842.
(4) (a) Wigginton, J. R.; Trepanier, S. J.; McDonald, R.; Ferguson, M. J.;
Cowie, M. Organometallics 2005, 24, 6194. (b) Rowsell, B. D.; McDonald,
R.; Cowie, M. Organometallics 2004, 23, 3873. (c) Trepanier, S. J.;
McDonald, R.; Cowie, M. Organometallics 2003, 22, 2638.
(5) (a) Davies, D. L.; Gracey, B. P.; Guerchais, V.; Knox, S. A. R.;
Orpen, A. G. J. Chem. Soc., Chem. Commun. 1984, 841. (b) Casey, C. P.;
Fagan, P. J.; Miles, W. H. J. Am. Chem. Soc. 1982, 104, 1134. (c) Dawkins,
G. M.; Green, M.; Orpen, A. G.; Stone, F. G. A. J. Chem. Soc., Chem.
Commun. 1982, 41.
(8) Apparently only a few complexes (all dichromium ones) have been
reported to have alkyl bridges across multiple metal-metal bonds: (a)
Heintz, R. A.; Ostrander, R. L.; Rheingold, A. L.; Theopold, K. H. J. Am.
Chem. Soc. 1994, 116, 11387. (b) Morse, P. M.; Spencer, M. D.; Wilson,
S. R.; Girolami, G. S. Organometallics 1994, 13, 1646. (c) Andersen, R. A.;
Jones, R. A.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1978, 446.
(9) (a) Garc´ıa, M. E.; Ramos, A.; Ruiz, M. A.; Lanfranchi, M.; Marchio,
L. Organometallics 2007, 26, 6197. (b) Garc´ıa; M. E.; Melo´n, S.; Ramos,
A.; Riera, V.; Ruiz, M. A.; Belletti, D.; Graiff, C.; Tiripicchio, A.
Organometallics 2003, 22, 1983.
(6) (a) Carlucci, L.; Proserpio, D. M.; D’Alfonso, G. Organometallics
1999, 18, 2091. (b) Noh, S. K.; Sendlinger, S. C.; Janiak, C.; Theopold,
K. H. J. Am. Chem. Soc. 1989, 111, 9127.
(7) Gao, Y.; Jennings, M. C.; Puddephatt, R. J. Organometallics 2001,
20, 1882.
10.1021/om800061z CCC: $40.75
2008 American Chemical Society
Publication on Web 04/03/2008

1974 Organometallics, Vol. 27, No. 9, 2008
Communications
Scheme 1
Figure 1. DFT optimized structure of compound 5, with H atoms
(except those of the methyl group) and Cy rings (except the C1
atoms) removed for clarity, and the corresponding electron density
map in the Mo2C(methyl) plane, with nuclear positions (b), bond
critical points (2), and bond critical paths (s) indicated. Selected
bond lengths (Å): Mo1-Mo2 ) 2.485; Mo1-CO ) 2.181;
Mo2-CO ) 2.032; Mo1-CH3) 2.434; Mo2-CH3 ) 2.195;
Mo1-HCH2 ) 1.982; Mo1-P ) 2.408; Mo2-P ) 2.431.
373 K of the complexes [MCpR(CO)3] (M ) Mo, W, R ) Et,
Ph, CH2Ph) to give hexacarbonyl dimers of the type
[M2Cp2(CO)6] having one or two η5-C5H4R ligands.16 Further
studies to better understand these singular reactions, including
those of the benzyl analogue of 1,9a are now in progress.
Even when the removal of a ligand from a complex having
an R-agostic methyl bridge might force the oxidative addition
of the agostic C-H bond to yield a less unsaturated methylene
hydride derivative, this turned out to be a difficult process in
the case of 1. Indeed, a carbonyl ligand can be cleanly removed
from 1 upon irradiation with visible-UV light in toluene
solution, but this gives the extremely air-sensitive monocarbonyl
derivative [Mo2Cp2(µ-η1:η2-CH3)(µ-PCy2)(µ-CO)] (5), still
retaining the R-agostic interaction. The averaged 1H NMR
methyl resonance for this fluxional complex (δH -1.60 ppm,
JCH ) 116 Hz)17 is comparable to that observed for 1 (δH -0.77
ppm, JCH ) 124 Hz),9 but its C-H coupling is significantly
lower, thus suggesting a stronger C-H-Mo agostic interaction.
To better define the hapticity of this metal-methyl interaction,
we carried out a DFT calculation on 5, and this yielded a
minimized structure with a quite asymmetric R-agostic bridging
methyl group (Mo1-H ) 1.982 Å, Mo1-C ) 2.434 Å,
Mo2-C ) 2.195 Å; see Figure 1 and the Supporting Informa-
tion).18 This suggests that the agostic interaction in 5 is much
stronger than that in 1,9a thus effectively yielding a 30-electron
complex with a triple intermetallic bond (Mo-Mo ) 2.485 Å).
In agreement with this, an AIM analysis of the topology of the
electron density in 5 revealed the presence of a bond path (and
the corresponding bond critical point) connecting the Mo1 and
H (but not C) atoms (Figure 1 and the Supporting Information),
as previously observed for strong ꢀ-agostic interactions in
mononuclear complexes19 but not for the weak interaction in
1.9a We note that this is the first time that such a topology has
been reported for a bridging R-agostic alkyl ligand.
The acetyl complex 2 is structurally related to its methoxy-
carbyne isomer [Mo2Cp2(µ-COMe)(µ-PCy2)(CO)3] recently
prepared by us10 but has a more complex structure in solution,
this involving cis/trans isomerism.11,12 The presence of the acyl
ligand is denoted by the appearance of a C-O stretching band
at 1583 cm-1 in the IR spectrum. On the other hand, the
spectroscopic data for the hydride complexes 313 and 414 clearly
establish that they have the same structure as the bis(cyclopen-
tadienyl) complex [Mo2Cp2(µ-H)(µ-PCy2)(CO)4],15 except for
the presence of a methyl and acetyl substituent, respectively,
in one of the Cp rings. The formation of compounds 3 and 4
requires a complex sequence of steps resulting in the exchange
of positions between a metal-bound methyl or acetyl ligand and
the H atom of a cyclopentadienyl ligand and is itself unprec-
edented. To our knowledge, the closest migratory transforma-
tions reported previously are the thermal decompositions above
(10) Garc´ıa, M. E.; Garc´ıa-Vivo´, D.; Ruiz, M. A.; Aullo´n, G.; Alvarez,
S. Organometallics 2007, 26, 5912.
(11) Selected data for 2: ν(CO) (CH2Cl2) 1919 (s), 1849 (vs), 1816 (m),
1583 (w, CdO) cm-1 31P{1H} NMR (121.52 MHz, 290 K, CDCl3) δ 264.5
;
(s, br, isomer cis), 217.6 (s, isomer trans).
(12) Cis/trans isomerism in alkenyl-bridged dimolybdenum complexes
isoelectronic with compound 2 has been recently discussed by us; see:
Alvarez, M. A.; Garc´ıa, M. E.; Ramos, A.; Ruiz, M. A.; Lanfranchi, M.;
Tiripicchio, A. Organometallics 2007, 26, 5454.
(13) Selected data for 3: ν(CO) (CH2Cl2) 1953 (m, sh), 1924 (vs), 1852
(s), 1819 (w, sh) cm-1 1H NMR (300.13 MHz, 290 K, CDCl3) δ 5.37,
;
Having established the photochemical stability of the methyl
bridge in 1, we turned to study its photochemical reactions with
metal-carbonyl complexes of the type [M(CO)x] or [MCp-
(CO)y] (M ) Cr, Mo, W, Mn, Fe, Ru; x, y ) 3-6) to examine
the synthetic potential of 1 in the preparation of heterometallic
clusters. To our surprise, compound 1 underwent dehydroge-
5.17, 5.04, 4.93 (4 × m, 4 × 1H, C5H4), 5.32 (s, 5H, Cp), 2.27 (s, 3H,
Me), -13.18 (d, 1H, JPH ) 35 Hz, µ-H).
(14) Selected data for 4: ν(CO) (CH2Cl2) 1950 (m, sh), 1933 (vs), 1864
(s), 1678 (w, CdO) cm-1; 1H NMR (300.13 MHz, 290 K, CDCl3) δ 6.03,
5.81, 5.37, 5.08 (4 × m, 4 × 1H, C5H4), 5.32 (s, 5H, Cp), 2.50 (s, 3H,
Me), -13.31 (d, 1H, JPH ) 35 Hz, µ-H); 13C{1H} NMR (100.63 MHz,
213 K) δ 243.0, 242.3 (2 × d, JCP ) 27 Hz, MoCO), 236.0, 233.8 (2 × s,
MoCO), 195.8 (s, C(O)Me).
(15) Garc´ıa; M. E.; Riera, V.; Ruiz, M. A.; Rueda, M. T.; Sa´ez, D.
Organometallics 2002, 21, 5515.
(18) Density functional calculations were performed with the GAUSS-
IAN03 program package using the hybrid method B3LYP, together with a
standard 6-311G* basis set on all atoms except Mo, for which a valence
double-ꢁ-quality basis set and LANL2DZ effective core potentials were
used. See the Supporting Information for further details.
(19) Popelier, P. L. A.; Logothetis, G. J. Organomet. Chem. 1998, 555,
101.
(16) Pittman, C. U., Jr.; Felis, R. F. J. Organomet. Chem. 1974, 72,
399.
(17) Selected data for 5: ν(CO) (toluene) 1697 (s) cm-1 31P{1H} NMR
;
(121.52 MHz, 290 K, toluene-d8) δ 231.9 (s); 1H NMR (300.13 MHz, 290
K, toluene-d8) δ 5.54 (s, 10H, Cp), -1.60 (s, 3H, JHC ) 116 Hz, µ-η1:η2-
CH3).

Communications
Organometallics, Vol. 27, No. 9, 2008 1975
Figure 3. ORTEP drawing (30% probability) of compound 7,
with H atoms (except that on C9) and cyclohexyl rings (except
the C1 atoms) omitted for clarity. Selected bond lengths (Å):
Mo1-Mo2 ) 2.835(1), Mo1-Fe1 ) 2.725(1), Mo1-Fe2 )
2.707(1), Mo2-Fe1 ) 2.770(1), Fe1-Fe2 ) 2.537(1), Mo1-C9
) 2.144(6), Mo2-C9 ) 2.113(6), Fe1-C9 ) 1.906(6), Fe2-C9
) 2.001(6), Mo1-P(1) ) 2.476(2), Mo2-P1 ) 2.392(1).
Figure 2. ORTEP drawing (30% probability) of compound 6, with
H atoms (except that on C8) and cyclohexyl rings (except the C1
atoms) omitted for clarity. Selected bond lengths (Å) and angles
(deg): Mo1-Mo2 ) 2.9283(3), Mo1-Mo3 ) 3.1245(3), Mo2-Mo3
) 3.0938(3), Mo1-C8 ) 2.040(3), Mo2-C8 ) 2.053(3), Mo3-C8
) 2.316(3), Mo1-P1 ) 2.410(1), Mo2-P1 ) 2.425(1); C1-Mo1-
Mo2 ) 86.8(1), C2-Mo2-Mo1 ) 87.0(1).
lengths of reference, this indicating a delocalization of such an
unsaturation. It is interesting to note that only a few similar
“butterfly” methylidyne clusters, [Fe4(µ-CH)(µ-H)(CO)12] and
[MFe3(µ-CH)(CO)x]n (M ) Cr, W, Mn, Rh; x ) 12, 13; n ) 0,
1-),25 have been reported previously. These, however, display
CH ligands involved in agostic C-H-M interactions, therefore
yielding electron-precise (62-electron) “butterfly” structures.
Methylidyne clusters are relatively scarce molecules related to
the surface species following methane activation by metal atoms
and surfaces, and to those involved in relevant heterogeneously
catalyzed industrial processes such as the Fischer-Tropsch
synthesis,26 and further work on the synthesis and chemical
behavior of these heterometallic methylidyne derivatives of 1
is currently in progress.
In summary, we have shown that the unsaturated R-agostic
methyl complex 1 displays reactivity patterns not observed in
related electron-precise binuclear complexes, such as an un-
precedented migratory behavior under mild conditions, yielding
an overall exchange between methyl and H(cyclopentadienyl)
positions, and easy dehydrogenation in the presence of
metal-carbonyl fragments, thus providing a new and rational
route to novel heterometallic clusters having methylidyne
bridges.
nation in many of these reactions to give methylidyne-bridged
clusters. For instance, the reaction of 1 and [Mo(CO)6] under
visible-UV irradiation gives the trimetallic cluster [Mo3Cp2(µ3-
CH)(µ-PCy2)(CO)7] (6) in high yield.20 The structure of 6 results
from the addition of a Mo(CO)5 fragment to the unsaturated
Mo2 center of 1, this being accompanied by dehydrogenation
of the methyl ligand and trans to cis isomerization of the original
Mo2(CO)2 moiety (Figure 2).21 The methylidyne ligand in this
electron-precise (48-electron) cluster (δC 244.7 ppm, JCH ) 162
Hz) exhibits a quite asymmetric coordination, it being strongly
bound to the MoCp centers (Mo-C ) ca. 2.06 Å) and more
weakly to the Mo(CO)5 fragment (Mo-C ) ca. 2.32 Å). We
note here that only one other methylidyne-bridged Mo3 cluster
appears to have been described previously.22 The most remark-
able reaction, however, occurs upon irradiation of toluene
solutions of 1 and [Fe2(CO)9], since this allows the incorporation
of two Fe(CO)3 moieties to the unsaturated Mo2 center and
dehydrogenation of the methyl ligand to give the methylidyne
cluster [Fe2Mo2Cp2(µ4-CH)(µ-PCy2)(CO)8] (7) in high yield.23
Unexpectedly, this 60-electron cluster does not display a
tetrahedral structure but a “butterfly” one with the CH ligand
(δC 243.8 ppm, JCH ) 143 Hz) bridging the four metal atoms
(Figure 3),24 and therefore the molecule must be considered as
unsaturated. In agreement with this, the intermetallic lengths
are 0.1-0.2 Å shorter than the corresponding single-bond
Acknowledgment. We thank the MEC of Spain for
financial support (Project CTQ2006-01207) and the Unidad
de Rayos X at the Universidad de Santiago de Compostela
and at the Universidad de Oviedo (Oviedo, Spain) for the
acquisition of the diffraction data reported here.
(20) Selected data for 6: ν(CO) (CH2Cl2) 2042 (s), 1949 (s, sh), 1935
(vs), 1825 (w) cm-1 31P{1H} NMR (121.52 MHz, 290 K, CD2Cl2) δ 172.8
;
Supporting Information Available: Text, tables, and figures
giving experimental procedures and spectroscopic data for new
compounds and details of DFT calculations on compound 5 and CIF
files giving crystallographic data for compounds 6 and 7. This material
is available free of charge via the Internet at http://pubs.acs.org.
(s); 1H NMR (300.13 MHz, 290 K, CD2Cl2) δ 12.26 (d, JPH ) 5, JCH
)
162 Hz, 1H, µ-CH), 5.25 (d, JPH ) 1 Hz, 10H, Cp); 13C{1H} NMR (75.48
MHz, 290 K, CD2Cl2) δ 244.7 (d, JCP ) 21 Hz, µ-CH).
(21) X-ray data for 6: brown crystals, monoclinic (P21/c), a ) 10.0881(2)
Å, b ) 15.5800(3) Å, c ) 19.2306(4) Å, ꢀ ) 95.895(1)°, V ) 3006.5(1)
Å3, T ) 100 K, Z ) 4, R ) 0.0342 (observed data with I > 2σ(I)), GOF
) 1.04.
OM800061Z
(22) Akita, M.; Noda, K.; Moro-oka, Y. Organometallics 1994, 13, 4145.
(23) Selected data for 7: ν(CO) (CH2Cl2) 2022 (s), 1984 (m, sh), 1972
(vs), 1944 (s), 1911 (m), 1870 (w) cm-1
;
31P{1H} NMR (121.52 MHz,
(25) (a) Tachikawa, M.; Muetterties, E. L. J. Am. Chem. Soc. 1980,
102, 4541. (b) Hriljac, J. A.; Harris, S.; Shriver, D. F. Inorg. Chem. 1988,
27, 816.
290 K, CD2Cl2) δ 164.2 (s); 1H NMR (400.13 MHz, 290 K, CD2Cl2) δ
5.46, 5.00 (2 × s, 2 × 5H, Cp), 4.24 (d, 1H, JPH ) 2 Hz, µ-CH); 13C{1H}
NMR (100.63 MHz, CD2Cl2) δ 234.3 (d, JCP ) 11 Hz, µ-CH).
(24) X-ray data for 7: black crystals, monoclinic (P21/c), a ) 13.3752(2)
Å, b ) 11.7509(2) Å, c ) 25.3894(4) Å, ꢀ ) 119.262(1)°, V ) 3481.3(1)
Å3, T ) 100 K, Z ) 2, R ) 0.0476 (observed data with I > 2σ(I)), GOF
) 1.19.
(26) (a) Cho, H.-G.; Lester, A. J. Phys. Chem. A 2006, 110, 3886. (b)
Marsh, A. L.; Becraft, K. A.; Somorjai, G. A. J. Phys. Chem. B 2005, 109,
13619. (c) Maitlis, P. M. J. Organomet. Chem. 2004, 689, 4366. (d) Maitlis,
P. M. J. Mol. Catal. A 2003, 204-205, 55. (e) Dry, M. E. Catal. Today
2002, 71, 227.