Transformations and agostic interactions of hydrocarbyl ligands bonded to the sulfur-rich dimolybdenum site {Mo2Cp2(μ-SMe) 3}: Chemical and electrochemical formation of μ-alkyl and μ-vinyl compounds from a μ-alkylidene derivative

Article abstract of DOI:10.1021/om050761g

A series of chemical and electrochemical transformations of systems in which a {Mo₂Cp₂(μ-SMe)₃} core is bridged by a μ-C₂H_nR ligand (n = 0-4) are described in this paper. The reaction of the alkylidene complex [Mo₂Cp₂(μ-SMe) ₃(μ-η¹:η²-CHCH₂Tol)] (BF₄) (1) with LiBuⁿ at 0°C produces the μ-σ,π-vinyl complex [Mo₂Cp₂(μ-SMe) ₃(μ-η¹:η²-CH=CHTol] (2) in good yield. The molecular structure of 2 has been confirmed by X-ray analysis. Upon treatment with NaBEL₄ 1 is readily converted into the semibridging alkyl species [Mo₂Cp₂(μ-SMe)₃(μ-CH ₂CH₂Tol)] (3), which is also formed by electrochemical reduction of 1 in acidic medium. NMR and X-ray diffraction studies of 3 are consistent with, but do not definitively establish, the presence of a η¹ α-agostic interaction. Density functional theory has been used to confirm the presence of agostic interactions in both 1 and 3 and also to explore the exchange pathways for these hydrocarbyl dimolybdenum systems. Electrochemical transformation of the μ-alkylidene complex 1 gives 3 as the major product when acid is present and a mixture of 2 and 3 when acid is absent, production of 2 being favored by low initial concentrations of 1. Theoretical, spectroscopic, and diffraction data are used to explain the formation and structures of closely related [Mo₂Cp ₂(μ-SMe)₃(μ-C₂H_nR] ^z- complexes (n = 0-4 and z = 0, 1), including 1-3.

Full text of DOI:10.1021/om050761g

6268
Organometallics 2005, 24, 6268-6278
Transformations and Agostic Interactions of
Hydrocarbyl Ligands Bonded to the Sulfur-Rich
Dimolybdenum Site {Mo2Cp2(µ-SMe)3}: Chemical and
Electrochemical Formation of µ-Alkyl and µ-Vinyl
Compounds from a µ-Alkylidene Derivative
Nolwenn Cabon, Alan Le Goff, Christine Le Roy, Franc¸ois Y. Pe´tillon,*
Philippe Schollhammer,* and Jean Talarmin*
UMR CNRS 6521, Chimie, Electrochimie Mole´culaires et Chimie Analytique, UFR Sciences et
Techniques, Universite´ de Bretagne Occidentale, CS 93837, 29238 Brest-Cedex 3, France
John E. McGrady*
Department of Chemistry, University of York, Heslington, York Y010 5DD, U.K.
Kenneth W. Muir*
Chemistry Department, University of Glasgow, Glasgow G12 8QQ, U.K.
Received September 2, 2005
A series of chemical and electrochemical transformations of systems in which a {Mo2Cp2-
(µ-SMe)3} core is bridged by a µ-C2HnR ligand (n ) 0-4) are described in this paper. The
reaction of the alkylidene complex [Mo2Cp2(µ-SMe)3(µ-η1:η2-CHCH2Tol)](BF4) (1) with LiBun
at 0 °C produces the µ-σ,π-vinyl complex [Mo2Cp2(µ-SMe)3((µ-η1:η2-CHdCHTol] (2) in good
yield. The molecular structure of 2 has been confirmed by X-ray analysis. Upon treatment
with NaBH4 1 is readily converted into the semibridging alkyl species [Mo2Cp2(µ-SMe)3-
(µ-CH2CH2Tol)] (3), which is also formed by electrochemical reduction of 1 in acidic medium.
NMR and X-ray diffraction studies of 3 are consistent with, but do not definitively establish,
the presence of a η1 R-agostic interaction. Density functional theory has been used to confirm
the presence of agostic interactions in both 1 and 3 and also to explore the exchange pathways
for these hydrocarbyl dimolybdenum systems. Electrochemical transformation of the
µ-alkylidene complex 1 gives 3 as the major product when acid is present and a mixture of
2 and 3 when acid is absent, production of 2 being favored by low initial concentrations of
1. Theoretical, spectroscopic, and diffraction data are used to explain the formation and
structures of closely related [Mo2Cp2(µ-SMe)3(µ-C2HnR]z+ complexes (n ) 0-4 and z ) 0, 1),
including 1-3.
Introduction
transformations of unsaturated dinitrogenous sub-
strates (RNdN, RNdNH) at a sulfur-rich dimolybde-
The search for new tools to activate molecules and
for models which mimic the behavior of less accessible
biological or industrial catalysts inspires current inter-
est in the chemistry of polymetallic compounds.1 For
example, the bis(cyclopentadienyl) bimetallic fragments
{M2Cp′2} (Cp′ ) η5-C5R5) exhibit significant metal-
metal interactions and have a rich and original chem-
istry which merits further exploration.2 As an illustra-
tion of this approach, we reported a sequence of
(2) For recent examples and reviews: (a) Alvarez, C. M.; Alvarez,
M. A.; Garcia, M. E.; Ramos, A.; Ruiz, M. A.; Lanfranchi, M.;
Tiripicchio, A. Organometallics 2005, 24, 7. (b) Alvarez, C. M.; Garcia,
M. E.; Rueda, M. T.; Ruiz, M. A.; Saez, D.; Connelly, N. G. Organo-
metallics 2005, 24, 650. (c) Alvarez, M. A.; Anaya, Y.; Garcia, M. E.;
Ruiz, M. A. Organometallics 2004, 23, 3950. (d) Alonso, M.; Garcia,
M. E.; Ruiz, M. A.; Hamidov, H.; Jeffery, J. C. J. Am. Chem. Soc. 2004,
126, 13610. (e) Cabon, N.; Pe´tillon, F. Y.; Schollhammer, P.; Talarmin,
J.; Muir, K. W. Dalton Trans. 2004, 2708. (f) Nishibayashi, Y.; Imajima,
H.; Onodera, G.; Hidai, M.; Uemura, S. Organometallics 2004, 23, 26.
(g) Nishioka, T.; Kitayama, H.; Breedlove, B. K.; Shiomi, K.; Kinoshita,
I.; Isobe, K. Inorg. Chem. 2004, 43, 5688. (h) Adams, H.; Morris, M.
J.; Morris, S. A.; Motley, J. C. J. Organomet. Chem. 2004, 689, 522. (i)
Cremer, C.; Burger, P. Chem. Eur. J. 2003, 9, 3583. (j) Nishibayashi,
Y.; Yoshikawa, M.; Inada, Y.; Hidai, M.; Uemura, S. J. Am. Chem.
Soc. 2002, 124, 11846. (k) Bridgeman, A. J.; Mays, M. J.; Woods, A. D.
Organometallics 2001, 20, 2076. (l) Hobert, S. E.; Bruce, C.; Noll, B.
C.; Rakowski DuBois, M. Organometallics 2001, 20, 1370. (m) Adams,
H.; Allott, C.; Bancroft, M. N.; Morris, M. J. Dalton Trans. 2000, 4145.
(n) Pe´tillon, F. Y.; Schollhammer, P.; Talarmin, J.; Muir, K. W. Coord.
Chem. Rev. 1998, 178-180, 203. (o) Rakowski DuBois, M. Polyhedron
1997, 16, 3089. (p) Rakowski DuBois, M. J. Cluster Sci. 1996, 7, 293.
(q) Hidai, M.; Mizobe, Y.; Matsuzaka, H. J. Organomet. Chem. 1994,
473, 1. (r) Winter, M. J. Adv. Organomet. Chem. 1989, 29, 101. (s)
Curtis, M. D. Polyhedron 1987, 6, 759.
* To whom correspondence should be addressed. E-mail:
franc¸ois.petillon@univ-brest.fr (F.Y.P.); schollha@univ-brest.fr (P.S.);
jean.talarmin@univ-brest.fr (J.T.); jem15@york.ac.uk (J.E.M.); ken@
chem.gla.ac.uk (K.W.M.).
(1) (a) Catalysis by Di- and Polynuclear Metal Cluster Complexes;
Adams, R. D., Cotton, F. A., Eds.; Wiley-VCH: New York, 1998. (b)
Braunstein, P.; Rose, J. In Chemical Bonds-Better Ways to Make Them
and Break Them; Bernal, I., Ed.; Elsevier: Amsterdam, 1989; p 5. (c)
Hidai, M.; Mizobe, Y. In Transition Metal Sulfur Chemistry-Biological
and Industrial Significance; Stiefel, E. I., Matsumoto, K., Eds.; ACS
Symposium Series 653; American Chemical Society: Washington, DC,
1996; p 310. (d) Hidai, M.; Mizobe, Y.; Matsuzaka , H. J. Organomet.
Chem. 1994, 473, 1 and references therein.
10.1021/om050761g CCC: $30.25 © 2005 American Chemical Society
Publication on Web 11/04/2005

Hydrocarbyl Ligands Bonded to {Mo2Cp2(µ-SMe)3}
Organometallics, Vol. 24, No. 25, 2005 6269
(Scheme 1) was the starting point for new studies6
focused on the transformations of hydrocarbyl groups
at this sulfur-rich site. There have been few systematic
studies of complexes featuring {M2(µ-η1:η2-CdCHR)}
groups,9 and it remains unclear whether the η1:η2
coordination mode induces novel reactivity in the vi-
nylidene moiety or is, instead, a dead end in its
transformation pathway. In addition to the fundamental
interest in this chemistry, the transformations of RCt
CH may also shed some light on the biologically and
industrially critical activation of the isoelectronic Nt
N bond. The investigation of reactions of D with various
nucleophilic reagents has been the focus of our studies
so far.6a,c The formal addition of hydride to the outer
carbon atom of the vinylidene bridge in D resulted in
the formation of µ-alkylidyne derivatives [Mo2Cp2(µ-
SMe)3(µ-CCH2R)] (E). The electrochemical conversion
Scheme 1. Successive Transformations of
Hydrocarbyl Bridges at the {Mo2Cp2(µ-SMe)3}
Group
of D into E will be reported in a separate paper.10
E
was readily protonated, forming the µ-alkylidene species
[Mo2(η5-C5H5)2(µ-SMe)3(µ-η1:η2-CHCH2R)](BF4) (1), which,
according to NMR evidence, exhibits R-agostic inter-
actions.6a
The reactivity of the µ-alkylidene derivative [Mo2Cp2-
(µ-SMe)3(µ-η1:η2-CHCH2R)](BF4) (1; R ) Ph, tolyl) forms
the basis of the current paper. We describe the depro-
tonation of 1 by LiBun, leading to the µ-σ,π-vinyl
complex [Mo2Cp2(µ-SMe)3((µ-η1:η2-CHdCHR] (2), as well
as its chemical reduction with sodium borohydride
(R ) Tol) and its electrochemical reduction in acidic
solution (R ) Ph), leading to the semibridging alkyl
derivative [Mo2Cp2(µ-SMe)3(µ-CH2CH2R)] (3). NMR evi-
dence suggests that 3, like 1, is stabilized by R-agostic
interactions. The remarkable diversity of hydrocarbyl
coordination environments illustrated in this and previ-
ous papers, along with their apparently facile intercon-
version, has also prompted us to use theoretical methods
to investigate their structure and dynamics.
num site {Mo2Cp2(µ-SMe)3}3 which suggests a com-
pletely novel pathway for the reduction of N2 at a
dinuclear site of FeMoco.3a,b,4 Very recently we have
used density functional theory to explore the electronic
pathways involved in these reduction processes.5
We have also described the activation of other small,
unsaturated molecules, such as terminal alkynes (HCt
CR) (Scheme 1),6 isocyanides (RNtC),7 and nitriles
(RCtN), by the {MoIII2Cp2} system.8 The conversion of
the alkyne adduct [Mo2Cp2(µ-SMe)3(HCtCR)](BF4) (B;
Cp ) η5-C5H5) into the readily accessible vinylidene
derivative [Mo2Cp2(µ-SMe)3(µ-η1:η2-CdCHR)](BF4) (D)
Results and Discussion
1. Computed Structure of the Alkylidene Cation
[Mo2Cp2(µ-SH)3(µ-CHCH2Ph)]+. As noted above, the
alkylidene species [Mo2Cp2(µ-SMe)3(µ-CHCH2Tol)](BF4)
(1) forms the foundation of the current studies. Although
we have characterized 1 thoroughly by NMR spectros-
copy, we have been unable to obtain crystals suitable
for X-ray analysis.
A logical starting point for the computational compo-
nent of the study is therefore the structure of the model
cation [Mo2Cp2(µ-SH)3(µ-CHCH2Ph)]+ (1′), where the
methyl substituents on the phenyl and sulfido ligands
have been removed for computational expedience. Our
previous experience in closely related systems suggests
that this simplification should not have a dramatic
influence on the structure.5 In all cases, the prime (′)
signifies that the methyl groups have been removed. The
optimized structure of 1′, shown in Figure 1, confirms
the highly asymmetric nature of the alkylidene bridge,
with Mo(1)-CR and Mo(2)-CR bond lengths of 1.95 and
2.39 Å, respectively. The very short bond length is
indicative of substantial ModC multiple bonding, simi-
(3) (a) Le Grand, N.; Muir, K. W.; Pe´tillon, F. Y.; Pickett, C. J.;
Schollhammer, P.; Talarmin, J. Chem. Eur. J. 2002, 8, 3115. (b)
Pe´tillon, F. Y.; Schollhammer, P.; Talarmin, J. Inorg. Chem. 1999, 38,
1954. (c) Schollhammer, P.; Didier, B.; Le Grand, N.; Pe´tillon, F. Y.;
Talarmin, J.; Muir, K. W.; Teat, S. J. Eur. J. Inorg. Chem. 2002, 658.
(d) Schollhammer, P.; Gue´nin, E.; Pe´tillon, F. Y.; Talarmin, J.; Muir,
K. W. Organometallics 1998, 17, 1922.
(4) (a) Durrant, M. C. Inorg. Chem. Commun. 2001, 4, 60. (b)
Barrie`re, F. Coord. Chem. Rev. 2003, 236, 71.
(5) McGrady, J. E.; Padden Metzker, J. K. Chem. Eur. J. 2004, 10,
6447.
(6) (a) Cabon, N.; Schollhammer, P.; Pe´tillon, F. Y.; Talarmin, J.;
Muir, K. W. Organometallics 2002, 21, 448. (b) Schollhammer, P.;
Cabon, N.; Capon, J. F.; Pe´tillon, F. Y.; Talarmin, J.; Muir, K. W.
Organometallics 2001, 20, 1230. (c) Schollhammer, P.; Cabon, N.;
Kervella, A.-C.; Pe´tillon, F. Y.; Rumin, R.; Talarmin, J.; Muir, K. W.
Inorg. Chim. Acta 2003, 350, 495.
(7) Cabon, N.; Paugam, E.; Pe´tillon, F. Y.; Schollhammer, P.;
Talarmin, J.; Muir, K. W. Organometallics 2003, 22, 4178.
(8) (a) Schollhammer, P.; Le He´nanf, M.; Le Roy-Le Floch, C.;
Pe´tillon, F. Y.; Talarmin, J.; Muir, K. W. Dalton Trans. 2001, 1573.
(b) Schollhammer, P.; Pichon, M.; Muir, K. W.; Pe´tillon, F. Y.; Pichon,
R.; Talarmin, J. Eur. J. Inorg. Chem. 1999, 221.
(9) (a) Bruce, M. I. Chem. Rev. 1991, 91, 197. (b) El Amouri, H.;
Gruselle, M. Chem. Rev. 1996, 96, 1077.
(10) Le Goff, A.; Le Roy, C.; Pe´tillon, F. Y.; Schollhammer, P.;
Talarmin, J., Manuscript in preparation.

6270 Organometallics, Vol. 24, No. 25, 2005
Cabon et al.
Figure 2. ORTEP drawing of complex 2 showing 20%
probability ellipsoids. The hydrogen atom on C(4) was not
located.
Figure 1. Optimized structure of [Mo2Cp2(µ-SH)3(µ-CHCH2-
Ph)]+ (1′).
Table 1. Selected Bond Lengths (Å) and Angles
(deg) for [Mo2Cp2(µ-SMe)3(µ-η1:η2-CHdCHTol)] (2)
lar to that observed in the related alkylidyne ([Mo2-
Cp2(µ-SMe)3(µ-CCH2n-Pr)]) and vinylidene ([Mo2Cp2(µ-
SMe)3(µ-CCHTol)]+) species.6a,b The elongated CR-H
bond (1.18 Å) and the acute Mo(2)-CR-H angle (50.1°)
are indicative of a significant agostic interaction with
the electron-deficient molybdenum center. However, it
should be noted that the Mo(1)-CR-H angle (122°) is
nearly identical with the ideal sp2 value of 120°,
suggesting that the acute Mo(2)-CR-H angle is a
consequence of the optimization of the Mo(1)-CR bond-
ing rather than an indication of any intrinsically strong
interaction between the C-H group and the metal
center. Similar features were noted in one of the earliest
studies of agostic bonding, where Eisenstein and co-
workers showed that the R-agostic distortion in H5-
Mo(1)-S(1)
Mo(1)-S(2)
Mo(1)-S(3)
Mo(1)-C(4)
Mo(1)-C(5)
Mo(1)-Mo(2)
2.482(2)
2.533(2)
2.448(2)
2.320(9)
2.528(9)
2.601(1)
Mo(2)-S(1)
Mo(2)-S(2)
Mo(2)-S(3)
Mo(2)-C(4)
C(4)-C(5)
2.455(2)
2.445(3)
2.457(2)
2.204(9)
1.371(12)
1.510(9)
C(5)-C(31)
Mo(1)-C(4)-C(5)
82.1(5)
Mo(2)-C(4)-C(5)
C(4)-C(5)-C(31)
133.6(7)
123.9(7)
Mo(1)-C(4)-C(5)-C(31)
C(4)-C(5)-C(31)-C(32)
Mo(2)-C(4)-C(5)-C(31)
-117.5(7)
-156.4(7)
-170.2(6)
enyl groups, in CD2Cl2 down to 220 K, indicating that
the dynamic process remains rapid at low temperature
and, hence, cannot be studied on the NMR time scale.
The vinyl bridge (µ-η1:η2-CHdCHTol) was characterized
by the observation of two doublets at 7.18 and 6.73 ppm
-
TaCH3 is driven by the optimization of M-C, rather
than C-H‚‚‚M bonding.11 As noted in ref 6a, only a
single Cp resonance was observed down to 213 K for 1,
suggesting that the complex is highly fluxional. We will
return to this issue following a discussion of the forma-
tion of 2 and 3 from 1.
3
with a coupling constant JHH of 15 Hz, indicating a
trans disposition of the vinyl hydrogen atoms. Two-
1
dimensional heteronuclear inverse-correlation H-13C
experiments (HMQC, HMBC) allowed further charac-
terization of 2. The 13C chemical shifts for σ-bonded (CR)
and non-σ-bonded (Câ) carbon atoms of the µ-vinyl group
appear at 153.8 and 104.8 ppm, respectively, suggesting
there is little contribution from a carbene-like canonical
form to 2.12 The structure of 2 (Figure 2 and Table 1)
was confirmed by X-ray analysis of a single crystal
obtained from a diethyl ether solution at room temper-
ature. This establishes that 2 contains a unsymmetrical
σ,π-CHdCHTol group which bridges the Mo-Mo bond
of the {MoIII2Cp2(µ-SMe)3} core. The length of the Mo-
Mo bond (2.601(1) Å) is unexceptional, as are those of
the µ-Mo-S bonds, which fall in the range 2.445(3)-
2.482(2) Å, apart from the somewhat longer Mo(1)-S(2)
distance of 2.533(2) Å.2n The three-electron-donor vinyl
ligand bonds terminally to Mo(2) (Mo(2)-C(4) ) 2.204-
(9) Å) and via a weak unsymmetrical π-interaction to
Mo(1) (Mo(1)-C(4) ) 2.320(9) and Mo(1)-C(5) ) 2.528-
(9) Å). The C(4)-C(5) vinyl bond length (1.371(12) Å)
agrees with values in related vinylidene species (1.356-
2. Reaction of [Mo2Cp2(µ-SMe)3(µ-CHCH2Tol)]-
(BF4) (1) with LiBun. Spectroscopic Characteriza-
tion and Molecular Structure of [Mo2Cp2(µ-SMe)3-
(µ-η1:η2-CHdCHTol)] (2). The reaction of the µ-alkyli-
dene compound [Mo2Cp2(µ-SMe)3(µ-CHCH2Tol)](BF4),
(1), with a slight excess of LiBun (1.5 equiv) in tetrahy-
drofuran at 273 K, resulted in the formation of the
µ-vinyl derivative [Mo2Cp2(µ-SMe)3((µ-η1:η2-CHdCHTol)]
(2), which was isolated in good yield (91%) (reaction 1).
2 was characterized by elemental analysis and 1H
NMR spectroscopy, which exhibits the sets of resonances
expected for the {Mo2Cp2(µ-SMe)3}core and the tolyl
group. Only one signal was observed for cyclopentadi-
(12) Schollhammer, P.; Pe´tillon, F. Y.; Poder-Guillou, S.; Talarmin,
J.; Muir, K. W.; Yufit, D. S. J. Organomet. Chem. 1995, 513, 181 and
references therein.
(11) Demolliens, A.; Jean, Y.; Eisenstein, O. Organometallics 1986,
5, 1457.

Hydrocarbyl Ligands Bonded to {Mo2Cp2(µ-SMe)3}
Organometallics, Vol. 24, No. 25, 2005 6271
Table 2. Selected Bond Lengths (Å) and Angles
(deg) for [Mo2Cp2(µ-SMe)3(µ-CH2CH2Tol)] (3)
Mo(1)-S(1)
Mo(1)-S(2)
Mo(1)-S(3)
Mo(1)-C(4)
Mo(1)-Mo(2)
2.441(2)
2.454(2)
2.447(2)
2.448(8)
2.573(1)
Mo(2)-S(1)
Mo(2)-S(2)
Mo(2)-S(3)
Mo(2)-C(4)
C(4)-C(5)
C(5)-C(6)
2.438(2)
2.449(2)
2.467(3)
2.285(7)
1.509(11)
1.495(12)
Mo(1)-C(4)-C(5)
C(4)-C(5)-C(6)
127.0(5)
111.9(7)
Mo(2)-C(4)-C(5)
C(5)-C(6)-C(7)
122.8(5)
121.8(8)
Mo(1)-C(4)-C(5)-C(6)
C(4)-C(5)-C(6)-C(7)
Mo(2)-C(4)-C(5)-C(6)
108.1(7)
-97.7(10)
-169.1(5)
(6) Å) (Table 3) and in other µ-σ,π-vinyl complexes.12
The Mo(2)-C(4)-C(5)-C(31) torsion angle of -170.2-
(6)° is consistent with the trans arrangement of protons
attached to C(4) and C(5) deduced from NMR. Com-
parison of the structural features of the {Mo2(µ-η1:η2-
CRHdCâHR)} core of 2 with that of the vinylidene
moiety {Mo2(µ-η1:η2-CRdCâHR)} in [Mo2Cp2(µ-SMe)3(µ-
η1:η2-CdCHPh)](BF4) (D) (Table 3) reveals that the
formal addition of H- to CR in D to give 2 increases the
Mo(2)-CR bond length from 1.894(5) Å in D to 2.204(9)
Å in 2. As expected, the Mo(2)-CR-Câ angle of 164.6-
(4)° in D decreases to 133.6(7)° in 2. However, the Mo-
CR and Mo-Câ lengths of 2.268(5) and 2.589(5) Å in D
are broadly comparable with the corresponding values
in 2.
Figure 3. Optimized structures of [Mo2Cp2(µ-SH)3(µ-η1:
η2-CHdCHPh)] (2′) and [Mo2Cp2(µ-SH)3(µ-CCH2Ph)] (E′).
Mo-C bonds are much less asymmetric in 2′ than in
1′, indicating much reduced ModC π bonding in the
µ-vinyl species. Perhaps the most interesting aspect of
the electronic structure of 2 is its comparison with the
isomeric alkylidyne [Mo2Cp2(µ-SMe)3((µ-CCH2Tol)] (E),
reported in ref 6a. 2 is formed by deprotonation of 1,
which is itself synthesized by the protonation of E. The
optimized structure of the model alkylidyne [Mo2Cp2-
(µ-SH)3(µ-CCH2Ph)] (E′) is also shown in Figure 3, and
the structure of the core is also in excellent agreement
with the available X-ray data (for [Mo2Cp2(µ-SMe)3(µ-
CCH2Prn)]).6a The µ-vinyl species 2′ is more stable than
The optimized structure of the model µ-vinyl com-
pound [Mo2Cp2(µ-SH)3((µ-η1:η2-CHdCHPh)] (2′) is sum-
marized in Figure 3, and all bond lengths are in good
agreement with the X-ray structure. Significantly, the
the µ-alkylidyne isomer, but only by 7 kJ mol-1
.
However, if the phenyl group is replaced by hydrogen,
the order of stability is reversed, with the µ-alkylidyne
species being 28 kJ mol-1 lower than its µ-vinyl coun-
Table 3. Selected Distances (Å) and Angles (deg) in Hydrocarbyl Groups Bridging a [Mo2Cp2(µ-SMe)3]
Corea
a Data are taken from the following: C and D, ref 6b; E, ref 6a; 2 and 3, this work.

6272 Organometallics, Vol. 24, No. 25, 2005
Cabon et al.
terpart, presumably due to the absence of conjugation
of the CdC double bond. The computed energies there-
fore suggest that the balance between the µ-alkylidyne
and µ-vinyl isomers is a delicate one and that subtle
changes in composition are sufficient to tip the balance
in favor of one or the other.
3. Reaction of [Mo2Cp2(µ-SMe)3(µ-CHCH2Tol)]-
(BF4) (1) with NaBH4. Spectroscopic Character-
ization and Molecular Structure of [Mo2Cp2(µ-
SMe)3(µ-CH2CH2Tol)] (3). Compound 1 reacted with
an excess of sodium borohydride in tetrahydrofuran at
room temperature to afford a mixture of three products,
[Mo2Cp2(µ-SMe)3(µ-CHdCHTol)] (2) and [Mo2Cp2(µ-
SMe)3(µ-CCH2Tol)] (E), both of which were character-
ized by comparison of their 1H NMR spectra with those
of authentic samples,6 along with the novel species [Mo2-
Cp2(µ-SMe)3(µ-CH2CH2Tol)] (3) (reaction 2). The ratio
Figure 4. ORTEP drawing of complex 3 showing 20%
probability ellipsoids. The hydrogen atoms on C(4) were
not located.
Chart 1
tadienyl groups, suggesting either that the alkyl bridge
is symmetrically bonded to the Mo2 unit or that this
group is fluxional in solution at room temperature. The
available spectroscopic data are therefore consistent
with the presence of an alkyl bridge and also suggest
the presence of an R-agostic interaction but are insuf-
ficient to determine unambiguously the coordination
mode of the -CH2CH2Tol group. A few complexes in
which an alkyl group (R ) CH3, generally) bridges
middle or late transition metals have been reported and
structurally characterized.15 Of the various modes of
coordination which have been proposed for the µ-alkyl
ligand, only three could be consistent with the semibridg-
ing alkyl ligand in 3 (Chart 1): (a) symmetric pyramidal,
(b) η1 agostic, and (c) η2 agostic. Note that only in (a)
and (c) does the R-C bond lie in the Mo2(µ-C) plane.
An X-ray analysis of a single crystal of 3 (Figure 4
and Table 3) obtained from a diethyl ether solution at
room temperature confirmed that the bridging carbon
is notably asymmetric (Mo(2)-C(4) ) 2.285(7) Å and
Mo(1)-C(4) ) 2.448(8) Å), thereby excluding the sym-
metric pyramidal mode of coordination (a) and confirm-
ing the semibridging nature of the alkyl bridge. Other
distances and angles (Table 2) are consistent with the
proposed formulation [Mo2Cp2(µ-SMe)3(µ-CH2CH2Tol)].
Although the analysis did not allow the positions of the
two hydrogen atoms attached to C(4) to be determined
experimentally, it does establish that the CH2Tol group
is oriented so that the C(4)-C(5) bond is nearly normal
to the Mo2C(4) plane (Mo(2)-Mo(1)-C(4)-C(5) ) 113.8-
(7)°), consistent only with (b) in Chart 1. This implies
of the three species, 2:E:3, is 33:10:57, confirming that
the reduced species is the dominant product.
We assume that the formation of the side product 2
arose from â-deprotonation of 1, as described previously,
while E could form either by competitive R-deprotona-
tion or by the reduction of E+, which is formed as a
byproduct (in low yields) of the protonation of E in the
preparation of 1.13 The transfer of a hydride to the
bridging carbene of 1 gives rise to the novel derivative
3. This complex was isolated as a green powder in only
moderate yields (29%) following chromatographic work-
up, due to partial decomposition on the silica gel column.
3 was formulated as [Mo2Cp2(µ-SMe)3(µ-CH2CH2Tol)] on
the basis of its spectroscopic and analytical data and
solid-state structure. In addition to the expected signals
for a {Mo2Cp2(µ-SMe)3} core and the tolyl group, the 1H
NMR spectrum displayed two signals attributable to two
methylene groups. One of these signals is observed at
2.42 ppm as a pseudo-triplet with a coupling constant
(JHH) of 6.6 Hz, while the second appears as a broad,
unresolved resonance at -5.57 ppm. 13C NMR and
1
HMQC H-13C experiments suggested the presence of
1
a semibridging alkyl ligand. The HMQC H-13C spec-
trum indicated that the two protons at -5.57 ppm were
attached to a shielded carbon atom which resonates at
1.9 ppm. The value of the coupling constant 1JCH (112.2
Hz) of the R-methylenic group in the {Mo2(µ-CRH2Câ-
H2R)} backbone, determined by recording the 13C NMR
1
(14) Martin, M. L.; Martin, C. J. In Manuel de re´sonnance magne´-
tique nucle´aire; Azoulay, Ed.; Paris, 1971.
spectrum of 3 without H decoupling, is low compared
1
with both the JCH value found for the â-methylenic
(15) (a) Shin, J. H.; Parkin, G. Chem. Commun. 1998, 1273. (b) Baik,
M.-H.; Friesner, R. A.; Parkin, G. Polyhedron 2004, 2879 and references
therein. (c) Braunstein, P.; Boag, N. M. Angew. Chem., Int. Ed. 2001,
40, 2427 and references therein. (d) Bursten, B. E.; Cayton, R. H. J.
Am. Chem. Soc. 1986, 5, 1051. (e) Garcia, M. A.; Melon, S.; Ramos, A.;
Riera, V.; Ruiz, M. A.; Belleti, D.; Graiff, C.; Tiripicchio, A. Organo-
metallics 2003, 22, 1983. (f) Dawkins, G. M.; Green, M.; Orpen, A. G.;
Stone, F. G. A. J. Chem. Soc., Chem. Commun. 1982, 41.
group of the {Mo2(µ-CRH2CâH2R)} bridge (129.2 Hz) and
with values generally observed for C(sp3)-H bonds.14,15a
A single resonance was observed for the two cyclopen-
(13) Le Goff, A.; Le Roy, C.; Pe´tillon, F. Y.; Schollhammer, P.;
Talarmin, J. Manuscript in preparation.

Hydrocarbyl Ligands Bonded to {Mo2Cp2(µ-SMe)3}
Organometallics, Vol. 24, No. 25, 2005 6273
Figure 6. Cyclic voltammetry of [Mo2Cp2(µ-SMe)3(µ-η1:
η2-CHCH2Ph)]+ (1; ca. 1 mM) in CH2Cl2-[NBu4][PF6]
(vitreous carbon electrode, v ) 0.2 V s-1).
tion, which makes both the C-H bonds and the Cp
groups equivalent. TS2 lies only 13 kJ mol-1 above the
ground state, and so is fully consistent with the persis-
tence of fluxionality even at low temperatures.
4. Reductive Electrochemistry of [Mo2Cp2(µ-
SMe)3(µ-CHCH2R)](BF4) (1). We have previously re-
ported the electrochemistry of various nitrogenous
ligands bound to the {Mo2Cp2(µ-SMe)3} core and dem-
onstrated that the NdN bond of a µ-η1:η1-phenyldiazene
bridge could be cleaved by controlled-potential reduction
of the phenyldiazene complex in acidic solution.3
The synthesis of a wide variety of compounds bearing
hydrocarbyl ligands resulting from the progressive
reduction of a CtC triple bond (µ-acetylide or µ-alkyne
complexes)6 gives us the opportunity to investigate their
interconversion by electrochemical routes. This section
focuses on one particular step of the overall CtC
reduction process: the electrochemical reduction of a
µ-alkylidene derivative 1 to µ-vinyl (2) and µ-alkyl (3)
complexes. The electrochemistry of µ-acetylide, µ-vi-
nylidene, and µ-alkylidyne complexes will be reported
separately.10,13
Figure 5. Potential energy surface for exchange of agostic
and nonagostic C-H bonds in [Mo2Cp2(µ-SH)3(µ-CH2CH2-
Ph)].
that only one C-H bond interacts with a Mo center. A
survey of the potential energy surface of the model alkyl
complex [Mo2Cp2(µ-SH)3(µ-CH2CH2Ph)] (3′) confirms
that, of the three possibilities shown in Chart 1, only
(b) corresponds to a minimum. The optimized structure
(Figure 5) is fully consistent with the X-ray data and,
most significantly, firmly locates one C-H bond parallel
to the Mo-Mo axis (C-H ) 1.14 Å vs 1.10 Å for its
nonagostic counterpart). A second R-C-H agostic mini-
mum, related to 3′ by a clockwise 120° rotation of the
bridging -CH2CH2Ph group shown in Figure 5, has also
been located only 2 kJ mol-1 higher in energy. The small
energy difference between the two isomers arises from
greater steric repulsions between the phenyl and S-H
groups in the less stable isomer. In both isomers, the
Mo(1)-CR-H angle is 116°, only slightly increased from
the ideal sp3 angle of 109.5°, suggesting that the tilting
of the alkyl group is a consequence of the optimization
of the Mo(1)-CR bonding rather than any intrinsically
strong C-H‚‚‚Mo interaction.
In contrast, structure (c) in Chart 1, formed by an
anticlockwise 60° rotation of the bridging -CH2CH2Ph
group, corresponds to a transition state (TS1) on the
potential energy surface, linking the two R-C-H agostic
minima described above. However, this transition state
cannot account for the observed fluxionality, because it
does not interconvert the two metal centers (and hence
the Cp groups). Moreover, TS1 lies 46 kJ mol-1 above
the minimum, a significant barrier that is inconsistent
with the observation of equivalent Cp groups down to
213 K. We have, however, also located a second transi-
tion state, TS2, corresponding to a clockwise 60° rota-
Although it is clear that the electrochemical conver-
sion of a µ-alkylidene bridge to an alkyl ligand will take
place more favorably in the presence of protons, we will
first present the electrochemistry of the µ-alkylidene
complex 1 in the absence of acid.
The µ-alkylidene complex 1 with R ) Ph or R ) Tol
presents similar cyclic voltammetry in CH2Cl2-[NBu4]-
[PF6]. The detailed electrochemical study was carried
out with the phenyl-substituted derivative. This com-
plex undergoes two one-electron reduction steps, at
red1
red2
E1/2
) -0.95 V and E1/2
) -1.72 V (Figure 6,
Scheme 2), as well as two irreversible oxidations at
Epox1 ) 0.56 V and Epox2 ) 0.81 V.16 Both reduction steps
are essentially reversible electrochemically with ∆Ep )
80 mV at v ) 0.2 V s-1, but measurements of the peak
(16) The parameters ip and Ep are respectively the peak current and
the peak potential of a redox process: E1/2 ) (Epa + Epc)/2; Epa, ipa and
c
Ep , ipc are respectively the potential and the current of the anodic peak
a
and of the cathodic peak of a reversible process. ∆Ep ) Ep - Epc. An
EC process comprises an electron-transfer step (E) followed by a
chemical reaction (C). CV stands for cyclic voltammetry, and v (V s-1
is the scan rate in CV experiments.
)

6274 Organometallics, Vol. 24, No. 25, 2005
Cabon et al.
Scheme 2a
a Cp rings and SMe bridges are omitted.
Table 4. Crystal Data for Complexes 2 and 3
2
3
formula
Mr
C
22H28Mo2S3
C22H30Mo2S3
580.50
582.52
0.20 × 0.18 ×
0.15
cryst size/mm
0.40 × 0.07 ×
0.06
cryst syst
orthorhombic
P212121
7.9181(2)
13.6483(4)
20.6495(6)
monoclinic
P21/n
Space group
a/Å
9.269(5)
8.706(2)
29.196(5)
93.622(4)
2351(1)
4
b/Å
c/Å
â/deg
V/Å3
2231.6(1)
4
Z
Dc/Mg m-3
1.728
1.646
T/K
100
293
Figure 7. Scan rate dependence of the current function
µ/mm-1
1.410
1.339
c
[ip /v1/2 red1
]
for a CH2Cl2-[NBu4][PF6] solution of [Mo2Cp2-
(µ-SMe)3(µ-η1:η2-CHCH2Ph)]+ (1): (a; b) in the absence of
2 equiv of HBF4/Et2O; (b; 2) in the presence of 2 equiv of
HBF4/Et2O.
θ(Mo KR)max/deg
no. of rflns collected
no. of unique data/params
Rint
24.9
28.4
9106
11 234
5005/244
0.149
2211/209
0.047
no. of obsd rflns (I > 2σ(I))
R1 (I > 2σ(I))
R1 (all data)
wR2 (all data)
∆Fmax, ∆Fmin/e Å-3
2125
2765
complex E18 and of [Mo2Cp2(µ-SMe)3(µ-Cl)].2e,19 The
latter probably arose from reaction of either 2 or 3 with
the dichloromethane solvent.
0.0378
0.0398
0.0952
0.63, -0.93
0.070
0.137
0.209
1.75, -0.89
Since the major products formed during the electroly-
sis of 1 were shown above to result either from the
current ratio ((ip /ipc)red < 1, (ip /ipc)red approaches unity
with increasing scan rate) indicate that the electron
transfers are followed by chemical reactions (EC pro-
cess)17 which give products detected by the appearance
of oxidation peaks around -0.7 V on the reverse scan.
The small reduction observed around -0.5 V (Figure 6)
is assigned to the presence of E+ as an inseparable
impurity (about 5-10%) in the sample of 1.13
a
a
deprotonation of 1 or from the addition of H- (or H+
+
2e) to 1, an ECE mechanism involving a “father-son”,
acid-base reaction between 1 and its reduced form can
be envisaged (Scheme 3). This type of reaction, which
may involve a proton-transfer relay,20 results from an
increase in the basicity of the complex as a consequence
of one-electron reduction.
The current function ipred/v1/2 is essentially indepen-
dent of scan rate (Figure 7a) and shows no clear
evidence for an ECE process. However, such a process
could be obscured by the effect on the magnitude of the
reduction current (ipred) both of the chemical step, which
consumes starting material, and of the reduction of 3+,
which causes the transfer of a second electron. The
mechanism in Scheme 3 was therefore investigated by
digital simulations of CV at different scan rates using
DigiElch.21 This showed that the current function was
almost independent of v for several sets of values for
the equilibrium and rate constants of the “father-son”
Controlled-potential reduction of 1 carried out at
273 K at a potential 200 mV negative to the first
reduction process was complete after the passage of ca.
1 F mol-1 of 1. Cyclic voltammetry of the catholyte
shows the formation of one or more products having
ox1
ox2
oxidation processes at E1/2 ) -0.68 V and E1/2
)
0.5 V: i.e., similar to those of the µ-σ,π vinyl (2) and
µ-alkyl complexes (3) (Table 5). It should be noted that
the second oxidation of 2 is reversible, while that of 3
is not. The nature of the products formed upon electro-
chemical reduction of 1 was confirmed by comparison
1
of the H NMR spectrum of the solid isolated from the
catholyte after removal of the supporting electrolyte
(18) The presence of E in the catholyte results from the reduction
of E+, present as an inseparable impurity in the samples of 1.13
(19) (a) Gomes de Lima, M. B.; Guerchais, J. E.; Mercier, R.; Pe´tillon,
F. Y. Organometallics 1986, 5, 1952. (b) Barrie`re, F.; Le Mest, Y.;
Pe´tillon, F. Y.; Poder-Guillou, S.; Schollhammer, P.; Talarmin, J. J.
Chem. Soc., Dalton Trans. 1996, 3967.
1
with H NMR spectra of authentic samples of 2 and 3
with R ) Ph. The 1H NMR spectrum showed the
presence of both 2 and 3 (60% and 35%, respectively),
along with minor amounts (ca. 5%) of the µ-alkylidyne
(20) Alias, Y.; Ibrahim, S. K.; Queiros, M. A.; Fonseca, A.; Talarmin,
J.; Volant, F.; Pickett, C. J. J. Chem. Soc. Dalton Trans. 1997, 4807.
(21) (a) DigiElch can be downloaded from www.DigiElch.de. (b)
Rudolph, M. J. Electroanal. Chem. 2003, 543, 23. (c) Rudolph, M. J.
Electroanal. Chem. 2004, 571, 289. (d) Rudolph, M. J. Comput. Chem.
2005, 26, 619. (e) Rudolph, M. J. Comput. Chem. 2005, 26, 633.
(17) (a) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods:
Fundamentals and Applications; Wiley: New York, 1980; Chapter 11,
p 429. (b) Brown, E. R.; Large, R. F. In Techniques of Chemistry;
Weissberger, A., Ed.; Wiley: New York, 1971; Vol. 1 (Physical Methods
of Chemistry), Part II A, Chapter 6, p 423.

Hydrocarbyl Ligands Bonded to {Mo2Cp2(µ-SMe)3}
Organometallics, Vol. 24, No. 25, 2005 6275
Table 5. Redox Potentials of Complexes 1-3 As Measured by Cyclic Voltammetry in CH2Cl2-[NBu4][PF6]a
complex
E1/2red1/V
-0.95
E1/2red2/V
-1.72
E1/2ox1/V
E1/2ox2/V
[Mo2Cp2(µ-SMe)3(µ-η1:η2-CHCH2Ph)]+ (1)
[Mo2Cp2(µ-SMe)3(µ-η1:η2-CHdCHPh] (2)
[Mo2Cp2(µ-SMe)3(µ-CH2CH2Ph)] (3)
0.56 (irr)
-0.68
-0.68
0.81 (irr)
0.5
0.5 (irr)
a Scan rate 0.2 V s-1; potentials are in V vs Fc.
Scheme 3
Scheme 4
reaction. The occurrence of an ECE-type process there-
fore cannot be ruled out solely on the basis of the scan-
rate dependence of the current function. However, this
process would produce equivalent amounts of 2 and 3,
which is not consistent with the experimental results.
An alternative explanation for the formation of 2 and
3 involves H atom transfers. Previously it was reported
that the electrochemical reduction of a µ2-ethylidyne
cation gave rise to neutral µ2-vinylidene and µ2-eth-
ylidene species via hydrogen atom transfer between two
µ2-ethylidyne radicals.22 In the present case H atom
transfer between two µ-alkylidene radicals would pro-
duce equivalent amounts of 2 and 3, while H atom loss
(22) Aase, T.; Tilset, M.; Parker, V. D. Organometallics 1989, 8,
1558.

6276 Organometallics, Vol. 24, No. 25, 2005
Cabon et al.
Scheme 5
from the reduced form of 1 might contribute to the
formation of 2 in more than 50% yield (Scheme 4). This
possibility is supported by the results of controlled-
potential electrolysis using a solution in which the
concentration of 1 had been halved; under these cir-
tion of 1 in the presence of acid occurs according to an
ECE process (Scheme 5).17
Controlled-potential reduction of 1 in the presence of
1 equiv of HBF4/Et2O (1.7 F mol-1 of 1) leads to the
formation of 3 as the major product (80%), as shown by
1
1
the H NMR spectrum of the solid residue separated
cumstances 2 was shown by H NMR to be formed in
from the supporting electrolyte. The presence of com-
plexes 2, E,18 and [Mo2Cp2(µ-SMe)3(µ-Cl)] (ca. 10%, 5%,
and 5%, respectively) in the residue was also detected.
Electrolyses performed in the presence of 2 equiv of
HBF4/Et2O produced 3 (ca. 70%) and [Mo2Cp2(µ-SMe)3-
(µ-Cl)] (30%) after the transfer of 2.2 F mol-1 of 1. The
increase in the amount of the µ-chloro complex is
tentatively assigned to reactions of 2 and 3 with protons
leading to the release of the hydrocarbyl ligands and
binding of a chlorine atom from the solvent. In agree-
ment with this, controlled-potential electrolysis of 1 in
the presence of 2 equiv of CF3CO2H produces 3 (75%)23
and the known trifluoroacetato-bridged complex [Mo2-
about 80% yield along with [Mo2Cp2(µ-SMe)3(µ-Cl)] (ca
20%). 3 could not be detected by 1H NMR of the residue
under these conditions.
It is difficult to discriminate between the mechanisms
in Schemes 3 and 4 when the electrochemical reduction
of 1 is carried out in the absence of acid; it is possible
that they may both operate. However, an ECE mecha-
nism is expected when the reaction occurs in the
presence of protons. Indeed, upon addition of acid (1
equiv of HBF4/Et2O) to a CH2Cl2-[NBu4][PF6] solution
of 1, CV shows that the current of the first reduction
increases while that of the second decreases, consistent
with the occurrence of an ECE process (Figure 8b).
Cp2(µ-SMe)3(µ-η1:η1-OCOCF3)] (25%),23 characterized by
red
Furthermore, the variations of the current function ip
/
its first oxidation at E1/2
) -0.35 V,24 after the
ox1
v1/2 against v deviate markedly from linearity at slow
passage of ca. 1.9 F mol-1 of 1. The formation of the
latter indicates the lability of the hydrocarbyl ligand in
species formed as intermediates or product (2) of
controlled-potential electrolyses of 1.
scan rates (Figure 7b), again confirming that the reduc-
Conclusion
We have shown that well-defined transformations of
the bridging alkylidene complex [Mo2Cp2(µ-SMe)3(µ-η1:
η2-CHCH2Tol)](BF4) (1) can be carried out by chemical
methods to afford the µ-vinyl and µ-alkyl species 2 and
3. Spectroscopic, X-ray diffraction, and theoretical meth-
ods lead to consistent conclusions regarding the struc-
tures of the new complexes. In particular, theoretical
(23) The yields of the products formed upon controlled-potential
reduction were calculated by comparing the CV peak currents of the
products to the reduction peak current of 1 before addition of acid and
electrolysis, assuming identical diffusion coefficients.
(24) Le He´nanf, M.; Le Roy, C.; Muir, K. W.; Pe´tillon, F. Y.;
Schollhammer, P.; Talarmin, J. Eur. J. Inorg. Chem. 2004, 1687.
Figure 8. Cyclic voltammetry of [Mo2Cp2(µ-SMe)3(µ-η1:
η2-CHCH2Ph)]+ (1; 1 mM) at 0 °C (vitreous carbon elec-
trode, v ) 0.2 V s-1): (a) in the absence of acid; (b) after
addition of 1 equiv of HBF4/Et2O in CH2Cl2-[NBu4][PF6].

Hydrocarbyl Ligands Bonded to {Mo2Cp2(µ-SMe)3}
Organometallics, Vol. 24, No. 25, 2005 6277
calculations support strongly a η1 agostic mode of
coordination in 1, for which a diffraction study was
unobtainable. For the semibridging alkyl complex [Mo2-
Cp2(µ-SMe)3(µ-CH2CH2Tol)] (3) theoretical calculations
confirm the heavy-atom skeleton determined by X-ray
diffraction and predict a η1 agostic coordination for the
bridging group. They also allow the fluxionality of 3 to
be rationalized. Studies of the electrochemical reduction
of the µ-alkylidene derivative 1 in the presence or
absence of protons reveal its relationships with the
µ-vinyl and µ-alkyl complexes. The products of the
electrochemical reduction of the µ-alkylidene derivative
1 depend on whether hydrogen ions are freely avail-
able: in the presence of acid the µ-alkyl complex 3 is
the major product; in the absence of acid the µ-vinyl
species 2 is formed in addition to 3 and the product ratio
2:3 increases as the starting concentration of 1 is
decreased.
presence of 3 equiv of NaBH4 (0.017 g) for 1 h at room
temperature. The solution readily turned color from brown to
green. The solvent was then removed under vacuum, and the
organometallic products were extracted with diethyl ether (2
× 20 mL). After evaporation of the diethyl ether, the residue
was chromatographed on a silica gel column. Elution with a
mixture of CH2Cl2/hexane (1:4) afforded a green solution of 3,
which was evaporated under vacuum. Compound 3 was
obtained as a green powder (0.025 g, 29% yield).
3. 1H NMR (toluene-d8, 25 °C; δ): 6.96 (d, 2H, JHH ) 7.8
Hz, CH3C6H4), 6.85 (d, 2H, JHH ) 7.8 Hz, CH3C6H4), 5.05 (s,
10H, C5H5), 2.42 (pt, 2H, JHH ) 6.6 Hz, Tol-CH2CH2-Mo) 2.15
(s, 3H, CH3C6H4), 1.71 (s, 3H, SCH3), 1.64 (s, 3H, SCH3), 1.17
(s, 3H, SCH3), -5.57 (br, 2H, Tol-CH2CH2-Mo). 13C{1H} NMR
(toluene-d8, 25 °C; δ): [139-125] (CH3C6H4), 90.0 (C5H5), 34.4
(Tol-CH2CH2-Mo), 21.1 (CH3C6H4), 12.3 (SCH3), 11.8 (SCH3),
10.9 (SCH3), 1.9 (Tol-CH2CH2-Mo). Anal. Calcd for C22H30-
Mo2S3, 1/4 CH2Cl2: C, 44.2; H, 5.1. Found C, 44.0; H, 5.2.
3 (R ) Ph).1H NMR (tol-d8, 25 °C; δ): 7.12-6.90 (m, 5H,
-C6H5), 5.04 (s, 10H, C5H5), 2.42 (pt, 2H, Ph-CH2CH2-Mo),
1.71 (s, 3H, SCH3), 1.65 (s, 3H, SCH3), 1.15 (s, 3H, SCH3),
-5.58 (br, 2H, Ph-CH2CH2-Mo).
Experimental Section
Controlled-Potential Electrolyses of 1. (a) In the
Absence of Acid. A 16.5 mg portion (2.5 × 10-5 mol) of
complex 1 was dissolved in 15 mL of CH2Cl2-[NBu4][PF6]
under N2, and the potential of the Pt cathode was set at -1.2
V. The electrolysis was completed after transfer of 2.2 C (0.9
F mol-1 of 1). The catholyte was cannulated in a Schlenk flask
and the solvent removed under reduced pressure. A 10 mL
portion of pentane was then added to the solid residue, and
the mixture was stirred for 10 min before being filtered. The
filtrate was dried under vacuum. The solid residue was
dissolved in C6D6, and the products formed by electrolysis were
General Procedures and Materials. All reactions were
performed under an atmosphere of argon or dinitrogen using
conventional Schlenk techniques. Solvents were deoxygenated
and dried by standard methods. Literature methods were used
for the preparation of [Mo2Cp2(µ-SMe)3(µ-CHCH2R)]BF4 (1).6a
All other reagents were purchased commercially. Yields of all
products are relative to the starting dimolybdenum complexes.
Column chromatography was carried out with silica gel
purchased from SDS. Chemical analyses were performed
either by the Service de Microanalyse ICSN, Gif sur Yvette,
France, or by the Centre de Microanalyses du CNRS, Vernai-
son, France. IR spectra were recorded on a Nicolet-Nexus FT
IR spectrometer from KBr pellets. The NMR spectra (1H, 13C),
in CDCl3 or C6D6 solutions, were recorded with a Bruker AC
300 or AMX 400 spectrometer and were referenced to SiMe4.
1H-13C 2D experiments were carried out on a Bruker DRX
500 spectrometer. The preparation and the purification of the
supporting electrolyte [NBu4][PF6] and the electrochemical
equipment were as described previously.25 All of the potentials
(text, tables, figures) are quoted against the ferrocene-
ferrocenium couple; ferrocene was added as an internal
standard at the end of the experiments.
1
characterized by H NMR.
(b) In the Presence of HBF4/Et2O. In a typical experi-
ment, to 13.9 mg (2.1 × 10-5 mol) of 1 dissolved in 15 mL of
CH2Cl2-[NBu4][PF6] under N2 was added 2 equiv of HBF4/
Et2O. The controlled-potential electrolysis was performed at
-1.2 V. The catholyte was treated as above.
Crystal Structure Determinations of 2 and 3. Pertinent
data are summarized in Table 4. Measurements for 2 were
made at 100 K on a Nonius KappaCCD diffractometer, while
those for 3 were made at room temperature on a Nonius CAD4
diffractometer. Mo KR radiation (λ ) 0.710 73 Å) was used in
both experiments. The structures were solved and refined by
standard procedures.26 H atoms were positioned using stereo-
chemical considerations, the orientations of methyl groups
being initially obtained from difference maps, and then rode
on their parent carbon atoms.
Molecules of 2 are disordered over two sites related ap-
proximately by the pseudosymmetrical mirror operation
1.5 - x, y, z. The main structure with occupancy 0.764(3) is
shown in Figure 2. Cp and phenyl rings were refined as regular
polygons, and anisotropic vibration tensors were refined only
for Mo and S atoms. This model, the best of several that were
considered, was refined satisfactorily with data averaged
assuming mmm Laue symmetry (see Table 4). Although the
overlapping of the molecular images obscures the finer details
of the structure, in particular the position of the H atom
attached to C(4), we consider that the main features of the
heavy-atom skeleton are reliably established. The disorder
seems to be a general feature of the crystals, since data sets
from three different specimens gave essentially the same
results. For 3 the crystals were small and of poor quality.
Refinement using a conventional model (Uij tensors for all
non-H atoms) established the heavy-atom skeleton but did not
yield experimental positions for the C(4) H atoms.
Reaction of 1 with Butyllithium. Complex 1 (0.1 g, 1.5
mmol) and 1.5 equiv of n-butyllithium (V ) 900 µL, 2.5 M
solution in hexane) were stirred in tetrahydrofuran (30 mL)
at 0 °C for 20 min. The solution turned color from brown to
purple. After evaporation of the solvents, 2 was extracted with
diethyl ether (3 × 10 mL). Evaporation of the volatiles and
washing of the residue with cold pentane afforded 2 as a purple
solid (0.08 g, 92% yield).
2. 1H NMR (C6D6, 25 °C; δ): 7.18 (d, 1H, JHH ) 15.0 Hz,
Tol-CHdCH-Mo), 7.04 (d, 2H, JHH ) 7.8 Hz, CH3C6H4), 6.92
(d, 2H, JHH ) 7.8 Hz, CH3C6H4), 6.73 (d, 1H, JHH ) 15.0 Hz,
Tol-CHdCH-Mo), 4.97 (s, 10H, C5H5), 2.06 (s, 3H, CH3C6H4)
1.70 (s, 3H, SCH3), 1.58 (s, 3H, SCH3), 1.54 (s, 3H, SCH3). 13C-
{1H} NMR (C6D6, 25 °C; δ): 153.8 (Tol-CHdCH-Mo), 139.1,
136.6, 129.4, 127.5 (CH3C6H4), 104.8 (Tol-CHdCH-Mo), 91.0
(C5H5), 21.1 (CH3C6H4), 16.3 (SCH3), 10.2 (SCH3), 9.8 (SCH3).
Anal. Calcd for C22H28Mo2S3: C, 45.5; H, 4.9. Found: C, 45.5;
H, 4.9.
1
2 (R ) Ph). H NMR (C6D6, 25 °C; δ): 7.21 (d, 1H, JHH
)
15.0 Hz, Ph-CHdCH-Mo), 7.16-7.06 (m, 5H, C6H5), 6.68 (d,
1H, JHH ) 15.0 Hz, Ph-CHdCH-Mo), 4.97 (s, 10H, C5H5), 1.72
(s, 3H, SCH3), 1.54 (s, 3H, SCH3), 1.49 (s, 3H, SCH3).
Reaction of 1 with NaBH4. A solution of complex 1 (0.1
g, 0.15 mmol) in tetrahydrofuran (30 mL) was stirred in the
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6278 Organometallics, Vol. 24, No. 25, 2005
Cabon et al.
Computational Methods. All calculations were carried out
using the Gaussian 03 program, Revision A.7.27 The B.03LYP
functional was used throughout,28 and the molybdenum and
sulfur atoms were represented by the LANL2 effective core
potential and its associated basis set,29 augmented by f and d
polarization functions, respectively.30 The 6-31G* basis set was
used for all other atoms, with the exception of the hydrogens
on the R-carbon (those potentially involved in agostic interac-
tions), which were described with a 6-311G** basis. Full
optimizations were performed without constraint, and vibra-
tional frequencies were calculated to confirm the nature of the
stationary points in each case.
Acknowledgment. We are grateful to N. Kervarec
and Dr. R. Pichon for recording two-dimensional NMR
spectra on a Bruker DRX 500 (500 MHz) spectrometer.
We thank the CNRS, the EPSRC, and the Universities
of Glasgow, York, and Brest for financial support.
(27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
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V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
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O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
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Supporting Information Available: Tables giving
structures and total energies of all stationary points reported
and CIF files giving crystallographic data for 2 and 3. This
material is available free of charge via the Internet at http://
pubs.acs.org. Crystallographic data for 2 and 3 have also been
deposited with the Cambridge Crystallographic Data Centre
as CCDC 279840 and 279841. Copies can be obtained free of
charge on application to the CCDC, 12 Union Road, Cambridge
CB2 1EZ, U.K. (fax, international + 44(0) 1223/336-033;
e-mail, deposit@ccdc.com.ac.uk).
OM050761G
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