Cyclopentadienylmolybdenum(II) and -(III) Complexes Containing Diene and Allyl Ligands. 1. Isomeric Preferences and Isomerization Rates in a Pair of Redox-Related Organometallic Complexes

Treatment of
CpMoCl2(η-C4H6)
(1) with 1 equiv of allylmagnesium bromide yields the
dinuclear complexes
[CpMo(η-C4H6)(μ-Br)]2
(2, major) and
Cp2Mo2(η-C4H6)2(μ-Br)(μ-Cl)
(3, minor). A solid solution of compounds
2
and 3 adopts an anti geometry in the solid state,
as shown by X-ray crystallography, whereas both anti and
syn
isomers are observed in benzene solution by 1H-NMR
spectroscopy. The reaction of 1 with 2 equiv of
allylmagnesium
bromide yields
[CpMo(η-C3H5)(η-C4H6)]
as an equilibrium mixture of a major (98%, 4a) and a minor
(2%, 4b)
isomer. NOE−NMR studies indicate the
CpMo(prone-C3H5)(supine-C4H6)
orientation for the major isomer 4a,
which is also found in the solid state by X-ray crystallography.
The orientation of 4b is suggested by the
1H-NMR
chemical shifts as
CpMo(supine-C3H5)(supine-C4H6).
Oxidation of 4a/b by ferrocenium hexafluorophosphate
in
dichloromethane gives the 17-electron compounds
[CpMo(η-C3H5)(η-C4H6)][PF6]
(5a/b). The green compound 5a
converts into the more stable red-violet 5b with an
estimated half-life of <20 s in THF. It can be observed,
however,
at low temperature by EPR spectroscopy. The
[CpMo(supine-η-C3H5)(supine-η-C4H6)][PF6]
configuration for 5b
has been confirmed by X-ray diffraction methods. Upon reduction
with cobaltocene, 5b is converted selectively
to
4b, followed by slow equilibration
(t1/2 = 6.5 h) with 4a.
Refluxing or photolyzing a solution of 4a/b in
benzene
generates a third isomer, 4c, which adopts a
CpMo(supine-C3H5)(s-trans-C4H6)
configuration as confirmed by an
X-ray analysis. The distribution of 4a and
4c at equilibrium is approximately 1:1 by starting either
from 4a/b or
from pure 4c and independent of the equilibration method
(thermal/photochemical). Oxidation of 4c generates
the
corresponding 1-electron oxidation product 5c, which rapidly
isomerizes to 5b. It can be observed, however,
by
EPR in THF together with a fourth isomer, 5d (ca. 1:1),
believed to differ from 5c only in the orientation of the
allyl
ligand. Equilibrium, rate, and electrochemical data allow most of
the thermodynamic and kinetic parameters related
to the transformation of the different compounds to be sorted out.
The faster s-trans- to
s-cis-butadiene isomerization
for 5 relative to 4 indicates the easier
accessibility of the unsaturated 15-electron vs 16-electron
intermediate. Possible
reasons for this trend are analyzed.