Dialkyl(butadiene)cyclopentadienylmolybdenum(III) Complexes. Synthesis, Characterization, and Reactivity

Treatment of CpMo(η4-diene)Cl2 (diene = 1,3-butadiene, C4H6, 1‘; isoprene, C5H8, 1‘ ‘; 2,3-dimethyl-1,3-butadiene, C6H10, 1‘ ‘) in diethyl ether at low temperature with 2 equiv of
alkylmagnesium RMgX reagents affords the corresponding dialkyl complexes CpMo(η4-1,3-diene)R2 (2, 2‘, 2‘ ‘, R = CH3, a; CH2Ph, b; CH2SiMe3, c). These species are isolable in moderate
yields and have been fully characterized by EPR, elemental analyses, and cyclic voltammetry.
They all show a reversible reduction process at relatively low potentials and an irreversible
oxidation. The structure of 2‘ ‘a was confirmed by single-crystal X-ray diffraction. The mixed
complex CpMo(η4-C4H6)Cl(CH3), 3, has also been obtained by selective monomethylation of
1. A slow ligand redistribution process occurs between equivalent amounts of 1 and 2a to
afford 3 quantitatively. Compound 3 slowly decomposes by elimination of a CH3 group,
yielding compound [CpMo(η4-C4H6)Cl]2, 4, which has been structurally characterized by X-ray
crystallography. Arylation with PhMgBr or (C6H2Me3-2,4,6)MgBr provides EPR evidence
for formation of arylated Mo(III) products. However, the isolation and crystallographic
characterization of [CpMo(η4-C4H4Me2-2,3)Br0.77Cl0.23]2, 4‘ ‘, for the mesityl reaction indicates
that halide exchange and electron transfer processes also take place competitively. Complex
2a does not react with Lewis bases such as phosphines or carbon monoxide nor with weak
Brønsted acids (MeOH, H2O, CH3COOH, H3PO4, H3O+). Protonolysis of the Mo−R bond could
only be observed by interaction with HCl, HBF4, and CF3COOD, with formation of CH4 or
CH3D. Compounds CpMo(η4-C6H10)(CH3)(OCOCF3), 5, and CpMo(η4-C6H10)(OCOCF3)2, 6,
have been isolated from the reaction between 2‘ ‘a and 1 or 2 equiv of CF3COOD, respectively.
Compound 6 has been structurally characterized by X-ray crystallography.