Reactions of co-ordinated ligands. Part 31. The synthesis, structure, and protonation of the octamethyloctatrienediylidenedimolybdenum complex [Mo2(µ-C8Me8)(η-C5H5)2], evidence for three-centre carbon–hydrogen–molybdenum interactions

Abstract

Reaction of [Mo(NCMe)(MeC2Me)2(η-C5H5)][BF4] with Na[Fe(CO)2(η5-C5H5)] in tetrahydrofuran affords [Fe2(CO)4(η-C5H5)2] and the octamethyloctatrienediylidenedimolybdenum complex [Mo2(µ-C8Me8)(η-C5H5)2](1) identified by X-ray diffraction studies. Complex (1) crystallizes in the triclinic space group P, with a= 8.449(1), b= 10.114(2), c= 14.872(3)A, α= 86.66(2), β= 80.88(1), γ= 63.78(1), and Z= 2. The structure was refined to R= 0.020 and R′= 0.025. The molecular structure of complex (1) shows non-crystallographic Cs symmetry, and there is a formal MoMo double bond [MoMo 2.595(1)A]. The C8 carbon chain binds to the dimolybdenum unit in an σ, η3, η2, η3, σ fashion such that it forms σ bonds to one molybdenum atom of mean length 2.092(2)A, each molybdenum atom carrying an η5-cyclopentadienyl group. Extended-Huckel molecular orbital calculations on complex (1) indicate an accumulation of electron density on the carbon atoms of the C8 chain, and in agreement with this finding protonation with HBF4·OEt2 or CF3CO2H affords purple crystals of [Mo2(µ-C8Me8)(µMo,c-H)(η-C5H5)2]BF4(2) and [Mo2(µ-C8Me8)(µMo,c-H)(η-C5H5)2][(CF3CO2)2H](3). The 1H n.m.r. spectra of these cations showed a high-field signal due to the proton derived from the acid indicating the presence of a CHMo system. This was confirmed by an X-ray crystallographic study on complex (3), which crystallizes in the monoclinic space group P21/c, with a= 10.648(2), b= 15.119(4), c= 19.655(6)A, β= 105.30(2)°, and Z= 4; the structure was refined to R= 0.029 and R′= 0.034. The crystal structure of complex (3) contains isolated [Mo2(µ-C8Me8)(µMo,c-H)(η-C5H5)2]+ cations and [(CF3CO2)2H]– anions. The cation non-hydrogen atom geometry is closely similar to that of complex (1), with the exception that one of the terminal Mo–C σ bonds is markedly extended [to 2.196(5)A]. This bond is bridged by a hydrogen atom which shows Mo–H and C–H distances of 1.88(8) and 0.89(8)A, and an Mo–H–C angle 99(5)°. The complex anion contains two CF3CO2 units linked by a strong, short hydrogen bond [O ⋯ O 2.429(8), O(311)–H(42) 1.35(7), and O(412)–H(42) 1.08(7)A]. Examination of the variable-temperature (192–295 K)13C n.m.r. spectra of the cation [Mo2(µ-C8Me8)(µMo,c-H)(η-C5H5)2]+ suggests that in solution the bridging proton undergoes site exchange such that it spends some time attached to the terminal carbon atom (Cα) of the C8 chain, and the remainder attached to one of the olefinic carbon atoms (Cδ), in both cases bridging from these to the molybdenum atom.