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)Å, 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 C_s symmetry, and there is a formal MoMo double bond [MoMo 2.595(1)Å]. 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)Å, each molybdenum atom carrying an η⁵-cyclopentadienyl group. Extended-Hückel 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)(µ_Mo,c-H)(η-C₅H₅)₂]BF₄(2) and [Mo₂(µ-C₈Me₈)(µ_Mo,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)Å, 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)(µ_Mo,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)Å]. This bond is bridged by a hydrogen atom which shows Mo–H and C–H distances of 1.88(8) and 0.89(8)Å, 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)Å]. Examination of the variable-temperature (192–295 K)¹³C n.m.r. spectra of the cation [Mo₂(µ-C₈Me₈)(µ_Mo,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 (C_α) of the C₈ chain, and the remainder attached to one of the olefinic carbon atoms (C_δ), in both cases bridging from these to the molybdenum atom.