Reactions of co-ordinated ligands. Part 29. The formation of a 1,3-diene at a molybdenum centre by linking of but-2-yne with an alkene; X-ray crystal structures of [Mo(η2-o-Ph2PC6H4CHCH2)(η2-MeC2Me)(η-C5H5)][BF4] and [Mo(NCMe)(η4-o-MeCHCMe–CHCHC6H4PPh2)(η-C5H5)][BF4]

Abstract

Reaction of [Mo(CO)(η²-RC₂R′)₂(η-C₅H₅)][BF₄](R = R′= Me; R = H, R′= Bu^t) with o-diphenylphosphinostyrene (dpps) in CH₂Cl₂ affords the alkyne–alkene complex [Mo(dpps)(η²-RC₂R′)(η-C₅H₅)][BF₄]. The structure of [Mo(η²-dpps)(η²-MeC₂Me)(η-C₅H₅)][BF₄] was established by X-ray crystallography. Crystals are orthorhombic, space group Pna2₁ with Z= 4 in a unit cell of dimensions a= 22.324(5), b= 9.249(3), c= 14.566(3)Å. The structure has been refined to R= 0.040 (R′= 0.041) for 2 228 reflections at 293 K. The molybdenum atom is bonded to an η-C₅H₅, an η²-bonded but-2-yne, and chelating o-diphenylphosphinostyrene ligands, where the C–C vectors of the co-ordinated alkyne and alkene lie essentially parallel to the Mo–P axis. In refluxing acetonitrile the but-2-yne cation is transformed via C–C bond formation and a 1,3-H shift into a 1,3-diene complex [Mo(NCMe)(η⁴-MeCHCMe–CHCHC₆H₄PPh₂-o)(η-C₅H₅)][BF₄] identified by X-ray crystallography. Crystals are monoclinic, space group P2₁/c with Z= 4 in a unit cell of dimensions a= 10.836(2), b= 8.551 (2), c= 30.896(6)Å, and β= 94.80(2)°. The structure has been refined to R= 0.029 (R′= 0.030) for 3 828 reflections at 293 K. The cation contains a molybdenum atom bound to an η-C₅H₅ ligand, an N-bonded acetonitrile, and a ligand formally derived from but-2-yne and o-diphenylphosphinostyrene, which is η⁴-bonded to the Mo via a 1,3-diene moiety. The co-ordinated acetonitrile is displaced by Bu^tCN, CO, and P(OMe)₃ to form the cations [Mo(L)(η⁴-MeCHCMe–CHCHC₆H₄PPh₂-o)(η-C₅H₅)][BF₄][L = Bu^tCN, CO, or P(OMe)₃]. The nitrile-substituted cations exist in solution as a mixture of two conformational isomers, whereas the CO- and P(OMe)₃-substituted cations are present in only one isomeric form. The conformations of these species are discussed in terms of frontier molecular orbitals, and the nature of the 1,3-H shift process has been examined using deuterium-labelled o-diphenylphosphinostyrene.