Mechanisms of alkyne trimerisation at molybdenum and tungsten centres leading to novel metallacycles: crystal and molecular structures of two isomeric forms of [Mo{C(CF3)C(CF3)C(CF3)C(CF3)C(CO2Me)C(CO2Me)(SPri)}(η5-C5H5)]

Abstract

The η²-C,C vinyl complexes [M{η³-C(CF₃)C(CF₃)SR}(CF₃CCCF₃)(η⁵-C₅H₅)]1(M = Mo, R = Prⁱ; M = W, R = Me, Et or Prⁱ) reacted with dimethyl acetylenedicarboxylate R′CCR′(R′= CO₂Me) to give products of alkyne trimerisation containing two different oligomerisation sequences depending on whether R′CCR′ occupies a terminal or central position in the alkyne trimer chain, i.e. C(CF₃)C(CF₃)C(CF₃)C(CF₃)C(R′)C(R′) and C(CF₃)C(CF₃)C(R′)C(R′)C(CF₃)C(CF₃). With M = Mo, kinetic ¹⁹F NMR studies established a reaction sequence proceeding above ca.–30 °C via two different butadienyl complexes [M{η²-C(CF₃)C(CF₃)C(R′)C(R′)SPrⁱ}(CF₃CCCF₃)(η⁵-C₅H₅)]2 and 3 to give isomeric trienyl derivatives [Mo{η⁶-C(CF₃)C(CF₃)C(CF₃)C(CF₃)C(R′)C(R′)SPrⁱ}(η⁵-C₅H₅)]4 and [Mo{η⁵-C(CF₃)C(CF₃)C(CF₃)C(CF₃)C(R′)SPrⁱC(R′)}(η⁵-C₅H₅)]5 which were isolated at room temperature. X-Ray diffraction studies established that in each case the trienyl ligand is bound to the metal via an η²-C,C alkenyl linkage, and two σ carbon–metal bonds. In 4 this is accompanied by co-ordination of a CO unit of a carbomethoxy carbonyl whereas in 5 the thiolate sulfur is bonded to the metal. Above room temperature 5 isomerises to give species 6 and ultimately 7 which appears to result from thiolate transfer to the metal to give a seven-membered metallacycle. With M = W, the reactions also proceed via a butadienyl complex 2 but this isomerises via two other intermediates to give trienyl products [W{η⁶-C(CF₃)C(CF₃)C(R′)C(R′)C(CF₃)C(CF₃)SR}(η⁵-C₅H₅)] and [WF{η⁵-C(CF₂)C(CF₃)C(R′)C(R′)C(CF₃)C(CF₃)SR}(η⁵-C₅H₅)]. X-Ray diffraction studies established that the trienyl ligand in the former is co-ordinated via one σ, two π alkene linkages and a thiolate sulfur. Two isomeric forms were structurally characterised which differ only in the orientation of the thiolate isopropyl substituent which can adopt syn or anti positions. In the fluoride a CF₃ fluorine has been transferred to the metal and this has generated a hexatrienyl attachment via a quasi π-allylic interaction and through W–C and W–S σ bonds.