Synthesis, reactions, and dynamic nuclear magnetic resonance studies of η2-vinyl complexes of molybdenum and tungsten, [M{η3-C(CF3)C(CF3)(L–L)}(CF3CCCF3)(η5-C5H5)](L–L = MeCO2–, Me2PS2–, Et2NCS2–, or Me2NCS2–)

Abstract

Reactions of bis(hexafluorobut-2-yne) complexes [MCl(CF₃CCCF₃)₂(η⁵-C₅H₅)](M = Mo or W) with anions L–L = MeCO₂^–, Me₂PS₂^–, and R₂NCS₂^–(R = Me or Et) give adducts [M(L–L)(CF₃C₂CF₃)₂(η⁵-C₅H₅)] which exhibit a bis(alkyne) structure containing a monodentate ligand L–L and/or an isomeric η²-vinyl structure in which the ligand L–L forms a bridge between the metal and the η²-vinyl ligand. In the cases M = Mo or W, L–L = R₂NCS₂(R = Me or Et) the η²-vinyl structure is stereochemically rigid whereas with M = Mo, L–L = MeCO₂^– or Me₂PS₂^–; M = W, L–L = Me₂PS₂^–, ¹⁹F n.m.r. studies reveal fluxional behaviour attributed to reversible cleavage of the (L–L)–η²-vinyl linkage which regenerates the bis(alkyne) structure. Thermolysis of η²-vinyl complexes [Mo{η³-C(CF₃)C(CF₃)(L–L)}(CF₃CCCF₃)(η⁵-C₅H₅)] gives tetrakis(trifluoromethyl)cyclobutadiene derivatives [Mo(L–L){η⁴-C₄(CF₃)₄}(η⁵-C₅H₅)](L–L = S₂CNR₂) in which the cyclobutadiene ring exhibits high barriers to rotation about the metal–ligand axis. Complexes of this type can also be obtained from the metathetical reactions of [MoI(CO){η⁴-C₄(CF₃)₄}(η⁵-C₅H₅)] with metal salts of (L–L)^–(S₂CNMe₂ or pyridine-2-thiolate). Thermolysis of [Mo{η³-C(CF₃)C(CF₃)S₂CNR₂}(CF₃CCCF₃)(η⁵-C₅H₅)] also gives small quantities of the η²-vinyl complex [Mo{η³-C(CF₃)C(CF₃)C(CF₃)C(CF₃)S}(η²-R₂NCS)(η⁵-C₅H₅)] resulting from condensation of the two fluorocarbon units and fragmentation of the R₂NCS₂ moiety. This type of complex is the major product when the η²-vinyl complex [Mo{η³-C(CF₃)C(CF₃)S₂CNR₂}(CF₃CCCF₃)(η⁵-C₅H₅)] is photolysed in solution.