Synthesis, reactions, and dynamic nuclear magnetic resonance studies of η2-vinyl complexes of molybdenum and tungsten, [M{η3-C(CF3)C(CF3)(L–L)}(CF3CCCF3)(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.