Co-ordinatively unsaturated alkyne complexes of tungsten: synthesis and dynamic nuclear magnetic resonance studies of some but-2-yne derivatives

Abstract

Reactions of [{W(µ-Br)Br(CO)(MeCCMe)₂}₂](1) with one molar equivalent of Na(S₂CNMe₂), Na(S₂PMe₂), or Tl(acac)(acac = acetylacetonate) give mononuclear derivatives [WBr(L–L)(CO)(MeCCMe)₂](2)(L–L = S₂CNMe₂, S₂PMe₂, or acac). The ¹H and ¹³C n.m.r. spectra of the acac complex are temperature independent whereas dynamic ¹H n.m.r. studies of the S₂CNMe₂ and S₂PMe₂ derivatives have been interpreted in terms of a molecular rearrangement involving dechelation of the bidentate ligand which exchanges the alkyne environments and the ends of the dithiolate ligand. The reaction of (1) with two molar equivalents of Na(S₂PMe₂) gives the bis-alkyne complex [W(S₂PMe₂)₂(MeCCMe)₂](3) whereas Na[S₂P(OMe)₂] and TI(acac) give mono-alkyne derivatives [W(L–L)₂(CO)(MeCCMe)](4)[L–L = S₂P(OMe)₂ or acac]. ¹H N.m.r. studies of (3) suggest fluxional behaviour involving dechelation of S₂PMe₂ which exchanges both the S₂PMe₂ methyls and the alkyne methyls without necessarily requiring alkyne propeller rotation. In contrast, fluxional behaviour in (4)[L–L = S₂P(OMe)₂] clearly involves alkyne propeller rotation in addition to two other processes, one of which exchanges the methyl groups of one S₂P(OMe)₂ group. However, the only dynamic process observed with (4)(L–L = acac) appears to involve alkyne propeller rotation.