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

Reaction of [{W(μ-Br)Br(CO)(MeC≡CMe)2}2] (I) with one molar equiv of Na(S2CNMe2), Na(S2PMe2), or Tl(acac) (acac = acetylacetonate) gives mononuclear derivs. [WBr(L2)(CO)(MeC≡CMe)2] (L2 = S2CNMe2, S2PMe2, acac). The 1H and 13C NMR spectra of the acac complex are temp. independent whereas dynamic 1H NMR studies of the S2CNMe2 and S2PMe2 derivs. have been interpreted in terms of a mol. rearrangement involving dechelation of the bidentate ligand which exchanges the alkyne environments and the ends of the dithiolate ligand. The reaction of I with two molar equiv of Na(S2PMe2) gives the bis-alkyne complex [W(S2PMe2)2(MeC≡CMe)2] (II) whereas Na[S2P(OMe)2] and Tl(acac) give mono-alkyne derivs. [W(L2)2(CO)(MeC≡CMe)] [III; L2 = S2P(OMe)2 or acac]. 1H NMR studies of II suggest fluxional behavior involving dechelation of S2PMe2 which exchanges both the S2PMe2 methyls and the alkyne methyls without necessarily requiring alkyne propeller rotation. In contrast, fluxional behavior in III [L2 = S2P(OMe)2] clearly involves alkyne propeller rotation in addn. to two other processes, one of which exchanges the Me groups of one S2P(OMe)2 group. However, the only dynamic process obsd. with III (L2 = acac) appears to involve alkyne propeller rotation.