Alkyne coupling reactions mediated by tris(pyrazolyl)borate ruthenium vinylidene complexes: X-ray crystal structures of [TpRu=C=CHPh(PEt3)(2)][BPh4] and [TpRu=C=C(COOMe)CH=CHCOOMe(PEt3)(2)][BPh4]

A series of cationic vinylidene complexes of the type [TpRu=C=CHR(PR3)(2)][ BPh4] (Tp = hydrotris(pyrazolyl)borate; PR3 = PEt3, (PMePr2)-Pr-1; R = Ph, Bu-t, COOMe) have been prepared. The reaction of [TpRu=C=CHCOOMe(PEt3)(2)][ BPh4] with a further HC=CCOOMe molecule yields the E-stereoisomer of the vi nylvinylidene derivative [TpRu=C=C(COOMe)CH= CHCOOMe(PEt3)(2)][BPh4], which has been structurally characterized. This C-C coupling reaction has been i nterpreted in terms of a [2 + 2] cycloaddition bf the alkyne to the C-alpha -C-beta bond of the vinylidene ligand to yield a cyclobutenylidene intermed iate, followed by a concerted ring opening. The neutral sigma-eta(1)-butadi enyl complex [TpRu(CH=C(COOMe)CH= CHCOOMe(PEt3)(2)] was prepared by reactio n of the vinylvinylidene complex with NaBH4 in MeOH. The reaction of [TpRuC l((PMePr2)-Pr-i)(2)] with l-alkynes leads to neutral vinylidene derivatives [TpRu=C=CHR(Cl)((PMePr2)-Pr-i)] (R = Ph, But, SiMe3, H). These complexes r eact with LDA and diphenylacetylene yielding the enynyl species [TpRu(PhC=C (Ph)C=CR)-((PMePr2)-Pr-i)] (R = Ph, But). These complexes, which were isola ted and characterized, are efficient catalysts for alkyne dimerization reac tions, as well as for cross-coupling reaction of terminal alkynes.