Reactions of 2,4-hexadiyne-1,6-diol with [H2Os3(CO)9(PR3)] clusters. Cyclization of the diyne and reversible exchange of the phosphine ligands between different positions of the “Os3C3” framework

Abstract

Reactions between unsaturated [H₂Os₃(CO)₉(PR₃)] clusters (PR₃ = PPh₃, P(4-CF₃–C₆H₄)₃, PEt₃) and 2,4-hexadiyne-1,6-diol have been studied. It was found that the diyne ligand easily reacts with all these complexes to give [HOs₃(CO)₈(PR₃){μ₃,η¹:η³:η¹-(CH₃–C–CCH–CHC–O)}] complexes (V, VI and VII, respectively) containing the “Os₃C₃” pentagonal pyramid cluster framework. This structural pattern is formed through the diyne cyclization, dissociation of a CO ligand and eventual coordination of the cyclized organic moiety to the osmium triangle in the μ₃,η¹:η³:η¹ manner. In the case of the PEt₃ substituted cluster the second hydride transfer onto the organic fragment occurs to afford the nonhydride [Os₃(CO)₈(PR₃){μ₃,η¹:η²:η¹-(CH₃–CH–CCH–CHC–O)}] cluster, VIII, containing distorted pentagonal pyramid framework with a broken Os–C bond. Heating of V, VI and VII in hexane solutions results in formation of the regioisomers (Va, VIa and VIIa) with the phosphine ligand located at adjacent osmium atoms across the Os–Os bond bridged by the coordinated organic fragment. The most probable mechanism of the isomerization includes reversible phosphine migration between these metal centres. Solid-state structure of V, Va, VI, VIIa and VIII have been established by single crystal X-ray diffraction. A general mechanistic scheme for the diyne ligand cyclization and cluster framework transformations is suggested and discussed.