Cluster core geometrical variation in heterometallic boride clusters containing RhRu4 skeletons: crystal structures of [RhRu4H2(η5-C5Me5)(µ-Cl)(CO)12B] and [RhRu4H(nbd)(CO)12B(AuPPh3)](nbd = norbornadiene)

Abstract

The reaction of the butterfly anion [Ru₄H(CO)₁₂BH]^– with [{Rh(η⁵-C₅Me₅)Cl₂}₂] gave the novel boron-containing cluster [RhRu₄H₂(η⁵-C₅Me₅)(µ-Cl)(CO)₁₂B] which has been characterised spectroscopically and by X-ray crystallography. Consistent with the 78 valence-electron (v.e.) count, the five metal atoms adopt an open skeletal structure which can be described as an ‘envelope’(or an edge-bridged square) geometry. In contrast, when [Ru₄H(CO)₁₂BH]^– reacted with [{Rh(nbd)Cl}₂](nbd = norbornadiene) and [Au(PR₃)Cl](R = Ph, C₆H₁₁ or 2-MeC₆H₄), boride clusters containing both RhRu₄B and Rh₂Ru₄B skeletons were obtained. The presence of the gold(I) phosphine is necessary to trap the RhRu₄B-containing product. The Rh₂Ru₄B clusters possess octahedral frameworks and are related to the previously reported [Rh₂Ru₄(CO)₁₆B(AuPR₃)] compounds. However, the incorporation of only one rhodium fragment leads to the clusters [RhRu₄H(nbd)(CO)₁₂B(AuPR₃)], and a square-based pyramidal RhRu₄ framework (consistent with a 74 v.e. count) has been confirmed by the results of a single-crystal X-ray diffraction study of [RhRu₄H(nbd)(CO)₁₂B(AuPPh₃)]. Variable-temperature ¹H NMR spectroscopic data for the clusters [RhRu₄H(nbd)(CO)₁₂B(AuPR₃)](R = Ph, C₆H₁₁ or 2-MeC₆H₄) indicate fluxional behaviour involving the norbornadiene ligand and the gold(I) phosphine unit. The reaction of [RhRu₄H(nbd)(CO)₁₂B(AuPR₃)](R = Ph or C₆H₁₁) with CO gave [RhRu₄H(CO)₁₄B(AuPR₃)].