Synthesis of bis[µ-bis(diphenylphosphino)methane]-tri-µ-carbonyl-tricarbonyl-triangulo-trirhodium(1+) perchlorate, [Rh3(CO)3(µ-CO)3(µ-dppm)2]ClO4. An unusual 46-electron cluster possessing the A-frame structure with a bridging Rh(CO)3 fragment
Abstract
The A-frame complexes [Rh2(µ-OR)(CO)2(µ-dppm)2]ClO4(R = H, Me, or Et; dppm = Ph2PCH2PPh2) react under carbon monoxide to give the 46-electron, trinuclear cluster [Rh3(CO)3(µ-CO)3(µ-dppm)2]ClO4 whose structure resembles that of an A-frame complex with a bridging Rh(CO)3group. The unique terminal carbonyl on the bridging rhodium atom undergoes rapid exchange with 13CO and is readily substituted by tertiary phosphines PR3(R3= MePh2, Et3, or Ph3; PR3= dppm). Unlike the parent cluster, the phosphine-substituted complexes [Rh3(CO)2(µ-CO)3(Pr3)(µ-dppm)2]ClO4 undergo rapid exchange of the 1,3 and 2,3 bridging carbonyl groups only. In addition, [Rh3(CO)2(µ-CO)3(dppm)(µ-dppm)2]ClO4 is fluxional, showing relatively rapid flipping between one donor atom and the other in the diphosphine. All complexes have been characterized on the basis of i.r., 1H, 13C, and 31P n.m.r. spectroscopy. The structure of [Rh3(CO)3(µ-CO)3(µ-dppm)2]ClO4 is discussed and a rationale proposed for the stability of the 46-electron cluster by comparison to the expected 48-electron species [Rh3(CO)7(µ-dppm)2]+.
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