Molecules, Vol. 25, Pages 519: Exopolyhedral Ligand Orientation Controls Diastereoisomer in Mixed-Metal Bis(carboranes) (Molecules)

Heterobimetallic derivatives of a bis(carborane), [μ7,8-(1′,3′−3′-Cl-3′-PPh3-closo-3′,1′,2′-RhC2B9H10)-2-(p-cymene)-closo-2,1,8-RuC2B9H10] (1) and [μ7,8-(1′,3′−3′-Cl-3′-PPh3-closo-3′,1′,2′-RhC2B9H10)-2-Cp-closo-2,1,8-CoC2B9H10] (2) have been synthesised and characterised, including crystallographic studies. A minor co-product during the synthesis of compound 2 is the new species [8-{8′-2′-H-2′,2′-(PPh3)2-closo-2′,1′,8′-RhC2B9H10}-2-Cp-closo-2,1,8-CoC2B9H10] (3), isolated as a mixture of diastereoisomers. Although, in principle, compounds 1 and 2 could also exist as two diastereoisomers, only one (the same in both cases) is formed. It is suggested that the preferred exopolyhedral ligand orientation in the rhodacarboranes in the non-observed diastereoisomers would lead to unacceptable steric crowding between the PPh3 ligand and either the p-cymene (compound 1) or Cp (compound 2) ligand of the ruthenacarborane or cobaltacarborane, respectively.