Coordinating Ability of the Iminophosphorane Group in ortho-Carborane Derivatives

Abstract

The coordinating ability of the nitrogen donor atom of C-carboranyl iminophosphoranes was studied in different ligand systems. The analysis of organotin derivatives of the unsubstituted iminophosphorane I1 (SnMe3I1 and SnClMe2I1) and the comparison with the non-carboranyl analog (SnClMe2I3), reveals the reduced donor capacity of the nitrogen atom due to the closo-carborane group. To promote the coordination of this weak donor atom, new phosphine–iminophosphorane ligands (IP1 and IP2), with an extra –PPh2 group on the other cage carbon atom, were synthesized and structurally characterized. The analysis of the products formed by the reaction of these ligands with the metal precursor cis-[PdCl2(PhCN)2] reveals that the phosphine group promotes the coordination of the iminophosphorane nitrogen atom, yielding (P, N) chelates. However, this coordination mode activates the closo-carborane ligands towards deboronation, and the complexes evolve to the nido derivatives in response to the polar solvents. The activation is increased by the presence of extra donor groups, like in the tridentate ligand IP2, with an extra thioether group. The deboronation by coordination of (P, N) bidentate carboranyl ligands has never been reported, although it has been studied for other ligands, especially for chelating carboranyl diphosphines.