Iminophosphorane-based [P2N2] rhodium complexes: synthesis, reactivity, and application in catalysed transfer hydrogenation of polar bonds

Abstract

[Rh(P₂N₂)X] complexes (2-X, X = Cl, BF₄) featuring a tetradentate iminophosphorane phosphine ligand were synthesised and characterised. X-Ray analysis provides evidence for a square planar geometry without coordination of the chloride anion. These complexes proved to be air-sensitive, and their oxidation to Rh(III) complexes 3-X was observed in air. The controlled reaction of 2-BF₄ with one equivalent of hexachloroethane yielded [Rh(P₂N₂)Cl₂(BF₄)] (3-BF₄). Direct synthesis of 3-Cl can also be achieved by coordination of the [P₂N₂] ligand to [RhCl(THT)₃] (THT = tetrahydrothiophene). The reactivity of Rh(I) complexes 2 was further investigated, and no reaction was observed with silanes, aryl halides, or pinacolborane, although decomposition was observed under 1 atm of H₂ upon prolonged heating. Interestingly, reduction of complex 3-Cl was observed by NMR upon treatment with silanes or sodium isopropoxide. Therefore, complex 3-Cl was used for catalytic transfer hydrogenation of polar bonds. The reduction of aromatic and aliphatic ketones can be carried out using 1% catalyst and 10% sodium isopropoxide, while imines are partially reduced under these conditions.