Mono- and binuclear complexes of rhodium involving a new series of hemilabile o-phosphinoaniline ligands

Abstract

Monophosphines of the type Ph_xPAr_3−x (x = 0, 1 or 2, Ar = o-N-methylanilinyl) and the diphosphine, Ar₂PCH₂PAr₂ (mapm) have been synthesized for use as chelating and/or bridging P,N-ligands within mono- and binuclear rhodium(I) complexes, respectively. The previously prepared phosphines, Ph_xPAr′_3-x (x = 0, 1 or 2, Ar′ = o-N,N-dimethylanilinyl) and Ar′₂PCH₂PAr′₂ (dmapm), have also been used to prepare analogous mono- and binuclear complexes. Variable temperature ¹H NMR spectroscopy of the mononuclear complexes, [RhCl(CO)(L)] (L = PhPAr₂, PhPAr′₂, PAr₃ and PAr′₃), and line-shape analyses of the resultant spectra indicate the substantially increased lability of the N,N-dimethylanilinyl donors relative to the related monomethylanilinyl groups. X-Ray structural analyses of the mononuclear complexes suggest that the enhanced Type II hemilability in the dimethylanilinyl complexes compared to their monomethyl analogues results from increased steric interactions involving the coordinated dimethylanilinyl substituents. In the case of the binuclear, dmapm-bridged compound [Rh₂Cl₂(CO)₂(μ-dmapm)], there are additional transannular repulsions between the chloro ligand on one metal and the coordinated dimethylanilinyl group on the other, which result in a Rh–Rh separation of over 4.1 Å. For the analogous mapm-bridged species, the transannular interactions between the chloro ligands and the amine hydrogens are in fact attractive, resulting in a much closer Rh–Rh separation (3.450 Å). The chloride substituents of [Rh₂Cl₂(CO)₂(μ-mapm)] can be replaced to generate the complexes, [Rh₂(X)₂(CO)₂(μ-mapm)] (X = I, CF₃SO₃, CH₃CO₂), the last of which also exhibits pronounced transannular hydrogen-bonding interactions in the solid state.