Preparation of a rhodium(iii) cis-diaquacomplex by protic acid induced oxalate-release from mer-[Rh(C2O4)Cl(py)3]

Abstract

The neutral rhodium(III) mer-[Rh(C₂O₄)Cl(py)₃] (1) complex was found to release oxalate in concentrated aqueous solutions of strong acids (HNO₃, HClO₄, and CF₃SO₃H), generating the heteroleptic cis-diaquacomplex cis,mer-[RhCl(H₂O)₂(py)₃]²⁺ (3). The process proceeded via formation of intermediate complex 2 ([Rh(HC₂O₄)Cl(H₂O)(py)₃]⁺) with the monodentately coordinated oxalato ligand. The structure of the intermediate complex 2 was probed using density functional theory (DFT) calculations. Experimental ¹³C nuclear magnetic resonance (NMR) data on the kinetics of oxalate release at 25 °C were evaluated within a consecutive reaction model with two pseudo-first order stages. The logarithms of the rate constants linearly depended on the Hammett acidity function of the medium. Cationic cis-diaquacomplex 3 was found to act as an active water reduction catalyst (WRC) under visible light irradiation when paired with the [Ir(ppy)₂bpy]⁺ photosensitizer (PS).