Rhodium(III) complexes of a substituted 2,2′-bisoxazoline

Reaction of 4,4,4′,4′-tetramethyl-2,2′-bisoxazoline (L) with RhCl3 in methanol followed by addition of excess NaClO4 yields cis-[RhL2Cl2]ClO4 (1). Complex 1 hydrolyses to yield the mixed ligand Rh(III) complex trans-[RhL(L′H)Cl2]ClO4·2H2O (2), where L′H is 2-amino-2-methyl-1-propanol (H: dissociable alcoholic proton). The X-ray crystal structures of 1 and 2 have been determined. The aminoalcohol L′H binds the metal in 2 in bidentate mode through the N and the O ends. The 1H NMR of 2 in CD3CN shows that the alcoholic proton dissociates in solution. DFT calculations at the B3LYP/LanL2DZ level reveal that the dissociation of the alcoholic proton in 2 is highly exothermic and accompanied with elimination of a chloride ion from the metal coordination sphere giving rise to a square pyramidal five-coordinate Rh(III) species. The relevant pKa value has been determined in acetonitrile from solution conductance as 8.98 (±0.27). Both the complexes 1 and 2 display an irreversible Rh(III/I) couple in cyclic voltammetry in acetonitrile at a glassy carbon electrode around −0.35 V versus NHE.