Spectroscopic and electrochemical properties of the isomeric bidiazine complexes [(C5Me5)CiRh(bdz)]+ and (C5Me5)Rh(bdz) and their relevance to the catalysis of the 2 H+ → H2 reaction by 2,2′-bipyridine analogues

Abstract Electronic structures and ligand effects have been studied for the isomeric Rh III/I complexes [(C 5 Me 5 )ClRh(bdz)](X) and (C 5 Me 5 )Rh(bdz); X - = Cl - ,PF 6 - ; bdz = bidiazines (3.3′-bipyridazine, 2.2′-bipyrazine, 2,2′- and 4,4′-bipyrimidine). Comparative NMR and UV-VIS spectroscopic as well as cyclic voltammetric measurements in aprotic solvents have allowed definition of a frontier MO situation that has been correlated with the reactivity of the [(C 5 Me 5 )ClRh(bpy)] + system as a catalyst for the evolution of hydrogen from water. Thus, while the potential for the two-electron reduction of the Rh 111 halide precursor depends on the ligand basicity, the MO description shows that there is extremely strong π-back donation and orbital mixing between the d orbitals of the H + -accepting (C 5 Me 5 )Rh 1 fragment and the acceptor level of the heterocyclic ligand.