Probing into the electrochemistry of four nickel(II) and cobalt(II) complexes with azadiphosphine ligands (PNP) and their catalysis on proton reduction

Abstract

In this work, four complexes of Co(II) and Ni(II), [Co(L1)2(CH3CN)](ClO4)2 (1), [Ni(L1)2](ClO4)2 (2), [Co(L2)2(CH3CN)](ClO4)2 (3) and [Ni(L2)2](ClO4)2 (4) were synthesized by reacting the perchlorate hydrate of M(II) (M = Co and Ni) with the ligands, Ph2PN(cyclohexyl)PPh2 (L1) and Ph2PN(pyridin-2-ylmethyl)PPh2 (L2), respectively. The complexes were fully characterized, and their absolute structures were determined using X-ray single crystallographic analysis. The cobalt complexes (1 and 3) exhibited two reductions in acetonitrile, one reversible reduction at about −0.65 V and the other at about −2.0 V at which catalytic reduction of proton was observed. The two reductions were assigned to Co(II) → Co(I) and Co(I) → Co(0) processes, respectively. The reduction of Ni(II) → Ni(I) was negatively shifted by about 300 mV compared to the cobalt analogues and the process showed two closely located reversible reductions due to geometric isomerization between the square planar (Sp) and tetrahedral (Td) geometries, of which the Td isomer was first reduced due to the lower energy level of its frontier orbitals. The second reduction of the Ni(II) complexes (2 and 4) was invisible in the potential range examined. Upon addition of acetic acid, it shifted positively by over 300 mV and showed catalysis on proton reduction. The shift was attributed to proton-coupled electron transfer (PCET) effect which was further enhanced by the pendant pyridyl group in ligand L2. The nickel complexes showed much higher activity than cobalt analogues in the catalysis on proton reduction and the pendant base group could synergistically enhance the activity of the complex (4).