Reduction of Schiff-base macrocyclic complexes. Stabilisation of nickel(I) conjugated macrocyclic complexes via a reversible ligand-to-metal electron-transfer process

Abstract

Reductive cyclic voltammetry of two Ni^II macrocyclic complexes [Ni^IIL]²⁺ in acetonitrile each shows two reversible one-electron reduction waves near ¹E_½=–1.0 V and ²E_½=–1.55 V. Electrochemical reduction by controlled-potential electrolysis at these potentials and investigation of the reduced products by e.s.r. spectroscopy show the two reduction waves to correspond to the formation of [Ni^II(L˙^–)]⁺ and [Ni^I(L˙^–)]⁰ respectively. In the presence of axial ligands X = P(OMe)₃ PPh₃, or CO, cyclic voltammetry of [Ni^IIL]²⁺ shows the primary reduction wave to shift to lower negative potentials, the magnitude of ¹E_½ increasing in the order P(OMe)₃ < CO < PPh₃ < no ligand added; the magnitude of ²E_½ was found to decrease along this series. Controlled-potential electrolysis of [Ni^IIL]²⁺ at the primary reduction potential in the presence of these ligands leads to the formation of the metal-reduced nickel(I) complexes, [Ni^ILX]⁺. Likewise, addition of these ligands to solutions of [Ni^II(L˙^–)]⁺ leads to the stabilisation of the same nickel(I) products. An intramolecular macrocyclic ligand-to-metal electron transfer is thought to be involved in the latter process and in the case of CO this electron transfer is found to be reversible. Reduction of [Ni^IIL]²⁺ at the secondary reduction potential in the absence or in the presence of CO, PPh₃, or P(OMe)₃ leads to the formation of nickel(I) ligand radical species; no nickel(0) products could be isolated.