NiII complexes of 1,4,8,11-tetraazacyclotetradec-4,11-diene and 1,4,8,11-tetraazacyclotetradecane bearing C-styrenyl: Synthesis, structures and voltammetry

Abstract

The reaction between [Ni(HMD)](OTf)2 (HMD = 5,7,7,12,14,14-hexamethyl-1,4,8,11-teraazacyclotetradeca-4,11-diene) and benzaldehyde in the presence of NaOH afforded the 5,7-di(styrenyl) derivative [Ni(HMD-Styr)](OTf)2 (1), which was converted to the corresponding cyclam complex [Ni(HMC-Styr)](OTf)2 (2) upon NaBH4 reduction. Addition of NaBPh4 to the reduction reaction mixture led to the isolation of [Ni(HMC-Styr)](BPh4)2 (2′). Single crystal X-ray diffraction studies of 1 and 2′ revealed a square planar NiII center in both [1]2+ and [2]2+, which is characteristic of a low spin NiIId8 center. Complexes 1 and 2 remain low spin in acetonitrile solution as evidenced by their 1H NMR spectra and Vis absorption spectra. Complex 1 displayed two irreversible one-electron reductions between −1.4 and −1.8 V (versus Fc(+1/0)) under Ar in 20% aqueous acetonitrile, and exhibited a new peak around −1.9 V under CO2, indicating a mild activity towards CO2 reduction. In contrast, complex 2 displayed a reversible one-electron reduction at −1.59 V (versus Fc(+1/0)) under Ar, and a less reversible reduction under CO2 without shift in potential, revealing the lack of activity towards CO2.