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

The reaction between [Ni(HMD)1(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 Ni-II center in both [1](2+) and [2](2+), which is characteristic of a low spin Ni-II d(8) center. Complexes 1 and 2 remain low spin in acetonitrile solution as evidenced by their H-1 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. (C) 2019 Elsevier Ltd. All rights reserved.