An economical synthesis of dibenzocyclamnickel(II) and a metal-free macrocycle with tetramethyl substituents

Abstract

The cationic dibenzocyclamnickel(II) complex, [Ni(Me4Bzo2[14]aneN4)]2+, was obtained in good yield by Fe/HCl reduction of the corresponding tetraazaannulene complex [Ni(Me4taa)], (1) {Me4Bzo2[14]aneN4 = 5,7,12,14-tetramethyldibenzo[b,i]-1,4,8,11-tetraazacyclotetradecane; Me4taa = 5,7,12,14-tetramethyldibenzo[b,i]-1,4,8,11-tetraazaannulene(2-)}. The orange–red product was isolated as the chloride (2) and perchlorate (3) salts. Analogous reduction with Zn/HCl yielded a diprotonated silky-white product [Ni(Me4Bzo2[14]aneN4-H2)][ZnCl4]2, (4). In the dry state, complex (4) is stable only under an HCl atmosphere and readily dissociates to give a solution of (2) when dissolved in polar solvents. Complexes (2) and (3), upon treatment with an excess of aqueous NaCN, undergo facile demetallation yielding the metal free macrocycle Me4Bzo2[14]aneN4, (5). These compounds were characterized using a combination of i.r., u.v.–vis., 1H-n.m.r., mass spectroscopy and voltammetry techniques. Unlike the parent tetraazaannulene complex (1), the reduced macrocycle complex, [Ni(Me4Bzo2[14]aneN4)]2+ exhibits mild catalytic activity towards electro-reduction of CO2 in MeCN solution.