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

The cationic dibenzocyclamnickel(II) complex, [Ni(Me(4)Bzo(2)[14]aneN(4))](2+), was obtained in good yield by Fe/HCl reduction of the corresponding tetraazaannulene complex [Ni(Me(4)taa)], (1) {Me(4)Bzo(2)[14]aneN(4) = 5,7,12,14-tetramethyldibenzo[b,i]-1,4,8,11-tetraazacyclotetradecane; Me(4)taa = 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(Me(4)Bzo(2)[14]aneN(4)-H-2)][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 Me(4)Bzo(2)[14]aneN(4), (5). These compounds were characterized using a combination of i.r., u.v.-vis., H-1-n.m.r., mass spectroscopy and voltammetry techniques. Unlike the parent tetraazaannulene complex (1), the reduced macrocycle complex, [Ni(Me(4)Bzo(2)[14]aneN(4))](2+) exhibits mild catalytic activity towards electro-reduction of CO2 in MeCN solution.