THE PREPARATION, CHEMISTRY AND CRYSTAL-STRUCTURE OF THE NICKEL(II) COMPLEX OF N-HYDROXYETHYLAZACYCLAM HYDROXYETHYL)-1,3,5,8,12-PENTA-AZACYCLOTETRADECANE NICKEL(II) PERCHLORATE] - A NEW ELECTROCATALYST FOR CO2 REDUCTION

The reaction of formaldehyde and ethanolamine with the nickel(II) comp lex of 1,9-diamino-3,7-diazanonane (2,3,2-tet) gives the nickel(II) co mplex of the macrocycle hydroxyethyl-1,3,5,8,12-penta-aza-cyclotetrade cane (I) which can be readily isolated as the perchlorate salt. The cr ystal structure of [NIL] (ClO4)(2) has been determined. The nickel ato m is in an essentially planar environment with N-1, N-5, N-8 and N-12 acting as donors with Ni-N bond distances in the range 1.930-1.938 Ang strom. The ligand has a trans III configuration of the sec-NH centres with chair six-membered and gauche five-membered chelate rings. The hy droxyethyl group on N; is axial. The two perchlorate anions lie in the axial sites but the Ni-O(4) and the Ni-O(7) distances of 2.836(3) and 3.028(3) Angstrom indicate that there is no bonding interaction betwe en these centres. In aqueous solution the complex is predominantly squ are planar. Addition of HCl leads to axial addition of chloride while addition of thiocyanate gives the trans-[NiL(NCS)(2)] complex. The lig and readily folds to give cis-complexes. Thus addition of ethylenediam ine to [NiL](2+) gives cis-[NiL(en)](2+) in solution. The electrochemi stry of [NiL](2+) has been studied in detail. The complex is a good el ectrocatalyst for the reduction of CO2. The complex was less active fo r hydrogen evolution in an acetate electrolyte than [Ni(cyclam)](2+) a nd thus appeared to be more active than the cyclam complex for CO2 red uction under these conditions. (C) 1997 Elsevier Science Ltd.