The preparation, chemistry and crystal structure of the nickel(II) complex of N-hydroxyethylazacyclam [3-(2'-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) complex of 1,9-diamino-3,7-diazanonane (2,3,2-tet) gives the nickel(II) complex of the macrocycle 3-hydroxyethyl-1,3,5,8,12-penta-aza-cyclotetradecane (I) which can be readily isolated as the perchlorate salt. The crystal structure of [NIL] (ClO4)(2) has been determined. The nickel atom 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 Angstrom. The ligand has a trans III configuration of the sec-NH centres with chair six-membered and gauche five-membered chelate rings. The hydroxyethyl 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 between these centres. In aqueous solution the complex is predominantly square planar. Addition of HCl leads to axial addition of chloride while addition of thiocyanate gives the trans-[NiL(NCS)(2)] complex. The ligand readily folds to give cis-complexes. Thus addition of ethylenediamine to [NiL](2+) gives cis-[NiL(en)](2+) in solution. The electrochemistry of [NiL](2+) has been studied in detail. The complex is a good electrocatalyst for the reduction of CO2. The complex was less active for hydrogen evolution in an acetate electrolyte than [Ni(cyclam)](2+) and thus appeared to be more active than the cyclam complex for CO2 reduction under these conditions. (C) 1997 Elsevier Science Ltd.