Experimental and computer-modelling studies of carbon-supported metal complexes. Part 1.—Adsorption isotherms of nickel tetraaza[14]annulenes on graphitised carbon: relevance to catalytic electroreduction of dioxygen

Abstract

Metal complexes of macrocycles, deposited on a carbon electrode, are used as catalysts in the electroreduction of dioxygen. We have studied the interaction between the macrocycles and a carbon substrate. Isotherms were measured for adsorption by a graphitised carbon of 5, 14-dihydrodibenzo[b,i][1, 4, 8, 11]tetraazacyclotetradecine (1)(a planar macrocycle), 6, 8, 15, 17-tetramethyl-5, 14-dihydrodibenzo[b,i][1, 4, 8, 11]tetraazacyclotetradecine (3)(non-planar or ‘buckled’), and their nickel derivatives (2 and 4). The carbon was graphitised XC72 (BET area 75 m² g^–1) and the solvents were 1, 2-dichloroethane and N,N-dimethylformamide (DMF).

The adsorption data for the metal-free compounds (1 and 3) obeyed the Langmuir isotherm, while the data for the metal complexes (2 and 4) could be fitted to a multiple Langmuir isotherm. Surface coverages and adsorption affinities were calculated from linear plots of the isotherms; hence the free energies of adsorption were obtained. It was found that (a) the surface coverage and free energy of adsorption are greater for the planar macrocycles (1 and 3) than for the buckled compounds (2 and 4); (b) there are extra binding sites available to the nickel complexes. The binding sites of the nickel complexes have greater free energy than the binding sites of the metal-free compounds.

We propose binding between the carbon surface and the macrocyle ring, and, for the nickel complexes, formation of a bond between the nickel ion and oxygen edge sites.