Experimental and computer modelling studies of carbon-supported metal complexes. Part 2.—Molecular mechanics study of the adsorption of tetraaza[14] annulenes and their nickel(II) complexes by a carbon surface

Abstract

A computer model based on molecular mechanics with the addition of terms to take account of π–π interactions has been developed for the study of the adsorption of molecules on an idealised graphite surface. This model has been validated by calculation of the adsorption energy of benzene on graphite; our value compares well with experimental and previously calculated values. Our model was then used to study the adsorption by graphite of the planar macrocycle 5,14-dihydrodibenzo[b,i][1,4,8,11]tetraazacyclotetradecine (1) and the saddle-shaped molecule 6,8,15,17-tetramethyl-5,14-dihydrodibenzo[b,i]–[1,4,8,11]tetraazacyclotetradecine (3), and their nickel complexes (2 and 4). The interaction energies calculated by the model agree with the trend in experimental adsorption free energies: 2 > 1 > 4 > 3.

The minimum energy orientation of the planar macrocycles (1 and 2) is parallel to the graphite surface and approximately staggered with respect to the C₆ hexagons of the graphite. The attractive interaction is the London dispersion energy. However, the electrostatic interactions (between the π-clouds of the macrocycle and the graphite) are repulsive and are responsible for the staggering of the macrocycles which orientate to minimise the π–π repulsions with the surface. The planar nickel complex 2 is optimally orientated with nickel above a graphite carbon, an attractive Ni–π interaction contributing to the bonding. The saddle-shaped compounds 3 and 4 prefer to orientate with their CH₃ groups next to the surface, a consequence of an attractive CH₃–π interaction, and with the molecule centres above the graphite ring centres.