EXPERIMENTAL AND COMPUTER MODELING STUDIES OF CARBON-SUPPORTED METAL-COMPLEXES .2. MOLECULAR MECHANICS STUDY OF THE ADSORPTION OF TETRAAZA[14]ANNULENES AND THEIR NICKEL(II) COMPLEXES BY A CARBON SURFACE

A computer model based on molecular mechanics with the addition of ter ms to take account of pi-pi interactions has been developed for the st udy of the adsorption of molecules on an idealised graphite surface. T his model has been validated by calculation of the adsorption energy o f benzene on graphite; our value compares well with experimental and p reviously calculated values. Our model was then used to study the adso rption by graphite of the planar macrocycle ydrodibenzo[b,i][1,4,8,11] tetraazacyclotetradecine (1) and the saddle-shaped molecule drodibenzo [b,i]-[1,4,8,11]tetraazacyclotetradecine (3), and their nickel complex es (2 and 4). The interaction energies calculated by the model agree w ith 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 res pect to the C6 hexagons of the graphite. The attractive interaction is the London dispersion energy. However, the electrostatic interactions (between the pi-clouds of the macrocycle and the graphite) are repuls ive and are responsible for the staggering of the macrocycles which or ientate to minimise the pi-pi repulsions with the surface. The planar nickel complex 2 is optimally orientated with nickel above a graphite carbon, an attractive Ni-pi interaction contributing to the bonding. T he saddle-shaped compounds 3 and 4 prefer to orientate with their CH3 groups next to the surface, a consequence of an attractive CH3-pi inte raction, and with the molecule centres above the graphite ring centres .