COMPARATIVE MOLECULAR MECHANICS STUDY OF THE LOW-SPIN NICKEL(II) COMPLEXES OF AN EXTENDED SERIES OF TETRAAZA MACROCYCLES

Molecular mechanics calculations of metal complex structures are often not straight forward because of uncertainties concerning the appropriate force-field parameters for structural elements involving the metal. A systematic study aimed at extending the Allinger MM2 force field for use with N4-macrocyclic complexes of low-spin Ni(II) is reported. Although molecular mechanics investigations of particular low-spin Ni(II) complexes of tetraaza macrocycles have been performed previously, no calibration of the force field for such complexes with a wide range of compounds of this type has been reported. X-ray data for twenty low-spin Ni(II) complexes incorporating solely sp3 nitrogen donors and six complexes also containing sp2 nitrogens have been used in the calibration of the extended force field. Inclusion of an out-of-plane bending term, which restricts the degree to which the Ni(II) atom can deviate from the least-squares plane of the donor atoms, was generally beneficial in modeling the respective coordination planes. The application of the extended force field to the re-examination of the configurational isomers of [Ni(cyclam)]2+ and their N,N,N,N-tetramethylated derivatives is reported.