Macromolecule-metal complexes: ligand field stabilization and thermophysical property modification

When transition metal cations coordinate to ligands in the sidegroup of a p olymer and modify the thermal response of a macromolecular complex, the enh ancement in T-g can be explained by focusing on ligand field stabilization of the metal d-electrons. The methodology to identify attractive coordinati on complexes and predict relative increases in T-g is described in terms of the local symmetry of the complex, the molecular orbital pattern, and the d-electron configuration. Interelectronic repulsion is considered for pseud o-octahedral d(6) and d(7) complexes in the glassy state when there is ambi guity in the order in which the d-orbitals are populated. Ligand field stab ilization energies are calculated for simple octahedral geometries, as well as 5-coordinate complexes with reduced symmetry, such as square pyramidal, trigonal bipyramidal, and pentagonal planar. If the transition metal catio n bridges two different macromolecules in the glassy state via coordination crosslinks, then 5-coordinate complexes with one surviving metal-polymer b ond above T-g represent reasonable geometries in the molten state. This mod el of thermochemical synergy in macromolecule-metal complexes with no adjus table parameters considers the glass transition as an endothermic process i n which sufficient thermal energy must be supplied to dissociate intermolec ular bridges or coordination crosslinks and produce coordinatively unsatura ted molten state complexes. The enhancement in T-g correlates well with the difference between ligand field stabilization energies in the glassy and m olten states for Ru2+ (d(6)), Co2+ (d(7)), and Ni2+ (d(8)) complexes with e ither poly(4-vinylpyridine), or poly(L-histidine). Larger increases in T-g are measured in complexes with the synthetic poly(alpha -amino acid) relati ve to those with poly(4-vinylpyridine), but the universality of the model i s not sufficient to predict relative T-g enhancements in complexes with dif ferent polymers. (C) 2001 Elsevier Science Ltd. All rights reserved.