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

When transition metal cations coordinate to ligands in the sidegroup of a polymer and modify the thermal response of a macromolecular complex, the enhancement in Tg can be explained by focusing on ligand field stabilization of the metal d-electrons. The methodology to identify attractive coordination complexes and predict relative increases in Tg 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 pseudo-octahedral d6 and d7 complexes in the glassy state when there is ambiguity in the order in which the d-orbitals are populated. Ligand field stabilization 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 cation bridges two different macromolecules in the glassy state via coordination crosslinks, then 5-coordinate complexes with one surviving metal–polymer bond above Tg represent reasonable geometries in the molten state. This model of thermochemical synergy in macromolecule–metal complexes with no adjustable parameters considers the glass transition as an endothermic process in which sufficient thermal energy must be supplied to dissociate intermolecular bridges or coordination crosslinks and produce coordinatively unsaturated molten state complexes. The enhancement in Tg correlates well with the difference between ligand field stabilization energies in the glassy and molten states for Ru2+(d6), Co2+(d7), and Ni2+(d8) complexes with either poly(4-vinylpyridine), or poly(l-histidine). Larger increases in Tg are measured in complexes with the synthetic poly(α-amino acid) relative to those with poly(4-vinylpyridine), but the universality of the model is not sufficient to predict relative Tg enhancements in complexes with different polymers.