Microstructure of N-Picolylpolyurethane Transition Metal Complexes

Abstract

Spectroscopic methods are used to investigate coordination structure of N-picolylpolyurethane transition metal complexes (PUPYM, M = Co2+ and Ni2+). Geometrical arrangement of ligands in first-shell coordination sphere of metal ions is postulated to be tetrahedral CoL2Cl2 and octahedral NiL2Cl2Z2, where L is the picolyl group and Z is a hydrate. From extended X-ray absorption fine structure (EXAFS) analysis, bond lengths for metal−chlorine and metal−ligand of PUPYM are similar to those of small molecular weight transition metal complexes. A two-phase model of PUPYM, which best describes the experimental data of DMTA and SAXS, is proposed. One microphase is the hard domain of self-segregated hard segments brought about by metal−ligand interaction, and the other phase is the matrix of soft segments. Transition metal ion−ligand moieties and their interactions dominate the macroscopic thermal behavior of PUPYM. The ligand field stabilization energy difference (ΔLFSE) between metal d-electrons in complexes with two picolyl ligands in the coordination sphere of metal ions and complexes maintaining one picolyl ligand as coordination pendent group is calculated on the basis of observed coordination structure, and it represents the energy supplied to split coordination cross-links. ΔLFSE of polyurethane nickel(II) complex is larger than that of the cobalt(II) complex. Since the mobility of hard segments is in inverse proportion to the strength of coordination cross-links, a higher α-transition temperature of PUPYNi2+ with respect to PUPYCo2+ is found as expected.