Structure characterization of the copper(II) complex of poly(4-vinylpyridine) by a combination of EPR, ENDOR, and molecular modeling techniques

A combination of EPR and orientation-selective pulse ENDOR methods provides enough constraints to characterize the structure of the metal center in the copper(II) complex of poly(4-vinylpyridine). The structure derives from the propeller structure of the tetrapyridino copper(II) complex in pyridine glass determined by the same methods. It features more disorder with respect to the angle between the pyridine ring and complex plane but only moderate tetrahedral distortion. The enhanced resolution of high-field EPR allows one to detect another metal center at higher copper loading of the polymer that probably involves only three pyridyl side group ligands. No evidence is found for an inhomogeneous metal distribution in the polymer if care is taken to equilibrate the structure during preparation. The EPR and ENDOR results are supported by molecular modeling based on density functional and force-field calculations. This combination of experimental and theoretical techniques provides a more detailed account of the principles governing the structure of this macromolecule-metal complex than EPR and ENDOR alone.