EPR, ENDOR, and DFT Studies on (β‐Octahalo‐meso‐tetraarylporphyrin)copper Complexes: Characterization of the Metal(d)−Porphyrin(a2u) Orbital Interaction

Abstract

AbstractA series of planar (porphyrin)copper(II) complexes and their β‐octahalogenated saddled derivatives have been studied by Electron Paramagnetic Resonance (EPR) spectroscopy, Electron Nuclear DOuble Resonance (ENDOR) spectroscopy, and Density Functional Theoretical (DFT) calculations. Both EPR/ENDOR spectroscopy and DFT calculations indicate a decrease in spin density on the central copper(II) ion and on the nitrogen atoms in the saddled compounds relative to the planar complexes. The EPR/ENDOR measurements show that the hyperfine coupling decreases by 12% on the nitrogen atoms and 9% on the copper ion, in going from planar (5,10,15,20‐tetraphenylporphyrin)copper (Cu[TPP]) to saddled (2,3,7,8,12,13,17,18‐octabromo‐5,10,15,20‐tetraphenylporphyrin)copper (Cu[Br8TPP]). Accordingly, saddling results in a decrease in the spin density on the copper ion and on the nitrogen atoms. DFT calculations on Cu[Br8TPP] yield spin populations of 42.4% on the copper ion, 9.9% on each nitrogen atom and 4.9% on each meso carbon atom, relative to DFT spin populations of 62, 10.2 and 0.3% on the copper ion, each nitrogen and each meso carbon atom, respectively, for porphinecopper (Cu[P]). These calculations further indicate that the decrease in spin density on the copper ion in the saddled complexes results from a saddling‐induced Cu(d)−porphyrin(a2u) orbital overlap whereby some of the Cu spin density is delocalized onto the porphyrin ring. The decrease in nitrogen spin population with saddling appears to be a more subtle effect caused by a superposition of two opposing factors. Saddling decreases the overlap between the nitrogen lone pairs and the Cu d orbital on one hand and enhances the overlap between the copper d orbital and the porphyrin a2u HOMO on the other. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)