EPR, ENDOR, and DFT Studies on (β‐Octahalo‐ meso ‐tetraarylporphyrin)copper Complexes: Characterization of the Metal(d x 2 −y 2 )−Porphyrin(a 2u ) Orbital Interaction

A 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[Br 8 TPP]). Accordingly, saddling results in a decrease in the spin density on the copper ion and on the nitrogen atoms. DFT calculations on Cu[Br 8 TPP] 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   x   2 −y   2)−porphyrin(a 2u ) 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   x   2 −y   2orbital on one hand and enhances the overlap between the copper d   x   2 −y   2orbital and the porphyrin a 2u HOMO on the other. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)