EPR, ENDOR, and DFT Studies on (β‐Octahalo‐meso‐tetraarylporphyrin)copper Complexes: Characterization of the Metal(d x 2−y 2)−Porphyrin(a2u) Orbital Interaction Academic Article uri icon

abstract

  • 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[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(dx2-y2)-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 dx2-y2 orbital on one hand and enhances the overlap between the copper dx2-y2 orbital and the porphyrin a2u HOMO on the other. © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

author list (cited authors)

  • Shao, J., Steene, E., Hoffman, B. M., & Ghosh, A.

citation count

  • 17

publication date

  • April 2005

publisher