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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
Author(s) -
Shao Junlong,
Steene Erik,
Hoffman Brian M.,
Ghosh Abhik
Publication year - 2005
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.200400549
Subject(s) - chemistry , electron paramagnetic resonance , copper , delocalized electron , tetraphenylporphyrin , porphyrin , density functional theory , crystallography , hyperfine structure , ion , unpaired electron , spectroscopy , inorganic chemistry , photochemistry , nuclear magnetic resonance , computational chemistry , molecule , atomic physics , organic chemistry , physics , quantum mechanics
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)