EPR, ENDOR, and DFT studies on (β-octahalo-mesotetraarylporphyrin) copper complexes: Characterization of the metal(d_x² _-y²)-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[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_x²_-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 d_x²_-y² orbital on one hand and enhances the overlap between the copper d_x ²_-y² orbital and the porphyrin a_2u HOMO on the other.