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 dx 2-y2 orbital and the porphyrin a2u HOMO on the other.
- Density functional calculations
- ENDOR spectroscopy
- EPR spectroscopy
ASJC Scopus subject areas
- Inorganic Chemistry