EPR, ENDOR, and DFT studies on (β-octahalo-mesotetraarylporphyrin) copper complexes: Characterization of the metal(dx2 -y2)-porphyrin(a2u) orbital interaction

Junlong Shao, Erik Steene, Brian M Hoffman*, Abhik Ghosh

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

27 Scopus citations

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 dx 2-y2 orbital and the porphyrin a2u HOMO on the other.

Original languageEnglish (US)
Pages (from-to)1609-1615
Number of pages7
JournalEuropean Journal of Inorganic Chemistry
Issue number8
DOIs
StatePublished - Apr 22 2005

Keywords

  • Copper
  • Density functional calculations
  • ENDOR spectroscopy
  • EPR spectroscopy
  • Porphyrins

ASJC Scopus subject areas

  • Inorganic Chemistry

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