Crystal structure, dynamic and dipolar effects in EPR spectra of Cu(2-benzoylpyridine) 2(ClO 4) 2 [Cu(C 12H 9NO) 2(ClO 4) 2] crystals with negligible exchange interaction

Abstract

The monoclinic crystal unit cell with space group P2 1/ c and dimensions a = 13.343 A ̊ , b = 21.018 A ̊ , c = 9.609 A ̊ and β = 100.99 ° contains four molecules. Copper(II) ions are coordinated by two bidentate 2-benzoylpyridine ligands in trans configuration with two weakly bonded ClO 4 ligands at apical positions forming a distorted octahedron. There is no chemical contact between octahedrons and electron paramagnetic resonance (EPR) spectra show that the exchange coupling between Cu(II) ions is negligible down to the lowest temperatures. At helium temperature, however, a shift and broadening of EPR lines appear, suggesting the appearance of a weak exchange coupling between Cu(II) which seems to be via direct through-space exchange. In EPR spectra both hyperfine splitting and dipolar splitting are observed which leads to complicated spectral patterns depending on the crystal orientation. This is analysed by using structural data. Electronic structure molecular orbtial parameters calculated from the experimental spin Hamiltonian parameters show a considerable mixing of the d z 2 state into the ground d x 2 − y 2 state of the Cu(II) ion owing to a low crystal-field symmetry. For temperatures above about 200 K an averaging of the line splitting appears which seems to be a consequence of the ligand dynamics.