CRYSTAL-STRUCTURE DYNAMICAL AND DIPOLAR EFFECTS IN EPR-SPECTRA OF CU(2-BENZOYLPYRIDINE)(2)(CLO4)(2) [CU(C12H9NO)(2)(CLO4)(2)] CRYSTAL WITH NEGLIGIBLE EXCHANGE INTERACTION

The monoclinic crystal unit cell with space group P2(1)/c and dimensio ns a = 13.343 Angstrom, b = 21.018 Angstrom, c = 9.609 Angstrom and be ta = 100.99 degrees contains four molecules. Copper(II) ions are coord inated by two bidentate 2-benzoylpyridine ligands in trans configurati on with two weakly bonded ClO4 ligands at apical positions forming a d istorted octahedron. There is no chemical contact between octahedrons and electron paramagnetic resonance (EPR) spectra show that the exchan ge coupling between Cu(II) ions is negligible down to the lowest tempe ratures. At helium temperature, however, a shift and broadening of EPR lines appear, suggesting the appearance of a weak exchange coupling b etween Cu(II) which seems to be via direct through-space exchange. In EPR spectra both hyperfine splitting and dipolar splitting are observe d which leads to complicated spectral patterns depending on the crysta l orientation. This is analysed by using structural data. Electronic s tructure molecular orbtial parameters calculated from the experimental spin Hamiltonian parameters show a considerable mixing of the d(z2) s tate into the ground d(x2-y2) state of the Cu(II) ion owing to a low c rystal-held symmetry. For temperatures above about 200 K an averaging of the line splitting appears which seems to be a consequence of the l igand dynamics. (C) 1997 Elsevier Science Ltd. All rights reserved.