Anisotropic exchange in transition-metal dinuclear complexes. 5. Bis(-.mu.-hydroxo)bis[(2-methylimidazole)copper(II)] diperchlorate dihydrate

Bis(~-hydroxo)bis[bis(2-methylimidazole)copper(II)] diperchlorate dihydrate, C~~(Meiz)~(OH)~(Cl0~)w~a.s 2pHre~p0ar,e d according to literature methods, and its crystal structure was solved at room temperature. It c stallizes in the monoclinic system, space group C2/m, with Z = 2. The lattice constants are a = 14.920 (6) A, b = 13.768 (7)r#, c = 7.482 (4) A, and 6 = 103.2 (3)O. Least-squares refinement of the structure led to a conventional R factor of 0.034. The structure consists of centrosymmetric [ C ~ ~ (Me i z ) ~ ( 0 Hc)a~ti]o~ns+, perchlorate anions, and water molecules. The bonds connecting the copper ions to the bridging oxygen atoms are longer than usually observed for the series of bis(p-hydroxo)-bridged complexes; as a consequence the observed singlet-triplet splitting does not fit the previously suggested linear relationship with the C u C u distance or the Cu-O-Cu angle. Single-crystal EPR spectra yielded the zero-field splitting tensor, which was found to have its largest compontnt roughly orthogonal to the equatorial coordination planes of the copper ions, thus showing the presence of a dominant exchange contribution. The observed zero-field splitting is related to the Cu-Cu distance in a series of homologous complexes, showing that D, decreases as r increases.