STRONG ANTIFERROMAGNETIC COUPLING IN CIS-MU-1,3-AZIDE-BRIDGED COPPER(II) DIMERS - TRIGONAL-BIPYRAMIDAL VS QUADRATIC PLANAR COORDINATION WITH A NEARLY IDENTICAL BINUCLEATING LIGAND SYSTEM

The structure of the complex [Cu-2(L-Et)(N-3)](BF4)(2) (L-Et is the anion of N,N,N',N'-tetrakis[2-(1-ethylbenzimidazolyl)]-2-hydroxy-1,3-diaminopropane) has been redetermined and has been found to deviate significantly from that published earlier (McKee, V.; Zvagulis, M.; Dagdigian, J. V.; Patch, M. G.; Reed, C. A. J. Am. Chem. Sec. 1984, 106, 4765). The space group of our product is P2(1)/n with the lattice parameters a = 14.104(2) Angstrom, b = 21.406(4) Angstrom, c = 15.989(2) Angstrom, and beta = 98.05(2)degrees (Z = 4), which cannot be transformed into those given by McKee et al. (C2/m, a = 19.082 Angstrom, b = 23.896 Angstrom, c = 13.230, and beta = 116.21 degrees). In addition, the copper coordination in the complex cation is found to be close to trigonal-bipyramidal whereas the geometry in the structure of McKee et al. is more square-pyramidal. However, strong antiferromagnetic coupling (-2J > 1200 cm(-1)) is also found for our product which is close to the value obtained earlier (-2J > 1100 cm(-1)). Hence, a cis mu-1,3 bridging azide group is able to mediate strong superexchange without contribution of a second bridging ligand (alkoxide), and a tetragonal ground state is not a necessary condition far strong antiferromagnetism as stated by McKee et al. To our knowledge. the -2J value of > 1200 cm(-1) represents a new lower limit for single bridging azide in copper dimers.