Crystal structures and magnetic properties of uniform and alternating azido-bridged (2,2′-bipyridyl)copper(II) chains

Single crystals of four copper(II) complexes of formula [Cu(bipy)(N-3)(2)](n) (1,2), [Cu(bipy)(2)(N-3) (C1O4)] (3) and [Cu(bipy)(2)(N-3)]ClO4 (4) (bipy = 2,2'-bipyridine) were obtained from aqueous solutions containing (2,2'-bipyridyl)copper(II) perchlorate and sodium azide, and their crystal structures were determined by X-ray diffraction methods. The structures of compounds 1 and 4 were already known, but the better accuracy of our structural determination of complex 1 led us to reconsider it here. The structures of compounds 1 and 2 are made up of neutral chains of copper(II) ions bridged by two azide groups exhibiting asymmetrical end-on (1) and alternating asymmetrical end-on and end-to-end bridging (2) modes. The copper atoms in 1 and 2 exhibit a distorted CuN6 octahedral environment: two bipyridyl nitrogen atoms from a chelating bipy molecule and two azide nitrogens, either from two bridging azide (I) or from one bridging azide and a terminal one (2), occupy the equatorial positions whereas two azide nitrogens from the two neighbouring neutral [Cu(bipy) (N-3)(2)] units fill the axial positions. The intrachain metal-metal separation is 3.516(1) and 3.497(1) Angstrom in 1 and 3.434(1) and 5.378(1) Angstrom in 2. The equatorial Cu-N bond lengths vary in the ranges 1.953(2)-2.029(2) (1) and 1.953(4)-2.029(3) a (2). The axial bonds are significantly longer, 2.683(3) and 2.684(3) Angstrom for 1 and 2.604(3) and 2.849(4) Angstrom for 2. The structure of complex 3 consists of neutral [Cu(bipy)(2)(N-3) (ClO4)] units where the metal atom has a highly distorted CuN5O octahedral environment: two chelating bipy in cis positions, a terminal azido group and a weakly coordinated perchlorate form the coordination polyhedron. The basal plane is composed of four nitrogen atoms, three from two bipy molecules and one from the azido group (the metal to nitrogen bonds varying in the range 1.971(5)-2.030(5) Angstrom, whereas the apical positions are occupied by a bipy nitrogen (2.218(5) Angstrom) and a perchlorate oxygen (2.950(5) Angstrom) atom. The magnetic properties of 1 and 2 have been investigated in the temperature range 2-300 K. Weak intrachain antiferro- (1) and alternating antiferro- and ferromagnetic (2) interactions have been observed and the efficiency of the out-of-plane exchange pathway through the azido bridge is analysed and discussed in the light of the available magneto-structural parameters. (C) 1998 Elsevier Science S.A. All rights reserved.