LINKAGE ISOMERISM IN EXCHANGE-COUPLED DINUCLEAR NI(II) COMPLEXES WITH3 AZIDE BRIDGES - TRIS(MU-1,1-AZIDO) VERSUS TRIS(MU-1,3-AZIDO) BINDING - CRYSTAL-STRUCTURES OF [L(2)NI(2)(MU-1,1-N-3)(3)]CLO4 AND [L(2)NI(2)(N-3)(2)-(MU-1,3-N-3)(2)] (L=1,4,7-TRIMETHYL-1,4,7-TRIAZACYCLONONANE)

Two linkage isomers of Ni(II), [L(2)Ni(2)(N-3)(3)]ClO4, containing eit her three end-to-end (mu-1,3-azido) 1, or three end-on (mu-1,1-azido) 2 bridging azide ligands and 1,4,7-trimethyl-1,4,7-triazacyclononane ( L) as the capping ligands have been reported. The compounds have been characterized on the basis of IR, UV-Vis spectroscopy and variable-tem perature (2-290 K) magnetic susceptibility measurements. The crystal s tructure of 1 has been reported earlier (J. Chem. Sec., Chem. Commun., (1985) 1618). 2 crystallizes in the monoclinic space group P2(1)/c wi th cell constants a = 14.989(1), b = 14.610(2), c = 14.756(2) Angstrom , beta = 111.12(1)degrees, V = 3014.5 Angstrom(3) and Z = 4. The struc ture of 2 consists of a six-coordinated dinickel(II) center with three azide ions linking two Ni(II) ions by adopting the end-on coordinatio n mode. Analysis of the susceptibility data yields a moderately strong antiferromagnetic interaction between nickel centers in 1, but a ferr omagnetic interaction in 2. The crystal structure of [L(2)Ni(2)(N-3)(2 )(mu-1,3-N-3)(2)] (3) has been determined at 193 K. Crystals of 3 . CH 3OH have the following cell parameters: monoclinic, C2/c; a = 24.302(2 ), b = 11.276(1), c = 11.437(1) Angstrom; beta = 109.70(1)degrees; V = 2950.8; Z = 4. The structure of 3 consists of a six-coordinated dinic kel(II) center with two end-to-end azide ions linking two Ni(II) ions. Magnetic susceptibility measurements have shown the existence of anti ferromagnetic exchange interaction between the nickel centers (2J = -7 2.8 cm(-1)). The magnetic behavior of 1, 2 and 3 together with other r elated complexes is discussed to show the difficulty of rationalizing the magnetostructural trends.