Magnetostructural correlations in parallel square-planar halo-bridged copper(II) dimers. Part II: Structure and magnetic properties of [Cu 2(terpy) 2Br 2](PF 6) 2

Abstract

The crystal and molecular structure of bis-μ- bromo-bis [(2,2′:6′,2″ - terpyridyl) copper (II) ] bis (hexafluorophosphate) has been determined from three dimensional single-crystal X-ray diffraction data, collected by counter techniques. The blue crystals belong to the monoclinic system, space group P2 1 /a, with four formula units in a unit cell of dimensions a=20.40(2), b=13.35(2), c=6.453(8) Å and β=97.74(9)°. The structure was solved using a refinement procedure and starting with the atomic positions of the isostructural complex [Cu(terpy)- Cl] 2(PF 6) 2, to final agreement factors R=0.071 ( R w=0.077). The structure consists of dimeric molecules bridged by two bromine atoms. The coordination geometry about the copper(II) ion is based on a distorted square pyramid with the bridging ligands occupying the apical and equatorial positions. The other three basal coordination positions are occupied by nitrogen atoms from the terpiridyl ligand. The apical CuBr bond distance is 2.83(9) Å, and the equatorial CuBr bond distance is 2.36(4) Å. The three copper-nitrogen bond lengths are 1.90(4), 2.01(4) and 2.01(8) Å. Both, magnetic susceptibility and ESR data evidenced the existence of exchange coupling. The best fit of the susceptibility data to the susceptibility expression for a pair of exchange coupled S=1/2 ions leads to an evaluation of the singlet-triplet energy gap ( 2J=−7.3 cm -1). A correlation is proposcd relating the magnetic behaviour to the topology of the coordination polyhedron about the copper(II) ion in bis (μ-halo)- bridged parallel planar dimers.