Synthesis, crystal structures and magnetic properties of linear and bent trinuclear complexes formed by hexacyanometallates and copper(ii) complexes

Abstract

The bimetallic trinuclear complexes, [{(tpa)Cu(NC)}2Fe(CN)4]ClO4·6H2O (1), [{(tren)Cu(NC)}2Fe(CN)4]ClO4·4H2O (2), [{Cu(tpa)NC}2Cr(CN)4]ClO4·8H2O (3) and [{Cu([15]aneN4)NC}2Cr(CN)4]ClO4·4H2O (4) [tpa = tris(2-pyridylmethyl)amine, tren = tris(2-aminoethyl)amine, ([15]aneN4 = 1,5,9,12-tetraazacyclopentadecane] were prepared by addition of solutions of the CuII tpa, tren and [15]aneN4 complexes to solutions of hexacyanometallate, [M(CN)6]3− (M = Fe or Cr), in a 2:1 molar ratio. For 1, an oxidant was added to prevent reduction of the FeIII centre, while for 2, the synthesis was carried out in acetonitrile. 1 Consists of quasi-linear trinuclear units of two trans-oriented [Cu(tpa)]2+ ions linked to the central FeIII core via two bridging cyano groups with the CuII centres residing in a distorted trigonal bipyramidal geometry. Use of a bis(bidentate) Cu(II) chelate, [Cu(H2LN4)2](NO3)2 (H2LN4 = 1,1′,4,4′,5,5′,6,6′-octahydro-2,2′-bipyrimidine), yielded the first example of a non-linear cis-oriented trinuclear complex, [{(H2LN4)2Cu(NC)}2Fe(CN)4]NO3·3H2O (5), in which the CuII centres have a square pyramidal geometry. Magnetic studies on complex 1 show a rapid increase in the magnetic moment below 25 K, consistent with the presence of ferromagnetic coupling between the FeIII (LS, S = ½) and CuII (S = ½) metal centres, where gCu = gFe = 2.16, J12 = 4.3 cm−1, J13 = −0.18 cm−1. The cis-complex, 5, showed close to Curie-like behaviour and gave a good fit to the trimer model using gCu = gFe = 2.31, J12 = 1.95 cm−1, J13 = −2.1 cm−1. The CuIICrIIICuII complex 4 also displayed ferromagnetic coupling across the Cr–CN–Cu bridges because of orthogonal t2g(Cr)/eg(Cu) orbital overlap. Best-fit parameters are g = 2.1, JCuCr = 1.31 cm−1, JCuCu = 0 cm−1. Complexes 2 and 3 showed related ferromagnetic behaviour but which fitted poorly to the trimer model.