Heterometallic trinuclear (CuM2III)-M-II (M = Fe or Cr) complexes with novel bridges and unusual magnetic properties

Three new trinuclear complexes based on [Cu(L-n)](2+) (L-1 = 1,5,8,12-tetramethyl-1,3,6,8,10,13-hexaazacyclotetradecane, L-2 = 1,3-propanediamine (tn)) and [M(bpb)(CN)(2)](-) (bpb(2-) = 1,2-bis(pyridine-2-carboxamido)benzenate, M = Fe or Cr) have been synthesized and characterized structurally and magnetically. The Cu(II) ions adopt an axially-elongated octahedron with four N atoms from the L-n ligands occupying the equatorial plane and another two atoms (cyano nitrogen atoms or amide oxygen atoms) from two [M(bpb)(CN)(2)](-) moieties at the axial positions, resulting in a neutral linear trinuclear complex. Magnetic studies show that the CuFe2 compound bridged by the amido group exhibits ferromagnetic behaviour. The two cyano-bridged CuFe2 or CuCr2 compounds show unusual magnetic properties: weak antiferromagnetic Cu(II)-Fe(III) coupling and negligible Cu(II)-Cr(III) magnetic exchange, proved by the results of the field dependence of magnetization.