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

The bimetallic trinuclear complexes, [{(tpa)Cu(NC)})₂Fe(CN)₄]ClO₄·6H ₂O (1), [{(tren)Cu(NC)})₂(Fe(CN)₄]ClO)4.4H₂O (2), [{Cu(tpa)NC})₂Cr(CN)₄]ClO₄·8H ₂O (3) and [{Cu([15]aneN₄)NC}₂(Cr(CN)₄]ClO ₄·4H₂O (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 Cu^II 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 Fe_III 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 Fe^III core via two bridging cyano groups with the Cu ^II 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, [{(H)₂LN₄)₂Cu(NC)})₂Fe(CN) ₄]NO₃·3H₂O (5), in which the Cu^II 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 Fe^III (LS, S = 1/2) and Cu_II (S = 1/2) where g_Cu = g_fe 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 g_Cu = g_Fe = 2.31, J₁₂ = 1.95 cm_-1, J₁₃ = 2.1 cm^-1. The Cu^IICr^IIICu^II complex 4 also displayed ferromagnetic coupling across the Cr-CN-Cu bridges because of orthogonal t_2g(Cr)/e_g(Cu) orbital overlap. Best-fit parameters areg = 2.1, J_CuCr = 1.31 cm^-1, J^CuCu = 0 cm^-1. Complexes 2 and 3 showed related ferromagnetic behaviour but which fitted poorly to the trimer model.