FURTHER NEIGHBOR MAGNETIC EXCHANGE INTERACTION IN A NOVEL PSEUDOLINEAR TETRAMER OF COPPER(II)

The synthesis, crystal structure, and magnetic properties are reported for the novel compound Cu4L2(CH3COO)6, where LH = 1-(5-bromosalicylaldimino)-3-(2-methylpiperidino)propane. The compound, Cu4C44H62N4O14Br2, crystallizes in the monoclinic space group P2(1)/n. Cell dimensions are as follows: a = 11.627 (3) angstrom, b = 22.442(3) angstrom, c = 9.693(3) angstrom, beta = 94.46(2)-degrees, and Z = 2. The structure was refined to an R(unweighted) value of 0.076. The tetranuclear unit of the complex is centrosymmetric and has an almost linear geometry. The separation between the ''central'' Cu(2) and Cu(2)' atoms is 2.63 angstrom and that between the end Cu(1) and Cu(1)' atoms is 11.81 angstrom. The Cu(1)-Cu(2) and Cu(2)-Cu(2)' directions form an angle of 164.5-degrees. Cu(2) and Cu(2)' have a square pyramidal environment and are bridged by four acetate groups in a syn-syn conformation, as in the well-known copper(II) acetate monohydrate dimer. Two additional acetate groups bridge, by adopting a syn-anti conformation, from the axial position of a central copper atom to a pseudo-basal position of the adjacent terminal copper atom. The end copper atoms have a (4 + 1) coordination geometry that is intermediate between square pyramidal and capped tetrahedral. The X-ray analysis of the compound does not reveal any bonding interactions between tetramers. The magnetic susceptibility of the compound has been measured between 2 and 290 K. At room temperature the chi(M)(4Cu) T value is slightly lower than expected for four uncoupled S=1/2 spins (1.29 vs 1.50 emu.mol-1.K). When the temperature is lowered, chi(M)T first decreases, reaching a value appropriate for two uncorrelated S = 1/2 spins (0.8 emu.mol-1.K) at about 80 K, then remains quite constant until ca. 10 K, and ultimately tends to zero as the temperature approaches zero. These properties can be nicely accounted for by a model in which there is very weak magnetic exchange between the outer pairs of copper atoms, strong antiferromagnetic exchange (-2J2 approximately 300 cm-1) between the central pair, and an individual antiferromagnetic coupling (-2J4 = 3.5 cm-1) between the terminal copper atoms. An orbital mechanism for the end-end interaction is proposed.