STRONG FERROMAGNETISM BETWEEN COPPER(II) IONS SEPARATED BY 6.7 A IN A NEW PHTHALATO-BRIDGED COPPER(II) BINUCLEAR COMPLEX

The crystal structure of [(dien)(ClO4)Cu(1)(mu-Phth)Cu(2)(dien)]ClO4), where Phth2- is the dianion of phthalic acid and dien is diethylenetriamine, has been determined by direct X-ray methods. The complex crystallizes in the monoclinic space group P2(1) with two formula units in a unit cell of dimensions a = 12.101 (6) angstrom, b = 7.805 (4) angstrom, c = 15.415 (7) angstrom, beta = 109.82 (4)-degrees, and rho-calcd = 1.690 g/cm3. The structure was refined to conventional discrepancy factors of R = 0.054 and R(w) = 0.060 for 2173 observed reflections. The acentric monoclinic cell contains binuclear cations [(dien)(ClO4)Cu(1)(mu-Phth)Cu(2)(dien)]+ and isolated perchlorate anions. The Cu centers are bridged by Phth dianions coordinated in both uni- and bidentate fashions to Cu(1) and Cu(2), respectively, through their carboxylic oxygen donor atoms. The two copper environments are dissimilar and irregular. The structure around Cu(1) is close to a square-based pyramid with one of the oxygen atoms of the perchlorate anions in the apical position. The Cu(2) environment is close to a severely distorted octahedron. An oxygen atom of a carboxylato group bridges two Cu(2) atoms of two different binuclear units, rather loosely [Cu(2)-O(22) = 2.318 (8) angstrom], to infinite chains of binuclear units around a screw axis 2(1). Least-squares fitting of the variable-temperature (4.2-295 K) magnetic susceptibility experimental data to Bleaney-Bowers equation with a molecular field approximation led to a singlet-triplet energy gap of 80 +/- 10 cm-1, g = 2.10, and zJ' = -1.81 cm-1. The EPR spectrum does not exhibit any evidence of triplet state. From an orbital interpretation of the coupling stems a new situation leading to ferromagnetic interaction between magnetic centers separated by multiatom-bridging units.