Intramolecular long-range exchange coupling in dinuclear copper(II) complexes with Cu center dot center dot center dot Cu separations greater than 10 angstrom

Magnetic long-range exchange coupling between two unpaired electrons of two copper(II) ions in a square-pyramidal ligand environment (d(x2-y2))(1), which are rigidly separated by > 10 Angstrom by an organic spacer ligand (e.g., a mu-dicarboxylato ligand) has been systematically studied. The following complexes have been synthesized: [{LCu(OH2)}(2)(mu-terephthalato)](ClO4)(2) (1), [LCu(II)(OH2)(benzoato)]-(ClO4) (2), [{LCu(II)(NCS)}(2)(mu-terephthalato)]. 2CH(3)OH (3), [{LCu(II)(NCS)}(2)-(mu-4,4'-oxybisbenzoato)]. 3CH(3)OH (4), [{LCu(II)(OH2)}(2)(mu-tetrafluoroterephthalato)](ClO4)(2) (5), [{LCu(II)(OH2)}(2){LCu(II)}(2)-(mu-tetrafluorobenzoato)(3)](ClO4)(2) . 6H(2)O (6), [{LCu(II)(OH2)}(2)(mu-biphenyl-4,4'-dicarboxylato)](ClO4)(2) (7), [LCu(II)(mu-biphenyl-4,4'-dicarboxylato)](n)(H2O)(2n) (8), [{LCu(II)(NCS)}(2)(mu-biphenyl-4,4'-dicarboxylato)] (9), [{LCu(OH2)}(2)(mu-trans,trans-2,4-hexadien-1,6-dicarboxylato)](ClO4)(2) (10) (L = 1,4,7-trimethyl-1,4,7-triazacyclononane). Complexes 1, 2, 3, 4, and 6 have been characterized by X-ray crystallography; the structure of 1 has been described previously.([15]) Temperature-dependent magnetic susceptibility data for all complexes have been measured over the range 2.0-298 K. In complexes 2, 3, 4, 6, 8, and 9 only very weak, if any. intermolecular antiferromagnetic or, in the case of 6, ferromagnetic coupling has been observed (\J\ less than or equal to cm(-1); H = - 2JS(1) . S-2, S-1 = S-2 = 1/2). In contrast, in complexes 5 (2 J = - 80 cm(-1)), 7 (2J = - 62 cm(-1)), and 10 (2J = - 100 cm(-1)) relatively strong intramolecular antiferromagnetic exchange coupling has been detected. The intramolecular Cu...Cu separation is 11.3 Angstrom in 5, 15.6 Angstrom in 7, and approximately 12.0 Angstrom in 10. These data demonstrate that intramolecular antiferromagnetic exchange coupling of considerable magnitude is possible over distances >15 Angstrom, provided that the effective magnetic orbitals are favorably aligned.