Intramolecular Long-Range Exchange Coupling in Dinuclear Copper(II) Complexes with Cu…Cu Separations Greater than 10Å

Magnetic long-range exchange coupling between two unpaired electrons of two copper(II) ions in a square-pyramidal ligand environment (d)1, which are rigidly separated by > 10 A by an organic spacer ligand (e.g., a μ-dicarboxylato ligand) has been systematically studied. The following complexes have been synthesized: [{LCu(OH2)}2(μ-terephthalato)](ClO4)2 (I), [LCuII(OH2)(benzoato)](CIO4) (2), [{LCuII(NCS)}2(μ-terephthalato)]-2 CH3OH (3), [{LCuII(NCS)}2 (μ-4,4′-oxybisbenzoato)] · 3 CH3OH (4), [{LCuII(OH2)}2 (μ-tetrafluoroterephthalato)](CIO4)2 (5), [{LCuII(OH2)}2{LCuII}2 (μ-tetrafluorobenzoato)3](CIO4)2 · 6 H2O (6), [{LCuII(OH2)}2 (μ-biphenyl-4,4′-dicarboxylato)](CIO4)2 (7), [LCuII(μ-biphenyl-4,4′-dicarboxylato)]n (HZO)2n (8), [{LCuII(NCS)}2 (μ-biphenyl-4.4′-dicarboxylato)] (9), [{LCu(OH2)}2, (μ-trans,trans-2,4-hexadien1,6-dicarboxylato)] (CIO4)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|⩽2cm−1; H= −2JS1 S2 S1 = S2 = 1/2). In contrast, in complexes 5 (2J=−80cm−1), 7(2J=−62cm−1), and 10 (2J = −100 cm−1) relatively strong intramolecular antiferromagnetic exchange coupling has been detected. The intramolecular Cu…Cu separation is 11.3A in 5, 15.6 A in 7, and approximately 12.0A in 10. These data demonstrate that intramolecular antiferromagnetic exchange coupling of considerable magnitude is possible over distances > 15 A, provided that the effective magnetic orbitals are favorably aligned.

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