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   x   2 −y   2) 1 , which are rigidly separated by > 10 Å by an organic spacer ligand (e.g., a μ‐dicarboxylato ligand) has been systematically studied. The following complexes have been synthesized: [{LCu(OH 2 )} 2 (μ‐terephthalato)](ClO 4 ) 2 (I), [LCu II (OH 2 )(benzoato)](CIO 4 ) ( 2 ), [{LCu II (NCS)} 2 (μ‐terephthalato)]‐2 CH 3 OH ( 3 ), [{LCu II (NCS)} 2 (μ‐4,4′‐oxybisbenzoato)] · 3 CH 3 OH ( 4 ), [{LCu II (OH 2 )} 2 (μ‐tetrafluoroterephthalato)](CIO 4 ) 2 ( 5 ), [{LCu II (OH 2 )} 2 {LCu II } 2 (μ‐tetrafluorobenzoato) 3 ](CIO 4 ) 2 · 6 H 2 O ( 6 ), [{LCu II (OH 2 )} 2 (μ‐biphenyl‐4,4′‐dicarboxylato)](CIO 4 ) 2 ( 7 ), [LCu II (μ‐biphenyl‐4,4′‐dicarboxylato)] n (H Z O) 2n ( 8 ), [{LCu II (NCS)} 2 (μ‐biphenyl‐4.4′‐dicarboxylato)] ( 9 ), [{LCu(OH 2 )} 2 , (μ‐ trans , trans ‐2,4‐hexadien1,6‐dicarboxylato)] (CIO 4 ) 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 = −2 J S 1 S 2 S 1 = S 2 = 1/2). In contrast, in complexes 5 (2 J =−80cm −1 ), 7 (2 J =−62cm −1 ), and 10 (2 J = −100 cm −1 ) relatively strong intramolecular antiferromagnetic exchange coupling has been detected. The intramolecular Cu…Cu separation is 11.3Å in 5 , 15.6 Å in 7 , and approximately 12.0Å in 10 . These data demonstrate that intramolecular antiferromagnetic exchange coupling of considerable magnitude is possible over distances > 15 Å, provided that the effective magnetic orbitals are favorably aligned.