Tuning the nature of the exchange interaction in out-of-plane oximato-bridged dinuclear copper(II) complexes

The dinuclear copper(II) complexes of formula [{Cu(Hdmg)(2)}(2)] 1, [{Cu(Hbdmg)}(2)][ClO4](2) 2, [{Cu(Hdeg)(2)}(2)] 3 and [{Cu(Hchd)(2)}(2)] 4 (H(2)dmg, H(2)bdmg, H(2)deg and H(2)chd = dimethylglyoxime, 3,10-dimethyl-4,9-diazadodeca-3,9-diene-2, 11-dione dioxime, diethylglyoxime and cyclohexane-l,2-dione dioxime) have been synthesized. The structures of 3 and 4 have been determined by single-crystal X-ray diffraction methods. Both consist of. centrosymmetric dinuclear bis(alkyl)glyoximatocopper(II) entities where the units are staggered so that the copper atom of one unit is directly opposite to the oxime-oxygen atom of the other, as previously found for 1 and 2. Each metal atom in 3 and 4 is five-co-ordinate with four imine nitrogen atoms comprising the basal plane and an oximate-oxygen atom in the apical position. The copper-imine nitrogen bond lengths (average 1.957 and 1.953 Angstrom. in 3 and 4, respectively) are shorter than that of the axial copper-oximate oxygen [2.263(3) (3) and 2.242(3) Angstrom (4)]. An oxime proton is lost from the ligand in the complex formation, the remaining oxime proton being involved in a hydrogen bond between the peripheral oxime oxygens of the same bis(alkyl)glyoximatocopper(II) unit. The intramolecular copper-copper separation is 3.898(1) (3)and 3.825(1) Angstrom (4). Variable-temperature magnetic susceptibility measurements showed the occurrence of intramolecular ferro-(1, 3 and 4) and antiferro-magnetic exchange interactions (2), the singlet-triplet energy gap J being +9.1 (1), -1.9 (2), +1.0 (3) and +3.1 cm(-1) (4). The analysis of the exchange pathway through the out-of-plane oximato bond in;this family has been substantiated by extended-Huckel calculations-and a quasi-linear correlation between the value of J and the angle at the Cu-O-N (alpha) has been found. The influence of the size of the imine-carbon alkyl substituents on both the nature and magnitude of J is discussed in the light of the available structural information.