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

Abstract

The dinuclear copper(II) complexes of formula [{Cu(Hdmg) ₂ } ₂ ] 1, [{Cu(Hbdmg)} ₂ ][ClO ₄ ] 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-1,2-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 Å in 3 and 4, respectively) are shorter than that of the axial copper–oximate oxygen [2.263(3) (3) and 2.242(3) Å (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) Å (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-Hückel calculations and a quasi-linear correlation between the value of J and the angle at the Cu–O–N (α) 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.