Crystal structures and magnetism of binuclear copper(II) complexes with alkoxide bridges. Importance of orbital complementarity in spin coupling through two different bridging groups

Abstract

Binuclear copper(II) complexes, [Cu₂(L¹)(O₂CMe)]·H₂O (1), [Cu₂(L⁵)(O₂CMe)]·MeOH (2), [Cu₂(L⁵)(O₂CPh)]·H₂O (3), [Cu₂(L³)(O₂CMe)](4), [Cu₂(L²)(O₂CMe)](5), and [Cu₂(L⁴)(OMe)(MeOH)](6) were prepared, where the ligands H₃Lⁿ are 1 : 2 Schiff bases derived from 1,3-diaminopropan-2-ol and the carbonyl compounds acetylacetone (for L¹), benzoylacetone (for L²), 3-ethoxymethylenepentane-2,4-dione (for L³), methyl acetoacetate (for L⁴), and salicylaldehyde (for L⁵). The crystal structures of (1)–(4) and (6) were determined by X-ray analysis. The results revealed that all the complexes are binuclear and bridged by alkoxide and carboxylate oxygens except for (6) in which the exogenous bridging group comprises two hydrogen-bridged methoxide ions. In all complexes co-ordination geometries are essentially planar, and the angle formed by the two co-ordination planes falls in the range 5–19° for (1)–(3) and (6), whereas the angle in (4) is 54.6° indicating a large distortion from a coplanar structure. Antiferromagnetic coupling is strong for (6)(–2J= 635 cm^–1) as expected from its large Cu–O–Cu angle (137.7°). However, the other complexes showed much lower antiferromagnetism though their Cu–O–Cu angles differ little from that of (6). The low antiferromagnetism of (1)–(5) was rationalised in terms of countercomplementarity of the orbitals between two bridging groups which participate in the superexchange interaction.