Synthesis, structures and magnetochemistry of binuclear cobalt(II), nickel(II) and copper(II) complexes of 2,6-diformyl-4-methylphenol dioxime

Reaction of 2,6-diformyl- and 2,6-diacetyl-4-methylphenol with a large excess of both NH2OH . HCl and CH3CO2K in EtOH affords high yields of 2,6-diformyl-4-methylphenol dioxime (2-hydroxy-5-methylbenzenedicarbaldehyde dioxime) (H3L1) and 2,6-diacetyl-4-methylphenol dioxime (H3L2), respectively. The crystal structure of (H3L2) shows intramolecular hydrogen bonding with long-range intermolecular pi-stacking interactions and an extended intermolecular hydrogen-bonding network. The binuclear complexes of Co-II, Ni-II and Cu-II-[Co-2(H2L1)(2)(MeOH)(2)(H2O)(2)]Cl-2. 2MeOH, [Ni-2(H2L1)(2)(H2O)(4)][ClO4](2). 2H(2)O and [Cu-2(H2L1)(2)(ClO4)(2)] respectively-derived from the dioxime ligand (H3L1) have been synthesized and characterised and their single-crystal structures determined. The structure of [Co-2(H2L1)(2)(MeOH)(2)(H2O)(2)](2+) shows each high-spin Co-II to be six-co-ordinate and bound to an N2O4-donor array presented by two dioxime ligands and axially co-ordinated H2O and MeOH molecules, the dioxime ligands co-ordinating via the imino N- and phenoxy O-donors. The structure of [Ni-2(H2L1)(2)(H2O)(4)](2+) shows two octahedrally co-ordinated Ni-II each with an N2O4 donor set similar to that in [Co-2(H2L1)(2)(MeOH)(2)(H2O)(2)](2+) except that the co-ordination sphere of each Ni-II is completed by axial ligation to two H2O molecules. The structure of [Cu-2(H2L1)(2)(ClO4)(2)] confirms N2O4 donation at CUII With two bidentate ClO4- anions, Cu ... O 2.51(2), 2.76(2) Angstrom, interacting with the metal centres on either side of the planar oxime-phenolate array. In all three complexes the two dioxime ligands are monodeprotonated at the phenolic oxygen, and the oximes are linked by hydrogen bonds, which results in a pseudo-macrocyclic framework. Magnetic susceptibility measurements on the complexes over the range 2.5-340 K confirm that the complexes are antiferromagnetically coupled with values for the magnetic exchange constant J of -6.9 +/- 0.1, -16.0 +/- 0.6, and -452 +/- 4 cm(-1) for [Co-2(H2L1)(2)(MeOH)(2)(H2O)(2)]Cl-2, [Ni-2(H2L1)(2)(H2O)(4)][ClO4](2) and [Cu-2(H2L1)(2)(ClO4)(2)] respectively.