PHENOXO-BRIDGED DINICKEL(II) COMPLEXES OF A MACROCYCLIC LIGAND - SYNTHESIS, STEREOCHEMICAL EQUILIBRIA AND STRUCTURE

Dinuclear nickel(II) complexes of a tetraaminediphenol macrocyclic ligand (H2L) of composition [Ni2L(B)2][ClO4]2 (B = NH3, imidazole, pyridine, or pyrazine) have been synthesised and characterized. With pyrazole (Hpz) [Ni2L(pz)(H2O)2][ClO4] and [Ni2L(pz)(Hpz)(H2O)][ClO4] complexes have been obtained in which pz- appears to act as a bridging ligand. The equilibrium constants for the reaction [Ni2L(MeOH)4]2+ + 2B half-arrow-right-over-half-arrow-left [Ni2L(B)2]2+ + 4MeOH [B = pyridine, 2-methylpyridine (2Me-py), 3Me-py, 4Me-py, imidazole, or pyrazine] in methanol solutions have been determined spectrophotometrically at room temperature. The X-ray structure of [Ni2L(py)2][ClO4]2 has been determined: monoclinic, space group P2(1)/n, with a = 9.251(1), b = 11.457(2), c = 18.555(2) angstrom, beta = 100.21(1)-degrees, and Z = 2. The structure was determined by direct methods and refined to R = 0.053 and R' = 0.039. In the complex cation the two nickel(II) centres are in square-pyramidal configuration with N2O2 in-plane donor sets, involving two phenoxide bridges and two trans-axially disposed pyridines. The distance between the two nickel centres is 3.206(5) angstrom.