Phenoxo-bridged dinickel(II) complexes of a macrocyclic ligand: synthesis, stereochemical equilibria and structure

Abstract

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 ⇌[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 P2₁/n, with a= 9.251(1), b= 11.457(2), c= 18.555(2)Å, β= 100.21(1)°, 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)Å.