Synthesis, crystal structure and properties of trigonal bipyramidal [M(L5)2(H2O)]·H2O complexes [M = cobalt(II) (S = 3/2) or copper(II) (S = 1/2); HL5 = N-(2-chloro-6-methylphenyl)pyridine-2-carboxamide

Abstract

Using a bidentate ligand N-(2-chloro-6-methylphenyl)pyridine-2-carboxamide (HL5), in its deprotonated form, two new five-co-ordinate complexes of composition [M(L5)2(H2O)]·H2O (M = CoII 1 or CuII 2) have been prepared and characterized including X-ray crystallography. The co-ordination geometry at CoII and CuII is approximately trigonal bipyramidal (two deprotonated amide nitrogens and a water molecule in the equatorial plane and two pyridines in the axial positions), being more distorted in the case of CuII. The observed distortion is caused by (i) a small bite angle of the chelating ligand and (ii) the presence of two ortho substituents, a chloro and a methyl group, on the phenyl ring (steric effect). To the best of our knowledge, 1 represents the first structurally characterized mononuclear high-spin cobalt(II) complex with a pyridine amide ligand. The magnetic moments of 1 and 2 at 300 K reveal that the compounds are paramagnetic (1 has S = 3/2 and 2 has S = 1/2), both as solids and in dmf solution. Temperature dependent magnetic susceptibility measurements confirmed their spin state. The stereochemistry of the cobalt(II) centre in 1 does not change to any measureable extent on dissolution in dmf (cf. solid and solution state absorption spectra). The geometry of the copper(II) centre in 2 observed in the solid state is not retained in dmf solution (absorption spectra), changing to a tetragonal stereochemistry. Cyclic voltammetric measurements (dmf solution; glassy carbon electrode) on 1 reveal an oxidative response at 0.48 V vs. saturated calomel electrode (SCE) and a reductive response at –1.66 V corresponding to CoIII–CoII and CoII–CoI redox couples, respectively. For 2 the CuII–CuI process was observed at –0.53 V vs. SCE.