Synthesis, crystal structure and properties of trigonal bipyramidal [M(L⁵)₂(H₂O)]·H₂O complexes [M = cobalt(II) (S = 3/2) or copper(II) (S =½); 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 = Co^II 1 or Cu^II 2) have been prepared and characterized including X-ray crystallography. The co-ordination geometry at Co^II and Cu^II 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 Cu^II. 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 =½), 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 Co^III-Co^II and Co^II-Co^I redox couples, respectively. For 2 the Cu^II-Cu^I process was observed at -0.53 V vs. SCE.