Five-coordinate cobalt(II) complexes of tris(2-pyridylmethyl)amine (TPA): Synthesis, structural and magnetic characterization of a terephthalato-bridged dinuclear cobalt(II) complex

Abstract

The cobalt(II) complexes [Co(TPA)Cl]ClO 4 ( 1), [Co(TPA)Br]ClO 4 ( 2), [Co(TPA)(H 2O)]Cl(ClO 4) ( 3) and [Co 2(TPA) 2(μ-tp)](ClO 4) 2 · 2H 2O ( 4) (TPA = tris(2-methylpyridyl)amine and tp = terephthalate dianion) were synthesized and structurally characterized by UV–vis and IR spectroscopy. The molecular structures of complexes 1 and 4 were determined by X-ray crystallography and their magnetic properties were measured over the temperature range 2–300 K. The coordination geometry around the central Co(II) in these compounds has a distorted trigonal bipyamidal geometry with four nitrogen atoms from the TPA ligand and the fifth coordination site is occupied by Cl − ion in 1, Br − ion in 2, coordinated oxygen atom from H 2O in 3 and by an oxygen atom supplied by the carboxylate group of the bridged terephthalato ligand in 4. The visible spectra of the complexes 1– 3 in MeOH show strong distortion toward tetrahedral geometry. For complex 4, analysis of the infrared spectral data for the ν(COO −) stretching frequencies of the tp-carboxalato groups reveals the existence of the bis(monodentate) coordination mode for the bridged tp. X-ray data for 1 and 4 show that the former is mononuclear while the latter is dinuclear. The electronic spectrum of 4 in MeOH is in complete agreement with the assigned X-ray geometry around the Co(II) centers. The magnetic behavior of the mononuclear complex 1 is indicative of a high-spin compound with zero-field splitting. The best fit was obtained with ∣ D∣ = 7.3 cm −1, g = 2.25. The dinuclear complex 4 exhibits weak antiferromagnetic coupling with a coupling constant J = −0.8 cm −1. The magnetic properties and the structural parameters of 4 are discussed in relation to the other related μ-terephthalato dinuclear Co(II) compounds. The geometry of the coordination sphere around 4 is unique – the CSD compilation listing only one other compound with such a geometry around the dinuclear Co(II) complex and its composition is far different from that in 4. However, they share a common feature of having a weakly antiferromagnetic coupling between Co(II) centers.