Copper(II) azide complexes of aliphatic and aromatic amine based tridentate ligands: novel structure, spectroscopy, and magnetic properties

Abstract

Copper(II) azide complexes of three tridentate ligands namely 2,6-(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L), 2,6-(pyrazol-1-ylmethyl)pyridine (L'), and dipropylenetriamine (dpt) yield three kinds of complexes with different azide-binding modes. The ligand L forms two end-on-end (µ -1,3) diazido-bridged binuclear complexes, [CuL(µ-N₃)]₂(ClO₄)₂ (1) and [CuL(µ-N₃)(ClO₄)]₂·2CH₃CN (2), and L' forms a perchlorato-bridged quasi-one-dimensional chain complex, [CuL'(N₃)(ClO₄)]_n (3) with monodentate azide coordination. The ligation of dipropylenetriamine (dpt) gives a end-on (µ-1,1) diazido-bridged binuclear copper complex [Cu(dpt)(µ-N₃)]₂(ClO₄)₂ (4). The crystal and molecular structures of these complexes have been solved. Variable-temperature EPR results of 1 and 2 are identical and indicate the presence of both ferromagnetic and antiferromagnetic interactions within the dimer, the former dominating at low temperatures and the latter at high temperatures. The unusual temperature-dependent magnetic moment and EPR spectra of this dimer reveal the presence of temperature-dependent population of two triplet states, one being caused by antiferromagnetic and the other by ferromagnetic interaction, the former transforming to the latter on cooling. While the interaction of ground spin doublets of the two metal centers gives rise to a ferromagnetic coupling of J_g = 90.73 cm^-1, the other coupling of J_e = -185.64 cm^-1 is suggested to be caused by the interaction between an electron in one metal center and an electron from the azide of the other monomer by excitation of a d-electron to the empty ligand orbital. The ferromagnetic state is energetically favored by 104.39 cm^-1. Compound 3 exhibits axial spectra at room temperature and 77 K, and variable-temperature magnetic susceptibility data indicate that the copper centers form a weakly antiferromagnetic one-dimensional chain with J = -0.11 cm^-1. In the case of 4, the unique presence of two nonidentical dimeric units with different bond lengths and bond angles within the unit cell as inferred by crystal structure is proved by single-crystal EPR spectroscopy.