Copper(II) Azide Complexes of Aliphatic and Aromatic Amine Based Tridentate Ligands: Novel Structure, Spectroscopy, and Magnetic Properties

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 Jg = 90.73 cm-1, the other coupling of Je = −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.