Chain of dimers to assembly of trimers: temperature and ligand influenced formation of novel supramolecular assemblies of Cu(ii) with isomeric (aminomethyl) pyridines and azide

Abstract

Five new complexes, viz., [Ni(2-aminomethylpyridine)₂(N₃)₂] (1), [Cu(2-aminomethylpyridine)(N₃)₂]_n (2), [Cu(2-aminomethylpyridine)₂(N₃)₂] (3), [Cu₃(3-aminomethylpyridine)₂(N₃)₆]_n (4) and [Zn(2-aminomethylpyridine)₂(N₃)₂] (5), were synthesized and structurally characterized to probe the role of temperature as well as ligand geometry in the formation of novel supramolecular assemblies. The influential and preferential role of temperature as well as ligand geometry is observed in the supramolecular structure formation of Cu(II) complexes alone, with isomeric (aminomethyl)pyridines and azide. With 2-amp as the ligand, temperature greatly influences the product formation, resulting in thermodynamically stable 2 at RT and kinetically stable 3 at LTs. However, such a temperature influence is not evident with the analogous 3-amp derivative. Interestingly, the change in ligand geometry from 2-amp to 3-amp has resulted in drastic changes in the final supramolecular structure formation. The change in ligand geometry from chelating in 2-amp, to bridging in 3-amp leads to a 1-D polymeric chain of dimers bridged by asymmetric end-on azides in 2 to a 1-D assembly of trimers bridged by symmetric end-on azides in 4. The change in the ligand geometry appears to be responsible for the changes in the metal ion's geometrical preferences from square pyramidal in 2 to simultaneous octahedral and square planar in 4, assisted by a change in azide's bridging modes. Magnetic studies on both 2 and 4 revealed antiferromagnetic Cu–Cu interactions with J = −2.26 cm⁻¹ (H = −JS₁S₂) and −7.5 cm⁻¹ (H = −J(S₁S₂ + S₂S₃)) respectively. While the observed AFM interactions in 4 are in accordance with the theoretical predictions based on the crossover in the magnetic property from FM to AFM at α > 104°, in asymmetric end-on azide bridged dimers of transition metal complexes, the magnetic interactions in 2 are contrary to the predictions. The results once again emphasize that the predictions are valid only for the basal–basal mode of azide bridging, but not for basal–apical mode of bridging in square pyramidal complexes.