Hydrothermal syntheses, crystal structures and physical properties of a new family of energetic coordination polymers with nitrogen-rich ligandN-[2-(1H-tetrazol-5-yl)ethyl]glycine

Abstract

Hydrothermal reaction of N-(2-cyanoethyl)glycine and NaN₃ with corresponding metal salts yielded three energetic coordination polymers: [Zn(tzeg)]_n1 and [M(tzeg)(H₂O)]_n (M = Cd²⁺ for 2, Cu²⁺ for 3) (H₂tzeg = N-[2-(1H-tetrazol-5-yl)ethyl]glycine), which were characterized by single-crystal X-ray diffraction. Nitrogen-rich tzeg²⁻ ligand is a new in situ generated organic compound through [2 + 3] cycloaddition reaction of nitrile and azide, which adopts two different coordination modes in 1–3. Polymer 1 features an achiral 3D network built up by interweaving of left/right-handed helical channels. Isostructural 2 and 3 exhibit an achiral 2D network formed alternately by left/right-handed helical chains. As nitrogen-rich energetic materials, 1 and 3 possess higher enthalpies of combustion (ΔH_c), which are −12.054 and −11.583 kJ g⁻¹, respectively, than the classical energetic compounds 1,3,5-trinitro-1,3,5,-triazine (RDX, −9.6 kJ g⁻¹) and 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX, −9.44 to −9.88 kJ g⁻¹). The solid-state excitation and emission spectra for 1–3 were investigated at room temperature, which reveals ligand-centered luminescent emissions in the intensity order of 2 > 1 > 3. Variable-temperature magnetic susceptibility data show that 3 displays a weak antiferromagnetic interaction mainly resulting from magnetic exchange coupling between Cu²⁺ ions through double tetrazolato bridges.