Synthesis, structure and characterization of neutral coordination polymers of 5,5′-bistetrazole with copper(II), zinc(II) and cadmium(II): A new route to reconcile oxygen balance and nitrogen content of high-energy MOFs

Hydrothermal reactions of Cu(ii)/Zn(ii)/Cd(ii) with 5,5′-bistetrazole (H₂BT) lead to three new energetic coordination polymers: [CuBT(H₂O)]_n (1), [ZnBT(H₂O)₂]_n (2), and [CdBT(H₂O)₂]_n (3). These crystal structures were determined by single X-ray diffraction. Compound 1 forms regular and compact 3-D frameworks and compounds 2-3 are 1-D chain structures. These compounds show prominent thermostability (T_dec = 349.1 °C for 1, 334.8 °C for 2, and 394.2 °C for 3) investigated by using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Sensitivities towards impact and friction were measured. Compound 1 is sensitive to both impact and friction (100% explosion under the test conditions), while compounds 2-3 are sensitive to neither (0% explosion under the test conditions). The heats of detonation (ΔH_det) of 1-3 were calculated based on density functional theory (DFT). Compound 1 possesses the highest calculated ΔH_det (26.7267 kJ g^-1) among the reported energetic MOFs. Moreover, compared with the reported energetic MOFs, compound 1 also has a good balance of high nitrogen content (51.46%) and high oxygen balance (-36.76%) as well as a very high crystal density of 2.505 g cm^-3.