Theoretical characterization of pentazole anion with metal counter ions. Calculated and experimental 15N shifts of aryldiazonium, -azide and -pentazole systems†

Theoretical studies of proposed structures for NaN5, KN5, Mg(N5)2, Ca(N5)2, and Zn(N5)2 metal complexed pentazole anions have been carried out with the RHF, MP2, MCSCF, and DFT theoretical methods. Additional DFT calculations were performed on MgN5Cl, CaN5Cl, and ZnN5Cl pentazoles. The structures considered are unidentate I, bidentate II, and metallocene-like III. For Mg, Na, K, and Ca pentazoles at every level of theory, II is the most energetically favoured, followed by I, then III. Complex I is preferred with Zn complexes due to favourable d orbital interactions. For double ring complexes only II (I for Zn) with perpendicular rings has all positive vibrational frequencies. For single ring complexes, both II (I for Zn) and III have all positive vibrations. Structure I (II for Zn) is a transition state structure for metal ion rotation around the ring (Ea 5–10 kcal mol−1). N atom chemical shifts relative to NH3 and nitromethane were calculated for each species using the lowest energy configuration and the B3LYP//6-311++G(2d,p) method on the B3LYP//6-31G(d) optimised geometry. Additional calculations were done for 1-arylpentazoles, 1-arylpentazene, aryl azides, and aryldiazonium ions. Calculated 15N NMR shifts were within 20 ppm of experiment. Time dependent B3LYP/6-31G(d) and B3LYP/6-311+G(d) calculations were performed on all stable species. All 1(π,π) transitions were calculated to be below 180 nm, while the 1(n,π) transitions were below 210 nm. The lowest energy transitions are from the lone pairs to the empty metal s orbital. For Mg and Zn these transitions are at ∼220 nm. For Na, Ca and K the transitions are considerably lower in energy, ∼250 nm.