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

Abstract

Theoretical studies of proposed structures for NaN 5 , KN 5 , Mg(N 5 ) 2 , Ca(N 5 ) 2 , and Zn(N 5 ) 2 metal complexed pentazole anions have been carried out with the RHF, MP2, MCSCF, and DFT theoretical methods. Additional DFT calculations were performed on MgN 5 Cl, CaN 5 Cl, and ZnN 5 Cl 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 (E a 5-10 kcal mol Ϫ1 ). N atom chemical shifts relative to NH 3 and nitromethane were calculated for each species using the lowest energy configuration and the B3LYP//6-311ϩϩG(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 15 N NMR shifts were within 20 ppm of experiment. Time dependent B3LYP/6-31G(d) and B3LYP/6-311ϩG(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.

First Generation of Pentazole (HN5, Pentazolic Acid), the Final Azole, and a Zinc Pentazolate Salt in Solution: A New N‐Dearylation of 1‐(p‐Methoxyphenyl) Pyrazoles, a 2‐(p‐Methoxyphenyl) Tetrazole and Application of the Methodology to 1‐(p‐Methoxyphenyl) Pentazole.

“…The calculated 15 N shift for Zn(N 5 )Cl is 216.6ppm. 13 This is an average of the five values reported for the 15 N shifts. Geometry optimization of Zn(N 5 )NO 3 using the same procedure 13 leads to the unidentate, planar configuration and an average 15 N shift of 217.5 ppm.…”

“…13 This is an average of the five values reported for the 15 N shifts. Geometry optimization of Zn(N 5 )NO 3 using the same procedure 13 leads to the unidentate, planar configuration and an average 15 N shift of 217.5 ppm. The spread in the signal position, 210.0 + 2.0 ppm in different runs, may be due to the mix of ions present in the solution.…”

“…An analysis of LCAO-MO coefficients showed considerable interaction between Zn d orbitals and the N 5 2 lone pairs and p MOs. 13 A Mulliken population analysis indicated much more of a covalent interaction between Zn and N 5 2 than the other metals. The Mulliken charges on the metal calculated with the basis set used in our study are Na +0.805, K +0.914 for MN 5 , and Mg +0.756, Ca +0.869, Zn +0.300 for M(N 5 )Cl.…”

“…[8][9][10][11][12] Metallocene pentazole derivatives have been predicted as viable forms of N 5 2 , but bidentate forms MN 5 (M = Na, K, MgCl + , CaCl + ) and M(N 5 ) 2 (M = Na, K, Mg, Ca) and unidentate Zn(N 5 )Cl and Zn(N 5 ) 2 are calculated to have lower energy. 13 Recently it has been suggested the N 5 + .N 5 2 is a viable entity. 14 Following our recent establishment of the mechanism of formation of aryl pentazoles from aryldiazonium ions and azide ion, 15 we have been endeavouring to generate HN 5 and its anion N 5 2 .…”

“…A theoretical study was carried out 13 to determine the 15 N NMR shift expected for N 5 2 and to identify the best cation to retain it, which proved to be Zn 2+ . An analysis of LCAO-MO coefficients showed considerable interaction between Zn d orbitals and the N 5 2 lone pairs and p MOs.…”