In situ synthesis of 5-substituted-tetrazoles and metallosupramolecular co-ordination polymers

Abstract

A series of metal–organic supramolecular co-ordination polymers [Zn(HPO₄)(Hpytz)] (1), [Cd(Hpytz)₂(H₂PO₄)₂(H₂O)₂] (2), [Zn(pytz)₂(H₂O)₄]·2H₂O (3) [pytz⁻ = 5-(4-pyridyl)tetrazolate], [M(tzpyo)₂(H₂O)₄] [M = Zn (4), Ni (5)] [tzpyo⁻ = 4-tetrazolate-pyridine N-oxide] incorporating tetrazolyl ligands have been synthesized via in situ [2 + 3] cycloaddition reactions of 4-cyanopyridine, 4-cyanopyridine N-oxide with sodium azide in the presence of zinc, cadmium or nickel salts under hydrothermal reaction conditions at 130–160 °C. Use of higher temperatures leads to hydrolysis of nitriles to carboxylates and subsequently to decarboxylation, resulting in two unexpected byproducts, [inaH]⁺(NO₃⁻) (6) [ina = isonicotinic acid] and [(4,4′-bipyH₂)(H₂PO₄)₂(H₂O)₂] (7). Single crystal X-ray structure analyses of 1 and 2, prepared under hydrothermal conditions in the presence of H₃PO₄, reveal κ₁-co-ordination of the N-donor at the 2-position of the tetrazolyl ligands binding in a TZ-mode. The pyridyl N-centres in 1 and 2 are protonated resulting in the formation of N_py–H⋯O hydrogen-bonds within the solid-state structures. In the absence of H₃PO₄ similar hydrothermal reactions afforded 3, in which the pyridyl group is directly bound to Zn(II) and tetrazolate-ring is unco-ordinated (PY-mode). Single crystal X-ray structures of 4 and 5 show a TZ-binding mode analogous to that for 1 and 2, with the pyridyl N-oxide moiety forming hydrogen-bonds with co-ordinated water molecules to give supramolecular pillar-layered frameworks. Complex 2 exhibits a distorted 6-connected α-Po net of 4¹²·6³ topology; 3 possesses an attractive 3D (4,5,10)-connected trinary (3·4²·5³)₂(3·4⁴·5⁴·6)₄(3²·4⁸·5¹⁴.6¹²·7⁷·8²) net, while the isostructural complexes 4 and 5 show 3D (4,8)-connected layer-pillared network structures with a unique (4⁶)₂(4¹²·6¹²·8⁴) topology. In 6, all hydrogen atoms of inaH⁺ cations are involved in the formation of hydrogen-bonds with either nitrate or other inaH⁺ cations resulting in a 2D (4,6)-connected binary net with (3²·4²·5²)(3⁴·4⁴·5⁴·6³) topology. In 7, the dihydrogenphosphate anions are arranged viahydrogen-bonds into an unusual (4,4) grid-type sheet, and these layers are further pillared by organobipyridine cations to form a 3D open framework with channels occupied by helical chains of water dimers. The thermal stability of these metal-tetrazolate complexes has been studied by TGA and powder X-ray diffraction, and the effect of proton source (H₃PO₄) on the co-ordination mode of these tetrazolyl ligands is discussed.