In Situ Solvothermal Generation of 1,2,4-Triazolates and Related Compounds from Organonitrile and Hydrazine Hydrate: A Mechanism Study

The facile and effective one-pot solvothermal syntheses of 3,5-disubstituted 1,2,4-triazole and its derivatives (substituted group = alkyl, aryl, and pyridyl) through cyclocondensations of organonitriles and hydrazine hydrate in the absence/presence of metal salts have been established. By control of the solvothermal conditions and/or the addition of counteranions, different intermediates and final products were derived from various organonitriles, in which an intermediate N,N'-bis(picolinamide)azine (H4bpa) has been successfully trapped in its neutral manganese(II) complexes. A systematical study shows that, after the initial formation of 2-pyridylamidrazone from 2-cyanopyridine and hydrazine, two reaction paths are involved in the formation of 1,2,4-triazoles: via the formation of 3,6-bis(2-pyridyl)-1,2-dihydro-1,2,4,5-tetrazine and H4bpa as intermediates. The tetrazine and H4bpa paths are preferred in the absence and presence of metal ions, respectively. In the presence of metal ions, metal ion binding can stabilize the tautomers, enhance the nucleophilic reactivity of the imino C atom, and inhibit the tautomerization of H4bpa, hence leading to the formation of 1,2,4-triazolates or H4bpa in complexed forms. The in situ cyclocondensation reactions of 2-pyridylamidrazone and carboxylate into asymmetric 3,5-disubstituted 1,2,4-triazolates under solvothermal conditions have also been observed for the first time. Crystal structures of the crystalline metal complexes have been obtained, including dinuclear [Mn2(bpt)2Cl2(H2O)2] (1) and [Mn2(bpt)2(SCN)2(H2O)2] (3; Hbpt = 3,5-bis(2-pyridyl)-4H-1,2,4-triazole), tetranuclear [Mn4(H3bpa)2(mpt)4(N3)2].2H2O (5; Hmpt = 3-methyl-5-(2-pyridyl)-1H-1,2,4-triazole), [Mn4(H3bpa)2(pt)4(N3)2].2C2H5OH (6; Hpt = 5-(2-pyridyl)-1H-1,2,4-triazole), and [Mn4(H3bpa)4(SCN)4].2C2H5OH (7), as well as helical [Cu(bpt)] (2). Among them, 7 is the first example of a neutral tetranuclear [2 x 2] grid manganese(II) complex. Both 5 and 7 exhibit antiferromagnetic interactions.