4-Substituted and 4,5-Disubstituted 3,6-Di(2-pyridyl)pyridazines: Ligands for Supramolecular Assemblies

Triazine Chalcogenones from Thiocyanate or Selenocyanate Addition to Tetrazine Ligands in Ruthenium Arene Complexes

The chemistry of 1,2,4,5-tetrazines has attracted considerable interest both from a synthetic and applicative standpoint. Recently, regioselective reactions with alkynes and alkenes have been reported to be favored once the tetrazine ring is coordinated to Re(I), Ru(II), and Ir(III) centers. Aiming to further explore the effects of metal coordination, herein, we unveil the unexplored reactivity of tetrazines with chalcogenocyanate anions. Thus, ruthenium(II) tetrazine complexes, [RuCl{κ²N-3-(2-pyridyl)-6-R-1,2,4,5-tetrazine}(η⁶-arene)]⁺ (arene = p-cymene, R = H, [1a]⁺, R = Me, [1b]⁺, R = 2-pyridyl, [1c]⁺; arene = C₆Me₆, R = H, [1d]⁺, R = Me, [1e]⁺; PF₆^- salts), reacted quantitatively and in mild conditions with M(ECN) salts (M = Na, K, Bu₄N; E = O, S, Se). The addition of thiocyanate or selenocyanate to the tetrazine ligand is regioselective and afforded, via N₂ release, 1,2,4-triazine-5-chalcogenone heterocycles, the one with selenium being unprecedented. The novel ruthenium complexes [RuCl{κ²N-(2-pyridyl)}{triazine chalcogenone}(η⁶-arene)] 2a-e (sulfur), 3b, 3d, and 3e (selenium) were characterized by analytical (CHNS analyses, conductivity), spectroscopic (IR, multinuclear and two-dimensional (2D) NMR), and spectrometric (electrospray ionization mass spectrometry (ESI-MS)) techniques. According to density functional theory (DFT) calculations, the nucleophilic attack of SCN^- on the tetrazine ring is kinetically driven. Compound 2b is selectively and reversibly mono-protonated on the triazine ring by HCl or other strong acids, affording a single tautomer. When reactions of chalcogenocyanates were performed on the 2,2'-bipyridine (bpy) complex [RuCl(bpy)(η⁶-p-cymene)]⁺, the chloride substitution products [Ru(ECN)(bpy)(η⁶-p-cymene)]⁺ (E = O, [4]⁺; E = S, [5]⁺; E = Se, [6]⁺) were obtained in 82-90% yields (PF₆^- salts). Combined spectroscopic data (IR, ¹H/¹³C/⁷⁷Se NMR) was revealed to be a useful tool to study the linkage isomerism of the chalcogenocyanate ligand in [4-6]⁺.

Autoxidation of 4-Hydrazinylquinolin-2(1H)-one; Synthesis of Pyridazino[4,3-c:5,6-c']diquinoline-6,7(5H,8H)-diones

An efficient synthesis of a series of pyridazino[4,3-c:5,6-c']diquinolines was achieved via the autoxidation of 4-hydrazinylquinolin-2(1H)-ones. IR, NMR (¹H and ¹³C), mass spectral data, and elemental analysis were used to fit and elucidate the structures of the newly synthesized compounds. X-ray structure analysis and theoretical calculations unequivocally proved the formation of the structure. The possible mechanism for the reaction is also discussed.

Crystal structure of 3,6-di-2-pyridinyl-4-pyridazine carbonitrile, C15H9N5

C15H9N5, monoclinic, P21/c (no. 14), a = 8.9817(4) Å, b = 11.5791(19) Å, c = 12.065(2) Å, β = 102.289(4)°, V = 1226.03(3) Å3, Z = 4, R gt (F) = 0.0533, wR ref (F 2 ) = 0.1512, T = 173(2) K.

4-Substituted 3,6-Bis(2-pyridyl)pyridazines: Silver(I) Complexes of 4-Cyano- and 4-(4-Bromophenyl)-3,6-bis(2-pyridyl)pyridazine and Pseudopolymorphs of 1,3,5-Tris{3,6-bis(2-pyridyl)pyridazin-4-yl}benzene

Metal:ligand complexes are crystallized from reaction mixtures of equimolar amounts of 4-cyano-3,6-bis(2-pyridyl)pyridazine 2 or 4-(4-bromophenyl)-3,6-bis(2-pyridyl)pyridazine 3 and silver(i) triflate. In [Ag2(2)2]2+, the two ligands adopt a head-to-tail arrangement, while a head-to-head motif is confirmed for the solid state structure of [Ag2(3)2]2+. In solution, one ligand environment is observed in each case. Silver(i) reacts with 1,3,5-tris{3,6-bis(2-pyridyl)pyridazin-4-yl}benzene 4 to give highly insoluble powders. The single crystal structures of the pseudopolymorphs (4)·Et2O and 2(4)·2MeCN·H2O are reported; in each structure, the ligand adopts the same conformation, derived from a Cs rather than C3v model structure.