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

Abstract

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{κ2 N-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{κ2 N-(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]+.

N^N^C-Cyclometalated rhodium(iii) complexes with isomeric pyrimidine-based ligands: unveiling the impact of isomerism on structural motifs, luminescence and cytotoxicity

“…the ability of compounds with identical molecular formulae to exist in spatially different forms is one of the most fundamental ideas in chemistry. [1][2][3] Revealing the impact of isomerism on crystal structures, [4][5][6][7][8][9][10][11][12] and chemical properties, [13][14][15][16][17][18][19][20][21] as well as physical [22][23][24][25][26][27][28][29][30][31][32][33][34] and biological properties [35][36][37][38][39] is of particular importance for establishing structure-property relationships which pave the way toward the more predictable design of substances with desirable properties. Despite various kinds of constitutional and configurational isomerism of metal complexes being well known, 40 ligand isomerism, which belongs to constitutional isomerism, is documented in literature to a considerably lesser extent than other types of the isomerism of complexes.…”