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,viaN2release,
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, and3e(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. Compound2bis 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]+.