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]+.
