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]+.