[(η-Cymene)[3-(pyrid-2-yl)-1,2,4,5-tetrazine]chlororuthenium(II)], Redox Noninnocence and Dienophile Addition to Coordinated Tetrazine.

The bidentate ligand 3-(pyrid-2-yl)-1,2,4,5-tetrazine (TzPy) coordinated in the complex [CyRuCl(TzPy)]PF ([]; Cy = η--cymene) shows noninnocent behavior and can be modified through the addition of dienophiles, vinylferrocene (ViFc) or ethynylferrocene (EthFc). The kinetics and transition-state thermodynamic analysis of the reaction of [] + ViFc found Δ(298 K) = 67 kJ mol, while that of [] + EthFc was Δ(298 K) = 83 kJ mol. The room temperature second-order rate of [] + EthFc, = 1.51(4) × 10 M s, was 3 orders of magnitude faster than that of EthFc + TzPy, = 1.05(15) × 10 M s. The [HFc] complex was converted to [Fc] by oxidation with oxygen and 3,5-di--butyl--quinone, and the molecular structure of [Fc] was determined by single-crystal X-ray diffraction. The title complex [] showed a quasi-reversible reduction in the cyclic voltammogram, and the electrochemical reduction mechanism was determined by UV-vis spectroelectrochemistry (SEC) experiments, as well as supported by density functional theory (DFT) calculations. The dihydropyridazine [HFc] and pyridazine [Fc] states of the ligand showed ligand noninnocence similar to that of the parent tetrazine but at a cathodically shifted potential. The dihydropyridazine [HFc] showed a mixture of several products; however, upon oxidation, only a single product, [Fc], was formed from the addition of the dienophile to []. The electrochemical mechanism of [Fc] was also studied by cyclic voltammetry and UV-vis SEC experiments, as well as supported by DFT calculations.