[(η6-p-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)]PF6 ([1](+); Cy = eta(6)-p-cymene) shows noninnocent behavior and can be modified through the addition of dienophiles, vinylferrocene (ViFc) or ethynylferrocene (EthFc). The kinetics and transition-state thermo-dynamic analysis of the reaction of [1](+) + ViFc found Delta G double dagger(298 K) = 67 kJ mol(-1), while that of [1](+) + EthFc was Delta G double dagger(298 K) = 83 kJ mol(-1). The room temperature second-order rate of [1](+) + EthFc, k(2) = 1.51(4) x 10(-2) M-1 s(-1), was 3 orders of magnitude faster than that of EthFc + TzPy, k(2) = 1.05(15) x 10(-4) M-1 s(-1). The [1H(2)Fc](+) complex was converted to [1Fc](+) by oxidation with oxygen and 3,5-di-tert-butyl-o-quinone, and the molecular structure of [1Fc](+) was determined by single-crystal X-ray diffraction. The title complex [1](+) 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 [1H(2)Fc](+) and pyridazine [1Fc](+) states of the ligand showed ligand noninnocence similar to that of the parent tetrazine but at a cathodically shifted potential. The dihydropyridazine [1H(2)Fc](+) showed a mixture of several products; however, upon oxidation, only a single product, [1Fc](+), was formed from the endo addition of the dienophile to [1](+). The electrochemical mechanism of [1Fc](+) was also studied by cyclic voltammetry and UV-vis SEC experiments, as well as supported by DFT calculations.

Automated summary

The bidentate ligand TzPy showed noninnocent behavior and could be modified by dienophiles. The kinetic analysis of the reaction between [1](+) and ViFc yielded Delta G double dagger(298 K) = 67 kJ mol(-1), while that of [1](+) and EthFc was Delta G double dagger(298 K) = 83 kJ mol(-1). The electrochemical reduction mechanism of [1Fc](+) was studied using cyclic voltammetry and UV-vis SEC experiments, supported by DFT calculations.