Spectral, structural and DFT studies of platinum group metal 3,6-bis(2-pyridyl)-4-phenylpyridazine complexes and their ligand bonding modes

[(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 = η⁶-p-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 [1]⁺ + ViFc found ΔG^⧧(298 K) = 67 kJ mol^-1, while that of [1]⁺ + EthFc was ΔG^⧧(298 K) = 83 kJ mol^-1. The room temperature second-order rate of [1]⁺ + EthFc, k₂ = 1.51(4) × 10^-2 M^-1 s^-1, was 3 orders of magnitude faster than that of EthFc + TzPy, k₂ = 1.05(15) × 10^-4 M^-1 s^-1. The [1H₂Fc]⁺ 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 [1H₂Fc]⁺ 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 [1H₂Fc]⁺ 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.