[(η⁶-p-Cymene)[3-(pyrid-2-yl)-1,2,4,5-tetrazine]chlororuthenium(II)]⁺, Redox Noninnocence and Dienophile Addition to Coordinated Tetrazine

Abstract

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, k 2 = 1.51(4) × 10–2 M–1 s–1, was 3 orders of magnitude faster than that of EthFc + TzPy, k 2 = 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.

Origin of Increased Reactivity in Rhenium-Mediated Cycloadditions of Tetrazines

“…This reaction allows for efficient labeling with the metal complex, opening possible applications in the fields of chemical biology and medicine. − Interestingly, Ringenberg et al describe an increase in rate constants of 2 orders of magnitude compared to the reaction of tetrazine itself. The same behavior was observed by the same group for a ruthenium complex . This acceleration makes these systems particularly interesting as it would allow for labeling at ultralow concentrations.…”

Electrochemical Degradation of a Dicationic Rhenium Complex via Hoffman-Type Elimination

“…Rhenium tricarbonyldiimine complexes are an attractive class of electrocatalysts for probing the mechanistic pathways of reductive CO 2 activation owing to the wealth of literature precedent and their facile syntheses. − The canonical [Re­(bpy)­(CO) 3 Br] ( 0 , where bpy = 2, 2′-bipyridine; Scheme ), for instance, has been shown to reduce CO 2 to carbon monoxide (CO) with 100% selectivity over proton reduction. ,, A two-electron reduction of the rhenium­(I) precursor yields a catalytically active anionic species that possesses a delocalized electron cloud and available π interactions from the equatorial carbonyl ligands, inducing the preferred formation of Re-CO 2 adducts over Re–H intermediates. , There is also a rich body of literature describing the in situ modification of redox-active unsaturated nitrogen-donor ligands under reducing or thermal conditions. ,− …”

(Spectro)Electrochemistry of 3‐(Pyrid‐2‐yl)‐s‐Tetrazine‐ and 1,2‐ (Dihydro)pyridazine Tricarbonylrhenium(I)chloride

“…The bulky ferrocenyl moiety was at the 5-position due to trapping of the kinetic product. [28] In a similar report on the addition of ViFc to [CyRuCl(TzPy)] + , Cy = η 6 -cymene, we found that dihydropyridazine could be oxidized to the pyridazine state of the ligand, [29] where [1H 2 Fc] did not show this apparent reactivity but did appear to slowly oxidize over the course of several weeks to [1Fc]. The pyridazine could, however, be generated by adding ethynylferrocene (EthFc) to [1] to give [1Fc] (Scheme 1).…”