Electrochemistry of Ruthenium Bis(imino)pyridine Compounds: Evidence for an ECE Mechanism and Isolation of Mono and Dicationic Complexes

Abstract

The electrochemical properties of a series of ruthenium complexes bearing the redox noninnocent bis(imino)pyridine ligand were investigated with cyclic voltammetry. An unexpected electrochemical feature in one of the complexes indicated a possible electrochemical-chemical-electrochemical (ECE) process. Further investigation into the oxidation mechanism included the synthesis and isolation of cationic and dicationic analogues of the complex. Density functional theory calculations suggested that a solvent interaction facilitated the ECE mechanism, so a solvent adduct was also synthesized and isolated. Calculations of this series of compounds were used to investigate the stabilizing role of the ligands over various oxidation states. As the overall charge of the complex was varied from anion to dication, a relatively constant Ru metal center oxidation state was found over all four states probed. This result suggests the strong involvement of the coligands in the redox behavior of the complexes investigated.

Experimental and Theoretical Insights into the Electronic Properties of Anionic N-Heterocyclic Dicarbenes through the Rational Synthesis of Their Transition Metal Complexes

“…While the cyclic voltammogram (CV) of Mn­(I) IMes complex 1 in acetonitrile reveals a fully reversible one-electron oxidation at E 1/2 {Mn­(I)/Mn­(II)} = −0.26 V vs Fc (Figure S9), its cationic Fe­(II) analogue, [ 6 ]­(OTf), is under the same conditions as an irreversible oxidation at E p.a. {Fe­(II)/Fe­(III)} = +1.44 V (Figure S13), which is probably due to the occurrence of an ECE-type process that is rather typical for this coordinating solvent . The electrochemical behavior of Fe­(II) a IMes derivative [ 4 ]­(OTf) was found to be intermediate compared to that of its normal IMes analogues, and a quasi-reversible ( I a / I c = 0.6) one-electron oxidation wave was observed at E 1/2 {Fe­(II)/Fe­(III)} = +1.12 V (Figure S11), in full agreement with a stronger donation of the a IMes ligand.…”

Effects of Ancillary Ligands on Redox and Chemical Properties of Ruthenium Coordinated Azoaromatic Pincer

“…On the other hand, electrogenerated complex [ 3 ] − is EPR inactive. The DFT analysis indicates a spin pairing situation over the coordinated L. It is well-documented that the bpy is a poorer , acceptor than pcp, and thus, the second reduction occurred on coordinated ligand L •– rather than bpy, resulting in the formation of a spin-coupled S = 0 state . On the other hand, the highest occupied molecular orbital (HOMO) of the isolated complexes [ 2 ] + and [ 3 ] + are primarily localized on the metal center.…”

Bis-Ferrocenyl-Pyridinediimine Trinuclear Mixed-Valent Complexes with Metal-Binding Dependent Electronic Coupling: Synthesis, Structures, and Redox-Spectroscopic Characterization

“…With these considerations in mind, CV was performed in DCM because coordinating solvents promote ECE mechanisms in PDI complexes, while TBA-BArF electrolyte was employed both to improve the electrochemistry in this nonpolar solvent and also because the PF 6 salts of these complexes are insoluble, precipitating onto the electrode. Figure shows that the free ligand displays a concerted two-electron wave, suggesting weak to no coupling.…”

“…Thus, the separation between ligand oxidations can be correlated as a function of metalation (e.g., with FeCl 2 , CoCl 2 , ZnCl 2 , or MgCl 2 ) with the delocalization of charge between the two ferrocene groups. 11a,b,19b With these considerations in mind, CV was performed in DCM because coordinating solvents promote ECE mechanisms in PDI complexes, 46 while TBA-BArF electrolyte was employed both to improve the electrochemistry in this nonpolar solvent 37 and also because the PF 6 salts of these complexes are insoluble, precipitating onto the electrode. Figure 2 shows that the free ligand displays a concerted twoelectron wave, suggesting weak to no coupling.…”