Redox Behavior of a Dinuclear Ruthenium(II) Complex Bearing an Uncommon Bridging Ligand: Insights from High-Pressure Electrochemistry

Abstract

A dinuclear ruthenium complex bridged by 2,3,5,6-pyrazinetetracarboxylic acid (μ-LH) was synthesized and characterized by X-ray crystallography, cyclic voltammetry under ambient and elevated pressures, electron paramagnetic resonance (EPR) and UV/vis-NIR (NIR = near-infrared) spectroelectrochemistry, pulse radiolysis, and computational methods. We probed for the first time in the field of mixed-valency the use of high-pressure electrochemical methods. The investigations were directed toward the influence of the protonation state of the bridging ligand on the electronic communication between the ruthenium ions, since such behavior is interesting in terms of modulating redox chemistry by pH. Starting from the [Ru(μ-LH)Ru] configuration, which shows an intense metal-to-ligand charge transfer absorption band at 600 nm, cyclic voltammetry revealed a pH-independent, reversible one-electron reduction and a protonation-state-dependent (proton coupled electron transfer, PCET) reversible oxidation. Deeper insight into the electrode reactions was provided by pressure-dependent cyclic voltammetry up to 150 MPa, providing insight into the conformational changes, the protonation state, and the environment of the molecule during the redox processes. Spectroelectrochemical investigations (EPR, UV/vis-NIR) of the respective redox reactions suggest a ligand-centered radical anion [Ru(μ-LH)Ru] upon reduction (EPR Δg = 0.042) and an ambiguous, EPR-silent one-electron oxidized state. In both cases, the absence of the otherwise typical broad intervalence charge transfer bands in the NIR region for mixed-valent complexes support the formulation as radical anionic bridged compound. However, on the basis of high-pressure electrochemical data and density functional theory calculations the one-electron oxidized form could be assigned as a charge-delocalized [Ru(μ-LH)Ru] valence tautomer rather than [Ru(μ-LH)Ru]. Deprotonation of the bridging ligand causes a severe shift of the redox potential for the metal-based oxidation toward lower potentials, yielding the charge-localized [Ru(μ-LH)Ru] complex. This PCET process is accompanied by large intrinsic volume changes. All findings are supported by computational methods (geometry optimization, spin population analysis). For all redox processes, valence alternatives are discussed.

Self-assembly of a new class of rhenium(i)-based double stranded dinuclear monohelicates with their photophysical and electrochemical studies

“…The cyclic voltammogram of 2 shows one reversible reduction at E 1/2 = −0.88 V and two quasi reversible reductions each at E 1/2 = −1.54 V and E 1/2 = −1.79 V. Similar to complex 2 , complex 4 shows one reversible redox process at E 1/2 = −0.90 V along with one reversible reduction at E 1/2 = −1.74 V and one irreversible reduction at E pc = −1.55 V. The reversible redox process is likely to be associated with the pyrazine core-based ligand motif because the LUMO and LUMO+1 are mostly centered on the pyrazine motif as evident from TD-DFT studies. 8 c ,14 Further another reversible redox process at E 1/2 = −2.06 V is observed in 4 , which may be attributed to the L 2 motif.…”

“…The metallomacrocycles of the quinonoid-based motif are known to display rich electrochemical properties. 12–14 The redox behaviour of the fac -Re(CO) 3 core-based metallomacrocycles having 1,4-dioxido-9,10-anthraquinone and 6,11-dioxido-5,12-naphthacenedione are thoroughly studied. 12 b Complexes 1–4 contain bis-chelating 2,5-dioxido-1,4-benzoquinone (dhbq 2− ) or 2,5-pyrazine dicarboxylate (pydc 2− ) and are expected to show redox properties.…”

“…12 b Complexes 1–4 contain bis-chelating 2,5-dioxido-1,4-benzoquinone (dhbq 2− ) or 2,5-pyrazine dicarboxylate (pydc 2− ) and are expected to show redox properties. 8 c ,13,14 The redox behaviours of the metallomacrocycles were studied using cyclic voltammetry and the redox potentials are quoted against ferrocene as a reference. Cyclic voltammograms of 1–4 were recorded using a glassy carbon electrode in DMSO solution with 0.1 M NBu 4 PF 6 as the supporting electrolyte under a N 2 atmosphere (Fig.…”

“…Crystal data. C 93 H 80 N 8 O 20 Re 4 , M = 2374.45, triclinic, a = 8.9621(4), b = 10.8991(3), c = 23.4881(7) Å, U = 2240.59(14) …”

The Excited‐State Creutz–Taube Ion

“…MV systems offer a unique playground for chemists to study the fundamentals of intramolecular electron transfer under a variety of different conditions. Systematic variations of temperature, [13] pressure, [14] solvent identity, [15,16] redox-active site potentials [17][18][19] and bridges connecting those redox sites [12,[20][21][22] are usually translated into clear experimental observables. Therefore MV systems in general, and the prototypical CT 5 + ion in particular, serve as models for how those variations affect electron transfer reactions.…”