Mixed-Valent Ruthenocene–Vinylruthenium Conjugates: Valence Delocalization Despite Chemically Different Redox Sites

Abstract

Ruthenocene–vinylruthenium conjugates Rc/Rc*–CHCH–Ru­(CO)­(L)­(P i Pr3)2 (Rc = (η5-C5H5)­Ru­(η5-C5H4); Rc* = (η5-C5Me5)­Ru­(η5-C5H4); L = Cl or κO,O′-acetylacetonato) have been prepared and investigated in their neutral, mono-, and dioxidized states by cyclic voltammetry, IR and UV/vis/NIR spectroelectrochemistry, and EPR spectroscopy. Their corresponding radical cations are (almost) completely delocalized mixed-valent systems as indicated by the low half-widths, the absence of solvatochromism, and the low-energy cutoff of their IVCT bands in the near-infrared (NIR) and their IR and EPR spectroscopic signatures. The degree of electronic coupling even exceeds that of their ferrocene analogs despite comparable differences between the intrinsic half-wave potentials of the vinylruthenium and the metallocenyl entities and substantially smaller half-wave potential splittings, ΔE 1/2, in the ruthenocene congeners. All experimental results are backed by quantum chemical calculations.

Redox Isomeric Ferrocenyl Styrylruthenium Radical Cations with Diphenyl-Substituted β-Ketoenolato Ligands

“…This is typical of oxidized styrylruthenium complexes with (partial) charge delocalization onto a secondary redox site. 37,51,55,57,58 The behavior of the present complexes matches that of the previously reported acetylacetonato derivative 2-acac. Here, two different isomers with distinct Ru(CO) bands, which differ with respect to whether the ferrocenyl or the styrylruthenium entity is oxidized first, were found to coexist in solution.…”

“…71−74 We have recently reported similar findings for mixed-valent alkenylruthenium complexes with a triarylamine or ruthenocene as the secondary redox− active entity. 37,56 In the present radical cations, the low-energy cutoff signals that the notion of ferrocenium-or styrylruthenium-based oxidations is not wholly adequate. It oversimplifies an even more complex situation, where the common styryl "bridge" constitutes an integral part of both chemically different redox systems and where at least the isomer denoted as Ru-Sty •+ has electronically strongly coupled redox sites.…”

“…This is evident from the consistently smaller blue shift of the Ru(CO) stretching band of 28−42 cm −1 when compared to that of 57−72 cm −1 for simple styrylruthenium complexes, 41 and the similarity of this shift to that in related styrylruthenium complexes with two electronically coupled redox sites. 37,51,55,57,58 An alternative description of the different isomers is therefore also possible, as two coexisting mixedvalent species that differ with respect to the extent of electron delocalization. We note in this context that mixed-valent 1,4diethynylphenylene-bridged diruthenium complexes exist as several conformers, which also differ with respect to their electronic coupling strengths.…”

Modulation of Electrochemical and Spectroscopic Properties in Ru(II)‐Terpyridyl End‐Capped Homobimetallic Organometallic Complexes by Varying π‐Conjugated Organic Spacers

“…This class of rigid wire‐like linear π‐conjugated systems combined with two redox‐active metal termini, [ML x n ‐μ‐(BL)‐ML x n+1 ] (BL=bridging ligand, M=redox active metal termini), generates mixed‐valence state when one of the metal center undergoes a chemically or electrochemically reversible redox process generating different oxidation states in the two metal termini. Since the pioneering discovery of the binuclear Ru 2 (II,III) complex, [(NH 3 ) 5 Ru‐pyrazine‐Ru(NH 3 ) 5 ] 5+ by Creutz and Taube, [30–31] a significant number of mixed‐valence metal complexes have been intensively investigated by varying the redox‐active metallic termini and also modifying the π‐conjugated bridges in order to tune the intermolecular electronic communication along the molecular backbone [32–60] …”

Unusually Strong Long-range Electronic Coupling Across Redox-Active Bridges in M3+/M4+ Mixed-Valence Complexes of Group 4 Congeners

“…[29][30] Overwhelmingly, inorganic/organic hybrid species developed as molecular wires feature so-called redox-active ligands as the bridging species. 5,[31][32][33][34][35] This trend is likely due to noninnocence displayed by these systems, whereby admixing of the ligand frontier orbitals and d-orbitals of the coordinated metal ion yield the requisite electronic delocalization for wire-like behavior, [36][37][38][39][40][41][42] and electronic cooperativity between the metal and ligand can provide additional stability during electron transfer. [43][44][45][46][47] The ligands of choice for these systems largely feature endocyclic imine moieties, such as pyrazine, 5,[48][49][50][51] pyridine, 22,[25][26][52][53][54][55][56] and porphyrin functionalities.…”