Inorganic Electron Transfer: Sharpening a Fuzzy Border in Mixed Valency and Extending Mixed Valency across Supramolecular Systems

Abstract

This article describes research from our laboratory on the chemistry and spectroscopic properties of inorganic mixed-valence complexes. After a brief review of the seminal work of Taube, Creutz, Day, Robin, Hush, and others in the 1960s and the confounding efforts to identify the borderline between class II and III mixed-valence systems in the 1990s and early 2000s, we describe our first experiments to observe and analyze the coalescence of ν(CO) band shapes in the 1D IR spectra of mixed-valence complexes of the type {[Ru3O(OAc)6(CO)(L)]2-BL}(-), where L = a pyridyl ligand and BL = pyrazine or 4,4'-bipyridine, to estimate rate constants of intramolecular electron transfer (ET). The strong involvement of the bridging ligands in mixed-valence complexes of this type was first identified in the appearance of totally symmetric vibrational modes of pyrazine bridging ligands in the IR because of strong vibronic coupling within a three-state metal cluster-bridge-metal cluster model. Application of the Brunschwig-Creutz-Sutin semiclassical three-state model of mixed valency accounts well for the appearance of two intervalence charge-transfer bands that are observed in the near-IR region of the electronic absorption spectra of these mixed-valence ions. The direct spectroscopic observation of "mixed-valence isomers", the two alternate charge distributions of a mixed-valence ion, are described. The equilibrium constants of mixed-valence isomers provide quantitative thermodynamic estimates of the electronic coupling, H(ab). The extent of delocalization in many of the mixed-valent bridged dimers of triruthenium clusters produces unusual behavior, especially rate constants for ET that are independent of normal solvent reorganization energies but do depend on solvent dynamical dipolar reorientation times. The strong dependence of rates on solvent dynamics is also found to produce a non-Arrhenius dependence of ET rate constants on temperature, faster rates in frozen solutions compared to fluid solutions. The studies of these mixed-valence systems have provided generalized guidelines for establishing where a particular mixed-valence system lies along the class II/III delocalization transition, and they have increased our understanding of the ET dynamics at the delocalization threshold.

Electron Localization of Anions Probed by Nitrile Vibrations

“…Kubiak and Ito have further applied IR to inorganic mixed-valence complexes in the form of IR spectroelectrochemistry 67,68. In their studies of inorganic mixed-valence compounds, they identified localization and delocalization of an exchanging electron by the coalescence of…”

Transition from a Metal‐Localized Mixed‐Valence Compound to a Fully Delocalized and Bridge‐Biased Electrophore in a Ruthenium–Amine–Ruthenium Tricenter System

“…Three-center mixed-valence systems are the focus of a number of studies and they are interesting from both a theoretical and applied perspective. [17][18][19][20][21][22] Depending on the relative redox energy of the metal site and the bridge component, the free spin can be either localized on the redox termini or bridge component or delocalized across the whole molecule. As a continuation of our recent interests in mixedvalence diruthenium complexes, [23] we present herein a ruthenium-amine-ruthenium tricenter system in which a transition from a metal-localized mixed-valence compound to a fully delocalized electrophore with extended conjugation, followed by a bridge-biased electrophore, is realized by simply varying the terminal ligands of the ruthenium components and the substituent on the central amine segment.…”

“…As a continuation of our recent interests in mixedvalence diruthenium complexes, [23] we present herein a ruthenium-amine-ruthenium tricenter system in which a transition from a metal-localized mixed-valence compound to a fully delocalized electrophore with extended conjugation, followed by a bridge-biased electrophore, is realized by simply varying the terminal ligands of the ruthenium components and the substituent on the central amine segment. Although known threecenter systems were reported to show different forms of charge delocalization, [17][18][19][20][21][22] the realization of the three situations in one single system has not been documented to date.For this purpose, seven amine-bridged diruthenium complexes 1(PF 6 ) 2 -7(PF 6 ) 2 were prepared (Figure 1). Each ruthenium component contains a RuÀC bond, which makes the Ru III/II potential comparable to the amine oxidation potential.…”

“…However, when a redox-active or -noninnocent bridge is used, [17][18][19] a three-center diruthenium complex is obtained. Three-center mixed-valence systems are the focus of a number of studies and they are interesting from both a theoretical and applied perspective.…”

Class III Delocalization in a Cyanide‐Bridged Trimetallic Mixed‐Valence Complex

“…This behavior has been demonstrated experimentally, [8,9] and the rate constant for electron transfer between the isomers has been measured. [10] In many of these nearly delocalized mixed-valence systems, the bridge plays an important role in the spectroscopy, [11][12][13] which requires a three-state model for an accurate interpretation. [14,15] Another focus has been the characterization of higher nuclearity systems, [16] which also present challenging spectroscopy.…”