Tuning Through-Bond Fe(III)/Fe(II) Coupling by Solvent Manipulation of a Central Ruthenium Redox Couple

Abstract

The relationships between the intervalence energy (E(IT)) and the free energy difference (DeltaG) that exists between the minima of redox isomers (Fe(II)-Ru(III)/Fe(III)-Ru(II)) for various heterobimetallic complexes [(R-Fcpy)Ru(NH(3))(5)](2+/3+) (R = H, ethyl, Br, actyl; Fcpy = (4-pyridyl)ferrocenyl; Ru(NH(3))(5) = pentaam(m)ineruthenium) were examined. The changes in DeltaG for the complexes in various solvents were due to the effects of both solvent donicity and the substituents. The intervalence energy versus DeltaG, DeltaG approximately FDeltaE(1/2) (DeltaE(1/2) = E(1/2)(Fe(III/II)) - E(1/2)(Ru(III/II))), plots for the complexes in various solvents suggest a nuclear reorganization energy (lambda) of approximately 6000 cm(-1) (Chen et al. Inorg. Chem. 2000, 39, 189). For [(R-Fcpy)Ru(NH(3))(5)](2+) and [(et-Fcpy)Ru(NH(3))(4)(py)](2+) (Ru(NH(3))(4) = trans-tetraam(m)ineruthenium; py = pyridine) in various solvents, the E(1/2)(Ru(III/II)) of rutheniumam(m)ine typically was less than the E(1/2)(Fe(III/II)) of the ferrocenyl moiety. However, the low-donicity solvents resulted in relatively large values of E(1/2)(Ru(III/II)) for [(et-Fcpy)Ru(NH(3))(4)(py)](2+/3+/4+). Under our unique solvent conditions, a dramatic end-to-end interaction was observed for the trimetal cation, [(et-Fcpy)(2)Ru(NH(3))(4)](4+), in which the [(et-Fcpy)(2)Ru(NH(3))(4)](4+) included a central trans-tetraam(m)ineruthenium(III) and a terminal Fe(II)/Fe(III) pair. In general, results of electrochemical studies of [(et-Fcpy)(2)Ru(NH(3))(4)](2+) indicated both solvent-tunable E(1/2)(Ru(III/II)) (1 e(-)) and solvent-insensitive E(1/2)(Fe(III/II)) (2 e(-)) redox centers. However, in nitriles, two E(1/2)(Fe(III/II)) peaks were found with DeltaE(1/2)(Fe(III/II) - Fe(III/II)) ranging between 83 and 108 mV at a terminal metal-to-metal distance of up to 15.6 A. Furthermore, the bridging dpi orbital of the ruthenium center mediated efficient end-to-end interaction between the combinations of the terminal Fe(II)-Fe(III)/Fe(III)-Fe(II) pair. To our knowledge, this is the first example of solvent-tunable end-to-end interactions in multimetal complexes.

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

“…In a two-step redox system with two chemically inequivalent redox sites, several factors contribute to Δ G comp (eq ). ,− Of these, the statistical term Δ G stat of 36 mV, the inductive term Δ G ind ,, and the magnetic exchange term Δ G exc , usually account for only some tens of millivolts. They can thus not explain the large differences of more than 400 mV of the Δ E 1/2 values of related ferrocene and ruthenocene vinylruthenium complexes.…”

Electronically Strongly Coupled Divinylheterocyclic‐Bridged Diruthenium Complexes

“…(1)]. [74][75][76][77] In the context of MV species, the most relevant contributor amongst these is the resonance contribution DG res ,w hich corresponds to intrinsic ground-state delocalization (or electronic coupling) in that state. In many systemso f similar overall architecture DE8' is, however,d ominated by the electrostatic contribution DG e ,w hich originates from increasing Coulombic repulsion on further chargingo famolecule.…”