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

The relationships between the intervalence energy (EIT) and the free energy difference (ΔG) that exists between the minima of redox isomers (FeII−RuIII/FeIII−RuII) for various heterobimetallic complexes [(R-Fcpy)Ru(NH3)5]2+/3+ (R = H, ethyl, Br, actyl; Fcpy = (4-pyridyl)ferrocenyl; Ru(NH3)5 = pentaam(m)ineruthenium) were examined. The changes in ΔG for the complexes in various solvents were due to the effects of both solvent donicity and the substituents. The intervalence energy versus ΔG, ΔG ≈ FΔE1/2 (ΔE1/2 = E1/2(FeIII/II) − E1/2(RuIII/II)), plots for the complexes in various solvents suggest a nuclear reorganization energy (λ) of ≈6000 cm−1 (Chen et al. Inorg. Chem. 2000, 39, 189). For [(R-Fcpy)Ru(NH3)5]2+ and [(et-Fcpy)Ru(NH3)4(py)]2+ (Ru(NH3)4 = trans-tetraam(m)ineruthenium; py = pyridine) in various solvents, the E1/2(RuIII/II) of rutheniumam(m)ine typically was less than the E1/2(FeIII/II) of the ferrocenyl moiety. However, the low-donicity solvents resulted in relatively large values of E1/2(RuIII/II) for [(et-Fcpy)Ru(NH3)4(py)]2+/3+/4+. Under our unique solvent conditions, a dramatic end-to-end interaction was observed for the trimetal cation, [(et-Fcpy)2Ru(NH3)4]4+, in which the [(et-Fcpy)2Ru(NH3)4]4+ included a central trans-tetraam(m)ineruthenium(III) and a terminal FeII/FeIII pair. In general, results of electrochemical studies of [(et-Fcpy)2Ru(NH3)4]2+ indicated both solvent-tunable E1/2(RuIII/II) (1 e−) and solvent-insensitive E1/2(FeIII/II) (2 e−) redox centers. However, in nitriles, two E1/2(FeIII/II) peaks were found with ΔE1/2(FeIII/II − FeIII/II) ranging between 83 and 108 mV at a terminal metal-to-metal distance of up to 15.6 Å. Furthermore, the bridging dπ orbital of the ruthenium center mediated efficient end-to-end interaction between the combinations of the terminal FeII−FeIII/FeIII−FeII pair. To our knowledge, this is the first example of solvent-tunable end-to-end interactions in multimetal complexes.