Comprehensive Experimental and Computational Spectroscopic Study of Electronic Coupling in Solvated Mixed Valence Multimetal Complexes Using Photons from the IR through X-rays

Abstract We present a comprehensive experimental and computational spectroscopic study of the trinuclear cyanide-bridged mixed-valence complex trans-[RuIIL4{(μ-NC)FeIII(CN)5}2]4– (FeRuFe), where L = pyridine or 4-methoxypyridine dissolved in methanol and water. Four complementary spectroscopies spanning 250 meV to 7.1 keV are combined to probe the electronic structure at all three metal sites. These include infrared, visible/near IR, Ru L-edge, and Fe K-edge X-ray absorption spectroscopy probing the cyanide stretches, the metal-to-metal charge transfer transition, and electronic structure around the Ru and Fe atoms, respectively. The experimental spectra show that the two terminal Fe sites, which are crystallographically equivalent in the solid state, become inequivalent in solution. The IR spectra reveal that the solvent-exposed terminal cyanide broadens markedly in water relative to methanol. The solvation environment removes the degeneracy of the near IR metal-to-metal charge transfer transition and we measure two components separated by 190–340 meV. A weak pre-edge feature at the Ru L3-edge, present in three of the four systems, reports on Fe–Ru coupling through the cyanide bridge. To interpret the experimental spectra, we performed TDDFT calculations using complementary descriptions of the solvated complex. In one approach, the transition metal complex is treated quantum mechanically (QM) and the solvent is treated at the molecular mechanics (MM) level within a QM/MM framework. We also performed targeted calculations in which molecules in the first solvation shell are included explicitly in the QM region along with the solute, while the remaining solvent is represented as background point charges taken from the QM/MM force field. We find that core transitions localized at Fe or Ru sites are well reproduced within the QM/MM framework, while spectroscopic observables sensitive to intersite coupling or specific solute–solvent interactions benefit from explicit quantum mechanical treatment of the first solvation shell. This work establishes a systematic framework for simulating spectroscopic observables from the IR through the X-rays to characterize electronic coupling in multimetallic mixed-valence complexes mediated by the solvent.