Quantum Interference in Mixed-Valence Complexes: Tuning Electronic Coupling Through Substituent Effects.

Whilst 2- or 5-OMe groups on the bridging phenylene ring in [{Cp*(dppe)RuC≡C} ₂ (μ-1,3-C ₆ H ₄ )] ⁺ have little influence on the electronic structure of this weakly coupled mixed-valence complex, a 4-OMe substituent enhances ground state electron delocalization, and increases the intensity of the IVCT transition. Vibrational frequency and TDDFT calculations (LH20t-D3(BJ), def2-SVP, COSMO (CH ₂ Cl ₂ )) on ([{Cp*(dppe)RuC≡C} ₂ (μ-1,3-C ₆ H ₃ -n-OMe)] ⁺ (n=2, 4, 5) models are in excellent agreement with the experimental results. The stronger ground state coupling is attributed to the change in composition of the β-HOSO brought about by the 4-OMe group, which is ortho or para to each of the metal fragments. The intensity of the IVCT transition increases with the greater overlap of the β-HOSO and β-LUSO, whilst the relative phases of the β-HOSO and β-LUSO in the 4-OMe substituted complex are consistent with predictions of constructive quantum interference from molecular circuit rules.
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