Protonation‐Induced Reversible Switching of Electronic Delocalization in a Linear Ru‐Ru‐Ru Complex

In this work, we report the first example of a molecular switch between electronic delocalization and electronic localization in cyanidometal‐bridged trinuclear mixed‐valence (MV) complex controlled by a protonation reaction. A MV complex with the terminal 2,3‐bis(diphenylphosphino)pyridine (L) ligand which has a higher proton‐accepting character, LRuIII‐NC‐RuII‐CN‐RuIIL (13+) was synthesized and well characterized. The investigations indicate that the interaction between the two terminal Ru centers of 13+ is so strong that it is fully delocalized and can be described as LRuII+0.5‐NC‐RuII‐CN‐RuII+0.5L. Most interestingly, 13+ can be reversibly protonated to 13+‐2H coupled with an electron transfer from the central RuII to the terminal RuIII, resulting in a change in electronic structure from LRuII+0.5‐NC‐RuII‐CN‐RuII+0.5L to HLRuII‐NC‐RuIII‐CN‐RuIILH. Cyclic voltammetry of 13+‐2H showed very weak interaction between the two terminal Ru centers. The remarkable change in electronic states by protonation‐induced electron transfer is also strongly supported by the (TD)DFT calculations. Complex 13+ may be potential for an excellent candidate of molecular switch.