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, LRu^III-NC-Ru^II-CN-Ru^IIL (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 LRu^II+0.5-NC-Ru^II-CN-Ru^II+0.5L. Most interestingly, 1³⁺ can be reversibly protonated to 1³⁺-2H coupled with an electron transfer from the central Ru^II to the terminal Ru^III, resulting in a change in electronic structure from LRu^II+0.5-NC-Ru^II-CN-Ru^II+0.5L to HLRu^II-NC-Ru^III-CN-Ru^IILH. Cyclic voltammogram 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.