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

Abstract 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 II L ( 1 3+ ) was synthesized and well characterized. The investigations indicate that the interaction between the two terminal Ru centers of 1 3+ is so strong that it is fully delocalized and can be described as LRu II+0.5 ‐NC‐Ru II ‐CN‐Ru II+0.5 L. Most interestingly, 1 3+ can be reversibly protonated to 1 3+ ‐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+0.5 L to HLRu II ‐NC‐Ru III ‐CN‐Ru II LH. Cyclic voltammogram of 1 3+ ‐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 1 3+ may be potential for an excellent candidate of molecular switch.