Brønsted Acid/Base Site Isolated in a Pentanuclear Scaffold.

The concept of Brønsted‐Lowry acids and bases is long and widely recognized as the most reasonable theory to explain the behavior of H+ ions. Here, we report a Brønsted acid/base pair that does not follow this theory. Two heteronuclear metal complexes, in which Brønsted acid/base sites are sterically isolated, were synthesized and characterized. These sterically isolated sites exhibited anomalous behavior, wherein the H+ species encapsulated in the Brønsted acid site did not undergo a deprotonation reaction, and the corresponding protonation reaction at the Brønsted base site failed to proceed. As a result, two states that are in a relationship of a Brønsted acid/base pair stably exist over a wide pH range without any interconversion, generating a thermodynamically metastable state. Additionally, these two states exhibited distinct electron transfer abilities and reactivities. The system presented in this study is in sharp contrast with the traditional concept of Brønsted–Lowry acids and bases. [ABSTRACT FROM AUTHOR]