The Role of Substrate Acidity in PN3P–Ru Pincer Complex Catalyzed Formic Acid Dehydrogenation: Pseudo-Dearomatization vs Non-Dearomatization Pathways

Abstract

Metal–ligand cooperation (MLC) plays a crucial role in catalysis by enabling substrate activation through coordinated interactions between the metal center and ligand framework. To elucidate the mechanism for the PN³P–Ru-pincer complex catalyzed formic acid (HCOOH) dehydrogenation (FADH) reaction, pseudo-dearomatization and non-dearomatization pathways were investigated. HCOOH acidity is a key determinant with acid–base interactions favoring a non-dearomatization pathway. The base, triethylamine (Et₃N), facilitates the formate ion generation, followed by the catalyst activation. Stability analysis indicates that the five-coordinated Ru species with a Cl^– counterion is the most stable form in solution compared to its six-coordinated species. Under FADH conditions, decarboxylation is identified as the rate-determining step (RDS), consistent with experimental kinetic isotope effect (KIE) observations. Although the dearomatized PN³P–Ru pincer complex can be synthesized under basic conditions without HCOOH, it rapidly rearomatizes in the presence of HCOOH and remains in its aromatic form throughout the catalytic cycle. Our findings suggest that dearomatization/rearomatization may not be essential for these pincer systems. These insights refine our understanding of the aromatization/dearomatization-based MLC in homogeneous FADH reactions catalyzed by the PN³P–Ru pincer complexes and provide guiding principles for the development of next-generation catalysts for energy-related transformations.