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The novel tridentate PNN pincer ligand L features a reactive 2-hydroxypyridine functionality as well as a bipyridyl-methylphosphine skeleton for meridional coordination. This proton-responsive ligand coordinates in a straightforward manner to RuCl(CO)(H)(PPh3)3 to generate complex 1. The methoxy-protected analogue L Me was also coordinated to Ru(II) for comparison. Both species have been crystallographically characterized. Site-selective deprotonation of the 2-hydroxypyridine functionality to give 1′ was achieved using both mild (DBU) and strong bases (KOBu and KHMDS), with no sign of involvement of the phosphinomethyl side arm that was previously established as the reactive fragment. Complex 1′ is catalytically active in the dehydrogenation of formic acid to generate CO-free hydrogen in three consecutive runs as well as for the dehydrogenative coupling of alcohols, giving high conversions to different esters and outperforming structurally related PNN ligands lacking the N fragment. DFT calculations suggest more favorable release of H2 through reversible reactivity of the hydroxypyridine functionality relative to the phosphinomethyl side arm. ■ INTRODUCTION Reactive ligand design has opened up fundamentally novel pathways for bond activation, small-molecule functionalization, and homogeneous catalysis. Tridentate pincer ligands that feature a reactive site have gained a great deal of interest during the past decade. Among several design types, lutidine-based PNP systems (Figure 1a) and analogues thereof have particularly gained momentum as ligands of choice for a range of transformations, because of their ability to undergo facile and reversible deprotonation of the phosphinomethyl side arm, generating a formally dearomatized N-heterocycle. Recently, also bis(pyrazole)-appended pyridines (Figure 1b) and pyridone-containing (NNN) derivatives (Figure 1c) have attracted much attention as pH-responsive pincer frameworks in the context of metal−ligand bifunctional

Ruthenium PNN(O) Complexes: Cooperative Reactivity and Application as Catalysts for Acceptorless Dehydrogenative Coupling Reactions