Divergent Ring-Opening Functionalization and Reductive Coupling of Pyridines via Zirconium Alkylidene Complexes

The transformation of N-heterocycles such as pyridines is of fundamental and practical importance, yet direct cleavage of their aromatic C─N bonds remains challenging due to strong electronic delocalization and intrinsic stability. Here we report an unprecedented nonreductive cleavage of the aromatic C─N bond in pyridines via direct ring-opening metathesis with PCP-ligated (PCP = 2,6-(^tBu₂PCH₂)₂-C₆H₃) zirconium alkylidene complexes under mild conditions. Experimental and computational studies reveal that these transformations proceed via highly reactive [Zr]═C species instead of alkylidyne pathways, forming acyclic Zr-imido-triene complexes that undergo imido/oxo exchange with acetone. This enables a concise conversion of pyridines into conjugated iminotrienes that otherwise require multistep syntheses. The ancillary ligand dictates divergent pathways: alkyl-, alkoxide-, or aryloxide-containing alkylidenes selectively promote ring-opening functionalization, whereas the chloride analogue triggers rare reductive homocoupling of pyridines to bipyridine dianions. This ligand-controlled reactivity arises from steric and electronic effects of the chloride variant, facilitating C─H activation and C─C bond formation. These findings demonstrate a novel approach to aromatic C─N bond scission and dual functionalization at both termini of the opened pyridine rings, along with a reductive homocoupling pathway to bipyridine dianions, providing new opportunities for catalytic transformations of aromatic N-heterocycles.