An Isolable Fused‐Metallacyclobutadiene Intermediate Enables Z ‐Selective Alkyne Dimerization via a Non‐Classical CCC ‐Pincer Platform

Comprehensive Summary Pincer complexes are privileged platforms in homogeneous catalysis, yet genuinely all‐carbon (CCC) variants remain uncommon and generally rely on N‐heterocyclic carbene auxiliaries for structural stabilization. This scarcity has limited understanding of how a meridional all‐carbon donor array can regulate substrate activation and stereocontrol. Here, osmapentalyne Os‐1 is employed as a non‐classical CCC‐pincer precatalyst for the head‐to‐head dimerization of terminal aryl alkynes. Under mild conditions (50 °C), a broad range of functionalized aryl and heteroaryl alkynes is converted into ( Z )‐1,3‐enynes in moderate to excellent yields with generally high Z ‐selectivity. Halide, CF 3 , CO 2 Me, OH, and NMe 2 substituents, as well as fused‐aryl, heteroaryl, and complex chiral fragments, are tolerated. High yield and selectivity are retained upon addition of 1 vol% water, whereas an NNN‐osmium complex and OsCl 2 (PPh 3 ) 3 are inactive under the optimized conditions, supporting a specific role for the carbolong framework. Mechanistically, stoichiometric [2+2] cycloaddition of a terminal alkyne with Os‐1 affords the fused‐metallacyclobutadiene A1 , which was isolated, crystallographically characterized, and independently shown to be catalytically competent. 31 P NMR and HRMS monitoring identifies A1 as the predominant complex after reaction and supports its assignment as an on‐cycle resting state, while allowing reversible PPh 3 dissociation later in turnover. Calculations further show that base‐assisted C–H activation from the fused CCCC‐Os species is substantially more favorable than a pathway initiated by PPh 3 dissociation. The rigid, highly planar fused framework provides a plausible structural basis for substrate‐dependent stereochemical discrimination: the equatorial CCC donor array shapes the axial reaction environment without requiring bond formation at a donor carbon. To our knowledge, A1 is the first crystallographically characterized fused‐metallacyclobutadiene shown to be catalytically competent in alkyne dimerization. This study therefore extends carbolong chemistry from stoichiometric reactivity to selective C–C bond‐forming catalysis and establishes an experimentally accessible platform for interrogating all‐carbon pincer function and informing future catalyst design.