Mechanochemical Solvent‐Free C–S Cross‐Coupling Enabled by Air‐Stable Pd(II)–BIAN–NHC Dimers

ABSTRACT

A series of sterically hindered Pd(II)–NHC and Pd(II)–BIAN–NHC dimeric complexes with the general formula [Pd(μ‐Cl)(NHC)Cl] ₂ were synthesized through a simple transformation of Pd(NHC)Cl(allyl/cinnamyl) precursors with HCl (4 M in 1,4‐dioxane). This method effectively forms air‐ and moisture‐stable dimers from bulky, electron‐rich ligands such as IMes, IPr, SIPr, IPr*, and BIAN–IPr. Systematic catalyst screening and optimization throughout this series have shown that BIAN‐containing compounds have greater catalytic efficiency than their equivalents without BIAN. Pd(II)–BIAN–NHC complex is the most active of these in C–S cross‐coupling reactions, facilitating the coupling of electronically and sterically demanding aryl chlorides with a wide variety of aryl and alkyl thiols under solvent‐free mechanochemical conditions (ball milling, room temperature), producing thioethers in good to excellent yields within short reaction times (∼2 h) at very low catalyst loadings (0.25–0.5 mol%). Excellent functional group tolerance is demonstrated by this method in 38 examples. The improved performance is explained by the BIAN–IPr ligand framework's robust σ‐donating capacity and adjustable steric profile. DFT calculations support the proposed catalytic mechanism, showing a reduced activation barrier for the rate‐determining step in the Pd‐BIAN‐IPr system and a favorable pathway for C─S bond formation. Overall, this study demonstrates the potential of BIAN–NHC‐based palladium dimers in green cross‐coupling chemistry and establishes a viable, effective, and sustainable substitute for traditional solution‐phase techniques.