Bimetallic Catalysis Involving Dipalladium(I) and Diruthenium(I) Complexes

Abstract

Dipalladium(I) and diruthenium(I) compounds bridged by two [{(5,7-dimethyl-1,8-naphthyridin-2-yl)amino}carbonyl]ferrocene (L) ligands have been synthesized. The X-ray structures of [Pd₂L₂][BF₄]₂ (1) and [Ru₂L₂(CO)₄][BF₄]₂ (2) reveal dinuclear structures with short metal–metal distances. In both of these structures, naphthyridine bridges the dimetal unit, and the site trans to the metal–metal bond is occupied by weakly coordinating oxygen from the amido fragment. The catalytic utilities of these bimetallic compounds are evaluated. Compound 1 is an excellent catalyst for phosphine-free, Suzuki cross-coupling reactions of aryl bromides with arylboronic acids and provides high yields in short reaction times. Compound 1 is also found to be catalytically active for aryl chlorides, although the corresponding yields are lower. A bimetallic mechanism is proposed, which involves the oxidative addition of aryl bromide across the PdPd bond and the bimetallic reductive elimination of the product. Compound 1 is also an efficient catalyst for the Heck cross-coupling of aryl bromides with styrenes. The mechanism for aldehyde olefination with ethyl diazoacetate (EDA) and PPh₃, catalyzed by 2, has been fully elucidated. It is demonstrated that 2 catalyzes the formation of phosphorane utilizing EDA and PPh₃, which subsequently reacts with aldehyde to produce a new olefin and phosphine oxide. The efficacy of bimetallic complexes in catalytic organic transformations is illustrated in this work.