Development of ruthenium(II) bis(N-Heterocyclic Carbene) complexes for homogeneous catalysis

• Synthetic Innovation : developed a novel synthetic pathway for ruthenium(II) organonitrile complexes with bis(NHC) ligands and various bidentate ligands, enabling access to a broad library of complexes. • Catalyst Isolation : successfully isolated the reactive, air-sensitive tetrakis(acetonitrile) ruthenium(II) bis(NHC) intermediate complex and demonstrated its use as a catalyst. • Mechanistic Insight : identified triethylamine as a dual-function agent, serving as both a base and an activator in the synthetic process. • Complex Selectivity : achieved selective isolation of both kinetic (trans) and thermodynamic ( cis ) isomers through the synthetic protocol. • Direct Arylation : demonstrated the catalysts’ efficiency in direct arylation reactions, with evidence suggesting the common pathway through bis(cyclometallated) intermediates is unlikely given the catalyst structure. • Transfer Hydrogenation : showed catalytic activity in transfer hydrogenation, these results surprisingly indicate that an inner-sphere mechanism predominates over the typical outer-sphere pathway involving amino ligands. A tetrakis(acetonitrile) complex bearing a bidentate bis(N-Heterocyclic Carbene) ligand was synthesized, and its formation was extensively studied. Triethylamine played a pivotal role in the reaction, initially forming a ruthenium(II)-triethylamino complex with the precursor. It then functioned as a base to produce the ruthenium(II) bis(NHC) complex, which was found to be very active in catalyzing the direct ortho-arylation of 2-phenylpyridine, with loadings as low as 2 % (a significant result considering that most reported examples in the literature require loadings as high as 10 % and are typically limited to monoarylations). Additionally, the reactive complex was successfully functionalized with various bidentate ligands of the amino, pyridine, and phosphine types, yielding a series of novel ruthenium(II) bis(NHC) dicationic organonitrile complexes. These complexes were evaluated in the catalytic transfer hydrogenation reaction, demonstrating notable activity (TOF of 50,000 h -1 ). It is notable that within the pool of reported ruthenium(II) complexes featuring bidentate bis(NHC) ligands, cis- [Ru(CH 3 CN) 2 (1,1′-bis(3-methyl-imidazol-2-ylidene)methylene))(2–2′Bipyridine)](PF 6 ) 2 demonstrates exceptional high rate, marking it as one of the fastest known. Overall, the synthesized tetrakis(acetonitrile) complex proves to be a promising platform for the development of future catalysts.