Coordination properties of 2,5-dimesitylpyridine: an encumbering and versatile ligand for transition-metal chemistry

To overcome the unfavorable steric pressures associated with 2,6-disubstitution in encumbering pyridine ligands, the coordination chemistry of a 2,5-disubstituted variant, namely, 2,5-dimesitylpyridine (2,5-Mes(2)py), is reported. This diaryl pyridine shows good binding ability to a range of transition-metal fragments with varying formal oxidation states and coligands. Treatment of 2.0 equiv of 2,5-Mes(2)py with monovalent Cu and Ag triflate sources generates complexes of the type M(2,5-Mes(2)py)(2)]OTf (M = Cu, Ag; OTf = OSO(2)CF(3)), which feature long M-OTf distances and a substrate-accessible primary coordination sphere. Combination of 2,5-Mes(2)py with Cu(OTf)(2) and Pd(OAc)(2) produces four-coordinate complexes featuring cis- and trans-2,5-Mes(2)py orientations, respectively. The four-coordinate palladium complex Pd(OAc)(2)(2,5-Mes(2)py)(2) is found to resist py-ligand dissociation at room temperature in solution, but functions as a precatalyst for the aerobic C-H bond olefination of benzene at elevated temperatures. This C-H bond activation chemistry is compared with a similar Pd-based system featuring 2,6-disubstituted pyridines. 2,5-Mes(2)py also readily supports mono- and dinuclear divalent Co complexes, and the solution-phase equilibria between such species are detailed. The coordination studies presented highlight the potential of 2,5-Mes(2)py to function as an encumbering ancillary for the stabilization of low-coordinate complexes and as a supporting ligand for metal-mediated transformations.