Synthesis, crystal structures, reactivity, and theoretical evaluations of aromatic pincer ligands with OCO-chelation motifs

Abstract

Pincer ligands bearing XCX-type scaffolds (X = N, P, S) have gained a great deal of attention as they allow the isolation of well-designed metal complexes, resulting in appealing structural and stereoelectronic features. In the present work, we report on the synthesis and characterization of a novel series of OCO conformation pincer ligands based on aryl benzyl ether structures, [(ROCH2)2C6H4], which contain unreported aromatic terminal units, where R = 2,6-dimethylphenyl (3a), 2,5-dimethylphenyl (3b), 2,4-dimethylphenyl (3c), [1,1′-biphenyl]-3-yl] (3d), [1,1′-biphenyl]-4-yl] (3e), and pentachlorophenyl (3f). Our rationale for choosing these fragments of aromatic derivatives (􀀀 OR terminal units) is to provide points of electronic modulation on the ligands as well as thermodynamic and kinetic properties. We accomplished the molecular structure of compounds 3a and 3e in the crystalline state by single-crystal X-ray structural analysis. Further, we emonstrate the reactivity of pincer ligand 3a in the presence of an organolithium reagent to afford the corresponding organolithium pincer compound 2,6-bis(2,6-dimethylphenoxy)methyl)phenyllithium (4). Interestingly, 4 proved highly stable over time, existing as dimers both in solid- and in solution-state as revealed by crystal structure and multinuclear and 2D NMR spectroscopy experiments, respectively. Finally, DFT calculations further assess the bonding situation for compounds 3a and 3e.