Cyclometalation and coupling of a rigid 4,5-bis(imino)acridanide pincer ligand on yttrium

Abstract

An extremely rigid NNN-donor proligand, 4,5-bis{(diphenylmethylene)amino}-2,7,9,9-tetramethylacridan, H[AIm₂] was prepared in five steps starting from 5-methyl-2-aminobenzoic acid and 4-bromotoluene. Reaction of intensely orange H[AIm₂] with LiCH₂SiMe₃ formed deep blue Li_x[AIm₂]_x (x = 2 in the solid state), while reaction with [Y(CH₂SiMe₃)₃(THF)₂] (0.5 equiv.) afforded deep blue [Y(AIm₂)(AIm^′₂)] (1; AIm^′₂ = an AIm₂ ligand cyclometalated at the ortho-position of one of the phenyl rings). Compound 1 slowly isomerizes to form green-brown 2, which contains a single trianionic, hexadentate ligand that features one amine, two imine, and three amido donors. The acridanide backbone and one imine group in each of the original AIm₂ ligands is intact, but the two acridanide backbones are now linked by an isoindoline heterocycle. Yttrium in 2 is coordinated to six nitrogen donors and the ortho carbon of an isoindoline phenyl substituent. The intense colours of H[AIm₂], Li_x[AIm₂]_x and 1 were shown by TD-DFT calculations to arise from charge transfer transitions from the HOMO, which is localized on the acridanide ligand backbone, to the LUMO and LUMO+1, which are localized on the imine substituents. The conversion of 1 to 2 was studied by UV-Visible absorption spectroscopy and is first-order with a half-life of 7.8 hours at room temperature.