Group 3 dialkyl complexes of a rigid monoanionic NNN-donor pincer ligand: synthesis, structures, unexpected reactivity with CPh3+, and hydroamination catalysis

Abstract

Palladium-catalyzed coupling of 4,5-dibromo-2,7,9,9-tetramethylacridan with two equivalents of 1,3-diisopropylimidazolin-2-imine afforded 4,5-bis(1,3-diisopropylimidazolin-2-imino)-2,7,9,9-tetramethylacridan, H[AII₂]. Reaction of the H[AII₂] pro-ligand with one equivalent of [M(CH₂SiMe₃)₃(THF)₂] (M = Y or Sc) yielded the base-free neutral dialkyl complexes [(AII₂)M(CH₂SiMe₃)₂] {M = Y (1) and Sc (2)}. The rigid AII₂ pincer ligand affords a similar steric profile to the previously reported XA₂ pincer ligand, but is monoanionic rather than dianionic. Reaction of 1 with one equiv. of [CPh₃][B(C₆F₅)₄] in C₆D₅Br generated a highly active catalyst for intramolecular alkene hydroamination. However, rather than forming the expected monoalkyl cation, this reaction afforded a diamagnetic product which was identified as [(AII₂-CH₂SiMe₃)Y(CH₂SiMe₃)₂][B(C₆F₅)₄] (3; AII₂-CH₂SiMe₃ is a neutral tridentate ligand with a central amine donor flanked by imidazolin-2-imine groups) in approx. 20% yield, accompanied by HCPh₃ (∼2 equiv. relative to 3), an unidentified paramagnetic product (detected by EPR spectroscopy), and a small amount of colourless precipitate. The unexpected reactivity of 1 with CPh₃⁺ is thought to involve initial AII₂ ligand backbone oxidation, given that the zwitterionic form of the ligand contains a phenylene ring with two adjacent anionic nitrogen donors, similar to a redox-non-innocent, dianionic ortho-phenylenediamido ligand.