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

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[AII 2 ]. Reaction of the H[AII 2 ] pro-ligand with one equivalent of [M(CH 2 SiMe 3 ) 3 (THF) 2 ] (M = Y or Sc) yielded the base-free neutral dialkyl complexes [(AII 2 )M(CH 2 SiMe 3 ) 2 ] {M = Y (1) and Sc (2)}. The rigid AII 2 pincer ligand affords a similar steric profile to the previously reported XA 2 pincer ligand, but is monoanionic rather than dianionic. Reaction of 1 with one equiv. of [CPh 3 ][B(C 6 F 5 ) 4 ] in C 6 D 5 Br 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 [(AII 2 -CH 2 SiMe 3 )Y(CH 2 SiMe 3 ) 2 ][B(C 6 F 5 ) 4 ] (3; AII 2 -CH 2 SiMe 3 is a neutral tridentate ligand with a central amine donor flanked by imidazolin-2-imine groups) in approx. 20% yield, accompanied by HCPh 3 (∼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 CPh 3 + is thought to involve initial AII 2 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.