Cyclometallation following coordination of anionic and neutral Lewis bases to a uranium(IV) dialkyl complex

Addition of 3.05 equiv of dme (1,2-dimethoxyethane) to an n-pentane solution of [(XA2)U(CH2SiMe3)2] (1; XA2 = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) and LiCH2SiMe3 (∼1:1) precipitated the tris(alkyl) 'ate' complex, [Li(dme)3][(XA2)U(CH2SiMe3)3] (2-dme). Sterically encumbered 2-dme is thermally unstable in solution, eliminating SiMe4 to form [Li(dme)3][(XA2*)U(CH2SiMe3)2] (3-dme; XA2* = XA2 cyclometallated at the methine carbon of an isopropyl group) as the major product. Compound 1 did not react with PMe3, 2,2ʹ-bipyridine (bipy), or quinuclidine in benzene at 40 °C. However, 1 reacted with 2.1 equiv of 4-(dimethylamino)pyridine (DMAP) or 9-azajulolidine (AJ; a DMAP derivative featuring a fused tricyclic structure) at 22 °C to afford [(XA2)U(CH2SiMe3)(η2 CN-DMAP*)(DMAP)] (4) and [(XA2)U(CH2SiMe3)(η2 CN-AJ*)(AJ)] (5), respectively, where DMAP* and AJ* are DMAP and AJ ligands cyclometallated at the 2-position. Reaction of 1 with DMAP-d 2 (deuterated in the ortho positions) confirmed that 4 is formed via a direct σ-bond metathesis mechanism. All newly-isolated uranium complexes (2–5) were crystallographically characterized.