Extremely stable thorium(IV) dialkyl complexes supported by rigid tridentate 4,5-bis(anilido)xanthene and 2,6-bis(anilidomethyl)pyridine ligands

A new NON-donor ligand, 4,5-bis(2,6-diisopropylanilino)-2,7-di-tert-butyl-9,9-dimethylxanthene (H-2[XA(2)], 1), was prepared by palladium-catalyzed coupling of 2,6-diisopropylaniline with the appropriate dibromoxanthene precursor. Stable K-2(dme)(2)[XA(2)] (2) and Na-2[XA(2)] (3) salts were accessible by deprotonation of H-2[XA(2)] with KH in dme or NaH in toluene. The thermally unstable lithium salt of McConville's 2,6-bis(2,6-diisopropylanilidomethyl)pyridine ligand (Li-2[BDPP], 4) was isolated by deprotonation with nBuLi or LiCH2SiMe3 in hexanes at low temperature. Reaction of [ThCl4(dme)(2)] with Li-2[BDPP] or M-2(dme)(n)[XA(2)] resulted in the formation of pentagonal bipyramidal [LThCl2(dme)] complexes (L = BDPP, 5; XA(2), 6). Subsequent reaction of 5 or 6 with LiCH2SiMe3 gave base- and salt-free dialkyl complexes, [LTh(CH2SiMe3)(2)] (L = BDPP, 7; XA(2), 8), which are stable for days in solution at 90 and 70 degrees C, respectively. Complexes 5, 7, and 8 were also accessible by initial combination of 2 or 4 equiv of LiCH2SiMe3 with [ThCl4(dme)(2)], followed by addition of H2L. These reactions likely proceed by alkane elimination, but dialkyl or tetraalkyl thorium intermediates were not identified. The X-ray crystal structure of 8 suggests the presence of alpha-agostic C-H-Th interactions for both alkyl groups. In solution, 7 and 8 exhibit temperature-dependent (1)J(C,H) coupling constants for ThCH2, demonstrating the presence of alpha-agostic interactions which become increasingly favored at lower temperature. Reaction of 5 with Li-2[BDPP] at 0 degrees C or 7 with H-2[BDPP] at 100 degrees C resulted in the formation of extremely sterically encumbered [Th(BDPP)(2)] (9), which adopts a highly distorted six-coordinate geometry with the four anilido groups arranged in an approximate tetrahedron around thorium. Bis-ligand complexes were not accessible with the XA(2) platform, presumably due to increased ligand rigidity.