Intramolecular C-H Bond Activation by Lanthanoid Complexes Bearing a Bulky Aminopyridinato Ligand

The present work is aimed towards the synthesis of C-H activation products of various group 3 and lanthanoid metals bearing a bulky aminopyridinato ligand, (2,6-diisopropylphenyl)[6-(2,6-dimethylphenyl)pyridin-2-yl]amine (1, Ap'H). Deprotonation of I using KH leads to polymeric [Ap'K](n) (2), which undergoes clean salt metathesis reaction with MX3 [M = Sc, Nd and Sm, and X = Cl or M = La and X = Br] forming mono thf adducts [Ap'2ScCl(thf)] (3), [Ap'2LaBr(thf)] (4), [Ap'2NdCl(thf)] (5), and [Ap'2SmCl(thf)] (6). However, reacting 2 with LuCl3 leads to mono- as well as bis(aminopyridinato)lutetium complexes [Ap'LuCl2(thf)(2)] (7) and [Ap'2LuCl(thf)] (8), respectively, while the analogous reaction with LaCl3 at 50 degrees C produces the tris(aminopyridinato)lanthanum complex [Ap'La-3] (9). For the selective synthesis of 8 in good yield amine elimination route was adopted. X-ray diffraction studies revealed a distorted octahedral coordination for the bis(aminopyridinato) complexes 3, 4 and 6, despite the differences in their ionic radii, Alkylation of the bis(aminopyridinato) monohalide complexes with equimolar amounts of LiCH2SiMe3, in hexane allowed the isolation of the corresponding alkyl derivatives. For the smaller metals like Sc and Lu affording [Ap'2ScCH2-SiMe3(thf)] (10) and [Ap'2LuCH2SiMe3(thf)] (11), respectively. However, lanthanoids with large ionic radii such as La and Nd resulted in the formation of methyl group C-H bond activation products [Ap'(Ap'(-H))La(thf)(2)] (12) and [Ap'-(Ap'(-H))Nd(thf)] (13), respectively. Most likely an alkyl species was formed which then undergoes intramolecular C-H activation and C-H activation runs fast with regard to the rate of alkyl complex formation. The alkylation of 6 (Sm) with LiCH2SiMe3 did not give a clear product. The reaction of 11 with PhSiH3 (Ph = phenyl) led via intramolecular C-H bond activation to [Ap'(Ap'(-H))Lu(thf)] (14). In this case most likely a hydride species was formed which then undergoes rapid C-H activation. The alkyl complex 10 (Sc) did not react with PhSiH3. The molecular structures of 11, 12 and 13 have been confirmed by X-ray crystal structure analysis.