Rare-Earth Metal Alkyl and Hydrido Complexes Containing a Thioether-Functionalized Bis(phenolato) Ligand: Efficient Catalysts for Olefin Hydrosilylation

Rare-earth metal alkyl complexes with tridentate [OSO]-type and tetradentate [OSSO]-type bis(phenolato) ligands, [Ln(L)(CH2SiMe3)(THF)n] (LH2 = 2,2′-thiobis(6-tert-butyl-4-methylphenol) (tbmpH2), 1,3-dithiapropanediylbis(6-tert-butyl-4-methylphenol) (mtbmpH2), 1,4-dithiabutanediylbis(6-tert-butyl-4-methylphenol) (etbmpH2); Ln = Y (1−3), Sc (4, 5), Lu (7), Ho (9, 10)), were synthesized from the reactions of the tris(alkyl) complexes [Ln(CH2SiMe3)3(THF)2] with the corresponding bis(phenol) via alkane elimination. The alkyl complexes were characterized by NMR spectroscopy (Y, Sc, Lu) and elemental analysis as well as by X-ray crystal structure analysis (5, 7). The reaction of [Lu(CH2SiMe3)3(THF)2] with H2etbmp in a 1:2 ratio led to the formation of the bis(phenolato)-bridged dinuclear complex [Lu2(etbmp)3(THF)2] (8). The reaction of the holmium alkyl complexes 9 and 10 with PhSiH3 resulted in the formation of the corresponding hydrido complexes [Ho(L)(μ-H)(THF)n]2 (L = tbmp, n = 3, 11; L = etbmp, n = 2, 12). The formation of the yttrium analogues could be observed by NMR spectroscopy. Complexes 2, 4, and 5 were tested in the hydrosilylation of a wide variety of aliphatic and aromatic 1-alkenes and 1,5-hexadiene with various silanes (PhSiH3, nBuSiH3, and Ph2SiH2). In the case of terminal aliphatic alkenes an anti-Markovnikov (1,2) addition takes place with 80−99% regioselectivity. The hydrosilylation of styrene afforded the Markovnikov (2,1) addition product PhHC(SiH2Ph)Me with 97% regioselectivity. The hydrosilylation of 1,5-hexadiene by PhSiH3 catalyzed by 2 resulted in the formation of a linear product, 1,6-bis(phenylsilyl)hexane (ca. 90%), and a cyclic product, (phenylsilylmethyl)cyclopentane (ca. 10%), whereas with nBuSiH3 84% of the cyclic product was obtained.