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

Abstract

Rare-earth metal alkyl complexes with tridentate [OSO]-type and tetradentate [OSSO]-type bis(phenolato) ligands, [Ln(L)(CH 2 SiMe 3 )(THF) n ] (LH 2 = 2,2′-thiobis(6- tert -butyl-4-methylphenol) (tbmpH 2 ), 1,3-dithiapropanediylbis(6- tert -butyl-4-methylphenol) (mtbmpH 2 ), 1,4-dithiabutanediylbis(6- tert -butyl-4-methylphenol) (etbmpH 2 ); Ln = Y ( 1 − 3 ), Sc ( 4, 5 ), Lu ( 7 ), Ho ( 9, 10 )), were synthesized from the reactions of the tris(alkyl) complexes [Ln(CH 2 SiMe 3 ) 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(CH 2 SiMe 3 ) 3 (THF) 2 ] with H 2 etbmp in a 1:2 ratio led to the formation of the bis(phenolato)-bridged dinuclear complex [Lu 2 (etbmp) 3 (THF) 2 ] ( 8 ). The reaction of the holmium alkyl complexes 9 and 10 with PhSiH 3 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 (PhSiH 3, n BuSiH 3, and Ph 2 SiH 2 ). 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(SiH 2 Ph)Me with 97% regioselectivity. The hydrosilylation of 1,5-hexadiene by PhSiH 3 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 n BuSiH 3 84% of the cyclic product was obtained.