Surface Organozirconium Electrophiles Activated by Chemisorption on “Super Acidic” Sulfated Zirconia as Hydrogenation and Polymerization Catalysts. A Synthetic, Structural, and Mechanistic Catalytic Study

Abstract

Structural studies including 13C CPMAS NMR spectroscopy of the 13Cα-enriched model adsorbates Cp‘2Th(13CH3)2 (1*) and CpTi(13CH3)3 (2*) (Cp = η5-C5H5, Cp‘= η5-(CH3)5C5), and organozirconium adsorbates Cp2Zr(13CH3)2 (3*), Cp‘Zr(13CH3)3 (4*), and Zr(13CH2tBu)4 (6*) chemisorbed on superacidic sulfated zirconia (ZRSx; x = activation temperature) reveal that all adsorbates undergo a new molecular chemisorptive process: protonolytic M−C σ-bond cleavage at the very strong surface Brønsted acid sites to yield “cation-like” organometallic electrophiles. A kinetic and mechanistic study is reported for olefin and arene hydrogenation and α-olefin homopolymerization meditated by the catalysts formed by chemisorption of Cp2Zr(CH3)2 (3), Cp‘Zr(CH3)3 (4), Zr(CH2TMS)4 (5), Zr(CH2tBu)4 (6), and Zr(CH2Ph)4 (7), onto zirconia (ZR) or ZRSx. At 25 °C, 14.7 psi H2, 1-hexene hydrogenation activity follows the order 4/ZRS400 ≫ 3/ZRS400 ≥ 3/ZRS300 ≫ 3/ZRS720 ≫ 3/ZR ∼ 0. Benzene hydrogenation rates (25 °C, 14.7 psi H2) follow the order 4/ZRS400 ≫ 5/ZRS400 > 6/ZRS400 > 7/ZRS400, with Nt = 970 h-1 for 4/ZRS400 making this the most active benzene hydrogenation catalyst yet discovered. As a function of arene substituent(s), 4/ZRS400 exhibits high chemoselectivity, with hydrogenation rates following the order benzene ≫ toluene ≫ p-xylene ∼ 0. For benzene hydrogenation by 6/ZRS400, kinetic data obey the rate law Nt = kobs[arene]0[PH2]1 with Ea = 10.3(8) kcal mol-1. Partially hydrogenated products are not detected at partial conversions, with H2 delivered pairwise to both faces of C6D6, forming all-cis and cis, cis, trans, cis, trans isotopomers (1:3.1). Protonolytic poisoning experiments reveal that a maximum of ∼68% of Zr sites in 4/ZRS400 are catalytically significant for benzene hydrogenation. Relative homopolymerization rates are 7/ZRS400 > 5/ZRS400 > 6/ZRS400 > 4/ZRS400 for both ethylene (150 psi C2H4, 60 °C) and liquid propylene (20 °C).