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 13 C CPMAS NMR spectroscopy of the 13 C α -enriched model adsorbates Cp‘ 2 Th( 13 CH 3 ) 2 ( 1 *) and CpTi( 13 CH 3 ) 3 ( 2 *) (Cp = η 5 -C 5 H 5, Cp‘= η 5 -(CH 3 ) 5 C 5 ), and organozirconium adsorbates Cp 2 Zr( 13 CH 3 ) 2 ( 3 *), Cp ‘ Zr( 13 CH 3 ) 3 ( 4 *), and Zr( 13 CH 2 tBu) 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 Cp 2 Zr(CH 3 ) 2 ( 3 ), Cp‘Zr(CH 3 ) 3 ( 4 ), Zr(CH 2 TMS) 4 ( 5 ), Zr(CH 2 tBu) 4 ( 6 ), and Zr(CH 2 Ph) 4 ( 7 ), onto zirconia (ZR) or ZRSx. At 25 °C, 14.7 psi H 2, 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 H 2 ) follow the order 4 /ZRS400 ≫ 5 /ZRS400 > 6 /ZRS400 > 7 /ZRS400, with N t = 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 N t = k obs [arene] 0 [ P H 2 ] 1 with E a = 10.3(8) kcal mol - 1 . Partially hydrogenated products are not detected at partial conversions, with H 2 delivered pairwise to both faces of C 6 D 6, 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 C 2 H 4, 60 °C) and liquid propylene (20 °C).