Structural characterization of the catalytically significant sites on solid catalyst surfaces is frequently tenuous because their fraction, among all sites, typically is quite low. Here we report the combined application of solid-state 13 C-cross-polarization magic angle spinning nuclear magnetic resonance ( 13 C-CPMAS-NMR) spectroscopy, density functional theory (DFT), and Zr X-ray absorption spectroscopy (XAS) to characterize the adsorption products and surface chemistry of the precatalysts (η 5 O rganometallic molecule-derived heterogeneous catalysts are of increasing interest owing to their enhanced thermal stability and activity vs. their homogeneous analogs, and their atomically precise tailorable metal-ligand structures vs. other heterogeneous catalysts (1, 2). Furthermore, it is becoming increasingly evident that the inorganic support in many systems is noninnocent and can function as both a ligand and an activator, with the chemically important but poorly understood nature of the catalyst-support interaction strongly modulating catalytic activity and selectivity (3, 4). When adsorbed on Lewis acidic, dehydroxylated alumina surfaces, group 4 complexes such as Cp 2 ZrR 2 (Cp = η 5 -C 5 H 5 ; A, R = H; B, R = CH 3 ) and Cp*Zr (CH 3 ) 3 [C, Cp* = η 5 -C 5 (CH 3 ) 5 ] were argued on the basis of highresolution solid-state NMR spectroscopy to transfer an alkyl anion to unsaturated, Lewis acidic surface sites as in Fig. 1 (complexes B, C → qualitative model D) (5, 6). The resulting catalysts are extremely active for olefin hydrogenation and polymerization, and analogous ion-paired species form the basis for large-scale industrial polymerization processes (7,8). However, kinetic poisoning experiments in which the catalytic sites are titrated in situ with H 2 O or t BuCH 2 OH indicate that ≤5% of D-type sites are catalytically significant, likely reflecting, among other factors, the established heterogeneity of alumina surfaces (5, 6, 9), hence rendering active site structural and chemical descriptions necessarily imprecise. In contrast to these results, chemisorption of such organozirconium precursors on SiO 2 , Al 2 O 3 , and SiO 2 -Al 2 O 3 surfaces having appreciable coverage by weakly acidic OH groups predominantly yields covalently bound, poorly electrophilic Etype species via Zr-CH 3 protonolysis with CH 4 evolution (5, 6, 10, 11). Although the E-type sites may be characterized in some detail by high-resolution solid-state NMR and extended X-ray adsorption fine structure spectroscopy (EXAFS), they display minimal catalytic turnover in the absence of added, complicating activators [e.g., methylalumoxane or B(C 6 F 5 ) 3 ], and the fraction of catalytically significant sites is unknown (12, 13). In such situations, it is experimentally impossible to unambiguously distinguish catalytically significant sites from inactive "spectator" sites, hence to fully understand the catalytic chemistry.-In marked contrast to the above results, chemisorption of these same organozirconium molecules on highly Brønsted "super...

Mono-and binuclear "constrained-geometry catalyst" (CGC) group 4 hydrocarbyls Me 2 Si(Me 5 C 5 )-, 4] undergo rapid chemisorption on highly Brønsted acidic sulfated alumina (AlS) surfaces. 13 C CPMAS NMR spectroscopy of the chemisorbed 13 C R H 3 -enriched complexes EICGCZr 13 Me 2 /AlS (2*/AlS) and EBICGC(Zr 13 Me 2 ) 2 /AlS (3*/AlS) reveals that chemisorption involves two processes, M-C σ-bond protonolysis at the strong surface Brønsted acid sites and heterolytic M-C scission with methide transfer to strong surface Lewis acid sites, forming similar "cation-like" electrophilic organo-group 4 complexes such as EICGCM 13 Me + . Relative rates of ethylene homopolymerization mediated by the catalysts prepared via chemisorption on AlS are 4/AlS > 2/AlS > 3/AlS > 1/AlS, for ethylene polymerization at 75 psi ethylene and 25 °C. Ethylene/1-hexene copolymerizations mediated by the same set of catalysts display relative polymerization rates of 4/AlS > 3/AlS > 2/AlS > 1/AlS, for copolymerizations at 75 psi ethylene, 0.8 M 1-hexene, and 25 °C.

Sulfated alumina nanoparticles (n-AlS) have been prepared as a highsurface-area support/activator for organozirconium catalysts. The large external surface area allows chemisorption of ca. 3 times more organozirconium catalyst per square nanometer than previously possible on bulk sulfated alumina. It is found that Cp*ZrMe 2 /n-AlS (Cp* = η 5 -(CH 3 ) 5 C 5 , Me = CH 3 ) and ZrBz 3 /n-AlS (Bz = CH 2 C 6 H 5 ) are active catalysts for arene hydrogenation as well as for olefin polymerizations. The less sterically encumbered ZrBz 3 /n-AlS exhibits higher polymerization activity and a greater arene hydrogenation turnover frequency than the slightly more encumbered Cp*ZrMe 2 /n-AlS. Catalyst ZrBz 3 /n-AlS also displays a larger tolerance for sterically demanding hydrogenation substrates as well as increased 1-hexene incorporation in ethylene/1-hexene copolymerizations.

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