NOE and PGSE NMR Spectroscopic Studies of Solution Structure and Aggregation in Metallocenium Ion-Pairs

Zuccaccia, Cristiano; Stahl, Nicholas G.; Macchioni, Alceo; Chen, Ming Chou; Roberts, John A.; Marks, Tobin J.

doi:10.1021/ja0387296

Cited by 164 publications

(170 citation statements)

References 97 publications

(112 reference statements)

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“…As is discussed below, these results are in good agreement with the EXAFS data, and represent significantly longer distances than in typical molecular covalent Zr(IV)-OR bonds (1.94-2.01 Å), † suggesting very weak ion pairing. Moreover, the computational models of C/AlS yield Cp*Zr(CH 3 ) 2 + ···O AlS − structures in excellent agreement with the present solid-state NMR spectroscopic data and NMR data for metallocenium electrophiles in solution (20,26,29).…”

Section: Resultssupporting

confidence: 85%

“…2C) (24), whereas exposure of C/AlS to 13 C-enriched benzene yields a downfield shifted resonance at δ 127.5 ppm, consistent with benzene coordination to a cation-like d 0 species (Fig. 2D) (25)(26)(27). Subsequent treatment of this species with substoichiometric H 2 in a benzene slurry yields a signal assignable to cyclohexane at δ 28.5 ppm, presumably physisorbed on the AlS surface ( Fig.…”

Section: Resultsmentioning

confidence: 85%

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Surface structural-chemical characterization of a single-site d ⁰ heterogeneous arene hydrogenation catalyst having 100% active sites

Williams

Guo

Motta

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

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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...

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Section: Resultssupporting

confidence: 85%

Section: Resultsmentioning

confidence: 85%

Surface structural-chemical characterization of a single-site d ⁰ heterogeneous arene hydrogenation catalyst having 100% active sites

Williams

Guo

Motta

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

Self Cite

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“…This type of concentration dependence has been documented on several occasions. 22,23 An appreciation of the 1 H, 19 F HOESY results for 2 requires a detailed assignment of the most important protons (Table 2) plus an understanding of the molecular structure of the cation. Normally, a -allyl ligand will have the allyl anion not quite perpendicular to the plane defined by the two P atoms and the palladium atom.…”

Section: Nmr Results Formentioning

confidence: 99%

“…Moreover, the value '6', in the denominator of the equation, has been criticized. 23 Nevertheless, measured PGSE data for the individual ions offer a rapid, facile method of recognizing ion pairing.…”

Section: Introductionmentioning

confidence: 99%

¹H and ¹⁹F PGSE diffusion and HOESY NMR studies on cationic palladium (II) 1,3‐diphenylallyl complexes in THF solution

Fernández

Pregosin

2005

Magnetic Reson in Chemistry

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THF solutions of the cationic chiral 1,3-diphenylallyl bidentate phosphine complexes [Pd(eta(3)-PhCHCHCHPh)(Duphos)](CF(3)SO(3)), Duphos = 1,2-Bis-((2R,5R)-2,5-dimethylphospholano)benzene), 2, and [Pd(eta(3)-PhCHCHCHPh)(P,S)]BF(4), 4, P,S = [8-((o-(diphenylphosphino)benzyl) thiomethyl]-(7,7'-dimethyl)-exo-norborneol, have been studied via pulsed gradient spin-echo (PGSE) diffusion, (1)H, (19)F HOESY and a variety of other multi-dimensional NMR methods. On the basis of the (1)H, (19)F HOESY data, the anions show a preference for a specific structural position with respect to the eta(3)-PhCHCHCHPh allyl ligand, i.e. the anion does not move evenly around the periphery of the cation. THF is shown to promote significant ion pairing, although neither 2 nor 4 shows 100% ion pairing.

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“…8 (2) and 2.1100(2) Å] due to the increased metal electrophilic character, whereas the Ti-Me(bridging) separation is 0.230 Å longer, reflecting the largely ionic character of the ion pair interaction. NMR spectroscopy reveals characteristic fingerprints of a very coordinatively unsaturated, electrophilic metal center, as well as strong but coordinatively flexible ion pairing and negligible ion pair aggregation in the concentration ranges used for typical polymerization (23,24).…”

Section: Binuclear Perfluorophenyl Bisborane and Bisborate Cocatalystmentioning

confidence: 99%

Nuclearity and cooperativity effects in binuclear catalysts and cocatalysts for olefin polymerization

Marks

2006

Proc. Natl. Acad. Sci. U.S.A.

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A series of bimetallic organo-group 4 ''constrained geometry'' catalysts and binuclear bisborane and bisborate cocatalysts have been synthesized to probe catalyst center-catalyst center cooperativity effects on olefin enchainment in homogenous olefin polymerization and copolymerization processes. Significant nuclearity effects are found versus mononuclear controls, and the effect can be correlated with metal-metal approach distances and ion pairing effects. Novel polymer structures can be obtained by using such binuclear catalyst͞cocatalyst systems.catalysis ͉ polymer ͉ polyolefin E nzymes achieve superior reactivity and selectivity, in part, by their efficacy in creating high local reagent concentrations and special, conformationally advantageous active site-substrate proximities and interactions (1-4). In this regard, the possibility of unique and more efficient abiotic catalytic transformations based on cooperative effects between adjacent active centers in multinuclear transition metal complexes is currently of great interest. Within the context of the rapidly advancing and technologically significant field of homogeneous single-site olefin polymerization catalysis (5-10), the conjecture that cooperative effects involving two or more metal centers in close proximity might achieve more efficient chain propagation and͞or novel polymer architectures motivated the research that is the subject of this contribution. Single-Site Olefin Polymerization ProcessesOne of the most exciting developments in the areas of catalysis, organometallic chemistry, and polymer science in recent years has been the intense development of new polymerization technologies based on well defined single-site metallocene and coordination complex olefin polymerization catalysts (5-10). The active catalytic species is typically generated by combining a transition-metal organometallic precursor with an activator or cocatalyst. The resulting electrophilic͞coordinatively unsaturated, strongly ionpaired species then undergoes rapid olefin enchainment. In optimum cases (e.g., group 4 metal and cyclopentadienyl-type ligand), catalysts with truly exceptional productivities and selectivities for high-molecular-weight polyolefins are produced. Constrained Geometry Catalysts: Testbeds for Multinuclear Cooperativity EffectsGroup 4 ''constrained geometry'' catalysts (CGCs) (A; Fig. 1) are well known single-site polymerization agents (16, 17) that produce unusual branched, hence far more processable, polyethylenes with high productivity and selectivity. One of the key features of these catalysts is the coordinatively open nature of the active site, which allows rapid enchainment of sterically encumbered olefin comonomers into the polyethylene backbone. Under certain conditions, these catalysts yield vinyl-terminated, chain-transferred macromonomers (a macromolecule that acts as a monomer) that diffuse away and then undergo competitive reinsertion into a growing polymer chain to produce macromolecules with long chain branching (Fig. 19, which is published ...

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NOE and PGSE NMR Spectroscopic Studies of Solution Structure and Aggregation in Metallocenium Ion-Pairs

Cited by 164 publications

References 97 publications

Surface structural-chemical characterization of a single-site d ⁰ heterogeneous arene hydrogenation catalyst having 100% active sites

Surface structural-chemical characterization of a single-site d ⁰ heterogeneous arene hydrogenation catalyst having 100% active sites

¹H and ¹⁹F PGSE diffusion and HOESY NMR studies on cationic palladium (II) 1,3‐diphenylallyl complexes in THF solution

Nuclearity and cooperativity effects in binuclear catalysts and cocatalysts for olefin polymerization

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NOE and PGSE NMR Spectroscopic Studies of Solution Structure and Aggregation in Metallocenium Ion-Pairs

Cited by 164 publications

References 97 publications

Surface structural-chemical characterization of a single-site d 0 heterogeneous arene hydrogenation catalyst having 100% active sites

Surface structural-chemical characterization of a single-site d 0 heterogeneous arene hydrogenation catalyst having 100% active sites

1H and 19F PGSE diffusion and HOESY NMR studies on cationic palladium (II) 1,3‐diphenylallyl complexes in THF solution

Nuclearity and cooperativity effects in binuclear catalysts and cocatalysts for olefin polymerization

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Surface structural-chemical characterization of a single-site d ⁰ heterogeneous arene hydrogenation catalyst having 100% active sites

Surface structural-chemical characterization of a single-site d ⁰ heterogeneous arene hydrogenation catalyst having 100% active sites

¹H and ¹⁹F PGSE diffusion and HOESY NMR studies on cationic palladium (II) 1,3‐diphenylallyl complexes in THF solution