Carbon‐Hydrogen‐Transition Metal Bonds

Brookhart, Maurice; Green, Malcolm L. H.; Wong, Luet‐Lok

doi:10.1002/9780470166376.ch1

Cited by 755 publications

(151 citation statements)

References 234 publications

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“…The lack of well characterized examples of species with M⅐⅐⅐C-C interactions is in contrast with M⅐⅐⅐HC agostic complexes that are well established (42,43), some of which go on to C-H activate (44).…”

mentioning

confidence: 92%

C–C σ complexes of rhodium

Brayshaw

Sceats²,

Green³

et al. 2007

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

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In this article, the complexes [Rh(Binor-S )(PR3)][BAr F 4 ] (R ‫؍‬ i Pr, Cy, C 5H9) are described. A combination of x-ray crystallography, NMR spectroscopy, density functional theory, and ''atoms in molecules'' calculations unequivocally demonstrates that the complexes contain rare examples of metal⅐⅐⅐C-C agostic interactions. Moreover, they are fluxional on the NMR time scale, undergoing rapid and reversible C-C activation. Kinetic data and calculations point to a bismetallacyclobutane, Rh(V), intermediate. C-C activation ͉ density functional theory

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mentioning

confidence: 92%

C–C σ complexes of rhodium

Brayshaw

Sceats²,

Green³

et al. 2007

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

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“…11. In this scenario, the eliminated oligomeric or polymeric vinyl macromonomer chain produced at one catalytic center is detained by binuclear interactions (presumably C-H⅐⅐⅐M ϩ agostic interactions between the electrophilic metal center and electron-rich C-H bonds-the importance of mononuclear agostic examples is of course well established in cationic d 0 polymerizations (29)(30)(31), involving the adjacent electrophilic metal center, and the weakly bound oligomeric͞ polymeric ␣-olefinic fragment therefore has an enhanced probability of subsequent intramolecular re-enchainment with 1,2 regiochemistry at a proximate Zr-ethyl ϩ or Zr-PЈ ϩ site. This model is consistent with an observed increase in product molecular weight with increasing catalyst͞cocatalyst nuclearity, with conversiondependent experiments revealing significant quantities of ethyl branches (but few other branches) produced in the Zr 2 ϩ B 2 polymerization system, even in the very earliest stages of the polymerization.…”

Section: Binuclear Structures and Macromolecular Branch Formation In mentioning

confidence: 99%

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

Marks

2006

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

224

109

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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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“…Thus, agostic M-H-C interactions are characterized by relatively short M-H distances (Ϸ1.8-2.3 Å) and small M-H-C bond angles (Ϸ90-140°), whereas anagostic interactions are characterized by relatively long M⅐⅐⅐H distances (Ϸ2.3-2.9 Å) and large M-H-C bond angles (Ϸ110-170°) (40,41). With respect to 1 H NMR spectroscopy, a signature of an agostic interaction is an unusually low 1 J CH value, which can range from 50 to 100 Hz (13,14). Furthermore, the chemical shifts of agostic hydrogen atoms are typically observed upfield of the uncoordinated group, whereas anagostic hydrogen atoms are typically observed downfield; the latter observation is in accord with the hydrogen-bonded description of the interaction.…”

Section: Agostic and Anagostic Interactionsmentioning

confidence: 99%

“…To emphasize this point, a review on the subject was published in 1983 (13), after which the area developed rapidly, and a second review was published in 1988 (14). These reviews have been cited 1,097 and 715 times, respectively (as of November 2006).…”

Section: Introduction and Historical Perspectivementioning

confidence: 99%