Oxidative homolysis of pentaaquoorganochromium(III) cations induced by macrocyclic nickel(III) complexes

Katsuyama, Tetsuo; Bakač, Andreja; Espenson, James H.

doi:10.1021/ic00301a037

Cited by 13 publications

(11 citation statements)

References 1 publication

(1 reference statement)

Supporting

Mentioning

Contrasting

Order By: Relevance

“…A polymerization-ready (organo)Cr III site would result from homolytic cleavage of one of the Cr–C bonds in 3 B.IVa . A similar reaction occurs readily in the molecular complex ions [CrR(H 2 O) 5 n + ] ( n = 2, 3) and has been observed in organometallic complexes such as CpCr[(ArNCMe) 2 CH]R . Liberation of an ethenyl radical to give 4 B.VIIIa has a high free energy change of 151 kJ/mol (Figure S3 in the Supporting Information), but ethyl radical release to give 4 B.Va is more favorable, requiring just 98 kJ/mol (Figure ).…”

Section: Resultssupporting

confidence: 53%

“…Molecular [Cr III R(H 2 O) 5 2+ ] complexes with primary alkyl ligands undergo Cr–C bond homolysis slowly in aqueous solution at room temperature . However, the reaction becomes very fast following one-electron oxidation to [Cr IV R(H 2 O) 5 3+ ], in a process described as “oxidative homolysis” . We considered how this facile reaction mechanism (denoted A for aqueous) might be relevant to the (ethenyl)(ethyl)Cr IV site 3 B.IVa , for which Cr–C homolysis is predicted to have a high energy barrier (172 kJ/mol).…”

Section: Resultsmentioning

confidence: 99%

“…We also consider possible alternatives which could account for formation of the first (organo)Cr III sites during Phillips initiation. In particular, bis(alkyl)Cr IV precursors might undergo Cr–C bond homolysis to give mono (alkyl)Cr III active sites, such as molecular [CrR(H 2 O) 5 n + ] complexes ( n = 2, 3) that liberate R • readily . The individual reaction steps considered here are summarized in Figure .…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

One-Electron-Redox Activation of the Reduced Phillips Polymerization Catalyst, via Alkylchromium(IV) Homolysis: A Computational Assessment

2016

View full text Add to dashboard Cite

In ethylene polymerization by the Phillips catalyst, inorganic Cr(II) sites are believed to be activated by reaction with ethylene to form (alkyl)CrIII sites, in a process that takes about 1 h at ca. 373 K. The detailed mechanism of this spontaneous self-initiation has long remained unknown. It must account both for the formation of the first Cr–C bond and for the one-electron oxidation of Cr(II) to Cr(III). In this study, we used density functional theory to investigate a two-step initiation mechanism by which ethylene oxidative addition leads first to various (organo)CrIV sites, and subsequent Cr–C bond homolysis gives (organo)CrIII sites capable of polymerizing ethylene. Pathways involving spin crossing, C–H oxidative addition, H atom transfer, and Cr–C bond homolytic cleavage were explored using a chromasiloxane cluster model. In particular, we used classical variational transition theory to compute free energy barriers and estimate rates for bond homolysis. A viable route to a four-coordinate bis(alkyl)CrIV site was found via spin crossing in a bis(ethylene)CrII complex followed by intramolecular H atom transfer. However, the barrier for subsequent Cr–C bond homolysis is a formidable 209 kJ/mol. Increasing the Cr coordination number to 6 with additional siloxane ligands lowers the homolysis barrier to just 47 kJ/mol, similar to reported homolysis paths in molecular [CrR(H2O)5 3+] complexes. However, siloxane coordination also raises the barrier for the prior oxidative addition step to form the bis(alkyl)CrIV site. Thus, we suggest that hemilability in the silica “ligand” may facilitate the homolysis step while still allowing the oxidative addition of ethylene.

show abstract

Section: Resultssupporting

confidence: 53%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

One-Electron-Redox Activation of the Reduced Phillips Polymerization Catalyst, via Alkylchromium(IV) Homolysis: A Computational Assessment

2016

View full text Add to dashboard Cite

show abstract

“…Previously, we proposed that a vinylCr III site could form by Cr–C bond homolysis (step 4) . Such reactions are well-documented and extremely rapid in at least some organoCr IV complexes. − The higher bond strength of the Cr-vinyl bond relative to the Cr-ethyl bond suggests that an ethyl radical would be extruded preferentially from an (ethyl)(vinyl)Cr IV intermediate. , We computed free energies for homolytic bond dissociation in (ethyl)(vinyl)Cr IV complexes with a bis(silanolate) supporting ligand at 100 °C . The results depend strongly on the Cr coordination number, varying from 98 kJ/mol for four-coordinate Cr to just 5 kJ/mol for 6-coordinate Cr, where the additional spectator ligands are provided by siloxanes (ethylene appears to be ineffective as a spectator ligand in enhancing the homolysis rate).…”

Section: Resultsmentioning

confidence: 99%

Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Ethylene Activation by Cr(II) and the Structure of the Resulting Active Site

et al. 2017

View full text Add to dashboard Cite

The structure and mechanism of the formation of sites which initiate ethylene polymerization in the atomically dispersed Phillips catalyst (Cr/ SiO 2 ) are two of the great unsolved mysteries of heterogeneous catalysis. After CO or C 2 H 4 reduction of silica-supported Cr VI ions to Cr II ions in the precatalyst, exposure to ethylene results in the formation of organoCr III sites that are capable of initiating polymerization without recourse to an external alkylating cocatalyst. In this work, a Phillips catalyst prepared, via sol−gel chemistry, as a mesoporous, optically transparent monolith was reduced with CO to the spectroscopically determined Cr II end point. Ethylene causes rapid reoxidation of these Cr II sites to Cr III , even at low temperatures. Solid-state 13 C CP-MAS NMR, IR, and Raman spectroscopies reveal that the resulting sites contain a vinyl ligand, described as (≡SiO) 2 Cr III −CHCH 2 although likely with a higher coordination number, which are capable of initiating polymerization. The formation of these vinyl sites is an incommensurate redox reaction involving one-electron oxidation of Cr II via ethylene disproportionation. The accompanying formation of organic radical intermediates and their characteristic reaction products suggest that the key step is homolysis of a Cr−ethyl bond. Plausible pathways for the initiation mechanism are suggested.

show abstract

“…Ethylene disproportionation to give (SiO) 2 Cr IV (CH 2 CH 3 )(CHCH 2 ) sites, followed by ethyl radical extrusion, was found to be consistent with the observed formation of organic radicals during initiation. 22,23 Thus, ab initio calculations, 8 as well as experimental observations 22,23 and precedents from molecular chromium chemistry, 24,25 all suggest a probable role for homolysis pathways in the initiation mechanism. However, our previous computational investigation using simple, rigid cluster models predicted that a two-step mechanism involving activation of coordinated ethylene to give an (ethyl)(vinyl)Cr(IV) site, followed by homolytic Cr−C bond cleavage, would require hemilabile coordination of siloxane ligands, and no suitable arrangement of ligands was found to allow both steps to proceed at appreciable rates.…”

mentioning

confidence: 92%

Computational Support for Phillips Catalyst Initiation via Cr–C Bond Homolysis in a Chromacyclopentane Site

et al. 2018

View full text Add to dashboard Cite

Using density functional theory, we examine a possible homolysis initiation mechanism for the Phillips catalyst, starting from CrII sites exposed to ethylene. Spin-crossing in an abundant quintet bis(ethylene) CrII site leads to cycloaddition to form a chromacyclopentane site. One Cr–C bond then homolyzes to generate a tethered n-butyl radical: [Cr(CH2)3CH2 •]. If the radical attaches to a nearby inorganic Cr site, it yields two alkylCrIII sites capable of Cossee–Arlman polymerization. The overall computed barrier for this initiation process is 132 kJ/mol, which is comparable to the 120 kJ/mol value that we estimated from reported initiation times in industrial reactors. Poisson statistics suggest that this mechanism could activate ∼35% of Cr sites on a commercial catalyst with a loading of 0.4 Cr/nm2. Pairwise Cr grafting, amplification by complementary initiation reactions, or the creation of dangling bonds that form as the silica support fractures, might explain the apparent increase in per-site activity at lower Cr loadings.

show abstract

Oxidative homolysis of pentaaquoorganochromium(III) cations induced by macrocyclic nickel(III) complexes

Cited by 13 publications

References 1 publication

One-Electron-Redox Activation of the Reduced Phillips Polymerization Catalyst, via Alkylchromium(IV) Homolysis: A Computational Assessment

One-Electron-Redox Activation of the Reduced Phillips Polymerization Catalyst, via Alkylchromium(IV) Homolysis: A Computational Assessment

Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Ethylene Activation by Cr(II) and the Structure of the Resulting Active Site

Computational Support for Phillips Catalyst Initiation via Cr–C Bond Homolysis in a Chromacyclopentane Site

Contact Info

Product

Resources

About