Kinetics and Mechanisms of Thermally Induced Alkane Eliminations from Silica-Supported Bis(alkyl)chromium(IV) and -vanadium(IV) Complexes

Abstract

Silica-supported bis(alkyl) complexes of Cr(IV) and V(IV), (tSiO) 2 M(CH 2 R) 2 , undergo surprisingly clean thermal transformations to generate supported alkylidene complexes, (tSiO) 2 MdCHR, with concurrent liberation of RCH 3 . The reactions are quantitative and kinetically first order, suggesting that all surface organometallic complexes possess or can access very similar coordination environments. Isotope-labeling and kinetics experiments support a mechanistic assignment of intramolecular R-H elimination. In the case of the Cr complexes, the temperature dependences of the first-order rate constants give nonlinear Eyring plots that are concave downward. A two-step mechanism of reversible (alkyl)(alkylidene)Cr VI hydride formation followed by reductive elimination of alkane is proposed. This mechanism is not accessible for V(IV), therefore concerted R-H elimination is considered more likely. The activation parameters for the thermolysis of (tSiO) 2 V(CH 2 -SiMe 3 ) 2 , obtained from the linear Eyring plot, are ∆H q ) (11.0 ( 1.0) kcal/mol and ∆S ‡ ) (-43 ( 3) cal K -1 mol -1 . The large negative value of ∆S ‡ is suggestive of surface assistance for R-H elimination.

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

“…In the presence of ethylene,t he reduced Cr II site is proposed to form Cr IV -alkyl complexes that could coordinate and insert ethylene (Scheme 17). [259,[273][274][275][276][277][278][279][280] Another possibility is the oxidative coupling of ethylene on aC r II site to form aC r IV -metallacyclopentane (Scheme 17) [262,281] that can grow into ap olymer by ring expansion [282] or by b-H elimination to form aC r IV -allyl hydride (SiO) 2 Cr(H)(h 3 -allyl) that is able to insert ethylene and start polymer growth. Insertion of ethylene in either the Cr-vinyl or the Cr À Hb ond of this species can initiate the polymerization.…”

“…[278,279,[298][299][300][301] Thep utative (SiO) 2 Cr(=CHEMe 3 )s pecies is proposed to react with ethylene to form oligomers and polymers (Scheme 19). Heating ( SiO) 2 Cr(CH 2 EMe 3 ) 2 to 70 8 8Co r150 8 8C results in the elimination of 1equiv of the corresponding alkane per Cr, which suggests the formation of ( SiO) 2 Cr-(=CHEMe 3 )t hrough a-H abstraction.…”

“…Insertion of ethylene in either the Cr‐vinyl or the Cr−H bond of this species can initiate the polymerization . It could also isomerize to Cr IV ‐alkylidene (≡SiO) 2 Cr(=CHMe), which could propagate polymer growth through a Green–Rooney mechanism . Another possibility is the oxidative coupling of ethylene on a Cr II site to form a Cr IV ‐metallacyclopentane (Scheme ) that can grow into a polymer by ring expansion or by β‐H elimination to form a Cr IV ‐allyl hydride (≡SiO) 2 Cr(H)(η 3 ‐allyl) that is able to insert ethylene and start polymer growth .…”

Eine Brücke zwischen industriellen und wohldefinierten Trägerkatalysatoren

“…Durch Ethyleninsertion in die Cr‐Vinyl‐ oder die Cr‐H‐Bindung kann diese Spezies die Polymerisation einleiten . Sie könnte auch zu dem Cr IV ‐Alkyliden (≡SiO) 2 Cr(=CHMe) isomerisieren, das durch einen Green‐Rooney‐Mechanismus zu Polymerwachstum führen könnte . Eine weitere Möglichkeit ist die oxidative Kupplung von Ethylen an ein Cr II ‐Zentrum zu einem Cr IV ‐Metallacyclopentan (Schema ), von dem aus durch Ringerweiterung oder nach β‐H‐Eliminierung zu einem Cr IV ‐Allyl‐Hydrid (≡SiO) 2 Cr(H)(η 3 ‐Allyl) unter Ethyleninsertion ein Polymer wachsen kann .…”

“…Cr(CH 2 EMe 3 ) 4 (E=C oder Si) reagiert mit teildehydroxyliertem Siliciumdioxid zu (≡SiO) 2 Cr(CH 2 EMe 3 ) 2 (Schema b). Erhitzen von (≡SiO) 2 Cr(CH 2 EMe 3 ) 2 auf 70 °C oder 150 °C führt zur Eliminierung von 1 Äquiv des entsprechenden Alkans pro Cr, ein Hinweis auf die Entstehung von (≡SiO) 2 Cr(=CHEMe 3 ) durch α‐H‐Abstraktion . Es wird vermutet, dass die Spezies (≡SiO) 2 Cr(=CHEMe 3 ) mit Ethylen reagiert, um Oligomere und Polymere zu bilden (Schema ).…”