Formation of High-Oxidation-State Metal–Carbon Double Bonds

Abstract

This tutorial explores the major pathways of forming metal–carbon double bonds in high-oxidation-state alkylidene complexes that began with the alkylidene chemistry of tantalum complexes in the 1970s and continued with the organometallic chemistry of Mo, W, and Re and the development of homogeneous catalysts for the metathesis of olefins. It also explores recent findings in surface organometallic chemistry and discusses the link between molecularly defined and heterogeneous catalysts. Recent results suggest that heterogeneous olefin metathesis catalysts that are activated toward metathesis upon exposure to olefins produce a d0 alkylidene through formation of a metallacyclopentane ring at d2 metal sites followed by “a ring contraction” to a metallacyclobutane, a reaction that was first observed in tantalum chemistry.

Molecular and Silica‐Supported Mo and W d0 Imido‐Methoxybenzylidene Complexes: Structure and Metathesis Activity

“…Olefin metathesis has developed in full bloom from the initial discovery of ill‐defined heterogeneous catalysts in the sixties to latest development of highly active and stereoselective homogeneous systems . They are nowadays complemented by well‐defined and highly active heterogeneous catalysts prepared through Surface Organometallic Chemistry ( SOMC ) . Between the two most versatile classes of homogeneous olefin metathesis catalysts ( Figure ), Schrock ‐type group 6 d 0 alkylidene complexes often display very high activities.…”

“…[2 -5] They are nowadays complemented by well-defined and highly active heterogeneous catalysts prepared through Surface Organometallic Chemistry (SOMC). [6][7][8][9][10] Between the two most versatile classes of homogeneous olefin metathesis catalysts (Figure 1), Schrock-type group 6 d 0 alkylidene complexes often display very high activities. However, an important criterion for the use of catalysts is their stability that provides easier handling, possible recyclability and higher efficiency as judged from higher turnover numbers per metal site.…”

Bis(η5:η1‐pentafulvene)niobium(V) Complexes: Efficient Synthons for Niobium Carbene and Imido Derivatives

“…This complex serves as precursor for an ovel synthetic route to Schrock carbene complexes employing the nucleophilic attack onto av inyl unit at the C b position (Scheme 2, C). This reaction pathway is completely unprecedented in comparison to classical a-hydrogen elimination reactions (A) [15] or thermolysis of diazo compounds (B) [16] to Schrock carbene complexes.…”

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

“…c) Proposed active sites of the industrial WO 3 /SiO 2 catalysts and their structural mimics prepared by SOMC. [516] This study provides am olecular rationale on how alkylidenes could form from oxo species in the classical industrial catalysts. oxo species.W ell-defined W VI -oxo species have recently been prepared by grafting WO 2 (OSi(OtBu) 3 ) 2 (DME) [581] onto SiO 2-700 followed by thermal treatment to yield well-defined ( SiO) 2 W(=O) 2 and ( SiO) 4 W( = O).…”

“…[446] The molecular structures of fully oxidized Mo catalysts on different supports (SiO 2 ,A l 2 O 3 ,a nd SiO 2 -Al 2 O 3 )w ere also extensively studied by in situ Raman, IR, and UV/Vis spectroscopy as well as DRS and XAS.T hese methods [507] as well as DFT calculations [508,509] indicate that surface di-oxo 4 Mo-( = O) sites are formed at higher coverage. [2,442,515,516] Most of them involve reduced metal sites that are possibly formed by reduction of M VI -oxo species in the presence of an olefin under the reaction conditions. [514] Anumber of mechanisms for the formation of alkylidenes from surface oxo species have been proposed.…”