Metallacyclobutanes from Schrock-Type d⁰ Metal Alkylidene Catalysts: Structural Preferences and Consequences in Alkene Metathesis

Abstract

International audienceMetallacyclobutanes have been observed as intermediates in the alkene metathesis reaction, with the 5-coordinated metal center, in a trigonal bipyramidal or a square-planar coordination. Previous calculations have shown that only the trigonal bipyramidal form is directly on the reaction pathway of alkene metathesis. The square-pyramidal form, which is obtained by a trigonal-bipyramidal (TBP)-square-based pyramidal (SP) interconversion at the metal, is an intermediate that can be responsible for catalyst deactivation. This computational study aimed at establishing the factors that control the properties of the two metallacyclobutanes (structural and 13C NMR features, stability, and TBP-SP interconversion) that can influence the efficiency of the metathesis reaction. Metallacyclobutanes resulting from the addition of ethene to a large set of methylidene complexes where the metal fragment is M(E)(X)(Y) (M = Mo or W; E = alkyl imido, aryl imido, or oxo), (X and Y) = alkyl, pyrrolyl, alkoxy and fluoroalkoxy, and large monoaryloxy) have been studied by density functional calculations (B3PW91, and M06). From the study of these numerous complexes that include in particular all characterized complexes, properties of the metallacyclobutanes could be derived: Metallacyclobutanes with W are more stable than those with Mo relative to reactants; electron withdrawing ligands stabilize the two isomeric forms of the metallacyclobutane but more the TBP than the SP form, and conversely, electron donating ligands destabilize both forms but more the TBP isomer. The energy barrier for the TBP-SP interconversion is found to be lower than that for the productive ethene metathesis pathway for W complexes but is generally higher for Mo species. These facts rationalize the experimental evidence and accounts in particular for the high efficiency of the Mo catalysts

Promoting Terminal Olefin Metathesis with a Supported Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Catalyst

“…30 equiv) provides am aterial whose 13 Cs olid-state NMR spectrum displays two very sharp peaks at d = 110 ppm and 0.6 ppm, which are attributed to the a and b carbon atoms of atrigonal bipyramidal (TBP) metallacycle (Scheme 3a,b). [16] Furthermore,o bserving an induction period for internal olefins is also remarkable and shows that initiation is particularly difficult for such olefins with these catalysts. However,t wo broad and very intense features at d = 70 and 62 ppm are additionally observed;t hey are attributed to ethylene coordinated to aM o IV center.…”

Metathesis by Molybdenum and Tungsten Catalysts

“…52 The role of dispersive forces in sterically crowded molecules of the type described here is a potential further complication that remains to be considered. 53 Although progress in the last several years has been significant, many problems remain to be solved in order to take full advantage of what can be achieved.…”

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

“…[477] Several other combinations of Re precursors and silica (Scheme 34 b) are inactive in metathesis, thus illustrating the effect of the ancillary ligand set. [481][482][483][484][485][486] This dissymmetry induces adistortion of the tetracoordinated metal site towards at rigonal-pyramidal geometry,w hich facilitates olefin coordination;t his also destabilizes metal-lacyclobutane intermediates and thus results in efficient metathesis catalysis.T his mechanism is general and applies to all tetracoordinated d 0 -based metathesis catalysts,i ncluding homogeneous ones. [481][482][483][484][485][486] This dissymmetry induces adistortion of the tetracoordinated metal site towards at rigonal-pyramidal geometry,w hich facilitates olefin coordination;t his also destabilizes metal-lacyclobutane intermediates and thus results in efficient metathesis catalysis.T his mechanism is general and applies to all tetracoordinated d 0 -based metathesis catalysts,i ncluding homogeneous ones.…”

“…[454] Symmetrical Mo( = NR 1 )( = CHR 2 )(OR 3 ) 2 complexes containing electron-withdrawing alkoxide ligands resulted in more active catalysts than those containing electron-donating alkoxides. [482][483][484][485][486] Thef irst fully characterized heterogeneous Mo catalyst was obtained by grafting Mo( = NAr)-(=CHtBu)(CH 2 tBu) 2 (Ar = 2,6-iPr 2 C 6 H 3 )o nto SiO 2-700 to form welldefined (SiO)Mo(=NAr)(=CHtBu)-(CH 2 tBu). [482][483][484][485][486] Thef irst fully characterized heterogeneous Mo catalyst was obtained by grafting Mo( = NAr)-(=CHtBu)(CH 2 tBu) 2 (Ar = 2,6-iPr 2 C 6 H 3 )o nto SiO 2-700 to form welldefined (SiO)Mo(=NAr)(=CHtBu)-(CH 2 tBu).…”

“…Thes ame difference in activity was observed for the metathesis of 1-nonene in batch conditions. According to DFT calculations,S Pm etallacycles are the resting states in the case of W, [486] and were observed experimentally by solid-state NMR spectroscopy using labeled ethylene. According to DFT calculations,S Pm etallacycles are the resting states in the case of W, [486] and were observed experimentally by solid-state NMR spectroscopy using labeled ethylene.…”