Silica‐Supported Molybdenum Alkylidyne N‐Heterocyclic Carbene Catalysts: Relevance of Site Isolation to Catalytic Performance

Abstract

The immobilization of the molybdenum alkylidyne complex [(p‐OMe‐C6H4C≡)Mo(OCMe(CF3)2)3⋅DME] (DME=1,2‐dimethoxyethane) on silica using a silica‐supported N‐heterocyclic carbene (NHC), that is, 1,3‐di(isopropyl)imidazol‐2‐ylidene, is reported. Two different types of silica, that is, silica 60 and partially dehydroxylated silica 60 obtained via heating to 650 °C (SiO2‐650) were used. In both cases, excess silanol groups were end‐capped with trimethylsilyl chloride. Immobilization of both the NHC precursor and the metal alkylidyne was followed by 19F magic angle spinning (MAS) and 13C cross‐polarization magic angle spinning (CPMAS) NMR and inductively‐coupled optical emission spectroscopy (ICP–OES). The metal alkylidyne supported on both types of silica was active in alkyne metathesis and showed activity for a series of functional 2‐alkynes. Most importantly, the metal alkylidyne supported on SiO2‐650 displayed a substantially higher alkyne metathesis activity than the homogeneous analog or the one supported on regular silica, which is attributed to a site isolation effect and, as a result, reduced bimolecular decomposition.

Well‐Defined Alkyne Metathesis Catalysts: Developments and Recent Applications

“…Later,t he concept of NHC coordination was extended to silica-supported NHC 41:T he corresponding NHC precursors were first immobilizedo nto partially dehydroxylated silica (SiO 2-650 )a nd subsequently coordinated to the molybdenum benzylidyne complex 38 (Scheme 13). [96] The immobilized catalyst 42 as well as the homogeneous analogue 40 were tested in the metathesis of various phenylpropyne derivatives. Amongt he tested compounds, the benzylidyne complex 42 graftedo nto SiO 2-650 showed the best catalytic performance (TON = 990) for 1-phenyl-1-propyne even surpassing the homogeneousa lkylidyne NHC complex 40, [96] although not reaching the TONs of silica-supported complex MoF6/SiO 2-700 (TON = 1990).…”

“…The group proposed a cationic molybdenum alkylidyne NHC complex as the active species for their catalyst systems. Later, the concept of NHC coordination was extended to silica‐supported NHC 41 : The corresponding NHC precursors were first immobilized onto partially dehydroxylated silica (SiO 2–650 ) and subsequently coordinated to the molybdenum benzylidyne complex 38 (Scheme ) . The immobilized catalyst 42 as well as the homogeneous analogue 40 were tested in the metathesis of various phenylpropyne derivatives.…”

“…The immobilized catalyst 42 as well as the homogeneous analogue 40 were tested in the metathesis of various phenylpropyne derivatives. Among the tested compounds, the benzylidyne complex 42 grafted onto SiO 2‐650 showed the best catalytic performance (TON=990) for 1‐phenyl‐1‐propyne even surpassing the homogeneous alkylidyne NHC complex 40 , although not reaching the TONs of silica‐supported complex MoF6/SiO 2 – 700 (TON=1990) …”

Catalytic Alkyne and Diyne Metathesis with Mixed Fluoroalkoxy-Siloxy Molybdenum Alkylidyne Complexes

“…16,19 Convenient access to these complexes was provided by the development of reliable protocols for their preparation from Mo(CO)6 or W(CO)6, respectively, 20,21 which also furnished MoF6-type initiators for ring-opening alkyne metathesis polymerization (ROAMP) [22][23][24] as well as N-heterocyclic carbene (NHC) adducts of related molybdenum and tungsten benzylidyne complexes. [25][26][27][28] Recently, related molybdenum benzylidyne complexes were even used for olefin metathesis reactions. 29 Siloxide-supported alkylidyne complexes are another important class of alkyne metathesis catalysts, 30 and the triphenylsiloxy molybdenum complex I is probably the most frequently used species, 31,32 in particular for natural product synthesis through ring-closing alkyne metathesis (RCAM).…”

SBA-15 as a Support for Effective Olefin Metathesis Catalysts

“…Another major challenge is the development of heterogeneous catalysts for alkyne and enyne metathesis. For example, Buchmeiser has recently developed silica-supported NHC Mo alkylidyne complexes used in the metathesis of various 2-alkynes [90]. For development of these new hybrid catalysts highly effective in metathesis of a variety of substrates, SBA-15 can be a good choice.…”