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

Abstract

Silica‐supported cationic Mo‐imido alkylidene N‐heterocyclic carbene catalysts, prepared by surface organometallic chemistry, display contrasting olefin metathesis activity for terminal and internal olefins. The high metathesis activity towards terminal alkenes is attributed to the strong σ‐donating property of the NHC ancillary ligand, which disfavors the formation of the parent square‐planar metallacyclobutane, an off‐cycle reaction intermediate resulting from the reaction with ethylene, one of the metathesis products. This tailored ligand environment also allowed the first trigonal bipyramidal (TBP) metallacyclobutane reaction intermediate for supported Mo metathesis catalysts to be identified.

Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Catalysts

“…Coordination always occurs trans to the NHC. This is supported by the single crystal X‐ray structures reported here and several other already reported crystal structures of similar compounds, recently published mechanistic studies from our group and mechanistic studies for MAP (monoalkoxide pyrrolide) type Schrock‐catalysts, which indicate that coordination occurs preferably trans to the strongest σ‐donor ligand . Hence, the NHC dictates the coordination of molecules to the metal center.…”

A Formulation Protocol with Pyridine to Enable DNP-SENS on Reactive Surface Sites: Case Study with Olefin Polymerization and Metathesis Catalysts

“…4d), besides the signals of Py (147, 136 and 126 ppm) and TCE (75 ppm), these at 20, 130, 142 and 324 ppm are consistent with the presence of the cationic Mo alkylidenes, in view of their similar chemical shift with the molecular compound. [38] One should note the alkylidene signal is slightly more deshielded than in the molecular complex (δ = 313 ppm [38]), which is consistent with coordination of Py, a -acceptor ligand, to the Mo-sites [40]. In fact, addition of pyridine to the molecular complex leads to similar observation (Fig.…”

“…To further evaluate the applicability of utilizing pre-adsorbed pyridine (Py) as catalyst surface passivation method to prevent radicals from being quenched, we evaluated this approach on two other types of highly air, moisture and DNP formulation sensitive supported catalysts: 1) alkyl zircononene supported on sulfated zirconia (ZrMe + @SZO300), that are well-known highly active olefin polymerization and hydrogenation catalysts, [36,37] and 2) a recently developed supported olefin metathesis catalyst based on well-defined silica-supported cationic Moalkylidenes (Mo=CHR + @SiO2-700) that show outstanding activity towards terminal olefins. [38] These materials were prepared as previously reported (see experimental details): ZrMe + @SZO300 was prepared by grafting of (C5H5)2Zr( 13 CH3)2 onto sulfated zirconia pretreated at 300 o C in air followed by evacuation at 10 -5 mbar, and Mo=CHR + @SiO2-700 by grafting of Mo + (=N-Ar)(= 13 CH-C(CH3)2Ph)( 13 C-1,3-(2,4,6)-(Me)3C6H2-imidazol-2-ylidene)(OCH(CF3)2) B (Ar F )4)onto SiO2-700 -silica pretreated at 700 o C under 10 -5 mbar for 48 hours. In order to adequately evaluate the DNP performance and obtain reliable data, we resorted to utilizing 13 C-labeled materials (100% enrichment of methyl groups for ZrMe + @SZO300 and 30% enrichment of the alkylidene carbon in Mo=CHR + @SiO2-700) for this study.…”

Metal‐Surface Interactions and Surface Heterogeneity in ‘Well‐Defined’ Silica‐Supported Alkene Metathesis Catalysts: Evidences and Consequences

“…Today, catalysts prepared by SOMC feature high turnover frequencies, turnover numbers and selectivities for metathesis of both internal and terminal olefins . DFT calculations have shown that the very high activities of these catalysts of the general formula (≡SiO)(X)M(E)(=CHR) – where X is usually an anionic ligand (alkyl, alkoxy, amido), E=oxo, imido or alkylidyne, M=Mo, W or Re – originate from the asymmetric electrophilic metal center, a configuration that is ideal for easy coordination/de‐coordination of the olefin as well as for the cycloaddition/cycloreversion steps .…”

“…Strategies to enhance catalyst performance for terminal olefins have relied on either the constant removal of ethylene (such as reactive distillation), or the incorporation of stronger σ ‐donor ligands that destabilize this metallacyclobutane . The latter approach has recently led to the development of a new class of molecular and supported catalysts bearing very strong neutral σ ‐donor N‐ heterocyclic carbene (NHC) ligands. The supported cationic W oxo‐alkylidene NHC complex [(≡SiO)(NHC)W(O)(=CHR)] + [B(3,5‐(CF 3 ) 2 C 6 H 3 ) 4 ] − ( 1 @SiO 2 , Figure ) is currently one of the most active and stable metathesis catalysts .…”