Molybdenum Imido Alkylidene N-Heterocyclic Carbene Research Articles

Neutral and Cationic Molybdenum Imido Alkylidene Cyclic Alkyl Amino Carbene (CAAC) Complexes for Olefin Metathesis.

The first neutral and cationic Mo imido alkylidene cyclic alkyl amino carbene (CAAC) complexes of the general formulae [Mo(N-Ar)(CHCMe2Ph)(X)2(CAAC)] and [Mo(N-Ar)(CHCMe2Ph)(X)(CAAC)][B(ArF)4] (X = Br, Cl, OTf, OC6F5; CAAC = 1-(2,6-iPr2-C6H3)-3,3,5,5-tetramethyltetrahydropyrrol-2-ylidene) have been synthesized from molybdenum imido bishalide alkylidene DME precursors. Different combinations of the imido and "X" ligands have been employed to understand synthetic peculiarities. Selected complexes have been characterized by single crystal X-ray analysis. Due to the pronounced σ-donor/π-acceptor characteristics of CAACs, the corresponding neutral and cationic molybdenum imido alkylidene CAAC complexes do not require the presence of stabilizing donor ligands such as nitriles. Calculations on the PBE0-D3BJ/def2-TZVP level for PBE0-D3BJ/def2-SVP optimized geometries revealed partial charges at molybdenum similar to the corresponding molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes with a slightly higher polarization of the molybdenum alkylidene bond in the CAAC complexes. All cationic complexes have been tested in olefin metathesis reactions and showed improved activity compared to the analogous NHC complexes for hydrocarbon-based substrates, allowing for turnover numbers (TONs) up to 9500 even at room temperature. Some Mo imido alkylidene CAAC complexes are tolerant towards functional groups like thioethers and sulfonamides.

Tuning the Latent Behavior of Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes in Dicyclopentadiene Polymerization

We report the synthesis of O- and N-chelated hexacoordinated molybdenum imido alkylidene N-heterocyclic carbene (NHC) bistriflate and pentacoordinated molybdenum imido alkylidene NHC monotriflate monoalkoxide complexes and their use as thermally latent and in some cases air-stable precatalysts in the ring-opening metathesis polymerization (ROMP) of dicyclopentadiene (DCPD). Introduction of electron- withdrawing and electron-donating groups at the O-chelated alkylidene ligand allowed for the tuning of both the onset temperature of polymerization (Tonset) and the temperature of the exotherm maximum (Texo,max). In addition, N-chelated complexes were synthesized using easily available 2-vinylpyridine or 2-vinyl- N,N-dimethylaniline to yield five-membered and, for the first time, four-membered molybdenum imido alkylidene NHC chelates. With these precatalysts, Tonset and Texo,max could be varied between 52 and 142 °C and between 99 and 174 °C, respectively. All primary data files and processed data of the journal article from the Buchmeiser group. Tuning the Latent Behavior of Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes in Dicyclopentadiene Polymerization. Nuclear magnetic resonance (NMR) spectra are uploaded. Also differential scanning calorimetry (DSC) data are available.

Reaction Mechanism of Ring-Closing Metathesis with a Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Catalyst

DFT calculations were carried out to explore all relevant pathways for the ring-closing metathesis (RCM) reaction of a cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) catalyst wit...

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

The origin of hydroxyl group tolerance in neutral and especially cationic molybdenum imido alkylidene N‐heterocyclic carbene (NHC) complexes has been investigated. A wide range of catalysts was prepared and tested. Most cationic complexes can be handled in air without difficulty and display an unprecedented stability towards water and alcohols. NHC complexes were successfully used with substrates containing the hydroxyl functionality in acyclic diene metathesis polymerization, homo‐, cross and ring‐opening cross metathesis reactions. The catalysts remain active even in 2‐PrOH and are applicable in ring‐opening metathesis polymerization and alkene homometathesis using alcohols as solvent. The use of weakly basic bidentate, hemilabile anionic ligands such as triflate or pentafluorobenzoate and weakly basic aromatic imido ligands in combination with a sterically demanding 1,3‐dimesitylimidazol‐2‐ylidene NHC ligand was found essential for reactive and yet robust catalysts.

Regio- and Stereospecific Cyclopolymerization of α,ω-Diynes by Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes.

Both solvent-free and acetonitrile-containing cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes of the general formula [Mo(NR')(CHCMe2 R)(NHC)(X)+ A- ] (R' = 2,6-Cl2 -C6 H3 , tBu, 2-CF3 -C6 H4 , 2-tBu-C6 H4 , 2,6-iPr2 -C6 H3 , 2,6-Me2 -C6 H3 ; R = Me, Ph; NHC = 1,3-dimesitylimidazol-2-ylidene (IMes), 1,3-di-iPr-imidazol-2-ylidene (IPr), 1,3,5-triphenyl-1,3,4-triazol-2-ylidene); X = CF3 SO3 , C6 F5 O, OCH(CF3 )2 , OC(CF3 )3 , pyrrolide, C6 F5 COO, 2,6-(CF3 )2 -C6 H3 COO; A- = B(ArF )4 - , Al(OC(CF3 )3 )4 - ), have been investigated for their propensity to cyclopolymerize 4,4-disubstituted 1,6-heptadiynes. All metal complexes contain a stereogenic (chiral) metal center, which accounts for the high reactivity and high regioselectivity of insertion (>99%) that are observed for all metal complexes, leading to highly conjugated, α-insertion-derived polyenes that are based on a highly regular polymer backbone and that show absorption maxima close to 600 nm. With the chiral monomer 4-(ethoxycarbonyl)-4-(1S,2R,5S)-(-)-menthoxycarbonyl-1,6-heptadiyne, high syndiospecifity (>99% syndiotactic) is observed. A mechanism explaining the high regio- and stereoselectivity is presented. Thus, α-addition of the monomers proceeds chain-end-controlled trans to the NHC and is preferred over β-addition through intramolecular Mo-O chelation. Insertion of the monomers entails double inversion at the stereogenic metal center in the course of one complete monomer insertion.

Mechanism of Olefin Metathesis with Neutral and Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes.

A series of neutral molybdenum imido alkylidene N-heterocyclic carbene (NHC) bistriflate and monotriflate monoalkoxide complexes as well as cationic molybdenum imido alkylidene triflate complexes have been subjected to NMR spectroscopic, X-ray crystallographic, and reaction kinetic measurements in order to gain a comprehensive understanding about the underlying mechanism in olefin metathesis of this new type of catalysts. On the basis of experimental evidence and on DFT calculations (BP86/def2-TZVP/D3/cosmo) for the entire mechanism, olefinic substrates coordinate trans to the NHC of neutral 16-electron complexes via an associative mechanism, followed by dissociation of an anionic ligand (e.g., triflate) and formation of an intermediary molybdacyclobutane trans to the NHC. Formation of a cationic complex is crucial in order to become olefin metathesis active. Variations in the NHC, the imido, the alkoxide, and the noncoordinating anion revealed their influence on reactivity. The reaction of neutral 16-electron complexes with 2-methoxystyrene is faster for catalysts bearing one triflate and one fluorinated alkoxide than for catalysts bearing two triflate ligands. This is also reflected by the Gibbs free energy values for the transition states, ΔG‡303, which are significantly lower for catalysts bearing only one triflate than for the corresponding bistriflate complexes. Reaction of a solvent-stabilized cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) monotriflate complex with 2-methoxystyrene proceeded via an associative mechanism too. Reaction rates of both solvent-free and solvent-stabilized cationic Mo imido alkylidene NHC catalysts with 2-methoxystyrene are controlled by the cross-metathesis step but not by adduct formation.

Stereoselective Ring-Opening Metathesis Polymerization with Molybdenum Imido Alkylidenes Containing O-Chelating N-Heterocyclic Carbenes: Influence of Syn/Anti Interconversion and Polymerization Rates on Polymer Structure

The structures of the first insertion products (syn/anti, cis/trans) in the ring-opening metathesis polymerization (ROMP) of norbornene derivatives using both neutral and cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes based on an O-chelating NHC, i.e., [Mo(N-2,6-Me2-C6H3)(N-mesityl-N′-2-O-1-C6H4-imidazolin-2-ylidene)(CHCMe2Ph)(OTf)] (1), [Mo(N-2,6-iPr2-C6H3)(N-2,6-iPr2-C6H3-N′-2-O-1-C6H4-imidazolin-2-ylidene)(CHCMe2Ph)(OTf)] (2), and [Mo(N-2,6-iPr2-C6H3)(N-2,6-iPr2-C6H3-N′-2-O-1-C6H4-imidazolin-2-ylidene)(CH3CN)(CHCMe2Ph)+ (B(ArF)4–)] (3), have been identified. Also, syn/anti interconversion rates of catalysts 1–3 have been determined in acetonitrile. Correlation of these values with the rate constants of polymerization revealed the importance of a balanced ratio between these two values. Disrupting that balance by changing the solvent or the monomer or by switching to a similar, but more ROMP-active catalyst leads to significant changes in the cis/trans contents of the result...

Neutral and Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes: Reactivity in Selected Olefin Metathesis Reactions and Immobilization on Silica.

The synthesis and single-crystal X-ray structures of the novel molybdenum imido alkylidene N-heterocyclic carbene complexes [Mo(N-2,6-Me2C6H3)(IMesH2)(CHCMe2Ph)(OTf)2] (3), [Mo(N-2,6-Me2C6H3)(IMes)(CHCMe2Ph)(OTf)2] (4), [Mo(N-2,6-Me2C6H3)(IMesH2)(CHCMe2Ph)(OTf){OCH(CF3)2}] (5), [Mo(N-2,6-Me2C6H3)(CH3CN)(IMesH2)(CHCMe2Ph)(OTf)](+)BArF(-) (6), [Mo(N-2,6-Cl2C6H3)(IMesH2)(CHCMe3)(OTf)2] (7) and [Mo(N-2,6-Cl2C6H3)(IMes)(CHCMe3)(OTf)2] (8) are reported (IMesH2=1,3-dimesitylimidazolidin-2-ylidene, IMes=1,3-dimesitylimidazolin-2-ylidene, BArF(-)=tetrakis-[3,5-bis(trifluoromethyl)phenyl] borate, OTf=CF3SO3(-)). Also, silica-immobilized versions I1 and I2 were prepared. Catalysts 3-8, I1 and I2 were used in homo-, cross-, and ring-closing metathesis (RCM) reactions and in the cyclopolymerization of α,ω-diynes. In the RCM of α,ω-dienes, in the homometathesis of 1-alkenes, and in the ethenolysis of cyclooctene, turnover numbers (TONs) up to 100,000, 210,000 and 30,000, respectively, were achieved. With I1 and I2, virtually Mo-free products were obtained (<3 ppm Mo). With 1,6-hepta- and 1,7-octadiynes, catalysts 3, 4, and 5 allowed for the regioselective cyclopolymerization of 4,4-bis(ethoxycarbonyl)-1,6-heptadiyne, 4,4-bis(hydroxymethyl)-1,6-heptadiyne, 4,4-bis[(3,5-diethoxybenzoyloxy)methyl]-1,6-heptadiyne, 4,4,5,5-tetrakis(ethoxycarbonyl)-1,7-octadiyne, and 1,6-heptadiyne-4-carboxylic acid, underlining the high functional-group tolerance of these novel Group 6 metal alkylidenes.