A Ruthenium Catalyst for Olefin Metathesis Featuring an Anti-Bredt N-Heterocyclic Carbene Ligand

N-Heterocyclic Carbenes with Polyfluorinated Groups at the 4- and 5-Positions from [3 + 2] Cycloadditions between Formamidinates and cis-1,2-Difluoroalkene Derivatives

We herein report the formation of fluorinated N-heterocyclic carbenes (NHCFs) that bear fluorine atoms at the 4- and 5-positions of the imidazol-2-ylidene ring. Treatment of sodium N,N'-bis(aryl)formamidinates with tetrafluoroethylene followed by the addition of LiBF4 induced a [3 + 2] cycloaddition to afford 4,5-difluorinated imidazolium salts, which served as the precursors for 4,5-difluorinated NHCs. A key feature of this procedure is its applicability to other perfluorinated compounds, which enabled us to incorporate polyfluorinated functional groups at 4- and 5-positions on the imidazol-2-ylidene skeleton. Thus, employing octafluorocyclopentene and hexafluorobenzene led to the formation of 4,4,5,5,6,6-hexafluoro-1,3-diaryl-3,4,5,6-tetrahydrocyclopenta[d]imidazolium (CypIPrF·HBF4) and 4,5,6,7-tetrafluoro-1,3-diarylbenzimidazolium (BIPrF·HBF4) salts, respectively. A thorough NMR analysis of these NHCFs, their selenium adducts, and their tricarbonyl nickel complexes, (NHCF)Ni(CO)3, demonstrated that the fluorine substituents, contrary to expectations, tend to act as electron donors owing to the considerable positive mesomeric effect, while the perfluorocyclopentene-fused and tetrafluorobenzo-fused rings are pure electron acceptors due to their strong negative inductive effect. The unique and increased π-accepting character of the perfluorocyclopentene-fused and tetrafluorobenzo-fused NHCFs in both stoichiometric and catalytic reactions is further demonstrated by employing (NHCF)Ni(CO)3 and (NHCF)AuCl species, respectively. Moreover, an analysis of the % buried volume (%Vbur) values clearly suggests that the modification of the NHC backbone with polyfluorinated groups can drastically alter the electronic properties of the NHC ligand without substantially changing its steric properties. Our experimental results were further corroborated by a series of computational calculations.

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)(CHCMe2 Ph)(X)2 (CAAC)] and [Mo(N-Ar)(CHCMe2 Ph)(X)(CAAC)][B(ArF )4 ] (X=Br, Cl, OTf, OC6 F5 ; CAAC=1-(2,6-iPr2 -C6 H3 )-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.

Stabilization of group 14 elements E = C, Si, Ge by hetero-bileptic ligands cAAC, MCOn with push-pull mechanism

The stability and bonding of a series of hetero-diatomic molecules with general formula (cAAC)EM(CO)_n , where cAAC = cyclic alkyl(amino) carbene; E = group 14 elements (C, Si, and Ge); M = transition metal (Ni, Fe, and Cr) have been studied by quantum chemical calculations using density functional theory (DFT) and energy decomposition analysis-natural orbital chemical valence (EDA-NOCV). The equilibrium geometries were calculated at the BP86/def2-TZVPP level of theory. The tri-coordinated group 14 complex (1a, 4a, and 7a) in which one of the CO groups is migrated to the central group 14 element from adjacent metal is theoretically found to be more stable when the central atom (E) is carbon. On the other hand, the two-coordinate group 14 element containing metal-complexes (2, 5, 8, 3, 6, and 9) are found to be more stable with their corresponding heavier analogues. The electronic structures of all the molecules have been analyzed by molecular orbital, topological analysis of electron density and natural bond orbital (NBO) analysis at the M06/def2-TZVPP//BP86/def2-TZVPP level of theory. The nature of the cAACE and EM bonds has been studied by EDA-NOCV calculations at BP86-D3(BJ)/TZ2P level of theory. The EDA analysis suggests that the bonding of cAACC(CO) can be best represented by electron sharing σ and π interactions, whereas, C(CO)M(CO)_n-1 by dative σ and π interactions. On the other hand, EDA-NOCV calculations suggests both dative σ and π interactions for cAACE and EM(CO)_n bonds of the corresponding Si and Ge analogues having stronger σ- and relatively weaker π-bonds. The topological analysis of electron density supports the closed-shell interaction for the Si and Ge complexes and open-shell interaction for the carbon complexes. The calculated proton affinity and hydride affinity values corroborated well with the present bonding description. This class of complexes might act as efficient future catalysts for different organic transformations due to the presence of electron rich group 14 element and metal carbonyl.

In situ transformations of Pd/NHC complexes with N-heterocyclic carbene ligands of different nature into colloidal Pd nanoparticles

A series of Pd/NHC species undergoes R–NHC coupling reaction forming catalytically active NHC-free complexes and/or colloidal Pd nanoparticles.

Design of Heterogeneous Hoveyda-Grubbs Second-Generation Catalyst-Lipase Conjugates

Heterogeneous catalysts have been synthesized by the conjugation of Hoveyda–Grubbs second-generation catalyst with a lipase. The catalytic properties of the organometallic compound in solution were firstly optimized, evaluating the activity of Ru in the ring-closing metathesis of diethyldiallymalonate at 25 °C at different solvents and in the presence of different additives. The best result was found using tetrahydrofuran as a solvent. Some additives such as phenylboronic acid or polyetheneglycol slightly improved the activity of the Ru catalyst whereas others, such as pyridine or dipeptides affected it negatively. The organometallic compound immobilized on functionalized-surface materials activated with boronic acid or epoxy groups (around 50–60 µg per mg support) and showed 50% conversion at 24 h in the ring-closing metathesis. Cross-linked enzyme aggregates (CLEA’s) of the Hoveyda–Grubbs second-generation catalyst with Candida antarctica lipase (CAL-B) were prepared, although low Ru catalyst was found to be translated in low conversion. Therefore, a sol–gel preparation of the Hoveyda–Grubbs second-generation and CAL-B was performed. This catalyst exhibited good activity in the metathesis of diethyldiallymalonate in toluene and in aqueous media. Finally, a new sustainable approach was used by the conjugation lipase–Grubbs in solid phase in aqueous media. Two strategies were used: one using lipase previously covalently immobilized on an epoxy-Sepharose support (hydrophilic matrix) and then conjugated with grubbs; and in the second, the free lipase was incubated with organometallic in aqueous solution and then immobilized on epoxy-Sepharose. The different catalysts showed excellent conversion values in the ring-closing metathesis of diethyldiallymalonate in aqueous media at 25 °C.