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The reaction of organoberyllium compounds with hexaphenylcarbodiphosphorane yields mono-ortho-beryllated complexes, which feature a double dative Be=C bond. The bonding situation in these compounds together with a simple carbodiphosphorane and an N-heterocyclic carbene adduct was analysed with energy decomposition analysis in combination with natural orbital for chemical valence as well as with quantum theory of atoms-in-molecules. Furthermore, the driving forces accountable for mono-ortho-beryllation were elucidated along with the reactivity of the Be=C bond.
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Various pathways for the synthesis of beryllium triflate were investigated. The reaction of triflic acid or trimethylsilyl triflate with beryllium metal in liquid ammonia led to the formation of mono-, di-, and tetra-nuclear beryllium ammine complexes. Utilization of SMe2 as a solvent gave homoleptic Be(OTf)2. Various beryllium triflate complexes with N- and O-donor ligands as well as the complex anions [Be(OTf)4]2- and [Be2(OTf)6]2- were synthesized to evaluate the reactivity and solution properties of beryllium triflate. This showed that OTf- is not a weakly coordinating anion for Be2+ cations and that it exhibits good bridging properties.
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Attaching a nitrene precursor to an intramolecular nucleophile allows for a catalytic asymmetric intramolecular oxyamination of alkenes in which the nucleophile adds in an endocyclic position and the amine in an exocyclic fashion. Using chiral-at-ruthenium catalysts, chiral γ-aminomethyl-γ-lactones containing a quaternary carbon in γ-position are provided in high yields (up to 99 %) and with excellent enantioselectivities (up to 99 % ee). DFT calculations support the possibility of both a singlet (concerted oxyamination of the alkene) and triplet pathway (stepwise oxyamination) for the formation of the predominant stereoisomer. γ-Aminomethyl-γ-lactones are versatile chiral building blocks and can be converted to other heterocycles such as δ-lactams, 2-oxazolidinones, and tetrahydrofurans.
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The ambiphilicity of pseudo-halides has been the object of extensive debate. Herein, we use a series of trispyrazolylborato beryllium pseudo-halido complexes [TpBe(X')] with X' = CN-, N3-, NCO- and NCS- to explore the origins of the preferred isomers. Thus, we have synthesised and characterised through NMR and IR spectroscopy as well as single crystal X-ray diffraction these complexes. A combination with quantum chemical calculations within the DFT framework enabled an in-depth understanding of the bonding modes and preferences of the investigated pseudo-halido ligands.
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The reaction of homoleptic beryllium halide with diphenyl beryllium complexes leads to the clean formation of heteroleptic beryllium Grignard compounds [(L)1-2 BePhX]1-2 (X=Cl, Br, I; L=C-, N-, O-donor ligand). The influence of ligands and solvent on these compounds, their formation and exchange equilibria in solution were investigated, together with the factors determining the complex constitution.
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A series of stereogenic-at-metal iron complexes comprising a non-C2 -symmetric chiral topology is introduced and applied to asymmetric 3d-transition metal catalysis. The chiral iron(II) complexes are built from chiral tetradentate N4-ligands containing a proline-derived amino pyrrolidinyl backbone which controls the relative (cis-α coordination) and absolute metal-centered configuration (Λ vs. Δ). Two chloride ligands complement the octahedral coordination sphere. The modular composition of the tetradentate ligands facilitates the straightforward incorporation of different terminal coordinating heteroaromatic groups into the scaffold. The influence of various combinations was evaluated in an asymmetric ring contraction of isoxazoles to 2H-azirines revealing that a decrease of symmetry is beneficial for the stereoinduction to obtain chiral products in up to 99 % yield and with up to 92 % ee. Conveniently, iron catalysis is feasible under open flask conditions with the bench-stable dichloro complexes exhibiting high robustness towards oxidative or hydrolytic decomposition. The versatility of non-racemic 2H-azirines was subsequently showcased with the conversion into a variety of quaternary α-amino acid derivatives.
Assuntos
Complexos de Coordenação , Modelos Moleculares , Ligantes , Complexos de Coordenação/química , Ferro , Catálise , Compostos FerrososRESUMO
[Mn3 O(OAc)7 (HOAc)]6 â x AcOH (x=6-9) represents a rare example of a compound containing molecular Mn18 -rings. These are formed by Mn3 (µ3 -O) subunits in which the high-spin Mn(III) centers are bridged by three pairs of acetate anions (AcO- ). An AcOH molecule coordinates to one of the Mn atoms leading to [Mn3 (µ3 -O)(µ2 -OAc)6 (AcOH)]-units, designated in short as Mn3 -units, that are interconnected by acetate anions via the other two Mn atoms to form Mn18 -rings. Magnetic measurements show weak ferromagnetic interactions between them that are suppressed in strong magnetic field. Quantum-chemical calculations on Mn3 model complexes using independently DFT and ab-initio multi reference methods (CASSCF/NEVPT2) show a correlation between the orientation of the pseudo-Jahn-Teller axes of pairs of Mn(III) magnetic centers and corresponding exchange coupling energies. Weak coupling between Mn3 -units within the Mn18 -ring allowed to simulate the magnetic susceptibility versus temperature dependence in terms of basically uncoupled magnetic moments of each Mn3 -unit within the ring.
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Mono and dinuclear phosphine complexes of beryllium halides [(PMe3)2BeX2], [(PMe3)BeX2]2 and [(PCy3)BeX2]2 (X = Cl, Br, I) were synthesised and characterised via NMR and IR spectroscopy as well as single crystal X-ray diffraction experiments. Dissociation and ligand exchange processes at these complexes were investigated through variable temperature NMR experiments in combination with line shape analysis and complemented by quantum chemical calculations. The PMe3 dissociation energy is smallest in [(PMe3)2BeCl2], while PMe3 exchange is similar in energy in all mononuclear [(PMe3)2BeX2] complexes and follows an interchange mechanism. While [(PMe3)BeX2]2 dissociates homolytically, [(PCy3)BeX2]2 cleaves one phosphine ligand. These distinctive dissociation processes account for the different chemical behaviour of these complexes.
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The synthesis of non-racemic 5-membered cyclic carbonates from abundant alcohols is reported. Conversion of the alcohol into an azanyl carbonate is followed by a chiral-at-ruthenium catalyzed cyclization to provide chiral cyclic carbonates in yields of up to 95 % and with up to 99 % ee. This new synthetic method is proposed to proceed through a nitrene-mediated intramolecular C(sp3 )-H oxygenation which includes an unusual 1,7-hydrogen atom transfer within a ruthenium nitrene intermediate. The method is applicable to the synthesis of non-racemic chiral mono-, di- and trisubstituted cyclic alkylene carbonates.
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Merging electrochemistry with asymmetric catalysis promises to provide an environmentally friendly and efficient strategy for the construction of nonracemic chiral molecules. However, in practice, significant challenges arise from the instability or incompatibility of the chiral catalysts under the electrochemical conditions at the interface of electrode and solution. Herein, we report a catalytic asymmetric indirect electrolysis employing the combination of a redox mediator and a chiral-at-rhodium Lewis acid, which achieves a previously elusive enantioselective nucleophilic α-C(sp3)-H alkenylation of ketones. Specifically, 2-acyl imidazoles react with potassium alkenyl trifluoroborates in high yields (up to 94%) and with exceptional enantioselectivities (27 examples with ≥99% ee) without the need for any additional stoichiometric oxidants (overall 40 examples). The new indirect electrosynthesis can be scaled to gram quantities and was applied to the straightforward synthesis of intermediates of the natural product cryptophycin A and a cathepsin K inhibitor.
Assuntos
Imidazóis , Ródio , Imidazóis/química , Cetonas/química , Ácidos de Lewis/química , Ródio/química , EstereoisomerismoRESUMO
The reaction of the uranium(IV) halides UCl4, UBr4, or UI4 with ethyl acetate (EtOAc) leads to the formation of the complexes [UX3(EtOAc)4][UX5(EtOAc)] (X = Cl, Br) or [UI4(EtOAc)3]. Thus, both UCl4 and UBr4 show self-ionization in ethyl acetate to a distorted pentagonal bipyramidal [UX3(EtOAc)4]+ cation and a distorted octahedral [UX5(EtOAc)]- anion. Surprisingly, the chloride and bromide compounds are not isotypic. While [UCl3(EtOAc)4][UCl5(EtOAc)] crystallizes in the orthorhombic crystal system, space group P212121 at 250 K, the bromide compound crystallizes in the monoclinic crystal system, P121/n1 at 100 K. Unexpectedly, UI4 does not show self-ionization but forms [UI4(EtOAc)3] molecules, which crystallize in the monoclinic crystal system, P21/c, at 100 K. The compounds were characterized by single-crystal X-ray diffraction, IR, Raman, and NMR spectroscopy, as well as molecular quantum chemical calculations using solvent models.
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Diphenylberyllium [Be3 Ph6 ] is shown here to react cleanly as a Brønsted base with a vast variety of protic compounds. Through the addition of the simple molecules tBuOH, HNPh2 and HPPh2 , as well as the more complex 1,3-bis-(2,6-diisopropylphenyl)imidazolinium chloride, one or two phenyl groups in diphenylberyllium were protonated. As a result, the long-postulated structures of [Be3 (OtBu)6 ] and [Be(µ-NPh2 )Ph]2 have finally been verified and shown to be static in solution. Additionally [Be(µ-PPh2 )(HPPh2 )Ph]2 was generated, which is only the second beryllium-phospanide to be prepared; the stark differences between its behaviour and that of the analogous amide were also examined. The first crystalline example of a beryllium Grignard reagent with a non-bulky aryl group has also been prepared; it is stabilised with an N-heterocyclic carbene.
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Recently, we introduced the first example of chiral-at-iron catalysts in which two achiral N-(2-pyridyl)-substituted N-heterocyclic carbene (NHC) ligands in addition to two labile acetonitriles are coordinated around a central iron, to generate a stereogenic metal center [Hong Y.Chiral-at-Iron Catalyst: Expanding the Chemical Space for Asymmetric Earth-Abundant Metal Catalysis. J. Am. Chem. Soc.2019, 141, 4569-4572]. A more facile synthesis of such chiral-at-iron catalysts was developed, which omits the use of expensive silver salts and an elaborate electrochemical setup. Configurational robustness was improved by replacing the imidazol-2-ylidene carbene moieties with benzimidazol-2-ylidenes. The π-acceptor properties of the altered NHCs were investigated by Ganter's 77Se NMR method. The obtained benzimidazol-2-ylidene chiral-at-iron complex is an excellent catalyst for an asymmetric hetero-Diels-Alder reaction under open-flask conditions.
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Optoelectronic properties of molecular solids are important for organic electronic devices and are largely determined by the adopted molecular packing motifs. In this study, we analyzed such structure-property relationships for the partially regioselective fluorinated tetracenes 1,2,12-trifluorotetracene, 1,2,10,12-tetrafluorotetracene and 1,2,9,10,11-pentafluorotetracene that were further compared with tetracene and perfluoro-tetracene. Quantum chemical DFT calculations in combination with optical absorption spectroscopy data show that the frontier orbital energies are lowered with the degree of fluorination, while their optical gap is barely affected. However, the crystal structure changes from a herringbone packing motif of tetracene towards a planar stacking motif of the fluorinated tetracene derivatives, which is accompanied by the formation of excimers and leads to strongly red-shifted photoluminescence with larger lifetimes.
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It is common knowledge that metal-to-ligand π back-donation requires filled atomic orbitals at the metal center. However, we show through a combined experimental and theoretical approach that Be(II)âN-heterocyclic carbene (NHC) π back-donation is present in the two carbene adducts [(iPr)BeBr2] (1) and [(iPr)2BeBr2] (2) (iPr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene). These complexes were characterized with NMR, IR, and Raman spectroscopy as well as with single-crystal X-ray diffractometry. The unusual bonding situation is understood from the results of energy decomposition analysis in combination with natural orbital for chemical valence and quantum theory of atoms-in-molecules analysis. The obtained findings shed light on the unusually high Be-C bond strength in carbene adducts to beryllium compounds and rationalize their geometry and reactivity.
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UIr has been discussed as a rare example of a noncentrosymmetric, ferromagnetic superconductor crystallizing in the acentric PdBi structure type (P21, mP16). Here we present a new structure model for UIr. By means of single-crystal and powder X-ray diffraction we find UIr to crystallize in the centrosymmetric space group P21/c, in line with previous ab initio calculations. The discrepancy with the previous noncentrosymmetric model in space group P21 is explained by the occurrence of twinning. The observed twinning hints toward a high-temperature displacive phase transition of UIr to the CrB structure type (Cmcm, oS8): we discuss the lattice dynamics corresponding to this transition by crystallographic symmetry mode analysis and by density functional theory (DFT). We find that spin-orbit coupling is essential to understand this phase transition. We apply our theoretical considerations for a critical judgment of the structure models of UPt and NpIr that have been reported to crystallize isotypically with UIr. We confirm that UPt is isotypic to UIr (P21/c), whereas we predict NpIr to crystallize in the CrB structure type. Our report on the centrosymmetric crystal structure of UIr has an effect on all those theoretical models that investigated potentially novel superconducting coupling mechanisms of this compound on the basis of the noncentrosymmetric structure model.
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We obtained single crystals of the binary mixed-valent fluorides Mn2F5 and Mn3F8 using a high-pressure/high-temperature approach. Mn2F5 crystallizes isotypic to CaCrF5 in the monoclinic space group C2/c (No. 15), with a = 8.7078(8) Å, b = 6.1473(6) Å, c = 7.7817(7) Å, ß = 117.41(1)°, V = 369.80(6) Å3, Z = 4, and mC28 at T = 173 K. Mn3F8 crystallizes in the monoclinic space group P21 (No. 4) with a = 5.5253(2) Å, b = 4.8786(2) Å, c = 9.9124(4) Å, ß = 92.608(2)°, V = 266.92(2) Å3, Z = 2, and mP22 at T = 183 K and presents a new structure type. Crystal-chemical reasoning, CHARDI calculations, and quantum-chemical calculations allowed for the assignment of the oxidation states of the Mn atoms. In both bulk compounds, MnF2 was present as an impurity, as evidenced by powder X-ray diffraction and IR and Raman spectroscopy.
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This study reports the catalytic deracemization of ketones bearing stereocenters in the α-position in a single reaction via deprotonation, followed by enantioselective protonation. The principle of microscopic reversibility, which has previously rendered this strategy elusive, is overcome by a photoredox deprotonation through single electron transfer and subsequent hydrogen atom transfer (HAT). Specifically, the irradiation of racemic pyridylketones in the presence of a single photocatalyst and a tertiary amine provides nonracemic carbonyl compounds with up to 97% enantiomeric excess. The photocatalyst harvests the visible light, induces the redox process, and is responsible for the asymmetric induction, while the amine serves as a single electron donor, HAT reagent, and proton source. This conceptually simple light-driven strategy of coupling a photoredox deprotonation with a stereocontrolled protonation, in conjunction with an enrichment process, serves as a blueprint for other deracemizations of ubiquitous carbonyl compounds.
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Single crystals of barium bis-[tetra-fluorido-bromate(III)], Ba[BrF4]2, were obtained in the form of tiny blocks. Crystal-structure refinement of Ba[BrF4]2 from single-crystal X-ray diffraction data confirmed the previous model obtained on the basis of powder data [Ivlev et al. (2014 â¸). Eur. J. Inorg. Chem. pp. 6261-6267], but with all atoms refined with anisotropic displacement parameters. The crystal structure consists of two symmetry-independent barium cations that are each coordinated by twelve fluorine atoms, forming edge-sharing polyhedra, and an almost square-planar [BrF4]- anion. The compound crystallizes in the Ba[AuF4]2 structure type.
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The rapid development of organic electronics is closely related to the availability of molecular materials with specific electronic properties. Here, we introduce a novel synthetic route enabling a unilateral functionalization of acenes along their long side, which is demonstrated by the synthesis of 1,2,10,11,12,14-hexafluoropentacene (1) and the related 1,2,9,10,11-pentafluorotetracene (2). Quantum chemical DFT calculations in combination with optical and X-ray absorption spectroscopy data indicate that the single-molecule properties of 1 are a connecting link between the organic semiconductor model systems pentacene (PEN) and perfluoropentacene (PFP). In contrast, the crystal structure analysis reveals a different packing motif than for the parent molecules. This can be related to distinct Fâ â â H interactions identified in the corresponding Hirshfeld surface analysis and also affects solid-state properties such as the exciton binding energy and the sublimation enthalpy.