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1.
J Phys Chem Lett ; : 3490-3496, 2021 Apr 01.
Artigo em Inglês | MEDLINE | ID: mdl-33792315

RESUMO

In light of the current energy requirements, the conversion of CO2 and N2 into useful C-N bond-containing products under mild conditions has become an area of intense research. However, the inert nature of N2 and CO2 renders their coupling extremely challenging. Herein, nitrogen and carbon atoms originating from N2 and CO2, respectively, are fixed sequentially by NbH2- anions in the gas phase at room temperature. Isocyanate and NbO2CN- anions were formed under thermal collision conditions, thus achieving the formation of new C-N bonds directly from simple N2 and CO2. The anion structures and reaction details were studied by mass spectrometry, photoelectron spectroscopy, and quantum chemical calculations. A novel N2 activation mode (metal-ligand activation, MLA) and a related mechanism for constructing C-N bonds mediated by a single non-noble metal atom are proposed. In this MLA mode, the C atom originating from CO2 serves as an electron reservoir to accept and donate electrons.

2.
J Am Chem Soc ; 142(30): 13102-13111, 2020 Jul 29.
Artigo em Inglês | MEDLINE | ID: mdl-32536163

RESUMO

The iron-catalyzed hydroarylation of allenes was accomplished by weak phenone assistance. The C-H activation proceeded with excellent efficacy and high ortho-regioselectivity in proximity to the weakly coordinating carbonyl group for a range of substituted phenones and allenes. Detailed mechanistic studies, including the isolation of key intermediates, the structural characterization of an iron-metallacycle, and kinetic analysis, allowed the sound elucidation of a plausible catalytic working mode. This mechanistic rationale is supported by detailed computational density functional theory studies, which fully address multi-spin-state reactivity. Furthermore, in operando nuclear magnetic resonance monitoring of the catalytic reaction provided detailed insights into the mode of action of the iron-catalyzed C-H alkylation with allenes.

3.
Angew Chem Int Ed Engl ; 59(3): 1176-1180, 2020 01 13.
Artigo em Inglês | MEDLINE | ID: mdl-31778293

RESUMO

A cooperative Cu/Pd-catalyzed enantioselective synthesis of multisubstituted allenes is established. By employing chiral sulfoxide phosphine (SOP)/Cu and PdCl2 (dppf) complexes as catalysts, the 1,4-arylboration of 1,3-enynes provides an efficient approach to trisubstituted chiral allenes with up to 92 % yield and 97:3 er. Furthermore, by using 2-substituted 1,3-enynes as substrates, the tetrasubstituted chiral allenes were successfully generated using this strategy. Finally, theoretical calculations indicate that the transmetallation of the allenylcopper species is the rate-limiting step of this transformation.

4.
Angew Chem Int Ed Engl ; 59(8): 3178-3183, 2020 Feb 17.
Artigo em Inglês | MEDLINE | ID: mdl-31729814

RESUMO

Nickela-electrooxidative C-H alkoxylations with challenging secondary alcohols were accomplished in a fully dehydrogenative fashion, thereby avoiding stoichiometric chemical oxidants, with H2 as the only stoichiometric byproduct. The nickela-electrocatalyzed oxygenation proved viable with various (hetero)arenes, including naturally occurring secondary alcohols, without racemization. Detailed mechanistic investigation, including DFT calculations and cyclovoltammetric studies of a well-defined C-H activated nickel(III) intermediate, suggest an oxidation-induced reductive elimination at nickel(III).

6.
J Am Chem Soc ; 141(32): 12592-12600, 2019 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-31314515

RESUMO

Dinitrogen activation and reduction is one of the most challenging and important subjects in chemistry. Herein, we report the N2 binding and reduction at the well-defined Ta3N3H- and Ta3N3- gas-phase clusters by using mass spectrometry (MS), anion photoelectron spectroscopy (PES), and quantum-chemical calculations. The PES and calculation results show clear evidence that N2 can be adsorbed and completely activated by Ta3N3H- and Ta3N3- clusters, yielding to the products Ta3N5H- and Ta3N5-, but the reactivity of Ta3N3H- is five times higher than that of the dehydrogenated Ta3N3- clusters. The detailed mechanistic investigations further indicate that a dissociative mechanism dominates the N2 activation reactions mediated by Ta3N3H- and Ta3N3-; two and three Ta atoms are active sites and also electron donors for the N2 reduction, respectively. Although the hydrogen atom in Ta3N3H- is not directly involved in the reaction, its very presence modifies the charge distribution and the geometry of Ta3N3H-, which is crucial to increase the reactivity. The mechanisms revealed in this gas-phase study stress the fundamental rules for N2 activation and the important role of transition metals as active sites as well as the new significant role of metal hydride bonds in the process of N2 reduction, which provides molecular-level insights into the rational design of tantalum nitride-based catalysts for N2 fixation and activation or NH3 synthesis.

7.
Angew Chem Int Ed Engl ; 58(35): 12264-12270, 2019 08 26.
Artigo em Inglês | MEDLINE | ID: mdl-31267622

RESUMO

A highly enantioselective thiocarbonylation of styrenes with CO and thiols has been achieved by Pd catalysis, providing highly enantioenriched thioesters in good to excellent yields. Key to the successful execution of this reaction is the use of a chiral sulfoxide-(P-dialkyl)-phosphine (SOP) ligands. This thiocarbonylation proceeds smoothly under mild reaction conditions (1 atm CO and 0 °C) and displays broad substrate scope. Also demonstrated is that this transformation can be conducted using surrogates of CO, greatly increasing the safety aspects of running the reaction. The generality and utility of the method is manifested by its application to the synthetic transformations of thioester products and the direct acylation of cysteine-containing dipeptides. A primary mechanism was investigated and a plausible catalytic cycle was proposed.

8.
J Am Chem Soc ; 139(44): 15564-15567, 2017 11 08.
Artigo em Inglês | MEDLINE | ID: mdl-29063756

RESUMO

Iron-catalyzed alkene [2+2] cycloaddition reactions represent a promising stepwise pathway to effect the kinetically hindered concerted [2+2] cycloaddition. However, the fundamental reactivity paradigm of these reactions remains unclear. Based on high level combined CASPT2/DFT modelings, herein we reveal an unprecedented substrate-dependent two-state reactivity scenario for the key C-C coupling in this iron catalysis, in which the representative substrates of mono-olefins only and mono-olefin plus 1,3-diene exhibit different reactivity paradigms. The role of the redox-active ligand is found to generate a ferric oxidation state for the metallacyclic intermediate of C-C coupling, thereby rendering a thermodynamically more accessible FeIII/FeI reductive elimination process compared with the otherwise FeII/Fe0 one. The enhancement of the spin state transition efficiency between the singlet and triplet states is predicted as an alternative way to increase the C-C coupling reactivity in the cross [2+2] cycloaddition reactions between mono-olefins and dienes. This work highlights the ab initio multi-reference method in describing very complicated open-shell iron catalysis.

9.
J Chem Theory Comput ; 13(10): 4841-4853, 2017 Oct 10.
Artigo em Inglês | MEDLINE | ID: mdl-28881134

RESUMO

Accurate modelings of reactions involving 3d transition metals (TMs) are very challenging to both ab initio and DFT approaches. To gain more knowledge in this field, we herein explored typical σ-bond activations of H-H, C-H, C-Cl, and C-C bonds promoted by nickel(0), a low-valent late 3d TM. For the key parameters of activation energy (ΔE‡) and reaction energy (ΔER) for these reactions, various issues related to the computational accuracy were systematically investigated. From the scrutiny of convergence issue with one-electron basis set, augmented (A) basis functions are found to be important, and the CCSD(T)/CBS level with complete basis set (CBS) limit extrapolation based on augmented double-ζ and triple-ζ basis pair (ADZ and ATZ), which produces deviations below 1 kcal/mol from the reference, is recommended for larger systems. As an alternative, the explicitly correlated F12 method can accelerate the basis set convergence further, especially after its CBS extrapolations. Thus, the CCSD(T)-F12/CBS(ADZ-ATZ) level with computational cost comparable to the conventional CCSD(T)/CBS(ADZ-ATZ) level, is found to reach the accuracy of the conventional CCSD(T)/A5Z level, which produces deviations below 0.5 kcal/mol from the reference, and is also highly recommendable. Scalar relativistic effects and 3s3p core-valence correlation are non-negligible for achieving chemical accuracy of around 1 kcal/mol. From the scrutiny of convergence issue with the N-electron basis set, in comparison with the reference CCSDTQ result, CCSD(T) is found to be able to calculate ΔE‡ quite accurately, which is not true for the ΔER calculations. Using highest-level CCSD(T) results of ΔE‡ in this work as references, we tested 18 DFT methods and found that PBE0 and CAM-B3LYP are among the three best performing functionals, irrespective of DFT empirical dispersion correction. With empirical dispersion correction included, ωB97XD is also recommendable due to its improved performance.

10.
J Am Chem Soc ; 139(10): 3876-3888, 2017 03 15.
Artigo em Inglês | MEDLINE | ID: mdl-28222257

RESUMO

Transition-metal alkylidenes are important reactive organometallic intermediates, and our current knowledge on them has been mainly restricted to those with closed-shell electronic configurations. In this study, we present an exploration on open-shell iron alkylidenes with a weak-field tripodal amido-phosphine-amido ligand. We found that a high-spin (amido-phosphine-amido)iron(II) complex can react with (p-tolyl)2CN2 to afford a high-spin (amido-ylide-amido)iron(II) complex, 2, which could transfer its alkylidene moiety to a variety of alkenes, either the electron-rich or electron-deficient ones, to form cyclopropane derivatives. The reaction of 2 with cis-ß-deuterio-styrene gave deuterated cyclopropane derivatives with partial loss of the stereochemical integrity with respect to the cis-styrene. Kinetic study on the cyclopropanation reaction of 2 with 4-fluoro-styrene disclosed the activation parameters of ΔH⧧ = 23 ± 1 kcal/mol and ΔS⧧ = -20 ± 3 cal/mol/K, which are comparable to those of the cyclopropanation reactions involving transition-metal alkylidenes. However, the cyclopropanation of para-substituted styrenes by 2 shows a nonlinear Hammett plot of log(kX/kH) vs σp. By introduction of a radical parameter, a linear plot of log(kX/kH) vs 0.59σp + 0.55σc• was obtained, which suggests the "nucleophilic" radical nature of the transition state of the cyclopropanation step. In corroboration with the experimental observations, density functional theory calculation on the reaction of 2 with styrene suggests the involvement of an open-shell (amido-phosphine-amido)iron alkylidene intermediate that is higher in energy than its (amido-ylide-amido)iron(II) precursor and an "outer-sphere" radical-type mechanism for the cyclopropanation step. The negative charge distribution on the alkylidene carbon atoms of the open-shell states (S = 2 and 1) explains the high activity of the cyclopropanation reaction toward electron-deficient alkenes. The study demonstrates the unique activity of open-shell iron alkylidene species beyond its closed-shell analogues, thus pointing out their potential synthetic usage in catalysis.

11.
J Phys Chem Lett ; 7(21): 4427-4432, 2016 Nov 03.
Artigo em Inglês | MEDLINE | ID: mdl-27775357

RESUMO

Cyanobacterial aldehyde-deformylating oxygenase (cADO) is a nonheme diiron enzyme that catalyzes the conversion of aldehyde to alk(a/e)ne, an important transformation in biofuel research. In this work, we report a highly desired computational study for probing the mechanism of cADO. By combining our QM/MM results with the available 57Fe Mössbauer spectroscopic data, the gained detailed structural information suggests construction of asymmetry from the symmetric diiron cofactor in an aldehyde substrate and O2 activation. His160, one of the two iron-coordinate histidine residues in cADO, plays a pivotal role in this asymmetric aldehyde activation process by unprecedented reversible dissociation from the diiron cofactor, a behavior unknown in any other nonheme dinuclear or mononuclear enzymes. The revealed intrinsically asymmetric interactions of the substrate/O2 with the symmetric cofactor in cADO are inspirational for exploring diiron subsite resolution in other nonheme diiron enzymes.


Assuntos
Aldeídos/química , Cianobactérias/química , Oxigenases/química , Catálise , Oxirredução
12.
J Am Chem Soc ; 138(24): 7753-7762, 2016 06 22.
Artigo em Inglês | MEDLINE | ID: mdl-27221953

RESUMO

Metal-nitrosyl complexes are key intermediates involved in many biological and physiological processes of nitric oxide (NO) activation by metalloproteins. In this study, we report the reactivities of mononuclear cobalt(III)-nitrosyl complexes bearing N-tetramethylated cyclam (TMC) ligands, [(14-TMC)Co(III)(NO)](2+) and [(12-TMC)Co(III)(NO)](2+), in NO-transfer and dioxygenation reactions. The Co(III)-nitrosyl complex bearing 14-TMC ligand, [(14-TMC)Co(III)(NO)](2+), transfers the bound nitrosyl ligand to [(12-TMC)Co(II)](2+) via a dissociative pathway, {[(14-TMC)Co(III)(NO)](2+) → {(14-TMC)Co···NO}(2+)}, thus affording [(12-TMC)Co(III)(NO)](2+) and [(14-TMC)Co(II)](2+) as products. The dissociation of NO from the [(14-TMC)Co(III)(NO)](2+) complex prior to NO-transfer is supported experimentally and theoretically. In contrast, the reverse reaction, which is the NO-transfer from [(12-TMC)Co(III)(NO)](2+) to [(14-TMC)Co(II)](2+), does not occur. In addition to the NO-transfer reaction, dioxygenation of [(14-TMC)Co(III)(NO)](2+) by O2 produces [(14-TMC)Co(II)(NO3)](+), which possesses an O,O-chelated nitrato ligand and where, based on an experiment using (18)O-labeled O2, two of the three O-atoms in the [(14-TMC)Co(II)(NO3)](+) product derive from O2. The dioxygenation reaction is proposed to occur via a dissociative pathway, as proposed in the NO-transfer reaction, and via the formation of a Co(II)-peroxynitrite intermediate, based on the observation of phenol ring nitration. In contrast, [(12-TMC)Co(III)(NO)](2+) does not react with O2. Thus, the present results demonstrate unambiguously that the NO-transfer/dioxygenation reactivity of the cobalt(III)-nitrosyl complexes bearing TMC ligands is significantly influenced by the ring size of the TMC ligands and/or the spin state of the cobalt ion.

13.
Phys Chem Chem Phys ; 18(30): 20321-9, 2016 Jul 27.
Artigo em Inglês | MEDLINE | ID: mdl-27066757

RESUMO

Gold-doped germanium clusters, AuGen(-) (n = 2-12), were investigated by using anion photoelectron spectroscopy in combination with ab initio calculations. Their geometric structures were determined by comparison of the theoretical calculations with the experimental results. The results show that the most stable isomers of AuGen(-) with n = 2-10 are all exohedral structures with the Au atom capping the vertex, edge or face of Gen clusters, while AuGe11(-) is found to be the critical size of the endohedral structure. Interestingly, AuGe12(-) has an Ih symmetric icosahedral structure with the Au atom located at the center. The molecular orbital analysis of the AuGe12(-) cluster suggests that the interactions between the 5d orbitals of the Au atom and the 4s4p hybridized orbitals of the Ge atoms may stabilize the Ih symmetric icosahedral cage and promote the Au atom to be encapsulated in the cage of Ge12. The NICS(0) and NICS(1) values are calculated to be -143.7 ppm and -36.3 ppm, respectively, indicating that the icosahedral AuGe12(-) cluster is significantly aromatic.

14.
J Am Chem Soc ; 138(11): 3715-30, 2016 Mar 23.
Artigo em Inglês | MEDLINE | ID: mdl-26907535

RESUMO

C-H bond activation/functionalization promoted by low-valent iron complexes has recently emerged as a promising approach for the utilization of earth-abundant first-row transition metals to carry out this difficult transformation. Herein we use extensive density functional theory and high-level ab initio coupled cluster calculations to shed light on the mechanism of these intriguing reactions. Our key mechanistic discovery for C-H arylation reactions reveals a two-state reactivity (TSR) scenario in which the low-spin Fe(II) singlet state, which is initially an excited state, crosses over the high-spin ground state and promotes C-H bond cleavage. Subsequently, aryl transmetalation occurs, followed by oxidation of Fe(II) to Fe(III) in a single-electron transfer (SET) step in which dichloroalkane serves as an oxidant, thus promoting the final C-C coupling and finalizing the C-H functionalization. Regeneration of the Fe(II) catalyst for the next round of C-H activation involves SET oxidation of the Fe(I) species generated after the C-C bond coupling. The ligand sphere of iron is found to play a crucial role in the TSR mechanism by stabilization of the reactive low-spin state that mediates the C-H activation. This is the first time that the successful TSR concept conceived for high-valent iron chemistry is shown to successfully rationalize the reactivity for a reaction promoted by low-valent iron complexes. A comparative study involving other divalent middle and late first-row transition metals implicates iron as the optimum metal in this TSR mechanism for C-H activation. It is predicted that stabilization of low-spin Mn(II) using an appropriate ligand sphere should produce another promising candidate for efficient C-H bond activation. This new TSR scenario therefore emerges as a new strategy for using low-valent first-row transition metals for C-H activation reactions.

15.
J Chem Theory Comput ; 11(10): 4601-14, 2015 Oct 13.
Artigo em Inglês | MEDLINE | ID: mdl-26574251

RESUMO

Using high level ab initio coupled cluster calculations as reference, the performances of 15 commonly used density functionals (DFs) on activation energy calculations for typical Mo/W-mediated reactions have been systematically assessed for the first time in this work. The selected representative Mo/W-mediated reactions cover a wide range from enzymatic reactions to organometallic reactions, which include Mo-catalyzed aldehyde oxidation (aldehyde oxidoreductase), Mo-catalyzed dimethyl sulfoxide (DMSO) reduction (DMSO reductase), W-catalyzed acetylene hydration (acetylene hydratase), Mo/W-mediated olefin metathesis, Mo/W-mediated olefin epoxidation, W-mediated alkyne metathesis, and W-mediated C-H bond activation. Covering both Mo- and W-mediated reactions, four DFs of B2GP-PLYP, M06, B2-PLYP, and B3LYP are uniformly recommended with and without DFT empirical dispersion correction. Among these four DFs, B3LYP is notably improved in performance by DFT empirical dispersion correction. In addition to the absolute value of calculation error, if the trend of DFT results is also a consideration, B2GP-PLYP, B2-PLYP, and M06 keep better performance than other functionals tested and constitute our final recommendation of DFs for both Mo- and W-mediated reactions.

16.
J Chem Theory Comput ; 11(4): 1428-38, 2015 Apr 14.
Artigo em Inglês | MEDLINE | ID: mdl-26574354

RESUMO

In this work, the performances of 19 density functional theory (DFT) methods are calibrated comparatively on Ru- and Rh-promoted σ-bond (C-H, O-H, and H-H) activations. DFT calibration reference is generated from explicitly correlated coupled cluster CCSD(T)-F12 calculations, and the 4s4p core-valence correlation effect of the two 4d platinum group transition metals is also included. Generally, the errors of DFT methods for calculating energetics of Ru-/Rh-mediated reactions appear to correlate more with the magnitude of energetics itself than other factors such as metal identity. For activation energy calculations, the best performing functionals for both Ru and Rh systems are MN12SX < CAM-B3LYP < M06-L < MN12L < M06 < ωB97X < B3LYP < LC-ωPBE (in the order of increasing mean unsigned deviations, MUDs, of less than 2 kcal/mol). For reaction energy calculations, best functionals with MUDs less than 2 kcal/mol are PBE0 < CAM-B3LYP ≈ N12SX. The effect of the DFT empirical dispersion correction on the performance of the DFT methods is beneficial for most density functionals tested in this work, reducing their MUDs to different extents. After including empirical dispersion correction, ωB97XD, B3LYP-D3, and CAM-B3LYP-D3 (PBE0-D3, B3LYP-D3, and ωB97XD) are the three best performing DFs for activation energy (reaction energy) calculations, from which B3LYP-D3 and ωB97XD can notably be recommended uniformly for both the reaction energy and reaction barrier calculations. The good performance of B3LYP-D3 in quantitative description of the energetic trends further adds value to B3LYP-D3 and singles this functional out as a reasonable choice in the Ru/Rh-promoted σ-bond activation processes.

17.
Chemistry ; 21(49): 17748-56, 2015 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-26490554

RESUMO

Although early transition metal (ETM) carbides can activate CH bonds in condensed-phase systems, the electronic-level mechanism is unclear. Atomic clusters are ideal model systems for understanding the mechanisms of bond activation. For the first time, CH activation of a simple alkane (ethane) by an ETM carbide cluster anion (MoC3 (-) ) under thermal-collision conditions has been identified by using high-resolution mass spectrometry, photoelectron imaging spectroscopy, and high-level quantum chemical calculations. Dehydrogenation and ethene elimination were observed in the reaction of MoC3 (-) with C2 H6 . The CH activation follows a mechanism of oxidative addition that is much more favorable in the carbon-stabilized low-spin ground electronic state than in the high-spin excited state. The reaction efficiency between the MoC3 (-) anion and C2 H6 is low (0.23±0.05) %. A comparison between the anionic and a highly efficient cationic reaction system (Pt(+) +C2 H6 ) was made. It turned out that the potential-energy surfaces for the entrance channels of the anionic and cationic reaction systems can be very different.

18.
J Am Chem Soc ; 137(44): 14196-207, 2015 Nov 11.
Artigo em Inglês | MEDLINE | ID: mdl-26505122

RESUMO

High-valent iron imido species are implicated as reactive intermediates in many iron-catalyzed transformations. However, isolable complexes of this type are rare, and their reactivity is poorly understood. Herein, we report the synthesis, characterization, and reactivity studies on novel three-coordinate iron(IV) bisimido complexes with aminocarbene ligation. Using our recently reported synthetic method for [LFe(NDipp)2] (L = IMes, 1; Me2-cAAC, 2), four new iron(IV) imido complexes, [(IPr)Fe(NDipp)2] (3) and [(Me2-cAAC)Fe(NR)2] (R = Mes, 4; Ad, 5; CMe2CH2Ph, 6), were prepared from the reactions of three-coordinate iron(0) compounds with organic azides. Characterization data acquired from (1)H and (13)C NMR spectroscopy, (57)Fe Mössbauer spectroscopy, and X-ray diffraction studies suggest a low-spin singlet ground state for these iron(IV) complexes and the multiple-bond character of their Fe-N bonds. A reactivity study taking the reactions of 1 as representative revealed an intramolecular alkane dehydrogenation of 1 to produce the iron(II) complex [(IMes)Fe(NHDipp)(NHC6H3-2-Pr(i)-6-CMe═CH2)] (7), a Si-H bond activation reaction of 1 with PhSiH3 to produce the iron(II) complex [(IMes)Fe(NHDipp)(NDippSiPhH2)] (8), and a [2+2]-addition reaction of 1 with PhNCNPh and p-Pr(i)C6H4NCO to form the corresponding open-shell formal iron(IV) monoimido complexes [(IMes)Fe(NDipp)(N(Dipp)C(NPh)(═NPh))] (9) and [(IMes)Fe(NDipp)(N(Dipp)C(O)N(p-Pr(i)C6H4))] (10), as well as [NDipp]-group-transfer reactions with CO and Bu(t)NC. Density functional theory calculations suggested that the alkane chain dehydrogenation reaction starts with a hydrogen atom abstraction mechanism, whereas the Si-H activation reaction proceeds in a [2π+2σ]-addition manner. Both reactions have the pathways at the triplet potential energy surfaces being energetically preferred, and have formal iron(IV) hydride and iron(IV) silyl species as intermediates, respectively. The low-coordinate nature and low d-electron count (d(4)) of iron(IV) imido complexes are thought to be the key features endowing their unique reactivity.

19.
J Chem Phys ; 142(16): 164301, 2015 Apr 28.
Artigo em Inglês | MEDLINE | ID: mdl-25933758

RESUMO

The isoeletronic diatomic MoC(-) and NbN(-) anions have been prepared by laser ablation and studied by photoelectron imaging spectroscopy combined with quantum chemistry calculations. The photoelectron spectra of NbN(-) can be very well assigned on the basis of literature reported optical spectroscopy of NbN. In contrast, the photoelectron spectra of MoC(-) are rather complex and the assignments suffered from the presence of many electronically hot bands and limited information from the reported optical spectroscopy of MoC. The electron affinities of NbN and MoC have been determined to be 1.450 ± 0.003 eV and 1.360 ± 0.003 eV, respectively. The good resolution of the imaging spectroscopy provided a chance to resolve the Ω splittings of the X(3)Σ(-) (Ω = 0 and 1) state of MoC and the X(4)Σ(-) (Ω = 1/2 and 3/2) state of MoC(-) for the first time. The spin-orbit splittings of the X(2)Δ state of NbN(-) and the a(2)Δ state of MoC(-) were also determined. The similarities and differences between the electronic structures of the NbN and MoC systems were discussed.

20.
Chemistry ; 21(8): 3495-501, 2015 Feb 16.
Artigo em Inglês | MEDLINE | ID: mdl-25589473

RESUMO

The stereoselective hydrogenation of alkynes to alkenes is an extremely useful transformation in synthetic chemistry. Despite numerous reports for the synthesis of Z-alkenes, the hydrogenation of alkynes to give E-alkenes is still not well resolved. In particular, selective preparation of both Z- and E-alkenes by the same catalytic hydrogenation system using molecular H2 has rarely been reported. In this paper, a novel strategy of using simple alkenes as promoters for the HB(C6 F5 )2 -catalyzed metal-free hydrogenation of alkynes was adopted. Significantly, both Z- and E-alkenes can be furnished by hydrogenation with molecular H2 in high yields with excellent stereoselectivities. Further experimental and theoretical mechanistic studies suggest that interactions between H and F atoms of the alkene promoter, borane intermediate, and H2 play an essential role in promoting the hydrogenolysis reaction.

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