Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 18 de 18
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
1.
J Am Chem Soc ; 142(18): 8147-8159, 2020 May 06.
Artigo em Inglês | MEDLINE | ID: mdl-32239927

RESUMO

A rare example of a dinuclear iron core with a non-linearly bridged dinitrogen ligand is reported in this work. One-electron reduction of [(tBupyrr2py)Fe(OEt2)] (1) (tBupyrr2py2- = 2,6-bis((3,5-di-tert-butyl)pyrrol-2-yl)pyridine) with KC8 yields the complex [K]2[(tBupyrr2py)Fe]2(µ2-η1:η1-N2) (2), where the unusual cis-divacant octahedral coordination geometry about each iron and the η5-cation-π coordination of two potassium ions with four pyrrolyl units of the ligand cause distortion of the bridging end-on µ-N2 about the FeN2Fe core. Attempts to generate a Et2O-free version of 1 resulted instead in a dinuclear helical dimer, [(tBupyrr2py)Fe]2 (3), via bridging of the pyridine moieties of the ligand. Reduction of 3 by two electrons under N2 does not break up the dimer, nor does it result in formation of 2 but instead formation of the ate-complex [K(OEt2)]2[(tBupyrr2py)Fe]2 (4). Reduction of 1 by two electrons and in the presence of crown-ether forms the tetraanionic N2 complex [K2][K(18-crown-6)]2(tBupyrr2py)Fe]2(µ2-η1:η1-N2) (5), also having a distorted FeN2Fe moiety akin to 2. Complex 2 is thermally unstable and loses N2, disproportionating to Fe nanoparticles among other products. A combination of single-crystal X-ray diffraction studies, solution and solid-state magnetic studies, and 57Fe Mössbauer spectroscopy has been applied to characterize complexes 2-5, whereas DFT studies have been used to help explain the bonding and electronic structure in these unique diiron-N2 complexes 2 and 5.

2.
J Phys Chem Lett ; : 3401-3407, 2020 Apr 17.
Artigo em Inglês | MEDLINE | ID: mdl-32271018

RESUMO

Dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP-SENS) has emerged as a powerful characterization tool in material chemistry and heterogeneous catalysis by dramatically increasing, by up to 2 orders of magnitude, the NMR signals associated with surface sites. DNP-SENS mostly relies on using exogenous polarizing agents (PAs), typically dinitroxyl radicals, to boost the NMR signals. However, the PAs may interact with the surface or even react with surface sites, thus leading to loss or quenching of DNP enhancements. Herein, we describe the development of a DNP-SENS formulation that allows broadening the application of DNP-SENS to samples containing highly reactive surface sites, namely a Ziegler-Natta propylene polymerization catalyst, a sulfated zirconia-supported metallocene, and a silica-supported cationic Mo alkylidene. The protocol consists of adsorbing pyridine prior to the DNP formulation (TEKPol/TCE). The addition of pyridine not only preserves the PAs and thereby restores the DNP enhancement but also allows probing Lewis/Brønsted acid surface sites that are often present on these catalysts.

3.
Angew Chem Int Ed Engl ; 58(29): 9841-9845, 2019 Jul 15.
Artigo em Inglês | MEDLINE | ID: mdl-31069914

RESUMO

Monomeric CuII sites supported on alumina, prepared using surface organometallic chemistry, convert CH4 to CH3 OH selectively. This reaction takes place by formation of CH3 O surface species with the concomitant reduction of two monomeric CuII sites to CuI , according to mass balance analysis, infrared, solid-state nuclear magnetic resonance, X-ray absorption, and electron paramagnetic resonance spectroscopy studies. This material contains a significant fraction of Cu active sites (22 %) and displays a selectivity for CH3 OH exceeding 83 %, based on the number of electrons involved in the transformation. These alumina-supported CuII sites reveal that C-H bond activation, along with the formation of CH3 O- surface species, can occur on pairs of proximal monomeric CuII sites in a short reaction time.

4.
ChemSusChem ; 12(5): 968-972, 2019 Mar 07.
Artigo em Inglês | MEDLINE | ID: mdl-30644172

RESUMO

Small and narrowly distributed Cu nanoparticles, supported on SiO2 decorated with isolated TiIV sites, prepared through surface organometallic chemistry, showed significantly improved CO2 hydrogenation activity and CH3 OH selectivity compared to the corresponding Cu nanoparticles supported on SiO2 . These isolated Lewis acid TiIV sites, evidenced by UV/Vis spectroscopy, are proposed to stabilize surface intermediates at the interface between Cu nanoparticles and the support.

5.
J Am Chem Soc ; 140(37): 11674-11679, 2018 09 19.
Artigo em Inglês | MEDLINE | ID: mdl-30145890

RESUMO

The development of more effective alkane dehydrogenation catalysts is a key technological challenge for the production of olefins from shale gas, an abundant source of light hydrocarbons. Surface organometallic chemistry provides an original approach to generate nanometric Ga-Pt bimetallic particles supported on partially dehydroxylated silica containing gallium single-sites, which displays high activity, selectivity, and stability in propane dehydrogenation. This catalyst was prepared via sequential grafting of a platinum precursor onto silica possessing site-isolated gallium sites followed by H2 reduction. Monitoring generation of the reduced species, Gaδ+Pt0/SiO2, via in situ X-ray absorption spectroscopy reveals formation of a Ga xPt (0.5 < x < 0.9) alloy with a fraction of gallium remaining as isolated sites. This bimetallic material exhibits catalytic performance that far surpasses each of the individual components and other reported Ga-Pt based catalysts; this is attributed to the highly dispersed Ga xPt alloyed structure on a support with low Brønsted acidity containing gallium single-sites.

6.
Chem Sci ; 9(7): 1912-1918, 2018 Feb 21.
Artigo em Inglês | MEDLINE | ID: mdl-29675237

RESUMO

Metal alkylidenes, which are key organometallic intermediates in reactions such as olefination or alkene and alkane metathesis, are typically generated from metal dialkyl compounds [M](CH2R)2 that show distinctively deshielded chemical shifts for their α-carbons. Experimental solid-state NMR measurements combined with DFT/ZORA calculations and a chemical shift tensor analysis reveal that this remarkable deshielding originates from an empty metal d-orbital oriented in the M-Cα-Cα' plane, interacting with the Cα p-orbital lying in the same plane. This π-type interaction inscribes some alkylidene character into Cα that favors alkylidene generation via α-H abstraction. The extent of the deshielding and the anisotropy of the alkyl chemical shift tensors distinguishes [M](CH2R)2 compounds that form alkylidenes from those that do not, relating the reactivity to molecular orbitals of the respective molecules. The α-carbon chemical shifts and tensor orientations thus predict the reactivity of metal alkyl compounds towards alkylidene generation.

7.
Angew Chem Int Ed Engl ; 57(22): 6398-6440, 2018 05 28.
Artigo em Inglês | MEDLINE | ID: mdl-28685920

RESUMO

Many industrial catalysts contain isolated metal sites on the surface of oxide supports. Although such catalysts have been used in a broad range of processes for more than 40 years, there is often a very limited understanding about the structure of the catalytically active sites. This Review discusses how surface organometallic chemistry (SOMC) engineers surface sites with well-defined structures and provides insight into the nature of the active sites of industrial catalysts; the Review focuses in particular on olefin production and conversion processes.

8.
Chem Sci ; 8(4): 2661-2666, 2017 Apr 01.
Artigo em Inglês | MEDLINE | ID: mdl-28553501

RESUMO

Single-site gallium centers on the surface of silica are prepared via grafting of [Ga(OSi(OtBu)3)3(THF)] on SiO2-700 followed by a thermolysis step. The resulting surface species corresponds to well-defined tetra-coordinate gallium single-sites, [( 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 SiO)3Ga(XOSi)] (X = -H or Si) according to IR, X-ray absorption near-edge structure and extended X-ray absorption fine structure analysis. These gallium sites show high activity, selectivity and stability for propane dehydrogenation with an initial turnover frequency of 20 per h per gallium center, propylene selectivity of ≥93% and remarkable stability over 20 h. The stability of the catalyst probably results from site-isolation of the active site on a non-reducible support such as silica, diminishing facile reduction typical of Ga2O3-based catalysts.

9.
Chem Rev ; 116(15): 8463-505, 2016 Aug 10.
Artigo em Inglês | MEDLINE | ID: mdl-27398777

RESUMO

Surface hydrides are ubiquitous in catalysis. However, their structures and properties are not as well-understood as those of their molecular counterparts, which have been extensively studied for the past 70 years. Hydrides isolated on surfaces have been characterized as stable entities on oxide surfaces or in zeolites. They have also been proposed as reaction intermediates in numerous catalytic processes (hydrogenation, hydrogenolysis, etc.). They have also been prepared via surface organometallic chemistry. In this review, we describe their key structural features and spectroscopic signatures. We discuss their reactivity and stability and also point out unexplored areas.

10.
Angew Chem Int Ed Engl ; 55(23): 6642-5, 2016 06 01.
Artigo em Inglês | MEDLINE | ID: mdl-27110689

RESUMO

The niobium methylidene [{(Ar'O)2 Nb}2 (µ2 -Cl)2 (µ2 -CH2 )] (2) can be cleanly prepared via thermolysis or photolysis of [(Ar'O)2 Nb(CH3 )2 Cl] (1) (OAr'=2,6-bis(diphenylmethyl)-4-tert-butylphenoxide). Reduction of 2 with two equivalents of KC8 results in formation of the first niobium methylidyne [K][{(Ar'O)2 Nb}2 (µ2 -CH)(µ2 -H)(µ2 -Cl)] (3) via a binuclear α-hydrogen elimination. Oxidation of 3 with two equiv of ClCPh3 reforms 2. In addition to solid state X-ray analysis, all these complexes were elucidated via multinuclear NMR experiments and isotopic labelling studies, including a crossover experiment, support the notion for a radical mechanism as well as a binuclear α-hydrogen abstraction pathway being operative in the formation of 2 from 1.

11.
J Am Chem Soc ; 137(37): 11872-5, 2015 Sep 23.
Artigo em Inglês | MEDLINE | ID: mdl-26302157

RESUMO

The ethylene complex (PNP)Ti(η(2)-H2C═CH2)(CH2(t)Bu) or (PNP)Ti═CH(t)Bu(CH2(t)Bu) (PNP(-) = N[2-P(CHMe2)2-4-methylphenyl]2) reacts with H2CPPh3 to form the κ(2)-phosphinoalkylidene (PNP)Ti═CHPPh2(Ph) (1). Compound 1 activates benzene via the transient intermediate [(PNP)Ti≡CPPh2] (C). By treatment of (PNP)Ti═CH(t)Bu(OTf) with LiCH2PPh2, 1 or its isotopologue (PNP)Ti═CDPPh2(C6D5) (1-d6) can be produced by an independent route involving intermediate C, which activates benzene or benzene-d6 and dehydrogenates cyclohexane-d12. Addition of MeOTf to 1 results in elimination of benzene concomitant with the formation of the phosphonioalkylidyne complex, [(PNP)Ti≡CPPh2Me(OTf) (2). Theoretical studies of 2 suggest a resonance structure having dominant Ti-C triple-bond character with some contribution also from a C-P multiple bond.

12.
Chem Commun (Camb) ; 51(17): 3526-8, 2015 Feb 28.
Artigo em Inglês | MEDLINE | ID: mdl-25642464

RESUMO

The easy-to-prepare Nb(V) aryloxide complex [(ArO)2Nb(µ-Cl)Cl2]2 (OAr = 2,6-bis(diphenylmethyl)-4-tert-butylphenoxide) is a precursor to both Nb(IV), [trans-(ArO)2NbCl2(THF)2], and Nb(III), K3[(ArO)4Nb2(µ-Cl)3Cl2], molecules. The Nb(IV) and (V) complexes readily split atmospheric nitrogen at room temperature and 1 atmosphere, under reducing conditions, to produce the low-coordinate nitride dimer [(ArO)2Nb(µ-N)]2 and its radical anion, K[(ArO)2Nb(µ-N)]2.

13.
Angew Chem Int Ed Engl ; 53(51): 14139-43, 2014 Dec 15.
Artigo em Inglês | MEDLINE | ID: mdl-25327475

RESUMO

A rare, low-spin Fe(IV)  imide complex [(pyrr2py)Fe=NAd] (pyrr2 py(2-) = bis(pyrrolyl)pyridine; Ad = 1-adamantyl) confined to a cis-divacant octahedral geometry, was prepared by reduction of N3Ad by the Fe(II)  precursor [(pyrr2py)Fe(OEt2)]. The imide complex is low-spin with temperature-independent paramagnetism. In comparison to an authentic Fe(III) complex, such as [(pyrr2py)FeCl], the pyrr2py(2-) ligand is virtually redox innocent.


Assuntos
Imidas/química , Compostos de Ferro/química , Compostos de Ferro/síntese química , Estrutura Molecular , Teoria Quântica , Estereoisomerismo
14.
Chem Commun (Camb) ; 50(47): 6267-9, 2014 Jun 14.
Artigo em Inglês | MEDLINE | ID: mdl-24788367

RESUMO

The first structurally characterized niobium(v) complex possessing a terminal methylidene ligand is reported in high yield from the reaction of [(Ar'O)2Nb(CH3)2Cl] (Ar' = (2,6-CHPh2)2-4-tBu-C6H2) and two equivalents of H2CPPh3.

15.
Inorg Chem ; 52(19): 11126-35, 2013 Oct 07.
Artigo em Inglês | MEDLINE | ID: mdl-24093483

RESUMO

The bulky aryloxide 2,6-bis(diphenylmethyl)-4-tert-butylphenol [HOAr(tBu)] (1) can be synthesized from 4-tert-butylphenol and benzhydrol in solvent-free conditions and obtained pure in 91% yield. Deprotonation of HOAr(tBu) is accomplished with M(N(SiMe3)2) (M = Na, Li), yielding the corresponding salts of the aryloxide [MOAr(tBu)] (M(+) = Na (2), Li(3)) in 83% and 73% yield, respectively. Facile salt formation of the aryloxide ligand allows for transmetalation to a variety of metal halides. Through transmetalation reactions involving two aryloxides, mononuclear complexes of the type [M'(OAr(tBu))2Cl(THF)2] (M' = Sc (4), V (5), Cr (6), Ti (7)) can be prepared from the corresponding metal halide precursor MCl3(THF)3. Additionally, two aryloxides can be coordinated to Ti(IV) via a protonolysis route of Ti(NMe2)2Cl2 and 2 equiv of HOAr(tBu) to yield [Ti(OAr(tBu))2Cl2(NHMe2)] (8) in 72% isolated yield. Single-crystal X-ray diffraction studies of 1, 2, and the 3d metal complexes 5-8 clearly show the steric demand of the bulky ligand, whereas in transition metal complexes we do not observe the formation of mononuclear tris-aryloxide complexes.

16.
Inorg Chem ; 52(9): 5611-9, 2013 May 06.
Artigo em Inglês | MEDLINE | ID: mdl-23601057

RESUMO

The potential redox activity of the 2,2'-pyridylpyrrolide ligand carrying two CF3 substituents (L(2)) is investigated. Synthesis and characterization of d(6) and d(7) species M(L(2))2 for M = Fe and Co are described (both are nonplanar, but not tetrahedral), as are the Lewis acidity of each. In spite of CV evidence for quasireversible reductions to form M(L(2))2(q-) where q = 1 and 2, chemical reductants instead yield divalent metal complexes KM(L(2))3, which show attractive interactions of K(+) to pyrrolide, to F, and to lattice toluene π cloud. The collected evidence on these products indicates that pyridylpyrrolide is a weak field ligand here, but CO can force spin pairing in Fe(L(2))2(CO)2. Evidence is presented that the overall reductive reaction yields 33 mol % of bulk metal, which is the fate of the reducing equivalents, and a mechanism for this ligand redistribution is proposed. Analogous ligand redistribution behavior is also seen for nickel and for trimeric monovalent copper analogues; reduction of Cu(L(2))2 simply forms Cu(L(2))2(-).

17.
J Chem Phys ; 139(24): 244703, 2013 Dec 28.
Artigo em Inglês | MEDLINE | ID: mdl-24387384

RESUMO

Liquid-like and liquid water at and near the surface of water-ice and frozen aqueous sodium chloride films were observed using attenuated total reflection infrared spectroscopy (ATR-IR). The concentration of NaCl ranged from 0.0001 to 0.01 M and the temperature varied from the melting point of water down to 256 K. The amount of liquid brine at the interface of the frozen films with the germanium ATR crystal increased with salt concentration and temperature. Experimental spectra are compared to reflection spectra calculated for a simplified morphology of a uniform liquid layer between the germanium crystal and the frozen film. This morphology allows for the amount of liquid observed in an experimental spectrum to be converted to the thickness of a homogenous layer with an equivalent amount of liquid. These equivalent thickness ranges from a nanometer for water-ice at 260 K to 170 nm for 0.01 M NaCl close to the melting point. The amounts of brine observed are over an order of magnitude less than the total liquid predicted by equilibrium thermodynamic models, implying that the vast majority of the liquid fraction of frozen solutions may be found in internal inclusions, grain boundaries, and the like. Thus, the amount of liquid and the solutes dissolved in them that are available to react with atmospheric gases on the surfaces of snow and ice are not well described by thermodynamic equilibrium models which assume the liquid phase is located entirely at the surface.

18.
Dalton Trans ; 41(32): 9619-22, 2012 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-22766760

RESUMO

The synthesis and characterization of a series of iridium(I) and iridium(III) complexes supported by a 3,5-bis(trifluoromethyl)-2-(2'-pyridyl)pyrrole (L(-)) ligand are reported. Compound [Ir(L)(COD)] (COD = 1,5-cyclooctadiene) demonstrates reversible binding of hydrogen to form a dihydride complex [Ir(L)(H)(2)(COD)] with no hydrogenation of COD.

SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA