16 research outputs found
Theoretical study of reactivity and dynamics of hybride-bridged diruthenium complexes and silylium
Esta tesis presenta un estudio computacional de los sistemas con hidruros puente. En la primera parte se estudia la química de complejos de dirutenio con cuatro hidruros puente. Esto incluye las siguientes reacciones: el intercambio del hidruro con hidrógeno molecular; la activación del enlace C-H del etileno para formar el complejo de bis(vinilo)-etileno; el acoplamiento C-C entre el etileno coordinado y dos ligandos vinilo para producir el complejo rutenaciclopentadieno. Al final de esta parte, se discuten a detalle los mecanismos de estas reacciones. Además, se demostró la importancia de la flexibilidad de los ligandos hidruro y la cooperación entre los dos centros metálicos. En la segunda parte, se estudió el comportamiento fluxional de dos complejos μ-silileno y de un catión sililio. Con esto, se estableció la ruta más favorable en donde se realiza el intercambio de los ligandos hidruro y de los grupos metilo en los complejos μ-silileno. Finalmente, se encontró que hay dos posibles rutas relativas al cambio en la posición del puente Si-H-Si en el cation sililio poliagóstico, asociadas con la rotación interna de los grupos sililo.The thesis presents a computational study of hydride bridged systems. The chemistry of diruthenium tetrahydride-bridged complex was studied in the first part. It includes the next reactions: the hydride exchange with dihydrogen; the C-H bond activation in ethylene to yield bis(vinyl) ethylene complex; the C-C coupling between coordinated ethy¬lene and two vinyl ligands to yield ruthenacyclopentadiene complex. The detailed mechanism of these reactions has been determined. The importance of flexibility of hydride ligands and cooperation between two metal centers has been demonstrated. The fluxional behavior of two μ-silylene complexes and a silylium cation was studied in the second part. The pathway responsible for the site-exchange of hydride ligands and methyl groups in the μ-silylene complexes has been established. Two possible mecha¬nisms of the shift of Si-H-Si bridge position in the polyagostic silylium cation, associated with internal rota¬tion of si¬lyl groups have been found
A mechanistic study of the utilization of arachno-diruthenaborane [(Cp*RuCO)<sub>2</sub>B<sub>2</sub>H<sub>6</sub>] as an active alkyne-cyclotrimerization catalyst
The reaction of nido-[1,2-(Cp*RuH)<sub>2</sub>B<sub>3</sub>H<sub>7</sub>] (1 a, Cp*=η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>) with [Mo(CO)<sub>3</sub>(CH<sub>3</sub>CN)<sub>3</sub>] under mild conditions yields the new metallaborane arachno-[(Cp*RuCO)<sub>2</sub>B<sub>2</sub>H<sub>6</sub>] (2). Compound 2 catalyzes the cyclotrimerization of a variety of internal- and terminal alkynes to yield mixtures of 1,3,5- and 1,2,4-substituted benzenes. The reactivities of nido-1 a and arachno-2 with alkynes demonstrates that a change in geometry from nido to arachno drives a change in the reaction from alkyne-insertion to catalytic cyclotrimerization, respectively. Density functional calculations have been used to evaluate the reaction pathways of the cyclotrimerization of alkynes catalyzed by compound 2. The reaction involves the formation of a ruthenacyclic intermediate and the subsequent alkyne-insertion step is initiated by a [2+2] cycloaddition between this intermediate and an alkyne. The experimental and quantum-chemical results also show that the stability of the metallacyclic intermediate is strongly dependent on the nature of the substituents that are present on the alkyne
Chemoselective Coupling of 1,1-Bis[(pinacolato)boryl]alkanes for the Transition-Metal-Free Borylation of Aryl and Vinyl Halides: A Combined Experimental and Theoretical Investigation
A new transition-metal-free borylation of aryl and vinyl halides using 1,1-bis[(pinacolato)boryl]alkanes as boron sources is described. In this transformation one of the boron groups from 1,1-bis[(pinacolato)boryl]alkanes is selectively transferred to aryl and vinyl halides in the presence of sodium tert-butoxide as the only activator to form organoboronate esters. Under the developed borylation conditions, a broad range of organohalides are borylated with excellent chemoselectivity and functional group compatibility, thus offering a rare example of a transition-metal-free borylation protocol. Experimental and theoretical studies have been performed to elucidate the reaction mechanism, revealing the unusual formation of Lewis acid/base adduct between organohalides and α-borylcarbanion, generated in situ from the reaction of 1,1-bis[(pinacolato)boryl]alkanes with an alkoxide base, to facilitate the borylation reactions. © 2016 American Chemical Society201
Computer-aided rational design of Fe(iii)-catalysts for the selective formation of cyclic carbonates from CO2 and internal epoxides
The catalytic mechanism of the cyclic carbonate formation reaction between CO2 and internal epoxides promoted by Fe-salen and the Kleij catalyst was examined in detail to better understand how the catalytic efficiency can be increased. Specifically, we aimed to make the catalyst more chemoselective towards forming cyclic carbonates and preventing the competing side reaction leading to polycarbonates via ring-opening polymerization. A few rational design principles were derived and first tested using computer models based on density functional theory. The most promising candidate that was identified in the computer model was then prepared and found to display significantly enhanced reactivity towards forming the cyclic carbonates, supporting the validity of the mechanistic insights deduced from the computer simulations. We propose that a cyclic carbonate is formed most efficiently via an inner-sphere mechanism where both the CO2 and epoxide substrates utilize the metal center for the key bond forming events. In contrast, the ring-opening polymerization uses an outer-sphere mechanism, where a carbonate attacks and ring-opens the epoxide bound to the metal without engaging the metal directly. These mechanistic differences are exploited to implement a chemoselective catalyst by enhancing the rate of the cyclic carbonate formation reaction while leaving the polymerization pathway largely unaffected. © 2017 The Royal Society of Chemistry2
Synthesis and reactivity of a mononuclear non- haem cobalt(IV)-oxo complex
Terminal cobalt(IV)-oxo (Co-IV-O) species have been implicated as key intermediates in various cobalt- mediated oxidation reactions. Herein we report the photocatalytic generation of a mononuclear non- haem [(13-TMC) Co-IV(O)] (2+) (2) by irradiating [ Co-II(13-TMC)(CF3SO3)] (+) (1) in the presence of [Ru-II( bpy)(3)] (2+), Na2S2O8, and water as an oxygen source. The intermediate 2 was also obtained by reacting 1 with an artificial oxidant (that is, iodosylbenzene) and characterized by various spectroscopic techniques. In particular, the resonance Raman spectrum of 2 reveals a diatomic Co-O vibration band at 770 cm (-1), which provides the conclusive evidence for the presence of a terminal Co-O bond. In reactivity studies, 2 was shown to be a competent oxidant in an intermetal oxygen atom transfer, C-H bond activation and olefin epoxidation reactions. The present results lend strong credence to the intermediacy of CoIV-O species in cobalt-catalysed oxidation of organic substrates as well as in the catalytic oxidation of water that evolves molecular oxygen ⓒThe Author(s) 201711sciescopu
Insight into the Oriented Growth of Surface-Attached Metal–Organic Frameworks: Surface Functionality, Deposition Temperature, and First Layer Order
The
layer-by-layer growth of a surface-attached metal–organic
framework (SURMOF), [Cu<sub>2</sub>(F<sub>4</sub>bdc)<sub>2</sub>(dabco)] (F<sub>4</sub>bdc = tetrafluorobenzene-1,4-dicarboxylate
and dabco = 1,4-diazabicyclo-[2.2.2]octane), on carboxylate- and pyridine-terminated
surfaces has been investigated by various surface characterization
techniques. Particular attention was paid to the dependency of the
crystal orientation and morphology on surface functionality, deposition
temperature, and first layer order. For the fully oriented deposition
of SURMOFs, not only a suitable surface chemistry but also the appropriate
temperature has to be chosen. In the case of carboxylate-terminated
surfaces, the expected [100] oriented [Cu<sub>2</sub>(F<sub>4</sub>bdc)<sub>2</sub>(dabco)] SURMOF can be achieved at low temperatures
(5 °C). In contrast, the predicted [001] oriented SURMOF on pyridine-terminated
surface was obtained only at high deposition temperatures (60 °C).
Interestingly, we found that rearrangement processes in the very first
layer determine the final orientation (distribution) of the growing
crystals. These effects could be explained by a surprisingly hampered
substitution at the apical position of the Cu<sub>2</sub>-paddle wheel
units, which requires significant thermal activation, as supported
by quantum-chemical calculations
A Hafnium-Based Metal–Organic Framework as an Efficient and Multifunctional Catalyst for Facile CO<sub>2</sub> Fixation and Regioselective and Enantioretentive Epoxide Activation
Porous heterogeneous
catalysts play a pivotal role in the chemical
industry. Herein a new Hf-based metal–organic framework (Hf-NU-1000)
incorporating Hf<sub>6</sub> clusters is reported. It demonstrates
high catalytic efficiency for the activation of epoxides, facilitating
the quantitative chemical fixation of CO<sub>2</sub> into five-membered
cyclic carbonates under ambient conditions, rendering this material
an excellent catalyst. As a multifunctional catalyst, Hf-NU-1000 is
also efficient for other epoxide activations, leading to the regioselective
and enantioretentive formation of 1,2-bifuctionalized systems via
solvolytic nucleophilic ring opening
Single-Site Organozirconium Catalyst Embedded in a Metal–Organic Framework
A structurally well-defined mesoporous
Hf-based metal–organic
framework (Hf-NU-1000) is employed as a well-defined scaffold for
a highly electrophilic single-site d<sup>0</sup> Zr–benzyl
catalytic center. This new material Hf-NU-1000-ZrBn is fully characterized
by a variety of spectroscopic techniques and DFT computation. Hf-NU-1000-ZrBn
is found to be a promising single-component catalyst (i.e., not requiring
a catalyst/activator) for ethylene and stereoregular 1-hexene polymerization
Targeted Single-Site MOF Node Modification: Trivalent Metal Loading via Atomic Layer Deposition
Postsynthetic functionalization of
metal organic frameworks (MOFs)
enables the controlled, high-density incorporation of new atoms on
a crystallographically precise framework. Leveraging the broad palette
of known atomic layer deposition (ALD) chemistries, <u>A</u>LD <u>i</u>n <u>M</u>OFs (AIM) is
one such targeted approach to construct diverse, highly functional,
few-atom clusters. We here demonstrate the saturating reaction of
trimethylindium (InMe<sub>3</sub>) with the node hydroxyls and ligated
water of NU-1000, which takes place without significant loss of MOF
crystallinity or internal surface area. We computationally identify
the elementary steps by which trimethylated trivalent metal compounds
(ALD precursors) react with this Zr-based MOF node to generate a uniform
and well characterized new surface layer on the node itself, and we
predict a final structure that is fully consistent with experimental
X-ray pair distribution function (PDF) analysis. We further demonstrate
tunable metal loading through controlled number density of the reactive
handles (−OH and −OH<sub>2</sub>) achieved through node
dehydration at elevated temperatures