Combined experimental and computational investigations of rhodium-catalysed C-H functionalisation of pyrazoles with alkenes

Detailed experimental and computational studies have been carried out on the oxidative coupling of the alkenes C(2)H(3)Y (Y=CO(2)Me (a), Ph (b), C(O)Me (c)) with 3-aryl-5-R-pyrazoles (R=Me (1 a), Ph (1 b), CF(3) (1 c)) using a [Rh(MeCN)(3)Cp*][PF(6)](2)/Cu(OAc)(2)⋅H(2)O catalyst system. In the reaction of methyl acrylate with 1 a, up to five products (2 aa–6 aa) were formed, including the trans monovinyl product, either complexed within a novel Cu(I) dimer (2 aa) or as the free species (3 aa), and a divinyl species (6 aa); both 3 aa and 6 aa underwent cyclisation by an aza-Michael reaction to give fused heterocycles 4 aa and 5 aa, respectively. With styrene, only trans mono- and divinylation products were observed, whereas with methyl vinyl ketone, a stronger Michael acceptor, only cyclised oxidative coupling products were formed. Density functional theory calculations were performed to characterise the different migratory insertion and β-H transfer steps implicated in the reactions of 1 a with methyl acrylate and styrene. The calculations showed a clear kinetic preference for 2,1-insertion and the formation of trans vinyl products, consistent with the experimental results

Combined kinetic and DFT studies on the stabilization of the pyramidal form of H3PO2 at the heterometal site of [Mo3M’S4(H2O)10]4+ clusters (M’= Pd, Ni)

Kinetic and DFT studies have been carried out on the reaction of the [Mo3M’S4(H2O)10]4+ clusters (M’= Pd, Ni) with H3PO2 to form the [Mo3M’(pyr-H3PO2)S4(H2O)9]4+ complexes, in which the rare pyramidal form of H3PO2 is stabilized by coordination to the M’ site of the clusters. The reaction proceeds with biphasic kinetics, both steps showing a first order dependence with respect to H3PO2. These results are interpreted in terms of a mechanism that involves an initial substitution step in which one tetrahedral H3PO2 molecule coordinates to M’ through the oxygen atom of the P=O bond, followed by a second step that consists in tautomerization of coordinated H3PO2 assisted by a second H3PO2 molecule. DFT studies have been carried out to obtain information on the details of both kinetic steps, the major finding being that the role of the additional H3PO2 molecule in the second step consists in catalysing a hydrogen shift from phosphorus to oxygen in O-coordinated H3PO2, which is made possible by its capability of accepting a proton from P-H to form H4PO2 + and then transfer it to the oxygen. DFT studies have been also carried out on the reaction at the Mo centres to understand the reasons that make these metal centres ineffective for promoting tautomerizatio

Catalytic Hydrogenation of Azobenzene in the Presence of a Cuboidal Mo3S4 Cluster via an Uncommon Sulfur-Based H2 Activation Mechanism

Azobenzene hydrogenation is catalyzed under moderate conditions by a cuboidal Mo3(μ3-S)(μ-S)3 diamino complex via a cluster catalysis mechanism. Dihydrogen activation by the molecular [Mo3(μ3-S)(μ-S)3Cl3(dmen)3]+ cluster cation takes place at the μ-S bridging atoms without direct participation of the metals in clear contrast with classical concepts. The reaction occurs with the formation of 1,2-diphenylhydrazine as an intermediate with similar appearance and disappearance rate constants. On the basis of DFT calculations, a mechanism is proposed in which formation of 1,2-diphenylhydrazine and aniline occurs through two interconnected catalytic cycles that share a common reaction step that involves H2 addition to two of the bridging sulfur atoms of the catalyst to form a dithiolate Mo3(μ3-S)(μ-SH)2)(μ-S) adduct. Both catalytic cycles have similar activation barriers, in agreement with the experimental observation of close rate constant values. Microkinetic modeling of the process leads to computed concentration–time profiles in excellent agreement with the experimental ones providing additional support to the calculated reaction mechanism. Slight modifications on the experimental conditions of the catalytic protocol in combination with theoretical calculations discard a direct participation of the metal on the reaction mechanism. The effect of the ancillary ligands on the catalytic activity of the cluster fully agrees with the present mechanistic proposal. The results herein demonstrate the capability of molybdenum sulfide materials to activate hydrogen through an uncommon sulfur based mechanism opening attractive possibilities toward their applications as catalysts in other hydrogenation processes

Fe(II) complexes of pyridine-substituted thiosemicarbazone ligands as catalysts for oxidations with hydrogen peroxide

La reacción de tres complejos [FeII(TSC)2], donde TSC es una ligando de tipo tiosemicarbazona sustituido por piridina, con H2O2 en acetonitrilo no permitía acumular los correspondientes complejos de Fe(III), [FeIII(TSC)2]+. En su lugar, se generaba una mezcla de especies de Fe(II) diamagnéticas de bajo espín. Según los espectros obtenidos por espectrometría de masas, estas especies eran el resultado de la adición secuencial de hasta cinco átomos de oxígeno al complejo. Esta capacidad para la adición de átomos de oxígeno sugirió que dichas especies podrían ser activas para la transferencia de átomos de oxígeno a sustratos externos. Por ello, se evaluó la capacidad de estos complejos para la oxidación de tioanisol y estireno empleando H2O2 como oxidante inicial. Los complejos fueron activos tanto en la oxidación de tioanisol a su sulfóxido como en la de estireno a benzaldehído, con escalas temporales que indicaban la participación de las especies intermedias que contenían los átomos de oxígeno añadidos. Curiosamente, los ligandos libres y el complejo [Zn(Dp44mT)2] también catalizaban la sulfoxidación selectiva del tioanisol, pero eran ineficaces para catalizar la oxidación del estireno a benzaldehído. Estos hallazgos abren nuevas vías para el desarrollo de catalizadores metálicos basados en tiosemicarbazonas en procesos de oxidación de gran interés

Benchmarking of DFT methods using experimental free energies and volumes of activation for the cycloaddition of alkynes to cuboidal Mo3S4 clusters

Here, the kinetics of the concerted [3 + 2] cycloaddition reaction between the [Mo3(μ3‐S)(μ‐S)3Cl3(dmen)3]+ (dmen = N,N′‐dimethyl‐ethylenediamine) ([1]+) cluster and various alkynes to form dithiolene derivatives is thoroughly studied, with measurements at different temperatures and pressures allowing the determination of the free energies and volumes of activation. These parameters, together with the available single‐crystal X‐ray diffraction structures, are used to test a number of commonly used density functional theory (DFT) methods from Jacob's ladder, as well as the effects associated with the size of the basis sets, the way in which solvent effects are taken into account, or the inclusion of dispersion effects. Overall, a protocol that leads to average deviations between experimental and computed ΔV and ΔG values similar to the uncertainty of the experimental measurements is obtained

Benchmarking of DFTmethods using experimental free energies and volumes of activation for the cycloaddition of alkynes to cuboidalMo(3)S(4)clusters

Here, the kinetics of the concerted [3 + 2] cycloaddition reaction between the [Mo3(μ3‐S)(μ‐S)3Cl3(dmen)3]+ (dmen = N,N′‐dimethyl‐ethylenediamine) ([1]+) cluster and various alkynes to form dithiolene derivatives is thoroughly studied, with measurements at different temperatures and pressures allowing the determination of the free energies and volumes of activation. These parameters, together with the available single‐crystal X‐ray diffraction structures, are used to test a number of commonly used density functional theory (DFT) methods from Jacob's ladder, as well as the effects associated with the size of the basis sets, the way in which solvent effects are taken into account, or the inclusion of dispersion effects. Overall, a protocol that leads to average deviations between experimental and computed ΔV‡ and ΔG‡ values similar to the uncertainty of the experimental measurements is obtained

Base-Free Catalytic Hydrogen Production from Formic Acid Mediated by a Cubane-Type Mo3S4 Cluster Hydride

Formic acid (FA) dehydrogenation is an attractive process in the implementation of a hydrogen economy. To make this process greener and less costly, the interest nowadays is moving toward non-noble metal catalysts and additive-free protocols. Efficient protocols using earth abundant first row transition metals, mostly iron, have been developed, but other metals, such as molybdenum, remain practically unexplored. Herein, we present the transformation of FA to form H2 and CO2 through a cluster catalysis mechanism mediated by a cuboidal [Mo3S4H3(dmpe)3]+ hydride cluster in the absence of base or any other additive. Our catalyst has proved to be more active and selective than the other molybdenum compounds reported to date for this purpose. Kinetic studies, reaction monitoring, and isolation of the [Mo3S4(OCHO)3(dmpe)3]+ formate reaction intermediate, in combination with DFT calculations, have allowed us to formulate an unambiguous mechanism of FA dehydrogenation. Kinetic studies indicate that the reaction at temperatures up to 60 °C ends at the triformate complex and occurs in a single kinetic step, which can be interpreted in terms of statistical kinetics at the three metal centers. The process starts with the formation of a dihydrogen-bonded species with Mo–H···HOOCH bonds, detected by NMR techniques, followed by hydrogen release and formate coordination. Whereas this process is favored at temperatures up to 60 °C, the subsequent β-hydride elimination that allows for the CO2 release and closes the catalytic cycle is only completed at higher temperatures. The cycle also operates starting from the [Mo3S4(OCHO)3(dmpe)3]+ formate intermediate, again with preservation of the cluster integrity, which adds our proposal to the list of the infrequent cluster catalysis reaction mechanisms.Funding for open access charge: CRUE-Universitat Jaume

Efficient (Z)-selective semihydrogenation of alkynes catalyzed by air-stable imidazolyl amino molybdenum cluster sulfides

Imidazolyl amino cuboidal Mo3(μ3-S)(μ-S)3 clusters have been investigated as catalysts for the semihydrogenation of alkynes. For that purpose, three new air-stable cluster salts [Mo3S4Cl3(ImNH2)3]BF4 ([1]BF4), [Mo3S4Cl3(ImNH(CH3))3]BF4 ([2]BF4) and [Mo3S4Cl3(ImN(CH3)2)3]BF4 ([3]BF4) have been isolated in moderate to high yields and fully characterized. Crystal structures of complexes [1]PF6 and [2]Cl confirm the formation of a single isomer in which the nitrogen atoms of the three imidazolyl groups of the ligands are located trans to the capping sulfur atom which leaves the three bridging sulfur centers on one side of the trimetallic plane while the amino groups lie on the opposite side. Kinetic studies show that the cluster bridging sulfurs react with diphenylacetylene (dpa) in a reversible equilibrium to form the corresponding dithiolene adduct. Formation of this adduct is postulated as the first step in the catalytic semihydrogenation of alkynes mediated by molybdenum sulfides. These complexes catalyze the (Z)-selective semihydrogenation of diphenylacetylene (dpa) under hydrogen in the absence of any additives. The catalytic activity lowers sequentially upon replacement of the hydrogen atoms of the N–H2 moiety in 1+ without reaching inhibition. Mechanistic experiments support a sulfur centered mechanism without participation of the amino groups. Different diphenylacetylene derivatives are selectively hydrogenated using complex 1+ to their corresponding Z-alkenes in excellent yields. Extension of this protocol to 3,7,11,15-tetramethylhexadec-1-yn-3-ol, an essential intermediate for the production of vitamin E, affords the semihydrogenation product in very good yield

Kinetic and DFT Studies on the Mechanism of C−S Bond Formation by Alkyne Addition to the [Mo3S4(H2O)9]4+ Cluster

Reaction of [Mo3(μ3-S)(μ-S)3] clusters with alkynes usually leads to formation of two C−S bonds between the alkyne and two of the bridging sulfides. The resulting compounds contain a bridging alkenedithiolate ligand, and the metal centers appear to play a passive role despite reactions at those sites being well illustrated for this kind of cluster. A detailed study including kinetic measurements and DFT calculations has been carried out to understand the mechanism of reaction of the [Mo3(μ3-S)(μ-S)3(H2O)9]4+ (1) cluster with two different alkynes, 2-butyne-1,4-diol and acetylenedicarboxylic acid. Stoppedflow experiments indicate that the reaction involves the appearance in a single kinetic step of a band at 855 or 875 nm, depending on the alkyne used, a position typical of clusters with two C−S bonds. The effects of the concentrations of the reagents, the acidity, and the reaction medium on the rate of reaction have been analyzed. DFT and TD-DFT calculations provide information on the nature of the product formed, its electronic spectrum and the energy profile for the reaction. The structure of the transition state indicates that the alkyne approaches the cluster in a lateral way and both C−S bonds are formed simultaneously

Bifunctional W/NH Cuboidal Aminophosphino W3S4 Cluster Hydrides: The Puzzling Behaviour behind the Hydridic-Protonic Interplay

The novel [W3S4H3(edpp)3]+ (edpp=(2-aminoethyl)diphenylphosphine) (1+) cluster hydride with an acidic −NH2 functionality has been synthetized and studied. Its crystal structure shows the characteristic incomplete W3S4 cubane core with the outer positions occupied by the P and N atoms of the edpp ligands. Although no signal due to the hydride ligands is observed in the 1H NMR spectrum, hydride assignment is supported by 1H-15N HSQC techniques, the changes in the 31P{1H} NMR chemical shift, and FT-IR spectra in the W−H region of the deuterated [W3S4D2H(edpp)3]+ (1+-d2) samples. Moreover, all NMR evidences suggest that one of the hydrogen atoms of the NH2 group in 1+ is rapidly exchanging with the hydride. The reaction of 1+ with acids (HCl, HBr and DCl) features complex polyphasic kinetics with zero-order dependence with respect to the acid concentration, being also independent of the solvent nature. This behavior differs from that of their diphosphino analogues, suggesting a different mechanism