RESUMO

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 ß-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.

RESUMO

The synthesis, crystal structure and solution characterization of the cubane-type [Mo(3)(FeCl)S(4)(dmpe)(3)Cl(3)] (1) (dmpe = 1,2-bis(dimethylphophane-ethane)) cluster are reported and the ligand substitution processes of chloride by thiophenolate investigated. The kinetics and the intimate mechanism of these substitutions reveal that compound 1 undergoes a number of Fe and Mo site specific ligand substitution reactions in acetonitrile solutions. In particular, PhS(-) coordination at the tetrahedral Fe site proceeds in a single resolved kinetic step whereas such substitutions at the Mo sites proceed more slowly. The effect of the presence of acids in the reaction media is also investigated and reveals that an acid excess hinders substitution reactions both at the Fe and Mo sites; however, an acid-promoted solvolysis of the Fe-Cl bonds is observed. Electrospray ionization (ESI) and tandem (ESI-MS/MS) mass spectrometry allow the identification of all the reaction intermediates proposed on the basis of stopped-flow measurements. The distinctive site specific reactivity made it possible to isolate two new clusters of the Mo(3)FeS(4)(4+) family featuring mixed chlorine/thiophenolate ligands, namely Mo(3)S(4)(FeSPh)(dmpe)(3)Cl(3) (2) and [Mo(3)S(4)(FeSPh)(dmpe)(3)(SPh)(3)] (3). A detailed computational study has also been carried out to understand the details of the mechanism of substitution at the M-Cl (M = Mo and Fe) bonds as well as the solvolysis at the Fe-Cl sites, with particular emphasis on the role of acids on the substitution process. The results of the calculations are in agreement with the experimental observations, thus justifying the non-existence of an accelerating effect of acids on the thiophenolate substitution reaction, which differs from previous proposals for the Fe(4)S(4) and MoFe(3)S(4) clusters and some related compounds.

Assuntos

RESUMO

The kinetics of reaction of the [W(3)PdS(4)H(3)(dmpe)(3)(CO)](+) hydride cluster (1(+)) with HCl has been measured in dichloromethane, and a second-order dependence with respect to the acid is found for the initial step. In the presence of added BF(4) (-) the second-order dependence is maintained, but there is a deceleration that becomes more evident as the acid concentration increases. DFT calculations indicate that these results can be rationalized on the basis of the mechanism previously proposed for the same reaction of the closely related [W(3)S(4)H(3)(dmpe)(3)](+) cluster, which involves parallel first- and second-order pathways in which the coordinated hydride interacts with one and two acid molecules, and ion pairing to BF(4) (-) hinders formation of dihydrogen bonded adducts able to evolve to the products of proton transfer. Additional DFT calculations are reported to understand the behavior of the cluster in neat acetonitrile and acetonitrile-water mixtures. The interaction of the HCl molecule with CH(3)CN is stronger than the W-H...HCl dihydrogen bond and so the reaction pathways operating in dichloromethane become inefficient, in agreement with the lack of reaction between 1(+) and HCl in neat acetonitrile. However, the attacking species in acetonitrile-water mixtures is the solvated proton, and DFT calculations indicate that the reaction can then go through pathways involving solvent attack to the W centers, while still maintaining the coordinated hydride, which is made possible by the capability of the cluster to undergo structural changes in its core.

Assuntos

RESUMO

Opening the cluster core: Substitution of the chloride ligand in the novel cationic cluster [W(3)CuS(4)H(3)Cl(dmpe)(3)](+) (see figure; dmpe=1,2-bis(dimethylphosphino)ethane) by acetonitrile is promoted by water addition. Kinetic and density functional theory studies lead to a mechanistic proposal in which acetonitrile or water attack causes the opening of the cluster core with dissociation of one of the Cu--S bonds to accommodate the entering ligand.Reaction of the incomplete cuboidal cationic cluster [W(3)S(4)H(3)(dmpe)(3)](+) (dmpe=1,2-bis(dimethylphosphino)ethane) with Cu(I) compounds produces rare examples of cationic heterodimetallic hydrido clusters of formula [W(3)CuClS(4)H(3)(dmpe)(3)](+) ([1](+)) and [W(3)Cu(CH(3)CN)S(4)H(3)(dmpe)(3)](2+) ([2](2+)). An unexpected conversion of [1](+) into [2](2+), which involves substitution of chloride by CH(3)CN at the copper centre, has been observed in CH(3)CN/H(2)O mixtures. Surprisingly, formation of the acetonitrile complex does not occur in neat acetonitrile and requires the presence of water. The kinetics of this reaction has been studied and the results indicate that the process is accelerated when the water concentration increases and is retarded in the presence of added chloride. Computational studies have also been carried out and a mechanism for the substitution reaction is proposed in which attack at the copper centre by acetonitrile or water causes disruption of the cubane-type core. ESI-MS experiments support the formation of intermediates with an open-core cluster structure. This kind of process is unprecedented in the chemistry of M(3)M'Q(4) (M=Mo, W; Q=S, Se) clusters, and allows for the transient appearance of a new coordination site at the M' site which could explain some aspects of the reactivity and catalytic properties of this kind of clusters.

Assuntos

RESUMO

Reaction of the incomplete cuboidal [W3S4H3(dmpe)3]+ cluster with a Pd(0) complex under a CO atmosphere produces a rare example of a heterodimetallic hydrido cluster of formula [W3PdS4H3(dmpe)3(CO)]+ ([1]+). There are not significant changes in the W-W bond lengths on going from the trinuclear to the tetranuclear cluster. The average W-W and W-Pd bond distances of 2.769[10] and 2.90[2] A, respectively, are consistent with the presence of single bonds between metal atoms. The heterodimetallic [1]+ complex is easier to oxidize and more difficult to reduce than its trinuclear precursor, which reflects the electron-donating capability of the Pd(CO) fragment. However, mechanistic studies on the reaction of [1]+ with acids show a lower basicity for this complex in comparison with that of its trinuclear precursor, so there is a major electron-density rearrangement within the cluster core upon Pd(CO) coordination. This rearrangement is also reflected in an unusual expansion of the sulfur tetrahedron within the W3PdS4 core with the concomitant elongation of the W-S bond distances by 0.04 A with respect to the analogous bond lengths in the trinuclear precursor. For those thermodynamically favored proton-transfer processes, the reaction mechanism of [1]+ with acids is quite similar to that observed for the incomplete trinuclear cluster, with only small changes in the rate constants. The reaction of [1]+ with HCl in acetonitrile/water mixtures produces [W3PdS4Cl3(dmpe)3(CO)]+ ([2]+) in two kinetically distinguishable steps. Proton transfer occurs in the initial step, in which the W-H bonds are attacked by the acid to yield dihydrogen-bonded adducts that are further attacked by an acetonitrile molecule to give [W3PdS4(CH3CN)3(dmpe)3(CO)]4+ and dihydrogen. The nature of processes involved in the second step are not well-understood with the present data, although it is very likely that these correspond to some secondary processes. In the third resolved step, the coordinated CH3CN ligands in [W3PdS4(CH3CN)3(dmpe)3(CO)]4+ are substituted by Cl- to afford the final [2]+ product. No reaction is observed between [1]+ and HCl in neat acetonitrile, whereas the product of the reaction of [1]+ with HBF4 or Hpts (pts- = p-toluenesulfonate) in this solvent is [W3PdS4(CH3CN)3(dmpe)3(CO)]4+. The reaction occurs in a single kinetic step with a first- (Hpts) or second-order (HBF4) dependence with respect to the acid. The first- and second-order acid dependences can be interpreted through the initial formation of dihydrogen adducts with one or two acid molecules, respectively.