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

X-ray structural data for the cubane-type clusters [Mo3CuS4(dmpe)3Cl4](+) and Mo3NiS4(dmpe)3Cl4 (dmpe = 1,2-bis(dimethylphosphino)ethane) with 16 metal electrons have been compared with optimized structural parameters calculated using "ab initio" methodologies. Compound Mo3NiS4(dmpe)3Cl4 crystallizes in the cubic noncentrosymmetric space group P213 with a Mo-Ni distance of 2.647 Angstrom, that is 0.2 Angstrom shorter than the Mo-Cu bond length in the isoelectronic copper cluster. The best agreement between theory and experiments has been obtained using the B3P86 method. In order to validate the B3P86 results, accurate infrared and Raman spectra have been acquired and the vibrational modes associated to the cubane-type Mo3M'S4 (M' = Cu or Ni) unit have been assigned theoretically. The electronic changes taking place when incorporating the M' into the Mo3S4 unit have been analyzed from a theoretical and experimental perspective. The bond dissociation energies between M'-Cl and Mo3S4 fragments show that formation of [Mo3CuS4(dmpe)3Cl4](+) is 135 kcal/mol energetically less favorable than the Ni incorporation. The more robust nature of the Mo3NiS4 fragment has been confirmed by mass spectrometry. The X-ray photoelectron spectroscopy (XPS) spectra of the trimetallic and tetrametallic complexes have been measured and the obtained binding energies compared with the computed electronic populations based on topological approaches of the electron localization function (ELF). The energies and shapes of the Cu 2p and Ni 2p lines indicate formal oxidation states of Cu(I) and Ni(II). However, the reductive addition of nickel into [Mo3S4(dmpe)3Cl3](+) causes a small decrease in the Mo 3d binding energies. This fact prevents an unambiguous assignment of an oxidation state in a conventional way, a circumstance that has been analyzed through the covariance of the electronic populations associated to the C(M') core and V(Mo3Ni) and V(S(2)') valence basins where Mo3NiS4 is a particularly electronically delocalized chemical entity.

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.

RESUMO

Heterodimetallic cubane-type complexes coordinated to diphosphanes [Mo(3)CoS(4)(dmpe)(3)Cl(4)](+) ([1](+)) (dmpe=1,2-bis(dimethylphosphanyl)ethane), [Mo(3)CoS(4)(dmpe)(3)Cl(4)] (1) and [Mo(3)CoS(4)(dmpe)(3)Cl(3)(CO)] (2) with 14, 15 and 16 metal electrons, respectively, have been prepared from the [Mo(3)S(4)(dmpe)(3)Cl(3)](+) trinuclear precursor using [Co(2)(CO)(8)] or CoCl(2) as cobalt source. Cluster complexes [1](+) and 1 are easily interconverted chemically and electrochemically. The Co-Cl distance increases upon electron addition and substitution of the chlorine atom coordinated to cobalt with CO only takes place in presence of a reducing agent to give complex 2. Structural changes in the intermetallic distances agree with the entering electrons occupying an orbital which is basically Mo-Mo non-bonding and slightly Mo-Co bonding. Magnetic susceptibility measurements for [1](+) and 1 are consistent with the presence of two and one unpaired electrons, respectively and therefore with an "e" character for the HOMO orbital. Oxidation of 1 with TCNQ results in the formation of a charge transfer salt formulated as [1](+)[TCNQ](-) with alternate layers of paramagnetic cluster cations and also paramagnetic organic anions. There is no magnetic interaction between layers and the thermal variation of the magnetic susceptibility has been modelled as a S= 1/2 TCNQ antiferromagnetic chain plus a S=1 cluster monomer with zero field splitting.