A combined stopped-flow, electrospray ionization mass spectrometry and³¹P NMR study on the acetic acid-mediated fragmentation of the hydroxo-chalcogenide cluster [W₃Se₄(OH)₃(dmpe)₃]⁺(dmpe = 1,2-bis(dimethylphosphanyl)ethane) to yield the dinuclear [W₂Se₂(µ-Se)₂(µ-CH₃CO₂)(dmpe)₂]⁺complex

Abstract

The reaction of the incomplete-cuboidal [W(3)Se(4)(OH)(3)(dmpe)(3)](+) ([1](+)) cluster with acetic acid in acetonitrile solution leads to cluster fragmentation with formation of the dinuclear [W(2)Se(2)(mu-Se)(2)(mu-CH(3)CO(2))(dmpe)(2)](+) ([2](+)) complex. The X-ray structure of [2]PF(6) presents two equivalent metal centres bridged by one acetate ligand. Each W atom is additionally coordinated by one terminal selenium atom, two bridging selenido and two diphosphane phosphorus atoms in an essentially octahedral environment. Stopped-flow and conventional UV-vis studies indicate that fragmentation of [1](+) into [2](+) occurs through a complex mechanism. Three steps can be distinguished in the stopped-flow time scale, all of them showing a first order dependence with respect to the acetic acid concentration, followed by very slow spectral changes that lead to the formation of [2](+). Phosphorus NMR, electrospray ionization mass spectrometry (ESI-MS) and tandem mass spectrometry (ESI-MS/MS) have been used to identify the nature of the reaction intermediates formed in the different steps. These studies indicate that the first two steps correspond to the formal substitutions of the hydroxo ligands at two metal centres by terminal acetate ligands. The third step involves bridging of one of the terminal acetate ligands, which actually prepares the trinuclear cluster to afford the acetate-bridged [W(2)Se(2)(mu-Se)(2)(mu-CH(3)CO(2))(dmpe)(2)](+) ([2](+)) complex. Although the precise details of the final conversion to [2](+) have not been established, the results obtained by combination of the different experimental techniques provide a complete picture of the speciation of the cluster [1](+) in acetonitrile solutions containing acetic acid.

Synthesis, Crystal Structure, Aqueous Speciation, and Kinetics of Substitution Reactions in a Water-Soluble Mo3S4 Cluster Bearing Hydroxymethyl Diphosphine Ligands

“…These deviations indicate that there is some degree of electronic communication between the metal centers in this kind of cluster. The present results, when taken together with other recent observations showing deviations from statistical kinetics in some reactions of related symmetrical clusters in nonaqueous solution, indicate that the behavior of the three metals as isolated or interacting reaction centers can to some extent be modulated by subtle factors such as the solvent or the ancillary ligands. Despite the fact that more work is required to achieve a precise understanding of the kinetic features associated with these interactions, it is now evident that the classical description of these clusters as undergoing statistically controlled reactions needs to be expanded.…”

“…This experimental evidence put forward an active role of the solvent or the diphosphine on the solution structure of the trinuclear [ 1 ]Cl dhmpe derivative. The appearance of six signals in the NMR spectra has been previously reported for intermediates formed in reactions of related clusters containing coordinated diphosphines, and they have been interpreted as the result of the nonequivalence of the three metal centers due to partial substitution at only one or two of the metals. ,,, This lowering of symmetry has also been observed in other idealized C 3 symmetry Mo 3 S 4 complexes upon dissolving in some coordinating solvents. For example, excision of the Mo 3 S 7 Cl 4/2 Cl 2 polymeric phase with PPh 3 in pyridine affords the Mo 3 S 4 Cl 4 (py) 5 complex where two metal centers contain a chlorine and two pyridine molecules while the third one is coordinated to two chlorine and one pyridine ligands …”

“…An inspection of the k r i and K i values in Table indicates that similar deviations from statistical predictions also occur for the rates of the reverse step and the equilibrium constants. We have recently observed these deviations from the statistical predictions in the kinetics of other reactions of this kind of cluster, , but this is to our knowledge the first time that they are detected for reactions of homometallic M 3 Q 4 clusters in aqueous solution.…”

“…In our case, all attempts to monitor the solution and reactivity behavior of [ 1 ] + using proton NMR were not conclusive because the spectra are dominated by broad overlapping signals, a behavior that has been previously observed in metal complexes with these kinds of phosphines . In this context, ESI-MS was used for the characterization and reactivity studies, as it has proved to be very fruitful in related M 3 Q 4 complexes. , The ESI-MS spectra of aqueous solutions of [ 1 ] Cl are presented in Figure together with the chemical formula employed to reproduce the isotopic distribution of the most intense peaks.

2 ESI-MS spectra of water solutions of compound [ 1 ]Cl at U c = 10 V together with the simulated isotopic distribution for [Mo 3 S 4 L(L-H) 2 ] 2+ .

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