Synthesis and structure of the incomplete cuboidal clusters [W3Se4H3(dmpe)3]+, [W3Se4H3−x(OH)x(dmpe)3]+ and [W3Se4(OH)3(dmpe)3]+, and the mechanism of the acid-assisted substitution of the coordinated hydrides
The novel incomplete cuboidal cluster [W3Se4H3(dmpe)3](PF6), (PF6), has been prepared by reduction of [W3Se4Br3(dmpe)3](PF6) with LiBH4 in THF solution. The trihydroxo complex [W3Se4(OH)3(dmpe)3](PF6), (PF6), was obtained by reacting [W3Se4Br3(dmpe)3](PF6) with NaOH in MeCN–H2O solution. The complexes (PF6) and (PF6) were converted to their BPh4− salts by treatment with NaBPh4. Recrystallisation of (BPh4) in the presence of traces of water affords the mixed dihydride hydroxo complex [W3Se4H2(OH)(dmpe)3](BPh4). The crystal structures of (BPh4), (BPh4) and [W3Se4H2(OH)(dmpe)3](BPh4) have been resolved. Although the + trihydride does not react with an excess of halide salts, reaction with HX leads to [W3Se4X3(dmpe)3]+ (X = Cl, Br). The kinetics of this reaction has been studied at 25 °C in MeCN–H2O solution (1 ∶ 1, v/v) and found to occur with two consecutive kinetic steps. The first step is independent of the nature and concentration of the X− anion but shows a first order dependence on the concentration of acid (k1 = 12.0 mol−1 dm3 s−1), whereas the second one is independent of the nature and concentration of both the acid and added salts (k2 = 0.024 s−1). In contrast, the reaction of + with acids occurs in a single step with kobs = 0.63 s−1 (HCl) and 0.17 s−1 (HBr). These kinetic results are discussed on the basis of the mechanism previously proposed for the reactions of the analogous [W3S4H3(dmpe)3]+ cluster, with special emphasis on the effects caused by the change of S by Se on the rate constants for the different processes involved.