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

Abstract

The novel incomplete cuboidal cluster [W₃Se₄H₃(dmpe)₃](PF₆), [1](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), [2](PF₆), was obtained by reacting [W₃Se₄Br₃(dmpe)₃](PF₆) with NaOH in MeCN–H₂O solution. The complexes [1](PF₆) and [2](PF₆) were converted to their BPh₄⁻ salts by treatment with NaBPh₄. Recrystallisation of [1](BPh₄) in the presence of traces of water affords the mixed dihydride hydroxo complex [W₃Se₄H₂(OH)(dmpe)₃](BPh₄). The crystal structures of [1](BPh₄), [2](BPh₄) and [W₃Se₄H₂(OH)(dmpe)₃](BPh₄) have been resolved. Although the [1]⁺ 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 [2]⁺ with acids occurs in a single step with k_obs = 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.