The synthesis of NR4[(Ta6Cl12)(H2O)6]X4(R = Me or Et, X = Cl or Br) by the spontaneous reduction of [Ta6Cl12]4+ to [Ta6Cl12]3+ in acidic media: X-ray structure analysis of NMe4[(Ta6Cl12)(H2O)6]Br4

Abstract

The treatment of an air-oxidized alkaline methanol–water or aqueous solution of [Ta₆Cl₁₂]⁴⁺ with concentrated hydrochloric or hydrobromic acid results in spontaneous reduction to [Ta₆Cl₁₂]³⁺; oxidation of some cluster units to hydrated tantalum(V) oxide occurs simultaneously. The isomorphous, paramagnetic series of clusters, NR₄[(Ta₆Cl₁₂)(H₂O)₆]X₄(R = Me or Et, X = Cl or Br) has been prepared. Aspects of the reduction of the [Ta₆Cl₁₂]⁴⁺ unit in acidic media are discussed. The crystal structure of NMe₄[(Ta₆Cl₁₂)(H₂O)₆] Br₄ has been determined by X-ray structure analysis. The compound crystallizes in the cubic space group Fd3m, with a= 19.926(5)Å and Z= 8. The structure has been determined by Patterson and Fourier methods and refined by full-matrix least squares to R= 0.042. It comprises [(Ta₆Cl₁₂)(H₂O)₆]³⁺ octahedra connected by O–H …Br hydrogen bonds, NMe₄⁺, and Br^– ions. Relevant bond distances are Ta–Ta 2.9059(8), Ta–Cl 2.449(2), and Ta–O 2.22(2)Å, respectively.