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.