Cu[B2 (SO4 )4 ] and Cu[B(SO4 )2 (HSO4 )]-Two Silicate Analogue Borosulfates Differing in their Dimensionality: A Comparative Study of Stability and Acidity.

Borosulfates are an ever-expanding class of compounds and the extent of their properties is still elusive. Herein, the first two copper borosulfates Cu[B2 (SO4 )4 ] and Cu[B(SO4 )2 (HSO4 )] are presented, which are structurally related but show different dimensionalities in their substructure: While Cu[B2 (SO4 )4 ] reveals an anionic chain, ∞1 [B(SO4 )4/2 ]- , with both a twisted and a unique chair conformation of the B(SO4 )2 B subunits, Cu[B(SO4 )2 (HSO4 )] reveals isolated [B2 (SO4 )4 (HSO4 )2 ]4- anions showing exclusively a twisted conformation. The complex anion can figuratively be obtained as a cut-out from the anionic chain by protons. Comparative DFT calculations based on magnetochemical measurements complement the experimental studies. Calculation of the pKa  values of the two conformers of the [B2 (SO4 )4 (HSO4 )2 ]4- anion revealed them to be more similar to silicic than to sulfuric acid, highlighting the close relationship to silicates.