Mössbauer Spectroscopy on Antimony Borosulfates Reveals Weak Coordination Behavior

With M^IIIM^I[B(SO₄)₂]₄ (M^III = Bi³⁺, Sb³⁺, Lu³⁺; M^I = H₃O⁺, NO₂ ⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺), we recently described the first modular system within borosulfate chemistry comprising a three-dimensional anion. Herein, we shed light on the respective series of antimony compounds SbX[B(SO₄)₂]₄ (X = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, Ag⁺, Tl⁺, NO⁺, NH₄ ⁺). While maintaining the same anionic topology, the compounds crystallize in the space groups I $\overline{4}$ (no. 82), P $\overline{4}$ (no. 81), and C2 (no. 5) and are strongly influenced by the lone pair of antimony as well as the size of the monovalent cations. In the course of this investigation SbX[B₄O₂(SO₄)₆] (X = Li⁺, Na⁺) were discovered. This borosulfate with a one-dimensional anion comprising B─O─B bridges crystallizes in the space group Pnma (no. 62) and features a new structure type. ¹²¹Sb Mössbauer spectra revealed negative isomer shifts of almost -22 mm·s^-1 not observed before and hinting towards a very weak coordination behavior of the borosulfate anion. The spectra are confirmed by DFT calculations. Furthermore, single crystal X-ray diffraction, infrared spectroscopy, thermal analysis, and temperature programmed X-ray diffraction experiments were carried out.