Formation of monomeric Sn(ii) and Sn(iv) perfluoropinacolate complexes and their characterization by 119Sn Mössbauer and 119Sn NMR spectroscopies

Abstract

The synthesis and characterization of a series of Sn(II) and Sn(IV) complexes supported by the highly electron-withdrawing dianionic perfluoropinacolate (pin^F) ligand are reported herein. Three analogs of [Sn^IV(pin^F)₃]²⁻ with NEt₃H⁺ (1), K⁺ (2), and {K(18C6)}⁺ (3) counter cations and two analogs of [Sn^II(pin^F)₂]²⁻ with K⁺ (4) and {K(15C5)₂}⁺ (5) counter cations were prepared and characterized by standard analytical methods, single-crystal X-ray diffraction, and ¹¹⁹Sn Mössbauer and NMR spectroscopies. The six-coordinate Sn^IV(pin^F) complexes display ¹¹⁹Sn NMR resonances and ¹¹⁹Sn Mössbauer spectra similar to SnO₂ (cassiterite). In contrast, the four-coordinate Sn^II(pin^F) complexes, featuring a stereochemically-active lone pair, possess low ¹¹⁹Sn NMR chemical shifts and relatively high quadrupolar splitting. Furthermore, the Sn(II) complexes are unreactive towards both Lewis bases (pyridine, NEt₃) and acids (BX₃, Et₃NH⁺). Calculations confirm that the Sn(II) lone pair is localized within the 5s orbital and reveal that the Sn 5p_x LUMO is energetically inaccessible, which effectively abates reactivity.