K–H₃C and K–Sn Interactions in Potassium Trimethylstannyl Complexes: A Structural, Mechanochemical, and NMR Study

Abstract

A series of trimethylstannyl potassium complexes [K­(L)­SnMe₃] with different auxiliary ligands L (L = 18-C-6, (TMEDA)₂ (TMEDA = tetramethylethylenediamine), and (12-C-4)₂) were synthesized by alkoxide-induced B–Sn bond cleavage. X-ray structure determinations were performed for all these complexes, and the structural chemistry was studied in detail. For L = 18-C-6 and (TMEDA)₂ the solid state structures comprise polymeric [K­(L)­SnMe₃]_<i>n</i> chains containing bidentate trimethylstannyl anions bridging two [K­(L)]⁺ ions, featuring unsymmetrical coordination of the [K­(L)]⁺ ion by K–Sn and K–H₃C interactions as a central structural motif. In contrast, for L = (12-C-4)₂, separated [K­(12-C-4)₂]⁺ and [SnMe₃]⁻ ions are observed. Unexpectedly, in the presence of tetrahydrofuran (THF), [K­(18-C-6)­SnMe₃]_<i>n</i> forms upon crystallization a new species consisting of separated [K­(18-C-6)­(THF)₂]⁺ and [(Me₂SnCH₃)­K­(18-C-6)­SnMe₃]⁻ ions. In this unsymmetrical anion two trimethylstannyl anions coordinate a single [K­(18-C-6)]⁺ ion; one trimethylstannyl anion coordinates via a K–Sn interaction, and the second coordinates via a K–H₃C interaction. Simulations of the mechanochemical properties (compliance constants) applying approximated density functional theory revealed that both interactions are very soft and are of comparable strength. Moreover, according to our gas phase simulations the unsymmetrically coordinated [(Me₂SnCH₃)­K­(18-C-6)­SnMe₃]⁻ is indeed thermodynamically favored over both possible symmetrical isomers with either K–Sn or K–H₃C coordination. Furthermore, the existence of multiple species due to the two coordination modes and aggregates of [K­(18-C-6)­SnMe₃] in solution is suggested by NMR spectroscopic studies using ¹H, NOESY/ROESY, and ¹H pulsed field gradient diffusion experiments