Stereochemically (In)active lone pairs in Sb(iii) complexes of a soft scorpionate ligand

Abstract

Bis-homoleptic antimony complexes of hydrotris(3-methyl-2-thiooxoimidazolyl) borate (TMe) crystallize as either a penta- or hexa-coordinated isomer of [Sb(TMe)₂]⁺; [Sb(κ³-TMe)(κ²-TMe)]⁺ or [Sb(κ³-TMe)₂]⁺, depending on the counter anion. The known pentacoordinated [Sb(κ³-TMe)(κ²-TMe)]⁺ crystallizes in its TMe salt, while [Sb(κ³-TMe)₂]⁺, which displays octahedral geometry, is isolated in a trifluoroacetate or PF₆⁻ salt. The non-coordinated arm of the bidentate TMe in [Sb(κ³-TMe)(κ²-TMe)]⁺ cannot be brought into the coordination sphere without a conformational rearrangement of the TMe amounting to inversion at the bridgehead BH group. Variable temperature ¹H-NMR and ¹¹B-NMR spectroscopy shows that [Sb(TMe)₂]⁺ is labile in solution in the presence of the counter anionic TMe. The exchange process between the coordinated TMe and the TMe counter anion can be partially slowed by cooling the sample. DFT calculations support these observations with only very small energetic difference (2.22 kcal mol⁻¹) between the two isomers, [Sb(κ³-TMe)₂]⁺ and [Sb(κ³-TMe)(κ²-TMe)]⁺. The stereochemically active lone pair suggested by the geometry of [Sb(κ³-TMe)(κ²-TMe)]⁺ is supported by Electron Localisation Function plots for the two structures, showing that the Sb(III) lone pair can impose stereochemical constraints for the [Sb(κ³-TMe)(κ²-TMe)]⁺ ion, but that these are not present in the octahedral [Sb(κ³-TMe)₂]⁺. A tetranuclear Sb(III) chloride complex of TMe and a dimetallic Sb(III) complex containing three TMe ligands and one methyl–imidazolethione originating from the hydrolysis of TMe have also been structurally characterized.