s-Block chalcogenoether chemistry – thio- and selenoether coordination with hard Group 2 ions

Abstract

A highly unusual series of Group 2 complexes with soft thio- and selenoether coordination, [MI₂([18]aneO₄E₂)] (M = Ca or Sr; E = S or Se), [CaI₂([18]aneO₂S₄)] and [MI₂([15]aneO₃S₂)], has been prepared by reaction of anhydrous MI₂ with the macrocycle in dry MeCN solution. The complexes have been characterised via¹H NMR and IR spectroscopy, microanalysis and crystallographic studies which provide unambiguous confirmation of the M–S/Se coordination. The neutral complexes are seven- or eight-coordinate with the iodo ligands cis. The long M–E bond distances of ∼3.0 Å indicate weak interactions, but they are significantly less than the sum of the van der Waals radii for M and E, and are important in facilitating isolation of the complexes. Trace hydrolysis of [MI₂([18]aneO₄E₂)] and [SrI₂([15]aneO₃S₂)] leads, unexpectedly, to displacement of the iodo ligands rather than the S/Se donor functions, and the resulting dicationic [Ca(H₂O)₂([18]aneO₄S₂)]I₂, [Sr(H₂O)₃([18]aneO₄S₂)]I₂·H₂O, [Sr(H₂O)₃([18]aneO₄Se₂)]I₂ and [Sr(H₂O)₃([15]aneO₃S₂)]I₂ complexes have been structurally characterised, forming eight- and nine-coordinate cations, with all the macrocyclic donor atoms coordinated. Reaction of Ca(CF₃SO₃)₂ with [18]aneO₄S₂ in anhydrous MeCN solution similarly affords [Ca(CF₃SO₃)₂([18]aneO₄S₂)], albeit in low yield, also proven crystallographically. Using the MI₂ precursors provides a general entry into this area of coordination chemistry of these Group 2 ions, owing in part at least to their higher solubility in the weak donor (weakly competing) MeCN solvent. While CaCl₂ reacts with 18-crown-6 either directly in MeCN giving [CaCl₂(18-crown-6)], or in the presence of SbCl₅ (to form trans-[Ca(MeCN)₂(18-crown-6)][SbCl₆]₂), neither of these routes works with the oxa-thia or oxa-selena crowns.