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

A highly unusual series of Group 2 complexes with soft thio-and selenoether coordination, [MI2([18]aneO(4)E(2))] (M = Ca or Sr; E = S or Se), [CaI2([18]aneO(2)S(4))] and [MI2([15]aneO(3)S(2))], has been prepared by reaction of anhydrous MI2 with the macrocycle in dry MeCN solution. The complexes have been characterised via H-1 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 similar to 3.0 angstrom 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]aneO(4)E(2))] and [SrI2([15]aneO(3)S(2))] leads, unexpectedly, to displacement of the iodo ligands rather than the S/Se donor functions, and the resulting dicationic [Ca(H2O)(2)([18]aneO(4)S(2))]I-2, [Sr(H2O)(3)([18]aneO(4)S(2))]I-2 center dot H2O, [Sr(H2O)(3)([18]aneO(4)Se(2))]I-2 and [Sr(H2O)(3)([15]aneO(3)S(2))]I-2 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]aneO(4)S(2) in anhydrous MeCN solution similarly affords [Ca(CF3SO3)(2)([18]aneO(4)S(2))], 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.