Oxa-thia-, oxa-selena and crown ether macrocyclic complexes of tin(ii) tetrafluoroborate and hexafluorophosphate – synthesis, properties and structures

Abstract

The reactions of Sn(BF₄)₂ and Sn(PF₆)₂ with crown ethers and oxa-thia- or oxa-selena-macrocycles are complex, with examples of fragmentation of the fluoroanions, and cleavage of the ligands observed, in addition to adduct formation. The reaction of Sn(BF₄)₂ with 15-crown-5 or 18-crown-6 produced the sandwich complex [Sn(15-crown-5)₂][BF₄]₂ with 10-coordinate tin, and [Sn(18-crown-6)(H₂O)][BF₄]₂·2H₂O which has an hexagonal pyramidal tin centre with two long contacts to lattice water molecules (overall 7 + 2 coordination). [Sn(18-crown-6)(PF₆)][PF₆] is formed from 18-crown-6 and Sn(PF₆)₂, but the hexafluorophosphate ions hydrolyse readily in these systems to produce F⁻ which coordinates to the tin to produce [Sn(18-crown-6)F][PF₆], which can also be made directly from Sn(PF₆)₂, 18-crown-6 and KF in MeCN. The structure contains a hexagonal pyramidal coordinated Sn(II) cation with an apical fluoride. The oxa-thia macrocycle [18]aneO₄S₂ forms [Sn([18]aneO₄S₂)(H₂O)₂(PF₆)][PF₆], from which some crystals of composition [Sn([18]aneO₄S₂)(H₂O)₂(PF₆)]₂[PF₆][F] were obtained. The cation contains an approximately planar O₄S₂ coordinated macrocycle, with two coordinated water molecules on one side of the plane and a weakly bound (κ²) PF₆⁻ group on the opposite face, and with the fluoride ion hydrogen bonded to the coordinated water molecules. In contrast, the oxa-selena macrocycle, [18]aneO₄Se₂, produces an anhydrous complex [Sn([18]aneO₄Se₂)(PF₆)₂] which probably contains coordinated anions, although it decomposes quite rapidly in solution, depositing elemental Se, and hence crystals for an X-ray study were not obtained. Reacting Sn(BF₄)₂ and [18]aneO₄Se₂ or [18]aneO₄S₂ also causes rapid decomposition, but from the latter reaction crystals of the 1,2-ethanediol complex [Sn([18]aneO₄S₂){C₂H₄(OH)₂}][BF₄]₂ were isolated. The structure reveals the coordinated macrocycle and a chelating diol, with the O–H protons of the latter hydrogen bonded to the [BF₄]⁻ anions. This is a very rare, structurally authenticated example of ring opening/cleavage of an oxa-thia macrocycle. The new complexes were characterised by microanalysis, IR, ¹H, ¹⁹F{¹H} and ³¹P{¹H} NMR spectroscopy as appropriate, and X-ray structures are reported for [Sn(15-crown-5)₂][BF₄]₃[H₃O]·H₂O, [Sn(18-crown-6)(H₂O)][BF₄]₂·2H₂O, [Sn(18-crown-6)F][PF₆], [Sn([18]aneO₄S₂)(H₂O)₂(PF₆)]₂[PF₆][F] and [Sn([18]aneO₄S₂){C₂H₄(OH)₂}][BF₄]₂. The complexes are compared and contrasted with chloro-tin(II) complexes of crown ethers, germanium(II) and lead(II) analogues.