Lead(ii) tetrafluoroborate and hexafluorophosphate complexes with crown ethers, mixed O/S- and O/Se-donor macrocycles and unusual [BF4]− and [PF6]− coordination

Abstract

The reaction of Pb[BF₄]₂ in H₂O/MeCN solution with the macrocycle 18-crown-6 gave the dinuclear complex [{Pb(18-crown-6)(H₂O)(μ²-BF₄)}₂][BF₄]₂, containing two nine-coordinate lead centres, each bound to all six oxygens of a crown ligand, one water molecule and bridged by two μ²-BF₄ groups. In contrast, the oxa-thia crown [18]aneO₄S₂ gave the mononuclear [Pb([18]aneO₄S₂)(H₂O)₂(BF₄)][BF₄] in which the lead is coordinated O₄S₂ within the puckered ring of the macrocycle, and with two water molecules on one side of the plane and a chelating (κ²) BF₄⁻ on the other. The [Pb([18]aneO₄Se₂)(BF₄)₂] has the two BF₄⁻ groups arranged mutually cis and with the macrocycle folded; within each BF₄⁻ group the Pb–F distances differ by ∼0.5 Å, producing a very unsymmetrical chelate. The 15-membered ring macrocycles 15-crown-5 and [15]aneO₃S₂ produce sandwich complexes [Pb(macrocycle)₂][BF₄]₂ which contain 10-coordinate lead centres. Pb[PF₆]₂ in H₂O/MeCN solution formed [Pb(18-crown-6)(H₂O)₂(PF₆)][PF₆] and [Pb([18]aneO₄S₂)(H₂O)₂(PF₆)][PF₆] which contain weak κ²-coordination of the PF₆⁻ group on the opposite side of the macrocyclic ring to two coordinated water molecules, giving 10-coordinate lead. In contrast, [Pb([18]aneO₄Se₂)(PF₆)₂] has two κ²-coordinated PF₆⁻ groups disposed cis, with a very folded macrocycle conformation. In [Pb(18-crown-6)(NO₃)(PF₆)] a chelating nitrate group occupies the coordination sites at Pb(II) instead of the two water molecules, and the weakly coordinating PF₆⁻ group is tridentate. The crystal structures of the lead nitrate complexes, [Pb(15-crown-5)(NO₃)₂] and [Pb([18]aneO₄Se₂)(NO₃)₂], containing nine- and 10-coordinate lead respectively, are also reported. In solution the complexes are labile, and both conductivity and ¹⁹F NMR spectroscopic studies show the BF₄⁻ and PF₆⁻ groups are dissociated, whereas in the nitrate complexes the anion coordination is retained in solution. The identification of the coordination modes of the NO₃⁻ and BF₄⁻ groups in the solid complexes by IR spectroscopy is discussed.