The first lead perrhenate crown ether complex [Pb(12-crown-4)(H2O)(ReO4)2]: from rotation of the ReO4 tetrahedra in the (3 + 1)D space using constrained Legendre functions to rotation in the 3D space

Abstract

A unique lead perrhenate crown ether complex, [Pb(12-crown-4)(H₂O)(ReO₄)₂], exhibits an incommensurately modulated crystal structure at 101 K. In this (3 + 1)D superspace description, the ReO₄⁻ tetrahedra undergo continuous rotation. Describing their continuous rotation required introducing several constraints into the Legendre polynomials describing the displacive modulation of these atoms. At 296 K, the rotation of these tetrahedra is characterized by jumps between certain “stationary” positions. The average structure is monoclinic, P2₁/c with Z = 4, and can be described as comprising [Pb(C₈H₁₆O₄)(ReO₄)₂(H₂O)] ribbons. One half of the Pb²⁺ coordination sphere is occupied by the crown ether ligand, while the other is occupied by the oxygen atoms of water molecules and the perrhenate groups. The latter play two crystallographic roles: one bridging the PbO₈ polyhedra and the other acting as a terminal ligand with a very rare monodentate (κ¹) coordination. This is probably the reason for the nearly free rotation of these terminal ReO₄ groups along the Pb–O–Re axis. The structural chemistry of lead crown ether complexes, even those with relatively simple counteranions, is expected to yield a wide variety of unique and complex structures.