Metal-binding studies of linear rigid-axle [2]pseudorotaxanes with in situ generated anionic metal halide complexes

Abstract

A systematic study of metal–pseudorotaxane binding, using the linear N,N′-bis(4-pyridyl)-4,4′-bipyridinium axle (PyBP²⁺) and aromatic crown ethers bis(1,5-naphtho)-32-crown-8 (BN32C8), bis(1,5-naphtho)-38-crown-10 (BN38C10) and bis-para-phenylene-34-crown-10 (BPP34C10), is presented. The three corresponding [2]pseudorotaxanes, including the novel, fully characterized (visible absorption and nuclear magnetic resonance spectra, association constant, electrochemistry, crystal structure) [PyBP/BPP34C10]²⁺ system, were each reacted with MX₂ (M = Zn²⁺, Cd²⁺, Hg²⁺; X = Cl⁻, Br⁻, I⁻). Of the twenty-seven different pseudorotaxane/MX₂ combinations explored, fifteen yielded single-crystals containing a pseudorotaxane unit, and were characterized by X-ray diffraction; in eight of those crystals metal–pseudorotaxane binding was observed. The results reflect the lower association constant of [PyBP/BPP34C10]²⁺ (170 M⁻¹) than the corresponding ones of [PyBP/BN32C8]²⁺ (870 M⁻¹) and [PyBP/BN38C10]²⁺ (420 M⁻¹) in solution, where pseudorotaxanes are in equilibrium with the corresponding free axle and wheel components. In all cases, the isolated crystals contain anionic metal halide species, such as singly charged MX₃⁻ (M = Zn, Cd; X = Cl, Br, I) or doubly charged CdX₄²⁻ (X = Br, I) and Hg₂X₆²⁻ (X = Cl, Br, I). Ordered 3D-arrays of perfectly aligned pseudorotaxane units are found in the dichroic mercury-based crystals. Our study demonstrates that although specific intermolecular interactions (collectively called crystal packing forces), may occasionally interfere with the crystallization of metal–pseudorotaxane complexes based on pseudorotaxanes with low association constants, the use of pseudorotaxanes instead of synthetically more challenging rotaxanes is a promising approach for the construction of metal–organic rotaxane frameworks (MORFs).