Tuning the architectures of polyoxometalate-templated complexes by changing the spacer lengths of bis-pyridyl-bis-amide ligands (L): from 1D chains to 2D networks based on different (CuL)n loops

Abstract

A series of metal–organic complexes based on Keggin-type polyoxometalate (POM) templates and bis-pyridyl-bis-amide ligands with tuned spacer lengths have been hydrothermally synthesized and structurally characterized: [Cu₂(L¹)₃(SiMo₁₂O₄₀)(H₂O)₆]·2H₂O (1), [Cu₂(L²)₃(SiMo₁₂O₄₀)(H₂O)₆]·9H₂O (2), [Cu₂(L²)₃(SiW₁₂O₄₀)(H₂O)₆]·6H₂O (3), [Cu₂(L³)₃(SiMo₁₂O₄₀)(H₂O)₆]·4H₂O (4) and [Cu₂(L³)₃(SiW₁₂O₄₀)(H₂O)₆]·4H₂O (5) [L¹ = N,N′-bis(3-pyridinecarboxamide)-1,2-ethane, L² = N,N′-bis(3-pyridinecarboxamide)-1,4-butane, L³ = N,N′-bis(3-pyridinecarboxamide)-1,6-hexane]. Single-crystal X-ray analyses reveal that compound 1 is a two-dimensional (2D) supramolecular structure based on one-dimensional (1D) infinite chains containing quadrate Cu₂(L¹)₂ loops and Keggin polyanion templates. Compounds 2 and 3 are isostructural, and they display unusual 1D→2D poly-rotaxane frameworks containing large square Cu₂(L²)₂ loops, in which the Keggin polyanions are sandwiched between the parallel-packed 2D sheets. Compounds 2 and 3 represent the first POM-templated 2D metal–organic poly-rotaxane networks. Compounds 4 and 5 are isostructural, and they exhibit a 2D metal–organic architecture based on Cu₆(L³)₆ loops which are large enough to accommodate two Keggin polyanions, in which the polyanions are sandwiched among the stagger-packed cationic sheets. Compounds 1–5 represent examples of introducing flexible bis-pyridyl-bis-amide ligands into POM systems. The electrochemical properties of compounds 1 and 3 and the photocatalytic properties of the title compounds have been reported.