An electron-deficient and potentially chromic ligand has been utilized to impart redox activity, photo- and hydrochromism, and solvotomagnetism to metal-organic frameworks (MOFs). A pair of MOFs were constructed from the flexible zwitterionic viologen-tethered tetracarboxylate linker N,N'-bis(3,5-dicarboxylatobenzyl)-4,4'-bipyridinium (L): [Co(L)(N)] (1) and [Mn(L)(N)(HO)]·3HO (2). Both compounds show three-dimensional frameworks in which mixed azido- and carboxylato-bridged chains are connected through the electron-deficient viologen moieties. The chain in 1 is built from alternating bis(azide) and (azide)bis(carboxylate) bridges, while that in 2 contains uniform (azide) (carboxylate) bridges. The MOFs shows the characteristic redox properties of the viologen moieties. The redox activity affords the MOFs with different chromic properties, owing to subtle differences in chemical environments. 1 shows reversible photochromism, which is related to the radical formation through photoinduced electron transfer from azide-carboxylate to viologen according to UV-vis, X-ray photoelectron, and electron spin resonance spectroscopy and DFT calculations. 2 is nonphotochromic for lack of appropriate pathways for electron transfer. Unexpectedly, 2 shows a novel type of solid-state hydrochromism. Upon the removal and reabsorption of water, the compound shows remarkable color change because of reversible electron transfer accompanying a reversible structural transformation. The radical mechanism is distinct from those for traditional hydrochromic inorganic and organic materials. Magnetic studies indicate ferro- and antiferromagnetic coupling in 1 and 2, respectively. What's more, 2 shows marked magnetic response to the removal of water molecules owing to the formation of radicals. The compound illustrates a unique material exhibiting dual responses (color and magnetism) to water.

Polyoxometalate-functionalized metal-organic frameworks featuring uniform proton-conducting pathways in three orthogonal directions, good water retention and stability were prepared. The proton conductivity of the hybrid material was observed to increase by 5 orders of magnitude compared to that of the parent material HKUST-1.

The solvothermal reaction of Mn(ClO4)2, NaN3, and a rigid viologen-tethered tetracarboxylic acid (1,1'-bis(3,5-dicarboxyphenyl)-4,4'-bipyridinium chloride, [H4L]Cl2) led to a coordination polymer of formula [Mn4(L)(N3)6(H2O)2]n. X-ray analysis revealed a 3D coordination structure. The Mn(II) ions are connected by mixed azide and carboxylate bridges to give 2D layers, which are pillared by the viologen tether of the zwitterionic ligand. Magnetic analyses suggested that the compound features antiferromagnetism and field-induced metamagnetism. The compound also shows photochromic and photomagnetic properties. The long-range magnetic ordering is owed to the spin-canting structure of the Mn(II)-azide-carboxylate layer; the photochromism involves the formation of viologen radicals via photoinduced electron transfer, and the photomagnetism is related to the interactions between the metal ion and the photogenerated radicals. The study demonstrates a strategy for the design of new multifunctional materials with photoresponsive properties.

Soft–hard-donor-combined ligands are a type of promising extractant for actinide and lanthanide separation. In this work, the effects of counteranions (Cl–, NO3 –, and ClO4 –) on the extraction and complexation behaviors of a recently reported tetradentate phenanthroline-derived phosphonate (POPhen) ligand toward lanthanides were thoroughly investigated using solvent extraction, NMR titration, UV–vis titration, and single-crystal X-ray diffraction measurements. It is found that C4-POPhen showed excellent extraction and selectivity toward heavy lanthanides [Lu­(III)] compared to light lanthanides, particularly with the counterion of ClO4 – and at low acidity. NMR titration studies demonstrated that both 1:1 and 1:2 Lu­(III)/C4-POPhen complexes were formed in a CD3OD solution with all three counteranions and the 1:2 species was easier to form in a complexation of C4-POPhen with Lu­(ClO4)3 under the same conditions. Furthermore, the stability constants of Nd­(III) complexation with C4-POPhen in the counteranions of Cl–, NO3 –, and ClO4 – systems were determined through UV–vis titration, and a much larger value of log β of complexes was found in the ClO4 – system, which was in good agreement with the results of solvent extraction. In addition, the structures of C2-POPhen complexation with Ln­(NO3)3/Ln­(ClO4)3 in the solid state were clearly unraveled by the single-crystal X-ray diffraction technique. This work demonstrated that the solvent extraction and complexation mechanisms of POPhen ligands with Ln­(III) were significantly affected by the counteranions from both the solution and solid-state aspects, which might shed light on the lanthanide/actinide separation.

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