Synthesis,Structure And Properties Of The Novel Uranyl-Organic Polyrotaxane Frameworks

Actinide coordination polymers,which show fascinating topologies and functions as well as the intriguing 5f bonding features,has drawn extensive attention from chemists and material specialists.As a special supramolecular motif with unique mechanical properties,MIMs（Mechanically Interlocked Molecules）has been suggested to be organized inside the metal-organic polymer materials to achieve a higher level of molecular organization and create functional materials,which can also promote the application of supermolecular materials.Based on the above two aspects,we bone up on the study of supramolecular coordination polymers based on actinides,for which can help us to understand the important effects of 5f orbital on the oxidation state and bonding properties of actinides,and helpful for the further study of the coordination chemistry of actinides.The unique structure of the actinide coordination polymer may result in different material properties,which may provide functional materials with better performance and promote the development of metal-organic functional materials.In this work,a series of novol uranyl-organic polyrotaxane framework materials have been hydrothermally synthesized,and their properties were studied by a series of characterization methods.Firstly,an interesting reversible solid-state SCSC transformation from a seven-fold coordinated uranyl polyrotaxane,α-UP,to its six-fold coordinated supramolecular isomer,β-UP,has been achieved by temperature-induced isomerisation,which represents the first case of solid-state SCSC isomerisation of CPs mediated by the change of uranyl-ligand coordination mode.We have studied the transformation process by a series of experiments withvariable-temperaturesingle-crystalX-raydiffraction.The characterization method of infrared spectroscopy also provides intuitionistic evidence for the supramolecular isomerism.In addition,two theoretical models were established by DFT theory to prove thatα-UP with seven-fold coordinated U（VI）is more stable thanβ-UP with six-fold coordinated U（VI）in thermodynamics.The elevated temperature increases thermal fluctuation of atomic motion such as oxygen atoms of carboxyl group,and loosen the U-O bonding interaction,which offers the highly torsionalη²-carboxyl ofα-UP an opportunity to rotate back and regain a normal conformation and finally generate the with all-in-η¹-coordinatedβ-UP.Certainly,a similar procedure including the rotation of carboxyl as well as partial bond breakage and/or formation might occur whenβ-UP transforms intoα-UP.The particular rotaxane linker different from traditional small-molecule linker ensures highly correlated isomeric structures between different isomers of uranyl polyrotaxanes,and help to achieve the SCSC transformation by reducing the conversion barriers.Secondly,five new uranyl-based polyrotaxane compounds incorporating a sulfate or oxalate coligand have been hydrothermally synthesized via a mixed-ligandmethod.BasedonC6BPCA@CB6（C6BPCA=1,1’-（hexane-1,6-diyl）bis（4-（carbonyl）pyridin-1-ium）,CB6=cucurbit[6]uril）ligand,UPS1(UO₂（L）_0.5（SO₄）（H₂O）·2H₂O,L=C6BPCA@CB6)is formed by the alteration of initial aqueous solution pH to a higher acidity.The resulting 2D uranyl polyrotaxane sheet structure of UPS1 is based on uranyl-sulfate ribbons connected by the C6BPCA@CB6 pseudorotaxane linkers.By using oxalate ligand instead of sulfate,four oxalate-containing uranyl polyrotaxane compounds,UPO1-UPO4,have been acquired by tuning reaction pH and ligand concentration:UPO1(UO₂（L）_0.5（C₂O₄）_0.5（NO₃）·3H₂O)in one-dimensional chain was obtained at a low pH value range（1.47-1.89）and UPO2（UO₂（L）（C₂O₄）（H₂O）·7H₂O）obtained at a higher pH value range（4.31-7.21）.By lowering the amount of oxalate,another two uranyl polyrotaxane network UPO3(（UO₂）₂（L）_0.5（C₂O₄）₂（H₂O）)and UPO4(（UO₂）₂O（OH）（L）_0.5（C₂O₄）_0.5（H₂O）)could be acquired at a low pH value of1.98 and a higher pH value over 6,respectively.The UPO1-UPO4compounds,which display structural diversity via pH-dependent competitive effect of oxalate,represent the first series of mixed-ligand uranyl polyrotaxanes with organic ligand as the coligand.Moreover,the self-assembly process and its internal mechanism concerning pH-dependent competitive effect and other related factors such as concentration of the reagents,and coordination behaviors of the coligands were discussed in detail.