Synthesis,Structure And Properties Of Polyoxometalate-based Complexes Involving In Situ Transformation Of Single-branch N-donor Benzonitrile Ligands

In this paper,selecting the single-branch N-donor benzonitrile ligands,3/4-(benzoimidazol-1-ylmethyl)benzonitrile(3/4-BICN)and4-[2-(1H-pyrazol-3-yl)pyridyl-1-ylmethyl]benzonitrile(4-PPCN)as initial organic ligands,nine Keggin and Wells-Dawson type-based complexes have been prepared with an aim to synthesize the polyoxometalate-based complexes with novel structures and excellent performances,depending on the in situ hydrolysis of cyano to carboxy group under hydrothermal or solvothermal conditions.The structures of compounds 1-9 were characterized and analyzed by a series of methods.The photocatalytic and electrochemical properties of these compounds were also studied.1.Choosing the single-branch N-donor benzonitrile compounds,3/4-(benzimidazole-1-methylene)benzonitrile(3/4-BICN)as organic ligands,five Keggin/Wells-Dawson type-based complexes 1-5 were synthesized under hydrothermal condition by changing the types of metal ions and polyoxometalates.[Cu₂^ICu₂^II(H₂O)₄(4-BIBAH)₆(4-BIBA)₂(PMo₁₁^VIMo^VO₄₀)](1)(4-HBIBAH)₂[Cu₂(4-BIBAH)₄(SiMo₁₂O₄₀)][Cu₄(4-BIBAH)₈(SiMo₁₂O₄₀)]·6H₂O(2)[Cu₂^ICu₂^II(H₂O)₄(4-HBIBA)₆(4-BIBA)₂(SiMo₁₂O₄₀)](3)(4-HBICN)₂[Co₂(?₂-H₂O)(H₂O)₄(4-BICN)₄(P₂W₁₈O₆₂)](4)(3-HBIBA)₄[PMo₁₁^VIMo^VO₄₀]·4H₂O(5)Compounds 1-5 have supramolecular structures,in which the 3/4-(benzoimidazol-1-ylmethyl)benzoic acid(3/4-BIBA)was derived from the in situ transformation of the original ligand 3/4-BICN.When[PMo₁₂O₄₀]^3-was selected,compound 1 was obtained containing a bi-supported polyoxoanion modified by{Cu₂/(4-BIBA)₄}metal-organic fragments.The in situ formation of[SiMo₁₂O₄₀]^4-resulted in the coexistence of bi-and tetra-supported polyoxoanions modified by{Cu/(4-BIBA)₂}metal-organic fragments.But the use of pre-prepared[SiMo₁₂O₄₀]^4-polyoxometalate did not lead to the structure of 2.Compound 3 was obtained by tuning the p H value of reaction system.Compound 5 is a supramolecular structure composed of{(3-BIBA)₂H₂O}dimer supramolecular unit,which is connected on both sides of[PMo₁₂O₄₀]^3-polyoxometalate by the bridging effect of water molecules.The supramolecular structure of compound 4 was built from the bi-supported[P₂W₁₈O₄₀]^6-polyoxoanion modified by{Co/(4-BICN)₂}fragments.That is to say,the p H value and the type of polyoxometalate played important roles in modifying compounds'structures.Compounds 1,3 and 5exhibited better electrocatalytic reduction abilities and electrochemical sensing behaviors for Br O₃^-,Fe(III)and Cr(VI)ions.1 and 5 have good electrocatalytic reduction abilities and electrochemical sensing behaviors toward Fe(III)and Cr(VI)ions.1 and 3 showed exceptional photocatalytic performance and were used to degrade the organic dyes methylene blue and crystal violet.2.Four polyoxometalate-based complexes 6-9 were synthesized by adjusting the types of polyoxometalate and metal ions by using the single-branch N-donor benzonitrile compound,4-[2-(1H-pyrazol-3-yl)pyridyl-1-ylmethyl]benzonitrile(4-PPCN),as the initial organic ligand.[Cu₂(4-PPBAH)₂(4-PPBA)(H₂O)₂(PMo₁₂O₄₀)](6)[Cu^ICu^II(4-PPBA)(4-PPBAH)(H₂O)(PW₁₂O₄₀)](7)[Cu₂(4-PPCN)₄(Si W₁₂O₄₀)](8)[Co₂(H₂O)₄(4-PPCN)₂(Si W₁₂O₄₀)]·4H₂O(9)Compounds 6-9 have supramolecular structures.The hydrolysis of cyano group leaded to the in situ transformation of the 4-PPCN to 4-[2-(1H-pyrazol-3-yl)pyridyl-1-ylmethyl]benzoic acid(4-PPBA)in the structures of 6 and 7,which was not involved in the structures of 8 and 9.When[PMo₁₂O₄₀]^3-and Cu²⁺ions were introduced,compound 6 was obtained containing a mono-supported polyoxoanion modified by one{Cu₂/(4-PPBA)₃}fragments.Compound 7 accompanying with a mono-supported polyoxoanion modified by one{Cu₂/(4-PPBA)₂}was synthesized by transforming the category of polyoxometalate and the ratio of raw materials.It could be found that the different reaction conditions have important effects on the structures formation of polyoxometalate-based compounds.In 8 and 9,the 4-PPCN ligand was not transformed to the 4-PPBA ligand,but the cyano group of 4-PPCN ligand participated in formation of the structures by coordinating with the metal center.In 8,the coodiantion of 4-PPCN ligands and Cu²⁺ions formed a 1D metal-organic chain{Cu/4-PPCN},which was extended a supramolecular structure via[Si W₁₂O₄₀]^4-ion.But a 1D chain was generated from the binuclear cobalt metal-organic rings{Co₂/(4-PPCN)₂}and[Si W₁₂O₄₀]^4-ions in 9.Compounds 6-9dispiayed remarkable photocatalytic activity for degradation of organic dyes methylene blue,crystal violet and rhodamine B.Compounds 6,8 and 9 exhibited better electrocatalytic reduction abilities and electrochemical sensing behaviors for NO₂^-,Br O₃^-and H₂O₂.