ABSTRACT
Herein, two novel Evans-Showell-type polyoxometalate (POM)-based metal-organic complexes, namely, {[Cu(L)(H2O)3][Cu(L)0.5(H2O)][Cu(L)0.5(H2O)4][Co2Mo10H4O38]}·5H2O (1) and [(H2L)0.5]2{[Zn(L)0.5(H2O)4]2[Co2Mo10H4O38]}·2H2O (2) (L = N,N'-bis(3-pyridinecarboxamide)-piperazine), were hydrothermally synthesized using a semi-rigid bis-pyridyl-bis-amide ligand and structurally characterized via single-crystal X-ray diffraction, elemental analysis, IR spectroscopy, powder X-ray diffraction (PXRD) and thermogravimetric analyses (TGA). The single-crystal X-ray diffraction analysis shows that complex 1 is a 3D Evans-Showell-type POM-based metal-organic network. In complex 1, the 1D infinite double chain structure constructed from {{Cu[Co2Mo10H4O38]}4-L} units and the µ4-bridging L ligand are linked by quadrate Cu2L2 loops to form a 2D layer, which is further connected by µ2-bridging L ligands, forming a 3D (2,3,4)-connected metal-organic framework. Complex 2 displays 3D supramolecular networks based on 1D {[Co2Mo10H4O38]-Zn-L}n infinite chains, which are constructed from Evans-Showell-type polyoxoanions and µ2-bridging 3-bpfp ligands (via ligation of pyridyl nitrogen atoms). The different coordination modes of the POM polyanions, bis(pyridylformyl)piperazine ligands and ratios play key roles in the construction of the title complexes. Significantly, the ligand L shows a µ4-bridging coordination mode in complex 1, which is observed for the first time in a POM system. Compounds 1 and 2 represent the first examples of metal-organic complexes based on Evans-Showell-type polyoxoanion and transition metal-bis-pyrazine-bis-amide coordination complexes. The fluorescence properties of the title complexes are reported herein. In addition, the title complexes act as heterogeneous Lewis acid catalysts for the oxidation of benzyl alcohol, and can also be recovered and reused without any significant loss in activity. Significantly, compound 1 with a 3D metal-organic framework showed higher catalytic performance with 99.4% conversion and 98.8% selectivity for benzoic acid at 10 h than compound 2 with 3D supramolecular networks.
ABSTRACT
Two new Evans-Showell-type polyoxometalate (POM)-based metal-organic complexes, namely {Cu3(L1)1.5(H2O)5[Co2Mo10H4O38]}·5H2O (1), {[Cu(L2)0.5(H2O)2]2[Co2Mo10H4O38]}·6H2O (2) (L1 = N,N'-bis(2-pyrazinecarboxamide)-1,4-butane, L2 = N,N'-bis(2-pyrazinecarboxamide)-1,6-hexane), were successfully synthesized and structurally characterized by single-crystal X-ray diffraction, elemental analysis, IR spectroscopy, powder X-ray diffraction (PXRD) and thermogravimetric analyses (TGA). In complex 1, the adjacent [Co2Mo10H4O38]6- polyoxoanions are linked by CuII ions to form a 1D Cu-[Co2Mo10H4O38]6- inorganic chain, which is further linked by ligand L1 and [Co2Mo10H4O38]6- polyoxoanions, forming a 3D metal-organic framework. In complex 2, the adjacent [Co2Mo10H4O38]6- polyoxoanions link the CuII ions to generate a 2D Cu-[Co2Mo10H4O38]6- inorganic layer, which is further connected with bidentate ligands L2 to obtain a 3D metal-organic framework. The structural diversities of compounds 1 and 2 showed that the spacer lengths of the flexible bis-pyrazine-bis-amide ligands play important roles in tuning the structures of the title complexes. Compounds 1 and 2 represent the first examples of 3D frameworks based on the Evans-Showell-type polyoxoanions and Cu-bis-pyrazine-bis-amide coordination complexes. Moreover, the ligand L1 was first successfully introduced into the POM system. The electrochemical and fluorescence properties of compounds 1 and 2 were discussed. As heterogeneous catalysts, compounds 1 and 2 have good catalytic activity for the oxidation of benzyl alcohol. Moreover, compound 2 has higher catalytic performance with 100% conversion and 98.0% selectivity for benzoic acid at 10 h. The difference in their catalytic performance may be mainly due to the difference of their structures. The catalysts can be recovered and reused without displaying any significant loss of activity.
