Neutral Mononuclear, Dinuclear, Tetranuclear d7/d10 Metal Complexes Containing bis-Pyrazole/Pyridine Ligands Supported by 2,6-bis(3-Pyrazolyl)Pyridine: Synthesis, Structure, Spectra, and Catalytic Activity

A series of novel bis-pyrazole/pyridine complexes, [Zn2(HL1)2(μ2-SO4)]2·EtOH·H2O (1), [Co2(HL1)2(μ2-SO4)]2·2DMF·6H2O (2), [Zn4(HL1)4(μ4-SO4)][OH]2 (3), [Zn2(HL2)2(μ2-SO4)]·2H2O (4), [Zn(H2L2)(H2O)2](SO4)·0.87H2O (5) (H2L1 = 2,6-di-(5-phenyl-1H-pyrazol-3-yl)pyridine, H2L2 = 2,6-di-(5-methyl-1H-pyrazol-3-yl)pyridine), were synthesized hydrothermally from the self-assembly of Zn(II) or Co(II) with different types of bipyrazolyl/pyridine derivative ligands. All the complexes were characterized by elemental analysis, IR and UV–vis spectroscopy, powder X-ray diffraction (PXRD), and single-crystal X-ray diffraction. Structural analyses revealed that metal atoms (Zn and Co) in complexes 1 – 5 are five-coordination modes, forming slightly distorted trigonal bipyramidal geometries. In complexes 1–3, H2L1 ligand connected the two metal centers via the tetradentate fashion, and the same form of connection was found in complex 4 with H2L2 ligand. While in complex 5, H2L2 only connected with one metal center via the tridentate fashion, which was different from those in complexes 1–4. Additionally, there are abundant hydrogen bonding interactions in complexes 1–4. Interestingly, for hydrogen bonding connecting fashions being different, the molecules for the complexes 1 and 4 are held together by the hydrogen bond to form a 1D supramolecular structure, whereas complexes 2 and 3 are a hydrogen bonded dimer. In addition, quantum chemical calculations for 1, 3, and 4, thermal behaviors and photoluminescent properties for 1 and 3–5 were performed and discussed in detail. In the mean time, we found that these complexes had potential catalytic activity for the oxidation reaction of cyclohexane.