Two Ni(II) semiconducting metal-organic frameworks based on the tetrakis(4-carboxyphenyl)silane and an imidazole ligand: Syntheses, characterization, water stability and photoelectric properties

摘要

Moisture or water stable, visible light absorbing, semiconducting metal-organic frameworks are useful in utilizing the inexhaustible and clean solar energies. By applying mixed ligands of an imidazole, 1,4-bis(2-methylimidazol-1-yl)benzene (BMIB) and the tetrakis(4-carboxyphenyl)silane (H4TCS), we successfully synthesized a moisture stable, microporous two-fold 3D Ni(II)-MOF ([Ni2(TCS)(BMIB)2] (1)) and a water stable, nonporous 2D MOF ([Ni2(H2TCS)2(BMIB)2]·H2O (2·H2O)). Ni(II) ions in MOFs1and2have simple 6-coordianted geometry. The reason why MOF2is more stable than MOF1is likely due to its low porosity, which prevents the water molecules from approaching the M-O and M-N bonds. The imidazole BMIB was found to have similar electron donating ability as that of pyridyl N atoms, and can be used with carboxylate ligands to form moisture and even water stable Ni(II)-MOFs. The BET surface area of MOF1was determined to be 825?m2/g. The H2uptake is 1.43?wt% at 77?K under 1?atm, which is relatively high among MOF materials. The CO2uptakes at 273 and 298?K under 1?atm are 69.8 and 46.4?ml/g (STP), respectively; the CH4uptakes at 273 and 298?K under 1?atm are 12.1 and 11.1?ml/g (STP), respectively, which are moderate among MOF materials. MOF2does not absorb N2at 77?K and absorbs only 2.1?ml/g CO2at 298?K under 1?bar, consisting with its low porosity calculated by Platon (9.4%). It however can absorb 50.4?ml/g CO2at 298?K under 9?bar. The band gaps of MOF1and2were found to be 2.47 and 2.31?eV, respectively. The conduction-band (CB) and valence-band (VB) edges are determined to be ??1.15 and 1.32?V for MOF1, ??0.76 and 1.55?V for MOF2versus the normal hydrogen electrode (NHE). In addition, MOFs1and2were found to respond to visible light. This work and our previous results on Ni(II)-MOFs suggest that Ni(II) is beneficial to be used to synthesize moisture or water stable, visible-light absorbing, semiconducting MOFs.