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

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 MOFs 1 and 2 have simple 6-coordianted geometry. The reason why MOF 2 is more stable than MOF 1 is 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 MOF 1 was determined to be 825 m²/g. The H₂ uptake is 1.43 wt% at 77 K under 1 atm, which is relatively high among MOF materials. The CO₂ uptakes at 273 and 298 K under 1 atm are 69.8 and 46.4 ml/g (STP), respectively; the CH₄ uptakes at 273 and 298 K under 1 atm are 12.1 and 11.1 ml/g (STP), respectively, which are moderate among MOF materials. MOF 2 does not absorb N₂ at 77 K and absorbs only 2.1 ml/g CO₂ at 298 K under 1 bar, consisting with its low porosity calculated by Platon (9.4%). It however can absorb 50.4 ml/g CO₂ at 298 K under 9 bar. The band gaps of MOF 1 and 2 were 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 MOF 1, − 0.76 and 1.55 V for MOF 2 versus the normal hydrogen electrode (NHE). In addition, MOFs 1 and 2 were 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.