What can pKa and NBO charges of the ligands tell us about the water and thermal stability of metal organic frameworks?

Abstract

The hydrothermal stability of metal organic frameworks (MOFs) is critical for their applications in hydrogen storage and other applications. The pK_a¹ values of the conjugated acids of ligands and natural bond orbital (NBO) charges of the coordinating atoms of 32 multicarboxylate ligands and 31 N-heterocyclic ligands applied in the MOF syntheses were calculated at 298.15 K with the B3LYP/6-31+G(d,p) level of calculations. Solvation Gibbs energies (ΔG_solv) in aqueous solution were obtained using SMD modeling. NBO charges were calculated based on optimized structures in the gas phase. We found that the calculated pK_a¹ values have a linear relationship with the experimental results of a few known bidentate ligands, but pK_a² values do not. We demonstrated that NBO charges of the coordinating atoms of the ligands, like pK_a¹ values of the conjugated acids of the ligands, can reflect the relative coordination abilities of the ligands to metal ions and the strength of the resulting M–L bonds. To the best of our knowledge, systematic calculations of the pK_a values and NBO charges of multidentate ligands have not been reported. The calculated results can be easily expanded using this inexpensive calculation method. Some experimental results about water and thermal stability of MOF materials were successfully explained in terms of M–L bond strengths and polarity based on our calculated results. The results can be used to predict whether a targeted MOF structure would be water and thermally stable, predict the hydrothermal stability of known MOFs, design new hydrothermally stable MOFs and provide guidance for the synthetic procedures based on the calculated pK_a values. The limitations of applying these values are also given.