Complexation of Aluminum Ions with Formic Acid Under Acidic Condition: ²⁷Al NMR, ESI-MS, and DFT study

To understand the behavior of aluminum ions (Al³⁺) in strongly acidified soil environments, complexation between Al³⁺ and formic acid (FA), a natural monocarboxylic acid, at pH 3 was investigated using NMR, ESI-MS, and DFT techniques. In the presence of FA, two new ²⁷Al NMR peaks appeared at high magnetic fields, in addition to a peak due to Al(H₂O)₆³⁺, suggesting the formation of monodentate 1 : 1 (AlFA²⁺) and 1 : 2 (AlFA₂⁺) complexes. The trends observed in the ²⁷Al NMR analysis of the Al-FA complex was well represented by DFT calculations (i. e., move of the chemical shifts to the high magnetic field side, additivity low of the ²⁷Al NMR chemical shift, and change in electron density of each Al³⁺ nucleus). The average conditional formation constants of the Al-FA complexes were determined to be log K₁=1.17 and log K₂=0.91 for the 1 : 1 and 1 : 2 complexes. The formation constant of the acetate complex was also determined to be log K=1.80 (1 : 1 complex). These results strongly suggest that natural monocarboxylic acids produced from the decomposition of organic compounds and certain bacterial activities show potential for the detoxification of Al³⁺, that is, complex formation with toxic Al species in acidified soil.