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

Abstract To understand the behavior of aluminum ions (Al 3+ ) in strongly acidified soil environments, complexation between Al 3+ 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 27 Al NMR peaks appeared at high magnetic fields, in addition to a peak due to Al(H 2 O) 6 3+ , suggesting the formation of monodentate 1 : 1 (AlFA 2+ ) and 1 : 2 (AlFA 2 + ) complexes. The trends observed in the 27 Al 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 27 Al NMR chemical shift, and change in electron density of each Al 3+ nucleus). The average conditional formation constants of the Al‐FA complexes were determined to be log K 1 =1.17 and log K 2 =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 Al 3+ , that is, complex formation with toxic Al species in acidified soil.