Water exchange in fluoroaluminate complexes in aqueous solution: a variable temperature multinuclear NMR study

Area of Science:

• Inorganic Chemistry
• Solution Chemistry
• Nuclear Magnetic Resonance Spectroscopy

Background:

• Fluoroaluminate complexes are crucial in various chemical and biological processes.
• Understanding the dynamics of water exchange in these complexes is key to elucidating their reactivity and speciation.
• Previous studies have indicated a relationship between fluoride substitution and water exchange rates, but precise quantification was lacking.

Purpose of the Study:

• To quantitatively determine the effect of fluoride substitution on the solvent water exchange rate for aluminum complexes.
• To investigate the thermodynamic parameters (enthalpy and entropy of activation) for water exchange in fluoroaluminate species.
• To compare NMR-determined speciation with thermodynamic calculations and validate the use of paramagnetic relaxation agents.

Main Methods:

• Utilized Oxygen-17 (17O), Fluorine-19 (19F), and Aluminum-27 (27Al) Nuclear Magnetic Resonance (NMR) spectroscopy.
• Employed manganese(II) as a paramagnetic relaxation agent to resolve aluminum-27 NMR signals of different fluoroaluminate species.
• Performed variable temperature NMR experiments to determine rate coefficients and activation parameters for water exchange.

Main Results:

• Each fluoride ligand substitution increases the water exchange rate by approximately two orders of magnitude.
• New rate coefficients (kex) for AlF(H2O)5(2+) and AlF2(H2O)4(+) were determined, showing significantly faster exchange with increasing fluoride.
• Water exchange rates were found to be independent of fluoride and proton concentrations within the studied ranges.
• NMR-based speciation analysis was consistent with thermodynamic predictions.

Conclusions:

• The substitution of water by fluoride in aluminum complexes dramatically enhances the rate of inner-sphere water exchange.
• The determined kinetic and thermodynamic data provide critical insights into the dynamic behavior of fluoroaluminate species.
• NMR spectroscopy, aided by paramagnetic relaxation agents, is a powerful tool for resolving and characterizing metal complexes in solution.