19F NMR method for evaluation of the donor site basicity and coordination modes of chelating ligands in the reaction of α-picolinic acid and its derivatives with titanium tetrafluoride

Abstract

It has been shown by the example of TiF4 complexes with α-picolinic acid (C5H5NCOOH, picH) and its esters (picEt and picSiMe3) in CH3CN that the 19F NMR chemical shifts can be used for estimating the basic properties of nonequivalent donor sites of chelating ligands. TiF4 reacts with picEt to form adducts: the TiF4(η2-picEt) chelate and trans-TiF4(picEt)2. The reaction of TiF4 with an equimolar amount of picH yields the cis-TiF4 (η2-picH) complex with three nonequivalent fluorine positions. The introduction of a ligand excess leads to a change in the chemical shifts of the fluoro complex, which is interpreted to be due to the formation of H-bonded complexes TiF4(η2-picH…picH). The introduction of Et3N, which binds protons, shifts the equilibrium toward the [TiF4(η2-pic)]− anion. The reaction of TiF4 with picSiMe3 occurs in an essentially different way. The major species in solution is the [(μ-F)(μ-OOpic)2(TiF3)2]− dimer. Its possible isomeric structures are discussed. On the basis of comparison of the chemical shifts of the fluorine atoms trans to the oxygen and nitrogen atoms of the chelate ligand, the conclusion has been drawn that the basicity of the chelate donor sites increases in the series (η2-picEt)TiF4 < (η2-picH)TiF4 < [(η2-pic)TiF4]− and that for all the complexes, the basicity of the nitrogen atom of the chelate is higher than that of the oxygen atom.