Galactose oxidase models: insights from 19F NMR spectroscopy

Abstract

¹⁹F labelled tripodal ligands that possess a N₃O donor set (one phenol, one tertiary amine and either two pyridines or one pyridine and one quinoline) have been synthesized. The fluorine is incorporated either at the phenol O-donor (HL^F and HL^CF3) or at the quinoline N-donor (HLq^OMe and HLq^NO2). The copper(II)–phenol complexes (2H)²⁺, (1H)²⁺, (3H)²⁺ and (4H)²⁺ as well as the corresponding copper(II)–phenolate complexes have been characterized. X-Ray diffraction reveals an increase in the oxygen–copper bond distance of more than 0.4 Å upon protonation of the phenolate moiety of (4)⁺. Protonation is accompanied by an axial to equatorial isomerization of the quinoline group. DFT calculations show that stretching of the Cu–O(phenol) bond, π-stacking interactions and rotation of the pyridine are key steps in this isomerization process. Protonation, and thus changes in the oxygen–copper bond distance induce either a decrease ((1H)²⁺, (2H)²⁺) or an increase ((3H)²⁺ and (4H)²⁺) in the copper–fluorine distance that could be monitored by ¹⁹F NMR. In the former case, a broadening of the ¹⁹F NMR signal is observed, whereas a sharpening is observed in the latter case. Temperature dependent ¹⁹F NMR measurements on equimolar mixtures of the phenol and phenolate complexes of (3)⁺ and (4)⁺ reveal rate constants for proton transfer and/or isomerization of 3000 ± 100 s⁻¹ and 2900 ± 100 s⁻¹, respectively, at the coalescence temperature. This temperature was found to be strongly affected by the phenolpara-substituent as it is 226 K and ca. 330 K for (3)⁺ and (4)⁺, respectively. A phenoxyl radical species ((3˙)²⁺) could be generated and characterized for the first time by ¹⁹F NMR spectroscopy.