Galactose oxidase models: insights from 19F NMR spectroscopy
(19)F labelled tripodal ligands that possess a N(3)O 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 HL...
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Published in | Dalton transactions : an international journal of inorganic chemistry no. 5; pp. 832 - 842 |
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Main Authors | , , , , , |
Format | Journal Article |
Language | English |
Published |
England
Royal Society of Chemistry
07.02.2009
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Subjects | |
Online Access | Get full text |
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Summary: | (19)F labelled tripodal ligands that possess a N(3)O 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 A 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, pi-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 (19)F NMR. In the former case, a broadening of the (19)F NMR signal is observed, whereas a sharpening is observed in the latter case. Temperature dependent (19)F 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 phenol para-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 (19)F NMR spectroscopy. |
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Bibliography: | ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
ISSN: | 1477-9226 1477-9234 |
DOI: | 10.1039/B813036A |