Galactose oxidase models: tuning the properties of CuII-phenoxyl radicals.
Four tripodal ligands with an N(3)O coordination sphere were synthesized: (2-hydroxy-3-tert-butyl-5-nitrobenzyl)bis(2-pyridylmethyl)amine (LNO(2)H), (2-hydroxy-3-tert-butyl-5- fluorobenzyl)bis(2-pyridylmethyl)amine (LFH), (2-hydroxy-3,5-di-tert-butylbenzyl)bis(2-pyridylmethyl)amine (LtBuH) and (2-hydroxy-3-tert-butyl-5-methoxybenzyl)bis(2-pyridylmethyl)amine (LOMeH). Their square-pyramidal copper(II) complexes, in which the phenol subunit occupies an axial position, were prepared and characterized by X-ray crystallography and UV/Vis and EPR spectroscopy. The phenolate moieties of the copper(II) complexes of LtBuH and LOMeH were electrochemically oxidized to phenoxyl radicals. These complexes are EPR-active (S=1), highly stable (k(decay)=0.008 min(-1) for [Cu(II)(LOMe(.))(CH(3)CN)](2+)) and stoichiometrically oxidise benzyl alcohol. Two additional tripodal ligands providing an N(2)O(2) coordination sphere were also studied: (2-pyridylmethyl)(2-hydroxy-3-tert-butyl-5-methoxybenzyl)(2-hydroxy-3-tert-butyl-5-nitrobenzyl)amine (L'OMeNO(2)H(2)) and (2-pyridylmethyl)bis(2-hydroxy-3-tert-butyl-5- methoxy)benzylamine (L'OMe(2)H(2)). Their copper(II) complexes were isolated as dimers ([Cu(2II)(L'OMe(2))(2)], [Cu(2II)(L'OMeNO(2))(2)]) that are converted to monomers on addition of pyridine. The complexes were investigated by X-ray crystallography and UV/Vis and EPR spectroscopy. Their one-electron electrochemical oxidation leads to copper(II)-phenoxyl systems that are less stable than those of the N(3)O complexes. The N(2)O(2) complexes are more reactive than the N(3)O analogues: they aerobically oxidize benzyl alcohol to benzaldehyde at a higher rate, as well as ethanol to acetaldehyde (40-80 turnovers).
Philibert
Aurélie
A
Laboratoire de Chimie Biomimétique, LEDSS, UMR CNRS 5616 Université J Fourier, BP 53, 38041, Grenoble Cedex 9, France.
Thomas
Fabrice
F
Philouze
Christian
C
Hamman
Sylvain
S
Saint-Aman
Eric
E
Pierre
Jean-Louis
JL
D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D004789:Enzyme Activation; D005609:Free Radicals / Q000737:chemistry; D005692:Galactose Oxidase / Q000737:chemistry* / Q000378:metabolism; D008024:Ligands; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry; D010084:Oxidation-Reduction; D010636:Phenols / Q000737:chemistry* / Q000378:metabolism; D013056:Spectrophotometry, Ultraviolet
D005609:Free Radicals; D008024:Ligands; D009942:Organometallic Compounds; D010636:Phenols; C042329:phenoxy radical; D003300:Copper; D005692:Galactose Oxidase