Galactose oxidase models: creation and modification of proton transfer coupled to copper(II) coordination processes in pro-phenoxyl ligands.

Two tripodal ligands I (R = benzimidazol-2-yl; N-methylbenzimidazol-2-yl) (HLH and HLMe, resp.) were synthesized. They possess a N3O donor set that is known to stabilize phenoxyl radicals more efficiently than the corresponding N2O2 donor unit. Reaction of one molar equiv. of Cu(ClO4)2·6H2O with HLH or HLMe affords the zwitterionic benzimidazolium phenolate complexes [CuII(HLH)]2+ and [CuII(HLMe)]2+ via a proton transfer reaction coupled to Cu(II) coordination. Addn. of HClO4 to [CuII(HLH)]2+ and [CuII(HLMe)]2+ gave the benzimidazolium phenol complexes [CuII(H2LH)]3+ and [CuII(H2LMe)]2+, while addn. of NEt3 affords the benzimidazol-phenolate complexes [CuII(LH)]2+ and [CuII(LMe)]2+, resp. The phenol's pKa is remarkably low due to the strong withdrawing effect of the benzimidazolium substituent. X-ray crystallog. anal. of the Cu(II) complexes shows that deprotonation of the axial phenol forces the metal to move out of the square plane towards the O atom and one (or two) Cu-Npyridine equatorial bond length increases. The Cu(II) phenoxyl species [CuII(HLH)]·3+ and [CuII(HLMe)]·3+ were prepd. electrochem., or by addn. of two molar equiv. of Cu(II) to HLH or HLMe. Under these conditions, radical formation has never been obsd. for tripodal ligands contg. two pyridyl and one 2,4-di-tert-butylphenol group. This difference is explained in terms of the proton transfer mechanism and redox potentials. [on SciFinder(R)]


Soumis le 12 avril 2018