Catalytic oxidation processes using functionalised 0,0'-bidentate ligand copper complexes

[摘要] The aim of this studywas to imitatetheactivityand behaviour of theenzyme catechol oxidase byemployingsimplecoppernanomolecularmaterials,andtoinvestigateelectronicandsteric effects on this catalytic oxidation process. A range of O,O'-bidentate ligands were systematically synthesizedbyimpartingdifferentelectronicpropertiestotheligandsystems.Theseligands were then coordinated to copper(II)metal ions to form thecorresponding complexes. A total of sixligandswerecharacterizedandstudiedextensively,namely:MM(naltol)H,ME(naltol)H, MP(naltol)H,EM(naltol)H,EE(naltol)HandEP(naltol)H(3-hydroxy-1,2-dimethyl-4-pyridone, 1-ethyl-3-hydroxy-2-methyl-4-pyridone,3-hydroxy-2-methyl-1-isopropyl-4-pyridone,2-ethyl-3-hydroxy-1-methyl-4-pyridone, 1,2-diethyl-3-hydroxy-4-pyridone, 2-ethyl-3-hydroxy-1-isopropyl-4-pyridone). Onlyfivecrystalstructuresarereported:(MM(naltol)H,ME(naltol)H, EM(naltol)H,EE(naltol)HandEP(naltol)H).Sixcoppercomplexeswerealsosynthesizedand studied of which only two crystal structures were reported {[Cu(ME(naltol))2] and [Cu(EP(naltol))2]}, namely; [Cu(MM(naltol))2], [Cu(ME(naltol))2], [Cu(MP(naltol))2], [Cu(EM(naltol))2], [Cu(EE(naltol))2] and [Cu(EP(naltol))2]. Structuraldatarevealedthatalltheligandswereintheketo-enoltautomericforminthesolid stateandinallthecaseswhereaclearpackingorderwasobserved,weakhydrogenbondingis present.These interactions result in the formation of dimers, which stabilizes the structures. This dataalsoindicatedaC=Obondlengthincreasewithincreasingelectrondonationinthe synthesized O,O'-bidentate ligands systems. The synthesized copper complexes were planar with slightdeviationsfromplanarityandinboththecomplexesthecopperatomslieoninversion centers. These complexes exhibit strong intramolecular hydrogen interactions. The solution study results suggest that the complex with the least electron donating group on the ligandwasthe most effective catalyst; however, the same complex was coincidentally themost sterically demanding complex in the study. As the catechol oxidase is a macro-molecule which is very sterically crowded, the data suggests that steric effects play an important role in the catalytic process and this was successfullydemonstrated at a small-molecular levelofdetail viasolution modelling experiments. The two proposedmechanismsfavour theprocess equallyand none ispreferred over the other. In the first mechanism the first step involves the coordination of 3,5-di-tert-butylcatechol to the copper which results in the subsequent loss of one of the coordinated ligands. In the second step thesecond3,5-di-tert-butylcatecholcoordinatestothecopperwiththesubsequentlossofthe secondcoordinated ligand. Thethirdstepinvolves the interaction withoxygen andthe subsequentlossofone3,5-di-tert-butylcatecholmoiety.Thefourthstepistheratedetermining formationof3,5-di-tert-butylbenzoquinone,definedbytherateconstants k3and k-3,whichalso generatesthecatalyticspecies.Thesecondmechanismissimilartothefirstandincludesa reversible equilibrium between a two coordinated catechol species and a one catechol coordinatedspecies,aswellasadirectcoordinationofanewcatecholmoleculeonthenaltol-copper(II) species. The one catechol coordinated species oxidises the catechol to the correspondingquinoneandthecoppercenterisreducedintheprocess.Aninteractionwith molecularoxygenre-oxidisesthemetalcenterandgeneratesthecatalyticspecies,whichyields theproduct3,5-di-tert-butylquinone.TherateconstantsinMechanism2aredefinedby k8andk-8.For the complexes [Cu(ME(naltol))2], [Cu(MP(naltol))2] and [Cu(EP(naltol))2] (inmethanol at25°C)therateconstantsare kf (=k3 ork8)and kr(=k-3ork-8):(3.4±0.6)x10-4M-1.s-1and (3.6±0.4) x 10-6 M-1.s-1), (4.8±0.9) x 10-4 M-1.s-1 and (3.2±0.6) x 10-6 M-1.s-1), (8.7±0.7) x 10-4 M-1.s-1 and (6.15±0.02) x 10-6 M-1.s-1).

[发布机构] University of the Free State