Research On Catalytic Performance And Mechanism Of Oxamate Cobalt Complex

Efficient and selective catalytic performance of biological enzymes has been a concern for researchers.However,their active chemical properties determine the difficulty for practical applications,because complicated constituents always make them deactivated.Therefore,it possesses significance for researchers to develop artificial bio-enzyme mimic catalysts with stable chemical properties to realize selective oxidations in high backgrounds.Oxamate anionic cobalt(III)complex([CoIII(opba)]-),belonging to one of bio-enzyme mimics,is a planar tetragonal compound formed by the coordination of cobalt ion with N and O atom of oxamate ligand.For decades,this catalyst has been developed and explored by the field of catalysis.Like the metal in biological enzymes,the trivalent cobalt atom with d4 electrons in this complex,not only can coordinate with N2O2 structure,but also coordinate with other atoms with lone pair electrons from the top or bottom of the plane respectively.Hence,in the catalytic process,the oxidant coordinates with cobalt atoms from one side of the N2O2 plane,and the fifth ligand combines with the cobalt from the other side and forms axial ligand.Accordingly,we investigated the catalytic performance of [CoIII(opba)]-by introducing axial ligand and substituents on oxamate ligand,oxidants,substrates.In this paper,hyperoxides were used as oxidants,[CoIII(opba)]-was used as catalyst to oxidize different substrates such as acid red 1(AR1),benzimidazole compounds,etc.The experimental results showed that the substituent group on the macrocyclic oxamate and the axial fifth ligand held an important effect on the catalytic performance of [CoIII(opba)]-.The catalytic degradation of AR1 and p-chlorophenol was accelerated by nitro on the macrocyclic oxamate,and after introducing some pyridyl axial ligands,the oxidation efficiency [CoIII(opba)]-/H2O2 catalytic system was improved significantly,especially when 4-cyanopyridine with strong electronegativity or pyridine compound with 4th-position orientating substituent was introduced.In addition,the substituents of oxidants have a decisive influence on the selective catalysis of [CoIII(opba)]-.The experimental results showed that,under the catalysis of [CoIII(opba)]-,only peracetic acid could oxidize alcohol compounds to aldehydes/ketones without by-products.In order to explain these experimental phenomena,hydroxyl radical scavenger(NaCl,Urea,PEG(M=200))test was conducted,while these reagents didn't decreased the oxidation efficiency of AR1.Beside,EPR detection ruled out the possibility of hydroxyl radical or peroxyl radical mechanism.Thus,we proposed that cobalt-oxygen active species played the key role in the catalysis.To provide our speculation and to explain the mechanism of the above experimental phenomena,we employed Density Functional Theory(DFT)calculations.The calculation results showed that there might be two mechanisms: high cobalt oxygen species(LCoIV=O·)and trivalent cobalt-hydroxyl(·LCoIII-OH)mechanism(L means the macrocyclic ligand).Moreover,the electronegative substituents and axial fifth ligands reduced the energy levels of the highest single occupied molecular orbitals and the lowest unoccupied molecular orbitals and weakened the spintronic density of the active sites.The reduced electron cloud density of the active site enhanced the oxidation efficiency and oxidizing power of the active intermediates.This paper depicted the catalytic mechanisms of cobalt complexes from four aspects: macrocyclic substituents,axial ligands,oxidant substituents and the nature of substrates.The exploration might provide a new idea about the design and development of novel bio-enzyme mimics.