Catalytic Activity Of Transition Metal Schiff Base Complexes: A Quantum Chemistry Study

With the development of bio-inorganic chemistry, it has been found that, in life-form there are some oxygen carriers with significant functions in regulating life activities. Cytochrome P-450, a monooxygenase, belongs to the oxygen carriers. It can transfer and activate oxygen molecule with highly selectivity even under mild conditions. And its excellent catalysis derives from transition metal complexes with remarkable activation. Some researchers have successfully simulated monooxygenase enzymes using transition metal complex model, such as porphyrins and Schiff bases, to investigate the enzymes properties. In this paper, all calculations were carried out using Gaussian 03 package. The Density Functional Theory (DFT) and Linear Combination of Atomic Orbitals (LCAO) were performed for the balanced geometry structures, net charges of atom, mean component and coefficient and energy of frontier molecule orbit, and frequency calculation. We subsequently discussed the orders of catalytic activity on oxygen molecule and the effect of ligand and metal cation on transition metal Schiff base complexes. Furthermore, we investigated the complicated complex formed by transition metal Schiff base complexes with ethene and oxygen molecule, and triggered procedure of oxidation course of unsaturated molecules with those Schiff base complexes was discussed in thermodynamics aspect. The main conclusions are following:(1) The calculation results show that CoSalicyhexen Schiff base complex is nealy a plane configuration and has considerable catalytic activities on oxygen molecule.(2) Compared CoSalicyhexen, CoBrSalicyhexen, CoClSalicyhexen and CoNO₂Salicyhexen complexes which can be produced by substituting the H atom of phenyl with Br, Cl, NO₂ group, the calculation results indicate that these substituent electron-withdrawing groups on the phenyl of the Schiff base complex have increased the catalytic activity of complexes on oxygen molecule, and order of catalytic activity is that CoBrSalicyhexen > CoClSalicyhexen > CoNO₂ Salicyhexen > CoSalicyhexen.(3) By studying CoSalicyhexen, CoBrSalicyhexen and CoBr₂ Salicyhexen complexes, the catalytic activity increases with increasing the numbers of electron-withdrowing groups on the phenyl of present complex.(4) By investigating CoSalicyhexen,CoSalen,CoSalphen, the calculation results show that the catalytic activity of the three kind of complex is CoSalphen > CoSalen > CoSalicyhexen.(5) By comparing the calculation results of CoSalicyhexen, MnSalicyhexen and CuSalicyhexen which difference only lies in the metal cation, it has been shown the order of catalytic activity is MnSalicyhexen > CuSalicyhexen > CoSalicyhexen.(6) By comparing the calculation results of Cu₂(3-COO)Salen with Cu(3-COOH)Salen and CuNi(3-COO)Salen with Ni(3-C00H) Salen, it has been shown that dinuclear complex has more catalytic activity on oxygen molecule. It can be contributed to the synergistic effect between two metal cations of complex. Furthermore, Cu₂(3-COO)Salen has more catalytic activity than CuNi(3-COO)Salen.(7) By investigating the catalytic activities of CoSalicyhexen, CoBrSalicyhexen and CoBr₂Salicyhexen on ethene and oxygen molecule, it was shown the procedure is feasible in the thermodynamics. And the order of the three complexes activating ethene and oxygen molecule is CoBr₂Salicyhexen > CoBrSalicyhexen > CoSalicyhexen. Furthermore, we found that Gibbs free energy of complex reacting with ethene is larger than the Gibbs free energy of complex reacting with oxygen molecule. So it can be inferred that the complexes will react with ethene priorly but not the oxygen molecule, when both of two molecules react with complexes together.