Synthesis, Structural Characterization and Catalytic Alcohol Oxidation Reactions of Transition Metal Complexes with Tridentate Ligands Containing Hetero-Functional Donor Groups
Date Issued
2007
Date
2007
Author(s)
Li, Horng Yi
DOI
en-US
Abstract
The synthesis, structural characterization, and catalytic oxidation reactions of mononuclear transition metal compounds ligated by hetero-functional donor groups are reported in this thesis. A series of divalent metal complexes bearing with Schiff-base ligand L1 were prepared in one-pot synthesis with mixing carboxaldehyde, thioether/ether and metal salts in methanol. And the Schiff-base ligand (L1) serves as a tridentate ligand with their NN’X (X = S, O) sets donors. Their structures are characterized by FAB-MS, UV-vis, IR, and X-ray crystallography.
In this system, it is interesting to find that different metal salts may lead to different geometry for synthesized complexes. In the case of Co(ClO4)2 6H2O, a six-coordinate complex containing two identical tridentate ligands in the form of [(L1)2CoII](ClO4)2 is obtained. The reactions using the nitrate or chloride salt yield the complexes containing only one tridentate ligand. In a similar reaction using hydrated zinc triflate, two zinc complexes with or without coordinating water are acquired. Although the chelation effect of the multidentates is considered to the dominant for their complexes, the thioether and imine donors of the studied ligands have been found relatively labile.
Magnetic studies from SQUID find that ligand L1 bearing quinoline group reveals a weak field characteristic. And for this reason, all of the bond lengths are comparably longer than that in pyridine analogue. Also all of the metal complexes with ligand L1 bearing quinoline groups show high-spin configuration and air resistance.
In order to further investigate the extent of stabilization of CoII/CoIII state toward oxidation, cyclic voltammetric studies were performed. Each complex exhibits a quasi-reversible cyclic voltammetric response in CH2Cl2, corresponding to the CoII/CoIII redox process. The E1/2 value of complex 1a-1c [(L1)2CoII](ClO4)2 (X = S, R1=Me, Et, Bn,) is significantly higher (~1.0 V) than that for six-coordinate complexes of pyridine ligand appending thioether donor. These studies reveal that incorporation of quinoline group will enhance the lability of ligand, but increase the stability toward oxidation.
Complex 1 and 2 show moderate catalytic activity toward alcohol oxidation in the presence of 17 equivalent of hydrogen peroxide. And the activity will increase dramatically when the temperature is over 60 ℃. Reaction conditions are also varied, but limited changes of activity and aldehyde selectivity were observed.
Subjects
鈷
異原子
硫醚
三牙基
醇氧化催化
Cobalt
Hetero-functional donor
Thioether
Tridentate
Catalytic alcohol oxidation
Type
thesis
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