Syntheses and reactivity of transition metal complexes of mono- and bicyclo- macrocyclic ligands




Xu, Chao

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Optically-active bis ((-)-(R)-2-methyl-1, 4, 7-triazacyclononane) complexes of Ni(II)/ (III) have been synthesized. The molar optical rotation, [special characters omitted] of the Ni(II) complex is -1.3x103 deg. M-1m-1, and that of the Ni(III) complex +1.9x103 deg. [special characters omitted]. The Ni(II) complex is relatively stable in acidic media, and the rate of decomposition is first-order in [special characters omitted]. The kinetics of the redox reaction at the nickel centre were investigated using Marcus Cross-correlation, and the self-exchange rate determined to be [special characters omitted]. The synthesis of a macrobicyclic ligand containing 14- and 9-membered rings is described, which involves a Cu(II)- template reaction of bis-pendant [9]aneN20 with glyoxal to form a cyclam ring. The isolated condensation product was an amidol species formed from a series of proton-shifts of the initial Schiff-base intermediate. Insufficient hydrogenation of the amidol complex resulted in an enamine species. The crystal structures of tetra-protonated ligand perchlorate salt and its Cu(II) complex, together with the amidol and enamine reaction intermediates, were solved. The mechanisms involved in both processes are discussed. In all the Cu(II) complexes, the metal ion is in a square pyramidal geometry with the ether oxygen binding at the apical position and the metal ion sitting above the N4 plane towards the apical donor. The spectroscopic and electrochemical properties of square pyramidal Cu(II) complexes of bicycloN4X (X = 0, S or N) ligands were investigated. As the apical donor was changed from 0 to S and N, a correlation between the d-d transition band, A^, and the oxidation potential was observed, and rationalized in terms of σ-bonding strength. In the case of N40, a "square-scheme" involving both electrochemical and chemical processes was observed in the cyclic-voltammetric reduction, whereas in the case where X = S or N, the reduction is of reversible character. The macrobicycloN40 Ni(II) complex was synthesized. The crystal structure showed a distorted octahedral geometry with an axial coordination of the ether oxygen and a perchlorate anion, and the Ni(II) ion below the N4 plane towards the coordinated perchlorate. The substitution reactions of the [special characters omitted] complex with Cl- and SCN- in aqueous media were investigated. The reaction rate is dependent not only on [H+], but also the nature and concentration of both the entering ligand and the supporting electrolyte. A mecnanism involving ion-pairing equilibria and deprotonation of the coordinated water is proposed. The preparation and characterization of the ethylenebridged highly-conjugated binuclear Ni (II) complex, [special characters omitted] are described, and the mechanism involving radical intermediates was proposed accounting for the dehydrogenation and dimerization processes. Both the molecular structure and chemical reactivities of the compound showed "quasi-aromatic" nature. Hydrolysis of this compound was ligand-based, whereas its oxidation was metal-based. The electrochemical reduction of the compound was a one-electron/two-step process with [special characters omitted] of 0.19 V and -0.03 V [special characters omitted], respectively. The self-exchange rate of the first reduction was determined (using Marcus Cross-correlation) to be 5.1x10[special characters omitted], and that of the second 180 [special characters omitted]..



Transition metal complexes, Synthesis, Reactivity