Syntheses and reactivity of transition metal complexes of mono- and bicyclo- macrocyclic ligands
Date
2018-06-27
Authors
Xu, Chao
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Abstract
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]..
Description
Keywords
Transition metal complexes, Synthesis, Reactivity