Synthesis, characterization and metal complexation of novel pentacoordinate N4S-donor macrobicyclic ligands
Date
2018-08-01
Authors
Coulter, Kevin Robert
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Abstract
The syntheses of three structurally isomeric pentacoordinate [special characters omitted]-donor
macrobicyclic ligands are reported. The isomers differ only in the identity of the two
nitrogen atoms of the cyclam ring across which the [special characters omitted] fragment
("sulphur bridge") has been linked. The first isomer, 15-Thia-1,5,8,12-
tetraazabicyclo[10.5.2]nonadecane (L1), consists of a 9-membered [special characters omitted]-donor ring fused
to the 14-membered 1,5,8,11-tetraazacyclotetradecane (cyclam) ring. The ligand was
synthesized by a copper(II) templated ring closure followed by borohydride reduction. A
novel side product (4), a monoamide derivative of (L1), was also isolated and
characterized. The second isomer, 17-Thia-1,5,8,12-tetraazabicyck)[6.6.5]nonadecane
(L2), consists of linking the sulphur bridge across the diagonally opposed [1,8]-nitrogens.
The ligand was synthesized by cyclizing cyclam (1,4,8,11-tetraazacyclotetradecane) with
thiodiglycolic acid chloride under high dilution conditions. The third isomer, 14-Thia-
1,4,8, 11-tetraazabicyclo[9.5.3]nonadecane (L3), consists of linking the sulphur bridge
across the adjacent [1,5]-nitrogens of the cyclam ring. The ligand was synthesized by
cyclization of 1-thia-4,8-diazacyclododecane (20) with N,N’ –bis(α-chloro
amido)diaminopropane (27) which afforded the diamide derivative, 14-Thia-1,4,8,11-
tetraazabicyclo[9.5.3]nonadecane-3,9-dione (30), in high yield. Reduction of 30 with
diborane gave the third structural isomer, (L3).
A novel, convenient synthesis of 1-thia-4,8-diazacyclododecane-3,9-dione (23)
from diaminopropane and thiodiglycolic acid chloride was developed. It was shown that
high dilution conditions were necessary to obtain high product yields. The nature of the
product obtained from reduction of 23 was shown to depend on the conditions used. In
the presence of excess borane and methanol, the major product was a boron complex of
the reduced ligand. Several changes to the conditions used avoided production of the
boron complex and yielded the intended product (20) in sufficient quantity.
The parent 1-Thia-4,7-diazacyclononane ([9][special characters omitted]) and 1,4,8,11-
tetraazacyclotetradecane ([14][special characters omitted]) ligands exhibit first order coupling behaviour as a
result of time averaging of molecular motions. However, the ambient temperature [special characters omitted]
spectrum of the [9][special characters omitted] bicyclic free ligand (L1) exhibited second-order
couplings and line broadening indicative of an exchange process. This result indicates that
the rigidity of the macrobicyclic structure has hindered certain molecular motions. The
variable-temperature [special characters omitted] spectra of (L1) were recorded in deuterated chloroform. It
was found that the geminal coupling (> 15 Hz) of the [special characters omitted] (propylene fragment)
methylene group approached 0 Hz as the temperature was raised beyond 40°C. It is
believed that nitrogen inversion processes are responsible for the collapse of these signals.
In order to determine if the rigidity of the [special characters omitted] macrobicyclic free ligand structure
would have a significant effect on the chemistry of the metal complexes, the copper(II)
and nickel(II) complexes of each of the [special characters omitted] isomers (L1-L3) were prepared and
characterized. Any differences observed between these isomers can be attributed to ring
strain effects.
Electronic spectroscopy of the copper(II) complexes determined the absorption
maxima of the [special characters omitted] isomers to be 532,
532.5 and 603 nm respectively. ESR spectroscopy showed each complex to have similar
[special characters omitted] values (2.092, 2.090 and 2.089), and the frozen solution (77 K) spectra were
indicative of tetragonally elongated axial symmetry [special characters omitted] with the unpaired
electron predominantly in the [special characters omitted] orbital.
The synthesis and characterization of the corresponding nickel(II) complexes of
each ligand are reported. The electronic spectra of each of the [special characters omitted](acetate)2 and [special characters omitted] isomers were consistent with that expected for [special characters omitted]
coordination of nickel(II). Electrochemical studies of the nickel(II)/(III) oxidations
showed that there was a 170 mV difference between the [special characters omitted] values of the [special characters omitted] and [special characters omitted] complexes.
These results implied that the (L1) and (L3) bicycle isomers are similar in
coordination properties such that significant differences were not observed. The
coordination properties of the third [special characters omitted] bicycle (L2) differ markedly from those of the
other two isomers. That the unique diagonally bridged structure resulted in significantly
different absorption maxima and redox potentials, indicates that the coordination of the
[special characters omitted] donor set has been perturbed (relative to the (L1) and (L3) bicyclic ligands) by the
strain energies unique to that isomer.
The molecular structures of the [special characters omitted] and [special characters omitted] complexes were determined by X-ray crystallographic techniques. The cobalt(Ill) complex
was shown to contain "true" octahedral coordination. The palladium(ll) complex was
shown to consist of square planar [special characters omitted] coordination with the axial sulphur donor positioned
2.87Å from the palladium atom with the M-S vector deviating 16° from the perpendicular
to the [special characters omitted] plane.
Description
Keywords
Ligands, Macrocyclic compounds