The synthesis and characterization of macrocyclic ligands and investigations of the thermo and photo reactivity of their transition metal ion complexes
Date
2017-06-28
Authors
Mackay, Ian Douglas
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Abstract
The mono protection of 1,4,7-triazacyclononane ([9]-aneN₃) was accomplished efficiently through the formation of an orthoamide derivative. The orthoamide was used to form three mono protected derivatives of [9]-aneN₃ which contained either a formyl, methyl, or benzyl protecting group attached to one of the nitrogen donors. The macrobicyclic complexes bicycloN₅, Me-bicycloN₅, and Bz-bicycloN₅ were synthesized through the Michael addition of two functional arms to the mono protected derivatives of [9]-aneN₃ followed by a ring closure template reaction around copper
with glyoxal. Incomplete reduction by BH₃ THF led to the isolation of an enamine intermediate.
The solution behavior o f the Ni(II) and Ni(III) complexes of these macrobicyclic ligands is
presented. The methyl and benzyl derivatives were found to have similar abilities as the parent
bicycloN₅ ligand to stabilize the Ni(III) metal ion. Removal of the benzyl protecting group was achieved by reaction with formic acid in the presence of a Pd/C catalyst.
Attempts to couple two mono protected nonane molecules through the addition of
functional acid chloride arms under conditions of high dilution were unsuccessful.
Reaction of the benzyl protected bicyclic ligand Bz-bicycloN₅ with a bridging ligand in a
high dilution reaction did provide evidence for the formation of a small amount of the
novel macrotricyclic ligand tricyclo[9.14.9]N₆.
The Ni(II) complexes of the macrobicyclic ligands, and a series of other macrocyclic and related
Ni(II) complexes having varying NiIII/II redox potentials, were used to study the quenching of
the excited state of the platinum(II) dimeric complex Pt₂(pop)₄⁴− . The quenching rate constants
kq were determined, and quenching of the excited state *Pt₂(pop)₄⁴− by the nickel complexes was
found to proceed by reductive electron transfer. These results are discussed in terms of Marcus
Theory. A plot of logkq versus the ΔG for electron transfer was found to exhibit classic Rehm-
Weller behavior. The excited state potential Pt₂(pop)₄⁴⁻*/⁵- was estimated from this series of
quenching reactions and a range of 1.24 to 1.34 V (vs. NHE) was identified.
The photochemical and photophysical properties of the macrocyclic complex
Cr([18]-aneN₆)³+ ([18]-aneN₆ = 1,4,7,10,13,16-hexaazacyclooctadecane) were
investigated and compared to the properties of the photoreactive Cr(III) complex
Cr(sen)³+ (sen = 4,4',4"-ethylidynetris(3-azabutan-l-amine)). The complex Cr([18]-
aneN₆)³+ was found to be unreactive (Ørxn < 10⁻³) while the photoreactivity of Cr(sen)³+
was confirmed (Ørxn = 0.10). Both of these complexes have very short ambient ²E
emission lifetimes and this is discussed in terms of distortions imposed on the complexes
by the coordination of the ligands.
Direct irradiation into the doublet excited state of Cr(sen)3⁺ at 675 nm resulted in a decrease
in the quantum yield for the photoreaction of this complex from Ørxn=0.10 for quartet
irradiation to Ørxn=0.08 for doublet irradiation. A model is suggested in which there are two
competitive processes deactivating the doublet excited state; reverse intersystem crossing to
the lowest quartet excited state and nonradiative decay back to the ground state. The
temperature dependence of the ²E emission lifetime was fitted to a two-term Arrhenius function
to give estimates for the pre-exponential factors and activation energies of these two
deactivation processes. Values of A₁ = (1.2 ± 0.9) x 1O¹⁵ s⁻¹ and Eₐ₁ = 45 ± 1 kJ mol⁻¹,and A₂ =
(5.4 ± 1.2) x 10¹¹s⁻¹ and Eₐ2 = 29 ± 1 kJ mol⁻¹ were obtained.
The photostereochemistry of Cr(sen)³⁺ was investigated using a modified reversed phase HPLC
technique. A total of four photoproducts were identified from the photolysis of the resolved
stereoisomers of Cr(sen)³⁺ and a loss of optical activity was found to be associated with the
photoreaction. These results are discussed in terms of current models for predicting
photostereoreactivity of Cr(III) complexes.
Description
Keywords
Transition metal compounds, Ligands