Tris(thioether)-supported base metal complexes with redox-active dioxolene and α-diimine ligands
Date
2015
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
The examination of transition metal complexes containing redox-active ligands
such as α-diimines and o-dioxolenes are important research topics due to their
relevance to biochemistry, electronic functional materials and catalysis. They also
provide interesting challenges for the comprehensive understanding of the electronic
structures, which are of fundamental import to their reactivity.
In this context, tris(thioether)-supported α-diiminato-iron complexes [κ2-
PhTttBu]Fe(AdNCHCHNAd) (PhTttBu = phenyltris((tert-butylthio)methyl)borate; Ad =
adamantyl) and [κ2-PhTttBu]Fe(CyNCHCHNCy) (Cy = cyclohexyl) were prepared
through reduction of [PhTttBu]FeCl in the presence of AdNCHCHNAd and
CyNCHCHNCy, respectively. Both complexes were characterized by liquid injection
field desorption ionization (LIFDI) mass spectroscopy, single crystal X-ray
diffraction, 1H NMR spectroscopy, solution magnetic moment measurements,
electronic spectroscopy, cyclic voltammetry (CV) and differential pulse voltammetry
(DPV). In addition, [κ2-PhTttBu]Fe(AdNCHCHNAd) was also characterized by
Mössbauer spectroscopy. The combined spectroscopic and magnetic data suggest [κ2-
PhTttBu]Fe(RNCHCHNR) (R = Ad or Cy) are best described as high-spin ferrous
complexes with antiferromagnetically coupled α-diiminato π-radicals. This assignment
is further supported by the single-point DFT calculations of [κ2-
PhTttBu]Fe(AdNCHCHNAd) using the broken-symmetry (BS) approach. By
comparing the structural and spectroscopic features of [κ2-
PhTttBu]Fe(AdNCHCHNAd) to those of its isoelectronic analogue, [κ2-
PhTttBu]Fe(AdNNNNAd) prepared by M. T. Mock, it is further concluded that [κ2-
PhTttBu]Fe(AdNNNNAd) contains a high-spin ferrous center that is
antiferromagnetically coupled to the dialkyltetraazadiene π-radical anion.
A series of tris(thioether)-supported metal iodide complexes [PhTttBu]MI (M =
Mn, Fe, Co, Ni) were prepared and characterized by LIFDI mass spectroscopy, X-ray
crystallography, 1H NMR spectroscopy, magnetic measurements and electronic
spectroscopy. In two representative cases, the iodide complexes are better synthetic
precursors compared to the corresponding chloride analogues, due to iodide lability.
First, [PhTttBu]CoI was a better synthetic precursor compared to [PhTttBu]CoCl in
preparing [PhTttBu]Co(3,5-DBCatH). Second, [PhTttBu]FeI reacted with CO in the
presence of a reductant (KC8) affording [PhTttBu]Fe(CO)2, whereas [PhTttBu]FeCl did
not react with CO in the presence of KC8.
Furthermore, [PhTttBu]MI (M = Mn, Fe, Co, Ni) were utilized to generate MII-
semiquinonate complexes [PhTttBu]M(phenSQ) (M = Mn, Fe, Co, Ni) and
[PhTttBu]M(3,5-DBSQ) (M = Co, Ni), interesting synthetic targets because of their
relevance to the proposed or observed intermediates in the catalytic cycles of the
catechol dioxygenases. These MII-semiquinonate complexes were studied by
comprehensive spectroscopic techniques in conjunction with the broken-symmetry
DFT calculations. Interestingly, [PhTttBu]Co(phenSQ) exhibited temperature-induced
spin-crossover phenomenon, as supported by variable temperature X-ray diffraction
analysis, variable temperature electronic spectroscopy, and variable temperature
magnetic susceptibility measurements. [PhTttBu]Co(3,5-DBSQ) also exhibited spincrossover
behavior. However, its spin transition occurred at a higher temperature,
making [PhTttBu]Co(3,5-DBSQ) stay mostly low-spin at room temperature. Reaction
of [PhTttBu]M(phenSQ) (M = Fe, Co) and [PhTttBu]M(3,5-DBSQ) (M = Co, Ni) with
O2 was studied by 1H NMR, electronic and mass spectroscopies. [PhTttBu]Fe(phenSQ)
and [PhTttBu]Co(3,5-DBSQ) exhibited O2-promoted intradiol cleavage.
[PhTttBu]Co(phenSQ) reacted with O2 forming mostly the autoxidation product
phenanthrenequinone (phenQ). [PhTttBu]Ni(3,5-DBSQ) did not react with O2. It is thus
concluded that the metal ion, the dioxolene ligands, and the spin state of the complex
play vial roles in affecting the intradiol dioxygenase reactivity. Based on the 1H NMR,
electronic, and mass spectroscopic data, plausible mechanisms are provided for the O2
reactivity of [PhTttBu]Fe(phenSQ) and [PhTttBu]Co(phenSQ). O2 addition to
[PhTttBu]Co(phenSQ) forms a CoIII-superoxo species. The thermal decay of this
species led to the formation of phenQ instead of diphenic anhydride, an intradiol
product, through one-electron oxidation of phenSQ. On the other hand, O2 addition to
[PhTttBu]Fe(phenSQ) forms an FeIII-superoxo species. The thermal decay of this
species leads to the formation of phenQ and diphenic anhydride. The results provide
the first direct experimental evidence that the FeII-semiquinonate species may be
responsible for the intradiol reactivity.