Metal-Pyrrolide Complexes in Three-fold Symmetry: Synthesis, Structure, Reactivity and Magnetism
- Author(s): Harman, William Hill
- Advisor(s): Chang, Christopher J
- et al.
Given the prominence of six-coordinate pseudo-octahedral complexes in transition metal chemistry, lower coordinate complexes in tetrahedral, trigonal pyramidal and trigonal bipyramidal configurations have garnered increased interest due to the changes in electronic structure and reactivity resultant from three-fold symmetry relative to the four-fold symmetry commonly observed in their pseudo-octahedral counterparts. Herein, we report a range of such effects in a collection of transition metal complexes supported by chelating, trianionic trispyrrolide ligands [tpaR]3-. In chapter one, two trigonal pyramidal iron(II) complexes, [(tpaPh)Fe]- and [(tpaMes)Fe]-, are discussed which are capable of undergoing oxygen atom transfer reactions and subsequent activation of strong C-H bonds. Additionally, the mesityl derivative is capable of oxygen atom transfer from nitrous oxide, an attractive but notoriously unreactive molecule.
In chapter two, this family of iron(II) complexes is expanded to include tert-butyl, 2,4,6-triisopropylphenyl and 2,6-difluorophenyl substituted variants, and the magnetic properties of this homologous series of trigonal pyramidal iron(II) complexes are explored. Notably, a number of these complexes exhibit frequency dependent signals at low temperature in the imaginary component of ac magnetic susceptibility measurements performed in the presence of a small applied field. Further exploration of this behavior via magnetometry, high-field EPR and Mössbauer spectroscopy reveal a large barrier to magnetic relaxation in these complexes, in some cases comparable to the best known single molecule magnets. This phenomenon is accounted for by the unquenched orbital angular momentum present in a three-fold symmetric high spin d6 system, leading to significant uniaxial anisotropy and a barrier to spin inversion. Progress towards a magneto-structural analysis of this behavior is made for this series.
Chapter three contains a through treatment of the coordination, redox and group transfer chemistry of the iron(II) platform [(tpaMes)Fe]-. In particular, it is shown that this complex is capable of supporting both high and low spin five coordinate adducts, including a rare example of a paramagnetic carbonyl complex of iron(II), [(tpaMes)Fe(CO)]-. Furthermore, the redox chemistry of this system is illustrated in the synthesis of the neutral iron(III) complex (tpaMes)Fe(i-PrNH2). As an extensiont to the oxygen atom chemistry presented in chapter one, nitrene group transfer to [(tpaMes)Fe]- is described, resulting in the synthesis of an iron(III) amido complex. The identity of this species is confirmed by high resolution electrospray mass spectrometry as well as Mössbauer, x-ray absorption and EPR spectroscopies.
In chapter four, the vanadium chemistry of [tpaR]3- is introduced. In particular, the vanadium(III) complex (tpaMes)V(THF) is show to react with an aryl azide to yield an isolable vanadium(V) diazenylimido species, a member of a very rare structural type. This complex is shown to decompose unimolecularly by dinitrogen loss to generate the cooresponding vanadium(V) imido. In addition to the vanadium-nitrogen multiple bond chemistry described, the synthesis two pseudo-three-fold symmetric vanadium(IV) oxo species is discussed. Single crystal x-ray structures of both complexes reveal a distorted coordination geometry and a short vanadium-oxo distance, consistent with a triple bond. The oxidation of both of these complexes to the corresponding vanadium(V) oxo is performed and shown by x-ray crystallography to result in the relief of the distortion observed in the vanadium(IV) oxo complexes. Taken together, these results shed light on the nature of metal ligand multiple bonding in three-fold symmetric geometries.
Chapter five discusses the chromium complexes of [tpaMes]3-, in particular the group transfer and redox chemistry of the system. Both chromium(II) and chromium(III) complexes are available by direct metallation, and the chromium(II) complex [(tpaMes)Cr(DME)]- is shown to undergo both oxygen atom and nitrene group transfer reactions to yield chromium(IV) oxo and imido complexes respectively. The chromium(IV) imido complex exhibits rich redox chemistry, and its oxidation to the isostructural chromium(V) and chromium(VI) complexes are demonstrated.