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https://hdl.handle.net/2142/20107
Description
Title
Transition metal bis(porphyrin) complexes
Author(s)
Gorlin, Philip Anthony
Issue Date
1994
Doctoral Committee Chair(s)
Kenneth S. Suslick
Girolami, Gregory S.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Biochemistry
Chemistry, Inorganic
Chemistry, Physical
Language
eng
Abstract
Metal bis(porphyrin) complexes M(porph)$\sb2$ have received much attention in the last ten years due to their electronic and structural similarities to the Special Pair in photosynthetic bacteria. The work presented here covers the first transition metal bis(porphyrin) complexes. In addition, the first uranium(V) porphyrin complexes are described.
Zirconium and hafnium bis(porphyrin) complexes have been prepared and have the shortest interplanar separations (and thus the strongest $\pi$-$\pi$ interactions) of any bis(porphyrin) complexes to date. Zr(OEP)$\sb2$ is nearly 1 V easier to oxidize than mono-(porphyrin) analogs. Oxidation to the cationic Zr(porph)$\sb2\sp{+}$ results in a new absorption in the near-IR analogous to that observed in the Special Pair, but not seen in mono(porphyrin) cations. These observations are readily explained by a supermolecular approach which takes into account the overlap of the porphyrin subunit molecular orbitals.
Electron-donating and -withdrawing substituents have been substituted at the porphyrin $\beta$-pyrrole sites of the sandwich complexes. Crystallographic studies on Zr(TTP)(2-nitro-TTP) show that the nitro group does not greatly alter the structure of the molecule relative to the parent complex. Substitution does alter the redox potentials of the complexes. Zr(TPP)(2-nitro-TPP) is harder to oxidize and easier to reduce than Zr(TPP)$\sb2$. In contrast, Zr(TPP)(2-amino-TPP) is easier to oxidize but only slightly harder to reduce than Zr(TPP)$\sb2$. The different effect of the nitro and amino groups on the reduction potentials can be accounted for in terms of $pi$-resonance and inductive effects. Chloro-substituted sandwich complexes display a linear relationship between the number of chlorine atoms and the oxidation potentials.
The strong $\pi$-$\pi$ interactions present in the sandwich complexes give rise to new visible bands which are proposed to involve excited states with charge resonance (CR) character (A$\sp+$B$\sp-$ + A$\sp-$B$\sp+$, where A and B are the two porphyrin rings). These bands red-shift upon attaching either electron-donating or -withdrawing groups on the porphyrin periphery, which supports the assignment of these states as having CR character.
Uranium mono(porphyrin) complexes have also been investigated. The reaction of (TTP)U(tccat) (tccat = tetrachlorocatechol) with one equivalent of tetrachloroquinone results in eight-coordinate uranium(V) bis(dioxolene) complex (TTP)U$\sp{\rm V}$(tccat)(tcsq). Crystallographic and spectroscopic studies support the presence of one catecholate and one semiquinone coordinated to the metal. Magnetic studies show a drop in $\mu\sb{\rm eff}$ as the temperature is lowered, consistent with antiferromagnetic coupling between the f$\sp1$ uranium(V) center and the radical ligand.
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