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NMR characterization of a diiron macrocycle and structural characterization of a diketo derivative

Author

Claudia Lindblom Brackett, University of the Pacific

Date of Award

2001

Document Type

Thesis - Pacific Access Restricted

Degree Name

Master of Science (M.S.)

Department

Chemistry

First Advisor

Larry O. Spreer

First Committee Member

Lydia Fox

Second Committee Member

Vyacheslav V. Samoshin

Abstract

The time-dependent visible spectra and the crystal structure of [Fe₂(C₂₀H₂₄N₈O₂)(CH₃CN)₄]·PF₆ (diketo-dimer) were studied. The spectra showed that the most significant chemistry occurred during the initial 1.5 hours of the synthetic reaction. The starting materials 343 nm peak shifted to a lower energy, at 360 nm, and a new shoulder appeared at 490 nm. This change suggests the formation of a new intermediate whose spectrum has an exceptional resemblance to the starting materials mixed valent species, [Fe₂(TIED)(Cl)⁴]⁺¹ (TIED = tetraiminethylene dimacrocycles). Two isosbestic points were found at 538 and 371 nm.

The diketo-dimer's crystals appear to have individual colors, a physical characteristic called pleochroism. Pleochroism is a topic in the study of optical crystallography which is discussed and applied to the diketo-dimer. The extinction angle was estimated to be 14°, a value consistent for triclinic crystals. X-ray crystallography found that the diketo-dimer is triclinic, and has a space group of P-1. A noteworthy feature is the bond length, 1.406 Å, between the two linking bridgehead carbons. This bond length matches the value for partial double bonds of aromatic compounds. This argues for a delocalized electron circulating within the macrocycle.

The NMR spectra of a diiron macrocycle, [Fe₂(TIED)(CH₃CN)₄]⁴⁺, were examined. Temperature dependent, pH dependent, D⁺ substitution, selectively decoupled, and COSY ¹H NMR experiments were performed. Two sets of structural equilibria were found. One set is temperature dependent, and the other is pH dependent. Of particular interest are the peaks centered at 9.7 ppm and assigned to the imine carbon protons H2. Its resonance indicates an imine proton in an extensively conjugated aromatic environment with an electron deficient metal.

Pages

152

Recommended Citation

Brackett, Claudia Lindblom. (2001). NMR characterization of a diiron macrocycle and structural characterization of a diketo derivative. University of the Pacific, Thesis - Pacific Access Restricted. https://scholarlycommons.pacific.edu/uop_etds/554