Document Type

Dissertation

Date of Degree

Spring 2014

Degree Name

PhD (Doctor of Philosophy)

Degree In

Chemistry

First Advisor

Jan-Uwe Rohde

Abstract

Complexes of redox-active ligands have received increasing attention, in part, because they have been shown to facilitate a variety of bond-forming and bond-breaking reactions, including the activation of small molecules. Unique attributes of redox-active ligands include that they influence the electronic properties of transition metal complexes and may participate in their multielectron redox chemistry. In this work, the utility of Ni complexes of redox-active bis(arylimino)pyridine ligands for the activation of dioxygen was explored. A series of bis(arylimino)pyridine complexes [Ni{2,6-(ArN=CMe)2C5H3N}Cl2](where Ar = 4-X-2,6-iPr2C6H2 and X = H, Cl, Br, NO2, or N(CH3)2) were synthesized along with the deuterium-labeled complex [Ni{2,6-(ArN=CCD3)2C5H3N}Cl2](where Ar = 2,6-iPr2C6H3).Characterization by X-ray crystallography of three complexes confirmed coordination of the Ni center by the three nitrogen donor atoms of the bis(arylimino)pyridine ligand and two chloro ligands. The structural analyses further revealed a distorted square pyramidal geometry of the Ni centers. The reduced complexes[Ni{2,6-(ArN=CMe)2C5H3N}Cl](where Ar = 4-X-2,6-iPr2C6H2 and X = H, Cl, or Br) and [Ni{2,6-(ArN=CCD3)2C5H3N}Cl](where Ar = 2,6-iPr2C6H3) were synthesized by reaction of the corresponding dichloride complexes with a sodium amalgam. These are four-coordinate complexes with distorted square planar geometry and are best characterized as ligand-radical complexes of NiII as opposed to NiI complexes on the basis of X-ray crystallography and electron paramagnetic resonance spectroscopy of Ni{2,6-(2,6-iPr2C6H3N=CMe)2C5H3N}Cl]. Reactions of the ligand-radical NiII complexes with dioxygen were studied by UV-Vis spectroscopy, and the overall course of the reaction was similar for all four complexes. The half-life of the reaction was not affected by deuterium labeling of the acetimino methyl groups but was affected by different substituents in the 4-position of the phenyl groups of the ligand. The reactions caused intraligand C-C bond cleavage with formation of a new C=O double bond, resulting in the complexes [Ni{6-(ArN=CMe)C5H3N-2-C(O)NAr}Cl] (where Ar = 4-X-2,6-iPr2C6H2 and X = H, Cl, or Br) and [Ni{6-(ArN=CCD3)C5H3N-2-C(O)NAr}Cl](where Ar = 2,6-iPr2C6H3). The structures of the complexes [Ni{6-(ArN=CR)C5H3N-2-C(O)NAr}Cl] (where Ar = 2,6-iPr2C6H3 and R = CH3 or CD3) were determined by X-ray crystallography. For all four complexes, the carboxamidato group was identified by a strong band in the infrared spectra assignable to the CO stretching vibration. Furthermore, the new carboxamidato ligands were removed from the metal center and isolated in their neutral form as 6-(ArN=CMe)C5H3N-2-C(O)NHAr (where Ar = 4-X-2,6-iPr2C6H2 and X = H, Cl, or Br) and 6-(ArN=CCD3)C5H3N-2-C(O)NHAr(where Ar = 2,6-iPr2C6H3). Taken together, these results indicate that the activation of dioxygen occurs at the ligand and without an overall oxidation state change at the Ni center. The observed ligand-centered oxygenation in these reactions contrasts with the prevailing metal-centered chemistry of redox-active bis(arylimino)pyridine complexes and demonstrates that they can be directly involved in the activation and conversion of a small molecule.

Pages

xii, 111 pages

Bibliography

Includes bibliographical references (pages 108-111).

Copyright

Copyright 2014 Antonio D Manuel

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Chemistry Commons

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