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Macrocylic tetraamido-N ligands that stabilize high valent complexes of chromium, manganese, iron, cobalt, nickel, and copper

Citation

Uffelman, Erich Stuart (1992) Macrocylic tetraamido-N ligands that stabilize high valent complexes of chromium, manganese, iron, cobalt, nickel, and copper. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-06012005-153821

Abstract

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High valent middle and later transition metal centers tend to oxidatively degrade their ligands. A series of ligand structural features that prevent discovered decomposition routes is presented. The result of the iterative design, synthesis, and testing process described are the macrocyclic tetraamides H4MAC* and H4DEMAMPA-DCB. H4MAC* and H4DEMAMPA-DCB are the parent acids of the macrocyclic tetraamido-N ligands [...] and [...], which are shown to stabilize high valent middle and later transition metal complexes unavailable in other systems. The crystal structures of H4MAC* and a copper complex of one of its synthetic precursors reveal intramolecular and intermolecular hydrogen-bonding patterns which are relevant to recent developments in the ordering effects of hydrogen-bonding on solution and solid state structures. The synthetic value of these ordering effects is discussed.

Two chromium(V) oxo complexes, [...] and [...], have been synthesized and characterized by X-ray crystallography and IR and EPR spectroscopies. Because exchange of the oxo ligand with water is slow, the easily synthesized, stable, crystalline [...]O-labeled diperoxide [...], was prepared and used to conveniently synthesize [...]O-labeled oxo complexes in high yields. The bonding of the two unique oxidation resistant macrocyclic tetraamides to chromium is compared. The structural and EPR properties are consistent with a chromium centered radical in each case and suggest that a chromium(V) oxidation state assignment is equally appropriate whether the ancillary ligand is the innocent [...] or the potentially noninnocent [...]. Both oxo complexes contain nonplanar amide groups. The distortions in [...] are more marked, and it is a unique species in containing four distinctly nonplanar amides.

The synthesis and characterization of the first water-stable Mn(V) monooxo complex is described. [...] has been isolated as a green crystalline material and characterization includes [...] NMR, IR, resonance Raman, and an X-ray crystal structure determination. The first definitive assignment of a [...] IR stretch [...] is made.

The first mononuclear five-coordinate Fe(IV) complex to be isolated as a crystalline compound and structurally characterized, [...], is described. The species has a square pyramidal structure. Cyclic voltammetry of [...] shows a reversible [...] couple at [...] vs. [...] with [...] (0.1 M) as supporting electrolyte. The Mossbauer spectrum of [...] obtained at 150 K has [...] and an isomer shift, [...], lending film support to the iron(IV) oxidation state assignment. Data on [...] are provided for comparison.

The synthesis and characterization of the first macrocyclic square planar Co(III) complex is described. [...] has been isolated as a red crystalline material and characterization includes a [...] spectrum of the lithium salt and an X-ray crystal structure determination of the [...] salt. The cobalt complex contains one significantly nonplanar amido-N ligand where the source of the nonplanarity appears to be a simple mismatch between the geometrical features of the macrocycle and the structural requirements of the square planar metal center, a unique case of amide nonplanarity in inorganic chemistry.

The first crystallographically characterized neutral square planar complex of cobalt in an oxidation state higher than [...], [...], is reported. Structural data for this new class of compounds indicate that the macrocycle in [...] is noninnocent; however, EPR data in toluene at 5.9 K [...] show that the metal center is the primary residence site of the unpaired electron. [...] is a stable, yet potent, oxidant which is soluble in benzene and slightly soluble in pentane. The [...] couple is reversible and found at 0.550 V vs. [...] in [...]. [...] slowly oxidizes water, yielding [...], which may also be prepared by the reaction of [...] with [...]. Both the redox and the acid/base chemistries of [...] are reversible. Electrochemical and EPR data are also presented for other derivatives.

The first square planar nickel(III) complex with an innocent ligand to be structurally characterized, [...], is described: All four Ni(III)N bond distances are significantly shorter [...] than any known Ni(III)-N bond distance. The average Ni-N distance in [...] is [...] and the shortest of the four is 1.825(4). [...] EPR and UV-vis evidence are presented which indicate that the four-coordinate structure found for the solid state form is also present in solutions of non-coordinating solvents, and in noncoordinating glasses at 4 K. In [...], five-coordinate species can be produced at [...] with cyanide ion [...] = [...], and at 77 K with pyridine, 2,6-lutidine, and [...], but not with [...], [...], THF (neat), [...] (neat), MeCN (neat), [...] (neat), or acetone (neat) which do not coordinate. The EPR spectra of the four coordinate complex in glasses of [...]/toluene or [...] are of significance to nickel bioinorganic chemistry since [...], the opposite to the usually accepted EPR signature for square planar nickel(III). The Ni(III/II) couple of [...] is reversible and occurs at -0.58 V vs. [...] supporting electrolyte, 0.13 V vs. NHE showing that an abiological square planar ligand complement consisting of four deprotonated amides can produce a marked stabilization of four-coordinate nickel(III). Data on [...] are provided for comparison.

The synthesis of copper(III) complexes with reversible cathodic and anodic waves is described. The low values for the Cu(III/II) couples (-0.815 V vs. [...]) and the high potentials for the [...] couples (ca. +0.830 V vs. [...]) are presented.

The Dunitz amide nonplanarity analysis is applied to the crystal structures presented to illustrate the unique bonding that occurs in some of the macrocyclic compounds.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Unknown, Unknown
Thesis Committee:
  • Unknown, Unknown
Defense Date:19 August 1991
Record Number:CaltechETD:etd-06012005-153821
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-06012005-153821
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:2359
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:01 Jun 2005
Last Modified:26 Dec 2012 02:50

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