I. Studies on the Coordination of Bis-hydroxybenzamido Chelating Ligations to Chromium, Iron, and Osmium. II. A Survey of the Organometallic Chemistry of Osmium Tetraoxide

Citation

Spies, George Harlan (1985) I. Studies on the Coordination of Bis-hydroxybenzamido Chelating Ligations to Chromium, Iron, and Osmium. II. A Survey of the Organometallic Chemistry of Osmium Tetraoxide. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/s19j-2033. https://resolver.caltech.edu/CaltechTHESIS:01182019-103133548

Abstract

The coordination chemistry of chromium, iron, and osmium with bis-hydroxybenzamido chelating ligands was investigated. Two types of complexes were obtained on reacting CrCl₃ • 6H₂O with 1,2-bis(3,5-dichloro-2-hydroxybenzanido)-ethane ((H)₄CHBA-Et). A neutral dimeric complex, in which two chelates bridge two metal centers, was discovered and characterized by X-ray crystallography. This is the first molecule in which both forms of monodentate organic amide coordination (i.e., O-bound and N-bound coordination) have been found. The N-bound amide was shown to be a better σ-donor. This is the first well-characterized example of N-coordination to chromium(III).

A second complex, which is a precursor to the neutral dimer, has not been fully characterized. It may also be dimeric. Similar complexes, with different bis-hydroxybenzamido ligands, have been examined. Iron(III) species, synthesized from these ligands and FeCl₃, appear to be analogous to the latter chromium(III) compounds.

X-ray crystallographic studies have complemented investigations into the coordination chemistry of these bis-hydroxybenzamido ligands with several transition metals. In addition to the dimeric chromium(III) complex, in which the ligand bridges two metal centers, many osmium(IV) complexes were found with the bis-hydroxybenzamido ligands coordinated to one metal center as a planar tetradentate tetraanion. These structures exhibited the first examples of bonds between osmium(IV) and an N-coordinated organic amide ligand and a phenoxide ligand. One such complex, a µ-oxo dimer, was found with potassium ions in unusually low coordination environments (i.e., four- and five-coordinate). Oxidation of trans-Os( CHBA-Et)(py)₂ subsequently resulted in crystals of a new con1plex with a hydroxy and a methoxy group on the ethylene unit bridging the amide nitrogen atoms of the chelated ligand. The bond distances and angles for this molecule are very similar to those of its precursor, trans-Os(CHBA-Et)(py)₂. The ligand, (H)₄ CHBA-Et, was found to cocrystallize with pyridinium chloride. This structure has provided a benchmark for comparison with complexed chelates; few significant differences were observed.

The fusion of 2,3-bis(2-hydroxy-benzamido)-2,3-butane with (NH₄)₂OsCl₆ (T > 300°C) resulted in the formation of an osmium(IV)-imidazoline complex.

Also examined was the organometallic chemistry of osmium tetraoxide. Oxidation of low valent carbonyl complexes with the quinuclidine adduct of osmium tetraoxide resulted in attack of both the low valent metal and a carbonyl ligand with the formation of µ-CO₂ mixed-valent complexes. The surprising stability of this new type of CO₂ complex is kinetic in origin as den1onstrated by the chemistry of a series of compounds. Decomposition with loss of CO₂ was observed for more labile metal complexes. In the absence of nitrogenous bases, oxidation of Ru(CO)₃(PPh₃)₂ with osmium tetraoxide resulted in the formation of what is believed to be a tetranuclear complex. The low valent metal center and a coordinated carbonyl ligand were oxidized.

One other mode of reactivity was discovered. Oxidative addition of osmium tetraoxide to Pt(PPh₃)₂(C₂H₄) displaced the ethylene ligand with the formation of a µ-oxo bridged tetranuclear complex. These and other potential modes of OsO₄ reactivity with organometallic and inorganic complexes are discussed.

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