Title

Nucleophilic Attack On Organonitrile Complexes Of Pentaamminecobalt(iii) (azide, Base Hydrolysis, Hammett, Tetrazoles)

Date of Award

1985

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Chemistry

Abstract

The effect of coordination of a variety of p-substituted benzonitrile complexes on the ('13)C chemical shift of the nitrile carbon has been found to be constant throughout the range of complexes studied. This has led to a reexamination of the possibility of establishing a Linear Free Energy Relationship (LFER) for the base hydrolysis reaction. A LFER indeed exists if one disregards the p-cyanophenoxy and m-cyanophenoxy complexes. This discrepancy is attributed to the unreliability of the (sigma) values rather than any mechanistic anomaly for these two systems.The kinetics of the base hydrolysis reaction of the pentaamminecobalt(III) complexes of 5-cyanotetrazolate and pyridine bonded 4-cyanopyridine has been investigated and found to be first order in both hydroxide and complex concentration. The enhancement ratio over the free ligand reaction has been found to be rather modest (20 to 50), when compared to the enhancement observed with organonitrile complexes where the nitrile group is directly bonded to the metal (10('6)). This is consistent with the hypothesis that most of the enhancement of the base hydrolysis reaction is due to the metal's role as an electron sink stabilizing the negative charge on the developing imino anion.The kinetics of azide attack have been investigated for the pentaamminecobalt(III) complexes of p-tolunitrile, benzonitrile, p-chlorobenzonitrile, m-formylbenzonitrile, p-nitrobenzonitrile, and terephthalonitrile, and found to be first order in both anionic azide and coordinated nitrile leading to formation of the N-1 bonded tetrazolate complex. A linear free energy relationship has been established between the second order rate constant and Hammett's parameter. Two mechanisms have been proposed which are consistent with the kinetic observations, a concerted 1,3-cycloaddition of the azide across the nitrile bond, or attack of the nitrile carbon by azide forming the imidoyl azide intermediate which subsequently undergoes ring closure. The LFER can be utilized in order to synthesize N-1 bonded tetrazolate complexes more efficiently while minimizing the N-2 bonded impurity.

Keywords

Chemistry, Inorganic

Link to Full Text

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