Nuclear Magnetic Resonance and Mechanistic Studies of Cobalt(III) Complexes

Field: Chemistry (Physical Sciences)

<p>This thesis describes a number of closely related studies on aspects of the chemistry of Co(III) complexes. Of particular interest are μ-peroxy Co(III) complexes formed by the addition of molecular oxygen to Co(II) containing solutions. These compounds have attracted much attention, partly because they can serve as models for biological oxygen carriers and partly because of their intrinsic chemical interest. A review of some of the pertinent chemistry is contained in Chapter One. A number of interesting questions which remain unanswered, particularly with respect to the unexpectedly rapid rates of ligand exchange reactions and the mechanisms of electron transfer reactions will be addressed in the present thesis.</p> <p>There are also problems with respect to the characterisation of the complexes in solution particularly in cases where there are a number of isomeric possibilities. ⁵⁹Co NMR has not previously been applied to this area and the thesis reports the initial results of such a study. Some necessary NMR theory is reviewed in Chapter Two. Experimental details for both the chemical and the NMR studies are contained in Chapter Three.</p> <p>The application of ⁵⁹Co NMR to these di-oxygen complexes resulted in several interesting observations and NMR problems. These topics will be dealt with in Chapters Four, Five, and Six. In Chapter Seven, the application of model for assigning ⁵⁹Co resonances to Co di-oxygen complexes is presented. Finally, in Chapter Eight, some mechanistic studies on ligand exchange and electron transfer reactions are described. Two appendices give computer programmes and some results on the characterisation of Fe compounds produced by oxidation with the Co di-oxygen complexes.</p> <p>In order to achieve better resolution, high field experiments were performed using superconducting magnet instruments. In the course of these experiments an anomolous field dependence of the linewidths was observed. The interpretation of this effect is the subject of Chapters Four and Five. In the first of these chapters, linewidth measurements on 22 complexes at 3 different fields are reported. Qualitatively, the results are consistent with relaxation due to chemical shielding anisotropy but calculations show that the observed broadening is two to three orders of magnitude greater than expected from this source. Several other possible explanations are considered. It is concluded that the most likely origin of the effect lies in the contribution of antisymmetric components of the chemical shielding tensor, modulated by an abnormally long correlation time which is associated with the lifetimes of hydrogen bonding interactions. Although the existence of antisymmetric components in the shielding tensor has been recognised as theoretically possible for some time, there has been no previous experimental evidence for their importance. This model leads to two predictions - that there will be a large solvent dependence of the field effect and that the very unusual result of T₁ less than T₂ will be found if this relaxation mechanism is dominant. Measurements showing T₁ less than T₂ have not previously been reported. Chapter Five reports the results of experiments designed to check these predictions. The data support the proposed relaxation mechanism. Nine compounds have been examined in a number of different solvents and the results correlated with the Gutmann donor numbers of the solvents. The contributions of quadrupolar relaxation and scalar relaxation of the second kind to the linewidths are assessed. The measurement of very short T₁'s (where T₁ becomes comparable with the instrumental pulse widths) presents some hazards but the predicted values relative to T₂ have been observed. These results suggest a new approach to the study of short lived hydrogen bonded second sphere intermediates of kinetic significance in transition metal complex reactions.</p> <p>Chapter Six returns to the problem of assigning ⁵⁹Co resonances in complex mixtures. A model is developed for estimating both the chemical shift and the linewidth of an octahedrally coordinated Co(III) complex with any ligand arrangement. This model is tested by application to some 50 known complexes. ⁵⁹Co data on a number of these complexes have not been reported previously. The relative chemical shifts, particularly for complexes with similar ligands, can be calculated with some reliability. The agreement between calculated and observed linewidths is only qualitative but the results are nevertheless useful in making isomeric assignments. In Chapter Seven, this model is applied to Co di-oxygen complexes. Examples involving complex isomeric mixtures are discussed and its use in identifying intermediates produced during the oxygenation of Co(II) solutions containing a variety of ligands is demonstrated.</p> <p>Finally, in Chapter Eight, some mechanistic studies on Co di-oxygen complexes are described. The mechanism of ligand exchange reactions involving bis(salicylaldehyde)ethylenediimine Co(III) dioxygen complexes has been investigated by stopped-flaw spectrophotometry. The initial step is dissociative ligand exchange. This result is contrasted with literature reports on similar reactions involving complexes containing amine ligands for which the initial step is electron transfer to the Co. The reasons for the high lability of this class of complexes are discussed. The reduction of several amine containing Co di-oxygen complexes by Fe(II) has been followed by stopped-flow spectrophotometry and by ⁵⁹Co NMR. In this case, the mechanistic results agree with literature reports on similar complexes but the interesting observation has been made that the different isomers react at different rates. The structures of the isomers have been assigned using ⁵⁹Co NMR.</p> <p>Some of the results contained in this thesis are reported in the following publications:</p> <p>i) Steve C.F. Au-Yeung and Donald R. Eaton, "Characterization of Cobalt-dioxygen complexes by means of High Field ⁵⁹Co NMR" Inorg. Chim. Acta, 76.,L141-144 (l983).</p> <p>ii) Steve C.F. Au-Yeung and Donald R. Eaton, "Chemical Shift Anisotropy and Second Sphere Hydrogen Bonding in Co(III) Complexes". J. Magn. Reson., 52(3), 351-365 (1983).</p> <p>iii) Steve C.F. Au-Yeung and Donald R. Eaton. "The Solvent and Field Dependence of ⁵⁹Co NMR linewidths", J. Magn. Reson., 52(3), 366-373 (1983).</p> <p>iv) Steve C.F. Au-Yeung, Richard J. Buist and Donald R. Eaton. "Spin-Lattice Relaxation of Low Symmetry Co(IIl) Complexes", J. Magn. Reson., in press.</p> <p>v) Steve C.F. Au-Yeung and Donald R. Eaton. "A Model for Estimating ⁵⁹Co Chemical Shifts and Linewidths and its application to Cobalt di-oxygen complexes", Can. J. Chem., 61, 2431-41 (1983).</p> <p>vi) Steve C.F. Au-Yeung and Donald R. Eaton. "The Kinetics and Mechanism of the Reaction of μ-Peroxy-bis(Salicylaldehyde)ethylenediimine Co(IlI) with Cyanide and Thiocyanate Ions". Inorg. Chem., in press.</p>