Dynamic, Multinuclear NMR Spectroscopy of Transition Metal Complexes

Abstract

<p>The use of NMR spectroscopy in inorganic chemistry is growing steadily as high-field, (multinuclear, sophisticated NMR spectrometers become more readily available. NMR of transition metal complexes is developing rapidly as metal nuclei as well as nuclei of ligating atoms become more popular candidates for NMR experiments. In this thesis, dynamic, multinuclear NMR spectroscopy has been carried out on various complexes of Cobalt(III) and Zn(II) as well as on free ligand molecules found in these complexes.</p> <p>The second-sphere hydrogen-bonding interaction with the hexacyanocobaitate(III)anion of substituted phenols, anilines, and benzoic acids has been studied by monitoring Co-59 NMR chemical shifts and linewidths. Analysis of the chemical shifts in order to estimate equilibrium constants has been attempted. The second-sphere complex with para-nitrophenol has also been studied by measuring H-1 and C-13 Ts. It is shown that the lifetime of the complex is longer than its rotational correlation time indicating a complex of possible mechanistic significance.</p> <p>The hexanitrocobaltate(III) anion has been studied by Co-59 NMR, IR and Raman spectroscopies in the solid state and by Raman, Co-59, N-14 and 0-17 NMR, and UU-vis spectroscopies in solution. A comparison of the solid-state and solution results has shown that all six ligands are N-bonded. The primary decomposition step is concluded to be aquation with a lifetime of about three minutes. A charge transfer mechanism is suggested to explain this anomalously high rate of reaction. Analysis of the Co-59 NMR chemical shifts and optical data of related compounds shows that the ligand-field stength of the nitro ligand is markedly variable. An empirical model has been developed for predicting Co-59 chemical shifts of nitro-containing complexes and for complexes containing ligands with different orbital reduction factors. Anomalies in the field dependence of the second-order quadrupole effect in the polycrystalline Co-59 NMR of sodium cobaltinitrite are discussed. The shielding anisotropy of the same compound is shown to be less than 10 ppm.</p> <p>The first application of the Tıɒ method has been made to a metal nucleus, Co-59. Scalar relaxation in cobaltinitrite and trisethylenediaminecobalt(III)has been studied using this technique. Proton Tıɒ measurements have also been carried out to study scalar relaxation in free thiourea and in Zn(tu)₄(ClO₄)₂. The advantages and disadvantages of this method are discussed.</p>