Applications of Paramagnetic NMR Spectroscopy for Monitoring Transition Metal Complex Stoichiometry and Speciation

Abstract

Although it is well established that paramagnetic NMR spectroscopy is a powerful tool to derive structural information, the methodology is still not yet universally applied to paramagnetic small molecule complexes. In this paper paramagnetic 1H NMR spectroscopy is investigated as a convenient method for the experimental inorganic chemist to elucidate solution structures and speciation of small molecule metal complexes derived from 2,6-pyridinedicarboxylic acid as ligand. Spectra of complexes with Oh geometry, in which the spin states of the metal ion range from d3 (Cr3+), d5 (Fe3+), d6 (Fe2+), d7 (Co2+) to d8 (Ni2+), were recorded and analyzed. For all complexes the 1H NMR spectra give well-resolved, easy detectable lines, which depending on the spin state and electron relaxation time of the metal ion and the pH of the solution can be fairly broad. Regardless, the spectra allow complexes of 1:1 and 1:2 stoichiometries to be distinguished in spite of the metal nucleus short nuclear correlation and relaxation times, and the magnitude of the hyperfine shift spread. The pH stability profile and the ability of the complexes to undergo ligand exchange reactions were also investigated for each of the complexes. This work demonstrates that paramagnetic 1H NMR spectroscopy is very useful for characterizing small molecule complexes and their solution chemistry without requiring a detailed analysis of the hyperfine shifts and relaxivities.