Unpaired Spin Densities from NMR Shifts and Magnetic Anisotropies of Pseudotetrahedral Cobalt(II) and Nickel(II) Vinamidine Bis(chelates)

Abstract

The distribution of unpaired electron spin over all regions of the organic ligands was extracted from the large positive and negative 1H and 13C NMR paramagnetic shifts of the title complexes. Owing to benevolent line broadening and to very high sensitivities of approximately 254,000 and approximately 201,000 ppm/(unpaired electron spin) for Co(II) and Ni(II), respectively, at 298 K in these pseudotetrahedral bis(N,N'-chelates), spin transmission through the sigma- (and orthogonal pi)-bonding system of the ligands could be traced from the chelate ring over five to nine sigma bonds. Most of those "experimental" spin densities DeltarhoN (situated at the observed nuclei) agree reasonably well with quantum chemical DeltarhoDFT (DFT = density functional theory) values and provide an unsurpassed number of benchmark values for the quality of certain types of modern density functionals. The extraction of DeltarhoN became possible through the unequivocal separation of the nuclear Fermi contact shift components from the metal-centered pseudocontact shifts, which are proportional to the anisotropy Deltachi of the magnetic susceptibility: Experimental Deltachi values were obtained in solution from measured deuterium quadrupole splittings in the 2H NMR spectra of two deuterated model complexes and were found to be nonlinear functions of the reciprocal temperature. This provided the reliable basis for predicting metal-centered pseudocontact shifts for any position of a topologically well-defined ligand at varying temperatures. The related ligand-centered pseudocontact shifts were sought by using the criterion of their expected nonlinear dependence on the reciprocal temperature. However, their contributions could not be differentiated from other small effects close to the metal center; otherwise, they appeared to be smaller than the experimental uncertainties. The free activation energy of N-aryl rotation past a vicinal tert-butyl substituent in the Ni(II) vinamidine bis(N,N'-chelates) is DeltaG++(+74 degrees C) approximately 17.0 kcal/mol and past a vicinal methyl group DeltaG++(-6 degrees C) approximately 13.1 kcal/mol.

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