Nature of NMR Shifts in Paramagnetic Octahedral Ru(III) Complexes with Axial Pyridine-Based Ligands

In recent decades, transition-metal coordination compounds have been extensively studied for their antitumor and antimetastatic activities. In this work, we synthesized a set of symmetric and asymmetric Ru(III) and Rh(III) coordination compounds of the general structure (Na+/K+/PPh4+/LH+) [trans- MIIIL(eq)nL(ax)2] - (M = RuIII or RhIII; L(eq) = Cl, n = 4; L(eq) = ox, n = 2; L(ax) = 4-R-pyridine, R = CH3, H, C6H5, COOH, CF3, CN; L(ax) = DMSO-S) and systematically investigated their structure, stability, and NMR properties. 1H and 13C NMR spectra measured at various temperatures were used to break down the total NMR shifts into the orbital (temperature-independent) and hyperfine (temperature-dependent) contributions. The hyperfine NMR shifts for para-magnetic Ru(III) compounds were analyzed in detail using relativistic density functional theory (DFT). The effects of (i) the 4-R substituent of pyridine, (ii) the axial trans ligand L(ax), and (iii) the equatorial ligands L(eq) on the distribution of spin density reflected in the through-bond (contact) and the through-space (pseudocontact) contributions to the hyperfine NMR shifts of the individual atoms of the pyridine ligands are rationalized. Further, we demonstrate the large effects of the solvent on the hyperfine NMR shifts and discuss our observations in the general context of the paramagnetic NMR spectroscopy of transition-metal complexes.

Automated summary

In recent years, there has been extensive research on the antitumor and antimetastatic activities of transition-metal coordination compounds. This study focused on synthesizing and investigating the structure, stability, and NMR properties of Ru(III) and Rh(III) coordination compounds. By analyzing NMR spectra and using relativistic density functional theory (DFT), the effects of various factors on the hyperfine NMR shifts of para-magnetic Ru(III) compounds were examined. The study also highlighted the significant influence of solvents on hyperfine NMR shifts and discussed the broader implications for paramagnetic NMR spectroscopy of transition-metal complexes.