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

Abstract

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 + /PPh 4 + /LH + ) [trans-M III L(eq) n L(ax) 2 ] − (M = Ru III or Rh III ; L(eq) = Cl, n = 4; L(eq) = ox, n = 2; L(ax) = 4-R-pyridine, R = CH 3 , H, C 6 H 5 , COOH, CF 3 , CN; L(ax) = DMSO-S) and systematically investigated their structure, stability, and NMR properties. 1 H and 13 C 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 paramagnetic 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.

Mechanisms of Ligand Hyperfine Coupling in Transition-Metal Complexes: σ and π Transmission Pathways

“…Consequently, the unpaired electron resides in a SOMO of different spatial symmetry relative to the Ru–N bond, Figure . Specifically, the unpaired electron is situated in a d xz -type molecular spin–orbital (MSO) in compound 1 (X = F), whereas in compound 5 (X = BH 2 ), it occupies an equatorial d xy -based MSO, akin to NAMI analogs reported previously. ,,, …”

“…Specifically, the unpaired electron is situated in a d xz -type molecular spin−orbital (MSO) in compound 1 (X = F), whereas in compound 5 (X = BH 2 ), it occupies an equatorial d xy -based MSO, akin to NAMI analogs reported previously. 25,35,37,52 The difference in the orientation of the SOMO between compounds 5 and 1 reflects distinct Ru−N bonding characteristics in these two compounds, 53−55 and results in different transmission mechanisms and a reversed π-polarization of pyridine in these compounds (Figure 10). Note that the change in the Ru−N bond distance also leads to a variation in the orientation of the pyridine plane relative to that of the equatorial ligands (Cl and NH 3 ).…”

Paramagnetic Effects in NMR Spectroscopy of Transition-Metal Complexes: Principles and Chemical Concepts

“…The character of the ligands can influence the distribution of spin density around the magnetic M and the transmission of spin density toward the spectator atom(s). This can be classified as a cis or equatorial-to-axial ligand effect (weak) involving mostly neighbor-atom polarization and a hyperfine trans effect (strong) 40 with two ligands competing for one d orbital (e.g., d z 2 ) of the M. 52 This leads to changes in the M–L bond distance and covalency upon the change of the trans ligand and simultaneously modulates the mechanism of the spin transmission and the ligand hyperfine coupling, as shown by the example of the iridium compounds in Figure 12 . Because of the relatively weak bond between the trans chloride and the iridium in compound IrCl , the Ir–N1 bond is strong, with significant delocalization of the spin density into the π-space of N1.…”

“… Correlation of the (a) Hammett constant, (b) Hirschfeld charge, and (c) atomic spin population with the hyperfine 1 H or 13 C NMR shift. Adapted with permission from refs ( 52 ) and ( 1 ). Copyright 2016 and 2023 American Chemical Society.…”

“…Effect of Substituents on the Aromatic Ligand.The character (electronic properties) of the substituents influences the distribution of electron density on the ligand and by that also the spin transmission from M toward the ligand atoms. The effect of the substituent on the electron (and spin) density is shown by correlating the δ L HF with the Hammett constant,52 Hirshfeld charge, 1 and atomic spin population1 in Figure 13. Depending on the position of the electron donor (acceptor), the concentration (depletion) of the atomic electron density can enhance or diminish the spin polarization and the FC contribution to δ L HF .…”