Hyperfine Effects in Ligand NMR: Paramagnetic Ru(III) Complexes with 3-Substituted Pyridines

Abstract

NMR spectroscopy is an indispensable tool in characterizing molecular systems, including transition-metal complexes. However, paramagnetic transition-metal complexes such as those with ruthenium in the +3 oxidation state are troublemakers because their unpaired electrons induce a fast nuclear spin relaxation that significantly broadens their NMR resonances. We recently demonstrated that the electronic and spin structures of paramagnetic Ru(III) systems can be characterized in unprecedented details by combining experimental NMR results with relativistic density-functional theory ( Novotny et al. J. Am. Chem. Soc. 2016 , 138 , 8432 ). In this study we focus on paramagnetic analogs of NAMI with the general structure [3-R-pyH]trans-[RuCl(DMSO)(3-R-py)], where 3-R-py stands for a 3-substituted pyridine. The experimental NMR data are interpreted in terms of the contributions of hyperfine (HF) NMR shielding and the distribution of spin density calculated using relativistic DFT. The DFT computational methodology is evaluated, and the effects of substituents, environment, and relativity on the hyperfine shielding are discussed. Particular attention is paid to the analysis of the fundamental Fermi-contact (FC), spin-dipole (SD), and paramagnetic spin-orbit (PSO) terms that contribute to the hyperfine H andC NMR shifts of the individual atoms in the pyridine ligands and the spin-polarization effects in the ligand system that are linked to the character of the metal-ligand bond. The individual HF shielding terms are systematically discussed as they relate to the traditional, but somewhat mixed, contact and pseudocontact NMR contributions used extensively by experimental spectroscopists in biomolecular NMR and the development of PARACEST magnetic-resonance contrast agents.

Electron-Spin Structure and Metal–Ligand Bonding in Open-Shell Systems from Relativistic EPR and NMR: A Case Study of Square-Planar Iridium Catalysts

“…where L HFi (derived from the isotropic g and A) and L HFa (derived from the g and A anisotropy) contain the traditional contact and pseudocontact contributions, 6,27,40 respectively. Note that according to the splitting of the hyperfine coupling tensor L into FC, SD, and PSO terms, their individual contributions to the hyperfine NMR shift can be calculated and analyzed separately (for example, see ref.…”

“…Note that according to the splitting of the hyperfine coupling tensor L into FC, SD, and PSO terms, their individual contributions to the hyperfine NMR shift can be calculated and analyzed separately (for example, see ref. 27 ).…”

“…contributions. 27,29 The results are summarized in Table 5. Significant differences in the individual terms between compounds 1 and 3, including the opposite signs of PSO for ligand atom L4, opposite signs of FC for C2 and C3, and large hyperfine NMR shifts for H2 in 3 vs H3 in 1 are highlighted in bold.…”

Remarkable reversal of 13C-NMR assignment in d1, d2 compared to d8, d9 acetylacetonate complexes: analysis and explanation based on solid-state MAS NMR and computations

“…Marek and coworkers have extensively studied 4d and 5d metal-organic complexes to explain the observed pNMR shifts based on electronic structure. 37,38,43,44 Recently, by variation of both ligand and metal oxidation states, the effects on the NMR shifts could be explained based on the contribution of the different SOMOs and total spin density plots. 38 The objective of the present work is to investigate the effect of the 3d-electron configuration in a series of structurally related acetyl acetonate (acac) complexes with different transition metal ions depicted in Fig.…”

Ab initio paramagnetic NMR shifts via point-dipole approximation in a large magnetic-anisotropy Co(ii) complex

“…18 The importance of this correction for NMR shieldings in open-shell molecules has been recently pointed out, for example, by Marek and coworkers. 19 The HFC tensors, together with g-and ZFS tensors, are incorporated in the expression for pNMR shielding tensors as detailed in ref. 11 and 20.…”