Crystal and Substituent Effects on Paramagnetic NMR Shifts in Transition-Metal Complexes

Abstract

Nuclear magnetic resonance (NMR) spectroscopy of paramagnetic molecules provides detailed information about their molecular and electron-spin structure. The paramagnetic NMR spectrum is a very rich source of information about the hyperfine interaction between the atomic nuclei and the unpaired electron density. The Fermi-contact contribution to ligand hyperfine NMR shifts is particularly informative about the nature of the metal–ligand bonding and the structural arrangements of the ligands coordinated to the metal center. In this account, we provide a detailed experimental and theoretical NMR study of compounds of Cr­(III) and Cu­(II) coordinated with substituted acetylacetonate (acac) ligands in the solid state. For the first time, we report the experimental observation of extremely paramagnetically deshielded 13C NMR resonances for these compounds in the range of 900–1200 ppm. We demonstrate an excellent agreement between the experimental NMR shifts and those calculated using relativistic density-functional theory. Crystal packing is shown to significantly influence the NMR shifts in the solid state, as demonstrated by theoretical calculations of various supramolecular clusters. The resonances are assigned to individual atoms in octahedral Cr­(acac)3 and square-planar Cu­(acac)2 compounds and interpreted by different electron configurations and magnetizations at the central metal atoms resulting in different spin delocalizations and polarizations of the ligand atoms. Further, effects of substituents on the 13C NMR resonance of the ipso carbon atom reaching almost 700 ppm for Cr­(acac)3 compounds are interpreted based on the analysis of Fermi-contact hyperfine contributions.

Paramagnetic 7Li NMR Shifts and Magnetic Properties of Divalent Transition Metal Silylamide Ate Complexes [LiM{N(SiMe3)2}3] (M2+ = Mn, Fe, Co)

“…In that study, a pronounced effect of the exchange fraction on the hyperfine coupling was found. Nowotny et al employed embedded cluster models and the PBE0 hybrid functional to calculate NMR shifts of Cr­(III) and Cu­(II) compounds . Spin–orbit effects were taken into account with the ZORA approach.…”

“…Nowotny et al employed embedded cluster models and the PBE0 hybrid functional to calculate NMR shifts of Cr(III) and Cu(II) compounds. 70 Spin−orbit effects were taken into account with the ZORA approach. Ott et al calculated the diamagnetic orbital shift, the Fermi-contact shift, and the pseudocontact shift of iron (II) hydrido complexes in solution with B3LYP.…”

Assigning 1H Chemical Shifts in Paramagnetic Mono and Bimetallic Surface Sites using DFT: a Case Study on the Union Carbide Polymerization Catalyst

“…The 1 H MAS NMR spectrum of 1-SiO2-1 in Figure 2 shows a substantial sideband manifold, consistent with large 1 H chemical shift anisotropies associated with the paramagnetic nature of surface Cr sites. [13,15,24] Besides the spinning sidebands, the 1 H ss-NMR spectrum of 1-SiO2-1 shows a weak, highly deshielded signal at 318 ppm, together with a more intense signal around 155 ppm. The latter disappears after long acquisition times (~days) regardless of the temperature of the measurement, indicating the sensitivity of the associated surface species.…”

Solid‐State Characterization of Acetylpyridine Copper Complexes for the Activation of H2O2 in Advanced Oxidation Processes

“…Moreover, when single crystals cannot be obtained, the structure elucidation of the system becomes challenging due to the paramagnetic center, especially if solid-(ss-NMR) and solution state (ls-NMR) nuclear magnetic resonance methods are employed. [18][19][20][21][22][23] For that reason, in this work we report a simple method to obtain two water-soluble copper complexes derived from 3acetylpyridine (3-AcPy) and 4-acetylpyridine (4-AcPy). The characterization and determination of the catalytic activity of such complexes is also reported.…”

“…The synthesis of most of these complexes involves the derivatization of the pyridine ring to obtain Schiff bases, semicarbazones, thiosemicarbazones, etc., which is sometimes difficult and expensive. Moreover, when single crystals cannot be obtained, the structure elucidation of the system becomes challenging due to the paramagnetic center, especially if solid‐( ss ‐NMR) and solution state ( ls ‐NMR) nuclear magnetic resonance methods are employed [18–23] …”