Solid‐State NMR Characterization of Paramagnetic Bis(L‐valinato)copper(II) Stereoisomers – Effect of Conformational Disorder and Molecular Mobility on ¹³C and ²H Fast Magic‐Angle Spinning Spectra

Abstract

The solid‐state 13C and 2H NMR spectra of paramagnetic anhydrous trans‐bis(L‐valinato)copper(II) and cis‐aquabis(L‐valinato)copper(II) complexes have been obtained. Under the very fast MAS conditions, both the 13C and 2H MAS spectra were well enough resolved to allow the easy distinction between the trans and cis stereoisomers. The conformational disorder observed previously in the X‐ray structure of the cis isomer was also reflected in the 13C and 2H MAS spectra. Variable‐temperature 2H MAS spectra of differently deuterated ligand species, that is, the copper(II) complexes with L‐[D2]valine, L‐[D8]valine, and L‐[D10]valine, suggested the dynamic nature of this disorder in the aqua cis isomer and confirmed static ND2 deuterons in the anhydrous trans isomer. Quantum chemical DFT/B3LYP calculations of the 13C hyperfine (Fermi contact) shifts of the paramagnetic term were useful as assignment aids in the interpretation of the 13C MAS spectra.

Computation of NMR Shifts for Paramagnetic Solids Including Zero-Field-Splitting and Beyond-DFT Approaches. Application to LiMPO4 (M = Mn, Fe, Co, Ni) and MPO4 (M = Fe, Co)

“…NMR spectroscopy of extended paramagnetic solids is an important and active field that has received increasing interest in recent years due to various technical improvements, in particular the use of high-frequency methods and fast magic-angle spinning. − One particular field that has motivated our recent foray into computations of NMR shifts for paramagnetic solids (“pNMR shifts”) is lithium-ion battery materials, which feature paramagnetic 3d transition-metal centers. As the paramagnetic systems can feature large hyperfine shifts and paramagnetic line broadening, the interpretation of their NMR spectra is invariably more complicated than for diamagnetic materials.…”

Disorder at the Chiral Cα Center and Room-Temperature Solid-State cis–trans Isomerization; Synthesis and Structural Characterization of Copper(II) Complexes with d-allo,l-Isoleucine

“…Their aim was to complement the X-ray diffraction results and contribute to the method development for the characterization of low-molecular-weight paramagnetic compounds. So far, the MAS ssNMR spectra showed an ability to capture the internal rotations and geometric isomerism of the cis and trans copper­(II) complexes with l -valine, l -alanine, dl -alanine , and glycine …”

“…The DFT method with the unrestricted B3LYP hybrid density functional was used for the calculations with the following basis set combination: a Wachters’ basis (62111111/3311111/3111) for Cu, 6-311G* for O, N, and C atoms, 6-31G for H atoms of the central cluster unit, and 3-21G* for the noncentral cluster-unit atoms. The choice of the functional/basis set used is based on the study of Oldfield and co-workers on the principal electronic interactions in the MAS ssNMR shifts of trans -bis­( d,l -alaninato)­copper­(II) hydrate clusters and on our previous reassuring results for the prediction and assignment of 13 C and 1 H chemical shifts of the cis and trans copper­(II) complexes with l -valine, l -alanine, dl -alanine, and glycine . All DFT/B3LYP calculations were performed with the Gaussian 09 program package…”

“…The 13 C and 2 H NMR chemical shift can be related to the diamagnetic and FC terms if other contributions in the hyperfine shift (such as the pseudocontact term) can be neglected. ,− Accordingly, eq defines the calculated 13 C and 2 H chemical shift δ calcd as follows where δ diam is the diamagnetic shift. For δ diam , the experimental diamagnetic shift values of the crystalline free ligands are used.…”

“…For δ diam , the experimental diamagnetic shift values of the crystalline free ligands are used. Namely, it was previously demonstrated , that the diamagnetic shift values measured for the crystalline free ligands were a suitable approximation for δ diam in eq . Specifically, they yielded better reproduction of the measured hyperfine chemical shifts than the DFT/B3LYP calculated diamagnetic shifts for Cu­(aa) 2 did. , …”