Solid-state^185/187Re NMR and GIPAW DFT study of perrhenates and Re₂(CO)₁₀: chemical shift anisotropy, NMR crystallography, and a metal–metal bond

Abstract

Advances in solid-state nuclear magnetic resonance (SSNMR) methods, such as dynamic nuclear polarization (DNP), intricate pulse sequences, and increased applied magnetic fields, allow for the study of systems which even very recently would be impractical. However, SSNMR methods using certain quadrupolar probe nuclei (i.e., I > 1/2), such as (185/187)Re remain far from fully developed due to the exceedingly strong interaction between the quadrupole moment of these nuclei and local electric field gradients (EFGs). We present a detailed high-field (B0 = 21.1 T) experimental SSNMR study on several perrhenates (KReO4, AgReO4, Ca(ReO4)2·2H2O), as well as ReO3 and Re2(CO)10. We propose solid ReO3 as a new rhenium SSNMR chemical shift standard due to its reproducible and sharp (185/187)Re NMR resonances. We show that for KReO4, previously poorly understood high-order quadrupole-induced effects (HOQIE) on the satellite transitions can be used to measure the EFG tensor asymmetry (i.e., ηQ) to nearly an order-of-magnitude greater precision than competing SSNMR and nuclear quadrupole resonance (NQR) approaches. Samples of AgReO4 and Ca(ReO4)2·2H2O enable us to comment on the effects of counter-ions and hydration upon Re(vii) chemical shifts. Calcium-43 and (185/187)Re NMR tensor parameters allow us to conclude that two proposed crystal structures for Ca(ReO4)2·2H2O, which would be considered as distinct, are in fact the same structure. Study of Re2(CO)10 provides insights into the effects of Re-Re bonding on the rhenium NMR tensor parameters and rhenium oxidation state on the Re chemical shift value. As overtone NQR experiments allowed us to precisely measure the (185/187)Re EFG tensor of Re2(CO)10, we were able to measure rhenium chemical shift anisotropy (CSA) for the first time in a powdered sample. Experimental observations are supported by gauge-including projector augmented-wave (GIPAW) density functional theory (DFT) calculations, with NMR tensor calculations also provided for NH4ReO4, NaReO4 and RbReO4. These calculations are able to reproduce many of the experimental trends in rhenium δiso values and EFG tensor magnitudes. Using KReO4 as a prototypical perrhenate-containing system, we establish a correlation between the tetrahedral shear strain parameter (|ψ|) and the nuclear electric quadrupolar coupling constant (CQ), which enables the refinement of the structure of ND4ReO4. Shortcomings in traditional DFT approaches, even when including relativistic effects via the zeroth-order regular approximation (ZORA), for calculating rhenium NMR tensor parameters are identified for Re2(CO)10.

NMR crystallography: structure and properties of materials from solid-state nuclear magnetic resonance observables

“…This demonstrates the particular sensitivity of NMR interactions to the precise positions of atoms in the crystal. Other recent NMR crystallography examples highlight the combined utility of the quadrupolar interaction, chemical shift tensor, and dipolar coupling constant for providing structural insights in metal complexes and sodium diphosphates Widdifield et al, 2015).…”

Solid‐state nuclear magnetic resonance and nuclear quadrupole resonance as complementary tools to study quadrupolar nuclei in solids

“…For instance, for a spin 5/2 nucleus, the observed transitions are typically ±1/2 → ±3/2 and ±3/2 →±5/2. It is to be noted that the overtone transition, ±1/2 → ±5/2, can sometimes be observed . As the Boltzmann spin population is proportional to the difference in the energy levels, the sensitivity of NQR experiments generally increases with the magnitude of the QI.…”

Recent Advances in Theoretical and Physical Aspects of NMR Chemical Shifts

“…It is observed that the observed EFG tensor is quite sensitive to whether the chlorine ligand is bridging, terminal-axial or terminalequatorial. An experimental and theoretical NMR study of 185/187 Re has been carried out on several perrhenates (KReO4, AgReO4, Ca(ReO4)22H2O), ReO3, and Re2(CO)10 ( Widdifield et al, 2015). The 77 Se chemical shift tensor for L-selenocystine has also been measured in the solid state (Struppe et al, 2015).…”