⁵¹V Solid-State Magic Angle Spinning NMR Spectroscopy and DFT Studies of Oxovanadium(V) Complexes Mimicking the Active Site of Vanadium Haloperoxidases

Abstract

A series of 11 oxovanadium(V) complexes mimicking the active site of vanadium haloperoxidases have been investigated by (51)V magic angle spinning NMR spectroscopy and density functional theory (DFT). The MAS spectra are dominated by the anisotropic quadrupolar and chemical shielding interactions; for these compounds, C(Q) ranges from 3 to 8 MHz, and delta(sigma) is in the range 340-730 ppm. The quadrupolar coupling and chemical shielding tensors as well as their relative orientations have been determined by numerical simulations of the spectra. The spectroscopic NMR observables appear to be very sensitive to the details of the electronic and geometric environment of the vanadium center in these complexes. For the four crystallographically characterized compounds from the series, the quadrupolar and chemical shielding anisotropies were computed at the DFT level using two different basis sets, and the calculated tensors were in general agreement with the experimental solid-state NMR data. A combination of (51)V solid-state NMR and computational methods is thus beneficial for investigation of the electrostatic and geometric environment in diamagnetic vanadium systems with moderate quadrupolar anisotropies.

Investigating the Vanadium Environments in Hydroxylamido V(V) Dipicolinate Complexes Using 51V NMR Spectroscopy and Density Functional Theory

“…Note that δ 11 has a deviation within experimental error, δ 22 deviates by 100 to 150 ppm, while δ 33 deviates by 330 to 430 ppm. Previously, quantum chemical calculations of the chemical shift anisotropy of vanadium coordination complexes have shown good agreement with experiment for complexes with O and N coordinated to the V. 26 The orientation of the tensors in the molecular frame, which is not available from the NMR experiments, has also been obtained from the calculations. However, given the errors in the EFG asymmetry parameter the reliability of the EFG orientation obtained from the calculations is questionable.…”

“…Furthermore, the solution 51 V NMR studies of these compounds demonstrated that the isotropic chemical shifts change upon modification of the hydroxylamido ligand, whereas only small changes were observed when the dipicolinate group was functionalized. However, knowledge of the isotropic chemical shifts in solution are insufficient for deriving information about the three-dimensional electronic and geometric structure of the vanadium site, 20,26 and therefore the anisotropic EFG and chemical shift tensor parameters measured here by SSNMR and calculated using DFT are important.…”

“…26 The sign of C Q is another parameter not available from 51 V NMR spectroscopy but available from the DFT calculations. The calculations indicate that C Q is positive for all three complexes.…”

“…21,22,23 The investigations of vanadium organometallic and coordination compounds have illustrated that valuable information about the molecular and electronic structure can be obtained by measuring the anisotropy of the 51 V chemical shift tensor and the electric field gradient (EFG) tensor; 51 V is a spin I = 7/2 quadrupolar nucleus. 24,25,26 The 51 V chemical shifts span a range of approximately 3500 ppm making it a sensitive probe of structure. 19 Typical quadrupolar coupling constants, C Q , are small compared to many other half-integer quadrupolar nuclei because of the small nuclear quadrupole moment of 51 V, −4.8 fm 2 ; 27 C Q values for highly coordinated vanadium sites are typically less than 15 MHz.…”

“…Good agreement between experimental and calculated NMR parameters has been obtained for vanadium. 26 To better understand the relationships between the NMR parameters and the reactivity at the vanadium center, more experimental and computational NMR investigations of systems with variations in the electronic properties of ligands and vanadium are required.…”