51V solid-state NMR (SSNMR) studies of a series of non-innocent vanadium(V) catechol complexes have been conducted to evaluate the possibility that 51V NMR observables, quadrupolar and chemical shift anisotropies, and electronic structures of such compounds can be used to characterize these compounds. The vanadium(V) catechol complexes described in these studies have relatively small quadrupolar coupling constants, which cover a surprisingly small range from 3.4 to 4.2 MHz. On the other hand, isotropic 51V NMR chemical shifts cover a wide range from −200 ppm to 400 ppm in solution and from −219 to 530 ppm in the solid state. A linear correlation of 51V NMR isotropic solution and solid-state chemical shifts of complexes containing non-innocent ligands is observed. These experimental results provide the information needed for the application of 51V SSNMR spectroscopy in characterizing the electronic properties of a wide variety of vanadium-containing systems, and in particular those containing non-innocent ligands and that have chemical shifts outside the populated range of −300 ppm to −700 ppm. The studies presented in this report demonstrate that the small quadrupolar couplings covering a narrow range of values reflect the symmetric electronic charge distribution, which is also similar across these complexes. These quadrupolar interaction parameters alone are not sufficient to capture the rich electronic structure of these complexes. In contrast, the chemical shift anisotropy tensor elements accessible from 51V SSNMR experiments are a highly sensitive probe of subtle differences in electronic distribution and orbital occupancy in these compounds. Quantum chemical (DFT) calculations of NMR parameters for [VO(hshed)(Cat)] yield 51V CSA tensor in reasonable agreement with the experimental results, but surprisingly, the calculated quadrupolar coupling constant is significantly greater than the experimental value. The studies demonstrate that substitution of the catechol ligand with electron donating groups results in an increase in the HOMO-LUMO gap and can be directly followed by an upfield shift for the vanadium catechol complex. In contrast, substitution of the catechol ligand with electron withdrawing groups results in a decrease in the HOMO-LUMO gap and can directly be followed by a downfield shift for the complex. The vanadium catechol complexes were used in this work because the 51V is a half-integer quadrupolar nucleus whose NMR observables are highly sensitive to the local environment. However, the results are general and could be extended to other redox active complexes that exhibit similar coordination chemistry as the vanadium catechol complexes.

Most known vanadium(V) complexes with redox-active o-dioxolene ligands are non-innocent. Since the vanadium(V) complex [VO(gsal)(HCat)] is innocent, its properties were investigated by 51 V solid-state NMR spectroscopy and density functional theory (DFT). The innocent ligand behavior manifested itself by the upfield isotropic chemical shift and large negative chemical shift anisotropy observed in the solid state. The electronic structure and NMR spectroscopic parameters of this complex were addressed by DFT calculations and found to be consistent with the NMR spectroscopic observa-4646 asymmetry parameter (δ σ , η σ ), together with the relative quadrupolar and CSA tensor orientations described by the Euler angles (α, β, γ) were extracted by numerical simulations of the full manifold of the spinning sidebands corresponding to the central and satellite transitions with the SIMPSON [30] software package. The best-fit parameters are summarized in Table 1.In this work, we use the Haeberlen-Mehring-Spiess convention to define the CSA tensor elements, [14] where |δ xx -δ iso | Յ |δ yy -δ iso | Յ |δ zz -δ iso |, δ iso = (δ xx + δ yy + δ zz )/3, δ σ = δ zz -δ iso , and η σ = (δ yyδ xx )/(δ zz -δ iso ). In this representation, δ ii corresponds to the principal components of the chemical shift tensor. The electric field gradient (EFG) tensor parameters are defined as C Q = eQV ZZ /h and η Q = (V XX -V YY )/V ZZ where |V ZZ | Ն |V YY | Ն |V XX |, e is the electronic charge, and h is Planck's constant.DFT Calculations: Quantum chemical calculations of the NMR spectroscopic parameters for [VO(gsal)(HCat)] and the hypothetical deprotonated complex [VO(gsal)(Cat)]were performed with DFT in Gaussian09. [31] The 51 V magnetic shielding and EFG tensors for both molecules were computed by using B3LYP and BLYP functionals. [32][33][34] For each method, calculations were conducted with three basis sets: (i) 6-311+G, (ii) TZV, [35] and (iii) augmented Wachters basis set on V [36,37] and 6-31* [38][39][40] on all other elements. Calculations were carried out by using geometry-optimized structures at the B3LYP/TZV level, and the Cartesian coordinates for the geometry-optimized [VO(gsal)(HCat)] and [VO(gsal)(Cat)]are presented in Table 1. The nuclear magnetic shielding calculations