Characterization of Noninnocent Metal Complexes Using Solid-State NMR Spectroscopy: o-Dioxolene Vanadium Complexes

Abstract

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.

Effect of ancillary ligand on electronic structure as probed by 51V solid-state NMR spectroscopy for vanadium–o-dioxolene complexes

“…We note that except for compound 3 , the isotropic chemical shifts for these compounds fall outside of the range of shifts observed for the majority of vanadium(V) complexes, due to the non-innocent nature of the ligands. 33, 57 …”

“…1, 15, 33–39 A hybrid 51 V SSNMR / Density Functional Theory (DFT) approach provides highly sensitive information regarding electronic structure and geometry not only in coordination complexes 1, 15, 33–36, 41–44 but also in large proteins, such-as vanadium-containing haloperoxidases. 37 This sensitivity arises from the fact that 51 V is a half-integer quadrupolar nucleus (I=7/2).…”

“…34 In addition, 51 V SSNMR parameters were effectively used to describe the electronic properties of vanadium complexes containing o -dioxolene ligand and were reported to provide direct information regarding the changes in the HOMO-LUMO gap. 33 …”

“…Vanadium complexes generally have 51 V NMR chemical shifts in the range of −300 ppm to −700 ppm, and linear correlation has been demonstrated between isotropic chemical shifts in the solid state (δ iso ) and in solution (δ) suggesting the same structure of these complex in both phases and is important to the interpretation of the parameters shown here. 33 …”

“…Recent investigations showed that a series of related vanadium(V) compounds containing substituted ancillary ligand, hshed, extended the linear relationships between isotropic chemical shifts in solution (δ) and in the solid state (δ iso ). 33 Interestingly, discrepancies between experimental and calculated solid-state NMR parameters were observed, 33 however most recent work by M. Bühl and colleagues demonstrated that excellent agreement between experiment and theory is achieved when adaptive quantum mechanics/molecular mechanics (QM/MM) treatment is used in order to accurately recapitulate long-range electrostatic effects. 55 In light of these recent theoretical developments enabling accurate computation of 51 V solid-state NMR parameters it becomes particularly important to garner more information on related systems.…”