Characterization of noninnocent metal complexes using solid-state NMR spectroscopy: o-dioxolene vanadium complexes

Abstract

(51)V solid-state NMR (SSNMR) studies of a series of noninnocent vanadium(V) catechol complexes have been conducted to evaluate the possibility that (51)V 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 (51)V NMR chemical shifts cover a wide range from -200 to 400 ppm in solution and from -219 to 530 ppm in the solid state. A linear correlation of (51)V NMR isotropic solution and solid-state chemical shifts of complexes containing noninnocent ligands is observed. These experimental results provide the information needed for the application of (51)V SSNMR spectroscopy in characterizing the electronic properties of a wide variety of vanadium-containing systems and, in particular, those containing noninnocent ligands and that have chemical shifts outside the populated range of -300 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 (51)V SSNMR experiments are a highly sensitive probe of subtle differences in electronic distribution and orbital occupancy in these compounds. Quantum chemical (density functional theory) calculations of NMR parameters for [VO(hshed)(Cat)] yield a (51)V chemical shift anisotropy 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 (51)V 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 coordination chemistry similar to that of the vanadium catechol complexes.

Figure 1

Redox non-innocent vanadium(V)-catechol complex (3,5-di-tertiarybutylcatecholato- -{N-(2-methylpyridine)-3-methoxysalicylideneaminato} oxovanadium(V) (SJZ00108).

Figure 2

Comparison between ⁵¹V isotropic chemical shifts obtained from solid-state and solution NMR for all the compounds reported thus far in the literature demonstrating a missing domain in the solid-state NMR investigation.

Figure 3

Qualitative presentation of molecular orbital energies and electronic excitation in free and vanadium-coordinated o-dioxolenes. For comparison separation between HOMO-LUMO for redox innocent ligands is also shown (left). The solid arrows correspond to the lowest energy excitation in each system.

Figure 4

Molecular structures of the four vanadium(V)-o-dioxolene compounds under investigation. [VO(hshed)(Cat)] (1a), [VO(hshed)(DTBCat)] (1b), [VO(hshed)(TBCat)] (1c), and [VO(acac)(TCCat)] (2).

Figure 5

⁵¹V solid-state NMR spectra of the four vanadium(V)-o-dioxolene compounds of the series V^VO-hshed (1a – 1c) (a–c) and [VO(acac)(TCCat)] (2) (d) obtained at a magnetic field of 9.4 T with MAS frequency of 17 kHz. 8192 scans were accumulated for each spectrum, and the pulse delay was 1.0 s. Experimental spectra are shown in black and best-fit simulated spectra are shown in red. The simulated spectra were obtained from SIMPSON using the NMR parameters listed in Table 1.

Figure 6

Experimental (top) and simulated (bottom) ⁵¹V solid-state NMR spectra of [VO(hshed)(Cat)] acquired at the MAS frequencies of (a) 7 kHz, (b) 13 kHz, (c) 17 kHz and (d) 20 kHz. The spectra were simulated with the following parameters: C_Q = 4.0 ± 0.1 MHz; δ_σ= −243 ± 30 ppm; η_Q = 1.0 ± 0.05; η_σ = 0.93 ± 0.05; α = 81 ± 10; β= 70 ± 15; γ = 87 ± 15.

Figure 7

Plot of the solution ⁵¹V NMR isotropic chemical shifts vs. experimentally obtained solid-state NMR chemical shifts for all the compounds reported hitherto in the literature along with the results reported in this study establishing a linear correlation between the different methods of investigations; solid state versus solution.