4-Amino- and 4-Nitrodipicolinatovanadium(V) Complexes and Their Hydroxylamido Derivatives:  Synthesis, Aqueous, and Solid-State Properties

Abstract

A number of 4-substituted, dipicolinatodioxovanadium(V) complexes and their hydroxylamido derivatives were synthesized to characterize the solid state and solution properties of five- and seven-coordinate vanadium(V) complexes. The X-ray crystal structures of Na[VO2dipic-NH2].2H2O (2) and K[VO2dipic-NO2] (3) show the vanadium adopting a distorted, trigonal-bipyramidal coordination environment similar to the parent coordination complex, [VO2dipic]- (1), reported previously as the Cs+ salt. The observed differences in the chemical shifts of the complexes both in the 1H (ca. 0.7-1.4 ppm) and 51V (ca. 1-11 ppm) NMR spectra were consistent with the electron-donating or electron-withdrawing properties of the substituent groups, respectively. Stoichiometric addition of a series of hydroxylamine ligands (H2NOH, MeHNOH, Me2NOH, and Et2NOH) to complexes 1-3 led to the formation of seven-coordinate vanadium(V) complexes. The X-ray crystal structure of [VO(dipic)(Me2NO)(H2O)].0.5H2O (1c) was found to be similar to the previously characterized complexes [VO(dipic)(H2NO)(H2O)] (1a) and [VO(dipic)(OO-tBu)(H2O)]. While only slight differences in the 1H NMR spectra were observed upon addition of the hydroxylamido ligand, the signals in the 51V NMR spectra change by up to 100 ppm. The addition of the hydroxylamido ligand increased the complex stability of complexes 2 and 3. Evidence for a nonstoichiometric redox reaction was found for the monoalkyl hydroxylamine ligand. The reaction of an unsaturated five-coordinate species with a hydroxylamine to form a seven-coordinate vanadium complex will, in general, dramatically increase the amounts of the vanadium compound that remain intact at pH values near neutral.

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

“…Therefore, a comparison of the solid-state and solution chemical shifts is relevant; Table 2 contains selected δ iso values measured in aqueous solution. 13 Figure 9 shows a plot of solution vs. solid-state δ iso and reveals that the differences are all less than 30 ppm, likely due to solvent effects. It has been suggested that for vanadium complexes with weakly bound ligands, such as the H 2 O in these compounds, thermal and solvent effects are on the order of a few dozen ppm.…”

“…The structures used in the calculations were obtained from single-crystal x-ray diffraction data 37,13 or the geometry optimized structure of VOCl 3 ; optimization was performed using B3LYP/6-311+G(d,p). Carbon-hydrogen bond lengths were set to 1.08 and 1.09 Å for carbons bonded to three and four other atoms, respectively.…”

“…Details of the preparation and solution properties of this class of compounds have been described elsewhere. 13 However, in this manuscript we provide sufficient information to allow a comparison of the aqueous solution and solid state NMR chemical shifts. 13 …”

“…13,37 These structures were used to calculate the magnetic shielding and the EFG tensors at the vanadium nucleus. Overall, the results presented in Table 2 indicate that the calculations are in reasonable agreement with experiment.…”

“…12 The parent five-coordinate dipicolinate vanadium complexes can form adducts with small ligands such as peroxides and hydroxylamines. 1,13,14 Studies of systematic series of ternary vanadium complexes that are known to contain two different coordinating ligand systems can assist in gauging the effect of ligand substitution on the chemistry at the vanadium center. Solid-state 51 V NMR has the potential to yield a threedimensional description of the local vanadium environment and of the possible changes that take place in the molecular orbitals and electrostatic structure at the vanadium when the coordinating ligands are changed.…”