Solid-state 93Nb and 13C NMR investigations of half-sandwich niobium(I) and niobium(V) cyclopentadienyl complexes

Solid-state Nb-93 and C-13 NMR experiments, in combination with theoretical calculations of NMR tensors, and single-crystal and powder X-ray diffraction experiments, are applied for the comprehensive characterization of structure and dynamics in a series of organometallic niobium complexes. Half-sandwich niobium metallocenes of the forms Cp'Nb(I)(CO)(4) and CpNb(V)Cl-4 are investigated, where Cp = C5H5- and Cp' = C5H4R- with R = COMe, CO2Me, CO2Et, and COCH2Ph. Anisotropic quadrupolar and chemical shielding (CS) parameters are extracted from Nb-93 MAS and static NMR spectra for seven different complexes. It is demonstrated that Nb-93 NMR parameters are sensitive to changes in temperature and Cp' ring substitution in the Cp'Nb(I)(CO)(4) complexes. There are dramatic differences in the Nb-93 quadrupolar coupling constants (C-Q) between the Nb(I) and Nb(V) complexes, with CQ between 1.0 and 12.0 MHz for Cp'Nb(CO)(4) and C-Q = 54.5 MHz for CpNbCl4. The quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) pulse sequence is applied to rapidly acquire, in a piecewise fashion, a high signal-to-noise ultra-wide-line Nb-93 NMR spectrum of CpNbCl4, which has a breadth of ca. 400 kHz. Solid-state Nb-93 and C-13 NMR spectra and powder XRD data are used to identify a new metallocene adduct coordinated at the axial position of the metal site by a THF molecule: CpNb(V)(Cl4THF)-T-.. C-13 MAS and CP/MAS NMR experiments are used to assess the purity of samples, as well as for measuring carbon CS tensors and the rare instance of one-bond Nb-93, C-13 J-coupling, (1)J(Nb-93, C-13). Theoretically calculated CS and electric field gradient (EFG) tensors are utilized to determine relationships between tensor orientations, the principal components, and molecular structures.