Chlorodicyclopentadienyloxoniobium(V) complexes revisited: the origin of the asymmetry in the 1H- and 13C-NMR spectra, X-ray crystal structures and ab initio/HF and DFT/B3LYP calculations

1H- and 13C-NMR spectra of chlorodicyclopentadienyloxoniobium(V) complex I and its four 1,1′-dialkyl substituted derivatives II–V have been recorded and assigned based on DQF 1H,1H-COSY and PFG 1H,13C-HMQC and HMBC experiments. Non-equivalences of all cyclopentadienyl protons and carbons in II–V (as reflected by their different 1H- and 13C-NMR chemical shifts) are explained by synchronous and out-of-phase rotations of the substituted cyclopentadienyl rings. A non-equivalence of the methyls in III (1,1′-di-isopropyl) is explained by a detailed inspection of the rotamers of the isopropyl groups. The X-ray structural data show that III and IV (1-methyl-1′-tert-butyl) crystallize in the monoclinic P21/m no. 14 (with crystallographic mirror plane) and in the triclinic P1̄ no. 2 space groups, respectively. Ab initio/HF and DFT/B3LYP calculations gave energetically optimized structures close to those obtained by X-ray structural analyses. Further, calculated and experimental 13C-NMR chemical shifts are comparable for a majority of carbons.