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

H-1- and C-13-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 H-1,H-1-COSY and PFG H-1,C-13-HMQC and HMBC expe riments. Non-equivalences of all cyclopentadienyl protons and carbons in II -V (as reflected by their different H-1- and C-13-NMR chemical shifts) are explained by synchronous and out-of-phase rotations of the substituted cycl opentadienyl rings. A non-equivalence of the methyls in III (1,1'-di-isopro pyl) 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-b utyl) crystallize in the monoclinic P2(1)/m no. 14 (with crystallographic m irror plane) and in the triclinic P (1) over bar no. 2 space groups, respec tively. Ab initio/HF and DFT/B3LYP calculations gave energetically optimize d structures close to those obtained by X-ray structural analyses. Further, calculated and experimental C-13-NMR chemical shifts are comparable for a majority of carbons. (C) 2000 Elsevier Science B.V. All rights reserved.