Ring folding in cyclopentadienyl diazabutadiene complexes of group 4 and 5 transition metals

The syntheses of CpM(i-PrDAB)2 (M = Nb, Ta; Cp = (C5H5); i-PrDAB = bis-isopropyl-1,4-diazabuta-1,3-diene) are reported. Both show fluxional NMR spectra indicating that the two DAB rings differ. The X-ray crystal structure of CpNb(i-PrDAB)2 shows one ring to be more folded than the other. Density functional calculations have been used to investigate the degree of folding of the chelate ring in the compounds Cp2M(R-DAB), (M = Ti, Zr, Hf, Nb, Ta; R = H, i-Pr; DAB = 1,4-diazabuta-1,3-diene) and CpM(R-DAB)2 (M = Nb and Ta). For Cp2M(R-DAB) the group 4 compounds all have folded rings whereas the Nb and Ta compounds have planar rings. In all compounds the rings are reduced and the folding is driven by the electron number requirements of the metal centre.