Eight-coordinate chelate complexes of zirconium(IV) and niobium(IV): X-ray diffractometric and EPR investigations

The N,N-diethylcarbamato derivative of zirconium(IV), Zr(O2CNEt2)(4) has been studied by X-ray crystallography, Crystal data: C20H40N4O8Zr, monoclinic, space group C2/c, a=14.057(1), b=12.168(1), c=16.746(2) Angstrom, beta=108.071(4)degrees, Z=4, D-c=1.356, F(000) = 1168, T = 213 K. The compound is isotypic with the corresponding niobium(IV) derivative with a dodecahedral coordination at the zirconium atom. By reaction of NbCl4(THF)(2) with Tl(hfacac), the hexafluoroacetylacetonato derivative of niobium(IV), Nb(hfacac)(4), has been prepared and structurally characterized. The compound crystallizes in the orthorhombic space group Pna2(1) with the following cell constants: a = 10.399(4), b = 15.852(9), c = 119.073(1) Angstrom. It is not isotypic with the corresponding zirconium(IV) derivative, Zr(hfacac)(4). Crystal data: C20H4F24O8Zr, monoclinic, space group P2(1)/n, a=11.974(4), b =20.451(6), c=13.140(3) Angstrom, beta=104.487(11)degrees, Z=4, D-c = 1.960, F(000) = 1776, T = 223 K. Although in both compounds the central metal atom shows a square antiprismatic coordination, the coordination mode of the ligands is different and slight deviations from the D-4(llll) and C-2(llss) ideal geometries have been observed in the case of niobium and zirconium, respectively. An EPR study has been performed on the Nb(IV) derivatives as diluted solid solutions in frozen organic solvents or in the diamagnetic matrix of the corresponding zirconium(IV) compound. The EPR spectra have confirmed the presence of non-interacting paramagnets in the solid solutions and, in the case of Nb(O2CNEt2)(4), the point symmetry of the paramagnetic centre has been found to be in agreement with the results of the X-ray investigation. An EPR spectrum of rhombic symmetry has been observed for the hexafluoroacetylacetonato derivative of Nb(IV) when diluted in frozen THF solution or in Zr(hfacac)(4). (C) 1998 Elsevier Science S.A.