8-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) h as 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) An gstrom, 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) deriva tive with a dodecahedral coordination at the zirconium atom. By reacti on of NbCl4(THF)(2) with Tl(hfacac), the hexafluoroacetylacetonato der ivative of niobium(IV), Nb(hfacac)(4), has been prepared and structura lly characterized. The compound crystallizes in the orthorhombic space group Pna2(1) with the following cell constants: a = 10.399(4), b = 1 5.852(9), c = 119.073(1) Angstrom. It is not isotypic with the corresp onding zirconium(IV) derivative, Zr(hfacac)(4). Crystal data: C20H4F24 O8Zr, 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 sho ws a square antiprismatic coordination, the coordination mode of the l igands is different and slight deviations from the D-4(llll) and C-2(l lss) ideal geometries have been observed in the case of niobium and zi rconium, respectively. An EPR study has been performed on the Nb(IV) d erivatives as diluted solid solutions in frozen organic solvents or in the diamagnetic matrix of the corresponding zirconium(IV) compound. T he EPR spectra have confirmed the presence of non-interacting paramagn ets in the solid solutions and, in the case of Nb(O2CNEt2)(4), the poi nt symmetry of the paramagnetic centre has been found to be in agreeme nt with the results of the X-ray investigation. An EPR spectrum of rho mbic symmetry has been observed for the hexafluoroacetylacetonato deri vative of Nb(IV) when diluted in frozen THF solution or in Zr(hfacac)( 4). (C) 1998 Elsevier Science S.A.