Eight Coordinate Chelate Complexes of Zirconium(IV) and Niobium(IV): X-Ray Diffractometric and EPR Investigation

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) A, ?= 108.071(4) °, Z=4, Dc = 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. B y reaction of NbC14 (THF) 2 with TI (hfacac), the hexafluoroacetylacetonato deft vati ve of niobium ( IV ), Nb (hfacac) 4,
has been prepared and structurally characterized. The compound crystallizes in the orthorhombic space group Pna2~ with the following cell
constants: a = 10.399(4), b = 15.852(9), c = 119.073( 1 ) A. It is not isotypic with the corresponding zirconium(IV) derivative, Zr(hfacac)4.
Crystal data: C20H40N4O8Zr, monoclinic, space group P21/n, a = 11.974(4), b=20.451(6), c= 13.140(3) A, ? = 104.487(11) °, Z=4,
Dc = 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 D4(llll) and C2(llss) ideal geometries have been observed in the
case of niobium and zirconium, respectively. An EPR study has been performed on the Nb(1V) 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