Speciation and interconversion mechanism of mixed halo O,O' - and N,O-bidentate ligand complexes of hafnium

[摘要] English: Hafnium and zirconium show extremely similar chemical properties and occur together in nature. Zirconium ore (commonly referred to as zircon) always contains 1 �?3% hafnium, and the separation of hafnium and zirconium is very difficult due to the similarities in chemical behaviour. The aim of this study was to investigate the chelating behaviour of hafnium with different organic bidentate ligands e.g. trifluoroacetylacetone (tfaaH), hexafluoroacetylacetone (hfaaH) and 8-hydroxyquinoline (OxH) and characterizing new compounds obtained from this by means of single crystal X-ray crystallography, NMR- and UV/Vis spectroscopy. Any differences in solution behaviour, whether it being reaction mechanism, solubility, coordination modes, equilibrium behaviour, etc., could possibly be exploited in developing novel separation techniques for the two metals. The structures of three new complexes, namely the [Hf(tfaa)4], [Hf(OH)(hfaa)3]2 and [Hf(Ox)4] were solved. This enabled the identification of products for kinetic studies and increased the available pool of these rare compounds in literature. The crystallographic characterization of these complexes are presented and compared with literature. Both [Hf(tfaa)4] and [Hf(OH)(hfaa)3]2 crystallized in a monoclinic space group, C2/c with Z = 4. [Hf(Ox)4] crystallized in a triclinic space group, Pī, with Z = 2. All three the structures include solvent molecules as part of the basic molecular unit. The co-crystallisation of the solvent molecules does not observably restrict or interfere with any significant physical properties of the solvated crystalline moieties. [Hf(tfaa)4] was coordinated to eight O atoms of the four tfaa ligands with an average Hf─O distance of 2.172(26) Å and O─Hf─O bite angle of 75.62(7)°. The hfaa ligands in [Hf(OH)(hfaa)3]2 formed three six-membered metallocycles with an average Hf-O bond distance of 2.190(21) Å and O-Hf-O bite angle of 75.3(5)°. [Hf(Ox)4] was coordinated by four O and four N atoms which formed four fivemembered metallocycles with average Hf-O and Hf-N bond distances of 2.095(13) Å and 2.399(15) Å, respectively and an O-Hf-N bite angle of 70.92(3)°. As part of the kinetic investigation the substitution reactions between HfCl4 and OxH ligands were followed by means of UV/Vis- and stopped-flow spectroscopy. Five reactions in total were observed for the stepwise coordination of OxH to the hafnium metal ion. From this the following mechanism was proposed: The first reaction (1st and 2nd observable reactions) is a two-step reaction involving the stepwise rapid formation of [Hf(LL)2Cl2] with pre-equilibrium K1 = 40(9) M-1 and rate determining second step k2 = 90(15) M-1s-1. The third observable reaction followed the usual two-term rate law, kobs3 = k3[LL'] + k-3 with k3 = 6.5(3) M-1s-1 and k-3 = 1.4(1) x 10-3 s-1 yielding an equilibrium constant K3 = 4(3) x 103 M-1. The forth reaction is proposed to be the ring closure of the five membered chelated metallocycle with K4 = 6(2) x 102 M-1 and k3 = 5.80(7) x 10-3 M-1s-1. The fifth and final reaction was concluded to be the formation of the tetrakis complex, [Hf(L,L')4] that was isolated in the synthesis. The last process followed a normal two-term rate law, kobs5 = k5[LL'] + k-5 with k5 = 0.073(9) M-1s-1 and k-5 = 2.3(2) x 10-4 s-1 producing an equilibrium constant, K5 = 320(50) M-1.

[发布机构] University of the Free State