A solution and solid state study of niobium complexes

[摘要] English: This research project focused on the investigation and identification of variousniobium(V) complexes containing selected O,O'-bidentate ligands that couldpotentially be used for the selective separation of niobium from tantalum. Emphasiswas placed on acetylacetone (acacH) type of ligands due to the ease of varying theirelectronic and steric properties.The crystallographic characterization of three novel complexes, (acetylacetonato-κ2-O,O')chloridotrimethoxidoniobium(V) (1), the 'cage-like structure of tetrakis-(acetylacetonato-κ2-O,O')octakis(etoxy)tetrakis(μ2-oxo)tetraniobium(V) (2) and thetwo structures that were obtained from the same crystal, (1-phenyl-1,3-butanedionato-κ2-O,O')chloridotrimethoxidoniobium(V) (3a) and (1-phenyl-1,3-butanedionato-κ2-O,O')dichloridodimethoxidoniobium(V) (3b), is discussed andcompared to literature. Complex 1 crystallized in an orthorhombic crystal system andspace group Pbca, while complexes 2, 3a and 3b all crystallized in a monocliniccrystal system and a space group P21/c, for all. In general it was observed that thesemono substituted β-diketonato complexes of niobium(V) crystallized in a distortedoctahedral coordination polyhedron. The average O-Nb-O bite angle and Nb-O bonddistance for these complexes were determined as 80.5 (1) ° and 2.108 (2) Å,respectively.A kinetic investigation was conducted to follow the formation of the (acetylacetonato-κ2-O,O')chloridotrimethoxidoniobium(V) complex in methanol. The coordinationmechanism is postulated for the two observed steps of acacH coordination, of whichthe initial coordination of the ligand takes place in the first step. The equilibriumconstant, K1, was determined as 1975 (201) M-1 at 25.0 °C. The second, ratedetermining step is representative of the total reaction and includes the ring-closureof the acac ligand and yields K1 as 1403 (379) M-1. Within experimental error, thisvalue is in good agreement with that of the first step. When comparing the rateconstants, k1 and k2, it is found that the first reaction is roughly six orders ofmagnitude (106) faster than the slower, second reaction step. 93Nb NMR was successfully used in characterising the niobium(V) productssynthesised and played an important role in the kinetic study of the project. Withregards to the kinetic study; solvent coordination proceeded rapidly upon solvation ofthe dimeric starting material, [NbCl5]2, in methanol and the niobium(V) startingreagent was correctly determined as [NbCl2(OMe)3(MeOH)] through 93Nb NMR.

[发布机构] University of the Free State