A solution and solid state study of niobium complexes University of ...

Abstract
This research project focused on the investigation and identification of various
niobium(V) complexes containing selected O,O’-bidentate ligands that could
potentially be used for the selective separation of niobium from tantalum. Emphasis
was placed on acetylacetone (acacH) type of ligands due to the ease of varying their
electronic and steric properties.
The crystallographic characterization of three novel complexes, (acetylacetonato-κ 2 -
O,O’)chloridotrimethoxidoniobium(V) (1), the “cage”-like structure of tetrakis-
(acetylacetonato-κ 2 -O,O’)octakis(etoxy)tetrakis(µ 2 -oxo)tetraniobium(V) (2) and the
two structures that were obtained from the same crystal, (1-phenyl-1,3-
butanedionato-κ 2 -O,O’)chloridotrimethoxidoniobium(V) (3a) and (1-phenyl-1,3-
butanedionato-κ 2 -O,O’)dichloridodimethoxidoniobium(V) (3b), is discussed and
compared to literature. Complex 1 crystallized in an orthorhombic crystal system and
space group Pbca, while complexes 2, 3a and 3b all crystallized in a monoclinic
crystal system and a space group P21/c, for all. In general it was observed that these
mono substituted β-diketonato complexes of niobium(V) crystallized in a distorted
octahedral coordination polyhedron. The average O-Nb-O bite angle and Nb-O bond
distance 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-
κ 2 -O,O’)chloridotrimethoxidoniobium(V) complex in methanol. The coordination
mechanism is postulated for the two observed steps of acacH coordination, of which
the initial coordination of the ligand takes place in the first step. The equilibrium
constant, K1, was determined as 1975 (201) M -1 at 25.0 °C. The second, rate
determining step is representative of the total reaction and includes the ring-closure
of the acac ligand and yields K1 as 1403 (379) M -1 . Within experimental error, this
value is in good agreement with that of the first step. When comparing the rate
constants, k1 and k2, it is found that the first reaction is roughly six orders of
magnitude (10 6 ) faster than the slower, second reaction step.
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