Coordination chemistry of niobium(v) and tantalum(v) with hard o-donor ligands: a solution and solid state investigation

English: This thesis describes the study directed towards the synthesis, characterization and substitution kinetics of novel niobium(V) and tantalum(V) complexes containing selected O,O’-bidentate ligands that could potentially be used for the separation of niobium from tantalum. There is also a search for complexes that will be attractive and useful as advanced materials for high technology applications and in the nuclear power environment. The study included systematic assessment of the physical and chemical behaviour of the free ligands, co-crystals and complexes as well as the factors which govern their stability, reactivity, coordination geometry, bond lengths and angles, π-π stacking interaction and inter- and intramolecular networks of hydrogen bonds. It became evident that the crystal stabilization was influenced by intermolecular π-π interactions and inter- and intramolecular hydrogen and halogen bond networks, producing a stabilized crystal lattice. A range of Nb(V) and Ta(V) complexes such as cis-(Et4N)[NbO(ca)2(H2O)OPPh3]·3H2O-·THF (5), (Et4N)4[Nb4O4(ca)2(μ2-O)2C18]·2CH3CN (6), [NbO(cupf)3] (7), [NbO(hopo)3]. MeOH (8), [TaO(hopo)3]·MeOH (9), [Ta(hopo)4]C1·2MeCN·2H2O (10) (where ca = 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinonato, cupf = N-nitroso-N-phenylhydroxylaminato, hopo = 2-hydroxypyridinato-N-oxide) as well as two co-crystals [OPPh3½(caH2)] (2) and [(2½(caH2)(DMAP)6·11H2O)] (4) were prepared and characterised by IR (ATR), NMR (1H, 13C and 31P), UV-Vis spectroscopy and single crystal X-ray diffraction. Additionally, two free ligands, [½(caH2)·H2O] (1) and [hopoH] (3), were characterised by single crystal X-ray diffraction. The six-, seven- and eight coordinate geometries show different coordination modes as observed in complexes (5) and (7) (D5h pentagonal bipyramidal geometry), (6) (a regular octahedral coordination mode), (8) (C3v-capped octahedral geometry), (9) (C2-capped trigonal prismatic geometry) and (10) (D2-square antiprismatic geometry). Therefore, these different geometries found in complexes (5), (6), (7), (8), (9) and (10) describe the shape of complexes with six, seven and eight oxygen atoms bound to the niobium(V) and tantalum(V) metal centres, with the differences playing a possible key role to increase the potential of a new type of separation of niobium from tantalum. Starting with the selected complex, cis-[NbO(ca)2(H2O)(OPPh3)]-, the substitution of the triphenyl-phosphineoxide by pyridine-type ligands such as pyridine (py), 4-N,N-dimethyl-aminopyridine (DMAP), 4-methylpyridine (4-Mepy), 3-chloropyridine (3-Clpy) and 3-bromopyridine (3-Brpy) in acetonitrile media at 31.2°C were determined and evaluated. These entering monodentate ligands could be arranged in terms of their increasing reactivity values: 3-Brpy < 3-Clpy < 4-Mepy < py < DMAP, defined by the values of kf = 0.147(4) x 10-2; 0.206(7) x 10-2; 0.181(6) x 10-2; 0.131(2) x 10-2 and 2.032(2) x 10-2 M-1s-1 respectively. The pseudo first-order rate constants were dependent on concentration and temperature. From the observed kinetic results a suitable mechanism was proposed. Activation parameters were determined for the reaction of the complex, cis-[NbO(ca)2(H2O)(OPPh3)]-, with 4-N,N-dimethylaminopyridine (DMAP) as entering ligand. The activation parameters for these triphenylphosphineoxide substitution reactions were calculated as 51.5(9) k J mol-1 and -108(3) J K-1 mol-1 for the standard change of enthalpy (ΔH≠(kf)) and the standard change of entropy (ΔS≠(kf)) respectively. The negative entropy of activation implies a more ordered state and presumably this substitution reaction could involve an associative ligand substitution mechanism. Although only part of a preliminary study, a range of novel Nb(V) and Ta(V) complexes with O,O’-, O,N- and N,N’-bidentate ligands were synthesised and characterised by IR (ATR), NMR (1H and 13C) and UV-Vis spectroscopy. The reaction of [NbCl6]- with chloranilic acid (caH2) in solution gave cis-[NbO(ca)2(H2O)]-, which react further with added triphenylphosphineoxide (OPPh3) to give the final product, cis-[NbO(ca)2(H2O)OPPh3]-. Knowledge obtained from this study should contribute to the understanding of the chemical and physical properties of the various Nb(V) and Ta(V) complexes and identify possible differences to apply in future for developing potential separation methods for niobium from tantalum.