A solution and solid state study of niobium complexes by Leandra Herbst A dissertation submitted to meet the requirements for the degree of Magister Scientiae in the Department of Chemistry Faculty of Natural- and Agricultural Sciences at the University of the Free State Supervisor: Prof. Andreas Roodt Co-Supervisor: Prof. Hendrik G. Visser January 2012 Acknowledgements First and foremost, I thank God Almighty for the countless blessings he has given me. Thank you to Prof. Roodt for all the opportunities, patience and endless enthusiasm for chemistry, inspiring me to learn as much as I can. It is truly an honour to be known as one of your students. To Prof. Deon Visser, thank you for all your guidance and encouragement. Your neverending patience and willingness to give advice is what kept me motivated throughout my studies. To Dr. Linette Bennie, thank you for your guidance with the niobium NMR throughout this project. Thank you to all my colleagues in the Inorganic group for all the laughter and jokes. Thank you for sharing your knowledge and for your patience when having to explain something several times. Every one of you contributed to this study in some way and for that I thank you. To my father, Lucas Herbst, my sister, Shané Herbst, and my grandmother, Nellie Grobler, without your love, support, faith, sacrifices, understanding and continuous encouragement this would not be possible. Financial assistance from the Advanced Metals Initiative (AMI) of the Department of Science and Technology (DST) of South Africa, through The New Metals Development Network (NMDN) coordinated by the South African Nuclear Energy Corporation Limited (Necsa), and the University of the Free State are gratefully acknowledged. Table of contents Abbreviations and Symbols………………………………………………………………………………… V Abstract…………………………………………………………………………………………………………….. VI Opsomming……………………………………………………………………………………………….……. VIII 1 Introduction ......................................................................................... 1 1.1 History of Niobium .............................................................................................. 1 1.2 The Aim of this Study ......................................................................................... 2 2 Literature Review of Niobium ............................................................ 4 2.1 Introduction ........................................................................................................ 4 2.1.1 Properties of Niobium .................................................................................. 5 2.1.2 Uses ............................................................................................................ 8 2.2 Separation of Niobium and Tantalum………………………………………………..10 2.2.1 Marignac Process ...................................................................................... 10 2.2.2 Solvent Extraction ...................................................................................... 10 2.2.3 Current Research ...................................................................................... 11 2.3 Purification ....................................................................................................... 12 2.4 Chemistry of Niobium ....................................................................................... 13 2.4.1 Oxides ....................................................................................................... 13 2.4.2 Halides....................................................................................................... 14 2.4.3 Borides ...................................................................................................... 15 2.4.4 Carbides .................................................................................................... 15 2.4.5 Nitrides ...................................................................................................... 15 2.4.6 Water absorption ....................................................................................... 16 2.4.7 Silicates ..................................................................................................... 16 I 2.4.8 Cyclopentadienes ...................................................................................... 17 2.4.9 Phosphates ................................................................................................ 17 2.5 Niobium as Catalyst ......................................................................................... 18 2.6 β-diketone Complexes of Niobium ................................................................... 20 2.6.1 Tetrakis( β-diketonate) niobium(IV) complexes .......................................... 21 2.6.1.1 [Nb(thd) 4].............................................................................21 2.6.1.2 [Nb(hfacac)4]........................................................................22 2.6.2 Bis( β-diketonate) niobium(IV) complexes .................................................. 23 2.6.2.1 trans-[NbCl 2(thd) 2]...............................................................23 2.6.3 Mono( β-diketonate) niobium(V) complexes ............................................... 24 2.6.3.1 fac -[Nb(NCS)(Opr) 3(dbm)] and trans -[Nb (NCS) 2(OEt) 2(dbm)].............................................................24 2.6.3.2 [Nb(OEt)4(dbm)]...................................................................25 - 2.6.3.3 [NbCl 3O(ttbd) ].....................................................................26 2.6.3.4 trans-[NbCl 2(OEt)2(dbm)].....................................................26 2.6.4 Bridged niobium β-diketonate complexes .................................................. 27 2.6.4.1 [Nb 2(µ-S2)2(acac) 4] and [Nb 2(µ-S2)(acac) 4]..........................27 2.6.4.2 [Sr 2Nb 2O(thd) 3(OEt) 9(EtOH) 3] and n n [Sr 2Nb 2O(thd) 3(O Pr) 9( PrOH) 3]...........................................28 2.7 Alkoxides .......................................................................................................... 30 2.7.1 Preparation of Metal alkoxides .................................................................. 30 2.7.2 Niobium alkoxides ..................................................................................... 32 2.7.3 93 Nb NMR Studies ..................................................................................... 33 2.8 Kinetic Studies of Niobium Complexes ............................................................ 36 2.9 Conclusion ....................................................................................................... 41 3 Synthesis and Characterisation of Niobium(V) Complexes ........... 42 3.1 Introduction ...................................................................................................... 42 3.2 Nuclear Magnetic Resonance Spectroscopy (NMR) ........................................ 42 3.3 Ultraviolet-visible Spectroscopy (UV/Vis) ......................................................... 45 II 3.4 Infrared Spectroscopy (IR) ............................................................................... 45 3.5 X-Ray Diffraction (XRD) ................................................................................... 47 3.5.1 Bragg’s law ................................................................................................ 48 3.5.2 Structure factor .......................................................................................... 49 3.5.3 ‘Phase problem’ ......................................................................................... 50 3.5.3.1 Direct method......................................................................50 3.5.3.2 Patterson method................................................................50 3.5.4 Least-squares refinement .......................................................................... 51 3.6 Chemical Kinetics ............................................................................................. 51 3.6.1 Reaction rate and rate laws ....................................................................... 52 3.7 Synthesis and Spectroscopic Characterisation of Compounds ........................ 54 3.7.2 Synthetic Procedures ................................................................................ 54 3.7.2.1 Synthesis of [NbCl(acac)(OMe) 3].........................................54 3.7.2.2 Synthesis of [Nb(acac)(OEt) 2(O)] 4.......................................55 3.7.2.3 Synthesis of [NbCl(phacac)(OMe) 3] and [NbCl 2(phacac)(OMe) 2]........................................................55 3.7.2.4 Synthesis of [NbCl 4(acac)]...................................................55 3.7.2.5 Synthesis of [NbCl 4(hfacac)]................................................56 3.7.2.6 Synthesis of [NbCl(trop)(OMe) 3]..........................................56 3.7.3 Discussion ................................................................................................. 57 3.7.4 Conclusion ................................................................................................. 57 4 Crystallographic Characterisation of Niobium(V) β-diketonate Complexes .............................................................................................. 58 4.1 Introduction ...................................................................................................... 58 4.2 Experimental .................................................................................................... 59 2 4.3 Crystal Structure of [NbCl(acac-К