Abstract
The aim of this thesis is to expand knowledge of the coordination chemistry of vanadium(II) because of its importance in chemical and biological systems which reduce dinitrogen to ammonia. A vanadium(II) complex, used extensively in the investigation of the coordination chemistry of vanadium(II), is [VCl2(TMEDA)2], in which TMEDA is N,N,N,N-tetramethylethylenediamine. The corresponding dibromo- and diiodo-complexes were not previously known. Both [VCl2(TMEDA)2] and [VBr2(TMEDA)2] have been successfully prepared by a new method and the mononuclear structure of [VBr2(TMEDA)2] has been confirmed by X-ray crystallography. During attempts to synthesise the corresponding diiodo-complex, [Vl2(TMEDA)2], a whole new area of vanadium(II) chemistry emerged. Metathesis of [VCl2(TMEDA)2] with trimethylsilyl iodide afforded, instead of the expected mononuclear complex [Vl2(TMEDA)2], a novel trinuclear and antiferromagnetic vanadium(II) complex [V3(-C1)5(TMEDA)3]I. By a remarkable coincidence, the carbon, hydrogen and nitrogen analyses expected for both [Vl2(TMEDA)2] and [V3(-CI)5(TMEDA)3]l are identical. An X-ray crystal structure determination showed that [V3(-Cl)5(TMEDA)3]l contains bridging chlorides in the triangulo-[V3Cl3] unit and axial triply-bridged chlorides. An alternative method was then employed in an attempt to isolate [Vl2(TMEDA)2] because the trimethylsilyl iodide had clearly not removed all the chloride from [VCl2(TMEDA)2]. The vanadium(II) iodide [V(MeOH)6]l2, derived from [VCl2(EtOH)2], was treated with TMEDA but another trinuclear complex [V3(?-Cl)3(?-OMe)2(TMEDA)3]l was unexpectedly isolated. This complex contains the triangulo-[V3Cl3] unit and triply-bridged methoxy groups above and below the plane of the triangulo-unit. It seems that a triangulo-unit must also be present in [VCl2(EtOH)2] or formed from it and then carried through the experimental procedure in which [V(MeOH)6]l2 is an intermediate. Therefore, the nature of other complexes prepared from [V(MeOH)6]l2 has been reconsidered and a few preliminary observations with related systems discussed. There is no doubt that the triangulo-[V3Cl3] moiety with either chloride or methoxy ions in the axial positions forms spontaneously because it has considerable stability in vanadium(II) chemistry and that other examples will be found. The elemental analyses of a diiodo complex, prepared by an earlier worker from [V(MeOH)6]l2 and 1,2-bis(diphenylphosphino)ethane (depe), fitted the formulation [V2(depe)5l2]l2 rather than [Vl2(depe)2]. As it had been synthesised from [V(MeOH)6]l2, it was thought that it might be trinuclear. However, recrystallisation gave mononuclear [Vl2(depe)2] confirmed by analysis, mass spectrometry and an X-ray structure determination. This is the first structurally characterised trans diiodide vanadium(II) complex. Complexes of vanadium(II) with N,N-dimethylethylenediamine were successfully prepared and characterised by elemental analyses, infrared spectra and magnetic susceptibility measurements. Schiff bases have been employed to synthesise novel coordinatively unsaturated complexes of vanadium(II) which would contain vacant sites for the coordination of dinitrogen. Many attempts were made to prepare vanadium(II) complexes of tetradentate Schiff bases but in no cases were vanadium(II) products characterised. A reaction involving the bidentate Schiff base Hhappram gave, instead of the hoped for [V(happram)2], the vanadium(III) complex [V(happram)3], the structure of which was determined. Since the coordinatively unsaturated nature of any vanadium(II)-Schiff base complex could facilitate oxidation by impurities, attempts were made to prepare vanadium(II) complexes from heavily substituted bidentate Schiff bases and [VCl2(py)4] in the expectation that the remaining coordination sites would be occupied by pyridine which could be removed at a later stage. A series of novel vanadium(II)-Schiff base complexes was isolated and the corresponding complexes of vanadium (III) were prepared for comparison. Complexes of vanadium(II) with the tetradentate ligands N,N-bis(8-quinolyl)ethylenediamine and N,N-bis(8-quinolyl)1,3-propanediamine were successfully prepared and characterised by elemental analyses, infrared spectra and magnetic susceptibility measurements.