Aspects of vanadium, iron and titanium chemistry related to nitrogen fixation

• Main Author: De Souza, Jaisa Soares
• Published: University of Sussex 1995
• Subjects: 546; Inorganic chemistry
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.283147

This project concerned the basic chemistry of VD, Vm and FeD (iJ because of interest in the role of vanadium and iron in the electron-transfer processes catalysed by the nitrogenases, and (iiJ to try to rationalise the fragmented data available on the chemistry of low-oxidation-state vanadium and its reactions with iron(II) compounds. In order to increase the range of FeD materials available for reaction with VD, the dinuclear complex [{FeCI(tmeda)h{Jt-Cl)2] and the mononuclear [FeCI2(tmeda)2] (tmeda = N,N,N',N'-tetramethylethylenediamine) were synthesised and characterised by chemical and physical methods including X-ray diffraction. The reaction between vanadium(II) or iron(II) compounds, such as [MCI2(tmeda)z], and chloride-abstracting agents (e.g. M~SiI or NaBPh4) afforded trinuclear ionic complexes containing [M3Cl3Y2(LL)3]+ , where M = FeDor VD, Y = chloride and LL = bidentate amine or phosphine; complexes containing Y = methoxide and M = yD were isolated from methanol solutions. The planar {M3CI3} core is highly stable and remains intact during substitution of the axial Y groups and the neutral ligands. Mixed clusters containing both vanadium(II) and iron(II) can also apparently be synthesised, but the similarity of the metal radii in oxidation state II makes characterisation by X-ray diffraction analysis difficult. Attempts to produce yD_ or ym -bridging N2 compounds containing the ligand 2- [(N,N-dimethylamino)methyl]ferrocenyl (L) were not successful. In order to obtain additional information on the reactivity of L towards early transition metals, the cyclopentadienyltitanium (IV) complexes [Cp2Ti(L)CI] and [CpTiLxCl3.J (x = 1-3) were prepared. X-ray diffraction analyses of [Cp2Ti(L)CI] and [CpTi(L)CIJ have shown that L can be monodentate or bidentate, depending on steric constraints about the metal atom. The complex [CpTi(L)(L')] has also been synthesised; L' is a tridentate ligand apparently generated upon heating by intramolecular C-H activation of a methyl substituent on one of the tertiary amino groups of [CpTi(L)3], followed by elimination of neutral [(N,N-dimethylamino)methyl] ferrocene. The complexity of this system and the formation oftrinuclear vanadium(II) complexes possibly account for some of our difficulties in understanding vanadium-dinitrogen chemistry.