This thesis is primarily concerned with the synthesis and reactions of iron and ruthenium dinitrogen complexes of tripodal phosphine ligands. Of particular interest is the cationic dinitrogen bridged iron complex [(FeH(PP3))2(μ-N2)]2+ 23, containing the tetradentate ligand P(CH2CH2PMe2)3, PP3 1, and its potential for facilitating the reduction of the bound dinitrogen upon treatment with acid.
The synthesis of a selection of novel and known tripodal phosphine and amino phosphine ligands is described. New ligands N(CH2CH2CH2PMe2)3 N3P3 7 and P(CH2CH2CH2PiPr2)3 P3Pi3 11 were synthesised by nucleophilic displacement of bromide from the bromoalkylphosphine and bromoalkylamine precursors with the relevant phosphide. A new method for synthesis of known ligand P(CH2CH2CH2PMe2)3 P3P3 19 by the nucleophilic substitution of its chloroalkylphosphine oxide with dimethylphosphide and subsequent reduction is also reported.
The reaction of [(FeH(PP3))2(μ-N2)]2+ 23 with base produced the singly deprotonated mixed valence species [(FeH(PP3))(μ-N2)(Fe(PP3))]+ 37 and subsequently the iron(0) dinuclear species (Fe(PP3))2(μ-N2) 38 and mononuclear complex Fe(N2)(PP3) 44. The 15N labelling of complexes has allowed the 15N NMR spectra of 23, 37 and 44 to be reported along with the observation of a long-range 5JP-P coupling across the bridging dinitrogen of 37. Complexes 23 and 37 were also structurally characterised by X-ray crystallography. The treatment of a variety of iron PP3 1 dinitrogen complexes, including the mononuclear species [(Fe(N2)H(PP3)]+ 22, with acid, or base then acid, did not result in the formation of ammonia from reduction of the complexed dinitrogen.
The reactions of FeCl2(PP3) 24 and FeClH(PP3) 25 with ammonia and hydrazine afforded the complexes [FeCl(N2H4)(PP3)] 48, [FeH(N2H4)(PP3)] 47, [FeCl(NH3)(PP3)] 49 and [FeH(NH3)(PP3)] 46. Complexes 47 and 46 are considered potential intermediates in any reduction of the dinitrogen ligand of 23 to ammonia. Complexes 49 and 46 were also formed from the decomposition of the hydrazine complexes 48 and 47. The 15N NMR shifts, derived from both the 15N labelling of complexes and from 1H-15N 2D NMR experiments at natural abundance are reported. In addition, complex 47 was characterised by X-ray crystallography.
The novel ligand P(CH2CH2PiPr2)3 PPi3 12 was used in the successful synthesis of [FeCl(PPi3)]+ 51 and [RuCl(PPi3)]+ 56. Reduction of 51 and 56 with potassium graphite under dinitrogen afforded the complexes Fe(N2)(PPi3) 52 and Ru(N2)(PPi3) 57 respectively. This is the first report of a Ru(0) dinitrogen complex. Treatment of 52 and 57 with lutidinium tetrafluoroborate resulted in protonation and oxidation of the metal centre to afford the hydrido complexes [Fe(N2)H(PPi3)]+ 53 and [Ru(N2)H(PPi3)]+ 58 respectively. 15N labelled analogues of 52, 53, 57 and 58 were achieved by exchange reactions with 15N2 gas, allowing for analysis by 15N NMR spectroscopy. Species 52, 57 and 58 have also been structurally characterised by X-ray crystallography. Treatment of 52 with excess acid in THF afforded both 53 and the dihydrogen complex [Fe(H2)H(PPi3)]+ 54.
The mechanism of formation of 54 probably involves the C-H activation of the solvent THF.
The complex cation [RuCl(P3Pi3)]+ 65 was synthesised using the novel ligand P3Pi3 11. A polymeric iron(II) complex, [Fe2Cl4(N3P3)2]n 66, of the tridentate ligand N3P3 7 was also synthesised. Characterisation of both 65 and 66 by X-ray crystallography is reported. (FeCl)2(μ-Cl)2(μ-Pi2)2 68, an unusual bridged dimer of the known ligand CH2(PiPr2)2 Pi2 67, and iron(II) and iron(0) tetramers of the PP3 1 ligand, namely [Fe4Cl4(PP3)5]4+ 71 and Fe4(PP3)5 72 were also characterised by X-ray crystallography.