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Title: The synthesis of terminal uranium nitride complexes
Author: King, David M.
ISNI:       0000 0004 5353 4794
Awarding Body: University of Nottingham
Current Institution: University of Nottingham
Date of Award: 2014
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The study of metal-ligand multiple bonding has provided significant insights into the electronic structure and behaviour of the transition metals, however the chemistry of uranium has been restricted to U-L double bonds such as uranyl, terminal chalcogenides, imides and carbene complexes. Of particular note, is the absence of the identification and isolation of terminal uranium-nitride complexes, U=N. The uranium-nitride linkage not only presents the opportunity to advance our understanding of the 5f orbitals and their participation in bonding, but uranium nitride also offers potential industrial applications. Uranium-nitride is currently under consideration as a future nuclear fuel source in preference to the traditional uranium oxides, due to its higher melting point, density and thermal conductivity. However despite numerous attempts to isolate a terminal U=N bond, molecular methods have thus far proved unsuccessful. Herein the syntheses of the first terminal uranium-nitride complexes are reported. The reaction ofPyTMsH (p/MS = CsH4N-2-NSiMe3) with I1BuLi afforded the ligand transfer reagent [{ Li(Py TMS)(THF) } 2]' Salt metathesis reaction of three equivalents of [{Li(P/MS)(THF) h] with [UX4] eX = Cl, I) afforded the uranium(lV) halide complexes [U(X)(P/MS)3]. Attempts to reduce [U(X)(pyTMS)3] to the hypothetical uranium(III) species were unsuccessful and only the uranium(lV) homoleptic complex [U(P/MS)4] could be isolated. A novel sterically demanding ligand architecture Tren TIPSH3 [Tren {N(CH2CH2NR)3} 3-, TIPS = Siipr3] has been synthesised with the sole aim of stabilising uranium-ligand multiple bonding. The pro-ligand [Li3(Tren TIPs)] was synthesised in good yield and subsequently transferred to uranium through salt metathesis to afford the uranium halide complexes [U(X)(Tren TIPS)]. These uranium(IV) species can be reduced with potassium to afford the first crystallogaphically characterised Tren derived uranium(III) complex, [U(Tren TIPS)]. Uranium(IV) azide complexes have been noted as potential precursors to uraniumnitride formation through photolysis. Consequently a series of novel uranium(lV) azide complexes have been synthesised and characterised supported by the TrenR (R The Synthesis of Terminal Uranium Nitride Complexes = SiMe3, Si'BuMe2, Siipr3) ligand framework. Interestingly the steric bulk plays a key role in controlling the extent of aggregation. The photolysis of the uranium azide species [{U(TrenDMBS) }4(Il:r(r{N3)4] led to the isolation of the mixed valent bimetallic uranium (IV IV) nitride complex [{U(TrenDMBS) h(Il-N)], which is best formulated as containing a [U-N=U] core. In contrast, photolysis of the terminal azide complex [U(N3)(Tren TIPS)], resulted in intitial N2 evolution to yield a transient uranium(VI) nitride which quickly underwent C-H activation of the ligand to afford the uranium(IV) amido complex [U {(NHCMe2Siipr2NCH2CH2)N(CH2- CH2NSiipr3)2} ]. A two-electron redox reaction of the sterically demanding uranium(III) complex, [U(Tren TIPs)] with sodium azide afforded the dimeric bridging uranium(V) nitride species [{U(Il-N)(Il-K)(Tren TIPS) h]' Abstraction and encapsulation of the sodium cation was achieved through treatment with polydentate crown ether 12-crown-4 to afford the first example of a terminal uranium(V) nitride complex, [U (N)(Tren TIPs)] [N a(l2C4 h]. This species can be oxidised to the neutral terminal uranium(VI) nitride, [U(N)(Tren TIPS)], through oxidation with iodine. [U(N)(Tren TIPs)] exhibits an exceptionally short U=N bond length of 1.799(9) A, consistent with a triple bond. The U=N linkage has been analysed by X-ray diffractometry, 15N-isotopic labelling, electronic absorption and electron paramagnetic resonance spectroscopies, magnetometry, and electronic structure calculations. Additionally the uranium(V) imido complexes [U(NSiMe3)(Tren TIPS)] and [U(NAd)(Tren TIPS)] (Ad = adamantyl) were synthesised through two-electron oxidation of organic azides with [U(Tren TIPS)]. The terminal amido complex [U (NH2)(Tren TIPS)] was synthesised through the reaction of [U (Tren TIPs)] with [NI1BU4] [N3]. Single-molecular-magnets, defined as molecules that exhibit slow relaxation of the magnetisation of purely molecular origin, are intensively researched due to the prospective applications in molecular electronics. Recently chemists have reported a handful of uranium(III) complexes which exhibit SMM behaviour. However observation of slow relaxation is not limited to uranium(lII) species. In this work the first monometallic uranium(V) SMM, [U(O)(Tren TIPs)], synthesised through a two electron oxidation of [U(Tren TIPs)] with Me3NO, is reported. Attempts to synthesise ~ VI ~ n The Synthesis of Terminal Uranium Nitride Complexes the analogous terminal uranium sulphide through various routes only resulted in the isolation of the dinuclear uranium(IV) sulphido complex [{U(Tren TIPS) b(/l-S)]. The isolation of a uranium(V) SMM suggests that uranium may offer a substantial role in the future of SMM chemistry. ~ VII
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available