Cleaves, Peter A.
(2016)
Synthesis, reactivity, and electronic structure of molecular uranium nitrides.
PhD thesis, University of Nottingham.
Abstract
The study of metal-ligand multiple bonding offers insight into the electronic structure and bond of metal systems. Until recently, for uranium, such systems were limited to uranyl, and terminal chalcogenide, imide and carbene complexes. In 2012, this was extended to nitrides with the first preparation of a uranium–nitride (U≡N) species isolable under standard conditions, namely [U(TrenTIPS)(N)][Na(12C4)2] (52), which is prepared by the two-electron reduction of sodium azide with a trivalent uranium(III) precursor [U(TrenTIPS)] (15), and the subsequent sequestration and encapsulation of the loosely bound sodium cations.
In order to then fully explore the bonding within this newly isolated fragment, alternative routes to prepare uranium–nitrides were investigated, in order to both expand the family of known uranium–nitrides, as well as remove the synthetic bottleneck that occurs due to issues in scale-up. The reduction of the pre-installed azide ligand of [U(TrenTIPS)(N3)] (13) with one-electron external reductants, namely the alkali powders or metals (lithium and sodium, rubidium and caesium) and potassium graphite, affords the dinuclear uranium(V)–nitride species of the form [{U(TrenTIPS)(µ-N)(µ-M)}2] (M = Li, Na, K, Rb, Cs; 51, 106 – 109). Analogously to the preparation of the first terminal uranium–nitride system, encapsulation of the cation affords separated ion pair species of the form [U(TrenTIPS)(N)][M(crown)2] (M = Na, K, Rb, Cs; 52, 114 – 117), or if a slightly larger co-ligand is utilised, capped uranium–nitrides of the form [U(TrenTIPS){(µ-N)(µ-M)(crown)}] (M = Li, Na, K, Rb, Cs; 53, 111 – 113, 118, 119). Oxidation of the separated ion pair nitrides affords the neutral uranium(VI)–nitride, [U(TrenTIPS)(N)] (54).
Attempts to prepare dinuclear uranium–nitrides by the reduction of 13 with benzyl potassium (KCH2Ph) afforded instead the cyclometallated species [U{CH2CH(Me)Si(iPr)2NCH2CH2N(CH2CH2NSiiPr3)2}] (110). Unexpectedly, in an inversion of the anticipated reactivity trend, attempts to prepare a thorium congener of 110 did not initially form a cyclometallated species, with the reaction of [Th(TrenTIPS)(I)] (123) and KCH2Ph affording [Th(TrenTIPS)(CH2Ph)] (124); and cyclometallation was thermolytically induced. Computational calculations indicate that this is due to the stabilisation of the σ bond metathesis transition state in the uranium case.
A combination of experimental and computational studies allows for the determination of the electronic structure of the families of uranium(V)–nitrides, by considering all the spectroscopic data available for the dinuclear, terminal separated ion pair and capped species in conjunction with ab initio calculations. This approach then leads to a description of the ground and excited states of Tren–uranium(V) nitrides, where [{U(TrenTIPS)(µ-N)(µ-K)}2] (107) and [U(TrenTIPS){(µ N)(µ Na)(15C5)}] (53) exhibit a jz ≈ ±5/2 ground state doublet, with a jz ≈ ±3/2 first excited state doublet, in contrast to all other nitrides studied, where the reverse is the case. The excited states can be derived from spectroscopic data (EPR and UV-vis-NIR), which corroborate these findings.
A series of investigations into the small molecule activation chemistry of uranium–nitrides were instigated. The reaction of Tren–uranium(VI) or uranium(V)–nitrides (54, with carbon monoxide resulted in two-electron reductive carbonylation to afford the corresponding uranium(IV)– and uranium(III)–isocyanates, [U(TrenTIPS)(NCO)] (135), [U(TrenTIPS)(NCO)][K(Bn 15C5)2] (136), [U(TrenTIPS){(µ NCO)(µ K)(18C6)}] (137). Reduction of 135 with potassium graphite afforded complete nitrogen atom transfer to eliminate KOCN and generate 15. In the presence of crown, the uranium–nitrogen bond is retained and 136 or 137 can be isolated. A synthetic cycle for the conversion of NaN3 to NaOCN was investigated employing the UIII-UV redox couple by the reaction of 15, NaN3 and CO in pyridine, where one turnover was observed. DFT calculations were used to model these reactions, and they provided evidence for nucleophilic behaviour and explained the difference in the rates of reaction.
The reaction of heteroallenes (CE2, E = O, S) with Tren–uranium nitrides was also investigated. It was found that terminal uranium–nitrides react with CO2 to afford uranium–oxo–isocyanates, with retention of uranium oxidation state. In the case of uranium(V), [U(TrenTIPS)(O)(NCO)][K(Bn-15C5)2] (138) is stable, though for uranium(VI), [U(TrenTIPS)(O)(NCO)] (139), a cyanate radical is extruded, which decomposes via diisooxocyan to afford N2 and CO, preparing [U(TrenTIPS)(O)] (122). With CS2, uranium(V)–nitrides undergo overall disproportionation to afford uranium(IV)–trithiocarbonates [U(TrenTIPS)(κ2-CS3)][K(Bn-15C5)2] (140) and [{U(TrenTIPS)(µ-κ2:κ1-CS3)(µ-K)(Bn2-18C6)}2(µ-C6H6)] (141), and uranium(VI)–nitride (54), alongside the formation of [K(crown)n][SCN]. The reaction of 54, which cannot engage in disproportionation chemistry, with CS2 prepares a uranium(IV)–isothiocyanate, [U(TrenTIPS)(NCS)] (142), by the elimination of sulfur, which can be scavenged by triphenylphosphine. Calculations reproduced experimental outcomes, and show that while uranium(V)–nitrides (115) engage in outer sphere type reactivity, uranium(VI)–nitrides (54) instead react via inner sphere mechanisms.
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