Benedict M. Gardner

Homologation and functionalization of carbon monoxide by a recyclable uranium complex

Carbon monoxide (CO) is in principle an excellent resource from which to produce industrial hydrocarbon feedstocks as alternatives to crude oil; however, CO has proven remarkably resistant to selective homologation, and the few complexes that can effect this transformation cannot be recycled because liberation of the homologated product destroys the complexes or they are substitutionally inert. Here, we show that under mild conditions a simple triamidoamine uranium(III) complex can reductively homologate CO and be recycled for reuse. Following treatment with organosilyl halides, bis(organosiloxy)acetylenes, which readily convert to furanones, are produced, and this was confirmed by the use of isotopically 13C-labeled CO. The precursor to the triamido uranium(III) complex is formed concomitantly. These findings establish that, under appropriate conditions, uranium(III) can mediate a complete synthetic cycle for the homologation of CO to higher derivatives. This work may prove useful in spurring wider efforts in CO homologation, and the simplicity of this system suggests that catalytic CO functionalization may soon be within reach.

Triamidoamine–Uranium(IV)‐Stabilized Terminal Parent Phosphide and Phosphinidene Complexes

Reaction of [U(Tren(TIPS) )(THF)][BPh4 ] (1; Tren(TIPS) =N{CH2 CH2 NSi(iPr)3 }3 ) with NaPH2 afforded the novel f-block terminal parent phosphide complex [U(Tren(TIPS) )(PH2 )] (2; U-P=2.883(2) Å). Treatment of 2 with one equivalent of KCH2 C6 H5 and two equivalents of benzo-15-crown-5 ether (B15C5) afforded the unprecedented metal-stabilized terminal parent phosphinidene complex [U(Tren(TIPS) )(PH)][K(B15C5)2 ] (4; UP=2.613(2) Å). DFT calculations reveal a polarized-covalent UP bond with a Mayer bond order of 1.92.

Synthesis and structure of [{N(CH2CH2NSiMe3)3}URe(η5-C5H5)2]: a heterobimetallic complex with an unsupported uranium–rhenium bond

The first structurally authenticated molecular uranium-transition metal bond is reported; DFT studies show sigma- and pi-components in the U-Re bond and this is the first time that the latter component has been reported in an unsupported f-element-transition metal bond.

A Heterobimetallic Gallyl Complex Containing an Unsupported Ga−Y Bond

The synthesis and characterization of the first unsupported Ga-Y bond in [Y{Ga(NArCH)(2)}{C(PPh(2)NSiMe(3))(2)}(THF)(2)] (Ar = 2,6-diisopropylphenyl) is described; structural and computational analyses are consistent with a highly polarized covalent Ga-Y bond.

Triamidoamine uranium(IV)–arsenic complexes containing one-, two- and threefold U–As bonding interactions

To further our fundamental understanding of the nature and extent of covalency in uranium–ligand bonding, and the benefits that this may have for the design of new ligands for nuclear waste separation, there is burgeoning interest in the nature of uranium complexes with soft- and multiple-bond-donor ligands. Despite this, there have so far been no examples of structurally authenticated molecular uranium–arsenic bonds under ambient conditions. Here, we report molecular uranium(IV)–arsenic complexes featuring formal single, double and triple U–As bonding interactions. Compound formulations are supported by a range of characterization techniques, and theoretical calculations suggest the presence of polarized covalent one-, two- and threefold bonding interactions between uranium and arsenic in parent arsenide [U–AsH2], terminal arsinidene [U=AsH] and arsenido [U≡AsK2] complexes, respectively. These studies inform our understanding of the bonding of actinides with soft donor ligands and may be of use in future ligand design in this area. The nature of actinide–ligand bonding is attracting attention, in particular in the context of nuclear waste separations. Structurally authenticated one-, two- and threefold uranium–arsenic bonding interactions are now reported. Computational analysis suggests the presence of polarized σ2, σ2π2, and σ2π4 in the arsenide, terminal arsinidene, and arsenido complexes, respectively.

A Crystallizable Dinuclear Tuck-In-Tuck-Over Tuck-Over Dialkyl Tren Uranium Complex and Double Dearylation of BPh4−To Give the BPh2-Functionalized Metallocycle [U{N(CH2CH2NSiMe3)2(CH2CH2NSiMe2CHBPh2)}(THF)]

The unprecedented formation of a dinuclear tuck-in-tuck-over tuck-over dialkyl Tren-uranium(IV) complex and the first example of double dearylation of BPh(4)(-) in a molecular context to give a BPh(2)-functionalized uranium metallocycle are reported.

The role of 5f-orbital participation in unexpected inversion of the σ-bond metathesis reactivity trend of triamidoamine thorium(IV) and uranium(IV) alkyls

We report on the role of 5f-orbital participation in the unexpected inversion of the σ-bond metathesis reactivity trend of triamidoamine thorium(IV) and uranium(IV) alkyls. Reaction of KCH2Ph with [U(TrenTIPS)(I)] [2a, TrenTIPS = N(CH2CH2NSiPri3)33−] gave the cyclometallate [U{N(CH2CH2NSiPri3)2(CH2CH2NSiPri2C[H]MeCH2)}] (3a) with the intermediate benzyl complex not observable. In contrast, when [Th(TrenTIPS)(I)] (2b) was treated with KCH2Ph, [Th(TrenTIPS)(CH2Ph)] (4) was isolated; which is notable as Tren N-silylalkyl metal alkyls tend to spontaneously cyclometallate. Thermolysis of 4 results in the extrusion of toluene and formation of the cyclometallate [Th{N(CH2CH2NSiPri3)2(CH2CH2NSiPri2C[H]MeCH2)}] (3b). This reactivity is the reverse of what would be predicted. Since the bonding of thorium is mainly electrostatic it would be predicted to undergo facile cyclometallation, whereas the more covalent uranium system might be expected to form an isolable benzyl intermediate. The thermolysis of 4 follows well-defined first order kinetics with an activation energy of 22.3 ± 0.1 kcal mol−1, and Eyring analyses yields ΔH‡ = 21.7 ± 3.6 kcal mol−1 and ΔS‡ = −10.5 ± 3.1 cal K−1 mol−1, which is consistent with a σ-bond metathesis reaction. Computational examination of the reaction profile shows that the inversion of the reactivity trend can be attributed to the greater f-orbital participation of the bonding for uranium facilitating the σ-bond metathesis transition state whereas for thorium the transition state is more ionic resulting in an isolable benzyl complex. The activation barriers are computed to be 19.0 and 22.2 kcal mol−1 for the uranium and thorium cases, respectively, and the latter agrees excellently with the experimental value. Reductive decomposition of “[U(TrenTIPS)(CH2Ph)]” to [U(TrenTIPS)] and bibenzyl followed by cyclometallation to give 3a with elimination of dihydrogen was found to be endergonic by 4 kcal mol−1 which rules out a redox-based cyclometallation route for uranium.

An unsupported uranium-rhenium complex prepared by alkane elimination

Understanding the nature of metal–metal bonds is fundamentally important to furthering our understanding of chemical bonding. This is particularly relevant to the fblock elements because there is continued debate over the degree of covalency in interactions involving f-block metal centres. Although compounds containing metal–metal bonds involving dand p-block elements are well known, and now even examples from the s-block have been reported, examples of uranium–metal complexes remain scarce. Prior to our initiation of a programme of research investigating uranium–metal bonds, structurally characterised examples of species with U M bonds were limited to the p-block derivatives [{(Ar) ACHTUNGTRENNUNG(tBu)N}3USiACHTUNGTRENNUNG(SiMe3)3] (Ar=3,5-Me2C6H3), [(hC5H5)3USnPh3], [7] and [(h-C5H4SiMe3)3UE ACHTUNGTRENNUNG(h5-C5Me5)] (E= Al, Ga). We have been investigating the chemistry of uranium– metal complexes supported by tripodal triamido ligands, which has resulted in the characterisation of the U Ga complex [(Tren) ACHTUNGTRENNUNG(THF)UGa ACHTUNGTRENNUNG(NAr’CH)2] [1, Ar’=2,6iPr2C6H3; Tren TMS =N(CH2CH2NSiMe3)3], [10] the U Re complex [(Tren)URe ACHTUNGTRENNUNG(h5-C5H5)2] (2), and two U Re complexes [(Ts) ACHTUNGTRENNUNG(THF)URe ACHTUNGTRENNUNG(h5-C5H5)2] [3, Ts =HCACHTUNGTRENNUNG(SiMe2NAr)3; Ar=3,5-Me2C6H3) and [(Ts)URe ACHTUNGTRENNUNG(h5-C5H5)2] (4). Complexes 1–4 are noteworthy because they exhibit s and p contributions within the uranium–metal interactions, although the latter component is clearly very weak. Compounds 1 and 2 were prepared by salt elimination, whereas complexes 3 and 4 were prepared by amine elimination. Although alkane elimination has proven to be a very successful strategy for preparing rare earth–metal bonds, it has not yet been applied to the synthesis of uranium–metal bonds; uranium alkyls tend to suffer from thermal instability, and since alkyls are reducing, the redox chemistry of uranium might be anticipated to interfere with uranium– metal bond formation. In search of a suitable uranium–alkyl complex with which to test whether or not alkane elimination can be a useful strategy for constructing uranium–metal bonds, our attention was drawn to the cyclometallated alkyl triamidoamine complex [U{N(CH2CH2NSiMe2tBu)2(CH2CH2NSiMetBuCH2)}] (5) developed by Scott and co-workers. [15] Complex 5 is stable at room temperature, in an inert atmosphere, and straightforward to prepare. This presents an excellent opportunity to examine whether alkane elimination is compatible with the construction of uranium–metal bonds. Herein, we show for the first time that a uranium–metal bond can be prepared by formal alkane elimination. Additionally, we report that the U Re interaction is predominantly ionic with a weak p contribution involving the U and Re centres, as revealed by analyses of the calculated electron density and frontier Kohn–Sham orbitals. The addition of toluene to a mixture of purple 5 and yellow rhenocene hydride resulted in a dark red solution. Following work-up, red crystals of [(Tren)URe ACHTUNGTRENNUNG(h5C5H5)2] [6, Tren DMSB =N(CH2CH2NSiMe2tBu)3]) were isolated in 46 % yield (Scheme 1). The H NMR spectrum exhibits paramagnetically shifted resonances over the range d= 19.85 to +5.64 ppm. The meff of 6 ranged from 0.31 to 2.86 mB over the temperature range 1.8–300 K (Figure 1), and clearly tends to zero, which is characteristic of a H4 uranium(IV) complex. The crystal structure of complex 6 is illustrated in Figure 2 with selected bond lengths. The uranium centre is five-coordinate, but, setting the U(1) N(4) bond to one [a] B. M. Gardner, Dr. J. McMaster, F. Moro, Dr. W. Lewis, Prof. A. J. Blake, Dr. S. T. Liddle School of Chemistry, University of Nottingham University Park, Nottingham, NG7 2RD (UK) Fax: (+44) 115-951-3563 E-mail : stephen.liddle@nottingham.ac.uk Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201100682.

The nature of unsupported uranium-ruthenium bonds: a combined experimental and theoretical study.

Four new uranium-ruthenium complexes, [(Tren(TMS))URu(η(5)-C(5)H(5))(CO)(2)] (9), [(Tren(DMSB))URu(η(5)-C(5)H(5))(CO)(2)] (10), [(Ts(Tolyl))(THF)URu(η(5)-C(5)H(5))(CO)(2)] (11), and [(Ts(Xylyl))(THF)URu(η(5)-C(5)H(5))(CO)(2)] (12) [Tren(TMS)=N(CH(2)CH(2)NSiMe(3))(3); Tren(DMSB)=N(CH(2)CH(2)NSiMe(2)tBu)(3)]; Ts(Tolyl)=HC(SiMe(2)NC(6)H(4)-4-Me)(3); Ts(Xylyl)=HC(SiMe(2)NC(6)H(3)-3,5-Me(2))(3)], were prepared by a salt-elimination strategy. Structural, spectroscopic, and computational analyses of 9-12 shows: i) the formation of unsupported uranium-ruthenium bonds with no isocarbonyl linkages in the solid state; ii) ruthenium-carbonyl backbonding in the [Ru(η(5)-C(5)H(5))(CO)(2)](-) ions that is tempered by polarization of charge within the ruthenium fragments towards uranium; iii) closed-shell uranium-ruthenium interactions that can be classified as predominantly ionic with little covalent character. Comparison of the calculated U-Ru bond interaction energies (BIEs) of 9-12 with the BIE of [(η(5)-C(5)H(5))(3)URu(η(5)-C(5)H(5))(CO)(2)], for which an experimentally determined U-Ru bond disruption enthalpy (BDE) has been reported, suggests BDEs of approximately 150 kJ mol(-1) for 9-12.

Halide, amide, cationic, manganese carbonylate, and oxide derivatives of triamidosilylamine uranium complexes

Treatment of the complex [U(Tren(TMS))(Cl)(THF)] [1, Tren(TMS) = N(CH(2)CH(2)NSiMe(3))(3)] with Me(3)SiI at room temperature afforded known crystalline [U(Tren(TMS))(I)(THF)] (2), which is reported as a new polymorph. Sublimation of 2 at 160 °C and 10(-6) mmHg afforded the solvent-free dimer complex [{U(Tren(TMS))(μ-I)}(2)] (3), which crystallizes in two polymorphic forms. During routine preparations of 1, an additional complex identified as [U(Cl)(5)(THF)][Li(THF)(4)] (4) was isolated in very low yield due to the presence of a slight excess of [U(Cl)(4)(THF)(3)] in one batch. Reaction of 1 with one equivalent of lithium dicyclohexylamide or bis(trimethylsilyl)amide gave the corresponding amide complexes [U(Tren(TMS))(NR(2))] (5, R = cyclohexyl; 6, R = trimethylsilyl), which both afforded the cationic, separated ion pair complex [U(Tren(TMS))(THF)(2)][BPh(4)] (7) following treatment of the respective amides with Et(3)NH·BPh(4). The analogous reaction of 5 with Et(3)NH·BAr(f)(4) [Ar(f) = C(6)H(3)-3,5-(CF(3))(2)] afforded, following addition of 1 to give a crystallizable compound, the cationic, separated ion pair complex [{U(Tren(TMS))(THF)}(2)(μ-Cl)][BAr(f)(4)] (8). Reaction of 7 with K[Mn(CO)(5)] or 5 or 6 with [HMn(CO)(5)] in THF afforded [U(Tren(TMS))(THF)(μ-OC)Mn(CO)(4)] (9); when these reactions were repeated in the presence of 1,2-dimethoxyethane (DME), the separated ion pair [U(Tren(TMS))(DME)][Mn(CO)(5)] (10) was isolated instead. Reaction of 5 with [HMn(CO)(5)] in toluene afforded [{U(Tren(TMS))(μ-OC)(2)Mn(CO)(3)}(2)] (11). Similarly, reaction of the cyclometalated complex [U{N(CH(2)CH(2)NSiMe(2)Bu(t))(2)(CH(2)CH(2)NSiMeBu(t)CH(2))}] with [HMn(CO)(5)] gave [{U(Tren(DMSB))(μ-OC)(2)Mn(CO)(3)}(2)] [12, Tren(DMSB) = N(CH(2)CH(2)NSiMe(2)Bu(t))(3)]. Attempts to prepare the manganocene derivative [U(Tren(TMS))MnCp(2)] from 7 and K[MnCp(2)] were unsuccessful and resulted in formation of [{U(Tren(TMS))}(2)(μ-O)] (13) and [MnCp(2)]. Complexes 3-13 have been characterized by X-ray crystallography, (1)H NMR spectroscopy, FTIR spectroscopy, Evans method magnetic moment, and CHN microanalyses.