Arene-, Chlorido-, and Imido-Uranium Bis- and Tris(boryloxide) Complexes

We introduce the boryloxide ligand {(HCNDipp)2BO}− (NBODipp, Dipp = 2,6-di-isopropylphenyl) to actinide chemistry. Protonolysis of [U{N(SiMe3)2}3] with 3 equiv of NBODippH produced the uranium(III) tris(boryloxide) complex [U(NBODipp)3] (1). In contrast, treatment of UCl4 with 3 equiv of NBODippK in THF at room temperature or reflux conditions produced only [U(NBODipp)2(Cl)2(THF)2] (2) with 1 equiv of NBODippK remaining unreacted. However, refluxing the mixture of 2 and unreacted NBODippK in toluene instead of THF afforded the target complex [U(NBODipp)3(Cl)(THF)] (3). Two-electron oxidation of 1 with AdN3 (Ad = 1-adamantyl) afforded the uranium(V)–imido complex [U(NBODipp)3(NAd)] (4). The solid-state structure of 1 reveals a uranium–arene bonding motif, and structural, spectroscopic, and DFT calculations all suggest modest uranium–arene δ-back-bonding with approximately equal donation into the arene π4 and π5 δ-symmetry π* molecular orbitals. Complex 4 exhibits a short uranium(V)–imido distance, and computational modeling enabled its electronic structure to be compared to related uranium–imido and uranium–oxo complexes, revealing a substantial 5f-orbital crystal field splitting and extensive mixing of 5f |ml,ms⟩ states and mj projections. Complexes 1–4 have been variously characterized by single-crystal X-ray diffraction, 1H NMR, IR, UV/vis/NIR, and EPR spectroscopies, SQUID magnetometry, elemental analysis, and CONDON, F-shell, DFT, NLMO, and QTAIM crystal field and quantum chemical calculations.


■ INTRODUCTION
It is often stated that ligand−metal complementarity is an essential component of the recipe for controlling the behavior of metal ions in coordination and organometallic complexes.−14 In recent years, we have made extensive use of triamidoamine (Tren) ligands to stabilize a wide range of novel An−ligand multiple bonds, 15−28 An−metal bonds, 15,29−35 novel main group moieties, 36−43 uranyl activation, 44 small molecule activation, 45−48 single-molecule magnets (SMMs), 49 and novel photochemical rearrangements, 50 and provide insight into fundamental f-block phenomena such as disproportionation, the inverse trans influence, pushing from below, 29,49,51−54 and NMR covalency studies. 55,56However, there are situations where Tren ligands reach their limitations.For example, while high oxidation state uranium(V) and uranium(VI) oxos, imidos, and nitridos and even a neptunium(V)−oxo are now straightforwardly accessible with Tren ancillary ligands, 16,19,22,[26][27][28][44][45][46][47]49,51,52,56 to date, uranium(IV) has proven to be the highest accessible oxidation state when paired with softer elements such as phosphorus, arsenic, antimony, and bismuth, with all attempts to oxidize resulting in uranium(IV) products exclusively being isolated.17,24,36,40 Although alkoxides and aryloxides are logical alternatives on hard−soft acid−base (HSAB) grounds, they do not always possess the ideal steric profiles for specific applications.
We have previously utilized the boryloxide ligand {(HCNDipp) 2 BO} − (NBO Dipp , Dipp = 2,6-di-isopropylphenyl) 57,58 to prepare rare earth complexes 59 including the dysprosium complex [Dy{OB(NDippCH) 2 } 2 (THF) 4 ][BPh 4 ] that exhibits a notably high SMM relaxation energy barrier. 59aving considered the structure of that dysprosium complex, we surmised that assembling three of those NBO Dipp ligands at actinide ions might produce sterically encumbered complexes with one or at most two well-protected reaction pockets at the metal, resulting from a "picket-fence" arrangement of Dipp groups, while also being capable of stabilizing high oxidation state actinide ions on HSAB principles.We also considered that the boryl center could provide an electronic "buffer", where the oxide can modulate its donor strength to a metal and transfer any excess electron density into the vacant boryl p orbital.Alternatively, if the boryl center is still electron deficient, it could make up the difference with π donation from the two α-nitrogen atoms in the NBO scaffold or not accept any N atom π donation if already electronically saturated.Further motivation stemmed from the fact that there are relatively few f-element boryloxide complexes, 60,61 which as R 2 BO − species are distinct from the far more prevalent borate derivatives, 62−78 and they have very different steric profiles from NBO Dipp .This is because R 2 BO − ligands tend to have the ligand sterics pointing away from a coordinated metal, whereas with NBO Dipp , they point toward the metal.Thus, we considered the NBO Dipp ligand to have significant potential for metal−ligand complementarity.
Here, we report the introduction of the NBO Dipp ligand into actinide chemistry through the synthesis of uranium(III), uranium(IV), and uranium(V) derivatives which showcases the flexibility of the NBO Dipp ligand to stabilize a range of actinide oxidation states.The uranium(III) derivative exhibits a uranium−arene interaction, where structural, spectroscopic, and computational methods consistently suggest uranium− arene δ-back-bonding with approximately equal donation into the π 4 and π 5 δ-symmetry π* molecular orbital combinations of the arene.However, the uranium−arene interaction is evidently weak, as demonstrated by straightforward twoelectron oxidation to provide an imido complex.NIR spectroscopic and computational studies on the imido enable its contextualization with existing related uranium−imido, uranium−nitrido, and uranium−oxo derivatives.These new complexes thus represent a new family of synthetic precursors for further elaboration.

■ RESULTS AND DISCUSSION
Synthesis.Treatment of [U{N(SiMe 3 ) 2 } 3 ] 79,80 with 3 equiv of NBO Dipp H 57,58 results, after a 2 day stir and workup, in the uranium(III)−tris(boryloxide) complex [U(NBO Dipp ) 3 ] (1) as an analytically pure dark purple solid in 51% yield, Scheme 1a; the HN(SiMe 3 ) 2 byproduct is conveniently removed, either when the reaction solvent is removed in vacuo or when the resulting solid is washed with pentane.Although the reaction is evidently driven thermodynamically by the formation of U−O bonds and the respective pK a values of the protic components of the reaction, it is clearly kinetically sluggish.This likely reflects the kinetic barrier to installing three sterically demanding NBO Dipp ligands at a single metal center, even one as large as uranium(III) (Shannon uranium-(III) six-coordinate ionic radius = 1.025Å). 81 Support for this suggestion can be found in the issues encountered in the synthesis of the chlorido analogs (see below).
Compound 1 constitutes a potentially useful starting material, where the redox chemistry of uranium(III) may be utilized (see below for an example), and seeking to extend the range of uranium−NBO Dipp synthetic precursors, we targeted a uranium(IV)−chlorido derivative for applications in salt elimination chemistry, Scheme 1b.Accordingly, we treated UCl 4 with 3 equiv of NBO Dipp K in THF, and the green solution turned brown during a 16 h stir.After workup, we isolated yellow [U(NBO Dipp ) 2 (Cl) 2 (THF) 2 ] (2) in 80% crystalline yield.The third equivalent of unreacted NBO Dipp K was found to be in the mother liquor by 1 H NMR spectroscopy.The fact that only 2 equiv of NBO Dipp substituted onto uranium by salt elimination is consistent with the slow substitution by protonolysis that produces 1. Repeating the reaction but refluxing in THF made no difference to the reaction outcome.However, when first stirring the 3:1 NBO Dipp K:UCl 4 mixture in THF, then replacing the THF with toluene, and refluxing, a dark green solid was isolated after workup.Recrystallization from toluene afforded [U(NBO Dipp ) 3 (Cl)(THF)] (3) as yellow-green crystals in 71% yield.We suggest that in THF the coordination sphere of uranium is permanently saturated with THF molecules, blocking installation of the third NBO Dipp ligand but that in toluene any loss of THF is not compensated by the bulk solvent, thus opening up the coordination sphere at uranium to enable salt elimination and installation of the third NBO Dipp ligand at uranium to occur.Alternatively, or in addition to, the partial solubility of KCl in THF may establish an equilibrium that is driven to products by the insolubility of KCl in toluene.
Experiments to derivatize 2 and 3 are ongoing and will be reported in due course.However, in a preliminary demonstration of the utility of 1 as a useful synthetic precursor the brown uranium(V)−imido complex [U(NBO Dipp ) 3 (NAd)] (4, Ad = 1-adamantyl) was prepared in a straightforward and essentially quantitative two-electron oxidation of 1 with AdN 3 with concomitant elimination of N 2 (65% crystalline yield), Scheme 1a.Thus, although the coordinated arene in 1 may be regarded as providing steric protection of the uranium ion, it is labile enough to not impede further reactivity.
NMR and IR Spectroscopies. 1 H NMR and IR spectroscopic data for 1−4 can be found in the Supporting Information (Figures S1−S8).Despite the different coordination modes of the NBO Dipp ligands in the solid-state structure of 1, the 1 H NMR spectrum of 1 spans the range from −2 to 10 ppm and indicates a symmetrical species in solution on the NMR time scale with 3-fold symmetry; this was not probed by variable-temperature experiments due to precipitation of 1.However, two i Pr-Me resonances and hence environments are observed, reflecting 12 Me groups that point toward the uranium ion and 12 that point away due to restricted rotation of the Dipp-i Pr groups.The ATR-IR spectrum of 1 is unremarkable, as expected.
The 1 H NMR spectra of 2 and 3 span the ranges from −72 to 46 and from −20 to 13 ppm, which for the former is qualitatively an unusually large range for uranium(IV), but this likely reflects the varied coordination environment in that complex; the range for 3 is unexceptional for uranium(IV).The IR data for 2 and 3 are unremarkable but are consistent with their formulations.
The 1 H NMR spectrum of 4 spans the range from −1 to 17 ppm, which is qualitatively as expected for a uranium(V) complex.The IR spectrum of 4 is as expected, and we note that there are absorptions around 1056−972 and 707−648 cm −1 consistent with the prediction of U�N stretches at 1000 and 663 cm −1 from an analytical frequencies calculation; those vibrations are best described as AdN�U bond stretches where the entire AdN unit moves together and at lower energy stretches where the U and Ad groups stretch out and in together about the N center.
SQUID Magnetometry.Variable-temperature SQUID magnetometry data for 1−4 can be found in the Supporting Information (Figures S9−S16) The uranium(III) formulation of 1 is confirmed by variable-temperature SQUID magnetometry of powdered 1, Figure 5, where the effective magnetic moment is 2.50 μ B at 300 K, and this decreases smoothly to ∼30 K, at which point there is a more rapid decrease, reaching 1.59 μ B at 1.8 K; the high-temperature effective magnetic moment of 1 is low for uranium(III), but this has been observed for uranium(III)−aryloxides, 88,89 though otherwise this behavior is typical of uranium(III). 9,88,90Magnetization vs field experiments at 2 K are approaching but do not reach saturation at the highest available field (∼0.9 μ B , 7 T), which is consistent with a Kramers uranium(III) ion. 91he variable-temperature SQUID magnetometry results of 2 and 3, Figure 5, reveal effective magnetic moments of 2.31 and 2.31 μ B at 300 K, respectively, and these values decrease smoothly over the temperature range reaching 0.12 and 0.25 μ B at 1.8 K and tending to zero, which is typical of magnetic singlet uranium(IV) ions subject to modest temperatureindependent paramagnetism. 9,88However, we do note that the 300 K μ eff values for 2 and 3 are lower than the theoretical Russell−Saunders magnetic moment of 3.58 μ B for 3 H 4 ions, but this is known for uranium−aryloxides. 88,92Nevertheless, at 2 K the magnetization vs field data for 2 and 3 are both still  rising with no sign of approaching saturation up to the highest available field (∼0.09 μ B for 2 and 3, 7 T), which confirms the uranium(IV) formulations of 2 and 3. 93 The variable-temperature SQUID magnetometry data on 4, Figure 5, reveal an almost constant effective magnetic moment of 1.69 μ B across the temperature range measured, except for a small, decisive drop below 10 K to a final value of 1.60 μ B at 1.8 K, likely resulting from depopulation of some low-lying states at very low temperature.This is similar to [U(NAd)-(Tren TIPS )] 27 and [U(O)(Tren TIPS )], 49 86 which exhibit effective magnetic moments of 2.47 and 2.12 μ B .The flat μ eff vs T plot for 4 suggests that at high temperature either only the ground state is thermally populated or any excited states have similar effective magnetic moments.The magnetization vs field data for 4 at 2 K are approaching saturation, suggesting a well-isolated electronic ground state, though saturation is not quite achieved up to the highest available field strength available (∼0.8 μ B , 7 T).Based on our prior analysis of the electronic structure of uranium nitrido complexes as a guide 22 and noting that precise values of magnetization data will vary depending on the ligand field and the g values, magnetization data of ∼0.9 μ B imply a m j ground state of 5/2 for 4 rather than 3/2, which would have a lower magnetization value of ∼0.5 μ B .
EPR Spectroscopy.The X-band EPR spectrum of powdered 1, Figure 6a, is consistent with the rhombic formulation anticipated from arene coordination to uranium-(III) without a strong axial ligand, 89 exhibiting anisotropic g values of g x = 2.680, g y = 1.776, and g z = 1.039.In all samples examined there is also an isotropic feature at 2.002.We note that the g value for benzenide radical anions is also 2.002, 94 and given the relative absorption intensities, the feature at g = 2.002 is assigned as a minor radical impurity.From , the anisotropic EPR g values suggest that the magnetic moment of 1 at 5 K should be 1.68 μ eff , which is in good agreement with the measured effective magnetic moment at 5 K (1.73 μ B ).
The X-band EPR spectrum of powdered 4, Figure 6b, presents an axial-type spectrum with g values of g x = g y = 0.564 and g z = 3.045.A g = 2.002 feature is also observed but is assigned as a minor organic radical impurity, likely of benzenide origin. 94Using , the anisotropic EPR g values suggest that the magnetic moment of 4 at 5 K should be 1.57μ eff , which is in good agreement with the measured effective magnetic moment at 5 K (1.63 μ B ).In C 3v symmetry, a pure m j = 3/2 state would be EPR silent (g x = g y = 0), as found for [U(O)(Tren TIPS )], 49 because the J = ±1 EPR selection rule cannot be met; however, with changes to the geometry and ligand field and with m j mixing, that selection rule can be met and g values of g x = g y = 0.1−0.5 and g z = ∼2.4 would be anticipated.For m j = 5/2, that is typically mixed with m j = 7/2, g values of g x = g y = ∼0.3 and g z = ∼3.7 are usually found.Thus, the observed g z values for 4 can be compared to those of [U(NAd){N(SiMe 3 ) 2 } 3 ] 85 (g z = 3.60), [U(NSiMe 3 )-{N(SiMe 3 ) 2 } 3 ] 85 (g z = 2.50), and [U(O){N(SiMe 3 ) 2 } 3 ] 85,87 (g z = 2.17), suggesting that the ground state of 4 is m j = 5/2, which is also consistent with the magnetometry data.
UV/vis/NIR Spectroscopy.UV/vis/NIR spectra for 1−4 were collected (see Supporting Information, Figures S17− S20).All complexes exhibit absorptions in the range from 25 000 to 30 000 cm −1 , which correspond to π−π* transitions of the NBO Dipp ligands.The UV/vis/NIR spectrum of 1, Figure 7, exhibits absorptions in the range from 5500 to ∼26 000 cm −1 , which are assigned as f−f transitions due to their approximate intensities (ε = ∼250−800 M −1 cm −1 , these values should be regarded as upper bounds rather than intrinsic molar absorptivities). 9Above 26 000 cm −1 , the spectrum is dominated by charge transfer bands.It is notable that there are not any absorptions in the region from 15 000 to 22 000 cm −1 with large enough intensities (∼1500 M −1 cm −1 ) to be assigned as f−d transitions.This suggests little stabilization of the 6d orbitals by the ligand field of 1 and that the f−d absorptions of 1 reside under the charge transfer bands at higher energy.
The UV/vis/NIR spectra of 2 and 3, Figure 7, exhibit weak absorptions in the NIR region that are characteristic of uranium(IV) ions, 9 and then, charge transfer bands tail into the visible region from the UV zone.
While the zeroth-order analysis is intuitive and provides a useful initial electronic structure description, it neglects symmetry-allowed mixing of the states.In order to probe this aspect, magnetic data and energies were fitted simultaneously using CONDON 3.0 99,100 in C 3v symmetry (see Figure S21 for magnetic data fits), and F-shell 101 was used to determine the full composition of the states (see the SI for details about the modeling procedure).F-shell outputs the compositions in the |L, S, J, m j ⟩ basis (coupled basis); therefore, these were converted into the |m l , m s ⟩ basis (uncoupled basis) using Clebsch−Gordan coefficients 102 as reported in Table 1.The fit produced the crystal field parameters (expressed in Wybourne notation) B .In general, the experimental magnetic data and NIR energies are modeled well, especially considering that a single ζ value is used in the calculations, but in reality, this and the associated orbital angular momentum would vary across different orbitals depending on their nature; for example, orbital mixing and hence reduced ζ and orbital angular momentum would be ordered 5f σ > 5f π ≫ 5f ϕ ≈ 5f δ .
Since in 4 the SOC and CF parameters are comparable in strength, it would be predicted that there would be substantial mixing of the J = 7/2 and J = 5/2 manifolds.Inspection of Table 1 reveals that when mixing of states is modeled, the initial zeroth-order model is largely maintained.Indeed, the largest component of all states is the one predicted from the zeroth order except for the doublet at 7525 cm −1 , which is composed by similar amounts of |2,+1/2⟩ and |3,−1/2⟩.It is noticeable that the calculated extent of mixing of states in 4 appears to be significantly more than that reported for [U(NR){N(SiMe 3 ) 2 } 3 ] (R = Ad, SiMe 3 ), 85 which we suggest stems from a larger crystal field for 4 resulting in more mixing overall.Hence, it would seem that the NBO Dipp ligand brings more crystal field splitting than the bis(trimethylsilyl)amide, consistent with their O vs N donor natures.Therefore, Table 1 also lists each component as its Russell−Saunders state and m j projection, showing that they are indeed significantly mixed, and largely the calculated 22,85 again, where there are differences (the third and fourth absorptions), the second largest component of each m j projection is the same as that for [U(NAd){N(SiMe 3 ) 2 } 3 ] and [U(N)(Tren TIPS )] − .This emphasizes that although 4 has pseudo-C 3v symmetry (and is in reality lower symmetry), the strength of the imido ligand confers substantial effective axial symmetry, as found for [U(E){N(SiMe 3 ) 2 } 3 ] − (E = NSiMe 3 , O). 93 Although it should be acknowledged that there is some uncertainty in the precise CF splittings of 4 (∼17 500 cm −1 ), [U(NAd){N(SiMe 3 ) 2 } 3 ] (15 434 cm −1 ), [U(NSiMe 3 ){N-(SiMe 3 ) 2 } 3 ] (16 955 cm −1 ), and [U(O){N(SiMe 3 ) 2 } 3 ] (20 262 cm −1 ), 22,85 due to the broadness and shoulder character of their U−E σ* (E = NR, O) absorptions, it is instructive to consider them in qualitative terms.The larger CF splitting of 4 vs [U(NAd){N(SiMe 3 ) 2 } 3 ] can be primarily rationalized on the basis that the U�NAd distance in 4 is ∼0.04 Å shorter than that in the latter, 85 and then, secondarily, the NBO Dipp ligands also contribute to a larger CF.For 4 vs [U(NSiMe 3 ){N(SiMe 3 ) 2 } 3 ], the U�NR distances are essentially the same, 86 so even though the U�NSiMe 3 σ* antibonding state should be higher in energy than U�NAd on a like for like basis (because the silylimido is less strongly donating than the alkylimido so will have slightly more stabilized bonding orbitals), the similar U�NR distances  87 than that in 4, and this seems to outweigh any secondary NBO Dipp vs amide CF effects.
Computational Characterization.In order to further understand complexes 1−4, we performed GGA LDA BP86 DFT calculations on the whole structures (Tables S2−S5).The gas-phase geometry-optimized structures (Table S6) match the experimental solid structures well overall, with U− O NBO , U−O THF , U−Cl, B−O, and B−N distances computed to within 0.03 Å and 5°.Computed bond orders, charges, spin densities, natural localized molecular orbital (NLMO), and quantum theory of atoms in molecules (QTAIM) data are compiled in Tables S6−S8.We note a discrepancy between experimental and computed U−C arene distances in 1 (Δ = 0.13 Å), which likely arises from this being a relatively "soft" interaction and that the calculations are free of solid-state effects in the experimental structure of 1. Geometry optimization using the hybrid PBE0 functional only marginally improved the discrepancy of the U−C arene distance (Δ = 0.09 Å), so we examined the crystallographic coordinates with heavy atoms frozen and H atom positions geometry optimized and found only minor changes to the electronic structure of 1.Thus, to provide an internally consistent set of data, we focus on the geometry-optimized BP86 data here.We conclude that the BP86 data provide qualitatively reliable models of the electronic structures of these two complexes since prior work has shown good bond order and NBO/NLMO analysis agreement between BP86 and experimentally (NMR) benchmarked hybrid B3LYP chemical shift anisotropy calculations. 55,56,103he computed uranium MDC q charges for 1−4, Table S6, are 2.52, 2.24, 2.40, and 3.01 and are as anticipated for uranium ions in +3, +4, +4, and +5 oxidation states, respectively.The corresponding MDC m spin densities of −2.51, −2.24, −2.25, and −1.31 are consistent for 2, 3, and 4 in terms of their formal f n counts (n = 2, 2, and 1, respectively) augmented by net donation of electron density from the Magnetic data and energies were fitted simultaneously using CONDON 3.0 in C 3v symmetry, and F-shell was used to determine the composition of the states.F-shell outputs the composition as |L, S, J, m j ⟩, and these were converted into the |m l , m s ⟩ basis using Clebsch−Gordan coefficients.b Outside the measurement window.

Inorganic Chemistry
ligands.However, the spin density for 1 is less than the anticipated value of 3 for a 5f 3 ion, suggesting that the uranium ion in 1 is a net exporter of electron density.The arene bonded to uranium has a computed net MDC q charge of −0.S6.This shows that the NBO Dipp ligands are strong donors and reflects the increasing formal uranium charges on moving from trivalent 1 to tetravalent 2 and 3.The fall in U−O NBO bond order in 4 is likely due to the presence of the strong imido donor ligand.Reflecting that the U−arene δ-symmetry bonding interactions exhibit only modest carbon contributions, the U−C Nalewajski−Mrozek bond orders average 0.31, which is certainly consistent with those interactions being weak given the apparent loss of the uranium−arene interaction in solution as evidenced by 1 H NMR spectroscopy.Although the changes are small, Table S6, the variation of the B−O and B−N Nalewajski−Mrozek bond orders generally confirms that when the U−O NBO bond order increases/decreases, the B−O and B−N bond orders decrease/increase and increase/decrease, respectively.This reflects the aforementioned electronic buffering role of the boryl as the oxide donates less and more to electron-rich and -poor metals, respectively.The U− N imido Nalewajski−Mrozek bond order is 2.80, reflecting that the imido ligand is a triple-bond donor in the KSMO scheme.
Complexes 1−4 provide an opportunity to examine the variation of the U−O NBO bond as a function of the uranium oxidation state, and NLMO data are compiled in Table S7.The NLMO analysis reveals rather polar U−O NBO bonds, and although the entire range of uranium contributions is only 3− 8%, it can be generally noted that the uranium contributions increase with increasing oxidation state and the uranium 5f character tends to dominate over 6d contributions (5f:6d ≈ 2:1 for σ bonds and ∼3:1 for π bonds).Complex 2 exhibits the largest uranium contributions to the U−O NBO bonds, likely  reflecting the presence of two neutral THF donors.This can also be seen in the U−Cl bonds, where the uranium contributes more to the U−Cl bonds in 2 than 3. NLMO analysis of 4, Figure 10, reveals a U�N σ bond that is 11% uranium and 88% N character.The uranium component is composed of 1/3/38/59% 7s/7p/6d/5f character, and the nitrogen is 65:35 2s:2p character.The U�N π bonds are essentially the same and are composed of 22% and 77% uranium and nitrogen contributions, respectively.The uranium component is 0/0/13/87% 7s/7p/6d/5f character, and the nitrogen components are 100% 2p character.
To gain a topological-rather than orbital-based perspective, we examined the 3,−1 bond critical point QTAIM data for 1− 4, Table S8.As expected, the data reveal slightly polar− covalent B−O and B−N bonds (ρ av = 0.21 and 0.19 e per Bohr 3 ; H av = −0.15 and −0.16 hartree per Bohr 3 , respectively), whereas the U−O NBO bonds are borderline ionic (ρ av = 0.10 e per Bohr 3 ) and much weaker in terms of energy (H av = −0.04hartree per Bohr 3 ).In contrast, the U−N imido bond is a much stronger interaction, with ρ and H values of 0.19 e per Bohr 3 and −0.14 hartree per Bohr 3 , respectively, and being close to zero, the ε value of 0.05 confirms a U−N triple bond. 109verall, these values are similar to data previously calculated for [U(NSiMe 3 ){N(SiMe 3 ) 2 } 3 ] 27 but indicate a slightly stronger U�N bond than that calculated for [U(NR)-(Tren TIPS )] (R = SiMe 3 , Ad) 27 but, as expected, a weaker U�N bond than the oxo and nitrido bonds in [U(O){N-(SiMe 3 ) 2 } 3 ], 87 [U(O)(Tren TIPS )], 49 [U(N)(Tren TIPS )] n (n = 0, −1), 27,28 and [Np(O)(Tren TIPS )]. 16Lastly, the QTAIM data for the U−arene interactions in 1 reveal ρ av and H av values of 0.03 e per Bohr 3 and −0.01 hartree per Bohr 3 , respectively, and, hence, in agreement with the analysis earlier, rather weak uranium−arene interactions.The uranium−arene ε av value of 0.49 reflects the asymmetry of the individual orbital coefficient bonding interactions that construct the uranium−arene δ bonds.

■ CONCLUSIONS
In conclusion, furthering the application of the boryloxide NBO Dipp ligand in f-element chemistry, we have introduced this ligand to uranium chemistry.This has furnished a uranium(III)−tris(boryloxide) complex that features a uranium−arene interaction, two chlorido derivatives, and a uranium(V)−imido complex.Although the preparation of these complexes is thermodynamically favorable, it is notable that there is kinetic hindrance installing three sterically demanding NBO Dipp ligands at a single metal center, even one as large as uranium(III).The coordinated arene may provide a useful protecting-group role but is evidently flexible enough to not impede reactivity as evidenced by preliminary work that has accessed a uranium(V)−imido complex by a two-electron oxidation strategy.
DFT studies qualitatively evidence modest uranium(III)− arene δ bonding, which appears to be approximately equal donation into the π 4 and π 5 δ-symmetry π* orbitals of the arene; this accounts for an apparent lack of Jahn−Teller distortion of the arene despite the fact it is shown experimentally and computationally to be carrying excess open-shell spin density.NIR and computational analysis of the imido complex shows that it has a similar CF to [U(NSiMe 3 )-{N(SiMe 3 ) 2 } 3 ], a larger CF than [U(NAd){N(SiMe 3 ) 2 } 3 ], but, as expected, a weaker CF than [U(O){N(SiMe 3 ) 2 } 3 ].However, replacing amides with the NBO Dipp ligand results in much more mixing of the |m l ,m s ⟩ states and Russell− Saunders m j projections, but overall, it is the case that their orderings approximate well to a basic zeroth-order model that neglects mixing of states.
The synthesis of uranium(III), uranium(IV), and uranium-(V) complexes supported by a common NBO Dipp ligand demonstrates the versatility of this boryloxide ligand over several oxidation states of uranium and has provided an opportunity to examine the U−NBO Dipp interactions over three uranium oxidation states.This versatility can in part be ascribed to the "buffer" nature of the boryl enabling the NBO Dipp ligand to vary its electron donation as required.Hence, the NBO Dipp ligand promises to stabilize complementary or new bonding motifs in actinide chemistry compared to existing Tren, amide, and aryloxide ligand classes and also suggests potential for further elaboration in lanthanide chemistry.Certainly, the NBO ligand class demonstrates the importance of ligand−metal cooperativity to bring new vistas to f-element chemistry, and work in that regard is underway in our laboratory.

Complex 1 ,
Figure 1, crystallizes with two of NBO Dipp ligands coordinated in terminal monodentate modes with the third NBO Dipp ligand coordinated as an η 6 -κ 1 -chelate, coordinated through one of the Dipp rings as well as the boryloxide O atom.Thus, counting the centroid of the arene ring as a coordination point, 1 adopts a distorted tetrahedral coordination geometry at uranium with a piano-stool geometry defined by the η 6 -arene and the three boryloxide O-donor centers.The three U−O distances are 2.217(3), 2.183(3), and 2.173(3) Å, similar to other uranium−boryloxide distances, 60,61 with the longer U−O distance being associated with the chelating NBO Dipp ligand, reflecting the bent U−O−B angle of 127.6(3)°in the former compared to the latter two U−O−B angles of 162.1(3)°and 162.4(3)°which are much closer to being linear.Although care must be taken when invoking π donation as a function of bond angles, 48,82,83 the more linear U−O−B coordination linkages do, in principle, offer the possibility of stronger donation to uranium and hence shorter U−O bonds compared to the bent U−O−B linkage.The U−C distances span the range 2.913(4)−2.999(4)Å (average 2.970 Å) with a U−C centroid distance of 2.616 Å, which falls in the middle to upper range of uranium(III)−arene distances, 84 likely due to the strain of the chelate ring, which is evident in the slightly bent C centroid −C ipso −N NBO angle of 168.2°.The coordinated arene ring is essentially planar (maximum rms deviations from the arene plane from −0.016 to 0.026 Å), but its C−C distances span the range 1.382(6)− 1.421(6) Å (average 1.406 Å), which suggests some transfer of electron density into its π* orbitals because the average C−C distance in the other Dipp rings in 1 is ∼1.396Å, though the difference is marginal.Complex 2, Figure 2, exhibits a pseudo-octahedral uranium-(IV) ion situated on a crystallographic center of inversion, coordinated to two NBO Dipp , two chloride, and two THF molecules in an all-trans arrangement.The U−O NBO distances are 2.127(2) Å, which is ∼0.05Å shorter than the terminal U− O distances in 1; this is around one-half the difference in the ionic radii of six-coordinate uranium(III) and uranium(IV) (1.025 vs 0.89 Å, respectively), 81 but this likely reflects the very different formal coordination numbers and geometries of 1 vs 2. The U−Cl and U−O THF distances are unremarkable.The structure of 3, Figure 3, reveals a pseudo-trigonal bipyramidal geometry at uranium with the three NBO Dipp ligands positioned in the equatorial plane and the chloride and THF ligands occupying the axial sites.The U−O NBO distances span the range 2.124(2)−2.134(2)Å (average 2.130 Å) and by the 3σ criterion are statistically indistinguishable from the corresponding U−O NBO distances in 2. The U−Cl and U− O THF distances in 3 (2.568(7) and 2.442(2) Å) are unremarkable, though we note that they are shorter and longer, respectively, than the corresponding distances in 2 (U− Cl = 2.648(2) Å; U−O THF = 2.424(2) Å); acknowledging the different uranium coordination numbers of 2 and 3, this most likely reflects the trans-influence effects of Cl vs THF in 3 and Cl vs Cl and THF vs THF ligand arrangements in 2. Complex 4, Figure 4, contains a uranium(V) ion that is four coordinate, adopting a distorted tetrahedral geometry.The U− O distances span the range 2.129(5)−2.153(5)Å (average 2.140 Å) and by the 3σ criterion are indistinguishable from the

Figure 1 .
Figure 1.Molecular structure of 1 at 100 K with selected atom labels.Displacement ellipsoids are set at 40%, and hydrogen atoms and disordered components are omitted for clarity.

Figure 2 .
Figure 2. Molecular structure of 2 at 120 K with selected atom labels.Displacement ellipsoids are set at 40%, and hydrogen atoms and disordered components are omitted for clarity.

Figure 3 .
Figure 3. Molecular structure of 3 at 150 K with selected atom labels.Displacement ellipsoids are set at 40%, and hydrogen atoms and disordered components are omitted for clarity.

Figure 4 .
Figure 4. Molecular structure of 4 at 120 K with selected atom labels.Displacement ellipsoids are set at 30%, and hydrogen atoms and disordered components are omitted for clarity.

Figure 6 .
Figure 6.EPR X-band (∼9.4 GHz) spectra of powdered samples of 1 and 4 at 5 K. Black lines are experimental data, and red lines represent simulated data.(a) 1: rhombic-type spectrum with g values of g x = 2.680, g y = 1.776, and g z = 1.039 and a radical feature at g = 2.002 attributed to a minor organic radical impurity.(b) 4: axial spectrum with g x = g y = 0.564 and g z = 3.045.The sharp feature at g = 2.002 (marked with an asterisk (*)) is associated with a small portion of an organic radical impurity in the sample of 4.

Table 1 .
Calculated Energies and Principal Components of the 5f States for Complex 4 Derived from CONDON 3.0 and F Shell a