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Accessing a Highly Reducing Uranium(III) Complex through Cyclometalation

  • Dieuwertje K. Modder
    Dieuwertje K. Modder
    Group of Coordination Chemistry, Institut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
  • Rosario Scopelliti
    Rosario Scopelliti
    Group of Coordination Chemistry, Institut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
  • , and 
  • Marinella Mazzanti*
    Marinella Mazzanti
    Group of Coordination Chemistry, Institut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
    *Marinella Mazzanti Email: [email protected]
Cite this: Inorg. Chem. 2024, 63, 21, 9527–9538
Publication Date (Web):January 13, 2024
https://doi.org/10.1021/acs.inorgchem.3c03668

Copyright © 2024 American Chemical Society. This publication is licensed under these Terms of Use.

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Abstract

U(IV) cyclometalated complexes have shown rich reactivity, but their low oxidation state analogues still remain rare. Herein, we report the isolation of [K(2.2.2-cryptand)][UIII{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], 1, from the reduction of [UIII{N(SiMe)2}3] with KC8 and 2.2.2-cryptand at room temperature. Cyclic voltammetry studies demonstrate that 1 has a reduction potential similar to that of the previously reported [K(2.2.2-cryptand)][UII{N(SiMe)2}3] (Epc = −2.6 V versus Fc+/0 and Epc = −2.8 V versus Fc+/0, respectively). Complex 1, indeed, shows similar reducing abilities upon reactions with 4,4′-bipyridine, 2,2′-bipyridine, and 1-azidoadamantane. Interestingly, 1 was also found to be the first example of a mononuclear U(III) complex that is capable of reducing pyridine. In addition, it is shown that a wide variety of substrates can be inserted into the U–C bond, forming new U(III) metallacycles. These results highlight that cyclometalated U(III) complexes can serve as versatile precursors for a broad range of reactivity and for assembling a variety of novel chemical architectures.

This publication is licensed for personal use by The American Chemical Society.

SPECIAL ISSUE

This article is part of the Ligand-Metal Complementarity in Rare Earth and Actinide Chemistry special issue.

Synopsis

The cyclometalated complex [K(2.2.2-cryptand)][UIII{N(SiMe3)2}22-C,N−CH2SiMe2NSiMe3)] could be obtained from the reduction of [UIII{N(SiMe3)2}3] with KC8 and 2.2.2-cryptand at room temperature. It showed a remarkably low reduction potential which is 1.1 V lower than [UIII{N(SiMe3)2}3], also evidenced by the reduction of various substrates, notably pyridine. In addition, it is capable of interesting insertion reactivity with substrates containing triple bonds, demonstrating its versatility for a broad range of reactivity and for assembling a variety of novel chemical architectures.

Introduction

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The bis(trimethylsilylamide) ligand, N(SiMe3)2 (N*), has played an important role in the development of the coordination chemistry of the actinides (1,2) and of low valent uranium in particular, since the synthesis of the stable [UIII(N*)3] complex (Scheme 1) was reported more than 30 years ago. (3−5) Notably, because of the facile substitution of the N* ligand through protonolysis reactions, [UIII(N*)3] serves as an excellent precursor in uranium(III) chemistry. (6−10) The use of this simple “old” supporting ligand has led to the more recent discovery of U(III) unprecedented chemistry such as CO homologation, (11) arene reduction, and functionalization by [UIII(N*)3] (12) and to the stabilization of U═E (E = N, O, S) multiple bonds. (13−18) The “ate” analogue of [UIII(N*)3], complex [K(THF)6][UIII(N*)4], C (Scheme 1), (19,20) provided a rare example of a low coordinate U(III) complex with single molecule magnet behavior (21) and showed unusual reactivity with CO2, which inserts into the U–N bond, yielding a U(IV) isocyanate complex. (22)

Scheme 1

Scheme 1. Reactivity of Previously Reported Uranium Silylamide Complexes [U{N(SiMe3)2}3], A, [K(2.2.2-cryptand)][U{N(SiMe3)2}3], B, [K(18c6)][U{N(SiMe3)2}4], C, and [U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], D, with Various Substrates, and the Highlights of This Work
More recently seminal studies from the Evans group showed that the N(SiMe3)2 ligand can also stabilize uranium in the +II oxidation state. (23) Notably, [UIII(N*)3] can be reduced to yield the U(II) analogue, [K(2.2.2-cryptand)][UII(N*)3], B (Scheme 1), which can effect the reductive coupling of pyridine (E1/2 in DMF vs Ag/AgCl = −2.76 V). (24) Upon reaction with azobenzene, B is capable of transferring up to four electrons forming a bis-imido U(VI) complex. (25)
Furthermore, the N* ligand is also known to undergo γ-CH deprotonation in the presence of a strong base and/or upon heating, resulting in cyclometalated complexes such as [UIV(N*)22-C,N–CH2SiMe2NSiMe3)], D (Scheme 1), known since 1981. (26) This U(IV) complex was shown to undergo insertion reactions into the U–C bond and act as a nucleophile toward carbonyl molecules. (27,28) More recently, the protonation chemistry as well as the reactivity with metalazides of D were reported by Ephritikhine and co-workers. (29,30) Preliminary attempts to obtain the U(III) analogue of the metalated complex [UIV(N*)22-C,N–CH2SiMe2NSiMe3)] by reacting [UIII(N*)3] with NaN* or LiCH2SiMe3 only resulted in the isolation of the U(IV) bis-cyclometalated products, [M(THF)xUN*(CH2SiMe2N{SiMe3})2]n (M = Na, Li). Hayton and co-workers recently demonstrated that complex [UIV(N*)22-C,N–CH2SiMe2NSiMe3)] provides a valuable precursor for the synthesis of the first examples of homonuclear and heteronuclear nitride bridged thorium complexes. (31)
Although complexes of uranium in the +III and +II oxidation states have attracted large attention due to their high reducing power (U(III): E1/2 = −3.10 to −1.41 V vs Fc+/0; U(II): E1/2 = −3.11 to −2.26 V vs Fc+/0) and reactivity toward small molecules, (23,32−42) studies on cyclometalated complexes of uranium in low oxidation states remain rare. The first cyclometalated U(III) complex was isolated in 1999 using the tripodal triamidoamine N(CH2CH2NSiMe2But)3 ligand although the presence of U(III) was not unambiguously established. (43) More recently, two examples of cyclometalated imido-bridged mixed-oxidation state U(III)/U(IV) complexes were reported by our group, (44,45) which formed from the 1,2-addition (46) of the C–H bond of the N(SiMe3)2 ligand across the uranium–nitride moiety of nitride bridged U(III)/U(IV) complexes.
Seminal attempts to prepare U(III) cyclometalated products by reduction of the U(IV) complex [UIV(N*)2Cl2] (12) led to the isolation of multiple products including a trinuclear cyclometalated U(III) trimer that demonstrated limited reducing power. (47) However, in 2020, two mononuclear cyclometalated U(III) complexes were isolated in good yield and crystallographically characterized by Evans and co-workers, from the reduction of the heteroleptic complexes [(C5Me5)2UIII(N*)] and [(C5Me5)UIII(N*)2]. (48) The reaction was proposed, on the basis of UV and DFT data, to proceed via a putative U(II) analogue intermediate which readily underwent C–H bond activation of the N* ligand to yield the U(III) cyclometalated products. The reactivity and redox properties of the U(III) cyclometalated products were not reported. Similar C–H bond activation behavior has been observed in reactions of Ln(III) complexes containing the N* ligand. (49,50) Notably, the cyclometalated yttrium(III) complex, [Y(N*)22-C,N–CH2SiMe2NSiMe3)]1–, is the major component from the reduction of Y(N*)3 with KC8. (50)
Recently we reported that the U(II) complex [K(2.2.2-cryptand)][UII(N*)3], B, (23) affects the reductive coupling of pyridine to afford a dinuclear U(III) complex with a bridging (pyridine2)2– ligand. From the reduction, a few crystals of the U(IV) cyclometalated complex [K(2.2.2-cryptand)]2[((Me3Si)2N)22-C,N–CH2SiMe2NSiMe3)U}2(4,4′-bpy)], were also isolated. We reasoned that this product may be originated by a putative cyclometalated U(III) complex formed by C–H activation of the U(II) [K(2.2.2-cryptand)][U(N(SiMe3)2)3] complex which turned out to be very temperature sensitive. Here, we report the synthesis and reactivity of the U(III) cyclometalated complex[K(2.2.2-cryptand)][U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], 1.

Results and Discussion

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Synthesis of [UIII{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)]

Upon addition of a brown suspension of KC8 to a purple solution of [UIII(N*)3] in THF at room temperature, the color changed quickly to dark green, and it was stirred for 1 h. After filtering, diffusion of hexane into the THF reaction mixture at −40 °C resulted in dark green/black crystals that were characterized by X-ray diffraction as the U(III) cyclometalated complex [K(2.2.2-cryptand)][U(N*)22-C,N–CH2SiMe2NSiMe3)], 1, in 71% yield (Scheme 2). The room temperature 1H NMR spectrum of 1 shows three sharp signals between 11.5 and −11.6 ppm, assigned to the methyl groups, as well as two sharp signals assigned to the 2.2.2-cryptand. In addition, there is a signal at −97.7 ppm, corresponding to the methylene protons. Complex 1 is also formed as the main product in the reduction of the U(IV) cyclometalated complex [UIV(N*)22-C,N–CH2SiMe2NSiMe3)], D, but this route was not pursued since it requires an additional step compared to the direct reduction of complex [UIII(N*)3]. The activation of C–H bonds by putative U(II) complexes has been previously observed although the U(II) intermediates could not be isolated. (39,48) Complex 1, once formed, slowly decomposes in the THF solution at room temperature, with 80% remaining after 1 day, 60% after 5 days, and 30% after 14 days (Figure S4). The decomposition products identified by 1H NMR spectroscopy are the 2.2.2-cryptand analogue of [K(18c6)][UIII(N*)4], C, and KN*. Therefore, 1 is considerably more stable than the U(II) complex [K(2.2.2-cryptand)][UII(N*)3], B. Complex 1 is obtained in high yield by reducing [UIII(N*)3] at room temperature, while the reduction of [UIII(N*)3] at low temperature yields [K(2.2.2-cryptand)][UII(N*)3]. Complex 1 is formed from the intramolecular C–H activation of [K(2.2.2-cryptand)][UII(N*)3] in solution over time at −40 °C or upon an increase in temperature.

Scheme 2

Scheme 2. Synthesis of [K(2.2.2-cryptand)][U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], 1

Spectroscopy

The UV/vis/NIR (ultraviolet/visible/near-infrared) electronic absorption spectra were collected for 2.5–5.0 mM THF solutions of [K(2.2.2-cryptand)][U(N*)22-C,N–CH2SiMe2NSiMe3)], 1, and [U{N(SiMe3)2}3] in the spectral range of 250–2500 nm (Figures S36 and S37). Both complexes feature weak and ill-defined absorption bands in the NIR region, which are characteristic for Laporte-forbidden f → f transitions typically found in actinide complexes. (4,51−54) In addition, a strong charge transfer band can be observed for 1 around 365 nm (ε = 871 M–1 cm–1), accounting for its dark green color. Furthermore, 1 exhibits multiple overlapping absorption bands between 460 and 640 nm (ε = 250 to 500 M–1 cm–1). In contrast, [U{N(SiMe3)2}3] features two charge transfer bands around 520 nm (ε = 394 M–1 cm–1) and 635 nm (ε = 230 M–1 cm–1), consistent with its purple color. Similar bands were observed in the electronic absorption spectrum for [U{N(SiMe3)2}3] in toluene. (55,56)
The X-band EPR spectrum of 1, measured in the solid state (Figure 1), presents a narrow signal, around g1 = 2.69 (H0 ≈ 2500 Gs), as well as two overlapping broad signals around g2 = 2.07 (H0 ≈ 3250 Gs) and g3 = 1.95 (H0 ≈ 3450 Gs). These data are consistent with those reported for other U(III) complexes. (55,57−59) Moreover, measurements performed on 1 in a 2-methyl-THF glass at 6 K feature a similar EPR pattern (Figure S39), suggesting that the crystal field symmetry of the compound is retained in the solution.

Figure 1

Figure 1. Solid-state EPR spectrum of 1 recorded at 6 K.

Electrochemistry

Cyclic voltammetry data were recorded for complexes [K(2.2.2-cryptand)][U(N*)22-C,N–CH2SiMe2NSiMe3)], 1, [UIII(N*)3], A, [K(THF)6][UIII(N*)4], C, and [UIV(N*)22-C,N–CH2SiMe2NSiMe3)], D, to determine their redox properties (Figure 2). These data provide an important addition to the small number of electrochemistry data reported for U(III) complexes. (34) The cyclic voltammograms of 1 and D show two redox waves at E1/2 = −2.5 V (Epc = −2.6/2.7 V) and E1/2 = −0.2 V versus Fc+/0 (assuming Fc*+/0 is at −0.59 V versus Fc+/0), which are both electrochemically reversible, as shown by the voltammograms measured at different scan rates (Figures S40 and S41). The first redox event is assigned to the U(III)/U(IV) couple, while the second event is assigned to the U(IV)/U(V) couple. The latter is approximately in the middle of the range reported for other complexes in THF having a U(IV) charge neutral oxidation state (34) (U(IV)/U(V): −0.86 to 0.12 V versus Fc+/0), while the former is on the low end of the U(III)/U(IV) range (−3.05 to −1.12 V versus Fc+/0), indicating a strong reducing character. (34) In contrast, the cyclic voltammogram of [UIII(N*)3] shows an irreversible wave at Epc = −2.8 V versus Fc+/0 and a reversible wave at E1/2 = −1.3 V (Epc = −1.5 V) versus Fc+/0 assigned to the U(II)/U(III) and to the U(III)/U(IV) couples, respectively. Remarkably the value of the reduction potential of 1 is very negative and closer to that of the U(II) complex [K(2.2.2-cryptand)][UII(N*)3], B, rather than to the U(III) complexes [UIII(N*)3] and [K(18c6)][UIII(N*)4], C, as a result of the presence of the additional R ligand in 1 compared to [UIII(N*)3]. The cyclic voltammogram of complex C, which has four N* ligands, shows a redox wave at E1/2 = −2.0 V (Epc = −2.1 V) versus Fc+/0 indicating that replacing the N* ligand with a methylene group results in a significantly more negative redox potential.

Figure 2

Figure 2. Cyclic voltammograms of [UIII(N*)3], A, [K(18c6)][UIII{N(SiMe3)2}4], C, [K(2.2.2-cryptand)][UIII{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], 1, and [UIV{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], D, in THF/0.06 M [NBu4][BPh4] at a 100 mV s–1 scan rate versus Fc+/0 using a Pt0 disk as a working electrode (arrows indicate scan direction).

Redox Reactivity

Motivated by the electrochemistry studies, which indicated that complex [K(2.2.2-cryptand)][U(N*)22-C,N–CH2SiMe2NSiMe3)], 1, should act as a strong reducing agent, the reactivity of this complex with N-heterocycles was explored and compared to the analogous reactivity previously reported for [UIII(N*)3] and [K(2.2.2-cryptand)][UII(N*)3], B. Complex [UIII(N*)3] was capable of reducing 2,2′-bpy by one electron and 4,4′-bpy by two electrons, whereas B was capable of reducing 2,2′-bpy and 4,4′-bpy by two electrons, as well as of reductively coupling pyridine.
The addition of 4,4′-bipyridine (4,4′-bpy) at −40 °C to a dark green solution of 1 in THF-d8 led to an immediate color change to dark brown, and a new set of signals was observed in the −40 °C 1H NMR spectrum at 10.8, −9.8, and −25.0 ppm. These signals indeed correspond to those observed for the diuranium(IV) cyclometalated complex [K(2.2.2-cryptand)]2[{((Me3Si)2N)22-C,N–CH2SiMe2NSiMe3)U}2(4,4′-bpy)], 2 (Scheme 3), which was previously isolated as a side-product from the reaction of the U(II) complex [K(2.2.2-cryptand)][UII(N*)3], B, prepared in situ and left at −40 °C for a few hours, with 4,4′-bipyridine. Dark brown crystals of 2 could be cleanly isolated from the direct reaction of 1 with 4,4′-bpy by the diffusion of hexane into the THF reaction mixture in a 73% yield.

Scheme 3

Scheme 3. Reaction of [K(2.2.2-cryptand)][U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], 1, with 4,4′-Bipyridine, 2,2′-Bipyridine, Pyridine, Diphenylacetylene, and 1-Azidoadamantane
2,2′-Bipyridine (2,2′-bpy) is slightly harder to reduce than 4,4′-bpy (E1/2 in DMF vs Ag/AgCl: 2,2′-bpy = −2.19 and −2.76 V; 4,4′-bpy = 1.91 and −2.47 V, respectively) (60) but is also significantly bulkier and acts as a bidentate ligand. Compared to [K(2.2.2-cryptand)][UII(N*)3], B, the uranium center in the cyclometalated analogue [K(2.2.2-cryptand)][U(N*)22-C,N–CH2SiMe2NSiMe3)], 1, is more sterically hindered due to the extra −CH2 ligand bound. Nevertheless, complex 1 reacted readily with 2,2′-bpy at −40 °C in THF, as indicated by an immediate color change to dark orange/brown and the observation of a new set of signals in the 1H NMR spectrum at 57.7, 17.0, −9.1, and −24.9 ppm measured at −40 °C. The diffusion of hexane into the THF reaction mixture afforded X-ray suitable dark brown crystals of the U(IV) complex [K(2.2.2-cryptand)][U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)(2,2′-bpy)], 3, in 85% yield (Scheme 3). These results show that the uranium center remains accessible for bulky bidentate substrates. In both these reactions, the reactivity of 1 is similar to that of [UIII(N*)3].
Since the reduction potential of 1 is close to that of the U(II) complex [K(2.2.2-cryptand)][UII(N*)3], B, we were curious whether complex [K(2.2.2-cryptand)][U(N*)22-C,N–CH2SiMe2NSiMe3)], 1, could also reduce pyridine. Indeed, upon addition of pyridine, the color of the solution immediately changed to light brown, and a new set of signals could be observed in the room temperature 1H NMR spectrum at 122.3, 80.5, 10.5, −11.7, and −25.9 ppm. Crystals obtained by the diffusion of hexane into the THF reaction mixture were identified by X-ray diffraction as the diuranium(IV) complex [K(2.2.2-cryptand)]2[((Me3Si)2N)22-C,N–CH2SiMe2NSiMe3)U}2{μ-(pyr)2}], 4 (Scheme 3). This is the first example of a mononuclear U(III) that is able to reduce pyridine, highlighting the high reducing ability of 1, in contrast to [UIII(N*)3], which shows no reactivity toward pyridine. This is remarkable considering that 1 shows a relative high solution stability comparable to [UIII(N*)3] and is significantly higher than [K(2.2.2-cryptand)][UII(N*)3], B.
In all these reactions, the uranium center transfers one electron, although it can cooperate with a second equivalent of the uranium complex to achieve two-electron transfer. Therefore, we were interested in whether multiple-electron transfer was also accessible to 1 in the reaction with the two-electron transfer substrates diphenylacetylene and 1-azidoadamantane.
Upon addition of diphenylacetylene to a dark green solution of [K(2.2.2-cryptand)][U(N*)22-C,N–CH2SiMe2NSiMe3)], 1, in THF-d8 at −40 °C, the color turned dark brown, and the 1H NMR spectrum measured at −40 °C showed signals of the bis-cyclometalated complex [K(2.2.2-cryptand)][UN*(CH2SiMe2N{SiMe3})2]. (61) At 0 °C, signals belonging to the two-electron reduced diphenylacetylene U(IV) noncyclometalated complex [K(2.2.2-cryptand)][U(η2-C2Ph2){N(SiMe3)2}3], which is silent at low temperatures, were also observed. (25) This complex was previously reported as the product of the reaction of [K(2.2.2-cryptand)][UII(N*)3], B, with diphenylacetylene. The observed products indicate that the reduction proceeds with ligand scrambling, preventing the isolation of the desired cyclometalated diphenylacetylene complex. In contrast, no reaction is observed between [UIII(N*)3] and diphenylacetylene.
Upon addition of 1-azidoadamantane to a dark green solution of 1 in THF-d8 at −40 °C, the color turned dark red/brown, and the 1H NMR spectrum at −40 °C showed a new set of signals at 42.6, 22.6, 18.8, 14.9, −11.9, and −26.2 ppm. The diffusion of hexane into the THF reaction mixture resulted in X-ray suitable dark red/brown crystals of the expected cyclometalated U(V) imido complex [K(2.2.2-cryptand)][UV(NAd){N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], 5, in 77% yield (Scheme 3). Similarly, [UIII(N*)3] has been reported to react with 1-azidoadamantane, forming the noncyclometalated U(V) imido complex, [K(2.2.2-cryptand)][U(NAd){N(SiMe3)2}3]. (15)
1H NMR studies of solutions of complexes 2-5 in THF-d8 indicated that only very minor decomposition can be observed at room temperature for 2, 4, and 5 after 14 days yielding traces of KN(SiMe3)2 and the bis-cyclometalated [K(2.2.2-cryptand)][UN*(CH2SiMe2N{SiMe3})2]. Complex 3 was found to decompose more rapidly to the same products with complete decomposition occurring after 6 days.

Insertion Reactivity

Besides their redox reactivity, cyclometalated complexes are also interesting for the reactivity centered at the U–C bond, which has been shown to be prone to the protonation and insertion reactions.
We therefore decided to investigate the reactivity of complex [K(2.2.2-cryptand)][U(N*)22-C,N–CH2SiMe2NSiMe3)], 1, with CO, acetonitrile, and phenylacetylene. Exposing a dark green solution of 1 in THF-d8 to 1 atm of CO caused an immediate color change to dark red, and the appearance of a new set of signals in the −40 °C 1H NMR spectrum at 32.8, 13.7, 10.1, −14.5, and −22.2 ppm. Crystals were obtained from a THF/hexane mixture cooled to −40 °C and characterized by X-ray diffraction as the U(III) complex [K(2.2.2-cryptand)][(N*)2U(κ2-O,N-OC(CH2)SiMe2NSiMe3)], 6 (Scheme 4). Complex 6 results from insertion of CO into the U–C bond which was previously reported for the U(IV) complex [UIV(N*)22-C,N–CH2SiMe2NSiMe3)], D, where CO appears to formally insert into the Si–C bond, although the mechanism was proposed to involve CO coordination, followed by methylene migration. (27) Insertion of CO into the U–C bond has been previously reported for U(IV) alkyl complexes, (27,29,62,63) but this is the first example of CO insertion into a U(III)-C bond.

Scheme 4

Scheme 4. Reaction of [K(2.2.2-cryptand)][U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], 1, with CO, MeCN, and PhCCH, Resulting in C–O, C–N, and C–C Insertion Products, Respectively
Furthermore, [UIII(N*)3] was reported to effect reductive coupling of CO in nonpolar solvents at room temperature. (11) In contrast, reduction of CO by 1 could not be observed in THF nor toluene. In addition, the addition of CO to a toluene-d8 solution of 6 at −40 °C also did not result in the reduction of CO. 1H NMR spectroscopy studies did not show any further reactivity when the reaction mixture was warmed to room temperature. Furthermore, we studied the electrochemical properties of 6 by measuring cyclic voltammetry. A reversible oxidation wave is observed at E1/2 = −2.4 V (Epc = −2.55 V) versus Fc0/+, which is assigned to the U(III)/U(IV) couple (Figures S42 and S43). Complex 6 is thus only slightly less reducing than 1 but still considerably more reducing than [UIII(N*)3]. We hypothesize that even though the metal center in 1 and 6 should be capable of effecting CO reduction, it might be too electron rich to be able to bind CO, thus hindering its reduction.
Upon addition of acetonitrile to a dark green solution of 1 in THF at −40 °C, the color immediately changed to dark red/brown, and a new set of signals could be observed in the −40 °C 1H NMR spectrum at 5.9, −7.4, −7.6, and −11.7 ppm. X-ray suitable crystals of the U(III) complex [K(2.2.2-cryptand)][((Me3Si)2N)2U(κ2-N,N-NC(CH3)CH2SiMe2NSiMe3)], 7 (Scheme 4), were obtained in 74% yield by diffusion of hexane into the THF reaction mixture, showing the insertion of acetonitrile into the U–C bond. A similar reaction has been reported previously for [Cp3U{CHP(Ph2)CH3}], although in that case the insertion took place into a U═C bond. (64)
The addition of phenylacetylene to a dark green solution of 1 in THF at −40 °C caused an immediate color change to olive green. X-ray suitable crystals of the U(III) complex [K(2.2.2-cryptand)][((Me3Si)2N)2U(κ2-C,N-PhCCHCH2SiMe2NSiMe3)], 8 (Scheme 4), were obtained in 80% yield by diffusion of hexane into the THF reaction mixture. Interestingly, phenylacetylene does not protonate the cyclometalated group nor get reduced as observed for diphenylacetylene (Scheme 3) but instead inserts into the U–C bond, similarly to acetonitrile, while the uranium center remains in the +3 oxidation state. In contrast, [UIII(N*)3] did not show any reaction with phenylacetylene.
These results show that besides the fact that 1 is very reducing, interesting insertion reactions occur for substrates featuring triple bonds. Here, we have shown that various types of substrates can be inserted, demonstrating C–O, C–N, and C–C insertion.

Solid State Structures

Complex 1 crystallizes in the P-1 space group. The molecular structure shows the presence of the cyclometalate complex [K(2.2.2-cryptand)][U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)] (Figure 3), which consists of an anionic [U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)] and a [K(2.2.2-cryptand)] countercation. The uranium center is disordered across two positions, and the data were modeled with 81% occupancy above the plane (U1A) and 19% occupancy below the plane (U1B). This disorder is often observed for both cyclometalated (48) and noncyclometalated (23,65) complexes containing the N* ligand.

Figure 3

Figure 3. Molecular structure of the [U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)] anion in 1 with thermal ellipsoids drawn at the 50% probability level. Disordered atoms, hydrogen atoms, and the [K(2.2.2-cryptand)] counterion have been omitted for clarity. Selected bond lengths (Å): U1A–Namide 2.268(7) – 2.418(7) (avg: 2.33(7)), U1A–C1 2.511(10).

Even though complex [UIV(N*)22-C,N–CH2SiMe2NSiMe3)], D, the U(IV) analogue of [K(2.2.2-cryptand)][U(N*)22-C,N–CH2SiMe2NSiMe3)], 1, has been known since 1981, the crystal structure was never reported. Following the synthesis reported, single crystals suitable for X-ray crystallography could be obtained by cooling a concentrated hexane solution at −40 °C (Figure S35). The molecular structure of D shows a neutral molecule, in which the uranium center is four-coordinated in a distorted tetrahedral geometry by two monodentate N(SiMe3)2 ligands and one bidentate N(SiMe3)(SiMe2CH2) ligand. Similar to 1, complex [UIV(N*)22-C,N–CH2SiMe2NSiMe3)], D, also displays disorder of the uranium center across two positions, but in this case, the occupancies are closer to 50%.
Although the U–Namide bond lengths of the major components in 1 show a wide range of 2.268(7) to 2.418(7) Å, there is a clear increase from the U(IV) to the U(III) analogue (U–(Namide)avg 2.24(2) and 2.33(7) Å, respectively). The U1A–C1 bond length of 2.511(10) Å is similar to the only other known U(III) cyclometalated complexes (2.500(5), 2.558(6) Å). (48)
Complex 3 crystallizes in the Pbca space group. The molecular structure shows the presence of the cyclometalate complex [K(2.2.2-cryptand)][U(2,2′-bpy){N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)] (Figure 4), which consists of an anionic U(IV) complex and a [K(2.2.2-cryptand)] countercation. The uranium center is six-coordinated in a distorted octahedral geometry by two N(SiMe3)2 ligands, one bidentate N(SiMe3)(SiMe2CH2) ligand, and one bidentate 2,2′-bpy ligand. The Cpy–Cpy bond distance of 1.408(5) Å is close to the range of those reported previously for U(IV)-2,2′-bpy•– complexes (1.409(13)–1.474 Å), (24,66−70) supporting this oxidation state assignment. Furthermore, the average U–Namide of 2.37(3) is consistent with other U(IV) amide complexes, and the U–Nbpy bond lengths of 2.491(3) and 2.522(2) are slightly elongated compared to those reported for the noncyclometalated U(IV) complex, [UIV(2,2′-bpy)•–(N*)3] (2.464(2) Å), probably due to the increased sterics of the bidentate cyclometalated ligand.

Figure 4

Figure 4. Molecular structure of the [U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)(2,2′-bpy)] anion in 3 with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms and the [K(2.2.2)-cryptand] counterion have been omitted for clarity. Selected bond lengths (Å): U1–(Namide)avg 2.37(3), U1–N4 2.491(3), U1–N5 2.522(2), U1–C1 2.486(3), C23–C24 1.408(5).

Complex 4 crystallizes in the P-1 space group. The molecular structure shows the presence of the cyclometalate complex [K(2.2.2-cryptand)]2[((Me3Si)2N)22-C,N–CH2SiMe2NSiMe3)U}2{μ-(pyr)2}] (Figure 5), which consists of a dianionic dinuclear U(IV) complex and two [K(2.2.2-cryptand)]+ counter cations. The uranium centers are five-coordinated in a distorted trigonal bipyramidal geometry by two N(SiMe3)2 ligands, one bidentate N(SiMe3)(SiMe2CH2) ligand, and bridged by two reductively coupled pyridines. The Cpy–Cpy bond distance of 1.565(6) Å is in line with a single bond. The bond length is similar to the previously reported noncyclometalated analogue, [K(2.2.2-cryptand)]2[{((Me3Si)2N)3U}2{μ-(pyr)2}], 1.570(17), and close to the range reported for complexes based on other metals featuring this bridging ligand (1.559(4)–1.570(17) Å). (24,71−75) Furthermore, the U1–(Namide)avg distance of 2.307(6) Å and the U1–N1 distance of 2.346(3) Å are significantly shorter than in the noncyclometalated analogue (2.385(5) Å and 2.404(7) Å, respectively), supporting the assignment of the uranium centers in the +4 oxidation state. Finally, the U1–N1 and the U1–C18 (2.487(4) Å) distances are comparable to those reported for the analogous cyclometalated complex bridged by a 4,4′-bpy2– ligand, [K(2.2.2-cryptand)]2[((Me3Si)2N)22-C,N–CH2SiMe2NSiMe3)U}2(4,4′-bpy)], complex 2 (2.357(4) Å and 2.507(5) Å, respectively).

Figure 5

Figure 5. Molecular structure of the [((Me3Si)2N)22-C,N–CH2SiMe2NSiMe3)U}2{μ-(pyr)2}]2– anion in 4 with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms and the [K(2.2.2)-cryptand] counterions have been omitted for clarity. Selected bond lengths (Å): U1–(Namide)avg 2.310(8), U1–N1 2.346(3), U1–C18 2.487(4), C3–C3′ 1.565(6).

Complex 5 crystallizes in the P21/c space group. The molecular structure shows the presence of the cyclometalate complex [K(2.2.2-cryptand)][U(NAd){N(SiMe3)2}22-C,N–CH2SiMe2NvSiMe3)] (Figure 6), which consists of an anionic U(V) complex and a [K(2.2.2-cryptand)] countercation. The uranium center is five-coordinated in a distorted trigonal bipyramidal geometry by two N(SiMe3)2 ligands, one bidentate N(SiMe3)(SiMe2CH2) ligand, and one adamantylimido ligand. The U–Nimido bond length of 1.945(15) Å corresponds well with that reported for the noncyclometalated U(V) analogue (1.945(2) Å). However, the average U–Namide bond length of 2.35(2) Å is considerably longer than in the noncyclometalated U(V) analogue (2.270(2) Å), which could be due to the sterics of the methylene coordination. (15)

Figure 6

Figure 6. Molecular structure of the [U(NAd){N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)] anion in 5 with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms and the [K(2.2.2)-cryptand] counterion have been omitted for clarity. Selected bond lengths (Å): U1–(Namide)avg 2.35(2), U1–N1 1.945(15), U1–C16 2.425(19).

The molecular structures of 6, 7, and 8 (Figure 7) are similar, and all consist of an anionic U(III) complex with a [K(2.2.2-cryptand)] countercation. The uranium center is four-coordinated in a distorted tetrahedral geometry by two N(SiMe3)2 ligands and one bidentate OC(CH2)SiMe2NSiMe3, NC(CH3)CH2SiMe2NSiMe3, or PhCCHCH2SiMe2NSiMe3 ligand for 6, 7, and 8, respectively. The average U–Namide bond lengths (2.38(1), 2.38(1) and 2.38(4) Å) are longer than those in 2-5, in line with the +3 compared to the +4 oxidation state. Complex 6 contains disorder in the OC(CH2)SiMe2NSiMe3 ligand, making it difficult to distinguish between a mono- and bis-metallacycle. However, the latter has been reported previously with either a Na or [Na(18c6)] counterion, and these have considerably shorter (U–Namide)avg bond lengths of 2.320(7) and 2.32(3), respectively. In combination with 1H NMR spectroscopy, there is significant evidence for the proposed structure of 6.

Figure 7

Figure 7. Molecular structure of a) the [(N*)2U(κ2-O,N-OC(CH2)SiMe2NSiMe3)] anion in 6, b) [((Me3Si)2N)2U(κ2-N,N-NC(CH3)CH2SiMe2NSiMe3)] anion in 7, and c) [((Me3Si)2N)2U(κ2-C,N-PhCCHCH2SiMe2NSiMe3)] anion in 8 with thermal ellipsoids drawn at the 50% (a, b), or 30% (c) probability level. Hydrogen atoms, the [K(2.2.2)-cryptand] counterions, and disorder in the ligands have been omitted for clarity. Selected bond lengths (Å): a) U1–(Namide)avg 2.38(1), b) U1–(Namide)avg 2.38(1), U1–N1 2.299(8), N1–C1 1.401(12), c) U1–(Namide)avg 2.38(4), U1–C1 2.50(2).

The C–H···U lengths are in the range 2.53–2.58 Å for 1 and for [UIV(N*)22-C,N–CH2SiMe2NSiMe3)], D, 2.75 Å for 6, and 2.86 Å for 7, suggesting the presence of agostic interactions. Distances of 2.705 and 2.773 Å were assigned to agostic interactions in [U{N(SiMe2H)4]. (76)

Conclusion

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The new cyclometalated U(III) complex [K(2.2.2-cryptand)][U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)] can be readily obtained from the reduction of [U{N(SiMe3)2}3] at room temperature. It has a high reduction potential and can reduce bipyridines. More remarkably, it is the first example of a mononuclear U(III) species capable of reducing pyridine while also being moderately stable in solution at room temperature. Besides cooperative two-electron transfer (meaning two equivalents of 1 transferring one electron each to a substrate), complex 1 can also act as a two-electron oxidizing agent toward 1-azidoadamantane, giving the adamantyl-imido U(V) derivative. Additionally, the U–C bond of 1 also displays interesting insertion reactivity toward C–O, C–N, and C–C bonds, resulting in new low oxidation state metallacycles.
These findings present an interesting case of a highly reducing mononuclear U(III) cyclometalated complex that also manifests rich insertion chemistry toward different substrates. This further extends the use of cyclometalated species, long known for U(IV) but hitherto not explored for U(III). Akin to the reactivity reported for its U(IV) congener, the U(III) cyclometalated analogue provides a versatile precursor for a broad range of reactivity and for assembling a variety of novel chemical architectures, including new low-valent heterobimetallic and heteroatom-bridged complexes.

Experimental Section

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General Considerations

All manipulations were carried out under inert atmospheres using an MBraun glovebox equipped with a purifier unit and Schlenk line techniques. The water and oxygen levels were always kept at less than 1 ppm. Anhydrous solvents were purchased from Sigma-Aldrich and vacuum distilled under potassium/benzophenone (THF, pyridine, PhCCH) or sodium sand (hexane) or calcium hydride (acetonitrile). Depleted uranium turnings were purchased from IBILABS, Florida (USA). 2,2′-Bipyridine, 4,4′-bipyridine, 1-azidoadamantyl, PhCCPh, 2.2.2-cryptand, and 18-crown-6 were purchased from Sigma-Aldrich and dried.
[U{N(SiMe3)2}3], (3) [U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], (27) KC8, (77) and [K(18c6)][U{N(SiMe3)2)4] (21) were synthesized according to their respective literature procedures. Elemental analyses were performed under nitrogen with a Thermo Scientific Flash 2000 Organic Elemental Analyzer. NMR experiments were carried out using NMR tubes adapted with J. Young valves. NMR spectra were recorded on a Bruker 400 MHz spectrometer, and the chemical shifts are reported in ppm with residual proteo-solvent signals used as an internal reference.
Cyclic voltammetry experiments were carried out at room temperature in an argon-filled glovebox described above. Data were collected using a Biologic SP-300 potentiostat connected to a personal computer. All samples were 2 mM in complex with the 0.06 M [NBu4][BPh4] supporting electrolyte in the THF solution. The experiments were carried out with a platinum disk (d = 5 mm) working electrode, a platinum wire counter electrode, and an Ag/AgCl reference electrode. Potential calibration was performed at the end of each data collection cycle using the decamethylferrocene/decamethylferrocenium [(C5Me5)2Fe]+/0 couple as an internal standard. The values were subsequently adjusted to reflect the values versus the [(C5H5)2Fe]+/0 couple assuming that [(C5Me5)2Fe]+/0 appears at −0.59 V with respect to [(C5H5)2Fe]+/0.
Electron paramagnetic resonance (EPR) measurements were recorded with a Bruker Elexsys E500 spectrometer operating in the X-band frequency region with an Oxford ESR900 cryostat for 4–300 K operation. The complexes were ground in an agate mortar. Five mmol was loaded in 1 mm diameter quartz capillaries and then inserted into J-Young valve EPR tubes.
Caution: Depleted uranium (primary isotope 238U) is a weak α-emitter (4.197 MeV) with a half-life of 4.47 × 109 years. Manipulations and reactions should be carried out in monitored fume hoods or in an inert atmosphere glovebox in a radiation laboratory equipped with α- and β-counting equipment.

Synthesis [K(2.2.2-cryptand)][U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], Complex 1

From [U{N(SiMe3)2}3] Synthetic Scale

A suspension of KC8 (18.9 mg, 0.14 mmol, 1.0 equiv) in THF (1 mL) was added to a solution of [U{N(SiMe3)2}3] (100.0 mg, 0.14 mmol, 1.0 equiv) and 2.2.2-cryptand (52.5 mg, 0.14 mmol, 1.0 equiv) in THF (0.5 mL), causing an immediate color change to dark green. The mixture was stirred at room temperature for 1 h and filtered, resulting in a dark green filtrate. X-ray suitable dark green (almost black) crystals of complex 1 were obtained by slow diffusion of hexane into a concentrated THF solution (111.9 mg, 0.099 mmol, 71.0% yield).
1H NMR (THF-d8, 193 K): δ 18.6 (s, 6H, N(SiMe3)(SiMe2CH2)), 3.3 (m, 2.2.2-cryptand), −13.0 (m, 9H, N(SiMe3)(SiMe2CH2)), −21.0 (s, 36H, N(SiMe3)2)), −133.3 (s, 2H, N(SiMe3)(SiMe2CH2)) ppm. 1H NMR (THF-d8, 233 K): δ 15.0 (s, 6H, N(SiMe3)(SiMe2CH2)), 3.3 (d, 2.2.2-cryptand), 2.3 (s, 2.2.2-cryptand), −10.3 (m, 9H, N(SiMe3)(SiMe2CH2)), −16.2 (s, 36H, N(SiMe3)2)), −118.6 (s, 2H, N(SiMe3)(SiMe2CH2)) ppm. 1H NMR (THF-d8, 273 K): δ 12.7 (br s, 9H, N(SiMe3)(SiMe2CH2)), 3.3 (d, 2.2.2-cryptand), 2.3 (s, 2.2.2-cryptand), −8.5 (m, 9H, N(SiMe3)(SiMe2CH2)), −13.0 (s, 36H, N(SiMe3)2)), −105.4 (s, 2H, N(SiMe3)(SiMe2CH2)) ppm. 1H NMR (THF-d8, 298 K): δ 11.5 (s, 6H, N(SiMe3)(SiMe2CH2)), 3.3 (d, 2.2.2-cryptand), 2.3 (s, 2.2.2-cryptand), −7.5 (m, 9H, N(SiMe3)(SiMe2CH2)), −11.5 (s, 36H, N(SiMe3)2)), −97.7 (s, 2H, N(SiMe3)(SiMe2CH2)) ppm. 29Si{1H} NMR (THF-d8, 233 K): δ 39.8 (s), −160.2 (s), −277.1 (br s) ppm.
Anal. Calcd for C36H89KN5O6Si6U: C, 38.14; H, 7.91; N, 6.18. Found: C, 38.36; H, 7.88; N, 5.77.

From [U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)] NMR Scale

A cold (−40 °C) suspension of KC8 (1.0 mg, 7.4 μmol, 1.05 equiv) in THF-d8 (0.5 mL) was added to a cold (−40 °C) mixture of [U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)] (5.0 mg, 7.2 μmol, 1.0 equiv) and 2.2.2-cryptand (2.7 mg, 7.2 μmol, 1.0 equiv), resulting in a dark green mixture. The mixture was stirred at −40 °C for 30 min and filtered, resulting in a dark green filtrate. 1H NMR showed predominantly signals belonging to 1 (See Figure S2).

Synthesis [K(2.2.2-cryptand)]2[{((Me3Si)2N)22-C,N–CH2SiMe2NSiMe3)U}2(4,4′-bpy)], Complex 2

A cold (−40 °C) colorless solution of 4,4′-bipyridine (1.4 mg, 9.0 μmol, 0.5 equiv) in THF (0.5 mL) was added to a cold (−40 °C) dark green solution of 1 (20.3 mg, 17.9 μmol, 1.0 equiv) in THF (0.5 mL), causing an immediate color change to dark brown. The mixture was stirred at −40 °C for 30 min. X-ray suitable dark brown crystals of complex 2 were obtained by slow diffusion of hexane into a concentrated THF solution (15.8 mg, 6.5 μmol, 72.8% yield).
1H NMR (THF-d8, 193 K): δ 4.5 (m, 2.2.2-cryptand), −14.7 (br s, 6H, N(SiMe3)(SiMe2CH2)), −51.1 (s, 36H, N(SiMe3)2)) ppm. 1H NMR (THF-d8, 233 K): δ 10.6 (br s, 9H, N(SiMe3)(SiMe2CH2)), 4.5 (d, 2.2.2-cryptand), 3.6 (s, 2.2.2-cryptand), −8.4 (s, 6H, N(SiMe3)(SiMe2CH2)), −25.4 (s, 36H, N(SiMe3)2)) ppm. 1H NMR (THF-d8, 273 K): δ 9.1 (br s, 9H, N(SiMe3)(SiMe2CH2)), 4.5 (d, 2.2.2-cryptand), 3.4 (s, 2.2.2-cryptand), −3.3 (s, 6H, N(SiMe3)(SiMe2CH2)), −19.3 (s, 36H, N(SiMe3)2)) ppm. 1H NMR (THF-d8, 298 K): δ 8.4 (br s, 9H, N(SiMe3)(SiMe2CH2)), 4.3 (d, 2.2.2-cryptand), 3.2 (s, 2.2.2-cryptand), −1.3 (s, 6H, N(SiMe3)(SiMe2CH2)), −17.1 (s, 36H, N(SiMe3)2)) ppm. 29Si{1H} NMR (THF-d8, 233 K): silent. 29Si{1H} NMR (THF-d8, 298 K): −30.7 (s), −268.5 (s) ppm. Anal. Calcd for C82H186K2N12O12Si12U2: C, 40.64; H, 7.74; N, 6.93. Found: C, 40.77; H, 7.68; N, 7.01.

Synthesis [K(2.2.2-cryptand)]2[{((Me3Si)2N)22-C,N–CH2SiMe2NSiMe3)U}2(2,2′-bpy)], Complex 3

A cold (−40 °C) colorless solution of 2,2′-bipyridine (3.5 mg, 22.4 μmol, 1.0 equiv) in THF (0.5 mL) was added to a cold (−40 °C) dark green solution of 1 (24.9 mg, 21.2 μmol, 1.0 equiv) in THF (0.5 mL), causing an immediate color change to dark yellow/brown. The mixture was stirred at −40 °C for 30 min. X-ray suitable dark brown crystals of complex 3 were obtained by slow diffusion of hexane into a concentrated THF solution (23.2 mg, 18.0 μmol, 84.8% yield).
1H NMR (THF-d8, 193 K): δ 72.4 (m), 54.7 (s), 19.7 (s), 3.0 (m, 2.2.2-cryptand), −13.5 (s), −17.0 (s), −22.6 (s), −30.3 (s), −34.0 (s), −38.3 (s), −54.2 (s). 1H NMR (THF-d8, 233 K): δ 57.7 (s), 17.0 (s), 3.1 (d, 2.2.2-cryptand), 2.1 (s, 2.2.2-cryptand), −9.1 (s), −24.9 (s). 1H NMR (THF-d8, 273 K): δ 46.3 (s), 24.8 (br s), 14.9 (s), 3.1 (d, 2.2.2-cryptand), 2.2 (s, 2.2.2-cryptand), −6.5 (s), −13.9 (br s), −20.6 (s). 1H NMR (THF-d8, 298 K): δ 40.3 (s), 21.6 (br s), 13.8 (s), 3.2 (d, 2.2.2-cryptand), 2.2 (s, 2.2.2-cryptand), −5.7 (s), −12.1 (br s), −18.4 (br s). 29Si{1H} NMR (THF-d8, 233 K): δ −61.8 (s), −162.7 (s), −232.0 (s), −341.8 (s) ppm. Anal. Calcd for C46H97KN7O6Si6U: C, 42.83; H, 7.58; N, 7.60. Found: C, 42.71; H, 7.57; N, 7.28.

Synthesis [K(2.2.2-cryptand)]2[{((Me3Si)2N)22-C,N–CH2SiMe2NSiMe3)U}2{μ-(pyr)2}], Complex 4

Pyridine (2.9 μL, 36.0 μmol, 2.0 equiv) was added to a cold (−40 °C) dark green solution of 1 (20.3 mg, 17.9 μmol, 1.0 equiv) in THF (1 mL), causing an immediate color change to light brown. The mixture was stirred at −40 °C for 30 min. X-ray suitable light brown crystals of complex 4 were obtained by slow diffusion of hexane into a concentrated THF solution (15.9 mg, 6.6 μmol, 73.2% yield).
1H NMR (THF-d8, 193 K): δ 168.5 (br s, pyridine), 162.8 (br s, pyridine), 159.0 (br s, pyridine), 12.0 (br s, pyridine), 96.3 (br s, pyridine), 19.3 (br s, 6H, N(SiMe3)(SiMe2CH2)), 4.3 (m, 2.2.2-cryptand), 3.0 (s, 2.2.2-cryptand), −18.6 (br m, 9H, N(SiMe3)(SiMe2CH2)), −55.3 (s, 36H, N(SiMe3)2)) ppm. 1H NMR (THF-d8, 233 K): δ 112.3 (br s, pyridine), 80.5 (br s, pyridine), 10.5 (s, 6H, N(SiMe3)(SiMe2CH2)), 4.3 (m, 2.2.2-cryptand), 3.3 (s, 2.2.2-cryptand), −11.7 (s, 9H, N(SiMe3)(SiMe2CH2)), −25.9 (br s, 36H, N(SiMe3)2)) ppm. 1H NMR (THF-d8, 273 K): δ 94.2 (br s, pyridine), 61.3 (br s, pyridine), 9.0 (s, 6H, N(SiMe3)(SiMe2CH2)), 4.1 (m, 2.2.2-cryptand), 3.1 (s, 2.2.2-cryptand), −6.9 (s, 9H, N(SiMe3)(SiMe2CH2)), −20.1 (s, 36H, N(SiMe3)2)) ppm. 1H NMR (THF-d8, 298 K): δ 81.8 (s, pyridine), 78.7 (br s, pyridine), 52.7 (s, pyridine), 8.3 (s, 6H, N(SiMe3)(SiMe2CH2)), 4.0 (m, 2.2.2-cryptand), 3.0 (s, 2.2.2-cryptand), −4.8 (s, 9H, N(SiMe3)(SiMe2CH2)), −17.7 (s, 36H, N(SiMe3)2)) ppm. 29Si{1H} NMR (THF-d8, 233 K): δ −25.7 (s), −159.4 (s) ppm. Anal. Calcd for C82H188K2N12O12Si12U2: C, 40.60; H, 7.81; N, 6.93. Found: C, 40.29; H, 7.68; N, 6.64.

Synthesis [K(2.2.2-cryptand)][((Me3Si)2N)22-C,N–CH2SiMe2NSiMe3)U(NAd)], Complex 5

A cold (−40 °C) colorless solution of adamantylazide (3.9 mg, 22.0 μmol, 1.0 equiv) in THF (0.5 mL) was added to a cold (−40 °C) dark green solution of 1 (25.0 mg, 22.1 μmol, 1.0 equiv) in THF (0.5 mL), causing an immediate color change to dark red/brown. The mixture was stirred at −40 °C for 30 min. X-ray suitable dark red/brown crystals of complex 5 were obtained by slow diffusion of hexane into a concentrated THF solution (21.7 mg, 16.9 μmol, 76.7% yield).
1H NMR (THF-d8, 193 K): δ 54.6 (br s), 38.3 (br s), 29.9 (br s), 28.4 (s), 23.8 (s), 18.8 (s), 9.3 (br s), 3.8 (m, 2.2.2-cryptand), 2.2 (s, 2.2.2-cryptand), −10.3 (s), −15.3 (s), −21.1 (s), −33.5 (br s), −35.5 (br s), −49.8 (br s) ppm. 1H NMR (THF-d8, 233 K): δ 42.6 (br s), 22.6 (s), 18.8 (s), 14.9 (s), 3.7 (d, 2.2.2-cryptand), 2.7 (s, 2.2.2-cryptand), −11.9 (s), −26.2 (s) ppm. 1H NMR (THF-d8, 273 K): δ 35.0 (br s), 18.9 (s), 15.6 (s), 12.4 (s), 10.3 (br s), 3.7 (d, 2.2.2-cryptand), 2.7 (s, 2.2.2-cryptand), −9.6 (s), −12.5 (br s) ppm. 1H NMR (THF-d8, 298 K): δ 21.6 (br s), 17.3 (s), 14.2 (s), 11.3 (s), 9.3 (s), 3.7 (d, 2.2.2-cryptand), 2.7 (s, 2.2.2-cryptand), −8.5 (s), −11.1 (br s) ppm. 29Si{1H} NMR (THF-d8, 233 K): δ −67.9 (s) ppm. Anal. Calcd for C46H104KN6O6Si6U: C, 43.06; H, 8.17; N, 6.55. Found: C, 42.64; H, 8.09; N, 6.66.

Synthesis [K(2.2.2-cryptand)][U{N(SiMe3)2}22-O,N-OC(CH2)SiMe2NSiMe3)], Complex 6

A cold (−40 °C) dark green solution of 1 (25.0 mg, 22.1 μmol, 1.0 equiv) in THF (1 mL) was degassed by three freeze–pump–thaw cycles. Then, CO (1 atm) was added to the cold solution, causing an immediate color change to dark red. X-ray suitable dark red crystals of complex 6 were obtained by slow diffusion of hexane into a concentrated THF solution (19.9 mg, 17.1 μmol, 77.5% yield).
1H NMR (THF-d8, 193 K): δ 39.1 (s, 1H, N(SiMe3)(SiMe2C(CH2)O)), 15.9 (s, 1H, N(SiMe3)(SiMe2C(CH2)O)), 12.6 (s, 6H, N(SiMe3)(SiMe2C(CH2)O)), 3.2 (m, 2.2.2-cryptand), −19.0 (br s, 36H, N(SiMe3)2)), −28.2 (s, 9H, N(SiMe3)(SiMe2C(CH2)O)) ppm. 1H NMR (THF-d8, 233 K): δ 32.8 (s, 1H, N(SiMe3)(SiMe2C(CH2)O)), 13.7 (s, 1H, N(SiMe3)(SiMe2C(CH2)O)), 10.1 (s, 6H, N(SiMe3)(SiMe2C(CH2)O)), 3.3 (d, 2.2.2-cryptand), 2.2 (s, 2.2.2-cryptand), −14.5 (br s, 36H, N(SiMe3)2)), −22.2 (s, 9H, N(SiMe3)(SiMe2C(CH2)O)) ppm. 1H NMR (THF-d8, 273 K): δ 27.9 (s, 1H, N(SiMe3)(SiMe2C(CH2)O)), 11.7 (s, 1H, N(SiMe3)(SiMe2C(CH2)O)), 8.4 (s, 6H, N(SiMe3)(SiMe2C(CH2)O)), 3.3 (d, 2.2.2-cryptand), 2.3 (s, 2.2.2-cryptand), −11.5 (br s, 36H, N(SiMe3)2)), −18.1 (s, 9H, N(SiMe3)(SiMe2C(CH2)O)) ppm. 1H NMR (THF-d8, 298 K): δ 25.5 (s, 1H, N(SiMe3)(SiMe2C(CH2)O)), 10.7 (s, 1H, N(SiMe3)(SiMe2C(CH2)O)), 7.7 (s, 6H, N(SiMe3)(SiMe2C(CH2)O)), 3.3 (d, 2.2.2-cryptand), 2.3 (s, 2.2.2-cryptand), −10.2 (br s, 36H, N(SiMe3)2)), −16.1 (s, 9H, N(SiMe3)(SiMe2C(CH2)O)) ppm. 29Si{1H} NMR (THF-d8, 233 K): δ −160.9 (s), −182.8 (s), −267.8 (s) ppm. Anal. Calcd for C37H89KN5O7Si6U: C, 38.25; H, 7.72; N, 6.03. Found: C, 38.63; H, 7.75; N, 6.09.

Synthesis [K(2.2.2-cryptand)][U{N(SiMe3)2}22-N,N-NC(CH3)CH2SiMe2NSiMe3)], Complex 7

MeCN (1.2 μL, 23.0 μmol, 1.0 equiv) was added to a cold (−40 °C) dark green solution of 1 (25.0 mg, 22.1 μmol, 1.0 equiv) in THF (1 mL), causing an immediate color change to dark red/brown. The mixture was stirred at −40 °C for 30 min. X-ray suitable dark red crystals of complex 7 were obtained by slow diffusion of hexane into a concentrated THF solution (19.3 mg, 16.4 μmol, 74.3% yield).
1H NMR (THF-d8, 193 K): δ 11.3 (br s), 3.1 (m, 2.2.2-cryptand), −3.2 (s), −5.2 (br s), −8.3 (br s), −18.0 (br s) −20.7 (s), −25.0 (br s), −33.9 (s) ppm. 1H NMR (THF-d8, 233 K): δ 8.5 (s, 6H, N(SiMe3)(SiMe2CH2)C(CH3)N)), 3.3 (d, 2.2.2-cryptand), 2.3 (s, 2.2.2-cryptand), −10.4 (br s, 36H, N(SiMe3)2)), −16.5 (s, 9H, N(SiMe3)(SiMe2CH2)C(CH3)N)), −23.6 (s) ppm. 1H NMR (THF-d8, 273 K): δ 6.7 (s, 6H, N(SiMe3)(SiMe2CH2)C(CH3)N)), 3.4 (d, 2.2.2-cryptand), 2.3 (s, 2.2.2-cryptand), −8.4 (m, 36H, N(SiMe3)2)), −13.2 (s, 9H, N(SiMe3)(SiMe2CH2)C(CH3)N)), −16.8 (s) ppm. 1H NMR (THF-d8, 298 K): δ 5.9 (s, 6H, N(SiMe3)(SiMe2CH2)C(CH3)N)), 3.4 (d, 2.2.2-cryptand), 2.4 (s, 2.2.2-cryptand), −7.5 (m, 36H, N(SiMe3)2)), −11.7 (s, 9H, N(SiMe3)(SiMe2CH2)C(CH3)N)), −14.0 (s) ppm. 29Si{1H} NMR (THF-d8, 233 K): δ −161.6 (s), −164.7 (br s), −201.0 (s), −217.2 (s) ppm.
Anal. Calcd for C38H92KN6O6Si6U: C, 38.85; H, 7.89; N, 7.15. Found: C, 38.71; H, 7.90; N, 6.89.

Synthesis [K(2.2.2-cryptand)][U{N(SiMe3)2}22-C,N–C(Ph)CHCH2SiMe2NSiMe3)], Complex 8

PhCCH (2.4 μL, 21.9 μmol, 1.0 equiv) was added to a cold (−40 °C) dark green solution of 1 (25.0 mg, 22.1 μmol, 1.0 equiv) in THF (0.5 mL), causing an immediate color change to olive-green. The mixture was stirred at −40 °C for 30 min. X-ray suitable olive-green crystals were obtained by slow diffusion of hexane into a concentrated THF solution (21.8 mg, 17.6 μmol, 80.0% yield).
1H NMR (THF-d8, 193 K): δ 22.4 (s), 6.9 (s), 3.8 (s, 2.2.2-cryptand), 3.2 (s, 2.2.2-cryptand), 2.1 (s, 2.2.2-cryptand), −2.5 (br s), −4.5 (s), −5.8 (s), −9.5 (s), −18.9 (s), −29.4 (br s) ppm. 1H NMR (THF-d8, 233 K): δ 16.3 (s), 3.6 (s, 2.2.2-cryptand), 2.6 (s, 2.2.2-cryptand), −1.3 (s), −3.0 (s), −12.7 (s), −23.9 (s), −38.6 (br s) ppm. 1H NMR (THF-d8, 273 K): δ 12.9 (s), 3.6 (s, 2.2.2-cryptand), 2.6 (s, 2.2.2-cryptand), −1.5 (s), −3.3 (s), −6.3 (s), −9.3 (s), −10.3 (s), −20.1 (br s), −26.8 (br s) ppm. 1H NMR (THF-d8, 298 K): δ 11.7 (br s), 3.7 (d, 2.2.2-cryptand), 2.6 (s, 2.2.2-cryptand), −1.9 (br s), −5.8 (s), −7.7 (s), −9.3 (s), −13.1 (s), −19.3 (br s) ppm. 29Si{1H} NMR (THF-d8, 233 K): δ −160.6 (s), −167.0 (s) ppm. Anal. Calcd for C44H95KN5O6Si6U: C, 42.76; H, 7.75; N, 5.67. Found: C, 42.45; H, 7.67; N, 5.89.

Reaction of [K(2.2.2-cryptand)][U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], Complex 1, with Diphenylacetylene

A cold (−40 °C) colorless solution of diphenylacetylene in THF-d8 (0.5 mL) was added to 1, resulting in a dark brown solution. The 1H NMR spectrum showed signals corresponding to [K(2.2.2-cryptand)][U(η2-C2Ph2){N(SiMe3)2}3], [K(2.2.2-cryptand)][U(N(SiMe3)2)CH2SiMe2N{SiMe3})2], and other unidentified products.

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.3c03668.

  • NMR and UV/vis/NIR spectra, cyclic voltammetry diagram, additional crystallographic details, 1H NMR spectra, EPR spectra, magnetic data (PDF)

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CCDC 2260638, 22994922299496, and 23001302300131 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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  • Corresponding Author
  • Authors
    • Dieuwertje K. Modder - Group of Coordination Chemistry, Institut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
    • Rosario Scopelliti - Group of Coordination Chemistry, Institut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, SwitzerlandOrcidhttps://orcid.org/0000-0001-8161-8715
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We thank F. Fadaei-Tirani for important contributions to the X-ray single crystal structure analyses (collection and resolution of structures of 1, 4, and 7), Dr. Andrzej Sienkiewicz for support with EPR data collection, and Dr. Chad Palumbo for preliminary experiments. We acknowledge support from the Swiss National Science Foundation grant number 200020_ 212723 and the Ecole Polytechnique Fédérale de Lausanne (EPFL).

References

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  • Abstract

    Scheme 1

    Scheme 1. Reactivity of Previously Reported Uranium Silylamide Complexes [U{N(SiMe3)2}3], A, [K(2.2.2-cryptand)][U{N(SiMe3)2}3], B, [K(18c6)][U{N(SiMe3)2}4], C, and [U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], D, with Various Substrates, and the Highlights of This Work

    Scheme 2

    Scheme 2. Synthesis of [K(2.2.2-cryptand)][U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], 1

    Figure 1

    Figure 1. Solid-state EPR spectrum of 1 recorded at 6 K.

    Figure 2

    Figure 2. Cyclic voltammograms of [UIII(N*)3], A, [K(18c6)][UIII{N(SiMe3)2}4], C, [K(2.2.2-cryptand)][UIII{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], 1, and [UIV{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], D, in THF/0.06 M [NBu4][BPh4] at a 100 mV s–1 scan rate versus Fc+/0 using a Pt0 disk as a working electrode (arrows indicate scan direction).

    Scheme 3

    Scheme 3. Reaction of [K(2.2.2-cryptand)][U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], 1, with 4,4′-Bipyridine, 2,2′-Bipyridine, Pyridine, Diphenylacetylene, and 1-Azidoadamantane

    Scheme 4

    Scheme 4. Reaction of [K(2.2.2-cryptand)][U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)], 1, with CO, MeCN, and PhCCH, Resulting in C–O, C–N, and C–C Insertion Products, Respectively

    Figure 3

    Figure 3. Molecular structure of the [U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)] anion in 1 with thermal ellipsoids drawn at the 50% probability level. Disordered atoms, hydrogen atoms, and the [K(2.2.2-cryptand)] counterion have been omitted for clarity. Selected bond lengths (Å): U1A–Namide 2.268(7) – 2.418(7) (avg: 2.33(7)), U1A–C1 2.511(10).

    Figure 4

    Figure 4. Molecular structure of the [U{N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)(2,2′-bpy)] anion in 3 with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms and the [K(2.2.2)-cryptand] counterion have been omitted for clarity. Selected bond lengths (Å): U1–(Namide)avg 2.37(3), U1–N4 2.491(3), U1–N5 2.522(2), U1–C1 2.486(3), C23–C24 1.408(5).

    Figure 5

    Figure 5. Molecular structure of the [((Me3Si)2N)22-C,N–CH2SiMe2NSiMe3)U}2{μ-(pyr)2}]2– anion in 4 with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms and the [K(2.2.2)-cryptand] counterions have been omitted for clarity. Selected bond lengths (Å): U1–(Namide)avg 2.310(8), U1–N1 2.346(3), U1–C18 2.487(4), C3–C3′ 1.565(6).

    Figure 6

    Figure 6. Molecular structure of the [U(NAd){N(SiMe3)2}22-C,N–CH2SiMe2NSiMe3)] anion in 5 with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms and the [K(2.2.2)-cryptand] counterion have been omitted for clarity. Selected bond lengths (Å): U1–(Namide)avg 2.35(2), U1–N1 1.945(15), U1–C16 2.425(19).

    Figure 7

    Figure 7. Molecular structure of a) the [(N*)2U(κ2-O,N-OC(CH2)SiMe2NSiMe3)] anion in 6, b) [((Me3Si)2N)2U(κ2-N,N-NC(CH3)CH2SiMe2NSiMe3)] anion in 7, and c) [((Me3Si)2N)2U(κ2-C,N-PhCCHCH2SiMe2NSiMe3)] anion in 8 with thermal ellipsoids drawn at the 50% (a, b), or 30% (c) probability level. Hydrogen atoms, the [K(2.2.2)-cryptand] counterions, and disorder in the ligands have been omitted for clarity. Selected bond lengths (Å): a) U1–(Namide)avg 2.38(1), b) U1–(Namide)avg 2.38(1), U1–N1 2.299(8), N1–C1 1.401(12), c) U1–(Namide)avg 2.38(4), U1–C1 2.50(2).

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