A New Supporting Ligand in Actinide Chemistry ... - ACS Publications

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A New Supporting Ligand in Actinide Chemistry Leads to Reactive Bis(NHC)borate-Supported Thorium Complexes Mary E. Garner,† Stephan Hohloch,† Laurent Maron,*,‡ and John Arnold*,† †

Department of Chemistry, University of California, Berkeley, California 94720, United States LPCNO, Université de Toulouse, INSA Toulouse, 135 Avenue de Rangueil, 31077 Toulouse, France

‡

S Supporting Information *

ABSTRACT: A versatile, monoanionic, chelating (bis)carbene ligand (2) was used to prepare a thorium dihalide complex (3) and a direduced-bpy derivative (4). CASSCF calculations suggest the involvement of a multiconfigurational open-shell singlet, with the main configuration corresponding to a Th(III)bpy(−1) (f1π*1) electronic structure. The reactivity of 4 was explored in various transformations, including reactions with carbonyls and organic azides; the latter gave rise to an unusual terminal Th-imido bpy complex (6).

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INTRODUCTION Molecular organometallic chemistry of the actinides is a burgeoning area of inorganic chemistry.1−3 Their versatile coordination chemistry and ability to exist in various oxidation states make the actinide metals excellent candidates for the discovery of new structural types, reactivities, and physical and spectroscopic properties. The cyclopentadiene (C5R5, Cp) ligand has been integral to developing this field.3 Species bearing Cp ligands were some of the first reported actinide complexes,4 and they provided a framework to directly compare the coordination chemistry of actinide metals to that of transition metals, revealing parallelyet frequently divergent reactivity.5 Indeed, the rich reaction chemistry demonstrated by the actinide metallocenes still dominates this area and few alternative platforms have been reported. In contrast, for transition metals a variety of supporting ligands have been employed, exemplified by N-heterocyclic carbene (NHC) ligands. These strong σ donors are versatile ligands that can bind transition-metal ions across a range of oxidation states.6,7 In comparison, actinide-NHCs are rare8−14 and only a single thorium complex has been described15 (Figure 1). This limited number of examples is perhaps due to the notion that Nheterocyclic carbene ligands are “soft” donors that might accordingly bind only weakly to “hard” actinide ions. Initial reactivity studies on An-NHCs revealed that, rather than being strong supporting ligands, NHCs were relatively labile and easily displaced.13−15 We were motivated to determine whether this undesired role was attributable to some fundamental incompatibility between “soft” NHCs and “hard” Lewis acidic actinide ions or if an alternative NHC ligand design16 would provide a means to study actinide-NHC coordination chemistry comparable to that of the metallocenes. © XXXX American Chemical Society

Figure 1. Examples of An-NHC complexes reported alongside the new class described in this work. Abbreviations: Mes = mesityl, Dipp = diisopropylphenyl, iPr = isopropyl.

In this work we introduce a bulky monoanionic bis(carbene) ligand (2) and its Th-NHC bis(iodide) complex (3). To access uncharacteristic redox reactivity with this formally 6d05f0 thorium system, a reduced bipyridine ligand was installed on the metal center (4). We describe the reactivity of 4 with organic carbonyls and azides and report the isolation of an unusual terminal thorium imido complex (6) in which the bpy ligand remains bound to the metal center. To our knowledge, this is the first example of a thorium carbene complex performing these types of transformations and the first example Received: June 11, 2016

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DOI: 10.1021/acs.organomet.6b00467 Organometallics XXXX, XXX, XXX−XXX

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Organometallics of any terminally bound thorium imido complex bearing a redox-active ligand.

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RESULTS AND DISCUSSION Synthesis of Th(BcMes)2I2 (3). Inspired by the work of Smith and co-workers,17 we targeted the bulky, chelating monoanionic bis(N-heterocyclic carbene)borate ligand 2 (Figure 2), envisioning its mesityl substituents would afford

Figure 3. Molecular structure of 2·Li (thermal ellipsoids drawn at the 50% probability level).

Two distinct Ccarbene shifts were observed in the 13C NMR spectrum at 215.8 and 208.2 ppm and agree with the only other Th-NHC complex known (210.3 ppm).15 Compound 3 is just the second reported and structurally characterized Th-NHC complex (Figure 4) and the first Th-

Figure 2. General representation of 2 highlighting the attractive features of the mesityl-substituted bis(NHC)borate ligand scaffold for supporting thorium.

solubility to the complex and steric protection to the actinide center. Additionally, binding of the NHC to thorium would be enhanced electrostatically by the five-electron, chelating nature of the anionic “L2X” moiety. The thorium complex 3 was prepared using a two-step, onepot approach (Scheme 1). Double deprotonation with LDA followed by metalation with 0.5 equiv of ThCl4(DME)2 proceeded smoothly over the course of 48 h to form the diiodide 3, isolated in 79% yield. Scheme 1

Figure 4. Molecular structure of 3 (thermal ellipsoids drawn at the 50% probability level).

NHC compound bearing halide ligands. Moreover, the Th− Ccarbene bond lengths in 3 are significantly shorter (2.623(6)− 2.634(6) Å) than those of its predecessor (2.852(6)−2.884(5) Å). With the first Th-NHC halide complex in hand, we were poised to explore its substitution chemistry and compare the bis(NHC)borate framework to that of Cp in the organoactinide regime.3 Synthesis of Th[(BcMes)2(bpy)] (4). We sought to access uncharacteristic redox reactivity with this formally 6d05f0 thorium system. Treating 3 with excess KC8 in the presence of 1 equiv of 2,2′-bipyridine furnished 4 in good yield (76%) as a very dark green solid (Scheme 2).

Related halide exchange has been seen in other systems18,19 and is presumably driven by precipitation of the LiCl byproduct. Truncated reaction times resulted in mixtures of products. Additionally, as reported with similar ligands,19−21 deprotonation of the proligand imidazolium borate salt 1 is highly sensitive to the choice of base. For example, nBuLi, LiHMDS, NaHMDS, and KHMDS did not promote clean reactions. Although the instability of 2 precluded long-term storage, crystals suitable for single-crystal X-ray diffraction studies were successfully grown of 2·Li (Figure 3). 2·Li crystallizes as a solvent-free dimer, with one carbene atom bound terminally and the other bridging the two lithium atoms. For additional structural details see Tables S4 and S5 in the Supporting Information. The 1H NMR spectrum of 3 confirmed successful metalation by the absence of the diagnostic downfield C2 imidazolium proton resonance of 1 (δ 9.15 ppm in CDCl3). Notably, the spectrum contains six separate mesityl methyl resonances in toluene-d8 at 296 K. VT-NMR experiments revealed that at 303 K two of these resonances coalesce. This suggests that 3 could have hindered rotation about the N−Cipso bond of either mesityl substituent or that 3 undergoes a conformational change within the chelate ring upon increasing temperature.

Scheme 2

The 1H NMR spectrum of 4 in C6D6 (296 K) appears to be diamagnetic with all signals between 0 and 8 ppm. The bpy resonances are present in a 1:2 ratio with respect to the bis(NHC)borate ligand and are shifted significantly upfield relative to those of free bpy (δ 5.98−3.95 ppm vs 8.74−6.67 ppm, respectively), due to increased electron density within the bipyridine ring.22,23 As in 3, the six mesityl methyl groups in 4 resonate distinctly at room temperature. B


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Organometallics

294.9 K, uncorrected). Taken together, these spectroscopic data implicate an electronic structure comprising some degree of Th(III)-bpy(−1) character, a view supported by computational studies discussed below. Initial DFT calculations indicated that complex 4 has a Th(III)-bpy(−1) ground state with a Th(IV)-bpy(−2) excited state +9.9 kcal/mol higher in energy. The optimized geometry of 4 fits the experimental structure with the main geometrical features being well-reproduced (e.g., Th−Ccarbene 2.65 Å, C53− C54 1.37 Å). Moreover, the bipyridine ligand is κ2 (κ-N,κ-N) bonded (Figure 7) unlike the η4(N,C,C,N) coordination mode found in the Th(IV)-bpy metallocene complex reported by Walter and co-workers.31

Dark green block-shaped crystals of 4 were grown from a concentrated hexane solution stored at room temperature for 48 h; the results of an X-ray diffraction study are shown in Figure 5. The range of Th−Ccarbene bond lengths in 4 is wider


than that of 3, spanning 2.656(4)−2.755(3) Å, likely due to packing effects. Two carbene atoms (one on either ligand) and the thorium center are nearly linear with a C1−Th1−C3 angle of 172.4(1)°. Inspection of the inter-ring C53−C54 bond length (1.362(5) Å) suggests that the bpy ligand is direduced and bound to a thorium(IV) center, agreeing well with other reported An(IV)-bpy complexes.22,24,25 DC magnetic susceptibility studies revealed that, apart from a small paramagnetic impurity,26 4 displayed diamagnetic behavior (Figure S29 in the Supporting Information). However, the infrared spectrum of 4 showed diagnostic stretches at 1571 and 977 cm −1 attributable to ring deformations within a monoanionic radical bipyridine ligand (Figure S21 in the Supporting Information).27,28 Furthermore, in addition to the intense absorbances at λmax 316, 415, 450, 485, 592, and 636 nm (ε (103 L mol−1 cm−1) = 5.0, 5.3, 4.9, 4.2, 3.0, and 2.4, respectively), the room-temperature UV−vis spectrum of 4 in toluene (Figure 6) also showed features characteristic of the bipyridine radical anion27,29,30 at λmax 731 and 805 nm (ε (103 L mol−1 cm−1) = 0.35 and 0.24, respectively). Finally, a nonzero solution magnetic moment, μeff = 0.80(1) μB, was determined by the Evans method (C6D6,

Figure 7. DFT optimized structure of 4. Hydrogen atoms omitted for clarity.

Examination of the frontier orbitals (Figure 8) corroborated the unusual electronic configuration of 4.

Figure 8. Frontier orbitals of 4.

One SOMO (Figure 8a) is clearly metal centered (f1) and the second (Figure 8b) bpy centered (π*). To decipher the nature of the ground state, particularly the spin state, CASSCF calculations were carried out. These studies revealed that the electronic ground state of the bis(NHC)borate-supported Thbpy complex 4 is a multiconfigurational open-shell singlet. The main configuration (73%) corresponds to Th(III)-bpy(−1) (f1π*1) and a smaller contribution (27%) to Th(IV)-bpy(−2) (f0π*2). This is reminiscent of the results found for the Yb(bpy) systems of Booth et al.32 Interestingly, the first excited state is a triplet with a pure Th(III)-bpy(−1) structure only +2.7 kcal/mol higher in energy than the ground state, whereas the second excited state is a closed-shell singlet that is pure Th(IV)-bpy(−2) + 10.2 kcal/ mol higher in energy. Such a narrow range of energies allows for easy thermal exchange between the different configurations and is no doubt the source of incongruent infrared, UV−vis, and magnetic observations.

Figure 6. UV−vis spectrum of 4. Inset: magnified portion highlighting absorbances attributable to the bipyridine radical anion. C


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remained bound to the thorium center of 6, both in solution and in the solid state. This is likely due to the less sterically demanding nature of the bis(NHC)borate ligand in comparison to that of the tri-tert-butyl- and pentamethyl-substituted Cp derivatives used previously.41,45,47 NMR and X-ray data showed that the bpy ligand was fully oxidized. Resonances corresponding to the bound bpy ligand in the 1H NMR spectrum of 6 were shifted significantly downfield (9.94 and 8.75 ppm) compared to those of 4 and those of free bpy, indicating a loss of electron density within the bipyridyl rings.23 From X-ray crystallography, the elongated C54−C53 bond length (1.51(3) Å) relative to that in 4 (1.362(5) Å) indicates that the bpy ligand is in its neutral form (Figure 10).25

The cyclic voltammogram of 4 showed two reversible processes at −1.80 and −2.64 V, which conforms well to two bpy-centered oxidations (Figure 9).33−39

Figure 9. CV of 4 in 0.2 M TBAPF in THF. Scan rate: 25 mV/s.

Seeing the propensity for redox reactivity, we investigated the interaction of 4 with aldehydes and ketones (Scheme 3). These Scheme 3


Additionally, the Th1−N11 bond length (2.07(1) Å) and the nearly linear Th1−N11−C59 bond angle (170.1(1)°) are consistent with a ThN multiple bond and agree well with the limited examples of terminally bound thorium imido species in the literature.43,45,46,48 Notably, complex 6 represents the first example of a terminal imido thorium compound bearing a redox-active ligand in the coordination sphere. CV experiments on 6 (Figure S26 in the Supporting Information) show an irreversible reduction at −2.88 V vs Fc/[Fc]+; however, the electronic structure of the reduced form of 6 is currently under investigation.

reactions led to heterocoupled reduction products (5a−c) related to those seen with An-bpy-metallocene systems.31,40,41 Specifically, the carbon−oxygen double bond of the carbonylated molecule was reduced concomitant with C(sp3)−C(sp3) bond formation between the substrate and the bipyridine ligand. A dramatic color change from dark green to deep bluepurple (see the Supporting Information for UV−vis and X-ray data) took place as soon as the reagents were combined. 1H NMR monitoring of the reaction progress confirmed fast, clean reactions, with no detectable intermediates. We also probed the ability of 4 to mediate the two-electron oxidation of organic azides and the capability of the bis(NHC)borate ligand (2) to stabilize a rare terminal thorium imido compound. To date, only five terminally bound thorium imido complexes have been reported,42−46 four of which are supported by Cp-derived ligands. Utilizing the nitrene transfer route,45 we subjected 4 to 1.1 equiv of p-tolyl azide (Scheme 4). Compound 4 reacted immediately to form 6 accompanied by gas evolution (N2) and color change to brown; standard workup afforded 6 in 53% yield. In contrast to prior actinide imido metallocene complexes prepared via this method,45,47 the oxidized bipyridine ligand

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CONCLUSIONS We have demonstrated that careful ligand design affords reactive thorium-NHC compounds and creates the steric and electronic environment necessary to stabilize unusual molecular and electronic structures. The new mesityl-substituted bis(NHC)borate scaffold 2 not only supported a thorium-bpy complex (3) through transformations with carbonylated substrates and p-tolyl azide, but also furnished the first terminally bound thorium-imido compound bearing a redoxactive ligand (6). Moreover, CASSCF calculations on 4 revealed an uncommon multi-configurational open-shell singlet electronic ground state. Our group is continuing to explore the implications of this electronic structure, the reactivity of 4 with other small molecules, and the potential for redox non-innocent transformations by 6.

Scheme 4

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EXPERIMENTAL SECTION

General Considerations. Unless otherwise stated, all reactions were performed under an atmosphere of dry N2 using standard Schlenk line techniques or in an MBraun N2 atmosphere glovebox (

A New Supporting Ligand in Actinide Chemistry ... - ACS Publications

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