Carbene Complexes of Neptunium

Since the advent of organotransuranium chemistry six decades ago, structurally verified complexes remain restricted to π-bonded carbocycle and σ-bonded hydrocarbyl derivatives. Thus, transuranium-carbon multiple or dative bonds are yet to be reported. Here, utilizing diphosphoniomethanide precursors we report the synthesis and characterization of transuranium-carbene derivatives, namely, diphosphonio-alkylidene- and N-heterocyclic carbene–neptunium(III) complexes that exhibit polarized-covalent σ2π2 multiple and dative σ2 single transuranium-carbon bond interactions, respectively. The reaction of [NpIIII3(THF)4] with [Rb(BIPMTMSH)] (BIPMTMSH = {HC(PPh2NSiMe3)2}1–) affords [(BIPMTMSH)NpIII(I)2(THF)] (3Np) in situ, and subsequent treatment with the N-heterocyclic carbene {C(NMeCMe)2} (IMe4) allows isolation of [(BIPMTMSH)NpIII(I)2(IMe4)] (4Np). Separate treatment of in situ prepared 3Np with benzyl potassium in 1,2-dimethoxyethane (DME) affords [(BIPMTMS)NpIII(I)(DME)] (5Np, BIPMTMS = {C(PPh2NSiMe3)2}2–). Analogously, addition of benzyl potassium and IMe4 to 4Np gives [(BIPMTMS)NpIII(I)(IMe4)2] (6Np). The synthesis of 3Np–6Np was facilitated by adopting a scaled-down prechoreographed approach using cerium synthetic surrogates. The thorium(III) and uranium(III) analogues of these neptunium(III) complexes are currently unavailable, meaning that the synthesis of 4Np–6Np provides an example of experimental grounding of 5f- vs 5f- and 5f- vs 4f-element bonding and reactivity comparisons being led by nonaqueous transuranium chemistry rather than thorium and uranium congeners. Computational analysis suggests that these NpIII=C bonds are more covalent than UIII=C, CeIII=C, and PmIII=C congeners but comparable to analogous UIV=C bonds in terms of bond orders and total metal contributions to the M=C bonds. A preliminary assessment of NpIII=C reactivity has introduced multiple bond metathesis to transuranium chemistry, extending the range of known metallo-Wittig reactions to encompass actinide oxidation states III-VI.

days, a = 777 TBq g -1 ) and associated = g-ray emission (most significant g-branching ratio for 233 Pa is 39% for the 312 keV line). Hence, all studies that involved manipulation of 237 Np material were conducted in a specialist transuranium radiological designated area equipped with high efficiency particulate in air (HEPA) filtered hoods and in negative pressure gloveboxes. Safety controls included continuous air monitoring for airborne a-emitting particles and use of hand-held radiation monitoring equipment. Entrance to the laboratory space was controlled with a hand and foot radiation monitoring instrument and a full body personal contamination monitoring station. The handling of free-flowing solids was restricted to be within negative pressure gloveboxes equipped with HEPA filters. In addition to standard laboratory PPE, aqueous solutions were handled using multiple layers of gloves (of a material compatible with the chemicals being handled) combined with DuPont™ Tyvek® 400 sleeves to provide overlapping coverage of the arms. Due to these radiological hazards, elemental analyses were not possible.
Unless otherwise described, all syntheses and manipulations were conducted under UHP argon (AirGas) or UHP helium (AirGas) with rigorous exclusion of oxygen and water using Schlenk line and glove box techniques (employing a negative-pressure, transuranium-capable, MBraun LabMaster, helium atmosphere glovebox where required). 4 Å molecular sieves were activated by heating for 36 hrs at 200 °C, 10 -4 mbar. Anhydrous DME (Sigma Aldrich) was transferred onto activated 4 Å molecular sieves, stored for 1 week prior to use, and degassed before use. Anhydrous Et2O containing BHT (100 ppm, Sigma Aldrich) was degassed, distilled from Na2Ph2CO, stored over S3 activated 4 Å molecular sieves for 1 week and degassed again before use. d8-THF, d6-benzene, anhydrous n-hexane and anhydrous toluene (Sigma Aldrich) were stored over activated 4 Å molecular sieves and degassed before use. All solvents were tested with a dilute THF solution of Na2Ph2CO (150 mg Ph2CO in 20 mL of THF with an excess of Na metal) such that ethereal solvents (including d8-THF) required 1 drop / mL to retain purple coloration and hydrocarbon solvents (including d6benzene) required 1 drop / 2 mL. and that atmospheric O2/H2O levels are too high to be conducive to this chemistry, requiring removal to lower levels before performing reactions/exposing reagents to the glovebox atmosphere). All glassware, and glass-fiber filter discs, was stored in a vacuum oven (>150 °C) for 24 hrs prior to being brought into the glovebox, and FEP (fluorinated ethylene propylene) NMR tube liners were brought into the glovebox via overnight or multi-hr vacuum cycles in the antechamber port.
Crystals for single-crystal X-ray diffraction studies were mounted either in Fomblin oil on a micromount (2) or in Paratone-N or NVH oil inside 0.5 mm quartz capillaries (Charles Supper) (1, S4 3-6). The quartz capillaries were inserted through silicone stoppers and placed inside test tubes to allow handling inside the transuranium glovebox while mounting crystals without contaminating the exterior surface of the capillary. The capillaries were then cut with nail clippers to appropriate size for mounting on a goniometer. The ends of the cut capillaries were sealed with hot capillary wax before being removed from the glovebox for coating with clear nail varnish (Hard as Nails™) to provide shatter-resilience. 11 During the clipping and wax sealing steps, care must be taken to avoid the capillary touching any contaminated surfaces (this is achieved by the introduction of fresh petri dishes, forceps, clippers, and wax, as needed in conjunction with careful handling techniques to avoid contamination transfer). Following removal from the glovebox, the exterior surfaces of the capillaries were monitored with α-particle detection instruments prior to transport to the X-ray diffraction laboratory.
Solution phase electronic absorption spectra were collected at ambient temperature using a Varian Cary 6000i UV/vis/NIR spectrometer. The solution was contained in a low volume (1 mL) screwcapped quartz cuvette (1 cm path length) that was loaded in a transuranium glovebox using Parafilm™ to protect the exterior surface of the cuvette and cap from radioactive contamination (Parafilm™ is removed in a fume hood and exterior surfaces of the cuvettes were monitored with αparticle detection instruments prior to data acquisition). Data was collected from 40,000 to 6,250 cm −1 (250 to 1,600 nm).
For NMR spectroscopy, a solution was loaded into a fresh FEP NMR tube liner that was protected from surface contamination with Parafilm™ while inside a transuranium glovebox. The liner was sealed with two PTFE plugs, brought out of the glovebox and verified to be free of surface contamination after the Parafilm™ was removed (using α-particle detection instruments and a Ludlum 3030E instrumentto detect both α-and β-particles on smear surveys of the exterior surfaces) and the liner loaded into a J. Young tap appended 5 mm NMR tube. The headspace was then S5 evacuated and refilled with He to provide an inert atmosphere headspace above the sample and Where ε values are reported for molecular complexes below there is a modest error due to the small quantities of weighed material, as is nearly always the case when these values are reported from synthetic chemistry (as opposed more rigorous and quality-assured analytical determination methods that are generally unfeasible to apply to non-aqueous synthetic chemistry). Nonetheless, the ε values we determine herein are still useful metrics that we determine based on weight of crystal dissolved and solvent weight, but should not be used as analytically to assay these compunds.
ATR-IR spectra of 4Ce and 6Ce were recorded on a Bruker Alpha spectrometer with a Platinum-ATR module in the glovebox.
CHN microanalyses on 4Ce and 6Ce were carried out by London Metropolitan University.

Synthesis of [{(BIPM TMS )U IV }2(µ-Cl)6{Li(DME)}2] (2)
In a 20 mL glass scintillation vial with a PTFE-coated stirrer bar, Et2O (1 ml) was added to Since the sole purpose of this synthesis was to scope the analogous Np preparation no data other than the single-crystal X-ray diffraction molecular structure were collected.

Synthesis of [(BIPM TMS H)Ce III (I)2(THF)] (3Ce)
Complex 3Ce was synthesized as previously reported, 12 with some modifications. In a 20 mL glass scintillation vial with a PTFE-coated stirrer bar, CeI3 (23 mg, 44 µmol) was stirred with THF (1.5 mL) at room temperature for 5 minutes which gave a turbid colorless solution with white solids. Solid [Rb(BIPM TMS H)] (28.3 mg, 44 µmol, 1 equiv.) was added in a 2 portions which caused the mixture to immediately turn pale yellow with concomitant dissolution of the white solids, followed by rapid precipitation of fine white solids (presumably RbI). The mixture was stirred for 10 minutes and then reduced to dryness in vacuo. The yellow solids were suspended in toluene (1 mL) and warmed gently (45 °C on a hot plate). Once cooled to room temperature, the mixture was centrifuged (5 minutes, 5,000 rpm) and filtered into a 4 mL glass vial through two glass microfiber filter discs packed in a glass pipette. The yellow solution was concentrated to the point of incipient crystallization, warmed gently (45 °C on a hot plate) and then stored at room temperature overnight (16 hrs). Several flaky colorless crystals grew and these were inspected by single-crystal X-ray diffraction and found to be 3Ce. Characterization data on this material matched the previously reported data. 12 The poor quality of the crystals meant that this motif was not extended to studies with Np.

Synthesis of [(BIPM TMS )Ce III (I)(DME)] (5Ce)
In a modification of the previously reported procedure, 12  44 µmol, 1 equiv.) was added in a 2 portions which caused the mixture to immediately turn pale yellow with concomittent dissolution of the white solids, followed by rapid precipitation of fine white solids (presumably RbI). The mixture was stirred for 5 minutes and then dried in vacuo to a pale yellow powder. DME (1.5 mL) was added, and then solid KBn (5.7 mg, 44 µmol, 1 equiv.) was added to the pale yellow suspension in several portions which caused the mixture to immediately turn from pale yellow to a slightly more intense yellow -the vivid orange color of the KBn discharged rapidly as each portion dissolved. The cloudy mixture was stirred for a further 5 minutes and then reduced to a yellow powder in vacuo. Toluene (1.5 mL) and DME (3 drops) were added to the yellow solids, which was then warmed gently (45 °C on a hot plate). Once cooled to room temperature, the mixture was centrifuged (5 minutes, 5,000 rpm) and filtered into a 4 mL glass vial through two glass microfiber filter discs packed in a glass pipette. The yellow solution was concentrated to ~0.5 mL which caused a large quantity of pale yellow solids to form on the vial walls. The mixture was heated strongly (130 °C on a hot plate) and gently refluxed inside the vial for ~2 minutes which resulted in all of the solids redissolving. The yellow solution was stored at room temperature overnight (16 hrs).
Large yellow blocks formed and these were inspected by single-crystal X-ray diffraction and found to be 5Ce. Characterization data on this material matched the previously reported data. 12

Synthesis of [(BIPM TMS )Ce III (I)(I Me4 )2] (6Ce)
In a 20 mL glass scintillation vial with a PTFE-coated stirrer bar, CeI3 (23 mg, 44 µmol) was stirred with THF (1.5 mL) at room temperature for 5 minutes which gave a turbid colorless solution with white solids. Solid [Rb(BIPM TMS H)] (28.3 mg, 44 µmol, 1 equiv.) was added in a 2 portions which caused the mixture to immediately turn pale yellow with concomitant dissolution of the white solids, S10 followed by rapid precipitation of fine white solids (presumably RbI). The mixture was stirred for 10 minutes and then solid KBn (5.7 mg, 44 µmol, 1 equiv.) was added in several portions which caused the mixture to immediately turn from pale yellow to a slightly more intense yellow -the vivid orange color of the KBn discharged instantaneously as each portion dissolved. The cloudy mixture was stirred for a further 15 minutes and then reduced to a yellow powder in vacuo. The yellow solids were suspended in toluene (1.5 mL) and warmed gently (45 °C on a hot plate). Once cooled to room temperature, the mixture was centrifuged (5 minutes, 5,000 rpm) and filtered into a 4 mL glass vial through two glass microfiber filter discs packed in a glass pipette.

Synthesis of [(BIPM TMS H)Np III (I)2(I Me4 )] (4Np)
In a 20 mL glass scintillation vial with a PTFE-coated stirrer bar, solid [Rb(BIPM TMS H)] ( and then cloudy and yellow with concomitant precipitation of fine white solids (presumably RbI).
The mixture was stirred for 5 minutes and then dried in vacuo to an orange powder. DME (1.5 mL) was added, and then solid KBn (5.7 mg, 44 µmol, 1 equiv.) was added to the orange suspension in several portions which caused the mixture to immediately turn from orange to red/purple -the vivid orange color of the KBn discharged instantaneously as each portion dissolved. The cloudy mixture was stirred for a further 5 minutes and then reduced to a red powder in vacuo. Toluene (1.5 mL) and DME (3 drops) were added to the orange solids and the mixture was stirred for 1 minute at room temperature. The mixture was filtered into a 4 mL glass vial through two glass microfiber filter discs packed in a glass pipette, then concentrated to ~0. 3

Synthesis of [(BIPM TMS )Np III (I)(I Me4 )2] (6Np)
In a 20 mL glass scintillation vial with a PTFE-coated stirrer bar, THF (1.5 mL) was added to solid Hz, 18 H, BIPM TMS Si(CH3)3 × 2) ppm. We tentatively assign this peak due to its intensity and position which is similar to that of 5Np. No other peak that could be definitively assigned to this complex rather than BIPM TMS H2, or a BIPM TMS H complex (like 4Np), could be identified. 13

Attempted Synthesis of [(BIPM TMS )U III (I)(DME)] (5U)
[(BIPM TMS H)UI2(THF)] was dissolved in DME (5 mL), stirred for several hrs, then all volatiles are removed in vacuo to give [(BIPM TMS H)UI2(DME)]. In a 20 mL glass scintillation vial with a PTFEcoated stirrer bar, [(BIPM TMS H)UI2(DME)] (55 mg, 49 µmol) was stirred in DME (5 mL) at room temperature for 5 minutes to give a dark blue solution. KBn (6.5 mg, 50 µmol, 1 equiv.) was added portionwise upon which a color change to brown was observed. The resulting mixture was stirred for 15 minutes after which time all the volatiles were removed in vacuo to afford a brown solid, which was extracted into toluene and filtered through a syringe filter to afford a pale brown solution.
Attempts to prepare crystalline material was unsuccessful despite several attempts using different solvents and solvent ratios (Toluene, DME).

Attempted Synthesis of [(BIPM TMS )U III (I)(I Me4 )2] (6U)
Attempt 1. In a 20 mL glass scintillation vial with a PTFE-coated stirrer bar, [(BIPM TMS H)UI2(THF)] (36 mg, 33 µmol) was stirred with toluene (3 mL) at room temperature for 5 minutes to give a dark blue solution. Solid I Me4 (4.2 mg, 33 µmol, 1 equiv.) was added and the resulting mixture stirred for 5 minutes to afford a dark blue/purple solution. KBn (4.3 mg, 33 µmol, 1 equiv.) was added and the resulting mixture stirred for 5 minutes after which all the KBn appeared to be consumed and the solution changed to a dark brown color with a colorless precipitate. The mixture was filtered through a syringe filter to afford a pale brown solution. Attempts to prepare crystalline material was unsuccessful despite several attempts using different solvents and solvent ratios (Toluene, Et2O, THF).

Attempt 2. In a 20 mL glass scintillation vial with a PTFE-coated stirrer bar, [(BIPM TMS H)UI2(THF)]
(36 mg, 33 µmol) was stirred with toluene (5 mL) at room temperature for 5 minutes to give a dark blue solution. Solid I Me4 (8.4 mg, 66 µmol, 2 equiv.) was added and the resulting mixture stirred for 30 minutes to afford a dark blue/purple solution. The solution was then stored at -30 °C for 30 minutes. KBn (4.3 mg, 33 µmol, 1 equiv.) was added and the resulting mixture stirred for 20 minutes after which time all the KBn appeared to be consumed, affording a dark blue/ brown solution with precipitation of a pale solid. Volatiles were removed from the solution to afford a sticky brown solid, which was extracted into toluene and filtered through a syringe filter to afford a pale brown solution.
Attempts to prepare crystalline material was unsuccessful despite several attempts using different solvents and solvent ratios (Toluene, Et2O, THF).

Reaction of 5Np with PhCHO to give PhC(H)=C(PPh2NSiMe3)2 (7)
In a 5 mL glass scintillation vial with a glass stirrer bar, 5Np ( ( 1 H and 31 P) matches that of the previously reported alkene, 13   2) The same material as on the left, but after addition of I Me4 . The pale precipitate can be clearly seen.
More of this same material formed after subsequent concentration and storage at -35 °C. Only once all of this material was removed could crystals of 4Np be isolated. S18 Figure S3. Crystals of 4Np under hexane during the washing procedure. The same material as on the left, but after addition of KBn.  in toluene for UV-vis-NIR spectroscopy.

General considerations
The crystal data for all complexes are compiled in Table S1 to   (2) 9.7913 (7) b, Å 17.7775 (3) 14.1981 (2) 11.9571 (8) c, Å 43.6609 (7) 14.6654 (2) 19.1734 (12) α, ° 89.2160 (10) 77.161 (5) β, ° 97.229 (2) 73.7920 (10) 77.102 (5) γ, ° 87.5900 (10)                           We were unable to obtain magnetic susceptibility measurements for complex 4Np due to the very limited sample quantity combined with difficulty fully solubilizing the sample in d6-benene at room temperature (for example, it crystallizes overnight from d6-benzene solutions in NMR tubes). As our sample sealing procedure makes it impractical to safely bring the sample back into the glovebox without decomposition, we were unable to add additional solvent (e.g. some d8-THF) to ensure complete dissolution of the sample, and we did not have enough material to gather new NMR spectra in a different solvent.

S9. Computational studies
Geometry optimizations for 4Np-6Np were performed using coordinates derived from their crystal all of the calculations. 27 Generalized gradient approximation corrections were performed using the functionals of Becke and Perdew. 28,29 Natural Bond Order (NBO) analyses were carried out with NBO 6.0.19. 30 The Quantum Theory of Atoms in Molecules analysis 31,32 was carried out within the ADF program, and those data were checked by comparing to values computed with Xaim-1.0 33 using WFN files generated by ADF. The ADF-GUI (ADFview) was used to prepare the three-dimensional plots of the electron density. In all cases, Aubau formulations were found with the appropriate spin formulations (5f 4 Np(III), quintet; 5f 3 U(III), quarted, 4f 1 Ce(III), doublet, 4f 4 Pm(III), quintet).