A Structurally Characterized Organometallic Plutonium(IV) Complex

Abstract The blood‐red plutonocene complex Pu(1,3‐COT′′)(1,4‐COT′′) (4; COT′′=η8‐bis(trimethylsilyl)cyclooctatetraenyl) has been synthesized by oxidation of the anionic sandwich complex Li[Pu(1,4‐COT′′)2] (3) with anhydrous cobalt(II) chloride. The first crystal structure determination of an organoplutonium(IV) complex revealed an asymmetric sandwich structure for 4 where one COT′′ ring is 1,3‐substituted while the other retains the original 1,4‐substitution pattern. The electronic structure of 4 has been elucidated by a computational study, revealing a probable cause for the unexpected silyl group migration.

. 1 H NMR and 13 C NMR data for 4 4 Figure S1. 1 H/ 13 C-HSQC spectrum (full) 4 Figure S2. Zoom into the aromatic region of the 1 H/ 13 C-HSQC spectrum and the 1H-NMR spectrum 5 Crystal structure report 5 Figure S3. Molecular structure of the title compound in the crystal 6 Table S2. Crystallographic experimental details 7 Table S3. Atomic coordinates and equivalent isotropic displacement parameters 8 Table S4. Interatomic distances 9 Table S5. Selected Interatomic angles 9 Table S6. Anisotropic displacement parameters 10 Table S7. Derived atomic coordinates and displacement parameters for hydrogen atoms 11 Computational study details 12 Table S8. Structural parameters of plutonocene (Pu(COT) 2 ) 13

Synthesis and spectroscopic characterization
General: Caution! All Pu isotopes are radioactive. 239 Pu is furthermore a fissile material and underlies therefor special control, regulations and restrictions. Actinides are in general α emitters and therefore any contamination or incorporation has to be avoided for sure.
The manipulations with radionuclides were conducted in the radiochemical laboratories at the Joint Research Centre (JRC) -Karlsruhe, Germany. Unsealed transuranium compounds were manipulated in dinitrogen filled (99+%), negative-pressure radiological gloveboxes.
The glovebox for preparative chemistry was fitted with an automated dual vacuum/argon manifold and standard Schlenk techniques were used. NMR spectra were recorded on the Bruker Ascent™ 400 MHz WB NMR/DNP spectrometer equipped with a Bruker Triple Resonance Broad Band Probe (TBI). Degassed fluoropolymer NMR tube liners (4 mm nominal O.D.; 140°C, 6 × 10 -4 mbar, 12 h) were charged with the liquid samples ensuring that the outer surface remained free from contamination, and sealed. The sealed liner was then transferred into a standard borosilicate glass NMR tube placed in a PVC bag, which was sealed by welding. Chemical shifts were calibrated against residual protio solvent signal and are reported relative to tetramethylsilane (δ = 0 ppm).
Commercially available reagents and solvents were obtained from Sigma-Aldrich Co., ACROS Organics, STREM Chemicals Inc. or ABCR GmbH & Co. KG and used as received unless otherwise stated. Argon 6.0 N was supplied by basi Schöberl GmbH & Co. KG and purified with Agilent Technologies Big Moisture and Oxygen Traps to achieve sub-5 and sub-1 ppb levels of the corresponding impurities respectively. Anhydrous grade solvents were purified immediately before use by distillation from appropriate drying agents under argon. PuCl 3 was on stock but might be prepared according to (1,2).
Compound: [Pu{η 8 -C 8 H 6 -1,3-(SiMe 3 ) 2 }{η 8 -C 8 H 6 -1,4-(SiMe 3 ) 2 }] (4) 140 mg (0.56 mmol) C 8 H 6 -1,4-(SiMe 3 ) 2 were dissolved in 5 ml THF. 0.8 ml of 1.6 molar n-BuLi (1.28 mmol) in hexanes are added at RT. The solution turns deep orange and warms up slightly. 97.2 mg (0.28 mmol) PuCl 3 were added as a solid. The brown-green reaction mixture was stirred at r.t. for 7d. The solvent was removed and the residue was dissolved in 10 ml of toluene forming a green solution. Addition of 20 mg (0.15 mmol) water-free CoCl 2 turned the reaction dark red. It was stirred at RT for 5h and refluxed for 15h to complete the reaction. The solvent was removed and the raw product was extracted with n-pentane, followed by removing the insoluble matter by filtration. Removal of the n-pentane from the filtrate yielded 170 mg (84%) of a waxy product which crystallised overnight under formation of dark red crystals with a low melting point of < 60°C. Pu content: found 31.0 % (α-spectroscopy), calculated for C 28 H 48 PuSi 4 : 33.1%.  Figure S1. 1 H/ 13 C-HSQC spectrum (full). Figure S2. Zoom into the aromatic region of the 1 H/ 13 C-HSQC spectrum (left) and the 1H-NMR spectrum (right). The CH correlated signal of the weak because very broad resonance at 9.78 ppm assigned to the proton in 2-position of 1,3-substituted COT ligand is resolved.

Crystal structure report
Single crystal analysis of 4 was performed on a Bruker Apex II Quazar diffractometer at 100 K collecting four spheres of data with an irradiation time of 10 to 20 s per frame applying a combination of ω-and ϕ-scans. For more information be referred to table S1.
The measurement was controlled with the APEX II software 11 , integration of the data proceeded with SAINT 11 , the data were corrected for Lorentz-and polarisation effects, and an experimental absorption correction with SADABS was performed 12

Computational study details
In (4), the ground state was assumed to have the same multiplicity as plutonocene with ground state 5 A g . 9,10  (7) and (8). The dihedral angle CCH smaller than 180° indicates a bending of the C-H bond towards the Pu atom (see Fig. 1).