Steric control of redox events in organo-uranium chemistry: synthesis and characterisation of U(v) oxo and nitrido complexes

Controlling the steric environment in U(η8-C8H6(1,4-SiR3)2)(η5-Cp*)] enables selective formation of either mononuclear U(v) or dinuclear U(iv) oxo and nitrido complexes.

Independent Synthesis of  Pr 3 ) 2 ](η 5 -Cp * )} 2 (μ-Ο) (5) from (1) and N 2 O: 210 mg (0.245 mmol) of (1) were dissolved in d 8 -toluene (ca. 1 mL) in a 50 mL Young's ampule with a capillary sidearm, connected to a Töpler line and degassed at -78 ºC for 5 minutes. This solution was treated with exactly 0.5 mol eq of N 2 O at this temperature with stirring and the reaction was left to equilibrate at RT over 2 hours to produce an intense red solution. 1 H-NMR spectroscopy showed conversion to (6) in ca. 90%. Volatiles were removed in vacuum and the red residue was dissolved in t BuOMe (ca. 5 mL) and upon cooling at 5 ºC produced crystals of the title compound as the t BuOMe solvate that were isolated by filtration and dried in vacuum. Yield: 140 mg (79.1%). Spectroscopic data were identical to the ones previously reported. NOTE: If an excess of N 2 O is used, the reaction yields brown green solids that are insoluble in organic solvents.
NMR scale reaction of (3) with (1): A Young's NMR tube was charged with 15 mg (0.0186 mmol) of (3) and 16 mg (1 mol eq) of (1) and the two solids were dissolved in C 6 D 6 and the NMR spectra recorded.

Discussion of the molecular structure of (7):
Crystals suitable for single crystal XRD were obtained from the absolute minimum of SiMe 4 (ca. 0.5 mL). An ORTEP diagram of its molecular structure is presented in Figure SI1. Figure SI1: ORTEP diagram of the molecular structure of (7) displaying 50% probability ellipsoids Hydrogen atoms and i Pr groups have been omitted for clarity. Selected bond lengths (Å) and angles (º): The only salient feature of this molecular structure is the rather long reason for this is to reduce the steric congestion due to the bulky Si i Pr 3 substituted COT ligand. All other metric features are as expected and warrant no further discussion.
Synthesis of [U{η 8 -C 8 H 6 (1,4-Si i Pr 3 ) 2 }(η 5 -Cp * )NNa(OEt 2 ) 2 ](9): 320 mg (0.38 mmol) of (1) and 26 mg (1.02 mol eq) of NaN 3 were charged in a Young's ampule in an Ar box and toluene and THF was added to the solids (ca. 2:1 v/v 10 mL total) at RT and the reaction mixture was stirred overnight. Volatiles were removed in vacuum to leave a brown-red residue that was extracted in warm (almost boiling) Et 2 O filtered hot (3x5 mL) followed by extraction and filtration of the remainding residue with hot toluene (not boiling) (ca 5 mL). Upon standing at RT the filtrate starts depositing crystals. The volume was slowly reduced to ca. 5 mL and refrigerated at -45 ºC overnight to complete crystallisation. The crystals were isolated by filtration cold -45 ºC and washed with cold (-45 ºC) nC 5 and dried in vacuum to yield the title compound. A second crop (ca 10-20 mg) was isolated by refrigeration (-45 ºC) of the combined wash and mother-liquor but was not as pure as the first one. Crystals suitable for X-ray diffraction were grown by slow evaporation of a saturated solution of (9)

SQUID MAGNETOMETRY:
Magnetic measurements of polycrystalline (10') were carried out using a Quantum Design MPMS-5 SQUID magnetometer at 0.1 Tesla in the range 2 -300 K, and for (9) and (3) using a Quantum Design MPMS-XL SQUID magnetometer at 0.1 Tesla in the range 5 -300 K. The accurately weighed samples (20 -40 mg) were placed into a gelatine capsule and then loaded into a nonmagnetic plastic straw before being lowered into the cryostat. Values of the magnetic susceptibility were corrected for the underlying diamagnetic increment by using tabulated Pascal's constants, viii and the effect of the blank sample holders (gelatin capsule/straw).   Figure SI5: Temperature dependence of the solid state susceptibility χ m of (9) at 0.1 Tesla. Figure SI6: Temperature dependence of the solid state χ m T product of (2) at 0.1 Tesla. Figure SI7: Temperature dependence of the inverse susceptibility χ m -1 of (9) at 0.1 Tesla. Figure SI8: Temperature dependence of the solid state χ m T product of (10') at 0.1 Tesla.

Cyclic Voltametry:
Cyclic Voltametry studies were performed in an Ar glovebox using a BASi-Epsilon potentiostat under computer control. IR drop was compensated using the feedback method. CV experiments were performed using the three electrode method with glassy carbon disk (7.0 mm 2 ) as the working electrode, Pt wire as the counter electrode and Ag wire as the pseudoreference electrode. Sample solutions were prepared by dissolving the appropriate supporting electrolyte in 1mL of solvent followed by addition of the analyte to give a concentration of ca 5mM. The reported half potential are referenced to Fc 0/+ redox couple, which was measured by adding ferrocene (ca 1mg) to the sample solution. In the case of (10') no processes could be observed using THF and either [N( n Bu) 4 ][B(C 6 F 5 ) 4 ] or [N( n Bu) 4 ]PF 6 as the supporting electrolyte or by using Au or Pt as the working electrode. When CH 3 CN (dried over CaH 2 under an Ar, distilled and kept over activated 3 Å molecular sieves) employed as a solvent and [N( n Bu) 4 ]PF 6 as electrolyte we managed to observe current responses upon scanning voltages over the solvent window. It has to be noted though that (10') reacts slowly with CH 3 CN (t 1/2 of approximately 3 hours) and as a result the experiment must be performed as quickly as possible. The product of this reaction is currently under investigation. In the case of (1) we used a similar procedure for the reasons previously described and its voltammogram had the same features as the ones observed for complexes of the type [U{η 8 -C 8 H 6 (1,4-SiMe 3 ) 2 }(η 5 -Cp Me4R )THF] (R = Me, Et, i Pr, t Bu).

S10
As can be seen from Figure SI9 another reversible process at ca -1.8 V is present that disappears when a smaller window is scanned. We tentatively assign this process to an electrochemically generated species. More in depth analysis was not possible due to the instability of (10') in MeCN.   Figure SI12: Determination of peak potentials and currents for 3. S11 Figure SI13: Determination of peak potentials and currents for 9. Figure SI14: Determination of peak potentials and currents for 10.