Electrochemical studies of tris(cyclopentadienyl)thorium and uranium complexes in the +2, +3, and +4 oxidation states†

Electrochemical measurements on tris(cyclopentadienyl)thorium and uranium compounds in the +2, +3, and +4 oxidation states are reported with C5H3(SiMe3)2, C5H4SiMe3, and C5Me4H ligands. The reduction potentials for both U and Th complexes trend with the electron donating abilities of the cyclopentadienyl ligand. Thorium complexes have more negative An(iii)/An(ii) reduction potentials than the uranium analogs. Electrochemical measurements of isolated Th(ii) complexes indicated that the Th(iii)/Th(ii) couple was surprisingly similar to the Th(iv)/Th(iii) couple in Cp′′-ligated complexes. This suggested that Th(ii) complexes could be prepared from Th(iv) precursors and this was demonstrated synthetically by isolation of directly from UV-visible spectroelectrochemical measurements and reactions of with elemental barium indicated that the thorium system undergoes sequential one electron transformations.


References and Definitions S84
S4

Experimental Details
Caution! 232 Th and 238 U are α emitters with half-lives of approximately 1.41×10 10 and 4.47 x 10 9 years, respectively. Samples should be prepared and handled only in laboratories appropriately equipped to handle radioactive materials.
All syntheses and manipulations were conducted under an Ar atmosphere with rigorous exclusion of air and water using standard glovebox and vacuum line techniques. Solvents were sparged with UHP argon and dried by passage through columns containing Q-5 and molecular sieves prior to use. Deuterated NMR solvents were dried over NaK alloy, degassed by three freezepump-thaw cycles, and vacuum transferred prior to use. NMR spectra were recorded on an AVANCE600 MHz spectrometer at 298 K and referenced to residual proteo-solvent resonances.
Electrochemical measurements were collected with a freshly made THF solution of supporting electrolyte with a glassy carbon working electrode, platinum wire counter electrode, and silver wire pseudo-reference electrode with a Princeton Applied Research PARSTAT 2273 Advanced Electrochemical System and referenced with internal standard (C5Me5)2Fe. Internal resistance was S5 measured for each solution and resistance was manually compensated by approximately 90% of the measured value. All scans were measured in the cathodic direction except for the isolated U (II) and Th(II) complexes and KC5R5 compounds which were measured in the anodic direction. UVvisible spectroelectrochemical measurements were made using a Pine Instruments UV-visible kit with a Pt working and counter electrode and Ag wire pseudo-reference and an Agilent Cary 60 UV-visible spectrophotometer fitted with an Agilent fiber optic coupler connected to an Ocean Optics CUV 1 cm cuvette holder inside the glovebox. UV-visible measurements were made using an Agilent Cary 60 spectrophotometer in THF in a 1 mm cuvette.                                 Table S2 and Figure S56. The irreversibility of these events is consistent with a chemical process occurring after oxidation such as dimerization of the in-situ generated radical. 18 This series of reduction potentials for simple potassium cyclopentadienyl salts does not match the trend observed in the thorium complexes above and in related zirconium systems. 19 However, it was noted that trends in cyclopentadienyl donor strength are system dependent. Addition of one equivalent of crown to KCp″ shifts the event slightly negative, Figure S57,

X-ray Data Collection, Structure Solution and Refinement for [K(crown)][Cp″].
A blue crystal of approximate dimensions 0.129 x 0.144 x 0.191 mm was mounted in a cryoloop and transferred to a Bruker SMART APEX II diffractometer. The APEX2 32 program package was used to determine the unit-cell parameters and for data collection (180 sec/frame scan time for a hemisphere of diffraction data). The raw frame data was processed using SAINT 33 and SADABS 34 to yield the reflection data file. Subsequent calculations were carried out using the SHELXTL 35 program. The diffraction symmetry was 2/m and the systematic absences were consistent with the monoclinic space group P21/c that was later determined to be correct.
The structure was solved by direct methods and refined on F 2 by full-matrix least-squares techniques. The analytical scattering factors 36 for neutral atoms were used throughout the analysis.
Hydrogen atoms were included using a riding model. There were two molecules of the formulaunit present (Z = 8). C(41) and C(42) were disordered and included using multiple components with partial site-occupancy factors.
Hydrogen atoms have been removed for clarity.

X-ray Data Collection, Structure Solution and Refinement for [Na(crown)2][Cp″3Th].
A red crystal of approximate dimensions 0.153 x 0.258 x 0.289 mm was mounted in a cryoloop and transferred to a Bruker SMART APEX II diffractometer. The APEX2 32 program package was used to determine the unit-cell parameters and for data collection (30 sec/frame scan time). The raw frame data was processed using SAINT 33 and SADABS 34 to yield the reflection data file. Subsequent calculations were carried out using the SHELXTL 35 program package. There were no systematic absences nor any diffraction symmetry other than the Friedel condition. The centrosymmetric triclinic space group P1 was assigned and later determined to be correct.
The structure was solved by direct methods and refined on F 2 by full-matrix least-squares techniques. The analytical scattering factors 36 for neutral atoms were used throughout the analysis.
The structure was solved by direct methods and refined on F 2 by full-matrix least-squares techniques. The analytical scattering factors 36 for neutral atoms were used throughout the analysis.
Hydrogen atoms were included using a riding model.

X-ray Data Collection, Structure Solution and Refinement for [Cs(crypt)][Cp″3Th].
A blue crystal of approximate dimensions 0.082 x 0.110 x 0.182 mm was mounted in a cryoloop and transferred to a Bruker SMART APEX II diffractometer. The APEX2 32 program package was used to determine the unit-cell parameters and for data collection (120 sec/frame scan time). The raw frame data was processed using SAINT 33 and SADABS 34 to yield the reflection S78 data file. Subsequent calculations were carried out using the SHELXTL 35 program package. There were no systematic absences nor any diffraction symmetry other than the Friedel condition. The centrosymmetric triclinic space group P1 was assigned and later determined to be correct.
The structure was solved by direct methods and refined on F 2 by full-matrix least-squares techniques. The analytical scattering factors 36 for neutral atoms were used throughout the analysis.
Hydrogen atoms were included using a riding model.