Preparation, Solution Behavior, and Solid-State Structures of (1,3-R 2 C 5 H 3 ) 2 UX 2 , Where R Is CMe 3 or SiMe 3 and X Is a One-Electron Ligand

High-yield preparations of the uranium metallocenes [1,3-(Me 3 Si) 2 C 5 H 3 ] 2 UCl 2 (Cp‘ ‘ 2 UCl 2 ) and [1,3-(Me 3 C) 2 C 5 H 3 ] 2 UCl 2 (Cp ⧧ 2 UCl 2 ) have been developed from the reaction of UCl 4 and the corresponding magnesocenes Cp‘ ‘ 2 Mg and Cp ⧧ 2 Mg in diethyl ether. The chloride ligands can be exchanged with either Me 3 SiBr or Me 3 SiI to give the uranium metallocene bromides or iodides. The fluorides were prepared by the reaction of BF 3 ·OEt 2 with Cp‘ ‘ 2 U(NMe) 2, Cp ⧧ 2 U(OMe) 2, or Cp ⧧ 2 UMe 2 . The crystal structures of Cp‘ ‘ 2 UCl 2, Cp ⧧ 2 UCl 2, Cp‘ ‘ 2 UMe 2, Cp ⧧ 2 UF 2, and dimeric (Cp‘ ‘ 2 UF 2 ) 2 are reported. The idealized symmetry of the monomers Cp‘ ‘ 2 UX 2 and Cp ⧧ 2 UCl 2 is C 2 v when X is F, Cl, or Br and C 2 when X is I or Me; in the dimer, the idealized symmetry is C i . This preference is rationalized by intramolecular and intermolecular steric effects. The solution ring conformations and intramolecular exchange processes have been studied by variable-temperature 1 H NMR spectroscopy. In all cases, the low-temperature limiting spectra are consistent with the idealized symmetry observed in the solid state.