Syntheses, structures, and solid-state carbon-13 NMR of two .eta.6-arene uranium(IV) complexes, [U(C6Me6)Cl2(.mu.-Cl)3UCl2(C6Me6)]AlCl4 and (C6Me6)UCl2(.mu.-Cl)3UCl2(.mu.-Cl)3UCl2(C6Me6)