The Nature of Bis(cyclopentadienyl)uranium Dichloride and Cyclopentadienyluranium Trichloride in Solution and in the Solid State

The composition of the compound “(C5H5)2UCl2”, prepared from uranium tetrachloride and thallium cyclopenta-dienide in DME (1, 2-dimethoxyethane), has been investigated in the solid state by infrared spectroscopy and X-ray powder diffraction, and in solution by electronic and ¹H NMR spectroscopy. All results indicate that (C5H5)2UCl2 is a mixture of the known compounds (C5H5)3UCl and (C5H5)UCl3(DME). The structure of this latter species is proposed to be mer pseudoocta-hedral on the basis of solution NMR studies (which also show it to be stereochemically dynamic) and by analogy to the related complex, (CH3C5H4)UCl3(THF)2, the solid-state molecular structure of which has been determined using single-crystal X-ray diffraction techniques. This compound crystallizes in the monoclinic space group P2₁/n with four molecules in a cell of dimensions a = 10.481 (2) Å, b = 12.981 (2) Å, c = 13.834 (2) Å, and β = 105.86 (1)°. Least-squares refinement of the 181 variables led to a value for the conventional R index (on F) of 0.058 for 3698 independent reflections having 2θMokα < 60.4° and 1 > 3σ(1). The coordination geometry about the uranium ion is approximately mer octahedral with the n5-CH3C5H4 and one coordinated THF occupying trans axial positions, and the other THF and three chlorine atoms occupying equatorial positions. The U-C(cyclopentadienyl), U-Cl, and U-O distances average 2.720 (14), 2.623 (3), and 2.450 (8) Å, respectively. The principal distortion of the coordination geometry from octahedral is a 0.47-Å displacement of the uranium atom out of the equatorial plane toward the cyclopentadienyl ring. It is found that the THF ligands of (CH3C5H4)UCl3(THF)2 are readily displaced by nitrogenous bases such as 2, 2'-bipyridyl and 1, 10-phenanthroline.