The Role of Neutral Coligands on the Stabilization of Mono-Tp^{^<i>i</i>Pr}_² U(III) Complexes

The reaction of [UI3(THF)4] with 1 equiv of KTpiPr2 in toluene in the presence of several neutral coligands allowed the synthesis of a novel family of mono-TpiPr2 complexes, [UI2TpiPr2(L)x] [L = OPPh3, x = 1 (3); L = C5H5N, x = 2 (4); L = HpztBu,Me, x = 2 (5); and L = bipy, x = 1 (6)]. The adduct with THF, [UI2TpiPr2(THF)2-3] (1), could also be isolated by reacting [UI3(THF)4] with 1 equiv of KTpiPr2 in tetrahydrofuran. However, complex 1 is not a good starting material to enter into the mono-TpiPr2 U(III) complexes as it decomposes in solution, leading to mixtures of U(III) species coordinated with HpziPr2. The solid-state structures of 3, 4, and 6 were determined by single-crystal X-ray diffraction and revealed that this family of mono-TpiPr2 complexes can be six- (3) or seven-coordinated (4 and 6), depending on the nature of the neutral coligand. Complex 3 displays distorted octahedral coordination geometry, while 4 and 6 display distorted pentagonal bipyramid and capped octahedral geometries, respectively. Complexes 3 and 6 are static in solution, and the patterns of the 1H NMR spectra are consistent with the Cs symmetry found in the solid state. The other complexes (1, 4, and 5) are fluxional, but the dynamic processes involved can be slowed by decreasing the temperature.