Lanthanide(III)/Actinide(III) Differentiation in Mixed Cyclopentadienyl/Dithiolene Compounds from X-ray Diffraction and Density Functional Theory Analysis

Treatment of [U(Cp*)2Cl2] with Na2dddt in thf afforded the “ate” complex [U(Cp*)2Cl(dddt)Na(thf)2] (1), and the salt-free compound [U(Cp*)2(dddt)] (2) could be extracted from
1 with toluene (Cp* = η-C5Me5; dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate). Reduction of
2 with Na(Hg) or addition of Na2dddt to [U(Cp*)2Cl2Na(thf)x] in the presence of 18-crown-6
gave the first uranium(III) dithiolene compound, [Na(18-crown-6)(thf)2][U(Cp*)2(dddt)] (4).
The dimeric lanthanide complexes [{Ln(Cp*)2(dddt)K(thf)2}2] (Ln = Ce (5), Nd (6)) were
prepared by reaction of [Ln(Cp*)2Cl2K] with K2dddt, and in the presence of 15-crown-5, they
were transformed into the cation−anion pairs [K(15-crown-5)2][Ln(Cp*)2(dddt)] (Ln = Ce
(7), Nd (8)). The crystal structures of 2, 4·thf, 5−7, 7·0.5(pentane), and 8·0.5(pentane) were
determined by X-ray diffraction analysis. Comparison of the structural parameters of the
anions [M(Cp*)2(dddt)]- (M = U, Ce, Nd) revealed that the U−S and U−C(Cp*) distances
are shorter than those expected from a purely ionic bonding model; the relatively small folding
of the dddt ligand suggests that the interaction between the CC double bond and the metal
center is weak, in agreement with the NMR observations in solution. The structural data
obtained from molecular geometry optimizations on the complexes [M(Cp*)2(dddt)]-,0
(M = Ce, U) using relativistic density functional theory (DFT) calculations reproduce
experimental trends. A detailed orbital analysis shows that the contraction of the metal−sulfur bond lengths when passing from [Ce(Cp*)2(dddt)]- to [U(Cp*)2(dddt)]- is partly related
to the uranium 5f orbital−ligand mixing, which is greater than the cerium 4f orbital−ligand
mixing. The comparison of the two [U(Cp*)2(dddt)]-,0 species reveals a higher ligand-to-metal donation in the case of the U(IV) complex.