U(SMes*)<i>_n</i>, (<i>n</i> = 3, 4) and Ln(SMes*)₃ (Ln = La, Ce, Pr, Nd): Lanthanide(III)/Actinide(III) Differentiation in Agostic Interactions and an Unprecedented η³ Ligation Mode of the Arylthiolate Ligand, from X-ray Diffraction and DFT Analysis

Reaction of U(NEt2)4 with HS-2,4,6-tBu3C6H2 (HSMes*) gave U(SMes*)3(NEt2)(py) (1), whereas similar treatment of U[N(SiMe3)SiMe2CH2][N(SiMe3)2]2 afforded U(SMes*)[N(SiMe3)2]3 (2) and U(SMes*)3[N(SiMe3)2]. The first neutral homoleptic uranium(IV) thiolate to have been crystallographically characterized, U(SMes*)4 (4), was isolated from the reaction of U(BH4)4 and KSMes*. The first homoleptic thiolate complex of uranium(III), U(SMes*)3 (5), was synthesized by protonolysis of U[N(SiMe3)2]3 with HSMes* in cyclohexane. The crystal structure of 5 exhibits the novel η3 ligation mode for the arylthiolate ligand. Comparison of the crystal structure of 5 with those of the isomorphous lanthanide congeners Ln(SMes*)3 (Ln = La, Ce, Pr, and Nd) indicates that the U−S, U−Cipso, and U−Cortho bond lengths are shorter than the corresponding ones in the 4f-element analogues, when taking into account the variation in the ionic radii of the metals. The distance between the uranium and the carbon atoms involved in the U···H−C ε agostic interaction of each thiolate ligand is shorter, by ∼0.05 Å, than that expected from a purely ionic bonding model. The lanthanide(III)/actinide(III) differentiation was analyzed by density functional theory (DFT). The nature of the M−S bond is shown to be ionic strongly polarized at the sulfur for M = U and iono-covalent (i.e. strongly ionic with low orbital interaction), for M = Ln. The strength of the U···H−C ε agostic interaction is proposed to be controlled by the maximization of the interaction between U+ and S- under steric constraints. The η3 ligation mode of the arylthiolate ligand is also obtained from DFT.