Structural Variability of 4f and 5f Thiocyanate Complexes and Dissociation of Uranium(III)-Thiocyanate Bonds with Increased Ionicity

摘要

A series of complexes [Et4N] [Ln(NCS)(4)(H2O)(4)] (Ln = Pr, Tb, Dy, Ho, Yb) have been structurally characterized, all showing the same structure, namely a distorted square antiprismatic coordination geometry, and the Ln-O and Ln-N bond lengths following the expected lanthanide contraction. When the counterion is Cs+, a different structural motif is observed and the eight-coordinate complex Cs-5[Nd(NCS)(8)] isolated. The thorium compounds [Me4N](4)[Th(NCS)(7)(NO3)] and [Me4N](4)[Th(NCS)(6)(NO3)(2)] have been characterized, and high coordination numbers are also observed. Finally, attempts to synthesize a U(III) thiocyanate compound has been unsuccessful; from the reaction mixture, a heterocycle formed by condensation of five MeCN solvent molecules, possibly promoted by U(III), was isolated and structurally characterized. To rationalize the inability to isolate U(III) thiocyanate compounds, thin-layer cyclic voltammetry and IR spectroelectrochemistry have been utilized to explore the cathodic behavior of [Et4N](4)[U(NCS)(8)] and [Et4N][U(NCS)(5)(bipy)(2)] along with a related uranyl compound [Et4N](3)[UO2(NCS)(5)]. In all examples, the reduction triggers a rapid dissociation of [NCS](-) ions and decomposition. Interestingly, the oxidation chemistry of [Et4N](3)[UO2(NCS)(5)] in the presence of bipy gives the U(IV) compound [Et4N](4)[U(NCS)(8)], an unusual example of a ligand-based oxidation triggering a metal-based reduction. The experimental results have been augmented by a computational investigation, concluding that the U(III)-NCS bond is more ionic than the U(IV)-NCS bond.