Selective complexation of uranium(iii) over lanthanide(iii) triflates by 2,2′:6′,2″-terpyridine. X-Ray crystal structures of [M(OTf)3(terpy)2] and [M(OTf)2(terpy)2(py)][OTf] (M = Nd, Ce, U) and of polynuclear µ-oxo uranium(iv) complexes resulting from hydrolysis

Abstract

Reactions of Ln(OTf)₃ (Ln = Ce, Nd) or [U(OTf)₃(dme)₂] (OTf = OSO₂CF₃, dme = dimethoxyethane) with 2 mol equivalents of 2,2′:6′,2″-terpyridine (terpy) in pyridine or acetonitrile led to the quantitative formation of the bis(terpy) complexes which crystallized as the discrete cation–anion pairs [M(OTf)₂(terpy)₂(py)][OTf]·0.5py from pyridine or neutral derivatives [M(OTf)₃(terpy)₂]·nMeCN from acetonitrile (M = Ce, Nd, U). The crystal structures of these complexes show the differences in the M–O bond lengths to follow the variation of the ionic radii of the metals, while the U–N(terpy) and U–N(py) bonds are shorter than those expected from a purely ionic bonding model. The better affinity of terpy for U^III over Ce^III and Nd^III was evidenced by the thermodynamic parameters (K, ΔH, ΔS) corresponding to the equilibrium between the bis- and tris(terpy) complexes in acetonitrile. Hydrolysis of the bis(terpy) compounds followed different courses; whereas the aquo compound [Ce(OTf)₂(terpy)₂(H₂O)][OTf] crystallized readily from pyridine, the uranium complexes [UX₂(terpy)₂(py)]X (X = I, OTf) were oxidized into the tri- and tetranuclear µ-oxo U(IV) compounds [{UI(terpy)₂(µ-O)}₂{UI₂(terpy)}]I₄·2MeCN·H₂O and [{U(OTf)(terpy)₂(µ-O)(µ-OTf)U(terpy)}₂(µ-OTf)₂(µ-O)][OTf]₄·py·MeCN. The crystal structures of these first examples of uranium(IV) compounds with terpy ligands show the almost linear arrangement of the metal atoms.