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

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, DeltaH, DeltaS) 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 mu-oxo U(IV) compounds [{UI(terpy)(2)(mu-O)}(2) {UI2(terpy)}]I-4.2MeCN.H2O and [{U(OTf)(terpy)(2)(mu-O)(mu-OTf)U(terpy)}(2)(mu-OTf)(2)(mu-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.