Yttrium and ytterbium(III) complexes with ansa-linked bis(amidinate) ligand containing conformationally rigid o-phenylene bridge

Abstract

The new tert butoxide yttrium complex [C 6 H 4 1,2 {NC(Bu t )N(2,6 Me 2 C 6 H 3 )} 2 ]YOBu t (DME) containing the dianionic ansa linked bis(amidinate) ligand with the rigid o phenylene linker was synthesized by the metathesis of the chloride complex [C 6 H 4 1,2 {NC(Bu t )N(2,6 Me 2 C 6 H 3 )} 2 ]Y(THF)( 2 Cl) 2 Li(THF) 2 with potassium tert butoxide and structurally charac terized. The synthesized yttrium bis(amidinate) tert butoxide complex exhibits a moderate catalytic activity in the polymerization of rac lactide under mild conditions but makes it possi ble to carry out the process with high control and obtain atactic polylactide with a narrow molecular weight distribution (M n /M w = 1.55) and good correspondence of the calculated and experimentally determined M n . It was found by X ray diffraction analysis that the substitution in the coordination sphere of the ytterbium atom in complex [C 6 H 4 1,2 {NC(Bu t )N (2,6 Me 2 C 6 H 3 )} 2 ]YbCl(DME) of a DME molecule for two THF molecules resulted in the migration of the chloride ligand from the apical to equatorial position.Organic derivatives of rare earth metals possess a unique complex of properties and are of doubtless inter est for catalysis. 1,2 High values of ionic radii of these ele ments 3 in combination with Lewis acidity and the pres ence of unoccupied 5d and 6s orbitals (for Ln 3+ ions) provide their compounds with a pronounced tendency to complex formation and, correspondingly, high coordina tion numbers. The redox 4 and chemical properties of rare earth elements are similar, while the ionic radii in their series change substantially (from 0.885 Å for Sc to 1.172 Å for La), 3 which gives a unique possibility to optimize the reactivity of the metal complex by both construction of the coordination sphere of the metal and choosing the radius of the metal cation according to the specifics of the catalyzed reaction. Since rare earth metals have long radii and the metal-ligand bond is significantly ionic, the sta bility and reactivity of their organic derivatives are deter mined, to a great extent, by the degree of steric and coor dination saturation of their sphere, which the problem of design of new ligand systems especially important. At the same time, the design of the coordination sphere of rare earth metal ions is of doubtless interest from the viewpoint of studying the influence of the coordination environment of the metal ion on the reactivity of the complexes, in cluding their catalytic activity.We have earlier synthesized the new dianionic bis (amidinate) ligand in which two amidinate groups are linked by the conformationally rigid o phenylene linker. 5 It was demonstrated that the use of the new ligand system makes it possible to obtain only thermally stable bis (amidinate) alkyl derivatives of rare earth metals (REM). 5 An interesting feature of the new ligand system is the di versity of variants of its coordination with metal ions de termined by both the volume of substituents at the nitro gen atoms of the amidinate groups and the ionic ra...