Tris(benzhydryl) and Cationic Bis(benzhydryl) Ln(III) Complexes: Exceptional Thermostability and Catalytic Activity in Olefin Hydroarylation and Hydrobenzylation with Substituted Pyridines

A series of Ln(III) tris(benzhydryl) complexes [(p-tBu-C6H4)(2)CH](3)Ln (Ln=La (1), Nd (2), Y (3)) were synthesized by the salt metathesis reactions of LnHal(3)(THF)(3.5)(Ln=La, Nd, Hal=Cl; Ln=Y, Hal=I) and [(p-tBu-C6H4)(2)CH]Na. In1-3the benzhydryl ligands are linked with the metal centres in eta(4)-coordination mode. For diamagnetic complexes1and3a fluxional behaviour was detected in solution. Complexes1-3proved to be thermally stable: no decomposition was observed even after heating their solutions in toluene-d(8)at 140 degrees C during 72 h. The reactions of1and2with B(C6F5)(3)allowed for the synthesis of base-free cationic complexes [(p-tBu-C6H4)(2)CH](2)Ln[(p-tBu-C6H4)(2)CHB(C6F5)(3)] (Ln=La (4), Nd (5)) which adopt the structure of a contact ion pair. Combinations of1-3and borane ((B(C6F5)(3), [Me2NHPh][B(C6F5)(4)], [Ph3C][B(C6F5)(4)]) as well as4and5were found to be highly efficient, regio- and chemoselective catalysts for hydroarylation and hydrobenzylation of C=C bonds of a variety of substrates with substituted pyridines. These catalysts enable highly challenging transformations such as hydrobenzylation of 1,1-disubstituted and internal C=C bonds.