Rare-earth metal bis(tetramethylaluminate) complexes supported by a sterically crowded triazenido ligand

Complexes [NNN]Ln(AlMe4)2 (Ln = Y, La, Nd, Lu) bearing the sterically demanding aryl-substituted triazenido ligand [(Tph)2N3] (Tph = [2-(2,4,6-iPr3C6H2)C6H4]) can be obtained from homoleptic complexes Ln(AlMe4)3 in moderate yields, both via protonolysis with [(Tph)2N3]H and a salt metathesis reaction pathway utilizing [(Tph)2N3]K. In the solid state the Y and Lu derivatives are isostructural, with both tetramethylaluminate groups coordinated in an η2 fashion, while one of the [AlMe4] ligands of the Nd derivative features a distorted η2 coordination mode. Due to the high affinity of the triazenido ligand toward the more Lewis-acidic and harder aluminium cation compared to the softer rare-earth metal centres, ligand redistribution is observed in solution and formation of byproduct [(Tph)2N3]AlMe2 is prominent. While the monoanionic triazenido ligand coordinates the rare-earth metal centres in an asymmetrical syn/anti fashion, it adopts an almost symmetric syn/syn configuration in the aluminium complex. Attempts were also made to produce putative dimethyl complexes {[(Tph)2N3]LnMe2} (Ln = Y, Lu) via cleavage of the aluminate moieties with diethyl ether. Furthermore, the intrinsic redistribution reactions are proposed to affect the performance of complexes [(Tph)2N3]Ln(AlMe4)2 in isoprene polymerization.