Monomeric Tetraalkylaluminates of Divalent Ytterbium Stabilized by a Bulky Tris(pyrazolyl)borate Ligand

Mononuclear heterobimetallic complexes (Tp(tBu,Me))Yb(AlR4) (R = Me, Et) were obtained in high yields according to a silylamide elimination protocol employing (Tp(tBu,Me))Yb[N(SiMe3)(2)] and AlR3. The solid-state structures revealed kappa(3)-coordinated Tp(tBu,Me) ligands and eta(2)-coordinated [AlR4] moieties, with additional Yb center dot center dot center dot H agostic interactions from the bridging methyl and methylene groups of the [AlR4] ligands. These agostic interactions however are not strong enough to maintain a rigid structure in solution. H-1 NMR spectroscopy showed a highly fluxional ligand environment with only one set of signals for both Tp(tBu,Me) and [AlR4] ligands at temperatures down to -80 degrees C.