Dialkylscandium Complexes Supported by β-Diketiminato Ligands:  Synthesis, Characterization, and Thermal Stability of a New Family of Organoscandium Complexes

Several diorganoscandium complexes stabilized by the β-diketiminato ligands (Ar)NC(R)CHC(R)N(Ar) (Ar = 2,6-iPr-C₆H₃; R = CH₃ (ligand a), R = tBu (ligand b)) have been synthesized. Reaction of the lithium salts of the ligands with ScCl₃·3THF leads to the complexes LScCl₂(THF)_n, which may be readily alkylated to form the dialkyl derivatives. Most are isolated as base-free, four-coordinate complexes. Several have been characterized via X-ray crystallography, and a detailed discussion of their structures is presented. Steric interactions between Ar and the Sc−alkyl groups force the scandium to adopt an out-of-plane bonding mode. In solution, this is manifested via a fluxional process which equilibrates the two diastereotopic alkyl groups and ligand groups as well. The barriers to this process roughly correlate with the steric bulk of the alkyl substituents. At elevated temperatures, the dialkyl derivatives LScR₂ undergo a metalation process whereby one of the alkyl groups is eliminated as RH, and a ligand iPr group is metalated in the methyl position. These reactions are first order in scandium complex, and activation parameters of ΔH^⧧ = 19.7(6) kcal mol^-1 and ΔS^⧧ = −17(2) cal mol^-1 K^-1 were measured for the loss of Me₄Si from (Ligb)Sc(CH₂SiMe₃)₂.