Open-Shell Early Lanthanide Terminal Imides

Group 3- and 4f-element organometallic chemistry and reactivity are decisively driven by the rare-earth-metal/lanthanide (Ln) ion size and associated electronegativity/ionicity/Lewis acidity criteria. For these reasons, the synthesis of terminal "unsupported" imides [Ln═NR] of the smaller, closed-shell Sc(III), Lu(III), Y(III), and increasingly covalent Ce(IV) has involved distinct reaction protocols while derivatives of the "early" large Ln(III) have remained elusive. Herein, we report such terminal imides of open-shell lanthanide cations Ce(III), Nd(III), and Sm(III) according to a new reaction protocol. Lewis-acid-stabilized methylidene complexes [TpLn(μ₃-CH₂){(μ₂-Me)MMe₂}₂] (Ln = Ce, Nd, Sm; M = Al, Ga) react with 2,6-diisopropylaniline (H₂NAr) via methane elimination. The formation of arylimide complexes is governed by the Ln(III) size, the Lewis acidity of the group 13 metal alkyl, steric factors, the presence of a donor solvent, and the sterics and acidity (pK_a) of the aromatic amine. Crucially, terminal arylimides [TpLn(═NAr)(THF)₂] (Ln = Ce, Nd, Sm) are formed only for M = Ga, and for M = Al, the Lewis-acid-stabilized imides [TpLn(NAr)(AlMe₃)] (Ln = Ce, Nd, Sm) are persistent. In stark contrast, the [GaMe₃]-stabilized imide [TpLn(NAr)(GaMe₃)] (Ln = Nd, Sm) is reversibly formed in noncoordinating solvents.