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 [LnNR] 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 [Tp
tBu,MeLn(μ3-CH2){(μ2-Me)MMe2}2] (Ln = Ce,
Nd, Sm; M = Al, Ga) react with 2,6-diisopropylaniline
(H2NAr
iPr) 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 [Tp
tBu,MeLn(NAr
iPr)(THF)2] (Ln = Ce, Nd, Sm) are
formed only for M = Ga, and for M = Al, the Lewis-acid-stabilized
imides [Tp
tBu,MeLn(NAr
iPr)(AlMe3)] (Ln = Ce, Nd, Sm) are persistent. In
stark contrast, the [GaMe3]-stabilized imide [Tp
tBu,MeLn(NAr
iPr)(GaMe3)] (Ln = Nd, Sm) is reversibly formed in noncoordinating solvents.
