Synthesis of Bis(NHC)-Based CNC-Pincer Rare-Earth-Metal Amido Complexes and Their Application for the Hydrophosphination of Heterocumulenes

The
bis­(NHC) (NHC = N-heterocyclic carbene)-based CNC-pincer rare-earth-metal
amido complexes LRE­[N­(SiMe3)2]2 (L = 4-CH3-2-{R-[N­(CH)2CN]}­C6H3]2N; L2, R = CH3; L3, R = CH­(CH3)2) were synthesized
and characterized, and their catalytic activities toward hydrophosphination
of heterocumulenes were developed. Reactions of bis­[2-(3-methylimidazolium)-4-methylphenyl]­amine
diiodide (H3L2I2) or bis­[2-(3-isopropylimidazolium)-4-methylphenyl]­amine diiodide
(H3L3I2) with 5 equiv of NaN­(SiMe3)2 followed by treatment
with 1 equiv of RECl3 in THF at −78 °C afforded
the bis­(NHC)-based CNC-pincer rare-earth-metal amido complexes LRE­[N­(SiMe3)2]2 (L2 = [4-CH3-2-{CH3-[N­(CH)2CN]}­C6H3]2N, RE = Y (1), Eu (2), Er (3); L3 = [4-CH3-2-{(CH3)2CH-[N­(CH)2CN]}­C6H3]2N, RE = Y (4), Er (5), Yb (6)). Complexes 4–6 can also be prepared
by stepwise reactions of H3L3I2 with n-BuLi in THF followed
by reactions with [(Me3Si)2N]3RE­(μ-Cl)­Li­(THF)3. Stepwise reactions of H3L2I2 with n-BuLi in THF followed
by treatment with [(Me3Si)2N]3RE­(μ-Cl)­Li­(THF)3 generated the bis­(NHC)-based CNC-pincer rare-earth-metal
amido complexes L2RE­[N­(SiMe3)2]2 (RE = Y (1), Er (3)) together with the fused-heterocyclic compound 3,8,9-trimethyl-8a,9-dihydro-8H-benzo­[4,5]­imidazo­[2′,1′:2,3]­imidazo­[1,2-a]­imidazo­[2,1-c]­quinoxaline (7), which formed through carbene C–C and C–N coupling.
Attempts to prepare complexes of the type LRE­[N­(SiMe3)2]2 by reaction of H3L3I2 with [(Me3Si)2N]3Yb­(μ-Cl)­Li­(THF)3 in
THF, however, afforded mixed complexes of the bis­(NHC)-based CNC-pincer
ytterbium complex L3Yb­[N­(SiMe3)2]2 (6) and the unexpected
bis­(NHC)-based CNC-pincer monoamido ytterbium iodide L3YbI­[N­(SiMe3)2] (8). Investigation of the catalytic activity of complexes 1–6 and 8 indicated that
all complexes displayed high activity toward the addition of the phosphine
P–H bond to heterocumulenes, producing the corresponding phosphaguanidines,
phosphaureas, and phosphathioureas, which represents the first example
of bis­(NHC)-based CNC-pincer type rare-earth-metal amido complexes
as catalysts for the catalytic addition of the phosphine P–H
bond to heterocumulenes with high efficiency in the presence of a
low catalyst loading at room temperature.