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

Abstract

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; L 2 , R = CH3; L 3 , 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 (H3 L 2 I2) or bis­[2-(3-isopropylimidazolium)-4-methylphenyl]­amine diiodide (H3 L 3 I2) 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 (L 2 = [4-CH3-2-{CH3-[N­(CH)2CN]}­C6H3]2N, RE = Y (1), Eu (2), Er (3); L 3 = [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 H3 L 3 I2 with n-BuLi in THF followed by reactions with [(Me3Si)2N]3RE­(μ-Cl)­Li­(THF)3. Stepwise reactions of H3 L 2 I2 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 L 2 RE­[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 H3 L 3 I2 with [(Me3Si)2N]3Yb­(μ-Cl)­Li­(THF)3 in THF, however, afforded mixed complexes of the bis­(NHC)-based CNC-pincer ytterbium complex L 3 Yb­[N­(SiMe3)2]2 (6) and the unexpected bis­(NHC)-based CNC-pincer monoamido ytterbium iodide L 3 YbI­[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.

Challenges in Catalytic Hydrophosphination

“…()] . Ytterbium compounds have a rich history as hydrophosphination catalysts, and among this family are rare examples of heterocumulene and alkyne hydrophosphination catalysts . The breadth of substrates that have been addressed with lanthanide catalysts demonstrates their success as hydrophosphination catalysts, and the more recent reports of these transformations suggests that they have yet more to offer.…”

Recent Advances in Rare Earth Complexes Containing N-Heterocyclic Carbenes: Synthesis, Reactivity, and Applications in Polymerization

“…Shen, Ni, and their coworkers have designed and synthesized two chelating bis(phenolate)-NHC (BisP-NHC) ligands for Ln complexation [83][84][85]. The BisP-NHC proligands featuring a five-membered (denoted as BisP-NHC-5 hereafter) and a six-membered (denoted as BisP-NHC-6 hereafter) imidazolinium rings were prepared using the corresponding H (81)(82)(83)(84)(85) containing bridging chlorides and imidazolinium-bridged bis(phenolate) ligands ( Figure 16).…”

“…Similarly, the reaction of the BisP-NHC-6 proligand with KN(SiMe3)2 and [Ln{N(SiMe3)2}3] resulted in the formation of heterobimetallic Ln/K complexes Ln(BisP-NHC-6)K(THF) (Ln = Nd (79), Y (80)). In the absence of KN(SiMe3)2, the reactions of BisP-NHC-5 or BisP-NHC-6 proligands with [Ln{N(SiMe3)2}3] or [(Me3Si)2N]3Ln(μ-Cl)Li(THF)3 only gave rise to dinculear Ln complexes (81)(82)(83)(84)(85) containing bridging chlorides and imidazolinium-bridged bis(phenolate) ligands ( Figure 16).…”

Facile amidation of esters with aromatic amines promoted by lanthanide tris (amide) complexes

“…Lanthanide tris (amide) complexes Ln[N (SiMe 3 ) 2 ] 3 ( μ ‐Cl)Li (THF) 3 are easily available, and have been widely used as starting materials to prepare lanthanide derivatives . Besides, these simple lanthanide tris (amide) complexes are catalytic active for various organic transformations, such as hydroelementation, hydrophosphonylation, amidation of aldehydes with amines, addition of alkynes to nitriles, aza‐Henry reaction, guanylation of amines, cross‐diinsertion of C ≡ N/C ≡ C into an N‐H bond, cycloamidination of aminoalkenes with nitriles, C‐C bond cleavage reaction, addition of alcohols to carbodiimides, guanylation/cyclization of amino acid esters and carbodiimides, alkene hydroamination, tandem insertion of secondary amines with 2‐alkynylbenzonitriles, addition of thiols to isocyanates, the Tishchenko reaction, and dehydrocoupling of alanes with amines, etc.…”