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

Abstract

N-heterocyclic carbenes (NHCs) are ubiquitous ancillary ligands employed in metal-catalyzed homogeneous reactions and polymerization reactions. Of significance is the use of NHCs as the supporting ligand in second- and third-generation Grubbs catalysts for their application in olefin metathesis and ring-opening metathesis polymerization. While the applications of transition metal catalysts ligated with NHCs in polymerization chemistry are well-documented, the use of analogous rare earth (Ln = Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) catalysts in this area remains under-developed, despite the unique role of rare earth elements in regio- and stereo-specific (co)polymerization reactions. By using hetero-atom-tethered chelating NHCs and, more recently, the employment of other structurally related NHCs, NHC-ligated Ln complexes have proven to be promising and fruitful catalysts for selective polymerization reactions. This review summarizes the recent developments in the coordination chemistry of Ln complexes containing NHCs and their catalytic performance in polymerization.

N-Heterocyclic Carbene-Coordinated M(II) (M = Yb, Sm, Ca) Bisamides: Expanding the Limits of Intermolecular Alkene Hydrophosphination

“…The first and only example of late transition-metal-mediated addition of PH 3 to activated substrate molecules (acrylonitrile, ethyl acrylate, and formaldehyde) has been reported by Pringle et al Homo- and heteroleptic Ca­(II) and Ln­(II) amido compounds, which are suitable catalyst precursors for hydrophosphination of styrene with Ph 2 PH and PhPH 2 , were totally inert when PH 3 was used. ,, Only the amido compound {LO NO4 }­Yb­{N­(SiMe 3 ) 2 } coordinated by a phenolate ligand with pendant aza-crown macrocycle allowed for the preparation of phenethylphosphines starting from PH 3 and styrenes with excellent anti-Markovnikov regioselectivity, high conversions, and reaction rates . Surprisingly, regardless of the impressive progress achieved in the catalysis of various chemical transformations due to the application of transition-metal compounds with N -heterocyclic carbene ligands (NHCs), the catalytic applications of rare-earth metal NHC complexes still are poorly explored. , Recently, we reported that amido complexes bearing NHC ligands (NHC) n M­[N­(SiMe 3 ) 2 ] 2 [M = Yb­(II), Sm­(II), Ca­(II)] provide a unique opportunity to combine high catalytic activity and perfect chemoselectivity in the addition of PH 3 to styrene and phenylacetylene . Factors such as electron-donating ability, denticity of the electron pair, donor ligand attached to the metal ion, and its ability to increase the stability and solubility of true catalytic species are known to have a crucial impact on the catalytic performance of compounds in intermolecular alkene hydrophosphination. , Herein, we report on the preparation and characterization of a large series of NHC metal complexes (NHC) n M­[N­(SiMe 3 ) 2 ] 2 (M = Yb, Sm, Ca; n = 1, 2), their catalytic activity, regioselectivity, and chemoselectivity in intermolecular hydrophosphination of a range of alkene and alkyne substrates with primary and secondary organophosphines, as well as with PH 3 .…”

Conversion of CO2 into Cyclic Carbonates under Ambient Conditions Catalyzed by Rare-Earth Metal Complexes Bearing Poly(phenolato) Ligand

“…Rare-earth metals (RE) find wide applications in various fields due to their distinctive physical and chemical properties. RE-containing compounds have been used as optical, electrical, and magnetic materials. , They have also been developed as catalysts, possessing advantages of good activity and selectivity . Rare-earth metals are highly oxyphilic and Lewis acidic, which makes their complexes potentially useful for the cycloaddition of CO 2 and epoxides. , To date, cyclic carbonate synthesis catalyzed by a RE-based complex still requires high temperature and pressure, which include SmOCl (supercritical CO 2 , 200 °C), Sc­(OTf) 2 (20 bar CO 2 , 120 °C), and polynuclear lanthanide complexes (10–20 bar CO 2 , 120 °C), ,, Our group has been devoted to exploiting the application of RE complexes in CO 2 transformations. ,,,,− We reported a RE complex bearing ethylenediamine-bridged poly­(phenolato) ligands, which is the first RE complex that catalyzed the cycloaddition reaction under mild conditions (85 °C and 1 bar CO 2 for monosubstituted epoxide or 10 bar CO 2 for disubstituted epoxide) .…”

“…68,69 They have also been developed as catalysts, 70 possessing advantages of good activity and selectivity. 71 Rare-earth metals are highly oxyphilic and Lewis acidic, which makes their complexes potentially useful for the cycloaddition of CO 2 and epoxides. 72,73 To date, cyclic carbonate synthesis catalyzed by a RE-based complex still requires high temperature and pressure, which include SmOCl (supercritical CO 2 , 200 °C), 47 Sc(OTf) 2 (20 bar CO 2 , 120 °C), 48 and polynuclear lanthanide complexes (10−20 bar CO 2 , 120 °C), 51,59,60 Our group has been devoted to exploiting the application of RE complexes in CO 2 transformations.…”

Electronic Effect of the [Au(PPh3)]+ Fragment in the Stabilization of Imidazolium Salts and the Destabilization of NHCs

“…[1][2][3][4] The increasing tendency of the use of such ligands in many important applications is mainly due to their great versatility and stability towards metal and non-metal coordination compounds. [5][6][7] In addition to the stability resulting from the electronic delocalization in the NCHN moiety, the electron pair is stabilized by the relatively easy electronic modulation and the steric effect induced by the functional groups anchored in the nitrogen atoms. [5] The design of an appropriate imidazolium salt used as a precursor for the synthesis of carbene ligands takes a crucial role.…”