Divalent lanthanide complex

Recent developments in the chemistry of divalent lanthanide complexes of simple cyclopentadienyl ligands have involved new syntheses and structural characterization. As described in Section VII, by using the 5Mes ligand, advances in reactivity as well as synthesis and structure have been made. 

Transmetallation is a more convenient method to obtaining divalent organolanthanide complexes from lanthanide metals. Reaction of lanthanide metal powder with a mercury alkyl or aryl complex affords the corresponding divalent lanthanide complex (Equation 8.28) [94]. The preparation of divalent perfluorophenyl lanthanide complexes Ln(C6F5)2(THF) (Ln = Eu, n = 5 Ln = Yb, n=4) is a typical example. In most cases, the addition of a small amount of Lnl3 leads to acceleration of the reaction [95]. 

An alternative route to divalent lanthanide complexes is a metallation reaction the acid-base reaction between a divalent organolanthanide complex with an acidic substrate... 

The divalent lanthanide monohalides, which are suitable precursors for mixed-ligand divalent lanthanide complexes, can also be synthesized via metathesis reaction. For example, the divalent samarium and ytterbium monohalides stabilized by a triazacyclononane-functionalized tetramethylcyclopentadienyl group have been prepared (Equation 8.31) [97]. 

The non-classical divalent lanthanide complexes have stronger reducing power than divalent samarium complexes because of their higher reduction potentials. Dinitrogen is not an inert atmosphere for these non-classical divalent lanthanide complexes. Therefore, attempts to prepare non-classical divalent organolanthanide complexes by metathesis reactions in dinitrogen atmosphere have been unsuccessful, and the dinitrogen-activated products were isolated. A typical example is shown in Equation 8.36 [112]. 

Divalent organolanthanide complexes can also initiate MMA polymerization. A divalent lanthanide complex, as a single-electron transfer reagent, can readily react with the monomer to generate a radical anion species, which subsequently couples into a bimetallic trivalent lanthanide enolate intermediate, which is the active center. Therefore, divalent organolanthanide complexes serve as bisinitiators for MMA polymerization [160]. 

The synthesis and characterization of divalent lanthanide complexes of a triazacyclononane-functionalized tetramethylcyclopentadienyl ligand have been reported.200 201 Addition of LnI2(THF)2 (Ln = Sm, Yb) to K[CsMe4SiMe2(Pr 2-tacn)] (Pr 2-tacn = l,4-diisopropyl-l,4,7-triazacyclononane) in THF yielded the monomeric organolanthanide [CsMe4SiMe2(Pr 2-tacn)]SmI (dark red crystals, 85% yield) and [C5Me4SiMe2(Pr 2-tacn)]YbI (red blocks, 80% yield) (Scheme 40). The crystal structures of both compounds have been explored.200... 

Reduction of dinitrogen molecule by divalent lanthanide complexes has been a popular subject in coordination and organometallic chemistry of the lanthanide elements. In the course of these studies, a large number of polynuclear lanthanide clusters featuring a core of lanthanide atoms organized around a dinitrogen unit have been obtained and structurally characterized. 

A U(III) pentalene complex has been shown to be capable of binding and reducing N2, in a similar manner to the divalent lanthanide complexes reported by Evans. However, the reactivity of the COT complexes towards CO is completely novel for organometalUc compounds of any type, and shows considerable promise for the future design of a system capable of producing oxocarbon products from CO or CO2 catalytically. 

The first definitive descriptions of organometallic divalent lanthanide complexes (Evans et al., 1970,1971) also alluded to applications similar to Grignard methodology [eq. (69)]. 

Divalent lanthanide complexes with coronands and one podand. ... 

Theoretical approaches have been extensively used to predict the nature of the ground state of several Yb(II) complexes, using the multireference CASSCF method. However, this computational approach is hardly tractable for bimetallic complexes and reactivity studies involving divalent lanthanide complexes. There was thus a need for a simplified approach but without, if possible, too much loss of precision. 

Non-classical divalent lanthanide complexes, F. Nief, Dalton Transactions 2010, 39, 6589. 

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