Divalent metallocenes of the lanthanides - a guideline to properties and reactivity

Since the discovery in the early 1980s, the soluble divalent metallocenes of lanthanides have become a steadily growing field in organometallic chemistry. The predominant part of the investigation has been performed with samarium, europium, and ytterbium, whereas only a few reports dealing with other rare earth elements were disclosed. Reactions of these metallocenes can be divided into two major categories: (1) formation of Lewis acid-base complexes, in which the oxidation state remains +II; and (2) single electron transfer (SET) reductions with the ultimate formation of Ln(III) complexes. Due to the increasing reducing character from Eu(II) over Yb(II) to Sm(II), the plethora of literature concerning redox reactions revolves around the metallocenes of Sm and Yb. In addition, a few reactivity studies on Nd(II), Dy(II) and mainly Tm(II) metallocenes were published. These compounds are even stronger reducing agents but significantly more difficult to handle. In most cases, the metals are ligated by the versatile pentamethylcyclopentadienyl ligand: (C₅Me₅). Other cyclopentadienyl ligands are fully covered but only discussed in detail, if the ligand causes differences in synthesis or reactivity. Thus, the focus lays on three compounds: [(C₅Me₅)₂Sm], [(C₅Me₅)₂Eu] and [(C₅Me₅)₂Yb] and their solvates. We discuss the synthesis and physical properties of divalent lanthanide metallocenes first, followed by an overview of the reactivity rendering the full potential of these versatile reactants.

Isolation of C1 through C4 derivatives from CO using heteroleptic uranium(iii) metallocene aryloxide complexes

The conversion of C1 feedstock molecules such as CO into commodity chemicals is a desirable, but challenging, endeavour. When the U(iii) complex, [(C₅Me₅)₂U(O-2,6- ^t Bu₂-4-MeC₆H₂)], is exposed to 1 atm of CO, only coordination is observed by IR spectroscopy as well as X-ray crystallography, unveiling a rare structurally characterized f element carbonyl. However, using [(C₅Me₅)₂(MesO)U (THF)], Mes = 2,4,6-Me₃C₆H₂, reaction with CO forms the bridging ethynediolate species, [{(C₅Me₅)₂(MesO)U}₂(μ₂-OCCO)]. While ethynediolate complexes are known, their reactivity has not been reported in much detail to afford further functionalization. For example, addition of more CO to the ethynediolate complex with heating forms a ketene carboxylate, [{(C₅Me₅)₂(MesO)U}₂(μ ₂:κ ²:η ¹-C₃O₃)], which can be further reacted with CO₂ to yield a ketene dicarboxylate complex, [{(C₅Me₅)₂(MesO)U}₂(μ ₂:κ ²:κ ²-C₄O₅)]. Since the ethynediolate showed reactivity with more CO, we explored its reactivity further. A [2 + 2] cycloaddition is observed with diphenylketene to yield [{(C₅Me₅)₂U}₂(OC(CPh₂)C([double bond, length as m-dash]O)CO)] with concomitant formation of [(C₅Me₅)₂U(OMes)₂]. Surprisingly, reaction with SO₂ shows rare S-O bond cleavage to yield the unusual [(O₂CC(O)(SO)]^2- bridging ligand between two U(iv) centres. All complexes have been characterized using spectroscopic and structural methods, and the reaction of the ethynediolate with CO to form the ketene carboxylate has been investigated computationally as well as the reaction with SO₂.

Exploring the reactivity of homoleptic organozincs towards SO2: Synthesis and structure of a homologous series of organozinc sulfinates

Studies on the reactivity of zinc alkyl compounds towards SO2 are relatively less explored than either the oxygenation or hydrolysis reactions. We report on environmentally friendly and efficient syntheses of...

Electrochemical Characterization of Isolated Nitrogenase Cofactors from Azotobacter vinelandii

The nitrogenase cofactors are structurally and functionally unique in biological chemistry. Despite a substantial amount of spectroscopic characterization of protein-bound and isolated nitrogenase cofactors, electrochemical characterization of these cofactors and their related species is far from complete. Herein we present voltammetric studies of three isolated nitrogenase cofactor species: the iron-molybdenum cofactor (M-cluster), iron-vanadium cofactor (V-cluster), and a homologue to the iron-iron cofactor (L-cluster). We observe two reductive events in the redox profiles of all three cofactors. Of the three, the V-cluster is the most reducing. The reduction potentials of the isolated cofactors are significantly more negative than previously measured values within the molybdenum-iron and vanadium-iron proteins. The outcome of this study provides insight into the importance of the heterometal identity, the overall ligation of the cluster, and the impact of the protein scaffolds on the overall electronic structures of the cofactors.

SO2 and SO3 reactions with [(C5Me5)2Sm-O-Sm(C5Me5)2]: a DFT investigation and comparison with CO2 reactivity

Samarocene oxide [Cp*₂Sm-O-SmCp*₂] with Cp* = C₅Me₅, often seen as an undesirable product resulting from the oxidation of samarocene, has been reported to react with organic and inorganic anhydrides.

Activation of SO2 by [Zn(Cp*)2] and [(Cp*)ZnI–ZnI(Cp*)]

The reactions of SO₂ with [Zn(Cp*)₂] and [(Cp*)Zn^I–Zn^I(Cp*)] proceeded with insertion of SO₂ into the Zn–C bonds, and oxidation of the Zn^I–Zn^I bond in the reaction with [(Cp*)Zn^I–Zn^I(Cp*)].

Samarocene oxide: from an undesired decomposition product to a new reagent

Samarocene oxide [Cp*₂Sm-O-SmCp*₂], which is mostly considered as an undesired decomposition product, may act as a mild oxide base and thus is a valuable synthetic equivalent for “O²⁻”.