F block metallocene

History

The first prepared and well-characterized f-block metallocenes were the tris(cyclopentadienyl) lanthanide complexes, (C₅H₅)₃Ln (Ln = La, Ce, Pr, Nd, Sm and Gd). However, their significance is limited more to their existences and structures than to their reactivity. The cyclopentadienyl ligands of f-block metallocenes were considered as inert ancillary ligands, only capable of enhancing their stability and solubility, but not their reactivity. In addition, only late and small metals in the lanthanide series, i.e., elements from Sm to Lu, are trivalent metallocene complexes, [(C₅H₅)₂LnZ]_n In 1980, the pentamethylcyclopentadienyl ligand, C₅Me−
5, was introduced to prepare the lanthanide complexes with all metals in the series. Apart from improving the stability and solubility of the complexes, it was demonstrated to participate in organometallic reactions. Subsequently, Evans, W. J. and his coworkers successfully isolated (C₅Me₅)₂Sm(THF)₂ and (C₅Me₅)₂Sm, making a breakthrough in f-block metallocenes, since both of these two organosamarium(II) complexes were unexpectedly found to participate in the coordination, activation and transformation of a variety of unsaturated compounds, including olefins, dinitrogen, internal alkynens, phosphaalkynes, carbon monoxide, carbon dioxide, isonitriles, diazine derivatives, imines and polycyclic aromatic hydrocarbons (PAHs). Moreover, due to its strong reducing potential, it was used to synthesize [(C₅Me₅)₂Sm(µ-H)]₂ and other trivalent f-block element complexes. Subsequently, tris(pentamethylcyclopentadienyl) lanthanide complexes, (C₅Me₅)₃Ln, and their relevant complexes were synthesized from Sm²⁺ complexes. These metallocenes included (C₅Me₅)₃Sm, [(C₅H₃(SiMe₃)₂]₃Sm, (C₅Me₅)₂Sm(C₅H₅), [(C₅Me₅)₂Sm]₂(µ-C₅H₅). Later, one tris(pentamethylcyclopentadienyl) f-element halide complex, (C₅Me₅)₃UCl, was successfully isolated as the intermediate of the formation of (C₅Me₅)₂UCl₂. It is worthy mentioning that (C₅Me₅)₃UCl has a very similar structure as (C₅Me₅)₃U and its uranium-chloride bond (2.90 Å) is relatively longer than the uranium-chloride bonds of other analogues. Its existence also indicates that the larger f-block elements are capable of accommodating additional ligands in addition to the three cyclopentadienyl ligands resulting in the isolation of the following complexes: (C₅Me₅)₃UF, (C₅Me₅)₃THz and (C₅Me₅)₃MH.

Synthesis

I. the synthesis of the first f-block metallocenes is described by following equation:

II. Preparation of (C₅Me₅)₃Sm:

(i) the first (C₅Me₅)₃Sm was prepared via exporatory Sm²⁺ chemistry with cyclooctatetraene:

(ii) Similar to method (i), (C₅Me₅)₃Sm can be efficiently synthesized from a Sm²⁺ precursor and (C₅Me₅)₂Pb:

In this pathway, (C₅Me₅)₂Sm(OEt₂) is used since it is more readily available than (C₅Me₅)₃Sm and does not react THF.

(iii) additionally, (C₅Me₅)₃Sm can also be prepared from trivalent percursors, without ring opening THF.

This solvated cation route generally allows the preparation of all (C₅Me₅)₃Ln complexes since [(C₅Me₅)₃LnH]_x is known for all lanthanide elements.

An alternative unsolvated cation pathway prohibits THF during the reaction since (C₅Me₅)₃Sm can ring open THF.

III. Generally, in order to synthesize (C₅Me₅)₃M, the starting materials and the reaction conditions require optimizing to ensure (C₅Me₅)₃M is the most favored product. In addition, compounds capable of reacting with (C₅Me₅)₃M, such as THF, nitirles or isolnitriles, should be avoided. Therefore, the following routes are possible options:

(i) For M=Ln including La, Ce, Pr, Nd and Gd, unsolvated cation route is preferred since [(C₅Me₅)₂LnH]_x complexes are too reactive.

Notably, the synthesis of (C₅Me₅)₃La and (C₅Me₅)₃Ce requires the usage of silylated glassware since they are easily oxidized.

(ii) For M=actinide like U, solvated cation route can be used.

IV. Synthesis of (C₅Me₅)₃MZ with Z=X, H, etc.

Reactivity

Unlike d-block elements, f-block elements do not follow 18-electron rule due to their f-orbitals. The following complexes, (C₅H₄SiMe₃)₃Ln, have extremely negative reduction potentials of -2.7 to -3.9 Volts versus the standard hydrogen electrode (NHE). Furthermore, in comparison with d-orbitals of transition metals, the radial extension of their 4f-orbitals are really small and limited, which greatly reduces the orbital effects. More specifically, its 4fn electron configurations have almost no effect on its chemical reactivity and its electrostatic interactions require optimizing through ligand geometries. Moreover, the reactivity of the f-block element complexes relies heavily on their sterics. In other words, a sterically saturated structure offers the best stability, and so, both ligand size or metal size can be altered to modify the reactivity. These special properties allow the following reactions to occur.

Alkyl-like Reactivity

Like alkyl group, the electron-rich ligand of f-block metallocenes can act as a nucleophile during organometallic reactions. For example, they can polymerize olefins, and participate in ring opening polymerizations, etc.

Ligand Cleavage

Especially in the presence of Lewis acids like B(C₆F₅)₃ or Al₂Me₆, the Cp and other similar ligands can be removed in the following way.

Insertions

The f-block metallocenes are able to undergo insertion reactions of compounds like carbon monoxide, nitriles or isocyanates.

Ordinary reductions

Since f-block metallocenes are very electron-rich, they tend to lose one electron and a pentamethylcyclopentadienyl ligand.

Sterically induced reduction (SIR)

Sterically crowded complexes like (C₅Me₅)₃Sm are able to provide strong reductivity and so this type of reaction was named as SIR. Due to the strong steric hindrance, one ligand cannot bind to the metal center at the ideal distance and so the complex is not stable. Thus, the anion is more inclined to become oxidized and leave the complex, resulting in a highly reducing metal complex.