F-block metallocene: Difference between revisions

f-Block Metallocene

f-Block Metallocene refers to a class of organometallic sandwich compounds with f-Block metal and electron-rich ligands like cyclopentadienyl anion. It is marked for its strong reductivity.

History

The first prepared and well-characterized f-block metallocenes were tris(cyclopentadienyl) lanthanide complexes, (C₅H₅)₃Ln (Ln = La, Ce, Pr, Nd, Sm and Gd).^[1][2][3] 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 being able to enhance 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, have trivalent metallocene complex, [(C₅H₅)₂LnZ]_n^[4,5][4] In 1980, the pentamethylcyclopentadienyl ligand, C₅Me₅^-_, was introduced to prepare the lanthanide complexes with all metals in the series.^[4,6-9] Apart from improving stability and solubility of the complex better, it was demonstrated to participate in the organometallic reaction. Subsequently, Evans, W. J. and his coworkers successfully isolated (C₅Me₅)₂Sm(THF)₂^[10] and (C₅Me₅)₂Sm,^[11] making a breakthrough in f-block metallocene, 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,^[10,12-15] dinitrogen,^[16] internal alkynens,^[17,18] phosphaalkynes,^[19] carbon monoxide,^[20] carbon dioxide,^[21] isonitriles,^[22] diazine derivatives,^[23-25] imines^[26] and polycyclic aromatic hydrocarbons (PAHs).^[27] Moreover, due to its strong reducing potential, it was used to synthesize [(C₅Me₅)₂Sm(µ-H)]₂ and other the trivalent f-block element complexes.^[17] Subsequently, tris(pentamethylcyclopentadienyl) lanthanide complexes, (C₅Me₅)₃Ln, and their relevant complexes were synthesized from the reductive Sm²⁺ chemistry. These metallocenes included (C₅Me₅)₃Sm, [(C₅H₃(SiMe₃)₂]₃Sm, (C₅Me₅)₂Sm(C₅H₅), [(C₅Me₅)₂Sm]₂(µ-C₅H₅).^[28-30] Later, one tris(pentamethylcyclopentadienyl) f-element halide complexes, (C₅Me₅)₃UCl, were successfully isolated as the intermediate of the formation of (C₅Me₅)₂UCl₂.^[31] 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.^[32] Its existence also indicates that the steric crowding around the center metal is still large enough to accommodate four large ligands and similar complexes of the larger f-block elements or other ligands (Z) are sterically possible: (C₅Me₅)₃UF,^[31] (C₅Me₅)₃ThZ^[33] and (C₅Me₅)₃MH^[34] were subsequently synthesized. Similarly, the crystallographs demonstrate that (C₅Me₅)₃MZ is structurally similar to (C₅Me₅)₃M and its M-Z bond distance of is obviously larger than the M-C bond distance.^[32]

Synthesis

I. the first f-block metallocene was synthesized through the following equation:^[1,2]

II. (C₅Me₅)₃Sm has the following three synthetic pathways:

(i) the first (C₅Me₅)₃Sm was prepared via exporatory Sm²⁺ chemistry with cyclopoctadiene:^[28]

(ii) Similar to method (i), (C₅Me₅)₃Sm can be efficiently synthesized from Sm²⁺ and (C₅Me₅)₂Pb:^[35]

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

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

Actually, this solvated cation routine generally allows the preparation of all (C₅Me₅)₃Ln since [(C₅Me₅)₃LnH]_x exist for all lanthanide elements.

An alternative unsolvated cation pathway prohibits THF during the reaction since (C₅Me₅)₃Sm can ring open THF.^[36,37]

III. Generally, in order to synthesize (C₅Me₅)₃M, the starting materials and the reaction conditions requires optimizing to ensure (C₅Me₅)₃M to be the most favored product.^[32] In addition, the compounds reacting with (C₅Me₅)₃M like THF, nitirles or isolnitriles, should be avoided.^[32] Therefore, the following routines 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 extremely easy to be oxidized.

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

IV. for the synthesis of (C₅Me₅)₃MZ with Z=X, H, etc., they could be prepared through the following pathways.^[31,34] It is worth noting that (C₅Me₅)₃UCl is isolated from the synthetic reaction of (C₅Me₅)₃UCl₂.^[31]

Reactivity

Unlike d-block elements, f-block elements do not follow 18-electron rules due to their f-orbitals.^[38] As a result, they are more electron-rich and so much more reductive. For example, (C₅H₄SiMe₃)₃Ln have extremely negative reduction potentials that are -2.7 to -3.9 Volts versus the standard hydrogen electrode (NHE).^[39] It is even able to reduce some d-block metallocenes like ferrocene. 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.^[40] 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 twisted to modify the reactivity.^[40] These special properties allow the following reactions to occur.

I.              Alkyl-like Reactivity

Like alkyl group, the electron-rich ligand of f-block metallocenes can act as a nucleophile during organometallic reactions. For example, it can polymerize double bonds,^[41] open rings,^{[37, 42]} etc.

II.            Ligand Cleavage

Especially in the presence of Lewis acids like B(C₆F₅)₃ or Al₂Me₆, the Cp and other similar ligands can be removed through the following reactions.^[43] This indicates that Lewis acids probably can be used to activate f-block metallocene for organometallic reactions like polymerizations.

III.          Insertions

The f-block metallocenes are able to undergo insertion reactions of compounds like carbon monoxide,^[44] nitriles or isocyanates.^[37]

IV.          Reductivity

(i)            Ordinary reductions

Since the Cp or other similar ligand of f-block metallocenes are very electropositive, it tends to lose one electron and pentamethylcyclopentadiene, which results in the strong reductivity of f-block metallocenes.

(ii)          Sterically induced reduction (SIR)

Sterically crowded complexes like (C₅Me₅)₃Sm are able to provided strong reductivity and so this type of reaction was named as SIR.^[45] Due to the strong steric hindrance, one ligand cannot bind to the metal center at the ideal distance and so the complex is not electrostatically stable.^[32] Thus, the anion is more inclined to become oxidized and leave the complex. This tendency gives it strong reductivity.

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