Tuck-in Complex

In organometallic chemistry, a tuck-in complex usually refers to derivatives of Cp* ligands wherein a methyl group is deprotonated and the resulting methylene attaches to the metal. The C₅–CH₂–M angle is acute. The term "tucked in" was coined to describe derivatives of organotungsten complexes. Although most "tucked-in" complexes are derived from Cp* ligands, other pi-bonded rings undergo similar reactions.

Scope and bonding

The "tuck-in" process is related to ortho-metalation in the sense that it is an intramolecular cyclometalation. Tuck-in complexes derived from Cp* ligands are derivatives of tetramethylfulvene, sometimes abbreviated Me₄Fv. A variety of complexes are known for Me₄Fv and related ligands. In these complexes, the Fv can serve as a 4-electron or as a 6-electron ligand.

Examples

The original example proceeded via sequential loss of two equivalents of H₂ from decamethyltungstocene dihydride, Cp*₂WH₂. The first dehydrogenation step affords a simple tuck-in complex:
:(C₅Me₅)₂WH₂ → (C₅Me₅)(''η''⁶-C₅Me₄CH₂)WH + H₂
The second dehydrogenation step affords a double tuck-in complex:
:(C₅Me₅)(''η''⁶-C₅Me₄CH₂)WH → (C₅Me₅)(''η''⁷-C₅Me₃(CH₂)₂)W + H₂

In organouranium chemistry, both tuck-in and tuck-over complexes are recognized, for example in the dihydrido diuranium complex p*₃(''η''⁷-C₅Me₃(CH₂))U₂H₂ In this complex the two methylene groups bind to different uranium centers. The tuck-over mode is binding of the Cp* methylene to a metal center elsewhere in the molecule rather than the one coordinated to that Cp* ligand.

Reactions

Tuck-in complexes retain nucleophilicity at the methylene carbon. They can be activated by Lewis acids to generate active catalysts for use in Ziegler–Natta catalysis. The Lewis acid attaches to the CH₂ group, exposing a vacant site on the electrophilic Zr(IV) centre.

References

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Organometallic chemistry