# Trimethylenemethane complexes

Trimethylenemethane complexes are metal complexes of the organic compound trimethylenemethane. Several examples are known, and some have been employed in organic synthesis.

## History

The synthesis of cyclobutadieneiron tricarbonyl pointed to the possible existence of related complexes of elusive organic compounds. Trimethylenemethane (TMM) has a natural connection to cyclobutadiene, and, in 1966, Emerson and co-workers reported the first trimethylenemethane (TMM) transition metal complex, η$^4$-[C(CH$_2$)$_3$]Fe(CO)$_3$. This compound became the starting point for extensive studies.

## Synthesis

Generally speaking, trimethylenemethane complexes are synthesized in the following four ways: (A) the dehalogenation of α, α'-dihalosubstituted precursors, (B) the thermal extrusion of XY (XY = HCl, Br$_2$, and CH$_4$,) from η$^3$-methylallyl complexes, (C) the ring opening of alkylidenecyclopropanes, and (D) the elimination of Me$_3$SiX [X = OAc, Cl, OS(O)$_2$Me] from functionalized allylsilanes (Figure 1).

### Dehalogenation of α, α'-dihalosubstituted precursors

η$^4$-[C(CH$_2$)$_3$]Fe(CO)$_3$, the first trimethylenemethane metal complex to be reported, was obtained from the reaction of 3-chloro-2-chloromethylprop-1-ene with Fe$_2$(CO)$_9$ or Na$_2$[Fe(CO)$_4$]. Followed by this result, a number of substituted trimethylenemethane iron complexes have been prepared.
The thermal extrusion from η$^3$-methylallyl complexes was reported by Emerson.The iron allyl complex, obtained from the reaction of 3-chloro-2-methylprop-1-ene with [Fe$_2$(CO)$_9$], decomposed on heating to afford the iron trimethylenemethane complex.

### Ring opening of alkylidenecyclopropanes

In the presence of [Fe$_2$(CO)$_9$], the ring opening of 2-substituted methylenecyclopropanes leads to the formation of various η$^4$-trimethylenemethane complexes containing different functional groups, such as (R1 = H, R2 = Ph), (R1 = Me, R2 = Ph), (R1 = R2 = Ph), and (R1 = H, R2 = CH=CH2). The stereochemistry has been elucidated by deuterium-labeling experiments.

### Elimination of Me$_3$SiX [X = OAc, Cl, OS(O)$_2$Me] from functionalized allylsilanes

tetrakis(triphenylphosphine)palladium(0) is a precursor to highly reactive η$^3$-trimethylenemethane complexes. Allylsilanes oxidatively add to some low-valent d$^8$ complexes resulting in the formation of an η$^1$-allyl complexes, followed by the formation of an η$^3$-allyl complex, and finally elimination of Me$_3$SiX to yield the η$^4$-trimethylenemethane complex. The isolation of the proposed intermidate further confirmed the mechanism.
    IrCl(CO)(PPh$_3$)$_2$ + CH$_2$=C(CH$_2$Cl)(CH$_2$tms) → η$^4$-[C(CH$_2$)$_3$]IrCl(PPh$_3$)(CO) + tmsCl + PPh$_3$ (Ph = C6H5)

## Structure

According to gas phase electron diffraction, η$^4$-C(CH$_2$)$_3$]Fe(CO)$_3$ adopts a staggered conformation about the iron center. The ligands, which include carbonyl and a trigonal-pyramidal trimethylenemethane, are arranged in the usual umbrella-type configuration. The central carbon of the trimethylenemethane ligand is closer to the iron center compared to the outer methylene carbons. This was confirmed by the Fe-C(central) distance measuring 1.94(1) Å, while the Fe-CH distances were measured at 2.12 Å. Moreover, this result has also been confirmed by X-ray diffraction and vibrational spectrum.
The primary bonding interaction occurs between the 2e set of the Fe(CO)$_3$ fragment and e" on the trimethylenemethane ligand. However, if the metal-trimethylenemethane axis is rotated by 60° into an eclipsed geometry, the interaction between 2e and e" is minimized, which results in an increase in the energy of the HOMO in the complex, which is a significant factor that provides a barrier to rotation, as shown in Figure 6b.
Extended Huckel calculations give a barrier of 87 KJ mol$^{−1}$ using a planar trimethylenemethane ligand. Introducing a puckered conformation to the trimethylenemethane ligand, which resembles the experimental geometry, leads to an increase in the calculated barrier to 98.6 kJ mol$^{−1}$. This puckering induces mixing of s character into e" orbitals, causing a more pronounced orientation toward the metal center. Consequently, the overlap between e" and 2e orbitals is enhanced. The degree of puckering, characterized by θ, falls within the range of 12°. The mixing of s character into e" also results in the H-C-H plane being tipped away from the metal. The angle β, between C-1 and C-2 and the plane H-C-H, is typically about 15°.

## Reactions

Trimethylenemethane complexes undergo a wide variety of reactions including those with electrophiles, nucleophiles as well as redox reactions.
η$^4$-C(CH$_2$)$_3$]Fe(CO)$_3$ adds hydrogen chloride to yield η$^3$-CH$_3$C(CH$_2$)]Fe(CO)$_3$. Substituted trimethylenemethane iron complexes, on the other hand, react with strong acids to produce cross-conjugated dienyl iron cations and η$^4$-diene complexes. η$^4$-C(CH$_2$)$_3$]Mo(CO)$_2$(C$_5$H$_5$)$^+$ add nucleophiles to give charge-neutral η$^3$-allyl complexes.
η$^4$-[C(CH$_2$)$_3$]Fe(PR$_3$)$_3$ (PR$_3$ = PMe$_3$ or PMe$_2$Ph) is oxidized by silver trifluoromethanesulfonate to give the 17-electron cation.