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 possibility complexes of other organic ligands that are elusive in their free state. Trimethylenemethane (TMM) has a natural connection to cyclobutadiene, and, in 1966, Emerson and co-workers reported the first trimethylenemethane (TMM) transition metal complex, (CO)3FeC(CH2)3, which 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, Br2, and CH4,) from η3-methylallyl complexes, (C) the ring opening of alkylidenecyclopropanes, and (D) the elimination of Me3SiX [X = OAc, Cl, OS(O)2Me] from functionalized allylsilanes (Figure 1).

Dehalogenation of α, α'-dihalosubstituted precursors
η4-[C(CH2)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 Fe2(CO)9 or Na2[Fe(CO)4]. Followed by this result, a number of substituted trimethylenemethane iron complexes have been prepared.

The thermal extrusion from η3-methylallyl complexes was reported by Emerson.The iron allyl complex, obtained from the reaction of 3-chloro-2-methylprop-1-ene with [Fe2(CO)9], decomposed on heating to afford the iron trimethylenemethane complex.

Ring opening of alkylidenecyclopropanes
In the presence of [Fe2(CO)9], the ring opening of 2-substituted methylenecyclopropanes leads to the formation of various η4-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 Me3SiX [X = OAc, Cl, OS(O)2Me] from functionalized allylsilanes
Pd(PPh3)4 is a precursor to highly reactive η3-trimethylenemethane complexes. Allylsilanes oxidatively add to some low-valent d8 complexes resulting in the formation of an η1-allyl complexes, followed by the formation of an η3-allyl complex, and finally elimination of Me3SiX to yield the η4-trimethylenemethane complex. The isolation of the proposed intermidate further confirmed the mechanism.
 * IrCl(CO)(PPh3)2 + CH2\dC(CH2Cl)(CH2tms)  ->  η4-[C(CH2)3]IrCl(PPh3)(CO) +  tmsCl  +  PPh3 (Ph = C6H5)

Structure and bonding
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According to gas phase electron diffraction, η4-C(CH2)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.

η4-C(CH2)3]Fe(CO)3 adds hydrogen chloride to yield η3-CH3C(CH2)]Fe(CO)3. Substituted trimethylenemethane iron complexes, on the other hand, react with strong acids to produce cross-conjugated dienyl iron cations and η4-diene complexes. η4-C(CH2)3]Mo(CO)2(C5H5)+ add nucleophiles to give charge-neutral η3-allyl complexes.

[Fe{η4-C(CH2)3(L)3] (L = PMe or PMe2Ph) (complex 4) reacts with silver trifluoromethanesulfonate to give the 17-electron cation (Figure 7).