Trimethylenemethane complexes are metal complexes of the organic compound trimethylenemethane. Several examples are known, and some have been employed in organic synthesis.[1]
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.
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)<sub>2</sub>Me] from functionalized allylsilanes (Figure 1).
η4-, 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)<sub>4</sub>].[2] Followed by this result, a number of substituted trimethylenemethane iron complexes have been prepared.[3] [4] [5]
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 [Fe<sub>2</sub>(CO)<sub>9</sub>], decomposed on heating to afford the iron trimethylenemethane complex.[6]
In the presence of [Fe<sub>2</sub>(CO)<sub>9</sub>], 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).[7] The stereochemistry has been elucidated by deuterium-labeling experiments.
Pd(PPh3)4 is a precursor to highly reactive η3-trimethylenemethane complexes.[1] 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.[8]
η4- (Ph = C6H5)
According to gas phase electron diffraction, η4- 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.[9] Moreover, this result has also been confirmed by X-ray diffraction and vibrational spectrum.[10]
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.[11] 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°.[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°.
Trimethylenemethane complexes undergo a wide variety of reactions including those with electrophiles, nucleophiles as well as redox reactions.
η4- adds hydrogen chloride to yield η3-. Substituted trimethylenemethane iron complexes, on the other hand, react with strong acids to produce cross-conjugated dienyl iron cations and η4-diene complexes.[13] η4- add nucleophiles to give charge-neutral η3-allyl complexes.[14]
[Fe{η<sup>4</sup>-C(CH<sub>2</sub>)<sub>3</sub>(L)<sub>3</sub>] (L = PMe or PMe2Ph) (complex 4) reacts with silver trifluoromethanesulfonate to give the 17-electron cation (Figure 7).[5]