Trimethylenemethane complexes

Figure 2

Tris(carbonyl)-η⁴-trimethylenemethaneiron(0) was the first trimethylenemethane metal complex to be synthesized and was obtained from the reaction of 3-chloro-2-chloromethylprop-1-ene with [Fe₂(CO)₉,] or Na₂[Fe(CO)₄].[7] Followed by this result, A number of substituted trimethylenemethane iron complexes have been prepared by similar methods to those used to prepare tris(carbonyl)-η⁴-trimethylenemethaneiron(0).[8][9][10]

This reaction was first 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.[11]

Figure 3

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).[12] The most interesting part of this strategy is the stereochemistry and it had been elucidated by stereospecific deuterium labeling experiments. (Figure 3) [13]

Figure 4

The use of [Pd(PPh₃)₄] to facilitate the elimination of Me₃SiX (X=OAc) and generate a highly reactive η³-trimethylenemethane palladium was first reported by Trost and Chan.[14] Subsequently, it was discovered that the reaction of allylsilanes with a d⁸ metal center proceeds via oxidative addition to the metal, resulting in the formation of an η¹-allyl complex, followed by the formation of an η³-allyl complex, and finally elimination of Me₃SiX to yield the η⁴-trimethylenemethane complex (Figure 4). The isolation of the proposed intermidate further confirmed the mechanism.[15]

In a study using gas phase electron diffraction, it was determined that tris(carbonyl)-η⁴-trimethylenemethaneiron 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. These findings are presented in Figure 5.[16] Moreover, this result has also been confirmed by X-ray diffraction and vibrational spectrum.[17]

Figure 6a

Figure 6b

The orbital interaction diagram presented in Figure 6a illustrates the key bonding interaction in a complex formed between a planar trimethylenemethane ligand and an Fe(CO)₃ fragment in a staggered geometry. Specifically, the diagram highlights that 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 reduced. This reduction 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.[18] 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°.[19] 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°.

With a good π system, iron trimethylenemethane complexes are susceptible to electrophilic attack. The addition of hydrochloric acid to tris(carbonyl)-η⁴-trimethylenemethaneiron yields chlorotris(carbonyl)-2-methylallyliron(II). Substituted trimethylenemethane iron complexes, on the other hand, react with strong acids to produce cross-conjugated dienyl iron cations and η⁴-diene complexes.[20]

The same as the reactions with electronphiles, the cationic molybdenum trimethylenemethane complex has been found to exhibit susceptibility to nucleophilic attack at the trimethylenemethane ligand. This leads to the formation of neutral η³-allyl complexes.[21]

The cationic complex [Ir{η⁴-C(CH₂)₃}(CO)(PPh₃)₂]PF₆ (complex 1) exhibits rapid reactivity towards various tertiary phosphine ligands at room temperature. Upon treatment with 1 mol equivalent of ethyldiphenylphosphine, a 1:1:1 mixture of complexes [Ir{η⁴-C(CH₂)₃}(CO)(PMePh₂)₂]PF₆ (complex 2), [Ir{η⁴-C(CH₂)₃}(CO)(PMePh₂)(PPh₃)]PF₆ (complex 3), and [Ir{η⁴-C(CH₂)₃}(CO)(PPh₃)₂]PF₆ (complex 1) is formed. Addition of 2 mol equivalents of methyldiphenylphosphine to complex 1 affords complex complex 2 in high yield. Similarly, complex 1 reacts with 1,2-bis(diphenylphosphino)ethane (dppe) to yield [Ir{η⁴-C(CH₂)₃}(CO)dppe]PF₆. These reactions demonstrate that the phosphine ligands in these cationic iridium complexes are labile. Moreover, addition of 1 mol equivalent of methyldiphenylphosphine to the complex [Ir{η⁴-C(CH₂)₃}(CO)₂(PPh₃)]PF₆ displaces a carbonyl ligand, yielding a 1:1:1 mixture of complexes 1,2 and 3.[22]

Figure 7

Up to now, redox reactions of trimethylenemethane complexes are still limited. One iconic example is the electron-rich complexes [Fe{η⁴-C(CH₂)₃(L)₃] (L = PMe or PMe₂Ph) (complex 4). Upon oxidation with 1 equivalent of silver trifluoromethanesulfonate, the neutral trimethylenemethane complexes 4 undergo transformation into their respective cationic 17-electron complexes 5.[23] These green oxidation complexes, can be regenerated back to their original neutral trimethylenemethane forms through reactions with sodium diethylmalonate or sodium borohydride (Figure 7).