Electron-reservoir complex

[1][2] From Astruc's discoveries, a whole family of thermally stable, neutral, 19-electron iron(I) organometallic complexes were isolated and characterized, and found to have applications in redox catalysis and electrocatalysis.

The parent complex, C5H5FeC6H6, is observed to undergo decomplexation and dimerization, whereas analogues containing six alkyl groups on the benzene ring exhibit stability in many solvents and remain catalytically active (written as η5-CpFe-η6-arene).

[9] Detailed EPR studies of the Fe(I) sandwiches in frozen solution, in the solid state, and in diamagnetic matrices show dynamic rhombic distortion, high degree of covalency, and spin-lattice relaxation.

When one electron is added to the dication, (C6Me6)2Fe2+, a stable 19-electron complex is formed, a compound stabilized only when the arene is endowed with multiple methyl groups.

In 1979, Didier et al. reported the activation of the arene group in η5-C5H5Fe-η6-C6(CH3)6 by dioxygen, O2, through an electron transfer mechanism to form the superoxide radical anion, O2–•.

Characterization by mass spectrometry, nuclear magnetic resonance, and X-ray crystallography revealed that the structure of 2 is best described as a cyclohexadienyl ligand coordinated in a pentahapto fashion to η5-C5H5Fe and bearing an exocyclic double bond (Figure 5).

[13] Reacting a stoichiometric amount of CpFe-C6(CH3)6 in THF in the presence of air with imidazolium salts, quickly results in the soluble carbenes, which are visible by the color change from deep-green to yellow (Figure 7).

A decade prior, Venkatesan et al. investigated a series of electron-rich manganese(I) half-sandwich complexes for applications as molecular batteries.

The study highlighted the possibility of using the C–C bonds in these vinylidene systems as electron reservoirs, enabling their potential as essential components in nano devices.

Figure 1. Synthesis of CpFe(arene) using ferrocene
Figure 2. Synthesis of bis(arene) iron dications
Figure 3. Molecular orbital diagram bis(arene) transition metal sandwiches
Figure 4. Three isostructural complexes of (C 6 Me 6 ) 2 Fe n + ( n = 0, 1, 2)
Figure 5. Hydrogen atom abstraction by O 2 from a methyl group on the arene ligand and benzylic activation by CX 2
Figure 6. O 2 -induced dimerization
Figure 7. Deprotonation of imidazolium salts to afford N-heterocyclic carbenes