Dimers of nineteen-electron sandwich compounds: Crystal and electronic structures, and comparison of reducing strengths

The dimers of some Group 8 metal cyclopentadienyl/arene complexes and Group 9 metallocenes can be handled in air, yet are strongly reducing, making them useful n-dopants in organic electronics. In this work, the X-ray molecular structures are shown to resemble those of Group 8 metal cyclopentadienyl/pentadienyl or Group 9 metal cyclopentadienyl/diene model compounds. Compared to those of the model compounds, the DFT HOMOs of the dimers are significantly destabilized by interactions between the metal and the central C-C σ-bonding orbital, accounting for the facile oxidation of the dimers. The lengths of these C-C bonds (X-ray or DFT) do not correlate with DFT dissociation energies, the latter depending strongly on the monomer stabilities. Ru and Ir monomers are more reducing than their Fe and Rh analogues, but the corresponding dimers also exhibit much higher dissociation energies, so the estimated monomer cation/neutral dimer potentials are, with the exception of that of [RhCp₂]₂, rather similar (-1.97 to -2.15 V vs. FeCp₂ ^+/0 in THF). The consequences of the variations in bond strength and redox potentials for the reactivity of the dimers are discussed. Dopant dissociation: Dimers of Group 9 metallocenes and Group 8 MCp(arene) (Cp=cyclopentadienyl) derivatives are strongly reducing yet relatively inert in air, making them useful in organic electronics. X-ray and DFT methods gave insights into the molecular structures of these compounds and their relation to electronic structure and reactivity. The figure shows how the variations in the central bond lengths do not reflect that in dissociation energies.