Is indenyl a stronger or weaker electron donor ligand than cyclopentadienyl? Opposing effects of indenyl electron density and ring slipping on electrochemical potentials
Nguyen, K. T.; Lane, E. E.; McMillen, C. D.; Pienkos, J. A.; Wagenknecht, P. S. "Is indenyl a stronger or weaker electron donor ligand than cyclopentadienyl? Opposing effects of indenyl electron density and ring slipping on electrochemical potentials" Organometallics 2020, 39, 670-678.
Complexes of the type RCp2Ti(C2Fc)2 (where RCp = cyclopentadienyl or a substituted cyclopentadienyl) have high molar absorptivity FeII to TiIV metal-to-metal charge transfer (MMCT) absorptions and have been investigated for possible use as photoredox catalysts. Relative to Cp2Ti(C2Fc)2, the MMCT shifts to higher or lower energy when RCp is either a stronger or weaker donor ligand than Cp, respectively. The indenyl ligand (Ind) has been described in the literature as both a stronger and weaker donor ligand than Cp. Herein, we report the preparation of Ind2Ti(C2Fc)2 and the complex with CuBr bound between the two alkynes, Ind2Ti(C2Fc)2CuBr, in order to determine the effect that replacing Cp with Ind has on the MMCT absorption energy. The complexes are characterized by NMR and UV–vis spectroscopy, cyclic voltammetry, and X-ray crystallography in the case of Ind2Ti(C2Fc)2CuBr. Relative to Cp2Ti(C2Fc)2, the FeIII/II potential of Ind2Ti(C2Fc)2 shifts cathodically and the TiIV/III potential shifts anodically, resulting in a lower energy MMCT absorption. The TiIV/III reduction wave is also chemically irreversible, with ipa/ipc approaching unity as the scan rate is increased from 10 to 500 mV/s. Examination of the literature reviewed in this report demonstrates that, in all cases, replacement of Cp with Ind leads to a cathodic shift of the potential for metal oxidation. However, replacement of Cp with Ind typically leads to an anodic shift of the reduction potential, leading some to suggest that Ind is a poorer e– donor than Cp. In the context of the literature, these results are interpreted as indicating that Ind is a better electron donor than Cp, but that reduction induces an η5 to η3 haptotropic shift that is rapid on the electrochemical time scale, stabilizing the reduction product, resulting in an anodic shift of the TiIV/III reduction. Furthermore, these opposing effects are not unique to this system.