Organometallic Fe-Fe Interactions: Beyond Common Metal-Metal Bonds and Inverse Mixed-Valent Charge Transfer

The compounds [Fe(CO)₃ (dRpf)]ⁿ⁺ , n=0, 1, 2 and dRpf=1,1'-bis(dicyclohexylphosphino)ferrocene ([1]ⁿ⁺ ) or 1,1'-bis(diisopropylphosphino)ferrocene ([2]ⁿ⁺ ), were obtained as two-step reversible redox systems by photolytic and redox reactions. The iron-iron distance decreases from about 4 Å to about 3 Å on oxidation, which takes place primarily at the tricarbonyliron moiety. Whereas ferrocene oxidation is calculated to occur only in excited states, the near infrared absorptions of the mixed-valent monocations are due to an unprecedented "inverse" inter-valence charge transfer from the electron-rich iron(II) in the ferrocene backbone to the electron-deficient tricarbonyliron(I). Protonation of complex 1 results in the formation of the structurally characterized hydride [1H]BF₄ , which reacts with acetone to form the dication, 1²⁺ , and isopropanol. While the hydride [2H]BF₄ was found to be unstable, protonation of 2 in acetone resulted in the clean formation of 2²⁺_, formally a hydrogen transfer.