IR sensing of the electronic structure in the mixed-valence states of iron carbonyl-attached biferrocene and terferrocene

Ferrocene, 1,1,"-biferrocene and 1,1 ":1 "',1 ""-terferrocene derivatives involving a (cycopentadiene)Fe(CO)(3) moiety were synthesized; the nu(CO) peak frequency shifts during their redox processes were monitored by the IRAS technique. All the complexes give two nu(CO) peaks, which shift to higher wavenumbers (12-18 cm(-1)) by the oxidation from the most reduced (all Fe's are Fe(II)) to the most oxidized (all Fe's are Fe(III)) forms. The biferrocene derivative in the monocationic form gives peaks at wavenumbers between those for neutral and dicationic forms, probably because the rate of electron-transfer affording two electronic isomers, Red-Ox-[FeCO] and Ox-Red-[FeCO], where Red, Ox, and [FeCO] refer to reduced and oxidized forms of a ferrocene unit, and a [(eta(4)-cyclopentadiene)Fe(CO)(3)] moiety, respectively, is slower than or comparable to the time scale of IR (10(11)-10(12) s(-1)). The terferrocene derivative shows nu(CO) frequency changes corresponding to the three-step 1e(-) oxidation processes. The shift is most significant in the second oxidation step, implying that the largest change in charge is that of the ferrocene unit nearest to Fe(CO)(3). This is in principle consistent with the elucidation of the oxidation process via thermodynamically favorable electronic isomers based on the interaction energy between neighboring oxidized and reduced sites: Red-Red-Red-[FeCO] --> Red-Ox-Red-[FeCO] --> Ox-Red-Ox-[FeCO] --> Ox-Ox-Ox-[FeCO].