Electrochemistry of chlorinated ferrocenes: stability of chlorinated ferrocenium ions

Abstract

The cyclic voltammetries of 1,2,3,4,5-pentachloro-and decachloro-ferrocene have been studied in acetonitrile. The complexes undergo an irreversible two-electron oxidation consistent with an electrochemical-chemical-electrochemical mechanism at scan rates up to 1 0 V s-l. However, at the faster scan rates (up to 1 6 0 V s-l) available to electrodes of small radius and microelectrodes, chemically reversible one-electron oxidations to the chlorinated ferrocenium ions, [C,,H,,-,CI,Fe] + are obtained under ambient conditions. The reversible [C,,H,,-,CI,Fe] +lo couples when x = 1 0 . 5 , 2 and 1 are observed at +1.246, +0.774, +0.315 and +0.168 V vs. ferrocenium-ferrocene. respectively. A plot of €, vs. C o p (op = Hammett para coefficient for the chloro substituent) shows that the neutral molecules are stabilised with respect to the corresponding ferrocenium cations by 0.1 6-0.1 2 V per CI. The rate constants of decomposition of the [Fe(q-C,CI,),] and [Fe(q-C,H,) (q-C,CI,)] + cations were calculated by both digital simulation and the method of Nicholson and Shain to be 40 k 2 0 and 200 k 50 s-l, respectively, at room temperature (ca. 2 0 "C). The complexes [Fe(q-C,H,CI),J and [Fe(q-C,H,) (q-C5H4Cl)] exhibit reversible oxidations at all scan rates down to 0.100 V s-' under the same conditions. Both [Fe(q-C,CI,),] and [Fe(q-C,H,) (q-C5C15) J undergo a series of irreversible two-electron reductions at potentials negative of -1 . 8 V, which lead to reductive dechlorination consistent with an electrochemical-chemicalelectrochemical-chemical reaction.

The Unusual Redox Properties of Fluoroferrocenes Revealed through a Comprehensive Study of the Haloferrocenes

“…It is apparent that no simple correlation exists between substituent electronegativity and E 1/2 in these complexes. Studies elsewhere have made similar observations -1,2,3,4,5-pentachloroferrocene ([FeCp(C 5 Cl 5 )], E 0 = 0.77 V vs. FcH/FcH + ; reversible only at high scan rates) 33 is reportedly much more difficult to oxidize than 1,2,3,4,5-pentafluoroferrocene ([FeCp(C 5 F 5 )], E 0 = 0.01 V vs. FcH/FcH + ). 6 [FcB(C 6 Cl 5 ) 2 ] has also been observed to exhibit a higher equilibrium potential than [FcB(C 6 F 5 ) 2 ] (E 1/2 = 0.55 V vs. 0.45 V, respectively; though the C 6 Cl 5 group is in this case considered more electron withdrawing).…”

Outer-Sphere Redox Couples as Shuttles in Dye-Sensitized Solar Cells. Performance Enhancement Based on Photoelectrode Modification via Atomic Layer Deposition

“…As illustrated by the cyclic voltammograms in Figure 6a, the redox couples are reversible, with the formal potentials of the chloroand dichloro-derivatives being 0.17 and 0.31 V more positive, respectively, than the parent redox couple, as expected. 58 , respectively, compared to Fc +/0 (Figure 6b). Deviation from these values, however, provides insight into the relative rates of recombination and interception.…”

Synthesis, chemical reactivity and electrochemical behaviour of mono- and difluoro metallocenes

“…This also allows a comparison of all mono-and dihalogenated ferrocenes ( Figure 2, Table 1). [23,33] All compounds in this work reveal quasi-reversible one-electron waves with peak-current ratios I P Ox / I P Red of about 1 and a distorted quasi-Nernstian behavior is observed (∆E p > 59 mV). However, Randles-Sevcik plots of compounds 1 -4 showed a good reversibility at the five different scan rates 50, 100, 250, 500, and 1000 mV * s -1 ( Figure S1).…”

“…[23] Likewise, the half-wave potential of about 256 mV is obtained from the sum of the I and F increments, and reasonable values can also be derived for the chlorinated species FcCl (168 mV) and FcCl 2 (315 mV). [33] Although fluorine is the most electronegative substituent, the half-wave potential ∆E 0 f of the fluorinated metallocenes (106 mV for 1 and 220 mV for 2) are actually lower compared than those of the other halogenated ferrocenes (160 -170 mV and 290 -320 mV; Table 1). One possible explanation is that in case of halogens besides the inductive effect (-I) a strong resonance effect (+M) plays a role as well.…”