A DFT-Elucidated Comparison of the Solution-Phase and SAM Electrochemical Properties of Short-Chain Mercaptoalkylferrocenes: Synthetic and Spectroscopic Aspects, and the Structure of Fc–CH2CH2–S–S–CH2CH2–Fc
journal contributionposted on 22.02.2016, 00:00 by Jan P. Lewtak, Marilé Landman, Israel Fernández, Jannie C. Swarts
Facile synthetic procedures to synthesize a series of difficult-to-obtain mercaptoalkylferrocenes, namely, Fc(CH2)nSH, where n = 1 (1), 2 (2), 3 (3), or 4 (4) and Fc = Fe(η5-C5H5)(η5-C5H4), are reported. Dimerization of 1–4 to the corresponding disulfides 19–22 was observed in air. Dimer 20 (Z = 2) crystallized in the triclinic space group P1̅. Dimers 20–22 could be reduced back to the original Fc(CH2)nSH derivatives with LiAlH4 in refluxing tetrahydrofuran. Density functional theory (DFT) calculations showed that the highest occupied molecular orbital of 1–4 lies exclusively on the ferrocenyl group implying that the electrochemical oxidation observed at ca. −15 < Epa < 76 mV versus FcH/FcH+ involves exclusively an Fe(II) to Fe(III) process. Further DFT calculations showed this one-electron oxidation is followed by proton loss on the thiol group to generate a radical, Fc(CH2)nS•, with spin density mainly located on the sulfur. Rapid exothermic dimerization leads to the observed dimers, Fc(CH2)n–S–S–(CH 2)nFc. Reduction of the ferrocenium groups on the dimer occurs at potentials that still showed the ferrocenyl group ΔE = Epa,monomer – Epc,dimer ≤ 78 mV, indicating that the redox properties of the ferrocenyl group on the mercaptans are very similar to those of the dimer. 1H NMR measurements showed that, like ferrocenyl oxidation, the resonance position of the sulfhydryl proton, SH, and others, are dependent on −(CH2)n– chain length. Self-assembled monolayers (SAMs) on gold were generated to investigate the electrochemical behavior of 1–4 in the absence of diffusion. Under these conditions, ΔE approached 0 mV for the longer chain derivatives at slow scan rates. The surface-bound ferrocenyl group of the metal-thioether, Fc(CH2)n −S–Au, is oxidized at approximately equal potentials as the equivalent CH2Cl2-dissolved ferrocenyl species 1–4. Surface coverage by the SAMs is dependent on alkyl chain length with the largest coverage obtained for 4, while the rate of heterogeneous electron transfer between SAM substrate and electrode was the fastest for the shortest chain derivative, Fc-CH2–S–Au.