Lewtak Jan P, Landman Marilé, Fernández Israel, Swarts Jannie C
Department of Chemistry, University of the Free State , P.O. Box 339, Bloemfontein 9300, Republic of South Africa.
Department of Chemistry, University of Pretoria , Private Bag X20, Hatfield 0028, Republic of South Africa.
Inorg Chem. 2016 Mar 7;55(5):2584-96. doi: 10.1021/acs.inorgchem.5b02936. Epub 2016 Feb 22.
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(η(5)-C5H5)(η(5)-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. (1)H 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.
报道了一系列难以获得的巯基烷基二茂铁，即Fc(CH₂)ₙSH（其中n = 1 (1)、2 (2)、3 (3) 或 4 (4)，且Fc = Fe(η⁵-C₅H₅)(η⁵-C₅H₄)）的简便合成方法。在空气中观察到1 - 4二聚形成相应的二硫化物19 - 22。二聚体20（Z = 2）以三斜空间群P1̅结晶。二聚体20 - 22可以在回流的四氢呋喃中用LiAlH₄还原回原来的Fc(CH₂)ₙSH衍生物。密度泛函理论（DFT）计算表明，1 - 4的最高占据分子轨道仅位于二茂铁基上，这意味着相对于FcH/FcH⁺在约 - 15 < Epa < 76 mV观察到的电化学氧化仅涉及Fe(II)到Fe(III)的过程。进一步的DFT计算表明，这种单电子氧化之后是硫醇基团上的质子损失，生成一个自由基Fc(CH₂)ₙS(•)，其自旋密度主要位于硫上。快速的放热二聚反应导致观察到二聚体Fc(CH₂)ₙ - S - S - (CH₂)ₙFc。二聚体上二茂铁鎓基团的还原发生在仍显示二茂铁基ΔE = Epa,单体 - Epc,二聚体≤ 78 mV的电位下，这表明硫醇上二茂铁基的氧化还原性质与二聚体的非常相似。¹H NMR测量表明，与二茂铁基氧化一样，巯基质子SH等的共振位置取决于 - (CH₂)ₙ - 链长。在金上生成自组装单分子层（SAMs）以研究1 - 4在无扩散情况下的电化学行为。在这些条件下，对于较长链衍生物，在慢扫描速率下ΔE接近0 mV。金属硫醚Fc(CH₂)ₙ - S - Au的表面结合二茂铁基在与等效的CH₂Cl₂溶解的二茂铁基物种1 - 4大致相同的电位下被氧化。SAMs的表面覆盖率取决于烷基链长，4的覆盖率最大，而对于最短链衍生物Fc - CH₂ - S - Au，SAM底物与电极之间的异相电子转移速率最快。
