Bitter Stefan, Kunkel Marius, Burkart Lisa, Mang André, Winter Rainer F, Polarz Sebastian
University of Konstanz, Universitätsstrasse 10, 78457 Konstanz, Germany.
ACS Omega. 2018 Aug 31;3(8):8854-8864. doi: 10.1021/acsomega.8b01405. Epub 2018 Aug 9.
Surfactants are functional molecules comprising a water-compatible head group and a hydrophobic tail. One of their features is the formation of self-assembled structures in contact with water, for instance, micelles, vesicles, or lyotropic liquid crystals. One way to increase the functionality of surfactants is to implement moieties containing transition-metal species. Ferrocene-based surfactants represent an excellent example because of the distinguished redox features. In most existing ferrocene-based amphiphiles, an alkyl chain is classically used as the hydrophobic tail. We report the synthesis and properties of 1-triisopropylsilylethynyl-1'-trimethylammoniummethylferrocene (FcNMeTIPS). In FcNMeTIPS, ferrocene is part of the head group (Gemini design) but is also attached to a (protected) π-conjugated ethynyl group. Although this architecture differs from that of classical amphiphiles and those of other ferrocene-based amphiphiles, the compound shows marked surfactant properties comparable to those of lipids, exhibiting a very low value of critical aggregation concentration in water (cac = 0.03 mM). It forms classical micelles only in a very narrow concentration range, which then convert into monolayer vesicles. Unlike classical surfactants, aggregates already form at a very low concentration, far beneath that required for the formation of a monolayer at the air-water interface. At even higher concentration, FcNMeTIPS forms lyotropic liquid crystals, not only in contact with water, but also in a variety of organic solvents. As an additional intriguing feature, FcNMeTIPS is amenable to a range of further modification reactions. The TIPS group is easily cleaved, and the resulting ethynyl function can be used to construct heterobimetallic platinum-ferrocene conjugates with -Pt(PEt)X (X = Cl, I) complex entities, leading to a heterobimetallic surfactant. We also found that the benzylic α-position of FcNMeTIPS is rather reactive and that the attached ammonium group can be exchanged by other substituents (e.g., -CN), which offers additional opportunities for further functionalization. Although FcNMeTIPS is reversibly oxidized in voltammetric and UV-vis spectroelectrochemical experiments, the high reactivity at the α-position is also responsible for the instability of the corresponding ferrocenium ion, leading to a polymerization reaction.
表面活性剂是一类功能分子，由亲水头基和疏水尾组成。其特点之一是在与水接触时能形成自组装结构，例如胶束、囊泡或溶致液晶。增加表面活性剂功能的一种方法是引入含过渡金属物种的部分。基于二茂铁的表面活性剂就是一个很好的例子，因为其具有独特的氧化还原特性。在大多数现有的基于二茂铁的两亲分子中，烷基链通常用作疏水尾。我们报道了1-三异丙基硅乙炔基-1'-三甲基铵甲基二茂铁（FcNMeTIPS）的合成及其性质。在FcNMeTIPS中，二茂铁是头基的一部分（双子星设计），但也连接到一个（受保护的）π共轭乙炔基上。尽管这种结构与传统两亲分子以及其他基于二茂铁的两亲分子不同，但该化合物表现出与脂质相当的显著表面活性剂性质，在水中的临界聚集浓度非常低（cac = 0.03 mM）。它仅在非常窄的浓度范围内形成经典胶束，然后转变为单层囊泡。与传统表面活性剂不同，聚集体在极低浓度下就已形成，远低于在气-水界面形成单分子层所需的浓度。在更高浓度下，FcNMeTIPS不仅在与水接触时形成溶致液晶，还能在多种有机溶剂中形成。作为另一个有趣的特性，FcNMeTIPS适合进行一系列进一步的修饰反应。TIPS基团很容易裂解，生成的乙炔官能团可用于构建与 -Pt(PEt)X（X = Cl，I）络合物实体的异双金属铂-二茂铁共轭物，从而得到一种异双金属表面活性剂。我们还发现，FcNMeTIPS的苄基α位相当活泼，连接的铵基团可被其他取代基（例如 -CN）取代，这为进一步功能化提供了额外的机会。尽管在伏安法和紫外-可见光谱电化学实验中，FcNMeTIPS可被可逆氧化，但α位的高反应活性也导致相应二茂铁鎓离子不稳定，引发聚合反应。
