Metal coordination to assist molecular gelation
Metal coordination to assist molecular gelation
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DOI：
10.1002/anie.200503704
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发表时间：
2006-01-01
影响因子：
16.6
通讯作者：
Fages, F
中科院分区：
文献类型：
--
作者：
Fages, F
分子凝胶是近年来发展起来的一种新型纳米材料，具有广阔的应用前景。[1]它们的形成源于低分子量化合物自发但受控的自组装成纤维状结构，纤维状结构反过来又形成了包裹溶剂分子的缠结三维网络。与其聚合物对应物的情况相反，分子凝胶涉及具有明确定义的化学结构的离散分子组分。因此，胶凝因子骨架的化学组成的轻微变化允许聚集体的形态、手性和尺寸，以及最终凝胶的宏观性质被精细地控制和调节。因此，分子凝胶的形成代表了一种用于制造纳米级材料的优雅的自下而上的方法。到目前为止，存在丰富多样的有机和水凝胶分子结构，能够分别在极低浓度下使有机流体或水凝胶化。[1]这种系统的特征在于通过多个非共价键的组合单向自组装的强烈倾向。色散力，π-π堆积，静电相互作用，主要是氢键已被大量利用。尽管金属配位在超分子化学中起着重要作用，[2]令人惊讶的是，直到最近，金属-配体相互作用在分子凝胶领域的应用才被忽视。脂肪酸的金属配合物[3]、卟啉[4]和酞菁[5]早已被认为可以产生凝胶，但这些例子代表了迄今为止可用的配体结构的巨大库中的相当狭窄的选择。在过去的两年里，人们对使用配位化学作为具有不寻常功能特性的自组装凝胶的合理设计路线产生了浓厚的兴趣。金属离子与含有附加配体位点的胶凝剂分子的结合可以影响自聚集模式并允许精细地调节胶凝能力。这种金属响应性有机胶凝剂的早期实例由胆固醇基胶凝剂1提供，其含有冠醚部分。事实上，1在甲基环己烷/苯中的凝胶化温度被证明取决于添加的碱金属阳离子的浓度。[6]化合物2具有两个通过长烷基链连接到双-（酰胺基）环己烷凝胶支架的β-二酮配体，其在甲醇中形成弱凝胶，在添加二价过渡金属离子后变得坚固。[7]结果表明，双（β-二酮基）铜（II）单元的形成起到了交联自组装纤维的作用。相反，金属络合可以诱导胶凝能力的完全丧失，如在含有菲咯啉3 [8a]或2，2 ′-联吡啶4 [8b]部分的胶凝剂的情况下所观察到的。自由配体的甘氨酰化需要在酰胺基团之间建立扩展的分子间氢键阵列。在金属络合时，后者成为分子内缔合的，这抑制了分子间网络的形成，从而抑制了凝胶化。与此同时-
Molecular gels have recently evolved as a fascinating class of smart and functional nanoscale materials with high potential for a wide range of advanced applications.[1] Their formation stems from the spontaneous but controlled self-assembly of low-molecular-weight compounds into fibrous architectures, which, in turn, form entangled threedimensional networks entrapping solvent molecules. In contrast to the case of their polymeric counterparts, molecular gels involve discrete molecular components with well-defined chemical structures. As a consequence, slight changes in the chemical composition of the backbone of the gelator allow the morphology, chirality, and size of the aggregates, and ultimately the macroscopic properties of the gel, to be exquisitely controlled and tuned. As such, the formation of molecular gels represents an elegant bottom-up approach for fabricating nanoscale materials. So far, there exists a rich variety of organo-and hydrogelator molecular structures that are able to immobilize organic fluids or water, respectively, at extremely low concentrations.[1] Such systems are characterized by a strong propensity to self-assemble unidirectionally through a combination of multiple noncovalent bonds. Dispersion forces, π–π stacking, electrostatic interactions, and mainly hydrogen bonding have been largely exploited. Despite the major role that metal coordination plays in supramolecular chemistry,[2] it is surprising that the use of metal–ligand interactions in the area of molecular gels has been somewhat neglected until very recently. Metal complexes of fatty acids,[3] porphyrins,[4] and phthalocyanins [5] have been long known to produce gels, but these examples represent a rather narrow selection from the huge library of ligand structures available to date. In the last two years, there has been a surge of interest in the use of coordination chemistry as a rational design route toward self-assembled gels with unusual functional properties. Binding of a metal ion to a gelator molecule containing an appended ligand site can affect self-aggregation modes and allow the gelation ability to be finely tuned. An early example of such metal-responsive organogelators was provided by the cholesterol-based gelator 1, which contains a crown ether moiety. Indeed, the gelation temperature of 1 in methylcyclohexane/benzene was shown to depend on the concentration of alkali metal cations added.[6] Compound 2, which has two β-diketonate ligands linked to a bis-(amido) cyclohexane gelating scaffold through long alkyl chains, formed weak gels in methanol that became robust upon addition of divalent transitionmetal ions.[7] It was suggested that the formation of bis (β-diketonato) copper (ii) units served to cross-link the self-assembled fibers. Metal complexation can instead induce the complete loss of gelation ability, as observed in the case of gelators that contain either phenanthroline 3 [8a] or 2, 2о-bipyridine 4 [8b] moieties. Gelation by the free ligands requires the establishment of extended arrays of intermolecular hydrogen bonds between amide groups. Upon metal complexation, the latter become associated intramolecularly, which inhibits the formation of intermolecular networks and thereby gelation. In con-