Challenges in the Structure Determination of Self-Assembled Metallacages: What Do Cage Cavities Contain, Internal Vapor Bubbles or Solvent and/or Counterions?
Challenges in the Structure Determination of Self-Assembled Metallacages: What Do Cage Cavities Contain, Internal Vapor Bubbles or Solvent and/or Counterions?
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DOI：
10.1021/jacs.5b12050
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发表时间：
2016-05-25
影响因子：
15
通讯作者：
Michl, Josef
中科院分区：
文献类型：
--
作者：
Givelet, Cecile C.;Dron, Paul I.;Michl, Josef
证明通过金属离子配位驱动的溶液自组装获得的带电金属络合物的结构具有挑战性，并且常规核磁共振波谱和质谱的常用方法是不可靠的。通过扩散有序质子磁共振（DOSY NMR）仔细确定了六个尺寸差异很大的笼的扩散系数，使我们提出将两个分子笼的结构重新分配，从以前喜欢的三聚体到五聚体或六聚体，另一个从三聚体到更高的低聚物，可能是一个有趣的十四聚体。在前一种情况下，通过观察最初形成的笼子在稀释后缓慢可逆转变为较小但光谱非常相似的笼子，为重新分配到更大的笼子提供了强有力的支持。在后一种情况下，冷冻断裂透射电子显微照片表明至少一些溶液是胶体的，并且高分辨率电子透射和原子力显微镜图像与十四聚体兼容，但与三聚体不兼容。将溶液密度测量推导出的溶质偏摩尔体积与分子模型预测的体积进行比较，强烈反对溶解在硝基甲烷中的笼中存在大空隙（溶剂蒸气泡）。先前提出了气泡的存在，试图解释其中一个笼中吡啶配体边缘交换反应的速率常数的艾林图的双线性性质以及高温状态下不寻常的活化参数。现在提出了另一种解释。
Proving the structures of charged metallacages obtained by metal ion coordination-driven solution self-assembly is challenging, and the common use of routine NMR spectroscopy and mass spectrometry is unreliable. Carefully determined diffusion coefficients from diffusion-ordered proton magnetic resonance (DOSY NMR) for six cages of widely differing sizes lead us to propose a structural reassignment of two molecular cages from a previously favored trimer to a pentamer or hexamer, and another from a trimer to a much higher oligomer, possibly an intriguing tetradecamer. In the former case, strong support for the reassignment to a larger cage is provided by an observation of a slow reversible transformation of the initially formed cage into a smaller but spectrally very similar one upon dilution. In the latter case, freeze-fracture transmission electron micrographs demonstrate that at least some of the solutions are colloidal, and high-resolution electron transmission and atomic force microscopy images are compatible with a tetradecamer but not a trimer. Comparison of solute partial molar volumes deduced from measurement of solution density with volumes anticipated from molecular models argues strongly against the presence of large voids (solvent vapor bubbles) in cages dissolved in nitromethane. The presence of bubbles was previously proposed in an attempt to account for the bilinear nature of the Eyring plot of the rate constant for pyridine ligand edge exchange reaction in one of the cages and for the unusual activation parameters in the high temperature regime. An alternative interpretation is proposed now.