Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California, USA.
Dalton Trans. 2013 Sep 21;42(35):12762-71. doi: 10.1039/c3dt50804h.
We investigate the synthesis of accessible calix[4]arene-bound gold clusters consisting of open "coordinatively unsaturated" active sites, using a comparative approach that relies on calix[4]arene ligands with various upper- and lower-rim substituents. In contrast with a reported Au(I)-tert-butyl-calixarene phosphine complex, which exhibits a single cone conformer in solution, the H upper-rim analog exhibits multiple conformers in solution. This contrasts with observations of the tert-butyl upper-rim analog, which exhibits a single cone conformer in solution under similar conditions. In the solid state, as determined by single-crystal X-ray diffraction, both H and tert-butyl upper-rim analogs exhibit exclusively cone conformer. A detailed structural analysis of these two solid-state structures highlights a CH-π interaction involving a methoxy lower-rim substituent and phenyl substituent on P as the key feature that enforces a tight configuration of Au(I) atoms on the same side of the calix[4]arene lower-rim plane. We hypothesize that such a configuration promotes chelation of the ligand to a gold surface and facilitates the synthesis of small Au11-sized clusters after reduction of both complexes. The new cluster, like the one reported with the tert-butyl analog, has an extraordinary 25% of surface atoms that are open and accessible to a 2-NT (2-naphthalenethiol) probe in solution. We also investigated the effect of calix[4]arene lower-rim substituents that coordinate to the metal, by using N-heterocyclic carbene (NHC) functional groups rather than phosphines. Four small (<1.6 nm diameter) calix[4]arene NHC-bound gold clusters were synthesized, including three using novel calix[4]arene NHC ligands. The smallest calix[4]arene NHC-bound Au cluster consisted of a 1.2 nm gold core, and its number density of accessible and open surface sites was measured. This required development of a new titration method for open sites on gold clusters, using a SAMSA fluorescein dye molecule, which excites and emits at lower energy relative to the previously used 2-NT probe. The number density of open sites on the new calix[4]arene NHC-bound gold cluster measured by the SAMSA fluorescein probe strongly supports the generality of a mechanical model of accessibility, which does not depend on the functional group involved in binding to the gold surface and rather depends on the relative radii of curvature of bound ligands and the gold cluster core.
我们研究了由开放的“配位不饱和”活性位点组成的易于接近的杯[4]芳烃结合金簇的合成，使用了一种比较方法，该方法依赖于具有各种上边缘和下边缘取代基的杯[4]芳烃配体。与报道的 Au(I)-叔丁基杯[4]芳烃膦配合物相反，该配合物在溶液中表现出单一的锥形构象，H 上边缘类似物在溶液中表现出多种构象。这与叔丁基上边缘类似物的观察结果形成对比，在相似条件下，该类似物在溶液中表现出单一的锥形构象。在固态中，通过单晶 X 射线衍射确定，H 和叔丁基上边缘类似物均仅表现出锥形构象。对这两种固态结构的详细结构分析突出了涉及甲氧基下边缘取代基和 P 上苯基取代基的 CH-π 相互作用，这是将 Au(I)原子固定在杯[4]芳烃下边缘平面同一侧的关键特征。我们假设这种构象促进了配体与金表面的螯合，并有利于两种配合物还原后小 Au11 尺寸簇的合成。新的簇，与叔丁基类似物报道的簇一样，有 25%的表面原子是开放的，可在溶液中用 2-NT（2-萘硫醇）探针探测。我们还研究了通过使用氮杂环卡宾（NHC）官能团而不是膦来配位金属的杯[4]芳烃下边缘取代基的影响。合成了四个小于 1.6nm 直径的杯[4]芳烃 NHC 结合的金簇，其中三个使用新型杯[4]芳烃 NHC 配体。最小的杯[4]芳烃 NHC 结合的 Au 簇由 1.2nm 金核组成，并且测量了其可及和开放表面位点的数密度。这需要开发一种用于金簇上开放位点的新滴定方法，使用 SAMSA 荧光素染料分子，该分子的激发和发射能量比以前使用的 2-NT 探针低。通过 SAMSA 荧光素探针测量的新型杯[4]芳烃 NHC 结合的金簇上开放位点的数密度强烈支持可及性的机械模型的通用性，该模型不依赖于与金表面结合的官能团，而是取决于结合配体和金簇核的相对曲率半径。
