Notestein Justin M, Andrini Leandro R, Kalchenko Vitaly I, Requejo Felix G, Katz Alexander, Iglesia Enrique
Department of Chemical Engineering, University of California at Berkeley, Berkeley, California 94720, USA.
J Am Chem Soc. 2007 Feb 7;129(5):1122-31. doi: 10.1021/ja065830c.
Calixarene-Ti complexes were grafted onto SiO2 (0.18-0.24 Ti nm-2) to form isolated and accessible Ti centers persistently coordinated to multidentate calixarene ligands. Grafted Ti-tert-butylcalix[4]arenes gave Ti K-edge absorption spectra with pre-edge features at 4968.6-4968.9 eV, independently of Ti surface density and of their use in epoxidation catalysis. The structure and reactivity of grafted Ti-calix[4]arenes were weakly dependent on thermal treatment below 573 K, and the relative epoxidation rates of trans- and cis-alkenes showed that calixarene ligands did not restrict access to Ti centers more than corresponding calcined Ti-SiO2 materials. For all materials, 13C NMR and UV-visible spectroscopies confirmed the presence of Ti-O-Si connectivity and identical ligand-to-metal transitions. Grafted Ti-homooxacalix[3]arene complexes, however, gave weaker pre-edge features at higher energies ( approximately 4969.5 eV), consistent with greater Ti 3d occupancy and coordination numbers greater than four, and 20-fold lower cyclohexene epoxidation rate constants (per Ti) than on calix[4]arene-based materials. These different rates and near-edge spectra result from aldehyde formation caused by unimolecular cleavage of ether linkages in homooxacalix[3]arene ligands during grafting, leading to higher coordination and electron density at Ti centers. Materials based on tert-butylcalix[4]arene and homooxacalix[3]arenes led to similar epoxidation rates and near-edge spectra after calcination, consistent with the conversion of both materials to isolated Ti centers with identical structure. These materials provide a systematic approach for relating oxidation reactivity to Ti 3d occupancy, a descriptor of Lewis acid strength, and Ti coordination, because they provide Ti centers with varying electron density and coordination, but maintain accessible active centers with uniform structure and unrestricted access to reactants.
杯芳烃 - 钛配合物被接枝到二氧化硅（0.18 - 0.24 Ti nm-2）上，形成与多齿杯芳烃配体持续配位的孤立且可及的钛中心。接枝的叔丁基杯[4]芳烃钛给出的钛K边吸收光谱在4968.6 - 4968.9 eV处有预边特征，与钛表面密度及其在环氧化催化中的用途无关。接枝的钛 - 杯[4]芳烃的结构和反应性对573 K以下的热处理弱依赖，反式和顺式烯烃的相对环氧化速率表明杯芳烃配体对钛中心的可及性限制不比相应的煅烧钛 - 二氧化硅材料更大。对于所有材料，13C NMR和紫外 - 可见光谱证实了钛 - 氧 - 硅连接性的存在以及相同的配体 - 金属跃迁。然而，接枝的钛 - 同氧杂杯[3]芳烃配合物在更高能量（约4969.5 eV）处给出较弱的预边特征，这与更大的钛3d占据和大于4的配位数一致，并且环己烯环氧化速率常数（每钛）比基于杯[4]芳烃的材料低20倍。这些不同的速率和近边光谱是由于在接枝过程中同氧杂杯[3]芳烃配体中的醚键单分子裂解导致醛的形成，从而导致钛中心处更高的配位和电子密度。基于叔丁基杯[4]芳烃和同氧杂杯[3]芳烃的材料在煅烧后导致相似的环氧化速率和近边光谱，这与两种材料都转化为具有相同结构的孤立钛中心一致。这些材料提供了一种系统的方法，用于将氧化反应性与钛3d占据（路易斯酸强度的描述符）和钛配位相关联，因为它们为钛中心提供了不同的电子密度和配位，但保持了具有均匀结构且反应物可及不受限制的可及活性中心。
