Complex Structural Landscape of Titanium Organophosphonates: Isolation of Structurally Related Ti 4 , Ti 5 , and Ti 6 Species and Mechanistic Insights

• Published in: Inorganic chemistry 2017-11, Vol.56 (21), p.12848-12858
• Main Authors: Sharma, Kamna, Antony, Rajendran, Kalita, Alok Ch, Gupta, Sandeep Kumar, Davis, Paul, Murugavel, Ramaswamy
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.7b01651

[Ti(acac) (O Pr) ] reacts with tert-butylphosphonic acid to yield a series of titanium organophosphonates such as tetranuclear [Ti (acac) (μ-O) (μ- BuPO ) (μ- BuPO H) ]·2CH CN (1), pentanuclear [Ti (acac) (μ-O) (O Pr)(μ- BuPO ) (μ- BuPO H) ] (2), hexanuclear [Ti (acac) (μ-O) (O Pr) (μ- BuPO ) ] (3), or [Ti (acac) (μ-O) (O Pr)(μ- BuPO ) (μ- BuPO H)]·2CH CN (4). The isolation of each of these products in pure form depends on the molar ratio of the reactants or the solvent medium. Among these, 3 is obtained as the only product when the reaction is conducted in CH Cl . The structural analysis reveals that a simple cluster growth route relates the clusters 1-4 to each other and that a reactive cyclic single-4-ring titanophosphonate [Ti(acac)(O Pr) ( BuPO H)] is the fundamental building block. While the tetranuclear 1 has structural resemblance to the D4R building block of zeolites, the hexanuclear clusters 3 and 4 have the shape of zeolitic D6R building blocks. The presence of adventitious water in the phosphonic acid (arising from small quantities of hydrogen-bonded water) results in the formation of μ-O bridges across an adjacent pair of titanium centers in clusters 1-4. To further verify the stability of the hexanuclear cluster over other structural forms, the reaction of BuPO H was performed with [Ti(acac) (O)], instead of Ti(acac) (O Pr) , in CH CN to yield [Ti (acac) (μ-O) (μ- BuPO ) (μ- BuPO H) ]·2CH CN (5). Compound 5 exhibits a core structure similar to those of 3 and 4 with small variations in the intracluster Ti-O-Ti linkage. Compound 3 is an efficient and selective catalyst for olefin epoxidation under both homogeneous and heterogeneous conditions.