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

[Ti(acac)2 (Oi Pr)2 ] reacts with tert-butylphosphonic acid to yield a series of titanium organophosphonates such as tetranuclear [Ti4 (acac)4 (μ-O)2 (μ-t BuPO3 )2 (μ-t BuPO3 H)4 ]·2CH3 CN (1), pentanuclear [Ti5 (acac)5 (μ-O)2 (Oi Pr)(μ-t BuPO3 )4 (μ-t BuPO3 H)2 ] (2), hexanuclear [Ti6 (acac)6 (μ-O)2 (Oi Pr)2 (μ-t BuPO3 )6 ] (3), or [Ti6 (acac)6 (μ-O)3 (Oi Pr)(μ-t BuPO3 )5 (μ-t BuPO3 H)]·2CH3 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 CH2 Cl2 . 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)(Oi Pr)2 (t BuPO3 H)]2 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-O2- 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 oft BuPO3 H2 was performed with [Ti(acac)2 (O)], instead of Ti(acac)2 (Oi Pr)2 , in CH3 CN to yield [Ti6 (acac)6 (μ-O)4 (μ-t BuPO3 )4 (μ-t BuPO3 H)2 ]·2CH3 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.