Complex Structural Landscape of Titanium Organophosphonates: Isolation of Structurally Related Ti4, Ti5, and Ti6 Species and Mechanistic Insights.

Abstract

[Ti(acac)2(OiPr)2] reacts with tert-butylphosphonic acid to yield a series of titanium organophosphonates such as tetranuclear [Ti4(acac)4(μ-O)2(μ-tBuPO3)2(μ-tBuPO3H)4]·2CH3CN (1), pentanuclear [Ti5(acac)5(μ-O)2(OiPr)(μ-tBuPO3)4(μ-tBuPO3H)2] (2), hexanuclear [Ti6(acac)6(μ-O)2(OiPr)2(μ-tBuPO3)6] (3), or [Ti6(acac)6(μ-O)3(OiPr)(μ-tBuPO3)5(μ-tBuPO3H)]·2CH3CN (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 CH2Cl2. 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)(OiPr)2(tBuPO3H)]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 of tBuPO3H2 was performed with [Ti(acac)2(O)], instead of Ti(acac)2(OiPr)2, in CH3CN to yield [Ti6(acac)6(μ-O)4(μ-tBuPO3)4(μ-tBuPO3H)2]·2CH3CN (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.