Oxovanadium(V) Tetrathiacalix[4]arene Complexes and Their Activity as Oxidation Catalysts

With the aim of modeling reactive moieties and relevant intermediates on the surfaces of vanadium oxide based catalysts during oxygenation/dehydrogenation of organic substrates, mono‐ and dinuclear vanadium oxo complexes of doubly deprotonated p‐tert ‐butylated tetrathiacalix[4]arene (H 4 TC ) have been synthesized and characterized: PPh 4 [(H 2 TC )VOCl 2 ] ( 1 ) and (PPh 4 ) 2 [{(H 2 TC )V(O)(μ‐O)} 2 ] ( 2 ). According to the NMR spectra of the dissolved complexes they both retain the structures adopted in the crystalline state, as revealed by single‐crystal X‐ray crystallography. Compounds 1 and 2 were tested as catalysts for the oxidation of alcohols with O 2 at 80 °C. Both 1 and 2 efficiently catalyze the oxidation of benzyl alcohol, crotyl alcohol, 1‐phenyl‐1‐propanol, and fluorenol, and in most cases dinuclear complex 2 is more active than mononuclear complex 1 . Moreover, the two thiacalixarene complexes 1 and 2 are in many instances more active than oxovanadium(V) complexes containing “classical” calixarene ligands tested previously. Complexes 1 and 2 also show significant activity in the oxidation of dihydroanthracene. Further investigations led to the conclusion that 1 acts as precatalyst that is converted to the active species PPh 4 [( TC )VO] ( 3 ) at 80 °C by double intramolecular HCl elimination. For complex 2 , the results of mechanistic investigations indicated that the oxidation chemistry takes place at the bridging oxo ligands and that the two vanadium centers cooperate during the process. The intermediate (PPh 4 ) 2 [{H 2 TC V(O)} 2 (μ‐OH)(μ‐OC 13 H 9 )] ( 4 ) was isolated and characterized, also with respect to its reactivity, and the results afforded a mechanistic proposal for a reasonable catalytic cycle. The implications which these findings gathered in solution may have for oxidation mechanisms on the surfaces of V‐based heterogeneous catalysts are discussed.