Mono- and Dinuclear Oxovanadium(V)calixarene Complexes and Their Activity as Oxidation Catalysts

Abstract

The background of the investigation is constituted by reactive moieties and intermediates playing relevant roles on the surfaces of vanadiumoxide-based catalysts during the oxygenation/dehydrogenation of organic substrates. With the aim of modeling such species, a series of mono- and dinuclear charged and uncharged vanadium oxo complexes containing p-tert-butylated calix[4]arene and calix[8]arene ligands (denoted H4B and H8B' ', respectively, in the protonated forms) has been synthesized and characterized: PPh4[O=VB] ((PPh41), O=VB(OAc) (2), PPh4[O2V2HB' '] (3), and [mu-O(O=V(OMe))2B(Me2)] (4), where superscripts OAc and Me2 indicate that one or two protons of H4B are substituted by these residues, respectively. These compounds were analyzed both in solution and by means of single-crystal X-ray crystallography; it turned out that the crystal structures are retained on dissolution (2 changed only from the paco to the cone structure). In the case of 4, it could be shown that the bulk product consists of a mixture of two isomers (4t and 4c) differing in the relative positions of the vanadium-bound methoxy groups. Subsequently, all compounds were tested as catalysts for the oxidation of alcohols with O2. It turned out that the two dinuclear complexes efficiently catalyze the oxidation of 1-phenyl-1-propargyl alcohol and fluorenol; in addition, they even show some activity with respect to the oxidation of dihydroanthracene. This may hint to a higher activity of dinuclear sites on the surfaces of heterogeneous catalysts as well.

From Surface‐Inspired Oxovanadium Silsesquioxane Models to Active Catalysts for the Oxidation of Alcohols with O2—The Cinnamic Acid/Metavanadate System

“…Naturally, the results of corresponding studies can also provide a stimulus for the synthesis of molecular model compounds. [3,4] These can then be investigated in the homogeneous phase and may serve to support or disprove proposals made concerning the structure, function, or the spectroscopic signatures of corresponding surface species. The construction of molecular models for metal-oxo surface sites requires ligands that are capable of mimicking the environments that metaloxo moieties experience on oxidic support or bulk materials, and for the modeling of silica supports in particular silsesquioxanebased ligands have established themselves.…”

Calix[8]arenes Solid-State Structures: Derivatization and Crystallization Strategies

“…The bimetallic V 5+ -imido (imido = N-p-tolyl 2-) complex has a structure similar to that reported for the Ti 4+ complex described above, with bridging phenoxides in transpositions relative to the imido groups, thus precluding coordination of acetonitrile solvent molecules (Gibson et al, 2001). Likewise, the analogous vanadyl derivative is characterized by bridging phenoxides trans to the oxo ligands (Hoppe et al, 2006); this configuration appears to preclude the presence of voids, thus favoring crystallization from acetonitrile solution upon cooling. The niobium complex reported was prepared from NbCl 5 as metal source, and crystallized from hot acetonitrile solution as two polymorphs, only one of which was refined as (NbCl 2 ) 2 (p-tert-butylcalix[8]arene-6H).…”

Ab Initio Investigating the Bonding and Electronic Structure in Bismuth Calixarene Complexes

“…As shown in the diagram, calixarenes can change not only the R-group, but also the oxygen to ester to accommodate the different oxidation states of metals. As a result, chemists have been focusing on synthesis, fine-tuning, and structural characterization of new metallocalixarenes for their potential applications in homogeneous catalysts and novel materials with transition metals and heavy main group elements [4][5][6][7]. While the bismuth complex with aryloxides and alkoxides demonstrated their extensive applications in catalysts, medicine, superconductor and ferroelectric materials [8][9][10], the recent work by Hanna et al showed the different kind of binding mode of bismuth Calixarenes complexes and their chemistry properties are quite different from that of their lighter congeners [2,[11][12][13][14][15].…”