Synthesis and catalytic property of inorganic-organic hybrid copper vanadate Cu2(eIM)6[V4O12] in selective oxidation of sulfides

Polyoxometallates (POMs) a re familiar inorganic building blocks for their rich structural diversities and potential applications in many fields, such as catalysis, magnetism, and energy, etc. As an important subclass of POMs, vanadates are a promising candidate to construct POMs-based hybrid materials due to their unprecedented structural diversity, mixed valency, and variable coordination geometries. Inorganic-organic hybrid vanadates with first-row transition metals show very rich structural diversities. Further, the hybrid vanadates are expected to exhibit improved properties by the synergetic interaction between the metal-organic subunits and the vanadium oxide. Thus, it is essential but remains challenging to control the synthesis of inorganic-organic hybrid vanadates rationally. Recently, our group reported several examples of inorganic-organic hybrid vanadates by controlling the synthetic reaction conditions and further explored their catalytic properties in the oxidation of sulfides, alcohols and olefin, etc. In order to continue our research in the controlling synthesis and catalytic properties of inorganic-organic hybrid vanadates, here we synthesized a hybrid copper vanadate Cu2(eIM)6[V4O12] (1). In the synthetic procesure, a neutral V6 cluster VIV2VV4O14(eIM)8 (eIM=1-ethylimidazole) which was reported in our earlier research, was selected as vanadium source and reacted with Cu(OAc)2·H2O in the presence of triethylamine by rationally controlling the reaction conditions. Compound 1 was fully characterzized by single-crystal X-ray diffraction (SXRD) and powder X-ray diffraction (PXRD) analyses, FT-IR spectroscopy and element analyses, etc. SXRD analyse shows that the molecular struture of 1 contains a [V4O12]4- anion cluster and two types of copper complex cation, Cu[(eIM)2]2+ and Cu[(eIM)4]2+. All the [V4O12]4- anion clusters were further connected to a two- dimensional (2D) plane network supramolecular structure by four- and six-coordinated copper complexes cations with the eIM ligands. Two types of coordination modes existing in the same tructure is rare in complexes of inorganic-organic hybrid vanadates. Selective oxidation of organic compounds, such as sulfides, alcoholes, aromatic and aliphatic hydrocarbons, etc., is one of the most vital transformations for upgrading raw starting materials into high value added products. Vanadium-containing materials show excellent catalytic performance in selective oxidation of these compounds. Given the importance of sulfoxides and sulfones as intermediates of biologic molecules, chiral auxiliaries and oxo-transfer reagents, the high selective oxidation of sulfides to obtain these compounds is more desirable. As a green oxidant, H2O2 offers significant enviromental and economic benefits over traditional stoichiometric oxidants. First, we selected the oxidation of methyl phenyl sulfide as a model reaction to evaluate the catalytic activities of compound 1. We evaluated the influencing factors of the ammount of catalyst and oxidant, the reaction temperature and time. Under the optimized conditions, the conversion of methyl phenyl sulfide and the selectivity to corresponding sulfoxide up to 98.5% and 97.5%, respectively. Other aromatic sulfides were also examined and no obvious decrease of the catalytic activity was observed when electron donor or acceptor groups with less steric hindrance were introduced in the monophenyl sulfides (conversion from 87.4% to 96.5%; selectivity toward corresponding sulfoxides from 97.5% to 98.5%). When the reaction conditions were applied to aliphatic sulfides including n-butyl sulfide, 2-chloroethyl ethyl sulfide and dimethyl sulfide, the high conversion (from 92.2% to 98.7%) and selectivity toward corresponding sulfoxides (from 83.5% to 95.5%) were also abtained. We also probed the role of each active center of compound 1 and found that the cooperation between copper complex cations and vanadate anions played some synergistic role in the sulfonation process, which can improve the catalytic performance. As a heterogeneous catalyst, 1 was also selected to examine the long-term stability and after at least 3 runs still maintained its activity without significant changes in its structure which characterized by FT-IR spectra and PXRD analyses, respectively. It is worth pointing out that by controlling the catalytic reaction conditions and using the same oxidant, we can prepare sulfones by various sulfides oxidation with excellent conversion and selectivity up to >99% and 97.5%, respectively.