Vanadyl cationic complexes as catalysts in olefin oxidation

resumo

Three new mononuclear oxovanadium(IV) complexes [VO(acac)(R-BIAN)]Cl (BIAN = 1,2-bis{(R-phenyl)imino} acenaphthene, R = H, 1; CH3, 2; Cl, 3) were prepared and characterized. They promoted the catalytic oxidation of olefins such as cyclohexene, cis-cyclooctene, and styrene with both tbhp (tert-butylhydroperoxide) and H2O2, and of enantiopure olefins (S(-)- and R(+)-pinene, and S(-)- and R(+)-limonene) selectively to their epoxides, with tbhp as the oxidant. The TOFs for styrene epoxidation promoted by complex 3 with H2O2 (290 mol mol(-1)V h(-1)) and for cis-cyclooctene epoxidation by 2 with tbhp (248 mol mol(V)(-1) h(-1)) are particularly good. Conversions reached 90% for several systems with tbhp, and were lower with H2O2. A preference for the internal C=C bond, rather than the terminal one, was found for limonene. Kinetic data indicate an associative process as the first step of the reaction and complex [VO(acac)(H-BIAN)](+) (1(+)) was isolated in an FTICR cell after adding tbhp to 1. EPR studies provide evidence for the presence of a V(IV) species in solution, until at least 48 hours after the addition of tbhp and cis-cyclooctene, and cyclic voltammetry studies revealed an oxidation potential above 1 V for complex 1. DFT calculations suggest that a [VO(H-BIAN)(MeOO)](+) complex is the likely active V(IV) species in the catalytic cycle from which two competitive mechanisms for the reaction proceed, an outer sphere path with an external attack of the olefin at the coordinated peroxide, and an inner sphere mechanism starting with a complex with the olefin coordinated to vanadium.

palavras-chave

MOLECULAR-ORBITAL METHODS; EFFECTIVE CORE POTENTIALS; TERT-BUTYL HYDROPEROXIDE; CONTINUUM SOLVATION MODELS; OXYGEN-TRANSFER REACTIONS; TRANSITION-METAL ATOMS; GAUSSIAN-BASIS SETS; AB-INITIO; OXOVANADIUM(IV) COMPLEXES; EQUILIBRIUM GEOMETRIES

categoria

Chemistry

autores

Nunes, CD; Vaz, PD; Felix, V; Veiros, LF; Moniz, T; Rangel, M; Realista, S; Mourato, AC; Calhorda, MJ

nossos autores

agradecimentos

We thank FCT, POCI, and FEDER for the funding (projects POCI/QUI/58925/2004, PEst-OE/QUI/UI0100/2014, and PEst-OE/QUI/UI0612/2014). The authors also thank the Portuguese National Mass Spectrometry Network (REDE/1501/REM/2005). The Bruker ELEXYS EPR spectrometer was purchased under the framework of QREN, through project NORTE-07-0162-FEDER-000048. The authors are greatly indebted to all the financing sources.

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