Article pubs.acs.org/IC
DFT Studies on the Mechanism of the Vanadium-Catalyzed Deoxydehydration of Diols Agustín Galindo* Departamento de Química Inorgánica, Facultad de Química, Universidad de Sevilla, Aptdo 1203, 41071 Sevilla, Spain S Supporting Information *
ABSTRACT: The mechanism of the vanadium-catalyzed deoxydehydration of glycols to alkenes by phosphanes (Chapman, G.; Nicholas, K. M. Chem. Commun. 2013, 49, 8199−8201) has been investigated with density functional calculations. Two alternative pathways, A and B, have been evaluated, and the three stages generally recognized in DODH processes have been identified: (i) activation of the diol by condensation on the vanadium complex and eventual formation of vanadium-glycolate, (ii) oxygen atom transfer to phosphane with reduction to a vanadium(III) species, and, finally, (iii) alkene extrusion from the vanadium-glycolate with regeneration of the starting catalyst. Pathway B, in which the reduction by the phosphane from vanadium(V) to V(III) species occurred before the diol condensation, is energetically preferred to pathway A, in which the diol activation preceded the oxo-transfer to the phosphane reductant.
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INTRODUCTION The development of efficient processes for biomass conversion into added-value chemicals is a topic of extraordinary importance.1 In particular, the metal-catalyzed deoxydehydration (DODH) of vicinal diols into alkenes, by using a reductant (“red” in Scheme 1), is a promising research field.2 Although
workers, and a stage of oxidative cleavage of the diolate ligand has also been proposed.18 Nicholas and Chapman reported in 2013 the deoxydehydration (DODH) of glycols to olefins catalyzed by oxido-vanadium derivatives with triphenylphosphine or sodium sulfite as reductants. 5 The most effective catalyst was [Bu 4 N][VO2(dipic)] complex (dipic = pyridine-2,6-dicarboxylate), and two catalytic pathways were tentatively proposed for such a reaction (Scheme 2). In pathway A, the diol activation by condensation occurs before the oxygen atom transfer to the phosphane reductant, while the opposite situation takes place in pathway B, namely reduction by the phosphane from vanadium(V) to V(III) species and then diol condensation with concomitant oxidation to vanadium(V). Both pathways have in common the final step corresponding to the ethylene extrusion from the vanadium glycolate species. This is the only example of vanadium-catalyzed DODH reaction, and, in order to gain further knowledge about the reaction mechanism, DFT calculations were carried out and the results are reported here.
Scheme 1. Metal-Catalyzed DODH of Vicinal Diols
some molybdenum3,4 and vanadium5 complexes have been reported as catalyst for this reaction, the rhenium derivatives are the DODH catalysts most investigated.2a,b,6 From the first example reported by Cook and Andrews employing Cp*ReO3 as the catalyst with PPh3 as the reductant,7 the expansion of the rhenium-system has been largely increased. New rheniumcatalysts are now known,8 several reductants have been studied,9−11 and the range of substrates2f,12 for DODH has been enlarged. The mechanism of rhenium-catalyzed DODH of diols has been the subject of a number of studies, both from experimental2g,13,14 and computational15−17 points of view, and three stages have been generally identified in this process. They are (i) activation of the diol by condensation to the rhenium complex, (ii) reduction of the rhenium center by an oxygen atom transfer to the reductant, and (iii) extrusion of the alkene from diolate. Recently, the mechanism of Mo-catalyzed DODH of diols has been theoretically investigated by Fristrup and co© XXXX American Chemical Society
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EXPERIMENTAL SECTION
The electronic structure and geometries of the model compounds were computed using density functional theory at the B3LYP level,19 with the 6-311+G* basis set for all the atoms. This combination of method and basis sets provides a good structural description of the vanadium complexes according to the comparison of the structural parameters of the optimized structure of the anion [VO2(dipic)]−, 1, with those of the crystal structures reported in the CSD (codes Received: November 19, 2015
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DOI: 10.1021/acs.inorgchem.5b02649 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Scheme 2. Proposed Catalytic Cycles for the DODH Reaction Catalyzed by [Bu4N][VO2(dipic)]5
Scheme 3. Pathway A: Catalytic Cycle Theoretically Investigated for the DODH of Ethylene Glycol with PPh3 Catalyzed by [VO2(dipic)]−
LEPJEV,20 ODPICV,21 PERGUP,22 PERHAW,22 QAHQAS,23 and YIDQAE24) containing the same anion. Molecular geometries of all model complexes were optimized without symmetry constraints. Frequency calculations were carried out at the same level of theory to identify all of the stationary points as transition states (one imaginary frequency) or as minima (zero imaginary frequencies) and to provide the thermal correction to free energies at 298.15 K and 1 atm. In some cases, a structure was considered a minimum in spite of a very low imaginary frequency (
