Abstract
The detailed catalytic mechanism of a series of paddlewheel complexes [Mo2L4] featuring Mo-Mo quadruply-bond on radical addition of CCl4 to 1-hexene was studied using density functional theory. Different ligands of Mo-Mo bond are investigated to illustrate the ligand effect on the catalytic activity. The results show that the Mo-Mo quadruply-bond paddlewheel complexes have high catalytic activities on the title reaction. The whole reaction involves 4 steps. Firstly, the C-Cl bond of first CCl4 is activated by [Mo2L4] catalyst, and [Mo2L3Cl] and CH3COOCCl3 are obtained. Then the second CCl4 adds to [Mo2L3Cl] to produce [Mo2L3Cl2] and·CCl3 radical;·CCl3 radical interacts with 1-hexene to get an addition, the addition product which reacts with one Cl atom of [Mo2L3Cl2] to get the last product nBuCHClCH2CCl3 and regenerate [Mo2L3Cl]. The addition of the first CCl4 to [Mo2L4] catalyst is the rate-determining step of the whole reaction. Because this step is not in the catalytic cycle, the reaction would speed up after a certain period of time. The catalytic activity of dimolybdenum paddlewheel complex is depended on the natural population analysis (NPA) charge of Mo and the redox potential E(Mo24+/Mo25+). The higher NPA of Mo atom and higher E(Mo24+/Mo25+) of the catalyst, the higher catalytic activity it has. Our results provide an explanation for experimental observations and useful insights for further development of bimetallic catalysts in radical addition reactions.
Highlights
Since the first dimolybdenum complex (molybdenum(II) acetate) containing Mo-Mo multiple bond was isolated and characterized [1], a large number of polymetal-metal bonding complexes have been synthesized [2,3,4,5,6,7,8]
The results show that the Mo-Mo quadruply-bond paddlewheel complexes have high catalytic activity on the title reaction
The catalytic activity of dimolybdenum paddlewheel complex is depended on the natural population analysis (NPA) charge of Mo and the redox potential E(Mo24+/Mo25+)
Summary
Since the first dimolybdenum complex (molybdenum(II) acetate) containing Mo-Mo multiple bond was isolated and characterized [1], a large number of polymetal-metal bonding complexes have been synthesized [2,3,4,5,6,7,8]. The Mo2L4 paddlewheel compounds possess a quadruple metal-metal bond (σ2π4δ2) supported by four bridging ligands, such as carboxylates, amides, pyrrolidinates, etc [2, 22,23,24] These compounds exhibits one-electron redox properties in radical transformation processes, which can result in reductively cleaving a carbon-halogen bond in organic halides to generate organic radicals [2, 4, 18, 24,25,26]. Mashima et al showed that Mo2(OCOAr) (Ar=2,4,6-iPr3C6H2) was used for a catalytic radical addition reaction of CCl4 to 1-hexene in THF-d8 at 80°C to give 1, 1, 1, 3-tetrachloroheptane regioselectively in 84% yield They found that the redox properties of [Mo2] complexes are changed by replacing the ligands, the catalytic activity for the radical addition reaction strongly depended on the redox potential of the [Mo2] complexes [4]. Patmore found that the electronic structure of Mo-Mo quadruple bonds can be tuned through O/S substitution of N by lowering the HOMO energy of Mo2 and reducing the Mo24+/5+ oxidation potential [27]
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