Abstract
The prevalence of metalloenzymes with multinuclear metal complexes in their active sites inspires chemists’ interest in the development of multinuclear catalysts. Studies in this area commonly focus on binuclear catalysts containing either metal-metal bond or electronically discrete, conformationally advantageous metal centres connected by multidentate ligands, while in many multinuclear metalloenzymes the metal centres are bridged through μ2-ligands without a metal-metal bond. We report herein a μ2-iodide-bridged binuclear palladium catalyst which accelerates the C-H nitrosation/annulation reaction and significantly enhances its yield compared with palladium acetate catalyst. The superior activity of this binuclear palladium catalyst is attributed to the trans effect-relay through the iodide bridge from one palladium sphere to the other palladium sphere, which facilitates dissociation of the stable six-membered chelating ring in palladium intermediate and accelerates the catalytic cycle. Such a trans effect-relay represents a bimetallic cooperation mode and may open an avenue to design and develop multinuclear catalysts.
Highlights
The prevalence of metalloenzymes with multinuclear metal complexes in their active sites inspires chemists’ interest in the development of multinuclear catalysts
Two types of bimetallic cooperation are commonly encountered in the catalytic processes of these binuclear catalysts: one is the simultaneous activation of two reaction partners by the binuclear catalysts in which multidentate ligands hold two metal centres in close proximity to create two electronically discrete, conformationally advantageous active sites for binding the corresponding substrate molecules[1,4,5,13,14], as illustrated by olefin polymerization[13] or enantioselective binuclear catalysts[5,14]; the other is that two metal centres of the metal–metal bond-containing binuclear catalyst synergistically participate in elemental redox steps in catalytic pathways[6,9,10,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32], as exemplified by Pd(III) dimer-catalysed oxidative C–H functionalization[17,18,19] and dirhodium-catalysed reactions for carbene[20,21,22] or nitrene insertion[23] into C–H bond
Due to the rapid equilibration of the multinuclear complexes with other metal species in the catalytic conditions, it has been highly challenging to identify the active species and reveal the responsible cooperation mechanism in a definitive manner[7], which is the reason why the promising binuclear or multinuclear metal catalysts are relatively underdeveloped compared with the mononuclear metal catalysts that dominate in homogenous catalysis
Summary
The prevalence of metalloenzymes with multinuclear metal complexes in their active sites inspires chemists’ interest in the development of multinuclear catalysts. The treatment of Pd(OAc)[2] with one equivalent of TBAI and 12 equivalents of 4,4′-di-n-butyl-azobenzene (1b) in DCB at 80 °C for 12 h afforded a soluble analogue of 4a, an iodide-bridged binuclear palladium complex of cyclopalladated 4,4′-di-n-butyl-azobenzene ligand (4b) in 65% yield with concomitant formation of [Pd2I6](n-Bu4N)[247] in 11% yield (Fig. 2b).
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