Mechanism of Mechanochemical C–H Bond Activation in an Azobenzene Substrate by Pd(II) Catalysts

Bjelopetrović, Alen; Lukin, Stipe; Halasz, Ivan; Užarević, Krunoslav; Đilović, Ivica; Barišić, Dajana; Budimir, Ana; Juribašić, Marina; Ćurić, Manda (2018) Mechanism of Mechanochemical C–H Bond Activation in an Azobenzene Substrate by Pd(II) Catalysts. Chemistry - A European Journal, 24 (42). pp. 10672-10682. ISSN 0947-6539

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Official URL: https://doi.org/10.1002/chem.201802403

Abstract

Mechanism of solid-state C–H bond activation by various Pd(II) catalysts under milling conditions has been studied by in situ Raman spectroscopy. Common Pd(II) precursors, i.e. PdCl2, [Pd(OAc)2]3, PdCl2(MeCN)2 and [Pd(MeCN)4][BF4]2, have been employed for solid-state activation of one or two C–H bonds in an asymmetric azobenzene substrate. The C–H activation was achieved by all used Pd(II) precursors and their reactivity increases in the order [Pd(OAc)2]3 < PdCl2(MeCN)2 < PdCl2 < [Pd(MeCN)4][BF4]2. In situ Raman monitoring in combination with stepwise ex situ NMR, IR and PXRD experiments has provided direct probing of the reaction mechanism and kinetics, and revealed how liquids of different acid-base properties and proticity as well as selected solids used as additives modify precursors or intermediates and their reactivity. Reaction intermediates that were isolated and structurally characterized agree with the observed species during reaction. In situ Raman spectroscopy has also enabled the derivation of reaction profiles suggesting an electrophilic process which proceeds via a coordination complex (adduct) undergoing deprotonation by a bound or an external base depending on the used Pd(II) precursor. Slow step of the first palladation for two chloride precursors and [Pd(MeCN)4][BF4]2 is the C–H bond cleavage whereas palladation using [Pd(OAc)2]3 depends primarily on breaking of its trimeric structure by the azobenzene substrate and/or liquid additives.

Item Type:

Article

Additional Information:

We acknowledge Mr. Sc. Zeljko Marinic and Nikolina Visic for NMR measurements. Computations were done on the Isabella cluster at SRCE, Zagreb. This work has been supported by the Croatian Science Foundation under projects IP-2014-09-7984 and UIP-2014-09-4744. Alen Bjelopetrovic and Stipe Lukin have been supported by the Croatian Science Foundation.

Uncontrolled Keywords:

mechanochemistry; organometals; palladium; C-H bond activation

Subjects:

NATURAL SCIENCES > Chemistry
NATURAL SCIENCES > Chemistry > Inorganic Chemistry

Divisions:

Division of Physical Chemistry

Projects: