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Study of Deactivation in Suzuki Reaction of Polymer-Stabilized Pd Nanocatalysts

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Study of Deactivation in Suzuki Reaction of Polymer-Stabilized Pd Nanocatalysts processes Article Study of Deactivation in Suzuki Reaction of Polymer-Stabilized Pd Nanocatalysts Linda Nikoshvili 1,* , Elena S. Bakhvalova 2, Alexey V. Bykov 1, Alexander I. Sidorov 1, Alexander L. Vasiliev 3, Valentina G. Matveeva 1,2, Mikhail G. Sulman 1, Valentin N. Sapunov 4 and Lioubov Kiwi-Minsker 2,5,* 1 Department of Biotechnology, Chemistry and Standardization, Tver State Technical University, A. Nikitina str., 22, 170026 Tver, Russia; [email protected] (A.V.B.); [email protected] (A.I.S.); [email protected] (V.G.M.); [email protected] (M.G.S.) 2 Regional Technological Centre, Tver State University, Zhelyabova str., 33, 170100 Tver, Russia; [email protected] 3 National Research Centre “Kurchatov Institute”, 123182 Moscow, Russia; [email protected] 4 Department of Chemical Technology of Basic Organic and Petrochemical Synthesis, D. Mendeleev University of Chemical Technology of Russia, Miusskaya pl., 9, 125047 Moscow, Russia; [email protected] 5 Ecole Polytechnique Fédérale de Lausanne, GGRC-ISIC-EPFL, CH-1015 Lausanne, Switzerland * Correspondence: [email protected] (L.N.); lioubov.kiwi-minsker@epfl.ch (L.K.-M.); Tel.: +7-904-005-7791 (L.N.); +41-21-693-3182 (L.K.-M.) Received: 1 December 2020; Accepted: 14 December 2020; Published: 15 December 2020 Abstract: This work is addressed to the phenomenon of catalyst deactivation taking place during the repeated uses in the reaction of Suzuki-Miyaura (S-M) cross-coupling, which is widely applied in industry for C-C bond formation. Ligandless catalysts based on Pd(0) NPs supported on hyper-cross-linked polystyrene (HPS) of two types (non-functionalized and bearing tertiary amino groups) were studied in a model S-M reaction between 4-bromoanisole and phenylboronic acid. Synthesized catalysts were shown to be highly active under mild reaction conditions. HPS allows stabilization of Pd(0) NPs and prevents their agglomeration and detectable Pd leaching. However, the loss of catalytic activity was observed during recycling. The deactivation issue was assigned to the hydrophobic nature of non-functionalized HPS, which allowed a strong adsorption of cross-coupling product during the catalyst separation procedure. A thorough washing of Pd/HPS catalyst by hydrophobic solvent was found to improve to the big extent the observed catalytic activity, while the replacement of non-functionalized HPS by a one containing amino groups increased the catalyst stability at the expense of their activity. Keywords: Suzuki cross-coupling; hyper-cross-linked polystyrene; palladium nanoparticles; catalyst stability 1. Introduction The reaction of Suzuki-Miyaura (S-M) cross-coupling between aryl halides and arylboronic acids is one of the most common and effective methods for the synthesis of biaryls, which are important intermediates in the synthesis of pharmaceuticals, ligands, and polymers [1,2]. It is known that this reaction is a palladium-catalyzed condensation reaction, which proceeds via a Pd(0)/Pd(II) cycle involving the oxidative addition of the starting components and the reductive elimination of the resulting reaction products. The loss of the catalyst activity is an important issue of all the cross-coupling reactions. Various processes (e.g., aggregation, dissociation, leaching, etc.) can contribute to changes of the catalyst nature and, as a result, its activity and selectivity [3–5]. Aryl halides cause very low Processes 2020, 8, 1653; doi:10.3390/pr8121653 www.mdpi.com/journal/processes Processes 2020, 8, x FOR PEER REVIEW 2 of 16 Processes 2020, 8, 1653 2 of 14 contribute to changes of the catalyst nature and, as a result, its activity and selectivity [3–5]. Aryl halides cause very low palladium dissolution, while arylboronic acids, on the contrary, causes palladiumsignificant dissolution,palladium whileleaching arylboronic accompanied acids, by on theits reduction contrary, causes and formation significant of palladium palladium leaching black. accompaniedHowever, in bythe itspresence reduction of andboth formation substrates, of Pd(0 palladium) formed black. in the However, solution in with the presence participation of both of substrates,arylboronic Pd(0) acid is formed converted in the by solution aryl halide with without participation precipitation of arylboronic of nanoparticles acid is converted (NPs) [6]. by aryl halideThis without work precipitation addresses of nanoparticlesthe catalytic (NPs)behavior [6]. of palladium NPs stabilized within hyper-cross-linkedThis work addresses polystyrene the catalytic (HPS) behavior support ofduring palladium the repeated NPs stabilized reaction within runs. hyper-cross-linkedHPS of two types polystyrenewas used, non-functionalized (HPS) support during (MN270) the repeated and reaction bearing runs. tertiary HPS ofamino two types groups was (MN100), used, non-functionalized both relate to (MN270)so-called and “Davankov-type” bearing tertiary aminoresins groups[7,8]. In (MN100), our previous both relateworks to we so-called have shown “Davankov-type” that Pd/HPS resins catalysts [7,8]. Inare our highly previous active works and we selective have shown in thatS-M Pdreaction/HPS catalysts between are highlyphenylboronic active and acid selective (PBA) in S-Mand reaction4-bromoanisole between phenylboronic(BrAn) as model acid (PBA) compounds and 4-bromoanisole [9–11]. However, (BrAn) as modeldue compoundsto the complex [9–11]. However,homo-heterogeneous due to the complex mechanism homo-heterogeneous [12] of this reaction mechanism, especially [12 in] of the this case reaction, of so-called especially “cocktail” in the casetype ofcatalysts so-called (containing “cocktail” metal type catalystsNPs as reservoirs (containing for metalcatalytically NPs as active reservoirs species for [4,13]), catalytically the question active speciesabout deactivation [4,13]), the questionduring the about reaction deactivation course an duringd the reasons the reaction for such course behavior and the remains reasons open. for such The behaviorterm “cocktail” remains catalyst open. Thewas term proposed “cocktail” by Ananikov catalyst was and proposed coworkers by [3,4] Ananikov for cross-coupling and coworkers processes [3,4] for cross-couplingand describes catalytic processes systems and describes containing catalytic more systems than one containing form of morepalladium than one including form of Pd(0) palladium NPs, includingwere the latter Pd(0) serve NPs, wereas pre-catalysts the latter serve generating as pre-catalysts active species generating during active the reaction. species during For such the catalysts reaction. Forit was such established catalysts it that was establishedvirtually any that form virtually of Pd any (small form clusters, of Pd (small NPs, clusters, inorganic NPs, salts, inorganic complexes salts, complexescontaining containingpalladium palladiumions or atoms) ions ormight atoms) start might catalytic start cycle. catalytic General cycle. scheme General of scheme catalytic ofcatalytic cycle of cycleS-M cross-coupling of S-M cross-coupling is presented is presented in Figure in 1. Figure 1. FigureFigure 1.1. SimplifiedSimplified scheme of proposed catalytic cycle taking place while usingusing PdPd/HPS/HPS inin S-MS-M cross-couplingcross-coupling betweenbetween BrAnBrAn (designated(designated herehere asas Br-Ph-OCHBr-Ph-OCH33)) andand PBAPBA (Ph-B(OH)(Ph-B(OH)22)) resultingresulting inin formation of 4-methoxybiphenyl (designated here as (Ph) -OCH ). formation of 4-methoxybiphenyl (designated here as (Ph)22-OCH33). In many works [4,6,14–16], the leaching of Pd and the formation of inert Pd(0)NPs are considered In many works [4,6,14–16], the leaching of Pd and the formation of inert Pd(0)NPs are as the main reasons hampering catalyst reuse. There are two main ways to study catalyst deactivation considered as the main reasons hampering catalyst reuse. There are two main ways to study catalyst and the nature (homogeneous vs. heterogeneous) of the active species: (i) hot filtration test and deactivation and the nature (homogeneous vs. heterogeneous) of the active species: (i) hot filtration (ii) mercury test [16]. Each of them has certain disadvantages when working with a complex “cocktail” test and (ii) mercury test [16]. Each of them has certain disadvantages when working with a complex type catalyst with high porosity support. The pores of polymeric support can play a role of nanoreactors, “cocktail” type catalyst with high porosity support. The pores of polymeric support can play a role in which Pd NPs can undergo dissolution, migration, reprecipitation and aggregation during the of nanoreactors, in which Pd NPs can undergo dissolution, migration, reprecipitation and reaction course; hence actually the reaction location may shift to the polymer volume rather than aggregation during the reaction course; hence actually the reaction location may shift to the polymer being catalyzed by soluble Pd species in the reaction mixture. In this study, we chose the hot filtration volume rather than being catalyzed by soluble Pd species in the reaction mixture. In this study, we test. We carried it out with the samples of Pd/HPS preliminarily reduced in hydrogen to form Pd(0) Processes 2020, 8, x FOR PEER REVIEW 3 of 16 Processeschose 2020the, hot8, 1653 filtration test. We carried it out with the samples of Pd/HPS preliminarily reduced3 of 14 in hydrogen to form Pd(0) NPs, in order to see if any loss of Pd species takes place. However, the nature of deactivation was completely different and didn’t rely on Pd
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