Anomalous Efficacy of Bimetallic Au/Pd Nanoclusters in C–Cl Bond Activation and Formal Metathesis-type C–B Bond Activation at Room Temperature

Abstract

Au/Pd alloy nanoclusters stabilized by poly(N-vinylpyrrolidone) catalyze two different reactions of phenylboronic acid with 4-chlorobenzoic acid at room temperature in a single reaction cycle, cross coupling and metathesis-type homocoupling that is normally inaccessible through conventional catalysis.Bimetallic nanoclusters with an alloy structure have attracted a great deal of interest during the past decade owing to their enhanced catalytic activities, which are superior to those of the corresponding monometallic counterparts.1,2 Among the various bimetallic nanoclusters fabricated to date, Au/Pd alloy clusters are particularly fascinating because of their excellent catalytic activities for a diversity of chemical transformations. Recent research reports have, however, dealt mainly with the significant extension of catalytic reactivity and selectivity of monometallic nanoclusters.3,4 Herein, we report the peculiar catalytic activity of bimetallic Au/Pd alloyed nanoclusters in coupling between 4-chlorobenzoic acid (CBA) and phenylboronic acid (PhB(OH) 2 ) under aqueous conditions at room temperature.Among the organic halides including triflates, organic chlorides are less reactive due to relatively high CCl bond strength and their reluctance to undergo oxidative addition. Recently significant progress has been made toward activation of aryl chlorides by the use of highly electron-donative and bulky ligands.5 These ligands make the metal more nucleophilic, which facilitates oxidative addition by less active CCl bonds, the initial step of cross-coupling reaction. We previously reported that colloidal gold nanoclusters, such as poly(Nvinylpyrrolidone)-protected gold (Au:PVP) or chitosan [¢-1,4-linked poly(D-glucosamine)]-protected gold (Au:chit), are excellent catalysts for aerobic oxidation including homocoupling reactions of arylboronic acids and their derivatives (cycle B in Scheme 1) in water under ambient conditions. 6,7 One characteristic feature of colloidal nanogold is that a negatively charged surface is generated due to the size effect of the clusters as well as the donation effect from the matrices, realizing the facile adsorption of molecular oxygen on its surface.8 Such electronically enriched cluster surface would also be expected to accelerate the oxidative addition process in the SuzukiMiyaura cross coupling (cycle A in Scheme 1). Since Pd always favors the cross coupling against homocoupling as compared with Au, we started to develop PVP-stabilized bimetallic Au/Pd alloy clusters for the activation of CCl bond under mild conditions in the SuzukiMiyaura reaction. The PVP-stabilized Au and Pd monometallic and bimetallic alloy clusters containing 1:1 ratios of two metals (Au 0.5 Pd 0.5 :PVP) were prepared according to our previous procedure. First, the coupling reactions between CBA and PhB(OH) 2 by monometallic clusters were investigated. Equimolar amounts of CBA and PhB(OH) 2 were used as reactants. The crosscoupling product, biphenyl-4-carboxylic acid (1) was not observed, but biphenyl (2) and phe...

Gold/Palladium Bimetallic Nanoclusters for C-X Bond Activation: A Unique Effect of Gold

“…As a model an equivalent amount of chlorobenzoic acid and phenylboronic acid was reacted in the presence of 300 mol% K 2 CO 3 at room temperature (Scheme 3). 18 To avoid oxidative homocoupling of PhB(OH) 2 , the reaction was performed under degassed (freeze pump thaw cycle) conditions. No coupling was observed with pure Au cluster, suggesting that no oxidant (O 2 ) was available in the reaction because an external oxidant such as oxygen is necessary for homocoupling to occur.…”

“…In addition, tetrahydroxydiboron, a side product derived from the phenylboronic acid during the course of reaction, 18 was detected by 11 B NMR, which indicates that the homocoupling product (2) was formed through a formal metathesis type reaction. To elucidate whether or not the catalytic process is truly heterogeneous the metal content of the ltrate after the reaction was analyzed by ICP AES.…”

“…Any leached metals (Au and Pd) are present at under their detection limits ( 50 ppb). 18 Although the details of the reaction mechanisms are still unclear, the mechanism involving a single metal (Au or Pd) catalytic site has been excluded and instead a new model of oxidative addition to the C Cl bond on the surface of Au/Pd (intermediate E, Figure 7) has been proposed. The thus formed spillover intermediate E, reacts heterogeneously to give two equivalent of cross coupling product (1) and one equivalent of biphenyl (2).…”

“…33,34 Therefore as shown in Figure 16, we considered some model systems to throw light on the catalytic center and to elucidate the role of Au and Pd atoms. The Pd, Pd Pd, and Au sites in Au 10 Pd 10 , and Au sites in Au 18 Pd 2 and Au 20 were examined. The Au 18 Pd 2 NC has a core shell structure, though it is a small NC example.…”

“…The Pd, Pd Pd, and Au sites in Au 10 Pd 10 , and Au sites in Au 18 Pd 2 and Au 20 were examined. The Au 18 Pd 2 NC has a core shell structure, though it is a small NC example. The calculations showed that the activation energy barrier of the oxidative addition is generally high at Au sites, namely more than 116 kJ/mol in Au 10 Pd 10 NC, while it is very low at the Pd and Pd Pd sites, at around 46 and 34 kJ/mol, respectively.…”