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Reduction of Nitrobenzene to Aniline by CO/H2O in the Presence of Palladium Nanoparticles

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Reduction of Nitrobenzene to Aniline by CO/H2O in the Presence of Palladium Nanoparticles Communicationcatalysts 2 ReductionCommunication of Nitrobenzene to Aniline by CO/H O in theReduction Presence of of Nitrobenzene Palladium N toanoparticles Aniline by CO/H2O in Agnieszkathe Presence Krogul-Sobczak,* of Palladium Jakub Cedrowski, NanoparticlesPatrycja Kasperska and Grzegorz Litwinienko AgnieszkaFaculty of Krogul-SobczakChemistry, University *, Jakub of Warsaw, Cedrowski Pasteura, Patrycja 1, 02- Kasperska093 Warsaw and Poland; [email protected] Litwinienko (J.C.); [email protected] (P.K.); [email protected] (G.L.) Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland; *[email protected] Correspondence: [email protected] (J.C.); [email protected] (P.K.); [email protected] (G.L.) Received:* Correspondence: 18 March 2019 [email protected]; Accepted: 24 April 2019; Published: date AbstractReceived:: 18The March transformation 2019; Accepted: of aromatic 24 April 2019; nitrocompounds Published: 30 Aprilinto amines 2019 by CO/H2O is catalyzed by palladium(II) complexes. Recently, we have proposed that the catalytic cycle includes Pd0 as the Abstract: The transformation of aromatic nitrocompounds into amines by CO/H O is catalyzed transient intermediate and herein, for the first time, we describe the application2 of palladium by palladium(II) complexes. Recently, we have proposed that the catalytic cycle includes Pd0 as nanoparticles (PdNPs) stabilized by monodentate N-heterocyclic ligands as nanocatalysts the transient intermediate and herein, for the first time, we describe the application of palladium facilitating the reduction of Ar–NO2 into Ar–NH2 by CO/H2O. Among the series—Pd(II) nanoparticles (PdNPs) stabilized by monodentate N-heterocyclic ligands as nanocatalysts facilitating complexes, PdNPs and commercial Pdblack—the highest catalytic activity was observed for PdNPs the reduction of Ar–NO into Ar–NH by CO/H O. Among the series—Pd(II) complexes, PdNPs and (3.0 0.5 nm) stabilized2 by 4-Me-pyridine2 in 2the presence of 2-Cl-pyridine. The results may be commercial Pd —the highest catalytic activity was observed for PdNPs (3.0 0.5 nm) stabilized helpful for mechanisticblack considerations on the role of metallic nanoparticles as± active species in by 4-Me-pyridine in the presence of 2-Cl-pyridine. The results may be helpful for mechanistic other organic processes. considerations on the role of metallic nanoparticles as active species in other organic processes. Keywords: aromatic nitrocompounds; aromatic amines; palladium nanoparticles; catalyst; Keywords: aromatic nitrocompounds; aromatic amines; palladium nanoparticles; catalyst; reduction; reduction; carbon monoxide; pyridine derivatives carbon monoxide; pyridine derivatives 1.1. Introduction Introduction AromaticAromatic aminesamines areare important important intermediates intermediates and and the finalthe productsfinal products in the chemicalin the chemical and polymer and polymerindustry industry [1–5]. Among [1–5]. the Among commonly the commonly applied methods applied of methods their synthesis, of their thesynthesis, reduction the of reduction nitroarenes of nitroarenesis the most convenientis the most approach. convenient Direct approach. hydrogenation Direct hydrogenatio of nitrocompoundsn of nitrocompounds undergoes minimal undergoes waste minimalgeneration waste [6] but generation is nonselective [6] but and is expensivenonselective [7,8 ],and thus, expensive other methods [7,8], thus, of reduction other methods are needed of reduction are needed for large scale production. One of the most promising methods utilizes the for large scale production. One of the most promising methods utilizes the mixture of CO/H2O as mixturean easily of accessible CO/H2O reducing as an easily agent accessible [7–11] and reducing is generally agent described [7–11] byand Equation is generally (1). described by Equation (1). (1) (1) Recently, we have developed the method of reduction of aromatic nitrocompounds to amines Recently, we have developed the method of reduction of aromatic nitrocompounds to amines with with CO/H2O, employing PdCl2(XnPy)2 complexes as pre-catalysts (X = Cl or CH3, n = 0 − 2) in the CO/H O, employing PdCl (XnPy) complexes as pre-catalysts (X = Cl or CH , n = 0 2) in the presence presence2 of Fe powder and2 I2 as co2 -catalyst and we proposed a plausible mechanism3 − of this process [of11,12 Fe]. powder Initially, and the I2 astraces co-catalyst of aniline and weare proposedgenerated a plausiblefrom water mechanism or ethanol of thisand process nitrobenzene [11,12]. (noInitially,-catalyst the is traces needed, of aniline thus, this are generatedstep is not from included water in or the ethanol main andcyclic nitrobenzene mechanism (no-catalyst presented in is Schemeneeded, 1) thus, and this the stepformed is not aniline included reacts in with the mainthe Pd(II) cyclic complex mechanism [11–14 presented]: in Scheme1) and the formed aniline reacts with the Pd(II) complex [11–14]: PdCl2(XnPy2)2 + 2PhNH2 → PdCl2(PhNH2)2 + 2XnPy (2) PdCl (XnPy ) + 2PhNH PdCl (PhNH ) + 2XnPy (2) The subsequent reaction with2 CO gives2 2 N,N'-diphenylurea2 ! 2 and trace2 2 amounts of Pd0 (Equation (3)): 0 The subsequentPdCl2 reaction(PhNH2) with2 + (zCO + 1)CO gives + N,N’-diphenylureamXnPy → (PhNH)2 andCO + trace Pd0(X amountsnPy)m(CO) of Pdz + 2HCl(Equation (3)):(3) 0 Catalysts 2019PdCl, 9, x; (PhNHdoi: FOR PEER) + (zREVIEW+ 1)CO + mXnPy (PhNH) CO + Pd (XnPy)www.mdpi.com/journal/m(CO)z + 2HClcatalysts (3) 2 2 2 ! 2 Catalysts 2019, 9, 404; doi:10.3390/catal9050404 www.mdpi.com/journal/catalysts Catalysts 2019, 9, x FOR PEER REVIEW 2 of 11 where m and z depend on the coordinating ability of Pd(0), amount of XnPy, and pressure of CO. Pdblack is oxidized to Pd2+: Fe2+ + 5CO + Pd(black) → Fe(CO)5 + Pd2+ (4) by Fe2+ cations generated from Fe (powder) and I2 i.e. co-catalysts [15]. One of the proofs of the Pd0 presence in the system is the partial precipitation of Pdblack [16]. Complex Pd0(XnPy)m(CO)z formed in reaction (3) reacts with nitrobenzene and carbon monoxide and, at this moment, the catalytic cycle starts; see Scheme 1. Because nitroaromatic compounds having electron deficient Y substituents (Y-C6H4NO2) undergo a faster reaction, we postulated that the electron transfer from Pd0 to Y-C6H4NO2 is the rate determining step (RDS). If it is true, the more basic XnPy ligands (derivatives of pyridine) in PdCl2(XnPy)2 should accelerate the catalytic cycle in Scheme 1. Surprisingly, the overall conversion and yield increased with the decreasing basicity of XnPy, i.e., complexes of chloropyridines were better than methylpyridines [11]. We suppose that increasing the basicity of XnPy is beneficial for the stability of Pd0 during RDS described by Equation (5) (see also step I in Scheme 1) but, on the other hand, the very basic pyridine ligand competes with Y-C6H4NO2 for access to the catalytic centre. In this work, we are testing the hypothesis that Pd0 is involved in the RDS and we are trying to explain Catalysts 2019, 9, 404 2 of 11 the role of XnPy in RDS. Pd0(XnPy)m + CO + YC6H4NO2 [Pd2+(XnPy)2(YC6H4NO2)(CO)] + (m − 2)XnPy (5) where m and z depend on the coordinating ability of Pd(0), amount of XnPy, and pressure of CO. 0 2+ PdFormationblack is oxidized of Pd in to situ Pd as: heterogeneous catalysts is in agreement with the mechanisms postulated for other reactions (mainly, hydrogenations) mediated by transition-metal complexes [17] and the common features of such a kindFe2+ of+ 5COcatalysis+ Pd are (i) applicationFe(CO) + ofPd easily2+ reduced pre-catalysts, (4)(ii) (black) ! 5 forcing reaction conditions, and (iii) the presence of nanocluster stabilizers (here: Pd complexes, 180 2+ 0 by°C, Fe 4MPa,cations and N generated-donor ligands, from Fe respectively). (powder) and In Iorder2 i.e. co-catalyststo verify this [15 hypothesis,]. One of the we proofs directly of theapplied Pd 0 presencePd nanoparticles in the system (PdNPs). is the The partial introduction precipitation of Pd of0 to Pd theblack reaction[16]. Complex system would Pd (Xn increasePy)m(CO) thez formedrate, as inthe reaction step of (3)nucleation reacts with and nitrobenzene formation of andPd0 carbon(Reactions monoxide (2) and and, (3)) atis omitted this moment,, and it the can catalytic give a proof cycle starts;that PdNPs see Scheme are essential1. for the catalytic process described by Scheme 1. SchemeScheme 1.1. TheThe proposedproposed mechanismmechanism ofof thethe reductionreduction ofof nitrobenzenenitrobenzene byby COCO/H/H22O inin thethe presencepresence ofof 00 00 PdPd formedformed fromfrom PdClPdCl22(X(XnnPy)2 complexes [[1111].]. PdPd (XnPy)Py)mm(CO)(CO)z zisis a a hypothetical hypothetical species (with(with oneone atom of Pd0 or it might also be a Pd0 nanoparticle or cluster stabilized by a non-stoichiometric number 2+ of XnPy ligands and/or CO) that is converted (step I) into a complex of Pd as the central ion. Because nitroaromatic compounds having electron deficient Y substituents (Y-C6H4NO2) undergo 0 a faster reaction, we postulated that the electron transfer from Pd to Y-C6H4NO2 is the rate determining step (RDS). If it is true, the more basic XnPy ligands (derivatives of pyridine) in PdCl2(XnPy)2 should accelerate the catalytic cycle in Scheme1. Surprisingly, the overall conversion and yield increased with the decreasing basicity of XnPy, i.e., complexes of chloropyridines were better than methylpyridines [11]. 0 We suppose that increasing the basicity of XnPy is beneficial for the stability
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