"Copper-in-charcoal" has been shown to be a versatile catalytic source of supported copper for a variety of important synthetic transformations, as well as in other fields such as energy. We herein report the characterization of this material and the implications that its preparation has on catalysis, thus providing a greater understanding of the scope and limitations of this catalyst system. (Chemical Equation Presented)

Benjamin Buckley (1249668)

Emma C. Stubbs (1351794)

Harry Heaney (1351788)

R. Butterworth (7163564)

Sandie Dann (1253892)

English

Loughborough University Institutional Repository

“Copper-in-Charcoal” Revisited: Delineating the Nature of theCopper Species and Its Role in CatalysisBenjamin R. Buckley,* Rachel Butterworth, Sandra E. Dann, Harry Heaney, and Emma C. StubbsDepartment of Chemistry, Loughborough University, Loughborough, Leicestershire, LE11 3TU, U.K.*S Supporting InformationABSTRACT: “Copper-in-charcoal” has been shown to be aversatile catalytic source of supported copper for a variety ofimportant synthetic transformations, as well as in other fieldssuch as energy. We herein report the characterization of thismaterial and the implications that its preparation has oncatalysis, thus providing a greater understanding of the scopeand limitations of this catalyst system.KEYWORDS: copper, charcoal, triazole, click, heterogeneousOver the past few decades, a range of catalysts have beenheterogenized onto solid supports (e.g., SiO2, Al2O3, molsieves, etc.), and activated carbon is one of the most commonlyemployed.1 These heterogeneous catalysts have received muchattention due to their notable increase in reactivity andrecyclability when compared to traditional transition-metal-based catalysts.2 However, research in this area has been largelyfocused on applying the heterogeneous catalyst systems to avariety of applications rather than characterizing and improvingthem, probably due in part to the expertise in the researchgroups reporting the work lying toward organic synthesis ratherthan materials characterization.3The Lipshutz group, has pioneered the use of metal-basedcatalysts on carbon supports, for example, Ni/C,4 Ni/Cg,5 Cu/C,6 and mixed metal systems,8 for a variety of reactions. Inparticular, they have used copper nanoparticles supportedwithin the pores of activated carbon to act as a source ofcopper(I) or (II) for a range of heterogeneously catalyzedreactions. Oxidized copper nanoparticles on activated carbonhave also been used in CuAAC reactions.9Several different protocols have been reported for thepreparation of Cu/C catalysts. For example, Mehandjievreported a procedure in which the final step involved thecopper-in-charcoal material being calcined at 503 K for 2 h,10a asimilar procedure was reported by Liu with heating at 523 K10band Wey reported the heating of the copper-in-charcoalmaterial at >700 K before the material was analyzed:10c allthree groups report X-ray powder diffraction data showing thepresence of CuO with lesser amounts of Cu2O in the charcoalmatrix. The material prepared and used by the Lipshutz groupinvolved the combination of activated carbon (Darco-KB) andan aqueous solution of copper(II) nitrate using sonification,distillation of water followed by azeotropic drying with tolueneto afford the catalyst (Scheme 1).6a,b Transmission electronmicroscopy (TEM) was also used to record 2D images of thecarbon material, from which the group proposed a relativelyeven distribution of the copper nanoparticles within thecharcoal matrix;6b lastly, quantitative inductively coupledplasma atomic emission spectrometry (ICP-AES) was used todetermine the loading of copper within the charcoal, which wasfound to be 0.344 mmol Cu/g catalyst, as well as the extent ofcopper bleed into solution during the reactions.6b Importantly,we noted that leaching of copper(II) from the carbon matrixwas reported during azide formation,6d and under flowconditions.7Of particular note, Lipshutz was able to show that high yieldsin a series of copper assisted Huisgen [3 + 2] cycloadditionreactions could be obtained (Scheme 2). They also showed thatthe copper-in-charcoal material was particularly robust andwere able to use a wide range of solvents using hightemperature microwave conditions, with no solubility issues.The catalysts could also be reused (for at least three catalyticcycles) without loss of activity.6aReceived: November 17, 2014Revised: December 16, 2014Published: December 22, 2014Scheme 1. Lipshutz’s Preparation of “Copper-in-Charcoal”Scheme 2. Lipshutz’s “Copper-in-Charcoal” CatalyzedTriazole Click ReactionLetterpubs.acs.org/acscatalysis© 2014 American Chemical Society 793 DOI: 10.1021/cs501826sACS Catal. 2015, 5, 793−796This is an open access article published under a Creative Commons Attribution (CC-BY)License, which permits unrestricted use, distribution and reproduction in any medium,provided the author and source are cited.Our multidisciplinary group is interested in the use ofheterogeneous catalysts for a variety of applications, and wehave previously investigated the role of copper(II) compoundsas precatalysts in copper alkyne azide cycloaddition (CuAAC)reactions.11 In those reactions, the copper(II) species act as acatalyst in Glaser coupling reactions and is thereby reduced tocopper(I) with concomitant formation of copper(I) acetylideswhich are subsequently involved in catalysis of the CuAACreaction.11a,c,12 A dicopper(II)-substituted γ-keggin silcotung-state has been used in Glaser coupling reactions13 and is alsoreduced to act as a precatalyst in CuAAC reactions.14Copper(I) acetylides can also be useful for the A3 multi-component synthesis of a range of tertiary amines,15,16 and theyare also involved in the ligand accelerated Stephens−Castrocoupling of terminal alkynes and aryl iodides17a andSonagashira−Hagihara reactions.17bWe decided to use the versatile supported copper system andinitiated the synthesis of the copper-in-charcoal materialfollowing the Lipshutz protocol, outlined in Scheme 1. Thepreparation went as expected, and we obtained a black powder,which we analyzed using X-ray powder diffraction, in order toconfirm its identity before use (Figure 1). Previous reports6a,dassumed, as a result of earlier work,10 that the Cu/C catalystwas a mixture of CuO/Cu2O bound to the surface of thecarbon support: instead we found that the mineral Gerhardite(Cu2(OH)3NO3) had been formed as a distinct material, notbound to the support. We then became aware that the copper-in-charcoal material was commercially available,18 and wedecided to analyze that material in order to compare it with ourinitial material. The two materials were found to match eachother; both contained a quantity of copper(II) hydroxynitrate,with a small amount of an unidentified impurity: the impuritywas not present when we prepared Gerhardite on Norit ASUPRA as the carbon support, or by omitting the carbon fromLipshutz’s procedure for “Cu/C” formation (Scheme 3)In order to prove that this impurity did not arise from thecopper nitrate starting material, a series of reactions werecarried out to prepare the copper-in-charcoal (Darco-KB) witha higher loading of copper, where the copper nitrate loadingwas increased to 2×, 5×, and 10× from the original reaction.The X-ray powder diffraction patterns of the solid supportmaterials prepared in this way are shown in Figure 2.As expected, the different copper loadings did not showmuch variation on the quantity of the impurity that formed.However, the reaction using the 2× loading was carried out at alower final temperature and did not result in the formation ofGerhardite: the impurity phase was the only copper materialpresent. The lack of Gerhardite proved to be an advantage,giving a clearer XRD pattern of the impurity, which allowed abetter library search of that material. A perfect match was madewith the mineral Libethenite (Cu2(PO4)OH), a phosphorus-containing species, which explains why the copper loading didnot affect the quantity of the impurity formed. A sample ofLibethenite was then prepared independently from the supportmaterial for XRD analysis19 and gave an even clearer match tothe impurities present in the Darco-KB copper-in-charcoalsupport materials.We believe Libethenite is formed due to the nature of thecarbon support; that is, Darco-KB is formed through thechemical activation of wood using the phosphoric acidprocess.20 The residual phosphate on the carbon could interactwith the copper nitrate during the reaction to prepare thecopper-in-charcoal support material. Libethenite is notobserved using Norit A SUPRA as this carbon material is asteam-activated carbon of natural origin.21In order to delineate the role of Libethenite and Gerharditein the catalytic processes reported by Lipshutz for “Cu/C”, wetested our copper containing materials in the CuAAC reaction(Table 1). Treatment of phenyl acetylene and benzyl azideunder microwave conditions employing a 10 mol % catalystloading generally afforded good to excellent yields of a singleregioisomer of the triazole.22 Gerhardite, without the charcoalsupport (entry 5), performed equally as well as “Cu/C” andwas reduced to copper(I) to produced the required polymericcopper(I) acetylide precatalyst, which was easily removed at theend of the reaction by filtration.23 Under the conditionsreported in Table 1, Et3N was not required for the reactions toproceed, as noted by Lipshutz for “Cu/C”.6a,24 Importantly,Figure 1. XRD results for the copper-in-charcoal material (top)compared to copper nitrate starting material (blue) andCu2(OH)3(NO3) (gray).Scheme 3. Gerhardite Formation Using Lipshutz’sConditions Omitting the Carbon SupportFigure 2. XRD patterns showing the results of the higher copperloading, 2× (Green), 5× (Red), and 10× (Black). Blue tickscorrespond to Gerhardite.ACS Catalysis LetterDOI: 10.1021/cs501826sACS Catal. 2015, 5, 793−796794Libethenite was found to be inactive in the CuAAC reaction(entry 6). For reasons which are unclear at present, thecommercial “Cu/C” performed poorly in this reaction, perhapsdue to degradation of the catalyst, although XRD analysis didshow good correlation between commercial and our preparedmaterial.25Having noted previously that materials such asCu2(OH)3OAc react with alkynes to initially form copper(I)acetylides,11d we were interested in examining the supportedmaterials after the CuAAC reaction. While there was someconversion shown from Gerhardite to copper(I) phenyl-acetylide during the reaction, it was not a full conversion so aseries of time controlled reactions were performed to try todrive the conversion of Gerhardite to copper(I) phenyl-acetylide.26 Using the conditions shown in Table 1 the triazoleclick reaction was catalyzed using a mixture of Gerhardite andLibethenite without the solid support material present. Webelieved that this would help to remove some of thebackground noise from the XRD patterns making interpretationeasier. The results are shown in Figure 3. The XRD patternsfrom 0 h to 10 days show the gradual formation of thecopper(I) phenylacetylide and the corresponding gradualdisappearance of Gerhardite as it is consumed. The Libetheniteremains present in all XRD patterns, which shows that it takesno part in the formation of the copper(I) species and hencecannot take part in catalysis of triazole formation.These observations led us to question why we did notobserve CuO or Cu2O, as had been previously assumed6 andreported independently by Mehandjiev, Liu, and Wey.9 Thedetailed procedures used by the research groups reveal a crucialdifference: in the Mehandjiev, Liu, and Wey reports, the finalstep in the experimental procedure involved the copper-in-charcoal material being calcined at either 503 K,10a 523 K,10b or>700 K10c before the material was analyzed by X-ray powderdiffraction. Under the Lipshutz conditions, that final step wasnot carried out, and as a result, the inorganic salts were notdecomposed but remained in the form of Gerhardite andLibethenite.In summary, we have shown that the procedure6 used toprepare copper-in-charcoal does not afford CuO or Cu2Obound to the charcoal matrix as previously assumed. However,Gerhardite is easily prepared and can be stored for long periodswithout decomposition, as also can, for example, the copper(I)acetylide polymer precatalysts. Hence, the copper species usingCu/C, that is responsible for reactions such as the CuAACprocess, has been identified as Gerhardite (Cu2(OH)3NO3),which acts as a precatalyst and is reduced under the reactionconditions to a copper(I) acetylide in order to carry out thetriazole forming step. Additionally, we observed an impurityphase in the materials prepared using the Darco-KB activatedcarbon. This impurity was identified as Libethenite, Cu2(PO4)-OH, and was shown to be inactive in the CuAAC reaction. Weare currently investigating the application of Gerhardite toother catalyed processes and early indications show that bothGerhardite and Libethenite can catalyze Ullmann aryl ethersynthesis. A full investigation will be published in due course.We anticipate that understanding of the catalyst involved in thisheterogeneous system will inspire further studies and advance-ments in this area.■ ASSOCIATED CONTENT*S Supporting InformationThe following file is available free of charge on the ACSPublications website at DOI: 10.1021/cs501826s.Experimental procedures, additional XRD data, andadditional experiments (PDF)■ AUTHOR INFORMATIONCorresponding Author*E-mail: b.r.buckley@lboro.ac.uk.NotesThe authors declare no competing financial interest.Table 1. Triazole Formation Using Gerhardite andLebethenite Supported and Unsupported Materialsaentry material (10 mol %) yield (%)b1 “Cu/C” 99c2 commercial “Cu/C” 54d3 prepared “Cu/C” 854 Gerhardite on NoritA SUPRA 825 Gerhardite 89 (97)e6 Libethenite <57 control <5faGeneral reaction conditions: catalyst (10 mol %), benzyl azide (1.7mmol), phenyl acetylene (2.6 mmol), MeCN, microwave at 100 °C,10 min. bIsolated yield after column chromatography. cYield reportedin ref 6a when using dioxane as solvent and Et3N (1.1 equiv).dCommercial material, see ref 14 for details. eReaction carried outusing dioxane as solvent. fReaction carried out in the absence of anycatalysts.Figure 3. XRD results from the series of time controlled clickreactions,26 showing the formation of copper(I) phenylacetylide(orange) from Gerhardite (blue) not Libethenite (green).ACS Catalysis LetterDOI: 10.1021/cs501826sACS Catal. 2015, 5, 793−796795■ ACKNOWLEDGMENTSThis work was supported by Loughborough University (PhDstudentship to E.C.S.). B.R.B. thanks the Research CouncilsU.K. for a RCUK Fellowship and the EPSRC for anundergraduate summer research bursary to RB.■ REFERENCES(1) Marsh, H.; Reinoso, F. R. in Activated Carbon; Elsevier Ltd.:Oxford, 2006.(2) Climent, M. J.; Corma, A.; Iborra, S. Chem. Rev. 2011, 111,1072−1133.(3) For example, X-ray powder diffraction has proven to be apowerful technique for characterizing heterogeneous catalysts:(a) Bordiga, S.; Groppo, E.; Agostini, G.; van Bokhoven, J. A.;Lamberti, C. Chem. Rev. 2013, 113, 1736−1850. (b) Handbook ofheterogeneous catalysis; Ertl, G., Knözinger, H., Weitkamp, J., Eds.;Wiley VCH: Weinheim, 1997; Vol. 2.(4) (a) Lipshutz, B. H.; Blomgren, P. A. J. Am. Chem. Soc. 1999, 121,5819−5820. (b) Lipshutz, B. H.; Tomioka, T.; Blomgren, P. A.;Sclafani, J. Inorg. Chim. Acta 1999, 296, 164−169. (c) Lipshutz, B. H.;Sclafani, J. A.; Blomgren, P. A. Tetrahedron 2000, 56, 2779−2788.(d) Lipshutz, B. H. Adv. Syn. Catal. 2001, 343, 313. (e) Lipshutz, B.H.; Tasler, S. Adv. Syn. Catal. 2001, 343, 327. (f) Lipshutz, B. H.;Tomioka, T.; Sato, K. Synlett 2001, 970−973. (g) Lipshutz, B. H.;Tasler, S.; Chrisman, W.; Spliethoff, B.; Tesche, B. J. Org. Chem. 2003,68, 1177−1189. (h) Tasler, S.; Lipshutz, B. H. J. Org. Chem. 2003, 68,1190−1199. (i) Frieman, B. A.; Taft, B. R.; Lee, C.-T.; Butler, T.;Lipshutz, B. H. Synthesis 2005, 2989−2993. (j) Lipshutz, B. H.;Frieman, B. A.; Lee, C.-T.; Lower, A.; Nihan, D. M.; Taft, B. R.Chem.Asian J. 2006, 1, 417−429.(5) (a) Lipshutz, B. H.; Frieman, B. A.; Butler, T.; Kogan, V. Angew.Chem., Int. Ed. 2006, 45, 800−803. (b) Lipshutz, B. H.; Butler, T.;Swift, E. Org. Biomol. Chem. 2008, 6, 19−25. (c) Lipshutz, B. H.;Butler, T.; Swift, E. Org. Lett. 2008, 10, 697−700.(6) (a) Lipshutz, B. H.; Taft, B. R. Angew. Chem., Int. Ed. 2006, 45,8235−8238. (b) Lipshutz, B. H.; Frieman, B. A.; Tomaso, A. E., Jr.Angew. Chem., Int. Ed. 2006, 45, 1259−1264. (c) Lipshutz, B. H.;Unger, J. B.; Taft, B. R. Org. Lett. 2007, 9, 1089−1092. (d) Lee, C.-T.;Huang, S.; Lipshutz, B. H. Adv. Syn. Catal. 2009, 351, 3139−3142.(7) Fuchs, M.; Goessler, W.; Pilger, C.; Kappe, C. O. Adv. Synth.Catal. 2010, 352, 323−328.(8) Lipshutz, B. H.; Nihan, D. M.; Vinogradova, E.; Taft, B. R.;Boskovic, Z. V. Org. Lett. 2008, 10, 4279−4282.(9) (a) Alonso, F.; Moglie, Y.; Radivoy, G.; Yus, M. Adv. Synth. Catal.2010, 352, 3208−3214. (b) Alonso, F.; Moglie, Y.; Radivoy, G.; Yus,M. Org. Biomol. Chem. 2011, 9, 6385−6395.(10) (a) Tsoncheva, T.; Vankova, S.; Mehandjiev, D. Fuel 2003, 82,755−763. (b) Zhao, H. H.; Liu, Z. Y.; Sun, D. K. J. Catal. 2004, 227,297−303. (c) Tseng, H. H.; Wey, M. Y. Carbon 2004, 42, 2269−2278.(11) (a) Buckley, B. R.; Heaney, H. in Topics in HeterocyclicChemistry; Springer Berlin Heidelberg, Berlin, Heidelberg, 2012; pp1−29;. (b) Buckley, B. R.; Dann, S. E.; Heaney, H.; Stubbs, E. C. Eur.J. Org. Chem. 2011, 770−776. (c) Buckley, B. R.; Dann, S. E.; Heaney,H. Chem.Eur. J. 2010, 16, 6278−6284. (d) Buckley, B. R.; Dann, S.E.; Harris, D. P.; Heaney, H.; Stubbs, E. C. Chem. Commun. 2010, 46,2274−2276.(12) Berg, R.; Straub, B. F. Beilstein J. Org. Chem. 2013, 9, 2715−2750.(13) Kamata, K.; Yamguchi, S.; Kotani, M.; Yamaguchi, K.; Mizuno,M. Angew. Chem., Int. Ed. 2008, 47, 2407−2410.(14) Kamata, K.; Nkaagawa, Y. M.; Yamguchi, K.; Mizuno, M. J. Am.Chem. Soc. 2008, 130, 15304−15310.(15) Albaladejo, M. J.; Alonso, F.; Moglie, Y.; Yus, M. Eur. J. Org.Chem. 2012, 2012, 3093−3104.(16) Buckley, B. R.; Khan, A. N.; Heaney, H. Chem.Eur. J. 2012,18, 3855−3858.(17) (a) Zuidema, E.; Bolm, C. Chem.Eur. J. 2010, 16, 4181−4185.(b) Zou, L.-H.; Johansson, A. J.; Zuidema, E.; Bolm, C. Chem.Eur. J.2013, 19, 8144−8152.(18) Copper in charcoal, 3 wt %, SKU number 709107, Sigma-Aldrich.(19) The X-ray powder diffraction pattern obtained from thisreaction can be found in the Supporting Information.(20) See product information page: http://www.norit.com/products-and-services/products/darco-kb-g/.(21) See product information page: http://www.norit.com/products-and-services/products/norit-a-supra-eur/.(22) See Table S1 in the Supporting Information for other examplesemploying Gerhardite as precatalyst in the CuAAC reaction.(23) The solid material obtained was isolated as a mixture ofGerhardite and the copper(I) acetylide.(24) See Table S1, entry 2 for a direct comparison of reactivitybetween “Cu/C” (reported in ref 6a) and Gerhardite.(25) One could also speculate that this commercial material mayhave a lower copper loading than the prepared material.(26) Details of the reactions carried out can be found in theSupporting Information.ACS Catalysis LetterDOI: 10.1021/cs501826sACS Catal. 2015, 5, 793−796796

"Copper-in-charcoal" revisited: delineating the nature of the copper species and its role in catalysis

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"Copper-in-charcoal" revisited: delineating the nature of the copper species and its role in catalysis

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