Wu_et_al-2013-Angewandte_Chemie_(International_ed._in_English)

Gold Chemistry

DOI:10.1002/anie.201207016

Activation of Multiple C àH Bonds Promoted by Gold in AuNbO 3+Clusters**

Xiao-Nan Wu,Xiao-Na Li,Xun-Lei Ding,and Sheng-Gui He*

The success of generating more efficient catalysts lies in the improvement of their activity without sacrificing their selec-tivity,primarily through the identification of active sites and mechanisms that govern the reaction.The development of catalysts for the selective activation of carbon àhydrogen (C àH)bonds is a challenging task in chemistry.[1]Earlier condensed-phase studies postulated that oxygen-centered radicals (O C à)[2]are active sites for the activation of C àH bonds of alkane molecules.[3]Recent investigations on oxide clusters in the gas phase [4]that can be handled under isolated,controlled,and reproducible conditions confirm that O C àradicals are indeed very crucial for C àH activation of methane,[5]ethane,[6]n -butane,[7]etc.under thermal collision conditions.The reported activations of alkane molecules by O C àover atomic clusters usually produce products of single hydrogen atom abstraction (HAA;products:alkyl radicals).Herein,we report that when O C à-containing clusters are doped with gold atoms,it is possible to observe activation of multiple (double and triple)C àH bonds of one alkane molecule with high selectivity.The activation of multiple C àH bonds is important as it directly generates alkenes,which are value-added products,from alkanes or alkenyl (other than alkyl)radicals,which can be intermediates for further chemical transformation.[8]

Gold catalysts,such as Au I and Au III salts,are powerful for C àH activation,[9]in which the Au I /Au III catalytic cycle is very likely to be involved.[10]Gold species in different charge states (cationic,neutral,and anionic)[11]are often identified in supported gold catalysts.These findings suggest that gold may switch its role between electron donator and electron acceptor in a chemical reaction.As a result,doping atomic clusters (metal oxide clusters in this study)with gold atoms may cause significant charge redistribution within the clusters and during reactions.The C àH activation,which depends heavily on the effects of local charges,[12]can thus be effectively tuned by the introduction of gold into clusters.

Gold clusters and gold-containing heteroatomic systems [13,14]in the gas phase have been extensively studied,and the bonding nature of gold,including the relativistic effect,[15]was demonstrated.Investigations on gold-containing clusters of mixed oxides are very limited,[16]but important in order to understand the mechanistic details of reactions catalyzed by oxide-supported gold,which have been under debate for many years.[11,17]Taking into account that niobium oxides have extraordinary catalytic properties in selective oxidation reactions,including hydrocarbon conversions,[18]we gener-ated the mixed-oxide clusters Au x Nb y O z +by laser ablation and studied their reactivity with small alkane molecules by mass spectrometry.

The mass spectra shown in Figure 1indicate that AuNbO 3+cluster (m /z =338)can abstract one,two,and three H atoms from methane,ethane,and n -butane,respec-tively:

AuNbO 3ttCH 4!AuNbO 3H ttCH 3e1TAuNbO 3ttC 2H 6!AuNbO 3H 2ttC 2H 4e2TAuNbO 3ttn -C 4H 10!AuNbO 3H 3ttC 4H 7

e3

T

Figure 1.Selected time-of-flight mass spectra for interactions of Au x Nb y O z +with a)methane,b)ethane,and c)n -butane.Reference spectra without hydrocarbons in the reaction cell are shown in a1,b1,and c1.The reactant gases in the cell are:a2)CH 4(0.25Pa),a3)CH 4(0.35Pa),a4)CD 4(0.35Pa);b2)C 2H 6(0.17Pa),b3)C 2H 6(0.28Pa),b4)C 2D 6(0.28Pa);and c2)n -C 4H 10(0.014Pa),c3)n -C 4H 10(0.017Pa),c4)n -C 4D 10(0.017Pa).“x ,y ,z ”denotes Au x Nb y O z +.The “+H”,“+D”,etc.mark the product signals with respect to AuNbO 3+or Nb 2O 5+.Most of the Au x Nb y O z +clusters pick up the hydrocarbon molecules in the reaction cell (see the Supporting Information for complete

spectra).The Nb 2O 6C 4H 10+signal overlaps with AuNbO 3H 2+(c2and c3).

[*]Dr.X.-N.Wu,Dr.X.-N.Li,Dr.X.-L.Ding,Prof.Dr.S.-G.He Beijing National Laboratory for Molecular Science,State Key

Laboratory for Structural Chemistry of Unstable and Stable Species,Institute of Chemistry,Chinese Academy of Sciences Beijing 100190(P.R.China)E-mail:shengguihe@fc0f522a84868762cbaed51c [**]This work is supported by the Chinese Academy of Science

(Knowledge Innovation Program No.KJCX2-EW-H01),the National Natural Science Foundation of China (Nos.20933008and

21173233),the Major Research Plan of China (No.2011CB932302),and the China Postdoctoral Science Foundation (Nos.20110490601and

2012T50138).

Supporting information for this article is available on the WWW under fc0f522a84868762cbaed51c/10.1002/anie.201207016.

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The above reactions were confirmed by isotopic labeling experiments(Figure1,a4,b4,and c4).It is important to point out that production of AuNbO3H2+from ethane and AuNbO3H3+from n-butane are very selective,because product signals of AuNbO3H+(with C2H6and n-C4H10)and AuNbO3D2+(with n-C4D10)are relatively weak.The reaction of AuNbO3+with propane(C3H8,see Figure S3in the Supporting Information)generates both the double and triple HAA products,thus indicating a transition from selective double(reaction(2))to selective triple HAA (reaction(3))reactivity.Figure1c and Figures S2–S4indicate that the double HAA from the C2–C4alkanes by homonuclear oxide cluster Nb2O5+(m/z=266)[5e]can also be observed, although this is only a minor reaction pathway compared with the single HAA.

In addition to HAA,molecular association is also observed for the reactions of AuNbO3+with alkane mole-cules.Table1lists the total and HAA rate constants.[6c]The derived reaction efficiencies(f)[19]of AuNbO3+with CD4, C2H6,C3D8,and n-C4H10are0.49,0.55,0.78,and 3.0, respectively.Note that the ionic products of multiple HAAs

in reactions(2)and(3)are not a result of secondary reactions, such as AuNbO3H++C2H6or AuNbO3H++n-C4H10,because the relative signal intensities of products of double or triple HAA with respect to AuNbO3H+do not change when the concentration of the reactant gases is lowered.

The observation of the highly selective double HAA (reaction(2))is interesting but not unexpected,because it terminates with the generation of a stable molecule(C2H4).[20] However,the triple HAA(reaction(3))is very surprising, because reaction(2)would suggest that AuNbO3++n-C4H10 should terminate with AuNbO3H2++C4H8.

Density functional theory(DFT)calculations were per-formed for the structures of AuNbO3+(Figure S7)and the reaction mechanisms with CH4,C2H6,and n-C4H10(Fig-ure2b–d).The Au atom is terminally bonded with one O atom in the isomer of AuNbO3+with the lowest energy, which contains an oxygen-centered radical O Cà(Figure2a). Such a radical can abstract one H atom from CH4,C2H6,or n-C4H10very easily.The HAA from CH4terminates with the generation of a CH3radical(D H0K=à0.75eV)and the formation of double HAA species CH2is endothermic (D H0K=0.96eV).The DFT study indicates that CH4can also be nondissociatively absorbed on AuNbO3+(see XCH4in Figure2b,where X denotes AuNbO3+),which can be confirmed by a collision-induced dissociation experiment (Figure S6).

For AuNbO3++C2H6(Figure2c),the C2H5radical gen-erated in the HAA by O Càtends to bind strongly with the Nb atom(I3!TS2!I4)rather than to form free C2H5(I3! XH+C2H5).The high energy that is released(2.33eV)after formation of I4drives the second HAA(I4!TS3!I5)from C2H5by a non-radical O2àion,which finally leads to the highly exothermic double HAA(reaction(2),D H0K=à2.56eV).Because TS2(à1.18eV)is significantly lower in energy than XH+C2H5(à0.96eV),the double HAA can be highly selective with respect to the single HAA,which agrees with the experiments.

For AuNbO3++n-C4H10(Figure2d),formation of the double HAA intermediate I9with the C4H8moiety is more favorable than butyl generation(XH+1-C4H9and XH+2-

Table1:Bimolecular rate constants(k1in10à10cm3sà1,with pseudo-first-order kinetic approximation)for total and HAA reactions of AuNbO3+with small molecules.

Entry Molecules[a]k1total k1HAA 1CD4 4.10.2 2C2H6 5.0 2.9 3C2D6 4.6 1.8 4C3D8 6.6 4.0 5n-C4H102820 [a]Rate constants for reactions with CH4,C3H8,and n-C4D10were not derived because AuNbO4+,AuNbO4N2+,and Nb3O8+signals overlap with AuNbO3CH4+,AuNbO3C3H8+,and AuNbO3C4D10+,respectively (Figures1,S3,and S4).The uncertainties of the relative k1values are within30%and the absolute rate constants can be systematically under-or over-estimated by a factor of five.[5e,

6a]Figure2.The DFT-calculated structure of AuNbO3+(a)and potential

energy profiles for reactions of AuNbO3+(denoted as X)with CH4(b),

C2H6(c),and n-C4H10(d).The structures of the reaction intermediates

(I1–I12)are given,those of the transition states(TS1–TS9)and

products can be found in the Supporting Information(Figure S8–S10).

The relative D H0K energies(in eV),bond lengths(in pm),and profiles

for unpaired spin densities are shown.

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C4H9),which is similar to the favorable formation of I5rather than XH+C2H5(Figure2c).After formation of I9,the Au atom is transferred from O to make chemical bonds with Nb and C atoms(I9!TS7!I10!TS8!I11).This process releases additional energy to break the third CàH bond (I11!TS9!I12).The energies of TS7–TS9and XH3+C4H7 (C4H7=CH2CHCHCH3)are well below that of XH2+2-C4H8,so the triple HAA(reaction(3))is the major pathway for AuNbO3++n-C4H10.This result agrees well with the experiments(Figure1c).Note that H atom transfers starting from many other intermediates and alternative carbon sites were also tested(Figure S11),while Figure2d provides the most favorable pathway for reaction(3).

The Au atom is transferred from O to Nb during reaction(3).Such a gold atom transfer(GAT)is crucial because it makes space on the O atom,which accepts the third H atom.The lowest lying isomer of AuNbO3H3+is a singlet with three OHàgroups and one AuàNb chemical bond (Figure S13).Only formation of this AuàNb(OH)3structure

can satisfy the favorable thermodynamics to selectively produce the triple rather than the double HAA products:

D H0KeAuNbO3H3ttC4H7T(D H0KeAuNbO3H2tt2-C4H8Te4T

During reaction(3),Au atoms are involved in significant charge(Table2)and spin(Figure2d)transfers.The natural charge on Au decreases monotonically from0.81j e j in I6to 0.16j e j in I12.In contrast,the charge variation of the Nb atom is small.The gold atom in I6–I8can be considered to be Au1+(5d106s0),which is consistent with no distribution of unpaired spin density(UPSD)on Au in these cluster species. Upon the second HAA and the GAT(I9!I10),more and more UPSD is transferred to Au and most of the UPSD (%1m B)of I10is localized on Au,which can be considered to be Au0(5d106s1).This result is also consistent with loosely-bonded nature of Au in I10.Insertion of Au atom into Nb and C atoms(I10!I11)causes significant UPSD transfer from Au to C atoms.The third HAA(I11!I12)further eliminates the UPSD on Au and results in sharing of one pair of valence electrons between Au and Nb(Figure3).For the free product AuNb(OH)3+,the total AuàNb bond order amounts to1.42 (Table3)implying that this pair of electrons is well stored between the two metal atoms.Because of the strong relativistic effect,[15]the Au atom has a contracted and stabilized6s orbital,which tends to accept an electron.[21] This result is consistent with a high percentage(62%)of Au 6s in the highest occupied molecular orbital(HOMO, Figure3,left)as well as significant(45%)ionic bonding between Au and Nb(Table3,last column).

During the second and the third HAA,the protons(H= p++eà)are transferred to non-radical O2àsites and the electrons reduce metal atoms(Figure2d,see entry3in Table2).Such a process has been well described[22]as proton-coupled electron transfer and was identified in reactions of many transition-metal complexes.Both the proton and electron are transferred to a radical O Càsite during the first HAA(I6!I7),the barrier of which(0.25eV) is much smaller than those of the second(0.78eV)and third (0.70eV)HAAs.

To further demonstrate that the multiple HAA reactivity is due to the unique property of gold,the reactivity of the silver-containing counterpart AgNbO3+was considered the-oretically and experimentally.In contrast to the highly localized HOMO(>80%on Au)of AuNbO3H3+,the HOMO of AgNbO3H3+is almost equally delocalized around Ag and Nb atoms.The bond order of AgàNb is much lower than that of AuàNb(Table3),and the calculated bond energy of AgàNbO3H3+(1.87eV)is also smaller than that of AuàNbO3H3+(2.35eV).As a result,the triple HAA can be more exothermic than the double HAA for AuNbO3++n-C4H10[equation(4)].For the silver system, the tendency is reverse:D H0K(AgNbO3H2++2-C4H8) (à2.80eV)is more negative than D H0K(AgNbO3H3++ C4H7)(à2.25eV),a result that is consistent with our experi-ment that the triple HAA can not be observed for AgNbO3++n-C4H10.It can be concluded that the triple HAA reactivity(reaction(3))is promoted by gold.

In AuNbO3++C2H6,the double HAA is highly selective with respect to the single HAA(Figure1b).This observation is in sharp contrast with the result for AgNbO3++C2H6,in which the single HAA dominates the reaction(Figure S5). The reaction profile of AgNbO3++C2H6is very similar to that of the gold system(Figure S9).However,the C2H5radical generated from the first HAA is more loosely bounded in AgNbO3H·C2H5+(I3’in Figure4)than in AuNbO3H·C2H5+ (I3in Figure2c)because the former has lower binding energy (0.25versus0.30eV)and longer OH–C2H5distance(197 versus191pm).As a result,the C2H5radical in AgNb-O3H·C2H5+tends to dissociate into the single HAA products,

Table2:Natural charges(in j e j)on metal atoms in I6–12of Figure2d.

First HAA Second HAA Third HAA Entry I6I7I8I9I10I11I12 1Q Au0.810.780.770.400.280.270.16 2Q Nb 1.65 1.78 1.76 1.75 1.54 1.63 1.51 3Q Au+Q Nb 2.46 2.56 2.53 2.15 1.82 1.90

1.67Figure3.The HOMOs of AuNbO3H3+and AgNbO3H3+obtained through DFT calculations.

Table3:MàNb bond orders of MNbO3H3+(M=Au,Ag,and Cl)by natural bond orbital analysis.

Entry Covalent Ionic Total Ionic[%] 1AuàNb0.780.64 1.4245

2AgàNb0.650.320.9733

3ClàNb0.48 1.07 1.5569

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while AuNbO 3H·C 2H 5+has more chance to trap C 2H 5for further conversion into C 2H 4,which interprets the exper-imental results.The net charge in AgNbO 3H +is mostly

shifted onto Ag atom (0.94j e j ,Figure 4)and the NbO 3H moiety is almost neutral (0.06j e j ).In contrast,the ionization energy of Au (9.22eV)is higher (because of the stabilized Au 6s orbital)[15]than that of Ag (7.58eV),so the Au atom in AuNbO 3H +is less charged (0.83j e j )and the remaining NbO 3H moiety has a net charge of 0.17j e j .Thus,when C 2H 5approaches MNbO 3H +(M =Au/Ag)through the NbO 3H side

(I3in Figure 2c and I3’in Figure 4),the gold system will have relatively strong polarization [19b,c]of C 2H 5by NbO 3H and thus a good chance of trapping C 2H 5for further chemical conversion.It can be concluded that the highly selective double HAA reactivity (reaction (2))is also promoted by gold.Great interests have been devoted to gold catalysts,such as Au/MgO,Au/TiO 2,and many others that show excellent

performances in important reactions,including CO oxida-tion [11]and selective oxidation of organic molecules [23](e.g.,alkenes,alkanes,etc.).The interpretation of gold oxidation states in the catalysts is often controversial.Some reports emphasized the importance of cationic gold,[11a]while others,such as those based on theoretical studies,demonstrated the importance of anionic gold (e.g.,in O 2activation).[11c]It is inappropriate to state that the oxidation state of Au in AuNbO 3H 3+(Figure 3)is anionic because Au carries a pos-itive natural charge (0.18j e j ).However,the Au atom acts as electron acceptor during the triple HAA (Table 2,entry 1)and helps to store a pair of valence electrons between two metal atoms (Au and Nb).This pair of electrons may be released to activate and reduce O 2(Figure S15).We thus propose that storage (starting from Au 1+)and release of a pair of valence electrons around an Au atom can be an important mechanism during gold-catalyzed oxidation reactions,partic-ularly in cases when isolated mononuclear gold species [11a,24]on oxides are prepared as catalysts.It is interesting to point out that multiple HAAs with C 2H 6or n -C 4H 10(Figure 1and 2)are initiated by highly reactive oxygen-centered radicals.Such radical initiation is often proposed for alkane transformations,such as the selective oxidation of cyclohexane to produce cyclohexanone and cyclohexanol over cobalt [25]and gold [23b,26]catalysts.The cluster reactions thus show similar behavior to condensed-phase systems and provide insights into the possible activa-tion of multiple C àH bonds at the molecular level,which can be difficult in condensed-phase studies.In conclusion,the highly selective activation of multiple C àH bonds initiated by oxygen-centered radicals and pro-moted by gold is demonstrated.Gold acts as electron acceptor,and a pair of valence electrons can be stored between gold and metal (niobium)atoms in the triple HAA.This can be an important step to understand gold chemistry in many chemical processes.

Experimental Section

A reflectron time-of-flight mass spectrometer (TOF-MS)coupled with a laser ablation/supersonic expansion cluster source and a fast-

flow reactor was used for the experiments (see Ref.[27]for details).The M x Nb y O z +

clusters (M =Au or Ag)were generated by laser

ablation of an M/Nb mixed metal disk (molar ratios:Au:Nb =3:7and

Ag:Nb =1:5)in the presence of O 2(0.3%)seeded He carrier gas

(8atm).The generated clusters are reacted with CH

4,C 2H 6,C 3H 8,

n -C 4H 10,and their deuterated compounds (0.01–0.5Pa)in the

reaction cell for about 60m s under thermal collision conditions.The reflectron TOF-MS is used to measure the cluster abundances before and after the reactions.A separated TOF/TOF-MS (tandem MS)[28]employing a crossed He beam is used to study collision-induced

dissociation for mass-selected AuNbO 3CD 4+from reaction of

AuNbO 3+with CD 4.The method to derive the rate constants

(Table 1)is described in Ref.[6c].The details of DFT calculations

are given in the Supporting Information.Received:August 30,2012Revised:December 14,2012Published online:January 25,2013

.Keywords:C àH activation ·cluster compounds ·

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Communications

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