Computational Investigation of Alkynols and Alkyndiols Hydrogenation

Dec 19, 2013 - palladium clusters could be selective on the partial hydrogenation of triple bonds. The first energy barrier for the hydrogenation of t...
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Computational Investigation of Alkynols and Alkyndiols Hydrogenation on a Palladium Cluster Francesco Ferrante,* Antonio Prestianni, and Dario Duca Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Viale delle Scienze, Parco d’Orleans II, Ed. 17, 90128, Palermo, Italy S Supporting Information *

ABSTRACT: The reaction path leading to the partial and total reduction of alkynols and alkyndiols with general formula R−CH2−CC−CH(OH)−R′ and R−CH(OH)−CC−CH(OH)−R′ (R, R′ = H, CH3) on a D3h symmetry Pd9 cluster where atomic hydrogen is available have been analyzed by means of calculations based on density functional theory. The results, analyzed and discussed in atomistic details, suggest that small palladium clusters could be selective on the partial hydrogenation of triple bonds. The first energy barrier for the hydrogenation of the alkene derivatives is in fact much higher than the one corresponding to the hydrogenation of the triple bond. Further, the products of the partial hydrogenation, also when adsorbed on the Pd9 cluster, are largely more stable than the corresponding alkynol or alkyndiol parents.

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INTRODUCTION Catalytic hydrogenation of alkynols is nowadays considered a basic process for producing fine and intermediate chemicals.1−3 Two important examples are represented by the production, through highly selective processes, of fragrant substances, used in perfume and cosmetics, and of intermediate precursors in the synthesis of the vitamins E and K and of the provitamin βcarotene.4,5 In particular, but-2-yne-1,4-diol is an important precursor in the industrial synthesis of vitamins A and B6.2,6 Palladium catalysts, as already shown for other acetylene derivatives, of interest in fine and intermediate industry, have been individuated as the most successful for obtaining alkynol hydrogenation derivatives of practical importance.7 Several factors affect activity and selectivity of catalyzed reactions, involving alkynol to alkenol and alkenol to alcohol single processes. The latter, in particular, are specially influenced by the adsorption strength of the CC moieties on the palladium sites.8 Indeed, catalytic acetylene alcohols transformations seem also to be affected by the size of the organic substrate4 and, likely, the catalytic hydrogenation of other acetylene derivatives seem to be affected by the size and shape of the metallic crystallites (nanoparticles) employed,9−11 the hydride and carbide phases formation,12,13 and the carbonaceous byproducts, which can be generated on the metallic surface.14−16 These phenomena can cooperatively originate both electronic and decoration effects. The latter, which at atomistic level could be anyway considered as related to the former, are actually related to the formation of characteristic molecular aggregates on the surface, which clearly could affect the catalytic properties of the metallic particles.15−17 More specifically, the electronic effects, due to morphological and geometrical, as well as inner (bulk) and outer (surface) metal particle alterations, © 2013 American Chemical Society

generate metal band changes that could (in some cases, also strongly) affect the catalytic properties of the metal particles.15,18 In conclusion, due to the large number of convoluted phenomena, it is generally not easy to isolate and analyze, by simple experimental techniques, atomistic aspects related to the elementary events occurring in the title reactions. Approaches based on computational chemistry could, on the contrary, overcome these difficulties, being effective in separating, hence, in singly analyzing, the convoluted phenomena.19,20 Computational modeling, in the frame of both simulative and quantum-mechanical methods, is indeed quite important in several aspects of the catalysis research and development.21,22 Among the main aims of the computational modeling, the engineering and the structural and energetic characterization of the catalytic sites involved in the heterogeneous processes as well as the molecular level studies on the interactions and transformations of the catalytic substrates adsorbed on the nanoparticle and on the cluster sites are of primary importance. With respect to this, we have recently reported on several palladium systems of interest in catalysis, taking into account cluster applications.23−35 Besides other potential applications, concerning as an example the development of magnetic26 and photonics27 devices, the very high surface area characterizing supported transition metal clusters is extremely interesting considering their potential industrial catalytic utilization. In fact, metal clusters opportunely built could gather-up the advantages of both homogeneous and heterogeneous catalysts, having Received: November 5, 2013 Revised: December 18, 2013 Published: December 19, 2013 551

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with the more expensive coupled cluster method42 and could give results in agreement with experimental evidence derived from kinetic investigations.43,44 The nature of minima and transition states on the potential energy surface has been checked by inspection of the harmonic vibrational frequencies. All energy values here reported have been corrected for the zero point vibrational contribution; essentially the same conclusions are achieved if Gibbs free energies are considered instead (see Tables S1−S3 in Supporting Information).

selectivity comparable to that of the former and, when supported, viability and manageability of the latter.28,29 Due to their morphological and electronic characteristics, they may have even special properties,30−33 which could result tunable by changing sizes and shapes of the same clusters.34 In this paper we report a computational study performed on the hydrogenation of a series of n-alkynols and n-alkyndiols on a small palladium cluster where atomic hydrogen is considered as easily available. The investigation, in particular, regards the -ol and -diol derivatives of n-alkynes with four to six carbon atoms, which bear the −OH group attached to a carbon atom adiacent to the unsaturate bond. The following standard nomenclature will be used along with the text to indicate the involved compounds: alk-(n+1)-yne-n-ol and alk-(n+1)-yne-n, (n+3)-diol, and so on for the corresponding -ene molecules. The proper -ynol and -enol forms, R−CC−OH and R− CHCH−OH, have been excluded from the present investigation since their chemistry is known to be fundamentally different from that of the compounds here chosen. Models and computational details are summarized in the next section, while the results with the related discussion are reported in the following section, which includes two subsections concerning alkynols and alkyndiols adsorption and hydrogenation to alkenols and alkendiols and two other subsections concerning the adsorption of the latters and their hydrogenation to the corresponding alcohol derivatives.

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RESULTS AND DISCUSSION In a recent computational investigation,24 it has been reported that the fragmentation of the hydrogen molecule on a bare Pd9 cluster should occur with energy barriers in the 25−35 kJ mol−1 range, depending on the H2 adsorption site. Further, the energy barriers for the diffusion of the atomic hydrogen along the faces of the small cluster should not exceed 20 kJ mol−1. These results lead to hypothesize that on the Pd9 cluster faces there should be a similar and easy availability of atomic hydrogen, irrespective of the cluster site.24,25 This assumption is in force in the present work. The results obtained in this work for the alkynol and alkyndiol hydrogenation energetics are collected in Tables 1−3. Table 1. Vibrational Zero-Point Corrected Energetics of the Alkynols and Alkyndiols to Alkenols and Alkendiols Catalytic Hydrogenation of Symmetric and Nonsymmetric Molecules on the Pd9 Cluster

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MODELS AND COMPUTATIONAL DETAILS The palladium substrate used is a Pd9 cluster having a trigonal prismatic geometry with centered square faces (D3h symmetry). It has three Pd atoms with coordination number 6 (A-type sites) and six Pd atoms with coordination number 4 (B-type sites). Previous investigations23 indicated that it has an electronic ground state with quintet spin multiplicity and it is more stable than a hypothetical C4v symmetry Pd9 cluster obtainable by simple truncation of palladium bulk. This particular geometry of the Pd9 has been shown to be that which better interacts with graphyte and carbon nanotubes,23 taking into account the peculiar interaction geometry between palladium and graphytic carbon. Hydrogen fragmentation on the Pd9 cluster leads the multiplicity lowering to the triplet state,24 likely due to spin uncoupling, as happens in the triplet/singlet change following hydrogen fragmentation that occurs on tetrahedrical Pd435 and on the Pt(H2) system.36 Finally, according to D’Anna et al.,25 all the hydrogenation reactions have been investigated on the triplet surface. The elementary events following the alkynol catalytic hydrogenation were investigated hypothesizing a Langmuir−Hinshelwood mechanism.8 The Gaussian 09 suite of programs37 has been employed for all the calculations here reported. All species have been investigated in the density functional theory framework by using the hybrid B3LYP functional38−40 in its unrestricted form, the cc-pVDZ-PP effective core potential for palladium41 and the cc-pvdz basis set for light atoms. The pseudopotential on the palladium atoms has been developed to take into account scalar relativistic effects to some extent; we expect that the neglect of spin−orbit interactions should not affect sensibly the conclusion reported in this work, which are based on large energy differences. In regard to the choice of the B3LYP exchange-correlation functional, very recent studies have demonstrated that, for reactions occurring on metal clusters, it can reproduce satisfactorily well energy barriers calculated

ΔEb (kJ mol−1) a

starting reactant

TS1

int1

TS2

-ene-

but-2-yne-1-ol pent-2-yne-1-ol pent-3-yne-2-ol hex-2-yne-1-ol hex-3-yne-2-ol hex-4-yne-3-ol but-2-yne-1,4-diol pent-2-yne-1,4-diol hex-2-yne-1,4-diol hex-3-yne-2,5-diol

43.6 44.6 46.0 45.1 56.6 50.0 34.3 38.2 39.1 41.0

−17.8 −17.0 −15.6 −16.8 −14.0 −12.0 −34.1 −33.7 −32.6 −30.1

92.9 94.6 94.7 94.6 96.4 94.4 95.3 87.4 96.3 97.3

−91.3 −91.2 −72.0 −90.9 −73.0 −69.6 −85.2 −67.0 −64.9 −64.5

For nonsymmetric molecules, energetics pertaining to β-type mechanisms are reported: Here, β-type mechanism states that the hydrogen transferred from the cluster reacts with carbon atoms in the β position with respect to the hydroxyl group. In the alkyndiol systems, the considered hydroxyl group is that in position 1 (see text). b The energy of each starting reactant is taken as reference for any of its hydrogenated compound derivative. a

Symmetric and nonsymmetric species are mentioned in Tables 1−3. It is here important to stress that this symmetry concerns the configuration (irrespective of the conformation) of the two moieties separated by the triple bond. Thus, alkynols are always nonsymmetric, while, as an example, but-2-yne-1,4-diol and pent-2-yne-1,4-diol are symmetric and nonsymmetric, respectively. In Figure 1 the mechanism applied to the but-2-yne-1,4-diol molecule is schematically represented. In fact, all the alkynol and alkyndiol derivatives show essentially the same structures, in regard to intermediates and transition states, with slight and obvious differences caused by local morphological changes determined by the presence (or the absence) of one or more groups. 552

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Table 2. Vibrational Zero-Point Corrected Energetics of the Alkynols and Alkyndiols to Alkenols and Alkendiols Catalytic Hydrogenation of Nonsymmetric Molecules on the Pd9 Cluster ΔEb (kJ mol−1) starting reactanta

TS1

int1

TS2

-ene-

but-2-yne-1-ol pent-2-yne-1-ol pent-3-yne-2-ol hex-2-yne-1-ol hex-3-yne-2-ol hex-4-yne-3-ol pent-2-yne-1,4-diol hex-2-yne-1,4-diol

42.0 44.4 40.8 45.7 43.6 41.1 50.5 55.0

−29.0 −30.2 −27.6 −28.3 −28.5 −27.6 −23.7 −24.0

87.1 88.2 88.3 90.0 89.7 90.5 102.5 106.3

−83.2 −82.1 −83.6 −80.5 −83.5 −79.8 −75.6 −70.5

For all the molecules, energetics pertaining to γ-type mechanisms are reported: Here, γ-type mechanism states that the hydrogen transferred from the cluster reacts with carbon atoms in γ position with respect to the hydroxyl group. In the alkyndiol systems, the considered hydroxyl group is that in position 1 (see text). bThe energy of each starting reactant is taken as reference for any of its hydrogenated compound derivative. a

Table 3. Vibrational Zero-Point Corrected Energetics of the Alkenols and Alkendiols to Alkanols and Alkandiols Catalytic Hydrogenation on the Pd9 Cluster ΔEa (kJ mol−1) starting reactant

TS3

int2

TS4

-ane-

but-2-ene-1-ol pent-2-ene-1-ol pent-3-ene-2-ol hex-2-ene-1-ol hex-3-ene-2-ol hex-4-ene-3-ol but-2-ene-1,4-diol pent-2-ene-1,4-diol hex-2-ene-1,4-diol hex-3-ene-2,5-diol

95.2 95.4 94.3 95.6 90.6 96.3 92.8 89.7 89.2 87.8

88.3 86.8 77.4 87.6 74.8 77.0 83.0 77.5 77.3 77.6

177.3 179.2 164.8 179.6 165.3 163.0 166.2 148.8 151.6 149.2

37.5 25.7 18.6 28.7 26.5 21.8 0.8 −13.3 −10.0 −14.5

Figure 1. Hydrogenation mechanisms on the Pd9(H)2 cluster, involving the but-2-yne-1,4-diol molecule as the starting reactant: Both the reaction paths are representative of all the investigated starting species. Upper and lower diagrams illustrate the hydrogenation from the triple to the double bond and from the double to the single bond, respectively.

a

The energy of each starting reactant is taken as reference for any of its hydrogenated compound derivative.

Adsorption of Alkynols and Alkyndiols on Pd9(H)2. The first undertaken step was the investigation of the preferred adsorption geometries of the alkynols and alkyndiols on the Pd9(H)2 cluster. The starting adsorbate geometries were taken from a preliminary study on the adsorption of acetylene on Pd9 and Pd9(H)2. According to our calculations, acetylene should adsorb on the bare palladium cluster by forming a four center ring, characterized by the sequence PdA−C−C−PdB (see Figure 2) and leaving the system on the quintet state. The adsorption causes major distortions in the acetylene molecule, whose CC bond length stretches to 1.311 Å, while the C−H bond becomes 1.099 Å (to be respectively compared with the values 1.210 and 1.072 Å, characterizing the isolated acetylene at the same level of theory) and the C−C−H bond angle closes to 134°. The adsorbed molecule remains almost planar and only slight distortions are observed into the cluster, decreasing the PdA−PdB bond length involved in the palladium adsorption sites of about 0.1 Å. The C−Pd distances in this system were, to end, 1.992 and 1.969 Å for C−PdA and C−PdB, respectively. When fragmented hydrogen is present on the cluster, located on the favorable BB′B″ and A″BB″ faces, no sensible

Figure 2. Optimized structure of the acetylene/Pd9 system, used as a template for the starting geometries of alkynols and alkyndiols. On the cluster, the Pd atom labeling, used throughout, is reported.

distorsions can be detected for the acetylene adsorption geometry, except for the C−PdB distance, which is increased to 2.000 Å, due to the crowding around the PdB center (the C− PdA distance, conversely, increases by only 0.005 Å). Thus, the presence of fragmented hydrogen coadsorbed on the palladium cluster should not produce large alteration to the adsorption characteristics of acetylene. The BB′B″−A″BB″ configuration for the (H)2 moiety is the most stable also when alkynols and alkyndiols are coadsorbed 553

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alkyndiol series does not seem to have significant effects. As a final consideration, it must be said that either alkynols or alkyndiols could also interact with the Pd9 cluster through oxygen atoms. If the species where the carbon atoms involved in the triple bond are attached to PdA and PdB, the interaction of the oxygen with palladium strongly depends on the conformation of the adsorbed molecule. It was found that an interaction O−Pd can occur involving the PdB site opposite to the one occupied by the strongly bonded carbon atom. In this case, however, a sensible distortion of the adsorbed molecule is required. The adsorbate should indeed bend by about 20° with respect to the ABB′ face, losing the very efficient interaction occurring between the triple bond and the Pd9 cluster but, in any case, resulting in a net stabilization of about 20 kJ mol−1. The conformation chosen for the starting reactants, used throughout, do not show O−Pd interactions, since neither the first transition states nor the first semihydrogenated intermediate could, for geometrical reasons, allow it. In order to investigate the energetics of the pure hydrogen shift, we preferred to start from the conformations that are the most closely related to those of the transition states. This modeling perspective would imply that, before the occurrence of the reaction, the adsorbate should, in a preceding step, change its conformation. There is, however, an issue that is worth noting. Some of the investigated compounds have one or two chiral carbon atoms. Let us take the pent-2-yne-1−4-diol as representative (see Figure 4). Here, the carbon atom in position 4 is chiral and the (R) and (S) stereisomers may exist. When the (R)-pent-2-yne1−4-diol is adsorbed on Pd9 in the configuration originating the γ-mechanism, the oxygen of the −OH group in position 4 could interact with PdB, with a stabilization of 21.9 kJ mol−1 with respect to the conformation without interaction. If the (S)pent-2-yne-1−4-diol is considered instead, the same interaction is less efficient because of unfavorable steric hindrance arising from the methyl group in position 4. In any case, the stabilization with respect to the conformation without O−Pd interaction is reduced to 9.6 kJ mol−1. This suggest that the Pd9 cluster (the one having D3h symmetry is indeed chiral) would discriminate the (R) and (S) stereoisomers through the adsorption strength and, if the O−Pd interaction is not present in the adsorbate species subject to the reaction, the (R) isomer would react slower than the (S) one. In this work, however, only the (S) stereisomer, the (2S,5S) in the particular case of hex-3-yne-2,5 diol, has been considered when the molecule possessed a chiral center. First Semihydrogenated Intermediate. The migration of a hydrogen atom from the palladium cluster to one adsorbed alkynol or alkyndiol molecule gives rise to the corresponding semihydrogenated intermediate. The carbon atom involved in the H-shift is invariably the one bonded to the PdA site; it is the carbon atom in the β position for the β-type mechanism or the one in the γ position for the γ-type mechanism. Since no Hshift has ever been obtained from the cluster to the unsaturated carbon atom bonded to the PdB site, the discrimination between β- and γ-type mechanisms seems to occur solely on the basis of the adsorption geometry of alkynols and alkyndiols. Calculations revealed that the intermediates are always more stable than the corresponding reactant. In the β-mechanism the stabilization is evaluated in the range 14−18 kJ mol−1, considering alkynols, to over 30 kJ mol−1, considering alkyndiols. In the γ-mechanism the intermediates derived from alkynols, lowering the energy by a little less than 30 kJ

on the cluster, but this configuration cannot be used as reactant since (as discussed below) the formation of the first reaction intermediate must occur through the hydrogenation of the carbon atom bonded to the central PdA site. The configuration BB′B″−ABB′ can be used instead, which is, on the average, 15 kJ mol−1 less stable than the BB′B″−A″BB″ configuration. According to D’Anna et al., less stable configurations of hydrogen atoms on the cluster can be, indeed, assumed since H-diffusion on Pd9 is an event requiring relatively small energetic expense.24 The energetics of all the transition states and intermediates in the partial hydrogenation of alkynols and alkyndiols will be therefore reported, considering the corresponding starting reactants with hydrogen atoms in the BB′B″−ABB′ configuration. When nonsymmetric alkynols and alkyndiols were considered, two adsorption geometries of the starting compound have been taken into account. In Figure 3 both geometries are

Figure 3. Two adsorption geometries originating β- (left) and γ-type (right) mechanisms, when nonsymmetric alkynols and alkyndiols are considered. The carbon atom accepting the first hydrogen from the cluster is always bonded to the PdA site. In this example, the es-2-yne1-ol/Pd9 systems are illustrated.

reported for the representative es-2-yne-1-ol case. In fact, the first migration of a hydrogen atom from the cluster to the adsorbed molecule, hence, to some extent the whole transformation mechanism that follows, can be dependent on whether the accepting carbon atom is in β or γ position with respect to the hydroxyl group (those in position 1 for the alkyndiol systems); we will refer to the two mechanisms as βtype and γ-type, respectively. The energy differences between the adsorption geometries of reactants in the β-type and the γtype mechanisms are reported in Table 4. The starting configuration is regularly more stable, from about 5 to 11 kJ mol−1, when the Cγ is interacting with a PdA site. The presence of the second hydroxyl group in the Table 4. Vibrational Zero-Point Corrected Energy Differences (ΔEβγ = Ezpvc − Ezpvc γ β ) Occurring between the Same Starting Reactants when Involved in Either β- or γType Mechanisms reactant

ΔEβγ (kJ mol−1)

reactant

ΔEβγ (kJ mol−1)

but-2-yne-1-ol pent-2-yne-1-ol pent-3-yne-2-ol hex-2-yne-1-ol

−8.1 −9.1 −5.1 −10.4

hex-3-yne-2-ol hex-4-yne-3-ol pent-2-yne-1,4-diol hex-2-yne-1,4-diol

−7.2 −5.9 −6.9 −10.8 554

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Figure 4. Optimized geometries of (R)-pent-2-yne-1−4-diol/Pd9 (a) and (S)-pent-2-yne-1−4-diol/Pd9 (b, c). The latter is in a conformation showing a nonefficient O−Pd interaction.

mol−1 with respect to the corresponding reactants, are slightly more stabilized (on the average, by about 5 kJ mol−1) than those derived by alkyndiols. In regard to the geometry of the intermediates (see Figure 5), the carbon atom that accepted the hydrogen remains atop

prefer the γ-mechanism by 9−13 kJ mol−1, and the nonsymmetric diols, which do prefer the β-mechanism by 12−16 kJ mol−1. These differences could be tentatively explained by taking into account a subtle balance between weak long-range O−Pd interactions (which realizes in alkynols, following γmechanism and all alkyndiols) and small steric hindrances mainly caused by CH(OH)CH3 or CH(OH)CH2CH3 groups, located too much close to the cluster (which realizes in the hexyne series). In order to achieve partial hydrogenation of the triple bond, a second hydrogen atom must migrate from the cluster to the carbon atom bonded to the PdB site. This second hydrogenation seems to be, in some way, more difficult than the first one, since it involves a transition state (TS2) very high in energy. The situation is worsened by the stability of the semihydrogenated intermediate. In order to transform into the product, the energy barrier that the semihydrogenated intermediate above has to overcome is about 110 or 120 kJ mol−1 for the alkynols, following β-mechanism or γ-mechanism, respectively. The energy barrier value reaches 130 kJ mol−1 for the alkyndiols. This is likely due to the fact that the structure and the orientation corresponding to TS2 can be achieved by the adsorbate only if the C−PdA bond is broken. Furthermore, the distortions cannot be compensated either by the appearance of O−Pd interactions (see Figure 5) or by the restoring of the original D3h-type symmetry of the Pd9 cluster. Processes end with the formation of the alkenols and alkendiols, which locate their double bond atop the PdB site. The geometries along the reaction path on the Pd9 cluster, that is, the migration of the first hydrogen from the ABB′ face and the migration of the second hydrogen from the BB′B″ face, forces the alkenols and alkendiols to be produced in the cis isomer form. Since no more hydrogen is fragmented on the cluster, the original quintet spin multiplicity of the Pd9 cluster is restored: In the triplet state the calculated energy rose by more than 20 kJ mol−1, in most cases. Pathways of β- and γ-type join to this point and this rules out the necessity to consider two reaction paths in the following. In fact, alkenols and alkendiols place themselves with their double bond axis almost perpendicular to the C3 symmetry axis of the Pd9 cluster. This placement is typical of molecular interactions involving CC bonds and sites of palladium clusters in the quintet spin multiplicity.25 Therefore, if the alkene species is formed from β- or γ-mechanisms, it can locate on the cluster in a way that would allow the oxygen atom to interact with PdA or PdA″, respectively. Thus, the γ-mechanism would be, in this

Figure 5. Schematization of the first transition state (TS1), the semihydrogenated intermediate (int1), and the second transition state (TS2) occurring along with alkynol and alkyndiol partial hydrogenation paths. The case of but-2-yne-1,4-diol/Pd9(H)2 was taken as representative and the reported distances (in Å) correspond to its reaction path. In the starting reactant of the same system, the CC, C−PdA, C−PdB, and PdA−PdB distances are 1.316, 2.026, 2.019, and 2.649 Å, respectively.

the PdA site while the one that was on the PdB site is now bridged almost symmetrically between the PdA (at distance 2.077 Å in the but-2-yne-1,4-diol system) and the PdB (at distance 1.997 Å). This causes a major rearrangement of the molecule on the cluster and a distortion of the cluster itself, essentially localized on the elongation of the PdB−PdB″ distance. The hydrogen atom once in the PdB−PdB′−PdB″ face, in turn, locates in the PdB′−PdB″ bridged position. The energy balance between the distortions and the strong chemisorption of the semihydrogenated alkyne is evidently in favor of the latter. The energetic trends reported in Tables 1 and 2 could be explained in terms of a weak interaction of the oxygen atom with the PdB where the carbon atom is bonded to, which can occur only in the alkynols following the γmechanism and alkyndiols (in both the mechanisms). The calculation results on the transition states, leading from the reactant to the semihydrogenated intermediate (TS1), show a relatively low energy barrier to be guessed. The lowest and the highest energy barriers are predicted to be those corresponding to the hydrogenation of but-2-yne-1−4-diol (34.3 kJ mol−1) and of hex-3-yne-2-ol following the βmechanism (56.6 kJ mol−1), respectively. As a matter of fact, the only significant differences between β-mechanism and γmechanism is the hex-3-yne-2-ol and hex-4-yne-3-ol, which 555

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case, specular to the β-one, due to the particular symmetry of the Pd9 cluster. By comparing with a conformation that does not allow the O−Pd interaction, the stabilization produced by this interaction has been calculated equal to 34 kJ mol−1 in the case of but-2ene-1-ol (see Figure S1 in Supporting Information). The stabilization above would have been even larger if the (R) stereoisomers were considered, where this can apply, since the H···CH3 or H···CH2CH3 steric repulsion should vanish. In point of fact, according to our calculations, the (R)-pent-3-ene2-ol/Pd9 and (R)-hex-4-ene-3-ol/Pd9 systems, for example, resulted in 16.7 and 16.0 kJ mol−1 more stable than their (S) counterparts, respectively. The H···CH3 and H···CH2CH3 steric repulsions are also responsible for the lower stability of secondary alkenols adsorbed on Pd9 with respect to the primary alkenols. Alkenols and Alkendiols on Pd9(H)2. Once triple bonds had been hydrogenated to double bonds, the following reduction of the corresponding alkene was investigated. Hence, a second hydrogen molecule was postulated to fragment on the cluster. This produces new systems (characterized by new potential energy surfaces) with the resulting two H atoms always in BB′B″ and ABB′ positions, again showing triplet spin multiplicity. The conformation of the alkenols and alkendiols is, anyway, the one directly achieved by the partial hydrogenation. The introduction of hydrogen atoms in the Pd9 position above causes, however, sensible distortions to the adsorption geometry of the alkenols and alkendiols. In fact, being in the triplet spin multiplicity, the axis of the double bond rearranges now almost parallel to the C3 axis. Finally, since it was found that also the transition states leading to the semihydrogenated intermediates show O−Pd interaction, no conformational changes occur in this case. Second Semihydrogenated Intermediate. The energy barriers characterizing the migration of the hydrogen atom from the cluster to the adsorbed alkenol and alkendiol molecules are in the narrow range 88−96 kJ mol−1, with the hex-3-ene-2,5diol/Pd9 system having the lowest barrier (87.8 kJ mol−1) and the hex-4-ene-3-ol/Pd9 one the highest (96.3 kJ mol−1). These values are in agreement with those already found in the case of the hydroisomerization of simple but-2-ene,25 and this seem to happen because the reactants have adsorption geometries that closely resemble those of the transition states (Figure 6), that is, no sensible distortions must come into play other than those obviously involving the shifting of the

hydrogen atom and a lenghtening of the CC bond by about 0.03 Å. As a matter of fact, the transition states show an O−Pd interaction almost unaltered with respect to the corresponding reactants. It is to say that on this particlar Pd9 cluster, the migration of hydrogen can occur solely from the ABB′ face to the carbon atom in γ position with respect to the one bearing the −OH groups interacting with Pd. The energy of the semihydrogenated intermediate, on the other hand, is strongly affected by the disappearance of this very same O−PdA″ interaction. In the alkendiols cases another interaction appears, now with PdB″, which does not seem efficient as regards both the O−Pd distance and orientation (secondary alchoholic groups shows a slightly stronger interaction). This of course affects the conclusion about the whole hydrogenation reaction paths. In the semihydrogenated intermediate there is the possibility of a relatively free rotation around what once was the double bond. The rotation would lead to a conformation that would evolve into the trans isomer of both the alkenol and alkendiol derivatives by back-donating the hydrogen atom to the cluster. By considering the but-2-ene-1,4-diol case, calculations reveal that the cis to trans hydroisomerization is a relatively easy task on the Pd9 cluster, having the transition state from int-2 to the trans isomer essentially the same energy of TS3 and being the reaction path almost specular with respect to that of int-2 (Figure 7). The energy barriers from int-2 to the cis or trans

Figure 7. cis−trans Hydroisomerization of but-2-ene-1,4-diol catalyzed by the Pd9 cluster. The reported energies are expressed in kJ mol−1.

form is very low; namely, about 35 kJ mol−1 lower than the corresponding barriers estimated for the but-2-ene hydroisomerization.25 This could once again be attributable to the fleeting nature of the semihydrogenated intermediate, which is destabilized with respect to the reactants and the transition states by the absence of efficient O−Pd interactions. Irrispective of the cis or trans isomer used as reactant, in the very last step the double bond is being completely saturated. It was found that the last migration of the hydrogen atom from the cluster involve exclusively the BB′B″ face, so that the O− PdA″ interaction must be definitively broken; in the diol cases, the O−PdB″ is even weakened. This yields to very high energy transition states, that lead to energy barriers ranging from 70 to 90 kJ mol−1. The whole hydrogenation of alkynols and alkyndiols ends up with alcohols and diols, once again in the quintet multiplicity surface. These compounds are free to adopt a conformation that adsorb on the Pd9 cluster efficiently (but almost exclusively) with (both) the hydroxyl group(s). The systems

Figure 6. Schematization of the first transition state (TS3), the semihydrogenated intermediate (int2), and the second transition state (TS4) occurring along the alkenol and alkendiol hydrogenation pathways. The case of but-2-ene-1,4-diol/Pd9(H)2 was taken as representative, and the reported distances (in Å) correspond to its reaction path. In the reactant of the same system, the CC bond length is 1.398 Å and the C−PdB distances are 2.181 and 2.194 Å. There is, finally, also an O−PdA″ interaction with O−Pd distance of 2.298 Å. 556

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The Journal of Physical Chemistry C formed by the alcohols adsorbed on Pd9 are less stable than the alkenol systems from which they derive for an amount of energy ranging from about 20 to about 40 kJ mol−1; secondary alchools resulted able to interact slightly stronger with the Pd9 cluster. As regard the alkandiols, on the other hand, the strong O−Pd interactions make their adsorption as stable (or even more, given the presence of supplementary secondary −OH group) as that of the alkendiols.

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REFERENCES

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CONCLUSIONS The results obtained in this work show that, on a small palladium cluster, the hydrogenation of the triple bond in alkynols and alkyndiols occurs through the formation of a stable first semihydrogenated intermediate by overcoming an energy barrier in the range of 35−55 kJ mol−1, being the diols in the lower end of the interval. The hydrogenation of the corresponding alkenols and alkendiols, on the other hand, occurs by the formation of fleeting intermediates obtained by overcoming energy barriers of about 90 kJ mol−1, irrespective of the presence of one or two −OH groups. The nature of the semihydrogenated intermediate in the alkynols and alkyndiols case is mirrored on the second energy barrier to be overcome: Its value is well beyond the 100 kJ mol−1 for all the compounds, letting one presume that the formation of the alkene-type derivatives, more difficult to be obtained from the point of view of the energy barrier, is thermodynamically driven by the fact that the final products are extremely stabilized with respect to the initial reactants. On this basis, the difference of 50 kJ mol−1, on the average, between the energy barriers corresponding to the first hydrogenation step of the triple and double bond would suggest that small palladium clusters could be particularly active for the partial hydrogenation of alkynols and alkyndiols, which is industrially desiderable. The role of the O−Pd interactions is very important in ruling the reaction pathways. However, they are, at least for the cases here studied, not able to alter the ratio generally characterizing the hydrogenation rates of triple and double bonds, being the former always more reactive. Finally, the whole energetic behavior seems to indicate that the title reaction is not strongly molecular sensitive. However, some differences emerge for the studied species that could be related to the peculiar interactions, which characterize the different adsorption modes of the moieties present in the different starting reactants with the cluster sites, once more, showing the importance of studying catalytic reactions in their specific complexity, in this case, for example, taking into consideration all the Pd9 cluster details. ASSOCIATED CONTENT

S Supporting Information *

Gibbs free energy profiles and Cartesian coordinates of all minima and transition states in the reaction paths. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

This work was supported by the POLYCAT project (Modern polymer-based catalysts and microflow conditions as key elements of innovations in fine chemical synthesis), funded by the 7th Framework Programme of the European Community; G.A. No. CP-IP 246095; http://polycat-fp7.eu/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +39 091 23897979. Fax: +39 091 590015. Notes

The authors declare no competing financial interest. 557

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The Journal of Physical Chemistry C

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