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J. Phys. Chem. C 2009, 113, 8794–8805
Mode of Adsorption of (CH3)2Au(acac) onto Partially Dehydroxylated Silica Miyako Hisamoto,† Ryan C. Nelson,† Ming-Yung Lee,† Juergen Eckert,‡ and Susannah L. Scott*,†,§ Department of Chemical Engineering, UniVersity of California, Santa Barbara, California 93106, Materials Research Laboratory, UniVersity of California, Santa Barbara, California 93106, and Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93106 ReceiVed: NoVember 15, 2008; ReVised Manuscript ReceiVed: March 5, 2009
The deposition of volatile cis-(CH3)2Au(O,O′-acac) onto silica partially dehydroxylated at 400 °C leads to molecular dispersion of the organogold complex. X-ray absorption near-edge structure demonstrates that the gold retains its oxidation state upon binding. IR and 1H magic-angle spinning NMR spectra suggest that the molecular framework also remains intact. Computational models involving hydroxyl-terminated octasilsesquioxane cube clusters to represent silica predict the most energetically favorable interaction to be hydrogenbonding between two surface hydroxyls and the oxygen donor atoms of the chelated acetylacetonate ligand. This mode of adsorption was confirmed by analysis of the IR and the Au LIII edge extended X-ray absorption fine structure. The surface hydroxyls most likely to participate in the attachment of the gold complex to silica are vicinal silanol pairs that are not involved in hydrogen bonding to other silanols. Introduction Small gold particles dispersed on silica were shown to be weakly active for the hydrogenation of olefins as early as 1973.1 A little more than a decade later, Haruta et al. reported that supported gold catalysts are highly active for CO oxidation at temperatures as low as -70 °C,2,3 provided the average particle diameter falls within a narrow range (3-10 nm).4 A computational investigation of the activity of noble metal nanoparticles revealed that the Sabatier rate for CO oxidation is a maximum for gold because of the increased adsorption energies on highly coordinatively unsaturated sites relative to close-packed surfaces.5 Gold support interactions have also been suggested to be crucial to catalytic activity, with the interface as the location of the most active sites and reducible oxide supports such as Fe2O3, TiO2, and CeO2 giving more active catalysts.6-8 However, the different sizes and/or morphologies of gold nanoparticles in different catalyst preparations may also contribute to changes in activity,9,10 and Au/SiO2 has been reported to be as active as Au/TiO2 when the gold particle size is suitably small.11,12 Metal acetylacetonates, including gold-based complexes,13 are widely used as chloride-free precursors in the chemical vapor deposition of thin metal films14 and in nanoparticle synthesis,15 the latter being of particular interest for the preparation of highly dispersed catalysts on a variety of solid supports.16-19 Mild thermolysis of supported metal acetylacetonates generally leads to better metal dispersions than is possible starting from metal salts.20,21 This effect has been attributed to stabilization of the molecularly dispersed precursor on the support via specific chemical interactions between the metal complex and functional groups on the surface. For acetylacetonates adsorbed on oxides, two such interactions are frequently proposed: hydrogen bonding * To whom correspondence should be addressed. E-mail: sscott@ engineering.ucsb.edu. Fax: 1-805-893-4731. † Department of Chemical Engineering, University of California, Santa Barbara. ‡ Materials Research Laboratory, University of California, Santa Barbara. § Department of Chemistry and Biochemistry, University of California, Santa Barbara.
between the ligand and the surface hydroxyls, and ligand displacement involving the formation of new, covalent metalsupport bonds with liberation of acetylacetone. The dominant path depends on the nature of the support and its hydroxyl population, the nature of the metal complex, its degree of coordinative unsaturation, as well as the temperature of the deposition reaction.22 The preparation of gold catalysts by chemical vapor deposition is useful because very small (12 h at room temperature). This suggests that a fraction of the surface hydroxyls are acidic enough to form strong hydrogen bonds. The difference IR spectrum of adsorbed acacH is compared with that of adsorbed (CH3)2Au(acac) in Figure 3. As expected,54 the CsH stretching modes of acacH are much weaker than those of the [(CH3)2Au]+ fragment. The presence of acacH causes the tSiOsH stretching vibration to shift to 3439 cm-1 (not shown), because of hydrogen-bonding. The position of the keto-enol tautomer equilibrium in β-diketones is strongly influenced by solvent polarity. The less polar enol form is favored for neat acacH, in both the gas and liquid phases as well as in nonpolar solvents. However, the keto form becomes more stable in protic environments, including water.61 Both keto and enol forms of acacH coexist on silica, Scheme 1. These structures and their relative energy are consistent with computational models for adducts with methanol62 as well as with octasilsesquioxane silanols (see below). In Figure 3, the vibrations at 1724 and 1695 cm-1 are assigned to symmetric and asymmetric carbonyl stretching, respectively, of the keto tautomer,63 while the bands at 1620 and 1590 cm-1
Figure 3. Comparison of transmission IR spectra for (a) (CH3)2Au(acac) grafted onto A380-400 (blue), (b) polycrystalline (CH3)2Au(acac) (red), and (c) acacH adsorbed on silica (black), for (i) the C-H stretching region, showing assignments to the modes of the [(CH3)2Au]+ fragment, and (ii) the C-O and C-C stretching regions, showing assignments to acac modes. Spectra a and c are difference spectra obtained by subtracting the silica spectrum. They were normalized to the same intensity of the silica overtone at 1980 cm-1. Spectra a and b were normalized to the same intensity of the mode at ca. 2900 cm-1.
In the region from 1800 to 1300 cm-1, which contains the C-O and C-C stretching modes of (CH3)2Au(acac) as well as its CH3 deformations, the positions and relative intensities of the IR-active modes are also seen to change upon adsorption. Narrowing of the bands is attributed to the absence of intermolecular interactions in the molecularly dispersed material. The most intense vibrations of polycrystalline (CH3)2Au(acac), at 1595 and 1518 cm-1, are νs(CO) and the mixed mode δ(CH) + νa(CCC), respectively, of the chelated acetylacetonate ligand, Table 2.53,55,56 Since no fundamental is predicted by DFT to occur between them, the band at 1543 cm-1 was assigned to the first overtone of the strong out-of-plane methine bending mode γ(CH), whose fundamental was observed at 777 cm-1.53
TABLE 1: Calculateda and Observed IR Frequencies (cm-1) and Their Assignments,b in the Range 4000 to 2800 cm-1, for (CH3)2Au(acac) and acacH Adsorbed on A380-400 and on Selected Hydroxyl-Terminated Octasilsesquioxane Cubes (CH3)2Au(acac) c
calcd
3217 3164 3162 3162
(13) (18) (7) (18)
obsd
(CH3)2Au(acac)/SiO2 d
3085 2984
3152 (1) 3151 (32) 3119 (15)
IVb 3582 3567 3223 3182 3171 3164 3157 3157 3156 3121
(1141) (171) (7) (16) (5) (9) (4) (6) (14) (12)
obsd 3354
acacH/SiO2 IIa
IIb
obsd
3431 (1265)
3392 (1962)
3439
ν(tSiO-H · · · OC)
3238 (5)
n.d.
ν(CH), methine νa(CH3), (CH3)2Au
3163 3155 3125 3117
2957 2928
νa(CH3), CCH3
2928 n.d.
νa(CH2) ν (CO-H · · · OC) νs(CH3), (CH3)2Au
2857
νs(CH3), CCH3
2857
νs(CH2) 2δa(CH3), (CH3)2Au
n.d. 3003
2974 sh
3170 3166 3117 3114 3131
(6) (6) (5) (3) (1)
(8) (20) (4) (7)
3117 (389) 3060 (28) 3057 (14) 3054 (5)
2911
2808
3061 (15) 3058 (11) 3052 (4)
assignments
2922 2855 2816
3051 (7) 3048 (2) 3043 (2)
3057 (3) 3056 (2)
a Calculated frequencies are unscaled, with relative intensities given in parentheses. b On the basis of DFT vibrational analysis and D-labeling of the molecular complex.53 Symbols: ν, stretching; δ, in-plane bending; n.d., not detected. c Isolated molecule. d Polycrystalline material, recorded as a KBr disk.
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TABLE 2: Calculateda and Observed IR Frequencies (cm-1) and Their Assignments,b in the Range 1800 to 1300 cm-1, for (CH3)2Au(acac) and acacH adsorbed on A380-400 and on Selected Hydroxyl-Terminated Octasilsesquioxane Cubes (CH3)2Au(acac) calcdc
obsdd
(CH3)2Au(acac)/SiO2 IVb
acacH/SiO2
obsd
IIa
obsd
1680 (433)
1722 1695 1620
1631 (383)
1590
1492 (7)
n.d. 1422
1811 (107) 1775 (276) 1637 (575)
1595
1568 (340) 1521 (13) 1501 (69)
1543 1518 n.d. n.d.
1496 (9) 1494 (10)
n.d.
1460 (225) 1417 (1) 1416 (10)
1391 1352
1618 (511) 1572 1514 1501 1496 1494 1481 1476 1444 1418 1417
(362) (2) (51) (9) (39) (8) (8) (272) (2) (18)
assignments
IIb
1584 1566 1522 n.d. 1431
1492 (19)
1431 1389 n.d.
νs(CO) νa(CO) νa(CdCsCdO) + δ(OH) νs(CdO) ν (CdO) + δ(OH) 2γ(CH) δ(CH) + νa(CCC) δa(CH3), CCH3 + δ(CH) + νa(CO) δa(CH3), CCH3 δa(CH3), Au(CH3)2
1486 (10) 1407 (50) 1473 (7)
1482 (121) 1429 (45) 1402 (230)
1366
νa(CO) + νa(CCC) + δa(CH3), CCH3 δs(CH3), CCH3
1366 n.d.
δs(CH2) + δa(CH3), CCH3 δ(OH)
a Calculated frequencies are unscaled, with relative intensities given in parentheses. b On the basis of DFT vibrational analysis and D-labeling of the molecular complex.53 For mixed modes, contributing vibrations are indicated with “+”. Symbols: ν, stretching; δ, in-plane bending; γ, out-of-plane bending; n.d., not detected. c Isolated molecule. d Polycrystalline material, recorded as a KBr disk.
SCHEME 1: Acetylacetone Tautomer Equilibrium Calculated on Silica
are the CdO stretching modes, strongly mixed with δ(OH),54 of the enol tautomer.64,65 Since almost none of the keto tautomer of acacH is detected in the IR spectrum of adsorbed (CH3)2Au(acac) (Figure 2d), the IR experiments confirm our solid-state 1H MAS NMR finding that acetylacetonate is retained in the coordination sphere of gold. Effect of Grafting on Electronic and Geometric Structure. The effect of hydrogen-bonding on the structure of (CH3)2Au(acac) was explored using XAS. The X-ray absorption near-edge structure (XANES) region is sensitive to even small changes in electron density at the metal. The 2pf5d electronic transition at the Au LIII edge gives rise to a whiteline or peak superimposed on the absorption edge.66 Its intensity reflects the density of unoccupied d states, which is highest for Au(III) compounds, and decreases as the formal oxidation state is reduced.10 The XANES for polycrystalline (CH3)2Au(acac), AuCl3, and Au foil are shown in Figure 4. The foil spectrum has very little whiteline intensity, indicating almost fully occupied 5d states in bulk Au(0).10 The spectra of both (CH3)2Au(acac) and AuCl3 exhibit intense whitelines at ca. 11.92 keV, consistent with the reduced d-band occupancy of Au(III). The whiteline for (CH3)2Au(acac) is slightly less intense and is blue-shifted, relative to that of AuCl3. The XANES region beyond the whiteline is also very sensitive to the nature and geometric arrangement of the ligands. Although (CH3)2Au(acac)
Figure 4. Comparison of XANES at the Au LIII edge. Frame (i): (a) polycrystalline (CH3)2Au(acac) (red); (b) AuCl3 standard (blue); (c) Au foil (black). Frame (ii): (a) polycrystalline (CH3)2Au(acac) (red); (d) (CH3)2Au(acac)/S952-400 (0.7 wt % Au, blue).
and solid AuCl3 (i.e., [Cl2Au(µ-Cl)]2)67 are both square-planar at gold, their XANES fingerprints are noticeably different. The XANES for polycrystalline (CH3)2Au(acac) and for (CH3)2Au(acac) grafted onto S952-400 are also compared in
Adsorption of (CH3)2Au(acac) on Silica CHART 1: Octasilsesquioxane Cube Models for the Non-Hydrogen-Bonded Isolated (Ia), Vicinal (Ib, Ic), and Geminal (Id, Id′) Hydroxyls of Partially Dehydroxylated Silicaa
a The dotted line in Id′ denotes the hydrogen bond. Color scheme: O (red); Si (blue); H (white).
Figure 4. Because deposition of the organogold complex onto silica results in little change in whiteline intensity, we infer that the formal oxidation state of gold is retained. The small increase in whiteline intensity upon grafting suggests slightly reduced electron density, arising from more unoccupied 5d states in the supported complex relative to the bulk, polycrystalline material (see below).10 However, striking similarities in the postedge XANES confirm that there are only minor changes in the gold
J. Phys. Chem. C, Vol. 113, No. 20, 2009 8799 coordination environment due to its interaction with silica. The structural origin of these differences is best explored using EXAFS, for which curvefitting models were obtained by computational analysis of the organogold-silica interaction. Computational Models for the Silica Surface. Amorphous silica was modeled using simple octasilsesquioxane cubes bearing mixed hydride/hydroxyl termination, Chart 1. Dangling hydroxyl groups attached to these clusters approximate the Brønsted acidity of the noninteracting hydroxyl groups on silica gel as well as their spectroscopic properties (e.g., vibrational frequencies, 1H and 29Si NMR chemical shifts), better than simpler silanol models, presumably due to ring strain present in the cubes.68-70 Ia represents an isolated hydroxyl group, Ib a pair of vicinal hydroxyls, and Ic a trisilanol. In the octasilsesquioxane cube Id, one of the siloxane edges has been broken in order to create a geminal pair of silanols. Although IR spectroscopy reveals that most of the hydroxyl groups on silicas pretreated at 400 °C are noninteracting (Figure 2a), vicinal and geminal silanol pairs may still be present, or even dominant,71 since neither is capable of “intra-pair” hydrogen-bonding.72 Computational Models for Grafted Organogold(III) Complexes. The interactions of (CH3)2Au(acac) with the silsesquioxane clusters were investigated in order to identify energetically preferred adsorption modes as well as to provide model input for the EXAFS curvefitting. A computational model for an isolated molecule of (CH3)2Au(acac) with C2V symmetry is shown in Chart 2. The atomic polar tensor (APT) charges73 show that both the oxygen atoms and the methine carbon of the acac ligand are sites of appreciable negative charge density. Square-planar Pd(acac)2 and Pt(acac)2, which share the preference of Au(III) for square-planar coordination, do not adhere to silica.74 In contrast, the affinity of silica for isostructural Cu(acac)2 was attributed to the willingness of the latter to adopt an increased coordination number. Analogous coordination of (CH3)2Au(acac) directly to the terminal silanol group of the cluster Ia, resulting in an increased coordination number at gold, was found to be highly unstable relative to the isolated molecules. A similar result was reported in a computational study of the interaction of square-planar [Au(en)2]3+ with both the neutral cluster Si4O6(OH)4 and its deprotonated analog.25
CHART 2: Atom-Labeling Scheme for (CH3)2Au(acac)a
a (left) Structure of isolated molecule, showing calculated APT atomic charges73 (with hydrogen atom charges summed into heavy atoms); (right) side-view of crystal structure,88 Showing head-to-tail stacking and atoms contributing to intermolecular extended X-ray absorption fine structure (EXAFS) scattering paths in the bulk material.
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CHART 3: DFT-Calculated Structures and Reaction Energies for (CH3)2Au(acac) Interacting with a Single Silanol of an Octasilsesquioxane Cubea
a Displacement of the acac ligand on the trisilanol Ic is shown with retention of acacH via hydrogen-bonding in either the keto (IIa) or enol (IIb) form. Simple hydrogen bonding of monosilanol (Ia) to either the methine carbon (IIIa) or an oxygen atom (IIIb) of the coordinated acac ligand is also considered. Color scheme: Au (yellow), C (green), O (red), Si (blue), H (white).
Protonation of acetylacetonate by a surface silanol, resulting in coordination of the [(CH3)2Au(III)]+ fragment to a silanolate oxygen, was explored, as seen in Chart 3. Similar protonolysis reactions have been suggested for (CH3)2Au(acac) on the hydroxyl-terminated surfaces of alumina, magnesia, titania, and lanthana.75-79 In model II, a bond between gold and a bridging oxygen (Si-O-Si) adjacent to the silanolate is present. However, displacement of acetylacetone is endothermic, even when acacH becomes hydrogen-bonded to a nearby silanol in either the keto (IIa) or enol (IIb) form. The small energy difference between the two structures, 8 kJ/mol, reflects the relative stabilities of the adsorbed acetylacetone tautomers, Scheme 1. In contrast, hydrogen-bonding of a silanol to the chelated acetylacetonate ligand is energetically favorable, as seen in Chart 3. This “outer-sphere coordination”, involving one or more ligand sites rather than the metal itself, finds precedent in the specific solvation of transition metal β-diketonates by both protic and polar solvents, including alcohols.80 Two possible sites were considered for hydrogen-bonding: the oxygen donor atoms and the pseudoaromatic π-system of the chelated acetylacetonate ring.81 Hydrogen-bonding to aromatic rings is well-established, with interaction energies that are in general about half that of hydrogen bonds to heteroatoms.82 However, metal-containing rings involving the acac ligand are not strongly aromatic.83 As a result, the silanol proton in IIIa is not centered beneath the metallacycle but is located directly beneath the methine carbon. This is consistent with the APT charge distribution73 in (CH3)2Au(acac), as seen in Chart 2, which makes the methine
Hisamoto et al. CHART 4: DFT-Calculated Structures and Reaction Energies for (CH3)2Au(acac) Interacting with Octasilsesquioxane Disilanol Cubes by Hydrogen Bondinga
a A vicinal silanol pair (Ib) interacting via the methine carbon and one oxygen of the acac ligand (IVa), or via both oxygens of the acac ligand (IVb), or with a geminal silanol pair (Id and Id′, differing by an intramolecular hydrogen bond) via both oxygens of the acac ligand (Va) and to a single oxygen of the acac ligand (Vb) (color scheme: Au (yellow); C (green); O (red); Si (blue); H (white)).
susceptible to electrophilic attack. This type of interaction is rather less stable (-25 kJ/mol) than hydrogen-bonding to an oxygen donor atom of the coordinated acac ligand (IIIb, -40 kJ/mol), consistent with solution-state studies of outer-sphere coordination in acetylacetonato complexes.80 If a second silanol is present at an appropriate distance, the formation of a second hydrogen-bond is possible. Chart 4 shows two structures that differ in the orientation of (CH3)2Au(acac) with respect to the second silanol, which can form a hydrogenbond either to the methine carbon of the acac ligand (IVa, -52 kJ/mol) or to its second oxygen atom (IVb, -64 kJ/mol). The latter is the most energetically favored structure found for (CH3)2Au(acac) interacting with the octasilsesquioxane cube models. However, the energy difference between IVa and IVb is small enough to make interconversion likely at room temperature. Hydrogen-bonding of (CH3)2Au(acac) to geminal silanols, corresponding to a previously proposed mode of attachment of (CH3)2Au(acac) to silica,33 was also investigated. Both silanols can form hydrogen bonds to the oxygen atoms of the coordinated acac ligand, as shown in model Va. However, this interaction is less favorable (-41 kJ/mol) than for a pair of vicinal silanols (IVb, -64 kJ/mol). Since the O-H · · · O angles are similar in both IVb and Va (158-164°), the energy difference is attributed to the greater acidity of vicinal silanols. However, when one silanol of a geminal pair is hydrogen-bonded to a nearby bridging oxygen (Si-O-Si), the hydrogen bond formed by the second silanol to the acac ligand is twice as strong (Vb, -43
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TABLE 3: Curvefit Parameters to the Fourier-Filtered Au LIII Edge EXAFS for Shells in the First Coordination Sphere of Au(III) with Equivalent (fit a) and Inequivalent (fit b) Au-C Distancesa (CH3)2Au(acac)b
(CH3)2Au(acac)/SiO2c σ2/Å2
R/Å
σ2/Å2
path
N
R/Å
fit a Au-C1 Au-O1
2 2
2.07 0.0030 2.09 0.0160 ∆E0 ) 3.7 eV; residual ) 1.4
2.01 0.0043 2.12 0.0058 ∆E0 ) 2.5 eV; residual ) 2.8
fit b Au-C1 Au-C2 Au-O1
1 1 2
1.95 0.0088 2.05 0.0005 2.07 0.0071 ∆E0 ) 1.7 eV; residual ) 0.66
1.96 0.0042 2.24 0.0057 2.06 0.0064 ∆E0 ) 2.2 eV; residual ) 2.1
a The value of S02 was fixed at 0.93, calculated by FEFF for an isolated (CH3)2Au(acac) molecule. Coordination numbers/path degeneracies were fixed at the integer values shown for each fit. The numbers of free parameters used are 5 and 7 for fits a and b, respectively. b Nidp ) 6.6. c Nidp ) 7.3.
kJ/mol) and is comparable to the strength of the hydrogen bond formed by the isolated silanol in IIIb. The calculated O · · · O distances for the hydrogen bonds in IIIb, IVa,b, and Vb range from 2.727 to 2.794 Å, Table S1. These distances are similar to that in ice Ih (2.75 Å),84 for which the hydrogen-bond energy (including nonbonded interactions) was estimated to be 28 kJ/ mol.85 Outer-sphere coordination such as the hydrogen-bonding interactions described above results in small but predictable changes in metal-ligand bond distances.80 In each cluster model, formation of a hydrogen-bond causes elongation of the Au-O distance by ca. 0.03 Å relative to the isolated molecule, as seen in Table S1 of the Supporting Information. It is also consistent with decreased electron density at gold, manifested as an increase in the XANES whiteline intensity (see above). A similar effect of hydrogen-bonding was observed in the Pt LIII edge XANES of Pt(acac)2/Al2O3.86 Predicted Effect of Hydrogen Bonding on IR Spectra. Vibrational frequencies in the region 1650-1540 cm-1 are expected to be most sensitive to hydrogen bonding involving the acetylacetonate ligand. The predicted IR spectra for IIIb, IVa, and IVb, each containing a silanol hydrogen bonded to an acetylacetonate oxygen of (CH3)2Au(acac), are compared with the calculated spectrum of the isolated molecule in Figure S2 and Table S2 of the Supporting Information. The νs(CO) mode at ca. 1630 cm-1 shifts red for IVb, while the frequency of this mode is virtually unchanged for IIIb and IVa. Experimentally, the peak assigned to νs(CO) is observed to undergo a strong red shift, Figure 3. Hydrogen-bonding interactions cause a blue-shift in 2γ(CH) for all three models, with the largest change for IVb and the smallest for IIIb. The experimental band assigned to this mode is observed to undergo a strong blue shift when (CH3)2Au(acac) is grafted onto silica. Thus model IVb, with two hydrogen bonds between each member of a pair of vicinal silanols and the acetylacetonate oxygen donor atoms, is both the most energetically favorable structure and the most consistent with the experimental IR spectrum. The calculated IR spectra of IVb and an isolated (CH3)2Au(acac) molecule are compared more extensively and quantitatively with each other and with the experimental spectra in Tables 1 and 2 and Figure S3 of the Supporting Information. In the computational model, the O-H stretching mode of silanols interacting with the organogold complex undergoes a 260 cm-1 red shift, compared to ca. 400 cm-1 observed experimentally on both A380-400 and S952-400. The difference reflects the lower acidity of the cube Ib relative to the hydroxyl groups of silica. Hydrogen bonding also results in a
blue shift of ca. 20 cm-1 for the νa(CH3) modes of the [(CH3)2Au(III)]+ fragment (compared to +19 cm-1, observed experimentally). Both νs(CO) and νa(CO) modes of (CH3)2Au(acac) are predicted to undergo substantial red shifts (-19 and -16 cm-1, respectively), due to hydrogen bonding. This is observed most clearly in the experimental spectra for νs(CO), which shifts from 1595 to 1584 cm-1 when (CH3)2Au(acac) is deposited on silica. Methine out-of-plane bending, γ(CH), although not observed directly for (CH3)2Au(acac)/SiO2 due to strong absorption by the silica below 1300 cm-1, is predicted to shift from 804 cm-1 for (CH3)2Au(acac) to 813 cm-1 in IVb; this is consistent with the observed shift of 2γ(CH) from 1543 to 1566 cm-1 upon adsorption of (CH3)2Au(acac) on silica. A small blue shift, +4 cm-1, is predicted for the methine in-plane bending mode, δ(CH), which is strongly mixed with the νa(CCC) mode of the acac ligand. This shift is reflected in its observed positions at 1518 and 1522 cm-1 in the spectra of (CH3)2Au(acac) and (CH3)2Au(acac)/SiO2, respectively. Thus the calculated spectra accurately predict the changes in the IR spectra which occur upon grafting (CH3)2Au(acac) onto partially dehydroxylated silica via hydrogen bonding. EXAFS of (CH3)2Au(acac) before and after Grafting. EXAFS can reveal subtle changes in the structural parameters of grafted metal complexes, provided the sites are locally uniform, and possibly the number and location of the hydrogenbonds. That structural changes occur upon grafting (CH3)2Au(acac) onto silica is evident in the comparison of the EXAFS in Figure 5. The peak at 1.65 Å in the Fourier transform magnitude corresponds to scattering by atoms in the first coordination sphere of gold, i.e., C and O atoms bonded directly to Au. Its intensity declines 37% upon grafting. Although this could result from a decrease in the gold coordination number, a more likely explanation is greater destructive interference in the short-range scattering paths. Figure 5 shows that long-range scattering paths in the range 2.0 < (R + R) < 4.0 Å, reflecting structural changes occurring beyond the first-coordination sphere of gold, are also altered upon grafting. This observation confirms that adsorption does not simply involve the formation of small crystallites of headto-tail stacked (CH3)2Au(acac) (Chart 2). Features seen at (R + R) > 2.0 Å arise from single-scattering paths involving nonbonded atoms (i.e., in the acetylacetonate ligand and/or the silica) as well as multiple-scattering paths. The latter were shown to be essential in the analysis of the EXAFS for square-planar metal complexes,87 and acetylacetonates in particular.18,86
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Figure 5. Comparison of EXAFS spectra of polycrystalline (CH3)2Au(acac) (red) and (CH3)2Au(acac) grafted onto S952-400 silica (0.7 wt % Au, black) in (a) k3-weighted k-space; and (b) nonphasecorrected R-space. The R-space spectrum was created by Fourier transforming over the range 2.0-13.2 Å-1 in k-space; the k-space data is Fourier-filtered (back-Fourier-transformed) over the range 1.0-4.0 Å in R-space.
Curve-Fitting Analysis of the First Coordination Sphere of Gold. Single-scattering paths arising from atoms directly bonded to gold were refined to the Fourier-filtered EXAFS of polycrystalline (CH3)2Au(acac) in the region 1.13 < (R + R) < 2.05 Å. Fit a, representing an isolated molecule of C2V symmetry with only paths (Au-C, N ) 2; Au-O, N ) 2), is shown in Figure S5 of the Supporting Information. However, the fitted Au-C distance (2.07 Å) is longer than either the Au-C1 and Au-C2 distances obtained by single-crystal X-ray diffraction (1.96 and 2.04 Å, respectively),88 Table 3. Although the incorrect Au-C bond length would not be apparent without a priori knowledge of the crystallographic distances, the unreasonably large Debye-Waller factor for the Au-O path (0.016 Å2) indicates that this EXAFS model is inadequate and suggests the need for more scattering paths in the first coordination sphere. Allowing the Au-O1 and Au-O2 paths to vary independently is not justified by the small difference in their crystallographic bond distances, 0.017(12) Å.88 However, the Au-C single-scattering paths are different enough to refine separately (∆d ) 0.08 Å).88 The corresponding fit b is shown in Figure 6. The fitted bond distances in Table 3 match their crystal structure values, within the accepted error of the EXAFS technique.89 The Fourier-filtered EXAFS for (CH3)2Au(acac) on S952-400 is shown in Figure 7. Fit a gives reasonable Debye-Waller factors for both paths, Table 3. Inclusion of a third singlescattering path (i.e., fit b, Figure S6 of the Supporting Information) leads to an unreasonably long Au-C2 distance and was therefore discarded. We conclude that the inequivalence in the Au-C1 and Au-C2 paths that is caused by crystal packing forces in the polycrystalline material is eliminated upon grafting (CH3)2Au(acac) onto silica. The origin of the decrease in R-space intensity in Figure 5 in the presence of silica is therefore confirmed to be greater destructive interference in the first-
Hisamoto et al.
Figure 6. Fourier-filtered (1.13-2.05 Å) EXAFS of polycrystalline (CH3)2Au(acac) in (a) k3-weighted k-space (red) and (b) non-phasecorrected R-space (FT magnitude, red points; imaginary FT, black points). Parameters for the curve fit (solid blue lines in frames a and b) to three single-scattering shells (fit b) are given in Table 3. Frames (c) and (d) show the contributions of the individual paths to the k-space and R-space curvefits, respectively.
TABLE 4: Comparison of Distances Obtained for Crystalline (CH3)2Au(acac) by Single-Crystal X-ray Diffractiona and by EXAFS Curvefittingb single-crystal X-ray diffraction88 d/Å
EXAFS d/Å
Au-C1 Au-C2 Au-O1 Au-O2 Au-C3 Au-C4 Au-C5 Au-C3-O1 Au-C4-O2
intramolecular paths 1.960 1.99 2.038 2.05 2.085 2.09 2.102 2.979 2.94 2.981 3.300 3.12 3.164 3.35 3.168
Au-C5′ Au-C3′ Au-C4′
intermolecular paths 3.486 3.27 3.860 3.76 3.895
σ2/Å2
N
0.0085 0.0000 0.0077
1 1 2
0.0084
2
0.0005 0.011
1 4
0.0054 0.0128
2 4
a The atom labeling scheme is shown in Chart 2. b The values of N were not refined in order to give physical meaning to the Debye-Waller factors. S02 was fixed at its FEFF-predicted value, 0.93. A single phase shift, ∆E0, was refined at 2.7 eV for all paths; the residual is 6.1, using 17 of 21 possible free parameters.
coordination sphere paths upon grafting (compare Figures 6b and 7b), which is a direct consequence of the elongation of the Au-O bonds (from 2.07 to 2.12 Å). This elongation is consistent with our computational models, in which hydrogen bonding between the silanols and the oxygen donor atoms of the
Adsorption of (CH3)2Au(acac) on Silica
Figure 7. Fourier-filtered (1.15-2.35 Å) EXAFS of (CH3)2Au(acac)/ S952-400 (0.7 wt % Au) in (a) k3-weighted k-space (red) and (b) nonphase-corrected R-space (FT magnitude, red points; imaginary FT, black points). Parameters for the curve fit (solid blue lines in frames a and b) to two single-scattering shells (fit a) are given in Table 3. Frames (c) and (d) show the contributions of the individual paths to the k- and R-space curvefits, respectively.
acetylacetonate ligands alters the Au-O distances without significantly affecting the Au-C distances (Table S1 of the Supporting Information). Curve-Fitting Analysis of Longer-Range Scattering Paths. Analysis of the unfiltered EXAFS for polycrystalline (CH3)2Au(acac) was attempted using only intramolecular paths, including all single-scattering paths with pathlengths less than 4.0 Å and one multiple-scattering path.90 However, we were unable to reproduce the long-range features in R-space in this way. Consideration of the packing diagram for crystalline (CH3)2Au(acac), Chart 2, reveals two additional, intermolecular single-scattering paths at distances less than 4.0 Å (i.e., Au-C5′ and Au-C3′/C4′).88 When these intermolecular paths were included, we obtained an excellent fit, shown in Figure 8 and Table 4. A similar need for intermolecular paths was reported for the analysis of the EXAFS of Pd(acac)2.18 The changes in the EXAFS of (CH3)2Au(acac) in the range 2.0 < (R + R) < 4.0 Å in the presence of silica were first considered in terms of a possible loss of the acac ligand. A single-scattering curvefit based on model II generates the needed long-range features in R-space, but there are obvious discrepancies between the curve-fit and the data in k-space, Figure S7 of the Supporting Information, and the agreement between the model distances and the fit parameters is poor, Table S3 of the Supporting Information. The failure of this fit is consistent with our computational analysis, which predicts the formation of acacH from (CH3)2Au(acac)/silica to be endothermic, and suggests instead that adsorption involves the intact molecular complex. We also attempted a curvefit using the intramolecular
J. Phys. Chem. C, Vol. 113, No. 20, 2009 8803
Figure 8. Complete EXAFS (unfiltered) of polycrystalline (CH3)2Au(acac) in (a) k3-weighted k-space (red) and (b) non-phasecorrected R-space (FT magnitude, red points; imaginary FT, black points). The inset shows the expanded region from 2.0-3.6 Å. Parameters for the curve-fit (blue) to a model with both intra- and intermolecular paths are given in Table 4, using the atom-labeling scheme shown in Chart 2.
single-scattering paths and triple-scattering path used to fit the EXAFS of the crystalline organogold complex, while omitting intermolecular paths present only in the crystal. This fit fails to reproduce the long-range paths. Thus we infer that the organogold-support interaction generates new scattering paths. A successful curve-fit was achieved using paths calculated for the most energetically favored model IVb. An additional triple-scattering path (Au-C5-C1/Au-C5-C2) is predicted by FEFF to have significant intensity for IVb and was included in the fit. The curve fit is shown in Figure 9; the fit parameters are given in Table 5. Considering the number of variables, the agreement between the calculated and experimental distances is very good. In particular, the curvefit returns a distance for the Au-O3/Au-O4 shell (involving the silanol oxygens of the silica surface) of 3.70 Å, compared to 3.72 and 3.73 Å in model IVb. The number of hydrogen-bonds that link (CH3)2Au(acac) to the silica surface was investigated further. With N ) 1, a negative Debye-Waller factor was obtained for the combined Au-O3/O4 shell, as well as for the Au-C-O triple-scattering path, Table S4. In addition, the fitted Au-O3 distance (3.70 Å) is a poor match to model IIIb (4.17 Å). The much longer distance in IIIb relative to IVb is a consequence of the nearperpendicular orientation of planar (CH3)2Au(acac) relative to the face of the octasilsesquioxane cube in IIIb, compared to the coplanar orientation required by the two hydrogen-bonds in IVb. We conclude that N ) 2 provides a more appropriate description of the Au-O3/O4 shell, and therefore of the number of hydrogen-bonds formed.
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Hisamoto et al. their mutual condensation involves the formation of a highly strained Si2O2 ring,94-96 they persist at temperatures much higher than the 400 °C used here to pretreat the silica.71 Indeed, the interaction of (CH3)2Au(acac) with such hydroxyl groups should be more favorable than with silanols that are mutually hydrogenbonded, for which the formation of new hydrogen bonds must occur at the expense of the existing ones. Only a fraction of the surface hydroxyls on partially dehydroxylated silica retain (CH3)2Au(acac) strongly, reflecting heterogeneity in the silanol population. The silica pretreatment temperature, which affects both the acidity and the spatial distribution of the hydroxyls, influences its ability to adsorb and retain (CH3)2Au(acac). The participation of the hydroxyl groups in the subsequent transformation of grafted organogold complexes to Au(0) will be described separately. Conclusion Deposition of (CH3)2Au(acac) onto the surfaces of dehydrated and partially dehydroxylated silicas leaves the organogold complex intact but attached to surface hydroxyl groups via robust hydrogen bonds. A combination of computational modeling and spectroscopic analysis suggests that two such hydrogenbonds are formed, between a pair of vicinal surface silanols and the oxygen donor atoms of the chelated acetylacetonate ligand, resulting in predicted changes in the IR spectrum. Controlled decomposition of the grafted complexes to supported gold nanoparticles should therefore be possible by manipulation of the surface interactions.
Figure 9. Complete EXAFS (unfiltered) of (CH3)2Au(acac)/S952-400 (0.7 wt % Au) in (a) k3-weighted k-space (red) and (b) non-phasecorrected R-space (FT magnitude, red points; imaginary FT, black points). Parameters for the curve-fit (blue) to a model for (CH3)2Au(acac) hydrogen-bonded to two surface hydroxyl groups (IVb) are given in Table 5, using the atom-labeling scheme shown in Chart 2.
TABLE 5: Comparison of EXAFS Curve-Fit Parameters for (CH3)2Au(acac)/S952-400, with DFT-Calculated Bond Distances for Model IVb EXAFSb
DFT patha Au-C1, Au-C2 Au-O1, Au-O2 Au-C3, Au-C4 Au-C5 Au-C3-O1, Au-C4-O2 Au-C5-C1, Au-C5-C2 Au-O3, Au-O4
d/Å
R/Å
σ2/Å2
N
2.044, 2.047 2.180 3.098 3.436 3.281 3.478 3.716, 3.728
2.01 2.13 3.10
0.0016 0.0031 0.0096
2 2 2 1 4 2 2
c
c
3.30 3.44 3.70
0.0019 0.0121 0.0034
a The atom-labeling scheme corresponds to that shown in Chart 2, with the silanol oxygens of cube Ib labeled O3 and O4. b The values of N were not refined in order to give physical meaning to the Debye-Waller factors. S02 was fixed at its FEFF-predicted value, 0.93. A single phase shift, ∆E0, was refined at 4.8 eV for all paths; the residual is 10.0 using 17 of 21 possible free parameters. c Not included in the fit, due to its low intensity predicted by FEFF.
The Nature of the Interacting Hydroxyl Pair. The need for two spatially adjacent hydroxyls to create two hydrogen bonds to (CH3)2Au(acac), predicted computationally and confirmed by EXAFS, is consistent with their identification as a vicinal pair (i.e., located on silicon atoms sharing an bridging oxygen). Such vicinal silanols interact weakly, if at all, with each other,91-93 consistent with the small fraction of hydrogenbonded silanols observed in the 1H MAS NMR and IR spectra of the partially dehydroxylated silicas, Figures 1 and 2. Since
Acknowledgment. This work was funded by the U.S. Department of Energy, Basic Energy Sciences, under Catalysis Science Grant No. DE-FG02-03ER15467. A portion of this work was performed at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. Portions of this work made use of MRL Central Facilities supported by the MRSEC Program of the National Science Foundation, under award No. DMR05-20415. Supporting Information Available: Detailed description of the X-ray absorption spectroscopy experiments; raw X-ray absorption spectra for polycrystalline (CH3)2Au(acac) and (CH3)2Au(acac)/SiO2; additional experimental IR spectra; calculated bond lengths and IR spectra; additional EXAFS curvefits and parameters; Cartesian coordinates and calculated energies (in Hartrees) for all DFT model structures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Bond, G. C.; Sermon, P. A. Gold Bull. 1973, 6, 102. (2) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 405, 1. (3) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301. (4) Haruta, M. Catal. SurV. Japan 1997, 1, 61. (5) Falsig, H.; Hvolboeg, B.; Kristensen, I. S.; Jiang, T.; Bligaard, T.; Christensen, C. H.; Nørskov, J. H. Angew. Chem., Int. Ed. 2008, 47, 4835. (6) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175. (7) Haruta, M. Catal. Today 1997, 36, 153. (8) Schubert, M. M.; Hackenberg, S.; van Veen, A. C.; Muhler, M.; Plzak, V.; Behm, R. J. J. Catal. 2001, 197, 113. (9) Chen, M.; Goodman, D. W. Acc. Chem. Res. 2006, 39, 739. (10) van Bokhoven, J. A.; Miller, J. T. J. Phys. Chem. C 2007, 111, 9245. (11) Okumura, M.; Tsubota, S.; Haruta, M. J. Mol. Catal. A: Chem. 2003, 199, 73. (12) Budroni, G.; Corma, A. Angew. Chem., Int. Ed. 2006, 45, 3328.
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