Initial Growth Mechanisms of Gold-Phosphine Clusters - American

ARTICLE pubs.acs.org/JPCC

Initial Growth Mechanisms of Gold-Phosphine Clusters Emilie B. Guidez, Allison Hadley, and Christine M. Aikens* Department of Chemistry, Kansas State University, 213 CBC Building, Manhattan, Kansas 66506, United States

bS Supporting Information ABSTRACT: Gold-phosphine cluster growth was studied at the BP86/TZV level of theory, starting from the precursor AuClPR3 (R = H, Me, and Ph), up to clusters containing four gold atoms. For all three substituents, AuClPR3 is linear. For R = H and R = Me, the anionic species AuClPR3 are bent, whereas for R = Ph, it is linear because of the addition of the electron in a π* orbital mainly localized on the benzyl rings. Chloride ion dissociation, especially from negatively charged clusters, is very common during cluster growth. Phosphine groups can also dissociate, although this becomes less favorable as the R group becomes larger. The growth products obtained strongly depend on the relative orientation of the reactants, leading to a wide variety of clusters. The inclusion of a classical dispersion term gives similar monomers and dimers to regular BP86 but may lead to different trimers. Most reactions involved in cluster growth are exothermic. Dispersion forces tend to increase the reaction energies involving a low charge and decrease the reaction energies involving a high negative charge. Trimers tend to have a triangular core and be negatively charged. Tetramers have a diamond, tetrahedral, or Y-shaped core. Diamonds and tetrahedra mostly have a positively charged or neutral core, as opposed to Y shapes, which have a negatively charged or neutral core. The optimized tetrahedral structure Au4Cl2(PH3)4 was found to be similar to the experimental cluster [Au4(μ-I)2(PPh3)4] and to be one of the lowest energy structures.

’ INTRODUCTION Gold clusters and nanoparticles have a number of useful applications. Contrary to the bulk material, gold nanoparticles show unique catalytic properties.1,2 Gold compounds have been used in the medical field to treat rheumatoid arthritis but also can potentially treat other diseases like bronchial asthma or malaria.3 Recent studies show that gold nanoparticles can improve anticancer drug delivery4 or even target cancer cells and induce cytokinesis arrest.5 In the past three decades, ligand-passivated gold(I) compounds (the most common ligands include phosphines, thiolates and halides) have generated a strong interest because of their characteristic luminescence, catalytic properties, and biological applications. The luminescence features of these complexes can be attributed to metal-centered transitions, ligand to gold charge transfer or even the aurophilic interaction between two neighboring complexes.68 Their catalytic applications are also diverse and include the synthesis of tri- and tetra-substituted furans,9the oxidation of styrene into benzaldehyde,10 and the cycloaddition of allenenes.11 One of the most promising applications for these gold clusters is cancer therapy because they show antitumoral activity.3,12 Many of these gold compounds with various stoichiometries have been synthesized and their crystal structures determined.1328 The basic way to synthesize these clusters is to combine a gold salt, the ligands of interest, and a strong reducing agent such as NaBH4 in a solvent like dichloromethane or chloroform. The smallest polyhedral gold-phosphine complex isolated and characterized is the tetrahedral structure Au4(PR3)42þ and the r 2011 American Chemical Society

related Au4I2(PR3)4 complexes.13,15,29 For these small clusters, an electron-donating phosphine ligand is necessary to stabilize the positive charge of the core.30 Although luminescence studies on these complexes29 have been performed as well as theoretical calculations to explain their stability,30,31 little is known about their formation mechanism. In this work, we study the growth of gold-phosphine clusters, starting with AuClPR3 (R = H, Me, Ph) as a precursor, a reducing agent and chloroform as a solvent. To do so, we determine the thermodynamic stability of one-, two-, three-, and four-goldatom clusters. Previous theoretical work has been done on small neutral and anionic bare gold clusters to determine their structures.1926 For neutral three-gold-atom clusters, the possible structures are linear, triangular, and obtuse-angle triangular. It is, however, uncertain which one is the most stable.4043 For neutral fourgold-atom clusters, diamond is the most favorable structure,44 followed by a higher energy Y-shape.4042 The anionic tetramer tends to be Y-shaped.44 All calculations in this work are performed using density functional theory. However, standard density functional theory does not include long distance dispersion interactions between atoms. Because of the large electron cloud around the gold atoms, these interactions can be important. A classical energy correction term of the form Received: December 30, 2010 Revised: February 22, 2011 Published: March 14, 2011 6305

dx.doi.org/10.1021/jp112388q | J. Phys. Chem. C 2011, 115, 6305–6316

The Journal of Physical Chemistry C N at 1

Edisp ¼ s6

N at

ARTICLE

∑ ∑ C66 fdmp ðRijÞ i¼ 1 j¼ iþ1 R ij

ij

is added to the KohnSham energy to obtain the total energy.45 Nat is the number of atoms in the system, C6ij is the dispersion coefficient for the atom pair ij; Rij is the distance between atoms i and j; f is a damping function, and s6 is a scaling coefficient.

’ COMPUTATIONAL METHODS The Amsterdam density functional package (ADF)46 was used to perform all calculations in this work. A polarized triple-ζ (TZV) basis set is used with a [1s24f14] frozen core for gold and a [1s22p6] frozen core for phosphorus and chlorine. A continuum chloroform solvent is added using the conductor-like screening model (COSMO).47,48 Scalar relativistic effects are included by adopting the zero-order regular approximation (ZORA).49,50 The generalized gradient approximation (GGA) Becke-Perdew (BP86)51,52 exchange-correlation functional is used to optimize the structures. Open-shell structures are run using unrestricted calculations. Calculations including dispersion forces45 (BP86-D) are run for the optimizations of one-, two-, and three-gold-atom systems. We note that the dispersion parameters for gold are not well-tested. Only the standard BP86 rather than BP86-D is used for four-gold-atom structures. Both the neutral and negatively charged monomers are optimized; negatively charged systems are considered to take into account the reducing agent. All possible reactions involving two monomers are simulated to give potential dimers. Reaction products are also optimized with an extra negative charge. Trimers are obtained by calculating all possible reactions between previously optimized dimers and monomers. The resulting products are also optimized with an extra negative charge to take the reducing agent into account. Tetramers are obtained with a reaction between a trimer and a monomer or between two dimers. When chloride ions or phosphine groups dissociate from a given cluster, these atoms are removed, the charge on the cluster is adjusted (for chloride dissociation), and the remaining cluster is further optimized. Although R = Ph is a more common substituent experimentally, to simplify the calculations, we considered only R = H for the structures with three or more gold atoms, which is sufficient for the geometry. Nevertheless, we must keep in mind that the nature of the substituent R can influence the accuracy of the resulting energy properties.53 All geometry optimizations employ an energy convergence criterion of 105 a.u. and a gradient convergence criterion of 104 a.u./Å except the clusters involving two triphenylphosphine groups where, because of the large system size, the energy convergence criterion is 104 a.u. and the gradient convergence criterion is 103 a.u./Å. ’ RESULTS AND DISCUSSION A. Precursor Molecule. The precursor molecules studied here are AuClPR3, with R = H, Me, and Ph. Under experimental growth conditions, a reducing agent needs to be present in solution. Therefore, reduced forms of AuClPR3 are also considered in this work. A wide variety of polar organic solvents is used experimentally for gold nanoparticle growth. In this work, chloroform is used. Geometry optimizations were performed for

Figure 1. Optimized structures of AuClPH3 (0) and AuClPH3(1). Key: Au, yellow; Cl, green; P, purple; H, white.

Figure 2. (a) HOMO of AuClPH3, (b) SOMO of AuClPH3, (c) HOMO of AuClPPh3, and (d) SOMO of AuClPPh3. Red and blue orbitals are doubly occupied; turquoise and orange orbitals are singly occupied. Contour value = 0.03.

the species AuClPR3, AuClPR3, and AuClPR32 with R = H, Me, and Ph. For all three substituents, AuClPR32 decomposes and will therefore not be further discussed here. Figure 1 shows the optimized structures of AuClPH3 (0) and AuClPH3 (1). The highest-occupied molecular orbital (HOMO) for AuClPH3 is mainly composed of a chlorine p orbital with contributions from a gold d orbital (Figure 2a). The bond lengths are much larger for the ion than for the neutral molecule as shown in Table 1. This is likely due to the fact that the added electron goes into an antibonding σ orbital (Figure 2b) leading to a destabilization of the bonds. The PAuCl angle is almost linear for the neutral molecule, but it is bent for the negatively charged ion, as shown in Table 2. This bending is due to the addition of the electron in the σ* orbital located mainly on the gold atom, which is now Au(0) instead of Au(I). To reduce the electronic repulsion around the metal, the angle decreases. Similar effects are observed for R = Me. However, for R = Me, the AuCl distance is much longer than that for R = H. The PAuCl angle is also significantly smaller. We notice that contrary to R = H and R = Me, the HOMO of AuClPPh3 is mainly located on the gold metal and the aromatic rings (Figure 2c). The additional electron in AuClPPh3 goes into an orbital on the triphenylphosphine group with π* character (Figure 2d). The aromatic ring can accommodate the extra electron so that the electronic repulsion around the metal is not as important as that for R = H or Me, and consequently, no bending is observed. The bond lengths are also similar to those of the neutral system. A single-point energy calculation on the bent AuClPPh3 (with a PAuCl angle of 147.0°) shows a LUMO of σ* character located on the gold atom, which matches the SOMO 6306

dx.doi.org/10.1021/jp112388q |J. Phys. Chem. C 2011, 115, 6305–6316

The Journal of Physical Chemistry C

ARTICLE

Table 1. Bond Lengths (in angstroms) for AuClPR3 and AuClPR3 with R = H, Me, and Ph Using BP86/TZV

a

AuClPH3

AuClPH3

AuClPMe3

AuClPMe3

AuClPPh3

AuClPPh3 expa

AuClPPh3

AuCl

2.325

2.537

2.343

2.792

2.336

2.279

2.373

PAu

2.255

2.430

2.269

2.395

2.272

2.235

2.292

PAt(1)b

1.416

1.532

1.833

1.850

1.835

1.803

1.822

PAt(2)b

1.416

1.426

1.833

1.850

1.835

1.866

1.815

PAt(3)b

1.416

1.426

1.833

1.872

1.835

1.792

1.850

Experimental bond lengths (ref 54). b For R = H, each PH bond length is reported. For R = Me and Ph, the PC bond lengths are reported.

Table 2. Bond Angles (in degrees) for AuClPR3 and AuClPR3 with R = H, Me and Ph using BP86/TZV

PAuCl a

AuClPH3

AuClPH3

AuClPMe3

AuClPMe3

AuClPPh3

AuClPPh3 expa

AuClPPh3

179.84

170.31

179.86

137.48

179.86

179.68

177.96

Experimental bond angles (ref 54).

of AuClPR3 (R = H, Me). Therefore, the bending greatly influences the ordering of the orbitals. The bond lengths and angles obtained experimentally54 for AuClPPh3 correspond to those obtained with our calculations to within 0.05 Å and 0.18°, as shown in Tables 1 and 2. A geometry optimization for all of these precursors is also run including dispersion forces. The resulting geometries are the same to within 0.021 Å and 0.83°, except for AuClPMe3, where the chlorine atom dissociates. B. Dimer. The possible ways that two of the precursors discussed previously could react to make a dimer are studied. Calculations including dispersion forces are run in addition to standard BP86 calculations. The resulting structures are summarized in Figure 3. Their energies of formation are given in Table 3. In general, the inclusion of dispersion forces leads to more negative energies of formation except for the reactions involving two negatively charged molecules, where the reaction energies are more positive. Like the precursors, the geometries obtained with BP86-D are very similar except for (3) and (5)H, as explained in the following discussion. We first notice that two neutral AuClPH3 molecules do not react with one another, which shows the necessity of the presence of a reducing agent in solution. However, we observe that the two neutral molecules align, as shown in Figure 3. The negative value of the energy found for the formation of the trans complexes (2)H, (2)Me, and (2)Ph, where the subscript refers to the R ligand, shows that there is a significant interaction that is possibly due to the favorable antiparallel alignment of the dipoles and also the dispersion-based aurophilic interaction, as will be discussed later. As R becomes larger, the formation of (2) becomes less favorable. This can be explained by the increasing steric hindrance of the ligand, which constrains the geometry of the complex. In fact, as R becomes larger, it is harder for the dipoledipole and aurophilic interactions to compensate for this effect. With BP86-D, the energy of formation of those complexes becomes more negative as expected. Also, for R = Ph, the optimized structure ((2)Ph BP86-D in Figure 3) is not the same as the one obtained with BP86. In fact, it is a perpendicular dimer. This can be explained by the favorable dispersion-based interaction between phenyl rings. As R becomes larger, the formation of (2) becomes more favorable with BP86-D because of the increasing contribution of dispersion forces. The inclusion of the dispersion forces here inverses the trend. The formation of the cis aurophilic complex (3)H is endothermic because of the steric repulsion

involved and the parallel alignment of the dipole moments. If dispersion forces are included, only the trans complex is stable. For R = Me, only the trans isomer is stable as well. For R = Ph, the cis isomer was not considered because of the bulkiness of the phenyl rings. For the reaction of anionic AuClPH3 with neutral AuClPH3, the formation of trans-Au2Cl2(PH3)2 (4)H is more favorable than cis-Au2Cl2(PH3)2 (5)H due to the lack of steric repulsion between the two phosphine groups. Contrary to the neutral trans isomer (2)H, there is a bond between the two gold atoms of the negatively charged (4)H. In fact, the (4)H goldgold distance is 2.81 Å, whereas the (2)H goldgold distance is 3.34 Å (with standard BP86). In addition, the PAuCl angles are bent. As shown in Figure 4, the singly occupied molecular orbital (SOMO) of (4)H is composed of a σ orbital between the gold atoms. Similar structures are optimized for the trans isomers (4)Me and (4)Ph. The goldgold distance in (4)Ph is 2.75 Å versus 3.61 Å in (2)Ph. The energies of formation of (4)H and (4)Me are more negative than the energies of formation of (2)H and (2)Me because there is a bonding interaction that is stronger than the dipoledipole or aurophilic interaction. The formation of (4)Ph is less favorable than the formation of (4)H and (4)Me. This might be due to the distortion induced by some steric hindrance between the chlorine atoms and phenyl groups. The cis isomer of Au2Cl2(PMe3)2, does not form, which is likely due to the strong steric repulsion between the two methyl groups. Instead, a chlorine ion is ejected leaving the neutral molecule Au2Cl(PMe3)2 (5)Me. The cis isomer of Au2Cl2(PPh3)2 is again not considered. The addition of dispersion forces gives more negative energies for this reaction. With BP86-D, (5)H does not form, and just like (2)Ph, (4)Ph is a perpendicular dimer. The reaction of two negatively charged AuClPH3 molecules leads to three different potential products depending on the orientation of the reactants. Because of the high electronic charge involved, the cis and trans isomers of Au2Cl2(PH3)22 are not stable: two chloride ions ((6)H), a chloride ion, and a phosphine group ((7)H) or two phosphine groups ((8)) spontaneously dissociate. The products formed are very sensitive to the starting geometry. As shown by the energies of formation, the removal of a chloride anion is thermodynamically more favorable than the removal of a phosphine group. The (6)Me, (6)Ph, (7)Me, and (7)Ph products were optimized. As the R group gets bigger, the dissociation of a phosphine group becomes less favorable. 6307



ARTICLE

Figure 3. (a) Aurophilic complexes, (b) (0)þ(1) products, (c) (1)þ(1) products, and (d) reduced forms of (6)H and (7)H. Key: Au, yellow; Cl, green; P, purple; H, white; C, black.

Additionally, the increase in the energy of formation of (7) and (8) with BP86-D, particularly for R = Me and Ph, suggests that the dispersion favors the retention of the phosphine ligand. In most experimental compounds, the phosphine groups outnumber the halide ligands.2641 In the remainder of this Article, we will focus on R = H. However, it should be stated that energies and dissociation products will be dependent on R. The dimer structures can potentially obtain an additional electron from the reducing agent in solution. Reduction of (2) and (3) leads to (4) and (5). Reduction of (4) and (5) leads to (6) þ 2Cl, (7) þ PH3þ Cl, or (8) þ 2PH3. The reduced

species of (6), (7), and (8) are labeled (6)0 , (7)0 , and (8)0 . The goldgold distance in (6) does not vary much with the addition of an electron (0.08 Å). The gold-phosphine length increases quite dramatically (0.19 Å): the extra electron goes in an orbital mainly located on the AuP bonds. The hydrogen atoms in the (6)0 dimer rearrange in a “trans” fashion. The reduction of (7) to (7)0 leads to elongations of 0.22 Å of the gold-phosphine bond and 0.25 Å of the gold-chloride bond. The length between the two gold atoms is increased by only 0.1 Å. As (8) is reduced to (8)0 , it loses two chloride ions. The vertical electron affinities of (6), (7), and (8) are 125.05, 57.48, and 101.28 kJ/mol, 6308



ARTICLE

Table 3. Energy of Formation of Two-Gold-Atom Structures in Kilojoules Per Mole Using BP86/TZV and BP86-D/TZV R=H reaction 0þ0

1þ1

R = Ph

products

BP86

BP86-D

BP86

BP86-D

aurophilic complex-trans (2)

15.36

68.22

11.12

89.72

5.34

79.93

51.67

85.46

3.87

60.36

69.11

aurophilic complex-cis (3) 0þ1

R = Me BP86

BP86-D 112.75

3.94

trans-Au2Cl2(PR3)2 (4)

46.17

cis-Au2Cl2(PR3)2(5)

34.07

Au2(PR3)2 (6) þ 2Cl

238.93

240.58

292.53

245.10

179.65

184.00

Au2ClPR3 (7) þ PR3þ Cl

221.51

214.65

218.04

145.65

143.14

94.42

Au2Cl22 (8) þ 2PR3

153.05

136.72

93.78

10.82

56.5

51.28

Table 4. Energy of the Aurophilic Interaction in the [ClAuPH3]2 Perpendicular Dimer method

a

Figure 4. SOMO of (4). Contour value = 0.03.

Figure 5. [ClAuPH3]2 perpendicular dimer. The color scheme is the same as that previously used.

respectively. The negative energies show that the reduction is favorable for (6) and (7). Considering a low concentration of reducing agent in solution and the fact that (6)0 and (7)0 have negative charges, we will ignore further reduction of these two compounds. Because the reduction of (8) is unfavorable, (8)0 will not be considered for the formation of trimers. Similar reduced structures and energies are obtained when considering dispersion. C. Aurophilic Interaction in the [ClAuPH3]2 Perpendicular Dimer. Gold(I) complexes do not repel each other. Because of electron correlation and relativistic effects,5557 an attractive force exists. The strength of the aurophilic interaction depends on the distance between the two gold atoms and the softness of the ligands.55 In (2), the predicted BP86/TZV distance between the two gold atoms is 3.34 Å, which is within the range of Au 3 3 3 Au distances determined experimentally.5860 To determine the magnitude of the aurophilic interaction in (2), the [ClAuPH3]2 perpendicular dimer (Figure 5) was modeled with an AuAu distance of 3.13 Å, and the energy of interaction was determined using the same methods as above. This perpendicular model allows us to eliminate the electrostatic dipole dipole interaction. The results are reported in Table 4.

AuAu bond length (Å)

ΔE (kJ/mol)

BP86/TZV

3.13

11.31

BP86-D/TZV

3.13

33.74

MP2/Def2-TZVPa

3.15

expb

2.753.72

14.70 29 to 46

Ref 61. b Refs 57, 59, and 60.

The energy obtained with a BP86/TZV level of theory is positive, which means that the two gold complexes repel each other at this distance, which does not concord with experimental trends.59,60 In comparison, the BP86-D/TZV value of 33.74 kJ/mol obtained is qualitatively similar to but larger than the value found by Pyykk€o and Zaleski-Ejgierd61 for this particular system using the MP2/def2-TZVP level of theory. D. Trimer. The formation of three-gold-atom structures for R = H was studied both including and not including dispersion forces. If the structures differ, the BP86-D derived structures are denoted with a subscript D. Multiple starting orientations of the reactants have been considered for each reaction. The product structures are given in Figure 6, and the reaction energies leading to these trimers are given in Table 5. Depending on their orientation, a given set of two molecules can react in different ways and yield many possible products. This is consistent with the wide variety of gold clusters observed experimentally. In the gold-phosphine trimers examined here, the three gold atoms generally make triangular structures that are neutral or negatively charged. Nevertheless, (12), (20), and (22) are obtuse angle triangles, and (20)D and (21) are linear. The formation of neutral trimers is thermodynamically preferred, but most structures have a (1) charge. Four trimer structures were found to carry a (2) charge: (14), (16)D, (20)D, (21), and (22). (21) is a linear structure where the three gold atoms have a single negative charge (4 s electrons). The distance between two gold atoms in this system is 2.62 Å. The linear Au3 cluster obtained by H€akkinen and Landman44 has been predicted to have a bond length of 2.60 Å at the BO-LSD-MD/PBE level of theory. As for the dimers, chloride ions and phosphine groups are found to dissociate spontaneously depending on the charge of the reaction. In general, chloride ions are more likely to leave than phosphine groups: the most exothermic reactions are those where a chloride ion is lost. Except for the reactions involving the loss of a phosphine group, the inclusion of dispersion leads to more negative energies of reaction. In some cases, the dispersion 6309



ARTICLE

Figure 6. Trimer structures resulting from the reaction of a dimer and a monomer. D subscript indicates that the structure was obtained including dispersion. The color scheme is the same as that previously used.

forces give very different structures. The reaction of (4) and (1) is a very good illustration of this point. When dispersion is not included, two chloride anions leave; however, when dispersion is included, a phosphine and a chloride leave. In other cases, the structures are very similar. For example, the bond lengths and angles of (14) and (14)D are similar to within 0.07 Å and 7.5°, respectively. All other results fall between these two extreme cases. (23) and (24) are trimers that are obtained from reactions involving four gold atoms. Their charges are þ1 and 0, respectively. This will be discussed in Section E. To compare all of these structures, we calculated the energy of formation of each trimer resulting from a hypothetical reaction between three monomers. To do this, additional chloride ions, phosphine groups, or both were added to balance the reaction and have a total of three chloride ions, three phosphine groups, and three gold atoms. Depending on the total amount of charge

involved in the products, the relative number of neutral and negatively charged monomers involved in the reaction was selected. The reaction energy was calculated by adding the energy of the trimer and added ligand and subtracting the energy of the three monomers involved. These energies are of course different from those listed in Table 5. The results are summarized in Figure 7. The dispersion products were not considered here. Three neutral monomers lead to no trimer, which shows again the importance of the presence of a reducing agent in solution. Most structures have a (þ1) gold core, and the reaction energies are between 200 and 275 kJ/mol. Four structures have a (þ2) gold core with reaction energies between 77 and 65 kJ/mol. These structures have more ligands due to the higher positive charge of the core, and the reaction energies are less exothermic. One structure was found to have a neutral core and a reaction energy of 294.17 kJ/mol, which is very exothermic. In general, the 6310



ARTICLE

Table 5. Formation of Three Gold Atom Structures reaction

product

charge of trimer

ΔE (kJ/mol)

product (dispersion)

charge of trimer

ΔE (kJ/mol) (dispersion)

(6) þ (0)

(9)

0

19.28

(4) þ (1)

(9) þ 2Cl

0

213.03

(9)0 D þ Cl þ PH3

(5) þ (1)

(10) þ 2Cl þ PH3

0

215.83

(9)D þ 2Cl

0

251.88

(4) þ (0)

(11)

1

31.04

(11)D

1

109.33

(4) þ (0)

(12)

1

19.28

(12)D

1

113.94

(5) þ (0)

(13)

1

37.56

(13)D

1

116.55

(5) þ (0)

(18)

1

42.55

(13)D

1

116.55

(7) þ (0) (7) þ (1)

(15) (16) þ Cl

1 1

26.57 72.66

(15)D (16)D

1 2

102.18 32.30

(8) þ (0)

(17) þ Cl

1

95.57

(17) þ Cl

1

107.20

(8) þ (0)

(14)

2

51.59

(14)

2

(5) þ (1)

(0) þ (7) þ PH3 þ Cl

(5) þ (1)

(19) þ PH3 þ Cl

(4) þ (1)

(0) þ (6) þ 2Cl

(4) þ (1)

(0) þ 2PH3 þ (8)

106.88

(8) þ (0) þ 2PH3

(6)0 þ (1) (7)0 þ (1)

(20) þ Cl (21) þ PH3 þ Cl

1 2

185.59 202.05

(20)D þ 3PH3 (22) þ PH3 þ Cl

2 2

205.19 219.60

(7)0 þ (1)

(22) þ PH3 þ Cl

2

194.78

(22) þ PH3 þ Cl

2

219.60

0

91.26

1

236.59

76.93

(7) þ (0) þ PH3 þ Cl

134.68

231.42

(7) þ (0) þ PH3 þ Cl

134.68

192.76

(8) þ (0) þ 2PH3

187.44 1

(9)D

56.79 56.79

Figure 7. Comparison of trimer energies (BP86/TZP) with respect to three monomers. Not to scale.

energy increases as the charge of the core decreases. We note that (20), (21), and (22) do not fit in any category because of their negatively charged core. Some reactions do not yield a trimer but a neutral monomer and a different dimer than the one that reacted. They all involve a reaction between two negatively charged molecules. It is important to note that the addition of dispersion gives less negative energy values for these cases.

Reduction of Three Gold Atom Structures. All of these trimers may be reduced in solution before they react with a monomer to form a tetramer. The reduction products and the vertical excitation energies are summarized in Table 6. We note that dispersion forces are not included in these calculations. New trimer structures are shown in Figure 8. No decomposition into dimer plus monomer is found despite the highly negative charges involved upon gaining an additional electron. Most structures obtained are identical to those 6311



ARTICLE

Table 6. Reduction of Trimers reacting structure

product

(9)

(23)0 þ Cl

(10)

(16)

(11)

charge of trimer product

vertical electron affinity (kJ/mol)

0

173.66

1

190.50

(24) þ 2Cl

0

225.43

(12)

(9) þ 2Cl

0

182.93

(13)

(24) þ 2Cl

0

207.45

(14)

(17) þ 2Cl

1

3.15

(15)

(16) þ Cl

1

88.88

(16) (17)

(25) þ Cl (17)0

1 2

164.57 108.80

(18)

(24) þ 2Cl

0

223.18

(19)

(16) þ Cl

1

98.44

(20)

(20)0 þ 3PH3

2

65.00

(21)

(20)0 þ PH3 þ Cl

2

5.62

(22)

(20)0 þ PH3 þ Cl

2

(23)

(23)0

0

257.86

(24)

(23)0 þ Cl

0

164.98

Figure 8. Structures resulting from the reduction of trimers. The color scheme is the same as that previously used.

observed in the previous section, which suggests that these trimers are very stable. It also should be emphasized that all reductions except for (17) and (10) lead to the spontaneous release of a chloride ion, which reduces the negative charge in the core. (23)0 is very similar to (23) but is not quite as symmetric. In fact, in (23), all AuAu lengths are equal to 2.70 Å; all AuP lengths are equal to 2.33 Å and all PAuAu angles are equal to 150°. In (23)0 , the distances between gold atoms are 2.63, 2.72, and 2.88 Å. The PAuAu angles are 136.45, 160.54, and 174.22°. The LUMO and LUMOþ1 of (23) are almost degenerate: E(LUMOþ1) E(LUMO) = 0.034 eV. Consequently, the addition of an electron creates a JahnTeller distortion. Similarly, we observe that the extra electron in (17)0 disturbed the symmetry of (17). Adding an electron to (16) leads to the breaking of the AuCl bond ((25)). The addition of an electron to the trimers is generally favorable and is endothermic only for (22). The ease of reduction of these compounds may be due to the phosphine group which, compared with a triphenylphosphine group, does not donate as much electron density to the gold atom, which has a relatively high electron affinity.30 The reduced structures are still considered for the formation of tetramers. The reduction of (20) gives (20)0 plus three phosphine groups. It should be noted that (20)0 is a similar linear Au32 complex as (20)D. The goldgold bond length is 2.71 Å for (20)0 and 2.62 Å for (20)D. This result is in accordance with the fact that dispersion forces lead to stronger bonds.

13.47

(17)0 has a neutral gold core and therefore can be put in the same category as (16) in Figure 7. The reaction energy among three negatively charged monomers leading to the formation of (17)0 is 214.38 kJ/mol. (23)0 and (25) have a negatively charged core and cannot be classified in Figure 7. E. Tetramer. The tetramers were obtained using the same procedure as above. Because of the large number of reactions studied for the tetramer, these reactions are studied only with the standard BP86 functional. Nevertheless, we have to keep in mind that the energies and products might differ if dispersion is added just like in the trimer case. Tetrahedra, diamonds (both planar and bent), and Y-shaped tetramers with a large variety of charges (from þ1 to 2) are observed as well as one linear gold nanowire. Each of these shapes will be described later in this section. Figure 9 shows the classification of the tetramers, similarly to what was done with the trimers. Structures are numbered consecutively based on the reactants (cf. Supporting Information). Figure 10 shows the selected structures from Figure 9. Because of the large number of tetramers obtained, only the lowest energy structures will be shown in this section. For a complete list of reaction energies and structures, see the Supporting Information. The following observations are consistent with the trimer results: (1) No tetramer with a (þ4) gold core was obtained. (2) The more positively charged the gold core, the more ligands are retained in the cluster. (3) The less negative the charge of the gold core, the lower the energy of the structure. The structures involved here are mainly diamonds and tetrahedra. However, three Y-shaped structures with a neutral gold core were obtained. For each type of shape (diamond, tetrahedron, and Y-shape), the most stable structure with a neutral gold core only has phosphine ligands. A few structures with one or more negative charges on the core were also observed. These are mainly the remaining Y-shaped structures. The linear nanowire also falls in that category. Diamonds. The majority of the tetramer structures have a diamond-like core (32 found in this work). This is in accordance with the greater thermodynamic stability of diamond structures 4042 compared with the tetrahedral and Y-shaped structures for bare gold clusters. Among those diamonds, 2 of them have a gold 6312



ARTICLE

Figure 9. Comparison of tetramer energies (BP86/TZP) with respect to four monomers. Not to scale. Structures in magenta are shown in Figure 10.

Figure 10. Lowest energy structures with (a) (þ3) gold core, (b) (þ2) gold core, (c) (þ1) gold core, and (d) neutral gold core.

core with a (þ3) charge, 12 of them have a core with a (þ2) charge, 8 of them have a core with a (þ1) charge, 9 of them are neutral, and 1 has a (1) charge. The core charges are determined by taking the cluster charge and subtracting one electron for each chlorine ligand. The reactions considered in this section involve the previously optimized trimers or dimers. (See the Supporting Information.) The computed reaction energies are not the same as those listed in Figure 9, which involve the hypothetical reaction between three monomers. As a general trend, the higher the positive charge is,

the less exothermic the formation of the tetramer. In fact, the diamonds with a (þ3) gold core have energies of formation between 4.30 and þ1.28 kJ/mol. Those with a (þ2) core have energies of formation between 27.10 and 16.34 kJ/ mol. Clusters with a (þ1) core have energies of formation between 137.62 and 35.68 kJ/mol. Finally, the neutral ones have energies of formation between 330.7 and 86.86 kJ/ mol, except for (79), which has a positive energy of formation. However, the release of chloride ions can dramatically increase or decrease these energies. In fact, the formation of a positively 6313



ARTICLE

Table 7. Bond Lengths (angstroms) for (74) and [Au4(μI)2(PPh3)4] bond

a

(74) (Å)

Table 8. Trimers Resulting from Four Gold Atom Decomposition Reactions

[Au4(μ-I)2(PPh3)4] (Å)a

reactants

charge of trimer ΔE (kJ/mol)

products

Au(8)Au(2)

2.719

2.649

(11) þ (1) (23) þ (0) þ 3Cl

þ1

121.93

Au(7)Au(2) Au(8)Au(1)

2.840 2.842

2.744 2.771

(4) þ (4) (23) þ (0) þ 3Cl (11) þ (1) (24) þ (0) þ 2Cl

þ1 0

106.80 197.98

Au(2)Au(1)

2.866

2.828

(13) þ (1) (24) þ (0) þ 2Cl

0

203.55

0

177.97

(18) þ (1) (10) þ (0) þ PH3 þ 2Cl

0

173.28

(18) þ (1) (24) þ (0) þ 2Cl

0

198.56

(4) þ (5)

Ref 13.

(9) þ (0) þ 2Cl

(15) þ (0) (19) þ (0)

1

20.50

(19) þ (1) (16) þ (0) þ Cl

1

28.68

Table 9. Dimers Resulting from Four Gold Atom Decomposition Reactions reactants

(6) þ 2(0) þ 2Cl (7) þ 2(0) þ Cl þ PH3

146.59 141.27

(6) þ 2(0) þ 2Cl

158.70

(4) þ (6)

(6) þ (8) þ 2PH3

142.29

(5) þ (5)

(6) þ 2(0) þ 2Cl

170.80

(5) þ (6)0

(6) þ (7) þ Cl þ PH3

222.86

(12) þ (1)

(6) þ 2(0) þ 2Cl

173.48

(7) þ 2(0) þ Cl þ PH3

156.06

(7)0 þ (5) (7)0 þ (4)

2(7) þ Cl þ PH3 (6) þ (7) þ 2Cl

284.19 289.50

(7)0 þ (6)0

Au22 þ (7) þ 2PH3

(4) þ (4) (4) þ (5) 0

Figure 11. Au4Cl2(PH3)4.

charged cluster by the release of a chloride ion is less exothermic than the formation of anionic or neutral clusters. The diamond structure with a negatively charged gold core (60) has an energy of formation of 87.88 kJ/mol. The average AuAu bond lengths for structures with a neutral gold core are between 2.75 and 2.80 Å, which is slightly longer than the 2.696 Å value determined by Li et al.41 at the PW91/ LANL2DZ level of theory. Tetrahedra. Thirteen of the observed structures have tetrahedral cores. Two of them have a (þ3) charged gold core, two have a (þ2) gold core, four have a (þ1) core, three have a neutral core, one has a (1) core, and one has a (2) core. The energies of formation range from 226.47 to þ12.29 kJ/mol, with the most exothermic reactions correlating with the release of one or more chloride ions and the endothermic reaction corresponding to the release of a chloride ion from a positively charged structure. One interesting structure is the tetrahedron [Au4(PH3)4]þ (69), which can be reduced to [Au4(PH3)4] (75). Both structures are shown in Figure 10. (69) has one positive charge. Experimentally, tetrahedral gold clusters with a (þ2) charge have been isolated.15 These clusters can be synthesized directly from the precursor molecules16 or from the reaction between [Au9(PR3)8(NO3)5] and potassium iodide.13 In addition, these clusters have substituted phosphine groups, which can donate electrons to the gold and therefore stabilize higher positively charged cores.30 Tetramer (74) is similar to the [Au4(μ-I)2(PPh3)4] tetramer isolated by Demartin et al.13 Bond angles and bond distances are compared in Table 7. (74) with labeled atoms is shown in Figure 11. The calculated bond lengths tend to be overestimated (between 0.04 and 0.1 Å) in comparison with the crystal structure, which is common for GGAs like BP86. Y-Shaped. Twelve tetramer structures are Y-shaped. Seven of them have a (1) charged gold core, which compares nicely with predictions by H€akkinen and Landman that the Y-shaped structure

ΔE (kJ/mol)

products

74.45

is the lowest energy isomer for the bare Au4 ion.44 Three of the tetramers have a neutral core, and two of them have a (2) core. This trend differs from the diamonds and tetrahedrons, which mostly tend to be positively charged. Linear Nanowire. One bare nanowire with two negative charges was optimized. The distance between the two central atoms is 2.60 Å, and the distance between the outer atoms and the central atoms is 2.67 Å. These values differ slightly from the values obtained for the neutral tetramer by Xiao et al.,43 which are 2.65 and 2.60 Å, respectively. In the Au42 system, the two additional electrons occupy an orbital with bonding character between the two central gold atoms and antibonding character between the outer and central atoms, which explains the difference in bond length. Decomposition Reactions. Not all reactions involving four gold atoms lead to the formation of tetramers but instead lead to decomposition into [trimer þ monomer] or [dimer þ two monomers], as summarized in Tables 8 and 9. In the former case, the trimers that are formed are either neutral or positively charged contrary to the [dimer þ monomer] reactions, where most of the trimers are negatively charged. The new trimer structures are shown in Figure 6. In the latter case, (6) is the dimer most likely to be formed. In both cases, the reactions are very exothermic. In addition, dissociation of chlorine ions, phosphine groups, or both is common. Overall, these reactions show that a significant number of species may be present during the growth of gold-phosphine clusters. We shall note that no reaction happens between (21) þ (1), (22) þ (1), (7) þ (4) and (8) þ (8). 6314



’ CONCLUSIONS The growth of gold-phosphine clusters from AuClPR3 is studied using density functional theory. The gold atom in the neutral monomer is linear for R = H, Me, and Ph. However, it is bent in the anionic monomer for R = H and Me due to the addition of an electron on a σ* orbital localized on the gold atom. Because the extra electron goes into a π* orbital on the phenyl rings, the anionic monomer remains linear for R = Ph. Two neutral monomers can form an aurophilic complex, but a reducing agent is necessary to observe a reaction, as evidenced by shorter AuAu bonds and occupied AuAu bonding orbitals. The addition of dispersion forces gives similar structures but different energies of formation for the dimer depending on the number of negative charges involved. The trimer structures obtained are triangular, obtuse angle triangular, and linear. They mainly tend to be negatively charged and have very negative energies of formation. The inclusion of dispersion forces can yield different structures and also tends to give higher energies of formation. The tetramers can have four different shapes: diamond, tetrahedron, Y-shape, and linear shape. Whereas gold cores in the diamonds and tetrahedra are mostly positively charged, the Y-shaped and linear clusters cores are negatively charged. The core shapes agree with previous structural predictions of bare gold clusters having similar core charges. The large variety of structures obtained in this simulation shows the complexity of gold-phosphine cluster growth mechanism. The relative orientation of the reactants can greatly influence the nature of the product. Because most of the reactions are very exothermic, we expect a poor selectivity. Not all reactions lead to cluster growth because a number of decomposition reactions are found to be favorable. This further increases the number of possible products. During the growth of gold clusters, chloride ions tend to dissociate. This is very favorable for neutral and negatively charged clusters but unfavorable for positively charged cores. Phosphine ligands also dissociate, although this is less favorable for PPh3 than PH3. Overall, this work shows that the presence of the reducing agent is required to have enough electrons to initiate formation of the first AuAu bond. Further reactions between reduced species often lead to spontaneous dissociation of ligands. Cl ligands dissociate preferentially over phosphine ligands, which aids in removing excess negative charge from the cluster core. In consequence, this also explains why Aux(PR3)yClz clusters are generally observed with formulas of x > y > z. Similar mechanisms are likely to play a role in the growth of gold-phosphine nanoparticles under comparable reaction conditions. ’ ASSOCIATED CONTENT

bS

Supporting Information. Reaction energies and products for tetramer formation. This material is available free of charge via the Internet at http://pubs.acs.org.

’ ACKNOWLEDGMENT We thank the Air Force Office of Scientific Research and the Defense Advance Research Projects Agency for funding research on the structure and optical properties of noble metal nanoparticles through grant FA9550-09-1-0451.

ARTICLE

’ REFERENCES (1) Turner, M.; Golovko, V. B.; Vaughan, O. P. H.; Abdulkin, P.; Berenguer-Murcia, A.; Tikhov, M. S.; Johnson, B. F. G.; Lambert, R. M. Nature 2008, 454, 981–982,983. (2) Zhang, C.; Yoon, B.; Landman, U. J. Am. Chem. Soc. 2007, 129, 2228–2229. (3) Shaw, C. F. Chem. Rev. 1999, 99, 2589–2600. (4) Brown, S. D.; Nativo, P.; Smith, J.; Stirling, D.; Edwards, P. R.; Venugopal, B.; Flint, D. J.; Plumb, J. A.; Graham, D.; Wheate, N. J. J. Am. Chem. Soc. 2010, 132, 4678–4684. (5) Kang, B.; Mackey, M. A.; El-Sayed, M. J. Am. Chem. Soc. 2010, 132, 1517–1519. (6) Forward, J. M.; Bohmann, D.; Fackler, J. P.; Staples, R. J. Inorg. Chem. 1995, 34, 6330–6336. (7) Gao, L.; Partyka, D. V.; Updegraff, J. B.; Deligonul, N.; Gray, T. G. Eur. J. Inorg. Chem. 2009, 2009, 2711–2719. (8) Tiekink, E. R. T.; Kang, J. Coord. Chem. Rev. 2009, 253, 1627–1648. (9) Suhre, M. H.; Reif, M.; Kirsch, S. F. Org. Lett. 2005, 7, 3925–3927. (10) Pei, Y.; Shao, N.; Gao, Y.; Zeng, X. C. ACS Nano 2010, 4, 2009–2020. ~ 3n, P.; Toste, F. D. J. Am. Chem. Soc. (11) Luzung, M. R.; MauleA 2007, 129, 12402–12403. (12) Mirabell, C. K.; Johnson, R. K.; Hill, D. T.; Faucette, L. F.; Girard, G. R.; Kuo, G. Y.; Sung, C. M.; Crooke, S. T. J. Med. Chem. 1986, 29, 218–223. (13) Demartin, F.; Manassero, M.; Naldini, L.; Ruggeri, R.; Sansoni, M. J. Chem. Soc., Chem. Commun. 1981, 222–223. (14) Mingos, D. M. P.; Powell, H. R.; Stolberg, T. L. Transition Met. Chem. 1992, 17, 334–334, 335, 336, 337. (15) Zeller, E.; Beruda, H.; Schmidbaur, H. Inorg. Chem. 1993, 32, 3203–3204. (16) Yang, Y.; Sharp, P. R. J. Am. Chem. Soc. 1994, 116, 6983–6984. (17) Bellon, P.; Manassero, M.; Sansoni, M. J. Chem. Soc., Dalton Trans. 1973, 2423–2427. (18) Briant, C. E.; Hall, K. P.; Mingos, D. M. P. J. Organomet. Chem. 1983, 254, C18–C20. (19) Van, d. V.; Bour, J. J.; Bosman, W. P.; Noordik, J. H. Inorg. Chem. 1983, 22, 1913–1918. (20) Schulz-Dobrick, M.; Jansen, M. Z. Anorg. Allg. Chem. 2007, 633, 2326. (21) McPartlin, M.; Mason, R.; Malatesta, L. J. Chem. Soc. D 1969, 334–334. (22) Bellon, P.; Manassero, M.; Sansoni, M. J. Chem. Soc., Dalton Trans. 1972, 1481–1487. (23) Nunokawa, K.; Onaka, S.; Ito, M.; Horibe, M.; Yonezawa, T.; Nishihara, H.; Ozeki, T.; Chiba, H.; Watase, S.; Nakamoto, M. J. Organomet. Chem. 2006, 691, 638–642. (24) Copley, R. C. B.; Mingos, D. M. J. Chem. Soc., Dalton Trans. 1996, 479–489. (25) Briant, C. E.; Theobald, B. R. C.; White, J. W.; Bell, L. K.; Mingos, D. M.; Welch, A. J. J. Chem. Soc., Chem. Commun. 1981, 201–202. (26) Van Attekum, P. M. T. M.; Van, d. V.; Trooster, J. M. Inorg. Chem. 1980, 19, 701–704. (27) Zhang, H.; Stender, M.; Zhang, R.; Wang, C.; Li, J.; Wang, L. J. Phys. Chem. B 2004, 108, 12259–12263. (28) Teo, B. K.; Shi, X.; Zhang, H. J. Am. Chem. Soc. 1992, 114, 2743–2745. (29) Calhorda, M. J.; Crespo, O.; Gimeno, M. C.; Jones, P. G.; Laguna, A.; Lopez-de-Luzuriaga, J. M.; Perez, J. L.; Ramon, M. A.; Veiros, L. F. Inorg. Chem. 2000, 39, 4280–4285. (30) Boca, R. J. Chem. Soc., Dalton Trans. 1994, 2061–2064. (31) Pyykko, P.; Runeberg, N. J. Chem. Soc., Chem. Commun. 1993, 1812–1813. (32) Bulusu, S.; Li, X.; Wang, L.; Zeng, X. C. J. Phys. Chem. C 2007, 111, 4190–4198. 6315



ARTICLE

(33) Bulusu, S.; Zeng, X. C. J. Chem. Phys. 2006, 125, 154303. (34) Gu, X.; Bulusu, S.; Li, X.; Zeng, X. C.; Li, J.; Gong, X. G.; Wang, L. J. Phys. Chem. C 2007, 111, 8228–8232. (35) Gao, Y.; Zeng, X. C. J. Am. Chem. Soc. 2005, 127, 3698–3699. (36) Olson, R. M.; Gordon, M. S. J. Chem. Phys. 2007, 126, 214310. (37) Olson, R. M.; Varganov, S.; Gordon, M. S.; Metiu, H.; Chretien, S.; Piecuch, P.; Kowalski, K.; Kucharski, S. A.; Musial, M. J. Am. Chem. Soc. 2005, 127, 1049–1052. (38) Han, Y. J. Chem. Phys. 2006, 124, 024316. (39) Diefenbach, M.; Kim, K. S. J. Phys. Chem. B 2006, 110, 21639–21642. (40) Wang, J.; Wang, G.; Zhao, J. Phys. Rev. B 2002, 66, 035418. (41) Li, X.; Wang, H.; Yang, X.; Zhu, Z.; Tang, Y. J. Chem. Phys. 2007, 126, 084505. (42) Walker, A. V. J. Chem. Phys. 2005, 122, 094310. (43) Xiao, L.; Tollberg, B.; Hu, X.; Wang, L. J. Chem. Phys. 2006, 124, 114309. (44) H€akkinen, H.; Landman, U. Phys. Rev. B 2000, 62, R2287. (45) Grimme, S. J. Comput. Chem. 2006, 27, 1787–1799. (46) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra, C.; van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22, 931–967. (47) Klamt, A.; Schuurmann, G. J. Chem. Soc., Perkin Trans. 2 1993, 799–805. (48) Klamt, A.; Jonas, V. J. Chem. Phys. 1996, 105, 9972–9981. (49) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1994, 101, 9783–9792. (50) van Lenthe, E.; Ehlers, A.; Baerends, E. J. Chem. Phys. 1999, 110, 8943–8953. (51) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (52) Perdew, J. P. Phys. Rev. B 1986, 33, 8822. (53) Haeberlen, O. D.; Roesch, N. J. Phys. Chem. 1993, 97, 4970–4973. (54) Baenziger, N. C.; Bennett, W. E.; Soborofe, D. M. Acta Crystallogr., Sect. B 1976, 32, 962–963. (55) Pyykk€o, P.; Li, J.; Runeberg, N. Chem. Phys. Lett. 1994, 218, 133–138. (56) Pyykk€o, P.; Zhao, Y. Angew. Chem. Int. Ed., Engl. 1991, 30, 604–605. (57) Li, J.; Pyykk€o, P. Chem. Phys. Lett. 1992, 197, 586–590. (58) Schmidbaur, H.; Weidenhiller, G.; Steigelmann, O.; Muller, G. Chem. Ber. 1989, 123, 285–287. (59) Schmidbaur, H.; Graf, W.; M€uller, G. Angew. Chem. Int. Ed., Engl. 1988, 27, 417–419. (60) Pyykko, P. Chem. Rev. 1997, 97, 597–636. (61) Pyykko, P.; Zaleski-Ejgierd, P. J. Chem. Phys. 2008, 128, 124309.

6316


Initial Growth Mechanisms of Gold-Phosphine Clusters - American

Recommend Documents