Understanding Planar Ligand-Supported MAu5 and MAu6 Cores

Article pubs.acs.org/JPCC

Understanding Planar Ligand-Supported MAu5 and MAu6 Cores. Theoretical Survey of [MAu5(Mes)5] and [MAu6(Mes)6] (M = Cu, Ag, Au; Mes = 2,4,6-Me3C6H2) Under the Planar Superatom Model A. Muñoz-Castro*,†,‡ and R. Guajardo Maturana‡ †

Direccion de Postgrado e Investigacion, Universidad Autonoma de Chile, Carlos Antunez 1920, Santiago, Chile Doctorado en Fisico-Quimica Molecular, Universidad Andres Bello, Republica 275, Santiago, Chile

‡

S Supporting Information *

ABSTRACT: The planar superatom model has been applied to the case of planar ligand-supported MAu5 and MAu6 cores, where M = Cu(I), Ag(I), and Au(I), in order to increase the understanding of the electronic structure and bonding properties of planar golden clusters. The study of [Au5(Mes)5], [Au6(Mes)6], [MAu5(Mes)5], and [MAu6(Mes)6] has been carried out by using relativistic DFT calculations, which describe the short d10−d10 contacts due to the bonding stabilization within the Aun core in addition to the respective aurophilic phenomena. The results under the planar superatom approach allow us to characterize the electronic structure in all the systems as formally 10 valence electron cores, depicting an overall 1s21px,y41dxy,x2−y24 configuration as a result of the ligand−metal interaction. The inclusion of the respective M(I) closed shell center increases the number of superatomic shells as 1s1p1d → 1s1p1d2s, denoting the interaction between each concentric section. Our results suggest that the MAun cores could be conveniently viewed as the combination of concentric structures denoted by [M@Aun]. In addition, the role of the inclusion of the spin−orbit term into the planar superatom model is discussed.

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the resulting enhanced stability.38−42 However, despite the comprehensive works exploring the electronic structure of three-dimensional (3D) clusters, the use of the planar superatom model (2D) has been studied to a small extent.43,44 Two-dimensional arrangements of such closed-shell complexes are of particular importance because they offer reliable models to gain a better understanding of the interaction between substrates and coinage−metal surfaces, in heterogeneous catalysts and metal surface reactions.45,46 The synthesis of such molecular frameworks involves the use of ligands or metal moieties acting as electron donor fragments toward the multinuclear core ([M(I)n]n+), leading to ligand-supported stabilized macrocycle clusters.47−50 In this respect, Floriani and co-workers51 have reported the early synthesis of a ligand-supported pentanuclear Au(I) ring, namely, [Au5(Mes)5] (Mes = 2,4,6-Me3C6H2), among the Cu(I) and Ag(I) counterparts, which is stabilized by mesityl ligands. The fascinating ability to incorporate a heteroatom inside the metallic ring or core in such systems has been characterized in further studies by Laguna and Hursthouse.52 Such authors describe the formation of penta- and hexagold rings displaying a centered Cu(I) or Ag(I) atom, which represent interesting cases of ligand-stabilized two-dimensional

INTRODUCTION Molecular entities involving several redox centers have attracted considerable attention because of their unique properties toward the formation of supramolecular materials. Such molecular arrays allow the obtention of novel functional building blocks for devices in molecular electronics,1−3 biosensors, and catalysts, among others, exhibiting a wide range of current and potential in technological issues.4−17 In this concern, the chemistry of group XI elements has been widely developed, revealing the rich structural versatility of closed-shell monovalent Cu(I), Ag(I), and Au(I) centers resulting in a plethora of discrete and extended structures.18−26 Their general tendency to form aggregates supporting d10− 10 d closed-shell interactions27−30 has been attributed to electronic correlation being reinforced by relativistic effects, particularly relevant for gold counterparts. Usually, d10−d10 contacts exhibit distances shorter than the sum of the van der Waals radii of the involved centers, resulting in a situation which has been coined as metallophilic interaction, leading to interesting properties.27−30 The extensive formation of gold nanostructures displaying a central metallic core protected by stabilizing ligands provides a significant contribution to various areas of science and technology.31−37 These structures have been rationalized in terms of the superatom model, where the central core describes an electronic shell closing in accordance to the spherical jellium model leading to a 1s2, 1p6, 1d10, ... shell order, accounting for © 2014 American Chemical Society

Received: June 10, 2014 Revised: August 21, 2014 Published: August 22, 2014 21185

dx.doi.org/10.1021/jp5057557 | J. Phys. Chem. C 2014, 118, 21185−21191

The Journal of Physical Chemistry C

Article

coinage metal clusters. Such planar structures,51,52 similarly to their three-dimensional counterparts, can be rationalized in analogy to the electron shell filling of the valence electrons within a two-dimensional electron gas according to the planar jellium model, namely, the planar superatom model.44 The latter point prompted us to carry out a rationalization of these planar stabilized cores including all the group IX elements as centered atoms (MAu5 and MAu6, M = Cu(I), Ag(I), and Au(I)) in order to extend the wide use of the superatom model in spherical-like clusters to the two-dimensional case. Herein, we explore electronic and bonding properties of [MAu5(Mes)5]+ and [MAu6(Mes)6]+ (M = Cu(I), Ag(I), and Au(I)) by using relativistic density functional theory (DFT) calculations.53,54 We focus on the [Aun(Mes)n]−M+ and [MAun]n+1−[(Mes)n]n− bonding schemes in order to gain insights into the formation of these clusters according to the planar superatom model. In addition, the population of the ns-based levels of the closed-shell core has been described as a useful characteristic to denote both short or long d10−d10 contacts,44,55 where in the former case the bonding combinations are mainly populated leading to distances sizably shorter than the sum of the van der Waals radii. Thus, we describe the population of the respective levels in order to account for the short d10−d10 distance of about 2.7−2.8 Å as has been described in [Au5(Mes)5] and [AgAu6(Trip)6] (Trip = 2,4,6-i-Pr3C6H2).51,52

characterization of [AgAu5(Mes)5]+, [CuAu5(Mes)5]+, and [AgAu6(Trip)5]+ via mass spectrometry and NMR experiments, obtaining in addition suitable crystals for X-ray diffraction for the latter cluster. The calculated structures for [MAu5(Mes)5]+ (M = Cu(I) (1), Ag(I) (2), and Au(I) (3)) and [MAu6(Mes)6]+ (M = Cu(I) (4), Ag(I) (5), and Au(I) (6)) are schematically represented in Figure 1 (Supporting Information), which

Figure 1. Representation of the optimized structures for [MAu5(Mes)5]+ and [MAu6(Mes)6]+ (M = Cu, Ag, Au).

belong to the C1 (1, 2, and 3) and D3 point groups (4, 5, and 6). The obtained structures for [MAu6(Trip)6]+ (M = Cu(I) (7), Ag(I) (8), and Au(I) (9)) are given in the Supporting Information. Selected calculated structural parameters are summarized in Table 1, denoting the agreement to the available

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Table 1. Selected Distances (Å) of the Studied Complexes

COMPUTATIONAL DETAILS Relativistic density functional theory calculations53 were carried out by using the ADF code,56 incorporating scalar relativistic effects via the ZORA Hamiltonian.54 The triple-ξ Slater basis set, plus two polarization functions (STO-TZ2P) for valence electrons, were employed within the generalized gradient approximation (GGA) according to the Perdew−Burke− Ernzerhof (PBE) nonlocal exchange-correlation functional,57,58 due to its improved performance on long-range interactions and relatively low computational cost for larger molecules.59,60 The frozen core approximation was applied to the [1s2−4f14] core for Au, [1s2−4p6] for Ag, [1s2−2p6] for Cu, and [1s2] for C, leaving the remaining electrons to be treated variationally. Geometry optimizations were done without any symmetry restrain, via the analytical energy gradient method implemented by Verluis and Ziegler,61 by using the experimentally characterized planar clusters51,52 as starting structures. All the obtained structures (Supporting Information) lie in a stationary point on the potential energy surface (PES) as determined by the vibrational analyses, which describe negative frequencies (∼ −20 to −30 cm−1) corresponding to the rotation of the methyl groups from the 2,4,6-Me3C6H2 (Mes) ligand.

[Au5(Mes)5] [CuAu5(Mes)5]+ [AgAu5(Mes)5]+ [AuAu5(Mes)5]+ [Au6(Mes)6] [CuAu6(Mes)6]+ [AgAu6(Mes)6]+ [AuAu6(Mes)6]+ [Au6(Trip)6] [CuAu6(Trip)6]+ [AgAu6(Trip)6]+ [AuAu6(Trip)6]+

exp.a

theor.

b

Au−Au

Au−Au

M−Au

2.700

2.776 3.009 2.999 3.152 2.790 2.795 2.875 2.862

2.803

2.819 2.906 2.882

C−Au

2.794 2.876 2.862

2.182 2.169 2.168 2.176 2.182 2.181 2.176 2.177

2.817 2.918 2.891

2.165 2.146 2.161

2.568 2.767b 2.688

qM(I)c 0.39 0.50 0.18 0.48 0.46 0.23

a

Experimental data from refs 51 and 52. bCentral silver atom located 1.090 Å above the Au5 ring (see Figure 1). cNatural population analysis of the central M(I) atom.

experimental structures.51,52 The structures exhibit a central ring composed by gold atoms, with Au−Au distances (Table 1) below the sum of their van der Waals radii (3.32 Å), denoting the enhanced interaction between these formally d10 closedshell centers. The systems involving the Trip ligands exhibit results similar to those obtained with Mes; thus, in the following, we focus our analysis on the results provided by the [Aun(Mes)n] and [MAun(Mes)n]+ (n = 5 and 6) calculations. The inclusion of the central M(I) atom increase the Au−Au distance of the five-membered case from 2.75 to ∼3.10 Å, resulting in a centered structure for the Cu(I) and Au(I) cases (1 and 3). For 2, the silver atom is located at 1.10 Å above the Au5 plane, avoiding a longer Au−Au separation within the ring (3.00 Å for 2). In contrast, the systems involving the Au6 ring (4, 5, and 6) exhibit a slight variation of the Au−Au distance (Au−Au = 2.78−2.86 Å), and the ring radius is enough to allow

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RESULTS AND DISCUSSION The early obtention of a series of molecular rings retaining d10− d10 metallophilic interactions has been reported by the group of Floriani51 denoting the structure of multinuclear aryl complexes of Cu(I), Ag(I), and Au(I). Their solid-state structures, obtained from toluene solutions, show clusters with high nuclearity, namely, [Cu(Mes)]5, [Ag(Mes)]4, and [Au(Mes)]5 (Mes = 2,4,6-Me3C6H2), describing stable homoleptic macrocycles which exhibit quite symmetric arrays roughly considered as D5h and D4h structures, respectively. Laguna and coworkers52 have reported a synthetic approach to obtain heteroleptic counterparts of such systems, resulting in the 21186



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[Au(Mes)]6, the respective levels are populated with 1.66 e̅ (1s-[Au6]), 2.22 e̅ (1px,y-[Au6]), 1.06 e̅ (1dxy,x2−y2-[Au6]), and 0.28 e̅ (1fx(x2−3y2)-[Au6]). Thus, these results denote a bonding situation within such metallic arrays, resulting in a short Au(I)− Au(I) contact (short d10−d10) depicting short aurophilic distances of about 2.8 Å which are considerably lower than the sum of their van der Waals radii.27−30 The inclusion of a central coinage atom into the [Au5(Mes)5] and [Au6(Mes)6] macrocycles leads to the formation of energetically favorable complexes (Figure 1) as summarized in Table 3. The total interaction energy (ΔEtot)

the inclusion of M(I). The M(I)−Aun distance increases from 2.57 to 2.78 Å for the [MAu5(Mes)5]+ series, being longer for the silver counterpart (2). A similar trend is denoted for the [MAu6(Mes)6]+ series, where a smaller variation is observed whithin the range of 2.79−2.88 Å. The electronic structure and bonding of [Au(Mes)]5 and the hypothetical [Au(Mes)]6 are revisited in analogy to the level sequence resulting from a planar superatom model which is applied for the Au5 or Au6 ring. This approach can be coined as planar superatom model, a two-dimensional case (i.e., neglecting the z-axis)43,44 related to the superatom model widely applied to spherical clusters.38−42 For the Au5 core, the molecular levels resulting from the combination between the five 6s-Au atomic functions span as a′1 ⊕ e′1 ⊕ e′2 (D5h point group) which resembles the 1s, 1px,y, 1dxy,x2−y2 set of electronic shells (hereafter, 1s-[Au5], 1px,y-[Au5], etc.). Similarly, the Au6 core under the D6h point group (Γ6s‑Au = a1g ⊕ e1u ⊕ e2g ⊕ b1u) describes the 1s, 1px,y, 1dxy,x2−y2, and 1fx(x2−3y2) (Figure 2) planar

Table 3. Energy Decomposition Analysis (EDA, kcal/mol) for [MAun(Mes)n], Denoting the M(I) and [Aun(Mes)n] Interaction

Figure 2. Frost diagram denoting qualitatively the relative bonding/ antibonding character of the ns combinations in Au5 and Au6 rings.

superatomic shells, denoted as 1s-[Au6], 1px,y-[Au6], ..., repectively. Under this framework we revise the stabilization of [Au(Mes)]5 and the hypothetical [Au(Mes)]6, which has been studied previously.62,63 In order to determine the relative bonding, nonbonding, and antibonding character of the respective superatomic shell, it is useful to use the Frost circle mnemonic diagrams64 considering one 6s-Au atomic function per vertex (Figure 2). Hence for the Au5 and Au6 cores or rings, the 1s-[Aun] is the totally bonding combination; the 1px,y-[Aun] is of lower bonding character; and 1dxy,x2−y2 is of large antibonding character. Lastly, for Au6, the 1fx(x2−3y2) is a totally antibonding level. As a result of the bonding interaction between the ligand and the metallic core, such 6s-based shells are populated as has been described previously.62,63 Despite the decrease in symmetry due to the formation of the ligand-protected cluster (C1 for 1−3 and D3 for 4−6), the electronic shells of the metallic core are labeled according to the superatomic shells described above. In [Au(Mes)]5, the 1s-[Au5], 1px,y-[Au5], and 1dxy,x2−y2-[Au5] levels are populated with 1.69 e,̅ 2.13 e,̅ and 0.66 e,̅ respectively, as given by the analysis of the coefficients of the respective wave functions in the overall cluster (Table 2). Similarly, for

1 2 3 4 5 6

1s

1p

1d

1f

1.69 1.98 1.85 1.98 1.66 1.97 1.89 1.98

2.15 2.24 2.30 2.14 2.22 2.36 2.35 2.20

0.78 0.97 0.95 0.98 1.06 1.02 0.99 0.97

0.28 0.34 0.32 0.31

2

3

Eprep Epauli Eorb Eelstat Eint

12.78 131.65 −113.71 −154.39 −123.67 4

11.51 112.07 −92.34 −123.38 −92.14 5

20.94 206.69 −153.27 −212.11 −137.75 6

Eprep Epauli Eorb Eelstat Eint

0.02 69.83 −98.30 −104.00 −132.45

42.4% 57.6%

48.6% 51.4%

1.86 93.12 −91.38 −113.44 −109.84

42.8% 57.2%

44.6% 55.4%

1.82 134.30 −135.05 −149.73 −148.66

41.9% 58.1%

47.4% 52.6%

ranges from −92.14 to −148.66 kcal/mol, which increases in the following order Ag+ < Cu+ < Au+, denoting the more stable situation for the central gold counterpart. In order to gain more insight into the different terms contributing to Etot, we performed an energy decomposition analysis within the Morokuma−Ziegler scheme.65,66 In this framework, Etot can be decomposed into several terms according to ΔEtot = ΔEprep + ΔE int = ΔEprep + ΔEelstat + ΔE Pauli + ΔEorb

The ΔEprep refers to the preparation energy involving the conformational changes of the respective [Aun(Mes)n] metallacycle from their equilibrium conformation to the geometries in the final [MAun(Mes)n] complexes. The ΔEelstat term is computed by considering each fragment (namely, A and B) in its unperturbed (frozen) electron density as isolated species (ΨAΨB), which accounts for the electrostatic character of the interaction. The ΔEPauli term comprises the destabilizing fourelectron two-orbital interactions between occupied orbitals, which is calculated from the energy change upon antisymmetrization and renormalization of the overlapped fragment densities (Ψ0 = NÂ {ΨAΨB}), denoting the steric hindrance. Lastly, the ΔEorb term is obtained when the densities of the constituent fragments are allowed to relax into the final molecular orbitals (ΨAB), which accounts for the covalent character of the interaction. The formation of [MAu5(Mes)5] (Figure 1 and Table 1) involves structural changes within the [Au5(Mes)5] metallacycle which accounts for the deformation or preparation energy (Eprep) term which range from 11.5 to 20.9 kcal/mol, denoting the deviation from the equilibrium geometry of [Au5(Mes)5]. In the Cu(I) and Au(I) cases (1 and 3), the central coinage

Table 2. Population of Selected Planar Superatomic Levels Denoting the 1s, 1p, 1d, and 1f Jellium-Like Shells

[Au5(Mes)5] [CuAu5(Mes)5]+ [AgAu5(Mes)5]+ [AuAu5(Mes)5]+ [Au6(Mes)6] [CuAu6(Mes)6]+ [AgAu6(Mes)6]+ [AuAu6(Mes)6]+

1

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atom is located at the Au5 plane, which increases the ring Au− Au distance (Table 1) resulting in a preparation energy of 12.78 and 20.94 kcal/mol, respectively. In contrast, in system 2 the central Ag(I) atom is located above the Au5 ring minimizing the structural rearrangement, resulting in a destabilization of about 10 kcal/mol for the [Au 5 (Mes) 5 ] structure. For the [Au6(Mes)6] case, lower values of the Eprep (∼1.5 kcal/mol) are obtained because of the small steric hindrance given by the size of the six-membered ring (see above). The resulting interaction energy (Eint) accounts for the favorable inclusion of M(I) into the [Au5(Mes)5] metallacycle, which increases as Ag+ < Cu+ < Au+ (Table 3) in the range of −92.14 to −137.75 kcal/mol depicting a slightly electrostatic character (∼58%) given by the relation between stabilizing terms (namely, Eorb and Eelect). The [MAu6(Mes)6] complexes denote a slighter more favorable Eint, due to the lesser steric hindrance as a consequence of the larger size of the Au6 ring. The molecular orbital diagrams in Figures 3 and 4 describe the representative interaction between M(I) and [Aun(Mes)n].

Figure 5. Isosurface (±0.02 au) representation of selected levels for [AgAu5(Mes)5]+ and [AgAu6(Mes)6]+.

complex in all the studied systems, which is of main M(I) character. The inclusion of the central M(I) increases the ligand−core ((Mes)n/MAun) interaction, as given by the variation of Eint summarized in Table S1 (Supporting Information). In the [MAu5(Mes)5] case, the net ligand donation increases from 2.37 e̅ (for [Au5(Mes)5]) to ∼3.00 e̅ and, for [MAu6(Mes)6], from −2.83 e̅ (for [Au6(Mes)6]) to ∼3.40 e.̅ The natural population analyses67 support the charge transfer between the [Aun(Mes)n] complex toward the M(I) center resulting in a charge of: Cu0.39+, Ag0.50+, and Au0.18+, for 1, 2, and 3, respectively; and Cu0.48+, Ag0.46+, and Au0.23+, for 4, 5, and 6 (Table 1). The population of the core-based levels in the studied complexes (Table 2) resembles the sequence for a planar superatom model which is similar to the discussion given above for [Aun(Mes)n], where the levels of bonding character are populated to a large extent in comparison to the antibonding combinations (Figure 5). Hence, the MAun core behaves as a whole cluster entity, which can be described by the M@Aun form, in analogy to three-dimensional endohedral clusters such as [M@Au12] among others,68−71 because the resulting electronic shells retain the characteristics given by the concentric structures, namely, M(I) and Aun. According to the planar superatom model, the electronic structure of the ligand-protected M@Au 5 and M@Au 6 complexes can be described by a formal 10 valence electron core, depicting an overall 1s21px,y41dxy,x2−y24 configuration. The electron count is suggested from the obtained population of the relevant shells which describes an almost fully occupied 1s shell (average 97%), whereas for 1px,y and 1dxy,x2−y2 shells the population rises up to an averaged value of 57% and 50%, respectively, for systems 1−6 (Table 2 and Table S3, Supporting Information). This fact denotes the effect of the charge transfer from the protecting ligands toward the metallic core which requires a sizable share of electron density in order to ensure the stability of the overall planar structure. Interestingly, the hypothetical [Au@Au6(Mes)6]+ compound can be related to the 6-ve Au7+ (D6h) bare cluster72 in order to gain more insights for the above-described discussion. Both planar seven-membered rings describe a 1s1px,y1dxy,x2−y22s electronic shell order (Figure 6); however, such structures denote differences in their formal electron count of 6-ve and 10ve for Au7+ and [Au@Au6(Mes)6]+, respectively. The overall population of all the relevant ns-based shells sums up to 6.00 e̅ for the bare cluster and 5.46 e̅ for the ligand-protected counterpart, denoting a rather similar amount, but with

Figure 3. Simplified molecular orbital diagram of [AgAu5(Mes)5]+ denoting the contribution from Ag(I) and [Au5(Mes)5]. Isosurface value ±0.02 au.

Figure 4. Simplified molecular orbital diagram of [AgAu6(Mes)6]+ denoting the contribution from Ag(I) and [Au6(Mes)6]. Isosurface value ±0.02 au.

The use of the planar superatom model in the studied systems simplifies the analysis of such an interaction despite their different point groups or symmetries. From Figures 3 and 4, it can be seen that the bonding scheme is given by the s-type interaction between the 1s-[Aun(Mes)n] (n = 5, 6) and the unfilled ns atomic shell of the M(I) center. Such interaction leads to the bonding 1s-[MAun(Mes)n] shell and the respective antibonding counterpart, namely, 2s-[MAun(Mes)n], which remain unoccupied (Figure 5). The bonding combination is contributed mainly by the [Aun(Mes)n] fragment, which is responsible for the charge transfer toward the M(I) closed-shell center. Moreover, the respective antibonding combination gives rise to the lowest unoccupied molecular orbital of the overall 21188



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each concentric section, namely, M(I) and Aun(Mes)n (n = 5 or 6), which increases the ligand charge donation toward the metallic core. For [Au 5 (Mes) 5 ], [Au 6 (Mes) 6 ], [MAu 5 (Mes) 5 ], and [MAu6(Mes)6], the interaction between the formally closedshell centers can be ascribed to a short d10−d10 contact, due to the bonding stabilization within the Aun core in addition to the respective aurophilic phenomena. Our results suggest that the MAun cores could be conveniently viewed as the combination of concentric structures denoted by [M@Aun] because the overall MAun core retaina the electronic shell characteristics of the involved concentric fragments. Thus, the studied compounds can be described by the [M@Au5(Mes)5] and [M@Au6(Mes)6] forms.

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Figure 6. Isosurface (±0.02 au) representation of selected levels for Au7+ and [AuAu6(Mes)6]+.

ASSOCIATED CONTENT

S Supporting Information *

Structural geometries for [Au6(Mes)6]+ (M = Cu, Ag, and Au) and selected levels for [Au5(Mes)5] and [Au6(Mes)6], denoting the planar superatomic levels. This material is available free of charge via the Internet at http://pubs.acs.org.

different distributions over their electronic shells. The formal electronic configuration for Au7+ is given by 1s21px,y4 and for [Au@Au6(Mes)6]+ is 1s21px,y41dxy,x2−y24, which differs because the ligand charge donation toward the metallic core involves several bonding combinations which are not only confined to the 1s and 1px,y shells, in order to ensure the ligand−core interaction which requires the sharing of electrons. Such an observation is an interesting difference which can be useful to increase the understanding between related ligand-protected and bare structures. In addition, the calculated Au+/Au6 interaction energy (Eint = −142.40 kcal/mol) in Au7+ is close to that obtained for [Au@Au6(Mes)6]+ (−148.66 kcal/mol). Thus, the structurally characterized [Ag@Au6(Trip)6]+ cluster can be considered as a ligand-protected counterpart of the AgAu6 cluster. Lastly, as part of our interest in the inclusion of the spin− orbit coupling into the description of the electronic structure and related molecular properties,43,69−75 we describe the electronic configuration in the relativistic case including the spin−orbit term. As has been discussed recently,44 in the planar case the levels with l ≠ 0, namely, 1px,y and 1dxy,x2−y2, are splitted, resulting in an electronic configuration given by 1s1/221p3/221p1/221d3/221d5/24, which is useful to describe the electronic structure of planar clusters under the relativistic framework, involving molecular spinors (j = l ± s) instead of molecular orbitals (pure l). As final words, the use of the planar superatomic levels allows a clear comparison between such related structures, avoiding the possible difficulty given by the different point groups. Thus, the application of the approach here given could be useful for the better understanding and comparison of planar clusters.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author thanks the financial support of FONDECYT Grant 11400359 and PROJECT MILLENNIUM RC120001. REFERENCES

(1) Moskovits, M. Metal Clusters; John Wiley & Sons: New York, 1986. (2) Braunstein, P.; Oro, L. A.; Raithby, P. R. Metal Clusters in Chemistry; Wiley-VCH: Weinheim, 1999. (3) Shriver, D. F.; Kaesz, H. D.; Adams, R. D. The Chemistry of Metal Cluster Complexes; VCH Publishers, Inc.: New York, 1990. (4) Mingos, D. M. P.; Wales, D. J., Eds. Introduction to Cluster Chemistry; Prentice Hall International, Inc.: Englewood Cliffs, NJ, 1990. (5) Corbett, J. D. In Modern Perspectives in Inorganic Crystal Chemistry; Parthé, E., Ed. Kluwer: Dortrecht, The Netherlands, 1992. (6) Zheng, Z.; Long, J. R.; Holm, R. H. A Basis Set of Re6Se8 Cluster Building Blocks and Demonstration of Their Linking Capability: Directed Synthesis of a Re12Se16 Dicluster. J. Am. Chem. Soc. 1997, 119, 2163−2171. (7) Wei, W.; Lu, Y.; Chen, W.; Chen, S. One-Pot Synthesis, Photoluminescence, and Electrocatalytic Properties of SubnanometerSized Copper Clusters. J. Am. Chem. Soc. 2011, 133, 2060−2063. (8) Alivisatos, A. P. Semiconductor Clusters, Nanocrystals, and Quantum Dots. Science 1996, 271, 933−937. (9) Tolles, W. M. In Nanotechnology: Molecularly Designed Materials; Chow, G., Gonsalves, K. E., Eds.; ACS Symposium Series 622; American Chemical Society: Washington, DC, 1995. (10) Adams, R. D.; Captain, B. Unusual Structures and Reactivity of Mixed Metal Cluster Complexes Containing the Palladium/Platinum Tri-t-butylphosphine Grouping. Acc. Chem. Res. 2009, 42, 409−419. (11) Sivaramakrishna, A.; Clayton, H. S.; Makhubela, B. C. E.; Moss, J. R. Platinum Based Mixed-Metal Clusters (PtnMm(CO)xLy, M = Ru or Os; n+m= 2 to 10 and Ly= other ligands). Synthesis, Structure, Reactivity and Applications. Coord. Chem. Rev. 2008, 252, 1460−1485. (12) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. MonolayerProtected Cluster Molecules. Acc. Chem. Res. 2000, 33, 27−36.

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CONCLUSIONS The study of [MAu 5 (Mes) 5 ] + (M = Cu, Ag) and [MAu 6 (Mes) 6 ] + (M = Ag) and the hypothetical [MAu5(Mes)5]+ (M = Au) and [MAu6(Mes)6]+ (M = Cu, Au) allows the evaluation of a complete series of planar MAu5 and MAu6 ligand-stabilized cores. The planar superatom model has been applied denoting that the electronic structure in all the studied systems can be described by formally 10 valence electron cores, depicting an overall 1s21px,y41dxy,x2−y24 configuration, which in the case of [AuAu6(Mes)6]+ can be related to the Au7+ (D6h) bare cluster. The inclusion of the respective M(I) closed shell center increases the number of superatomic shells as 1s1p1d → 1s1p1d2s, denoting the interaction between 21189



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Understanding Planar Ligand-Supported MAu5 and MAu6 Cores

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