Computational Insight into the Covalent OrganicâInorganic Interface

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Computational Insight into the Covalent Organic−Inorganic Interface† †

This Perspective is part of the Up-and-Coming series.

Qing Tang and De-en Jiang* Department of Chemistry, University of California, Riverside, California 92521, United States ABSTRACT: From chemical functionalization of an inorganic substrate to protection/passivation of a nanocluster/nanoparticle surface, the organic−inorganic interface is key to chemistry of many materials. Herein computational insights into the structure, bonding, and energetics of several representative covalent organic−inorganic interfaces are reviewed mainly from the authors’ work and based on density functional theory. We start with the zero-curvature substrates including graphene, metal surfaces, and MoS2 and discuss their covalent functionalization by the attachment of the aryl group via the diazonium chemistry. We then move on to large-curvature, ligand-protected gold nanoclusters where we focus on the ligand−gold interface. The theoretical studies have produced both general trends that help understand the experimental results and specific predictions that have been confirmed or are yet to be verified. Opportunities abound in coupling theory and experiment to understand the roles of the covalent organic−inorganic interfaces in synthesis and application of nanomaterials.

1. INTRODUCTION Inorganic surfaces functionalized by a layer of organic groups (Figure 1a) and nanocluster/nanoparticles capped by a monolayer of organic groups (Figure 1b) are ubiquitous in materials chemistry and have diverse applications in molecular electronics, sensors, catalysis, and biorecognition.1,2 The strongly bound, covalent organic−inorganic interface is key to both types of materials chemistry. A comparison of the intrinsic characteristics for the inorganic and organic components of an organic−inorganic interface is summarized in Table 1. The inorganic part serves as the stable and hard core of the system, possessing compact and welldefined structures of strong covalent, ionic or metallic bonds. The organic peripheries are soft and flexible, providing a protective role and also a bridge to communicate with another chemical system. The organic layers can be further modified by chemistry of the end groups. By incorporating the advantages of both the inorganic and organic parts, the physicochemical properties of the material system can be tailored for specific applications. Formation of the organic layer depends on the energetic balance between the headgroup−substrate interaction and the van der Waals (vdW) interactions among organic tail groups. Examples of organic groups that readily form self-assembly layers on inorganic nanocrystals include thiolates,2 phosphines,3,4 aryl radicals,5,6 alkynes,7,8 and some nitrogen-containing carbenes.9−11 Several key issues and challenges in understanding the covalent organic−inorganic interface include: (i) precise determination of the bonding geometries and energetic stability of the interfacial

motifs; (ii) the energetic competition between the ligand− substrate bonds and ligand−ligand interaction; (iii) the reaction and dynamics of monolayer formation of the organic ligands on the substrate surface. Great progress in characterizing the nature of the bonding at the organic−inorganic interfaces has been made recently, especially at the thiolate−Au interface due to the successful total-structure determination of about 20 Aun(SR)m nanoclusters.12−29 The well-defined nature of the thiolate−Au interface allows theory to not only explain the known facts but also explore energetics and dynamics of the interface and the organic layer. State-of-the-art density functional theory (DFT) approach that combines good accuracy with efficiency has proven to be a powerful tool for providing reliable thermodynamic, structural, magnetic, and electronic properties of molecules and extended metallic or semiconducting systems. In particular, the Perdew− Burke−Ernzerhof (PBE) functional of the generalized gradient approximation (GGA)30 has been used extensively to give a good representation of the properties of the whole systems, such as self-assembly of thiol31 or ethynylbenzene32 on Au surfaces. DFT-PBE has also been routinely applied to predict the atomic structures,33,34 optical absorption,13 circular dichroism (CD) spectrum,35 ligand exchange,36 and vibrational spectroscopy37 of the thiolated gold clusters. Although standard DFT cannot properly account for the dispersion interactions, both empirical and first-principles approaches have been implemented to include the vdW interaction at the DFT level such as the commonly used Received: April 28, 2016 Revised: June 29, 2016

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This Perspective is part of the Up-and-Coming series. © XXXX American Chemical Society

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DOI: 10.1021/acs.chemmater.6b01740 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 1. Schematics of two typical covalent organic−inorganic interfaces: (a) zero-curvature planar substrates functionalized with an organic layer; (b) large-curvature nanoclusters/nanoparticles capped by a layer of organic groups.

2. FUNCTIONALIZATION OF A SUBSTRATE BASED ON THE DIAZONIUM CHEMISTRY First discovered by Pinson and co-workers,44 the diazonium chemistry provides a powerful way to functionalize the surface of a variety of substrates with the aryl groups. The approach was first applied to the glassy carbon electrodes electrochemically,44 and then extended to other types of carbons45−48 and metallic surfaces.5 2.1. Phenyl−Carbon Interface. The covalent modification of carbon surfaces by grafting of aryl radicals via the electrochemical or thermal reduction of diazonium salts was proposed to be a two-step process: first, the aryl diazonium cation obtains one electron from the electrode or substrate and subsequently decomposes into a highly reactive aryl radical (e.g., phenyl, nitrophenyl) and a free N2 molecule; then, the radical covalently attaches to the electrode or substrate surface forming a densely packed monolayer on carbon. This type of chemistry has been successfully applied to the modification of all kinds of sp2-carbon surfaces, such as glass carbon (GC),44,49 highly oriented pyrolytic graphite (HOPG),45 carbon nanotubes,46,47,50 mesoporous carbon,51,52 carbon felts,53 carbon fibers,48 and pyrolyzed photoresist film.54,55 Graphene can be viewed as the structural basis to build up sp2-carbon-based materials. DFT calculations on the structure and energetics of the phenyl/graphene interface showed that the binding interaction of an isolated C6H5 radical to the graphene basal plane is very weak (about 0.25 eV), comparable to the typical physisorption.56 The weak bonding can be attributed to the intrinsic stability of the extended delocalized π-electron system, where formation of a new C−C bond by converting the sp2 graphene carbon to sp3 carbon would destabilize the π-electron system and cause strain in the graphene sheet. Interestingly, bonding of the C6H5 groups on the basal plane can be strengthened significantly when the two phenyl groups are attached to the para-positions of the same graphene ring to form a pair (Figure 2a), yielding a binding energy of 1.27 eV per phenyl group. Compared to the basal plane, the edge sites show much higher reactivity for phenyl attachments.56 Specifically, attaching one phenyl group leads to an energy gain of 1.11 eV for the armchair edge, whereas a much larger value of 2.75 eV is found for the zigzag edge. This relatively higher affinity of the zigzag edge for an isolated phenyl group results directly from the presence of a

Table 1. Comparison of the Inorganic and Organic Parts of a Typical Organic−Inorganic Interface inorganic part

organic layer

hard well-defined structures atoms stay relatively in place more electronic states at the Fermi level many choices of elements covalent/ionic/metallic bonds

soft floppy conformations chemical groups are dynamic less electronic states at the Fermi level many choices of functional groups van der Waals interactions among groups

DFT-D238 and DFT-D3 methods.39 This gives computational chemists a powerful way to address the ligand−ligand interactions. In this perspective, we present an overview of structure, bonding, and energetics at the strongly bound, covalent organic− inorganic interfaces from a computational point of view based on the DFT-PBE method and mainly from our recent work. First, we focus on covalent functionalization of the zero-curvature planar substrates where we could employ periodic boundary conditions: starting with the attachment of the aryl group to graphene (the most studied 2D material) and metal surfaces via the diazonium chemistry (section 2), followed by functionalization of MoS2 (section 3), the most important member of transitionmetal dichalcogenides. Then we move on to the large-curvature, ligand-protected gold nanoclusters, one of the most important types of monolayer-protected clusters. First, the thiolate−gold and alkynyl−gold interfacial features will be discussed (section 4), followed by energetic competition between the metal center and the organic ligands in dictating the ground state structure of a single nanocluster (section 5). Next we examine the interdigitation of organic ligands to link nanoclusters together to form polymers (section 6). Last but not the least, catalytic applications of the ligand-stabilized gold nanoclusters will be discussed in terms of accessibility of the surface sites with the presence of the ligand layer (section 7). Finally, we give a summary and outlook for the interplay of theory and experiment in understanding the strongly bound, covalent organic−inorganic interfaces. Other important organic−inorganic interfaces such as those based on vdW interaction are not discussed here and the reader can refer to some recent reviews.40−43 B


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Figure 2. Optimized structures of two phenyl groups attached onto the graphene basal plane (a), armchair edge (b), and zigzag edge (c); the local density of states of carbon atoms on the zigzag and armchair edge (d), energy is relative to the Fermi level. The edges are terminated by hydrogen atoms (blue). Adapted from ref 56 with permission. Copyright 2006 American Chemical Society.

or tilted fashion with C6H5 σ-bonded to the atop site (Figure 3). For the transition metals in the middle such as Fe, the flat-lying configuration of C6H5 is about equally stable as the upright one on Fe(110). The DFT modeling revealed that the phenyl groups can form strong and versatile surface bonding to metal surfaces. Experimental techniques such as X-ray photoelectron spectroscopy studies and high-resolution electron energy loss spectroscopy (HREELS) can further confirm the existence of such metal−C covalent bonds.

localized state near the Fermi level (Figure 2d), which is absent in the armchair edge and the pristine graphene sheet. The DFT studies lend support to the Raman and scanning tunneling microscopy (STM) observations that on graphite, the diazonium-grafting reaction is faster at the edges.57−59 On the basal plane, the predicted para-position pairwise addition (Figure 2a) still waits to be experimentally confirmed. 2.2. Phenyl−Metal Interface. Grafting organic molecules on metal surfaces not only provides an efficient protection of metal against corrosion but also tunes the properties of the surfaces. Metals including Fe, Co, Ni, Pd, Pt, Zn, Cu, and Au have been aryl-grafted by diazonium salts.60−65 A key question here was the nature of the bond at the aryl/metal interface. To answer this question, several metals were selected across the periodic table (Ti, Fe, Pd, Cu, and Au) for a DFT study.66 The interactions of a phenyl radical with Ti(0001), Fe(110), Cu(111), Au(111), and Pd(111) surfaces were all found to be chemical in nature. The calculated binding energy of C6H5 group ranges from 1.02 eV on Au(111) to 1.78 eV on Fe(110) and up to 4.64 eV on Ti(0001). The phenyl−metal binding strength decreases from Ti to Cu and Au, which can be explained by the theory of the d-band center: with the increasing number of d-electrons from left to right, the metal d-band shifts further below the Fermi level, thus decreasing the bonding between surfaces and adsorbates.

3. FUNCTIONALIZATION OF MOS2 Moving beyond the graphene and metal substrates, we now discuss functionalization of binary compounds. Molybdenum disulfide (MoS2) is a prototypical member of the layered transition metal dichalcogenides (TMDs). 2D MoS2 nanosheets of single layer to few layers have attracted tremendous interest recently.67,68 MoS2 has two common crystal phases: the thermodynamically more stable 2H phase and the metastable 1T phase (Figure 4a insets). In the 2H phase, each Mo center is prismatically coordinated by six surrounding S atoms, with the S atoms in the upper layer lying directly above those of the lower layer; in the 1T phase, the Mo atom is octahedrally coordinated to six neighboring S atoms and the two S layers stack in the A-B packing mode. The 1T MoS2 phase can be made via intercalation of the 2H MoS2 lattice with alkali metals to induce phase transformation.69 The 2H and 1T phases of MoS2 have drastically different electronic properties: 2H MoS2 is semiconducting and photoluminescent, whereas 1T MoS2 is metallic. Although most studies center on 2H MoS2, 1T MoS2 has begun to attract interest due to its high electrical conductivity,70 high catalytic property for hydrogen evolution reaction (HER),71−74 and interesting electrochemical behaviors as supercapacitor electrode materials.75 As discussed above for graphene, surface modification by covalent attachment of organic groups is a commonly used strategy to tune the surface chemistry of 2D materials. However, the basal plane of the pristine 2H MoS2 is rather inert. By introducing sulfur vacancies at the surface or edges, the defected 2H MoS2 nanosheets then become chemically reactive for surface modification and functionalization.76−78 Another promising and more general method to functionalize MoS2 without relying on the creation of lattice defects has been established recently by Chhowalla et al.79 and Backes et al.80 The functionalization reaction was facilitated by electron transfer between the electron-rich metallic 1T phase and the strong electrophiles such as organic halides and diazonium salts, resulting in attachment of functional groups (e.g., −CH3, −CH2CONH2, 4-methoxyphenyl) to the surface S atoms via covalent C−S bonds. Surface

Figure 3. Optimized structures of phenyl groups on metal surfaces. Color code: C, blue; H, red; metal, green. Adapted from ref 66 with permission. Copyright 2006 American Chemical Society.

This trend also correlates with the bonding geometries of C6H5 on the metal surfaces: early transition metals (such as Ti) favor the flat-lying (parallel), π-bonded configuration of C6H5, whereas late transition metals (such as Cu and Au) prefer the upright C


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Figure 4. (a) Energetic difference between 1T- and 2H MoS2 phases (ΔE, eV per MoS2 unit) as a function of the CH3 coverage. (b) Computed band gaps of CH3-functionalized 1T MoS2 at different surface coverages based on the valence band maximum and conduction band minimum from the calculated density of states at the GGA-PBE level. (Color mode: C, black; H, purple; Mo, cyan; front S layer, yellow; back S layer, orange). Adapted from ref 81 with permission. Copyright 2015 American Chemical Society.

Aun(SG)m clusters by employing polyacrylamide gel electrophoresis (PAGE) and electrospray ionization (ESI) mass spectroscopy.91 The structural nature of the S−Au anchor bond of the thiolated gold clusters had remained unclear until the breakthrough brought about by the structure determination of Au102(SR)44 in 2007.12 Au102(SR)44 comprises a 79-atom gold core capped by 19 [RS−Au−RS] and 2 [RS−Au−RS−Au−RS] units, called the

functionalization significantly altered the electronic and optical properties of 1T MoS2, rendering it semiconducting and giving strong and tunable photoluminescence. Inspired by the experimental results, DFT studies were used to provide deeper insights into the bonding and band-structure modification of the MoS2 monolayer due to covalent functionalization.81 It was found that functional groups (including H, CH3, CF3, OCH3, and NH2) bind strongly to the 1T MoS2 via the surface S atom (4−5 eV), but very weakly on the 2H phase (0.1−0.4 eV). The higher surface reactivity of 1T-MoS2 is closely related to its metallicity and partially filled Mo 4d states, which provide the unpaired electrons to pair up with the chemical groups, thus leading to strong covalent interaction. As surface coverage increases, the 1T phase, which is metastable when unfunctionalized, is greatly stabilized and becomes more stable than the 2H phase after a crossover coverage (Figure 4a). Similar to the experimental finding, surface functionalization results in dramatic changes to the electronic properties of 1T MoS2. The band gap of the 1T MoS2 monolayer shows strong dependence on the coverage and can be tuned from 0 to ∼1 eV (Figure 4b). These encouraging results indicate that covalent functionalization holds great promise to control MoS2’s phase stability and electronic properties. It is hoped that carefully designed experiments can test the band gap dependence on the functionalgroup coverage and that the covalent chemistry via the surface S layers will be expanded to other TMDs and binary materials.

Figure 5. Schematics for the construction of a Aun(SR)m cluster (a) with the monomer and dimer staple motifs (b); spontaneous formation of a linear CH3S−Au−SCH3 staple motif on a Au38 cluster after DFT geometry optimization (c). (Color code: Au, yellow; S, blue; C, red; H, green). Panel c is adapted from ref 93 with permission. Copyright 2008 American Chemical Society.

4. LIGAND-PROTECTED METAL NANOCLUSTERS AND NANOPARTICLES Up to this point, we have discussed the functionalization of zerocurvature, planar substrates by organic groups. As the curvature increases, the surface of the inorganic center can accommodate more organic groups on a per-unit-area basis, thereby enabling the synthesis of ligand-capped nanoparticles of different sizes or curvatures. This type of the covalent organic−inorganic interface is created during the synthesis of the nanoclusters and nanoparticles starting with solvent-dissolved precursors. The advantage of this approach is that one can obtain the wet-chemistry-prepared, molecularly pure material in a single-crystal form for totalstructure determination.82,83 The thiolate-protected Au nanoclusters are probably the most successfully example for the past decade.84−86 4.1. Thiolate−Gold Interface. The field of thiolated gold clusters and nanoparticles was pioneered by Brust,87 Whetten,88 Murray,89,90 among others. In 2005, Tsukuda et al. accurately identified many compositions of glutathionate(SG)-protected

staple motifs. The linear [RS−Au−RS] motif has also been observed on Au-adatom-induced self-assembly of methylthiolate species on Au(111).92 The core-staple construction turns out to be a general feature of Aun(SR)m nanoclusters (Figure 5a).94 From an energetic perspective, the formation of staple motifs (Figure 5b) around a gold core has three contributions to its stability: formation of RS(AuSR)x (x = 1, 2, 3, ...) covalent bonds; the bonding between the S atoms of the two terminal thiolate groups and the surface Au atoms of the core; the Au−Au aurophilic interaction between the Au core and Au in the RS(AuSR)x staple motifs. Figure 5c shows a simple computational test from DFT-PBE to demonstrate the energetic stability of forming a staple motif.93 Starting from two CH3S− groups adsorbed closely on a bare Au38 cluster, D


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that on the Au(111) surface, the presence of a Au adatom induces formation of the flat-lying PhCC−Auadatom−CCPh motif with an energy gain of 1.58 eV (Figure 6a), very similar to the linear RS−Au−SR motif observed at the gold−thiolate interface.104 But different from the typical RS−Au−SR staple motifs formed only via the Au−S σ bonds, the PhCC−Auadatom−CCPh motif consists of additional π bonding of the CC bond to the surface Au atom in addition to the σ bonding of terminal C to the staple Au atom. In other words, the CC group can function as both σ and π donors in coordination to gold, thereby providing a bonding flexibility at the interface that can lead to novel structures. DFT simulated annealing further showed that this new type of π staple motif is also energetically favored on the Au20 cluster (Figure 6b), in which the CC/Au π interaction drives the formation of the π staple motif. The π-type bonding is due to the back-donation of d electrons of the substrate Au to the π* orbitals of the CC moiety. In thiolate−gold nanoclusters, longer staple motifs such as dimer motif [RS−Au−SR−Au−SR] are also common (Figure 5a,b). Interestingly, it is found that the dimeric [PhCC−Au−CCPh−Au−CCPh] π staple motif (Figure 6b) could also exist as a bonding motif of the Au20 cluster. 2015 saw major breakthroughs in the crystallization of alkynyldominated gold nanoclusters from two research groups. Wang et al. first crystallized the [Au19(PhCC)9(Hdppa)3]2+ (Hdppa = N,N-bis(diphenylphosphino)amine),105 then [Au23(PhC C)9(Ph3P)6]2+,106 and [Au24(PhCC)14(PPh3)4]2+.107 Zheng et al. reported Au24Ag20(2-SPy)4(PhCC)20Cl2 (SPy = 2-pyridylthiolate) in 2015108 and Au34Ag28(PhCC)34 in 2016.109 The Au19 and Au23 clusters feature three dimeric [PhCC−Au−CCPh−Au−CCPh] motifs (Figure 7a,b), exactly as predicted from DFT (Figure 6b).104 This exciting development suggests a parallel between the Au−thiolate and Au−alkynyl nanosystems. We have every reason to postulate that more Au− alkynyl nanoclusters will be discovered together with more studies of self-assembled monolayers of alkynyl ligands on Au substrates.

the Au atom in-between the two CH3S− groups was found to be pulled out of the outer shell after structural relaxation, and a linear CH3S−Au−SCH3 staple motif formed spontaneously, accompanied by a drastic change to the structure of the underlying cluster and a large energy gain. The three contributions to the cluster stability due to the formation of the staple motif can be clearly seen in the final relaxed structure: linear S−Austaple−S bond, S−Aucore bond, and Austaple−Aucore interaction. The staple motifs have been demonstrated to be the key feature of the gold−thiolate interface and have served as an important hypothesis for structure prediction of thiolated gold nanoclusters.94,95 The Au core usually has high symmetry, and clusters with lower Au/SR ratios typically need longer RS(AuSR)x staples (x ≥ 3). During the past few years, this hypothesis, as an inherent structural rule, has been applied to the structural predictions of a variety of Aun(SR)m clusters with known compositions but unknown structures.33,34,96−99 Particularly, some of the predictions, such as Au25(SR)18−,33 Au38(SR)24,34 and Au24(SR)2096 were quite close to the experimental X-ray structures.13−16 4.2. Alkynyl−Gold Interface. Beyond the thiolate−ligand paradigm, the alkynyl groups (RCC−) have emerged as a new class of ligands for making atomically precise gold nanoclusters. For instance, direct ligation of phenylacetylene (PA-H) to preformed polyvinylpyrrolidone (PVP)-stabilized Au clusters led to well-defined compositions including Au34(PA)16, Au43(PA)22, and Au54(PA)26.100−102 Unfortunately, their structures remain unsolved. On a different but related front, it has been experimentally demonstrated that the terminal alkynyl group can bind to Au(111) as an anchoring group to form highly ordered self-assembled monolayers very similar to those of alkanethiolates on Au(111).103

5. ENERGETIC COMPETITION BETWEEN THE METAL CENTER AND THE ORGANIC LIGANDS Above, we have discussed some unique compositions of ligandprotected gold nanoclusters. These “magic” compositions of the atomically precise metal clusters have higher relative stability. The energy of a ligand-protected cluster has contributions from the core, the interface, and the ligand layer. Most DFT calculations used the methyl group to replace the real ligands to save the computational cost. This simplification assumed the contribution of the ligand−ligand interaction to the stability is minimal. However, in many experimental cases, the ligand may change the structure of a cluster; in other words, the ligand−ligand interaction can change the energetic landscape of a cluster.110−112 In the case of Au24(SR)20, two ligands have been used experimentally, −SCH2Ph-tBu and −SCH2CH2Ph. The structure of Au24(SCH2Ph-tBu)20 (J model; Figure 8a) has been solved,19 whereas Au24(SCH2CH2Ph)20 has not.113 The best model from DFT predictions was based on Au24(SCH3)20 (the P model; Figure 8b), which is different from the J model.96 The question is: would Au24(SCH2CH2Ph)20 prefer the P or J model? By comparing the relative energy of the two candidate isomers with dispersion-corrected DFT,114 it was found that when R = −CH3, the two isomers have comparable stabilities. For R = −CH2CH2Ph, P isomer is more stable by 1.6 eV, whereas for R = −CH2Ph-tBu, the energy order is reversed: J isomer is more stable by 1.0 eV. This reversal of stability suggests that the isomer stability depends on the specific ligands. It also indicates that Au24(SCH2CH2Ph)20 is likely

Figure 6. (a) Schematic showing the energetic of formation of the PhCC−Auadatom−CCPh π motif relative to isolated Au adatom and PhCC− groups on Au(111) from DFT (inset: a side view of the PhCC− Auadatom−CCPh π staple motif). (b) DFT-optimized structures of monomeric [PhCC−Au−CCPh] (left) and dimeric [PhCC−Au− CCPh−Au−CCPh] (right) π staple motifs on the model Au20 cluster. Adapted from ref 104 with permission. Copyright 2014 American Chemical Society.

The staple motif is the key feature of the gold−thiolate interface. What about the gold−alkynyl interface? Simulated annealing from DFT-based molecular dynamics (MD) showed E


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Figure 7. (a) Left panel: top, crystal structure of the [Au19(PhCC)9(Hdppa)3]2+ cluster; lower left, R− groups omitted to show clearly the framework; lower right, one PhCC−Au−CCPh−Au−CCPh motif highlighted. (b) Right panel: top, crystal structure of the [Au23(Ph3P)6(PhCC)9]2+ cluster (the three PhCC−Au−CCPh−Au−CCPh motifs are colored green, yellow, and red); lower left, six Ph3P ligands removed; lower middle, the coordination of the six phosphine ligands; lower right, the Au17 kernel with the three shared Au atoms highlighted in yellow. Panel a is adapted from ref 105 with permission by American Chemical Society, Copyright 2014. Panel b is adapted from ref 106 with permission by Wiley-VCH Verlag GmbH & Co., Copyright 2015.

demonstrates that the ligand can greatly affect the final structure via the ligand−ligand interaction to the total energy. More recently, Jin et al. presented another interesting example of a reversible structural isomerism between Au28(S−c-C6H11)20 (where −c-C6H11 = cyclohexyl) and Au28(SPh-tBu)20,22 where both structures were experimentally determined. The two structures are both thermodynamically stable and can be reversibly transformed into each other via thermal ligand exchange. Figure 9 shows that both structures feature a rod-shaped Au20 kernel (A vs C) and eight bridging thiolates (B vs D). Their structural difference mainly lies in the gold−sulfur interface: in Au28(S−c-C6H11)20, the eight gold atoms and 12 thiolate ligands are arranged into two trimeric staple motifs and two monomeric staple motifs (E−G); in Au28(SPh-tBu)20, they were arranged into four dimeric staple motifs (H−J). DFT calculations confirmed that both the experimental structures are indeed thermodynamically more stable than the isomers when the ligands are switched.22 In the case of the S−c-C6H11 ligand, the experimental structure wins due to its much lower DFT energy of the Au−S framework. In the case of the SPh-tBu ligand, the experimental structure is more stable due to the more favorable vdW interaction from the packing of ligands. To examine further the flexible and dynamic nature of the ligands on the cluster surface, a force-field approach was employed to explore the ligand-conformation energy landscapes and how they affect the relative stability of the whole cluster.115 Using Au25(SC2H4Ph)18 as an example, three conformational degrees of freedom were considered (Figure 10a) and over half a million configurations were searched. Many conformations with energy lower than the experimental structures in a crystalline state were found, and Figure 10b−e shows a few such examples. This insight suggests that the experimental conformation from a crystalline state is not the most stable state in the gas or solution phase and that crystalline packing can change the ligand conformation.

Figure 8. Structures of two Au24(SR)20 isomers: (a) the J model based on the experimental structure of Au24(SCH2Ph-tBu)20;19 (b) P model based on the DFT prediction for Au24(SCH3)20.96 Color code: core Au, pink; S, green; staple Au, yellow; R−, gray. Adapted from ref 114 with permission. Copyright 2014 Royal Society of Chemistry.

to have the P structure. By dividing the total energy into DFT and vdW contributions, the higher stability of the P isomer in the case of Au24(SCH2CH2Ph)20 was found to stem from the stronger vdW interactions among −CH2CH2Ph groups, whereas the higher stability of J isomer in the case of Au24(SCH2Ph-tBu)20 was found to be due to its better capacity to accommodate the steric effect of bulkier −CH2Ph-tBu groups. This example nicely F


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Figure 9. Structural comparison of Au28(S−c-C6H11)20 (A, B, E−G) and Au28(SPh-tBu)20 (C, D, H−J). (A, B, E−G) Au20 kernel (magenta), eight bridging thiolates (orange), two trimeric staples (light blue) and two monomeric staples (light green) in Au28(S−c-C6H11)20; (C, D, H−J) Au20 kernel (blue), eight bridging thiolates (yellow), four dimeric staples (red) in Au28(SPh-tBu)20. Adapted from ref 22 with permission. Copyright 2016 American Chemical Society.

Figure 10. (a) Schematic of the S−Au−S cis/trans (bottom), S−C anti/gauche (middle), and C−C anti/gauche (top) conformations of the SC2H4Ph ligand. (b−e) four lowest energy isomers of Au25(SC2H4Ph)18 from conformational search. Adapted from ref 115 with permission. Copyright 2015 American Chemical Society.

Although the charge-neutral Au25(SBu)180 monomer itself is a paramagnetic molecule in its ground state, nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) analysis revealed that the Au25(SBu)180 polymer has a nonmagnetic ground state instead, which can be described as a 1D antiferromagnetic system. DFT calculations indicated that the unpaired electrons of interacting Au25(SBu)180 clusters pair up as they are closely interconnected, leading to a fully occupied valence band (VB) and an empty conduction band (CB) with an energy gap of about 0.12 eV (Figure 12a).116 The energetic preference for the nonmagnetic state is closely related to the close intercluster Au−Au contact in the Au25(SBu)180 crystal, which is made possible by vdW attractions among SBu ligands of neighboring clusters. As the Au−Au distance increases (stretch the polymer along the chain direction to decrease the intercluster interaction), a nonmagnetic-to-magnetic transition can be observed after a crossover distance (Figure 12b). This interesting work shows that the intercluster ligand−ligand interaction can be used to direct the assembly of nanoclusters into extended systems.

The intercluster packing via ligand−ligand interaction will be explored in the next section.

6. INTERDIGITATION OF ORGANIC LIGANDS TO LINK NANOCLUSTERS TOGETHER The preceding section shows the importance of ligand−ligand vdW interaction on a single cluster. Not only can the ligands protect a single nanocluster but they can also be used to link together individual nanoclusters into an assembly of 1D, 2D, or even 3D frameworks. For example, it was found recently that Au25(SBu)18 clusters (SBu = n-butanethiolate) form a linear polymer of Au25 clusters connected via intercluster Au−Au bonds in the solid-state phase (Figure 11).116 The distance of the two staple Au atoms linking two neighboring Au25 clusters together is about 3.15 Å (Figure 11a). This indicates a strong intermolecular interaction between the adjacent Au25 nanoclusters. In this case, the interactive S−Au−S staples are oriented almost perpendicular to each other (torsional angle 83°), allowing for interdigitation of ligands and a short distance of two neighboring central Au atoms (Figure 11b). G


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Figure 12. (a) Electronic density of states of the optimized Au25(SBu)18 crystal in the nonmagnetic state at 0 K. Inset shows a zoom in of the region near the Fermi energy, with the valence band (VB) and conduction band (CB) labeled. (b) Relative energy of the nonmagnetic and magnetic states of the one-dimensional Au25(SCH3)18 polymer as a function of the distance between the central atoms of two neighboring clusters (Au = yellow, S = red, central Au = green, methyl groups removed for clarity). The arrow indicates the spin crossover. Adapted from ref 116 with permission. Copyright 2014 American Chemical Society.

Figure 11. Structure of the Au25(SBu)18 wire: (a) 1D polymer chain along the (111) direction, connecting Au−Au bonds highlighted in green; (b) Au25S18 skeleton of Au25(SBu)18 with relevant distances and torsional angle; (c) interdigitation of SBu ligands. Adapted from ref 116 with permission. Copyright 2014 American Chemical Society.

We expect that more new materials may be realized through this approach.

the open facet gold of Au25(SR)18 nanocluster is accessible to reactants even with the presence of real ligands. In addition to the coupling reaction, Jin and co-workers showed that both the free and oxide-supported Au25(SR)18 can catalyze selective hydrogenation of α,β-unsaturated ketones to α,β-unsaturated alcohols,121 hydrogenation of aldehydes to alcohols,122 and semihydrogenation of terminal alkynes into alkenes.123 Intriguingly, it was found that hydrogenation of α,β-unsaturated ketone preferentially occurs at the CO bond over the CC bond, and a complete (100%) chemoselectivity for the unsaturated alcohol can be achieved (Figure 14a).121 To unravel the origin of this interesting catalytic property, hydrogenation of benzalacetone was chosen as a testing reaction to investigate computationally the catalytic mechanism of Au25(SR)18 using the simplified Au25(SCH3)18 model.124 The hydrogenation activity was found to be determined by the heterolytic cleavage of H2 between one staple Au atom of the open facet and the carbonyl oxygen (CO) of the reactant (transition state shown in Figure 14b), followed by the facile transfer of H from Au to the carbonyl C to form unsaturated alcohol. In the rate-determining step, the addition of H atom to CO has lower barrier than that to CC, which agrees with the experimentally observed selectivity. Alkynyl-protected nanoclusters have also been exploited as potential catalysts. Zheng and co-workers have recently synthesized an all-alkynyl-protected nanocluster, Au34Ag28(PA)34, and evaluated its catalytic activity for hydrolytic oxidation of organosilanes to silanols.109 The alkynyl-capped Au34Ag28(PA)34

7. CATALYTIC APPLICATIONS: ACCESSIBILITY OF THE INORGANIC SURFACE The well-defined structures and the many available size/ compositions of the atomically precise thiolate−gold nanoclusters provide an ideal platform to study nanocatalysis by the interplay of theory and experiment.117−119 The experimentally observed catalytic properties of Aun(SR)m clusters are rather interesting and surprising, since it is generally considered that the ligands present in a nanocluster would block the active sites and reduce the catalytic activity. Thus, one important issue here is whether the inorganic core is still accessible to reactants despite the presence of surface capping ligands. Jin and co-workers have experimentally discovered that oxide-supported Au25(CH2CH2Ph)18 cluster can effectively catalyze Sonogashira cross-coupling reaction between phenylacetylene and p-iodoanisole (Scheme 1) to generate diphenylacetylene product with excellent selectivity.120 The structure of Au25(CH2CH2Ph)18 consists of a Au13 core protected by six −SR−Au−SR−Au−SR− dimeric staple motifs. Two facets comprising three surface gold atoms in three separate staple motifs are well exposed with little steric hindrance for reactant access (Figure 13a,b). The DFT-D2 calculations of the reactant adsorption on Au25(CH2CH2Ph)18 surface showed that both phenylacetylene and iodobenzene prefer to adsorb on the gold atoms of the open facet with their phenyl ring facing the staple gold atom (adsorption energy of −0.40 and −0.48 eV, respectively; Figure 13c,d).120 The DFT modeling indicated that H


Perspective

Chemistry of Materials Scheme 1. Nanogold-Catalyzed Sonogashira Coupling between p-Iodoanisole (R = OMe) and Phenylacetylenea

a

Adapted from ref 120 with permission. Copyright 2013 Elsevier Inc.

Figure 13. (a) Top view of one of the two open facets of Au25(CH2CH2Ph)18 clusters where three surface gold atoms are exposed; the other facet is on the back side. (b) Side view of the two facets which are at top and bottom. (c) Top view and (d) side view of the coadsorption of phenylacetylene and iodobenzene on the surface of the Au25(SCH2CH2Ph)18 clusters. Au, yellow; S, blue; C, gray; H, white; I, green. Adapted from ref 120 with permission. Copyright 2013 Elsevier Inc.

Figure 14. (a) Hydrogenation of α,β-unsaturated ketone to form saturated ketone or unsaturated alcohol. (b) Transition state structure for H2 cleavage between the staple Au atom of Au25(SCH3)18 and the carbonyl oxygen of benzalacetone. Panel b is adapted from ref 124 with permission. Copyright 2015 American Chemical Society.

materials systems. Advances in targeted synthesis and structural characterization have motivated quantum mechanical calculations with DFT that not only provided insights into the interface but also in many cases predicted new structures and properties that have been or yet to be verified. In this perspective, we reviewed two important cases of the covalently bound organic−inorganic interfaces regarding their structures, bonding, and interfacial properties: (a) chemical functionalization of zero-curvature, flat substrates; (b) the ligand− gold interface at the large curvature, monolayer-protected or

nanoclusters exhibited better catalytic performances than clusters with surface ligands partially or completely removed. Future work is needed to explore the mechanism of the promoting effect of alkynyl ligands during the catalytic process.

8. SUMMARY AND OUTLOOK Understanding and control of the covalent organic−inorganic interfaces and their bonding properties is the key to exploring the functionalities and to realizing the full potential of many I


Perspective

Chemistry of Materials capped metal nanoparticles or nanoclusters. For the first case, we discussed (i) chemical modification of conducting surfaces (carbons such as graphene and metals such as Au, Pd, Ti) by aryl groups via the diazonium chemistry and (ii) covalent chemistry via the S layer of the MoS2 monolayer. Importantly, the DFT predictions of pairwise addition of aryl groups on the graphene sheet and band gap variation with functional group coverage on the 1T MoS2 remain to be experimentally verified. For the case of the organic−inorganic interface in metal nanoparticles or nanoclusters, we examined in detail (i) the thiolate−gold interface in comparison with the alkynyl−gold interface; (ii) the ligand−ligand interaction in dictating the stability of a cluster; (iii) the intercluster assembly via ligand− ligand interdigitation; (iv) the accessibility of surface gold atoms for catalysis despite the presence of capping ligands. The DFT prediction of the PhCC−Au−CCPh−Au−CCPh motif on the gold cluster has been nicely confirmed in a single crystal structure of the [Au19(PhCC)9(Hdppa)3]2+ cluster for the first time, while our prediction of the PhCC−Au−CCPh motif on Au(111) still awaits experimental confirmation. Dispersion-corrected DFT calculations allowed us to assess the vdW interaction among ligands in determining the ground-state structure of Aun(SR)m clusters. In terms of catalysis, the calculations now could provide a mechanistic explanation of the accessibility of the active site and the selectivity in terms of selective hydrogenation of α,β-unsaturated ketone. Looking ahead into the future, the strongly bound, covalent organic−inorganic interface remains central to many materials chemistry systems. One important challenge is to make the wetchemistry synthesis of nanomaterials a “Science” instead of an “Art”. Here the monolayer-protected metal clusters can serve as an example to understand the growth and formation processes that lead to “magic” or stable compositions. We envision that a multiscale simulation approach would be essential to reveal how the covalent inorganic−organic interface is formed and how it directs the growth of nanoclusters/nanoparticles of different curvatures or sizes. The second question is how dynamic the covalent organic−inorganic interface is under the solution and reaction conditions. This question concerns both the chemical tunability of the interface (for example, via ligand exchange) and the long-term stability of the interface during device or catalytic applications. In essence, the chemistry of the covalent inorganic− organic interface should be simulated under the real or reaction conditions. The third question is about the influence of the interface on the interaction of the nanosystems with light. This aspect is especially important in the emission properties where simulations significantly lag behind experiments. One hopes that semiempirical methods coupled with electron dynamics can help address this problem for realistic experimental systems instead of small toy models. Moore’s Law has been powering the computing hardware for the past half century and the silicon-based transistor technology is now close to the 10 nm.125 As it further shrinks in size, the inorganic-based semiconductor will reach the size of a typical organic molecule, ∼1 nm. Here a bright future of interfacing chemistry (organic) and electronics (inorganic) awaits, where the interplay of theory and experiment is sure to bring unexpected results.

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Notes

The authors declare no competing financial interest. Biographies Qing Tang received her B.S. in Chemistry from Shenzhen University (2009) and Ph.D. in Inorganic Chemistry from Nankai University (2014), following which she joined the group of Prof. De-en Jiang as a postdoctoral fellow at University of California, Riverside. Her research interests relate to the surface and interface chemistry of nanomaterials. De-en Jiang is an assistant professor in Department of Chemistry and a participating faculty member in Materials Science and Engineering, University of California, Riverside. He received his B.S. degree in 1997 and M.S. degree in 2000 both from Peking University and his Ph.D. degree in 2005 from UCLA, all in chemistry. He joined Oak Ridge National Laboratory (ORNL) first as a postdoctoral research associate and then became a research staff member in 2006. He moved to University of California, Riverside in July 2014. His research focuses on computational materials chemistry.

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ACKNOWLEDGMENTS This work was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy and University of California, Riverside. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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AUTHOR INFORMATION

Corresponding Author

*D.-e. Jiang. E-mail: [email protected]. Phone: +1-951-827-4430. J


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