How Do Secondary Phosphine Oxides Interact with Silver Nanoclusters?

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

How Do Secondary Phosphine Oxides Interact with Silver Nanoclusters? Insights from Computation Felipe Silveira de Souza Schneider, Maximiliano Segala, Giovanni Finoto Caramori, Eder Henrique da Silva, Renato Luis Tame Parreira, Henri Stephan Schrekker, and Piet W. N. M. van Leeuwen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06244 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 29, 2018

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The Journal of Physical Chemistry

How Do Secondary Phosphine Oxides Interact with Silver Nanoclusters? Insights from Computation Felipe S. S. Schneider,† Maximiliano Segala,∗,‡ Giovanni F. Caramori,† Eder Henrique da Silva,¶ Renato L. T. Parreira,¶ Henri S. Schrekker,‡ and Piet W. N. M. van Leeuwen§ †Departamento de Química, Universidade Federal de Santa Catarina, Campus Universitário Trindade, 88040-900, Florianópolis, SC, Brazil. ‡Departamento de Físico-Química, Instituto de Química, Universidade Federal do Rio Grande do Sul, 91501-970, Porto Alegre, RS, Brazil. ¶Núcleo de Pesquisa em Ciências Exatas e Tecnológicas, Universidade de Franca, 14404-600, Franca, SP, Brazil. §Institut National des Sciences Appliquées, 135 Avenue de Rangueil, 31077 Toulouse Cedex 4, France. E-mail: [email protected]

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Abstract Air-stable nanoparticles stabilized with secondary phosphine oxides (SPOs) can have a remarkable role as catalysts. Since size and activity depend both on the clusterligand interaction and the nature of the employed ligand, the present work provides a fresh perspective on the prospects of bonding analysis, rationalizing for the first time the physical nature of interactions between silver nanoclusters (SNCs) and secondary phosphine oxides (SPOs) in the light of energy decomposition (EDA-NOCV) and noncovalent interaction (NCI) analyses, using the silver core of the X-ray structure for a highly symmetric, ligand-decorated Ag44 cluster available in the literature. Our findings reveal that the coordinating electron lone-pair of phosphorus is stabilized by aliphatic substituents, in contrast to aromatic groups, and that SPOs containing aromatic moieties become aligned to the silver surface upon adsorption. This alignment of aromatic substituents substantially contribute to dispersion interactions, which is observed by both EDA-NOCV and NCI analyses, showing ligand to metal donations and fragment polarizations, as well as metal to ligand backdonations in some cases. Adsorption conformations bearing hydrogen bonds to the surface were found to be the most favorable ones, and models for SPO-cluster hydrogen transfer are in line with previous experimental findings on gold nanoparticles. The present contribution will help pave the way for the rational development of new SPO-protected nanoparticles and nanoclusters, which may become cheaper and more efficient catalysts in the future.

Introduction The synthesis of air-stable gold nanoparticles 1 with application as catalysts for the chemoselective hydrogenation of substituted aldehydes has recently been reported and the proposed mechanism has shown that secondary phosphine oxides (SPOs), when employed as ligands, have a remarkable role in the chemoselectivity. Since the applicability of catalysis depends on the variability of catalysts, there is a strong need for research on variations on known successful catalysts. The use of other metals can not only be cheaper, but also provide 2


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novel insights into catalysis. A simple, yet powerful, exploration is the use of silver for preparing nanoparticles, which is especially practical after the synthesis of an ultrastable thiolate-protected silver nanocluster (SNC) has been reported. 2 The stability of such SNC is claimed to be related to the 32-silver-atoms core, composed of a hollow Ag12 icosahedral motif encapsulated by a 20-silver-atoms dodecahedron. Since size and activity depend on both the cluster-ligand interaction and the nature of the employed ligand, the present work aims at rationalizing for the first time the physical nature of interactions that occur between SNCs and SPOs 1–6 (Figure 1). The theoretical description, in the light of bonding energy decompositions and Non-covalent Interactions, of the Ag−P bonding situation enables the rational design of new SPO-protected nanoclusters, which will help to pave the way for the preparation of potentially more efficient molecular catalysts in the near future. SPOs have been successfully employed as ligands in complexes used in homogeneous catalysis of enantioselective hydrogenation of ketones, 3 and also as ligands in heterogenous nanoparticle catalysis, studied and employed in selective hydrogenation of substituted aldehydes. 1,4,5 It has been suggested that SPOs might also be used as homogeneous catalysts for enyne cycloisomerization and hydroxy- and methoxycyclization reactions as well. 6 Two different applications of SPOs as ligands in heterogenous nanoparticle catalysts are highlighted in Figure 1a. 1,7 Coinage metals (copper, silver, and gold) are known as being chemically inert; notwithstanding, catalytic activity of their nanoparticles has been reported. 8 Particularly, the dramatic change in atomic packing of nanoclusters (< 2 nm) is a major factor for their activity. Recently, a routinely-applicable synthetic protocol for the preparation of a very stable –4 thiolate-protected silver nanocluster M+ (with M+ = alkali counterion 4 [Ag44 (p-MBA)30 ]

and p−MBA = p-mercaptobenzoic acid) that leads to monodisperse reaction product in quantitative yields (above 95%), and do not require any size sorting, has been reported. 2 The ultrastability of this SNC was attributed not only to the choice of protective ligands, but also to the Ag32 excavated-dodecahedron core (called in this work I@II, which consists of

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Figure 1: Examples from the literature of catalytic hydrogenations with SPO-stabilized (1– 6) metal nanoparticles. (a) Surface reaction mechanism proposed by Cano 1 (adapted with permission, copyright 2017 American Chemical Society) for reactions in (b), where chemoselective hydrogenation of aldehydes with SPO-stabilized gold nanoparticles takes place.

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a hollow Ag12 icosahedral motif (I) encapsulated by a Ag20 dodecahedral-shaped outer-shell (II), (Figure 2). The rest of the silver atoms in this structure is found in Ag2 S5 capping structures that lie on top of the silver core. Such SNCs have been found to be even more stable than the highly stable Au25 (SG)18 gold nanocluster (SG = glutathionate). 2,9 The preparation of similar SPO protected silver nanoclusters may certainly be achieved either by adapting the synthesis of Desireddy, or by using a highly efficient, atomically precise ligand exchange procedure, such as recently developed by Bootharaju, 10 who makes further progress by proceeding at room temperature.

Figure 2: Models of silver nanoclusters (SNCs) used in this work. I@II is the Ag32 excavateddodecahedron core determined by Desireddy et al.. 2 I@II can be seen as the result of the coverage of the Ag12 icosahedron center I by the outer Ag20 dodecahedral shell II. This schematic representation has been chosen to facilitate the visualization. Due to their high affinity towards silver, 11 thiolates are frequently employed to passivate silver nanoclusters. 10,12,13 While thiolate-protected gold nanoclusters show normally staple structures such as −RS−Au−SR− and −RS−Au−SR−Au−SR−, 12 thiolate-protected silver nanoclusters show a rich cordination chemistry, with each surface thiolate binding two, three or four silver atoms. 12 In a similar way, phosphorus in SPOs have strong charge donation capacity, exhibiting ligand properties analogous to S in thiolates 1 but, in gold nanoclusters, Au−P bonds are known to be weaker than Au−S bonds. 14 Furthermore, SPOs are commonly bulkier than thiolates, and contain three substituent groups, while thiolates accommodate 5



only a single one. The comprehension of metal-ligand interaction in SPO-protected silver nanoclusters has thus become far-reaching, enabling the applicability of SPOs as ligands, leading to a wide scope in different areas, since similarities and differences between thiolates and SPOs as ligands could be rationalized. In order to provide a deep insight into the bonding situations between silver nanoclusters and SPOs, the present work provides a different perspective on the prospects of bonding analysis, rationalizing them, for the first time, in terms of their physical nature by means the Energy Decomposition Analysis combined with the Natural Orbitals for Chemical Valence (EDA-NOCV) and Non-covalent Interactions (NCI). All SPOs were chosen from experimental investigations conducted by van Leeuwen et al. 1,3,7 on the use of ligand-protected metal nanoparticles (MNPs) as catalysts, and therefore a set of six SPOs (1–6) with broad electron-donating and -withdrawing characteristics was systematically investigated (Figure 3). We surveyed not only the form of coordination and nature of interaction of the Ag−P bond in SPO-nanocluster systems, but also the effect of P−H bond dissociation of SPOs may have on the interaction. It is believed that both aspects are crucial for understanding catalysis in these systems. 1,4,5

Computational methods Geometry optimizations were carried out using density functional theory (DFT) 15,16 as implemented in the ORCA quantum chemistry package. 17,18 SPOs were allowed to relax in all cases, but silver clusters were used as found in the X-ray structure for the highly symmetric, thiolate-decorated Ag44 cluster synthetized by Desireddy et al.. 2 The exchange and correlation functionals were chosen to be B88 19 and P86, 20 respectively, and the basis set used was the Gaussian-type triple-ζ quality def2-TZVP. 21,22 Relativistic-optimised Stuttgart effective core potentials (ECP) were used for all silver atoms. 22,23 Atom pairwise dispersion correction (D3BJ) was also employed. 24,25 The Natural Population (NPA) and Natural Bonding Orbital (NBO) analyses 26 (version

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5.9 compiled with GAMESS 27 version 2013), were employed in order to describe the electronic structure of isolated SPOs. Electrostatic potential maps of the SPOs were obtained with the software MultiWFN 28 (version 3.3.9). In order to understand how the interaction between SPOs and SNCs proceeded, the Energy Decomposition Analysis (EDA) coupled with the Natural Orbitals for Chemical Valence (NOCV) 29 was carried out by using optimized structures from ORCA (i.e., [ADF]BP86-D3/TZ2P//[ORCA]BP86-D3/def2-TZVP). EDANOCV was employed as implemented in the ADF2016 quantum chemistry package, 30–32 with the same density functional and dispersion correction as before (BP86-D3). Full-electron calculations were carried out using the zeroth-order regular approximation (ZORA). 33,34 The basis set used was the Slater-type triple-ζ quality TZ2P. 35 In EDA-NOCV, the interaction energy (∆Eint ) between two fragments of a “supermolecule” is decomposed, in a Morokumalike scheme, 36 into four physically meaningful components as expressed in Equation 1. (elec)

∆Eint = ∆Eint

(disp)

+ ∆Eint

(disp)

= ∆Eelstat + ∆EPauli + ∆Eorb + ∆Eint

(1)

Here, ∆Eelstat stands for the quasiclassical electrostatic interaction between the interacting fragments with their frozen charge distribution at the geometry of the supermolecule; ∆EPauli is the Pauli repulsion between the occupied orbitals of the interacting fragments; and ∆Eorb is the orbital interaction, which describes charge transfer processes (interactions between occupied molecular orbitals of one fragment with the unoccupied orbitals of the other) and polarizations (empty and occupied orbital mixing on the same fragment). These (elec)

(disp)

first three constributions are summed up in ∆Eint . ∆Eint

is also obtained since we are

using an atom-pairwise empirical dispersion correction; 24,25 it represents the difference in dispersion correction between the supermolecule and its fragments. The differential density ∆ρ(~r) can be decomposed in the NOCV scheme into deformation densities ∆ρi (~r) such that P ∆ρ(~r) = ∆ρi (~r), which provide information about the directionality and magnitude of charge flow. The orbital component in EDA-NOCV has thus an associated representation

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∆Eorb =

P

∆Eorb in which every ∆Eorb is related to a charge transfer channel ∆ρi (~r). A i i

more detailed description of the EDA-NOCV scheme can be found in the original work of Mitoraj. 29 Non-covalent Interactions (NCI) were revealed using the method of Johnson 37 and the NCIPLOT package. 38 NCI plots were generated with VMD 39 (version 1.9.2) and its Tachyon rendering engine. 40 General molecular graphics were produced with the UCSF Chimera 41 package (version 1.11.1, build 41268). Model building and visualization were employed using the softwares Avogadro 42 (version 1.1.1) and Chemcraft 43 (version 1.8, build 489).

Results and Discussion Proposed models for SPO structures SPOs occur in two tautomeric forms: (i ) the trivalent P(III) (1(III)–6(III)) and (ii ) the pentavalent P(V) (1(V)–6(V)). 3 Although strongly electron withdrawing groups give measurable amounts of P(III) tautomer, 44 P(V) tautomers are energetically favored in all cases (Figure 3). On the other hand, P(III) tautomers coordinate more readily to metal centres (Figure 4). For that reason, when coordination takes place such tautomerism is shifted towards P(III). Geometries of P(V) and P(III) tautomers were first optimised and characterised as local minima on the potential energy surface (PES) by the absence of imaginary eigenvalues in the Hessian matrix. The obtained geometries for the coordinating P(III) along with energetics relative to their P(V) tautomers are presented in Figure 3. Substituent groups attached to SPOs have a remarkable effect on the the tautomeric equilibrium of the studied compounds: phenyl(1-naphthyl)phosphine oxide (1), diphenylphosphine oxide (2), diethylphosphine oxide (3), dicyclohexylphosphine oxide (4), tert-butyl(1naphthyl)phosphine oxide (5) and di-tert-butylphosphine oxide (6). According to Figure 3, the most stable free form is indeed P(V) (i.e., ∆E = E P(III) − E P(V) > 0 in all cases), as expected due to the absence of electron lone-pairs on phosphorus. 1,4,45,46 These energy differ8


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Figure 3: Differences in electronic energies (kcal·mol−1 ; E P(III) − E P(V) ) between secondary phosphine oxides (SPOs) investigated in this study, in their coordinating P(III) phosphinous acid form (1(III)–6(III)) and in the P(V) phosphorus oxide form (1(V)–6(V)). The energy values are shown in scale in the diagram.

Figure 4: Tautomerism between the pentavalent P(V) phosphorus oxide and the trivalent P(III) phosphinous acid, which has a lone electron pair on phosphorus and can thus chelate transition metals (M). 9



ences tell us how accessible the coordinating species are. For instance, when only purely aromatic substituents as naphthyl and phenyl are employed (1(V)/1(III)) and (2(V)/2(III)), the observed differences of energy are 7.4 and 7.5 kcal·mol−1 , respectively, corresponding to the most significant energy stabilization values for P(III) tautomer. On the other hand, when aliphatic groups ethyl, cyclohexyl and tert-butyl are used (3(V)/3(III) – 6(V)/6(III)), ∆E values range from 9.4 to 12.0 kcal·mol−1 ), indicating that such aliphatic groups do not stabilize P(III) forms in a significant way. Such behaviour can be rationalised in terms of the electron pushing and pulling abilities of substituents and clearly correlates with the difference in electronic energy between tautomers. Particularly, this energy difference is largest for the bulky tert-butyl group (compounds 5(V)/5(III) and 6(V)/6(III), ∆E = 11.2 and 12.0 kcal·mol−1 , respectively). Recently, Cano et al. observed that gold nanoparticles passivated by 5 are very successful in the chemoselectivite hydrogenation of aldehydes. 4,7 In fact, 5 contains both naphthyl and tert-butyl groups, which are groups that contribute, according to Figure 3, to stabilisation and destabilisation, respectively, of the trivalent tautomer, playing thus opposite roles in this respect. Electrostatic potential maps (Figure S1 in supporting information material) of tautomers P(III) and P(V) reveal a negative electrostatic potential accumulation on the oxygen in 1(V)–6(V), while phosphorus atom is sterically hindered. Conversely, 1(III)–6(III) show a less negative electrostatic potential on oxygen, but a very significant positive electrostatic potential on the hydrogen bound to oxygen, when compared to 1(V)–6(V). In compounds 1(III)–6(III) the phosphorus is less sterically hindered. The effect of substituents in POH group was rationalized in terms of NBOs (Table S1 in supporting information material), and the results point out that the increase in electronic population of the P lone-pair is stabilized by aliphatic substituents (3(III), 4(III), 6(III)) in contrast to aromatic groups (1(III), 2(III), 5(III)), which decrease the non-bonding electron pair density of P. Delocalisation effects of aromatic groups and inductive effects of aliphatic groups were already suggested as reasoning for this finding, as exemplified by SPOs bearing tert-butyl and phenyl in Segala’s

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work about Hammett constants. 46

The coordination modes of SPOs on SNC surfaces The coordination of SPOs on SNC surface was modeled by using the fixed structure of I@II as silver nanocluster model (Figure 2), which was synthetized and crystallographically determined by Desireddy et al.. 2 The relative effect of both moieties composing I@II (Ag12 icosahedral motif, I, and Ag20 dodecahedral-shaped outer-shell, II) on the SPOs coordination was addressed by considering their chemisorption (evaluating adsorption energies) through the hypothetical reaction (Figure 5) in which 1(V)–6(V) react with the surface of substrates I@II, I, or II, respectively. Although SPO geometries were allowed to completely relax on the surface of SNCs in all cases, surface reconstruction and internal degrees of freedom of I@II, I, and II were not considered during geometry optimization procedures. This methodology was compared to the more costly procedure of optimizing the whole structure (Figure S2 in supporting information material). Despite the fact that the core/shell structure becomes distorted, indicating that more ligands are needed to maintain the SNC structure, we kept the experimentally found silver structure since adsorption energies were found to be similar. A detailed analysis of both possible coordination modes Ag−P and Ag−O was thus undertaken, by considering a set of proposed adsorption conformers, which were generated by combining the model structures (I, II and I@II) with SPOs (1(V)–6(V) and 1(III)– 6(III)), and by setting the hydrogen bound to P or O) in the SPO to point towards the SNC surface (Ht) or away from it (Ha), as exemplified in Figure 6. The results reveal that Ht structures, with the hydrogen atom pointing towards the metallic surface, are in general favored, and I@II-3(III)-Ht is energetically more favorable than I@II-3(V)-Ht by approximately 6.8 kcal·mol−1 . Hydrogen atoms are 2.64 Å distant from the closest cluster silver atom in I@II-3(III)-Ht and 2.40 Å in I@II-3(V)-Ht. Nonetheless, the directionality of the hydrogen, bound to oxygen (P(III)) or to phosphorus (P(V)), not always followed the 11



expectation, particularly for the smallest cluster (Figure S3).

Figure 5: Chemisorption process of SPOs on studied SNCs, here exemplified on top of SNC I@II. The chirality of P in SPOs 1 and 5 (Figures 3 and S4) doubled the number of adsorption structures screened for SPOs, but only the most stable structures for each type were considered in this study. The most favorable structures for each conformation of 1 and 5 are shown in Figures S5 and 7, respectively. The most stable conformation of 1 on the top of the cluster model was I@II-1(III)-Ht, in which the naphthyl group becomes aligned with the metal surface, while the phenyl group is displaced away from the cluster, in order to maximize the dispersion interaction. When 1 adsorbs on top of II, the obtained conformations are similar to those with the same SPO on top of I@II, the alignment of the naphthyl group to the metal surface occurs similarly, and II-1(III)-Ht is also the most stable conformer. The adsorption energy of II-1(III)-Ht is less significant than I@II-1(III)-Ht by 16.7 kcal·mol−1 , confirming the role of the central icosahedral I in the SPO-cluster interac12


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Figure 6: Most stable adsorption structures of 3 on I@II for the four conformers considered: tautomers P(V) (first and third, I@II-3(V)-Ha and I@II-3(V)-Ht) and P(III) (second and fourth, I@II-3(III)-Ha and I@II-3(III)-Ht), in which the P−H or P−OH hydrogen was pointing, at the beginning of the optimization, either towards (last two structures) or away from the SNC surface (first two). Adsorption energies (−BDE, kcal·mol−1 ) are also shown, and the value of the most stable conformer is given in bold. tion. 2 Furthermore, the interaction in I-1(III)-Ht is more favorable than in II-1(III)-Ht by 9.9 kcal·mol−1 . The naphthyl group does not become completely aligned on the surface of I, which stems from the smaller surface area available in I. The alignment of naphthyl group arising mainly from dispersion interactions is observed when structures of 5 are adsorbed on top of I@II (Figure 7), prompting the tert-butyl group to point away from the silver surface. Naphthyl group alignment with SNC surface is observed for all conformations of the considered nanocluster models (e.g., Figures S5 and 7). On the other hand, such alignment of naphthyl is less effective on top of I due to the smaller surface area of this cluster. The Ag−P bond length values in the most stable conformations of I-5(III)-Ht and II-5(III)-Ht were 2.38 Å and 2.40 Å, respectively. The Ag−H distances for those structures were 3.15 Å and 2.19 Å, respectively. The dissociation energy of I-5(III)Ht is 11.1 kcal·mol−1 larger in magnitude than that of II-5(III)-Ht, confirming again the crucial role of I on the adsorption of SPOs. Despite the fact that all most stable adsorptions found involve a trivalent coordinate P atom (Table S2), the most stable adsorption of 5 on

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top of I@II occurs through oxygen, I@II-5(V)-Ht, being 3.9 kcal·mol−1 more stable than I@II-5(III)-Ht and 3.6 kcal·mol−1 more stable than I-5(III)-Ht. This seems to be due to the capability of oxygen coordination to enable a larger aromatic-surface interaction than P coordination.

Figure 7: Most stable adsorption structures of 5 on I@II for the four conformers screened: tautomers P(V) (first and third, I@II-5(V)-Ha and I@II-5(V)-Ht) and P(III) (second and fourth, I@II-5(III)-Ha and I@II-5(III)-Ht), in which the P−H or P−OH hydrogen was pointing, at the beginning of the optimization, either towards (last two structures) or away from the SNC surface (first two structures). Adsorption energies (−BDE, kcal·mol−1 ) are also shown, and the value of the most stable conformer is given in bold.

A stable structure for II-4(III)-Ha, in which the hydrogen atom points away from the SNC surface, could not be obtained (Figure S6). In fact, all tentative geometries converged to conformations with hydrogen pointing towards the surface, whose results agreed in geometry and energy with II-4(III)-Ht. II-4(III)-Ht was the most stable structure found, differing by 6.4 kcal·mol−1 from the most favorable II-4(V). The Ag−H distances found for II4(III)-Ht and II-4(V)-Ht were 2.55 Å and 2.64 Å, respectively. The Ag−P bond length in II-4(III)-Ht was of 2.39 Å. The coordination in II-4(V)-Ht is bidentate, with Ag−O bond lengths of 2.41–2.42 Å. The most stable structures for all determined interactions are shown in Figure 8. According to Figures 7 and 8, the most stable adsorption mode of SPOs on SNCs is 14


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Figure 8: Adsorption structures of I@II with the P(III) tautomers and the P−OH hydrogen pointing in the direction of the cluster surface. The most stable conformation (smallest −BDE) for all the structures evaluated is shown, except for I@II-5(V)-Ht (see Figure 7).

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P(III), with I@II-5(V)-Ht being the only exception (Figure 7). The results make clear that P(III)-SPOs with aromatic substituents present longer Ag−P bond lengths (I@II1(III)-Ht (2.38 Å), I@II-5(III)-Ht (2.38 Å), I@II-2(III)-Ht (2.37 Å), I@II-3(III)-Ht (2.37 Å), I@II-4(III)-Ht (2.36 Å) and I@II-6(III)-Ht (2.36 Å)). They also reveal that aliphatic substituents (3, 5 and 6), which have more electron-donating capacity, stabilize the interaction between the cluster surface and the hydrogen, as indicated by the shortest Ag−H distances (I@II-1(III)-Ht (3.74 Å), I@II-2(III)-Ht (3.57 Å), I@II-4(III)-Ht (3.27 Å), I@II-3(III)-Ht (2.64 Å), I@II-6(III)-Ht (2.50 Å), and I@II-5(III)-Ht (2.34 Å)). On the other hand, I@II-5(V)-Ht, which is the most stable structure for 5 on top of I@II, has shown Ag−O and Ag−H distances equal to 2.29 Å and 2.95 Å, respectively. The dissociation energies values between SPOs and I@II cluster model reveal that the icosahedral core I, due to its compact geometry, provides the most significant contribution to the cluster stability. For instance, the dissociation energy of I@II-1(III)Ht is -62.7 kcal·mol−1 , while for I-1(III)-Ht is -55.8 kcal·mol−1 and for II-1(III)-Ht, -45.9 kcal·mol−1 . This can also be rationalized by the formation of stronger Ag−P bonds with I@II and its icosahedral structure I. That is totally in line with calculated Ag−P bond lengths. The alignment of the naphthyl group on the surface of SNCs to maximize the dispersion interaction is confirmed for I@II-5(III)-Ht and I@II-5(V)-Ht with the help of NCI graphs (Figure 9). NCI graphs reveal weak dispersion interactions on a large surface area for the aromatic group in green, while repulsive interactions are also presented on the border of the clusters, suggesting a role in the interaction played by the curvature of the silver surface. NCI graphs for other minima structures can be seen in Figure S7. Strong interactions between hydrogen atoms in SPOs and the silver surface are observed in almost all cases, indicating that hydrogen bonds are very significant to the SPO-SNC interaction, playing an interplay with dispersion interactions when SPOs contain aromatic moieties.

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Figure 9: Graphs for Non-covalent Interactions (NCI) for the conformations of I@II-5, for which I@II-5(V)-Ht is the most stable structure. Weak attractive interactions are shown in green, strong attractive interactions in dark blue. Repulsive interactions are shown in red.

Decomposing the adsorption energy between SPOs and SNCs The adsorption energy −BDE can be decomposed into two different contributions: −BDE = ∆Eint + ∆Estrain , where ∆Eint expresses the instantaneous interaction between two or more fragments composing a system, while ∆Estrain , called strain energy, represents the energy cost associated with the geometrical and electronic rearrangements that both isolated fragments have to overcome to form an interacting complex. In the present study, both interacting fragments (SPO and SNC) were considered as singlets, and the interaction energy calculated with EDA-NOCV methodology. Favorable interactions fulfill the following conditions ∆Eint < 0 and ∆Estrain > 0. Interaction energy decomposition (Equation 1), strain energies, and Hirshfeld charges of fragments for the most favorable adsorptions of SPOs on SNC are gathered in Table 1. EDA-NOCV results show that among SPOs 1(III)–6(III), those containing aromatic moieties interact more strongly with SNCs than SPOs containing aliphatic groups (Table 1). For instance, the following ∆Eint values I@II-1(III)-Ht (-73.0 kcal·mol−1 ), I@II-2(III)-Ht (-61.7 kcal·mol−1 ), and I@II-5(III)-Ht (-71.1 kcal·mol−1 ) are observed for SPOs containing aromatic groups, while the following ∆Eint values are observed for SPOs containing aliphatic groups I@II-3(III)-Ht (-49.7 kcal·mol−1 ), I@II-4(III)-Ht (-55.5 kcal·mol−1 ), and I@II17



Page 18 of 34

Table 1: EDA-NOCV interaction energies (∆Eint ) in kcal·mol−1 and their decomposition for the most stable adsorption conformers of 1(V)–6(V)/1(III)–6(III) on top of I@II, II and I. The fragmentation was taken between SPO and SNC, both singlets. (elec)

∆Eint

(disp)

qSNC

qSPO

(15.79%) (15.05%) (10.59%) (14.24%) (18.68%) (14.92%) (11.50%)

-0.21 -0.23 -0.16 -0.24 -0.11 -0.23 -0.19

0.21 0.22 0.16 0.24 0.11 0.23 0.19

-40.1 (12.77%) -32.3 (11.83%) -20.2 ( 9.01%) -25.6 (10.34%) -37.0 (11.93%) -22.1 ( 9.28%)

-0.33 -0.30 -0.27 -0.31 -0.35 -0.29

0.33 0.30 0.27 0.31 0.35 0.29

-38.6 -35.7 ( 9.10%) -29.7 -32.5 ( 8.74%) -24.8 -20.6 ( 8.08%) -24.0 -28.8 (10.67%) -40.4 -40.0 ( 9.44%) -26.3 -26.9 ( 9.73%)

-0.26 -0.27 -0.31 -0.28 -0.24 -0.34

0.27 0.27 0.31 0.28 0.24 0.33

∆Estrain

∆Eint

∆EPauli

I@II-1(III)-Ht I@II-2(III)-Ht I@II-3(III)-Ht I@II-4(III)-Ht I@II-5(V)-Ht I@II-5(III)-Hta I@II-6(III)-Ht

10.3 10.2 11.5 10.1 3.3 19.8 13.1

-73.0 -61.7 -49.7 -55.5 -58.5 -71.1 -51.2

312.8 246.2 177.6 199.8 188.5 253.8 186.6

-221.9 -180.1 -140.8 -156.1 -132.1 -190.5 -147.6

(57.52%) -103.0 (26.69%) (58.48%) -81.5 (26.47%) (61.96%) -62.4 (27.45%) (61.14%) -62.8 (24.61%) (53.47%) -68.8 (27.84%) (58.62%) -86.0 (26.46%) (62.08%) -62.8 (26.42%)

-324.9 -261.6 -203.2 -218.9 -200.9 -276.4 -210.5

(84.21%) (84.95%) (89.41%) (85.75%) (81.31%) (85.08%) (88.50%)

-12.1 -15.4 -25.6 -19.1 -12.4 -22.6 -23.8

-60.9 -46.3 -24.1 -36.4 -46.2 -48.5 -27.4

II-1(III)-Ht II-2(III)-Ht II-3(III)-Ht II-4(III)-Ht II-5(III)-Ht II-6(III)-Ht

9.7 13.2 10.4 10.6 16.6 15.3

-55.7 -55.0 -44.5 -50.3 -57.1 -47.5

258.5 218.0 179.1 197.4 253.1 190.8

-191.7 -157.5 -139.7 -149.5 -185.3 -147.5

(61.01%) (57.68%) (62.48%) (60.34%) (59.74%) (61.89%)

(26.22%) (30.49%) (28.51%) (29.31%) (28.33%) (28.83%)

-274.1 -240.7 -203.4 -222.0 -273.2 -216.2

(87.23%) (88.17%) (90.99%) (89.65%) (88.07%) (90.72%)

-15.6 -22.7 -24.4 -24.6 -20.1 -25.4

I-1(III)-Ht I-2(III)-Ht I-3(III)-Ht I-4(III)-Ht I-5(III)-Ht I-6(III)-Ht

18.7 10.3 5.5 8.2 28.7 9.5

-74.3 -62.2 -45.4 -52.8 -80.4 -53.2

317.8 310.2 209.4 217.5 343.3 223.3

-228.2 -221.1 -154.5 -163.9 -242.9 -166.9

(58.20%) -128.2 (32.70%) (59.38%) -118.7 (31.89%) (60.64%) -79.7 (31.28%) (60.64%) -77.6 (28.69%) (57.34%) -140.8 (33.22%) (60.38%) -82.7 (29.90%)

-356.4 -339.8 -234.2 -241.5 -383.7 -249.6

(90.90%) (91.27%) (91.92%) (89.33%) (90.56%) (90.28%)

SNC-n

a

∆Eelstat

∆Eorb

-82.4 -83.2 -63.8 -72.6 -87.9 -68.7

∆Eelstat + ∆Eorb

∆Eint

Not the most stable conformer, here shown for comparison.

6(III)-Ht (-51.2 kcal·mol−1 ). The energy decomposition reveals that the electrostatic term is the most significant contribution to the interaction, followed by orbital and dispersion contributions, which are much more significant in aromatic SPOs. It becomes clear that the presence of aromatic moieties in SPOs favors their adsorption in a significant way, since the electron density becomes enhanced, which amplifies both orbital and dispersion contributions, and consequently leads Pauli repulsion to increase. For instance, in systems I@II-1(III)-Ht, I@II-2(III)-Ht and (disp)

I@II-5(III)-Ht, ∆Eorb varies between -81.5 and -103.0 kcal·mol−1 and dispersion ∆Eint

between -46.3 and -60.9 kcal·mol−1 . In contrast to that, in I@II-3(III)-Ht, I@II-4(III)-Ht (disp)

and I@II-6(III)-Ht, ∆Eorb varies from -62.4 to -63.8 kcal·mol−1 and ∆Eint

from -24.1 to

-36.4 kcal·mol−1 . The key role played by the dispersion term is further highlighted by the sum (elec)

∆Eint

= ∆Eelstat + ∆EPauli + ∆Eorb : while it stays between -12.1 and -25.6 kcal·mol−1 for

I@II-n(III)-Ht (n = 1–6), the dispersion term is found between -24.1 and -60.9 kcal·mol−1 for the same structures, being more strongly dependent on the SPOs and larger for aromatic moieties. 18


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The strain energy values, ∆Estrain , in systems I@II-1(III)-Ht–I@II-4(III)-Ht are quite similar, ranging from 10.1 to 11.5 kcal·mol−1 . An interesting situation is observed between structures I@II-5(III)-Ht and I@II-5(V)-Ht: as presented in Figure 7, the adsorption energy values (−BDE) for I@II-5(III)-Ht and I@II-5(V)-Ht are -51.3 and -55.2 kcal·mol−1 , respectively, while according to EDA-NOCV, ∆Eint values are -71.1 and -58.5 kcal·mol−1 , respectively. This indicates that the interaction between 5(III) and I@II is much more stabilizing than with 5(V), but 5(III) demands a significant strain energy (∆Estrain = 19.8 kcal·mol−1 ), while 5(V) needs only 3.3 kcal·mol−1 . The fact that 5(III) has lower adsorption energy than 5(V) comes thus from the fact that 5(III) needs to deform much more than 5(V) to maximize its interaction with the SNC, which leads to a significant increase of strain energy. When Ag12 icosahedral motif (I) is removed from the Ag20 dodecahedral outer-shell (II), the interaction energy values between SPOs 1(III)–6(III) and II decreases in a considerable way, totally in line with (−BDE) values, which suggests that the icosahedral I is crucial for (elec)

the SPO-SNC interaction. Table 1 shows that, although ∆Eint

becomes more significant

from I@II to II and to I, the dispersion contribution decreases much more strongly in proportion, and in general, ∆Estrain for I is smaller than for II or I@II. This can be rationalised by the smaller steric volume of I. There are two exceptions to this result, however, both of which, 1(III) and 5(III), have visibly distorted naphthyl groups on the surface of I (Figure S3), leading to larger strain energies. This way, I-5(III)-Ht presents the largest ∆Estrain calculated in this study (28.7 kcal·mol−1 ). It confirms that much more energy is necessary to provide a better alignment of aromatic groups to more curved surfaces. Structures I@II-5(III)-Ht and II-5(III)-Ht presented the largest ∆Estrain also for their respective nanoclusters I@II and II (19.8 kcal·mol−1 and 16.6 kcal·mol−1 , respectively), further stressing the substitution effect in the strain energy. In contrast, 1(III) presents a large decrease in ∆Estrain when I-1(III)-Ht (18.7 kcal·mol−1 ) is compared with II-1(III)Ht and I@II-1(III)-Ht (9.7 and 10.3 kcal·mol−1 , respectively). Similarly to 5(V)/5(III),

19



Page 20 of 34

1(V)/1(III) has a naphthyl moiety attached, but no bulky tert-butyl (Figure 3). For 6(V)/6(III), which has only attached tert-butyl groups, ∆Estrain does not differ by more than ≈ 2.2 kcal·mol−1 for both II-6(III)-Ht and I@II-6(III)-Ht (15.3 kcal·mol−1 and 13.1 kcal·mol−1 , respectively). In contrast, ∆Estrain equals 9.5 kcal·mol−1 for I-6(III)-Ht. Therefore, the curvature of I requires less deformation of the more bulky ligands. The larger interaction energy values observed for SPOs bearing aromatic moieties is consistent not only with the NCIs (Figures 9 and S7) but also with the EDA-NOCV deformation density channels, which maps the electron density inflow and outflow between the interacting fragments (e.g., Figures 10 and 11). For instance, I@II-1(III)-Ht and I@II-2(III)-Ht present ligand→metal donations (L→M; ∆Eorb,3 = −18.1 and ∆Eorb,2 = −17.1 kcal·mol−1 , respectively) and metal→ligand backdonations (M→L; ∆Eorb,1 = −13.6 and ∆Eorb,1 = −13.1 kcal·mol−1 , respectively). Comparing I@II-1(III)-Ht with I@II-2(III)-Ht, we observed that the naphthyl group (present in I@II-1(III)-Ht) contributes more substantially (disp)

to both donations and backdonations, and increased ∆Eint , ∆Eorb and ∆Eint

, but also in-

creased Pauli repulsions (∆EPauli ). Other deformation density channels of I@II-1(III)-Ht and I@II-3(III)-Ht show L→M donations and fragment polarizations. The density deformation channels of I@II-5(III)-Ht (Figure 10) reveal that L→M donation, M→L back-donation and also polarizations take place. However, L→M donations are more significant. For instance, channels 2, 3 and 5 shown in Figure 10 show L→M donations, which stem from 2s occupied states of naphthyl carbons towards silver unoccupied p states (e.g., ∆Eorb,2 = −19.9 kcal·mol−1 ). M→L back-donation, observed in channel 1, involves donation from occupied silver p states to unoccupied carbon 3s states (q1 = 1.64 e). Channel 4 also exhibits polarization of the ligand, which involves occupied 2s and unoccupied 5s states of carbons. I@II-5(V)-Ht (Figure 11), the most stable structure for this SNC/SPO pair, shows a direct cluster–naphthyl M→L interaction (∆Eorb,1 = −9.5 kcal·mol−1 ). Other deformation density channels also indicate charge polarizations (∆Eorb,4 = −6.9 and ∆Eorb,5 = −4.2 kcal·mol−1 ) and Ag−O bond formation

20


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Figure 10: EDA-NOCV deformation density channels (cutoff of 0.001 au) for I@II-5(III)Ht, which is not the most stable conformation for this pair. Values are depicted in Table 1. Charge deficient regions are shown in red, charge enriched regions are shown in blue. Energies in kcal·mol−1 .

Figure 11: EDA-NOCV deformation density channels (cutoff of 0.001 au) for I@II-5(V)-Ht, the most stable conformation for this pair. Values are depicted in Table 1. Charge deficient regions are shown in red, charge enriched regions are shown in blue. Energies in kcal·mol−1 .

21



(∆Eorb,2 = −8.5 and ∆Eorb,3 = −9.6 kcal·mol−1 ). The analysis of density deformation channels are entirely consistent with the orbital contributions reported in Table 1, and give support to the fact that the Ag12 icosahedral motif (I, Table 1 and Figure 2) amplifies the orbital interactions, increasing donation, backdonation, and polarization. For instance, deformation density channels for adsorptions on top of I reveals that L→M donations coupled with polarizations are the most significant mode of interaction for all structures. I-1(III)-Ht presents an interaction, with charge flowing from the ligand and redistributing itself inside the cluster, of magnitude equal to ∆Eorb,1 = −49.2 kcal·mol−1 , while a similar interaction is presented by I-3(III)-Ht (∆Eorb,2 = −33.6 kcal·mol−1 ) and other structures (−19.7 — −53.3 kcal·mol−1 ). Backdonations are also observed in structures whose ligands have aromatic moieties such as I-1(III)-Ht (∆Eorb,2 = −19.0 kcal·mol−1 ), I-2(III)-Ht (∆Eorb,2 = −21.9 kcal·mol−1 ) and I-5(III)-Ht (∆Eorb,2 = −21.8 kcal·mol−1 ). On the other hand, when the Ag12 icosahedral core (I) is removed from the Ag20 (II) the charge redistribution becomes less effective, and it does not contribute to the total interaction in a significant way.

The proton transfer from SPOs to SNC According to Cano, 1 it is feasible to consider that, under specific conditions, SPOs release a proton, possibly first to the cluster surface, and then to the medium. Furthermore, it’s known that non-protonated SPO-cluster systems play a role in catalysis. 1,4 In order to shed light on the nature of the SPO-cluster interaction in such cases, a series of adsorbed structures of I@II where hydrogen is attached to the cluster surface were optimised, and we report here the most stable ones found (named H-I@II-n with n = 1–6, Figure 12). Adsorption energies are taken as the difference in energy between the most stable adsorption structure with hydrogen transfered (shown in Table 2), and optimised SPOs and fixed I@II. Hydrogen transfer energies were calculated as the difference in energy between the most stable structures with hydrogen bound to the SPO and the most stable structures with it bound to the 22


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cluster surface.

Figure 12: Adsorption structures for I@II with SPO and hydrogen on the silver surface (named H-I@II-n with n = 1–6). Hydrogen adsorption energies are shown below each structure.

All −BDE values fall in the same range as when hydrogen is attached to the SPO, but in most cases the transfer is favored by 1.0–7.3 kcal·mol−1 . The only exception is H-I@II-4, for which the most stable structure found was one with the hydrogen very close to the phosphorus and unfavorably (4.8 kcal·mol−1 ) “releases” hydrogen from 4. Furthermore, H-I@II-3 presents hydrogen transfer energy close to zero (−0.7 kcal·mol−1 ). Therefore, cyclohexyl or ethyl substituents are not expected to favor hydrogen release, showing hydrogen transfer en23



Page 24 of 34

Table 2: Adsorption energies (in kcal·mol−1 ) for each conformation obtained for n = 1–6 on I@II after hydrogen transfer. SNC-n H-I@II-1 H-I@II-2 H-I@II-3 H-I@II-4 H-I@II-5 H-I@II-6

−BDE

H transfer (kcal·mol−1 )

-64.7 -53.7 -38.9 -40.5 -62.6 -43.3

-2.1 -2.1 -0.7 4.8 -7.3 -5.3

ergies between −0.7 and +4.8 kcal·mol−1 (3 and 4, respectively). Aromatic moieties (1 and 2) release hydrogen favorably to the surface by approximately −2.1 kcal·mol−1 . tert-Butyl further enhances hydrogen delivery, since 5 and 6 show hydrogen transfer energies of −7.3 and −5.3 kcal·mol−1 , respectively. This is in line with experimental findings for these ligands in the role of gold nanoparticle passivators. It was previously observed that gold NPs made with 1, 2 and 5 show no O−H stretchings in FT-IR, in contrast with 3 and 4, which indicates that SPOs with aromatic moieties lose hydrogen upon adsorption on gold nanoparticles. 1 Note that 6 is unsuitable for the production of gold nanoparticles, 1 in agreement with our results for silver nanoparticles, which suggest that hydrogen transfer energies for H-I@II-3 and H-I@II-4 are less favorable and that the determinant step for proton release might be its transfer to the metal surface. The EDA-NOCV analysis (Table 3) for protonated species reveals that the most electrostatic interactions are due to aliphatic substituents (both 67% for ethyl and tert-butyl of 3 and 6, respectively), while at least one aromatic moiety already decreases this term to (disp)

62–63% (1, 2 and 5) and increases ∆Eint

from 6–7% (aliphatic moieties only) to 9–11%

(at least one aromatic moiety). The impact of electrostatic interactions may be rationalised by a M→L charge transfer of 0.23–0.41 e in 1–3, 5 and 6, which agrees with the electrostatic potential maps of those structures (Figure S8). H-I@II-4 shows mainly orbital interactions (54%) and a ten-fold smaller M→L charge donation (0.04 e), while other structures present ∆Eorb of 26–29%. Furthermore, the addition of a second, adjacent SPO 2 in H-I@II-2 24


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showed minor differences in the EDA-NOCV analysis (H-I@II-22 in Table 3), which emphasizes that our models with a single SPO can be employed to address the interaction issue in the fully populated cluster. Table 3: EDA-NOCV interaction energies (∆Eint ) in kcal·mol−1 and their decomposition for the most stable adsorption conformers of SPOs n = 1–6 on SNC models with hydrogen on the silver surface. Here the fragmentation was taken between the negatively charged unhydrogenated SPO and singlet hydrogenated positively charged SNC. ∆Eint

∆EPauli

H-I@II-1 -169.1 H-I@II-2 -171.7 H-I@II-22 * -158.6 * -166.0 H-I@II-3 -149.5 H-I@II-4 -190.3 H-I@II-5 -172.0 H-I@II-6 -163.2

299.8 268.8 270.2 278.4 262.7 396.5 302.1 261.7

SNC-n

∗

∆Eelstat -294.7 -274.1 -262.0 -270.7 -277.5 -234.8 -298.1 -283.9

(62.86%) (62.23%) (61.09%) (60.93%) (67.32%) (40.01%) (62.86%) (66.81%)

∆Eorb -124.2 -126.5 -112.9 -119.7 -110.3 -316.7 -125.5 -109.9

(26.48%) (28.72%) (26.33%) (26.94%) (26.75%) (53.98%) (26.46%) (25.86%)

∆Eelstat + ∆Eorb -418.9 -400.6 -374.9 -390.4 -387.8 -551.5 -423.5 -393.8

(89.34%) (90.95%) (87.42%) (87.87%) (94.07%) (93.99%) (89.32%) (92.67%)

(elec)

∆Eint

-119.1 -131.8 -104.7 -112.1 -125.1 -155.1 -121.4 -132.1

(disp)

qSNC

qSPO

-50.0 (10.65%) -39.9 ( 9.06%) -53.9 (12.56%) -53.9 (12.12%) -24.4 ( 5.93%) -35.2 ( 5.99%) -50.6 (10.68%) -31.1 ( 7.32%)

0.26 0.33 0.35 0.32 0.38 0.03 0.23 0.41

-0.26 -0.33 -0.35 -0.33 -0.38 -0.04 -0.23 -0.41

∆Eint

These two lines are associated with the two different fragmentations possible with two adjacent 2 SPOs on the surface of H-I@II.

In this context, the Reverse Ostwald Ripening model of Burlakov 47 suggests that clusters and nanoparticles with similar radii can be obtained from two different ligands if their metal-ligand binding energies are similar, 47 i.e., radii correlate with ligand-metal interaction energies. Cano et al. remark that SPOs produced gold nanoparticles with radii in the order 5 < 4 ≈ 2 < 1 ≤ 3. 1 This is in agreement with our interaction energies, except that 3 and 4 are misplaced. The misplacement of 3 and 4 may be rationalised by the fact that a model without a O−H bond is not a good model for either one, as discussed above.

Conclusions The physical nature of the interaction between secondary phosphine oxides (1–6) and experimentally determined, 2 unoptimized silver nanocluster models (I@II, II and I) was investigated. It was shown that substituents bound to phosphorus in the SPOs have a crucial influence on the stability of each tautomer. Furthermore, aliphatic and aromatic substituents play opposite roles, the former donating charge density and favoring the trivalent tautomer, 25



which is coordinating, and the latter withdrawing charge density and favoring P(V), as can be seen by the substituent effect on the natural charges of the P−OH group. As shown by NPA charges and electrostatic potential maps, there is a correlation between the energy difference E P(III) − E P(V) and the negative charge concentration on oxygen. The most favorable coordination mode was found to be in trivalent form of SPOs, with formation of a Ag−P bond. The exception was 5 (HP(−O)tert−BuNaph) adsorbed on Ag32 (I@II), which showed the most stable coordination through the pentavalent tautomer and formation of a Ag−O bond. This took place due to a prohibitive strain energy for the I@II-5(III)-Ht adsorption. In spite of this, EDA-NOCV interaction energies are most significant when coordination is through a trivalent SPO, even when I@II-5(V)-Ht and I@II-5(III)-Ht are compared. Aromatic substituents become aligned to the silver surface upon adsorption. From the strain energy analysis, and orbital and dispersion contributions to the interaction energies, it was observed that bulky SPO substituents such as tert-butyl are less distorted on top of the smaller cluster, while aromatic moieties such as naphthyl and phenyl are less distorted on more planar surfaces. The high strain energy, on top of all model clusters, for adsorptions of 5, which has naphthyl and tert-butyl as substituents, could then be explained. According to EDA-NOCV, aromatic substituents also contribute more significantly to orbital and dispersion interactions, which is in total agreement with the analysis of Noncovalent Interactions. According to EDA-NOCV deformation density channels, the most significative interactions are ligand→metal donations and fragment polarizations, but aromatic moieties also contribute to significant metal→ligand backdonations. The most relevant dissociation energies takes place through adsorptions on top of I@II and the least significative on top of II. Interaction energies also follow the same trend, suggesting an active role of the central icosahedral on the interaction. Our models for SPO-cluster hydrogen transfer agree with the experimental finding that O−H stretching was not found for gold nanoparticles bound by 1, 2 and 5. Furthermore, the

26


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experimental radii found for gold nanoparticles passivated by 1, 2 and 5 follows the silverSPO interaction energies found in this study. It is accepted that knowing the physical nature of the ligand-nanocluster interaction makes it possible to control the particle diameter by choosing adequate ligands. 47 The agreement of our results with experimental radii indicate that our models are thus very reliable and can enable the rational development of new SPO-protected nanoparticles and nanoclusters.

Author Information Corresponding Author Correspondence should be addressed to M. S..

Author Contributions F. S. S. S. and E. H. S. carried out the calculations. F. S. S. S., M. S. and G. F. C. analysed the data. F. S. S. S., M. S., G. F. C. and R. L. T. P. wrote the manuscript. M. S. proposed the initial idea. M. S., G. F. C. and H. S. S. conceived and designed the project. M. S. and G. F. C. supervised and coordinated the project. H. S. S. and P. W. N. M. L. provided advice and critically reviewed the manuscript.

Notes The authors declare no competing financial interests.

Acknowledgement F. S. S. S. and G. F. C. thank the Brazilian National Council for Scientific and Technological Development (CNPq) for financial support (grants 302408/2014-2 and 311963/2017-0, respectively). F. S. S. S., M. S. and G. F. C. thank the excellent computational services 27



provided by Centro Nacional de Supercomputação (CESUP/UFRGS). R. L. T. P. thanks FAPESP for financial support (grant 2011/07623-8). P. W. N. M. van L. thanks CAPES for financial support (grant A029_2013).

Supporting Information Available Supporting Information, containing electrostatic potential maps and NPA/NBO analyses of secondary phosphine oxides, adsorption geometries and their respective dissociation energies, non-covalent interaction analysis of adsorbed geometries and electrostatic potential maps for H-I@II-1–H-I@II-6, is available free of charge.

References (1) Cano, I.; Huertos, M. A.; Chapman, A. M.; Buntkowsky, G.; Gutmann, T.; Groszewicz, P. B.; van Leeuwen, P. W. N. M. Air-Stable Gold Nanoparticles Ligated By Secondary Phosphine Oxides As Catalyst For The Chemoselective Hydrogenation Of Substituted Aldehydes: A Remarkable Ligand Effect. J. Am. Chem. Soc. 2015, 137, 7718–7727. (2) Desireddy, A.; Conn, B. E.; Guo, J.; Yoon, B.; Barnett, R. N.; Monahan, B. M.; Kirschbaum, K.; Griffith, W. P.; Whetten, R. L.; Landman, U. et al. Ultrastable Silver Nanoparticles. Nature 2013, 501, 399–402. (3) Castro, P. M.; Gulyás, H.; Benet-Buchholz, J.; Bo, C.; Freixa, Z.; van Leeuwen, P. W. N. M. SPOs As New Ligands In Rh(iii) Catalyzed Enantioselective Transfer Hydrogenation. Catal. Sci. Technol. 2011, 1, 401. (4) Cano, I.; Chapman, A. M.; Urakawa, A.; van Leeuwen, P. W. N. M. Air-Stable Gold Nanoparticles Ligated By Secondary Phosphine Oxides For The Chemoselective Hydro-

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How Do Secondary Phosphine Oxides Interact with Silver Nanoclusters?

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