Discovery and Evolution of Polyoxopalladates - Accounts of Chemical

Article Cite This: Acc. Chem. Res. 2018, 51, 1599−1608

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Discovery and Evolution of Polyoxopalladates Peng Yang and Ulrich Kortz*

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Department of Life Sciences and Chemistry, Jacobs University, Campus Ring 1, 28759 Bremen, Germany CONSPECTUS: Noble metal catalysts, in particular palladium-containing materials, are of prime commercial interest, because of their role as oxidation catalysts in automobile emission-control systems and reforming catalysts for the production of high-octane gasoline. However, despite almost two centuries of research, the precise structure of such materials is still ill-defined on the subnanometer scale, which severely limits the understanding of the underlying catalytic mechanisms. As a burgeoning class of structurally well-defined noble metal oxide nanoclusters, polyoxopalladates (POPs) have been highly rated as ideal models to fully decipher the molecular mechanism of noble metal-based catalysis. Being at the frontier of polyoxometalates (POMs), the chemistry of POPs, which are based exclusively on PdII centers as addenda is currently progressing rapidly, owing to their structural and compositional novelty, high solution stability, combined with promising applications especially as noble metal-based catalysts. Controlled hydrolysis−condensation processes of square-planar PdIIO4 units in the presence of external oxyacid heterogroups (e.g., AsO43−, PO43−, and SeO32−) drive the selfassembly of such discrete, polynuclear PdII-oxo nanoclusters in facile one-pot reactions using aqueous solvents. By now, more than 70 POPs have been discovered, encompassing a large structural variety, including cube, star, bowl, dumbbell, wheel, and open-shell archetypes. Moreover, the POP cages can serve as adaptable molecular containers for encapsulation/interaction with a range of metallic elements across the s, p, d, and f blocks of the periodic table, resulting in a library of host−guest assemblies of varying shapes and sizes. Besides a delicate balance of experimental variables, the fine-tuning of POP structure, composition, and properties is possible by systematic replacement of the metal ion guest and/or the capping heterogroups. Besides, nearly all POPs obtained so far could be perfectly rationalized by theoretical calculations, and even prediction of the design and synthesis of new POP structures is possible. The excellent stability of POPs in the solid state and in solution (both aqueous and organic media) and gas phase allows for applications mainly in homo- and heterogeneous catalysis or as molecular precursors for monodisperse nanoparticles via an ingenious bottom-up route for functional nanotechnology. Apart from catalysis, owing to the unique structural features of POPs, other areas of interest exist, for example, in magnetism as molecular spin qubits and in biology as aqueous-phase macromolecular models. Overall, as a distinct subclass of POMs, POPs not only integrate the advantages of tunable shape, size, composition, solution stability, redox activity, and facile synthetic procedures, but drive immense potential for achieving an atom-to-atom fabrication and modulation of nanostructures as well, thereby providing models for unveiling mechanistic insight of noble metal-based catalysis at the molecular level, which will, in turn, guide the programmed assembly of nanomaterials with improved performance in a controllable manner. This Account is directed to cover the main structural types of POPs and to discuss the structuredirecting template effects induced by the guest ions on the resultant host−guest assemblies.

1. INTRODUCTION

has largely been confined to conventional POMs (comprising early d-block addenda), and efforts to extend this research realm to the platinum group metals have been of limited success. In the early 19th century, Döbereiner first proposed an ideal model to decode the intricate molecular mechanism of noble metal-based catalysis, that is, structurally well-defined noble metal oxide clusters.4 To this end, the introduction of noble metals in POMs has been provoking extensive attention, as a result of not only the precise local structure determination, but also the integration of the intrinsic catalytic properties of noble

Polyoxometalates (POMs), a large class of discrete metal−oxo anions of early d-block elements in high oxidation states (VV, NbV, TaV, MoVI, and WVI), cover an exceptional range of structural and compositional diversity as well as the ability to generate dynamic structures that can range in size from the nano- to micrometer scale.1 Owing to the unique ensemble of features such as structural robustness, oxidative and thermal stability, and tunability of acidity and redox activity, POMs possess great prospects as functional devices in fundamental and applied areas as diverse as catalysis, magnetism, electro/ photochromism, bio- and nanotechnology, medicine, and materials sciences.2,3 Albeit a remarkable development in POM chemistry during the last several decades, this progress © 2018 American Chemical Society

Received: February 25, 2018 Published: June 18, 2018 1599

DOI: 10.1021/acs.accounts.8b00082 Acc. Chem. Res. 2018, 51, 1599−1608

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Accounts of Chemical Research metals with the ability of POMs to activate the easily accessible “green” oxidants H2O2 and O2 toward, for example, the direct synthesis of acetic acid from ethylene.5 However, the proportion of noble metals based on the overall metal content of the reported POM frameworks is far below 50%, which severely restrict their catalytic performance.6,7 In light of this, the exploration of POMs being composed exclusively of precious metal centers as addenda may be en route to not just competitive systems with immense catalytic potential, but also promising models to understand the mechanism of noble metal-based oxidation processes.8 The area of noble metal-based POMs, or polyoxo-noblemetalates, was initiated by Wickleder and co-workers in 2004 with the preparation of the polyoxo-12-platinate(III) [PtIII12O8(SO4)12]4−.9 Over the past decade, polyoxopalladates(II) (POPs) and polyoxoaurates(III) (POAs) have been at the forefront of research in this emerging field ever since the pioneering discovery of the first POP, [H6PdII13O8(AsO4)8]8−,10 and POA, [AuIII4O4(AsO4)4]8−,11 by Kortz and co-workers in 2008 and 2010, respectively.12,13 The chemistry of POPs, inter alia, has witnessed an impressive development in recent years, primarily engineered with a view to their salient physiochemical properties as well as broad applications, especially in noble metal-based catalysis.10,14,15 In contrast to the earlier comprehensive reviews on noble metal-containing POMs,6,7 herein the focus is fully on the family of POPs, which nowadays constitute the largest subclass (>70 compounds) of polyoxo-noble-metalates and are continuously growing. The structural variety is enormous and key building units can be identified, which have allowed to construct cube, 10,16 star, 14,17,18 bowl, 19 dumbbell, 20,21 wheel,22−24 and open-shell archetypes.25−27 On the other hand, around 30 metal ion guests across the s, p, d, and f blocks of the periodic table of elements could be successfully incorporated into the POP host shells of various shape and size, resulting in prospective candidates for tunable composite catalysts, molecular spin qubits, and precursors for core−shell nanoparticles coupled with a fascinating host−guest chemistry.13−15,17,20,21,25−32 Besides experimental techniques, systematic theoretical calculations have also been employed, in a quest to completely understand the formation mechanism underlying POP chemistry, as well as predicting fundamentally novel structures. In this Account, we provide an overview of the main structural types of POPs and discuss the template effects induced by the guest ions on the resultant host−guest assemblies.

Figure 1. (a) Ball-and-stick representation of Pd13As8. Color code: Pd, blue; As, green; O, red. (b−e) Representation of the onion-type shell structure of Pd13As8. (f) Similar addenda arrangement in Pd13As8 (left) and Keggin ion (right).

parts (from inside out): (i) the central PdII atom; (ii) the 8 internal oxygen atoms (4 μ3- and 4 μ4-O) situated at the vertices of a distorted O8 cube (Figure 1c); (iii) the 12 remaining PdII centers surrounding the inner PdO8 moiety, form an approximately icosahedral arrangement (distorted toward a cuboctahedron, Figure 1d); (iv) the 24 outer μ2-oxo groups form a O24 truncated-cube-shaped shell (Figure 1e); (v) the 8 terminal (AsO)3+ groups capping the Pd13O32 assembly. The structural features of Pd13As8 allow to postulate a resemblance to the well-known Keggin ion XM12O40 (X = heteroatom, M = MoVI or WVI), as both have high symmetry and can be considered as plenary (closed shell) POMs, allowing (at least theoretically) for lacunary (vacant) derivatives. In Pd13As8 the 12 peripheral PdII centers form a distorted cuboctahedron, which resembles the arrangement of the 12 addenda sites in the Keggin ion with idealized Td symmetry (Figure 1f). However, in marked contrast to conventional POMs such as Keggin, which are typically comprised of (distorted) octahedral MO6 building blocks and internal heterogroups, in Pd13As8 all 13 PdII ions retain square-planar oxo-coordination geometry and the heterogroups are external. Subsequent studies on replacing the arsenate (AsO43−) heterogroups in Pd13As8 by the lone pair-containing selenite (SeO32−) and phenylarsonate (PhAsO32−) has led to two Pd13 derivatives, [PdII13O8(SeO3)8]6− (Pd13Se8) and [PdII13O8(PhAsO3)8]6− (Pd13(PhAs)8), respectively.16 Interestingly, the coordination number (and geometry) of the central PdII ion in Pd13As8, Pd13Se8, and Pd13(PhAs)8 changes from 4 (square-planar), to 6 (octahedral), and 8 (cubic), as shown by X-ray crystallography and magnetic measurements. It is evident that the nature of the selected capping group (e.g., charge, X− O bond length, geometry, and steric hindrance) allows to modulate the POP structure, and in particular the detailed geometry of the central cavity (Figure 2). Being motivated by such observations, we set out to synthesize the hypothetical Pd13P8 analogue by employing phosphate as heterogroups, and unexpectedly, a pentagonal star-shaped polyoxo-15-palladate, [Pd II 15 O 10 (PO 4 ) 10 ] 20− (Pd15P10, Figure 3a), with an approximate diameter of 1.5 nm was obtained.14 Employing again the multishell approach we used for Pd13As8, it is possible to distinguish four layers (from

2. STRUCTURAL DIVERSITY OF POPs As a distinct category of molecular palladium(II)-oxide nanoclusters, POPs are preferably prepared by self-condensation in aqueous media of square-planar PdIIO4 building units and this process being terminated by external heterogroups (e.g., AsO43−, PO43−, and SeO32−) capping the discrete palladium-oxo assembly. The cuboid-shaped [H6PdII13O8(AsO4)8]8− (Pd13As8, Figure 1a) represents the first example of a POP, prepared by a self-assembly reaction of PdCl2 and As2O5 in aqueous acetate medium.10 This Pd13As8 anion is a distorted “nanocube” due to edge lengths of ca. 1 nm. The structure of Pd13As8 can be described in terms of Müller’s “Keplerate” terminology, as it is comprised of symmetrical Platonic and Archimedean solids of different size arranged within each other, resembling a Russian “Matryoshka” doll (Figure 1b). Specifically, Pd13As8 can be dissected into five 1600


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Figure 2. Coordination geometry of the central PdII ion in Pd13As8 (left), Pd13Se8 (middle), and Pd13(PhAs)8 (right). Color code: Pd, blue; As, green; Se, orange; O, red; C, black. Hydrogen atoms omitted for clarity.

Figure 4. Basic building unit {Pd3O8(XR)2} in the construction of Pd12 (top) and Pd15 (down) archetypes.

By now, it has been evidenced that the structural intricacy of POPs is directly related to a complex interplay of experimental variables involving pH, temperature, heterogroups, ratio of reagents, concentration, ionic strength, and counter cations. A handful of POPs bearing novel structural topologies have been successfully isolated by virtue of delicate control of the reaction parameters, and several representative examples are listed below: (i) pH and temperature: (a) Despite the same ratio and concentration of the starting reagents, at slightly lower pH with gentle heating (pH = 5.0, 50 °C), the star-shaped Pd15Se1030 is formed instead of the cube-shaped Pd13Se8 (pH = 6.9, 80 °C).16 (b) With respect to the phosphate-capped POPs, slow hydrolysis−condensation processes (room temperature, 20 h) could significantly facilitate the isolation of the gigantic wheelshaped {Pdx}L (x = 84, L = acetate or glycolate; x = 72, L = propionate) nanoclusters rather than the low-nuclearity Pd15P10 (80 °C, 90 min).22−24 Compared to other large molecular wheels such as Müller’s polyoxomolybdate {Mo154}33 and Christou et al.’s manganese-oxo cluster {Mn84},34 the family of {Pdx}L represents the first metal-oxo macrocycles being constructed solely by late transition metals that enable nuclearity control via auxiliary ligand replacement. As an example, the {Pd84}OAc ([PdII84O42(PO4)42(OAc)28]70−) ring22 consists of seven {[PdII6(μ4-O) 2(μ2-O)(PO4)3 (OAc)2 ]2} ({Pd6}2) repeat units, where every six PdII centers in each Pd6 subunit are ligated via two μ4-O ligands, with an overall D7d symmetry and a C7 (S14) axis (Figure 5). The 14 Pd6 subunits are directly interconnected by μ2-oxo groups and further bridged by four acetate and two phosphate ligands. The PO4 heterogroups herein play the role as either capping (two per Pd6) or bridging the neighboring subunits. Fourteen acetate functions are decorated on the inner rim resulting in a hydrophobic cavity (diameter of ca. 1 nm), with the remaining acetates residing around the outside to complete the wheel archetype (diameter of ca. 3 nm) containing the largest number of PdII ions yet found in POP chemistry. It should be noted here that all our efforts in the last years to reproduce Cronin’s {Pd84} wheel according to references 22 and 23 have failed. (ii) Heterogroups: It should be noted that tetrahedral heterogroups have been extensively used in POP synthesis. The deliberate employment of vanadate ions (which can exhibit tetrahedral VVO4 as well as square-pyramidal VVO5 geometries) as heterogroups has resulted in a unique bowl-shaped, polyoxo-

Figure 3. (a) Ball-and-stick top and side views of Pd15P10. Color code: Pd, blue; P, fuchsia; O, red. (b−e) Representation of the onion-type shell structure of Pd15P10.

inside out) in the onionlike Pd15P10 structure (Figure 3b): (i) 10 internal oxygens forming an O10 pentagonal prism (Figure 3c); (ii) 15 PdII atoms forming a distorted pentacapped pentagonal prism (Figure 3d); (iii) 30 external μ2-oxo ligands forming an O30 truncated pentagonal prism (Figure 3e); (iv) 10 (PO)3+ capping groups. Overall, the Pd15P10 structure displays pseudo-D5h symmetry, and the central vacancy spanned by the O10 donors allows to encapsulate NaI or PdII cation guests.14,18 Later, its selenite-analogue [PdII15O10(SeO3)10]10− (Pd15Se10) was reported.17,30 Based on the above, some structural analogies between the Pd12 (Pd13 backbone without the central PdII ion) and Pd15 archetypes can be identified. Both cages constitute the same Pd3O8(XR)2 (X = AsV, PV, R = O, Ph; X = SeIV, R = lone pair) building blocks. Two Pd−O and two X−O bridges link these units into a cycle around the symmetry axis Cn (n = 4 for Pd12 and n = 5 for Pd15, Figure 4). The shell of Pd12/Pd15 can hence be described as a tetramer/pentamer of the building block by applying the general formula {Pd3O8(XR)2}4/5. Therefore, some magic numbers for POPs can be proposed, such as Pd9X6, Pd12X8, Pd15X10, Pd18X12, Pd21X14,... , Pd30X20,... , Pd60X40,... , Pd72X48,... , Pd84X56,... , Pd90X60, etc.), predicting species which have not been identified yet. 1601


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Figure 5. Ball-and-stick representation of {Pd84}OAc and the {Pd6}2 repeat unit. Color code: Pd, blue; P, fuchsia; O, red; C, black; H, gray; the μ4-O and μ2-O groups in {Pd6}2 are highlighted in green and yellow color, respectively.

instead of PdII, and two NaPd11As7 fragments are fused to a dimeric assembly via two extra μ4-AsO4 bridges (Figure 6b). As based on the above it can be concluded that all reported POP archetypes are structurally related. The condensation process of PdIIO4 units is largely affected by the combinatorial complexity associated with multiple reaction parameters, in particular pH. However, as the final products of pH-dependent self-assembly processes, most POPs possess high aqueous stability within a large pH window. For example, Pd13As8 is water-stable at least from pH 3 to 9, which is far superior to most conventional POMs.10 In addition, the structure-directing role of the heterogroups has a profound impact when tailoring POPs with a desired nuclearity and shape. Recently, a POP building block has been found in a PdII-containing heteropolytungstate,35,36 which merges the young and emerging field of POPs with the classical and well-developed field of POMs, and thereby proposes an even larger scope of structures and applications.

6-vanado-7-palladate, [PdII7VV6O24(OH)2]6− (Pd7V6, Figure 6a).19 In Pd7V6, two Pd3V3O11 half units are bridged through four μ2-oxo groups with one remaining PdII center sealing the bottom of the “bowl”. This reemphasizes the strong structuredirecting effect of the capping group in POP chemistry. (iii) PdII concentration: Keeping the reaction conditions of Pd13As8 constant, by lowering the PdII concentration from 0.125 to 0.063 M, a new dumbbell-shaped POP, [HNa2PdII22O12(AsO4)16]25− (Na2Pd22As16), has been isolated in the pH range of 7.5−8.0.21 In this case, the structure can be viewed as two defect, monolacunary [NaPdII11O8(AsO4)7]14− ({NaPd11As7}) fragments, each derived from the plenary Pd13As8 cube, where one PdAsO4 corner unit has been removed. In this case, the central position is occupied by NaI

3. HOST−GUEST ASSEMBLY IN POPs The multitude of POP structural motifs (vide supra) can be predominantly classified into two classes, namely, the cubeshaped Pd12 and the star-shaped Pd15 cages. Meanwhile it has been well established that the central cavity inside the Pd12 and Pd15 nanoshell hosts can accommodate not only PdII ions, but also various other guest metal ions, resulting in an expanded class of host−guest assemblies with the general formula [MzO8PdII12L8]n− (MPd12L8)15,25,26,28−31 and [MzO10PdII15L10]n− (MPd15L10),14,17,25,32 (Mz = guest metal ion, L = heterogroup), as well as a few other derivatives (see Table 1). Such POPs were synthesized by direct addition of the appropriate M salt into the PdII/L reaction mixture, where M covers a large range of elements across the periodic table from

Figure 6. (a) Ball-and-stick representation of Pd7V6 and the {Pd3V3O11} half unit. (b) Combined polyhedral/ball-and-stick illustration of the Na2Pd22As16 assembly process. Color code: Pd, blue; V (tetrahedral), magenta; V (square-pyramidal), yellow; Na, purple; O, red balls; {AsO4}, green tetrahedra; the two bridging {μ4-AsO4} groups are highlighted in orange color. 1602


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Accounts of Chemical Research Table 1. Structural Types of Guest Metal Ion-Encapsulated POPs

suggesting the structural and electronic flexibility of the POP containers. Several specific cases are addressed below. (i) As opposed to the 3d metal-centered MPd12P8 (M = MnII, FeIII, CoII, ZnII) analogues, the CuIIO8 cube in CuPd12P8 is elongated along one of the two diagonal O4 planes, reflecting the Jahn−Teller distortion of the encapsulated CuII guest ion.15 By pH adjustment (pH = 6.9−7.2), CuPd12P8 dissociates one Pd(PO4)2 edge of the cube, resulting in a monolacunary {CuPd11P6} fragment, two of which can be fused by four hydroxo-bridges to yield the double-cube [H4CuII2PdII22O16(PO4)12(OH)4]20− (Cu2Pd22P12, Figure 7),20 which is reminiscent of the aforementioned Na2Pd22As16.21 The presence of bridging hydroxyl groups, which supposedly drive the dimerization of the monolacunary {CuPd11P6} motifs, supports again the crucial role of pH in the formation of POPs. Also, the formation of lacunary derivatives of plenary POP

Group 1 to 13 with distinct ionic radii and charges. With respect to catalysis, we believe that the encapsulated metal ion guests allow for fine-tuning the catalytic activity of POPs. Detailed studies on this topic are currently underway in our laboratory. Regarding the MPd12L8 nanocube, in principle, the central metal ion guest M is coordinated by eight μ4-O atoms to form a cubic MO8 arrangement. Such a high coordination number is rather unusual, especially for 3d metal ions and has important consequences for their magnetic and spectroscopic properties.15,26,28,29,37 By contrast, in view of the larger size of the central cavity in the MPd15L10 nanostar, the guest ions NaI and especially PdII reside on the side of the pentagonal channel,14 whereas the larger BaII is located exactly at the body center.25 Detailed inspection shows that the M−O bond lengths and Pd···Pd distances vary with the encapsulated guest M, 1603


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nanostar (BaPd15(PhAs)10), disclosing an intriguing structuredirecting template effect evoked by the size of the guest cation, which determines what kind of palladium-oxo shell is formed around it. In stark contrast to the plenary structures of CaPd12(PhAs)8 and BaPd15(PhAs)10, the SrII-derivative SrPd12(PhAs)6(OAc)3 features an unprecedented open-shell type configuration. Such structural type, MPd12L6L′3, can be viewed as an intermediate between MPd12L8 and MPd15L10. It appears that the ionic radius of SrII is too large for the formation of the Pd12 shell and too small for Pd15, hence three PdII centers and four PhAsO3 heterogroups from the hypothetical SrPd15(PhAs)10 are removed, and the corresponding empty positions are occupied by acetate ligands, to yield a novel trilacunary open-shell type framework with the SrII guest being encapsulated in a lowsymmetry O9 ligand field (Figure 8). Shortly thereafter, the LaIII-incorporated analogue, LaPd12(PhAs)6(OAc)3, was obtained, which represents the second example of such open-shell prototype.26 It should be stressed that up to now all reported POPs had been isolated as hydrated alkali metal salts with good solubility in aqueous, but not organic media. Very recently it was shown that the lability of the three acetate caps on SrPd12(PhAs)6(OAc)3 allows for reversible post-synthetic substitution by longer chain carboxylates, resulting in a family of organically functionalized POPs of the formula [SrIIPdII12O6(OH)3(PhAsO3)6(CnH2n+1COO)3]4− (n = 2− 5).27 These surfactant-type POPs with a hydrophilic metaloxo core and three pending hydrophobic alkyl arms could be crystallized as mixed Na-TBA (tetra-n-butylammonium) salts, which are solution-stable not only in water, but in various organic solvents as well. This work is a door-opener for POP chemistry in organic solvents and profoundly escalates the use of POPs in homogeneous catalysis. (iii) The existence of POPs and POAs raises the question if mixed POP−POAs exist, and if other noble metals can also

Figure 7. Combined polyhedral/ball-and-stick illustration of the Cu2Pd22P12 assembly process. The long Cu−O bonds are indicated by dotted lines. Color code: Pd, blue; Cu, turquoise; O, red balls; {PO4}, fuchsia tetrahedra; the four bridging {μ2−OH} groups are highlighted in green color.

structures (albeit in situ), suggests an analogy to classical POM chemistry. (ii) In the course of investigating alkaline earth metal ions as guests in POP chemistry, a family of three structurally distinct M-centered, phenylarsonate-capped POPs, [CaIIPdII12O8(PhAsO3)8]6− (CaPd12(PhAs)8), [SrIIPdII12O6(OH)3(PhAsO3)6(OAc)3]4− (SrPd12(PhAs)6(OAc)3), and [BaIIPdII15O10(PhAsVO3)10]8− (BaPd15(PhAs)10) have been isolated (Figure 8).25 As the radius of the guest cation increases from CaII to SrII and BaII, the POP cage gradually transforms from nanocube (CaPd12(PhAs)8) via open-shell (SrPd12(PhAs)6(OAc)3) to

Figure 8. (a) Ball-and-stick representation of CaPd12(PhAs)8 (left), SrPd12(PhAs)6(OAc)3 (middle), and BaPd15(PhAs)10 (right). Color code: Pd, blue; As, green; Ca, magenta; Sr, turquoise; Ba, orange; O, red; C, black. Hydrogen atoms omitted for clarity. (b) Structural evolution of the SrPd12(PhAs)6(OAc)3 archetype from two different views. The building blocks to be removed and the newly coordinated acetate ligands are highlighted in yellow and fuchsia color, respectively. 1604


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Accounts of Chemical Research form polyoxo-noble-metalates. Attempts to introduce silver and gold as guest metals in POPs have afforded the first examples of AgI- and AuIII-containing POPs in a fully inorganic assembly. As for the former, [{Ag(H2O)3}4PdII13O8(AsO4)8]10− (Ag4Pd13) and [{Ag4.25(H2O)2}AgPdII15O10(PO4)10]14.75− (Ag5Pd15), have been isolated, featuring an unprecedented host−guest modality comprising hetero- and homometallic Ag−Pd and Ag−Ag bonding interactions (Figure 9a and b).32 The AgI guest ions in

Figure 10. Ecom of the cationic guest M encapsulated in the Pd12(PhAs)8 nanocube from monovalent to tetravalent at B3LYP (black), M06 (red), and ωB97XD (green) functional level, respectively. The Ecom for RaII, UIV, and ThIV at ωB97XD level is not included (square) due to the unavailable van der Waals radius for these elements. Adapted with permission from ref 26. Copyright 2017 Royal Society of Chemistry.

higher affinity of Pd12 for the respective guest metal ion. Setting the Ecom of PdII (−54.6 kcal·mol−1) as a reference, the values below the reference manifest that from a thermodynamic point of view, the respective metal ion guests are more favorable in stabilizing the Pd12 host than PdII. It is exciting to note that the theoretical data is perfectly consistent with the experimental results obtained hitherto. In particular, (i) compared to the parent PdII, the larger SrII ion is incorporated into the openshell instead of the cubic shell in that the former ensemble (Ecom = −50.0 kcal·mol−1) is slightly more exothermic than the latter (Ecom = −45.4 kcal·mol−1); (ii) combined with the consideration of size matching, the less negative Ecom for AgI (−29.7 kcal·mol−1) and BaII (−11.9 kcal·mol−1) indicates that the star-shaped Pd15 cage suits such large guests better; (iii) there is a clear trend of increasing encapsulation ability of the Pd12 cage going down Group IIIA, which is in full agreement with the preferential uptake of InIII (Ecom = −137.1 kcal·mol−1) rather than GaIII (Ecom = −115.8 kcal·mol−1) in the abovementioned competition studies. Also, a few potential tetravalent guest candidates (e.g., SnIV, ZrIV, UIV) for encapsulation inside the Pd 12 shell have been predicted. Guided by the computations, some of these POPs have been isolated very recently and will be published in the near future. Besides the template effect of the guest metal M, the role of the heterogroups must also be considered. Comparing the phosphate- and phosphonate-capped nanocubes MPd12P8 and MPd12(PhP)8 to the arsenate- and arsonate analogues MPd12As8 and MPd12(PhAs)8, it is evident that the shorter P−O(Pd) bonds (ca. 1.5 Å) compared to the longer As− O(Pd) bonds (ca. 1.7 Å) result in smaller outer dimensions of the respective cuboid MPd12P8 shell (compared to MPd12As8), and also giving rise to a smaller central cavity for the former. Consequently, the smaller cavity in MPd12P8 can only accommodate first-row transition metals, but not the larger lanthanide and PdII ions.15,28,29

Figure 9. Ball-and-stick perspective views of Ag4Pd13 (a), Ag5Pd15 (b), and the two lowest energy positional isomers for Au4Pd8As8 (c). Color code: Pd, blue; As, green; P, fuchsia; Ag, orange; Au, yellow; Na, purple; O, red; Ag−Pd interaction, yellow bond; Ag−Ag interaction, green bond.

Ag5Pd15 are stabilized not only in the central channel by complexation with the internal oxo groups, but also externally grafted onto the surface of the POP cluster via metal−metal bonding exerted by cation confinement. In the case of AuIII, it was demonstrated that it is indeed possible to merge POPs and POAs into one polyanion, [NaAu III 4 Pd II 8 O 8 (AsO 4 ) 8 ] 11− (Au4Pd8As8), of which the structure and composition has been ascertained by both experimental and computational techniques (Figure 9c).13 These mixed-noble metalates could be considered as molecular precursors for composite (core− shell) noble metal nanoparticles with uniform size and welldefined composition. (iv) To further extend our research on M-centered POPs by using p-block elements as guests, two [MzO8PdII12(PhAsO3)8]5− (Mz = GaIII, GaPd12(PhAs)8; Mz = InIII, InPd12(PhAs)8) archetypal polyanions have been designed and then prepared.26 Interestingly, an unusual feature of selective entrapment of main Group IIIA elements within the phenylarsonate-capped Pd12 cage has been discovered. If equimolar quantities of GaIII and InIII ions are both present in the same reaction system, then only InPd12(PhAs)8 is formed, as based on a multitude of physicochemical techniques as well as computational methods (vide infra), thereby confirming the thermodynamic control of the host−guest formation mechanism in MPd12(PhAs)8. It is worth mentioning that systematic theoretical analysis has always been performed to unveil the basic principles underlying POP chemistry, especially for the host−guest assemblies.13,19,21,25,26,28,29,32 From an energetic point of view, the simulated complexation energies (Ecom) of the guest Mn+ (n = 1−4) could nicely predict whether a certain metal ion can be entrapped in the cuboid Pd12 cage or not.26 As shown in Figure 10, the more negative values of Ecom (exothermic) indicate a

4. CONCLUSIONS Noble metal-based nanomaterials, in particular nanosized palladium oxides, are of fundamental importance for boosting catalytic activities for a large number of industrial chemical tran sformations. The breakth roug h d isco very of polyoxopalladates(II) (POPs) opens a new era as such molecular palladium-oxide nanoclusters are ideal models to 1605


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for the development of rational synthetic procedures, thereby aiding the transition from discovery to design of POPs with desired applications in fields of current interest.

shed light on the mechanism of noble metal-based catalysis. The synthesis of POPs is usually accomplished in facile one-pot reactions by controlled hydrolysis and condensation of aquapalladate(II) precursors in the presence of heterogroups. The self-assembly of discrete palladium(II)-oxo clusters can be orchestrated by a set of important reaction parameters such as pH, temperature, and ratio/concentration of reagents, but equally important are the proper selection of the guest metal ion and the type of heterogroup. To date, more than 70 POPs have been isolated covering a range of structural types from compact, cagelike cube, star, dumbbell, and open-shell-shaped matrix suitable for the uptake of cationic guests, to open bowland giant wheel-like motifs. From a structural point of view, POPs exhibit some principles of conventional POMs, promising that the former may showcase a chemistry as rich and diverse as the latter: (i) the similar addenda arrangement within the Keggin XM12 and Kortz MPd12 ions; (ii) the extreme pH sensitivity allowing to remove fragments to form lacunary (e.g., {XM11} vs {CuPd11}) moieties; (iii) the central position in both Keggin and Kortz ions can be occupied by a wide range of elements across the periodic table. Finally, the distinct square-planar PdO4 addenda units in POPs allow for the formation of unprecedented structures that are impossible to prepare using traditional MO6 octahedral synthons. The reported studies on POPs have mainly focused on solidstate structures and in-silico modeling, but also on the solution behavior. It has been demonstrated experimentally that the structures of most POPs remain intact in aqueous solution (over a large pH range), as well as in organic media, which is an excellent basis for post-synthetic modification, homo- and heterogeneous catalysis, or as precursors for monodisperse nanoparticles via an elegant bottom-up approach. Indeed, as ecofriendly catalysts, the parent species Pd13As8 and Pd15P10 have shown exceptional stability and selectivity in homogeneous alcohol oxidation,10,14 whereas FePd12P8 and CuPd12P8 have been recognized to be precatalysts for olefin hydrogenation.15 Some of the water-stable POPs have also been successfully used to fabricate Pd0-modified electrodes via electrochemical reduction, where the deposition process exhibits faster electrochemical kinetic behaviors, allowing to use such films in order to improve the kinetics of electrocatalytic processes, such as hydrazine oxidation and dioxygen reduction.10,14−16,20,28,29 A range of other highlights have also been documented: (i) the bonding of lanthanide ions in LnPd12(PhAs)8 involves a rare case of LnIII in a cubic ligand field. Magnetic studies performed on such systems revealed a unique behavior, which makes them ideal candidates for molecular spin qubits;38 (ii) as structurally well-defined polyanions of tunable size and charge, POPs have been applied as biological models to obtain deeper insight into the formation of ion-pairs and hydration shells in solution,39 as well as supramolecular self-aggregation properties;27,40 (iii) very recently, the first example of a MOF constructed by POPs has been discovered, creating the area of POP-MOFs, and such materials possess exciting structural and physisorption properties.41 It can be foreseen that, apart from the seminal achievements received so far, precise engineering of POPs is nonetheless far from reach, and hence we are still a long way off from completely understanding the elusive formation mechanisms in POP chemistry. The complementary combination of experimental and computational techniques will open fruitful vistas

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. ORCID

Ulrich Kortz: 0000-0002-5472-3058 Notes

The authors declare no competing financial interest. Biographies Peng Yang received his Ph.D. in Chemistry at Jacobs University Bremen (Germany) in 2016 under the supervision of Ulrich Kortz. Currently he is a postdoctoral fellow at King Abdullah University of Science & Technology (KAUST) in Saudi Arabia, performing research focused on the design and synthesis of crystalline porous materials. Ulrich Kortz received his Ph.D. in Chemistry at Georgetown University (Washington, DC) in 1995 under the supervision of Michael T. Pope. After postdoctoral studies in Florence (Italy) and Versailles (France) he started his independent academic career in 1997 at the American University of Beirut (Lebanon). In 2002, he joined International University Bremen (now Jacobs University Bremen) in Germany. His research interests include synthetic inorganic and organometallic chemistry, structural inorganic chemistry, polyoxometalates, MOFs, catalysis, magnetism, and electrochemistry.

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ACKNOWLEDGMENTS U.K. thanks the German Research Council (DFG, KO-2288/ 26-1, KO-2288/20-1, KO-2288/16-1), Jacobs University, and CMST COST Action CM1203 (PoCheMoN) for support. P.Y. sincerely acknowledges CSC (China Scholarship Council) for a doctoral fellowship. We thank Dr. Saurav Bhattacharya for proof-reading the manuscript and galley proofs.

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DEDICATION Dedicated to Professor Michael T. Pope on the occasion of his 85th birthday.

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REFERENCES

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Accounts of Chemical Research (41) Bhattacharya, S.; Ayass, W. W.; Taffa, D. H.; Schneemann, A.; Semrau, A. L.; Wannapaiboon, S.; Altmann, P. J.; Pöthig, A.; Nisar, T.; Balster, T.; Wagner, V.; Fischer, R. A.; Wark, M.; Kortz, U. Synthesis of Porous Polyoxopalladate-Based MOFs and Use for Efficient Heterogeneous Suzuki-Miyaura Coupling Reactions. Submitted for publication.

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Discovery and Evolution of Polyoxopalladates - Accounts of Chemical

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