Alkaline earth guests in polyoxopalladate chemistry: from nanocube to nanostar via an open-shell structure

Discrete Platinum(II/IV)-Arsenito Clusters with Pt-As and Pt-O Bonding: [PtIV(As3O6)2]2-, [Pt4II(H2AsO3)6(HAsO3)2]2-, and [Pt2IIAs6W4O28]10

The first two discrete, fully inorganic platinum(II/IV)-arsenito clusters, [fac-PtIV(As3O6)2]2- (PtAs6) and [Pt4II(H2AsO3)6(HAsO3)2]2- (Pt4As8), as well as the platinum(II)-arsenito heteropolytungstate [Pt2IIAs6W4O28]10- (Pt2As6W4), have been synthesized in aqueous media using simple one-pot reaction conditions. In PtAs6, a PtIV ion is coordinated to two cyclic, tridentate As3O6 units via oxo-donation (PtIV-O ∼ 2.02 Å). In Pt4As8, each PtII ion is coordinated to four AsO3 ligands via two oxygens and two AsIII atoms in a square-planar fashion (PtII-AsIII 2.31 Å, PtII-O 2.07 Å), resulting in an open cage-like structure, which forms a strong tetrameric assembly in the solid state mediated by two K+ counterions. In Pt2As6W4, each PtII ion is coordinated by the As atoms of three AsO3 ligands (PtII-AsIII 2.38 Å) and an oxo group (PtII-O 2.07 Å) in addition to bridging two tungsten ions, and this polyanion was characterized in solution by 195Pt NMR.

Binary Heterogroup-Templated Scaffolds of Polyoxopalladates as Precatalysts for Plasma-Assisted Ammonia Synthesis

In addition to improving the synthetic efficiency, the template method can do a lot more in the chemistry of polyoxopalladates (POPs), such as the establishment of novel metal-oxo scaffolds. In this endeavor, a binary system comprising heterogroups of nonmetallic {As/SiO4} and metallic {VO4/5} successfully fulfills the templated growth of two POPs with unprecedented seesaw- and spindle-like prototypes. Of these, self-aggregation of heterogroups beacons an effective route to break the highly symmetrical PdII-oxo matrix and to force the arrangement of addenda in a nonconventional manner. Aside from the interest in their structural features, the as-made POPs are available for immobilization on the mesoporous SBA-15 as precatalysts for ammonia synthesis. The outer cover of heterogroups in the POP precursors contributes to the ultrafine size and uniform distribution of derived Pd0 nanoparticles (PdNPs). With the help of plasma activation on H2 and N2, such PdNPs-SBA15 catalysts significantly improve the production performance of NH3, showcasing the maximum synthesis rate of 64.42 μmol/(min·gcat) with the corresponding energy yield as high as 4.38 g-NH3/kWh.

Gallium(III)- and Thallium(III)-Encapsulated Polyoxopalladates: Synthesis, Structure, Multinuclear NMR, and Biological Activity Studies

Three gallium(III)- and thallium(III)-containing polyoxopalladates (POPs) have been synthesized and structurally characterized in the solid state and in solution, namely, the phosphate-capped 12-palladate nanocubes [XPd12O8(PO4)8]13- (X = GaIII, GaPd12P8; X = TlIII, TlPd12P8) and the 23-palladate double-cube [Tl2IIIPd23P14O70(OH)2]20- (Tl2Pd23P14). The cuboid POPs, GaPd12P8 and TlPd12P8, are solution stable as verified by the respective 31P, 71Ga, and 205Tl nuclear magnetic resonance (NMR) spectra. Of prime interest, the spin-spin coupling schemes allowed for an intimate study of the solution behavior of the TlIII-containing POPs via a combination of 31P and 205Tl NMR, including the stoichiometry of the major fragments of Tl2Pd23P14. Moreover, biological studies demonstrated the antitumor and antiviral activity of GaPd12P8 and TlPd12P8, which were validated to be as efficient as cis-platinum against human melanoma and acute promyelocytic leukemia cells. Furthermore, GaPd12P8 and TlPd12P8 exerted inhibitory activity against two herpetic viruses, HSV-2 and HCMV, in a dose-response manner.

PtIV-Containing Hexaplatinate(II) [PtIVPtII6O6(AsO2(CH3)2)6]2- and Hexapalladate(II) [PtIVPdII6O6(AsO2(CH3)2)6]2

The first PtIV-containing discrete polyoxoplatinate(II) [PtIVPtII6O6(AsO2(CH3)2)6]2- (Pt7) and polyoxopalladate(II) [PtIVPdII6O6(AsO2(CH3)2)6]2- (PtPd6) have been prepared and characterized in the solid state, in solution, and in the gas phase. The molecular structures of the noble metal-oxo clusters Pt7 and PtPd6 comprise a central, octahedral PtIVO6 hetero group surrounded by six square-planar MO4 (M = PtII, PdII) units, which are capped by six dimethylarsinate ligands. The polyanions were prepared under simple one-pot aqueous solution conditions by reacting H2Pt(OH)6 with either K2PtCl4 or Pd(NO3)2 in sodium dimethylarsinate buffer (pH 7) at 80 °C. Catalytic studies were performed on Pt7 supported on SBA15-apts for o-xylene hydrogenation at 300 °C and 90 bar H2 pressure and indicated excellent activity and recyclability with low activation temperature.

Mixed-valent palladium(IV/II)-oxoanion, [PdIVO6PdII6((CH3)2AsO2)6]2

We report on the first example of a Pd^IV-containing polyoxopalladate(II). The discrete mixed-valent polyoxopalladate(IV/II), [Pd^IVPdII6O₆((CH₃)₂AsO₂)₆]^2-, comprising a central Pd^IV ion that is surrounded by a six-membered Pd^II-oxo ring capped by six dimethylarsinate groups, was synthesized and structurally characterized in the solid state, in solution and in the gas phase by multiple analytical techniques.

Discovery of a Neutral 40-PdII-Oxo Molecular Disk, [Pd40O24(OH)16{(CH3)2AsO2}16]: Synthesis, Structural Characterization, and Catalytic Studies

We report on the synthesis and structural characterization of a giant, discrete, and neutral molecular disk, [Pd₄₀O₂₄(OH)₁₆{(CH₃)₂AsO₂}₁₆] (Pd₄₀), comprising a 40-palladium-oxo core that is capped by 16 dimethylarsinate moieties, resulting in a palladium-oxo cluster (POC) with a diameter of ∼2 nm. Pd₄₀, which is the largest known neutral Pd-based oxo cluster, can be isolated either as a discrete species or constituting a 3D H-bonded organic-inorganic framework (HOIF) with a 12-tungstate Keggin ion, [SiW₁₂O₄₀]^4- or [GeW₁₂O₄₀]^4-. ¹H and ¹³C NMR as well as ¹H-DOSY NMR studies indicate that Pd₄₀ is stable in aqueous solution, which is also confirmed by ESI-MS studies. Pd₄₀ was also immobilized on a mesoporous support (SBA15) followed by the generation of size-controlled Pd nanoparticles (diameter ∼2-6 nm, as based on HR-TEM), leading to an effective heterogeneous hydrogenation catalyst for the transformation of various arenes to saturated carbocycles.

Polyoxoplatinates as covalently dynamic electron sponges and molecular electronics materials

In organic systems, dynamic covalent chemistry provides an adaptive approach (i.e., "covalent dynamics") where thermodynamic equilibria are used to tailor structural and electronic changes in molecular assemblies. The covalent dynamics finds utility in the design of novel self-healing materials, sensors, and actuators. Herein, using density functional theory (DFT) we explore the structural, electronic and transport properties of the Pt-based polyoxometalate (POM) [Pt^III ₁₂O₈(SO₄)₁₂]^4- and its derivatives. The latter POM has six redox responsive {O-Pt-Pt-O} moieties and prospects for storage of up to twelve electrons, thus exemplifying how dynamic covalent chemistry may manifest itself in fully inorganic systems. Simulations of the Au/POM/Au junction show that the electron conduction strongly depends on the redox of the POM but more weakly on its rotations with respect to the Au surface. Moreover, the POM shows promising spin-polarized current behaviour, which can be modulated using bias and gate voltages.

Selected polyoxopalladates as promising and selective antitumor drug candidates

Polyoxo-noble-metalates (PONMs), a class of molecular noble metal-oxo nanoclusters that combine features of both polyoxometalates and noble metals, are a promising platform for the development of next-generation antitumor metallodrugs. This study aimed to evaluate the antitumor potential against human neuroblastoma cells (SH-SY5Y), as well as toxicity towards healthy human peripheral blood cells (HPBCs), of five polyoxopalladates(II): (Na₈[Pd₁₃As₈O₃₄(OH)₆]·42H₂O (Pd₁₃), Na₄[SrPd₁₂O₆(OH)₃(PhAsO₃)₆(OAc)₃]·2NaOAc·32H₂O (SrPd₁₂), Na₆[Pd₁₃(AsPh)₈O₃₂]·23H₂O (Pd₁₃L), Na₁₂[SnO₈Pd₁₂(PO₄)₈]·43H₂O (SnPd₁₂), and Na₁₂[PbO₈Pd₁₂(PO₄)₈]·38H₂O (PbPd₁₂)), as the largest subset of PONMs. A pure inorganic, Pd₁₃, was found as the most potent and selective antineuroblastoma agent with IC₅₀ values (µM) of 7.2 ± 2.2 and 4.4 ± 1.2 for 24 and 48 h treatment, respectively, even lower than cisplatin (28.4 ± 7.4 and 11.6 ± 0.8). The obtained IC₅₀ values (µM) for 24/48 h treatment with SrPd₁₂ and Pd₁₃L were 75.8 ± 6.7/76.7 ± 22.9 and 63.8 ± 3.6/21.4 ± 10.8, respectively, whereas SnPd₁₂ and PbPd₁₂ did not remarkably affect the SH-SY5Y viability (IC₅₀ > > 100 µM). Pd₁₃ caused depolarisation of inner mitochondrial membrane prior to superoxide ion hyperproduction, followed by caspase activation, DNA fragmentation and cell cycle arrest, all hallmarks of apoptotic cell death, and accompanied by an increase in acidic vesicles content, suggestive of autophagy induction. Importantly, Pd₁₃ demonstrated the antitumor effect at concentrations not cytogenotoxic for normal HPBCs. On the contrary, SrPd₁₂ and Pd₁₃L at concentrations ≥ 1/3 IC₅₀ (24 h) decreased HPBC viability and increased % tail DNA up to 42% and 3.05 times, respectively, related to control. SnPd₁₂ and PbPd_12, previously confirmed promising antileukemic agents, did not exhibit cytogenotoxicity to HPBCs, and thus could be regarded as tumor cell specific and selective drug candidates.

Discovery and Supramolecular Interactions of Neutral Palladium-Oxo Clusters Pd16 and Pd24

We report on the synthesis, structure, and physicochemical characterization of the first three examples of neutral palladium-oxo clusters (POCs). The 16-palladium(II)-oxo cluster [Pd16 O24 (OH)8 ((CH3 )2 As)8 ] (Pd16 ) comprises a cyclic palladium-oxo unit capped by eight dimethylarsinate groups. The chloro-derivative [Pd16 Na2 O26 (OH)3  Cl3  ((CH3 )2  As)8 ] (Pd16 Cl) was also prepared, which forms a highly stable 3D supramolecular lattice via strong intermolecular interactions. The 24-palladium(II)-oxo cluster [Pd24 O44 (OH)8 ((CH3 )2 As)16 ] (Pd24 ) can be considered as a bicapped derivative of Pd16 with a tetra-palladium-oxo unit grafted on either side. The three compounds were fully characterized 1) in the solid state by single-crystal and powder XRD, IR, TGA, and solid-state 1 H and 13 C NMR spectroscopy, 2) in solution by 1 H, 13 C NMR and 1 H DOSY spectroscopic methods, and 3) in the gas phase by electrospray ionization mass spectrometry (ESI-MS).

Shape and Size Tuning of BiIII-Centered Polyoxopalladates: High Resolution 209Bi NMR and 205/206Bi Radiolabeling for Potential Pharmaceutical Applications

We have discovered five bismuth(III)-containing polyoxopalladates (POPs) which were fully characterized by solution and solid-state physicochemical techniques: the cube-shaped [BiPd₁₂O₃₂(AsPh)₈]^5- (BiPd₁₂AsL), [BiPd₁₂O₃₂(AsC₆H₄N₃)₈]^5- (BiPd₁₂AsL_N), and [BiPd₁₂O₃₂(AsC₆H₄COO)₈]^13- (BiPd₁₂AsL_C) as well as the star-shaped [BiPd₁₅O₄₀(PO)₁₀H₆]^11- (BiPd₁₅P) and [BiPd₁₅O₄₀(PPh)₁₀]^7- (BiPd₁₅PL), respectively. The organically modified capping groups phenylarsonate, p-azidophenylarsonate, and p-carboxyphenylarsonate were chosen as the azido (-N₃) and carboxyl (-COOH) groups open up opportunities to covalently conjugate (via click reaction, amide coupling, etc.) with targeting vectors. The synthesis of p-azidophenylarsonate is reported here for the first time. The effects of the Bi^III template and the organoarsonate vs -posphonate capping groups on the resulting POP shape (cube vs star) are discussed. The ²⁰⁹Bi NMR (I = 9/2) spectra of BiPd₁₂AsL, BiPd₁₂AsL_N, and BiPd₁₂AsL_C revealed narrow peaks (ν_1/2 ∼ 200 Hz) at 5470 ppm with a longitudinal relaxation time in the millisecond range (at 8.46 T). The absence of a quadrupolar relaxation contribution could be attributed to the allocation of Bi^III in the highly symmetrical cuboid POP host cage. Similar peaks were absent in the ²⁰⁹Bi-NMR spectra of the star-shaped POPs BiPd₁₅P and BiPd₁₅PL due to the less symmetric coordination environment around the central Bi^III ion. Further, ^205/206Bi-radiolabeled POPs have been synthesized by incorporating a ^205/206Bi^III ion in the center of the POP structures. Carrier-free ^205/206Bi radioisotopes (as surrogates of α-emitting ²¹³Bi) were incorporated into the POP host-cage for the preparation of ^205/206BiPd₁₂AsL, ^205/206BiPd₁₂AsL_N, ^205/206BiPd₁₂AsL_C, and ^205/206BiPd₁₅PL, respectively. The radiometal incorporation was complete (>99% radiochemical yield) in 10 min according to radio-thin-layer chromatography. The ^205/206BiPd₁₂AsL polyanion was purified by solid-phase extraction. The incubation in rat serum showed the formation of a ^205/206BiPd₁₂AsL-protein aggregate.

Co-ion Effects in the Self-Assembly of Macroions: From Co-ions to Co-macroions and to the Unique Feature of Self-Recognition

Macroions, as soluble ions with a size on the nanometer scale, show unique solution behavior different from those of simple ions and large colloidal suspensions. In macroionic solutions, the counterions are known to be important and well-explored. However, the role of co-ions (ions carrying the same type of charge as the macroions) is often ignored. Here, through experimental and simulation studies, we demonstrate the role of co-ions as a function of co-ion size on their interaction with the macroions (using {Mo₇₂Fe₃₀} and {SrPd₁₂} as models) and the related self-assembly into blackberry-type structures in dilute solutions. Several regimes of unique co-ion effects are clearly identified: small ions (halides, oxoacid ions), subnanometer-scaled bulky ions (lacunary Keggin and dodecaborate ions), and those with sizes comparable to the macroions. Small co-ions have no observable effect on the self-assembly of fully hydrophilic {Mo₇₂Fe₃₀}, while due to hydrophobic interaction and intermolecular hydrogen bonds, the small co-ions show influences on the self-assembly of hydrophobic {SrPd₁₂}. Subnanometer ions, a.k.a. "superchaotropic ions", are still too small to assemble into a blackberry by themselves, but they can coassemble with the macroions, showing a strong interaction with the macroionic system. When the co-ion size is comparable to that of the macroions, they assemble independently instead of assembling with the macroions, leading to the previously reported unique self-recognition phenomenon for macroions.

Indium in Polyoxopalladate(II) Chemistry: Synthesis of All-Acetate-Capped [InPd12O8(OAc)16]5- and Controlled Transformation to Phosphate-Capped Double-Cube and Monocube

We have prepared the indium(III)-centered, all-acetate-capped polyoxopalladate(II) nanocube [InPd₁₂O₈(OAc)₁₆]^5- (InPd₁₂Ac₁₆), which can be further used as precursor to form the phosphate-capped (i) double-cube [In₂Pd₂₃O₁₇(OH)(PO₄)₁₂(PO₃OH)]^21- (In₂Pd₂₃P₁₃) and (ii) monocube [InPd₁₂O₈(PO₄)₈]^13- (InPd₁₂P₈). All three novel polyoxopalladates (POPs) were synthesized using conventional one-pot techniques in aqueous solution and characterized in the solid state (single-crystal XRD, IR, elemental analysis), in solution (¹¹⁵In, ³¹P, and ¹³C NMR), and in the gas phase (ESI-MS).

Frontiers and progress in cation-uptake and exchange chemistry of polyoxometalate-based compounds

Cation-uptake and exchange has been an important topic in both basic and applied chemistry relevant to life and materials science. For example, living cells contain appreciable amounts of Na+ and K+, and their concentrations are regulated by the sodium-potassium pump. Solid-state cation-exchangers such as clays and zeolites both natural and synthetic have been used widely in water softening and purification, separation of metal ions and biomolecules, etc. Polyoxometalates (POMs) are robust, discrete, and structurally well-defined metal-oxide cluster anions, and have stimulated research in broad fields of sciences. In this perspective, cation-uptake and exchange in POM and POM-based compounds are categorized and reviewed in three groups: (i) POMs as inorganic crown ethers and cryptands, (ii) POM-based ionic solids as cation-exchangers, and (iii) reduction-induced cation-uptake in POM-based ionic solids, which is based on a feature of POMs that they are redox-active and multi-electron transfer occurs reversibly in multiple steps. This method can be utilized to synthesize mixed-valence metal clusters in metal ion-exchanged POM-based ionic solids.

Tetravalent Metal Ion Guests in Polyoxopalladate Chemistry: Synthesis and Anticancer Activity of [MO8Pd12(PO4)8]12- (M = SnIV, PbIV)

The first two examples of polyoxopalladates(II) (POPs) containing tetravalent metal ion guests, [MO₈Pd₁₂(PO₄)₈]^12- (M = Sn^IV, Pb^IV), have been prepared and structurally characterized in the solid state, solution, and gas phase. The interactions of the metal ion guests and the palladium-oxo shell were studied by theoretical calculations. The POPs were shown to possess anticancer activity by causing oxidative stress inducing caspase activation and consecutive apoptosis of leukemic cells.

A 224Ra-labeled polyoxopalladate as a putative radiopharmaceutical

Despite their attractive properties, internal targeted alpha therapies using 223/224Ra are limited to bone-seeking applications. As there is no suitable chelator available, the search for new carriers to stably bind Ra2+ and to connect it to biological target molecules is necessary. Polyoxopalladates represent a class of compounds where Ra2+ can be easily introduced into the Pd-POM core during a facile one-pot preparation. Due to the formation of a protein corona, the connection to other targeting (bio)macromolecules is possible.

Discrete Polyoxopalladates as Molecular Precursors for Supported Palladium Metal Nanoparticles as Hydrogenation Catalysts

We have used discrete polyoxopalladates(II) (POPs) of the MPd₁₂X₈ nanocube- and Pd₁₅X₁₀ nanostar-types (M = central metal ion, X = capping group) as molecular precursors (diameter ca. 1 nm) for the formation of supported (SBA-15) metallic nanoparticles. These materials proved to be highly active in the hydrogenation of o-xylene. The characterization of such hydrogenation catalysts revealed that the average size of the resulting alloy particles is quite uniform with diameters ranging from 1 to 3 nm (indicating little to no agglomeration). The central transition-metal ion M ⁿ⁺ (Mn^II, Fe^III, Co^II, Ni^II, Cu^II, Zn^II, Pd^II) in the POP structure and also the nature of the capping group (AsO₄^3-, SeO₃^2-, PO₄^3-, phenyl-AsO₃^2-) influence the resulting catalytic performance.

Polyoxometalate-Cyclodextrin Metal-Organic Frameworks: From Tunable Structure to Customized Storage Functionality

Self-assembly allows structures to organize themselves into regular patterns by using local forces to find the lowest-energy configuration. However, assembling organic and inorganic building blocks in an ordered framework remains challenging due to  difficulties in rationally interfacing two dissimilar materials. Herein, the ensemble of polyoxometalates (POMs) and cyclodextrins (CDs) as molecular building blocks (MBBs) has yielded two unprecedented POM-CD-MOFs, namely [PW₁₂O₄₀]^3- and α-CD MOF (POT-CD) as well as [P₁₀Pd_15.5O₅₀]^19- and γ-CD MOF (POP-CD), with distinct properties not shared by their isolated parent MBBs. Markedly, the POT-CD features a nontraditional enhanced Li storage behavior by virtue of a unique "amorphization and pulverization" process. This opens the door to a new generation of hybrid materials with tuned structures and customized functionalities.

Host-Guest-Induced Environment Tuning of 3d Ions in a Polyoxopalladate Matrix

A series of unprecedented supramolecular associates of phenylarsonate-capped {M^II Pd^II ₁₂ O₈ }-type (M=Co, Ni and Zn) polyoxopalladates with α-cyclodextrins (α-CD) was obtained and characterized in the solid state (single-crystal X-ray diffraction (XRD), FT-IR spectroscopy, elemental and thermogravimetric (TGA) analyses), in aqueous solution (¹ H and ¹³ C NMR) and in the gas phase (ESI-MS). The non-covalent host-guest interactions between the organopolyoxoanions and α-CD rings alter the O₈ coordination environment of a 3d transition metal ion (M^II ) situated at the center of a cuboid polyoxododecapalladate shell. This synthetically controlled "chemical pressure" effectively induces axial distortion of the otherwise cubic polyoxopalladate environment between two trans-positioned α-CD moieties. Its effect on the magnetic properties and the electronic structure of the Co^II derivative was assessed in a combined SQUID magnetometry, EPR, X-ray magnetic circular/linear dichroism (XMCD/XMLD), and X-ray absorption near-edge structure (XANES) spectroscopy study.

Discovery and Evolution of Polyoxopalladates

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 sub-nanometer 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 Pd^II 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 Pd^IIO₄ units in the presence of external oxyacid heterogroups (e.g., AsO₄^3-, PO₄^3-, and SeO₃^2-) drive the self-assembly of such discrete, polynuclear Pd^II-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 structure-directing template effects induced by the guest ions on the resultant host-guest assemblies.

Rational Design of Organically Functionalized Polyoxopalladates and Their Supramolecular Properties

The Sr^II -centered 12-palladate(II) open-cube {SrPd₁₂ (OAc)₃ } has been systematically evolved by substitution of the three acetate ligands by a library of saturated carboxylic acids with increasing chain lengths leading to four novel polyoxopalladates(II) with the formula [SrPd₁₂ O₆ (OH)₃ (PhAsO₃ )₆ (L)₃ ]^4- (SrPd₁₂ L₃ , L=C_n H_2n+1 COO, n=2 to 5). These first examples of surfactant-type polyoxopalladates with a hydrophilic metal-oxo unit and three hydrophobic alkyl chains were characterized in the solid state (single-crystal XRD, FTIR, TGA), in solution (¹ H, ¹³ C NMR spectroscopy), and in the gas phase (ESI-MS). The two polyanions SrPd₁₂ L₃ with chain lengths of 5 and 6 are the first examples of polyoxopalladates that are soluble and stable in organic media. The Na salts of the amphiphilic polyoxopalladates SrPd₁₂ L₃ were shown to self-assemble into "blackberry"-type spherical supramolecular structures in dilute solutions, of which an unusual "volcano"-shaped trend of assembly size versus solvent polarity is chiefly influenced by directional hydrogen bonding interactions.

Size and charge effect of guest cations in the formation of polyoxopalladates: a theoretical and experimental study

The development of rational synthetic procedures with desired nuclearity and high selectivity is a critical issue in inorganic chemistry. Here we demonstrate a comprehensive understanding of the template effect induced by metal cations in the formation mechanism of the class of polyoxopalladates ({MPd12L8} nanocube and {MPd15L10} nanostar) by combining computational and experimental techniques. The capture of a M n+ guest ion by a peripheral palladium(ii)-oxo shell leads to a competition between the parent Pd2+ addenda ion and the respective guest metal ion. The present study reveals that (i) the selection of the incorporated guest ion has a thermodynamic control, (ii) the main factors governing the formation of a particular polyanion are the charge and size of the guest cation, (iii) the electrostatic interaction between the cation and the surrounding oxo ligands and (iv) the dehydration ability of the cation. As expected from the number of observed {M n+Pd12L8} species, trivalent cations M3+ were found to be good templates resulting in several examples of {M3+Pd12L8}, whereas monovalent cations M+ are much less prone to form {M+Pd12L8}. For tetravalent cations the dehydration energies are very large, however, the formation of {M4+Pd12L8} nanocubes is found to be still energetic favourable. Fully consistent with computational predictions, four novel polyoxo-12-palladates were synthesized: the La3+-centered nanocube [LaPd12O8(PhAsO3)8]5- (LaPd12-closed), the La3+-centered "open" nanocube [LaPd12O6(OH)3(PhAsO3)6(OAc)3]3- (LaPd12-open), the Ga3+-centered [GaPd12O8(PhAsO3)8]5- (GaPd12 ), and the In3+-analogue [InPd12O8(PhAsO3)8]5- (InPd12 ). All four compounds were fully characterized in the solid state and in solution by a multitude of physicochemical techniques, including 71Ga and 115In NMR as well as mass spectrometry. The experimentally observed selective incorporation of only In3+ ions in the presence of Ga3+ and In3+ confirmed the thermodynamic control of the formation mechanism, which we had predicted by theory.

Low-cost p-type dye-sensitized solar cells based on Dawson-type transition metal-substituted polyoxometalate inorganic co-sensitizers

α₂-K₈P₂W₁₇O₆₁(Co²⁺·OH₂)·16H₂O(P₂W₁₇Co) and α₂-K₇P₂W₁₇O₆₁(Mn³⁺·OH₂)·12H₂O(P₂W₁₇Mn) are employed to construct a new inorganic co-sensitizer.

Classical/Non-classical Polyoxometalate Hybrids

Two polyanions [Se^I V₂ Pd^II₄ W^VI₁₄ O₅₆ H]^11- and [Se^I V₄ Pd^II₄ W^VI₂₈ O₁₀₈ H₁₂ ]^12- are the first hybrid polyoxometalates in which classical (Group 5/6 metal based) and non-classical (late transition-metal based) polyoxometalate units are joined. Requiring no supporting groups, this co-condensation of polyoxotungstate and isopolyoxopalladate constituents also provides a logical link between POM-Pd^II coordination complexes and the young subclass of polyoxopalladates. Solid-state, solution, and gas-phase studies suggest interesting specific reactivities for these hybrids and point to several potential derivatives and functionalization strategies.

Overcoming the Crystallization Bottleneck: A Family of Gigantic Inorganic {Pdx }(L) (x=84, 72) Palladium Macrocycles Discovered using Solution Techniques

The {Pd₈₄}^Ac wheel, initially discovered serendipitously, is the only reported giant palladium macrocycle—a unique structure that spontaneously assembles from small building blocks. Analogues of this structure are elusive. A new modular route to {Pd₈₄}^Ac is described, allowing incorporation of other ligands, and a new screening approach to cluster discovery. Structural assignments were made of new species from solution experiments, overcoming the need for crystallographic analysis. As a result, two new palladium macrocycles were discovered: a structural analogue of the existing {Pd₈₄}^Ac wheel with glycolate ligands, {Pd₈₄}^Gly, and the next in a magic number series for this cluster family—a new {Pd₇₂}^Prop wheel decorated with propionate ligands. These findings confirm predictions of a magic number rule for the family of {Pd_x} macrocycles. Furthermore, structures with variable fractions of functional ligands were obtained. Together these discoveries establish palladium clusters as a new class of tunable nanostructures. In facilitating the discovery of species that would not have been discovered by orthodox crystallization approaches, this work also demonstrates the value of solution‐based screening and characterization in cluster chemistry, as a means to decouple cluster formation, discovery, and isolation.

Chiral Dodecanuclear Palladium(II) Thio Cluster: Synthesis, Structure, and Formation Mechanism Explored by ESI-MS and DFT Calculations

The chiral dodecanuclear palladium(II) thio cluster LaPd12(C3H5NO2S)3(C3H6NO2S)21 (1) was prepared by reacting l-cysteine (l-Cys) with PdCl2 and La2O3 in aqueous solution under carefully controlled conditions. Compound 1 was structurally characterized by single-crystal XRD, TGA, IR, UV-vis, (13)C NMR, and CD spectroscopy. Insight into the dimerization process of 1 was obtained by ESI-MS and DFT calculations.

Complex solution chemistry behind the simple "one-pot" synthesis of vanadium-substituted polyoxometalates unraveled by electrospray ionization mass spectrometry

RATIONALE

The "one-pot" method is a deceptively simple and straightforward method for synthesis of polyoxometalates (POMs). However, the complex solution chemistry behind the simple method is neither clear nor elucidated thoroughly by a suitable method. In general, POM chemists are more focused on what final products are obtained by deliberately choosing the ingredients used in the "one-pot" process with a limited knowledge of what is actually happening in the reaction solution.

METHODS

Time-resolved electrospray ionization mass spectrometry (ESI-MS) was developed to monitor the dynamic solution speciation changes occurred in the reaction mixtures ({SiW12-x } (x = 1-3) + NaVO3 ) against reaction time. The reactions were conducted as normal, with 2 μL aliquots removed at specific time intervals during the reaction. The aliquots were immediately diluted with 1 mL of HPLC grade water and then analyzed by ESI-MS. As a control, solutions of each {SiW12-x } were analyzed alongside the vanadium reactions to corroborate which fragments were due to the lacunary precursors and which were due to the reactions with sodium metavanadate.

RESULTS

It was discovered that the reaction solution speciation was sensitively changed with the reaction conditions such as concentration, temperature, and reaction time. Spontaneous transformations from mono- into di- and eventually tri-substituted products ({SiW11 V} → {SiW10 V2 } → {SiW9 V3 }) were observed for the reactions of {SiW11 } + VO3 (-) and {SiW10 } + VO3 (-) , respectively. Higher concentration and temperature definitely accelerate the transformation reaction rates whereas different molar ratios of reactants have minimal effects on changing the solution speciation in the cases studied.

CONCLUSIONS

Time-resolved ESI-MS is effectively and successfully used in the monitoring of the dynamic speciation changes in the reaction mixtures consisting of lacunary POMs with transition salts. This study will provide guidance for the oriented design and controlled synthesis of POMs with novel structures. Copyright © 2016 John Wiley & Sons, Ltd.

Thermal Responsive Ion Selectivity of Uranyl Peroxide Nanocages: An Inorganic Mimic of K(+) Ion Channels

An actinyl peroxide cage cluster, Li48+m K12 (OH)m [UO2 (O2 )(OH)]60 (H2 O)n (m≈20 and n≈310; U60 ), discriminates precisely between Na(+) and K(+) ions when heated to certain temperatures, a most essential feature for K(+) selective filters. The U60 clusters demonstrate several other features in common with K(+) ion channels, including passive transport of K(+) ions, a high flux rate, and the dehydration of U60 and K(+) ions. These qualities make U60 (a pure inorganic cluster) a promising ion channel mimic in an aqueous environment. Laser light scattering (LLS) and isothermal titration calorimetry (ITC) studies revealed that the tailorable ion selectivity of U60 clusters is a result of the thermal responsiveness of the U60 hydration shells.

Assembly of inorganic [Mo2S2O2]2+ panels connected by selenite anions to nanoscale chalcogenide-polyoxometalate clusters

We describe how supramolecular assembly, mediated by control of the ratio of the hetero-atoms in the units [Mo₂S₂O₂]²⁺ and SeO₃^2- leads to the formation of new types of building blocks, [(Mo₂O₂S₂)₃(OH)₄(H₂O)₆(SeO₃)] = {Mo₆} and [(Mo₂O₂S₂)₂(OH)₂(H₂O)₄(SeO₃)] = {Mo₄} which are linked in an type of inorganic 'panelling' to the assembly of a range of new clusters 1-3 with the general formula {(Mo₂O₂S₂) _x (OH) _y (SeO₃) _z (H₂O) _w } ^n-, where x, y, z, w, n = [8, 0, 20, 8, 24] for 1, [14, 14, 17, 8, 20] for 2 and [8, 8, 8, 0, 8] for 3. Cluster 1, a rare example of inorganic cryptand, exhibits an elliptical "endo" motif defining an anisotropic ellipse with the dimensions 1.7 × 1.0 nm, with pores ranging from 5.3 to 6.4 Å and site selective cation recognition properties; cluster 2 exhibits an "exo" structural motif constructed by 3 × {Mo₆} and 2 × {Mo₄} panels spanning a cross shape 2.4 × 2.0 nm and cluster 3 a ring shaped structure of a 1.5 nm in diameter. The control of endo vs. exo topology as a function of the Se : Mo ratio is reflected to the difference in surface area of ca. 500 Ų between clusters 1 and 2 intermolecular interactions and proton conduction properties, and this work shows that very simple synthetic parameters can critically change the structure and properties of all-inorganic nanoscale chalcogenide-polyoxometalates.

Molecular spin qubits based on lanthanide ions encapsulated in cubic polyoxopalladates: design criteria to enhance quantum coherence

Axial compression and a magnetic field can help to get coherent spin qubits.