Classical/Non-classical Polyoxometalate Hybrids.

Two polyanions [SeI V2 PdII4 WVI14 O56 H]11- and [SeI V4 PdII4 WVI28 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-PdII 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.

The chemistry of palladium‐containing polyoxometalates (POMs) has experienced impressive development over the past decade,1 with progress primarily concentrated on two areas. The first is defined by conventional PdII coordination complexes of lacunary polyoxotungstates (POTs), [PdII (XWO)r], where PdII ions in square‐planar environments coordinate oxygen atoms of vacant sites of POT ligands, resulting in a diverse range of structures incorporating one to four PdII centers.2 Such species are convenient precursors for highly stable suspensions of POT‐stabilized Pd0 nanoparticles, which can be obtained at mild conditions in aqueous media.3 Some of the Pd‐POT complexes were also shown to act as pre‐catalysts for various organic transformations.2m, 4 In these complexes, the PdII centers typically lack a direct connection, with the only exception in [PdII 4(α‐P2W15O56)2]16−, where two out of four PdII ions are bridged via O atoms of two phosphate groups.2p

In the second main area, formed by so‐called polyoxopalladates (POPds), the PdII centers, in contrast, act as addenda ions themselves. Here, the elementary PdO4 building blocks are condensed via corners and edges, typically also involving external RXO3 heterogroups stabilizing the discrete {PdO} entity.5, 6 About 50 of these non‐classical POMs are known today, incorporating up to 84 PdII ions. One of the most stable POPds archetype comprises species of general composition [MPdII 12O8(RXO3)8] ({MPd12}), where a heterometal ion M in a cubic O8 environment is encapsulated in the cuboid‐shaped {PdII 12O8(RXO3)8} shell (RX=SeIV, OAsV, PhAsV, OPV, PhPV).6

Recently, Cronin and co‐workers also reported several polyanions that can be considered as complexes of seleno‐ and tellurotungstates {XWO} with selenite‐ or tellurate‐supported multinuclear PdII‐based fragments.7 In two isomeric [HPdII 10SeI V 10W52O206](40− polyanions, two {Pd5Se2O2} units are coordinated to {B‐α‐SeW9O33} and {γ‐Se2W14O56} POT moieties. In [PdII 6TeI V 19W42O190]40− two identical {Pd3Te3O3} groups are stabilized by six {α‐TeW7O27} lacunary POTs.7

Yet, up to now there was no systematic investigation on how to achieve commensurate reaction conditions that allow to co‐condense, and thus cleanly interface, classical POTs and non‐classical POPds. We thus explored the possibility to prepare hybrid polyoxopalladatotungstates [XPdII WIV O], where both PdII and WVI centers act as addenda centers of their individual POM units, without the need for any additional external stabilizing groups. Herein we report two first examples of such hybrid palladatotungstates, [SeI V 2PdII 4WVI 14O56H]11− (1) and [SeI V 4PdII 4WVI 28O108H12]12− (2), crystallized as hydrated mixed cesium/sodium salts Cs4Na3H4[Se2Pd4W14O56H]⋅ 18 H2O⋅0.3 CsOAc⋅0.2 NaOAc (CsNa‐1; OAc−=acetate) and Cs9.5Na2.5[Se4Pd4W28O108H12]⋅30 H2O (CsNa‐2), respectively, and their characterization in the solid state, aqueous solutions, and gas phase.

The polyanions 1 and 2 have been prepared in reactions of [SeI V 6WVI 39O141(H2O)3]24− ({Se6W39})8 with PdII nitrate in different aqueous media (Supporting Information, Scheme S1). The {Se6W39} precursor possesses a cyclic structure, where three {γ‐Se2W12O46} units are alternating with three trans‐{O=W(H2O)} groups. In aqueous solution it slowly decomposes, releasing {SeWO} fragments8 and thus could act as a source for preparation of diverse tungstoselenite complexes.9 The Cs+ counterions seem to play an important role for isolation of 1 and 2 as pure crystalline materials owing to relatively low solubility of the hydrated Cs+ salts. Alternatively, a Rb+/Na+ salt of 1 can be successfully prepared by replacing CsNO3 with RbNO3 in the synthesis of CsNa‐1. With no additional counterions only the hydrated sodium salt of paratungstate‐B ([H2W12O42]10−) could be isolated from the reaction medium for preparation of 1 as a crystalline product. The paratungstate‐B salt is also sometimes present as an impurity to CsNa‐1, which could be purified in this case by recrystallization from 0.25 m NaOAc aqueous solution (pH 6.7). Similar recrystallization of CsNa‐2 leads to formation of a mixture of CsNa‐1, CsNa‐2, and other undefined products. The purity and composition of the compounds was further confirmed by elemental analysis, PXRD, TGA, and XPS (see the Supporting Information for details).

CsNa‐1 crystallizes in the orthorhombic space group Pnnm. The polyanion 1 exhibits idealized C 2 symmetry and comprises an [α‐Se2W14O52]12− POT moiety ({α‐Se2W14}) supporting a {Pd4O4} fragment (Figure 1).

Figure 1

Structure of 1 (a) and the {(H2O)3Na}‐1 associate (b); comparison with the {Pd4O4(RXO3)4} fragment (c) in the cuboid‐shaped polyoxopalladate [MPd12O8(RXO3)8] (d). WO6 lime green, PdO4 blue polyhedra; Pd blue, Se/X yellow, O red, Na purple, M light blue. The R groups in {MPd12} are omitted for clarity.

The {α‐Se2W14} unit can be compared to a hypothetical tetralacunary Wells–Dawson‐type {α‐P2W14O54} fragment (Supporting Information, Figure S3), with two neighboring {W2O10} groups, composed of two edge‐shared {WO6} octahedra, removed from the inner {W6} belts of [α‐P2W18O62]6− ({α‐P2W18}; Supporting Information, Figure S3a/b), each one from one belt. The SeIV ions in {α‐Se2W14} adopt a trigonal pyramidal environment with the outwards oriented lone pair (Supporting Information, Figure S3d; Se−O 1.677(16)–1.725(15) Å). The formation of {α‐Se2W14} from the {γ‐Se2W12} building blocks of the {Se6W39} precursor requires attachment of two additional WVI ions to {γ‐Se2W12}, each of which is completing the outer {W3} cap of the POT fragment, combined with {γ‐Se2W14} isomerization by rotation of both {W3} caps by 60° (Supporting Information, Figure S3). The same {α‐Se2W14} building blocks have been recently observed in [Fe6Se6W34O124(OH)16]18− polyanions.9 At the same time, the arrangement of WVI centers in {α‐Se2W14} is different from that in the actual {α‐P2W14O54} moieties that, for example, form [H12Fe8P4W28O120]16− and [(W4Mn4O12)(P2W14O54)2]20−complexes.10 In fact, these {α‐P2W14O54} building blocks are the structural isomers to the hypothetical {α‐P2W14} units discussed above, and can be obtained from {α‐P2W18} polyanions by removing not {W2O10} but rather corner‐sharing {W2O11} units from its inner belts (Supporting Information, Figure S3c). It is also different in {γ‐Se2W14} moieties constructing the reported [HPd10Se10W52O206] (see above) and [Fe10Se8W62O222(OH)18(H2O)4]28−[9] complexes where the two {W3} caps are rotated by 60° relative to their orientation in the α isomer (Supporting Information, Figure S3e).

The four PdII centers in the {Pd4O4} fragment form a rectangle (Pd⋅⋅⋅Pd 3.360(2)–3.375(2) Å) and are linked by four μ2‐O sites (Figure 1 a). All square‐planar PdIIO4 (Pd−O 1.976(14)‐2.010(15) Å) include two cis‐positioned μ2‐O of the {Pd4O4} fragment as well as two OPOT atoms: two PdII centers bind to the {W3} caps and two to the belts of {α‐Se2W14} (Figure 1 a). Based on bond valence sums, the proton in 1 is disordered over the four μ2‐O atoms linking the PdII centers. These oxygens also coordinate to a {Na(OH2)3}+ counterion (Figure 1 b; Na−O 2.42(2)–2.53(2) Å).

The direct connection between the PdII centers by oxo ligands as well as the complete integration of the POPd {Pd4O4} moiety in the POM framework allow to consider 1 as a genuine hybrid polyoxopalladatotungstate. Interestingly, the structure of {Pd4O4} unit in 1 compares to the {Pd4O4(RXO3)4} face in the cuboid‐shaped {MPd12} POPds (Figure 1 c/d1), with the RXO3 groups stabilizing the {MPd12O8} core replaced by {α‐Se2W14}. Moreover, the Na+ attachment to {Pd4O4} in 1 is similar to the connection mode between the central M ion and the {Pd4O4(RXO3)4} face in {MPd12} nanocubes (Figure 1 b/d1). This suggests that the {Pd4O4} group in 1 possesses reactivity towards oxophilic heterometals.

The total number of metal centers in 1 allows for an analogy between 1 and Wells–Dawson‐type polyanions {α‐P2W18}.11 Both POMs comprise two central heteroatoms surrounded by 18 addenda metal ions. However the {Pd4} rectangle in 1 is rotated by 45° in comparison to the {WVI 4} rectangle in {α‐P2W18} if the latter is formally decomposed into the above‐mentioned hypothetical {α‐P2W14} fragment and four WVI centers (Supporting Information, Figure S4), possibly enforced by the square‐planar Pd coordination mode in 1 relative to the octahedral WVIO6 groups. This analogy prompted us to probe the possibility to form lacunary derivatives of 1 at conditions similar to those for formation of {α2‐P2W17} and {α‐P2W15} from {α‐P2W18}. These experiments, however, only resulted in Cs2Na3[H5Pd15Se10O10(SeO3)10]⋅ca. 20 H2O⋅POPd,12 which suggests that decomposition of 1 proceeds first through release of PdII ions, followed by POT decomposition.

However the possibility of existence of unstable lacunary derivatives of {α/β/γ‐Se2Pd4W14} polyanions is evident from the structure of 2 obtained indirectly by reaction of {Se6W39} with PdII in water. The compound CsNa‐2 crystallizes in the triclinic space group P . The unit cell in CsNa‐2 contains two identical polyanions 2, each of which can be imagined as a dimer of two γ‐{(H2O)(OH)2PdII 2SeI V 2W13O49} ({γ‐Pd2Se2W13}) units connected by two trans‐{O=W(H2O)} groups (Figure 2). In line with the previous discussion, the {γ‐Se2W13} structure can be understood as a {γ‐Se2W12} unit, present in {Se6W39}, binding a WVI to complete one of the {W3} caps or, alternatively, as {γ‐Se2W14} (Supporting Information, Figure S3e), missing one WVI ion in its {W3} cap. The two PdII ions in {γ‐Pd2Se2W13} assume a square planar environment, each coordinating two cis‐positioned oxygens of {γ‐Se2W13}: one from the WVI ion in the {W3} cap and one from the {W4} belt (Figure 2 a). Furthermore, the two PdII ions are μ2‐OH‐bridged. One of the PdII ions additionally coordinates a terminal H2O, and its μ2‐O (Pd, W) ion in the trans‐position to the aqua ligand is protonated (Figure 2 a/c2; Supporting Information, Table S4). The second PdII ion is bound to trans‐{O=W(H2O)} group through the μ2‐O (Figure 2 b).

Figure 2

The structure of a {γ‐Pd2Se2W13} monomer (a) and a γ‐Pd2Se2W13{O=W(H2O)}2 moiety (b) in the polyanion 2 (c). WO6 lime green octahedra, PdO4 blue squares; Pd blue, Se yellow, O red spheres. The monoprotonated O atoms in the structure of 2 are highlighted in light purple, while aquo ligands are shown in pink.

Thus, the {γ‐Pd2Se2W13} fragment can be considered as a lacunary derivative of a hypothetical plenary {γ‐Pd4Se2W14} polyanion, lacking two PdII and one WVI centers. It is interesting to note that the orientation of {Pd4O4} fragment in this {γ‐Pd4Se2W14} POM, in case it exists, would be similar to that in {α‐P2W18} and not in {α‐Pd4Se2W14}. Along with the μ2‐O ligand connecting it to PdII (see above), the WVI center of each trans‐{O=W(H2O)} group also binds to an O atom of the neighboring {W4} belt of the same {γ‐Pd2Se2W13} monomeric unit as well as to the two O atoms of the incomplete {W2} cap group of the second {γ‐Pd2Se2W13} monomer, each of which belongs to different WVI ions (Figure 2 b/c2). Interestingly, one of the H2O ligands of the trans‐{O=W(H2O)} groups is directed inward the polyanion, while the second one is pointed outward (Figure 2 c). Thus, considering the protonation sites, 2 is of C 1 symmetry. Otherwise, it would possess a C 2 axis passing through the center of a line connecting the WVI centers of the two {O=W(H2O)} groups (Figure 2 c).

Owing to the presence of large Cs+ cations, the compounds CsNa‐1 and CsNa‐2 are only slightly soluble in water; however, their solubility is significantly increased in 0.25–0.5 m sodium and lithium acetate solutions (pH 6–7), especially upon heating to 65–70 °C. This allowed assessment of the solution behavior of 1 and 2 by 77Se NMR and UV/Vis spectroscopy (see the Supporting Information). Room‐temperature 77Se NMR of 1 in 0.25 m LiOAc solution (pH 6.2) exhibits a singlet at 1225.3 ppm (Figure 3), consistent with the presence of only one symmetrically non‐equivalent SeIV ion in the crystal structure of CsNa‐1 and with the observation of a singlet at 1202 ppm in the 77Se MAS NMR for this compound (Supporting Information, Figure S12). This indicates stability of 1 in aqueous medium in saturated solutions. The observed chemical shift is commensurate with those of ZnII (1222.5 ppm) and LuIII (1223.8 ppm)‐centered cuboid {MPd12Se8} POPds6c and is significantly upfield‐shifted compared to an aqueous SeO2 solution (pH 6.4; 1316.3 ppm). For comparison with other tungstoselenites, the {Se6W39} precursor (unstable in solution) gives a broad peak centered at 1289.1 ppm in 77Se MAS NMR.8a

Figure 3

Room‐temperature 77Se NMR spectrum of CsNa‐1 dissolved in 0.25 m LiOAc solution in H2O/D2O (pH 6.2).

The 77Se MAS NMR of CsNa‐2 (Supporting Information, Figure S13) shows two broad signals centered at 1255 and 1187 ppm (verified for two different spinning frequencies), in line with the symmetry of 2. Based on literature data for {Se6W39}8a and the data obtained for CsNa‐1 (see above), we tentatively assign the upfield signal to SeIV ions of the {Pd2SeW7} half of the {γ‐Pd2Se2W13} subunit (Figure 2 a), and the 1255 ppm peak to the SeIV ions positioned in the PdII‐free {SeW6} part of this motif. In contrast to 1, solution 77Se NMR of 2 exhibits two main signals at 1316.5 ppm and 1226.8 ppm with 1.8:1 relative intensities (Supporting Information, Figure S14). The chemical shifts of the signals are evident of decomposition of the polyanions with the release of selenite ions (signal at 1316.5 ppm) concurrent with formation of 1 (singlet at 1226.8 ppm), in line with the formation of CsNa‐1 crystals after recrystallization of CsNa‐2 from aqueous acetate solutions. These solution stability observations for 1 and 2 are further supported by SEM images obtained after drop‐casting of 10−4  m CsNa‐1 and CsNa‐2 solutions in ultra‐pure water onto HOPG surface (Supporting Information, Figure S5).

The exact composition of ion pairs based on 1 and 2 that potentially exist in solutions and gas phase was probed by mass spectrometry. The negative‐ion‐mode ESI‐MS spectrum of 1 (Figure 4) shows a set of peaks (III–VII), which can be attributed to various ion pairs {HNa[Se2Pd4W14O56H]}3− based on the intact polyanion 1 (Table 1), by virtue of their m/z values and analysis of the corresponding calculated and observed isotope envelopes (see Figure 4, inset; Supporting Information, Figures S17–S24). Peak II could be attributed to an ion pair based on a monovacant derivative of 1, where one of the PdII centers is missing, while peak I belongs to a dilacunary species lacking two PdII ions with the μ2‐briding oxygen ion linking these metal ions together. This suggest that decomposition of 1 in gas phase (and possibly also in solution) proceeds via release of PdII centers in a first step.

Figure 4

ESI mass spectrum of 1 in H2O/acetone (80:20 vol %) solution in negative‐ion mode. Inset: comparison of the calculated and experimentally observed isotope envelopes for the most intense signal (III).

Table 1

Assignment of the peaks observed in the ESI‐MS spectrum of 1.[a]

Peak	Formula	m/z (calcd)	m/z (found)
I	{H9Na2[Se2Pd2W14O55]}3−	1293.23	1293.54
II	{H8Na3[Se2Pd3W14O56]}3−	1341.36	1342.52
III	{H8[Se2Pd4W14O56H]}3−	1354.18	1354.19
IV	{H7Na[Se2Pd4W14O56H]}3−	1361.51	1361.52
V	{H6Na2[Se2Pd4W14O56H]}3−	1368.83	1369.18
VI	{H2Na6[Se2Pd4W14O56H]}3−	1397.14	1397.83
VII	{Na8[Se2Pd4W14O56H]}3− or {CsH5Na2[Se2Pd4W14O56H]}3−	1412.80 1412.80	1413.99

[a] Values are given for the most abundant isotopologue (see Figure 4). The small discrepancy in the experimental and calculated m/z values is due to the average element isotope composition was taken for the calculation of the masses. The precise assignment of the signals is made by comparison of the observed and calculated isotope envelopes (see the Supporting Information for details).

This is consistent with our observations of loss of PdII ions and the following POT moiety decomposition during our attempts to prepare lacunary derivatives of 1, but also suggests that such species could in principle exist if adequately stabilized. The ESI‐MS spectrum of 2 recorded at similar conditions (Supporting Information, Figure S25) only exhibits peaks attributed to singly charged POM decomposition products (see the Supporting Information for details), consistent with our NMR observations.

In summary, we have isolated and characterized two polyanions [SeI V 2PdII 4WVI 14O56H]11− and [SeI V 4PdII 4WVI 28O108H12]12− comprising both WVI and PdII addenda sites. As such, the new hybrid species bridge the conventional POT‐PdII coordination complexes and POPds. The analysis of the structural data for CsNa‐1 suggests reactivity of μ2‐O ions bridging PdII ions in its {Pd4O4} fragment towards oxophilic metals. Hence, the {Pd4O4} site in 1 could serve an analogy to a vacant site of lacunary POTs, that, in combination with solution stability of 1, could lead to a novel rich class of heterometal derivatives of mixed palladate–tungstates. On the other hand, the ESI‐MS results display a possibility for existence of lacunary species for 1 at appropriate conditions, with one or two PdII centers missing. This hypothesis is further supported by isolation of polyanion 2 which could be imagined as a dimer of two lacunary derivatives of hypothetical {γ‐Pd4Se2W14} species. Follow‐up work will focus on these possibilities.

Experimental Section

Synthesis of CsNa‐1: Samples of Na24[H6Se6W39O144]⋅74 H2O8a (0.500 g, 0.042 mmol) and Pd(NO3)2⋅H2O (0.105 g, 0.423 mmol) were dissolved in 5 mL of aqueous 0.5 m NaOAc solution (prepared by addition of solid NaOH into 0.5 m HOAc solution in water until pH reaches 6.7) under vigorous stirring and heating at about 50–60 °C. The obtained clear dark‐red reaction mixture was stirred at 50 °C for 30 min and then cooled to room temperature. After that 0.5 mL of 1 m CsNO3 solution in H2O was added to the reaction mixture under stirring leading to immediate formation of light‐brown precipitate. The precipitate was collected by filtration and recrystallized from warm 0.25 m NaOAc (pH 6.7) resulting in an orange solution. Needle‐like brown‐yellow crystals of CsNa‐1 form within several days. The filtrate produced additional portion of CsNa‐1, although often contaminated by hydrated Cs/Na salt of paratungstate‐B (based on IR and single‐crystal XRD). In this case purification is achieved by recrystallization of the obtained solid material from 0.25 m NaOAc medium (pH 6.7). The crystals of the product were collected by filtration and washed with small amount of ice cold water. Total yield: 0.177 g (33 % based on Pd).

Elemental analysis calcd (%) for C1H42.5Cs4.3Na3.2O75Pd4Se2W14: Cs 11.30, Na 1.45, Pd 8.42, Se 3.12, W 50.89; found: Cs 11.53, Na 1.51, Pd 7.89, Se 3.11, W 51.64. IR (KBr pellet), [cm−1]: 3424 (s, br); 1625 (m); 1420 (w); 1108 (w); 943 (s); 902 (s, sh); 874 (s); 840 (s); 819 (s); 774 (s); 713 (s), 676 (s); 502 (s); 451 (s). Raman (solid sample, λ e=1064 nm), [cm−1]: 958 (s); 891 (m); 872 (m); 835 (m); 787 (w); 582 (w); 507 (w, br); 241 (w, br); 197 (m); 161 (m, br); 130 (m); 100 (m); 75 (m). 77Se NMR (H2O/D2O): 1225.3 ppm. 77Se MAS NMR: 1202 ppm. UV/Vis (0.25 m NaOAc buffer solution, pH 6.7): λ max (ϵ)=227 (74450), 273 (shoulder, 34153), 414 nm (1484 mol−1 dm−3 cm−1). CSD no.: 431484.

Synthesis of CsNa‐2: Na24[H6Se6W39O144]⋅74 H2O8a (0.200 g, 0.017 mmol) and Pd(NO3)2⋅H2O (0.026 g, 0.105 mmol) were dissolved in 2 mL of H2O under vigorous stirring and heating at about 50–60 °C. After the dissolution of all the reagents, the reaction mixture was stirred and further heated for 1 h and then cooled to room temperature and filtered. Three drops of 1 m aqueous CsNO3 solution were added to the obtained dark red–brown filtrate. The obtained pale brown precipitate13 was filtered and the evaporation of the resulting solution at room temperature led to brown crystalline material of CsNa‐2 within 1–3 days. Crystals were collected by filtration, washed with ice‐cold water and dried in air. Yield: 0.040 g (17 % based on W).

Elemental analysis calcd (%) for H72Cs9.5Na2.5O138Pd4Se4W28: Cs 13.31, Na 0.61, Pd 4.49, Se 3.33, W 54.24; found: Cs 13.22, Na 0.61, Pd 4.49, Se 3.39, W 54.2. IR (KBr pellet), [cm−1]: 3423 (s, br); 1614 (s); 954 (s); 843 (s); 768 (s); 704 (s); 662 (s, br); 491 (m); 427 (s). Raman (solid sample, λ e=1064 nm), [cm−1]: 970 (s); 914 (m); 902 (m); 885 (m); 866 (w, sh); 812 (m); 717 (w); 660 (m); 646 (m); 513 (w); 503 (w); 216 (m); 110 (m); 75 (m). 77Se MAS NMR: 1255 and 1187 ppm. CSD no.: 431485.

The Supporting Information for this article includes experimental and crystallographic details, powder X‐ray diffraction, XPS/SEM data, bond valence sum values; IR, Raman, UV/Vis, 77Se MAS and solution NMR spectra, and ESI‐MS with simulations.

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Supplementary

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