Classical/Non‐classical Polyoxometalate Hybrids

Abstract Two polyanions [SeI V 2PdII 4WVI 14O56H]11− and [SeI V 4PdII 4WVI 28O108H12]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.

could be isolated from the reactionm ediumf or preparation of 1 as ac rystalline product. The paratungstate-B salt is also sometimes presenta sa ni mpurity to CsNa-1,w hich could be purified in this case by recrystallization from 0.25 m NaOAc aqueous solution( pH 6.7). Similarr ecrystallization of CsNa-2 leads to formation of am ixture of CsNa-1, CsNa-2,a nd other undefined products.T he purity and composition of the compounds was further confirmed by elementala nalysis, PXRD, TGA, and XPS (see the Supporting Information for details).
The {a-Se 2 W 14 }u nit can be compared to ah ypothetical tetralacunary Wells-Dawson-type {a-P 2 W 14 O 54 }f ragment (Supporting Information, Figure S3), with two neighboring{ W 2 O 10 } groups,c omposed of two edge-shared {WO 6 }o ctahedra, removed from the inner {W 6 }b elts of [a-P 2 W 18 O 62 ] 6À ({a-P 2 W 18 }; Supporting Information, Figure S3a/b), each one from one belt. The Se IV ions in {a-Se 2 W 14 }a dopt at rigonal pyramidal environment with the outwards oriented lone pair (Supporting Information, Figure S3d;S e ÀO1 .677(16)-1.725 (15) ). The formation of {a-Se 2 W 14 }f rom the {g-Se 2 W 12 }b uildingb locks of the {Se 6 W 39 }p recursor requires attachment of two additional W VI ions to {g-Se 2 W 12 }, each of which is completing the outer {W 3 } cap of the POT fragment, combined with {g-Se 2 W 14 }i somerization by rotation of both {W 3 }c aps by 608 (Supporting Information, Figure S3) [9] complexesw here the two {W 3 }c aps are rotatedb y6 0 8 relative to their orientation in the a isomer (Supporting Information, Figure S3e).
The direct connection between the Pd II centers by oxo ligands as well as the complete integration of the POPd {Pd 4 O 4 } moiety in the POM framework allow to consider 1 as agenuine hybrid polyoxopalladatotungstate. Interestingly,t he structure of {Pd 4  The total number of metal centersi n1 allows for an analogy between 1 and Wells-Dawson-type polyanions {a-P 2 W 18 }. [11] Both POMs comprise two central heteroatoms surroundedb y 18 addenda metal ions. Howevert he {Pd 4 }r ectangle in 1 is rotated by 458 in comparison to the {W VI 4 }r ectangle in {a-P 2 W 18 } if the latter is formally decomposed into the above-mentioned hypothetical {a-P 2 W 14 }f ragment and four W VI centers (Supporting Information, Figure S4), possibly enforced by the squareplanar Pd coordinationm ode in 1 relative to the octahedral W VI O 6 groups. This analogy prompted us to probe the possibility to form lacunary derivativeso f1 at conditions similart o those for formation of {a 2 -P 2 W 17 }a nd {a-P 2 W 15 }f rom {a-P 2 W 18 }. These experiments, however,o nly resulted in Cs 2 Na 3 [H 5 Pd 15 Se 10 O 10 (SeO 3 ) 10 ]·ca. 20 H 2 O·POPd, [12] which suggests that decomposition of 1 proceeds first through release of Pd II ions, followed by POT decomposition.
However the possibility of existence of unstable lacunary derivatives of {a/b/g-Se 2 Pd 4 W 14 }p olyanions is evident from the structure of 2 obtainedi ndirectly by reaction of {Se 6 W 39 }w ith Pd II in water.T he compound CsNa-2 crystallizes in the triclinic space group P1 .T he unit cell in CsNa-2 contains two identical polyanions 2,e ach of which can be imagined as ad imer of   FigureS12). This indicates stability of 1 in aqueous medium in saturated solutions. The observed chemical shift is commensurate with those of Zn II (1222.5 ppm) and Lu III (1223.8 ppm)-centered cuboid {MPd 12 Se 8 }P OPds [6c] and is significantly upfieldshifted compared to an aqueous SeO 2 solution (pH 6.4; 1316.3 ppm). For comparison with other tungstoselenites, the {Se 6 W 39 }p recursor (unstablei ns olution) gives ab road peak centered at 1289.1 ppm in 77 Se MAS NMR. [8a] The 77 Se 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),i n line with the symmetry of 2.B ased on literature data for {Se 6 W 39 } [8a] and the data obtained for CsNa-1 (see above), we tentatively assign the upfield signal to Se IV ions of the {Pd 2 SeW 7 }h alf of the {g-Pd 2 Se 2 W 13 }s ubunit (Figure 2a), and the 1255 ppm peak to the Se IV ions positioned in the Pd II -free {SeW 6 }p art of this motif. In contrastt o1,s olution 77 Se NMR of 2 exhibits two main signals at 1316.5ppm 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 releaseo fs elenitei ons (signal at 1316.5 ppm) concurrent with formation of 1 (singlet at 1226.8 ppm), in line with the formationo fCsNa-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 À4 m CsNa-1 and CsNa-2 solutions in ultra-pure water onto HOPG surface (Supporting Information, Figure S5).
The exact compositiono fi on pairs based on 1 and 2 that potentially exist in solutions and gas phase was probedb y   (Table 1), by virtue of their m/z values and analysis of the corresponding calculated and observed isotope envelopes( see Figure 4, inset;S upporting Information, Figures S17-S24). Peak II could be attributed to an ion pair based on am onovacant derivativeo f1,w here one of the Pd II centers is missing, while peak Ib elongs to ad ilacunary species lacking two Pd II ions with the m 2 -briding oxygen ion linking these metal ions together.T his suggest that decomposition of 1 in gas phase (and possibly also in solution)p roceeds via release of Pd II centers in af irst step. This is consistentw ith our observations of loss of Pd II ions and the following POT moietyd ecomposition during our attempts to preparel acunary derivatives of 1,b ut also suggests that such speciesc ould in principle exist if adequately stabilized. The ESI-MS spectrumo f2 recorded at similar conditions (Supporting Information, Figure S25) only exhibits peaks attrib-uted to singly charged POM decomposition products (see the Supporting Information for details), consistentw ith our NMR observations.
In summary,w eh ave isolated and characterizedt wo polyan- } site in 1 could serve an analogyt oavacant site of lacunary POTs, that, in combination with solution stability of 1,c ould lead to an ovel rich class of heterometal derivatives of mixed palladate-tungstates. On the other hand, the ESI-MS results display ap ossibility for existenceo fl acunary speciesf or 1 at appropriate conditions, with one or two Pd II centers missing. This hypothesis is further supportedb yi solation of polyanion 2 whichc ould be imagined as ad imer of two lacunary derivatives of hypothetical {g-Pd 4 Se 2 W 14 }s pecies. Follow-up work will focus on these possibilities.

Experimental Section
Synthesis of CsNa-1:S amples of Na 24 [H 6 Se 6 W 39 O 144 ]·74 H 2 O [8a] (0.500 g, 0.042 mmol) and Pd(NO 3 ) 2 ·H 2 O( 0.105 g, 0.423 mmol) were dissolved in 5mLo fa queous 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 8C. The obtained clear dark-red reaction mixture was stirred at 50 8C for 30 min and then cooled to room temperature. After that 0.5 mL of 1 m CsNO 3 solution in H 2 Ow as 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,a lthough 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.T otal yield:0 .177 g( 33 %b ased on Pd).  After the dissolution of all the reagents, the reaction mixture was stirred and further heated for 1h and then cooled to room temperature and filtered. Three drops of 1 m aqueous CsNO 3 solution were added to the obtained dark red-brown filtrate. The  obtained pale brown precipitate [13] 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).  (