Zhongling
Lang§
a,
Peng
Yang§
b,
Zhengguo
Lin
b,
Likai
Yan
c,
Ming-Xing
Li
d,
Jorge J.
Carbó
a,
Ulrich
Kortz
*b and
Josep M.
Poblet
*a
aDepartament de Química Física i Inorgànica, Universitat Rovira i Virgili, c/Marcel lí Domingo 1, 43007 Tarragona, Spain. E-mail: josepmaria.poblet@urv.cat
bDepartment of Life Sciences and Chemistry, Jacobs University, Campus Ring 1, 28759 Bremen, Germany. E-mail: u.kortz@jacobs-university.de
cInstitute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, P. R. China
dDepartment of Chemistry, College of Sciences, Shanghai University, Shanghai 200444, P. R. China
First published on 25th September 2017
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 Mn+ 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 {Mn+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.
Besides classical POMs,3 in the last decade or so an “unconventional” POM family based exclusively on PdII, PtIII, or AuIII addenda has been developed.4 Since noble metals are well-known active ingredients of many catalysts, the study of noble metal-containing POMs is a particularly interesting topic. In 2004, Wickleder's group synthesized the first polyoxoplatinate exclusively based on d7 addenda ions, [PtIII12O8(SVIO4)12]4−.5 Since then the Kortz group has pioneered the class of polyoxopalladates(II) in 2008,6 and the class of polyoxoaurates(III) in 2010.7a The first polyoxopalladate was the [H6Pd13O8(AsO4)8]8− (Pd13) nanocube, prepared by simple condensation of Pd2+ and arsenate (AsVO43−) ions in aqueous medium.6 In the following years it was demonstrated that the eight arsenate capping groups can be easily replaced by other heterogroups, such as selenite ([Pd13O8(SeO3)8]6−, Pd13Se) or phenylarsonate ([Pd13O8(PhAsO3)8]6−, Pd13AsPh).8 Interestingly, substitution of the capping groups is accompanied by an increase of the coordination number of the central Pd2+ ion from 4 (Pd13) to 6 (Pd13Se) and even 8 (Pd13AsPh).
In addition to the capping groups the central palladium(II) ion in the nanocube {MPd12L8} (Fig. 1a) can also be replaced by other metal ion guests, including trivalent lanthanide ions (Ln3+ = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and 3d transition metal ions (Sc3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+).9 Interestingly, the nanostar {MPd15L10} (Fig. 1b) can be formed only in the presence of Na+, K+, Ag+, or Ba2+,10 whereas in the presence of Sr2+ ions the “open” nanocube {SrPd12L6L′3} (L = phenylarsonate, L′ = acetate) is formed.10e Such observations bear similarities with the important template role of alkali and alkaline earth ions in the formation of various organic macrocycle-based structures (crown ethers etc.).11 In addition to the above-mentioned nanocube, nanostar and open-nanocube structural types, some additional polyoxopalladates with unexpected geometries have been obtained, such as the bowl-shaped palladovanadate {Pd7V6},12a the double cuboid-shaped 22-palladates {Cu2Pd22}12b and {Na2Pd22},12c as well as palladate macrocycles {Pdn} (n = 60, 72, 84, 96, 108).13
It is evident that the central metal ion guest as well as the capping groups play a key role in the formation mechanism of polyoxopalladates, but details remain unknown. Hence the rational synthesis of novel polyoxopalladate structural types of desired shape, size and composition is virtually impossible. On the other hand, density functional theory (DFT) methods have been applied to POMs significantly in the last two decades, in particular with respect to (i) electronic structure, (ii) rationalization of physicochemical properties, and (iii) reactivity as a function of shape and composition.14 In order to obtain more insight into polyoxopalladate chemistry, in particular with respect to factors that govern guest metal ion encapsulation and to perhaps shed more light on selectivity issues, we have decided to perform systematic theoretical analysis for a series of 35 metal ion guests M involved in the formation of the {MPd12L8} nanocube and {MPd15L10} nanostar polyoxopalladate structural types.
The encapsulated cations were selected by considering both their charge and size, which range from alkali and alkaline earth ions to transition metal ions, as well as trivalent and tetravalent main group cations. We have discovered a remarkable competition between Pd2+ ions and other cations, which is key for the formation of a specific polyoxopalladate structural type. With a focus on eventually being able to computationally predict experimental results, we have carefully studied experimentally (i) the capture of the largest trivalent cation La3+ inside a polyoxopalladate, and (ii) the selective incorporation of In3+vs. Ga3+ in a polyoxopalladate.
As shown in Fig. 1, the nanocube {MPd12L8} and nanostar {MPd15L10} were selected, in which the central metal ion usually has an oxo-coordination number of 8 and 10, respectively. In order to evaluate the selective encapsulation of different guest metal ions, the reaction mechanism is simulated by scheme (1), and the complexation energy (Ecom) was calculated by eqn (2) and (3):
![]() | (1) |
Ecom = E(Mn+Pd12/15) + E(6H2O) − E(Pd12/15) − E(Mn+(H2O)6) | (2) |
Ecom = ΔEbind + ΔEdef + ΔEdehyd | (3) |
For efficient encapsulation, the guest cation and the cavity in the palladate cage should match well. Due to the small flexibility of the {O8} coordination shell, all guest cations are optimally located in the centre of the cube. The average Oc–Oc distance generally elongates as the effective ionic radius of Mn+ increases, and this trend is perfectly in line with an increase of the M–Oc bond distances. To gain insight into the effect of cation size on the geometry of the cage, the Oc–Oc distortions after encapsulation of the cation are plotted in Fig. 2, by referring to the distance in the empty cage Pd12 (dOc–Oc = 2.728 Å). We found that guest cations with a radius smaller than 1.12 Å could induce a contraction of the {O8} cavity (e.g. Co2+, Ni2+, Cu2+, Zn2+, Mn2+, Sc3+, Fe3+, Lu3+) and also guest ions that have not yet been incorporated in the Pd12 cage experimentally, such as Li+, Be2+, Mg2+, Ga3+, In3+, Sn4+, Zr4+, and Hf4+. In particular, Be2+, Fe3+, Ga3+, and Sn4+ seem too small to be hosted efficiently, and consequently a large contraction occurs to maximize the M–Oc interactions. When the ion size is between 1.13 and 1.26 Å, a small expansion with Δd less than 0.1 Å is needed, except for Th4+. However, a significant distortion of {O8} was detected for ions larger than 1.28 Å, such as Ce3+, La3+ Sr2+, Ag+, K+, Rb+, Cs+, Ba2+, and Ra2+, where for some cases elongations larger than 0.4 Å were observed.
It is remarkable that for most of the experimentally observed palladate nanocubes, the distortion induced by guest metal ion encapsulation is no larger than 0.1 Å, and therefore the size matching between the cation guests and the cavity of the Pd12 cage is an important factor that must be considered. Fig. 2 also suggests that guest cations with a size ranging from 0.97–1.26 Å fit well within the {O8} cavity in Pd12 regardless of the charge (except Th4+). In contrast, smaller (r ≤ 0.95 Å) and larger (r ≥ 1.28 Å) cations are probably poor candidates for constructing {MPd12L8} palladate nanocubes from a size-matching point of view, with the optimal cation size being in the 0.97–1.26 Å range.
Large guest cations are more likely to accommodate themselves in larger polyoxopalladate assemblies with larger cavities. In order to compare with our computational results on the Pd12 nanocube, we inserted some selected cations (Li+, Na+, K+, Rb+, Cs+, Ag+, Be2+, Mg2+, Ca2+, Sr2+, Ba2+, Zn2+, Pd2+) also in the Pd15 nanostar cage. The Pd15 host provides a pentagonal-prismatic {O10} inner coordination sphere, which appears too large for encapsulation of small guest cations, such as Li+, Be2+, Mg2+, and Zn2+ (r < 1.2 Å). In this case the cations move away from the C5 symmetry axis of the Pd15 cage and coordinate to less than 10 oxo-ligands. Two types of such off-center coordination modes were observed from our DFT calculations, C4 and C5 as depicted in Fig. 3. For example, Li+ could coordinate to five Oc from a Pd5O5 fragment (2.107, 2.276, 2.261, 2.457, 2.336 Å) or to four Oc from a Pd4O4 fragment (2.002, 2.023, 2.005, 2.022 Å). Although the former configuration includes five coordinated oxygen atoms, the shorter bonds in the latter situation indicate stronger interactions. As expected, the energy differences illustrate that the C4 mode is more stable than C5 by 6.1 kcal mol−1 (Table S2‡). On the other hand, encapsulation of Na+, K+, Rb+, Cs+, Ag+ Ca2+, Sr2+, Ba2+, and Ra2+ (r > 1.2 Å) is expected to work well for the Pd15 nanostar cage. Some of them are strongly supported by experiments, which show that NaPd15, KPd15, BaPd15, and AgPd15 can be easily observed in the presence of Na+, K+, Ba2+, and Ag+.10 It is worth mentioning that Na+ prefers to coordinate to a Pd5O5 face (C5) rather than sitting at the body centre of Pd15, as suggested by experiment.10a Interestingly, Pd2+ prefers a C4 mode, binding to only four Oc of the Pd5O5 face, as shown by XRD.10a,c The good reproducibility of the experimental results by DFT reemphasizes that the size of the cation guest indeed plays an important role in the formation mechanism of the resulting palladate structure.
Following such strategy, all cations were divided into two domains with Ecom of Pd2+ as a reference (dashed grey line). The Ecom values below the reference line indicate that from a thermodynamic point of view, the respective ions are more favourable to stabilize the Pd12 host cage than the reference ion Pd2+. Thus, polyanion nanocubes of the type {MPd12(AsPh)8} are preferentially formed as compared to {Pd13(AsPh)8} (Pd13AsPh). For instance, encapsulation of Fe3+ and Sc3+ ions inside the Pd12 shell has associated complexation energies of −124.1 ({FePd12(AsPh)8}) and −116.2 kcal mol−1 ({ScPd12(AsPh)8}), respectively, both being significantly more exothermic than Pd2+ (−54.6 kcal mol−1, Pd13AsPh). Indeed, the nanocube family {MPd12(AsPh)8} with M = Ca2+, Co2+, Cu2+, Ni2+, Zn2+, Mn2+, Sc3+, Fe3+, Y3+, Yb3+, Lu3+, and Eu3+ has already been synthesized by using similar synthetic procedures.9 In contrast, encapsulation of M = alkali metal ions, Ag+, Be2+, Sr2+, Ba2+, and Ra2+ in {MPd12(AsPh)8} is expected to be difficult due to the less favourable complexation energy with respect to Pd2+, in spite of some of them (Li+) having a suitable size. In fact, these hypothetical polyoxopalladates have not been synthesized yet in the laboratory. Such conclusions are also valid for nanocube derivatives with other capping groups, such as arsenate (AsO43−) and selenite (SeO32−), see Fig. S3.‡ The absolute values of Ecom seem to be influenced by the charge of the capping group. Interestingly, almost identical complexation energies were obtained when replacing PhAsO32− by SeO32−, which have the same charge and the AsV–O and SeIV–O distances are similar. As based on size only, Ag+, K+, Rb+, Cs+, and Ba2+ can be excluded as guests for the nanocube cage Pd12.
We also decided to consider computationally the encapsulation of large guest cations by the 15-palladate nanostar cage {MPd15(AsPh)10} (Fig. S4‡), and then compare to Pd12. As expected, the larger guest ions Na+, Ag+, K+, Rb+, Ba2+, and Ra2+ ions were calculated to be both geometrically and energetically suitable for the Pd15 nanostar rather than the Pd12 nanocube, and these results are in full agreement with the experimental facts.10 Most of the smaller cations such as Mg2+ and Zn2+ do not fit geometrically and are also energetically unfavourable in Pd15. On the other hand, Pd2+ shows a similar ability to be encapsulated by the Pd12 nanocube and the Pd15 nanostar, which is consistent with the experimentally observed nanocube Pd13AsPh,8 as well as the mono- and di-palladium-centered nanostar derivatives {Pd ⊂ Pd15(PhAsO3)10} and {Pd2 ⊂ Pd15(PhAsO3)10}.10a,c The medium-sized Sr2+ ion was shown experimentally to form an unexpected ‘open-nanocube’ structure [SrPd12O6(OH)3(PhAsO3)6(OAc)3]4− (SrPd12-open),10e which inspired us to compute the complexation energies for both the open and closed nanocubes Pd12, see Table 1. The energy required for encapsulating Sr2+ into the open form SrPd12-open was computed to be −50.0 kcal mol−1, and for the closed form [SrPd12O8(PhAsO3)8]6− (SrPd12-closed) it was slightly less exothermic, −45.4 kcal mol−1. Therefore, computationally it is predicted that the open form SrPd12-open is preferentially formed compared to SrPd12-closed. Amongst the unfavourable guest ions for the closed nanocube shell Pd12-closed, Sr2+ shows the smallest Ecom difference compared to Pd2+ with only 9.2 kcal mol−1 (and only 4.6 kcal mol−1 for Pd12-open) at the B3LYP level, and even smaller differences at the M06 and ωB97XD levels. Thus, a competition between Pd2+ and Sr2+ guest ions is predicated computationally for such reactions. Experiments showed that only 2% of SrPd12-closed is formed, and that the Sr2+ ion can be substituted by Pd2+ to form Pd13AsPh by simply increasing the pH of the solution.10e On the other hand, SrPd12-open can indeed be isolated in clean form, but to date the Pd13-open structural type has not been prepared yet.
B3LYP | M06 | ωB97XD | Exp. ratio% | ||
---|---|---|---|---|---|
Pd2+ | Closed | −54.6 | −60.8 | −76.4 | 100% |
Sr2+ | Open | −49.9 | −61.0 | −75.8 | 98% |
Closed | −45.4 | −52.9 | −70.9 | 2% | |
La3+ | Open | −70.6 | −74.7 | −105.7 | 40% |
Closed | −74.2 | −75.2 | −105.9 | 60% |
It is interesting to note that Ecom for La3+ is very close to that calculated for Pd2+, which may lead to mixed products LaPd12-closed and Pd13AsPh. In contrast to Sr2+, calculations suggest that for La3+ the closed nanocube (LaPd12-closed) is slightly more favourable than the open one (LaPd12-open), see Table 1. Accordingly, the nature and size of the guest cation directly influences the shape of the resulting poly-oxopalladate and this in turn strongly suggests a template effect of the cation in polypalladate synthesis.
To date there is a dominance of 3d transition metal and lanthanide elements as central guests of the polyoxo-12-palladate nanocube family {MPd12L8}. Much less attention has been paid on p-block elements. We calculated the encapsulation of Ga3+, In3+, and Tl3+ as very exothermic with energies of −115.8, −137.1 and −155.1 kcal mol−1, respectively. For all three cations, the encapsulation is predicted to be more favourable than for the already reported Sc3+ derivative [ScPd12O8(PhAsO3)8]5−.9b In addition, there is a clear trend of increasing encapsulation ability going down group IIIA (B, Al, Ga, In, Tl).
On the other hand, to date no cation with a charge larger than 3+ has been encapsulated in the Pd12 nanocube shell. Therefore, we analysed computationally the encapsulation of several tetravalent cations, such as Sn4+, Zr4+, Th4+, U4+, and Hf4+. As expected, all tetravalent cations exhibit much favourable complexation energies due to the large anion–cation electrostatic interactions. The small Sn4+ ion has the lowest energy of all computed tetravalent ions and is hence the most promising candidate for encapsulation.
In order to verify the various theoretical predictions experimentally, we designed several key experiments, which concern mainly (i) encapsulation of p-block elements in the Pd12 nanocube shell, (ii) synthesis of open- and closed-nanocube isomers for La3+, and (iii) competition of three guest cations for Pd12 nanocube shell.
The 13C and 1H NMR spectra indicate good aqueous stability of all four polyanions (Fig. S5 and S6‡). Moreover, we also performed 71Ga and 115In NMR studies on solutions of Na-GaPd12 and Na-InPd12, respectively. The observed singlets in 71Ga NMR centred at 48.9 ppm (GaPd12, Fig. S7‡) and the singlet in 115In NMR centred at 247.7 ppm (InPd12, Fig. S8‡) are in full agreement with the solid-state structures. The spectra are clean, indicating that no impurities are present, and the signals are rather narrow, in spite of the quadrupolar nature of both isotopes (71Ga, S = 3/2; 115In, S = 9/2), which is a result of the cubic coordination environment around the metal ions combined with the highly symmetrical (cuboctahedral) structure of the overall polyanion, rendering the electric field gradient virtually zero.
We also performed ESI-MS studies in order to study the solution and gas phase properties of GaPd12 and InPd12. All peaks shown in the spectra can be assigned to species related to GaPd12 and InPd12, with different numbers of protons or sodium ions attached. For instance, the major envelopes centred at m/z = 1025.45 (Fig. S9a‡) and m/z = 1041.45 (Fig. S9b‡) can be attributed to the triply negatively charged [H2GaPd12]3− and [H2InPd12]3−. Additional MS assignments are summarized in Table S5.‡
For the same mixed, equimolar Ga/In system, the filtrated mother solution was allowed to evaporate until the maximum amount of crystals had formed, which were isolated when still covered by mother liquor. These crystals were analysed by ESI-MS and the spectrum obtained showed peaks corresponding exclusively to InPd12-related species (Fig. 6c). Notably, the envelopes appearing at m/z = 1021.46 and 1027.74 can be unequivocally ascribed to {Na[InPd12O8(C6H5AsO3)7(AsO3)+]}3− and {Na[InPd12O8(C6H5AsO3)7(AsO3)+](H2O)}3−, which may form from the plenary InPd12 cluster by losing one (C6H5)− fragment, perhaps during the electrospray ionization processes.
We also performed additional competition experiments, for example for the template pairs Ga3+/Sc3+ and In3+/Sc3+, respectively. Both 71Ga and 45Sc NMR signals could be detected for the Ga3+/Sc3+ system after the reaction, indicating that GaPd12 and ScPd12 are both formed and coexist in solution (Fig. S10 and S11‡). For the In3+/Sc3+ system, the 115In NMR signal for InPd12 could be detected after a few seconds; whereas the 45Sc signal for ScPd12 could only be obtained overnight. These results indicate that selective encapsulation features exist for the central cation guest M of the Pd12 nanocube, which fit well with the trends of the computed complexation energies shown in Fig. 4. Combining the theoretical and experimental results, we obtain a selectivity order of In3+ > Ga3+ ≈ Sc3+. The apparent Ecom difference between In3+ (−137.1 kcal mol−1) and Ga3+/Sc3+ (−116.2/−115.8 kcal mol−1) leads indeed to a pronounced encapsulation selectivity for In3+, whereas Sc3+ and Ga3+ are more difficult to be separated by polyoxopalladate formation, due to similar complexation energies.
![]() | ||
Fig. 7 Complexation energy Ecom (in kcal mol−1) of Mn+ encapsulated in Pd12L8 (L = PhAs) host shell and its decomposition terms ΔEdehyd, ΔEbind and ΔEdef. |
The dehydration energy (ΔEdehyd) of the cation guest M and the electrostatic interaction (ΔEbind) between M and the Pd12 nanocage exhibit large values, and in all cases ΔEbind is larger than the sum ΔEdehyd + ΔEdef, consequently, the Ecom term is always negative and hence exothermic. However, this does not mean that ΔEbind alone is sufficient to describe the Ecom trend. In Table 2 three examples are shown indicating that in absolute value ΔEbind is indeed dominant, but this term alone does not allow predicting the correct trend for Ecom. For example, let us consider the cation guest pair Ga3+/In3+, for which experiments clearly demonstrated that In3+ is captured preferentially over Ga3+. The more negative value for Ecom for In3+ originates essentially from the large deformation energy of 25.7 kcal mol−1 for Ga3+ (vs. 8.4 kcal mol−1 for In3+). The other two energy terms (ΔEbind and ΔEdehyd) are significantly larger in absolute terms for Ga3+ than In3+, but they cancel each other out. Notice that the Pd12 host cage deforms significantly more for Ga3+ than In3+, because the former is rather small.
Mn+ | E com | ΔEdef | ΔEbind | ΔEdehyd | Ionic radius | Critical energy term |
---|---|---|---|---|---|---|
Ga3+ | −115.8 | 25.7 | −1267.1 | 1125.5 | 0.92 | ΔEdef |
In3+ | −137.1 | 8.4 | −1146.9 | 1001.5 | 1.06 | |
Lu3+ | −115.0 | 5.7 | −1021.2 | 900.5 | 1.12 | ΔEdehyd |
Yb3+ | −103.0 | 4.9 | −1018.4 | 910.5 | 1.13 | |
Ce3+ | −95.5 | 5.8 | −935.2 | 833.9 | 1.28 | ΔEbind |
La3+ | −86.3 | 3.9 | −920.6 | 830.4 | 1.30 |
For the Lu3+/Yb3+ pair, it can be noticed from the values in Table 2 that Lu3+ has a more negative complexation energy by 12 kcal mol−1. The radii of both ions are virtually identical and the deformation and binding energies are rather similar, and so it can be concluded that the dehydration energy is the critical term in this case.
Finally, the third ion pair Ce3+/La3+ allows to identify the relevance of electronic structure. The La3+ and Ce3+ ions have the same charge and essentially identical ionic radii, but the larger atomic number for Ce3+ leads to a higher effective nuclear charge and due to the low shielding of f electrons Ce3+ has a larger (more negative) binding energy than La3+, which in turn leads to a higher (more negative) complexation energy for the former.
In summary, Ecom depends mainly on the following four properties of the metal ion guest: (i) effective ionic radius, (ii) valence state, (iii) dehydration ability, and (iv) electronic configuration and resulting charge-accepting ability. The selectivity for a given cation guest is the result of a delicate balance between the cation–polyoxopalladate and the cation–solvent interactions.
The prototype Pd13 is formed by condensation of [Pd(H2O)4]2+ complex cations in the presence of arsenate anion heterogroups. Nevertheless, if the solution contains other cations Mn+, then in principle {MPd12L8} type species may also be formed, determined by the favourable complexation energy and the relative competition with respect to Pd2+ ions. After an exhaustive computational analysis of complexation and dehydration energies for a series of cation guests we were able to identify the most promising cations to be encapsulated inside the Pd12 nanocube shell. Trivalent and tetravalent cations are easily trapped inside Pd12, whereas monovalent cations are largely elusive. As based on the calculations, we also performed target-oriented synthetic studies and we were able to isolate four novel polyoxopalladates: (i) 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 [GaPd12O8(PhAsO3)8]5− (InPd12). All four compounds were fully characterized in the solid state, in solution, and in the gas phase. In particular 115In and 75Ga NMR combined with mass spectrometry were very useful, as these techniques allowed to perform speciation studies. We demonstrated that in solutions containing In3+ and Ga3+ ions only the former is incorporated in the Pd12 shell, due to its more suitable size and higher complexation energy. DFT method also predicted that the large La3+ ion should fit in the Pd12 host shell. The experimental work following the computations indeed resulted in the successful synthesis of the regular nanocube LaPd12-closed as well as the open-shell structure LaPd12-open. These results reemphasize that size and dehydration energy of the cation guest are the key driving forces in the formation mechanism of nanocuboid polyoxopalladates of the type {MPd12L8}. Our work has demonstrated how powerful the interplay between theory and experiment can be. We predict that other cations such as Cd2+, Tl3+, Sn4+, Zr4+, Hf4+, U4+, and Th4+ amongst others are potential candidates for encapsulation inside the Pd12 host and our efforts are geared in this direction.
Footnotes |
† Dedicated to Professor Walter Klemperer on the occasion of his 70th birthday. |
‡ Electronic supplementary information (ESI) available: Details on experimental methods; the experimental and computational related tables and figures. CCDC 1555480–1555483. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7sc03441e |
§ Z. L. and P. Y. equally contributed to the work. |
This journal is © The Royal Society of Chemistry 2017 |