Crystallizing Elusive Chromium Polycations
Nyman and colleagues elucidate a mechanism for metal-oxo cluster growth and crystallization that avoids the predominance of fully formed clusters in solution. Rather, only monomer and dimer precursors to the 14-metal-center Keggin ion are observed spectroscopically. Cluster self-assembly and crystallization occur simultaneously at the solution surface with evaporation of HNO 3 -H 2 O azeotrope. This pathway allows the isolation of hydrolytically unstable clusters, including the Zn 2+ -Cr 3+ Al 3+ polyoxocation structurally described here.
INTRODUCTION<!>The Bigger Picture<!>Structure<!>Computational Studies<!>Solution Characterization<!>DISCUSSION<!>X-Ray Scattering Analysis<!>EXPERIMENTAL PROCEDURES<!>Monitoring Reaction Solutions with Heating<!>Electron Microprobe Analysis<!>Determination of Water Content and Moiety Formula<!>ESI-MS<!>Computational Details<!>ACCESSION NUMBERS
<p>Simple molecular oxo-hydroxo metal clusters, assembled in water without stabilizing organic ligands, are building blocks for both natural and synthetic metal oxides. [1][2][3][4] Molecular clusters function as homogeneous catalysts; [5][6][7] as separation reagents for environmental, 8 energy, 9 and biomedical applications; 10 and in the crystallization of biomolecules. 11 Controlling cluster reactivity has led to breakthroughs in their use as precursors for thin-film deposition 12 and high-resolution nanopatterning. 13 Among all metals in the periodic table, only oxo-hydroxo clusters of Al 3+ , Ga 3+ , U 6+ , and the d 0 transition metals V 5+ , Nb 5+ , Ta 5+ , Mo 6+ , and W 6+ are readily isolated in stable forms without supporting organic or inorganic ligands. [14][15][16] Certainly, ligation plays an important role in inorganic and materials chemistry, as well as geochemistry. For example, organic ligands provide a mechanism to stabilize clusters that form in water, yielding structural information that is important for elucidating aqueous reactivity and speciation of metal cations. 4,17 Organic ligands are historically and currently valuable for isolating transition-metal and lanthanide single-molecule magnet clusters that possess unique magnetic behavior. [18][19][20] Inorganic ligands (polyoxometalate fragments and oxoanions) are also used for capturing transient polynuclear forms to elucidate speciation [21][22][23] and provide water-soluble cluster forms for catalysis, for instance. 7,24 Inorganic oxoanion ligands link clusters into functional open-framework materials. 25 However, many applications are hindered by ligands. To advance the use of discrete clusters in medicine, 26 technology, 27,28 and other important applications, we must determine how to isolate clusters of any metal in the periodic table without the use of chemistry-limiting ligands.</p><!><p>Aqueous metal-oxide clusters are important in natural processes, in synthesis, and in technology. In nature, they are central to contaminant transport, mineral growth, photosynthesis, and biomineralization. In synthesis, they provide nanometric forms with size-dependent properties, and they are pre-assembled building blocks for materials. Fabricating functional metaloxide clusters and cluster-based materials in water minimizes environmental impact. Clusters synthesized and manipulated in water provide an understanding of natural processes and inspire biomimetic materials, i.e., for artificial photosynthesis.</p><p>Although synthesis of functional clusters can be accomplished with great control in organic solvents, it doesn't offer the simplicity and sustainability of aqueous synthesis. However, aqueous syntheses that provide wellcontrolled cluster and material growth are not well developed. The unique cluster-formation processes created in this study open the door to a new understanding of natural processes and the control of aqueous synthesis. pH-dependent speciation diagrams from decades of research 29 convincingly show that (1) cluster assembly in water is controlled predominantly by pH and concentration, (2) clusters generally revert to monomers or aggregates and precipitates outside a specific pH stability window, and (3) clusters must be abundant in solution before crystallization (as an important purification step) can proceed. Hence, isolating clusters existing only in narrow pH and concentration windows is very difficult. Clusters of open-shell transition metals fall into this class, because the partially filled d shell promotes the reactivity of coordinated aqua ligands 30 and thus causes uncontrolled hydrolysis and condensation.</p><p>Here, we demonstrate isolation via crystal growth of a discrete mixed-metal cluster containing the open-shell transition metal Cr 3+ . We show that the cationic cluster [ZnO 4 Al 5.1 Cr 6.9 (OH) 24 (H 2 O) 12 Zn(H 2 O) 3 ] 8+ , hereafter abbreviated as Zn(CrAl) 12 , is readily isolated in single-crystal form as a nitrate salt, but we never observed pre-assembly in solution. Cluster formation in solution is suppressed by excess nitrate, which stabilizes monomeric or dimeric aqua metal cations instead. Removal of nitrate by evaporation of the HNO 3 -H 2 O azeotrope drives cluster formation only as a crystalline solid. This ''classical'' growth mechanism, a description of metal-oxide formation without polynuclear cluster assembly prior to crystallization, bypasses the unstable solution cluster state that usually results in aggregation and production of an insoluble hydroxide. [1][2][3]31 Crystallization of clusters without initial persistence in solution has not previously been considered a way to isolate highly reactive clusters. The synthetic strategy employed here exploits both a slow hydrolysis process (by oxidative dissolution of zinc in the metal nitrate solution) and heterogeneous solution conditions that presumably allow condensation reactions only at the liquid-air interface, where rapid crystallization is also seeded. However, once the Zn(CrAl) 12 cluster is isolated in the solid-state lattice, it is stable and can be redissolved, characterized, and exploited in different solvents, including water and dimethylformamide.</p><p>On the other hand, isolation of the Zn(CrAl) 12 cluster simply provides two important advances in polyoxocation chemistry. First, the Cr polycation Keggin ion (or related cluster) was long thought to exist as an aqueous species, but prior attempts of its isolation proved unsuccessful. 32 Second, Zn in the center of the Keggin ion was predicted to offer stabilization of polycationic Keggin ions, 8,33 but again, this could not be proven without definite structural evidence. Isolation and structural characterization of Zn(CrAl) 12 has now provided an opportunity to probe via computation and experiment the stability of chromium polycations, as well as the effect of the Zn center, the Zn cap, and more broadly, the relationship between capping Keggin ions and rotational isomerization.</p><!><p>The relationship between structure (Figures S1-S3 and Data S1) and composition of Zn(CrAl) 12 provides insight into its formation, as will be discussed later. It is a Zncentered, Zn-capped d-Keggin ion (Figure 1), represented as a stacking of mixed-occupancy Cr/Al-centered octahedra: basal (blue), equatorial (magenta), and apical (teal). The occupancies of Cr and Al in these sites were determined from the X-ray structure and derived bond valence sums (BVSs; Tables S1-S16). Free refinement gives Cr/Al ratios of 0.37:0.63 for the CrAl3 basal plane, 0.67:0.33 for the CrAl2 equatorial plane, and 0.89:0.11 for the CrAl1 apical plane. Hence, the average Cr/Al ratio is 6.9:5.1, consistent with electron microprobe analysis on the bulk material (Cr/Al = 6.8:5.2; Tables S17 and S18). Metal-oxygen bond distances increase from CrAl3 (1.90-1.94 [3] A ˚) to CrAl2 (1.95-1.98 [3] A ˚) to CrAl1 (1.94-2.00 [3] A ˚)-all correlated to the larger six-coordinate ionic radius of Cr 3+ (r = 0.76 A ˚)</p><p>and Al 3+ (r = 0.68 A ˚). From BVS analysis, H 2 O occupies the terminal positions of the Cr/Al octahedra and the Zn cap (oxygen BVS = 0.4-0.5), OH À bridges the m 2 and m 3 modes (oxygen BVS $ 1.0), and O 2À bridges the central Zn and neighboring Cr/Al sites (oxygen BVS $ 2.0).</p><!><p>Computational studies were executed to elucidate the origins of the observed (Figure 1B) preferential occupancy in the CrAl position next to the Zn cap. These studies also yielded a broader understanding of Keggin ion rotational isomerization.</p><p>The site preferences of Cr 3+ among the three nonequivalent octahedral positions have been elucidated by theory. The observed $7:5 Cr/Al ratio leads to an intractable number of isomers for computation. Therefore, we modeled the cluster as a d-Keggin with the metal composition ZnAl 6 Cr 6 . This formulation is similar to the experimentally determined composition, but it sets a manageable number of isomers: 16 (Table 1). The degeneracy of each isomer is derived from combinatorial algebra: with m Cr atoms in CrAl3, n in CrAl2, and l in CrAl1, the total number of isoenergetic structures will be (6/m)3(3/n)3(3/l), where n + l + m = 6. All positional isomers and corresponding degeneracy (number of Cr/Al arrangements within the three layers; Figures S1 and S2), relative energy, and Boltzmann weight are compiled in Table 1. Boltzmann weight is defined in the Supplemental Information; additional isomers that were considered for those lowest-energy structures are also provided, confirming the hypothesis that Cr/Al arrangement within layers is less important.</p><p>The energetically most stable arrangements correspond to those with the highest Cr contents in the equatorial and apical layers (0/3/3, 1/2/3, and 1/3/2). Analysis employing the Boltzmann distribution yielded a Cr population of 92% for the three lowest-energy isomers. The highest Boltzmann weight (55.3%) does not correspond to the lowest energy structure because of the large number of degenerate structures. Considering the Boltzmann weight of a given structure, the mean theoretical occupancies of each metal atom per layer can be determined (see Supplemental Information). The agreement between simulated and experimental compositions is remarkable (Table 2). The largest deviation is in the basal layer, where a simulated Cr occupancy of 15% compares with an experimental value of 37%. This result is an artifact of our simplified computational model, which assumes a lower Al content than observed experimentally.</p><p>Hirshfeld charges, q, 34 of the hydroxo ligands in the apical plane of simulated Al 13 , ZnAl 12 , and ZnCr 12 d isomers were slightly more negative (q o = À0.302) than those of the other hydroxo ligands in the cluster (q o = À0.285). This result is illustrated visually in the molecular electrostatic potential (MEP) of d-ZnAl 12 (Figure 1C). The capping of the cluster by (Zn(H 2 O) 3 ) 2+ can therefore be rationalized as a consequence of the additional nucleophilic character of the hydroxo ligands in the apical trimer. Moreover, the computed nucleophilicity of the hydroxo ligands in the apical plane is consistent with the observation that all d-Keggin polycation isomers and derivatives are capped by an additional metal ion. 35,36 To further consider the selectivity of Cr over Al in the apical plane, we performed fragment analyses of hypothetical d-ZnAl 12 and d-ZnCr 12 with the (Zn(H 2 O) 3 ) 2+ cap. These analyses revealed a considerable difference in interaction energies, i.e., 152 kJ$mol À1 in favor of d-Zn 2 Cr 12 over d-Zn 2 Al 12 . This result indicates a thermodynamic preference for association between the (Zn(H 2 O) 3 ) 2+ cap and a Cr-rich apical plane. Overall, the computations show that the preferred apical-plane occupation by Cr drives the experimentally observed site ordering.</p><p>We conclude the computational discussion with consideration of the stabilization effect of ZnO 4 as a templating central polyhedron of the Keggin ion in comparison to AlO 4 (Table 3). For all rotational isomers, the AlO 4 -centered cluster is more stable than the ZnO 4 -centered cluster. Although this calculation is consistent with the fact that the ZnO 4 -centered Keggin ion could not be previously isolated, it is inconsistent with the results of our synthetic experiments, in which only ZnO 4 occupied the center of the Keggin ion. This is a fact confirmed by 27 Al NMR of the dissolved Zn(CrAl) 12 crystals, which did not show the typical tetrahedral AlO 4 at 62 ppm observed in 27 Al NMR spectra of the aqueous tridecameric Al 13 Keggin ion. The pH of the reaction solution (2.0-2.5) is probably too low to allow abundant AlO 4 , whereas AlO 4 -centered aluminum polycation Keggin ion can be isolated from pH 4.0-4.5 solutions. This is also the emerging explanation for conditions that favor different tridecameric Al polycations: the ''flat'' Al 13 (discussed below) containing only octahedral aluminum forms at pH 2-3, and the Keggin Al 13 forms at pH 4-5. However, deprotonation of a Zn-H 2 O complex is considerably less favorable than an Al-H 2 O complex, discussed later in relation to the pKa of metal-bound water. On the other hand, structural and related computational studies of Zn in protein complexes, for example, show a preference for tetrahedral coordination. 16</p><!><p>Zn dissolution in an acidic metal nitrate solution to drive condensation of monomers to clusters was first demonstrated for synthesis of the ''flat'' Al 13 cluster [Al 13 (OH) 24 (H 2 O) 24 ] 15+ from Al(NO 3 ) 3 . 37 The acidity of the reaction solution before the addition of zinc (pH 2.2) results from deprotonation of water molecules bound to the metal cations (M = AlCr):</p><p>where y is some fraction of the total metal cation concentration. In this reaction, Zn metal could reduce either H + or NO 3 À with the following potentials:</p><p>Zn 0 / Zn 2+ + 2e À (À0.76 V) (Equation 2) 3)</p><p>(Equation 4)</p><p>Equations 3 and 4 both consume protons and increase the pH, and the reduction potential suggests that Equation 3 most likely dominates over Equation 4. The pH increases in the reaction solution with increasing additions of zinc metal (Figure 2A; Table S19). With increased Zn addition, which drives Equation 1 to the right as protons are consumed, oxolation (Equation 5) or olation (Equation 6) occurs between hydrated and partially deprotonated metal cations, i.e.: 5) 6)  7)</p><p>After the solution was allowed to evaporate in open air at room temperature for 2 weeks, the solution's pH dropped from 3 to 2, and the blue-green crystals of Zn(CrAl) 12 were observed. The decrease in pH is owed to the release of protons upon cluster assembly (Equation 7) and an increase in solution concentration with evaporation. The reaction solutions with variable zinc concentration were characterized by small-and wide-angle X-ray scattering (SWAXS). With increasing zinc concentration, scattering intensity increased, indicating linking of metal cations to form larger dissolved species, i.e., Equations 5, 6, and 7 (Figures 2A; Figures S3  and S4). We prepared a control experiment by dissolving Zn nitrate in the Al/Cr nitrate solution instead of zinc metal (control 5, Table S19). This solution did not exhibit the increase in scattering intensity (Figures S3 and S4) that is consistent with the hypothesis that oxidative dissolution of Zn 0 drives pH increase and thus leads to hydrolysis and condensation reactions. On the other hand, X-ray scattering intensity of the reaction solution decreased with aging (Figure S5; Figure 2B), consistent with loss of the larger scattering species (as crystallization begins).</p><p>Despite the qualitative increase in scattering intensity with increased Zn to drive hydrolysis, we were surprised that the SWAXS data of the reaction solutions never indicated the formation of a fully intact cluster, even at the point where Zn(CrAl) 12 crystals were abundant. In fact, the scattering was very weak; this is emphasized in Figure S7 of the data without background subtraction, allowing comparison of the scattering intensity with the solvent (water) peak.</p><p>However, isolating the crystals and redissolving them in water (Figure 3) led to scattering, which is expected and usually observed in solutions of fully formed clusters. X-ray scattering of dissolved Zn(CrAl) 12 crystals was more similar to that of the simulated Zn(CrAl) 12 , as were the radii of gyration R g (experiment) = 4.1 A ˚and R g (simulated) = 3.9 A ˚, determined by a Guinier analysis (q = 0.25-0.35 A ˚À1 ). On the other hand, the Guinier analyses of all reaction-solution scattering curves yielded R g = 2.7 A ˚(R g measured at q = 0.3-0.5 A ˚À1 ; Figure S8). However, the intensity of the initial scattering decreased with increasing time and decreasing pH (Figure 2), meaning that the size of the species remained constant while the concentration varied. In summary, although X-ray scattering suggested that hydrolysis and condensation reactions occurred in the reaction solutions, full cluster formation was never observed before or in tandem with crystallization, which is unexpected given our experience with other metal-oxo cluster systems. 4,38,39 For an additional control experiment, we prepared a solution of the Al 13 Keggin ion as per the well-known synthesis. 40 In this synthesis, 2.6 equiv of NaOH was added dropwise to a heated aqueous solution of 0.1 M aluminum nitrate. This solution indeed yielded X-ray scattering, as expected from a solution of fully formed clusters (Figure 4). Note, however, that this reaction solution contained $1/10 th of the concentration of the cluster-forming metal in the optimal synthesis of Zn(CrAl) 12 . This is an important point that is discussed later.</p><p>Electrospray ionization mass spectrometry (ESI-MS) gave an independent assessment of the reaction solutions. Even at the point of saturation and growth of Zn(CrAl) 12 crystals, ESI-MS revealed predominantly aqua monomers of Al, Zn, and Cr (Table S20) and dimers of Al (Figure 5A; Figure S6) with small error (G0.001) between experimental and calculated mass/charge ratios of assigned peaks. The spectrum in Figure 5B was collected for reaction solution 4 (Table S19) at pH 3.0 with 1 day of aging. Spectra were collected every day until crystallization, and no change was observed in the identified species. ESI-MS (Figure 5B) of Zn(CrAl) 12 crystals dissolved in water showed a complicated mixture of clusters with a range of Cr/Al ratios, representing parent clusters and their daughter ionization fragments. The tridecamer with and without the Zn cap (Table S21) was observed. The natural isotopic distribution of both Zn and Cr rendered signature peak envelopes that could be simulated, greatly improving confidence in species identification in both the reaction and dissolved-cluster solutions (Figure S7). These data show that (1) the Cr/Al ratios of the observed compositions correlate well with the bulk composition determined from structure, electron microprobe, and theory and (2) clusters with higher Cr content are more extensively fragmented by the ionization process, providing evidence for their lower kinetic stability. [40][41][42] In contrast to the reaction solutions, clusters were readily detected in solutions prepared by dissolution of Zn(CrAl) 12 crystals. These data confirmed our SWAXS observations: clusters were not present in the reaction solutions at the point of saturation and crystallization of Zn(CrAl) 12 . Together, ESI-MS and SWAXS provide a consistent picture of a reaction solution dominated by monomers However, we cannot ignore that monomers were also observed in solutions of dissolved Zn(CrAl) 12 crystals. Although ESI-MS peak intensities do not reliably quantify species concentrations, the ratio of cluster-peak intensity to monomerpeak intensity was $1.6. Likewise, we cannot ignore the mismatch between the calculated and experimental scattering data in Figure 3, particularly between 0.5 and 0.8 A ˚À1 . The shallower slope of the experimental scattering curve indicates smaller species that scatter at higher q, such as the monomers observed in the mass spectral data. These observations show that the clusters are not entirely stable in neat water. However, aging of the solution does not result in any major change.</p><!><p>Proposed Pathway for Zn(CrAl) 12 Cluster Growth How do Zn(CrAl) 12 crystals grow from a saturated solution when clusters are not detected in the reaction solution? Perhaps the clusters are highly insoluble in the reaction solution as they form; they might be rapidly consumed via crystal growth.</p><p>To test solubility, we submerged Zn(CrAl) 12 crystals in a reaction solution. The crystals dissolved quickly, and SWAXS measurements revealed that the Zn(CrAl) 12 clusters reverted to small polydisperse species, shown in Figure 4. This result is consistent with the acidity constants of the aqua ions:  S19) and solution of dissolved crystals. Intensity [I(q)] is normalized for ease of comparison. Zn(CrAl) 12 crystals were actively growing from the reaction solution.</p><p>pK a [Cr(H 2 O) 6 ] 3+ = 4</p><p>The Zn aqua ion is the least acidic, and the Cr aqua ion is the most acidic. According to the pK a values of all the metals and the low pH (2-3) of the reaction solution, the bridging hydroxyl linkages are expected to hydrolyze, which would then dismantle the clusters. Consequently, the reaction solution does not support fully formed clusters.</p><p>We hypothesize that, besides pH, the NO 3 À (aq) concentration also orchestrates cluster formation. Zn(CrAl) 12 crystals only grow with evaporation-a capped solution does not produce crystals. According to electrochemical potentials, Zn metal is expected to primarily reduce H + (aq). Accordingly, the NO 3 À (aq) concentration remains constant and high throughout the Zn dissolution process. This high concentration further inhibits condensation. To reduce the NO 3 À (aq), we heated the reaction solution in a distillation setup to rapidly evolve nitrate as the H 2 O-HNO 3 azeotrope (bp = 85 C at 1 atm). The distillate tested acidic with a pH strip, and its Raman spectrum matched that of 15.7 N HNO 3 (aq) (see Figure S8). These results show that HNO 3 (aq) evolves as expected via azeotropic evaporation. The controlled reduction in the NO 3 À (aq) concentration also prompts and expedites crystal growth.</p><p>Because of the loss of HNO 3 (aq) on heating, the pH of the reaction solution is expected to increase, but it decreased from 3 to 2. Solution hydrolysis (Equation 1) and condensation reactions (Equations 5, 6, and 7) reduce the pH. ESI-MS data (Equation 4) showed that condensation produced mixed metal dimers (see also Figure 6 and Table S22). Similarly, SWAXS data in Figure S9 revealed weak scattering and average R g = 2.7 A ˚over the q range 0.3-0.5 A ˚À1 , results consistent with the ESI-MS study. Heating might be expected to produce some uncontrolled condensation and hydroxide precipitation. However, with heating, we observed only limited</p><!><p>The scattering curves of the reaction solutions of Zn(CrAl) 12 are compared with that of Keggin Al 13 , as well as those of Zn(CrAl) 12 crystals redissolved in a fresh reaction solution. I 0 intensities are scaled to match for ease of comparison.</p><p>solution cloudiness as the characteristic blue crystals of Zn(CrAl) 12 grew. SWAXS data revealed a change in scattering intensity below q = 0.1 A ˚À1 , consistent with aggregation and the observed cloudiness of the solution. Also, an increase in the slope of the scattering curves near q = 1 A ˚À1 at elevated temperatures indicates increasing condensation products, consistent with the ESI-MS results. With heat, the crystals grow in 1 day, whereas they take several weeks under room-temperature evaporation.</p><p>During the initial stages of Zn(CrAl) 12 synthesis by Zn metal dissolution, we suggest that the combination of low pH ($2) and high nitrate concentration hinders complete cluster assembly. Instead, monomeric aqua ions and small oligomers exist and form in the reaction solution. The persistence of larger clusters in solution commonly produces aggregation and precipitation of metal oxides or hydroxides. For example, iron oxide and calcium carbonate readily precipitate from aqueous solutions with very low concentrations of cations and carbonate, after clusters are observed in solution. 2,31 These solids precipitate from pre-nucleation clusters at concentrations below 10 mM metal. The conventional Al 13 Keggin synthesis with NaOH(aq) requires relatively low metal concentrations (0.1 M), given that synthesis with concentrated solutions near 1.01 M Al produces aggregates and precipitates, as does the analogous NaOH(aq) addition to 1.01 M Zn/Cr/Al nitrate solutions. These syntheses highlight the importance of a preparative strategy to prevent aggregation in the bulk solution. We have done so with the Zn(CrAl) 12 synthesis at 1.01 M metal through (1) pH regulation via Zn dissolution to minimize rapid, uncontrolled hydrolysis and condensation reactions and (2) controlled nitrate evolution via azeotropic evaporation.</p><p>Finally, we consider where the Zn(CrAl) 12 clusters assemble and the crystals nucleate, if not in the bulk solution. We suggest two possibilities: (1) the clusters nucleate and grow directly on the surface of the container from monomers and small, oligomeric species in the bulk solution or (2) pre-nucleation clusters or complete Zn(CrAl) 12 clusters form at the water-air interface.</p><p>We grew crystals in different types of containers, i.e., glass, Teflon, and plastic. We observed no difference in crystal morphology, growth rate, or composition. Also, we observed crystals only at the bottom of each container and on the surface of the  S19).</p><p>solution, suggesting that crystals might not nucleate on the container surfaces. Consequently, proposal 1 is not supported by these observations.</p><p>As for proposal 2, Antonio and co-workers 43 recently characterized both the surface and the bulk of a drop of an aqueous Er(NO 3 ) 3 solution by X-ray absorption spectroscopy. The spectra show clusters near the surface, whereas only monomers existed in the bulk. Ions, including hydrolyzed metal aqua ions, tend to concentrate near the aqueous-air interface, 44 which then favors condensation and cluster formation. In the Zn(CrAl) 12 system, we propose that similar effects produce high concentrations of hydrolyzed aqua ions near the solution surface. Their condensation is enhanced by reduction of the nitrate concentration via evaporation of the nitric-acid-water azeotrope. Collectively, these processes most likely produce low concentrations of high nuclearity clusters that then can contribute to crystal growth. We propose that X-ray absorption measurements could reveal the presence of clusters near the water-air interface in the Zn(CrAl) 12 system. These measurements and others will help inform our understanding of the pathways by which such clusters form and crystallize.</p><!><p>Synthesis of Zn(CrAl) 12  For preparation of the reaction solution, Al(NO 3 ) 3 $9H ). Zinc metal powder (0.20 g, 0.3M concentration) (Alfa Aesar, À140+325 mesh, 99.9% metals basis) was then added to this clear solution. The molar ratio between Zn and (Al+Cr) was 1:2, and the total metal concentration was 1.01 M. The reaction mixture was stirred overnight, and the clear solution was then filtered into a small vial and placed in ambient conditions for evaporation. Numerous dark-green crystals were formed after 2-3 weeks. Yield was $70% on the basis of Cr.</p><!><p>To investigate the reaction pathway, we also carried out the synthesis with heating (40 C, 50 C, 60 C, 70 C, and 80 C). Both X-ray scattering and ESI-MS (see below) were used for monitoring these reaction solutions. We also collected the vapors that distilled from these heated solutions via condensation, and this solution was characterized by Raman spectroscopy (Thermo Scientific DXR SmartRaman spectrometer, 780 nm laser source). Solutions contained in a glass vial were placed on the sample window for data collection (step size = 1 cm À1 , range = 600-1,000 cm À1 , number of scans = 8, and exposure time = 2 s).  S10 for details). We could not find a solution for this disorder, and the disordered NO 3 anions and solvent water in the structure were treated by SQUEEZE. 45 Correction of the X-ray data by SQUEEZE yielded 289 electrons, and the required value for eight NO 3 À anions is 256 electrons in the symmetrically independent part. The structure has been deposited in the Inorganic Crystal Structure Database under accession number ICSD: CSD-428054.</p><!><p>Data were collected with a Cameca SX-50 with wavelength dispersive spectrometers and gas-flow proportional detectors with P-10 gas; electrons were accelerated under 10, 15, and 20 kV, and the generated Ka line intensity of Zn, Al, Cr, N, and O was recorded during the measurement. Measurement was done at six different positions on the crystal. Zn metal, Al metal, Cr metal, AlN, and MgO were used as standards, and a background correction was applied as described in detail by Donovan and Tingle. 46 Data analysis was done with ProbeWin software. Note that with the proper references, these data yielded a composition consistent with the BVS from crystallography and free refinement of mixed-occupancy Cr/Al sites (according to thermal parameters). Therefore, this method is considered acceptable for compositional analysis.</p><!><p>The solvent content in Zn(CrAl) 12 crystals was determined by thermogravimetric analysis (TGA) with a Q50 TGA analyzer (TA instruments). Samples were run at a heating rate of 10 C/min up to 850 C in a nitrogen environment with a flow rate of 100 mL/min. The total weight loss was 55%, consistent with 16 water molecules. The structure had a free void space (from the checkcif) of</p><!><p>ESI-MS was carried out with an Agilent 6230 TOF mass spectrometer. Reaction solutions were diluted 10-fold, and 10 mL aliquots were injected with water as the mobile phase and flow rate of 0.4 mL/min. Fragmentation voltage, cone voltage, and octopole voltage were set to 100V, 50V, and 750V, respectively. SWAXS X-ray scattering data were collected on an Anton Paar SAXSess instrument utilizing Cu-Ka radiation (1.54 A ˚) and line collimation and equipped with a 2D image plate for data collection in the q = 0.018-2.5 A ˚À1 range. The sample to image-plate distance was 26.1 cm. All solutions were contained in a sealed disposable 1.5-mm-diameter borosilicate capillary tube, and the data-collection time was 30 min. All solutions were filtered before analysis for the removal of large particles, e.g., dust. Likewise, background solutions were measured for 30 min. SAXSquant software was used for data collection and treatment (normalization, background subtraction, desmearing, and smoothing to remove extra noise created by the desmearing routine). Differences in scattering intensity in the high q range ($q > 1 A ˚À1 ) from similar solutions were the result of imperfect background subtraction, a universal problem with these small clusters. However, we found that varying processing parameters (i.e., normalization of scattering curves before background subtraction) gave similar results. Therefore, the data interpretation is robust. All analyses were carried out with the IRENA 47 macros within IgorPro 6.3 (Wavemetrics) software. Simulation of scattering data was performed with SolX. 48,49 Initially, an .xyz file of the cluster-only was prepared (with no symmetry elements) and converted to .pdh format for import into SolX.</p><!><p>The Amsterdam Density Functional (ADF) program package (Scientific Computing and Modeling ADF-2013.01, http://www.scm.com) version 2013.01 was used throughout to optimize the geometries of all species considered herein. The Becke 50 and Perdew 51,52 gradient-corrected exchange and correlation functionals (BP86), respectively, were used in the calculations. The ZORA 53,54 scalar relativistic Hamiltonian was employed with a triple zeta Slater-type orbital 53 augmented with one polarization function (TZP) for all atoms. A small frozen core was used for all elements (1s2 shell for O and C and 2p6 shell for Al and Zn) except hydrogen. The geometry optimizations were performed via the default numerical integration scheme of Becke. 55 Stationary points were located with the default integration accuracy and employed the COSMO implicit solvation scheme. No symmetry constraints were used. Because magnetic couplings are small and in the order of cm À1 , all Cr(III) derivatives were calculated as high-spin open-shell systems.</p><!><p>The crystal structure for the nitrate salt of [ZnO 4 Al</p>