From Ordinary to Extraordinary: Insights into the Formation Mechanism

Jul 14, 2016 - Nanojars are large (2 nm wide) anion-incarcerating coordination ... in sync with the coordination preference of metals, lead to complex...
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From Ordinary to Extraordinary: Insights into the Formation Mechanism and pH-Dependent Assembly/Disassembly of Nanojars Basil M. Ahmed and Gellert Mezei* Department of Chemistry, Western Michigan University, Kalamazoo, Michigan 49008-5413, United States S Supporting Information *

ABSTRACT: Nanojars are large (2 nm wide) anion-incarcerating coordination complexes of the composition [anion⊂{Cu(μOH)(μ-pz)}n] (n = 27−36), formed by the self-assembly of simple Cu2+, HO−, and pyrazolate (pz− = C3H3N2−) ions in the presence of certain anions with large hydration energy (e.g., CO32−, SO42−, PO43−, HPO42−). Nanojars display spectacular chemical properties, such as unparalleled anion binding strength and, as shown herein, extraordinary resistance to extreme alkalinities (10 M NaOH). To shed light on the mechanism of the self-assembly process leading to these distinctive constructs, we employed an array of complementary techniques including mass spectrometry, pH titration, UV−vis and NMR spectroscopies, chemical synthesis, and single-crystal X-ray diffraction. In the reaction of Cu(NO3)2, pyrazole, NaOH, and Na2CO3 in tetrahydrofuran (THF), the first major intermediate is a trinuclear copper pyrazolate complex, [Cu3(μ3-OH)(μ-pz)3(NO3)2(H2O)], which was separately isolated and characterized. As the THFinsoluble NaOH slowly reacts, the nitrate ions are gradually precipitated out as NaNO3 and replaced by hydroxide ions. The resulting species, [Cu3(μ3-OH)(μ-pz)3(OH)x(NO3)3−x]− (x = 1−3), have unstable terminal Cu−OH groups and react with each other to yield OH-bridged units, such as [Cu3(μ3-OH)(μ-pz)3(NO3)2]2(μ-OH) and then [{Cu3(μ3-OH)(μ-pz)3(μOH)2}x(NaNO3)y(Na2CO3)z] oligomers. The Cu3(OH)3(pz)3 repeating units of these oligomers have the same composition as the [Cu(OH)(pz)]n (n = 3x) nanojars and rearrange to the final products, Na2[CO3⊂{Cu(μ-OH)(μ-pz)}n] (n = 27, 29, 31), while eliminating the last amounts of NaNO3. pH titration, UV−vis monitoring, and chemical synthesis also confirm the formation of the trinuclear intermediate, followed by its clean transformation to nanojars. While displaying an unusual stability to high pH, nanojars are sensitive to acids stronger than water, a property exploitable for the recovery of the incarcerated anion. On lowering the pH, nanojars first break down to trinuclear complexes and finally to copper ions and pyrazole. This process is fully reversible, and nanojars are reassembled as pH is increased.

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INTRODUTION The self-assembly of large metal−organic coordination architectures is a fascinating, intricate process whereby the intrinsic structural characteristics of multitopic ligand molecules, in sync with the coordination preference of metals, lead to complex assemblies via multiple ligand−metal coordinate bond formation.1 Depending on the geometric placement of the donor atoms and on the flexibility of the ligand, the resulting coordination architecture can either assume a discrete often highly symmetric structure (such as polygons and polyhedra)2 or extend into a 1D, 2D, or 3D framework.3 Whereas the geometry of mononuclear metal complexes can be more easily predicted based on the coordination requirements of the metal and the directionality of metal−ligand bonds,4 the structure of novel, self-assembled multinuclear complexes is incomparably harder to foresee. Indeed, even in the absence of obvious internal degrees of freedom of the ligand (such as rotation around a C−C single bond), coordinate bond and ligand flexibility that are negligible on a mononuclear level can add up and become increasingly more significant as the size of the complex increases, resulting in unpredictable outcomes. In addition, two or more alternate architectures with similar © XXXX American Chemical Society

thermodynamic stability yet completely different structure may exist for a given system, in which case subtle changes in reaction conditions will determine the outcome.5 The structure of the final product may be further complicated in practice by possible intermolecular interactions, with either solvent molecules or counterions in solution, or neighboring molecules in the crystalline state.6 Not surprisingly, the discovery of novel types of large polynuclear self-assembled coordination architectures is most often serendipitous.7 The initial serendipitous discovery is usually followed by rational design and leads to the development of new classes of coordination compounds. Understanding the stepwise mechanism of formation of large selfassembled architectures, however, is not a straightforward task, as numerous intermediates can coexist during the self-assembly process, and many are often very short lived. UV−vis spectroscopy,8−11 electrospray-ionization mass spectrometry (ESI-MS),8,11−15 and NMR spectroscopy,10,13,15−19 occasionally aided by equilibrium-restricted factor analysis,9,10 n−k Received: May 12, 2016

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DOI: 10.1021/acs.inorgchem.6b01172 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Time-dependent ESI-MS(−) spectra of a solution (in CH3CN) sampled periodically from a mixture of Cu(NO3)2·2.5H2O, pyrazole, NaOH, and Na2CO3·H2O (1:1:2:1 molar ratio) stirred in THF. The spectrum at time zero is of the solution of the first two, THF-soluble reactants, just before addition of the insoluble sodium hydroxide and carbonate.

analysis,16,17 or molecular dynamics simulations20 have been used in the investigation of the self-assembly mechanisms during the formation of multimetallic coordination architectures. The structural complexity of metal−organic architectures is not always correlated to building block complexity, unlike in biological systems, where self-assembly usually involves rather intricate building blocks. For instance, pyrazole, a deceptively simple, small organic molecule, self-assembles with Cu2+ and OH− ion in the presence of anions with large hydration energies to yield a series of ∼2 nm wide, complex architectures.21−25 These assemblies, termed nanojars, have the formula [anion⊂{Cu(μ-OH)(μ-pz)}n] (anion = CO32−, SO42−, PO43−, HPO42−, AsO43−, HAsO42−, Cl−) and contain n

= 27−36 repeating units of [Cu(OH)(pz)]. These repeating units are arranged into three or four stacked metallamacrocycles, which create a hydrophilic central cavity with an incarcerated anion. Originally obtained serendipitously from the reaction of trinuclear complexes [Cu3(μ3-O)(μ-pz)3Cl3]2− and [Cu3(μ3-Cl)2(μ-pz)3Cl3]2− with KReO4 and Ag2CO3, respectively, nanojars have since been prepared rationally from Cu(OH)2 and pyrazole and are currently synthesized by the direct self-assembly of pyrazole with Cu2+ and OH− ions. To study the intermediates and the mechanism by which these entities self-assemble into complex nanojars, we use a set of complementary techniques including mass spectrometry, pH titration, UV−vis and NMR spectroscopies, chemical synthesis, and single-crystal X-ray diffraction. The results also reveal a pHB

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Inorganic Chemistry Scheme 1. Proposed Mechanism of Formation of Nanojars

dependent equilibrium between Cu2+ ions and pyrazole, a trinuclear copper pyrazolate complex, and nanojars.

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mass of the most abundant peak within the isotope pattern; all observed values match calculated values). The presence of the latter complex indicates that pyrazole is partially deprotonated even in the absence of a base. This suggests that a solution of copper nitrate should become more acidic upon addition of pyrazole, as some H3O+ ions should form along with the trinuclear complex, which is indeed the case (see pH Titration and UV−vis Monitoring section below). As soon as NaOH/Na2CO3 is added (not soluble in THF), the abundance of the mononuclear species decreases whereas the abundance of the trinuclear species increases. After 25 min of stirring, the mononuclear species are mostly replaced by trinuclear species, as indicated by peaks corresponding to [Cu3O(pz)2(NO3)3]− (m/z 526.8) and [Cu3(OH)3(pz)2(NO3)2]− (m/z 499.8) and smaller amounts of [Cu3(OH)3(pz)(NO3)2]− (m/z 432.8), [Cu3(OH)4(pz)3]− (m/z 459.9), [Cu3(OH)4(pz)2(NO3)(H2O)]− (m/z 472.9),

RESULTS AND DISCUSSION

Reaction Monitoring by Mass Spectrometry Snapshots. The reaction of copper nitrate and pyrazole with NaOH (1:1:2 molar ratio) in the presence of sodium carbonate in THF was monitored by ESI-MS(−) (Figures 1 and S1−S14). The reaction mixture (which does not dissolve the NaOH and Na2CO3 reactants or the NaNO3 byproduct) was sampled periodically. Before the addition of any base, the equimolar mixture of Cu(NO3)2 and pyrazole (Hpz) gives rise to peaks corresponding to mononuclear [Cu(NO3)2]− (m/z 186.9), [Cu(NO3)3]− (m/z 248.9), and [CuO(NO3)]− (m/z 140.9) as major species along with a significant peak at m/z 526.8 corresponding to [Cu3O(pz)2(NO3)3]− (m/z values represent monoisotopic C

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Figure 2. Illustration of the relationship between trinuclear [Cu(OH)(pz)]3 units and [Cu(OH)(pz)]x rings of identical composition found in nanojars (a ring combination with x = 6 and 12 is shown).

[Cu 3 (OH) 3 (pz) 3 (NO 3 )] − (m/z 504.9), and [Cu 3 O(pz)3(NO3)2]− (m/z 531.8). After 45 min, in addition to the trinuclear species, tetranuclear [Cu4O2(pz)2(NO3)3]− (m/z 605.7), [NaCu4O2(OH)(pz)3(NO3)2]− (m/z 650.8), and [Na2Cu4O3(pz)4(NO3)]− (m/z 677.8) as well as hexanuclear [Cu6O2(pz)5(NO3)3(OH)]− (m/z 951.7), [Cu6O2(pz)6(NO3)2(H2O)(OH)]− (m/z 974.7), [Cu6O2(pz)5(NO3)4]− (m/z 996.7), [Cu6O2(pz)6(NO3)3]− (m/z 1001.7), [Cu 6 (OH) 3 (pz) 6 (NO 3 ) 3 (HCO 3 )] − (m/z 1081.7), and [Cu6(OH)3(pz)6(NO3)4]− (m/z 1082.7) species appear. After 85 min, the major species identified are [Cu3(OH)5(pz)2(H2O)2]− (m/z 445.9), [NaCu3(OH)6(pz)2]− (m/z 449.9), [Cu3(OH)4(pz)2(NO3)(H2O)]− (m/z 472.9), [Cu3(OH)4(pz)3(H2O)]− (m/z 477.9), [Cu3(OH)3(pz)2(NO3)2]− (m/z 499.8), [Cu3(OH)3(pz)3(NO3)]− (m/z 504.9), [Cu3O(pz)2(NO3)3]− (m/z 526.8), [Cu 3 O(pz) 3 (NO 3 ) 2 ] − (m/z 531.8), [Cu 4 O 2 (pz) 2 (NO 3 ) 3 (H 2 O)] − (m/z 623.7), and [NaCu4O2(pz)3(NO3)2(OH)]− (m/z 650.8). Most intriguingly, peaks corresponding to sodium nitrate clusters, [Nax(NO3)x+1]− (x = 0−22; m/z 62.0−1931.5) start appearing at this point, together with traces of intermediates [{Cu3(OH)3(pz)3}n/3(NaNO3)yCO3]2− (n = 27, 29, 31; y = 1−13; m/z 2064.5−2869.8) and nanojars [CO3⊂{Cu(OH)(pz)}n]2− (n = 27, m/z 2022.0; n = 29, m/z 2169.9; n = 31, m/z 2317.4). The presence of sodium nitrate clusters is unexpected, as NaNO3 is insoluble in both THF and CH3CN, as indicated by the absence of any sodium nitrate clusters in the ESI-MS spectra of THF or CH3CN stirred with NaNO3. It is also important to notice that after the peaks of the [{Cu3(OH)3(pz)3}n/3(NaNO3)yCO3]2− intermediates disappear and the nanojars are fully formed (after ∼4 h of stirring), the peaks corresponding to NaNO3 clusters can no longer be observed in the ESI-MS spectra. Therefore, we conclude that the NaNO3 clusters detected by ESI-MS are formed during sample injection as the [{Cu3(OH)3(pz)3}n/3(NaNO3)yCO3]2− intermediates eliminate NaNO3 and transform to the final products, [CO3⊂{Cu(OH)(pz)}n]2−. On the basis of these observations, the following mechanism is proposed (Scheme 1). As the NaOH pellets gradually react with the THF solution of copper nitrate and pyrazole, the trinuclear complex [Cu3(μ3-OH)(μ-pz)3(NO3)2(H2O)] (1a) forms. This complex was isolated and characterized in detail (see below). Further reaction of this complex with NaOH leads to replacement of the H2O molecule and NO3− ions with HO−

ions, forming [Cu3(μ3-OH)(μ-pz)3(OH)x(NO3)3‑x]− (x = 1, m/z 549.9 (2); x = 2, m/z 504.9 (3); x = 3, m/z 459.9 (4)) species (species wherein the OH group is deprotonated or pyrazolate is replaced by nitrate are also observed in the mass spectra). These species are unstable: unsupported, terminal CuII−OH groups are virtually absent in copper−pyrazole chemistry. Of the ∼500 crystal structures in the current Cambridge Structural Database (CSD) containing a [Cu(OH)(pz)] moiety, only three have terminal CuII−OH groups. In two of these, the terminal CuII−OH group is supported by intramolecular H bonding,26,27 whereas in the third one it is supported by steric crowding around the OH group;28 the rest contain far more common bridging μ2-OH and/or μ3-OH groups. In other copper(II) complexes, terminal Cu−OH groups are similarly stabilized by hydrogen bonding29−32 and/ or steric crowding,33−38 which prevent bridging/oligomerization. In the next step, the unstable species 2, 3, and 4 form transient tetranuclear and hexanuclear species. For example, [Cu6(OH)3(pz)6(NO3)4]− (5), detected at m/z 1082.7, can be viewed as two trinuclear [Cu3(μ3-OH)(μ-pz)3(NO3)2] units bridged by a μ-OH group. Also, the first carbonateincorporating species, [Cu6(OH)2(H2O)(pz)6(NO3)3(CO3)]−, is observed at m/z 1081.7. As more and more NaOH reacts, increasing numbers of trinuclear units end up bridged together, forming large intermediates with the formula [{Cu3(OH)3(pz)3}x(NaNO3)y(Na2CO3)z] (6). These intermediates are short lived and will eliminate NaNO3 while transforming into the final products Na2[CO3⊂{Cu(OH)(pz)}n] (n = 27, 29, 31). It is important to notice that the oligomeric intermediates [Cu3(OH)3(pz)3]x, based on trinuclear repeating units, have the same composition as the nanojars, [Cu(OH)(pz)]n (n = 3x) (Figure 2). Although the exact structure of the intermediates 6 cannot be established, their existence is unequivocally demonstrated by the following experiment. Mixing an aqueous solution of copper nitrate and pyrazole with an aqueous solution of NaOH (with small amounts of Na2CO3) in a 1:1:2 molar ratio under vigorous stirring leads to the immediate, quantitative precipitation of a dark-blue solid (intermediate 6). If this solid is stirred in its mother liquor for several days, it slowly changes color to purple, as it transforms into the polymeric [trans-Cu(μ-OH)(μ-pz)]∞.22 This material is completely insoluble in THF and in other solvents. In contrast, a completely different outcome is observed if the dark-blue solid (intermediate 6) is immediately filtered out of the D

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Inorganic Chemistry solution upon its precipitation. Although initially completely insoluble (indicating that this material contains neither the trinuclear intermediate nor the nanojars), when stirred with THF, it slowly dissolves and yields a mixture of nanojars Na2[CO3⊂{cis-Cu(μ-OH)(μ-pz)}n] (n = 27, 29, 31). It becomes apparent that in order for the intermediates 6 to convert to nanojars, the reaction medium must dissolve the final product. Indeed, if a THF solution of Bu4NOH (NaOH is insoluble in THF) is added at once to a vigorously stirred THF solution of copper nitrate and pyrazole, nanojars instantly form (as evidenced by ESI-MS). Immediately upon the addition of the Bu4NOH solution, the reaction is quenched by pouring the deep-blue solution into a large amount of water under stirring. The blue solid formed is filtered out immediately, rinsed with water, and dried. This product, when added to THF, dissolves instantly and completely, and nanojars (Bu4N)2[CO3⊂{Cu(OH)(pz)}n] (n = 27, 29, 31) are identified in the resulting solution by ESI-MS. When the reaction with Bu4NOH is carried out in H2O, similar results are obtained as with NaOH: a dark-blue precipitate forms upon mixing the aqueous solutions of Bu4NOH and copper nitrate/pyrazole, which after immediate filtration from the solution, followed by rinsing with water and drying, is at first insoluble in THF. Upon continued stirring, the product slowly dissolves in THF and fully transforms into nanojars (Bu4N)2[CO3⊂{Cu(OH)(pz)}n] (n = 27, 29, 31), as shown by ESI-MS. It is conceivable that when the reaction is carried out in THF with soluble Bu4NOH, the intermediate [Cu3(OH)3(pz)3]x has a size comparable to that of the final nanojars (x = n/3), and being soluble in THF, it quickly converts to the final product. In contrast, if the reaction is carried out in H2O, in which neither the intermediate nor the nanojars are soluble, a polymeric intermediate (large x) is produced, which is also insoluble in THF. This polymeric intermediate will slowly break down and transform into THF-soluble nanojars upon stirring with THF. pH Titration and UV−vis Monitoring. An aqueous solution of Cu(NO3)2 is acidic, due to the hydrolysis of the hydrated Cu2+ ion (pKa = 7.5).39 Pyrazole is a very weak base (N: pKb 11.43) and a very weak acid (N−H: pKa = 14.21),40 and its aqueous solution is practically neutral. Upon addition of an equimolar amount of pyrazole to an aqueous Cu(NO3)2 solution (1.72 × 10−2 M), the pH drops from 4.55 to 3.88, accompanied by a change in the wavelength of the absorbance maximum (λmax) from 800 to 768 nm. These changes indicate not only that pyrazole (Hpz) binds to copper but also that it partially deprotonates yielding H3O+ ions, along with pyrazolate ion (pz−) in the resulting product(s). The presence of a trinuclear copper pyrazolate species in this solution is clearly established by ESI-MS (see above). Upon addition of increasing amounts of NaOH (with traces of Na2CO3) to this solution, the pH gradually increases (Figure 3, Table S1), while λmax continually shifts toward lower wavelengths (Figure 4, Table S1). At a Cu:Hpz:NaOH molar ratio of 3:3:4, the absorbance reaches a maximum at λmax = 668 nm at a pH of 6.23. At this point, the soluble, trinuclear complex [Cu3(μ3OH)(μ-pz)3(NO3)2(H2O)] (1a) is fully formed. As more NaOH is added to the solution, the pH slowly increases and the absorbance gradually decreases, accompanied by precipitation of a dark-blue solid (filtered out of the solution prior to the measurements). When a Cu:Hpz:NaOH molar ratio of 3:3:6 is reached, the absorbance at 668 nm drops to zero and the pH becomes neutral. At t his point, all [Cu 3 (OH)(pz)3(NO3)2(H2O)] (1a) is converted to the insoluble

Figure 3. pH titration curve (A) and accompanying absorbance change at 668 nm (B) of an aqueous solution of copper nitrate and pyrazole (1.72 × 10−2 M each) with increasing amounts of NaOH (containing Na2CO3) from a Cu:Hpz:NaOH molar ratio of 3:3:0 to 3:3:9. At 4 equiv of NaOH, the trinuclear complex Cu3(OH)(pz)3(NO3)2(H2O) is fully formed, whereas at 6 equiv of NaOH the trinuclear complex is fully transformed into insoluble [Cu(OH)(pz)]∞, which is filtered out of the solution prior to the measurements.

[{Cu3(OH)3(pz)3}x(NaNO3)y(Na2CO3)z]∞ (6). Addition of further amounts of NaOH to the colorless solution leads to an abrupt increase in pH, indicating that no other species are left in solution that would react with NaOH. The two inflection points on the pH titration curve at Cu:Hpz:NaOH molar ratios of 3:3:4 and 3:3:6, along with the accompanying UV−vis spectral changes, suggest the reaction sequence shown in Scheme 2. As demonstrated below, nanojars can be transformed back to the trinuclear copper pyrazolate complex and ultimately to Cu2+ ions and pyrazole upon addition of increasing amounts of acids. Stepwise Synthesis of Nanojars. To obtain further proof of the identity of the trinuclear complex intermediate in the formation of the nanojars, a reaction using Cu(NO3)2, pyrazole, and NaOH in a 3:3:4 molar ratio is carried out in tetrahydrofuran (THF). After filtering out the insoluble NaNO3 byproduct and evaporating the solvent, the isolated E

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Figure 4. Electronic absorption spectra obtained during the titration of an aqueous solution of copper nitrate and pyrazole (1.72 × 10−2 M each) with NaOH (containing Na2CO3) from a Cu:Hpz:NaOH molar ratio of 3:3:0 to 3:3:4 (traces a to h in A) and from 3:3:4 to 3:3:6 (traces h to q in B). [Cu(OH)(pz)]∞ is insoluble and filtered out of the solution. Spectra from the titration of the copper nitrate/pyrazole solution with HNO3 to a Cu:Hpz:HNO3 molar ratio of 3:3:1 (trace b′ in A; pH = 2.34) and 3:3:18 (trace c′ in A; pH = 1.08) are also shown.

Scheme 2. pH-Dependent Equilibrium between Cu2+ Ions and Pyrazole, Trinuclear Copper Pyrazolate Complex, and Nanojarsa

a

Only the one with n = 27 is shown. The stepwise formation of nanojars with increasing amounts of base is reversible upon lowering the pH.

Figure 5. ESI-MS(−) evidence for the clean synthesis of either trinuclear complex or nanojars based on the Cu:Hpz:NaOH ratio employed. The lower spectrum corresponds to the product obtained from the isolated trinuclear complex upon treatment with 2 equiv of NaOH.

NaOH (and small amounts of Na2CO3) in THF, the color of the solution changes from dark green-blue to deep blue and a NaNO3 precipitate is filtered out. After evaporation of the solvent, the product is identified as pure nanojars Na2[CO3⊂{Cu(OH)(pz)}n] (n = 27, 29, 31) based on ESIMS, 1H NMR, and single-crystal X-ray diffraction studies.24

product (near-quantitative yield) is identified as pure [Cu3(μ3OH)(μ-pz)3(NO3)2(H2O)] (1a) with no trace of nanojars, based on single-crystal X-ray diffraction, ESI-MS, UV−vis, and 1 H NMR studies (see next section). Next, we demonstrate that the trinuclear intermediate 1a is fully convertible to nanojars. Upon stirring 1a with 2 equiv of F

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Figure 6. (Left) Crystal structure of Cu3(μ3-OH)(μ-pz)3(NO3)2(CH3CN) (1b). Only one position of the disordered nitrate (50/50) and hydroxide ions (50/50) is shown for clarity. Symmetry code: (a) y, x, −z. Bond lengths (Å): Cu1−O1, 2.011(4); Cu1−N1, 1.952(3); Cu1−N5, 2.027(4); Cu2−O1, 2.042(5); Cu2−N2, 1.919(3); Cu2−N3, 1.927(3); Cu2−O2, 1.922(12). (Right) Helical arrangement of the trinuclear units around the crystallographic 43 screw axis (C−H hydrogens, nitrate, and acetonitrile units not shown).

z 683.8), [Cu6(OH)2(pz)6(NO3)3]+ (m/z 1003.7), and [{Cu6(OH)2(pz)6(NO3)3}2(NO3)]+ (m/z 2069.4) are also present. In the ESI-MS(−), the base peak observed at m/z 526.8 corresponds to [Cu3O(pz)2(NO3)3]−; a smaller peak attributable to [Cu3(OH)3(pz)2(NO3)2]− (m/z 499.8), as well as traces of [Cu 3 (OH)4 (pz) 3 ] − (m/z 459.9), [Cu 3 O(pz)3(NO3)2]− (m/z 531.9), [Cu4O2(pz)2(NO3)3]− (m/z 605.7), [Cu4O2(pz)4(NO3)2]− (m/z 677.8), [Cu6O2(pz)4(NO3)5]− (m/z 991.6), and [Cu6O 2(pz)5(NO3)4]− (m/z 996.6) are also identified. Importantly, no traces of nanojars could be detected in either negative or positive mode. The 1H NMR spectrum of 1a shows two peaks in a 1:2 integrated ratio at 40.90 and 35.19 ppm in (CD3)2CO at 25 °C, attributable to the pyrazolate protons in the 4 and 3,5 positions, respectively (Figures S16−S20). The observed signals are relatively sharp for a paramagnetic complex, suggesting strong antiferromagnetic coupling between the Cu(II) centers.47 Similar values are obtained with recrystallized 1b (40.99 and 35.13 ppm in (CD3)2CO at 25 °C). These results show that in contrast to the solid state structure, in solution the complex assumes 3-fold symmetry, as a result of fast exchange of the nitrate ions with the water (1a) or acetonitrile (1b) moieties. Complex 1b retains its 3-fold symmetry in (CD3)2CO solution down to −100 °C (Figure 7). 1a, however, partially dimerizes in (CD3)2CO solution, as suggested by the appearance of a second set of peaks. The ratio of dimer to trinuclear complex increases as the temperature is lowered (Figure 7). Dimerization leads to a decrease of the unpaired electron density on the complex, resulting in the upfield shift of the peaks, along with lowering the symmetry from C3v to C2h, resulting in a larger number of peaks. The proposed structure of the dimer with bridging H2O molecules is shown in Figure 8. Similar structures, with carboxylate bridges instead of water and

Thus, we observe a clean transformation of the intermediate to nanojars, as no trinuclear complex nor any other copper complex is detectable in the final product (Figure 5). Characterization of the Trinuclear Intermediate. The crystal structure of the trinuclear complex [Cu3(μ3-OH)(μpz)3(NO3)2(CH3CN)] (1b), obtained by diethyl ether vapor diffusion into an acetonitrile solution of 1a, is shown in Figure 6 (see Figure S21 for thermogravimetric analysis of 1b). The complex, crystallized in the chiral space group P43212, is located on a C2 axis running through the linear CH3CN molecule and symmetrically bisecting the pyrazolate moiety on the opposite side of the trinuclear unit. The Cu3(μ-pz)3 framework is nearly planar, with the pyrazolate moieties twisted by 3.8(1)° and 7.2(2)° relative to the Cu3(pz)3 mean plane. The μ3-OH unit is located 0.509(4) Å above this mean plane and is disordered (50/50) about the C2 axis (Figure S15). The nitrate ion, located on a general position, is also disordered over two positions (50/50). To date, only one of the structurally characterized Cu3(μ-pz)3 frameworks has nitrate as sole terminal ligand for the Cu centers;41 all others contain pyrazole42−45 or pyridine46 ligands in addition to the nitrate ions. In 1b, the coordinating O atom of the nitrate ion forms a longer, axial bond to the CH3CN-bound Cu atom of an adjacent trinuclear unit (O2−Cu1′, 2.403(12) Å), whereas another O atom of the same nitrate ion forms a hydrogen bond with the μ3-OH group of the same adjacent trinuclear unit (O3···O1′, 1.967(10) Å). Interatomic distances are similar to related trinuclear copper pyrazolate complexes.41−46 Within the crystal lattice, the trinuclear units are found in a helical arrangement around the 43 screw axis (Figure 6). In the ESI-MS(+) spectrum of 1a in CH3CN, the base peak observed at m/z 407.9 corresponds to [Cu3O(pz)3]+ (due to loss of H2O, NO3−, and HNO3 upon ionization); smaller peaks attributable to [Cu4O2(pz)4]+ (m/z 553.8), [Cu5O2(pz)5]+ (m/ G

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Figure 7. Variable-temperature 1H NMR spectra of [Cu3(μ3-OH)(μ-pz)3(NO3)2(H2O)] (1a, left) and [Cu3(μ3-OH)(μ-pz)3(NO3)2(CH3CN)] (1b, right) in (CD3)2CO. Peaks corresponding to the trinuclear complexes are indicated in blue, those of the dimer in red, those of H2O in cyan, and those of CH3CN in violet.

H2O or CH3CN molecules in 1a and 1b, respectively: as the temperature is lowered, the corresponding peaks display increasing hyperfine shifts and shorter relaxation times (evidenced by downfield shifts and broadening), similar to the pyrazolate peaks. Resistance to High pH. Nanojars (Bu4N)2[CO3⊂{Cu(OH)(pz)}n] are exceptionally stable under alkaline conditions: no changes are observed in the ESI-MS and UV−vis spectra of a solution of nanojars in CH2Cl2 after 20 weeks of vigorous stirring with an aqueous 1 M NaOH solution (pH 14). Amazingly, nanojars are resistant even to extreme alkalinities: both a solid sample and a THF solution of the nanojars are recovered unchanged after stirring for 5 weeks with a 10 M NaOH (30%) solution in water. Extraction of Carbonate at pH > 14. When an aqueous 10 M NaOH solution is added to a THF solution of Cu(NO3)2 and pyrazole (NaOH:Cu:Hpz molar ratio of ∼50:1:1), the color of the THF solution immediately turns deep blue. After 5 min of stirring, ESI-MS of this solution shows the presence of nanojars [CO3⊂{Cu(OH)(pz)}n]2− (n = 27, 29, 31), with no traces of trinuclear or any other copper complexes. After 16 h of stirring, the THF solution is still deep blue and shows nanojars as the only product. The source of the carbonate in these nanojars is Na2CO3. According to the manufacturer, NaOH contains up to 0.5% Na2CO3. At that concentration, the

Figure 8. Proposed structure of the dimer formed by 1a in acetone solution. Color scheme: Cu, blue; O, red; N, light blue; C, black (H atoms not shown).

carboxylate or pyrazole ligands instead of nitrate, have been characterized crystallographically.48−51 The variable-temperature 1H NMR studies also provide proof for the coordinated H

DOI: 10.1021/acs.inorgchem.6b01172 Inorg. Chem. XXXX, XXX, XXX−XXX

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initially blue methanolic solution of nanojars gradually turns colorless and deposits an insoluble, brown solid. Ethanol (pKa 15.9) and higher alcohols (pKa ≥ 16.0), which have lower acidity than water (pKa 15.7), dissolve the nanojars without decomposition, as attested by ESI-MS in ethanol and isopropanol, which show only intact nanojars, [CO3⊂{Cu(OH)(pz)}n]2−.

amount of NaOH employed in preparing the 10 M solution contains ∼2.6 times the required amount of carbonate for the formation of the nanojars. More carbonate can form as a result of the reaction of NaOH with CO2 from air. Carbonate is easily extracted from water when present as hydrogen carbonate (pH range ca. 4−12), as HCO3− has a small hydration energy (ΔGh° = −343 kJ mol−1).52 Above pH ≈ 12, carbonate is found exclusively as CO32−, which is much harder to extract from water due to its very large hydration energy (ΔGh° = −1324 kJ mol−1).53 Moreover, under highly alkaline conditions, known anion extraction agents decompose or become ineffective due to either breakdown of the organic ligand (hydrolysis or deprotonation leading to loss of affinity for the anion) or loss of the metal component (in the case of metal−organic complexes) by precipitation of the metal hydroxide or formation of hydroxometalates. Nanojars represent the first class of anion extraction agents capable of extracting CO32− from highly alkaline aqueous solutions into aliphatic solvents.25 Reactivity at pH < 7. In contrast to neutral or basic pH, nanojars are unstable under acidic pH, and even mildly acidic conditions lead to protonation of their OH− (pKa of water, 15.7) and the pz− (pKa of pyrazole, 14.2) moieties (Scheme 2). Weak acids, such as hydrated transition metal ions M2+(aq) (M = Cu, Pb, Ni, Cd, Mn; pKa 7.5, 7.6, 9.9, 10.1 and 10.6, respectively)39 and NH4+(aq) ions (pKa 9.2) as their nitrate salts in wet THF ([M(H2O)n]x+ + H2O → [M(OH)(H2O)n−1](x−1)+ + H3O+; NH4+ + H2O → NH3 + H3O+), break down nanojars into trinuclear species, as indicated by the [Cu3O(pz)3(NO3)2]− (m/z 531.9) and [Cu3O(pz)2(NO3)3]− (m/z 526.8) ions observed by ESI-MS(−), as well as the [Cu3O(pz)3]+ (m/z 407.9) ion observed by ESI-MS(+). The breakdown occurs even with extremely weak acids, such as Mg2+(aq) and Ca2+(aq) (pKa 11.2 and 12.7, respectively); however, nanojars reassemble and precipitate out upon addition of excess water to the corresponding THF solutions. Stronger acids, such as Fe3+(aq), Hg2+(aq), and Al3+(aq) (pKa 2.2, 3.4 and 5.0, respectively), lead to complete breakdown of the nanojars to Cu2+ and pyrazole, so that only mononuclear species, such as [Cu(NO3)3]− (m/z 248.9), [Cu(NO3)2]− (m/z 188) and [CuO(NO3)]− (m/z 142), are observed in the ESIMS(−) spectra of the resulting solutions. As shown above (Scheme 2), these reactions are reversible, and addition of sufficient amounts of a strong base restores the nanojars. Nanojars are not affected by Tl+(aq), Sr2+(aq), Ba2+(aq), Li+(aq), Na+(aq), K+(aq), and Bu4N+(aq) nitrates (pKa of the hydrated metal cations is 13.2, 13.2, 13.4, 13.6, 13.9, and 14.0, respectively; Bu4NOH is a strong base similar to NaOH). Other weak acids, such as alkylcarboxylic acids (pKa ≈ 5), phenol (pKa 10.0), alkanethiols (pKa 10.4−10.7), and even methanol (pKa 15.5), also lead to breakdown of the nanojars. Indeed, ESI-MS of (Bu4N)2[CO3⊂{Cu(OH)(pz)}n] (n = 27, 29, 31) in CH3CN/CH3OH (2:1) shows nanojars in which an HO− group is substituted by CH3O−, [CO3⊂{Cun(OCH3)(OH)n−1(pz)n}]2− (n = 27, m/z 2029.0; n = 29, m/z 2176.9; n = 31, m/z 2324.4). At higher methanol concentrations (CH3CN/CH3OH = 1:1), both an HO− and a pz− group are and substituted by CH3O− [CO3⊂{Cun(OCH3)2(OH)n−1(pz)n−1}]2− species are observed (n = 27, m/z 2011.0; n = 29, m/z 2158.9; n = 31, m/z 2306.4). Upon further substitution, the nanojars break down to small fragments, so that in a neat CH3OH solution no nanojar species can be detected, even when freshly prepared. On standing, the

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CONCLUSIONS Supported by mass spectrometric, UV−vis and NMR spectroscopic, pH titration, and crystallographic evidence, a mechanism for the formation of nanojars from Cu(NO3)2, pyrazole, and NaOH (in a 1:1:2 molar ratio) in the presence of Na2CO3 is proposed. When the reaction is carried out in THF, in which NaOH and Na2CO3 are insoluble and will only gradually react with the copper−pyrazole complex, a trinuclear intermediate [Cu3(μ3-OH)(μ-pz)3(NO3)2(H2O)] forms first. As more NaOH reacts, the nitrate ions are gradually replaced by hydroxide ions and precipitated out as NaNO3. Terminal Cu−OH groups are reactive; therefore, the subsequent [Cu3(μ3-OH)(μ-pz)3(OH)x(NO3)3−x]− (x = 1−3) intermediates condense and form [Cu3(μ3-OH)(μ-pz)3(NO3)2]2(μ-OH) and ultimately [{Cu3(μ3-OH)(μ-pz)3(μOH)2}x(NaNO3)y(Na2CO3)z] oligomers. These oligomers rearrange, while eliminating the peripheral nitrate ions, to form the final nanojar products, Na2[CO3⊂{Cu(μ-OH)(μpz)}n] (n = 27, 29, 31). When carried out in water, in which all reactants including NaOH and Na2CO3 are soluble, the reaction is very fast, and an insoluble, polymeric [{Cu 3 (OH) 3 (pz) 3 } x (NaNO 3 ) y (Na 2 CO 3 ) z ] ∞ intermediate forms instantly. The fate of this intermediate depends on the reaction conditions. If it is further stirred in water, it slowly eliminates NaNO3 and Na2CO3 and transforms into intractable [trans-Cu(μ-OH)(μ-pz)]∞. In contrast, if it is immediately removed from water and then stirred with THF, it breaks down and rearranges to form THF-soluble Na2[CO3⊂{cis-Cu(μOH)(μ-pz)}n] nanojars. Finally, when the reaction is carried out so that all reactants and products are soluble in the reaction medium (using Cu(NO 3 ) 2 , pyrazole, and Bu 4 NOH/ (Bu4N)2CO3 in THF), (Bu4N)2[CO3⊂{cis-Cu(μ-OH)(μpz)}n] nanojars are instantly obtained, reflecting the extreme coordinative lability of Cu2+ ions. pH titration and UV−vis monitoring of the accompanying absorbance changes confirms the formation of a trinuclear intermediate at a Cu:Hpz:NaOH molar ratio of 3:3:4, which was isolated and fully characterized. This intermediate transforms cleanly into nanojars upon addition of two more equivalents of NaOH (at a Cu:Hpz:NaOH molar ratio of 3:3:6). The unusual stability of nanojars to 10 M NaOH as well as Ba(OH)2 solutions points to a remarkable robustness of these anion-incarcerating, supramolecular coordination complexes. In stark contrast, nanojars are acid sensitive and even very weak acids break down nanojars to trinuclear copper pyrazolate complexes and ultimately to copper ions and pyrazole. These properties render nanojars ideal for the extraction of anions with large hydration energies from alkaline aqueous solutions, a particularly difficult task.25 The extracted anion is conveniently recovered under slightly acidic conditions. Moreover, the extraction process is repeatable, as the nanojars fully reassemble when the pH is made alkaline. I

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immediately filtered out (colorless filtrate), followed by rinsing with water and drying in the air. The resulting powder is instantly soluble in THF (20 mL). The deep-blue solution is filtered, and the solvent is evaporated, affording 650 mg of dark blue solid, identified as (Bu4N)2[CO3⊂{Cu(OH)(pz)}n] by ESI-MS (n = 27, m/z 2022.0; n = 29, m/z 2169.9; n = 31, m/z 2317.4). Method B. Cu(NO3)2·2.5H2O (1.000 g, 4.30 mmol) and pyrazole (293 mg, 4.30 mmol) are dissolved in H2O (20 mL). To this solution is added under vigorous stirring a solution of Bu4NOH (55% in H2O, 4.325 g, 9.17 mmol) in H2O (15 mL), which was previously exposed to the air for 15 min. A dark-blue precipitate forms, which is filtered out immediately (colorless filtrate), followed by rinsing with water and drying in the air. At first, the resulting powder is insoluble in THF (20 mL), but it gradually dissolves completely by stirring overnight. The deep-blue solution is filtered, and the solvent is evaporated, affording 711 mg of dark blue solid identified as (Bu4N)2[CO3⊂{Cu(OH)(pz)}n] by ESI-MS (n = 27, m/z 2022.0; n = 29, m/z 2169.9; n = 31, m/z 2317.4). Reaction with Ammonium and Metal Nitrates. An amount of NH4NO3, Bu4NNO3, Cu(NO3)2·2.5H2O, Pb(NO3)2, Ni(NO3)2· 6H2O, Cd(NO3)2·4H2O, Mn(NO3)2·4H2O, Mg(NO3)2·6H2O, Ca(NO3)2·4H2O, Fe(NO3)3·9H2O, Hg(NO3)2·H2O, Al(NO3)3·9H2O, TlNO3, Sr(NO3)2, Ba(NO3)2, LiNO3, NaNO3, or KNO3 (300 μmol) is added to a nanojar mixture (Bu4N)2[CO3⊂{Cu(OH)(pz)}n] (50 mg, ∼10 μmol) in THF (10 mL) and H2O (∼120 mg). After stirring for 3 days, the solution is filtered and analyzed by ESI-MS. Reaction with Ba(OH)2. An aqueous Ba(OH)2 solution (∼0.16 M, pH 13.5, 10 mL) is added to a solution of nanojars (Bu4N)2[CO3⊂{Cu(OH)(pz)}n] (50 mg) in CH2Cl2 (10 mL), and the mixture is stirred vigorously in a closed flask. The CH2Cl2 layer is sampled periodically and analyzed by ESI-MS. Only the original nanojars, [CO3⊂{Cu(OH)(pz)}n]2− (n = 27, m/z 2022.0; n = 29, m/z 2169.9; n = 31, m/z 2317.4) are observed even after 8 weeks of stirring. Reaction with NaOH. Method A. A solution of nanojars (Bu4N)2[CO3⊂{Cu(OH)(pz)}n] (n = 27, 29, 31) (50 mg) in CH2Cl2 (10 mL) is added to an aqueous NaOH solution (1 M, 10 mL; pH 10) and stirred vigorously for 20 weeks in a closed flask. The CH2Cl2 layer is sampled periodically and analyzed by ESI-SMS. No change is observed. Method B. Nanojars (Bu4N)2[CO3⊂{Cu(OH)(pz)}n] (500 mg) are added to an aqueous NaOH solution (10 M, 20 mL), and the mixture is stirred vigorously for 5 weeks in a closed flask. The solid is filtered out, washed thoroughly with water, dried in the air, and analyzed by ESI-MS. No change is observed. The solid recovered from above is redissolved in THF (20 mL) and stirred vigorously with an aqueous NaOH solution (10 M, 20 mL) for 5 weeks in a closed flask. The THF layer is sampled periodically and analyzed by ESI-SMS. Again, no change is observed. Extraction of Carbonate at pH > 14. A 10 M aqueous NaOH solution (prepared by dissolving 8.344 g NaOH in water and diluting to 20 mL in a volumetric flask; 209 mmol) is added to a THF solution of Cu(NO3)2 (1.000 g, 4.30 mmol) and pyrazole (293 mg, 4.30 mmol). The blue color of the THF solution immediately turns very deep blue. The mixture is stirred vigorously for 16 h, followed by separation of the two layers using a separatory funnel. The THF layer is filtered, and the solvent is evaporated. Na2[CO3⊂{Cu(OH)(pz)}n] is obtained as dark-blue poweder (553 mg, ∼85% yield). ESI-MS shows characteristic peaks for [CO3⊂{Cu(OH)(pz)}n]2− (n = 27, m/z 2022.0; n = 29, m/z 2169.9; n = 31, m/z 2317.4). Mass Spectrometry. Mass spectrometric analyses are performed on a Waters Synapt G1 HDMS instrument using electrospray ionization (ESI). Sample solutions (10−4−10−5 M in CH3CN) are infused by a syringe pump at 5 μL/min, and nitrogen is supplied as nebulizing gas at 500 L/h. The electrospray capillary voltage is set to −2.5 or +3.0 kV, respectively, with a desolvation temperature of 110 °C. The sampling and extraction cones are maintained at 40 and 3.0 V, respectively, at 80 °C. Reported m/z values represent monoisotopic mass of the most abundant peak within the isotope pattern. All

EXPERIMENTAL SECTION

General. All reagents and solvents are commercially available and used as received. THF is inhibited with 250 ppm BHT. Water was deionized (for synthesis) or distilled under N2 (for pH and UV−vis measurements) before use. NMR spectra are collected on a Jeol JNMECP400 instrument, and UV−vis measurements are carried out on a Shimadzu UV-1650PC spectrophotometer. pH is measured with a Mettler Toledo S20 SevenEasy pH-meter, calibrated with pH 4.01, 7.00, and 10.01 standard buffer solutions. Stepwise Synthesis of Na2[CO3⊂{Cu(OH)(pz)}n] (n = 27, 29, 31). a. Synthesis of [Cu3(μ3-OH)(μ-pz)3(NO3)2(H2O)] (1a). Cu(NO3)2· 2.5H2O (3.000 g, 12.90 mmol), pyrazole (878 mg, 12.90 mmol), and NaOH pellets (688 mg, 17.2 mmol) are stirred in THF (75 mL) for 3 days. The dark blue-green solution is filtered, and the solvent is removed. The oily residue is triturated with Et2O (100 mL) until it solidifies; then the dark blue powder is filtered out, rinsed with Et2O, and dried in the air. Yield: 2.561 g. After several hours under high vacuum, the product still retains some THF and Et2O, as evidenced by 1 H NMR (Figures S16−S18) and thermogravimetric analysis (Figure S22). 1H NMR (400 MHz, (CD3)2CO, 25 °C): δ 40.90 (s, 3H, 4-Hpz), 35.19 (s, 6H, 3,5-H-pz), 4.01 (s, 2H, H2O) ppm. 1H NMR (400 MHz, D2O, 25 °C): δ 39.93 (s, 3H, 4-H-pz), 33.61 (s, 6H, 3,5-H-pz) ppm. 1H NMR (400 MHz, CD3CN, 25 °C): δ 40.59 (s, 3H, 4-H-pz), 33.73 (s, 6H, 3,5-H-pz) ppm. The reaction can also be carried out in H2O as solvent; however, as the NaNO3 byproduct is water soluble as well, the product can only be isolated after evaporation of the water and extraction of the product with THF. 1a is soluble in H2O, THF, CH3CN, and acetone, sparingly soluble in CH2Cl2, and insoluble in CHCl3, diethyl ether, toluene, and hexane. Recrystallization from CH3CN solution by Et2O vapor diffusion provides pure [Cu3(μ3OH)(μ-pz)3(NO3)2(CH3CN)] (1b). Anal. Calcd for C11H13Cu3N9O7: C, 23.02; H, 2.28; N, 21.97. Found: C, 23.46; H, 2.14; N, 21.41. b. Reaction of 1a with NaOH/Na2CO3. The material obtained above is dissolved in THF (75 mL) and stirred with NaOH (435 mg, 10.9 mmol) and Na2CO3·H2O (1.599 g, 12.9 mmol) for 1 day. ESIMS indicates no apparent reaction. After the addition of 1 mL of H2O, stirring is continued for 1 day. The color of the solution gradually changes from dark green-blue to deep blue, and ESI-MS shows complete conversion of the trinuclear complex to nanojars [CO3⊂{Cu(OH)(pz)}n]2− (n = 27, m/z 2022.0; n = 29, m/z 2169.9; n = 31, m/z 2317.4). After filtration and evaporation of the solvent, 1.800 g of dark blue powder is obtained (yield ≈ 93%). Direct Synthesis of Na2[CO3⊂{Cu(OH)(pz)}n] (n = 27, 29, 31). Cu(NO3)2·2.5H2O (1.000 g, 4.30 mmol) and pyrazole (293 mg, 4.30 mmol) are dissolved in H2O (20 mL). To this solution is added under vigorous stirring a solution of NaOH (344 mg, 8.60 mmol) and Na2CO3·H2O (20 mg, 0.16 mmol) in H2O (15 mL). A dark-blue precipitate forms, which is filtered out immediately (colorless filtrate), followed by rinsing with water and drying in the air. At first, the resulting powder is completely insoluble in THF (20 mL), but it gradually dissolves over 3 h of stirring. The deep-blue solution is filtered, and the solvent is evaporated, affording 565 mg of dark blue solid identified as Na2[CO3⊂{Cu(OH)(pz)}n] by ESI-MS (n = 27, m/ z 2022.0; n = 29, m/z 2169.9; n = 31, m/z 2317.4). Synthesis of [Cu(OH)(pz)]∞. The synthesis is carried out exactly as described above, but instead of filtering the dark-blue precipitate out immediately, it is stirred for 7 days before filtering. In contrast to the previous experiment, the product obtained has a purple-blue color (as opposed to dark-blue) and is completely insoluble in THF even after days of stirring. This product is identical to the one obtained previously from CuSO4, pyrazole, and KOH.22 Direct Synthesis of (Bu4N)2[CO3⊂{Cu(OH)(pz)}n] (n = 27, 29, 31). Method A. Cu(NO3)2·2.5H2O (1.000 g, 4.30 mmol) and pyrazole (293 mg, 4.30 mmol) are dissolved in THF (20 mL). To this solution is added under vigorous stirring a solution of Bu4NOH (55% in H2O, 4.325 g, 9.17 mmol) in THF (15 mL), which was previously exposed to the air for 15 min. A deep-blue solution forms (a minute amount is quickly drawn for ESI-MS analysis), which is immediately poured into H2O (400 mL) under vigorous stirring. The blue precipitate is J

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observed isotope patterns match the corresponding predicted ones (examples are shown in Figures S23−S26, and in ref 24). The ESI-MS monitored reaction was carried out as follows. In a 200 mL round-bottom flask, Cu(NO3)2·2.5H2O (5.000 g, 21.5 mmol) and pyrazole (1.464 g, 21.5 mmol) were dissolved in tetrahydrofuran (60 mL). The solution was sampled with the tip of a Pasteur pipet, which was then dipped in 2 mL of LC-MS-grade CH3CN, followed by immediately filtering the solution and injecting it into the mass spectrometer. After addition of sodium hydroxide pellets (1.720 g, 43.0 mmol) and Na2CO3·H2O (91.9 mg, 0.741 mmol), the mixture was stirred and sampled periodically (after 5 min first, then every 20 min for 6 h, and finally after 3 days). pH Titration and UV−vis Monitoring. Stock solutions of Cu(NO3)2/pyrazole (1:1 molar ratio) and NaOH/Na2CO3 are prepared by dissolving in water 9.7862 g of Cu(NO3)2·2.5H2O (42.07 mmol) and 2.8644 g (42.07 mmol) of pyrazole and 4.5377 g of NaOH (113.4 mmol) and 0.2398 g (1.934 mmol) of Na2CO3·H2O, respectively, in 1000.0 ± 0.3 mL volumetric flasks. The concentration of NaOH is verified by titration with potassium hydrogen phthalate (0.1122 ± 0.0003 M), and the solution is protected from atmospheric CO2. Samples for pH and UV−vis measurements are prepared in 25.00 ± 0.06 mL volumetric flasks by using 10.00 mL of a Cu(NO3)2/ pyrazole (1:1 molar ratio) stock solution and increasing amounts of the NaOH/Na2CO3 stock solution (up to a Cu:Hpz:NaOH molar ratio of 1:1:3) or HNO3 (up to a Cu:Hpz:HNO3 molar ratio of 3:3:18), completed to 25.00 mL with water followed by filtration. Crystal Growing. An oily material repeatedly forms upon attempts of crystal growing from a THF solution of [Cu3(μ3-OH)(μpz)3(NO3)2(H2O)] (1a) by Et2O vapor diffusion or by evaporation of its aqueous solution. Nevertheless, single crystals of [Cu3(μ3OH)(μ-pz)3(NO3)2(CH3CN)] (1b) can be obtained in close to quantitative yield by Et2O vapor diffusion into a CH3CN solution or from a sonicated suspension in CH2Cl2 that contains small amounts of CH3CN on standing. For 1b: 1H NMR (400 MHz, D2O, 25 °C) δ 39.95 (s, 3H, 4-H-pz), 33.73 (s, 6H, 3,5-H-pz) ppm. 1H NMR (400 MHz, (CD3)2CO, 25 °C): δ 40.99 (s, 3H, 4-H-pz), 35.13 (s, 6H, 3,5H-pz), 6.37 (s, 3H, CH3CN) ppm. X-ray Crystallography. X-ray diffraction data are collected at room temperature from a single crystal mounted atop a glass fiber with cyanoacrylate glue, with a Bruker SMART APEX II diffractometer using graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation. The structures are solved by employing SHELXTL direct methods and refined by full-matrix least-squares on F2 using the APEX2 v2014.9-0 software package.54 All non-H atoms are refined with independent anisotropic displacement parameters. C−H hydrogen atoms are placed in idealized positions and refined using the riding model. The O−H hydrogen atom is located from the difference Fourier maps; its displacement parameter is fixed to be 20% larger than that of the attached O atom. For the disordered nitrate molecules, geometrical restraints are used. A thermal ellipsoid plot is shown in the Supporting Information (Figure S15). Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre (deposition number CCDC 1478231). Copies of the data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge, CB2 1EZ, UK; fax: +44 1223 336033; e-mail: deposit@ccdc. cam.ac.uk). Summary of Crystallographic Data. Chemical formula, C11H13Cu3N9O7; formula weight, 573.92; crystal system, tetragonal; space group, P43212 (No. 96); a = b = 11.4466(1) Å; c = 14.7076(2) Å; α = β = γ = 90°; V = 1927.06(4) Å3; Z = 4; Dcalcd = 1.978 g/cm3; μ = 3.337 mm−1; no. of reflns collected, 50 415; no of unique reflns, 3323; no. of obsd reflns [I > 2σ(I)], 2322; R(int), 0.0311; data/ parameters/restrains, 3323/183/9; goodness-of-fit (on F2), 1.030; R(F) [I > 2σ(I)], 0.0284; Rw(F) [I > 2σ(I)], 0.0579; R(F) (all data), 0.0573; Rw(F) (all data), 0.0663; residual electron density, max/min (e/Å3), 0.312/−0.234.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01172. pH titration, UV−vis, ESI-MS, NMR, and TGA data; thermal ellipsoid plot for 1b (PDF) X-ray crystallographic data for 1b (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grant No. CHE-1404730. REFERENCES

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