Mapping the Intricate Reactivity of Nanojars toward Molecules of

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Mapping the Intricate Reactivity of Nanojars toward Molecules of Varying Acidity and Their Conjugate Bases Leading To Exchange of Pyrazolate Ligands Christian K. Hartman and Gellert Mezei* Department of Chemistry, Western Michigan University, Kalamazoo 49008, Michigan, United States S Supporting Information *

ABSTRACT: A comprehensive reactivity study of nanojars toward 18 different acidic compounds with varying pKa, including 12 different carboxylic acids (both aliphatic and aromatic mono- and dicarboxylic acids), p-toluenesulfonic acid, hydrogen sulfate, hydrogen carbonate, carbonic acid, 1decanethiol, and methanol, as well as four different conjugate bases (formate, acetate, benzoate, 2-bromoethanesulfonate) is carried out with the aid of electrospray-ionization mass spectrometry. Thus, the effect on nanojar substitution and breakdown pattern of a number of variables, such as concentration of reagent (acid or conjugate base), acidity of reagent (pKa), effect of acid vs conjugate base, steric effects, aromaticity, incarcerated anion and size of the nanojar, is evaluated. Of the substitution and breakdown products identified by mass spectrometry, acetate-substituted nanojars (Bu4N)2[CO3⊂{Cu27(μ-OH)27(μ-pz)27−x(μ-CH3COO)x}] (x = 1 and 2), as well as dimeric complexes (Bu4N)2[Cu2(μ-pz)2A2] (A = CO32− and SO42−) have been isolated and characterized by single-crystal Xray diffraction. This study offers a detailed understanding of the behavior of nanojars of various sizes and with different incarcerated anions in the presence of the above-mentioned compounds at varying concentrations and tests the limits of the pyrazolate/carboxylate structural analogy in multinuclear metal complexes. The results point to the possibility of obtaining functionalized nanojars via pyrazolate/carboxylate ligand exchange, an aid in the design of anion extraction processes using nanojars or similar complexes as extracting agents.

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INTRODUCTION Self-assembled metal−organic complexes with potential anion recognition, sensing, and separation applications are an integral part of supramolecular anion binding research efforts and continue to generate sustained interest.1 Nanojars have recently been introduced as a new class of neutral anion-incarcerating agents with several outstanding features.2−11 Self-assembled from Cu(II) ions and pyrazole (Hpz) in the presence of a base and the anion to be incarcerated, nanojars of the formula [A⊂{Cu(μ-OH)(μ-pz)}n]2− (CunA; A = anion, n = 22−33) are comprised of three [Cu(μ-OH)(μ-pz)]x rings (Cux; x = 6−14, except 11). Nanojars are totally selective for doubly charged anions (such as CO32−, SO42−, HPO42−, and HAsO42−) in the presence of singly charged anions (such as NO3− and ClO4−) in large excess.3,4,6,8 An extremely efficient binding of the incarcerated anion is demonstrated by the inability of an aqueous Ba2+ solution to precipitate the highly insoluble barium salt of the anion (e.g., BaSO4, Ksp = 1.08 × 10−10 at 25 °C in H2O).3,4 Indeed, nanojars completely surround and isolate the incarcerated anion from its surroundings, as revealed by crystallographic analysis.2−6,8 Another advantage of nanojars as anion extraction agents is their exceptional stability to highly alkaline conditions. It has been shown, for instance, that CO32− © XXXX American Chemical Society

can be extracted from a 10 M NaOH solution (pH > 14) into organic solvents using nanojars.6,7 In contrast to the extraordinary resistance to high alkalinity, nanojars are vulnerable under acidic conditions.2,7,9,10 We have shown that under acidic conditions, nanojars break down into trinuclear or mononuclear copper complexes, depending on the acidity of the solution.7 A very different reactivity toward acidic compounds was discovered serendipitously: we have repeatedly observed nanojars with one or more pyrazolates substituted with formate during mass spectrometric analysis of nanojar solutions.6 Formic acid is commonly used in mass spectrometry as an additive to promote ionization by producing [M + H]+ ions. Apparently, nanojars easily pick up traces of residual HCOOH in the mass spectrometer. These observations pointed to the possibility of peripheral reaction of nanojars with acids without breakdown of the underlying nanojar framework and prompted a more in-depth investigation. The ability of substituting pyrazolate ligands by certain acidic compounds offers not only the possibility of using nanojars as anion extraction agents in the presence of such compounds, but also the opportunity of Received: June 21, 2017

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DOI: 10.1021/acs.inorgchem.7b01593 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

evaporated using a rotary evaporator, and the viscous, colorless liquid residue is dried under high vacuum for 2 days. A white, free-flowing crystalline product is obtained in quantitative yield, which is extremely hygroscopic and is stored under dry N2. Synthesis of (Bu4N)HCOO. Formic acid (88% in H2O, 1.22 g/mL; 1.50 mL, 34.99 mmol) is added dropwise to (Bu4N)OH (56% in H2O; 16.117 g, 34.79 mmol) under stirring with cooling (exothermic). Water is removed from the resulting colorless, viscous solution by drying under high vacuum for several days. Tetrabutylammonium formate is obtained quantitatively as an extremely hygroscopic, white powder (stored under dry N2). Synthesis of (Bu4N)2Cu2(μ-pz)2(CO3)2. (Bu4N)2[CO3⊂{Cu(μOH)(μ-pz)}n] (0.5000 g; 0.1036 mmol, based on an average n = 29) is dissolved in toluene (50 mL) in a 100 mL round-bottom flask. To this solution, (Bu4N)HCO3 (0.9747 g, 3.212 mmol) is added, and the suspension is stirred for 1 week. The nanojars react according to the following equation: 2 (Bu4N)2[CO3⊂{Cu(μ-OH)(μ-pz)}n] + 2n (Bu4N)HCO3 → n (Bu4N)2Cu2(μ-pz)2(CO3)2 + 2n H2O + 2 (Bu4N)2CO3. The blue color of the toluene solution gradually decreases in intensity and a purple solid deposits on the walls of the flask. The sticky solid is filtered out and is washed with toluene to remove any remaining nanojars. If more (Bu4N)HCO3 is added to the filtrate (which is still blue), an additional amount of purple solid is obtained upon stirring. The product is triturated three times with diethyl ether, until a free-flowing, dark purple solid is obtained. Further purification by recrystallization from acetone using diethyl ether vapor diffusion produces plum-purple crystals (1.1449 g, 88%). ESI-MS(−): m/z 622.2, [(Bu 4 N)Cu 2 (pz) 2 (CO 3 ) 2 ] − ; m/z 1488.6, [(Bu 4 N) 3 {Cu 2 (pz) 2 (CO3)2}2]−. Anal. calcd for C40H78Cu2N6O6: C, 55.47; H, 9.08; N, 9.70. Found: C, 55.86; H, 8.97; N, 9.88. UV−vis in CH3CN, λmax/nm (ε/M−1 cm−1): 558 (2.5 × 102). The product is soluble in polar solvents, including dimethylformamide, acetonitrile, acetone, dichloromethane, and chloroform; it is slightly soluble in tetrahydrofuran, and is insoluble in less polar solvents, such as diethyl or diisopropyl ether, diglyme, toluene, and hexanes. It is also insoluble in water (remains unchanged after sitting in water for 30 min). Solvents such as methanol, isopropanol, ethyl acetate, 2-butanone, and 0.1 M aqueous HCl break down the product and give a blue solution. A 0.1 M aqueous NaOH solution also changes the color of the solid from dark purple to blue but it does not dissolve it, whereas a saturated solution of KOH in isopropanol produces a green solution. Synthesis of (Bu4N)2Cu2(μ-pz)2(SO4)2. Prepared similarly to the carbonate analogue, using (Bu 4 N) 2[SO4⊂{Cu(μ-OH)(μ-pz)} n] (0.5366 g; 0.1071 mmol, based on an average n = 30) and (Bu4N)HSO4 (1.1273 g, 3.320 mmol). Recrystallization from acetone by diethyl ether vapor diffusion yields indigo-violet, needle-shaped crystals (1.3869 g, 92%). ESI-MS(−): m/z 694.1 [(Bu4N)Cu2(pz)2(SO4)2]−; m/z 1632.5, [(Bu 4 N) 3 {Cu 2 (pz) 2 (SO 4 ) 2 } 2 ] − . ESI-MS(+): m/z 1178.7, [(Bu4N)3Cu2(pz)2(SO4)2]+. Anal. Calcd for C40H78Cu2N6O6: C, 48.64; H, 8.38; N, 8.96. Found: C, 48.54; H, 8.04; N, 9.04. UV−vis in CH3CN, λmax/nm (ε/M−1 cm−1): 600 (1.7 × 102). The product is soluble in acetonitrile, acetone, tetrahydrofuran, and chloroform, and is insoluble in toluene, ethers, and hexanes. It breaks down similarly to the carbonate analogue in alcohols, ethyl acetate, dimethylformamide, and dimethyl sulfoxide, giving a blue-green solution. It also breaks down in water (color of solid immediately changes to light blue, then a bluegreen solution forms) and 0.1 M aqueous NaOH (a blue suspension forms). Synthesis of (Bu4N)2Cu2(μ-pz)2(CO3)(SO4). Prepared similarly to the dicarbonate and disulfate analogues, using (Bu4N)2[CO3⊂{Cu(μOH)(μ-pz)}n] (0.1000 g; 0.0207 mmol, based on an average n = 29), (Bu4N)HCO3 (0.0880 g, 0.290 mmol) and (Bu4N)HSO4 (0.1055 g, 0.3107 mmol) in toluene (20 mL). ESI-MS(−) indicates that the product consist of a mixture of [(Bu4N)Cu2(pz)2(CO3)(SO4)]− (m/z 658.1) with [(Bu4N)Cu2(pz)2(CO3)2]− (m/z 622.2) and [(Bu4N)Cu2(pz)2(SO4)2]− (m/z 694.1). Synthesis of Nanojars from Copper Chloride, Bromide, or Acetate. Copper salt (4.01 mmol; CuCl2·2H2O: 684 mg; CuBr2: 896 mg; Cu(CH3COO)2·H2O: 801 mg), pyrazole (273 mg, 4.01 mmol), NaOH (309 mg, 7.73 mmol), Bu4NOH (1 M in H2O; 276 mg, 0.276

peripheral decoration of nanojars with various functionalities by simple ligand exchange. The study presented herein aims at answering the following questions: (1) How many pyrazolate ligands can be exchanged with a carboxylate or other ligand before the nanojar breaks down? (2) What is the effect of acid/conjugate base concentration? (3) What is the effect of different acidity (pKa), steric bulkiness, and aromatic character of the reactant acid? (4) Can the same substitution patterns be achieved using the conjugate bases of the acids? (5) Do different nanojar sizes, such as Cu27, Cu29, Cu31, etc., have different reactivity? (6) Do nanojars of the same size but with different incarcerated ions (such as CO32− versus SO42−) have different reactivity? (7) Can nanojars be tethered together by dicarboxylates? To answer these questions, we carried out a comprehensive study using 12 different carboxylic acids (including both aliphatic and aromatic mono- and dicarboxylic acids), as well as other compounds with varying acidity: p-toluenesulfonic acid, hydrogen sulfate, hydrogen carbonate, carbonic acid, 1-decanethiol, and methanol. In addition, we also investigated the reactivity of nanojars toward conjugate bases of different acids, including formate, acetate, benzoate, 2-bromoethanesulfonate, sulfate, and carbonate. Titrations with different reactants were monitored by electrospray ionization mass spectrometry (ESI-MS); in addition, details about the specific location of substitution within nanojars were obtained using single-crystal X-ray diffraction. Another aim of this work is to test the limits of the analogy between pyrazolate and carboxylate ligands in the coordination chemistry of multinuclear metal complexes,12 and to attempt to prepare nanojars with all carboxylate ligands. Examples of complexes that have been prepared with both pyrazolate and carboxylate ligands include a trinuclear Co(III) complex with all pyrazolate ligands, [Co 3 (μ 3 -O)(μ-4-NO 2 pz) 6 L 3 ]2− (L = NO2−),12e all acetate ligands, [Co3(μ3-O)(μ-OAc)6L3]+ (L = py),13 and also with various mixtures of pyrazolate/acetate ligands, [Co3(μ3-O)(μ-pz)6−x(μ-OAc)xL3]+ (x = 2−5; L = Hpz),14 tetranuclear complexes with a tetrahedral [Co4(μ4-O)]6+ core based on pyrazolates15 or carboxylates16 (which can also coexist in the same structure),17 tetranuclear complexes with planar [Cu4(μ4-L)]7+ cores (L = OH or Cl) with either pyrazolate,11 carboxylate18 or carbonate ligands,19 hexanuclear copper complexes in which two trinuclear units are bridged by either three pyrazolates20 or three carboxylates,21 pyrazolatebased [Ni8(OH)6(pz)12]22 vs carboxylate-based [Ni8(OH)4(H2O)2(Me3CCOO)12] cube-like octanuclear complexes,23 as well as larger multinuclear metallacycles with either pyrazolate12e,24 or carboxylate ligands.25 There would be an obvious advantage to obtaining nanojars with carboxylate instead of pyrazolate ligands, especially when it comes to large scale applications, as variously substituted carboxylate ligands are incomparably more accessible and inexpensive than the corresponding pyrazolate ligands.

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EXPERIMENTAL SECTION

General Methods. All reagents and solvents are commercially available and are used as received (THF is stabilized with 250 ppm BHT). (Bu4N)2[CO3⊂{Cu(μ-OH)(μ-pz)}n] (n = 27, 29, 30, 31) and (Bu4N)2[SO4⊂{Cu(μ-OH)(μ-pz)}n] (n = 27, 28, 29, 31, 32) are prepared according to published procedures.5,8 NMR and UV−vis spectra are collected on a JEOL model JNMECP400 and a Shimadzu UV-1650PC instrument, respectively. Synthesis of (Bu4N)HCO3. (Bu4N)OH (54.9% in H2O; 10.5157 g) is dissolved in methanol (50 mL) in a 200 mL round-bottom flask, and excess CO2 is passed through the solution for 1.5 h. The solvent is B


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Inorganic Chemistry Table 1. Crystallographic Data for 1−4 formula FW (g·mol−1) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g·cm−3) μ (mm−1) θ range (deg) reflns collected Rint obsd reflns [I > 2σ(I)] data/restraints/parameters GOF (on F2) R factors [I > 2 σ(I)] R factors (all data) maximum peak/hole (e·Å−3) CCDC number

1

2

3

4

C267H408Cu54Nl06O66 9590.13 monoclinic P21/n 35.5582(5) 25.409l(4) 39.608l(6) 90.000 97.831(1) 90.000 35452.3(9) 4 1.797 3.241 1.15−20.96 253564 0.0813 26795 37748/55/4158 1.277 R1 = 0.0554 WR2 = 0.1528 R1 = 0.0930 WR2 = 0.1843 1.431/−0.686 1557569

C38H78Cu2N6O8S2 938.26 monoclinic P21/n 8.3744(1) 22.5315(3) 13.2157(1) 90.000 98.284(1) 90.000 2467.62(5) 2 1.263 0.996 1.80−27.88 59643 0.0303 4801 5879/0/257 1.036 R1 = 0.0308 WR2 = 0.0822 R1 = 0.0417 WR2 = 0.0894 0.400/−0.201 1557568

C40H78Cu2N6O6 866.16 monoclinic C2/c 22.1910(4) 13.4674(4) 16.2485(4) 90.000 95.881(2) 90.000 4830.4(2) 4 1.191 0.926 1.77−20.36 22872 0.0376 1818 2377/10/284 0.914 R1 = 0.0486 WR2 = 0.1378 R1 = 0.0657 WR2 = 0.1582 0.518/−0.220 1557566

C40H82Cu2N6O8S 934.25 triclinic P1̅ 12.3414(14) 14.1065(15) 15.9544(18) 87.868(7) 77.864(7) 74.006(7) 2609.7(5) 2 1.189 0.903 1.31−25.02 106542 0.0717 5151 9205/39/495 1.066 R1 = 0.0776 WR2 = 0.2499 R1 = 0.1287 WR2 = 0.3147 0.805/−0.279 1557567

mmol), and Na2CO3·H2O (497 mg, 4.01 mmol) are stirred in THF (20 mL) for 3 days. The deep blue solution is filtered and the solvent is removed in a vacuum to yield a blue powder (700, 676, and 639 mg, respectively). ESI-MS indicates that the products consist of (Bu4N)2[{Cu(OH)(pz)}nCO3], (Bu4N)2[{Cun(OH)n(pz)n−x(Brpz)x(CO3)] and (Bu4N)2[{Cu(OH)(pz)}nCO3], respectively (n = 27, 29, 30, 31; x = 0−3). Reactions with Carboxylates, Halides, and Other Anions. (Bu4N)2[{Cu(μ-OH)(μ-pz)}nCO3] (50.0 mg, 10.4 μmol, based on an average n = 29) is stirred with (Bu4N)HCOO (179 mg, 623 μmol), NaHCOO (42.3 mg, 622 μmol), (Bu4N)CH3COO (187 mg, 620 μmol), NaCH3COO·3H2O (84.6 mg, 622 μmol), Ba(CH3COO)2 (79.4 mg, 311 μmol; 622 μmol acetate), Cd(CH3COO)2·2H2O (82.8 mg, 311 μmol; 622 μmol acetate), Pb(CH3COO)2·3H2O (117.9 mg, 311 μmol; 622 μmol acetate), Na(C 6 H 5 COO) (89.6 mg, 622 μmol), NH4(C6H5COO) (86.5 mg, 622 μmol), (Bu4N)NO3 (315 mg, 1.04 mmol), (Bu4N)ClO4 (354 mg, 1.04 mmol), (Bu4N)BF4 (341 mg, 1.04 mmol), Bu4NF (271 mg, 1.04 mmol), Bu4NCl (288 mg, 1.04 mmol), Bu4NBr (334 mg, 1.04 mmol), or Bu4NI (383 mg, 1.04 mmol) in THF (10 mL). Samples for ESI-MS analysis are taken periodically. In the case of the Bu4N-salts, the reaction mixture is poured into H2O (100 mL) after 1 day of stirring. Then, the blue precipitate is filtered out, washed with water, and dried. The initial nanojar mixture is recovered in all cases (as shown by ESI-MS), except the formate and acetate reactions, where carboxylate-substituted nanojars are obtained and the filtrate retains an amount of water-soluble trinuclear species, (Bu4N)[Cu3(μ3-OH)(μpz)3(RCOO)3] (R = HCOO or CH3COO). Solution Preparation for ESI-MS Analysis. A 1.0 × 10−4 M solution of nanojars is prepared by dissolving (Bu4N)2[CO3⊂{Cu(μOH)(μ-pz)}n] (24.1 mg, 4.99 μmol, based on an average n = 29) or (Bu4N)2[SO4⊂{Cu(μ-OH)(μ-pz)}n] (25.1 mg, 5.01 μmol, based on an average n = 30) in mass spectrometric grade acetonitrile and diluted to volume in a 50 mL volumetric flask. Stock solutions (1.0 × 10−1 M) are prepared in mass spectrometric grade acetonitrile (in the case of formic acid, acetic acid, pivalic acid, benzoic acid, oxalic acid, malonic acid, glutaric acid, methanol, (Bu4N)HSO4), or spectrophotometric grade dimethylformamide (in

the case of stearic acid, succinic acid, adipic acid, sebacic acid, terephthalic acid, p-toluenesulfonic acid, sodium 2-bromoethanesulfonate, and 1-decanethiol, which are insoluble or not sufficiently soluble in acetonitrile), by diluting to volume in a 10 mL volumetric flask. The (Bu4N)HCO3 solution is prepared by passing a steady stream of CO2 (produced by sublimation of dry ice in an Erlenmeyer flask with side arm) through a solution of (Bu4N)OH (1.0 M in H2O; 1000 μL) in CH3CN (9.0 mL) in a 50 mL beaker for 30 min; the solution is quantitatively transferred to a 10 mL volumetric flask and diluted to volume with CH3CN. Two milliliters of the nanojar solution is transferred to a 1 dram vial via 1000 μL micropipette. One microliter of 1.0 × 10−1 M stock solution is required per 1 mL of 1.0 × 10−4 M nanojar solution for each molar equivalent; thus, 2 μL of stock solution is required for 2 mL of nanojar solution to conduct a reaction with 1 mol equiv of substrate. A series of solutions containing 1−10, 15, 20, 25, 31, 100, and 200 mol equiv of substrate are prepared in each case (except for the (Bu4N)HCO3 and (Bu4N)HSO4 experiments, when the 2−4 and 6−9 equiv are omitted). In addition, solutions containing 0.5 equiv of succinic, glutaric, adipic, and sebacic acid are also prepared. After addition of the substrate, the vial is capped, swirled, and allowed to stand overnight (approximately 17 h) at ambient laboratory conditions. The following day, any solids formed are filtered before analysis by ESI mass spectrometry. For the reaction with carbonic acid, excess CO2 gas is passed through a 1.0 × 10−4 M nanojar solution in CH3CN (10 mL) for 30 min (as described above for the preparation of the (Bu4N)HCO3 solution). The resulting solution (no discernible changes) is analyzed directly by ESIMS. Mass Spectrometry. Mass spectrometric analyses are performed on a Waters Synapt G1 HDMS instrument, using electrospray ionization (ESI). Sample solutions are infused by a syringe pump at 10 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. All ESI-MS spectra are recorded in CH3CN solutions; some samples contain small amounts of DMF from the stock solutions, or small amounts of THF when C


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Inorganic Chemistry sampled directly from a reaction mixture in THF. Peak assignment is performed using isotope pattern analysis, as described before; all observed isotope patterns match the corresponding predicted ones within m/z ± 0.1 (reported m/z values represent monoisotopic mass of the most abundant peak within the isotope pattern).7,8,11 Some of the low-nuclearity breakdown products are fully or partially reduced species; it has been documented that Cu(II) is easily reduced to Cu(I) during ESI-MS(−) analysis.26 The parent nanojar peaks are as follows: [CO3⊂{Cu(OH)(pz)}n]2− (n = 27, m/z 2022.0; n = 29, m/z 2169.9; n = 30, m/z 2243.9; n = 31, m/z 2317.4); [SO4⊂{Cu(OH)(pz)}n]2− (n = 27, m/z 2040.0; n = 28, m/z 2113.4; n = 29, m/z 2187.9; n = 31, m/z 2335.4; n = 32, m/z 2409.9; n = 33, m/z 2483.3). Minor peaks corresponding to [(Bu4N)A⊂{Cu(OH)(pz)}n]− (A = CO32− or SO42−) nanojars and their substituted analogues are also observed above m/z 4000 (not shown). Occasionally, minor contamination due to detergent residues, such as [C10H21C6H4SO3]− (m/z 297.2), [C11H23C6H4SO3]− (m/z 311.2), [C12H25C6H4SO3]− (m/z 325.2), [C13H27C6H4SO3]− (m/z 339.2), solvent stabilizer (BHT−, m/z 219.3) or other unidentified contaminants (e.g., m/z 233) is observed. Figure S75 demonstrates that dilution does not lead to nanojar breakdown. X-ray Crystallography. Single crystals of (Bu4N)2[CO3⊂{Cu27(μOH)27(μ-pz)27−x(μ-CH3COO)x}](C6H5CH3)6 (x = 1 and 2) (1) are grown from a toluene solution by hexane vapor diffusion at room temperature. Once removed from the mother liquor, the crystals are sensitive to solvent loss at ambient conditions and are quickly mounted under a cryostream (100 K) to prevent decomposition. Single crystals of (Bu4N)2[Cu2(μ-pz)2(SO4)2] (2), (Bu4N)2[Cu2(μ-pz)2(CO3)2] (3) and (Bu4N)2[Cu2(μ-pz)2(CO3)(SO4)]·CH3OH (4) are grown from an acetone solution (acetone/CH3OH for the latter) by diethyl ether vapor diffusion at room temperature. X-ray diffraction data are collected at 100 K (nanojar) or room temperature (dinuclear complexes) from a single-crystal mounted atop a glass fiber under Paratone-N oil, 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 leastsquares on F2, using the APEX2 software package.27 All non-H atoms are refined with independent anisotropic displacement parameters (except for some of the disordered atoms noted below). C−H hydrogen atoms are placed in idealized positions and refined using the riding model, whereas O−H hydrogen atoms are located from the difference Fourier maps; their displacement parameters are fixed to be 20% larger than those of the attached O atoms. In 1, one pyrazolate moiety is disordered with an acetate moiety (50/50) in each crystallographically independent nanojar unit; in one of the two units, another pyrazolate moiety is disordered over two positions (50/50). Several CH3 and CH2 endgroups of the tetrabutylammonium counterions are also disordered over two positions (50/50). Disordered atoms and solvent molecules were refined with isotropic ADPs; no H atoms were assigned for the disordered atoms, solvent molecules, and counterions. In 3, one of the two pyrazolate units is disordered over two positions (50/50); these disordered groups are refined with isotropic ADPs, without H atoms. Also, two CH2CH3 end-groups of the Bu4N+ counterions are disordered over two positions (50/50). In 4, one of the two Bu4N+ counterions and the methanol solvent molecule are disordered over two positions (60/ 40) and are refined with isotropic ADPs, without H atoms, using geometric restrains. Crystallographic details are summarized in Table 1.

28, 29, 31, 32) will be employed to abbreviate different carbonate- and sulfate-incarcerating nanojars (including substituted ones). As a general trend, the deep-blue color of the nanojar solutions gradually fades as increasing amounts of acid are added (0−31 equiv). In the case of carboxylic acid reagents, increasing amounts of a purple-gray precipitate also forms concomitantly above certain concentrations of acid (see Supporting Information for details). On the basis of its insolubility in all common solvents, including DMF and DMSO, the precipitate is most likely polymeric [Cu(μ-pz)x(μRCOO)y]∞ (Scheme 1), similar to [Cu(μ-OH)(μ-pz)]∞ and Scheme 1. Examples of Possible Repeating Units in the [Cu(μpz)x(μ-RCOO)y]∞ Polymer

[Cu(μ-pz)2]∞.3,28 In most cases, no precipitate forms at higher acidities (100 and/or 200 equiv of acid); however, those solutions change color from blue to seafoam green. In the case of dicarboxylic acids, precipitates (very pale blue) only form at higher equivalents of acid. Finally, in the case of the stronger oxalic and p-toluenesulfonic acids, as well as the tetrabutylammonium salts (bicarbonate and bisulfate), no precipitates form at any equivalents of acid. The following section describes the composition of the filtered solutions with varying equivalents of acid, based on ESI-MS analysis (both negative and positive mode). Nanojars are not detectable in ESI-MS(+), whereas lownuclearity breakdown products are detectable both in ESIMS(−) and ESI-MS(+). Therefore, emphasis is placed on ESIMS(−), where both nanojars and their breakdown products are visible. Examples of peaks observed in ESI-MS(+), with counterparts in ESI-MS(−), are given in the case of formic acid (see below). Examples of the structure of nanojars and lower nuclearity breakdown products are illustrated in Figure 1. Formic acid, HCOOH (pKa = 3.75 in H2O).29 At 1 equiv of HCOOH, up to three pyrazolates are substituted with formates in both Cu27CO3 and Cu29CO3, whereas in Cu30CO3 and Cu31CO3 only one pyrazolate is substituted. At 2 equiv and

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RESULTS Effect of Acids and Their Conjugate Bases on Nanojars. To assess the effect of acids at different concentrations on nanojars, titration experiments are carried out with 0−200 equiv of acid in acetonitrile solution, and the results are monitored both visually and by ESI-MS spectrometry. Reactivity toward conjugate bases is tested similarly, if the salt of the conjugate base is soluble in acetonitrile. Otherwise, a solution of nanojars in wet THF is stirred with the suspended salt for several days. In each case, an as-synthesized mixture of nanojars is used. Hereafter, CunCO3 (n = 27, 29, 30, 31) and CunSO4 (n = 27,

Figure 1. Illustration of the structure of nanojars and its tri- and hexanuclear copper-pyrazolate breakdown products. Color code for atoms: Cu − blue; O − red; C − black. D


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Inorganic Chemistry

Figure 2. ESI-MS(−) spectra of CunCO3 and CunSO4 with varying amounts of formic acid. Color code for [A⊂Cun(OH)n(pz)n−x(HCOO)x]2−: x = 0 (red), x = 1 (cyan), x = 2 (orange), x = 3 (green), x = 4 (violet), x = 5 (magenta), x = 6 (blue), and x = 7 (brown) (see also Figures S1−S6).

above, substitution of up to four pyrazolates (up to five in Cu29CO3) is indicated by the presence of [CO3⊂{Cun(OH)n(pz)n−x(HCOO)x}]2− (n = 27, 29−31; x = 0−5) species in the ESI-MS(−) spectrum (Figures 2, S1−S3, and Table 2), along with gradually increasing amounts of the hexanuclear [Cu6O2(pz)6(HCOO)3]− and small amounts of lower nuclearity breakdown products (Table S1). Examples of breakdown products identified in ESI-MS(+) are [Cu3O(pz)(HCOO)2]+ (m/z 363.8), [Cu3O(pz)2(HCOO)]+ (m/z 385.8), [Cu3O(pz)3]+ (m/z 407.9), [Cu3(OH)(pz)3(HCOO)]+ (m/z 453.9), [Cu3(OH)(pz)4]+ (m/z 475.9), [(Bu4N)2Cu3O(pz) 3 (HCOO) 2 ] + (m/z 982.4), [(Bu 4 N) 2 Cu 6 O 2 (pz) 6 (HCOO)3]+ (m/z 1435.3). At 6 equiv of HCOOH, Cu30CO3 and Cu31CO3 are absent; all nanojars break down completely at 20 equiv of acid. Trinuclear and hexanuclear species are stable even at 200 equiv of HCOOH as indicated by peaks in the ESIMS spectrum corresponding to [Cu3O(pz)3−x(HCOO)x]+ (x = 0−2), [Cu6O2(pz)9‑x(HCOO)x]− (x = 3−9) and [Cu6O3(HCOO)7]− (Table S1). The reactivity of CunSO4 displays significant differences compared to the analogous CunCO3 (Figures 2, S4−S6 and Table 2). Solids only start forming at 20 equiv of HCOOH, above which all nanojars break down into lower nuclearity complexes described above (Cu28SO4 and Cu31SO4 break down completely above 3 and 6 equiv, respectively). Up to seven pyrazolates are substituted with formates in Cu27SO4 and Cu29SO4, whereas only three are substituted in Cu28SO4 and Cu31SO4. Between 6−20 equivalents, [Cu33(OH)33(pz)33−x(HCOO)xSO4]2− (x = 0−4) species are also observed. Acetic acid, CH3COOH (pKa = 4.76 in H2O,29 23.51 in CH3CN).30 The reactivity of CunCO3 toward acetic acid is similar to its reactivity to formic acid (Figures S7−S9, Table 2),

although insoluble solids, as well as soluble breakdown products, such as [Cu3O(pz)2(CH3COO)3]−, [Cu3O(pz)3(CH3COO)2]− and [Cu6O2(pz)6(CH3COO)3]− are obtained from the first equivalent of CH3COOH added (Table S1). Co-crystallized acetate-substituted nanojars (Bu4N)2[CO3⊂{Cu27(μ-OH)27(μpz)27−x(μ-CH3COO)x}] (x = 1 and 2) have been obtained as single crystals; X-ray diffraction reveals that substitution preferentially occurs at the Cu9-ring (see Crystallography section below). Cu31CO3 is the most sensitive nanojar, which first breaks down into Cu30CO3. As with HCOOH, both Cu30CO3 and Cu31CO3 completely disappear above 5 equiv, and all nanojars break down above 20 equiv. Trinuclear and hexanuclear species are present at 100 and 200 equiv, along with small amounts of mononuclear complexes. Unlike CunCO3, CunSO4 nanojars do not form an insoluble breakdown product with any equivalents of acetic acid. Instead, CunSO4 allows more pyrazolate ligands to be exchanged with acetate: up to seven acetates are observed with Cu27SO4 and Cu29SO4, up to three with Cu28SO4 and up to five with Cu31SO4 (Figures S10−S12, Table 2). As seen before with formic acid, CunSO4 nanojars are also more resistant to acidity: Cu28SO4 and Cu31SO4 survive up to 5 and 15 equiv, respectively, whereas Cu27SO4 and Cu29SO4 are present up to 31 equiv. Breakdown products similar to the ones observed with CunCO3 are present above 2 equiv of acid. Above 3 equiv, hitherto unidentified groups of multiply substituted nanojars appear at m/z 2037− 2050, 2512−2526, and 3020−3040; these nanojars are exceptionally stable to CH3COOH, as shown by their presence, in small amounts, even at 100 equiv of acid. Stearic acid, CH3(CH2)16COOH (pKa ≈ 5.0 in H2O). Although color changes observed upon addition of increasing amounts of stearic acid are similar to the case of formic acid E


Article

Inorganic Chemistry Table 2. Carboxylate-Substituted Nanojar Species (m/z) Observed by ESI-MS(−) carboxylate-substituted nanojars, [A⊂Cun(OH)n(pz)n−xLx]2− (A = CO32− or SO42−) acid (HL) HCOOH (A = CO3)

HCOOH (A = SO4)

CH3COOH (A = CO3)

CH3COOH (A = SO4)

CH3(CH2)16COOH (A = CO3)

(CH3)3CCOOH (A = CO3)

(CH3)3CCOOH (A = SO4)

C6H5COOH (A = CO3)

C6H5COOH (A = SO4)

n = 27 n = 29 n = 30 n = 31 n = 27 n = 28 n = 29 n = 31 n = 33 n = 27 n = 29 n = 30 n = 31 n = 27 n = 28 n = 29 n = 31 n = 27 n = 29 n = 30 n = 31 n = 27 n = 28 n = 29 n = 30 n = 31 n = 32 n = 27 n = 28 n = 29 n = 31 n = 27 n = 29 n = 30 n = 31 n = 27 n = 28 n = 29 n = 31

x=1

x=2

x=3

x=4

2011.5 2158.9 2232.4 2305.9 2029.9 2102.4 2176.9 2323.9 2472.3 2018.0 2165.9 2239.4 2313.4 2036.9 2109.4 2183.9 2330.9 2130.6 2278.6 2352.0 2425.5 2039.0

2000.4 2147.9 2221.4 2294.9 2017.9 2091.4 2165.9 2312.8 2461.3 2014.5 2161.9 2235.4 2308.9 2031.9 2105.4 2179.9 2326.9 2238.7 2387.0 2460.2 2533.6 2056.0

1977.9 2126.4 2199.3 2272.8 1995.8 2143.9

2187.0 2260.9 2334.4

2204.0 2277.9 2351.4

2057.0 2130.5 2204.9 2352.4 2049.0 2196.9 2270.9 2344.4 2067.0 2140.4 2214.9 2362.4

2074.0 2147.5 2222.4 2369.4 2076.0 2224.4 2297.9 2371.4 2094.0 2167.4 2242.4 2389.4

1989.9 2136.9 2210.4 2283.8 2006.9 2079.4 2154.9 2301.8 2450.3 2009.9 2157.9 2231.4 2305.4 2028.4 2102.4 2175.9 2322.9 2346.8 2494.8 2568.3 2641.7 2073.0 2147.0 2221.5 2295.0 2368.4 2442.9 2091.0 2164.5 2239.5 2386.4 2103.5 2251.4 2324.9 2398.4 2121.5

x=6

x=7

x=8

x=9

x = 10

1984.9

1973.9

1962.8

2133.3

2121.8

2110.8

2020.4

2015.9

2011.9

2167.9

2163.9

2159.9

2107.0 2181.0 2255.5 2329.0

2124.6 2198.0 2272.5 2346.0

2141.6 2215.1 2289.0 2363.0

2158.6 2232.1

2249.1

2266.1

2380.0

2397.1

2414.1

2459.9 2108.0

2477.0 2125.5

2494.0 2142.5

2511.0 2159.6

2528.0 2176.6

2545.0 2193.6

2256.5 2403.4 2130.5 2278.4 2351.9

2273.5 2420.5 2157.5 2305.4

2290.5 2437.5 2184.5 2332.4

2307.5 2454.5

2324.5 2471.5

2341.6 2488.5

2359.4

2148.5

2175.5

2202.5

2229.5

2556.5

2283.5

2269.4 2416.4

2296.4 2443.4

2323.4 2470.4

2350.4 2497.4

2377.4

2404.4

2432.4

2438.8 2006.4 2153.9 2227.4 2300.9 2024.4 2171.9 2318.8 2454.9 2602.9 2676.4 2749.9 2090.0 2164.0 2238.5 2312.0

x=5 2114.9

2149.9

2563.1 2711.0

absent and increasing amounts of Cu30CO3 form. Unexpectedly, substituted Cu28CO3 and Cu32CO3 nanojars start appearing at 3 and 10 equiv of pivalic acid, respectively. Cu30CO3 and Cu32CO3 are present up to 31 equiv; Cu30CO3 is the most resistant nanojar to pivalic acid. In the case of CunSO4 nanojars, substituted Cu27SO4, Cu29SO4, and Cu31SO4 species are still present at 31 equiv of acid (Figures S19−S21, Table 2). Benzoic acid, C6H5COOH (pKa = 4.20 in H2O, 21.51 in CH3CN). As with pivalic acid, breakdown products start forming from the first equivalent; however, a smaller number of pyrazolates are substituted in the CunCO3 nanojars (Figures S22−S24, Table 2). Above 3 equiv of acid, Cu31CO3 species are absent; at the same time, the amount of Cu30CO3 nanojars increases temporarily up to 6 equiv. At 10 equiv, Cu29CO3 and small amounts of Cu27CO3 nanojars are detectable; all nanojars break down above 10 equiv, and additional breakdown species become visible in the ESI-MS spectrum (Table S1). In the case of CunSO4, Cu28SO4 nanojars break down above 3 equiv, whereas

(precipitates form from 5 equiv), a significantly different reactivity is observed. The most striking difference is that significant amounts of breakdown products are only observed above 5 equiv of acid (Figures S13−S15). In this case, a trinuclear complex is the only major breakdown product, as evidenced by the presence of [Cu3O(pz)2(C17H35COO)3]− and [Cu3(OH)(pz)3(C17H35COO)3]− between 5−31 equiv (Table S1). At all equivalents, there is a larger amount of nanojars present than in the corresponding cases with formic or acetic acids. Cu30CO3 and Cu31CO3 only survive up to 3 equiv of stearic acid, whereas Cu29CO3 and small amounts of Cu27CO3 are detectable up to 31 equiv. Up to six and seven pyrazolates are substituted in Cu27CO3 and Cu29CO3, respectively, and up to three in Cu30CO3 and Cu31CO3 (Table 2). Above 100 equiv, only stearate and its cluster ions are observed (Table S1). Pivalic acid, (CH3)3CCOOH (pKa = 5.03 in H2O). From the first equivalent, tri- and hexanuclear breakdown species are observed along with substituted nanojars (Figures 3 and S16− S18, Tables 2 and S1). Above 3 equiv, Cu31CO3 nanojars are F


Article

Inorganic Chemistry

and 1878.8, for instance, correspond to a complex composed of oxalate-tethered tri- and hexanuclear species, [{Cu6(OH)2(pz)6(HC2O4)2}(C2O4){Cu3O(pz)2(HC2O4)2(OH)2}]2−, and its Bu4N-adduct. Oxalate-substituted nanojars are not observed as free-standing species but only tethered to breakdown products, as 2−, 3−, and 4− species (Table 3). Nanojars tethered to both tri- and hexanuclear species are also observed at m/z 1966.3. At higher equivalents of acid, increasing amounts of mono-, di-, tri-, and tetranuclear breakdown products are observed. All nanojars are broken down at and above 25 equiv. At 100 and 200 equiv of oxalic acid no copper complexes, only oxalate species, are detected (Table S2). Malonic acid, CH2(COOH)2 (pKa1 = 2.85; pKa2 = 5.70 in H2O; pKa1 = 15.3 in CH3CN). From the first equivalent of acid, nanojars in which malonate substitutes either one or two OH groups, or one pyrazolate group are observed (Figures S34−S36, Table 3). Increasing amounts of breakdown products are observed at higher equivalents. Substituted Cu31CO3 nanojars (m/z 2413.4 and 2455.4), as well as a [Cu27(OH)26O(pz)26CO3]3− (m/z 1989.0) are observed up to 9 equiv. Above 10 equiv all nanojars break down, and [OOCCH2COO]− and [Cu4O(OH)2(pz)4(HOOCCH2COO)]− become the dominant peaks. At 100 and 200 equiv, malonate species are dominant, with small amounts of [Cu(HOOCCH2COO)2]− (m/z 268.9) (Table S2). Succinic acid, (CH2)2(COOH)2 (pKa1 = 4.21; pKa2 = 5.64 in H2O; pKa1 = 17.6 in CH3CN). Starting with succinic acid, a novel species appears at m/z 2504.3; this peak is attributed to a triply substituted nanojar species, [CO3⊂{Cu27(OH)27(pz)24}{OOC(CH2)2COO}3{Cu6O(OH)(pz)6}]2−, in which three pyrazolate groups are substituted with succinate moieties; the succinate arms act as tethers to the hexanuclear breakdown product (Figures 4 and S37−S39, Table 3). On the basis of the crystal structure of acetate-substituted Cu27CO3 (see Crystallography section below), it is likely that substitution with the dicarboxylate ligand occurs symmetrically at every third pyrazolate group of the Cu9 ring. A doubly substituted Cu31CO3 nanojar (m/z 2417.4), where two OH groups are substituted with succinate moieties, also forms from 0.5 equiv and up. At 3 equiv and above, the freestanding hexanuclear breakdown product [Cu6O2(pz)5{OOC(CH2)2COO}2]− (m/z 980.7) is also observed. The Cu30CO3 nanojar is absent above 2 equiv; between 6−10 equiv, Cu27CO3+Cu6 is the only major nanojar species observed, along with the Cu6 breakdown product. At 15 equiv and up, all nanojars and the Cu6-complex break down; instead, lower nuclearity species are present. At 100 and 200 equiv, only succinate and its cluster ions are observed (Table S3). Glutaric acid, (CH2)3(COOH)2 (pKa1 = 4.32; pKa2 = 5.42 in H2O; pKa1 = 19.2 in CH3CN). As with succinic acid, [CO3⊂{Cu27(OH)27(pz)24}{OOC(CH2)3COO}3{Cu6O(OH)(pz)6}]2− (m/z 2525.3) begins to form at 0.5 equiv (Figures S40−S42, Table 3). A peak corresponding to the novel species [CO3⊂Cun(OH)n(pz)n−2{OOC(CH2)3COO}]2−, where the dicarboxylate acts as a ditopic ligand substituting two pyrazolate groups, is only observed for n = 27 (m/z 2020.0); this is completely transformed to the m/z 2525.3 species above 2 equiv. At that point, free [Cu6O2(pz)5{OOC(CH2)3COO}2]− (m/z 1008.8) is also observed, along with [Cu6O2(OH)(pz)4{OOC(CH2)3COO}2]− (m/z 958.7). Above 4 equiv, Cu30CO3 and Cu31CO3 nanojars are absent, whereas above 6 equiv the m/z 2525.3 species is virtually the only nanojar present. At 15 equiv and up, all nanojars are completely broken down; instead, lower

Figure 3. ESI-MS(−) spectra of CunCO3 with varying amounts of pivalic acid. Color code for [CO3⊂{Cun(OH)n(pz)n−x(pivalate)x}]2−: x = 0 (red), x = 1 (cyan), x = 2 (orange), x = 3 (green), x = 4 (violet), x = 5 (magenta), x = 6 (blue), x = 7 (brown), x = 8 (pink), x = 9 (lime), x = 10 (teal) (see also Figures S16−S18).

Cu27SO4, Cu29SO4, and Cu31SO4 species resist up to 20 equiv of acid (Figures S25−S27, Table 2). Terephthalic acid, C6H4(COOH)2 (pKa1 = 3.54, pKa2 = 4.34 in H2O; pKa1 = 19.7 in CH3CN). Compared to benzoic acid, the presence of two carboxylic acid groups in terephthalic acid leads to significant differences in reactivity toward nanojars (Figures S28−S30, Tables 3 and S2). Only mono- and disubstituted nanojars are observed, with either singly or doubly deprotonated terephthalate. Interestingly, a hitherto unidentified nanojar at m/ z 2848 (present in trace amounts in the parent nanojar mixture) is amplified and becomes the base peak in the spectrum at 5 equiv of terephthalic acid. In contrast to benzoic acid, insoluble breakdown products are not observed up to 100 equiv, and nanojar species dominate over breakdown products up to 20 equiv. Whereas with benzoic acid all nanojars break down above 10 equiv of acid, in the case of terephthalic acid [CO3⊂{Cu(OH)(pz)}29]2− and [CO3⊂{Cu29(OH)29(pz)28(OOCC6H4COO)}]3− survive and become the major peaks in the ESI-MS spectrum at 20 equiv of acid. Oxalic acid, (COOH)2 (pKa1 = 1.25; pKa2 = 3.81 in H2O; pKa1 = 14.5 in CH3CN). Being a relatively strong acid, oxalic acid starts breaking down nanojars from the first equivalent (Figures S31−S33, Tables 3 and S2). In contrast to monocarboxylic acids, oxalic acid only forms soluble breakdown products at all equivalents. The most prominent breakdown products are trinuclear and hexanuclear complexes, which, due to the ditopic nature of oxalate, are not only found individually, but also tethered to each other as well as to nanojars. Peaks at m/z 818.3 G


H

(CH2)8(COOH)2, pKa1 = 4.59; pKa2 = 5.59

(CH2)2(COOH)2, pKa1 = 4.21; pKa2 = 5.64 (CH2)3(COOH)2, pKa1 = 4.32; pKa2 = 5.42 (CH2)4(COOH)2, pKa1 = 4.41; pKa2 = 5.41

CH2(COOH)2, pKa1 = 2.85; pKa2 = 5.70

(COOH)2 pKa1 = 1.25; pKa2 = 3.81

C6H4(COOH)2, pKa1 = 3.54, pKa2 = 4.34

acid (H2L)

[Cun(OH)n(pz)n−1(L)CO3]3− (n = 29, 1458.3; n = 31, 1556.6; n = 32, 1606.2) [Cu31(OH)29(pz)31CO3{HL)2]2− (2417.4)

[Cu31(OH)31−x(pz)31(HL)xCO3(H2L)]2− (x = 1, 2413.4; x = 2, 2455.4)

[Cun(OH)n(pz)n−2(HL)2CO3]2− (n = 27, 2120.0; n = 29, 2268.4; n = 30, 2341.9; n = 31, 2415.4) [Cun(OH)n(pz)n−1(L)CO3]3− (n = 27, 1380.3; n = 29, 1479.3; n = 30, 1528.3; n = 31, 1577.3)

[Cun(OH)n(pz)n−1(HL)CO3] (n = 27, 2071.0; n = 29, 2219.4; n = 30, 2292.9; n = 31, 2366.4)

2−

nanojar with monotopic dicarboxylate (m/z)

dicarboxylate-substituted nanojars

[Cu27(OH)27(pz)25(L)CO3]2− (2020.0)

nanojar with ditopic dicarboxylate (m/z)

[Cun(OH)n(pz)n−2xLxCO3]2−, (n = 27: x = 1, 2055.0; x = 2, 2088.0; n = 29: x = 1, 2203.0; x = 2, 2236.5; x = 3, 2269.5; n = 30: x = 1, 2276.9; n = 31: x = 1, 2350.4)

Table 3. Dicarboxylate-Substituted Nanojar Species Observed by ESI-MS(−)

[{Cu27(OH)27(pz)24CO3L3}{Cu6O2(pz)6}]3− (1697.2) [(Bu4N)2{Cu27(OH)27(pz)24CO3L3}{Cu6O2(OH)(pz)7}]3− (1886.1) [{Cu27(OH)27(pz)24CO3L3}{Cu6O(OH)(pz)6}]2− (2630.5)

[{Cu27(OH)27(pz)24CO3L3}{Cu6O(OH)(pz)6}]2− (2546.4)



[(Bu4N){Cun(OH)n(pz)n−1CO3}{Cu3(OH)(pz)3L2}]2− (n = 27, 2402.5; n = 29, 2550.5; n = 31, 2697.4) [(Bu4N)2{Cu27(OH)27(pz)26CO3}{Cu6O2(pz)6L3}]3− (m/z 1847.4) [(Bu4N){Cu27(OH)27−xOx(pz)26−xCO3}{Cu6O2(pz)6L3}]4− (x = 0, 1325.0; x = 2, 1291.0; x = 4, 1256.9; x = 6, 1222.9) [(Bu4N){Cu27(OH)27−xOx(pz)28−xCO3}{Cu6O2(pz)6L2}]4− (x = 0, 1336.2; x = 2, 1302.5; x = 4, 1268.5; x = 6, 1234.4) [(Bu4N)2{Cu27(OH)27(pz)26CO3}{Cu6O2(pz)6L3} {Cu3O2(pz)2}]3− (1966.3)

[(Bu4N)2{Cun(OH)n(pz)n−1CO3}{Cu6O(OH)(pz)6L3}]2− (n = 27, 2885.6; n = 29, 3033.6; n = 30, 3107.1; n = 31, 3180.6)

nanojar tethered to tri- or hexanuclear complex (m/z)

Inorganic Chemistry Article


Article

Inorganic Chemistry

6 equiv all nanojars break down. Cu29CO3 is the most stable nanojar to sebacic acid. At 100 and 200 equiv, only sebacate and its cluster ions are observed (Table S3). p-Toluenesulfonic acid, CH3C6H4SO3H (pKa = −2.8 in H2O, 8.6 in CH3CN). At 1 equiv of acid, the nanojar peak intensities decline and mono- and trinuclear breakdown products are observed, along with metal-free p-toluenesulfonate species (Figures S49−S51, Table S1). At 2 equiv, all nanojars completely break down and [Cu(CH3C6H4SO3)2]− (m/z 405.0) becomes the base peak; small amounts of tetranuclear and hexanuclear species also appear. Above 3 equiv, the trinuclear breakdown product becomes the major species present and the color of the solution gradually turns blue-green. At 31 equiv, [Cu3O(pz)2(CH3C6H4SO3)3]− (m/z 853.9) is still the dominant species, and [Cu2(pz)2(CH3C6H4SO3)3]− (m/z 773.0) is also observable. At 100 and 200 equiv, the solutions are almost colorless and only cluster ions [Cux(CH3C6H4SO3)2x+1]− (x = 1−4) are observed along with p-toluenesulfonate and its cluster ions. In contrast to carboxylates, no sulfonate-substituted nanojars are observed at any equivalents of acid. Sodium 2-bromoethanesulfonate, BrCH2CH2SO3Na. As with ptoluenesulfonic acid, no sulfonate-substituted nanojars are observed at any equivalents (Figures S52−S54). Above 15 equiv, the color of the solutions gradually decreases and a blue solid precipitates out. All parent CunCO3 nanojars are present in solution up to 200 equiv, along with cluster ions [CO3⊂{Cu(OH)(pz)}n(BrCH2CH2SO3Na)x]2− (x = 1−5; n = 27, m/z 2126.9−2548.7; n = 29, m/z 2274.9−2697.2; n = 30, m/z 2348.9−2770.7; n = 31, m/z 2422.3−2844.1), [Nax(BrCH2CH2SO3)x+1]− (x = 0−12; m/z 186.9−2717.6), [Na(BrCH 2 CH 2 SO 3 )Br] − (m/z 290.8) and [Bu 4 N(BrCH2CH2SO3)2]− (m/z 618.1). At higher equivalents, [Nax(BrCH2CH2SO3)x+2]− (x = 13−21; m/z 1558.2−2402.8) clusters are also observed. 1-Decanethiol, CH3(CH2)9SH (pKa ≈ 10.5 in H2O). As with sulfonates, no substituted nanojars are observed with 1decanethiol (Figures S55−S57). Instead, an etching effect similar to the action of NH3 and pyridine described earlier, is observed.4 Whereas NH3 led to the transformation of all larger CunCO3 nanojars into Cu27CO3, in this case an amplification of the amount of Cu30CO3 occurs (becomes base peak at 31 equiv), at the expense of other CunCO3 nanojars (n = 27, 29, 31). Even more unexpectedly, small amounts of the extremely rare [CO 3 ⊂{Cu(OH)(pz)} 28 ] 2− and the never-before-seen [CO3⊂{Cu(OH)(pz)}36]2− nanojars appear (at m/z 2095.5 and 2686.8, respectively). Above 15 equiv the blue color of nanojars gradually decreases in intensity, and increasing amounts of breakdown products, including [Cu6O2(pz)9]− (m/z 1016.9), are detected; at 100 and 200 equiv, the solution becomes colorless and a white colloidal solid forms. Methanol, CH3OH (pKa = 15.5 in H2O). Methanol does not have an observable effect on CunCO3 nanojars between 1−200 equiv (Figures S58−S60). It has been shown previously, however, that nanojars do break down at higher methanol concentrations, such as in a CH3CN/CH3OH (2:1) solution.6 Carbonic acid, H2CO3 (pKa1 = 6.35; pKa2 = 10.33 in H2O). When CO2 is passed through a solution of nanojars in wet acetonitrile, an etching effect identical to the one of NH3 is observed.4 Thus, CunCO3 nanojars are converted to Cu27CO3, whereas CunSO4 nanojars are converted into Cu31SO4 (Figure S61). Bicarbonate, HCO3− (pKa = 10.33 in H2O). At 1 equiv of (Bu4N)HCO3, the soluble dinuclear complex

Figure 4. ESI-MS(−) spectra of CunCO3 with varying amounts of succinic acid (H2L). Color code: red − parent CunCO3 nanojars; green − [CO3⊂{Cu31(OH)29(pz)31(HL)2}]2−; cyan − [CO3⊂{Cu27(OH)27(pz)24L3}{Cu6O(OH)(pz)6}]2− (see also Figures S37−S39).

nuclearity species are present. At 100 and 200 equiv, only glutarate and its cluster ions are observed (Table S3). Adipic acid, (CH2)4(COOH)2 (pKa1 = 4.41; pKa2 = 5.41 in H2O; pKa1 = 20.3 in CH3CN). At 0.5 equiv, [CO3⊂{Cu27(OH)27(pz)24}{OOC(CH2)4COO}3{Cu6O(OH)(pz)6}]2− (m/z 2546.4) begins to form (Figures S43−S45, Table 3). Above 2 equiv a related complex [(Bu 4 N) 2 CO 3 ⊂{Cu 27 (OH) 27 (pz) 24 }{OOC(CH 2 ) 4 COO} 3 {Cu6O2(OH)(pz)7}]3− (m/z 1886.1) is detected. Between 7−10 equiv, these two species are virtually the only nanojars present; triply charged [CO3⊂{Cu27(OH)27(pz)24}{OOC(CH2)4COO}3{Cu6O2(pz)6}]3− (m/z 1697.2) is also detected. At 15 equiv and up, all nanojars are completely broken down to lower nuclearity species. At 100 and 200 equiv, only adipate and its cluster ions are observed (Table S3). Sebacic acid, (CH2)8(COOH)2 (pKa1 = 4.59; pKa2 = 5.59 in H2O). As with succinic, glutaric, and adipic acids, [CO3⊂{Cu27(OH) 27 (pz) 24 }{OOC(CH 2 ) 8 COO} 3 {Cu 6 O(OH)(pz) 6 }] 2− (m/z 2630.5) begins to form at 0.5 equiv. Nanojar species, where up to three sebacates act as ditopic ligands substituting two pyrazolate moieties each, are observed; the prevalence of these species is much more pronounced in the case of Cu29CO3 and Cu27CO3, than Cu31CO3 and Cu30CO3 (Figures S46−S48, Table 3). From the first equivalent of acid and up, gradually increasing amounts of lower nuclearity breakdown products are observed. Cu31CO3 and Cu30CO3 are absent above 2 equiv, whereas above I


Article

Inorganic Chemistry

Figure 5. ESI-MS(−) spectra of CunCO3 with varying amounts of (Bu4N)HCO3 (left) and (Bu4N)HSO4 (right). Color code: red − parent CunCO3 nanojars; cyan − [(Bu4N)CO3⊂{Cun(OH)n(pz)n−1(SO4)}]2−; orange − [(Bu4N)2{CO3⊂Cun(OH)n(pz)n−2(SO4)2}]2−; magenta − [SO4⊂{Cu(OH)(pz)}31]2−; olive − [(Bu4N)SO4⊂{Cun(OH)n(pz)n−1(SO4)}]2−; brown − [(Bu4N)SO4⊂{Cun(OH)n(pz)n−2(SO4)2}]2−.

[Cu2(pz)2(CO3)2]2− begins to form (Figures 5 and S62). The ESI-MS peak corresponding to [(Bu4N)Cu2(pz)2(CO3)2]− (m/ z 622.2) becomes the base peak at 2 equiv; [(Bu4N)2Cu2(pz)2(CO3)2(HCO3)]− (m/z 925.5) is also detected, along with HCO3− (m/z 61.0), [(Bu4N)(HCO3)2]− (m/z 364.3) and small amounts of larger [(Bu4N)x(HCO3)x+1]− cluster ions. At increasing equivalents of HCO3−, the amount of nanojars gradually decreases and the amount of dimer increases (Figure S63). Nanojars are still detected at 100 and 200 equiv, while the spectra are dominated by [(Bu4N)x(HCO3)x+1]− cluster ions (x = 0−9, m/z 61.0−2791.5). No nanojar substitution or precipitate formation is observed at any equivalents; however, the solutions gradually turn purple, the color of the dinuclear complex. CunSO4 nanojars react similarly, although Cu28SO4 is not stable and it breaks down completely at 10 equiv and above (Figures S64 and S65). Also, additional cluster ions are observed: [(Bu4N)x+1Cu2(pz)2(CO3)2(HCO3)x]− (x = 1−6, m/z 925.5− 2442.8), [(Bu4N)3Cu4(pz)4(CO3)4]− (m/z 1488.6), [(Bu4N)x(HCO3)x+1]− (x = 1−9, m/z 364.3−2791.5). Bisulfate, HSO4− (pKa = 1.99 in H2O). At 1 equiv of (Bu4N)HSO4, each Cu31CO3 nanojar begins to exchange one pyrazolate with an SO42− ion: besides the major set of peaks corresponding to the parent nanojars, a new set of peaks corresponding to [(Bu4N)CO3⊂{Cun(OH)n(pz)n−1(SO4)}]2− (n = 27, m/z 2158.1; n = 29, m/z 2306.0; n = 30, m/z 2379.5; n = 31, m/z 2453.0) are observed (Figures 5 and S66). In addition, some of the Cu31CO3 nanojar is converted to [SO4⊂{Cu(OH)(pz)}31]2− (m/z 2335.4), [SO4⊂{Cu31(OH)31(pz)30(SO4)}]3− (m/z 1566.2) and [(Bu4N)SO4⊂{Cu31(OH)31(pz)30(SO4)}]2− (m/z 2471.0). None of these two types of reactivity was observed with bicarbonate, as no pyrazolate ligand nor incarcerated sulfate ion was found to exchange with carbonate ion. As with (Bu4N)HCO3, a dinuclear species is

present, attested by a peak at m/z 694.1 corresponding to [(Bu4N)Cu2(pz)2(SO4)2]−. At 5 equiv of bisulfate, all initial CunCO3 nanojars are absent (except small amounts of Cu27CO3), and new peaks corresponding to [(Bu4N)2{CO3⊂Cun(OH)n(pz)n−2(SO4)2}]2− (n = 27, m/z 2293.7; n = 29, m/z 2441.6; n = 30, m/z 2515.1), [(Bu 4 N) 2 SO 4 ⊂{Cu 3 1 (OH) 3 1 (pz) 2 9 (SO 4 ) 2 }] 2 − (m/z 2606.6) and [(Bu4N)x−1SO4⊂Cu31(OH)31(pz)31−x(SO4)x]3− (x = 1, m/z 1566.2; x = 2, m/z 1657.0; x = 3, m/z 1747.4) appear. Significant amounts of [HSO4]− (m/z 97.0) are also detected. At 10 equiv of HSO4−, small amounts of [(Bu4N)3SO4⊂{Cu31(OH)31(pz)28(SO4)3}]2− (m/z 2742.2) and [(Bu4N)2Cu2(pz)2(SO4)2(HSO4)]− (m/z 1033.4) are also present. At increasing equivalents of HSO4− (15−31), the amount of nanojars gradually decreases while the amount of dinuclear complex increases, and increasing amounts of cluster ions [(Bu4N)x(HSO4)x+1]− (x = 0−8, m/z 97.0−2811.9) and [(Bu4N)x+1(HSO4)x(SO4)]− (x = 2, m/z 1016.7; x = 3, m/z 1356.0) are observed (Figure S67). At 100 and 200 equiv, the nanojars are virtually completely converted to [Cu2(pz)2(SO4)2]2−: the peak corresponding to [(Bu4N)Cu2(pz)2(SO4)2]− (m/z 694.1) dominates the spectrum (along with tetrabutylammonium hydrogen sulfate/sulfate cluster ions), and the color of the solution turns to a distinct purple. No insoluble breakdown products form at any equivalents. In the case of the CunSO4 nanojars, 1 equiv of (Bu4N)HSO4 causes each nanojar to begin exchanging one or two pyrazolates with sulfate ions, and peaks corresponding to [(Bu4N)SO4⊂{Cun(OH)n(pz)n−1(SO4)}]2− (n = 27, m/z 2176.1; n = 28, m/z 2249.5; n = 29, m/z 2324.0; n = 31, m/z 2471.0; n = 32, m/z 2545.5), [SO4⊂Cun(OH)n(pz)n−1(SO4)]3− (n = 27, m/z 1369.9; n = 29, m/z 1468.3; n = 31, m/z 1566.2; n = 32, m/z 1616.2) and [(Bu4N)2SO4⊂{Cun(OH)n(pz)n−2(SO4)2}]2− (n = 27, m/z J


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Inorganic Chemistry

Scheme 2. Illustration of the Reaction of Nanojars (Left; Only One Ring Is Shown for Clarity; m = 1−9, n = 27, 29, 30, 31) with Carboxylate Ions, Leading to Carboxylate-Substituted Nanojars (center; x = 1−4) and Ultimately to Trinuclear Complexes (R = H, CH3, C6H5); Grey Sphere Denotes CO32−

2311.7; n = 29, m/z 2459.6) are observed besides the parent nanojars (Figure S68). The base peak corresponds to [(Bu4N)Cu2(pz)2(SO4)2]− (m/z 694.1), and the cluster ion [(Bu4N)Cu4(pz)4(SO4)3]− (m/z 1052.0) is also observed. At 5 equiv of HSO4− only small amounts of unsubstituted Cu27SO4 and Cu31SO4 are left, and the major nanojar peaks in the ESI-MS spectrum correspond to mono- and disubstituted Cu27SO4 (m/z 2176.1 and 2311.7) and monosubstituted Cu31SO4 (m/z 2471.0). Smaller amounts of mono- and disubstituted Cu29SO4 (m/z 2324.0 and 2459.6) are also visible, along with new peaks corresponding to [(Bu4N)2SO4⊂{Cu31(OH)31(pz)29(SO4)2}]2− (m/z 2606.6), [(Bu 4 N) 2 Cu 2 (pz)2 (SO 4 ) 2 (HSO 4 )]− (m/z 1033.4) and [(Bu4N)3Cu4(pz)4(SO4)4]− (m/z 1632.5). Also, the disubstituted [(Bu4N)SO4⊂{Cun(OH)n(pz)n−2(SO4)2}]3− (n = 27, m/z 1460.3; n = 29, m/z 1559.0; n = 31, m/z 1657.0) nanojars become more abundant than the monosubstituted ones, and trisubstituted species [(Bu4N)2SO4⊂{Cun(OH)n(pz)n−3(SO4)3}]3− (n = 27, m/z 1550.8; n = 29, m/z 1649.4) are also detected. At 10 equiv of HSO4−, disubstituted Cu27SO4 (m/z 2311.7) is the major nanojar species, which is still present in small amounts at 31 equiv of HSO4− along with [(Bu4N)3SO4⊂Cun(OH)n(pz)n−3(SO4)3]2− (n = 27, m/z 2447.3; n = 29, m/z 2595.2; n = 31, m/z 2742.2) and its corresponding (3−) peaks and a few other nanojar species. At 100 and 200 equiv, the spectrum is dominated by the dinuclear complex and its cluster ions, including [(Bu 4 N) 3 Cu 2 (pz)(SO 4 ) 3 (HSO 4 )] − (m/z 1304.6) and [(Bu4N)3Cu2(pz)2(SO4)3]− (m/z 1274.6), as well as by tetrabutylammonium hydrogen sulfate/sulfate cluster ions (Figure S69). Carboxylates, RCOO− (R = H, CH3, C6H5). Similarly to carboxylic acids, carboxylate ions also react with nanojars and replace one or more pyrazolate moieties. In the ESI-MS of CunCO3 treated with excess NaRCOO or Ba(RCOO)2 in wet THF (R = H or CH3; not soluble in the THF reaction solvent), [CO3⊂{Cun(OH)n(pz)n−x(RCOO)x}]2− species (x = 1−3) are observed exclusively (Figures S70, S71). If more pyrazolates are substituted by carboxylates, the nanojars become unstable and break down into hexanuclear [{Cu3(μ3-OH/μ3-O)(μ-pz)3}2(RCOO)3]± and/or trinuclear [Cu3(μ3-OH/μ3-O)(μ-pz)3(RCOO)3]1−/2− species. Treatment with THF-soluble (Bu4N)RCOO leads to complete breakdown into trinuclear species: in the case of (Bu4N)CH3COO, ESI-MS(−) shows only [Cu3O(pz)3(CH3COO)2]− (m/z 525.9) and [(Bu4N)Cu3O(pz)3(CH3COO)3]− (m/z 827.2) species in the negative mode, and [(Bu4N)2Cu3(OH)(pz)3(CH3COO)3]+ (m/z 1070.5) and [(Bu4N)3Cu3O(pz)3(CH3COO)3]+ (m/z 1311.8) in the positive mode. Similar results are obtained with (Bu4N)HCOO.

Dilution of the resulting THF solutions with excess water regenerates the nanojars as [CO 3 ⊂{Cu n (OH) n (pz) n−x (RCOO)x}]2− (x = 1−3) species, which precipitate out of aqueous solutions (Scheme 2). As opposed to the Na+ and Ba2+ salts, Pb(CH3COO)2·3H2O or Cd(CH3COO)2·2H2O quickly break down nanojars due to the acidity of hydrated Cd2+ and Pb2+ ions (pKa 10.1 and 7.6, respectively, in H2O). In these cases, hexanuclear [Cu6O2(pz)6(CH3COO)3]− (m/z 992.8) species are observed in addition to the trinuclear species in the corresponding ESI-MS spectra (Figure S71). The reaction of CunCO3 with sodium benzoate in THF is sampled periodically and analyzed by ESI-MS (Figure S72). After 1 day, substituted nanojars [CO 3 ⊂Cu n (OH) n (pz) n−x (C6H5COO)x]2− (n = 27 and 29, x = 1−4; n = 30, x = 1−2; n = 31, x = 1−3) are observed. After 3 days, trinuclear breakdown products [Cu3(OH)3(pz)2(C6H5COO)2]− (m/z 617.9), [Cu3O(pz)3(C6H5COO)2]− (m/z 649.9), [Cu3O(pz)2(C6H5COO)3]− (m/z 703.9), [Cu3(OH)(pz)4(C6H5COO)2]− (m/z 718.0), [Cu3O(pz)3(C6H5COO)2(NaNO3)]− (m/z 734.9), [NaCu3O(pz)3(C6H5COO)3]− (m/z 793.9) are present along with substituted nanojars. After 6 days, all nanojars break down and only trinuclear species are observed. Similarly, NH4(C6H5COO) (pKa of NH4+ = 9.2 in H2O) completely breaks down nanojars to trinuclear species and small amounts of hexanuclear complex [Cu6O2(pz)6(C6H5COO)3]− (m/z 1178.8) after 3 days of stirring. Halides and other anions: F−, Cl−, Br−, I−, NO3−, ClO4−, BF4−, CO32−, SO42−. The effect of various anions was tested by treating CunCO3 nanojars with a 100-fold molar excess of THF-soluble Bu4N-salts of those anions and reprecipitating the nanojars by pouring the solution in excess water. ESI-MS analysis shows no reaction with halides, nitrate, perchlorate, and tetrafluoroborate. Reactivity toward carbonate and sulfate was described earlier;7 carbonate reacts only with Cu27SO4 and converts it into Cu27CO3, whereas sulfate reacts only with Cu31CO3 and converts it into Cu31SO4. As observed with bisulfate, sulfate can replace up to three pyrazolate ions in both CunCO3 and CunSO4. To further test the effect of various anions, nanojars were prepared using CuCl2, CuBr2, and Cu(CH3COO)2 as copper sources. It has already been established that pure CunCO3 nanojars can be prepared from Cu(NO3)2 or Cu(ClO4)2 in the presence of carbonate ions,3 whereas CuSO4 leads to a mixture of CunCO3 and CunSO4 under similar conditions.8 CuCl2, CuBr2, and Cu(CH3COO)2 all lead to pure CunCO3 (n = 27, 29, 30, 31) nanojars, with only carbonate as an incarcerated ion (Figure S73). In the case of CuBr2, however, partial bromination of the pyrazole ligands is observed, as indicated by the following peaks K


Article

Inorganic Chemistry in the ESI-MS(−) spectrum: [CO3⊂{Cun(OH)n(pz)n−x(Brpz)x}]2− (n = 27: x = 1, m/z 2060.9; x = 2, m/z 2100.9; x = 3, m/z 2139.8; n = 29: x = 1, m/z 2208.9; x = 2, m/z 2248.8; x = 3, m/z 2287.8; n = 30: x = 1, m/z 2282.4; x = 2, m/z 2322.3; x = 3, m/z 2361.3; n = 31: x = 1, m/z 2356.4; x = 2, m/z 2395.8; x = 3, m/z 2435.3). Indeed, it is known that pyrazole is easily brominated at its 4-position.31 Crystallographic Description. Single-crystal X-ray diffraction plays a crucial role in the elucidation of the structure of nanojars, and despite the extreme instability (due to rapid solvent loss) and typically low quality of the corresponding crystals, structural characterization of nanojars with incarcerated carbonate,2,5,6 sulfate,2,3,8 phosphate,4 hydrogen phosphate,4 arsenate,4 hydrogenarsenate,4 and chloride2 has been achieved. Of the various nanojar sizes (Cu26−Cu36, except Cu35) obtained so far, the following have been crystallographically characterized: Cu27,2,4,5 Cu28,3 Cu29,5,8 Cu30,6 Cu31,2−4 Cu36.2 In here we describe the structure of Cu27CO3 nanojars in which one or two pyrazolates are substituted by acetate ligands, along with three dinuclear copper-pyrazolate complexes formed as breakdown products of nanojars in the presence of hydrogen carbonate and/ or hydrogen sulfate ions. (Bu 4N) 2 [CO 3 ⊂{Cu 27(μ-OH) 27 (μ-pz) 27−x (μ-CH 3 COO) x }] (C6H5CH3)6 (x = 1 and 2) (1). The title compound was separated from other [CO 3 ⊂{Cu n (μ-OH) n (μ-pz) n−x (μCH3COO)x}] (n = 29−31, x = 1, 2) nanojars by crystallization from neat Et3N (Figure S74) followed by recrystallization from toluene/hexane, and consist of cocrystallized [CO3⊂{Cu27(μOH)27(μ-pz)26(μ-CH3COO)}]2− and [CO3⊂{Cu27(μ-OH)27(μ-pz)25(μ-CH3COO)2}]2−. As illustrated in Figures 6 and S76,

(Bu4N)2[Cu2(μ-pz)2(SO4)2] (2). Located on an inversion center (Figure 7), the dinuclear complex in 2 contains Cu atoms

Figure 7. Thermal ellipsoid plot (50% probability) of the crystal structure of 2. Counterions are omitted for clarity. Symmetry operator (′): −x, −y + 1, −z + 1.

(Cu···Cu distance: 3.7719(9) Å) in distorted square planar coordination geometry, with structural parameter τ4 and τ4′ values of 0.225 and 0.224, respectively (τ4 = τ 4′ = 0 for square planar and τ4 = τ4′ = 1 for tetrahedral).32 The small bite angle of the sulfate ion is manifested in an acute O−Cu−O angle of 71.75(6)°, whereas the O−Cu−N and N−Cu−N angles are closer to 90° (Table 4). The CuO2 and CuN2 planes formed by the Cu-center with the chelating sulfate ligand and the two cispyrazolate ligands, respectively, form a dihedral angle of 6.01(7)°. The complex resembles the dinuclear compound (Bu4N)2[Cu2(μ-3-Mepz)2Cl4], in which two chlorido ligands are found in place of the chelating sulfato ligand.33 (Bu4N)2[Cu2(pz)2(CO3)2] (3). The complex molecule in 3 is located on a C2 rotation axis that bisects the pyrazolate ligands (one is disordered 50/50 about the C2-axis) (Figure 8). As in the case of the sulfate analogue, the Cu atoms have distorted square planar coordination geometry, with τ4 and τ4′ values of 0.26/0.28 and 0.25/0.28 (Cu···Cu distance: 3.781(1) Å). An acute O−Cu− O angle of only 66.5(2)° is observed, whereas the O−Cu−N and N−Cu−N angles are closer to 90° (Table 4). The CuO2 and CuN2 planes are at dihedral angles of 6.4(3) and 14.3(4)° (for the Cu1N1N2a and Cu1N1N2b planes). (Bu4N)2[Cu2(pz)2(CO3)(SO4)]·CH3OH (4). Whereas the dinuclear complexes are located on special positions within the monoclinic crystal lattices of the disulfate and dicarbonate analogues, the mixed carbonate-sulfate complex in 4 is located on a general position within a triclinic lattice (Figure 9). The Cuatoms have distorted square planar coordination geometry, with τ4 and τ4′ values of 0.25 and 0.25 for Cu1 (CO3), and 0.24 and 0.24 for Cu2 (SO4). A Cu···Cu distance of 3.776(1) Å, similar to the ones in 2 and 3, is observed. The bite angles of the sulfate and carbonate ions in this complex are 70.6(2) and 67.2(2)°, respectively, whereas the deviation of the O−Cu−N and N− Cu−N angles from 90° is less pronounced (Table 4). The CuO2 and CuN2 planes form dihedral angles of 6.7(3) and 7.9(2)° in the case of the sulfate and carbonate ligands, respectively.

Figure 6. Crystal structure of 1 showing the two crystallographically independent [CO3⊂{Cu27(OH)27(pz)27−x(CH3COO)x}]2− (x = 1 and 2) units; the acetate-substituted Cu9-rings are highlighted. H atoms, counterions, and solvent molecules are omitted for clarity. Color code for atoms: Cu − blue; O − red; N − light blue; C − black.

the structure of the underlying nanojar framework is virtually identical to the ones previously observed in [CO3⊂{Cu(μOH)(μ-pz)} 27 ] 2− and [CO 3 ⊂{Cu 27 (μ-OH) 27 (μ-pz) 26 (μCH3COO)}]2−.2,5 The Cu···Cu separations of 3.300(2) and 3.268(2) Å in the case of the acetate-bridged pairs of copper atoms in the two crystallographically independent nanojar units of 1 are within the range of 3.124(2)−3.401(2) Å (average: 3.301(2) Å) of Cu···Cu distances between pyrazolate-bridged copper atoms. Both mono- and disubstitution (as shown in Figure 6) lower the point group symmetry of the nanojar from C3v to Cs. Symmetrical trisubstitution, as is most likely the case with [OOC(CH2)xCOO]2− (x = 2−8) tethering a [Cu6(μ3O)2(μ-pz)6]2+ unit to the nanojar (see above), restores the C3v symmetry of the complex.

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DISCUSSION The following variables have been identified to influence the reactivity of nanojars and to affect the course and degree of their substitution and breakdown. L


Article

Inorganic Chemistry Table 4. Selected Bond Lengths (Å) and Angles (deg) for the Dinuclear Complexes 2−4 (Bu4N)2[Cu2(pz)2(SO4)2] Cu1−O1 1.976(1) Cu1−O2 1.964(1)

Cu1−N1 1.923(1) Cu1−N2 1.928(1)

Cu1−O1 1.931(4) Cu1−O2 1.917(4)

Cu1−N1 1.934(4) Cu1−N2a 1.89(1) Cu1−N2b 1.95(1)

Cu1−O1 1.954(4) Cu1−O2 1.940(5) Cu2−O4 1.937(5) Cu2−O5 1.953(5)

Cu1−N1 1.928(5) Cu1−N4 1.942(5) Cu2−N2 1.925(5) Cu1−N2 1.926(5)

N1−Cu1−N2′ 102.40(6) N1−Cu1−O1 92.67(6) (Bu4N)2[Cu2(pz)2(CO3)2]

N2′−Cu1−O2 93.45(6) O1−Cu1−O2 71.75(6)

N1−Cu1−O2 163.96(7) N2′−Cu1−O1 164.30(7)

N1−Cu1−N2a 101.5(4) N2a−Cu1−O1 96.9(4) N1−Cu1−N2b 101.5(4) N2b−Cu1−O1 97.7(4) N1−Cu1−O2 94.7(2) O1−Cu1−O2 66.5(2) (Bu4N)2[Cu2(pz)2(CO3)(SO4)]·CH3OH

N1−Cu1−O1 160.9(2) N2a−Cu1−O2 162.9(4) N2b−Cu1−O2 159.0(4)

N1−Cu1−N4 100.6(3) N1−Cu1−O1 96.0(2) N2−Cu2−N3 102.9(2) N2−Cu2−O4 92.6(2)

N4−Cu1−O2 96.7(2) O1−Cu1−O2 67.2(2) N3−Cu2−O5 93.7(2) O4−Cu2−O5 70.6(2)

N1−Cu1−O2 162.6(2) N4−Cu1−O1 162.3(2) N2−Cu2−O5 163.3(2) N3−Cu2−O4 163.1(3)

equiv of acid (except in the case of oxalic acid, which is considerably stronger than the other dicarboxylic acids). Conjugate bases of monocarboxylic acids also lead to nanojar substitution at lower concentration and to tri-/hexanuclear breakdown products at higher concentrations; however, no insoluble/polymeric breakdown product forms at any concentration. No breakdown products of any kind are obtained with carbonic acid; an etching effect similar to the one of NH3 is observed instead, leading to conversion of all nanojars to [CO3⊂{Cu(OH)(pz)}27]2− and [SO4⊂{Cu(OH)(pz)}31]2− in the case of carbonate- and sulfate incarcerating nanojars, respectively. Bicarbonate and bisulfate, as well as their conjugate bases carbonate and sulfate,8 lead to the breakdown of nanojars to dimeric complexes exclusively; only sulfate is able to exchange with pyrazolate ligands (up to three). 1-Decanethiol (up to 31 equiv) has an etching effect on [CO3⊂{Cu(OH)(pz)}n]2− (n = 27, 29, 30, 31), favoring the formation of [CO3⊂{Cu(OH)(pz)}30]2−; at higher concentrations of 1-decanethiol, however, nanojars completely break down into reduced, insoluble Cu(I) species. Methanol does not affect nanojars up to 200 equiv; however, substitution is observed at much higher methanol concentrations, such as in a CH3CN/CH3OH (2:1) solution, and complete breakdown occurs in neat methanol solution.7 Sulfonate does not substitute pyrazolate ligands, nor does it lead to breakdown of nanojars; instead, it has a salting-out effect at higher concentrations. Acidity of Reagent (pKa in H2O Is Used for Comparison Purposes). Increasing reactivity of nanojars is observed with increasing reagent acidity. In the presence of strong acids, such as p-toluenesulfonic acid (pKa = −2.8), nanojars easily break down; no nanojars are detected in the case of CunCO3 solutions above 2 equiv of acid. With weaker acids, such as monocarboxylic (pKa = 3.75−5.03) and dicarboxylic acids (pKa1 = 1.25−4.59), varying degrees of nanojar substitution occur before breakdown. CunCO3 nanojars are still present in solution with 31 equiv of pivalic acid (pKa = 5.03), whereas the same nanojars break down completely above 15 equiv of formic acid (pKa = 3.75). The formation of the polymeric breakdown product, [Cu(μ-pz)(μRCOO)]∞, is prohibited at high acidities; thus, no insoluble precipitate is obtained in the case of oxalic and p-toluenesulfonic acids at any concentration. Protonation of the nanojars’ OH− (pKa of water = 15.7) and pz− (pKa of pyrazole = 14.2) moieties can also occur with very weak acids, such as HCO3− (pKa = 10.3) and aqueous metal ions,7 and even with CH3OH (pKa = 15.5). Ethanol (pKa = 15.9) and higher alcohols (pKa ≥ 16.0), which have lower acidity than water, dissolve nanojars without decomposition, as attested by ESI-MS of neat ethanol or isopropanol solutions.7 This study also indicates that the tri- and

Figure 8. Thermal ellipsoid plot (20% probability) of the crystal structure of 3. One of the pyrazolate units is disordered over two positions (50/50, refined isotropically). H atoms and counterions are omitted for clarity. Symmetry operator (′): −x + 1, y, −z + 3/2.

Figure 9. Thermal ellipsoid plot (20% probability) of the crystal structure of 4. Counterions and the CH3OH solvent molecule are omitted for clarity.

Concentration of Reagent (Acid or Conjugate Base). The degree of nanojar substitution increases with increasing reagent concentration in all cases investigated. Yet at the same time, the amount of breakdown products also increases. The maximum number of pyrazolate substitutions observed is 10, in the case of pivalic acid. In the case of monocarboxylic acids, the amount of insoluble, polymeric breakdown product also increases with increasing molar equivalents (up to 31) of acid. At higher concentrations of acid, the polymeric product redissolves; the dissolution is more facile in the case of acids with smaller pKa. In contrast, dicarboxylic acids only form precipitates at higher concentrations that do not dissolve at 200 M


Article

Inorganic Chemistry

seen [CO3⊂{Cu(OH)(pz)}32]2− form at increasing amounts of pivalic acid, and Cu30CO3, which is a minor component of the assynthesized CunCO3 nanojar mixture, becomes the dominant nanojar. With monocarboxylic acids, exchange of up to 10 pyrazolates with carboxylates has been found, whereas with dicarboxylic acids up to a maximum of three pyrazolates are exchanged (Tables 2 and 3). In the case of dicarboxylic acids, different substitution patterns have been observed. Besides a simple pyrazolate/monotopic carboxylate exchange, substitution of one or two OH− groups with monotopic carboxylates (no pyrazolates exchanged) has also been observed in the case of malonic and succinic acids. Because of their ditopic nature, dicarboxylates, once attached to the nanojar with one end, can form a bridge to the breakdown products. Thus, nanojars tethered by a singledicarboxylate unit to tri- and/or hexanuclear breakdown products have been observed with terephthalic and oxalic acids. In the case of [OOC(CH2)xCOO]2− ligands with x ≥ 2, ternary tethered [{CO 3 ⊂Cu 2 7 (OH) 2 7 (pz) 2 4 }{OOC(CH2)xCOO}{Cu6O(OH)(pz)6}]2− species are observed instead. Finally, nanojar species wherein a ditopic dicarboxylate ligand exchanges two pyrazolate units are found in the case of glutaric and sebacic acids. Evidently, the C−C chain connecting the two carboxylate moieties needs to be long enough to allow for this type of coordination to nanojars. No tethered nanojars have been detected in any case. While the carboxylate-substituted nanojars presented here offer further examples of the analogy between pyrazolate and carboxylate ligands, the lack of mostly carboxylate based (and eventually carboxylate-only) nanojars points to a limitation of the analogy. Steric factors alone are unlikely to be responsible, as nanojars substituted with more trimethyl acetate (pivalate) than acetate ligands have been observed (Table 2). Prohibitive electrostatic repulsions between electron-rich O atoms of closely spaced carboxylate moieties are also implausible, as cyclic hexanuclear Cu(II) complexes are known with contiguous ciscarboxylates, such as phenoxyacetate,34 3,4,5-tri(ethoxy)benzoate,35 and 2-phenyl-3,6,9-trioxadecanoate.36 Likewise, cyclic tetranuclear and higher nuclearity complexes with contiguous cis-carboxylates are also known.18,23,25 Therefore, we tentatively suggest that the absence of highly substituted nanojars (x > 10) is attributable to the diminishing of the stabilizing π−π interactions offered by pyrazolate ligands.

hexanuclear copper-pyrazolate complexes [Cu3(OH)(pz)3(RCOO)3]− and [Cu6O2(pz)6(RCOO)3]− are much more resistant to protonation than nanojars. Acid vs Conjugate Base. Similar nanojar substitution patterns have been observed with different carboxylic acids and their corresponding conjugate bases, although with a major difference in the overall breakdown pattern of nanojars: acids lead to significant amounts of the insoluble, polymeric breakdown product, [Cu(μ-pz)(μ-RCOO)]∞, whereas their conjugate bases convert nanojars solely to [Cu3(OH)(pz)3(RCOO)3]−, even when used in large excess. Steric Effects. Steric bulk does not appear to hinder the exchange of pyrazolate units with carboxylates. Formic, acetic, and stearic acids, for instance, provide identical substitution patterns. Interestingly, the considerably bulkier trimethyl acetate (pivalate) is capable of replacing 10 pyrazolate units, whereas acetate is only capable of replacing 5. Aromaticity. Carboxylates with aromatic substituents lead to more exchange than the ones without substituent or with alkyl substituent. Thus, benzoate (pKb = 9.80) substitutes up to seven pyrazolates in CunCO3 nanojars, whereas the less basic formate (pKb = 10.25) or the more basic acetate (pKb = 9.24) substitute only up to five. Incarcerated Anion. CO32− (pKb = 3.67) is much more basic than SO42− (pKb = 12.01); consequently, carbonateincarcerating nanojars are in general less stable to acidity than sulfate-incarcerating nanojars. In the case of 6 equiv of formic acid, for example, almost all CunCO3 nanojars are broken down, whereas under the same conditions CunSO4 nanojars still dominate the ESI-MS spectrum. Similarly, the insoluble polymeric breakdown product, [Cu(μ-pz)(μ-RCOO)]∞, forms at 4 equiv of formic acid in the case of CunCO3, but only at 20 equiv in the case of CunSO4. Therefore, a higher degree of substitution is usually attained with CunSO4 than with CunCO3. With formic acid, for instance, up to seven pyrazolates are exchanged with formate in Cu27SO4, and only four in Cu27CO3 (Table 2). Size of the Nanojar. Nanojars of different sizes have different stability to acids. Cu28SO4 is the most sensitive nanojaralthough it accounts for the base peak in the ESI-MS spectrum of the CunSO4 mixture, it completely breaks down above 2 equiv of acid. Cu30CO3 and Cu31CO3 are in general less stable than Cu27CO3 and Cu29CO3. A notable exception is the case of pivalic acid, where Cu30CO3 is the most stable nanojar; in fact, nanojars of other sizes transform into Cu30CO3 with increasing amounts of pivalic acid, and Cu30CO3 is the only carbonate-incarcerating nanojar that survives at 31 equiv of acid. Nanojar Substitution Pattern. Nanojars do not react stoichiometrically with acids. With 1 equiv of acid, a mixture of unreacted nanojars and multiply substituted nanojars is obtained, together, in some cases, with breakdown products. In the case of 1 equiv of pivalic acid, for example, Cu27SO4 exchanges up to four pyrazolates with pivalates, whereas Cu31SO4 exchanges only up to two pyrazolates, and it is found mostly unsubstituted. With increasing equivalents of acid, the parent nanojars are eventually converted completely to nanojars with increasing numbers of carboxylates and then break down into lower-nuclearity complexes. Conversely, not all nanojars are broken down with 31 equiv of acid, which is in fact sufficient to protonate all OH groups of the nanojars. At 31 equiv of pivalic acid, [SO4⊂{Cun(OH)n(pz)n−x(RCOO)x}]2− (n = 27, 29, 31; x = 6−9) species are still present. Interestingly, in the case of CunCO3 nanojars extremely rare [CO3⊂{Cu(OH)(pz)}28]2− and never-before-

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CONCLUSIONS Our comprehensive study using 18 different acidic compounds with varying pKa, as well as six different conjugate bases, provides a detailed picture of the intricate reactivity of [CO3⊂{Cu(OH)(pz)}n]2− (n = 27, 29, 30, 31) and [SO4⊂{Cu(OH)(pz)}n]2− (n = 27, 28, 29, 31, 32) nanojars toward these compounds. Depending on the nature and amount of a given reagent, nanojars react to form variously substituted nanojars and/or different low-nuclearity (Cu1−Cu9) and polymeric breakdown products, according to the following general equation: (Bu4N)2[CO3⊂{Cu(OH)pz}n] + RCOOH → (Bu4N)2[CO3⊂{Cun(OH)n(pz)n−x(RCOO)x}] + (Bu4N)[Cu3(OH)(pz)3(RCOO)3] + (Bu4N)[Cu6O2(pz)6(RCOO)3] + [Cu(pz)(RCOO)]∞ + Bu4N(RCOO) + H2O + CO2 (unbalanced; only major breakdown products are shown). In the case of bicarbonate and bisulfate, distinct dimeric complexes, [Cu2(μpz)2X2]2− (X = CO32− or SO42−) are obtained as major breakdown products. In addition, nanojars of certain sizes can also rearrange into nanojars of a different size. N


Article

Inorganic Chemistry

(2) Mezei, G.; Baran, P.; Raptis, R. G. Anion Encapsulation by Neutral Supramolecular Assemblies of Cyclic CuII Complexes: A Series of Five Polymerization Isomers, [{cis-CuII(μ-OH)(μ-pz)}n], n = 6, 8, 9, 12, and 14. Angew. Chem., Int. Ed. 2004, 43, 574−577. (3) Fernando, I. R.; Surmann, S. A.; Urech, A. A.; Poulsen, A. M.; Mezei, G. Selective total encapsulation of the sulfate anion by neutral nano-jars. Chem. Commun. 2012, 48, 6860−6862. (4) Mezei, G. Incarceration of one or two phosphate or arsenate species within nanojars, capped nanojars and nanohelicages: helical chirality from two closely-spaced, head-to-head PO43− or AsO43− ions. Chem. Commun. 2015, 51, 10341−10344. (5) Ahmed, B. M.; Szymczyna, B. R.; Jianrattanasawat, S.; Surmann, S. A.; Mezei, G. Survival of the Fittest Nanojar: Stepwise Breakdown of Polydisperse Cu27−Cu31 Nanojar Mixtures into Monodisperse Cu27(CO3) and Cu31(SO4) Nanojars. Chem. - Eur. J. 2016, 22, 5499− 5503. (6) Ahmed, B. M.; Calco, B.; Mezei, G. Tuning the structure and solubility of nanojars by peripheral ligand substitution, leading to unprecedented liquid−liquid extraction of the carbonate ion from water into aliphatic solvents. Dalton Trans. 2016, 45, 8327−8339. (7) Ahmed, B. M.; Mezei, G. From Ordinary to Extraordinary: Insights into the Formation Mechanism and pH-Dependent Assembly/ Disassembly of Nanojars. Inorg. Chem. 2016, 55, 7717−7728. (8) Ahmed, B. M.; Hartman, C. K.; Mezei, G. Sulfate-Incarcerating Nanojars: Solution and Solid-State Studies, Sulfate Extraction from Water, and Anion Exchange with Carbonate. Inorg. Chem. 2016, 55, 10666−10679. (9) Mezei, G. Sulfate-bridged dimeric trinuclear copper(II)− pyrazolate complex with three different terminal ligands. Acta Crystallogr. 2016, 72, 1064−1067. (10) Surmann, S. A.; Mezei, G. Halogen-bonded network of trinuclear copper(II) 4-iodopyrazolate complexes formed by mutual breakdown of chloroform and nanojars. Acta Crystallogr. 2016, 72, 1517−1520. (11) Ahmed, B. M.; Mezei, G. Accessing the inaccessible: discrete multinuclear coordination complexes and selective anion binding attainable only by tethering ligands together. Chem. Commun. 2017, 53, 1029−1032. (12) (a) Baran, P.; Marrero, C. M.; Pérez, S.; Raptis, R. G. Stepwise, ring-closure synthesis and characterization of a homoleptic palladium(II)-pyrazolato cyclic trimer. Chem. Commun. 2002, 1012−1013. (b) Umakoshi, K.; Yamauchi, Y.; Nakamiya, K.; Kojima, T.; Yamasaki, M.; Kawano, H.; Onishi, M. Pyrazolato-Bridged Polynuclear Palladium and Platinum Complexes. Synthesis, Structure, and Reactivity. Inorg. Chem. 2003, 42, 3907−3916. (c) Piñero, D.; Baran, P.; Boca, R.; Herchel, R.; Klein, M.; Raptis, R. G.; Renz, F.; Sanakis, Y. A PyrazolateSupported Fe3(μ3-O) Core: Structural, Spectroscopic, Electrochemical, and Magnetic Study. Inorg. Chem. 2007, 46, 10981−10989. (d) Miras, H. N.; Zhao, H.; Herchel, R.; Rinaldi, C.; Pérez, S.; Raptis, R. G. Synthesis and Characterization of Linear Trinuclear Pd, Co, and Pd/Co Pyrazolate Complexes. Eur. J. Inorg. Chem. 2008, 2008, 4745−4755. (e) Miras, H. N.; Chakraborty, I.; Raptis, R. G. Tri-, deca- and dodecanuclear Co(III)−pyrazolate metallacycles. Chem. Commun. 2010, 46, 2569−2571. (13) (a) Sumner, C. E., Jr.; Steinmetz, G. R. Isolation of Oxo-Centered Cobalt(III) Clusters and Their Role in the Cobalt Bromide Catalyzed Autoxidation of Aromatic Hydrocarbons. J. Am. Chem. Soc. 1985, 107, 6124−6126. (b) Beattie, J. K.; Hambley, T. W.; Klepetko, J. A.; Masters, A. F.; Turner, P. The chemistry of cobalt acetate: The isolation and crystal structure of the symmetric trimer, hexakis(μ-acetato)-μ3-oxotris(pyridine)tricobalt(III) perchlorate water solvate, [Co 3 O(CH3CO2)6(C5H5N)3][ClO4]·H2O. Polyhedron 1996, 15, 2141−2150. (14) Yoshida, J.; Kondo, S.; Yuge, H. A synthetic strategy for a new series of oxo-centered tricobalt complexes with mixed bridging ligands of acetate and pyrazolate anions. Dalton Trans. 2013, 42, 2406−2413. (15) (a) Ehlert, M. K.; Rettig, S. J.; Storr, A.; Thompson, R. C.; Trotter, J. μ4-Oxo-hexakis(μ-3,5-dimethylpyrazolato-N,N′)tetracobalt(II). Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1994, 50, 1023−1026. (16) (a) Jaitner, P.; Veciana, J.; Sporer, C.; Kopacka, H.; Wurst, K.; Ruiz-Molina, D. Synthesis, Crystal Structure, and Spectroscopic and

This study reveals that acidic compounds or their conjugate bases can have profound implications if present during anion extraction processes that use nanojars as extraction agents. Understanding the reactivity of nanojars toward these compounds, nonetheless, facilitates the design of anion extraction processes. Furthermore, controlled exchange of pyrazolate with carboxylate ligands opens up the possibility of introducing various functionalities to nanojars via carboxylate-bearing molecules.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01593. Detailed visual observations during the titration experiments, tables of nanojar breakdown products observed, ESI-MS spectra and comparison of substituted and nonsubstituted nanojar crystal structures (PDF) Accession Codes

CCDC 1557566−1557569 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Gellert Mezei: 0000-0002-3120-3084 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based on work supported by the National Science Foundation under Grant No. CHE-1404730. DEDICATION Dedicated to Professor Raphael G. Raptis on the occasion of his 60th birthday. REFERENCES

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