Mapping the Intricate Reactivity of Nanojars toward Molecules of Varying Acidity and Their Conjugate Bases Leading to Exchange of Pyrazolate Ligands

Article abstract of DOI:10.1021/acs.inorgchem.7b01593

A comprehensive reactivity study of nanojars toward 18 different acidic compounds with varying pK_a, including 12 different carboxylic acids (both aliphatic and aromatic mono- and dicarboxylic acids), p-toluenesulfonic acid, hydrogen sulfate, hydrogen carbonate, carbonic acid, 1-decanethiol, 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 (pK_a), 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 = CO₃^2- and SO₄^2-) have been isolated and characterized by single-crystal X-ray 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.

Full text of DOI:10.1021/acs.inorgchem.7b01593

Article
pubs.acs.org/IC
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, 1-
decanethiol, 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 X-
ray 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.
can be extracted from a 10 M NaOH solution (pH > 14) into
organic solvents using nanojars.6,7
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
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
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
Received: June 21, 2017
2−
alkaline conditions. It has been shown, for instance, that CO3
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01593
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Article
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).
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, hydro-
gen 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 electro-
spray ionization mass spectrometry (ESI-MS); in addition,
details about the specific location of substitution within nanojars
were obtained using single-crystal X-ray diffraction.
Synthesis of (Bu4N)2Cu2(μ-pz)2(CO3)2. (Bu4N)2[CO3⊂{Cu(μ-
OH)(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,
[(Bu4N)Cu2(pz)2(CO3)2]−; m/z 1488.6, [(Bu4N)3{Cu2(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 (Bu4N)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,
[(Bu4N)3{Cu2(pz)2(SO4)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 blue-
green solution forms) and 0.1 M aqueous NaOH (a blue suspension
forms).
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, [Co3(μ3-O)(μ-4-NO2pz)6L3]2− (L =
12e
NO2 ), 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 pyrazolate-
2 2
based [Ni8 (OH)6 (pz)1 2
]
vs carboxylate-based
[Ni8(OH)4(H2O)2(Me3CCOO)12] cube-like octanuclear com-
plexes,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.
Synthesis of (Bu4N)2Cu2(μ-pz)2(CO3)(SO4). Prepared similarly to
the dicarbonate and disulfate analogues, using (Bu4N)2[CO3⊂{Cu(μ-
OH)(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
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
DOI: 10.1021/acs.inorgchem.7b01593
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Article
Table 1. Crystallographic Data for 1−4
1
2
3
4
formula
FW (g·mol−1
C
267H408Cu54Nl06O66
C38H78Cu2N6O8S2
938.26
C40H78Cu2N6O6
866.16
C40H82Cu2N6O8S
934.25
)
9590.13
crystal system
space group
a (Å)
monoclinic
P21/n
monoclinic
P21/n
monoclinic
C2/c
triclinic
P1
̅
35.5582(5)
25.409l(4)
39.608l(6)
90.000
8.3744(1)
22.5315(3)
13.2157(1)
90.000
22.1910(4)
13.4674(4)
16.2485(4)
90.000
12.3414(14)
14.1065(15)
15.9544(18)
87.868(7)
77.864(7)
74.006(7)
2609.7(5)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
97.831(1)
90.000
98.284(1)
90.000
95.881(2)
90.000
35452.3(9)
4
2467.62(5)
2
4830.4(2)
4
D
calc (g·cm−3
μ (mm−1
)
1.797
1.263
1.191
1.189
)
3.241
0.996
0.926
0.903
θ range (deg)
1.15−20.96
253564
1.80−27.88
59643
1.77−20.36
22872
1.31−25.02
106542
reflns collected
Rint
0.0813
0.0303
0.0376
0.0717
obsd reflns [I > 2σ(I)]
data/restraints/parameters
GOF (on F2)
26795
4801
1818
5151
37748/55/4158
1.277
5879/0/257
1.036
2377/10/284
0.914
9205/39/495
1.066
R factors [I > 2 σ(I)]
R1 = 0.0554
WR2 = 0.1528
R1 = 0.0930
WR2 = 0.1843
1.431/−0.686
1557569
R1 = 0.0308
WR2 = 0.0822
R1 = 0.0417
WR2 = 0.0894
0.400/−0.201
1557568
R1 = 0.0486
WR2 = 0.1378
R1 = 0.0657
WR2 = 0.1582
0.518/−0.220
1557566
R1 = 0.0776
WR2 = 0.2499
R1 = 0.1287
WR2 = 0.3147
0.805/−0.279
1557567
R factors (all data)
maximum peak/hole (e·Å−3
CCDC number
)
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).
the case of stearic acid, succinic acid, adipic acid, sebacic acid,
terephthalic acid, p-toluenesulfonic acid, sodium 2-bromoethanesulfo-
nate, 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 ESI-
MS.
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(C6H5COO) (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)(μ-
pz)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(μ-
OH)(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.
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
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
C
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Article
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
28, 29, 31, 32) will be employed to abbreviate different
carbonate- and sulfate-incarcerating nanojars (including sub-
stituted 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(μ-
RCOO)y]∞ (Scheme 1), similar to [Cu(μ-OH)(μ-pz)]∞ and
2335.4; n = 32, m/z 2409.9; n = 33, m/z 2483.3). Minor peaks
2−
corresponding to [(Bu4N)A⊂{Cu(OH)(pz)}n]− (A = CO32− or SO4
)
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.
Scheme 1. Examples of Possible Repeating Units in the [Cu(μ-
pz)x(μ-RCOO)y]∞ Polymer
X-ray Crystallography. Single crystals of (Bu4N)2[CO3⊂{Cu27(μ-
OH)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 least-
squares 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 end-
groups 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.
[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 tetrabutylam-
monium 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 low-
nuclearity breakdown products are detectable both in ESI-
MS(−) and ESI-MS(+). Therefore, emphasis is placed on ESI-
MS(−), 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
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.
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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), [(Bu4 N)2 Cu3 O-
(pz)3(HCOO)2]+ (m/z 982.4), [(Bu4N)2Cu6O2(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 ESI-
MS 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
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(μ-
pz)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 excep-
tionally stable to CH3COOH, as shown by their presence, in
small amounts, even at 100 equiv of acid.
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),
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
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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 SO4
)
2−
acid (HL)
x = 1
x = 2
x = 3
x = 4
x = 5
x = 6
x = 7
x = 8
x = 9
x = 10
HCOOH (A = CO3)
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
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
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
1977.9
2126.4
2199.3
2272.8
1995.8
2114.9
HCOOH (A = SO4)
1984.9
2133.3
1973.9
2121.8
1962.8
2110.8
2143.9
2438.8
2006.4
2153.9
2227.4
2300.9
2024.4
CH3COOH (A = CO3)
2149.9
CH3COOH (A = SO4)
2020.4
2167.9
2015.9
2163.9
2011.9
2159.9
2171.9
2318.8
2454.9
2602.9
2676.4
2749.9
2090.0
2164.0
2238.5
2312.0
CH3(CH2)16COOH (A = CO3)
(CH3)3CCOOH (A = CO3)
2563.1
2711.0
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
2397.1
2266.1
2414.1
2187.0
2260.9
2334.4
2204.0
2277.9
2351.4
2380.0
2459.9
2108.0
2477.0
2125.5
2494.0
2142.5
2511.0
2159.6
2528.0
2176.6
2545.0
2193.6
(CH3)3CCOOH (A = SO4)
C6H5COOH (A = CO3)
C6H5COOH (A = SO4)
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
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
2377.4
2556.5
2404.4
2283.5
2432.4
2269.4
2416.4
2296.4
2443.4
2323.4
2470.4
2350.4
2497.4
(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).
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
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
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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 free-
standing 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-
2 −
(OH)(pz)}2 9
]
and [CO3 ⊂{Cu2 9 (OH)2 9 (pz)2 8 -
(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
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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 p-
toluenesulfonic 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(BrCH2CH2SO3)Br]− (m/z 290.8) and [Bu4N-
(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.
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).
1-Decanethiol, CH3(CH2)9SH (pKa ≈ 10.5 in H2O). As with
sulfonates, no substituted nanojars are observed with 1-
decanethiol (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
[CO3⊂{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.
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
H2 O; pKa 1 = 20.3 in CH3 CN). 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 ) . A b o v e
2
e q u i v
a
r e l a t e d c o m p l e x
[(Bu4N)2CO3⊂{Cu27(OH)27(pz)24}{OOC(CH2)4COO}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 ⊂{Cu2 7 (OH)2 7 (pz)2 4 }{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(CH2)8COO}3{Cu6O(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
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
(Bu4 N)HCO3 , the soluble dinuclear complex
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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
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), [(Bu4N)2SO4-
⊂{Cu31(OH)31(pz)29(SO4)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
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).
−
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 spec-
trum (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.
−
Bisulfate, HSO4 (pKa = 1.99 in H2O). At 1 equiv of
(Bu4N)HSO4, each Cu31CO3 nanojar begins to exchange one
2−
pyrazolate with an SO4 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
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
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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,
2−
CH3, C6H5); Grey Sphere Denotes CO3
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
Dilution of the resulting THF solutions with excess water
regenerates the nanojars as [CO3⊂{Cun(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).
−
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), [(Bu4N)2Cu2(pz)2(SO4)2(HSO4)]− (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,
The reaction of CunCO3 with sodium benzoate in THF is
sampled periodically and analyzed by ESI-MS (Figure S72). After
1 day, substituted nanojars [CO3⊂Cun(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)
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 [(Bu4N)3Cu2(pz)(SO4)3(HSO4)]− (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).
+
(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.
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 pos-
itive mode. Similar results are obtained with (Bu4N)HCOO.
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
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in the ESI-MS(−) spectrum: [CO3⊂{Cun(OH)n(pz)n−x
-
(Bu4N)2[Cu2(μ-pz)2(SO4)2] (2). Located on an inversion
center (Figure 7), the dinuclear complex in 2 contains Cu atoms
(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 diffrac-
tion 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.
(Bu4N)2[CO3⊂{Cu27(μ-OH)27(μ-pz)27−x(μ-CH3COO)x}]-
(C6H5CH3)6 (x = 1 and 2) (1). The title compound was
separated from other [CO3⊂{Cun(μ-OH)n(μ-pz)n−x(μ-
CH3COO)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(μ-
OH)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,
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 cis-
pyrazolate 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 Cu-
atoms 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(μ-
OH)(μ-pz)}27]2− and [CO3⊂{Cu27(μ-OH)27(μ-pz)26(μ-
CH3COO)}]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(μ3-
O)2(μ-pz)6]2+ unit to the nanojar (see above), restores the C3v
symmetry of the complex.
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
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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)
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)
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)
N1−Cu1−N2a 101.5(4)
N1−Cu1−N2b 101.5(4)
N1−Cu1−O2 94.7(2)
N2a−Cu1−O1 96.9(4)
N2b−Cu1−O1 97.7(4)
O1−Cu1−O2 66.5(2)
N1−Cu1−O1 160.9(2)
N2a−Cu1−O2 162.9(4)
N2b−Cu1−O2 159.0(4)
(Bu4N)2[Cu2(pz)2(CO3)(SO4)]·CH3OH
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−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)
N1−Cu1−O2 162.6(2)
N4−Cu1−O1 162.3(2)
N2−Cu2−O5 163.3(2)
N3−Cu2−O4 163.1(3)
O1−Cu1−O2 67.2(2)
N3−Cu2−O5 93.7(2)
O4−Cu2−O5 70.6(2)
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 concen-
tration. 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.
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.
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)(μ-
RCOO)]∞, 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 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
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hexanuclear copper-pyrazolate complexes [Cu3(OH)(pz)3-
(RCOO)3]− and [Cu6O2(pz)6(RCOO)3]− are much more
resistant to protonation than nanojars.
seen [CO3⊂{Cu(OH)(pz)}32]2− form at increasing amounts of
pivalic acid, and Cu30CO3, which is a minor component of the as-
synthesized CunCO3 nanojar mixture, becomes the dominant
nanojar.
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.
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 single-
dicarboxylate 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 [{CO3⊂Cu27(OH)27(pz)24}{OOC-
(CH2)xCOO}{Cu6O(OH)(pz)6}]2− species are observed in-
stead. 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 cis-
carboxylates, 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.
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.
2−
Incarcerated Anion. CO3 (pKb = 3.67) is much more
2−
basic than SO4 (pKb = 12.01); consequently, carbonate-
incarcerating 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-
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(μ-
2−
pz)2X2]2− (X = CO3 or SO42−) are obtained as major
breakdown products. In addition, nanojars of certain sizes can
also rearrange into nanojars of a different size.
N
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(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.
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 com-
pounds, 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.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01593.
(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.
Detailed visual observations during the titration experi-
ments, 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 crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
(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.
AUTHOR INFORMATION
Corresponding Author
*E-mail: gellert.mezei@wmich.edu.
ORCID
Gellert Mezei: 0000-0002-3120-3084
Notes
The authors declare no competing financial interest.
■
́
(12) (a) Baran, P.; Marrero, C. M.; Perez, 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.
ACKNOWLEDGMENTS
This material is based on work supported by the National Science
Foundation under Grant No. CHE-1404730.
■
Chem. 2003, 42, 3907−3916. (c) Pinero, D.; Baran, P.; Boca, R.;
̃
Herchel, R.; Klein, M.; Raptis, R. G.; Renz, F.; Sanakis, Y. A Pyrazolate-
Supported Fe3(μ3-O) Core: Structural, Spectroscopic, Electrochemical,
and Magnetic Study. Inorg. Chem. 2007, 46, 10981−10989. (d) Miras,
DEDICATION
■
Dedicated to Professor Raphael G. Raptis on the occasion of his
60th birthday.
́
H. N.; Zhao, H.; Herchel, R.; Rinaldi, C.; Perez, 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.
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DOI: 10.1021/acs.inorgchem.7b01593
Inorg. Chem. XXXX, XXX, XXX−XXX