Doubling the Carbonate-Binding Capacity of Nanojars by the Formation of Expanded Nanojars

Anion binding and extraction from solutions is currently a dynamic research topic in the field of supramolecular chemistry. A particularly challenging task is the extraction of anions with large hydration energies, such as the carbonate ion. Carbonate-binding complexes are also receiving increased interest due to their relevance to atmospheric CO2 fixation. Nanojars are a class of self-assembled, supramolecular coordination complexes that have been shown to bind highly hydrophilic anions and to extract even the most hydrophilic ones, including carbonate, from water into aliphatic solvents. Here we present an expanded nanojar that is able to bind two carbonate ions, thus doubling the previously reported carbonate-binding capacity of nanojars. The new nanojar is characterized by detailed single-crystal X-ray crystallographic studies in the solid state and electrospray ionization mass spectrometric (including tandem MS/MS) studies in solution.


Introduction
Nanojars are a family of supramolecular coordination complexes that form from a solution of Cu 2+ , OH − and pyrazolate (pz = C 3 H 3 N 2 − ) ions in the presence of a hydrophilic anion, such as carbonate [1], sulfate [2], phosphate [3], arsenate [3] or chloride [4]. The anion templates the formation of {cis-Cu II (µ-OH)(µ-pz)} x metallamacrocycles (x = 6-14, except 11). Three (in the case of carbonate, sulfate, phosphate, arsenate) or four (in the case of chloride) of these metallamacrocycles self-assemble around a central anion into nanojars of the formula [anion⊂{Cu II (µ-OH)(µ-pz)} n ] (n = 27-33), via inter-metallamacrocycle and anion-metallamacrocycle hydrogen bonding, as well as inter-metallamacrocycle Cu···O interactions. The incarcerated anion appears to be crucial for the formation of nanojars, as the neutral nanojar host does not exist on its own without an anion guest. Figure 1 illustrates the structure of the nanojar with n = 27.
The recognition and binding of anions has been receiving increased interest in recent years [5][6][7], as the supramolecular binding of anions finds applications in anion sensing, extraction and separation of anions, transmembrane anion transport and anion-driven architectonics and organocatalysis [8]. We have recently shown that nanojars bind the incarcerated oxoanions (carbonate, sulfate, phosphate, arsenate) with unprecedented strength by wrapping a multitude of hydrogen bonds around the anion and totally isolating it from its surrounding medium (as in the sulfate [9] and phosphate [10] binding proteins). Indeed, an aqueous Ba 2+ solution is unable to precipitate the corresponding barium salt (e.g., BaSO 4 , K sp = 1.08 × 10 −10 at 25 • C in H 2 O) when stirred with a solution of the nanojars. We have also demonstrated that nanojars are able to transfer these anions, including one of the most hydrophilic ones, carbonate, from water into aliphatic solvents [11]. Thus, nanojars can be used as extraction agents for the removal of such anions from contaminated aqueous media by liquid-liquid extraction [12].
Herein we report the serendipitous discovery that upon addition of 1,10-phenathroline into the nanojar-forming reaction mixture, expanded nanojars form that bind two carbonate The recognition and binding of anions has been receiving increased interest in recent years [5][6][7], as the supramolecular binding of anions finds applications in anion sensing, extraction and separation of anions, transmembrane anion transport and anion-driven architectonics and organocatalysis [8]. We have recently shown that nanojars bind the incarcerated oxoanions (carbonate, sulfate, phosphate, arsenate) with unprecedented strength by wrapping a multitude of hydrogen bonds around the anion and totally isolating it from its surrounding medium (as in the sulfate [9] and phosphate [10] binding proteins). Indeed, an aqueous Ba 2+ solution is unable to precipitate the corresponding barium salt (e.g., BaSO4, Ksp = 1.08 × 10 −10 at 25 °C in H2O) when stirred with a solution of the nanojars. We have also demonstrated that nanojars are able to transfer these anions, including one of the most hydrophilic ones, carbonate, from water into aliphatic solvents [11]. Thus, nanojars can be used as extraction agents for the removal of such anions from contaminated aqueous media by liquid-liquid extraction [12].
Herein we report the serendipitous discovery that upon addition of 1,10-phenathroline into the nanojar-forming reaction mixture, expanded nanojars form that bind two carbonate ions instead of one, thus doubling the carbonate-binding capacity of nanojars. As described below, the binding of the second carbonate ion by four copper-centers (μ4-CO3) provides for an interesting new example of an inverse coordination complex, wherein the bridging ligand is the coordination center surrounded by metal ions [13][14][15][16][17][18][19][20].

Crystallographic Description
Located on a general position, nanojar 2 (triclinic, Pī) has pseudo-mirror symmetry ( Figure 2). Its structure is closely related to that of 1, in which three neutral [cis-Cu II (µ-OH)(µ-pz)] n rings, with a larger one (n = 12) sandwiched by two smaller ones (n = 6 and 9), define the nanojar, with its cavity occupied by an incarcerated carbonate ion (Figure 1). The same Cu 6 + Cu 12 + Cu 9 ring combination is found in both 1 and 2, with the exception that one OH − group of the Cu 9 -ring in 2 is replaced by an O-atom of a second CO 3 2− ion. The central, larger ring is approximately flat, with the pyrazolate units symmetrically alternating slightly above and below the ring mean-plane and not forming hydrogen bonds to the carbonate ion. The smaller side-rings are bowl-shaped, with their pyrazolate moieties pointing away from the central ring and their OH groups pointing toward the center of the nanojar and forming multiple hydrogen bonds with the incarcerated CO 3 2− ion. Although there is no direct bonding between the two smaller rings, they are both involved in multiple H-bonds and weak axial Cu-O interactions with the larger central ring. In the [cis-Cu II (µ-OH)(µ-pz)] n rings, Cu-O and Cu-N bond-lengths are within normal ranges, 1.893(3)-2.007(3) and 1.943(6)-2.06(2) Å, respectively (Table 1). While in 1 the 2-charge of the incarcerated carbonate ion is balanced by two Bu 4 N + counterions, in 2 it is the additional bonded [Cu 2 (phen) 2 CO 3 ] 2+ moiety that renders the assembly neutral. that one OH − group of the Cu9-ring in 2 is replaced by an O-atom of a second CO3 2− ion. The central, larger ring is approximately flat, with the pyrazolate units symmetrically alternating slightly above and below the ring mean-plane and not forming hydrogen bonds to the carbonate ion. The smaller side-rings are bowl-shaped, with their pyrazolate moieties pointing away from the central ring and their OH groups pointing toward the center of the nanojar and forming multiple hydrogen bonds with the incarcerated CO3 2− ion. Although there is no direct bonding between the two smaller rings, they are both involved in multiple H-bonds and weak axial Cu-O interactions with the larger central ring. In the [cis-Cu II (μ-OH)(μ-pz)]n rings, Cu-O and Cu-N bond-lengths are within normal ranges, 1.893(3)-2.007(3) and 1.943(6)-2.06(2) Å , respectively (Table 1). While in 1 the 2-charge of the incarcerated carbonate ion is balanced by two Bu4N + counterions, in 2 it is the additional bonded [Cu2(phen)2CO3] 2+ moiety that renders the assembly neutral.   In 2, two O-atoms of the additional carbonate ion are bound to two Cu II (1,10-phen) units, which are bridged by a OH − group (O28) and form weak Cu-O bonds (2.296(3) and 2.268(3) Å) with the Cu 9 -ring ( Figure 3). The OH − group (O28) is H-bonded to the central carbonate ion. As a consequence of binding the second carbonate ion, a pyrazolate group of the Cu 9 -ring is pulled away from the Cu 12 -ring, opening up a cavity that becomes occupied by a water molecule. This H 2 O molecule bridges two Cu-atoms of the Cu 9 -ring (Cu···O: 2.419(3) and 2.432(3) Å) and donates a H-bond to a OH-group of the Cu 12 -ring (O40···O8: 2.684(5) Å).
As in 1, the OH-groups of the Cu 12 -ring in 2 donate twelve alternating H-bonds, six to the Cu 6   In addition to the two carbonate ions, nanojar 2 also binds a nitrobenzene solvent molecule in the outer cavity of the Cu 6 -ring ( Figure 5) by a close π-π stacking interaction between the phenyl group and a pyrazolate moiety (centroid···centroid: 3.593(3) Å, angle between mean-planes: 6.7(2) • ) and by weak, bifurcated interactions between the O-atoms of the nitro group and four Cu-atoms (Cu···O: 2.647(4) and 3.033(4) Å, and 2.827(4) and 2.844(4) Å, respectively). As shown in Figure 6, the close-packing of nanojars leaves relatively large void spaces in the crystal lattice, which are filled by multiple solvent molecules (see also Figure 7). In addition to the nitrobenzene molecule bound in the outer cavity of the Cu 6 -ring of the nanojar, there are five more nitrobenzene molecules filling up the void spaces, as well as a seventh nitrobenzene molecule disordered with a heptane molecule. The presence of aromatic moieties in the included solvent molecules appears to be crucial for the formation of nanojar crystals, as they form multiple aromatic interactions with the nanojar molecules and with each other. Nevertheless, the crystal lattice is not robust: the crystals quickly become opaque and disintegrate if removed from the mother liquor at ambient conditions, requiring low-temperature conditions for X-ray diffraction measurement. carbonate ion. As a consequence of binding the second carbonate ion, a pyrazolate group of the Cu9-ring is pulled away from the Cu12-ring, opening up a cavity that becomes occupied by a water molecule. This H2O molecule bridges two Cu-atoms of the Cu9-ring (Cu···O: 2.419(3) and 2.432(3) Å ) and donates a H-bond to a OH-group of the Cu12-ring (O40···O8: 2.684(5) Å ).  While in 1 the central carbonate ion is approximately parallel to the [Cu(μ-OH)(μpz)]n rings, in 2 it is found tilted: the angle between the CO3 2− and Cu12-ring mean-planes is 2.2(1) in 1 and 22.2(2) in 2 ( Figure 4). As a consequence of the tilting, some of the Hbonding distances to CO3 2− in 2 ( Table 2) are shorter (down to 2.657(5) Å ) and others are longer (up to 3.088(5) Å ) than in 1 (2.746(5)-2.915(5) Å ). Nonetheless, the average of the twelve H-bonds to carbonate (four to each O-atom) is virtually identical in 1 (2.842(5) Å ) and 2 (2.838(5) Å ). The second CO3 2− ion in 2 is coordinate-covalently bound to the Cu9 ring and the two additional Cu-atoms, almost parallel to the central CO3 2− ion (angle between mean-planes: 6.6(2)), with a C···C separation of only 3.071(6) Å . Another very closely-spaced, head-to-head pair of CO3 2− ions (C···C: 3.664(1), O···O: 1.946(1) Å ) has been reported in which both CO3 2− ions are bound to multiple metal centers [21]. The tetranuclear Cu4(μ4-CO3) moiety has also been reported with a few other ligand systems [22][23][24][25][26][27][28][29].
In addition to the two carbonate ions, nanojar 2 also binds a nitrobenzene solvent molecule in the outer cavity of the Cu6-ring ( Figure 5) by a close - stacking interaction between the phenyl group and a pyrazolate moiety (centroid···centroid: 3.593(3) Å , angle between mean-planes: 6.7(2)) and by weak, bifurcated interactions between the O-atoms of the nitro group and four Cu-atoms (Cu···O: 2.647(4) and 3.033(4) Å , and 2.827(4) and 2.844(4) Å , respectively).  As shown in Figure 6, the close-packing of nanojars leaves relatively large void spaces in the crystal lattice, which are filled by multiple solvent molecules (see also Figure  7). In addition to the nitrobenzene molecule bound in the outer cavity of the Cu6-ring of the nanojar, there are five more nitrobenzene molecules filling up the void spaces, as well as a seventh nitrobenzene molecule disordered with a heptane molecule. The presence of aromatic moieties in the included solvent molecules appears to be crucial for the formation of nanojar crystals, as they form multiple aromatic interactions with the nanojar molecules and with each other. Nevertheless, the crystal lattice is not robust: the crystals quickly become opaque and disintegrate if removed from the mother liquor at ambient conditions, requiring low-temperature conditions for X-ray diffraction measurement.

Materials and Methods
All commercially available chemicals were used as received. Reactions were carried out in closed vessels, but not under strictly air-free conditions.

Materials and Methods
All commercially available chemicals were used as received. Reactions were carried out in closed vessels, but not under strictly air-free conditions.

Mass Spectrometry
Mass spectrometric analysis of the nanojars was performed with a Waters Synapt G1 HDMS instrument using electrospray ionization (ESI). 10 −4 -10 −5 M solutions were prepared in N,N-dimethylformamide (DMF). Samples were infused by a syringe pump at 5 µL/min, and nitrogen was supplied as nebulizing gas at 500 L/h. The electrospray capillary voltage was set to -2.5 or +3.0 kV, respectively, with a desolvation temperature of 150 • C. The sampling and extraction cones were maintained at 40 V and 4.0 V, respectively, at 80 • C. The MS/MS conditions were the same, except the transfer collision energy was 5 V and the trap collision energies were 5, 30, 40, 50, 60 and 70 V.

X-ray Crystallography
A few single-crystals of 2 were grown from a nitrobenzene solution by heptane vapor diffusion. Once removed from the mother liquor, the crystals are very sensitive to solvent loss at ambient conditions and were mounted quickly under a cryostream (100 K) to prevent decomposition. X-ray diffraction data were collected at 100 K 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 structure was solved by employing SHELXTL direct methods and refined by full-matrix least squares on F 2 using the APEX2 v2014.9-0 software package [30]. C-H hydrogen atoms were placed in idealized positions and refined using the riding model. Hydroxyl and water H atom positions were located from difference density maps and were refined with O-H distance restraints of 0.82(2) Å. A pyrazolate ligand was refined as disordered. The two disordered moieties were restrained to have similar geometries. U ij components of ADPs for disordered atoms closer to each other than 2.0 Å were restrained to be similar. Subject to these conditions, the occupancy ratio refined to 0.805(13)/0.195(4). The oxygen atoms of the carbonate ion were refined as disordered. The two disordered moieties were restrained to have similar geometries. U ij components of ADPs for disordered atoms closer to each other than 2.0 Å were restrained to be similar. Subject to these conditions, the occupancy ratio refined to 0.913(4)/0.087(4). Three nitrobenzene solvate molecules were disordered with two alternative orientations (one by two-fold symmetry, two in general positions), one with three orientations, and one was disordered with a heptane molecule. The disordered nitrobenzene moieties were restrained to have similar geometries (SAME commands). C926 of one nitrobenzene moiety was restrained to be coplanar with its neighboring atoms. Bond distances within the heptane molecule were restrained to be similar to each other (SADI command). U ij components of ADPs for disordered atoms closer to each other than 2.0 Å were restrained to be similar. Subject to these conditions, the occupancy rates refined to 0.910(4)/0.090(4) for the two moieties of the nitrobenzene of N60, to 0.489(14)/0.511 (14) for the two moieties of the nitrobenzene of N62, to 0.502(3)/0.313(3)/0.185(3) for the three moieties of the nitrobenzene of N64 and to 0.765(6)/0.235 (6) for the disorder of heptane and nitrobenzene (in favor of heptane). A thermal ellipsoid plot of the crystal structure is shown in Figure 7.