From a Well-Defined Organozinc Precursor to Diverse Luminescent Coordination Polymers Based on Zn(II)-Quinolinate Building Units Interconnected by Mixed Ligand Systems.

Introduction of photoactive building blocks into mixed-ligand coordination polymers appears to be a promising way to produce new advanced luminescent materials. However, rational design and self-assembly of the multi-component supramolecular systems is challenging from both a conceptual and synthetic perspective. Here, we report exploratory studies that investigate the potential of [Zn(q)2]2[tBuZn(OH)]2 complex (q = deprotonated 8-hydroxyquinoline) as an organozinc precursor as well as a mixed-ligand synthetic strategy for the preparation of new luminescent coordination polymers (CPs). As a result we present three new 2D mixed-ligand Zn(II)-quinolinate coordination polymers which are based on various zinc quinolinate secondary building units interconnected by two different organic linker types, i.e., deprotonated 4,4'-oxybisbenzoic acid (H2obc) as a flexible dicarboxylate linker and/or selected bipyridines (bipy). Remarkably, using the title organozinc precursors in a combination with H2obc and 4,4'-bipyridine, a novel molecular zinc quinolinate building unit, [Zn4(q)6(bipy)2(obc)2], was obtained which self-assembled into a chain-type hydrogen-bonded network. The application of the organometallic precursor allowed for its direct reaction with the selected ligands at ambient temperature, avoiding the use of both solvothermal conditions and additional base reagents. In turn, the reaction involving Zn(NO3)2, as a classical inorganic precursor, in a combination with H2obc and bipy led to a novel 1D coordination polymer [Zn2(q)2(NO3)2(bipy)]. While the presence of H2obc was essential for the formation of this coordination polymer, this ditopic linker was not incorporated into the isolated product, which indicates its templating behavior. The reported compounds were characterized by single-crystal and powder X-ray diffraction, elemental analysis as well as UV-Vis and photoluminescence spectroscopy.

1. Introduction

Coordination polymers (CPs), a class of crystalline porous materials made up of metal clusters or ions linked via organic ligands, represent a rapidly growing area in modern chemistry and materials science due to their wide range of applications [1,2,3,4,5]. In particular, luminescent CPs have attracted considerable attention in chemical sensing [6,7,8], biomedical imaging [9,10], nonlinear optics [11,12], as well as in light-emitting and display devices [13,14,15]. The tunable hybrid structure of CPs provides opportunities to manipulate their properties towards unusual luminescence behavior by varying the building blocks or structural connectivity [16,17,18]. Preparation of luminescent CPs has been achieved by several approaches, e.g., by application of photoactive metal cations, organic moieties, metal-organic charge transfer species or guest molecules [19,20,21]. One of the ligand systems known for its capability to form luminescent molecular complexes [22] and metal-organic materials [23] is 8-hydroxyquinoline (Hq) and its derivatives, which have been successfully applied in biomedicine [24] and the fabrication of optoelectronic devices such as organic light emitting diodes or solar cells [25,26]. Moreover, extended organic ligands based on the quinoline core have been applied in the development of luminescent coordination polymers, which represents an efficient strategy, albeit, often requiring multi-step preparation of organic ligand systems [27,28,29,30,31,32,33,34]. Alternatively, simple quinolinate ligands could be incorporated into mixed-ligand CPs as co-ligands, an approach which has the challenge of designing and controlling the self-assembly of multicomponent supramolecular systems [35], therefore it is not surprising that this approach has been explored to a lesser extent and only a few examples of well-defined functional quinolinate-based CPs [36,37,38], including materials with valuable luminescent properties [39,40], are reported.

In our previous reports we have demonstrated a series of novel molecular organozinc 8-hydroxyquinolinate complexes with high propensity to noncovalent interactions-driven self-organization into luminescent porous materials [41,42]. With this in mind and in continuation of our interest in both multifaceted organozinc chemistry [43,44,45] and the development of new synthetic approaches for the preparation of Zn-based functional materials [46,47,48,49,50,51], herein we report on the synthesis and characterization of a series of coordination polymers using a mixed-ligand synthetic strategy [35] and a [Zn(q)2]2[tBuZn(OH)]2 complex as a versatile organozinc precursor. The application of this well-defined organometallic precursor allowed for direct reaction with a ditopic organic acid and selected bipyridine ligands at ambient temperature, thus avoiding the use of both solvothermal conditions and additional base reagents. Using the mixed-ligand approach, we explore the influence of the metal precursor type and organic linker selection on both the molecular structure of quinolinate-based secondary building units (SBUs), their supramolecular architectures and the luminescence properties of the resulting CPs. We demonstrate the remarkable superiority of an organozinc precursor over the classic inorganic salt, regarding the structure-related luminescent properties of the resultant CPs.

2. Results and Discussion

2.1. Synthesis of Molecular Building Units and Coordination Polymers Based on Zn(II)-Quinolinate Species

Self-assembly of pre-designed functional molecular building units using mixed-ligand systems has arisen as a powerful tool for the development of novel porous materials due to the variety of supramolecular architectures possible [35]. In this regard, we selected the previously reported quinoline-based zinc complex [Zn(q)2]2[tBuZn(OH)]2 (q = deprotonated 8-hydroxyquinoline) [41,42] as a model luminescent core for exploratory studies investigating the potential of organozinc precursors and a mixed-ligand synthetic strategy for the preparation of new luminescent coordination polymers (CPs). In particular, these investigations concern the formation of CPs by self-assembly involving two different linker types: a flexible 4,4′-oxybisbenzoate linker and various bipyridine ligands (Scheme 1).

Scheme 1

Representation of the synthetic pathways leading to luminescencent mixed-ligand Zn(II)-quinolinate coordination polymers: [Zn4(q)4(obc)2(DEF)2] (a); [Zn2(q)2(bpe)(obc)], [Zn2(q)2(bpene)(obc)] (b) and molecular cluster [Zn4(q)6(bipy)2(obc)2] (c). Hydrogen bonds in HP4 are marked with blue dashed lines.

Initially, a reference reaction involving only [Zn(q)2]2[tBuZn(OH)]2 and 4,4′-oxybisbenzoic acid (H2obc) in diethylformamide (DEF) was conducted, resulting in the formation of a 2D-coordination polymer [(Zn4(q)4(obc)2(DEF)2] (CP1), which is based on tetranuclear quinolinate-based SBU interconnected by the ditopic carboxylate linkers. (Scheme 1a). Moreover, two of the zinc centres in the CP1 SBU are coordinated by solvent molecules, which suggests that introduction of additional neutral organic linkers could lead to new supramolecular architectures. Thus, in the next steps different bipyridne ligands were employed in the reaction with [Zn(q)2]2[tBuZn(OH)]2 and H2obc acid in order to evaluate the influence of the neutral ditopic linkers on the resulting CPs. In case of 1,2-bis(4-pyridyl)ethane (bpe), used as a flexible bipyridine ligand, a 2D coordination polymer [(Znq)2(bpe)(obc)] (CP2) was formed. This polymer incorporates dimeric [(Zn2(q)2] nodes interconnected by the ditopic obc and bpe linkers (Scheme 1b). When a more rigid 1,2-bis(4-pyridyl)ethylene (bpene) ligand was used, the reaction afforded a new 2D coordination polymer, [(Znq)2(bpene)(obc)] (CP3), containing the same dimeric SBU as CP2, but adopting different supramolecular architecture. Remarkably, using 4,4′-bipyridine (bipy) in the analogous reaction with [Zn(q)2]2[tBuZn(OH)]2 and H2obc led to completely different product: a molecular cluster [Zn4(q)6(bipy)2(obc)2] (HP4) self-assembled into chain-type hydrogen-bonded network (Scheme 1c). Finally, the applicability of zinc nitrate as an inorganic precursor in a combination with the same mixed organic ligand system was investigated. The reaction involving Zn(NO3)2, H2obc and bipy afforded a novel 1D coordination polymer [Zn2(q)2(NO3)2(bipy)] (CP5) (Scheme 2). While the presence of H2obc was essential for the product formation, this ditopic linker was not incorporated into the isolated product, which indicates its templating behaviour during the CP5 formation. Strikingly, in contrast to luminescent zinc-quinolinate coordination polymers CP1, CP2, CP3 and HP4, CP5 was not luminescent (vide infra) despite involving the zinc quinolinate moieties; possibly on account of quenching behavior of nitrates (these aspects are a subject of our further ongoing investigations). The structures and composition of all of the resulting CPs were elucidated by means of elemental analysis and FTIR spectroscopy, single-crystal X-ray diffraction studies (SCXRD) and powder X-ray diffraction (PXRD), and they photophysical properties were tested using UV-Vis and photoluminescence (PL) spectroscopy. All the developed materials exhibited high stability upon prolonged storage (at least 3 months) under ambient conditions. Moreover, thermal stability of CP1, as a selected example, was investigated using thermogravimetric analysis (TGA/DTG), which revealed a mult-stage decomposition pathway with the first major step at ca. 193 °C (Figure S2).

Scheme 2

The inorganic synthetic approach to CP5 with quenched luminescence.

2.2. Single Crystal X-ray Diffraction and Structure Analysis

According to the SCXRD analysis, CP1 crystallizes in the monoclinic space group P21/c. The tetranuclear SBU in CP1 includes 5- and 6-coordinated zinc centres stabilized by the bridging quinolinate ligands as well as solvating DEF molecules connected to the terminal Zn atoms (Figure 1a). The extended supramolecular architecture of CP1 involves connection of each of the SBU clusters with four bifunctional obc ligands, leading to the formation of 2D layers, which are arranged perpendicular to the crystallographic a axis and stacked in a staggered geometry with ca. 10.6 Å distance between adjacent layers (Figure 1a). The 2D framework of CP1 exhibits the sql topology with Schläfli point symbol of {44·62} calculated for the clusters simplified as a 4-connecting node. Within each individual 2D layer of the polymer there are π···π interactions between the quinoline aromatic rings present in the central subunit [Zn2(q)2] of adjacent nodes. Moreover, the interactions between adjacent polymer layers are also based on the set of π···π contacts between the quinoline rings of the terminal fragments [Zn(q)(DEF)]. Interestingly, the molecular structure contains system of parallel 1D channels filled with solvent molecules. The channels are directed along the crystallographic b axis, parallel to the 2D layers in CP1. To provide the upper limit to space accessible to guest molecules in CP1, we have used procrystal electron density approach implemented in CrystalExplorer [52]. According to this calculations, CP1 exhibit procrystal surface area of 1102 m2/g and the largest pore volumes of 543 Å3 per unit cell corresponding to 14% of the total material volume.

Figure 1

Molecular and supramolecular structures of CP1 (a), CP2 (b), CP3 (c) and CP5 (d). Zinc = violet, Oxygen = red, Nitrogen = blue, Carbon = grey. Most of the hydrogen atoms have been omitted for clarity.

Compound CP2 crystallizes in a monoclinic P21/n space group. The dinuclear SBU in CP2 comprises two Zn centres (5- and 6-coodrinated) stabilized by the quinolinate ligands as well as obc and bpe linkers (Figure 1b). The supramolecular arrangement of CP2 involves the dinuclear SBU clusters, each connected by two pairs of bifunctional ligands: obc and bpe, leading to formation of extended 2D-layered structure directed perpendicular to the crystallographic b axis with a sql rhombic-grid topology, described with Schläfli symbol of {44.62} (Figure 1b). The supramolecular arrangement of the adjacent 2D layers can be described as an ABA sentence, with the interlayer distance about 10.8 Å, which is maintained by a set of cooperative noncovalent interactions. Moreover, the CP2 structure includes a 2D system of pores filled with solvent molecules, extending along the b and c crystallographic axes of the structure. CP2 has the procrystal surface area of about 2514 m2/g with pore volumes of 1494 Å3 (26% of total cell unit volume).

CP3 crystallizes in the triclinic P-1 group and is composed of dinuclear SBUs interconnected by obc and bpene ligands (Figure 1c). The SBUs in CP3 and CP2 are structurally similar with the same connectivity of organic ligands, involving 5- and 6-coordinated zinc centers stabilized by quinolinate ligands. Interconnection of the dinuclear SBUs in CP3 by the mixed-ligand systems leads to formation of extended 2D network layers with ellipsoidal openings approximately 11 × 21 Å in size. The adjacent polymer layers form a set of interlayer non-covalent interactions, which directs the layer stacking into the ABA sequence. The network topology on CP3 is described with the point symbol {44.62}, which corresponds to the sql rhombic grid topology, similarly to CP2. However, the use of a more flexible bpene connector results in the adjacent layers forming more closely packed structure in CP3, compared to CP2. This, combined with the numerous non-covalent (C-H type) interactions between the individual polymer layers in CP3, creates a more compact 3D supramolecular structure. The supramolecular structure of CP3 involves a system of 1D bottle-neck channels filled with solvent molecules, involving larger voids connected by narrower channels. The calculated procrystal surface area of CP3 is 1754 m2/g and the cell consist of 20% (505 Å3) of free volume.

Compound HP4, with the chemical formula [Zn4(q)6(bipy)2(Hobca)2], crystallizes in the monoclinic C2/c space group and exhibits clear differences in comparison to the CP1–CP3 in the character of molecular and supramolecular structure (Figure 2). The molecular cluster [Zn4(q)6(bipy)2(Hobca)2] exhibits a macrocyclic structure with two [Zn2(q)3(Hobc)] interconnected by two bipyridine ligands (Figure 2). The supramolecular structure of HP4 is governed through cooperative O-H···O hydrogen interactions and π-π interactions. The molecular units of HP4 self-assemble via cooperative O-H···O hydrogen interactions (with the O···O distance of 2.469 Å) mediated by the distal carboxylic groups and affording a 1D chain-type structure. Additionally, the 1D chains are self-organized by π-π interactions of the quinolinate aromatic ring into a 2D supramolecular system with open channels extending in the direction perpendicular to the crystallo-graphic c axis and occupied by solvent molecules. HP4 exhibits procrystal surface area of 1902 m2/g and the largest pore volumes of 2611 Å3 per unit cell corresponding to 22% of the total material volume.

Figure 2

Molecular and supramolecular structure of HP4; zinc = violet, oxygen = red, nitrogen = blue, carbon = grey. Hydrogen bonds are marked with blue dashed lines. Hydrogen atoms have been omitted for clarity.

As evidenced by the SCXRD, compound CP5 (space group P-1) is a 1D coordination polymer formed as a result of the bridging of each dimeric node [(Zn2(q)2(NO3)2] by two linear bipy ligands (Figure 1d). The [(Znq)2(NO3)2] SBU in CP5 contains the [Zn2(q)2] core involving two zinc centres with octahedral geometry, each coordinated by a nitrate group. Analysis of the supramolecular structure of CP5 revealed that the adjacent polymer chains are of the distance 2.377 Å between oxygen of the nitrate group and hydrogen of the pyridyl ring of bipy of adjacent polymer chain. Thus, defined 2D adjacent layers of CP5, stacked together by CH···π interactions (2.830 Å) between hydrogen of quinoline ligand and electrons of pyridyl ring of bipy and by π···π interactions (3.371 Å) between quinoline rings create 3D supramolecular architecture with slip along the crystallographic b axis. Furthermore, packing of the layers generates a 2D pore system filled with solvent molecules within the crystal structure. The calculated procrystal surface area of CP5 is 1585 m2/g and pore volume is 172 Å3 (19% of the total volume of the unit cell).

We also note that to evaluate the phase purity of the developed products, powder X-ray diffraction (PXRD) experiments were conducted. For all products the experimental PXRD patterns displayed characteristics closely corresponding to the reference patterns simulated from the collected single crystal data (Figure S1), while some minor differences could be attributed to the different measurement temperatures as well as texturing of powder samples.

2.3. FTIR Spectroscopic Study

In order to further investigate the coordination modes of carboxylate and quinolinate moieties in compounds CP1–CP5 and HP4 their FTIR spectra in solid state (Figures S3–S7) were analysed and compared to the spectra of selected reference compounds: [Zn(q)2]2[tBuZn(OH)]2 precursor, Hq ligand and H2obc linker (Figures S8–S10) as well as the literature references [53,54]. As a result, we found that the bands related to the quinolinate ligand (most characteristic bands observed in the 1580–1322 cm−1 and 825–730 cm−1 ranges) in the spectra of CP1–CP5 and HP4 were shifted to the higher frequencies in comparison to the free Hq ligand, which is in agreement with the spectrum of the [Zn(q)2]2[tBuZn(OH)]2 precursor [41,42]. Moreover, the spectrum of CP1 contains a sharp high intensity band at 1379 cm−1 corresponding to the symmetric vibrations of the carboxyl group and two bands at 1598, cm−1 representing symmetric vibrations of the carboxylate group. These results indicate presence of two different bidentate modes of the carboxylate binding in the structure of CP1. The asymmetric and symmetric stretching vibrational modes of the carboxylate ligand for both complexes CP2 and CP3 are further split into two peaks, respectively: 1615, 1597 cm−1 and 1381, 1367 cm−1 (in CP2); and 1600, 1578 cm−1 and 1378, 1363 cm−1 (in CP3). These observations confirm the existence of monodentate and chelating carboxylate group in the structures of CP2 and CP3. The absence of band at ~1700 cm−1 for CP1–CP3 compounds confirm complete deprotonation of carboxyl groups. For compound HP4 the asymmetric and symmetric vibrations of the carboxylate species were observed at 1595 and 1381 cm−1, respectively, while a band at 1694 cm−1 corresponds to the stretching vibrations of C=O carbonyl group in HP4. The nitrate ion stretching vibration bands for CP5 were found at 1464 and 1298 cm−1. The separation of 166 cm−1 between them is consistent with a bidentate coordination of the nitrate ion. Stretching vibrations of the carbonyl group from DEF molecules coordinated to the Zn center in CP1 gives band at 1634 cm−1, while the guest molecules located in the free space of the structure exhibit the band at 1670 cm−1. The solvent guest DMF molecules in CP2–CP5 and HP4 are represented by stretching C=O bands at 1666–1672 cm−1. Importantly, the observation of the characteristic bands related to the carboxylate moieties in CP1–CP3 and HP4 well corroborate with the coordination modes of carboxylate ligands judged from the X-ray diffraction study.

2.4. Photophysical Characterization

Solid-state UV-Vis spectra of the developed materials were measured in diffusive reflectance geometry and transformed using Kubelka-Munk equation. As shown in Figure 3a and Figure S11, UV-Vis absorption maxima appear at 404 nm for CP1, 393 nm for CP2, 371 nm for CP3, 392 nm for HP4, and 370 nm for CP5, showing blue shifts compared to that observed for the [Zn(q)2]2[tBuZn(OH)]2 precursor (absorption maximum at 410 nm). It should be noted that the solid-state UV-Vis absorption spectrum of CP3 exhibited long tail extending through visible region possibly due to the scattering effect. Under the same excitation conditions (λex: 400 nm), CP1–CP3 and HP4 polymers exhibit intense photoluminescence with the emission maximum: 520 nm (CP1), 505 nm (CP2), 511 nm (with a shoulder peak at ~590 nm) (CP3) and 537 nm (with a shoulder peak at ~590 nm) (HP4) (Figure 3b). In turn, CP5 does not exhibit luminescent properties. The maxima of the absorption bands of the CP1, CP2, and CP3 polymers are blue-shifted, while the emission band of HP4 is red-shifted, in comparison to that observed for [Zn(q)2]2[tBuZn(OH)]2 (emission maximum at 530 nm). The observed photoluminescence maxima for these coordination polymers are also blue-shifted compared to other organozinc 8-hydroxyquinolinate complexes: [tBuZn(q)]3 (emission band at 555 nm for λex = 350 nm) and [(tBu)3Zn5(µ4-O)(q)5] (emission band at 535 nm for λex = 350 nm) [41]. The emission maxima of CP1, CP2 and CP3 are also different from the PL of the well-studied compound Zn(q)2 [55] (536 nm for λex = 400 nm), being blues-hifted by 16, 31 and 25 nm, respectively. Maximum of the HP4 emission spectra is comparable with that for Zn(q)2. In turn, the emission maxima of CP1, CP3 and HP4 are red-shifted in comparison to emission maximum of α-Al(q)3 crystal phase (504 nm for λex = 365 nm) while the emission maximum of CP2 is comparable [56]. The resulting spectra clearly indicate that the introduction of various bipyridine ligands allows for tailoring both the absorption and emission properties of the developed materials. Moreover, the data lead to the conclusion that increase of the coordination framework rigidity through crosslinking of metal centers with dicarboxylate and bipyridine ligands affects blueshift of the emission maximum within this class of materials. Surprisingly, the CP5 exhibits no luminescence, despite its great structural similarities to the other studied luminescent materials (CP1–CP3 and HP4), which could be related to the quenching effect of the inorganic nitrate ligands present in the structure of CP5. This intriguing issue is a subject of our ongoing investigations which will be published due course.

Figure 3

Normalized (a) UV-Vis absorption measured in diffusive reflectance of compound powders dispersed with BaSO4, (b) luminescent spectra measured in the solid-state of the [Zn(q)2]2[tBuZn(OH)]2 precursor and CP1–CP5 and HP4 coordination polymers at room temperature; excitation at 400 nm.

3. Conclusions

The multifaceted chemistry of molecular organometallic complexes incorporating bidentate quinolinate ligands as potential precursors of functional materials has remained a largely undeveloped area of research, which significantly hampers the possibility of obtaining novel fluorescent hybrid materials. Herein, we demonstrate a new synthetic approach to photoluminescent quinolinate ligand-based CPs that allowed for the isolation of a series of structurally diverse hybrid organic-inorganic materials. The application of well-defined organozinc precursor allowed for direct reaction with a ditopic organic acid and a selected bipyridine ligand at ambient temperature, avoiding the use of both solvothermal conditions and additional base reagents. Using the mixed-ligand approach we explored the influence of the metal precursor type and organic linker selection on both the molecular structure of quinolinate-based SBUs and supramolecular architectures alongside luminescence properties of the resulting CPs. The presented approach appears as an intriguing strategy for the design and construction of new luminescent materials based on 8-hydroxyquinoline derivatives. We believe that the reported synthetic strategy unveils new prospects for the design of reaction systems leading to a variety of novel luminescent CP networks of interest in sensing, optoelectronics, and (photo) catalysis.

4. Materials and Methods

4.1. General Information

All chemical reagents, including 8-hydroxyquinoline (Hq) and 4,4′-oxybisbenzoic acid (H2obc), 1,2-bis(4-pyridyl)ethane (bpe), 1,2-bis(4-pyridyl)ethylene (bpene), 4,4′-bipyridine (bipy) and Zn(q)2 were purchased from commercial vendors. Solvents were dried and distilled prior to use. All of the described synthetic procedures were carried out under a nitrogen atmosphere using standard Schlenk and glovebox techniques. Synthesis of CP5 was performed under an air atmosphere. The [Zn(q)2]2[tBuZn(OH)]2 precursor was obtained according to our previously reported procedure [41]. The reported synthetic procedures demonstrate the most efficient ways (in terms of yield and product purity) for preparation of the discussed products, that we were able to find during the study.

Fourier Transform Infrared Attenuated Total Reflectance (FTIR ATR) were recorded (500–4000 cm−1 region) on a Tensor instrument (Bruker, Billerica, MA, USA) equipped with an ATR accessory. Powder X-ray diffraction (PXRD) data were collected on Empyrean diffractometer (PANalytical, Almelo, Netherlands). Measurements employed Ni-filtered Cu Kα radiation of a copper sealed tube charged with 40 kV voltage and 40 mA current and Bragg Brentano geometry and a Si zero-background holder. Diffraction patterns were measured in the range of 2θ = 3−60° of scattering angle by step scanning with step of 0.02°. Thermogravimetric and derivative thermogravimetric analyses (TGA/DTG) were recorded on Q600 apparatus (TA Instruments, Milford, MA, USA). Samples for thermogravimetric characterization was placed in alumina crucibles under an argon atmosphere at heating rate of 5 °C/min. Optical absorption (UV-Vis) measurements were carried out using a UV-2600 spectrophotometer (Shimadzu, Kyoto, Japan). Photoluminescence (PL) spectra were recorded on F-7000 fluorescence spectrophotometer (Hitachi, Tokyo, Japan).

4.2. Synthesis of [(Zn4(q)4(obc)2(DEF)2] (CP1)

A THF (5 mL) solution of [Zn(q)2]2[tBuZn(OH)]2 (0.090 mg, 0.091 mmol) was cooled to −78 °C and H2obc (0.024 g, 0.091 mmol) in THF (5 mL) was added using a syringe. The reaction mixture was stirred for 15 min to warm it up to room temperature. The solid residue obtained after removal of the solvent under reduced pressure using a high vacuum pump, was recrystallized from DEF (5 mL). The crystallization overnight at 4 °C resulted in formation yellow crystals of polymer CP1. The crystal product was decanted and the solution was removed using a syringe. Then solid product was washed with DEF (3 mL) and CH2Cl2 (3 mL). The washed product was dried at room temperature using vacuum pump at 10−3 mbar for 3 h. Isolated yield: 72%. IR (ATR): ṽ [cm−1] = 1670 (m), 1634 (m), 1598 (s), 1579 (m), 1556 (m), 1496 (s), 1467 (s), 1379 (s), 1322 (s), 1278 (m), 1261 (m), 1236 (s), 1159 (m), 1126 (w), 1106 (s), 1035 (w), 1012 (m), 959 (w), 944 (w), 908 (w), 876 (m), 863 (w), 824 (s), 804 (m), 782 (s), 770 (m), 750 (m), 735 (s), 709 (w), 696 (w), 658 (m), 643 (w), 626 (m), 607 (w), 593 (w), 518 (w), 503 (m), 421 (m). Elemental analysis (%) calcd for [C74H60N6O16Zn4·2(C5H11NO)]: C 57.55, H 4.71, N 6.39; found C 57.25, H 4.67, N 6.48.

4.3. Synthesis of [(Znq)2(bpe)(obc)] (CP2)

A DMF (12 mL) solution of [Zn(q)2]2[tBuZn(OH)]2 (0.030 g, 0.030 mmol) was cooled to −78 °C and mixture of H2obc (0.016 g, 0.061 mmol) and bpe (0.011 g, 0.061 mmol) in DMF (4 mL) was added using a syringe. The reaction mixture was stirred for 15 min to warm to room temperature and the resultant mixture was left overnight at 4 °C. The resulting precipitate was removed by filtration using a syringe filter (pore size 0.45 µm) and then the solution was concentrated to ~10 mL at a vacuum pump for ~2 h. Overnight crystallization at room temperature resulted in the formation of yellow crystals of polymer CP2. The crystal product was decanted and the solution was removed using a syringe. The product was washed three times with DMF (3 mL). The washed product was dried at room temperature using a vacuum pump (10−3 mbar) for 3 h. Isolated yield: 58%. IR (ATR): ṽ [cm−1] = 1672 (s), 1615 (m), 1597 (s), 1573 (m), 1549 (m), 1494 (s), 1462 (s), 1434 (m), 1406 (m), 1381 (s), 1367 (s), 1323 (s), 1279 (m), 1228 (s), 1160 (m), 1135 (w), 1108 (s), 1089 (s), 1073 (m), 1062 (m), 1028 (m), 1011 (w), 907 (w), 874 (m), 824 (s), 803 (m), 784 (s), 768 (m), 731 (s), 713 (w), 697 (w), 659 (m), 648 (m), 632 (w), 618 (w), 606 (w), 579 (w), 554 (w), 540 (w), 500 (m), 440 (w). Elemental analysis (%) calcd for [C44H32N4O7Zn2·3(C3H7NO)]: C 58.98, H 5.00, N 9.08; found C 58.82, H 4.94, N 9.17.

4.4. Synthesis of [(Znq)2(bpene)(obc)] (CP3)

A DMF (4 mL) solution of [Zn(q)2]2[tBuZn(OH)]2 (0.040 g, 0.041 mmol) was cooled to −78 °C and H2obc (0.005 g, 0.020 mmol) in DMF (3 mL) and bpene (0.002 g, 0.010 mmol) in DMF (3 mL) was added using a syringe. The reaction mixture was then stirred for 15 min while warming to room temperature. Crystallization at room temperature resulted in the formation of yellow crystals of polymer CP3 after three days. The crystalline product was decanted and the solution was removed using a syringe. The product was washed three times with DMF (3 mL). The washed product was dried for 3 h at room temperature using a high-vacuum pump (10−3 mbar). Isolated yield: 64%. IR (ATR): ṽ [cm−1] = 1668 (m), 1634 (m), 1600 (s), 1578 (s), 1556 (m), 1497 (s), 1466 (s), 1378 (s), 1363 (m), 1322 (s), 1277 (m), 1236 (s), 1214 (m), 1159 (m), 1138 (w), 1106 (s), 1068 (w), 1035 (w), 1011 (w), 945 (w), 908 (w), 877 (m), 864 (w), 825 (s), 804 (m), 783 (s), 769 (m), 747 (m), 734 (s), 696 (w), 658 (m), 644 (w), 626 (w), 596 (w), 568 (w), 504 (m), 491 (m). Elemental analysis (%) calcd for [C44H30N4O7Zn2·(C3H7NO)]: C 60.66, H 4.01, N 7.53; found C 60.45, H 3.96, N 7.68.

4.5. Synthesis of the Molecular Cluster [Zn4(q)6(bipy)2(obc)2] (HP4)

A DMF (8 mL) solution of [Zn(q)2]2[tBuZn(OH)]2 (0.090 g, 0.091 mmol) was cooled to −78 °C and mixture of H2obc (0.047 g, 0.182 mmol) and bipy (0.028 g, 0.182 mmol) in DMF (4 mL) was added using a syringe. The reaction mixture was stirred for 10 min while it warmed to room temperature and the resultant suspension was dissolved by addition of DMF (8 mL) and stirred at 60 °C for a few minutes until the precipitate dissolved. The obtained solution was filtered using a syringe filter (pore size 0.45 µm) and concentrated to ~7 mL. The crystallization at 4 °C resulted in the formation of yellow crystals of compound HP4 after two days. The crystalline product was decanted and the solution was removed using a syringe. The product was washed three times with DMF (3 mL). The washed product was dried at room temperature using vacuum pump at 10−3 mbar for 2 h. Isolated yield: 52%. IR (ATR): ṽ [cm−1] = 1666 (m), 1595 (m), 1571 (m), 1535 (w), 1495 (s), 1460 (s), 1407 (w), 1381 (s), 1324 (s), 1280 (m), 1235 (s), 1218 (s), 1176 (w), 1159 (m), 1108 (s), 1096 (m), 1065 (m), 1045 (w), 1032 (m), 1010 (m), 990 (w), 964 (w), 906 (w), 876 (m), 856 (w), 824 (m), 803 (s), 788 (s), 748 (m), 730 (s), 694 (m), 674 (w), 648 (m), 632 (m), 605 (m), 584 (m), 514 (m), 495 (s), 454 (m). Elemental analysis (%) calcd for [C102H70N10O16Zn4·2(C3H7NO)]: C 61.79, H 4.03, N 8.01; found C 61.65, H 3.97, N 8.09.

4.6. Synthesis of [Zn2(q)2(NO3)2(bipy)] (CP5)

Hq (0.100 g, 0,689 mmol) and Zn(NO3)2·6H2O (0.205 g, 0.689 mmol) were dissolved in DMF (3 mL) and the reaction mixture was stirred for few minutes at room temperature, then the mixture of H2obc (0.089 g, 0.344 mmol) and bipy (0.054 g, 0.344 mmol) in DMF (3 mL) was added using syringe. The crystallization at room temperature resulted in formation yellow crystals of compound CP5 after two days. The crystal product was decanted and the solution was removed using a syringe. The product was washed three times with DMF (2 mL). The washed product was dried at room temperature using vacuum pump at 10−3 mbar for 1 h. Isolated yield: 46%. IR (ATR): ṽ [cm−1] = 1666 (m), 1609 (m), 1580 (m), 1538 (w), 1495 (m), 1464 (s), 1418 (m), 1382 (s), 1322 (s), 1298 (s), 1276 (s), 1242 (m), 1229 (m), 1177 (w), 1136 (w), 1108 (s), 1077 (m), 1048 (w), 1031 (m), 1013 (m), 980 (w), 962 (w), 911 (w), 869 (w), 822 (s), 802 (m), 787 (m), 747 (m), 733 (s), 661 (m), 639 (s), 608 (m), 594 (m), 573 (m), 510 (m), 501 (s), 468 (m). Elemental analysis (%) calcd for [C28H20N6O8Zn2·(C3H7NO)]: C 48.21, H 3.52, N 12.70; found C 48.08, H 3.47, N 12.83.

4.7. Single-Crystal X-ray Diffraction Studies

The crystals were selected under Paratone-N oil, mounted on nylon loops and positioned in the cold stream of the diffractometer. The X-ray data for complex CP1 were collected on a Nonius Kappa CCD diffractometer (Billerica, MA, USA) using graphite monochromated MoKα radiation (λ = 0.71073 Å). The data were processed with DENZO and SCALEPACK (HKL2000 package) [57]. The X-ray data for complexes CP2, CP3, HP4, and CP5 were collected at 100(2) K on a SuperNova diffractometer (Agilent, Santa Clara, CA, USA) using MoKα radiation (λ = 0.71073 Å). The data were processed with CrysAlisPro [58]. Structures were solved by direct methods and refined using SHELXL-2016/4 [59]. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were added to the structure model at geometrically idealized coordinates and refined as riding atoms.

For compounds CP2, CP3 and HP4 we observed significant residual electron densities within the pores from the solvent molecule. The solvent molecules appeared to be highly disordered and it was difficult to reliably model their positions and distribution, therefore, the SQUEEZE function of PLATON [60] was used to eliminate the contribution of the electron density in the solvent region from the intensity data. See Supporting Information for more details on the SCXRD experiments and data refinement.