Two-step synthesis of heterometallic coordination polymers using a polyazamacrocyclic linker †

A new macrocyclic linker 1,4,7,10-tetraazacyclododecane-1,7-bis(4-acetamidobenzoic)-4,10-diacetic acid (H4L1) was synthesized and characterized. This linker was used to create two heterometallic coordination polymers following a two-step synthesis. This synthesis consisted of first combining this polyazamacrocyclic linker with Ni(II) or Mn(II) ions to obtain the corresponding metallomacrocyclic complexes showing non-coordinated carboxylic groups. In a second step, these metallated macrocycles were used as building units to construct two heterometallic Ni(II)–Zn(II) and Mn(II)–Zn(II) coordination polymers when combined with Zn(II) ions. In addition, a third Zn(II)–Zn(II) coordination polymer could also be synthesized by direct mixing of H4L1 with Zn(II) ions. Interestingly, the Mn(II)–Zn(II) coordination polymer exhibits a reversible type-I “crystal-to-amorphous transformation” upon water sorption/desorption.


Introduction
Coordination polymers have become an in-depth thread of study during the last few decades mainly because of their myriad potential properties and applications. [1][2][3] In this field, the strategic design of the organic linkers to be used (known as the linker design approach) has proven critical for tuning the composition and structure of coordination polymers and thus, for achieving the desired functionality for a target application. [4][5][6] For example, Kitagawa et al. designed a luminescent porous coordination polymer-based sensor by using a dipyridyl derivative of the photoactive 1,4,5,8-naphthalenediimide. 7 The material showed strong shifting on the emitted wavelength when different volatile organic compounds were adsorbed on the framework. In a similar way, Zhou et al. designed a porous coordination polymer with lightcontrolled reversible alteration of its adsorbing properties through the design of a photosensitive linker. 8 The continuous investigation of new organic linkers is thus essential for accessing to novel coordination polymers with enhanced performances in numerous applications as, for example, in catalysis, gas sorption, separation, sensors, pollutant removal, and antibacterials.
Polyazamacrocycles (cyclen and cyclam derivatives) are a family of heteromolecules with high affinity to chelate metal ions in their cavities thanks to the preorganization of their binding sites and their kinetic and thermodynamic stability induced by the macrocyclic effect. 9,10 Because of this affinity, these macrocyclic ligands have already shown promise in different fields ranging from contrast agents 11 and catalysis 12,13 to sensors. 14 Recently, the functionalization of the reactive amine positions of polyazamacrocycles with pendant arms incorporating functional groups (e.g. carboxylates, amides, and phosphonates) [15][16][17][18] has opened the possibility to use them as multitopic organic linkers. These organic linkers have the potential to coordinate a metal ion inside the inner cavity and different ones through these pendant arms, extending the framework and thus forming heterometallic coordination polymers. However, their use as linkers for the synthesis of coordination polymers is up to date limited; only a few examples of homometallic, [19][20][21][22][23][24][25][26] and even less of heterometallic 27 coordination polymers are found in the literature. This scarcity of structures can be attributed to their high degree of flexibility that difficult obtaining suitable crystals for single-crystal X-ray diffraction, to their sensitivity to harsh reaction media, and to the tendency of their pendant arms to coordinate the metal ions within the macrocyclic cavity, 28 thereby promoting the generation of mononuclear compounds rather than coordination polymers.
Herein, we present the stepwise synthesis of heterometallic coordination polymers using polyazamacrocycles as organic linkers. Our strategy consists on first preparing complexes of the polyazamacrocycle 1,4,7,10-tetraazacyclododecane-1,7-bis(4acetamidobenzoic)-4,10-diacetic acid (H 4 L1) with Mn(II) and Ni(II) ions, and then using these metallated macrocycles as secondary building units to create heterometallic coordination polymers. We show that the coordination of these Mn(II)/Ni(II) building units showing free carboxylic groups with Zn(II) ions allows the synthesis of two bimetallic Zn(II)-Mn(II) and Zn(II)-Ni(II) coordination polymers (Scheme 1). We also show that a Zn(II)-Zn(II) coordination polymer could be synthesized by direct mixing of H4L1 with Zn(II) ions. Interestingly, the Mn(II)-Zn(II) coordination polymer shows a reversible Type-I "crystal-to-amorphous transformation" upon water sorption/desorption. 29
DO2AtBu (a.k.a. di-tert-butyl 2,2'-(1,4,7,10tetraazacyclododecane-1,7-diyl)diacetate) was purchased from CheMatech. All solvents were purchased from Fisher Scientific and Scharlab. Solvents and reagents were used as received unless otherwise stated. 1 H and 13 C NMR spectra of intermediates 1 and 2 were recorded on a Bruker Avance DPX-360 spectrometer operating at 360 and 90 MHz, respectively. 1 H and 13 C NMR spectra of H4L1 were recorded on a Bruker Avance III 400 SB operating at 400 and 100 MHz respectively. Samples were dissolved in CDCl3 or D2O after purification. Mass spectra were acquired on a micrOTOF-QII ESI-MS instrument. Purity of all bulk material batches was confirmed by Xray powder diffraction (XRPD) patterns collected on an X'Pert PRO MPD analytical diffractometer (Panalytical) at 45 kV, 40 mA using Cu Kα radiation (λ 1.5419 Å), and compared with single crystal simulated patterns. Thermogravimetric analyses were performed under nitrogen flow using a STA 449 F1 Jupiter-Simultaneous TGA-DSC from NETZSCH with a heat rate of 5°C/min. IR spectra were recorded in transmission mode on a Bruker Tensor 27FTIR equipped with a Golden Gate diamond ATR cell. Gravimetric water sorption measurements were performed at 298 K using a flow of nitrogen, up to relative humidity of 90 % with a DVS-Advantage-1 (Surface Measurements Systems). Elemental Analysis measurements were performed on a Flash EA 2000 CHNS, Thermo Fisher Scientific analyser.

Synthesis of the linker Scheme 1.
Illustration showing the two-step strategy followed to synthesize the heterometallic coordination polymers 6 and 7.

Synthesis of discrete complexes Synthesis of [Mn(H1L1)]·[Na(H2O)6]·4H2O (3).
A stirred mixture of H4L1 (1.0 g, 1.56 mmol) in water (100 mL) was basified to pH = 8.0 with 1M sodium hydroxide. Then, manganese chloride (195.8 mg, 1.56 mmol) in water (2 mL) was added dropwise into the 4 | Table 1. Crystallographic data of H4L1, discrete complexes 3 and 4; and coordination polymers 5, 6 and 7. previous solution. After this, the pH of the stirred mixture was readjusted again to 8.0, and the mixture was left at room temperature under stirring for 18 h. The pH of the suspension was again adjusted to pH = 12 to precipitate the excess of manganese chloride in the form of insoluble manganese hydroxides. The mixture was filtered and the solids discarded. The filtrate was then acidified to pH = 3 with 1 M hydrochloric acid to precipitate 3. The precipitated complex was centrifuged, the solvent was exchanged by fresh deionized water, and the solid was resuspended. This process was repeated twice. Finally, the suspension was filtered and the pale yellow solid was air-dried. Yield: 1.

X-Ray Crystallography
Refinement and structural information is shown in Table 1. Crystallographic data for 3-5 were collected at 293 K at I19 beamline at Diamond synchrotron ( = 0.6889 Å). These data were indexed, integrated and scaled using CrystalClear software from Rigaku group programs. Absorption correction was performed by Multi-Scan method giving max./min. transmission factors of 1.000/0.923 (3), 1.000/0.654 (4), and 1.000/0.448 (5). Crystallographic data for 6 and 7 were collected at 100 K at XALOC beamline at ALBA synchrotron ( = 0.71049 Å). These data were indexed, integrated and scaled using the XDS and IMOSFLM programs. 30 Absorption correction was not applied. Crystallographic data for H4L1 was collected at 293 K on a Bruker AXS SMART Apex diffractometer using graphite monochromated Mo-K radiation (= 0.71073 Å), and it was corrected for Lorentz and polarisation effects. The frames were integrated with the Bruker SAINT software package. Absorption corrections were applied using SADABS giving max./min. transmission factors of 1.000/0.270. The structures were solved by direct methods and subsequently refined by correction of F2 against all reflections using SHELXS2013 and SHELXL2013 within the WinGX package. 31 All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix least-squares calculations on F2 using SHELXL2013. Hydrogen atoms were inserted at calculated positions and constrained with isotropic thermal parameters. The hydrogen of the different hydratation water molecules present in all six structures were not located on Fourier map, but were added in the empirical formula to allow accurate determination of density and absorption coefficient. The structure of 3 contains disordered [Na(H2O)6] + . Two sites were identified corresponding to disordered partial-occupancy sodium atoms (one in special position) and were refined with occupancy factors of 0.75/0.25. Due to this disorder, the oxygen atoms coordinated to these sodium ions were located and refined with different occupancy factors to correlate the different occupancies to the presence of this [Na(H2O)6] + . CCDC 1443836-1443841.

Crystal structures of H4L1 and complexes 3 and 4 H 4 L1·8H 2 O. Suitable crystals of H 4 L1
for single-crystal X-ray diffraction were obtained by diffusion of acidic vapours into a solution of H 4 L1 (5 mg/mL) in water placed in the fridge at 5 °C for one week. The structure was solved and refined in the monoclinic space group C2/c. The asymmetric unit was found to contain one half of the macrocyclic H 4 L1 together with four water molecules; being the remaining macrocyclic atomic positions generated by a twofold rotation axis piercing through the central cavity (Figure 1a). Charge balance considerations and the location of residual Fourier peaks suggest that H4L1 is protonated on two of the four central nitrogen atoms (N2 and N2#) and on two of the four carboxylic moieties (O2 and O2#). The macrocycles H4L1 are linked by hydrogen-bonded interactions between them and water molecules to form 1D chains extending along the ac plane ( Figure S9, ESI †). Water molecules act as connectors between two NH of the amide groups (O1W···H-N3, 2.156 (7)

[Mn(H1L1)]·[Na(H2O)6]·4H2O (3).
Suitable crystals for single-crystal X-ray diffraction were obtained by slow diffusion of acid vapours into a basic solution of 3 in water (5 mg/mL, pH = 8.5) at 5 °C for one week. Analysis of these crystals by single crystal X-ray diffraction gave a structure model in the monoclinic space group P21/c, where the asymmetric unit was found to contain one L1 macrocycle coordinated to a single Mn(II) ion ( Figure 1b) and one disordered [Na(H 2 O) 6 ] + counterion. The Mn(II) ion is placed within the macrocyclic cavity in a square antiprismatic molecular geometry. The square antiprism has one plane defined by the four aza nitrogen atoms from the macrocyclic core, and the other plane defined by O1 and O6 of the amide groups and O4 and O9 of the acetate pendant arms. Mn(II) ion lies 1.324(0) Å above the plane formed by the four aza nitrogen atoms and 1.219(3) Å below the plane defined by the four oxygen atoms. The presence of a Na cation means that 3 must negatively charged. By comparing the C-O distances of both benzene carboxylic acid groups, it can be noted that C28-O7 distance (1.319 (7) Figure S10, ESI + ). In these layers, two equivalent protonated benzenecarboxylic aromatic rings also form an offset face-to-face - interaction with a distance of 3.701(6) Å from centroid to centroid.
[Ni(H2L1)]·2H2O (4). Crystals of 4 suitable for single-crystal X-ray diffraction were obtained by slow diffusion of acid vapours into a basic solution of 4 in water (5 mg/mL, pH = 8.5) at 5 °C for one week. The structure was solved and refined in the monoclinic space group C2/c, where the asymmetric unit was found to contain one half of the complex. L1 coordinates to a single Ni(II) ion within the macrocyclic cavity in a highly distorted octahedral geometry (Fig.  1c). In this case, the four nitrogen atoms are far from being coplanar, laying at 0.397(4) Å from the average plane. The coordination sphere of Ni(II) is formed by the four aza nitrogen atoms and two equivalent O4 atoms of the acetate pendant arms. Contrary to 3, the oxygen atoms of the amide groups are not involved on any coordination bond, leaving both pendant arms free to rotate. The absence of any counterion and the symmetry equivalence between the two amide pendant arms indicate that both benzene carboxylic acid groups are protonated. These complexes are linked by hydrogen bonds to form layers along the ac plane ( Figure S11 Crystal structures of coordination polymers [Zn(ZnL1)H2O]·3H2O (5). 5 crystallises in the monoclinic space group P21/n forming a one-dimensional coordination polymer. Its asymmetric unit contains two Zn(II) ions exhibiting two different coordination environments. Zn1 is chelated within the macrocyclic cavity through the four aza nitrogen atoms, two acetate (O4 and O9) pendant arms and O1 of the amide pendant arm in a capped trigonal prism molecular geometry (Figure 2a). Zn2 adopts a distorted square pyramidal geometry coordinated to two benzene carboxylate groups in a bidentate (O7 and O8) and monodentate (O2) fashion, one acetate group (O5) that bridges Zn1 and Zn2, and one water molecule (Figure 2b). The structure of 5 is constructed by the connection of Zn2 ions through three different L1 linkers, forming a double-strand chain in a zigzag conformation that extends along the b axis (Figures 2c,d). The double-strand chains are then connected to each other along the a direction through hydrogen bonds (Figure 2d and Fig S12, ESI †). These hydrogen [Zn(NiL1)H2O]·7H2O (6). 6 also crystallises in the monoclinic space group P21/n, forming a one-dimensional coordination polymer. Its asymmetric unit contains a slightly modified building unit 4 (only the benzene rings are slightly rotated) and one Zn(II) ion connecting these units. As in 4, Ni(II) ion shows a distorted octahedral geometry coordinated by the four aza nitrogen atoms and O4 and O9 of two acetate pendant arms (Figure 3a). On the other hand, Zn(II) ion is tetrahedrally coordinated to two benzene carboxylate groups (O3 and O7) acting both in a monodentate fashion, one O5 of the acetate pendant arm and one water molecule (Figure 3b). In this structure, each Zn(II) ion connects three different nickelated L1 units creating a double strand chain extending along the [110] and [-110] directions (Figures 3c,d). The different chains form a crosslinked structure in which the water molecule O2W acts as the connector between two different double-strand chains, forming hydrogen bonds with one of the amide groups (O2W···H-N6, 2.023(3) Å; O2W-H6N, 170.46(6)°) and one oxygen atom from the benzene carboxylate moiety (O2W···O2, 2.818(1) Å; O2W-O2-C17, 105.58(0)°). Also, the coordinated water molecule forms a hydrogen bond with the acetate pendant arm (O1W···O10, 2.591(0) Å; O1W-O10-C30, 140.07(4)°).
[Zn(MnL1)H2O]·9H2O (7). 7 crystallises in the monoclinic space group C2\c, forming a two-dimensional coordination polymer. The structural model shows a modified building unit 3 and one Zn(II) connecting these units. Indeed, if we compare the initial conformation of the building unit 3 with that found in 7, a clear difference is that the Mn(II) ion has changed its coordination number from eight to seven. This change leaves one amide pendant arm free to be accommodated in an extended position and thus, facilitates the coordination of Zn(II) ions with the two benzene carboxylic acid groups. In 7, the Mn(II) ion adopts a capped trigonal prismatic geometry coordinated to the four aza nitrogen atoms, O4 and O9 atoms of two acetate pendant arms and O1 of the amide pendant arm (Figure 4a). Similar to 6, Zn(II) ion is tetrahedrally coordinated to two benzene carboxylate groups (O2 and O7) acting both in a monodentate fashion, one O5 of the acetate pendant arm and one water molecule (Figure 4b). The coordination structure is built up from connecting each Mn(II)-L1 unit through three different Zn2 ions (Figure 4c), forming corrugated layers that grow along the (-101) plane. The different layers are packed in an ABAB sequence connected via a water molecule, which directly connects these layers through two hydrogen bond interactions involving one amide pendant arm (O4W···H-N5, 2.130 (5)    that run parallel to the plane (-101).

Structural integrity and water adsorption of 7.
The high content of water molecules in the framework of 7 prompted us to investigate its behaviour upon water sorption /desorption. Initially, we performed a thermogravimetric analysis (TGA) on 7, showing a first weight loss of 21.2 % from 20 to 120 °C. This loss was attributed to the loss of all water molecules (19.2 %) according to the molecular formula [Zn(MnL1)H2O]·9H2O ( Figure  5a). A second weight loss was found in the temperature range of 340-470 °C corresponding to the decomposition of the framework.
To evaluate the water-vapour sorption properties, 7 was first activated at 120 °C overnight under vacuum, and the activated sample (7') was characterized by TGA and PXRD (Figures 5a,b). TGA revealed the absence of any weight loss from 20 to 340 °C, confirming that 7 ' was free of guest water molecules. On the other hand, the experimental PXRD showed that 7 ' becomes an amorphous material after evacuating the water molecules. This behaviour is not unexpected as layers of 7 are three-dimensionally packed via hydrogen-bonded water molecules, so that their removal is likely inducing their disordering and loss of crystallinity. Remarkably, the framework of 7 was recovered when 7' was reimmersed in water or exposed to water vapors (Figure 5b), as confirmed by X-ray powder diffraction (XRPD) patterns. This reversible behavior is typical of a Type-I "crystal-to-amorphous transformation" as described by Kitagawa et al. 29 Dynamic water sorption measurements performed on 7' showed an adsorption branch with a sigmoidal shape representative of a type II trend. 32 The water uptake reaches a plateau at a relative humidity (RH) of 60 %, achieving an uptake of 0.23 g H2O / g 7'. The maximum water uptake was 0.26 g H2O / g 7' at a RH of 90 %. This weight increase can be attributed to 10 water molecules (calculated on the basis of the dehydrated formula, theoretical = 24 %), in agreement with TGA results and the initial hydrated crystal structure. In addition, we observed hysteresis loop between the adsorption and desorption branches, from which a 5.1 % in weight was found that is not desorbed. This percentage can be attributed to two water molecules (calculated on the basis of the dehydrated formula, theoretical = 4.7 %) that cannot be desorbed without heating this material. We can tentatively ascribe these two water molecules to those that are more strongly bonded in the framework of 7; that is, O1W that is coordinated to the Zn(II) ions (Figure 4b), and O4W that is the responsible of three-dimensionally connecting the coordination layers through several hydrogen bond interactions ( Figure S14, ESI †). In fact, when performing an XRPD of the sample resulting from the desorption process, we could observe a certain degree of crystallinity, thus confirming that this sample shows a certain ordering between layers ( Figure S15, ESI †).
We then performed six water sorption/desorption cycles by  alternatively exposing the outgassed material to humid (60 % RH) and dry (2 % RH) environments (Figure 5d). Remarkably, the maximum uptake at 60 % RH (ca. 23 %) was not significantly modified with the number of cycles, confirming the stability of the material to water sorption/desorption processes. Furthermore, it should be noted that the 5 % in weight of water molecules that remains in the material after desorption reduces its working capacity. However, as counterpart, it makes the cycles narrower, meaning that this slightly crystalline material shows a faster response to water sorption than its amorphous analogue 7'.

Conclusions
In conclusion, we have synthesised a tetraazacyclododecane-based linker H4L1, and demonstrated its versatility in the formation of two discrete Mn(II) and Ni(II) chelates showing free carboxylic groups. These complexes were further used, in a second step, as metallated building units to create two heterometallic Zn(II)-Ni(II) (6) and Zn(II)-Mn(II) (7) coordination polymers. In addition, a third homometallic Zn(II)-based coordination polymer (5) was synthesized by direct mixing H4L1 with Zn(II) ions. 5 and 6 show extended double strand chains stacked to each other through hydrogen bonds, whereas 7 forms corrugated layers packed in an ABAB fashion also containing independent H-bonded networks of water molecules. Interestingly, this coordination polymer exhibits a reversible Type-I "crystal-to-amorphous transformation" upon water sorption/desorption, a plateau uptake of 23 % in weight at 60 % RH, and is stable to at least six water sorption/desorption cycles.