Dimensionality Control in Crystalline Zinc(II) and Silver(I) Complexes with Ditopic Benzothiadiazole-Dipyridine Ligands

Three 2,1,3-benzothiadiazole-based ligands decorated with two pyridyl groups, 4,7-di(2-pyridyl)-2,1,3-benzothiadiazol (2-PyBTD), 4,7-di(3-pyridyl)-2,1,3-benzothiadiazol (3-PyBTD) and 4,7-di(4-pyridyl)-2,1,3 benzothiadiazol (4-PyBTD), generate ZnII and AgI complexes with a rich structural variety: [Zn(hfac)2(2-PyBTD)] 1, [Zn2(hfac)4(2-PyBTD)] 2, [Ag(CF3SO3)(2-PyBTD)]23, [Ag(2-PyBTD)]2(SbF6)24, [Ag2(NO3)2(2-PyBTD)(CH3CN)] 5, [Zn(hfac)2(3-PyBTD)] 6, [Zn(hfac)2(4-PyBTD)] 7, [ZnCl2(4-PyBTD)2] 8 and [ZnCl2(4-PyBTD)] 9 (hfac = hexafluoroacetylacetonato). The nature of the resulting complexes (discrete species or coordination polymers) is influenced by the relative position of the pyridyl nitrogen atoms, the nature of the starting metal precursors, as well as by the synthetic conditions. Compounds 1 and 8 are mononuclear and 2, 3 and 4 are binuclear species. Compounds 6, 7 and 9 are 1D coordination polymers, while compound 5 is a 2D coordination polymer, the metal ions being bridged by 2-PyBTD and nitrato ligands. The solid-state architectures are sustained by intermolecular π–π stacking interactions established between the pyridyl group and the benzene ring from the benzothiadiazol moiety. Compounds 1, 2, 7–9 show luminescence in the visible range. Density Functional Theory (DFT) and Time Dependent Density Functional Theory (TD-DFT) calculations have been performed on the ZnII complexes 1 and 2 in order to disclose the nature of the electronic transitions and to have an insight on the modulation of the photophysical properties upon complexation.

We report herein a first combined coordination chemistry study of the three ditopic ligands 2-PyBTD, 3-PyBTD and 4-PyBTD (Scheme 1) towards zinc(II) and silver(I) based fragments with the objective to explore the topicity of the ligands and the dimensionality of the resulting complexes, for which the crystal structures are described in detail, with a special focus on the supramolecular interactions in the solid state. Moreover, the photophysical properties of the Zn II complexes are reported and compared to those of the ligands.

Materials and Methods
The reagents employed were purchased from commercial sources and used without further purification.
[Zn(hfac)2(3-PyBTD)] 6: A solution of 0.0041 mmol (0.0012g) 3-PyBTD in 4 mL CHCl3 is added to a solution containing 0.0081 mmol [Zn(hfac)2(H2O)2]•H2O (0.0042 g) in 2 mL of methanol. The mixture is stirred for 2 h and then filtered. Yellow single crystals of 6 are formed in few days by slow evaporation of the solvent (0.0013 g, yield 42%). Selected IR data (KBr, cm [ZnCl2(4-PyBTD)2] 8: 0.0024g (0.0082 mmol) 4-PyBTD in 4 mL CH2Cl2 and 2 mL methanol were added to a solution of 0.0012 g (0.0088 mmol) of ZnCl2 in 2 mL methanol and stirred for 10 min, when the mixture became opaque. After adding 2 mL DMF and heating at 60 °C for 30 min, the precipitate was solubilized and the light-yellow solution was left to slowly evaporate. Dark-yellow single crystals were formed within a few days (0.0025 g, yield 85%). Selected IR data (KBr, cm [ZnCl2(4-PyBTD)] 9: Light yellow single crystals of 9 were synthesized in the same manner as 8, except that the opaque solution was further stirred for one hour before 2 mL DMF were added, followed by two hours stirring at 60 °C (0.0019 g, yield 54%). Selected IR data (KBr, cm IR spectra (KBr pellets) were recorded on a Bruker Tensor 37 spectrophotometer in 4000 to 400 cm −1 frequencies range. Diffuse reflectance spectra were performed on a JASCO V-670 spectrophotometer. Elemental analysis was carried out on a EuroEa Elemental Analyzer. The fluorescence spectra were carried out on a JASCO FP-6500 spectrofluorimeter.
X-Ray data for compounds 2-6 and 8 were collected on an Agilent Supernova diffractometer with CuKα (λ = 1.54184 Å) and for compounds 1, 7 and 9 on a Rigaku XtaLAB Synergy, Single source at offset/far, HyPix diffractometer using a graphite-monochromated Mo Kα radiation source (λ = 0.71073 Å). The structures were solved by direct methods and refined by full-matrix least squares techniques based on F 2 . The non-H atoms were refined with anisotropic displacement parameters. Calculations were performed using SHELXT and SHELXL-2015 crystallographic software packages [33,34]. A summary of the crystallographic data and the structure refinement for crystals 1-9 is given in Tables S1 and S2.
DFT and TD-DFT calculations have been performed with the Gaussian 09 program [35] using the DFT method with the PBE1PBE functional and the augmented and polarized Ahlrichs triple-zeta basis set TZVP [36]. The full molecular reports, molecular orbitals, electron density differences pictures and calculated spectra have been automatically generated by a homemade Python program, quchemreport [37], based on cclib [38].

Results and Discussion
The ligands 2-PyBTD, 3-PyBTD and 4-PyBTD (Scheme 1) have been synthesized according to the literature procedure described by Yamashita et al. [30]. To compare their coordination modes, the precursor {Zn(hfac)2} (hfac = hexafluoroacetylacetonate) has been used throughout the whole series, thus ensuring, in principle, the preparation of neutral complexes and the coordination of two additional ligands in either cis or trans positions. The coordination ability of the metal center is enhanced thanks to the electron withdrawing effect of the fluorinated hfac ligands [39,40]. Furthermore, while the Zn II ion provides a more rigid system upon coordination, it does not present any d-d transition which could interfere with the ligand based emission. Then, the scope of the study has been extended towards the use of ZnCl2, as it provides tetrahedral ZnCl2L2 complexes with pyridine based ligands [41], at the difference with the {Zn(hfac)2} fragment which favors the formation of octahedral Zn(hfac)2L2 complexes. Finally, silver(I) precursors have been evaluated in coordination with 2-PyBTD in order to take advantage of the propensity of this metal center to adopt various coordination modes as a consequence of the interplay between the coordination flexibility of the 2-PyBTD ligand, that is, chelating vs. divergent ditopic and the coordination behavior of the anionic ligand of the Ag I precursor. In the following discussion we describe in detail the crystal structures of the complexes obtained with each of the three ligands, together with the photophysical properties of the zinc(II) complexes with 2-PyBTD and 4-PyBTD.

Discrete Complexes and Coordination Polymers with the 2-PyBTD Ligand
The reaction between 2-PyBTD and the [Zn(hfac)2(H2O)2]•H2O precursor in a ratio of 1:2 afforded either the mononuclear complex 1 or the binuclear complex 2 depending on the reaction solvent mixture, that is, methanol/chloroform and heptane/methylene chloride, respectively. Complex 1 crystallized in the monoclinic space group P21/c, with one independent complex molecule in the asymmetric unit. The metal center shows octahedral coordination stereochemistry, with the coordination sphere generated by four oxygen atoms (O1-O4) from the hfac − anionic ligands and two nitrogen atoms from a pyridine (N1) and the BTD unit (N2) (Figure 1a, Tables 1 and S3).  The pyridine units of the ligand are cis-trans oriented and are slightly twisted with respect to the BTD plane, with values of 14.1° (PyN1) and 15.3° (PyN4) for the corresponding dihedral angles. The mononuclear units arrange in supramolecular dimers, with opposite chiralities Δ and Λ of the octahedral configuration of Zn II , thanks to π stacking interactions between the uncoordinated pyridine and BTD phenyl rings (Figure 1b).
The binuclear complex 2 crystallized in the monoclinic space group C2/c with half a molecule of complex in the asymmetric unit, the other half being generated by the C2 axis crossing the BTD unit through the S atom located on a special position (Table 1 and Figure  2). Zn-O and Zn-N bond lengths are comparable in the two complexes. However, the Py rings in 2, having now a cis-cis arrangement due to the bis-chelation, are more strongly twisted when compared to 1, as attested by the value of ±26.9° for the Py-BTD dihedral angle. Whereas both metal centers of the binuclear unit show the same Δ or Λ configuration, the complexes form supramolecular chains through offset π-π stacking interactions between the pyridyl rings (centroid-centroid distances = 3.64 Å), with an alternation of Δ and Λ configurations (Figure 2b).
Reaction between 2-PyBTD and AgNO3 provided a 2D coordination polymer with a 2:1 metal to ligand ratio. The compound crystallized in the monoclinic space group P21/c, with one ligand, two silver(I), two nitrate ions and one acetonitrile molecule in the asymmetric unit. The structure of the coordination polymer can be described as follows: the nitrate anions link the silver(I) ions in double chains ( Figure 4a) and 2-PyBTD acts as tridentate ligand, thus connecting the inorganic chains in bidimensional layers (Figure 4b).
The two metal centers are crystallographically inequivalent and present different coordination geometry. Accordingly, Ag1 ions are hexacoordinated and their coordination stereochemistry can be at best described as pentagonal pyramid following the SHAPE analysis [45,46] (Table S5) . The BTD ligand adopts a cis-trans conformation and shows a tridentate coordination mode, connecting two metal centers of the same double chain through a pyridine (N1) and a thiadiazolyl (N2) nitrogen atom and a third silver ion from a neighboring chain through the second pyridine nitrogen atom (N4) (Figure 4c). The two nitrate anions adopt as well different coordination modes, with N5 being bidentate chelate to Ag1 and N6 linking four metal centers. Within a chain π-π stacking contacts establish between pyridine rings (centroidcentroid distance of 3.54 Å) and also short Ag•••Ag distances of 3.55 Å for Ag1•••Ag2 are observed.
The metal nodes within the chains show opposite alternated chirality and the chains are associated in supramolecular layers thanks to π-π stacking between the pyridine (3.50 Å) and benzene (3.69 Å) rings (Figures 5b and S10).

Coordination Polymer [Zn(hfac)2(4-PyBTD)] 7
Compound 7 has been obtained as yellow crystals in a similar manner as compound 6. It crystallized in the triclinic space group P-1, with one independent ligand 4-PyBTD in general position, two Zn(II) ions located on inversion centers and one hfac ligand on each metal center, the other hfac ligands being generated through the inversion centers. At the difference with complex 6, in 7 the pyridine ligands are located in trans positions ( Figure  6). As expected, the coordination geometry around the metal centers is octahedral, with

Coordination Complexes of 4-PyBTD with ZnCl2
The reaction of 4-PyBTD with ZnCl2 afforded either the mononuclear complex [ZnCl2(4-PyBTD)2] 8 or the coordination polymer [ZnCl2(4-PyBTD)] 9 by slightly varying the experimental conditions. The former crystallized in the monoclinic space group P21/c with one complex in general position in the asymmetric unit, while the latter crystallized in the non-centrosymmetric orthorhombic space group P212121, with one 4-PyBTD ligand and one ZnCl2 fragment in the asymmetric unit. In both complexes the coordination geometry around the metal center is tetrahedral, with values of the bond lengths Zn-Cl and Zn-N in the usual range (Tables 6 and S9).   (Figure S11a), and, comparable, much weaker distortions in the coordination polymer 9 (23.2° (N1) and 16.9° (N4)) ( Figure 7). In the structure of 8 a 2D network is established thanks to π-π stacking interactions between pyridine rings, with centroidcentroid distances of 3.79 and 3.78 Å ( Figure S11b). In the coordination polymer 9, the bidentate tecton connects the metal ions in zig-zag chains (Figure 7a). The chains arrange in layers upon π-π stacking interactions between pyridine and benzene rings, with an interplanar distance of 3.89 Å (Figure 7b).
The purity of the different complexes have been checked, when the amounts were sufficient, by elemental analysis and powder X-ray diffraction (PXRD) analysis, by comparing the experimental PXRD diffractograms with those simulated from the single crystal X-ray data (Figures S12-S16).

Photophysical Properties
As mentioned in the Introduction, the Py-BTD ligands are luminescent. Their coordination to metal ions with d 10 configurations such as Zn(II) should provide luminescent complexes with, in principle, an increase of the emission efficiency as a consequence of a more rigid structure [47][48][49][50]. Moreover, emission wavelength could vary because of the different dihedral angles between the pyridine and BTD units, influencing the extension of the π-conjugated system. We have therefore investigated the photophysical properties of the Zn(II) complexes of ligands 2-PyBTD and 4-Py-BTD and compared them with those of the ligands, previously reported [30]. In order to have reliable comparisons between ligands, molecular complexes and coordination polymers, all the measurements have been performed in the solid state.

Complexes 1 and 2
The ligand 2-PyBTD presents in the UV-Vis absorption spectrum a very broad intense band with a maximum at 420 nm and another distinct weaker band at 258 nm encompassing several π-π* transitions [30] (Figure S17). Upon excitation at λex = 400 nm, an intense emission band centered at λem = 523 nm is observed, which, interestingly, is strongly bathochromically shifted when compared to the value λem = 469 nm (λex = 299 nm) measured in acetonitrile solutions [30] (see Figure S18 for spectra measured in CH2Cl2 solutions). Clearly, packing effects have a strong influence on the absorption and emission wavelengths. However, a direct comparison between the photophysical properties of 2-Py-BTD and its zinc(II) complexes 1 and 2 is not straightforward since in the former the pyridine rings adopt a trans-trans conformation [30], while in the complexes the respective conformations are cis-trans and cis-cis.
The Zn(II) ion does not possess any d-d transition, therefore, following its coordination to 2-PyBTD, ligand based π-π* transitions centered at 247, 343 and 440 nm are observed in the UV-Vis spectrum of complex 1 (Figure 8). In its emission spectrum, the band observed at λem = 526 nm upon excitation at λex = 440 nm is much more intense than the one of the free ligand. Coordination of two Zn(hfac)2 fragments induces massive changes in the photophysical properties of complex 2 compared to the free ligand and complex 1. In the diffused reflexion UV-Vis absorption spectrum of 2, three bands at 379, 313 and 226 nm are observed, while the emission spectrum shows an intense band centered at λem = 495 nm upon excitation at λex = 380 nm (Figure 8). The blue-shift of the absorption and emission bands of complex 2 compared to complex 1 can be tentatively explained, on the one hand, by the different conformations adopted by the pyridine rings, that is, cis-cis in 2 and cis-trans in 1 and also by the decrease of planarity in the former system upon coordination of 2-PyBTD to two metal centers. Indeed, in complex 2 the twist between the two pyridine rings amounts to 52.9° compared to 7.3° in 1 and the distortion with respect to the BTD chromophore are 26.9° in 2 and 14.1° and 15.3° in 1 (see Figures 1 and 2). On the other hand, photophysical properties measured in the solid state can be strongly influenced by the intermolecular interactions in the packing. As discussed above, in complex 1 there is formation of supramolecular dimers through π-π stacking interactions between the uncoordinated pyridine and BTD, while in 2 the complexes form supramolecular chains through π-π stacking between the pyridine units (vide supra). The influence of all these factors on the absorption/emission properties is difficult to be disclosed.
In order to have an insight on the nature of the electronic transitions involved in the absorption and emission bands, we have performed DFT and TD-DFT calculations on both complexes 1 and 2 (see the details in the SI). The optimized geometries are in agreement with the experimental ones obtained by X-ray structure analysis. The simulated UV-Vis absorption spectra of complexes 1 and 2 are shown in Figure 9. The first calculated allowed transition in complex 1, occurring at 413 nm (SI), corresponds to a HOMO  LUMO excitation of π-π* type (Figure 10 left), HOMO being based mainly on the pyridine and benzene rings, while LUMO is mainly delocalized on the BTD unit, with a large participation of the thiadiazole ring. The corresponding calculated emission wavelength values amounts to 478 nm, hence blue-shifted compared to the experimental value measured in the solid state but in agreement with the value of 467 nm for the emission in CH2Cl2 solution ( Figure S19). One can thus hypothesize that the red-shift for the absorption/emission bands observed in the solid state is the consequence of intermolecular π-π stacking interactions, all the more since HOMO and LUMO are localized on the π system involved in the corresponding electronic transition. In complex 2 the highest four occupied orbitals, that is, from HOMO-3 to HOMO, are in-phase and out-ofphase combinations of hfac based orbitals, while the LUMO has a similar distribution as in complex 1, namely on the BTD unit, with a large participation of the thiadiazole ring. Consequently, the first four singlet excited states, which are HOMO-n  LUMO excitations (n = 0-3), present very weak oscillator strengths. The first intense absorption band is described by a HOMO-4  LUMO excitation, as shown by the electron density difference (EDD) between the 5th excited state and the ground state (Figure 10 right) and corresponds, as the S1-S0 transition in complex 1, to a charge transfer from the benzene-dipyridine skeleton to thiadiazole. The calculated wavelength value for this absorption is 397 nm, thus slightly blue-shifted compared to 1, yet remaining consistent with the experimental absorption band measured in CH2Cl2 solution occurring at λmax = 387 nm ( Figure  S20). Unfortunately, the optimization of the excited emissive state for complex 2 could not be carried out due to interstate mixing. It should be emphasized once again, that it is not straightforward to directly compare the absorption/emission wavelengths values between ligand 2-PyBTD, complex 1 and complex 2, as they show the three possible conformations trans-trans, cis-trans and cis-cis, respectively. However, as stated above, the blue-shift observed in the solid state for the absorption/emission bands of 2 compared to those of 1 is very likely due to the different π-π stacking interactions involving the aromatic rings responsible for the corresponding electronic transitions.

Complexes 7-9
The UV-Vis spectrum of 4-PyBTD ligand, which provided complexes 7-9, shows the presence of one very broad band with a maximum at 397 nm and an additional weaker band at 260 nm. In the emission spectrum an intense band centered at λem = 472 nm (λex = 370 nm) is observed ( Figure S21), in agreement with a previous report [31]. At the difference with ligand 2-PyBTD (vide supra), a much weaker bathochromic shift is observed now for the solid state emission of 4-PyBTD compared to the emission band in acetonitrile solution occurring at λem = 448 nm (λex = 299 nm) [30].
Coordination of Zn(hfac)2 fragments to provide the coordination polymer 7 is accompanied by a strong bathochromic shift of the luminescence, as witnessed by the emission band centered at 527 nm (λex = 460 nm) and an increase of the emission intensity compared to the free ligand ( Figure 11). A similar bathochromic shift has been observed for a mixed coordination polymer of Zn(II), 4-PyBTD and isophthalic acid [31].  Figure S22 for the absorption spectra), respectively, comparable to the emission wavelength of the free ligand (vide supra). The excitation spectra of both complexes overlap with the corresponding wavelength regions of the absorption spectra. The absence of bathochromic shift of the luminescence in the case of complexes 8 and 9 when compared to complex 7, might have its origin, besides the different molecular packing in the solid state, in the difference of molecular orbital levels involved in the emission, as a consequence of the strong electron withdrawing effect exerted by the hfac ligands with respect to the chloride ligands which have a π-donating effect.

Conclusions
4,7-dipyridyl-2,1,3-benzothiadiazol ligands 2-PyBTD, 3-PyBTD and 4-PyBTD have been used to prepare coordination complexes with d 10 transition metals group in order to take advantage of their flexible topicity and luminescence properties. Compounds 1-5 with 2-PyBTD and 6 with 3-PyBTD represent the first reported complexes based on these ligands. While ligand 2-PyBTD showed chelating behavior towards zinc(II) and silver(I) ions, involving coordination by the pyridine and thiadiazole nitrogen atoms, in the zinc(II) complexes based on 3-PyBTD and 4-PyBTD the ligands act as bridges through the pyridine nitrogen atoms to provide the coordination polymers 6, 7 and 9. Moreover, the mononuclear complex [ZnCl2(4-PyBTD)2] 8 has been prepared as well, as a brick model for the coordination polymer 9. In the crystal structures of all the complexes the occurrence of supramolecular interactions directing the overall architecture has been thoroughly discussed. A very peculiar coordination behavior of 2-PyBTD has been revealed in the coordination polymer 5 with silver nitrate, where the ligand shows a tridentate coordination mode leading to the formation of an original 2D structure. A striking difference has been observed between 3-PyBTD and 4-PyBTD towards the Zn(hfac)2 fragments in complexes 6 and 7, since in the former the pyridine nitrogen atoms are in cis configuration, while in the latter they coordinate the metal ion in axial positions. Solid state photophysical properties of the zinc(II) complexes of ligands 2-PyBTD and 4-PyBTD indicate an enhancement of the emission intensity when compared to the free ligands. The emission wavelength shows modulation with the nuclearity of the complexes, that is, blue-shift in 2 compared to 1 very likely because of the different intermolecular π-π stacking interactions, and with the coordinated fragment, that is, red-shift in 7 compared to 8 and 9. DFT calculations on complexes 1 and 2 shed light on the nature of the electronic transitions involved in the low energy absorption band responsible for the emissive properties. This first coordination chemistry study of PyBTD with d 10 metals clearly highlight the interest of these ligands for the access to multifunctional ligands and open the way towards the preparation of whole series of complexes. Particularly attractive, for example, is the use of 2-PyBTD ligand as bridge between paramagnetic metal centers where the magnetic interaction can be mediated by the oxidation state of the ligand [51], when considering the possibility to reduce the BTD unit into radical anion species. These directions are currently explored in our groups.