Syntheses, crystal structures and thermal properties of catena-poly[cadmium(II)-di-μ-bromido-μ-pyridazine-κ2 N 1:N 2] and catena-poly[cadmium(II)-di-μ-iodido-μ-pyridazine-κ2 N 1:N 2]

In the crystal structures of the title compounds, the cadmium cations are octahedrally coordinated by four halide anions and two pyridazine ligands in a trans-CdX 4N2 (X = Br, I) arrangement and are linked into chains by the halide anions and the pyridazine ligands.

The reactions of cadmium bromide and cadmium iodide with pyridazine (C 4 H 4 N 2 ) in ethanol under solvothermal conditions led to the formation of crystals of [CdBr 2 (pyridazine)] n (1) and [CdI 2 (pyridazine)] n (2), which were characterized by single-crystal X-ray diffraction. The asymmetric units of both compounds consist of a cadmium cation located on the intersection point of a twofold screw axis and a mirror plane (2/m), a halide anion that is located on a mirror plane and a pyridazine ligand, with all atoms occupying Wyckoff position 4e (mm2). These compounds are isotypic and consist of cadmium cations that are octahedrally coordinated by four halide anions and two pyridazine ligands and are linked into [100] chains by pairs of -1,1-bridging halide anions and bridging pyridazine ligands. In the crystals, the pyridazine ligands of neighboring chains are stacked onto each other, indicatinginteractions. Larger amounts of pure samples can also be obtained by stirring at room-temperature, as proven by powder X-ray diffraction. Measurements using thermogravimetry and differential thermoanalysis (TG-DTA) reveal that upon heating all the pyridiazine ligands are removed in one step, which leads to the formation of CdBr 2 or CdI 2 .

Chemical context
Coordination polymers based on transition-metal halides show a versatile structural behavior and can form networks of different dimensionalities (Peng et al., 2010). This is especially valid for compounds based on Cu I , which show different CuX substructures (X = Cl, Br, I) such as, for example, dimeric units, chains or layers that can be additionally connected by bridging neutral coligands (Peng et al., 2010). These compounds are of additional interest because of their luminescence behavior (Gibbons et al., 2017;Mensah et al., 2022). For one particular metal halide and coligand, compounds of different stoichiometry are frequently observed. In most cases they were synthesized in the liquid state, but in some cases the coligand-deficient phases cannot be obtained from solution or are obtained only as mixtures with coligand-rich phases.
We have been interested in the structural properties of such compounds for several years and have found that upon heating most of the coligand-rich compounds lose their coligands stepwise and transform into new coligand-deficient compounds that show condensed copper-halide networks (Nä ther & Jess, 2004;Nä ther et al., 2001Nä ther et al., , 2007. The advantage of this method is the fact that this reaction is irreversible, and that the new compounds are obtained in quantitative yields. Moreover, in some cases, metastable polymorphs or isomers can also be obtained  and this method can also be used for the synthesis of new coordination polymers with other bridging anionic ligands such as, for example, thioor selenocyanates (Werner et al., 2015;Wriedt & Nä ther, 2010).
We subsequently found that transition-metal halide compounds with twofold positively charged cations such as Cd II that also show a pronounced structural variability can be obtained by this route (Nä ther et al., 2017;Jess et al., 2020). In most cases, discrete CdX 2 complexes are observed (Ghanbari et al., 2017;Liu, 2011), but these units can also condense into dinuclear (Santra et al., 2016;Xie et al., 2003) and tetranuclear units (Zhu, 2011) or polymers (Nezhadali Baghan et al., 2021;Satoh et al., 2001), where the latter can be further linked by the coligands into layers (Hu et al., 2009;Marchetti et al., 2011).
In this context, we have reported on CdX 2 coordination polymers with 2-chloro and 2-methylpyrazine with the composition CdX 2 (L) 2 with X = Cl, Br, I and L = 2-chloro or 2-methylpyrazine). These compounds consists of CdX 2 chains in which the Cd cations are linked by two pairs of -1,1bridging halide anions (Nä ther et al., 2017). Surprisingly, upon heating, the compounds with 2-chloropyrazine lose all the coligands in one single step, whereas decomposition of the 2-methylpyrazine compounds leads to the formation of compounds with the composition CdX 2 (2-methylpyrazine), in which the CDX 2 chains are linked into layers by the 2-methylpyrazine ligands. These compounds can also be obtained if the discrete complex CdI 2 (2-methylpyrazine) 2 (H 2 O) is thermally decomposed. In further work we investigated similar compounds with 2-cyanopyrazine as coligand, where we observed a different thermal reactivity as a function of the nature of the halide anions (Jess et al., 2020).
In the course of our investigations we also became interested in compounds with pyridazine as coligand. A search in the CCDC database revealed that several transition-metal halide coordination compounds with this ligand have already been reported in the literature (see Database survey). With cadmium, one compound with the composition CdCl 2 (pyridazine) is reported, in which the Cd II cations are linked by -1,1-bridging chloride anions into chains, in which each two Cd II cations are additionally connected by the pyridazine ligands (Pazderski et al., 2004a). As this compound is isotypic to many other MX 2 (pyridazine) coordination compounds, one can assume that this structure represents a very stable arrangement. On the other hand, compounds with this composition have also been reported with ZnX 2 . In contrast to the bromide and iodide compounds, the chloride analog crystallizes in three different modifications, which indicates that the structural behavior also depends on the nature of the halide anion (Bhosekar et al., 2006a,b;Pazderski et al., 2004b;Bhosekar et al., 2007). Moreover, even if in the majority of compounds Nezhadali acts as a bridging ligands, some examples have been reported in which this ligand is coordinated to metal cations with only one of the two N atoms, thereby forming discrete complexes, which also include transition-metal halide complexes (Handy et al., 2017;Boeckmann et al., 2011;Laramé e & Hanan, 2014;Yang, 2017;Harvey et al., 2004).
Based on all these findings, we reacted CdBr 2 and CdI 2 in different molar ratios with pyridazine in several solvents to investigate whether compounds with a different ratio between CdX 2 and pyridazine can be prepared, which also might include pyridazine-rich discrete complexes that upon heating might transform into new compounds with a more condensed network. However, independent of the reaction conditions and the stoichiometric ratio, we always obtained the same crystalline phases, as proven by powder X-ray diffraction (PXRD). Crystals of both compounds were obtained at elevated temperatures and structure analysis proves that compounds with the composition CdBr 2 (pyridazine) (1) and CdI 2 (pyridazine) (2) were obtained. Comparison of the experimental PXRD patterns with those calculated from the results of the structure determinations, prove that both compounds were obtained as pure phases (Figs. S1 and S2). Measurements using thermogravimetry and differential thermoanalysis reveal that both compounds decompose in one step, which is accompanied with an endothermic event in the DTA curve (Figs. S3 and S4). The experimental mass losses of 22.9% for 1 and 18.1% for 2 are in good agreement with those calculated for the removal of one pyridazine ligand (Ám calc. = 22.7% for 1 and 17.9% for 2), indicating that CdBr 2 and CdI 2 , respectively, have formed.
In this context, it is noted that the formation of a more pyridazine-deficient compound with a more condensed network is not expected, because for M 2+ cations, the network should be negatively charged. This is impossible in this case, but it is noted that one compound with CdCl 2 and a more condensed metal-halide network is reported in the literature (Jin et al., 2014).

Structural commentary
The reaction of cadmium dibromide or cadmium diiodide with pyridazine leads to the formation of crystals of CdBr 2 (pyridazine) (1) and CdI 2 (pyridazine) (2). Both compounds are isotypic to their CdCl 2 analog already reported in the literature (Pazderski et al., 2004a). In this context, it is noted that for compound 2 a pseudo-translation along the crystallographic baxis is detected, leading to half of the unit cell and space group Cmmm but the refinement clearly shows that the present unit cell and space group is correct (see Refinement). Both compounds are also isotypic to a number of other metalhalide coordination polymers, indicating that this is a very stable arrangement (see Database survey).
The asymmetric units of compound 1 and 2 consist of a cadmium cation located on the intersection point of a twofold screw axis and a mirror plane (Wyckoff site 4c, symmetry 2/m), as well as a bromide or iodide anion lying on a mirror plane (Wyckoff site 8h) and a pyridazine ligand, with all atoms located on Wyckoff position 4e (mm2) (Fig. 1). In both compounds, the Cd II cations are octahedrally in a trans-CdX 4 N 2 arrangement, coordinated by four halide anions and two pyridazine ligands, and are linked by pairs of -1,1bridging halide anions into chains that propagate in the crys-tallographic a-axis direction (Fig. 2). The pyridazine ligands also act as bridging ligands, connecting two neighboring Cd II cations (Fig. 2). Within the chains, all of the pyridazine ligands are coplanar. (Fig. 2).
The Cd-N bond lengths to the pyridazine ligand are slightly longer in the iodide compound 2 compared to compound 1, which might be traced back to some crowding of the bulky iodide anion. In agreement, this distance is the shortest in the corresponding chloride compound (Pazderski et al., 2004a) reported in the literature (Tables 1 and 2). The N-Cd-Br and N-Cd-I bond angles are comparable, which is also valid for that in the chloride compound (Pazderski et al., 2004a). As expected, the intrachain CdÁ Á ÁCd distance increases from Cl [CdÁ Á ÁCd = 3.5280 (5)

Figure 3
Arrangement of the chains in the crystal structure of 1 in a view along the crystallographic b-axis direction.

Figure 2
Fragment of a [100] polymeric chain in the crystal structure of 1.
along the crystallographic b-axis direction (Fig. 3). The angle between two neighboring pyridazine ligands is 180 in both compounds, which is also valid for the chloride analog (Pazderski et al., 2004a). The distance between the centroids of adjacent pyridazine rings is 3.724 Å for the chloride, 3.8623 (1) Å (slippage = 0.095 Å ) for the bromide and 4.1551 (1) Å (0.226 Å ) for the iodide, consistent withinteractions (Fig. 4), although they must be weak for the iodide. There are no directional intermolecular interactions such as intermolecular C-HÁ Á ÁX hydrogen bonding. As mentioned above, this structure type is common for the majority of transition-metal pyridazine coordination compounds with such a metal-to-pyridazine ratio, indicating thatinteractions might also be responsible for this obviously very stable arrangement.

Database survey
A search in the CCDC database (version 5.43, last update November 2022; Groom et al., 2016) revealed that some compounds with the general composition MX 2 (pyridazine) (M = transition metal and X = halide anion) have already been reported in the literature. The compounds with NiCl 2 (CSD refcode POPCIG) and NiBr 2 (POPCOM) were structurally characterized by Rietveld refinements using laboratory X-ray powder diffraction data and are isotypic to the title compounds (Masciocchi et al., 1994). In this contribution, the compounds with Mn, Fe, Co, Cu and Zn with chloride and bromide as anions were also synthesized, and their lattice parameters determined from their powder patterns, indicating that the compounds with Mn, Fe and Co are isotypic to the Ni compound, which is not the case for the compounds with Cu and Zn (Masciocchi et al., 1994). The compounds MCl 2 (pyr-(pyridazine) with Mn (LANJEQ) and Fe (LANJAM) were later determined by single-crystal X-ray diffraction, which definitely proves that they crystallize in space group Immm (Yi et al., 2002). In this context it is noted that three compounds containing diamagnetic Zn II cations have been reported, which consist of discrete complexes with a tetrahedral coordination, viz. ZnI 2 (pyridazine) 2 (MENSUU; Bhosekar et al., 2006a), ZnBr 2 (pyridazine) 2 (VEMBEV; Bhosekar et al., 2006b) and three modifications of CuCl 2 (pyridazine) 2 (YAFYOU, YAFYOU01, YAFYOU02 and YAFYOU03; Pazderski et al., 2004b andBhosekar et al., 2007). Surprisingly, none of the different forms are isotypic to the chloride and bromide compounds reported by Masciocchi et al. (1994) based on XRPD patterns.

Synthesis
CdBr 2 , CdI 2 and pyridazine were purchased from Sigma-Aldrich. All chemicals were used without further purification.
Colorless single crystals of compound 1 and 2 were obtained by the reaction of 0.500 mmol of CdBr 2 or 0.500 mmol of CdI 2 with 0.500 mmol of pyridazine in 1 ml of ethanol. The reaction mixtures were sealed in glass tubes and heated at 388 K for 1 d and finally cooled down to room temperature.
Larger amounts of a microcrystalline powder of 1 and 2 were obtained stirring the same amount of reactants in ethanol or water at room temperature for 1 d. For the IR spectra of 1 and 2 see Figs. S5 and S6.

Experimental details
The IR spectra were measured using an ATI Mattson Genesis Series FTIR Spectrometer, control software: WINFIRST, from ATI Mattson. The PXRD measurements were performed with Cu K 1 radiation ( = 1.540598 Å ) using a Stoe Transmission Powder Diffraction System (STADI P) equipped with a MYTHEN 1K detector and a Johansson-type Ge(111) monochromator. Thermogravimetry and differential thermoanalysis (TG-DTA) measurements were performed in a dynamic nitrogen atmosphere in Al 2 O 3 crucibles using a STA-PT 1000 thermobalance from Linseis. The instrument was calibrated using standard reference materials. Arrangement of neighboring pyridazine rings in 1 showingstacking interactions.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The C-bound hydrogen atoms were positioned with idealized geometry and refined with U iso (H) = 1.2U eq (C). For compound 2, PLATON (Spek, 2020) suggested a pseudo-translation along the b-axis with a fit of 80%. If the structure is determined in a unit cell with half of the b-axis, space group Cmmm is suggested. The structure can easily be solved in this space group but the refinement leads to only very poor reliability factors (R1 = 11.5%). Moreover, in this case, disorder of the nitrogen atoms of the pyridazine ring is observed, because the N atoms of the pyridazine rings of neighboring chains are superimposed.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq