Synthesis, crystal structure and thermal properties of poly[[μ-1,2-bis(pyridin-4-yl)ethene-κ2 N:N′-μ-bromido-copper(I)] 1,2-bis(pyridin-4-yl)ethene 0.25-solvate]

In the crystal structure of the title compound, the copper(I) cations are tetrahededrally coordinated and linked by pairs of bromide anions into dinuclear units that are further connected into layers by the 1,2-bis(pyridin-4-yl)ethene (4-bpe) coligands. The layers are stacked so that cavities are formed in which disordered 4-bpe molecules are embedded.

The reaction of copper(I) bromide with 1,2-bis(pyridin-4-yl)ethene in acetonitrile leads to the formation of the title compound, {[CuBr(C 12 H 10 N 2 )]•-0.25C12 H 10 N 2 } n or CuBr(4-bpe)•0.25(4-bpe) [4-bpe = 1,2-bis(pyridin-4-yl)ethene].The asymmetric unit consists of one copper(I) cation and one bromide anion in general positions as well as two crystallographically independent half 4-bpe ligands and a quarter of a disordered 4-bpe solvate molecule that are completed by centers of inversion.The copper(I) cations are tetrahededrally coordinated as CuBr 2 N 2 and linked by pairs of �-1,1-bridging bromide anions into centrosymmetric dinuclear units that are further connected into layers by the 4-bpe coligands.Between the layers, interlayer C-H� � �Br hydrogen bonding is observed.The layers are arranged in such a way that cavities are formed in which the disordered 4-bpe solvate molecules are located.Powder X-ray (PXRD) investigations reveal that a pure sample has been obtained.Thermogravimetric (TG) and differential thermoanalysis (DTA) measurements show two mass losses that are accompanied by endothermic events in the DTA curve.The first mass loss correspond to the removal of 0.75 4-bpe molecules, leading to the formation of (CuBr) 2 (4-bpe), already reported in the literature as proven by PXRD.

Chemical context
Coordination polymers based on copper(I) halides show a large structural variability and are of interest, for example, regarding their luminescence behavior (Jess et al., 2007;Peng et al., 2010;Gibbons et al., 2017;Jia et al., 2018;Nitsch et al., 2015;Mensah et al., 2021).They consist of CuX substructures including monomeric and dimeric units, chains, double chains and layers, which can be further connected into one-, two-and three-dimensional networks if bridging coligands are present (Peng et al., 2010;Na ¨ther et al., 2007;Kromp et al., 2003).For a pairing of a particular copper(I) halide and coligand, frequently two or more compounds with a different ratio between the copper(I) halide and the coligand are found.
In previous investigations we have found that the coligandrich compounds usually lose their coligands stepwise, which lead to the irreversible formation of ligand-deficient intermediates that are obtained in quantitative yield (Na ¨ther & Jess, 2004;Na ¨ther et al., 2002).In the course of this reaction, compounds with more condensed CuX substructures are formed.This is the case, e.g., for coordination compounds based on pyrazine and 4,4 0 -bipyridine.With pyrazine, one compound with the composition CuCl(pyrazine) is known in which the copper(I) cations are linked by the chloride anions into chains, which are further connected into layers by the pyrazine ligands (Moreno et al., 1995).Upon heating, half of the pyrazine ligands are removed, leading to a compound with the composition (CuCl) 2 (pyrazine), in which the Cu I cations are linked by �-1,1 bridging chloride anions into double chains, which are further connected into layers by the coligands (Kawata et al., 1998;Na ¨ther et al., 2001).4,4 0 -Bipyridine compounds with the composition CuX(4,4 0 -bipyridine) (X = Cl, Br, I) have been reported in which the copper cations are connected into (CuX) 2 dimeric units, which are further linked into layers by the 4,4 0 -bipyridine ligands (Yaghi & Li, 1995;Batten et al., 1999;Lu et al., 1999).Thermogravimetric experiments prove that the coligands are removed in a stepwise fashion leading to compounds with the composition (CuX) 2 (4,4 0 -bipyridine) (X = Cl, Br, I), in which the Cu I cations are linked into double chains, which are further connected into layers by bridging 4,4 0 -bipyridine ligands (Yaghi & Li, 1995;Na ¨ther & Jess, 2001).
A further bridging coligand is 1,2-bis(pyridin-4-yl)ethene, for which some compounds have already been reported in the literature (see Database survey).These includes three liganddeficient compounds with the composition (CuX) 2 (4-bpe) (X = Cl, Br, I) in which the copper(I) cations are linked by the halide anions into chains, which are further connected into layers by the 4-bpe ligand (Li et al., 2006;Yang & Li, 2006;Chen et al., 2008;Wang, 2016;Shen & Lush, 2010;Blake et al., 1999;Neal et al., 2019).With CuI, a ligand-rich compound with the composition CuI(4-bpe)•0.25 4-bpe has already been reported, which is not known for CuBr and CuI (Hoffman et al., 2020).This compound consists of layers that are stacked in such a way that pores are formed, in which 4-bpe solvate molecules are located.A very similar structure is found for (CuCl) 2 (4-bpe)•4H 2 O, but in this compound the pores are filled with water, instead of 4-bpe (Mohapatra & Maji, 2010).Based on these findings, one can assume that a similar compound might also exist with CuBr.Moreover, for such a compound it is highly likely that upon heating it will transform into the ligand-deficient compound (CuBr) 2 (4-bpe) already reported in the literature.Therefore, we reacted CuBr with 4-bpe in different solvents and from acetonitrile we obtained a new crystalline phase that was characterized by single-crystal X-ray diffraction and thermoanalytical measurements.

Structural commentary
The title compound is isotypic to CuI(4-bpe)•0.25 4-bpe already reported in the literature (Hoffman et al., 2020).Its asymmetric unit consists of one Cu I cation and one bromide anion in general positions as well as two crystallographically independent half 4-bpe ligands that are completed by inversion symmetry (Fig. 1).There is one quarter of an additional bpe solvate molecule that is disordered around a center of inversion (Fig. 2 and see Refinement section).Because of the disorder, this ligand is not fully occupied and was refined using a split model (Fig. 3).
The copper(I) cations are tetrahedrally coordinated by two symmetry-equivalent bromide anions and two N atoms of two crystallographically independent 4-bpe ligands (Fig. 1).From the bond lengths and angles (Table 1), it is apparent that the tetrahedra are slightly distorted.Pairs of Cu I cations are linked by two �-1,1 bridging bromide anions into dimeric (CuBr) 2 units that are located on centers of inversion and are further connected by the 4-bpe ligands into layers (Fig. 4).

Supramolecular features
In the crystal structure of the title compound, the layers are arranged in such a way that cavities are formed, which proceed along the a-axis direction, in which the disordered 4-bpe Crystal structure of the title compound with labeling and displacement ellipsoids drawn at the 50% probability level.Symmetry codes for the generation of equivalent atoms:

Figure 2
Crystal structure of the solvate 4-bpe molecule with labeling and displacement ellipsoids drawn at the 50% probability level.Symmetry code for the generation of equivalent atoms: (iv) À x + 2, À y + 1, À z. solvate molecules are embedded (Fig. 5).The layers are connected via intermolecular C-H� � �Br hydrogen bonding (Table 2).The C-H� � �Br angle is close to linearity, indicating that this is a significant interaction.There are additional C-H� � �Br interactions, between the C-H groupings of the solvate 4-bpe ligands and the bromide ions (Table 2).
In all these compounds, the copper(I) cations are tetrahedrally coordinated by three bromide anions and one 4-bpe ligand.The copper(I) cations are linked by the three �-1,1,1 bridging halide anions into chains that are further linked into layers by the 4-bpe coligands.The chloride and iodide compounds are isotypic, which is not the case for the bromide compound.There is one compound of the composition (CuI)(4-bpe)•0.25 4-bpe that is isotypic to the title compound (TUYRAJ; Hoffman et al., 2020).Another compound of the composition (CuCl) 2 (4-bpe)•4H 2 O has similar unit-cell parameters as well as the same space group, which indicates that this compound may also be isotypic to the title compound (HUTXIE; Mohapatra & Maji, 2010).

Thermoanalytical investigations
Comparison of the experimental powder pattern with that calculated from single-crystal data reveals that the title compound was obtained as a pure phase (Fig. S1).The title compound was characterized for its thermal properties by simultaneous thermogravimetry and differential thermoanalysis (TG-DTA).Upon heating, two mass losses are observed in the TG curve that are accompanied by endothermic events in the DTA curve (Fig. 6).From the first derivative of the TG curve (DTG curve), it is obvious that both mass losses are well resolved (Fig. 6).The first mass loss of 36.4% is in good agreement with that calculated for the removal of 0.75 4-bpe ligands (�m calc.= 36.8%),whereas the second mass loss of 19.7% is much lower than that expected

Table 1
Selected geometric parameters (A ˚, � ). for the loss of the remaining 4-bpe ligands (�m calc.= 24.5%),indicating that in this step the coligands are not completely removed.However, the first observation indicates that after the first mass loss a compound with the composition (CuBr) 2 (4-bpe) has been formed.To prove this assumption, a second TG measurement was performed, in which the residue formed after the first mass loss was isolated and investigated by PXRD.Comparison of the experimental pattern with that calculated for (CuBr) 2 (4-bpe) reported in the literature (Shen et al., 2010) proves that this compound was obtained (Fig. S2).

Synthesis and crystallization
CuBr was purchased from Riedel de Hae ¨n.4-bpe was purchased from Sigma-Aldrich.A microcrystalline powder was obtained by the reaction of 0.5 mmol CuBr (71.75 mg) and 1.0 mmol 4-bpe (182.2 mg) in 3 ml of MeCN.The mixture was stirred for 4 d at room temperature and filtered off.Crystals suitable for single-crystal X-ray diffraction were obtained under hydrothermal conditions for 4 d at 403 K using 0.5 mmol of CuBr (71.75 mg), 2.0 mmol of 4-bpe (364.4 mg) in 3 ml of MeCN as a solvent.An IR spectrum of the title compound can be found in Fig. S3.

Experimental details
The XRPD measurements were performed with a Stoe Transmission Powder Diffraction System (STADI P) equipped with a MYTHEN 1K detector and a Johansson-type Ge(111) monochromator using Cu K� 1 radiation (� = 1.540598A ˚).The IR spectra were measured using an ATI Mattson Genesis Series FTIR Spectrometer, control software: WINFIRST, from ATI Mattson.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.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.The C-bound H atoms were positioned with idealized geometry and were refined isotropically with U ı ˜so (H) = 1.2U eq (C) using a riding model.The solvate 4-bpe molecule is disordered around a center of inversion.Therefore, it was refined using a split model with restraints for the geometry (SAME) and half occupancy for all atoms.(Brandenburg & Putz, 1999); software used to prepare material for publication: publCIF (Westrip, 2010). Poly

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes. Fractional

Figure 3
Figure 3Crystal structure of the title compound showing the disorder of the solvate 4-bpe molecule.

Figure 4
Figure 4Crystal structure of the title compound with a view of one CuBr(4-bpe) layer along the crystallographic b-axis direction.The disordered 4-bpe solvate molecule is not shown for clarity.

Figure 6
Figure 6 DTG, TG and DTA curve for the title compound, measured with a 4 � C min À 1 heating rate.

Table 3
Experimental details.