Chalcogen S ··· S Bonding in Supramolecular Assemblies of Cadmium(II) Coordination Polymers with Pyridine-Based Ligands

: Two cadmium(II) coordination polymers, with thiocyanate and pyridine-based ligands e.g., 3-acetamidopyridine (3-Acpy) and niazid (nicotinic acid hydrazide, nia), namely one-dimensional {[Cd(SCN) 2 (3-Acpy)]} n ( 1 ) and two-dimensional {[Cd(SCN) 2 (nia)]} n ( 2 ), are prepared in the mixture of water and ethanol. The adjacent cadmium(II) ions in 1 are bridged by two N , S -thiocyanate ions and an N , O -bridging 3-Acpy molecule, forming inﬁnite one-dimensional polymeric chains, which are assembled by the intermolecular N–H ··· S hydrogen bonds in one direction and by the intermolecular S ··· S chalcogen bonds in another direction. Within the coordination network of 2 , the adjacent cadmium(II) ions are bridged by N , S -thiocyanate ions in one direction and by N , O , N’ -chelating and bridging nia molecules in another direction. The coordination networks of 2 are assembled by the intermolecular N–H ··· S and N–H ··· N hydrogen bonds and S ··· S chalcogen bonds. Being the only supramolecular interactions responsible for assembling the polymer chains of 1 in the particular direction, the chalcogen S ··· S bonds are more signiﬁcant in the structure of 1 , whilst the chalcogen S ··· S bonds which act in cooperation with the N–H ··· S and N–H ··· N hydrogen bonds are of less signiﬁcance in the structure of 2


Introduction
Although not yet as established and studied as hydrogen and halogen bonding, chalcogen bonding (ChB) has recently gained more attention as an important supramolecular interaction with possible applications in biochemistry, crystal engineering, the design of new materials, and catalysis [1][2][3][4][5]. The chalcogen bond (D-Ch···A) is a non-covalent interaction between a chalcogen bond donor D and a chalcogen bond acceptor A. Chalcogen bond donor D is a Lewis acid and acceptor A is a Lewis base, while Ch is a chalcogen atom (of group 16). The chalcogen bonds belong to the group of σ-hole interactions. The σ-hole interactions, particularly halogen bonds, have become more popular in the last two decades, particularly due to their strength and high directionality. Nowadays, however, chalcogen bonds attract more attention, though their nature and mechanism of formation still remain unclear [1,5,6]. In the case of the chalcogen bonds, the interactions are formed between a positive (electrophilic) region on the chalcogen atom and a negative electron density (nucleophilic region) on the chalcogen bond acceptor [1,4]. The anisotropy in the surface electrostatic potential on chalcogen atoms is crucial for the formation of chalcogen bonds, which become more evident when the radius of chalcogen atoms increases. The main characteristic of the chalcogen bonds, as opposed to hydrogen bonds, is their directional nature, which is a consequence of the aforementioned anisotropy [7]. The strength of the chalcogen bonds seems to be dependent on the electron-accepting capability of the respective chalcogen molecule. Although understood from the crystal structures of various chalcogenated compounds for some time, the chalcogen bonds did not gain desired importance and become relevant supramolecular interactions in crystal engineering, in the way that halogen bonds did [4,6].
Niazid (nia, nicotinic acid hydrazide), equipped with an N,O-donor set of atoms, could be a useful ligand for the design of coordination polymers of various dimensionalities due to its ability to chelately bind to metal ions via hydrazide N and carbonyl O atoms and, as well as bridge via pyridine N atom [8]. However, in spite of having such the potential to make coordination polymers, only a few coordination polymers with the described coordination mode of nia have been reported, e.g., coordination polymers of nickel(II) [9], lead(II) [10], and cadmium(II) [11]. Three different cadmium(II) coordination polymers of nia have been prepared and described, depending on the metal-to-ligand molar ratios and the solvents used [11]. Although 3-acetamidopyridine (3-Acpy) is structurally similar to nia, also containing the N,O-set of atoms as possible donors, it lacks bridging and chelating potential due to the spatial arrangement of N and O atoms. Indeed, only a few metal complexes of 3-Acpy are known, each of them containing 3-Acpy ligand coordinated in an N-monodentate fashion (via pyridine N atom) [12,13].
We wanted to explore the possibilities of delivering more diverse cadmium(II) coordination polymers by using mixed ligands-organic pyridine-based ligands (nia or 3-Acpy) and highly versatile thiocyanate ion as an inorganic linker. The thiocyanate ion is able to coordinate to the same metal ion via both N and S atoms, enabling its bridging capacity. Additionally, we wanted to examine if the introduction of the thiocyanate could increase the dimensionality of the cadmium(II) coordination polymer formed, especially when combined with chelating and bridging nia, as opposed to the combination with monodentate 3-Acpy. However our main purpose was to explore the potential for linking metal-organic building units via chalcogen bonds upon introduction of the sulfur-containing ligand that might enable the formation of the chalcogen S···S bonding in the structures of the prepared cadmium(II) coordination polymers. In the construction of coordination polymers, we opted for a cadmium(II) ion due to its highly unpredictable and unreliable coordination environment, as this would increase our chances of obtaining more diverse coordination polymers. In line with these guidelines, we prepared a one-dimensional (1-D) cadmium(II) coordination polymer with 3-Acpy and thiocyanate and two-dimensional (2-D) cadmium(II) coordination polymer with nia and thiocyanate under the same experimental conditions (Scheme 1), namely {[Cd(SCN) 2 (3-Acpy)]} n (1) and {[Cd(SCN) 2 (nia)]} n (2). We then specifically checked if the chalcogen S···S bonds appeared in the structures of 1 and 2. Furthermore, we determined their role and significance in the supramolecular assemblies of the discussed cadmium(II) coordination polymers.
108.79(6)°) pairs of the ligating atoms indicate ( Table 1). The reason for such a distortion is the N,O-bidentate binding of the nia ligand, leading to the formation of the five-membered chelate ring (Figure 2c) with the very small N5 iii -Cd1-O1 iii angle (70.14(7)°).

Conclusions
We cadmium(II) ions in 2 are bridged by thiocyanate ions in one direction, but also by chelating and bridging nia in another direction, leading to the formation of a coordination network. The coordination mode of 3-Acpy found in 1 (bridging via its pyridine N and carbonyl O atoms) is unprecedented, as until now, 3-Acpy acted exclusively as an N-monodentate ligand in the reported metal-containing compounds. On the other hand, the coordination mode of nia found in 2 was the expected one, previously established in the known coordination polymers. We did confirm that the dimensionality of a coordination polymer can be increased (from 1D in the case of 1 to 2D in the case of 2) if two bridging ligands (thiocyanate and nia) are used in the case of nia in 2. However, it was revealed that 3-Acpy also acts as a bridging ligand in 1, yielding a lower-dimensionality polymer than in the case of 2. As 1 and 2 both contain bridging thiocyanates and bridging 3-Acpy or nia, respectively, the difference in their dimensionality can solely be ascribed to the chelating ability of nia. Nia can, therefore, more efficiently bridge the cadmium(II) ions in a different direction (as compared to the direction the thiocyanate bridge along), thus leading to the higher dimensionality of 2, as opposed to 1. The introduction of thiocyanate ions indeed enabled the expected formation of the chalcogen S···S bonds in the crystal packings of 1 and 2, though this was not of the same role or significance. It could be noted that the intermolecular chalcogen S···S bonds are more significant in the structure of 1, as they are the only supramolecular interactions responsible for assembling the polymeric chains of 1 in this particular direction. In the structure of 2, the intermolecular chalcogen S···S bonds also assemble the coordination networks of 2 in a particular direction, but this time in cooperation with the intermolecular N-H···S and N-H···N hydrogen bonds, acting along the same direction. In both structures, the employment of the strongest acceptor atoms (O atoms), in the coordination with the neighboring cadmium(II) ions, reduces the potential for establishing stronger supramolecular links, thus facilitating the supramolecular connectivity of metal-containing building units via desired S···S links.

Materials and Methods
All commercially available chemicals were of reagent grade and were used as received, without further purification. 3-Acetamidopyridine (3-Acpy) was prepared according to the previously reported method [23,24]. CHN elemental analyses were carried out with a Perkin-Elmer 2400 Series II CHNS analyzer in Analytical Services Laboratories of the Ruder Bošković Institute, Zagreb, Croatia. The IR spectra were obtained from KBr pellets in the range 4000-400 cm -1 on a Perkin-Elmer Spectrum Two FT-IR spectrometer.

Synthesis of {[Cd(SCN) 2 (nia)]} n (2) in Bulk
Cadmium(II) nitrate tetrahydrate (0.07 g; 0.23 mmol) was dissolved in ethanol (2 mL) and niazid (0.07 g; 0.51 mmol) was dissolved in 3 mL of water-ethanol mixture (1:2, v/v). The two solutions were mixed together and added to the solution of sodium thiocyanate (0.04 g; 0.49 mmol) in ethanol (3 mL). A white powder of 2 was formed in a period of 3-4 days, filtered off, washed with small amounts of cold water and dried in vacuo. Yield: 50% Cadmium(II) nitrate tetrahydrate (0.07 g; 0.23 mmol) was dissolved in ethanol (2.5 mL) and niazid (0.07 g; 0.51 mmol) was dissolved in 3 mL of water-ethanol mixture (1:2, v/v). The solutions were mixed and transferred to a test tube and carefully layered with 1 mL of ethanol, followed by the careful addition of 2.5 mL of ethanol solution of sodium thiocyanate (0.04 g; 0.49 mmol). Colorless crystals of 2, suitable for single-crystal X-ray structure determination, were formed in a couple of weeks.

X-ray Crystallographic Analysis
The suitable single crystals of 1 and 2 were selected and mounted in Paratone-N oil onto thin glass fibers. The data collection was carried out on an Oxford Diffraction Xcalibur four-circle kappa geometry diffractometer with Xcalibur Sapphire 3 CCD detector, using graphite monochromated MoKα (λ = 0.71073 Å) radiation at room temperature (296(2) K) and by applying the CrysAlis PRO Software system [25]. The data reduction and cell refinement were performed by the CrysAlis PRO Software system [25]. The structures were solved by SHELXT [26] and refined by SHELXL-2018/3 [27]. The refinement procedure was done by full-matrix least-squares methods based on F 2 values against all reflections. The figures were made with MERCURY (Version 2020.2.0) [28]. The crystallographic data for 1 and 2 are summarized in Table 3.