Silver(I) 2,2'-(1,2-Phenylenedisulfanediyl)diacetic Acid as a Molecular Building Block for a Silver(I)-Cadmium(II) Coordination Polymer

Starting from heterotopic multidentate ligand 2,2'-(1,2-phenylenedisulfanediyl)diacetic acid, (RS,RS,RS,RS/SS,SS,SS,SS)-[Ag{1,2-C6H4(SCH2COOH)2-κ2S,S'}2]BF4 (1) was prepared and further used as a building block for the synthesis of heterobimetallic Ag-Cd coordination polymer [Ag2Cd2{1,2-(OOCCH2S)2C6H4}3(H2O)3 5H2O]n (2). Both complexes were characterized by X-ray structure analysis and conventional spectroscopic techniques.


Introduction
Although the chemistry of coordination polymers has received much attention during the last decade, not only due to their interesting architectures [1] but also because of their potential applications in gas storage [2][3][4], nonlinear optics [5,6], or catalysis [7,8], and interesting magnetic [9,10] or luminescence [11] properties, the synthesis of such polymers with predictable geometries and structures is still a challenge [12,13]. Increasing attention has been paid in recent years to rational synthetic approaches for the assembly of target structures [14,15]. The key step of this approach is the design of molecular building blocks which can direct the formation of the desired architecture and functionality of the target compound.
Here, we report the synthesis of a mononuclear silver(I) complex from heterotopic multidentate ligand 2,2'-(1,2-phenylenedisulfanediyl)diacetic acid, which was further used as a molecular building block for the synthesis of a Ag-Cd heterobimetallic coordination polymer.
Complex 1 was characterized by 1 H, 11 B{ 1 H}, 19 F{ 1 H}, 13 C{ 1 H} NMR and IR spectroscopy and elemental analysis, and insoluble polymer 2 by IR spectroscopy and elemental analysis. Furthermore, single crystals suitable for X-ray crystallography could be obtained for both compounds.

Molecular Structures of 1 and 2
Suitable crystals of 1 for X-ray structure analysis were obtained from thf at −20 °C. Complex 1 crystallizes in the tetragonal space group 4 P n2 with two molecules in the unit cell. In addition, eight thf molecules are also present in the unit cell. The structural motif of the [Ag{1   Colorless crystals of 2 were obtained at room temperature in three days from a solvent mixture of dmf, MeOH, and H2O. Polymer 2 crystallizes in the monoclinic space group P21 with two formula units in the unit cell. Five water molecules are present in the asymmetric unit.
Polymer 2 contains three types of C6H4(SCH2COO)2 2− anions ( Figure 2) which differ from each other in their coordination mode with the metal ions (Cd 2+ is exclusively coordinated by oxygen, whereas Ag + is preferably coordinated by sulfur), as well as in the deviation of the SCH2COO groups from the plane of the benzene ring. The silver ions are no longer coordinated in a chelating fashion by four sulfur atoms of two neutral ligands but by three sulfur atoms from three different C6H4(SCH2COO)2 2− dianions and by one carboxylate oxygen atom.
In type 1, the C6H4(SCH2COO)2 2− anion acts as a tetradentate ligand, bridging two silver and two cadmium ions. Each sulfur atom coordinates to a silver ion. One of the silver ions is also coordinated in a chelating fashion by one carboxylate oxygen atom, which also coordinates to a cadmium ion. The second cadmium ion is coordinated by an oxygen atom from the second carboxylate group. One SCH2COO group is almost coplanar with the benzene ring, while the other is located out of the plane, and torsion angles Caryl-S-Calkyl-C are 169. 16(1)° and -85.94(1)°, respectively. The type 2, a hexadentate C6H4(SCH2COO)2 2− anion bridges two silver and four cadmium ions, each oxygen atom being involved in coordination with a different cadmium ion, while each sulfur atom coordinates to a silver ion. Like in type 1, only one SCH2COO group is almost coplanar with the plane of the benzene ring, and the torsion angles Caryl-S-Calkyl-C are 165.04(1)° and -68. 38(1)°. In the third type of coordination, type 3, the C6H4(SCH2COO)2 2− anion acts as a pentadentate ligand bridging two silver and two cadmium ions. Each sulfur atom coordinates to a silver ion. One of the silver ions is also coordinated in a chelating fashion by one carboxylate oxygen atom, and the second oxygen atom of this carboxylate group coordinates to a cadmium ion. The second cadmium ion is coordinated by an oxygen atom from the second carboxylate group. Again, one SCH2COO group is almost coplanar with the benzene ring, while the other is located out of the plane, and the torsion angles Caryl-S-Calkyl-C are -175.14(1)° and 81.22(1)°, respectively. In 2, two types of Cd 2+ ions (Cd(1) and Cd (2)) and two types of Ag + ions (Ag(1) and Ag (2)) are present. The Cd(1) ions are pentacoordinated in a square-pyramidal fashion by three carboxylate oxygen atoms from three different C6H4(SCH2COO)2 2− anions and by two oxygen atoms from two coordinating water molecules. The Cd(2) ions are hexacoordinated in a distorted octahedral fashion by four carboxylate oxygen atoms from four different C6H4(SCH2COO)2 2− anions and two oxygen atoms from two coordinating water molecules. The Cd(1) and Cd (2) ions are bridged by one oxygen atom (O(14)) from a water molecule which coordinates to both metal ions ( Figure 3, Table 2). The Cd-Ocarboxylate bond lengths range from 2.255(2) to 2.295(2) Å and are in agreement with those observed for similar compounds [31][32][33]. The Cd-Owater bond lengths vary from 2.247(2) Å (Cd (1) (2) 106.58 (2) Both silver ions Ag(1) and Ag (2) are tetracoordinated in a distorted trigonal-pyramidal fashion by two sulfur atoms from two different C6H4(SCH2COO)2 2− and by one sulfur atom and one carboxylate oxygen atom from a third dicarboxylate anion. The ions Ag(1) and Ag(2) are bridged by two sulfur atoms of the same ligand molecule (Ag(1)···Ag (2) -[(μ2-benzene-1,3-dicarboxylato)bis(μ2-3,3',5,5'-tetramethyl-4,4'-bipyrazole)disilver(I)] (2.615(2) Å and 2.672(2) Å) [34].
The structure extends to a two-dimensional network parallel to the C face ( Figure 4). The water molecules of solvation interconnect the two-dimensional sheets via hydrogen bonding, giving rise to a three-dimensional supramolecular network (Figure 5a,b). The five noncoordinating water molecules are involved in hydrogen bonding between themselves (H···O 1.69 Å to 1.93 Å and O···O 2.649 (5)

General Procedures
All manipulations for the synthesis of compound 1 were carried out in an inert atmosphere of dry nitrogen and in the absence of light; the synthesis of 2 was performed in air. 2,2'-(1,2-Phenylenedisulfanediyl)diacetic acid [35,36] was prepared according to literature methods; AgBF4 and Cd(OAc)2·3H2O were commercially available. The thf was dried over sodium/benzophenone, distilled under an atmosphere of dry argon, and stored over potassium mirror. CD3OD for NMR spectroscopy was kept in an inert atmosphere of dry argon over molecular sieves. The NMR spectra were recorded on a Bruker Avance DRX-400 spectrometer. Chemical shifts are quoted in ppm relative to tetramethylsilane. Elemental analysis was performed with a Vario EL-Heraeus microanalyzer. IR spectra were recorded with a Perkin-Elmer System 2000 in the range 4000-400 cm −1 in KBr pellets.

X-ray Structure Determination
Data for 1 were collected on a Siemens CCD diffractometer (SMART) [37] in ϕ-scan mode using MoKα radiation (λ = 0.71073 Å). Data reduction was performed with SAINT [38], including the program SADABS [39] for empirical absorption correction. Data for 2 were collected on a CCD Gemini diffractometer (Agilent Technologies) in ω-scan mode using MoKα radiation (λ = 0.71073 Å). Data reduction was performed with CrysAlis Pro including analytical numeric absorption correction using a multifaceted crystal model based on expressions derived by Clark and Reid [40]. All structures were solved by direct methods [41,42] and the refinement of all atoms was performed with SHELXL-97 [31]. With the exception of thf molecules, all hydrogen atoms for 1 were located in difference Fourier maps calculated at the final stage of structure refinement. For 2, all H atoms except those of the water molecules H2O(13) and H2O (14) were calculated on idealized positions. Structure figures were generated with ORTEP [43] and DIAMOND-3 [44]. CCDC 877467 (1) and 877468 (2) contain the supplementary crystallographic data for this paper [45]. A summary of the data collection, structure solution, and refinement details for compounds 1 and 2 is given in Table 3.

Conclusions
The reaction of 2,2'-(1,2-phenylenedisulfanediyl)diacetic acid with AgBF4 produced mononuclear silver(I) complex 1, which was further used as a building block for the synthesis of two-dimensional heterobimetallic Ag-Cd coordination polymer 2. In the discrete silver(I) complex cation in 1, the four COOH groups are not coordinated to the metal center but are involved in hydrogen bonding with one thf molecule each. In 2, the water molecules of solvation interconnect the two-dimensional sheets via hydrogen bonding, giving rise to a three-dimensional supramolecular network.