N , N -Diarylformamidine Dithiocarbamate Ag(I) Cluster and Coordination Polymer

: An Ag(I)formamidine cluster Ag 6 L1 6 ( 1) and an Ag(I)formamidine coordination polymer Ag 7 ( L2 ) 2 2 ( L1 = N , N (cid:48) -bis(2,6-disopropylphenyl) formamidine dithiocarbamate and L2 = N , N (cid:48) mesityl formamidine dithiocarbamate) have been synthesized from the reactions of L1 and L2 with AgNO 3 respectively. The complexes were characterized using spectroscopic and analytical methods, including single-crystal X-ray diffraction. In the structure of 1 , a six vertex distorted square bipyramidal octahedron is formed from an Ag 6 core. The N , N (cid:48) -bis(2,6-disopropylphenyl) formamidine dithiocarbamate ligands stabilize this core through two main –CS 2 bridging modes giving a propeller like structure. In the structure of 2 , each of the two Ag(I) centers are bridged by two N , N (cid:48) -mesityl formamidine dithiocarbamate ligands forming 8-member Ag 2 (CS 2 ) 2 metallacycles with an inversion center in the middle of the Ag—Ag argentophilic bond. The metallacycles are connected through Ag—S bonds forming ribbons in the crystallographic a -axis. The Ag(I) centers are coordinated to two N , N (cid:48) -mesitylformamidine dithiocarbamates through the dithiocarbamate S atoms. The thermal decomposition of complexes 1 and 2 had similar thermograms with one major weight loss activity and the formation of elemental silver particles thereafter.


Introduction
Silver(I) complexes have been applied in various fields, therefore, attracting great attention due to their structural features and functional considerations [1][2][3]. They have been tested as fluorescent materials [4], semiconductors [5,6], source of nanomaterials [7] and biological imaging agents [8] due to their interesting and fascinating physicochemical properties. Ag(I) complexes have also shown potential medical applications, most especially as antibacterial [9,10], antifungal [11], anticancer [12], and antimalarial agents [13]. Argentophilic bonding interactions which exist between seemingly closed-shell silver(I) atoms center have been reported to be responsible for their significant physical properties as well as their structural details [2]. Structurally, they possess coordination geometries ranging from two-coordinate (linear) to eight-coordinate (tetragonal prism) [14]. Factors, such as reaction conditions, as well as alkyl substitution effects might be responsible for the formation of a variety of complexes with unprecedented structures [15,16]. In the past decades, monomeric, dimeric, hexameric and polymeric structures have been reported [17].
Dithiocarbamates (R 2 CNS 2 − ) belong to a class of mono anionic 1,1-dithiolate ligands, and they are often prepared by nucleophilic addition reaction of primary or secondary amines and carbon disulfide in the presence of a base as a proton acceptor [18]. Dithiocarbamates have been reported to react with silver(I) ions to form Ag(I) complexes of various geometries [19,20] and also serve as efficient stabilizers for silver nanoparticles [21]. Silver(I) dithiocarbamates, [Ag(S 2 CNR 2 )] n have been known as far back as the 1950s, but little work Molbank 2022, 2022, M1327 2 of 10 had been done on them probably due to their low solubility [17]. They have been widely used as a precursor to acanthite (α-Ag 2 S), a potential material for microelectronics [22,23]. Early crystallographic studies revealed the hexameric nature of Ag-S 2 CNR 2 (R = Et, Pr, and n-Bu) in the solid states. They have a polymeric chain structure, in which the silver atom is bonded in a distorted manner by three dithiocarbamate ligand, two acting as µ 2 -bridges and one acting as a chelate [15,24]. Yin et al. reported the crystal structure of poly [(µ 3 -N,N-dibenzyldithiocarbamatoκ 4 S,S':S:S') silver(I)] [17]. Each Ag(I) cation in this complex was bonded to two pairs of sulfur atoms from three N,N -dibenzyldithiocarbamate ligands, conforming to distorted tetrahedral geometry. The crystal structure of hexakis (µ 3 -N,N-diisopropyldithiocarbamato) hexasilver(I) had also been reported by Yin and co-workers [25]. The metal center of the complex was centrosymmetric, and its hexanuclear structure entails two cyclohexane-like Ag 3 S 3 units which are joined together by the S atoms of the thiocarbamate groups with the Ag...Ag distances ranging from 3.0382(5) to 3.0985(5) Å. Herein, we report the synthesis, characterization and thermal analysis of silver(I) dithiocarbamate complexes derived from symmetrical N,N -diarylformidine dithiocarbamate ligands.

Preparation of Complexes
The synthesis of N,N -bis(2,6-disopropylphenyl) formamidine dithiocarbamate and N,N -mesityl formamidine dithiocarbamate salts have been reported in our previous work [26]. The complexes were synthesized by dissolving two equivalents of the potassium dithiocarbamate salts in 15 mL of acetonitrile into which a solution 1 equivalent of AgNO 3 in 5 mL of H 2 O was added drop-wise with stirring for 30 min at room temperature. The resultant yellow solids were collected by filtration washed three times with diethyl ether, and dried in the oven at 50 • C.

Single Crystal X-ray Diffraction
The structure refinement parameter, as well as the crystallographic data for complexes 1 and 2, are given in Table 1. Evaluation of crystals and collection of data was done on a Bruker Smart APEXII diffractometer with Mo Kα radiation (I = 0.71073 Å) equipped with an Oxford Cryostream low-temperature apparatus operating at 101 K for all samples. Reflections were collected at different starting angles, and the APEXII program suite was used to index the reflections [27]. Reduction of data was carried out using the SAINT software [28], and the absorption corrections and scaling were applied using the SADABS multi-scan technique [29]. The two structures were solved by the direct method using the SHELXS program and refined using the SHELXL program [30]. Graphics of the crystal structures were drawn using Mercury software [31]. Non-hydrogen atoms were first refined isotropically and then by anisotropic refinement with the full-matrix least square method based on F 2 using SHELXL. All hydrogen atoms were positioned geometrically, allowed to ride on their parent atoms, and refined isotropically. The crystallographic data and structure refinement parameters for complexes 1 and 2 are given in Table 1. Table 1. The summary of X-ray crystal data collection and structure refinement parameters for complexes 1 and 2.

Synthesis of N,N -Diarylformamidines Dithiocarbamate Ag(I) Complexes
Complexes 1 and 2 were synthesized by reacting acetonitrile potassium salt solutions of L1 and L2 ( Figure 1) with aqueous solutions of AgNO 3 in a 2:1 ratio. Both complexes were obtained as thermally stable yellow solids with melting points ranging between 214 and 222 • C, 1 having a lower melting point than 2. Both complexes are soluble in dichloromethane, chloroform, toluene and benzene.

Synthesis of N,N'-Diarylformamidines Dithiocarbamate Ag(I) Complexes
Complexes 1 and 2 were synthesized by reacting acetonitrile potassium salt solutions of L1 and L2 (Figure 1) with aqueous solutions of AgNO3 in a 2:1 ratio. Both complexes were obtained as thermally stable yellow solids with melting points ranging between 214 and 222 °C, 1 having a lower melting point than 2. Both complexes are soluble in dichloromethane, chloroform, toluene and benzene.

Spectroscopic Studies
The 1 H and 13 C NMR spectra for complexes 1 and 2 obtained in chloroform had signature peaks for the methane proton of the L1 and L2 as confirmed using their 2D NMR spectra. The azomethine proton of 1 and 2 was observed at 9.38 and 9.43 ppm in the spectra of L1 and L2, respectively, an upfield shift from 10.15 and 9.92 ppm [26]. The signals of aliphatic protons in the spectra of the complexes shifted noticeably downfield. For example, methyl protons in 2 appeared at 1.94, 2.03, and 2.15 ppm but were at 1.99, 2.11, and 2.22 ppm in L2. The downfield shift is a result of the drifts of electron density towards the metal ion center [32,33]. There were similar observations in the 13 C-NMR spectra of 1 and 2, and an upfield shift of the carbon atom of -NCS2 moiety to 214.15 and 214.01 ppm from 220.94 and 218.95 ppm in the spectra of L1 and L2, corroborating coordination of the S atom to Ag.
The FT-IR spectra of complexes I and 2 showed a strong absorption band at 1474 and 1476 cm −1 , stretching bands for the C-N bond of the thiouride group. These stretching frequencies are intermediate of a typical C-N single bond (1250-1360 cm −1 ) and a double bond (1640 cm −1 ), an indication of a partial double bond character of the thiouride bond [34,35]. The two vibrational bands around 1252-1246 cm −1 and 982-983 cm −1 in the spectra of 1 and 2 are assigned to the asymmetric C-(S)-S and the symmetric C-(S)-S moiety confirming the asymmetric linking of the sulfur atoms to the silver atoms [36]. The vibrational bands for υ(C=Nstr) of the azomethine (C(H)=N) in the formamidine backbone of the complexes were observed around 1634-1639 cm −1 .

Spectroscopic Studies
The 1 H and 13 C NMR spectra for complexes 1 and 2 obtained in chloroform had signature peaks for the methane proton of the L1 and L2 as confirmed using their 2D NMR spectra. The azomethine proton of 1 and 2 was observed at 9.38 and 9.43 ppm in the spectra of L1 and L2, respectively, an upfield shift from 10.15 and 9.92 ppm [26]. The signals of aliphatic protons in the spectra of the complexes shifted noticeably downfield. For example, methyl protons in 2 appeared at 1.94, 2.03, and 2.15 ppm but were at 1.99, 2.11, and 2.22 ppm in L2. The downfield shift is a result of the drifts of electron density towards the metal ion center [32,33]. There were similar observations in the 13 C-NMR spectra of 1 and 2, and an upfield shift of the carbon atom of -NCS 2 moiety to 214.15 and 214.01 ppm from 220.94 and 218.95 ppm in the spectra of L1 and L2, corroborating coordination of the S atom to Ag.
The FT-IR spectra of complexes I and 2 showed a strong absorption band at 1474 and 1476 cm −1 , stretching bands for the C-N bond of the thiouride group. These stretching frequencies are intermediate of a typical C-N single bond (1250-1360 cm −1 ) and a double bond (1640 cm −1 ), an indication of a partial double bond character of the thiouride bond [34,35]. The two vibrational bands around 1252-1246 cm −1 and 982-983 cm −1 in the spectra of 1 and 2 are assigned to the asymmetric C-(S)-S and the symmetric C-(S)-S moiety confirming the asymmetric linking of the sulfur atoms to the silver atoms [36]. The vibrational bands for υ(C=Nstr) of the azomethine (C(H)=N) in the formamidine backbone of the complexes were observed around 1634-1639 cm −1 .
In the electronic spectra of 1 and 2, two bands were observed at 238 and 312, and 314 nm and these are attributed to intraligand π→π* transition associated with N-C=S and π→π* transition within S-C=S groups of the coordinated dithiocarbamate ligands [37].

X-ray Structural Analysis
Suitable crystals for single-crystal X-ray diffraction analysis were obtained by slow diffusion of hexane into a chloroform solution of complexes 1 and 2, each. While complex 1 is an Ag(I) diarylformamidines dithiocarbamate cluster, 2 is an Ag(I) diarylformamidines dithiocarbamate coordination polymer. Complex 1 is a polymorph of the previously reported structure of Ag 6 [CS 2 (2,6-i Pr 2 C 6 H 3 NC(H)=NC 6 H 3 -i Pr 2 )] 6 which crystallized in the Ia3 cubic space group ( Table 2). The previously reported structure was synthesized by the insertion of CS 2 into a dinuclear Ag(I) complex of Ag 2 [2,6-i Pr 2 C 6 H 3 N) 2 C-(H)] 2 using toluene as a solvent for the reaction [38]. Table 2. Comparison of some selected X-ray crystal data collection and structure refinement parameters for complex 1 and its polymorph. Complex 1 is assembled through six Ag(I) centers and six diarylformamidines dithiocarbamate ligands (1:1 ratio). The asymmetric unit contains one-half of the cluster and is made up of three Ag(I) centers, coordinated through bridging by three diarylformamidines dithiocarbamate molecules (Figure 2a). The other half is generated through an inversion center. In the crystal, a six-vertex distorted square bipyramid octahedron is formed in which Ag3 forms the apexes of the pyramid while Ag1 2 Ag2 2 (red dashed lines in Figure 2b) for the base of the bipyramid (Figure 2b). The square bipyramid is stabilized by six L1 ligands which have the coordination modes µ 1µ 1 -for four of the ligands while for the other two have a µ 1µ 2 -coordination mode. With the CS 2 moieties added on, a propeller like core is formed (Figure 2c). In the electronic spectra of 1 and 2, two bands were observed at 238 and 312, and 314 nm and these are attributed to intraligand π→π* transition associated with N-C=S and π→π* transition within S-C=S groups of the coordinated dithiocarbamate ligands [37].

X-ray Structural Analysis
Suitable crystals for single-crystal X-ray diffraction analysis were obtained by slow diffusion of hexane into a chloroform solution of complexes 1 and 2, each. While complex 1 is an Ag(I) diarylformamidines dithiocarbamate cluster, 2 is an Ag(I) diarylformamidines dithiocarbamate coordination polymer. Complex 1 is a polymorph of the previously reported structure of Ag6[CS2(2,6i Pr2C6H3NC(H)=NC6H3i Pr2)]6 which crystallized in the Ia3 cubic space group ( Table 2). The previously reported structure was synthesized by the insertion of CS2 into a dinuclear Ag(I) complex of Ag2[2,6i Pr2C6H3N)2C-(H)]2 using toluene as a solvent for the reaction [38].
Complex 1 is assembled through six Ag(I) centers and six diarylformamidines dithiocarbamate ligands (1:1 ratio). The asymmetric unit contains one-half of the cluster and is made up of three Ag(I) centers, coordinated through bridging by three diarylformamidines dithiocarbamate molecules (Figure 2a). The other half is generated through an inversion center. In the crystal, a six-vertex distorted square bipyramid octahedron is formed in which Ag3 forms the apexes of the pyramid while Ag12Ag22 (red dashed lines in Figure2b) for the base of the bipyramid (Figure 2b). The square bipyramid is stabilized by six L1 ligands which have the coordination modes μ 1 -μ 1 -for four of the ligands while for the other two have a μ 1 -μ 2 -coordination mode. With the CS2 moieties added on, a propeller like core is formed (Figure 2c). Comparison of 1 to Ag6[CS2 (2,6- The literature structure is solvated with one molecule of chloroform, while 1 is not. The Ag(I) octahedron core of the literature structure forms a perfect square bipyramid, whereas that of 1 is slightly distorted. The Ag-Ag distances in 1 range between 2.9204(4) and 3.3452(4) Å while the distances are between 3.0001(3) and 3.3178(3) Å for literature structure. In both complexes, the Ag-Ag argentophilic distances are all less than the sum of the van der Waals radii of two Ag atoms, 3.44 Å [23,24].
Each Ag(I) center in 1 is coordinating to three sulfur atoms in a κ 1 κ 1 κ 1 -S fashion, a geometry around Ag(I) that can be described as distorted trigonal pyramidal in which the Ag(I) center serves as the apex of the pyramid. The S-Ag-S bond angles range between 107.07(4) and 123.40(3)° in 1 while it ranges between 109.29(3) and 127.14(3) in the literature structure. The Ag-S bond distances are 2.4857(11), 2.5231(10), and 2.5322(9) in comparison to 2.4779(3) and 2.5008(2) for the literature structure. Table 2. Comparison of some selected X-ray crystal data collection and structure refinement parameters for complex 1 and its polymorph.  The literature structure is solvated with one molecule of chloroform, while 1 is not. The Ag(I) octahedron core of the literature structure forms a perfect square bipyramid, whereas that of 1 is slightly distorted. The Ag-Ag distances in 1 range between 2.9204(4) and 3.3452(4) Å while the distances are between 3.0001(3) and 3.3178(3) Å for literature structure. In both complexes, the Ag-Ag argentophilic distances are all less than the sum of the van der Waals radii of two Ag atoms, 3.44 Å [23,24].
Each Ag(I) center in 1 is coordinating to three sulfur atoms in a κ 1 κ 1 κ 1 -S fashion, a geometry around Ag(I) that can be described as distorted trigonal pyramidal in which the Ag(I) center serves as the apex of the pyramid. The S-Ag-S bond angles range between 107.07(4) and 123.40(3) • in 1 while it ranges between 109.29(3) and 127.14(3) in the literature structure. The Ag-S bond distances are 2.4857(11), 2.5231(10), and 2.5322(9) in comparison to 2.4779(3) and 2.5008(2) for the literature structure.
In the crystal of complex 2, the asymmetric unit consists of two diarylformamidines dithiocarbamate ligands (L2) and two Ag(I) centers (Figure 3a). The Ag(I) centers are bridged by two diarylformamidine dithiocarbamate units in such a way that two Ag1s are paired and likewise two Ag2s. In doing so, each bridged Ag(I) pairs with an 8-member bimetallocycle, and an inversion center is formed. The metallacycles are connected through two centrosymmetrically related Ag-S bonds leading to a ribbon that runs in the acrystallographic axis (Figure 3b).
Just like in 1, the Ag(I) center is coordinated to three S atoms giving a trigonal pyramidal geometry around the metal center. This differs from the coordination polymer of [Ag(dibenzyldithiocarbamate)]∞ [17] where each Ag(I) cation is bonded to two pairs of sulfur atoms from three dithiocarbamate ligands resulting in a distorted tetrahedral geometry. The S-Ag-S bond angles range from 102.58 ( (Table 3) and comparatively, they are similar to those previously reported related complexes [39,40]. The Ag-Ag bond lengths in complex 2 are identical to those observed in 1. The C-S bond distances in the dithiocarbamate ligand (L2) backbone of complex 2, fall between ideal single and double C-S bonds indicating partial delocalization of π-electron density over the entire S 2 CN fragments in the complex [26,41].
through two centrosymmetrically related Ag-S bonds leading to a ribbon that runs the a-crystallographic axis (Figure 3b).
Just like in 1, the Ag(I) center is coordinated to three S atoms giving a trigonal p ramidal geometry around the metal center. This differs from the coordination polymer [Ag(dibenzyldithiocarbamate)]∞ [17] where each Ag(I) cation is bonded to two pairs sulfur atoms from three dithiocarbamate ligands resulting in a distorted tetrahedral ometry. The S-Ag-S bond angles range from 102.58 (3)

Thermal Decomposition Studies
The thermal decomposition of the two silver complexes was examined by thermogravimetric (TG) analysis, and the superimposed TG graphs are represented in Figure 4. It is observed from the TGA curve that the thermal decomposition of the complexes shows one step that is equivalent to about 78% weight loss associated with the decomposition of the dithiocarbamate backbone ligand and the dominant weight loss of complex 1 occur in the temperature region of 250-325 • C while the one for 2 occurs at 250-350 • C. In both complexes, there is almost no weight loss below 100 • C and above 350 • C. The decomposition of 1 and 2 led to the formation of elemental silver residue with about 22% residue, which was about 2% more than the calculated value of 20% for 1 and 1% less than the calculated value of 23% for 2.

Thermal Decomposition Studies
The thermal decomposition of the two silver complexes was examined by thermogravimetric (TG) analysis, and the superimposed TG graphs are represented in Figure 4 It is observed from the TGA curve that the thermal decomposition of the complexes shows one step that is equivalent to about 78% weight loss associated with the decomposition of the dithiocarbamate backbone ligand and the dominant weight loss of complex 1 occur in the temperature region of 250-325 °C while the one for 2 occurs at 250-350 °C. In both complexes, there is almost no weight loss below 100 °C and above 350 °C. The decomposition of 1 and 2 led to the formation of elemental silver residue with about 22% residue, which was about 2% more than the calculated value of 20% for 1 and 1% less than the calculated value of 23% for 2.

Conclusions
Conclusively, Ag(I)formamidine cluster and Ag(I)formamidine coordination polymer has been synthesized and characterized by means of thermogravimetric analysis together with FT-IR, UV-vis, 1 H, and 13 C NMR spectroscopy. X-ray structural analysis showed that the Ag 6 core of structure 1 formed a six vertex distorted square bi-pyramidal octahedron while in structure 2, each of the two Ag(I) centers are bridged by two N,N -mesityl formamidine dithiocarbamate ligands to form an 8-member Ag 2 (CS 2 ) 2 metallacycles with an inversion center in the middle of the Ag-Ag argentophilic bond. The thermal analysis of complexes 1 and 2 led to the formation of the elemental silver residue.