Polyhedron 48 (2012) 245–252

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Polyhedron

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Novel silver(I) pyrazole-based coordination polymers: Synthetic andstructural studies

Azizolla Beheshti a,⇑, Hamid Reza Zafarian a, Rahman Khorramdin a, Mohammad Fattahi Monavvar a,Carmel T. Abrahams b

a Department of Chemistry, Faculty of Sciences, Shahid Chamran University, Ahvaz, Iranb Department of Chemistry, La Trobe University, Bundoora Victoria 3086, Australia

a r t i c l e i n f o

Article history:Received 1 July 2012Accepted 28 August 2012Available online 7 September 2012

Keywords:Crystal structuresSilver complexesFlexible N-donor ligandCoordination polymerPyrazole ligandsPolymorphism

0277-5387/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.poly.2012.08.068

⇑ Corresponding author. Tel./fax: +98 611 333 1042E-mail address: beheshti_a@hotmail.com (A. Behe

a b s t r a c t

The reaction of 1,1,3,3-tetrakis(3,5-dimethyl-1-pyrazole)propane (tdmpp) with silver(I) salts (AgSCN orAgI) leads to the formation of [Ag2(l2-SCN)2(l2-tdmpp)]n (1) and [{Ag2(l2-I)2(l2-tdmpp)}.DMF]n (2)complexes. These compounds have been characterized using single crystal X-ray diffraction, elementalanalysis and infrared spectroscopy. In both complexes, the Ag(I) centers are in distorted tetrahedral envi-ronments with Ag centers bridged alternately by tdmpp bridges and pairs of anions. The crystals of poly-morphs [Ag(dmpzH)2]2Cr2O7 (3) and (4), were generated by the reaction of silver(I) chromate withdmpzH (3,5-dimethylpyrazole) in acetone. Recrystallization from acetone/water yielded (3) and recrys-tallization from thf gave (4). In each structure, two monodentate pyrazole ligands bind to the Ag(I) cen-ters in a linear arrangement. Longer, weak interactions, with the dichromate anions, lead to the formationof 1D polymers in each case.

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1. Introduction

In recent years the design, synthesis and structural character-ization of one, two and three-dimensional inorganic/organic hy-brid networks formed by the self-assembly of organic andmetallic components have attracted considerable interest. Gener-ally, the chemical structures of these materials are influenced bymany factors, such as the crystallization solvent, the nature ofthe counter anion, the preferred coordination environment of themetal center and the location and orientation of ligating atomswithin a polynucleating ligand. The ability of such ligands to par-ticipate in non-covalent bonds, such as hydrogen-bonding andp� � �p stacking interactions is also a significant structure-directinginfluence. Factors such as ligand flexibility and the metal/ligand ra-tio all play important roles in the generation of novel coordinationpolymers [1].

The soft d10Ag+ ion, has been commonly used as a metal centerin the construction of coordination polymers [2]. The absence ofcrystal field stabilization for the d10 monovalent silver cation,coupled with its ability to tolerate a wide range of coordinationgeometries (with coordination numbers spanning 2–7) leads toconsiderable variation in the geometry and connectivity of the net-works that can be formed when Ag(I) centers act as connectors [3].

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.shti).

Pyrazole and its derivatives are important heterocyclic mole-cules that exhibit the ability to coordinate to metal centers andparticipate in hydrogen bonding interactions [4]. They show versa-tility in their chemical behavior and constitute the active moietiesof several biochemical systems and ligands of coordination com-plexes [5]. Pyrazole-based ligands may be used as synthetic ana-logs of imidazole and have been used to mimic the coordinatinggroups of metal enzymes or metalloproteins [6]. Ligands incorpo-rating five-membered heterocyclic rings such as pyrazole andimidazole have been widely used in the construction of coordina-tion polymers (commonly referred to as MOFs). Recently, pyra-zole-based moieties have been incorporated into neutral ligandssuch as 1,4-bis(3,5-dimethylpyrazol-1-yl)butane [7] and 1,1,3,3-tetrakis(3,5-dimethyl-1-pyrazole)propane [8] with a view to gen-erating ligands that are capable of linking metal centers withincoordination networks. We have published a number of papersinvolving the synthesis and structural characterization of pyra-zole-based metal complexes. In these studies we have investigatedthe effect of the ligand structure, the metal/ligand ratio and co-li-gands on the topology and geometry of the networks [5a,b,7,9].In a continuation of this work, we present here the preparation,spectroscopic characterization (FT-IR and UV–Vis) and X-raystructures of two new Ag(I) complexes involving the ligand,1,1,3,3-tetrakis(3,5-dimethyl-1-pyrazole)propane(tdmpp). Thefour dimethylpyrazole groups of this potentially tetradentate li-gand (Scheme 1) give it the ability to simultaneously chelate andbridge Ag(I) centers. This study has been extended to include

Scheme 1.

246 A. Beheshti et al. / Polyhedron 48 (2012) 245–252

Ag(I) complexes with the discrete ligand 1,3-dimethylpyrazole(dmpzH), in order to determine the effect of a spacer group be-tween the pyrazole rings on the topology of complexes. In the caseof Ag(I) complexes with dmpzH, we have also investigated theinfluence of the crystallization solvent on the molecular structuresof the complexes. It is hoped that this current work will assist inidentifying factors that govern the formation and structure of poly-meric Ag(I)–pyrazole based compounds.

2. Experimental

2.1. Materials and instrumentation

All synthetic procedures were performed without precautionsto exclude air. Starting materials were purchased from commercialsources and used without further purification; tdmpp was pre-pared by published methods [8]. The infrared spectra (4000–400 cm�1) were recorded on KBr disks with an FT-IR model BOMENMB102 spectrometer. Electronic spectra were recorded on a GBCCintral 101 spectrophotometer from samples in DMF over therange of 700–270 nm. Chemical analyses for C, H and N were per-formed by the Department of Chemistry, Shiraz University, Iran.

2.1.1. Preparation of [Ag2(l2-SCN)2(l2-tdmpp)]n (1)AgSCN (0.332 g, 2.00 mmol) was added to DMSO (30 ml) and

the mixture was stirred for ca. 20 min at room temperature. tdmpp(0.420 g, 1.00 mmol) was added to this solution and the mixturewas stirred for 10 h. The precipitate was centrifuged and filteredoff. The residue was washed with ethanol (2 � 2 ml) and diethylether (2 � 5 ml) and dried in vacuum to give a white powder ofthe product (570 mg, yield: 78% based on Ag). Colorless single crys-tals suitable for X-ray studies were obtained from the filtrate, byslow evaporation of the solvent after 5 days. Anal. Calc. for Ag2C25-

H32N10S2: C, 39.90; H, 4.29; N, 18.61. Found: C, 40.22; H, 3.91; N,18.34%. IR(KBr, cm�1): 3126 (m), 2915 (m), 2101 (s), 1635 (m),1558 (s), 1458 (s), 1418 (s), 1387 (m), 1319 (m), 1297 (m), 1034(s), 790 (s), 681 (m), 565 (m).

2.1.2. Preparation of [{Ag2(l2-I)2(l2-tdmpp)}.DMF]n (2)Compound 2 was prepared and crystallized in a manner similar

to that for complex 1, but using AgI (0.470 g, 2 mmol) and tdmpp(0.420 g, 1 mmol) in DMF (30 ml) to give 613 mg of the product(yield: 64% based on Ag). Anal. Calc. for Ag2I2C26H39N9O: C,32.42; H, 4.08; N, 13.09. Found: C, 33.06; H, 3.57; N, 13.48%. IR(KBr,cm�1): 3120 (m), 2916 (m), 1677 (s), 1557 (s), 1461 (s), 1418 (s),1381 (m), 1323 (m), 1295 (m), 1030 (s), 784 (s), 684 (m), 565(m). The air-stable colorless single crystals of 1 and 2 are insolublein common organic solvents such as acetone, MeCN and CH2Cl2,but they exhibit limited solubility in DMF and DMSO.

2.1.3. Preparation of [Ag(dmpzH)2]2Cr2O7 (3) and (4)Ag2CrO4 (0.332 g, 1.0 mmol) was dissolved in acetone (20 ml)

and solid dmpzH (0.384 g, 4 mmol) was added. After stirring for24 h, the yellow precipitate was collected by filtration. The solid

was washed with diethyl ether (3 � 5 ml) and dried in vacuo togive [Ag(dmpzH)2]2Cr2O7 (544 mg, yield: 76% based on Ag) as ayellow powder. [Ag(dmpzH)2]2Cr2O7 was dissolved in a mixtureof acetone/water (5 � 2, 20 ml). The yellow solution was stirredfor 5 min and then filtered. Red needle-shaped single crystals suit-able for X-ray crystallography of 3 were obtained by slow evapora-tion of the above solution after three days. In the case of 4, cubiccrystals were obtained by slow evaporation of the yellow solutionof THF (20 ml) after three days. IR(KBr, cm�1) for 3: 543(w),770(m), 945(s), 1048(w), 1298(w), 1418(m), 1582(m), 2873(s),3038(s), 3132(s), 3200(s); Decomposition point: 175 �C. IR(KBr,cm�1) For 4: 542(w), 770(m), 943(s), 1047(w), 1298(w), 1417(m),1572(m), 2874(s), 3038(s), 3132(s), 3199(s); Decomposition point:175 �C .

2.2. Physical measurements

2.2.1. X-ray crystallographyThe intensity data for 1–3 were collected using an Oxford Dif-

fraction, Supernova diffractometer. The intensity data for 4 werecollected using an Oxford Diffraction, Xcalibur diffractometer.Crystals were transferred directly from the mother liquid to a pro-tective oil before being cooled to 130 K in a stream of nitrogen. Thestructures were solved using direct methods and refined using afull-matrix least squares procedure based on F2 using all data[10]. Hydrogen atoms were placed at geometrically estimated posi-tions. Crystallographic data for compounds 1–4 are given inTable 1.

3. Results and discussion

3.1. Description of structures

3.1.1. Crystal structure of [Ag2(l2-SCN)2 (l2-tdmpp)]n (1)Single crystal X-ray studies of [Ag2(l2-SCN)2(l2-tdmpp)]n, 1,

indicate that the compound crystallizes in the monoclinic spacegroup C2/c. The asymmetric unit consists of one Ag, one SCN- anionand one half of a 1,1,3,3-tetrakis(3,5-dimethyl-1-pyrazole)propane(tdmpp) ligand which has the central atom of the propylene bridgelocated on a twofold axis. Each tdmpp ligand bridges a pair of crys-tallographically related Ag(I) centers with each Ag(I) chelated bytwo pyrazole groups (Fig. 1a). A distorted tetrahedral coordinationenvironment is completed around each Ag(I) center by a sulfur do-nor and a nitrogen donor belonging to a pair of centrosymmetrical-ly related thiocyanate anions that act as a double bridge to anotherAg(I) center (Fig. 1b). The result is a sinusoidal 1D polymer that ex-tends along the direction of the c-axis with Ag(I) centers linkedalternately by tdmpp ligands and double SCN- bridges. Within thischain the 8-membered rings formed by the two SCN- anions andtwo Ag(I) centers are close to planar but along the length of thechain these two rings alternate in orientation as can be seen inFig. 1b. The affinity of Ag(I) for both N and S donors makes the thio-cyanate double bridge a potentially useful motif for extending thedimensionality of Ag(I)-based supramolecular systems; a numberof examples of Ag2(SCN)2 units have been reported in the literature[11].

In the extended structure all polymer chains are parallel andeach chain is surrounded by six equivalent chains which arerelated to each other by pure translations, as indicated in Fig. 1c.The sinusoidal chains stack on top of each other as indicated inFig. 1d.

3.1.2. Crystal structure of [{Ag2(l2-I)2(l2-tdmpp)}.DMF]n (2)Compound 2 crystallizes in the monoclinic system with space

group P21/c. The tdmpp ligand adopts a similar conformation to

Table 1Crystallographic and data collection parameters for compounds 1-4.

[Ag2(l2-SCN)2(l2-tdmpp)]n (1) [{Ag2(l2-I)2(l2-tdmpp)}.DMF]n (2) [Ag(dmpzH)2]2Cr2O7 (3) [Ag(dmpzH)2]2Cr2O7 (4)

Chemical Formula C25H32Ag2N10S2 C26H39Ag2I2N9O C20H32Ag2Cr2N8O7 C20H32Ag2Cr2N8O7

Formula Weight 752.47 963.20 816.28 816.28Crystal system monoclinic monoclinic monoclinic monoclinicSpace Group C2/c P21/c P21/n C2/ca(Å) 22.256(3) 11.7314(12) 13.5624(5) 19.9694(8)b(Å) 7.9245(6) 19.0187(15) 15.3087(5) 8.7060(2)c(Å) 19.196(2) 15.2179(15) 13.8451(4) 17.4856(7)a(�) 90 90 90 90.002b(�) 121.38(2) 95.864(9) 101.587(4) 109.925(4)c(�) 90 90 90 90V(Å3) 2890.4(5) 3377.6(5) 2816.0(2) 2858.0(2)Z 4 4 4 4Temperature (K) 130(1) 130(1) 130(1) 130(1)k, (MoKa/CuKa) (Å) 0.71073 1.54184 0.71073 0.71073qcalc. (g cm�3) 1.729 1.894 1.925 1.897F(000) 1512 1864 1624 1624l (cm�1) 1.534 23.925 2.175 2.143Crystal size (mm) 0.03 � 0.089 � 0.098 0.028 � 0.043 � 0.152 0.044 � 0.052 � 0.382 0.07 � 0.08 � 0.1Reflections measured 6592 10636 17626 8366Unique reflections [Rint] 2547 5753 6316 4320No. of Parameters 181 371 360 181wR2 (all data) 0.1266 0.2200 0.0868 0.0581R1 (I > 2r(I)) 0.0551 0.0764 0.0325 0.0243Completeness of data 99.8% 100% 97.7% 99.74%

A. Beheshti et al. / Polyhedron 48 (2012) 245–252 247

that seen in 1 however there is no crystallographic 2-fold axispassing through the central propylene carbon atom (Fig. 2a). Thestructure consists of 1D polymeric chains similar to 1, but withdouble iodide bridges in place of the double thiocyanate bridges(Fig. 2b). In contrast to 1, the asymmetric unit consists of a fulltdmpp ligand which bridges two distinct Ag(I) centers (Ag1 andAg2) that are in similar tetrahedral coordination environmentsformed by two iodide ions and a pair of pyrazole nitrogen atoms(Fig. 2b). The two crystallographically distinct Ag2(l2-I)2 rings areeach centered on centers of inversion and in each case the Ag cen-ters are less than 2.82 Å apart. Similar separations between Ag(I)centres have been attributed to weak Ag–Ag bonds but in thesecases [12] pairs of Ag(I) centres are unsupported by bridging an-ions. The double iodide bridge plays a similar role to the thiocya-nate bridges in 1. A number of examples of the Ag2(l2-I)2 motifhave been reported [13].

Similar to 1, in the extended structure of 2, all the chains areparallel and each is surrounded by six equivalent chains as indi-cated in Fig. 2c. In contrast to 1 however, the chains are in two dis-tinct orientations. The top and central chains from Fig. 2c aredepicted in Fig. 2d, which shows a side-on view of the chains aswell as the location of DMF molecules, which occupy spacesbetween adjacent polymer chains.

3.1.3. Crystal structure of [Ag(dmpzH)2]2Cr2O7 (3)The single crystal structure determination of 3 reveals two dis-

tinct Ag(I) centers, (Ag1 and Ag2), four 3,5-dimethylpyrazoleligands and a single dichromate ion in the asymmetric unit. Eachof the silver centers is coordinated by a pair of nitrogen atomsbelonging to two 3,5-dimethylpyrazole ligands (Fig. 3a and b).The Ag–N bond lengths fall within the range of 2.101(2)–2.116(2) Å and the N–Ag–N angles are close to linear [N–Ag1–N169.9(1)� and N–Ag2–N 171.3(1)�]. The pair of pyrazole ligandsbound to each Ag center are close to coplanar and are arrangedsuch that the protonated nitrogen atoms in each complex lie onthe same side of the almost co-linear Ag–N bonds.

Dichromate oxygen atoms interact only weakly with the Agcenters with Ag� � �O distances close to 3 Å, a separation which isjust under the sum of the van der Waals radii for silver and oxygen(3.14 Å) [14]. Whilst such separations would normally be consid-

ered too long to be coordinate bonds, such interactions have beenshown to exert a significant structure-directing influence in vari-ous coordination polymers [12,15]. For this reason these interac-tions have been included in the discussion of the coordinationgeometries of the Ag(I) centers. In the case of Ag1, two oxygenatoms (O3 and O4I) belonging to two different dichromate anionsbind to the metal center with separations of 2.850(2) Å (Ag1–O3)and 3.064(3) Å (Ag1–O4I). If only the shorter of the two interac-tions is considered then the coordination geometry of the Ag1 cen-ter is T-shaped. If the two Ag–O interactions are taken into accountthen the Ag1 center has an irregular 4-coordinate geometry thatmay be described as a distorted ‘‘see-saw’’ geometry [16](Fig. 3a). A pair of dichromate oxygen atoms from two distinct an-ions bind to each Ag2 center with Ag–O separations significantlyshorter than those observed with the Ag1 center (Ag2–O7:2.651(2) Å, Ag2–O7I: 2.756(2) Å). As with Ag1 the inclusion ofthese two Ag–O interactions leads to a 4-coordinate geometry thatmay be described as ‘‘see-saw’’. The two O atoms bound to Ag2 arepart of a 4-membered ring as indicated in Fig. 3c. A center ofinversion lies at the center of this 4-membered ring. The two sym-metry-related Ag2 centers are 3.398 Å apart. If all the long Ag–Ointeractions are taken into account the result is a 1-D polymericstructure (Fig. 3d) in which dichromate anions link an infinite ser-ies of bis(pyrazolyl)silver complexes that extend in the a-direction.Whilst the Ag2 centers are bridged by a pair of oxygen atoms, theAg1 centers are bridged by a pair of O–Cr–O connections, resultingin the formation of Ag(O–Cr–O)2Ag, 8-membered rings; a secondcenter of inversion lies at the center of this ring. Ag(I) complexeswith the dichromate anion are rare [17].

Each dichromate anion forms links to four Ag centers and eachAg center is linked to two dichromate anions located on either sideof the chain. Along the length of the chain, the location of thedichromate alternates from front to back as shown in Fig. 3d. Thepyrazole rings lie approximately parallel to the bc-plane, extendingabove and below the Ag centers. Although offset from each other tosome extent, parallel pyrazole groups form infinite face-to-facestacks that extend along the length of the polymer.

Both protonated pyrazole groups bound to a single Ag centerparticipate in H-bonding interactions with dichromate oxygenatoms as indicated in Fig. 3a and b. It is these interactions that

Fig. 1. The structure of 1. (a) The coordination mode of the tdmpp ligand. (b) The 1D polymeric chain, showing the coordination environment of the Ag center. For clarity,methyl groups and hydrogen atoms have been omitted in panels (a and b). Color codes for panels (a and b): C black; N blue, Ag pink, S yellow, coordinate bonds are indicatedby pink connections. (c) A stick representation of seven equivalent parallel chains, related to each other by pure translations, viewed down the c-axis; the central chain isindicated in blue. (d) The top, bottom and central chains from panel (c) viewed side-on. (Colour online.)

248 A. Beheshti et al. / Polyhedron 48 (2012) 245–252

appear to be responsible for directing the orientation of the proton-ated pyrazole nitrogen atoms to the same side of the silver center.

3.1.4. Crystal structure of [Ag(dmpzH)2]2Cr2O7 (4)The asymmetric unit of 4 consists of a single Ag(I) center, two

3,5-dimethylpyrazoles and half a dichromate anion. A pair of nitro-gen atoms, belonging to two 3,5-dimethylpyrazole ligands, bind tothe Ag center in a near linear geometry (N–Ag1–N 167.0(1)�). TheAg–N bond lengths lie in the range of 2.111(2)–2.117(2) Å. The twopyrazole ligands bound to each Ag center are essentially coplanarand are arranged such that the protonated nitrogen atoms are on

the same side of the almost co-linear Ag–N bonds. Both of the pro-tonated nitrogen atoms participate in hydrogen bonds with oxygenatoms of a dichromate anion as indicated in Fig. 4a. In addition tothe two nitrogen atoms, two oxygen atoms belonging to two dis-tinct dichromate anions are bound to the Ag center (Fig. 4a) withAg–O distances of 2.639(2) and 2.850(2) Å. These four donor atomsprovide a see-saw coordination geometry around the Ag center.The coordinated oxygen atoms depicted in Fig. 4a are also boundto a symmetry-related Ag center. A center of inversion lies at thecenter of the 4-membered (Ag–O)2 ring (Fig. 4b). This arrangementbrings the two silver centers to within 3.0900(4) Å of each other; a

Fig. 2. The structure of 2. (a) The coordination mode of the tdmpp ligand. (b) The 1D polymeric chain. For clarity, methyl groups and hydrogen atoms have been omitted.Color code (atoms): C black; N blue, Ag pink, I purple, coordinate bonds are indicated by pink connections. (c) A stick representation of seven equivalent parallel chains viewedalong the direction of the polymer; the central chain is indicated in blue. (d) The top and central chains from panel (c) viewed side-on; dimethylformamide moleculesbetween the two adjacent polymer chains are indicated by green connections. (Colour online.)

A. Beheshti et al. / Polyhedron 48 (2012) 245–252 249

Fig. 3. The structure of 3. (a) The coordination environment of Ag1. Hydrogen bonds are indicated by striped connections; N4� � �O4I 2.747(3), N2� � �O3I 2.864(3) Å. (b) Thecoordination environment of Ag2. Hydrogen bonds are indicated by striped connections; N8� � �O1II 2.845(3), N6� � �O7II 2.964 (4) Å. (c) The 4-membered (Ag2–O7)2 ring. (d)alternating 4- and 8-membered rings in 1D polymer. For clarity, hydrogen atoms in have been omitted in panel (d). Color code: C black; H pale pink; N blue, O red, Cr green, Agpurple. (I 2-x, -y, 2-z; II 1-x, -y, 2-z). (Colour online.)

250 A. Beheshti et al. / Polyhedron 48 (2012) 245–252

separation consistent with a weak Ag���Ag interaction [12]. The aro-matic ligands within the binuclear unit make close face-to-facecontact. The binuclear units are bridged by dichromate anionsand form a 1D polymer that extends in the c-direction (seeFig. 4c). The plane of the 4-membered (Ag–O)2 ring alternates inorientation along the length of the chain.

The structure of 4 provides an interesting contrast with its poly-morph 3. The local geometry of the Ag centers in 3 and 4 is similar

as is apparent by comparison of Fig. 3a and b with Fig. 4a. This sim-ilarity would appear to reflect a strong preference for trans coordi-nation of the 1,3-dimethylpyrazole ligands, favorable hydrogenbonding involving the protonated ligand and the anion and p�pstacking interactions involving coordinated 1,3-dimethylpyrazoleon neighboring Ag(I) centers. Although the similarities between 3and 4 extend to the presence of polymeric chains, differences be-tween the two polymorphs are most evident in their extended

Fig. 4. The structure of 4. (a) The Ag coordination environment with hydrogen bonds indicated by striped connections; N2� � �O2 2.856(2), N4� � �O3I 2.991(3) Å. (b) The 4-membered (Ag–O)2 ring. (c) The 1D coordination polymer with the 4-membered (Ag–O)2 rings alternating in orientation, along the length of the polymer. For clarity,hydrogen atoms have been omitted in panels (b and c). Color code: As in Fig. 3. (I 1-x, y, 1/2-z; II 1-x, -y, 1-z). (Colour online.)

A. Beheshti et al. / Polyhedron 48 (2012) 245–252 251

structures (Figs. 3d and 4c). In 3, all of the Ag–N bonds are close toparallel while in 4 the Ag–N bonds are in two distinct orientations.Furthermore within the 1D polymeric structure (Ag–O)2 4-mem-bered rings in 3 (Fig. 3c) have a Ag–Ag separation which is signif-icantly longer (3.398 Å) than that observed in 4.

3.2. Spectroscopic characterization

The IR spectra of all the complexes showed strong bands corre-sponding to the stretching vibration of the C@N bonds typical ofthe pyrazole rings at the region of 1555–1595(s) cm�1. This bandis shifted to lower frequencies with respect to the spectrum ofthe free dmpzH ligand (1595 cm�1). In the infrared spectrum ofcomplex 1 only one band is found for the m(CN) stretching modeat 2101 cm�1, which is located 48 cm�1 higher than that expectedfor the free NCS� ligand. This suggests that the NCS� ligands are allbridging (Ag–NCS–Ag) in the complex [5b]. The infrared spectrumof 2 shows a band at 1677 cm�1 due to the C@O group of theuncoordinated DMF molecule. The FT–IR spectrum of complex 3exhibits a band at 3200 cm�1 characteristic of N-H stretchingvibrations. A heteroaromatic molecule such as 3,5-dimethylpyra-zole containing an N–H group shows its stretching absorption inthe region of 3500–3220 cm�1 [18]. The relatively low value ofthe t(N–H) frequency in the infrared spectrum of 3 provides evi-dence of the presence of N–H� � �O hydrogen bonds in the complex.In addition, in the infrared spectrum of the complex, the t(Cr–O)stretching frequencies were observed at 945 cm�1, which is shiftedto a lower frequency relative to the parent [Cr2O7]2�dianion(950 cm�1) [19]. This indicates that the [Cr2O7]2� dianion is hydro-gen bonded to the N–H and C–H groups of the 3,5-dimethylpyra-zole ligands. These results are consistent with crystal structureanalysis of the complex.

The UV–Vis absorption spectra of freshly prepared samples ofcomplexes 3 and 4 in dilute solution in DMF show typical absorp-tion peaks at 274 and 375 nm, due to the ligand to metal chargetransfer transition (O ? Cr) in the [Cr2O7]2� dianion. These bandsare red-shifted compared to the free [Cr2O7]2� dianion.

4. Concluding remarks

When discrete pyrazole ligands are present in the silver com-plexes considered here, a linear coordination at the silver centerappears to be favoured. However, when the tetra-pyrazole ligand,1,1,3,3-tetrakis(3,5-dimethyl-1-pyrazole)propane (tdmpp) is em-ployed, two of the pyrazole groups chelate a metal center, allowinganions to approach the Ag center more closely, thus forming moresignificant coordinate interactions. In all cases 1-4 we have seenthe anions act as bridging units in the generation of 1D polymers,even though in the case of dichromate the anion–Ag interactionsare quite weak (3 and 4). It is noted that in the case of the com-plexes containing the dimethylpyrazole ligand, choice of the crys-tallization solvent and H-bonding with the anion appears to beimportant to the resulting structures.

Acknowledgement

We thank Shahid Chamran University of Ahvaz for financialsupport.

Appendix A. Supplementary data

CCDC 886797–886800 contain the supplementary crystallo-graphic data for this paper. These data can be obtained free of

252 A. Beheshti et al. / Polyhedron 48 (2012) 245–252

charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, orfrom the Cambridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail:deposit@ccdc.cam.ac.uk.

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