SYNTHESIS AND SPECTROSCOPIC CHARACTERIZATION OF POLYNUCLEAR SILVER(I) COMPLEX WITH 2,2’-BIQUINOLINE

Polynuclear silver(I) complex, [Ag(NO3-O)(2,2’-bq-N,N’)]n, was synthesized by the reaction of equimolar amount of silver(I) nitrate and 2,2’-biquinoline (2,2’-bq) in ethanol at room temperature. The characterization of the complex was established on the basis of elemental microanalysis, IR, NMR (H and C) and UV-Vis spectroscopic techniques. The results of spectroscopic analyses revealed that in [Ag(NO3-O)(2,2’-bq-N,N’)]n complex, 2,2’-bq ligand behaves as a chelate, while the remaining coordination sites are occupied by the oxygen atoms of two nitrates.

The second important reason for investigation of silver(I) chemistry with aromatic N-heterocycles stems from the findings that the Ag(I) ion coordinated by these ligands is a favorable building block for coordination polymers, having potential applications for design of innovative materials, including liquid crystals (Khlobystov et al., 2001;Yeşilel et al., 2010;Pucci et al., 2011). In coordination polymers, Ag(I) ion can have coordination numbers between two and six, adopting various geometries, such as linear, bent, trigonal, T-shaped, tetrahedral, trigonal pyramidal and octahedral. Moreover, weak contacts, such as argentophilic Ag ... Ag, Ag ... π and Ag ... solvent/counterion interactions, have significant influence on the structural *Corresponding author: dejan.guresic@pr.ac.rs properties of silver(I) coordination polymers in the solid state (Khlobystov et al., 2001;Fik et al., 2014).
In the design of silver(I) coordination polymers, various bridging and chelating aromatic N-heterocycles have been used. Among them, 2,2'-bipyridine (2,2'-bipy) and its derivatives have been a subject of research due to their coordination versatility which allowed the tuning of the topology, the supramolecular architectures and other features in a series of silver(I) complexes (Pucci et al., 2011;Bellusci et al., 2008;Bowmaker et al., 2005). Herein, 2,2'-biquinoline (2,2'-bq), which is a larger and more rigid π system than 2,2'-bipy, was used as a ligand for complexation to Ag(I) ion. This ligand has two nitrogen-donors and the two flexible quinoline moiety linked together by a single C-C bond, which allow its different coordination behaviour towards metal ions (Pucci et al., 2011). Considering this, the reaction of silver(I) nitrate with an equimolar amount of 2,2'biquinoline (2,2'-bq) was performed and polynuclear silver(I) complex, [Ag(NO 3 -O)(2,2'-bq-N,N')] n , was isolated and characterized by spectroscopy (IR, 1 H and 13 C NMR and UV-Vis).

Measurements
Elemental microanalysis of the silver(I) complex for carbon, hydrogen and nitrogen was performed by the Microanalytical Laboratory, Faculty of Chemistry, University of Belgrade. All NMR spectra were recorded at 25 C on a Bruker Avance III 400 MHz spectrometer ( 1 H at 400 MHz, 13 C at 101 MHz). 5 mg of 2,2'-bq and its silver(I) complex was dissolved in 0.6 mL of DMF-d 7 and transferred into a 5 mm NMR tube. Chemical shifts are expressed in ppm ( / ppm) and scalar couplings are reported in Hertz (J / Hz). Chemical shifts were calibrated relative to those of the solvent. The IR spectra were recorded as KBr pellets on a Perkin-Elmer Spectrum One FT-IR spectrometer over the wavenumber range 4000 -450 cm -1 . The UV-Vis spectra were recorded over the wavelenth range of 900 -200 nm on a Shimadzu UV-1800 spectrophotometer after dissolving 2,2'-bq and its silver(I) complex in DMF. The concentration was 1.9 . 10 -5 M.

Synthesis and structural features of [Ag(NO 3 -O)(2,2'-bq-N,N')] n complex
Silver(I) complex with 2,2'-biquinoline (2,2'-bq) was synthesized according to the route presented in Figure 1. The reaction of AgNO 3 and 2,2'-bq in 1 : 1 mole ratio in ethanol at room temperature yielded polynuclear [Ag(NO 3 -O)(2,2'-bq-N,N')] n complex. The composition and structural formula of this silver(I) complex was consistent with elemental analysis, IR, solution NMR ( 1 H and 13 C) and UV-Vis spectroscopic results. We isolated the crystals of the complex suitable for X-ray analysis, however, the crystallographic results indicated that the crystal structure of [Ag(NO 3 -O)(2,2'-bq-N,N')] n was very similar to that for the complex obtained in the reaction of AgNO 3 and 2,2'-bq in acetonitrile (Bowmaker et al., 2005). Considering this, the crystal structure of polynuclear [Ag(NO 3 -O)(2,2'-bq-N,N')] n complex will not be discussed herein.

Spectroscopic characterization
The IR, NMR ( 1 H and 13 C), and UV-Vis spectroscopic data for 2,2'-bq and [Ag(NO 3 -O)(2,2'-bq-N,N')] n complex are listed in the Experimental section. In the IR spectrum of [Ag(NO 3 -O)(2,2'-bq-N,N')] n complex, two strong bands at 1384 and 1302 cm -1 due to the nitrate asymmetric stretching vibrations are observed (Table 1), indicating that nitrate is coordinated to Ag(I) ion (Potapov et al., 2015). The splitting of the nitrate asymmetric stretching vibrations in the IR spectrum of [Ag(NO 3 -O)(2,2'-bq-N,N')] n is in accordance with that observed in the spectra of polynuclear [Ag(NO 3 -O)(qz)] n (qz is quinazoline) (Savić et al., 2016) and [Ag(NO 3 -O)(L-N4) 2 ] n complexes, L is 1-benzyl-1Htetrazole (bntz), 1-benzyl-1H-tetrazol-5-amine (bntza) and 1-(4methoxybenzyl)-1H-tetrazol-5-amine (mbntza) (Andrejević et al., 2018), all containing nitrate as a bridging ligand between two Ag(I) ions (Table 1). Solution state 1 H and 13 C NMR spectra were measured in deuterated DMF in order to confirm the bidentate coordination of 2,2'-bq to the Ag(I) ion. The spectra of the [Ag(NO 3 -O)(2,2'-bq-N,N')] n complex were compared with those for the corresponding ligand. Numbering scheme of carbon atoms in 2,2'-bq is presented in Figure 1. In the 1 H NMR spectrum of the silver(I) complex, there are noticeable downfield shifts (+0.16 and +0.37 ppm) for the H-3 and H-4 protons of the pyridyl moiety of 2,2'bq in respect to those for these protons of the free ligand ( Figure  2). The shifting of the resonance due to the H-3 proton is thought to be a consequence of a change of 2,2'-bq configuration upon its coordination from preferred transoid form to a cisoid one (Starosta et al., 2013). Furthermore, the resonances of the H-5 -H-8 protons are slightly shifted compared to those of the uncoordinated 2,2'-bq. The small coordination shifts observed for these protons in [Ag(NO 3 -O)(2,2'-bq-N,N')] n complex are in agreement with the spectroscopic features of silver(I) complexes with aromatic nitrogen-containing heterocyclic ligands, due to the fast ligand exchange phenomenon on the NMR timescale (Kalinowska-Lis et al., 2015). For the presently investigated silver(I) complex, the order of 1 H resonances is in accordance with those for the mononuclear [Ag(tsac-S) (2,2'-bq-N,N')] . CH 3 CN complex, which spectrum is recorded in DMSOd 6 (tsac is thiosaccharinate anion) (Burrow et al., 2016). The latter complex was previously obtained in the reaction of hexameric [Ag 6 (tsac) 6 ] complex with 2,2'-bq in acetonitrile as solvent (Burrow et al., 2016).
The 13 C NMR spectrum of [Ag(NO 3 -O)(2,2'-bq-N,N')] n in DMF-d 7 displays nine signals differently positioned from those of the uncoordinated ligand. Within the 2,2'-biquinoline, the nitrogen-adjacent C-atoms, i.e. C-2 and C-8a are shielded (-3.32 ppm for C2 and -1.26 ppm for C8a), while the more far-distant ring carbons are deshielded (up to +2.94 ppm for C4).  As can be seen, the spectral features of this complex are similar with those of the 2,2'-bq, differing only in the appearance of a shoulder in the spectrum of the complex. Considering this and the fact that the d 10 metal coordination does not influence the electronic transitions already active in the ligand, the three bands at λ = 315.0, 326.0 and 339.0 nm can be attributed to the π → π* transitions of the aromatic rings of the 2,2'-bq and originate from an intraligand charge transfer (ILCT) (Pucci et al., 2011). On the other hand, the appearance of a shoulder at λ = 355.0 nm in the spectrum of the complex is attributed to the complexation process and can be assigned to the charge transfer processes between silver(I) ion and 2,2'-bq ligand (Tailor et al., 2015;Rusu et al., 2016;Kharat et al., 2011). No d → d transitions are expected for a complex of d 10 metal ion.