Synthesis, crystal structure, and theoretical studies of a macrocyclic silver(I) complex of imino-pyridyl Schiff base ligand European Journal of Chemistry 12 (3) (2021) 248-255 European Journal of Chemistry ISSN 2153-2249 (Print) / ISSN 2153-2257 (Online) – Copyright © 2021 The Authors – Atlanta Publishing House LLC – Printed in the USA. This work is published and licensed by Atlanta Publishing House LLC – CC BY NC – Some Rights Reserved. https://dx.doi.org/10.5155/eurjchem.12.3.248-255.2091 European Journal of Chemistry View Journal Online View Article Online Synthesis, crystal structure, and theoretical studies of a macrocyclic silver(I) complex of imino-pyridyl Schiff base ligand Jahangir Mondal 1, Meman Sahu 1, Bhaskar Sharma 1, Rakesh Ganguly 2, Shubhamoy Chowdhury 3 and Goutam Kumar Patra 1,* 1 Department of Chemistry, Faculty of Physical Sciences, Guru Ghasidas Vishwavidyalaya, Bilaspur, Chhattisgarh, 495009, India mailtobapi88@gmail.com (J.M.), memansahu8@gmail.com (M.S.), bsharma05@gmail.com (B.S.), goutam.patra@ggu.ac.in (G.K.P.) 2 Shiv Nadar University, Greater Noida, Gautam Buddha Nagar, Uttar Pradesh, 201314, India rakesh.ganguly@snu.edu.in (R.G.) 3 Department of Chemistry, University of Gour Banga, Malda, West Bengal, 732103, India shubha103@yahoo.com (S.C.) * Corresponding author at: Department of Chemistry, Faculty of Physical Sciences, Guru Ghasidas Vishwavidyalaya, Bilaspur, Chhattisgarh, 495009, India. e-mail: goutam.patra@ggu.ac.in (G.K. Patra). 10.5155/eurjchem.12.3.248-255.2091 Received: 29 January 2021 Received in revised form: 14 March 2021 Accepted: 30 April 2021 Published online: 30 September 2021 Printed: 30 September 2021 The synthesis and characterization of an imino-pyridyl ligand N,N'-(butane-1,4-diyl)bis(1- (pyridin-2-yl)methanimine) (L) and its Ag(I)ClO4 complex (I) by various spectroscopic techniques and elemental analyses is presented in this study. X-ray single crystal structure of complex I revealed that in complex I, each Ag(I) ion is tetra coordinated with two pyridine N-atoms and two imine N-atoms of the ligand L, forming a macrocyclic dimeric Ag(I) grid. In the macrocyclic dimer complex I, Ag-Ag separation along the chain is 5.318 Å. The Ag-Npy average distance is 2.396 Å and that of the Ag-Nim is 2.257 Å. The macrocyclic dimer complex I is supramolecularly arranged by π-stacking interactions. Computational, Hirshfeld surface analysis and photophysical studies on ligand L and complex I have also been performed. Crystal data for C32H36Ag2Cl2N8O8 (M =947.33 g/mol): Triclinic, space group P-1 (no. 2), a = 9.1714(12) Å, b = 10.4373(14) Å, c = 10.8297(14) Å, α = 112.317(3)°, β = 91.391(3)°, γ = 92.353(3)°, V = 957.3(2) Å3, Z = 1, T = 293.15 K, μ(MoKα) = 1.220 mm-1, Dcalc = 1.643 g/cm3, 10248 reflections measured (4.07° ≤ 2Θ ≤ 53.098°), 3966 unique (Rint = 0.0280, Rsigma = 0.0331) which were used in all calculations. The final R1 was 0.0722 (I > 2σ(I)) and wR2 was 0.2229 (all data). DFT Ag(I) dimer Photophysics Imino-pyridyl ligand X-ray crystal structure Hirshfeld surface studies Cite this: Eur. J. Chem. 2021, 12(3), 248-255 Journal website: www.eurjchem.com 1. Introduction Schiff base complexes have remained an imperative and popular area of research due to their simple synthesis, versa- tility and diverse range of applications [1]. Among coordination compounds, d10 metal complexes are of interest due to their high thermal stability and good photoluminescent and electro- luminescent properties [2-7]. Silver is an important precious metal and it has been widely used in industry, for example electrical and electronic applica- tions, photographic production and the manufacturing of fungicides [8-17]. The complexation of Ag(I) in a range of supramolecular materials has received increasing attention, due to the coordinative flexibility of the d10 ion as well as to its well-documented tendency to form strong complexes with nitrogen donor ligands [18]. Many topologically promising architectures have been constructed for the Ag(I) ion with bidentate building blocks containing a nitrogen donor [19,20]. Silver(I) possesses high affinity towards N, P, and O donor ligands. However, it prefers N donors over O donor sites in presence of other hard metal ions and therefore, various heterometallic or bimetallic coordination polymers containing Ag(I) have been successfully synthesized [21-24]. Like 2,2'- bipyridine and 1,10-phenanthroline, Schiff bases derived from 2-pyridine-carboxaldehyde provide the p-acidic α,α’-diimine fragment for metal coordination [25]. Among the coordination compounds of various metals, silver(I) complexes are a theme of interest as luminescent compounds with a wide range of photophysical and photo- chemical properties. Sensitivity of silver(I) complexes to such external stimuli as solvent vapors, mechanical grinding, and temperature make them appealing candidates for the design of smart luminescent materials. Various relaxation mechanisms of the excited states have been established for silver(I) complexes, e.g., fluorescence, phosphorescence, and thermally activated delayed fluorescence (TADF). The excited state lifetimes of silver(I) complexes range from nanoseconds up to micro- and milli-seconds. ABSTRACT RESEARCH ARTICLE KEYWORDS https://dx.doi.org/10.5155/eurjchem.12.3.248-255.2091 https://www.eurjchem.com/ https://dx.doi.org/10.5155/eurjchem.12.3.248-255.2091 mailto:mailtobapi88@gmail.com mailto:memansahu8@gmail.com mailto:bsharma05@gmail.com mailto:goutam.patra@ggu.ac.in mailto:rakesh.ganguly@snu.edu.in mailto:shubha103@yahoo.com mailto:goutam.patra@ggu.ac.in http://www.eurjchem.com/ https://crossmark.crossref.org/dialog/?doi=10.5155/eurjchem.12.3.248-255.2091&domain=pdf&date_stamp=2021-09-30 Mondal et al. / European Journal of Chemistry 12 (3) (2021) 248-255 249 2021 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.12.3.248-255.2091 N N N N Scheme 1. Ligand system (L) used in the study. In comparison with other types of emission sensitivity to external stimuli, the excitation wavelength-dependent photo- luminescence is a rare property for silver(I) complexes. Analysis of the self-assembly through various inter- molecular interactions like hydrogen bonding or π-π stacking is important to understand how molecules interact with their direct environment and focus on insights into crystal packing behavior. Hirshfeld surface-based tools appear as a novel approach to this end. The central element in this method is the derivation of the Hirshfeld surface and the immediately interpretable visualization of a molecule within its environ- ment, and the decomposition of this surface to provide a directly accessible 2D map as “molecular fingerprint”. As a part of our ongoing research to study a series of supramolecular complexes of imino-pyridyl Schiff-base ligand and d10-metal ions as building blocks [26-28], we have presen- ted here the synthesis, structure, photophysical properties and computational study of a novel N,N-donor Schiff base imino- pyridyl ligand L (Scheme 1) and its macrocyclic Ag(I) complex (I). 2. Experimental 2.1. Materials and physical measurements All chemicals used in this study were purchased from Aldrich Chemical Company, USA, and Acros Chemical Company, USA, and used without further purification unless otherwise mentioned. The melting point was determined by an electro- thermal IA9000 series digital melting point apparatus and is uncorrected. Microanalyses were carried out using a Perkin- Elmer 2400II elemental analyzer. Infrared (IR) spectra and solution electronic spectra were recorded on Nicolet Magna IR (Series II) and Shimadzu UV-160A spectrophotometers, respectively. 1H NMR spectra and electro-spray ionization mass (ESI-MS) measurements were made using a Bruker Advance 400 MHz NMR Spectrometer and Finnigan LCQ Decaxp MAX mass spectrometer, respectively. Fluorescence spectra were recorded on a Perkin Elmer LS55 Luminescence Spectrometer. 2.2. Synthesis of the ligand Ligand was prepared by following a reported procedure [26]. 1,4-Diaminobutane (0.088 g, 1 mmol) was dissolved in anhydrous methanol (15 mL) and to this solution 2 mmol (0.215 g) 2-pyridinecarboxaldehyde was added. The reaction mixture was heated under reflux maintaining a dry condition for 3 h. Then it was kept in air for 24 h. The obtained off-white solid was re-crystallized from methanol. N,N'-(Butane-1,4-diyl)bis(1-(pyridin-2-yl)methanimine) (L): Color: Off-white. Yield: 72%. M.p.: 62-64 °C. FT-IR (KBr, ν, cm- 1): 3431 (b), 3055(b), 2937(m), 2854(m), 1649(s), 1587(s), 1469(m), 1437 (s), 1342(m), 1305(s), 1257(m), 1143(vs), 1122(vs), 1087(vs), 1001(m), 867(m), 775(vs), 744(s), 628(vs), 493(s), 406(s). 1H NMR (400 MHz, CDCl3, δ, ppm):8.57 (d, 2H, Ar-H), 8.31 (s, 2H, -CH=N), 7.92 (d, 2H, Ar-H), 7.67 (t, 2H, Ar-H), 7.22-7.27 (m, 2H, Ar-H), 3.60 (t, 4H, -CH2), 1.66 (q, 4H, - CH2). MS (EI, m/z (%)):267.31 (LH+, 100%). Anal. calcd. for C16H18N4: C, 72.15; H, 6.81; N, 21.04. Found: C, 71.96; H, 6.69; N, 22.04%. 2.3. Synthesis of the complex [Ag2L2](ClO4)2(I) To 20 mL off-white methanol solution of L (0.27 g, 1 mmol) was added solid AgClO4 (0.21 g, 1 mmol). The reaction mixture was stirred for 3 h. Light yellow colored precipitate obtained was filtered off and dried in air. It was then dissolved in acetonitrile and kept in the refrigerator overnight. Light yellow crystalline complexes suitable for X-ray analysis were obtained, filtered off, washed with 5 mL methanol dried in vacuum over fused CaCl2. Color: Light yellow. Yield: 60%. FT-IR (KBr, ν, cm- 1): 3429 (wb), 3037 (w), 2943 (m), 2856 (m), 2015 (w) 1645 (s), 1615 (s), 1458 (w), 1412 (m), 1318 (w), 1245 (m), 1107 (s), 825 (s), 629 (s), 498 (w). Anal. calcd. for C32H36N8Ag2Cl2O8: C, 40.57; H, 3.83; N, 11.83. Found: C, 40.65; H, 3.92; N, 11.76%. CAUTION! Although while working with the perchlorate complex described here, we have not met with any incident, care should be taken in handling them as perchlorates are potentially explosive. They should not be prepared and stored in large amounts. 2.4. X-ray crystallography X-ray single-crystal data are collected using MoKα (λ = 0.7107 Å) radiation on a Bruker APEX II diffractometer equipped with CCD area detector. Data collection, data reduction, structure solution/refinement are carried out using the software package of SMART APEX [29]. The structures are solved by direct methods (SHELXS-97) and standard Fourier techniques and refined on F2 using full-matrix least-squares procedures (SHELXL-97) using the SHELX-97 package [30] incorporated in WinGX [31]. Generally, non-hydrogen atoms are considered anisotropically. In the crystal structure of complex I, hydrogen atoms were introduced in the riding mode. In other cases, the hydrogen atoms are geometrically fixed. The crystallographic details of complex I have been summarized in Table 1, and the selected bond lengths and angles of complex I have been listed in Table 2. 2.5. Theoretical calculations The program package GAUSSIAN-09 Revision C.01 was used for all calculations [32]. The gas phase geometries of the compound were fully optimized symmetry restrictions in the singlet ground state with the gradient-corrected DFT level coupled with B3LYP [33]. The LanL2DZ basis set was used for the L and I [34]. The HOMOs and LUMOs of the L and I were calculated with the TD-DFT method, and the solvent effect (in acetonitrile) was simulated using the polarizing continuum model with the integral equation formalism (C-PCM) [35,36]. 2.6. Hirshfeld surface calculation For obtaining additional insight into the intermolecular interaction of molecular crystals, Hirshfeld surface analysis helps as a powerful set-up. The size and shape of Hirshfeld surface allow the qualitative and quantitative study and imagining of intermolecular close contacts in molecular crystals [37]. The Hirshfeld surface enclosing a molecule is defined by a set of points in 3D space where the contribution to the electron density from the molecule of interest is equal to the contri- bution from all other molecules. 250 Mondal et al. / European Journal of Chemistry 12 (3) (2021) 248-255 2021 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.12.3.248-255.2091 Table.1. Crystal data and structure refinement for complex I. Empirical formula C32H36Ag2Cl2N8O8 Formula weight 947.33 Temperature (K) 293.15 Crystal system Triclinic Space group P-1 a (Å) 9.1714(12) b (Å) 10.4373(14) c (Å) 10.8297(14) α (°) 112.317(3) β (°) 91.391(3) γ (°) 92.353(3) Volume (Å3) 957.3(2) Z 1 ρcalc (g/cm3) 1.643 μ (mm-1) 1.220 F(000) 476.0 Crystal size (mm3) 0.16 × 0.14 × 0.1 Radiation MoKα (λ = 0.71073) 2Θ range for data collection (°) 4.07 to 53.098 Index ranges -11 ≤ h ≤ 11, -13 ≤ k ≤ 13, -13 ≤ l ≤ 13 Reflections collected 10248 Independent reflections 3966 [Rint = 0.0280, Rsigma = 0.0331] Data/restraints/parameters 3966/1124/422 Goodness-of-fit on F2 1.046 Final R indexes [I≥2σ (I)] R1 = 0.0722, wR2 = 0.2098 Final R indexes [all data] R1 = 0.0853, wR2 = 0.2229 Largest diff. peak/hole (e Å-3) 1.16/-0.77 CCDC number 2058296 Table 2. Experimental (X-ray) and theoretical bond distances (Å) and angles (°) for complex I. Atom Atom Length (Å) Calc. (Å) Atom Atom Atom Angle (°) Calc. (°) Atom Atom Atom Angle (°) Calc. (°) Ag1 N1 2.426(12) 2.4533 N13 Ag1 N1 107.1(6) 141.301 C14 C13 C12 120.0 119.079 Ag1 N13 2.302(13) 2.3327 N13 Ag1 N14 71.9(4) 72.007 C15 C14 C13 120.0 118.825 Ag1 N2 2.234(13) 2.3337 N2 Ag1 N1 72.6(4) 72.017 C14 C15 C16 120.0 118.791 Ag1 N14 2.335(12) 2.4550 N2 Ag1 N13 158.9(7) 136.136 C15 C16 N4 120.0 122.481 N1 C1 1.3900 1.3518 N2 Ag1 N14 120.3(6) 141.168 N3 C11 C12 124.9(13) 122.567 N1 C5 1.3900 1.3658 N14 Ag1 N1 146.8(6) 101.846 C10 N3 Ag1A 127.0(10) 124.672 C1 C2 1.3900 1.4112 C1 N1 Ag1 129.4(5) 129.203 C11 N3 C10 115.4(13) 119.793 C2 C3 1.3900 1.4112 C1 N1 C5 120.0 118.934 C11 N3 Ag1A 116.5(10) 115.533 C3 C4 1.3900 1.4079 C5 N1 Ag1 109.3(5) 111.510 N12 C110 C109 93.7(11) 110.301 C4 C5 1.3900 1.4089 N1 C1 C2 120.0 122.481 N13 C107 C108 86.2(10) 110.301 C5 C6 1.396(15) 1.4788 C3 C2 C1 120.0 118.791 C109 C108 C107 118.0(12) 112.106 C6 N2 1.260(16) 1.2949 C4 C3 C2 120.0 118.825 C110 C109 C108 125.4(12) 112.107 C7 C8 1.450(16) 1.5499 C3 C4 C5 120.0 119.079 C6 N2 Ag1 115.4(9) 115.536 C7 N2 1.386(16) 1.4826 N1 C5 C6 116.1(8) 117.819 C6 N2 C7 120.2(13) 119.795 C8 C9 1.522(14) 1.5445 C4 C5 N1 120.0 121.890 C7 N2 Ag1 121.1(11) 124.669 C9 C10 1.518(16) 1.5498 C4 C5 C6 123.8(8) 120.286 C101 N14 Ag1 127.3(6) 129.249 C10 N3 1.55(3) 1.4826 N2 C6 C5 125.9(11) 122.563 C101 N14 C105 120.0 118.930 N13 C107 1.347(16) 1.4827 N2 C7 C8 89.8(11) 110.298 C105 N14 Ag1 112.6(6) 111.468 N13 C106 1.236(17) 1.2949 C7 C8 C9 123.5(12) 112.107 C102 C101 N14 120.0 121.893 N12 C111 1.291(16) 1.2949 C10 C9 C8 118.4(12) 112.108 C103 C102 C101 120.0 118.824 N12 Ag1A 2.312(15) 2.3333 C9 C10 N3 90.7(14) 110.299 C102 C103 C104 120.0 118.791 N12 C110 1.409(16) 1.4826 C107 N13 Ag1 125.1(13) 124.629 C103 C104 C105 120.0 119.078 N11 C112 1.3900 1.3658 C106 N13 Ag1 115.1(9) 115.580 N14 C105 C106 116.1(8) 117.821 N11 C116 1.3900 1.3518 C106 N13 C107 117.2(14) 119.791 C104 C105 N14 120.0 121.893 N11 Ag1A 2.411(13) 2.4542 C111 N12 Ag1A 114.3(9) 115.552 C104 C105 C106 123.8(8) 120.282 C112 C113 1.3900 1.4089 C111 N12 C110 119.2(12) 119.794 N13 C106 C105 123.4(11) 122.570 C112 C111 1.394(16) 1.4787 C110 N12 Ag1A 125.3(10) 124.653 C113 C114 1.3900 1.4079 C112 N11 C116 120.0 118.932 C114 C115 1.3900 1.4057 C112 N11 Ag1A 110.9(6) 111.485 C115 C116 1.3900 1.4112 C116 N11 Ag1A 128.4(6) 129.228 Ag1A N4 2.414(12) 2.4535 N11 C112 C113 120.0 121.891 Ag1A N3 2.180(16) 2.3337 N11 C112 C111 117.8(9) 117.823 N4 C12 1.3900 1.3658 C113 C112 C111 122.1(9) 120.282 N4 C16 1.3900 1.3518 C114 C113 C112 120.0 119.079 C12 C13 1.3900 1.4089 C115 C114 C113 120.0 118.824 C12 C11 1.409(16) 1.4788 C114 C115 C116 120.0 118.791 C13 C14 1.3900 1.4079 C115 C116 N11 120.0 122.482 C14 C15 1.3900 1.4057 N12 C111 C112 124.0(12) 122.569 C15 C16 1.3900 1.4112 N12 Ag1A N11 72.0(5) 72.013 C11 N3 1.230(18) 1.2949 N12 Ag1A N4 109.8(5) 136.116 C110 C109 1.488(16) 1.5498 N11 Ag1A N4 147.2(6) 141.212 C107 C108 1.530(16) 1.5499 N3 Ag1A N12 163.7(7) 141.269 C108 C109 1.494(14) 1.5445 N3 Ag1A N11 114.7(5) 101.862 N14 C101 1.3900 1.3518 N3 Ag1A N4 73.1(5) 72.017 N14 C105 1.3900 1.3658 C12 N4 Ag1A 108.0(6) 111.504 C101 C102 1.3900 1.4112 C12 N4 C16 120.0 118.933 C102 C103 1.3900 1.4057 C16 N4 Ag1A 131.6(6) 129.209 C103 C104 1.3900 1.4079 N4 C12 C13 120.0 121.890 C104 C105 1.3900 1.4089 N4 C12 C11 115.9(8) 117.822 C105 C106 1.419(16) 1.4787 C13 C12 C11 124.1(8) 135.930 Mondal et al. / European Journal of Chemistry 12 (3) (2021) 248-255 251 2021 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.12.3.248-255.2091 N N N N AgClO4 RT 1 L + CH3OH N N N N N NN N (ClO4)2Ag Ag 2+ Scheme 2. The synthesis of the complex I. Molecular Hirshfeld surfaces are built based on electron distribution calculated as the sum of spherical atom electron densities [38,39]. Thus, an isosurface is obtained, and for each point of the isosurface, two distances can be defined: de, the distance from the point to the nearest atom outside to the surface, and di, the distance to the nearest atom inside to the surface. Furthermore, the identification of the regions of particular importance to intermolecular interactions is achieved by mapping normalized contact distance (dnorm), expressed as: dnorm = (di-rivdw)/rivdw+(de-revdw)/revdw; where rivdw and revdw are the van der Waals radii of the atoms [40]. The value of dnorm is negative or positive when intermolecular contacts are shorter or longer than rvdw, respectively. The graphical plots of the molecular Hirshfeld surfaces mapped with dnorm employ the red-white-blue color scheme, where red color indicates the shorter intermolecular contacts, the white color shows the contacts around the rvdw separation, and blue color is used to point out the longer contact distances. Due to the symmetry between de and di in the expression for dnorm, where two Hirshfeld surfaces touch, both will display a red spot identical in color intensity as well as size and shape [41]. The mixture of de and di in the form of a 2D fingerprint plot provides a summary of intermolecular contacts in the crystal and are in complement to the Hirshfeld surfaces [40]. The information about the intermolecular interactions in the immediate environment of each molecule in the asymmetric unit is achieved by such plots. In addition, the close contacts between particular atom types can be highlighted in so-called resolved fingerprint plots [42], which allow the facile assignment of an intermolecular contact to a certain type of interaction and quantitatively summarize the nature and type of intermolecular contacts. Two additional colored properties (shape index and curvedness) based on the local curvature of the surface can also be specified [43]. The Hirshfeld surfaces are mapped with dnorm, shape-index, curvedness and 2D fingerprint plots (full and resolved) repor- ted in this manuscript were generated using Crystal-Explorer 3.1 [44]. 2.7. Molecular electrostatic potential (MEP) The molecular electrostatic potential at a given point around a molecule can be defined in terms of total charge distribution of the molecule and related with the dipole moments. It supplies a method to understand the electron density which is useful for determining the electrophilic reactivity and nucleophilic reactivity along with hydrogen- bonding interactions [45,46]. 3. Results and discussion 3.1. Synthetic aspects The imino-pyridyl ligand L is a 1+2 condensates of 1,4- diaminobutane and 2-pyridine carboxaldehyde. The Ag(I) complex of L has been synthesized in good yields by reacting the AgClO4 with the corresponding ligand at room temperature in an equimolar proportion. The synthesis of the complex I has been summarized in Scheme 2. Complex I is stable in solid state for about 2-3 weeks in air and CH3OH and CH3CN solution for about 6 h. 3.2. IR and NMR spectroscopy The complex I under study shows characteristic peaks due to the ligation of the ligand to the metal center in the KBr-phase IR spectra. In the IR spectrum of the ligand L, the characteristic band at 1649 cm-1 is assigned to the imine (C=N) stretching frequencies. These bands due to C=N stretching have been shifted and appear at around 1615 cm-1 in complex I. The bands due to ClO4- ions appear at 1107 and 825 cm-1 for complex I. The 1H NMR spectra of the ligand L in CDCl3 showed a singlet at δ 8.31 ppm due to (HC=N) proton, methylene protons resonates at δ 3.6 and 1.7 ppm. These protons appeared downfield for complex I. 3.3. Structural description of complex I The X-ray crystallographic studies of complex I revealed that in the presence of Ag(I)ions, the ligand L gives rise to 20- membered macrocyclic dimer by Ag(I) grid (Figure 1). Two CH2 groups in the asymmetric unit of complex I are disordered and located at three sites. For clarity, only one orientation of the four methylene groups are shown in Figure 1. In each complex, the tetradentate ligand binds the metal ion through two pyridine-N and two imine-N atoms. In the macrocyclic dimer I, each Ag(I) ion is tetra coordinated with two pyridine N-atoms and two imine N-atoms of the two different molecules of the spacer ligand L. Both the Ag(I) adopt distorted tetrahedral geometry. There exists weak Ag-Ag interaction having Ag-Ag separation of 5.318 Å along the chain. The Ag-Npy distances fall in the range 2.335-2.426 Å and the Ag-Nim distances fall in the range 2.180-2.312 Å (Table 2). The angles between the coordinating atoms centering Ag1 are 107.1(6), 72.6(4), 120.3(6), and 71.9(4)°; and Ag(I) sits 0.12 Å above the plane. The macrocyclic dimer in complex I supramolecularly arranged by π-stacking interactions. We have checked PLATON as well as Mercury, there are no Voids, if default probe radius of 1.2 Å is considered. In general, there are no classical H-bond in complex I. 3.4. Theoretical investigations The optimized bond length and angles for the Ag(I) complex I are well replicated with the experimental single crystal X-ray diffraction structure data. Calculated bond parameters by DFT are consistent with the crystallographic data of complex I and are given in Table 2. The calculated metal ligand bonds Ag1–N1, Ag1–N2, Ag1–N13, and Ag1–N14 bond distances are a little bit longer than their respective X-ray crystallographic data. For complex I, the computed angles have less than ±3° deviation from their respective experimental values. 252 Mondal et al. / European Journal of Chemistry 12 (3) (2021) 248-255 2021 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.12.3.248-255.2091 Figure 1. Molecular structure of [Ag2(µL)2](ClO4)2 (I). Figure 2. Optimized molecular structure of the ligand L and complex I. Figure 3. Surface plots of HOMO and LUMO of the ligand L and Ag(I) complex I. The optimized bond parameters of free ligand (L) and both optimized as well as experimental bond parameters of coordi- nated ligand are almost same. Thus, our calculated bond lengths and angles are in agreement with the experimental data. The overall variations are reasonable, since the performed quan- tum mechanical optimization is not exactly comparable with the experimental data as it was carried out in vacuum at 0 K. Thus, the deviation of the calculated bond parameters from the experimental data may arise due to conformational changes induced by the crystal field and temperature effect as well as the basis sets chosen for calculations [47]. The optimized structure of ligand L and the Ag(I) complex I have been shown in Figure 2. The surface plots of HOMO and LUMO of the ligand L and complex I have been depicted in Figure 3. 3.5. Molecular Hirshfeld surfaces The Hirshfeld surface is a suitable tool for describing the surface characteristics of molecules. The molecular Hirshfeld surface of complex I was generated using a standard (high) surface resolution with the 3D dnorm surfaces mapped over a fixed color scale of -0.22 (red) to 1.4 Å (blue). Mondal et al. / European Journal of Chemistry 12 (3) (2021) 248-255 253 2021 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.12.3.248-255.2091 (a) (b) (c) Figure 4. Hirshfeld surfaces of complex I, (a) 3D dnorm surface, (b) shape index, (c) curvedness. (a) (b) (c) (d) Figure 5. 2D fingerprint plots of complex I: (a) standard full and (b) resolved into C···C and (c) resolved into H···H (d) O···H contacts, showing the percentages of contacts contributing to the total Hirshfeld surface area of the molecule. The shape index mapped in the color range of -0.99 to 1.0, and Curvedness was in the range of -4.0 to 0.4. The surfaces were shown to be transparent to allow visualization of the molecular moiety in a similar orientation for all of the structures around which they were calculated. The molecular Hirshfeld surface (dnorm, Shape index and Curvedness) of complex I has been shown in Figure 4. The Hirshfeld surface analysis of complex I shows that C···C, H···H, and O···H interactions of 16.6, 12.7, and 11.1%, respect- tively, which revealed that the main intermolecular interactions were C···C intermolecular interactions. Both the H···H and O···H interactions were represented almost same area by a small area in the right side of the top in the 2D fingerprint map, whereas the C···C interactions were represented by the largest in the fingerprint plot (Figure 5) and thus had the most significant contribution to the total Hirshfeld surfaces (16.6 %). 3.6. Molecular electrostatic potential (MEP) The potential increases in the order red < yellow < green < blue < pink < white. Red and yellow represent the regions of most negative electrostatic potential which is related to electrophilic reactivity, white represents the region of most positive electrostatic potential which is related to nucleophilic reactivity and blue represents the region of zero potential. Molecular electrostatic potential (MEP) of ligand L has been shown in Figure 6. 3.7. Photophysical studies Photoluminescence properties of d10 metal coordination compounds have been extensively studied due to their potential applications as luminescent materials. Coordination com- pounds have been reported to have ability to adjust the emission wavelength of organic substances through the incorporation of metal centers and the anions. Therefore, it is important to investigate the luminescence properties of coordi- nation compounds in view of their potential applications as light-emitting diodes (LEDs). The luminescent behaviors of the ligand and its complex I have been studied in the CH3CN solution at room temperature with the emission maxima at 448 nm. Intense photoluminescence was observed only for complex I and the ligand L does not show any emission (Figure 7). The mechanism of fluorescence properties of complex I can be predicted by TD-DFT calculations. The energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of L and I is 5.113 and 3.796 eV, respectively. The silver complex I compared to L are established to exhibit an easy electronic transition, resulting in their further stability (Figure 3). 254 Mondal et al. / European Journal of Chemistry 12 (3) (2021) 248-255 2021 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.12.3.248-255.2091 Figure 6. Molecular electrostatic potential of L. Figure 7. Emission spectra of complex I in acetonitrile at room temperature with excitation at 340 nm. The contours of the electronic distribution in the HOMO and LUMO states of these molecules suggest significant energy difference of 1.317 eV between L and I. The L is not response to photoluminescent but its Ag(I) complex displays photo- luminescence excited at 340 nm. The contours of HOMO and LUMO of complex I clearly show chelation of L with Ag(I). The origin of the fluorescence properties of I probably because of the CHEF (chelation-enhanced fluorescence) processes [48]. 4. Conclusion Herein, a bis Schiff base imino pyridyl ligand L and its dimeric silver(I) complex I have been reported. Complex I represents a 20-membered macrocyclic cage via Ag(I) grid with the imino-pyridyl ligand L. A somewhat distorted tetrahedral geometry around each Ag(I) has been observed in its X-ray single crystal structure. DFT studies reveals that the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of L and [Ag2L2]2+ (I) is 5.113 and 3.796 eV, respectively. The contours of the electronic distribution in the HOMO and LUMO states of these molecules suggest a significant energy difference of 1.317 eV between L and I. The ligand L is non-photoluminescent, but its Ag(I) complex (I) displays photoluminescence on excitation at 340 nm with the emission maxima at 450 nm; which is supposed to be because of the easy electronic transition. The Hirshfeld surface analysis of complex I show that C···C, H···H, and O···H interactions of 16.6, 12.7, and 11.1%; respectively, which exposed that the main intermolecular interactions were C···C intermolecular interactions. There is no void, if default probe radius of 1.2 Å is considered. In general, there is no classical H-bond in the complex I. Acknowledgements Goutam Kumar Patra would like to thank the Department of Science and Technology (SR/FST/CSI-264/2014 and EMR/ 2017/0001789) and Department of Biotechnology, Govern- ment of India, New Delhi for financial support. Meman Sahu thanks the Council for Scientific and Industrial Research, Government of India, for financial support in the form of research fellowships. Supporting information CCDC-2058296 contains the supplementary crystallo- graphic data for this paper. These data can be obtained free of charge viahttps://www.ccdc.cam.ac.uk/structures/, or by e- mailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44(0)1223-336033. Disclosure statement Conflict of interests: The authors declare that they have no conflict of interest. Author contributions: All authors contributed equally to this work. Ethical approval: All ethical guidelines have been adhered. Sample availability: Samples of the compounds are available from the author. ORCID Jahangir Mondal https://orcid.org/0000-0001-9867-5193 MemanSahu https://orcid.org/0000-0001-9397-4805 Bhaskar Sharma https://orcid.org/0000-0001-7944-4955 Rakesh Ganguly https://orcid.org/0000-0002-9523-6918 Shubhamoy Chowdhury https://orcid.org/0000-0003-1545-9693 Goutam Kumar Patra https://orcid.org/0000-0003-3151-0284 https://www.ccdc.cam.ac.uk/structures/ mailto:data_request@ccdc.cam.ac.uk https://orcid.org/0000-0001-9867-5193 https://orcid.org/0000-0001-9397-4805 https://orcid.org/0000-0001-7944-4955 https://orcid.org/0000-0002-9523-6918 https://orcid.org/0000-0003-1545-9693 https://orcid.org/0000-0003-3151-0284 Mondal et al. / European Journal of Chemistry 12 (3) (2021) 248-255 255 2021 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.12.3.248-255.2091 References [1]. Yamada, S. Coord. Chem. Rev.1999, 190–192, 537–555. [2]. 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The full terms of this license are available at http://www.eurjchem.com/index.php/eurjchem/pages/view/terms and incorporate the Creative Commons Attribution-Non Commercial (CC BY NC) (International, v4.0) License (http://creativecommons.org/licenses/by-nc/4.0). By accessing the work, you hereby accept the Terms. This is an open access article distributed under the terms and conditions of the CC BY NC License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited without any further permission from Atlanta Publishing House LLC (European Journal of Chemistry). No use, distribution or reproduction is permitted which does not comply with these terms. Permissions for commercial use of this work beyond the scope of the License (http://www.eurjchem.com/index.php/eurjchem/pages/view/terms) are administered by Atlanta Publishing House LLC (European Journal of Chemistry). http://www.eurjchem.com/index.php/eurjchem/pages/view/terms http://creativecommons.org/licenses/by-nc/4.0 http://www.eurjchem.com/index.php/eurjchem/pages/view/terms 1. Introduction 2. Experimental 2.1. Materials and physical measurements 2.2. Synthesis of the ligand 2.3. Synthesis of the complex [Ag2L2](ClO4)2(I) 2.4. X-ray crystallography 2.5. Theoretical calculations 2.6. Hirshfeld surface calculation 2.7. Molecular electrostatic potential (MEP) 3. Results and discussion 3.1. Synthetic aspects 3.2. IR and NMR spectroscopy 3.3. Structural description of complex I 3.4. Theoretical investigations 3.5. Molecular Hirshfeld surfaces 3.6. Molecular electrostatic potential (MEP) 3.7. Photophysical studies 4. Conclusion Acknowledgements Supporting information Disclosure statement ORCID References PrintField10: PrintField11: PrintField12: PrintField13: PrintField14: PrintField15: PrintField16: PrintField17: PrintField20: PrintField21: PrintField22: PrintField23: PrintField24: PrintField25: PrintField26: PrintField27: