Comparative Study of Structures and Luminescent Properties of Three

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Comparative Study of Structures and Luminescent Properties of Three Ag(I) Complexes Utilizing an Achiral Byridyl-N or Its Axially Chiral N-Oxide Analogue Quan-Quan Li, Zeng-Yin Chao, Cong-Yan Liu, Wen-Qian Zhang, Qing Liu, Wang-Zhao Cai, Ping Liu, and Yao-Yu Wang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01531 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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Crystal Growth & Design

Comparative Study of Structures and Luminescent Properties of Three Ag(I) Complexes Utilizing an Achiral Byridyl-N or Its Axially Chiral N-Oxide Analogue Quan-Quan Li, Zeng-Yin Chao, Cong-Yan Liu, Wen-Qian Zhang, Qing Liu, Wang-Zhao Cai, Ping Liu*, Yao-Yu Wang

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi'an, 710127, P. R. China

ABSTRACT: An achiral 2,2'-bipyridyl-3,3'-dicarboxylate (H2LN) can be oxidized to a C2 axially chiral N-oxide analogy (R,S)-2,2'-bipyridyl-3,3'-dicarboxylate-1,1'-dioxide ((R,S)-H2LNO), converting normal pyridyl-N to charge-separated N-oxide group. The achiral H2LN connects the Ag(I) ions into a 4-connected mesomeric 3D network [Ag4(LN)2·(H2O)]·3H2O (1). However, as expected, with the introduction of N-oxide groups, {Ag4[(R,S)-LNO]2·2(H2O)}·H2O (2) and {Ag2[(R,S)-LNO]}·H2O (3) contain right- and left-handedness homochiral layers. Such opposite handedness layers are linked together to give a racemic compound in 2 but a meso-double layers in 3. Notably, both ligands in 1-3 support argentophilic interactions. Moreover, a

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

luminescent comparison of 1-3, pyridyl-N and its functional N-oxide ligands is also investigated in detail.

INTRODUCTION The chemical diversities of complexes depended on ligand functionality remain an attractive spot in crystal engineer and material chemistry, because slight changes of ligand’s substituents might affect the final structures and properties.1-8 In this respect, the functionalization of the pyridyl-N into the pyridyl N-oxide ligand has been proven to ameliorate the properties of complexes.4-6,9-11 For instance, Su has reported that the introduction of N-oxide groups of T-shaped linkers improves the adsorption selectivity of complexes compared with ones with pyridyl donors.4,5 Besides, some groups have demonstrated that the encapsulation of the pyridyl N-oxide groups enhances the quantum yields of Eu(III)-complexes in contrast with pyridyl-N ones.12,13 Further, Amoroso has shown that a mono-N-oxide (terpyridine-1-oxide) complex exhibits the most intense luminescence than that of pyridyl-N, bis-N-oxide and tris-N-oxide ones.10 Beyond these examples, although some complexes formed by N-oxide/pyridyl-N ligands and metal ions (such as Tb(III), Ag(I), Zn(II), Cd(II)) are also investigated, it is difficult to systematically compare and summary their structural and property differences caused by the N-oxide groups and pyridyl-N, for there are only one of a pair comparable complexes.11,14-16 Therefore, further investigation and exploration are required. All above trigger us to give a comparison of structure and property of complexes by utilizing byridyl-N and its functional ligand with N-oxide group, which shows an amusing trend.


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The (R,S)-2,2'-bipyridyl-3,3'-dicarboxylate-1,1'-dioxide ((R,S)-H2LNO) with N-oxide group, a good candidate ligand for building complexes with attractive structures and multitude properties,14,17-23 is employed in our previous study to investigate the complexes’ structure-property relationships.18,19 Such (R,S)-H2LNO can be obtained by oxidizing the 2,2'-bipyridyl-3,3'-dicarboxylate (H2LN).24,25 As shown in scheme 1, the brilliant oxidation of the pyridyl-N into pyridyl N-oxide brings a series of changes: 1) pyridine-N becomes charge-separated N-oxide group; 2) the coordination modes become more versatile due to that the N-oxide donor possesses two lone pairs of electrons in different orientations while N has one pair (Figure S1a and S1b);4,5,23,26 3) the borderline bases pyridyl-N turn into hard O donors; and 4) the conformations of ligands vary from achiral H2LN to C2 axially chiral (R,S)-H2LNO.

Ag(I) ion has received attention for its flexible coordination sphere and argentophilic interactions.27-31 As a soft Lewis acid, it is a good native match for the flexible coordinative tendencies of pyridyl ligands.31 However, as we’ve seen, until now, only one example of one-dimensional Ag(I)-complex with H2LN has been reported,32 but its N-oxide Ag(I)-complex still does not exist. Here, based on such two ligands, three complexes, namely, [Ag4(LN)2·(H2O)]·3H2O (1), {Ag4[(R,S)-LNO]2·2(H2O)}·H2O (2) and {Ag2[(R,S)-LNO]}·H2O (3) have been prepared. And a comparison between their structures and properties are studied, further exploring their diversities supported by slight changes of ligand’s substituents.



Scheme 1. The chemical drawings of H2LN and (R,S)-H2LNO.

EXPERIMENTAL SECTION Materials and general methods. The (R,S)-H2LNO is prepared in the light of the reported works.18 And all the solvents and reagents employed in this work are purchased and they need not further treatment. Infrared spectra are measured with KBr pellets on a Nicolet Avatar 360 FTIR spectrometer (4000-400 cm-1). Elemental analyses of C, H and N atoms are measured using a Perkin-Elmer 2400C Elemental analyzer. Thermogravimetric analyses (TGA) are tested on a NETZSCH TG209F3 under N2 atmosphere between 30 and 800 °C (heating rate: 10 °C min-1). The powder X-ray diffraction patterns (PXRD) are tested applying a Bruker D8 ADVANCE X-ray powder diffractometer (Cu-Kα, 1.5418 Å). Luminescence spectra are determined on a Perkin-Elmer LS55 luminescence spectrometer (for solid state) and a Hitachi F-4500 fluorescence spectrophotometer (for ethanol suspension state) at room temperature. Luminescent lifetime was recorded on Edinburgh FLsp920 with excitation at 374.4 nm.

Synthesis of [Ag4(LN)2·(H2O)]·3H2O (1). H2LN (0.0244 g, 0.1 mmol) and AgNO3·6H2O (0.0169 g, 0.1 mmol) are mixed in distilled H2O/CH3OH (8 mL/2 mL). The mixture seals in a 19 mL Teﬂon-liner


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autoclave with 1 mol L-1 KOH solution (5 drops, pH ~ 6), which heated at 105 °C for 60 h, then cooled to room temperature, and block colorless crystals are obtained. Yield: 62 % (Based on Ag(NO3)2·6H2O). Elemental analysis (%): Calcd for Ag4C24H20O12N4 (987.91): C, 29.18; H, 2.04; N, 5.67. Found: C, 29.16; H, 2.09; N, 5.68. IR (KBr, cm-1): 3493(m), 3383(m), 2958(w), 2900(w), 1600(m), 1561(m), 1394(s), 1348(w), 1311(w), 1163(w), 1050(w), 1017(w), 978(w), 854(w), 760(m), 659(w), 574(w), 527(w).

Synthesis of {Ag4[(R,S)-LNO]2·2(H2O)}·H2O (2). Method 1: The synthetic method of 2 is simialr to that of 1 except that the H2LN is replaced by (R,S)-H2LNO (0.0276 g, 0.1 mmol). Yield: 67 % (Based on Ag(NO3)2·6H2O). Elemental analysis (%): Calcd for Ag4C24H18O15N4 (1033.89): C, 27.88; H, 1.75; N, 5.42. Found: C, 27.91; H, 1.80; N, 5.39. IR (KBr cm-1): 3471(m), 1612(s), 1381(s), 1240(s), 1026(w), 968(w), 810(w), 780(w), 681(w), 549(w), 454(w).

Method 2: A mixture of (R,S)-H2LNO (0.0138 g, 0.05 mmol) and H2O (3 mL) is stirred with triethyle/water (1:20) (2 drops) of pH ~ 6 under air atmosphere for 5 min and pour it into the test tube. The other mixture of AgNO3·6H2O (0.0169 g, 0.1 mmol), DMF (2 mL) and THF (1 mL) is stirred under air atmosphere for 5 min. Then, the metal salt solution is slowly layered onto the mixture of ligand via syringe over 10 min. One week later, 2 are collected. Yield: 77 % (Based on Ag(NO3)2·6H2O).

Method 3: The synthetic method is simialr to that of complex 3 except that the triethyle/water was replaced by 1,2-diaminopropane/water (1:20) (3 drops) or



diethylamine/water (1:20) (2 drops) (pH ~ 7). Two weeks later, 2 are obtained. Yield: 52 % for 1,2-diaminopropane/water and 63 % for diethylamine/water (Based on Ag(NO3)2·6H2O).

Synthesis of {Ag2[(R,S)-LNO]}·H2O (3). 3 shows block corlorless crystals and is gained applying a similar procedure with Method 2 of complex 2 except that the 2,2'-dipyridine (2,2'-dpy) (0.0078 g, 0.05 mmol) was added into the ligand solution, and the pH was adjusted to ca. 7 with triethyle/water (1:20) (2 drops). Yield: 76 % (Based on Ag(NO3)2·6H2O). Elemental analysis (%): Calcd for Ag2C12H8O7N2 (507.94): C, 28.38; H, 1.59; N, 5.52. Found: C, 28.36; H, 1.62; N, 5.55. IR (KBr cm-1): 3386(s), 2206 (vw), 1608(s), 1571 (s), 1389(s), 1240(s), 1100 (vw), 1026(m), 952(m), 807(w), 780(m), 743(w), 685(vw), 605(vw), 551(m), 455(w).

X-ray Crystallography. Single crystal X-ray diffraction analyses of 1-3 are operated on a Bruker SMART APEX II CCD diffractometer with a graphite monochromated Mo-Kα radiation (λ = 0.071073 nm) at ca. 296 K. All structures are analyzed and reﬁned via direct and full-matrix least-squares method on F2 with anisotropic thermal parameters for all non-hydrogen atoms (SHELXL-97), respectively.33-35 Hydrogen atoms of H2O in 1-3 are located in successive different Fourier maps and no constraint is applied, hydrogen atoms of LN2- and (R,S)-LNO2- are placed in calculated positions and treated with a riding method. It should be noted that the C12 and O6 atoms in 3 are refined


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utilizing the similar ADP restraint SIMU to make the ADP values of them more reasonable. Further details of structural analyses of 1-3 are recorded in Table 1. Selected bond lengths and angles are listed in Table S1. And hydrogen-bonding lengths (Å) and angles (˚) of 1-3 are listed in Table S2. CCDC: 1504117-1504119 for complexes 1-3, respectively.

Table 1. Crystallographic data and structure refinement parameters for 1-3. Complex chemical formula formula weight crystal shape crystal color temperature (K) crystal system space group a/Ǻ b/Ǻ c/Ǻ α (°) β (°) γ (°) V/Ǻ3 Z density (mg/m3) µ (mm-1) F(000) reflections collected Rint number of parameters Goodness-of-fit on F^2 R1a, wR2b [I>2σ(I) ] R1, wR2 (all data)

1 C12H10Ag2N2O6 493.96 block colorless 296(2) Orthorhombic Pbcn 13.889(4) 13.470(4) 13.852(4) 90 90 90 2591.6(13) 8 2.532 3.056 1904 12094 0.0720 200 0.998 0.0512, 0.1391 0.0700, 0.1580

2 C24H18Ag4N4O15 1033.90 block colorless 296(2) Orthorhombic Pbcn 16.6836(2) 12.3270(1) 13.5641(1) 90 90 90 2789.6(5) 4 2.462 2.852 1992 13144 0.0308 214 1.065 0.0445, 0.1125 0.0535, 0.1192


3 C12H8Ag2N2O7 507.94 block colorless 296(2) Triclinic P-1 6.663(2) 9.053(3) 11.386(4) 91.826(6) 98.048(5) 106.330(4) 650.7(4) 2 2.591 3.050 488 3553 0.0152 208 0.998 0.0707, 0.2214 0.0964, 0.2990


RESULTS AND DISCUSSION Syntheses and crystal structures of three Ag(I)-complexes. Scheme 2. The simplified synthetic route for 1-3.

The H2LN reacts with silver nitrate in solvothermal conditions, producing an X-ray-quality crystalline product 1 (Scheme 2). The X-ray crystallographic study shows that 1 with formula [Ag4(LN)2·(H2O)]·3H2O is a mesomeric 3D framework and in the orthorhombic with space group Pbcn. The asymmetric unit consists of two crystallographically independent Ag(I) ions (Ag1 and Ag2), one LN2- ligand, half coordinated H2O, and one and a half free H2O. As shown in Figure 1a, Ag1 center is three-coordinated and exhibits irregular Y-shaped coordination geometry completed by two carboxylate O atoms (O1A and O2B) and one pyridine N atom (N1) from three LN2- ligands with normal bond distances and angles.31,36-38 Meanwhile, there exist week interactions between Ag1 and free water molecular O6 (Ag1···O6 = 2.829 Å), which are outside the typical range of 2.3-2.6 Å while shorter than the corresponding van der Waals contacts (3.24 Å).39 The Ag1···Ag1 distance is 2.902(8) Å, slightly longer than the Ag-Ag contact (2.89 Å) but below the sum of the van der Waals radii of two Ag (3.44 Å), suggesting the argentophilic interaction (Figure S2).27,31,37,40,41 Ag2 center shows a distorted triangular pyramid completed by two


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carboxyl O atoms (O3D and O4B), one pyridine N atom (N2) from three LN2- ligands, and one coordinated water molecule (O5). The distortion of this tetrahedron (τ4 = 0.9) is in part attribute to the inherent deficiency of crystal field stabilization energy that arises from a spherical d10 electronic configuration.42 The Ag2···Ag2 distance of 3.169(5) Å demonstrates the argentophilic interaction (Figure S2).27,31,37,40,41 And the Ag2-O and Ag2-N bond lengths fall into the normal range.31,36-38 Each LN2- connects six Ag centers (three Ag1 and three Ag2), in which both carboxylate groups take µ2-ƞ1:ƞ1 coordination mode and two pyridine N atoms adopt monodentate mode (Figure S1). Such metal-ligand coordination “lock” the C-C bond rotation between two pyridine rings, leading to axially chiral conformational isomers of the pyridine-pyridine unit (Figure 1) as reported in the literature, and the dihedral angle between two pyridine rings is ca. 56° (the angle mentioned later refers to one between two planes position to the carboxylate groups), which is smaller than the reported values (ca. 65°).43 So the formation of the chiral building units from the achiral ligands is very interesting and also a good strategy for the assembly of chiral building units.44 Notably, such coordination mode of LN2- that both nitrogen atoms adopt monodentate mode to coordinate metal ions is uncommon in previously reported.



Figure 1. (a) The coordination environment of Ag(I) ions in 1. Symmetric mode: A = -x+3/2, y-1/2, z; B = x-1/2, -y+1/2, -z+2; C = -x+3/2, -y+1/2, z+1/2; D = x-0.5, -y+0.5, -z+2. (b) Two mirror images of the chiral building blocks in 1. (R-conformation in blue, and S-conformation in yellow.)

As shown in Figure 2a, two Ag1 centers are interlinked together through two carboxylate groups of two LN2- ligands to form [Ag2(COO)2] SBU, which connects adjacent four LN2- ligands via carboxylate oxygen atoms (O1 and O2) and pyridine nitrogen atom (N1) to build a 2D layer down the c-axis. And the rest carboxyl oxygen atoms (O3 and O4) and nitrogen atoms (N2) of LN2- connect the [Ag2(COO)2(H2O)] SBUs constructed by two carboxylate groups (O3 and O4) and one µ2-water molecule (O5), forming a 3D framework (Figure 2c). From another perspective, the same conformational pyridine-pyridine units (R- or S-conformal) are assembled to form 2D layers (Figure 2b), which are staked together to give mesomeric 3D network (Figure 2c). Besides, the hydrogen bonds between the carboxyl oxygen atoms and the water


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molecules exist in the structure, playing a significant role in stabilizing the final structure. To simplify this complicated framework, the topological approach is needed. All the [Ag2(COO)2], [Ag2(COO)2(H2O)] SBU and LN2- can be viewed as one node. Each LN2- links two [Ag2(COO)2] SBUs and two [Ag2(COO)2(H2O)] SBUs, and each [Ag2(COO)2] SBU/[Ag2(COO)2(H2O)] SBU links four LN2- ligands. So, this complicated framework can be viewed as a (4,4,4)-connected network topology with Point (Schläfli) symbol of {4^2.8^4} (Figure 2f).

Figure 2. (a) The [Ag2(COO)2] SBUs connect adjacent four LN2- ligands to build a [Ag2(COO)2]-LN layer down the c-axis and (d) its topological network, LN2- in yellow and [Ag2(COO)2] SBU in blue; (b) the R-conformational pyridine-pyridine units are assembled to form 2D layers, and (e) its topological network, [Ag2(COO)2(H2O)] SBUs in pink; (c) The 3D framework of 1 can be viewed as the combination of (a) and (b), the R-conformational pyridine-pyridine units in blue, and S-conformational



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ones in yellow, and (f) its topological network, the green dotted lines represent hydrogen bonding interactions.

In order to explore the changes after introducing the N-oxide groups into the H2LN, the ligand H2LN is replaced by (R,S)-H2LNO under the synthetic condition of 1, and fortunately,

we

obtained

an

X-ray-quality

crystalline

2

of

formula

{Ag4[(R,S)-LNO]2·2(H2O)}·H2O. Complex 2 has the same space group with that of 1 and its asymmetric unit consists of two crystallographically independent Ag(I) ions (one Ag1, half Ag2 and half Ag3), one (R,S)-LNO2- ligand, one coordinated and half free H2O. As depicted in Figure 3a, three Ag(I) ions possess various coordination numbers: 2, 3 and 4. Each Ag1 center coordinates to two carboxylate oxygen atoms (O3D and O4) and N-oxide (O6E) of three (R,S)-LNO2- ligands, exhibiting a triangular geometry. The Ag1···O8 distance of 2.700 (1) Å is beyond the normal Ag-O bonds but within the range of weak Ag···O interactions.39 Ag2 center connects to two carboxylate O atoms (O1 and O1A) and two water molecules (O7 and O7A), showing a distorted tetrahedral geometry with τ4 being 0.7. Each Ag3 center is coordinated by two carboxylate O atoms (O2 and O2A) from two (R,S)-LNO2- with the bond angle of O2–Ag3–O2A being ca. 172°. And Ag3 links two adjacent Ag1 centers to form a isosceles triangle geometry (Ag3···Ag1B = Ag3···Ag1C = 3.212(1) Å, Ag1B···Ag1C = 2.947(1) Å). The Ag2···Ag3 (3.018(2) Å), Ag1···Ag3 and Ag1···Ag1 distances indicate the argentophilic interactions.27,31,37,40,41


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Figure 3. (a) The coordination environment of Ag centers of 2. Symmetric codes: A = -x+1, y, -z+3/2; B = x, -y, z-1/2; C = -x+1, -y, -z+2; D = -x+1, y, -z+5/2; E = x-1/2, -y+1/2, -z+2. (b) The R- and S-conformational LNO2- ligands exist in the structure of 2, the coordinated H2O are omitted for clarity. (R-LNO2- in blue, and S-LNO2- in yellow) The (R,S)-LNO2-, existing as R-LNO2- or S-LNO2-, exhibits µ5-bridged mode in which both carboxylate groups adopt µ2-η1:η1 bridging mode and one N-oxide adopts monodentate mode (Figure 3b, S1d and S3). To the best of our knowledge, there is no precedent of (R,S)-LNO2- with such coordination mode during the assemblies of complexes. The dihedral angle between two pyridine rings of (R,S)-LNO2- is ca. 81°. In 2, as shown in Figure 4c, there exist two SBUs of [Ag2(COO)2(H2O)] and [Ag2(COO)2], and the adjacent S-LNO2- (or R-LNO2-) ligands are connected by such two SBUs to form a 1D right- (or left-) handed helix along the c axis with the pitch of 13.564(1) Å, and the helixes of same handedness are further orderly arranged by Ag1-O6 coordination bonds to give a homochiral 2D sheet (Figure 4a, 4b and S4). Within this 2D sheet, there are O-H···O hydrogen bonds. Further, the opposite handedness homochiral sheets are hold together to form a racemic compound via the



Ag1···Ag3 interaction and O-H···O hydrogen bonds (Figure 4d). The topological analysis is used here to simplify this complicated structure, a Ag4 SBU constituted by four Ag(I) ions (two Ag1, one Ag2 and one Ag3) and four carboxylate groups from four different (R,S)-LNO2- can be viewed as a node to link six (R,S)-LNO2-, and each (R,S)-LNO2- acts as a node to connect three Ag4 SBUs, resulting in a (3,6)-connected net with Point (Schlafli) symbol for net: {4.6^2}2{4^2.6^9.8^4} (Figure 4e).

Figure 4. (a) A view of a 1D right-helical chain constructed by S-LNO2- and two SBUs of [Ag2(COO)2(H2O)] and [Ag2(COO)2], sectional coordinated H2O are omitted for clarity; (b) The right-helixes are further connected by Ag1-O6 coordination bonds to


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form homochiral 2D layer, sectional coordinated H2O are omitted for clarity; (c) Within the homochiral layer, there exist two SBUs of [Ag2(COO)2(H2O)] and [Ag2(COO)2], and O-H···O hydrogen bonds; (d) The 3D supramolecular architecture constructed by the Ag1···Ag3 interaction and hydrogen bonds, and (e) its topological structure, Ag4 SBUs in purple and (R,S)-LNO2- in green.

Interestingly, besides the solvothermal conditions, 2 can also be obtained via the diffusional method (Scheme 2). But surprisely, in the diffusional system of the 2,2'-dpy and triethyle, a new complex 3 of formular {Ag2[(R,S)-LNO]}·H2O was acquired from diffusional system only. Complex 3 is meso-double 2D layer constructed by [Ag4(COO)4(H2O)2] SBU and (R,S)-LNO2- and crystallizes in a space group P-1 with equal R- and S-configured LNO2- ligands (Figure 5). The asymmetric unit consists of two crystallographically independent Ag(I) ions, one (R,S)-LNO2ligand and one free water molecule. As shown in Figure 5, Ag1 center is surrounded by one carboxylate oxygen atom (O1) and one N-oxide oxygen atom (O3A) from two (R,S)-LNO2- ligands with bond angle O1-Ag1-O3A being ca. 166°. Ag2 center shows a scalene triangle and is bound by three carboxylate oxygen atoms (O2D, O5 and O6C) from three (R,S)-LNO2- ligands. Besides, there are exist Ag···O weak interactions and argentophilic interactions between two Ag centers in this structure.27,31,37,39-41



Figure 5. The coordination environment of Ag(I) ions in 3, and symmetric mode: A = x+1, y+1, z; B = x+1, y, z; C = -x, -y+1, -z+1; D = x-1, y, z. The R- and S-conformational LNO2- ligands exist in the structure of 3. (R-LNO2- in blue, and S-LNO2- in yellow.)

Figure 6. (a) The carboxylate groups of R-LNO2- or S-LNO2- connect Ag2 atoms to


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form left- or right-helical chains; (b) The helixes of the same handedness are linked together to form left- or right-handed layers through Ag1-O1 and Ag1-O3 coordination bonds; (c) The meso double-layer is constructed by the stacking of two homochiral layers of the opposite handedness via Ag2-O2 and Ag2-O6 coordination bonds (Red and black dotted lines represent Ag···O weak interactions and O-H···O hydrogen bonds, respectively); (d) A view of 3D supramolecular structure of 3, the black dotted lines represent C-H···O weak interactions.

The coordination modes and dihedral angles between two pyridines rings of (R,S)-LNO2- ligand are same to those of 2 (Figure S1d). The carboxylate groups of R-LNO2- (or S-LNO2-) connect Ag2 to form left- (or right-) handed helical chains propagating along a axis with the pitch of 6.664(3) Å (Figure 6a). The helixes of same handedness are linked together to form homochiral 2D layers through Ag1-O1 and Ag1-O3 coordination bonds (Figure 6b), and the adjacent two types of homochiral layers (left- and right-handed) connect together to form meso double-layer via Ag2-O2 and Ag2-O6 coordination bonds (Figure 6c). Besides, the O-H···O hydrogen bonds exist within the achiral 2D layers. Furthermore, the achiral 2D layers are held together into 3D supramolecular structure via C-H···O weak interactions (Figure 6d).

Effect of ligands on the final structures of Ag(I)-complexes. In order to investigate the role of slight changes of ligand’s substituents on the structures of Ag(I)-complexes, it becomes significant to compare their structures in detail. Firstly, in the H2LN and (R,S)-H2LNO, even though their carboxylate groups all



take µ2-ƞ1:ƞ1 coordination mode, their main differences lie on the pyridyl-N and N-oxide groups: for LN2-, both pyridine-N atoms assume monodentate mode; for (R,S)-LNO2-, one N-oxide connects one metal ion, while the other is still pendent. That is to say, the coordinative ability of N-oxide atoms of (R,S)-H2LNO to Ag(I) seems to be weaker than that of pyridyl-N atoms of H2LN in 1-3.45-47 Secondly, for the (R,S)-LNO2-, its dihedral angles between two pyridine rings are ca. 81° in 2 and 3; While achiral H2LN’s free rotational C-C bond between two pyridines rings is “locked” after coordinating with Ag(I), showing R- and S-conformational pyridine-pyridine unit (Figure 1) with dihedral angle of ca. 56°, which is smaller than that of (R,S)-LNO2-. Thirdly, the assembly of achiral H2LN and Ag(I) produces mesomeric 3D network structure, but there exist homochiral 2D layers when (R,S)-H2LNO coordinates with Ag(I). Fourthly, as depicted in Figure 7, 1-3 possess three kinds of SBUs: [Ag2(COO)2] and [Ag2(COO)2(H2O)] SBUs for 1 and 2, and [Ag4(COO)4(H2O)2] SBU for 3. Fifthly, although 1 and 2 have same SBUs, such SBUs connect the H2LN or (R,S)-H2LNO in a diverse way at the axial positions. In [Ag2(COO)2(H2O)] SBUs, each Ag atom shows a distorted triangular pyramid (including Ag···Owater weak interactions of 2) with three oxygen atoms forming the equatorial plane. In 1, the pyridine is almost perpendicular to the plane with a slope being ca. 17 ° (the angle refers to the angle between N-Ag and a line through Ag atom to the equatorial plane) (Figure 7a), nevertheless, in 2, the N-oxide groups are bent with a slope of ca. 7 ° and N-O-Ag angle of ca. 120 ° (Figure 7c).4 Besides, compared to the pyridine N atoms at both ends of [Ag2(COO)2] in 1 (Figure 7b), the H2O or Ag


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atoms attack the N-oxide groups and occupy the axial positions in 2 (Figure 7d). Thus, the O atoms of N-oxide group can be act as potential “Open Donor Sites” (ODSs) and prone to be beat by some aggressive coordination atoms of other molecules, which perhaps result in the poor thermal stability of complexes (see thermal stability).4

Figure 7. The coordination environments of Ag(I) ions and the bent configuration of N-oxide ligand, and three kinds of SBUs exist in 1-3. Although 2 and 3 are assembled of Ag(I) and (R,S)-H2LNO, and the (R,S)-LNO2- show the same coordination modes, their structures exhibit discriminable patterns in the simulated PXRD (Figure S5). Further structural observation shows the following differences: 1) they crystallize in Pbcn and P-1 space group, respectively; 2) The Ag atoms in 2 have three different coordination spheres, while two ones in 3; 3) 2 has two kinds of SBUs, but 3 possesses one [Ag4(COO)4(H2O)2] SBU, and interestingly, [Ag4(COO)4(H2O)2] SBU can be taken as the combination of one [Ag2(COO)2] and two half of [Ag2(COO)2(H2O)] SBUs (Figure 7); 4) both 2 and 3 contain right- and



left-handedness homochiral layers, but the linkage modes of such homochiral layers are different. As depicted in Figure 8, in 2, the homochiral layers of the opposite handedness are connected alternately through the Ag···Ag interaction and hydrogen bonds to form a racemic compound, but in 3, they are arranged to give a meso-double-layer via coordination bonds.

Figure 8. The simplified homochiral sheet of 2 and meso double-layer of 3.

Purity and thermal stability. The purities of powder samples 1-3 are demonstrated through PXRD analyses (Figure S6). And their thermal stabilities are examined via testing the temperatures that lead to structural chemical decomposition, and this process involves the bond breaking which can be reckon by TGA measurement (Figure 9). The TGA curves of 1 and 2 exhibit some similarities and reveal that the frameworks are stable up to 125 °C for 1 and 102 °C for 2, and the coordinated and free water molecules are removed below 180 °C for 1 and 175 °C for 2 (obsd. 7.27%, calcd. 7.29% for 1, and obsd. 5.20%, calcd. 5.23% for 2), and it is stable up to 240 °C for 1 and 205 °C for 2, then the overall framework begins to decompose. Obviously, the temperature of structural


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chemical decomposition of 1 is higher than that of 2, demonstrating the existence of N-oxide groups can cause the poor thermal stability of complexes. The TGA curve of 3 reveals that one free water molecule was removed below 165 °C (obsd. 3.60%, calcd. 3.54%), and then followed by the collapse of the structure.

Figure 9. The TGA curves of complexes 1-3.

Luminescent properties. In order to compare luminescent properties of 1-3, we study their luminescent properties in solid and suspension state, as well as luminescent lifetimes at room temperature (Figure 10, S8 and S10, ethanol-suspension luminescent test is described in supporting information, and the stability of samples in ethanol was proved by PXRD in Figure S9). The excitation and emission wavelengths of H2LN, (R,S)-H2LNO and 1-3 in solid state are listed in Table S3. When excited at 332 nm in the solid state, the free H2LN shows an emission maximum at 432 nm and a very weak peak at 471 nm (Figure 10). Whereas with the introduction of stronger electron-withdrawing N-oxide group (compared with pyridyl-N), the emission maximum of (R,S)-H2LNO



shows a blue-shift of 31 nm to 401 nm relative to H2LN (λex = 320 nm), which is probably due to the electron-withdrawing properties.48,49 The spectra of H2LN and (R,S)-H2LNO are ascribed to the π→π* electronic transitions.50-53 As shown in Figure S8, when excitation at λex = 300 nm in ethanol-suspension state, the emission bands of both ligands (432 and 471 nm for H2LN, 401 and 476 nm for (R,S)-H2LNO) show same to that in solid state. And the luminescence intensity of (R,S)-H2LNO is stronger than that of H2LN, which can be attributed to the presence of N-oxide group and increasing conformational rigidity.

Figure 10. Luminescence spetra of 1-3, H2LN and (R,S)-H2LNO in solid state.

After the coordination of such two ligands with Ag(I) salt, some differences of emission spectra are observed. Excitations at 300 nm for 1-3 in the solid state result in emission peaks at 417 and 471 nm for 1, 391 and 471 nm for 2, and 393 and 472 nm for 3 (Figure 10). These complexes show some low-energy emission bands whose shapes and positions are similar to that of their corresponding ligands, indicating that their emissions mainly come from intraligand (π→π*) luminescent emission.43,54-56


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And the maximum emissions of 1-3 present various degrees of blue shifts (15, 10 and 8nm) compared with their respective ligands (432 nm and 401 nm for H2LN and (R,S)-H2LNO, respectively), which is due to the increasing energy gaps between π and π* molecular orbitals after the ligands coordinating with Ag(I).57,58 However, upon excited at 300 nm in ethanol-suspension state, as shown in Figure S8, 1-3 respective exhibit a weak broad emission at 428, 425 and 424 nm, a strong emission at 477, 475 and 489 nm, showing various degrees of red shifts compared with that in the solid state. This is possibly ascribe to the ethanol polarity.59 Meanwhile, compared to the free H2LN, the enhanced luminescence intensity of 1 is mostly due to the coordination of Ag(I) to H2LN and argentophilic interactions, effectively resulting in the increasing conformational rigidity of LN2- and inhibition of radiationless decay process.30,40,42 However, the luminescent intensities of 2 and 3 become weaker in contrast with (R,S)-H2LNO and 1, which is much different from the previously reported Eu(III)-complexes and Ag(I)-complex with N-oxide group that show more intense luminescent emission than that of pyridyl-N,10,11,60 which perhaps because the weakened conjugacy after coordination.31,54,61-64 Moreover, the enhanced intensity of 2 compared with 3 is probably due to the increasing of amounts of water molecules supporting the luminescence, the rigidity difference of their host frameworks, and the coordination diversity of Ag(I) ions.62,65,66

The luminescence decay curves of 1-3 were obtained at room temperature (Figure S10), and can be fitted to double-exponential decay (χ2 > 1) with the different lifetimes (τ1 = 0.81 ns, 52.22 %, τ2 = 5.51 ns, 47.78 % for 1; τ1 = 0.32 ns, 89.36 %, τ2



= 4.45 ns, 10.64 % for 2 and τ1 = 0.33 ns, 90.30 %, τ2 = 4.26 ns, 9.70 % for 3) and the average lifetimes (τavg = 3.06 ns for 1; τavg = 0.76 ns for 2 and τavg = 0.71 ns for 3). And the results show that the average lifetime of 1 is longer than that of 2 and 3.

CONCLUSIONS In summary, via successive modification of the pyridyl-N into the pyridyl N-oxide groups, three Ag(I)-complexes have been prepared. The effect of two comparable ligands on the varieties of structures and luminescence of these Ag(I)-complexes has been appraised. And the study results are following: 1) the introduction of N-oxide groups leads to charge variation and bent-binding, and the conformations of ligands vary from an achiral H2LN to an axially chiral (R,S)-H2LNO; 2) Complex 1 exhibit a mesomeric 3D network structure, while both 2 and 3 contain homochiral 2D layer, and such opposite handedness layers are staked together to form a racemic compound in 2 but a meso-double layers in 3; 3) The study of thermal stabilities of 1-3 reveals that the presence of N-oxide group might cause the complexes with poor stability; 4) A comparative study of luminescent properties of 1-3, H2LN and (R,S)-H2LNO shows that for (R,S)-H2LNO, its maximum emission band is blue-shift and luminescent intensity becomes stronger compared with H2LN, while its Ag(I)-complexes show weaker emission intensity than H2LN ones; 5) 1 has longer luminescent lifetime than that of 2 and 3.

ASSOCIATED CONTENT Supporting Information. Crystallographic data, IR spetra, PXRD patterns, additional


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tables, structural figures, luminescent properties, and luminescent lifetime. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *(P. Liu): [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by the NSF of China (No. 21673173, 21572177, 21371142, and 21531007), the NSF of Shaanxi Province of China (No. 2016JZ004 and 2015JZ003),

the

Xi'an

City

Science

and

Technology

Project

(No.

2017085CG/RC048(XBDX004)), the Northwest University Science Foundation for Postgraduate Students (No. YZZ15040) and the Chinese National Innovation Experiment Program for University Students (G201710697034 and G201710697041) for financial support.

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“For Table of Contents Use Only”

Comparative Study of Structures and Luminescent Properties of Three Ag(I) Complexes Utilizing an Achiral Byridyl-N or Its Axially Chiral N-Oxide Analogue

Quan-Quan Li, Zeng-Yin Chao, Cong-Yan Liu, Wen-Qian Zhang, Qing Liu, Wang-Zhao Cai, Ping Liu*, Yao-Yu Wang

The functionalization of an achiral 2,2'-bipyridyl-3,3'-dicarboxylate with N-oxide groups into C2 axially chiral N-oxide analogy, and their self-assembly with Ag(I) ions are investigated in detail to give a comparison study of their structures and luminescent properties supported by a pair of pyridyl N-oxide and pyridyl-N ligands.


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Scheme 1



Scheme 2


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Figure 1



Figure 2


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Figure 3



Figure 4


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Figure 5



Figure 6


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Figure 7



Figure 8


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Figure 9



Figure 10


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Comparative Study of Structures and Luminescent Properties of Three

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