Homochiral Erbium Coordination Polymers: Salt-Assisted Conversion

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Homochiral Erbium Coordination Polymers: Saltassisted Conversion from Triple to Quadruple Helices Yan Xu, Song-Song Bao, Xin-Da Huang, and Li-Min Zheng Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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Crystal Growth & Design

For Table of Contents Use Only

Homochiral Erbium Coordination Polymers: Salt-assisted Conversion from Triple to Quadruple Helices Yan Xu, Song-Song Bao, Xin-Da Huang & Li-Min Zheng*

Homochiral erbium coordination polymers with quadruple-stranded helical structures can be obtained via salt-assisted transformation of the related compounds with triple-stranded helical structures. The increased ionic strength plays a key role in this process.

ACS Paragon Plus Environment

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

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Homochiral Erbium Coordination Polymers: Salt-assisted Conversion from Triple to Quadruple Helices

Yan Xu1,2, Song-Song Bao1, Xin-Da Huang1 & Li-Min Zheng1* 1

State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, China. 2 Institute of Information Engineering, Suqian College, Suqian, 223800, P. R. China. ABSTRACT: Self-assembly of multi-stranded helical metal coordination polymers has attracted considerable attentions in order to mimic natural helices such as DNA. Of particular interest is the conversion of helical structures from low-stranded to high-stranded helices. In this paper, we report four pairs of homochiral erbium phosphonates with formulae R- or S-[Er4(pempH)12]·7H2O (1), R- or S-[Er3(pempH)7(pempH2)2](NO3)2·2H2O (2), R- or S-[Er2(pempH)4(NO3)Cl] (3) and R- or S-[Er2(pempH)4Cl2] (4), where R- or S-pempH2 represents R- or S-(1-phenylethylamino)methylphosphonic acid. Compounds 1 and 2 show triple-stranded helical chain structures, whereas compounds 3 and 4 exhibit quadruple-stranded helical chain structures. Remarkably, the triple helical structures of 1 and 2 can be transformed into quadruple helical structures of 3 simply by adding NaCl. A systematic study by varying the amount of NaCl and using different metal salts as additives reveals that the increased ionic strength plays a key role in the transformation of the helical chain structures from triple- to quadruple-stranded. The UV-vis absorption and near-infrared emission properties are also discussed. INTRODUCTION Homochiral helical architectures are prominently present in nature, such as the single, double and triple helices of DNA. The four-stranded DNA, or so-called G-quadruplex, has received particular interest in recent years because it was found in human cells and involved in the process of DNA replication which is pivotal to cell division and production1-3. As a mimic of natural helices, the self-assembly of discrete linear multi-stranded helical metal coordination compounds (helicates) or helical coordination polymers (hCPs) has become a topic of major importance in supramolecular chemistry4-7. Following the seminal work of Lehn and co-workers on the self-assembly triple-stranded

of

metal-oligopyridine

helicates

or

hCPs

double-stranded have

been

helicates8-9,

documented4-7,

numerous

10-17.

However,

single-, the

double-,

construction

and of

quadruple-stranded helicates18-19 or hCPs20 remains a challenge due to the increasing complexity of the architecture. Notably, homochiral hCPs with quadruple helical structures have never been reported. G-quadruplex structures of DNA contain G-quartets made up of four Hoogsteen hydrogen-bonded guanine residues21. The four strands can be formed intramolecularly from a single nucleic acid sequence or intermolecularly from two or more sequences. Interestingly, the coordination of monovalent metal ions (Na +, K+) in the quadruplex is essential for its formation, and determines the stability and polymorphism of the quadruplex


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structures22. For the artificial systems, the construction of quadruple helicates often requires transition metal ions with square-planar geometry23, 24, or lanthanide25-27 and actinide28 ions with high coordination numbers to connect suitable mono- or bi-dentate ligands. The role that monovalent alkali metal ions or anions may participate in the formation of quadruple helicates or hCPs has not so far been explored. In particular, the conversion from low stranded (single, double, triple) to quadruple-stranded helical structures remains a challenging task. In a previous work, we found that the optically active R- or S-(1-phenylethylamino)methylphosphonic acid (Ror S-pempH2) can react with metal ions forming homochiral chain or layer structures29-33. By taking the advantage of the high coordination numbers of lanthanide ions, herein we report four pairs of enantiopure Er/pemp compounds, namely, R- or S-[Er4(pempH)12]7H2O (1) and R- or S-[Er3(pempH)7(pempH2)2] (NO3)22H2O (2) with triple-stranded helical chain structures, and R- or S-[Er2(pempH)4(NO3)Cl] (3) and R- or S-[Er2(pempH)4Cl2] (4) with quadruple helical chain structures (Scheme 1). Remarkably, the triple helical structures of 1 and 2 can be transformed into quadruple helical structures of 3, simply by adding NaCl. The optical properties of 1-4 are also studied owing to the fascinating luminescent behavior of the erbium-based materials in the near-infrared (NIR) region which are important for biological and technological applications34-35. EXPERIMENTAL SECTION Materials and physical measurements. R- and S-1-phenylethylamine were purchased from Aldrich without further puriﬁcation, and all the other starting materials were of reagent grade quality. R- and S-(1-phenylethylamino)methylphosphonic acid (pempH2) were prepared according to the literature method.32 Elemental analyses for C, N and H were determined with a Perkin Elmer 240C elemental analyzer. Infrared spectra were measured on a Bruker TENSOR 27 IR spectrometer with pressed KBr pellets in the range of 400-4000 cm-1. Thermogravimetric analysis (TGA) were performed on a Mettler-Toledo TGA/DSC STARe thermal analyzer in the range of 25-500 C under a nitrogen flow at a heating rate of 10 C/min. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu-K) at room temperature. The circular dichroism spectra were recorded on a JASCO ̶1500 spectropolarimeter using KCl pellets at room temperature. Ultraviolet–visible absorption spectra were recorded on a Perkin Elmer Lambda 950 spectrometer. Steady-state emission spectra were recorded on a FLS920 fluorescence spectrometer (Edinburgh Instrument) equipped with a 450-W continuous wavelength Xe lamp (range from 230 to 900 nm), using Hamamatsu R928 (visible) or R5509-72 (Vis and NIR range) photomultipliers. Synthesis of R- or S-[Er4(pempH)12]·7H2O (R-1 or S-1). Compounds R-1 and S-1 were synthesized under similar experimental conditions except that the R-pempH2 and S-pempH2 were used, respectively, as the starting material. A typical procedure for the preparation of R-1 is as follows. A mixture of Er(NO3)3·6H2O (0.1 mmol, 0.0461 g) and R-pempH2 (0.5 mmol, 0.1080 g) in 9 mL of H2O, adjusted to pH 3.7 with 0.5 mol/L NaOH, was kept in a Teflon-lined autoclave at 120 °C for 2 days. After cooling to room temperature, colorless rod-like crystals were obtained. Yield: 55.3% based on Er. Elemental analysis (%) calcd for C108H156N12O36P12Er4·7H2O: C 38.54, H 5.09, N



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4.99; Found: C 38.92, H 4.99, N 5.09. IR (KBr, cm-1): 3060(s), 3035(m), 2985(m), 2785(m), 2526(m), 2418(w), 1614 (m), 1494 (m), 1456 (m), 1456 (w), 1384 (w), 1274 (m), 1163 (s), 1120 (s), 1020 (s), 989 (s), 763 (m), 702 (m), 565 (m), 536 (m), 509(m), 481(m). For S-1: Yield: 58.2% based on Er. Elemental analysis (%) calcd for C108H156N12O36P12Er4·7H2O: C 38.54, H 5.09, N 4.99; Found: C 38.04; H, 5.04; N, 5.03. IR (KBr, cm -1): 2985(s), 1614(w), 1456(m), 1384(m), 1296(m), 1163(s), 1120(s), 1082(s), 1020(s), 989(s), 763(s), 536(m), 426(w). Synthesis of R- or S-[Er3(pempH)7(pempH2)2](NO3)2·2H2O (R-2 or S-2 ). Compounds R-2 and S-2 were synthesized under similar experimental conditions except that the R-pempH2 and S-pempH2 were used, respectively, as the starting material. A typical procedure for the preparation of R-2 is described as below. A mixture of Er(NO3)3·6H2O (0.1 mmol, 0.0461 g) and R-pempH2 (0.5 mmol, 0.1080 g) in 9 mL of H2O, adjusted to pH 3.0 with 0.5 mol/L NaOH, was kept in a Teflon-lined autoclave at 120 °C for 2 days. After cooling to room temperature, colorless block-like crystals were obtained. Yield: 35.3% based on Er. Elemental analysis calcd (%) for C81H119N11O33P9Er3·2H2O: C 37.54, H 4.78, N 5.95; Found: C 37.13, H 4.79, N 5.77. IR (KBr, cm-1): 3357(s), 3060(m), 2985(m), 2783(m), 2524(m), 2414(w), 1614(m), 1494(m), 1456(m), 1427(w), 1384(w), 1296(m), 1163(s), 1120(s), 1020(s), 989(s), 763(m), 702(m), 565(m), 536(m), 505(m), 474(m). For S-2: Yield: 39.2% based on Er. Elemental analysis (%) calcd for C81H119N11O33P9Er3·2H2O: C 37.54, H 4.78, N 5.95; Found: C, 37.28; H, 4.78; N, 5.81. IR (KBr, cm -1): 3060(s), 2985(w), 2785(m), 2526(m), 1614(m), 1456(w), 1384(w), 1296(w), 1163(s), 1082(s), 1020(s), 989(s), 763(s), 702(s), 565(m), 536 (m). Synthesis of R- or S-[Er2(pempH)4(NO3)(Cl)] (R-3 or S-3). Compounds R-3 and S-3 were synthesized under similar experimental conditions except that the R-pempH2 and S-pempH2 were used, respectively, as the starting material. A typical procedure for the preparation of R-3 is described as below. A mixture of Er(NO3)3·6H2O (0.1 mmol, 0.0461 g), R-pempH2 (0.5 mmol, 0.1080 g) and NaCl (2 mmol, 0.1168 g) in 9 mL of H2O, adjusted to pH 3.7 with 0.5 mol/L NaOH, was kept in a Teflon-lined autoclave at 120 °C for 2 days. After cooling to room temperature, colorless rectangle-like crystals suitable for single crystal structural analysis were obtained as a major phase together with a small amount of R-2, confirmed by the PXRD measurements. A pure phase of R-3 can be isolated when the amount of NaCl increases to 5 mmol. Yield: 35.7% based on Er. Elemental analysis (%) calcd for C36H52N5O15P4ClEr2: C 33.55, H 4.07, N 5.43; found: C 33.03, H 4.12, N 5.44. IR (KBr, cm -1): 3384(s), 3116(m), 2987(m), 2785(m), 2511(m), 2405(w), 1589 (m), 1494 (m), 1452 (m), 1384(w), 1311(w), 1265 (m), 1193 (s), 1128 (s), 1082 (s), 1018 (s), 756 (s), 702 (m), 574 (m), 504 (m), 476 (m). For S-3: Yield: 38.1% based on Er. Elemental analysis (%) calcd for C36H52N5O15P4ClEr2: C 33.55, H 4.07, N 5.43; Found: C, 33.24; H, 4.16; N, 4.48. IR (KBr, cm-1): 3388(s), 3118(m), 2991(m), 2802(m), 2515(m), 2360(w), 1589(m), 1492(m), 1452(m), 1384(w), 1311(w), 1267 (m), 1191(s), 1128(s), 1082(s),1020(s), 754(s), 702(m), 574(m), 513(m), 476(m). Synthesis of R- or S-[Er2(pempH)4(Cl)2] (R-4 or S-4). Compounds R-4 and S-4 were synthesized under similar experimental conditions except that the R-pempH2 and S-pempH2 were used, respectively, as the starting material. A typical procedure for the preparation of R-4 is as follows. A mixture of ErCl3·6H2O (0.1 mmol, 0.0381 g), R-pempH2 (0.5 mmol, 0.1080 g) and NaCl (2 mmol, 0.1168 g) in 9 mL of H 2O, adjusted to pH 3.7 with 0.5


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mol/L NaOH, was kept in a Teflon-lined autoclave at 120°C for 2 days. After cooling to room temperature, colorless rectangle-like crystals were obtained. Yield: 37.3% based on Er. Elemental analysis calcd (%) for C36H52N4O12P4Cl2Er2: C 34.26, H 4.15, N 4.44; found: C 33.62; H 4.30; N 4.88. IR (KBr, cm -1): 3409(s), 3048(m), 2985(m), 2784(m), 2518(m), 2404(w), 1620(m), 1492(m), 1457(m), 1423(w), 1382(w), 1273(m), 1149(s), 1079(s), 1022(s), 987(s), 765(m), 701(m), 567 (m), 536(m), 509(m), 476(m). For S-4: Yield: 38.1% based on Er. Elemental analysis (%) calcd for C36H52N4O12P4Cl2Er2: C 34.26, H 4.15, N 4.44; Found: C, 33.38; H, 4.26; N, 4.82. IR (KBr, cm-1): 3111(s), 1612(w), 1507(m), 1462(m), 1444(m), 1386(w), 1262(w), 1196(w), 1140(s), 1092(s), 747(s), 606(m), 426(w). Structure Determinations. Single crystals of compounds 1-4 were used for data collection on a Bruker APEXII (for R-1 and S-3), Bruker APEX DUO (for S-1 and R-3) or D8 (for R-2, S-2 and R-4) diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 296 K (for R-1, S-1 and S-3) or 123 K (for R-2, S-2, R-3 and R-4). A hemisphere of data was collected in the θ range of 0.7-26.0° for R-1, 0.7-26.0° for S-1, 2.0-26.0° for R-2, 2.1-26.0° for S-2, 1.8-27.7° for R-3, 1.4-26.0° for S-3, and 2.2-27.5° for R-4. The data were integrated using the Siemens SAINT program36, with the intensities corrected for Lorentz factor, polarization, air absorption, and absorption due to variation in the path length through the detector faceplate. Empirical absorption and extinction corrections were applied. The structures were solved by direct method and refined on F2 by full-matrix least squares using SHELXTL37. All the non-hydrogen atoms were refined anisotropically. All the hydrogen atoms except those attached to water molecules were put in calculated positions and refined isotropically with the isotropic vibration parameters related to the non-hydrogen atoms to which they are bonded. The hydrogen atoms attached to water molecules were found from Fourier maps and were refined isotropically. The crystallographic data are given in Table 1. RESULTS AND DISCUSSION Structures of 1 and 2. Compounds 1 and 2 were synthesized following a similar procedure by reacting Er(NO3)3·6H2O and R- or S-pempH2 under hydrothermal conditions at 120 C for 2 days except that the pH of the reaction mixtures was ca. 3.8 for 1 and 3.0 for 2. The R- and S-isomers are a pair of enantiomers, confirmed by powder X-ray diffraction (PXRD) patterns (Figures S1-S2), infrared (IR) (Figures S3-S4) and circular dichroism (CD) spectra (Figure 1 and Figures S5-S6). So only single crystal structures of R-1 and R-2 are described in detail as representatives. Scheme 1. Syntheses of compounds 1-4.



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Table 1. Crystallographic data for compounds R-1, S-1, R-2, S-2, R-3, S-3 and R-4. Compound Formula M crystal size [mm] crystal system space group T (K) a [Å] b [Å] c [Å] V [Å 3], Z Dc [g cm-3]  [mm-1] F(000) Rint Tmax,Tmin GoF on F2 R1, wR2a[I>2(I)] (all data) Flack parameter ()max, min/e Å -3 CCDC number

R-1 C108H156N12O36P12Er4 3351.12 0.30× 0.15× 0.15 hexagonal P65 296 31.817(1) 31.817(1) 24.3689(18) 21364(2), 6 1.563 2.547 10104 0.079 0.775, 0.700 1.05 0.0607, 0.1377 0.0940, 0.1597 -0.007(6) 2.90, -1.42 1539563

S-1 C108H156N12O36P12Er4 3351.12 0.4×0.1× 0.1 hexagonal P61 296 31.8494(9) 31.8494(9) 24.3632(14) 21403(2), 6 1.560 2.542 10104 0.109 0.776，0.740 1.04 0.0610，0.1468 0.0824，0.1650 0.005(15) 4.18, -2.16 1540219

Compound R-3 S-3 Formula C36H52N5O15P4ClEr2 C36H52N5O15P4ClEr2 M 1288.67 1288.67 crystal size [mm] 0.05 x 0.05 x 0.10 0.05 x 0.10 x 0.10 crystal system orthorhombic orthorhombic space group P21212 P21212 T (K) 123(2) 296 a [Å] 15.880(2) 15.9865(15) b [Å] 31.885(4) 32.069(3) c [Å] 9.1544(12) 9.1593(8) V [Å 3], Z 4635.2(10), 4 4695.7(7), 4 Dc [g cm-3] 1.847 1.823 -1  [mm ] 3.862 3.812 F(000) 2544 2544 Rint 0.132 0.026 Tmax,Tmin 0.830, 0.699 0.832, 0.700 GoF on F2 0.98 1.01 [a] R1, wR2 [I>2(I)] 0.0391, 0.0740 0.0213, 0.0517 (all data) 0.0666, 0.0813 0.0230, 0.0526 Flack parameter -0.003(12) 0.007(9) ()max, min/e Å -3 2.14, -2.35 0.96, -1.02 CCDC number 1540224 1540225 aR = F F /F . bwR = [w(F 2-F 2)2/w(F 2)2]1/2 1 o c o 2 o c o

R-2 C81H123N11O35P9Er3 2591.41 0.30 x 0.30 x 0.25 orthorhombic P212121 123(2) 17.0734(15) 24.139(2) 25.726(2) 10602.6(15), 4 1.623 2.591 5220 0.064 0.566, 0.500 1.03 0.0398，0.0791 0.0614, 0.0882 -0.008(4) 2.11,-1.66 1540220 R-4 C36H52N4O12P4Cl2Er2 1262.11 0.10 x 0.20 x 0.20 orthorhombic P21212 123(2) 15.858(4) 31.752(6) 9.130(2) 4597.2(18), 4 1.824 3.943 2488 0.049 0.694, 0.500 1.00 0.0348, 0.0800 0.0391, 0.0818 0.016(12) 2.12, -1.43 1540226


S-2 C81H123N11O35P9Er3 2591.41 0.40 x 0.40 x 0.30 orthorhombic P212121 123(2) 16.9757(6) 24.1073(11) 25.705(1) 10519.5(7), 4 1.636 2.571 5220 0.072 0.510, 0.420 1.04 0.0632,0.1194 0.0980, 0.1321 0.003(14) 2.46,-2.01 1540221

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Figure 1. Circular dichroism spectra of compounds 1, 2 (left) and 3, 4 (right) in the solid state. Compound R-1 crystallizes in the hexagonal chiral space group P65 (no. 170). The asymmetric unit contains four independent Er3+, twelve R-pempH- and seven lattice water molecules (Figure S9). Two different neutral chains are identified in R-1. Chain-I has the composition of [Er(R-pempH)3] and contains only one Er atom (Er1). The Er1 atom is seven-coordinated, surrounded by seven phosphonate oxygen atoms (O1, O3, O4, O6, O7, O8, O1A) from six phosphonate ligands (Figure S9a). The Er-O bond lengths are 2.231(19)-2.447(12) Å and the O-Er1-O bond angles are 58.9(4)-168.0(4). There are three crystallographically distinguished R-pempH- ligands in chain-I. One of them (P1) bridges two equivalent Er atoms through a 3-O(P) atom (O1). While the remaining two (P2 and P3) connect the equivalent Er atoms through O-P-O units. Consequently, neighbouring Er atoms are triply bridged by one 3-O(P) and two O-P-O units, forming an infinite chain running along the c-axis (Figure 2a). The chain contains triple helical strands made up of -Er1-O1-P1-O3-Er1-, -Er1-O4-P2-O6-Er1- and -Er1-O7-P3-O8-Er1- with a pitch of 24.37 Å and a diameter of 17.96 Å (Figure 2e, Figure S10). Chain-II has the composition of [Er3(R-pempH)9] and contains three distinct Er atoms (Er2, Er3, Er4). Two of them (Er2, Er3) are eight-coordinated, whereas the remaining one (Er4) is seven-coordinated by phosphonate oxygen atoms (Figure S9b). The Er-O bond lengths are 2.215(12) - 2.664(13) Å , and O-Er-O bond angles are 58.1(4)-171.4(4). A trimer unit can be identified within the chain, where the Er atoms are triply bridged by two μ3-O(P) (O23 and O25 for Er2-Er3, O28 and O35 for Er3-Er4) and one O-P-O units (O19-P7-O20 for Er2-Er3, O31-P11-O32 for Er3-Er4). The trimers are cross-linked by one 3-O(P) (O16) and two O-P-O units (O10-P4-O12 and O13-P5-O15) between Er2 and Er4 atoms, forming infinite chains running along the c-axis (Figure 2b). Chain-II

also

contains

triple

helical

strands

of

Er4-O12-P4-O10-Er2-O19-P7-O20-Er3-O31-P11-O32-Er4-,-Er4-O15-P5-O13-Er2-O22-P8-O23-Er3-O34-P12-O35-Er 4-, -Er4-O16-P6-O17-Er2-O25-P9-O26-Er3-O28-P10-O29-Er4- with a pitch of 24.37 Å and a diameter of 18.56 Å (Figure 2g). Chain-I and chain-II are packed in the lattice with interchain distances of 15.909 Å . Van der Waals interactions are dominant between the phenyl rings of the adjacent chain (Figures S10, S11). Unlike R-1, R-2 exists as a cationic chain of R-[Er3(pempH)7(pempH2)2]2+, with two nitrates as counter ions and two water molecules as solvent of crystallization. It crystallizes in the orthorhombic space group P212121 (no. 19), and is isostructural to a terbium analogue33. The chemical composition and structure of R-2 are similar to that of chain-II in R-1, except that two zwitterionic R-pempH2 ligands are present in R-2 in addition to seven



Page 8 of 19

R-pempH- ligands. A trimer unit is again observed, where the Er atoms are connected by two μ3-O(P) and one O-P-O units (Figure S12). The trimers are fused by one μ3-O(P) and two O-P-O units, thus leading to an infinite chain containing triple helical strands (Figure 2c, 2g). Compared to the chain-II in R-1, the helical chain in R-2 is more bent with a slightly shortened pitch of 24.13 Å and an expanded diameter of 18.85 Å (Figure S13). This could be related to the presence of the protonated phosphonate oxygen atoms (O6 and O24) in the zwitterionic R-pempH2 ligands, which are involved in the hydrogen-bond interactions with two NO3- counter anions [O6···O32i: 3.20(2) Å ; O6···O33i: 2.57(3) Å (i: -1+x, y, z); O24···O28: 3.33(2) Å , O24···O30: 2.54(2) Å ] (Table S6, Figure S14). The isolation of compounds 1 and 2 at different pH is interesting. A systematic study on Er(NO3)3 / R-pempH2 reaction system reveals that pure phases of R-1 and R-2 can be obtained in the pH ranges 3.7-5.0 and 2.6-3.2, respectively (Figures S15, S16). When the pH was in between (3.3-3.6), a mixture of R-1 and R-2 is formed (Figures S15, S16). Apparently, a lower pH promotes the partial protonation of the phosphonate oxygen atoms, and thus the formation of R-2. Further study shows that compound R-1 can transform into R-2 when the pH of the mother liquid containing R-1 is lowered to 2.2. In contrast, compound R-2 can transform into R-1 when the pH of the mother liquid containing R-2 is increased to 4.5 (Figure S17).

(a)

(b)

(e)

(c)

(f)

(d)

(g)

(h)

Figure 2. Inorganic chains of compounds R-1 (a, b), R-2 (c) and R-3 (d). Pairs of chains showing three- or four-stranded helices in compounds R-1 and S-1 (e, chain I; f, chain II), compounds R-2 and S-2 (g) and compounds R-3 and S-3 (h).


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Structures of 3 and 4. In order to investigate whether the presence of an alkali metal salt may affect the formation of triple helical structures of 1 and 2, we performed the same synthetic reaction of Er(NO3)3 and pempH2 at pH 3.8 except with additional NaCl (20 equiv of Er, ca. 0.55 mol/L). Surprisingly, compounds R- or S-[Er2(pempH)4(NO3)Cl] (R-3 or S-3) with quadruple-stranded helical structures were obtained. Compounds R-3 and S-3 are a pair of enantiomers, confirmed by the PXRD, IR and CD spectra (Figures S18-S20). Hence R-3 is selected for a detailed structural description. Compound R-3 crystallizes in the orthorhombic system space group P21212 (no. 18). The asymmetric unit is comprised of two independent Er3+, four R-pempH-, one NO3- and one Cl- (Figure S22). Both Er1 and Er2 have distorted octahedral environments. The six coordination sites of Er1 are provided by one Cl and five phosphonate oxygen atoms [Er1-O(Cl): 2.193(7)-2.687(7) Å , O-Er1-O(Cl): 86.5(2)-176.8(2)]. While those of Er2 are filled with one nitrate oxygen and five phosphonate oxygen atoms [Er2-O: 2.187(6) - 2.620(2) Å , O-Er1-O: 48.0(5)-174.7(2)]. The Er1 and Er2 atoms are connected by a pair of O-P-O units in an alternative manner along the c-axis, whereas the equivalent Er1 or Er2 atoms are linked by a pair of O-P-O units along the b-direction (Figure 2d). An infinite chain running along the c-axis is thus constructed,

which

contains

quadruple-stranded

helices

made

up

of

-O5-P2-O4-Er1-O11-P4-O10-Er2-O8-P3-O9-Er1-O1-P1--O2-Er2- (Figure 2h). The diameter and pitch of the helical chains in R-3 are 19.35 Å and 36.62 Å , respectively (Figure S23). The latter is significantly larger than those in R-1 (24.37 Å ) and R-2 (24.11 Å ). Compared to R-1 and R-2, R-3 has several significant features: 1) The molar ratio of Er:pemp in the molecular formula of R-3 becomes 1:2 instead of 1:3 as in the cases of R-1 and R-2; 2) The Er atoms are all six-coordinated instead of seven- or eight-coordinated as in the cases of R-1 and R-2; 3) Neighbouring Er atoms are bridged purely by O-P-O units instead of both μ3-O(P) and O-P-O units as in the cases of R-1 and R-2; 4) Half of the four independent R-pempH- ligands each bridges three Er atoms by using its three phosphonate oxygen atoms, instead of bridging two Er atoms via two of its three phosphonate oxygen atoms as in the cases of R-1 and R-2; 5) The chloride and nitrate anions are involved in the coordination with metal ions. The incorporation of coordinated NO3- in R-3 is evidenced by the emergence of a peak at ca. 1312 cm-1 in the IR spectrum (Figure S19). If ErCl3·6H2O instead of Er(NO3)3·6H2O was used as the starting material to react with pempH2 in the presence of NaCl (20 equiv of Er), compounds R- or S-[Er2(pempH)4Cl2] (4) was isolated as a major phase. Unfortunately, single crystal structure was solved only for R-4 due to the poor crystal quality of the crystals of S-4. Compounds R-4 and S-4 are a pair of enantiomers, confirmed by the PXRD, IR and CD spectra (Figures S24-S26). Compound R-4 also crystallizes in the orthorhombic chiral space group P21212 (no. 18). Its structure is identical to that of R-3 except that the coordinated NO3- anion in R-3 is substituted by a Cl- anion (Figures S28, S29). It is worth mentioned that although the enantiomeric nature of the R- and S-isomers of compounds 1-4 is confirmed by CD spectra which exhibit mirrored responses with Cotton effects of opposite signs, the spectra of compounds 2 are different from the others. Take the R-isomers as examples. For R-1, one positive and three negative peaks appear at 228, 255, 262 and 269 nm with a crossover at 238 nm. Compounds R-3 and R-4 have identical CD spectra. In contrast, compound R-2 shows a broad band between 220 and 270 nm which contains



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several negative peaks. Further, the signals of Cotton effects in all compounds appear below 300 nm, demonstrating that the chirality of the complexes stems from the chiral phosphonate ligand. The lack of CD signals in the visible region suggests that the chiral transformation from the ligand to the metal center is not evident. The role of salt. The formation of a quadruple-stranded helical chain structure of R-3 in the presence of NaCl is unexpected. Sodium chloride is well known to play important roles in biological systems. In synthetic chemistry, however, NaCl or other salt-assisted formation of a quadruple helical structure has never been documented, even though salts were previously used as additives to control the size, morphology and structure of inorganic nanocrystals38-40. Now the question is: which kind of role NaCl plays in the assembly of quadruple-stranded helical structures of R-3 or R-4? As a strong electrolyte, NaCl can completely dissociate in an aqueous solution. The role of NaCl could be two-fold: (1) The dissociated sodium or chloride ions can participate in the coordination with the phosphonate ligand or erbium(III) ion; (2) The ionic strength of the solution is increased upon the addition of NaCl. To check whether the Na+ or Cl- ion is specific for the formation of the quadruple-stranded helices, we conducted the same synthetic reactions of Er(NO3)3 and R-pempH2 at pH 3.8 with the addition of other monovalent salts such as LiCl, KCl and NH4Cl (50 equiv of Er). In all cases, compound R-3 was obtained as a pure phase (Figures S30, S31). We also performed the same synthetic reactions with the addition of other sodium salts such as NaNO3, NaClO4 and Na2SO4 (50 equiv of Er). The PXRD and IR results suggest that the product using NaNO3 is isostructural with R-3 (Figures S32, S33). The presence of the coordinated NO3- anion is confirmed by the increased peak intensity at 1312 cm-1. For the product using NaClO4, although the PXRD pattern is different, its IR spectrum is very similar to that of R-3 indicating that the two structures are closely related to each other. The enhanced intensity at 1118 cm-1 and 630 cm-1 are attributed to the ClO4- anion. The presence of ClO4- is also confirmed by the EDX measurement (Figure S34). According to the elemental analyses, the formulae can be proposed as R-[Er2(pempH)4(NO3)2] and R-[Er2(pempH)4(ClO4)2], respectively (Table S12). In contrast, the addition of a dianion salt Na2SO4 resulted in a new but unrecognized phase. Obviously, strong electrolytes based on other monovalent metal chlorides, nitrates and perchlorate can play the same role as NaCl in the formation of the quadruple helical structures of R-3. Therefore, we suppose that the ionic strength is the key in the self-assembly of triple or quadruple helical structures of the Er/pemp system. The ionic strength (I) is a function of the concentration of all ions present in the solution, e.g. I = 1/2cizi2. It is related to the concentration (ci) and charge (zi) of all ions instead of the nature of these ions. This can explain why other monovalent metal or ammonium salts with the same concentration behave like NaCl in the formation of R-3. Assuming that the concentrations of Er3+ and R-pempH- remain the same in the reaction mixture, the addition of NaCl will increase the ionic strength of the solution. The increased ionic strength should then increase the Debye-Hückel shielding, and thus reduce the electrostatic attraction between Er 3+/H+ and pempH-. This may explain the decrease of the Er:pemp molar ratio from 1:3 in R-1 and R-2 to 1:2 in R-3.


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Figure 3. The products after hydrothermal reactions of Er(NO3)3 and pempH2 at different pH with the addition of different amount of NaCl (1-100 equiv of Er) at 120 °C for 2d. To determine the effect of the concentration of NaCl on the structure of the final products, we conducted similar reactions (120°C, 2d) at different pH with the addition of different amount of NaCl (1-100 equiv of Er) (Figure 3). When the molar ratio of Er:NaCl is 1:1 (ca. 0.01 mol/L NaCl), R-1, R-2 and a mixture of R-1 and R-2 were obtained at pH 4.0-5.0, 2.7-3.4 and 3.6-3.8, respectively (Figures S35,S36). When the ratio of Er:NaCl is 1:(5-20) (ca. 0.06-0.22 mol/L NaCl), compound R-3 appears at pH range 2.56-3.40 (for 1:5), 2.56-3.40 (for 1:10) and 2.60-3.84 (for 1:20), respectively (Figures S37-S42). When the ratio of Er:NaCl is 1:50 (ca. 0.55 mol/L NaCl), a pure phase of compound R-3 can be obtained at a wider pH range of 2.80-3.81 (Figures S43, S44). When the ratio is 1:100 (ca. 1.1 mol/L NaCl), pure R-3 was isolated at pH 2.60-5.00 (Figures S45, S46). Clearly, the concentration of NaCl in the reaction mixture and hence the ionic strength is very important for the formation of the quadruple-stranded helical structures of R-3. In order to estimate the ionic strength in solution quantitatively, we performed electrical conductivity measurements in aqueous solutions of Er(NO3)3 (0.1 mmol ) and R-pempH2 (0.5 mmol) in the presence of different amount of NaCl (0-100 equiv of Er). As expected, the conductivity increases linearly with increasing amount of NaCl, from 2.63 mS/cm (for zero equiv of Er) to 64.4 mS/cm (for 100 equiv of Er) (Figure S47). Structural transformation. We next ask whether R-1 can be transformed into R-3 simply by adding a suitable amount of NaCl. The experiments were carried out as follows. First, a mixture of Er(NO 3)3·6H2O and R-pempH2 was allowed to react under hydrothermal conditions at 120°C for 2 days at pH 3.8. After the autoclave was cooled to room temperature, rod-like crystals of compound R-1 can be found in the mother liquid. Then, different amount of NaCl (20-100 equiv to Er) was added to the reaction mixture, and the hydrothermal reaction was continued for different periods of time (1 - 16 d). Figure 4 shows the PXRD patterns and photographs of the products after adding 20 equiv NaCl. Interestingly, a two-step transformation process is clearly observed, e.g. from R-1 to R-2, and then to R-3. The transformation from R-1 to R-2 completes within 4 d. Then R-3 appears together with R-2 in the final products. Although the amount of R-3 increases and that of R-2 decreases with



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prolonged reaction time, the full conversion from R-2 to R-3 is not achieved after 16 d. The IR spectra of the products after different period of reaction time are shown in Figure S49. The band at 702 cm-1 is assigned to the characteristic vibration of mono-substituted phenyl which can be used as the reference band, and the peak at 1312 cm-1 corresponds to the vibration of nitrate anion in the product. The conversion rate from R-1 to R-3 is estimated to be 98.1% after 16 days of reaction. The situation is not significantly improved when 50 equiv. NaCl was added to the reaction mixture (Figures S51-S53). When 100 equiv. NaCl was used, a complete transformation from R-1 to R-3 is found within 8 d (Figures S54-S56).

(a)

(b)

Figure 4. PXRD patterns (a) and photographs (b) of the reaction products after adding 20 equiv NaCl to the mother liquid containing crystals of R-1 and then reacting for different period of time under hydrothermal conditions (120oC, 2 d). We also tested the transformation from R-2 to R-3. For this purpose, the pH of the reaction mixture of Er(NO3)3·6H2O and R-pempH2 was adjusted to 2.8-2.9 in order to obtain compound R-2 first. Remarkably, with the aid of 20 or 50 equiv. NaCl, R-2 can be completely transformed into R-3 within 2 d (Figures S57-S62). The results demonstrate that the triple-stranded helical structures of R-1 and R-2 can be successfully converted into quadruple-stranded helical structure of R-3 simply by adding suitable amount of NaCl. The conversion rate, however, could be dependent on both the particular ionic strength raised by the salt and the initial pH value of the reaction mixture. The transformation mechanism concerns with the construction of Er-Cl bonds, accompanied by the dissociation of part of the Er-O(P) bonds. Absorption spectra. Lanthanide ions with unique electronic configurations possess fascinating optical properties that provide new possibilities for a broad range of applications in lighting devices 41, telecommunications42 and biophotonic applications43. The erbium-based materials are of particular interest


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owing to their energy level profile spanning from the near-infrared (NIR) to the visible region and long-lived excited states due to Laporte forbidden electric dipole transitions 34. The solid state UV-vis absorption spectra of complexes R-1, R-2, R-3 and R-4 are shown in Figures S64-S67, translated from the diffuse reflectance spectra using equation F(R) = (1-R)2/2R. The strong absorption bands in the 200–350 nm range are attributed to the singlet–singlet (1π–π*) transitions in the phenyl rings of the organic ligand. Meanwhile, the Er3+ f–f transitions are also observed in all four cases, from the fundamental state 4I15/2 to 4G11/2 (∼377 nm), 2H9/2 (∼405 nm), (4F5/2, 4F3/2) (∼450 nm), 4F7/2 (∼488 nm), 2H11/2 (∼520 nm), 4S3/2 (∼544 nm) and 4F9/2 (∼655 nm) states44,45. Notably, the absorption spectra of R-1 and R-2 are similar, while those of R-3 and R-4 are close to each other. This is in accordance with their structures, e.g. both R-1 and R-2 with Er:pemp ratio of 1:3 show triple-stranded helical chain structures, while R-3 and R-4 with Er:pemp ratio of 1:2 show quadruple-stranded helical chain structures. Figure 5a compares the normalized absorption spectra of R-1 and R-3. The most significant difference is found below 350 nm. There are three maxima appearing at ca. 220, 260 and 310 nm for R-1, which move to 238, 270 and 295 nm for R-3. The difference may originate from the different coordination modes and protonation extent of the phosphonate ligands in the two structures. The Er3+ f–f transitions are quite similar in the two cases except the intensities of 4I15/2  4G11/2 at ca. 377 nm and 4I15/2  2H11/2 at ca. 520 nm. The two transitions are known to exhibit hypersensitive behaviour, satisfying electric quadrupole selection rules in the intermediate-coupling scheme46, 47. The oscillator strengths of these transitions are sensitive to the structural details and chemical nature of the ligand environment. According to the Continuous Shape Measure (CShM) analysis48, the Er atoms in R-1 have distorted polyhedral geometries closer to D4d (for Er1 and Er2), D2d (for Er3) or C2v (for Er4) symmetries. While those in R-3 have slightly distorted octahedral geometries with Oh symmetry (Table S13). Indeed, the intensity of the 4I15/22H11/2 transition is much stronger in R-3 than that in R-1, attributed to the different coordination geometries of the Er3+ ions in the two structures.

Figure 5. (a) UV-Vis absorption spectra for R-1 and R-3 in the solid state. (b) Emission spectra excited under 377 nm and (c) luminescent decay profiles monitored at 1532 nm for R-1 and R-2, and 1540 nm for R-3 and R-4 in the solid state at room temperature. Emission spectra. Figure 5b shows the normalized emission spectra of compounds R-1, R-2, R-3 and R-4 in the solid state at room temperature. All exhibit the well-resolved multiline emission bands in the near infrared



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region ranging from 1450 to 1630 nm. For R-1 and R-2, there appears four distinct peaks at ca. 1490, 1532, 1560 and 1608 nm with the most intensive one at 1532 nm. The multiline emission bands are attributed to the transition from the first excited state (4I13/2) to the manifold ±MJ sublevels of the ground state (4I15/2) of the Er3+ ion as a consequence of ligand-field effects.16 Interestingly, although the overall profiles for compounds R-3 and R-4 are similar to those of R-1 and R-2, the strongest peak is centered at 1540 nm and the peaks at ca. 1490, 1560 and 1608 nm are largely reduced or even invisible. Such a difference arises from the different coordination geometries in these compounds, e.g. seven- or eight-coordinated Er atoms in compounds R-1 and R-2 and six-coordinated Er atoms in compounds R-3 and R-4, which lead to different ligand-field effects. Moreover, noting that most popular ligands are UV-excitable which is harmful for living tissue and with very weak penetration in biological tissue, the capability of exciting the emission of compounds 1-4 under visible light at 520 nm suggests feasible bio-applications. The decay curves monitored at 1532 nm for R-1 and R-2 and 1540 nm for R-3 and R-4 can be fitted to a bi-exponential process, affording lifetimes () of 1.35 μs (89.5%) and 10.6 μs (10.5%) for R-1, 1.45 μs (90.6%) and 11.04 μs (9.4%) for R-2, 2.33 μs (93.7%) and 17.85 μs (6.3%) for R-3 and 2.02 μs (91.8%) and 13.42 μs (8.2%) for R-4, respectively (Figure 5c, Figure S71). The relatively longer lifetimes of compounds R-3 and R-4 compared with those of compounds R-1 and R-2 can be well interpreted by the elimination of solvent water molecules in the structures which significantly reduces the degree of vibrational quenching49, 50. In summary, four pairs of homochiral erbium compounds are reported, namely, R- or S-[Er4(pempH)12]·7H2O (1) and R- or S-[Er3(pempH)7(pempH2)2](NO3)2·2H2O (2) with triple-stranded helical chain structures, and R- or S-[Er2(pempH)4(NO3)Cl] (3) and R- or S-[Er2(pempH)4Cl2] (4) with quadruple-stranded helical chain structures. Compounds 3 and 4 are the first examples of homochiral coordination polymers that show quadruple-stranded helical structures. More significantly, the conversion from triple-stranded helical structures of 1 and 2 into quadruple-stranded helical structures of 3 can be achieved simply by raising the ionic strength of the solution via the addition of different salts. This work provides a new route to control the formation of multi-stranded helical coordination polymers, which may be extended to other self-assembled coordination systems. Optical studies reveal that compounds 1-4 can be excited under visible light and emit near infrared luminescence which provide potential applications in material science and as biological probes. Additional information Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Accession Codes CCDC 1539563, 1540219-154021, 1540224-1540226 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.


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AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS Financial support by the National Natural Science Foundation of China (21731003, U1532110), the National Key R&D Program of China (2017YFA0303203), and the Natural Science Foundation of Suqian college (2015KY26) is acknowledged. We thank M. Kurmoo in Strasberg University of France for proof reading the manuscript. DEDICATION Dedicated to Prof. Xin-Tao Wu on the occasion of his 80th birthday. References (1) Biffi, G.; Tannahill, D.; McCafferty, J.; Balasubramanian, S. Quantitative Visualization of DNA G-quadruplex Structures in Human Cells. Nat. Chem., 2013, 5, 182-186. (2) Mendoza, O.; Bourdoncle, A.; Boulé,J.-B; BroshJr, R. M.; Mergny, J.-L. G-quadruplexes and Helicases. Nucleic Acids Res., 2016, 44, 1989-2006. (3) Maizels, N. G4-associated Human Diseases. EMBO reports, 2015, 16, 910-922. (4) Albrecht, M. “Let's twist again” Double-stranded, Triple-stranded, and Circular Helicates. Chem. Rev., 2001, 101, 3457-3497. (5) Saalfrank, R. W.; Maid, H.; Scheurer, A. Scheurer, Supramolecular Coordination Chemistry: The Synergistic Effect of Serendipity and Rational Design. Angew. Chem. Int. Ed., 2008, 47, 8794-8824. (6) Miyake, H.; Tsukube, H. Coordination Chemistry Strategies for Dynamic Helicates: Time-programmable Chirality Switching with Labile and Inert Metal Helicates. Chem. Soc. Rev., 2012, 41, 6977-6991. (7) Boiocchi, M.; Fabbrizzi, L. Double-stranded Dimetallic Helicates: Assembling–disassembling Driven by the CuI / CuII Redox Change and the Principle of Homochiral Recognition. Chem. Soc. Rev., 2014, 43, 1835-1847. (8) Lehn,J.-M.; Rigault, A.; Siegel, J.; Harrowfield, J.; Chevriert, B.; Morast, D. Spontaneous Assembly of Double-stranded Helicates from Oligobipyriding Ligands and Copper(I) Cations-structure of an Inorganic Double Helix. Proc. Natl. Acad. Sci. USA, 1987, 84, 2565-2569. (9) Kramer, R.; Lehn, J.-M.; Marquis-Rigault, A. Self-recognition in Helicate Self-assembly: Spontaneous Formation of Helical Metal Complexes from Mixtures of Ligands and Metal Ions. Proc. Natl. Acad. Sci. USA, 1993, 90, 5394-5398. (10) Stadler, A.-M.; Burg, C.; Ramírez, J.; Lehn, J.-M. Grid–double-helicate Interconversion. Chem. Commun., 2013, 49, 5733-5735.



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Homochiral Erbium Coordination Polymers: Salt-Assisted Conversion

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