Rationally designed and controlled syntheses of different series of 4d–4f heterometallic coordination frameworks based on lanthanide carboxylate and Ag(IN)₂ substructures

Abstract

Different series of Ln(III)–Ag(I) heterometallic coordination frameworks, namely LnAg(OAc)(IN)₂(H₂O)·(ClO₄) [Ln = Tb (1a); Ho (1b), HOAc = acetic acid, HIN = isonicotinic acid], LnAg₂(OX)_0.5(IN)₃(H₂O)_1.5·(ClO₄) [Ln = Nd (2a); Eu (2b) Tb (2c), OX = oxalate], Eu₂Ag₃(OX)_0.5(IN)₆(NO₃)(ClO₄)(H₂O)₃·2(H₂O) (3), LnAg(OX) (IN)₂(H₂O) [Ln = Ce (4a); Sm (4b)], LnAg(Mal)(IN)₂(H₂O) [Ln = La (5a); Nd (5b), H₂Mal = malonic acid], Ln₃Ag₄(Lac)₂(IN)₈(H₂O)₅·2(ClO₄)·2.5(H₂O) [Ln = La (6a); Lu (6b), HLac = lactic acid], Sm₃Ag₄(Lac)₂(IN)₈(H₂O)₄·2(ClO₄)·2.5(H₂O) (7), and Eu₂Ag₃(S-Lac)(IN)₆(H2O)4·2(ClO₄)·4(H₂O) (8) were successfully synthesized by systematic variation of reaction parameters such as initial reactants, reaction time, pH, temperature, etc. Compounds 1a and 1b represent 3D coordination frameworks that are constructed from adjacent lanthanide carboxylate layers and pillared Ag(IN)₂ units. Compounds 2a, 2b and 2c comprise of 2D Ln-carboxylate-Ag layered networks and pillared Ag(IN)₂ units. Compound 3 exhibits 3D coordination frameworks that are built up from the assembly of europium-carboxylate layers and pillared Ag(IN)₂ units. 3D coordination networks of compounds 4a and 4b containing tetranuclear Ln₂Ag₂ cores are constructed from 2D cerium oxalate layers and tilted pillared Ag(IN)₂ subunits. Compounds 5a and 5b display attractive 3D coordination frameworks constructed of 2D lanthanum malonate chains and pillared Ag(IN)₂ units. Compounds 6a and 6b are 2D coordination frameworks comprised of 1D racemic lanthanum carboxylate chains and pillared Ag(IN)₂ units. The crystal structure of 7 is almost the same as that of 6a and 6b, only one less coordination water molecule is coordinated to its metal centre. Compound 8, which spontaneously resolved upon crystallization, represents the first example of a 3D Ln–Ag coordination framework built up from 1D europium-carboxylate chains containing chiral molecules and pillared Ag(IN)₂ units.

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Introduction

The study of lanthanide-transition metal complexes has gained great recognition over the last decade owing to their fascinating structural diversity and the intriguing topological networks they form.¹ They were also studied to investigate potentially multifunctional materials with useful structural properties, such as porosity, gas storage, and ion exchange, or additional physical properties such as magnetism and photoluminescence.^2–6 Many heterometallic complexes have been obtained to date by allowing the assembly of mixed metal ions and organic ligands containing mixed-donor atoms such as pyridinecarboxylate, carbonyl, CN group, amino acids, etc.^7–10 However, the construction of unusual multidimensional heterometallic coordination frameworks with useful physico-chemical properties and intriguing structural topologies is still a challenging task, the reason being the choice of organic ligands, the low stereochemical preference and the variable and versatile coordination numbers for lanthanide metals, and competitive reactions between lanthanide and transition metals coordinated to organic ligands.¹¹ Fortunately, the characteristics of lanthanide and transition metal ions have different affinities for N and O donors, which may provide a new impetus for the rational construction of novel heterometallic coordination frameworks.¹²

Chiral metal–organic coordination polymers have currently found wide applications in enantioselective synthesis, asymmetric catalysis, and nonlinear optical materials (second harmonic generation (SHG)).^13–15 Enantioseparation is an important but very difficult task in biochemistry, agrochemistry, materials science and, in particular, pharmaceutics.^{15a, 16} Employing crystal-engineering strategies can be prone to achieve separation in a more convenient way.

As we know, the lanthanide ions have a strong tendency to bind to ligands containing O-donor atoms to form lanthanide carboxylate subunits, while soft transition metal ions, such as the Ag(I) or Cu(I) ion easily bond to organic ligands containing N-donors to form lanthanide carboxylate subunits.¹⁷ Hydrothermal synthesis has several tremendous advantages over other methods for the synthesis of heterometallic coordination frameworks.²⁹ Among them is the possibility to use lanthanide oxides rather than salts such as lanthanide perchlorates or nitrates as the starting materials, and in our previous work we introduced perchloric acid as a perchlorate source in the formation of lanthanide coordination frameworks. The perchlorate ion prefers to be situated between silver ions, and acts as a mutifunctional linear linker from silver ions coordinated to two IN ligands (IN = isonicotinate).¹⁸ Owing to the high coordination numbers for lanthanide ions, the presence of void filling perchlorates may provide an impetus for the coordination from some small molecules containing carboxylate groups, such as acetate and oxalate to form lanthanide carboxylate subunits.^{19,17e, 17g}

These tendencies and our further studies showed that by manipulating the appropriate reaction parameters such as initial reactants, reaction time, pH value, temperature, etc, different series of lanthanide-silver heterometallic coordination polymers could be selectively synthesized.²⁰ In this full paper, a systematic study has been performed to investigate the controlled synthesis of lanthanide-silver heterometallic coordination polymers by trying to introduce different small molecules containing carboxylate groups, and a suitable mechanism has been proposed for their formation. In this context, all IN ligands coordinate in a linear fashion to one silver ion which will be regarded in brief as the [Ag(IN)₂] coordination mode (Chart 1a).

Chart 1 Coordination modes of Ag(IN)₂ in compounds 1–8.

Experimental

Materials and instruments

All materials and reagents were obtained commercially and used without further purification. Elemental (C, H, N) analyses were performed on a Perkin–Elmer 2400 element analyzer. FT-infrared (IR) samples were prepared as KBr pellets, and spectra were obtained in the 4000 − 400 cm⁻¹ range using a Nicolet Avatar 360 FT-IR spectrophotometer. Thermogravimetric analysis (TGA) experiments were carried out on a Perkin–Elmer TGA 7 thermogravimetric analyzer with a heating rate of 10 °C/min from rt to 850 °C under nitrogen atmosphere. Fluorescence spectra were recorded using an Edinburgh F900 FL Spectrophotometer analyzer.

Syntheses of 1a–1b: An aqueous mixture (10 mL) of isonicotinic acid (0.123 g), AgNO₃ (0.0845 g), Ln₂O₃ (Tb₄O₇ 0.093 g; Ho₂O₃ 0.094 g), CH₃COONa (0.068 g) and HClO₄ (1.90 mmol) in a ratio of 2:1:0.5:1 was placed in a 23 ml Teflon-lined stainless-steel autoclave and heated to 150 °C for 5 d. Colorless prismatic single crystals of 1a–1b were obtained from cooling the solution to room temperature at 10 °C h⁻¹. Compound 1a: Yield (based on Tb): 31%; elemental analysis calcd (%) for C₁₄H₁₃TbAgClN₂O₁₁: C, 24.46; H, 1.90; N, 4.08; found C, 24.50; H, 1.82; N, 4.14. IR(KBr, cm⁻¹): 3442, 1589, 1544, 1390, 1222, 1122, 1049, 769, 700, 617. Compound 1b: Yield (based on Ho): 35%; elemental analysis calcd (%) for C₁₄H₁₃HoAgClN₂O₁₁: C, 24.25; H, 1.89; N, 4.04; found C, 24.34; H, 1.80; N, 4.17. IR(KBr, cm⁻¹): 3442, 1589, 1544, 1386, 1222, 1122, 1053, 765, 700, 617.

Synthesis of 2c: An aqueous mixture (10 mL) of isonicotinic acid (0.123 g), AgNO₃ (0.118 g), Tb₄O₇ (0.056 g), H₂ox (0.025 g) and HClO₄ (1.90 mmol) in the ratio of 3:2:0.5:0.5 was placed in a 23 ml Teflon-lined stainless-steel autoclave and heated to 140 °C for 4 d. Plate-like single crystals of 2c were obtained from cooling the solution to room temperature at 10 °C h⁻¹. Compound 2a: Yield (based on Nd): 40%; elemental analysis calcd (%) for C₃₈H₃₀Nd₂Ag₄Cl₂N₆O₂₇: C, 25.45; H, 1.68; N, 4.68; found C, 25.20; H, 1.79; N, 4.80. IR(KBr, cm⁻¹): 3447, 1592, 1544, 1395, 1118, 1055, 778, 696. Compound 2b: Yield (based on Eu): 29%; elemental analysis calcd (%) for C₃₈H₃₀Eu₂Ag₄Cl₂N₆O₂₇: C, 25.23; H, 1.67; N, 4.64, found C, 25.14; H, 1.79; N, 4.48. IR(KBr, cm⁻¹): 3435, 1590, 1541, 1394, 1120, 1050, 774, 694. Compound 2c: Yield (based on Tb): 33%; elemental analysis calcd (%) for C₃₈H₃₀Tb₂Ag₄Cl₂N₆O₂₇: C, 25.04; H, 1.66; N, 4.61; found C, 25.20; H, 1.49; N, 4.70. IR(KBr, cm⁻¹): 3437, 1589, 1544, 1390, 1122, 1053, 775, 690, 617.

Synthesis of 3: An aqueous mixture (10 mL) of isonicotinic acid (0.123 g), AgNO₃ (0.085g), Eu₂O₃ (0.0525 g), H₂ox (0.020 g) and HClO₄ (1.90 mmol) in the ratio of 6:3:1:1:1 was placed in a 23 ml Teflon-lined stainless-steel autoclave and heated to 150 °C for 5 d. Colorless prismatic single crystals of 3 were obtained from cooling the solution to room temperature at 10 °C h⁻¹. Compound 3: Yield (based on Eu): 27%; elemental analysis calcd (%) for C₃₇H₃₄Ag₃ClEu₂N₇O₂₆: C, 26.79; H, 2.05; N, 5.90. found C, 26.84; H, 2.07; N, 5.92. IR(KBr, cm⁻¹): 3392, 1604, 1548, 1386, 1226, 1116, 1087, 844, 765, 686.

Syntheses of 4a–4b: An aqueous mixture (10 mL) of isonicotinic acid (0.123 g), AgNO₃ (0.085g), Ln(NO₃)₃·6H₂O [Ce(NO₃)₃·6H₂O 0.217 g; Sm(NO₃)₃·6H₂O, 0.222 g] and H₂ox (0.063 g) in the ratio of 2:1:1:1 was placed in a 23 ml Teflon-lined stainless-steel autoclave and heated to 140 °C for 4 d. Prismatic single crystals of (4a–4b) were obtained from cooling the solution to room temperature at 10 °C h⁻¹. Compound 4a: Yield (based on Ce): 42%; elemental analysis calcd (%) for C₁₄H₁₀CeAgN₂O₉: C, 28.11; H, 1.68; N, 4.68; found C, 28.04; H, 1.76;N, 4.80. IR(KBr, cm⁻¹): 3431, 1595, 1544, 1400, 1311, 1055, 860, 790, 769. Compound 4b: Yield (based on Sm): 35%; elemental analysis calcd (%) for C₁₄H₁₀SmAgN₂O₉: C, 27.64; H, 1.66; N, 4.60. found C, 27.70; H, 1.75;N, 4.46. IR(KBr, cm⁻¹): 3435, 1590, 1546, 1404, 1310, 1052, 864, 781, 769, 712.

Syntheses of 5a–5b: An aqueous mixture (10 mL) of isonicotinic acid (0.123 g), AgNO₃ (0.085 g), Ln₂O₃ (La₂O₃ 0.081 g, Nd₂O₃ 0.084 g), malonic acid (0.052 g) and HClO₄ (1.90 mmol) in the ratio of 2:1:0.5:1 was placed in a 23 ml Teflon-lined stainless-steel autoclave and heated to 130 °C, for 50 h. Plate-like single crystals of 5a–5b were obtained from cooling the solution to room temperature at 10 °C h⁻¹. Compound 5a: Yield (based on La): 25%; elemental analysis calcd (%) C₁₅H₁₂ LaAgN₂O₉: C, 29.48; H, 1.98; N, 4.58. found C, 29.60; H, 1.85; N, 4.64. IR(KBr, cm⁻¹): 3465, 2950, 1633, 1602, 1544, 1396, 1342, 1265, 1222, 1116, 1087, 1049, 854, 771. Compound 5b: Yield (based on Nd): 21%; elemental analysis calcd (%) for C₁₅H₁₂AgN₂NdO₉: C, 29.23; H, 1.96; N, 4.54; found C, 29.34; H, 2.07; N, 4.41. IR(KBr, cm⁻¹): 3481, 2965, 1604, 1544, 1390, 1261, 1089, 804, 765.

Syntheses of 6a–6b: An aqueous mixture (10 mL) of isonicotinic acid (0.123 g), AgNO₃ (0.085 g), Ln₂O₃ (La₂O₃ 0.082 g; Lu₂O₃ 0.099 g, racemic lactic acid (0.040 g) and HClO₄ (1.90 mmol) in a ratio of 4:2:1:1.5 was placed in a 23 ml Teflon-lined stainless-steel autoclave and heated to 140 °C for 5 d. Prismatic single crystals of 6a–6b were obtained from cooling the solution to room temperature at 10 °C h⁻¹. Compound 6a: Yield (based on La): 32%; elemental analysis calcd (%) for C₅₄H₅₆La₃Ag₄Cl₂N₈O_37.50: C, 27.76; H, 2.42; N, 4.80; found C, 27.80; H, 2.57; N, 4.69. IR(KBr, cm⁻¹): 3437, 3120, 1595, 1543, 1409, 1226, 1062, 864, 775, 690, 617, 542, 418. Compound 6b: Yield (based on Lu): 27%; elemental analysis calcd (%) for C₅₄H₅₆Lu₃Ag₄Cl₂N₈O₃₇: C, 26.70; H, 2.25; N, 4.68; found C, 26.62; H, 2.32; N, 4.60. IR(KBr, cm⁻¹): 3437, 3112, 1608, 1548, 1390, 1226, 1112, 1087, 1055, 846, 765, 690.

Synthesis of 7: An aqueous mixture (10 mL) of isonicotinic acid (0.123 g), AgNO₃ (0.085 g), Sm₂O₃ (0.087 g), racemic lactic acid (0.040 g) and HClO₄ (1.90 mmol) in a ratio of 4:2:1:1.5 was placed in a 23 ml Teflon-lined stainless-steel autoclave and heated to 140 °C for 5 d. Prismatic single crystals of 7 were obtained from cooling the solution to room temperature at 10 °C h⁻¹. Compound 7: Yield (based on Sm): 26%; elemental analysis calcd (%) for C₅₄H_52.50Sm₃Ag₄Cl₂N₈O_35.75: C, 27.73; H, 2.24; N, 4.79. found C, 27.82; H, 2.20; N, 4.70. IR(KBr, cm⁻¹): 3304, 3110, 1598, 1544, 1390, 1222, 1122, 765, 686.

Synthesis of 8: An aqueous mixture (10 mL) of isonicotinic acid (0.123 g), AgNO₃ (0.085 g), Eu₂O₃ (0.06 g), racemic lactic acid (0.020 g) and HClO₄ (1.90 mmol) in a ratio of 3:1.5:1:0.5 was placed in a 23 ml Teflon-lined stainless-steel autoclave and heated to 140 °C for 4 d. Prismatic single crystals of 8 were obtained from cooling the solution to room temperature at 10 °C h⁻¹. Compound 8: Yield (based on Eu): 20%; elemental analysis calcd (%) for C₃₉H₄₅Eu₂Ag₃Cl₂N₆O₃₁: C, 27.23; H, 2.45; N, 4.72; found C, 27.14; H, 2.53; N, 4.69. IR(KBr, cm⁻¹): 3379, 1608, 1548, 1386, 1222, 1122, 1087, 846, 765.

Crystallographic studies: Single crystal X-ray diffraction data collections of 1–8 were performed on a Bruker Apex II CCD diffractometer operating at 50 kV and 30 mA using Mo Ka radiation (λ = 0.71073 Å). Data collection and reduction were performed using the APEX II software.³³ Multi-scan absorption corrections were applied for all data sets using the APEX II program.³³ All structures were solved by direct methods and refined by full-matrix least squares on F² using the SHELXTL program package.³³ Crystallographic data for 1–8 are listed in Table 1 and selected bond lengths for all compounds are given in Table 2.

Table 1 Data details of the structure determination for complexes 1–8

a R 1 = ∑||F0| – |Fc||/∑|F0|. b wR 2 = {∑[wR(F0^2 − Fc^2)^2]/ ∑(F0^2)^2}^1/2.
	1a	1b	2a	2b	2c
Empirical formula	C14H13TbAgClN2O11	C14H13HoAgClN2O11	C38H30Nd2Ag4Cl2N6O27	C38H30Eu2Ag4Cl2N6O27	C38H30Tb2Ag4Cl2N6O27
Formula weight	687.50	693.51	1793.54	1808.98	1822.90
Crystal system	Monoclinic	Monoclinic	Triclinic	Triclinic	Triclinic
Space group	P21/c	P21/c	P-1	P-1	P-1
a/Å	16.2107(4)	16.2194(2)	8.3173(3)	8.3118(1)	8.3112(4)
b/Å	14.9781(4)	14.8870(2)	10.0725(4)	10.0115(1)	9.9605(4)
c/Å	7.9573(2)	7.92870(10)	16.622(2)	16.5213(2)	16.4964(7)
α/°			91.248(2)	90.820(1)	90.699(2)
β/°	92.0690(10)	91.7020(10)	102.677(2)	103.299(1)	103.392(2)
γ/°			110.576(2)	110.498(1)	110.548(2)
V/Å^3	1930.82(9)	1913.61(4)	1264.46(17)	1246.41(2)	1237.41(9)
Z	4	4	1.095	1.057	1.131
R int	0.0251	0.0357	0.0671	0.0254	0.0319
R 1 [I > 2σ(I)]^a	0.0189	0.0235	0.0693	0.0312	0.0631
wR 2 (all data)^b	0.0490	0.0521	0.2043	0.0844	0.1889

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	3	4a	4b	5a	5b
Empirical formula	C37H34Ag3ClEu2N7O26	C14H10CeAgN2O9	C14H10SmAgN2O9	C15H12 LaAgN2O9	C15H12AgN2NdO9
Formula weight	1655.69	598.23	608.46	611.05	616.38
Crystal system	Triclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P-1	C2/c	C2/c	P21/c	P21/c
a/Å	8.3624(5)	21.728(2)	22.0591(2)	7.68150(10)	7.59830(10)
b/Å	16.903(1)	9.3675(10)	9.22690(10)	18.9433(4)	18.8221(2)
c/Å	17.3261(10)	17.4117(18)	17.1097(3)	12.1566(2)	12.1000(2)
α/°	93.234(4)
β/°	97.017(4)	109.202(2)	108.214(1)	106.9290(10)	106.6480(10)
γ/°	100.010(4)
V/Å^3	2386.1(2)	3346.8(6)	3307.97(8)	1692.29(5)	1657.96(4)
Z	2	8	8	4	4
R int	0.0720	0.0837	0.0416	0.0381	0.0266
GOF	1.131	1.062	1.032	1.119	1.058
R 1 [I > 2σ(I)]^a	0.1008	0.0448	0.0258	0.0324	0.0217
wR 2 (all data)^b	0.2664	0.1048	0.0575	0.0740	0.0483

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	6a	6b	7	8
Empirical formula	C54H56La3Ag4Cl2N8O37.50	C54H56Lu3Ag4Cl2N8O37	C54H52.50Sm3Ag4Cl2N8O35.75	C39H45Eu2Ag3Cl2N6O31
Formula weight	2336.18	2436.36	2338.97	1792.24
Crystal system	Triclinic	Triclinic	Triclinic	Triclinic
Space group	P-1	P-1	P-1	P-1
a/Å	14.3670(6)	14.3880(2)	14.0535(4)	8.0578(2)
b/Å	15.0764(6)	15.0923(2)	14.8033(4)	10.6254(2)
c/Å	17.3874(6)	17.3671(2)	17.3184(4)	17.1987(3)
α/°	85.911(2)	86.0380(10)	86.265(2)	82.3430(10)
β/°	83.182(3)	83.1440(10)	84.502(2)	78.9970(10)
γ/°	84.224(3)	84.3150(10)	84.233(2)	71.0530(10)
V/Å^3	3713.8(2)	3719.58(8)	3562.55(16)	1363.09(5)
Z	2	2	2	1
R int	0.0754	0.0664	0.0758	0.0264
GOF	0.974	1.097	0.972	0.977
R 1 [I > 2σ(I)]^a	0.0494	0.0795	0.0586	0.0261
wR 2 (all data)^b	0.1159	0.2597	0.1460	0.0536

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Table 2 Selected bond lengths (Å) for 1–8^a

a Symmetry codes: 1a: #1: −x, y + 1/2, −z + 1/2; #2: −x + 1, −y + 1, −z; #3: −x, −y + 1, −z + 1; 1b: #1 − x, y + 1/2, −z + 1/2; #2 −x + 1, −y + 1,−z; #3 −x, −y + 1,−z + 1; 2a: #1: −x + 1, −y + 2, −z + 1; #2: −x + 1, −y + 1, −z + 1; #3: −x + 2, −y + 2, −z + 1; #4: x, y, z + 1; #5: −x + 2, −y + 1, −z + 1; 2b: #1: −x + 2, −y, −z + 1; #2: x + 1, y, z; #3: x, y, z − 1 #4: −x + 2, −y + 1, −z + 1; #5: −x + 1, −y, −z + 1; #6: −x + 1, −y + 1, −z + 1; 2c: #1: −x + 2, −y, −z + 2; #2: x + 1, y, z; #3: −x + 2, −y + 1, −z + 2; #4: −x + 1, −y + 1, −z + 1; #5: x, y, z − 1; #6: −x + 1, −y, −z + 2; 3: #1 −x + 1, −y, −z; #2: −x, −y, −z; #3: x, y − 1, z; #4: −x, −y, −z + 1; #5 −x, −y + 1, −z + 1 #6: x, y + 1, z; #7: −x + 1, −y + 1, −z. 4a: #1: −x + 1/2, y + 1/2, −z + 1/2; #2: −x, −y + 1, −z + 1; #3: −x, y, −z + 3/2; #4: −x, −y, −z + 1; #5: x + 1/2, −y + 1/2, z − 1/2; 4b: #1: −x + 1/2, y + 1/2, −z + 1/2; #2: −x, −y + 1, −z + 1; #3: −x, y, −z + 3/2; #4: −x, −y, −z + 1; #5: x + 1/2, −y + 1/2, z − 1/2; 5a: #1: −x + 1, y − 1/2, −z + 3/2; #2: −x − 1, −y, −z + 1; #3: −x, −y, −z + 1; #4: −x + 1, −y + 1, −z + 1; #5: x − 1, y, z; 5b: #1: −x + 1, y − 1/2, −z + 3/2; #2: −x − 1, −y, −z + 1; #3: −x, −y, −z + 1; #4: −x + 1, −y + 1, −z + 1; #5: x − 1, y, z; 6a: #1: x, y + 1, z − 1; #2: x, y, z − 1; #3: −x + 1, −y + 2, −z + 1; #4: x, y − 1, z; #5: −x + 1, −y + 2, −z + 2; 6b: #1: x, y + 1, z, #2: −x + 1,−y + 2,−z + 2; #3: x, y + 1, z − 1; #4: x, y, z − 1; #5: −x + 1, −y + 2, −z + 1; 7: #1: x, y + 1, z − 1; #2: x, y, z − 1; #3: −x + 1, −y + 2, −z + 1; #4: x, y − 1, z; #5: −x + 1, −y + 2, −z + 2; 8: #1: x − 2, y, z + 1; #2: x − 1, y, z; #3: x − 1, y + 1, z + 1; #4: x − 1, y, z + 1.
Complex 1a
Ag(1)–N(1)	2.154(2)	Ag(1)–N(2)	2.155(2)
Tb(1)–O(1)^#3	2.338(2)	Tb(1)–O(2)^#1	2.299(2)
Tb(1)–O(3)^#2	2.333(2)	Tb(1)–O(4)	2.321(2)
Tb(1)–O(5)	2.489(2)	Tb(1)–O(5)^#2	2.376(2)
Tb(1)–O(6)	2.439(2)	Tb(1)–O(1w)	2.404(2)
Complex 1b
Ag(1)–N(1)	2.160(3)	Ag(1)–N(2)	2.166(3)
Ho(1)–O(1)^#3	2.327(2)	Ho(1)–O(2)^#1	2.277(2)
Ho(1)–O(3)^#2	2.315(2)	Ho(1)–O(4)	2.307(2)
Ho(1)–O(5)^#2	2.368(2)	Ho(1)–O(5)	2.473(2)
Ho(1)–O(6)	2.412(3)	Ho(1)–O(1w)	2.394(2)
Complex 2a
Ag(1)–O(4)	2.542(10)	Ag(1)–O(6)	2.498(14)
Ag(1)–O(6)^#5	2.324(13)	Ag(1)–O(2w)	2.64(4)
Ag(1)–Ag(1)^#5	2.533(8)	Ag(2)–N(1)	2.127(12)
Ag(2)–N(3)^#4	2.136(12)	Ag(3)–N(2)	2.140(11)
Nd(1)–O(1)	2.403(10)	Nd(1)–O(2)^#1	2.446(9)
Nd(1)–O(3)	2.380(8)	Nd(1)–O(4)^#2	2.466(9)
Nd(1)–O(5)	2.378(9)	Nd(1)–O(7)^#3	2.500(9)
Nd(1)–O(8)	2.515(10)	Nd(1)–O(1w)	2.513(9)
Complex 2b
Ag(1)–O(4)	2.239(4)	Ag(1)–O(4)^#1	2.401(4)
Ag(1)–O(5)^#2	2.476(3)	Ag(1)–O(2w)	2.510(8)
Ag(1)–Ag(1)^#1	2.5781(18)	Ag(2)–N(2)	2.131(3)
Ag(2)–N(3)^#3	2.129(3)	Ag(3)–N(1)	2.146(3)
Eu(1)–O(1)	2.355(3)	Eu(1)–O(2)^#6	2.391(3)
Eu(1)–O(3)	2.326(3)	Eu(1)–O(5)	2.441(3)
Eu(1)–O(6)^#5	2.325(3)	Eu(1)–O(7)	2.446(3)
Eu(1)–O(8)^#4	2.476(3)	Eu(1)–O(1w)	2.475(3)
Complex 2c
Ag(1)–O(4)	2.22(2)	Ag(1)–O(4)^#1	2.37(2)
Ag(1)–O(5)^#2	2.466(15)	Ag(1)–O(2w)	2.50(5)
Ag(1)–Ag(1)^#1	2.599(9)	Ag(2)–N(1)	2.134(19)
Ag(2)–N(2)	2.136(19)	Ag(3)–N(3)	2.152(17)
O(1)–Tb(1)^#4	2.352(15)	O(2)–Tb(1)^#5	2.346(15)
O(3)–Tb(1)	2.301(14)	O(6)–Tb(1)^#6	2.298(14)
O(5)–Tb(1)	2.417(15)	O(8)–Tb(1)^#3	2.450(14)
O(7)–Tb(1)	2.432(16)	O(1w)–Tb(1)	2.458(14)
Complex 3
Eu(1)–O(5)	2.372(13)	Eu(1)–O(7)	2.391(13)
Eu(1)–O(8)^#1	2.324(15)	Eu(1)–O(10)	2.362(13)
Eu(1)–O(12)	2.353(13)	Eu(1)–O(21)	2.472(12)
Eu(1)–O(22)^#2	2.470(15)	Eu(1)–O(1w)	2.499(12)
Eu(2)–O(1)^#4	2.405(14)	Eu(2)–O(2)	2.409(14)
Eu(2)–O(3)^#5	2.438(13)	Eu(2)–O(4)^#3	2.376(13)
Eu(2)–O(9)	2.369(13)	Eu(2)–O(11)	2.362(13)
Eu(2)–O(2w)	2.410(12)	Eu(2)–O(3w)	2.417(15)
Ag(1)–N(1)	2.216(18)	Ag(1)–N(2)	2.190(19)
Ag(2)–N(3)	2.188(18)	Ag(3)–N(4)^#7	2.119(17)
Ag(3)–N(5)	2.108(17)	Ag(2)–N(6)^#6	2.177(19)
Complex 4a
Ag(1)–N(1)	2.181(6)	Ag(1)–N(2)	2.173(6)
O(1)–Ce(1)	2.493(4)	O(2)–Ce(1)^#4	2.437(4)
O(3)–Ce(1)^#1	2.587(5)	O(4)–Ce(1)^#5	2.539(4)
O(4)–Ce(1)^#1	2.765(4)	O(5)–Ce(1)^#2	2.503(4)
O(6)–Ce(1)	2.503(4)	O(7)–Ce(1)	2.544(4)
O(8)–Ce(1)^#3	2.544(4)
Complex 4b
Ag(1)–N(1)	2.184(3)	Ag(1)–N(2)	2.184(3)
O(1)–Sm(1)	2.428(2)	O(2)–Sm(1)^#4	2.384(3)
O(3)–Sm(1)^#1	2.507(3)	O(4)–Sm(1)^#5	2.450(3)
O(4)–Sm(1)^#1	2.780(3)	O(5)–Sm(1)^#2	2.447(3)
O(6)–Sm(1)	2.460(3)	O(7)–Sm(1)	2.490(3)
O(8)–Sm(1)^#3	2.478(3)
Complex 5a
Ag(1)–N(1)	2.182(5)	Ag(1)–N(2)	2.167(5)
Ag(1)–O(8)^#1	2.600(4)	Ag(1)–Ag(1)^#2	3.1839(11)
La(1)–O(1)	2.630(4)	La(1)–O(2)	2.634(4)
La(1)–O(3)^#3	2.498(4)	La(1)–O(5)	2.639(4)
La(1)–O(6)	2.684(4)	La(1)–O(6)^#4	2.523(4)
La(1)–O(7)^#4	2.571(4)	La(1)–O(7)^#5	2.621(4)
La(1)–O(8)^#5	2.733(4)	La(1)–O(1w)	2.559(4)
Complex 5b
Ag(1)–N(1)	2.178(3)	Ag(1)–N(2)	2.168(3)
Ag(1)–O(8)^#1	2.585(2)	Ag(1)–Ag(1)^#2	3.1694(6)
Nd(1)–O(1)	2.564(2)	Nd(1)–O(2)	2.585(2)
Nd(1)–O(3)^#3	2.450(2)	Nd(1)–O(5)	2.574(2)
Nd(1)–O(6)	2.655(2)	Nd(1)–O(6)^#4	2.459(2)
Nd(1)–O(7)^#4	2.537(2)	Nd(1)–O(7)^#5	2.570(2)
Nd(1)–O(8)^#5	2.705(3)	Nd(1)–O(1w)	2.491(2)
Complex 6a
Ag(1)–N(1)	2.149(6)	Ag(1)–N(2)^#1	2.128(6)
Ag(2)–N(7)	2.138(6)	Ag(2)–N(8)^#2	2.127(6)
Ag(2)–O(5)	2.564(18)	Ag(3)–N(5)	2.178(6)
Ag(3)–N(6)^#3	2.169(6)	Ag(4)–N(3)^#2	2.173(6)
Ag(4)–N(4)	2.164(6)	La(1)–O(18)	2.444(5)
La(1)–O(20)	2.492(4)	La(1)–O(21)	2.589(5)
La(1)–O(23)	2.483(5)	La(1)–O(30)	2.556(5)
La(1)–O(31)	2.591(5)	La(1)–O(32)	2.573(5)
La(1)–O(34)	2.520(6)	La(1)–O(2w)	2.642(5)
La(2)^#4–O(14)	2.482(5)	La(2)–O(16)	2.433(5)
La(2)^#5–O(22)	2.616(5)	La(2)–O(24)	2.515(5)
La(2)^#4–O(25)	2.522(5)	La(2)–O(29)	2.574(5)
La(2)–O(30)	2.763(5)	La(2)–O(1w)	2.564(5)
La(2)–O(6w)	2.592(6)	La(3)–O(13)	2.549(5)
La(3)–O(17)	2.455(5)	La(3)–O(19)	2.561(4)
La(3)–O(26)	2.449(5)	La(3)–O(27)	2.487(5)
La(3)–O(32)	2.678(5)	La(3)–O(33)	2.598(5)
Complex 6b
Ag(1)–N(1)	2.151(12)	Ag(1)–N(2)^#3	2.123(12)
Ag(2)–N(7)	2.141(12)	Ag(2)–N(8)^#4	2.124(13)
Ag(2)–O(5)	2.54(2)	Ag(3)–N(5)	2.161(12)
Ag(3)–N(6)^#5	2.189(13)	Ag(4)–N(3)^#4	2.179(13)
Ag(4)–N(4)	2.158(12)	Lu(2)–O(14)^#1	2.515(10)
Lu(1)–O(18)	2.430(10)	Lu(1)–O(20)	2.511(9)
Lu(1)–O(21)	2.581(9)	Lu(1)–O(23)	2.488(9)
Lu(1)–O(30)	2.550(8)	Lu(1)–O(31)	2.601(9)
Lu(1)–O(32)	2.562(8)	Lu(1)–O(34)	2.536(10)
Lu(1)–O(2w)	2.645(10)	Lu(2)–O(16)	2.435(10)
Lu(2)–O(22)^#2	2.613(9)	Lu(2)–O(24)	2.508(10)
Lu(2)–O(25)^#1	2.536(9)	Lu(2)–O(29)	2.567(9)
Lu(2)–O(30)	2.769(8)	Lu(2)–O(1w)	2.597(9)
Lu(2)–O(6w)	2.592(10)	Lu(3)–O(13)	2.560(9)
Lu(3)–O(17)	2.430(10)	Lu(3)–O(19)	2.568(8)
Lu(3)–O(26)	2.462(9)	Lu(3)–O(27)	2.498(9)
Lu(3)–O(32)	2.700(8)	Lu(3)–O(33)	2.606(9)
Lu(3)–O(3w)	2.722(9)	Lu(3)–O(4w)	2.603(9)
Complex 7
Ag(1)–N(1)	2.141(9)	Ag(1)–N(2)^#1	2.173(8)
Ag(2)–N(7)	2.121(8)	Ag(2)–N(8)^#2	2.127(8)
Ag(2)–O(5)	2.441(8)	Ag(3)–N(5)	2.165(8)
Ag(3)–N(6)^#3	2.165(8)	Ag(4)–N(3)^#2	2.175(8)
Sm(1)–O(18)	2.364(6)	Sm(1)–O(20)	2.403(6)
Sm(1)–O(21)	2.574(6)	Sm(1)–O(23)	2.415(6)
Sm(1)–O(30)	2.485(6)	Sm(1)–O(31)	2.535(6)
Sm(1)–O(32)	2.486(6)	Sm(1)–O(34)	2.430(6)
Sm(1)–O(2w)	2.519(6)	Sm(2)^#4–O(14)	2.358(6)
Sm(2)–O(16)	2.331(7)	Sm(2)^#5–O(22)	2.467(6)
Sm(2)–O(24)	2.402(6)	Sm(2)^#4–O(25)	2.388(7)
Sm(2)–O(29)	2.445(7)	Sm(2)–O(30)	2.636(6)
Sm(2)–O(1w)	2.467(6)	Sm(3)–O(13)	2.477(6)
Sm(3)–O(17)	2.381(6)	Sm(3)–O(19)	2.493(6)
Sm(3)–O(26)	2.321(6)	Sm(3)–O(27)	2.385(6)
Sm(3)–O(32)	2.627(6)	Sm(3)–O(33)	2.496(6)
Complex 8
Ag(1)–N(1)	2.150(5)	Ag(1)–N(2)	2.152(5)
Ag(2)–N(3)	2.130(5)	Ag(2)–N(4)	2.132(5)
Ag(3)–N(5)	2.122(5)	Ag(3)–N(6)	2.124(5)
Eu(1)–O(1)	2.680(3)	Eu(1)–O(2)	2.483(4)
Eu(1)–O(4)	2.5005(8)	Eu(1)–O(5)	2.3932(7)
Eu(1)–O(8)^#1	2.295(4)	Eu(1)–O(10)^#2	2.374(4)
Eu(1)–O(15)^#4	2.441(4)	Eu(1)–O(19)^#3	2.457(4)
Eu(1)–O(18)^#4	2.727(5)	Eu(2)–O(1)	2.457(3)
Eu(2)–O(3)	2.424(3)	Eu(2)–O(6)	2.422(4)
Eu(2)–O(7)	2.4548(7)	Eu(2)–O(9)^#4	2.398(4)
Eu(2)–O(11)	2.337(4)	Eu(2)–O(12)	2.440(4)
Eu(2)–O(13)	2.951(5)	Eu(2)–O(16)	2.428(4)

---

Results and discussion

Description of crystal structures

Single-crystal X-ray diffraction analyses reveal that compounds 1a–b, 2a–c, 4a–b, 5a–b, 6a–b are isomorphous. Thus, only the representative complexes 1a, 2c, 4a, 5a and 6a are discussed in detail.

Crystal structure of 1a

Single-crystal X-ray diffraction analysis reveals that structure 1a crystallizes in the monoclinic space group P2₁/c and represents a 3D coordination framework constructed from 2D lanthanide carboxylate layers and Ag(IN)₂ subunits. The asymmetric unit of 1a contains one unique Tb³⁺ ion, one Ag⁺ ion, two crystallographically unique IN ligands, one OAc ligand, one ClO₄⁻ ion and one coordination water molecule. The Tb³⁺ ion is eight-coordinated and has a bicapped trigonal prismatic coordination geometry with three oxygen atoms from two OAc ligands, four oxygen atoms from four bridging IN ligands and one coordinating water molecule (Fig. 1a). The Tb–O bond lengths range from 2.299(2) to 2.489(2) Å, all of which are within the range of those observed for other eight-coordinate Tb(III) compounds with oxygen donor ligands.²¹ The Ag⁺ ion is coordinated by two nitrogen atoms from two IN ligands with Ag–N distances of 2.154(2) and 2.155(2) Å and a bond angle of 165.8(1)°. The Ag–N bond distances are shorter than those found in the structure of LnAg(OAc)(IN)₃.^17e In the structure of 1a, Ag(IN)₂ only acts in a tetradentate coordination mode with four oxygen atoms coordinating to four Tb³⁺ ions (chart 1b). The OAc ligand exhibits a chelating and bridging mode (Chart 2a). In the structure of LnAg(OAc)(IN)₃, on the other hand, the Ag(IN)₂ units have two types of coordination modes, and OAc adopts a different chelating and bridging modes with a carboxylate group coordinating to three lanthanide ions.

Chart 2 Coordination modes of small molecular acids in compounds 1–8.

Fig. 1 (a) Perspective view of the asymmetric unit of 1a. (b) View of a 2D smooth layered network of 1a comprised of dinuclear terbium bricks in the bc plane. (c) View of 3D coordination framework of 1a constructed by adjacent terbium carboxylate layers and pillared Ag(IN)₂ units (all H atoms were omitted for clarity).

From the connection of the two organic ligands, a 2D terbium carboxylate layer forms. The OAc ligands bridge two symmetrically-related Tb atoms with a separation of 3.883(2) Å to form a dinuclear terbium brick. These bricks are interconnected to extend in the bc plane by carboxylates of IN ligands, leading to the formation of a 2D smooth layered network (Fig. 1b). These layers are further interlinked through pillared Ag(IN)₂ units to generate a 3D coordination framework (Fig. 1c). The structure is strictly arrayed in an ABAB sequence: the layer A may be viewed as a smooth terbium-carboxylate layer. The B layer contains silver and perchlorate ions with weak interactions as a ···Ag···ClO₄···Ag···ClO₄··· array. All pillared Ag(IN)₂ units are not parallel to each other, but they are also not intersecting. These adjacent Ag(IN)₂ units along the c-axis of the unit cell do not exhibit strong π···π stacking and Ag···Ag contacts, and are well separated by ClO₄⁻ ions. The motif of the 3D coordination framework of 1a is distinct from that of the structure LnAg(OAc)(IN)₃, the former crystallizes in the centrosymmetric space group P2₁/c, the latter crystallizes in the chiral space group P6₁22 and possesses a homochiral 3D heterometallic coordination framework constructed from inorganic right-handed helical chains and IN linkers. Their difference thus implies that the ClO₄⁻ ions introduced play a critical role in the formation of a new coordination framework.

Crystal structure of 2c

Single-crystal X-ray diffraction analysis reveals that structure 2c crystallizes in the triclinic space group P-1 and represents a 3D coordination framework constructed by europium–silver–carboxylate layers, silver(I) centers, and IN ligands. In our previous report, the structures of 2a and 2b have been introduced in detail. In order to give a complete overview in this systematic study and investigate the difference with other correlative 4d–4f systems, we give a further description of their structures in this paper. The asymmetric unit of 2c contains one Tb³⁺ ion, two (1 + 0.5 + 0.5) Ag⁺ ions, half an oxalate ligand, one ClO₄⁻ ion, three crystallogaphically unique IN ligands, and one-and-a-half water molecules (Fig. 2a). The Tb³⁺ ion is surrounded by eight oxygen atoms and exhibits a bicapped trigonal prismatic coordination geometry: five oxygen atoms from five IN ligands, two oxygen atoms from one chelating oxalate ligand and one water molecule. The Tb–O bond distances [2.298(14)–2.466(15) Å] are in the normal range. Two of the silver ions, Ag2 and Ag3, are both coordinated in a linear fashion each by two pyridyl nitrogen atoms of the IN ligands. Silver ion Ag2, located on a general position, has Ag–N distances of 2.134(2) and 2.136(2) Å and an N–Ag–N angle of 168.8(8)°; silver ion Ag3 lies on a center of symmetry with coplanar pyridyl rings with an N–Ag–N angle of 180°, the Ag–N distance is 2.152(2) Å. The Ag1 ion, on the other hand, exhibits a different chemical environment with a coordination sphere of four oxygen atoms from three carboxylate groups and one water molecule. Both the silver ion and also the coordinated water molecule are disordered around an inversion center with each site being exactly half occupied. In the structure of 2c, Ag(IN)₂ units act in two types of tetradentate coordination modes with three oxygen atoms coordinating to three Tb³⁺ ions and one oxygen atom coordinating to a Ag⁺ ion, (Chart 1c) or with one oxygen atom coordinating to a Tb³⁺ ion and a Ag⁺ ion (Chart 1d). The oxalate ligand adopts chelating and bridging coordination modes to two Tb³⁺ ions (Chart 2b).

Fig. 2 (a) Perspective view of the asymmetric unit of 2c. (b) View of a 2D Ln-carboxylate-Ag layer of 2c; (b) A polyhedral view of the 3D coordination framework of 2c constructed by the Ln-carboxylate-Ag layers and Ag(IN)₂ units (all H atoms were omitted for clarity).

The Ag(IN)₂ coordination modes in 2c differ from previous reports and from the other compounds described here, which leads to the formation of Ln-carboxylate-Ag layered network (Fig. 2b). Similar to the structure of 1a, a 3D coordination framework of 2c is constructed by pillared Ag(IN)₂ units via the assembly of the 2D Ln-carboxylate-Ag layered network (Fig. 2c), where the layer is formed via the connection among the carboxylate groups of IN ligands, the oxalate-connected dimeric terbium units and the Ag(OH₂) moieties. The layer extends in the ab plane of the unit cell. These stacking pillared Ag(IN)₂ units have centroid to centroid distances of about 3.57 or 3.76 Å between neighboring pyridyl rings, and are also well separated by ClO₄⁻ ions. The linear coordinated Ag2 and Ag3 ions are arranged in an array in the sequence ···Ag2···Ag2···Ag3···Ag2···Ag2···. The Ag2···Ag3 distance is 3.535(2) Å, which is a little larger than the sum of the van der Waals radii for two silver atoms (3.44 Å),²² whereas the Ag2···Ag2 distance is 3.427(2) Å, which, being below this value, shows Ag···Ag interactions.

Crystal structure of 3

Complex 3 crystallizes in the triclinic space group P-1 and represents a 3D coordination framework constructed from the linkage between europium-carboxylate layers and Ag(IN)₂ units. In the asymmetric unit of 3, there are two unique Eu³⁺ ion, three Ag⁺ ions, six crystallogaphically unique IN ligands, half an oxalate ligand, one ClO₄⁻ ion, one NO₃⁻ ion, and three coordinated water molecules and two noncoordinated water molecules (Fig. 3). The two unique Eu³⁺ ions are eight-coordinated and exhibit a bicapped trigonal prismatic coordination geometry: Eu1 is surrounded by five oxygen donors of five IN ligands, two oxygen donors of an ox ligand and one water molecule; Eu2 is coordinated by six oxygen donors of six IN ligands and two water molecules. The Eu–O bond distances [2.324(15)–2.499(12) Å] are in the normal range.²³ Of the three unique silver ions, Ag1 is four-coordinated by two pyridyl nitrogen atoms of the IN ligands and two NO₃⁻ ions; Ag2 is T-shaped coordinated by two pyridyl nitrogen atoms of the IN ligands and one ClO₄⁻ ion; Ag3 is linear coordinated by two pyridyl nitrogen atoms of the IN ligands. The Ag–N bond lengths range from 2.108(17) to 2.216(18) Å, and the N–Ag–N bond angles fall between 171.3(8) and 176.2(8)°. The Ag–O distances are around 2.72 Å, thus showing weak interactions between them. In the structure of 3, Ag(IN)₂ units act in two types of coordination modes: one uses four oxygen atoms to bind to four Eu³⁺ ions (Chart 1b), the other uses three oxygen atoms to coordinate to three Eu ³⁺ ions (Chart 1e). The oxalate ligand adopts a chelating and bridging coordination mode to link two Eu³⁺ ions (Chart 2b).

Fig. 3 (a) Perspective view of the asymmetric unit of 3. (b) View of an europium-carboxylate chain of 3 constructed from the carboxylate groups of IN and ox ligands connecting to metal centers. (b) A polyhedral view of the 3D coordination network of 3 comprised of the europium-carboxylate chains and Ag(IN)₂ units (all H atoms were omitted for clarity).

On the basis of the two types of Ag(IN)₂ coordination modes, a layered sheet is formed and extended in the b-axis direction (Fig. 3b). These sheets are further interconnected by oxalate ligands to construct a 3D coordination framework (Fig. 6b). The interlayered π–π stacking and H-bonded interactions also enhance the structural stability. These stacking pillared Ag(IN)₂ units, have centroid to centroid distances of about 3.71 or 3.89 Å between neighboring pyridyl rings. The 3D coordination polymer can be viewed as the assembly of europium-carboxylate layers and pillared Ag(IN)₂ units. It should be noted that the distance between neighboring europium-carboxylate layers of 16.903 Å is equal to the length of Ag(IN)₂ coordination mode as well as to the unit length of the b axis.

Crystal structure of 4a

Single-crystal X-ray diffraction analysis reveals that structure 4a crystallizes in the monoclinic space group C2/c and displays a 3D coordination framework constructed by 2D cerium-oxalate layers and tilt pillared Ag(IN)₂ subunits. The asymmetric unit of 4a contains one unique Ce³⁺ ion, one Ag⁺ ion, two unique IN ligands, one (0.5 + 0.5) ox ligand, and one lattice water molecule (Fig. 4a). The Ce³⁺ ion is nine-coordinated and has distorted monocapped square antiprismatic coordination geometry by four oxygen atoms from two ox ligands and five oxygen atoms from four IN ligands. The Ce–O bond lengths range from 2.493(4) to 2.765(4) Å. The Ag⁺ ion is in a ‘T-shaped’ configuration and is defined by two nitrogen atoms from two different IN ligands, with bond distances of 2.173(6) and 2.181(6) Å, and by one oxygen atom from one ox ligand. The Ag–O bond distance of 2.632(2) Å is shorter than that of distances reported for [C₂₄H₃₆Ag₂N₄O₆](ClO₄)₂ complexes [Ag–O = 2.708(2) Å].²⁴ In the structure of 4a, the Ag(IN)₂ unit acts in one coordination mode (Chart 1f) with one carboxylate coordinating to two Ce³⁺ ions via a bismonodentate coordination, another carboxylate coordinating to two Ce³⁺ ions via chelating and bridging coordinations. The two OX ligands also adopt two coordination modes: one acts as a chelating and bridging ligand to connect to two Ce³⁺ ions (Chart 2b), the other connects to two Ce³⁺ ions as well as two Ag⁺ ions (Chart 2c).

Fig. 4 (a) Perspective view of the asymmetric unit of 4a. (b) View of a 2D cerium oxalate layered network of 4a. (c) A polyhedral view of 3D Ce–Ag coordination framework of 4a constructed from 2D cerium-oxalate layers and tilt pillared Ag(IN)₂ subunits (all H atoms were omitted for clarity).

On the basis of these connection modes of the organic ligands, a heteronuclear Ce₂Ag₂ core is built up from two edge-sharing Ce³⁺ polyhedra and two corner-sharing trigonalhedra. The Ce···Ce and Ce···Ag separations are 4.230(2) and 5.099(3) Å, respectively. The OX ligands bridge cerium centers to form a 2D cerium oxalate layered network, which can be observed in other analogous lanthanide oxalate complexes (Fig. 4b). The 3D Ce–Ag coordination framework is constructed from 2D cerium-oxalate layers and tilt pillared Ag(IN)₂ subunits (Fig. 4c). The motif of tilt pillared Ag(IN)₂ subunits provides an appropriate coordination site for the silver ion to link the third donor. Lattice water molecules connected via H-bonded interactions [O···O = 2. 897(2) and 2.908(2) Å] are arranged in the channels along the b- and c-axis of the unit cell. To our knowledge, a Ag–Ce coordination framework with a tetranuclear Ce₂Ag₂ core has not been reported to date. From our observation for the above-mentioned structures containing ClO₄⁻ ions, the tilt motif of the Ag(IN)₂ units may also give a hint towards the construction of other novel coordination frameworks.

Crystal structure of 5a

Single-crystal X-ray diffraction analysis reveals that structure 5a crystallizes in the monoclinic space group P2₁/c and displays a novel 3D coordination frameworks constructed by 2D lanthanum malonate chains and pillared Ag(IN)₂ units. The asymmetric unit of 5a contains one unique La³⁺ ion, one Ag⁺ ion, two crystallographically unique IN ligands, one malonate ligand, and one coordinating water molecule (Fig. 5a). The La³⁺ ion is ten-coordinated by six oxygen atoms from three malonate ligands, three oxygen atoms from two IN ligands and one water molecule. The La–O bond distances range from 2.498(4) to 2.733(4) Å. The Ag⁺ ion is in a trigonal configuration and defined by two nitrogen atoms from two different IN ligands with bond distances of 2.167(5) and 2.182(5) Å and one oxygen atom from one malonate ligand. The Ag–O bond distance of 2.600(4) Å is comparable to those of structures 4a and 4b and other analogous complexes.²⁵ The Ag···Ag distance of 3.1839(11) Å is shorter than that in the analogous heteronuclear complexes [Ln₂Ag₄(ina)₈(H₂O)₁₀][NO₃]₂·4H₂O [Ln = Sm, Eu, Dy].²⁶ The value is well below the sum of the van der Waals radii for two silver atoms (3.44 Å), thus showing strong Ag···Ag contacts. In the structure of 5a, Ag(IN)₂ units act in one coordination mode (Chart 1g) with one carboxylate coordinating to one La³⁺ ion via bismonodentate coordination, and another carboxylate coordinating to one La³⁺ ion via bisdentate chelating coordination. The malonate ligand acts in a chelating and bridging coordination mode with two carboxylates coordinating to three La³⁺ ions and one Ag⁺ ion (Chart 2d).

Fig. 5 (a) Perspective view of the asymmetric unit of 5a. (b) View of a 2D polynuclear La–Ag layer of 5a. (c) View of a 3D coordination framework of 5a assembled by 2D La–Ag heteronuclear layers and pillared Ag(IN)₂ subunits containing strong Ag⋯Ag bonds (all H atoms were omitted for clarity).

On the basis of these connections of IN and malonate ligands, a 2D polynuclear La–Ag layer is built up from edge-sharing La³⁺ chains and corner-sharing Ag⁺ trigonalhedra completed by Ag···Ag contacts (Fig. 5b). The separations of La···La and La···Ag atoms are 4.427(2) and 5.095(3) Å, respectively. It should be noted that the strong Ag···Ag interactions play an important role in the assembly process. To our knowledge, such polyheterometallic layers have never been reported to date. The 2D polynuclear La–Ag heterometallic layers are linked by IN and malonate ligands to form a 3D coordination framework (Fig. 5c), in which the flexible malonate ligands in turn connect lanthanum centers, extending to form a lanthanum malonate chain in the a-axis direction. This arrangement, which is rare even in a homometallic lanthanide malonate framework, can provide important coordination sites to bind to Ag⁺ ions. To our knowledge, the structure is the first 3D Ln–Ag coordination framework built up from polyheteronuclear 2D layers and mixed organic ligands.

Crystal structure of 6a

Single-crystal X-ray diffraction analysis revealed that structure 6a crystallizes in the triclinic space group P-1 and displays a novel 2D coordination framework constructed by two 1D racemic lanthanum carboxylate chains and pillared Ag(IN)₂ units. The asymmetric unit of 6a contains three La³⁺ ions, four Ag⁺ ions, eight crystallographically unique IN ligands, two lactate ligands, two ClO₄⁻ ions, five coordination water molecules and two-and-a-half interstitial water molecules (Fig. 6a). The La1, La2 and La3 ions all are nine-coordinate and display a tricapped trigonal prismatic coordination environment: four oxygen atoms from four IN ligands, four oxygen atoms from two lactate ligands, and one water molecule for the La1 ion; and five oxygen atoms from five IN ligands, two oxygen atoms from one lactate ligand, and two water molecules for the La2 and La3 ions. The La–O bond distances range from 2.433(5) to 2.763(5) Å, thus being in the normal range observed in other complexes.²⁷ All four silver ions are coordinated in a linear fashion each by two pyridyl nitrogen atoms of the IN ligands and located on general positions. The Ag–N distances range from 2.128(6) to 2.178(6) Å, all within the range of those observed for other linear coordinate silver compounds and our previous reports.²⁸ In the structure of 6a, Ag(IN)₂ units act as two types of coordination modes: one acts as a tetramonodentate ligand coordinating to four La³⁺ ions (Chart 1b), the other acts as a trimonodentate ligand coordinating to three La³⁺ ions (Chart 1e). The two lactate ligands adopt a chelating and bridging coordination mode to coordinate to two La³⁺ ions (Chart 2e).

The racemic lactate ligands in the asymmetric unit connect three lanthanum ions, which can be regarded as a building block with a symmetrical centre through the La1 atoms. These building blocks are in turn interconnected by carboxylates of IN ligands to form an infinite chain in the b-axis direction (Fig. 6b). It should be noted that the 1D chains are interconnected by bridging carboxylate groups of IN ligands to 2D double chains (Fig. 6c), which are further assembled via Ag(IN)₂ units to form a 2D network in the ac-plane (Fig. 6d). The configuration can be regarded as an assembly of pillared Ag(IN)₂ units and 1D double chains.

Fig. 6 Perspective view of the asymmetric unit of 6a. (b) View of a 1D chain of 6a involving ramemic lactate groups. (c) View of a double chain of 6a. (c) 2D coordination network of 6a in the ac-plane constructed by the double chains and pillared Ag(IN)₂ subunits (all H atoms were omitted for clarity).

The crystal structure 7 is almost the same as that of 6a, the difference being that the Sm2³⁺ center is eight-coordinate, with one less coordination water molecule coordinated to its metal center. The reason is that the radius of Sm³⁺ ions is smaller than that of La³⁺, leading to the Sm–O bond distances in 7 are shorter than those of 6a with no space left for coordination of another water molecule.

Crystal structure of 8

Single-crystal X-ray diffraction analysis reveals that compound 8 spontaneously resolved into chiral triclinic crystals in the space group P1 (Flack parameter of 0.035(6)) despite the use of racemic lactic acid as the starting material. It displays an unprecedented 2D coordination framework constructed from pillared Ag(IN)₂ units, europium centers and one lactate anion (S-lactate in the crystal analyzed). The asymmetric unit of 8 contains two unique Eu³⁺ ions, three Ag⁺ ions, six crystallographically unique IN ligands, one lactate ligand, two ClO₄⁻ ions, and four coordination water molecules and four interstitial water molecules (Fig. 7a), For two of the Eu³⁺ ions, Eu1 and Eu2, the Eu1³⁺ ion is nine-coordinated by five oxygen atoms from four IN ligands, two oxygen atoms from one S-lactate ligand, and two water molecules, and displays a tricapped trigonal prismatic coordination geometry. Eu2 ion is eight-coordinate and surrounded by eight oxygen atoms and exhibits a bicapped trigonal prismatic coordination geometry: four oxygen atoms from four IN ligands, two oxygen atoms from one S-lactate ligands and two water molecules. The Eu–O bond distances range from 2.295(4) to 2.727(5) Å. All three silver ions are coordinated in a linear fashion each by two pyridyl nitrogen atoms of the IN ligands and are located in general positions. One of the Ag(IN)₂ units is disordered over two sites in a ratio of 0.594(3):0.406(3). The Ag–N distances range from 2.122(5) to 2.163(7) Å, and the N–Ag–N bond angles are in the range of 172.8(2)–176.1(5)°. In the structure of 8, Ag(IN)₂ acts in three types of coordination modes: one uses two opposite carboxylates to bind to two Eu³⁺ ions via monodentate coordination (Chart 1g); the second uses one carboxylate to coordinate to one Eu³⁺ ion via monodentate coordination, the other carboxylate to coordinate to one Eu³⁺ ion via bisdentate chelating coordination (Chart 1h); the third acts in tetramonodentate bridging mode (Chart 1b). One lactate ligand adopts a chelating and bridging coordination mode to coordinate to two Eu³⁺ ions (Chart 2e).

Fig. 7 Perspective view of the asymmetric unit of 8. (b) View of a helical chain of 8 constructed from the carboxylate groups of IN and S-lactate ligands and metal ions. (c) View of a 2D coordination network composed of 1D helical chains and pillared Ag(IN)₂ subunits (all H atoms were omitted for clarity).

On the basis of these connections of IN and lactate ligands, two europium ions are linked by one S-lactate ligand to form a chiral building block as in a knot with a Eu···Eu separation of 4.956(2) Å. These knots are connected by two pairs of Ag(IN)₂ units in an antiparallel orientation to form a zigzag chain in the b-axis direction (Fig. 7b), in which two adjacent Ag(IN)₂ units are stacked atop each other. The interplanar and centroid to centroid distances are 3.66(5) and 3.73(4) Å, respectively. These 1D zigzag chains can be regarded as supramolecular second building units, which are further cross-linked via Ag(IN)₂ coordinations (Chart 1b) to form a 3D coordination framework (Fig. 7c). These dinuclear chiral building blocks are interconnected via bridging carboxylate groups of IN ligands to form an S-helical chain, extending in the c-axis direction. These helices are further assembled via pillared Ag(IN)₂ units into a 3D pillared coordination framework. To our knowledge, compound 8 is the first 3D Ln–Ag coordination framework built up from 1D helical chains containing chiral molecules and pillared Ag(IN)₂ units.

Summary

In the syntheses of these compounds, the reaction pH, time, temperature and initial reactants have a considerable influence on the growth of the single crystals. As we know, hydrothermal synthesis shows great advantages over other methods for the synthesis of heterometallic coordination frameworks.²⁹ By introducing this technique, we can use lanthanide oxides as the lanthanide source, so that lanthanide ions are slowly released to lower the polymerization rate which allows to obtain high-quality single crystals. In our previous work, we introduced perchloric acid as a perchlorate source. This has proven to play a significant role in the construction of Ln–Ag coordination frameworks. The perchlorate ion prefers to be situated between silver ions, and acts a mutifunctional linear linker from silver ions coordinated to two IN ligands (IN = isonicotinate).¹⁸ Owing to high coordination numbers for lanthanide ions, the presence of the void filling perchlorates did provide an impetus for the formation of novel lanthanide frameworks by rationally selecting small molecular acids in initial reactants. By optimizing the reaction temperature, time and initial reactants, compounds 5a and 5b were isolated from the hydrothermal synthesis at lower temperature. At temperatures over 140 °C and with times over 3 days compounds 5a and 5b can not be obtained, as malonic acid does decompose into oxalate, acetate, etc. Although malonic acid had been used in an analogous reaction by Han, they had not been able to isolate these frameworks, most likely due to decomposition of malonate at the high reaction temperature used.²⁶ The pH in these systems also plays a key role in the construction of Ln–M heterometallic coordination polymers, as lanthanide ions frameworks prefer to form at lower pH values.

While racemic lactates were used in the synthesis of compounds 6a, 6b, 7, as well as 8, spontaneous resolution into chiral crystals did only happen for the crystals of 8. This indicates that the Ag(IN)₂ units in the Eu system might be able to play an important role in enantioseparation of racemic lactic acid.

IR spectroscopy

The IR spectra of 1–8 are similar. The broad absorption bands in the range of 3300–3500 cm⁻¹ in 1–8 are assigned to the O–H vibration of water molecules. For 1–8, the characteristic bands of carboxylate groups are shown at 1589–1608 cm⁻¹ for asymmetric stretching and 1386–1409 cm⁻¹ for symmetric stretching. The absence of strong bands around 1700 cm⁻¹ indicates that all carboxyl groups of organic moieties in 1–8 are deprotonated. The strong vibrations appearing in the region of 1122–1049 cm⁻¹ correspond to the ClO₄⁻ stretching vibrations.³⁰

Thermal properties

The thermal stabilities of all complexes have been studied by thermal gravimetric analysis in N₂ atmosphere (See ESI).† The first weight losses below 210 °C for all complexes correspond to removal of the noncoordination and coordination water molecules. The second sharp weight losses above 310 °C for all complexes except compounds 5a and 5b are attributed to decomposition of the perchlorate or nitrate ions, while the frameworks of compounds 5a and 5b are stable up to 370 °C. Decomposition of the perchlorate anions is also evident in the differential scanning calorimetry curves which exhibit sharp exothermic peaks at the same temperatures.

Luminescence properties

Due to the excellent luminescent properties of Tb(III) and Eu(III) ions, the photoluminescence of 1a, 2b, 2c, 3 and 8 in the solid state was investigated at room temperature. The luminescence spectra of these complexes were shown in Fig. 8,9 and S3–5 (see ESI).† The emission spectra upon 330 nm excitation for Tb-complexes (Fig. 8 and S3†) consist of four bands located at about 489, 545, 584 and 621 nm. The four main peaks can be attributed to the ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅, ⁵D₄ → ⁷F₄ and ⁵D₄ → ⁷F₃ transitions, respectively.³¹ The emission spectra for Eu-complexes (Fig. 9 and S4,5†) excited at 396 nm exhibit characteristic transitions of Eu ions, the peaks at 594, 616, 650 and 698 nm correspond to the ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃ and ⁵D₀ → ⁷F₄ transition of Eu ions, respectively, which implies an efficient energy transfer from organic ligands to the metal ion (LMCT).³² The emission peak ⁵D₀ → ⁷F₁ is a magnetic dipole transition and its intensity should vary with the crystal field strength acting on the Eu(III) ion. The ⁵D₀ → ⁷F₂ transition is an electric dipole transition and the intensity of the ⁵D₀ → ⁷F₂ transition increases as the site symmetry of Eu(III) ions decreases. Unfortunately, there was no detectable photoluminescence for other complexes at room temperature.

Fig. 8 Solid-state emission spectrum of 2c at room temperature.

Fig. 9 Solid-state emission spectrum of 3 at room temperature.

Conclusion

In summary, we have succeeded in utilizing isonicotinic acid in combination with small molecular acids as ideal bridging units under hydrothermal synthesis conditions to prepare novel 4d–4f complexes. The most intriguing structural feature is that all IN ligands have a linear fashion to coordinate to one silver ion, and all compounds use Ag(IN)₂ units to construct these coordination frameworks. Our successful synthetic route may be applicable to the preparation of other novel high-dimensional lanthanide-transition metal coordination polymers with interesting structural, topologies and functional properties.

Acknowledgements

This work was supported by the National Natural Science Foundation of China, Grant No. 20871048.

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Footnote

† Electronic supplementary information (ESI) available: X-Ray crystallographic file (CIF) for complexes 1–8, TGA curves of complexes 1–8 and solid-state emission spectrum of complexes 1a, 2b,8. CCDC reference numbers 713246–725765. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b915025k

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