Four new silver phosphonates constructed from semi-rigid phosphonate ligands: synthesis, structure and properties

Abstract

Four new silver phosphonates, namely, Ag₃(H₃L¹) (1), Ag₂(H₂L²) (2), Ag₂(H₂L²)(pyz) (3) and Ag₂(H₄L²)(H₂L²)(4,4′-bipy)(H₂O) (4) (H₆L¹ = (benzene-1,3,5-triyltris(methylene))tris(phosphonic acid); H₄L² = (2,5-dimethyl-1,4-phenylene)bis(phosphonic acid); pyz = pyrazine; 4,4′-bipy = 4,4′-bipyridine) have been synthesized from the reactions of silver oxide (Ag₂O) and tris or bisphosphonate ligands with or without the existence of second auxiliary ligands via hydrothermal methods. These silver phosphonates were systematically characterized using single-crystal and powder X-ray diffraction, IR, EA, TGA and fluorescence technologies. The phosphonate ligands adopt diverse coordination modes and coordinate to the silver ions with O–Ag bonds as well as C–Ag bonds, forming various coordination configurations around the silver ions. Compound 1 features a pillar-layered structure with silver and oxygen atoms as the layer and the organic linkers as pillars. Compound 2 displays a layer structure which can assemble into a three-dimensional supramolecular structure with the aid of O–H⋯O interactions. In compound 3, silver ions are linked into layers and further connected to form a pillar-layered structure with the pyrazine molecules as pillars. Compound 4 also displays a layer structure which is constructed from ladders and organic phosphonate ligands. The photophysical measurements suggested that these compounds all displayed intraligand fluorescence.

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Introduction

The coordination chemistry of silver(I) has been of great interest due to the fantastic structures and potential applications of silver(I) compounds in the fields of optical,¹ semiconductor,² medicinal,³ catalysis⁴ and so on. To assemble a silver coordination compound for a specific function, it is of great significance to predict the crystal structure of silver compounds. The crystal structure of silver compounds is subject to many factors which include the synthesis conditions (temperature, pH value, ligand/metal ratio, etc.), the coordination modes of the ligands and the preferential coordination configuration of the metal center. It is particular difficult to predict the crystal structures of silver(I) compounds in part due to the lack of stereochemical preference.⁵ Silver(I) displays various coordination configurations with the coordination numbers ranging from two to eight depending on the selection of organic ligands. Usually, high-nuclearity clusters could be isolated with the employment of ethynide-containing ligands or bulky ligands with steric hindrance,⁶ while bridging ligands with multi-functional groups or at the presence of N-heterocyclic auxiliary ligands tend to form three-dimensional architectures or supramolecular structures.⁷ In high-nuclearity clusters, Ag–Ag interactions or Ag–C bonds occur widely but are rarely found in three-dimensional architectures or supramolecular structures, in which Ag–O and Ag–N are the dominant bonding interactions.

In the past years, carboxylic acid,⁸ sulfonic acid,⁹ phosphonic acid,¹⁰ N-heterocyclic ligand,¹¹ ethynide-containing ligand,¹² carbene,¹³ etc. have been employed for the construction of silver(I) coordination compounds. Among these compounds, silver phosphonates have been less exploited but are gaining more and more attention. Comparing with carboxylic acid analogues, phosphonic acid ligand possesses an additional oxygen atom which endows it with more coordination modes and enriches the crystal structures. It can be used as co-ligands in the construction of silver(I) clusters. For example, Mak et al. reported three high-nuclearity silver ethynide clusters by reacting t-BuPO₃H₂ with AgC≡CtBu, AgBF₄ and (Et₄N)VO₃.¹⁴ Also from this group, seven silver(I) multiple salts of Ag₂C₂ were synthesized by incorporating phosphonate ligands into the silver–ethynediide system via hydrothermal method.¹⁵ By decorating with Lewis basic group (amine,¹⁶ pyridine,¹⁷ pyrimidine,¹⁸ nitrile,¹⁹ and so on), which is excellent platform for coordinating to silver(I) ion, intricate silver phosphonates can be assembled. For example, the introduction of nitrile group to methylene-bis(phosphonate) ligand, heterometallic phosphonate could be constructed in non-coordinating solvents.¹⁹ By tuning the pH value with urea and varying the position of carboxylic acid group of phosphonobenzoic acid, silver(I) carboxy-phosphonates with different topologies could be isolated.^20,21 These compounds showed a green luminescence response to laser excitation²⁰ and could serve as silver reservoir for bactericidal usage.²¹ The employment of flexible phosphonate ligands would enrich the coordination modes greatly and resulted in new silver compounds with novel luminescence²² and conductivity properties.²³

In our former work, we synthesized many metal phosphonates and investigated the role of rigidity/flexibility, decoration of methyl group and configuration of the ligands on the structural formation of metal phosphonates.²⁴ To have a better understanding of structure–property relationship and put forward the structural prediction of silver phosphonates, we continued our work by reacting two semi-rigid phosphonate ligands with silver oxide (Ag₂O) and successfully obtained four new silver(I) phosphonates, namely, Ag₃(H₃L¹) (1), Ag₂(H₂L²) (2), Ag₂(H₂L²)(pyz) (3) and Ag₂(H₄L²)(H₂L²)(4,4′-bipy)(H₂O) (4) (H₆L¹ = (benzene-1,3,5-triyltris-(methylene))tris(phosphonic acid); H₄L² = (2,5-dimethyl-1,4-phenylene)bis(phosphonic acid); pyz = pyrazine; 4,4′-bipy = 4,4′-bipyridine). Here, we report on their syntheses, structures and photophysical properties.

Experimental

Materials and instruments

The synthesis of H₆L¹ has been reported elsewhere.^24b The C, H, N contents were determined on a Vario EL III elemental analyzer. IR spectra were recorded on a Nicolet 6700 FTIR Spectrometer with KBr pellets in the range of 4000–400 cm⁻¹. Powder X-ray diffraction patterns were recorded on a Bruker D8 Advance diffractometer using Cu Kα radiation. Solution ¹H and ³¹P NMR spectra were recorded on a Bruker AVANCE-III NMR (600 MHz). H₃PO₄ was used as standard reference. Thermogravimetric analyses (TGA) were carried out on a NETZSCH STA 449C unit at a heating rate of 10 °C min⁻¹ under a nitrogen atmosphere. Fluorescent analyses of the free phosphonate ligands (H₆L¹ and H₄L²), pyz, 4,4′-bipy and compounds 1 to 4 were performed on a Fluoromax-4 spectrofluorometer.

Single-crystal structure determination

The diffraction intensity data sets of compounds 1–4 were collected on a Bruker SMART APEX II CCD diffractometer (Mo Kα radiation, λ = 0.71073 Å) at room temperature. SAINT was used for integration of intensity of reflections and scaling.²⁵ Absorption corrections were carried out with the program SADABS.²⁶ Crystal structures were solved by direct methods using SHELXS.²⁷ Subsequent difference Fourier analyses and least squares refinement with SHELXL-97 (ref. 28) allowed for the location of the atom positions. All non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms on the water molecules of 4 were located from the difference Fourier map and not included in the refinements. The crystallographic details are summarized in Table 1. The data have been deposited in the Cambridge Crystallographic Data Centre (CCDC), deposition numbers CCDC 1498976–1498979 for compounds 1–4.†

Table 1 Crystallographic parameters of compounds 1–4^ab

a R1 = Σ||Fo| − |Fc||/Σ|Fo|.b wR2 = {Σ[w(|Fo|^2 − |Fc|^2)^2]/Σw(|Fo|^2)^2}^1/2.
Compound	1	2	3	4
Formula	C9H12Ag3O9P3	C10H14Ag2O6P2	C14H18Ag2N2O6P2	C35H40Ag2N4O10P3
CCDC no.	1498977	1498976	1498979	1498978
fw	680.71	507.89	587.98	985.36
Space group	P1̄	P21/c	P2/c	P1̄
a (Å)	8.3751(9)	6.747(2)	11.7570(14)	10.4999(7)
b (Å)	9.2734(9)	6.1736(16)	4.6361(5)	12.1860(8)
c (Å)	10.7848(13)	15.970(5)	16.3047(18)	15.1329(9)
α (deg)	86.327(4)	90	90	97.139(2)
β (deg)	89.491(4)	94.412(13)	102.766(4)	103.613(2)
γ (deg)	63.771(3)	90	90	99.718(2)
V (Å^3)	749.64(14)	663.2(4)	866.75(17)	1827.5(2)
Z	2	2	2	2
Dcalcd/g cm^−3	3.016	2.543	2.253	1.791
Abs coeff/mm^−1	4.246	3.214	2.480	1.267
F(000)	648	492	576	994
Theta range	3.199–26.481	3.028–24.998	3.43–24.99	3.035–25.000
Completeness/%	98.6	95.8	99.3	95.5
Reflns collected	8206	3501	4858	14 209
Independent reflns/Rint	2951/0.0588	1144/0.0310	1520/0.0245	6327/0.0289
GOF on F^2	1.006	1.060	1.064	1.031
Final R indices [I > 2σ(I)]: R1, wR2	0.0386, 0.0580	0.0329, 0.0737	0.0261, 0.0565	0.0458, 0.0999
R indices (all data): R1, wR2	0.0799, 0.0670	0.0477, 0.0794	0.0387, 0.0616	0.0747, 0.1157

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Synthesis of ligand H₄L²

The ligand H₄L² has been synthesized using the same method used for ligand H₆L¹ by reacting 1,4-bis(bromomethyl)-2,5-dimethylbenzene²⁹ with triethylphosphite (Aldrich). ¹H NMR (DMSO-d⁶) data for H₄L²: δ 6.73 (Ph-H, s, 2H), 3.92–3.88 (–CH₂–, m, 2H); 2.25 (–CH₃, s, 6H). ³¹P NMR (DMSO-d⁶) shows only single pink at 23.81 ppm.

Synthesis of compound Ag₃(H₃L¹) (1)

H₆L¹ (0.090 g, 0.25 mmol), Ag₂O (0.0869 g, 0.375 mmol) and 10 mL of water were mixed and stirred for 30 min in a Teflon-lined autoclave. Afterward, it was sealed and heated at 160 °C for 3 days and allowed to cool to room temperature in a time period of 24 hours. Colorless plate crystals (0.045 g, yield: 26.44%, based on metal source) were collected manually from unidentified particles and washed with deionized water. Elemental analysis (%) calcd for C₉H₁₂Ag₃O₉P₃ (680.71): C 15.88, H 1.78%; found: C 15.79, H 1.83%. IR (KBr, cm⁻¹): 3300.5 (s), 2386.5 (w), 1666.4 (m), 1599.6 (m), 1455.9 (w), 1412.3 (w), 1238.7 (m), 1156.9 (s), 1056.7 (s), 970.8 (m), 761.1 (m), 719.9 (m), 637.7 (m), 533.4 (m), 491.7 (m).

Synthesis of compound Ag₂(H₂L²) (2)

The synthesis of compound 2 is similar to that of compound 1 via the reaction of equal molar of H₄L² (0.0736 g, 0.25 mmol) and Ag₂O (0.0579 g, yield: 0.25 mmol). Colorless plate crystals (0.036 g, 28.35%, based on metal source) were collected manually from unidentified particles and washed with deionized water. Elemental analysis (%) calcd for C₁₀H₁₄Ag₂O₆P₂ (507.89): C 23.65, H 2.78%; found: C 23.61, H 2.89%. IR(KBr, cm⁻¹): 3482.9 (s), 3284.6 (m), 3009.9 (w), 2954.2 (w), 2924.2 (w), 2923.7 (w), 1624.7 (m), 1508.2 (w), 1444.1 (w), 1411.7 (m), 1395.6 (w), 1300.8 (w), 1247.6 (s), 1232.3 (s), 1147.2 (vs), 1101.9 (s), 1030.0 (vs), 905.0 (s), 811.1 (m), 853.0 (w), 752.0 (w), 704.9 (w), 612.6 (m), 517.8 (m), 468.2 (w), 440.8 (m).

Synthesis of compound Ag₂(H₂L²)(pyz) (3)

H₄L² (0.0736 g, 0.25 mmol), Ag₂O (0.0579 g, 0.25 mmol), pyz (0.020 g, 0.25 mmol) and 10 mL of water were mixed and stirred in a Teflon-lined autoclave. Afterward, it was sealed and heated at 160 °C for 3 days and allowed to cool to room temperature in a time period of 24 hours. Colorless block crystals (0.056 g) were collected and washed with deionized water in satisfying yield (38.10%, based on metal source). Elemental analysis (%) calcd for C₁₄H₁₈Ag₂N₂O₆P₂ (587.98): C 28.60, H 3.09, N 4.76%; found: C 28.66, H 3.19, N 4.83%. IR(KBr, cm⁻¹): 3483.6 (m), 3281.1 (w), 3009.1 (w), 2921.2 (w), 2284.9 (w), 1624.7 (w), 1508.0 (w), 1444.9 (vw), 1411.3 (w), 1247.3 (m), 1232.3 (m), 1147.5 (vw), 1101.4 (m), 1029.4 (vs), 905.1 (s), 880.7 (m), 751.8 (w), 704.2 (vw), 611.3 (m), 518.1 (m), 440.8 (m).

Synthesis of compound Ag₂(H₄L²)(H₂L²)(4,4′-bipy)(H₂O) (4)

The synthesis of compound 4 is similar to that of compound 3 except the replacement of pyz with 4,4′-bipy. Colorless block crystals (0.049 g) were collected and washed with deionized water in satisfying yield (39.77%, based on metal source). Elemental analysis (%) calcd for C₃₅H₄₀Ag₂N₄O₁₀P₃ (985.36): C 42.66, H 4.09, N 5.69%; found: C 28.66, H 3.19, N 5.61%. IR(KBr, cm⁻¹): 3484.7 (m), 3281.7 (m), 3009.2 (m), 2923.0 (m), 2317.0 (w), 1598.1 (m), 1530.2 (w), 1509.1 (w), 1410.8 (m), 1393.3 (w), 1247.3 (m), 1232.1 (m), 1147.7 (s), 1101.9 (m), 1028.0 (vs), 905.3 (s), 880.8 (m), 853.6 (w), 804.4 (m), 766.0 (w), 751.8 (w), 613.2 (m), 519.3 (m), 468.5 (w), 441.0 (m).

Results and discussion

The syntheses of silver(I) phosphonates

Different silver salts were tested to crystallize silver phosphonates. It was found that with AgNO₃ or Ag(OAc) as silver source only powder or tiny crystals could be obtained. With Ag₂O as the silver source, plate crystals with optimal size for single-crystal measurement could be obtained but unreacted Ag₂O and unidentified powder could also be found in the products. However, the introduction of pyrazine and 4,4′-bipy could result in pure block crystals, probably these N-containing auxiliary ligands could keep the concentration of free silver(I) ions at a low level. As a result, the nucleation rate is low and yield rather large single crystals instead of powder samples.

Structure description of compound 1

Compound 1 crystallizes in triclinic space group P1̄ with two molecules in each unit cell. There are one triply deprotonated trisphosphonate ligand (H₃L¹)³⁻ and three crystallographic independent silver(I) ions in each asymmetric unit, indicating a formula of Ag₃(H₃L¹). Each phosphonate group in the trisphosphonate ligand is singly deprotonated. The coordination mode of the trisphosphonate ligand can be denoted as μ³:η⁰:η⁰:η¹:η⁰:η⁰:η¹:η⁰:η¹:η¹, which means that the trisphosphonate ligand binds to three silver(I) ions with its four phosphonate oxygen atoms (see Scheme 1(a) and Fig. 1). Among the three crystallographic independent silver ions, Ag1 (Wyckoff position: 2i; site symmetry: 1) is five-coordinated whereas Ag2 (Wyckoff position: 2i; site symmetry: 1) and Ag3 (Wyckoff position: 2i; site symmetry: 1) are four coordinated without considering the Ag⋯Ag interactions. The Ag⋯Ag distances are observed in the range of 3.0107(10)–3.3469(8) Å, longer than the Ag–Ag distance of 2.889 Å in metallic silver³⁰ and comparable to those in other silver phosphonates.¹⁵ It should be noted that these distances are shorter than twice the van der Waals radius of silver(I) ion (3.44 Å),³¹ suggesting significant argentophilic interactions between silver(I) ions. Ag1 is five-coordinated by five phosphonate oxygen atoms from four phosphonic acid groups of three trisphosphonate ligands. Its distortion parameter (τ₅) is calculated to be 0.13, indicating a distorted square pyramidal coordination manner.³² Ag2 is four-coordinated by four phosphonate oxygen atoms from four phosphonic acid groups of four trisphosphonate ligands. Ag3 is also four-coordinated by four phosphonate oxygen atoms, but the oxygen atoms are from three trisphosphonate ligands. The four-coordinate distortion indices (τ₄) of Ag2 and Ag3 are determined to be 0.71 and 0.57, respectively, indicating distorted tetrahedral coordination spheres.³³ The Ag–O distances are found in the range of 2.267(4)–2.725(4) Å (see Table 2), which are comparable to those of other silver phosphonates.¹⁹

Scheme 1 Coordination modes of the phosphonate ligands in compounds 1–4.

Fig. 1 Coordination environment of the trisphosphonate ligand (H₃L¹)³⁻ in compound 1 with 30% probability, the Ag⋯Ag interactions are presented in dash lines. Selected interatomic distances [Å]: Ag(1)⋯Ag(1)C 3.0106(10), Ag(3)⋯Ag(2)E 3.1691(8), Ag(1)⋯Ag(3)B 3.2891(8), Ag(3)⋯Ag(1)B 3.2892(8), Ag(2)⋯Ag(3)E 3.1691(8), Ag(3)⋯Ag(2)J 3.3469(8), Ag(2)⋯Ag(3)G 3.3469(8). Symmetry transformations used to generate equivalent atoms: (A) x, y, −1 + z; (B) 1 − x, 1 − y, 1 − z; (C) 1 − x, 1 − y, −z; (D) 2 − x, 1 − y, 1 − z; (E) 2 − x, 1 − y, 2 − z; (F) 1 − x, 1 − y, 2 − z; (G) x, 1 + y, z; (H) x, y, 1 + z; (I) 2 − x, −y, 1 − z; (J) x, y − 1, z.

Table 2 Selected bonds (Å) in compounds 1–4^a

a Symmetry transformations used to generate equivalent atoms: For 1: #1 −x + 1, −y + 1, −z + 1; #2 x, y, z − 1; #3 −x + 1, −y + 1, −z; #4 −x + 1, −y + 1, −z + 2; #5 −x + 2, −y + 1, −z + 1; #6 −x + 2, −y + 1, −z + 2; #7 x, y + 1, z; #8 −x + 2, −y, −z + 1; #9 x, y, z + 1; #10 x, y − 1, z. For 2: #1 x, y − 1, z; #2 −x + 2, y − 1/2, −z + 1/2. For 3: #1 −x + 1, y + 1/2, −z + 3/2; #2 −x + 1, y − 1/2, −z + 3/2. For 4: #1 −x + 1, −y + 1, −z; #2 −x + 2, −y, −z + 1.
Compound 1
Ag(1)–O(8)#1	2.283(4)	Ag(2)–O(5)	2.367(4)
Ag(1)–O(1)	2.442(4)	Ag(2)–O(4)#6	2.535(4)
Ag(1)–O(6)#2	2.488(4)	Ag(3)–O(3)#8	2.275(3)
Ag(1)–O(6)#1	2.714(4)	Ag(3)–O(6)	2.287(4)
Ag(1)–O(5)#2	2.725(4)	Ag(3)–O(8)	2.410(4)
Ag(2)–O(9)#4	2.267(4)	Ag(3)–O(3)#9	2.642(4)
Ag(2)–O(3)#5	2.341(4)
Ag(1)⋯Ag(1)#3	3.0106(10)	Ag(3)⋯Ag(2)#6	3.1691(8)
Ag(1)⋯Ag(3)#1	3.2891(8)	Ag(3)⋯Ag(1)#1	3.2892(8)
Ag(2)⋯Ag(3)#6	3.1691(8)	Ag(3)⋯Ag(2)#10	3.3469(8)
Ag(2)⋯Ag(3)#7	3.3469(8)
Compound 2
Ag(1)–O(3)	2.290(4)	Ag(1)–O(1)#2	2.360(4)
Ag(1)–O(1)#1	2.316(4)	Ag(1)–C(4)#1	2.584(5)
Compound 3
Ag(1)–O(1)	2.180(2)	Ag(1)⋯Ag(1)#2	3.2260(5)
Ag(1)–O(2)#1	2.187(2)	Ag(1)⋯Ag(1)#1	3.2260(5)
Ag(1)–N(1)	2.395(3)
Compound 4
Ag(1)–N(1)	2.190(4)	Ag(1)–O(2)	2.594(4)
Ag(1)–N(3)	2.192(4)	Ag(2)–N(2)#2	2.206(4)
Ag(2)–N(4)#1	2.202(4)	Ag(2)–O(8)	2.730(5)
Ag(1)⋯Ag(2)	3.1819(8)

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The edge-sharing Ag(3)O₄ and Ag(1)O₄ tetrahedrons connect into chains along (1, −1, 0) direction and are further assembled into a layer in ab-plane via the linkage of Ag(2)O₄ tetrahedrons in corner-sharing manner (see Fig. 2a). The organic trisphosphonate ligands behave as linkers which connect these layers of silver and oxygen atoms to form a pillar-layered structure (see Fig. 2b). Three phosphonate oxygen atoms (O2, O4 and O7) are protonated and behave as hydrogen donor atoms to form O–H⋯O bonds with neighboring phosphonate oxygen atoms (see Table 3).

Fig. 2 The layer structure in ab-plane and three-dimensional packing structure viewing along a-axis of compound 1. The AgO_x (x = 4 or 5) polyhedra and PO₃C tetrahedrons are shaded in yellow green and pink, respectively.

Table 3 O–H⋯O hydrogen bonds in compounds 1–4^a

	O–H (Å)	H⋯O (Å)	O⋯O (Å)	∠O–H⋯O (°)
a Symmetry transformations used to generate equivalent atoms: #1 1 − x, −y, 1 − z; #2 2 − x, 1 − y, 1 − z; #3 x, −1 + y, z; #4 3 − x, 1/2 + y, 1/2 − z; #5 1 + x, y, z; #6 1 − x, −y, −z; #7 1 − x, 1 − y, 1 − z.
Compound 1
O2–H2B⋯O9#1	0.8200	1.7600	2.578(5)	178.00
O4–H4B⋯O1#2	0.8200	1.7200	2.530(6)	170.00
O7–H7C⋯O5#3	0.8200	1.7600	2.578(5)	175.00
Compound 2
O2–H2A⋯O3#4	0.8200	1.7500	2.557(5)	169.00
Compound 3
O3–H3A⋯O1#3	0.8200	2.2000	2.593(3)	110.00
Compound 4
O1–H1B⋯O7#5	0.8200	1.6200	2.421(6)	165.00
O1W–H2W⋯O2#6	0.8500	2.0300	2.855(10)	164.00
O3–H3A⋯O6#5	0.8200	1.6800	2.492(6)	173.00
O4–H4A⋯O5#7	0.8200	1.8200	2.635(6)	176.00
O9–H9B⋯O5	0.8200	1.9100	2.682(7)	157.00

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Structure description of compound 2

Compound 2 crystallizes in monoclinic space group P2₁/c. Its asymmetric unit is comprised of one silver ion, half double deprotonated diphosphonate ligand (H₂L²)²⁻, corresponding to a formula of Ag₂(H₂L²) (see Scheme 1b and Fig. 3). Each phosphonic acid group in the bisphosphonate ligand is singly deprotonated and binds with three silver ions with its two phosphonate oxygen atoms. Its coordination mode can be denoted as: μ⁶:η¹:η⁰:η²:η¹:η⁰:η². The silver ion is four-coordinated by three phosphonate oxygen atoms from three bisphosphonate ligands and one carbon atom from the benzene core with a distortion index of 0.70, which indicates a distorted tetrahedron coordination sphere. The Ag–O bond lengths are found in the range of 2.290(4)–2.360(4) Å, which are comparable to those of other silver phosphonates.¹⁹ The Ag–C bond length is 2.584(5) Å, which is comparable to those of Ag–C π bonds but longer than those of Ag–C σ bonds found in silver ethynide complexes.¹² It is worth noting that it is very rare the benzene carbon atom is involved in the coordination of silver complexes isolated from water.

Fig. 3 Coordination environment of the bisphosphonate ligand (H₂L²)²⁻ in compound 2 with 30% probability, the O–H⋯O interactions are presented in dash lines. Symmetry transformations used to generate equivalent atoms: (A) x, −1 + y, z; (B) 2 − x, −0.5 + y, 0.5 − z; (C) x, 1 + y, z; (D) 2 − x, 0.5 + y, 0.5 − z; (E) 3 − x, 0.5 + y, 0.5 − z.

The silver ions are bridged by the bisphosphonate ligands to generate a layer in bc-plane (see Fig. 4a). The centroid to centroid distance of neighboring benzene rings is about 6.174 Å, indicating no π⋯π interactions. The protonated phosphonate oxygen atoms (O2) behave as hydrogen donor atoms and form inter-layer hydrogen bonds with O3 (see Table 3), which assemble the layers into a three-dimensional supramolecular structure (see Fig. 4b).

Fig. 4 The layer structure in bc-plane (a) and three-dimensional supramolecular packing structure viewing along b-direction (b) of compound 2. The π⋯π and O–H⋯O interactions are presented in blue and red dash lines, respectively.

Structure description of compound 3

Compound 3 crystallizes in the monoclinic space group P2/c. The asymmetric unit is comprised of one crystallographic independent Ag(I) ion, half pyrazine molecule and half bisphosphonate ligand, suggesting a formula of Ag₂(H₂L²)(pyz). The bisphosphonate ligand is also doubly deprotonated. It is tetradentate and its coordination mode can be denoted as: μ⁴:η¹:η¹:η⁰:η¹:η¹:η⁰ (see Scheme 1c and Fig. 5). The silver ion is triply coordinated by two oxygen atoms from two bisphosphonate ligands and one nitrogen atom from one pyrazine molecule, displaying a Y shaped arrangement with the largest angle being 150.24(10)° (see Table S1†). The Ag–O and Ag–N bond lengths are found in the range of 2.180(2)–2.187(2) Å and 2.395(3) Å, respectively, which are all in the expected ranges.¹⁷ The Ag⋯Ag distances are found to be 3.2260(5) Å which is shorter than the twice the van der Waals radius of silver(I) ion and comparable to those of other silver phosphonates.¹⁵ Taking into account of the Ag⋯Ag interactions in compound 3, the coordination sphere of the Ag(I) ion can also be viewed as a square pyramidal coordination manner with a τ₅ parameter of 0.33. It is noted that these silver ions are bridged by the tetradentate bisphosphonate ligands and argentophilic interactions into a layer in the ab-plane (see Fig. 6a). Short distances (4.636(1) Å) between the benzene rings of the bisphosphonate ligands are observed, indicating significant inner-layer π⋯π interactions. Additionally, O–H⋯O and C–H⋯O hydrogen bonds are also formed between neighboring phosphonate ligands (Tables 3 and S2†). These layers are further linked by the pyrazine auxiliary ligands to generate a pillar-layered structure (see Fig. 6b). The centroid to centroid distances of neighboring pyrazine molecules are exactly the same of the distance between benzene rings of bisphosphonate ligands from neighboring layers.

Fig. 5 Coordination environment of the bisphosphonate ligand (H₂L²)²⁻ in compound 3 with 30% probability, the O–H⋯O and Ag⋯Ag interactions are presented in dash lines. Selected interatomic distances [Å]: Ag(1)⋯Ag(1)A 3.2260(5), Ag(1)⋯Ag(1)B 3.2260(5). Symmetry transformations used to generate equivalent atoms: (A) −x + 1, y − 1/2, −z + 3/2; (B) −x + 1, y + 1/2, −z + 3/2; (C) x, −1 + y, z.

Fig. 6 The layer in ab-plane (a) and three-dimensional packing structure viewing along b-axis (b) of compound 3.

Structure description of compound 4

The replacement of pyz with 4,4′-bipy leads to the formation of compound 4. It crystallizes in triclinic P1̄ space group (see Table 1) and exhibits a three-dimensional supramolecular structure. In the asymmetric unit, there exists two crystallographic independent silver ions, two 4,4′-bipy molecules, three halves of bisphosphonate ligands and one aqua ligand (see Fig. 7), corresponding a formula of Ag₂(H₄L²)(H₂L²)(4,4′-bipy)(H₂O). Both the two crystallographic independent silver ions adopt T-shaped coordination configurations. The coordination sphere is filled by two pyridine nitrogen atoms aligning in the horizontal direction and one phosphonate oxygen atom in the axial direction. The Ag–N and Ag–O are found in the ranges of 2.190(4)–2.206(4) Å and 2.594(4)–2.730(5) Å, respectively, which are all comparable to those of other silver phosphonates.¹⁷ The Ag⋯Ag distance is found to be 3.1819(8) Å (see Table 2), suggesting the existence of argentophilic interaction in compound 4. Two arrays of silver ions are linked by parallel 4,4′-bipy molecules into ladder like structures with the aid of argentophilic interactions and π⋯π interactions between pyridine rings of 4,4′-bipy (centroid to centroid: 3.710(6) and 3.754(6) Å) (see Fig. 8a). The three bisphosphonate moieties adopt different coordination modes and deprotonation states from the charge balance and P–O bond distances (see Scheme 1d and e). One kind of ligands are neutral and bidentate, binding two silver ions with their phosphoryl oxygen atoms (O2, see Scheme 1d). The second kind of ligands are also bidentate but are doubly deprotonated (see Scheme 1f). These bidentate bisphosphonate ligands further assemble the ladders into a corrugated layer (see Fig. 8b). The third kind of bisphosphonate ligands are also doubly deprotonated but are not involved in the coordination with the silver ions (see Scheme 1e) and behave as hydrogen donors and acceptors of H-bonds with phosphonate oxygen atoms of neighboring layers, leading to the formation of three-dimensional packing structure (see Fig. 8c). The lattice water molecules (O1W) are also accommodated between the layers and are involved in the formation of O–H⋯O interactions with phosphonate oxygen atoms (see Table 3). This kind of packing structure contributes to the formation of C–H⋯O interactions which are formed between the pyridine and benzene groups with the phosphonate oxygen atoms (see Table S2†).

Fig. 7 Coordination environment of the bisphosphonate ligands⁻ in compound 4 with 30% probability, the O–H⋯O and Ag⋯Ag interactions are presented in dash lines. Selected interatomic distances [Å]: Ag(1)⋯Ag(2) 3.1819(8). Symmetry transformations used to generate equivalent atoms: (A) 2 − x, −y, 1 − z; (B) 1 − x, −y, 1 − z; (C) 1 + x, y, z; (D) 1 − x, 1 − y, 1 − z; (E) 1 − x, −y, −z.

Fig. 8 The ladder like structure (a), layer (b) and three-dimensional supramolecular packing structure (c) of compound 4. The O–H⋯O and Ag⋯Ag interactions are presented in dash lines.

Structure discussion

From above structural descriptions it can be found that silver phosphonates display various crystal structures in which the phosphonate ligands and silver ions adopt variable coordination modes and coordination spheres. The phosphonate groups are mainly single deprotonated but can also be neutral. The coordination numbers of the silver ions in these compounds are usually four or five, displaying distorted tetrahedron or tetragonal pyramid coordination manner, respectively. Besides the familiar Ag–O, Ag–N and Ag–Ag bonds, Ag–C is also observed, suggesting that Ag–C bond can also be formed in silver compounds synthesized by hydrothermal method. In this work, the silver compounds mainly display two-dimensional layered structures.

Thermogravimetric analyses (TGA)

TGA measurements were carried out on the powder samples of compounds 1–4 to investigate their thermal stabilities (see Fig. 9). From the TGA traces of compounds 1–3, it is found that there is no obvious weight loss below 280 °C for compounds 1 and 2, and 200 °C for compound 3, indicating their good thermal stabilities. This is what we expect because the structural characterizations have revealed that there is no lattice or coordinated water molecules in compounds 1–3. With the further increasing of temperature, compounds 1 and 2 start to decompose. For compound 3, a steep weight loss about 12.74% happens in the range of 200–310 °C. This value is very close to the mass ratio of one pyz ligand in each formula, therefore, it can be ascribed to the removal of pyz ligand. The removal of pyz ligand leads to the decomposition of the framework which gets drastic above 380 °C. In comparison with compounds 1–3, compound 4 starts to lose weight much earlier. The weight loss (about 1.99%) in the range of 50–120 °C, corresponding to the removal of one water molecule in each formula, matches well with the theoretical value (1.83%). When the temperature is higher than 180 °C, it starts to lose weight drastically, suggesting the collapse of the supramolecular structure.

Fig. 9 TGA diagrams of compounds 1–4.

Photophysical properties

As one kind of d¹⁰ coordination compounds, the photophysical properties of silver compounds are of particular interest because of their potential applications in the fields of chemical sensor, photochemistry and light-emitting diode (LED).^32–38 The luminescence properties of compounds 1–4 and related free ligands were investigated in the solid state at room temperature (see Fig. 10). The phosphonate ligands (H₆L¹ and H₄L²) display broad emission peaks at about 376 and 370 nm with shoulders at 398.5 and 398 nm, respectively. The emission peaks of pyz and 4,4′-bipy are found at 359.5 and 430 nm, respectively. These emissions could be ascribed to the transitions of π* → n or π* → π. Similar emission spectra are observed with the maxima localized at 373 and 398.5 nm for 1, 351.5 and 398 nm for 2, 374 and 398.5 nm for 3, 375 and 398.5 nm for 4, respectively. The resemblance between compounds 1–4 and the corresponding free ligands suggest that these emission transitions are originated from the intraligand π* → π transitions. The slight blue or red shifts might be due to their different coordination configurations and diverse Ag⋯Ag interactions which have great effect on the energy gaps.⁸

Fig. 10 Photoluminescent properties of compounds 1–4.

Conclusions

Four silver(I) phosphonates have been synthesized hydrothermally from the reaction of two semi-rigid phosphonate ligands and silver oxide with or without the presence of N-heterocyclic auxiliary ligands. The introduction of N-heterocyclic auxiliary ligands can greatly enhance the crystallinity of silver(I) phosphonates. These compounds possess either layer or pillar-layered structures in which the silver ions adopt diverse coordination manners and the phosphonate ligands possess numerous coordination modes. The phosphonate ligands can coordinate with the silver centers via Ag–O and Ag–N bonds as well as Ag–C bonds. These compounds mainly display intraligand emissions. Further work will be undertaken to tune the photophysical properties of silver phosphonates by elaborate selection of the organic ligands.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21171173).

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Footnote

† Electronic supplementary information (ESI) available: PXRD diagrams and specified C–H⋯O interactions. CCDC 1498976–1498979. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra21382k

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