Spontaneous chiral resolution and hierarchical directing effects of two-winged propeller-like SDAs on the construction of noncentrosymmetric iodoargentates/iodocuprates

Abstract

Directed by in situ benzylated pyridine derivatives with potential conformational chirality, four inorganic–organic hybrids, [BCP]₂[Ag₄I₆] (1), [BCP][Cu^I₂Cu^III₅]·2H₂O (2), [BQL][Ag₄I₅] (3), [BQL]₂[Cu₅I₇] (4) (BCP⁺ = N-benzyl-4-cyanopyridinium, BQL⁺ = N-benzyl-quinolinium) have been synthesized solvothermally. The contrast between spontaneous chiral resolution for BCP⁺ in 1 and 2 and racemic BQL⁺ aggregates in 3 and 4, and remarkable structural correlations between organic and inorganic species exhibit interesting chiral structure directing effects, which are mainly attributed to the variation of organic motif, substituent, and consequently flexible supramolecular assemblies, as well as hierarchical matching ability between inorganic and organic species. 1 also exhibits second harmonic generation (SHG) responses in both visible and ultraviolet regions.

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Introduction

The construction of noncentrosymmetric (NCS) inorganic–organic hybrids¹ is of great interest due to their diverse composition-structure advantages and potential applications in enantioselective sorption, separation, catalysis,² and their second-order nonlinear optical behavior, pyroelectricity, piezoelectricity, ferroelectricity and triboluminescence.³ Although much effort has been made, it is still a challenge to effectively construct chiral or acentric inorganic frameworks from achiral primary building units because of the dominant occurrence of the centric symmetric space group.⁴ Structural directing strategies have been paid intense attention in this domain.⁵ One interesting strategy is based on the occasionally spontaneous chiral resolution of inorganic components in the absence of chiral precursors, including [CN₃H₆][Sn₄P₃O₁₂] and [Me₂-DABCO][M₂(HPO₃)₃].⁶ However, the more effective strategy is exploiting the optically pure chiral template to impart their chirality and facilitate the formation of a NCS framework, such as enantiopure metal complex d-Co(en)₃I₃,⁷ (1S,2S)-1,2-diaminocyclohexane⁸ and (R)-2-methylpiperazine.⁹ In addition, in situ synthetic enantiomeric chiral–metal complexes can also exhibit spontaneous resolution in the process of crystallization and the chiral information is transferred into the inorganic framework.¹⁰ Noteworthy, some of two-winged propeller-like organic molecules can adopt specifically chiral conformation while ‘being frozen’ in the organic crystal lattice,¹¹ and give rise to conglomerate (spontaneous chiral resolution) or racemate on the basis of various non-covalent intermolecular interactions.¹² Obviously, such a crystallizing behavior can also be used in the structural directing construction of noncentrosymmetric inorganic framework, in which potentially conformational chiral organic cations^1a,13 and its supramolecular aggregates^14,15 would take the structural directing roles to provide hierarchical directing environment through flexible modification on spatial and electronic features.

Our work is focusing on the family of organic-templated iodometallates because of their structural diversification, ranging from 0D clusters, 1D chains, 2D layers and 3D open frameworks, and the useful physical properties associated with hyperpolarizable organic and inorganic components,^3f such as SHG responses. However, chiral or acentric silver/copper iodide hybrids which also exhibit symmetry-dependent properties are rarely reported.¹⁶

As an extension of our previous studies of the N-benzyl-pyridinium directed noncentrosymmetric iodometallates,¹⁶ two-winged propeller-like N-benzyl-4-cyanopyridinium and N-benzyl-quinolinium as chirally conformational cations were introduced in the construction of iodometallates. Herein, [BCP]₂[Ag₄I₆] (1), [BCP][Cu^I₂Cu^III₅]·2H₂O (2), [BQL][Ag₄I₅] (3), and [BQL]₂[Cu₅I₇] (4) were synthesized, exhibiting remarkable component dependent spontaneous chiral resolution and hierarchical directing effects of organic structure directing agents (SDAs).

Experimental

Materials and methods

All starting materials used in the synthesis were purchased from commercial sources without further purification. The purity of the title compounds has been proved by X-ray powder diffraction (XRPD) by using a Rigaku Ultima IV-185 diffractometer, in which the experimental pattern agrees with the simulated one (Fig. S5†). Elemental analyses (C, H, N) were performed on a Perkin-Elmer 240 elemental analyzer. Inductively coupled plasma (ICP) analyses (Ag, Cu, I) were performed on a Perkin-Elmer 2400 elemental analyzer. The IR spectra were measured with a Nicolet 5DX spectrometer as KBr disks (4000–400 cm⁻¹) (Fig. S6†). Electron paramagnetic resonance (EPR) spectrum was recorded on a Bruker A300-10/12 spectrometer at room temperature (RT). Thermal analyses were tested in air using HTG-3 equipment in the range of 25–900 °C with a heating rate of 10 °C min⁻¹. UV-vis diffuse reflectance spectrum was conducted on a Varian Cary 5000 UV-vis spectrophotometer at RT and 77 K.

Preparation of {[BCP]₂[Ag₄I₆]}_n (1)

In situ reaction of 4-cyanopyridine (0.068 g, 0.65 mmol), AgI (0.57 g, 3 mmol), concentrated HI (0.37 mL, 45%), benzyl alcohol (2 mL), and acetone (4 mL) was stirred in air and then transferred into a 15 mL Teflon-lined stainless steel reactor, which was heated at 110 °C for 3 days. After cooling to ambient temperature, orange crystals of 1 were obtained in 56% yield based on Ag. Chemical analysis indicates the contents of Ag and I as 27.45 and 48.11 wt% (calculated: 27.26 and 48.08 wt%), giving the Ag/I molar ratio of 2 : 3. Anal. calcd for C₂₆H₂₂N₄Ag₄I₆: C, 19.72; H, 1.40; N, 3.54%. Found: C, 19.42; H, 1.28; N, 3.74%. IR (KBr, cm⁻¹): ν 3109(w), 3046(w), 2926(w), 2852(w), 1636(s), 1505(m), 1453(s), 1289(w), 1135(w), 821(w), 724(m), 605(m).

Preparation of {[BCP][Cu^I₂Cu^III₅]·2H₂O}_n (2)

A mixture of 4-cyanopyridine (0.068 g, 0.65 mmol), concentrated HI (0.37 mL, 45%), benzyl alcohol (1 mL), and acetonitrile (4 mL) at 110 °C for 2 days yielded the black-red clear solution containing iodide of BCP⁺. After cooling, CuI (0.38 g, 2 mmol), and concentrated HI (0.37 mL, 45%) was added to the resulting solution and then the clear solution were kept at RT. Black-red plate-like crystals of 2 grew overnight, which were separated by filtration. Chemical analysis indicates the contents of Cu and I as 18.15 and 60.33 wt% (calculated: 18.05 and 60.06 wt%), giving the Cu/I molar ratio of 3 : 5. Anal. calcd for C₁₃H₁₅N₂O₂Cu₃I₅: C, 14.78; H, 1.43; N, 2.65; O, 3.03%. Found: C, 14.53; H, 1.38; N, 2.45; O, 3.12%. IR (KBr, cm⁻¹): ν 3188(m), 3035(w), 2967(w), 2858(w), 1636(s), 1505(m), 1408(s), 1266(w), 1102(m), 811(w), 729(w), 598(w).

Preparation of {[BQL][Ag₄I₅]}_n (3)

Compound 3 was prepared from one-pot reaction involving quinoline (0.129 g, 1.0 mmol), AgI (0.470 g, 2 mmol), NaI (0.075 g, 0.5 mmol), concentrated HNO₃ (0.5 mL, 65%), benzyl alcohol (2 mL), and acetone (5 mL). The subsequent procedure was similar to 1 and dark-yellow stick crystals were achieved in 40.12% yield (based on Ag). Chemical analysis indicates the contents of Ag and I as 33.87 and 49.58 wt% (calculated: 33.56 and 49.32 wt%), giving the Ag/I molar ratio of 4 : 5. Anal. calcd for C₁₆H₁₄NAg₄I₅: C, 14.94; H, 1.10; N, 1.10%. Found: C, 14.32; H, 1.05; N, 1.18%. IR (KBr, cm⁻¹): ν 3064(w), 2966(w), 2919(m), 2852(w), 2356(w), 1625(s), 1523(s), 1453(m), 1381(w), 1157(w), 1049(w), 815(w), 776(w), 596(w).

Preparation of {[BQL]₂[Cu₅I₇]}_n (4)

The dark-red crystals of 4 were synthesized by the similar procedure to the synthesis of 3 except that NaI, concentrated HNO₃, and AgI was replaced by concentrated HI (0.37 mL, 45%), and CuI (0.380 g, 2 mmol). Yield: 46.23% (based on CuI). Chemical analysis indicates the contents of Cu and I as 19.12 and 53.61 wt% (calculated: 19.30 and 53.95 wt%), giving the Cu/I molar ratio of 5 : 7. Anal. calcd for C₃₂H₂₈N₂Cu₅I₇: C, 23.34; H, 1.71; N, 1.70%. Found: C, 23.73; H, 1.58; N, 1.96%. IR (KBr, cm⁻¹): ν 3036(w), 2966(w), 2926(m), 2851(w), 2362(w), 1624(s), 1527(s), 1460(m), 1386(w), 1164(w), 1038(m), 781(w), 718(w), 582(w).

X-ray crystallography

Single-crystal X-ray diffraction data for 1–4 were collected at 293 K on an Oxford Gemini diffractometer using a fine-focus sealed-tube X-ray source (Cu-Kα radiation for 1, Mo-Kα radiation for 2–4, graphite monochromator). SCALE3 ABSPACK scaling algorithm was used for empirical absorption correction with spherical harmonics.¹⁷ Four structures were solved by direct method and refined by full-matrix least-squares techniques on F2 using SHELXTL-97 program.¹⁸ Non-hydrogen atoms were treated with anisotropic displacement parameters based on fixed thermal factors. The crystal structure of 2 contains serious disordered solvent molecules, which was assigned to lattice water molecules. There are strong hydrogen bonds between O(1) and O(2) in 2, and the distance between them is reasonable for donor and acceptor atoms with strong hydrogen bonding interaction. Details of single crystal data are summarized in Table 1, and selected bond lengths and bond angles are given in Table S1 of the ESI.† CCDC-1440184 (1), -1440188 (2), -1440189 (3) and -1440191 (4) contain the supplementary crystallographic data for this paper.

Table 1 Crystal data and structure refinement for compounds 1–4

a R1 = ∑||Fo| − |Fc||/∑|Fo|.b wR2 = [∑w(Fo^2 − Fc^2)^2/∑w(Fo^2)^2]^1/2.
Compound	1	2	3	4
CCDC code	1440184	1440188	1440189	1440191
Temperature	293(2) K	293(2) K	293(2) K	293(2) K
Formula	C26H22N4Ag4I6	C13H15N2O2Cu3I5	C16H14NAg4I5	C32H28N2Cu5I7
Formula weight	1583.36	1056.37	1286.26	1646.56
Crystal size (mm)	0.18 × 0.10 × 0.09	0.18 × 0.11 × 0.10	0.18 × 0.10 × 0.04	0.18 × 0.15 × 0.05
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	Cc	P21	Pn	C2/c
a (Å)	20.3555(4)	8.7403(3)	12.9435(11)	27.5988(8)
b (Å)	16.0201(3)	10.9631(4)	4.5716(3)	10.9911(3)
c (Å)	15.0540(3)	13.0161(5)	20.997(2)	27.3011(7)
α (deg)	90.00	90.00	90.00	90.00
β (deg)	131.898(2)	106.319(4)	101.605(9)	105.089(3)
γ (deg)	90.00	90.00	90.00	90.00
V (Å^3)	3653.99(12)	1196.96(7)	1217.05(18)	7996.0(4)
Z	4	2	2	8
Dc (g cm^−3)	2.878	2.920	3.510	2.736
F (000)	2848	942	1140.0	6000.0
μ (mm^−1)	56.929	9.098	9.515	8.047
Reflections collected	10 169	5233	5125	17 909
Unique reflections	5115	3167	3293	7866
Rint	0.0914	0.0258	0.0322	0.0268
Goodness-of-fit on F^2	1.032	1.012	1.030	1.092
R1/wR2, [I ≥ 2σI]^a^,^b	0.0892, 0.2190	0.0319, 0.0622	0.0441, 0.1029	0.0341, 0.0627
R1/wR2, (all data)	0.0930, 0.2271	0.0380, 0.0651	0.0493, 0.1078	0.0490, 0.0678
Δρmax, Δρmin (e Å^−3)	3.456, −3.181	1.801, −1.256	2.571, −1.707	1.233, −1.157

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Second harmonic generation (SHG) measurements

The SHG properties of 1 were tested on the polycrystalline powder samples by the modified method of Kurtz and Perry using a Q-switched Nd:YAG laser at the fundamental wavelength of 1064 nm and its SHG at 532 nm.¹⁹ The sample was ground and sieved into several distinct particle sizes ranging from 25–40, 40–63, 63–80, 80–125, 125–150, 150–200, and 200–300 μm, respectively, while the KDP powders of similar particle sizes were serve as the reference. Finally, the KDP powder sieved as 200–300 μm was used as a reference to assume the ratio of the SHG efficiencies.

Results and discussion

Description of structures

{[BCP]₂[Ag₄I₆]}_n (1). Compound 1 crystallizes in acentric space group Cc and the asymmetric unit contains four silver(I) atoms and six iodine atoms together with two BCP⁺ cations (Fig. S1†). Each Ag(I) atom is coordinated by one μ-I and three μ₃-I atoms with a distorted tetrahedral coordination geometry. The Ag–μ-I bond distances range from 2.7551(10) to 2.8326(9) Å, the Ag–μ₃-I bond lengths are in the range of 2.8525(10)–2.9654(10) Å, and I–Ag–I bond angles vary between 102.55(3) and 122.69(3)°. 1 presents a 3D open framework built up from four-connected cubane-like [Ag₄(μ₃-I)₄] core (Fig. 1a), in which Ag⋯Ag contact (3.1828(10)–3.4380(12) Å) show the presence of weak argentophilic interactions (longer than that of metallic silver (2.88 Å) but shorter than the van der Waals radius sum of silver (3.44 Å)). BCP⁺ cations are enclosed in ovary channels (Fig. 1b). So far, a variety of silver iodide clusters, such as [AgI₄]³⁻, [Ag₄I₈]⁴⁻, [Ag₅I₉]⁴⁻, [Ag₅I₁₀]⁵⁻, [Ag₆I₁₂]⁶⁻, [Ag₇I₁₃]⁶⁻, [Ag₁₂I₁₂@I]⁻, [Ag₁₄I₁₆]²⁻, and [Ag₈I₆]²⁺,²⁰ have been used as building blocks to obtain high-dimensional open framework structures. However, three-dimensional iodoargentate frameworks constructed from Ag₄I₈⁴⁻ clusters containing a cubane-like Ag₄I₄ core is relatively rare. Topological analysis for 1 reveals a novel 3D topological network of β-cristobalite, different from the similar reported iodoargentate framework (α-cristobalite) directed by N-alkyl-4-cyanopyridinium and iodocuprate framework by N-methyl-4-cyanopyridium.²¹ BCP⁺ cations adopt non-enantiomerically chiral conformations and aggregate as cationic dimers through three weak interactions: (a) the π⋯π stacking interaction with centroid-to-centroid distances, (b) the p⋯π interaction (N⋯π), (c) the C–H⋯π interaction (Fig. 1c and Table 2).

Fig. 1 (a) View of the cubane-like Ag₄I₄ clusters in 1 and the 4-connectivity between the adjacent Ag₄I₄ clusters. (b) View of BCP⁺ intercalated in ovary channels formed by the 3D iodoargentate framework of 1. (c) View of cationic dimers via weak interactions (H atoms are omitted for clarity).

Table 2 The torsion angles (°) of crystallographic independent molecules and π-stacking interactions (Å) in 1–4

	Crystallographic independent molecule 1		Crystallographic independent molecule 2		π-Stacking interactions
1	N2–C7–C8–C9	60.86	N4–C20–C21–C26	84.88	π⋯π	3.772(8)
	C8–C7–N2–C5	−105.69	C21–C20–N4–C19	−75.38	p⋯π	3.175(13)
					C(24)H⋯π	3.022(0)
2	N2–C7–C8–C9	105.88			π⋯π	3.929(1)
	C8–C7–N2–C5	−77.43			p⋯π	3.345(3), 3.728(15)
3	N1–C10–C11–C16	−53.94			π⋯π	3.659(2)
	C11–C10–N1–C5	99.07			C(10)H⋯π	2.440(3)
4	N1–C10–C11–C12	−54.38	N2–C26–C27–C28	−61.59	π⋯π	3.753(1), 3.608(5), 3.901(7), 3.938(9), 3.967(8)
	C11–C10–N1–C1	−32.62	C27–C26–N2–C17	97.00

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{[BCP][Cu^I₂Cu^III₅]·2H₂O}_n (2). Compound 2 crystallizes in chiral space group P2₁ and the asymmetric unit contains three copper atoms and five iodine atoms together with one BCP⁺ cation and two lattice water molecules (Fig. S1†). The unusual mixed-valence coppers meet the requirement of charge-balance, as confirmed by a broad EPR signal centred at g = 2.16, which rarely appears in the copper iodide clusters (Fig. S2†).²² Each Cu center is four-coordinated in a distorted tetrahedral geometry in which the terminal Cu–I bond distance is 2.5842(17) Å, the Cu–μ-I bond lengths vary between 2.6137(15) and 2.6470(16) Å, the Cu–μ₃-I bond lengths range from 2.6662(18) to 2.822(2) Å, and the Cu–I bond distances vary between 2.5842(17) and 2.822(2) Å and I–Cu–I bond angles range from 101.04(5) to 118.91(6) Å, deviating slightly from an ideal tetrahedron. The Cu⋯Cu distances (2.715(2) Å) are shorter than the van der Waals radius sum of copper (2.8 Å), suggesting Cu–Cu weak interaction. Cu(1), Cu(2) and Cu(3) are bridged into an incomplete cubane-like Cu₃I₇ secondary building units (SBU), similar to the reported [N-Bz-Py]₂[Cu₆I₈]^16a (Fig. 2b). Further connection via sharing two iodine atoms leads to the first homochiral two-fold helical [Cu^I₂Cu^III₅]_n chains along the b axes, which is separated by lattice water molecules (Fig. 2a and b) along the a axes and BCP⁺ cations along the c axes. Relatively short O(w1)⋯O(w2) distances (2.833(3) Å) indicate the existence of water dimers via the strong hydrogen bond interaction. The BCP⁺ cations adopt homochiral conformation and are linked into homochiral supramolecular layers through π⋯π interaction and p⋯π interaction (N⋯π) (Fig. 2c and Table 2).

Fig. 2 (a) View of the packing diagram of 2 made up of anionic chains which are separated by water dimers and cationic supramolecular layers. (b) View of a two-fold helical iodocuprate chain. (c) View of a homochiral cationic layer. All hydrogen atoms are omitted for charity.

{[BQL] [Ag₄I₅]}_n (3). Complex 3 crystallizes in a central space group Pn and presents a layered iodoargentate structure. The asymmetric unit of 3 contains four silver(I) atoms, five iodine atoms, and one BQL⁺ cation (Fig. S1†). Each Ag atom adopts a distorted tetrahedral geometry with the Ag–μ₃-I bond distances ranging from 2.7732(8) to 2.9126(8) Å, Ag–μ₄-I bond lengths varying between 2.8385(9) and 3.1036(9) Å, and the bond angles from 97.59(3) to 144.47(3)°. The Ag⋯Ag distances (3.1522(11) and 3.1614(9) Å) imply Ag–Ag weak interactions. The inorganic layer [Ag₄I₅]_n can be viewed as alternative connections of ladder-like [Ag₂I₄]_n chain (A) and belt-like [Ag₂I₄]_n chain (B) along the c axis (Fig. 3a). The BQL⁺ cations lie between the anionic layers, presenting enantiomeric conformations (Fig. 3b). Noteworthy, BQL⁺ with the same chirality form into a supramolecular chain along the b axis via π⋯π interactions and C–H⋯π interactions and exhibit an alternative arrangement with the opposite chiral ones along the c axis (Fig. 3 and Table 2). Meanwhile, the spatial correspondence between quinoline rings and chains (A) as well as benzyl group and chains (B), show the remarkable structural matching ability of organic and inorganic components (Fig. 3a).

Fig. 3 (a) View of the [Ag₄I₅]_n layer of 3 composed of [Ag₂I₄]_n chain (A) and [Ag₂I₄]_n chain (B) and the position with BQL⁺ cations. (b) The packing diagram of 3 showing the alternative enantiomeric BQL⁺ supramolecular chains. All hydrogen atoms are omitted for charity.

{[BQL]₂[Cu₅I₇]}_n (4). Complex 4 crystallizes in the space group C2/c and presents 1D iodocuprate chains. The asymmetric unit contains five copper(I) atom, seven iodine atoms, and two BQL⁺ cations (Fig. S1†). Cu(1)–Cu(4) atoms adopt distorted tetrahedral geometries and Cu(5) is pseudo-trigonal pyramid geometry with the Cu–μ-I bond distances ranging from 2.5071(10) to 2.6336(10) Å, the Cu–μ₃-I bond distances varying between 2.6320(10) and 2.7131(10) Å, the Cu–μ₄-I bond lengths being in the range of 2.6593(10)–3.0797(12) Å, and the bond angles from 94.52(3) to 125.93(4)°. There is shorter distance between Cu(1) and Cu(2) (2.5498(13) Å), implying the Cu–Cu weak interactions. The [Cu₅I₇]_n chains, isostructural in inorganic moiety with the reported [(N-Bz-3-MePy)₂(Cu₅I₇)]_n^16b and [(ipq)₂(Cu₅I₇)]_n,²³ present alternative right- and left-handed helical chains along the b axis, yielding an overall racemic packing arrangement (Fig. 4a and b). In contrast to 3, the BQL⁺ adopt non-enantiomerical chiral conformations in a relatively opposite manner and further assemble into a 3D (10,3)-d supramolecular network,²⁴ in which there exist various π⋯π interactions between aromatic rings of 3-connected BQL⁺ nodes, featured as a alternatively handed helical channel occupied by anionic chains (Fig. 4a and c).

Fig. 4 (a) View of the overall racemic packing arrangement. (b) 1D right- and left-handed helical iodocuprate chains. (c) 3D (10,3)-d racemic supramolecular net generated by the 3-connected BQL⁺ nodes. All hydrogen atoms are omitted for charity.

Influences of the organic motif, substituent of the cations on packing structures of hybrids

The spontaneous chiral resolution of two-winged propeller-like organic molecules, such as benzophenone^11b and N-sulfonylpyrimidine derivatives,^11c and diaryl sulfide,^11d,11e has been a commonly known phenomenon in organic chemistry. Meanwhile, conformational chirality has important implications as supramolecular synthons in crystal engineering,²⁵ in which the occurrence of spontaneous chiral resolution depends on the size, shape, and intermolecular interactions of the starting materials.¹² In organic–inorganic hybrids, inorganic components will simultaneously affect the process of spontaneous chiral resolution of organic moiety, which conversely plays structural directing roles in the construction of multifunctional inorganic framework.²⁶

In contrast to the central symmetric compounds 3 and 4, explicit spontaneous chiral resolution in noncentrosymmetric 1 and 2 reveals the fixation of specific chiral conformation of organic cations and structural correlation between organic components and iodometallate frameworks, implying the special spatial directing effect of BCP⁺ on NCS iodometallates. Meanwhile, chiral conformational information in a molecular level can be further amplified and transferred via supramolecular arrangements (accentric in 1, chiral in 2, racemic in 3 and 4), which provide electronically and spatially defined crystallizing environment and dictate the construction of inorganic framework in a collective level. Furthermore, the flexible supramolecular arrangements with multiple weak interactions can be influenced by inorganic moieties (1 vs. 2 or 3 vs. 4), and organic molecular architecture (shape, size, local charge distribution and type of substituent) (1 vs. 3 or 2 vs. 4),²⁷ which lay an excellent basis on the hierarchical structural directing ability. Compared with the reported N-alkyl-4-cyanopyridinium iodoargentate (P2₁2₁2₁),²¹ the change of space group in 1 further verifies the substituent and supramolecular directing effects on the inorganic framework. However, the occurrence of cubane like [Ag₄(μ₃-I)₄] core in N-alkyl-4-cyanopyridinium iodoargentate, and incomplete cubane-like Cu₃I₇ SBU in N-benzyl-pyridinium iodocuprate¹⁶ and 2 reveal the dominant effect of basic organic motif on the inorganic crystallization. In contrast to the reported homochiral cationic supramolecular (10,3)-a net in N-benzyl-pyridinium iodocuprate,^16a the racemic (10,3)-d net in 4 presents another spatial and electronic effects of organic motif due to the coexistence of opposite non-enantiomers and various π–π interactions.

Thermogravimetric (TG) analyses

As shown in Fig. S3,† 1, 3, and 4 are thermally stable up to 200 °C. The weight loss in the range of 238–634 °C for 1 (calcd: 38.26%; found: 35.03%), 243–652 °C for 3 (calcd: 25.49%; found: 21.01%), and 205–443 °C for 4 (calcd: 39.83%; found: 37.11%) correspond to the decomposition of BCP⁺ or BQL⁺ iodide.^16b Besides, the last stage mainly involves the extreme burning of the iodometallate framework and the residue is Ag for 1 (calcd: 27.26%; found: 24.20%), 3 (calcd: 33.56%; found: 30.80%), and CuO for 4 (calcd: 24.16%; found: 22.91%).²⁸ Since 2 contains lattice water molecules, the first stage occurs in the range of 90–122 °C, which is attributed to the loss of lattice water molecules (calcd: 3.41%; found: 2.12%), and the subsequent weight loss in the range of 200–483 °C is assigned to the decomposition of [BCP]I (calcd: 28.66%; found: 32.05%),^16b and further weight loss of 483–615 °C corresponds to the decomposition of remaining iodocuprate framework, and the final residue is CuO (calcd: 22.60%; found: 17.69%).

Second harmonic generation measurement

Because of the dark color for 2, only 1 was tested as the candidate for SHG response. As shown in Fig. 5, 1 exhibits type-I phase-matching behavior in both visible regions (approximately 1.14 times that of KDP), and ultraviolet region (approximately 0.24 times that of KDP),²⁹ proving it's accentricity and a potential second-order NLO material. Notably, the relative SHG intensities in the UV region differ significantly from those in the visible region, which may be resulted from its Ag₄I₄ cubane with a large polarizability³⁰ and 3D inorganic diamondoid network (similar to the prototypical NLO-active material, KDP-type structure)³¹ and additionally affected by possible intermolecular charge separation between anionic framework and cations.³² Compared with other SHG of nonlinear optical (NLO) active materials, such as metal iodates, metal borates, metal oxides, metal selenites and metal phosphates etc.,³³ the acentric structure of 1 exhibits only weak optical activity, which is probably due to cancellation of the dipole moments of iodoargentate.

Fig. 5 Phase-matching curves for 1 with KDP as reference at the wavelengths of 532 nm (a) and 1064 nm (b).

Optical absorption spectra and thermochromic property

The UV-vis diffuse-reflection spectra of 1–4 were measured at RT and the absorption (a/S) data were obtained by the reflectance data using the Kubelka–Munk function (Fig. 6).³⁴ The band gaps (2.63 eV for 1, 1.91 eV for 2, 2.54 eV for 3, and 2.22 eV for 4) were evaluated with linear extrapolation of the linear portion of the absorption edges, suggesting the semiconductor nature (Fig. S4†). Except for mixed-valence Cu(I)/Cu(II) in 2, the red shift of 1, 3 and 4 with respect to that of β-AgI (2.81 eV) or CuI (2.95 eV) is due to high electron affinity of aromatic 4-cyanopyridiniums and quinolinium motifs³⁵ and consequent intermolecular charge transfer (CT) effects.^20,36 Meanwhile, relative blue shift of 4 with respect to those of 1 and 3 is probably due to the strong donating ability of iodocuprate framework.^16,37 Compared with colorless [ipq]₄[Ag₄I₈] and {[pql][Ag₂I₃],³⁸ the red shift of 3 is probably ascribed to the consecutive π-stacking cationic supramolecular aggregates which could improve their charge-carrier mobility.³⁹

Fig. 6 Solid state optical diffuse-reflection spectra of 1–4.

Upon being immersed into liquid nitrogen (77 K), crystals of 1–4 turned to bright yellow, red, sand yellow and blood red respectively (Fig. 7), and gradually recovered to initial colors with the temperature increasing to 293 K, indicative of reversible thermochromic property of 1–4. The agreement of XRPD patterns at RT and 100 K implies no phase change (Fig. S5†). Besides, no obvious shift of absorption edge of UV-vis absorption spectra at RT and 77 K imply the lattice contraction of iodoargentates and iodocuprate contributes little to the thermochromism, which is different from the thermochromic iodobismuthates or iodoplumbates hybrids.⁴⁰ On the basis of our previous work,^16,21a the thermochromic mechanism of 1–4 could be attributed to the cold-induced decrease of intermolecular charge transfer (CT) population rather than structural variations, which is also confirmed by remarkably different absorption intensity in the UV-vis absorption spectra of 1 and 2, as the representative, at 77 K and RT (Fig. 3c).

Fig. 7 Thermochromic behavior of compounds 1–4: digital photographs of crystals taken at RT (293 K) and temperature of liquid nitrogen (77 K).

Conclusions

In summary, directed by two-winged propeller-like organic SDAs, a series of iodometallate hybrids have been successfully synthesized. Interestingly, on the basis of the variation of organic motif and modulation of substituent, organic cations with potentially conformational chiralilty and consequently flexible supramolecular assemblies via multiple noncovalent interactions can be used as an effective strategy for the construction of multifunctional noncentrosymmetric materials. Further studies are still ongoing.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No. 21171110), the Research Fund for the Doctoral Program of Higher Education of China (No. 20131404110001) and the Natural Science Foundation of Shanxi Normal University (No. ZR1502).

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Footnote

† Electronic supplementary information (ESI) available: Asymmetric unit diagram (Fig. S1), EPR spectrum (Fig. S2), TG analyses (Fig. S3), solid state optical diffuse-reflection spectra (Fig. S4), X-ray powder diffraction (XRPD) patterns (Fig. S5), infrared spectroscopy (Fig. S6), and selected bond lengths (Å) and angles (°) for 1–4 (Tables S1). CCDC 1440184, 1440188, 1440189 and 1440191. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra15159k

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