Three Iodoargentate-Based Hybrids Decorated by Metal Complexes: Structures, Optical/Photoelectric Properties and Theoretical Studies

Abstract

So far, the development of new iodoargentate-based hybrids, especially those compounds with metal complex cations, and the understanding of their structure–activity relationships have been of vital importance but full of challenges. Herein, using the in-situ-generated metal complex cations as structural directing agents, three new iodoargentate-based hybrids, namely, [Co(phen)₃]Ag₂PbI₆ (phen = 1,10-phenanthroline; 1), [Ni(5,5-dmpy)₃]Ag₇I₉·CH₃CN (5,5-dmpy = 5,5-dimethyl-2,2-bipyridine; 2) and [Co(5,5-dmpy)₃]Ag₅I₈ (3), have been solvothermally prepared and then structurally characterized. Compound 1 represents one new heterometallic Ag–Pb–I compound characteristic of the chain-like [Ag₂PbI₆]_n²ⁿ⁻ anions. Compound 2 features the straight one-dimensional (1D) [Ag₇I₉]_n²ⁿ⁻ anionic moieties, while compound 3 contains infrequent two types of curved [Ag₅I₈]_n³ⁿ⁻ anions. Optical properties reveal that the title compounds exhibit interesting semiconductor behaviors with the band gaps of 1.59–2.78 eV, which endow them with good photoelectric switching performances under the alternate light irradiations. We also present their Hirshfeld surface analyses, and the theoretical studies (band structures, density of states (DOS) and partial density of states (PDOS)).

1. Introduction

Organic–inorganic hybrid metal halides continue to fascinate researchers, which not only profit from their diversified structures, but also from their unusual photophysical performances inherited from the synergistic combinations of both organic and inorganic components [1,2,3]. Among them, iodoargentate-based hybrids are of special interest by virtue of the wide applications in many fields, such as white light emission, thermo-/photo-chromism, photocurrent response, dye sorption/separation and photocatalysis, gaining more and more attention in recent years [4,5,6,7,8,9].

From the standpoint of anionic moieties, Ag⁺ ion possesses the flexible coordination modes comprising the [AgI₂] dumbbell, [AgI₃] triangle, [AgI₄] tetrahedron and [AgI₆] octahedron [10,11]. More attractively, these primary building units also feature high self-assembly characterizations, which provide the abundant possibilities of generating numerous secondary building units (SBUs). By sharing of corner/edge/face, together with the short interactions of Ag⋯Ag, a large number of SBUs, such as [Ag₅I₆]⁻, [Ag₃I₇]⁴⁻, [Ag₅I₉]⁴⁻, [Ag₆I₁₀]⁴⁻, [Ag₆I₁₁]⁵⁻, [Ag₃I₈]⁵⁻, [Ag₅I₁₀]⁵⁻, [Ag₇I₁₂]⁵⁻, [Ag₄I₁₀]⁶⁻, [Ag₆I₁₂]⁶⁻, [Ag₈I₁₄]⁶⁻, [Ag₁₁I₁₈]⁷⁻, [Ag₁₀I₁₇]⁷⁻, and [Ag₁₀I₁₈]⁸⁻ have been successively found [10,11]. From the perspective of cationic components, hitherto, multiple types of templates (e.g., aliphatic amines, aromatic amines and metal complexes) have been broadly exploited to decorate and prepare the novel iodoargentate-based hybrids, which range from the zero-dimensional (0D) discrete clusters, one-dimensional (1D) chains, two-dimensional (2D) layers, and three-dimensional (3D) frameworks [10,11]. Of these, the metal complex cations have received extensive research interest due to the stable and strong structural directing effects. More importantly, most of them are optically active or photosensitive, and their introduction may endow the as-synthesized materials with intriguing physical and chemical properties. Representative examples are [M(bipy)₃]Ag₃I₅ (M = Zn, Ni, Co, Fe, Mn; bipy = 2,2-bipyridine), K[M(bipy)₃]₂Ag₆I₁₁ (M = Zn, Ni, Co, Fe, Mn), [(Ni(bipy)₃][H-bipy]Ag₃I₆, [M(bipy)₃]Ag₅I₇ (M = Co, Zn, Ni), [Co(bipy)₃]Ag₃I₆, {[Ni(bipy)(THF)₂(H₂O)₂](Ag₁₀I₁₂)₃·2DMF}_n (THF = tetrahydrofuran, DMF = N,N-dimethylformamide), [Co(phen)₃]Ag₂I₄·3DMF, and [Cd(phen)₃]₂Ag₁₃I₁₇ [9,12,13,14,15,16,17]. In addition, some heterometallic phases including [Co(bipy)₃]AgPb₂I₇, [Ni(phen)₃]Ag₂PbI₆, [Ni(bipy)₃]AgBiI₆, [Co(bipy)₃]₂Ag₄Bi₂I₁₆, etc., have also been isolated [18,19,20,21]. Among them, compound K[Fe(bipy)₃]₂Ag₆I₁₁ exhibits the visible light-driven photocatalytic decontamination of organic pollutant, while compound [Ni(bipy)₃]AgBiI₆ displays the photocurrent response behavior under the alternating light radiation [13,20]. It is worth noting that most existing metal complex-decorated iodoargentate-based hybrids focus on the bipy ligands. Relatively speaking, the instances with other types of ligands are rarely documented. Therefore, enriching the new members of metal complex-templated iodoargentate family and further exploring their structure–activity relationships are still of great significance but challenging.

In general, the former transition metals (e.g., Ni, Co, Mn, and Fe) are easy to be coordinated by the chelated amines or organic ligands. As such, with the in-situ-formed metal complexes as charge-compensating agents, we successfully isolated three new iodoargentate-based hybrids, namely [Co(phen)₃]Ag₂PbI₆ (1), [Ni(5,5-dmpy)₃]Ag₇I₉·CH₃CN (2) and [Co(5,5-dmpy)₃]Ag₅I₈ (3). Further studies show that the title compounds possess the chain-like structures, exhibiting the semiconductor behaviors with the optical band gaps of 1.59–2.78 eV. In addition, they also display rapid and stable photoelectric converting performances under the alternate light illuminations, with photocurrent densities comparable to those of many halide analogues. This work also provides Hirshfeld surface analyses and theoretical results.

2. Results and Discussion

2.1. Crystal Structures

2.1.1. Structural Description of [Co(phen)₃]Ag₂PbI₆ (1)

X-ray crystallographic analyses showed that compound 1 has the 1D [Ag₂PbI₆]_n²ⁿ⁻ anionic chains that were charge balanced by the [Co(phen)₃]²⁺ cations (Figure 1). The asymmetric unit of compound 1 is described in Figure 1a, consisting of one in-situ-generated [Co(phen)₃]²⁺ cation, two Ag⁺ ions, one Pb²⁺ ion and six I⁻ ions. In 1, Co²⁺ ions feature the octahedral coordination configurations, with Co–N bond lengths and N–Co–N angles of 2.134(13) to 2.198(13) Å and 76.9(5) to 168.3(6)°, respectively. In the 1D anionic moiety of [Ag₂PbI₆]_n²ⁿ⁻, each Ag atom is 4-coordinated, giving the distorted tetrahedral configuration. The Ag–I bond lengths and I–Ag–I angles vary from 2.780(2)–2.991(2) Å and 98.14(7)–124.21(8)°, respectively. The Pb²⁺ ions lie in the ψ-{PbI₄} coordinated environment with Pb–I bond lengths of 2.832(19)–3.4246(15) Å and I–Pb–I bond angles of 89.32(4)–162.62(4)°, respectively. Interestingly, the I atoms exhibit three coordination modes: I(2) and I(3) act as the terminal atoms, I(4) is the μ₃-I atom connecting two Ag⁺ ions and one Pb²⁺ ion, and I(1), I(5) and I(6) serve as bridging modes linking two metal centers (Figure 1b). The above-mentioned coordination styles and observed values are normal and are comparable to some related analogues, such as [(Me)₂DABCO]Ag₂PbBr₆, [Ni(bipy)₃]AgPb₂Br₇ and [La(DMSO)₈]Ag₂Pb₂I₉ [18,22,23].

As presented in Figure 1b, two [AgI₄] tetrahedra are condensed by edge-sharing to form the [Ag₂I₆] dimer. Noteworthily, the distance of Ag⋯Ag in the [Ag₂I₆] dimer reaches 3.234(4) Å, which is less than the sum of the van der Waals radii of Ag (3.44 Å), suggesting the significant metal–metal interactions [24]. This phenomenon is also observed in the case of [Fe(bipy)₃]AgBiI₆, [Zn(phen)₃]₂Ag₂Bi₂I₁₂, [Co(bipy)₃]₂Ag₄Bi₂I₁₆, [Ce(DMF)₈]₂Ag₁₀Pb₃I₂₂, [Ni(bipy)₃]AgPb₂I₇, [Fe(H₂O)₆]Ag₁₅I₁₈, [Cd(phen)₃]₂Ag₁₃I₁₇ and [Co(bipy)₃]Ag₃I₆ [15,17,18,20,21,25,26,27]. Then, one [Ag₂I₆] unit joins two ψ-{PbI₄} blocks through the I(4) atoms to generate the [Ag₂Pb₂I₁₂]⁶⁻ moiety. Every two adjacent [Ag₂Pb₂I₁₂]⁶⁻ units are further connected by the [Ag₂I₄] units to result in the curve-like [Ag₂PbI₆]_n²ⁿ⁻ anionic chain extended along the a-axis. The in-situ-produced [Co(phen)₃]²⁺ complexes serve as the structural directors and charge balancers, separating the anionic components and forming the 3D supramolecular framework through the extensive hydrogen bond contacts (Figure 1c). The C–H⋯I hydrogen bond lengths and C–H⋯I hydrogen bond angles are between 3.78(2)–3.968(16) Å and 129.9–155.0°. In addition to the C–H⋯I hydrogen bonds, compound 1 also contains the π⋯π and C–H⋯π interactions (Figure 1d,e).

2.1.2. Structural Description of [Ni(5,5-dmpy)₃]Ag₇I₉·CH₃CN (2)

X-ray crystallographic studies revealed that compound 2 features the straight 1D [Ag₇I₉]_n²ⁿ⁻ anionic chains that were isolated by the [Ni(5,5-dmpy)₃]²⁺ metal complexes (Figure 2). The asymmetric unit of compound 2 includes one formula unit, that is, seven Ag⁺ ions, nine I⁻ ions, one in-situ-formed [Ni(5,5-dmpy)₃]²⁺ cation and one free CH₃CN molecule (Figure 2a). In the anionic moiety of [Ag₇I₉]_n²ⁿ⁻, except for the triangular Ag(4) atom, the other Ag⁺ ions adopt the distorted tetrahedral coordination styles, with the Ag–I bond distances and I–Ag–I angles lying in the range of 2.6972(19)–3.1850(2) Å and 87.86(5)–130.84(7)°, respectively. For I atoms, interestingly, three different coordination modes are presented in compound 2: I(1), I(7) and I(9) act as the bridging atoms; I(4), I(6) and I(8) are the μ₃-I atoms linking three Ag⁺ ions; I(2), I(3) and I(5) are the μ₄-I atoms that were bonded to four Ag⁺ ions. Like the case of 1, the cationic metal centers are surrounded by six N atoms from three organic ligands, with Ni–N bond lengths and N–Ni–N angles of 2.0310(9) to 2.1010(11) Å and 78.6(4) to 177.2(4)°, respectively.

As shown in Figure S2a, edge-sharing of four [AgI₄] tetrahedra to produce the [Ag₄I₉] unit, which was encapsulated by two [AgI₄] tetrahedra and one [AgI₃] triangle by sharing the I(2), I(3), I(4) and I(5) atoms to obtain the [Ag₇I₁₃] moiety (Figure S2b). In the [Ag₇I₁₃] moiety, noteworthily, the shortest Ag⋯Ag separation achieves 2.9610(2) Å, implying the obvious d¹⁰-d¹⁰ argentophilic interactions [24]. Every two centrosymmetrical [Ag₇I₁₃] moieties are further condensed by the I(1), I(2), I(7) and I(8) atoms to give the [Ag₁₄I₂₂] moieties (Figure S2c), which further link the adjacent ones to get the complex [Ag₇I₉]_n²ⁿ⁻ anionic chains that straightly extended along the b axis (Figure 2b). The [Ag₇I₉]_n²ⁿ⁻ anionic moieties collaborate with the [Ni(5,5-dmpy)₃]²⁺ cations and lattice CH₃CN molecules, resulting in the 3D supramolecular framework with the help of extensive C–H⋯I hydrogen bonds (Figure 2c). The C–H⋯I hydrogen bond lengths and C–H⋯I hydrogen bond angles scatter from 3.857(13)–3.912(13) Å and 123.2–147.1°, respectively.

2.1.3. Structural Description of [Co(5,5-dmpy)₃]Ag₅I₈ (3)

Compound 3 exhibits two types of curved [Ag₅I₈]_n³ⁿ⁻ anions (Figure 3). As described in Figure S3, its asymmetric unit was composed of four Ag⁺ ions (Ag(2), Ag(3), Ag(4) and Ag(5)), two halves of Ag⁺ ions (Ag(1) and Ag(6)), seven I⁻ ions (I(1), I(2), I(3), I(6), I(7), I(8) and I(9)), two halves of I⁻ ions (I(4) and I(5)) and one in-situ-formed [Co(5,5-dmpy)₃]³⁺ cation. In compound 3, except for the Ag(6), the other Ag⁺ ions are surrounded by four I⁻ ions, while the I⁻ ions present two different bonding patterns (I(1), I(3), I(4), I(5), I(6), I(8) and I(9) are μ₂-I atoms; I(2) and I(7) are μ₄-I atoms). The coordination modes of Co³⁺ ions, the observed bond lengths of Ag–I and Co–N, and the angles of I–Ag–I and N–Co–N are also unexceptional and coincide with those in the literature [9,13,15].

Compound 3 exhibits a captivating structural characteristic, wherein it encompasses two distinct types of anionic chains. As depicted in Figure S4a, face-sharing of three [AgI₄] tetrahedra to produce a [Ag₃I₆] trimer, which connects a [Ag₂I₇] unit by I(2) and I(3) atoms to form a [Ag₅I₉] moiety. Every two neighboring [Ag₅I₉] moieties are interconnected via I(5) atoms to obtain the [Ag₅I₈]_n³ⁿ⁻ anionic chain denoted as “type A” (Figure 3a). Two [AgI₄] tetrahedra and one [AgI₃] triangle are condensed by I(7) and I(9) atoms to generate another [Ag₃I₆] trimer, which connects two [AgI₄] tetrahedra through the edge-sharing to create the [Ag₅I₁₀] unit (Figure S4b). Such [Ag₅I₁₀] units are further interconnected via edge-sharing to result in the final [Ag₅I₈]_n³ⁿ⁻ anionic chain denoted as “type B” (Figure 3b). The [Co(5,5-dmpy)₃]³⁺ cation acting as the charge-balancing agents isolated the above-mentioned two types of anionic chains, forming the extensive C–H⋯I hydrogen bonding interactions, with the bond lengths and angles in the range of 3.71(2)–4.00(2) Å and 127.6–172.5°, respectively (Figure 3c).

2.2. Hirshfeld Surface Analyses

In order to visually and quantitatively determine the percentage of the area occupied by different types of intermolecular interactions, we performed the Hirshfeld surface analyses for title compounds. The full interactions were depicted in Figure 4a–c, with a scattering range of 2.6 ≤ d_e + d_i ≤ 5.8 Å for 1, 2.4 ≤ d_e + d_i ≤ 5.8 Å for 2, 2.0 ≤ d_e + d_i ≤ 5.8 Å for 3, respectively. The decomposed 2D fingerprint plots showed that the H⋯I/I⋯H hydrogen bonds emerged like two wings of a bird, which accounted for the highest proportion of the total Hirshfeld surface area (35.1% for 1, 32.0% for 2 and 40.8% for 3; Figure 4d–f). For compound 1, the ratio of H⋯I contacts is 13.9% (top left), which is smaller than the ratio of I⋯H contacts (21.2%, bottom right). Similar case has also been observed in compounds 2 and 3. Further investigations revealed that the second contributors are due to the H⋯H contacts. As shown in Figure 4g–i, they mainly occurred in the intermediate region, making up 20.7% (1), 31.0% (2) and 26.2% (3) of the total surface. Furthermore, the crystal packing of compound 1 is significantly influenced by the C⋯H/H⋯C and C⋯C contacts. In general, these contacts are frequently employed to highlight the C–H⋯π (17.2%) and π⋯π (3.2%) interactions, thus underscoring their vital significance in the overall structure (Figure S5a,c). Although compound [Ni(phen)₃]Ag₂PbI₆ is isomorphic with 1, the C–H⋯π and π⋯π interactions have different contribution proportions, accounting for 10.0% and 16.0% of total Hirshfeld surface, respectively [19]. It is noteworthy that the π⋯π interactions are not obvious in compounds 2 and 3. These findings are consistent with the results of crystal structure analyses. The comparative contributions from other interactions are illustrated in Figure 4j–l. Clearly, the interactions associated with hydrogen atoms contribute greatly to their structural stabilities. The occurrence of this phenomenon is unsurprising and frequently observed in numerous metal halides, like [NH₄][Fe(bipy)₃]₂[Ag₆Br₁₁], [Zn(bipy)₃]₂Ag₂BiI₆(I)_1.355(I₃)_1.645, [Fe(phen)₃]₂Ag₃Pb₂I₁₁, [Ni(phen)₃]Ag₂PbI₆ and [Ni(5,5′-dmbpy)₃]₂Ag_4.9I_8.9·4H₂O [19,28,29,30,31]. More Hirshfeld surface comparisons of compounds 1–3 with some related analogues are listed in Table S10.

2.3. Optical Properties

The UV–Vis diffuse reflectance spectra, obtained from powder samples at room temperature, show that the optical absorption edges of title compounds are estimated to be 2.24 eV for 1, 2.78 eV for 2, and 1.59 eV for 3, respectively (Figure 5a–c). This indicates that the title compounds are potential visible light responsive semiconductors. In addition, these band values are very consistent with their respective crystal colors and match well with those of some reported Ag-based metal halide analogues, e.g., K[Fe(bipy)₃]₂Ag₆I₁₁ (1.66 eV), K[Co(bipy)₃]₂Ag₆I₁₁ (1.75 eV), [Fe(phen)₃]₂Ag₃I₇ (1.78 eV), [NH₄][Fe(bipy)₃]₂Ag₆Br₁₁ (1.90 eV), [Ni(bipy)₃]AgPb₂I₇ (1.92 eV), [Co(bipy)₃]Ag₃I₆ (2.03 eV), [Ni(5,5-dmbpy)₃]₂Ag_4.9I_8.9·4H₂O (2.07 eV), [(Me)₂-DABCO]₂Ag₅Pb₂I₁₃ (2.33 eV), [Ni(dien)(MeCN)₃]Ag₂Pb₃I₁₀·MeCN (2.42 eV), [V(DMSO)₅(H₂O)]Ag₆I₈ (2.61 eV), [Ni(phen)₃]Ag₂I₄·3DMF (2.64 eV) and [Co(phen)₃]₂Ag₁₁I₁₅·H₂O (2.84 eV) [9,13,15,17,18,22,28,31,32,33,34].

2.4. Photocurrent Responses

Inspired by the semiconductor nature of title compounds, we have further tested their photoelectric performances in a KCl solution using a classic three-electrode configuration, which were commonly used to evaluate the potential applications in the photovoltaic field. The photocurrent–time curves with switching interval of 20 s were recorded in Figure 6. It can be seen that they have the obvious photocurrent response behaviors under the alternating light irradiation. The average visible light photocurrent densities of compounds 1–3 are 0.16, 0.14 and 0.14 μA cm⁻² (Figure 6a–c), respectively, which are well comparable with some high-performance metal halides, such as [Ag₂I₂(phen)]_n, {[Nd₂(dpdo)(DMF)₁₄](Ag₁₂I₁₈)}_n, [Ni(phen)₃]Pb₂I₆·CH₃CN, [Co(bipy)₃]₂Pb₈I₂₁, {[La(dpdo)(DMF)₆](Bi₂I₉)₂}_n, [Ni(bipy)₃]AgBiI₆, and [Zn(bipy)₃]₂Ag₂BiI₆(I)_1.355(I₃)_1.645 [20,29,35,36,37,38]. High photocurrent densities indicate that they possess the satisfactory transfer capacity of charge carriers. In addition, there are no substantial declines of the photocurrent switching ratios after multiple cycles, which prove the excellent stability and repeatability. This is significantly superior to [CH₃NH₃]PbI₃, a classic perovskite photovoltaic material, which usually became unstable due to the hydrolysis instability when exposed to the light irradiation. More importantly, their photoelectric switching abilities could be further enhanced with the conversion of visible light to the full spectrum condition (Figure 6d–f). Such a phenomenon is normal and has also been observed in the case of [NH₄][Fe(bipy)₃]₂Ag₆Br₁₁, [Pb(MCP)₂I]PbI₃, [Zn(phen)₃]₂Ag₂Bi₂I₁₂, and [Co(bipy)₃]₂Ag₄Bi₂I₁₆ [21,25,28,39].

2.5. Theoretical Studies

In order to gain a deeper understanding of the optical properties and photoelectric behaviors of title compounds, this study utilizes density functional theory (DFT) and the first-principles approach to analyze their electronic structures (Figure 7). According to the Figure 7a, the valence band (VB) maximum and the conduction band (CB) minimum of compound 1 are located at the same symmetric point (Γ), indicating the direct band gap semiconductor character. Compounds 2 and 3, in contrast to 1, are indirect band gap materials. In detail, the VB maximum are both situated at Γ point, while the CB minimum appear at the D for 2 and C points for 3, respectively (Figure 7b,c). Specially, compounds 2 and 3 exhibit obvious band dispersions, generally meaning the small effective mass and the good carrier transport. According to theoretical calculations, the band gap values of compounds 1–3 are found to be 1.29, 1.82, and 1.06 eV, respectively. However, upon comparison with the experimental results of 2.24, 2.78, and 1.59 eV, it is evident that the theoretical values underestimate the true values. This underestimation is a common phenomenon and can be attributed to the limitations of DFT calculations [40,41].

By examining the DOS and PDOS diagrams of compound 1, it can be observed that the bottom of CB is primarily influenced by the Co–3d, N–2p and C–2p orbitals through local interactions (Figure 7d and Figure S15). The VB near the Fermi level is mainly contributed by the 3d states of Co and Ag mixed with the 5p state of I. Note that the contribution of Pb²⁺ ions to the front orbitals near the Fermi level is negligible. In addition, the contribution in the region of −4 to −15 eV is almost from the C–2p and I–5s states. For compound 2, the top part of VB mainly originates from 5p state of I and 3d states of Co and Ag, while the CB minimum is mostly constituted by Ni–3d with small amounts of N–2p states (Figure 7e and Figure S16). For compound 3, the mixing of I–5p state makes up the bottom of the CB, and the VB is dominated by Co–3d, C–2p and N–2p states (Figure 7f and Figure S17). The results of this study indicate that the optical properties of the compounds mentioned are influenced by both organic and inorganic components, particularly the photosensitive metal complex cations. This discovery is consistent with previous studies on metal halides with optical activities, such as [Ni(phen)₃]Ag₂PbI₆, [Fe(bipy)₃]AgPb₂Br₇, [NH₄][Ni(phen)₃]BiI₆, [Co(phen)₃]Pb₂Br₆, [Ni(bipy)₃]AgBiI₆, [Zn(bipy)₃]₂Ag₂BiI₆(I)_1.355(I₃)_1.645 and [Co(bipy)₃]₂Ag₄Bi₂I₁₆ [18,19,20,21,29,42,43].

3. Materials and Methods

3.1. General Information

Silver iodide (AgI, Adamas, 98%), lead iodide (PbI₂, Aladdin, 98%), cobalt chloride hexahydrate (CoCl₂·6H₂O, Sinopharm, 99.9%), nickel chloride hexahydrate (NiCl₂·6H₂O, Sinopharm, 99.9%), potassium iodide (KI, Greagent, 99%), 1,10-phenanthroline (phen, Aladdin, 99%), 5,5-dimethyl-2,2-dipyridine (5,5-dmpy, Adamas, 98%), acetonitrile (CH₃CN, Kermel, 99.5%) and hydriodic acid (HI, Adamas, 55–57 wt %). All the starting chemicals in this study were commercially available and need not be further disposal when used.

Purity identifications of samples were conducted by a SmartLab diffractometer. Thermogravimeric curves of title compounds were obtained using a NETZSCH STA449C unit (N₂ atmosphere, 10 K/min). A Thermo Fisher GX4 scanning electron microscope and a 3600 SHIMADZU spectrometer were utilized to acquire the energy-dispersive X-ray (EDX) spectra and the solid optical diffuse reflectance data, respectively.

3.2. Syntheses

Synthesis of [Co(phen)₃]Ag₂PbI₆ (1). The mixtures of PbI₂ (0.332 g, 0.7 mmol), AgI (0.187 g, 0.8 mmol), KI (0.332 g, 2 mmol), CoCl₂·6H₂O (0.118 g, 0.5 mmol), phen (0.297 g, 1.5 mmol), hydroiodic acid (55–57 wt.%, 1 mL) and acetonitrile (99.5%, 4 mL) were placed in a 23 mL polytetrafluoroethylene-lined stainless steel autoclave. Subsequently, the temperature of resulting mixtures was maintained at 140 °C for 5 days. After the cooling and washing treatment, brown blocky crystals of 1 were harvested (38% yield based on PbI₂). Elemental analysis calcd (%) for C₃₆H₂₄Ag₂CoI₆N₆Pb (1): C 24.24, H 1.36, N 4.71; found: C 23.66, H 1.20, N 4.57.

Synthesis of [Ni(5,5-dmpy)₃]Ag₇I₉·CH₃CN (2). Compound 2 was also prepared by a typical solvothermal method. In detail, the chemicals including AgI (0.4108 g, 1.75 mmol), KI (0.2905 g, 1.75 mmol), NiCl₂·6H₂O (0.0594 g, 0.25 mmol), 5,5-dmpy (0.138 g, 0.75 mmol), hydroiodic acid (55–57 wt.%, 1 mL) and acetonitrile (99.5%, 4 mL) were added into a 23 mL polytetrafluoroethylene-lined container, which was kept at 140 °C for 5 days. After the filtration and the ethanol washing, pale-red block-like crystals were obtained by manual separation (yield: 36%, based on AgI). Elemental analysis calcd (%) for C₃₈H₃₉Ag₇I₉N₇Ni (2): C 17.90, H 1.54, N 3.85; found: C 16.95, H 1.50, N 3.70.

Synthesis of [Co(5,5-dmpy)₃]Ag₅I₈ (3). Compound 3 has the same synthesis process as compound 2, but with different materials and dosages. Specifically, the reagents of AgI (0.3228 g, 1.375 mmol), CoCl₂·6H₂O (0.0595 g, 0.25 mmol), 5,5-dmpy (0.138 g, 0.75 mmol), hydroiodic acid (55–57 wt.%, 1 mL) and acetonitrile (99.5%, 4 mL) were put in a 23 mL polytetrafluoroethylene-lined stainless steel autoclave. Then, the mixture was heated at 140 °C for 5 days, followed by the gradual cooling to room temperature. Using a similar processing method mentioned above, black block-typed crystals of 3 were found, with the yield of 23% based on CoCl₂·6H₂O. Elemental analysis calcd (%) for C₃₆H₃₆Ag₅CoI₈N₆ (3): C 19.96, H 1.68, N 3.88; found: C 19.92, H 1.61, N 3.76. The solvothermal syntheses of title compounds are summarized in Scheme 1.

3.3. X-ray Crystallography

Intensity data of title compounds were gathered on a Bruker SMART diffractometer (1, 3) with an APEX II CCD detector and an Xcalibur E Oxford diffractometer (2) with an Atlas CCD detector using the graphite monochromatic Mo-Kα radiation (λ = 0.71073 Å). Their structures were analyzed by a direct method and optimized on F² by full-matrix least-squares technique using the SHELXTL–2014 program [44]. All non-hydrogen atoms were arranged anisotropically, and the hydrogen linked to carbon atoms were presented geometrically and refined isotropically under a fixed thermal factor. Their crystal data and structure refinement details are summarized in

Table 1. Crystallographic data of compounds 1–3.

	1	2	3
formula	C36H24Ag2CoI6N6Pb	C38H39Ag7I9N7Ni	C36H36Ag5CoI8N6
fw	1783.87	2549.66	2166.19
crystal system	triclinic	monoclinic	monoclinic
space group	P-1 (No. 2)	P21/n (No. 14)	P2/c (No. 13)
a/Ǻ	12.0376(11)	17.9597(8)	22.867(2)
b/Ǻ	13.8472(12)	13.1590(5)	13.4056(14)
c/Ǻ	14.8853(13)	24.1497(11)	17.2141(18)
α/°	112.785(4)	90	90
β/°	94.948(2)	101.825(4)	100.819(4)
γ/°	101.542(3)	90	90
V/Å3	2204.2(3)	5586.2(4)	5183.0(9)
Z	2	4	4
T/K	298(2)	293(2)	293(2)
λ/Ǻ	0.71073	0.71073	0.71073
ρcalcd/g cm−3	2.688	3.032	2.776
F(000)	1606	4600	3920
Rint	0.0465	0.0396	0.1065
GOF	1.091	1.032	1.029
R1 [I > 2s(I)]	0.0725	0.0315	0.0776
wR(F2) [I > 2s(I)]	0.2053	0.0667	0.1942
CCDC	2,282,554	2,282,555	2,282,556

. The selected bond lengths and angles, hydrogen bond data, C–H⋯π interactions and π⋯π interactions are listed in Tables S1–S9.

3.4. Photocurrent Measurements

The photocurrent experiments of title compounds were carried out on a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). In this study, we used a typical three-electrode configuration (the sample-coated ITO glass was the working electrode, the platinum wire was the counter electrode, and Ag/AgCl was the reference electrode). The sample/ITO electrode was prepared by the solution coating method. First, 5 mg of powder samples were added into a mixed solvent containing 475 µL ethanol and 25 µL Nafion. Ultrasonic treatment was carried out for 2 h, and the obtained solution was dropped on the surface of the pre-polished ITO glass, and then dried at room temperature. A 300-W Xenon lamp equipped with/without a 420-nm cut-off filter was used for the visible light and full spectrum light source. The electrolyte solution is the KCl solution (0.1 M).

3.5. Calculation Details

The calculation results including band structures, DOS and PDOS were obtained by density functional theory (DFT) using the VASP program. The exchange correlation function adopts the Perdew Burke Ernzerhof (PBE) function of the Generalized Gradient Approximation (GGA) [40,41]. Herein, H–1s¹, C–2s²2p², N–2s²2p³, Co–3d⁷4s², Ni–3d⁸4s², Ag–4d¹⁰5s¹, Pb–6s²6p² and I–5s²5p⁵ were served as the valence electrons. A cutoff energy of 500 eV was applied to define the number of plane waves. The numerical integration of the Brillouin zone was performed using Monkhorst-Pack k–point sampling of 3 × 3 × 3 for 1, 3 × 5 × 3 for 2 and 1 × 2 × 2 for 3, respectively.

4. Conclusions

In summary, the solvothermal syntheses, crystal structures, optical/photoelectric properties and theoretical studies of three new iodoargentate-based hybrids with metal complex cations were reported here. Compounds 1–3 possess chain-like structures, exhibiting semiconductor behaviors with the band gaps of 1.59–2.78 eV. Furthermore, the title compounds display interesting photoelectric switching performances upon the alternate light irradiations. Further work will focus on the fabrications of more metal complex-directed halide analogues and the in-depth understandings of their structure–activity relationships.