Cs₂CO₃ (17.0 mg, 0.052 mmol), CoCl₂·6H₂O (24.0 mg, 0.077 mmol), As₂S₃ (12.0 mg, 0.049 mmol) and Se (18.0 mg, 0.228 mmol), and a mixed solvent of tren (500 mg, 3.424 mmol) and C₂H₅OH (250 mg, 5.434 mmol) were added to Pyrex glass tube. The glass tube was sealed with a 10% filling, placed into a Teflon-lined stainless steel autoclave and heated at 150°C for 7 d. The products were washed with ethanol and deionized water, respectively, and dark yellow blocks crystal were obtained (27% yield based Se). Elemental analysis for 1: C 19.31%, H 4.92%, N 15.05%. Calc.: C 19.38%, H 4.97%, N 15.07%.

NiCl₂·6H₂O (24.0 mg, 0.101 mmol), As₂S₃ (12.0 mg, 0.049 mmol), Se (16.0 mg, 0.203 mmol), and a mixed solvent of dien (630 mg, 4.315 mmol) and CH₃OH (240 mg, 7.490 mmol) were added to Pyrex glass tube. The glass tube was sealed with a 10% filling, placed into a Teflon-lined stainless steel autoclave and heated at 150°C for 7 d. The products were washed with ethanol and deionized water, respectively, and red blocks crystal were obtained (29% yield based Se). Elemental analysis for 2: C 17.84%, H 4.79%, N 15.58%. Calc.: C 17.90%, H 4.84%, N 15.67%.

Cs₂CO₃ (17.0 mg, 0.052 mmol), Zn(Ac)₂·2H₂O (22.0 mg, 0.100 mmol), As₂S₃ (12.0 mg, 0.049 mmol), Se (16.0 mg, 0.203 mmol), and a mixed solvent of tren (800 mg, 5.479 mmol) and PEG-400 (250 mg, 4.03 mmol) were added to Pyrex glass tube. The glass tube was sealed with a 10% filling, placed into a Teflon-lined stainless steel autoclave and heated at 160°C for 6 d. The products were washed with ethanol and deionized water, respectively, and yellow blocks crystal were obtained (31% yield based Se). Elemental analysis for 3: C 14.81%, H 3.66%, N 11.52%, calc.: C 14.88%, H 3.71%, N 11.57%.

K₂CO₃ (14.0 mg, 0.111 mmol), MnCl₂·4H₂O (12.0 mg, 0.061 mmol), As₂S₃ (12.0 mg, 0.049 mmol), Se (16.0 mg, 0.203 mmol), and tren of solvent (600 mg, 4.109 mmol) to a Pyrex glass tube. The glass tube was sealed with a 10% filling, placed into a Teflon-lined stainless steel autoclave and heated at 150°C for 7 d. The products were washed with ethanol and deionized water, respectively, and yellow rodlike crystal were obtained (21% yield based Se). Elemental analysis for 4: C 15.16%, H 3.77%, N 11.77%, Calc: C 15.21%, H 3.80%, N 11.83%.

It has been widely observed that transition metal complexes are useful as template or structural-directing agents in the synthesis of chalcogenides. In this work, we have synthesized successfully four novel selenoarsenates in amine-alcohol system by solvothermal method, [Co(tren)₂H]AsSe₄ (1); [Ni₂(dien)₄][As₂Se₅] (2); [Zn(tren)]₂[As₂Se₅] (3) and [Mn(tren)]₂[As₂Se₅] (4). When organic amines and second agents (e.g., methanol, ethanol, polyethylene glycol) are used as mixed solvents in the synthesis of compounds 1, 2 and 3, their effects on crystallization are favorable. Conversely, when the second agent was not present, there was a significant loss of yield. In all probability, this is due to the drastic changes in some physical properties of the solvent (e.g., pH, density, viscosity, and diffusion coefficient), which contribute to the increased solubility and diffusivity of the reactants, as well as crystal growth. Furthermore, we found that adding K₂CO₃/Cs₂CO₃ was necessary for the synthesis of compounds 1, 3 and 4. If K₂CO₃/Cs₂CO₃ is removed from the reaction system, then the target product is not obtained, indicating its role as a mineralizer. The mineralizer may not be involved in the crystal structure but is crucial to the preparation of chalcogenides.

Compound 1 crystallizes in the monoclinic crystal system in space group P2₁/n, and it consist of discrete tetrahedral AsSe₄ and trigonal-bipyramid [Co(tren)₂]²⁺ (Figure 1). The arsenic atoms have the pentavalent state, and they form tetrahedra AsSe₄ by bonding with four Se atoms. Co²⁺ coordinates with four N atoms of one tren ligand and one N atom of another tren ligand to form a trigonal-bipyramid [Co(tren)₂]²⁺ complex cation. In the compound [Co(phen)₃][As₂Se₂(μ-Se₃) (μ-Se₅)] (Zhao et al., 2011a), however, arsenic atoms adopt a trivalent state. And the [As₂Se₂(μ-Se₃) (μ-Se₅)]²⁻ anion contains two crystallographically As³⁺ centres, and each is coordinated by a terminal Se²⁻ to give AsSe⁺ units. The AsSe⁺ units are joined together by μ-Se₃ ²⁻ and μ-Se₅ ²⁻ bridging ligands to give rise to a one-dimensional chain [As₂Se₂(μ-Se₃) (μ-Se₅)]²⁻ (Du et al., 2019). Moreover, in the compound [Co(peha)][Co(As₃S₃)₂] (Han et al., 2016), the arsenic atom binds three S²⁻ ions, forming a typical trigonal pyramid AsS₃. The adjacent trigonal pyramid AsS₃ unit is connected by an arsenic atom via two S²⁻ ions, forming As₃S₃ aggregation. Also the two As₃S₃ aggregates coordinate to the Co²⁺ ion via two arsenic atoms and an S atom to form [Co(As₃S₃)₂]₂ cluster. For compound 1, the As-Se bonds range between 2.2679(13) and 2.2932(11) Å, while Se-As-Se bond angles range from 106.74(5) to 112.48(5) ^°. The Co-N bond length ranges from 2.057(8) to 2.258(6) Å with N-Co-N bond angles from 79.3(3) to 178.4(2) ^°. The bond lengths and angles reported here are similar to those reported previously (Zhao et al., 2011a; Han et al., 2016).

Compound 2 crystallizes in the monoclinic crystal system in space group P2₁/n. Two [AsSe₃] trigonal pyramids are joined via corner sharing to form the [As₂Se₅]⁴⁻ anion (Figure 2A). The As(1), Se(2) and Se(3) atoms are disordered. As shown in Figure 2B, the Ni²⁺ ion is coordinated by six N atoms to produce octahedral [Ni(dien)₂]²⁺. Supplementary Figure S1 shows the cluster structure of the compound 2. Unlike the compound 2, [Ni(en)₃]₂[As₂S₅] contains arsenic atoms that adopt a pyramidal coordination geometry by bonding with three S atoms to form the [AsS₃] pyramid. Two [AsS₃] pyramids form dimeric [As₂S₅]⁴⁻ anion by corner-sharing, and the two AsS₃ pyramids are in cis-conformation (Jia et al., 2006). For compound 2, the As-Se bonds range from 1.8755(18) to 2.4404(19) Å and Se-As-Se bond angles range from 52.91(8) to 133.15(8)^°. The Ni-N bond lengths range from 2.081(6) to 2.135(6) Å while the N-Ni-N bond angles range from 82.2(2) to 178.9(2)^°. According to literature, bond lengths and angles are consistent (Jia et al., 2006; Du et al., 2019).

Compound 3 crystallizes in the monoclinic crystal system in space group C2/c. In contrast to compound 1 and 2, in compound 3 the arsenic atoms are trivalent and they join three Se²⁻ ions to form pyramid AsSe₃, followed by two trigonal pyramid AsSe₃ units are linked to μ ₂ -Se(3) to form dimers [As₂Se₅]⁴⁻. Figure 3 shows the trigonal-bipyramidal unsaturated [Zn(tren)]²⁺ caiton formed by the coordination of the Zn²⁺ ion with four N atoms of the tren ligands and one μ ₂ -Se atom. And two unsaturated [Zn(tren)]²⁺ caiton forms a cluster structure via dimeric [As₂Se₅]⁴⁻ linkage between them. The cluster structure of compound 3 is shown in Supplementary Figure S2. Comparison with only one [As₂Se₆]²⁻ anion type of zinc selenoarsenate [Zn(phen) (dien)][As₂Se₆]·2phen is quite different (Jia et al., 2011). In the [Zn(phen) (dien)][As₂Se₆]·2phen, the AsSe₃ pyramids are tied together with two Se-Se bonds, forming the dimeric anion [As₂Se₆]²⁻, each [As₂Se₆]²⁻ anion contains a six-membered ring. With compound 3, As-Se bond lengths range from 2.3181(10) to 2.4685(10) Å; Se-As-Se bond angles are between 86.92(3) and 105.74(4) ^°; and Zn-N bond lengths are between 2.074(6) and 2.416(6) Å; N-Zn-N bond angles are between 76.8(2) and 171.07(14) ^°. The bond lengths and angles are consistent with those reported in the literature (Jia et al., 2011; (Teri et al., 2021b). To the best of our knowledge, compound 3 is the first zinc selenoarsenate containing [As₂Se₅]^4- anion type.

Compound 4 crystallizes in the triclinic crystal system in space group P 1 ¯ . The arsenic atoms in compound 4 are coordinated in the same way as those in compound 3 and have the same dimeric [As₂Se₅]⁴⁻ unit. There are two kinds of coordination modes for Mn²⁺ ions: (1) μ ₅ -Mn(2)²⁺ ion is coordinated by four N atoms of one tren ligands and one μ ₂ -Se(1) atom to form trigonal-bipyramidal [Mn(tren)]Se (the [Mn(tren)]Se is outlined by a dashed line area in Figure 4); (2) μ ₆ -Mn(1)²⁺ ion is coordinated by four N atoms of one tren ligands and two μ ₃ -Se(5) atoms to formed octahedral [Mn(tren)]Se₂. Firstly, two octahedra [Mn(tren)]Se₂ sharing edges, forming a unsaturated [Mn(tren)]²⁺ caiton (the unsaturated [Mn(tren)]²⁺ caiton is outlined by a solid line area). Further, the cluster is connected to two trigone-bipyramidal [Mn(tren)]Se via the dimer [As₂Se₅]⁴⁻ to form [Mn(tren)]₄[As₄Se₁₀]. The cluster structure of compound 4 is shown in Supplementary Figure S3. When compared to any manganese selenoarsenate of the [As₂Q₅]⁴⁻ anion type, the structures are quite different. As an example, [{Mn(terpy)}₂(μ-As₂Se₅)] consists of dipyramidal [As₂Se₅]⁴⁻ ligands that span three Mn^II atoms in a tetradentate pattern of μ ₃-1κ ²Se¹, Se²:2κSe⁴: 3κSe⁵. Tetranuclear complexes are centrosymmetric and exhibit an 8-member ring (MnSeAsSe)₂ (Kromm and Sheldrick, 2008). Nevertheless, in [Mn(en)₃]₂As₂Se₅, there are isolated anions [As₂Se₅]⁴⁻ and transition metal cations [Mn(en)₃]²⁺. Initially, the dimeric [As₂Se₅]⁴⁻ anion was isolated and co-crystallized with transition metal complex cations as counterions, composed of two corner-sharing AsSe₃ trigonal pyramids (Jia et al., 2006). It has been proposed that compound Mn₂(2,2′-bipy)As₂ ^IIIS₅ is formed from four-cubane [Mn₆(2,2′-bipy)₄As₆ ^IIIS₁₄]²⁺, which are interlinked by face-sharing to form a two-dimensional network. There are also two coordination environments for the Mn atom. The Mn atom is coordinated by six sulfur atoms from three [As₂ ^IIIS₅]⁴⁻ groups; the Mn atom is chelated by two 2,2-bipy ligands and coordinated by four sulfur atoms from two [As₂ ^IIIS₅]⁴⁻ groups (Fu et al., 2006). In compound 4, As-Se bonds range from 2.3291(17) to 2.4628(17) Å with Se-As-Se angles between 102.04(6) and 105.29(6) ^°. Mn-N bond lengths range from 2.192(10) to 2.368(9) Å and N-Mn-N bond angles range from 75.8(4) to 117.8(4) ^°. According to the published literature, these bond lengths and angles are similar (Fu et al., 2006; Jia et al., 2006). We found that in compound 4, Mn²⁺ ions had two coordination modes, which was unusual in previous reports.

Compounds 3 and 4 have some special structural features. First, in compounds 3 and 4, it is the dimeric [As₂Se₅]⁴⁻ unit that is linked to the transition metal complex. To the best of our knowledge, this type of connection mode has never been done before. It appears that most of them are directly linked to transition metal complexes via the AsSe_x (x = 3,4) units (Chen et al., 2021; Zhao et al., 2011b). Second, [As₂Se₆]²⁻ anion type zinc selenoarsenate was synthesized in 2011 by Jia’s group (Jia et al., 2011). In addition, compound 3 is the first [As₂Se₅]⁴⁻ anion type of zinc selenoarsenate, which lays a foundation for future work. Lastly, in compound 4, the five- and six-coordinate manganese atoms are linked by dimeric [As₂Se₅]⁴⁻ units to form [Mn(tren)]₄[As₄Se₁₀] clusters. Therefore, compounds 3 and 4 present a new structural pattern.

In Supplementary Figure S4, the position of the 2θ diffraction peak obtained by the experiment is consistent with simulation results of the analysis of a single-crystal structure, showing that the products of the compounds are very pure, and all samples can be used for further study. The thermal stability of compounds 1-4 was studied by thermogravimetric and differential thermal analysis (Supplementary Figure S5). During the test, compound 1, the weight loss rate of 14% in the range of 181–286°C, which was consistent with the loss of one molecule of H₂Se (theoretical weight loss rate of 10.89%), and at approximately 400–550°C have a significant weight loss rate of 42% (the theoretical weight loss rate of 40.65%), which may be due to the loss of two molecules of tren organic amine, and was accompanied by an endothermic peak at 300 and 578°C in the DTA curve. The weight loss rate of compound 2 is 36% between 346 and 451°C, consistent with the loss of four molecules of dien ligand (theoretical weight loss rate of 38.48%), and the DTA curve shows an endothermic peak at 398°C. Compound 3 has a weight loss of 31% between 252 and 305°C, which is consistent with the loss rate of two molecules of tren ligand (30.22%), and has an endothermic peak at 323°C in the DTA curve. Compound 4 has a weight loss rate of 32% between 200 and 295°C, which is consistent with the loss of a molecule of tren ligand (theoretical weight loss rate of 30.88%). At the same time, the endothermic peak occurs at 249°C on the DTA curve. The appearance of an endothermic peak can be attributed to the formation of amorphous material by structural collapse at a given temperature.

FT-IR spectrum (Supplementary Figure S6) shows that compounds 1, 3, 4 mainly originate from organic solvent tren, and the absorption peaks of compound 2 are mainly due to dien. In the 1, 3, 4, a strong absorption peak between 3,196 cm⁻¹ and 3,092 cm⁻¹ is due to the stretching vibration of the N-H bond; the strong N-H bending vibration appears separately at 1,567, 1,624, 1,601 cm⁻¹; the C-H stretching vibration occurs between 2,968 and 2,838 cm⁻¹; the -CH₂- bending vibration peak individually appears at 1,466, 1,466, 1,452 cm⁻¹; C-C, C-N stretching vibration absorption peaks are located in the region of 1,389–1,002 cm⁻¹; weak N-H, C-H bending vibration peaks are located in the region of 994–523 cm⁻¹. In compound 2, the absorption peak at 3,196 cm⁻¹ and 3,104 cm⁻¹ is caused by N-H stretching vibrations. There is a weak C-H stretching vibration absorption peak in the range of 2,911–2,857 cm⁻¹; at 1,452 cm⁻¹, there is a strong -CH₂- bending vibration absorption peak; the absorption peaks in the range of 1,384–1,077 cm⁻¹ are the result of C-C and C-N stretching vibrations; there are weak N-H and C-H bending vibration absorption peaks in the range of 951–527 cm⁻¹.

Using a 300 W xenon lamp exposed to visible light (λ ≥ 420 nm) for photocurrent measurements, repeatable responses were observed for compounds 1–4. Compounds 1-4 exhibit good photocurrent profiles under visible light illumination as shown in Figure 5. Compounds 2 and 4 have twice the photocurrent density of 1 and 3. A high photocurrent indicates that the compound has a high photoelectron transfer efficiency, which provides evidence for the application of their photocatalytic properties. The photocurrent densities of compound 2 and 4 are much higher than those of other chalcogenides, including [pipH₂]₂[pipH]₂[In₂As^III ₂As^V ₂S_10.2Se_3.1(Se₂)_0.7] (ca. 47 nA/cm²) (Wang et al., 2020), Rb₂Ba₃Cu₂Sb₂S₁₀ (ca. 6 nA/cm²) (Liu et al., 2020), K₃Mn₂Sb₃S₈ (ca. 6 nA/cm²) (Xiao et al., 2021), K₂HgSnSe₄ (ca. 3 μA/cm²) (Teri et al., 2021a), Cs₂Ag₆As₂S₇ (ca. 5 μA/cm²) (Li et al., 2021), and [Zn(tren)₂H]SbSe₄ (ca. 10 μA/cm²) (Shele et al., 2021).

The variable-temperature magnetic susceptibility data were collected for compounds at an applied dc field of 1,000 Oe in the 2–300 K temperature range. The χ _M T vs T and χ _M vs. T plots for 1 and 2 are shown in Figure 6. For compound 1, the χ _M T value at 300 K is 2.51 cm³ mol⁻¹ K, and after cooling, the χ _M T value falls to 1.48 cm³ mol⁻¹ K at 2 K. Meanwhile, χ _M gradually increases from 0.008 cm³ mol⁻¹ at 300 K to a value of 0.739 cm³ mol⁻¹ at about 2 K. This character suggests the dominant antiferromagnetic interaction between the Co^II centers in 1. The 1/χ _M vs T curve above 50 K obeys the Curie-Weiss law with C = 2.54 cm³ mol⁻¹ K and θ = -0.65 K (Figure 6, insert). The negative θ value further confirms the antiferromagnetic coupling among the Co²⁺ ions. In compound 2, reached a maximum χ _M T value of 0.66 cm³ mol⁻¹ K at 300 K and a minimum value of 0.40 cm³ mol⁻¹ K at 2 K as temperature decreased. Furthermore, χ _M gradually increases from 0.002 cm³ mol⁻¹ at 300 K to a value of 0.203 cm³ mol⁻¹ at about 2 K. This character suggests the dominant antiferromagnetic interaction between the Ni^II centers in 2. The 1/χ _M vs T curve above 60 K obeys the Curie-Weiss law with C = 0.56 cm³ mol⁻¹ K and θ = -4.45 K. The negative θ value further confirms the antiferromagnetic coupling among the Ni²⁺ ions.