Synthesis and Characterisation of Iodobismuthates Containing N-substituted 1,4-Diazabicyclo[2.2.2]octane

Two new hybrid iodobismuthates, [C8H17N2][C10H22N2][BiI6] (1) and [C6H12N2]0.5[C10H22N2]3.5[Bi2I10][Bi2I9] (2), have been prepared by solvothermal synthesis in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO) and ethanol. Both compounds have been characterized by single-crystal and powder X-ray diffraction, infrared and UV–Vis spectroscopies and thermogravimetric analysis. Structure determination reveals that the crystal structure of 1 contains mononuclear [BiI6]3− anions, whilst 2 contains an unusual combination of dinuclear anions, [Bi2I9]3− and [Bi2I10]4−, consisting of two edge- and two face-sharing [BiI6]3− octahedra, respectively. Mono- and diethylated derivatives of DABCO, which are formed in situ under solvothermal conditions, act as countercations and are located between the discrete anions. The optical band gaps of 1 and 2, which are 2.29(1) and 2.03(2) eV respectively, are consistent with the red color of these compounds, and are comparable to the band gaps measured for other iodobismuthates containing discrete anions. Two new iodobismuthates, [C8H17N2][C10H22N2][BiI6] (1) and [C6H12N2]0.5[C10H22N2]3.5[Bi2I10][Bi2I9] (2), have been synthesized under solvothermal conditions, and their crystal structures determined by single-crystal X-ray diffraction.


Introduction
As an important class of low-dimensional hybrid materials, organic-inorganic halides of the type R x M y X z (where R is a protonated amine, M is a main-group metal and X is a halide) have attracted considerable interest, due to their structural diversity and optical and electronic properties [1,2]. This class of materials is exemplified by the lead perovskite MAPbI 3 (where MA = CH 3 NH 3 + ). MAPbI 3 has been found to be a remarkable photovoltaic material [3][4][5], which when used in single-junction solar cells has a conversion efficiency of 25% [6], comparable to those of commercial silicon-based solar cells. Given the lower toxicity of bismuth when compared to that of its neighbors in the periodic table [7], bismuth-based organic-inorganic halides, which show better stability under ambient atmosphere than lead perovskites, are attracting interest as environmentally friendly materials for optoelectronic applications [8][9][10].
In hybrid iodobismuthates, the Bi 3+ cation usually adopts a distorted octahedral coordination, with BiI 6 octahedra linked by vertex-, edge-or face-sharing into polynuclear anions [11][12][13][14][15][16]. A variety of polymeric, discrete polynuclear and mononuclear anionic units have been described previously [17]. The structures of the iodobismuthate anions formed are dependent on the size and shape of the organic countercations, as well as the synthetic conditions used [18,19]. Protonated 1,4-diazabicyclo [2.2.2] octane (DABCO) and its N-substituted derivatives have been found to act as countercations in a number of iodobismuthates, as exemplified by [C 6 [21]. We have recently shown that, in the absence of hydroiodic acid in solvothermal reactions, DABCO can also act as a linker, as exemplified by (C 6 H 12 N 2 )BiI 3 , in which pairs of edgesharing bismuth octahedra are linked by DABCO ligands into hybrid ribbons [24].
Here, we describe the solvothermal synthesis and characterization of two new hybrid iodobismuthates containing N-substituted DABCO (Scheme 1

Experimental Section
All compounds were synthesized in 23 mL Teflon-lined stainless-steel autoclaves. Ethanol (> 99.8%), BiI 3 (99%), AgI (99.9%), KI (≥ 99%), and DABCO (≥ 99%) were obtained from Sigma-Aldrich. For each reaction, the reagents were loaded into the Teflon liner and stirred for approximately 10 min, prior to the reaction vessel being sealed into the autoclave and heated. The heating and cooling rates were 0.83 °C min −1 . After cooling to room temperature, the products were collected by vacuum filtration and washed with ethanol and deionized water.

Single-Crystal X-ray Diffraction
Single-crystal X-ray diffraction data (Table 1) were collected using Mo Kα radiation (λ = 0.71073 Å) using an Agilent Gemini S Ultra diffractometer for 1 and a Rigaku XtaLAB Synergy diffractometer for 2. Preliminary data for the impurity present in the bulk sample of compound 2 were collected at the UK National Crystallography Service (Southampton, UK) [25] using a Rigaku XtaLAB Diffractometer with an AFC12 goniometer and a rotating anode Mo source. Data reduction was carried out in each case using CrysAlisPro [26]. The structures were solved using Superflip [27] and refined against F using the program CRYSTALS [28]. The crystal of 1, solved in space group P2 1 3, was found to be an inversion twin with a Flack parameter of 0.48 (2). Although Platon/ADDSYM suggests Pa3 as a possible space group for 1, refinements in this space group were not successful. The crystal structure in this space group contains only a single DABCO moiety and attached ethyl groups could not be modelled satisfactorily. As described below, two distinct DABCO moieties, with one and two ethyl groups attached, are identified in the final structure in P2 1 3 and are necessary in order to achieve charge balance.
Data for compound 2 were treated with SQUEEZE [29] to correct for the effects of disordered organic cations. SQUEEZE found a total void volume of 423 Å per unit cell, which contained 45 electrons. This is consistent with the presence of half a [Et 2 DABCO] 2+ moiety per unit cell (48 electrons).

Characterization
Powder X-ray diffraction patterns were collected at room temperature for the as-synthesized materials and for finelyground hand-picked crystals of 1 and 2 using a Bruker D8 Advance powder X-ray diffractometer (Cu K α1 radiation, λ = 1.5406 Å). Pawley refinements were performed within Topas [30] in order to confirm the identity of the crystals formed in the two reactions.  Additional characterization measurements were carried out on ground hand-picked crystals of 1 and 2. Thermogravimetric Analysis (TGA) was performed on a TA-TGA Q50 instrument, operating under a flowing nitrogen atmosphere. Data were collected from room temperature to 650 °C, at a rate of 10 °C/min. UV-Vis diffuse reflectance data were collected using a Perkin Elmer Lambda 35 UV-Vis spectrometer equipped with an integrating sphere and using BaSO 4 as a standard. The absorption data were calculated using the Kubelka-Munk function [31]. Fourier Transform infrared spectra were collected using a Perkin Elmer Spectrum 100 FT-IR spectrometer. Elemental analysis was carried out by MEDAC LTD.

Results and Discussion
Analysis of the powder X-ray diffraction data for the assynthesized products of the reactions described above indicates that, in addition to compounds 1 and 2, bismuth metal is always present (Supplementary Information), indicating that these reactions involve redox processes. In the case of the reaction producing 2, the solid product also contained a small amount of an unidentified impurity. Preliminary single-crystal diffraction data collected on this impurity indicates that it crystallises in the space group P 2 1 /c, with lattice parameters a = 9.0406(1), b = 16.0735(2), c = 34.1517(6) Å and β = 92.1522(13)°. Attempts to produce larger amounts of this impurity, to enable its full characterization, have so far been unsuccessful.
As illustrated in Fig. 1, there is good agreement between the experimental and calculated powder X-ray patterns, based on the structures determined for these compounds using single-crystal X-ray diffraction. The lattice parameters determined from the powder diffraction data agree well with those determined by single-crystal diffraction (Supplementary Information). FTIR data collected on handpicked crystals of 1 and 2 (Supplementary Information) are in good agreement with previous literature reports for DABCO [32,33], confirming its presence in the products. At high wavenumbers, the absorption centered at around 2900 cm −1 is assignable to CH 2 stretches, while bands at 1300-1500 cm −1 can be associated with the CH 2 deformation (δ) and CH 2 deformation (t-w) modes.

Crystal Structure of [C 8 H 17 N 2 ][C 10 H 22 N 2 ][BiI 6 ] (1)
The asymmetric unit of 1 (Supplementary Information) contains one third of a [BiI 6 ] 3− octahedron, and one third of two N-substituted DABCO cations. Although the reagents used were DABCO and ethanol, an alkylation reaction has taken place under the solvothermal conditions, and the product of this reaction contains ethylated DABCO cations. In-situ alkylation reactions have been previously observed in solvothermal reactions involving amines and alcohols [24,34].   (Fig. 2a), each [BiI 6 ] 3− anion is surrounded by eight [Et 2 DABCO] 2+ cations. Figure 2b shows a slice of the crystal structure, parallel to the (010) planes, in which each [BiI 6 ] 3− anion is surrounded by two diethylated and two monoethylated DABCO cations in the same plane. Two additional ethylated DABCO cations are located above each [BiI 6 ] 3− anion, and two below. There are no short I···I distances (below the van der Waals' radii for two iodine atoms, 3.96 Å) [41], between the [BiI 6 ] 3− anions. Instead, there are a number of short C-H···I contacts (Supplementary Information), which are likely to contribute to the stabilization of this crystal structure.  (Fig. 3a). The coexistence of two types of dimeric anions in the structure of iodobismuthates is quite unusual, with only one previous example containing [ Bi [23]. In the [Bi 2 I 10 ] 4− dimers, which contain two bridging μ 2 -I − , the Bi-I bond distances vary between 2.9515(4) and 3.2597(4) Å while the I-Bi-I angles range from 80.401(9)° to 99.599(9)°. These values are similar to those found in   (Fig. 3b). There are a number of I···I contacts between the anions over the range 3.91 to 4.19 Å (Supplementary Information), comparable to the sum of the van der Waals' radii for two iodine atoms [41], which link the discrete anions into a pseudo-two-dimensional structure (Fig. 3c). It has been suggested that in crystal structures containing discrete iodobismuthate anions, the presence of I···I contacts might lead to increased band dispersion, particularly when the I···I distances are comparatively short [44]. In addition to the I···I contacts, in the crystal structure of 2 there are also many short C-H···I distances at under 3.4 Å, which are also likely to stabilize the crystal structure.

Thermal Stability and UV-Vis Diffuse Reflectance
Thermogravimetric data (Fig. 4a) indicate that, under a nitrogen atmosphere, both compounds are stable up to approximately 280 °C, with decomposition occurring in each case in a single step. UV-Vis diffuse reflectance data collected on ground crystals of 1 and 2 are shown in Fig. 4b. Absorption peaks observed at approximately 2.6 and 2.3 eV for 1 and 2, respectively, might be attributed to an exciton. These are often observed in the UV-Vis absorption spectra  Fig. 1 (Color figure online) of iodobismuthates, even when measurements are performed at room temperature [45,46].

Conclusions
In summary, two new iodobismuthates have been synthesized under solvothermal conditions in the presence of DABCO and ethanol, and their crystal structures determined by single-crystal X-ray diffraction. Although we have previously shown that DABCO can act as a linker between iodobismuthate moieties [24], in-situ alkylation of DABCO is occurring under the solvothermal conditions described here. Compounds 1 and 2 contain discrete mononuclear and dinuclear anions, rather than polymeric units, and exhibit  2.16 [21] optical band gaps comparable to those of other iodobismuthates containing discrete anions.
Acknowledgements The University of Reading is acknowledged for access to the Chemical Analysis Facility. Mr Nick Spencer is thanked for his assistance with the collection of single-crystal X-ray diffraction data and Dr Pedro Rivas Ruiz for help with FT-IR and Thermogravimetric Analysis.
Funding No funding was received for conducting this study.
Data Availability All data generated or analyzed for this work are included in the article and the Supplementary Information

Conflict of interest
The authors have no conflict of interest to declare that are relevant to the content of this paper.
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