17 Complexes as Molecular Models for Metal Halide Double

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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11/15/17 Complexes as Molecular Models for Metal Halide Double Perovskite Materials Jakob Möbs and Johanna Heine* Department of Chemistry and Material Sciences Center, Philipps-Universität Marburg, Hans-Meerwein-Straße, 35043 Marburg, Germany

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S Supporting Information *

ABSTRACT: Thirteen neutral complexes [E x M y X z (PPh3)n(L)m] (E = Bi, Sb; M = Cu, Ag; X = Cl, Br, I; L = solvent) featuring three different motifs were prepared and characterized regarding their structure, stability, and absorption spectra. While not identical in structural motif, the compounds can serve as models for the influence of the composition E/M/X on the optical properties of double perovskites A2EMX6 (A = Cs, CH3NH3).

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INTRODUCTION Lead halide perovskites APbX3 (A = monovalent organic or inorganic cation; X = Cl, Br, I) have emerged as a new class of materials with possible applications ranging from the prominent use in solar cells with record efficiencies above 20%1,2 to solid state lighting,3 lasers,4 and photocatalysis.5 Their ease of fabrication and excellent, robust optical properties have opened up a whole field of research in the past few years.6 Concerns over the long-term stability of lead halide perovskites are being addressed by creating more advanced materials featuring a mixture of cations A, the use of nanoparticles, and different encapsulation techniques.7−10 Yet, the toxicity of lead compounds remains a serious issue with a view toward large-scale application.11 Different approaches have been taken to replace lead in metal halide perovskites.12 The use of the nontoxic heavy metal bismuth13 appears promising, but the difference in charge between Pb2+ and Bi3+ precludes a direct substitution. Iodido bismuthates AxBiyIz rarely feature anions of a dimensionality of more than one14,15 and a “3D” anionic motif is unknown.16−18 As a consequence of this, compounds such as (CH3NH3)3Bi2I9, which features a molecular Bi2I93− anion, show much more localized exciton dynamics19 than (CH3NH3)PbI3, and the efficiencies of solar cells fabricated from (CH3NH3)3Bi2I9 and related bismuthates remain low despite significant efforts to improve thin film quality.20 Inspired by classic solid state chemistry techniques, researchers have used heterovalent substitution of Pb2+ by Bi3+ and a monovalent cation M+ such as Ag+21−24 or Tl+25,26 to create double perovskites A2BiMX6. The most widely studied compound of this class, Cs2BiAgBr6, has already been tested in photovoltaic devices, showing a promising efficiency of 2.5%.27 This has spurred much activity in this subfield of metal halide perovskite chemistry, including a number of © XXXX American Chemical Society

quantum chemical investigations charting hypothetical double perovskites that could show even better performance.28,29 Not all of these compounds will be easily available, as the decomposition into simple ternary compounds AxBiyXz and AxMyXz,30 disproportionation of less stable oxidation states such as In(I),31 and preferences in the coordination environment unfavorable for the double perovskite structure, for example, for Cu(I),32 are an issue. Still, a growing number of new double perovskites are being realized in addition to compounds known from older works.33 Nonlinear effects of doping34 and alloying35 in these compounds open up additional ways to influence and optimize the optical properties of double perovskites. Most recently, the growing current interest in two-dimensional (2D) perovskites36 has prompted researchers to create layered double perovskites as well.37 In this work, we present the synthesis, structure, and optical properties of a series of 13 neutral complexes [ExMyXz(PPh3)n(L)m] (E = Bi, Sb; M = Cu, Ag; X = Cl, Br, I; L = acetonitrile, acetone; see Scheme 1) that can serve as molecular models with the same element combinations as those found in double perovskites currently investigated as lead halide perovskite replacements. The series consists of three different types of complexes: Type I: [Bi2Cu2Cl8(PPh3)4(Me2CO)]·2Me2CO, type II: [EM2X5(PPh3)4(L)] and type III: [EM3X6(PPh3)6] and its solvate [EM3X6(PPh3)6]·4MeCN. Two of these complexes, 5a and 8a, have been described before, in both cases as side products in the synthesis of other compounds.38,39 We discuss the similarities and differences of our model compounds with respect to the double perovskites and point Received: February 13, 2019

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DOI: 10.1021/acs.inorgchem.9b00429 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Synthesis. All compounds can be obtained by heating a mixture of EX3, MX, and PPh3 in DME, MeCN, or Me2CO. Three representative procedures are given below, additional descriptions for the synthesis of all compounds are given in the Supporting Information (SI). [Bi2Cu2Cl8(PPh3)4(Me2CO)2]·2 Me2CO (1). A total of 32 mg of BiCl3 (0.1 mmol, 1 equiv), 30 mg of CuCl (0.3 mmol, 3 equiv), and 157 mg of PPh3 (0.6 mmol, 6 equiv) were dissolved in 10 mL of acetone and heated to 65 °C for 1 h, resulting in a yellow solution and a colorless precipitate. The precipitate was removed by filtration and identified by powder diffraction as [Cu2Cl2(PPh3)3].43 The filtrate was left to crystallize for 1 week at room temperature and then concentrated to half its original volume and stored at 5 °C. Within 2 weeks, colorless needles of [Cu2Cl2(PPh3)3] (identified via SCXRD and their bright blue luminescence under UV light) had formed along with yellow blocks of 1. A few of these blocks were hand-picked, washed with hexane, and dried in a vacuum. Bulk analysis showed that the dried product was not identical to 1, containing 1.5 molecules less acetone according to CHN analysis resulting in a composition of [Bi2Cu2Cl8(PPh3)4(Me2CO)2] ·0.5 Me2CO and displaying a different powder pattern (see SI for details). [BiAg2Cl5(PPh3)4(Me2CO)] (2). A total of 47 mg of BiCl3 (0.15 mmol, 1 equiv) and 43 mg of AgCl (0.3 mmol, 2 equiv) were suspended in 10 mL of acetone. A total of 157 mg of PPh3 (0.6 mmol, 4 equiv) was added resulting in an initially clear solution. The mixture was heated for 4 h at 70 °C and left to cool to room temperature. A colorless precipitate of 2 had formed and was filtered off, washed three times with 5 mL of pentane, and dried in a vacuum. Yield: 169 mg (67%). CHN (calculated for [BiAg2Cl5(PPh3)4(Me2CO)0.66]): C 52.26 (52.60), H 3.845 (3.82), N 0 (0). Small, colorless single crystal of 2 were grown from the filtrate within 5 days at 5 °C. Powder patterns of the dried precipitate accord well with the simulation from SCXRD data, and thermal analysis confirms the partial solvent loss observed in the CHN analysis (see SI for details). [BiCu3I6(PPh3)6] (8a) and [BiCu3I6(PPh3)6]·4 MeCN (8b). A total of 59 mg of BiI3 (0.1 mmol, 1 equiv), 57 mg of CuI (0.3 mmol, 3 equiv) and 157 mg of PPh3 (0.6 mmol, 6 equiv) were suspended in 10 mL of acetonitrile and heated to 90 °C for 40 min, resulting in a red solution and a small amount of solids that were subsequently identified as metallic bismuth. The reaction mixture was filtered while hot, and the filtrate was stored at −26 °C for 2 days. Dark red rods of 8a and dark red blocks of 8b were formed as a mixture that had to be separated by hand. While it is easy to obtain a sufficient amount of each compound for full characterization this way due to the formation of large crystals, the yields of each species appeared to vary from batch to batch, apparently depending on factors like the exact filtration temperature and crystallization vessel surface roughness. 8a: CHN (calculated): C 47.64 (47.43), H 3.330 (3.32), N 0.17 (0). 8b: CHN (calculated): C 47.91 (48.06), H 3.501 (3.55), N 1.91 (1.93) The synthesis of 8a was also successfully performed in acetone and DME; yet in this case the colorless blocks of [Cu2I2(PPh3)3]44 crystallized along with dark red rods of 8a. X-ray Crystallography. Single crystal X-ray determination was performed at 100 K on a Bruker Quest D8 diffractometer with microfocus MoKα radiation and a Photon 100 (CMOS) detector, a STOE IPDS2 diffractometer equipped with an imaging plate detector system using MoKα radiation with graphite monochromatization, or a STOE StadiVari diffractometer using CuKα radiation from an X-ray micro source with X-ray optics and a Pilatus 300 K Si hybrid pixel array detector. The structures were solved using direct methods, refined by fullmatrix least-squares techniques, and expanded using Fourier techniques, using the ShelX software package45−47 within the OLEX2 suite.48 All non-hydrogen atoms were refined anisotropically unless otherwise indicated. Hydrogen atoms were assigned to idealized geometric positions and included in structure factors calculations. Pictures of the crystal structures were created using DIAMOND.49 Additional details on individual refinements are reported in the SI. The data for compounds 1, 2, 3a, 4, 5b, 6, 7, 8a, 8b, and 9b were deposited as CCDC 1891815−1891824.

Scheme 1. Overview of the Three Different Types of Complexes Obtained in This Work, Together with the E/ M/X Element Combinations Available for Each Type

out aspects where trends found for our models are seemingly at odds with them, underlining the need for further work in this area.

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EXPERIMENTAL SECTION

General. SbI3 and BiI3 were synthesized from the elements according to literature procedures.40,41 AgCl and AgBr were prepared by mixing aqueous solutions of AgNO3 and KCl or KBr. The resulting precipitate was isolated by centrifugation, washed with water, dried under a vacuum, and stored in the dark. Copper halides, as well as AgI, BiBr3, BiCl3, and PPh3, were used as supplied from commercial sources. Reactions were performed under inert conditions using acetone (Me2CO), acetonitrile (MeCN), or 1,2-dimethoxyethane (DME) dried over a 3 Å mole sieve to avoid the formation of hydrolysis or oxidation products during the reaction in solution. The dried products are generally air-stable unless indicated otherwise. CHN analysis was carried out on an Elementar CHN analyzer. Micro X-ray fluorescence spectroscopy (μ-XRF) was carried out using a Bruker M4 Tornado spectrometer with a Rh target X-ray tube, polycapillary optics, and a Si drift detector. Thermal analysis was carried out by simultaneous DTA/TG on a NETZSCH STA 409 C/ CD in the temperature range of 25−1000 °C with a heating rate of 10 °C min−1 in a constant flow of 80 mL min−1 N2. Powder patterns were recorded on a STADI MP (STOE Darmstadt) powder diffractometer, with CuKα1 radiation with λ = 1.54056 Å at room temperature in transmission mode. IR spectra were measured on a Bruker Tensor 37 FT-IR spectrometer equipped with an ATRplatinum measuring unit. Optical absorption spectra were recorded on a Varian Cary 5000 UV/vis/NIR spectrometer in the range of 300− 800 nm in diffuse reflectance mode employing a Praying Mantis accessory (Harrick). For ease of viewing, raw data were transformed from %reflectance R to absorbance A according to A = log(1/R), which yields estimates comparable to the widely used Kubelka−Munk relation.42 Photoluminescence (PL) was recorded with a Horiba Jobin Yvon Spex Fluorolog 3 (Horiba Jobin Yvon, France) equipped with a 450 W Xe-lamp and double grating excitation and emission monochromators. B


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RESULTS AND DISCUSSION Synthesis. Compounds 1−9b were obtained from the reaction of EX3 with MX and PPh3 in MeCN, acetone, or 1,2dimethoxyethane (DME). While few binary E/X/PR 3 complexes are known50,51 and were generally not observed in our experiments, the cocrystallization of a variety of M/X/ PPh3 complexes occurred frequently and required careful adjustment of the reaction and crystallization conditions, as well as the crystal harvesting times. Fortunately, the E/M/X/ PPh3 complexes grew as sizable single crystals in most cases and could be separated by hand from byproducts when necessary. For the colorless or light yellow chloride and bromide complexes 1−5b, the well-known, intense luminescence of the binary side products M/X/PPh3 under the light of a hand-held UV lamp52 could be used to identify the ternary compounds. In some cases an excess of EX3 can be used to suppress the formation of M/X/PPh3 complexes. Despite our efforts to expand the series toward more antimony compounds, [SbCu3I6(PPh3)6] (7) remained the only Sb complex we could isolate. This appears to be a consequence of a better solubility of the antimony complexes, leading to an even more pronounced crystallization of M/X/PPh3 complexes than in case of the bismuth compounds. Similarly, we have not yet found conditions that allow for the extension toward gold or thallium compounds. Under the tested reaction conditions, type III complexes [EM3X6(PPh3)6] do not form for X = Cl, while type I or II complexes are not found for iodides. This suggests the steric crowding of the PPh3 groups in type III complexes as a factor in the compounds’ formation. For the combination Bi/Ag/Br type II (3a and 3b) and III (5a and 5b) complexes could be synthesized, depending on the reaction stoichiometry and solvent. In acetone, the type II complex [BiAg2Br5(PPh3)4(Me2CO)] (3b) could be isolated in good yield. In acetonitrile, the type II complex [BiAg2Br5(PPh3)4(MeCN)] (3a) can be isolated in good yield when the reaction stoichiometry is adjusted to the product composition of BiBr3/AgBr/PPh3 1:2:4. In contrast to this, only a few single crystals of the type III complex [BiAg3Br6(PPh3)6]·4MeCN (5b) were obtained along with significant amounts of 3a when using the stoichiometry BiBr3/AgBr/PPh3 1:3:6. In DME the rapid formation of [BiAg3Br6(PPh3)6] (5a) is observed, likely because DME does not act as a monodentate ligand with similar ease as Me2CO or MeCN to form a type II complex. We also tried to produce quarternary complexes and could successfully synthesize the mixed halide complex [BiCu3Br3I3(PPh3)6] (6) from BiBr3 and CuI. Interestingly, no significant halide scrambling occurred in this case, and the composition of the product matched well with the formula shown above, even when an excess of BiBr3 was used to suppress the formation of Cu/X/PPh3 complexes. In contrast to this, no compounds featuring both Cu and Ag could be synthesized. Only the copper complex 8a could be obtained when using copper and silver iodide in the reaction mixture due to a depletion of AgI, which was found in the precipitate along with [Ag4I4(PPh3)4]. X-ray Crystallography. Two complexes of the series [EM3X6(PPh3)6], 5a and 8a, have been described before, obtained as byproducts in the synthesis of a chalcogenolate cluster38 and ionic iodido bismuthates,39 respectively. The other two motifs were previously unknown and point to a much richer structural chemistry of phosphine-terminated 11/ 15/17 complexes than anticipatedaside from 5a and 8a only

a single example of a neutral, nonchalcogenolate M/X/E/PR3 complex, (Ph3P)CuCl·SbCl3, has been reported.53 The singular example of a type I complex, [Bi2Cu2Cl8(PPh3)4(Me2CO)2]·2Me2CO (1) crystallizes in the space group P1 (No. 2) in the triclinic crystal system as light yellow rods. The molecular structure is shown in Figure 1A. A central

Figure 1. Molecular structures of [Bi2Cu2Cl8(PPh3)4(Me2CO)2]·2 Me2CO (1) (A) and [BiAg2Br5(PPh3)4(MeCN)0.7] (3a) (B). Bond length given in Å, hydrogen atoms and disordered parts omitted for clarity.

[Bi2Cl8(Me2CO)2]2− unit composed of two edge-sharing BiCl5(Me2CO) octahedra is decorated by two [Cu(PPh3)2]+ units on opposite edges, forming an overall neutral complex. The compound represents the second example of a Cu−Cl−Bi unit in a coordination complex.54 Bi−Cl interatomic distances lie between 2.490 and 3.050 Å, a large range that is nonetheless typical for bismuth halide compounds18 and can be observed in a similar fashion for the anionic building block in [PPh4]2[Bi2Cl8(Me2CO)2] (Bi−Cl distances from 2.515 to 2.854 Å).55 A close inspection of the bond lengths reveals two very different Cu−Cl interatomic distances of 2.335 and 2.798 Å, indicating that the latter is better described as a contact than a bond, since Cu-μ-Cl bonds in copper chloride phosphine complexes typically range between 2.23 and 2.50 Å.56,57 This accords well with the overall coordination sphere of the Cu atoms, which is nearly trigonal. A similar Cu···Cl contact has been observed in the adduct [(InCl3)(Cu(SEt)(PPh3)2].58 As an example of a type II complex [BiAg2Br5(PPh3)4(MeCN)0.7] (3a) will be discussed. Both 2 and 3b are isostructural to 3a (see SI for details). 3a crystallizes in the triclinic space group P1 (No. 2) as yellow plates. The molecular structure is shown in Figure 1B. An octahedral [BiBr5(MeCN)]2− unit is decorated with two [Ag(PPh3)2]+ C


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Inorganic Chemistry

anion in [2,6-diisopropylanilinium]4[BiI6][I].65 Cu−I distances of 2.665−2.727 Å are similar to those found in [Cu2I2(PPh3)3] (2.50−2.82 Å).44 In contrast, the overall coordination environment of Bi shows a large degree of distortion with respect to the expected octahedral one, as evidenced by trans I−Bi−I angles of 152−157°. The average twist angle φ (as defined by Stiefel66 to distinguish between an octahedral and trigonal prismatic coordination environment in complexes featuring three bidentate ligands) between the two triangles orthogonal to the 3-fold axis formed by the iodine atoms is 28°, halfway between the value of 60° expected for an ideal octahedron and 0° expected for an ideal trigonal prism, as previously noted by Pike and co-workers.39 [BiCu3I6(PPh3)6]·4MeCN (8b) crystallizes in the trigonal space group R3c (No. 161) as dark red blocks. The main motif of the ternary complex is identical to that found in 8a, and the coordination environment around the Bi atom is similarly distorted. The solvent molecules are disordered about the 3fold axis along c between the phenyl groups of adjacent complexes in the a−b plane, as shown in Figure 3. Since 8a and

units on trans-standing edges. The two terminal bromo and acetonitrile ligands are disordered, with the bromo ligand showing a 50% occupancy and the acetonitrile a 33% occupancy on each position. This indicates that a third of the complexes have a central square-pyramidal [BiBr5]2− unit. While bismuth halide complexes typically display an octahedral coordination sphere about the bismuth atom, examples of the square-pyramidal59,60 and even trigonal− bipyramidal coordination61 are known, underlining the flexibility of these complexes. Additionally, a related In(III) complex, [InCu2Cl5(PPh3)4]·THF, has been reported that also features a distorted square-pyramidal motif.62 The Bi−Br distances in 3a are more uniform than the Bi−Cl distances found in 1, ranging between 2.831 and 2.858 Å, similar to those found in compounds containing [BiBr6]3− anions (e.g., 2.755−2.981 Å in [(ArNH3)3(BiBr6)] with Ar = 2,6diisopropylphenyl63). Once again, two fairly unequal M−X bond length of 2.715 and 2.956 Å are observed together with a nearly trigonal-planar coordination environment at the silver atom, indicating a 3 + 1 coordination sphere, as seen in 1, although the difference between a longer and shorter M−X bond is less pronounced here and rather close to the range observed for silver bromide phosphine complexes with typical Ag−Br distances between 2.60 and 2.93 Å.64 For the type III complexes, two different crystal structures, that of the regular [EM3X6(PPh3)6] complex and that of its solvate [EM3X6(PPh3)6]·4 MeCN are observed. To allow an easy comparison, we discuss the elemental combination Bi/ Cu/I in the following, since both forms were available as single crystals in this case. [BiCu3I6(PPh3)6] (8a) crystallizes in the orthorhombic space group Pna21 (No. 33) as dark red rods. The molecular structure is shown in Figure 2. Here, a central [BiI6]3− unit’s edges are decorated with three [Cu(PPh3)2]+ units generating an optically active complex with an idealized D3 point group. Bismuth− and copper−iodide distances show little variation within the complex. Bi−I distances range between 3.012 and 3.177 Å in good accordance with values of 3.07−3.09 Å found in the [BiI6]3−

Figure 3. Excerpt of the crystal structure of [BiCu3I6(PPh3)6]· 4MeCN (8b). Hydrogen atoms omitted for clarity, disordered MeCN molecules highlighted in cyan.

8b feature an identical complex composition, this allows for a facile comparison of the packing in the solvent free and solvate compound. The inspection of the overall packing in the two forms reveals that in both cases the complexes form layers in the a−b plane and that the packing in the solvate is less dense to accommodate the additional acetonitrile molecules, while in the nonsolvated form two different orientations are found within the layer to allow for a more efficient packing (Figure 4). In the other windmill-like type III complexes, similar observations can be made, as summarized in Tables S12 and S13 in the SI. Interestingly, the twist angle φ appears to depend mainly on the group 11 metal, as the copper compounds display angles of 28−30° and the silver compounds of 39−42° Overall it has to be emphasized that while the compounds reported here feature a composition related to metal halide

Figure 2. Molecular structure of [BiCu3I6(PPh3)6] (8a). Hydrogen atoms omitted for clarity. D


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Inorganic Chemistry

Figure 5. Absorption spectra of the Bi/Ag/Br complexes 3a, 3b, and 5a, recorded in diffuse reflectance, with single crystals of 3a shown as an inset.

Table 1. Onset of Absorption As Estimated from Diffuse Reflectance Measurementsa Eonset/eV [BiAg2Cl5(PPh3)4(Me2CO)] (2) [BiAg2Br5(PPh3)4(MeCN)] (3a) [BiAg2Br5(PPh3)4(Me2CO)] (3b) [BiCu3Br6(PPh3)6] (4) [BiAg3Br6(PPh3)6] (5a) [BiCu3I3Br3(PPh3)6] (6) [SbCu3I6(PPh3)6] (7) [BiCu3I6(PPh3)6] (8a) [BiCu3I6(PPh3)6]·4MeCN (8b) [BiAg3I6(PPh3)6]·4MeCN (9b)

3.0 2.6 2.6 2.1 2.6 1.8 1.9 1.6 1.6 1.9

Figure 4. Excerpts of the crystal structure of [BiCu3I6(PPh3)6] (8a) (A) and [BiCu3I6(PPh3)6]·4MeCN (8b) (B). Hydrogen and solvent atoms omitted for clarity, different orientations in A marked by red and blue polyhedra.

a

double perovskites, the structural motifs are quite different, with trigonal or tetrahedral coordination environments around the group 11 metals and edge-sharing between M−X and E−X units instead of the corner-sharing octahedra characteristic of cubic perovskites. Thermal Analysis. All phase pure compounds display good thermal stability, with the onset of decomposition for the desolvated forms observed above 200 °C in all cases (see SI for details). It would be desirable to produce ternary phases ExMyXz via thermal decomposition of our compounds, as these are currently discussed as solar cell materials.67 However, exemplary analysis of the TGA residues of compounds 4 and 5a by PXRD shows that the copper compound forms Cu3P, while the silver compound is decomposed into Ag and AgBr, both in addition to unidentified amorphous phases (see Figures S33 and S34). Optical Properties. Absorption spectra were recorded for all phase pure compounds of the series (Figure S37). Due to the comparatively large number of similar compounds at hand, a few trends can be established from this data set. As a first observation, the complex type and solvation appear to have only little influence on the onset of absorption, as similar values for the different Bi/Ag/Br complexes 3a, 3b, 5a (see Figure 5) and Bi/Cu/I complexes with and without solvate acetonitrile 8a and 8b show (see also Table 1). As expected from the optical properties of halogenido bismuthates68 and related metalates,69 the halogen X has a large influence on the onset of absorption. Here, the Bi/Ag/X

compounds 2, 3a, and 9b provide a good reference series, demonstrating that, going from Cl to Br and I, the onset is shifted to significantly lower energies (Figure 6A). Additionally, this shift is continuous in nature, as the series of Bi/Cu/X compounds 4, 6, and 8a shows, where the mixed halogen compound 6 lies right between the parent bromine and iodine compounds (Figure 6B). The influence of the group 11 metal M is also significant, as a comparison of the type III Bi/M/Br complexes 4 and 5a, as well as the Bi/M/I complexes 8b and 9b show (Figure 7). While no absorption spectrum of 1 could be obtained due to the compound’s transformation upon drying, a comparison of microscopic photographs of the crystals of Bi/M/Cl complexes 1 and 2 (Figures S38 and S39) also suggests a red-shift of the onset of absorption going form Ag to Cu. This effect has been observed before in compounds with ternary iodido bismuthate anions and was attributed to the different influence of the Cu and Ag states on the valence band maximum of the respective compounds.70,71 Only two compounds provide insight into the effect of the group 15 metal E: A comparison of the type III complexes E/ Cu/I 7 and 8a (Figure 8) shows that a shift in the onset of absorption toward lower energies when going from Sb to Bi, as might be expected when comparing the color and band gaps of SbI3 and BiI 3 (red and black and 2.2 and 1.7 eV, respectively).72,73 Yet this difference is not always so clearcut, as the comparatively similar band gaps in the isostructural E

See also Figure S37.


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Inorganic Chemistry

Figure 8. Absorption spectra of the E/Cu/I complexes 7 and 8b with single crystals of 7 shown as an inset.

band structure effects like the generation of defects by doping which can be challenging to handle.75 We do not want to draw a direct correlation between the onset of absorption we observe and the nature of the compounds’ band gap. Likely, the absorption processes in our complexes are better described as a CT band due to their molecular nature. Nonetheless, any effects we observe should be consistent throughout the series, and trends should translate to metal halide double perovskites as far as the influence of the composition is concerned. It would be desirable to draw a direct comparison between our findings and the experimental data on A2MEX6 double perovskite compounds reported in the literature, but this is not easily done: Values reported for the same compound can vary due to differences in measurement protocols (e.g., with band gaps of 1.95 eV21 and 2.2 eV22 for Cs2AgBiBr6). Additionally, not all compositions are available, as for example the Cu(I) compounds have been shown to be unstable.32,30 The preparation of nanoparticles allows access to a broader range of double perovskites, albeit at the price of possibly introducing additional effects.76 Thus, we have chosen the quantum chemical investigation of the series Cs2MEX6 by Giustino and Snaith as our point of comparison.28 The two general trends observed in our work are reflected in the computed band gap values, with red-shifts in the series Cl → Br → I and Ag → Cu. Interestingly, the red-shift we observed for Sb → Bi is not found in these investigations. Instead, substituting antimony for bismuth appears to cause a narrowing of the band gap, as has also been shown experimentally by Mitzi and Hutter in the solid solution series Cs2AgBi1−xSbxBr6.35,77 In contrast, Deng and Han synthesized (CH3NH3)2AgEI6 (E = Sb, Bi) and found little difference in band gaps between the antimonate and bismuthate (1.93 eV for E = Sb and 1.96 eV for E = Bi).78,79 These peculiarities suggest that future work on antimony based double perovskites as well as quarternary halogenido antimonates in general may prove to be a valuable addition to the current interest in the related bismuthates. In contrast to many Cu(I)halide phosphine complexes, none of the ternary complexes show bright luminescence when excited with a hand-held UV lamp. Nonetheless, we have investigated the photoluminescence (PL) properties of 5a as a representative example that shows a close relationship to the most intensely investigated double perovskite composition

Figure 6. Absorption spectra of the Bi/Ag/X complexes 2, 3a, and 9b with single crystals of 9b shown as an inset (A) and absorption spectra of the Bi/Cu/X complexes 4, 6, and 8b with single crystals of the mixed halide complex 6 shown as an inset. All spectra recorded in diffuse reflectance.

Figure 7. Absorption spectra of the Bi/M/X complexes 4, 5a, 8b, and 9b with single crystals of 4 shown as an inset.

iodido pentelates Rb3E2I9 and Cs3Sb2I9 show (2.03 eV for Rb3Sb2I9, 1.93 eV for Rb3Bi2I9 and 1.89 eV for Cs3Sb2I9).74 Overall, our measurements provide the basic effects of composition E/M/X on the compounds’ absorption, free of F


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Inorganic Chemistry Accession Codes

Cs2BiAgBr6 (Figure 9). 5a shows a weak, broad luminescence band with a maximum at 449 nm in the visible part of the

CCDC 1891815−1891824 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Johanna Heine: 0000-0002-6795-5288 Funding

German Research Foundation DFG (HE 8018/1). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.H. thanks Prof. Stefanie Dehnen for her support. The authors thank Dr. Carsten Donsbach for his help in obtaining one of the SCXRD datasets, Marvin Szabo for his help with some of the synthetic work, and Dr. Frank Tambornino for his help in the treatment of PXRD data. The authors thank Lilli Neumeier, Dr. Silke Wolf, and Prof. Claus Feldmann of the Karlsruhe Institute of Technology for measuring photoluminescence spectra of compound 5a on very short notice during the revision process.

Figure 9. Photoluminescence (red) and excitation (black) spectra of 5a.

spectrum and a more pronounced band at 335 nm. The excitation spectrum recorded at an emission of 449 nm features two bands at 370 and 306 nm. A comparison with literature indicates that the bands in the PL spectrum can likely be attributed to transitions centered on the PPh3 ligands80 and that no metal-centered processes are involved here. This is consistent with the interpretation that PL processes observed in the double perovskite Cs2BiAgBr6,26 the (001) double perovskite (C4H12N)4BiAgBr8 (C4H12N = butylammonium),37 and the layered nonperovskite Rb4BiAg2Br981 are based on excitons and defects, which would not be available in a molecular model such as [BiAg3Br6(PPh3)6].

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CONCLUSION A series of 13 different neutral complexes E/M/X (E = Sb, Bi; M = Cu, Ag; X = Cl, Br, I) were prepared and characterized. A change in structural motif is observed when changing the composition from Cl to Br and I, with the heavier halogens supporting a windmill-like complex. The compounds’ optical properties are determined by their composition, with each component E, M, and X showing a distinct effect on the onset of absorption. A comparison of these effects with recent findings on the properties of double perovskites highlights that future work on antimonates may help to get a clearer picture of the effect of substituting Bi with Sb. [(PR3)2M]+ units may also prove to be of utility in double perovskite nanoparticle termination or modification. A future extension toward smaller phosphine ligands may also allow access to larger E/M/X complexes, which have remained unknown until now.

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17 Complexes as Molecular Models for Metal Halide Double

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