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Ternary group 11/15 iodido metalates as building blocks for new materials
In this work, the synthesis and properties of new halogenido antimonates and bismuthates were investigated. The focus lay on iodido complexes and heterometallic compounds that feature coinage metal atoms in the anionic motif alongside antimony or bismuth. The motivation for this line of research is...
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| Format: | Dissertation |
| Sprache: | Englisch |
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Philipps-Universität Marburg
2023
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| Zusammenfassung: | In this work, the synthesis and properties of new halogenido antimonates and bismuthates were investigated. The focus lay on iodido complexes and heterometallic compounds that feature coinage metal atoms in the anionic motif alongside antimony or bismuth. The motivation for this line of research is based on the success of halogenido plumbates in semiconductor applications. With methyl ammonium lead iodide, for example, outstanding efficiencies in a new generation of solar cells have been achieved, but issues of stability as well as the toxicity of lead are obstacles for large-scale application. The heavy group 15 elements present a good alternative to lead since they show similar chemistry with regard to halogenido metalates while being less toxic and the resulting materials being generally more stable in terms of temperature and moisture resistance. The inclusion of coinage metals can further improve the optical and electrical properties of the heavy group 15 metalates.
This class of heterometallic compounds is still quite small and especially structure-property relationships are not yet well understood. Therefore, different reaction conditions and a variety of small organic cations were explored to synthesize new heterometallic metalates. Here, especially the use of cations derived by protonation of secondary amines and tertiary phosphines proved successful. Either by direct use or in situ deprotonation, the parent bases are available in the reaction and can coordinate to the coinage metal, which stabilizes heterometallic motifs. This led to the discovery of several stable and easy to prepare heterometallic compounds.
For the pair pyridine/pyridinium a copper iodido bismuthate with a chain-like anion and a remarkably small optical band gap was found. It also shows photoconductivity which makes this compound a prime candidate for experimental solar cells in the future. The use
of P(o-tol)3 / HP(o-tol)3 yielded a series of discrete heterometallic compounds for various of group 11 and group 15 elements. These compounds are structurally very similar to a series of PPh3-complexes, which are the result of previous work but differ greatly in optical properties. This allowed for a computational comparison of the absorption mechanisms, which revealed the electronic states involved to be strongly dependent on the fine changes in the coordination environment of the coinage metal atoms. This high sensitivity was further highlighted in the investigation of the also newly synthesized bismuth/silver compound [SMe3]2[Bi2Ag2I10]. Here all ligands are the same, namely iodide ions, as in a small number of previously known compounds with the same nominal composition of the anion. Still, the optical band gap of [SMe3]2[Bi2Ag2I10] is significantly smaller than observed for the comparable compounds, simply due to a slightly different connectivity of the {BiI6} and {AgI4} building blocks.
Aside from these heterometallic compounds, metalates featuring interesting homometallic anionic motifs have been prepared as well. The discovery of [Hpyz]4[Sb10I34], which contains the largest discrete halogenido pentelate anion reported to date, allowed for the experimental as well as computational investigation of the relationship between anion size and optical band gap in a series of closely related antimonates. With these results, the concept that larger anionic motifs lead to smaller optical band gaps, which is well established for other classes of semiconductors, could be expanded to group 15 iodido metalates. It was also shown that seemingly contrasting cases can be traced back to additional effects like charge-transfer excitation.
Furthermore, a new iodido bismuthate with a chain-like anionic motif threaded through a 2d-network of a cationic copper/pyrazine coordination polymer was discovered. Although it does not feature any chiral building blocks, it crystallizes in a non-centrosymmetric space group. In addition to a narrow band gap, it shows non-linear optical properties like strong circular dichroism and circularly polarized emission. These properties are generally overlooked for this class of compounds and associated only with metalates featuring chiral organic cations. An oversight that should be avoided in the future.
To further broaden the class of heterometallic metalates in the future and especially towards anions with higher dimensionality, the group of cations used in this work should be expanded to still small but doubly charged ions. This leaves more room for the anionic framework as less space is occupied by the cations. Exploratory experiments with piperazinium derivatives have already yielded promising results, which, however, are not part of this thesis. |
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| DOI: | 10.17192/z2023.0668 |