Development of Coinage Metal Sulfide Clusters Protected by Octahedral Metal Complexes with Aminothiolates

概要

Coinage metal sulfide clusters are promising candidates for optoelectronic materials due to their less toxic nature and quantum confinement properties for band gap tuning. However, hampered by their synthetic difficulties, research on coinage metal sulfide cluster are less. The formation of metal sulfide cluster is highly sensitive to the synthetic conditions. A careful choice of capping ligands and sulfide source is crucial. To date, the effects of ligand on the structure or properties of metal sulfide clusters are rarely discussed. Our group focuses on the usage of aminothiolato metalloligand for the formation of polynuclear species. The thiolate lone pairs in the metalloligands are well-directed, which allows us to predict the structures of polynuclear species. Although metalloligands are known to be useful in forming polynuclear complexes, their utility in the formation of metal sulfide clusters are less explored. In this thesis, the syntheses and effect of different octahedral aminothiolato metalloligands on the cluster structure or properties are studied.

The first study features a silver sulfide cluster covered by the 5-membered rings metalloligand [M(aet)3] (M = RhIII/IrIII, Haet = 2-aminoethanethiol). Previously, it was found that the reaction of [Rh(aet)3] with Ag+ and D-pen (D-penicillaminate) yields a 60-nuclear silver sulfide cluster, [Ag46S13{Rh(aet)3}14]20+ ([1Rh]20+). In my thesis, I investigated the effect of the metal center in metalloligand and heterometallation on the photophysical properties of the silver sulfide cluster. Firstly, the photoluminescence of the analogous [Ag46S13{Ir(aet)3}14]20+ ([1Ir]20+) was studied. It was found that changing the metalloligand metal centre from Rh to Ir helps to improve the quantum yield of the cluster without causing much changes in the absorption and emission wavelengths. Owing to the higher d-d transition of IrIII compared to that of RhIII, the non-emissive energy transfer to the d-d excited state was avoided, thus improving the emission quantum yield of the silver sulfide cluster. Next, heterometallation of [1Ir]20+ with CuI or AuI was carried out to yield isostructural mixed-metal cluster, [Ag43Cu3S13{Ir(aet)3}14]20+ ([2Ir]20+) and [Ag43Au3S13{Ir(aet)3}14]20+ ([3Ir]20+). The quantum yield of the silver sulfide clusters decreased upon reaction with CuI, while reaction with AuI improved the quantum yield. These changes are ascribed to the smaller [AgCu3S]2+ providing more structural flexibility that is detrimental to the luminescence intensity, while the higher quantum yield of [ 3Ir]20+ is attributed to the presence of the more emissive Au3S moiety in the [AgAu3S]2+ core.

Additionally, the effect of chelate ring size toward the silver sulfide clusters formed was investigated. Three silver sulfide clusters with different nuclearities, 17-nuclear ([4]9+), 19-nuclear ([5]9+), and 41-nuclear ([6]17+) could be formed using the bulkier [Rh(apt)3] metalloligand with 6-membered chelate rings. The lower nuclearity [4]9+ was formed when D-pen was employed as sulfide source which is reasoned to the chelating ability of D-pen and also the slower introduction of sulfide to the reaction system. Whereas the higher nuclearity [5]11+ was obtained along with a small amount of [6]17+ when the direct sulfide source of NaSH was used. Furthermore, conversion of [4]9+ with an [Ag5S]3+ core to [5]11+ with an [Ag4S]2+ core was achieved simply just by addition of Ag+ ions. [5]11+ could be partially converted back to [4]9+ by the treatment of D-pen. Such conversion is possible because of the same ligand shell involved in both [4]9+ and [5]11+. Compare to the [M(aet)3] metalloligands with 5-membered chelate rings, the bulkier [Rh(apt)3] yields lower nuclearity clusters. However, because of their more flexible carbon backbone and narrower chelating angles of thiolate groups, three silver sulfide clusters of different structures could be formed.

In the third chapter, pentanuclear copper complexes of [M(aet)3] (M = RhIII/IrIII) and [Rh(apt)3] were treated with NaSH. The 5-membered rings copper complexes, [Cu3I{M(aet)3}2(CH3CN)]3+, allows for the formation of decanuclear copper sulfide cluster ([7Rh/Ir]4+), upon treatment with NaSH. In contrast, the reaction of the 6-membered rings copper complex, [Cu3{Rh(apt)3}2]3+, with NaSH produced a tetranuclear complex ([8]2+) via a demetallation reaction. Due to the shorter Cu– S bond length and bulky chelate rings on [Rh(apt)3], the formation of copper sulfide cluster was not achieved.

In short, the effect of metalloligands towards the structure or photophysical properties of clusters were studied. The IrIII metalloligand with a higher d-d transition was found to be beneficial for the photoluminescence properties of clusters. Heterometallation allowed for the tuning of emission intensities and emission wavelengths. The usage of bulkier [Rh(apt)3] was found to yield lower nuclearity structures due to the steric hindrance in the arrangement of metalloligand on the cluster surface. But the more flexible chelate ring was found to be advantages for forming cluster of different structures. Post synthetic reactions such as heterometallation or structural conversion were achieved without changing the number of metalloligands. Allowing us to change the composition of cluster while restricting the size of clusters. The formation of copper sulfide cluster was found to be achievable with the smaller [M(aet)3] metalloligands. Once again showing that the [M(aet)3] is more practical in the synthesis of higher nuclearity structures. For the bulkier [Rh(apt)3] metalloligand, it is not achieved due to the steric effects. These findings would contribute to the further development of synthetic route to obtain metal sulfide clusters protected by metalloligands. I hope that these results would pave way for the future design of clusters with tunable properties via post synthetic modifications.