Synthesis and photophysical properties of neutral and cationic octahedral tungsten iodide clusters

Abstract

In the course of this thesis, the first crystal structures of cationic and neutral tungsten iodide clusters were solved and published. In the first step the compound [(W6I8)I3(CH3CN)3]I7·I2 featuring the heteroleptic cluster cation [(W6I8)I3(CH3CN)3]+ was synthesized as bulk material. Follow-up experiments starting from [(W6I8)I3(CH3CN)3]I7·I2 yielded the two species [(W6I8)I(CH3CN)5](I3)2(BF4) and [(W6I8)(CH3CN)6](I3)(BF4)3·(H2O). The structures of these two compounds feature the cluster cations [(W6I8)I(CH3CN)5]3+ and [(W6I8)(CH3CN)6]4+. An-ions in the three mentioned compounds are either I3- or I7-, which cause a dark crystal or pow-der color and no obvious photoluminescence. Further conducted reactions gave [W6I8(CH3CN)6](BF4)4·(CH3CN)2 as a yellow powder or plated-shaped crystals. The crystalline compound shows the typical octahedral metal halide cluster photoluminescence with characteristic broad excitation and emission bands. However, in contrast to species bearing electron-withdrawing ligands, emission lifetimes are short, and no quenching in the presence of molecular oxygen is observed. The calculated Electron Local-ization Function (ELF) revealed significantly higher d-electron density above the tungsten atoms and reduced ionicity of the W–N bond for [W6I8(CH3CN)6](BF4)4·(CH3CN)2 compared to (TBA)2[W6I8(CO2C3F7)6], which is a strong quencher. This leads to the conclusion that the ionicity of the W–Ligand bond influences the energetic splitting of the emitting triplet sublevels and therefore also emission lifetimes. Besides the mentioned cationic clusters, the neutral tungsten iodide cluster [W6I12(NCC6H5)2] was also synthesized and characterized for its properties. Due to two benzonitrile and four iodide as apical ligands, the compound is a heteroleptic cluster species. It shows a good hy-drolysis stability and remarkable temperature stability up to 400 °C. Photoluminescence in the solid state is only weakly pronounced with low emission intensity, short lifetime and negligi-ble quantum yield. However, the good hydrolysis and temperature stability as well as the opti-cal band gap of 2.17 eV make the compound a perfect candidate for application in photoca-talysis. This was confirmed in an experiment observing the photocatalytic decomposition of Rhodamine B (Rh B) in aqueous solution. Additional experiments yielded the compounds [W6I8(CH3CN)6](ClO4)4·(CH3CN)2 and [W6I8(DMSO)6](I3)4 featuring cationic clusters in their structures. Further reactions with dif-ferent solvents suggest a high potential to produce many other cationic and neutral tungsten iodide clusters. This would allow for new insights into the photophysical properties of this type of cluster as well as the possibility to discover other highly stable compounds. In addition, with the compounds (nBu4N)2[M6I8(NCO)6] (M = Mo, W), new metal iodide clus-ters are reported. In their structure, NCO- anions are connected to the [M6I8] core as apical ligands via the nitrogen atoms. In solid state, they show the typical cluster photoluminescence with weakly pronounced oxygen quenching. For comparison of photoluminescence properties with the analogue iodides, the compounds (PPN)2[W6Cl14]·(CH2Cl2)2 and (PPh4)2[W6Cl14]·(C3H6O)2 were synthesized. In solid state, the corresponding octahedral tungsten iodide clusters show high quantum yields. In contrast to this, photoluminescence of the synthesized tungsten chloride species is only weakly pro-nounced with short lifetimes and unquantifiable quantum yields. Another contribution of this thesis concerns the synthesis of compounds combining tungsten clusters with metal ions onto which an energy transfer is possible. Reactions with RE-chlorides (RE = Gd, Tb, Eu) produced GdW6Cl15, TbW6Cl15 and EuW6Cl14 as products. Magnetic measurements revealed paramagnetism in correspondence to the magnetic moment of the RE-ion. Photoluminescence spectra recorded of crystalline samples show no emission of the RE-ions. Cooperation with Dr. T. Maulbetsch lead to the synthesis of the compound [Fe(CTP)]2[W6I14]. Photoluminescence experiments at room temperature showed no emission. Experiments con-ducted between 3 K and 100 K in order to clarify if an energy transfer occurs yielded the same results. Further experiments towards the synthesis of supramolecular [W6I14]-metal complex com-pounds lead to crystals of [Fe(CH3CN)6][W6I14]. They show no sign of luminescence but re-veal potential for the synthesis of other compounds based on a metal complex and the [W6I14]2- anion.