Synthesis, crystal structure, and photophysical properties of acid (H3O)2[{W6Br8}Br6] · 4H2O

Abstract

It is well known that upon UV irradiation many chalcogenide and halide hexanuclear clusters of tran sition metals, for example, [{Re 6 Q 8 }L 6 ] (Q = S, Se) and [{M 6 X 8 }L 6 ] (M = Mo, W; X = Cl, Br), where L are various inorganic and organic ligands, luminesce in the red spectral range with microsecond emission life times [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Phosphorescence in the red spectral range and, as a consequence, the ability to generate singlet oxygen, as well as specific features of the struc tures of similar octahedral clusters of heavy metals, suggest that these compounds are potentially promis ing for the use as photosensitizers in photodynamic therapy and as X ray contrast preparations in diagnos tics of oncological diseases. The experimental studies and electron density calculations showed that the pho tophysical properties (the position of the spectral maximum, quantum yield, and emission lifetime) depended, to a considerable extent, on the type of internal (Q, X) and apical (L) ligands.Tungsten(II) bromide has been known since the end of the 19th century [19,20], but the cluster nature of W 6 Br 12 and derivatives was established much later, in 1967 [21]. It should be mentioned that the lumines cence properties of the compounds based on the {W 6 Br 8 } 2+ cluster were studied very rarely in spite of their many year history [12,13,18,22].In the present work, we describe the synthesis, structure, and luminescence properties of the finely crystalline sample (H 3 O) 2 [{W 6 Br 8 }Br 6 ] ⋅ 4H 2 O (I). EXPERIMENTALTungsten powder, dibromine, ethanol, and con centrated HBr (not lower than analytical grade) were used as received. Powder diffraction patterns were recorded at room temperature on a Philips APD 1700 diffractometer (CuK α radiation, graphite monochro mator). For compound I, the ratio of heavy elements was determined by energy dispersive X ray analysis (EDXA) using a JSM 6700F scanning electron microscope. The IR spectrum was recorded on a Scimitar FTS 2000 spectrometer (pellets with KBr). Thermal gravimetric analysis (TGA) was carried out on a Netzch TG 209 F1 Iris instrument: the sample was heated in a helium flow from 25 to 400°С with a rate of 10°C/min. To study the luminescence proper ties, the pure crystalline substance was divided in an agate mortar, and the obtained powder was placed between two non fluorescing glass plates. The cor rected emission spectrum and the emission decay were recorded at room temperature using a Hamamatsu Photonics PMA 11 multichannel photodetector and a Hamamatsu Photonics C4334 streak camera, respec tively. When recording the luminescence spectrum and analyzing the emission decay, the sample was excited by a LOTIS TII LS 2137/3 ND:YAG pulse laser (pulse duration 6 ns) with the wavelength 355 nm. The luminescence quantum yield was detected using a Hamamatsu Photonics C9920 03 complex setup for the determination of the absolute photoluminescence quantum yield. The setup consists of a xenon lamp as an excitation source (excitatio...

Complexes of {W6I8}4+ Clusters with Carboxylates: Preparation, Electrochemistry, and Luminescence

“…Octahedral halide cluster complexes of Mo II and W II , [{M 6 (µ 3 ‐X) 8 }L 6 ] (M = Mo or W; X = Cl, Br or I; L = neutral or anionic ligands), are chemically robust constructions with outstanding photophysical properties; therefore, they are potentially useful for various applications . The most studied are the derivatives of molybdenum chloride and bromide (M = Mo; X = Cl, Br).…”

Improved Synthesis of (TBA)2[W6Br14] Paving the Way to Further Study of Bromide Cluster Complexes

“…32 Thus, the region of tungsten bromide cluster complexes is "terra incognita" on the map of the properties of {M 6 X 8 } 4+ cluster complexes, which can be attributed to more labor-intensive and expensive methods for obtaining the initial compounds (TBA) 2 [W 6 Br 14 ] or W 6 Br 12 in gram amounts in comparison with the molybdenum ones 38,39 or tungsten chloride and iodide cluster complexes. 3,40,41 Three main approaches to the formation of the bromide cluster core {W 6 Br 8 } 4+ are presented in the literature: (1) the reduction of higher halides WBr 5 or WBr 6 with various reducing agents, such as hydrogen, 42,43 tungsten, 44 and aluminum 45−47 as well as bismuth, 48 antimony, 37,49 and red phosphorus, 50 which were used to reduce WBr 6 in Meyer's group; (2) the interaction of tungsten and bromine, which was first proposed in the work of Schafer in 1896 51−53 and developed later; 54 (3) the WBr 4 disproportionation described by Brown in his dissertation of 1963 55 and the work of 1964. 56 In addition, the synthesis of bromide cluster complexes as a result of the replacement of terminal and internal chloride ligands of polymeric W 6 Cl 12 with a LiBr/KBr salt melt (20-fold excess, 40:60 ratio) at 350 °C has been described.…”

“…Three main approaches to the formation of the bromide cluster core {W 6 Br 8 } 4+ are presented in the literature: (1) the reduction of higher halides WBr 5 or WBr 6 with various reducing agents, such as hydrogen, , tungsten, and aluminum − as well as bismuth, antimony, , and red phosphorus, which were used to reduce WBr 6 in Meyer’s group; (2) the interaction of tungsten and bromine, which was first proposed in the work of Schafer in 1896 − and developed later; (3) the WBr 4 disproportionation described by Brown in his dissertation of 1963 and the work of 1964 . In addition, the synthesis of bromide cluster complexes as a result of the replacement of terminal and internal chloride ligands of polymeric W 6 Cl 12 with a LiBr/KBr salt melt (20-fold excess, 40:60 ratio) at 350 °C has been described .…”