The Remarkably Robust, Photoactive Tungsten Iodide Cluster [W6I12(NCC6H5)2]

The new heteroleptic tungsten iodide cluster compound [W 6 I 12 (NCC 6 H 5 ) 2 ] is presented. The synthesis is carried-out from Cs 2 W 6 I 14 and ZnI 2 under solvothermal conditions in benzonitrile solution, yielding red cube-shaped crystals. [W 6 I 12 (NCC 6 H 5 ) 2 ] represents a heteroleptic [W 6 I 8 ]-type cluster bearing four apical iodides and two benzonitrile ligands. Molecular [W 6 I 12 (NCC 6 H 5 ) 2 ] clusters form a robust hydrogen bridged crystal structure with high thermal stability and high resistibility against hydrolysis. The electronic structure is analyzed by quantum chemical methods of the calculated electron localization function (ELF) and the band structure. Photoluminescence measurements are performed to verify and describe the photophysical properties of [W 6 I 12 (NCC 6 H 5 ) 2 ]. Finally, the photocatalytic properties of [W 6 I 12 (NCC 6 H 5 ) 2 ] are evaluated as a proof-of-concept.


Introduction
Metal halide clusters M 6 X 12 (M = Mo, W; X = Cl, Br, I) consist of an octahedral metal core surrounded by eight inner halide ligands located above the faces of the octahedron, and six apical halide ligands above the edges (four of them shared between clusters).The structure of these binary metal halides was first reported in the 1960s by Schäfer et al. for Mo 6 Cl 12 . [1]Based on this work, structures have also been reported for other octahedral tungsten and molybdenum metal halides. [2]Since then, it has been discovered that these clusters show a bright red to orange luminescence based on their [M 6 X 8 ] moieties, which is caused by excitation of an electron from the ground state S 0 into an excited singlet state S n , followed by intersystem crossing (ISC) into excited triplet states and subsequent relaxation into the ground state through phosphorescence. [3]ontinuing research discovered the energy transfer from these excited triplet states onto molecular oxygen to form singlet oxygen. [4]These and other optical properties make cluster compounds based on the [M 6 X 8 ] type interesting for applications in various different fields like in medicine (photodynamic therapy, [5] X-ray contrast agent, surface disinfection [6] ), photocatalysis, [7] solar energy harvesting, [8] oxygen sensing [9] and so on.
Typically, these octahedral metal halide clusters are semiconductors featuring optical bandgaps around 2 eV, [7d,10] qualifying them for applications in photocatalysis.Their potential in this field has been proven by water purification from organic dyes, as has been reported for two cluster species, showing good activity in the decomposition of rhodamine B. [7c,d,10c] However, stability is always a concern when using tungsten or molybdenum halide clusters in reactions or applications involving aqueous media.Thus, the field of possible candidates is limited.Species bearing organic anions are prone to undergo ligand exchange with OH À . [5,11]esearch into the photophysical properties of the octahedral metal halide clusters has mostly been undertaken on anionic cluster species of the form [M 6 X 8 L 6 ] 2À (M = Mo, W; X = Cl, Br, I; L = Cl, Br, I, organic anions) [10a,12] accompanied by large organic cations like tetrabutylammonium (TBA), triphenyl phosphonium (PPh 4 ) or bis(triphenylphosphine)iminium (PPN). [13]However, investigations regarding the luminescence of neutral and cationic cluster species are scarce, even though first examples were reported decades ago. [14]10b,15] Herein we report on the synthesis and structure of the new neutral octahedral tungsten iodide cluster [W 6 I 12 (NCC 6 H 5 ) 2 ], which bears two benzonitrile ligands, and subsequent investigations into it's thermal stability and stability in aqueous media, bonding and electronic conditions, photoluminescence studies and on the photophysical decomposition of rhodamine B.

Results and Discussion
15b] Reactions starting from the soluble cluster species W 6 I 22 under solvothermal conditions in benzonitrile, performed in a fused quartz ampoule yielded [W 6 I 12 (NCC 6 H 5 ) 2 ] (1, Figure 1, left side) as black cube-shaped crystals (Figure 1, middle).We attributed the black body color to the excess of iodine in the reaction and thus tried to remove it by rinsing or redissolving the crystals.Unfortunately, the compound is insoluble in all common solvents including benzonitrile.Subsequent synthesis approaches departed from Cs 2 W 6 I 14 in benzonitrile, which could be expected to form [W 6 I 12 (NCC 6 H 5 ) 2 ] plus CsI straight forwardly.However, the resulting product was not our desired compound 1 and remained unidentified.Trying to capture CsI, we added ZnI 2 to obtain Cs 2 ZnI 4 as byproduct.The reaction mixture of Cs 2 W 6 I 14 , ZnI 2 in benzonitrile (solvothermal conditions at 200 °C) yielded 1 as orange cube-shaped crystals with a red to orange photoluminescence (Figure 1, right side) and Cs 2 ZnI 4 as byproduct.
Like other octahedral tungsten halide compounds, 1 features the characteristic [W 6 X 8 ]-type cluster with six ligands attached at the corners of the octahedral tungsten core (Figure 1, left side).Four iodide and two benzonitrile ligands in these positions represent a heteroleptic cluster species.15b] [W 6 I 12 (NCC 6 H 5 ) 2 ] crystallizes in the monoclinic space group P2 1 /c, with the clusters oriented in layers in the bc plane (Figure 2, left side).The isolated cluster units are alternately tilted to the right and left (Figure 2, right side), to allow a tight packing arrangement of molecular clusters without the presence of any co-crystallized solvent.Some crystallographic data are presented in Table 1.

Electron localization function and band structure
10a,12c,19] For this reason we had a closer look at the nature of the bonding with apical ligands.For this purpose, the electron localization function Along the WÀ N bond four local maxima are visible, while in case of the WÀ O bond only two are present, and the WÀ I bond shows three.The number of local maxima is related to the degree of covalency, [20] and suggests higher covalency in 1.Nevertheless, all shown bonds between WÀ N/O/I are primarily ionic, as evidenced by the presence of a nodal plane at the midpoint between the atomic centers.
The calculated electronic band structure of 1 (Figure S1) shows it to be a semiconductor with an indirect band gap of 1.9 eV, corresponding well with the experimental optical band gap of 2.17 eV.Note, the value derived from DFT is likely an underestimate, due to the well-known band gap problem.

Stability
The employment of a given material, like octahedral metal halide clusters in fields like oxygen sensing, [9b,21] photocatalysis [7,22] or solar cells [8b,23] requires long-term stability.Particularly three key aspects need to be avoided, namely thermal decomposition, oxidation in air, and hydrolysis.Therefore, one important property is thermal stability.Binary tungsten or molybdenum halides M 6 X 12 (X = Cl, Br, I) are stable at high temperatures. [1,24]This changes after outer halide ligands are substituted by organic ligands.Investigations into the thermal stability of 1 by thermogravimetric (TG) analysis under argon (Figure 5) shows the compound to be stable until 400 °C without any mass loss beforehand.Above 400 °C a mass loss of around 7 % occurs, which corresponds to the mass of benzonitrile (7.3 %).The remaining W 6 I 12 begins to decompose around 650 °C.
Stability up to 400 °C is previously unreported in any tungsten halide cluster bearing organic ligands.Related molybdenum cluster compounds have shown lower stabilities. [25]10b] Aside from temperature, stability against hydrolysis is also a concern for octahedral metal halide clusters.Exposed to aqueous media the octahedral tungsten and molybdenum iodide cluster species tend to partially exchange apical ligands by hydroxyl groups. [5,11]With the goal of long-term stability and retention of the clusters photophysical properties this is disadvantageous.Therefore, it was interesting to see if the cluster can remain intact upon continuous contact with water.For that purpose, a small amount of 1 was dispersed into water for over a month.Afterwards the body color of the powder was unchanged, the PXRD pattern showed phase pure 1 (Figure S3), suggesting hydrolytic stability in aqueous media.

Luminescence studies
10a,12,19,26] Typically, these clusters are anionic species and only few examples exist for neutral or cationic species.Hence investigations concerning their luminescence are rare.However, it is assumed that [Mo 6 Cl 12 (CH 3 CN) 2 ] is an intermediate in the production of the oxygen sensor made from [Mo 6 Cl 12 ]. [9]10b] Like other octahedral metal halide clusters, 1 shows a broad excitation band between 250 nm and 550 nm (18200 cm À 1 ) and a broad emission band peaking at around 630 nm (15900 cm À 1 ) with a FWHM of 3000 cm À 1 (Figure 6).The photoluminescence emission is rather weak with a low quantum yield (0.4 %) and short lifetimes of τ 1 = 0.48 μs (29 %) and τ 2 = 1.57μs (71 %) (Figure S6).We attributed the short triplet lifetimes to a small energetic splitting between the emitting triplet sublevels with the highest lying sublevel possessing an allowed transition towards the ground state. [27]Low energetic splitting is explained to be a consequence of high d-electron density on the metal atoms. [27]The observed electron density in the ELF is comparable to the one of (TBA) 2 [W 6 I 14 ] and far higher than for (TBA) 2 [W 6 I 8 (CO 2 C 3 F 7 ) 6 ] (Figure 4).Additionally, due to the observed dense packing of cluster units, very short decay times and low intensities can be expected.15a,28]

Photocatalysis
Purifying wastewater from persistent organic pollutants is an extensively investigated topic.Many studies deal with removing or breaking down organic pollutants for example, estrogen or triclosan. [29]Heterogenous photocatalysis is considered as a promising approach using TiO 2 or other semiconducting materials as catalysts.Upon irradiation with UV radiation or visible light these materials produce reactive oxygen species. [30]deally, the pollutant is completely broken down to CO 2 and H 2 O.However, TiO 2 with a band gap of about 3.2 eV uses only a small portion of the solar light spectrum. [31]7d,10] Consequently, they can potentially be used for photocatalytic applications.It has been shown that some compounds are active in the photocatalytic decomposition of rhodamine B (RhB).For example, this is reported for Na 2 [Mo 6 Br 8 (N 3 ) 6 ], [7d] nanocomposites out of gold nanoparticles, graphene oxide and Na 2 [Mo 6 Br 8 (N 3 ) 6 ] [7c] and [Mo 6 I 8 (H 2 O) 2 (OH) 4 ] supported on hÀ BN. [10c] Different studies also showed the potential of these clusters in the photocatalytic water reduction. [22]s has been mentioned, stability is a concern for molybdenum and tungsten halide clusters.From this perspective the durability of 1 in the presence of water or temperature is remarkable and make it a suitable candidate for photocatalytic applications.Thus, we attempted, in a proof of concept, to photocatalytically decompose RhB in water with 1 as a photocatalyst.During the experiment we used the decay of the absorption band of RhB at ca. 554 nm to monitor the decomposition of the dye against irradiation time.We recorded the changes during a time span of 180 min, while we stirred the suspension/solution in the dark for 30 min to establish an adsorption and desorption equilibrium and subsequent UV irradiation for 150 min.The control experiment, with a pure RhB solution showed less then 10 % degradation after 150 min irradiation with UV radiation and otherwise identical conditions.
Figure 7a shows the recorded UV/Vis spectra of the RhB solution with 1 as photocatalyst during the experiment.The decreasing intensity of the rhodamine B absorption and the characteristic hypsochromic shift of the absorption maximum over time are clearly visible.This shift can be observed due to  the stepwise de-ethylation of the RhB-molecule.In Figure 7b the change of concentration is plotted in dependence of the irradiation time.After 30 min without irradiation, an equilibrium of desorption and adsorption of the dye on the particle surface of 1 is achieved.Around 22 % of the dye is adsorbed onto the particle surface.Over the 150 min of irradiation time a continuous decrease of the rhodamine B absorption is visible.Afterwards, more than 40 % of the dye have been decomposed.Decomposition was also observed in an additional experiment using small screwcap vials filled with a suspension of 5 mg of 1 in 10 mL of the RhB solution (Figure S4).During the experiment the vials were placed in front of the window in the daylight (conducted during winter).After 2-3 days the solution was completely colorless, indicating complete decomposition of the RhB molecules.This was repeated for two times with similar results.The decomposition of RhB is reported to show three different stages of decomposition, with the first being formation of de-ethylated rhodamine B species, the second features different compounds resulting from a cleaved chromophore system and the third with complete cleavage into different aliphatic alcohols and carboxylic acids. [32]A cleavage of the chromophore structure is reported to be the dominant step.7d,30] The process may involve hydroxyl radicals (OH * ), superoxide radicals (O 2 *À ) or singlet oxygen.

Conclusions
The new [W 6 I 12 (NCC 6 H 5 ) 2 ] is a remarkable compound because it demonstrates a number of important features in cluster chemistry.Due to the neutral charge of this species the crystal structure shows molecular clusters.Despite this molecular appearance, these clusters are densely packed and strongly interconnected via hydrogen bonding in the crystal.This bridging of adjacent clusters adds to the stability of the compound and allows decomposition temperatures above 400 °C.
Normally, ligand substituted tungsten or molybdenum halide clusters show low stability against water.However, [W 6 I 12 (NCC 6 H 5 ) 2 ] displays no sign of hydrolysis, even after being suspended into water for over a month.Thermal and hydrolytic stability and a narrow band gap of 2.17 eV allow for the compound to be used as a photocatalyst for excitation at wavelengths below 570 nm.
[W 6 I 12 (NCC 6 H 5 ) 2 ] successfully acts as a photocatalyst in the decomposition of rhodamine B in water.Repeated photocatalysis experiments using solar light demonstrate continuous activity without visible hydrolysis.
The preparation of the starting materials Cs 2 W 6 I 14 [12c] or W 6 I 22 [33] is reported elsewhere.
Thermoanalysis.Differential thermal analysis (DTA) was performed with a STA 449F3 Jupiter apparatus (Netzsch, Selb, Germany).Samples were filled into corundum containers and analyzed between room temperature and 1000 °C with a heating and cooling rate of 2 °C/min under argon stream.
Single-crystal X-ray Diffraction.The single-crystal X-ray diffraction (XRD) study on [W 6 I 12 (NCC 6 H 5 ) 2 ] was performed using a Rigaku XtaLab Synergy-S diffractometer with MoÀ K α radiation (λ = 0.71073 Å) and a mirror monochromator.A cube-shaped singlecrystal was used for the measurement under N 2 cooling at 100 K. Corrections for absorption effects were applied with CrysAlisPro 1.171.41.65a (Rigaku Oxford Diffraction, 2020).The structure was solved by direct methods (SHELXS), [35] and full-matrix least-squares structure refinements were performed with SHELXL-2014 [36] implemented in Olex2 1.3-ac4.Deposition Number 2084368 (for 1) contains the supplementary crystallographic data for this paper.These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.
Luminescence.Excitation and emission spectra of [W 6 I 12 (NCC 6 H 5 ) 2 ] were recorded by using a fluorescence spectrometer FLS920 (Edinburgh Instruments) equipped with a 450 W xenon discharge lamp (OSRAM).Additionally, a mirror optic for powder samples was mounted inside the sample chamber.For detection, an R2658P single-photon-counting photomultiplier tube (Hamamatsu) was used.All photoluminescence spectra were recorded with a spectral resolution of 1 nm, a dwell time of 0.5 s in 1 nm steps and 2 repeats.The photoluminescence decay curve was measured with the same spectrometer using a 445 nm picosecond laser.

Photocatalysis.
The photocatalysis experiments were carried out with a standard UV radiation with a wavelength of 395 nm (5 W). 40 mL of a 40 μmol/l solution of rhodamine B dissolved in distilled water was transferred into a beaker and 20 mg of [W 6 I 12 (NCC 6 H 5 ) 2 ] powder was added.After À 30, À 15, 0, 10, 20, 30, 45, 60, 75, 90, 120 and 150 min of irradiation time a sample of 3 mL was centrifuged, transferred into a glass cuvette and UV/Vis spectra were recorded in the range between 200 nm and 800 nm.For recording the spectra, a Cary 60 UV-Vis spectrophotometer from Agilent was used.Photocatalysis experiments with solar light were carried out by dispersing 5 mg of 1 in 10 mL of the rhodamine B solution in a screwcap vial placed in front of the window.
UV-Vis in diffuse reflectance.For recording the spectra a Maya 2000 Pro spectrometer from OceanOptics equipped with a praying mantis sample chamber from Harrick and a DH-2000-BAL (Deuterium-Tungsten-Lamp) as a light source from OceanOptics.As a software OceanView 1.6.7 (lite) from OceanOptics was used with following settings: scans to average = 10, boxcarwidth = 5 and integration time = 115 ms.The band gap E g of 1 was calculated using following equation (αhν) 1/n = A • (hν-E g ), where α is the absorption coefficient, h is the Planck constant, A is a material related constant and hν is the photon energy.For an indirect band gap n = 2.
DFT. Density functional theory (DFT) was used to calculate the ELF of 1 and (TBA) 2 [W 6 I 14 ], using the Abinit software package. [37]alculations were performed using a 36 Ha plane-wave basis set energy cutoff and 4 × 4 × 4 (1), and 2 × 2 × 1 (TBA) 2 [W 6 I 14 ] Monkhorst-Pack grids [38] of k points.Norm-conserving pseudopotentials were used as received from the Abinit library.Example input files are available as part of the Supporting Information.
Elemental analyses.Elemental Analyses (C, H, N) was obtained using a UNICUBE apparatus from ELEMENTAR.
EDX Measurement.The EDX spectroscopy data was collected using a Hitachi SU8030 scanning electron microscope equipped with a Bruker QUANTAX 6G EDX detector.
Electron Microscopy.Scanning electron microscopy (SEM) was carried out on a Hitachi SU8030 scanning electron microscope equipped with a Bruker QUANTAX 6G EDX detector.
The authors declare no competing financial interest.

Figure 1 .
Figure 1.Structure of the neutral tungsten iodide cluster 1 (left side) with tungsten atoms as turquoise, iodine as pink, nitrogen as blue, carbon as brown, and hydrogen as black balls.Single crystals of 1 resulting from the synthesis from [W 6 I 22 ] (middle) and from Cs 2 W 6 I 14 with addition of ZnI 2 (right side).

Figure 2 .
Figure 2. Projection of the crystal structure of 1 in the ab (left side) and bc plane (right side) with W atoms as turquoise, I as pink, carbon as brown, nitrogen as blue and hydrogen as black spheres.
(ELF) was calculated to investigate the WÀ N bonding of 1 and compared with the WÀ I bonding in (TBA) 2 [W 6 I 14 ] and the WÀ O bonding in (TBA) 2 [W 6 I 8 (CO 2 C 3 F 7 ) 6 (Figure 4).The surfaces of the ELF show high values about the metal centers for both 1 and (TBA) 2 [W 6 I 14 ].Significantly lower values are obtained in (TBA) 2 [W 6 I 8 (CO 2 C 3 F 7 ) 6 ] featuring a strong electron withdrawing ligand, implying a reduced ionicity in 1 and (TBA) 2 [W 6 I 14 ].

Figure 3 .
Figure 3. Projection of the crystal structure of 1 on the bc plane with close contacts between hydride and iodide marked as red dashed bonds.W atoms are displayed as turquoise, I as pink, carbon as brown, nitrogen as blue and hydrogen as black spheres.

Figure 5 .
Figure 5. Thermal analysis of crystalline material of 1 (see inset of a SEM micrograph) under argon flow with the thermogravimetric (TG) curve displayed in black and the differential thermal analysis (DTA) curve in red.

Figure 6 .
Figure 6.Excitation (black curve) and emission (blue curve) spectrum of crystalline 1 recorded at room temperature.

Figure 7 .
Figure 7. UV/Vis absorption spectra of the photocatalytic decomposition of RhB with 1 as catalyst starting from À 30 min to 150 min irradiation time (left side).Time-dependent course of the RhB concentration during the photocatalysis experiment with and without 1 as photocatalyst.