A Neutral Heteroleptic Molybdenum Cluster trans-[{Mo₆I₈}(py)₂I₄]

Abstract

Despite that the chemistry of octahedral cluster complexes has been actively developed recently, there are still a lot of unexplored areas. For example, to date, only a few halide M₆-clusters with N-heterocycles are known. Here, we obtained an apically heteroleptic octahedral iodide molybdenum cluster complex with pyridine ligands—trans-[{Mo₆I₈}(py)₂I₄] by the direct substitution of iodide apical ligands of [{Mo₆I₈}I₆]^2– in a pyridine solution. The compound co-crystalized with a monosubstituted form [{Mo₆I₈}(py)I₅]^– in the ratio of 1:4, and thus, can be described by the formula (pyH)_0.2[{Mo₆I₈}(py)_1.8I_4.2]·1.8py. The composition was studied using XRPD, elemental analyses, and ¹H-NMR and IR spectroscopies. According to the absorption and luminescence data, the partial substitution of apical ligands weakly affects optical properties.

1. Introduction

Octahedral halide M₆-clusters of the general formula [{M₆X₈}L₆]ⁿ (M = Mo, W, X = halogen, L—ligand of any nature) are unique compounds possessing a number of valuable practically oriented properties—this is the main point to consider when regarding compounds as promising components of different multifunctional materials. Among these properties are (i) bright luminescence in red/near infrared spectral regions (with microsecond lifetimes and high quantum yields) under the excitation of ultraviolet/visible light (up to 550 nm) or X-rays [1,2,3,4,5,6,7,8]; (ii) the efficient photosensitization of the reactive oxygen species generation process (mainly singlet oxygen, ¹O₂) [4,8,9,10]; (iii) high radiopacity due to a high local concentration of heavy atoms in a cluster core [7,11]. Thus, different groups have demonstrated the utilization of cluster complexes individually or as active components of the materials in numerous areas, such as catalytic systems [12,13,14,15,16], liquid crystals [17,18], oxygen sensors [19,20,21,22], solar concentrators [16,23,24], an active layer of solar cells [25,26], bioimaging, photodynamic therapy (PDT) [27,28,29,30,31,32,33], as components of self-sterilizing materials [34,35,36,37], or as radiocontrast agents [11]. The abundance of publications on the practical application of the clusters to some extent confirms their high potential. However, terra incognita areas are still presented in cluster chemistry, especially in the case of molybdenum and tungsten halide clusters. One of the almost unknown parts correspond to Mo₆ and W₆ clusters with nitrogen heterocycles. N-heterocycles are a huge class of organic compounds that have great structural diversity, functionalization potential, and possess various biological activities [38,39]. To date, only a few examples of such clusters with N-heterocyclic ligands are reported and only three of them are structurally characterized [40,41]. On the contrary, a fairly large number of complexes with coordinated heterocycles have been obtained in the case of rhenium clusters [42,43], indicating that N-heterocyclic ligands can preserve their useful properties upon coordination to metal centers. For example, clusters [{Re₆Q₈}(btrz)₆]^4– (Q = S, Se; btrz is benzotriazolate) similar to pure btrzH are able to bind directly to DNA [42].

Herein, we present the preparation of the new iodide Mo₆-cluster with N-heterocyclic ligand, namely, pyridine—trans-[{Mo₆I₈}(py)₂I₄]. The compound composition was confirmed using SCXRD (single crystal X-ray diffraction), XRPD (X-ray powder diffraction), elemental analyses, and ¹H-NMR and IR spectroscopies. Optical properties, such as absorption and luminescence, were studied in a solid state and in a solution.

2. Materials and Methods

All reagents and solvents employed were commercially available and used as received without further purification. (Bu₄N)₂[{Mo₆I₈}I₆] was prepared by a metathesis reaction of Cs₂[{Mo₆I₈}I₆] and Bu₄NI in acetone [44]. Cs₂[{Mo₆I₈}I₆] was synthesized from Mo, I₂, and CsI by a high-temperature reaction according to the procedure described in [44].

Elemental analyses were obtained using a EuroVector EA3000 Elemental Analyser (S.p.A., Milan, Italy). FTIR spectra in the range of 4000–400 cm⁻¹ for solid (Bu₄N)₂[{Mo₆I₈}I₆] and (pyH)_0.2[{Mo₆I₈}(py)_1.8I_4.2]·1.8py as the KBr disk or for liquid pyridine placed between the two aperture plates (KBr) were recorded on a Scimitar FTS 2000 (Digilab LLC, Canton, MA, USA). An energy-dispersive X-ray spectroscopy (EDS) was performed on a Hitachi TM3000 TableTop SEM (Hitachi High-Technologies Corporation, Tokyo, Japan) with Bruker QUANTAX 70 EDS equipment. X-ray powder diffraction (XRPD) patterns were recorded on a Shimadzu XRD 7000S diffractometer (Shimadzu, Kyoto, Japan) (Cu Kα radiation, graphite monochromator, and silicon plate as an external standard). The ¹H-NMR spectra were recorded from a saturated DMSO-d₆ solution at room temperature on a Bruker Avance III 500 FT-spectrometer (Bruker BioSpin AG, Faellanden, Switzerland) with working frequencies of 499.93 MHz. The chemical shifts were reported in ppm of the δ scale and referred to the signal of residual protons of solvent: δ = 2.5 ppm. Thermal properties were studied on a Thermo Microbalance TG 209 F3 Tarsus (NETZSCH, Selb, Germany) from 30 to 650 °C, at a rate of 10 °C min⁻¹ in He flow (30 mL min⁻¹). Mass spectrometric (MS) detection was performed with a direct injection of liquid samples via an automatic syringe pump KDS 100 (KD scientific Inc., Holliston, MA, USA) at a 180 μL h^–1 rate by an electrospray ionization quadrupole time-of-flight (ESI-q-TOF) high resolution mass spectrometer Maxis 4G (Bruker Daltonics, Bremen, Germany). Mass spectra were recorded in the negative mode within the 700–4500 m/z range. The MS calibration was performed externally using an ESI-L calibration mix (Agilent Technologies, Santa Clara, CA, USA); the typical resolution was ca. 50,000, and the accuracy was <1 ppm. Diffuse reflectance spectra were recorded using a UV-Vis-NIR 3101 PC spectrophotometer (Shimadzu Corporation, Kyoto, Japan). The excitation (monitored at λ_em = 700 nm) and emission (excitation wavelength was 350 nm) spectra were recorded by Cary Eclipse Fluorescence Spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) for compounds in the solid state and in pyridine solutions. The absorbance of pyridine solutions was set at <0.1 at 355 nm.

2.1. Synthesis

(Bu₄N)₂[{Mo₆I₈}I₆] (300 mg, 0.105 mmol) was dissolved in 3 mL of pyridine (99.5%, 0.037 mol). The resulting mixture was heated in a sealed glass tube at 110 °C for 48 h. During the reaction, a crystalline powder was formed on the walls of the tube. The powder was then separated from the solution, washed 3 times with acetone and air dried. Yield (calcd for [{Mo₆I₈}(py)₂I₄]): 56 mg (23%). EDS: Mo/I = 6:13.2. FTIR (KBr, cm⁻¹): ν_s(CH) = 2953, 2920, 2849, 1217, δ_s(CC) = 1599, 1483, 1439, 1356, δ(CH) = 1150, 1065, 1007, δ_s(CH) = 752, 696, 631. ¹H NMR ([{Mo₆I₈}(py)I₅]⁻, 500 MHz, DMSO) δ 7.54 (t, 3,5-Py), 8.12 (t, 4-Py), 9.38 (d, J = 5.4 Hz, 2,6-Py), ¹H NMR (trans-[{Mo₆I₈}(py)₂I₄], 500 MHz, DMSO) δ 7.53 (t, 3,5-Py), 8.12 (t, 4-Py), 9.38 (d, J = 5.3 Hz, 2,6-Py). A single crystal suitable for X-ray structural analysis was grown by the slow cooling of the tube during 24 h.

2.2. Crystallography

Single-crystal X-ray diffraction data were collected using a Bruker Nonius X8 Apex 4K CCD (Bruker Corporation, Billerica, MA, USA) diffractometer with graphite monochromatized MoKα radiation (λ = 0.71073 Å). Absorption corrections were made empirically using the SADABS program [45]. The structures were solved by the direct method and further refined by the full-matrix least-squares method using the SHELXTL program package [45,46]. All non-hydrogen atoms were refined anisotropically. Table S1 summarizes crystallographic data, while CCDC 2207425 contains the supplementary crystallographic data for this paper.

3. Results and Discussion

3.1. Crystal Structure and Literture Analysis

The polycrystalline product formed after the slow cooling of the reaction mixture which contained single crystals suitable for SCXRD. The compound crystalizes as solvate with two pyridines in monoclinic space group P 2₁/c (Z = 2). The crystallographic data are summarized in Table S1. The unit cell contains three independent Mo atoms and three inner Iⁱ atoms belonging to the same cluster unit. All atoms belonging to cluster core {Mo₆Iⁱ₈} are located in general positions while the center of the cluster coincides with the special centrosymmetrical position (0, 0, ½). Two of molybdenum atoms are coordinated to apical iodine ligand (I^a) and other one by pyridine (py^a) or iodine (I^a) with occupancies 0.9 and 0.1, respectively. Thus, the cluster unit represents superposition of at least two cluster complexes, namely anionic [{Mo₆I₈}(py)I₅]^– and neutral [{Mo₆I₈}(py)₂I₄] (Figure 1B,C). According to the symmetry of the cluster unit (C_i site symmetry) the neutral cluster complex [{Mo₆I₈}(py)₂I₄] is trans-isomer. In addition, such position symmetry assumes the presence of third possible cluster unit—the initial [{Mo₆I₈}I₆]^2– (Figure 1A). Based on the atoms occupancies and C_i symmetry, the possible ratio of cluster units [{Mo₆I₈}I₆]^2–:[{Mo₆I₈}(py)I₅]^–:[{Mo₆I₈}(py)₂I₄] is 0.01:0.18:0.81. According to probability calculation, the content of [{Mo₆I₈}I₆]^2– is low, and thus, the formula of the compound can be designated as (pyH)_0.2[{Mo₆I₈}(py)_1.8I_4.2]·1.8py.

Additionally, solvate pyridine molecule/pyridinium cation is located in the unit cell and interacts with terminal ligands (pyridine or I^a, respectively). The N···H–C hydrogen contacts (py^solv···py^coord) are about 3.31 Å and pyH···I^a are about 4.22 Å. The cluster unit has a typical for this type of clusters geometry, and Mo–Mo, Mo–Iⁱ and Mo–I^a distances are in good agreement with the literature data (

Table 1. Interatomic distances in trans-[{Mo₆I₈}(py)₂I₄] and related compounds.

Cluster Complex	Interatomic Distances, Å				References
	Mo–Mo	Mo–Ii	Mo–Ia	Mo–N
trans-[{Mo6I8}(py)2I4]	2.675(2)– 2.6900(17)	2.7656(18)– 2.7891(16)	2.75(2)– 2.8432(19)	2.22(3)	this work
[{Mo6I8}(N3C2H(COOCH3))6]2−	2.6721(7)– 2.6794(6)	2.7590(6)– 2.7815(6)	-	2.199(4)	[40]
[{Mo6I8}(N4C(C6H5))6]2−	2.6728(10)– 2.6856(9)	2.7588(8)– 2.7737(9)	2.844(12)– 2.995(8) #	2.230(8)– 2.238(9)	[41]
[{Mo6I8}(N4C(C6F5))6]2−	2.6703(12)– 2.6771(12)	2.7557(9)– 2.7760(11)	-	2.220(9)– 2.259(9)

^# Partially substituted forms.

) [40,41].

To the best of our knowledge, only four articles on preparation of halide Mo₆-clusters with N-heterocyclic ligands are reported. J. Kraft and H. Schäfer in 1985 described preparation of heteroleptic clusters [{Mo₆X₈}(py)₂X₄] (X = Cl, Br) and [{Mo₆Cl₈}(N-base)₂X₄] (N-base = 2,2′-bipyridine and 2-phenylpyridine; X = Cl, Br, I) [47]. These compounds were obtained by direct reaction of [{Mo₆Cl₈}X₂X_4/2] with corresponding N-heterocycle in ethanol or chloroform under reflux conditions. All compounds obtained were characterized only by elemental and thermogravimetric (TG) analyses, while no single-crystal data were obtained. Nevertheless, the authors managed to determine the crystal lattice parameters of compounds with pyridine and 2,2′-bipyridine ligands (Table S2). The crystal lattice parameters for [{Mo₆X₈}(py)₂X₄] are different from reported here, presumably due to the presence of solvate pyridine molecules in (pyH)_0.2[{Mo₆I₈}(py)_1.8I_4.2]·1.8py.

Another article by C. Perrin with co-authors dating from 2006 devoted to the preparation of heteroleptic (Bu₄N)[{Mo₆X₈}(R-py)Br₅] and homoleptic [{Mo₆X₈}(R-py)₆](CF₃SO₃)₄ clusters (X = Br, I, R = H, 4-tert-Bu, 4-vinyl; in some cases R-py—dendrons of complicate composition) from (Bu₄N)₂[{Mo₆Br₈}(CF₃SO₃)Br₅] and (Bu₄N)₂[Mo₆Br₈}(CF₃SO₃)₆], respectively [48]. These compounds were well characterized by NMR and mass-spectrometry, however, single crystals data even for complexes with simple pyridine ligands were not obtained.

The first crystallographic data of Mo₆-halide clusters were obtained only in 2020 by M. Sokolov et al. for homoleptic {Mo₆I₈}-cluster with triazole ligands, namely, (Bu₄N)₂[{Mo₆I₈(N₃C₂H(COOCH₃))₆] [40]. This compound was obtained by [2+3] cycloaddition of CH₃O(O)C≡C(O)OCH₃ to hexaazide cluster (Bu₄N)₂[{Mo₆I₈}(N₃)₆]. Later, this scientific group obtained two other clusters with tetrazole ligands, [{Mo₆I₈}(N₄C(C₆H₅))₆]^2– and [{Mo₆I₈}(N₄C(C₆F₅))₆]^2–, using the same method [41]. Mo–N bond lengths in [{Mo₆I₈}(py)₂I₄] are similar to those for triazole- and tetrazole-coordinated Mo₆-clusters (Table 1).

3.2. Synthetic Features and Characterization

Heating a dark red solution of (Bu₄N)₂[{Mo₆I₈}I₆] in pyridine at 110 °C for 48 h leads to the precipitation of a dark red crystalline product. Single-crystal analysis as well as elemental analysis (CHN, EDS) revealed the formation of a new cluster compound with the formula (pyH)_0.2[{Mo₆I₈}(py)_1.8I_4.2]·1.8py, which can be considered as a co-crystallization of two cluster complexes in the ratio of 1:4—anionic [{Mo₆I₈}(py)I₅]^– (Figure 1B) and neutral trans-[{Mo₆I₈}(py)₂I₄] (Figure 1C), i.e., {(pyH)[{Mo₆I₈}(py)I₅]·py}_0.2{trans-[{Mo₆I₈}(py)₂I₄]·2py}_0.8. Thus, during the reaction, the substitution of one or two terminal I^–-ligands occurs. A low reaction yield (about 23%) is probably associated with the equilibrium in the synthesis process presented in the following equations:

[{Mo₆I₈}I₆]²⁻ + py ⇌ [{Mo₆I₈}(py)I₅]⁻ + I⁻

(1)

[{Mo₆I₈}(py)I₅]⁻ + py ⇌ [{Mo₆I₈}(py)₂I₄] + I⁻

(2)

Iodine anion, released into the solution during the substitution reaction, shifts the equilibrium to the reagent’s side. Moreover, the formation of soluble products also shifts the equilibrium to the same side. Neutral trans-[{Mo₆I₈}(py)₂I₄] is partially soluble in hot pyridine, while [{Mo₆I₈}(py)I₅]⁻, as an ionic compound, should be relatively soluble in pyridine even at room temperature. Indeed, HR-MS of mother liquor showed the presence of monosubstituted form besides unreacted [{Mo₆I₈}I₆]^2– (Figure S2). Thus, when the concentration of I^– and [{Mo₆I₈}(py)I₅]⁻ exceeds some limit, the reaction completely stops, which results in the low yield of the desired neutral cluster. This suggestion was supported by additional experiments. Subsequent heating of the mother liquor after the removal of the solid (pyH)_0.2[{Mo₆I₈}(py)_1.8I_4.2]·1.8py, as well as heating the (Bu₄N)₂[{Mo₆I₈}I₆] solution in pyridine in the presence of an excess of I^– (in the form of Bu₄NI), did not lead to the formation of any precipitate of (pyH)_0.2[{Mo₆I₈}(py)_1.8I_4.2]·1.8py. At the same time, heating (pyH)_0.2[{Mo₆I₈}(py)_1.8I_4.2]·1.8py in pure pyridine in the presence of Bu₄NI resulted in the complete dissolution of the solid, confirming the reversibility of the process.

Phase purity and composition of the compound obtained was confirmed by XRPD, FTIR, and ¹H-NMR. Diffractogram of the powder product almost fully coincides with calculated data, except for several low intensity reflexes at low angles (Figure 2A). We believe that these signals could correspond to the admixture of pure (pyH)[{Mo₆I₈}(py)I₅]. According to FTIR, all ligand-related bands are preserved in (pyH)_0.2[{Mo₆I₈}(py)_1.8I_4.2]·1.8py (Figure 2B). The coordination of the pyridine ligands was also confirmed using NMR spectroscopy. Due to the low solubility of products (less than 1 mg in 0.6 mL of solvent), only the ¹H NMR and 2D ¹H-¹H COSY spectra were obtained. According to the NMR data, three series of the signal in the aromatic area with a ratio of 1:0.4:0.2 were observed (Figure 2C and Figure S3). The main components are the free-pyridine molecules with chemical shifts at 7.39, 7.78, and 8.58 ppm for meta-, para-, and ortho-protons, respectively. The high content of free pyridine can be explained by its leaching from the powdered complex. The coordination of pyridine to molybdenum atoms in the cluster core causes downfield shifts of the signals due to the electron-withdrawing nature of the {Mo₆I₈}⁴⁺ motif. As expected, the greatest shift in comparison with pure pyridine was observed for 2,6-atoms of hydrogen: 0.65 and 0.81 ppm. It is highly likely that the most downfield signal (9.38 ppm) refers to monosubstituted form [{Mo₆I₈}(py)I₅]⁻, whereas the second signal (9.22 ppm) refers to the main product, trans-[{Mo₆I₈}(py)₂I₄], since the electron-withdrawing effect of the {Mo₆I₈}⁴⁺ core is probably decreased with the addition of pyridine ligands. Chemical shifts for other hydrogen atoms are close, and the signals from mono- and di-substituted forms overlap (Figure S3) at 8.12 ppm for para-positions (downshift relative to the free pyridine by 0.24 ppm) and 7.53 for meta-positions (downshift by 0.15 ppm). In an attempt to increase the cluster concentration in DMSO-d₆, we heated the solution at 80 °C for 30 min. This, indeed, led to a more intense color of the solution, while ¹H NMR spectra contained only the forms that referred to free-pyridine molecules (Figure S4), thus, indicating the complete substitution of py ligands with DMSO.

According to the literature, known [{Mo₆X₈}(N-base)₂X₄] compounds lose two pyridine molecules at about 500 °C with the formation of Mo₆X₁₂ [47]. The compound obtained here, (pyH)_0.2[{Mo₆I₈}(py)_1.8I_4.2]·1.8py, demonstrates three stages of temperature decomposition: a gradual mass loss (≈3%) up to 360 °C, a sharp mass loss in temperature interval of 360–460 °C (≈7%), and a gradual mass loss up to 550 °C (≈2%) (Figure 2D). In total, the compound loss of approximately 12% of mass corresponds to 3.8 molecules of pyridine (calcd. 12.4%) and correlates well with crystal data and elemental analyses. A slight increase in the decomposition temperature in comparison with [{Mo₆X₈}(N-base)₂X₄] (X = Cl, Br) may indicate stronger Mo-N bonds in the case of (pyH)_0.2[{Mo₆I₈}(py)_1.8I_4.2]·1.8py, however, the lack of crystalline structure data do not allow us to analyze bond lengths.

3.3. Optical Properties

The absorption properties of (pyH)_0.2[{Mo₆I₈}(py)_1.8I_4.2]·1.8py were studied both in the solution and solid state and were compared to the initial cluster (Bu₄N)₂[{Mo₆I₈}I₆] (Figure 3). According to NMR data, DMSO can substitute pyridine ligands in the cluster (Figure S4), thus, pyridine was used as a solvent in order to prevent even a partial substitution. One can see that in the pyridine solution, both clusters have almost similar absorption profiles extended up to ~550 nm, though (pyH)_0.2[{Mo₆I₈}(py)_1.8I_4.2]·1.8py demonstrates more pronounced peaks at ~351 and ~500 nm, and a redshifted peak at ~426 nm in comparison to (Bu₄N)₂[{Mo₆I₈}I₆] (~420 nm) (Figure 3A). Overall, these changes are caused by changes in the ligand’s environment, yet this effect is weak due to preservation of the majority of apical iodide ligands. Even less differences were observed in the case of the absorption in the solid state (Figure 3B). One can see that the profile shapes of both compounds are almost similar. The optical energy gaps (E_g), referred to as spin-allowed transitions (singlet state to singlet state), were calculated via the Tauc plot (see the inset in Figure 3). The E_g values of (pyH)_0.2[{Mo₆I₈}(py)_1.8I_4.2]·1.8py and (Bu₄N)₂[{Mo₆I₈}I₆] are close and equal to 1.75 and 1.6 eV, respectively.

Powdered (pyH)_0.2[{Mo₆I₈}(py)_1.8I_4.2]·1.8py exhibits very low emissions, but its maximum coincides with that of (Bu₄N)₂[{Mo₆I₈}I₆] (λ_em ~ 700 nm) (Figure 4A). Interestingly, the reverse situation is observed in the solution in pyridine, and the complex obtained here exhibits an emission intensity comparable to that of the initial complex (Figure 4B). This behavior is explained by a decrease in inter-cluster interactions in the solution, thus, reducing the efficiency of non-radiative energy transfer [2,49]. Indeed, according to crystal data, the distances between apical and inner (I^a-Iⁱ and Iⁱ-Iⁱ) iodine ligands belonging to neighboring cluster units are 3.800–3.875 Å (Figures S5 and S6), which is lower than the sum of van der Waals radii (1.98 Å), thus, indicating the formation of weak halogen (I-I) bonds [50]. Obviously, these interactions do not exist in the solution, resulting in the appearance of emissions after dissolution. Due to the presence of a large cation in (Bu₄N)₂[{Mo₆I₈}I₆], there are no noticeable inter-cluster interactions, and thus, the differences in its emission in the solution and solid state are not so significant. It should be noted that the excitation and emission spectra of both compounds have an identical shape, which is consistent with the weak effect of the partial substitution on the optical properties.

4. Conclusions

To conclude, here, by simple heating (Bu₄N)₂[{Mo₆I₈}I₆] in pyridine, we obtained a rare example of apically heteroleptic neutral octahedral iodide molybdenum cluster complexes with N-base ligand—trans-[{Mo₆I₈}(py)₂I₄]. Due to the reversibility of the reaction process, the iodine anions released during the synthesis shift the equilibrium to the reagent’s side, stopping the reaction, and thus, resulting in a low yield of the desired product. According to SCXRD, the entitled compound co-crystallizes with anionic monosubstituted form [{Mo₆I₈}(py)I₅]⁻ in the ratio of 1:4. TGA data revealed higher temperatures of pyridine ligand loss in comparison with the literature, thus, indicating stronger Mo-N bonds in the case of Mo₆ iodide clusters. This can be an additional driving force of the reaction, despite the natural competition of iodine to be bonded to Mo₆I₈ units. The formation of [{Mo₆I₈}(py)I₅]⁻ was also confirmed by HR-MS and ¹H-NMR. The absorption of the entitled compound is almost similar to the initial cluster both in the solution and solid state, with optical bandgap values of 1.75 vs. 1.6 eV for (pyH)_0.2[{Mo₆I₈}(py)_1.8I_4.2]·1.8py and (Bu₄N)₂[{Mo₆I₈}I₆], respectively. Similar behavior was observed in the case of luminescent properties in the pyridine solution, while in the solid state, the pyridine-substituted cluster showed almost no emissions. Overall, this indicates a weak effect of the partial ligand substitution on optical properties.