First Pentanuclear Molybdenum Iodide Cluster (Bu4N)[Mo5OI13]: Synthesis and Structure

The first iodide cluster of molybdenum with the metal cage in the form of a tetragonal pyramid (Bu4N)[Mo5OI13]·THF (I) and cocrystallizate (Bu4N){[Mo5OI13]0.9[Mo6I14]·THF (II) are synthesized for the first time by heating a LiI–I2–Mo mixture in a temperature range of 300–400°С followed by the extraction of the product. Complexes I and II are studied by X-ray diffraction analysis (CIF files CCDC nos. 2063029 (I) and 2063030 (II)). The molybdenum atoms in [Mo5OI13] form a square pyramid with Mo–Mo distances of 2.67 Å between the basal molybdenum atoms and the Mo–Mo distances equal to 2.72 Å between the apical and basal molybdenum atoms. The oxygen atom is coordinated to the pyramid base (Mo–μ4-O 2.10 Å). The cluster anion [Mo5OI13] can be presented as the octahedral cluster anion [Mo6I14] in which the position of the {MoI}– fragment (d0, 6e) is occupied by the isoelectronic oxygen atom (s2p4, 6e). In the structure of compound II, the [Mo5OI13] and [Mo6I14] cluster anions occupy close atomic positions.


INTRODUCTION
The formation of stable cluster groups with metalmetal bonds of various multiplicity is a characteristic feature of lower molybdenum and tungsten halides [1]. The clusters containing the cluster cores {M 6 X 8 } 4+ (M = Mo, W; X = Cl, Br, I) are most studied [2]. Active interest in them is presently evoked by the ability of these clusters to exhibit bright red phosphorescence, which can find use in diverse areas [3]. As a rule, the structures of metal cages of the clusters of lower nuclearity (3-5 metal atoms) can formally be removed from the octahedron by the removal of one (square pyramid), two ("butterfly" or square), and three (triangle) vertices ( Fig. 1), although the tetrahedral clusters are also known [4,5]. The pentanuclear clusters with the square pyramid structure are of interest as possible precursors of heterometallic octahedral clusters of the {M 5 M'X 8 } 4+ type (M' is heteroatom), especially in the case of X = I, since these are the iodide cluster complexes of molybdenum [Mo 6 I 8 L 6 ] z (L are terminal ligands, and z is the charge of the coordination sphere) that demonstrate the highest quan-tum yields and lifetimes [3]. It can be expected that the replacement of one of the molybdenum atom by a heterometal would increase the range of quantitative characteristics of phosphorescence (maximum of emission wavelength, quantum yield, and lifetime). The pentanuclear iodide clusters both in the composition of binary phases [5] and as anionic complexes [W 5 I 13 ] -/2and [W 5 (μ 4 -C)I 13 ] - [6] were described for tungsten, while only the chloride and bromide clusters [Mo 5 X 13 ] 2-(X = Cl, Br) were synthesized for molybdenum [7][8][9][10].
The first example of the molybdenum iodide cluster (Bu 4 N)[Mo 5 OI 13 ]·THF with the metal cage structure as a square pyramid was obtained and studied by X-ray diffraction analysis (XRD) in this work.

EXPERIMENTAL
Organic solvents (tetrahydrofuran (THF), acetonitrile, and ethanol) were purified using standard procedures, and hexane (special purity grade) and diethyl ether were used as received. The starting reagents (Mo, I 2 , LiI, and Bu 4 NI) with the purity not lower than analytical grade were purchased from commercial sources. High-temperature syntheses were carried out in evacuated and sealed glass ampules. After the ampules were opened, all manipulations were carried out in air. Anhydrous LiI was stored in a dry box.
IR spectra were recorded on VERTEX 80 (600-100 cm -1 ) and SCIMITAR FTS 2000 (4000-400 cm -1 ) spectrometers. Raman spectra were recorded on a LabRAM HR Evolution spectrometer (Horiba, Japan) at the 632.8 nm line of a He-Ne laser. Analysis by energy dispersive X-ray spectroscopy (EDXA) was performed on a Hitachi Tabletop Microscope TM-3000 spectrometer equipped with a QUANTAX 70 system. Elemental analysis was conducted on a Euro EA 3000 CHNS analyzer at the Analytical Laboratory of the Nikolaev Institute of Inorganic Chemistry (Siberian Branch, Russian Academy of Sciences).
Mass spectrometric data were obtained on a 6130 Quadrupole MS, 1260 infinity LC liquid chromato-graph (Agilent) combined with a mass spectrometer (LC-MS). The LC-MS analysis was carried out in a range of (m/z) 350-3000 for both positively and negatively charged ions in the scan mode. Electrospray was used as the ionization source. A gaseous nitrogen flow served as a drying agent (350°С, flow rate 7 L/min, pressure on the sprayer (nitrogen) 60 psi, and voltage on the capillary 4000 V). In order to retain weakly bound forms in the mass spectra, the voltage on the fragmentor was zero in all experiments. A solution of the studied compound (5 μL) in deuterated acetonitrile with a concentration of ~10-4 g/mL was injected into the mobile phase (special purity grade acetonitrile) with a flow rate of 0.4 mL/min, sprayed, and ionized. The experimental peaks were compared with the calculated ones, including those by the isotope distribution. The Molecular Weight Calculator by Matthew Monroe program was used for the calculations.

Synthesis of (Bu 4 N)[Mo 5 OI 13 ]·THF (I).
A mixture of the Mo powder (1.403 g, 0.014 mol), I 2 (4 g,  0.016 mol), and LiI (0.767 g, 0.0057 mol) in a dry box was placed in a glass ampule, which was evacuated, sealed, and placed in a muffle furnace, which was heated to 400°С for 4 h. This temperature was maintained for 48 h. After the ampule was cooled to room temperature, the melt was taken out and triturated with a Bu 4 NI powder (6 g, 0.016 mol), and the resulting mixture was heated to 150°С in a glass ampule for 24 h. The cooled melt was washed with ethanol to remove Bu 4 NI excess and dissolved in acetone. The residue was extracted with THF, combined with the acetone extract, and evaporated. The solid product was washed with ethanol and diethyl ether on a porous glass filter. The weight of the washed and dried (with an air flow on the glass filter) chocolate-colored powder was 3.330 g. The product contained a mixture of (Bu 4 N) 2  XRD of the samples of compounds I and II was carried out at 150 K on a Bruker D8 Venture diffractometer (Center for Collective Use of the Nikolaev Institute of Inorganic Chemistry (Siberian Branch, Russian Academy of Sciences)) equipped with a CMOS PHOTON III detector and an IμS 3.0 microfocus source (MoK α , Montel focusing mirrors). The data were reduced using the APEX3 program package [11]. The structures were solved and refined using the SHELXT [12] and SHELXL [13] programs and the Olex2 software shell [14]. The studied sample of compound I was a joint of two main crystalline domains. The data with allowance for the both domains were not reduced correctly because of a too high difference in intensities of the low-and high-angle reflections. Data reduction for the main domain and the subsequent introduction of the twinning matrix did not allow us to completely avoid the presence of the artifact residual electron density, which resulted in a relatively high final R factor. The structure of compound II exhibited the disordering of the O and {MoI}fragments of two different clusters. The Bu 4 N + atoms should also occupy different positions according to different steric effects of the clusters, but the disordering of Bu 4 N + was not introduced because of the low population of alternative atomic positions and the absence of the corresponding pronounced peaks. The crystallographic data and experimental and structure refinement details for compounds I and II are given in Table 1.
The XRD data were deposited with the Cambridge Crystallographic Data Centre (CIF files ССDC nos. 2063029 (I) and 2063030 (II); http://www.ccdc.cam. ac.uk/conts/retrieving.html) and are available at request of the authors.

RESULTS AND DISCUSSION
The oxoiodide cluster complex (Bu 4 N)[Mo 5 OI 13 ]· THF (I) was synthesized in two stages. At the first stage, a mixture of Mo, I, and LiI was heated at 400°С for 48 h in the ratio assuming the formation of the cluster salt of the pyramidal Li 2 Mo 5 I 13 cluster, which is presumably formed by analogy to the known [W 5 I 13 ] 2salts [6]. At the second stage, the cation was replaced by tetrabutylammonium by heating the reaction product with Bu 4 NI at the melting point of the latter (150°С) for 24 h. The final product turned out to be the oxoiodide cluster isolated from a THF solution as solvate I after double recrystallization. The cluster anion of compound I is shown in Fig. 2. In addition, (Bu 4 N) 2 Mo 6 I 14 was isolated, which assumes the formation of Mo 6 I 12 as one of the products of the hightemperature reaction (it becomes the single product when the reaction occurs at temperatures higher than 500°С). It can be assumed that the second product is [Mo 5 I 13 ] 2-, which is oxidized to [Mo 5 OI 13 ] 2upon extraction in air.
A temperature of 400°С was chosen on the basis of an assumption that the formation of the pyramidal iodide cluster of Mo should be expected at relatively low temperatures in a range of 300-500°С, while the octahedral {Mo 6 I 8 } 4+ clusters will be formed at the temperatures higher than 500°С. The preliminarily experiments showed that at 300°С Mo reacted with iodine to an insignificant extent (a minor amount of I 2 remains in the ampule), although compound I can be isolated as particular single crystals under these conditions. The temperature 400°С seems to be optimal, and the fraction of (Bu 4 N) 2 [Mo 6 I 14 ] will increase with increasing temperature. Under the chosen synthesis conditions, the amount of formed (Bu 4 N)[Mo 5 OI 13 ] is by ~2 times higher (by weight) than that of (Bu 4 N) 2 [Mo 6 I 14 ]. On the one hand, no formation of molybdenum iodide clusters of lower nuclearity was observed at Т > 300°С. On the other hand, it is known that the tetranuclear molybdenum cluster [Mo 4 OI 12 ] 2will be the final product if another molybdenum source (Mo(CO) 6 ) would be used and the reaction with iodine would be carried out at a substantially lower temperature (110°С) in diglyme [15]. An increase in nuclearity in the series [Mo 4 OI 12 ] 2--[Mo 5 OI 13 ] --[Mo 6 I 14 ] 2is parallel with a decrease in the oxidation state of molybdenum (+3.0, +2.8, and +2.0, respectively). A correlation between the degree of nuclearity of the cluster cores (or the number of metal atoms in the metal cage of the cluster cores) and synthesis temperatures is observed for the syntheses of the iodide clusters of W and can be described as follows: (a) the clusters of higher nuclearity are formed at higher temperatures and (b) the clusters of lower nuclearity can be transformed into the clusters of higher nuclearity with increasing temperature [5,6].
The (Bu 4 N) 2 [Mo 6 I 14 ] cluster is readily separable from the mixture due to a substantial difference in solubility of tetrabutylammonium cluster salts in acetonitrile. The (Bu 4 N)[Mo 5 OI 13 ] cluster is poorly soluble in acetonitrile but is highly soluble in THF, the phase of compound I can be obtained in the pure state after recrystallization from THF, and the purity of the   (Fig. 4).
According to the XRD data, the molybdenum atoms in compound I (Fig. 2) [16,17]. This indicates that the replacement of the {MoI}fragment (d 0 , 6e) by the isoelectronic oxygen atom (s 2 p 4 , 6e) does not substantially affect the bonding cluster skeletal orbitals responsible for the formation of metal-metal bonds. In the oxoiodide cluster (Bu 4 N) 2   Three MOs of the previous octahedral cluster become antibonding toward the М-М bonding, although one electron can occupy the lowest-lying molecular orbital of these MOs. These orbitals are shown in Fig. 5. Evidently, the inverse coordination to the square face of the {MX} metal fragments, which are isoelectronic to {MoI} -(e.g., {NbI} 2-) [18], recovers the scheme of binding with 12 bonding MOs. In the case of nonmetal (O or C), only two orbitals (е set) are appropriate in symmetry for the interaction with р orbitals (e.g., p x and p y ), which additionally provides two bonding orbitals, the capacity of which is four electrons. This does not change the number of skeletal electrons (18) but totally gives 72 cluster valence electrons if μ 4 -С and μ 4 -О are considered as donors of four electrons. Unlike C and O, {MoI}and {NbI} -(formally d 6 ) use the d orbitals, the symmetry properties of which assume binding with both the е and b 1 orbitals using all the six electrons. To conclude, it can be assumed that the nido-clusters [Mo 5 Х 13 ]would form stable closoclusters upon the addition of the metal fragments {MX} with the d 6 electronic configuration or capped ligands capable of donating four electrons. In the first case, the pentanuclear cluster would act as a unique cluster ligand (nucleophile), whereas the second case is the polynuclear complex (electrophile) exhibiting the dual nature like the well-known cluster boron hydrides [19].