Bis[tris(diisobutyldithiocarbamato)-μ3-sulfido-tri-μ2-disulfido-trimolybdenum(IV)] sulfide tetrahydrofuran monosolvate

The structure of [Mo3S7(S2CN i Bu2)3]2(μ6-S) features a μ6S2 2− anion asymmetrically wedged between two [Mo3S7(S2CN i Bu2)3]+ cations with close μ6-S2−⋯S2 2− contacts that indicate significant covalency to the interactions.


Chemical context
Triangular molybdenum sulfide clusters of the form [Mo 3 S 7 (S 2 CNR 2 ) 3 ] + I À (R = alkyl group) function as precatalysts for an H 2 evolving system under both photolytic and electrolytic conditions with H 2 O serving as source of protons (Fontenot et al., 2019).In the photolysis system, rapid mass spectrometry assays in the first moments of irradiation reveal the loss of atomic sulfur from the bridging S 2 2À ligands to form monosulfido bridges and an [Mo 3 S 4 ] 4+ core prior to the onset of H 2 evolution.In a bulk electrolysis of [Mo 3 S 7 (S 2 CN i Bu 2 ) 3 ] + •I À in the presence of H 2 O, the Faradaic efficiency is observed to be only about 37%.Because the same system and set of conditions reduced methyl viologen with much higher Faradaic efficiency, it is probable the the extruded elemental sulfur is competing for reducing equivalents.
As a means of developing further insight into this system, we undertook a preparative scale reduction of [Mo 3 S 7 (S 2 -CN i Bu 2 ) 3 ] + •I À using the prototypical outer-sphere reductant Cp 2 Co.While the initial reaction was marked by a darkening in color, the work-up and subsequent crystallization identified yellow [Mo 3 S 7 (S 2 CN i Bu 2 ) 3 ] 2 (� 6 -S) as the dominant isolable species.The presence of the sulfido counter-anion, which forms close S� � �S contacts with the axial S atoms of the bridging disulfide ligands of two different [Mo 3 S 7 (S 2 C-N i Bu 2 ) 3 ] + clusters, confirms the diversion of electrons to free S 0 in competition with H + reduction in the bulk electrolysis.In this article, we detail the structural features of [Mo 3 S 7 (S 2 C-N i Bu 2 ) 3 ] 2 (� 6 -S), (I).

Structural commentary
The [Mo 3 S 7 (S 2 CN i Bu 2 ) 3 ] 2 (� 6 -S) structure comprises two [Mo 3 S 7 (S 2 CN i Bu 2 ) 3 ] + cations between which is ensconced an S 2À counter-anion (S27).The asymmetric joining of the two Mo 3 clusters, as if by a hinge at S27, produces a half-opened clamshell-like appearance to the compound (Fig. 1).The angle at which these two Mo 3 planes are disposed is 40.637(15) � with a distance of 6.88 A ˚between the centroids of the two Mo 3 triangles.
A general observation in the structures of [Mo 3 E 7 (S 2 CNR 2 ) 3 ] + (E = S or Se; R = alkyl group) complexes is that soft monoatomic counter-anions situate themselves at the 'underside' of the cluster cation opposite to the unique � 3 -E ligand and in close proximity to the 'axial' chalcogen atom of the bridging dichalcogenide (Fig. 2) (Zimmermann et al., 1991;Fedin et al., 1992;Il'inchuk et al., 2002;Lu et al., 1993).
These anion� � �E ax contacts are typically less that the sum of the van der Waals radii, a fact attributed to an electrophilic character of the E ax atom and the felicitous nature of the 'softsoft' E ax � � �anion interaction.In [Mo 3 S 7 (S 2 CN i Bu 2 ) 3 ] 2 (� 6 -S), the S27� � �S ax interatomic distances partition into two sets: the S27-S3 distance at 2.4849 ( 14) A ˚and the remaining five, which are in the range 2.7252 (13)-2.8077( 14) A ˚, all of which are substantially less than twice the crystallographic radius for sulfur (3.6A ˚; Batsanov, 2001) and therefore indicative of appreciable covalency to the interactions.The markedly stronger interaction of S27 with the S3-S4 disulfide ligand is   manifested in the S3-S4 distance being significantly longer [2.2414 (13) A ˚] than the remaining S-S distances in the �-S 2 2À ligands, which range from 2.0671 (13)-2.1198(13) A ånd average as 2.0857 (6) A ˚.This comparative elongation of the S3-S4 bond length is consistent with the proposal, as advanced in a review of the structural chemistry of [M 3 X 7 ] 4+ and [M 3 X 4 ] 4+ (M = Mo, W; X = O, S, Se) clusters (Virovets & Podberezskaya, 1993), that the sulfide counter-anion (S27) infuses electron density into the S3-S4 �* orbital by overlap with one of its electron lone pairs.
The packing arrangement for [Mo 3 S 7 (S 2 CN i Bu 2 ) 3 ] 2 (� 6 -S) places the assembly into columnar stacks along the a axis of the cell (Fig. 3).The isobutyl substituents of the i Bu 2 NCS 2 À ligands project into the spacings between these columns and likely play a decisive role in guiding the formation of this pattern by virtue of favorable dispersion-type attractive forces.

Synthesis and crystallization
A solution of [Mo 3 S 7 (S 2 CN i Bu 2 ) 3 ]I (0.049 g, 0.0039 mmol) in tetrahydrofuran (THF) was cooled to 195 K in the cold well of a glove-box.Upon cooling, a solution of cobaltocene in THF (0.0183 g, 0.0968 mol) was added dropwise to the stirring solution.This reaction mixture was stirred at 243 K for 30 min and then was removed from the cold well and warmed to room temperature with continued stirring.Upon attaining room temperature, the solution was filtered through Celite, and the volatiles were removed under reduced pressure.The oily residue was then dissolved in 20% THF in hexanes and passed through a 3 cm pad of silica in a glass pipette.All volatiles were then removed under reduced pressure to yield a darkorange-brown oil.Crystals suitable for X-ray diffraction were grown by layering a concentrated THF solution with hexanes and maintaining the layered mixture at 243 K. Yield: 70%. 1 H NMR (300 MHz; �, ppm in CDCl 3 ): 3.59 (dd, J = 24 Hz, 7.5 Hz, 2H, CH 2 ), 2.22 (m, 1H, CH), 0.95 (d, J = 6.6 Hz, 6H, CH 3 ).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1.An initial solution for [Mo 3 S 7 (S 2 CN i Bu 2 ) 3 ] 2 (� 6 -S) was obtained by direct methods and revealed the positions of most of the non-H atoms except for some peripheral C atoms of the isobutyl groups.Subsequent cycles of least-squares refinement revealed several isobutyl groups that suffered a static disorder over two posi-   (Bruker, 2021), SHELXT (Sheldrick, 2015a), SHELXL2018 (Sheldrick, 2015b) and SHELXTL (Sheldrick, 2008).
tions.This disorder was treated with a split atom model that attained a best fit distribution in each case.All non-H atoms were refined anisotropically, but the disordered C atoms were treated with SIMU and RIGU restraints.All H atoms were refined isotropically as riding atoms with displacement parameters 1.2-1.5 times those of the C atoms to which they were attached.In the final difference maps, two positions occupied by disordered solvent molecules were identified.These severely disordered solvent molecules, which presented an electron density attributable to 367 electrons in a solventaccessible volume of 1692 A ˚3 per unit cell, have been masked using the SQUEEZE routine (Spek, 2015) in PLATON (Spek, 2020).

Figure 1
Figure 1Displacement ellipsoid plot (50% probability level) of [Mo 3 S 7 (S 2 CN i Bu 2 ) 3 ] 2 (� 6 -S) with complete atom labeling.For greater clarity, all H atoms and one of the two disordered parts of each disordered isobutyl group are removed.

Figure 2
Figure 2Illustration of the structural distinction between axial and equatorial sulfur atoms of the �-S 2 2À ligands in [Mo 3 S 7 (S 2 CNR 2 ) 3 ] + structures, with anion position in proximity to the axial S atoms.

Figure 3
Figure 3Packing arrangement for [Mo 3 S 7 (S 2 CN i Bu 2 ) 3 ] 2 (� 6 -S) viewed down the a axis of the unit cell.Displacement ellipsoids are presented at the 50% probability level, and all H atoms are omitted for clarity.

Table 1
Experimental details.
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