A three-dimensional adamantane-like nanoscopic cage built from four iodide-bridged triangular Mo 3 S 7 cluster units w

Metal-directed self-assembly is a versatile synthetic approach to produce discrete three dimensional polyhedral or cage compounds. Metal centers bring a variety of coordination sites and bond angles to a supramolecular framework, and in combination with different bridging ligands provide great versatility for chemical and structural modification of the inner cavity. Recently, the use of multi-nuclear systems with labile positions (for example dimeric M2, where M=Mo, Rh) to build 3D supramolecular cages has also been explored, and new perspectives have been obtained on the facility for fine tuning the net charge, the magnetic properties and the electronic communication between the metal units. In this context, the family of triangular [Mo3(m3-S)(m-S2)3X6] 2 (X = halide) clusters are promising candidates as building blocks for multi-nuclear aggregates. This is due to the presence of two labile binding sites in a cis configuration (an X–Mo–X angle of ca. 901, see Scheme 1a), which can be easily occupied by a wide spectrum of ligands. In addition, ligand substitution in the [Mo3S7X6] 2 dianion can be carried out selectively, affecting all six halide ligands or only the three halides cis to the m3-S sulfur atom, 3 demonstrating the geometry-guiding versatility of the metal centers (see Scheme 1b and c). Therefore, the Mo3S7 cluster can be regarded as a tritopic building block to construct convex corners in tetrahedral symmetry-like supramolecular capsules or cages that enclose an isolated nanospace. To date, all molecular architectures based on Mo3S7 cluster entities possess 1 or 2D character (open structures), including the well known polymeric {M3S7X2X4/2}n solid phases (M = Mo, W), where each Mo3S7 unit is covalently linked to its neighbors through four halide bridges forming zigzag chains. Herein, we report the synthesis of a novel 3D cage based on a dodecanuclear Mo12 complex adopting an adamantanoid structure built from four trinuclear Mo3S7 entities bridged by six m-I atoms and encapsulating one iodide anion. This discrete cluster was first obtained as an unexpected product, but a detailed study has since revealed the experimental factors leading to its efficient self-assembly. In recent years, our research interests have focused on the coordination of redox-active ligands such as 1,2-dithiolenes to the trinuclear Mo3S7 cluster, 6 to afford dianionic [Mo3S7(1,2-dithiolene)3] 2 complexes, which after chemical or electrochemical oxidation afford single-component conductors. During the course of this work we have explored the coordination chemistry of 1,2-dioxolenes. The compound (PPh4)2[Mo3S7(Cl4cat)3] ((PPh4)2[1]) (Cl4cat = tetrachlorocatecholate) is prepared in high yield by treatment of [Mo3S7Br6] 2 with an excess of Cl4catH2 in the presence of triethylamine.w Unexpectedly, the slow diffusion of an excess of iodine into (PPh4)2[1] solutions produces (PPh4)3[Mo3S7I3]4(m-I)6I ((PPh4)3[2]). Optimal conditions for its preparation were found using a 3 : 1 iodine to (PPh4)2[1] ratio, which led to compound (PPh4)3[2] in moderate yield (ca. 40%).w The use of low iodine to (PPh4)2[1] ratios (typically below 3 : 1) reduced the yield of ((PPh4)3[2]), while the use of higher ratios did not improve the yield. On the basis of the ratios of Mo3S7 : I2 used for optimal preparation of (PPh4)3[2], the proposed stoichiometry is shown in eqn (1).

Metal-directed self-assembly is a versatile synthetic approach to produce discrete three dimensional polyhedral or cage compounds.Metal centers bring a variety of coordination sites and bond angles to a supramolecular framework, and in combination with different bridging ligands provide great versatility for chemical and structural modification of the inner cavity. 1 Recently, the use of multi-nuclear systems with labile positions (for example dimeric M 2 , where M = Mo, Rh) to build 3D supramolecular cages has also been explored, and new perspectives have been obtained on the facility for fine tuning the net charge, the magnetic properties and the electronic communication between the metal units. 2 In this context, the family of triangular [Mo 3 (m 3 -S)-(m-S 2 ) 3 X 6 ] 2À (X = halide) clusters are promising candidates as building blocks for multi-nuclear aggregates.This is due to the presence of two labile binding sites in a cis configuration (an X-Mo-X angle of ca.901, see Scheme 1a), which can be easily occupied by a wide spectrum of ligands.In addition, ligand substitution in the [Mo 3 S 7 X 6 ] 2À dianion can be carried out selectively, affecting all six halide ligands or only the three halides cis to the m 3 -S sulfur atom, 3 demonstrating the geometry-guiding versatility of the metal centers (see Scheme 1b and c).Therefore, the Mo 3 S 7 cluster can be regarded as a tritopic building block to construct convex corners in tetrahedral symmetry-like supramolecular capsules or cages that enclose an isolated nanospace. 4To date, all molecular architectures based on Mo 3 S 7 cluster entities possess 1 or 2D character (open structures), including the well known polymeric {M 3 S 7 X 2 X 4/2 } n solid phases (M = Mo, W), where each Mo 3 S 7 unit is covalently linked to its neighbors through four halide bridges forming zigzag chains. 5erein, we report the synthesis of a novel 3D cage based on a dodecanuclear Mo 12 complex adopting an adamantanoid structure built from four trinuclear Mo 3 S 7 entities bridged by six m-I atoms and encapsulating one iodide anion.This discrete cluster was first obtained as an unexpected product, but a detailed study has since revealed the experimental factors leading to its efficient self-assembly.

4(PPh
The molecular structure of (PPh 4 ) 3 [2] has been determined by X-ray diffraction methods.zThe compound crystallizes in the acentric cubic space group P 43n and is comprised of dodecanuclear trianion units built from four Mo 3 S 7 trinuclear clusters (Fig. 1).The repeating metal core can be written as Mo 3 S 7 I 3 I 3/2 , where three bridging iodides connect the trinuclear Mo 3 S 7 units.Each trinuclear unit consists of an equilateral Mo 3 triangle capped by a m 3 -S 2À atom that lies above the Mo 3 plane.Additionally, the three Mo-Mo edges are bridged by three m-S 2 2À groups, with sulfur atoms occupying equatorial positions (S eq , those labeled S(2)), essentially in the Mo 3 plane, and axial positions (S ax , labeled S(1)) located out of the metal plane.Two iodide ligands fill the remaining two positions of the seven-coordinate molybdenum atoms and are oriented almost perpendicular to the Mo 3 plane.The bridging iodides define the octahedral assembly of a discrete molecular cage that closely resembles an adamantine-like structure.This octahedron circumscribes two different cuboctahedrons defined by the 12 S ax or S eq atoms (see Fig. S2w) and also the tetrahedron defined by the centroids of the four Mo 3 units. 8he formation of the discrete dodecanuclear cluster (PPh 4 ) 3 [2] is unprecedented despite the rich structural chemistry of Mo 3 Q 7 complexes (Q = S, Se).Mo 3 Q 7 (Q = S, Se) based supramolecular aggregates are ubiquitous in the solid state owing to the prominent electrophilic character of the three axial sulfur atoms.In the present work, the presence of an interstitial iodide-anion close to the ''electrophilic'' axial sulfur atoms d(S ax Á Á ÁI interstitial ) = 3.293(4)A ˚clearly illustrate this behaviour.From this, the Mo 3 Q 7 cores derive the ability to form non-covalent links to other anions, giving di-, tri-, tetra-or polymeric clusters of discrete Mo 3 Q 7 modules.
With the aim of acquiring further insight into the experimental factors leading to the self-assembly of compound (PPh 4 ) 3 [2], we sought additional ways to synthesize (PPh 4 ) 3 [2] by starting from (PPh 4 ) 2 [1] and using different experimental conditions.The reaction of other non-oxidizing iodine sources, namely PPh 4 I or (n-Bu 4 N)I with cluster (PPh 4 ) 2 [1] in different solvents did not lead to Cl 4 cat replacement, highlighting the fact that Cl 4 cat to iodide replacement is triggered by ligand oxidation.Catecholate displacement in the complex [Mn(Br 4 cat) 3 ] 3À (Br 4 cat = tetrabromocatecholate) also requires the presence of an external oxidant such as air to afford the [Mn(Br 4 cat) 2 (solvent) 2 ] À anion. 9It has to be pointed out that metal-directed self-assembly typically involves simple procedures and gives high yields, which is ascribed to the thermodynamic stability of the 3D cage compound.However, in the present work, direct mixing of dichloromethane solutions of the cluster (PPh 4 ) 2 [1] and iodine at room temperature or under reflux conditions did not lead to the formation of the cage (PPh 4 ) 3 [2] complex; the trinuclear (PPh 4 ) 2 [Mo 3 S 7 I 6 ] ((PPh 4 ) 2 [3]) compound being obtained instead in an almost quantitative yield.wThis distinctive reactivity represents a unique example where two different products, (PPh 4 ) 3 [2] and (PPh 4 ) 2 [3], can be isolated under a set of experimental conditions that differs in the rate of Cl 4 cat oxidation.We thus suggest that slow iodine oxidation of (PPh 4 ) 2 [1] yields, after a relatively short reaction time, the adamantane-like cage (PPh 4 ) 3 [2], which can be trapped from the reaction mixture as the PPh 4 + salt.Conversely, the trinuclear (PPh 4 ) 2 [3] is obtained after prolonged heating and therefore represents the thermodynamically favoured product (see Scheme 2).
We have also investigated the effect of the counter-cation as the structural template for the formation of the (PPh 4 ) 3 [2]  complex.Oxidation of other salts of the 1 2À cluster , such as the n-Bu 4 N + , Et 4 N + or PPN + (PPN = bis(triphenyl-phosphine)iminium) salts did not lead to the characteristic crystalization of the 2 3À cage complex.Only substitution of PPh 4 + by the topologically related AsPh 4 + cation afforded, upon reaction with iodine, the isostructural (AsPh 4 ) 3 [2] cage species, as confirmed by their solid state UV-vis spectra and XPD patterns.wThis result is a good indication that the solubility of the cage is affected by the presence of PPh 4 + or AsPh 4 + cations, which do ''intercept'' (and precipitate) the 3D cage complex.Finally, it is also reasonable to hypothesize that the S ax Á Á ÁI interstitial interaction observed in the solid state structure of 2 3À might play a significant role at pre-organizing the tetrahedral configuration of the Mo 3 S 7 clusters around the iodide-templating atom.To evaluate this possibility we have carried out the reaction of the starting (PPh 4 ) 2 [1] with I 2 in the presence of PPh 4 X (X = Cl, Br) salts and we observed that the characteristic appearance of single-crystals of (PPh 4 ) 3 [2]  was dramatically reduced.This suggests that the non-covalent S ax Á Á ÁI interstitial interaction plays an important role in promoting the self-assembly of the 3D (PPh 4 ) 3 [2] cage.A closely related example that emphasizes the importance of this S ax Á Á ÁI interstitial interaction has been recently reported, namely {[Mo 3 S 7 (dtp) 3 ] 4 ÁI(HgI 3 ) 3 } in which four Mo 3 S 7 clusters with dithiophosphate ligands are arranged around an interstitial iodide atom. 10he thermal stability of compounds (PPh 4 ) 3 [2] and (PPh 4 ) 2 [3] upon heating are very similar, with basically flat TG-DTA curves up to ca. 300 1C and hence a remarkable thermal stability.At higher temperatures, between 300 and 600 1C, exothermic processes occur with mass losses of ca.65% (for (PPh 4 ) 3 [2]) and 60% (for (PPh 4 ) 2 [3]).Furthermore, XPD experiments at different temperatures for compound (PPh 4 ) 3 [2] showed a gradual loss of crystallinity starting from 250 1C to yield an amorphous solid above 300 1C.Both TG-DTA and XPD measurements rule out a plausible interconversion from (PPh 4 ) 3 [2] to (PPh 4 ) 2 [3] in the solid state.
In summary, we report the first example of polyhedral molecules assembled from trinuclear corner pieces and iodide linkers which were obtained via spontaneous self-assembly.It is clear that in the self-assembly of complex architectures, a number of kinetic products may appear before formation of the thermodynamic product(s).In the present system, both the kinetic and thermodynamic products have been successfully isolated and characterized, mainly because of their distinctive solubility.It is shown that iodine oxidation of the dioxolene [Mo 3 S 7 (Cl 4 cat) 3 ] 2À complex forms the thermodynamically stable [Mo 3 S 7 I 6 ] 2À (3 2À ) dianion by way of a metastable adamantine-like cage trianion, namely 2 3À , which can be intercepted fleetingly as a PPh 4 + and AsPh 4 + salt.Besides the importance of the counter-cation, our results also reveal that the presence of iodide either as bridging-ligands or interstitials play a crucial role in the self-assembly of the 3D (PPh 4 ) 3 [2]