“Wheeling Templates” in Molecular Oxothiomolybdate Rings: Syntheses, Structures, and Dynamics

Abstract

The influence of three dicarboxylate anions, oxalate, (C2O4)2-, glutarate, (H6C5O4)2-, and pimelate, (H10C7O4)2-, on the self-condensation process of the [Mo2S2O2]2+ dithiocation has been investigated. [Mo8S8O8(OH)8(C2O4)]2- ([Mo8-ox]2-), [Mo10S10O10(OH)10(H6C5O4)]2- ([Mo10-glu]2-), and [Mo12S12O12(OH)12(H10C7O4)]2- ([Mo12-pim]2-) have been prepared and characterized in aqueous solution by 1H NMR and electrospray mass spectroscopy and in the solid state by elemental analyses, X-ray crystallography, and infrared spectroscopy. The molecular arrangement of [Mo8-ox]2-, [Mo10-glu]2-, and [Mo12-pim]2- exhibits the same type of topology derived from the neutral cyclic {Mo2 n S2 n O2 n (OH)2 n } backbone. {Mo2 n S2 n O2 n (OH)2 n } inorganic rings encapsulate the organic guest with direct covalent interactions between Mo centers and carboxylate groups. Excess of glutarate does not change the molecular arrangement of the anion but modifies the solid-state arrangement of [Mo10-glu]2-. Rb4(C5H6O4)[Mo10S10O10(OH)10(H6C5O4)]·5H2O (Rb4glu[Mo10-glu]·5H2O) has been crystallized and structurally characterized by X-ray diffraction. The structure of Rb4glu[Mo10-glu]·5H2O contains the same decameric molecular anion as that found in Cs2[Mo10-glu]·20H2O with an additional glutarate capping the molecular wheel. The supramolecular interactions developed in Rb4glu[Mo10-glu]·5H2O are ensured by hydrogen bonds involving the hydroxo bridges of the inorganic ring and the carboxylate groups of the additional glutarate. Negative-ion electrospray mass spectroscopy (ESMS) has been performed on aqueous solution containing the {Mo2S2O2} building block and the templating agent at pH 4.5. The anions [Mo8-ox]2-, [Mo10-glu]2-, and [Mo12-pim]2- were clearly identified by their parent peaks observed in the ESMS spectra. Experimental m/z ratios are fully consistent with the corresponding X-ray diffraction results. The templated anionic Mo2 n rings have been characterized by 1H NMR spectroscopy in aqueous solution and in acetonitrile-d 3. The 1H NMR spectra of [Mo8-ox]2-, [Mo10-glu]2-, and [Mo12-pim]2- as Li+ salts in CD3CN unambiguously contain the resonances of the hydroxo bridges belonging to the ring and those of the inner CH2 groups of the alkyl chain in [Mo10-glu]2- and [Mo12-pim]2-. Variable-temperature 1H NMR spectra recorded in CD3CN revealed the fluxionality of [Mo10-glu]2- and [Mo12-pim]2- in contrast to the rigidity of [Mo8-ox]2-. At low temperature (T = 226 K), the motion of the guest, i.e., the central template in Mo10 and Mo12 rings, becomes slow enough on the NMR time scale to postulate the presence of conformational isomers in solution.

Structure, Formation, and Dynamics of Mo12 and Mo16 Oxothiomolybdenum Rings Containing Terephtalate Derivatives

“…Linear saturated dicarboxylate ions with various lengths of the alkyl chain (from C2 to C8) gave highly flexible and fluxional host-guest architectures with nuclearity varying in the Mo8-Mo12 range. [28,32] These results highlight a mutual adaptability and concerted dynamics between the host and guest. The use of rigid templates (unsaturated [27] or aromatic polycarboxylate ions [29] ) is expected to bring rigidity to the host-guest system.…”

“…[19,20] This system was shown to be fruitful for developing extended cyclic inorganic clusters, differing in their nuclearity and shape by the nature of the encapsulated substrate, being either mono-or polyphosphates, [21][22][23] metalates, [24][25][26] or polycarboxylates. [27][28][29] The flexible and adaptable cyclic oxothiomolybdenum rings exhibiting an open inner available cavity, display attractive properties for bioin- 16 …”

“…An additional signal, at about d = 8 ppm corresponds to the DMF formamide proton, present as solvate in the crystals of Cs 2 A C H T U N G T R E N N U N G [Mo 12 TerP]. The sharpness of the resonances of the inorganic and organic protons of the anion [Mo 12 TerP] 2À , compared to those arising from host-guest systems containing flexible linear carboxylate ions [28] is related to the rigidity of the central TerP 2À ion, which prevents any observable dynamics on the NMR time scale. In D 2 O (see Figure S2 in the Supporting Information), the resonances related to the hydroxo bridges disappeared because fast proton-deuterium exchanges occur in aqueous solvent.…”

Building Block Libraries and Structural Considerations in the Self-assembly of Polyoxometalate and Polyoxothiometalate Systems

“…The isolation of this cluster highlighted the potential of using organic templates of various lengths, flexibility and coordination ability to influence the self-condensation process of [Mo2O2S2] 2+ units. Indeed, Cadot and co-workers using a templated synthesis approach have reported the constructive utilization of organic carboxylate ligands to template the self-condensation of [Mo2O2S2] 2+ [54]. More specifically, a family of ring-shaped POTM clusters exhibiting a variety of nuclearities, shapes and symmetries have been synthesized and characterized as host-guest assemblies (see Fig.…”

“…9 Å. Interestingly, the cluster incorporates 24 outward-oriented acetate ligands which can be substituted by specific organic moieties, which paves the way for post-functionalization and design tailored for specific applications. According to previously reported examples, hydrolytic condensation of the {Mo2O2S2(H2O)6} 2+ cation produced a rich class of cyclic host-guest compounds, with the shape and size of the host cyclic rings being dependent on the size of flexible organic carboxylates [54]. However, this approach led to limited structural diversity due to the lack of a building block library of sufficient complexity.…”