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

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 {Mo2nS2nO2n(OH)2n} backbone. {Mo2nS2nO2n(OH)2n} 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 Mo2n rings have been characterized by 1H NMR spectroscopy in aqueous solution and in acetonitrile-d3. 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.