Linking the Inner Isophtalate Guests Within Hexadeca-Oxothiomolybdenum Cyclic Arrangements. Synthesis, Structures and Stability in Solution

The complexes [Mo16O16S16(OH)16(abtc)(H2O)2]4− (1) and [Mo16O16S16(OH)16(abtc)2(H2O)2]8− (2) (abtc4− = 3,5-dicarboxyl-(3′,5′-dicarboxyazophenyl)benzene) have been obtained from the condensation of the [Mo2O2S2]2+ cluster in the presence of abtc4− ligand. Compounds 1 and 2 were characterized structurally, revealing host–guest architectures containing one or two encapsulated abtc4− ligands. In 1, the guest abtc4− moieties appears firmly coordinated to the ring by the four carboxylato groups, giving a nearly planar conformation which fall in the plane of the sixteen Mo atoms of the inorganic macrocycle. Conversely, the two equivalent abtc4− ligands in 2 exhibit an asymmetric mode of coordination, where two carboxylato groups are coordinated to the {Mo16} ring and the two remaining hang outer the open cavity. Anions 1 and 2 have been characterized by 1H NMR in solution, revealing for 1 an enhancement of the symmetry likely due to some molecular dynamic, while the compound 2 easily converts into 1 with a consecutive release of one abtc4− ion. Furthermore, a solution study showed that the formal linkage of the two isophatlate moieties through an azo {N=N} unit brings a drastic stability increase of the host–guest assembly compared to the bis-isophtalate containing {Mo16} ring. Synthesis, elemental analysis, and infrared spectra are also supplied.