Sulfur-Bridged Incomplete Cubane-Type Iminodiacetato Molybdenum Clusters: Geometrical Isomers and Their Isomerization

Abstract Reaction of the incomplete cubane-type aqua clusters [Mo3(μ3-S)(μ-S)3(H2O)9]4+ (A) with H2ida ( = iminodiacetic acid) followed by ion exchange separation gave four geometrical isomers of [Mo3(μ3-S)(μ-S)3(ida)3]2− (hereafter [Mo3S4(ida)3]2−; A1, A2, A3, and A4 based on the elution sequence), which were isolated as solid samples: Ca[Mo3S4(ida)3]·12H2O (A1′), Ca[Mo3S4(ida)3]·21H2O (A2′), Ca[Mo3S4(ida)3]·10H2O (A3′), and Ca[Mo3S4(ida)3]·14H2O (A4′), respectively. The X-ray structural analyses of A2′ and A4′ revealed that the three nitrogen atoms of three ida ligands in each isomer have the configuration of “trans, trans, cis” and “cis, cis, cis” to μ3-S’s, respectively. The X-ray structure of A1′ has already been determined to have “trans, trans, trans” configuration, therefore, that of A3′ is estimated to have “trans, cis, cis” configuration. HPLC analysis reveals that each isomer isomerizes, cis–transconversion of the ida ligand occurring one by one, and that in equilibrium the anions A1 (18%), A2 (68%), and A3 (14%) exist and A4 does not exist. The advent of A4 is possible only in the course of preparation from A and H2ida.