Geometrically Constrained Molybdenum(VI) Metallosupramolecular Architectures: Conventional Synthesis versus Vapor and Thermally Induced Solid-State Structural Transformations

This contribution describes the synthesis, characterization, and catalytic implementation of the highly constrained molybdenum­(VI) polymers [MoO2(LR)]n (with R = H (1), 3OMe (2), and 4OMe (3)) and cyclic tetramer [MoO2(L4OMe)]4­·0.5MeCN (3′·0.5MeCN) with nicotinoyl hydrazonato ligands (LR) derived from salicylaldehyde (LH), 3-methoxy-2-hydroxybenaldehyde (L3OMe), or 4-methoxy-2-hydroxybenzaldehyde (L4OMe). In order to determine the most stable geometries, conformational analysis of the free and coordinated ligands in these assemblies was carried out through the potential energy surface scans using quantum-chemical methods. The mononuclear analogues [MoO2(HLR)­(MeOH)]Cl (R = H (1a) and 3OMe (2a)), [MoO2(HLR)­Cl] (R = 3OMe (2b) and 4OMe (3b)), and [MoO2(LR)­(MeOH)] (R = H (1c), and 4OMe (3c-α and 3c-β)) were also synthesized and evaluated as potential precursors for reactions in the solid state. Vapor induced transformation of 2a quantitatively afforded complex 2b. The chemometric analysis using principal component analysis was applied to provide insight into the reaction profile. Thermally induced solid-state reactions resulted in the transformation of metallocycle 3′·0.5MeCN and monomeric complexes 1c, 2a, 3c-α, and 3c-β into the corresponding coordination polymers. The structural features responsible for these conversions are discussed in detail. Lastly, the metallosupramolecular architectures were tested as catalysts for cyclooctene epoxidation by using tert-butyl hydroperoxide as an oxidant in water or decane. Compound 3′ is shown to be the most effective and selective catalyst