Discrete and polymeric ensembles based on dinuclear molybdenum(vi) building blocks with adaptive carbohydrazide ligands: from the design to catalytic epoxidation

Abstract

Symmetrical disubstituted carbohydrazides (H₄L^1–6) when reacted with [MoO₂(acac)₂] (in 1 : 2 molar ratio) afford the corresponding discrete complexes, [(MoO₂)₂L^1,5,6(MeOH)₂], [(MoO₂)₂L^1,5,6(Im)₂] (Im = imidazole), and/or polymeric ensembles, [(MoO₂)₂L^1–6]_n, depending on the reaction solvent, the ligand framework and the presence of ancillary coordinating species. In these assemblies, asymmetrical dimolybdenum(VI) entities, [(MoO₂)₂L^1–6], wherein the corresponding carbohydrazide acts as an adaptable two-compartment ligand offering ONO and ONN coordination environments, serve as the main building blocks. The solid-state structures of the obtained materials were elucidated via X-ray diffraction and infrared spectroscopy, whereas the behaviour of polymeric ensembles in DMSO-d₆ solution was investigated by nuclear magnetic resonance. Thermal studies revealed that the discrete species transform to the corresponding polymeric ensembles upon heating, whereas upon grinding they either remain intact or afford their reactive and coordinatively unsaturated (pentacoordinated) counterparts. While the discrete methanol-based complexes have a comparable spatial arrangement of ancillary ligands, imidazole-based ones offer substantially different scenario which was addressed in detail via density functional theory calculations. Finally, the polymeric ensembles were shown to be efficient catalysts for cyclooctene epoxidation under eco-friendly conditions while employing aqueous tert-butyl hydroperoxide as an oxidant.