New aqua-soluble dicopper(ii) aminoalcoholate cores for mild and water-assisted catalytic oxidation of alkanes

Abstract

Two new dicopper(II) aminoalcoholate compounds, namely a discrete 0D complex [{Cu(Hdea)(H₂dea)}₂(μ₂-H₂pma)]·3H₂O (1) and a 1D coordination polymer [Cu₂(μ₂-H₂tea)₂(μ₂-nda)]_n·2nH₂O (2), were self-assembled from copper(II) nitrate, an aminoalcohol {diethanolamine (H₂dea) or triethanolamine (H₃tea)} and carboxylic acid {pyromellitic (H₄pma) or 2,6-naphthalenedicarboxylic (H₂nda)} building blocks. Both products were isolated as stable and water-soluble microcrystalline solids and fully characterized by IR spectroscopy, ESI-MS(±), elemental analysis and single-crystal X-ray diffraction, revealing distinct types of dicopper(II) units. The structures of 1 and 2 are further extended by intermolecular hydrogen bonds giving rise to 3D or 2D H-bonded nets with a pcu and sql topology, respectively. Both compounds act as efficient pre-catalysts for the mild homogeneous oxidation, by aqueous H₂O₂ in MeCN/H₂O medium, of different cycloalkanes to the corresponding cyclic alcohols and ketones, which are obtained in up to 34% total yields (based on alkanes). The influence of various reaction parameters such as the amount and type of pre-catalyst and acid promoter, solvent composition and substrate scope were investigated, which also show that the oxidation of cycloalkanes and the activity of a catalytic system can be restarted upon addition of new portions of oxidant and substrate. Besides, an unusual promoting role of water was found when using both pre-catalysts 1 and 2. The bond-, regio- and stereoselectivity parameters were established using different alkane substrates, supporting a free radical oxidation mechanism. The model 1/H⁺/H₂O₂ and 2/H⁺/H₂O₂ solutions were investigated by ESI-MS(±), allowing us to propose some possible catalytically active species.