Formation and Reactivity of Copper Acetylacetone Bis(Thiosemicarbazone) Complexes

Abstract

The complexation reaction mechanism of acetylacetone bis-thiosemicarbazone ligands (H 2 acacR) with Cu II is explored using a variety of physical methods. The complexes form via a complicated multistep mechanism that is initiated by ring opening of the pyrazoline form of the ligand and leads, ultimately in air, to an oxidised ketone form of the ligand. Tetradentate N 2 S 2 coordinated forms of the intermediate [Cu (acacR)] [a]

Copper Complexes of Benzoylacetone Bis-Thiosemicarbazones: Metal and Ligand Based Redox Reactivity

“…5a, b) are very similar and the most prominent features to emerge in the final products are a very broad maximum around 780 nm and a shoulder around 440 nm, which was also seen in the [Cu-(acacR)]series. [4] For [Cu II (banphe)] - (Figs 4c and 5c) a much more intense maximum appears at 440 nm with a shoulder at 460 nm. This transition must be linked with the N-phenyl chromophore.…”

“…Previous work with the H 3 acacR ligands [4] has shown that their complexation reactions with Cu II are complicated and involve several steps. They are also affected by the presence of oxygen.…”

“…Investigations were carried out in DMSO solution, as preliminary studies in DMF resulted in precipitation of some compounds upon oxidation. To eliminate complications from different protonated and tautomeric forms, all data shown were acquired with three equivalents of Et 3 N added in line with the studies reported previously [4] generating a trianionic ligand initially coordinated to Cu II (Fig. 2).…”

“…Assignment of the structures of each intermediate was assisted by comparisons with the [Cu II -(acacR)]series as a point of reference. [4] Time Resolved UV-Vis Spectroscopy UV-Vis spectroscopy is well suited to studying copper thiosemicarbazone complex formation as the complexes exhibit some quite intense ligand-to-metal charge transfer absorption bands (e . 1000 M À1 cm À1 ) that are characteristic of the metal in different oxidation states.…”

“…Our own interest has been on the ligands derived from acetylacetone where we have reported some remarkably facile redox reactions on the coordinated ligand; in particular conversion of the apical C-atom into a carbonyl group in the presence of oxygen as the sole oxidant. [4] These reactions do not take place in the absence of metal (Ni or Cu) and intermediates observed over the time course of the reaction suggest that both metalcentred and ligand-centred redox reactions occur in parallel. This provides a contrast between the [Cu(ATSM)] and [Cu-(acacR)] (ATSM ¼ diacetyl-bis(N 4 -methylthiosemicarbazone; HacacR ¼ acetylacetone-bis(thiosemicarbazone)) families despite their structural similarities.…”