Anthraphen: A Salphen‐Like Non‐Innocent Tetradentate Anthraquinone Imine Dye – Coordination and Electrochemistry

Abstract

A new, highly redox‐active chromophore ligand, H2(anthraphen) (1), containing two o‐phenylenediamine‐linked anthraquinone imine units has been synthesized in a three‐step synthesis and fully characterized by X‐ray crystallography, UV/Vis spectroscopy, and cyclic voltammetry. The dark‐red dye 1 shows molar extinction coefficients of up to 47000 L mol–1 cm–1 and four reversible reduction processes. This dianionic N2O2 ligand, with a significantly extended π system, offers a salphen‐like binding cavity for metal coordination, as has been demonstrated by the synthesis of [K2(anthraphen)] (2) and the transition‐metal complexes of TiIV (3), VIV (4), FeII (5), FeIII (6), CoII (7), NiII (8), CuII (9), PdII (10), PtII (11), and ZnII (12). These metal chromophores have colors ranging from dark‐red, violet, green to black. Their optical and electrochemical properties were investigated and compared with those of the diprotic ligand. Coordination led to an increase in the molar extinction coefficients up to 66400 L mol–1 cm–1 in the case of [V(anthraphen)O] (4), broadening of the absorption bands in the visible‐light region as well as a redshift of the lowest‐energy absorption band. The lowest‐energy absorption observed for these complexes to date is for [Ni(anthraphen)] (8) at 784 nm. The metal chromophores exhibit up to five fully reversible oxidation and reduction processes.

Juglophen: a tetradentate non-innocent electron sponge naphthoquinone-imine ligand and its reduced and oxidized nickel complexes [Ni(jp)]−,0,+

“…The highly reactive Michael-acceptor CC functionality also precluded following a strategy of selectively converting the keto functionality into an activated electrophilic ketal form, a protocol successfully used in our synthesis of related ligand anthraphen and other anthraquinoneimines. 25,39 The adaption of this three-step synthesis is shown in Scheme S1 in the ESI †. Key of this strategy is the aim of using an auxiliary O -acetyl group in 5 40 to activate the neighbouring carbonyl group for a regioselective acetalization.…”

“…2 A series of salphen-related quinoid electron sponge ligands. 25 Scheme 1 Synthesis of juglophen 1 and its isomer benzophenazin 1'.…”

“…25 1 displays an absorption maximum at 227 nm hypsochromically shifted compared to anthraphen with an absorption maximum at 252 nm. 25 This observation supports the postulate of Prinzisky et al who considered the strongest band as aromatic π → π* transition. The larger the π system, the more bathochromic the expected shift of such absorption.…”

“…2). 25 Hydroxy-anthraquinones are prominent dyes already used in early human civilization 26 as well as in modern applications and in chemical research. [27][28][29][30][31] Variation of the hydroxy-anthraquinone-imine structural motif has led to new chromophoric ligands and TiO 2 sensitizers for investigation of opto-and photoelectronic applications.…”

“…[27][28][29][30][31] Variation of the hydroxy-anthraquinone-imine structural motif has led to new chromophoric ligands and TiO 2 sensitizers for investigation of opto-and photoelectronic applications. 25,32 In this paper we introduce a tetradentate chromophoric, noninnocent naphthoquinone imine "electron sponge" ligand able in taking up four electrons and also capable in releasing electrons. The dye juglone, [33][34][35][36][37] naturally occurring in leaves, roots, husks, fruit, and bark of plants, particularly of the black walnut, was identified as lead structure for our new hybrid ligand juglophen.…”