Valence tautomerism induced nucleophilic ipso substitution in a coordinated tetrabromocatecholate ligand and diverse catalytic activity mimicking the function of phenoxazinone synthase

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Highlights

  • Manganese(III) complex with redox-noninnocent tetrabromocatecholate ligand.

  • Valence tautomerism induced aromatic nucleophilic substitution of a tetrabromocatecholate ligand by pyridine, supported by DFT calculations.

  • The first example of valence tautomerism induced nucleophilic ipso substitution by a nitrogen containing ligand.

  • Diverse catalytic activity mimicking the function of phenoxazinone synthase.

Abstract

Two new manganese(III) complexes, [pyH][Mn(Br4Cat)2(py)] (1) and [Mn(Br4Cat)(Br3pyCat)(py)2] (2), where py is pyridine, Br4CatH2 is tetrabromocatechol and Br3pyCatH2 is 3,5,6-tribromo-4-pyridiniumcatechol, derived from redox ‘noninnocent’ bromo-substituted catecholate ligands are reported. Both complexes were characterized by various spectroscopic techniques in addition to the single crystal X-ray crystallography, and the electrochemical behavior of these complexes was investigated by cyclic voltammetry. Variable temperature UV–vis spectral studies for complex 1 reveal an unprecedented observation in which a dramatic increase of the ligand-to-metal charge transfer band at 592 nm associated with concomitant change in color of the solution from olive-green to dark-green is noticed with increase in temperature. This unprecedented spectral feature is consistent with the formation of a new species 2 in which valence tautomerism induced aromatic nucleophilic substitution of a tetrabromocatecholate ligand by pyridine is observed. DFT calculations have been used to rationalize this unexpected result. To the best of our knowledge, the nucleophilic aromatic substitution of tetrabromosemiquinone by pyridine to generate this pyridinium-containing catecholate ligand is the first example of valence tautomerism induced nucleophilic ipso substitution by a nitrogen containing ligand. Although the electrochemical behavior of both complexes is similar, the probable role of a positive charge on the ligand backbone has been discussed in order to justify the significantly higher catalytic activity of complex 2 over complex 1.

Graphical abstract

The nucleophilic aromatic substitution of tetrabromosemiquinone by pyridine to generate a pyridinium-containing catecholate ligand is the first example of valence tautomerism induced nucleophilic ipso substitution by a nitrogen containing ligand. The probable role of a positive charge on the ligand backbone on diverse catalytic activity has been explored.

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Introduction

Metalloenzymes play vital roles in dioxygen binding and oxidation of various biologically important small molecules, in which they facilitate the spin-forbidden interaction between dioxygen and organic compounds [1], [2], [3]. These enzymes may act as oxygenases, the functions of which are the incorporation of one or both oxygen atoms from molecular oxygen into organic substrates, or oxidases, which impart substrate oxidation with concomitant reduction of molecular oxygen to water or hydrogen peroxide [1], [2], [3]. Bioinorganic chemists have paid attention to the functioning mode of such enzymes and hence to their structures and spectroscopic characterizations. Modeling of such enzymes through the characterization of low-molecular-weight metal complexes is a way to gain structural and functional insights into the active sites of enzymes. As a consequence, the development of new redox catalysts that mimic the functions of these enzymes for the selective oxidation reactions of organic substances in industrial and synthetic processes has been an ongoing study [4], [5], [6], [7], [8]. Traditionally, inorganic and organometallic redox catalysts are derived from second- and third-row transition-metal ions with redox-inert ancillary ligands [4], [5], [6], [7], [8]. In these systems, the multielectron redox activity is entirely metal-driven. In contrast, redox-active “noninnocent” ligands may impart a multielectron redox capacity to mononuclear first-row metal complexes, which typically prefer only single-electron redox changes [9], [10]. Aside from metalloporphyrin complexes, such ligand-derived multielectron reaction chemistry has been largely unexplored for redox transformations of small-molecule substrates, but has received increasing attention [9], [10], [11], [12], [13].

Research on valence tautomeric (VT) complexes of transition metal containing redox noninnocent dioxolene ligands is an important topic because of their potential applications in future bistable molecular switching materials and devices [14], [15], [16]. In addition to the materials perspective, naturally occurring bromopyrocatechols are found in marine life and frequently isolated from red algae of the family Rhodomelaceae [17]. Most of these compounds have important biological activities. For example, some bromopyrocatechols exhibit enzyme inhibition, cytotoxicity, feeding deterrent and microbial activities [18], [19], [20], [21]. Protein tyrosine phosphatase inhibitory activity of bromopyrocatechols has also been reported [22]. Therefore, synthesis of bromo-derivatives of pyrocatechols is of immense importance for their biological application. One of the ways to derive halogen-substituted catechols is the metal catalyzed ipso substitution of aromatic halides, and in those systems metal-assisted electron deficiency in aromatic ring favors substitution of halides [23]. Furthermore, electron deficient catechols such as tetrachlorocatechol bound manganese(III) complexes were proved to be efficient catalysts for the production of hydrogen peroxide from O2 in the presence of hydroxylamine or hydrazine as a sacrificial reductant [24], [25]. Thus, we speculated that such dioxolene-based manganese(III) complexes may provide a robust platform to exhibit oxidase activity toward the oxidation of small molecule organic substances. As a part of our ongoing study of transition-metal dioxolene chemistry [26], [27], [28], [29] and functional mimics of phenoxazinone synthase [30], [31], [32], [33], we report herein the syntheses, structures and spectroscopic investigations of mononuclear manganese(III) complexes, [pyH][Mn(Br4Cat)2(py)] (1) and [Mn(Br4Cat)(Br3pyCat)(py)2] (2), where py is pyridine, Br4CatH2 is tetrabromocatechol and Br3pyCatH2 is 3,5,6-tribromo-4-pyridiniumcatechol. The present report demonstrates, for the first time, the valance tautomerism induced nucleophilic substitution reaction of an aromatic Br-ion by a nitrogen containing ligand, supported by DFT calculations. The effect of the positive charge at the ligand backbone on the diverse catalytic activity modeling the function of phenoxazinone synthase has also been investigated.

Section snippets

Materials and physical measurements

Tetrabromocatechol monohydrate, manganese(II) chloride tetrahydrate and pyridine (py) were of commercially available reagent or analytical grade chemicals and were used as received. Other chemicals and solvents were of reagent or analytical grade and used without further purification.

Elemental analyses for C, H, and N were performed in a PerkinElmer 240C elemental analyzer. The infrared spectra of the samples were recorded in the range of 400–4000 cm−1 on a PerkinElmer Spectrum Two Infrared

Syntheses and general characterizations

Reaction of manganese(II) chloride tetrahydrate and tetrabromocatechol in a 1: 2 molar ratio in methanol in the presence of pyridine under aerobic conditions resulted in an olive-green solution, which upon standing at 4 °C afforded olive-green crystals of 1 overnight in high yield in which manganese(II) is converted to manganese(III) by aerial oxidation. A similar reaction in the presence of excess pyridine at elevated temperature ultimately results in complex 2 within a couple of days, in which

Conclusions

In summary, a mononuclear manganese(III)-catecholate complex 1 has been synthesized by the reaction of tetrabromocatechol and MnCl2 under aerobic condition in the presence of pyridine. In contrast to the literature reports of similar systems, variable temperature UV–vis spectral studies of complex 1 reveal unprecedented spectral changes in which a dramatic increase in the ligand-to-metal charge transfer band at 592 nm associated with concomitant change of color of the solution from olive-green

Acknowledgements

A.P. would like to thank the Department of Science and Technology (DST), New Delhi, under FAST Track Scheme (Order No. SB/FT/CS-016/2012, dated 20/12/2013) for financial support. The authors thank the IISER, Kolkata for providing the EPR facility. The authors also thank Dr. J. P. Naskar, Department of Chemistry, Jadavpur University for the cyclic voltammetry experiment. A.F. thanks the DGICYT of Spain (projects CTQ2014-57393-C2-1-P and CONSOLIDER INGENIO 2010CSD2010-00065, FEDER funds).

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