Scheme 2.1: APD catalyzed CC bond cleavage of 2˗aminophenol
In 2013, Li et al3 have reported the crystal structures of APD from Comamonas sp. strain CNB-1 as the apoenzyme, the holoenzyme and as complexes with the lactone intermediate (4Z,6Z)-3-iminooxepin-2(3H)-one, the product 2-aminomuconic-6-semialdehyde and with the suicide inhibitor 4-nitrocatechol. The active site of APD contains a mononuclear nonheme iron centre. The iron is in +II oxidation state3 and the “His13-His62-Glu251 facial triad” occupies the coordination sites.3 During the oxidation catalysis, both N and O atoms from the substrate 2- aminophenol derivative and an oxygen atom from a dioxygen molecule bind to the iron centre form the other face. Thus, a six-coordinate intermediate forms. The catalytic path for CC bond cleavage of 2-aminophenol via the incorporation of two oxygen atoms is being proposed to follow the mechanism similar to that of extradiol cleavage by catechol dioxygenases.4
In general, it has been observed that redox-active 2-aminophenol or its derivatives upon coordination to iron center do not undergo aromatic CC bond cleavage in the presence of dioxygen.5 Rather, it exists mainly in its one-electron oxidized 2-iminobenzosemiquinonato π- radical state in the coordination complexes.5 Thus, biomimetic model complexes for APD are very rare.6 In the model complexes, either tridentate or tetradentate ligand scaffolds along with
substrate 2-aminophenol derivatives are being employed to form the corresponding five- coordinate and six-coordinate complexes under inert atmosphere. Dioxygen reactivity of those complexes is then studied for the mechanistic understanding of APD.6 In all the reports, the aminophenol-derived cleavage products are isolated via acidic work up of the reaction solutions.
In the procedure, the possibility of unexpected condensation and/or ring opening of the actual oxidative CC cleavage species via hydrolysis cannot be revoked. Thus, a direct method for identification of the CC bond cleavage product was necessary. In this context, we have incorporated a 3,5-di-tert-butyl-2-aminophenol unit at the ortho-position of a tripodal N,N- bis(pyridine-2-ylmethyl)benzylamine ligand scaffold (Scheme 2.2). Thus the designed ligand, designated here as H2GanAP, contains both substrate 2-aminophenol and tripodal N3 iron coordination site. The ligand would provide a mononuclear five˗coordinate iron complex that could allow its reaction toward dioxygen. Thus complex finally could provide the two oxygen atoms˗incorporated CC cleavage product of 3,5-di-tert-butyl-2-aminophenol unit. X-ray crystallographic analysis of the final complex would then provide crystallographic identification of the aromatic oxidative CC cleavage product.
Manganese ions are omnipresent in living systems and catalyze a wide range of biological transformations, which include: water-splitting to oxygen molecule in photosynthesis (PS II);7 the reduction of nucleotides to their corresponding deoxynucleotides for DNA replication and repair of all organisms (Ribonucleotide Reductase);8 dismutation of toxic superoxide to hydrogen peroxide and oxygen molecule (Mn-Superoxide Dismutase);9 decomposition of hydrogen peroxide to water and oxygen molecule (Catalase);10 etc. In all the enzymatic activities, the manganese ions shuttle between various oxidation states. In addition to the involvement of manganese ions of different oxidation states, the participation and the pivotal role of a tyrosine radical in PS II are equivocally establishes. Recently, Stubbe and co-workers8b have concluded the presence of a mono(-oxo)-bridged Mn2III
-tyrosine radical unit as the active site in the cofactor of Class Ib Ribonucleotide Reductase. Hence, the synthesis, as well as structural and spectroscopic characterization of mono(-oxo)-bridged, radical-containing binuclear Mn(III) complexes have attained a special interest.
Bis(-oxo)-bridged binuclear Mn-complex are well documented.11a-f Mono(-oxo)- bridged binuclear Mn(III) complexes and radical-conatining Mn(III) complexes are scarce.
While, to the best of our knowledge, mono(-oxo)-bridged binuclear Mn(III) complex coordinated to -radical anions is yet to be reported. Herein, we have initiated the plausible synthesis of a mono(oxo)-bridged, radical-containing binuclear Mn(III) complex.
2.2 Synthesis and Characterization of Five-coordinate Pyridine based Aminophenol Appended Non-innocent Ligand H2
A schematic representation for the synthesis of ligand H2GanAP is shown in Scheme 2.2.
Scheme 2.2: Synthetic route for the preparation of H2GanAP.
A reaction between 1:1 2-picolyl amine (I) with pyridinecarboxaldehyde (II) in CH3OH followed by NaBH4 reduction gave bis(2-pyridylmethyl)amine (III).11g 2-[bis(2- pyridylmethyl)aminomethyl]-nitrobenzene (V) was formed by reaction between bis(2- pyridylmethyl)amine (III) with 2-nitrobenzyl bromide (IV) in dry CH3CN in the presence of K2CO3, which upon reduction by Pd/C in the presence of NaBH4 provided 2-[bis(2- pyridylmethyl)aminomethyl)]aniline (VI).11h-i The ligand H2GanAP was was obtain in 50% yield (Scheme 2.2) by reacting equimolar amounts of 2-[bis(2-pyridylmethyl)aminomethyl]aniline (VI) and 3,5-di-tert-butyl catechol (VII) in hexane in the presence of Et3N under air. The ligand was characterized by using FT-IR spectroscopy, NMR spectroscopy and mass spectrometry techniques.
Initially, in the presence of Et3N and air, 3, 5-di-tert-butyl catechol (VII) gets oxidized to 3, 5-di-tert-butyl benzoquinone (BQ). 2-[bis(2-pyridylmethyl)aminomethyl]aniline(VI) then reacted with the generated 3,5-di-tert-butyl benzoquinone (BQ) and provided the compound (GanIBQ), which will get reduced by an equivalent amount of 3,5-di-tert-butyl catechol(VII) and consequently, provided the ligand H2GanAP (Scheme 2.3).11j
Scheme 2.3: Probable mechanism for the formation of H2GanAP from 3,5-di-tert-butyl catechol.
The 2-aminophenol unit of the synthesized ligand H2GanAP might behave as non-