Conversion of Cobalt(II) Porphyrin into a Helical Cobalt(III) Complex of Acyclic Pentapyrrole

Abstract

Conversion of artificial porphyrin to acyclic polypyrrole by oxidation is important not only for model reactions of heme oxygenase (HO), which converts heme to biliverdin, [1][2][3] but also for the syntheses of new acyclic polypyrroles. [4][5][6][7][8][9][10] Biliverdin-type ligands are nonplanar tetrapyrrole derivatives and have been utilized for the syntheses of metal complexes with helical structures that are easily racemized, owing to its low steric hindrance for interconversion. [11][12][13][14][15][16] In contrast, metal complexes bearing acyclic polypyrroles constructed of more than five pyrroles are still rare because of the limited availability of synthetic routes to prepare them. [17][18][19][20][21][22] As an exceptional example, a new dicopper(II) complex bearing acyclic heptapyrrolic helices has been prepared from dicopper(II) heptaphyrin by reaction with O 2 . [7] In this reaction, O 2 was not activated at the metal center but reacted with the heptaphyrin ligand directly.Two intrinsic effects of the coordination environment around heme on the activation of the O 2 molecule have been proposed in the biological system. One is the electron donation from coordinating bases such as imidazole and thiolate at the fifth coordination site to the iron center, and the other is the hydrogen bond from the histidine residue to the O 2 molecule bound at the metal center. When these two effects contribute cooperatively, this activation mechanism is called a "push-pull" mechanism. [3,23] To mimic the pull effects, some porphyrin ligands bearing carboxylic acid, which could form a hydrogen bond to the bound O 2 , have been designed and synthesized. [4,[23][24][25] For example, Chang and co-workers have shown that Co II porphyrin complex bearing naphthoic acid, [Co(npca-por)], converted to metal-free biliverdin-type acyclic polypyrrole by reaction with O 2 . This reaction proceeds under ambient condition without coordination of any bases at the axial site. [4] The authors proposed that in this system, the carboxylic acid group protonated the coordinating O 2 molecule to yield a Co III À OOH species as an intermediate. Although this system mimicked the pull effect well, porphyrin complexes which require both push and pull effects for the cleavage of the porphyrin ring are still not known.Recently, we designed a new porphyrin ligand (amtpp; Scheme 1) that has an amide group in the ortho position of a phenyl ring of tetraphenylporphyrin (H 2 tpp) to mimic the pull effect by histidine residue in the biological system. We found that the imidazole adduct [Co(amtpp)(1-MeIm)] (1-MeIm = 1-methylimidazole) activated O 2 , thus converting to a new Co III helical complex with a coiled acyclic pentapyrrole-type ligand, despite the fact that the Co II complex [Co(amtpp)] (1) was inert to air (Scheme 1). Moreover, we have found that chiral axial ligands (S)-nicotine and (S)-cotinine induced the (M)-helical form preferentially in this conversion system. Herein, we report the synthesis and structure of the Co-amtpp complex and the un...

A Supramolecular Palladium Catalyst Displaying Substrate Selectivity by Remote Control

“…The free-base porphyrin for L6 was synthesized by following ar eported procedure. [30] All the other employed chemicals were purchased from commercial sources and used as received. Unless otherwise specified, reactions were carried out under argon atmosphere by employing standard Schlenk and vacuum-line techniques.…”

Conformational Changes and Redox Properties of Bimetallic Single Helicates of Hexapyrrole‐α,ω‐dicarbaldehydes

“…[2] Moreover,t he strongly metal-coordinating nature of oligopyrroles enables generating av ariety of mono-and multi-nuclearh elical metal complexes of single-and multi-stranded structures, M m L n (1 m, n 3). [3][4][5][6][7][8][9][10] Although an umber of those helicates of openchain tetrapyrroles including bile pigments have been known, [3][4][5][6] studies on metal complexes of long-chain oligopyrroles with more than four pyrrolic units remain rather limited, [7][8][9][10] mainly because such ligands are not easy to synthesize. [2d, 11-13] The coordination chemistry of hexapyrrolic ligands (L) was studied by Dolphina nd co-workers and they reported the double-stranded helicates Zn 2 L 2 and Zn 3 L 2 .…”

Single Helicates of Dipalladium(II) Hexapyrroles: Helicity Induction and Redox Tuning of Chiroptical Properties

“…A number of mononuclear complexes of tetrapyrroles with helical structures have been reported,6 and a few examples of multinuclear tetrapyrroles are also known 7. However, control of their helical handedness is difficult, and successful examples have been limited 8–10…”