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 uni...

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

“…The agostic metal–H geometry observed for 1 a Cu is consistent with the high‐frequency shift observed for 1 a Ni. Actually, we have previously reported that the hydrogen atom in the vicinity (≈2.6 Å) of the Ni atom underwent a remarkable high‐frequency shift (Δ δ ≈2 ppm) in the 1 H NMR spectrum of bisnickel(II) complexes of 1,3‐phenylenehexapyrroles 2 Ni . This singlet (H14) of 1 a Ni shifted gradually to an even higher‐frequency region ( δ =9.4 ppm) at −60 °C, which may be explained in terms of an equilibrium between a homohelical C 2 ‐symmetric open form and a heterohelical C i ‐symmetric open form, as depicted in Figure .…”

“…The bimetallic complexes of the hexapyrrole‐α,ω‐dicarbaldehydes could be regarded as a unique scaffold in developing functional materials of variable helical conformations and of redox‐tunable spectroscopic properties. In fact, the terminal formyl groups can be conveniently employed to introduce a chiral source that should induce helical sense bias in the oligopyrrole chain . That study will be reported in detail as a separate work.…”

“…The pyrrole NH hydrogen atoms play a key role in building helical conformations owing to intramolecular and supramolecular hydrogen‐bonding networks . Moreover, the strongly metal‐coordinating nature of oligopyrroles enables generating a variety of mono‐ and multi‐nuclear helical metal complexes of single‐ and multi‐stranded structures, M m L n (1≤ m , n ≤3) . Although a number of those helicates of open‐chain tetrapyrroles including bile pigments have been known, studies on metal complexes of long‐chain oligopyrroles with more than four pyrrolic units remain rather limited, mainly because such ligands are not easy to synthesize …”

“…The coordination compound 9 Pd of a hexapyrrolic ligand was structurally characterized for the first time by Bröring and co‐workers (Scheme ) . Monometallic helical complexes, such as 10 Fe and 11 Co, were also prepared by oxidative metallation of the precursor ligands . As for bimetallic helicates, we have prepared the bisPd II complex 1 a Pd and the bisNi II complex 2 Ni of partially π‐conjugated hexapyrrolic ligands ,.…”

Photocatalytic Degradation of Methyl Orange over Metalloporphyrins Supported on TiO2 Degussa P25

“…In the past decades, metalloporphyrins have been used as cytochrome P-450 models for the highly efficient homogeneous or heterogeneous oxidation of organic compounds [ 11 , 12 , 13 , 14 , 15 , 16 ]. Moreover, they are recognized to be the most promising photosensitizers due to their very strong absorption in the 400–500 nm region (Soret band) and in the 500–700 nm region (Q bands) [ 17 ].…”