A Linear Tetrapyrrole as a Binucleating Ligand with Copper(II). Coordination beyond the Usual M-N(4) Bonding

Synthesis of a donor-acceptor heterodimer via trifunctional completive self-sorting

Selective self-assembly of heterodimers consisting of two non-identical subunits plays important roles in Nature but is rarely encountered in synthetic supramolecular systems. Here we show that photocleavage of a donor–acceptor porphyrin complex produces an heterodimeric structure with surprising selectivity. The system forms via a multi-step sequence that starts with an oxidative ring opening, which produces an equimolar mixture of two isomeric degradation products (zinc(II) bilatrien-abc-ones, BTOs). These two isomers are susceptible to water addition, yielding the corresponding zinc(II) 15-hydroxybiladien-ab-ones (HBDOs). However, in the photocleavage experiment only one HBDO isomer is formed, and it quantitatively combines with the remaining BTO isomer. The resulting heterodimer is stabilized by a Zn–O coordination bond and extended dispersion interactions between the overlapping π-surfaces of the monomers. The observed selectivity can be seen as a case of completive self-sorting, simultaneously controlled by three types of complementary interactions.

The preparation of heterodimeric structures via self-assembly processes is challenging. Here, the authors report the photooxidation of a donor–acceptor metalloporphyrin, which enables a self-sorting process that yields an heterodimer quantitatively.

Oxygenation of Phenanthriporphyrin and Copper(III) Phenanthriporphyrin: An Efficient Route to Phenanthribilinones

Photooxidation of copper(III) 5,6-dimethoxyphenathriporphyrin and copper(III) 5,6-dioxophenanthriporphyrin, which contain phenanthrene or dioxophenathrene moieties built into the macrocyclic frameworks, resulted in the regioselective cleavage that afforded organometallic copper(III) complexes of open-chain phenanthribilinone-type acyclic ligands terminated by carbonyl groups. The copper(III) coordinates two carbon atoms of phenantherene (dioxophenanthrene) and two nitrogen atoms of pyrrole and pyrrolone units, preserving the donor sets of the paternal complexes. The primary dioxygen attack is located at the meso carbon atom adjacent to the phenanthrene moiety. Demetalation of copper(III) 21-benzoyl-phenanthribilin-1-one and copper(III) 21-benzoyl-dioxophenanthribilin-1-one yielded mainly two diastereomers [15Z, 20E] and [15Z, 20Z], which differ in the configurations at two C_α-C_meso double bonds. The regioselectivity of the cleavage, detected in the course of experimental studies, has been substantiated by DFT investigations. The regioselective cleavage of 5,6-dimethoxyphenanthriporphyrin in reaction with basic iron(III) acetate was detected, providing the synthetically efficient methodology to produce 21-benzoyl-dioxophenanthribilin-1-one.

Organocopper(III) Phenanthriporphyrin-Exocyclic Transformations

5,6-Dimethoxyphenanthriporphyrin 1 and 5,6-dioxophenanthriporphyrin 2 act as suitable organometallic ligands for copper(III), adopting trianionic [CCNN] coordination cores. Under oxidizing conditions, in the presence of methanol, copper(III) phenanthriporphyrin 1-Cu undergoes transformation to copper(III) phenanthriporphodimethene with methoxy substituents attached to two trans meso positions. Addition of acids to 1-Cu yields two isomeric copper(III) isophenanthriporphyrins protonated on one of the meso carbon atoms. Protonation of copper(III) 5,6-dioxophenanthriporphyrin 2-Cu yields the aromatic diprotonated complex 2-Cu-H₂²⁺. In the presence of HBF₄ 2-Cu undergoes borylation at the carbonyl oxygen atoms, forming an aromatic exocyclic boron(III) complex.

Electrochemical, Spectroscopic, and 1O2 Sensitization Characteristics of Synthetically Accessible Linear Tetrapyrrole Complexes of Palladium and Platinum

The synthesis, electrochemistry, and photophysical characterization of a 10,10-dimethyl-5,15-bis(pentafluorophenyl)biladiene (DMBil1) linear tetrapyrrole supporting Pd^II or Pt^II centers is presented. Both of these nonmacrocyclic tetrapyrrole platforms are robust and easily prepared via modular routes. X-ray diffraction experiments reveal that the Pd[DMBil1] and Pt[DMBil1] complexes adopt similar structures and incorporate a single Pd^II and Pt^II center, respectively. Additionally, electrochemical experiments revealed that both Pd[DMBil1] and Pt[DMBil1] can undergo two discrete oxidation and reduction processes. Spectroscopic experiments carried out for Pd[DMBil1] and Pt[DMBil1] provide further understanding of the electronic structure of these systems. Both complexes strongly absorb light in the UV-visible region, especially in the 350-600 nm range. Both Pd[DMBil1] and Pt[DMBil1] are luminescent under a nitrogen atmosphere. Upon photoexcitation of Pd[DMBil1], two emission bands are observed; fluorescence is detected from ∼500-700 nm and phosphorescence from ∼700-875 nm. Photoexcitation of Pt[DMBil1] leads only to phosphorescence, presumably due to enhanced intersystem crossing imparted by the heavier Pt^II center. Phosphorescence from both complexes is quenched under air due to energy transfer from the excited triplet state to ground state oxygen. Accordingly, irradiation with light of λ ≥ 500 nm prompts Pd[DMBil1] and Pt[DMBil1] to photosensitize the generation of ¹O₂ (singlet oxygen) with impressive quantum yields of 80% and 78%, respectively. The synthetic accessibility of these complexes coupled with their ability to efficiently photosensitize ¹O₂ may make them attractive platforms for development of new agents for photodynamic therapy.

Electrochemical, spectroscopic, and (1)O2 sensitization characteristics of 10,10-dimethylbiladiene complexes of zinc and copper

The synthesis, electrochemistry, and photophysical characterization of a 10,10-dimethylbiladiene tetrapyrrole bearing ancillary pentafluorophenyl groups at the 5- and 15-meso positions (DMBil1) is presented. This nonmacrocyclic tetrapyrrole platform is robust and can serve as an excellent ligand scaffold for Zn²⁺ and Cu²⁺ centers. X-ray diffraction studies conducted for DMBil1 along with the corresponding Zn[DMBil1] and Cu[DMBil1] complexes show that this ligand scaffold binds a single metal ion within the tetrapyrrole core. Additionally, electrochemical experiments revealed that all three of the aforementioned compounds display an interesting redox chemistry as the DMBil1 framework can be both oxidized and reduced by two electrons. Spectroscopic and photophysical experiments carried out for DMBil1, Zn[DMBil1], and Cu[DMBil1] provide a basic picture of the electronic properties of these platforms. All three biladiene derivatives strongly absorb light in the visible region and are weakly emissive. The ability of these compounds to sensitize the formation of ¹O₂ at wavelengths longer than 500 nm was probed. Both the free base and Zn²⁺ 10,10-dimethylbiladiene architectures show modest efficiencies for ¹O₂ sensitization. The combination of structural, electrochemical, and photophysical data detailed herein provides a basis for the design of additional biladiene constructs for the activation of O₂ and other small molecules.

Nucleophilic ring opening of meso-substituted 5-oxaporphyrin by oxygen, nitrogen, sulfur, and carbon nucleophiles

Nucleophilic ring opening of 23H-[21,23-didehydro-10,15,20-tris(4-methoxycarbonylphenyl)-5-oxaporphyrinato](trifluoroacetato)zinc(II) with various nucleophiles such as alkoxide, amine, thiolate, and enolate gave 19-substituted bilinone zinc complexes, and they were isolated as free base bilinones. An X-ray crystallographic study demonstrated that the product of 5-oxaporphyrin with sodium methoxide was 21H,23H-(4Z,9Z,15Z)-1,21-dihydro-19-methoxy-5,10,15-tris(4-methoxycarbonylphenyl)bilin-1-one with a helicoidal conformation. The structure of the product of 5-oxaporphyrin with an enolate of ethyl acetoacetate was 21H,22H,24H-(4Z,9Z,15Z,19E)-19-(1-ethoxycarbonyl-2-oxopropylidene)-5,10,15-tris(4-methoxycarbonylphenyl)-1,19,21,24-tetrahydrobilin-1-one, with three inner NH groups. The product with SH(-) was also the same tautomer, 21H,22H,24H-19-thioxo-bilin-1-one, with three NH groups, while the products with RO(-), RNH2, and RS(-) nucleophiles were 21H,23H-bilin-1-ones with two inner NH groups. The first-order rate constants of the ring opening reaction of 5-oxaporphyrin with 1 M BnOH and BnSH in toluene at 303 K were 3.0 × 10(-4) and 6.1 × 10(-4) s(-1), respectively. The ratio of the rate of alcohol to thiol was much higher than that with methyl iodide, suggesting that 5-oxaporphyrin reacted as a hard electrophile in comparison to methyl iodide. UV-visible spectra of 19-substituted bilinones in CHCl3 at 298 K showed that the absorption maximum of the lower energy band was red-shifted in increasing order of O-substituted (645 nm), S-substituted (668 nm), N-substituted (699 nm), and C-substituted bilinones (706 nm).

Bilitrienones from the chemical oxidation of dodecasubstituted porphyrins

The structure of the ring-opened product from direct oxidation of meso-tetraarylporphyrins has been controversial for three decades. Herein we show that bilitrienones 2 are obtained from oxidation of metal-free dodecasubstituted porphyrins 1 in the presence of sodium nitrite, trifluoroacetic acid and air oxygen. The presence of the para-nonyl groups in 1b stabilized the corresponding bilitrienone 2b, which was characterized by X-ray crystallography. In the absence of the para-nonyl groups bilitrienone 2a undergoes a rapid hydration reaction, giving biladienone 3a as the major isolated product. The molecular structures of 2b and 3a, and. the photochemical isomerization of 3a are discussed.

A facile and versatile preparation of bilindiones and biladienones from tetraarylporphyrins

Bilindiones and biladienones carrying aryl groups at the meso positions were prepared using coupled oxidation reactions of iron tetraarylporphyrins in 20-63% yield.

A rare ether-bridged cobalt complex which gives rise to an unusual 'serpentine' metal-ligand binding motif

An unusual metal-ligand binding motif is found in dimeric cobalt(II) complexes coordinated by diamidoether ligands that bridge the metals in a 'serpentine' fashion through the ether donors of the ligand backbone rather than the amido groups.

Regioselective oxidative liberation of aryl-substituted tripyrrinone metal complexes from N-confused porphyrin

[reaction: see text] A Cu(II) complex of the first aryl-substituted tripyrrinone derivative, 14-benzoyl-5,10-diphenyl-1-oxo-tripyrrinato copper(II) (3-Cu) was obtained in the reaction of N-confused tetraphenylporphyrin (NCTPP) and Cu(OAc)(2) in refluxing toluene, and subsequent treatment with acid afforded a free tripyrrolic ligand that could bind a variety of transition metals.

Biladienones from the photooxidation of a meso-gem-disubstituted phlorin: crystal and molecular structures of the 3N + O coordinated nickel(II) and copper(II) complexes

The photooxidation of a meso-gem-disubstituted phlorin gave two isomeric biladienones in an equilibrium involving a Z-E double bond photoisomerism. The structures of these bile pigments were elucidated using NMR techniques and show terminal benzoyl and pyrrolone moieties. Complexation with divalent cations (nickel(II), copper (II)) gave stable compounds whose crystal and molecular structure could be determined by X-ray diffraction. The metal is coordinated with three pyrrole nitrogen atoms and one oxygen atom of a terminal benzoyl group. In the crystal, the molecules are arranged in pairs through hydrogen bonds between the free terminal pyrrolones.

Manganese(III) biliverdin IX dimethyl ester: a powerful catalytic scavenger of superoxide employing the Mn(III)/Mn(IV) redox couple

A manganese(III) complex of biliverdin IX dimethyl ester, (MnIIIBVDME)2, was prepared and characterized by elemental analysis, UV/vis spectroscopy, cyclic voltammetry, chronocoulometry, electrospray mass spectrometry, freezing-point depression, magnetic susceptibility, and catalytic dismuting of superoxide anion (O2.-). In a dimeric conformation each trivalent manganese is bound to four pyrrolic nitrogens of one biliverdin dimethyl ester molecule and to the enolic oxygen of another molecule. This type of coordination stabilizes the +4 metal oxidation state, whereby the +3/+4 redox cycling of the manganese in aqueous medium was found to be at E1/2 = +0.45 V vs NHE. This potential allows the Mn(III)/Mn(IV) couple to efficiently catalyze the dismutation of O2.- with the catalytic rate constant of kcat = 5.0 x 10(7) M-1 s-1 (concentration calculated per manganese) obtained by cytochrome c assay at pH 7.8 and 25 degrees C. The fifth coordination site of the manganese is occupied by an enolic oxygen, which precludes binding of NO., thus enhancing the specificity of the metal center toward O2.-. For the same reason the (MnIIIBVDME)2 is resistant to attack by H2O2. The compound also proved to be an efficient SOD mimic in vivo, facilitating the aerobic growth of SOD-deficient Escherichia coli.