Covalently linked bisporphyrins bearing tetraphenylporphyrin and perbromoporphyrin units: Synthesis and their properties

A series of covalently linked bisporphyrins bearingmeso-tetraphenylporphyrin (TPP) and octabromotetraphenylporphyrin (OBTPP) units have been synthesised and characterised. Electrochemical studies on these bisporphyrins showed an anodic shift (∼ 30–60 mV) of the TPP unit and a cathodic shift (∼40-80 mV) of OBTPP in redox potentials. Further, steady-state fluorescence studies on bisporphyrins indicated dramatic decrease in fluorescence quantum yields of the TPP moiety. Electrochemical redox and fluorescence data seem to suggest the possible existence of intramolecular interactions in these bisporphyrins     

DABCO-directed self-assembly of bisporphyrins (DABCO=1,4-diazabicyclo[2.2.2]octane)

Three isomeric zinc bisporphyrins have been prepared by covalently linking together two aminoporphyrins with an isophthalic acid derivative. The porphyrins differ in the substitution pattern on the meso phenyl groups, that is, ortho, meta, or para. Titrations carried out by UV-visible and 1H NMR spectroscopy have been used to map out the stabilities and the stoichiometries of the complexes formed with 1,4-diazabicyclo[2.2.2]octane (DABCO) in chloroform. The ortho- and meta-substituted bisporphyrins form 1:1 intramolecular sandwich complexes. The para-substituted bisporphyrin cannot adopt the cofacial conformation required for this type of complex and forms a higher order 2:2 intermolecular assembly, which is stable over a wide range of DABCO concentrations.     

双核金属卟啉的合成及其催化羟基化环己烷的研究

合成了以乙基、丙基、丁基在卟啉5,5′－苯基邻位通过醚键联结的不同链长的双核金属铁（Ⅲ）、锰（Ⅲ）卟啉,并用NMR、元素分析和UV－Vis光谱进行了验证。以它们为催化剂,亚碘酰苯为氧化剂获得氧化环己烷的最好产率,环己醇为35．83％,环己酮为23．41％。通过产率比较,初步定性地获得了双核金属卟啉的活性和稳定性的规律,即双铁亚乙基卟啉＞双铁亚丙基卟啉＞双铁亚丁基卟啉＞单铁卟啉和单锰卟啉＞双锰亚丁基卟啉＞双锰双丙基卟啉＞双锰亚乙基卟啉,由于相同的条件下铁和锰的规律相反,推测在以双卟啉模型中存在着金属之间的相互作用,铁的作用和锰的作用对双卟啉的催化性能影响相反。     

Intermolecular complexes of singly linked bisporphyrins with trinitrobenzene

Several covalently linked bisporphyrin systems, free-base (H₂P---H₂P), hybrid bisporphyrins (Zn---H₂P) and Zn(II) dimers (ZnP---ZnP) and their 1:1 molecular complexes with sym 1,3,5-trinitrobenzene have been investigated by optical absorption and emission, and magnetic resonance spectroscopic methods. In these systems, two porphyrin units are linked singly through one of the meso aryl groups via ether linkages of variable length. The bisporphyrins cooperatively bind a molecule of a π-acceptor; 1,3,5-trinitrobenzene (TNB). The binding constant values vary with interchromophore separation. Maximum binding is observed in the bisporphyrin bearing a two-ether covalent linkage. It is found that TNB quenches the fluorescence of the two porphyrine units in a selective manner. It is suggested that a critical distance between the two porphyrin units is necessary for the observance of maximum cooperative intermolecular binding with an acceptor.     

Equilibrium, photophysical, photochemical, and quantum chemical examination of anionic mercury(II) mono- and bisporphyrins

Mercury(II) ion and 5,10,15,20-tetrakis(parasulfonato-phenyl)porphyrin anion can form 1:1, 2:2, and 3:2 (metal ion/porphyrin) out-of-plane (OOP) complexes, from which Hg2P2(8-) has not been identified until now. Identification of this species significantly promoted the confirmation of the composition and the precise elucidation of the equilibrium of Hg3P2(6-). Since the formation of each complex is too fast, their kinetic behavior was studied from the side of dissociation. The rate-determining step in dissociations, as well as in the formation of the 2:2 complex, that is, the dimerization of 1:1 complex, proved to be virtually first-order under these conditions, while the consecutive formations of HgP(4-) and Hg3P2(6-) are second-order reactions. The equilibria can be spectrophotometrically investigated because the Soret- as well as the Q-absorption bands of the free-base ligand are more and more red-shifted in the series of 1:1, 2:2, and 3:2 complexes, and the split of Q-bands disappears as the singlet-1 excited states become degenerate; in the case of bisporphyrins, the bands broaden, especially in the longer-wavelength region of the spectra. The quantum yield and the lifetime of S1-fluorescence from the macrocycle is decreased by the insertion of a mercury(II) ion due to distortion, and in bisporphyrins the luminescence totally ceases because their more complicated structure promotes other ways of energy dissipation. The lifetime of the triplet excited-state is also reduced by metalation. The transient absorption measured upon excitation of Hg3P2(6-) probably originates from Hg2P2(8-) formed by efficient photodissocation during the laser pulse. This photoinduced dissociation is characteristic to out-of-plane complexes, but in metallo-monoporphyrins it needs the energetically higher Soret-excitation; in bisporphyrins, it can take place during irradiation at the longer Q-wavelengths. Investigation of the intramolecular photoredox reactions has proved that for the increased efficiency of the indirect photoinduced LMCT, not the redox potential, but the position of the metal center is responsible. The two orders of magnitude higher photoredux quantum yield for the 3:2 complex, compared to that of the 2:2 species, can be explained by the repulsive effect of the inner mercury(II) ion pushing the other two farther out of the ligand cavity. In bisporphyrins the second excited states are photochemically more reactive than the first ones, while most of the photochemical processes of HgP(4-) originate from the first excited state. According to our quantum chemical calculations, the mercury(II) ion causes the expansion of the porphyrin-cavity; therefore its out-of-plane position is smaller than the value expected based on its ionic radius. In the hitherto unknown 2:2 dimer two 1:1 saucer-shaped monomers are kept together by secondary forces, mostly by pi-pi interaction, but their relative arrangement was not unequivocally determined by the two DFT functionals used. The arrangements with a symmetry axis or plane perpendicular to both rings are not favored; instead, the two monomers are shifted along the porphyrin planes, either in a Hg-P-Hg-P or a Hg-P-P-Hg order. Our time-dependent density functional theory (TD-DFT) calculations indicate that the electronic spectra are not very sensitive to the structure of the dimer, even though the environment of the porphyrin rings is quite different if one of the metal ions is between or outside of both macrocycles. The calculated spectral shifts agree only partially with the experimental data. The TD-DFT calculations suggest that the chromophores are not fully independent in the bisporphyrins and that the observed spectral shift cannot be uniquely assigned to the geometrical distortion of the porphyrin macrocyle.     

Noncovalent endo-binding of fullerenes to diprotonated bisporphyrins

Noncovalent binding of fullerenes to bisporphyrins was studied in the gas phase by energy-dependent collision-induced dissociation (CID) with Xe under single-collision conditions. The electrospray ionization mass spectra of calix[4]arene-linked bisporphyrins show that bisporphyrins take up to 3-4 protons, depending on the type of meso-substituents. Of the protonated bisporphyrins, the diprotonated species form stable 1:1 complexes with fullerenes (C(60) and C(70)). CID cracking patterns of the diprotonated bisporphyrins indicate that each monomeric porphyrin moiety is singly protonated. CID yield-energy curves obtained from the 1:1 diprotonated bisporphyrin-fullerene complexes suggest that a fullerene occupies the endo-binding site intercalated between the two singly protonated porphyrin moieties. In the cases of 1:2 diprotonated bisporphyrin-fullerene complexes, CID results show that one fullerene binds inside (endo-binding) and the other outside (exo-binding). The exo-binding mode is energetically almost identical to the binding of fullerenes to singly protonated porphyrin monomers. The endo-binding energy is at least twice the exo-binding energy. To gain insights into the binding mode, we optimized structures of diprotonated bisporphyrins and their 1:1 endo-complexes with fullerenes, and calculated the endo-binding energy for C(60), C(70) (end-on), and C(70) (side-on). The endo-binding of fullerenes to diprotonated bisporphyrins nearly doubles the π-π interactions while reducing the electrostatic repulsion between the two singly protonated porphyrin moieties. The side-on binding of C(70) is favored over the end-on binding because the former exerts less steric strain to the lower rim of calixarene.     

Complexes between hydrogen bonded bisporphyrin tweezers and cholesterol-appended fullerenes as organogelators and liquid crystals

This paper reports the gelation and liquid crystal properties of a class of new complexes between zinc and copper bisporphyrin and C₆₀ derivatives. The bisporphyrins are induced by intramolecular hydrogen bonding to adopt a preorganized ‘U’-shaped conformation and therefore efficiently complex the C₆₀ derivatives. As a result, the capacity of their mixtures to gelate alkanes is increased notably. The bisporphyrins themselves and their complexes with the C₆₀ derivatives form the smectic liquid crystal phase. However, the glassy transition temperature of the complexes decreases considerably.     

Diiron(III)–μ‐Fluoro Bisporphyrins: Effect of Bridging Ligand on the Metal Spin State

A hitherto unknown family of diiron(III)–μ‐fluoro bisporphyrins has been synthesized and structurally characterized. Fluoride abstraction from SbF₆^? and BF₄^? resulted in the synthesis of the μ‐fluoro complexes of ethane‐ and ethene‐bridged diiron(III) bisporphyrins. Two such complexes were structurally characterized, which revealed a single fluoro bridge between two iron centers with a remarkably bent Fe‐F‐Fe unit. Although isoelectronic with the μ‐hydroxo complexes, the μ‐fluoro species are quite divergent in terms of the electronic structure and properties. UV/Vis spectroscopy of the μ‐fluoro complex exhibits a large redshift (ca. 18 nm) of the Soret band in comparison to their μ‐hydroxo analog. Combined analysis by single crystal X‐ray structure determination and Mössbauer and ¹H NMR spectroscopy revealed the presence of two equivalent iron(III) centers in the μ‐fluoro complexes in both solid and solution phases. In contrast, the iron(III) centers of the μ‐hydroxo complexes are known to be inequivalent. Variable‐temperature magnetic studies show a weak antiferromagnetic interaction between the iron(III) centers of the μ‐fluoro complexes with coupling constants (J) ranging from ?33 to ?40 cm^?1. The experimental results were further supported by DFT calculations.     

Synthesis of an anthracenyl bridged porphyrin–corrole bismacrocycle. Physicochemical and electrochemical characterisation of the biscobalt μ-superoxo derivative

Dicobalt or heterobimetallic cofacial bisporphyrins are up till now amongst the very few molecular electrocatalysts able to promote the direct reduction of dioxygen to water via a four-electron process in acidic medium. Numerous studies have been devoted to elucidate the key steps of this catalytic reaction and an important result has revealed an unexpected high dioxygen affinity for a mixed valence Co(II)/Co(III) cofacial porphyrin, the key intermediate complex being a μ-superoxo derivative. At the same time, the great importance assumed by ‘Pacman’ porphyrins and the recent developments in corrole chemistry have provided the stimulation to synthesise porphyrin–corrole dyads which might also transport and/or activate dioxygen. In the present paper, we report the stepwise synthesis and characterisation of a cofacial porphyrin–corrole bearing an anthracenyl bridge, (PCA)H₅ where PCA is the pentaanion of 1-(13,17-diethyl-2,3,7,8,12,18-hexamethylporphyrin–5-yl)-8-(7,8,12,13-tetramethyl-2,3,17,18-tetraphenylcorrol-10-yl) anthracene. The synthesis and characterisation of the μ-superoxo Co(III)/Co(III) complex 〚(PCA)Co₂Im₂〛(μ-O₂) is also described.     

Supramolecular inclusion complexes of two cyclic zinc bisporphyrins with C60 and C70: structural, thermodynamic, and photophysical characterization

The formation of thermodynamically stable inclusion complexes between two cyclic zinc bisporphyrins, differing in the saturation degree of the hydrocarbon linkers that connect their porphyrin units, and the fullerenes C(60) and C(70) is described. Binding and photophysical studies were performed in two solvents of very different polarity: toluene and dichloromethane. UV/Vis and fluorescence titration experiments showed π-π interactions between the cyclic zinc bisporphyrins and the fullerenes. Solid-state structures were determined by X-ray diffraction analysis and gave valuable insight into the different complexation behaviors of the two macrocyclic systems towards the fullerenes. NMR titrations were also helpful in understanding the geometry of the complexes in solution. Upon fullerene complexation, the two macrocyclic bisporphyrins adopt very distinct conformations. Charge-transfer absorption bands point to ground-state interactions, and quenching of the porphyrin component luminescence indicates fast reactivity in the excited states. Energy transfer plus HOMO-HOMO and LUMO-LUMO electron-transfer processes occur within the complexes. Charge-separated states characterized by a reduced fullerene and an oxidized porphyrin radical, with lifetimes in the order of several hundred picoseconds, are detected.     

Cyclic porphyrin dimers as hosts for coordinating ligands

Bicovalently linked tetraphenylporphyrins bearing dioxypentane groups at the opposite (transoid, H₄A) and adjacent (cisoid, H₄B) aryl groups have been synthesised. Protonation of the free-base porphyrins leads to fully protonated species H₈A⁴⁺/H₈A⁴⁺ accompanied by expansion of cavity size of the bisporphyrins. The electrochemical redox studies of these porphyrins and their Zinc(II) derivatives revealed that the first ring oxidation proceeds through a two-electron process while the second ring oxidation occurs at two distinct one-electron steps indicating unsymmetrical charge distribution in the oxidized intermediate. The axial ligation properties of the Zinc(Il) derivatives of H₄A/H₄B with DABCO and PMDA investigated by spectroscopic and single crystal X-ray diffraction studies showed predominant existence of 1: I complex. The Zn₂A.DABCO complex assumes an interesting eclipsed structure wherein DABCO is located inside the cavity between the two porphyrin planes with Zn-N distances at 2.08 and 2.22 ?. The Zn atoms are pulled into the cavity due to coordination towards nitrogen atoms of DABCO and deviate from the mean porphyrin plane by 0.35 ?. The electrochemical redox potentials of the axially ligated metal derivatives are found to be sensitive function of the relative coordinating ability of the ligands and the conformation of the hosts.     

A phototriggered molecular spring for aerobic catalytic oxidation reactions

The first catalytic oxidation reactions with cofacial bisporphyrins using molecular oxygen as the terminal oxidant are presented. The photocatalytic reactions proceed under mild conditions (ambient temperature and pressure) without the need for a co-reductant. A systematic reactivity study of the photooxidation of dimethyl sulfide (DMS) with a homologous series of bisiron(III) mu-oxo porphyrins containing dibenzofuran, xanthene, or no bridge reveals that the facility of these processes is markedly dependent on the vertical flexibility of the catalyst to provide a phototriggered molecular spring.     

Meso-tetraaryl cofacial bisporphyrins delivered by Suzuki cross-coupling

The Suzuki cross-coupling methodology provides a facile synthetic approach for the modular preparation of meso-tetraaryl cofacial bisporphyrins anchored by xanthene and dibenzofuran. This synthetic method furnishes cofacial bisporphyrin templates with enhanced steric and electronic protection from mu-oxo formation and oxidative degradation. The ability of these platforms to support multielectron oxidation chemistry mediated by proton-coupled electron transfer (PCET) is demonstrated by their reactivity for the catalytic disproportionation of hydrogen peroxide to oxygen and water.     

Fluorescence Quenching Study of Zinc Bisporphyrins by Fulleropyrrolidines and Their N-Oxides

Novel phenylene-bridged zinc bisporphyrins （1-4）, fulleropyrrolidines （C60-m, C60-h） and their N-oxides （C60-mo, C60-ho） were synthesized. The fluorescence quenching processes of bisporphyrins in toluene solution by fulleropyrrolidines and their N-oxides were investigated by steady-state fluorescence spectra. The fluorescence quenching constants proved that the fluorescence quenching ability was decreased as reduction of the pyrrolidine functional groups of fullerene surface： C60-h〉C60-m〉C60, and the fluorescence quenching ability was increased about 1.3-7.4 times by utilizing fulleropyrrolidine N-oxides （C60-mo, C60-ho） compared to fulleropyrrolidine compounds （C60-m, C60-h）. The results revealed photoinduced electron transfer （PET） efficiency between bispor-phyrin and fullerene derivatives could be tunable by change of functional groups on fullerene surface.     

Chiroptical properties of an alternatingly functionalized cellotriose bearing two porphyrin groups

Right-handedness derived from bisporphyrins attached to a cellotriose backbone at O-6 and O'-6 positions is revealed for the first time. This cellotriose is proposed as a model of alternatingly functionalized cellulosics, which have promising properties for applications in optoelectronics and molecular receptors owing to the chirality and rigid backbone effects.     

Effect of Inter‐Porphyrin Distance on Spin‐State in Diiron(III) μ‐Hydroxo Bisporphyrins

The synthesis, structure, and properties of bischloro, μ‐oxo, and a family of μ‐hydroxo complexes (with BF₄^?, SbF₆^?, and PF₆^? counteranions) of diethylpyrrole‐bridged diiron(III) bisporphyrins are reported. Spectroscopic characterization has revealed that the iron centers of the bischloro and μ‐oxo complexes are in the high‐spin state (S=⁵/₂). However, the two iron centers in the diiron(III) μ‐hydroxo complexes are equivalent with high spin (S=⁵/₂) in the solid state and an intermediate‐spin state (S=³/₂) in solution. The molecules have been compared with previously known diiron(III) μ‐hydroxo complexes of ethane‐bridged bisporphyrin, in which two different spin states of iron were stabilized under the influence of counteranions. The dimanganese(III) analogues were also synthesized and spectroscopically characterized. A comparison of the X‐ray structural parameters between diethylpyrrole and ethane‐bridged μ‐hydroxo bisporphyrins suggest an increased separation, and hence, less interactions between the two heme units of the former. As a result, unlike the ethane‐bridged μ‐hydroxo complex, both iron centers become equivalent in the diethylpyrrole‐bridged complex and their spin state remains unresponsive to the change in counteranion. The iron(III) centers of the diethylpyrrole‐bridged diiron(III) μ‐oxo bisporphyrin undergo very strong antiferromagnetic interactions (J=?137.7 cm^?1), although the coupling constant is reduced to only a weak value in the μ‐hydroxo complexes (J=?42.2, ?44.1, and ?42.4 cm^?1 for the BF₄, SbF₆, and PF₆ complexes, respectively).     

Nickel(II) chloride complexes of 1,5-disubstituted 2,4-dithiobiurets

The complex compounds formed by the interaction of Nickel(II) chloride with 1,5-disubstituted 2,4-dithiobiurets, have been isolated and characterised. The structure of these complexes have been studied by i.r. spectra, conductance and magnetic susceptibility measurements. It has been found that coordination occurs in these ligands through thiocarbonyl sulphur atoms, present in ligand molecule, which is supported by the potentiometric titrations of ligands and complexes with iodine in THF.     

Synthesis of triazole- and pyridine-bridged porphyrin-chlorin and porphyrin dimers

The synthesis of 4-amino-1,2,4-triazole linked porphyrin-chlorin dimer and porphyrin dimer is described. New cationic porphyrin dimers were synthesized via the alkylation of the nitrogen atoms of 4,4′-bipyridine and 1,2-di-(4-pyridyl)ethene by meso-monobromomethylporphyrin. These cationic bisporphyrins can interact with DNA and have the ability to cleave it.     

Cyclometallated platinum(II) complexes: oxidation to, and C-H activation by, platinum(IV)

A series of cyclometallated phenylpyridine platinum(II) complexes have been synthesised with a systematic variation in both the phenylpyridine and the ancillary ligand. Oxidation of one of the cyclometallated species leads to a number of isomeric platinum(IV) complexes, all of which eventually isomerize to a single compound. The route to these new compounds has been demonstrated to involve an initial slow oxidation followed by a rapid C-H activation to give doubly cyclometallated complexes. The solid state structures of a number of both the platinum(II) and the platinum(IV) species have been solved; many of the structures exhibited extended interactions that result in complex three dimensional packing.     

Magnetic circular dichroism study of directly fused porphyrins.

The magnetic circular dichroism (MCD) spectra of doubly and triply linked fused bisporphyrins (2MD and 2MT, M = Ni, Zn, Cu, Pd, and H2) and triply linked higher oligomers (3ZnT and 4ZnT) have been measured, and their Q-bands assigned based on the results of INDO/s calculations. In contrast to the Faraday A term observed for the Q(0,0) band of Ni(II) tetraphenylporphyrin, a single positive Faraday B term was observed for the lowest energy transition of the fused systems. The calculations indicated that the molecular orbitals (MOs) of the directly fused porphyrins consist of linear combinations of the constituent monomeric MOs, and that the effect of lowering the symmetry is always larger on the lowest unoccupied molecular orbital (LUMO) than on the highest occupied molecular orbital (HOMO). On the basis of Michl's perimeter model, these features can be correlated with the observed positive MCD signs in the near infrared region. A weak absorption band at 600-700 nm for the fused dimers can be assigned to a short-axis polarized Q transition.