Porphyrin architectures bearing functionalized xanthene spacers

A modular synthetic strategy for the construction of cofacial porphyrin architectures bearing hydrogen-bond synthons on a xanthene platform is presented. The convergent approach is based on a xanthene aldehyde-ester building block that is easily obtainable on a multigram scale with minimal purification. Treatment of this xanthene derivative with a variety of aryl aldehydes and pyrrole under standard Lindsey conditions affords a family of meso-substituted porphyrins bearing a single functionalized xanthene spacer. Direct modification of the hydrogen-bond synthon after macrocyclization proceeds smoothly to furnish porphyrin systems with a variety of cofacial functionalities (e.g., carboxylic acid, ester, amide). Porphyrins bearing two trans-functionalized xanthene spacers are prepared by the MacDonald [2 + 2] condensation of the xanthene aldehyde-ester with readily available 5-aryl-substituted dipyrromethanes such as 5-mesityldipyrromethane to afford the pure alpha,alpha- and alpha,beta-porphyrin atropisomers after chromatographic separation. The versatility of this synthetic method offers intriguing opportunities for the use of these and related templates for the study of proton-coupled activation of small molecules.     

Long, directional interactions in cofacial silicon phthalocyanine oligomers

Single crystal structures have been determined for the three cofacial, oxygen-bridged, silicon phthalocyanine oligomers, [((CH(3))(3)SiO)(2)(CH(3))SiO](SiPcO)(2-4)[Si(CH(3))(OSi(CH(3))(3))(2)], and for the corresponding monomer. The data for the oligomers give structural parameters for a matching set of three cofacial, oxygen-bridged silicon phthalocyanine oligomers for the first time. The staggering angles between the six adjacent cofacial ring pairs in the three oligomers are not in a random distribution nor in a cluster at the intuitively expected angle of 45° but rather are in two clusters, one at an angle of 15° and the other at an angle of 41°. These two clusters lead to the conclusion that long, directional interactions (LDI) exist between the adjacent ring pairs. An understanding of these interactions is provided by atoms-in-molecules (AIM) and reduced-density-gradient (RDG) studies. A survey of the staggering angles in other single-atom-bridged, cofacial phthalocyanine oligomers provides further evidence for the existence of LDI between cofacial phthalocyanine ring pairs in single-atom-bridged phthalocyanine oligomers.     

The first versatile synthetic approach to cofacial bis-porphyrins with calixarene spacers

[structure: see text] A versatile stepwise synthetic approach to cofacial bis-porphyrin species with calix[4]arene spacers has been designed. The three examples described demonstrate that the method allows selection, in a tailor-made fashion, of the calix[4]arene conformation, the type of functionalization of the calixarene rims, and the anchoring point of the chromophores on the calix[4]arene spacer.     

A convergent synthetic approach using sterically demanding aryldipyrrylmethanes for tuning the pocket sizes of cofacial bisporphyrins

Meso substitution opposite to the spacer provides a convenient approach for tuning the pocket sizes of pillared cofacial bisporphyrins. The synthesis and coordination chemistry of xanthene and dibenzofuran anchored platforms structurally modified with 2,6-dimethoxyaryl groups are described. Comparative structural analysis of xanthene derivatives confirms the ability of the trans-aryl groups to adjust the vertical dimension of the cofacial cleft: 7 (C(97)H(106)Cl(4)N(8)O(5)), monoclinic, space group P2(1)/c, a = 28.8353(12) A, b = 17.1139(7) A, c = 17.5978(7) A, beta = 98.826(1) degrees, Z = 4; 8 (C(101)H(123)Cl(2)N(8)O(11.5)Zn(2)), monoclinic, space group P2(1)/n, a = 14.5517(6) A, b = 22.9226(10) A, c = 28.5155(13) A, beta = 90.312(14) degrees, Z = 4; 12 (C(99)H(102)Cl(14)N(8)O(5)Mn(2)), monoclinic, space group P2/c, a = 19.5891(3) A, b = 15.0741(2) A, c = 33.2019(6) A, beta = 91.947(10) degrees, Z = 4. The convenience and versatility of this synthetic method offers intriguing opportunities to specifically tailor the binding pockets of cofacial bisporphyrins for the study of small-molecule activation within a proton-coupled electron transfer framework.     

Long Directional Interactions (LDIs) in Oligomeric Cofacial Silicon Phthalocyanines and Other Oligomeric and Polymeric Cofacial Phthalocyanines

Crystal structures have been determined for the three-member set of cofacial silicon phthalocyanines, ((n-C(6)H(13))(3)SiO)[SiPcO](1-3)(Si(n-C(6)H(13))(3)). The staggering angles between adjacent rings in the dimer and trimer of this set are ～16°. The interactions leading to these angles have been investigated by the atoms-in-molecules (AIM) and reduced-density-gradient (RDG) methods. The results show that long directional interactions (LDIs) are responsible for these angles. A survey of the staggering angles in various cofacial phthalocyanines described in the literature has revealed the existence of significant LDIs in a number of them. It is apparent that in many cases the ability of LDIs to dominate the forces giving rise to the staggering angles observed in cofacial phthalocyanines depends on their inter-ring separations.     

Synthesis and Crystal Structures of Cofacial Bisoctaethylchlorins as Structural Models for the Special Pair*

Covalently linked bistetrapyrrole systems can be used to mimic structurally the arrangement of the two special pair bacteriochlorophylls in the bacterial photosynthetic reaction center. Such models require a cofacial arrangement of the two tetrapyrrole units with some overlap of the It systems and intramolecular stabilization by aggregation of the two macrocycles. Using the McMurry re-action for the coupling of metallo formyloctaethylchlorins, covalently linked cofacial bischlorins were prepared and characterized by spectroscopy and single crystal X-ray crystallography. Coupling of NI(II)5-formyloctaethylchlorin yielded a (Z)-ethene-bridged bischlorin with cofacial orientation of the subunits stabilized by intramolecular π-π interactions that structurally resembles the situation found for the special pair in bacterial reaction centers. In addition, two different atropisomers of the respective (E)-ethene-bridged bischlorin were formed. Use of Cu(II)5-formyloctaethylchlorin yielded similar products, however a crystal structure showed a (Z)-ethene-bridged bischlorin without intramolecular stabilization via aggregation effects. The bischlorins are easily oxidized to mixed chlorin-porphyrin species.     

A Mechanochromic Luminescent Dye Exhibiting On/Off Switching by Crystalline–Amorphous Transitions

A mechanochromic luminescent dye based on a simple aminomaleimide skeleton was readily synthesized in a one‐pot process. It exhibited an on/off mechanochromic luminescent switching property dependent on external stimuli, unlike a traditional mechanochromic color change. The green emission was turned on by grinding in a mortar and turned off by heating or treatment with dichloromethane. In the crystalline state, two molecules were stacked by cofacial π–π interactions, which caused concentration self‐quenching. The crystalline‐to‐amorphous transition induced by grinding removed cofacial π–π stacking, which led to intensive emission. Crystallizing processes recovered the cofacial π–π stacking, resulting in elimination of the emission. Theoretical calculations and X‐ray diffraction analyses revealed that the dye molecule was distorted in the crystalline state; thus even a mechanical stimulus caused the crystalline‐to‐amorphous transition.     

Mechanism of four-electron reduction of dioxygen to water by ferrocene derivatives in the presence of perchloric acid in benzonitrile, catalyzed by cofacial dicobalt porphyrins

The selective two-electron reduction of dioxygen occurs in the case of a monocobalt porphyrin [Co(OEP)], whereas the selective four-electron reduction of dioxygen occurs in the case of a cofacial dicobalt porphyrin [Co(2)(DPX)]. The other cofacial dicobalt porphyrins [Co(2)(DPA), Co(2)(DPB), and Co(2)(DPD)] also catalyze the two-electron reduction of dioxygen, but the four-electron reduction is not as efficient as in the case of Co(2)(DPX). The micro-superoxo species of cofacial dicobalt porphyrins were produced by the reactions of cofacial dicobalt(II) porphyrins with dioxygen in the presence of a bulky base and the subsequent one-electron oxidation of the resulting micro-peroxo species by iodine. The superhyperfine structure due to two equivalent cobalt nuclei was observed at room temperature in the ESR spectra of the micro-superoxo species. The superhyperfine coupling constant of the micro-superoxo species of Co(2)(DPX) is the largest among those of cofacial dicobalt porphyrins. This indicates that the efficient catalysis by Co(2)(DPX) for the four-electron reduction of dioxygen by Fe(C(5)H(4)Me)(2) results from the strong binding of the reduced oxygen with Co(2)(DPX) which has a subtle distance between two cobalt nuclei for the oxygen binding. Mechanisms of the catalytic two-electron and four-electron reduction of dioxygen by ferrocene derivatives will be discussed on the basis of detailed kinetics studies on the overall catalytic reactions as well as on each redox reaction in the catalytic cycle. The turnover-determining step in the Co(OEP)-catalyzed two-electron reduction of dioxygen is an electron transfer from ferrocene derivatives to Co(OEP)(+), whereas the turnover-determining step in the Co(2)(DPX)-catalyzed four-electron reduction of dioxygen changes from the electron transfer to the O-O bond cleavage of the peroxo species of Co(2)(DPX), depending on the electron donor ability of ferrocene derivatives.     

Structural and binding features of cofacial bis-porphyrins with calixarene spacers: pac-man porphyrins that can chew

Based on the efficient combination of calixarene spacers and acetylenic porphyrin derivatives, a new generation of cofacial bis-porphyrins has been synthesized. The first crystal structure of a cofacial bis-porphyrin-calixarene conjugate is reported. Their unique architectural features, analogous to those of pac-man-type bis-porphyrins, allow these calixarene-porphyrin conjugates to adapt their shape to the size of bidentate guests, such as diazabicyclo[2.2.2]octane (dabco) and 1,4-pyrazine. The predefined, cofacial arrangement of the porphyrin moieties observed in the solid state and in solution results in extremely high affinities (in the range of 10(9) M(-1)) for these guests. The 1,3-alternate calixarene conformations afford "open-mouth" pac-man structures whose ability to bite on nitrogen bidentates depends on their functionalization. A cone conformer provides a much more flexible structure that exhibits the highest affinity for dabco and pyrazine.     

The use of squaric acid as a scaffold for cofacial phenyl rings

[structure: see text] To examine the possibility of using squaric acid as a scaffold for organizing phenyl rings in a cofacial orientation, we undertook an investigation of the conformational preferences of secondary and tertiary N-phenylsquaramides. In secondary squaramides, the extended ZZ conformation is preferred, while in the N-methyl derivative, the folded EE conformation with cofacial phenyl rings is preferred. This conformational switch is likely driven by a combination of steric and electronic factors.     

Pt(II)- and Pt(IV)-bridged cofacial diporphyrins via carbon-transition metal sigma-bonds

A directly Pt(IV)-bridged cofacial diporphyrin has been synthesized by the cyclometalation reaction of beta-pyridylporphyrin with a Pt(IV) salt. Upon treatment with methylhydrazine, the Pt(IV) bridge is reduced to the Pt(II) center, resulting in a Pt(II)-bridged cofacial dimer with a helicity inversion of the complex as well as change in electronic communication through the metal bridge.     

Xanthene-bridged cofacial bisporphyrins

The synthesis and characterization of cofacial bisporphyrins juxtaposed by xanthene-bridged pillars are presented. The one-pot preparation of the xanthene dialdehyde avoids the lengthy bridge synthesis accompanying other cofacial porphyrin systems, thus allowing for the facile preparation of homobimetallic zinc (10), copper (11), and nickel (12) complexes. The cofacial orientation of the two porphyrin macrocycles was confirmed by X-ray crystallography. Structural data are provided for bisporphyrins 10-12: 10 (C79H82N8OZn2), triclinic, space group P1, a = 11.2671(2) A, b = 14.9809(2) A, c = 20.4852(2) A, alpha = 101.6680(10) degrees, beta = 100.8890(10) degrees, gamma = 101.8060(10) degrees, Z = 2; 11 (C79H82N8OCu2), triclinic, space group P1, a = 11.21410(10) A, b = 14.9539(5) A, c = 20.6915(7) A, alpha = 101.810(2) degrees, beta = 101.044(2) degrees, gamma = 101.722(2) degrees, Z = 2; 12 (C79H82N8ONi2), monoclinic, space group C2/c, a = 24.1671(4) A, b = 10.669 A, c = 50.5080(9) A, beta = 99.553(2) degrees, Z = 8. Exciton interactions between the porphyrin rings are apparent in electronic spectra, consistent with the cofacial superstructure. The combination of structural and spectroscopic data provides a basis for the design of additional metal derivatives for the activation of dioxygen and other small molecules.     

Transient absorption studies of the Pacman effect in spring-loaded diiron(III) mu-oxo bisporphyrins

Picosecond transient absorption spectroscopy of diiron(III) mu-oxo bisporphyrins appended to xanthene, (DPX)Fe2O and (DPXM)Fe2O, and dibenzofuran (DPD)Fe2O have been investigated in order to decipher the effect of a spring-loaded cleft on their photophysics and attendant oxidation photocatalysis. The tension of the cofacial pocket is systematically tuned with the bridge span and meso-substitution opposite to the bridge; the distances of the relaxed cofacial pockets and clamped Fe-O-Fe pockets are known from X-ray crystallography (Deltad(M-M)(relaxed-clamped)=4.271 A (DPD), 2.424 A (DPXM), 0.208 A (DPX)). The photophysical and chemical properties of these cofacial platforms are compared to the unbridged diiron(III) mu-oxo analogue, (Etio)2Fe2O. Photon absorption by the diiron(III) mu-oxo chromophore prompts Fe-O-Fe photocleavage to release the spring and present a PFeIVO/PFeII pair (P=porphyrin subunit); net photooxidation is observed when oxygen atom transfer to substrate occurs before the spring can reclamp to form the mu-oxo species. The inherent lifetimes of the PFeIVO/PFeII pairs for the four compounds are surprisingly similar (tau[(DPD)Fe2O]=1.36(3) ns, tau[(DPX)Fe2O]=1.26(5) ns, tau[(DPXM)Fe2O]=1.27(9) ns, and tau[(Etio)2Fe2O]=0.97(3) ns), considering the structural differences arising from tensely clamped (DPD and DPXM), relaxed (DPX), and unbridged (Etio) cofacial architectures. However, the rates of net oxygen atom transfer for (DPD)Fe2O and (Etio)2Fe2O are found to be 4 orders of magnitude greater than that of (DPX)Fe2O and 2 orders of magnitude greater than that of (DPXM)Fe2O. These results show that the spring action of the cleft, known as the Pacman effect, does little to impede reclamping to form the mu-oxo species but rather is manifest to opening the cofacial cleft to allow substrate access to the photogenerated oxidant. Consistent with this finding, photooxidation efficiencies decrease as the steric demand of substrates increase.     

Transannular pi-pi interactions in janusenes and in related rigid systems with cofacial aromatic rings; gauging aromaticity in the hydrocarbons and in model carbocations; a DFT study

The utility of NICS (nuclear independent chemical shift) as a probe for detecting/sensing variation in aromaticity due to transannular pi-pi interactions in janusene , a [3.3]orthocyclophane having two cofacial benzene rings within van der Waals distance, its tetrafluoro- and octafluoro-derivatives and , and in tropiliojanusene was studied by DFT at the B3LYP/6-31G(d) level. The related hydrocarbons and with a buried double-bond and their carbocations were also included in this study. Whereas NICS(0) and NICS(1) are rather insensitive to transannular interactions, computed NICS(1)(zz), values are larger and more negative for both pi-decks in the interannular space and this is consistent with increased transannular pi-pi interactions in the cofacial rings, previously shown in these systems via spectroscopic studies (UV and NMR), and by electrophilic chemistry. Transannular effects in , , and were also probed by examining the forms of HOMO-LUMOs. Attempts to measure donor-accepter interactions between electron rich/electron poor cofacial decks via NICS (1)(zz) through substituent effects proved unsuccessful, resulting in only very small changes. Protonation of the double-bond buried in between the two pi-decks in and results in internally pi-stabilized carbocations that exhibit more negative NICS(1) and NICS(1)(zz) values in the interannular space. GIAO NMR data were computed for the neutral hydrocarbons and their derived carbocations, as a guiding tool for planned experimental studies.     

Phthalocyanine-centred and naphthalocyanine-centred aryl ether dendrimers with oligo(ethyleneoxy) surface groups

The synthesis of the 1st, 2nd and 3rd generation phthalocyanine-centred and naphthalocyanine-centred poly(aryl ether) dendrimers possessing oligo(ethyleneoxy) surface groups is described. These materials are soluble in polar protic solvents. For both types of macrocycle, the tendency of the non-polar phthalocyanine core towards intermolecular cofacial aggregation is not reduced by peripheral dendritic substitution. However, the prohibition of cofacial aggregation can be achieved by placing the dendritic substituents at the axial sites of the silicon-containing macrocycle. A single crystal X-ray diffraction analysis of one of these compounds beautifully illustrates this concept.     

Aerobic catalytic photooxidation of olefins by an electron-deficient Pacman bisiron(III) mu-oxo porphyrin

The synthesis and oxygen atom transfer (OAT) photoreactivity of a diiron(III) mu-oxo meso-tripentafluorophenyl bisporphyrin appended to a dibenzofuran spacer are presented. Reaction of 4,6-diformyldibenzofuran under standard Lindsey conditions furnishes the parent cofacial porphyrin architecture in a single step. These cofacial porphyrins photocatalyze the oxidation of sulfides and olefins using visible light and molecular oxygen as the terminal oxidant. High turnover numbers reflect the enhanced stability of the electron-deficient diiron(III) mu-oxo bisporphyrin core appended to a dibenzofuran spacer under aerobic conditions.     

Intramolecular excimer formation for covalently linked boron dipyrromethene dyes

Photophysical properties have been recorded for a small series of covalently linked, symmetrical dimers formed around boron dipyrromethene (Bodipy) dyes. Within the series, a control dimer is unable to adopt a cofacial arrangement because of steric factors, while a second dimer possesses sufficient internal flexibility to form the cofacial geometry but with little overlap of the Bodipy units. The other three members of the series take up a cofacial arrangement with varying bite angles between the planes of the two Bodipy units. Fluorescence quantum yields and excited-state lifetimes indicate differing extents of electronic interaction between the two Bodipy head-groups, but only the compound with the smallest bite angle exhibits excimer emission in solution under ambient conditions. Time-resolved fluorescence studies show dual-exponential decay kinetics in each case, while temperature-dependent emission studies reveal reversible coupling between monomer and lower-energy excimer states. The latter is weakly fluorescent, at best, and is seen clearly only for dimers having small bite angles. The application of high pressure to dilute solutions of these dimers promotes excimer formation in certain cases and leads to loss of monomer-like fluorescence. Under high pressure, excimer emission is more evident, and the overall results can be discussed in terms of subtle structural rearrangements that favor excimer formation.     

Double-pillared cobalt Pacman complexes: synthesis, structures and oxygen reduction catalysis

The syntheses and structures of binuclear cobalt complexes of a double-pillared cofacial Schiff-base pyrrole macrocycle (L) were determined and their activity as catalysts for the oxygen reduction reaction evaluated. The new binuclear cobalt complex, [Co(2)(L)], 1 was formed in good yield using a salt-elimination method and was characterised as adopting a cofacial structure in solution by NMR spectroscopy and as its THF and pyridine solvates in the solid state by X-ray crystallography. Using a variety of spectroscopic techniques, this complex was found to react reversibly with dioxygen to form a new paramagnetic complex. Furthermore, the new aqua-hydroxy double salt [Co(2)(μ-H(3)O(2))(py)(2)(L)][BF(4)] 2 was characterised by X-ray crystallography. In acidified benzonitrile solution, 1 behaves as a catalyst for the selective four-electron reduction of dioxygen to water and showed a large improvement in efficacy compared to its o-phenylene Schiff-base analogues.     

The Pacman effect: a supramolecular strategy for controlling the excited-state dynamics of pillared cofacial bisporphyrins

The molecular recognition properties of dizinc(II) bisporphyrin anchored by dibenzofuran (DPD), Zn2(DPD) (1), were evaluated as a strategy for utilizing the Pacman effect to control the excited-state properties of cofacial bisporphyrin motifs. Crystallographic studies establish that DPD furnishes a cofacial system with vertical flexibility and horizontal preorganization. The structure determination of a substrate-bound DPD species, Zn2(DPD)(2-aminopyrimidine) (2), completes a set of structurally homologous zinc(II) porphyrin host and host-guest complexes, which offer a direct structural comparison for the Pacman effect upon substrate complexation. Binding studies reveal that pyrimidine encapsulation by the DPD framework is accompanied by a markedly reduced entropic penalty (approximately 60 J mol(-1)K(-1)) with respect to traditional face-to-face bisporphyrin systems, giving rise to a smaller conformational energy cost upon substrate binding. Transient absorption spectroscopy reveals that substrate encapsulation within the DPD cleft dramatically affects excited-state dynamics of cofacial bisporphyrins. The emission lifetime of host-guest complex 2 increases by more than an order of magnitude compared to free host 1. In the absence of the guest, the excited-state dynamics are governed by torsional motion of the porphyrin rings about the aryl ring of the DPD pillar. Host-guest binding attenuates this conformational flexibility, thereby removing efficient nonradiative decay pathways. Taken together, these findings support the exceptional ability of the DPD system to structurally accommodate reaction intermediates during catalytic turnover and provide a novel supramolecular approach toward developing a reaction chemistry derived directly from the excited states of Pacman constructs.     

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.