Saddle-distorted Porphyrin Research Articles

Dioxygen Activation by Gold(I)-Distorted Porphyrin Dinuclear Complexes.

Interactions between gold-based materials and dioxygen (O2) have motivated researchers to understand reaction mechanisms for O2 activation by homo- and heterogeneous gold catalysts. In this work, gold(I) porphyrin dinuclear complexes were synthesized with a saddle-distorted porphyrin ligand. The gold(I) porphyrin complexes showed unprecedented O2 activation in the presence of protic solvents to form gold(III) tetradentate porphyrin complexes. Mechanistic insights into the O2 activation by the gold(I) center were elucidated by spectroscopic measurements and theoretical calculations, revealing that dissociation of halides on the gold(I) center by alcohol solvents and hydrogen bonding of an N-H proton in the distorted porphyrin with dioxygen played important roles in establishing the unique reactivities of gold(I) complexes.

Catalytic and Photocatalytic Functionality of Diprotonated Porphyrins

Development of strategy in energy conversion and storage is indispensable for a sustainable society. So far, redox-active transition metal complexes have been used as efficient catalysts and photosensitizers in energy-converting reactions such as hydrogen evolution through thermal and photoinduced electron transfer (ET).We have been working on development of redox and photochemical functionality of diprotonated saddle-distorted porphyrins based on ET reactions. Herein we would like to highlight recent advance in our study of catalytic dioxygen reduction and hydrogen evolution using diprotonated forms of dodecaphenylporphyrin (H2DPP) derivatives as organocatalysts and photosensitizers.A diprotonated form of a water-soluble H2DPP derivative can act as a photosensitizer (PS) in photocatalytic oxidation of organic substrates in water using Na2S2O8 as an oxidant under visible-light irradiation.1 On the contrary, diprotonated H2DPP has been used as a PS in photocatalytic H2 evolution using platinum nanoparticles as catalysts under near-infrared (710 nm) irradiation in the quantum yield of 17%.2 Diprotonated H2DPP (H4DPP2+) can serve as an organo-photocatalyst in highly selective and efficient O2 reduction to afford H2O2 under photoirradiation at 480 nm; the quantum yield of H2O2 generation was 12% and turnover frequency was 500 h–1.3 Interconversion of O2 and H2O2 has been achieved using an N21,N23-dimethylated H2DPP derivative.4 The porphyrin is reduced by H2O2 to form the corresponding isophlorin derivative as a two-electron-reduced product, which is in turn oxidized by O2 to afford the porphyrin and H2O2. We also developed efficient O2 reduction using N21,N22- and N21,N23-dimethylated H2DPP derivatives as catalysts; in the catalysis, catalytic mechanisms and performance depend on the structures of catalysts used in terms of interaction between O2 and the corresponding isophlorins.5 References T. Ishizuka et al., Green Chem. 2018, 20, 1975.H. Kotani et al., ACS Appl. Energy Mater. 2020, 3, 3193.E. Aoki et al., Chem. Commun. 2019, 55, 4925.W. Suzuki et al., J. Am. Chem. Soc. 2019, 141, 5987.W. Suzuki et al., Chem. Eur. J. 2020, 26, 10480.

Ethyne Functionalized Meso-Phenothiazinyl-Phenyl-Porphyrins: Synthesis and Optical Properties of Free Base Versus Protonated Species.

Synthesis, structural characterization and photophysical properties for a series of new trans-A2B2- and A3B-type ethynyl functionalized meso-phenothiazinyl-phenyl porphyrin derivatives are described. The new compounds displayed the characteristic porphyrin absorption spectra slightly modified by weak auxochromic effects of the substituents and fluorescence emission in the range of 651–659 nm with 11–25% quantum yields. The changes recorded in the UV-vis absorption spectra in the presence of trifluoroacetic acid (TFA) are consistent with the protonation of the two internal nitrogen atoms of the free-base porphyrin (19 nm bathochromic shift of the strong Soret band and one long wave absorption maxima situated in the range of 665–695 nm). Protonation of the phenothiazine substituents required increased amounts of TFA and produced a distinct hypsochromic shift of the long wave absorption maxima. The density functional theory (DFT) calculations of a porphyrin dication pointed out a saddle-distorted porphyrin ring as the ground-state geometry.

Application of Saddle-Distorted Porphyrins to Dioxygen Reduction

Efficient and selective dioxygen (O2) reduction is an important process in biological dioxygen activation and also in fuel cells to gain a chemical potential. So far, O2 activation has been achieved using transition-metal complexes as observed in that by metalloenzymes such as cytochrome P-450. However, organic molecules have scarcely used for O2 reduction probably due to instability during redox reactions under oxic conditions. We report herein the utilization of diprotonated and saddle-distorted porphyrins for photocatalytic O2 reduction and unprecedented interconversion between O2 and H2O2.The photocatalytic O2 reduction was performed using diprotonated dodecaphenylporphyrin (H4DPP2+) as a photocatalyst, 10-methyl-9,10-dihydroacridine (AcrH2) as a two-electron donor in CH3CN in the presence of CH2ClCOOH as a proton source.1 Under photoirradiation at 480 nm, H2O2 was produced in turnover number of > 3000 and turnover frequency of 500 h–1 in the yield of 99% and the quantum yield of 12%. The efficiency is the highest among porphyrin photocatalysts reported so far. This is due to a low energy level of the triplet excited state of H4DPP2+ to avoid energy transfer to 3O2 for the formation of 1O2.On the other hand, an N22, N23-dimethylated H2DPP derivative (Me2Por) reacted with excess H2O2 to afford the corresponding isophlorin derivative (Me2IPh) and O2.2 The isophlorin derivative conversely reacted with O2 to form the dimethylated porphyrin and H2O2. This reversibility has been gained due to a small Gibbs free energy difference between the two states, Me2Por + H2O2 and Me2IPh + O2, and minimizing activation energy of the reaction. The hydrogen-bond formation, which is facilitated by the saddle distortion of the porphyrin to fix the direction of protons or lone pairs of the pyrroles, between Me2Por and H2O2 and between Me2IPh and O2 plays indispensable roles in each process to promote proton-coupled electron transfer. The interconversion can be repeated by converting the atmosphere between O2 and Ar at least 11 times without losing activity due to the robustness of the dimethylated porphyrin.

Efficient Near-Infrared Light-Driven Hydrogen Evolution Catalyzed by a Saddle-Distorted Porphyrin as a Photocatalyst

Development of near-infrared (NIR) light-induced hydrogen (H2) evolution system is indispensable to construct a sustainable society to maximize the utilization of solar energy. Here, we report NIR ...

Conformational Re‐engineering of Porphyrins as Receptors with Switchable N−H⋅⋅⋅X‐Type Binding Modes

The selectivity and functional variability of porphyrin cofactors are typically based on substrate binding of metalloporphyrins wherein the pyrrole nitrogen units only serve to chelate the metal ions. Yet, using the porphyrin inner core system for other functions is possible through conformational engineering. As a first step towards porphyrin “enzyme‐like” active centers, a structural and spectroscopic study of substrate binding to the inner core porphyrin system shows that a highly saddle‐distorted porphyrin with peripheral amino receptor groups (1, 2,3,7,8,12,13,17,18‐octaethyl‐5,10,15,20‐tetrakis(2‐aminophenyl)porphyrin) coordinates analytes in a switchable manner dependent on the acidity of the solution. The supramolecular ensemble exhibits exceptionally high affinity to and selectivity for the pyrophosphate anion (2.26±0.021)×109 m −1. 1H NMR spectroscopic studies provided insight into the likely mode of binding and the characterization of atropisomers, all four of which were also studied by X‐ray crystallography.

A Diprotonated Porphyrin as an Electron Mediator in Photoinduced Electron Transfer in Hydrogen-Bonded Supramolecular Assemblies

We have successfully constructed hydrogen-bonded supramolecular assemblies based on a diprotonated saddle-distorted porphyrin (H4DPP2+) and redox-active molecules bearing a carboxylate group, such ...

Dioxygen/Hydrogen Peroxide Interconversion Using Redox Couples of Saddle-Distorted Porphyrins and Isophlorins.

Interconversion between dioxygen (O2) and hydrogen peroxide (H2O2) has attracted much interest because of the growing importance of H2O2 as an energy source. There are many reports on O2 conversions to H2O2; however, no example has been reported on O2/H2O2 interconversion. Herein, we describe successful achievement of a reversible O2/H2O2 conversion based on an N21, N23-dimethylated saddle-distorted porphyrin and the corresponding two-electron-reduced porphyrin (isophlorin) for the first time. The isophlorin could react with O2 to afford the corresponding porphyrin and H2O2; conversely, the porphyrin also reacted with excess H2O2 to reproduce the corresponding isophlorin and O2. The isophlorin-O2/porphyrin-H2O2 interconversion was repeatedly proceeded by alternate bubbling of Ar or O2, although no reversible conversion was observed in the case of an N21, N22-dimethylated porphyrin as a structural isomer. Such a drastic change of the reversibility was derived from the directions of inner N H protons in hydrogen-bond formation of the isophlorin core with O2 as well as those of the lone pairs of the inner nitrogen atoms of the porphyrin core to form hydrogen bonds with H2O2. The intriguing isophlorin-O2/porphyrin-H2O2 interconversion was accomplished by introducing methyl groups at the inner nitrogen atoms to minimize the difference of the Gibbs free energy between isophlorin-O2/porphyrin-H2O2 states and the Gibbs activation energy of the interconversion. On the basis of the kinetic and thermodynamic analysis on the isophlorin-O2/porphyrin-H2O2 interconversion using 1H NMR and UV-vis spectroscopies and DFT calculations, we propose the formation of a two-point hydrogen-bonding adduct between the N21, N23-dimethylated porphyrin and H2O2 as an intermediate.

Efficient photocatalytic proton-coupled electron-transfer reduction of O2 using a saddle-distorted porphyrin as a photocatalyst.

Photocatalytic O2 reduction reactions proceeded to produce H2O2 using a diprotonated saddle-distorted dodecaphenylporphyrin as a photocatalyst. The quantum yield (12%), the turnover number (3000 for 6 h), and the turnover frequency (500 h-1) are achieved in photocatalytic systems based on free-base porphyrins for the first time. The photocatalytic reaction mechanism has been revealed by ns-laser flash photolysis and kinetic analysis.

Optical properties of β-brominated meso-tetraphenylporphyrins: Comparative experimental and computational studies

The UV-vis and fluorescence spectra of a series of [Formula: see text]-brominated derivatives of meso-tetraphenylporphyrin (H[Formula: see text]TPPBr[Formula: see text]; [Formula: see text] 0, 2, 4, 5, 6, 7, 8) were used to investigate the steroelectronic effects of bromine atoms on the optical properties of H[Formula: see text]TPP. The wavelength and intensity of the absorption and emission bands were found to be influenced by the bromine atoms. The degree of configuration interaction between the S[Formula: see text] and S[Formula: see text] excited states and fluorescence quantum yield and their dependence on the bromine atoms were also studied. Computational studies based on density functional theory and time-dependent density functional approach (DFT and TD-DFT) support and provide an explanation for the spectral changes associated with the bromination of H[Formula: see text]TPP. Furthermore, the solvent effect on the spectral properties of higher degrees of [Formula: see text]-bromination was attributed to the formation of relatively strong hydrogen bonds between their saddle-distorted porphyrin cores and the solvent molecules.

Formation of Hydrogen-Bonded Supramolecular Hetero-Triads with a Diprotonated Porphyrin

Supramolecular assemblies based on intermolecular hydrogen bonding (H bonding) have been intensively investigated to develop functional materials. Along this line, we have developed supramolecular assemblies using protonated and saddle-distorted porphyrins as H-bonding electron acceptors to gain unique photofunctionality of the assemblies.1 In order to expand the possibility of structures and components of the assemblies, it is required to develop a strategy to construct H-bonded supramolecular assemblies involving different components. Herein, we present selective formation of unusual H-bonded hetero-triads. We examined formation of hydrogen-bonded supramolecular assemblies using dodecaphenylporphyrin (H­2DPP) with various acids including carboxylic acids, sulfonic acids, and carboxylic acids with positively charged moieties. It was revealed that the stability of homo-assembly (H4DPPX2; X = conjugate base of an acid) can be controlled by the strength of the H bonding depending on the acidity of an acid used to protonate H2DPP and the electrostatic repulsion between the H4DPP2+ cation and the positively charged moiety of an acid used.2 By employing a carboxylic acid with a pyridinium moiety, the PF6 - salt of N-benzyl-4-carboxyphenylpyridinium (BnPy+-COOH•PF6 -) to react with H4DPPCl2 in chloroform, we succeeded in the formation and crystal structure determination of a supramolecular hetero-triad, [H4DPP2+(Cl–)(BnPy+-PhCOO–)](PF6 –). S. Fukuzumi, T. Honda, T. Kojima, Coord. Chem. Rev. 2012, 256, 2488-2502.W. Suzuki, H. Kotani, T. Ishizuka, Y. Shiota, K. Yoshizawa, T. Kojima, Chem. Commun. 2017, 53, 6359.

Supramolecular Assemblies of a Water-Soluble Diprotonated Porphyrin and Heteropolyoxometalates

Photocatalytic oxidation of organic substrates has been of great interest in terms of utilization of photon energy to produce useful materials from abundant feedstock. As a photocatalyst that can mediate electron transfer (ET) from an oxidation catalyst to an oxidant upon photoexcitation, porphyrins can be useful in light of the strong visible-light absorption and clear redox behavior. Although porphyrins have been used as photocatalysts for reduction of organic substrates, photocatalytic oxidation of organic substrates has been seldom reported by using porphyrins as photocatalysts. So far, we have established the utility of diprotonated porphyrins as electron acceptors in photoinduced ET.1 On the basis of the concept, we prepared a water-soluble and saddle-distorted porphyrin, which can act as an electron acceptor in photoinduced ET upon diprotonation. The porphyrin (H2P4+) possesses four N-methylpyridinium-4-yl substituents at the para positions of the four meso-phenyl groups of dodecaphenylporphyrin. We observed proton-coupled electron transfer processes of the diprotonated form of H2P4+, H4P6+, in water to establish the Pourbaix diagram of H4P6+. We also determined the pKa value of the first protonation/deprotonation equilibrium to form the monoprotonated H3P5+ (pKa1 = 4.04) and that of the second one to afford H2P4+ (pKa2 = 7.83). The highly positively charged H4P6+ was revealed to form supramolecular assemblies with negatively charged heteropolyoxometalates (POMs) in the ratio of H4P6+ : POM = 1 : 2 in water at pH 3 as indicated by Job's plots. Based on the redox properties of H4P4+ and the formation of supramolecular assemblies, we examined intra-supramolecular photoinduced ET from heteropolyoxometalates as electron donors to H4P6+ as an electron acceptor in water at pH 3 using femto-second laser flash photolysis upon photoexcitation at 393 nm. First-order rate constants of ET were analyzed on the basis of the Marcus theory of ET to determine the reorganization energy of ET to be 0.74 eV and the electronic coupling matrix element (V) to be 43 cm-1. We also examined its application to photocatalytic oxidation of organic substrates in water, using H4P6+ as a photocatalyst, Na2S2O6 as an oxidant, and a Ru-substituted POM (RuPOM) as an oxidation catalyst, under the visible light irradiation. Preliminary results on the photocatalytic oxidation of benzyl alcohol were 560 turnovers for 4h; blank tests clearly suggest that H4P6+, RuPOM, the oxidant, and photoirradiation are all required to perform the photocatalytic reaction.

(Invited) Stabilization of a Saddle-Distorted Porphyrin Monoacid

Porphyrins have been known to undergo diprotonation to form porphyrin diacids (diprotonated porphyrins) under acidic conditions without forming porphyrin monoacids (monoprotonated porphyrins). So far, chemistry of monoprotonated porphyrins has not yet to be sufficiently pursued due to the difficulty of the formation. It should be required to establish a solid preparation procedure of monoprotonated porphyrins in solution and in solid for the development of supramolecular assemblies based on monoprotonated porphyrins. Previously, saddle-distorted dodecaphenylporphyrin (H2DPP) allowed us to observe stepwise protonation in benzonitrile and isolation of monoprotonated H3DPP+ with use of 2-anthrathene sulfonic acid (2-AnSO3H) as a hydrogen-bonded supramolecular assembly, [H3DPP](2-AnSO3)(CH3OH), which was obtained by crystallization from dichloromethane/CH3OH [1]. In order to establish the requirements for the formation of monoprotonated porphyrin, we examined reactions of H2DPP with carboxylic acids, including trifluoroacetic acid (TFA), in acetone in the presence of methanol. In the course of titration of H2DPP using TFA in a mixture of acetone/methanol (3%), we could observe two-step spectral change showing different isosbestic points, indicating step-wise protonation of H2DPP. Upon addition of 1 eq of TFA to the solution of H2DPP in acetone-d6/methanol-d4 (3%), the 1H NMR spectrum of the mixture allowed us to observe signals assigned to the o-protons of the meso-phenyl groups of H3DPP+ at 8.1 ppm, those of H2DPP and H4DPP2+ at 7.7 and 8.3 ppm, respectively. We found that the amount of methanol added had a great impact on the yield of the monoacid relative to the initial concentration of H2DPP: The increase of the portion of methanol up to 10% gained the yield, reaching to 90%. These results indicate that the formation of monoprotonated porphyrin selectively occurs in the presence of a protic solvent such as methanol even using TFA, which affords only diprotonated species in acetone. The importance of methanol for the formation of H3DPP+ was demonstrated by the crystal structure of [H3DPP](CF3COO)(MeOH). As depicted in Figure 1, the crystal structure involves hydrogen bonding of both a methanol molecule and CF3COO– with H3DPP+. Thus, together with the impact of the amount of methanol on the formation yield of H3DPP+, the hydrogen bonding between MeOH and H3DPP+ should be important for the stabilization of the monoprotonated porphyrin. Addition of other protic solvents (3%) in acetone, such as water and 2,2,2-trifluoroethanol, also allowed us to observe the H3DPP+ formation with TFA; however, we just observed the formation of H4DPP2+ in the presence of acetonitrile (5%) as a polar aprotic solvent in place of methanol with comparable permittivity. Thus, we conclude that the hydrogen bond formation of a protic solvent molecule with H3DPP+ plays a crucial role to stabilize the monoacid. We also revealed that the acidity (pK a) of an acid used for the protonation is an important factor for the selective formation of H3DPP+: The smaller pK a value of an acid, the higher formation yield of H3DPP+. In the case of p-toluenesulfonic acid as a strong acid, the formation yield of the monoacid was 100%, indicating the complete selectivity in the monoacid formation. This result is consistent with the stepwise protonation of H2DPP in benzonitrile even in the absence of methanol [1]. The higher selectivity for a strong acid should stem from weak hydrogen bonding of the corresponding conjugate base with protonated species of H2DPP, allowing competitive interaction of the protic solvent molecule with H3DPP+. We will also report the thermodynamics of the equilibrium of the protonation of H2DPP in acetone-methanol mixed solvents. Figure 1. A side view of the crystal structure of [H3DPP](CF3COO)(CH3OH). Reference Honda, T.; Kojima, T.; Fukuzumi, S. Chem. Commun. 2009, 4994-4996. Figure 1

(Invited) Photoinduced Electron-Transfer Dynamics of a Monoprotonated Saddle-Distorted Porphyrin

Protonation of a free-base porphyrin (H2P) has been widely investigated because of the crucial influences on characteristic redox and photophysical properties of H2P. Protonation of H2P under strongly acidic conditions resulted in formation of diprotonated porphyrin (H4P2+) without any detectable monoprotonated porphyrin (H3P+) as an intermediate. Thus, there are only a few reports on the formation of H3P+ because of the higher basicity of saddle distorted H3P+ rather than that of planer H2P. On the other hand, dodecaphenylporphyrin (H2DPP),1 shows a saddle-distorted structure due to the steric repulsion among the phenyl rings. Previously, we reported a crystal structure of a monoprotonated dodecaphenylporphyrin (H3DPP+) by using 2-anthracene sulfonic acid (2-AN-SO3H) as a proton source.2 Herein, we report selective formation of H3DPP+ in an acetone solution with a polar protic solvent such as methanol. Hydrogen bonding between H3DPP+ and methanol has been revealed to be important for stabilization of the monoprotonated porphyrin. In this presentation, we will focus on photoinduced electron-transfer (ET) reactions from ferrocene derivatives to the triplet excited state of H3DPP+ (3[H3DPP+]*), which have been scrutinized by femto- and nano-second laser flash photolysis for the first time. Protonated products of saddle-distorted H2DPP with trifluoroacetic acid (TFA) in acetone were revealed to be controlled by addition of methanol as a protic polar solvent. Addition of 1 eq of TFA to an acetone solution containing H2DPP in the presence of 3% methanol (v/v) resulted in selective formation of a monoprotonated form (H3DPP+) as confirmed by spectroscopic measurements and X-ray crystallography. The crucial role of methanol for the selective H3DPP+ formation was interpreted as stabilization of H3DPP+ by hydrogen bonding with methanol; because a methanol molecule was found to form two-point hydrogen bonding with an NH proton and the non-protonated nitrogen atom of H3DPP+in the crystal. The reduction potential (E red, V vs Fc/Fc+) of H3DPP+(CF3COO–) was determined to be –1.05 V in acetone/methanol (3%) containing 0.1 M [(n-butyl)4N]BPh4 as an electrolyte at 298 K by electrochemical measurements. The E red value of H3DPP+(CF3COO–) was lower than that of H4DPP2+(CF3COO–)2 (–0.83 V) and higher than that of H2DPP (–1.41 V). Then, absorption and emission spectra of H3DPP+(CF3COO–) were measured to determine the energy levels of the singlet and triplet excited states of H3DPP+(CF3COO–) in acetone/methanol (3%). As a result, the energy levels of 1[H3DPP+]* and 3[H3DPP+]* were determined to be 1.66 eV and 1.44 eV, respectively. Femto- and nano-second laser flash photolysis was applied to elucidate the photodynamics of intermolecular photoinduced ET reactions from ferrocene derivatives as one-electron donors to H3DPP+ as an electron acceptor. Femto-second laser flash photolysis of H3DPP+ in acetone/methanol (3%) with laser excitation at 500 nm revealed a transient absorption spectrum with a peak at 575 nm assigned to the singlet excited state of H3DPP+ (1[H3DPP+]*). The decay time profile at 575 nm exhibited a mono-exponential decay with the lifetime of 200 ps, which is ascribed to intersystem crossing from 1[H3DPP+]* to the triplet excited state of H3DPP+ (3[H3DPP+]*). The decay time profiles of 3[H3DPP+]* were measured by nano-second laser flash photolysis in the absence and presence of ferrocene derivatives as one-electron donors. As a result, we obtained the lifetime of 3[H3DPP+]* (τ = 42 μs) and second-order rate constants (k et) of the ET reactions, respectively. The k et values were analyzed in light of the Marcus theory of electron transfer (Figure 1). The reorganization energy (λ) of electron transfer was determined to be 1.85 eV, which is slightly larger than that of H4DPP2+ in acetonitrile (1.69 eV),3 due to the larger structural change after electron transfer than that of H4DPP2+. In conclusion, we have succeeded in selective formation of H3DPP+ in the presence of methanol. We have also revealed the photodynamics of H3DPP+ in photoinduced ET by analyzing intermolecular ET from ferrocene derivatives as electron donor to 3[H3DPP+]* as an electron acceptor to determine the λ value of intermolecular ET for H3DPP+to be 1.85 eV. This is the first report on a reorganization energy of a monoprotonated porphyrin in ET. Figure 1. Driving-force dependence of log k et for ET from electron donors to 3[H3DPP+]* and a fitting curve based on the Marcus theory of ET (black). Gray: H4DPP2+(Cl–)2 in acetonitrile.3 1: Fc, 2: Me2Fc, 3: Me5Fc, 4: Me8Fc, 5: Me10Fc. References Medforth, C. J.; Senge, M. O.; Smith, K. M.; Sparks, L. D.; Shelnutt, J. A. J. Am. Chem. Soc. 1992, 114, 9859-9869.Honda, T.; Kojima, T.; Fukuzumi, S. Chem. Commun. 2009, 4994-4996.Nakanishi, T.; Ohkubo, K.; Kojima, T.; Fukuzumi, S. J. Am. Chem. Soc. 2009, 131, 577–584. Figure 1

ChemInform Abstract: Molecular Assemblies Based on Strong Axial Coordination in Metal Complexes of Saddle‐Distorted Dodecaphenylporphyrins

AbstractReview: 48 refs.

A samarium hybrid with porphyrin and amino acid as mixed ligands: Electrochemical and photophysical properties

A samarium hybrid with porphyrin and amino acid as mixed ligands {[Sm(His)(H2O)][Sm(H3O)3](H2TPPS)2} • 5H2O (1) (His = histidine; H6TPPS = tetra(4-sulfonatophenyl)porphyrin) has been synthesized via a hydrothermal reaction and characterized by single-crystal X-ray diffraction. Compound 1is crystallized in the space group [Formula: see text] of the triclinic system. Compound 1is characteristic of a three-dimensional (3-D) open framework with the samarium ions in two types of coordinating environments and a saddle-distorted nonplanar porphyrin macrocycle. Compound 1 exhibits fluorescence in the red region. The fluorescence lifetime of 1 is 15.65 ns and the emission quantum yield is 4.1%. The CV/DPV, UV-vis and FT-IR are also reported in detail.

Formation of Hydrogen-Bonded Supramolecular Assemblies Based on Functionalized Saddle-Distorted Porphyrins

Supramolecular assemblies consisting of porphyrins have been intensively investigated in the light of promising photo-functionality including photoinduced electron transfer, photo-conducting materials, dye-sensitized solar cells, and guest inclusion toward the emergence of novel functions on the basis of unique spatial host-guest arrangements and interactions. Along this line, we have developed a series of supramolecular assemblies using the diprotonated form of saddle-distorted dodecaphenylporphyrin (H2DPP) through intermolecular π-π interaction, such as Porphyrin Nanochannels, which include electron-donating guest molecules.1 In this presentation, we present formation and characterization of supramolecular assemblies based on H2DPP derivatives having hydrogen-bonding functional groups and their metal complexes. Introduction of carboxyl groups or hydroxyl groups to the meso-phenyl groups of H2DPP as hydrogen-bonding sites allows us to construct robust and well-organized supramolecular assemblies. A diprotonated tetracarboxyl-DPP derivative (1, Fig. 1a), for example, formed two-dimensional hydrogen-bonding networks (Fig. 1b) to form a porphyrin nanochannel structure, which were reminiscent of that formed by [H4DPP2+]Cl2 and was persistent up to 150°C, in contrast to that of [H4DPP2+]Cl2.2 To expand the size of the nanochannels, we also introduced 4-carboxylphenyl groups at the 4-positions of the meso-phenyl groups of H4DPP2+. A Zn(II) complex of H2DPP derivatives having hydroxyl groups at the 4-positions of the meso-phenyl groups (2) was prepared to form a dimer bridged by 4,4’-bypyridine.3 The dimer was revealed to form intermolecular hydrogen-bonding network among the hydroxyl groups in the crystal. In addition to the crystal structures, we will also mention adsorption of gaseous molecules such as N2 and CO2 with porous supramolecular assemblies of functionalized H2DPP derivatives in the solid state. References (a) R. Harada et al., Chem. Commun. 2005, 716. (b) T. Kojima et al., Chem.–Eur. J. 2007, 13, 8714. (c) T. Nakanishi et al., Chem. Mater. 2008, 21, 7492. (d) S. Fukuzumi et al., Coord. Chem. Rev. 2012, 256, 2488.T. Ishizuka et al. Chem. Commun. 2012, 48, 6481.T. Ishizuka et al. Dalton Trans. 2013, 42, 16073. Figure 1

Molecular assemblies based on strong axial coordination in metal complexes of saddle-distorted dodecaphenylporphyrins

In this mini-review, we have highlighted our works on metal complexes having saddle-distorted dodecaphenylporphyrin (DPP) and its derivative as ligands in the light of enhancement of the Lewis acidity of a metal center coordinated by the porphyrin. The important point through this mini-review is ill-overlap of the out-of-plane lone pairs of pyrrole nitrogen atoms with σ-orbitals of the metal center bound to the saddle-distorted porphyrin core. The enhanced Lewis acidity of the central metal ions enabled us to construct stable molecular complexes through axial coordination using metal–DPP (M(DPP)) moieties ( M = Mo V or Sn IV ) and molecular or ionic entities with Lewis-basic coordination sites, including Keggin-type polyoxometallates (POM), which are known to have weak Lewis basicity and thus hard to coordinate to metal ions. A discrete 1:2 complex with a Ru -substituted POM performs catalytic substrate oxidation reactions in organic solvents. A 1:1 complex between Sn IV ( DPP ) and a Keggin-type POM exhibited photoinduced electron transfer, in which the Sn IV ( DPP ) moiety acts as an electron donor and the POM as an electron acceptor. Besides POM, other electron acceptors, including μ3-oxo trinuclear Ru III clusters and anthraquinone, having carboxyl groups as a linker unit also formed stable complexes with DPP-metal complexes as axial ligands to perform photoinduced electron transfer. Successful photoreactions of the M(DPP)-acceptor complexes are mainly enabled by the enhanced Lewis acidity of the DPP-metal complexes for the stabilization of the assemblies and also by lowering the oxidation potential of the porphyrin ligand to gain larger driving force of electron transfer to form an electron-transfer state with avoiding intersystem crossing. The stability and photochemical behavior are in sharp contrast to those for metal complexes with planar porphyrins as ligands.

Control of the spatial arrangements of supramolecular networks based on saddle-distorted porphyrins by intermolecular hydrogen bonding

Supramolecular integration of a saddle-distorted zinc(II) porphyrin complex, which has hydroxyl groups at the para-position of the four meso-aryl groups, has been demonstrated on the basis of hydrogen bonding among the peripheral hydroxyl groups. The hydrogen-bonding patterns were controlled by the recrystallization solvents and additives, and particularly, addition of a bifunctional ligand such as 4,4'-bipyridine (bpy). The coordination of bpy to form dinuclear Zn(II)-porphyrin complexes causes a conformational difference: the dimeric complex with four hydroxyl groups is in an eclipsed form, however, a derivative without hydroxyl groups is in a staggered form due to the presence or absence of the intermolecular hydrogen bonding. In addition, the dimerization by the bpy coordination resulted in the expansion of the intermolecular space formed in the porphyrin networks, suggesting the potential to be applied for inclusion of guest molecules.

Switching Orientation of Two Axial Imidazole Ligands between Parallel and Perpendicular in Low-Spin Fe(III) and Fe(II) Nonplanar Porphyrinates

We have reported here the synthesis, structure, and properties of low-spin bis-imidazole-coordinated Fe(III) and Fe(II) complexes of 5,10,15,20-tetrakis(pentafluorophenyl)-2,3,7,8,12,13,17,18-octachloroporphyrin, [Fe(III)(TFPPCl(8))(L)(2)]ClO(4) and Fe(II)(TFPPCl(8))(L)(2) (L = 1-methylimidazole, 4-methylimidazole, imidazole). The X-ray structure of Fe(II)(TFPPCl(8))(1-MeIm)(2) is reported here, which demonstrated the near-perpendicular axial ligand orientation (dihedral angle between two 1-methylimidazoles is 80.7°) for Fe(II) porphyrins in a highly saddle-distorted macrocyclic environment. Oxidation of Fe(II)(TFPPCl(8))(L)(2) using thianthrenium perchlorate produces [Fe(III)(TFPPCl(8))(L)(2)]ClO(4), which was also isolated in the solid state and characterized spectroscopically. The complex gives rhombic EPR spectra in both solid and solution phases at 77 K and thus represents a rare example of nearly parallel axial ligand orientations for the unhindered imidazoles in a saddle-distorted porphyrin macrocycle. Geometry optimization using DFT also converged to the parallel axial alignment when 1-methylimidazole was used as the axial ligand (the dihedral angle between two axial ligands is 8.6°). The potential energy surface (PES) scan results also show that the relatively parallel axial orientations are energetically preferred for Fe(III), while perpendicular orientations are preferred for the Fe(II) complexes reported here. Bulk oxidation of Fe(II)(TFPPCl(8))(L)(2) in dichloromethane at a constant potential under nitrogen converts it to [Fe(III)(TFPPCl(8))(L)(2)]ClO(4), which gives identical EPR spectra at 77 K and which upon reduction regenerates Fe(II)(TFPPCl(8))(L)(2) again. Thus, we have demonstrated here very rare examples of Fe porphyrins in which the relative axial imidazole orientations switch between parallel and perpendicular just upon changing the oxidation states of iron from +3 to +2, respectively, in a nonplanar porphyrinic environment. These observations could be immensely important for understanding the possible effects of axial histidine orientations on similar macrocyclic deformations observed in various heme proteins.