Electronic Structures of Five-Coordinate Iron(III) Porphyrin Complexes with Highly Ruffled Porphyrin Ring

Nonplanar porphyrins: synthesis, properties, and unique functionalities

Porphyrins are variously substituted tetrapyrrolic macrocycles, with wide-ranging biological and chemical applications derived from metal chelation in the core and the 18π aromatic surface. Under suitable conditions, the porphyrin framework can deform significantly from regular planar shape, owing to steric overload on the porphyrin periphery or steric repulsion in the core, among other structure modulation strategies. Adopting this nonplanar porphyrin architecture allows guest molecules to interact directly with an exposed core, with guest-responsive and photoactive electronic states of the porphyrin allowing energy, information, atom and electron transfer within and between these species. This functionality can be incorporated and tuned by decoration of functional groups and electronic modifications, with individual deformation profiles adapted to specific key sensing and catalysis applications. Nonplanar porphyrins are assisting breakthroughs in molecular recognition, organo- and photoredox catalysis; simultaneously bio-inspired and distinctly synthetic, these molecules offer a new dimension in shape-responsive host-guest chemistry. In this review, we have summarized the synthetic methods and design aspects of nonplanar porphyrin formation, key properties, structure and functionality of the nonplanar aromatic framework, and the scope and utility of this emerging class towards outstanding scientific, industrial and environmental issues.

Linkage Isomers of 4-Methylimidazolate Mn(II) Porphyrinates: Hindered or Unhindered?

Three different manganese(II) porphyrins have been exploited to react with 4-methylimidazolate (4-MeIm^-), and the five-coordinate products are characterized by ultraviolet-visible, single-crystal X-ray, and electronic paramagnetic resonance spectroscopies. Interestingly, 4-MeIm^- is found to bond to the metal center through either of the two N atoms (N1 or N3), which yielded two linkage isomers with either an unhindered or a hindered ligand conformation, respectively. Investigations revealed it is the large metal out-of-plane displacements (Δ₂₄ and Δ₄ ≥ 0.59 Å) that have rendered the equivalence of two isomers with a small energy difference (5.2-8.3 kJ/mol). The nonbonded intra- and intermolecular interactions thus become crucial factors in the balance of linkage isomerization. All of the products in both solution and solid states show the same characteristic resonances of high-spin Mn(II) (S = ⁵/₂) with g_⊥ ≈ 5.9 and g_∥ ≈ 2.0 at 4 K, consistent with the weak effects of the axial ligand on core conformation and metal electronic configurations. Zero-field splitting parameters obtained through simulations are also reported.

Luminescent ruffled iridium(iii) porphyrin complexes containing N-heterocyclic carbene ligands: structures, spectroscopies and potent antitumor activities under dark and light irradiation conditions

A panel of iridium(iii) porphyrin complexes containing axial N-heterocyclic carbene (NHC) ligand(s) were synthesized and characterized. X-ray crystal structures of the bis-NHC complexes [Ir^III(ttp)(IMe)₂]⁺ (2a), [Ir^III(oep)(BIMe)₂]⁺ (2d), [Ir^III(oep)(I ⁱ Pr)₂]⁺ (2e) and [Ir^III(F₂₀tpp)(IMe)₂]⁺ (2f) display ruffled porphyrin rings with mesocarbon displacements of 0.483-0.594 Å and long Ir-C_NHC bonds of 2.100-2.152 Å. Variable-temperature ¹H NMR analysis of 2a reveals that the macrocycle porphyrin ring inversion takes place in solution with an activation barrier of 40 ± 1 kJ mol^-1. The UV-vis absorption spectra of Ir^III(por)-NHC complexes display split Soret bands. TD-DFT calculations and resonance Raman experiments show that the higher-energy Soret band is derived from the ¹MLCT dπ(Ir) → π*(por) transition. The near-infrared phosphorescence of Ir^III(por)-NHC complexes from the porphyrin-based ³(π, π*) state features broad emission bands at 701-754 nm with low emission quantum yields and short lifetimes (Φ _em < 0.01; τ < 4 μs). [Ir^III(por)(IMe)₂]⁺ complexes (por = ttp and oep) are efficient photosensitizers for ¹O₂ generation (Φ _so = 0.64 and 0.88) and are catalytically active in the light-induced aerobic oxidation of secondary amines and arylboronic acid. The bis-NHC complexes exhibit potent dark cytotoxicity towards a panel of cancer cells with IC₅₀ values at submicromolar levels. The cytotoxicity of these complexes could be further enhanced upon light irradiation with IC₅₀ values as low as nanomolar levels in association with the light-induced generation of reactive oxygen species (ROS). Bioimaging of [Ir^III(oep)(IMe)₂]⁺ (2c) treated cells indicates that this Ir complex mainly targets the endoplasmic reticulum. [Ir^III(oep)(IMe)₂]⁺ catalyzes the photoinduced generation of singlet oxygen and triggers protein oxidation, cell cycle arrest, apoptosis and the inhibition of angiogenesis. It also causes pronounced photoinduced inhibition of tumor growth in a mouse model of human cancer.

Hydration of iron-porphyrins: ab initio quantum mechanical charge field molecular dynamics simulation study

The ab initio quantum mechanical charge field molecular dynamics (QMCF-MD) simulation approach was successfully applied to Fe²⁺-P and Fe³⁺-P in water to evaluate their structural, dynamical and energetic properties. Based on the structural data, it was found that Fe²⁺-P accommodates one water molecule in the first coordination sphere of the Fe²⁺ ion including the four nitrogen atoms of the porphyrin system coordinating with central metal species. On the other hand, two water molecules were coordinated to Fe³⁺-P, thus forming a hexa-coordinated species. Comparison of dynamical properties such as the vibrational power spectrum and ligand mean residence times to other metal-free porphyrin systems demonstrate the ions' influence on the hydration structure, enabling a characterisation of the strong interaction of the ions which greatly reduces the hydrogen bonding potential of the complex. The association of water molecules with the metal ions in both solutes was quantified by computing the free energy of binding obtained via the potential of mean force. This further confirmed the strong association of water to the metal ions which was conversely weak as inferred from the energetic data for the Fe²⁺-P system.

Heterocorrole Conformations: Little Saddling, Much Ruffling

10-Heterocorrole complexes with oxygen, sulfur, and selenium at position 10 of the macrocycle and with the divalent ions of nickel, copper, and palladium were prepared and investigated. The focus was set on the size adaptation and matching mechanisms of cavity size versus ionic radius in corrole-type macrocycles. A full set of single-crystal X-ray analytical data revealed that in all but one case the N₄ binding site of the ring-contracted tetrapyrrole was larger than necessary to bind the metal ion without deformation. In-plane size adaptation through M-N bond-length elongation by 2.5-3.2 % was effective, as well as pronounced out-of-plane ruffling of the macrocycle for those compounds with a more severe size mismatch. Such ruffling had been excluded for corroles previously, but is apparently the most efficient mechanism to adapt to small central ions.

Hydroxo-bridged diiron(iii) and dimanganese(iii) bisporphyrins: modulation of metal spins by counter anions

A brief account has been presented on how the inter-heme interactions in μ-hydroxo diiron(iii) bisporphyrins and counter anions can induce significant change in the structure and properties including the iron spin state without affecting the overall topology.

Remarkable Anion-Dependent Spin-State Switching in Diiron(III) μ-Hydroxo Bisporphyrins: What Role do Counterions Play?

Addition of 2,4,6-trinitrophenol (HTNP) to an ethene-bridged diiron(III) μ-oxo bisporphyrin (1) in CH₂ Cl₂ initially leads to the formation of diiron(III) μ-hydroxo bisporphyrin (2⋅TNP) with a phenolate counterion that, after further addition of HTNP or dissolution in a nonpolar solvent, converts to a diiron(III) complex with axial phenoxide coordination (3⋅(TNP)₂ ). The progress of the reaction from μ-oxo to μ-hydroxo to axially ligated complex has been monitored in solution by using ¹ H NMR spectroscopy because their signals appear in three different and distinct spectral regions. The X-ray structure of 2⋅TNP revealed that the nearly planar TNP counterion fits perfectly within the bisporphyrin cavity to form a strong hydrogen bond with the μ-hydroxo group, which thus stabilizes the two equivalent iron centers. In contrast, such counterions as I₅ , I₃ , BF₄ , SbF₆ , and PF₆ are found to be tightly associated with one of the porphyrin rings and, therefore, stabilize two different spin states of iron in one molecule. A spectroscopic investigation of 2⋅TNP has revealed the presence of two equivalent iron centers with a high-spin state (S=5/2) in the solid state that converts to intermediate spin (S=3/2) in solution. An extensive computational study by using a range of DFT methods was performed on 2⋅TNP and 2⁺ , and clearly supports the experimentally observed spin flip triggered by hydrogen-bonding interactions. The counterion is shown to perturb the spin-state ordering through, for example, hydrogen-bonding interactions, switched positions between counterion and axial ligand, ion-pair interactions, and charge polarization. The present investigation thus provides a clear rationalization of the unusual counterion-specific spin states observed in the μ-hydroxo bisporphyrins that have so far remained the most outstanding issue.

Intercalation of Coordinatively Unsaturated Fe(III) Ion within Interpenetrated Metal-Organic Framework MOF-5

Coordinatively unsaturated Fe(III) metal sites were successfully incorporated into the iconic MOF-5 framework. This new structure, Fe(III) -iMOF-5, is the first example of an interpenetrated MOF linked through intercalated metal ions. Structural characterization was performed with single-crystal and powder XRD, followed by extensive analysis by spectroscopic methods and solid-state NMR, which reveals the paramagnetic ion through its interaction with the framework. EPR and Mössbauer spectroscopy confirmed that the intercalated ions were indeed Fe(III) , whereas DFT calculations were employed to ascertain the unique pentacoordinate architecture around the Fe(III) ion. Interestingly, this is also the first crystallographic evidence of pentacoordinate Zn(II) within the MOF-5 SBU. This new MOF structure displays the potential for metal-site addition as a framework connector, thus creating further opportunity for the innovative development of new MOF materials.

A convenient procedure for the synthesis of fluoro-iron(III) complexes of common synthetic porphyrinates

We report here an improved method for the preparation of fluoro-iron(III) porphyrinate complexes. Treatment of [ Fe 2( P )2(μ- O )] (P = tetraphenylporphyrinate {TPP}, tetra-p-tolylporphyrinate {TTP}, or octaethylporphyrinate {OEP}) or [ Fe ( OH )( OH 2)( TMP )] (TMP = tetramesitylporphyrinate) with the commercially available fluorinating agent, Et 3 N ·3 HF , in dichloromethane affords the desired [ FeF ( P )] complexes in a straightforward fashion and in good yield while avoiding the use of aqueous hydrofluoric acid. All fluoro-iron(III) complexes have been completely characterized by a series of different spectroscopic techniques including cyclic voltammetry. Reaction of a representative complex, [ FeF ( OEP )], with various chloride reagents demonstrates that halide exchange with chloride is facile, but only proceeds at an appreciable rate in the presence of proton sources. Unexpectedly, treatment of [ FeF ( OEP )] with NOBF 4 did not to lead formation of an oxidized species, but rather to formation of the { Fe – NO }6 complex, [ Fe ( NO )( OEP )]( BF 4).

Thermal decomposition of [Co(en)3][Fe(CN)6]∙ 2H2O: Topotactic dehydration process, valence and spin exchange mechanism elucidation

BACKGROUND

The Prussian blue analogues represent well-known and extensively studied group of coordination species which has many remarkable applications due to their ion-exchange, electron transfer or magnetic properties. Among them, Co-Fe Prussian blue analogues have been extensively studied due to the photoinduced magnetization. Surprisingly, their suitability as precursors for solid-state synthesis of magnetic nanoparticles is almost unexplored. In this paper, the mechanism of thermal decomposition of [Co(en)3][Fe(CN)6] ∙∙ 2H2O (1a) is elucidated, including the topotactic dehydration, valence and spins exchange mechanisms suggestion and the formation of a mixture of CoFe2O4-Co3O4 (3:1) as final products of thermal degradation.

RESULTS

The course of thermal decomposition of 1a in air atmosphere up to 600°C was monitored by TG/DSC techniques, (57)Fe Mössbauer and IR spectroscopy. As first, the topotactic dehydration of 1a to the hemihydrate [Co(en)3][Fe(CN)6] ∙∙ 1/2H2O (1b) occurred with preserving the single-crystal character as was confirmed by the X-ray diffraction analysis. The consequent thermal decomposition proceeded in further four stages including intermediates varying in valence and spin states of both transition metal ions in their structures, i.e. [Fe(II)(en)2(μ-NC)Co(III)(CN)4], Fe(III)(NH2CH2CH3)2(μ-NC)2Co(II)(CN)3] and Fe(III)[Co(II)(CN)5], which were suggested mainly from (57)Fe Mössbauer, IR spectral and elemental analyses data. Thermal decomposition was completed at 400°C when superparamagnetic phases of CoFe2O4 and Co3O4 in the molar ratio of 3:1 were formed. During further temperature increase (450 and 600°C), the ongoing crystallization process gave a new ferromagnetic phase attributed to the CoFe2O4-Co3O4 nanocomposite particles. Their formation was confirmed by XRD and TEM analyses. In-field (5 K / 5 T) Mössbauer spectrum revealed canting of Fe(III) spin in almost fully inverse spinel structure of CoFe2O4.

CONCLUSIONS

It has been found that the thermal decomposition of [Co(en)3][Fe(CN)6] ∙∙ 2H2O in air atmosphere is a gradual multiple process accompanied by the formation of intermediates with different composition, stereochemistry, oxidation as well as spin states of both the central transition metals. The decomposition is finished above 400°C and the ongoing heating to 600°C results in the formation of CoFe2O4-Co3O4 nanocomposite particles as the final decomposition product.

Effect of the Ruffled Porphyrin Ring on Electronic Structures: Structure and Characterization of [Fe(TalkylP)(OClO3)] and [Fe(TPrP)(THF)2]ClO4 (alkyl = Ethyl, Et and n-Propyl, Pr)

We report the synthesis of Fe(TalkylP)(OClO3)] (alkyl = ethyl and propyl) and [Fe(TPrP)(THF)2]ClO4, which are characterized by UV-vis, EPR, X-ray crystallography, and solid-state magnetic susceptibilities. The macrocycles of all three complexes are ruffled, all of the structural features for [Fe(TEtP(OClO3)] and [Fe(TPrP)(OClO3)] are characteristic of the nearly pure S = 3/2 state, while the structural parameters for [Fe(TPrP)(THF)2]ClO4 feature a pure intermediate-spin (S = 3/2) state, which are all consistent with EPR and magnetic data. It is clear from these studies that the ruffled conformation plays a significant role in affecting the extent of S = 3/2 character.

Insertion of heme b into the structure of the Cys34-carbamidomethylated human lipocalin α(1)-microglobulin: formation of a [(heme)(2) (α(1)-Microglobulin)](3) complex

α(1)-Microglobulin (α(1)m) is a 26 kDa plasma and tissue protein belonging to the lipocalin protein family. Previous investigations indicate that the protein interacts with heme and suggest that it has a function in heme metabolism. However, detailed characterizations of the α(1)m-heme interactions are lacking. Here, we report for the first time the preparation and analysis of a stable α(1)m-heme complex upon carbamidomethylation of the reactive Cys34 by using recombinantly expressed human α(1)m. Analytical size-exclusion chromatography coupled with a diode-array absorbance spectrophotometry demonstrates that at first an α(1)m-heme monomer is formed. Subsequently, a second heme triggers oligomerization that leads to trimerization. The resulting (α(1)m[heme](2))(3) complex was characterized by resonance Raman and EPR spectroscopy, which support the presence of two ferrihemes, thus indicating an unusual spin-state admixed ground state with S=(3)/(2), (5)/(2).

Kinetic Selectivity in the N-Alkylation of 2-Pyridyl Porphyrins: A Facile Approach to the ααββ Scaffold

meso-Tetrakis(2-pyridyl)-porphyrin (2-PyP) was tetra-N-alkylated with three different alpha-bromoacetamides to generate a series of water-soluble N-alkylpyridinium porphyrins (1-3). The product mixtures showed a marked preference for the formation of the alphaalphabetabeta atropisomer. With alpha-bromo-N-n-butylacetamide, the corresponding alphaalphabetabeta 2-tetrakis (N-n-butylacetamido)-pyridyl porphyrin (2-TnBuPyP, 3) was obtained in 69% isolated yield in a single step. Prolonged heating lead to equilibration of the rotational isomers for the less bulky alkyl groups, indicating that the observed preference is a kinetic effect. The intermediate products for the N-bornyl case were identified by LC/ESI-MS to deduce an explanation for the observed nonstatistical selectivity.

Built-in Axial Base Binding on Phenanthroline-Strapped Zinc(II) and Iron(III) Porphyrins

In addition to the need for functional models of cytochrome c oxidase, structural models are still required for a better understanding of the small reorganizations occurring during the catalytic cycle. An efficient synthetic approach has been designed to prepare several phenanthroline-strapped porphyrins, two of them bearing two pendant imidazoles. These built-in bases are both potentially able to act as axial bases for the metalloporphyrin and as complementary ligands for copper if necessary. Diamagnetic zinc(II) was used to demonstrate that the distal/proximal selectivity demonstrated by exogenic bases binding studies can be extended to the coordination of iron(III). Combination of EPR and paramagnetic 1H NMR shows that the imidazole binding on the zinc species can be further extended to the iron(III) species in dilute conditions.

Ethyne-Bridged (Porphinato)Zinc(II)−(Porphinato)Iron(III) Complexes: Phenomenological Dependence of Excited-State Dynamics upon (Porphinato)Iron Electronic Structure

We report the synthesis, spectroscopy, potentiometric properties, and excited-state dynamical studies of 5-[(10,20-di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]zinc(II)-[5'-[(10',20'- di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]iron(III)-chloride]ethyne (PZn-PFe-Cl), along with a series of related supermolecules ([PZn-PFe-(L)1,2]+ species) that possess a range of metal axial ligation environments (L = pyridine, 4-cyanopyridine, 2,4,6-trimethylpyridine (collidine), and 2,6-dimethylpyridine (2,6-lutidine)). Relevant monomeric [(porphinato)iron-(ligand)1,2]+ ([PFe(L)1,2]+) benchmarks have also been synthesized and fully characterized. Ultrafast pump-probe transient absorption spectroscopic experiments that interrogate the initially prepared electronically excited states of [PFe(L)1,2]+ species bearing nonhindered axial ligands demonstrated subpicosecond-to-picosecond relaxation dynamics to the ground electronic state. Comparative pump-probe transient absorption experiments that interrogate the initially prepared excited states of PZn-PFe-Cl, [PZn-PFe-(py)2]+, [PZn-PFe-(4-CN-py)2]+, [PZn-PFe-(collidine)]+, and [PZn-PFe-(2,6-lutidine)]+ demonstrate that the spectra of all these species are dominated by a broad, intense NIR S1 --> Sn transient absorption manifold. While PZn-PFe-Cl, [PZn-PFe-(py)2]+, and [PZn-PFe-(4-CN-py)2]+ evince subpicosecond and picosecond time-scale relaxation of their respective initially prepared electronically excited states to the ground state, the excited-state dynamics observed for [PZn-PFe-(2,6-lutidine)]+ and [PZn-PFe-(collidine)]+ show fast relaxation to a [PZn+-PFe(II)] charge-separated state having a lifetime of nearly 1 ns. Potentiometric data indicate that while DeltaGCS for [PZn-PFe-(L)1,2]+ species is strongly influenced by the PFe+ ligation state [ligand (DeltaGCS): 4-cyanopyridine (-0.79 eV) < pyridine (-1.04 eV) < collidine (-1.35 eV) < chloride (-1.40 eV); solvent = CH2Cl2], the pump-probe transient absorption dynamical data demonstrate that the nature of the dominant excited-state decay pathway is not correlated with the thermodynamic driving force for photoinduced charge separation, but depends on the ferric ion ligation mode. These data indicate that sterically bulky axial ligands that drive a pentacoordinate PFe center and a weak metal axial ligand interaction serve to sufficiently suppress the normally large magnitude nonradiative decay rate constants characteristic of (porphinato)iron(III) complexes, and thus make electron transfer a competitive excited-state deactivation pathway.

Electronic Structures of Six-Coordinate Ferric Porphyrin Complexes with Weak Axial Ligands: Usefulness of 13C NMR Chemical Shifts

1H NMR, (13)C NMR, and EPR spectra of six-coordinate ferric porphyrin complexes [Fe(Por)L2]ClO4 with different porphyrin structures are presented, where porphyrins (Por) are planar 5,10,15,20-tetraphenylporphyrin (TPP), ruffled 5,10,15,20-tetraisopropylporphyrin (TiPrP), and saddled 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (OETPP), and axial ligands (L) are weak oxygen ligands such as pyridine-N-oxide, substituted pyridine-N-oxide, DMSO, DMF, MeOH, THF, 2-MeTHF, and dioxane. These complexes exhibit the spin states ranging from an essentially pure high-spin (S = 5/2) to an essentially pure intermediate-spin (S = 3/2) state depending on the field strength of the axial ligands and the structure of the porphyrin rings. Reed and Guiset reported that the pyrrole-H chemical shift is a good probe to determine the spin state in the spin admixed S = 5/2,3/2 complexes (Reed, C. A.; Guiset, F. J. Am. Chem. Soc. 1996, 118, 3281-3282). In this paper, we report that the chemical shifts of the alpha- and beta-pyrrole carbons can also be good probes to determine the spin state because they have shown good correlation with those of the pyrrole-H or pyrrole-C(alpha). By putting the observed or assumed pyrrole-H or pyrrole-C(alpha) chemical shifts of the pure high-spin and pure intermediate-spin complexes into the correlation equations, we have estimated the carbon chemical shits of the corresponding complexes. The orbital interactions between iron(III) and porphyrin have been examined on the basis of these chemical shifts, from which we have found that both the d(xy)-a(2u) interaction in the ruffled Fe(T(i)PrP)L2+ and d(xy)-a(1u) interaction in the saddled Fe(OETPP)L2+ are quite weak in the high-spin and probably in the intermediate-spin complexes as well. Close inspection of the correlation lines has suggested that the electron configuration of an essentially pure intermediate-spin Fe(T(i)PrP)L2+ changes from (d(xy), d(yz))3(d(xy))1(d(z)2)1 to (d(xy))2(d(xz), d(yz))2(d(z)2)1 as the axial ligand (L) changes from DMF to MeOH, THF, 2-MeTHF, and then to dioxane. Although the DFT calculation has indicated that the highly saddled intermediate-spin Fe(OETPP)(THF)2+ should adopt (d(xy), d(yz))3(d(xy))1(d(z)2)1 rather than (d(xy))2(d(xz), d(yz))2(d(z)2)1 because of the strong d(xy)-a(1u) interaction (Cheng, R.-J.; Wang, Y.-K.; Chen, P.-Y.; Han, Y.-P.; Chang, C.-C. Chem. Commun. 2005, 1312-1314), our 13C NMR study again suggests that Fe(OETPP)(THF)2+ should be represented as (d(xy))2(d(xz), d(yz))2(d(z)2)1 because of the weak d(xy)-a(1u) interaction. The contribution of the S = 3/2 state in all types of the spin admixed S = 5/2,3/2 six-coordinate complexes has been determined on the basis of the (13)C NMR chemical shifts.