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

Spin-Flip via Subtle Electronic Perturbation in Axially Ligated Diiron(III) Porphyrin Dimer

Spin state switching in the metal center is a crucial phenomenon in many enzymatic reactions in biology. The spin state alteration, a critical step in cytochrome P450 catalysis, is driven most likely through a weak perturbation upon substrate binding in the enzyme, which is still not well clarified. In the current work, the spin state transition of iron(III) from high to intermediate via an admixed state is observed upon a subtle electronic perturbation to the sulphonate moieties coordinated axially to a diiron(III)porphyrin dimer. While electron-donating substituents stabilize the high-spin state of iron(III), strongly electron-withdrawing groups stabilize an intermediate-spin state, whereas the moderate electron-withdrawing nature of axial ligands resulted in an admixed state. Confirmation of the molecular structures and their spin states have been made utilizing single-crystal X-ray structure analysis, Mössbauer, magnetic, EPR, and 1H NMR spectroscopic investigations. The position of the signals of the porphyrin macrocycle in the paramagnetic 1H NMR is found to be very characteristic of the spin state of the iron center in solution. The Curie plot for the pure high-spin complexes shows the signals' temperature dependency in line with the Curie law. Conversely, the pure intermediate-spin state of iron exhibits an anti-Curie temperature dependence, whereas the admixed-spin state of iron displays significant curvature of the lines in the Curie plot. An extensive DFT analysis displays a linear dependence between the energy difference between dx 2 - y 2 ${{_{x{^{2}}- y{^{2}}}}}$ and dz 2 ${{_{z{^{2}}}}}$ orbital versus Fe-Npor distance for the complexes reported here. Furthermore, a strong linear correlation between the Fe-O distance and the spin density over the oxygen atom, as well as the Fe-Npor distance for the complexes, has been observed. Thus, a slight electronic perturbation at the axial ligand of the diheme resulted in a large change in the electronic structures with a spin-flip. This is at par with the metalloenzymes, which employ minute perturbations around the periphery of the active sites, leading to spin state transitions.

Coordination of ε-Caprolactone to a Cationic Niobium(V) Alkoxide Complex: Fundamental Insight into Ring-Opening Polymerization via Coordination-Insertion

We report three niobium-based initiators for the catalytic ring-opening polymerization (ROP) of ε-caprolactone, exhibiting good activity and molecular weight control. In particular, we have prepared on the gram-scale and fully characterized a monometallic cationic alkoxo-Nb(V) ε-caprolactone adduct representing, to the best of our knowledge, an unprecedented example of a metal complex with an intact lactone monomer and a functional ROP-initiating group simultaneously coordinated at the metal center. At 80 °C, all three systems initiate the immortal solution-state ROP of ε-caprolactone via a coordination-insertion mechanism, which has been confirmed through experimental studies, and is supported by computational data. Natural bond orbital calculations further indicate that polymerization may necessitate isomerization about the metal center between the alkoxide chain and the coordinated monomer. The observations made in this work are expected to inform mechanistic understanding both of amine tris(phenolate)-supported metal alkoxide ROP initiators, including various highly stereoselective systems for the polymerization of lactides and of coordination-insertion-type ROP protocols more broadly.

Evidence for a High-Valent Iron-Fluoride That Mediates Oxidative C(sp3)-H Fluorination

[Fe^II(NCCH₃)(NTB)](OTf)₂ (NTB = tris(2-benzimidazoylmethyl)amine, OTf = trifluoromethanesulfonate) was reacted with difluoro(phenyl)-λ³-iodane (PhIF₂) in the presence of a variety of saturated hydrocarbons, resulting in the oxidative fluorination of the hydrocarbons in moderate-to-good yields. Kinetic and product analysis point towards a hydrogen atom transfer oxidation prior to fluorine radical rebound to form the fluorinated product. The combined evidence supports the formation of a formally Fe^IV(F)₂ oxidant that performs hydrogen atom transfer followed by the formation of a dimeric μ-F-(Fe^III)₂ product that is a plausible fluorine atom transfer rebound reagent. This approach mimics the heme paradigm for hydrocarbon hydroxylation, opening up avenues for oxidative hydrocarbon halogenation.

Rapid Iron(III)-Fluoride-Mediated Hydrogen Atom Transfer

We anticipate high-valent metal-fluoride species will be highly effective hydrogen atom transfer (HAT) oxidants because of the magnitude of the H-F bond (in the product) that drives HAT oxidation. We prepared a dimeric FeIII (F)-F-FeIII (F) complex (1) by reacting [FeII (NCCH3 )2 (TPA)](ClO4 )2 (TPA=tris(2-pyridylmethyl)amine) with difluoro(phenyl)-λ3 -iodane (difluoroiodobenzene). 1 was a sluggish oxidant, however, it was readily activated by reaction with Lewis or Brønsted acids to yield a monomeric [FeIII (TPA)(F)(X)]+ complex (2) where X=F/OTf. 1 and 2 were characterized using NMR, EPR, UV/Vis, and FT-IR spectroscopies and mass spectrometry. 2 was a remarkably reactive FeIII reagent for oxidative C-H activation, demonstrating reaction rates for hydrocarbon HAT comparable to the most reactive FeIII and FeIV oxidants.

Halogen Transfer to Carbon Radicals by High-Valent Iron Chloride and Iron Fluoride Corroles

High-valent iron halide corroles were examined to determine their reactivity with carbon radicals and their ability to undergo radical rebound-like processes. Beginning with Fe(Cl)(ttppc) (1) (ttppc = 5,10,15-tris(2,4,6-triphenylphenyl)corrolato^3-), the new iron corroles Fe(OTf)(ttppc) (2), Fe(OTf)(ttppc)(AgOTf) (3), and Fe(F)(ttppc) (4) were synthesized. Complexes 3 and 4 are the first iron triflate and iron fluoride corroles to be structurally characterized by single crystal X-ray diffraction. The structure of 3 reveals an Ag^I-pyrrole (η²-π) interaction. The Fe(Cl)(ttppc) and Fe(F)(ttppc) complexes undergo halogen transfer to triarylmethyl radicals, and kinetic analysis of the reaction between (p-OMe-C₆H₄)₃C• and 1 gave k = 1.34(3) × 10³ M^-1 s^-1 at 23 °C and 2.2(2) M^-1 s^-1 at -60 °C, ΔH^⧧ = +9.8(3) kcal mol^-1, and ΔS^⧧ = -14(1) cal mol^-1 K^-1 through an Eyring analysis. Complex 4 is significantly more reactive, giving k = 1.16(6) × 10⁵ M^-1 s^-1 at 23 °C. The data point to a concerted mechanism and show the trend X = F^- > Cl^- > OH^- for Fe(X)(ttppc). This study provides mechanistic insights into halogen rebound for an iron porphyrinoid complex.

Self-assembly of a trigonal bipyramidal architecture with stabilisation of iron in three spin states

Self-assembly and characterisation of a supramolecular trigonal bipyramidal iron cage containing an [Fe^III(μ₂-F)₆(Fe^II)₃]³⁺ star motif at its core is reported. The complex can be formed in a one step reaction using an heterotopic ligand that supports site-specific incorporation of iron in three distinct electronic configurations: low-spin Fe^II, high-spin Fe^II and high-spin Fe^III, with iron(II) tetrafluoroborate as the source of the bridging fluorides. Formation of a μ₂-F bridged mixed-valence Fe^II-Fe^III star is unprecedented. The peripheral high-spin Fe^II centres of the mixed-valence tetranuclear star incorporated in the iron cage are highly anisotropic and engage in F-mediated antiferromagnetic exchange with the central Fe^III ion.

Heme-Heme Interactions in Diheme Cytochromes: Effect of Mixed-Axial Ligation on the Electronic Structure and Electrochemical Properties

Diheme cytochromes, the simplest members in the multiheme family, play substantial biochemical roles in enzymatic catalysis as well as in electron transfer. A series of diiron(III) porphyrin dimers have been synthesized as active site analogues of diheme cytochromes. The complexes contain six-coordinated iron(III) having thiophenol and imidazole at the fifth and sixth coordination sites, respectively. The iron centers in the complexes have been found to be in a low-spin state, as confirmed through solid-state Mössbauer and electron paramagnetic resonance (EPR) spectroscopic investigations. Mössbauer quadrupole splitting of complexes having mixed ligands is substantially larger than that observed when both axial ligands are the same. Rhombic types of EPR spectra with narrow separation between g_x, g_y, and g_z clearly distinguish heme thiolate coordination compared to bis(imidazole)-ligated low-spin heme centers. The redox potential in diheme cytochromes has been found to be tuned by interheme interactions along with the nature of axial ligands. The effect of mixed-axial ligation within the diiron(III) porphyrin dimers is demonstrated by a positive shift in the Fe(III)/Fe(II) redox couple upon thiophenolate coordination compared to their bis(imidazole) analogues. The pK_a of the imidazole also decides the extent of the shift for the Fe(III)/Fe(II) couple, while the potential of the mixed-ligated diiron(III) porphyrin dimer is more positive compared to their monomeric analogue. A variation of around 1.1 V for the Fe(III)/Fe(II) redox potential in the diiron(III) porphyrin dimer can be achieved with the combined effect of axial ligation and a metal spin state, while such a large variation in the redox potential, compared to their monomeric analogues, is attributed to the heme-heme interactions observed in dihemes. Moreover, theoretical calculations also support the experimental shifts in the redox potential values.

Through Bridge Spin Coupling in Homo- and Heterobimetallic Porphyrin Dimers upon Stepwise Oxidations: A Spectroscopic and Theoretical Investigation

We have described copper(II)-iron(III) and copper(II)-manganese(III) heterobimetallic porphyrin dimers and compared them with the corresponding homobimetallic analogs. UV-visible spectra are very distinct in the heterometallic species while electrochemical studies demonstrate that these species, as compared to the homobimetallic analog, are much easier to oxidize. Combined Mössbauer, EPR, NMR, magnetic and UV-visible spectroscopic studies show that upon 2e-oxidation of the heterobimetallic complexes only ring-centered oxidation occurs. The energy differences between HOMO and LUMO are linearly dependent with the low-energy NIR band obtained for the 2e-oxidized complexes. Also, strong electronic communication between two porphyrin rings through the bridge facilitates coupling between various unpaired spins present while the coupling model depends on the nature of metal ions used. While unpaired spins of Fe(III) and the porphyrin π-cation radical are strongly antiferromagnetically coupled, such coupling is rather weak between Mn(III) and a porphyrin π-cation radical. Moreover, the coupling between two π-cation radicals are much stronger in the 2e-oxidized complexes of dimanganese(III) and copper(II)-manganese(III) porphyrin dimers as compared to their diiron(III) and copper(II)-iron(III) analogs. Furthermore, coupling between the unpaired spins of a π-cation radical and copper(II) is much stronger in the 2e-oxidized complex of copper(II)-iron(III) porphyrin dimer as compared to its copper(II)-manganese(III) analog. The Mulliken spin density distributions in 2e-oxidized homo- and heterobimetallic complexes show symmetric and asymmetric spread between the two macrocycles, respectively. In both the 2e-oxidized heterobimetallic complexes, the Cu(II) porphyrin center acts as a charge donor while Fe(III)/Mn(III) porphyrin center act as a charge acceptor. The experimental observations are also strongly supported by DFT calculations.

Induction, control, and rationalization of supramolecular chirogenesis using metalloporphyrin tweezers: a structure-function correlation

Supramolecular chirogenesis is one of the most rudimentary topics in the interdisciplinary sciences and essentially deals with various natural processes and innovative modern technologies. A comprehensive and rigorous understanding of such phenomenon is necessary to have a clear insight into the fundamental mechanisms and the various controlling factors, which would eventually lead to a range of practical applications of chiral supramolecular science. Metalloporphyrin tweezers have been extensively employed for such chirogenic processes due to their exciting physicochemical and tunable spectral properties, large stabilities, easily available synthetic protocols, and excellent abilities to form molecular assemblies. During the last few decades, various metalloporphyrin tweezers have been developed and considerably utilized by several research groups for assigning the absolute configuration to a variety of chiral diamines, conjugates of primary and secondary amines, amino alcohols, secondary alcohols, α-chiral carboxylic acids, etc. Our group has been at the forefront in trying to establish the structure-property correlation in this important area of interdisciplinary research. A brief account of our systematic investigation for understanding the underpinning mechanism of chirality induction and control at the molecular level over the last few years is presented in this Perspective article. The comprehensive understanding of such mechanistic details will be helpful in understanding various natural processes and designing modern technologies for various chirogenic functions in the fields of molecular sensors, nanotechnology, and supramolecular chemistry.

Fluoro-bridged dimanganese(III) porphyrin dimer: Effect of intermacrocyclic interactions in modulating metal spin state

The synthesis, structure, and spectroscopic properties of a remarkably deformed dimanganese(III)-[Formula: see text]-fluoro porphyrin dimer in connection to its diiron(III) [Formula: see text]-fluoro and [Formula: see text]-hydroxo counterparts is reported. It is the most recent addition to the continued effort in understanding the effect of intermacrocyclic interaction on the molecular structure and properties of di-heme proteins. The dimanganese(III)-[Formula: see text]-fluoro porphyrin dimer, similar to its [Formula: see text]-hydroxo analogs, stabilizes two equivalent high-spin ([Formula: see text]= 2) manganese(III) centers. This is in contrast to the diiron(III) analogs where the [Formula: see text]-hydroxo complex stabilizes two inequivalent iron(III) centers with two different spin states of iron in the same molecule while the [Formula: see text]-fluoro complex stabilizes two equivalent metal centers with intermediate spin ([Formula: see text]= 3/2) state. The present results further establish the extreme interdependence of the energetically accessible spin states of the metal and the extent of porphyrin core deformation. It is very likely that nature utilizes the same powerful tool to modulate large change in the electronic structure and properties of the heme center needed for its function.

Equatorial ligand plane perturbations lead to a spin-state change in an iron(iii) porphyrin dimer

A complete reversal of the spin state of iron(iii) is observed upon a small change to the diporphyrin bridge from ethane to ethene by keeping all other factors intact.

Stepwise oxidations of a nickel(ii)–iron(iii) heterobimetallic porphyrin dimer: structure, spectroscopic and theoretical investigation

A novel nickel(ii)–iron(iii) heterobimetallic ethene-bridged porphyrin dimer has been synthesized which upon two-electron oxidation produces a nickel(ii)–iron(iii) dication diradical complex where radicals undergo extensive conjugation through the bridge for all possible interactions.

Multiheme proteins: effect of heme–heme interactions

This Frontier illustrates a brief personal account on the effect of heme–heme interactions in dihemes which thereby discloses some of the evolutionary design principles involved in multiheme proteins for their diverse structures and functions.

Synthesis and structural, photophysical, electrochemical redox and axial ligation properties of highly electron deficient perchlorometalloporphyrins and selective CN− sensing by Co(ii) complexes

Synthesis and characterization of MTPP(NO₂)Cl₇ have been reported. Zn^II porphyrins exhibited higher Lewis acidity as compared to perbromoporphyrins. Co^II porphyrins were utilized for the selective rapid visual detection of cyanide ions.

Target-Guided Synthesis and Antiplasmodial Evaluation of a New Fluorinated 3-Alkylpyridine Marine Alkaloid Analog

The need to develop new alternatives for antimalarial treatment is urgent. Herein, we report the synthesis and antimalarial evaluation of a small library of synthetic 3-alkylpyridine marine alkaloid (3-APA) analogs. First, the compounds were evaluated in vitro against Plasmodium falciparum. The most active compound 5c was selected for optimization of its antimalarial properties. An in silico approach was used based on pure ab initio electronic structure prediction, and the results indicated that a substitution of the hydroxyl group by a fluorine atom could favor a more stable complex with heme at a molecular ratio of 2:1 (heme/3-APA halogenated). A new fluorinated 3-APA analog was synthesized (compound 7), and its antimalarial activity was re-evaluated. Compound 7 exhibited optimized antimalarial properties (P. falciparum IC₅₀ = 2.5 μM), low genotoxicity, capacity to form a more stable heme/3-APA complex at a molecular ratio of 2:1, and conformity to RO5. The new compound, therefore, has great potential as a new lead antimalarial agent.

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.