Electronic Coupling and Electrocatalysis in Redox Active Fused Iron Corroles.

Conjugated arrays composed of corrole macrocycles are increasingly more common, but their chemistry still lags behind that of their porphyrin counterparts. Here, we report on the insertion of iron(III) into a ß,ß-fused corrole dimer and on the electronic effects that this redox active metal center has on the already rich coordination chemistry of [H3tpfc] COT, where COT = cyclo-octatetraene and tpfc = tris(pentafluorophenyl)corrole. Synthetic manipulations were performed for the isolation and full characterization of both the 5-coordinate [FeIIItpfc(py)]2COT and 6-coordinate [FeIIItpfc(py)2]2COT, with one and two axial pyridine ligands per metal, respectively. X-Ray crystallography reveals a dome-shaped structure for [FeIIItpfc(py)]2COT and a perfectly planar geometry which (surprisingly at first) is also characterized by shorter Fe-N (corrole) and Fe-N (pyridine) distances. Computational investigations clarify that the structural phenomena are due to a change in the iron(III) spin state from intermediate (S = 3/2) to low (S = 1/2), and that both the 5- and 6-coordinated complexes are enthalpically favored. Yet, in contrast to iron(III) porphyrins, the formation enthalpy for the coordination of the first pyridine to Fe(III) corrole is more negative than that of the second pyridine coordination. Possible interactions between the two corrole subunits and the chelated iron ions were examined by UV-Vis spectroscopy, electrochemical techniques, and density functional theory (DFT). The large differences in the electronic spectra of the dimer relative to the monomer are concluded to be due to a reduced electronic gap, owing to the extensive electron delocalization through the fusing bridge. A cathodic sweep for the dimer discloses two redox processes, separated by 230 mV. The DFT self-consistent charge density for the neutral and cationic states (1- and 2-electron oxidized) reveals that the holes are localized on the macrocycle. A different picture emerges from the reduction process, where both the electrochemistry and the calculated charge density point toward two consecutive electron transfers with similar energetics, indicative of very weak electron communication between the two redox active iron(III) sites. The binuclear complex was determined to be a much better catalyst for the electrochemical hydrogen evolution reaction (HER) than the analogous mononuclear corrole.

The Planar Cyclooctatetraene Bridge in Bis-Metallic Macrocycles: Isolating or Conjugating?

A minor modification of the reported procedure for the synthesis of a corrole dimer that is fused by the cyclooctatetraene (COT) unit, (H3tpfc)2COT, allowed for its isolation in 18% yield. Of the two redox isomers that this interesting macrocycle does form, the current focus is on the reduced form, in which each subunit resembles that of monomeric corroles with a trianionic N4 coordination core. The corresponding bis-gallium(III) complex was prepared as an entry into the potentially rich coordination chemistry of (H3tpfc)2COT. Both X-ray crystallography and DFT calculations disclosed that the COT moiety is essentially planar with very unusual nonalternating C-C bonds. The same holds true for the bis-gallium(III) complexes [(Ga-tpfc)2]COT(py)2 and [(Ga-tpfc)2]COT(py)4, obtained with one and two pyridine molecules coordinated to each metal ion, respectively. The electronic spectra of both the free base and the gallium(III) complexes display an extremely low energy band (λmax at 720-724 nm), which points toward extensive π delocalization through the COT bridge. This aspect was fully addressed by examining the interactions between the two corrole subunits in terms of electrochemistry and DFT calculations of the oxidized and reduced macrocycle. The new near-IR bands that appear upon both oxidation (λmax 1250 nm) and reduction (λmax 1780 nm) serve as additional supporting evidence for this conclusion.

Experimental and Theoretical Investigation of a Series of Novel Dimanganese(III) µ-Hydroxo Bisporphyrins: Magneto-Structural Correlation and Effect of Metal Spin on Porphyrin Core Deformation.

The synthesis, structure, and properties of a new family of five ethane-bridged dimanganese(III) µ-hydroxo bisporphyrins with the same core structure but different counteranions are reported here. Additions of 10% Brønsted acids such as HI, HBF4, HSbF6, HPF6, and HClO4 to a dichloromethane solution of the dichloro dimanganese(III) bisporphyrin produces complexes having a remarkably bent µ-hydroxo group with I3(-), BF4(-), SbF6(-), PF6(-), and ClO4(-) as counteranions, respectively. The X-ray structures of all complexes have been determined, which have revealed the presence of two equivalent high-spin manganese(III) centers with equally distorted porphyrin rings in the complexes, in sharp contrast with the case for the diiron(III) µ-hydroxo bisporphyrin analogues. (1)H NMR spectra have shown highly deshielded meso resonances, unlike the case for the diiron(III) analogues, where the meso resonances are highly shielded. The variable-temperature magnetic data have been subjected to a least-squares fit which provides a moderate antiferromagnetic coupling through the hydroxo bridge between two zero-field split Mn(III) centers with coupling constant (J) values ranging from -29.5 to -38.6 cm(-1). Fairly good correlations are observed for J with Mn-O(H) distances and Mn-O(H)-Mn angles for all the complexes except for that having an I3(-) counteranion. DFT calculations support the stabilization of two equivalent high-spin Mn(III) porphyrin cores in the complexes and have also explored the role of metal spin in controlling porphyrin ring deformation. Unlike diiron(III) µ-hydroxo bisporphyrin complexes, the dimanganese(III) analogues do not have easily accessible spin states of the metal attainable by subtle environmental perturbations and, therefore, can only stabilize the high-spin state with a variety of counteranions.

Hedgehog-shaped {Mo368} cluster: unique electronic/structural properties, surfactant encapsulation and related self-assembly into vesicles and films.

The hedgehog-shaped {Mo368} cluster shows unique electronic (extremely high extinction coefficient) and structural features, especially regarding its size, the high number of delocalized electrons which allows to measure the surface enhanced Raman scattering (SERS) spectrum and the option for coordination chemistry inside the cavity. Its relative instability in aqueous solution can be overcome by embedment in a hydrophobic shell of dimethyldioctadecylammonium cations. The resulting hybrid self-assembles into spherical vesicles in acetone-water mixtures, according to a process directed by hydrophobic-hydrophilic interactions. It also forms rather stable Langmuir monolayers while a second layer evolves under higher surface pressure, in accordance with a rather low alkyl surface density.

Unusual stabilization of an intermediate spin state of iron upon the axial phenoxide coordination of a diiron(III)-bisporphyrin: effect of heme-heme interactions.

The binding of a series of substituted phenols as axial ligands onto a diiron(III)bisporphyrin framework have been investigated. Spectroscopic characterization revealed high-spin states of the iron centers in all of the phenolate complexes, with one exception in the 2,4,6-trinitrophenolate complex of diiron(III)bisporphyrin, which only stabilized the pure intermediate-spin (S=3/2) state of the iron centers. The average FeN (porphyrin) and FeO (phenol) distances that were observed with the 2,4,6-trinitrophenolate complex were 1.972(3) Šand 2.000(2) Å, respectively, which are the shortest and longest distances reported so far for any Fe(III) porphyrin with phenoxide coordination. The alternating shift pattern, which shows opposite signs of the chemical shifts for the meta versus ortho/para protons, is attributed to negative and positive spin densities on the phenolate carbon atoms, respectively, and is indicative of π-spin delocalization onto the bound phenolate. Electrochemical data reveals that the E1/2 value for the Fe(III) /Fe(II) couple is positively shifted with increasing acidity of the phenol. However, a plot of the E1/2 values for the Fe(III) /Fe(II) couple versus the pKa values of the phenols shows a linear relationship for all of the complexes, except for the 2,4,6-trinitrophenolate complex. The large deviation from linearity is probably due to the change of spin for the complex. Although 2,4,6-trinitrophenol is the weakest axial ligand in the series, its similar binding with the corresponding Fe(III) monoporphyrin only results in stabilization of the high-spin state. The porphyrin macrocycle in the 2,4,6-trinitrophenolate complex of diiron(III)bisporphyrin is the most distorted, whilst the "ruffling" deformation affects the energy levels of the iron d orbitals. The larger size and weaker binding of 2,4,6-trinitrophenol, along with hemeheme interactions in the diiron(III)bisporphyrin, are responsible for the larger ring deformations and eventual stabilization of the pure intermediate-spin states of the iron centers in the complex.

Effect of heme-heme interactions and modulation of metal spins by counter anions in a series of diiron(III)-µ-hydroxo bisporphyrins: unusual stabilization of two different spins in a single molecular framework.

A new family of five ethene-bridged diiron(III)-µ-hydroxo bisporphyrins with the same core structure but different counter anions, represented by the general formula [Fe2 (bisporphyrin)]OH·X (X=counter anion), is reported herein. In these complexes, two different spin states of Fe are stabilized in a single molecular framework. Protonation of the oxo-bridged dimer 1 by strong Brønsted acids such as HI, HBF4, HPF6, HSbF6 , and HClO4 produces the µ-hydroxo complexes with I5(-)(2), BF4(-)(3), PF6(-)(4), SbF6(-)(5), and ClO4(-)(6) as counter anions, respectively. The X-ray structures of 2 and 6 have been determined, which provide a rare opportunity to investigate structural changes upon protonation. Spectroscopic characterization has revealed that the two iron(III) centers in 2 are nonequivalent with nearly high and admixed-intermediate spins in both the solid state and solution. Moreover, the two different Fe(III) centers of 3-5 are best described as having admixed-high and admixed-intermediate spins with variable contributions of S=5/2 and 3/2 for each state in the solid, but two different admixed-intermediate spins in solution. In contrast, the two Fe(III) centers in 6 are equivalent and are assigned as having high and intermediate spin states in the solid and solution, respectively. The X-ray structures reveal that the Fe-O bond length increases on going from the µ-oxo to the µ-hydroxo complexes, and the Fe-O(H)-Fe unit becomes more bent, with the dihedral angle decreasing from 150.9(2)° in 1 to 142.3(3)° and 143.85(2)° in 2 and 6, respectively. Variable-temperature magnetic data have been subjected to a least-squares fitting using the expressions derived from the spin Hamiltonians H=-2JS1·S2 -µ·B+D[S(2)(z) - 1/3S(S + 1)] (for 2, 3, 4, and 5) and H=-2JS1·S2 (for 6). The results show that strong antiferromagnetic coupling between the two Fe(III) centers in 1 is attenuated to nearly zero (-2.4 cm(-1)) in 2, whereas the values are -46, -32.6, -33.5, and -34 cm(-1) for 3, 4, 5, and 6, respectively.

Protonation of an oxo-bridged diiron unit gives two different iron centers: synthesis and structure of a new class of diiron(III)-µ-hydroxo bisporphyrins and the control of spin states by using counterions.

Reported herein is a hitherto unknown family of diiron(III)-µ-hydroxo bisporphyrins in which two different spin states of Fe are stabilized in a single molecular framework, although both cores have identical molecular structures. Protonation of the oxo-bridged dimer (2) by using strong Brønsted acids, such as HI, HBF(4), and HClO(4), produce red µ-hydroxo complexes with I(3)(-) (3), BF(4)(-) (4), and ClO(4)(-) (5) counterions, respectively. The X-ray structure of the molecule reveals that the Fe-O bond length increases on going from the µ-oxo to the hydroxo complex, whereas the Fe-O(H)-Fe unit becomes more bent, which results in the smallest known Fe-O(H)-Fe angles of 142.5(2) and 141.2(1)° for 3 and 5, respectively. In contrast, the Fe-O(H)-Fe angle remains unaltered in 4 from the corresponding µ-oxo complex. The close approach of two rings in a molecule results in unequal core deformations in 3 and 4, whereas the cores are deformed almost equally but to a lesser extent in 5. Although 3 was found to have nearly high-spin and admixed intermediate Fe spin states in cores I and II, respectively, two admixed intermediate spin states were observed in 4. Even though the cores have identical chemical structures, crucial bond parameters, such as the Fe-N(p), Fe-O, and Fe⋅⋅⋅Ct(p) bond lengths and the ring deformations, are all different between the two Fe(III) centers in 3 and 4, which leads to an eventual stabilization of two different spin states of Fe in each molecule. In contrast, the two Fe centers in 5 are equivalent and assigned to high and intermediate spin states in the solid and solution states, respectively. The spin states are thus found to be dependent on the counterions and can also be reversibly interconverted. Upon protonation, the strong antiferromagnetic coupling in the µ-oxo dimer (J, -126.6 cm(-1)) is attenuated to almost zero in the µ-hydroxo complex with the I(3)(-) counterion, whereas the values of J are -36 and -42 cm(-1), respectively, for complexes with BF(4)(-) and ClO(4)(-) counterions.

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. Combined analysis using single crystal X-ray structure determination, Mössbauer, variable-temperature magnetic, 1H NMR and EPR studies has confirmed the spin states of iron(iii) complexes both in solid and solution phases.

Oxidation triggers extensive conjugation and unusual stabilization of two di-heme dication diradical intermediates: role of bridging group for electronic communication.

MauG is a diheme enzyme that utilizes two covalently bound c-type hemes to catalyse the biosynthesis of the protein-derived cofactor tryptophan tryptophylquinone. The two hemes are physically separated by 14.5 Å and a hole-hopping mechanism is proposed in which a tryptophan residue located between the hemes undergoes reversible oxidation and reduction to increase the effective electronic coupling element and enhance the rate of reversible electron transfer between the hemes in bis-Fe(iv) MauG. The present work describes the structure and spectroscopic investigation of 2e-oxidations of the synthetic diheme analogs in which two heme centers are covalently connected through a conjugated ethylene bridge that leads to the stabilization of two unusual trans conformations (U and P' forms) with different and distinct spectroscopic and geometric features. Unlike in MauG, where the two oxidizing equivalents are distributed within the diheme system giving rise to the bis-Fe(iv) redox state, the synthetic analog stabilizes two ferric hemes, each coupled with a porphyrin cation radical, a scenario resembling the binuclear dication diradical complex. Interestingly, charge resonance-transition phenomena are observed here both in 1e and 2e-oxidised species from the same system, which are also clearly distinguishable by their relative position and intensity. Detailed UV-vis-NIR, X-ray, Mössbauer, EPR and 1H NMR spectroscopic investigations as well as variable temperature magnetic studies have unraveled strong electronic communications between two porphyrin π-cation radicals through the bridging ethylene group. The extensive π-conjugation also allows antiferromagnetic coupling between iron(iii) centers and porphyrin radical spins of both rings. DFT calculations revealed extended π-conjugation and H-bonding interaction as the major factors in controlling the stability of the conformers.

Control of spins by ring deformation in a diiron(III)bisporphyrin: reversal of ClO4⁻ and CF3SO3⁻ ligand field strength in the magnetochemical series.

The complete reversal of the ligand field strength of ClO(4)(-) and CF(3)SO(3)(-) in the magnetochemical series is observed in a diiron(III)bisporphyrin. While ClO(4)(-), as the axial ligand, gives a typical high-spin complex, just twisting the conformation of the porphyrin macrocycle gives properties of a pure intermediate-spin state with CF(3)SO(3)(-), although the axial ligand strengths suggest the reverse order of spin stabilization.

Effects of axial pyridine coordination on a saddle-distorted porphyrin macrocycle: stabilization of hexa-coordinated high-spin Fe(III) and air-stable low-spin iron(II) porphyrinates.

We have reported here the effect of axial ligand L (L: pyridine/substituted pyridine) on Fe(III)(tn-OEP)Cl/Fe(III)(tn-OEP)ClO(4) that first form high-spin Fe(III)(tn-OEP)(L)(2).X (X: Cl, ClO(4)) which, on longer exposure, spontaneously auto reduce to a series of air stable Fe(II)(tn-OEP)(L)(2) complexes. The introduction of four nitro groups into the meso-positions of octaethyl porphyrin (tn-OEP), severely distorts the porphyrin macrocycle which enables the facile isolation of a rare family of high-spin Fe(III)(tn-OEP)(L)(2)(+) in a saddle distorted macrocyclic environment. The synthesis and characterization of high-spin Fe(III)(tn-OEP)(L)(2).X and low-spin Fe(II)(tn-OEP)(L)(2) are reported. The X-ray structures of Fe(II)(tn-OEP)(py)(2), Fe(II)(tn-OEP)(4-CNpy)(2) and Fe(II)(tn-OEP)(3-Clpy)(2) have been determined in which the axial ligands are orientated nearly perpendicular to each other. Electrochemical data obtained from cyclic voltammetric study for Fe(II)(tn-OEP)(L)(2) reveals the one electron oxidations at very high positive potentials which readily explains why the complexes are so stable in air. However, spectroscopic characterizations such as magnetic and EPR measurements in both solid and solution, and (1)H NMR in solution demonstrates the high-spin nature of Fe(III)(tn-OEP)(L)(2).X. Molecular orbital calculations using DFT for five coordinate Fe(III)(tn-OEP)Cl shows a(2u)-like HOMO that is expected for a saddle distorted porphyrin but for six coordinate Fe(III)(tn-OEP)(L)(2).X results in switch of the HOMO from a(2u) to a(1u). However, metal d(x(2)-y(2)) and porphyrin a(1u) bonding interaction is symmetrically unfavorable and thus responsible for high-spin nature of the complexes reported here. The porphyrin cores (tn-OEP) are found to be least distorted in Fe(III)(tn-OEP)(H(2)O)(2).ClO(4) with a core size of 2.061 A while, for Fe(II)(tn-OEP)(py)(2), the macrocycle is distorted most with lowest core size of 1.961 A; thus shows a significant and unprecedented core expansion of 0.1 A in the series.