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.

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.

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.

A minor modification of the reported procedure for the synthesis of a corrole dimer that is fused by the cyclooctatetraene (COT) unit, (Htpfc)COT, 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 N coordination core. The corresponding bis-gallium(III) complex was prepared as an entry into the potentially rich coordination chemistry of (Htpfc)COT. 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)]COT(py) and [(Ga-tpfc)]COT(py), 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 (λ 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 (λ 1250 nm) and reduction (λ 1780 nm) serve as additional supporting evidence for this conclusion.

Synthetic analogs of diheme enzyme MauG have been reported. Unlike the bis-Fe(iv) state in MauG, the 2e-oxidation stabilizes two ferric hemes, each coupled with a porphyrin π-cation radical.

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.

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.

Synthesis, structure and properties of new five-coordinate phenolate complexes of di-iron(III) bisporphyrin are reported here, in which phenol binds in η 1-fashion as an axial ligand. The solid and solution EPR at 120 K and 1 H NMR spectral pattern in solution provide unequivocal evidence for the high spin (S = 5/2) nature of the complex. Mulliken spin density calculation using DFT demonstrates the positive spin densities at the meso carbons and negative spin densities at the methylene carbons and, as a result, the meso and methylene protons are shifted in the upfield and down field regions, respectively in the 1 H NMR spectra of the molecule. Also, the ortho-and para-protons of the phenolate ligands are observed to be shifted in the upfield region while meta-protons are shifted downfield. The alternating shift pattern, which is the opposite sign of the chemical shifts for meta-versus ortho-and para-protons, was also explained due to negative and positive spin densities, respectively on the carbons and indicative of π spin delocalization on the phenolate ligand. Thus, the calculated spin density maps accounted for the essential 1 H NMR spectroscopic features that are observed here for the phenolate complexes of di-iron(III) bisporphyrin. The temperature dependence of the signals follows the Curie law which is indicative of single spin state throughout the temperature range of −40 to +40 • C. The single crystal X-ray structure of the corresponding chloro derivative, trans 1,2-bis(chloroiron(III) octaethyl porphyrinyl)ethene, has also been reported here which authenticates the high-spin nature of the complex.