The synthesis and characterization of the Fe(III) complex of a novel crown ether-porphyrin conjugate, 52-N-(4-aza-18-crown-6)methyl-54,104,154,204-tetra-tert-butyl-56-methyl-5,10,15,20-tetraphenylporphyrin (H2Porph), as well as the corresponding hydroxo, dimeric, Fe(II), and peroxo species are reported. The crystal structure of [FeIII(Porph)Cl].H3O+.FeCl4-.C6H6.EtOH is also reported. [FeIII(Porph)(DMSO)2]+ and K[FeIII(Porph)(O22-)] are high-spin species (Mössbauer data: delta = 0.38 mm s(-1), DeltaEq = 0.83 mm s(-1) and delta = 0.41 mm s(-1), DeltaEq = 0.51 mm s(-1), respectively), whereas in a solution of reduced [FeIII(Porph)(DMSO)2]+ complex the low-spin [FeII(Porph)(DMSO)2] (delta = 0.44 mm s(-1), DeltaEq = 1.32 mm s(-1)) and high-spin [FeII(Porph)(DMSO)] (delta = 1.27 mm s(-1), DeltaEq = 3.13 mm s(-1)) iron(II) species are observed. The reaction of [FeIII(Porph)(DMSO)2]+ with KO2 in DMSO has been investigated. The first reaction step, involving reduction to [FeII(Porph)(DMSO)2], was not investigated in detail because of parallel formation of an Fe(III)-hydroxo species. The kinetics and thermodynamics of the second reaction step, reversible binding of superoxide to the Fe(II) complex and formation of an Fe(III)-peroxo species, were studied in detail (by stopped-flow time-resolved UV/vis measurements in DMSO at 25 degrees C), resulting in kon = 36 500 +/- 500 M(-1) s(-1), koff = 0.21 +/- 0.01 s(-1) (direct measurements using an acid as a superoxide scavenger), and KO2- = (1.7 +/- 0.2) x 10(5) (superoxide binding constant kinetically obtained as kon/koff), (1.4 +/- 0.1) x 10(5), and (9.0 +/- 0.1) x 10(4) M(-1) (thermodynamically obtained in the absence and in the presence of 0.1 M NBu4PF6, respectively). Temperature-dependent kinetic measurements for kon (-40 to 25 degrees C in 3:7 DMSO/CH3CN mixture) yielded the activation parameters DeltaH = 61.2 +/- 0.9 kJ mol(-1) and DeltaS = +48 +/- 3 J K(-1) mol(-1). The observed reversible binding of superoxide to the metal center and the obtained kinetic and thermodynamic parameters are unique. The finding that fine-tuning of the proton concentration can cause the Fe(III)-peroxo species to release O2- and form an Fe(II) species is of biological interest, since this process might occur under very specific physiological conditions.

New transitions: Low‐energy electronic transitions have been detected spectroscopically in the FeII–FeIII mixed‐valent biferrocenyl radical cation, but are absent in the spectra of the neutral analogue. They have been assigned by time‐dependent DFT calculations (squares in figure). Analogous investigations were performed for the bisfulvalenide FeII–FeIII radical cation.magnified imageUV‐visible/near‐IR (NIR)/mid‐IR (MIR) solution, solid‐state, and matrix‐isolation electronic absorption spectra of the FeII–FeIII mixed‐valent homobimetallic compounds biferrocenyl triiodide (1) and 1′,1′′′‐diethylbiferrocenyltriiodide (2) reveal the presence of a low‐energy transition in the MIR region that has not been reported before. The new absorption feature and the known NIR band are both assigned to intervalence charge‐transfer (IVCT) transitions. To obtain insight into the electronic structures of 1 and 2, DFT calculations with the BP86 functionals and different basis sets have been performed. Based on the molecular orbital scheme of cation 1, one band corresponds to the transition between the highest occupied d orbitals on the two iron centers, whereas the other one is assigned to a transition from a lower‐lying d orbital to the d orbital. For comparison, the doubly bridged bisfulvalenide diiron cation (3) has been investigated by optical absorption spectroscopy and DFT calculations. The experimental and theoretical results are discussed with respect to the degree of electron localization/delocalization in these systems.

Ferrocenyl cobaltocenium hexafluorophosphate (1) and ferrocenylene cobaltocenylenium hexafluorophosphate (2) are investigated by a range of spectroscopic methods. Both compounds are diamagnetic, in contrast to an earlier report indicating a temperature-dependent paramagnetism of 2. Electronic absorption spectra of 1 and 2 are presented and fully assigned up to 50 000 cm(-1) on the basis of electronic structure (DFT) calculations and spectral comparisons with ferrocene and cobaltocenium. The lowest-energy bands, I, of both 1 and 2 correspond to metal-to-metal CT (MMCT) transitions; further intermetallocene charge-transfer bands are identified at higher energy (bands III and V). On the basis of the spectroscopic properties, a trans geometry and a twisted structure are derived for 1 and 2, respectively, in solution. Analysis of the I bands gives orbital mixing coefficients, alpha, electronic-coupling matrix elements, V(AB), and reorganization energies, lambda. Importantly, alpha and V(AB) are larger for 1 than for 2 (0.07 and 1200 cm(-1) vs 0.04 and approximately 600 cm(-1), respectively), apparently in contrast to the presence of one bridge in 1 and two bridges in 2. This result is explained in terms of the respective electronic and geometric structures. Reorganization energies are determined to be 7600 cm(-1) for 1 and 4600 cm(-1) for 2, in qualitative agreement with the analogous Fe(II)-Fe(III) compounds. The general implications of these findings with respect to the spectroscopic and electron-transfer properties of bimetallocenes are discussed.

Ferrocene-ferrocenium dimers exhibit a double-peak intervalence charge-transfer (IVCT) band in the NIR/MIR region, which is analyzed in terms of a four-level, two-mode vibronic coupling configuration interaction (VCCI) scheme. Besides providing satisfactory fits of the measured spectra, the model also gives electronic and vibronic coupling parameters as well as CI mixing coefficients. A temperature-dependent asymmetry of the potential is introduced in order to describe the temperature dependence of the solid-state spectra and account for complementary Mossbauer data, which indicate a temperature-driven electron localization-delocalization transition in the singly bridged radical cations. The VCCI model is also successfully applied to the doubly bridged Fe(II)-Fe(III) bisfulvalenide dimer, which exhibits a double-peak IVCT transition as well. The VCCI analysis reveals that all dimers have a one-minimum potential in the ground state, leading in the absence of asymmetry to class III behavior (electron delocalization). If electron localization corresponding to class II characteristics occurs, it is due to an asymmetric (but still one-minimum) potential and not, as usual, to a double-minimum potential, explaining the class II-III borderline behavior observed for these systems.