Solution and Solid State Properties for Low-Spin Cobalt(II) Dibenzotetramethyltetraaza[14]annulene [(tmtaa)CoII] and the Monopyridine Complex.

The single-crystal X-ray structure of solvent-free (tmtaa)CoII reveals three different π-π intermacrocyclic interactions between tmtaa units (tmtaa = dibenzotetramethyltetraaza[14]annulene). Pairs of inequivalent (tmtaa)CoII units in the unit cell link into a one-dimensional π-π stacked array in the solid state. Magnetic susceptibility (χ) studies from 300 to 2 K reveal the effects of intermolecular interactions between (tmtaa)CoII units in the solid state. The effective magnetic moment per CoII center is constant at 2.83 µB from 300 to 100 K and begins to significantly decrease at lower temperatures. The magnetic data are fit to a singlet ( S = 0) ground state with a triplet ( S = 1) excited state that is 13 cm-1 higher in energy (-2 J = 13 cm-1). Toluene solutions of (tmtaa)CoII have 1H nuclear magnetic resonance (NMR) paramagnetic shifts, a solution-phase magnetic moment µeff (295 K) of 2.1 µB, and toluene glass electron paramagnetic resonance spectra that are most consistent with a low-spin ( S = 1/2) CoII with the unpaired electron located in the d yz orbital. Pyridine interacts with (tmtaa)CoII to form a five-coordinate monopyridine complex in which the unpaired electron is in the d z2 orbital. The five-coordinate complex has been structurally characterized by single-crystal X-ray diffraction, and the equilibrium constant for pyridine binding at 295 K has been evaluated by both electronic and 1H NMR spectra. Density functional theory computation using the UB3LYP hybrid functional places the unpaired electron for (tmtaa)CoII in the d yz orbital and that for the monopyridine complex in the d z2 orbital, consistent with spectroscopic observations.

Effect of Two Interacting Rings in Metalloporphyrin Dimers upon Stepwise Oxidations.

The interaction between two porphyrin macrocycles, connected covalently through either a rigid ethylene or a flexible ethane bridge, in the metalloporphyrin dimers (M: 2H, Zn(2+)) have been investigated upon stepwise oxidations. Upon 1e-oxidation, two porphyrin macrocycles come closer and cofacial to each other while 2e-oxidation forces them to be separated as far as possible. This has resulted in the conversion of the cis isomer to trans for the ethylene bridged porphyrin dimer with the stabilization of an unusual "U" form, which has unique spectral and geometrical features. Detailed ultraviolet-visible-near-infrared (UV-vis-NIR), infrared (IR), electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopic investigations, along with X-ray structure determination of the 2e-oxidized complexes, have demonstrated strong electronic communications between two porphyrin π-cation radicals through the bridging ethylene group. Such extensive π-conjugation also results in strong antiferromagnetic coupling between the radical spins of both of the macrocycles, which generates a diamagnetic compound. The experimental observations are also strongly supported by density functional theory (DFT) calculations.

A Highly Oxidized Cobalt Porphyrin Dimer: Spin Coupling and Stabilization of the Four-Electron Oxidation Product.

A highly oxidized cobalt porphyrin dimer is reported. Each cobalt(II) ion and porphyrin ring underwent 1e oxidation with iodine as the oxidant to give a 4e-oxidized cobalt(III) porphyrin π-cation radical dimer. The bridging ethylene group allows for substantial conjugation of the porphyrin macrocycles, thus leading to a strong antiferromagnetic coupling between the π-cation radicals and to stabilization of the singlet state. X-ray crystallography clearly showed that the complex may be considered as a real supramolecule rather than two cobalt(III) porphyrin π-cation radicals that interact through space.

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.

Syn-anti conformational switching in an ethane-bridged Co(II)bisporphyrin induced by external stimuli: effects of inter-macrocyclic interactions, axial ligation and chemical and electrochemical oxidations.

The syn-anti conformational switching has been demonstrated in the ethane-bridged dicobalt(II)bisporphyrin which is present in the syn-form only. The addition of either perylene or axial ligands to Co(II)(bisporphyrin) completely transforms the syn form into the anti because of strong π-π interaction and axial coordination, respectively. The complex undergoes four 1e-oxidations in CH2Cl2 which are indicative of strong through space interactions between the two cofacial Co-porphyrins at 295 K. The first oxidation is a metal centered one and occurs at a potential much lower than that of the monomeric analog. However, the second oxidation, which is again metal centered, was at a significantly higher potential. The large difference between the first two oxidations, as observed here, is due to much stronger inter-porphyrin interactions. The step-wise oxidations have been performed both chemically and electro-chemically while the progress of the reactions was monitored by UV-visible and (1)H NMR spectroscopy. After 1e-oxidation, a very broad (1)H NMR signal results with increased difference between two meso resonances, which indicates that the two macrocycles are in the syn-form with lesser interplanar separation as also observed by DFT. However, 2e-oxidation results in the stabilization of the anti form. The addition of axial ligands to Co(II)(bisporphyrin) also completely transforms the syn form into the anti form. While additions of THF and I2/I(-) both result in the formation of five-coordinate complexes, Co(II) is oxidized to Co(III) in the case of the latter. However, additions of 1-methylimidazole, pyridine and pyrazine as axial ligands result in the formation of a six-coordinate complex in which Co(II) is spontaneously oxidized to Co(III) in air.

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.

Switching orientation of two axial imidazole ligands between parallel and perpendicular in low-spin Fe(III) and Fe(II) nonplanar porphyrinates.

We have reported here the synthesis, structure, and properties of low-spin bis-imidazole-coordinated Fe(III) and Fe(II) complexes of 5,10,15,20-tetrakis(pentafluorophenyl)-2,3,7,8,12,13,17,18-octachloroporphyrin, [Fe(III)(TFPPCl(8))(L)(2)]ClO(4) and Fe(II)(TFPPCl(8))(L)(2) (L = 1-methylimidazole, 4-methylimidazole, imidazole). The X-ray structure of Fe(II)(TFPPCl(8))(1-MeIm)(2) is reported here, which demonstrated the near-perpendicular axial ligand orientation (dihedral angle between two 1-methylimidazoles is 80.7°) for Fe(II) porphyrins in a highly saddle-distorted macrocyclic environment. Oxidation of Fe(II)(TFPPCl(8))(L)(2) using thianthrenium perchlorate produces [Fe(III)(TFPPCl(8))(L)(2)]ClO(4), which was also isolated in the solid state and characterized spectroscopically. The complex gives rhombic EPR spectra in both solid and solution phases at 77 K and thus represents a rare example of nearly parallel axial ligand orientations for the unhindered imidazoles in a saddle-distorted porphyrin macrocycle. Geometry optimization using DFT also converged to the parallel axial alignment when 1-methylimidazole was used as the axial ligand (the dihedral angle between two axial ligands is 8.6°). The potential energy surface (PES) scan results also show that the relatively parallel axial orientations are energetically preferred for Fe(III), while perpendicular orientations are preferred for the Fe(II) complexes reported here. Bulk oxidation of Fe(II)(TFPPCl(8))(L)(2) in dichloromethane at a constant potential under nitrogen converts it to [Fe(III)(TFPPCl(8))(L)(2)]ClO(4), which gives identical EPR spectra at 77 K and which upon reduction regenerates Fe(II)(TFPPCl(8))(L)(2) again. Thus, we have demonstrated here very rare examples of Fe porphyrins in which the relative axial imidazole orientations switch between parallel and perpendicular just upon changing the oxidation states of iron from +3 to +2, respectively, in a nonplanar porphyrinic environment. These observations could be immensely important for understanding the possible effects of axial histidine orientations on similar macrocyclic deformations observed in various heme proteins.

Induction of supramolecular chirality in di-zinc(II) bisporphyrin via tweezer formation: synthesis, structure and rationalization of chirality.

Two new supramolecular complexes consisting of an achiral bisporphyrin host and a chiral diamine guest are reported. One shows a remarkably high amplitude bisignate CD signal while the other one shows a very low value. X-ray structure and other spectroscopic investigations of the tweezer complexes clearly rationalize the origin of the optical activity that has so far remained an unresolved issue.