Solution and Solid State Properties for Low-Spin Cobalt(II) Dibenzotetramethyltetraaza[14]annulene [(tmtaa)Co^II] 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 (−2J = 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 dyz orbital. Pyridine interacts with (tmtaa)­CoII to form a five-coordinate monopyridine complex in which the unpaired electron is in the dz2 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 dyz orbital and that for the monopyridine complex in the dz2 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, Zn2+) 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

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, Zn²⁺) 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 <i>cis</i> isomer to <i>trans</i> for the ethylene bridged porphyrin dimer with the stabilization of an unusual “<i>U</i>” 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

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(TFPPCl₈)­(L)₂]­ClO₄ and Fe^II(TFPPCl₈)­(L)₂ (L = 1-methylimidazole, 4-methylimidazole, imidazole). The X-ray structure of Fe^II(TFPPCl₈)­(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(TFPPCl₈)­(L)₂ using thianthrenium perchlorate produces [Fe^III(TFPPCl₈)­(L)₂]­ClO₄, 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(TFPPCl₈)­(L)₂ in dichloromethane at a constant potential under nitrogen converts it to [Fe^III(TFPPCl₈)­(L)₂]­ClO₄, which gives identical EPR spectra at 77 K and which upon reduction regenerates Fe^II(TFPPCl₈)­(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