Axial Mn–CCN Bonds of Cyano Manganese(II) Porphyrin Complexes: Flexible and Weak?

Molecular Engineering of CoII Porphyrins with Asymmetric Architecture for Improved Electrochemical CO2 Reduction

The electrochemical reduction of carbon dioxide (CO₂ ) based on molecular catalysts has attracted more attention, owing to their well-defined active sites and rational structural design. Metal porphyrins (PorMs) have the extended π-conjugated backbone with different transition metals, endowing them with unique CO₂ reduction properties. However, few works focus on the investigation of symmetric architecture of PorMs as well as their aggregation behavior to CO₂ reduction. In this work, a series of Co^II porphyrins (PorCos) with symmetric and asymmetric substituents were used as model of molecular catalysts for CO₂ reduction. Owing to the electron donating effect of 2,6-dimethylbenzene (DMB), bandgaps of the complexes became narrower with the increasing number of DMB. As electrocatalysts, all PorCos exhibited promising electrocatalytic CO₂ reduction performance. Among the three molecules, asymmetric Co^II porphyrin (as-PorCo) showed the lowest onset potential of -288 mV and faradaic efficiencies exceeding 93 % at -0.6 V vs. reversible hydrogen electrode, which is highly competitive among the reported state-of-art porphyrin-based electrocatalysts. The CO₂ reduction performance depended on π-π stacking between PorCo with carbon nanotubes (CNTs) and adjacent PorCos, which could be readily controlled by atomically positioned DMB in PorCo. Density functional theory calculations also suggested that the charge density between PorCo and CNT was highest due to the weak steric hindrance in as-PorCo, providing the new insight into molecular design of catalysts for efficient electrochemical CO₂ reduction.

Nitrato(5,10,15,20-tetra-phenyl-porphinato)manganese(III)-benzene-n-hexa-ne (2/1/1)

The crystal structure of solvated [Mn(TPP)(NO3)] (TPP = 5,10,15,20-tetra-phenyl-porphyrinato, C44H28N4), [Mn(C44H28N4O3)(NO3)]·0.5C6H14·0.5C6H6, has been determined in the space group Pccn. The MnIII atom has a distorted square-pyramidal environment, being coordinated by four pyrrole N atoms of the porphyrin ligand in the basal plane and an O atom of the nitrato ligand in the apical site. The MnIII atom is displaced out of the porphyrin plane by 0.22 (4) Å with the average Mn-Np distance being 2.011 (6) Å (where Np is a porphyrin N atom). The Mn-O bond length is 2.1246 (18) Å. Two kinds of inter-molecular C-H⋯O hydrogen bonds exist in the crystal structure, with the apical nitrato ligands inter-acting with solvent mol-ecules and adjacent mol-ecules, respectively.

([2.2.2]Cryptand)potassium (4-methyl-benzene-thiol-ato)[5,10,15,20-tetra-kis-(4-chloro-phen-yl)porphyr-inato]manganate(II) tetra-hydro-furan disolvate

Single crystals of the title compound, [K(C18H36N2O6)][Mn(C44H24Cl4N4)(C7H7S)]·2C4H8O, were obtained by the solvent evaporation method. The MnII cation is coordinated by four pyrrole N atoms (Np) of the porphyrin ring and one S atom of the apical 4-methyl-benzene-thiol-ate ligand with the average Mn-Np and the apical Mn-S bond lengths being 2.160 (9) and 2.4642 (8) Å, respectively. Two tetra-hydro-furan solvent mol-ecules and a potassium cation chelated inside a [2.2.2]cryptand (4,7,13,16,21,24-hexa-oxa-1,10-di-aza-bicyclo[8.8.8]hexa-cosa-ne) are also present.

Linkage Isomers of 4-Methylimidazolate Mn(II) Porphyrinates: Hindered or Unhindered?

Three different manganese(II) porphyrins have been exploited to react with 4-methylimidazolate (4-MeIm^-), and the five-coordinate products are characterized by ultraviolet-visible, single-crystal X-ray, and electronic paramagnetic resonance spectroscopies. Interestingly, 4-MeIm^- is found to bond to the metal center through either of the two N atoms (N1 or N3), which yielded two linkage isomers with either an unhindered or a hindered ligand conformation, respectively. Investigations revealed it is the large metal out-of-plane displacements (Δ₂₄ and Δ₄ ≥ 0.59 Å) that have rendered the equivalence of two isomers with a small energy difference (5.2-8.3 kJ/mol). The nonbonded intra- and intermolecular interactions thus become crucial factors in the balance of linkage isomerization. All of the products in both solution and solid states show the same characteristic resonances of high-spin Mn(II) (S = ⁵/₂) with g_⊥ ≈ 5.9 and g_∥ ≈ 2.0 at 4 K, consistent with the weak effects of the axial ligand on core conformation and metal electronic configurations. Zero-field splitting parameters obtained through simulations are also reported.

Intermolecular Interactions and Intramolecular Couplings of Binuclear Porphyrin Models for Cytochrome c Oxidase

Cytochrome c oxidase (CcO) has a binuclear active site composed of a high-spin heme group and a tris-histidine-ligated copper ion (Cu_B). By using two different porphyrin models derived by Gunter (H₂TPyPP) and us (H₂TImPP), we have isolated several mono- and binuclear complexes including one carbonyl and three chloride derivatives which are determined by 100 K single-crystal X-ray. Low-temperature (4 K) EPR and multitemperature (295-25 K) Mössbauer investigations on the products not only confirmed the spin states of the two metal ions (S = 5/2 Fe³⁺ and S = 1/2 Cu²⁺) but also revealed the intermolecular interactions and intramolecular couplings which are in accordance with the crystal structural features.

Synthesis, crystal structure and characterizations of di-μ-cyanido-1:2κ2 N:C;2:3κ2 C:N-bis-(4,7,13,16,21,24-hexa-oxa-1,10-di-aza-bicyclo-[8.8.8]hexacosa-ne)-1κ8 N 1,N 10,O 4,O 7,O 13,O 16,O 21,O 24;3κ8 N 1,N 10,O 4,O 7,O 13,O 16,O 21,O 24-[5,10,15,20-tetra-kis-(4-chloro-phen-yl)porphyrinato-2κ4 N]-2-iron(II)-1,3-dipotassium(I) tetra-hydro-furan disolvate with an unknown solvent

The title compound, [Fe(C44H24N8Cl4)(CN)2][K2(C18H36N2O6)2]·2C4H8O was synthesized and characterized by single-crystal X-ray diffraction as well as FTIR and UV-vis spectroscopy. The central FeII ion is coordinated by four pyrrole N atoms of the porphyrin core and two C atoms of the cyano groups in a slightly distorted octa-hedral coordination environment. The complex mol-ecule crystallizes with two tetra-hydro-furan solvent mol-ecules, one of which was refined as disordered over two sets of sites with refined occupancies of 0.619 (5) and 0.381 (5). It has a distorted porphyrin core with mean absolute core-atom displacements Ca, Cb, Cm and Cav of 0.32 (3), 0.22 (3), 0.56 (2) and 0.37 (14) Å, respectively. The axial Fe-Ccyano bond lengths are 1.991 (2) and 1.988 (2) Å. The average Fe-Np (Np is a porphyrin N atom) bond length is 1.964 (10) Å. One of the O atoms and several C atoms of the 222 moiety [222 = 4,7,13,16,21,24-hexa-oxa-1,10-di-aza-bicyclo-[8.8.8]hexa-cosa-ne] were refined as disordered over two sets of sites with occupancy ratios of 0.739 (6):0.261 (6) and 0.832 (4):0.168 (4). Additional solvent mol-ecules were found to be highly disordered and their contribution to the scattering was removed using the SQUEEZE procedure in PLATON [Spek (2015 ▸). Acta Cryst. C71, 9-18], which indicated a solvent cavity of volume 372 Å3 containing approximately 83 electrons. These solvent mol-ecules are not considered in the given chemical formula and other crystal data.

Spectral, Electrochemical, and ESR Characterization of Manganese Tetraarylporphyrins Containing Four β,β'-Pyrrole Fused Butano and Benzo Groups in Nonaqueous Media

Two series of β,β'-pyrrole butano- and benzo-substituted mangenese(III) tetraarylporphyrins were synthesized and characterized with regard to their spectral and electrochemical properties. The investigated compounds have the general formula butano(Ar)₄PorMnCl and benzo(Ar)₄PorMnCl, where Por is the dianion of the porphyrin and Ar is a p-CH₃Ph, Ph or p-ClPh group on each of the four meso-positions of the macrocycle. Each manganese(III) butano- or benzoporphyrin was examined in CH₂Cl₂ and/or pyridine containing 0.1 M tetra- n-butylammonium perchlorate and the data then were compared to that of the parent tetraarylporphyrins having the same meso-substituents. Up to four reductions are observed for each compound, the first being metal-centered to generate a Mn(II) porphyrin, and the second and third being porphyrin ring-centered to give a Mn(II) porphyrin π-anion radical and dianion, respectively. The one-electron reduced manganese porphyrins have an ESR spectrum with signals at g_⊥= 5.6-5.8 and g_// = 2.0, indicating a mixture of the four- and five-coordinated Mn(II) complexes in a high-spin state (3d⁵, S = 5/2, I = 5/2). Data from cyclic voltammetry and spectroelectrochemistry both suggest that formation of the porphyrin dianion is followed by a chemical reaction at the electrode surface to give an electroactive phlorin anion. The effects of solvent and porphyrin substituents on ultraviolet-visible light (UV-vis) spectra, redox potentials, and electron transfer mechanisms are discussed.

ESR analyses of picket fence MnII and 6th ligand coordinated FeIII porphyrins (S = 5/2) and a CoII(hfac) complex (S = 3/2) with sizable ZFS parameters revisited: a full spin Hamiltonian approach and quantum chemical calculations

The conventional X-band ESR spectra are fully reanalyzed by a full spin Hamiltonian approach.