Addition Of Axial Ligands Research Articles

Two-dimensional iron-phthalocyanine (Fe-Pc) monolayer as a promising single-atom-catalyst for oxygen reduction reaction: a computational study.

Searching for low-cost non-Pt catalysts for oxygen reduction reaction (ORR) has been a key scientific issue in the development of fuel cells. In this work, the potential of utilizing the experimentally available two-dimensional (2D) Fe-phthalocyanine (Fe-Pc) monolayer with precisely-controlled distribution of Fe atoms as a catalyst of ORR was systematically explored by means of comprehensive density functional theory computations. The computations revealed that O2 molecules can be sufficiently activated on the surface of the Fe-Pc monolayer, and the subsequent ORR steps prefer to proceed on the Fe-Pc monolayer through a more efficient 4e pathway with a considerable limiting potential of 0.68 V. Especially, the Fe-Pc monolayer is more stable than the Fe-Pc molecule in acidic medium, and can present good catalytic performance for ORR on the addition of axial ligands. Therefore, the Fe-Pc monolayer is quite a promising single-atom-catalyst with high efficiency for ORR in fuel cells.

Sawhorse-type diruthenium tetracarbonyl complexes derived from pyrenyl-carboxylic acids

A series of sawhorse-type diruthenium tetracarbonyl complexes containing pyrenyl groups in the equatorial positions of the diruthenium backbone has been synthesized and characterized. The reaction between dodecacarbonyltriruthenium Ru3(CO)12 and pyrene-carboxylic acids, 1-pyrenecarboxylic acid (C16H9COOH), 1-pyreneacetic acid (C16H9CH2COOH) and 1-pyrenebutyric acid (C16H9(CH2)3COOH), followed by addition of axial ligands (L), pyridine (a) and triphenylphosphine (b), affords the stable diruthenium tetracarbonyl complexes, Ru2(CO)4(μ2-η2-OOCC16H9)2(L)2 (1), Ru2(CO)4(μ2-η2-OOCCH2C16H9)2(L)2 (2) and Ru2(CO)4(μ2-η2-OOC(CH2)3C16H9)2(L)2 (3), respectively. The molecular structure of the triphenylphosphine derivatives Ru2(CO)4(μ2-η2-OOCC16H9)2(PPh3)2 (1b), Ru2(CO)4(μ2-η2-OOCCH2C16H9)2(PPh3)2 (2b) and Ru2(CO)4(μ2-η2-OOC(CH2)3C16H9)2(PPh3)2 (3b) was determined by single-crystal X-ray structure analysis and showed a typical diruthenium tetracarbonyl backbone bridged by the pyrene-carboxylato ligands and completed with PPh3 axial ligands. The molecular structure of complexes 2 and 3 reveals that the pyrenyl moieties are adequately positioned to potentially allow these bis-pyrenyl systems to act as molecular tweezers. However, these systems have showed no interaction with fullerene in solution, despite possessing, as suggested by molecular modeling, appropriate structural features.

Sawhorse-type diruthenium tetracarbonyl complexes containing biologically relevant acids

The reaction between the biologically active acids probenecid (HO2CC12H18NO2S), indomethacin (HO2CC18H15ClNO2) and sulindac (HO2CC19H16FOS) with Ru3(CO)12, followed by addition of axial ligands (L), such as pyridine, triphenylphosphine, or 5-(4-pyridyl)-10,15,20-triphenylporphyrin, generates a series of stable diruthenium tetracarbonyl complexes of the formula Ru2(CO)4(μ2-η2-O2CC12H18NO2S)2L2, Ru2(CO)4(μ2-η2-O2CC18H15ClNO2)2L2 and Ru2(CO)4(μ2-η2-O2CC19H16FOS)2L2, respectively. The molecular structure of 1a · C6H6 was solved by single-crystal X-ray structure analysis and a typical diruthenium tetracarbonyl backbone bridged by the carboxylato ligands and two axial triphenylphosphine ligands was revealed. The benzene molecule sits between two probenecid units, and is involved in π-stacking interactions with the aromatic part of probenecid. Despite the presence of biologically relevant derivatives and carbonyl groups within the sawhorse-type dinuclear complexes, all systems show no cytotoxicity towards human cancer cells, presumably due to the high lipophilicity of these neutral complexes.

Synthesis and structure of five or six-coordinate manganese deuteroporphyrin-niacin dyads with intramolecular axial pyridine

Abstract Deuteroporphyrin-niacin dyads with different chain lengths have been synthesized by modification of the propionate side chains of hemin. When a manganese ion was inserted into the porphyrin core, the UV spectral shift of manganese deuteroporphyrin-niacin dyads were experimentally demonstrated to mainly originate from the intramolecular coordination. In order to elaborate the intramolecular coordination, the spectra of single manganese porphyrin complexes in CH 2 Cl 2 solution were measured and compared to that of the addition of axial ligands (pyridine and methyl nicotinate). Among all the synthetic dyads, the compounds of 2,7,12,18-tetramethyl-13,17-di(3-hydroxypropyl nicotinate) porphyrin and 2,7,12,18-tetramethyl-13,17-di(3-aminoethyl nicotinate) porphyrin manganese bearing the short chains did not show intramolecular coordination of the terminal base on the metal ion. Other three compounds of the niacin moiety indirectly bonded to the propionate side chains of manganese porphyrin through the diols linkage exhibited optical spectra characteristic of five or six-coordinate manganese complexes. These results indicate that the niacin groups' access to the Mn(III) center depended on the chain lengths.

Computational Studies on Structural and Excited-State Properties of Modified Chlorophyll f with Various Axial Ligands

Time-dependent density functional theory (TDDFT) calculations have been used to understand the excited-state properties of modified chlorophyll f (Chlide f), Chlide a, Chlide b, and axial ligated (with imidazole, H(2)O, CH(3)OH, CH(3)COOH, C(6)H(5)OH) Chlide f molecules. The computed differences among the Q(x), Q(y), B(x), and B(y) band absorbance wavelengths of Chlide a, b, and f molecules are found to be comparable with the experimentally observed shifts for these bands in chlorophyll a (chl a), chl b, and chl f molecules. Our computations provide evidence that the red shift in the Q(y) band of chl f is due to the extended delocalization of macrocycle chlorin ring because of the presence of the -CHO group. The local contribution from the -CHO substituent to the macrocycle chlorin ring stabilizes the corresponding molecular orbitals (lowest unoccupied molecular orbital (LUMO) of the Chlide f and LUMO-1 of the Chlide b). All the absorption bands of Chlide f shift to higher wavelengths on the addition of axial ligands. Computed redox potentials show that, among the axial ligated Chlide f molecules, Chlide f-imidazole acts as a good electron donor and Chlide f-CH(3)COOH acts as a good electron acceptor.

A square-planar di- N-carboxamido, dithiolato–cobalt(III) complex related to nitrile hydratase metallic site. Addition of axial ligands and EXAFS study of the derived dicyano and diisocyanido complexes

A square-planar cobalt(III) complex with a N 2S 2 di- N-carboxamido di-thiolato tetradentate ligand, mimics the metal core of the cobalt containing nitrile hydratases. It forms hexacoordinated complexes only upon addition of ligands like cyanide or isocyanides. An EXAFS analysis confirmed that two axial ligands get bound, and lead to a significant increase of the bond lengths between the cobalt(III) and the equatorial ligands. Whereas the planar complex is stable in the presence of air or hydrogen peroxide, its hexacoordinated derivatives with strong σ-donor axial ligands undergo an oxygenation of the equatorial thiolate ligands.

The structure of dichromium tetraformate

Second-order perturbation theory (CASPT2) calculations using large basis sets have been used to study the CrCr bond in dichromium tetraformate, Cr 2(O 2CH) 4 The CrCr potential was found to have a double minimum with one short equilibrium at 1.94 Å, exhibiting considerable 3d-3d bonding, and a long equilibrium at 2.50 Å, which is essentially determined by the structure of the bridging ligands. The energy difference between the two minima is small (of the order of 0.1 eV) and the relative order depends on the level of theory used. Inclusion of relativistic effects and 3s,3p correlation (estimated from the corresponding effect in the 3 Σ u + state of Cr 2 locates the inner minimum 0.07 eV below the outer. The measured distance in Cr 2(O 2CCH 3) 4 is 1.966 Å. It is likely that addition of axial ligands like water will make the outer minimum more stable, thus explaining the variations in Cr{inCr} distances in dichromium complexes. The DFr approach is also applied and long, 2.296 and 2.487 \\rA, CrCr bond distances are found for Cr{in2}(O{in2}CH){in4} and Cr{in2}(O{in2}CCH{in3}){in4}, respectively. Addition of axial water ligands increases the CrCr distance by about 0.19 and 0.07 \\rA, respectively, for these two systems.

Structural and Electrochemical Comparison of NiII in an N2S2 Plane with Two, One and No Axial Ligands: Isolation and Characterization of a Five‐Coordinate Nickel(II) Complex

AbstractAddition of two equivalents of benzyl bromide to [N,N′‐bis(2‐mercaptoethyl)‐1,5‐diazacyclooctane]nickel(II) (Ni‐1) produced the green dibenzyl dithioether complex [Ni‐1‐Bzl2Br2] structurally characterized by X‐ray crystallography as a six‐coordinate octahedral complex in a monoclinic crystal system and the P21/c space group. Benzylation of the methyl thioether/thiolate [(Ni‐1‐Me)+I– yielded the mixed benzyl/methyl dithioether [(Ni‐1‐BzlMe)Br]I, structurally characterized by X‐ray crystallography as a five‐coordinate square pyramidal nickel(II) complex with a Ni–Br distance of 2.451 Å. The complex crystallizes in the orthorhombic Cmca space group. The latter is the first nickel(II) pentacoordinate complex isolated for the N,N′‐bis(2‐mercaptoethyl)‐1,5‐diazacyclooctane (daco) ligand. Structural comparison of these two complexes with the square‐planar dimethyl compound [Ni‐1‐Me2]I2 found an expansion of the N2S2 plane upon addition of axial ligands. The five‐coordinate nickel complex showed a displacement of the Ni ion from the N2S2 plane by 0.43 Å toward the axial bromide ligand, a common observation of pentacoordinate complexes of this ligand with metals such as Zn and Fe. Electrochemical studies as well as the X‐ray crystal structures for complex [Ni‐1‐Bzl2Br2] and [(Ni‐1‐BzlMe)Br]I are presented and discussed in detail.

Molecular orbital calculations of the active site complex of two iron ferredoxins

The results presented here are the first part of a systematic theoretical study of some of the physical and biochemical properties of two iron ferredoxins obtained by the use of an Extended Huckel Self-Consistent Charge iteration method of molecular orbital calculations. In this initial study, attention is focused on the calculation of electronic energies as a function of molecular geometry and the nature of the bonding ligands at the active site in order to determine the most stable form of the active site complex. Included in the active site complex are two iron atoms, two acid labile sulfur atoms of unknown inorganic origin and four sulfur atoms presumably from nearby cysteine residues. Fifteen chemical-conformational variations of this basic active site complex were considered. Among these conformational variations of the sulfur ligands, Fe-Fe distances, bond lengths and angles and chemical variations such as the effect of axial ligands, disulfide bonds and added protons were included. Our results indicate that that with all reasonable variations of the ligands, the preferred molecular geometry about 4-coordinated Fe is tetrahedral rather than planar. The planar conformation is somewhat stabilized by the addition of axial ligands, but is still less favorable than the tetrahedral conformation. In this model, interactions between the two iron atoms occur automatically since they are both part of the same active site complex. Hence the absence of low temperature paramagnetism in the oxidized state is readily explained. Preliminary investigations of the reduced state with one additional electron indicate that the odd electron is delocalized, as observed in both ESR and ENDOR. Its presence apparently substantially destabilizes all of the molecular orbital energies in accord with the observation that only one electron can be added to these proteins without decomposing them.