A pyridinic Fe-N4 macrocycle models the active sites in Fe/N-doped carbon electrocatalysts.

Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum catalysts for the oxygen reduction reaction (ORR) in fuel cells; however, their active site structures remain poorly understood. A leading postulate is that the iron-containing active sites exist primarily in a pyridinic Fe-N4 ligation environment, yet, molecular model catalysts generally feature pyrrolic coordination. Herein, we report a molecular pyridinic hexaazacyclophane macrocycle, (phen2N2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for ORR to a typical Fe-N-C material and prototypical pyrrolic iron macrocycles. N 1s XPS and XAS signatures for (phen2N2)Fe are remarkably similar to those of Fe-N-C. Electrochemical studies reveal that (phen2N2)Fe has a relatively high Fe(III/II) potential with a correlated ORR onset potential within 150 mV of Fe-N-C. Unlike the pyrrolic macrocycles, (phen2N2)Fe displays excellent selectivity for four-electron ORR, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data demonstrate that (phen2N2)Fe is a more effective model of Fe-N-C active sites relative to the pyrrolic iron macrocycles, thereby establishing a new molecular platform that can aid understanding of this important class of catalytic materials.

On the border between localization and delocalization: tris(iminoxolene)titanium(iv).

The tris(aminophenol) ligand tris(4-methyl-2-(3',5'-di-tert-butyl-2'-hydroxyphenylamino)phenyl)amine, MeClampH6, reacts with Ti(OiPr)4 to give, after exposure to air, the dark purple, neutral, diamagnetic complex (MeClamp)Ti. The compound is six-coordinate, with an uncoordinated central nitrogen (Ti-N = 2.8274(12) Å), and contains titanium(iv) and a doubly oxidized ligand, formally a bis(iminosemiquinone)-mono(amidophenoxide). The compound is unsymmetrical in the solid state, though the three ligands are equivalent on the NMR timescale in solution. Ab initio calculations indicate that the ground state is a multiconfigurational singlet, with a low-lying multiconfigurational triplet state. Variable-temperature NMR measurements are consistent with a singlet-triplet gap of 1200 ± 70 cm-1, in good agreement with calculations. The distortion from threefold symmetry allows a low-lying, partially populated ligand-centered π nonbonding orbital to mix with largely occupied metal-ligand π bonding orbitals. The energetic accessibility of this distortion is inversely related to the strength of the metal-ligand π bonding interaction.

Redox activity and π bonding in a tripodal seven-coordinate molybdenum(VI) tris(amidophenolate).

A tris(aminophenol), tris(2-(3',5'-di-tert-butyl-2'-hydroxyphenyl)amino-4-methylphenyl)amine, MeClampH6, is prepared in three steps from tri-p-tolylamine. The ligand reacts with dioxomolybdenum(VI) bis(acetylacetonate) to form an oxo-free heptadentate complex, (MeClamp)Mo, with a capped octahedral geometry. The molybdenum is formally in the +6 oxidation state, with significant π donation of the amidophenolates, as judged by intraligand bond distances. Two ligand-based oxidations and one metal-centered reduction are observed by cyclic voltammetry. Analysis of the optical spectrum of the compound gives an estimate of the energetic stabilization of the ligand π orbitals by bonding to the molybdenum of approximately 0.9 eV, corresponding to about 40 kcal mol(-1) per π bond.

Nonclassical oxygen atom transfer reactions of oxomolybdenum(VI) bis(catecholate).

Mechanistic studies indicate that the oxomolybdenum(vi) bis(3,5-di-tert-butylcatecholate) fragment deoxygenates pyridine-N-oxides in a reaction where the oxygen is delivered to molybdenum but the electrons for substrate reduction are drawn from the bound catecholate ligands, forming 3,5-di-tert-butyl-1,2-benzoquinone.