Selective formation of either Tröger’s base or spiro Tröger’s base derivatives from [2-aminoporphyrinato(2-)]nickel by choice of reaction conditions

Spectroscopic study on enantiomeric Ni(II) complexes of porphyrin-porphyrin Tröger's base and porphyrin-chlorin spiro-Tröger's base derivatives supported by DFT calculations

The chiral properties of nickel(II) complexes of porphyrin-porphyrin Tröger's base and porphyrin-chlorin spiro-Tröger's base with phenyl or 3-methoxyphenyl substitutions in their meso-positions were studied. Enantioseparation of racemic mixtures was investigated via high-performance liquid chromatography (HPLC) on an analytical ReproSil Chiral-NR column. The optimal conditions were utilized for a multimilligram scale isolation with a semipreparative column. The purity of the isolated enantiomers was determined by HPLC and UV-Vis spectroscopy. The absolute configurations of the isolated enantiomers were determined by evaluating the Cotton effect in electronic circular dichroism spectra. The determination was supported by TDDFT calculations, in which good agreement was achieved between the experimental and simulated spectra. The maximum molar ellipticity values, [θ]λmax given in deg ∙ cm2 ∙ dmol-1, were [θ]435 = 1.73 ∙ 107 for phenyl spiroTB and [θ]436 = 1.24 ∙ 107 and [θ]436 = 2.15 ∙ 107 for 3-methoxyphenyl TB and spiroTB, respectively.

Unexpected Solvent Effect in Electrocatalytic CO2 to CO Conversion Revealed Using Asymmetric Metalloporphyrins

Rapid and efficient electrochemical CO₂ reduction is an ongoing challenge for the production of sustainable fuels and chemicals. In this work, electrochemical CO₂ reduction is investigated using metalloporphyrin catalysts (metal = Mn, Fe, Co, Ni, Cu) that feature one hydroxyphenyl group, and three other phenyl groups, in the porphyrin heterocycle (5-(2-hydroxyphenyl)-10,15,20-triphenylporphyrin, TPOH). These complexes, which are minimal versions of related complexes bearing up to eight proton relays, were designed to allow more straightforward determination of the role of the 2-hydroxylphenyl functional group. The iron-substituted version of TPOH supports robust reduction of CO₂ in acetonitrile solvent, where carbon monoxide is the only detected product. Addition of weak Brønsted acids (1 M water or 8 mM phenol) gives rise to almost 100-fold enhancement in turnover frequency. Surprisingly, the iron analogue is a poor catalyst when the solvent is changed to dimethylformamide. These results lead to the proposal of a model where the hydroxyphenyl group behaves as a local proton source, a hydrogen bond donor to CO₂-bound intermediates, and a hydrogen bonding partner to Brønsted acids. The observations from this model suggest improvements for existing electrocatalytic CO₂ reduction systems.

Nitrogen-Bridged Metallodiazaporphyrin Dimers: Synergistic Effects of Nitrogen Bridges and meso-Nitrogen Atoms on Structure and Properties

NH-bridged and pyrazine-fused metallodiazaporphyrin dimers have been prepared from nickel(II) and copper(II) complexes of 3-amino-5,15-diazaporphyrin by Pd-catalyzed C-N cross-coupling and oxidative dimerization reactions, respectively. The synergistic effects of the nitrogen bridges and meso-nitrogen atoms play major roles in enhancing the light-harvesting properties and delocalization of an electron spin over the entire π-skeletons of the metallodiazaporphyrin dimers.

Accessing N-Stereogenicity through a Double Aza-Michael Reaction: Mechanistic Insights

Further development of the chemistry and applications of chiral compounds that possess configurationally stable stereogenic nitrogen atoms is hampered by the lack of efficient strategies to access such compounds in an enantiomerically pure form. Esters of propiolic acid and chiral alcohols were evaluated as cheap and readily available Michael acceptors in a diastereoselective synthesis of N-stereogenic compounds by means of a double aza-Michael conjugate addition. Diastereomeric ratios of up to 74:26 and high yields were achieved with (-)-menthyl propiolate as a substrate. Furthermore, a detailed mechanistic investigation was undertaken to shed some light on the course of this domino transformation. Kinetic studies revealed that the protic-solvent additive acts as a Brønsted acid and activates the ester toward the initial attack of the tetrahydrodiazocine partner. Conversely, acidic conditions proved unfavorable during the final cyclization step that provides the product.