All Four Atropisomers of Iron Tetra(o-N,N,N-trimethylanilinium)porphyrin in Both the Ferric and Ferrous States

Abstract

Electrostatic effects are key to many biological and (electro)­chemical transformations, especially those that involve charged species. The position and orientation of the electric field with respect to the molecules undergoing charge rearrangement are often crucial to the progress of the reaction. Recently, several molecular (electro)­catalysts have been designed to contain spatially positioned charged groups that can engage in specific intramolecular electrostatic interactions. For instance, iron complexes of the tetra­(o-N,N,N-trimethylanilinium)­porphyrin ligand, which has four cationic groups, have been used to great effect for both CO2 and O2 reduction. Because of the ortho-substitution pattern on the porphyrin ligand, there are four possible atropisomerssuch as the αβαβ isomer with trimethylanilinium groups on alternating faces of the porphyrinand thus four unique electrostatic environments. This study details the synthesis and characterization (1H NMR spectroscopy, single crystal X-ray diffraction, and cyclic voltammetry) of these four metalloporphyrin isomers in both the ferric (FeIII) and ferrous (FeII) forms by using a synthetic route that preserves atropisomeric purity. The atropisomers are different in some respects but show remarkable similarities in others, such as their reduction potentials. This study also shows that the widely-cited literature method used previously to prepare the molecular electrocatalyst for CO2 and O2 reduction yields a mixture of atropisomers rather than a single one, as was previously assumed. These results identify the ways in which intra- and intermolecular electrostatic effects affect both solution and solid-state properties as well underscoring the challenges associated with preparing metalloporphyrins with high atropisomeric purity.

Oriented internal electrostatic fields: an emerging design element in coordination chemistry and catalysis

“…11). [69][70][71][72] The authors found no catalytic improvement over other iron porphyrin electrocatalysts, corroborating the study discussed above in which O 2 binding was (nearly) unaffected by the presence of cationic groups in the secondary coordination sphere. 68 Previous studies with iron porphyrin ORR electrocatalysts established scaling relationships between log(TOF max ) and effective overpotential under various reaction conditions, wherein changes in acid concentration, acid pK a , O 2 concentration, or catalyst E 1/2 values led to increases in log(TOF max ) at the expense of effective overpotential, or vice versa.…”

Electric Fields in Catalysis: From Enzymes to Molecular Catalysts

“…Reported homogeneous transition metal complexes with charged functionalities that contribute to changes in electronic structure or reactivity. Left, top-to-bottom: Agapie (refs and ), Gilbertson (ref ), Mayer (ref and ), Groves (refs and ), Shaik (ref ), McCrory (ref ); Right, top-to-bottom: Tomson (ref ), Savéant (ref ), Warren (ref ), Tolman (ref ), Wiedner (ref ).…”

“…For the former, computational studies indicate the cationic functionalities stabilize anionic intermediates . In the latter example, the proximal cations play a complex role in pre-equilibria substrate binding due to through-space electrostatic and inductive contributions. , Additionally, Groves and co-workers reported Fe porphyrin complexes with cationic porphyrins that were exceptionally active for C–H activation. , A subsequent computational study by Shaik and co-workers indicated that the electric field generated by the cationic functionalities selectively destabilized the reactants more than the transition states, leading to the accelerated rate …”

Melding of Experiment and Theory Illuminates Mechanisms of Metal-Catalyzed Rearrangements: Computational Approaches and Caveats

“…Similar issues have been described for 4-fold symmetric iron complexes used for CO 2 and O 2 reduction. 122,123 One approach to avoiding the problem of conformational complexity is to carry out calculations using small, inflexible ligands, e.g., formate groups, acetate groups, or truncated chiral ligands. While this approach is often reasonable, one must be exceedingly cautious when employing it to assure that important substrate-ligand interactions are not missed and that the shape of the catalyst cavity is not compromised.…”