Elaboration on the Electronics of Salen Manganese Nitrides: Investigations into Alkoxy-Substituted Ligand Scaffolds

Abstract

The ligand electronics of salen manganese nitride complexes directly influence the locus of oxidation and, thus, the reactivity of the resulting oxidized species. This work investigates the influence of tert-butoxy, isopropoxy, and methoxy substituents on the electronics of salen manganese nitride species and includes the first documentation of the para Hammett value for the tert-butoxy substituent (σpara = −0.13 ± 0.03). Each alkoxy-substituted complex undergoes metal-based oxidation to form manganese­(VI), and the kinetics of bimolecular homocoupling to form N2 were assessed by cyclic voltammetry. Bis-oxidation of the manganese complexes was investigated at low temperature using cyclic voltammery and UV–vis–near-IR spectroscopy, and in combination with theoretical calculations, plausible electronic structures of the dications are provided.

Chromium Nitride Umpolung Tuned by the Locus of Oxidation

“…We have reported that oxidation of nitridomanganese­(V) salen complexes (“salen” being a common abbreviation for N 2 O 2 bis-phenolate bis-Schiff-base ligands) results in an electronic structure that can be tuned through the modulation of para -R phenolate substituents, without altering the geometry at the metal center. , Salen ligands exhibit redox-active behavior and have the potential to undergo oxidation (or reduction) at the ligand in place of the metal. − We, and others, have observed that the formation of a Mn VI salen nitride upon oxidation results in an activated MnN bond that rapidly homocouples to form N 2 , even at a low temperature. ,, Interestingly, para -R substitution with electron-donating substituents (R = NMe 2 ) resulted in ligand radical formation where homocoupling was not observed, even at room temperature . Recent reports have utilized Mn salen nitrides to study the hydrogen atom bond dissociation free energy of the associated imido complexes, formation of ammonia via proton-coupled electron transfer, − and as catalysts for ammonia oxidation .…”

Untangling ancillary ligand donation versus locus of oxidation effects on metal nitride reactivity

“…In accord with experimental data, calculations on the one-electron oxidized complexes predict a Mn VI electronic ground state for R = CF 3 , tBu, O i Pr, H, while a Mn V ligand radical ground state was predicted for R = NMe 2 , NEt 2 (vide supra). 62,63 We next evaluated the difference in nitride NPA charge between the neutral and oxidized forms, with the Mn VI derivatives exhibiting a much more signicant increase in nitride NPA charge and therefore electrophilicity upon oxidation in comparison to the Mn V ligand radical derivatives (Fig. 6).…”

“…Mn(Sal CF3 )N, Mn(Sal tBu )N, Mn(Sal OiPr )N and Mn(Sal NMe2 )N have been previously synthesized. 62,63 For the two new complexes, unsubstituted Mn(Sal H )N was prepared by the reaction of corresponding manganese chloride salt with NH 4 OH as reported by Carreira and coworkers (Scheme S1 and Fig. S1 †).…”

“…We have shown that the electronic structure of oxidized nitridomanganese(V) salen complexes (where "salen" represents N 2 O 2 bis-phenolate bis-Schiff-base ligands) can be tuned through the adjustment of the electron-donating ability of the para-R phenolate substituents, while preserving the geometry at the metal center. 62,63 Salen ligands exhibit redox-active behavior, enabling oxidation or reduction at the ligand in competition with the metal. [64][65][66][67][68][69][70] Upon oxidation, we and others observed the formation of a Mn VI salen nitride, leading to the activation of the Mn^N bond, and rapid nitride homocoupling to generate N 2 , even at low temperatures.…”

(Salen)Mn(III)-Catalyzed Enantioselective Intramolecular Haloamination of Alkenes through Chiral Aziridinium Ion Ring-Opening Sequence

“…When the solution was frozen after 10 min at 70K, it showed an EPR spectrum with peaks at effective gvalues (g ≈ 2) (Scheme 4, (e)). The aminium radical-cation (g = 2.0028) and triplet Mn ligand radical was observed for the spectrum of EPR, 18 which was consistent with the hypothesis that Mn−N intermediates would exist in triplet electronic ground states. At last, aziridine product 9 was assessed with styrene and p-toluenesulfonamide, 16 which revealed that the reaction of (Salen)Mn(III)-catalyzed enantioselective intramolecular haloamination of alkenes could generate a chiral aziridines intermediate.…”