Effect of Appended S‑Block Metal Ion Crown Ethers on Redox Properties and Catalytic Activity of Mn–Nitride Schiff Base Complexes: Methane Activation

Using density functional
theory (DFT), the effects of appended
s-block metal ion crown ethers upon the redox properties of the following
nitridomanganese­(V) salen complexes were investigated: [(salen)­MnV(N)­(Mn+-crown ether)]n+ (salen = N,N′-bis­(salicydene)­ethylenediamine;
M = Na+, K+, Ba2+, and Sr2+ for 1Na, 1K, 1Ba, and 1Sr, respectively; A = complex without Mn+-crown ether and B = without
Mn+). NBO analysis of the MnN bond orders,
optimized bond lengths, and stretching frequencies changes upon oxidation
for all species show that for A, B, and 1Na MnN has more nitridyl character while a nitride form is
more significant for 1K, 1Ba, and 1Sr. The results reveal that ΔGrxn(e–) and thus E1/2 are quite sensitive to the point charge (q) of
the s-block metal ions (1 for K+/Na+ and 2 for
Ba2+/Sr2+). Computations suggest that the degree
of delocalization of the HOMO electrons on the supporting ligand is
modified by the chelated s-block metal ion. Methane activation by A•+, 1K•+, and 1Ba•+ complexes proceeds via a hydrogen atom transfer (HAT) pathway
with reasonable barriers for all complexes with ∼4 kcal/mol
difference in energy. The molecular electrostatic potential (MEP)
maps indicate a shift in redox potential imposed by the nonredox active
cations by altering the electrostatic potential of the complexes.
Computations show that the complexes with higher point charge of the
incorporated metal ions result in higher N–H bond BDFEs. Changes
in predicted properties as a function of continuum solvent dielectric
constant suggest that the primary effect of the appended s-block ion
is via “through space” interactions.