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

Abstract

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)Mn V (N)(M n+ -crown ether)] n+ (salen = N,N′bis(salicydene)ethylenediamine; M = Na + , K + , Ba 2+ , and Sr 2+ for 1Na, 1K, 1Ba, and 1Sr, respectively; A = complex without M n+crown ether and B = without M n+ ). 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 ΔG rxn (e − ) and thus E 1/2 are quite sensitive to the point charge (q) of the s-block metal ions (1 for K + /Na + and 2 for Ba 2+ /Sr 2+ ). 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.

Cationic Effects on the Net Hydrogen Atom Bond Dissociation Free Energy of High-Valent Manganese Imido Complexes

“…A computational study by Cundari and co-workers on these complexes determined that an increase in charge at the bound cation resulted in an increase in the N−H BDFE and lower free energy barriers for hydrogen atom transfer (HAT) from methane. 72 Our experimentally determined BDFE values do not indicate an increasing N−H BDFE with higher cation charges. However, this may be due to our use of bracketed pK a values due to the instability of the imidos.…”

“…Although the determination of N–H bond strengths is challenging due to the reactive nature of the intermediate imido, these values are essential to predicting reactivity. A computational study by Cundari and co-workers on these complexes determined that an increase in charge at the bound cation resulted in an increase in the N–H BDFE and lower free energy barriers for hydrogen atom transfer (HAT) from methane . Our experimentally determined BDFE values do not indicate an increasing N–H BDFE with higher cation charges.…”

“…(a) Secondary coordination sphere effects on E 1/2 or p K a in metal hydroxides (refs and ). (b) Activation of salen manganese nitridos ((i) ref ; (ii) refs and ; (iii) ref and this work).…”

A turn-off fluorescent probe for the detection of Cu2+ based on a tetraphenylethylene-functionalized salicylaldehyde Schiff-base

“…Schiff-based compounds are excellent ligands to coordinate with metal ions (such as rare-earth metals, transition metals, alkali metals and alkali earth metals) with potential applications in organic catalysis, [29][30][31][32] and also have been widely explored as chemisensors for the detection of metal ions, due to their high binding affinity. 33,34 Herein, to test the detecting ability of the TPE-functionalized Schiff-base TPE-An-Py, the probe (1.00 Â 10 À4 mol L À1 ) with various cations (such as Cd 2+ , Cu 2+ , K + , Pb 2+ , Li + , Fe 3+ , Ca 2+ , Mg 2+ , Co 2+ , Cr 3+ , Al 3+ , Ba 2+ , Ni 2+ , Zn 2+ , Ag + , Na + , Hg + , [M] n+ = 2.00 Â 10 À4 mol L À1 ) was evaluated in mixtures of THF and water (V THF /V water = 4/1, pH = 7.00).…”

Cationic Effects on the Effective Hydrogen Atom Bond Dissociation Free Energy of High Valent Manganese Imido Complexes

“…In a recent computational study from Cundari and coworkers on these crown-appended manganese nitrido salen complexes, they determined that increase in charge at the bound cation resulted in an increase in N-H BDFE. 97 Further, an increase in charge of the cation was correlated to lowering of the free barrier energy for hydrogen atom transfer (HAT) from methane, thus indicating that this process should be kinetically facilitated by the presence of bound cation. Therefore, we explored the hydrogen atom abstraction (HAA) reactivity of the Mn(VI) complexes with a hydrogen atom donor, 9,10-dihydroanthracene (DHA, BDFE (DMSO) = 76 kcal/mol) 17 (Figure 4).…”