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

Abstract

Local electric fields can alter energy landscapes to impart enhanced reactivity in enzymes and at surfaces. Similar fields can be generated in molecular systems using charged functionalities. Manganese(V) salen nitrido complexes (salen = N,N′ethylenebis(salicylideneaminato)) appended with a crown ether unit containing Na + (1-Na), K + , (1-K), Ba 2+ (1-Ba), Sr 2+ (1-Sr), La 3+ (1-La), or Eu 3+ (1-Eu) cation were investigated to determine the effect of charge on pK a , E 1/2 , and the net bond dissociation free energy (BDFE) of N−H bonds. The series, which includes the manganese(V) salen nitrido without an appended crown, spans 4 units of charge. Bounds for the pK a values of the transient imido complexes were used with the Mn(VI/V) reduction potentials to calculate the N−H BDFEs of the imidos in acetonitrile. Despite a span of >700 mV and >9 pK a units across the series, the hydrogen atom BDFE only spans ∼6 kcal/mol (between 73 and 79 kcal/mol). These results suggest that the incorporation of cationic functionalities is an effective strategy for accessing wide ranges of reduction potentials and pK a values while minimally affecting the BDFE, which is essential to modulating electron, proton, or hydrogen atom transfer pathways.

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

“…Recently, the Yang group explored electrostatic effects on C-H activation for a series of manganese nitride complexes supported by their salen-based ligand scaffold. 91 By varying the charge in the crown ether moity from Na + to Eu 3+ , the reduction potentials of the complexes were found to shi anodically by upwards of 730 mV compared to the analogous non-crown salen manganese nitrido species. However, increasing the charge was shown to decrease the basicity of the manganese(v) nitride species, resulting in relatively consistent bond dissociation free energies across the series.…”

“…15 Mn V nitride complexes that were shown to display inverse scaling relationships between Mn V /Mn IV redox potentials and the rate of N 2 release upon oxidation. 87,91 Fig. 16 Top: major resonance contributors to the ground state of a novel phosphine selenide architecture bearing an appended -BF 3 À unit.…”

Electrostatic vs. inductive effects in phosphine ligand donor properties and reactivity

“…Indeed, there has been enormous interest in modeling or leveraging electrostatic interactions in organic and inorganic molecules. 24–46 Computations predict that electrostatic effects can have a large impact on reactivity and catalysis. 47–61…”

Thermochemical Studies of Nickel Hydride Complexes with Cationic Ligands in Aqueous and Organic Solvents

“…The desired solubility properties were achieved by adding charged functional groups, 19 which are commonly used to engender water solubility to organometallic complexes, although they can change other thermochemical properties. 20 Specifically, nickel complexes with two diphosphine ligands containing an amine in the backbone were singly or doubly methylated (Scheme 1). These highly charged nickel complexes enabled the study of nickel hydride BDFEs in aqueous and organic solvents.…”