Group 13 ion coordination to pyridyl breaks the reduction potential vs. hydricity scaling relationship for dihydropyridinates

Abstract

The relationship E_pvs. ΔG_H– correlates the applied potential (E_p) needed to drive organohydride formation with the strength of the hydride donor that is formed: in the absence of kinetic effects E_pvs. ΔG_H– should be linear but it would be more energy efficient if E_p could be shifted anodically using kinetic effects. Biological hydride transfers (HT) performed by cofactors including NADH and lactate racemase do occur at low potentials and functional modeling of those processes could lead to low energy HT reactions in electrosynthesis and to accurate models for cofactor chemistry. Herein we probe the influence of N-alkylation or N-metallation on ΔG_H– for dihydropyridinates (DHP⁻) and on E_p of the DHP⁻ precursors. We synthesized a series of DHP⁻ complexes of the form (pz₂^HP⁻)E via hydride transfer from their respective [(pz₂P)E]⁺ forms where E = AlCl₂⁺, GaCl₂⁺ or Me⁺. Relative ΔG_H– for the (pz₂^HP⁻)E series all fall within 1 kcal mol⁻¹, and ΔG_H– for (pz₂^HP)CH₃ was approximated as 47.5 ± 2.5 kcal mol⁻¹ in MeCN solution. Plots of E_pvs. ΔG_H– including [(pz₂P)E]⁺ suggest kinetic effects shift E_p anodically by ∼215 mV.