An electrochemical investigation of intermediates and processes involved in the catalyticreduction of dinitrogen by [HIPTN3N]Mo (HIPTN3N = (3,5-(2,4,6-i-Pr3C6H2)2C6H3NCH2CH2)3N)

Abstract

The redox properties of [HIPTN₃N]Mo complexes (where HIPTN₃N = (3,5-(2,4,6-i-Pr₃C₆H₂)₂C₆H₃NCH₂CH₂)₃N) involved in the catalytic dinitrogen reduction cycle were studied using cyclic voltammetry in fluorobenzene with Bu₄NPF₆ as the electrolyte. MoN₂ (Mo = [HIPTN₃N]Mo, E_1/2 = −1.96 V vs. Fc⁺/Fc at a Pt electrode), Mo≡N (E_1/2 = −2.68 V vs. Fc⁺/Fc (Pt)), and [Mo(NH₃)]BAr′₄ (Ar′ = 3,5-(CF₃)₂C₆H₃, E_1/2 = −1.53 V vs. Fc⁺/Fc (Pt)) each undergo a chemically reversible one-electron reduction, while [MoNNH₂]BAr′₄ (E_1/2 = −1.50 V vs. Fc⁺/Fc (Pt)) and [MoNH]BAr′₄ (E_1/2 = −1.26 V vs. Fc⁺/Fc (Pt)) each undergo a one-electron reduction with partial chemical reversibility. The acid employed in the catalytic reduction, [2,4,6-collidinium]BAr′₄, reduces irreversibly at −1.11 V vs. Fc⁺/Fc at Pt and at −2.10 V vs. Fc⁺/Fc at a glassy carbon electrode. The reduction peak potentials of the Mo complexes shift in the presence of acids. For example, the reduction peak for MoN₂ in the presence of [2,4,6-collidinium]BAr′₄ at a glassy carbon electrode shifts positively by 130 mV. The shift in reduction potential is explained in terms of reversible hydrogen bonding and/or protonation at a nitrogen site in Mo complexes. The significance of productive and unproductive proton-coupled electron transfer reactions in the catalytic dinitrogen reduction cycle is discussed.