Mechanism of Chemical and Electrochemical N₂ Splitting by a Rhenium Pincer Complex

Abstract

A comprehensive mechanistic study of N2 activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl2(PNP)] is presented (PNP– = N(CH2CH2PtBu2)2–). Low-temperature studies using chemical reductants enabled full characterization of the N2-bridged intermediate [{(PNP)ClRe}2(N2)] and kinetic analysis of the N–N bond scission process. Controlled potential electrolysis at room temperature also resulted in formation of the nitride product [Re(N)Cl(PNP)]. This first example of molecular electrochemical N2 splitting into nitride complexes enabled the use of cyclic voltammetry (CV) methods to establish the mechanism of reductive N2 activation to form the N2-bridged intermediate. CV data was acquired under Ar and N2, and with varying chloride concentration, rhenium concentration, and N2 pressure. A series of kinetic models was vetted against the CV data using digital simulations, leading to the assignment of an ECCEC mechanism (where “E” is an electrochemical step and “C” is a chemical step) for N2 activation that proceeds via initial reduction to ReII, N2 binding, chloride dissociation, and further reduction to ReI before formation of the N2-bridged, dinuclear intermediate by comproportionation with the ReIII precursor. Experimental kinetic data for all individual steps could be obtained. The mechanism is supported by density functional theory computations, which provide further insight into the electronic structure requirements for N2 splitting in the tetragonal frameworks enforced by rigid pincer ligands.

Biomimetic Nitrogen Fixation Catalyzed by Transition Metal Sulfide Surfaces in an Electrolytic Cell

“…However, there are no barriers on CrS and NbS (0.02 and0 .0 eV,r espectively), and the dissociation would be highly facile on these surfaces. The pathway towards ammonia formation by this mechanism is also explored on these sulfides by Equations (9)- (15). Figure 4s hows the free energy diagrams for the NRR to ammonia in which the activation energies of dinitrogen dissociation (E a, N-N )a re included.…”

“…[7][8][9][10] If electricity is supplied from clean energy sourcess uch as solar,w ind, geothermal, or hydropower,t he electrochemical process can successfully use renewable energy for ammonia synthesis. [11,12] Despite all the effortst oi mprovet he thermodynamics and optimize the rate and efficiency of NH 3 formation, [13][14][15][16][17][18][19][20][21][22][23][24] achieving production rates suitable for commercialization (4.3-8.7 10 À7 mol s À1 cm À2 with efficiencies > 50 %) [8] under ambient conditions is still a challenge. The nitrogen reduction reaction (NRR) to ammonia in aqueous electrolytes under ambient conditions is challenging owing to the exceptionals tability of the N 2 triple bond along with the presence of the hydrogen evolution reaction (HER) as ac ompeting side reaction.…”

Oxidative Coupling of Terminal Rhenium Pnictide Complexes

“…[17] Facile nitride formation by N À Nb ond cleavage of [(m-N 2 ){ReCl(PNP)} 2 ] emphasizes strong rhenium-to-pnictide multiple bonding and motivated the examination of higher homologues. [17] Facile nitride formation by N À Nb ond cleavage of [(m-N 2 ){ReCl(PNP)} 2 ] emphasizes strong rhenium-to-pnictide multiple bonding and motivated the examination of higher homologues.…”

“…Our group has examined the mechanism of dinitrogen splitting upon reduction of rhenium(III) pincer complex ReCl 2 (PNP) (PNP = N{CH 2 CH 2 P t Bu 2 } 2 ; 1 Cl ). [17] Facile nitride formation by N À Nb ond cleavage of [(m-N 2 ){ReCl(PNP)} 2 ] emphasizes strong rhenium-to-pnictide multiple bonding and motivated the examination of higher homologues. 1 Cl and the analogous iodide [ReI 2 (PNP)] (1 I )w ere reacted with the phosphide transfer reagent [Na(diox) x ]PCO.…”

“…[18] Similar potentials were found for 3 As ,y et with increased reversibility of the second wave. (17),R e1-N2 2.1208 (17),R e1-P1 2.4223(6), Re1-P2 2.4280(6), Re1-P3 2.0939(6);N1-Re1-N2 148.29 (7), N1-Re1-P3 110.66 (5), P1-Re1-P2 154.820 (19); 3 As :R e1-N1 1.986(2), Re1-N2 2.123(2), Re1-P1 2.4227(8), Re1-P2 2.4288 (7), Re1-As1 2.2021 (3); N1-Re1-N2 148.60 (9), N1-Re1-As1 110.60 (7), P1-Re1-P2 154.86(3); 5 As :Re1-N1 1.951(4), Re1-N2, 2.173 (3), Re1-N3 2.139 (3), Re1-P1 2.4482 (12), Re1-P2 2.4743 (12), Re1-As1 2.4421(5), As1-As2 2.2422 (7), As2-N4 1.943(4);N1-Re1-N2 90.98(13), N1-Re1-N3 162.94 (14), N1-Re1-As1 115.31 (10),R e1-As1-As2 109.83 (2), As1-As2-N4 93.75 (11), P1-Re1-P2 160.86(4).…”