Abstract
The splitting of N2 into well‐defined terminal nitride complexes is a key reaction for nitrogen fixation at ambient conditions. In continuation of our previous work on rhenium pincer mediated N2 splitting, nitrogen activation and cleavage upon (electro)chemical reduction of [ReCl2(L2)] {L2 = N(CHCHPtBu2)2 –} is reported. The electrochemical characterization of [ReCl2(L2)] and comparison with our previously reported platform [ReCl2(L1)] {L1 = N(CH2CH2PtBu2)2 –} provides mechanistic insight to rationalize the dependence of nitride yield on the reductant. Furthermore, the reactivity of N2 derived nitride complex [Re(N)Cl(L2)] with electrophiles is presented.
Highlights
Industrial ammonia synthesis by the Haber–Bosch process is carried out at a scale of 150 Mt/a, using hydrogen produced via steam reforming of fossil fuels that accounts for massive energy consumption and CO2 emission.[1]
In analogy to other rhenium(III) phosphine complexes and 1L1,[12h,17] the high-field shift is attributed to mixing of the ground-state with low-lying excited states leading to temperature independent paramagnetism (TIP),[19] as substantiated for 1L1 and 1L2 by SQUID magnetometry { M[10–6 × cm3 mol–1] = 280 (1L1), 300 (1L2); Figure S26}
A strong dependence of the nitrogen splitting yield on the nature of chemical reductants (CoCp*2: 60 %, Na/Hg: 30 %, KC8: 20 %) or electrolysis (15 %) was found, which markedly differs from parent 1L1 (CoCp*2: 75 %, Na/Hg: 80 %, electrolysis: 60 %)
Summary
Industrial ammonia synthesis by the Haber–Bosch process is carried out at a scale of 150 Mt/a, using hydrogen produced via steam reforming of fossil fuels that accounts for massive energy consumption and CO2 emission.[1] The replacement of H2 as reductant is highly desirable to enhance the sustainability of nitrogen fixation. The electrochemically driven nitrogen reduction reaction (NRR) is an appealing alternative to feed renewable energy from photovoltaic harvesting.[2] Electrocatalytic NRR has seen tremendous progress in recent years. Faradaic yields up to 73.3 % have been reported, yet with current densities far below the US Department of Energy targets.[3,4] the mechanistic basis of heterogeneous electrocatalysts remains comparatively ill-defined. Molecular NRR electrocatalysts are highly limited.[6]
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