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New in nitrogen reduction

19 September 2019 - The long game

miller 2019 nitrogen reduction.JPGNew nitrogen reduction paper out on ChemRxiv (Bruch et al. DOI). See earlier nitrogen reduction coverage here and here. Alexander Miller heads the Miller group at UNC and has been publishing about catalysis since 2005. The publication trail almost inevitably has been leading up to nitrogen reduction with articles covering topics such as reductions, photocatalysis and pincer compounds. The latest effort appears to be following a lead from a 2017 publication about ammonia synthesis from a pincer ruthenium nitride PCET reaction (DOI).
In the new work ammonia was synthesised from dinitrogen is four distinct steps. In the first catalyst forming step (acetonitrile)trichlorobis(triphenylphosphine)rhenium(III) was combined with pincer ligand bis(diisopropylphosphinito)pyridine (PONOP) to form the octahedral complex (PONOP)ReCl3 with Rhenium coordinating to three chlorine atoms, two phosphines and one pyridine nitrogen (70°C / 16 hours). This complex is found to be easily reducable and the PONOP part certainly helps. Lithium triethylborohydride is a well known reducing agent, with co-reagent dinitrogen a nitrogen-bridged complex was obtained with dinitrogen flanked by rhenium complexes with dichloride loss. This was again an isolable complex.
From there the next step was cleavage along the dinitrogen section to a rhenium nitride. Brute thermal force did not help except for cis-trans isomerisation to two distinct isomers. All theory and prior art had suggested the rhenium-nitrogen complex should have no trouble forming a nitride and one can imagine the researchers got stuck at this point. This cliffhanger moment passed when the complex was exposed not to heat but to photons. Under a blue led light the it took 9 days for only trace amount to form of the original isomer but the thermodynamically most stable isomer demonstrated the highest quantum yield and a 47% ultimate yield.
The final step required the nitride (nitrogen triple bonded to the metal) to liberate it's nitrogen as ammonia, a step involving proton-coupled electron transfer or PCET. This concept was introduced by T.J. Meyer in 1981 (DOI) who needed to describe a certain comproportionation reaction in which a Ru(IV)(O) complex reacted with a Ru(II)(OH2) complex with formation of Ru(III)(OH). The reaction type has been found to be a common one, water splitting is also a PCET reaction as is photosynthesis. In this reaction type the transfer of one electron and one proton is concerted, the kinetic isotope effect is large and the reaction does not tend to depend on the pH.
The 2019 PCET incarnation with 10 equivalents of a well-known reducing agent Samarium(II) iodide and 100 equivalents of water (though barely an acid) saw the nitrogen atom leaving with four protons to the ammonium ion and the rhenium atom leaving with one iodine and 4 hydrogen substituents. Overall ammonia yield: 74%.