Activation of ammonia and hydrazine by electron rich Fe() complexes supported by a dianionic pentadentate ligand platform through a common terminal Fe() amido intermediate

• Published in: Chemical science (Cambridge) 2020-12, Vol.12 (6), p.2231-2241
• Main Authors: Nurdin, Lucie, Yang, Yan, Neate, Peter G. N, Piers, Warren E, Maron, Laurent, Neidig, Michael L, Lin, Jian-Bin, Gelfand, Benjamin S
• Format: Article
• Language: English
• Subjects: Ammonia; Cartesian coordinates; Chemical Physics; Chemistry; Coordination compounds; Crystallography; Dimers; Hydrazines; Hydrogen atoms; Iron; Ligands; Nitrogen; Oxidation; Physics; Radicals; Room temperature; Transition metals
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d0sc06466a

We report the use of electron rich iron complexes supported by a dianionic diborate pentadentate ligand system, B 2 Pz 4 Py , for the coordination and activation of ammonia (NH 3 ) and hydrazine (NH 2 NH 2 ). For ammonia, coordination to neutral (B 2 Pz 4 Py)Fe( ii ) or cationic [(B 2 Pz 4 Py)Fe( iii )] + platforms leads to well characterized ammine complexes from which hydrogen atoms or protons can be removed to generate, fleetingly, a proposed (B 2 Pz 4 Py)Fe( iii )-NH 2 complex ( 3 Ar -NH 2 ). DFT computations suggest a high degree of spin density on the amido ligand, giving it significant aminyl radical character. It rapidly traps the H atom abstracting agent 2,4,6-tri- tert -butylphenoxy radical (ArO&z.rad;) to form a C-N bond in a fully characterized product ( 2 Ar ), or scavenges hydrogen atoms to return to the ammonia complex (B 2 Pz 4 Py)Fe( ii )-NH 3 ( 1 Ar -NH 3 ). Interestingly, when (B 2 Pz 4 Py)Fe( ii ) is reacted with NH 2 NH 2 , a hydrazine bridged dimer, (B 2 Pz 4 Py)Fe( ii )-NH 2 NH 2 -Fe( ii )(B 2 Pz 4 Py) ( (1 Ar ) 2 -NH 2 NH 2 ), is observed at −78 °C and converts to a fully characterized bridging diazene complex, 4 Ar , along with ammonia adduct 1 Ar -NH 3 as it is allowed to warm to room temperature. Experimental and computational evidence is presented to suggest that (B 2 Pz 4 Py)Fe( ii ) induces reductive cleavage of the N-N bond in hydrazine to produce the Fe( iii )-NH 2 complex 3 Ar -NH 2 , which abstracts H&z.rad; atoms from (1 Ar ) 2 -NH 2 NH 2 to generate the observed products. All of these transformations are relevant to proposed steps in the ammonia oxidation reaction, an important process for the use of nitrogen-based fuels enabled by abundant first row transition metals. Synopsis: a highly reactive Fe( iii )-NH 2 complex is generated via activation of ammonia or hydrazine in reactions of relevance to fundamental steps in ammonia oxidation processes mediated by an abundant, first row transition metal.