Activation of ammonia and hydrazine by electron rich Fe(ii) complexes supported by a dianionic pentadentate ligand...

We report the use of electron rich iron complexes supported by a dianionic diborate pentadentate ligand system, B2Pz4Py, for the coordination and activation of ammonia (NH₃) and hydrazine (NH₂NH₂). For ammonia, coordination to neutral (B₂Pz₄Py)Fe(ii) or cationic [(B₂Pz₄Py)Fe(iii)]⁺ platforms leads to well characterized ammine complexes from which hydrogen atoms or protons can be removed to generate, fleetingly, a proposed (B₂Pz₄Py)Fe(iii)-NH₂ complex (3Ar-NH2). 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˙) to form a C-N bond in a fully characterized product (2Ar), or scavenges hydrogen atoms to return to the ammonia complex (B₂Pz₄Py)Fe(ii)-NH₃ (1Ar-NH3). Interestingly, when (B₂Pz₄Py)Fe(ii) is reacted with NH₂NH₂, a hydrazine bridged dimer, (B₂Pz₄Py)Fe(ii)-NH₂NH₂-Fe(ii)(B₂Pz₄Py) ((1Ar)2-NH2NH2), is observed at -78 °C and converts to a fully characterized bridging diazene complex, 4Ar, along with ammonia adduct 1Ar-NH3 as it is allowed to warm to room temperature. Experimental and computational evidence is presented to suggest that (B₂Pz₄Py)Fe(ii) induces reductive cleavage of the N-N bond in hydrazine to produce the Fe(iii)-NH₂ complex 3Ar-NH2, which abstracts H˙ atoms from (1Ar)2-NH2NH2 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.