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

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˙) 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˙ 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.