Isolation of iron(II) aqua and hydroxyl complexes featuring a tripodal H-bond donor and acceptor ligand

A tripodal ligand platform, tris(5-cycloiminopyrrol-2-ylmethyl)amine (H3[N(pi(Cy))3]), that features a hydrogen bond-accepting secondary coordination sphere when bound anionically to an iron center is reported. Neutral coordination to iron affords ligand tautomerization, resulting in a hydrogen bond-donating secondary coordination sphere, and formation of the tris(5-cyclohexyl-amineazafulvene-2-methyl)amine, H3[N(afa(Cy))3], scaffold. Both binding motifs result in formation of stable, high-spin iron(II) complexes featuring ancillary water, triflate, or hydroxo ligands. Structural analysis reveals that these complexes exhibit distorted trigonal-bipyramidal geometries with coordination of the apical nitrogen to iron as well as three equatorial amine or imine nitrogens, depending on the axial ancillary ligand. Formation of the aqua complex K[(N(pi(Cy))3)Fe(OH2)] (3) illustrated the propensity of the ligand to be hydrogen bond-accepting, whereas the iron triflate species [N(afa(Cy))3Fe](OTf)2 (4) features a hydrogen bond-donating secondary coordination sphere. The ability of each of the three arms of the ligand to tautomerize independently was observed during the formation of the iron-hydroxyl species [N(afa(Cy))2(pi(Cy))]FeOH (5) and characterized by X-ray crystallography and IR spectroscopy. The combined data for the iron complexes established that each arm of the tripodal ligand can tautomerize independently and is likely dependent on the electronic needs of the iron center when binding various substrates.