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(piCy)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(afaCy)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(piCy)3)­Fe­(OH2)] (3) illustrated
the propensity of the ligand to be hydrogen bond-accepting, whereas
the iron triflate species [N­(afaCy)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(afaCy)2­(piCy)]­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.