Proton Control of Oxidation and Spin State in a Series of Iron Tripodal Imidazole Complexes

Reaction of iron salts with three tripodal imidazole ligands, H3(1), H3(2), H3(3), formed from the condensation of tris(2-aminoethyl)amine (tren) with 3 equiv of an imidazole carboxaldehyde yielded eight new cationic iron(III) and iron(II), [FeH3L]3+or2+, and neutral iron(III), FeL, complexes. All complexes were characterized by EA(CHN), IR, UV, Mössbauer, mass spectral techniques and cyclic voltammetry. Structures of three of the complexes, Fe(2)·3H2O (C18H27FeN10O3, a = b = c = 20.2707(5), cubic, I4̄3d, Z = 16), Fe(3)·4.5H2O (C18H30FeN10O4.5, a = 20.9986(10), b = 11.7098(5), c = 19.9405(9), β = 109.141(1), monoclinic, P2(1)/c), Z = 8), and [FeH3(3)](ClO4)2·H2O (C18H26Cl2FeN10O9, a = 9.4848(4), b = 23.2354(9), c = 12.2048(5), β = 111.147(1)°, monoclinic, P2(1)/n, Z = 4) were determined at 100 K. The structures are similar to one another and feature an octahedral iron with facial coordination of imidazoles and imine nitrogen atoms. The iron(III) complexes of the deprotonated ligands, Fe(1), Fe(2), and Fe(3), are low-spin while the protonated iron(III) cationic complexes, [FeH3(1)](ClO4)3 and [FeH3(2)](ClO4)3, are high-spin and spin-crossover, respectively. The iron(II) cationic complexes, [FeH3(1)]S4O6, [FeH3(2)](ClO4)2, [FeH3(3)](ClO4)2, and [FeH3(3)][B(C6H5)4]2 exhibit spin-crossover behavior. Cyclic voltammetric measurements on the series of complexes show that complete deprotonation of the ligands produces a negative shift in the Fe(III)/Fe(II) reduction potential of 981 mV on average. Deprotonation in air of either cationic iron(II) or iron(III) complexes, [FeH3L]3+or2+, yields the neutral iron(III) complex, FeL. The process is reversible for Fe(3), where protonation of Fe(3) yields [FeH3(3)]2+.