Proton control of oxidation and spin state in a series of iron tripodal imidazole complexes

Abstract

Reaction of iron salts with three tripodal imidazole ligands, H(3)(1), H(3)(2), H(3)(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), [FeH(3)L](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).3H(2)O (C(18)H(27)FeN(10)O(3), a = b = c = 20.2707(5), cubic, I3d, Z = 16), Fe(3).4.5H(2)O (C(18)H(30)FeN(10)O(4.5), a = 20.9986(10), b = 11.7098(5), c = 19.9405(9), beta = 109.141(1), monoclinic, P2(1)/c), Z = 8), and [FeH(3)(3)](ClO(4))(2).H(2)O (C(18)H(26)Cl(2)FeN(10)O(9), a = 9.4848(4), b = 23.2354(9), c = 12.2048(5), beta = 111.147(1) degrees, 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, [FeH(3)(1)](ClO(4))(3) and [FeH(3)(2)](ClO(4))(3), are high-spin and spin-crossover, respectively. The iron(II) cationic complexes, [FeH(3)(1)]S(4)O(6), [FeH(3)(2)](ClO(4))(2), [FeH(3)(3)](ClO(4))(2), and [FeH(3)(3)][B(C(6)H(5))(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, [FeH(3)L](3+or2+), yields the neutral iron(III) complex, FeL. The process is reversible for Fe(3), where protonation of Fe(3) yields [FeH(3)(3)](2+).

Figure 1

Line drawings of ligands.

Figure 2

Titration of [FeH₃(1)](ClO₄)₃ with standard NaOH. Darkened circles represent mV and open circles represent the second derivative. Conditions are as described in the Experimental Section.

Figure 3

Titration of [FeH₃(3)](BPh₄)₂with standard NaOH. Darkened circles represent mV and open circles represent the second derivative. Conditions are described in the Experimental Section.

Figure 4

Mössbauer spectrum of [FeH₃(2)](ClO₄)₂ at 295 K (top) and 77 K (bottom).

Figure 5

ORTEP diagram of Fe(2). Atoms are contoured at the 50% probability level, and hydrogen atoms have been omitted for clarity.

Figure 6

ORTEP diagram of two independent molecules of Fe(3). Atoms are contoured at the 50% probability level, and hydrogen atoms have been omitted for clarity.

Figure 7

ORTEP diagram of [FeH₃(3)](ClO₄)₂·H₂O. Atoms are contoured at the 50% probability level, and hydrogen atoms have been omitted for clarity. The ordered and disordered perchlorates are shown.