Photochemistry and Anion-Controlled Structure of Fe(III) Complexes with an α‑Hydroxy Acid-Containing Tripodal Amine Chelate

The tripodal amine chelate with two pyridyl groups and an α-hydroxy acid (AHA) group, Pyr-TPA-AHA, was synthesized. Different Fe­(III) complexes form with this chelate depending upon the counterion of the Fe­(III) source used in the synthesis. A dinuclear complex, Fe­(III)2(Pyr-TPA-AHA)2(μ-O), 1, and mononuclear complexes Fe­(III)­(Pyr-TPA-AHA)­X (X = Cl– or Br–, 2 and 3, respectively) were synthesized. 2 can be easily converted to 1 by addition of silver nitrate or a large excess of water. The structure of 1 was solved by X-ray crystallography (C32H34N6O7Fe2·13H2O, a = 14.1236(6) Å, b = 14.1236(6) Å, c = 21.7469(15) Å, α = β = γ = 90°, tetragonal, P42212, Z = 4). 2 and 3 each have simple quasireversible cyclic voltammograms with E 1/2 (vs aqueous Ag/AgCl) = +135 mV for 2 and +470 for 3 in acetonitrile. The cyclic voltammogram for 1 in acetonitrile has a quasireversible feature at E 1/2 = −285 mV and an irreversible cathodic feature at −1140 mV. All three complexes are photochemically active upon irradiation with UV light, resulting in cleavage of the AHA group and reduction of the iron to Fe­(II). Photolysis of 1 results in reduction of both Fe­(III) ions in the dinuclear complex for each AHA group that is cleaved, while photolysis of 2 and 3 results in reduction of a single Fe­(III) for each AHA cleavage. The quantum yields for 2 and 3 are significantly higher than that of 1.