Macrocycle- and metal-centered reduction of metal tetraphenylporphyrins where the metal is copper(ii), nickel(ii) and iron(ii)

Abstract

The reduction of metal(II) tetraphenylporphyrins, where metal(II) is copper, nickel or iron, has been performed in toluene solution in the presence of a cryptand. Cesium anthracenide was used as a reductant. Crystalline salts {cryptand(Cs⁺)}₂{Cu^II(TPP⁴⁻)}²⁻ (1) and {cryptand(Cs⁺)}{Ni^I(TPP²⁻)}⁻·C₆H₅CH₃ (2) have been obtained. The two-electron reduction of {Cu^II(TPP²⁻)}⁰ is centered on the macrocycle allowing one to study for the first time the structure and properties of the TPP⁴⁻ tetraanions in the solid state. Tetraanions have a diamagnetic state and show essential C–C_meso bond alternation. New bands attributed to TPP⁴⁻ appear at 670, 770 and 870 nm. Unpaired S = 1/2 spin is localized on Cu^II. The one-electron reduction of {Ni^II(TPP²⁻)}⁰ centered on nickel provides the formation of {Ni^I(TPP²⁻)}⁻ with unpaired S = 1/2 spin localized on Ni^I at 100(2) K. The effective magnetic moment of 2 is 1.68μ_B at 120 K and a broad asymmetric EPR signal characteristic of Ni^I is observed for 2 and also for (Bu₃MeP⁺){Ni^I(TPP²⁻)}⁻·C₆H₅CH₃ (3) in the 4.2–120 K range. Since dianionic TPP²⁻ macrocycles are present at 100(2) K, no alternation of C–C_meso bonds is observed in 2. The excited quartet S = 3/2 state according to the calculations is positioned close to the ground S = 1/2 state. In the excited state, charge transfer from Ni^I to the macrocycle takes place resulting in the formation of Ni^II with S = 1 and TPP˙³⁻ with S = 1/2 in the {Ni^II(TPP˙³⁻)}⁻ anions. Therefore, the increase in the magnetic moment of 2 above 150 K is attributed to the population of the excited quartet state with a gap of 750 K. Salt 2 is EPR silent above 150 K and manifests absorption bands characteristic of TPP˙³⁻ at RT. The reduction of Ni^II(TPP²⁻) and Fe^II(TPP²⁻) by cesium anthracenide in the presence of Bu₃MeP⁺ yields crystals of 3 and (Bu₃MeP⁺){Fe^I(TPP²⁻)}⁻·C₆H₅CH₃ (4) whose crystal structures and optical properties are also presented. DFT calculations have been carried out for {M^II(TPP²⁻)} (M = Cu, Ni and Fe) and their anions to interpret the experimental results obtained for 1–4.