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

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(TPP^4-)}^2- (1) and {cryptand(Cs⁺)}{Ni^I(TPP^2-)}^-·C₆H₅CH₃ (2) have been obtained. The two-electron reduction of {Cu^II(TPP^2-)}⁰ is centered on the macrocycle allowing one to study for the first time the structure and properties of the TPP^4- tetraanions in the solid state. Tetraanions have a diamagnetic state and show essential C-C_meso bond alternation. New bands attributed to TPP^4- appear at 670, 770 and 870 nm. Unpaired S = 1/2 spin is localized on Cu^II. The one-electron reduction of {Ni^II(TPP^2-)}⁰ centered on nickel provides the formation of {Ni^I(TPP^2-)}^- 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(TPP^2-)}^-·C₆H₅CH₃ (3) in the 4.2-120 K range. Since dianionic TPP^2- 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(TPP^2-) and Fe^II(TPP^2-) by cesium anthracenide in the presence of Bu₃MeP⁺ yields crystals of 3 and (Bu₃MeP⁺){Fe^I(TPP^2-)}^-·C₆H₅CH₃ (4) whose crystal structures and optical properties are also presented. DFT calculations have been carried out for {M^II(TPP^2-)} (M = Cu, Ni and Fe) and their anions to interpret the experimental results obtained for 1-4.

Reaction of GaIIIClPc, SnIVCl2TPP and BIIIClSubPc with cyanide anions: reduction of macrocycles vs. formation of cyano-containing macrocyclic anions

The reaction of GaIIIClPc, SnIVCl2TPP and BIIIClSubPc containing phthalocyanine (Pc), tetraphenylporphyrin (TPP) and subphthalocyanine (SubPc) macrocycles with cyanide in the presence of cryptand[2.2.2] under anaerobic conditions yields crystalline salts in which cyano anions substitute chloride anions at GaIII, SnIV or BIII, as well as reducing the macrocycles or adding one or two CN- to them. The reaction of GaIIICl(Pc2-) with CN- yields {crypt(K+)}{GaIIICN(Pc˙3-)}˙-·0.5C6H4Cl2 (1) in which the Pc2- macrocycle is reduced to Pc˙3-. Such reduction could probably occur through the addition of CN- to Pc2- forming {GaIII(CN)[Pc(CN)]3-}- which can decompose further interacting with an excess of CN-. As a result, Pc˙3- and cyanogene anions are formed. The interaction of SnIVCl2(TPP2-) with CN- is accompanied by the addition of CN- to the meso-carbon atom of porphyrin forming diamagnetic TPP(CN)3- macrocycles in {crypt(K+)}{SnIV(CN)2[TPP(CN)]3-}- (2). Salt 2 shows a strong NIR absorption band with the maximum at 854 nm whose intensity is comparable with that of the Soret band. The interaction of BIIICl(SubPc2-) with three equivalents of CN- is accompanied by the addition of two CN- to carbon atoms of SubPc2- closest to meso-nitrogen atoms forming {BIII(CN)[SubPc(CN)2]4-}2-. Most probably these dianions transfer electrons to C6H4Cl2 producing the {BIII(CN)[SubPc(CN)2]˙3-}˙- radical anions which form σ-bonded diamagnetic dianions in {crypt(K+)}2{BIII(CN)[SubPc(CN)2]}22-·3C6H4Cl2 (3). The remaining carbon atom closest to the meso-nitrogen atom is involved in this dimerization. According to the calculations, the energy of the C-CN bond is minimal for {GaIII(CN)[Pc(CN)]3-}- enabling further transformation of these anions to {GaIIICN(Pc˙3-)}˙- in 1, whereas cyano-containing anions in 2 and 3 with higher energy of this bond are stable towards the elimination of CN. Optical and magnetic properties of 1-3 together with their crystal and molecular structures are presented. The possible ways of the formation of 1-3 are discussed based on DFT calculations.