Tetra-2,3-pyrazinoporphyrazines with Externally Appended Pyridine Rings. 4. UV−Visible Spectral and Electrochemical Evidence of the Remarkable Electron-Deficient Properties of the New Tetrakis-2,3-[5,6-di{2-(N-methyl)pyridiniumyl}pyrazino]porphyrazinatometal Octacations, [(2-Mepy)₈TPyzPzM]⁸⁺ (M = Mg^II(H₂O), Co^II, Cu^II, Zn^II)

Abstract

Metal derivatives of the octacationic tetrakis-2,3-[5,6-di{2-(N-methyl)pyridiniumyl}pyrazino]porphyrazine macrocycle [(2-Mepy)(8)TPyzPzH(2)](8+) (2-Mepy = 2-(N-methyl)pyridiniumyl ring) isolated as water-soluble hydrated iodide salts of the general formula [(2-Mepy)(8)TPyzPzM](I(8)).xH(2)O, (M = Mg(II)(H(2)O), Co(II), Cu(II), Zn(II); x = 2-5) were prepared from the corresponding neutral complexes [Py(8)TPyzPzM].xH(2)O previously reported. Reaction of these complexes with CH(3)I in N,N-dimethylformamide under mild conditions led to full quaternization of all eight pyridine N atoms and formation of the octacations [(2-Mepy)(8)TPyzPzM](8+). Clathrated water molecules could be eliminated from the species [(2-Mepy)(8)TPyzPzM](I(8)).xH(2)O by mild heating (<or=100 degrees C) under vacuum, but the unsolvated species which were formed tended to rehydrate when exposed to air. Magnetic susceptibility measurements and EPR spectra prove that the Cu(II) and Co(II) complexes in the solid state are both paramagnetic with one unpaired electron, thus giving a low-spin state Co(II) for the latter compound. Studies of the charged species [(2-Mepy)(8)TPyzPzM](8+) in aqueous media at ca. 10(-)(5) M concentration provide evidence for the occurrence of molecular aggregation, similar to what is seen for the related free-base species [(2-Mepy)(8)TPyzPzH(2)](8+) (see part 3 of this series, preceding paper in this issue), but the formation of monomeric species is generally favored upon dilution of the solutions. The same octacations are essentially monomeric in solutions of pyridine or dimethyl sulfoxide (DMSO), but traces of aggregation, if occasionally present, vanish with the time. Changes in the UV-visible spectra are observed in the Q- and B-band regions as a result of the quaternization at the pyridine N atoms. Cyclic voltammetry and thin-layer spectroelectrochemical data in DMSO show well-resolved reversible multistep one-electron reductions for both the unmethylated and methylated complexes, all of which appear to be ligand-centered, the only exception being reduction of the Co(II) complex. For this species, the first one-electron reduction is a metal-centered Co(II) --> Co(I) process, but the site of electron transfer is reversed and the final product upon a further one-electron reduction is formulated as a Co(II) dianion as opposed to a Co(I) pi-anion radical. This sequence is similar to what was earlier reported for reduction of the same compound in pyridine. Reversible one-electron oxidations are also observed for the unmethylated species [Py(8)TPyzPzM].xH(2)O where M = Co(II) and Mn(II) in DMSO. Remarkably, the octacationic macrocycles [(2-Mepy)(8)TPyzPzM](I(8)).xH(2)O, (M = Mg(II)(H(2)O), Co(II), Cu(II), and Zn(II); x = 2-5) are more easily reduced at any step of the reduction than the corresponding unquaternized species with the same metal ion. This indicates a higher tendency to stepwise electron uptake after the quaternization process, which enhances the charge redistribution capability within the species formed by the ele...

Tetrapyrazinoporphyrazines with different number of peripheral pyridyl rings: Synthesis, photophysical and photochemical properties

“…Tetrapyrazinoporphyrazines (TPyzPA) with eight pyridyl moieties have been investigated for their remarkable electron-deficient properties in electrochemistry both free and quarternized which allowed water solubility [9][10][11][12]. The same compounds have also been investigated for their non-linear optical properties [13].…”

Tetra(pyrazino[2,3-b]pyrazino)porphyrazines: Synthesis, absorption, photophysical and electrochemical properties of strongly electron-deficient macrocycles

“…Its noteworthy, that the first reduction potential of the macrocycle core can be also considerably shifted by peripheral substitution, e.g. similarly as in remarkable electron-deficient TPyzPzs substituted with quaternized pyridyl substituents [20]. This method of tuning of the electrochemical parameters is, however, not applicable to the TPyzPyzPzs at this moment due to low synthetic availability as discussed above.…”

Tetrapyrazinoporphyrazines and their metal derivatives. Part I: Synthesis and basic structural information

“…mononuclear complexes [TPyz42PzM] (B) [204,207,209,210], the related quaternized octacations [Me 8 TPyz42PzM] 8+ (C) [206,207,209,210], the homo/hetero pentanuclear derivatives [(M'Cl 2 ) 4 TPyz42PzM] (D) [58,207,209,210], the neutral heterobinuclear [(M'Cl 2 )TPyz42PzM] (E) [56] and the externally monoplatinated quaternized hexacations, [(PtCl 2 )Me 6 TPyz42PzM] 6+ [56].…”

“…With reference to similar tetrapyrrolic porphyrazine analogs having Co II also located in-plane and with a d 7 low-spin configuration, this value is slightly longer than that found in the β-polymorphic form of Co II phthalocyanine (1.908(2) Å) [220] and practically coincident with that found (1.942(6) Å) for the Co II complex of tetrakis(thiadiazole)porphyrazine [221,222]. As to the non-coplanar positioning of the pyridine rings with respect to the quasi planar porphyrazine core, the dihedral angles between the pyridine and related pyrazine rings are within 35 The pyridine N atoms in the mononuclear metal complexes [TPyz42PzM] (M = Mg II (H 2 O), Zn II , Cu II , Co II ) can be methylated when treated with methyl iodide in DMF at 25-50°C for 1-2 days with formation of octaquaternized water soluble species [Me 8 TPyz42PzM] 8+ (neutralized by I -ions) [205,206] (see Scheme 31, C). Since a direct quaternization of the starting metal free [TPyz42PzH 2 ] by reaction with CH 3 I did not give reproducible results, the octaquaternized [Me 8 TPyz42PzH 2 ] 8+ was obtained by demetallation of the octacationic Mg II complex [Me 8 TPyz42PzMg] 8+ [205].…”