1,2,5-Thiadiazolo[3,4-b ]pyrazine-5,6-dicarbonitrile and derived porphyrazines: synthesis and electrochemical study

1,2,5-Thiadiazolo[3,4-b ]pyrazine-5,6-dicarbonitrile, obtained by reaction of 5,6-diaminopyrazine-2,3-dicarbo-nitrile with SOCl 2 , exhibits strongly electron-deficient properties and easily forms reduced species (the reduction potentials -0.01 and -1.02 V in MeCN vs Ag/AgCl). Template cyclotetramerization in the presence of Mg butoxide in n -butanol and with indium(III) chloride in 1,2,4-trichlorobenzene affords the corresponding complexes of tetrakis(1,2,5-thiadiazolo[3 ′ ,4 ′ :5,6]pyrazino)[2,3-b,g,l,q ]porphyrazine, [(TSN 2 PyzPA)M] (M = Mg(H 2 O), InCl). The Mg(II) complex is characterized by a Q-band at 657 and a Soret band at 370 nm (in DMSO) and shows three reduction processes at -0.38, -0.83 and


Porphyrazine complexes
The dinitrile 2 was used as precursor for preparation of tetrapyrazinoporphyrazines with fused 1,2,5-thiadiazole rings, [T(SN2Pyz)PAM].Already upon melting at 232 °C the dinitrile 2 turns greenish-brown due to selfmacrocyclization process.Template Linstead cyclotetramerization of 2 in the presence of Mg II butoxide in nbutanol under reflux affords the Mg(II) complex 3 only with 11% yield.Such low yield might be connected with participation of this strongly electron-deficient dinitrile in side reactions, e.g. in nucleophilic substitution of CN groups by butoxide as a strong nucleophile.Cyclotetramerization of the dinitrile 2 in non-nucleophilic conditions in the presence of In(III) chloride in 1,2,4-trichlorobenzene leads to the In(III) complex 4 with 28% yield.The obtained complexes [T(SN 2 Pyz)PAM], M = Mg (3), In(Cl) (4)) are practically insoluble in non-donor organic solvents and can be purified from organic impurities by extraction.The solubility in donor solvents (pyridine, DMSO) is higher and the obtained complexes were additionally reprecipitated from DMSO solutions.
The MALDI mass-spectra could be obtained only for the In(III) complex 4 (Figure 4).In the negative region the molecular ion peak [4-Cl+H] -is observed at 868 Da.In the positive region the peak is observed at m/z 783 Da.It might be very likely assigned to a defragmentation product with the loss of four S atoms and formation of positively charged ion of octaimino derivative [4 -4S + 9H] + .Interestingly, this peak has the isotopic distribution typical for the presence of chlorine atom, indicating that the In III complex 4 contains Cl as axial ligand which is retained in this positively charged ion.
Low solubility and/or strong tendency of porphyrazines 3 and 4 to aggregation prevent using NMR spectroscopy for their characterization.Freshly prepared solutions of complexes 3 and 4 in donor solvents (DMSO, pyridine) contain strongly broadened absorption bands in the visible region (600-700 nm) due to considerable aggregation effects.On prolonged staying the spectra of the diluted solutions become typical for non-aggregated porphyrazine complexes (Figure 5) and exhibit a single Q band in the visible region near 660 nm and more intense and broad Soret band in the UV-region (370 nm for 3 and 369 nm for 4).The Q-band maxima are shifted bathochromically by 15-25 nm as compared to the corresponding complexes of unsubstituted tetrapyrazinoporphyrazine [TPyzPAM] (M = Mg(II), In(III)Cl) 8 due to enlargement of the -chromophore by annulation of 1,2,5-thiadiazole rings.At the same time the maximum of the Q-band is shifted hypsochromically to ~660 nm as compared to complexes of the TPyzPA macrocycles with fused less electrondeficient heterocyclic fragments, e.g.pyrazine (700-720 nm), 17,19 isoindole (~715 nm) 13 or thianaphthene (~690 nm).The electrochemical study using cyclic voltammetry confirms that fusion of 1,2,5-thiadiazole rings increases the electron-deficient properties of the TPyzPA macrocycle.For the Mg(II) complex 3 three reduction waves are observed at -0.38, -0.83 and -1.21 V vs Ag/AgCl in DMSO (Figure 6).The 1 st reduction potential is shifted more positively by ~200-300 mV as compared to benzo and by ~100 mV as compared to pyrazine fused TPyzPA (for Zn(II) complexes of tert-butylsulfanyl substituted TPyzPA with four fused benzene and pyrazine rings E½ = -0.66 and -0.49V vs SCE in THF, respectively 18 ).

Experimental Section
General.The IR spectra were measured on an IR-spectrometer AVATAR 360 FT-IR using KBr pellets.UV-vis spectra were recorded using a Hitachi U-2000 spectrophotometer.Mass-spectrometric measurements were carried out on a MALDI-TOF Shimadzu Biotech Axima Confidence spectrometer in negative and positive modes.Elemental analyses were performed on a CHN analyser Flash EA 1112.Reduction potentials were measured in deoxygenated (Ar) dimethylsulfoxide (Aldrich) solution containing 0.1 M tetrabutylammonium perchlorate as supporting electrolyte on a potentiostat Elins P-4 equipped with a three electrode electrochemical cell with a glassy carbon working electrode, a Pt wire counter electrode and an Ag/AgCl reference electrode.The reference Fc/Fc+ couple was observed at +0.465 V vs Ag/AgCl.The obtained potential values are negatively shifted versus Standard Calomel Electrode (SCE) by 0.035 V (the recommended value of the Fc/Fc+ correction vs SCE in DMSO is +0.50 V 31 ).2), was prepared closely following the procedure suggested by Tong. 212,3-Diaminopyrazine-5,6-dicarbonitrile (2 g, 12.5 mmol, TCI Europe) was suspended in p-xylene (100 ml) and after addition of thionyl chloride (4 ml) the mixture was refluxed for 20 h.

Figure 4 .
Figure 4. MALDI mass-spectra (CHCA matrix) of the In III complex 4 in the negative (top) and positive (bottom) regions.Inserts display the experimental and theoretical isotopic distribution patterns.

Figure 5 .
Figure 5. UV-VIS spectra of the Mg(II) and In(III) complexes (3 and 4) in DMSO.Dotted line shows the spectrum of aggregated species in freshly prepared solution.