Super-Charged Porphyrazines: Synthesis and Physical Properties of Octacationic Tetraazaporphyrins

We report the synthesis of octacationic macrocycles based on the porphyrazine (tetraazaporphyrin) core. The first stage is the synthesis of Mg(II) 2,3,7,8,12,13,17,18-octakis(4-pyridyl)porphyrazine, [Mgpz(pyr)(8)], and the parent [H(2)pz(pyr)(8)]. Both compounds are freely soluble in aqueous acid due to protonation of the pyridyl groups but with concomitant loss of Mg(2+) by the latter. Methylation of [H(2)pz(pyr)(8)] gives the octacation, 2,3,7,8,12,13,17,18-octakis(N-methyl-4-pyridiniumyl)porphyrazine, [H(2)pz(Me-pyr)(8)](8+), which is freely soluble in water as the chloride salt. The peripheral charges greatly lower the pK(a) values of the pyrrole protons of [H(2)pz(Me-pyr)(8)](8+) in aqueous solution. Global fitting of optical pH titration curves gives pK(a) values for the first and second deprotonations of 6.3 and 8.8 in 2.0 M NaCl and 4.1 and 5.6 at 0.1 M NaCl. The analysis further gives the UV-visible spectra of the [H(2)pz(Me-pyr)(8)](8+), [Hpz(Me-pyr)(8)](7+), and [pz(Me-pyr)(8)](6+) species. The free-base compound [H(2)pz(Me-pyr)(8)](8+) rapidly incorporates a variety of metal ions (Ba(II), Cd(II), Co(II), Cu(II), Mn(III), Ni(II), Pd(II), and Zn(II)) to form [Mpz(Me-pyr)(8)](8+). We present the complete synthesis and characterization of the [Nipz(Me-pyr)(8)](8+) and [Cupz(Me-pyr)(8)](8+) derivatives.