Acid-base properties of porphyrins in nonaqueous solutions

Abstract

The study of acid-base properties of porphyrins have been started in our university under the guide of our teacher Professor B. D. Berezin in the beginning of the 60th. The obtained results can be considered as a significant contribution to the porphyrin chemistry. In proton-donor media porphyrins can be stepwise protonated forming the mono-and dications and deprotonated forming mono-and dianions. Acid-base forms of porphyrins can be stabilized by electronic, solvation and steric factors.[8-9] Electronic factors are characterised by changing of electronic density in the porphyrin macroring during protonation or deprotonation. Substituents in β-, meso-positions and at inner N-atoms play an important role in the redistrubution of the electron density. Stabilization of ionic forms in solution occur by solvation with the solvent molecules and by association with M+ or X-. Ability of the macrocycle to deformation is an important steric factor. Reaction with inorganic base in the polar solvent leads to formation of the ion-molecular associate N-H.OH.[6] Complete ionization of the N-H bond may occur only in non-aqueous media in tetraalkylammonium hydroxides or at high concentration of alkoxide-anions. [10-11] The negative charge of the monoanionic forms is delocalized in the coordination center and concentration of ionic forms is decreased in the order IV >V > VI.[12] Parameters of the acidic dissociation of the porphyrins in methanol obtained by method of solubility are listed in Table 1. The value of pK3 is increased in the order: H2TPP(p-OCH3) < H2TPP < H2DP < H2PP < H2EP < H2MP < H2Chl(b). Acid ionization of porphyrins in DMSO was studied by spectropotentiometric titration with [N(CH3)4]OH. The stability of monoanions in DMSO is increased in the order: H2P < H2TBP << H2TAP < H2P(SO3). The nature of the solvent have influence on acid properties of the porphyrins. The solvent can stabilise neutral and ionic forms of porphyrin. The monoanion is stabilised in DMF and MeOH, but stabilisation of HP-is absent in DMSO (ΔS ≈ 0).[12] So pK3 of H2TBP and H2TAP in DMF are lower than in DMSO. Substituents have considerable electronic influence on the NH-acid centers of porphyrin. The introduction of electron-acceptor substituent (Cl) into the phenyl rings of H2TPP increases acidity in DMSO by 1-1.3 order of magnitude, while tetra-meso-propyl substitution in H2P decreases acidity by 1.5 orders (Table 2). Basic properties of porphyrins in different solvents were also studied systematically.[20-28] Solvation has important role in stabilization of protonated forms. Thermodynamic parameters obtained for dissociation of the dicationic forms of porphyrins evidence, that acid ionization of H4P2+ leads to the increase of solvation (ΔS > 0). Polar solvents (DMSO, DMF, MeOH) have a good solvation capacity for cationic forms of porphyrins by means of specific solvation of NH-groups. ΔS value is increased in the the process of dissociatoin of protonated forms.[31-33] In acetonitrile and nitrobenzene specific solvation of the reaction center is decreased (Table 3). The constants of dissociation of protonated forms H2P are in accordance with the electronic nature of substituents. Protonation lead to distortion of the planar structure of the porphyrin molecule. Unlike porphyrins tetraazaporphyrins are not protonated on inner N-atoms. The thermodynamic compensation effect of energetic investments on ionization by solvation has the linear character (Figure 3). It shows, that energy required for ionization of the N-H bonds is compensated by salvation. © ISUCT Publishing.