944
IVANOVA et al.
where Kа, the combined acidity constant; Ind, the
indicator ratio, [P2–]/[H2P]; cDBU, concentration of
DBU in the system, mol/L. The combined acidity
constant of II equaled pKa 10.44 in CH3CN–DBU at
298 K. The current concentrations of deprotonated and
neutral forms of II were calculated accounting for the
material balance equation; at сDBU ≈ 6.5 × 10–5 mol/L
the porphyrin II was almost completely converted into
the doubly deprotonated form (Fig. 6).
conjugated acid was of рKа 13.2 in acetonitrile [24].
The solvents (benzene, dichloromethane, perchloric
acid, and acetonitrile) were purified as described
elsewhere [25].
5,10,15,20-Tetranitro-2,3,7,8,12,13,17,18-octa-
ethylporphyrin (I) was prepared as described in [26].
It was then purified via chromatography on Al2O3
(Brockmann grade III) with dichloromethane as eluent.
The product purity was monitored by TLC at Silufol
plates (0.5 mm, Merck, CH2Cl2 as eluent). Yield 60%.
IR spectrum, cm–1: 1533 [δas(NO2)], 1370 [δs(NO2)].
1Н NMR spectrum, δ, ppm: 3.49 (16Н), 1.43 (24Н).
Electronic absorption spectrum (CH2Cl2), λmax, nm
(log ε): 426 (4.82), 530 (3.96), 572 sh (3.57), 611
(3.57), 665 (3.03).
In the course of spectrophotometric titration of the
porphyrin I with the organic base, spectra of the anionic
forms were not detected at 298 K and at elevated
temperature. Thus, I was much less acidic than II.
Coordination of porphyrins with salts of transition
metals can occur either via molecular or via ionic route
[17]. Formation of complexes of the porphyrin II with
zinc ions was studied in the CH3CN–DBU–Zn(OAc)2
system at 100-fold excess of the salt with respect to the
porphyrin. The data collected in Table 2 revealed
acceleration of the reaction by more than an order of
magnitude when the neutral ligand II was transformed
into the ionic form IIa. Simultaneously, the activation
energy was down by 50 kJ/mol, and the activation
entropy was down by 120 J mol–1 K–1. That could be
due to no energy losses upon deformation and cleavage
of the N−H bonds of the reactive site as well as higher
polarization of the molecule leading to more solvated
state of the anionic form in the transition state.
REFERENCES
1. Lukin, J.A. and Ho, C., Chem. Rev., 2004, vol. 104,
no. 3, p. 1219.
2. Antonini, E. and Brunori, M., Hemoglobin and
Myoglobin in Their Reactions with Ligands,
Amsterdam: North-Holland Publ. Comp., 1971.
3. Collman, J.P., Eberspacher, T., Fu, L., and Herrmann, P.C.,
J. Mol. Catalysis (A), 1997, vol. 117, p. 9.
4. Golubchikov, O.A., Pukhovskaya, S.G., and Kuvshi-
nova, E.M., Russ. Chem. Rev., 2005, vol. 74, no. 3, p. 249.
5. Pukhovskaya, S.G., Guseva, L.Zh., Semeikin, A.S., and
Golubchikov, O.A., Kinet. Catal., 2007, vol. 48, no. 2,
p. 190.
NMR spectrum of the formed zinc complex
completely coincided that of the compound prepared
via reaction (1). However, the red shift (about 10 nm)
of all absorption bands in the electronic spectra was
revealed, due to additional coordination of the base
present in the solution with the complex (Fig. 7).
6. Pukhovskaya, S.G., Guseva, L.Zh., Mamardashvili, N.Zh.,
Kuvshinova, E.M., Semeiki, A.S., and Golubchikov, O.A.,
Russ. J. Coord. Chem., 1998, vol. 24, no. 11, p. 797.
7. Kuvshinova, E.M., Kuz’min, D.L., Dudkina, N.S.,
Pukhovskaya, S.G., Semeikin, A.S., and Golubchikov, O.A.,
Russ. J. Gen. Chem., 2002, vol. 72, no. 1, p. 133. DOI:
10.1023/A:1015322118490.
EXPERIMENTAL
8. Jentzen, W., Simpson, M.C., Jentzen, W., Hobbs, J.D.,
Song, X., Ema, T., Nelson, N.Y., Medforth, C.J., Smith, K.M.,
Veyrat, M., Mazzanti, M., Ramasseul, R., Marchon, J.-C.,
Takeuchi, T., Goddard, W.A., III, and Shelnutt, J.A.,
J. Am. Shem. Soc., 1995, vol. 117, no. 45, p. 11085.
Zinc acetate (analytical pure grade) was
recrystallized from aqueous acetic acid and dehydrated
at 380–390 K [23].
9. Parusel, A.B., Wondimagegn, T., and Ghosh, A., J. Am.
Chem. Soc., 2000, vol. 122, no. 27, p. 6371.
10. Pukhovskaya, S.G., Guseva, L.Zh., Malkova, O.V.,
Semeikin, A.S., and Golubchikov, O.A., Russ. J. Gen.
Chem., 2003, vol. 73, no. 3, p. 473. DOI: 10.1023/
A:1024930608567.
IR spectra were recorded using the Avatar 360
FTTIR ESP spectrometer (KBr). Н NMR spectra of
1
solutions in CDCl3 were recorded using the Bruker-
500 spectrometer (500 MHz, TMS as internal
reference). Electronic absorption spectra were recorded
using the Shimadzu UVV1800 and Hitachi UU2000
Cary 100 Varian spectrophotometers.
11. Berezin, D.B., Ivanova, Yu.B., and Sheinin, V.B., Russ.
J. Phys. Chem. A, 2007, vol. 81, no. 12, p. 1986.
1,8-Diazabicyclo[5.4.0]undec-7-ene (special pure
grade) was used as received. Ionization constant of its
12. Pukhovskaya, S.G., Efimovich, B.A., Semeikin, A.S.,
Kolodina, E.A., and Golubchikov, O.A., Russ. J. Gen.
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