1582
The Journal of Physical Chemistty, Vol. 83, No. 12, 1979
Mulazzani et al.
One-Electron Reduction of Aromatic Nitrogen Heterocycles in Aqueous Solution. 2,2'-Bipyridine and 1,I0-Phenanthroline' Q. G. MuIazzani,*2aS. Emmi,2aP. G. Fuoch/,28M. Venturl,2bM. 2. Hoffman,*2Cand M. G. SimicZd Laboratorio di Fotochimica e Radiazioni d' Alta Energia del C.N.R., 40 126 Bologna, Italy: Cattedra di Chlmica Generaie ed Inorganica, Facolti di Farmacia, Universiti di Bologna, 40 126 Bologna, Italy; Department of Chemistry, Boston University, Boston, Massachusetts 022 15; Food Engineering Laboratory, U S . Army Natick Research and Development Command, Natick, Massachusetts 0 1760 (Received November 20, 1978: Revised Manuscript Received March 8, 1979) Publication costs assisted by the National Science Foundation
The reaction of 2,2'-bipyridine (bpy), 1,lO-phenanthroline (phen), and their monoprotonated cations (bpyH+ and phenH+) with radiation-generated one-electron reducing agents yields radicals, the spectra, kinetics, and acid-base properties of which have been established. BpyH2+.has a pK, of 5.6 and exhibits an intense absorption maximum at 375 nm ( e 4.5 X lo4 M-l cm-'1; bpyH. shows A,, 365 nm ( E 3.0 X lo4 M-' cm-'). BpyH. decays via second-order kinetics (2h = 4.0 X lo9 M-' sd) representing primarily a disproportionation reaction. BpyH2+. appears to decay via concurrent first- and second-order reactions;the processes are complicated by unidentified dose-dependent sequences. Irrespective of pH, the final isolatable product is a pale yellow solid which can be identified by elemental analysis, molecular weight determination, and IR spectrum as a trimer of dihydrobipyridine isomers. PhenHz+.shows A,, 370 nm ( e 7.8 X lo3 M-' cm-l ) and 510 nm ( e 3.8 X lo3 M-l cm-' ) and pK, = 6.3 while phenH. exhibits A,, 350 nm ( E 5.8 X lo3 M-' cm-' ) and 475 nm (t 3.0 X lo3 M-' cm-l ). PhenH. protonates to form phenHz+-with k = 8.3 X lo9M-' s-' and phenHz+.decays via second-order kinetics (2k = 1.6 x lo9 M-l s-l ). In alkaline solution (pH 10-13) the decay of phenH. is first order ( k = 2 X IO39-l). The bimolecular reaction appears to proceed predominantly via combination; a product has been isolated with the empirical formula C12HllN2C104.The behavior of the one-electron reduction products is compared with that of species believed to be similar arising from the photoreduction of bpy and phen in aqueous solution.
Introduction The 2,2'-bipyridine (bpy) and 1,lO-phenanthroline (phen) complexes of transition metals have attracted wide interest as electron and energy transfer agents in their excited ~ t a t e s Reductive .~ quenching of * R ~ ( b p y ) ~for ~+, example, generates R ~ ( b p y ) ~there + ; ~is ample evidence to indicate that this species can be described as a one-electron reduced bpy ligand-radical complex with a Ru(I1) centerak8 Recently, the photoreductions of bpy and phen in aqueous solution have been reported to yield the corresponding one-electron reduced r a d i c a l ~ . ~ J ~ For these reasons, and as part of our continuing investigation of the interaction of radiation-generated free radicals with polypyridines and their coordination comp l e x e ~ , ' J -we ~ ~have examined in detail the spectral and kinetic behavior of the species formed from the oneelectron reduction of bpy and phen in aqueous solution. Because one of the experimental variables is the pH of the solution, it is important to keep in mind the acid-base properties of the various forms of the substrates: bpyH:+ (pK, = -0.2), bpyH+ (4.25), phenHzz+ (-1.6), phenH+ (5.46).15916 Experimental Section Materials. Bpy (Merck) and phen (Merck) were purified by zone refining. Methanol and 2-propanol were further purified by the method of Baxendale and Wardman.17 Sodium formate was recrystallized twice from water. Acetone was purified by refluxing with KMn04,distilling, and fractionating under N2. Nitrous oxide was passed through a column of NaOH pellets. Distilled water was further purified by distillation from acidic K2CrZO7, from alkaline KMn04,then fractionated in an all-silica apparatus. All other materials were of the highest purity available and were used without further treatment. The aqueous solutions were buffered with HC104,phosphate, or NaOH. 0022-3654/79/2083-1582$01 .OO/O
Radiation Techniques. Continuous radiolyses were performed at room temperature in a 6oCo-ysource with a dose rate of -2.5 krd min-l; the exact dose received by the solutions was determined by use of the Fricke chemical dosimeter.18 For spectrophotometric observations, the irradiations were carried out on -10-mL samples contained in cylindrical pyrex vessels fitted with 2-, 1-, 0.2-, or 0.1-cm silica glass optical cells on a side arm. Irradiations of volumes of solutions up to 1L were performed in order to generate sufficient amounts of products for analysis. The samples were deaerated by purging with Ar M) with NzO. or saturating (2.5 X Pulse radiolyses with optical absorption detection were performed using the facilities at the C.N.R. Laboratory (Bologna) which have already been des~ribed.'~ The bpy and phen solutions were protected, whenever possible, from the photolytic effect of the analyzing light by appropriate UV cutoff filters. A shutter, which opened a few seconds before the radiation pulse, was always in place between the sample and the analyzing light. The optical path length of the irradiation cell was generally 2 cm; an optical path length of 5 cm was used when low dose pulses were used. In that case, in order to achieve uniform irradiation, the geometry of the electron beam was adapted to the cell geometry by the use of a quadrupole magnet on the drift tube of the accelerator and a 1-mm A1 plate at the exit port. The dose delivered by each pulse was monitored with a charge collector placed behind the cell and coupled to a charge amplifier. The system was calibrated with 02-saturated0.1 M KSCN solution assuming G E = 2.15 X lo4 at 500 nm. Kinetic data analyses were performed with a Bradley 175 curve analyzer and a Hewlett-Packard 9830A calculator. Analyses. UV-visible spectra were obtained with Perkin-Elmer 402 and 139 or Beckman ACTA CIII spectrophotometers. The pH of the solutions was determined with Gibertini DP 100 or Orion 801 p H meters 0 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83,No. 12, 7979
One-Electron Reduction of Aromatic N Heterocycles
1583
TABLE I : Rate Constants for t h e Reduction of bpy, phen, and Their Monocations in Aqueous Solution h , M-I s-' radical
bPY" 5.0 X l o s a 3.5 x l O 8 b
(CH3)2C0.CH,OH .CH,O-
bPY
phenH+
2.5 X 10'oc 320 nm remains after this decay. In more acidic solutions the decay becomes faster; 2h = 3.0 X lo9 M-ls-l in 1 M HC104. In alkaline (pH 10-13) solutions containing (CH3)2CHOH,the initial transient decays via good first-order kinetics (k = 2 X lo3 9-l) at low (-0.2 krd) dose; this decay removes -25% of the maximum absorbance measured after the pulse. An increase in dose by a factor of -15 increases the rate of this first-order decay by a factor of -2.5. No further changes are ever observed on a longer time scale. However, a precipitate is observed to form and the solution becomes turbid. The nature of this precipitate from the pulse and the dose dependence of the decay in alkaline solution was not investigated further. The species formed from the reduction of bpy and phen can be scavenged by oxidizing agents. For example, re-
-
0.5
..
0.0 200
250
350
400
Flgure 6. Spectra from the continuous radiolysis of 0.44 mM bpyH' in 0.1 M (CH&CHOH at pH 2, Ar-purged solution, dose rate = 1.43 X loi7 eV mL-' rn1n-l. (a) Irradiation time: 0 rnin (-), 20 rnin (---), 40 min 0.2-cm optical cell pathlength. (b) Spectrum of the solution in (a) irradiated for 40 min after which NaOH is added to pH 10. (c) Spectrum (-) after complete extraction of unreacted bpy with nheptane from solution in (b); spectrum (.-) after this bask solutlon Is acidified to pH 1, 1-cm optical cell pathlength. (..e);
duced bpy reacts with benzoquinone (2 X M bpy, 1 X benzoquinone, 1 M tert-butyl alcohol, pH 7.0, Ar-purged solution) with k = 3.4 X lo9 M-l s-l to yield the reduced benzoquinone radical which is detected by its 430 nm).27 Recharacteristic absorption spectrum (A, M bpy, duced bpy also reacts with cytochrome c (2 X M cytochrome c, 0.2 M tert-butyl alcohol, pH 7.0, 4X Ar-purged solutions) with k = 5.7 X lo8 M-' s-l to yield reduced cytochrome c.28 Reduced bpy reacts with O2 at pH 2 (k = 1.5 X lo9 M-l s-l) and pH 7 (k = 3.0 X lo9 M-l SI); reduced phen reacts with O2 at pH 2 (k = 7.2 X lo8 M-l s-l) and pH 11 (k = 2.4 X lo9 M-l SI). Continuous Radiolysis. Continuous irradiation of acidic solutions containing bpyH+ and (CH3)2CHOHshows a linear decrease in the bpyH+ absorption maximum with increasing exposure time and, hence, with increasing radiation dose delivered to the solutions. From a knowledge of the dose rate of the radiation source and,,E for bpyH+, a value of G(-bpyH+) = 2.4 is calculated. The products of the reaction contribute only a negligible absorption in the region of bpyH+ absorption (-300 nm). Similarly, in neutral N20-saturated solutions containing bpy and (CH&CHOH or CH30H, G(-bpy) = 1.3. In this case, the G value obtained from the decrease of bpy absorption is in good agreement with the value obtained from the extraction of unreacted bpy from the irradiated samples. In N20-saturated alkaline solution in the presence of (CH3)2CHOH,G(-bpy) increases with increasing [bpy] up to a plateau value of 2.8. Figure 6a shows the spectral results from a typical continuous radiolysis experiment in (CH3),CHOH solution at pH 2. The spectrum of bpyH+ decreases linearly with time with isosbestic points at 255, 270, and 330 nm clearly resolved. After irradiation, introduction of air into the
1586
The Journal of Physical Chemistry, Vol. 83, No. 12,
Mulazzanl et al.
1979
1 4000
3000
2000
1800
1600 1400 frequency (cm-'I
1200
lo00
800
400
600
Flgure 7. IR spectrum of solid product from continuous radiolytic reduction of bpy.
solution has no effect on the spectrum. Addition of NaOH (to pH 10) to the irradiated acidic solution yields the spectrum shown in Figure 6b which has elements of unreacted bpy. Extraction of this alkaline solution with n-heptane (four cycles) removes completely the unreacted bpy and no other species; the spectrum of the resulting aqueous solution (Figure 6c) shows peaks at 255,262, and 268 nm. Acidification (to pH 1)of the alkaline aqueous solution changes the spectrum somewhat (Figure 64. The change in absorbance as a function of pH shows the existence of two pK,'s at -2 and -7. Aside from the protonation-deprotonation process, it appears that this final product is the same regardless of the pH of the irradiated solution. In neutral solution, this final product is only slightly soluble, affording the solution with some opalescence. It can be isolated as a pale yellow powder and purified as described in the Experimental Section. Its IR spectrum in KBr, shown in Figure 7, indicates the presence of an intact pyridine ring inasmuch as the bands in the 3050-, 1600-, and 800-700-cm-l regions can be attributed to v(aromatic CH), v(aromatic C=C), and out-of-plane bending of the aromatic CH, respectively. The bands in the 2900- and 1465-cm-l regions are consistent with the presence of aliphatic carbons. The bands at 3400 and 1640 cm-l are very similar to those shown by phen monohydrate and can be attributed to the presence of bound water. This water must be very strongly held inasmuch as, unlike phen monohydrate, these bands are not affected by vacuum heating treatments in the preparation of the samples for IR analysis. The molecular weight of the materials is -500 dalton and the elemental analysis is as follows: C = 69.4%, H = 6.9%, N = 13.3%, and 0 = 6.1%. A gray residue amounting to -4% of the product (not analyzed) is observed at the end of the analysis. The continuous irradiation of aqueous solutions of phen and phenH+ results in the decrease of the absorption of those species from which G(-phen) and G(-phenH+) are determined. The G values reach their maximum in the presence of (CH,)&HOH well below a substrate concentration of 4 X lo4 M. Plots of absorbance vs. dose give straight lines for more than 50% conversion.of substrate to products; the G values are, therefore, dose independent. At pH 4-13, the irradiated solutions are opalescent so that it is necessary to use a 1-mm spe'ctrophotometer cell in order to minimize the effect of that opalescence upon the absorption measurement. Figure 8 shows G(-substrate) M total for (CH&CHOH solutions containing 4 X phen as a function of pH, as determined in the two spectral regions where phen and phenH+ show their maximum
3t 1
o/
.
I
//"
I
0
A
A
A
The Journal of Physical Chemistty, Vol. 83, No. 72, 1979
One-Electron Reduction of Aromatic N Heterocycles
1587
Scheme I d--' H+
a
bpy/phen t e- + [bpy.-/phen.-]
_ _ f
zKa>14
bpyH./phenH.
lf
€I+ pKa=5. 6 / 6 . 3
HtI I P K ~ = ~ . Z 5.46 S/ H+
bpyH'/phenH+ t eH"
bpyH,'./phenH,'.
11
pKa= -0.2, - 1.6
H ' 1 1 pK,
