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H attack on 2,2'-bipyridine in acid aqueous solution
Radiat. Phys. Chem. Vol. 26, No. 1, pp. 109-116, 1985 Printed in the U.S.A.
0146-5724/85 $3.00 + .00 © Pergamon Press Ltd.
H ATTACK ON 2,2'-BIPYRIDINE IN ACID AQUEOUS
SOLUTIONt SONJA SOLAR Institut ftir Theoretische Chemie und Strahlenchemie der Universitiit Wien and Ludwig Boltzmann Institut for Strahlenchemie, A-1090 Wien, W~ihringerstr. 38, Austria
(Received 14 May 1984) Abstract--In deoxygenated acid aqueous solution (pH = 1) 70% H attack the N position of 2,2'bipyridine (2,2'-bpy-H +) with k = (1.4 --- 0.1) x 10s dm3 mol-t s-t, producing relatively long-lived species Rt (kmax = 365 nm, e365 = 4500 m2 mol-t); 30% H react on C-atom positions with k = (0.6 --- 0.1) x 108 dm3 mol- t s - t resulting in transients R2 (kmax = 330 nm, e33o = 9000 m2 mol- ~, decay 2k = (1.4 --- 0.2) x 109 dm3 mol -t s-t). The formation of Rt species by reaction of (CH3)2COH radicals with 2,2'-bpy-H ÷ was reinvestigated. For the reactivity of e~-qwith 2,2'-bpy a rate constant k(e~q + 2,2'-bpy) = (1.95 _+ 0.1) x 101°dm3 mol -t s - l a t pH = 8 was determined. An attempt has been made to analyse the site specificity of H addition to the 2,2'-bipyridine molecule in terms of the energetic stability of the possible isomeric transients as well as of the net atomic charges on all ring atoms of the substrate. The calculations were based on a semiempirical SCF-MO technique (MNDO).
1. INTRODUCTION
2. EXPERIMENTAL SECTION
2,2'-bipyridine (2,2'-bpy) and its transient metal complexes have been subject of various studies, particularly as energy and electron transfer agents in their excited states. They have been investigated photochemically (e.g. Refs. (1-6)) as well as by pulse radiolysis. ~7-9) Hoffman et al. ~s~ studied the reaction of 2,2'-bpy and its protonated form 2,2'bpy-H ÷ ( p K = 4.3) with various reducing radicals (Red) like eaq, C O 2 , (CH3)2COH, and (CH3)2CO-. The resulting reduced species were assumed to protonate immediately (2,2'-bpy + Red + H+--> 2,2'b p y - H , ts-l°)) yielding 2,2'-bpy-H" or 2,2'-bpy-tZI~( p K = 5.6) with absorption maxima at 365 (e = 3000 m 2 mol -~) and 375 nm (e = 4500 m E m o l - t ) , respectively. ~s) Simic and Ebert ~7) reported an absorption spectrum for transients obtained from reaction of 2,2'-bipyridine with e~q at pH = 7-12.7 (kmax = 365 and 450 nm) as well as with H atoms at pH - 1 (emax = 370 nm), however, no precise reaction mechanism has been given. Pulse radiolysis experiments were now undertaken in order to elucidate the reactivity of H atoms towards 2,2'-bpy-H + in more detail, particularly in respect to the specific H attack on the C and N position of this molecule.
The pulse radiolysis equipment (3 MeV Van de Graaff accelerator type K, High Voltage Engineering Co., Burlington, USA) (3t) combined with a Biomation 8100 transient recorder controlled by a PDP-11/10 computer) has been described previously.(~ 1-13) 2,2'-bpy was zone refined and all other chemicals (t-butanol, propanol-2, HC104, Ba(OH)2; p.A. Merck) were used without further purification. The solutions of 5 × 10 -5 to 5 x 10 -4 mol dm -3 2,2'bpy were prepared with at least four times distilled water and were deoxygenated by purging with high purity argon for about 1 h. The samples were irradiated in a 2 cm quartz cell (optical light path: 3 x 2 cm) with a dose of 5 to 10 J kg-a per 0.4 ixs pulse and the solution was exchanged after each single pulse. All computations in respect to the simulation and optimization of the kinetic and spectroscopic parameters were performed on the CDC computer (CYBER) of the Vienna University.
t The author would like to dedicate this paper to Professor Dr. K. Schl6gl on the occasion of his 60th anniversary.
3. RESULTS AND DISCUSSION 3.1 Reactivity of 2,2'-bpy with e~q The rate constant ofe~q with 2,2'-bpy (5 x 10 -5 mol dm -3 2,2'-bpy, 10 -2 mol dm -3 t-butanol, Ar) was determined at pH = 8 by following the pseudofirst order decay of e~q at 720 nm. Matrix correction
109
110
SONJA SOLAR
was applied, w h e r e k(OH + t-C4HgOH) = 5.5 x 108 dm 3 m o l - 1 s - i.(14) The m e a n value of 20 measu r e m e n t s was (1)
k(e~q
+ 2,2'-bpy) = (1.95 -+ 0.1) x
°11 \
0.08101° dm 3 m o l - ' s - I .
This v a l u e is in g o o d a g r e e m e n t with p r e v i o u s l y reported values (k = (1.8 - 0.2) × 10 I° d m 3 mo1-1 s - i at p H = 6 to 7 "5) and k = 2.5 × 101° d m 3 m o l - 1 s - I at p H = 9.2(7)). 3.2 Reactivity of 2,2'-bpy-H ÷ in the presence of
reducing radicals T h e f o r m a t i o n o f the o n e - e l e c t r o n r e d u c e d 2,2'bpy species was reinvestigated using d e o x y g e n a t e d solutions of 5 x 10 -4 mol d m -3 2 , 2 ' - b p y - H ÷ in the p r e s e n c e o f 0.2 mol dm -3 propanol-2 at p H = 1 to 2, adjusted with HCIO4. In this case e~q is c o n v e r t e d into H a t o m and both, H and O H , react with propanol-2 resulting in the strongly reducing species, (CHa)2CHO with G = Gr~ + Ge~ + GOH = 6.5: (2)
0D
cm
eZa + H + --~ H
0.06!
°oi:i: 350
450
550
650
nm
Fig. 1. Total transient absorption spectrum (A), obtained by H attack on 5 × 10 - 4 mol dm -3 2,2'-bpy-H + in the presence of 0.25 mol dm -3 t-butanol in aqueous deoxygenated solution, pH = 1 (OD/cm normalized to 11 J kg- ' per 0.4 txs pulse)• Spectrum (C) obtained by substraction of transient absorption spectrum (B) in insert II from spectrum A (A - B = C). Insert I: Absorption spectrum of 10 -4 mol dm -3 2,2'-bpy, pH = 1. Insert II: Absorption spectrum of 2,2'-bpy-l:I~- species (R0 obtained by one electron reduction of 2,2'-bpy (see the text)•
(k = 2.3 x 1010dm a m o l - ' s - l ( m ) , (3)
H (or O H ) + (CH3)2CHOH ~ -t- H2 (or H 2 0 )
(CH3)2COH
(k = 7 x l07 d m 3 m o l - 1
x s - l / 1 . 3 x 109 dm 3 mo1-1 s -1 .6.17)), (4)
0.9. T h e rest of O H as well as the H a t o m s are reacting with propanol-2 to f o r m (CH3)2COH with G = 5.6. U n d e r these e x p e r i m e n t a l conditions the d e c a y of 2,2'-bpy-H~- b e c o m e s significantly faster
(CH3)E~OH + 2 , 2 ' - b p y - H + ---> 2,2'-bpy-I:I~ + (CH3)2C~--------O (k = 3.5 x 10s dm 3 x m o l - 1 S - 1 (9)).
T h e resulting absorption s p e c t r u m o f 2,2'-bpy-H~exhibits t w o m a x i m a at 370 nm (e = 4500 m 2 m o l - ') and a b o u t 800 nm (e = 335 m 2 mol - ' ) as well as a shoulder at h = 420 nm (e = 630 m 2 m o l - ' ) . In Fig. 1, insert II only the first part of s p e c t r u m B is displayed. It is in fair a g r e e m e n t with the p r e v i o u s l y d e t e r m i n e d spectra, m•9) The 2,2'-bpy-H~- species are rather stable in the investigated time range (up to 2 ms; see also Fig. 2(A)). F o r their d e c a y at low doses (4 J kg - I ) a first o r d e r p r o c e s s has been reported (k - 0.2 s - i (s)). In o r d e r to c h e c k w h e t h e r t-butanol radicals (t(~4HsOH) are r e a c t i v e t o w a r d s 2 , 2 ' - b p y - I : I f , a d e o x y g e n a t e d solution o f a mixture o f 0.1 mol dm -3 propanol-2 and 0.1 mol d m -3 t-butanol in the prese n c e o f 2 × 10 -4 mol dm -3 2 , 2 ' - b p y - H + at p H = 1 was pulse radiolyzed. Since k ( O H + t-butanol) = 5.5 x l0 s d m 3 m o l - ~ s - 1,o4) about 30% O H are s c a v e n g e d by t-butanol leading to G(t-C4HsOH) =
ODret
A
A
365 nrn
420 n m
!
a
B
i
365
oDt "ll I
0
I
I
420 n m
nm i
800 IJs
I
0
I
I
I
800 Ns
Fig. 2. Influence of t-C4HsOH radical on the decay of 2,2'bpy-l:t~- in deoxygenated solutions. Solution A: 2 × 10 4 mol dm -3 2,2'-bpy-H ÷ , 0.1 tool dm -3 propanol-2, pH = 1. Solution B: as A, but containing 0.1 mol dm -3 tC4H9OH additionally.
111
H attack on 2,2'-bipyridine in acid aqueous solution within the first 400 Ixs as shown in Fig. 2(B). This fact indicates a reaction of t-C4HsOH with 2,2'-bpy-I:/~-, which is in contrast to previous observations.(S)
k.,ll-/r~ I B m
330nm
m
3.3 Reactivity o f H atoms with 2,2'-bpy-H ÷ The total absorption spectrum of transients, formed by reaction of H atoms with 2,2'-bpy-H ÷ was measured in the range from 330-820 nm. Using a deoxygenated solution of 5 x 10 - 4 mol d m - 3 2,2'b p y - H ÷ in the presence of 0.25 mol dm -3 t-butanol as OH scavenger (pH = 1; dose: 5 - 6 J k g - I per 0.4 VLspulse) the maximum transient absorption was attained at about 20 IXS after pulse. It is normalized to 11 J k g - ~ and is displayed as spectrum A in Fig. 1. The absorption of the starting compound is given for comparison in insert I, Fig. 1. The transient spectrum A exhibits doubtless the features of the 2,2'-bpy-H~- species, obtained by reaction of 2,2'bpy-H ÷ with (CH3)2(~OH, especially in the range 350-400 nm. At X < 350 nm and in the range from 380-450 nm the formation of a second type of transients is indicated. Since the absorption band at about 370 nm is known to represent the protonated form of the one-electron reduced species, 2,2'-bpy-I:I~- (paragraph 3.2), it can be assumed that under the present experimental conditions the same radical type (denoted as RI) is produced by H attack on the N position of the 2,2'-bpy-H ÷ molecule (reaction (5)). The remaining absorption bands at 330 nm and partly at 390 nm can be now attributed to a second type of transients (R2), resulting from the H attack on ring carbons of the substrate molecule (reaction (6)): (5)
ks. ~ ' ~ +
(6,
.-
H~ t
(R,; 2,2'-bpy-H~-) H
H+
- . .e.g. .+
(R2; adduct on ring carbon).
By substraction of the known absorption spectrum of RI species (2,2'-bpy-I:I~- ; spectrum B, Fig. 1) from the total absorption spectrum A, the spectrum C is obtained, which is assigned to the R2 transients. The formation of two kinds of species by the reaction of H with 2,2'-bpy-H ÷ is also demonstrated by the difference of the kinetical courses at the main absorption bands. At 330 nm (R2), the transient absorption disappears much faster than at 365 nm (RI), as shown in Fig. 3. It might be mentioned that the measured deca~ of Rt at 365 nm (Fig. 3) is somewhat faster, compared to the corresponding.curve A in Fig. 2, be-
m B m
I
I
0
160
320 ps
Fig. 3. Measured ODrel at 330 and 365 nm as a function of time (Ws) for transients resulting from reaction of H atoms with 2,2'-bpy-H ÷ (solution: 5 x 10 -5 mol dm -3 2,2'-bpy-H ÷, 2.5 x 10 -2 tool dm -3 t-butanol, pH = 1, airfree, dose: 5 J kg-l). cause of the reaction of t-CaHsOH with R~ (Fig. 2(B)). In order to varify the influence of t-C4HsOH on the decay of RI, separate experiments were performed in a special pressure cell, (18) whereby 5 x 10 -4 mol dm -3 2,2'-bpy-H ÷ (pH = 1) was irradiated in the presence of 140 bar H2 (0.11 mol dm-3). Under these conditions only H atoms can react with 2,2'-bpy-H ÷ , since OH radicals are converted into H atoms by following reaction: (7)
OH + H2--~ H + H20 (k = 2.5 x 107 dm 3 m o l - 1 s - 1 (16)).
The OD/cm values measured at 330 and 365 nm (19) were found to he rather similar to these in t-butanol solutions. The decay kinetics at 365 nm, however, were significantly different. Using H2 a s OH scavenger (pressure cell) the transient absorption is practically stable up to 500 I~s after pulse end (Fig. 4, curve A), whereas in the presence of t-C4HaOH radicals (curve B) a strong transient decay is obServed in the same time range. F o r the reaction of R1 with t-C4HsOH a second order rate constant k = 1.1 × 109 dm 3 mo1-1 s -1, was determined. A similar reaction of t-C4HsOH has been observed previously with methyl viologen cation radical (MV÷). (2°'21) U n d e r the reaction conditions in the pressure cell, for the R2 transient a second order decay (2k/e - 2 x 104 cm s - i ) could be established at ?30 nm. To analyse the kinetics of the simultaneous H attack on C and N position of the 2,2'-bpy-H ÷ molecule a previously described optimization proc e d u r e (22"23) has now been applied. Reactions and rate constants, which were regarded in the calculations are summarized in Table 1. The rate con-
112
SONJA SOLAR
ODre I
A
ODrel.
:
B
365 nm
I
0 0
I
0
I
50
I
I
l
160 ps I
I
100 150 200 laS
Fig. 4. ODr~lat 365 nm as a function of time (V.s) for deoxygenated solutions: (A) 5 x 10 - 4 tool dm -3 2,2'-bpy-H + , 0.11 mol dm -3 H2, pH = 1 and (B) 5 x 10 -4 mol dm -3 2,2'-bpy-H +, 0.25 mol dm -3 t-butanol, pH = 1. stanLk9 for the cross reaction R2 + t-C4HsOH, (9), was assumed to be in the same order of magnitude as ks for reaction (8). The decay of RI (k ~ 0.2 s-1 (9)) is not included in the reaction scheme, because o f its minor importance in the considered time range up to 2000 I~s after pulse end. The formation and decay rate constants for RI and R2, together with extinction coefficients at some wavelengths of
interest are presented in Table 2. As main product RI species (70%) are formed, whereas H adducts on ring carbons amount to only 30%. Comparing these with the results of H attack on 4,4'-bipyridine (4,4'-bpy) in acid, airfree aqueous solutions (24) it has to be stated that the positions of nitrogen in a bipyridine ring system have a considerable influence on the distribution of H atoms. In the case of 4,4'bpy ~ 5 5 % of H atoms are attacking the N position and - 4 5 % are leading to adducts on ring carbons. A reasonable explanation for this fact is probably the different protonation state of both bipyridine molecules. Whereas at pH = 1 4,4'-bpy exists i n its diprotonated form, in the case of 2,2'-bpy only one nitrogen is protonated. The unprotonated nitrogen of 2,2'-bpy is a place of considerably high electron density and therefore H will probably strongly favour this position. On the other hand in the 4,4'-bpy molecule the difference of the electron densities between ring carbons and its protonated nitrogens is not so pronounced and therefore the reactivity of H atoms towards nitrogen and ring carbons is very similar. An influence of the electron density on the reactivity of H atoms with aromatic and heterocyclic compounds, at least to some extent, has been observed previously.(25) In order to find out the most probable reaction sites for H atoms on 2,2'-bpy-H ÷ , calculations on the electron densities of the ring atoms on the substrate as well as on the energetic stabilities of the various formed isomeric H adducts were carried out. 3.4 MNDO calculations The site specificity of O H addition to substituted benzene and to pyridine systems has been refered
T a b l e I. REACTIONS (AND RATE CONSTANTS) CONSIDERED BY PULSE RADIOLYSIS OF 2,2'-BIPYRIDINE 1N THE PRESENCE OF t-BUTANOL IN ACID AQUEOUS SOLUTION ( p H = 1)
No.
Reaction
1
2,2'-bpy-H + + H - - ~ R1
2
R2
3 4
2R2 H + H
5
H + t-C4H9OH
6
O H + t-C4H9OH
7 8 9 10 11 12 13 14 15
2t-(~4HaOH t-~4HsOH + R1 t-C4HsOH + R2 OH + OH H + OH e~q + H + e~ + e~q e~ + H e~q + OH
) products ~ H~ ) t - C 4 H s O H + H2
~ t-C4HsOH + H20 ) (C4HaOH)2 ~ products ) products ) H202 )H20 ) H ) H2 + 2OH~ H2 + O H ~ OH-
Rate constant °4-16) (dm 3 mol- 1 s - 1) kl = ? k2 = .9
2k3 = ? 2k4 = 2.3 x 101° k5 = 3 x 105
k6 2k7 ks k9 2kid ku
= = = = = =
5.5 x 10s 1.4 x 109 1.1 x 109 1.1 x 109 1.2 × 101° 2 x 101° kl2 = 2.3 × 10I° 2kt3 = 1.2 x 101° k]4 --- 2.5 x 101° k15 = 3 x 10l°
H attack on 2,2'-bipyridine in acid aqueous solution
113
Table 2.
FORMATION AND DECAY RATE CONSTANTS ( k ) AND MOLAR EXTINCTION COEFFICIENTS (~) OF TRANSIENTS FORMED BY PULSE RADIOLYSIS OF 2 , 2 ' - b p y - H + IN AIRFREE ACID AQUEOUS SOLUTION IN THE PRESENCE O F / - B U T A N O L
Rate constants (in dm 3 m o l - ' s - I ) for: Transients
Formation
R1
kl = (1.4 ± 0.1) x l0 s
k' ~ 0.2 s -1 t
R2
k2 = (0.6 ± 0.1) x l0 s
2k3 = (1.4 --- 0.2)
Absorption characteristics
Decay
h (nm)
x 109
330 365 420 700 330 420 700
~ (m2 mol-~) 610 4500 640 268 9000 840 350
--- 50 ± 100 _+ 50 --- 20 ___ 500 --- 40 --- 25
t Reference (9). to the electrophilic character of OH (e.g. Refs. (26) and (27)). As a sequence of this the prefered reaction site of OH should be the least electron deficient position o f an aromatic ring. Anthony et al. (2s~ reported recently a new approach in analyzing the site specificity of OH for pyridine systems using ab initio S C F - M O calculations on the stabilities (total energies, heats of formation) of the isomeric radical products. The authors demonstrated that based on the electron densities on pyridine OH should react preferentially at the nitrogen position of this molecule, whereas calculating the relative energies o f the different OH adducts metaC ring carbon should be the favoured reaction site. Experimentally the meta OH adduct was found to be the main product (>80%). (2o Therefore, considerations on the stabilities of the formed transients seem to be a very promising way for estimating the site of radical attack. To gain a better information o f the prefered positions of H attack on 2,2'-bpy-H ÷ a semiempirical S C F - M O technique (MNDO (2m) was applied to calculate the net atomic charges on all ring atoms as well as the total energies and heats of formation ( A H s) of all possible isomeric radical products. F o r the calculations experimental parameters °°~ have been taken for the geometry of the pyridine ring system and its protonated form. The same geometries have been used as the initial guess for the geometry optimization for all different isomeric radical products. The series of all radical species studied in this work are shown in Fig. 5 ( X I I - X X I I I ) .
H Fig. 5. Numbering of the atoms in pyridine and 2,2'-bipyridine species.
F o r comparison theoretical calculations have also been carried out for the simplest of the heterocyclic compounds, namely pyridine and its protonated form, pyridinium ion, as well as for all possible H adducts of these molecules (Fig. 6, I - X ) . Addition of H to pyridine and pyridinium ion can occur at four different positions of the ring (nitrogen; carbons at the ortho, meta, and para position). The remaining two positions in the ring can be treated as symmetrically identical to one of these positions. The calculated net atomic charges on all ring atoms are presented in Table 3 for pyridine, pyridinium ion, and 2,2'-bpy-H ÷. In the first two systems the order of electron density on the ring atoms is calculated to be: N > meta-C >> para-C > ortho-C. By protonation of pyridine the sequence of ring atoms remains the same but it is obvious that the electron density on N position is stronger decreasing than on the meta-C position. Regarding the calculated total energies and heats of formation (Table 4) the order of the site preference of H addition to pyridine is nearly the same as obtained considering the electron densities. In the case of pyridinium ion, however, a complete change in site specificity takes place resulting in the sequence ortho-C > meta-C > para-C >> N.
Evidently, by protonation of pyridine, N atom changes from the prefered to the least favoured position of H attack. Considering these facts and in analogy to the OH reactivity towards pyridine systems (26,2s) it can be assumed that the site specificity of H addition is not determined predominantly by the prefered reaction at the least electron deficient ring positions. Rather, the energetic stability of the formed radicals appears to play a major role in determining the order of product preference. Comparing the results of pyridine and pyridinium ion with 2,2'-bpy-H + (Fig. 6, XI) the electron densities on the corresponding atoms are very similar (Table 3). Based on the net atomic charges cal-
114
SONJA SOLAR T a b l e 3. NET ATOMIC CHARGES ON RING ATOMS OF PYRIDINE. BIPYRIDINIUM ION, AND 2 , 2 ' BIPYRIDINE CATION
Atomt
Molecule pyridine
Ion pyridine
Atomt
2,2'-bipyridine cation
1 2 3 4 5 6
-0.2312 0.0544 -0.1241 -0.0074 -0.1241 0.0544
-0.1403 0.1439 -0.0855 0.1015 -0.0855 0.1439
l 2 3 4 5 6 1' 2' 3' 4' 5' 6'
-0.2513 -0.0295 -0.0248 -0.0123 -0.0461 0.0832 -0.1295 0.2239 -0.1000 0.0890 -0.0900 0.1321
t For the number sequence, see Fig. 5.
culated for 2,2'-bpy-H + the preferential order for H addition on the different positions of the two ring systems is N I ~ N v > C y > C 5' > C 5. It should be noted, that the differences in the electronic densities from N v to C 5 are very small, only the addition on N ~ position should be expected to be favoured to a considerable extent. Regarding the calculated total energies and heats of formation (Table 4) the order of the site preference should be N ~ > C 5 > C a > C 5'. Based on the differences in the heats of formation it can be concluded that only few of the isomeric products are of some importance, namely N I adduct (XII) and the isomers on C 5 (XIV) and C 3 (XVI). Comparing all these results for pyridine, pyridinium ion, and 2,2'-bpy-H +, it can be stated that
the unprotonated nitrogen is in all cases the most favoured reaction site for H atoms. Although possible effects due to solvent interaction have not been considered in the calculations these findings are in v e r y good agreement with the experimental observation that 70% of H atoms react with 2,2'bpy-H + under formation of an H adduct on N 1 position (RI, Table 2). Based on the fact that considerations on the energetic stabilities of the formed products deliver better results than those derived from charge distributions on the substrate molecule, (28) it can be assumed that R2 represents an H adduct on ring carbons of the nonprotonated ring of 2,2'-bpy-H +. Though the intermediates C 5 (XIV) and C 3 (XVI) exhibit rather small differences in its total energies as well as in its AHy (Table 4), an
Table 4. CALCULATED TOTAL ENERGIES AND HEATS OF FORMATION ( A n y ) OF PYRIDINE. PYRIDINIUM ION, 2,2'-BIPYRIDINE CATION AND ITS DIFFERENT ISOMERIC H ADDUCTS
Moleculest I II III IV
(meta) (ortho) (para)
V
(N)
VI VII VIII IX X
(meta) (ortho) (para) (N)
Total energy (eV) -916.618 -930.214 -930.003 -929.919 -930.214
AH: (Kcal/mol)
44 17 21 80 23
28.723 41.864 46.729 48.653 41.863
-923.887 12 -937.370 14 -937.463 93 - 937.342 98 -936.728 40
187.772 203.513 201.350 204.139 218.312
Structures of molecules I-XXIII are given in Fig. 6.
Moleculest XI XII XIII XIV XV XVI XVII XVIII XIX XX XXI XXII XXIII
All:
Total energy (eV)
(Kcal/mol)
- 1812.319 52 -1826.096 10 -1825.527 82 - 1826.073 31 - 1825.664 72 - 1825.982 14 -1825.384 13 -1825.723 12 - 1825.639 45 - 1825.681 02 - 1825.721 35 - 1824.360 21 - 1824.208 91
213.148 222.119 235.224 222.645 232.067 224.747 238.711 230.726 228.889 229.515 230.971 261.212 265.640
115
H attack on 2,2'-bipyridine in acid aqueous solution H
I
II
H
XI
~HH ~ III
IV
H
H
XII
H
~ H
~
V
Vl
~H ~HH H~ VII
H
XIII
VIII
IX
H
H
XIV
XV
H
H
H XVI
HH X
H/ \ H XVII
HH XVIII
H
H
XIX
XX
H
H XXI
H XXII
H XXIII
Fig. 6. Structures of molecules, ions, and radicals studied. assignment of R2 to C 5 (XIV) species seems more reasonable because of the high symmetry of this radical. This could eventually partly explain the rather high extinction coefficient found for the R2 transients.
fen for valuable advices regarding the efficient use of this program. The financial support of the Ludwig Boltzmann Institut fOr Strahlenchemie, Wien, is gratefully acknowledged.
REFERENCES CONCLUSION It can be stressed that the species produced by reaction of e~q with 2,2'-bpy, which protonate immediately, are resulting exclusively in an H adduct on N atom (R0. The same species are obtained by one-electron reduction of the substrate, using (CH3)2(2OH, (CH3)2CO-, etc. The reduction with H atoms, however, occurs only to 70% on N ~ position of 2,2'-bpy-H +, the remaining 30% H are leading to H adducts on ring carbons (R2), probably on C 5 position (XIV). The attribution to this carbon site is based on MNDO calculations. The results are of interest for solar energy utilizing systems containing 2,2'-bpy or its derivatives as sensitizer. Acknowledgments--The author appreciates very much the valuable discussions and encouragements by Professor Dr. N. Getoff during the course of this work. Thanks are also expressed to Professor Dr. D. Schulte-Frohlinde, Max-Planck-Institut for Strahlenchemie, MQlheim/Ruhr, F.R.G. for the permission to use the pulse-radiolysis facility. Gratitude is directed to Dr. K. Sehested for the use of the accelerator with pressure cell at RIS0, Denmark. The author is further indebted to Professor Dr. H. Lischka for the disposal of an MNDO program and to Dr. A. Karp-
1. N. SUTINand C. CREUTZ,Adv. Chem. Ser. 1978, 168, 1. 2. N. SUTXSand C. CREUTZ,Pure Appl. Chem. 1980, 52, 2717. 3. C. V. KRISHNAN, C. CREUTZ, D. MAHAJAN, H. A. SCHWARZand N. SUTIN,IsraelJ. Chem. 1982, 22, 98, and references therein. 4. D. MEISEL, M. S. MATHESON,W. A. MULACand J. RABANI, J. Phys. Chem. 1977, 81, 1449. 5. M. KIRCH, J. M. LEHN and J. P. SAUVAGE,Heir. Chim. Acta 1979, 62, 1345. 6. E. BORGARELLO,J. KIW1,E. PELIZETTI,M. VISCAand M. GRATZEL,J. Am. Chem. So¢. 1981, 103, 6324. 7. M. SIMICand M. EBERT,Int. J. Radiat. Phys. Chem. 1971, 3, 259. 8. M. Z. HOFFMAN,M. G. SIMIC, Q. R. MULAZZANI,S. EMIl, P. G. FUOCHIand M. VENTURI,Radiat. Phys. Chem. 1978, 12, 111. 9. Q. G. MULAZZANI,S. EMMI, P. G. FUOCHI,M. VENTURI, M. Z. HOFFMANand M. G. SIMIC, J. Phys. Chem. 1979, 83, 1582. 10. E. HAYONand M. SIM1C,ACC. Chem. Res. 1974, 7, 114. 11. N. GETOFFand F. SCHWORER,Radiat. Res. 1970, 41, 1. 12. N. GETOFFand F. SCHWORER,J. Radiat. Phys. Chem. 1971, 3, 429. 13. S. SOLAR,W. SOLARand N. GETO~, J. Phys. Chem. 1984, 88, 2091.
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14. M. GOrlN and N. GETOFF, J. Chem. Soc. Faraday Trans. 1 1977, 73, 1207. 15. M. ANBAR, M. BAMBENEKand A. B. Ross, Natl. Stand. Ref. Data Ser. Natl. Bur. Stand. No. 43 (1973). 16. M. ANBAR,FARHATAZXZand A. B. Ross, Natl. Stand. Ref. Data Ser. Natl. Bur. Stand. No. 51 (1975). 17. FARHATAZXZand A. B. Ross, Natl. Stand. Ref. Data Ser. Natl. Bur. Stand. No. 59 (1977). 18. H. CHRIS~r4SEN and K. SEHESTED, Radiat. Phys. Chem. 1980, 16, 184. 19. S. SOLAR, W. SOLAR, N. GETOFF, J. HOLCMANand K. SEHESTED(to be published). 20. A. LEDWITH,Acc. Chem. Res. 1972, 5, 133. 21. S. SOLAR, W. SOLAR, N. GETOFF, J. HOLCMANand K. SEHESTED,J. Chem. Soc. Faraday Trans. 1 1984, 80, 2929. 22. S. SOLAR, W. SOLARand N. GETOFF, Radiat. Phys. Chem. 1983, 21, 129.
23. S. SOLAR, W. SOLARand N. GETOFF, J. Chem. Soc. Faraday Trans. 2 1983, 79, 123. 24. S. SOt.AR,J. Phys. Chem. 1984, 88, 5624. 25. P. NETAand R. H. SCnULER,J. Am. Chem. Soc. 1972, 94, 1056. 26. S. STEm~KENand P. O'NEmL, J. Phys. Chem. 1978, 82, 372. 27. N. V. RAOHAVANand S. STEENKEN,J. Am. Chem. Soc. 1980, 102, 3495. 28. M.C. ANTHONY,W. L. WALTZand P. G. MEZEY,Can. J. Chem. 1982, 60, 813. 29. M. J. S. DEWARand W. TmEL, J. Am. Chem. Soc. 1977, 99, 4899, 4907. 30, Comprehensive Organic Chemistry. Vol. 4. (Edited by P. G. Sammes). Pergamon Press, Oxford, 1979. 31. The valuable help given by Mr. K.-H. Toepfer and Mr. F. Reikowski is very much appreciated.
0146-5724/85 $3.00 + .00 © Pergamon Press Ltd.
H ATTACK ON 2,2'-BIPYRIDINE IN ACID AQUEOUS
SOLUTIONt SONJA SOLAR Institut ftir Theoretische Chemie und Strahlenchemie der Universitiit Wien and Ludwig Boltzmann Institut for Strahlenchemie, A-1090 Wien, W~ihringerstr. 38, Austria
(Received 14 May 1984) Abstract--In deoxygenated acid aqueous solution (pH = 1) 70% H attack the N position of 2,2'bipyridine (2,2'-bpy-H +) with k = (1.4 --- 0.1) x 10s dm3 mol-t s-t, producing relatively long-lived species Rt (kmax = 365 nm, e365 = 4500 m2 mol-t); 30% H react on C-atom positions with k = (0.6 --- 0.1) x 108 dm3 mol- t s - t resulting in transients R2 (kmax = 330 nm, e33o = 9000 m2 mol- ~, decay 2k = (1.4 --- 0.2) x 109 dm3 mol -t s-t). The formation of Rt species by reaction of (CH3)2COH radicals with 2,2'-bpy-H ÷ was reinvestigated. For the reactivity of e~-qwith 2,2'-bpy a rate constant k(e~q + 2,2'-bpy) = (1.95 _+ 0.1) x 101°dm3 mol -t s - l a t pH = 8 was determined. An attempt has been made to analyse the site specificity of H addition to the 2,2'-bipyridine molecule in terms of the energetic stability of the possible isomeric transients as well as of the net atomic charges on all ring atoms of the substrate. The calculations were based on a semiempirical SCF-MO technique (MNDO).
1. INTRODUCTION
2. EXPERIMENTAL SECTION
2,2'-bipyridine (2,2'-bpy) and its transient metal complexes have been subject of various studies, particularly as energy and electron transfer agents in their excited states. They have been investigated photochemically (e.g. Refs. (1-6)) as well as by pulse radiolysis. ~7-9) Hoffman et al. ~s~ studied the reaction of 2,2'-bpy and its protonated form 2,2'bpy-H ÷ ( p K = 4.3) with various reducing radicals (Red) like eaq, C O 2 , (CH3)2COH, and (CH3)2CO-. The resulting reduced species were assumed to protonate immediately (2,2'-bpy + Red + H+--> 2,2'b p y - H , ts-l°)) yielding 2,2'-bpy-H" or 2,2'-bpy-tZI~( p K = 5.6) with absorption maxima at 365 (e = 3000 m 2 mol -~) and 375 nm (e = 4500 m E m o l - t ) , respectively. ~s) Simic and Ebert ~7) reported an absorption spectrum for transients obtained from reaction of 2,2'-bipyridine with e~q at pH = 7-12.7 (kmax = 365 and 450 nm) as well as with H atoms at pH - 1 (emax = 370 nm), however, no precise reaction mechanism has been given. Pulse radiolysis experiments were now undertaken in order to elucidate the reactivity of H atoms towards 2,2'-bpy-H + in more detail, particularly in respect to the specific H attack on the C and N position of this molecule.
The pulse radiolysis equipment (3 MeV Van de Graaff accelerator type K, High Voltage Engineering Co., Burlington, USA) (3t) combined with a Biomation 8100 transient recorder controlled by a PDP-11/10 computer) has been described previously.(~ 1-13) 2,2'-bpy was zone refined and all other chemicals (t-butanol, propanol-2, HC104, Ba(OH)2; p.A. Merck) were used without further purification. The solutions of 5 × 10 -5 to 5 x 10 -4 mol dm -3 2,2'bpy were prepared with at least four times distilled water and were deoxygenated by purging with high purity argon for about 1 h. The samples were irradiated in a 2 cm quartz cell (optical light path: 3 x 2 cm) with a dose of 5 to 10 J kg-a per 0.4 ixs pulse and the solution was exchanged after each single pulse. All computations in respect to the simulation and optimization of the kinetic and spectroscopic parameters were performed on the CDC computer (CYBER) of the Vienna University.
t The author would like to dedicate this paper to Professor Dr. K. Schl6gl on the occasion of his 60th anniversary.
3. RESULTS AND DISCUSSION 3.1 Reactivity of 2,2'-bpy with e~q The rate constant ofe~q with 2,2'-bpy (5 x 10 -5 mol dm -3 2,2'-bpy, 10 -2 mol dm -3 t-butanol, Ar) was determined at pH = 8 by following the pseudofirst order decay of e~q at 720 nm. Matrix correction
109
110
SONJA SOLAR
was applied, w h e r e k(OH + t-C4HgOH) = 5.5 x 108 dm 3 m o l - 1 s - i.(14) The m e a n value of 20 measu r e m e n t s was (1)
k(e~q
+ 2,2'-bpy) = (1.95 -+ 0.1) x
°11 \
0.08101° dm 3 m o l - ' s - I .
This v a l u e is in g o o d a g r e e m e n t with p r e v i o u s l y reported values (k = (1.8 - 0.2) × 10 I° d m 3 mo1-1 s - i at p H = 6 to 7 "5) and k = 2.5 × 101° d m 3 m o l - 1 s - I at p H = 9.2(7)). 3.2 Reactivity of 2,2'-bpy-H ÷ in the presence of
reducing radicals T h e f o r m a t i o n o f the o n e - e l e c t r o n r e d u c e d 2,2'bpy species was reinvestigated using d e o x y g e n a t e d solutions of 5 x 10 -4 mol d m -3 2 , 2 ' - b p y - H ÷ in the p r e s e n c e o f 0.2 mol dm -3 propanol-2 at p H = 1 to 2, adjusted with HCIO4. In this case e~q is c o n v e r t e d into H a t o m and both, H and O H , react with propanol-2 resulting in the strongly reducing species, (CHa)2CHO with G = Gr~ + Ge~ + GOH = 6.5: (2)
0D
cm
eZa + H + --~ H
0.06!
°oi:i: 350
450
550
650
nm
Fig. 1. Total transient absorption spectrum (A), obtained by H attack on 5 × 10 - 4 mol dm -3 2,2'-bpy-H + in the presence of 0.25 mol dm -3 t-butanol in aqueous deoxygenated solution, pH = 1 (OD/cm normalized to 11 J kg- ' per 0.4 txs pulse)• Spectrum (C) obtained by substraction of transient absorption spectrum (B) in insert II from spectrum A (A - B = C). Insert I: Absorption spectrum of 10 -4 mol dm -3 2,2'-bpy, pH = 1. Insert II: Absorption spectrum of 2,2'-bpy-l:I~- species (R0 obtained by one electron reduction of 2,2'-bpy (see the text)•
(k = 2.3 x 1010dm a m o l - ' s - l ( m ) , (3)
H (or O H ) + (CH3)2CHOH ~ -t- H2 (or H 2 0 )
(CH3)2COH
(k = 7 x l07 d m 3 m o l - 1
x s - l / 1 . 3 x 109 dm 3 mo1-1 s -1 .6.17)), (4)
0.9. T h e rest of O H as well as the H a t o m s are reacting with propanol-2 to f o r m (CH3)2COH with G = 5.6. U n d e r these e x p e r i m e n t a l conditions the d e c a y of 2,2'-bpy-H~- b e c o m e s significantly faster
(CH3)E~OH + 2 , 2 ' - b p y - H + ---> 2,2'-bpy-I:I~ + (CH3)2C~--------O (k = 3.5 x 10s dm 3 x m o l - 1 S - 1 (9)).
T h e resulting absorption s p e c t r u m o f 2,2'-bpy-H~exhibits t w o m a x i m a at 370 nm (e = 4500 m 2 m o l - ') and a b o u t 800 nm (e = 335 m 2 mol - ' ) as well as a shoulder at h = 420 nm (e = 630 m 2 m o l - ' ) . In Fig. 1, insert II only the first part of s p e c t r u m B is displayed. It is in fair a g r e e m e n t with the p r e v i o u s l y d e t e r m i n e d spectra, m•9) The 2,2'-bpy-H~- species are rather stable in the investigated time range (up to 2 ms; see also Fig. 2(A)). F o r their d e c a y at low doses (4 J kg - I ) a first o r d e r p r o c e s s has been reported (k - 0.2 s - i (s)). In o r d e r to c h e c k w h e t h e r t-butanol radicals (t(~4HsOH) are r e a c t i v e t o w a r d s 2 , 2 ' - b p y - I : I f , a d e o x y g e n a t e d solution o f a mixture o f 0.1 mol dm -3 propanol-2 and 0.1 mol d m -3 t-butanol in the prese n c e o f 2 × 10 -4 mol dm -3 2 , 2 ' - b p y - H + at p H = 1 was pulse radiolyzed. Since k ( O H + t-butanol) = 5.5 x l0 s d m 3 m o l - ~ s - 1,o4) about 30% O H are s c a v e n g e d by t-butanol leading to G(t-C4HsOH) =
ODret
A
A
365 nrn
420 n m
!
a
B
i
365
oDt "ll I
0
I
I
420 n m
nm i
800 IJs
I
0
I
I
I
800 Ns
Fig. 2. Influence of t-C4HsOH radical on the decay of 2,2'bpy-l:t~- in deoxygenated solutions. Solution A: 2 × 10 4 mol dm -3 2,2'-bpy-H ÷ , 0.1 tool dm -3 propanol-2, pH = 1. Solution B: as A, but containing 0.1 mol dm -3 tC4H9OH additionally.
111
H attack on 2,2'-bipyridine in acid aqueous solution within the first 400 Ixs as shown in Fig. 2(B). This fact indicates a reaction of t-C4HsOH with 2,2'-bpy-I:/~-, which is in contrast to previous observations.(S)
k.,ll-/r~ I B m
330nm
m
3.3 Reactivity o f H atoms with 2,2'-bpy-H ÷ The total absorption spectrum of transients, formed by reaction of H atoms with 2,2'-bpy-H ÷ was measured in the range from 330-820 nm. Using a deoxygenated solution of 5 x 10 - 4 mol d m - 3 2,2'b p y - H ÷ in the presence of 0.25 mol dm -3 t-butanol as OH scavenger (pH = 1; dose: 5 - 6 J k g - I per 0.4 VLspulse) the maximum transient absorption was attained at about 20 IXS after pulse. It is normalized to 11 J k g - ~ and is displayed as spectrum A in Fig. 1. The absorption of the starting compound is given for comparison in insert I, Fig. 1. The transient spectrum A exhibits doubtless the features of the 2,2'-bpy-H~- species, obtained by reaction of 2,2'bpy-H ÷ with (CH3)2(~OH, especially in the range 350-400 nm. At X < 350 nm and in the range from 380-450 nm the formation of a second type of transients is indicated. Since the absorption band at about 370 nm is known to represent the protonated form of the one-electron reduced species, 2,2'-bpy-I:I~- (paragraph 3.2), it can be assumed that under the present experimental conditions the same radical type (denoted as RI) is produced by H attack on the N position of the 2,2'-bpy-H ÷ molecule (reaction (5)). The remaining absorption bands at 330 nm and partly at 390 nm can be now attributed to a second type of transients (R2), resulting from the H attack on ring carbons of the substrate molecule (reaction (6)): (5)
ks. ~ ' ~ +
(6,
.-
H~ t
(R,; 2,2'-bpy-H~-) H
H+
- . .e.g. .+
(R2; adduct on ring carbon).
By substraction of the known absorption spectrum of RI species (2,2'-bpy-I:I~- ; spectrum B, Fig. 1) from the total absorption spectrum A, the spectrum C is obtained, which is assigned to the R2 transients. The formation of two kinds of species by the reaction of H with 2,2'-bpy-H ÷ is also demonstrated by the difference of the kinetical courses at the main absorption bands. At 330 nm (R2), the transient absorption disappears much faster than at 365 nm (RI), as shown in Fig. 3. It might be mentioned that the measured deca~ of Rt at 365 nm (Fig. 3) is somewhat faster, compared to the corresponding.curve A in Fig. 2, be-
m B m
I
I
0
160
320 ps
Fig. 3. Measured ODrel at 330 and 365 nm as a function of time (Ws) for transients resulting from reaction of H atoms with 2,2'-bpy-H ÷ (solution: 5 x 10 -5 mol dm -3 2,2'-bpy-H ÷, 2.5 x 10 -2 tool dm -3 t-butanol, pH = 1, airfree, dose: 5 J kg-l). cause of the reaction of t-CaHsOH with R~ (Fig. 2(B)). In order to varify the influence of t-C4HsOH on the decay of RI, separate experiments were performed in a special pressure cell, (18) whereby 5 x 10 -4 mol dm -3 2,2'-bpy-H ÷ (pH = 1) was irradiated in the presence of 140 bar H2 (0.11 mol dm-3). Under these conditions only H atoms can react with 2,2'-bpy-H ÷ , since OH radicals are converted into H atoms by following reaction: (7)
OH + H2--~ H + H20 (k = 2.5 x 107 dm 3 m o l - 1 s - 1 (16)).
The OD/cm values measured at 330 and 365 nm (19) were found to he rather similar to these in t-butanol solutions. The decay kinetics at 365 nm, however, were significantly different. Using H2 a s OH scavenger (pressure cell) the transient absorption is practically stable up to 500 I~s after pulse end (Fig. 4, curve A), whereas in the presence of t-C4HaOH radicals (curve B) a strong transient decay is obServed in the same time range. F o r the reaction of R1 with t-C4HsOH a second order rate constant k = 1.1 × 109 dm 3 mo1-1 s -1, was determined. A similar reaction of t-C4HsOH has been observed previously with methyl viologen cation radical (MV÷). (2°'21) U n d e r the reaction conditions in the pressure cell, for the R2 transient a second order decay (2k/e - 2 x 104 cm s - i ) could be established at ?30 nm. To analyse the kinetics of the simultaneous H attack on C and N position of the 2,2'-bpy-H ÷ molecule a previously described optimization proc e d u r e (22"23) has now been applied. Reactions and rate constants, which were regarded in the calculations are summarized in Table 1. The rate con-
112
SONJA SOLAR
ODre I
A
ODrel.
:
B
365 nm
I
0 0
I
0
I
50
I
I
l
160 ps I
I
100 150 200 laS
Fig. 4. ODr~lat 365 nm as a function of time (V.s) for deoxygenated solutions: (A) 5 x 10 - 4 tool dm -3 2,2'-bpy-H + , 0.11 mol dm -3 H2, pH = 1 and (B) 5 x 10 -4 mol dm -3 2,2'-bpy-H +, 0.25 mol dm -3 t-butanol, pH = 1. stanLk9 for the cross reaction R2 + t-C4HsOH, (9), was assumed to be in the same order of magnitude as ks for reaction (8). The decay of RI (k ~ 0.2 s-1 (9)) is not included in the reaction scheme, because o f its minor importance in the considered time range up to 2000 I~s after pulse end. The formation and decay rate constants for RI and R2, together with extinction coefficients at some wavelengths of
interest are presented in Table 2. As main product RI species (70%) are formed, whereas H adducts on ring carbons amount to only 30%. Comparing these with the results of H attack on 4,4'-bipyridine (4,4'-bpy) in acid, airfree aqueous solutions (24) it has to be stated that the positions of nitrogen in a bipyridine ring system have a considerable influence on the distribution of H atoms. In the case of 4,4'bpy ~ 5 5 % of H atoms are attacking the N position and - 4 5 % are leading to adducts on ring carbons. A reasonable explanation for this fact is probably the different protonation state of both bipyridine molecules. Whereas at pH = 1 4,4'-bpy exists i n its diprotonated form, in the case of 2,2'-bpy only one nitrogen is protonated. The unprotonated nitrogen of 2,2'-bpy is a place of considerably high electron density and therefore H will probably strongly favour this position. On the other hand in the 4,4'-bpy molecule the difference of the electron densities between ring carbons and its protonated nitrogens is not so pronounced and therefore the reactivity of H atoms towards nitrogen and ring carbons is very similar. An influence of the electron density on the reactivity of H atoms with aromatic and heterocyclic compounds, at least to some extent, has been observed previously.(25) In order to find out the most probable reaction sites for H atoms on 2,2'-bpy-H ÷ , calculations on the electron densities of the ring atoms on the substrate as well as on the energetic stabilities of the various formed isomeric H adducts were carried out. 3.4 MNDO calculations The site specificity of O H addition to substituted benzene and to pyridine systems has been refered
T a b l e I. REACTIONS (AND RATE CONSTANTS) CONSIDERED BY PULSE RADIOLYSIS OF 2,2'-BIPYRIDINE 1N THE PRESENCE OF t-BUTANOL IN ACID AQUEOUS SOLUTION ( p H = 1)
No.
Reaction
1
2,2'-bpy-H + + H - - ~ R1
2
R2
3 4
2R2 H + H
5
H + t-C4H9OH
6
O H + t-C4H9OH
7 8 9 10 11 12 13 14 15
2t-(~4HaOH t-~4HsOH + R1 t-C4HsOH + R2 OH + OH H + OH e~q + H + e~ + e~q e~ + H e~q + OH
) products ~ H~ ) t - C 4 H s O H + H2
~ t-C4HsOH + H20 ) (C4HaOH)2 ~ products ) products ) H202 )H20 ) H ) H2 + 2OH~ H2 + O H ~ OH-
Rate constant °4-16) (dm 3 mol- 1 s - 1) kl = ? k2 = .9
2k3 = ? 2k4 = 2.3 x 101° k5 = 3 x 105
k6 2k7 ks k9 2kid ku
= = = = = =
5.5 x 10s 1.4 x 109 1.1 x 109 1.1 x 109 1.2 × 101° 2 x 101° kl2 = 2.3 × 10I° 2kt3 = 1.2 x 101° k]4 --- 2.5 x 101° k15 = 3 x 10l°
H attack on 2,2'-bipyridine in acid aqueous solution
113
Table 2.
FORMATION AND DECAY RATE CONSTANTS ( k ) AND MOLAR EXTINCTION COEFFICIENTS (~) OF TRANSIENTS FORMED BY PULSE RADIOLYSIS OF 2 , 2 ' - b p y - H + IN AIRFREE ACID AQUEOUS SOLUTION IN THE PRESENCE O F / - B U T A N O L
Rate constants (in dm 3 m o l - ' s - I ) for: Transients
Formation
R1
kl = (1.4 ± 0.1) x l0 s
k' ~ 0.2 s -1 t
R2
k2 = (0.6 ± 0.1) x l0 s
2k3 = (1.4 --- 0.2)
Absorption characteristics
Decay
h (nm)
x 109
330 365 420 700 330 420 700
~ (m2 mol-~) 610 4500 640 268 9000 840 350
--- 50 ± 100 _+ 50 --- 20 ___ 500 --- 40 --- 25
t Reference (9). to the electrophilic character of OH (e.g. Refs. (26) and (27)). As a sequence of this the prefered reaction site of OH should be the least electron deficient position o f an aromatic ring. Anthony et al. (2s~ reported recently a new approach in analyzing the site specificity of OH for pyridine systems using ab initio S C F - M O calculations on the stabilities (total energies, heats of formation) of the isomeric radical products. The authors demonstrated that based on the electron densities on pyridine OH should react preferentially at the nitrogen position of this molecule, whereas calculating the relative energies o f the different OH adducts metaC ring carbon should be the favoured reaction site. Experimentally the meta OH adduct was found to be the main product (>80%). (2o Therefore, considerations on the stabilities of the formed transients seem to be a very promising way for estimating the site of radical attack. To gain a better information o f the prefered positions of H attack on 2,2'-bpy-H ÷ a semiempirical S C F - M O technique (MNDO (2m) was applied to calculate the net atomic charges on all ring atoms as well as the total energies and heats of formation ( A H s) of all possible isomeric radical products. F o r the calculations experimental parameters °°~ have been taken for the geometry of the pyridine ring system and its protonated form. The same geometries have been used as the initial guess for the geometry optimization for all different isomeric radical products. The series of all radical species studied in this work are shown in Fig. 5 ( X I I - X X I I I ) .
H Fig. 5. Numbering of the atoms in pyridine and 2,2'-bipyridine species.
F o r comparison theoretical calculations have also been carried out for the simplest of the heterocyclic compounds, namely pyridine and its protonated form, pyridinium ion, as well as for all possible H adducts of these molecules (Fig. 6, I - X ) . Addition of H to pyridine and pyridinium ion can occur at four different positions of the ring (nitrogen; carbons at the ortho, meta, and para position). The remaining two positions in the ring can be treated as symmetrically identical to one of these positions. The calculated net atomic charges on all ring atoms are presented in Table 3 for pyridine, pyridinium ion, and 2,2'-bpy-H ÷. In the first two systems the order of electron density on the ring atoms is calculated to be: N > meta-C >> para-C > ortho-C. By protonation of pyridine the sequence of ring atoms remains the same but it is obvious that the electron density on N position is stronger decreasing than on the meta-C position. Regarding the calculated total energies and heats of formation (Table 4) the order of the site preference of H addition to pyridine is nearly the same as obtained considering the electron densities. In the case of pyridinium ion, however, a complete change in site specificity takes place resulting in the sequence ortho-C > meta-C > para-C >> N.
Evidently, by protonation of pyridine, N atom changes from the prefered to the least favoured position of H attack. Considering these facts and in analogy to the OH reactivity towards pyridine systems (26,2s) it can be assumed that the site specificity of H addition is not determined predominantly by the prefered reaction at the least electron deficient ring positions. Rather, the energetic stability of the formed radicals appears to play a major role in determining the order of product preference. Comparing the results of pyridine and pyridinium ion with 2,2'-bpy-H + (Fig. 6, XI) the electron densities on the corresponding atoms are very similar (Table 3). Based on the net atomic charges cal-
114
SONJA SOLAR T a b l e 3. NET ATOMIC CHARGES ON RING ATOMS OF PYRIDINE. BIPYRIDINIUM ION, AND 2 , 2 ' BIPYRIDINE CATION
Atomt
Molecule pyridine
Ion pyridine
Atomt
2,2'-bipyridine cation
1 2 3 4 5 6
-0.2312 0.0544 -0.1241 -0.0074 -0.1241 0.0544
-0.1403 0.1439 -0.0855 0.1015 -0.0855 0.1439
l 2 3 4 5 6 1' 2' 3' 4' 5' 6'
-0.2513 -0.0295 -0.0248 -0.0123 -0.0461 0.0832 -0.1295 0.2239 -0.1000 0.0890 -0.0900 0.1321
t For the number sequence, see Fig. 5.
culated for 2,2'-bpy-H + the preferential order for H addition on the different positions of the two ring systems is N I ~ N v > C y > C 5' > C 5. It should be noted, that the differences in the electronic densities from N v to C 5 are very small, only the addition on N ~ position should be expected to be favoured to a considerable extent. Regarding the calculated total energies and heats of formation (Table 4) the order of the site preference should be N ~ > C 5 > C a > C 5'. Based on the differences in the heats of formation it can be concluded that only few of the isomeric products are of some importance, namely N I adduct (XII) and the isomers on C 5 (XIV) and C 3 (XVI). Comparing all these results for pyridine, pyridinium ion, and 2,2'-bpy-H +, it can be stated that
the unprotonated nitrogen is in all cases the most favoured reaction site for H atoms. Although possible effects due to solvent interaction have not been considered in the calculations these findings are in v e r y good agreement with the experimental observation that 70% of H atoms react with 2,2'bpy-H + under formation of an H adduct on N 1 position (RI, Table 2). Based on the fact that considerations on the energetic stabilities of the formed products deliver better results than those derived from charge distributions on the substrate molecule, (28) it can be assumed that R2 represents an H adduct on ring carbons of the nonprotonated ring of 2,2'-bpy-H +. Though the intermediates C 5 (XIV) and C 3 (XVI) exhibit rather small differences in its total energies as well as in its AHy (Table 4), an
Table 4. CALCULATED TOTAL ENERGIES AND HEATS OF FORMATION ( A n y ) OF PYRIDINE. PYRIDINIUM ION, 2,2'-BIPYRIDINE CATION AND ITS DIFFERENT ISOMERIC H ADDUCTS
Moleculest I II III IV
(meta) (ortho) (para)
V
(N)
VI VII VIII IX X
(meta) (ortho) (para) (N)
Total energy (eV) -916.618 -930.214 -930.003 -929.919 -930.214
AH: (Kcal/mol)
44 17 21 80 23
28.723 41.864 46.729 48.653 41.863
-923.887 12 -937.370 14 -937.463 93 - 937.342 98 -936.728 40
187.772 203.513 201.350 204.139 218.312
Structures of molecules I-XXIII are given in Fig. 6.
Moleculest XI XII XIII XIV XV XVI XVII XVIII XIX XX XXI XXII XXIII
All:
Total energy (eV)
(Kcal/mol)
- 1812.319 52 -1826.096 10 -1825.527 82 - 1826.073 31 - 1825.664 72 - 1825.982 14 -1825.384 13 -1825.723 12 - 1825.639 45 - 1825.681 02 - 1825.721 35 - 1824.360 21 - 1824.208 91
213.148 222.119 235.224 222.645 232.067 224.747 238.711 230.726 228.889 229.515 230.971 261.212 265.640
115
H attack on 2,2'-bipyridine in acid aqueous solution H
I
II
H
XI
~HH ~ III
IV
H
H
XII
H
~ H
~
V
Vl
~H ~HH H~ VII
H
XIII
VIII
IX
H
H
XIV
XV
H
H
H XVI
HH X
H/ \ H XVII
HH XVIII
H
H
XIX
XX
H
H XXI
H XXII
H XXIII
Fig. 6. Structures of molecules, ions, and radicals studied. assignment of R2 to C 5 (XIV) species seems more reasonable because of the high symmetry of this radical. This could eventually partly explain the rather high extinction coefficient found for the R2 transients.
fen for valuable advices regarding the efficient use of this program. The financial support of the Ludwig Boltzmann Institut fOr Strahlenchemie, Wien, is gratefully acknowledged.
REFERENCES CONCLUSION It can be stressed that the species produced by reaction of e~q with 2,2'-bpy, which protonate immediately, are resulting exclusively in an H adduct on N atom (R0. The same species are obtained by one-electron reduction of the substrate, using (CH3)2(2OH, (CH3)2CO-, etc. The reduction with H atoms, however, occurs only to 70% on N ~ position of 2,2'-bpy-H +, the remaining 30% H are leading to H adducts on ring carbons (R2), probably on C 5 position (XIV). The attribution to this carbon site is based on MNDO calculations. The results are of interest for solar energy utilizing systems containing 2,2'-bpy or its derivatives as sensitizer. Acknowledgments--The author appreciates very much the valuable discussions and encouragements by Professor Dr. N. Getoff during the course of this work. Thanks are also expressed to Professor Dr. D. Schulte-Frohlinde, Max-Planck-Institut for Strahlenchemie, MQlheim/Ruhr, F.R.G. for the permission to use the pulse-radiolysis facility. Gratitude is directed to Dr. K. Sehested for the use of the accelerator with pressure cell at RIS0, Denmark. The author is further indebted to Professor Dr. H. Lischka for the disposal of an MNDO program and to Dr. A. Karp-
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SONJA SOLAR
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