J. Phys. Chem. 1986, 90, 3691-3696
3691
Electron-Transfer Reactions Involving the Azidyl Radical M. S. Ram and David M. Stanbury* Department of Chemistry, Rice University, Houston, Texas 77251 (Received: February 25, 1986; In Final Form: March 27, 1986)
In aqueous solution at 25.0 OC azide is oxidized to N2 by IrCls2-, 1rBrs2-,and [ F e ( b ~ y ) ~ ] With ~ + . an excess of azide in all cases the kinetics is highly non-first-order and shows strong retardation by the corresponding reduced complexes. In the presence of the spin traps PBN and DMPO the reactions are much faster and pseudo first order; under these conditions the rates are independent of the nature or concentration of the spin traps, but they do show a first-order dependence on [N,]. The apparent second-order rate constants are 1.8 X lo2, 6.6 X lo', and 8.4 X 10" M-' s-l for IrClsz-, IrBrt-, and [Fe(bpy),]'+, respectively. Pulse radiolysis measurements have been made on the reaction of N3 with IrCl;-; the reaction produces IrClsz-, is first order with respect to N3 and IrClt-, and has a rate constant of 5.5 X lo8 M-' s-I. A mechanism is proposed for the oxidations of N3- in which N3 is produced reversibly by electron transfer to the complex; N3 is removed from the system by dimerization, formation of Ns-, and (possibly) oxidation of N6- by the complex. Combination of the reduction potential for IrCls2- with rate constants for the reaction of N3 with IrCls3- and of N3- with IrCls2- yields a value of 1.33 V for the reduction potential of the azidyl radical. The oxidation by N3 of all three complexes in their reduced states is significantly less than diffusion controlled. A self-exchange rate constant of =4 X lo4 M-' s-l for the N3/N3- couple is inferred from the cross-relationship of Marcus theory.
Prior reports have developed an understanding of the kinetics for electron-transfer reactions between substitution-inert coordination complexes and bent triatomic molecules such as NO2and It was found that treatment by the Marcus theory for outer-sphere reactions provided a good basis for predicting the rates of these reactions, that bond bending and stretching were important factors in determining the activation barriers, and that nuclear tunneling could be a substantial effect.2 The contribution of solvation to the activation barrier was left as an adjustable parameter, and the deduced contribution, comparable in magnitude to the contributions from bond bending and stretching, was deemed reasonable. In this context it is of interest to focus more closely on the contribution of solvation in electron-transfer reactions of small molecules. Candidate redox couples in such a study 'include S2032-/S203-, SO,"/SOy, I-/I, SCN-/SCN, and N and N< at pH 6.5 were prepared in conductivity water and saturated with N 2 0 . They were used within 1 h of preparation in order to minimize the effects of hydrolysis of IrCh3-. Transient absorbance measurements were made at 487 and 434 nm. Dosimetry was performed by SCN- method.24 Results Stoichiometry. Careful experiments were carried out only on the reaction of IrC16*- with N3-. A consumption ratio for AIrC162-/AN< of 1.O has been r e p ~ r t e dand , ~ we have confirmed this result. Our determination of the N2 yield gave a mole ratio for N2 produced to IrC162-consumed of 1.35 f 0.2. The iridium-containing product was identified as IrC16,-. The net reaction can thus be represented as 21rC162- + 2N3-
-
21rCi63-
+ 3N2
(1)
In the presence of the spin trap DMPO (2 X lo-, to 2 X M) a consumption ratio for Ahci6z-/AN3- of 2.3 f 0.2 was obtained with IrC162-(1 X M) in excess over N,- (1-2) X lo4 M), and a consumption ratio of 2.1 f 0.2 was obtained with N,- (1 X lo-, M) in excess over IrC162-(2-5) X lo-, M). From the kinetic studies described below it may be inferred that first the spin traps react with N, to form nitroxyl radicals; experiments with the stable nitroxyl radicals di-tert-butyl nitroxyl and Tempo showed that they were instantly oxidized by IrC162-,presumably (22) Melvin, W. S.; Haim, A. Inorg. Chem. 1977, 16, 2016. (23) Smith, R. M., Martell, A. E., Eds. Critical Stabiliry Consrants; Plenum: New York, 1976; Vol. 4, p 45. (24) Stanbury, D. M. Inorg. Chem. 1984, 23, 2879.
Electron-Transfer Reactions Involving Azidyl Radical
The Journal of Physical Chemistry, Vol. 90, No. 16, 1986 3693 TABLE I: Kinetic Data for Oxidation of N, by IrCb2-with Added Spin Trapsa
[NaNJ, M
\,
5 a- 0 , 7 1 \
1-1.2
0. I d 0. I d
[spin trap], M koM, s-’ Spin Trap PBN 7.2b 2.0 x 10-4 9.0 x 10-4 15 2.0 x 10-3 14.8 1.0 x 10-2 17.8 18.4 1.8 X 1.4 2.0 x 10-3 2.0 x 10-3 72 1.0 x 10-2 86 2.0 x 10-3 7.5 2.0 x 10-3 3.7 1.8 x 10-2 4.5
0.1 0.1 0.1 0.1 0.02 0.05 0.5
Spin Trap DMPO 13.6b 2.0 x 10-4 1.0 x 10-3 18.8 2.0 x 10-3 18.4 1.0 x 10-2 19.9 3.2 2.0 x 10-3 9.6 2.0 x 10-3 95 2.0 x 10-3
a
0.1 0.1
l3
0.1
I
- -03s -0.9c -1
0.1 0.1 0.01
.o-
0.5
-1.11
I
I
’
I
‘i52.8
IO 15 20 TIME, seconds
5
0
0.5 0.1c
Figure 1. log ( A - A,) vs. time for reaction of IrC12- with NC. [IrC1t-]o =8X M, [NC] = 0.1 M, pH 6.5. Upper trace also has [PBN] = 2 x 10-3 M.
I
0
,
0,2
I
,
I
,
72 150 148
178 184 140 144 172 75 37 45
136 188 184 199 160 192 190
= 1.0 M (NaC10,); 25.0 ‘C, pH 6.5; [IrC12-]o= 1 X lo4 M; PBN = N-terr-butyl-a-phenylnitrone;DMPO = 5,S-dimethyl-lpyrroline-N-oxide. Mild deviations toward second-orderdecay. pH 4.4. d u = 0.1 M. I
I
0,4 0,6 0.8 TIME, seconds
I
I
I,O
Figure 2. [hC162-]vs. time for reaction of IrC1:-
= 8.5 X M, [N3-] = 1.0 M, pH 6.5: line) simulation.
koM/[N3-],M-I s-I
(X)
with N3-. [IrC162-]o experimental data; (solid
1
gF
TABLE II: Kinetic Data for Oxidation of N< by [Fe(bpy)d+ with Added PBN‘
[N3-1, M 2.0 x 10-4 2.0 x 10-4 4.0 X lo4 7.0 X lo4 1.0 x 10-3 1.0 x 10-3 1.4 x 10-3
[PBNI, M 2 x 10-3 1 x 10-2 2 x 10-3 2 x 10-3 1 x 10-2 2 x 10-3 2 x 10-3
k o u . S-’
15.8 14.7 27
61 78 120 120
koM/NC, M-’ s-’ 1.9 x 104 7.35 x 104 6.75 x 104 8.71 x 104 7.8 x 104 1.2 x 105 8.57 x 104
“ p = 0.15 M (Na,S04); T = 25.0 ‘C; X = 522 nm; natural pH (= M; PBN = N-rert-butyl-ru-phenyl6.5); [Fe(bpy)3]3+]o= 3 X nitrone.
0
100
200 300 400 TIME, seconds
500
Figure 3. [IrCl6’-] vs. time for reaction of IrCI,” with N
