3137
driving force (expressed as AEO).21 On this basis a decrease in kl from 2 X lo4 to 4 X lo3 M-’ sec-‘ may be accounted for in the reaction of Fe(EDTA)2- with cyt c (III)N at pH 9, but this leaves a factor of about 150 in the observed decrease in kl to 2.67 X 10’ M-’ sec-’ unexplained. Further rate inhibition may arise from steric factors originating in the limited conformational change known to accompany formation of the high pH derivative.28 The heme pocket presumably is the region in the protein most directly affected by displacement of the Met-80 ligand, but structural perturbations in the vicinity of the heme edge are possible as well. More structural information as well as a better estimate of the Eo for the high pH cyt c (111) derivative will be required before a choice can be made among the several possible explanations of the observed rate decrease.
Acknowledgmen.ts. We thank Dr. Larry Bennett for helpful comments and suggestions. R. H. acknowledges the National Science Foundation for a Graduate Fellowship (1969-1971). H. B. G. held a John Simon Guggenheim Fellowship during 1972-1973. This research was additionally supported by a grant from the National Science Foundation. Supplementary Material Available. A listing of k&sd values will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X 148 mm, 24X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D. C. 20036. Remit check or money order for $3.00 for photocopy or $2.00 for microfiche, referring to code number JACS74-3132..
Reaction of Chromium( VI) with Hydrazinium Ion G. P. Haight, Jr.,*l Tracy J. Huang, and Harry Platt
Contribution from the School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801. Received November 16, 1973 Abstract: The study of the stoichiometry and kinetics of oxidation of hydrazinium ion (N2H5+)by chromium(V1) is extended to higher acidities than heretofore, The reaction proceeds through an N-bonded chromate ester via two equivalent redox steps. The reaction is catalyzed by acids H+, &Pod, HzP04-, and HCr04-. Cr(IV) is found to disproportionate, but it is not trapped by Cr(V1) as previously supposed. Revised estimates of Cr(VI)/ Cr(V) and Cr(VI)/Cr(IV)potentials are given. The Mn2+effect on Cr(V1) oxidations is critically examined.
C
hromium(V1) oxidations are characterized by preequilibria involving chromate(V1) esters or oneelectron reductions to c h r ~ m i u m ( V )and ~ by a variety of steps involving active intermediates Cr(V) and Cr(IV).2,3 Hydrazine is of interest as a reducing agent because of the absence of information on N-Cr bonded esters and because of its ability to discriminate between two-equivalent substrates4 NzH4 +NI
+ 4H+ + k-
(1)
and one-equivalent substrates4
+
N2H4----f 1/2N2 NH4+
+ e-
(2)
Durham and Beck5 have reported on the kinetics of the reaction over the pH range 1-3. We have reported 13H+
+ 4HCrOa- + 3N2Hr+
----f
4Cr3+
+ 3Nz + 16H20
(3)
on the fate of Cr(1V) intermediates as deduced from stoichiometric studies and Mn2+trapping experiments.6 We report here the kinetics of the reaction at high acidity, 2.2 M 2 [H+] 2 0.2 M , and in phosphoric acid buffers of pH 1.6-3.0 and reexamine inferences drawn from the Mn2+trapping experiments. (1) To whom correspondence should be addressed. (2) J. K. Beattie and G. P. Haight, Jr., in “Inorganic Reaction Mechanisms Part 11,” John 0. Edwards, Ed., Wiley, New York,N. Y.,1972, pp 93-145. (3) J. H. Espenson, Accounrs Chem. Res., 3,347 (1920). (4) (a) R. E.Kirk and A. W. Browne, J . Amer. Chem. SOC.,50, 337 (1928); (b) W. C. H. Higginson and J. W. Marshall, J. Chem. SOC., 447(1957). ( 5 ) M. T. Beck and D. A. Durham, J. Inorg. Nucl. Chem., 32, 1971 (1970). (6) G. P. Haight, Jr., T. J. Huang, and B. 2. Shakhashiri, J. Inorg. N i d . Chem., 33,2169 (1971).
Experimental Section Analytical grade reagents were used without further purification. Kinetics were measured by following the disappearance of Cr(V1) at the absorption maximum of 350 nm. Both HCr04- and H2Cr04 are present over much of the [H30+] range studied, so absolute determination of Cr(V1) was made after making solutions alkaline at 372 nm at the absorbance maximum of CrOa2-. Fast reactions were studied in a Sutin stopped-flow apparatus previously described.’ Detection of a chromium(V) intermediate was made by employing a continuous flow of reacting mixtures through the cavity of an esr spectrometer as previously described? Ionic strength was controlled using NaC104 (or LiCIO1) and HC104.
Results The Rate Law. Figure 1 shows that all runs with excess hydrazine exhibit first-order kinetics in Cr(V1) over several half-lives, giving the pseudo-first-order rate law
(4) Order in NH3NH2+. The slopes of the lines in Figure 1 are roughly proportional to [NH3NH2+]. However, the values of kobsd given in Table I show a small but distinct departure from first-order behavior at high [NH3NH2+]consistent with the equation
+
kb[NHaNH2+]/(1 3.2[NH3NH,+]) ( 5 ) This result is consistent with the formation of a kobsd
=
(7) S. A. Frennesson, J. K. Beattie, and G. P. Haight, Jr., J. Amer. Chem. SOC.,90,6018 (1968).
Haight, Huang, Platt / Reaction of Chromium( V I ) with Hydrazinium Ion
3138
-
0 0
.E
8
4
16
lo3 [H*]
+
Figure 3. [H+] effect in H3P04-H2P04- buffer: [H,PO4] [HzP04-]= 0.2 M , [N2HS+]= 5 X M , [Cr(VI)]o = 2 X lo-' M.
0
1
2
3
4
5
8
7
6
9
10
Time (sed
. I
Figure 1. First-order dependence of Cr(V1) on time. All runs in 1.0 M HC1O4. [NzHs+] = (A) 0.03, (B) 0.01, (C) 0.04, (D) 0.05, (E) 0.02 M.
[H+] but starts to level off at high [Hf]. The leveling off is consistent with the formation of H2Cr04 from HCrO*-. We postulate the mechanism HCr04HCr04-
1
+ H+
H2Cr04(K
+ NHaNHz+
=
0.15)
KO
+ HzO
h3NHCr03
(6)
(7)
(or NH2&H2Cr03)
35"
H+
ki + &H3NHCrO3----f products
(8)
-d[Cr(V1)l = klK0[HCrO4-][NH3NH2+][H+] dt [Cr(VI)] = [HCr04-]
+ [H2Cr04]+ [kH3NHCr031
[Cr(VI)] = [HCr04-](1
+ 0.15[H+I + Ko[NzHs+I)
- d[Cr(VI)] - k1Ko[Cr(VI)I[N2H~+1[H+l dt
1D
0.5
20
1.5
p =
2.2, [NzHs+]= 2 X
[H~Cr04]
M , [HCrO4-1o
[H+][HCrO 4-1
+
[NzH5+1[HCr04-1
0.96 1.38 1.87 3.51 4.95 6.73 8.57
0.93 1.38 1.84 3.37 5.22 6.79 8.28
a KO= 3.2, 1.1 = 1.0 M , klKo = 21.7 M-I sec-1, LH+] = 1.0 M , [Cr(VI)lo= 2 x 10-4 M , T = 25".
chromate ester of hydrazinium ion (NH3+NHCr03-) with Kt = 3.2 0.3. Effect of Hydrogen Ion Concentration on kb. Table I1 shows that kb increases linearly with [H+] at low
*
Table II. Variation of [H+]" 0.2 0.6 1 .o 1 .o 1 .o 1.4 1.8 2.2 o p =
=
2X
0.693 2.31 3.46 3.20 3.50 4.95 5.06 6.02
(9)
= 0.15
[H 3NNHCrO3-]
Table I. Variation of N2Hs+ in 1 M HC104a 0.500 0.750 1 .oo 2.00 3.00 4.00 5.00
+ 0.15[H+] + Ko[NzHs+]
Here
[H'l
Figure 2. [H+]effect: = 2 x 10-4 M.
1
0.739 2.10 3.32 3.32 3.32 4.43 5.44 6.37
3.47 11.55 17.30 16.00 17.50 24.75 25.30 30.10
2.2 M , Kokl = 20.2, [NzHs+]= 2 X M . NaC104was added to control p.
Journal of the American Chemical Society
M , [HCr04-lo
1 96:10
=
KO
Since Ko[NzH5+]
