2324
readily deactivated, through collisions with a suitable inert solvent, to triplet NCN which is incapable of stereospecific insertion3 into C-H bonds. In the absence of a solvent, singlet NCN is in all probability the reactive species also in other reactions such as additions to aromatic10 and olefinic" double bonds since these are in general less energetic processes than insertion. The results reported here point to Scheme I in which path a is followed in the absence of a solvent, path b in methylene bromide, and a combination of a and b in methylene chloride. l 2 Scheme I A
N3CN
+ 'NCN - Na
collisional
deactivation
/ CH3
R',\ 'C
R,/
'NCN
R?, C-CH3
f-
f '
'NHCN
Rz %, Rz,x
'C
cis-Cr(N&+
I'
Ri
+ Cr2+
NHCN
[cis-N3CrN3CrJ+]* + CrN32+ Crz+
+
+ N3-
(2)
A direct test of reaction 2 was made by preparing cisCT(N~)~+'O and allowing it to react with chromium(I1).
C-CH,
t-
I \ R1 NHCN
fJ.
Furthermore, we have demonstrated that, after the cisCr(N3)z+-Cr2+ exchange which proceeds via the diazidobridged transition state is substantially complete,3 it is possible to detect the c i ~ - C r ( N ~ ) ~ + - Creaction r~+ that makes use of a single azide bridge. When chromium(I1) was added to an excess of cisCO(NH,),(N~)~+, both CrN32+ and cis-Cr(N&+ were p r o d ~ c e d . ~ JThe ratio [ C ~ ~ - C ~ ( N , ) ~ + ] / [ Cwas ~N~~+] 0.6 f 0.1 at 0" in 0.010 to 0.10 M perchloric acid. This ratio was found to be reasonably reproducible if the cobalt(II1) complex was in excess. However, when an excess of chromium(I1) was added to cisCO(NH,),(N~)~+, the yield of cis-Cr(N&+ appeared to be variable. In general the yield of cis-Cr(N&+ was found to decrease with increasing [Cr2+],and with increasing contact times between the excess chromium(11) and the c i ~ - C r ( N ~ )produced ~+ in the cis-Co(NH3),(N&+-Cr 2+ reaction. These results suggested chromium(I1) catalysis of the aquation of cis-Cr(N&+ via the single-bridged electron-transfer pathg
NHCN
Table I. Kinetics of the Chromium(I1)-Catalyzed Aquation of cis-Cr(N&+ ([HCIO,] = 0.20 M , [ ~ i s - C r ( ? & ) ~=+ ]3-4 X loe6 M ) -
(10) F. D . Marsh and H. E. Simons, J . Am. Chem. SOC.,87, 3529 (1965). (11) A. G. Anastassiou, ibid., 87, 5512 (1965). (12) Within the frame of this mechanism, equilibration between the stereoisomeric radicals is a rapid process compared with recombination.
15"
c
[Crz+],M x 1036 1.26 2.92 2.92 5.84 1.14 2.32 2.58
A. G . Anastassiou Contribution No. 1193from the Central Research Department E. I . du Pont de Nemours and Company Experimental Station, Wilmington, Delaware 19898 Received February 21, 1966
---.
25 k , M-l sec-I
4.6 4.4 4.7 4.4 4.8c 4.44 4.4'3d Average 4 . 5
[Crz+J,M
x
103b
1.24 1.66 2.28 2.68 2.87 2.13 2.61
k , M-I sec8.2 1.1 1.5
1.4 7.6 1.9 7.9 Average 7 . 6
0 Except as noted, cis-Cr(N&+ was prepared in Jitu by the cisC O ( N H ~ ) ~ ( N ~ ) ~reaction. + - C ~ ~ +* Measured at the completion of the reaction from the increase in absorbance at 418 mp upon reaction with oxygen. cis-Cr(N3)*+prepared as indicated in footnote 10. d [HClOd] = 0.080 M .
Parallel Single- and Double-Bridged Activated Complexes in Electron-Transfer Reactions'
Sir : Although a double-bridged activated complex would (6) The separation of cis-Cr(Na)~+ and CrNa*+ was achieved by ionexchange chromatography. 3 At the concentrations used, both comappear to provide a reasonable path for an electronplexes were adsorbed on the resin (Dowex SOX-8, 5G100 mesh). Elutransfer reaction,2 such a path has been demonstrated tion with 0.2 and 1.5 M perchloric acid, followed by spectrophotometric only in the cis-Cr(N&+-Cr sf exchange reaction. measurements at 275 mp (e 5900) and 270 m p (e 3700), yielded the concentrations of cis-Cr(N&+ and CrN32+, respectively. A search for a double-bridged activated complex has (7) Hydrazoic acid was also formed under these conditions. The - C r ~ +amount also been made in the c i ~ - C o ( e n ) ~ ( O H ~ ) ~ ~ +and of hydrazoic acid formed was estimated from the absorbance at cis-CrFz+-Cr2+ system^,^^^ but the experimental evi260 mp in the fraction that passed directly through the column, and was found to be comparable to the CrNg2+ formed. This observation indidence indicates that transition states with a single cates that the chromium(I1)-hydrazoic acid reaction,3,*although rapid, bridging ligand are dominant in these cases. does not compete with the faster chromium(I1)-cis-Co(NHa)n(Na)z+ reaction. Furthermore, although it is possible that electron transfer In contrast with these results, we have found that occurs first to the azide bridge, the production of hydrazoic acid demonthe reaction of cis-Co(NH&(N&+ (and of cis-Costrates that the electron is ultimately transferred to the cobalt(II1) center. (en),(N&+) with chromium(I1) proceeds via parallel (8) M. Ardon and B. E. Mayer, J. Chem. Soc., 2861 (1962). (9) Chromium(I1) and azide ion are shown as products of reaction 2. single- and double-bridged activated complexes. kd
+
CrZ+
cis-Cr(N3b+ C~S-CO(NH~)~(N~)~+ CrN32+
(1)
(1) This work was supported by the National Science Foundation, Grant GP 2001. ( 2 ) H . Taube, Advan. Znorg. Radiochem., 1, 1 (1959). (3) R. Snellgrove and E. L. King, J . A m . Chem. Soc., 84, 4609 (1962). (4) W. Kruse and H. Taube, ibid., 82, 526 (1960). (5) Y.T. Chia and E. L. King, Discussions Faraday SOC.,29, 109 (1 960).
Journal of the American Chemical Society
1 88:lO 1 M a y
However, in the presence of excess chromium(II), hydrazoic acid is rapidly reduced. 3'8 The rate of this reaction was not measured. However, the following blank experiments were carried out to ascertain the effect of this reaction on the measurements presented in Table I. Hydrazoic acid in 0.20 M perchloric acid was generated in silu by the rapid reaction ( k > 103 M-1 sec-1) of chromium(I1) (-2 X M ) with trans-Co(NH~)4(Ns)r+(-3 x 10-5 M ) , and the absorbance at 275 mp was measured as a function of time. No change in absorbance was observed after the mixing time of 10 sec, and we conclude that, under these conditions, any changes in absorbance due to the chromium(I1)-hydrazoic acid reaction are too small to interfere with reaction 2. Furthermore, as expected, we conclude that cis-Cr(Ns)~+is not formed in the tran~-Co(NHs)a(Na)z+-Cr2~ reaction.
20, 1966
2325 Table 11. Rate Constants for Some Reactions of Chromium(I1) at 0
O
AH*, Reaction
+
cis-Cr(N&+ Crz+ CrN8*+ Cr2+ cis-CrFZ+ Cr2+ CrF2+ Cr 2+ cis-Co(NH&(N&+ cis-Co(enh(N3~+
+ + +
kd' M-1 sec-1 6oC
...
...
...
ksb,h M-l sec-l 1.9d 1.3
1 . 2 x 10-3 3 . 2 x 10-3d
>io3
>io3
>io3
>io3
kd/kBh 31
...
