6384 samples of 3 under argon a t room temperature turned cloudy after several days, but major decomposition did not occur. It is evident that 3 is rather stable thermally, although it is highly reactive toward common reagents. Our studies in this area are continuing with emphasis on the preparation of other silacyclopropenes and on a broad scope study of the chemical reactivity of this novel class of compounds. It will be of special interest to determine if the trimethylsilyl substituents play an important role in the stability of 3. We were prompted to disclose these preliminary results by a paper presented a t the 10th Organosilicon Award Symposium by Gaspar and ConlinI2 in which the synthesis of tetramethylsilirene by the flow pyrolysis of 1,2-dimethoxytetramethyldisilane in the presence of 2-butyne was claimed.
Acknowledgments.The authors are grateful to the U S . Air Force Office of Scientific Research (NC)-AFSC (Grant AF-AFOSR-76-2917) for generous support of this work and to Mara 0.Nestle and Joseph S. Merola for obtaining the I3C and z9SiN M R spectra. References and Notes
2
0
(1) (a) M. E. Vol'pin, Yu. D. Kweshkov. and D. N. KU~SB~OV, lzv. Akad Nauk SSSR. Old Khim. Nauk. 1355 119611: lbl M. E. Vol'oin. Yu. D. KOTBSMOV. ' v. G.D U I O V B , ~ ~ ~N.D K. U & + V , Tiwahadron. 18: io7(1982). (2)(a) F. J a h n m . R. S. Gohlke. and W. H. Narutavicus. J. Orqsnomel. ulsm.. 3.233(1965);(b)R.WeslandR.E.BaiIey. J.Am. Chem. Soc.,85,2871 (1963): (c)N.G.BokiiandYu.T.Struchkov. J. Smrct chsm USSR,6.548 (1965). (3) W. H.Atwell and D. R. Weyenberg. Inla-Sci. Chem. Rep, 7, NO. 4. 139 (1973): lor experimental details see W. H. Atwell. U.S. paten1 3 576 024 (April 20. 1971). (4) R. L. Lambert. Jr.. and D. Seyfsth. J. Am. Chem. Soc.. 94s 9246 (1972). (5) D. Seytedh. C. K. Haas. andD. C. Annarelli. J. hpmmsl. Chem. 56, C7 (1973). (6)D. Seyfenh andD. C. Annarelli. J. Am. Chem. Sm.. 97. 716211975) (7) D. Seyferth and D. C. Anmreili. J. Am. Chem. Sm.; 97; 2273 i1975). (8) (a) R. L. Scholl. G. E. Maciel. and W. K. Musker. J. Am. Chem. Sm.. 94, 6376(1972): (b)C.R. Ems1.L. Spianer.G. R. Buell,andD. L. Wilhte. ibM... 96,5375 (1974). (9)(a)G.L.ClossandL.E.Cloos.J.Am.Chem.Soc.,63,1W3(1961):(b)ib~. 85,99 11963):(c)G. L. Clo~s.L. E. Closs. and W. A. 8011, J. Am. wlem. Soc., 85,3796 (1963). (10) H.Bock and H. Seidl. J. Organomel. Chem.. 13.87 (1968). (11) W. H. Atwell and D. R. Weyenberg, J. Am. Chem. Sm.. 90, 3438 ~
~~
0
IO
20
TIME, p e c
-
Figure 1. Flash photolysis ofeuropium(ll)~Ru(bpy),'+ sduiion: (a) absorbance at 500 nm (AA 0.02 per major division) as a function of lime (0.2 ps per major division): (b) emission intensity a1 600 nm (Icm increases down the trace) in arbitrary units as a function of time (0.2 ps per major division); (c) same as (a), but 2 11s per major division.
(1966).
(12) P. P. Gaspar and R. T. Conlin, Abstracts. 1Mh organosilicon Award S y m posium. Windsor. Ontario, April 2-3. 1976, p 18: J. Am. Chem. Sm., 98, 3715 (1976).
Dietmar Seyferth,* Dennis C. Annarelli, Steven C. Vick Department of Chemistry Massachusetts Institute o/ Technology Combridre. Massachusetfs 02139 Receiuedfune 7.1976
Electron-Transfer Reactions of Excited States: Direct Evidence for Reduction of the Charge-Transfer Excited State of Tris(Z,Z'-bipyridine)ruthenium(Il) Sir:
The charge-transfer excited state of tris(2.2'-bipyridine)ruthenium(lI) (*Ru(bpy),2+)i is a more powerful reducing agent (eq 1 ) than the ground state molecule by -2.1 V, the excitation free Thus oxidants (e.g., C O " ' ( N H ~ ) ~ X ~Fe'+,S-7 + , ~ . ~ Ru(NH&,+ 2.6) can quench the emission from *Ru(bpy)12+ by an electron-transfer mechanism (eq 2) as has been demonstrated by product analyses: steady-state measurements,' flash-photolysis experiments.6 and rate ~omparisons.2J.~ Ru(bpy),j'+
+e
t
+
* R ~ ( h p y ) , , ~ +Q
-
* R ~ ( b p y ) , ~ + , E " = -0.R3V
Ru(bpy);'+
Journal o j t h e American Chemical Society
+ Q-
(11
(21
The same excited state is also expected to possess oxidizing properties (eq 3). Evidence for reduction of *Ru(bpy)]*+ (eq 4) to Ru(bpy),+ (which has previously been produced by
+
Ru(bpy);,+, E" = +O&V
*Ru(bpy):,2+ e
*Ru(bpy),,'+
+Q
-
Ru(bpy),l+
+ Q'
(3)
(41
e l e c t r o ~ h e m i c a and l ~ ~ ~pulse-radiolysis lotechniques) has recently been sought by studying the quenching of the *Ru(bpy)12+ emission by reductants such as E U ( I I ) . ~ ~ R u ( N H , ) ~ ~ + and metal cyanides (e.&, O S ( C N ) ~ ~ - ) .Here I~ we present results of flash-photolysis experiments which provide direct evidence for the reduction of the charge-transfer excited state to Ru(bpy)]+. Flash-photolysis studies using a frequency-doubled neodymium laser as the excitation source7b(A 530 nm, pulse width -30 ns) were undertaken for the Eu"-Ru(bpy),2+ system" (Eu(l1) 0.01-0.1 M: Ru(bpy),*+ 0.3-3 X M;excitation intensities 10-102 einstein cm-2 s-I; 0.5 M CI-, 0.05 M H+, 25 "C). Examples of the traces obtained are shown in Figure I . An increase in absorbance in the region 470-550 nm (Figure la) accompanied the decrease in emission intensity following the excitation flash (Figure Ib). The magnitude of the absorbance increase at 500 nm was linear in the excitation light intensity. The rate of decay of the transient (Figure IC) was first-order in the transient and first-order with respect to added Eu( I l I ) . l 3 These observations are interpreted in terms of the following sequence: reductive quenching of *Ru(bpy),2+ to yield Ru(bpy),+ and Eu(ll1) (eq 6, Figure l a ) is followed by Eu(ll1)-oxidation of R u ( b p y ) ~ +to regenerate the preflash
1 98:20 1 September 29, 1976
6385
+0'3r-----I I +0.1
+
A
2*Ru(bpy),'+ + R ~ ( b p y ) , , ~ + Ru( bpy),' (8) timated to be favored by -1.7 V. Evidence for the occurrence of reaction 8 is presently being sought a t high R ~ ( b p y ) 3 ~ + concentrations and high light intensities. The flash-photolysis technique described here thus offers a convenient means of probing the chemistry of both the excited state * R ~ ( b p y ) 3 ~and + the unstable oxidation state Ru(bpy)3+. Fundamental studies of this kind may also be of considerable practical import in solar energy conversion as complexes like R ~ ( b p y ) 3 ~have + properties which make them useful mediators in the photodissociation of water.l5Sl6
I
f
t
Acknowledgment. This work was performed under the auspices of the U S . Energy Research and Development Administration. References and Notes
! -0.2
410
1
1
I
450
500
550
(1) (a)J. N. Demas and G. A. Crosby, J. Am. Chem. SOC.,93, 2841 (1971); (b) G. D. Hager and G. A. Crosby, bid., 97, 7031 (1975). and references cited
(2) (3)
X,nm
Figure 2. Difference spectrum for Ru(bpy)3+-Ru(bpy)32+: 0.08 M Eu(ll), 1.4 X M Ru(bpy)32+, 0.5 M CI-, 0.05 M H+, 25 "C, 10 n m band pass for probe beam.
(4) (5) (6) (7)
solution absorbance (eq 7, Figure IC). At 25 OC,k , = 2.8 X 107 M-1 s-lI 1 and k7 = 2.7 X lo7 M-I s-l.
-
Ru(bpy),>+
A 530 nm
1
+ Eu(1I) Ru(bpy),+ + Eu(1II)
*Ru(bpyh'+
k k +
(9)
(5)
(10) (11) (12) (13)
TI
*Ru(bpy),2+
(8)
Ru(bpy),+
+ Eu(II1)
(6)
Ru(bpy),?+
+ Eu(I1)
(7)
The difference spectrum obtained for the transient is presented in Figure 2. The spectrum was the same a t 0.05 M H+ (shown), 0.4 M H+, and p H 3. The same transient is produced in smaller yields from reaction of * R ~ ( b p y ) 3 ~ +with R u ( N H ~ ) and ~ ~ +Fe(CN)64-. In addition, 500-nm-absorbing transients are produced when solutions containing Eu(I1) and RuL3*+ (L = 1,lO-phenanthroline, 4,4'-dimethyl-2,2'-bipyridine, 4,7-dimethyl- 1,lo-phenanthroline, and 5-chloro1,lO-phenanthroline) are flash photolyzed. Detailed observations for the latter systems will be reported elsewhere. Figure 2 is similar to, but not identical with, the spectrum reported by Baxendale and Fiti.Io The reduced species which they generated by reaction of Ru(bpy)j2+ with eaq- or Zn+ has,,A 510 nm ( E 1.2 X lo4 M-' cm-I), 530 nm (here 490,510 nm) with a second, less intense, maximum a t -610 nm which we do not observe. Using the reaction of *Ru(bpy)32+ with Fe3+ (yield of R ~ ( b p y ) 3 ~ +Fe2+, 0.8 f 0.27) to estimate the initial post-flash concentration of * R ~ ( b p y ) 3 ~ allowing +, for the fraction of excited state quenched, and assuming a yield of 1.0 for reaction 6, the molar absorptivity of Ru(bpy)3+ a t 490 nm is calculated to be (1.4 f 0.3) X lo4 M-' cm-l. Thus in intensity and band shape the visible spectrum of Ru(bpy)3+ strongly resembles that of R ~ ( b p y ) 3 ~ +and , ' ~it is possible that the 490-510-nm band in the spectrum of Ru(bpy)3+ (like the 450-nm band for R ~ ( b p y ) 3 ~ is + )associated with a metal-toligand charge-transfer (d6(H*)I d5(7r*)*) transition.I4 There is now ample evidence that the charge-transfer excited state of R ~ ( b p y ) 3 ~readily + undergoes both oxidation and reduction, as is predicted from thermodynamic and spectroscopic I One additional implication of such considerations can be seen by subtracting eq 1 from eq 3 to give eq 8. Disproportionation of the excited state (eq 8) is thus es-
(14)
3.5 X l o 8 tration [!?u(bpyh+] so that reaction 7 predominates and follows pseudo-first-order kinetics under ail conditions. By contrast, the lowest energy absorption maximum for 'Ru(bpy)3*+, which like Ru(bpyh+ has one electron in the ligand K. system but only five metal d electrons, occurs at -360 nm (C. Creutz, unpublished observations, d7 for the 1975). A referee has suggested that the assignment ds(,*)l 490-510-nm band of Ru(bpyh+ may more readily account for the positions of the band maxima in Ru(bpy)3+ and 'Ru(bpy)S2+. Although this assignmeni is not unreasonable on energetic grounds, the band seems remarkably intense ( e >IO4 M-' cm-l ) for this kind of transition. C. Creutz and N. Sutin, Proc. Natl. Acad. Sci. U.S.A., 72, 2858 (1975). G. Sprintschnik, H. W. Sprintschnik, P. P. Kirsch, and D. G. Whitten, J. Am. Chem. Soc., 98, 2337 (1976).
Homoallylic' Interaction Involving Nitrogen Lone Pairs, a Reexamination Sir:
Recently published work has examined the significance of homoallylic interaction between a nitrogen lone pair and a remote H bond in bicyclic amine^.^-^ Comparison with data from well-established model systems led us to reassign the photoelectron spectra (PES) of the bicyclic amines 5-8 of ref 2a and hence challenge the reported conclusions regarding the importance of homoallylic interaction in such systems. The recent report5 of the N M R data for 5-8 confirmed our skepConzmunications to the Editor
