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Organometallic free radicals in solution
journal of
MOLECULAR
LIQUIDS ELSEVIER
Journal of MolecularLiquids.65/66(1995) 205-212
Organometallic Free Radicals in Solution James H. Espenson Ames Laboratory and Department of Chemistry, Iowa State University, Ames Iowa 50011, USA Abstract Laser flash photolysis of [CpM(CO)3] 2 (M = W, Mo, and Cr) provides a convenient source of CpM(CO)3 ~ an organometallic free radical with 17 valence electrons. It is a transient and highly reactive spedes. D e p e n d i n g on the circumstances and the other reagents present, the radical will dimerize, u n d e r g o halogen and h y d r o g e n atom abstraction reactions, and electron transfer reactions. With t e t r a m e t h y l - p h e n y l e n e d i a m i n e , there is a cyclic process of electron transfer steps, the net result of which is the catalyzed disproportionation of the metal radical. I. Introduction Organometallic compounds of the transition metals with carbonyl and other ~ - a d d ligands are generally constrained to 18 electron configurations, if they are to be stable. Compounds with only 17e are the subject of this paper. Because such species are one electron short of a closed shell, they react like free radicals. The 17e compounds are generally transient entities, except for those cases w h e r e the steric bulk of some of the ligands p r e v e n t s their coupling. The transient entities are typified by the c o m p o u n d s that are the subject of this study, (rl5-C5H5)M(CO)3 . (M = Cr, Mo, W) To achieve stability, these reactive entities may u n d e r g o any n u m b e r of reactions. The purpose here is to enumerate what these reactions are and the mechanisms by which they occur. Moreover, an a t t e m p t is m a d e to show how the organometallic free radicals are analogous to organic radicals, which are in general more familiar. The 17e organometallic species are isolobal with alkyl radicals which are 7e species, also one electron shy of a closed shell. II. Reactions of an OrganometaUic Free Radical To outline the processes the reactive o r g a n o m e t a l l i c radical m a y undergo, we consider the following: (a) dimerization, so as to regenerate the stable dimer from which they were formed by photohomolysis; (b) halogen abstraction from alkyl halides; (c) halogen abstraction from transition metal halides; (d) O H g r o u p abstraction from peroxides; (e) h y d r o g e n atom 0167-7322/95/$09.50 9 1995ElsevierScienceB.V. All fightsreserved. 0167-7322(95) 00853-5
SSD!
206 abstraction from certain metal hydrides; (f) addition of 30 2 and subsequent reaction; (g) ligand-catalyzed disproportionation; (h) electron-transfer reduction; (i) electron-transfer oxidation, before or after ligand addition; and (j) electron-transfer induced disproportionation. These reactions are outlined in Scheme I. The occurrences and mechanisms of these transformations will be dealt with here.
Scheme I CpW(CO) 3- + CpW(CO)3PPh3 + ~ h CpW(CO)3 L+ ~
(
3P
(a~,, ~ o ~cpw(co)3
(i) CpW(CO)3-
1
~
30 CpW(CO)3OO'~f
[CpW(CO)3]2
\ (h)
OC /
CO
i CO
(b) ~-C p W (CO)3X+ R~ RX
[ ~ ) ~ " (ZpW(CO)3X,M"
(eC~pM ~(e (CO)3H ROOH ~
M = Rh(dmg~2PPh 3) CpW(CO)3OH + RO ~
CpW(CO)3H
III. Experimental Procedures The chemistry of the CpM(CO)3 ~ radicals occurs on the microsecond time scale, and flash photolysis with optical detection is the technique most generally applicable. The radical is created by the photohomolysis of the stable dimer, [CpM(CO)3] 2, available commercially for M = Mo and W. The excitation sources used were Nd-YAG (~'exc 532 nm) and flashlamp-pumped dye lasers (~'exc 490-525 nm). When shorter wavelengths are used, the desired process in accompanied by increasing amounts of CO loss; this is in general undesirable because it introduces other transients into the system. The methods of generation of the radical and of analysis of different types of kinetic data have been referred to. ~-6 Optical detection of species was made, primarily of the parent dimer itself or of certain products referred to later.
20? IV.
Results
Radical recombination. The results p e r t a i n i n g to m a n y of the reactions in S c h e m e I will n o w be presented, starting with (a). W h e n a solution of the metal dimer is subjected to a laser flash, -25% of the d i m e r is dissociated; in a typical experiment a 20 ~M solution of [CpM(CO)3] 2 (M = Mo, W) yields about 25 ~M of the dimer. A slightly different p r o c e d u r e was u s e d for M = Cr, but with comparable results. 7 The recombination of the radicals occurs o v e r about 300 I~s a n d follows s e c o n d - o r d e r kinetics. A typical e x p e r i m e n t for the m o l y b d e n u m radical and the fit to s e c o n d - o r d er kinetics is s h o w n in Figure 1. The rate constants (k/109 L mol d s -1) in acetonitrile at 23 ~ are: Cr: 0.27, Mo: 2.16, and W: 4.7. In other organic solvents the rate constants are c o m p a r a b l e to these, reflecting the relatively small differences in viscosity. In a q u e o u s solution (CsH4CO2-)Mo(CO)~" has k/109 L mol -I s "I = 3.0.
Absorbance
i
0
0
1 10-4 2 104 Time/s
Figmre 1. The results of a laser flash photolysis experiment, showing the combination reaction of CpMo(CO)3", as monitored by the recovery of the dimer at its 384 run maximum. The smooth curve through the points shows the fit to second-order kinetics. On closer inspection, the c o m b i n a t i o n rate constants are a b o u t 1 / 4 of the estimated diffusion-controlled rate constant. For acetonitrile, for example, kdc .- 2.9 x 1010 L mol "1 s -1 from the y o n S m o l u c h o w s k i e q u a t i o n 8 with a diffusion coefficient from a modified version of the Stokes-Einstein relation, D - . kT/4xTlr. 9,10 O w i n g to the restriction to singlet state recombination, an experimental rate constant 1 / 4 of kdc is quite reasonable. O n the other hand, for these h e a v y metals, the spin restriction m a y not apply, in which case one w o u l d a r g u e that the geometrical a n d o r i e n t a t i o n a l r e q u i r e m e n t s of these large species c o u l d well give r e c o m b i n a t i o n rates s o m e w h a t b e l o w the theoretical m a x i m u m .
208
Halogen atom abstraction. The metal radicals react with alkyl and aralkyl halides (RX, X = C1, Br, I) to yield CpM(CO)3X and R ~ The metal halide p r o d u c t is a species k n o w n i n d e p e n d e n t l y , and its f o r m a t i o n in these reactions was verified by IR. The organic free radicals w e r e not detected directly, but were inferred from the final organic products determined by GC; these products were consistent with those k n o w n to be f o r m e d when R ~ undergoes dimerization and disproportionation reactions. A large n u m b e r of rate constants for organic halides have been reported. 2 A small selection will be presented here. First, the trend with the group R, and then with X, using data for CpW(CO)3": Alkyl group trends
I XUrl: .22 •
[ Bu': 6.0 •
Halide trends AUyl, CH2CHCH2-X Trichloromethyl, C13C-X
k/L mol "1 s "1
Pr': 3.0 •
! Pr":
•
[M,, < 6 •
I
k/L mol "1 s -1 X = I: 1.22 x 108 X = Br: 5.4 x 104 X = Br: 5.3 x 108 X = CI: 2.92 x 104
These results suggest that the transition state features an incipient free radical (note the trends with R), and that the metal atom has begun to make a b o n d to the halogen of appreciable strength. The t r e n d with halogen substitution is a particularly pronounced one, since the trends in BDE(R-X) and BDE(M-X) are in the opposite direction (that is, the low-valent metal center is a soft acid). H a l o g e n atom abstraction occurs also with m e t a l halides, both ( N H 3 ) s C o l I I - x 2+ and X-RhIII(dmgH)2PPh3 having been studied. For the latter, kcl = 9.2 x 107 and kBr = 1.57 x 108 L mo1-1 s -1 for the reactions of C p W ( C O ) 3 ~ The i m m e d i a t e p r o d u c t of the r h o d i u m reaction is the i n d e p e n d e n t l y - k n o w n r h o d i u m radical, RhIl(dmgH)2PPh3 ~ a 17e species. It in turn dimerizes to yield the stable dimer, [Rh(dmgH)2PPh3] 2. Both of these p r o c e s s e s can be m o n i t o r e d d i r e c t l y , the b u i l d u p a n d decay of RhII(dmgH)2PPh3 ~ at 580 nm and the formation of the r h o d i u m dimer at 452 nm. The mechanistic discussion concerning the alkyl halides is pertinent here as well; the rhodium system allowed the direct detection of the radical product, which had only been inferred in the organic reactions.
Hydrogen atom transfer also occurs, as represented in one instance by the reaction of CpW(CO)3 ~ with CpMo(CO)3-H, a process driven by the higher bond energy of the tungsten hydride. 11 The atom transfer process is
209 not to be confused with proton transfer; the reaction of CpMo(CO) 3- with CpMo(CO)3-H is also known.
Reduction of the metal radicals. The anionic complexes CpM(CO) 3- are well k n o w n species; they are stable entities with 18 valence electrons. The standard reduction potential for the CpMo(CO)3~ - couple is -0.08 V vs SSCE. 12 The m o l y b d e n u m radical is thus a mild oxidizing agent; with suitable electron donors it can be reduced to the anion. For example, the radical oxidizes Fe(T15-CsMes)2 with a rate constant of 2.2 x 108 L mo1-1 s "1 in acetonitri]e at 23 ~ 6 Application of the Marcus equation for electron transfer affords the electron exchange rate of the molybdenum radical/anion couple. The value is kee = 3 x 107 L mo1-1 s-1. The high value argues that very little nuclear reorganization is needed to add an electron to the SOMO of the 17e radical. Oxidation of the metal radicals. The cationic d e r i v a t i v e s of these o r g a n o m e t a l l i c c o m p o u n d s contain an a d d i t i o n a l ligand. For example, CpM(CO)3PPh3 + and CpM(CO)3NCCH3 + are readily obtained; the latter can be g e n e r a t e d electrochemically in acetonitrile. The C p M o ( C O ) 3 N C C H 3 +CpMo(CO)3 ~ couple has a standard electrode potential of-0.50 V vs SSCE. 12, which shows that the metal radical is a strong reducing agent. For example, Fe(TIS-CsHs)2 + reacts with CpW(CO)3 ~ with a rate constant of 1.9 x 107 L mol d s "1 in acetonitrile at 23 ~ 6 As large as this this value is, it corresponds to an electron exchange rate between CpMo(CO)3NCCH3 + and CpM(CO)3 ~ of only -10 -12 L mol d s -1. The small value signals a large inner-shell reorganization; the two species differ by one coordinated molecule of solvent. Ligand-catalyzed disproportionation; the role of 19e radicals. There is now considerable evidence for the association of a 17e radical and Lewis bases, such as phosphines, pyridine, and even acetonitrile. The resulting species has the formula CpM(CO)3 L~ and is often referred to as a 19e radical. It is probably just that, but it might instead represent a species with a "slipped" Cp ring, T!3CsH 5. In any event, the 19e species is a much stronger electron donor than C p M ( C O ) 3 L itself. The complex CpW(CO)3PPh3 ~ which has a formation constant of 6 L mol d, reacts at a diffusion-controlled rate with CpW(CO)3 ~ The products are those of the pure electron transfer steps that occur without solvent or ligand redistribution and without W - W b o n d formation. The occurrence of electron transfer alone is the feature that m a k e s the reaction so efficient. The diagram in Scheme II details the various reactions
210 and it presents the electron count for the various species. The electron count is the guide to reactivity.
S c h e m e II:
A t o m Transfer vs Electron Transfer: Electron C o u n t s +L
CpMo(CO)3 ~
~
~-~
C p M o ( C O ) 3 L ~ FeCp2+ ~
CpMo(CO)3 L+
p2§ CpMo(CO)sX + R ~ CpMo(CO)3 + CpMo(CO)3NCCH3 +
Radical disproportionation induced by electron transfer. Several interesting and interrelated p h e n o m e n a occur when N,N,N',N'-tetramethyl1,4-phenylenediamine (TMPD) is a d d e d to the solution of [CpM(CO)3] 2 prior to the laser flash. As expectedfrom the electrode potential of TMPD ~ 0.16 V vs SSCE in acetonitrile, the first event is the rapid growth of the intense absorption band of the amine radical cation centered at 613 nm (e = 1.2 x 104 L mol "1 cm-l). This absorption then fades fairly rapidly. The fading was quite u n e x p e c t e d , since TMPD ~ n o r m a l l y persists indefinitely. This phenomenon is illustrated in Figure 2. Indeed, when independently-prepared T M P D ~ was injected into the cuvette after the laser flash its blue color persisted indefinitely. Clearly, the disappearance of TMPD ~ signals that some intermediate in the system is causing its destruction. Tests based on the kinetics and on the addition of CpMo(CO) 3- and CpMo(CO)3NCCH3 + to the reaction mixture and to samples of TMPD and TMPD ~ have revealed the sequence of chemical events. 4 The reaction occurs by a catalytic cycle of electron transfer events. The net result is the disproportionation of CpMo(CO)3 ~ Scheme III shows the catalytic cycle.
211
0.08
9
,
,
,
,
[Mo'l/gM a 40.5 b 28.4 c 9.76 d 4.43
0.06 ~ a ~ }b\\ 0.04 J \ \ 0.02 0.00 I 010 o
I
I
I
I
1 104
2 10-4
i
3 104
Time/s
Figure 2. Experimental absorbance changes following [TMPD~ at 613 nm in a series of experiments with different initial concentrations of [CpMo(CO)3]2. In each, [TMPD]0 --- 5.00 mM. (Mo~ = CpMo(CO)3"). The fitting to the model in Scheme III is shown by smooth curves.
S c h e m e III
CpMo(CO)3 +
CpMo(CO)3 ~/
CpMo(CO)3 ~
\
~
~, CpMo(CO) 3-
The t h e r m o c h e m i s t r y of this system, discussed intermittently in w h a t has c o m e before, can be s u m m a r i z e d here. D i s p r o p o r t i o n a t i o n is less favorable t h e r m o d y n a m i c a l l y than recombination; the e q u i l i b r i u m constant is 10 7 as c o mp a r e d to 1016. Nonetheless, disproportionation can be m a d e to be the more i m p o r t a n t reaction, even the nearly exclusive one, by virtue of the c o n t r o l e x e r c i s e d by t h e c o n c e n t r a t i o n of T M P D . T h e p r o d u c t s of disproportionation, M o - a n d M o - A N +, do n o t react r e a d i l y . The disproportionation products are " t r a p p e d " as such. In other words, the metal radicals are c a p t u r e d into an electron transfer cycle w h i c h p r e v e n t s their falling to the most stable products, the dimer. The energetics of this system are depicted in Figure 3.
212 Electron-Transfer Catalysis to the Disfavored Product 1:
~
.... 7--Yi
/
F
AG~= 32.1kJ
....... \
- '
\
,
Figure 3. The reaction coordinate diagram comparing the dimerization (left) and electron transfer (right) processes. Note that the ionic products of disproportionation, the products of photolysis in the presence of TMPD, are very slow to yield dimer. One should note that the light energy provided by the initial laser flash has thus been "trapped" in part in the form of the chemical energy of the ionic pair relative to the dimer. They lie in a free energy well above the thermodynamic product, the Mo 2 dimer, but do not rapidly revert to it.
Acknowledgment. This work was supported by the U. S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division under contract W-7405-Eng-82. The author is grateful to many individuals for their assistance: J. Balla, A. Bakac, W-J. Chen, O. Pestovsky, S. Scott, T-J. Won, Q. Yao, and Z. Zhu, and to the Yamada Foundation for sponsoring the conference for which this paper was presented. References (1) Scott, S. L.; Espenson, J. H. Organomet. 1993,12, 4077. (2) Scott, S. L.; Espenson, J. H.; Zhu, Z. J. Am. Chem. Soc. 1993,115, 1789. (3) Scott, S. L.; Bakac, A.; Espenson, J. H. Organomet. 1993, 12, 1044. (4) Balla, J.; Bakac, A.; Espenson, J. H. Organomet. 1994, 13, 1073-1074. (5) Zhu, Z.; Espenson, J. H. Organomet. 1994, 13, 1893-1989. (6) Scott, S. L.; Espenson, J. H.; Chen, W.-J. Organomet. 1993, 12, 4077. (7) Yao, Q.; Bakac, A.; Espenson, J. H. Organomet. 1993, 12, 2010. (8) vonSmoluchowski, M. Z. Phys. Chem. 1917, 92, 129. (9) Edward, J. T. J. Chem. Educ. 1970, 47, 261-270. (10) Beckwith, A. L. J.; Bowry, V. W.; Ingold, K. U. J. Am. Chem. Soc. 1992, 114, 4983-4992. (11) Pestovsky, O.; Espenson, J. H. unpublished results (12) Pugh, R. J.; Meyer, T. J. J. Am. Chem. Soc. 1992, 114, 3784.
MOLECULAR
LIQUIDS ELSEVIER
Journal of MolecularLiquids.65/66(1995) 205-212
Organometallic Free Radicals in Solution James H. Espenson Ames Laboratory and Department of Chemistry, Iowa State University, Ames Iowa 50011, USA Abstract Laser flash photolysis of [CpM(CO)3] 2 (M = W, Mo, and Cr) provides a convenient source of CpM(CO)3 ~ an organometallic free radical with 17 valence electrons. It is a transient and highly reactive spedes. D e p e n d i n g on the circumstances and the other reagents present, the radical will dimerize, u n d e r g o halogen and h y d r o g e n atom abstraction reactions, and electron transfer reactions. With t e t r a m e t h y l - p h e n y l e n e d i a m i n e , there is a cyclic process of electron transfer steps, the net result of which is the catalyzed disproportionation of the metal radical. I. Introduction Organometallic compounds of the transition metals with carbonyl and other ~ - a d d ligands are generally constrained to 18 electron configurations, if they are to be stable. Compounds with only 17e are the subject of this paper. Because such species are one electron short of a closed shell, they react like free radicals. The 17e compounds are generally transient entities, except for those cases w h e r e the steric bulk of some of the ligands p r e v e n t s their coupling. The transient entities are typified by the c o m p o u n d s that are the subject of this study, (rl5-C5H5)M(CO)3 . (M = Cr, Mo, W) To achieve stability, these reactive entities may u n d e r g o any n u m b e r of reactions. The purpose here is to enumerate what these reactions are and the mechanisms by which they occur. Moreover, an a t t e m p t is m a d e to show how the organometallic free radicals are analogous to organic radicals, which are in general more familiar. The 17e organometallic species are isolobal with alkyl radicals which are 7e species, also one electron shy of a closed shell. II. Reactions of an OrganometaUic Free Radical To outline the processes the reactive o r g a n o m e t a l l i c radical m a y undergo, we consider the following: (a) dimerization, so as to regenerate the stable dimer from which they were formed by photohomolysis; (b) halogen abstraction from alkyl halides; (c) halogen abstraction from transition metal halides; (d) O H g r o u p abstraction from peroxides; (e) h y d r o g e n atom 0167-7322/95/$09.50 9 1995ElsevierScienceB.V. All fightsreserved. 0167-7322(95) 00853-5
SSD!
206 abstraction from certain metal hydrides; (f) addition of 30 2 and subsequent reaction; (g) ligand-catalyzed disproportionation; (h) electron-transfer reduction; (i) electron-transfer oxidation, before or after ligand addition; and (j) electron-transfer induced disproportionation. These reactions are outlined in Scheme I. The occurrences and mechanisms of these transformations will be dealt with here.
Scheme I CpW(CO) 3- + CpW(CO)3PPh3 + ~ h CpW(CO)3 L+ ~
(
3P
(a~,, ~ o ~cpw(co)3
(i) CpW(CO)3-
1
~
30 CpW(CO)3OO'~f
[CpW(CO)3]2
\ (h)
OC /
CO
i CO
(b) ~-C p W (CO)3X+ R~ RX
[ ~ ) ~ " (ZpW(CO)3X,M"
(eC~pM ~(e (CO)3H ROOH ~
M = Rh(dmg~2PPh 3) CpW(CO)3OH + RO ~
CpW(CO)3H
III. Experimental Procedures The chemistry of the CpM(CO)3 ~ radicals occurs on the microsecond time scale, and flash photolysis with optical detection is the technique most generally applicable. The radical is created by the photohomolysis of the stable dimer, [CpM(CO)3] 2, available commercially for M = Mo and W. The excitation sources used were Nd-YAG (~'exc 532 nm) and flashlamp-pumped dye lasers (~'exc 490-525 nm). When shorter wavelengths are used, the desired process in accompanied by increasing amounts of CO loss; this is in general undesirable because it introduces other transients into the system. The methods of generation of the radical and of analysis of different types of kinetic data have been referred to. ~-6 Optical detection of species was made, primarily of the parent dimer itself or of certain products referred to later.
20? IV.
Results
Radical recombination. The results p e r t a i n i n g to m a n y of the reactions in S c h e m e I will n o w be presented, starting with (a). W h e n a solution of the metal dimer is subjected to a laser flash, -25% of the d i m e r is dissociated; in a typical experiment a 20 ~M solution of [CpM(CO)3] 2 (M = Mo, W) yields about 25 ~M of the dimer. A slightly different p r o c e d u r e was u s e d for M = Cr, but with comparable results. 7 The recombination of the radicals occurs o v e r about 300 I~s a n d follows s e c o n d - o r d e r kinetics. A typical e x p e r i m e n t for the m o l y b d e n u m radical and the fit to s e c o n d - o r d er kinetics is s h o w n in Figure 1. The rate constants (k/109 L mol d s -1) in acetonitrile at 23 ~ are: Cr: 0.27, Mo: 2.16, and W: 4.7. In other organic solvents the rate constants are c o m p a r a b l e to these, reflecting the relatively small differences in viscosity. In a q u e o u s solution (CsH4CO2-)Mo(CO)~" has k/109 L mol -I s "I = 3.0.
Absorbance
i
0
0
1 10-4 2 104 Time/s
Figmre 1. The results of a laser flash photolysis experiment, showing the combination reaction of CpMo(CO)3", as monitored by the recovery of the dimer at its 384 run maximum. The smooth curve through the points shows the fit to second-order kinetics. On closer inspection, the c o m b i n a t i o n rate constants are a b o u t 1 / 4 of the estimated diffusion-controlled rate constant. For acetonitrile, for example, kdc .- 2.9 x 1010 L mol "1 s -1 from the y o n S m o l u c h o w s k i e q u a t i o n 8 with a diffusion coefficient from a modified version of the Stokes-Einstein relation, D - . kT/4xTlr. 9,10 O w i n g to the restriction to singlet state recombination, an experimental rate constant 1 / 4 of kdc is quite reasonable. O n the other hand, for these h e a v y metals, the spin restriction m a y not apply, in which case one w o u l d a r g u e that the geometrical a n d o r i e n t a t i o n a l r e q u i r e m e n t s of these large species c o u l d well give r e c o m b i n a t i o n rates s o m e w h a t b e l o w the theoretical m a x i m u m .
208
Halogen atom abstraction. The metal radicals react with alkyl and aralkyl halides (RX, X = C1, Br, I) to yield CpM(CO)3X and R ~ The metal halide p r o d u c t is a species k n o w n i n d e p e n d e n t l y , and its f o r m a t i o n in these reactions was verified by IR. The organic free radicals w e r e not detected directly, but were inferred from the final organic products determined by GC; these products were consistent with those k n o w n to be f o r m e d when R ~ undergoes dimerization and disproportionation reactions. A large n u m b e r of rate constants for organic halides have been reported. 2 A small selection will be presented here. First, the trend with the group R, and then with X, using data for CpW(CO)3": Alkyl group trends
I XUrl: .22 •
[ Bu': 6.0 •
Halide trends AUyl, CH2CHCH2-X Trichloromethyl, C13C-X
k/L mol "1 s "1
Pr': 3.0 •
! Pr":
•
[M,, < 6 •
I
k/L mol "1 s -1 X = I: 1.22 x 108 X = Br: 5.4 x 104 X = Br: 5.3 x 108 X = CI: 2.92 x 104
These results suggest that the transition state features an incipient free radical (note the trends with R), and that the metal atom has begun to make a b o n d to the halogen of appreciable strength. The t r e n d with halogen substitution is a particularly pronounced one, since the trends in BDE(R-X) and BDE(M-X) are in the opposite direction (that is, the low-valent metal center is a soft acid). H a l o g e n atom abstraction occurs also with m e t a l halides, both ( N H 3 ) s C o l I I - x 2+ and X-RhIII(dmgH)2PPh3 having been studied. For the latter, kcl = 9.2 x 107 and kBr = 1.57 x 108 L mo1-1 s -1 for the reactions of C p W ( C O ) 3 ~ The i m m e d i a t e p r o d u c t of the r h o d i u m reaction is the i n d e p e n d e n t l y - k n o w n r h o d i u m radical, RhIl(dmgH)2PPh3 ~ a 17e species. It in turn dimerizes to yield the stable dimer, [Rh(dmgH)2PPh3] 2. Both of these p r o c e s s e s can be m o n i t o r e d d i r e c t l y , the b u i l d u p a n d decay of RhII(dmgH)2PPh3 ~ at 580 nm and the formation of the r h o d i u m dimer at 452 nm. The mechanistic discussion concerning the alkyl halides is pertinent here as well; the rhodium system allowed the direct detection of the radical product, which had only been inferred in the organic reactions.
Hydrogen atom transfer also occurs, as represented in one instance by the reaction of CpW(CO)3 ~ with CpMo(CO)3-H, a process driven by the higher bond energy of the tungsten hydride. 11 The atom transfer process is
209 not to be confused with proton transfer; the reaction of CpMo(CO) 3- with CpMo(CO)3-H is also known.
Reduction of the metal radicals. The anionic complexes CpM(CO) 3- are well k n o w n species; they are stable entities with 18 valence electrons. The standard reduction potential for the CpMo(CO)3~ - couple is -0.08 V vs SSCE. 12 The m o l y b d e n u m radical is thus a mild oxidizing agent; with suitable electron donors it can be reduced to the anion. For example, the radical oxidizes Fe(T15-CsMes)2 with a rate constant of 2.2 x 108 L mo1-1 s "1 in acetonitri]e at 23 ~ 6 Application of the Marcus equation for electron transfer affords the electron exchange rate of the molybdenum radical/anion couple. The value is kee = 3 x 107 L mo1-1 s-1. The high value argues that very little nuclear reorganization is needed to add an electron to the SOMO of the 17e radical. Oxidation of the metal radicals. The cationic d e r i v a t i v e s of these o r g a n o m e t a l l i c c o m p o u n d s contain an a d d i t i o n a l ligand. For example, CpM(CO)3PPh3 + and CpM(CO)3NCCH3 + are readily obtained; the latter can be g e n e r a t e d electrochemically in acetonitrile. The C p M o ( C O ) 3 N C C H 3 +CpMo(CO)3 ~ couple has a standard electrode potential of-0.50 V vs SSCE. 12, which shows that the metal radical is a strong reducing agent. For example, Fe(TIS-CsHs)2 + reacts with CpW(CO)3 ~ with a rate constant of 1.9 x 107 L mol d s "1 in acetonitrile at 23 ~ 6 As large as this this value is, it corresponds to an electron exchange rate between CpMo(CO)3NCCH3 + and CpM(CO)3 ~ of only -10 -12 L mol d s -1. The small value signals a large inner-shell reorganization; the two species differ by one coordinated molecule of solvent. Ligand-catalyzed disproportionation; the role of 19e radicals. There is now considerable evidence for the association of a 17e radical and Lewis bases, such as phosphines, pyridine, and even acetonitrile. The resulting species has the formula CpM(CO)3 L~ and is often referred to as a 19e radical. It is probably just that, but it might instead represent a species with a "slipped" Cp ring, T!3CsH 5. In any event, the 19e species is a much stronger electron donor than C p M ( C O ) 3 L itself. The complex CpW(CO)3PPh3 ~ which has a formation constant of 6 L mol d, reacts at a diffusion-controlled rate with CpW(CO)3 ~ The products are those of the pure electron transfer steps that occur without solvent or ligand redistribution and without W - W b o n d formation. The occurrence of electron transfer alone is the feature that m a k e s the reaction so efficient. The diagram in Scheme II details the various reactions
210 and it presents the electron count for the various species. The electron count is the guide to reactivity.
S c h e m e II:
A t o m Transfer vs Electron Transfer: Electron C o u n t s +L
CpMo(CO)3 ~
~
~-~
C p M o ( C O ) 3 L ~ FeCp2+ ~
CpMo(CO)3 L+
p2§ CpMo(CO)sX + R ~ CpMo(CO)3 + CpMo(CO)3NCCH3 +
Radical disproportionation induced by electron transfer. Several interesting and interrelated p h e n o m e n a occur when N,N,N',N'-tetramethyl1,4-phenylenediamine (TMPD) is a d d e d to the solution of [CpM(CO)3] 2 prior to the laser flash. As expectedfrom the electrode potential of TMPD ~ 0.16 V vs SSCE in acetonitrile, the first event is the rapid growth of the intense absorption band of the amine radical cation centered at 613 nm (e = 1.2 x 104 L mol "1 cm-l). This absorption then fades fairly rapidly. The fading was quite u n e x p e c t e d , since TMPD ~ n o r m a l l y persists indefinitely. This phenomenon is illustrated in Figure 2. Indeed, when independently-prepared T M P D ~ was injected into the cuvette after the laser flash its blue color persisted indefinitely. Clearly, the disappearance of TMPD ~ signals that some intermediate in the system is causing its destruction. Tests based on the kinetics and on the addition of CpMo(CO) 3- and CpMo(CO)3NCCH3 + to the reaction mixture and to samples of TMPD and TMPD ~ have revealed the sequence of chemical events. 4 The reaction occurs by a catalytic cycle of electron transfer events. The net result is the disproportionation of CpMo(CO)3 ~ Scheme III shows the catalytic cycle.
211
0.08
9
,
,
,
,
[Mo'l/gM a 40.5 b 28.4 c 9.76 d 4.43
0.06 ~ a ~ }b\\ 0.04 J \ \ 0.02 0.00 I 010 o
I
I
I
I
1 104
2 10-4
i
3 104
Time/s
Figure 2. Experimental absorbance changes following [TMPD~ at 613 nm in a series of experiments with different initial concentrations of [CpMo(CO)3]2. In each, [TMPD]0 --- 5.00 mM. (Mo~ = CpMo(CO)3"). The fitting to the model in Scheme III is shown by smooth curves.
S c h e m e III
CpMo(CO)3 +
CpMo(CO)3 ~/
CpMo(CO)3 ~
\
~
~, CpMo(CO) 3-
The t h e r m o c h e m i s t r y of this system, discussed intermittently in w h a t has c o m e before, can be s u m m a r i z e d here. D i s p r o p o r t i o n a t i o n is less favorable t h e r m o d y n a m i c a l l y than recombination; the e q u i l i b r i u m constant is 10 7 as c o mp a r e d to 1016. Nonetheless, disproportionation can be m a d e to be the more i m p o r t a n t reaction, even the nearly exclusive one, by virtue of the c o n t r o l e x e r c i s e d by t h e c o n c e n t r a t i o n of T M P D . T h e p r o d u c t s of disproportionation, M o - a n d M o - A N +, do n o t react r e a d i l y . The disproportionation products are " t r a p p e d " as such. In other words, the metal radicals are c a p t u r e d into an electron transfer cycle w h i c h p r e v e n t s their falling to the most stable products, the dimer. The energetics of this system are depicted in Figure 3.
212 Electron-Transfer Catalysis to the Disfavored Product 1:
~
.... 7--Yi
/
F
AG~= 32.1kJ
....... \
- '
\
,
Figure 3. The reaction coordinate diagram comparing the dimerization (left) and electron transfer (right) processes. Note that the ionic products of disproportionation, the products of photolysis in the presence of TMPD, are very slow to yield dimer. One should note that the light energy provided by the initial laser flash has thus been "trapped" in part in the form of the chemical energy of the ionic pair relative to the dimer. They lie in a free energy well above the thermodynamic product, the Mo 2 dimer, but do not rapidly revert to it.
Acknowledgment. This work was supported by the U. S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division under contract W-7405-Eng-82. The author is grateful to many individuals for their assistance: J. Balla, A. Bakac, W-J. Chen, O. Pestovsky, S. Scott, T-J. Won, Q. Yao, and Z. Zhu, and to the Yamada Foundation for sponsoring the conference for which this paper was presented. References (1) Scott, S. L.; Espenson, J. H. Organomet. 1993,12, 4077. (2) Scott, S. L.; Espenson, J. H.; Zhu, Z. J. Am. Chem. Soc. 1993,115, 1789. (3) Scott, S. L.; Bakac, A.; Espenson, J. H. Organomet. 1993, 12, 1044. (4) Balla, J.; Bakac, A.; Espenson, J. H. Organomet. 1994, 13, 1073-1074. (5) Zhu, Z.; Espenson, J. H. Organomet. 1994, 13, 1893-1989. (6) Scott, S. L.; Espenson, J. H.; Chen, W.-J. Organomet. 1993, 12, 4077. (7) Yao, Q.; Bakac, A.; Espenson, J. H. Organomet. 1993, 12, 2010. (8) vonSmoluchowski, M. Z. Phys. Chem. 1917, 92, 129. (9) Edward, J. T. J. Chem. Educ. 1970, 47, 261-270. (10) Beckwith, A. L. J.; Bowry, V. W.; Ingold, K. U. J. Am. Chem. Soc. 1992, 114, 4983-4992. (11) Pestovsky, O.; Espenson, J. H. unpublished results (12) Pugh, R. J.; Meyer, T. J. J. Am. Chem. Soc. 1992, 114, 3784.