2164
J . Am. Chem. SOC.1988, 110, 2764-2772
Effect of Long-Distance Electron Transfer on Chemiluminescence Efficiencies Robert D. Mussell and Daniel G. Nocera* Contribution from the Department of Chemistry, Michigan State University, East Lansing, Michigan 48824. Received March 23, 1987. Revised Manuscript Received October 23, 1987
Abstract: The electrogenerated chemiluminescence (ecl) of Mo,CI,,~- with three series of structurally and electronically related electroactive organic compounds in acetonitrile has been investigated. The yields for the formation of the electronically excited M0,5Cl14~-ion produced by the electron-transfer reaction of Mo6C1,,3- with aromatic amine radical cations (A’) and by the reaction of Mo6CII4-with nitroaromatic radical anions (D-) and pyridinium radicals (D)have been measured over a wide potential range by simply varying the reduction potential of the electroactiveorganic reagent. The dependence of the formation yield of MO,CI~.,~-*,d,, on the driving force of the annihilation reaction is similar for the three series. I # J ~ ~is immeasurable (> k , and 4, A), it follows directly from eq 13 that the increase of A, reactions with large exergonicities. 39a,80,8 I causes an enhancement of the electron-transfer rate in the inverted Calculations of the integral rates k , and k , for the Mo6ClI4*-/A region. These opposing effects of H A B and A, on the electronand D systems are similar to those of the Mo6C1142-/D+system; transfer rate are reflected in a maximum of kgs,difn at r = 15 A. however, the equilibrium pair distribution function must be The disparate behavior of differential excited-state and groundevaluated for the former series. Parallel to the results described state rates with separation distance has interesting implications above, although formation of excited-state Mo,C~,,~-is favored for the chemiluminescent reactivity of the for electron exchange between proximate reactants, the longsystem. As Figure 3 clearly illustrates, the contribution of the distance electron-transfer channel yielding ground-state products ground-state pathway to the overall rate comes from r > u, while contributes significantly to M O ~ C I , ~ ~ - and / A +Mo6Cl14-/D-anmost of the contribution for excited-state production comes from nihilation. Solving eq 10 and 11 with the experimentally measured r u. Thus, the appreciable values of kgs,difn at r > u suggest yields of the M O & ~ , ~ ~ - / and A + Mo6ClI4-/D- systems listed in that electron transfer to yield ground-state products is competitive Tables I and I1 gives reaction separation distances ranging from with excited-state production. 11 to 18 A. The integral (or overall) excited-state (kes)and ground-state Evaluation of eq 10 and 11 for the M ~ ~ C l , ~ ~ - / a c c e and ptor (kgs)rates are explicitly related to the experimentally measured donor systems necessarily relies on estimates of HABo and p. It chemiluminescence yields by eq 6. Accordingly, the reaction is satisfying that the general conclusions derived from Figures 3 distance for electron transfer can be determined by integrating and 4 do not significantly depend on these estimates. Specifically, eq 10 and 11 from r = m to a value of r that yields a k,/k,, ratio the relative dependence of the ground- and excited-state rates vary commensurate with that calculated from the observed chemiluonly marginally over the rather large interval 0.8 Ad < p < 1.8 minescence yields. For the M O ~ C ~ ~ ~ - /annihilation CMP reaction, A-l, which includes any reasonable value of /3 for the reactions an observed kes/krsratio of 0.033 yields an electron-transfer of the type described in our ecl studies. Furthermore, H A B o is a reaction distance of 13 A. This result clearly implies that approach constant, and therefore the excited- and ground-state electronof the electrogenerated reactants to a distance of closest approach transfer pathways exhibit a parallel dependence on the electronic ( u = 9.3 A) is impeded. Recent studies of outer-sphere eleccoupling element. This result is predicated on our tacit assumption tron-transfer reactions of inorganic metal complexes in nonaqueous that HABois similar for the ground- and excited-state pathways. solution have demonstrated that ion pairing decreases electrontransfer rates by increasing the electron-transfer d i s t a n ~ e . ~ ~ - ~ O Recently, the effect of disparate values of the electronic coupling element on chemiluminescent reactivity has been addressed for
results of the M ~ ~ C l , ~ ~ - / p y r i d i nseries i u m because calculations for the electron-transfer annihilation reactions of this system are simplified by the fact that ge(r) = 1. Figure 3 shows a plot of the excited-state and ground-state differential rate constants (kadifn and k,,difn, respectively) as a function of r for the electron-transfer reactions of Mo6Cli4-with C M P (eq 15 and 16). Equations 10
-
(88) (a) Borchardt, D.; Pool, K.; Wherland, S. Inorg. Chem. 1982, 21, 93-97. (b) Nielson, R. M.; Wherland, S. Inorg. Chem. 1984,23,1338-1344. ( c ) Borchardt, D.; Wherland, S. Znorg. Chem. 1984, 23, 2537-2542. (d) Nielson, R. M.; Wherland, S. Inorg. Chem. 1986, 25, 2437-2440.
(89) Kjaer, A. M.; Ulstrup, J. Inorg. Chem. 1986, 25, 644-651. (90) Truong, T.E. Pure Appl. Chem. 1986, 58, 1279-1284.
J . Am. Chem. SOC..Vol. 110, No. 9, 1988 2111
Chemiluminescence Efficiencies
1
I
(b)
8
r/ii Figure 3. Distance dependence of the differential bimolecular rate constant for the excited-state (es) and ground-state (gs) electron-transfer channels for the reaction between Mo6CIl,- and one-electron-reduced 4-cyano-N-methylpyridinium(CMP), calculated by solving q 8-14 between r and r 6r with fl = 1.2 A-' and HASo = 200 cal.
+
i
(C)
v)
::
Y
0 -0
annihilation reactions involving transition-metal polypyridyl complexes.22 Although the magnitudes of these effects have not explicitly been evaluated for metal polypyridyl complexes or for that matter any chemiluminescent system to date, the discrepancies in H A B o for the excited-state and ground-state pathways will be minimized for exchange reactions involving orbitals of similar parentages. This is the case for Mo6C11~-/acceptorand donor systems. As Figure 2 illustrates, annihilation involves electrons residing in the metal-based egand aZgorbitals of the cluster core. Simple group theoretical treatments reveals that the eB(HOMO) and aZg(LUMO) molecular orbitals are constructed from the linear combination of d, orbitals of adjacent metal atoms.68 These molecular orbitals are shown below. Owing to the similar radial
e
g
distributions of these metal-based orbitals, the electronic factors of the excited- and ground-state electron-transfer pathways are more closely related than those of any cl or ecl system studied to date. Nevertheless, our assumption of similar values of H A B o for the two reaction pathways, at best, is tenuous. The electron-transfer chemistry of Mo6Cl14-and Mo&1,4' ions can be described in terms of two competing reaction channels: a highly exergonic electron-transfer pathway yields ground-state products, and less exergonic exchange leads to the formation of electronically excited M O ~ C ~ion. , ~ The ~ - ratio of the electrontransfer rates for these two channels, deduced from measurements of ecl yields, is a powerful experimental quantity that has provided us with the opportunity to address fundamental aspects of electron transfer in highly exergonic regions. Specifically, the observation of ecl from M0~C1,,~-/acceptorand donor systems is evidence of the Marcus inverted region. Moreover, the cl electron-transfer chemistry, interpreted within the context of the theoretical prediction of Marcus and Side@' and of Brunschwig, Ehrenson, and Sutinsl that the electron-transfer rate in the inverted region will accelerate with increasing distance owing to an increase in the solvent reorganizational parameter, suggests that excited-state production yields of less than unity result from facile electron transfer over long distances. These ecl results imply that the most efficient cl or ecl systems will be those possessing annihilation
r/P4 Figure 4. Distance dependence of the differential bimolecular rate constant for the excited-state (es) and ground-state (gs) electron-transfer channels for the reaction of MogCII4-with (a) a hypothetical one-electron-reduced pyridinium species with AGg,O = -2.05 V and Aceso = -0.05 V; (b) a hypothetical one-electron-reduced pyridinium species with AG,O = -2.15 V and AG,O = -0.15 V;(c) 4-cyano-N-benzylpyridinium; (d) 4-cyano-N-methylpyridinium; (e) 4-carbethoxy-N-benzylpyridinium; and (f) 4-carbethoxy-N-methylpyridinium.The standard free energy driving forces for (c)-(f) are given in Table 111.
reactions between redox centers chemically linked over short separation distances.
Acknowledgment. We thank Professor R. I. Cukier for helpful discussions. Financial support f r o m the National Science Foundation (Grant CHE-8705871), the Presidential Young Investigator Program, and a Dreyfus Grant for Newly Appointed Young Faculty in Chemistry is acknowledged. The generous support provided by PYI industrial matching funds from the Dow Chemical Co. is also gratefully acknowledged. Registry No. Phenothiazine radical cation, 34559-26-7;1 O-methoxyphenothiazine radical cation, 34510-35-5;N,N-dimethyl-p-toluidine radical cation, 77133-47-2; tri-p-tolylamine radical cation, 34516-45-5; tris(4-bromopheny1)amine radical cation, 37881-41-7;p-benzoquinone radical anion, 3225-29-4;2,6-dimethyl-p-benzoquinone radical anion,
J. Am. Chem. SOC.1988, 110, 2712-2774
2772
34496-58-7; pdinitrobenzene radical anion, 34505-33-4; *dinitrobenzene radical anion, 34505-38-9; p-nitrobenzaldehyde radical anion, 345 1233-9; m-nitrobenzaldehyde radical anion, 40951-85-7; 1-chloro-4-nitrobenzene radical anion, 34473-09-1; 5-nitro-m-xylene radical anion, 39933-64-7; 3-nitro-o-xylene radical anion, 83-41-0; 4-cyano-N-benzyl-
pyridinium radical, 1 13249-25-5; 4-cyano-N-methylpyridiniumradical, 64365-84-0; 4-carbethoxy-N-benzylpyridiniumradical, 76036-35-6; 4carbethoxy-N-methylpyridinium radical, 75302-27- 1; 4-amido-Nbenzylpyridinium radical, 1 13249-26-6; 4-amido-N-methylpyridinium radical, 113249-24-4.
Insertion of Two CO Moieties into an Alkene Double Bond To Form a RCH =C(O)C( 0)=CHR2- Unit via Organosamarium Activation’ William J. Evans* and Donald I(. Drummond Contribution from the Department of Chemistry, University of California, Iruine, Irvine. California 9271 7. Received May 1 1 , 1987
Abstract: RCH=CHR ( R = 2-pyridyl) reacts with (C5Me5)2Sm(THF)2to form a red complex, which reacts with CO at 80 psi in toluene to form [(C5Me5)?Sm]2[p-q4-RCH..-C(0)C(O)4HR] (1) in 90% yield. 1 cocrystallizes with 2 molecules of toluene in space group C 2 / m with u = 15.818 (2) A, b =I 14.060 (2) A, c = 15.353 (2) A, B = 111.480 (12)O, and 2 = 2 for Dcalcd= 1.32 g ~ m - ~Least-squares . refinement on the basis of 2526 observed reflections led to a final R value of 0.045. The two (C5Me5)$m units are bridged by a tetradentate bisenolate ligand, RCH=C(O)C(O)=CHR*-, such that each S m is coordinated to one oxygen and the nitrogen atom of the pyridyl group closest to that oxygen. The Sm-0, Sm-N, and average Sm-C(ring) distances are 2.191 (6), 2.473 (7), and 2.71 (1) A, respectively.
T h e divalent organosamarium complex (C5Me5)2Sm(THF)22 has proven t o have a remarkable reductive chemistry with unsaturated substrates such as C S , 3 * 4R=R,&’ and RNNRe8s9 This powerful Sm(I1) reagent can induce facile multiple-bond cleavage and reorganization to provide unusual transformations of multiply bonded species. Three examples are shown in eq 1-3. If C=C double bonds could also be transformed in 4(C5Me5)2Sm(THF)2+ 6CO [[(C5Me5)2Sm]2[p-q3-02CC=C=O](THF)]2 + 6THF (1)3
-
2(C5Me5)2Sm(THF)2
+
C6H,CECC6H5 (C,Me,
+
-
2C0
Table I. Crystal Data and Summary of Data Collection and Structure Refinement for
(C5MeS),Sm[p-~4-(CsH4N)CH=C(0)C(O)=CH(C5H4N)]Sm(CSM~J)~.~C~H~ formula mol wt space gP 4A
b, A c, 8,
P, deg
v,A’
z
),S m 0
g/cm3 temp, “ C h(Mo Ka),A g, cm-I min-max transmissn coeff type of scan scan width, deg scan speed, deg/min bkgd counting data collecn range, deg total no. of unique data no. of unique data with I? 341) no. of parameters R(F) Doled.
A
OSm(C,Me,+ 2(C5Me5)2Sm(THF)2
+
C6H5N=NC6H5
4- 2 C 0
-
C6H5
I
c6H5
(1) Reported in part at the 2nd International Conference on the Chemistry and Technology of the Lanthanides and Actinides, Lisbon, Portugal, April 1987, L(I1)I. (2) Evans, W. J.; Grate, J. W.; Choi, H. W.;Bloom, I.; Hunter, W. E.; Atwood, J. L. J. Am. Chem. Soc. 1985,107,941-946. (3) Evans, W. J.; Grate, J. W.; Hughes, L. A.; Zhang, H.; Atwood, J. L. J. Am. Chem. Sw.1985, 107, 3728-3730. (4) Evans, W. J.; Hughes, L. A.; Drummond, D. K.; Zhang, H.; Atwood, J. L. J. Am. Chem. Soc. 1986, 108, 1722-1723. (5) Evans, W. J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. J. Am. Chem. SOC.1983, 105, 1401-1403. (6) Evans, W. J. Polyhedron 1987, 6, 803-835. (7) Evans, W. J. In High Energy Processes in Organometallic Chemistry; Suslick, K . S., Ed.; ACS Symposium Series 333; American Chemical Society: Washington, DC, 1987; pp 278-289. (8) Evans, W. J.; Drummond, D. K.;Bott, S. G.; Atwood, J. L. Organometallics 1986, 5, 2389-2391. (9) Evans, W. J.; Drummond, D. K. J. Am. Chem. Soc. 1986, 108, 7440-744 1.
RdF) GOF max A/u in final cycle
Sm2C68H86N202
1264.15 C2/m 15.818 (2) 14.060 (2) 15.353 (2) 111.480 (12) 3177 2 1.32 24 0.71 073; graphite monochromator 18.8 0.339-0.449 8-28 -1.2 in 28 from K a l to +1.2 from K a 2 3 evaluated from a 96-step peak profile 3-50 2955 2526 187 0.045 0.059 1.84 0.39
u n u s u a l ways by ( C 5 M e 5 ) 2 S m ( T H F ) z , t h e n this organosamarium(I1) approach to multiple-bond functionalization would apply to an even wider range of substrates. We report here the samarium-mediated functionalization of a C=C bond in which complete cleavage of t h e double-bond and double CO insertion is observed.
Experimental Section The complexes described below are extremely air- and moisture-sensitive. Therefore, both the syntheses and subsequent manipulations of these compounds were conducted under nitrogen with rigorous exclusion of air and water by Schlenk, vacuum-line, and glovebox techniques. The preparation of (C5MeS)2Sm(THF)2 and the methods for drying solvents
0002-7863/88/15 IO-2772%01.50/0 0 1988 American Chemical Society
