J . Phys. Chem. 1984, 88, 2875-2880
2875
Electrochemical and Electron Spin Resonance Studies of Fluorocyclooctatetraene Davis L. Taggart, Wayne Peppercorn, and Larry B. Anderson* Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 (Received: August 1 , 1983; In Final Form: December 12, 1983)
Fluorocyclooctatetraene(FCOT) shows two reduction waves in hexamethylphosphoramide (HMPA) solvent, forming stable products, the radical anion at -1.461 V vs. SCE and the dianion at -1.881 V vs. SCE. The anions are unusually stable to fluoro elimination, and this behavior is attributed to the nonbonding character of the LUMO of FCOT and the substantial resonance energy of the planar anions. Electron spin resonance spectra of solutions of the radical anion show the spin density to be distributed predominantly on the odd-numbered carbons of the FCOT ring. Monosubstitution on the eight-membered ring lifts the degeneracy of the nonbonding orbitals of the anion, causing the observed alternating pattern of spin density. Excellent correlations are found between the measured formal potentials of a number of substituted cyclooctatetraene(COT) molecules in HMPA medium and the 8 values for the substituents. These correlations give evidence of strong steric interaction between adjacent methyl groups in polymethylated COT dianions.
Introduction Electrochemical and chemical reduction of molecules containing the cyclooctatetraene ring system may produce radical anions and dianions of sufficient stability to allow their study by conventional spectroscopic m e t h ~ d s . l - ' ~ Electron paramagnetic resonance (ESR) spectroscopy is a sensitive method of investigating the electronic structure of species containing unpaired electrons and is well suited to observing products of electrochemical reactions at concentrations in the range from to M. This study reports electrochemical and ESR experiments on fluorocyclooctatetraene (FCOT) and its reduction products in hexamethylphosphoramide (HMPA) solvent by using tetrabutylammonium perchlorate (TBAP) as the background electrolyte. We compare these results with data on other substituted COT species. Stevenson and co-workers first used ESR spectra to estimate splitting of the two nearly degenerate nonbonding orbitals of radical anions of various substituted COT'S.*-" Other authors12-14 have extended such studies to additional COT systems. In all of these, marked substituent effects are observed on the distribution of electron spin density and some correlations with the Hammett u values have been attempted.1° Substantial solvent and ionpairing effects were reported. FCOT offers an interesting subject for electrochemical and ESR studies.14 Reversible reduction to the radical anion and dianion can be observed, and the stability of these anions to fluoro elimination is unusual. The measured ESR spectrum of the electrogenerated radical anion, FCOT--, has been used to estimate the orbital splitting, e, in this compound. We have compared tFCOTwith e values of other substituted cyclooctatetraenes and sought correlations be(1) Straws, H. L.; Katz, T. J.; Fraenkel, G. K. J . Am. Chem. SOC.1963, 85, 2360-4.
(2) (a) Carrington, A.; Todd, P. F. Mol. Phys. 1963, 7, 533-40. (b) Carrington, A,; Todd, P. F. Ibid. 1964, 8, 299. (3) Allendoerfer, R. D.; Rieger, P. H. J . Am. Chem. SOC.1965, 87, 2336-44. (4) Kimmel, P. J.; Straws, H. L. J . Phys. Chem. 1968, 72, 2813. (5) (a) Oth, J. F. M.; Baumann, H.; Giles, J.-M.; Schroder, G. J . Am. Chem. SOC.1972,94, 3498-512. (b) Oth, J. F. M.; Woo, E. P.; Sondheimer, F. Ibid. 1973, 95, 7337-45. (6) Shida, T.; Iwata, S . J . Am. Chem. SOC.1973, 95, 3473. (7) Rieke, R. D.; Copenhafer, R. A. J. Elecrroanal. Chem. 1974,56,408. (8) Stevenson, G. R.; Concepcion, J. G.; Echegoyen, L. J. Am. Chem. Soc. 1974, 96, 5452-5. (9) Stevenson, G. R.; Concepcion, J. G. J . Phys. Chem. 1974, 78, 90-1. (10) Stevenson, G. R.; Echegoyen, L. J . Phys. Chem. 1975, 79, 929-32. (11) Stevenson, G. R.; Forch, B. E. J . Phys. Chem. 1981, 85, 378-82. (12) Concepcion, S. G.: Vincow, G. J . Phys. Chem. 1975, 79, 2042-8. (13) Hammons, J. H.; Kresge, C. T.; Paquette, L. A. J. Am. Chem. SOC. 1976, 98, 8172-4. (14) Hammons, J. H.; Bernstein, M.; Myers, R. J. J . Phys. Chem. 1979, 83, 2034-40. (15) Paquette, L. A.; Ewing, G. D.; Traynor, S . G. J . Am. Chem. SOC. 1976, 98, 219. (16) Katz, T. J.; Straws, H. L. J . Chem. Phys. 1960, 32, 1873.
0022-3654/84/2088-2875.$01.50/0
TABLE I: Electrochemical Data for the Reduction of FCOT and COT in HMPA Media cyclic voltammetry polarography
1st wave 2nd wave 3rd wave COT' 1st wave 2nd wave 3rd wave
-1.461 -1.881 -2.4
2.084 2.005
0.36 0.38
-1.461 -1.881
-1.607 -1.923
2.32 1.94
0.40 0.37
-1.605 -1.920
#Reference electrode was Ag/Ag+ (0.1 M), E ' A ~ , A ~=+0.360 V fiA s1l6 M-' g-2/3cP'I2. vs. SCE, in HMPA-0.1 M TABP. cUnits: fiA cm-2 M-' V-'/' s1I2. dCFC~T= 1.74 X lo-' M. eCCOT = 4.70 x 10-3 M. tween e values and the reduction potentials and the Hammett u values for these compounds. Results Electrochemical reduction of FCOT in HMPA solvent occurs in two steps with formation of products stable for at least several seconds (Figure 1). Both cyclic voltammetry and polarography show limiting currents similar to those observed for COT in the same medium (Table I). The product of the first reduction step is apparently the radical anion FCOT-.. Controlled potential electrolysis at -1.95 V (on the plateau of the first wave) yielded a linear plot of log i vs. t with an nappof 0.73. A short time after start of electrolysis a pale green product became visible and the color darkened with further electrolysis. A sample of the deep green solution (approximately 50% reduced) was transferred, without violation of the inert atmosphere, to a quartz ESR flat cell. The X-band ESR spectrum is shown in Figure 2. This spectrum consists of two partially overlapping quartets due to one fluorine atom with a coupling constant of 12.9 G and three nearly equivalent protons with coupling constants of 5.8 G. The individual lines are quite broad (- 1 G), indicating additional unresolved structure may be present (possibly hyperfine splitting by the four remaining ring protons14). Discussion of this spectrum is expanded below. When a sample of the green electrolysis product was quenched with 10 pL of neat water, the color lightened but did not disappear. On exposure to the atmosphere the green color faded rapidly, leaving a pale yellow solution. GC-MS analysis of a pentane extract of the water and oxygen quenched solution showed only the parent, FCOT, present above the limit of detection ( 5%). Neither dihydro products nor elimination products were detected (e.g., COT or H,COT). N
0 1984 American Chemical Society
2816
The Journal of Physical Chemistry, Vol. 88, No. 13, 1984
Taggart et al.
TABLE II: ESR and Electrochemical Data from Various Sources for Substituted COT'S coupling constant, G potential,a V vs. SCE substituent asubstit aH3.5,7 an2,4,6,8 EO', Eor2 F 12.9 5.8 (3)b unresol (4) -1.461 -1.881 +13.01 -6.500 (3) 0.326 (2) 0.164 (2) C6HS 0.46 (0,p) (3) 2.25 (3) 3.85 (4) -1.612 -1.736 0.23 (m) (2) C6D5 none 2.38 (3) 3.68 (4) 1,3,5,7-tetraphenyl 0.40 (0,p) (12) none 3.30 (4) [-1.358 (2)] [-1.358 ( 2 ) ] 0.20 (m) (8) H none 3.18 (8) 3.18 (8) -1.605 -1.920 CH3 5.1 (3) 4.8 (3) 1.6 (4) -1.729 -1.948 +5.544 (3) -5.057 (3) -1.401 (4) 1,2-dimethyl -1.930 (2) 1,4-dimethyl +3.50 (6) -3.11 (6) -3.11 (6) [-2.221 [-3.021
1$-dimethyl
6.27 (6)
-5.85 (2)
+0.48 (4)
1,4-(CH2)9 CH30
0.826 (3)
-6.096 (3)
-0.431 (2) -0.326 (2)
none
0.42 (4) 0.45
1,2,3-trimethyl 1,3,5,7-tetramethyl 6.29 (12) 6.41 1,2,3,4-tetramethyI
-1.868 [-2.141 -1.936 [-1.871
-2.020 [-2.951 -2.28 (irr) [-2.051
[-2.228 (2)] -2.540 (2) -2.430 (2)
ref and comments this work 14 "3, -72 " C 7, HMPA 8, HMPA 7, HMPA (ESR) 7, AN (elect) 17, HMPA 2, THF, -100 OC 14, "3, -72 " C 13, HMPA 13, DMF, -55 "C 13, THF (elect) 13, DMF, -55 OC 13, THF (elect) 13, HMPA 14, "3, 75 O C 14, DMF (elect) 14, HMPA 8, HMPA, ambient 11, e = 0.078 eV 19, values are for two bond isomers
'Unless indicated, all potentials were obtained in HMPA solvent, 0.1 M TBAP electrolyte. bNumbers in parentheses indicate number of equivalent protons. cNumbers in square brackets indicate potentials measured in different solvent media.
i
n'
I
W
,,ll lil l l l l i1l l l l l 1l l 1l l l l
4c
1
-e
5 POTENTIAL,
0 v o l t s vs
I
I
I
I
-1
-2
5
S CE
Figure 1. Cyclic voltammetry and polarography of 1.74 X loT3M FCOT in HMPA-0.1 M TBAP.
Controlled potential electrolysis at -2.15 V (plateau of the second wave) gave a total integrated current equivalent to 2.06 electrons per mole of FCOT. The plot of log i vs. t was linear after the first 3 min of electrolysis and the slope corresponded to napp= 2.2 electrons. GC-MS analysis of a pentane extract of a fully reduced solution which had been quenched with oxygen-free water gave positive identification of C8HI0,FCsH,, and tributylamine in approximate ratios of 4:6:9. The electrochemical behavior of several other substituted COT molecules in HMPA medium has also been studied.35 The appropriate formal potential data have been summarized in Tables I1 and 111. Figure 3 illustrates the cyclic voltammetric behavior of these compounds under uniform conditions of solvent, temperature, and background electrolyte.
represented the wave function of the spin distribution as a linear combination of the wave function of the two nonbonding orbitals, \kA and \ks. Myers et al.14 find that this is a reasonable approximation; we have therefore followed this approach to analysis of the coupling constants for FCOT-e. It is assumed that the wave function, Qspin, of the unpaired spin may be represented by a two-parameter perturbation equation:
Discussion ESR Spectrum of FCOT.. To explain the even-odd distribution of spin density in substituted cyclooctatetraenes, Carrington and ToddZ assumed DShsymmetry for the radical anions and
In the parent COT, 9,, is doubly degenerate, and the probability of electron occupation of these orbitals, CAzand Cs2,is equal to 0.5 in each case. For substituted COT'S, where the degeneracy
-(IF-
Figure 2. X-Band ESR spectrum of a partially reduced solution of FCOT in HMPA-0.1 M TBAP.
*spin
= CA*A + CS*S
(1)
The Journal of Physical Chemistry, Vol. 88, No. 13, 1984 2877
Electrochemical and ESR Studies of FCOT
TABLE 111: Shifts of datum no. 1 2 3 4 5
6 7 8 9 10 11 12 13
Various Characteristic Potentials for Substituted COT'S with Normal Substituent Constants" substituent(s) g" A E o / I ,mV ~, AEotZ,R,mV AEO',,, mV AE0'1,2,mV F +O. 17 +144 +39 +92 420 Ph +0.01 -7 +184 +88 124 H 315 CH3 -0.13 -124 -28 -76 219 C(CH313 -0.20 -276 -7 5 176 114 1,5-(CH3)z -0.26 -249 -100 -174 166 1,2-(CH3)2 -0.26 -325 ( 2 ) b -210 0 1,4-(CH2)9 -331 (1) -346 350 -623 (2) -466 0 1,2,3-(CH3)3 -0.39 1,2,3,4-(CH3)4 -0.52 -935 (2) -778 0 -825 (2) -668 0 1,2,3,8-(CH3)4 -0.52 193,5,7-(CH3)3 -0.52 -800 OCH3 -0.11 -130 180
c
meV +73
E,
-1 8
+22 +47
+78 +48
'Values of u" are those complied by: Shorter, J. "Correlation Analysis in Organic Chemistry"; Clarendon Press: Oxford, 1973. bNumbers in parentheses indicate number of electrons. On application of this formalism to the ESR data in Figure 2, the spin densities may be calculated. If we take QHC-Hin the McConnell equation a~ = QHc-~pc
(4)
to be 25.4,17 the mean values of p1,3,5,7are estimated to be +0.228 in HMPA at 25 OC. The remaining spin density must be located on the F atom and the even-numbered carbons. We estimated the spin density on the fluorine atom, pF*, from the two-parameter equation suggested by Konishi and Morokuma: l 8
+
a(l0F) = 32pCff 225pFff
(5)
For FCOT-., pF* is then calculated to be 0.025 at 25 OC. Thus, the total spin density in \ks equals 0.937. We assume that the remaining spin density, 0.063, is to be found in \kA and may be taken as Cpeven.This assumption is at least consistent with the observed line width of 1 G. If 1 G represents the total width of unresolved fine structure due to an equal spin density on each of the even carbons, the resultant splitting of 0.25 G would yield an estimate of 0.04 for Cpcvenfrom eq 4. From the equation*
-
cZ ,
-I
6
-18
-20
-22
-18
-20
-P2
-24
Figure 3. Cyclic voltammogramsof several substituted cyclooctatetraenes in HMPA solvent. Concentrations and voltage scan rates are as follows: PhCOT, 0.21 mM, 20 mV/s; CH,COT, 1.45 mM, 100 mV/s; (CH,),CCOT, 0.90 mM, 150 mV/s; (CH2)&OT, 1.4 mM, 100 mV/s; 1,2(CH3),COT, 0.13 mM, 500 mV/s; 1,2,3-(CH3)$OT, 0.39 mM, 150
mV/s. is removed, CA2and Cs2 can be estimated from the ESR spin densities cS2
=
CPcdd/C(Pcdd
+ peven)
(2)
where x p d d is the sum of the a-spin densities on carbons 1, 3, 5, and 7, and x p e v e nthe sum for carbons 2, 4, 6, and 8. For the radical anion, Cp = 1.00, which may include spin densities on substituents as well as on the skeletal carbon atoms.
= e-f/kT/(1 + p / k T )
(6)
we estimate E to be 0.073 eV at 25 "C. Thus, while the fluorine substituent is electron withdrawing from the o-bond system of COT, coupling between the "nonbonding" a-orbital of fluorine and the ?r-type orbital of FCOT raises the energy of 'ksrelative to Spectra of F C O T . in liquid ammonia reported by Myers et al.I4 show additional hyperfine structure and might therefore allow direct estimation of However, at these lower temperatures the splitting between \kA and \ks is even greater, and Myers has discussed the possible assignment of negative spin density to the even-numbered carbons as a consequence of correlation effects between the electrons in these orbitals. E is at least as large as and probably larger than the 73 meV estimated for HMPA at 25 OC. Comparison of cFcoT with Other RCOTs. In Table I1 we have collected together data on the ESR coupling constants and electrochemical reduction potentials of a variety of substituted COT systems. In some cases, estimation of E from these data is straightforward, but ambiguities exist for several systems. We discuss our assumptions below. Phenylcyclooctatetraene. Using Q H ~ - H = 25.4, assign p1,3,5,7 = +0.0886, p2,4,6,8 = +0.1516; phenyl-ring pd2,4,6 = +0.0181 and (17) Stevenson, G. R.; Concepcion, J. G. J . Phys. Chem. 1972, 76, 2176. (18) Konishi, H.; Morokuma, K. J . Am. Chem. SOC.1972,94, 5603-12. (19) Domelsmith, L. N.; Houk, K. N.; Piedrahita, C.; Dolbier, W. J., Jr. J . Am. Chem. SOC.1978, 100, 6908. (20) Paquette, L. A.; Photis, J. M. J. Am. Chem. SOC.1976, 98, 4936-45.
2878
Taggart et al.
The Journal of Physical Chemistry,Vol. 88,No. 13, 1984
pdl,3,5= -0.0091 yields a consistent value of ptotal= 0.987. Values of Cs2 = 0.381 and CAz= 0.631 yield an estimate for c = -0.019 eV. If pmetais taken as positive, ptotal= 1.042. 1,3,5,7-Tetraphenylcyclooctatetraene. QHc-H = 25.4 yields Cpeven= 0.518. The spin densities on the phenyl carbons, with 6 = f0.2085, and xpdl,3,s = -0.1042. A on carbons 1, 3, and 5 of the phenyl rings, pmeta,is suggested by the analysis for phenylcyclooctatetraene given above. Then Cpdd = 0.378 and e = -0.008. The alternative assumption, that pmetais positive, yields Cpdd = 0.169 and E = -0.026. Again a substantial ambiguity of sign for pmetaremains. at Methylcyclooctatetraene. With QHC-H= 26.22 in = 0.7714 and Cp,,,, = 0.2137. This -72 OC, we assign gives a value of ptotal= 0.985. Thus, c = 0.022 eV. 1,5-Dimethylcyclooctatetraene.In DMF at -55 OC, QHc+ = 26.16,13and consequently Cpdd= 0.0734 and Cpeven = 0.894. These spin densities total 0.967, and t = 0.047 eV. Methoxycyclooctatetraene. QHc-H = 26.22, = +0.9300, x p z , 8= 0.0328, = 0.0249. Thus, if pevenare positive, ptatal = 0.988 and c = 0.048 eV. I ,3,5,7-Tetramethylcyclooctatetraene.A careful temperature study by Concepcion and VincowIz has estimated c = 0.078 eV. In FCOT the fluorine substituent raises the energy of *s, apparently by a-backbonding with this orbital. An increase in energy of \ks also appears to occur in CH3COT where the inductive effect of the substituent on the COT ring is considered to be electron donating through the u-bond system relative to hydrogen (Xunis positive). There does not seem to be any simple relationship between the splitting of these energy levels in the radical anion and the ease of addition or removal of an electron from these same orbitals. Indeed, these same two substituents cause opposite shifts in the reduction potentials. Because of the sensitivity of the formal potentials of substituted COT molecules to solvent, background electrolyte, electrode composition, and t e m p e r a t ~ r e we , ~ ~have measured the electrochemical behavior of a set of RCOT molecules under identical conditions. It is these values of Eo’which are correlated with the Hammett u value and ESR-determined c value below. Table I1 also contains Eo’information obtained in other solvent media when it is not available in HMPA. Energy-LevelCorrelations.Correlations have been reported between the formal potential, Eo’l, of the first electron addition to substituted COT’s and the sum of the unvalues for the substituents on the eight carbons of the ring.z1-z5 There are, in fact, several additional measures of the electronic energies in these compounds that may show correlative behavior with this common index of inductive effects. Successful correlation with one or more of these may lead to new understanding of the electron distribution in these compounds. Table I11 summarizes pertinent data, which are discussed further below. is proportional to the electron affinity of the olefin, and its shift may therefore reflect changes in the energy necessary to add an electron to the LUM0.z2 In most of the RCOT molecules in Table 111, this process involves flattening of the eight-membered
(21) Hall, D. W.; Russell, C. D. J. Am. Chem. SOC.1967, 89, 2316. (22) Zuman, P. “Substituent Effects in Organic Polarography”; Plenum Press: New York, 1967. (23) Kuwana, T.; Bublitz, D. E.; Hoh, G. J . Am. Chem. SOC.1960, 82, 5811 (24) Hoh, G. L. K.; McEwen, W. E.; Kleinberg, J. J. Am. Chem. SOC. 1961,83, 3949. (25) Paquette, L. A,; Ley, S . D.; Meisinger, R. H.; Russell, R. K.; Oku, M. J . Am. Chem. SOC.1974, 96, 5806-15 Paquette, L A.; Wright, C. D.; Traynor, S.G.; Taggart, D. L.; Ewing, G. D. Tetrahedron 1976,32,1885-91. (26) Nadjo, L.; Saveant, J. M. J . Electroanal. Chem. 1971, 30, 41-57. (27) Russell, G. A,; Gerlock, J. L.; Underwood, G. R. J . Am. Chem SOC. 1972, 94, 5209. Williams, L. F.; Yim, M. B.; Wood, D. E. Zbid. 1973, 95, 6475. (28) Alwair, K.; Grimshaw, J. J . Chem. SOC.,Perkin Trans. 2 1973, 1811, 1150. (29) Toriyama, K.; Iwasaki, M. J . Phys. Chem. 1972, 76, 1824. (30) van den Hamm, D. M. W.; Harrison, G F.; Spaans, A.; van der Meer, D.J. R . Neth. Chem. SOC.1975, 94, 168-73. (31) Yim, M. B.; Wood, D. E. J . Am. Chem. SOC.1976, 98, 2053-59. (32) Sales, K. D. Ado. Free-Radical Chem. 1969, 3, 184-97.
2001
1 to 2
I
I
1
0
-02
,
-04
-06
ZUn
Figure 4. Correlation between the shift in formal potential of the first reduction wave of substituted COT’s with the Hammett un values.
ring, reorganization of solvent molecules, and changes in ionic interactions with the electrolyte as well as reduction of the ring itself. In order to focus principally on the reduction process, we follow the common practicez2 of plotting the shift
AEO’I,,= Eo’l,R- Eo‘l,H
(7)
where is the experimentally estimated formal potential for is the potential for COT itself. the compound, RCOT, and Eo’l,H During the first reduction step, the electron is added to an orbital, \kcharge,that is a linear combination of ‘k, and \ks complementary to ‘kspinin eq 1. The energy of these nearly degenerate orbitals (\ksplnand *charge) should be quite sensitive to addition or withdrawal of electron density from the carbon atoms of the ring. The correlation of AEO‘1,R with x u n is shown in Figure 4 for 10 compounds. The data appear to define two straight line segments. For five compounds (F, Ph, CH3, 1,5-(CH&, and OCH3 substituents), the p value (slope) is 0.91 V with a correlation coefficient of 0.998. This p value is comparable to, but approximately double, the slope reported for reduction of a number of substituted ferrocene^.^^,^^ This may reflect the fact that the inductive source in the RCOT molecule is covalently attached to a specific ring carbon on which electron density is being modified during reduction. In ferrocenes the inductive source, R, is attached to a ring carbon which is in turn bonded to the point of electrochemical activity, the central iron ion. In most other electrochemical studies of this type,22the inductive source is a substituent on a benzene ring which can substantially isolate it from the point of electrochemical activity. The second straight line segment, defined by CH,, l,2-(CH3)2, 1,2,3-(CH3),, 1,2,3,8-(CH&, and 1,2,3,4-(CH3)4substituents, shows a p value of 1.98 V and a correlation coefficient of 0.991. This group of molecules has been discussed by Paquette and c o - w ~ r k e r swho , ~ showed ~ ~ ~ ~that ~ ~very ~ ~large ~ ~ steric interactions occur between adjacent methyl groups when the molecule approaches a planar configuration during reduction. Thus, the total energy of adding an electron contains at least two components:
AEo’l = AEo’I,R+ AEo’strain,CHj
(9)
where AEo’stralnis a measure of the incremental energy necessary to overcome the steric strain resulting from interaction between two adjacent methyl groups on the planar COT ring. Apparently, the steric interference is additive, and addition of each successive adjacent methyl group adds an increment of approximately -0.140 V to the formal reduction potential. This would indicate that approximately 3.3 kcal/mol is required to overcome steric strain (33) (a) Paquette, L. A.; Gardlik, J. M. J . Am. Chem. SOC.1980, 102, 5016-25. (b) Gardlik, J. M.; Paquette, L. A,; Gleiter, R. Ibid. 1979, 101, 16 17-20.
The Journal of Physical Chemistry, Vol. 88, No. 13, 1984 2879
Electrochemical and ESR Studies of FCOT
/ '3)4 H3)4
I
~
to2
00
-02
-04
-06
ZUn
Figure 5. Correlation between the shift in hE"',, and Ed'for substituted
COT's. induced when two adjacent methyl groups are forced into the planar or near-planar geometry of the radical anion. This compares with a value of 6-7 kcal/mol reported33bfor the increase in AG* for ring inversion for each adjacent methyl group on the ring. Equating these observed changes in AE",,R/Au" with steric strain can only be a crude approximation because among other things it does not take into account different nonbonded interactions between adjacent methyl groups in the parent hydroc a r b o n ~ . ~ "In, ~addition, ~ we face the problem that the cyclic voltammograms of all of the sterically hindered polymethylated COT's show evidence of irreversible chemical decomposition of the reduction product. This will cause E"'1,R estimated as the average of the peak potentials of the cyclic voltammogram to be more positive than the (unknown) true value, resulting in underestimation of AGstraln.As discussed below, the shift of E"'1,R to more negative values may cause the reduction of parent to radical anion to be more difficult than reduction of radical anion to dianion, corresponding to disproportionation of the radical anion. Again this results in underestimation of AGstraln by this technique. Finally, the assumption that the radical anion is a fully planar species may be a poorer and poorer approximation as more adjacent methyl groups are added to the ring and the resultant strain energy cannot be compared to the energy barrier to ring inversion. AE0'2,R, the shift in formal potential of the second electron addition to RCOT, also correlates with xu"but in a less certain fashion (Table 111). A trend is present ( p = 0.31 V; r = 0.973), but the data cannot include values for those molecules for which the first and second waves overlap. The four monosubstituted COT's gave fully reversible second waves and these data are the ones shown in Figure 5, but the polymethylated anion products showed some chemical instability. Because the two waves overlap completely for 1,2-dimethyl- and the more highly methyl-substituted species (see Figure 3), a more productive procedure for handling two-step reduction processes may be to correlate the average of the formal potentials of the two steps. In that case, the formal potential of the overall process, E'',,,R, is This corresponds to computing the E'' value for the two-electron-transfer reaction RCOT
+ 2e- F? RCOTZ-
One advantage of this formalism is that the E"' values of the two waves need not be measured separately in order to estimate EO',. In addition, while most COT radical anions are thought to be planar or nearly planar, it is quite certain that the dianion product of each reduction is fully planar and aromatic,2sand the energy
of its formation will include the full steric interactions mentioned. The correlation, shown in Figure 5, is excellent. For the first five points p = 0.65 V with r = 0.982. For the five sterically hindered methyl derivatives, p = 1.68 V with r = 0.990. This difference in slopes results in a dramatic increase in the electrochemically estimated effects of methyl-methyl interaction in the planar product. The steric shift of AE",, per adjacent methyl group is -0.138 V, giving a AG,,,,,, of 6.3 kcal/mol (here AGsterica 2AE0',,, for a two-electron process). This estimate is much closer to the more direct estimate of the effect of nonbonded methyl interactions on the free energy of activation for ring inversion in these compounds.33b Of the four causes given above for underestimating AGStraln by the disproportionation of RCOT-- and correlation of AE' uncertain planarity of the product seem to be compensated by correlating AE'',,. Errors due to irreversible electrochemical behavior and nonbonded steric interactions in the parent remain uncorrected. Table I1 also contains two very different measures of the relative energies of the nonbonding orbitals in COT and its radical anion, AED'l,2and t . The difference between the formal potentials of the first and second electron transfers measures the difference in energies between the LUMO of the neutral and the LUMO of the radical anion. AE0'l,2= E"'1,R - E"2,R
= R T / ( F In KD)
( 1 1)
This in turn is directly related to the disproportionation constant of the radical anion, KD. AE"'l,2 will be affected by t, the separation between 9, and \ks in the radical anion, the electronelectron repulsion energy, as well as by differences of solvation, steric effects of the methyl substituents, and ion pairing in the neutral, radical anion, and dianion. Some correlation is observed between AE0'1,2 and xu", with a p value of 0.53 V and a correlation coefficient of 0.973. With so many parameters affecting the correlation, it is not surprising that the u" value alone does not account for the observed shifts in AE0'1,2. AE0'1,2 would show a correlation with the ESR'>*-estimated value of if differences in ion pairing, solvation, and steric effects were constant. This correlation was attempted for data numbers 1, 2, 4, 5, 12, and 13 in Table 111, but no correlation was found ( p = 0.004, r = 0.03). Perhaps the fact that changes in t are only tens of millivolts whereas E"' values change by hundreds of millivolts means that any possible correlation is lost in the electrochemical noise. Ion pairing, solvation, and other secondary effects presumably are of a magnitude comparable to or larger than 6, and changes in the energies of both nonbonding orbitals probably greatly exceed changes in t alone. At the extremes of orbital splitting discussed here, near plus and minus 75 meV, our assumption of a two-state model of spin distribution appears to be simplistic. Electron correlation effects are calculated to be substantial for FCOT and CH3COT14and high-resolution spectra of FCOT., CH30COT., and N C C O T show additional ESR ~p1ittings.l~Both are indicative of the inadequacy of eq 1 to describe the electron spin density distribution accurately. Stability of FCOT Anions. Several factors may control whether or not fluoride ion will be eliminated from a given fluoro radical a n i ~ n . ~ ~At- ~a "minimum, rupture of the carbon-fluorine bond requires that the free energy of formation of the solvated F ion exceed the carbon-fluorine bond strength. It has been suggestedz8 that a negative correlation may exist between the stability of the C-X bond and the charge density on the carbon atom. However, van den Hamm et found no correlation between the ESRmeasured charge density and the electrochemically estimated stabilities of radical anions of several fluorinated azaromatics. The cyclic voltammogram in Figure 1 shows that reduction of FCOT to the radical anion and dianion is nearly Nernstian, the criterion used by van de Hamm3" to indicate stability. We estimate the charge density on carbon 1 to be less than 0.05 for FCOT-. and approximately 0.25 for FCOT2-. Thus, factors other than
J. Phys. Chem. 1984,88, 2880-2883
2880
charge density must be important. Fluoride ion elimination from FCOT anions also involves loss of *-delocalization energy and a decrease in bond angle strain. Therefore, for both the reactions
Experimental Section The electrochemical cell and its operation have been described e l s e ~ h e r e . ~Potential ~ , ~ ~ control and current measurement were
made with a PARC Model 173 potentiostat/galvanostat driven by a Wavetek Model 133 function generator. ESR samples were obtained by attaching an evacuated ESR flat cell to the electrochemical cell by a 5-mm Solv-Seal joint (Fisher-Porter). The attachment was provided with a Teflon stopcock to maintain evacuation of the ESR cell during electrolysis. The electrochemical cell was then filled as usual,34and, after an appropriate amount of electrolysis had been carried out, the stopcock was opened and a sample drawn into the ESR cell for analysis without violation of the inert atmosphere. ESR spectra were taken at 23 "C by using a Varian Model E4 X-band spectrometer. We would like to thank Prof. Lawrence Berliner for his aid in obtaining these ESR spectra. GC-MS analysis was carried out by using a Perkin-Elmer Model 990 gas chromatograph connected by a jet separator to a Du Pont Model 21-490 single focusing mass spectrometer. Electrolysis solutions (- 10 mL) were extracted with three 5-mL aliquots of pentane. The pentane extract was washed twice with water (to remove HMPA) and reduced at ice temperature to about 1 mL. The GC temperature program was as follows: 70 "C for 1 min followed by an increase of 16 "C/min to 210 OC. The flow rate was 30 mL of He/min. Samples of cyclooctatetraene and substituted cyclooctatetraenes were obtained through the courtesy of Prof. Leo Paquette. Samples were purified by G C immediately prior to preparation of the electrochemical solutions studied. Registry No. Fluorocyclooctatetraene, 1884-66-8;tert-butylcyclooctatetraene, 61593-18-8;1,2,3,8-tetramethylcyclooctatetraene, 5668376-2;fluorocyclooctatetraeneanion radical, 70741-95-6;fluorocyclooctatetraene dianion, 90064-78-1,
(34) Mills, J. L.: Nelson, R.; Shore, S.; Anderson, L. B. Anal. Chem. 1971, 43, 157-60.
(35) Taggart, D. L.Ph.D. Dissertation, The Ohio State University, Columbus, OH, 1975.
0.
-
the following inequality applies: AGrepul+ AGsolv+ AG,,,,i,
> AG$$
+ AGT
(14)
Bond angle strain is thought to be moderate, around 12.5-14.8 kcal/m01,~~ for planar eight-membered carbocyclic rings. The a-delocalization energy, 25 kcal/mol for FCOT, thus seems to act as a major stabilizing factor for its anions when compared to radical anions and dianions of fluoro-substituted benzenoid aromatics, where the extra electron (or electrons) occupies an antibonding orbital, making loss of F energetically more favorable.
~~
Thermochemical Properties of Gas-Phase Mixed Clusters: H20/C02 with Na+ K. I. Peterson? T. D. Mark,t R. G. Keesee, and A. W. Castleman, Jr.* Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: August 21, 1983; In Final Form: November 28, 1983)
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-
The gas-phase equilibria for the reaction Na+(COz)n+ COz Na+(C02)n+land those leading to mixed clusters Na+(H,O), + COz Na+(H20),(C02) were determined over a range of temperatures by high-pressure mass spectrometry. Enthalpy and entropy changes were deduced for the stepwise clustering reactions with n ranging from 0 to 3. From these data, earlier Na+ hydration results, and electrostatic calculations for the bond between Nat and various ligands, insight is gained about the effect of H20-C02interactionsin the formation, stability, and structure of the mixed clusters of the form Na+(HzO),(C02),, Le., COz is found to bind more strongly to Na+(H20)than to Na+(C02),whereas H 2 0 binds more strongly to Na+(CO,) than to Na+(H20).
Introduction Most ion-molecule clustering experiments have been concerned with the sequential attachment of one particular species to an ion. A few exceptions include the H z 0 / N H 3 and H20/H2Ssystems with the proton as the ion and HzO/C6H6 on K+.' Few comprehensive gas-phase thermodynamic studies have been done on mixed clusters involving two different ligands and a metal ion. Interest in this type of cluster arises from the possibility that, due to the close proximity of the ion, the high field experienced by the attached ligands might change their electronic structure in +Department of Chemistry, Harvard University, Cambridge, MA 021 15. f Permanent address: Inst. f. Experimentalphysik, Leopold Franzens Universitat, A-6020 Innsbruck, Austria. *Experimental work was conducted while the authors were at the Unversity of Colorado.
0022-3654/84/2088-2880%01.50/0
such a way as to render them more reactive to a different molecule, Le., the ion could serve as a catalyst.z Carbon dioxide complexes with H 2 0 in the gas phase with an enthalpy change (at 298 K) of -4.8 kcal/moL3 The formation of H2CO3 does not occur because of the presence of a high activation barrier.4 In biological systems, the reaction
(1) (a) J. D. Payzant, A. J. Cunningham, and P. Kebarle, Can. J . Chem., 51, 3242 (1973). (b) K. Hiraoka and P. Kebarle, Ibid.,55, 24 (1977). (c) J. Sunner, K. Nishizawa, and P. Kebarle, J . Phys. Chem., 85, 1814.(1981). (2) B. R.Rowe, A. A. Viggiano, F. C. Fehsenfeld, D. Fahey, and E. E. Ferguson, J . Chem. Phys., 76,742 (1982). (3) C.R.Coan and A. D. King, Jr., J . Am. Chem. Soc., 93,1857 (1971). (4)J. E. Coleman, J . B i d . Chem., 242, 5212 (1967).
0 1984 American Chemical Society
