1,2-Disubstituted CyclooctatetraeneAnion Radicals
1685
Electron Distribution in Some 1,P-Disubstituted Cyclooctatetraene Anion Radicals and Dianions Gerald R. Stevenson,* Marltra Colbn, lgnacio Ocaslo, Jesus Qllberto Concepcibn, Universlty of Puerto Rico, Department of Chemlstry, Rio Piedras, Puerto Rico 0093 1
and Arthur McB. Block Division of Physical Sclences and TerrestialEcology Program, Puerto Rico Nuclear Center, Rio Piedras, Puerto Rico (Received February 12, 1975) Publication costs assisted by the University of Puerto Rico
The enthalpies controlling the disproportionation equilibrium constant for the anion radicals of benzocyclooctatetraene and naphthocyclooctatetraene have been studied in hexamethylphosphoramide. Upon comparing these enthalpies with those for cyclooctatetraene, we find that the enthalpy term does not reflect a strong decrease in the electron-electron repulsion in the dianions due to the extended conjugation with the benzo and naphtho ring systems. However, some delocalization of the two extra electrons into the naphtho moiety was evidenced by the entropy term, which reflects a decrease in ion pairing. The odd electron in the anion radicals of benzo- and naphthocyclooctatetraene also resides predominantly in the eightmember ring system, and the spin distribution in these systems resembles that for the 1,2-dicarbomethoxycyclooctatetraene and cyclooctatrienyne anion radicals. The tendency of the added electrons in the fused cyclooctatetraene systems to remain in the eight-member ring system is probably due to the fact that the cyclooctatetraene moiety “needs” extra electrons to approach aromaticity and the benzene ring systems already have 4n 2 electrons.
+
I t is well known that the concentrations of the cyclooctatetraene (COT) and substituted cyclooctatetraene anion radical, dianion, and neutral molecule are controlled in solution by the disproportionation reaction1 n2-
+n
F!
2n.-
(1)
The thermodynamic parameters for reaction 1 have been studied for COT, bis(cyc1ooctatetraene) (BCOT), and phenylcyclooctatetraene (PCOT) under conditions where ion pairing is minimized, that is in hexamethylphosphoramide (HMPA). Comparing the enthalpies for reaction 1 for these three anion radicals it was noticed that the enthalpy increases with the size of the substituent, and thus with the degree of conjugation between the substituent and the COT ring.2 This effect has been attributed to the lower electron-electron repulsion in the dianions with more extensive conjugation. Thus it appears as if there is large conjugative decrease in the electron-electron repulsion due to the second COT moiety in BCOT but only a very small effect due to the phenyl group on PCOT. T o date there has been no report of the enthalpies of disproportionation for fused benzocyclooctatetraenes (benzoCOT). However, the spin distribution in the anion radical of benzo-COT has been of considerable recent interesL3q4 Anderson and Paquette3 have suggested that the anion radical of benzo-COT has a structure in which the benzo and hexatriene moieties are noninteracting, and the odd electron would exist only in the hexatriene moiety. However, Dodd4 has provided recent theoretical work coupled with the ESR pattern of the anion radical that indicates that this anion radical is planar. Further, Katz and coworkers6 have observed that the extra electrons in both the sym- dibenzcyclooctatetraene anion radical and dianion are appreciably delocalized about the central ring. Although the anions of COT and some substituted COT’s appear to be essentially planar and fully conjugated,
the neutral molecules generally exist in a tub type configuration. Anet and coworkers6 have been able to determine the barrier to ring inversion and thus bond isomerization for several 1,2-disubstituted COT’s. This work was possible due to the fact that 1,2-disubstituted COT’s can exist in two possible isomeric forms, one with a double bond between the two substituents and one with a single bond between the substituents.’ However, to our knowledge, there has been no report of a 1,2-disubstituted COT anion radical. Here we wish to report the enthalpies of disprortionation of benzo-COT and naphthocyclooctatetraene (NCOT) together with the spin distributions in the benzo-COT, NCOT, and 1,2-dicarbomethoxycyclooctatetraenesystems.
Results and Discussion Solutions of NCOT in HMPA will dissolve small amounts of potassium metal to form the respective anion radical. This anion radical solution yields a well-resolved ESR pattern, Figure 1, that consists of five doublet splittings each due to two equivalent protons. The experimental coupling constants together with those predicted by simple Huckel theory are given in Table I. The experimental P orbital spin densities were calculated using -23 G for Q. Addition of more potassium to this anion radical solution results in a decrease in the spin concentration due to the formation of the dianion. With the addition of about 2 mol of alkali metal the solution becomes diamagnetic. However, addition of the neutral molecule to this dianion solution results in a return of the ESR signal. The anion radical concentration for the NCOT-HMPAK system was determined using the system COT-HMPAK as a spin standard in the previously described technique.8 Assuming a lorentzian line shape, the anion radical concentration is proportional to the ESR peak amplitude ( A ) times the square of the extrema to extrema line width The Journal of Physical Chemistry, Vol. 79, No. 16. 1975
isas
Stevenson et al. I
1.8
1.9
1.7
10J/RT
Flgure 2. Plot of In Keq vs. llRTfor the system NCOT-HMPA-K.
where B is a proportionality constant. The equilibrium at 25". A plot of In constant was found to be (4 f 2) X K,, vs. 1/RT yields an enthalpy for the reverse disproportionation (comproportionation) of -4.7 f 0.04 kcal/mol. Since the concentration of the anion radical is much smaller than that for the dianion or neutral molecule and the line width for the overmodulated ESR line does not change with temperature, a simple plot of In A vs. 1/RT is also linear and has a slope of -&Yobsd0/2. Enthalpies determined in this manner are identical with those obtained by plotting the area under the integrated ESR spectrum (doubly integrated ESR line) vs. 1/RT. For these systems where the anion radical concentration is much smaller than that for the dianion or neutral molecule, the determination of M o b s d o is much simpler experimentally than the determination of the observed equilibrium constant (Kobsd). If &Yobsd" is the true thermodynamic enthalpy of comproportionation, the stoichiometry given in eq 1 must be correct and ion pairing must be minimal. Any dianion that is ion paired will undergo comproportionation with the following stoichiometry.
t
.lr2-,K+
Flgure 1. Low-field half of the ESR spectrum of NCOTe- in HMPA (upper), computer simulation (lower).
(Am). The equilibrium constant can be expressed by eq 2 Kobsd
= B 2 A 2 ( A ~ ) 4 /(a2-) (a)
The Journal of Physical Chemistry, Vol. 79, No. 16, 1975
(2)
t .lr
2 7 ~ -t K+
(3)
The presence of dianion ion pairs results in an increase in the observed e n t h a l ~ y . ~ The reduction of benzo-COT by potassium metal in HMPA also results in the formation of a solution yielding a well-resolved ESR pattern. This spectrum consists of five triplets each due to two equivalent protons, Table I. The extremely narrow line width allows the resolution of the splittings for the positions 3,6 and 10,ll. Previously only a quintet was observed for these four proton^.^ The enthalpy of comproportionation for the benzo-COT-HMPA-K system was found to be -5.7 f 0.08 kcal/mol, Table 11. The large error in this number and the fact that Kobsd could not be determined is due to the fact that the anion radical is not very stable in HMPA. As previously mentioned two factors can cause an increase in h H o b s d " , either an increase in ion pairing or a decrease in the electron-electron repulsion of the two electrons in the dianion. The entropy term, however, is effected predominantly by ion pairing.2 Since the enthalpies of comproportionation of NCOT and benzo-COT are not more positive than that for COT itself, it can be concluded that either the two electrons in the dianion reside predominantly in the eight-member ring, or the effect of decreased
1,P-Disubstituted Cyclooctatetraene Anion Radicals
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TABLE I: ESR Coupling Constants for the Systems NCOT-HMPA-K a n d benzo-COT-HMPA-K at 23"
Position
AH,G
PH
Pea
Position
AH,G
Pe
198 0.040 198 237 3.72' 0.16 0.157 277 3.70 0.161 376 1.42 0.062 0.077 396 2.00 0.087 495 2.90 0.126 0.118 435 3.12 0.136 9,16 1.01 0.044 0.055 9,12 0.364 0.016 10,15 0.025 10,ll 1.91 0.083 11,14 0.89 0.039 0.012 12,13
