EPR
Studies of Bridged Biaryl Cation Radicals
The Journal of Physical Chemistry, Vol. 82, No.
IO, 1978 1181
Electron Paramagnetic Resonance Studies of Bridged Biaryl Cation Radicals’ Paul D. Sullivan,* Joseph Y. Fong, Michael L. Williams, Depaflment of Chemistry, Ohio University, Athens, Ohio 4570 7
and Vernon D. Parker Faculty of Science, University of Lund, Lund, 7, Sweden (Received August 4, 1977) Publication costs assisted by the National Science Foundation
The EPR spectra of some bridged 2,2’-bipyridyl and biphenyl cation radicals have been investigated. For the monomethylene bridged species, the p methylene splittings are large for the bipyridyl(24.24 G) and small (0.14 G) for the biphenyl cation radicals. These results provide another example of the effects of orbital symmetry on methylene splittings. Barriers to inversion have been estimated from the EPR data for the dimethylene bridged compounds. An upper limit of 7 kcal/mol was found for the bipyridyl while the barrier for the corresponding biphenyl is found to be 9.8 f 1.4 kcal/mol. These experimental results are compared with those obtained for related compounds and with INDO calculations.
Introduction We have recently studied2” the effects of removal or addition of an electron on the structure of some biphenyls. It was predicted that biphenyl systems should become more planar on radical formation and that the 1,l’bond length would decrease and the energy difference between the planar and perpendicular conformations would increase. These predictions are justified by the available experimental data. If one now fixes the dihedral angle between the phenyl rings by bridging across the 2,2’ positions one can study the effects of electron loss or gain on the bridged biaryl system. This paper reports on our studies of the radical ions of bridged biphenyls (11) and
0‘
\
Ia, n = 2 Ib, n = 1
IIa, n = 2 IIb,n= 1 IIc, n = 0
bipyridyls (I). These compounds are also of interest for other reasons. The radical ions of the single methylene bridged compounds, Ib and IIb, provide another example of the effect of orbital symmetry on hyperconjugative interaction^.^ The bridged biphenyls are of interest because of their similarity to several naturally occurring compounds, some of which are reported to have antitumor and antileukemic a~tivities.,~~~ The bridged bipyridyls are of interest because of their herbicidal proper tie^,^ which are undoubtedly related to their structure and ease of reduction to the cation radical.s Experimental Section All tetramethoxy-bridged biphenyls were synthesized electro~hemically,~ the corresponding cation radicals were prepared by oxidation with AlC1, in nitromethane, or alternatively by reaction with trifluoroacetic acid (TFA) and trifluoroacetic anhydride (TFAn) as previously de~cribed.~ Dipyrido[l,2-c:2’,1’-e]imidazoliumiodide (DPI) was prepared from 2,2’-bipyridyl and methylene iodide by the method of Calder and Sasse.lo The cation radical of 60022-3654/78/2082-1181$01 .OO/O
hydrodipyrido[ 1,2-c:2’,lf-e]imidazole(DPI+-)was made by reduction of the parent compound with Zn in TFA.ll All EPR samples were submitted to several freezepump-thaw cycles before spectral measurements. The spectra were recorded on a Varian E-15 spectrometer equipped with a field frequency lock. Sample temperature was monitored with a microprobe thermocouple mounted directly in the EPR cavity and the perylene anion radical was used as a secondary standard for field and g value measurements. Simulations of spectra exhibiting ring inversion were carried out using the density matrix method12 and least-squares analysis of the splitting constants were carried out as previously described.13 Results 6-H y drod ipyrid o [ 1,2-c:2’,1 ‘-e]imidazol ium Cat i o n (DPP., Ib+). The cation radical Ib+ was produced from DPI by reduction with Zn dust in TFA. The first step involves the protonation of DPI to form the 6-hydro DPI dication,14which is then reduced by Zn to the 6-hydro DPI cation radical (eq 1). The EPR spectrum of Ib+ at 20 “C
Q=pQ-Qq=&rJ \, i
\c/
HI
‘C’ H’
DPI
H ‘
H’
\H
DPI .+
(1)
is shown in Figure 1. The spectrum is dominated by a very large splitting constant of 24.246 G from two equivalent protons. The asymmetry of the center group and the obvious deviation from the expected 1:2:1intensity ratio is due to a second-order splitting of approximately 0.14 G. The large splitting is readily assigned to the 3( methylene protons since no known mechanism could produce a splitting of this magnitude from an cy proton, Confirmation of this assignment is obtained by studying the cation radical in deuterated TFA. Deuterium exchange occurs rapidly at the methylene protons due to the reversibility of the first step in eq 1. The EPR spectrum is then drastically reduced in overall width due t o the much smaller deuterium splitting of 3.669 G. The other splitting constants were obtained from an expanded spectrum of the outer groups of lines. Four pairs of two equivalent ring protons with splittings of 0.229, 0 1978 American Chemical Soclety
1182
The Journal of Physical Chemistry,
Vol. 82, No.
Sullivan et al.
10, 1978
TABLE I: Summary of Splitting Constantsa for Bridged Biuryls
2.389d
0.64R
2.810
0.229
4.339
2.003lfi~
2.00343 Fluorene3.64 0.83 4.82 1.14 3.99 3.359 5.363 0.708 9.10-Dihydro1.481(av) 0.161 phenanthrene1.452 4.743 0.0s 4.101 5,5-Dimetboxy2.649 fluoreneNumbering is changed to be consistent for all compounds All splitting constants i9.01 C, or better.
C g
AIL g values tO.OOOO1.
Methoxyl splittings.
Flgure 1. The EPR
Ring splittings assigned on the,basis of INDO calculations. Reference 20. ' Reference 17. Assignments uncertain.
spectrum of Ib+ in TFA at +20
e
This work.
f
d
e h I
e
Reference 11.
J
OC.
0.648,2.389, and 2.810 G and two equivalent nitrogens of splittings 4.339 G were found. These splitting constants are very similar to those of the dihridged hipyridyl (see Table I) and simulations taking into account the second-order splittings are in good agreement with experiment. 6,7-Dihydrodipyrido[l ,2-a:2',lf-c]pyrazinediiumCation (Diquat, la+). The EPR spectrum of the Diquat cation radical has been previously reported and analyzed." The results are shown in Table I and the similarity between the ring and nitrogen splittings of Ia and Ih should be noted. 2,3,6,7-Tetramethoxybiphenylene Cation (IIc+). The EPR spectrum of this radical formed in A1Cl3-CH3NO2 was readily analyzed in terms of a group of 12 and a group of 4 equivalent protons (see Table I). 2,3,6,7-Tetramethoxyfluorene Cation (IIb+). The IIh+ radical was studied in both A1C13/CH3N02 and TFA/ TFAn/CH,NO, mixtures over a wide temperature range. The overall width of the spectrum was only 15.73 G. (See Figure 2.) Difficulties in analyzing the spectrum are caused by accidental degeneracies of the splitting constants. Two groups of six equivalent protons are assigned splittings of 1.601 and 0.802 G. A splitting of 0.375 G is assigned to a pair of protons and a splitting of 0.141 G is found for four equivalent protons in AlCl,/CH,NO,. A slightly different splitting of 0.401 G is found for the two equivalent protons in TFA/TFAn/CH,N02, the other splittings remaining unchanged within experimental error. Studies of IIb+ in TFA-d/TFAn/CH,NO, indicate that partial deuteration occurs at the positions occupied by the
Flgure 2. The EPR
spectrum
of IIb+ in AICI,/CH,NO,
Figure 3. The EPR spectrum of IIa+
at -10 "C.
in TFA/CH,NO, at -17 'C.
two equivalent protons of splitting 0.401 G. On chemical grounds one would predict that the most likely positions for deuterium exchange to 'occur are the 4 and 5 ring protons, hence the 0.401 G is assigned to these protons. The four protons of 0.141 G are therefore assigned to an accidental degeneracy of the 1,8 ring protons and the methylene protons. 2,3,6,7-Tetramethoxy-9,lO-dihydrophenanthrene Cation (IIa'). Radical IIa' was studied in A1C1,/CH3N02 and in TFA/TFAn/CH,NO, from -40 to +24 OC. The EPR spectrum observed a t -17 'C is shown in Figure 3. The spectrum shows three groups of lines separated by 10.4 G with an approximately L 4 1 intensity distribution. This pattern is attributed to an intermediate rate of axialequatorial methylene proton interconversion caused by inversion of the cyclohexadiene ring. Such a process occurs
EPR Studies of Bridged Biaryl Cation Radicals
for the 9,lO-dihydrophenanthreneanion radical15-17and is postulated to occur for Ia+.ll The rest of the spectrum is analyzed in terms of two sets of six equivalent protons of 1.505 and 0.848 G and a pair of protons of 0.424 G, which are assigned to protons at positions 4 and 5 on the basis of MO calculations and by analogy to IIb+. Simulations using these parameters are in good agreement with the experimental spectra and we therefore conclude that the splitting of the last pair of ring protons at positions 1and 8 must be considerably smaller than the line width (100 mG) Lowering the temperature should decrease the rate of inversion so that in the slow limit the splittings of the axial and equatorial protons should be resolved. Unfortunately, even at the lowest feasible temperatures the rate was not slowed sufficiently to resolve out the two isomers. Increasing the temperature should increase the rate of inversion so that in the fast limit four equivalent methylene protons will be observed. Experimentally it is possible to approach the fast limit, additional groups of lines at f5.20 G begin to grow in between -10 and +20 “C. Even though we do not know the axial and equatorial splitting constants one can guess that the ratio should lie somewhere between 4:1, as would be observed for exact axial and equatorial protons,l* and 1O:l which is close to the observed ratio found for 9,lO-dihydrophenanthreneanion radical.15-17 Combining these limits with the observed sum of the axial and equatorial splittings of 10.40 G gives possible splittings of either 8.32/2.08 or 9.45/0.94. Using these values as the slow exchange limit, simulations of the two-jump interconversion were carried out using a density matrix program developed by Heinzer.12 The simulations were matched to the experimental spectra to give two sets (one for each axial/equatorial pair) of rates of inversion vs. temperature. Arrhenius plots of this data gave the activation energy from the slopes as 9.80 f 1.39 kcal/mol. This activation energy was approximately independent of the axial/equatorial ratio chosen. The intercepts of the curves were however quite dependent on the chosen ratio and hence no further information can be obtained unless the correct ratio is known.
.
Molecular Orbital Calculations Mclachlan modified HMO calculations were performed on all the compounds shown in Table I using parameters which have been previously reported for analogous corn pound^.^^^^ INDO calculations were performed on Ia+ and Ib+ and for the fluorene and 9,lO-dihydrophenanthrene anion and cation radicals. The other compounds were too large for the INDO program. The atomic coordinates for Ia+ were those determined from an x-ray structure determinati0n;ll similar coordinates were used for Ib+ except for the methylene bridge, for which standard bond lengths were assumed. For the fluorene anion and cation radicals standard bond lengths and angles were used except for the 1,l’ bond length which was fixed a t 1.44 A (by analogy to the biphenyl radical ions2) and the angles of the five-membered ring which were adjusted to give a planar structure. For the 9,lO-dihydrophenanthreneanion and cation a partial geometry optimization was performed by varying the 1,l’bond length concomitant with the dihedral angle between the rings until an energy minimum was obtained. The results of these calculations are summarized in Table 11. Discussion Monomethylene Bridged Biaryls (Ib+,IIb+). These two compounds provide a striking example of the effects of orbital symmetry on the hyperconjugative coupling of
The Journal of Physical Chemistry, Vol. 82, No. 10, 1978 1103
methylene protons interacting with two spin sites of the same P system. The effect which was first invoked by Whiffen4 to explain the large methylene splitting in the cyclohexadienyl radical has since been observed in only a small number of molecules. Those which have been studied include the cycloheptatriene anion radical,lg 1,2-indandione anion,20 4,4-dimethyl-1,2-tetralindione anion,20 fluorene anion,20 cyclobutenyl radical,21 trans10b,10c-dihydropyrene anion,22 and some fluorinated cyclohexadienyl radicals.23 According to Whiffen the splitting constant of a proton /3 to two different spin sites should be given by = &p(ci
+ cj )*
(2)
where ci and cj are the coefficients of the P orbital on each of the two atoms. Thus if the coefficients are equal and opposite in sign no P-hyperconjugative coupling is predicted, whereas if they are equal and of the same sign a coupling four times as large as normal might be expected. In the case of IIb+ the molecular orbital containing the unpaired electron is antisymmetric with respect to a plane bisecting the u and P fragments. For Ib+, however, which is isoelectronic with the fluorene anion radical, the molecular orbital containing the unpaired electron is symmetric with respect to the same plane. Qualitatively one therefore predicts a small splitting arising from residual spin polarization for the methylene protons of IIb+ and a large splitting for the same protons of Ib+. The actual splittings of 0.141 and 24.24 G dramatically illustrate the effect. One would similarly predict a large methylene splitting for the anion radical of IIb. An attempt to make this radical by reduction with Na in T H F resulted in an EPR spectrum, after standing for about 1 h, which was consistent with the anion radical of 3,6-dimethoxyfluorene (or 5,5’-dimethoxyfluorene in the numbering system used in Table I). Such a cleavage reaction of the 4,4’-methoxy groups has been noted previously for 3,3’,4,4’-tetram e t h o ~ y b i p h e n y l . ~The ~ dimethoxyfluorene anion is similar to the unsubstituted anion radical and no conclusions may be drawn regarding the effects of the methoxyl groups. Since the fluorene cation radical has not so far been prepared there is still no example of a compound containing a bridging methylene group which can form both an anion and cation radical. The best comparison that can be made for the moment is between the fluorene anion radical and the tetramethoxyfluorene cation radical (IIb+). The molecular orbital calculations on Ib+ show a similar discrepancy between the HMO and INDO calculations as regards the ring splitting constants as previously found for Ia+.ll Using eq 1the HMO calculation predicts a splitting for the methylene protons in fair agreement with experiment whereas the INDO calculated value is ca. 40% too small. The fluorene anion and cation radicals give calculated ring splittings which do not greatly differ between the HMO and INDO methods and which are in fair agreement with the experimental values for the anion radicaLZ0The methylene splittings are predicted by HMO to be 12.7 and 0.0 G for the anion and cation and by INDO to be 22.16 and 3.54 G. These compare to the experimental value of 3.64 G for the unsubstituted anion radical and 0.141 G for the tetramethoxyfluorene cation radical IIb+. Similarly large errors in the INDO calculated methylene splittings of the cyclohexadienyl radical have been noted by Wood et aLZ3The MO calculation of such methylene splittings thus appears, at the moment, to be fraught with error.
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The Journal of Physical Chemistry, Vol. 82, No. 10, 1978
Sullivan et al.
TABLE 11: Summary of Molecular Orbital Calculations of Spin Densities and Splitting Constants Position 1,l' Ib+ HMO(sd)! HMO(sc)' INDO(sc)
0.1312
FluoreneHMO(sd) HMO(sc) INDO(SC)~
0.0979
Fluorene+ HMO(sd) HMO(sc) INDO(SC)~
0.1122
IIb' HMO(sd) HMO(sc)
0.0753
2,2' or N,N'
3,3'
4,4'
0.1507 3.77a 4.79
0.0112 0.30b - 1.64
0.0845 2.28 -0.56
0.0813 2.19 -2.16
0.0149 0.40 -0.12
21.09' 13.79
0.0908
-0.0145 0.39 1.88
0.2115 5.71 -5.20
- 0.0434
1.17 1.44
0.1408 3.80 - 3.03
12-71' 22.16
- 0.0307
0.2013 5.44 - 5.68
-0.0292 0.79 1.21
0.1262 3.41 - 3.04
0.0' 3.54
0.1219
0.83 2.08 0.1289
5,5'-Dimethoxyfluorene HMO(sd) 0.0917 HMO(sc)
0.0485
Ia+ g HMO(sd) HMO(sc) INDO(sc)
0.1470 3.67 5" 4.387
0.1586
9,lO-DihydrophenanthreneHMO(sd) 0.1032 0.1051 HMO(sc) INDO(sc)
9,1O-Dihydrophenanthrene+ HMO(sd) 0.1145 0.1522 HMO(sc) INDO(sc) IIa+ HMO(sd) HMO(sc)
0.0825
IIc+ HMO(sd) HMO(sc)
0.1252
0.1404
0.1252
5,5'
6,6'
CHa
0.36
0.0136 0. o c
-0.0092e 0.28f
0.191 5.16
6.78'
0.0660 1.78 0.45
0.1079 2.91 - 2.67
-0.0112 0.310 1.07
3.97h 2.78
-0.0228 0.62 1.990
0.2147 5.80 - 5.105
- 0.0395
1.07 1.265
0.1328 3.58 - 2.89
2.84h 7.58
- 0.0523
0.1776 4.80 - 4.24
0.0010 0.03 1.35
0.0858 2.32 - 2.28
4.11h 6.70
3.79h
0.0039 0.10
0.0594e 1.78f
0.0196 0.53
0.1992 5.38
0.0246 0.66 - 2.46
1.41 1.975
0.02 614e 0.7 8f
-0.0345 0.93
0.0462e 1.3865
0.0274e 0.820f
-0.019 0.52
-0.0291 0.78
0.0383e 0.93f
0.038 3e 0.93f
- 0.0291
0.78
U ~ C = H pCQHC~ where , Q H c=~27 G. ' Using Whiffen's formula U ~ C H =, Q ( q + a a" = pNQN, where QN = 25 G. Proton splittings only. e Oxygen spin densities on methoxyl oxygens. f Methoxyl splittings, C . ) a ,where Q = 35 G. akm,= p o Q H 0 ~ ,, Q H 0 a H=, 30 G. g Reference 11. aHp = QHppi where QHp = 27 G, p i = pc or p ~ . sd = spin densi-
ties; sc = splitting constants.
The large difference between the methylene splittings in the isoelectronic radicals Ib+ and fluorene- (24.24 and 3.64 G) is striking and may be due in part to a different spin density distribution in the two compounds. The difference may also arise from the relative energy levels of the MO containing the unpaired electron. The energy level of the HOMO for Ib+ is calculated to be of lower energy than that of the fluorene anion radical in both the HMO and INDO approximations. This indicates that the hyperconjugative interactions in Ib+ should be stronger than in the fluorene anion hence leading to a large methylene splitting constant. Dimethylene Bridged Biaryls (la+, IIa'). The ring splitting constants in these compounds are very similar to those of the monobridged species Ib+ and IIb+; the most interesting aspect of these compounds concerns the barrier to inversion of the cyclohexadienyl ring and the 0methylene splitting constants. As previously reported," the p protons of Ia+ show a 1:4:1 intensity pattern from -50 to +50 'C consistent with an intermediate rate of inversion over this temperature range. This result is only explained if the barrier to inversion is less than ca. 7 kcal/mol. For IIa+ the rate of inversion may be increased
sufficiently to show observable effects on the E P R spectrum and an activation energy of 9.80 f 1.40 kcal/mol has been obtained. These results may be compared with the barrier to inversion for the 9,lO-dihydrophenanthrene anion radical which has reported values between 4.6 and 6.3 kcal/mol.15-17 INDO calculations of the inversion barrier have previously been made for the diquat dication (6.0 kcal/mol) and cation radical (3.8 kcal/mol). We have carried out similar calculations for 9,lO-dihydrophenanthreneanion, cation, and neutral compounds. The barrier to inversion is taken to be the difference in total energy between the twisted and planar configurations using energy minimized values for the 1,l'bond length and dihedral angle. For the anion with a 1,l'bond length of 1.46 8, and a dihedral angle of 17.6' the barrier is calculated to be 8.0 kcal/mol, the cation with bond length 1.45 8, and angle 18.0' has a barrier of 7.8 kcal mol and the neutral compound with bond length 1.48 and angle 16.9' has a barrier of 10.7 kcal/mol. This latter value for the neutral molecule seems too high in view of the experimental NMR spectrum which predicts a barrier
