13152
J. Phys. Chem. 1993, 97, 13152-13157
Tetraarylborates ([ArIdB-j: Estimation of Oxidation Potentials and Reorganization Energies from Electron-Transfer Rated !%an T. Murphy, Cbaofeng Zou, Jeffrey B. Miers, Richard M. Ballew, Dana D. Dlott, and Gary B. Schuster' Department of Chemistry, Roger Adams Laboratory, University of Illinois, Urbana, Illinois 61801 Received: July 7, 1993; In Final Form: September 28, 1993'
Photoinduced electron transfer in ion pairs where the anion is a tetrasubstituted borate gives a boranyl radical. This species is reactive. When the boranyl radical contains an alkyl group, the carbon-boron bond cleaves in ca. 250 fs. The lifetime of tetraarylboranyl radicals is greater than 40 ps. The short lifetime of boranyl radicals prohibits measurement of the oxidation potentials of borates by electrochemical methods. These quantities were estimated by application of Marcus electron-transfer theory, which relates the energetics to the kinetics of the reaction. We find that the use of the empirical Rehm-Weller equation for this purpose is not appropriate and leads to errors in estimation of both the reorganization energy of the reaction and the oxidation potential of the borates. The oxidation potentials were determined in acetonitrile solution by this approach for two series of borates, (tolyl)n(Ph)knB-, where the tolyl group might be ortho or para substituted and n = 0-4. The data reveal that A, the reorganization energy, is 1.O eV. The oxidation potentials of the borates vary systematically in the p-tolyl series from 0.8 1 to 1.03 V vs SCE with decreasing number of tolyl groups on the borate. The o-tolyl series shows higher than expected oxidation potentials for the tetra- and tritolyl cases, which may be due to greater steric hindrance to the acceptor for these two borates.
Introduction Borates are anions for which the negative charge is associated with electrons in bonding orbitals. Consequently, oxidation of a borate will convert its two electron carbon-boron bonds to ones having fewer electrons resulting in a concomitant reduction in bond energy. For triarylalkylborates { [Ar3BR]-), one-electron oxidation forms a boranyl radical which undergoes very rapid cleavage of the alkyl carbon-boron bond yielding an alkyl radical and a triarylborane. This reaction sequence is summarized in eq 1. The one-electron oxidation of borates may be accomplished
photochemically in polar solvent with uncharged acceptors' or within an ion pair in a nonpolar solvent when the electron acceptor is positively charged.* The photoinitiated oxidation of borates has been applied to the solution of practical and theoretical problems. Paired with cationic visible-light-absorbing dyes, triarylalkylborates are commercially important initiators for polymerization reaction^.^.^ In phospholipid membranes, photoinitiated reactions of borates and cationic cyanine dyes have been used to probe vectorial electron t r a n ~ f e r .In ~ the solid state, light-initiated electron transfer in dye borates leads to selective bleaching and image formation.'j Photoinitiated transfer of an electron from a borate to an excited cationic acceptor within an ion pair permits investigation of intermolecularelectron-transfer reactions that occur with rates far above the diffusion limited value.' Consequently, these reactions provide an important opportunity to test theories of electron-transfer reactions. Marcus theory makes the remarkable prediction that the rate of an electron-transfer reaction will slow down when its driving force (AGT) becomes sufficiently exoergic, giving rise to the inverted region.8 Attempts to verify this prediction were unsuccessful for more than 20 years. Instead of inverted region behavior, for intermolecular cases, the reaction rate reaches a plateau value when the electron transfer is This paper is dedicated to the memory of Gerhard Closs. .Abstract published in Aduunce ACS Absrracts, November 15, 1993.
sufficiently exoergic, and further increase in the driving force does not result in a decrease in rate.9 However, pioneering discoveries by Closs and his co-workerslo showed clear experimental evidence for inverted region behavior in intramolecular reactions when the electron donor and acceptor are held some distance apart by a rigid spacer group. Photoinitiated electron transfer from a borate to a cationicacceptor in an ion pair enables a search for inverted region behavior in intermolecular electrontransfer reactions with high driving force. Indeed, just such behavior was observed for back electron-transfer to pyrylium radicals from a series of boranyl radicals but not for the forward electron transfer from the borate to the excited pyrylium cation.' Critically, application of the findings from electron-transfer reactionsof borates to tests of the energy and distancedependence predictions of Marcus theory requires accurate values for A G ~ T . These values are usually calculated by application of eq 2,where EOXand ERED are the oxidation potential and reduction potential of the donor and acceptor, respectively, Em is the singlet excitation energy of (in this case) the acceptor, ZI and Z2 are the charge numbers, e is the electronic charge, e is the dielectric constant of the medium, and rDA is the charge separation distance." All of the parameters needed to apply eq 2 to intra-ion pair electron transfer of borates are available except EOX.
One-electron oxidation of a borate results in the rapid chemical reaction of the boranyl radical. Consequently, all attempts to measure EOXby electrochemical methods, such as ultrafast cyclic voltammetry, give irreversible waves that reveal only peak potentials which are not thermodynamicallymeaningful values. We report herein the application of a kinetic method based on Marcus theory8 that provides a means to estimate thermodynamically meaningful values of EOXfor the borates. Results (1) Are Tetraarylboranyl Radicals Bound Species? A critical question to consider in attempting to determine the oxidation potential of tetraarylborates is whether their oxidized forms, the
0022-3654/93/209113 152$04.00/0 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 50, 1993 13153
Tetraarylborates { [ArIdB-) tetraarylboranyl radical { [Ar]4B'}, exist in a thermodynamic minimum. It is well-known that many oxidation and reduction reactions are dissociativeand that for these cases electron transfer is accompanied by concerted bond breaking.12 Importantly, Marcus theory presumes that the reactants and products are approximately described by harmonic oscillators and, thus, dissociative electron-transfer reactions should not be expected to show Marcus behavior. Furthermore, Marcus theory is based upon the assumption that the oxidized form of the donor and the reduced from of the acceptor have sufficient lifetimes to establish an equilibriumwith their precursor species. If the electron transfer is dissociative, this equilibrium cannot be established and applicationof Marcus theory may not be appropriate. We carried out a seriesof timeresolved spectroscopic measurements to assess the lifetime of boranyl radicals. Pyrylium cations [Py+]form ion pairs with borates in benzene solution. Visible-light irradiation of the ion pair gives the locally excited singlet state of the pyryliumcation, [Py+]* (eq 3), which is thermodynamically capable of oxidizing the adjacent borate to the boranyl radical and simultaneously forming the pyrylium radical [Py'] (eq 4). This process converts the initial pyrylium borate ion pair into a radical pair. The boranyl radical in this pair undergoes two competing processes. The first is an irreversible chemical reaction of the boranyl radical (carbonboron bond cleavage, for example) which does not result in consumption of [Py'] (eq 5). The second is back-electrontransfer to re-form the ground state of the ion pair which does consume [py'l (eq 6).
IPY+lWB7
IPY+14f[4B7 wIU4W wIff4B')
--
[py+14U4B7
(3)
wIU4B')
(4)
py'+k-Ar
(5)
IPY+lwl4~7
(6)
Thus, if the oxidation of the borate is dissociative (or nearly so), the boranyl radical lifetime will be very short and there will be little consumption of [Py'] by back-electron transfer. On the other hand, if the boranyl radical is a bound species with a measurable lifetime, then back-electron transfer in the radical pair will consume [Py']. Two pyrylium borate salts were prepared13 to study the lifetime of boranyl radicals. In the first, the anion is n-butyltriphenylborate (n-C4HgB[Ph]3-}, an alkyltriphenylborate. For the second, the anion is ( [p-MeOPhI4B-}, a tetraarylborate. These structures are shown in Chart I. Irradiation of [Py]+{n-C4HgB[Ph]3-}at 573 nm in benzene solution with a ca. 250-fs pulse results in the apparently instantaneous formation of [py'] which has an optical absorption spectrum with a strong band at 477 nm37 The time dependence of the absorption of the [py'] formed in this process is shown in Figure 1: the radical shows no measurable decay during the 70-ps analysis period. In contrast, the irradiation of [Py]+([pMeOPhIdB-) similarly gives an instantaneous formation of [Py'], but the absorption of this radical decays to 4% of its initial value with an apparent first-order rate constant of 3.0 X 10" s-l. Evidently,the rateof carbon-boron bond cleavage in {n-CdHgB[Ph]3*)is so fast that back-electron transfer to regenerate [Py+] is not competitive. Based on the assumption that a 10% change in the absorbance of [Py.] could be detected in Figure 1, the maximum lifetime possible for (n-C4H9B[Ph]s') is about 250 fs. Clearly, oxidation of (n-C4HgB[Ph],-) is dissociative (or very nearly so) and Marcus theory may not predict rates of its electrontransfer reactions. Back-electron transfer does compete successfully with the irreversiblechemical reactions of ([p-MeOPh]4B*),and thus this radical must have a finite lifetime. By assuming that the residual absorption of [Py.] at long times (70 ps) in Figure 1 is due to the
CHART I 1+
Ar
Ar
'4
GY
Naphthalene Acceptom
n
ACE
26N
23N
14N
2NA
1NA
NAP
irreversible consumption of ( [p-MeOPh]4B0) by a chemical reaction, the lifetime of this tetraarylboranyl radical is calculated to be 45 ps. Therefore,tetraarylboranyl radicals are bound species, and the rates of their electron-transfer reactions should obey Marcus theory. (2) Tetraarylborates are Quencbers of Aromatic Hydrocarbon Fluorescence. The fluorescence of aromatic hydrocarbons in acetonitrile solution is quenched by the addition of tetraarylborates. This quenching cannot occur by energy transfer since the singlet energies of the borates are above those of the fluorescers. However,electron-transferquenching seems possible in these cases based on the known reduction potentials and singlet energies of the fluorescers and estimates of the oxidation potentials of the borates from the peak currents observed in cyclic voltammetric measurements. Time-resolved spectroscopic experiments have verified this post~late.~ For example, the fluorescenceof chrysene in an acetonitrile solution is quenched by 0.016 M tetramethylammonium borate. Pulsed irradiation of this solution results in the formation of the chrysene radical anion, confirming electron transfer from the borate in this case. The dependence of the electron-transfer rate ET) on driving force for intermolecular photoinitiated electron-transfer reactions in polar solvents has been studied exten~ively.~J~ Although Marcus theory predicts inverted region behavior at sufficient driving force, this has not been clearly observed experimentally. The observed quenchingrates for electron transfer (b) generally increase with increasing driving force and then reach a maximum value determined by the diffusion limit of the solvent. This behavior can be fit by classical Marcus theory, modified by imposition of a diffusion limit, which relates the observed quenching rates to the free energy change of the electron-transfer reaction.8.9-15 In the normal region, where AGET is positive or only slightly negative, this modified Marcus equation reliably
Murphy et al.
13154 The Journal of Physical Chemistry, Vol. 97, No. 50, 1993
TABLE I: Thermodynamic and Kinetic Data for Hydrocarbon Acceptors Quenched by Tetra-ptolylborate Emob A& Em r d k&d AG* fluorcscep (V) (nm) (eV) (ns) (s-l M-I) (eV) 320.0 3.87 37.6 1.3 X lo9 0.116 ACE -2.67'
I
8 0.2 --
26N 23N 14N 2NA 1NA NAP
2 0
324.0 320.0 321.5 319.0 318.0 315.0
3.83 3.87 3.86 3.89 3.90 3.94
38.1 70.5 52.9 54.5 72.1 89.5
1.9 X 2.6 X 3.8 X 5.1 X 5.7 X 7.5 X
lo9 lo9 lo9 lo9 lo9 lo9
0.099 0.090 0.079 0.069 0.065 0.054
See Chart I for structures. Measured in DMF vs SCE. All except as noted from Streitwiescr and Schwager.l* Measured in DMF by: Pointeau, R. Ann. Chim. (Paris)1962,7,669. Measuredin acetonitrile.
0.0 .10
-2.62 -2.64 -2.57 -2.59 -2.57 -2.54
10
20
30
do
53
M
70
(PS)
Figure 1. Absorption of pyrylium radical produced in a ca.250-fs pulse. With (n-C,HgB[Ph]p-), solid symbols,the radical does not undergo backelectron transfer and therefore docs not decay. With (Ip-MeOPhIdB-), open symbols, the boranyl radical lives long enough to undergo backelectron transfer and a measurable decay is observed.
models the observed quenching reactions of the borates with aromatic hydrocarbons. We confined measurements of the electron-transfer rate constants of the borates with aromatic hydrocarbons to this region. In the first set of experiments, tetramethylammonium tetrap-tolylborate, was used to quench the fluorescence of a series of aromatic hydrocarbons in Nz-purged acetonitrile solution. The singlet lifetimes (7") of the fluorescers were measured under identical conditions by time-resolved techniques, and the data were analyzed according to the usual Stern-Volmer equationI6 to give &,a. In some cases kob was measured by direct determination of 'IFI at varying concentrations of the borate. The two techniques give identical results, within experimental error. The mechanism usually assumed for electron-transfer quenching in solution is shown in eq 7,and the relationship between kob and kET is shown in eq 8. The derivation of this equation requires the assumption that back-electron transfer to generate ground states is much faster than it is to generate excited states. This is expected to be true in the present case because the latter reaction is endogonic. The values obtained for kob are related to the activation free energy for electron transfer (AG') according to eq 9, where k,,, is the maximum electron-transfer rate, normally taken to be 10" M-I s-I, kdiffis the diffusion limited rate in acetonitrile solution, normally set at 2 X 10'0 M-1 s-I, and k-diff is the rate constant for diffusive separation set to 2 X 1Olo Since the value of kdin is normally estimated for neutral species, not for anions such as borates, we measured this quantity independently by studying the fluorescence quenching of 9-cyanoanthracene by tetra-p-tolylborate in acetonitrile solution. The driving force for this reaction (-AGET) is estimated to be at least 20 kcal/mol, so its rate is expected to be at the diffusion limit, and a value of 2.0 X 1010 M-l s-I is obtained in this case. The activation free energy calculated in eq 9 is related to AGBTby classical Marcus theory, eq 10. By combining eqs 9 and 10, one obtains the relationship between kob and AGETshown in eq 11. The thermodynamic and kinetic data with derived activation energies for quenching of a series of naphthalene-based aromatic hydrocarbons by tetra-p-tolylborate are summarized in Table I. In the next set of experiments,the rate constantsfor fluorescence quenching of the methyl substituted naphthalenes (Table I) were measured for tetraphenylborate, and for each of the 0- andp-tolyl borates in the series, the results are shown in Table 11. These rate constants were substituted into eq 9 to give a set of values for AG* for each borate-naphthalene pair. The series of p-tolylborates was fit to eq 11 by allowing the four oxidation potentials to vary but restricting the solution to find a singlecommonreorganization energy. The oxidation potentials are overdetermined by the data, so the reliability of this approach is supported by the consistency
D-
km kb* + A* kdirr F? [D-***A*]e D' + A'-- D- + A k-jm
(7)
kpr
of the oxidation potentials obtained with the various naphthalenes used. This fitting procedure produces a reorganization energy of 0.9 eV. The same technique was applied to the series of o-tolylborates, and the best fit reorganization energy was similarly found to be 1.2 eV. The quenching rate constants for all of the borates examined are combined in Figure 1; these data show a remarkably good fit to classical Marcus theory with a reorganization energy of 1.OeV. The oxidation potentials of the borates obtained by this approach are given in Table 11. Interestingly, for the p-tolyl series the borate oxidation potential decreases systematically with the number of tolyl groups; but this is not true in the o-tolyl series. In the latter case, addition of the third and fourth tolyl groups has a smaller affect on the oxidation potential than does the addition of the first and second groups. This may be a consequence of increased steric hindrance in the o-tolyl substituted borates.
Discussion A major motivation for this work is the desire to determine accurate, thermodynamicallymeaningful values for the oxidation potentials of tetraarylborates. The time-resolved spectroscopic measurements reported above show that the lifetimes for the oxidized form of the borate, tetraarylboranyl radicals, is much less than a nanosecond. There are presently no direct electrochemical methods that allow measurement of redox potentials when a chemical reaction following oxidation occurs on such a short time scale. Consequently, we are compelled to rely on indirect techniques to assess these values. Fortunately there is a substantial body of data and extensively developed theories which permit the correlation of the rate constants for electrontransfer reactions with their driving force. Therefore, under carefully defined conditions,and with the assumption of a reaction mechanism, it is possible to use kinetic measurementsto determine thermodynamicallymeaningful values for the oxidation potentials of tetraarylborates. (1) Models for Pbotoiaitiated Electron Transfer in Solution. The dynamics of many photoinitiated electron-transfer reactions in solution have been carefully examined by numerous investigators since it seemed likely that Marcus' inverted region would be readily found for these systems. It is now well-known that this
Tetraarylborates { [Ar],Bj
TABLE Ik
The Journal of Physical Chemistry, Vol. 97,No. 50, 1993 13155
Ouenchiap Rates for Borates' quenching rate for given fluorescer
ACE 1.3 x 109 b 5.2 X 108 b b
borate 4PB 3PB 2PB 1 PB 40B 30B 20B 1 OB PhB
26N 1.9 x 109 9.9 x 108 6.9 X 108 2.7 X lo8 3.1 X lo* 2.4 X 108 2.2 x 108 BDL BDL
b BDL BDL BDL
23N 2.6 x 109 1.6 x 109 1.0 x 109 5.7 x 108 2.7 X lo8 2.8 X lo8 2.8 X lo8 1.7 X lo8 BDL
2NA 5.1 x 109 3.6 x 109 2.1 x 109 1.2 x 109 6.0 X 108 7.1 X lo8 6.7 X 108 4.5 x 108 3.6 X 108
1NA 5.7 x 109 4.4 x 109 3.0 x 109 1.5 x 109 1.1 x 109 1.3 x 109 1.1 x 109 7.2 X 108 5.8 X 108
NAP 7.5 x 109
Eox
7.0 x 109 109
4.7 x 3.2 x 2.0 x 2.1 x 2.3 x 1.7 x 1.3 x
(V)
0.81 0.85 0.91 0.94 0.91 0.97 0.98 1.01 1.03
109 109 109 109 109 109
0 PB indicates a ptolylborate; OB, an o-tolyl substituted borate; the integer tells how many of these groups are present in the tetraarylborate. All rates have units of M-1 s-1. Not measured. BDL = below detection limit.
*
0.8
10.0
....___ ...-.._._
--
.-.
C..,
?.S
--
8.5
--
7.5
t
''.I.,
4
0.8
0.7
0.6
0.5
.0.4
-0.3
0.2
J
02 I
\
01
I
00
0.1
A 0 @V)
Figure2. Comparisonof Marcus theory and the Rehm-Weller equation. The dotted line represents the best fit to the data of Marcus theory. The reorganizationenergy was treated as varible but forced to the same value for all borate/naphthalene pairs. The solid line represents the best fit of the Rehm-Weller equation obtained by varying the reorganization energy with oxidation potentials determined by Marcus theory.
-
- - R e m wellel ec MalCut meow
0
10
l/AG* (eV) 15
20
25
Figure 3. Comparisonof methods to determine oxidationpotentialsfrom fluorescence quenching data. The data are presented in the method of Fukazumi18 which predicts a linear correlation based on the RehmWeller equation. The data do not show a linear correlation and instead fit Marcustheory (dotted line). Thedashed line represents the correlation predicted by the Rehm-Weller equationif the fitted reorganizationenergy from Marcustheory is used. The solid line shows the 0.4-V overestimation oftheoxidationpotentialfor4PBwhenthedataare fit tothelinepredicted by the Rehm-Weller equation instead of a curve predicted by Marcus theory.
is not the case, and several explanations have been offered to account for this o b ~ e r v a t i o n . ~Experimentally, ~~~ the actual dynamic dependenceon reaction driving force for these reactions is often found to obey the empirical Rehm-Weller r e l a t i ~ n s h i p . ~ * ~ ~of the linear form of the Rehrn-Weller equation is revealed by However, it has been found in this work that the application of the solid line in Figure 3. This line is fit to the quenching data the Rehm-Weller relationship leads to significantly different (shown as squares) for tetra-p-tolylborate (4PB, see Table I) estimates for the oxidation potentials compared with classical with the series of naphthalenes. These points do indeed seem to Marcus theory. follow the linear equation of Fukazumi and co-workers, but only Figure 2 shows the best-fit curve of the data to Marcus theory because the data cover a small section of a curve. Indeed, the and the curve of the Rehm-Weller equation fit to the data with intercept of this line leads tooverestimationof the borateoxidation values of EOXestimated from Marcus theory. It is evident that potential compared with application of the Marcus theory by ca. the curves are quite different and that the fit to Marcus theory 0.38 eV. Similarly, the slope of the linearized Rehm-Weller is superior. More importantly, the Rehm-Weller equation leads equation depends on which portion of the curve is probed and is to an overestimation of the oxidation potential for the borates in not strictly a measure of the reorganizationenergy for the electrontransfer reaction. this study. (2) The Oxidation Potentials of Tetraarylborates. Inspection Fukazumi and co-workers18 rearranged the Rehm-Weller of Figure 2 reveals that application of Marcus theory to the equation to produce plots of (AG* ERED Em) us l / A G * and determination of oxidation potentials of tetraarylborates gives to extract redox potentials from kinetic data. In this approach, precise results. The accuracy of these potentials is controlled the intercept of a linear plot of (AG* ERED+ Em) us l / A G * primarily by the accuracy in the reduction potentials of the excited gives the oxidation potential directly and the slope is equal to electron acceptors and the ability to keep A constant through the (X/4)*. Figure 3 shows the data for the quenching of the series. The reduction potential of the excited state is composed naphthalenes by the borates. In this presentation, EOXhas been of two parts: the singlet excited-state energies, which are known subtracted from the y-value so that the data should yield an with high precision and accuracy from absorptionand fluorescence intercept of zero. The dashed line in Figure 3 represents the measurements, and the reduction potentials which are measured Rehm-Weller equation with X = 1.0 eV, and although it does go electrochemically. Due to their relatively high reduction pothrough the origin, it does not contain any of the experimental tentials, it is difficult to measure this quantity for all of the points. The dotted line in this figure is the data fit to classical substituted naphthalenes used in this work in acetonitrile solution. Marcus theory which shows a curved dependence on driving force. Thus for these compounds, reduction potentials determined in It is clear by inspection that the data also show a curved energy dimethylformamide (DMF) solution were used. DMF is isodidependenceand that the fit to Marcus theory is much better than electric with acetonitrile, and, therefore,Igthe reduction potentials it is to the Rehm-Weller equation. A danger in the application
+
+
+
13156 The Journal of Physical Chemistry, Vol. 97, No. 50, 1993 should not depend on which of these two solvents is used. This was verified by comparing reduction potentials determined by Streitweiser and Schwager" in DMF solution (these data were used to construct Figure 3) with those determined in acetonitrile by Parker.21 The values of the reduction potentials for aromatic hydrocarbons from the two different solvents are in good agreement. On this basis, we conclude that the accuracy of the oxidation potentials of the borates determined by application of the Marcus theory is high and limited only by the accuracy in the reduction potentials of the electron acceptors used to develop the kinetic data and the assumption that A is constant. Conclusion Measurements of the oxidation potentials of tetraarylborates have been obtained by application of a kinetic method. We find that Marcus theory is appropriate for measuring redox potentials from quenching studies whereas the Rehm-Weller equation is not. A value for the reorganization of 1.0 eV was found for the borate electron transfer to methyl substituted naphthalenes. By means of Marcus theory and carefully controlling the structures of the acceptors and donors, accurate measures of the redox potentials can be made by the kinetic method. Experimental Section
General. Absorption spectra were recorded with a Varian Cary 1E UV-vis spectrophotometerin quartz cuvettes with 1-cm path length. 1H NMR spectra were taken with a 200-MHz Varian XL200 or a General Electric QE300 instrument as noted. Solvents were freshly distilled prior to use. The aromatic hydrocarbons used in this work were obtained from commercial sources and purified as necessary by distillation or recrystallization. Spectrophotometric grade acetonitrile (Aldrich) was used without further purification. Tetraphenylborate was purchased from the Aldrich Chemical Co. Fluorescence Quenching. Quenching of the hydrocarbon fluorescencewas monitored on a Spex Fluorolog F1 11fluorometer. The hydrocarbon fluoresce sample had an optical density of 0.050.07. All fluorescer solutions were deoxygenated by bubbling with nitrogen. Aliquots of the borate solution were added, and no significant change in the emission spectrum of the hydrocarbon was observed. After correcting for borate absorption in some cases, the data were analyzed according to the Stern-Volmer relationship. The lifetimes of the hydrocarbons were measured witha Photon Technology International LS-100lifetimeanalyzer using the stroboscopic technique. Tetratolylborate Salts. The tetratolylborates were prepared from the corresponding tolyl magnesium bromide and sodium tetrafluoroborate. The tolyl bromide was converted to the tolylmagnesium bromide using approximately 1 equiv of magnesium turnings by heating a THF solution at reflux. To this solution was added 0.95 equiv of sodium tetrafluoroborate, and the mixture was heated at reflux overnight. The reaction mixture was concentrated in vacuo, and the residue was dissolved in acetonitrile and water. This solution was extracted once with hexane, and the aqueous portion was filtered, if necessary, to remove any undissolvedsalts. To the aqueous solution was added 5-10 equiv of tetramethylammoniumchloride to exchange cations with the borate. The mixture was made more polar by passing a stream of nitrogen over the solution to evaporatethe acetonitrile and precipitate the tetramethylammonium borate. The borate was collected by vacuum filtration, dried under vacuum, and recrystallized from acetonitrile/water if necessary. Tetramethylammonium Tetra-etolylborate. 1H NMR (200 MHz, CD3CN): 6 7.30-7.20 (m, 4 H), 6.84-6.69 (m, 12 H), 3.05 (9, 12 H), 1.52 (9, 12 H). Anal. Calcd for C32HmNB: C, 85.51; H, 8.97; N, 3.12; B, 2.41. Found: C, 85.37; H, 8.94; N, 3.27; B, 2.44.
Murphy et al. Tetramethylammonium Tetra-ptolylborate. 1H NMR (300 MHz, CD3CN): 6 7.10 (m, 8 H), 6.78 (d, J = 7.6 Hz, 8 H), 3.04 (s, 12 H), 2.18 (s, 12 H). Anal. Calcd for C32HmNB: C, 85.51; H, 8.97; N, 3.12; B, 2.41. Found: C, 85.39; H, 9.02; N, 3.20; B, 2.38. Tritolylpbenylborate Salts. To an ether solution of tolylmagnesium bromide was added 0.5 equiv of dichlorophenyl boron, and the solution was heated at reflux overnight. The borate was then treated by the usual procedure described above. Tetramethylammonium M-etolylpbenyhte. 1HNMR (300 MHz, CD3CN): 6 7.30-7.20 (m, 3 H), 7.10-7.02 (m, 2 H), 6.93 (dd, J = 7 for both, 2 H), 6.90-6.72 (m, 10 H), 3.05 (s, 12 H), 1.57 (s, 9 H). Anal. Calcd for C31H38NB:C, 85.51; H, 8.80; N, 3.22; B, 2.48. Found: C, 85.51; H, 8.80; N, 3.29; B, 2.45. Tetramethylammonium Tri-ptolylpbenylborate. lH NMR (300MHz,CD$N): 67.26-7.20(m,2H),7.16-7.06(m,6H), 6.95 (dd, J = 7 Hz for both, 2 H), 6.84-6.76 (m, 7 H), 3.03 (5, 12 H), 2.18 (s, 9 H). Anal. Calcd for C31H38NB: C, 85.51; HI 8.80; N, 3.22; B, 2.48. Found: C, 85.43; H, 8.80; N, 3.27; B, 2.43. Dipbenylditolybrate Salts. Diphenyl(diethy1amino)boron was prepared by the method of Niedenzu and Dawson22by first heating a mixture of boron trichloride and diethylamineto form dichloro(diethy1amino)boron. To the dichloroboranewas added phenylmagnesium bromide formed in situ and heated at reflux in ether to form diphenyl(diethy1amino)boron. A 0.4 M solution of the amino borane in hexane was converted to diphenylchloroborane by stirring at -78 "C with 2.1 equiv of boron trichloride for 15 min. The solution was then allowed to warm to room temperature and stirred for 5 h. Finally, the solution was heated at reflux for 17 h. The solution was concentrated in vacuo, diphenylchloroborane was vacuum distilled at 0.4 Torr, and the fraction boiling at 90-93 "C was collected. The diphenylditolylborate was prepared by addition of tolylmagnesium bromide to the diphenylborane. The borate was then treated by the usual procedure described above. Tetramethylammonium Diphenyldi-etolylborate. lH NMR (300 MHz, CD3CN): 6 7.24-7.10 (m, 6 H), 6.95 (dd, J = 7 Hz for both, 4 H), 6.88-6.74 (m, 8 H), 3.02 (5, 12 H), 1.66 (s, 6 H). Anal. CalcdforCpoHp6NB: C,85.50;H,8.61;N,3.32;B,2.57. Found: C, 85.53; H, 8.63; N, 3.35; B, 2.65. Tetramethylammonium Diphenyl-ptolylborate. 1H NMR (300 MHz, CD3CN): 6 7.30-7.20 (m, 4 H), 7.18-7.08 (m, 4 H), 6.96 (dd, J = 7 Hz for both, 4 H), 6.88-6.76 (m, 6 H), 3.05 (s, 12 H), 2.18 (s, 6 H). Anal. Calcd for C30H36NB: C, 85.50; H, 8.61; N, 3.32; B, 2.57. Found: C, 85.50; H, 8.64; N, 3.37; B, 2.48. TripbenyltolylborateSalts. A 0.15 M solution of tolyl lithium was made by stirring tolyl bromide and 0.95 equiv of 1.6 M n-butyl lithium in ether at 0 "C for 31/2 h. The solution was cooled to -78 "C, and 0.65 equiv of a 0.1 M solution of triphenyboronin 3:l ether:benzenewas added. The solution was allowed to warm to room temperature and stirred overnight. The borate was then treated by the usual procedure described above. Tetrametbylammodum Triphenyl-etolylborate. lH NMR (300MHz,CD,CN): 67.26-7.14(m,6H),7.12-7.04(m, 1 H), 6.97 (dd, J = 7 Hz for both, 6H), 6.88-6.74 (m, 6 H), 3.04 (s, 12 H), 1.70 (s, 3 H). Anal. Calcd for C29H34NB: C, 85.50; H, 8.41; N, 3.44; B, 2.65. Found C, 85.51; H, 8.42; N, 3.43; B, 2.78. Tetramethylammonium Tripbenyl-ptolylborate. 'H NMR (300 MHz, CD3CN): 6 7.34-7.22 (m, 6 H), 7.20-7.10 (m, 2 H), 6.97 (dd, J = 7 Hz for both, 6H), 6.84-6.80 (m, 5 H), 3.05 (s, 12 H), 2.19 (s, 3 H). Anal. Calcd for C29H34NB: C, 85.50; H, 8.41; N, 3.44; B, 2.65. Found: C, 85.14; H, 8.64; N, 3.55; B, 2.72. Acknowledgment. This work was supported by a grant from the National Science Foundation (NSF Grant DMR 90-04130,
Tetraarylborates { [Ar]4Bj
NSF Grant CHE-88-20271) for which we are grateful. J.B.M. was supported by Grant PHS GM08276.
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