Measurements of standard potentials for nucleophiles by fast cyclic voltammetry

Journal of Ekctroanalyrical Chemistry,362 (1993) 109-118

109

JEC 02874

Measurements of standard potentials for nucleophiles by fast cyclic voltammetry I: 9-substituted fluorenide ions in dimethyl sulphoxide Torben Lund Instituteof Life Science and Chemistry, Universityof Rod&it, DK-4000 Roskikie (Denmark)

Steen Uttrup Pedersen Department of Organic Chemdy,

Universityof Aarhus, DK-8ooOA’rhus C (Denmark)

(Received 18 February 1993; in revised form 5 April.1993)

AbShWt Standard potentials E&.,,- for 12 different 9-substituted. fluorenide ions were measured in dimethyl sulphoxide (DIMSO) by fast cyclic voltammetry. 9-Amino-fluorenide ions are found to exhibit two reversible single-electron waves in cyclic voltammetry showimg that the corresponding 9-aminofluorenyl tadicals and fluorenium cations are stable. Standard potentials &r the 9-aminofluorenide ion and for the 9-aminofluorenyl radical were measured. For 9-alkyl-, phenyl-, methoxy-, phenylthiofluorenide ions, only one single-electron irreversible wave was found and linear sweep voltammetric results indicate a DIhk ‘mechanism for less sterically hindered fluorenyl radicals. For severely sterically hindered fluorenyl radicals a more compkatid mechanism was found to occur. The rate constants for the dimerization of the fluorenyl radicals were determined by wmparison.of experimental and simulated voltammograms, from the potential shift of oxidation potentials relative to standard potentials or by double potential step chronoamperometry.

1. IntroductIolI

Standard potentials El.,*- of anions are important thermodynamic parameters; however, only a few genuine standard potentials are known. This is due to the fact that the radical A obtained by oxidation of the anion A- in general is extremely reactive. The lifetime of A is often very much shorter than the time scale of traditional cyclic voltammetry (CV) and therefore only a limited number of anionic nucleophiles exhibit reversible voltammograms at normal scan rates that make measurement of standard potentials possible. An anion for which the standard potential is known is the enolate ion l- derived from 1,4dihydro-4-(methoxycarbonyl)-1-methyl-pyridinium salt through two oneelectron electrochemical reduction steps. The standard potential is easily measured for 1 - by CV because two reversible single-electron waves are observed, implying that both l- and 1’ are stable during the electrochemical experiment. The stability of 1’ is the basis of the

success of 1- as a nucleophile/ electron donor in numerous kinetic studies [l-71. Two alternative strategies are often used to avoid the experimental problems involved in measurements of standard potentials of anions. The first alternative is to obtain the standard potentials through thermodynamic calculations. It is, however, often difficult to find relevant data for larger organic molecules such as fluorenide ions. Most data available refer to gas phase or aqueous conditions and therefore have to be adjusted by the Gibbs energy of transfer AGE if Ei.,*- is needed for organic solvents

Bl. The other approach is to approximate standard potentials with irreversible (peak) oxidation potentials that include rapid follow-up chemistry as standard potentials. One should however be cautious with this strategy because oxidation potentials might be shifted substantially relative to standard potentials. For example in the case of second-order follow-up reactions Q 1993 - Elsevier Sequoia S.A. All rights resewed

110

,’

(DIMl), eqns. (1) and (2), the relation between the peak oxidation potential and the standard potential is given by eqn. (3) where [A-] is the substrate concentration and kdim is the rate constant for the second-order reaction [9,101. In the case of oxidation of anions the generated radicals are likely to dime&e (DIMl), as will be demonstrated for some of the 9-substituted fluorenyl radicals, and the rate constants for such radical dimerizations, where the activation barrier is approaching zero, are very close to diffusion controlled. Such rapid dimerizations can lead to a shift of the measured peak oxidation potential to values 100 mV more negative than the standard potential: A--

e- -A

RT In E E ca,p= E;.,A- - 3nF i( nF Equation

R

N,N-dimethylamioo NJ-isopropylamino

(2) (3)

Methyl Isopropyl

(7) (8)

Pyrl+&yl Piperidioyl Hydrogen

(4) (5) (6)

tcrt-Butyl Pheoyl Phenylthio

(9)

Methoxy

(12)

(W

(U)

Scheme 1

(1)

~A-lkam v

1

3.12

(3) 1

(3) is valid for DIM1 for xk = (RT/nF)

[A-]k,Jv > 10 where u is the scan rate in CV, n is the number of electrons involved in eqn. (11, T is the absolute temperature and R and F are the usual constants 19,101. Irreversible peak oxidation potentials might also be perturbed by slow heterogeneous kinetics in which case the potential is shifted p&tive relative to the standard potential. In this case the shift can be many hundreds of millivolts. Fast CV, applying scan rates up to 1 MV ‘s-l at ultramicroelectrodes, introduced and developed by Wightman and coworkers [ll-131, Amatore and coworkers [14,15], Sav&nt and coworkers [16,17] and many other groups [181, has dramatically increased the number of short-lived intermediates that can be observed and studied. Second-order reactions with rates close to diffusion control can at least theoretically be outrun with fast CV at scan rates up to a few hundred kilovolts per second. Unfortunately, limited rates of heterogeneous electron transfer do not always allow the application of such high scan rates. Fast CV has thus increased dramatically the number of standard potentials of anions that can be measured [19]. Standard potentials of phenolates obtained by fast cyclic voltammetry at ultramicroelectrodes were very recently presented by Hapiot et al. [201. The purpose of this study was to obtain genuine standard potentials for 9-substituted fluorenide ions (9-R-FL-, Scheme 1) by application of fast CV, and to investigate the follow-up kinetics with regard to mechanisms and rate constants. Oxidation potentials (peak) have previously been

measured by B&well and Bausch for a series of 9-substituted fluorenide ions by classical CV at a fixed scan rate of 0.1 V s-l [21]. The measured oxidation potentials of the fluorenide ions were used to correlated their basicity [21] and to calculate outer-sphere electron transfer (ET) rate constants according to the Marcus eqns. (4)-(6), where A is the reorganization energy, 2 is the collision number [22-251. Discussions of relative bond dissociation energies and radical stability have been based on the former correlation [21] AC0 = F( E; -E;)

(4)

(5) $[,RT]

(6)

It is to be expected that the kinetic perturbations included in the oxidation potentials relative to the thermodynamic standard potentials might endanger the previously drawn conclusion based on Bordwell’s correlations. We have therefore included a discussion of our standard potentials compared with &iordwell’soddation potentials and the possible consequences of differences.

2. Experimental 2.1. General analytical apparatus NMR spectra were obtained with a Varian Gemini 200 MHz spectrometer and GC-MS were recorded on a Hewlett Packard 589OA gas chromatograph equipped with a 5971A MSD.

2.2. Sohent DMSO (Aldrich) was stored under nitrogen (SureSeal bottles) and used without further purification. All

111

handling of DMSO solutions was performed uum line using normal syringe techniques.

on a vac-

2.3. Materials The supporting electrolyte tetrabutylammonium tetrafluoroborate (TBABFJ was prepared by standard methods and dried under vacuum at 60°C for at least 24 h before use. The fluorene was obtained from Aldrich. The 9-amino fluorenes were synthesized by the method of Bordwell et al. [26]. The 9-alkyl substituted fluorenes were obtained by reduction of the corresponding 9-alkyl substituted fluorenol in acetic acid containing I, and hypophosphorous acid [27]. The synthesis of 9-(phenylthio)fluorene was done according to Bavin [28]. Dimsyl potassium (KCH,SOCH,) was prepared by the method of Olmstead et al. [291. 2.4. Electroanalytical techniques Two different potentiostats, both home-made, were used in this study. A computer controlled three-electrode adder type potentiostat with positive feedback and ZR-compensation was used for slow CV (Y < 1000 V s-l) and for slow double potential step chronoamperometry (r > 1 ms). A faster potentiostat (higher bandwidth) based on current amplifiers, two electrodes and ZR-compensation was built according to the description of Garreau et al. [17]. This fast potentiostat was tested with anthracene in DMF + 0.6 M TBABF, and was found to work well up to 300 kV s-i. At scan rates beyond this limit the time delay induced by the operational amplifiers is comparable with the time width of the peaks in the voltammograms and therefore ZR-compensation is meaningless. The triangular ramp or the pulse signal was taken from an HP 8116A function generator controlled by a PC 80386~SX-16 through an IEEE 488 interface. The potential and current signal were recorded by a Tektronix RTD 710 digitizer. All traditional data manipulations \1yere possible on the PC and the voltammograms were plotted by an IBM 4019E laser priri&. The CV-cell was the same as used previously [30]. A 0.5 mm diameter gold disc electrode was used as the working electrode in slow CV and 25 pm or 33 pm diameter gold disc microelectrodes were used in fast CV. A platinum wire counter-electrode and. a silver wire reference electrode were used in slow CV whereas only the silver wire was used as the counter-electrode in fast CV. The supporting electrolyte, 0.3 M TBABF,, was weighed directly into the cell. The cell and supporting electrolyte were evacuated and heated with a hot air gun to remove residual water. After cooling, the cell was purged with, argon and a small over-pressure was maintained during the experiment. A typical measurement was made in the following

way. 1 g of TBABF, was weighed in the cell, evacuated and purged with argon. 70-180 mg of the 9-R-FLH was placed in a test tube equipped with a rubber septum and purged with argon for 15 min. DMSO (10 ml) was transferred by syringe to the test tube and stirred magnetically. Finally 1.5 ml of the prepared solution of CH,SOCH;K+ (0.3 MI was added and normally a deep red colour developed indicating the presence of the fluorenide ions. 10 ml of DMSO was transferred by syringe under argon to the cell and finally 0.25-1.0 ml of the fluorenide ion solution was added to obtain a 1-4 mM solution of the fluorenide ion. The solution was studied by traditional CV first and eventually, if needed, by fast CV. The scan rate was increased up to 50 kV s-l or until reversible voltammograms were obtained. After these experiments l-2 mM of ferrocene was added in order to refer all potentials to the ferrocenium/ ferrocene redox couple. 3. Potentials All potentials in this study were originally referenced to the standard potential of the ferrocenium/ ferrocene (fer(III)/fer(II)) couple. The latter potential was in DMSO + 0.1 M tetrabutylammonium iodide (TBAI) measured to 0.927 V vs. Ag/AgI/DMSG + 0.1 M TBAI. The Ag/AgI(DMSG) reference electrode was then assumed to have the same potential as Ag/ AgI(H,O) which probably is not quite true. The, latter electrode was then referenced to the SHE by adding (0.152 V) [31]. Tabulated potentials (vs. SHE) are therefore obtained by adding 0.775 V to the original potentials referenced to ferrocene. The uncertainty indicated in the tables concerns the potentials relative to ferrocene. Standard potentials are here always measured as the midpoint between the anodic and cathodic peak potentials. Bordwell et al. used the same redox couple to adjust their scale of potential and found that E&IIO,fcr(IIj= 0.875 V vs. Ag/AgI. Conversion to the SHE scale was done by adding -0.125 V. Bordwell’s oxidation potentials are therefore obtained by adding 0.750 V to the potentials originally referenced to ferrocene. 3.1. Preparative oxidation of 6, 7 and 12 Fluorene (55 mg, 0.33 mM) was oxidized under nitrogen in 25 ml DMSO + 0.1 M TBABF,.at 0.0 V vs. a silver wire in an H-cell containing a platinum anode: After the consumption of 30.8 C (n = 0.97) the elwtrolysis mixture was poured into water and. extracted with 50 ml of dietbyl ether. The ethereal layer ‘was washed three times with water and dried by 4 A

112

moke&ar sieves. The ether extract was analysed by ‘H-NMR and CC-MS. 3.U. oxidation of 6. The mainproduct was identified as PQ’difluorenyl. ‘H-NMR (CD&): 3.9 C2H,s), 7.2-7.8 Gn, 16I-D.MS (70 eV). m/z (%): 331 (4), 330 (171, 166 (111, 165 (100), 164@, 163 (4). 3.1.2. Oxiaktionof 12. The main product was identified as 9,9’-dimethoxy-9,9’-difluorenyl. ‘H-NMR (CDCI,): 2.84 (s, 6H), 7.0-7.6 (m, 16H). MS (20 eV). m/z (%I: 328 (201, 327 (441, 326 (111, 196 (321, 195 WIO), 180 (440, 152 (11).

E/mV

(a)

3.1.3. oxidation of 7. Three products were identified from the crude product by ‘H-NMR: 9,9’-dimethyl-9,9’-difiuorenyi (JO%), 9-methylfluorene (25%) and 9-methylfluorenol (25%). The latter two corn; pounds were identified by comparison with authentic spectra. The dimer was identified according to ‘HNMR (CDCI,): 1.80 (6H, s), 7.2-7.8 (16H, m). 4, Results and discussion The 9-substituted fluorenide ions (Scheme 1) can be separated into two groups according to their electrochemical appearance in traditional CV in DMSO. ZBvo reversible single-electron waves are observed in cyclic voltammograms at a scan rate of 1 V s-l when the substituent is an amine: 9_(iV,N-dimethylan&oMuorenide (2), 9-(NJ?-isopropylamino)_fluorenide (31, 9pyrrolidinylfluorenide (41, 9-piperidinylfluorenide (5). This means that the fluorenyl radicals as well as the fluorenium cation are stable in the time scale of the electrochemical experiment (t,, > 5 s). The voltammogram of 5 is shown in Fig. l(a). The other group, fluorenide (6), 9-methylfluorenide (71, 9-isopropylfluorenide (81, 9-tert-but+fluorenide (91, 9-phenylfluorenide (lo), 9-(phenylthio)fluorenide (11) and 9methoxyfluorenide (12) all exhibit one irreversible oxidative wave in their cyclic voltammogram at 1 V s- ‘. The voltaminogram of 9 is shown in Fig. l(b).

4.1. 9-aminofh4orenideions The standard potentials for the fluorenide ions E$,.,r,- and the fluorenyl radicals Ez+,r.,. are readily obtained for the first group of Paminofluorenide ions directly from the voltammograms and these values are given in Table l+ The reversibility of the voltammogram stems .from the stability of the fluorenyl radical which is caused by the spin delocalization at the nitrogen of the amina group as, indicated in Scheme 2 for 9-piperidinyhkorenide (5). Calculations using the semi-empirical VAMl approach 132,331have shown

E/mV (b)

Fig. 1. Cyclic vohammograms of (a) 9-piperidinylfhrorenide (10)and (b) 9-terr-butylfhmrenide (9) at a scan rate of 1 V s-l in 0.3 M TRARFZ +DMSO at a 0.5 mm gold disk electrode in the presence of ferrocene (the most positive redox wave).

that about 35% of the spin density is present at .the nitrogen in 2 and only minor density has spread to the phenyl groups. For the 9-methoxyfhrorenyl radical ., about 3OYqof the spin density is found on the oxygen, ~f but here no stabilization of the radical is indicated@, the experimental values. Finally the spin dens$y of ,

/ 3 TABLE 1. Standard potentials for 9-aminofluorenide ions E$/,and 9-aminofluorenyl radicals Ek+,,. vs. SHE in DMSO at room temperature R

E&,-/V

-N(Me), -N(i-Pr>; Pyrrolidinyl Piperidinyl

- 0.66OC5) -O&3(5) - 0.785(5) - 0.635(5)

a

4. /V b -0.660 - 0.485 -0.805 -0.643

A&,/V

E;+,,YV

0 -22 -20 -8

-0.166(S) - O.O5s(S) - 0.214(5) - 0.059(5)

a In volts. The potentials are reported relative to the standard hydrogen electrode (SHE) taken with the ferrocenium/ferrocene couple as an internal standard (E” = +0.775 V vs. SHE). b Published values of Rordweli and Hart&on 1221vs. SHE.

113 TABLE 2. Standard rate constants for the heterogeneous electron transfer to 9-aminofluorenide ions measured by cyclic voltammetry in DMSO + 0.3 M TEA3F, at a gold electrode R

v/v

Piperidinyl

40 1Oa 40 100 40 100

Pyrrolidinyl -N(Me),

s-l

AE,/mV 90(l) loo(l) 98(l) 990) 1040) 108(l)

a

&‘/cm s-* b 0.17 0.20 0.23 0.20 0.11 0.16

’ Separation of peak potentials. b Interpolated from Table 2 in ref. 34 assuming the transfer weft% cient to ,k 0.5 and the diffusion coefficient to be 1 X lo-’ cm2 s-l.

9-alkyl-fluorenyl radicals is almost 100% on the carbon atom in the 9 position. Apparent standard rate constants (without Frumkin correction) of the heterogeneous electron transfer reaction were measured for the three 9aminofluorenide ions and are shown in Table 2. The apparent standard rate constants for the three compounds are similar but signif~tly smaller than those measured for more extended conjugated systems such as anthracene and naphthalene [35]. This difference may reflect differences in the intramolecular reorganization energy which are large for the fluorenide ion/fluorenyl radical owing to the localization of the charge in the 9 position but also owing to the fact that a dipole is present in the amino compound and this will contribute to the solvent reorganization energy A(O). The solvation of dipoles is neglected in calculations using the Born equation for conducting spheres in homogeneous dielectrics. Calculations have shown that the solvation of dipoles contributes significantly to h(O) 1361. Bordwell has “corrected” the standard potentials to irreversible oxidation potentials whereas the opposite would be more reasonable. Bordwell writes that “One way to correct is by application of the Nicholson equa-

Scheme 2

tion; this leads to a value of 80 mV for the cathodic shift under the condition of our experiment”. [21,37]. The negative (cathodic) shift mentioned is the oxidation potential (peak) of 10 relative to 2, but it is unclear how this result is reached. It is well known that completely irreversible voltammograms will look the same as long as the kinetic dimensionless ‘parameter xk = (RT/nF)k,, [9-R-Fl-I/V is greater than 10. Bordwell et al. probably assumed that the dimerization of 10 proceeds by a DIM1 mechanism and that the dimerization is fast and near the diffusion limit and the rate constant is therefore set to 1 X lo9 M-i s-t. Equation (3) will then predict a negative shift of 80 mV but it is important to remember that this result is based on the assumption of a DIM1 mei;rhanism and a constant high dimerization rate, which we later will show is off by almost three orders of magnitude. The various forms of corrections introduced .by Bordwell et al. do not seem satisfactory, and in Bordwell’s following papers correction of the oxidation potentials is omitted. The difference between the standard potentW1E’ and the reversible peak oxidation potential should be -28.5 mV [31] if both potentials are referenced to the same ferrocenium/ ferrocene standard potential E&II),fer(II). However, it is unclear whether Bordwell’s potentials are referenced to the peak oxidation potential of the ferrocenium/ ferrocene couple or to the standard potential E&m~,fer(n~. If the former case. is true, Bordwell’s peak potentials EWm- are equal to for the aminofluorenides when referenced to JG,Flferrocenium/ ferrocene. The potentials are converted to the SHE scale by adding +775 mV (this work) and + 750 mV (- 875 mV + 125 mV> [381 respectively.. The difference AE,, = Eox,m-- E&.,m- is therefore expected to be equal to (750 - 775) mV = -25 mV, which is close to the observed values (see Table 1).

114 TABLE 3. Peak oxidation potentials Ear, and standard potentials E$,.,,for Fsubstituted fluorenide ions vs. SHE in DMSO at room temperature Gb EL-,-/V

R

E,,/V

4-i (a-) -CH, (7) -CH&fe), (8) -c(Me)s (9) -C,Hs (10) -s-c&Is (11) -0CHs (12)

-9.270(S) - 0.43%) -0.391(S) -0.348(S) - 0.225(5) -0.046(S) -0.525(J)

a Em/V’

- O.lM(20~ d - 0.357(20) d -0.375(S) - 0.3250) -0.203(15) +0.031(29) - 0.45so(20~d

- 0.319 = -0.480 -0.458 -0.409 - 0.278 -0.099 -0.574

4E,,/mV + 124 + 123 +83 +&Q +75 + 130 + 124

* In volts. The potentials are reported relative to the standard hydrogen electrode (SHE) taken with the ferricinium/ferrocene couple as an internal standard L!?’ = +0.775 V vs. SHE). b Peak oxidation potentials at 1 V s-l. c Published values of Bordwell and Harrelson 122,391 and V/vs. SHE. d Potentials were calculated from eqn. (3) using values of k&,, given in Table 6. c This potential was originally referenced to Ag/Agl[21] but is here referenced to SHE by adding -0.125 V.

Conclusions based on Bordwell’s peak oxidation potentials will for this group of 9-aminofluorenide ions not be altered by substitution with standard potentials. This result was also expected because peak reversible potentials are not shifted kinetically relative to standard potentials. 4.2. Other 9-subs~~-fl~renide ions Irreversible peak oxidation potentials E,,were measured for ail the compounds by traditional CV and the values are induded in Table 3. For this group of g-substituted Iluorenide ions traditional CV with scan rates less than 1000 V s-i was not sufficient to outrun the follow-up reactions and to reach reversible voltammograms. Only for 9 was reversibility observed at low scan rates and therefore fast CV had to be applied in order to reach reversibility for the other compounds in this group. The scan rate was increased for each compound until reversibility was visible, characterized by a reduction peak for the fluorenyl radical. For 6, 7, and 12 no sign of reversibility was detected before our limiting scan rate of 50 kV s-l. The fast cyclic voltammograrn of 9-(phenylthiohluorenide (11) is shown in Fig. 2. In all the cases where reversibility was reached, reversible standard potentials were measured as the potential midpoint between the peak oxidation potential and peak reduction potential. These standard potentials are tabulated in Table 3.’ The range of scan rates where the transition of the voltammograrn from irreversible to reversible is observed can be used to estimate the rate constant of the dimerization as soon as a proper mechanism has been suggested. Mechanistic possibilities are discussed in the following.

Preparative electrochemical oxidation of 6 and 12 (n = 1) gave the corresponding dimers in high yields. In the oxidation of 7 the dimer (50%) was obtained together with the biproducts 9-metbyltiorene (25%) and 9-methylfluorenol (25%). These biproducts are most probably formed by degradation of the 9,9-dimethyl-9,9-difluorene during the work up procedure. The preparative results thus support the simple DIM1 mechanism for the anions 6, 7 and 12. In the cases where reversibility could be reached at high scan rates it was possible to estimate the apparent standard rate constants k” for the heterogeneous electron transfer using the normal procedure of Nicholson [34,40]. The results are given in Table 4 and it is seen that all the non-amino substituted fluorenide ions have higher standard rate constants (OS-O.8 cm s-l) than the 9-aminofluorenide (0.14-0.22 cm s-l). One might expect that the more stable 9-aminofluorenide ions would have lower inner reorganization energy A(i) owing to extensive stabilization of the fluorenyl radical, but this lowering in A(i) is counterbalanced by ‘the contribution of the dipole in the latter compound to the solvent reorganization energy A(O). The higher heterogeneous rate constants for non-ammo substituted fluorenide ions permit the use. of higher scan rates in cyclic voltamrnetry without too serious interference of rnized heterogeneous and homogeneous kinetic control. The mechanism for this dimerization was examined further by linear sweep voltammetry (LW) by mea@uring the slopes H&,/a log Y, E,,&3 log c and the Table 5. The electrochemipeak width EmP - &,x,p,z, cal characteristics of different dimerization mecha-

I

o-

-._

;---; 500

600

700

800 - 900

1000

1100

1200

E/mV

Fig. 2. Qclic volmmmogmm of 9_(phenylthio)tluorenide (11) at a scan rate of 20000 V s-* in 0.3 M T3ABF.$ +DMSO at a 25 pm gold dish uhmmicmelectrode.

115

pear. However, it can be difficult to detect feeble peaks in cyclic voltammetry and therefore the tabulated rate constant might in the worst case be a factor P/an s-lb v/v s-1 AE, /mV * of 4 too large. The measured values of kdim are inR cluded in Table 6. 0.80 loo00 125(l) -CXMe)3 (9) 0.80 loo00 125(l) The standard potentials could not be measured di-CH(Me)2(s1 0.52 1900) -S-&H5 (11) 2fMOO rectly from fast CV for 6, 7 and 12 because full 0.73 loo00 1300) -C,H, (10) reversibility could not be reached. The rate constant a Separation of peak potentials. kdim can however be estimated with less than an order b interpolated from Table 2 in ref. 34 assuming the transfer q$fiof uncertainty which will give E&.,,with an uncercient to be 0.5 and the diffusion coefficient to be 1 X lo-’ ‘cm2 s-l. tainty of less than 20 mV (eqn. (3)). For 6 and 7 no reversibility was detected until scan rates of 50 and 40 nisms have previously been discussed intensively [9,101. kV s-r respectively. Equation (9) can be used to calculate a minimum value for kdh. However, in all The mostsimple suggestion for the dimerization mechsecond-order reactions the reactants have to diffuse anism is the dimerization of two fluorenyl radicals, together and therefore their rate is limited by the DIMl, eqns. (7) and (8). The LSV slopes for comdiffusion controlled rate constant k,, which can be pounds 6, 7 and 12 are in accordance with this simple calculated from the Smoiuchowski equation, eqn. (101, dimerization mechanism, %!$,._/a log v = 19.7 mV and to be 3.0 X 10’ M-’ s-l when the viscosity of DMSO aE_,/a i0g c = - 19.7 mV. The peak width E_ is 7 = 2.22 [42]. The uncertainty is minimized by tabu= 38.8 mV is however somewhat smaller than E lating the midpoint between the minimum value of th?~~erimentally observed values (47.4-49.0 mV). k,, and the upper limit kdif, Table 3. The heterogeneous electron transfer is not nernstian The calculated values of k,,, for 6, 7 and 12 can and therefore there is mixed control of charge transfer now be used together with irreversible peak oxidation and dimerization and the peak width will thus increase potentials in eqn. (3) to determine standard potentials. to 42 mV [41]. These potentials are included in Table 3. The LSV slopes and the half-peak widths shown in DIM1 Table 5 for 8, 9 and 11 indicate strongly a shift in FJl-- e- e Fl’ (7) mechanism away from DIMl. The mechanism seems to be the same for 8 and 9 and the explanation for the Fl’+Fl’- kti Fl, (8) shift in mechanism is probably that the rate of dimerF vi ization of two fluorenyl radicals is slowed down by k dim= 0.8RT [fl-] (9) steric hindrance at the 9 position owing to the bulky substituents. Other follow-up reactions of the fluorenyi 8RT k,-(10) radical might therefore compete. 37 In the preparative oxidation of 9 the main products The rate constant for the dimerization of two fluoobtained were 9-tett-butyl-fluorene and 9-tert-butyl-9renyl radicals (DIM11 can now be calculated. Digital fluorenol (1: 1) and only a very low yield of dimeric simulations have shown that a peak from the reduction products was observed. Attempts to synthesize the of fluorenyl radicals can be detected for xk = dimer of 9 by oxidation of 9-tert-butyl-9-fluorenol by I, (RT/F)k&[Fl-l/v = 0.8. The rate constant k,,, can and SnCl,/HCl in acetic acid according to the procetherefore be calculated from eqn. (9) by observing the dure described by Wawzonek and Dufek 1431 also scan rate vi where the first signs of reversibility apfailed. Similar results are obtained with compound 8. The products may either be formed directly after the electrochemical oxidation through disproportionation, TABLE 5. LSV slopes and peak width for 9-substituted fluorenide eqn. (121, followed by the reaction with residual water, ions in DMSO + 0.3 M TBABF4 eqns. (14) and (151, or from degradation of the steric constrained dimer by a heterolytic process, eqn. (151, in R * /mV (E,, - E,x,P,2)/mV the cell or during the work-up.

TABLE 4. Standard rate wWants fbr the heterogeneous electron transfer to 9-subsWed-fhwrenide ions measured by cyclic voltammetxy in DMSO + 0.3 M TBABF, at a 5 pm gold disc electrode

-H (6) -cl-i, (7) -WZIQ (9) -SPh (11) -0cH 3 (12)

21.5 19.2 21.4 17.1 23.4

22.8 23.0 41.0 31.8 21.2

53.3 47.7 45.5 47.4 49.0

Fl--

e- ----5Fl. Fl-+ Fl+

2Fl.w w2

-

Fl-+ Fl+

(11) (12) (13)

116

Fl-+ H,O -FlH+OH-

(14)

Fl++ H,O \

(15)

4, = exp

FlOH + H+

1

F( E&.,Fy- E$+,m.) RT

(16)

An LSV slope %!&,-/a logfFl-I equal to -40 mV decade- ’ is expected for reactions second order in [Fl ‘I and first order in [Fl-] according to eqn. (24) in ref. 44. In order to test this observation, reaction orders in the substrate were measured by double potential step chronoamperometry [31]. It was found for 9 that ri,* = 5.0 ms for [Fl-] = 1.0 mM and r1,2 = 1.2 ms for [Fl-] = 2.0 mM. The reaction order in the substrate Rm-,m. is determined from Rm-,n.= la log r&a log [Fl-] = 3.05 which clearly demonstrates the third-order bebaviour in the substrate [44]. The LSV slopes and the reaction order found for this reaction are both in accordance with the DIMZDISP2 mechanism. Equations (ll), (121, (14) and (15) are an example of a mechanism which can explain the products. However, this mechanism is not third order in substrate when eqn. (15) is the rate-determining step and eqn. (12) is an equilibrium, but only first order; furthermore, the measured rate is not consistent with this mechanism. Equation (12) is a disproportionation reaction with an equilibrium constant K,, determined by the difference in standard potentials of the fluorenide ion and the fluorenyl radical as stated in eqn. (16). The oxidation wave .of the fluorenyl radicals of 8 and 9 were however never observed, even at higher scan rates where the first oxidation eqn. (11) is reversible and at an extended scan range including the ferrocinium/ ferrocene couple. The standard potential of the fluorenyl radicals must therefore be at least 1 V more positive than the standard oxidation potential of 8 and 9. The equilibrium of eqn. (12) therefore lies completely to the left with K,, > 1 x 101’ = k,,/k_,,. Subsequently, k,, will never be able to exceed 10e7 M-l s-i, even when

k_,, is diffusion controlled, and therefore the measured rate is inconsistent with the .disproportionation reaction. If it is assumed however that the dimer is formed directly and the isolated products are formed by heterolytic cleavage of the dimer in the cell or more probably during the work-up, eqns. (131, (14) and (15), then the third-order in substrate behaviour and the products can be explained. The proposed mechanism would also explain the failure to syntheshe the dimer [43]. The dimerization may proceed by a DIMZDISP2 mechanism as outlined in eqns. (17)-(20) with eqn. (19) as the rate-determining step; .Digital simulations of this mechanism by Sav&nt et al. have given the following LSV slopes and half-peak width, %!&,,~CI log v = 29.7 mV, aE,,/a log c = - 39.4 mV and finally E_, E = 45.5 mV [lo]. For 9-tere-butylfluorenide ions <9~%se characteristic values fit completely with the experimentally observed values and it is therefore assumed that the DIMZDISP2 is a plausible mechanism for this compound. DIM2-DISP2 Fl-- e- .‘Fl. _

(17)

Fl’+ Fl- =

Fl;’

Fl;’ + Fl’ k,,

= K,,k,,

(18)

Fl, + Fl=

(19)

l.OL 2!RT [Fl-I2

The rate constants K&i, can now be calculated from the scan rate where the first signs of reversibility appear, Vi, and from the concentrations of 8 and 9. Digital simulations of this mechanism have shown that a small reduction wave from the fluorenyl radical is observable when xk = CRT/F) K,,k,rJFl-12/v G 1.0. The rate constants kdim can therefore be calculated from eqn. (20). The values of k,, are included in Table 6. Only for compound 11 does no simple mecanism seem to describe the LSV slopes and the peak

TABLE 6. Rate constants for the dimerization of 9-substituted fluorenyl radicals in DMSO + 0.3 M TBARF, R’

Mechanism

10s vi/v s- *

[Fl-l/mM

log&,/M-’

-H (6) -CH 3 (7) -CIWI-I,),

DIM1 DIM1 DIMZDISP2 DIMZ-DRIP2 DIM1 Unknown DIM1

> so >40 1.0 0.005 1.0 5.0 20

2.0 2.0 1.0 1.0 1.7 3.8 4.0

9.2fO.3) 9.RO.4) lO.S(O.1) c KZ(O.1) c 7.3(0.11 S.Z(O.1)

(8) (9) -C,H, (IO) -S-C,H, (11) -oCH, (12) -cm-I,),

s-l) a

log&,/M-'

s-l) b

9.NO.5) = 9.ZxO.S)c 6.SKbS) 9.RO.S)

a For DIM1 k,, = 31.1 vi/[n-], and for DIM2-DISPZ kh =Krskrs = 38.9 Vi/[Fl-]‘. b For DIM1 calculated from eqn. (3) using the measured peak oxidation potentials and standard potentials given in Table 3. The scan rate was always 1 V s-l and [Fl-1 = 1.0 mM. For DIMZDISP2 the equation similar to eqn. (3) taken from Table 3 in ref. 10 was used to calculate k&. ’ kdim = K,,k,,.

117

width and it is therefore not possible to calculate rate constants for this specific reaction. The standard potential is still correct with or without knowledge of the mechanisms of the follow-up reactions. In Table 6 the scan rate at which the first signs of reversibility appear is included. The results of this investigation show very clearly that for 9aminofluorenide ions where reversible voltammograms are found, the difference between the oxidation potentials of Bordwell and standard potentials are very modest whereas major differences (150 mV) are found in the other group of 9-substituted fluorenide ions exhibiting irreversible voltammograms at low scan rates. This of course reflects the shift in oxidation potentials caused by rapid follow-up kinetics with second-order rates close to the diffusion limit. It is important to notice that large variations in ‘the rates of dimerization exist and here it is mandatory to correct the misconceptions presented in ref. 21, p. 1983 that superimposable voltammograms means an equal rate of dimerization and that reversible potentials are “corrected” to irreversible potentials. Both actions are wrong! Bordwell used the “uncorrected” oxidation potentials of 9-substituted fluorenide ions to construct a correlation between their basicity and oxidation potential [211. This correlation should be improved if correct standard potentials, stripped from the kinetic perturbation, are used instead. Acidity-oxidation potentials (AOP) were calculated in order to discuss the polar and steric effects of the substituents on radical stability. The largest effect of using standard potentials instead of irreversible oxidation potentials will be seen when the differences in AOP between 9-aminofluorenide ions and other 9-substituted fluorenide ions are considered. The latter AOPs should be corrected by up to 9.6 kJ mol-‘, but this correction will only underline the conclusion that the order of radical-stabilizing effects is (CH,),N- > CH,O- > CH, > H. The success of the AOP method does depend however on the effects of the substitution being polar and not steric in nature. The breakdown of this method is illustrated in ref. 45 where differences in AOP values are discussed for 9-alkylfluorenide ions. Large differences in AOPs are observed between CH,-, (CH,),CHand (CH,),Cwhich stem from steric effects on both the acidity (steric inhibition of solvation) and the peak oxidation potential. The introduction of standard potentials eliminates the steric effects on the potentials and it is observed that the differences in AOPs are diminished (AAOP I 14.6 kJ mol-i). The remaining differences are probably due to steric effects on the pK, values.

Ekxdwell also used the oxidation potentials in the Marcus relation, eqn. (6) [22,39]. The corrected k,(corr), based on the standard potentials, may be calculated from the Bordwell values by a linearized approximation of the Marcus relation eqn. (21) (with the assumption (AG”/A)’ K 1): ln( k,)/k,(corr))

= FAE/2RT

(21) In eqn. (21) AE = E” -E,(V), where E” is the standard potential and E,,p is the peak oxidation potential obtained by Ekxdwell (see Table 3). For 9-methylfluorenide, AE = 0.123 V and k&x-r) = 0.09 km. The k, for the fluorenide ions calculated on the basis of irreversible oxidation potentials may therefore be too high by a factor of l-10. A correction in this range, however, will not affect the general conclusions obtained by the kinetic test of electron transfer theory [22,39]. 5. Summary 9aminofluorenyl radicals are found to be stable owing to spin delocalization to the nitrogen present and it was possible to measure the standard potentials for the 9-aminofluorenide ion and the 9aminofluorenyl radical. The former potentials are quite close to the oxidation potentials published by Bordwell. For the other group of 9-substituted fluorenide ions we find that the corresponding fluorenyl radicals dimerize. If the steric hindrance is not too severe, two fluorenyl radicals will dimerize and 9,9’-disubstituted9,9’-bifluorene is obtained. When steric hindrance becomes more important the dimerization in the 9-position becomes slower and an alternative dimerization of one fluorenyl radical and one fluorenide ion results in the anion radical of the bifluorene. This anion radical is finally oxidized by a fluorenyl radical to the bifluorene in a rate-determining solution electron transfer. References 1 K. Daasbjerg, S.U. Pedersen and H. Lund, Acta Chem. &and., 43 (1989) 876. 2 K. Daasbjerg, J.N. Hansen and H. Lund, Acta Chem. Stand., 44 (1990) 711. 3 T. Lund and H. Lund, Acta Chem. Stand. B, 40 (1986) 470. 4 M. Mohammad and E.M. Kosower, J. Am. Chem. Sot., 93 (1971) 2709. 5 M. Mohammad and E.M. Kosower, J. Am. Chem. Sot., 93 (1971) 2713. 6 T. Lund and H. Lund, Acta Chem. Stand. B, 41 (1987) 93. 7 K. Daasbjerg, T. Lund and H. Lund, Tetrahedron Lett., 30 (1989) 493. 8 L. Eberson, Electron Transfer Reactions in Organic Chemistry, Springer, Berlin, 1987. 9 E. Lamy, L. Nadjo and J.-M. Savbant, J. Electroanal. Chem., 42 (1973) 189.

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