J. Am. Chem. SOC.1993, 115, 12276-12289
12276
Polycyclic Bis( hydrazine) and Bis( hydrazyl) Radical Cations: High and Low Inner-Sphere Reorganization Energy Organic Intervalence Compounds Stephen F. Nelsen,' Hao Chang, J. Jens Wolff,' and Jan Adamus* Contribution from the S. M . McElvain Laboratories of Organic Chemistry, Department of Chemistry, University of Wisconsin, 1 1 01 West University Avenue, Madison, Wisconsin 53706- 1396 Received March 24, 1993. Revised Manuscript Received October 13, 1993"
Abstract: Four-u-bond-linked bis(hydrazine) radical cations s3'+, a3*+, and a8'+ show broad visible absorption bands with A, = 512-548 nm in CH3CN at room temperature, attributed to Hush-type charge-transfer bands (transition energies E,, = 52.2-55.8 kcal/mol). Thecorresponding bis(hydrazy1) radical cations s2*+, a2.+, and a?+ show near-IR absorption with A, = 1062-1 199 nm (E,, = 26.9-29.3 kcal/mol). The large difference in E,, is caused by innersphere reorganization energy differences, which are predicted well by AM 1 semiempirical molecular orbital calculations. Hush analysis of the absorption bands produces electronic coupling matrix elements J = 3.5 f 0.5 kcal/mol for these species, and Marcus-Hush theory predicts intramolecular electron-transfer rate constants which are consistent with the experimental observation that E T is slow on the ESR time scale for the hydrazines and fast for the hydrazyls. The bis-inner hydrazyl radical cation 13'+ exhibits a near-IR absorption band at A,, = 850 nm which is narrower than those of 2*+and 7*+and is concluded not to be a Hush-type charge-transfer band. Introduction
Scheme I
The free energy barrier (AG*)to thermal electron transfer (ET) in Marcus theory is one-quarter of the free energy barrier for vertical ET (A), in which the electron is transferred between the components without allowing any relaxation.' X is the sum and internal geometry reorgaof solvent reorganization (but) nization (Xin) terms (eq 1). We shall consider only the theoretically X = Aout
+ A,
(1)
simple case of "self-ET" reaction between a neutral molecule and its own radical cation, for which AG" = 0, and the electrontransfer rate constant k,, is given by the product of a preexponential term and the exponential term containing X (eq 2). We
k,, = PRE exp(-A/4RT)
1
I
MeLi
1
MeLi
(2)
have particularly studied self-ET reactions of hydrazines, which have an unusually large geometry change upon electron loss, causing a large Xi, value and hence unusually small k,, values.2 However, X cannot be directly evaluated from intermolecular ET work, as several other quantities also go into determining kex.An important step in separating these quantities is measuring X for hydrazine ET reactions, which we hoped to do by making the self-ET intramolecular instead of intermolecular. We recently reported a practical conversion of tetracyclic bis(azo) compound 1 to the bis(tert-butyldiazenium) salts 2 and Present address: Organisch-Chemisches Institut der Universitlt Heidelberg, Im Neuenheimer Feld 270, D-6900 Heidelberg, FRG. t Present address: Institute of Applied Radiation Chemistry, Technical University (Politechnika) tMZ, Wr6blewskiego 15, Poland. e Abstract published in Advance ACS Abstracts, December 1, 1993. (1) For recent eeviews of electron-transfer theory, see: (a) Sutin, N. Prog. Inorg. Chem. 1983,30,441. (b) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta. 1985, 811, 265. (2) For reviews of hydrazine electron-transfer work, see (a) Nelsen, S. F. Acc. Chem. Res. 1981, 14, 131. (b) Nelsen, S. F. In Molecular Structures and Energetics; Liebman, J. F., Greenberg, A., Eds.; VCH Publishers, Inc.: Deerfield Beach, FL, 1986; Vol. 3,Chapte.r 1, pp 1-86. (c) Nelsen, S. F. In Advances in Electron Transfer Chemistry, Vol. 3, Mariano, P.S., Ed..: JAI: Greeenwich, CT, in press.
bis( tert-butylmethylhydrazines) 3, as outlined in Scheme 1.3 These compounds are of interest here as precursors to radical cations which are organic analogues of the mixed-valence transitionmetal complexes4for which Hush theory5has proven so successful in understanding electron-transfer properties. According to Hush theory, a charge-transfer (CT) optical absorption corresponding to vertical electron transfer (ET) between the redox centers will be observed if electronic interaction at the transition state is large enough. The transition energy for the charge-transfer band, E,,, is the energy gap between the ground state, which has both redox centers and their solvation shells relaxed, and the C T state with an electron transferred but no relaxation. E,, is an experimental measure of the Marcus X value.' (3) Nelsen, S. F.; Wolff , J. J.; Chang, H. J. Am. Chem. SOC.1991, 113, 7882. (4) For a review, see: Creutz, C. Prog. Inorg. Chem. 1983, 30, 1. ( 5 ) (a) Hush, N. S. Trans. Faraday SOC.1961,57,557. (b) Allen, G. C.; Hush, N. S. Prog. Inorg. Chem. 1967, 8, 357; 391. (c) Hush, N. S. Coord. Chem. Rev. 1985,64, 135. (d) Hush, N. S. In Mixed Valence Compounds; Brown, D. B., Ed., Reidel: Dordrecht, the Netherlands, 1980; p 151.
0002-7863/93/1515-12276$04.00/00 1993 American Chemical Society
Polycyclic Bis(hydrazine) and - (hydrazyl) Radical Cations In this work we hoped to measure X using the Hush method for intramolecular ET in bis(hydrazine) radical cations, shown as A*+ below. Comparison of A*+ with bis(hydrazy1) radical
J . Am. Chem. SOC.,Vol. 115, No. 26, 1993 Scheme I1
1 B*+ cations Bo+was also an important goal of this work. The spinbearing dinitrogen units of A*+* and B + are similar, with the odd electron in a x*(NN) orbital. Their diamagnetic dinitrogen units are very different, the hydrazine unit of A’+ having pyramidal nitrogens and a long N-N single bond length and the diazenium cation unit of Bo+ a planar tert-butylated nitrogen and a short N-N double bond length. Clearly, smaller geometry change upon vertical ET occurs for Bo+than for A’+. The geometry change in B+ET ought to be more like the second (radical cation, dication) ET of hydrazines than the first ET, and Xi, should be smaller for B+.We showed in an intermolecular case that the second ET for a sesquibicyclic hydrazine is faster than the first despite the work term necessary for approach of a dication to a cation.’ We hoped to establish whether the prediction of very different optical properties for A*+and B’+ is true and to examine the application of Hush theory to these organic radical cations, which we expected to differ from mixed-valence transition-metal compounds by having much larger EOpvalues(for A’+) and higher frequency vibrational couplings. We previously investigated the oxidation of several bis(hydrazines),* but the connecting links between the nitrogens in the compounds studied were too flexible to define the radical cation geometry very well, and these studies were plagued by short radical cation lifetimes. The rigidity of the double, four-a-bond linkages of 2’+ and 3*+were expected to provide relatively long lifetimes. Here we study 22+and 3 and some analogues prepared from other bis(azo) compounds, comparing their ET thermodynamics measured by cyclic voltammetry and the spectral properties of their radical cation oxidation states with Marcus-Hush theory.
Compound Preparation As discussed previ~usly,~ we prepared 1 by Shen’s route,9 hydrogenating diene 4 (the basketene, bis(diethy1azodicarboxylate) adduct, see Scheme 11) before hydrolysis and oxidation. We also repeated Shen’s 2 2 photoclosure prior to azo compound formation, producing his hexacyclic bis(azo) compound 6, and converted 6 to the hexacyclic diazenium salts 72+ and hydrazines 8 as outlined in Scheme 11. Separation of the diazenium salts 72+ by crystallization proved more difficult than for the 22+saltsS3 Although a72+free from the syn isomer by I3C N M R was obtained by multiple crystallizations, our sample of s72+ contained about 15% of the anti isomer. Methylation of a72+ produces hexacyclic hydrazine a8, which in CDC13 is a mixture of two symmetrical conformations (eight carbons each by 13C) and a minor, unsymmetrical one (two tBu CCH3 by l3C NMR, several of the
+
(6) (a) Nelsen, S.F.; Landis, R. T., 11. J . Am. Chem. SOC.1973,95,2719. (b) Nelsen, S. F.; Landis, R. T., 11. J. Am. Chem. SOC.1974, 96, 1788. (c) Use of tetamethylammonium thiophenylate as the reductant for 21+ results in formation of a weak purple color caused by a species with A, of 520 nm, which is apparently not a radical because the ESR spectra are similar for the purplish solutions generated by thiophenylate reduction and the yellowish ones generated by electrolytic reduction. It seems possible that this color is caused by charge transfer from residual PhS-21+ pairs, but this point has not been investigated here. (7) Nelsen, S. F.;Blackstock, S.C.;Kim, Y. J. Am. Chem. SOC.1987,109, 671. (8) Nelsen, S. F.;Willi, M. R.; Mellor, J. M.; Smith, N. M. J . Ora. Chem. 1986,’51, 2081. (9) . , (a) . , Shen. K. W. J . Chem. Soc.. Chem. Commun. 1971.391. (b) Shen, K. W. J . Am. Chem. Soc. 1971, 93, 3064 ’
12211
light Y-E
l)KOH,iPrOH
:X@ZN-E
P)CuCI,;NH,OH
5
6
I
8
IBuOH,HBF,
72*
less intense of the 14 other carbons which must be present not clearly observed). Only trans-dialkylated hydrazine units are tau
a8:
Oi,Oi
l0,lo
Oi,lo
reasonable conformations for a8.3 The unsymmetrical conformation can only be Oi,Io (and its enantiomer Io,Oi), while the symmetrical ones are the bis-tert-butyl “Out” and “In” conformations OiOi and IOIO. Integrations of the IH N M R NCH3 and CCH3 signals were consistent with mole fractions 0.565:0.290: 0.145 f 0.005, making their relative free energies 0.0, 0.4, and 1.2 kcal/mol. These conformational isomers of a8 are much closer in energy than are thoseof a3, where only OiOi is detectably occupied by NMR,3 doubtless because ring torsion is less in the hexacyclic a8 than in the tetracyclic a3. MM2 molecular mechanics calculationslo for these conformations give the following relative strain energies: for a3,0i,Oi 0.0, 10,Io 1.6, and Io,Oi 4.3 kcal/mol; and for a8,0i,Oi 0.0, 10,Io 0.4, and Io,Oi 1.8 kcal/mol, in apparent excellent agreement with experiment.” We failed to separate pure enough s8 from the methylation product of the 85% s72+mixture for meaningful spectral measurements. 22+/3 and 72+/8 share the dinitrogen units being substituted outer on the bicyclo[2.2.2]octane units fused at the central C-C bond. We used Prinzbach’s proceedure to convert Vogel’s methano-bridged cyclodecapentenel2 9 to the bis(inner azo) compound l2,13 which was tert-butylated to the anti bis(diazenium) salt 13*+ (see Scheme 111). We did not observe N M R peaks attributable to either syn isomer of 132+,and we (10) (a) MM2 calculations: Allinger, N. L. J . Am. Chem. Soc. 1977, 99, 8127. Allinger, N. L.; Yuh, Y. QCPE Bull.1980, 12, 395. (b) The sucess of MM2 calculations on predicting relative energies of conformations of these compounds is a result of their special conformational constraints; see: Nelsen, S.F.; Wang, Y.; Powell, D. R.; Hayashi, R. K. manuscript submitted for publication. (1 1) (a) Unfortunately, the lack of resolution of the central bond and fourmembered-ring C H signals, even at 500 MHz, precludes confident use of NOSEY experiments, which were used to prove that a3 is in the Oi,Oi conformation.3 to experimentally verify the identification of A with Oi,Oi. The chemical shift difference between the four-membered-ring CH signals is very different for the two isomers (6 0.08 for A and 6 4.6 for B), while that between the CHIsignals of a 3 is 6 5.6 , which would seem more consistent with B being Oi,Oi than with A, but it does not prove it. (b) CCCC twist angles at thecentral bond: (83) Oi,Oi 11.9’, Io,Io9.0°, Io,Oi 3.5 and 9.5’; (88) Oi,Oi 4.9‘, 10,Io 4.9’, 10,Oi 1.1 and -0.9’. (12) Vogel, E.; Klug, W.; Breuer, A. Org. Synth. 1974, 54, 11. (13) Prinzbach, H.; Fischer, G.;Rihs, G.; Sedelmeier,G.; Heilbronner, E.; Yang, Z . Z. Tetrahedron Lett. 1982, 23, 1251. (14) Ammar, F.; SavCant, J. M. J . Electroanal. Chem. Interfacial Electrochem. 1973, 47, 115; 215.
12278 J. Am. Chem. SOC.,Vol. 115, No. 26, 1993
Nelsen et al. Table I. Cyclic Voltammetry Data in Acetonitrile
Scheme 111
AEIO b
compd 14 (“mon”) s3 (tet) a3 (tet) a8 (hex)
I 1 tBu
15 (“mon”) 16 s2 (tet) a2 (tet) 97 (hex)c a7 (hex) 13 (tet inner)
4.89 [4.85]/4.91 4.88 [4.81]/4.87 5.0114.98
-0.795, -0,645, -0).745,4.40 -0.755,-0).41 -0.765, -0.37 477,4395 -0.76, -0.04
+7.1 +7.1 +8.3 +8.2 +15.8
4.8514.83 4.85 [4.74]/4.84 4.9514.94 4.9514.95 2.9212.84
EO
+3.5 +1.6 +1.4 +1.2 +1.0 +1.3
tBu
were unsuccessful in attempts to isolate a hydrazine from the reaction of 132+with methyllithium.
Results end Discussion Cyclic Voltammetry. The thermodynamics for electron removal from A and B were studied by cyclic voltammetric determination of their formal redox potentials, Eo. Although the hydrazines were scanned through two oxidations followed by two reductions and the diazenium salts through two reductions followed by two oxidations, it is most convenient to compare them starting from the neutral forms. The electron transfers are all chemically reversible, exhibiting equal oxidation and reduction currents for both waves. They are electrochemically quasireversible, the hydrazines exhibiting 60-80 mV peak-to-peak separations (AE,,) and the diazenium salts 60-85-mV separations in acetonitrile a t E 3 / 2 for slow scan rates. The Eo values reported are (E: cases exhibiting well-separated waves and were determined by comparing the experimental curves with simulations of overlapping quasireversible waves when AE‘ was under 0.2V. The CV data in acetonitrile are summarized in Table I, and data in dimethyl sulfoxide (for B) and methylene chloride (for A) are discussed in the supplementary material. A compound with two equivalent, noninteracting redox sites shows a cyclic voltammogram having a single wave of the same AE,, and shape but twice the current as that for a compound with only one redox site at the same concentration. The observed wave is caused by the overlap of two one-electron waves having - E l 0 of 35.6 mV at 25 a statistically determined AEo = 0C.15J6We therefore tabulate values of AElo (eq 3) in Table
+
Alllo,V = Elo- Eo(mono) + 0.018
(3)
I to allow proper comparison of the ease of removal of the first electron from the bis systems with that for the“monomeric” single redox sitecompounds, hydrazine 14 and hydrazyl15’O. The AElo values are small and negative for the tetracyclic hydrazines s3 and a3, but the hexacyclic hydrazine a8 is 1.4 kcal/mol harder to oxidize than 14, possibly because the hexacyclic framework (15) Flanagan, J. B.; Mangel, S.; Bard, A. J.; Anson, F. C. J . Am. Chem. 1978, loo, 4248. (16) Rehm, D.; Weller, A. Z . Phys. Chem., N . E . 1970, 69, 183.
SOC.
+2.6 +2.6 +4.3
a In acetonitrile containing 0.1 M n-BurNC104, Pt electrode, room temperature,vs a saturated calomel electrode. b Difference in Elo from “monomeric”compound, correctedfor statistical effect using eq 3 of the text. Difference in Eo values for second and first electron removal, corrected for statistical effect using eq 4 of text. Average of closest N-N distances in neutral hydrazines and diazenium dications; after slash, in the radical cations. 15% a7 in sample.
U
tBuOH,HBF,
1s2+
-0.5 -0.9 +1.4
+0.06, +0.21 +0.04, +0.18 +0.14,+0.36
N-N distanced
(A), AMl[X-ray]
Hydrazyls
PdlC,H2
U
,ojc
(kcall (kcall Elo, E 2 O * (V) mol) mol) Hydrazines +0.10, EO
14
16+
15*
restricts torsion. A E 1 O values areal1 positive for the bis(hydrazy1s) and similar in size for the tetracyclic and hexacyclic compounds. A smaller Mor range seems reasonable because the hydrazyl/ diazenium cation electron loss causes less of a geometry change than the hydrazine/hydrazine radical cation oxidation, so differences in flexibility of the polycyclic compounds should be less important. The neutral tetracyclic and hexacyclic bis(hydrazy1s) differ from 15’0 by having hydrazyl nitrogens four u bonds away from the hydrazyl being oxidized. That detectable effects can be transmitted through four u bonds is indicated by the 3.5 kcal/ mol more positive Eovalue for the azo-substituted monohydrazyl derived by reduction of 16+ than for 15’O. The more planar hydrazyl nitrogens may be more electron-withdrawing than hydrazine nitrogens and also contribute to the observed more positive AEIo values for bis(hydrazy1s) than for bis(hydrazines). The differences between the two oxidation potentials, corrected (see eq 4), are for the statistical effect, which we will call Mor Mor, kcal/mol = 23.O6(E2O - E,’ - 0.036)
(4)
also given in Table I. Mor ought to be affected by electrostatics (removal of the second electron requires a “through space” electrostatic work term, AE,’, because the oxidized dinitrogen unit bears a positive charge), “through bond” effects the charges in the dication will also interact through the u-bonds connecting the dinitrogen units), and ionpairing/solvation effects, AEipo (see eq 5 ) .
+
+
A E O ’ = AE,o AEtbO AEipo (5) The distances between the dinitrogen units of these compounds are constrained by the polycyclic connecting units, and the N-N’ distances d estimated by A M 1 semiempirical molecular orbital calculations are compared with X-ray geometric data for the three available cases in the last column of Table I. The AM1 distances are slightly larger than the X-ray distances, but the differences are rather small, and the relative sizes of the d values should be reliable. The dinitrogen units are held nearly parallel to each other, and it makes no significant difference in estimating
Polycyclic Bis(hydrazine) and - (hydrazyl) Radical Cations
J. Am. Chem. SOC.,Vol. 115, No. 26, 1993
12279
Figure 1. (Top) ESR spectrum of the anti hexacyclic hydrazine radical cation a@+in 1:l acetonitrile/butyronitrileat 87 O C , and (bottom) a simulation for a slow exchange spectrum (see text).
the work term whether full charges at the average dinitrogen distancequoted or contributions of charges of 112 at each nitrogen are summed. We use the averaged AM1 d values in this work. The electrostatic work term for two point charges separated a t a distance d (A) in a medium of dielectric constant E is given by eq 6. It has been traditional to use thesolvent dielectric constant
W = e2/dc = 3 3 2 . 1 / d ~
(6)
E, and the distance between centers when the molecules touch in the work term for electron-transfer considerations, as for example in the Rehm-Weller equationI6 and in Marcus theory.' The c,(25 "C)value for CHsCN is 35.94 D. As Suppan has pointed out for C T ion pairs,'' it seems unrealistic to expect that an
intermolecular electrostatic work term would be as small as that obtained using the bulk solvent E ~because , if the molecules touch, there is no solvent between them. Suppan suggests employing the much larger number obtained by using a hydrocarbon e,value, about 2 D, as the effective dielectric constant, e C ~for , use in eq 6 toestimate the workterm. Thisappearsunrealistic tous because solvent is present to mediate the interaction between the charges, although not directly between the charged units. The large A E O ' for 13 is presumably principally caused by the close proximity of the inner diazenium cation units in the dication, which is only consistent with an ~,ffvaluesignificantly lower than that for bulk acetonitrile. Because the connecting u bonds are not in the favorable anti arrangement for strongest through bond inter(17) Suppan, P. J . Chem. SOC.,Faraday Trans. 1 1986, 82, 509.
Nelsen et al.
12280 J. Am. Chem. SOC.,Vol. 115, No. 26, 1993
07” -13OC
acetonitrile
Y
3530
3340
3350
3360
3370
3330
IC1
Figure 2. (Top) ESR spectrumof the anti hexacyclic hydrazyl radical cation a?+ in acetonitrileat -1 3 O C , with (bottom)a simulation for fast exchange [a(4N) = 5.5 G, line width 1.0 GI.
action,Sc the contribution of AEtboto A E O ’ cannot reasonably be larger than that for a2’+, and the minimum AEWocontribution ought to be ca. 11.3 kcal/mol. The minimum effective distance between the dinitrogen units reasonable to use is the 2.84-A value of d, so use of eq 6 to estimate the effective e value in acetonitrile produces e,ff 110.3 D, or 3 X lo8 6.8 X lo7
