Slow electron transfer reactions involving tetraisopropylhydrazine

Full text of DOI:10.1021/ja9537080

J. Am. Chem. Soc. 1996, 118, 1555-1556
1555
Chart 1
Slow Electron Transfer Reactions Involving
Tetraisopropylhydrazine
Stephen F. Nelsen,*,† Rustem F. Ismagilov,† Ling-Jen Chen,†
Jennifer L. Brandt,‡ Xi Chen,‡ and Jack R. Pladziewicz*,‡
S. M. McElVain Laboratories of Organic Chemistry
Department of Chemistry, UniVersity of Wisconsin
Madison, Wisconsin 53706-1396
Department of Chemistry, UniVersity of Wisconsin
Eau Claire, Wisconsin 54702
ReceiVed NoVember 3, 1995
Marcus pointed out in 1956 that the fundamental parameter
to know for predicting outer-sphere electron transfer (ET)
activation free energies is λ, the vertical free energy gap between
a non-interacting pair consisting of a neutral species M0 and
its own radical ion (M+ or M-) in solution, and the same pair
in which an electron has been transferred between the two
components without allowing any relaxation.1 If the relaxed,
solvated neutral and radical cation are designated by n and c,
and the charge present is shown as a superscript, this energy
gap for vertical “self-ET” between n0 and c+ may be written as
the sum of the relaxation energies for cationic and neutral
species (eq 1).2 λ corresponds to 4 times the thermal barrier
for ET,
removal has been documented,5 and k11 values have been
measured by slow exchange region NMR line broadening.7 The
nitrogen lone pair, lone pair dihedral angle, θ, is near 0° for
21/21 and 22/u22, which have the highest k11 values reported,
1.85 × 104 and 1.21 × 104 M-1 s-1, respectively.7c 21/21 has
k11 26 times that for the slightly twisted 22/227 and 8.4 times
that estimated for the θ ) 180° hydrazine 33N)2 from NMR
line broadening studies in CD2Cl2.8 λin should increase
significantly when θ changes greatly between n0 and c+, as
shown by measurements of the enthalpy portion of ∆Grel(cat)
using photoelectron spectroscopy and high-pressure mass
spectrometry,9 but how large the effect might be remained
unmeasured until this work. By these criteria, the θ ≈ 90°
tetraisopropylhydrazine, iPr2N)2, should have a larger λ and
lower k11. We recently showed by isolating iPr2N)2+ that it is
not necessary to have Bredt's rule protection to produce isolable
hydrazine radical cations; four R-branched substituents provide
sufficient kinetic protection from CR-H bond cleavage reac-
λ ) ∆Grel(cat) + ∆Grel(neu) ) [∆Gf(n+) - ∆Gf(c+)] +
[∆Gf(c0) - ∆Gf(n0)] (1)
assuming that ET is adiabatic. This concept has remained
essentially unchanged for 40 years, although more modern ET
theory has introduced other parameters which are also important
in determining the ET rate constant.1 λ is assumed to be the
sum of a solvation term, λout, which is determined by molecular
size and the solvent employed, and a solvent-independent
structural reorganization term, λin. λin is rather small compared
to λout for many aromatic organic molecules, which results in
self-ET rate constants k11 (rate constants reported here are at
25 °C in CH3CN containing 0.10 M tetrabutylammonium
perchlorate) above 108 M-1 s-1 for many aromatic compounds.3
For the particularly well studied tetramethyl-p-phenylenedi-
+
tions.10 The formal reduction potential for iPr2N)2 of +0.26
V (vs a saturated calomel electrode in acetonitrile containing
0.1 M tetraethylammonium perchlorate, abbreviated E°′ below)
is high enough for its radical cation to be a convenient oxidant
for other compounds, allowing the cross-ET rate constant k12
to be measured by stopped-flow techniques, as we have
previously reported for other ET reactions involving hydra-
zines.8,11 Marcus showed that if ET is assumed to be adiabatic
and λ12 for a cross-ET is assumed to be the average of λ11 and
amine, TMPD, k11 is 1.5 × 109 M-1 s-1 4
. λin increases as the
structural change between n0 and c+ increases, and tetraalkyl-
hydrazines undergo especially large structural changes upon
electron loss, resulting in far higher λin values and much smaller
k11 values.5 It is necessary to have mutual stability of both
oxidation states for direct measurement of k11, which requires
special alkyl groups for tetraalkylhydrazines. Several bis-
(bicycloalkyl) hydrazines which give isolable radical cations
because of Bredt’s rule protection from CR-H cleavage6 have
been made, the size of their structural changes upon electron
λ
22 for the two components, k12 is given by eqs 2 and 3, where
Z
12 is the preexponential factor for the mixed ET.12 Using eqs
k12(calc) ) (k11k22K12f12)1/2
(2)
(3)
ln f12 ) (ln K12)2/[4 ln(k11k22/Z122)]
2 and 3 with Z12 ) 1011 M-1 s-1 works rather well for ET
reactions between a wide variety of transition metal coordination
complexes for which the kii values were independently deter-
mined.1 Equation 2 is quite insensitive to the Z12 employed
unless K12 is very different from 1.8 Nevertheless, one might
expect deviations when the ET partners differ greatly electroni-
† UWsMadison.
‡ UWsEau Claire.
(1) For reviews of ET theory, see: (a) Marcus, R. A.; Sutin, N. Biochim.
Biophys. Acta 1985, 811, 265. (b) Sutin, N. Prog. Inorg. Chem. 1983, 30,
441.
(2) (a) Nelsen, S. F.; Blackstock, S. C.; Kim, Y. J. Am. Chem. Soc. 1987,
109, 677. (b) AM1 calculations give λ′in (the enthalpy portion of eq 1 in
the absence of solvent) of 35.2 kcal/mol for 21/21 and 55.5 for iPr2N)2, a
difference of 20.3 kcal/mol.
(3) Eberson, L. AdV. Phys. Org. Chem. 1982, 18, 79.
(4) (a) Grampp, G.; Jaenicke, W. Ber. Bunsen-Ges. Phys. Chem. 1984,
88, 325. (b) Grampp, G.; Jaenicke, W. Ibid. 1984, 88, 335. (c) Grampp,
G.; Jaenicke, W. Ibid. 1991, 95, 904. (d) Grampp, G.; Nelsen, S. F.
Unpublished results.
(5) For reviews of hydrazine ET chemistry, 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, Chapter 1, pp 1-86.
(6) Nelsen, S. F.; Kessel, C. R.; Brien, D. J. J. Am. Chem. Soc. 1980,
102, 702.
(7) (a) Nelsen, S. F.; Blackstock, S. C. J. Am. Chem. Soc. 1985, 107,
7189. (b) Nelsen, S. F.; Kim, Y.; Blackstock, S. C. Ibid. 1989, 111, 2045.
(c) Nelsen, S. F.; Wang, Y. J. Org. Chem. 1994, 59, 1655.
(8) (a) Nelsen, S. F.; Chen, L.-J.; Ramm, M. T.; Voy, G. T.; Powell, D.
R.; Accola, M. A.; Seehafer, T. R.; Sabelko, J. J.; Pladziewicz, J. R. J.
Org. Chem., in press. (b) The effect is largest for the smaller f12. Using
22/u22 as reductant, use of Z12 ) 109 M-1 s-1 increases k11(calc) by 59%,
and use of 1013 decreases it by 36%. The corresponding changes are e+12%
and -9% for the reductants with smaller ∆∆G°.
(9) Nelsen, S. F.; Rumack, D. T.; Meot-Ner (Mautner), M. J. Am. Chem.
Soc. 1988, 110, 7945.
(10) Nelsen, S. F.; Chen, L.-J.; Powell, D. R.; Neugebauer, F. A. J. Am.
Chem. Soc. 1995, 117, 11434.
0002-7863/96/1518-1555$12.00/0 © 1996 American Chemical Society

1556 J. Am. Chem. Soc., Vol. 118, No. 6, 1996
Communications to the Editor
+
0/+
Table 1. Comparison of Stopped-Flow Cross Rate Studies using iPr2N)2 as Oxidant with the Directly Measured Value for iPr2N)2
reductant
∆G0, kcal/mol
k22(obs), M-1 s-1
k12(obs), M-1 s-1
f12(calc)
k11(calc), M-1 s-1
TMPD
-3.2
-3.1
1.5 × 109 a
8.5 × 106 b
2.9 × 107 c
1.9 × 104 b
2.2 × 103 b
1.2 × 104 c
7.0 × 102 d
(1.6 ( 0.1) × 103
(1.2 ( 0.1) × 104
(4.9 ( 0.6) × 104
(2.6 ( 0.02) × 102
(1.1 ( 0.2) × 102
(1.1 ( 0.1) × 104
(3.2 ( 0.2) × 105
0.82
0.84
0.29
0.35
0.26
0.15
0.01
5.1 × 10-4
Cp*CpFe
18 × 10-4
Cp* Fe
-8.5
4.0 × 10-4
3.5 × 10-4
2.6 × 10-4
2.3 × 10-4
6.4 × 10-4
6.0 × 10-4 (av)
30 × 10-4 (obs)
2
21/21
33N)2
22/u22
22/22
-5.8
-6.2
-11.6
-18.2
iPr2N)2-d6
0
3.0 × 10-3
a From ref 4b. b From ref 8a. c From ref 11. d From refs 7.
Our work demonstrates that the use of Marcus cross rate theory
0/+
produces k11(calc) values for iPr2N)2 within a factor of 13
of the correct value for compounds which differ tremendously
in k22, range electronically from being diaminobenzenes to two-
atom π-centered to iron-centered radical cations, and range
sterically from having an unsubstituted cyclopentadienyl π-sys-
tem to having a dinitrogen π-system which is blocked by the
presence of four R-branched alkyl substituents. Electronic
factors appear to be surprisingly unimportant in determining
k12(obs), and the reaction with the aromatic amine TMPD, which
has k22(obs) 5 × 1011 times faster than iPr2N)2, gives a k11-
(calc) value which is closer to the experimental value than do
reactions with three of the four hydrazines studied, which are
far closer in structure to iPr2N)2. Higher k11(obs) values than
k11(calc) from cross rate studies have resulted for all hydrazines
studied.8,11 The smallest ratios of k11(obs) to k11(calc) have been
found for reactions involving Cp*CpFe: the ratios are 2.2 for
33N)2, 2.0 for 22/u22, and from this work, 1.7 for iPr2N)2. We
lack an internally self-consistent explanation13 for why k11(calc)
values are smaller than those obtained by direct measurement.
Recent work concludes14 that several classes of organic
electron transfer reactions occur by inner-sphere mechanisms,
including self-exchange reactions of alkyl halides,15 aromatic
radical anion/neutral exchange,15 and aromatic nitration.16 This
work compels us to consider the possibility that the reactions
considered here are influenced by partial bond formation in the
transition state sufficient for their classification as inner-sphere.
Several characteristics of the iPr2N)2 reactions make this
unlikely. In addition to there being no direct evidence for
bonded intermediates, the nitrogen atoms of iPr2N)2 are
sterically hindered by the isopropyl groups in a manner that
greatly restricts close approach of the reactants used in this
study. Moreover, the agreement of the directly measured k11
for iPr2N2 with those derived from the varied cross-reactions
would require that all of the cross-reactions have approximately
the same degree of bond formation in the transition state and
be slightly less bonded than the iPr2N)2 self-exchange. This
would be a remarkable coincidence and seems unlikely.
However, given the importance of this mechanistic question,
even more stringent tests of the assumptions implicit in our
treatment of the iPr2N)2 reactions are being sought and will be
discussed in the future.
+
Figure 1. Plot of ∆Gq11(calc) versus ∆G° for the reduction of iPr2N)2
by the seven reductants of Table 1, along with the observed value.
cally, because the parameters V and hνin used in more modern,
diabatic (“nonadiabatic”) ET theory1 ought to vary, causing the
preexponential factors for k11 and k22 to differ. It is not at all
clear how Z12 ought to be predicted in such a case. Hydrazines,
ferrocenes, and TMPD have all been argued to have significantly
diabatic ET,7c but eqs 2 and 3 with Z12 ) 1011 M-1 s-1 have
been shown to work as well for tetraalkylhydrazine-methylated
ferrocene cross-ET reactions8,11 as they do for reactions between
transition metal coordination complexes. The most stringent
tests of the λ12 ) (λ11 + λ22)/2 assumption occur when the
components differ greatly in λin, so that k22 is very different
from k11. This effect has been examined in this work, where
TMPD, methylated ferrocenes Cp*CpFe and Cp* Fe, and the
four hydrazines shown above have been used as reductants of
2
+
iPr2N)2
. The cross ET rate constants, k12(obs), for these
reactions are summarized in Table 1, and a plot of the self-ET
activation free energy, ∆Gq11(calc), versus cross-reaction free
energy, ∆G°, is shown in Figure 1. In spite of great differences
in the intrinsic reducing agent reactivity (the range of reducing
agent k22 values is a factor of 2.1 × 106), the k11(calc) values
derived for iPr2N)2 from k12(obs) are very similar, agreeing
within a factor of 8. Moreover, k11(calc) is quite insensitive to
the substantial differences in electronic and molecular structure
of the reductants, and there is no significant trend with reaction
∆G° for the k11(calc) values obtained. As observed previously,8
cross-reactions employing Cp* Fe produce slightly lower k11(calc)
2
values than do reactions using Cp*CpFe.
0/+
The self-exchange ET for iPr2N)2 is so slow that direct
measurement of k11 was possible. Acetone-d6 was employed
to introduce one CH(CD3)2 group, producing iPr2N)2-d6, and
the concentration of labeled neutral material was monitored as
Acknowledgment. We thank the National Science Foundation for
partial financial support of this work under Grants CHE-9504133
(J.R.P.) and -9105485 and -9417946 (S.F.N.). Acknowledgement is
also made to the donors of the Petroleum Research Fund, administered
by the American Chemical Society, for partial support of the research
under Grant ACS-PRF 29982-B4 (J.R.P.).
2
1
a function of time by both H- and H-NMR upon addition of
0/+
unlabeled radical cation. k11(obs) values for iPr2N)2 were
determined from four kinetic runs to be (3.0 ( 0.3) × 10-3
M-1 s-1, 5.0 times the average k11(calc) value.
This work extends the total k11 range measured for tetra-R-
branched tetraalkylhydrazines to a factor of 5.6 × 106, a ∆Gq
difference of 9.2 kcal/mol, between the θ ≈ 0° compound 21/
21 and the θ ≈ 90° iPr2N)2. This would correspond to a λin
difference of 36.8 kcal/mol if the preexponential factor and λout
were the same for both hydrazines (which might not be the case).
The effect of θ on k11, and thus presumably on λin, for hydrazines
is clearly much larger than that calculated by the AM1 method.2b
JA9537080
(13) Lowering Z12 slightly raises k11(calc), but the effect is small: the
average k11(calc) for the data of Table 1 is 6.9 × 10-4 using Z12 of 1010,
and 8.7 × 10-4 using 109. Differing precursor complex equilibrium constants
would also affect the rate, but they would have to be rather constant between
∆G° of -3 and -18 kcal/mol, yet all differ significantly from the value at
∆G° ) 0, which appears unlikely. Supporting electrolyte was not used in
the NMR measurement.
(14) Eberson, L. New J. Chem. 1992, 16, 151.
(15) Eberson, L.; Shaik, S. S. J. Am. Chem. Soc. 1990, 112, 4484.
(16) Kochi, J. K. Acc. Chem. Res. 1992, 25, 39.
(11) Nelsen, S. F.; Wang, Y.; Ramm, M. T.; Accola, M. A.; Pladziewicz,
J. R. J. Phys. Chem. 1992, 96, 10654.
(12) Marcus, R. A.; Sutin, N. Inorg. Chem. 1975, 14, 213.