Separated versus contact ion-pair structures in solution from their crystalline states: Dynamic effects on dinitrobenzenide as a mixed-valence anion

Article abstract of DOI:10.1021/ja043998x

Qualitative structural concepts about dynamic ion pairs, historically deduced in solution as labile solvent-separated and contact species, are now quantified by the low-temperature isolation of crystalline (reactive) salts suitable for direct X-ray analysis. Thus, dinitrobenzenide anion (DNB ^-) can be prepared in the two basic ion-paired forms by potassium-mirror reduction of p-dinitrobenzene in the presence of macrocyclic polyether ligands: L_C (cryptand) and L_E (crown-ethers). The crystalline separated ion-pair salt isolated as K(L _C)⁺//DNB^- is crystallographically differentiated from the contact ion-pair salt isolated as K(L _E)⁺DNB^- by their distinctive interionic separations. Spectral analysis reveals pronounced near-IR absorptions arising from intervalence transitions that characterize dinitrobenzenide to be a prototypical mixed-valence anion. Most importantly, the unique patterns of vibronic (fine-structure) progressions that also distinguish the separated from the contact ion pair in the crystalline solid state are the same as those dissolved into THF solvent and ensure that the same X-ray structures persist in solution. Moreover, these distinctive NIR patterns are assigned with the aid of Marcus-Hush (two-state) theory to the separated ion pair in which the unpaired electron is equally delocalized between both NO₂-centers in the symmetric ground state of dinitrobenzenide, and by contrast, the asymmetric electron distribution inherent to contact ion pairs favors only that single NO ₂-center intimately paired to the counterion. The labilities of these dynamic ion pairs in solution are thoroughly elucidated by temperature- dependent ESR spectral changes that provide intimate details of facile isomerizations, ionic separations, and counterion-mediated exchanges.

Full text of DOI:10.1021/ja043998x

Published on Web 01/25/2005
“Separated” versus “Contact” Ion-Pair Structures in Solution
from Their Crystalline States: Dynamic Effects on
Dinitrobenzenide as a Mixed-Valence Anion
Jian-Ming Lu¨, Sergiy V. Rosokha, Sergey V. Lindeman, Ivan S. Neretin, and
Jay K. Kochi*
Contribution from the Chemistry Department, UniVersity of Houston,
Houston, Texas 77204-5003
Received October 1, 2004; E-mail: jkochi@uh.edu
Abstract: Qualitative structural concepts about dynamic ion pairs, historically deduced in solution as labile
solvent-separated and contact species, are now quantified by the low-temperature isolation of crystalline
(reactive) salts suitable for direct X-ray analysis. Thus, dinitrobenzenide anion (DNB-) can be prepared in
the two basic ion-paired forms by potassium-mirror reduction of p-dinitrobenzene in the presence of
macrocyclic polyether ligands: LC (cryptand) and LE (crown-ethers). The crystalline “separated” ion-pair
salt isolated as K(LC)+//DNB- is crystallographically differentiated from the “contact” ion-pair salt isolated
as K(LE)+DNB- by their distinctive interionic separations. Spectral analysis reveals pronounced near-IR
absorptions arising from intervalence transitions that characterize dinitrobenzenide to be a prototypical
mixed-valence anion. Most importantly, the unique patterns of vibronic (fine-structure) progressions that
also distinguish the “separated” from the “contact” ion pair in the crystalline solid state are the same as
those dissolved into THF solvent and ensure that the same X-ray structures persist in solution. Moreover,
these distinctive NIR patterns are assigned with the aid of Marcus-Hush (two-state) theory to the “separated”
ion pair in which the unpaired electron is equally delocalized between both NO2-centers in the symmetric
ground state of dinitrobenzenide, and by contrast, the asymmetric electron distribution inherent to “contact”
ion pairs favors only that single NO2-center intimately paired to the counterion. The labilities of these dynamic
ion pairs in solution are thoroughly elucidated by temperature-dependent ESR spectral changes that provide
intimate details of facile isomerizations, ionic separations, and counterion-mediated exchanges.
and thermodynamics8 as well as (ii) partial electron transfer
Introduction
(delocalization)9 and reactivity of cationic (D/D+•) and anionic
Intramolecular electron transfer has been extensively studied
with different donor D and acceptor A dyads connected via
molecular spacers, i.e., D(bridge)A1 (also referred to as generic
D(b)A systems2) owing to their relevance to biochemical
assemblies, electron-transfer theory, molecular electronics,
etc.3-5 Of these, the mixed-valence systems with ∆G°ET ) 0
and comprised of single donor/acceptor entities provide the
opportunity to examine systematically the structural effects of
the redox centers as well as the bridge on electron-transfer rates
in D(b)D+• or A(b)A-• systems.6,7 Indeed, the discrete character
of such organic mixed-valence systems is particularly useful in
quantitatively elucidating the influence of (i) ion-pair structures
(A/A-•) redox centers.
One of the earliest and interesting examples of an organic
mixed-valence system is dinitrobenzene anion radical (with a
-•
p-phenylene bridge connecting the NO2/NO2 redox dyads),
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9
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J. AM. CHEM. SOC. 2005, 127, 1797-1809
1797

A R T I C L E S
Lu¨ et al.
Chart 1. a
and ESR studies over the past 50 years have led to the
provocative conclusion that at least two distinctive anions exist
in solution dependent on their mode of preparation with different
countercations (M+), solvent polarities, etc.10 Species A with
the unpaired electron equally delocalized between both nitro
groups was assigned to the “free” anion (DNB-•) and/or a
loosely bound, “separated” ion pair (M+//DNB-•).11 Conversely,
the ESR spectrum of the second species (B) consisting of a
major hyperfine splitting to only a single nitro-center was
assigned to a tightly bound, “contact” ion pair (M+DNB-•).12
Recently, Nelsen and co-workers13 showed that the diagnostic
near-IR absorption band can be quantitatively analyzed by the
two-state Marcus-Hush model as a delocalized system in which
the unpaired electron resides in a single potential-energy well.
Such a dinitrobenzenide anion would correspond to the free and/
or separated ion pair consistent with the delocalized species A
observed in the early ESR studies.10,11 If so, the identity and
nature of the contact ion pair (species B) remain open for
questions.14 Moreover, it is important to emphasize that the
generalized concept of “solvent-separated” and “contact” ion
pairs pertains to binary species extant in solution,15 and the
structures of such assemblies can only be qualitatively inferred
from the solution (spectral) measurements. By contrast, X-ray
crystallography of such highly reactive ion-pair salts can provide
definitive structural information but requires the difficult
isolation of single crystals and therefore have heretofore eluded
a direct (unambiguous) tie-in to the ubiquitous solution data.
a Note these space-filling representations of potassium cation show how
(A) its entombment by [2.2.2]cryptand effectively precludes direct interionic
contact and (B) its nesting in 18-crown-6 fully shields only the back face.
made possible by the use of cyclic polyether ligands (L) during
one-electron reduction of nitrobenzene with alkali-metal mir-
rors.18 Thus, ion-pair separation is enforced via complete K+
encapsulation with the three-dimensional [2,2,2] cryptand19 so
that the anion is effectively insulated from the complexed cation
in solvent-separated form (see K(LC)+, Chart 1A).
By analogous reasoning, the [1:1] complexation of K+ with
the two-dimensional macrocyclic polyether (18-crown-6)20 is
sufficient to protect only the back face of the complexed cation
which is then free to form the contact ion pair with nitrobenz-
enide anion from the unprotected front face (see K(LE)+, Chart
1B).
The properties of such well-defined separated ion pairs (SIP)
and contact ion pairs (CIP) as crystalline salts now provide the
opportunity to examine the intrinsic characteristics of various
other reactive anions, as they are affected by the countercation
in the solid state and in solution, and, hence, their chemical
reactivity.21 To this end, the one-electron reduction of nitroarenes
is an especially fortunate choice, since the electronic structure
of the resulting anion radical can also be simultaneously probed
by ESR in combination with IR and UV-vis spectroscopy.
Accordingly, we focus here on various crystalline ion-pair salts
derived from p-dinitrobenzene reduction to establish the direct
relationship between solid-state (X-ray) structures of SIPs and
CIPs with those responsible for the ESR and electronic
We showed in Part I16 how the isolation of alkali-metal salts
of the parent nitrobenzenide (NB-•) as both the crystalline
separated ion pair and the corresponding contact ion pair17 is
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(17) (a) Our operational classification of the contact ion pair herein requires
direct interionic contact between the positive and negative centers (within
their van der Waals radii). By contrast, such a direct electrostatic interaction
between cation/anion centers is modified in the separated ion pair by
intervention of one or more solvent, ligand, etc. (b) The absence of close
contacts (less than van der Waals separation) between the crown-ethers
(or cryptand) and dinitrobenzenide indicates that there are no special
(bonding) interactions of the anion with the ligand(s).
(18) For representative examples of isolated ion pairs of aromatic anion radicals
via alkali-metal reductions, see: (a) Mooij, J. J.; Klaassen, A. A. K.; de
Boer, E.; Degens, H. M. L.; van der Hark, T. E. M.; Noordik, J. H. J. Am.
Chem. Soc. 1976, 98, 680. (b) Jost, W.; Adam, M.; Enkelmann, V.; Muellen,
K. Angew. Chem., Int. Ed. Engl. 1992, 31, 878. (c) Janiak, C.; Hemling,
H. Chem. Ber. 1994, 127, 1251. (d) Bock, H.; Arad, C.; Naether, C.; Havlas,
Z. Chem. Commun. 1995, 2393. (e) Naether, C.; Bock, H.; Claridge, R. F.
C. HelV. Chim. Acta 1996, 79, 84. (f) Naether, C.; Bock, H.; Havlas, Z.;
Hauck, T. Organometallics 1998, 17, 4707. (g) Bock, H.; Gharagozloo-
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29, 1629.
(15) (a) Szwarc, M., Ed. Ions and Ion Pairs in Organic Reactions; Wiley-
Interscience: New York, 1972; Vols. 1 and 2. (b) Streitwieser, A., Jr.
SolVolytic Displacement Reactions; McGraw-Hill: New York, 1962. (c)
Gordon, J. E. Organic Chemistry of Electrolyte Solutions; Wiley: New
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15a.
(20) Cram, D. J.; Trueblood, K. N. Top. Curr. Chem. 1981, 98, 43.
(21) (a) Loupy, A.; Tchoubar, B. Effets des Sales en Chimie Organque et
Organometallique; Dunod University: Paris, 1988. (b) Kosower, E. M.
Introduction to Physical Organic Chemistry; Wiley: New York, 1968. (c)
Kaiser, E. T.; Kevan, L. Radical Ions; Interscience: New York, 1968. (d)
Peters, S. J.; Turk, M. R.; Kiesewetter, M. K.; Reiter, R. C.; Stevenson, C.
D. J. Am. Chem. Soc. 2003, 125, 11212. (e) Batz, M. L.; Garland, P. M.;
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Separated vs Contact Ion-Pair Structures in Solution
A R T I C L E S
Chart 2
Table 1. Principal Geometric Parameters of Dinitrobenzenide (DNB-) in Crystalline Ion-Pair Salts
R,
a deg
â,
b deg
r, Å
a, Å
b, Å
c, Å
d, Å
a. Separated Ion-Pair Salt
K(LC)+//DNB-
+//DNB-
A-NO2
B-NO2
A-NO2
B-NO2
2.8
4.821(1)
5.150(1)
5.339(1)
6.312(1)
1.259(1)
1.260(1)
1.255(1)
1.255(1)
1.403(1)
1.408(1)
1.406(2)
1.407(2)
1.409(1)
1.409(1)
1.407(1)
1.408(1)
1.372(1)
1.372(1)
2.9
3.7
2.0
K(LE1
)
2
b. Contact Ion-Pair Salt
3.191(3)
K(LE3)+DNB-
A-NO2
B-NO2
37.4
47.3
1.4
1.3
0.2
1.247(3)
1.277(3)
1.251(3)
1.246(3)
1.384(4)
1.408(4)
1.398(4)
1.412(4)
1.400(4)
1.397(4)
1.359(3)
2.699(3)
5.703(3)
7.461(3)
-0.5
c. Contact Dianion Triple Salt
[K(LE2)+]2DNB2-
A-NO2
B-NO2
6.7
5.5
1.8
5.4
2.737(1)
2.747(1)
1.296(2)
1.304(2)
1.353(4)
1.346(4)
1.434(3)
1.431(3)
1.357(3)
1.391
d. Neutral Acceptor
DNBc
10.2
1.228
1.470
1.390
a R is the out-of-plane angle of the K-O bond. b â is the torsion angle O-N-C-C. c Data from Cambridge Structural Database (ref. DNITBZ02).
(UV-vis) spectra in solution, especially insofar as the ac-
companying electronic changes in dinitrobenzenide as a mixed-
valence anion.
the bis-complex K(LE1)2+ as the separated ion pair (vide supra)
- - even during crystallization from THF solutions containing a
large deficit of the ligand (relative to K+).
The ternary ion-triplet salt of the diamagnetic p-dinitroben-
zene dianion (DNB2-) was prepared as the “contact” complex
consisting of the [2:1] salt/[K(LE2)+]2(DNB2-) via the low-
temperature reduction of p-dinitrobenzene in the presence of
potassium mirror and the 18-crown-6 ligand, both in excess.
Unfortunately, we were not able to isolate the separated ion
triplet based on the [2,2,2]cryptand ligand largely owing to the
insolubility of the separated ion-pair salt.
Results
I. Isolation of Dinitrobenzenide Anion as Crystalline Ion-
Pair Salts. The successful preparation of pure ion-pair salts was
critically dependent on the presence of the polyether ligands
depicted in Chart 2 during the potassium-mirror reduction of
p-dinitrobenzene in THF solution (see Experimental Section for
synthetic details).
“Separated” ion-pair salts of dinitrobenzene anion radical
obtained when the coordination sites of K+ were fully saturated
by the presence of either (a) 1 equiv of [2,2,2] cryptand as the
complexed cation K(LC)+ or (b) 2 equiv of the crown-ether
ligand benzo-15-crown-5 as K(LE1)2+ to simultaneously protect
both its back face and its front face.
“Contact” ion-pair salts were generally more labile than
the corresponding SIP salt, and we were able to isolate only
the [1:1] dicyclohexano-18-crown-6 (LE3) complex with K+ as
a single-crystal suitable for X-ray crystallography (Table 1).
Repeated attempts to utilize the other 18-crown-6 ligands, LE2
and LE4, afforded contact ion-pair salts that were crystalline and
useful for UV-vis spectral measurements (vide infra) but
unsuited for X-ray analysis.22 Interestingly, the smaller macro-
cyclic benzo-15-crown-5 ligand LE1 consistently afforded only
II. X-ray Crystallography of Pure Dinitrobenzenide Salts.
A. Ligated Potassium Salts as Crystalline “Separated” Ion
Pairs. The diffraction data of highly hygroscopic dark green
plates of the cryptand complex K(Lc)+//DNB-• were collected
at -150 °C, and the crystal structure was solved and refined
by conventional methods (see Experimental Section) to a uni-
form precision of 0.2-0.4 pm. A cursory glance of the packing
pattern in the monoclinic unit cell illustrated in Figure 1A
quickly establishes the “separated” dinitrobenzenide anion to
exist as an independent entity apart from its potassium coun-
terion as the cryptand complex K(LC)+. Indeed, even the inter-
ionic separation of potassium from the nearest NO2-center of
neighboring DNB anions (arbitrary taken as the K....O distance)
with r > 4.8 Å is significantly larger than the sum of their van
der Waals radii (4.3 Å). The same general picture emerges from
the packing observed in the triclinic unit cell of the bis-benzo-
15-crown-5 complex of potassium K(LE1)2+//DNB-• shown in
Figure 1B, and the interionic separation of r > 5 Å is even
(22) The related dibenzo-18-crown-6 ligand LE4 afforded from various solutions
only microscopic polycrystalline materials that were pure but unsuitable
for X-ray analysis even after repeated attempts. Likewise, the parent 18-
crown-6 ligand LE2 led to very thin plates.
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1799

A R T I C L E S
Lu¨ et al.
Figure 1. Unit cells of “separated” ion-pair salts: K (LC)+//DNB- (A) and K(LE1
)
+//DNB- (B) showing the packings of loosely associated cation/anion
2
pairs, each as discrete entities.
Figure 2. Unit cells of the “contact” ion-pair K(LE3)+DNB- (A) and “contact” ion triplet [K(LE2)+]2DNB2- (B) showing the tight association of cation/
anion pairs as indistinguishable entities.
larger than that in the cryptand complex. Both unit cells in
Figure 1 consist of close-packed quasi-spherical cations in which
the voids are occupied by individual dinitrobenzenide anions,
and an anionic NO2-center thus lies within van der Waals
distance of the outer lipophilic (cryptand or crown-ether) sheath
of K+.17b Such loose structures therefore represent true crystal-
line forms of what have been qualitatively described as “solvent-
separated” ion pairs in solution.15
K(LE)+ in Chart 1) to dinitrobenzenide then constitutes the
crystalline counterpart of the “contact” ion pair in solution.
The packing pattern in the monoclinic unit cell of the dark
yellow plate of the ternary dianion salt in Figure 2B also shows
the presence of only a single tightly bound ionic assembly of
[K(LE2)+]2DNB2-. Thus, both NO2-centers of the dinitroben-
zenide dianion are very close to a pair of complexed cations
K(18-crown-6)+ via their open front face (see Chart 1B). The
short interionic separation of r ≈ 2.7 Å in Table 1 is diagnostic
of contact ion pairing generally in the symmetric bidentate form,
(more or less) related to that in the monoanion salt described
above.
III. Optical Transitions in “Separated” and “Contact” Ion
Pairs from Dinitrobenzenide Anions in Solution and in the
Crystalline Solid State. A. Electronic Spectra in Solution.
Dissolution of the crystalline ion-pair salts of dinitrobenzenide
anion in Table 1 afforded yellow-green to green THF solutions
which consistently showed two groups of diagnostic absorption
bands in the UV-vis spectra (Table 2). The high-energy
envelope (I) with λmax ≈ 400 nm was singularly invariant with
B. Crystalline Dinitrobenzenide Salts as the “Contact” Ion
Pair and the Ion Triplet. A labile dark green needle of the
dinitrobenzenide salt of the dicyclohexano-18-crown-6 complex
K(LE3)+ was successfully mounted at -100 °C, and Figure 2A
shows the packing pattern in the orthorhombic unit cell to consist
of only a single type of tight ion-pair assembly. The interionic
separation between K+ and DNB- is quite intimate and highly
unsymmetrical in two ways. First, the two nitro groups in DNB-
are markedly nonequivalent, with one NO2-center (A) tightly
bonded with a very close K+....O contact of r1 ) 2.7 Å, whereas
the other NO2-center (B) is nonbonded with rB > 10 Å. Second,
NO2-center A is unsymmetrically bonded to potassium, with
the second oxygen showing a longer K+....O contact of r2 )
3.2 Å. As such, this tight and unsymmetrical binding of the
crown ether-ligated cation to only a single nitro-center is
generally in the form that is somewhat reminiscent of covalent
bonding in other σ-complexes.23As such, the direct coordination
of the crown ether-ligated K+ via its free front face (compare
(23) For other examples of similar bonding of crown ether-ligated potassium,
see: (a) Caswell, L. R.; Hardcastle, J. E.; Jordan, T. A.; Alam, I.; McDowell,
K. A.; Mahan, C. A.; Fronczek, F. R.; Gandour R. D. J. Inclusion Phenom.
Macrocyclic Chem. 1992, 13, 37. (b) Veya, P.; Floriani, C.; Chiesi-Villa,
A.; Rizzoli, C. Organometallics 1994, 13, 214. (c) Mlinaric-Majerski, K.;
Visnjevac, A.; Kragol, G.; Kojic-Prodic, B. J. Mol. Struct. 2000, 554, 279.
(d) Dyer, R. B.; Ghirardelli, R. G.; Palmer, R. A.; Holt, E. M Inorg. Chem.
1986, 25, 3184. (e) See also: Cambillau, C.; Bram, G.; Corset, J.; Riche,
C. Can. J. Chem. 1982, 60, 2554.
9
1800 J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005

Separated vs Contact Ion-Pair Structures in Solution
A R T I C L E S
ion-pair form, independent of whether K+ is complexed to
cryptand or two benzo-15-crown-5 ligands. Moreover, the
similarity of Figure 3a to the spectrum in the more polar solvent
(DMF)13 strongly suggests that it is spectrally indistinguishable
from that of free dinitrobenzenide anion in solution.
Dissolution of the dicyclohexano-18-crown-6 complex as the
crystalline contact ion-pair salt K(LE3)+DNB- into THF afforded
the UV-vis spectrum in Figure 3b showing partially resolved
envelope II to resemble that in Figure 3a, but with the marked
attenuation of the lowest energy vibronic band from λmax ) 915
nm (ꢀ 21 000 M-1 cm-1) to λmax ) 910 nm (ꢀ 11 000 M-1
cm-1). Essentially the same unique spectral progression was
observed in THF solutions resulting from the dissolution of the
crystalline 18-crown-6 complex K(LE2)+DNB-.
Table 2. Electronic Absorption Spectra of Dinitrobenzenide
Ion-Paired with Complexed K+a
λmax (ꢀ)
K(L)+
envelope I
envelope II
K(LC)+
396 (6.7) 420 (6.0)
400 (5.4) 420(5.4)
396 (6.3) 420sh
400 420
800 (7.0) 835 (8.8) 915 (21)
772sh 847 (7.0) 910(4.0)
800(7.7) 832sh 855(9.1) 910(11)
764 847 911
+
K(LE4
K(LE3
K+d
)
)
+ c
a In THF, at 20 °C, unless noted otherwise; λmax, in nm, ꢀ in 103 M-1
cm-1 b Spectral characteristics are the same for K(LE1 2+//DNB-• c Spectral
characteristics are the same for K(LE2)+DNB-• d DME solution.
.
)
.
.
Finally, the solution spectrum obtained from the crystalline
dibenzo-18-crown-6 complex K(LE4)+DNB-, as shown in
Figure 3c, is quite unique in that the lowest energy vibronic
component in envelope II is almost missing in Figure 3c, merely
showing up as a weak shoulder at λsh ) 910 nm.
B. Electronic Spectra in the Solid State. The diagnostic
intervalence transitions (envelope II) of crystalline ion pairs are
isolated in the inset of Figure 3. The three solid-state spectra
corresponding to the solution spectra of the prototypical salts
were taken as extremely hygroscopic powders finely suspended
in mineral oil and measured in absorption mode.25
Barring some minor complications from reflective dispersion
as well as crystal lattice effects, the solid-state spectra show
considerable resolution, certainly sufficient to observe the
retention of fine-structure characteristics in common with those
in the solution spectra. Most importantly, such an unambiguous
spectral correspondence establishes the direct structural relation-
ship between the separated and contact ion pairs measured as
dynamic species in solution vis a´ vis those defined by X-ray
analysis in the crystalline solid state.23e
Figure 3. UV-NIR spectra of ion-pair salts dissolved into THF (20 °C):
(a) K(LC)+ //DNB-, (b) K(LE3)+DNB-, and (c) K(LE4)+DNB-. Inset:
corresponding solid-state spectra of crystalline salts suspended in mineral
oil (see text).
IV. ESR Spectra of Dinitrobenzene Anion Radicals and
the Temperature-Dependent Dynamics of “Separated” and
“Contact” Ion Pairs. A. Hyperfine Differentiation in Static
ESR Spectra of Dinitrobenzenide Ion Pairs. Dissolution of
the crystalline dinitrobenzenide salts of K+ complexed to
cryptand and to two benzo-15-crown-5 ligands as the separated
ion pairs salts K(LC)+//DNB- and K(LE1)2+//DNB-, respec-
tively, into THF solvent afforded identical ESR spectra (Figure
4a) that are characterized by well-resolved hyperfine splittings
from two nitrogens and four hydrogens (Table 3). This ESR
spectrum was temperature-independent over the range 170-
300 K, and it thus corresponds to a static DNB-• structure (on
the ESR time scale of >10-8s) showing delocalization of the
unpaired electron between two equivalent NO2-centers, as
expected from a symmetrical DNB-• structure unencumbered
by its counterion, i.e., a “separated” ion pair.26
the changes in both the complexed K+ counterion and the
temperature. Moreover, the similarity of envelope I (though
slightly blue-shifted) to that observed in the parent nitroben-
zenide anion (NB-) indicated that it derives from local transi-
tions associated with the negatively charged nitroarene chro-
mophore. By way of contrast, the second, low-energy envelope
II in the near-IR region (650-950 nm and not present in NB-)
was both cation- and temperature-dependent as follows.
The UV-vis spectrum in Figure 3a obtained from the
dissolution of the crystalline cryptand salt K(LC)+//DNB- into
THF was quite similar but slightly blue-shifted relative to
dinitrobenzenide anion previously generated in situ via the
sodium-amalgam reduction of p-dinitrobenzene together with
[2,2,2]cryptand in DMF solution.24 Particularly relevant in
Figure 3a is the decreasing progression of resolved bands
assigned to splitting into vibronic components of the intra-
molecular charge-transfer (intervalence) transition (vide infra).13
Since the identical spectrum was also observed upon dissolution
of the [2:1] complex of benzo-15-crown-5 with K+ as the
crystalline salt K(LE1)2+DNB-, it is easy to conclude that
spectrum 3a represents dinitrobenzenide anion in the separated
By way of contrast, dissolution of the crystalline dibenzo-
18-crown-6 complex as the contact ion-pair salt K(LE4)+DNB-
(25) (a) Essentially the same spectra (although broadened) were obtained in
Al2O3 matrixes measured in the reflectance mode. (b) Note that dinitroben-
zenide salts are insoluble in hydrocarbon solvents and, thus, precludes their
dissolution in the mineral oil used in this study. Therefore, the spectra of
dinitrobenzenide anion salts finely ground in mineral oil (presented in the
inset of Figure 3) represent that of the pure solid state.
(26) (a) It is to be noted that the hyperfine splittings in Table 3 are significantly
less than a2N ) 1.74 G and a4H ) 1.12 G reported earlier for “delocalized”
p-dinitrobenzene anion radical generated electrochemically in the presence
of alkylammonium counterions.11a (b) Note that all ion-pair salts show broad
(unresolved) solid-state ESR spectra, even when very finely ground.
(24) For comparison, free nitrobenzenide prepared in situ in DMF by Nelsen
and co-workers13 was characterized by an intervalence (NIR) absorption
with maxima at 924 (20.3), 851 (11.9), and 815 (10.3) (in nm, in
parentheses; extinction coefficients, in 103 M-1 cm-1).
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1801

A R T I C L E S
Lu¨ et al.
Figure 4. ESR spectra of crystalline ion pairs dissolved into THF as the
(a) “separated” K(LC)+//DNB- showing hyperfine splittings a2N and a4H
and (b) “contact” K(LE4)+DNB- showing splittings a1N and a2H at 293 K.
Table 3. ESR Hyperfine Splittings (G) of Dinitrobenzene Anion
Radicals Ion-Paired with Complexed K+a
K(L)+
aN
aH
K(LC)+
1.23 (2N)
1.23 (2N)
1.11 (4H)
1.11 (4H)
2.30 (2H)b
2.5 (2H)c
+
K(LE1
K(LE4
)
)
2
+
4.56 (1N)b
5.0 (1N), 0.4 (1N)c
3.3 (1N)e
+d
+d
K(LE2
K(LE3
)
)
∼0.2 (2D)
3.2(1N)e
∼0.2 (2D)
a In THF. b CIP1. c CIP2. d DNB-d4
.
e At room temperature, the spectra
-
Figure 5. Temperature-dependent ESR spectrum of crystalline K(LE4)+DNB-
dissolved into THF showing equilibria between CIP1 and CIP2 with SIP in
eq 2. The 313 K spectrum corresponds to CIP1, and lowering the temperature
leads to increasing fraction of CIP2 with broader lines. Note the appearance
of SIP at lowest temperature.
are doubled due to intramolecular self-exchange (see text).
into THF at 313 K showed a narrow-line ESR spectrum (Figure
4b) with hyperfine splittings to only a single nitrogen (aN
)
4.56 G) and two hydrogens (a2H ) 2.30 G) that correspond to
electron localization only to a single NO2-center, as expected
from a unsymmetrical DNB-• structure with intimate K+
association to one NO2-center, i.e., a “contact” ion pair.
B. Temperature-Dependent Hyperfine Splittings of Dini-
trobenzenide Ion Pairs. The ESR spectra of contact ion-pair
salts dissolved in THF showed two types of temperature-
dependent behavior that are nominally classified as (i) increases
in the number of resolved hyperfine splittings and (ii) selective
line broadening of the hyperfine lines that are indicative of
thermodynamic equilibria and dynamic rate processes as follows.
(i) Thermodynamic equilibria between contact ion pairs are
best illustrated by following the temperature-dependent ESR
spectrum of the dibenzo-18-crown-6 complex K(LE4)+DNB-
shown in Figure 4b.27a The series of ESR spectra of this contact
ion pair in THF solution (Figure 5) show the gradual appearance
of new species that are first noticeable at 273 K and progres-
sively increase in intensity (at the expense of original spectrum)
as the temperature is lowered.27b Such a spectral change is
consistent with the relatively straightforward conversion of the
high-temperature species with hyperfine lines (aN ) 4.56 G,
a2H ) 2.30 G) to a somewhat similar species with hyperfine
lines (aN ) 5.0 G, a2H ) 2.5 G) that is prominent at <213 K.
Since this spectral change is reversible, it is reasonable to include
the existence of a temperature-dependent equilibrium between
a pair of isomeric contact ion pairs, i.e.,
CIP1 {K } CIP2
(1)
iso
Deconvolution of a composite spectrum leads to the isomer-
ization constant in eq 1 Kiso ) 5 at 293 K and the thermody-
namic parameters ∆Hiso ) -1.5 kcal mol-1 and ∆Siso ) -2 eu
in the temperature range 223-313 K (Figure S1 in Supporting
Information). Finally, at the lowest practicable temperature of
193 K, a small spectral admixture of a third species with
hyperfine splittings of a2N )1.23 G (the same as those observed
in Figure 4a) indicated further solvent separation, i.e.,
(27) (a) The peak-to-peak line widths (∆H ≈ 0.3 G) in the ESR spectra of species
1 were invariant between 253 and 313 K. (b) The line width of species 2
was significantly broader (∆H ≈ 1.3 G at 293 K) than that of species 1
and slightly temperature-dependent.
Ksep
CIP1 {K } CIP2 { } SIP
(2)
iso
(28) (a) Based on earlier ESR studies, the enthalpy change (∆H) in the CIP-
to-SIP interconversion is negative. See: (b) Hirota, N. J. Phys. Chem. 1967,
71, 127.
which is estimated to be ∼5% at the lowest temperature
studied.28
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1802 J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005

Separated vs Contact Ion-Pair Structures in Solution
A R T I C L E S
Figure 7. Temperature-dependence of the UV-NIR spectrum of
K(LE3)+DNB- in THF measured at (bottom-to-top) 295, 259, 233, 213,
and 190 K to reflect the SIP a CIP interchange in eq 4. Inset: fraction of
SIP fitted with ∆H ) -3.6 kcal/mol and ∆S ) -20 eu.
Such a dynamic behavior can be related to intramolecular spin-
exchange between two different NO2-centers in the contact ion
pair, e.g.,
Stated alternatively, the exchange in eq 3 can be sufficiently
slow at low temperature to generate the static ESR spectrum of
the “frozen” contact ion pair. With rising temperatures, the
interchange rate increases and leads to the doubling of the ESR
spectra (apparent in the 293 K spectrum). Dynamic ESR
simulation (Figure S3) of such a self-exchange between two
NO2-centers in p-dinitrobenzene anion radical that is closely
paired to K(LE3)+ yields the first-order rate constant kex ) 8 ×
108 s-1 at 293 K with the activation barrier ∆H*ex ) 0.8 kcal/
-
Figure 6. Temperature-dependent ESR spectrum of K(LE3)+DNB-d4
showing ion-pair exchange (eq 3) of CIP3 that leads to SIP at the lowest
temperature (see text).
(ii) Interchange dynamics of contact ion pairs are readily
evaluated in the temperature-dependent ESR spectrum of the
crystalline salt of the dicyclohexano-18-crown-6 complex
K(LE3)+DNB- dissolved into THF. Since the temperature-
dependent ESR spectral changes of this salt (Figure S2) were
difficult to decipher quantitatively, dinitrobenzene was converted
to the perdeuterated derivative (DNB-d4, as described in the
Experimental Section). Indeed, deuterium substitution simplified
the picture considerably, and the ESR spectrum of K(LE3)+-
DNB-d4- showed five major lines at 293 K (Figure 6). At first
glance, such an ESR spectrum might be assigned to a species
with the spin delocalized over two nitrogens. However, the
progressive lowering of the temperature resulted in the selective
broadening and gradual disappearance of two lines with mI )
(1, so at low temperature (213 K) the ESR spectrum consisted
predominantly of a three-line (1:1:1) splitting with aN ) 3.3 G.
Quantitative line broadening analysis29 of the high-temperature
spectra, particularly in the range 233-293 K, reproduced this
alternating line width broadening (see Figure S3 in Supporting
Information) arising from the contact ion pair with aN ) 3.3 G.
mol and ∆S* ) -5 eu. Finally, at the lowest (practicable)
ex
temperature of 173 K, additional hyperfine features are apparent
in Figure 6 (bottom spectrum) which can be readily assigned
to the ESR spectrum (a2N ) 1.23 G) corresponding to
completely delocalized dinitrobenzene anion radical (Figure 4a)
in reversible equilibrium with the contact ion pair, i.e.,
kex
CIP3 {\
} CIP3′ {Ksep} SIP
(4)
Additional experimental support for such an interconversion of
contact and separated ion pairs is obtained independently by
observing the temperature-dependent spectral (UV-vis) changes
under the same conditions. Thus Figure 7 confirms the spectral
interchange of contact and separated ion pairs derived from
dicyclohexano-18-crown-6 complex K+ salt in the temperature
range 295-190 K. The inset shows the fit of the spectral data
to the equilibrium in eq 4 with ∆Hsep ) -3.6 kcal/mol and
∆Ssep ) -20 eu. The preferential formation of this separated
ion pair at low temperatures is analogous to that observed with
dibenzo-18-crown-6 complexed K+ in eq 2, but its magnitude
is significantly larger, as indicated by comparative decon-
volutions of the low-temperature ESR spectra (Figures S4 and
S5).
(29) Heinzer, J. Quantum Chemistry Program Exchange 209, as modified by
Petillo, P. A. and Ismagilov, R. F. We thank Prof. S. F. Nelsen for kindly
providing us with a copy of this program.
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1803

A R T I C L E S
Lu¨ et al.
Finally, it is important to note that the temperature-dependent
ESR spectra in Figure 6 are essentially identical to those
obtained from the related contact ion-pair salt derived from the
18-crown-6 analogue K(LE2)+DNB-d4-, as well as those from
the undeuterated parent K(LE2)+DNB- (see Figures S6 and S7,
Supporting Information). Moreover, the self-exchange rate kex
) 10 × 108 s-1 at 293 K for the contact ion pair from (LE2)+-
DNB-d4- and activation barrier ∆H*ex ) 0.9 kcal/mol and ∆S*
ex
) -4 eu in THF solution are the same as those derived from
the dicyclohexano-18-crown-6 complex (vide supra). The latter
underscores the fact that spectral measurements can provide
ready diagnostic probes for precisely distinguishing various ion
pairs that are structurally defined by the counterion, even in
solution.
Figure 8. Correlation of the bond length (d) with reduced state of acceptor
(Z/N) for comparison to p-dinitrobenzene (black) and nitrobenzene (grey).
[Note that there are three black overlapping rhombics and three black circles
at Z/N ) 0.5, representing two SIPs and one CIP of DNB-•; two gray
rhombics and two gray circles at Z/N ) 1 represent CIP and SIP of NB-•.]
Discussion
The clear differentiation of ion-pair structures, particularly
in those aspects relevant to “separated” and “contact” in Figures
1 and 2, respectively, represents the unambiguous starting point
for their structural comparison with dynamic species extant in
solution, as deduced from less definitive (indirect) spectral
methods. Accordingly, let us first scrutinize our X-ray data and
glean from the precise interatomic distances and angles the
quantitative changes in dinitrobenzenide moieties attendant upon
formation of different ion pairs.
I. X-ray Structure Analysis: Charge Delocalization and
Electronic Coupling of NO2-Centers in Dinitrobenzenide Ion
Pairs. The X-ray parameters in Table 1 (entries 1 and 2)
establish the dinitrobenzenide moiety to exist within the
separated ion pair as an essentially planar D2h-symmetric anion.
Thus, upon one-electron reduction, the neutral p-dinitrobenzene
acceptor (Table 1, entry 5) undergoes planarization and a
symmetric molecular distortion accompanied by elongation of
all N-O bonds, marked contraction of both N-C bonds, and
elongation of the phenylenyl bridge. Such a highly symmetrical
structural change is magnified in dinitrobenzene dianion (Table
1, entry 4) by a factor of roughly 2.30
The contrasting formation of the contact ion pair (Table 1,
entry 3) is accompanied by the loss of all symmetry elements.
Although both NO2-centers show bond-length increases upon
reduction, the change is decidedly unsymmetrical, being more
pronounced at the coordinated site. Thus the average value of
the two N-O bonds in the latter is slightly longer (closer to
that in the dianion) than that in the uncoordinated NO2-center
(closer to that in the neutral acceptor). Furthermore, this
unsymmetric distortion also shows up in similar fashion in the
C-N bonds that are shorter at the coordinated site and longer
at the other C-N bond (both relative to that in the symmetrical
“separated” DNB-•). In other words, as the result of “contact”
ion pairing, the electron-density distribution over the entire
dinitrobenzenide moiety is skewed toward the site of the positive
charge.
Table 4. Estimation of Charge Distribution between Paired
NO2-Centers and Quinonoidal Distortion of the Phenylene Bridge
(dq) in the Dinitrobenzenide Ion Pairs and Dianion Triplet
b
ion-pair salt
q′
/qa
dq
K(LC)+DNB-
0.5/0.5
0.5/0.5
0.6/0.4c
1.0/1.0
0.0/0.0
0.25 (0.36)
0.25 (0.36)
0.32 (0.44)
0.52 (0.72)
0.0 (0.0)
K(LE1)
+DNB-
2
K(LE3)+DNB-
[K(LE2)+]2 DNB2-
DNB
a Charge on NO2-center in dinitrobenzenide dianion taken as unity. b For
quinonoidal distortion, the central C-C bond in benzoquinone is taken as
unity; in parentheses, values calculated taking into account the C-N bond
changes. c Coordinated/noncoordinated NO2-centers.
(i) The bond length dependence on the effective reduced state
of dinitrobenzene can be taken as Z/N, i.e., the electron excess
(Z) normalized to the NO2-centers (N).31 As such, the linear
trends observed in Figure 8 for both the nitrogen-oxygen and
the nitrogen-carbon bonds lead to the conclusion that (a) bond
length changes are directly related to the charge on the
dinitrobenzenide moiety in general and at the NO2-centers in
particular and (b) the fraction of charge on both NO2-centers is
essentially invariant. Quantitatively, we estimate the relative
charges (qi) on the individual NO2-centers from their bond
lengths with the aid of eq 5 which follows from the classic
Pauling bond length/bondorder relationship,32 i.e.,
qi ) 1/n Σ(di - d0)/(d1 - d0)
(5)
where di is either the N-O or the C-N bond length associated
with the relevant NO2-center, d0 are those in the neutral acceptor,
and d1 are those in the dinitrobenzenide dianion; the normaliza-
tion factor n accommodates the number of bonds employed in
the computation. Based on this analysis of N-O and C-N bond
lengths (Table 4), we conclude the fraction of charge q on the
To quantify such inherent differences between separated and
contact ion pairs, let us examine the structures of the dinitro-
benzenide moiety in terms of (i) selective bond-length/ charge-
density distributions, (ii) quinonoidal distortions of the phen-
ylenyl bridge, and (iii) deviations from planarity as follows.
NO2-centers in separated ion pairs K(LC)+//DNB- and K(LE1)2 //
+
DNB- shows the expected symmetrical 0.5/0.5 distribution. On
(31) Thus, Z/N ) 0, 0.5, and 1.0 for the neutral acceptor, anion radical, and
dianion, respectively.
(30) Although “contact” ion pairing pertains to the dianion salt, the bond length
changes are sufficiently small to approximate these changes in the
“separated” ion triplet.
(32) (a) Pauling, L. Nature of the Chemical Bond; Cornell: Ithaca, NY, 1960.
(b) Le Magueres, P.; Lindeman, S. V.; Kochi, J. K. J. Chem. Soc., Perkin
Trans. 2 2001, 1180.
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1804 J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005

Separated vs Contact Ion-Pair Structures in Solution
A R T I C L E S
the other hand, the charge distribution in the contact ion pair
K(LE3)+DNB- is calculated to be unsymmetrical, with the
charge on the coordinated NO2-center being about 0.6 and that
on the uncoordinated center about 0.4.
two-state (Marcus-Hush) model for dinitrobenzenide anion as
a mixed-valence (intervalence) system.37
II. Diagnostic Intervalence Transitions in Dinitroben-
zenide Ion Pairs as Mixed-Valence Systems. Electronic spectra
of separated and contact ion pairs as shown in Figure 3 form
the critical link between solid-state structures and solution
behavior insofar as they provide topological information about
the ground state of the dinitrobenzenide anion as it is subjected
to counterion effects. According to Nelsen and co-workers,13
dinitrobenzenide is a mixed-valence anion in which the paired
NO2/NO2- redox centers (with a reorganization energy of λv )
1050 cm-1) are strongly coupled by the sizable electronic
coupling element Hab ) 5400 cm-1. The large magnitude of
the ratio Hab/λ ) 5.1 predicts a Robin-Day Class III anion38
in which the interaction between redox sites is so large that
(ii) In a similar vein, quinonoidal distortion of the p-phenylene
bridge33 can be computed by focusing first on the contraction
of its central C-C bond (di) upon reduction, so that normaliza-
tion to the benzene(d0)/p-benzoquinone(d1) bond length changes
provides the measure of the distortion parameter dq listed in
column 3 of Table 4. The corresponding evaluation of qui-
nonoidal distortion based on N-C bond changes34 is indicated
in parentheses. If due cognizance is taken of the semiquantitative
nature of such an analysis, we can find no discernible difference
in quinonoidal distortion of the dinitrobenzenide moiety despite
the unsymmetrical distortion of the charge density between SIP
and CIP structures.
-
separate NO2/NO2 minima no longer exist, and the unpaired
electron is delocalized (barrierless) between both redox centers.39
Such a ground state is represented by the symmetric potential
energy well illustrated in Figure 9, and this single minimum is
also largely applicable to the loose dinitrobenzenide in separated
ion pairs, owing to minimal influences of the loosely bound
counterion.
(iii) Reduction of the p-dinitrobenzene to the anion radical
and dianion results in planarization. Thus, the torsion angles â
of both NO2-groups relative to the phenylene bridge in p-
dinitrobenzene (Table 1) deviate by approximately 10° from
planarity.35 However, every dinitrobenzenide moiety in Table
4 (entries 1-3) is entirely coplanar with â < 2°, and the sizable
conjugation of NO2-centers (vide supra) is essentially the same
in SIP and CIP ion pairs.
By way of contrast, the counterion cannot be ignored in
contact ion pairs, and cation binding differentiates the paired
NO2-centers sufficient to induce significant polarization.40 Such
an intimately cation-bound dinitrobenzenide remains a Class
III anion, with its asymmetric ground-state structure constructed
in Figure 9B with λv and Hab values that are those applicable to
“free” dinitrobenzenide.
Experimental support for the two-state representation of
nitrobenzenide ion pairs in Figure 9 (A and B) is best provided
by further analysis of the distinctive NIR envelopes (Figure 3)
for the separated ion pair versus the contact ion pair. In the
former, the most important spectral facet is the vibronic fine
structures in free dinitrobenzenide13 and is reproduced in the
separated anion as the characteristic decreasing progression in
Figure 3a in both the solution and solid-state spectrum.41 By
way of contrast, in the contact ion pair the vibronic fine
structures show a head-and-shoulders progression in which the
principal band is not the lowest-energy component but the
penultimate one.
Despite the large (overall) structural changes that accompany
SIP and CIP formations, we conclude from such a complete
analysis of the X-ray structural data that the electronic interaction
between the paired NO2-centers in dinitrobenzenide is largely
unaffected by contact ion pairing. Accordingly, we attribute the
observed spectral difference/changes to an electrostatics-induced
polarization of the electron-density distribution by the potassium
cation, as opposed to the cation-induced electron decoupling
between paired NO2-centers.36 To determine how such a
structural formulation can shed light on the complex spectral
changes in the ESR and UV-vis data in Figures 4(5) and 3,
respectively, we now return to the earlier development of the
(33) For previous evaluations of quinonoidal distortion in p-phenylene bridged
mixed-valence systems, see: Sun, D.-L.; Lindeman, S. V.; Rathore, R.;
Kochi, J. K. J. Chem Soc., Perkin Trans. 2 2001, 1585.
According to the two-state model for the free dinitroben-
zenide, the principal (0-0) vibronic transition proceeds from
the intervalence (delocalized) ground state and is shown in
Figure 9A as the most intense (heavy vertical arrow) owing to
favorable Franck-Condon factor for the symmetric potential-
energy well. The diminished overlap for the secondary (0-1
(34) (a) The values in parentheses were calculated via eq 5 using two C-N34b
and six C-C bond lengths of the corresponding dinitrobenzenide. (b) With
d0 ) 1.47 and d1 ) 1.36 Å (single and double C-N bonds, respectively).
(35) A reviewer has suggested that the slight reduction in the torsion angle â
that accompanies the reduction of p-dinitrobenzene can result from the
electrostatic attraction between the negatively charged NO2-group and ortho-
hydrogens.
(36) (a) Due to similarity of the geometries of “free” and ion-paired dinitroben-
zenide anions (distances between redox centers, planarities, quinonoidal
distortions, etc.), the electronic interactions between the NO2-centers are
expected to be comparable. Their reorganization energies (determined by
the same hypothetical noninteracted oxidized and reduced redox centers)
are unlikely to differ dramatically.36b As such, the Hab/λv ratio, which is
>10 for “free” dinitrobenzenide,13 should be significantly larger than unity
for ion-paired species. This ensures the absence of a ground-state barrier
for electron transfer in ion-paired p-dinitrobenzene anion radicals; i.e., they
belong to Class III. According to Robin and Day,38 the basic distinction
between mixed-valence complexes lies in their ground-state structure: Class
III lying into a single potential well as opposed to the two-well model
invoked for Class II. (b) Any variation of the reorganization energies and
coupling elements resulting from solvent and counterion effects are
moderate (mainly limited to 10-20%, as determined from the intervalence
optical transition).8,36c-e Thus, such effects can lead to the transformation
of Class III into Class II only in the case of borderline systems.5h,13 (c)
Nelsen, S. F.; Tran, H. Q. J. Phys. Chem. A 1999, 103, 8139. (d) Nelsen,
S. F.; Trieber, D. A., II; Ismagilov, R. F.; Teki, Y. J. Am. Chem. Soc,
2001, 123, 5684. (e) Nelsen, S. F.; Konradsson, A. E.; Clennan, E. L.;
Singleton, J. Org. Lett. 2004, 6, 285.
(37) (a) Marcus, R. A. Discuss. Faraday Soc. 1960, 29, 21. (b) Marcus, R. A.
J. Phys. Chem. 1963, 67, 853. (c) Marcus, R. A.; Sutin, N. Biochim.
Biophys. Acta 1985, 811, 265. (d) Hush, N. S. Trans. Faraday Soc. 1961,
57, 557. (e) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391. (f) Hush, N. S.
Electrochim. Acta 1968, 13, 1005. (g) Sutin, N. Prog. Inorg. Chem. 1983,
30, 441. (h) Brunschwig, B. S.; Sutin, N. Coord. Chem. ReV. 1999, 187,
233.
(38) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247.
(39) (a) Compare similar barrierless electron transfer in strongly coupled donor/
acceptor dyads in arene charge-transfer complexes with nitrosonium.39b,c
(b) Rosokha, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2001, 123, 8985. (c)
Rosokha, S. V.; Kochi, J. K. New J. Chem. 2002, 26, 851.
(40) For an earlier example of counterion induced polarization, see: Sun, D.-
L.; Rosokha, S. V.; Lindeman, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2003,
125, 15950.
(41) See: (a) Risko, C.; Barlow, S.; Coropceanu, V.; Halik, M.; Bre´das, J.-L.;
Marder, S. R. Chem. Commun. 2003, 194. (b) Bailey, S. E.; Zink, J. I.;
Nelsen, S. F. J. Am. Chem. Soc. 2003, 125, 5939 for examples of vibronic-
splitting analysis in mixed-valence systems.
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1805

A R T I C L E S
Lu¨ et al.
spectrum in Figure 3c.43 Finally, the intermediate NIR spectrum
of the contact ion pair for the dicyclohexano-18-crown-6
complex (Figure 3b) is a result of the partial charge redistribution
measured in the X-ray structure (Table 1, entry 3). By
interpolation, the constructed spectrum of K(LE3)+DNB- results
from the two-state model when the minor shift in the somewhat
asymmetric (0.6/0.4) ground state merely attenuates the (0-0)
transition, consistent with the experimental observation of the
reduced 910-nm intensity in Figure 3b.
The meaningful comparison of separated and contact ion pairs
provided by the success of the two-state model for the optical
changes ensures the direct applicability of the quantitative X-ray
structures to these highly labile species in solution. As such,
we can now turn to the ESR studies in Figures 5 and 6 with the
assurance that they faithfully reflect the structural changes
between separated and contact ion pairs in solution, especially
as their equilibria are finely modulated by rather small variations
in temperature.
III. Dynamic Equilibria of “Contact” Ion Pairs. The
selective (alternating) line broadening behavior in the ESR
spectrum of contact ion pairs derived from the dicyclohexano-
18-crown-6 and 18-crown-6 complexes K(LE3)+ and K(LE2)+,
respectively, in THF solution (Figure 6) reflects the fast
redistribution of partial spin densities between the two NO2/
-
NO2 centers. Phenomenologically, such a dynamic effect is
similar to a rapid intramolecular electron transfer, and the
temperature-dependent line broadening allows the self-exchange
process to be evaluated (vide supra). However, dinitrobenzenide
is a Robin-Day Class III anion, and the large electronic
coupling Hab relative to the reorganization energy λv is sufficient
to nullify any such (intra-anion) activation barrier,44 and the
spin redistribution must perforce be attributed to some other
time-dependent phenomenon. In view of the tight cation binding
in this system, we attribute the dynamic line broadening to the
migration of the potassium counterion from one “contact” nitro-
center to the other (uncoordinated) site, simultaneous with spin
redistribution. The interchange can be pictorially depicted as
Figure 9. Two-state representations of a (A) “separated” ion pair and (B)
“contact” ion pair of dinitrobenzenide showing the principal intervalence
transitions (heavy vertical arrows) from symmetric and asymmetric ground
states, respectively. The constructed NIR envelopes are illustrated by dashed
curves on the left sides of the drawings to be compared with the experimental
spectra in Figure 3a and c, respectively.
and 0-2) transitions predict the decreasing progression of
resolved vibronic bands found in the experimental spectrum,
as illustrated by the constructed NIR spectrum on the left side
of the drawing to be compared with the experimental spectrum
in Figure 3a.
and it is strongly reminiscent of an intramolecular self-
exchange rate process in a simple Class II anion, not mediated
by the counterion.45
The two-state model also accommodates the contact ion pair
via the distortion of the intervalence ground state by electrostatic
polarization (vide supra). The resulting asymmetric potential-
energy well is qualitatively drawn (Figure 9B) to depict the
The mechanism of the cation-interchange process in eq 6 can
occur via the continuous migration of the complexed K+ across
the surface of the anion by always maintaining its “contact”
characteristics intact. However, the ESR observation of the
-
skewed spin distribution between the paired NO2/NO2 redox
centers.42 Such a desymmetrization is tantamount to a shift of
the intervalence ground state (Figure 9B) so that the (0-1)
transition is the most probable, as depicted by the heavy vertical
arrow, and the associated (0-0 and 0-2) transitions are
consequently the minor ones. The constructed NIR envelope
of the “contact” dinitrobenzenide is illustrated on the left side
of the drawing to reflect its striking similarity to the experimental
(43) (a) The quantitative analysis of this vibronic splitting awaits time-dependent
quantum mechanical calculations such as those described earlier.13,41 See:
(b) Zink, J. I.; Shin, K.-S. AdVanced Photochemistry; Wiley: New York,
1991; Vol. 16, p 119. (c) Schatz, P. N. In Inorganic Electronic Structure
and Spectroscopy; Solomon, E. I., Lever, A. B. P., Eds.; Wiley: New York,
1999; Vol. II, p 175. (d) Piepho, S. B.; Krausz, E. R.; Schatz, P. M. J. Am.
Chem. Soc. 1978, 100, 2996. (e) Simoni, E.; Reber, C.; Talaga, D.; Zink,
J. I. J. Phys. Chem. 1993, 97, 12678. (f) Coropceanu, V.; Malagoli, M.;
Andre, J. M.; Bre´das, J. L. J. Chem. Phys. 2001, 115, 10409.
(44) (a) In the absence of the intrinsic barrier related to the Marcus reorganization
energy, the usual expression of the electron-transfer rate constant44b is not
applicable. (b) ket ) kνn exp(-∆G*/RT), where k is the electronic
transmission coefficient, νn is the nuclear vibration frequency, and ∆G* )
(λ - 2H)2/4λ is the free energy of activation.37
(45) Due to such a similarity, the (solvent-dependent) ESR line-broadening of
p-dinitrobenzene anion radical was earlier interpreted as Class II electron
transfer. See: Telo, J. P.; Grampp, G.; Shohoji, M. C. B. L. Phys. Chem.
Chem. Phys. 1999, 1, 99.
(42) Interaction of potassium with one of the NO2-centers in ion-paired mixed-
valence anions results in the stabilization (vertical shift) of one of the
(parabolic) diabatic states and leads to the corresponding transformations
of the adiabatic ground and excited states in Figure 9B (constructed with
the two-state model37 using the same Hab and λ values as those in the
symmetric system in Figure 9A).
9
1806 J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005

Separated vs Contact Ion-Pair Structures in Solution
A R T I C L E S
separated ion pair at very low temperatures shows that this loose
species is thermodynamically viable, most likely as a result of
the enhanced solvation of the complexed cations K(LE3)+ and
K(LE2)+ by THF solvent at low temperature.46 The latter favors
the dynamic spin redistribution to more likely be a two-state
process that proceeds via the separated ion pair, i.e.,
although their exchange is likely to proceed via a preequilibrium
dissociation observed in the low-temperature (193 K) spectrum
in Figure 5, i.e.48
-1
K
′
CIP1 {K } SIP { SEP } CIP2
(8)
SEP
If so, the rate of isomerization between CIP1 and CIP2 in eq 1
is controlled by their thermodynamics stabilization.
Summary and Conclusions
The distinctive structures of “separated” and “contact” ion
pairs are unambiguously identified in Figures 1 and 2, respec-
tively, through X-ray crystallography of dinitrobenzenide (DNB-)
salts of potassium complexed with either a three-dimensional
[2,2,2]-cryptand ligand (Lc) or various two-dimensional crown-
ethers (LE) in Chart 1. These ion-pair structures are also
spectrally characterized by their own unique NIR bands (Figure
3a-c) with resolved vibronic splittings arising from intervalence
transitions in dinitrobenzenide as a strongly coupled mixed-
valence anion. Most importantly, the same diagnostic vibrational
fine structures of each ion-pair salt dissolved into THF ensure
that these “separated” and “contact” ion-pair structures also
persist in solution.
Comparative spectral analysis shows that dinitrobenzenide
as “separated” ion pairs in THF solution is equivalent to the
“free” anion in polar DMF, previously assigned with the aid of
the Marcus-Hush two-state model to a strongly coupled
Robin-Day Class III intervalence system. It is thus noteworthy
that the experimental X-ray structure in Table 1 as well as the
ESR spectrum in Figure 4a confirms the single potential-energy
ground state for “separated” dinitrobenzenide, with its odd
electron lying in a symmetric minimum equally distributed
between both NO2-centers. Contrastingly, X-ray and ESR
structure analyses of the “contact” ion pair establish an
asymmetric ground state for dinitrobenzenide with the charge
electrostatically skewed (polarized) toward that NO2-center
directly paired to the potassium counterion. Otherwise, the two-
state analysis of the spectral NIR band in Figure 9B predicts
the electronic coupling between the paired NO2-centers to be
essentially the same as that in “free” dinitrobenzenide or as the
“separated” ion pair.
The relatively fast kinetics of the spin redistribution of kex
≈
109 s-1 and moderate thermodynamic parameters of ∆H*
)
ex
0.8 kcal mol-1 and ∆S*ex ) -5 eu suggest that the rate of ion-
pair dissociation is rate-limiting.
In ion pairs mediated by alkali-metal cations, such an ion
separation will be dependent on the polyether complexon, which
in our system is expected to be47
K(LE1)+ > K(LE2)+ ≈ K(LE3)+ > K(LE4)+
In qualitative accord with these expectations, the temperature-
dependent ESR spectrum of the dibenzo-18-crown-6 complex
as the contact ion pair salt K(LE4)+DNB-• dissolved into THF
(Figure 5) reveals the presence of two isomeric contact ion pairs
identified as CIP1 and CIP2 (eq 1) in equilibrium with the
separated ion pair (SIP). The absence of the temperature-
dependent line broadening in the ESR spectrum of CIP1 indicates
that ion-pair dissociation is slow relative to spin redistribution,
but the difference is less in CIP2 which clearly shows a limited
line broadening behavior (without alternation). We attribute this
difference to the existence of two types of ion-pairing binding
of the K(LE4)+ cation to DNB- anion in solution, with one type
showing tighter binding than the other. If so, tighter ion-pair
binding can be speculatively presented as the symmetrical
(bidentate) structure,23
Such intact characteristics of dinitrobenzenide anion upon
contact ion pairing with different complexed potassium coun-
terions K(LE)+ can be directly observed as at least four types
of dynamic effects in their temperature-dependent ESR spectra.
Thus, among the various crown-ether ligands, the dibenzo-18-
crown-6 complex as K(LE4)+ is the least prone to separate from
dinitrobenzenide anion, and two isomeric contact ion pairs (CIP1
and CIP2) undergo facile interchange via the separated ion pair
(SIP) in eq 2. On the other hand, K+ complexed to either 18-
crown-6 as K(LE2)+ or dicyclohexano-18-crown-6 as K(LE3)+
is substantially more prone to separate from dinitrobenzenide
anion. The fast dynamic process for CIP3 is observed in the
temperature-dependent ESR spectra (Figure 6) as a selective
line broadening leading to the site exchange in eq 3, and the
pathway for electron redistribution is again preferentially viewed
as ion-pair separation in eq 4. As expected for such differences
and the looser ion-pair binding is depicted on the right.
Experimentally, these types of symmetric and unsymmetric ion-
pair bindings are indeed inherent to the X-ray structures of the
“contact” dianion and anion, as illustrated in Figure 2B and A,
respectively. Furthermore, the activation barrier between CIP1
and CIP2 is sufficient to preclude a general ESR line broadening,
(46) (a) The low-temperature dissociation of ion-paired anion radicals can result
from favorable counterion solvation; see ref 28b. (b) In eqs 2 and 4, SIP
formation may result in the further deligation of K(LE)+ by THF as an
ethereal (donor) solvent. This is indicated by the value of formation constant
of K(LE4)+: log K ≈ 2 in 50% aqueous THF.46c (c) Rechnitz, G. A.; Eyal,
E. Anal. Chem. 1972, 44, 370.
(47) (a) Such an order agrees (i) with dissociation equilibria that show enhanced
+
+
shifts to “free” anion radicals in solution with K(LE3
) or K(LE2) as
compared to those with K(LE4)
+ and (ii) with the vibronic-splitting analysis
of the NIR intervalence absorption. Therefore, the rate of the intramolecular
exchange processes that are directly related to the strength of DNB-/K(L)+
interaction) is much slower in ion pairs with the K(LE4)
+ counterion. This
results in the weak line-broadening of the ESR spectrum of CIP2 as
compared to those in systems with either K(LE3
+ or K(LE2 + counterions.
(b) Similarly, picrate anion was found earlier to be bound more strongly
)
)
(48) Alternatively, the dissociation of one of the ionic K-O bonds may lead to
the direct transformation of CIP1 into CIP2, followed by further dissociation
into SIP.
+
to K(LE4
Dokl. Chem. 1983, 273, 414.
)
than to K(LE2)+. See: Yatsimirskii, K. B.; Talanova, G. G.
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1807

A R T I C L E S
Lu¨ et al.
Table 5. Crystallographic Parameters of the Ion-Pair Saltsa
b
-
parameter
K(LC)+DNB-
K(LE1
)
+DNB-
K(LE3)+DNB-
[K(LE2)+]2DNB2
2
a, Å
b, Å
c, Å
R, deg
â, deg
18.2320(9)
11.6937(6)
13.9494(7)
90
107.036(1)
90
12.1156(6)
12.8981(7)
12.9427(7)
111.682(1)
91.833(1)
110.057(1)
1736
16.027(4)
14.913(4)
23.619(8)
90
90
90
20.411(3)
8.321(1)
22.718(4)
90
103.938(4)
90
γ, deg
V, Å3
2844
5645
3745
F(000)
1244
786
2472
1648
µ, mm-1
F, g/cm3
reflections
independent
observed (F > 4σ)
parameters
R (F > 4σ)
wR2
0.25
1.363
9653
9251
8015
512
0.0386
0.1054
1.053
0.23
1.423
10834
10834
8786
636
0.0471
0.1240
1.045
0.25
0.32
1.375
8958
8459
5576
1.364
75582
5547
2194
352
0.0497
0.1107
0.740
659
0.0542
0.1513
1.030
GOF
a Data collected at 123 K, unless noted otherwise. b At 173 K.
between K(LE4)+ and either K(LE2)+ or K(LE3)+, the ultimate
ion-pair separation to SIP favors CIP3 over CIP1 or CIP2, as
established by the series of comparative (temperature-dependent)
ESR spectra in Figures 5 and 6 as well as the UV-vis spectral
changes in Figures 7 and S8.
In a more general context, the apparent spin “localization”
observed in the ESR spectra of dinitrobenzenide indicates that
strongly coupled Class III systems upon contact ion pairing can
exhibit spectral features that are commonly ascribed to localized
intervalence Class II systems. As such, it is important to
emphasize the caveat that dynamic ion-pairing effects must
always be quantitatively taken into account in the assignment/
interpretation of experimental data, especially of charged (ionic)
mixed-valence systems at the Class III/II border that have
recently received increased attention.4h,9
on the wall were apparent. In another Schlenk tube, a sample of
p-dinitrobenzene (30 mg, 0.178 mmol) together with a stoichiometric
amount (0.178 mmol) of the complexon (either cryptand or crown
ethers) was degassed by melting in high vacuo (10-3 Torr). The
degassed mixture was dissolved in 20-25 mL of dry THF, and the
solution was transferred into the Schlenk tube containing the metal
mirror. An intense green coloration appeared immediately. The solution
was allowed to stand at -20 °C for a few days until the reduction was
complete (potassium mirror disappeared completely). The resulting
greenish solution was carefully covered with a layer of dry hexane (20
mL) and placed for 2-3 days in a constant-temperature cooler
maintained at -60 °C. The reaction batch typically yielded 5-10 mg
of well-formed crystals growing on the walls at the interface where
the solvents diffuse together. The highly air-sensitive, dark green/or
brown crystals (Table S1) were carefully collected and then used for
the X-ray structural analysis and IR studies (see IR spectra in Figure
S9 in Supporting Information) as well as (after dissolving in dry THF)
for ESR and UV-vis spectroscopic measurements.
Experimental Section
Preparation of p-Dinitrobenzene Dianion Salt. The dinitroben-
zenide dianion was prepared by the procedure employed for the anion
radical except 2-3 molar equivalents of crown ether and potassium
relative to DNB were used.
Materials. Metallic potassium from Aldrich was stored in a drybox
and used without additional purification. The crown-ethers benzo-15-
crown-5, 18-crown-6, dicyclohexano-18-crown-6, dibenzo-18-crown-
6, and [2,2,2]cryptand (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo-
[8.8.8]hexacosane) from Aldrich were also used as received. p-Dinitro-
benzene (DNB) from Aldrich was sublimed twice and recrystallized
from ethanol and dried in vacuo. High purity tetrahydrofuran from
Merck was freshly distilled from sodium/benzophenone under an argon
atmosphere and briefly stored over potassium or sodium mirror. Hexane
from Merck was successively washed with concentrated HNO3/H2SO4,
H2O, NaOH, and H2O, dried over CaCl2 followed by distillation from
sodium/benzophenone, and stored under argon atmosphere. Ethylene
glycol dimethyl ether (DME) from Sigma-Aldrich was freshly distilled
from sodium/benzophenone and stored under an argon atmosphere.
p-Dinitrobenzene-d4 was synthesized from nitrobenzene-d5 (Aldrich,
99.5 atom % D) by standard reduction, acetylation, and nitration
procedures to afford p-nitroaniline-d4 followed by Cu catalyzed
reoxidation to p-dinitrobenzene-d4 in the presence of NaNO2 and
HBF4.49 Isolated yield: 40%; pale yellow crystals; m/z, 172; mp, 173-5
°C.
X-ray Crystallography of Ion-Pair Salts. The single crystals of
the ion-pair salts were obtained at -60 °C by the slow diffusion of
hexane carefully layered on the top of the corresponding saturated THF
solutions. The diffraction data for all the ion-pair salts were collected
with the aid of a Siemens/Bruker SMART diffractometer equipped with
an APEX CCD detector using Mo KR radiation (λ ) 0.710 73 Å),
either at -150 or -100 °C. In all cases, semiempirical absorption
correction was applied.50 The structures were solved by direct methods51
and refined by a full matrix least-squares procedure52 with IBM Pentium
and SGI O2 computers (see Table 5). [The X-ray structure details of
various compounds are on deposit and can be obtained from the
Cambridge Crystallographic Data Center, U.K.]
ESR Measurements. The electron spin resonance measurements
were performed on a Bruker ESP-300 X-band spectrometer with 100
kHz field modulation, 0.1 to 0.2 G modulation amplitude, and 20 mW
microwave power. The THF solutions were prepared in a Schlenk tube
Preparation of Ion-Pair Salts of p-Dinitrobenzene Radical
Anions: p-Dinitrobenzene was reduced with potassium mirror in the
presence of cryptand or crown ethers as follows. Potassium metal (7
mg, 0.179 mmol) was carefully cleaned mechanically in a glovebox
under an argon atmosphere and placed in a well-dried Schlenk tube
(50 mL). The tube was slowly heated with an oil bath in a vacuum
(10-3 Torr) until complete sublimation of the metal and mirror formation
(49) Blatt, A. H, Ed. Organic Synthesis; Wiley: New York, 1961; Vol. 2, p
225.
(50) Sheldrick, G. M. SADABS, version 2.03; Bruker/Siemens Area Detector
Absorption and Other Corrections. Bruker X-ray Analytical Systems:
Madison, Wisconsin, 2000.
(51) Sheldrick, G. M. SHELXS 97, Program for Crystal Structure Solutions;
University of Go¨ttingen: Germany, 1997.
(52) Sheldrick, G. M. SHELXL 97, Program for Crystal Structure Refinement;
University of Go¨ttingen: Germany, 1997.
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1808 J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005

Separated vs Contact Ion-Pair Structures in Solution
A R T I C L E S
kept at -40 °C then transferred into a quartz ESR tube (2-mm diameter)
under an argon atmosphere. The tube was placed in a quartz Dewar
set in the center of a rectangular cavity, with the temperature regulated
by an IBM control unit to (0.5 °C. Dry compressed nitrogen was
pumped through the cavity to avoid condensation of adventitious
moisture at the low (below 0 °C) temperature. The calculation of
hyperfine splitting constants was carried out by the computer simulation
of the ESR spectra with a Bruker WINEPR Simfonia program (version
1.25). Alternatively, the THF solution of crystals obtained from a series
of crown ethers also showed well-resolved ESR spectra, as those crystals
that are not stable at room temperature and also their solutions were
kept at -40 °C under an argon atmosphere for ESR measurements.
On the other hand, p-dinitrobenzene radical anion salts with different
crown ethers were also prepared in situ by exposing the DNB solution
to a potassium mirror attached to the wall of the ESR tube. Both
crystal and in situ prepared solutions of a patrticular radical anion
were measured by ESR, the results were in good agreement, and the
hyperfine splitting constants obtained from the spectra are listed in
Table 3.
Measurement of Electronic Spectra. All spectroscopic measure-
ments were performed under an argon atmosphere in a 1-mm or 1 cm
quartz cuvette on either a Hewlett-Packard 8453 diode-array or a Varian
Cary 5 spectrometer. In each case, freshly prepared THF solutions of
the corresponding ion-pairs salts were made up in a rigorously air-free
drybox, or in a Schlenk tube under argon at -40 °C. The samples for
in situ measurements were prepared by exposing the solution to
potassium mirror inside the UV cuvette at a temperature of -20 °C.
The reaction was spectroscopically monitored for completion by
following the changes in concentration of the neutral p-dinitrobenzene
which has an absorption peak at 266 nm. For solid-state measurements,
2 or 3 mg of crystals of the corresponding ion-pair salts were added to
a drop of mineral oil (ca. 0.1 mL) attached to a glass slide, and the
crystals were ground between two glass plates to a fine powder and
the absorption measurements were taken.
IR Measurements. Crystalline samples of the ion-pair salts were
mounted directly on the germanium sampling plate of the single-
reflection HATR (Smart Miracle, Pike Technology) under an argon
atmosphere, and the infrared spectra (Figure S9) were measured with
a Nexus 470 FT-IR (Thermo Nicolet). Alternatively, crystals for X-ray
crystallographic analysis suspended in mineral oil were used (and the
mineral oil background subtracted digitally).
Acknowledgment. We thank Professor S. F. Nelsen for kindly
providing us with a copy of the ESR-EXN program, M.G.
Davlieva for isolation of the crystalline salts of K(LC)+//DNB-
and [K(LE2)+]2DNB2-, S. M. Dibrov for crystallographic
assistance, and the R. A. Welch Foundation for financial support.
Supporting Information Available: X-ray crystallographic
files in CIF format. Crystallization details and physical pro-
perties of ion pair salts (Table S1); thermodynamics of the CIP1/
CIP2 isomerization of K(LE4)+DNB- (Figure S1); temperature-
dependent ESR spectrum of K(LE3)+DNB- (Figure S2); dy-
namic ESR simulation of alternating line-broadening for
K(LE3)+DNB- (Figure S3); simulation of low-temperature ESR
spectrum of K(LE3)+DNB- as superposition of CIP and SIP
ion pairs (Figure S4); simulation of low-temperature ESR
spectrum of K(LE4)+DNB- as superposition of CIP and SIP ion
pairs (Figure S5); temperature-dependent ESR spectrum of
K(LE2)+DNB- (Figure S6); temperature-dependent ESR spec-
-
trum of K(LE2)+DNB-d4 (Figure S7); temperature-dependent
electronic spectrum of K(LE2)+DNB- (Figure S8); and IR
spectra of crystalline ion pair salts (Figure S9). This material is
available free of charge via the Internet at http://pubs.acs.org.
JA043998X
9
J. AM. CHEM. SOC. VOL. 127, NO. 6, 2005 1809