Acid-base equilibrium between phenoxyl-nitronyl nitroxide biradical and closed-shell cation. A magnetic pH sensor

Full text of DOI:10.1021/ja963289m

J. Am. Chem. Soc. 1997, 119, 3625-3626
3625
Scheme 1
Acid-Base Equilibrium between
Phenoxyl-Nitronyl Nitroxide Biradical and
Closed-Shell Cation. A Magnetic pH Sensor
Katsuya Ishiguro, Mitsutoshi Ozaki, Nobuyuki Sekine, and
Yasuhiko Sawaki*
Department of Applied Chemistry, Faculty of Engineering
Nagoya UniVersity, Chikusa-ku, Nagoya 464-01, Japan
ReceiVed September 19, 1996
Spin-crossover, which means the switching between two
species or phases with different magnetic properties, have been
focused much attention, especially in developing a novel
functionality for magnetic materials.1 In relation to the recent
advances in the studies on organic ferromagnets,2 molecule-
based spin-crossovers may be also an attractive target in organic
chemistry, but a few related phenomena such as the photo-
isomerization of a carbene3a and the spin isomerism of a non-
Kekule´ molecule3b are known only under matrix-isolated
conditions.
An alternative model for spin-crossover molecules may be
given for a cross-conjugated unsymmetrical biradical with
donor-acceptor characters (D•-A•); if the singlet zwitterionic
state (D+-A-) is energetically close to the triplet biradical state,4
a large difference between them in dipole moments or acid-
base properties may bring about a novel reversed relative
stability by their intermolecular interaction. Typically, when
the zwitterionic state may be stabilized in acidic media by
forming D+-AH species, the spin-state may be controlled by
changing pH of solutions.
A•, 4), and anion radical (D•-A-, 5), were shown to persist
during the voltammetric analyses.
The absorption spectrum of 1 in acetonitrile showed maxi-
mum absorption bands at 283, 322, 365, and 615 nm,5 and those
of 2 were observed at 318, 359, and 604 nm (solid line in Figure
2B). On the other hand, the chemical one-electron oxidation
-
of 1 with Cu(ClO4)2 (Ered ) +1.0 V vs SCE)7a or NO+BF4
(Ered ) +1.3 V vs SCE)7b in acetonitrile resulted in almost
complete conversion of 1 but the formation of 2 was not
identified by absorption spectrum. Instead, an alternative
product was formed which showed the maximum absorption at
334 nm (solid line in Figure 2A). A similar absorption spectrum
could be obtained by the controlled potential electrolysis (+0.9
V vs SCE, ∼1 F/mol) of 1 in acetonitrile containing 0.1 M
n-Bu4NBF4. According to the cyclic voltammogram of 1, the
product at λmax ) 334 nm was assigned as the closed-shell cation
3 generated by the removal of unpaired electron from the
nitronyl nitroxide, which does not deprotonate the phenol proton
under the neutral conditions.
The interconversion between 2 and 3 was successfully
observed by the addition of acid or base. Spectral changes upon
the addition of pyridine to the solution of 3 prepared from 1
and Cu(ClO4)2 are shown as dashed lines in Figure 2A. The
clear isosbestic points observed at 322, 355, and 362 nm indicate
the quantitative nature of the transformation (3 f 2). The
absorption spectrum of 2 was also obtained by the electrolysis
of 1 followed by the deprotonation of 3 with pyridine. The
reverse transformation, i.e., the protonation of biradical 2 by
acids leading to the formation of cation 3, could be followed
by the changes in absorption spectra upon the addition of
trifluoroacetic acid to the solution of 2 (dashed lines in Figure
2B), indicating the equilibrated acid-base pair (2 and 3) with
different spin multiplicities.
Recently, we could prepare a cross-conjugated phenoxyl-
nitronyl nitroxide biradical, 2-(3′,5′-di-tert-butylphenyl-4′-oxy)-
4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl 3-oxide, 2,
by the PbO2 oxidation of the corresponding phenol-substituted
radical 1.5 In contrast to the triplet biradical structure of 2 as
confirmed by ESR spectroscopy,6 the one-electron oxidation of
1 in acetonitrile led to the generation of the corresponding
closed-shell cation (3), which was silent on ESR. We report
here an acid-base pair of 2 and 3 with different spin multiplici-
ties, both of which satisfies the requirement of kinetic stability
in solutions (Scheme 1), representing a novel pH sensor
responding by the change of magnetic property.
As reported before,5 the PbO2 oxidation of a phenol-
substituted nitronyl nitroxide radical (D•-AH, 1) led to the
formation of diradical 2 almost quantitatively, which was stable
in solution and could be isolated as a powder. The redox
properties of 1 and 2 were investigated by cyclic voltammetry
in acetonitrile containing 0.1 M n-Bu4NBF4 as a supporting
electrolyte. The voltammogram of 1 showed a reversible couple
at +0.71 V vs SCE (Figure 1a). While the oxidation of 2
occurred at +1.00 V vs SCE, the reduction of 2 showed a
reversible peak at -0.05 V vs SCE (Figure 1e). Thus, all of
the intermediates generated by the redox reactions of 1 and 2,
such as closed-shell cation (D+-AH, 3), cation radical (D+-
The interconversions were also observable by cyclic voltam-
metry in acetonitrile solutions. When 10.0 mM pyridine was
added to the solution of 1, the oxidation peak at +0.7 V became
irreversible and the formation of 2 was identified by the
appearance of its redox peak at +1.0 V (Figure 1d). The
formation of 2 decreased with decreasing basicity of pyridines
B
on the order of pyridine (pKa ) 12.3)8a > 3-chloropyridine
B
B
(pKa ) 9.0,8b Figure 1c) > 4-cyanopyridine (pKa ) 7.0,8c
Figure 1b). The voltammogram of 2 (Figure 1e) was also
affected by the presence of acids. When 10.0 mM acetic acid
(pKa ) 22.3, Figure 2f) was added,9 the oxidation peak of 2 at
+1.0 V remained but the reversible cathodic couple at -0.05
V vs SCE became irreversible, indicating that acetic acid is not
stronger acid than 3 but can protonate the anion radical 5 to
yield 1. With a stronger acid such as trifluoroacetic acid (pKa
(1) (a) Kahn, O. Molecular Magnetism; VCH Publishers: New York,
1993. (b) Gu¨tlich, P.; Hauser, A.; Spiering, H. Angew. Chem., Int. Ed. Engl,
1994, 33, 2024 and references cited therein.
(2) Iwamura, H. AdV. Phys. Org. Chem 1990, 26, 179 and references
cited therein.
(3) (a) Sander, W.; Bucher, G.; Reichel, F.; Cremer, D. J. Am. Chem.
Soc. 1991, 113, 5311. (b) Bush, L. C.; Heath, R. B.; Berson, J. A. J. Am.
Chem. Soc. 1993, 115, 9830.
(4) Salem, L.; Rowland, C. Angew. Chem., Int. Ed. Engl. 1972, 11, 92.
(5) Ishiguro, K.; Ozaki, M.; Kamekura, Y.; Sekine, N.; Sawaki, Y. Mol.
Cryst. Liq. Cryst. In press.
(6) The hyperfine coupling constant in toluene is aN ≈ 3.7 G (2N) at
room temperature and the zero-field splitting parameters at 77 K are |D/
hc| ) 0.107 cm-1 and |E/hc| ≈ 0.5 See Supporting Information for the
ESR spectra.
(7) (a) Bethell, D.; Handoo, K. L.; Fairhurst, S. A.; Sutcliffe, L. H. J.
Chem Soc., Perkin Trans. 2 1977, 707. (b) Lee, K. Y.; Kuchynka, D. J.;
Kochi, J. K. Inorg. Chem. 1990, 29, 4196.
(8) (a) Coetzee, J. F.; Padmanabhan, G. R. J. Am. Chem. Soc. 1965, 87,
5005. (b) Cauquis, G.; Deronzier, A.; Serve, D.; Vieil, E. J. Electroanal.
Chem. Interfacial Electrochem. 1975, 60, 205. (c) Schlesener, C. J.;
Amatore, C.; Kochi, J. K. J. Am. Chem. Soc. 1984, 106, 7472.
(9) Kolthoff, I. M.; Chantooni, Jr., M. K.; Bhowmik, S. J. Am. Chem.
Soc. 1968, 90, 23.
S0002-7863(96)03289-1 CCC: $14.00 © 1997 American Chemical Society

3626 J. Am. Chem. Soc., Vol. 119, No. 15, 1997
Communications to the Editor
Table 1. Reaction of Cation 3 with Substituted Pyridines
substituent
pKaB(pyridines)a
Kb
pKa(3)c
4-CN
3-Cl
3-F
8.0
9.0
9.4
(9.8 ( 0.2) × 10-4
(5.1 ( 0.6) × 10-2
(9.8 ( 0.6) × 10-3
average
10.8
10.2
11.2
10.7 ( 0.5
a pKaB values of substituted pyridines in acetonitrile at 25 °C.8
b Equilibrium constants for the reaction of 3 + Py a 2 + PyH+ at 0
°C. c pKa values of 3 at 25 °C.
Scheme 2
Figure 1. Cyclic voltammograms of 1.0 mM 1 in the absence (a) and
presence of 10.0 mM (b) 4-cyanopyridine, (c) 3-chloropyridine, or (d)
pyridine, and those of 1.0 mM 2 in the absence (e) and presence of
10.0 mM (f) acetic acid or (g) trifluoroacetic acid in acetonitrile
containing 0.1 M n-Bu4NBF4 at sweep rate ) 1 V/s.
(273.15/298.15) log(K). As listed in Table 1, the resulting
pKa(3) of 10.7 ( 0.5 is in good agreement to the observed effects
of acids or bases on cyclic voltammetry.
The unexpectedly high acidity of 3, which is intermediate
between those of common phenols (pKa ) 19-27)13 and phenol
radical cation (pKa ≈ 1)14 and is as high as that of picric acid
(pKa ) 11),13 may be explained by assuming that the actual
conjugate base of 3 is a singlet zwitterion 6 (D+-A-) rather
than 2. As shown in Scheme 2, the transition (6 f 2) gains an
additional stabilization energy due to the exothermic electron
transfer from A- to D+, where a large energy gap of ca. -1.0
eV (≈-16.9 pKa unit) is estimated from the Weller expression,
2
E
1/2(5/2) - E1/2(2/4) + e0 /ꢀa.15 The protonation of triplet 2 to
singlet 3 may take place either via 6, which may exist in
equilibrium with 2, or at the singlet-triplet surface crossing
point.16
In summary, the acid-base pair (2 and 3) with different spin
multiplicities can be prepared independently by the oxidation
of the parent phenol 1, and they are shown to be interconvertible
in solution, which may provide a basis for novel magnetic
switching devices or sensors for pH monitoring.
Figure 2. Absorption spectra of ca. 0.05 mM of (A) 3 and (B) 2 in
acetonitrile. Dashed lines show the spectral changes upon the addition
of (A) pyridine and (B) trifluoroacetic acid.
Acknowledgment. This work was supported in part by a Grant-
in-Aid for Scientific Research from the Ministry of Education, Science
and Culture of Japan.
≈ 13,10 Figure 1g), in accordance with the spectroscopic
observation, the formation of cation 3 was proved by the
appearance of the reduction peak at +0.7 V vs SCE.
Supporting Information Available: ESR spectra of 2 (1 page).
See any current masthead page for ordering and Internet access
instructions.
The equilibrium constants (K) for 3 with various substituted
pyridines were determined from spectral changes upon the
-
addition of pyridines to the 0.05 mM 3‚BF4 solution11 at 0
JA963289M
°C,12 and the pKavalues for the equilibrium between 3 and 2 at
(13) Kolthoff, I. M.; Chantooni, M. K., Jr. J. Am. Chem. Soc. 1969, 91,
4621.
B
25 °C were obtained according to pKa(3) ) pKa (pyridines) +
(14) (a) The acidity of phenol radical cation in MeCN was calculated
from the difference between oxidation potentials of phenol and phenoxide
(2.1 and 0.55 V vs Ag/AgI, respectively)14b and pKa of phenol (27).13 (b)
Bordwell, F. G.; Cheng, J.-P. J. Am. Chem. Soc. 1991, 113, 1736.
(15) (a) Weller. A. Z. Physik. Chem. NF 1982, 133, 93. (b) The value of
(10) (a) The pKa value of CF3CO2H in acetonitrile was estimated from
the pKa in dimethyl sulfoxide (3.45)10b and a typical difference in acidities
(9.7 pKa unit) of carboxylic acids in DMSO and in MeCN.9 (b) Bordwell,
F. G. Acc. Chem. Res. 1988, 21, 456.
2
(11) The oxidant-free solution of 3‚BF4- was prepared by the oxidation
e0 /ꢀa ) 0.06 eV is applied, which corresponds to the Coulombic energy
of 1 with equimolar of NO + BF4 followed by the removal of NO• by
of organic ion pair in acetonitrile at separation distance of 7 Å.
(16) (a) Berson, J. A. Rearrangements in Ground and Excited States;
de Mayo, P., Ed.; Academic Press: New York, 1980; Vol. I, p 346. (b)
We are thankful to a reviewer for the consideration on this point.
-
passing argon gas.
(12) The cation 3 decomposed slowly in acetonitrile at room temperature
but was stable for over several hours at 0 °C.