One-electron reduction of an antiferromagnetically coupled triradical yields a mixed-valent biradical with enhanced ferromagnetic coupling

Full text of DOI:10.1021/ja010630g

J. Am. Chem. Soc. 2001, 123, 6431-6432
One-Electron Reduction of an Antiferromagnetically
6431
Coupled Triradical Yields a Mixed-Valent Biradical
with Enhanced Ferromagnetic Coupling
David A. Shultz* and R. Krishna Kumar
Department of Chemistry
North Carolina State UniVersity
Raleigh, North Carolina 27695-8204
ReceiVed March 9, 2001
A variety of molecule-based magnetic materials1,2 has been
prepared in which the primary structural element is a high-spin
(S g 1) organic molecule.3-7 The most common high-spin
molecule design is to attach paramagnetic functional groups to a
π-system8,9 (a Coupler) to form a cross-conjugated, nondisjoint10
spin system. For example, the structural features of trimethylene-
methane and m-xylylene form the basis of a multitude of high-
spin molecules.6,9,11 However, since the beginning of high-spin
organic molecule research no new molecular design paradigm
has been proposed.
Certain mixed-valent metal complexes exhibit a spin-spin
coupling mechanism for which there is no organic counterpart:
double exchange, also called spin-dependent delocalization
(SDD).2 The salient feature of SDD is that the highest spin-
multiplicity state is lowest energy. An organic system that exhibits
true SDD may be difficult to design, but exchange-coupled,
mixed-valent organic species are not difficult to design. Despite
this fact, only a few examples of such species have been reported,
none of which relate either new coupling pathways or enhanced
ferromagnetic coupling.12-14 From these reports it appears that
mixed valency does not offer new possibilities for designing high-
spin organic molecules.
Nevertheless, we believe that there are opportunities, and to
begin our search for new design principles based on mixed
valency, we desired a three-spin system featuring an inherently
weak antiferromagnetic Coupler. We suspected that the effects
of mixed valency might be modest and difficult to observe in a
strongly coupled system. Moreover, we wanted to discover
enhanced ferromagnetic coupling as a consequence of mixed
valency. Herein we report the first example of enhanced ferro-
magnetic coupling in a mixed-valent molecule that lacks an
effective π-type ferromagnetic Coupler ([Na+][(LZn)3(SQ2Cat)]-),
formed from one-electron reduction of an antiferromagnetically
coupled triradical, (LZn)3(SQ3).15
Figure 1. 77 K EPR spectra (inserts show ∆ms g 2 transitions) for (a)
(LZn)3(SQ3), (b) incompletely oxidized reaction mixture containing a
large mole fraction of (LZn)3(SQ2HCat), (c) [Na+][(LZn)3(SQ2Cat)]-.
Asterisks (*) denote biradical species. All samples were ca. 10 mM
solutions in 2-MTHF. See Table 1 for zero-field splitting parameters.
Table 1. Zero-Field Splitting Parameters for Triradical and
Biradicalsa
complex
(LZn)3(SQ3)
|D/hc|/cm-1
0.00854
0.01417
0.01215
|E/hc|/cm-1
0
(LZn)3(SQ2HCat)
0.00108
0.00058
[Na+][(LZn)3(SQ2Cat)]-
a ZFS parameters determined by spectral simulation.17,20
As shown in Figure 1a, the triradical exhibits an axial quartet
powder EPR spectrum18,19 at 77 K in 2-methyltetrahydrofuran (2-
MTHF). Also visible are ∆ms ) 2 (with fine structure), and ∆ms
) 3 transitions, confirming the existence of the quartet state. The
∆ms ) 1 region was simulated17,20 to provide zero-field splitting
parameters, |D/hc| ) 0.00854 cm-1, and |E/hc| ) 0 cm-1 (see
also Table 1).
Figure 2a shows the EPR Curie plot for the triradical. Assuming
a Boltzmann distribution of spin states and regular-triangular21
coupling, the data were fit according to the expression:
4 exp(3J/kT)
C
T
Iquartet
)
[
]
4 + 4 exp(3J/kT)
The fit parameters give J ) -18 cm-1; that is, for (LZn)3(SQ3)
the three spins are antiferromagnetically coupled. This was both
desired and expected since (LZn)3(SQ3) lacks an effective
ferromagnetic Coupler.8,9
To demonstrate intrinsic antiferromagnetic coupling within this
molecular framework, we examined the spin-spin coupling in
the localized biradical, (LZn)3(SQ2HCat). If the oxidation
(9) Rajca, A. Chem. ReV. 1994, 94, 871.
(10) Borden, W. T.; Davidson, E. R. J. Am. Chem. Soc. 1977, 99, 4587.
(11) Berson, J. A. Diradicals; Wiley: New York, 1982.
(12) Nakamura, T.; Momose, T.; Shida, T.; Kinoshita, T.; Takui, T.; Teki,
Y.; Itoh, K. J. Am. Chem. Soc. 1995, 117, 11292.
(13) Rajca, S.; Rajca, A. J. Am. Chem. Soc. 1995, 117, 9172.
(14) Sedo´, J.; Ruiz, D.; VidalGancedo, J.; Rovira, C. J.; Bonvoisin, J.;
Launay, J. P.; Veciana, J. AdV. Mater. 1996, 8, 748.
(15) L ) hydrotris[3-(4′-chlorophenyl)-5-methylpyrazolyl]borate.
(16) Ruf, M.; Noll, B. C.; Groner, M. D.; Yee, G. T.; Pierpont, C. G. Inorg.
Chem. 1997, 36, 4860.
(17) See Supporting Information for synthetic details, electrochemistry, and
EPR simulations.
(18) Weltner, W., Jr. Magnetic Atoms and Molecules; Dover Publications:
New York, 1983.
Complex (LZn)3(SQ3) was prepared from the corresponding
tris(catechol) using methodology first reported by Pierpont.16,17
(1) Lahti, P. M. Magnetic Properties of Organic Materials; Marcel
Dekker: New York, 1999.
(2) Kahn, O. Molecular Magnetism; VCH: New York, 1993.
(3) Caneschi, A.; Gatteschi, D.; Sessoli, R.; Rey, P. Acc. Chem. Res. 1989,
22, 392.
(4) Miller, J. S.; Calabrese, J. C.; McLean, R. S.; Epstein, A. J. AdV. Mater.
1992, 4, 498.
(5) Luneau, D.; Laugier, J.; Rey, P.; Ulrich, G.; Ziessel, R.; Legoll, P.;
Drillon, M. J. Chem. Soc., Chem. Commun. 1994, 741.
(6) Iwamura, H.; Inoue, K.; Haymizu, T. Pure Appl. Chem. 1996, 68, 243.
(7) Shultz, D. A.; Bodnar, S. H.; Kumar, R. K.; Kampf, J. W. J. Am. Chem.
Soc. 1999, 121, 10664.
(19) Signals marked with an * in the EPR spectrum are for two different
S ) 1 species: (LZn)3(SQ2HCat) and [M+][(LZn)3(SQ2Cat)]-, see text.
(20) Bru¨ker WINEPR SimFonia; 1.25, Shareware Version; Bru¨ker Ana-
lytische Messtechnik GmbH: Karlsruhe, Germany, 1996.
(21) Kanno, F.; Inoue, K.; Koga, N.; Iwamura, H. J. Phys. Chem. 1993,
97, 13267.
(8) Dougherty, D. A. Acc. Chem. Res. 1991, 24, 88.
10.1021/ja010630g CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/08/2001

6432 J. Am. Chem. Soc., Vol. 123, No. 26, 2001
Communications to the Editor
∆ms ) 1 region gives zero-field splitting parameters, |D/hc| )
0.01215 cm-1, and |E/hc| ) 0.00058 cm-1 (Table 1). The D-value
for [Na+][(LZn)3(SQ2Cat)]- is slightly less than that of (LZn)3-
(SQ2HCat), suggesting some delocalization in mixed-valent
[Na+][(LZn)3(SQ2Cat)]-.25,26 Also, the E-value is much smaller
than that of the localized biradical (0.00108 cm-1), consistent with
higher symmetry in the mixed-valent biradical. When ferrocenium
tetrafluoroboratewasaddedtothesolutionof[Na+][(LZn)3(SQ2Cat)]-,
the triradical (LZn)3(SQ3) is slowly regenerated, confirming our
assignment.
The Curie plot for the mixed-valent biradical is shown in Figure
2c and is linear, consistent with J g 0 cm-1 22
. Thus, by virtue of
being mixed-valent, the intrinsic antiferromagnetic coupling has
been eclipsed by an increased ferromagnetic contribution.27
One possible explanation for the enhanced ferromagnetic
coupling in [Na+][(LZn)3(SQ2Cat)]- could be that the transfer
integral, |â|, that characterizes delocalization in the mixed-valent
biradical anion28 is greater than the intrinsic antiferromagnetic
coupling constant, |J|. From spectral data Veciana et al. calculated
an interaction parameter of 320 cm-1 for their mixed-valent
biradical anion (shown below),14 so a transfer integral g20 cm-1
might not be unreasonable for [Na+][(LZn)3(SQ2Cat)]-.29
Figure 2. Curie plots and fits for (a) (LZn)3(SQ3) (J ) -18 cm-1), (b)
(LZn)3(SQ2HCat) (J ) -24 cm-1), and (c) [Na+][(LZn)S(SQ2Cat)]-.
(J g 0). All samples were ca. 10 mM solutions in 2-MTHF.
reaction that provides (LZn)3(SQ3) is halted prematurely, the
majority of the reaction mixture is biradical (LZn)3(SQ2HCat).
Figure 1b shows the EPR spectrum of this biradical. Zero-field
splitting parameters are given in Table 1. Addition of the oxidant
PbO2 to the EPR tube results in the disappearance of signals
attributed to (LZn)3(SQ2HCat) and an increase in the quartet
signals, supporting our assignment. A small amount of a second
triplet, [M+][(LZn)3(SQ2Cat)]-, is also present, whose signals
are marked with * (M+ ) pyrazole-H+ or LZn+) in Figure 1b.
Thus, like SDD, delocalization stabilizes the high-spin state:
When the temperature of the incompletely oxidized sample is
lowered, ∆ms ) 1 signals of (LZn)3(SQ2HCat) decrease in
intensity, indicating a singlet ground state, while signals due to
[M+][(LZn)3(SQ2Cat)]- increase in intensity.
The EPR Curie plot using double integration of the coincident
∆ms ) 2 signals for these two biradicals is shown in Figure 2b
and was fit according to the expression:22
The graphic above is not intended to relate relative orbital
energies, only to suggest that delocalization favors a triplet spin-
state.
We demonstrated a new paradigm for high-spin molecule
design based on mixed-valency: delocalization enhances ferro-
magnetic coupling in a molecule that lacks an intrinsic ferro-
magnetic Coupler. Future work will focus on exploring the
generality of this design, spectroscopic study of this and similar
mixed-valent species with S > 1/2, and experimentally determining
the magnitude of the exchange parameter in such species.
3 exp(2J/kT)
C
T
C
T
Itriplet
)
+
+ b
(
)
[
]
1 + 3 exp(2J/kT)
The first right-hand term is for (LZn)3(SQ2HCat), and the second
is for [M+][(LZn)3(SQ2Cat)]- that was expected (see below) to
Acknowledgment. This paper is dedicated to the Chemistry faculty,
past and present, of Shippensburg University, Shippensburg, Pennsylvania.
This work was funded by the National Science Foundation (CHE-
9910076). D.A.S. thanks the Camille and Henry Dreyfus Foundation for
a Camille Dreyfus Teacher-Scholar Award. We thank Professor Stefan
Franzen for helpful discussions.
exhibit a linear Curie plot. Fit parameters give J ) -24 cm-1
;
that is in (LZn)3(SQ2HCat) the two spins are antiferromagneti-
cally coupled. The coupling in this localized biradical provides
strong evidence that the molecular geometry imposed by the
bulky, encapsulating L ligands enforces antiferromagnetic cou-
pling in isovalent species regardless of the number of unpaired
electrons.
Having demonstrated intrinsic antiferromagnetic coupling in
(LZn)3(SQ3) and (LZn)3(SQ2HCat), we are now ready to
examine coupling in the mixed-valent form. The 77 K EPR
spectrum recorded after exposing triradical (LZn)3(SQ3) to sodium
mirror in the presence of 18-crown-623 is shown in Figure 1c
and is also characteristic of an S ) 1 triplet.24 A ∆ms ) 2 transition
confirms the existence of the triplet state. Simulation17,20 of the
Supporting Information Available: Synthetic details, electrochem-
istry, and EPR simulations and spectra (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
JA010630G
(25) Shultz, D. A.; Boal, A. K.; Lee, H.; Farmer, G. T. J. Org. Chem.
1998, 63, 9462.
(26) Jain, R.; Sponsler, M. B.; Coms, F. D.; Dougherty, D. A. J. Am. Chem.
Soc. 1988, 110, 1356.
(27) The mixed-valent form was also prepared by CoCp2 reduction of
(LZn)3(SQ3), and by KH deprotonation of (LZn)3(SQ2HCat). In all cases,
the mixed-valent form exhibits a linear Curie plot, suggesting that counterions
do not play a significant role in the exchange coupling. See Supporting
Information.
(28) We have shown that stabilization of mixed-valent semiquinone-
catecholate species, as measured by redox splitting, is nearly identical to the
stabilization of mixed-valent quinone-semiquinone species.7 Redox splittings
in the tris(quinone) derived from (H2Cat)3 are ca. 215 mV.
(29) For now, the reactivity of [Na+][(LZn)3(SQ2Cat)]- precludes spectral
analysis that would facilitate estimation of delocalization. Future efforts will
include this area of study.
(22) Berson, J. A. The Chemistry of the Quinonoid Compounds; Patai, S.
R., Z., Ed.; John Wiley & Sons: New York, 1988; Vol. II, p 482.
(23) (LZn)3(SQ3) does not react with sodium mirror in the absence of the
crown ether due to inherently slow heterogeneous electron transfer caused by
the encapsulating ligands. The triradical also does not exhibit cyclic volta-
mmetric waves in the potential region characteristic of other Zn(semiquinone)
complexes prepared in our labs. Despite this, the mixed-valent biradical can
also be formed by reacting (LZn)3(SQ2HCat) with potassium hydride, and
(LZn)3(SQ3) can be prepared by oxidation of (LZn)3(SQ2HCat) with PbO2.
(24) Wasserman, E.; Snyder, L. C.; Yager, W. A. J. Chem. Phys. 1964,
41, 1763.