Trends in exchange coupling for trimethylenemethane-type bis(semiquinone) biradicals and correlation of magnetic exchange with mixed valency for cross-conjugated systems

Article abstract of DOI:10.1021/ja0367849

A magnetostructural correlation (conformational electron spin exchange modulation) within an isostructural series of biradical complexes is presented. X-ray crystal structures, variable-temperature electron paramagnetic resonance spectroscopy, zero-field

Full text of DOI:10.1021/ja0367849

Published on Web 08/29/2003
Trends in Exchange Coupling for Trimethylenemethane-Type
Bis(semiquinone) Biradicals and Correlation of Magnetic
Exchange with Mixed Valency for Cross-Conjugated Systems
David A. Shultz,*,§ Rosario M. Fico, Jr.,§ Scot H. Bodnar,§ R. Krishna Kumar,§
Kira E. Vostrikova,§ Jeff W. Kampf,‡ and Paul D. Boyle§
Contribution from the Department of Chemistry, North Carolina State UniVersity,
Raleigh, North Carolina 27695-8204, and Department of Chemistry, UniVersity of Michigan,
Ann Arbor, Michigan 48109-1055
Received June 19, 2003; E-mail: david_shultz@ncsu.edu
Abstract: A magnetostructural correlation (conformational electron spin exchange modulation) within an
isostructural series of biradical complexes is presented. X-ray crystal structures, variable-temperature
electron paramagnetic resonance spectroscopy, zero-field splitting parameters, and variable-temperature
magnetic susceptibility measurements were used to evaluate molecular conformation and electron spin
exchange coupling in this series of molecules. Our combined results indicate that the ferromagnetic portion
of the exchange couplings occurs via the cross-conjugated π-systems, while the antiferromagnetic portion
occurs through space and is equivalent to incipient bond formation. Thus, molecular conformation controls
the relative amounts of ferro- and antiferromagnetic contributions to exchange coupling. In fact, the exchange
parameter correlates with average semiquinone ring torsion angles via a Karplus-Conroy-type relation.
Because of the natural connection between electron spin exchange coupling and electronic coupling related
to electron transfer, we also correlate the exchange parameters in the biradical complexes to mixed valency
in the corresponding quinone-semiquinone radical anions. Our results suggest that delocalization in the
cross-conjugated, mixed-valent radical anions is proportional to the ferromagnetic contribution to the
exchange coupling in the biradical oxidation states.
Introduction
preceded by theoretical/computational studies that led to the
formation of a rubric for creating high-spin structures.13-16 This
Whether as transient reactive intermediates, as challenging
syntheses, as tests of bonding theories, or as objects of
theoretical interest, biradicals have held the fascination of
chemists for nearly a century.1-5 Over the past 20 years,
biradicals have been studied intensively from a magnetochem-
istry perspective, with a focus on understanding electron spin
exchange coupling. Indeed, some bi- and triradicals have found
utility as ligands in the construction of metal ion-containing
magnetically ordered solids.6,7 Chemists now have a thorough
understanding of the structural requirements for through-bond
electron spin exchange coupling.8-10 This understanding of
exchange coupling allows synthetic chemists to design and
prepare new multispin systems with very high-spin ground
states.11,12 In fact, many successful synthetic efforts were
involves attaching spin-containing moieties to a “Coupler” such
that the SOMOs in the resulting biradical are “nondisjoint”
(biradical obeys Hund’s first rule).14 Several saturated and
π-system Couplers have been identified and explored.8,9
To fully reveal the relationships between molecular structure
and ferromagnetic exchange coupling, one must understand the
effects of both substituents and conformation.17-19 We recently
presented the first experimental examples of substituent-
modulated exchange coupling in a series of triplet ground-state
biradicals 1(SQZnL)2 (Scheme 1),20 As for conformation-
dependent exchange coupling, it has been understood for some
(11) Rajca, A.; Wongsriratanakul, J.; Rajca, S. Science 2001, 294, 1503-1505.
(12) Rajca, A.; Rajca, S.; Wongsriratanakul, J. J. Am. Chem. Soc. 1999, 121,
6308-6309.
§ North Carolina State University.
(13) Jacobs, S. J.; Shultz, D. A.; Jain, R.; Novak, J.; Dougherty, D. A. J. Am.
Chem. Soc. 1993, 115, 1744-1753.
‡ University of Michigan.
(1) Chichibabin, A. E. Ber. Dtsch. Chem. Ges. 1907, 40, 1810-1819.
(2) Schlenk, W.; Brauns, M. Chem. Ber. 1915, 48, 661-669.
(3) Schlenk, W.; Brauns, M. Chem. Ber. 1915, 48, 716-728.
(4) Wentrup, C. Science 2002, 295, 1846-1847.
(5) Scheschkewitz, D.; Amii, H.; Gornitzka, H.; Schoeller, W. W.; Bourissou,
D.; Bertrand, G. Science 2002, 295, 1880-1881.
(6) Iwamura, H.; Inoue, K.; Haymizu, T. Pure Appl. Chem. 1996, 68, 243-
252.
(14) Borden, W. T.; Davidson, E. R. J. Am. Chem. Soc. 1977, 99, 4587-4594.
(15) Borden, W. T. Diradicals; Wiley: New York, 1982.
(16) Borden, W. T.; Iwamura, H.; Berson, J. A. Acc. Chem. Res. 1994, 27, 109-
116.
(17) For work on substituent and conformation effects on biradicals using
computational chemistry and EPR spectroscopy, see the following and ref
18: Silverman, S. K.; Dougherty, D. A. J. Phys. Chem. 1993, 97, 13273-
13283.
(7) Inoue, K.; Hayamizu, T.; Iwamura, H.; Hashizume, D.; Ohashi, Y. J. Am.
Chem. Soc. 1996, 118, 1803-1804.
(18) West, A. P., Jr.; Silverman, S. K.; Dougherty, D. A. J. Am. Chem. Soc.
1996, 118, 1452-1463.
(19) For electronic modulation of J for singlet ground-state biradicals, see:
Berson, J. A. Structural Determinants of the Chemical and Magnetic
Properties of Non-Kekule Molecules. In Magnetic Properties of Organic
Materials; Lahti, P. M., Ed.; Marcel Dekker: New York, 1999; pp 7-26.
(8) Dougherty, D. A. Acc. Chem. Res. 1991, 24, 88-94.
(9) Rajca, A. Chem. ReV. 1994, 94, 871-893.
(10) Lahti, P. M. Magnetic Properties of Organic Materials; Marcel Dekker:
New York, 1999.
9
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J. AM. CHEM. SOC. 2003, 125, 11761-11771
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A R T I C L E S
Shultz et al.
Scheme 1
Scheme 2
Results and Analysis
time that severe torsions of bonds that connect spin-containing
and Coupler fragments attenuate exchange coupling, albeit not
in an isostructural series, rather in a handful of disparate
examples.21 Herein, we report the first magnetostructural study
of conformational exchange modulation in an isostructural series
of bis(semiquinone) complexes.
Furthermore, we show how energy matching of Coupler and
semiquinone (SQ) orbitals can enhance exchange coupling.
Finally, because of the natural connection between electron spin
exchange coupling and electronic coupling matrix elements
pertinent to electron transfer, we correlate magnetic exchange
coupling with Coupler-modulated delocalization within mixed-
valent forms of our ligands.
Synthesis. The biradical complexes 2(SQZnL)2-7(SQZnL)2
presented here are shown in Scheme 1, left (R ) SQZnL).
Complex formation generally follows a procedure (Scheme 1,
right) reported previously by Pierpont,22 and adopted by us (see
Experimental Section for details). The bis(catechol) ligands, 3-
(CatH2)2-7(CatH2)2 have been reported previously,23,24 except
for 2(CatH2)2. The synthetic outline for preparation of new
molecules is given in Scheme 2.
Preparation of 2(CatH2)2 begins with carbinol 824 which was
oxidized to ketone 9 and subsequently transformed into thione
10 using P4S10.25 Reaction of thione 10 with diazo compound
(22) Ruf, M.; Noll, B. C.; Groner, M. D.; Yee, G. T.; Pierpont, C. G. Inorg.
(20) Shultz, D. A.; Bodnar, S. H.; Lee, H.; Kampf, J. W.; Incarvito, C. D.;
Rheingold, A. L. J. Am. Chem. Soc. 2002, 124, 10054-10061.
(21) Shultz, D. A. Conformational Exchange Modulation in Trimethylen-
emethane (TMM)-Type Biradicals. In Magnetic Properties of Organic
Materials; Lahti, P., Ed.; Marcel Dekker: New York, 1999.
Chem. 1997, 36, 4860-4865.
(23) Shultz, D. A.; Boal, A. K.; Lee, H.; Farmer, G. T. J. Org. Chem. 1998, 63,
9462-9469.
(24) Shultz, D. A.; Lee, H.; Fico, R. M., Jr. Tetrahedron 1999, 55, 12079-
12086.
9
11762 J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003

Trends in Exchange Coupling in Biradical Complexes
A R T I C L E S
Figure 1. Thermal ellipsoid plots of 2(SQZnL)2-4(SQZnL)2, 6(SQZnL)2, and 7(SQZnL)2. Ancillary ligands and hydrogens have been omitted for clarity.
11,26 followed by desulfurization with triphenylphosphine gave
alkene 12. Deprotection afforded 2(CatH2)2, and oxidation gave
bis(quinone) 2(Q)2.
2-MTHF. All spectra are consistent with randomly oriented
triplet species31 along with small amounts of doublet mono-
radical impurities.32 The zero-field-splitting parameters for the
biradicals, obtained by simulation,33 are given in Table 3. The
spectral shapes of all biradicals, except 3(SQZnL)2, are invariant
with temperature changes (4-77 K), suggesting the absence of
rotamers.34 The results for 3(SQZnL)2 are discussed below.
Variable-Temperature EPR Spectroscopy. Curie plots for
the doubly integrated ∆ms ) 2 signals of 2(SQZnL)2-7-
Molecular Structures and Conformations. The molecular
structures of 2(SQZnL)2-7(SQZnL)2 were determined by
X-ray crystallography, and thermal ellipsoid plots are shown
in Figure 1. The structure of 5(SQZnL)2 was reported previ-
ously.27 Important structural parameters are collected in Tables
1 and 2. All of the dioxolene bond lengths are consistent with
the SQ oxidation state as opposed to Cat or CatH.28-30 Other
bond lengths and angles are listed in the Supporting Informa-
tion.
(28) Pierpont, C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331-442.
(29) Ruf, M.; Lawrence, A. M.; Noll, B. C.; Pierpont, C. G. Inorg. Chem. 1998,
37, 1992-1999.
(30) Shultz, D. A.; Bodnar, S. H.; Kumar, R. K.; Lee, H.; Kampf, J. W. Inorg.
Chem. 2001, 40, 546-549.
Frozen-Solution EPR Spectroscopy. Figure 2 shows the
EPR spectra of 2(SQZnL)2-7(SQZnL)2 recorded at 77 K in
(31) Wasserman, E.; Snyder, L. C.; Yager, W. A. J. Chem. Phys. 1964, 41,
1763-1772.
(32) Some of the g ) 2 signal is a double-quantum transition as shown by power
dependence studies, see: deGroot, M. S.; van der Waals, J. H. Physica
1963, 29, 1128-1132.
(25) Scheeren, J. W.; Ooms, P. H. J.; Nivard, R. J. F. Synthesis 1973, 149-
151.
(26) Shultz, D. A.; Boal, A. K.; Lee, H.; Farmer, G. T. J. Org. Chem. 1999, 64,
(33) Bruker WINEPR SimFonia, 1.25, Shareware Version ed.; Bru¨ker Ana-
lytische Messtechnik GmbH: Karlsruhe, Germany, 1996.
(34) Shultz, D. A.; Boal, A. K.; Farmer, G. T. J. Am. Chem. Soc. 1997, 119,
3846-3847.
4386-4396.
(27) Shultz, D. A.; Bodnar, S. H.; Kampf, J. W. Chem. Commun. 2001, 93-
94.
9
J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003 11763

A R T I C L E S
Shultz et al.
Table 1. Important Bond Lengths
Table 2. Semiquinone Ring Torsion Anglesa-d
semiquinone ring
torsion angles (deg)a
average semiquinone
ring torsion angles (deg)
biradical
2(SQZnL)2
3(SQZnL)2
4(SQZnL)2b
5(SQZnL)2
6(SQZnL)2
7(SQZnL)2
64.10 ( 0.13, 78.04 ( 0.13
47.51 ( 0.18, 48.94 ( 0.27
48.37 ( 0.29
71.1 ( 3.5
48.23 ( 0.4
48.37(0.29
46.5 ( 3.9
7.3 ( 2.4
50.3 ( 0.3, 42.6 ( 0.8
4.9 ( 0.8, 9.7 ( 0.8
42.3 ( 0.9, 52.4 ( 0.1
47.4 ( 2.5
a Torsion angle defined as the angle between the plane of a SQ ring and
the plane containing the ethene Coupler. b Data from ref 27. c 7(SQZnL)2
torsion angles defined as bond torsion through the sp3 carbon of the fluorenyl
group.
(SQZnL)2 are shown in Figure 3, and are summarized in Table
4. Biradicals 4(SQZnL)2-7(SQZnL)2 gave linear responses,
consistent with J > 0 (ferromagnetic coupling) or J ) 0,35 and
are in agreement with our previous findings on bis(SQ)s
prepared by electrochemical and chemical reduction.23,24 The
intensity of ∆ms ) 2 signal of 2(SQZnL)2 varies with
decreasing temperature consistent with antiferromagnetic cou-
pling. The data were fit to eq 1:35
Figure 2. EPR spectra of biradicals 2(SQZnL)2-7(SQZnL)2 collected at
77 K in 2-MTHF.
where C is a constant and J is the exchange parameter. Best fit
-2J
kT
3 exp
results give J ) -35 cm-1
.
(
)
C
T
IEPR
)
(1)
-2J
kT
(35) Berson, J. A. The Chemistry of the Quinonoid Compounds; Patai, S. R.,
[
]
1 + 3 exp
(
)
Z., Ed.; John Wiley & Sons: New York, 1988; Vol. II, p 482.
9
11764 J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003

Trends in Exchange Coupling in Biradical Complexes
A R T I C L E S
Figure 3. EPR Curie plots for biradical complexes 2(SQZnL)2, 4(SQZnL)2-7(SQZnL)2 (A) and 3(SQZnL)2 (B).
Table 3. Zero-Field-Splitting Parameters
-1
-1
biradical
|D/hc| (cm
)
|E/hc| (cm
)
|E/D|
2(SQZnL)2
3(SQZnL)2
4(SQZnL)2
5(SQZnL)2
6(SQZnL)2
7(SQZnL)2
0.0124
0.0105
0.0098
0.0093
0.0068
0.0123
0.0011
0.0008
0.0006
0.0006
0.0017
0.0005
0.0887
0.0762
0.0612
0.0645
0.2500
0.0407
Table 4. Variable-Temperature Susceptibility Fit Parameters, and
Results from Variable-Temperature EPR Experimentsa
J
(cm
zJ′
EPR Curie plot,
J(cm-1
-1 b
biradical
)
(cm-1 c
)
)
2(SQZnL)2
3(SQZnL)2
4(SQZnL)2
-30.3 ( 0.8d
+24.4 ( 0.6
+87.0 ( 3.0
+0.011 ( 0.001
curved, -35
-0.032 ( 0.008 curved, -3,-9,-38
-0.024 ( 0.018
-0.029 ( 0.002
-0.080 ( 0.004
linear, J g 0
linear, J g 0
linear, J g 0
linear, J g 0
e
5(SQZnL)2 +209.4 ( 1.4
6(SQZnL)2 +163.6 ( 1.6
7(SQZnL)2
+0.99 ( 0.06 -0.503 ( 0.021
a Fits used g ) 2.002. b J > 0 for triplet ground-state. c Intermolecular
interaction. d J-value for average determined from øpara vs T plot fit
parameters and Tmax.
e Fit parameters from from ref 27.
The ∆ms ) 1 region of the EPR spectrum of biradical 3-
(SQZnL)2 shows that at least three rotamers characterized by
different zero-field splitting parameters are present (see Sup-
porting Information). By fitting either the intensities of the low-
field z-transition of the ∆ms ) 1 region, or by fitting the doubly
integrated ∆ms ) 2 region to eq 1, we find best fits for three
rotamers having J ≈ -3, -9, and -38 cm-1. This result differs
from the linear Curie plot for the sodium salt of the 3(SQ)2,36
but parallels the results for the corresponding bis(phenoxy)
radical.34
Figure 4. øparaT vs T plots: 2(SQZnL)2 (0); 3(SQZnL)2 (4); 4(SQZnL)2
([); 5(SQZnL)2 (2); 6(SQZnL)2 (O); 7(SQZnL)2 (1). Inset: øpara vs T
plot for 2(SQZnL)2.
2Ng2â2
k[3 + e-2J/kT
øT )
(2)
Solid-State Magnetic Susceptibility Measurements. The
magnetic susceptibility data for all of the compounds are plotted
in Figure 4. Modeling the temperature-dependent øparaT products
of S ) 1 molecules can be achieved by fitting to a field-
independent van Vleck expression, eq 2,37,38
]
where g is the isotropic Lande´ constant (g ) 2.0023), â is the
Bohr magneton, T is temperature in Kelvin, k is Boltzmann’s
constant, and J is the intramolecular SQ-SQ exchange coupling
parameter. The relative singlet- and triplet-state energies were
derived using the Hamiltonian, Η ) -2JSˆ1‚Sˆ2, where Sˆ1 and
Sˆ2 are the spin operators for the SQs. The decrease in the øT
data at low temperatures was accounted for with a Weiss
correction, using the expression øeff ) ø/(1 - ϑø), where ϑ )
(36) Shultz, D. A.; Boal, A. K.; Driscoll, D. J.; Farmer, G. T.; Hollomon, M.
G.; Kitchin, J. R.; Miller, D. B.; Tew, G. N. Mol. Cryst. Liq. Cryst. 1997,
305, 303-310.
(37) Kahn, O. Molecular Magnetism; VCH: New York, 1993; p 104.
(38) Bleaney, B.; Bowers, K. D. Proc. R. Soc. London 1952, A214, 451-465.
9
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A R T I C L E S
Shultz et al.
Table 5. Cyclic Voltammetric Data for Bis(quinones)a
containing units and the TMM functionality to provide confor-
mational modulation of exchange coupling.21
bis(quinone)
E
1/2(1)b
E
1/2(2)c
∆E
K
com
d
e
1/2
Possible capping groups include aryl groups and bicyclic or
tricyclic ring systems. Simple primary or secondary alkyl groups
were not considered since they contain allylic hydrogens that
are excellent candidates for abstraction and subsequent loss of
spin.42 However, the bridgehead C-H bonds of bicyclic or
tricyclic ring-capping groups are orthogonal to the ethene
Coupler π-bond. Moreover, these bridgehead hydrogens experi-
ence van der Waals repulsions with the SQ rings when the SQs
approach coplanarity with the ethene Coupler. This steric
interaction will result in conformational attenuation of ferro-
magnetic exchange coupling and should increase as the bond
angle θ increases. The success of this approach was first seen
in a series of isostructural dinitroxide biradicals reported
previously.26 In the present case, control over biradical confor-
mation for the bis(SQ)s via the capping group can be seen in
the SQ ring torsion angles relative to the ethene Couplers
determined by X-ray crystallographic analysis (Table 2). Gener-
ally, the SQ ring torsion angles increase in going from
6(SQZnL)2 (in which the conformation is held nearly planar
by covalent bonds) to 2(SQZnL)2. The similarity in the
conformations of 3(SQZnL)2 and 4(SQZnL)2 was unexpected,
as molecular mechanics predicted a ca. 10° difference in SQ
ring torsions.26 Nevertheless, a range of SQ ring torsions in
biradicals having the same Coupler is achieved.
As mentioned in the previous section, all dioxolene bond
lengths are in accord with the SQ oxidation state. Small
deviations from “standard”22 lengths are common for SQs with
conjugating substituents.27,43 In addition, the CdC Coupler bond
lengths for 2(SQZnL)2-4(SQZnL)2 and 6(SQZnL)2 are also
within the range of CdC bond lengths for 1,1-diaryl-, triaryl-,
and tetraarylethenes (1.343 ( 0.14 Å) gleaned from the
Cambridge Structural Database.44
Zero-Field Splitting Parameters as a Measure of Biradical
Electronic Structure and Conformation. Because the mech-
anism for zero-field splitting (zfs) in organic biradicals is dipolar,
zfs parameters (D and E) strongly correlate with molecular
conformation.45-48 Such is the case for 2(SQZnL)2-7-
(SQZnL)2: D and E are consistent with both their electronic
structures23,24 and D is a direct measure of the SQ ring
torsions.49-51 As can be seen in Figure 2, |D/hc| decreases in
the series 2(SQZnL)2-7(SQZnL)2 consistent with the corre-
sponding decrease in SQ ring torsions as suggested in Figure
5. Since the trend in experimental zfs parameters is consistent
with the conformations determined by the crystal structures, we
conclude that the conformations observed in the solid state are
similar to those in frozen solution. Our contention is supported
2(Q)2
3(Q)2
4(Q)2
5(Q)2
6(Q)2
7(Q)2
-0.81
-0.83
-0.77
-0.60
-0.82
-0.81
-1.00
-1.04
-0.99
-0.85
-1.09
-1.01
-0.19
-0.21
-0.22
-0.25
-0.27
-0.20
1625
3540
5225
16 790
36 560
2400
a 0.5 mM bis(quinone) in THF, 100 mM n-Bu4NPF6 electrolyte; potentials
in volts vs ferrocene; scan rate ) 200 mV/s; data for 2(Q)2-7(Q)2 from
ref 23. b Redox potential for bis(Q)/(Q-SQ) couple; E1/2 ) (Ep,c - Ep,a)/2.
c Redox potential for (Q-SQ)/bis(SQ) couple; E1/2 ) (Ep,c - Ep,a)/2. d ∆E1/2
) E1/2(2) - E1/2(1). e log Kcom ) 16.9∆E1/2
.
2zJ′/(Ng2â2).39 The origin of zJ′ may be zero-field splitting,
intermolecular interaction, saturation effects, or some combina-
tion of all three.40 The other terms have their usual meanings.39
All curve fit results are presented in Table 4.
The results for 2(SQZnL)2 are displayed as a øpara vs T plot
as the inset in Figure 4. For antiferromagnetically coupled
dimers, the øpara vs T plot displays a signature maximum with
Tmax ) 1.285 J/k.37 We were unable to satisfactorily fit the low-
temperature region of either the øparaT or øpara vs T plot.
Nevertheless, the J-value from the fit parameters (-29.5 cm-1
)
compares well with the value of J determined from Tmax (J )
-31 cm-1). The value for J given in Table 4 represents an
average of the values from the fit and from the value determined
from Tmax
.
Electrochemistry of Corresponding Bis(Quinone)s. The
cyclic voltammograms of 3(Q)2-7(Q)2 have been presented
previously.23,24 It was noted that the separation of the first and
second redox processes (corresponding to reversible production
of the mixed-valent quinone/SQ and isovalent bis(SQ)s) fol-
lowed a periodic trend. Pertinent data are reproduced in Table
5 along with new data for 2(Q)2. The difference in the first and
second redox potentials (∆E1/2) can be used to calculate the
equilibrium constant for the comproportionation reaction, eq 3.41
Thus, Kcom is a measure of the stability of the mixed-valent
semiquione-quinone (Q-SQ) species.41
Discussion
Biradical Design Elements, Molecular Structures, and
Conformations. Design elements of a generic trimethylen-
emethane-type (TMM-type)23 biradical have been presented
previously.26 These design elements are featured in our series
of bis(SQ) complexes. The π-systems are cross-conjugated and
therefore fulfill the π-connectivity requirements for ferromag-
netic coupling of the unpaired electrons of the SQ spin-
containing units.14 “Capping groups” provide steric protection
of the delocalized unpaired electrons. It is the steric interactions
between atoms in the capping group and those in the spin-
containing unit that disrupt conjugation between the spin-
(42) Anderson, K. K.; Shultz, D. A.; Dougherty, D. A. J. Org. Chem. 1997, 62,
7575-7584.
(43) Shultz, D. A.; Bodnar, S. H.; Kumar, R. K.; Kampf, J. W. J. Am. Chem.
Soc. 1999, 121, 10664-10665.
(44) Allen, F. H. Acta Crystallogr. 2002, B58, 380-388.
(45) Ullman, E. F.; Boocock, D. G. B. J. Chem. Soc., Chem. Commun. 1969,
20, 1161-1162.
(46) Mukai, K.; Tamaki, T. Bull. Chem. Soc. Jpn. 1977, 50, 1239-1244.
(47) Sandberg, K. A.; Shultz, D. A. J. Phys. Org. Chem. 1998, 11, 819-824.
(48) We were unsuccessful at calculating the zfs parameters for our biradicals
using the methods described in ref 47, possibly due to the fact several of
the biradicals are between the extremes of disjoint and nondisjoint.
(49) For beautiful examples of |D/hc| as a measure of delocalization, see the
following plus refs 50 and 51: Jain, R.; Sponsler, M. B.; Coms, F. D.;
Dougherty, D. A. J. Am. Chem. Soc. 1988, 110, 1356-1366.
(50) Rajca, A.; Utamapanya, S.; Xu, J. J. Am. Chem. Soc. 1991, 113, 9235-
9241.
(39) O’Connor, C. J. Prog. Inorg. Chem. 1982, 29, 203-283.
(40) Caneschi, A.; Dei, A.; Mussari, C. P.; Shultz, D. A.; Sorace, L.; Vostrikova,
K. E. Inorg. Chem. 2002, 41, 1086-1092.
(41) Ward, M. D. Chem. Soc. ReV. 1995, 24, 121-134.
(51) Rajca, A.; Utamapanya, S. J. Org. Chem. 1992, 57, 1760-1767.
9
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Trends in Exchange Coupling in Biradical Complexes
A R T I C L E S
Figure 5. How conformation manifests itself in the zero-field splitting parameters. Axes are for illustration purposes only, and are not meant necessarily
to represent principal axes of the D tensor.
by the similarity of the solution- and solid-state J-parameters
for 2(SQZnL)2 (Table 4). Moreover, we conclude that severe
torsions of the SQ rings (e.g., 2(SQZnL)2) cause the unpaired
electrons of the SQ π-systems to be “stacked” in such a way to
favor incipient σ-bond formation.52,53 Thus, the same structural
features that give rise to large |D/hc| as indicated in Figure 5,
also result in increased antiferromagnetic coupling in TMM-
type biradicals.
such that in the absence of severe bond torsions,21 the triplet
state is lower in energy than the singlet state. Perusal of Table
4 shows this to be the case: in the solid state, all of the biradicals
save 2(SQZnL)2 are triplet ground-state species but increased
SQ ring torsions make J less ferromagnetic (or more antifer-
romagnetic). Severe twisting, as in 2(SQZnL)2, gives rise to a
singlet ground-state biradical.
The molecular conformation of 5(SQZnL)2 is similar to that
of 3(SQZnL)2 and 4(SQZnL)2, yet the exchange parameter is
significantly greater for 5(SQZnL)2 than for the latter two. We
hypothesize that the orbitals of the quinone-methide Coupler
of 5(SQZnL)2 mix more effectively with those of the SQ groups
than do those of a simple ethene Coupler. Our hypothesis is
supported by the fact that the 5(Q)2 S 5(SQ)2 redox couples
are more positive than those of the corresponding couples for
molecules with a simple CdC Coupler (see Table 5). The more
positive redox potentials suggest that the redox orbitals are lower
in energy by virtue of stronger mixing with quinone-methide
(Coupler) orbitals. The result of this stronger mixing is greater
overlap density which in a biradical oxidation state implies a
larger exchange integral. This result parallels the findings for
bridge substitution in donor-acceptor molecular wires.58
The exchange parameter for the fluorenyl-linked biradical 7-
(SQZnL)2 is also noteworthy. This biradical is comprised of
two SQs linked in a geminal fashion to an sp3 carbon and
therefore gives us the opportunity to determine the overall
effectiveness of the CdC Coupler. As can be seen from the
meager J-parameter of 7(SQZnL)2 compared to that of 4-
(SQZnL)2 or to that of 6(SQZnL)2, the CdC Coupler is indeed
more effective at promoting ferromagnetic exchange coupling
than an sp3 carbon.59
We also note that 3(SQZnL)2 exhibits multiple EPR signals
as a function of temperature (see Supporting Information for
spectra) that are consistent with different rotamers having
different exchange parameters, J (Figure 3B). Similar behavior
was observed by us for the phenoxy biradical analogue of 3-
(SQZnL)2,34 and the nitroxide analogue of 3(SQZnL)2 under-
goes a hysteretic transition involving molecular conformation.54
It appears that the adamantyl Capping group results in shallow
conformational potential energy surface for 1,1-diarylethenes
that can accommodate several metastable conformations.
The Nature of the Exchange Couplings. The exchange
coupling parameter, J, is a sum of ferromagnetic and antifer-
romagnetic terms. As shown in eq 4, the ferromagnetic term
(JF) is composed of the exchange integral, k (the electron-
electron repulsion in the overlap region: k ) φ1(1)φ2(1)|e2/
r12|φ1(2)φ2(2) , where φ1(1)φ2(1) is the overlap density),
while the antiferromagnetic term (JAF) is the result of config-
uration interaction that arises from mixing of two singlet
states via transfer (resonance) integral â, and separated by
energy U.37,55,56 The integral l is the hybrid coulomb integral,
and represents the electron-electron repulsion between an
electron in an orbital with one in the overlap region (e.g., l )
φ1(1)φ2(1)|e2/r12|φ2(2)φ2(2) , where φ1(1)φ2(1) represents the
overlap density).37,57
How does one evaluate the antiferromagnetic contributions
to the exchange parameter in this series? Nominally, our TMM-
type bis(SQ)s 2(SQZnL)2-7(SQZnL)2 have C2 symmetry60 and
the two Hu¨ckel SOMOs transform as a and b irreducible
representations as shown in Figure 6.
4(â + l)2
J ) JF + JAF ≈ 2k -
(4)
U
The cross-conjugated π-connectivity of our biradicals ensures
The electronic configurations (a1b1), (a2b0), and (a0b2) form
nondisjoint SOMOs,14 and therefore a sizable exchange integral
3
four low-lying electronic states: two open-shell states, B and
1B, and two zwitterionic 1A states, respectively. At this
symmetry, configuration interaction cannot provide an antifer-
romagnetic contribution to the exchange parameter since the
lowest singlet is B and the zwitterionic singlets are A. Therefore,
(52) Hatfield, W. E. Chapter 7. Properties of Condensed Compounds (Com-
pounds with Spin Exchange). In Theory and Applications of Molecular
Paramagnetism; Boudreaux, E. A., Mulay, L. N., Eds.; Wiley-Inter-
science: New York, 1976; pp 381-385.
(53) Anderson, P. W. Phys. ReV. 1959, 115, 2-13.
(54) Shultz, D. A.; Fico, R. M., Jr.; Boyle, P. D.; Kampf, J. W. J. Am. Chem.
Soc. 2001, 123, 10403-10404.
(55) Hay, P. J.; Thibeault, J. C.; Hoffmann, R. J. Am. Chem. Soc. 1975, 97,
4884-4899.
(58) Davis, W. B.; Svec, W. A.; Ratner, M. A.; Wasielewski, M. R. Nature
1998, 396, 60-63.
(56) Kahn, O.; Briat, B. J. Chem. Soc., Faraday Trans. 2 1976, 72, 268-281.
(57) Michl, J.; Bonacic-Koutecky, V. Electronic Aspects of Organic Photo-
chemistry; Wiley-Interscience: New York, 1990.
(59) Goldberg, A. H.; Dougherty, D. A. J. Am. Chem. Soc. 1983, 105, 284-
290.
(60) C2V symmetry is precluded by steric interactions between the SQ groups.
9
J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003 11767

A R T I C L E S
Shultz et al.
Figure 6. TMM-type bis(SQ) Hu¨ckel SOMOs (triplet electronic config-
uration shown) and electronic states.
1
1B and A can mix only through b vibrations.61 That is to say
JAF is vibronic.62 Moreover, if it is true generally that vibronic
coupling matrix elements are smaller than configuration interac-
tion matrix elements, then C2-symmetric biradicals are less likely
to have singlet ground states since JAF is attenuated compared
to a higher-symmetry case where configuration interaction
contributes to JAF. Of course, configuration interaction will
operate when orbitals other than those that are singly occupied
are involved.
The preceding discussion notwithstanding, the crystal struc-
tures of 2(SQZnL)2-7(SQZnL)2 show that the molecules in
the solid-state are asymmetric.63 As such, configuration interac-
tion will contribute to JAF. However, there is another antifer-
romagnetic contribution: direct through-space interaction of SQ
groups. This contribution results from the close spatial proximity
of the geminal SQ groups (see also EPR section above) and is
equivalent to incipient bond formation.52,53 This through-space
antiferromagnetic contribution increases with incresasing SQ
torsions and is therefore greatest in 2(SQZnL)2. As noted in
the EPR section, the increased SQ torsions that give rise to large
|D| also result in increased antiferromagnetic coupling.
The exchange coupling parameters for those biradicals having
a simple CdC Coupler (2(SQZnL)2-4(SQZnL)2, 6(SQZnL)2)
Figure 7. “Karplus-Conroy-type” relationship for electron spin exchange
coupling in 2(SQZnL)2-4(SQZnL)2, and 6(SQZnL)2.
range in J-values for 2(SQZnL)2-4(SQZnL)2 and 6(SQZnL)2
is most likely greater than a corresponding breadth in related
m-phenylene-type biradicals, due to the larger contribution of
through-space contributions to JAF in TMM-type biradicals, and
the inherently stronger JF provided by the ethene Coupler.9,66
However, TMM-type biradicals, as assessed by our derivatives,
can exhibit net ferromagnetic exchange coupling over a broad
range of SQ ring torsions (e ∼60°).
The correlation of SQ ring torsion and exchange parameter
is not perfect since the J-values for 3(SQZnL)2 and 4(SQZnL)2
differ by a factor of 3.6, yet the SQ ring torsions are nearly
identical. We note however that the geometries of 3(SQZnL)2
and 4(SQZnL)2 are different: the former is asymmetric, while
the latter has (crystallographic) C2 symmetry. Clearly, there is
more to the exchange coupling in this series than SQ ring
torsion, as is the case for Karplus-Conroy NMR correlation.64
Nevertheless, our series of biradicals shows a clear variation
from strong ferromagnetic exchange coupling to moderate
antiferromagnetic coupling as SQ ring torsion increases.
can be correlated to the average SQ ring torsion angles (φave
via a “Karplus-Conroy-type” relation,64 eq 5:
)
J ) A cos2(φav) + B
(5)
Equation 5 contains a cos2 φ term because overlaps of 2p-π
orbitals of the SQ groups with those of the ethene Coupler vary
as the cosine of the torsion angles (φ) between the groups.65
Due to the small number of data points, and for comparison to
other systems, our primary concern is to keep eq 5 simple. To
achieve this, we consider only a single trigonometric function,
and we use an average of the two torsion angles. The data are
displayed in Figure 7. The “fit” to eq 5 gives A ) 213 cm-1
and B ) -44 cm-1. Given the simplicity of our model (eq 5),
the A- and B-terms should be considered semiquantitative at
best, as suggested by the thickness of the curve in Figure 7.
Generally, the A-term should be a function of both the Coupler
and the spin density in the spin-containing group, and in this
case, the B-term should be proportional to the through-space
antiferromagnetic interaction. In view of this, the ∼200 cm-1
Correlation of Exchange Coupling in Biradical Com-
plexes, and Mixed Valency in Quinone/Semiquinone Anions.
The correlation of exchange coupling parameters with electron-
transfer parameters is an area of intense, recent interest.67-72
We feel that biradicals having exchange parameters that can be
measured by magnetometry, and stability that permits X-ray
crystallographic analysis are perhaps the optimal molecules to
study the correlation of electron/energy transfer and exchange
coupling.
In the electrochemistry section above, we suggested that ∆E1/2
(redox splitting) is a measure of stabilization of the mixed-valent
(66) Wenthold, P. G.; Hu, J.; Squires, R. R.; Lineberger, W. C. J. Am. Chem.
Soc. 1996, 118, 475-476.
(61) There are other mechanisms by which these states can mix (e.g., hyperfine
interactions), but these interactions are expected to be even weaker than
vibronic mixing.
(67) Weiss, E. A.; Ratner, M. A.; Wasielewski, M. R. J. Phys. Chem. A 2003,
107, 3639-3647.
(62) This is similar to the case of cyclobutadiene, which distorts via a second-
order Jahn-Teller distortion. The notable difference is that the latter is
computed to be a singlet at all geometries when CI is included, see: Borden,
W. T.; Davidson, E. R. Acc. Chem. Res. 1981, 14, 69-76.
(63) This asymmetry is a consequence of the enantiomerization mechanism,
see: Kaftory, M.; Nugiel, D. A.; Biali, S. E.; Rappoport, Z. J. Am. Chem.
Soc. 1989, 111, 8181-8191.
(64) Karplus, M. J. Am. Chem. Soc. 1963, 85, 2870-2871.
(65) Streitwieser, A., Jr. Molecular Orbital Theory for Organic Chemists; Wiley
& Sons: New York, 1961; p 16.
(68) Lukas, A. S.; Bushard, P. J.; Weiss, E. A.; Wasielewski, M. R. J. Am.
Chem. Soc. 2003, 125, 3921-3930.
(69) Weldon, B. T.; Wheeler, D. E.; Kirby, J. P.; McCusker, J. K. Inorg. Chem.
2001, 40, 6802-6812.
(70) Brunold, T. C.; Gamelin, D. R.; Solomon, E. I. J. Am. Chem. Soc. 2000,
122, 8511-8523.
(71) Teki, Y.; Ismagilov, R. F.; Nelsen, S. F. Mol. Cryst. Liq. Crys. A 1999,
334, 313-322.
(72) Nelsen, S. F.; Ismagilov, R. F.; Teki, Y. J. Am. Chem. Soc. 1998, 120,
2200-2201.
9
11768 J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003

Trends in Exchange Coupling in Biradical Complexes
A R T I C L E S
Figure 8. Correlation of ∆E1/2 for 2(Q)2-7(Q)2 and |D/hc| for 2-
(SQZnL)2-7(SQZnL)2.
Figure 9. Correlation of log Kcom for 2(Q)2-7(Q)2 and J for 2(SQZnL)2-
7(SQZnL)2.
Q-SQ species. However, there are several contributors to redox
In our cross-conjugated, mixed-valent Q-SQ molecules, the
electron transfer is between two orthogonal molecular orbitals
(φ1 and φ2, Figure 6) for which the overlap integral is necessarily
zero. Since the Wolfsberg-Helmholz approximation65,76 relates
â to the overlap integral, when the latter is zero â vanishes but
splitting, most importantly electrostatic effects.73 Nevertheless,
we feel that our observed trend in ∆E1/2 (and therefore Kcom
)
does indeed reflect a trend in Coupler-modulated stabilization
in the mixed-valent Q-SQ species. Our assertion is based upon
the correlation displayed in Figure 8. As can be seen, the
biradicals that give smaller D-values correspond to bis(Q)
molecules that exhibit greater redox splitting (and therefore
larger log Kcom). If the redox splitting were due solely to
electrostatic effects, it would be expected that the greatest redox
splitting would correspond to a biradical with the greatest
D-value since |D| is inversely proportional to the cube of the
interelectronic separation.74 However, the exact opposite cor-
relation is observed. However, as we mentioned in the electro-
chemistry section, redox splitting is also proportional to
delocalization in mixed-valent species.41 Therefore, the observed
trend in ∆E1/2 reflects Coupler-modulated delocalization of the
mixed-valent Q-SQ anions rather than simple electrostatics.
To correlate delocalization in mixed-valent Q-SQ molecules
in our series with exchange coupling in the biradical forms, we
begin by describing the electronic coupling matrix element for
electron transfer in an isovalent biradical. This electronic
coupling matrix element is given by the numerator of JAF in
2
l does not, and |Vmixed-valent| ≈ 2l2. The integral l is the hybrid
coulomb integral, and describes the electron-electron repulsion
between the oVerlap region and each of the orbitals (vide
supra).37,57
However, our observed trend in Kcom (Table 5) as a measure
2
2
of |Vmixed-valent| suggests that |Vmixed-valent| scales with biradical
JF, and not with JAF, as seen graphically in Figure 9. It is the
positive slope of this plot that indicates that log Kcom is pro-
portional to JF, since JF makes a positive contribution to J (see
eq 4). This is initially counterintuitive but might be rationalized
as follows. We hypothesize that the observed trend in Kcom is a
manifestation of the fact that the hybrid coulomb integral, l,
scales with the exchange integral, k, since both integrals are
proportional to the overlap density.37,57 Thus, in cross-conjugated
2
systems such as those described here, |Vmixed-valent| scales with
JF, while in conjugated systems (or those with direct spatial
2
overlap),72 |Vmixed-valent| is proportional to JAF
.
Interestingly, the through-space overlap that makes a large
contribution to JAF does not appear to provide an effective
pathway for electron delocalization in the mixed-valent anions.
This might be the result of different conformations for the
biradical complexes and the mixed-valent anions so that the
through-space overlap in the biradicals is smaller in the mixed-
valent anions.
2
eq 4 (|V| is proportional to JAF) and is re-cast in eq 6.
2
2
|Visovalent| ) |2(â + l)|
(6)
As can be seen, electron transfer is most favorable when the
magnetic orbitals have the same symmetry and therefore a
nonzero transfer integral, â.75 The necessary key connection
between iso- and mixed-valent electronic coupling matrix
elements was recently derived by Solomon et al. for dinuclear
metal complexes.70 These authors showed that the electronic
coupling matrix elements for isovalent and mixed-valent forms
Conclusions
Our experimental results demonstrate that conformational
J-modulation is strong in TMM-type biradicals. The breadth in
J-values for the TMM-type biradicals is most likely greater than
the corresponding breadth in related m-phenylene-type biradi-
cals, although no magnetostructural study for a series of
biradicals in the latter case has been reported.21 The breadth in
J-values for TMM-type biradicals studied herein is augmented
by the larger contribution of through-space interaction to JAF
brought about by the close spatial proximity of the SQ groups.
In fact, spin density close to the Coupler is critical, and the
2
2
2
are directly related (|Visovalent| ≈ 2|Vmixed-valent| ) and again |V|
is proportional to JAF. For our purposes then, eq 6 is sufficient
for the following qualitative arguments concerning the relation
between exchange coupling in biradicals and electron transfer
in mixed-valent forms of cross-conjugated molecules.
(73) Ferrere, S.; Elliot, C. M. Inorg. Chem. 1995, 34, 5818-5824.
(74) Wertz, J. E.; Bolton, J. R. Electron Spin Resonance; Chapman and Hall:
New York, 1986.
(75) For an example of symmetry-controlled energy transfer, see: Holten, D.;
Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57-69.
(76) Wolfsberg, M.; Helmholtz, L. J. Chem. Phys. 1952, 20, 837-843.
9
J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003 11769

A R T I C L E S
Shultz et al.
5,5′-Di-tert-butyl-3,3′,4,4′-tetramethoxybenzophenone (9). A 100-
mL flask containing CrO3 (1.1 g, 11.0 mmol) was pump/purged three
times with nitrogen. Pyridine (1.7 mL, 21 mmol) was added, and the
mixture was dissolved in 20 mL of dry CH2Cl2. The solution was stirred
for 0.5 h, and carbinol 8 (727.5 mg, 1.75 mmol) dissolved in 20 mL of
CH2Cl2 was cannulated into the reaction. After stirring for 6 h, the
reaction was quenched with 1 mL of 1 M NaOH, washed with brine,
and dried over Na2SO4. Methanol was added to the resulting yellow
J-modulation in a similar isostructural series of bis(nitroxide)
biradicals26 is diminished compared to the present case.
Nevertheless, TMM-type biradicals, as assessed by our bis(SQ)
derivatives, can exhibit net ferromagnetic exchange coupling
over a broad range of conformations (0 e torsions e ca. 60°).
We also showed how energy matching of Coupler and SQ
orbitals can enhance exchange coupling, as demonstrated by
the fact that the conformation of 5(SQZnL)2 is similar to that
of 3(SQZnL)2, but J is nearly 1 order of magnitude greater for
the former.
Our results also indicate that for the molecules studied, there
is a qualitative correlation between the molecular conformations
observed in crystal structures, and those adopted in frozen
solution, as judged by EPR zero-field splitting parameters. This
relationship results in similar J-values for solution and solid-
state species. However, we note that linear EPR Curie plots
only indicate that J g 0. A notable exception to the correlation
between solution- and solid-state molecular conformation is the
adamantyl-capped biradical, 3(SQZnL)2. The adamantyl group
seems to impart interesting properties to TMM-type biradicals
since we reported solvent-dependent conformational J-modula-
tion for a bis(phenoxy) biradical analogue of 3(SQZnL)234 and
hysteresis for a bis(nitroxide) derivative of 3(SQZnL)2.54
Finally, we correlate magnetic exchange coupling with
Coupler-modulated delocalization within mixed-valent forms of
our ligands. This is the first example of such a correlation within
an isostructural series. Electron transfer in mixed-valent, cross-
conjugated systems correlates with JF, while theory indicates a
correlation with JAF for other systems. We propose that this
correlation is the result of the fact that the hybrid coulomb
1
oil to give a white solid (638 mg, 88%). H NMR (CDCl3) δ 7.39 (d,
2H, J ) 2.1 Hz), 7.37 (d, 2H, J ) 1.8 Hz), 3.96 (s, 6H), 3.92 (s, 6H),
1.38 (s, 18H). 13C NMR δ 195.6, 153.3, 152.5, 142.7, 132.4, 122.7,
112.1, 60.7, 56.1, 35.4, 30.6. IR (film from CH2Cl2) ν (cm-1): 2955.0,
1649.9, 1573.0, 1453.5, 1410.0, 1359.7, 1323.9, 1292.6, 1250.3, 1222.4,
1153.6, 1066.8, 1003.7, 762.3. HRMS (FAB): (m/z) C25H34O5 (M)+,
Calcd 414.2406. Found 414.2393.
5,5′-Di-tert-butyl-3,3′,4,4′-tetramethoxy-thiabenzophenone (10).
In a 100-mL flask benzophenone, 9, (81.3 mg, 0.20 mmol), phosphorus
pentasulfide (131.0 mg, 0.29 mmol), and sodium bicarbonate (100.8
mg, 1.20 mmol) were taken up in 10 mL of acetonitrile. The mixture
was refluxed for 4 h during which time the color of the mixture turned
blue. The mixture was cooled to 0 °C, quenched with saturated aqueous
NaHCO3, and washed twice with brine, and the blue solution was then
dried over Na2SO4. The solvent was removed, and the blue solid was
taken up in hot methanol and cooled to 0 °C to induce precipitation
(44.6 mg, 0.10 mmol) in 51% yield. 1H NMR (CDCl3) δ 7.41 (d, 2H,
J ) 1.8 Hz), 7.25 (d, 2H, J ) 2.1 Hz), 3.96 (s, 6H), 3.91 (s, 6H), 1.35
(s, 18H). 13C NMR δ 235.2, 153.0, 152.9, 142.3, 142.0, 122.2, 113.5,
60.7, 56.0, 35.4, 30.6. IR (film from CH2Cl2) ν (cm-1): 2998.4, 2956.1,
2866.9, 2832.6, 2600.7, 1995.7, 1582.8, 1567.3, 1478.5, 1450.4, 1405.5,
1359.7, 1314.4, 1288.8, 1249.4, 1205.7, 1151.3, 1073.7, 1060.9, 1002.9,
882.7, 860.0, 790.0, 704.5, 669.5. HRMS (FAB): (m/z) C25H34O4 (M
+ H)+ Calcd 414.2256. Found 414.2246. Anal. Calcd for C25H34O4:
C, 69.73; H, 7.96%. Found: C, 69.76; H, 7.88%.
integral, l, scales with the exchange integral, k. Thus, in cross-
11-[Bis-(3-tert-butyl-4,5-dimethoxyphenyl)-methylene]-bicyclo-
[4.4.1]undecane (12). In a 50-mL flask thione 10 (401.2 mg, 0.93
mmol) was dissolved in 15 of mL THF. The diazo 11, (182.2 mg, 0.94
mmol) was dissolved in 5 mL of THF and added dropwise to the stirring
benzothione 10 solution. Bubbling and a white precipitate that quickly
redissolved were observed. The blue solution was refluxed overnight.
The solvent was removed and the remaining solid was taken up in 15
mL of toluene. Triphenylphosphine (502.3 mg, 1.92 mmol) was added,
and the solution was refluxed for 2 days. Solvent was removed and
the mixture purified by radial chromatography (gradient elution;
petroleum ether to 10% ether/petroleum ether). The white solid was
precipitated from petroleum ether, producing 12 (396.2 mg, 0.72 mmol)
2
conjugated systems such as those described here, |Vmixed-valent
|
scales with JF, while in conjugated systems (or those with direct
2
spatial overlap),72 |Vmixed-valent| is proportional to JAF
.
Experimental Section
General Experimental. Unless noted otherwise, reactions were
carried out in oven-dried glassware under a nitrogen atmosphere. THF
and toluene were distilled under argon from sodium benzophenone
ketyl, and acetonitrile and methylene chloride were distilled from CaH2
under argon. tert-Butyllithium (1.5 M in pentane) was used as received
from Aldrich Chemical Co. Other chemicals were purchased from
Aldrich Chemical Co. X-Band EPR spectroscopy and electrochemistry
were performed as described previously.77 NMR spectra were recorded
1
in 77% yield. H NMR (CDCl3) δ 6.78 (d, 2H J ) 1.8 Hz), 6.62 (d,
2H J ) 1.5 Hz), 3.84 (s, 6H), 3.81 (s, 6H), 2.91 (pent., 2H, J ) 6.0
Hz), 1.68 (m, 4H), 1.55 (m, 12H), 1.36 (s, 18H). 13C NMR δ 153.0,
146.6, 146.5, 142.7, 139.9, 139.4, 119.1, 111.0, 60.5, 56.0, 41.0, 35.2,
32.7, 30.9, 27.2. IR (film from CH2Cl2) ν(cm-1): 2951.7, 2921.9,
2863.2, 1568.2, 1465.5, 1409.1, 1358.0, 1327.6, 1303.0, 1257.7, 1234.7,
1152.7, 1136.0, 1074.0, 1010.3, 937.1, 908.6, 878.7, 849.1, 780.0, 736.5,
704.4, 675.2. Anal. Calcd for C36H52O4: C, 78.79; H, 9.55%. Found:
C, 78.73; H, 9.76%.
1
at 300 MHz for H NMR (referenced to TMS) and 75 MHz for 13C
NMR. Elemental analyses were performed by Atlantic Microlab, Inc.,
Norcross, GA. Mass spectrometry was carried out at the NC State
University Mass Spectrometry Facility. Electronic absorption spectra
were collected on a Shimadzu UV-3101PC scanning spectrophotometer.
Other experimental details can be found in Supporting Information.
23,24
Synthesis. Catechols 3(CatH2)2-7(CatH2)2
and their corre-
11-[Bis-(3-tert-butyl-4,5-dihydroxyphenyl)-methylene]-bicyclo-
[4.4.1]undecane (2(CatH2)2). A dry 50-mL flask containing tet-
ramethoxyether 12 (153.6 mg, 0.28 mmol) and 30 mL of dry CH2Cl2
was pumped/purged three times with nitrogen. The solution was cooled
to -78 °C. Boron tribromide (1.0 M in CH2Cl2, 5.6 mL, 5.6 mmol)
was added dropwise, and the solution was stirred overnight (-78 °C
to 25 °C). The solution was poured onto ice, washed with brine, and
dried over Na2SO4. The crude catechol was used without further
sponding ZnII complexes22,27,78 were prepared as described previously,
except for 4(SQZnL)2 and 6(SQZnL)2 which were prepared by a
“comproportionation route” as described below for 2(SQZnL)2.
Complex 6(SQZnL)2 was prepared without the addition of KH. X-ray
quality crystals for biradical complexes (except 6(SQZnL)2) were
grown by layering methanol onto a methylene chloride solution of the
biradical. Crystals of complex 6(SQZnL)2 were grown in a glovebox
using an analogous procedure, but replacing methanol with n-heptane.
Carbinol 8 was prepared as previously described.24
1
purification. H NMR (CDCl3) δ 6.72 (d, 2H, J ) 1.5 Hz), 6.49 (d,
2H, J ) 1.8 Hz), 5.43 (s, 2H), 4.75 (s, 2H), 2.97 (pent., 2H, J ) 6
Hz), 1.67 (m, 4H), 1.53 (m, 12H) 1.39 (s, 18H). 13C NMR δ 146.7,
142.7, 141.4, 138.9, 136.3, 136.2, 119.7, 113.4, 41.0, 34.8, 32.6, 29.9,
(77) Shultz, D. A.; Farmer, G. T. J. Org. Chem. 1998, 63, 6254-6257.
(78) Shultz, D. A.; Bodnar, S. H. Inorg. Chem. 1999, 38, 591-594.
9
11770 J. AM. CHEM. SOC. VOL. 125, NO. 38, 2003

Trends in Exchange Coupling in Biradical Complexes
A R T I C L E S
27.2. IR (film from CH2Cl2) ν (cm-1): 3519.7, 2921.2, 1590.2, 1483.7,
1415.4, 1362.6, 1289.6, 1237.6, 1049.9, 963.9, 862.7, 801.7, 737.7,
705.3, 663.9. HRMS (FAB): (m/z) C32H44O4 (M)+ Calcd 492.3240.
Found 492.3238.
Crystallographic Structure Determinations. Pertinent information
can be found in Supporting Information.
Magnetic Susceptibility Measurements. Magnetic susceptibilities
were measured on a Quantum Design MPMS-XL7 SQUID magne-
tometer using an applied field of 1 T for Curie plots. Microcrystalline
samples were loaded into the sample space of a Delrin sample holder,
and mounted to the sample rod. Data were corrected for sample holder
and molecular diamagnetism. The decrease in the øT data at low
temperatures was accounted for with a Weiss correction, using the
expression øeff ) ø/(1 - ϑø), where ϑ ) 2zJi′nter/(Ng2â2).39 The origin
of Ji′nter may be zero-field splitting, intermolecular interaction, satura-
tion effects, or some combination of all three.40 The other terms have
their usual meaning.39 The data are plotted as øT vs T (Figure 2), except
for 2(SQZnL)2 which is also displayed as ø vs T in Figure 4.
Di(5-tert-butyl-3,4-hydroxyphenyl)methylene-bicyclo[4.4.1]-
undecane (2(Q)2). A 50-mL flask containing 2(CatH2)2 (73.9 mg, 0.15
mmol) and 522 mg Fetizon’s reagent was stirred with 25 mL of THF
overnight. The mixture was filtered through SiO2. Radial chromatog-
raphy (CH2Cl2) yielded a yellow/green solid (65.9 mg, 90%). 1H NMR
(CDCl3) δ 6.62 (d, 2H, J ) 1.8 Hz), 6.21 (d, 2H J ) 1.8 Hz), 3.07
(pent., 2H, J ) 5.4 Hz), 1.76 (m, 4H), 1.55 (m, 18H), 1.27 (s, 18H).
13C NMR δ 180.1, 179.3, 155.6, 152.9, 152.5, 136.0, 133.2, 127.2,
44.0, 35.9, 32.3, 28.4, 26.8. IR (film from CH2Cl2) ν (cm-1): 2923.6,
1660.4, 1613.5, 1469.4, 1367.6, 1266.0, 1238.6, 910.4, 834.1, 805.4,
731.0. HRMS (FAB): (m/z) C32H40O4 (M + H)+ Calcd 489.3005.
Found 489.3034.
Acknowledgment. This work is dedicated to Professor Dennis
A. Dougherty. D.A.S. acknowledges the National Science
Foundation (CHE-9910076; CHE-0091247, SQUID Magne-
tometer) for financial support and thanks the Camille and Henry
Dreyfus Foundation for a Camille Dreyfus Teacher-Scholar
Award. We thank Dr. Dadi Dai, Dr. Hyun-Joo Koo, Professor
Mike Whangbo, and Professor Stefan Franzen for helpful
discussions.
2(SQZnL)2. Inside a drybox, 2(CatH2)2 (27.9 mg, 0.057 mmol) and
2(Q)2 (27.7 mg, 0.057 mmol) were combined in THF with excess KH
and strirred for 15 min. In a separate vial KL79 (162.2 mg, 0.25 mmol)
and Zn(ClO4)2 6H2O (85.0 mg, 0.23 mmol) were stirred for 15 min.
The solution of the 2(SQ)2 and KH was filtered through a 0.2 µm
syringe filter into the stirring KL and Zn(ClO4)2‚6H2O. This solution
was stirred for 15 min and was filtered through a 0.2 µm syringe filter,
yielding the green biradical. IR (film from CH2Cl2) ν (cm-1): 3423.1,
2923.3, 2525.6, 1552.3, 1616.4, 1445.2, 1360.0, 1182.6, 1064.3, 833.7,
787.5.
Supporting Information Available: Crystallographic data (in
CIF format), electronic absorption spectra, EPR spectra for
3(SQZnL)2, and CV of 2(Q)2 (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
(79) Ruf, M.; Weis, K.; Vahrenkamp, H. J. Chem. Soc., Chem. Commun. 1994,
135-136.
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