X-ray crystallography and magnetic studies of a stable macrocyclic tetranitroxide. Intramolecular dimer of nitroxides in a constrained geometry of the upper rim of calix[4]arene

Article abstract of DOI:10.1021/ja034115p

Synthesis, crystallography, and magnetic characterization of a stable macrocyclic tetranitroxide 1, a calix[4]arene, which is functionalized with four tert-butylnitroxides at the upper rim, is described. In solution, 1 has a 4-fold symmetric fixed cone co

Full text of DOI:10.1021/ja034115p

Published on Web 06/13/2003
X-ray Crystallography and Magnetic Studies of a Stable
Macrocyclic Tetranitroxide. Intramolecular Dimer of Nitroxides
in a Constrained Geometry of the Upper Rim of Calix[4]arene
Andrzej Rajca,*,† Maren Pink,‡ Teerapat Rojsajjakul,† Kan Lu,† Hua Wang,† and
Suchada Rajca†
Contribution from the Department of Chemistry, UniVersity of Nebraska,
Lincoln, Nebraska 68588-0304, and X-ray Crystallographic Laboratory,
Department of Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455
Received January 10, 2003; E-mail: arajca1@unl.edu
Abstract: Synthesis, crystallography, and magnetic characterization of a stable macrocyclic tetranitroxide
1, a calix[4]arene, which is functionalized with four tert-butylnitroxides at the upper rim, is described. In
solution, 1 has a 4-fold symmetric fixed cone conformation on the NMR time scale and small, but
nonnegligible, exchange interactions between the radicals (30 K > |J/k| .1.8 mK). In the solid state,
dimerization of one diagonal pair of nitroxides leads to a pinched cone conformation for 1 with strong
intradimer antiferromagnetic coupling with |J/k| ) 200-300 K (singlet-triplet energy gap, ∆EST ≈ 1 kcal/
mol).
Introduction
Recent discovery of magnetic ordering in a conjugated
organic polymer was based upon a rational design of connectiv-
ity between radicals, including the ferrimagnetic coupling
scheme with antiferromagnetic coupling units (aCUs) mediating
exchange coupling between unequal spins.1,2 With stronger and
more reliable exchange couplings associated with such aCUs,3-5
higher ordering temperatures for such polymers may be feasible.
It is well-known that intermolecular π-dimerization of nitroxides
and other stable radicals may lead to relatively strong antifer-
romagnetic couplings.6 We envision that intramolecular dimers
of stable di- and polynitroxides, with a conformationally
restricted macrocyclic backbone, may provide novel aCUs. Such
coupling units are essential for effective approaches to polymer-
based organic magnets.1
crystallographic and magnetic characterizations of dimers of
stable organic radicals remain scarce and they are confined to
intermolecular dimers of mono- and diradicals.6,11,12
We report on the synthesis, X-ray crystallography, and
magnetic measurements of the ambient stable nitroxide tet-
raradical 1 based upon calix[4]arene, which is functionalized
with four nitroxides at the upper rim.13,14 Dimerization of one
diagonal pair of nitroxides leads to a pinched cone conformation
of 1 in the solid state and strong intradimer antiferromagnetic
coupling with |J/k| ) 200-300 K (singlet-triplet energy gap,
∆EST ≈ 1 kcal/mol).
Intramolecular dimers may also serve as a model for
dimerization of stable organic radicals in the solid state. For a
number of such solids, interesting magnetic properties, including
phase transitions with hysteresis of magnetic susceptibility vs
temperature, were found.7-10 However, the rigorous X-ray
Results and Discussion
† University of Nebraska.
Synthesis. The synthesis of tetranitroxide 1 is outlined in
Scheme 1. In the first step, the etherification of the lower rim
‡ University of Minnesota. Present address: IUMSC, Department of
Chemistry, Indiana University, Bloomington, IN 47405.
(1) Rajca, A.; Wongsriratanakul, J.; Rajca, S. Science 2001, 294, 1503-1505.
(2) Rajca, A.; Rajca, S.; Wongsriratanakul, J. J. Am. Chem. Soc. 1999, 121,
6308-6309.
(11) Capiomont, P. A.; Chion, B.; Lajzerowicz, J. Acta Crystallogr. 1971, B27,
322-326.
(3) Dougherty, D. A. Acc. Chem. Res. 1991, 24, 88-94.
(4) Rajca, A. Chem. ReV. 1994, 94, 871-893.
(12) Fremy salt: Blackstock, S., personal communication. Howie, R. A.; Glasser,
L. S. D.; Moser, W. J. Chem. Soc. A 1968, 3043-3047.
(13) Crystallography of tetranitroxides: Griesar, K.; Haase, W.; Svoboda, I.;
Fuess, H. Acta Crystallogr. 1997, C53, 1488-1490. Mathevet, F.; Luneau,
D. J. Am Chem. Soc. 2001, 123, 7465-7466 (as Mn(II) complex). Liao,
Y.; Baskett, M.; Lahti, P. M.; Palacio, F. Chem. Commun. 2002, 252-
253.
(5) Berson, J. A. Acc. Chem. Res. 1997, 30, 238-244.
(6) Sakane, A.; Kumada, H.; Karasawa, S.; Koga, N.; Iwamura, H. Inorg. Chem.
2000, 39, 2891-2896.
(7) Fujita, W.; Awaga, K. Science 1999, 286, 261-262.
(8) Itkis, M. E.; Chi, X.; Cordes, A. W.; Haddon, R. C. Science 2002, 296,
1443-1445.
(14) Nitroxides and nitronyl nitroxides on the upper and lower rim of calix[4]-
arene, respectively: Wang, Q.; Li, Y.; Wu, G. Chem. Commun. 2002,
1268-1269. Ulrich, G.; Turek, P.; Ziessel, R. Tetrahedron Lett. 1996, 37,
8755-8758.
(9) Sugano, T. Polyhedron 2001, 20, 1285-1289.
(10) Postulated dimer of dinitroxides: Schultz, D. A.; Fico, R. M., Jr.; Boyle,
P. D.; Kampf, J. W. J. Am. Chem. Soc. 2001, 123, 10403-10404.
9
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J. AM. CHEM. SOC. 2003, 125, 8534-8538
10.1021/ja034115p CCC: $25.00 © 2003 American Chemical Society

Nitroxide Dimer in the Upper Rim of Calix[4]arene
A R T I C L E S
Scheme 1. aSynthesis of Teranitroxide 1
a Conditions: (i) NaH, THF/DMF, 4-tert-butylbenzylbromide, 40-56%;
(ii) NBS, 2-butanone, 62-92%; (iii) t-BuLi (8 equiv), THF; (iv) [(CH3)3NO]2
(2.1 equiv), 14-43% from 4; (v) Ag2O, CHCl3, 30-70%.
Figure 2. ESR (X-Band) spectrum of 1 in CDCl3 at ambient temperature.
Black trace: experimental spectrum. Red trace: numerical fit (R ) 0.999)
with the following variable parameters [rel concentration, Lorentzian line
width, g-shift, 14N-splitting (spin, number), 1H-splitting (spin, number)]:
species no. 1 [77%, 0.285, -3.57, 0.328 (1, 4), 0.046 (0.5, 8)], species no.
2 [14%, 0.255, -3.40, 0.440 (1, 3), 0.058 (0.5, 6)], species no. 3 [9%,
0.265, -3.70, 0.690 (1, 2), 0.089 (0.5, 4)]. The line widths and hyperfine
splittings are converted to milliTesla.
Therefore, the strength of exchange coupling between nitroxides
in 1 in solution is well outside the 298-30 K range, providing
an upper bound for |J/k|, i.e., |J/k| < 30 K.
ESR (X-band) spectra for 1 show detectable exchange
coupling and provide the lower bound for |J|. At ambient
temperature in CDCl3, a poorly resolved multiplet (g ) 2.0058)
is obtained. The numerical fits (R ) 0.999) reveal a mixture of
oligonitroxides, containing about 75% of tetranitroxide 1 in
addition to the corresponding tri- and dinitroxide impurities
(Figure 2). A value of øT ≈ 1.37 calculated for such a mixture
is in excellent agreement with the magnetic susceptibility
measurements. For 1 at ambient temperature, numerical fitting
gives a nonet for the 14N hyperfine coupling with peak-to-peak
splitting Hpp ≈ 0.328 mT ) aN/4, i.e., aN ) 1.31 mT, which is
in a good agreement with aN ) 1.28 mT for 3,5-dimethyl-4-
methoxyphenyl-tert-butylnitroxide.20 This is consistent with
exchange-coupled nitroxides with a coupling constant signifi-
cantly larger than the 14N hyperfine coupling, |J/gµB| . aN,
i.e., |J/k| .1.8 mK.
Figure 1. 1H NMR (500 MHz) spectrum of 1 in CDCl3 at 293 K. The
tallest peak in the spectrum, the singlet at 1.40 ppm, is shown out of scale.
introduces a fixed cone conformation;15 both 3 and 4 show
4-fold symmetry on the 1H (500 MHz) and 13C (125 MHz) NMR
time scales at 295 K in CDCl3. For tetrakis(hydroxyamine) 5,
only 2-fold symmetry on the 1H NMR scale in CDCl3 is found,
as expected for a pinched cone conformation.16
1
Solution Studies of 1. H NMR (500 MHz) spectra of 1 in
CDCl3 in the 298-223 K range are compatible with 4-fold
symmetry. At 298 K, six broad resonances are observed; two
36-proton resonances at 1.40 and -4.5 ppm are assigned to the
tert-butyl groups at the benzyl and nitroxide moieties, respec-
tively (Figure 1). For the most downfield 8-proton 1H resonances
(8.39 and 7.48 ppm), which are tentatively assigned to the
aromatic protons of the benzyl groups, the plots of observed
frequency (Vobs) versus reciprocal temperature (1/T) give straight
lines (R ) 0.999 and 0.995).17 The bulk paramagnetic suscep-
tibility (ø) measurements for 1 in 2-methyltetrahydrofuran (2-
MeTHF) reveal a constant value of øT ≈ 1.3-1.4 emu K mol-1
in the 290-30 K range.18 A similar value of øT ≈ 1.34 ( 0.04
(i.e., 3.57 ( 0.09 unpaired electrons per molecule) is obtained
for 1 in chloroform using the 1H NMR-based Evans method.19
Both the smallness of the exchange coupling and the 4-fold
symmetry on the NMR time scale are more consistent with the
cone rather than pinched cone conformation for 1 in solution.
Additional evidence in support of this finding is provided by
the EPR spectroscopy of 1 in frozen 2-MeTHF at 77 K; the
∆ms ) 1 region of the spectrum consists of several peaks spread
over the spectral width (|2D|) of about 20 mT and the ∆ms )
2 signal is easily detectable. If 1 were in the pinched cone
conformation, two of the diagonally positioned nitroxides would
be in close proximity, leading to relatively large spectral width
derived from their magnetic dipole-dipole interaction. Accord-
ing to a simple point-dipole approximation, |2D| ≈ 20 mT
corresponds to an effective distance of about 6 Å between the
two magnetic dipoles.21 While the point-dipole approximation
is not strictly applicable to 1, delocalization of spin density into
the relatively proximate benzene rings should even further
(15) Gutsche, C. D.; Pagoria, P. F. J. Org. Chem. 1985, 50, 5795-5802.
(16) Pinched cone conformation for calix[4]arene with hydrogen-bonded groups
on the upper rim: Conner, M.; Janout, V.; Regen, S. L. J. Am. Chem. Soc.
1991, 113, 9670-9671.
1
(17) For the two most downfield 8-proton H resonances (8.39 and 7.48 ppm)
and assuming four S ) 1/2 radicals, numerical fitting of the observed
frequency (Vobs) to Vobs ) Vdia + [4S(S + 1)gµBH0(N)A]/[(γN/2π)3kT] )
Vdia + 15.777A/T, gives Vdia ) 3.0 kHz, A ) 22 kHz and Vdia ) 3.5 kHz,
A ) 3.5 kHz, respectively. Reference 4 and NMR of Paramagnetic
Molecules; La Mar, G. N., Horrocks, W. DeW., Jr., Holm, R. H., Eds.;
Academic: New York, 1973.
(19) Evans, D. F. J. Chem. Soc. 1959, 2003-2005. Live, D. H.; Chan, S. I.
Anal. Chem. 1970, 42, 791-792.
(20) Forrester, A. R.; Hepburn, S. P.; McConnachie, G. J. Chem. Soc., Perkin
Trans. I 1974, 2213-2219.
(18) The low temperature (below 30 K) behavior of 1 will be reported separately.
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A R T I C L E S
Rajca et al.
alternative refinements but the results were inconclusive. It is
also possible that crystal structure is of the solid solution type.
The molecules of 1 are packed as “bilayers” with the upper
and lower rims of calix[4]arenes forming separate regions.
Within each layer, the other, nondimerized, diagonally posi-
tioned pair of nitroxides in 1 is involved in short intermolecular
contacts with the N‚‚‚O distances of about 4.0 Å between the
molecules translated along the b axis (Figure 5).26
Another set of lesser quality X-ray crystallographic data on
a different single crystal at 95 K (i.e., temperature near the
plateau in the øT vs T plot) supports the structural findings at
173 K, except for small differences in disorder and methanol
content.23b The distances (and mutual orientations) of the
nitroxide moieties in the intramolecular dimer and intermolecular
close contacts in both structures are compatible with relatively
strong and weak antiferromagnetic couplings, respectively.
Numerical fits to the øT vs T data (Figure 3) are based upon
the model of two diradicals with variable number of molar
equivalents (N1 and N2) and coupling constants (J1/k and J2/k),
describing the intramolecular dimer and intermolecular N‚‚‚O
contacts in 1, respectively (eq 1).
Figure 3. Plot of øT vs T for polycrystalline 1. The numerical fit (solid
line) to eq 1 has the following variable parameters (and their parameter
dependencies): J1/k ) -253 K (0.90), J2/k ) -1.5 K (0.34), N1 ) 0.649
(0.88), N2 ) 1.141 (0.54); R2 ) 0.9993.
increase |2D|, compared to the case of localization of spin
density on the NO moieties. For localized dinitroxides, in which
the intramolecular distances between the midpoints of the N-O
bonds were approximately 6.2 and 4.7 Å, |2D| ≈ 25 mT and
|2D| ≈ 40 mT were measured.22
Solid State Studies of 1. A plot of øT vs. T for the solid of
1 shows drastically different behavior compared to that found
in solution (Figure 3). The high-temperature slope and low
temperature downward turn in the øT vs T plot suggest the
presence of strong (|J/k| > 100 K) and weak (|J/k| < 10 K)
antiferromagnetic exchange interactions, respectively. When
heating and cooling at various rates (e2 K/min) within 2-300
and 90-370 K ranges, no hysteresis for øT vs T is detectable;
only slow decomposition of 1 above 300 K is found.
øT ) (1.118T/H)[N1F1 + N2F2]
(1)
Fn ) [2 sinh(a)]/[1 + 2 cosh(a) + exp((-2Jn/k)/T)];
n ) 1, 2
a ) 1.345(H/T)
Fitting with N1 * N2 indirectly accommodates tri- and dini-
troxide impurities and other structures in which nitroxides are
weakly coupled. The intradimer J1/k has a substantial range
(-200 to -300 K, 4 samples), which may in part have its origin
in the variable occupancies for the crystallographically disor-
dered major and minor positions of the nitroxide moieties of
the dimer in polycrystalline samples used for magnetic measure-
ments; the relatively shorter N‚‚‚O contacts (and smaller N-O-
N-O torsional angles) in the minor dimer are likely to be
associated with stronger antiferromagnetic couplings.
The correlation between the magnetic data on 10-mg powder
samples and structural data on single crystals is difficult; because
of solvent loss, only broad powder diffraction patterns were
obtained at room temperature. Therefore, a 0.5-mg batch of
single crystals was selected under polarizing microscope.
Following determination of full datasets for two crystals and
cell constants for additional three crystals at 153 K (Table 1s,
Supporting Information), the øT vs T plot, characteristic of
tetranitroxide 1, is obtained. These five datasets and the
previously discussed dataset at 173 K possess essentially
identical cell constants. (For the refined structures, small
differences in methanol content were observed.) This suggests
reproducibility of crystal structure for single crystals and, in
conjunction with the high-temperature slope (and low temper-
ature downward turn) in øT, confirms the presence of the
intramolecular nitroxide dimer.
To understand the origin of the strong exchange interactions
in the solid state, the structure of 1 is determined by X-ray
crystallography at 173 K (Figure 4).23a One of the two pairs of
diagonally positioned nitroxides is found in a pararellogram
arrangement with close intramolecular contacts in the 2.6-2.8
Å range, well within the N-O van der Waals contact of 3.07
Å.24 In this nitroxide dimer, the oxygens are disordered with
occupancies of approximately two-thirds and one-third for the
major (O2A and O2C) and minor (O2F and O2G) positions,
respectively (Figure 4). The N-O bond lengths in this intra-
molecular dimer are elongated by 0.2-0.3 Å compared to those
in the intermolecular dimers of nitroxides (and monomeric
arylalkylnitroxides).6,11-13 These disorder affected, elongated
N-O bond lengths may accommodate distortion to form the
intramolecular dimer or be associated with hydroxyamines.25
The latter, i.e., dimers of hydroxyamines, was explored in
(21) Eaton, S. S.; More, K. M.; Sawant, B. M.; Eaton, G. R. J. Am. Chem. Soc.
1983, 105, 6560-6567. The dependence of the spectral width (|2D|) on
the inter-dipole distance (r) is calculated according to the following
equation: |2D| ) 3gâ/r3 ) 2.78 × 103 (g/r3), where |D| is in mT and r is
in Å. The r ) 5 and 4 Å would correspond to |2D| of about 40 and 80 mT,
respectively, for a g ≈ 2 diradical.
(22) Rohde, O.; Van, S. P.; Kester, W. R.; Griffith, O. H. J. Am Chem. Soc.
1974, 96, 5311-5318.
(23) Crystallographic data for tetranitroxide 1: (a) at 173 K (dataset A),
C
88H112N4O8‚2.31CH3OH, red crystal (0.17 × 0.09 × 0.04 mm3), T ) 173
K, triclinic, P1h , a ) 10.2842(7) Å, b ) 13.0555(8) Å, c ) 33.167(2) Å,
R ) 95.779(1)°, â ) 96.733(1)°, γ ) 100.260(1)°, V ) 4318.0(5) Å3, Z )
2, µ ) 0.071 mm-1, R ) 0.0640, wR2 ) 0.1680, GoF ) 0.926 (F2, all
data). (b) at 95 K (dataset B), C88H112N4O8‚2CH3OH, orange crystal (cut,
0.05 × 0.05 × 0.005 mm3), T ) 95 K, triclinic, P1h , a ) 10.260(2) Å, b
) 13.106(4) Å, c ) 32.814(11) Å, R ) 95.487(9)°, â ) 90.497(15)°, γ )
100.881(15)°, V ) 4311(2) Å3, Z ) 2, µ ) 0.045 mm-1, R ) 0.1253, wR2
) 0.3164, GoF ) 1.082 (F2, all data).
Conclusion
One pair of diagonal nitroxides on the calix[4]arene upper
rim is found to form a strong antiferromagnetic, intramolecular
(24) For intermolecular nitroxide dimers, typical N‚‚‚O contacts and N-O bond
lengths are 2.28-2.37 and 1.28 Å, respectively (refs 6, 11, 12).
(25) Inoue, K.; Koga, N.; Iwamura, H. J. Am. Chem. Soc. 1991, 113, 9803-
9810.
(26) The oxygens of one of the nitroxides are also disordered over two positions
(63:37) introducing the negligible difference of 0.03 Å for the two 4.0 Å
N‚‚‚O contacts; the N-O bond lengths are normal 1.28 Å.
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Nitroxide Dimer in the Upper Rim of Calix[4]arene
A R T I C L E S
Figure 4. Molecular structure of 1 at 173 K. (A) Ortep plot (50% probability level). (B) N-O distances for the intramolecular dimer of nitroxides (major
and minor occupancies); the O-N-O angles and the N-O-N-O torsions are in the 86.0-89.6° and 18.7-21.4° ranges, respectively.
(14%) of 5 were obtained. Anal. Calcd for C88H116N4O8: C, 77.83; H,
1
8.62. Found: C, 77.55; H, 8.31. H NMR (500 MHz, CDCl3): 8.873
(s, 2 H, exch D2O), 7.476, 7.450 (AB, J ) 8.1, 8 H), 7.041, 6.902
(AB, J ) 7.9, 8 H), 7.002 (bs, 2 H), 6.865 (bs, 2 H), 6.370 (bs, 2 H),
6.00 (bs, 2 H, exch D2O), 5.480 (bs, 2 H), 4.991 (s, 4 H), 4.67 (bs, 4
H), 4.239 (d, J ) 13.6, 4 H), 3.01 (bd, J ) 14.1, 2 H), 2.82 (bd, J )
13.3, 2 H), 1.347 (s, 9 H), 1.208 (s, 9 H), 1.170 (s, 9 H), 0.906 (s, 9
H). FABMS (3-NBA + Na+): m/z (% RA in the m/z 1377-1386 range)
at (M + Na)+, 1379.8 (100), 1380.8 (90), 1381.8 (48), 1382.8 (20);
calcd for C88H116N4O8Na1 1379.87 (98), 1380.87 (100), 1381.88 (52),
1382.88 (18). IR (cm-1): 3223, 2963, 2905, 2868, 1460, 1362, 1200.
5,11,17,23-Tetrakis(N-tert-butylaminoxyl)-25,26,27,28-tetrakis(4′-
tert-butylbenzyloxy)calix[4]arene (1). Hydroxyamine 5 (150 mg, 0.11
mmol) in CHCl3 (15 mL) was added to freshly prepared silver oxide
(0.5 g). After stirring at room temperature in darkness for 18 h, the
reaction mixture was filtered, and then the red filtrate was evaporated
to dryness. Column chromatography (50% ether in hexanes) gave a
red solid (126 mg). Recrystallization from MeOH gave red crystals
(103 mg, 69%). From several reactions on the 100-200-mg scale, 1
was obtained in 30-70% isolated yields. Mp 130 °C (under argon,
dec). Anal. Calcd for C88H112N4O8: C, 78.06; H, 8.34. Found: C, 78.83;
H, 8.39. 1H NMR (500 MHz, CDCl3, (150 ppm, 295 K): 8.39 (bs, 8
H), 7.48 (bs, 8 H), 1.40 (s, 36 H), 1.02 (bs, ∼8 H), 0.9 (br, ∼8 H),
-4.5 (br, 36 H). Evans method (4 measurements, CDCl3/CHCl3, 295
or 298 K), øT ) 1.3-1.4 emu K mol-1 (3.5, 3.6, 3.6, 3.7 unpaired
Figure 5. Crystal packing of 1 at 173 K, showing short intermolecular
electrons). FABMS (3-NBA + Na+), m/z (% RA in the m/z 1372-
contacts with the N‚‚‚O distances of about 4.0 Å. Hydrogen atoms and
solvent molecules are omitted for clarity.
1386 range) at (M + Na)+, 1376.0 (74), 1376.9 (100), 1378.0 (70),
1378.0 (36); calcd for C88H112N4O8Na1 1375.84 (98), 1376.84 (100),
1377.84 (51), 1378.85 (15). IR (cm-1): 2962, 1463, 1203, 977.
X-ray Crystallography for 1. Two datasets were collected: one
on a Bruker SMART system at 173 K at the University of Minnesota
(A) and the second at 95 K at beamline 15ID, ChemMatCARS, at the
Advanced Photon Source, Argonne National Laboratory in Chicago
(B). Final cell constants were calculated from the xyz centroids of strong
reflections from the actual data collection after integration (SAINT).
The intensity data were corrected for absorption (SADABS). The space
group P1h was determined on the basis of the lack of systematic absences
and on intensity statistics. The structures were solved with direct-
methods (SIR-92). Full-matrix least squares/difference Fourier cycles
were performed (SHELXL-97). All non-hydrogen atoms were refined
with anisotropic displacement parameters unless stated otherwise. All
hydrogen atoms were placed in ideal positions and refined as riding
atoms with relative isotropic displacement parameters.
dimer in the solid state. Such intramolecular dimers of nitroxides
may serve as antiferromagnetic coupling units, a promising
building block for exchange-coupled macromolecules.
Experimental Section
5,11,17,23-Tetrakis(N-tert-butylhydroxyamine)-25,26,27,28-tetrakis-
(4′-tert-butylbenzyloxy)calix[4]arene (5). t-BuLi in pentane (3.7 mL,
6.0 mmol) was added to a tetrabromocalix[4]arene 4 (1.000 g, 0.755
mmol) in THF (25 mL) at -78 °C. After 2 h at -78 °C, 2-methyl-2-
nitrosopropane dimer (0.3 g, 1.6 mmol) in THF (2 mL)) was added.
The reaction mixture was stirred for 30 min at -78 °C and then allowed
to warm to ambient temperature. After additional stirring for 3 h at
ambient temperature, the orange reaction mixture was quenched with
water (25 mL) and extracted with ether (2 × 25 mL). The organic
layer was dried over MgSO4 and evaporated to dryness. Column
chromatography (25% ether in hexanes) and recrystallization from
CHCl3/MeOH afforded an orange powder (0.446 g, 43%, mp ) 188
°C). From two reactions on the 1-g scale, 0.275 g (27%) and 0.143 g
Dataset A. A red plate (0.17 × 0.09 × 0.04 mm3) obtained from a
methanol solution by slow evaporation was selected for analysis. The
data collection was carried out using Mo KR radiation (graphite
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A R T I C L E S
Rajca et al.
monochromator). All non-hydrogen atoms were refined with anisotropic
displacement parameters, except for one partially occupied methanol
molecule (O4E, C24E). The final full matrix least squares refinement
converged to R1 ) 0.0640 (I > 2σ(I)) and wR2 ) 0.1994 (F2, all
data). The remaining electron density is located in the vicinity of the
disordered methanol molecules.
samples, the tetranitroxide (0.5-2 mg) was loaded to the tube, solvent
was vacuum transferred to form ca. 5-mm high band of solution of
tetranitroxide, and then the tube was flame sealed under vacuum. The
tubes were inserted to MPMS5S SQUID magnetometer at near the room
temperature (290-300 K); the initial sequence of measurements was
in the cooling mode to allow for condensation of the solvent. For solid-
state samples, the tetranitroxide was loaded to the tube, placed under
vacuum, and then flame sealed under partial pressure of helium gas
(about 100 mTorr). The sample tubes were stored in liquid nitrogen.
There are several incidents of disorder in the structure. First, the
nitroxide dimer (having a parallelogram arrangement) is composed of
two NO groups with oxygen atoms disordered over two positions
(67:33). Alternative arrangements of the NO-dimer were considered
(e.g., oxygen atoms interacting with each other). However, in a test
calculation the site occupancies were refined independently from each
other and resulted also in values of approximately two-thirds for O2A
and O2C and one-third for O2F and O2G. No constraints were applied
to any involved distances although it appears that the N-O distances
are about 0.2 Å longer than expected and found for the NO groups B
and D (approximately 1.28 Å). Second, another NO group is disordered
over two positions (N1D/N1E). The corresponding N-bound tert-butyl
group is disordered with 1.31 methanol molecules (O4D, C24D; O4E,
C24E; O5, C25). The methanol molecule O5, C25 is further disordered
over an inversion center and was refined with a partial site occupancy
considering both the inversion center and the disorder with the tert-
butyl group. Last, two of the tert-butyl groups connected to the phenyl
rings of the upper rim are disordered over two sites. Similarly, the
displacement parameters for the carbon atoms of a third tert-butyl group
(C19D, C20D, C21D, C22D) are rather large. However, attempts to
refine disorder failed; no definite second site was found.
Dataset B. An orange needle (0.05 × 0.05 × 0.005 mm3) obtained
from a methanol solution by slow evaporation was selected for analysis
at 95 K. The measurements were carried out using a Kappa SMART
diffractometer equipped with a two by two array of 1K CCD detectors
and a wavelength of 0.55942 (∼Ag KR). All non-hydrogen atoms were
refined with anisotropic displacement parameters with the exception
of the carbon and oxygen atoms of the disordered solvent molecules.
The final full matrix least squares refinement converged to R1 ) 0.1253
and wR2 ) 0.3361 (F2, all data). The remaining electron density is
located in the vicinity of the disordered methanol molecules. The
structure is similar to A, varying in site occupancies of the nitroxide
moieties (73:27 for the intramolecular nitroxide dimer) and solvent
content.
The magnetic data were corrected for diamagnetism as follows. For
solution samples, the ø vs 1/T plots were extrapolated from the
T ) 100-240 K range (cooling mode of the MPMS5S). For solid
samples, the tetranitroxide was removed from the sample tube, and
then each empty tube was subjected to the identical sequence of
measurements as the original sample. After the point-by-point correction
with these data, the molar susceptibilities were further corrected using
the Pascal constants (-9.24 × 10-3 emu mol-1).
Correlation between the Structural and Magnetic Data. A batch
of single crystals (0.5 mg) is selected under a polarizing microscope.
Datasets for two crystals and cell constants for three crystals at 153 K
are determined and compared to the dataset A (Table 1s, Supporting
Information).
Following single crystal determination of cell constants as described
above, the 0.5-mg batch of single crystals of tetranitroxide was loaded
to a gelatine capsule. Due to the unknown amount of diamagnetic oil,
used for mounting of crystals in diffractometer, only qualitative
magnetic data could be obtained.
Acknowledgment. This research was supported by the
National Science Foundation grants to A.R. (CHE-9806954 and
CHE-0107241). MS analyses were carried out at the Nebraska
Center for Mass Spectrometry. ChemMatCARS Sector 15 is
principally supported by the National Science Foundation/
Department of Energy under Grant CHE0087817 and by the
Illinois Board of Higher Education. The Advanced Photon
Source is supported by the U.S. Department of Energy, Basic
Energy Sciences, Office of Science, under Contract W-31-109-
Eng-38. We thank Professor Silas Blackstock for providing us
with the new crystallographic data on Fremy salt. We thank
Dr. Makoto Miyasaka and Jirawat Wongsriratanakul for growing
additional crystals of 1 and assistance with magnetic measure-
ments.
As a test for the alternative dimer of hydroxyamines structure,
refinement of dataset A was carried out using idealized OH group with
a tetrahedral N-O-H angle. The hydrogen was allowed to rotate about
the N-O bond; this motion was combined with the riding motion of
the H atom on its parent atom. Unreasonable displacement parameters
for the hydrogens were obtained, probably due to the vicinity of the
hydrogens to the disordered oxygen atoms. This result still does not
preclude the solid solution type of crystal structures.
Magnetic Studies. The sample vessels for the SQUID measurements
were based upon 5-mm OD EPR-quality quartz tubes (Wilmad),
modified to possess a thin bottom, which is 6 cm from the end of the
tube. This geometry allows for an approximate offset of the diamagnetic
background above and below the bottom of the tube. For solution
Supporting Information Available: Experimental section
(synthetic details for compounds 3 and 4, materials, special
procedures, instrumentation, numerical fitting), ESR spectrum
for 1 at 77 K, summary of cell constants, and X-ray crystal-
lographic files (datasets A and B in CIF format). This material
is available free of charge via the Internet at http://pubs.acs.org.
JA034115P
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8538 J. AM. CHEM. SOC. VOL. 125, NO. 28, 2003