Polyether-bridged bis(tert-butyl nitroxide) paramagnetic hosts showing receptor ability to calcium(II) and barium(II) ions

Article abstract of DOI:10.1016/j.tet.2012.05.065

The title compounds, Ph2bNO and Ph3bNO, were designed as a biradical paramagnetic host, and their chelation ability was confirmed by inclusion of a size-matched alkaline-earth metal ion. The crystal structures of [Ca(hfac) ₂(Ph2bNO)] and [Ba(hfac)₂(Ph3bNO)] were determined, where Hhfac stands for 1,1,1,5,5,5-hexafluoropentane-2,4-dione. The solution electron paramagnetic resonance spectra showed switching behavior. Five lines were found for Ph3bNO and charecterized as the hyperfine splitting due to the two nitrogen atoms. After addition of barium(II) ion, the spectrum turned to be three lines. Removal of the barium ion recovered the five line pattern. The present system can be regarded as a reversible magnetic-coupling switch by means of a supramolecular technique.

Full text of DOI:10.1016/j.tet.2012.05.065

Tetrahedron 68 (2012) 6193e6197
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Polyether-bridged bis(tert-butyl nitroxide) paramagnetic hosts showing receptor
ability to calcium(II) and barium(II) ions
*
Sayaka Osada, Naoki Hirosawa, Takayuki Ishida
Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 April 2012
Received in revised form 16 May 2012
Accepted 18 May 2012
Available online 26 May 2012
The title compounds, Ph2bNO and Ph3bNO, were designed as a biradical paramagnetic host, and their
chelation ability was confirmed by inclusion of a size-matched alkaline-earth metal ion. The crystal
structures of [Ca(hfac)2(Ph2bNO)] and [Ba(hfac)2(Ph3bNO)] were determined, where Hhfac stands for
1,1,1,5,5,5-hexafluoropentane-2,4-dione. The solution electron paramagnetic resonance spectra showed
switching behavior. Five lines were found for Ph3bNO and charecterized as the hyperfine splitting due to
the two nitrogen atoms. After addition of barium(II) ion, the spectrum turned to be three lines. Removal
of the barium ion recovered the five line pattern. The present system can be regarded as a reversible
magnetic-coupling switch by means of a supramolecular technique.
Keywords:
Radicals
Supramolecular chemistry
Hosteguest systems
Exchange interaction
Electron spin resonance
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
In this report we propose bis(tert-butyl nitroxide) compounds
concatenated with polyether bridges; namely, PhnbNO with n¼2
Supramolecular techniques, such as host-guest complex forma-
tion have been applied to tune molecule-based magnetic materials
toward the development of information storage and molecular
computing devices.1 We have proposed and actually prepared sev-
eral spin-labels, in which the paramagnetic centers, such as nitro-
xide (>NeOꢀ) directly coordinated to the metal ion (Mnþ) as a guest,
thus affording an OꢀeMnþeOꢀ superexchange pathway.2 As Scheme
1 shows, the magnetic exchange coupling can be changed depend-
ing on the ‘off’ and ‘on’ states of the guest ions. The nitronyl nitroxide
radical group (4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide)3 has
been the best investigated for metal-radical hybrid solids.4 We are
now focusing on aryl tert-butyl nitroxide derivatives,5 because the
nitroxide coordination has an advantage for the strong metal-
eradical exchange interaction in comparisonwith nitronyl nitroxide
and related spin-delocalized radicals.5,6
and 3 (Scheme 2). One may recall EGTA and BAPTA known as strong
Ca2þ ion receptors7 and suppose the possible utility for an ion
sensor or imaging agent, which will be detectable by means of
electron paramagnetic resonance (EPR) spectroscopy when the
host is paramagnetic. We describe the structural and magnetic
characterization and solution EPR analysis on the Ph2bNO,
Ph3bNO, and their metal-ion complexes. The advantage of the EPR
technique will be demonstrated in the study of subtle change of
exchange interaction for well-separated biradical systems.8
Scheme 2. Structural formulas of PhnbNO, EGTA, and BAPTA.
2. Results and discussion
Scheme 1. Open and folded forms of bis(radical-armed) polyethers.
2.1. Synthesis and characterization of the host compounds
Ph2bNO and Ph3bNO were prepared according to the conven-
tional procedure.5 Peripheral phenyl groups are introduced for the
improvement of stability under ambient conditions. The compounds
were purified by column chromatography and recrystallization.
* Corresponding author. Tel.: þ81 42 443 5490; fax: þ81 42 443 5501; e-mail
address: ishi@pc.uec.ac.jp (T. Ishida).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.05.065

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S. Osada et al. / Tetrahedron 68 (2012) 6193e6197
The EPR spectrum of Ph2bNO showed five lines with
As the X-ray diffraction study revealed (Fig. 2),1 and 2 crystallize
in orthorhombic P21212 and orthorhombic Pccn space groups, re-
spectively, and half a molecule is crystallographically independent.
Hosts Ph2bNO and Ph3bNO play the role of penta- and hexadentate
ligands, respectively, as designed. The coordination numbers of the
Ca2þ and Ba2þ ions are nine and ten, respectively. The two nitroxide
oxygen atoms are coordinated to the metal center, with the OꢀeM
aN(ꢁO)¼0.716 mT at g¼2.0062 (9.4 GHz in toluene at room tem-
perature), being typical of bisnitroxide compounds (Fig. 1). Ph3bNO
also showed five lines with aN(ꢁO)¼0.718 mT at g¼2.0062. The fast-
exchange limit would show five lines while the slow-exchange
limit three.9 Even when the polyether bridge was longer (Ph4bNO
etc.), five lines were still observed. No appreciable concentration
dependence of the line-shape was observed below 10ꢁ4 mol Lꢁ1
.
distances of 2.4558(18) A for 1 and 2.767(3) A for 2. Note that the
ꢀ
ꢀ
The exchange was supposed to occur in a through-space manner,
because the nitroxide groups have a dipolar character (>NþꢀeOꢁ)
favoring a head-to-tail dimerization.10 In a dilute solution, the
intramolecular nitroxide dimerization would take place with
a flexible polyether backbone bent to form a loop conformation, as
previously proposed for polymethylene-bridged bis(2,2,6,6-
tetramethylpieridin-1-oxyl) systems.11
nitroxide oxygen atoms are somewhat closely located to each other
ꢀ
ꢀ
ꢀ
with the O /O distances of 4.460(3) and 4.382(4) A for 1 and 2,
respectively. The OꢀeMeOꢀ angles are 130.50(8) and 104.75(8)ꢂ,
respectively.
Fig. 1. X-Band EPR spectra of (a) Ph2bNO and (b) Ph3bNO in toluene at room
temperature. The inset in (b) shows the spectrum of Ph3bNO measured at 380 K.
From a close look at the present spectra, we found that the
relative intensity was considerably deformed from the ideal 1/2/3/
2/1 hyperfine structure, suggesting the intermediate molecular
motion rate with respect to the EPR timescale. This hypothesis was
supported by variable-temperature EPR measurements; the in-
tensity ratio approached the ideal pattern when the sample solu-
tion was heated to 107 ꢂC in toluene (Fig. 1b, inset). Furthermore,
the EPR line-shape showed solvent-dependence; the broadening of
the second and fourth lines was clearly demonstrated in dichloro-
methane and chloroform (Supplementary data, Figs. S1 and S2).
The molar magnetic susceptibilities (cm) of polycrystalline
samples of Ph2bNO and Ph3bNO were measured on a SQUID
magnetometer. The data were analyzed on the empirical Curi-
Fig. 2. X-ray crystal structures of (a) 1 and (b) 2. Hydrogen and fluorine atoms are
omitted. Thermal ellipsoids are drawn at the 50% probability level. Symmetry
operation codes for # and * are (ꢁx, ꢁy, z) and (ꢁxþ1/2, ꢁyþ1/2, z), respectively.
eeWeiss equation (cm¼C/(Tꢁ
q); C and q stand for the Curie and
Weiss constants, respectively). The constants were optimized as
The radical sites did not decompose during the complexation, as
clarified by the spin entity determined by means of magnetic
measurements (Fig. 3). The cmT values of 1 and 2 monotonically
decreased on cooling. In these complexes, the hfac ligands mag-
netically insulate intermolecular interaction. The magnetic cou-
pling parameter was estimated from the BleaneyeBowers
equation,13 where the singlet-triplet energy gap is defined as 2J. The
optimized parameters are 2J/kB¼ꢁ1.51(2) K with g¼1.985(4) for 1
and 2J/kB¼ꢁ3.16(1) K with g¼1.986(3) for 2. Assuming the g values
of 2.006, the radical purities are estimated to be ca. 98% for both.
The antiferromagnetic couplings were assigned to the OꢀeMeOꢀ
superexchange together with a direct through-space Oꢀ/Oꢀ in-
teraction. Similar interactions have been reported recently.14,15
C¼0.78(2) cm3 K molꢁ1 and
q¼e2.61(7) K for Ph2bNO and
C¼0.784(1) cm3 K molꢁ1 and
¼ꢁ11.3(5) K for Ph3bNO. The spin-
q
only values are expected as Ccalcd¼0.75 cm3 K molꢁ1, and accord-
ingly the observed values imply the spin entity with a high purity.
The negative
q values indicate the presence of antiferromagnetic
interaction among the spins. The molecular structure of Ph3bNO
was determined by means of X-ray crystallographic analysis
(Fig. S3).
2.2. Synthesis and characterization of the host-guest
complexes
We tried to prepare host-guest complexes of Ph2bNO and
Ph3bNO with various metal ions, and eventually obtained good
crystalline compounds [Ca(hfac)2(Ph2bNO)] (1) and [Ba(h-
fac)2(Ph3bNO)] (2), where Hhfac stands for 1,1,1,5,5,5-
hexafluoropentane-2,4-dione. The effective ion radii of Ca2þ and
2.3. EPR switching behavior
We measured solution EPR of the paramagnetic host in the
presence of various metal ions, especially Ca2þ and Ba2þ ions. We
found drastic change of the EPR line-shape for the combination of
Ph3bNO and Ba(hfac)2 in toluene (Fig. 4). The line became a three-
line pattern just like a mononitroxide species (b). To eliminate the
12
Ba2þ are 1.00e1.34 and 1.35e1.61 A, respectively, and the suc-
cessful combination suggests the importance of size-matching,
though the hosts have an open chain structure.
ꢀ

S. Osada et al. / Tetrahedron 68 (2012) 6193e6197
6195
The g value was also switched (Fig. 5a). The complexation with
cation caused a slightly negative shift of the g value; namely, the
metal-free Ph3bNO showed the five lines at g¼2.0061(2) while the
three lines appeared at g¼2.0051(2) in the presence of Ba2þ ion. The
positive charge of the species may the negative shift of the g values,
as reported for various hydrocarbon anion and cation radicals.17 The
report on the g shift (
D
g¼0.000101) for the ascorbic acid radical
anion before and after Ca2þ complexation18 is a more closely
related work. These g shifts are induced by decrease or increase of
spin density on the radical oxygen or nitrogen atoms.
Fig. 3. Temperature dependence of cmT and cꢁm1 for 1 (top) and 2 (bottom). Solid lines
represent calculated curves based on the singlet-triplet model. For the optimized pa-
rameters, see the text.
Fig. 5. (a) The g value in the experiment of the addition/removal of Ba2þ (see Fig. 4 for
the conditions of aee). (b) The g value as a function of the metal/ligand (M/L) ratio with
the initial L concentration of 10ꢁ4 mol Lꢁ1
.
Though the presence or absence of the second and fourth lines is
a convenient marker for the exchange coupling switch, the g-shift
can also be easily monitored in the following EPR titration
experiments (Fig. 5b). Addition of Ba2þ gradually changed the line
shape and g value, and finally the addition of 3 equiv of Ba2þ
completed the complexation. The result implies that the com-
plexation equilibrium constant is not large. This argument is sup-
ported by the successful removal of Ba2þ ion with EGTA. The EGTA is
a good Ba2þ chelater (association equilibrium constant, log K¼8.32
in water), though it behaves as a much better Ca2þ chelater (log
K¼10.93 in water).19 The present competition experiment implies
that K between Ph3bNO and Ba2þ ion is smaller than that of EGTA
and Ba2þ in toluene. From Fig. 5b, K is roughly estimated as log
K¼4e5.20
The fast exchange coupling unexpectedly disappeared for the
Ph3bNO and Ba2þ complex. Ziessel and co-workers previously
reported the lack of fast exchange coupling of the bis(nitronyl
nitroxide) paramagnetic ligands after the complexation of dia-
magnetic ions, and explained this finding in terms of shielding of
exchange.21 A large guest cation provides a steric barrier that pre-
vents coupling between the terminal radical centers and rotating in
the flexible bridge. The present study has already revealed that an
appreciable exchange occurs through the OꢀeMeOꢀ pathway (Figs.
2 and 3). Consequently we can assume a slightly different config-
uration in solution (ex. Oꢀ/MeOꢀ or some high order structures).
In other words, the dimerized nitroxide unit, which is present even
at higher temperatures (Fig. 1), underwent facile dissociation by
insertion of the metal ion.
Fig. 4. EPR spectra of Ph3bNO in toluene at room temperature. The spectra were
successively recorded in a sequence of (a) only Ph3bNO, (b) addition of 3 equiv of
Ba(hfac)2, (c) addition of 3 equiv of EGTA, (d) addition of 10 equiv of Ba(hfac)2, and
finally (e) addition of 10 equiv of EGTA. The EGTA addition was given as a solution in
N,N-dimethylformamide. The arrows denote the signal center.
possibility of radical degradation, the Ba2þ ion was removed by ad-
dition of a stronger Ba2þ receptor, EGTA. The five-line shape was
recovered (c), indicating that the host-guest association and disso-
ciation are responsible for the reversible line-shape change. This
change can be repeated a few times (d and e), but the signals finally
became an intermediate and vague shape, because the solution is
diluted and competing equilibria occur. The combination of Ca2þ and
Ph2bNO exhibited a similar switching behavior of the EPR line shape,
but it was somewhat complex owing to spectrum superposition.16
The hyperfine coupling constant (aN) supports the complexa-
tion. The aN value of the triplet signal is slightly larger than twice
the aN value of the quintet, indicating the larger contribution of
a dipolar nature (>NþꢀeOꢁ) rather than a neutral one (>NeOꢀ) in
a coordination compound. The presence of a cation adjacent to
the nitroxide oxygen atom emphasizes the dipolar-ionic bonding
character.
3. Summary
We have developed PhnbNO (n¼2, 3) as paramagnetic host
compounds. They showed the geometrical change in the presence
of a suitable ion in size, accompanied by the change of exchange
coupling easily detectable in EPR.
One may wonder the inverse situation would be expected from
Scheme 1; that is, a three-line pattern would have appeared before
the complexation and the five-line pattern after. On the contrary,

6196
S. Osada et al. / Tetrahedron 68 (2012) 6193e6197
the observation is the other way round. The reason is the flexible
loop conformation before complexation and the exchange-
shielding after in a solution phase. The reversible magnetic-
coupling switch has been established in this way, and this work
can be regarded as a successful supramolecular control of organic/
molecule-based magnetic properties. The flexible nature of the
organic skeleton may play a key role in the application of organic
magnetism.
4.3. Preparation of Ca and Ba complexes
Complexation of Ph2bNO and Ca(hfac)2 in
a
dichlor-
omethaneeheptane mixed solution gave red platelet crystals of
[Ca(hfac)2(Ph2bNO)] (1) after the solvent was almost removed on
warming below 90 ꢂC. The yield was 43%. Mp 146e148 ꢂC. Anal.
Found: C; 53.58, H; 4.31, N, 2.76%. Calcd. C; 53.28, H; 4.28, N, 2.70%
for C46H44CaF12N2O9. IR (neat, ATR) 2940, 1673, 1533, 1147,
768 cmꢁ1. Complexation using Ph3bNO and Ba(hfac)2 in a similar
manner to that of 1 gave red prisms of [Ba(hfac)2(Ph3bNO)] (2) in
47% yield. Mp 161e164 ꢂC. Anal. Found: C; 48.94, H; 3.81, N, 2.38%.
Calcd: C; 48.93, H; 4.11, N, 2.38% for C48H48BaF12N2O10. IR (neat,
4. Experimental section
4.1. Preparation of PhnbBr (n[2 and 3)
ATR) 2947, 1671, 1528, 1119, 766 cmꢁ1
.
Precursory dibromides Ph2bBr and Ph3bBr (3-oxapentane-1,5-
diyldioxy-
and
3,6-dioxaoctane-1,8-diyldioxybis(2-bromo-4-
4.4. X-ray crystallographic analysis
phenylbenzene)s, respectively) were prepared from 2-bromo-4-
phenylphenol22 with diethylene and triethylene glycol tosylates,
respectively, according to the Williamson procedure (by using po-
tassium carbonate as a base in N,N-dimethylformamide; 120 ꢂC,
18 h). Ph2bBr: yield, 76%. Mp 78e80 ꢂC. 1H NMR (500 MHz, CDCl3)
X-ray diffraction data were collected on a Saturn70 CCD diffrac-
ꢀ
tometer with graphite monochromated MoK
a
radiation (
l
¼0.71073 A).
The structures were directly solved and expanded using Fourier tech-
niques in the CRYSTALSTRUCTURE 4.0 program package.24 Selected
d
7.77 (d, J¼2 Hz, 2H), 7.50 (d, J¼9 Hz, 4H), 7.45 (dd, J¼9 Hz, 2 Hz,
data for Ph3bNO: C38H46N2O6, monoclinic, P21/c, a¼12.320(7),
3
b¼18.04(2), c¼9.251(5) A,
b
¼125.945(7)ꢂ, V¼1664(3) A , Z¼2,
2H), 7.40 (t, J¼9 Hz, 4H), 7.32 (t, J¼9 Hz, 2H), 7.00 (d, J¼9 Hz, 2H),
ꢀ
ꢀ
4.27 (t, J¼5 Hz, 4H), 4.09 (t, J¼5 Hz, 4H). 13C NMR (125.8 MHz,
dcalcd¼1.251
g
cmꢁ3
,
m
(MoK
a
)¼0.0840 mmꢁ1
Rint¼0.042, R(F)
,
(I>2s
(I))¼0.0488, Rw(F2) (all data)¼0.1342, and T¼90 K for 3797
CDCl3)
d 154.8, 139.5, 135.6, 132.0, 128.9, 127.3, 127.0, 126.8, 114.0,
112.9, 70.2, 69.5. Ph3bBr: yield, 86%. Mp 82e83 ꢂC. 1H NMR
(500 MHz, CDCl3)
unique reflections. Selected data for 1: C46H44CaF12N2O9, orthorhom-
3
ꢀ
d
7.77 (d, J¼2 Hz, 2H), 7.50 (d, J¼9 Hz, 4H), 7.44
ꢀ
bic, P21212, a¼15.914(7), b¼12.739(6), c¼11.766(11) A, V¼2385(3) A ,
Z¼2, dcalcd¼1.444 g cmꢁ3
,
m
(MoK
a
)¼0.2338 mmꢁ1, Rint¼0.058, R(F)
(dd, J¼9, 2 Hz, 2H), 7.40 (t, J¼9 Hz, 4H), 7.32 (t, J¼9 Hz, 2H), 6.97 (d,
J¼9 Hz, 2H), 4.23 (t, J¼5 Hz, 4H), 3.96 (t, J¼5 Hz, 4H), 3.85 (s, 4H). 13C
(I>2s
(I))¼0.0565, Rw(F2) (all data)¼0.0941, and T¼100 K for 5461
NMR (125.8 MHz, CDCl3)
d 154.8, 139.4, 135.5, 132.0, 128.9, 127.3,
unique reflections. Selected data for 2: C48H48BaF12N2O10, ortho-
127.1, 126.8, 113.8, 112.8, 71.4, 69.7, 69.3.
ꢀ
rhombic, Pccn, a¼24.726(7), b¼12.402(7), c¼16.377(5) A,
V¼5022(4) A , Z¼4, dcalcd¼1.558 g cmꢁ3
,
m
(MoK
a
)¼0.893 mmꢁ1
,
3
ꢀ
Rint¼0.049, R(F)(I>2
s
(I))¼0.0582, Rw(F2) (all data)¼0.1739, and T¼90 K
4.2. Preparation of PhnbNO (n[2 and 3)
for 5748 unique reflections. CCDC reference numbers 869280e869282.
The hydroxylamines PhnbNOH (n¼2 and 3) were prepared from
the corresponding bromides by way of lithiation with butyl lithium
in THF followed by coupling with 2-methyl-2-nitrosopropane.
Ph2bNOH: yield, 79%. Mp 151e152 ꢂC (decomp.). 1H NMR
4.5. Physical measurements
The dc magnetic susceptibilities of polycrystalline specimens of
the present compounds were measured on a Quantum Design
SQUID magnetometer (MPMS-7) in a range of 1.8e300 K. A static
magnetic field of 5000 Oe was applied. The EPR spectra were
obtained on a Bruker ESP300E spectrometer equipped with an X-
band macrowave oscillator. The solution was degassed and mea-
(500 MHz, (CD3)2SO)
d
7.66 (d, J¼2 Hz, 2H), 7.52 (d, J¼8 Hz, 4H), 7.40
(t, J¼8 Hz, 4H), 7.33 (dd, J¼9, 2 Hz, 2H), 7.27 (t, J¼8 Hz, 2H), 6.97 (d,
J¼9 Hz, 2H), 4.11 (t, J¼5 Hz, 4H), 3.83 (t, J¼5 Hz, 4H),1.11 (s, 18H). 13C
NMR (125.8 MHz, (CD3)2SO) d 152.9, 140.9, 140.6, 132.6, 129.5, 127.2,
126.6, 125.4, 124.3, 113.8, 69.8, 67.9, 60.8, 25.9. IR (neat, ATR
(attenuated total reflectance)) 3201, 2972, 1486, 1135, 762 cmꢁ1
Ph3bNOH: yield, 73%. Mp 152e153 ꢂC (decomp.). 1H NMR
sured at room temperature. Typical concentration was 10ꢁ4 mol Lꢁ1
.
(500 MHz, DCON(CD3)2)
d
7.80 (d, J¼2 Hz, 2H), 7.59 (d, J¼8 Hz, 4H),
Acknowledgements
7.44 (t, J¼8 Hz, 4H), 7.39 (dd, J¼9, 2 Hz, 2H), 7.30 (t, J¼8 Hz, 2H), 7.05
(d, J¼9 Hz, 2H), 4.18 (t, J¼5 Hz, 4H), 3.84 (t, J¼5 Hz, 4H), 3.73 (s, 4H),
1.15 (s, 18H). 13C NMR (125.8 MHz, DCON(CD3)2)
d 153.3, 141.0,
This work was supported by Grants-in-Aids for Scientific
Research (Nos. 23110711 and 22350059) from the Ministry of
Education, Culture, Sports, Science and Technology, Japan. The
authors thank Prof. Daisuke Shiomi and Dr. Yuki Kanzaki (Osaka City
University) for fruitful discussion and Mr. Akira Nozawa (The Univ.
of Electro-Communications) for assistance in the ESI-MS
measurements.
140.8, 132.8, 129.0, 126.8, 126.4, 125.4, 124.1, 114.3, 70.7, 69.8, 68.3,
60.7, 25.3. IR (neat, ATR) 3243, 2967, 1487, 1138, 759 cmꢁ1. The
hydroxylamines were dissolved in dichloromethane (20 mL) and
oxidized with freshly prepared Ag2O.23 Final products PhnbNO
were purified by silica-gel column chromatography eluted with
dichloromethane. A red fraction was collected and concentrated
under a reduced pressure, giving red prisms. Ph2bNO: yield, 61%.
Mp 122e123 ꢂC. ESI-MS (methanol) m/z¼605.3 (100%, MþNaþ),
621.2 (29%, MþKþ). Calcd 582.31 for C36H42N2O5. IR (neat, ATR)
2973, 1485, 1278, 1142, 765 cmꢁ1. EPR (rt in toluene) g¼2.0062,
aN¼0.716 mT (quintet). Anal. Found: C, 74.43; H, 7.28; N, 4.80%.
Calcd: C, 74.20; H, 7.26; N, 4.81% for C36H42N2O5. Ph3bNO: yield,
85%. Mp 143e145 ꢂC. ESI-MS (1/1 methanol/chloroform) m/
z¼649.3 (100%, MþNaþ), 665.3 (73%, MþKþ). Calcd 626.34
for C38H46N2O6. IR (neat, ATR) 2866,1485,1279,1140, 769 cmꢁ1. EPR
(rt in toluene) g¼2.0062, aN¼0.718 mT (quintet). Anal. Found: C,
72.78; H, 7.20; N, 4.38%. Calcd: C, 72.82; H, 7.40; N; 4.47% for
C38H46N2O6.
Supplementary data
X-Band EPR spectra of Ph2bNO and Ph3bNO in dichloromethane
and chloroform and X-ray crystal structures of Ph3bNO, [Ca(h-
fac)2(Ph2bNO)] (1), and [Ba(hfac)2(Ph3bNO)] (2). CCDC 869280,
869281, and 869282 contain the supplementary crystallographic
data for Ph3bNO, 1, and 2, respectively. These data can be obtained
free of charge from via http://www.ccdc.cam.ac.uk/conts/retrie-
ving.html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB21 EZ, UK; fax: (þ44) 1223-336-033; or
e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated

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