Crystal structures and magnetic properties of nitronyl nitroxide radicals

Article abstract of DOI:10.1002/hlca.200390106

The crystal structures and magnetic properties of the nitronyl nitroxide radicals 4,5-dihydro-4,4,5,5-tetramethyl-3-oxido(1H-imidazol-1-yloxyl) (1), 4,5-dihydro-2,4,4,5,5-pentamethyl-3-oxido(1H-imidazol-1-yloxyl) (2), 2-(4-chlorophenyl)-4,5-dihydro-4,4,5,

Full text of DOI:10.1002/hlca.200390106

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Helvetica Chimica Acta ± Vol. 86 (2003)
Crystal Structures and Magnetic Properties of Nitronyl Nitroxide Radicals
a
a
a
a
a
by Alexander Zakrassov ), Vitaly Shteiman ), Yana Sheynin ), Mark Botoshansky ), Moshe Kapon ),
a
b
b
Menahem Kaftory* ), Rico E. Del Sesto ), and Joel S. Miller )
a) Department of Chemistry and Lise Meitner Minerva Center for the Computational and Quantum Chemistry,
Technion ± Israel Institute of Technology, Haifa 32000, Israel
(
phone: (972)-48295703; fax: (972)-48295703; e-mail: kaftory@tx.technion.ac.il)
b) Department of Chemistry, University of Utah, Salt Lake City, UT 84112-0850, USA
Dedicated to Professor Jack D. Dunitz on the occasion of his 80th birthday
The crystal structures and magnetic properties of the nitronyl nitroxide radicals 4,5-dihydro-4,4,5,5-
tetramethyl-3-oxido(1H-imidazol-1-yloxyl) (1), 4,5-dihydro-2,4,4,5,5-pentamethyl-3-oxido(1H-imidazol-1-
yloxyl) (2), 2-(4-chlorophenyl)-4,5-dihydro-4,4,5,5-tetramethyl-3-oxido(1H-imidazol-1-yloxyl) (3), and 4,5-
dihydro-2-(2-hydroxy-5-nitrophenyl)-4,4,5,5-tetramethyl-3-oxido(1H-imidazol-1-yloxyl) (4) are reported.
Compound 1 has two polymorphic forms: the a phase is monoclinic (P2
1
/n space group), with a single
molecule in the asymmetric unit, and the b phase is monoclinic (P2 /c space group), with four molecules in the
1
asymmetric unit. In the two polymorphs, the molecules are arranged in dimers formed by hydrogen bonds of the
type CÀH ¥¥¥ OÀN. The crystal structure of 3 contains layers of antiparallel ribbons of molecules. Compound 4
crystallizes with solvent molecules, and an intramolecular hydrogen bond is formed between the 2-OH group of
the phenyl ring and the nitroxide O-atom. Compound 4 also loses the two O-atoms of the nitroxide moiety upon
heating to 908. Magnetic measurements showed that both a and b polymorphs of 1 exhibit antiferromagnetic
coupling. The best fit to the experimental data was obtained using BleanyÀBower×s singlet-triplet model (H ˆ
À2JSaSb): J ˆ À11.2 K for the a phase and J ˆ À15.0 K for the b phase. Compounds 3 and 4 show no evidence
for spin coupling.
Introduction. ± The quest for organic materials exhibiting technologically useful
properties, such as electrical conductivity or ferromagnetism, started several decades
ago. The goal was to create an assembly of organic molecules or macromolecules
constructed from only light elements (C, H, N, O, S, etc.) and yet possessing the
properties similar to those of pure metals, e.g., to conduct electricity by electron
mobility or some metals to enable spin alignment. The possibility of ferromagnetic
coupling in organic radicals was theoretically predicted in 1963by McConnell [1]. The
first example of a purely organic bulk ferromagnet was reported in 1991 by Kinoshita
and co-workers [2] for crystals of the b phase of the 2-nitrophenyl nitronyl nitroxide
radical (I). The evidence for the existence of long-range ferromagnetic order for this
compound was achieved by electric-susceptibility and heat-capacity measurements at
low temperatures. The ferromagnetic behavior was detected below 0.60 K (T ). It is
c
interesting to note that the three other phases did not show ferromagnetic ordering,
which verifies that it is not sufficient to search for stable organic radicals, as magnetism
is a bulk property and, therefore, highly dependent on the solid-state structure.
Since 1991, there has been a tremendous effort by many research groups worldwide
to uncover the mystery of the dependence of spin coupling as a consequence of distinct
relative molecular geometries. The belief that even the most-sophisticated properties

Helvetica Chimica Acta ± Vol. 86 (2003)
1235
can be rationally designed by a systematic modification of organic molecular structures,
motivated and fueled by increased synthetic capabilities, enlarged the number of
groups working in this field. New stable organic radicals that undergo ferromagnetic
ordering were prepared, e.g., compounds of type II [3] and III [4]. However, the goal of
easily preparing compounds that display ferromagnetic properties at ambient temper-
atures is yet to be realized.
Ferro- or antiferromagnetic coupling of stable radicals depends on the spin density
on each atom and the interactions between them [5]. Since no correlation was found
between relative orientation of OÀNˆCÀNÀO groups and the types of coupling
[
6][7], it appears that ferro- or antiferromagnetic coupling in the nitronyl nitroxide
radicals depends on the shortest contacts between atoms (i.e., hydrogen and Van der
Waals bonds) and their spin densities.
H-Bonds play a crucial role in controlling the structural dimensionality of open-
shell molecules as a crystal-design element for organic molecular solids with
intermolecular ferromagnetic coupling [8][9]. Moreover, H-bonds have been proved
to transmit ferromagnetic coupling between spins in the different hydroxy- and alkyne-
substituted, ferromagnetically ordered nitroxide radicals [9 ± 11]. Also, there are some
reports about magnetic coupling via H-bonds in transition-metal complexes [12].
In the present work, we describe and discuss the crystal structures of four stable
nitronyl nitroxides radicals: 4,5-dihydro-4,4,5,5-tetramethyl-3-oxido(1H-imidazol-1-
yloxyl) (1), 4,5-dihydro-2,4,4,5,5-pentamethyl-3-oxido(1H-imidazol-1-yloxyl) (2), 2-
(4-chlorophenyl)-4,5-dihydro-4,4,5,5-tetramethyl-3-oxido(1H-imidazol-1-yloxyl) (3),
and 4,5-dihydro-2-(2-hydroxy-5-nitrophenyl)-4,4,5,5-tetramethyl-3-oxido(1H-imida-
zol-1-yloxyl) (4), as well as the magnetic properties of three of these compounds (1,

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3
, and 4). The crystal structure of the product obtained by heating compound 4 is also
discussed.
Results and Discussion. ± Crystal Structures. Compound 1 crystallizes in two
polymorphic forms (a and b). The crystal structures of both forms were solved [13], but
have never been published in detail. The a form crystallizes in the monoclinic crystal
system in space group P2 /n, with a single molecule in the asymmetric unit. The b form
1
crystallizes in the monoclinic crystal system in space group P2 /c, with four independent
1
molecules in the asymmetric unit. In both polymorphs, the molecules are arranged in
dimers formed through H-bonds of the type NÀO ¥¥¥ HÀC. In the a polymorph, there is
a single type of dimer formed between identical molecules related by an inversion
center (Fig. 1). Comparison of the H-bond geometries is given in Table 1. In the b
polymorph, there are three different dimers. There are two dimers at z ˆ 0.0, each is
made up from two identical molecules related by inversion centers (Fig. 2, left). The
third dimer at z ˆ 0.25 is made of two nonidentical molecules related via a pseudo
inversion center (Fig. 2, right).
Fig. 1. Packing of molecules of the a polymorph of 1
The different structures of the two polymorphs, and, in particular, the unique
structure of the b polymorph, were probed by crystal-structure prediction. The crystal
structure of the a phase was among the most stable ones produced by the polymorph
predictor [14]. Since it was suggested [15] that the b phase is metastable, its crystal
structure was not considered in such a prediction.

Fig. 2. View of a layer of molecules of the b polymorph of 1 for z ˆ 0.0 (left) and z ˆ 0.25 (right)

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Table 1. Comparison of H-Bonding Geometries in the Two Polymorphs of 1. −D× stands for −donor×, −A× for
acceptor×; bond lengths in ä, bond angles in degrees.
−
DÀH ¥¥¥ A
DÀH
H ¥¥¥ A
D ¥¥¥ A
DÀH ¥¥¥ A
a-1
C(1)ÀH(1) ¥¥¥ O(2)
0.94(3)
2.438(4)
3.347(4)
163.3(18)
b-1
C(1a)ÀH(1a) ¥¥¥ O(1b)
C(1b)ÀH(1b) ¥¥¥ O(2a)
C(1c)ÀH(1c) ¥¥¥ O(1c)
C(1d)ÀH(1d) ¥¥¥ O(2d)
0.87(3)
0.89(3)
0.90(3)
0.93(3)
2.241(4)
2.365(4)
2.255(4)
2.296(4)
3.094(4)
3.197(4)
3.135(4)
3.187(4)
164.4(18)
154.4(18)
164.3(18)
159.7(18)
Table 2. Comparison of Selected Bond Lengths [ä] and Angles [8]
1(a) 1(b)a 1(b)b 1(b)c 1(b)d
Atoms
2
3
4
O(1)ÀN(1)
O(2)ÀN(2)
N(1)ÀC(1)
N(2)ÀC(1)
N(1)ÀC(2)
N(2)ÀC(3)
C(2)ÀC(3)
1.272(3) 1.281(3) 1.280(3) 1.276(3) 1.278(3) 1.285(4) 1.279(2) 1.312(6)
1.276(3) 1.294(3) 1.282(3) 1.287(3) 1.279(3) 1.276(4) 1.285(2) 1.292(5)
1.327(4) 1.316(3) 1.329(3) 1.330(3) 1.325(3) 1.324(4) 1.351(3) 1.319(7)
1.327(4) 1.328(3) 1.333(3) 1.326(3) 1.325(3) 1.327(5) 1.343(3) 1.376(7)
1.508(4) 1.501(3) 1.490(3) 1.490(3) 1.505(3) 1.506(4) 1.494(3) 1.508(7)
1.512(4) 1.485(3) 1.505(3) 1.509(3) 1.494(3) 1.487(4) 1.496(3) 1.501(7)
1.559(4) 1.563(3) 1.567(3) 1.560(3) 1.576(3) 1.559(5) 1.555(3) 1.568(8)
O(1)ÀN(1)ÀC(1) 126.3(3) 125.6(2) 125.9(2) 126.0(2) 125.3(2) 125.8(3) 126.3(2) 124.4(4)
O(2)ÀN(2)ÀC(1) 126.2(3) 125.3(2) 125.8(2) 125.9(2) 125.8(2) 125.5(3) 126.1(2) 124.9(4)
O(1)ÀN(1)ÀC(2) 122.4(2) 122.2(2) 121.3(2) 121.8(2) 121.8(2) 122.1(3) 120.3(2) 119.8(4)
O(2)ÀN(2)ÀC(3) 122.4(2) 121.9(2) 122.6(2) 122.5(2) 122.1(2) 121.2(3) 120.1(2) 120.6(4)
C(1)ÀN(1)ÀC(2)
110.9(2) 112.2(2) 122.4(2) 112.0(2) 112.9(2) 111.8(3) 113.2(2) 115.7(4)
C(1)ÀN(2)ÀC(3) 111.1(2) 112.8(2) 111.3(2) 111.3(2) 112.1(2) 113.0(3) 113.6(2) 114.1(4)
N(1)ÀC(1)ÀN(2) 110.9(3) 111.0(2) 110.8(2) 110.7(2) 111.1(2) 109.9(3) 108.3(2) 106.9(4)
N(1)ÀC(2)ÀC(3) 100.5(2) 101.9(2) 101.0(2) 101.0(2) 101.6(2) 101.1(2) 102.0(2) 101.1(4)
N(2)ÀC(3)ÀC(2) 100.2(2) 101.7(2) 101.4(2) 101.0(2) 101.3(2) 100.5(2) 101.7(2) 101.2(4)
The packing of molecules in the crystal of compound 2 is shown in Fig. 3. Selected
bond lengths and angles are given in Table 2.
The structure of compound 3 is shown in Fig. 4, and selected bond lengths and
angles are also given in Table 2.
The molecular structure of 3 was known before from the crystal-structure
determination of its complex with copper [16] and gadolinium [17]. The molecules
are packed in layers perpendicular to the c-axis. A layer consists of ribbons related by
an inversion center. As a result of the packing, the intermolecular distance between two
Cl-atoms related by the inversion center is 3.360(2) ä, i.e., shorter than the sum of Van
der Waals radii (3.50 ä). The 4-chlorophenyl moiety is rotated by 25.28 with respect to
the imidazole ring.
Compound 4 crystallized with disordered solvent molecules from ethyl acetate. The
molecular packing in the unit cell is shown in Fig. 5, and selected bond lengths and
angles are given in Table 2.
Intramolecular H-bonding between the phenyl OH group and the nitroxide O-atom
determines the conformation of the molecule with respect to the rotation of the
phenyl ring relative to the plane formed by nitronyl nitroxide. The torsion angle

Helvetica Chimica Acta ± Vol. 86 (2003)
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Fig. 3. Packing of molecules of 2
Fig. 4. A layer in the crystal structure of 3
N(1)ÀC(1)ÀC(8)ÀC(13) is À35.7(9)8, and the O(1) ¥¥¥ H(3) distance is 1.58(8) ä. The
molecular structure may be compared with that of 2-(2,5-dihydroxyphenyl)-4,5-
dihydro-4,4,5,5-tetramethyl-3-oxido(1H-imidazolyl-1-oxyl) [10]. The conformations of

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Helvetica Chimica Acta ± Vol. 86 (2003)
Fig. 5. Packing of molecules of 4
the two polymorphs that are determined by the intramolecular H-bond are very
similar; the torsion angles are 36.68 and 38.28 in the two polymorphs, respectively, and
the corresponding O ¥¥¥ H distances are 1.59 and 1.73ä.
In the compounds investigated, most bond lengths and angles at the radical site are
practically equal, however some are significantly different. As a result of the
intramolecular H-bond, the bond lengths involving the O(1)ÀN(1)ÀC(1)ÀN(2)ÀO(2)
moiety is significantly different in 4. The O(1)-atom is involved in a H-bond with the
OH group and, therefore, the N(1)ÀO(1) bond is longer than N(2)ÀO(2) (1.312 and
1.292 ä, respectively), while the average of this bond in the other compounds is
1
.280(5) ä. The N(1)ÀC(1) bond shortens, and its double-bond character increases at
the expense of C(1)ÀN(2), whose double-bond character decreases (1.319 and 1.376 ä,
respectively), while the average of this bond in the other compounds is 1.329(8) ä. The
bond angles of the nitronyl nitroxide moiety in 3 and 4 are significantly different. The
average of the C(1)ÀN(1)ÀO(2) (equivalent to C(1)ÀN(2)ÀO(2)) bond angle is
125.8(3)8, while in 3 it is 126.2(2)8, and 124.6(4)8 in 4. The average inner-ring bond
angle at N(1), C(1)ÀN(1)ÀC(2) (and the equivalent C(1)ÀN(2)ÀC(3)) is 112.0(7)8,
while it is 113.4(2)8 in 3 and 114.9(4)8 in 4.
When 4 was heated to 1008, an exothermic process was observed by means of
differential scanning calorimetry (DSC) (Fig. 6) (DH ˆ 267.2 kJ/mol). The major
product (5) was isolated, crystallized, and its crystal structure was determined. As can
be seen from Fig. 7, the two NÀO O-atoms are lost. The phenyl ring and the five-
membered ring are almost coplanar (N(1)ÀC(1)ÀC(8)ÀC(13) torsion angle of
9
.5(5)8) due to the H-bond formed between N(1)ÀH and O(3) (N(1)ÀH ˆ 0.84(3) ä,
H ¥¥¥ O(3)ˆ 1.976(7) ä, N(1) ¥¥¥ O(3) ˆ 2.600(4) ä, and N(1)ÀH ¥¥¥ O(3) is 129.7(5)8).
However the most-striking structural parameters are the bond lengths within the five-

Helvetica Chimica Acta ± Vol. 86 (2003)
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membered ring of 5. The geometry may be compared with the average values obtained
from the similar compounds 6 ± 10. The C(8)ÀC(13) and C(13)ÀC(12) bonds (1.440(4)
and 1.421(4) ä, respectively) are longer than typical CÀC bonds in aromatic
compounds, the average in 6 ± 10 being 1.421(8) and 1.413(6) ä. The C(8)ÀC(9),
C(9)ÀC(10), and C(10)ÀC(11) bonds are normal (1.387(4), 1.370(4), and 1.355(4) ä,
respectively). Bonds N(1)ÀC(1) and N(2)ÀC(1) in the five-membered ring are too
3
2
short for an N(sp )ÀC(sp ) bond, and the average of the same bond in 6 ± 10 is
1
.298(16) ä. The O(3)ÀC(13) bond is longer (1.275(3) ä) than a classical carbonyl
bond, the average in 6 ± 10 is 1.307(10) ä. The H-bond between the charged O-atom
and partially charged N-atom is strong, resulting in a short O ¥¥¥ HN distance (1.98(3) ä
relative to 1.52(15) ä in 6 ± 10).
Fig. 6. DSC Thermograph of 4 at a heating rate of 38/min
Magnetic Properties. The magnetic measurements have been performed on a
SQUID magnetometer between 1.8 and 300 K using an externally applied magnetic
field of 500 Oe. The value of the effective magnetic moment (m ) of the b polymorph
eff
of 1 at room temperature, plotted per dimer (Fig. 8), is in reasonable agreement with
the theoretical value of 1.73  m for an unpaired spin (S ˆ 1/2). However, m (T)
B
eff
decreases with decreasing temperature, with m ˆ 0.4  m at 1.8 K. This suggests a
eff
B

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Helvetica Chimica Acta ± Vol. 86 (2003)
Fig. 7. Molecular structure of 5
dimer with a singlet ground-state and triplet excited state. The best fitting with the
experimental data (J ˆ À15.0 K) was obtained using BleanyÀBower×s singlet-triplet
model [18] (Eqn. 1) with the effective DiracÀVan VleckÀHeisenberg Spin Hamil-
tonian (H ˆ À2S J S S [19]).
i,j i,j
i j
2
2
B
Ng m
c ˆ
(1)
3
kBT ꢀ3 ‡ eÀ2J=kB T
†
c is the molar paramagnetic susceptibility, N is Avogadro×s number, g is the Zeeman g
factor, mB is the electronic Bohr magneton, kB is Boltzmann×s constant, T is the
temperature, and J is the isotropic magnetic exchange coupling constant. The J value
and meff of the a polymorph have been previously reported to be À11.2 K and 2.0 Â m ,
B
respectively [13]. We failed to reproduce the synthesis of the a polymorph to refine the
magnetic measurements. The larger J value for the b phase of 1 in comparison with the
a phase may be rationalized by the shorter H-bonds.
The dependence of the magnetic susceptibility on temperature for compounds 3
and 4 is shown in Fig. 9. The cT value for 3 and 4 (0.36 and 0.35 emu K/mol,
respectively) in the high-temperature region corresponds to the theoretical value of
0
.375 emu K/mol for unpaired spin (S ˆ 1/2). Compounds 3 and 4 show no evidence of
spin coupling (q ˆ 0), in accord with the absence of short intermolecular contacts.
-Chlorophenyl nitronly nitroxide radicals exhibit a weak antiferromagnetic
2
coupling with J ˆ À0.95 K [20], and the a and b phase of 2,5-dihydroxyphenyl nitronyl
nitroxide radicals exhibit a ferromagnetic coupling with J ˆ 0.95 and 5.0 K, respectively
(a phase ordered ferromagnetically below 0.5 K) [10].
Although the radicals 1, 3, and 4 form different types of H-bonds and Van der Waals
interactions, only 1 exhibits spin coupling due to the strong intermolecular H-bonds.
The weak Van der Waals forces in 3 and 4 do not transmit a similar magnetic coupling
observed for 2-chlorophenyl nitronyl nitroxide radicals [20]. The H-bonds of 1 are

Helvetica Chimica Acta ± Vol. 86 (2003)
1243
Fig. 8. Effective magnetic moment of 1 (b phase) as a function of temperature
À1
Fig. 9. Temperature dependence of cT and c for 3 and 4
stronger than those of the 2,5-dihydroxy nitronyl nitroxide radical, but due to the lack
of H-bonding networks, it does not exhibit any ferromagnetic coupling.
Experimental Part
Syntheses. Compounds 1 and 2 have been prepared according to literature procedures [21]. Compounds 3
and 4 were synthesized by the following procedure: a soln. of 2,3-bis(hydroxylamino)-2-3-dimethylbutane
(
1 mmol, 148 mg) and 1.1 equiv. of the corresponding aldehyde in MeOH (25 ml) was stirred at r.t. for 24 h.
Then, the imidazolidine was oxidized to nitronyl nitroxide with PbO (15 mmol, 3.59 g) for 2 ± 3 h. The residue
was separated and washed with MeOH. The solvent was removed in vacuo, and the residue was chromato-
2

Table 3. Crystallographic Data and Parameters for 1 ± 5
a-1
b-1
2
3
4
5
Formula
C
7
H
13
N
2
O
2
C
7
H
13
N
2
O
2
C
8
H
15
N
2
O
2
C
13
H
16ClN
2
O
2
C
13
H
16
N
3
O
5
C
13 17 3 3
H N O
M
r
157.19
157.19
red, plate
171.22
267.73
338.34
blue, needle
263.30
Crystal color, habit
Crystal dimensions
Crystal system
Space group
a [ä]
b [ä]
c [ä]
a [8]
light yellow, prism
0.60 Â 0.20 Â 0.15
monoclinic
dark red, plate
0.90 Â 0.60 Â 0.105
orthorhombic
Pbca
16.897(4)
10.242(4)
11.283(4)
90.00
dark blue prism
0.60 Â 0.30 Â 0.15
monoclinic
Colorless, crube
0.30 Â 0.30 Â 0.25
monoclinic
0.60 Â 0.30 Â 0.05
0.39 Â 0.13 Â 0.10
monoclinic
orthorhombic
P2
1
/n
P2
1
/c
P2
1
/c
Pna2
1
1
P2 /n
11.741(4)
11.502(4)
6.251(3)
90.00
12.065(2)
14.033(3)
19.785(4)
90.00
8.166(2)
18.841(4)
8.866(3)
90.00
7.169(2)
15.101(3)
14.999(3)
90.00
12.057(2)
10.381(3)
12.197(4)
90.00
b [8]
g [8]
V [ä ]
Z
104.18(3)
90.00
818.4(6)
4
1.276
0.094
93.02(2)
90.00
3345.0(2)
16
1.249
0.092
90.00
90.00
1952.6(11)
8
1.165
105.81(9)
90.00
1312.45(6)
4
1.355
0.287
90.00
90.00
1623.8(6)
4
1.384
117.22(3)
90.00
1257.6(7)
4
1.288
0.093
3
À3
D
calc [g cm
]
À1
m (MoK
F(000)
a
) [cm
]
0.084
744
0.108
716
340
1360
565
560
2
q
max [8]
50.06
1583
50.10
6317
46.00
2129
50.210
3032
50.06
4450
50.0
2657
Reflections collected
Independent reflections
Observed reflections
Largest difference peak [eä
1449
1178
0.212
5910
3361
0.569
1458
1357
0.232
2339
1999
0.451
2578
1466
0.295
2392
1505
0.184
À3
]
À3
Largest difference hole [eä
No. of parameters
]
À 0.270
À 0.369
À 0.214
À 0.358
À 0.243
À 0.188
109
430
114
183
218
184
a
R )
0.0618
0.1455
1.275
0.0588
0.1460
1.026
0.0696
0.1815
1.042
0.045
0.107
1.031
1
0.06030.063
0.14430.1276
1.031
a
wR )
GOF )
b
1.062
a) RˆSj jF
j À jF
j j/SjF
j ; wRˆ[Sw(j F
j À jF
j ) /SwjF
2
j ] . b) Goodness-of-fitˆ[Sw(j F
2
1/2
j À jF
j ) /(NOÀNV)]1/2, where NO is the number of observations
2
o
c
o
o
c
o
o
c
and NV is the number of variables.

Helvetica Chimica Acta ± Vol. 86 (2003)
1245
graphed (SiO ; CHCl ). Some of the compounds (expecially b-1) are unstable, and their crystals do not diffract
2 3
well, which affects the refinement.
X-Ray Crystallography. Crystals suitable for X-ray-diffraction studies were obtained by slow evaporation of
the reaction solvent. The diffraction-intensity measurements for a-1, 3, and 5 were carried out on a Philips PW-
1
100 four-circle diffractometer, and a Nonius KappaCCD diffractometer was used for compounds b-1, 2, and 4,
with graphite-monochromated MoK
a
radiation (l ˆ 0.71073ä). Unit-cell parameters were determined from 20
to 25 well-centered, intense reflections (Philips), or from all the data (Nonius). Structure solutions and full-matrix
least-squares refinements were accomplished with the SHELXS-97 PC package [22]. All non-H-atoms were
refined anisotropically, except for the disordered solvent present in 4. All methyl H-atom positions were calculated
geometrically, fixed at a CÀH distance of 0.96 ä (not refined). Other H-atoms were isotropically refined. Crystal
data and further data-collection parameters for the compounds studied are summarized in Table 3.
Crystallographic data (excluding structure factors) for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as deposition No. CCDC-201084 (3), CCDC-
2
01181 (4), CCDC-201182 (5), CCDC-201183( 2), CCDC-201184 (b-1), and CCDC-201185 (a-1). Copies of the
data can be obtained, free of charge, on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ UK
(
fax: ‡ 441223 336033; e-mail: deposit@ccdc.cam.ac.uk).
This research was supported mainly by the United States-Israel Binational Science Foundation, Grant No.
1
997389, and partially by the Technion V.P.R. Fund, and by the Fund for the Promotion of Research at the
Technion. R. E. D. S. and J. S. M. gratefully acknowledge support from the U.S. NSF (Grant No. CHE 0110685)
and U.S. DOE (Grant No. DE-FG-93ER45504).
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Received February 6, 2003

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