Persistent vinylnitroxides

Article abstract of DOI:10.1039/a900733d

A method for the synthesis of persistent vinylnitroxides (vinylaminoxyls) has been developed and, for one of the radicals synthesized, an extremely high energy of intermolecular exchange interaction (-101 K) has been found.

Full text of DOI:10.1039/a900733d

Persistent vinylnitroxides
Vladimir A. Reznikov,a Ivan V. Ovcharenko,b Natalia V. Pervukhina,a Vladimir N. Ikorskii,a Andre Grandc
and Victor I. Ovcharenkoa*
a International Tomography Center, 630090 Novosibirsk, Russia. E-mail: ovchar@tomo.nsc.ru
b Institute of Chemical Kinetics and Combustion, 630090 Novosibirsk, Russia
c CEA-CENG, Grenoble 38054, France
Received (in Cambridge, UK) 27th January 1999, Accepted 12th February 1999
A method for the synthesis of persistent vinylnitroxides
(vinylaminoxyls) has been developed and, for one of the
radicals synthesized, an extremely high energy of inter-
molecular exchange interaction (2101 K) has been found.
Vinylnitroxides are traditionally considered transient radicals,
since even EPR methods are often not powerful enough to
detect them without using spin traps.1 We succeeded in
synthesizing persistent vinylnitroxides 1 and 2 and isolating
them as individual solids (Scheme 1).†
•
R
OH
R
OH
R
O
N
N
N
NaCN
[O]
Cl
NC
NC
DMSO
O
O
O
3a R = Ph
b R = m-pyridyl
4a R = Ph
b R = m-pyridyl
1 R = Ph
2 R = m-pyridyl
Fig. 2 ORTEP plot of the structure of 1.
Scheme 1
experimental error (1.70 mB for 1 and 1.73 mB for 2) and do not
change when the temperature decreases. Thus the experiment
with vinylnitroxide solutions confirmed that strong spin–spin
interactions are inherent in the solids. This prompted us to study
the crystal structure of the vinylnitroxides. For 1 we succeeded
in growing a single crystal of good quality (Fig. 2).§ In solid 1,
the shortest intermolecular contacts are at least 3 Å. Therefore,
the structure of 1 should be considered molecular. However, the
magnetic structure of 1, as noted above, involves magnetic
chains. The shortest intermolecular contacts in solid 1 are
shown in Fig. 3. These are the nearly equal distances from the
nitroxyl oxygen atom of one 1 molecule to both vinyl carbon
•
The vinylnitroxides isolated are similar to nitronyl nitroxides,
whose synthesis was developed by Ullman et al.,2 in that they
have effective delocalization of spin density. The EPR data of
solutions of 1 and 2 indicate that the spin density in 1 and 2 is
significantly delocalized from the N–O• group to the nitrile
•
•
group [aN(N–O ) = 6.0 G, aN(CN) = 1.0 G for 1 and aN(N–O )
6.0 G, aN(CN) = 1.2 G for 2).
=
The magnetic properties‡ of solid 1 and 2 deserve special
attention. Treatment of the experimental dependence c(T) for 1
(Fig. 1) and 2 showed that the experimental curves perfectly fit
the exchange chain model3 with parameters g = 2.04, J/k =
2101 K and x = 0.02 for 1 and g = 2.01, J/k = 255 K and x
= 0.03 for 2. Due to these strong exchange interactions in solid
1 and 2, even at room temperature the meff values of the
compounds (1.49 mB for 1 and 1.58 mB for 2) are considerably
smaller than the theoretical value of 1.73 mB corresponding to
the presence of one unpaired electron per molecule. The
effective magnetic moments of 1 and 2 continuously decrease to
0.2 and 0.4 mB at 2 K, respectively. In CHCl3 solutions of 1 and
2, the magnetic moments equal the theoretical value within
atoms of the neighboring 1 molecule (dO(N–O )…b-C(CNC)
=
•
3.183 Å and dO(N–O )…a-C(CNC) = 3.296 Å). It is reasonable to
assume that the ‘magnetic chains’ are formed by these contacts
in solid 1 and are responsible for the strong exchange
interactions (2101 K) in 1.
To understand the magnetic behavior of 1 we carried out ab
initio calculations on the electronic structure and intermolecular
exchange interaction parameters in a two-radical model system
[Fig. 4(a)]. It was found that the traditional HF (ROHF, UHF)
and post-HF (direct CI, CASSCF) methods failed to give the
Fig. 1 Plots of effective magnetic moment (meff) (8) and magnetic
susceptibility (cM) (-) for 1.
Fig. 3 Structure of the exchange chain in solid 1 (H atoms and CH3 groups
are omitted for clarity).
Chem. Commun., 1999, 539–540
539

contacts the spin density localized on the nitroxyl group of the
neighboring molecule, causing an effective intermolecular
exchange interaction between the neighboring radicals.
Thus, we succeeded in synthesizing persistent vinylnitr-
oxides and revealed their surprising ability to form magnetic
chains with strong interchain exchange interactions at long
intermolecular distances ( > 3 Å) in formally molecular solids.
The quantum-chemical calculations adequately explain these
strong exchange interactions in solid vinylnitroxides. It ap-
peared that these strong interactions are favored by localization
of the spin density of the N–O• group of one nitroxide near the
b-carbon of the vinyl group bonded to the nitroxyl group of the
neighboring nitroxide.
Notes and references
† The starting 2-phenylpyrroline 3a was obtained according to V. A.
Reznikov, L. A. Vishnivetskaya and L. B. Volodarsky, Izv. Akad. Nauk.,
Ser. Khim., 1990, 390. The pyridine derivative was synthesized in a similar
manner.
Fig. 4 Truncated (two-radical) model system (a) and general view of the
spin density distribution for the model system (b).
Synthesis of 4a: 0.5 g (2 mmol) of chloropyrroline 3a was added over 15
min to a solution of 0.2 g of NaCN (4 mmol) in 5 ml of anhydrous DMSO
with stirring and cooling. The mixture was stirred further for 30 min at
20 °C and then diluted with 15 ml of water with cooling. The solution was
acidified with 5% HCl to pH 3. The product precipitate was filtered off,
washed with water and a 1:1 EtOAc–hexane mixture, and dried (80%); mp
213–216 °C (from EtOAc–MeOH); nmax(KBr)/cm21 2200 (C·N), 1675
(CNO); lmax(EtOH)/nm (log e) 245 (4.27), 346 (3.88); dH([2H6]DMSO)
1.35 [s, 6H, 5-(CH3)2], 7.64 (m, 3H), 7.84 (m, 2H, Ph); dC([2H6]DMSO)
20.75 [5-(CH3)2], 71.08 (C-5), 77.96 (C-3), 115.41 (C·N), 125.82, 128.45,
128.84, 132.64 (Ph), 170.92 (C-2), 194.96 (C-4). The pyridine derivative 4b
was obtained in a similar manner.
Synthesis of 1: A suspension of 0.2 g of the hydroxy compound 4b and 2 g
of MnO2 in 10 ml of CHCl3 was stirred for 30 min at 20 °C. The excess of
the oxidant was filtered off, the solution was evaporated, and the residue
was chromatographed on silica gel, with CHCl3 as eluent (60%), mp
143–145 °C (from hexane); nmax(KBr)/cm21 2200 (C·N), 1700 (CNO),
1540, 1600 (CNC); lmax(hexane)/nm (log e) 250 (4.17), 270 (4.11), 313
(3.73), 334 (3.93), 397 (3.68), 578 (3.26) [Found (calc.): C, 68.5 (68.7); H,
4.8 (4.8); N, 12.1% (12.1%)]. An analogous procedure with the pyridine
derivative 4b afforded the corresponding radical 2 in 40% yield, mp
113–115 °C (from hexane); nmax(KBr)/cm21 1705 (CNO), 1590 (CNC),
2200 (C·N); lmax(hexane)/nm (log e) 246 (4.14), 308 (3.48), 333 (3.67),
395 (3.48), 564 (3.05) [Found (calc.): C, 55.7 (55.6); H, 4.2 (4.5); N, 15.3%
(15.6%)].
Table 1 Exchange interaction parameters calculated in the framework of HF
and DFT methods
Method
Basis seta
(J/k)/K
ROHF UHF
CASSCF (4,6)
ROHF CI(2,2)
ROHF CI(2,4)
ROHF CI(4,6)
VWN
BP86
VWN
BP86
Experiment
BG
BG
BG
BG
BG
BG
BG
DZVP
DZVP
> 104
1.5
2.4
2.8
40.3
298.6
250.2
2190.0
2108.9
2101
a BG: see ref. 4. DZVP: see ref. 5.
Table 2 Main calculated atomic spin densities (VWN approach)
Basis set
Atom
DZVP
BG
‡ Magnetic susceptibility was measured with an MPMS-5S SQUID
magnetometer over 2–300 K.
O1
N1
O2
C8
0.420
0.214
0.101
0.237
0.093
0.437
0.201
0.098
0.235
0.088
0.547
0.124
0.107
0.212
0.074
0.548
0.119
0.106
0.211
0.074
§ Crystal data for 1: C13H11N2O2, M = 227.24, orthorhombic, space group
Aba2, a = 15.334(3), b = 18.203(4), c = 8.414(2) Å, V = 2348.6(9) Å3,
Z = 8, Dc = 1.285 g cm23, l(Mo-Ka), Enraf-Nonius CAD 4, q region 2.60
≤ q ≤ 23.48°, T = 293 K, 1512 reflections collected, 847 independent (Rint
= 0.0265), full-matrix least-squares on F2 (SHELX 97), GOF = 0.506,
final R values [847 Il > 2s(I) R1 = 0.0204, wR2 = 0.0251], extinction
coefficient 0.0056(4). CCDC 182/1172. Crystallographic data are available
in CIF format from the RSC web site, see: http://www.rsc.org/suppdata/CC/
1999/539/
N2
O1a
N1a
O2a
C8a
N2a
1 H. G. Aurich, K. Hahn and K. Stock, Chem. Ber., 1979, 112, 2776.
2 E. F. Ullman, J. H. Osiecki, D. G. B. Boocock and R. Darcy, J. Am. Chem.
Soc., 1972, 94, 7049.
3 J. C. Bonner and M. E. Fisher, Phys. Rev. A, 1964, 135, 640.
4 R. Poirier, R. Kari and I. G. Csizmadia, Handbook of Gaussian Basis
Sets, Elsevier, Netherlands, 1985.
5 E. R. Davidson and D. Feller, Chem. Rev., 1986, 86, 681; N. Godbout,
D. R. Salahub, J. Andzelm and E. Wimmer, Can. J. Chem., 1992, 70,
560.
sign of the exchange interaction parameter, whereas DFT/
Broken Symmetry methods gave the same sign and order of the
J parameter as in the experiment (Table 1). Analyzing the spin
density distribution [Table 2 and Fig. 2(b)] calculated by the
VWN method, one can assume that the most preferable
intermolecular exchange channel passes through the vinyl b-
carbon atom (C8a). The part (0.21–0.24) of the unpaired
electron spin density localized on the C8a atom in one molecule
Communication 9/00733D
540
Chem. Commun., 1999, 539–540