Structure of 2,4,4,5,5-pentamethyl-2-imidazoline-1-oxyl-3-oxide

Article abstract of DOI:10.1023/A:1025523429661

The crystal and molecular structures of the stable nitroxide radical 2,4,4,5,5-pentamethyl-2-imidazoline-1-oxyl-3-oxide was determined. The N-O bond lengths are 1.279(2) and 1.280(2) A, respectively. The O ^--N⁺=C-N-?O fragment is nearly planar with carbon atoms of the ethyl fragment that deviated from the O-N⁺=C-N-?O plane by -0.204(5) and +0.176(5) A. The minimum intermolecular distance between the oxygen atoms of NO groups is 4.094 A.

Full text of DOI:10.1023/A:1025523429661

Journal of Structural Chemistry, Vol. 44, No. 2, pp. 314–317, 2003
Original Russian Text Copyright @ 2003 by G. V. Romanenko, D. V. Ovcharenko, and S. F. Vasilevskii
STRUCTURE OF 2,4,4,5,5-PENTAMETHYL-2-IMIDAZOLINE-1-OXYL-3-OXIDE
G. V. Romanenko,1 D. V. Ovcharenko,2 and S. F. Vasilevskii2
UDC 548.737
The crystal and molecular structures of the stable nitroxide radical 2,4,4,5,5-pentamethyl-2-imidazoline-
˚
1-oxyl-3-oxide was determined. The N O bond lengths are 1.279(2) and 1.280(2) A, respectively.
The O− N+
C
N
•O fragment is nearly planar with carbon atoms of the ethyl fragment that
deviated from the O N+
C
N
•O plane by −0.204(5) and +0.176(5) A. The minimum inter-
˚
˚
molecular distance between the oxygen atoms of NO groups is 4.094 A.
Key words: nitroxide radicals, nitroxides, structure.
Recently, we described synthesis and structure of the first nitronyl nitroxide radicals containing various N-
alkylpyrazoles (NIT-Pz) [1] as substituents; these radicals were used to produce single crystals of chain polymer
heterospin Cu(hfac)2(NIT-Pz) complexes with unusual magnetic and mechanical properties [2]. As a continuation of
these studies, we undertook an attempt to develop a method for synthesizing dinitronyl nitroxide radicals containing
nitrogen unsubstituted pyrazole fragments whose structure favors the formation of high-dimension systems. At the
first stage of these studies, we unexpectedly synthesized single crystals of a simple classical nitronyl nitroxide,
2,4,4,5,5-pentamethyl-2-imidazoline-1-oxyl-3-oxide (1). The structure of this compound is described in the present
paper.
◦
˚
˚
˚
The crystals of 1 are monoclinic prisms: a = 6.235(1) A, b = 13.135(2) A, c = 11.844(2) A, β = 101.82(2) ,
V = 949.4(3) A , Z = 4 for C8H15N2O2, dcalc = 1.198 g/cm3, and µ(MoKα) = 0.087 mm−1. A set of reflections
3
˚
was obtained on a Bruker AXS P4 autodiffractometer using a standard technique [239 K, Mo radiation, θ/2θ scan
with a variable velocity of Vmin = 3 deg/min in the range 2.34 < θ < 25.02◦; 1815 Ihkl were measured, among which
1656 were independent (Rint = 0.0313)]. The structure was solved by direct methods and refined by a full-matrix
least-squares method in an anisotropic approximation for non-hydrogen atoms and in an isotropic approximation for
experimentally located hydrogen atoms up to R1 = 0.0551 and wR2 = 0.1584 for Ihkl > 2σ(I) and GOOF = 1.041.
The structural solution and refinement were performed using the SHELX-97 software package. Table 1 shows atomic
coordinates, and Table 2 lists the main bond lengths and angles.
TABLE 1
2
3
Atomic Coordinates ( × 104) and Equivalent Thermal Parameters Ueq = (U11 + U22 + U33)/3 (A × 10 )
˚
Atom
x
y
z
Ueq
Atom
x
y
z
Ueq
O(1)
N(1)
C(1)
C(11)
C(12)
C(2)
782(4)
551(3)
3145(2)
3417(1)
2845(2)
2624(3)
1845(2)
3575(2)
5501(2)
6506(2)
7568(2)
7575(3)
7511(3)
8533(2)
82(1)
52(1)
49(1)
78(1)
78(1)
45(1)
C(21)
C(22)
N(2)
389(5)
3044(4)
3085(3)
4286(2)
4218(1)
4839(2)
4134(2)
4736(3)
9536(3)
9009(3)
7867(2)
8353(2)
6733(2)
5865(3)
71(1)
72(1)
53(1)
84(1)
54(1)
86(1)
1595(3)
3976(4)
286(6)
−656(3)
−1764(3)
−821(4)
−2345(7)
O(2)
C(3)
1152(3)
C(31)
1International Tomography Center, Siberian Branch, Russian Academy of Sciences, Novosibirsk;
romanenk@tomo.nsc.ru. 2Institute of Chemical Kinetics and Combustion, Siberian Branch, Russian Academy
of Sciences, Novosibirsk. Translated from Zhurnal Strukturnoi Khimii, Vol. 44, No. 2, pp. 351–354, March–April,
2003. Original article submitted November 13, 2001.
c
314
0022-4766/03/4402-0314 $25.00 ꢀ 2003 Plenum Publishing Corporation

TABLE 2
˚
Bond Lengths [A] and Angles [deg] in 1
O(1) N(1)
N(1) C(3)
1.279(2)
1.336(3)
N(2) O(2)
N(2) C(3)
1.280(2)
1.331(3)
N(1) C(1)
C(1) C(11)
1.495(3)
1.511(3)
C(2) N(2)
C(2) C(21)
1.497(3)
1.511(3)
C(1) C(12)
C(1) C(2)
1.541(4)
1.558(3)
C(2) C(22)
C(3) C(31)
1.520(3)
1.478(3)
O(1) N(1) C(3)
O(1) N(1) C(1)
C(3) N(1) C(1)
N(1) C(1) C(11)
N(1) C(1) C(12)
C(11) C(1) C(12)
N(1) C(1) C(2)
C(11) C(1) C(2)
C(12) C(1) C(2)
N(2) C(3) N(1)
N(2) C(3) C(31)
125.67(19)
122.03(17)
111.77(17)
111.0(2)
O(2) N(2) C(3)
O(2) N(2) C(2)
C(3) N(2) C(2)
N(2) C(2) C(21)
N(2) C(2) C(22)
C(21) C(2) C(22)
N(2) C(2) C(1)
C(21) C(2) C(1)
C(22) C(2) C(1)
N(1) C(3) C(31)
124.70(19)
122.73(17)
112.30(17)
109.27(18)
106.96(19)
108.4(2)
104.83(19)
110.3(2)
101.34(15)
115.83(19)
112.6(2)
100.83(15)
116.5(2)
114.19(19)
125.7(2)
109.93(18)
124.3(2)
Diagram 1.
315

Fig. 1. Molecular structure of 1 and a (100) projection of the structure.
As shown in Diagram 1, the synthesis of the dinitronyl nitroxide radical containing the nitrogen unsubstituted
pyrazole fragment involved etherification of diacid 2, addition of diether 3 to vinyl ethyl ether, reduction of nitrogen
substituted diether 4, oxidation of dialcohol 5 to dialdehyde 6, and condensation of the resultant dialdehyde with
bishydroxylamine (BHA) by the classical Ulman procedure to the nitronyl nitroxide radical (NNR) and tetrahydroxy
derivative 7 followed by its oxidation to 8. The final stage envisaged removal of acetal protection by an acid.
However, only trace amounts of dinitroxide 8 were detected. It was found that the main reaction product
at the stage of production of 7, which was oxidized to 8 without isolation, was nitroxide 1. Its formation can be
explained by acidic hydrolysis of 6 and/or 7 under the conditions of production of 7 and splitting of the alcohol
molecule from the intermediate semiacetal followed by interaction of acetic aldehyde with BHA and its subsequent
oxidation to 1 under the conditions of production of 8. This also explains the minor amounts of 8 in the competitive
reactions of hydrolysis of N-acetal 6 and/or 7 and condensation of the carbonyl compound with BHA. The rate of
the first reaction is obviously higher, and the aliphatic aldehyde formed by acidic hydrolysis is more reactive than
the aromatic aldehyde. This explains why nitroxide 1 was obtained in the form of red crystals after recrystallization
of the final product from hexane.
316

Previously, 1 has been used in the synthesis of heterospin complexes [4–11] although its crystal structure
remained unknown. The structure of 1 is molecular (see Fig. 1). As in most nitronyl nitroxide radicals, the N
and N(2) O(2), C(3) N(1) and C(3) N(2), and C(1) N(1) and C(2) N(2) bond lengths are close: 1.279(2)
O
−
N+
C
C
N
N
•O fragment
˚
and 1.280(2), 1.336(3) and 1.331(3), and 1.495(3) and 1.497(3) A, respectively. The O
is nearly planar with carbon atoms of the ethyl fragments that deviated from the O− N+
•O plane by
˚
−0.204(5) and +0.176(5) A. The intermolecular distances between the oxygen atoms of the NO groups are rather
˚
large, the minimum of which is 4.094 A. Hydrogen bonds are absent. We also note that the distance to the carbon
atom of the methyl group at position 2 of the cycle is significantly shorter than the single bond length.
In metal complexes, the nitroxide structure differs significantly. In the examined complexes with 1, the
deviation of the carbon atoms at positions 4 and 5 of the heterocycle from the CN2O2 plane varies from 0.09 to
˚
˚
0.27 A; the bond lengths in coordinated N O groups lie in the interval 1.286–1.320 A, whereas in uncoordinated
˚
N
O groups, they are within 1.268–1.282 A. Correspondingly, the C N bond lengths in the CN2 fragment are
also different: the length is shorter for the coordinated group (1.310–1.335 A) than for the uncoordinated group
(1.328–1.363 A). In all complexes, the C C distance to the carbon atom of the methyl group at position 2 of the
cycle is short (1.433–1.490 A).
˚
˚
˚
This work was supported by the Civil Research and Development Foundation (Grant No. REC-008), the
Russian Foundation for Basic Research (Grant Nos. 02-03-32265 and 02-03-33112), and the Integration program.
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