Metal complex with the enaminoketone derivative of 2-imidazoline nitroxide

Article abstract of DOI:10.1070/MC2001v011n05ABEH001472

Functional derivatives of iminonitroxides were prepared by introducing a functional group into the side chain of 1-hydroxy-2-methyl-2-imidazoline; a K⁺ salt and a Cu²⁺ bischelate with the first enaminoketone derivative of 2-iminonitroxide were synthesised and structurally characterised.

Full text of DOI:10.1070/MC2001v011n05ABEH001472

, 2001, 11(5), 179–181
Metal complex with the enaminoketone derivative of 2-imidazoline nitroxide
Pavel A. Petrov,a Sergei V. Fokin,a Galina V. Romanenko,b Yuri G. Shvedenkov,a Vladimir A. Reznikova and
Victor I. Ovcharenko*b
a Department of Natural Sciences, Novosibirsk State University, 630090 Novosibirsk, Russian Federation
b International Tomography Centre, Siberian Branch of Russian Academy of Sciences, 630090 Novosibirsk, Russian Federation.
Fax: +7 3832 33 1399; e-mail: ovchar@tomo.nsc.ru
10.1070/MC2001v011n05ABEH001472
Functional derivatives of iminonitroxides were prepared by introducing a functional group into the side chain of 1-hydroxy-2-
methyl-2-imidazoline; a K+ salt and a Cu2+ bischelate with the first enaminoketone derivative of 2-iminonitroxide were synthesised
and structurally characterised.
Heterospin systems based on metal complexes with 2-imidazoline
nitroxides are widely used in molecular magnet design.1 How-
ever, the syntheses of nitronyl nitroxides 1 and iminonitroxides
2 containing functional groups R (Scheme 1), which are favour-
able for metal complex formation, were confined to only one
synthetic procedure. This method, which was proposed by Ullman,
First, we found an effective approach to the synthesis of 5.
The process looks like the ‘thermal dehydration’ of 4 to 5
(Scheme 1). It proceeds with a high yield in boiling heptane (or
toluene at 100 °C).† We found that the dehydration was not
accelerated in the presence of para-toluenesulfonic acid. The
conversion of 4 to 5 by thermal dehydration does not proceed in
an inert atmosphere. The presence of atmospheric oxygen is
indispensable. Thermal dehydration of 4 is a very effective one-
pot synthesis of 5.
This easy route to 5 permits one to use 2,4,4,5,5-penta-
methyl-1-hydroxy-2-imidazoline (5, R = Me) as a starting com-
pound for the syntheses of persistent enaminoketones of 2-imida-
zoline nitroxide. The reactions of 5 (R = Me) with esters in
the presence of lithium diisopropylamide give enaminoketones
6 (Scheme 2). The introduction of the nitrile substituent into
enaminoketones 6 (R1 = Ph, CF3) and further oxidation led to
relatively persistent nitroxides 7 (MS, m/z: 276 [M+] R1 = CF3;
m/z: 284 [M+] R1 = Ph). Nitroxide analogues of 7 with hydrogen
substituted for the nitrile group are unstable and quickly de-
compose in solution. Nitroxide 7 (R1 = CF3) turned out to be
the most long-living. However, we converted it every time to
potassium salt 8 persistent under normal conditions. Single crys-
tals of potassium salt 8‡ and Cu2+ complex 9§ with nitroxide 7
(R1 = CF3) were obtained, whose structures are shown in Figures
1 and 2, respectively. Synthesis¶ of 9 is actually a logical end of
the chain of transformations presented in Scheme 2.
OH
NHOH
N
N
N
N
RCHO
R
R
– H2O
NHOH
OH
OH
3
4
5
O
N
N
N
N
R
R
O
O
1
2
Scheme 1
involves the condensation of dihydroxyamine 3 with aldehydes
or their synthetic equivalents and the subsequent oxidation of
dihydroxyimidazolidines 4. Iminonitroxides 2 were generated by
the reduction of 1.2 In this work, a donor group R (enamino-
ketone fragment) was introduced at the 2-position of a heterocyclic
ring using another approach, the modification of 1-hydroxy-2-
methyl-2-imidazoline (5, R = Me).
†
A typical procedure includes the boiling of a suspension of 1 g of 4 in
30–40 ml of heptane for 10–12 h. The precipitate of 1-hydroxy-2-
imidazoline 5 was filtered off (85% yield for R = Me, 70% for R = Et
and 50% for R = Ph) after recrystallization from hexane–ethyl acetate.
5 (R = Me): mp 104–106 °C. 1H NMR ([2H6]DMSO) d: 1.08 (s, 6H),
1.09 (s, 6H, 4,5-Me2), 1.93 (s, 3H, 2-Me), 8.08 (br. s, OH). 13C NMR
([2H6]DMSO) d: 13.5 (2-Me), 18.6, 23.4 (4,5-Me2), 65.1, 69.8 (C-4, C-5),
162.1 (C-2). IR (KBr, n/cm–1): 3160, 2900–2600, 1616. Found (%): C,
61.5; H, 10.5; N, 17.9. Calc. for C8H16N2O (%): C, 61.5; H, 10.3; N, 17.9.
5 (R = Et): mp 121–123 °C. 1H NMR (CD3OD) d: 1.15 (t, 3H, CH2Me),
1.21 (s, 6H), 1.24 (s, 6H, 4,5-Me2), 2.45 (q, 2H, CH2Me). IR (KBr,
n/cm–1): 3110, 2900–2600, 1613. Found (%): C, 62.9; H, 11.1; N, 16.3.
Calc. for C9H18N2O (%): C, 63.5; H, 10.7; N, 16.5.
O
H
R1
N
N
N
i
N
OH
OH
1
5 (R = Ph): mp 190–191 °C. H NMR (CDCl3) d: 1.36 (s, 6H), 1.43
5
6
(s, 6H, 4,5-Me2), 7.25, 7.49 (2m, 5H, Ph), 8.33 (br. s, 1H, OH). IR (KBr,
n/cm–1): 3110, 2900–2600, 1611, 1591, 1573. Found (%): C, 71.3; H, 8.2;
N, 12.6. Calc. for C13H18N2O (%): C, 71.5; H, 8.3; N, 12.8.
R1 = CF3, Ph, CO2Et
ii–iv
1
Hydroxylamine precursors of 7 (R1 = Ph): mp 213–214 °C. H NMR
([2H6]DMSO–CD3COCD3) d: 1.18 (s, 6H), 1.29 (s, 6H, 4,5-Me2), 7.39–
7.71 (m, 5H, Ph), 9.6 (br. s, 1H, NH), 10.1 (s, 1H, OH). 13C NMR
(CD3OD) d: 18.5, 23.1 (4,5-Me2), 62.6 (C-4), 71.6 (C-5), 121.7 (CºN),
128.7, 128.9, 131.6, 141.5 (Ph), 166.3 (C-2), 194.0 (C=O). IR (KBr,
n/cm–1): 3200–2800, 2209 (CºN), 1600, 1577, 1534. Found (%): C, 67.3;
H, 6.8; N, 14.6. Calc. for C16H19N3O2 (%): C, 67.4; H, 6.7; N, 14.7.
(R1 = CF3): mp 189–191 °C. 1H NMR ([2H6]DMSO) d: 1.11 (s, 6H), 1.21
(s, 6H, 4,5-Me2), 9.30 (s, NH), 10.4 (s, OH). 13C NMR ([2H6]DMSO)
d: 18.3, 22.2 (4,5-Me2), 61.9 (C-4), 63.6 (=C–CN), 70.3 (C-5), 115.8
(CºN), 117.2 (q, CF3, JC–F 291 Hz), 162.8 (C-2), 173.8 (q, C=O, JC–F
32 Hz). IR (KBr, n/cm–1): 3400–3200, 2218 (CºN), 1620, 1547. Found
(%): C, 47.7; H, 4.5; N, 14.8. Calc. for C11H14N3O2F3 (%): C 47.7; H, 5.1;
N, 15.2.
M
O
O
H
R1
R1
N
N
N
N
CN
CN
O
O
R1 = CF3, M = K
8
7
9 R1 = CF3, M = Cu/2
R1 = CF3, Ph
Scheme 2 Reagents and conditions: i, LDA, R1CO2Et, Et2O, 0 ºC; ii,
NCS, CHCl3, room temperature; iii, NaCN, DMSO, room temperature; iv,
PbO2, CHCl3.
– 179 –

, 2001, 11(5), 179–181
F(5)
F(6)
C(52)
C(41)
C(42)
C(52)
F(4)
C(06)
O(11a)
C(51)
C(42)
C(4)
N(211)
C(6)
O(2)
C(51)
C(71)
C(41)
C(4)
N(11)
O(11)
C(5)
K
N(1)
N(1a)
Cu
C(7)
C(8)
N(11)
C(2a)
N(1)
C(2)
O(11)
N(2a)
O(1)
C(1)
C(3)
O(21)
C(91)
C(1a)
C(3)
N(111)
C(2)
N(2)
N(21)
N(11a)
O(1)
C(21)
C(1)
C(01)
C(10)
C(102)
C(11)
C(9)
F(3)
F(1)
F(2)
F(3)
C(101)
F(2)
C(92)
F(1)
Figure 2 Molecular structure of 9 with displacement ellipsoids drawn at a
35% probability level (hydrogen atoms omitted for clarity). Selected dis-
tances (Å) and angles (°): Cu–O(2) 1.953(7), Cu–O(1) 1.963(6), Cu–N(1)
2.024(6), Cu–N(2) 2.033(6), Cu–N(11a) 2.299(9), Cu–N(1a) 2.774(14),
O(1)–C(1) 1.290(10), C(1)–C(2) 1.368(11), C(2)–C(3) 1.437(11), C(2)–
C(21) 1.441(12), C(21)–N(111) 1.142(11), C(3)–N(1) 1.266(10), N(11)–
O(11) 1.264(10), O(2)–C(6) 1.263(11), C(6)–C(7) 1.386(14), C(7)–C(8)
1.409(14), C(7)–C(71) 1.435(15), C(71)–N(211) 1.148(16), C(8)–N(2)
1.296(10), N(21)–O(21) 1.269(10), N(1a)–C(1a) 1.071(17), C(1a)–C(2a)
1.43(2); O(1)–Cu–N(1) 87.9(3), O(2)–Cu–N(2) 88.5(3), N(11a)–Cu–N(1a)
178.0(4), C(1a)–N(1a)–Cu 139.8(13).
Figure 1 Molecular structure of 8 with displacement ellipsoids drawn at a
35% probability level (hydrogen atoms omitted for clarity). Selected dis-
tances (Å) and angles (°): K–O(11a) 2.688(4), K–O(1) 2.719(4), K–N(2a)
2.821(7), K–N(1) 2.854(5), O(1)–C(1) 1.255(7), C(1)–C(2) 1.343(7), C(2)–
C(21) 1.407(9), C(2)–C(3) 1.422(7), C(21)–N(2) 1.172(8), O(11)–N(11)
1.281(5), C(3)–N(1) 1.287(6); O(1)–K–N(1) 62.78(13), C(1)–O(1)–K
120.9(4), O(1)–C(1)–C(2) 126.9(6), C(1)–C(2)–C(3) 121.6(6), N(2)–C(21)–
C(2) 177.5(8), O(11)–N(11)–C(3) 126.4(5), O(11)–N(11)–C(4) 123.4(4),
C(3)–N(11)–C(4) 109.8(5), N(1)–C(3)–C(2) 127.6(5), C(3)–N(1)–K 124.9(4).
The structure of 8 is ionic. The distances between the para-
magnetic centres in 8 are at least 6.71 Å, leading to constant meff
(1.68 B.M.) in the temperature range from 300 to 10 K. In com-
plex 9, the distorted octahedral environment of the central atom
is formed by the O and N atoms of the two deprotonated
enaminoketone ligands and by the N atoms of acetonitrile, and
the nitrile group of the neighbouring bis-chelate molecule, lead-
ing to formation of polymer chains. The temperature dependence
of the magnetic susceptibility of 9 is presented in Figure 3. It is
adequately approximated by a cluster model with the parameters
g = 2.0, J = 90 cm–1 and nJ' = –0.3 cm–1. The J value is the
highest positive value of the intramolecular exchange interaction
energy among the known copper bis-chelates with stable nitroxides.
This value points to the presence of highly effective exchange
clusters inside the bis-chelate fragments.
Thus, a convenient route to a series of 4,4,5,5-tetramethyl-1-
hydroxy-2R-2-imidazolines (5, R = Me, Et, Ph) has been found.
Derivative 5 (R = Me) may be used as a substrate for syntheses
of persistent spin-labeled enaminoketones (7, R1 = CF3, Ph). The
first metal complex with the enaminoketone derivative of 2-imino-
nitroxide 9 has been isolated. A transition from metal com-
plexes with spin-labeled 3-imidazoline enaminoketones, charac-
terised by intramolecular exchange interaction energies of about
5–15 cm–1, to the complex with 2-imidazoline analogue allowed
us to achieve noticeably higher intramolecular exchange inter-
action energies between the unpaired electrons of paramagnetic
centres (90 cm–1). Studies are currently under way to synthesise
other metal complexes with the enaminoketone derivatives of
2-imidazoline nitroxide and to investigate their structure and
magnetic properties.
‡
Crystal data for 8: C11H12F3KN3O2, M = 314.34, monoclinic, a =
= 11.068(2) Å, b = 9.203(2) Å, c = 14.177(3) Å, b = 92.19(3)°, V =
= 1443.0(5) Å3, T = 293 K, space group P21/c (no. 14), Z = 4, dcalc
=
,
= 1.447 g cm–3, m(MoKα ) = 0.405 mm–1. 2194 Ihkl (2012 unique Ihkl
Rint = 0.1104) were measured on a Bruker AXS P4 four-circle automatic
diffractometer (lMoKα , graphite monochromator, q/2q-scan, 2.64 < q <
< 24.98°). The structure was solved using the SIR97 program and
refined by the full-matrix least-squares technique in an anisotropic ap-
proximation for all non-hydrogen atoms. All hydrogen atoms were
located in a difference Fourier map and then refined in an isotropic
approximation. The final R indexes are: R1 = 0.0542, wR2 = 0.1006 for
2012 unique Ihkl > 2s(I), GOOF = 0.664.
§
Crystal data for 9: C24H27CuF6N7O4, M = 655.07, orthorhombic, a =
= 14.045(2) Å, b = 12.342(2) Å, c = 17.259(3) Å, V = 2991.7(8) Å3, T =
= 293 K, space group Pca21 (no. 29), Z = 4, dcalc = 1.454 g cm–3,
m(MoKα ) = 0.809 mm–1. 2723 Ihkl (2723 unique Ihkl) were measured on
a Bruker AXS P4 four-circle automated diffractometer (lMoKα , graphite
monochromator, q/2q-scan, 2.36 < q < 24.99°, empirical absorption cor-
rection). The structure was solved by the SIR97 program and refined by
the full-matrix least-square technique in an anisotropic approximation for
all non-hydrogen atoms. Positions of all hydrogen atoms were located in
a difference Fourier map and than refined in an isotropic approximation.
The final R indexes for 406 refined parameters are: R1 = 0.0576, wR2 =
= 0.1427 for Ihkl > 2s(I), GOOF = 1.024. All calculations were carried
out using the SHELX97 program. Atomic coordinates, bond lengths, bond
angles and thermal parameters have been deposited at the Cambridge
Crystallographic Data Centre (CCDC). For details, see ‘Notice to
Authors’, Mendeleev Commun., Issue 1, 2001. Any request to the CCDC
for data should quote the full literature citation and the reference number
1135/98.
3.8
3.6
3.4
3.2
3.0
¶
For the synthesis of 8, a solution of KOH (4.0 mmol) in MeOH (5 ml)
was added to the solution of 7 (R1 = CF3) (1 g, 3.6 mmol) in ethyl acetate
(100 ml) with stirring. Salt 8 was precipitated from the reaction mixture
by adding toluene (50% yield). Single crystals were grown by slow dif-
fusion of benzene into an ethyl acetate solution of 8. To prepare 9, a
mixture of Cu(NO3)2(H2O)3 (39 mg, 0.16 mmol) and 8 (100 mg, 0.32 mmol)
was dissolved in 10 ml of dry acetonitrile, and the solution was allowed
to stand at –10 °C. After 2 days, the KNO3 precipitate was filtered off;
after one more day, dark red crystals of the complex were isolated (yield
45%). 8: mp 257–262 °C (decomp.). IR (KBr, n/cm–1): 2190 (CºN), 1603,
1553. Found (%): C, 41.8; H, 3.9; N, 13.1. Calc. for C11H12N3O2F3K
(%): C, 42.0; C, 3.9; N, 13.4. 9: mp 176–179 °C (decomp.). IR (KBr,
n/cm–1): 2220 (CºN), 1588, 1523. Found (%): C, 43.6; C, 4.0; N, 14.8.
Calc. for C24H27N7O4F6Cu (%): C, 44.0; H, 4.2; N, 15.0.
0
50
100
150
200
250
300
T/K
Figure 3 The temperature dependence of meff for 9.
This work was supported by CRDF (grant REC-008), Russian
Foundation for Basic Research (grant nos. 00-03-32987 and
00-03-04006) and the ‘Integration’ Foundation.
– 180 –

, 2001, 11(5), 179–181
1078; (c) D. G. B. Boocock, R. Darcy and E. F. Ullman, J. Am. Chem.
Soc., 1968, 90, 5945; (d) E. F. Ullman, J. H. Osiecki, D. G. B. Boocock
and R. Darcy, J. Am. Chem. Soc., 1972, 94, 7049.
References
1 (a) A. Caneschi, D. Gatteschi, R. Sessoli and P. Rey, Acc. Chem. Res.,
1989, 22, 392; (b) O. Kahn, Molecular Magnetism, VCH, New York,
1993; (c) V. I. Ovcharenko and R. Z. Sagdeev, Usp. Khim., 1999, 68, 381
(Russ. Chem. Rev., 1999, 68, 345).
2 (a) E. F. Ullman, L. Call and J. H. Osiecki, J. Org. Chem., 1970, 35,
3623; (b) J. H. Osiecki and E. F. Ullman, J. Am. Chem. Soc., 1968, 90,
Received: 7th May 2001; Com. 01/1798
– 181 –