Manganese (II) coordination complexes involving nitronyl nitroxide radicals

Article abstract of DOI:10.1021/ic9905751

Three-new complexes involving nitronyl nitroxide and PhCOO_- or N_3- ligands have been synthesized and structurally and magnetically characterized. These compounds are formulated as Mn(L)₄(X)₂.nH₂O, L = 2-(p-pyridyl)-4,4,5,5-tetramethylimidazoline-l-oxyl-3-oxide(PNN)or 2-(p-pyridyl)-4,4,5,5-tetramethylimidazoline-l-oxyl (PN) and X = PhCOO^- or N3^-. Compound A [Mn(PNN)₂(PhCOO)₂(H₂O)₂] presents the following structural parameters: triclinic, space group P1～ (No. 2), a = 6.859(1) ?, b = 11.271(2) ?, c = 13.978(6) ?, α = 88.88(2)°, β -89.38(2)°, γ = 78.28(2)°, Z = 1. The complex has a centrosymmetric distorted octahedral geometry in which the manganese ion is bound to two radical ligands through the nitrogen atom of the pyridine rings, two benzoate groups, and two water molecules. The two compounds B [Mn(PNN)₄(N₃)₂] and C [Mn(PN)₄(N₃)₂] are isostructural with the following structural parameters: B; triclinic, space group P1～ (No. 2), a = 7.177(4) ?, b = 13.767(3) ?, c = 13.928(4) ?, α = 90.20(2)°, β= 102.94(4)°, γ = 91.86(4)°, Z = 1; C, triclinic, space group P1～ (No. 2), a = 7.004(2) ?, b = 13.885(1) ?, c = 14.036(2) ?, α = 90.34(1)°, β= 101.42(2)°, γ = 92.92(1)°, Z = l. They adopt a centrosymmetric tetragonally distorted octahedral geometry in which the manganese ion is bound to four radical ligands through the nitrogen atom of the pyridine rings, and the azido groups occupy the apical positions. For all three compounds A, B, and C intramolecular ferromagnetic interactions between the Mn(II) ion and the nitronyl nitroxide radicals are observed, but at very low temperatures, intermolecular antiferromagnetic interactions dominate.

Full text of DOI:10.1021/ic9905751

Inorg. Chem. 1999, 38, 3967-3971
3967
Manganese (II) Coordination Complexes Involving Nitronyl Nitroxide Radicals
Mohammed Fettouhi,*,† Mazen Khaled,† Abdel Waheed,† Ste´phane Golhen,‡
Lahce`ne Ouahab,*,‡ Jean-Pascal Sutter,§ and Olivier Kahn§
Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia,
Laboratoire de Chimie du Solide et Inorganique Mole´culaire, UMR 6511, Universite´ de Rennes 1,
Campus de Beaulieu, 35042, Rennes Cedex, France, and Laboratoire des Sciences Mole´culaires,
ICMCB, UPR CNRS no 9048, 33608 Pessac, France
ReceiVed March 11, 1999
Three new complexes involving nitronyl nitroxide and PhCOO- or N3- ligands have been synthesized and
structurally and magnetically characterized. These compounds are formulated as Mn(L)4(X)2‚nH2O, L ) 2-(p-
pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PNN) or 2-(p-pyridyl)-4,4,5,5-tetramethylimidazoline-1-
oxyl (PN) and X ) PhCOO- or N3-. Compound A [Mn(PNN)2(PhCOO)2(H2O)2] presents the following structural
parameters: triclinic, space group P1h (No. 2), a ) 6.859(1) Å, b ) 11.271(2) Å, c ) 13.978(6) Å, R ) 88.88(2)°,
â ) 89.38(2)°, γ ) 78.28(2)°, Z ) 1. The complex has a centrosymmetric distorted octahedral geometry in
which the manganese ion is bound to two radical ligands through the nitrogen atom of the pyridine rings, two
benzoate groups, and two water molecules. The two compounds B [Mn(PNN)4(N3)2] and C [Mn(PN)4(N3)2] are
isostructural with the following structural parameters: B; triclinic, space group P1h (No. 2), a ) 7.177(4) Å, b )
13.767(3) Å, c ) 13.928(4) Å, R ) 90.20(2)°, â ) 102.94(4)°, γ ) 91.86(4)°, Z ) 1; C, triclinic, space group
P1h (No. 2), a ) 7.004(2) Å, b ) 13.885(1) Å, c ) 14.036(2) Å, R ) 90.34(1)°, â ) 101.42(2)°, γ ) 92.92(1)°,
Z ) 1. They adopt a centrosymmetric tetragonally distorted octahedral geometry in which the manganese ion is
bound to four radical ligands through the nitrogen atom of the pyridine rings, and the azido groups occupy the
apical positions. For all three compounds A, B, and C intramolecular ferromagnetic interactions between the
Mn(II) ion and the nitronyl nitroxide radicals are observed, but at very low temperatures, intermolecular
antiferromagnetic interactions dominate.
Introduction
early solution ESR studies of some metal-pyridyl nitronyl
16,17
nitroxide complexes,
the coordination chemistry of such
During the past two decades, the field of molecular magnets
has attracted scientists from different horizons. Their main
objectives are on one hand the chemical design of molecular
assemblies that exhibit a spontaneous magnetization and on the
other hand the rationalization of the magnetostructural correla-
tions.1
Among the ligands capable of building extended 1D, 2D, and
3D networks in association with open-shell metal ions, great
attention was devoted separately to both nitronyl nitroxides2-4
and acetate, carboxylate, and azido anions.1,5-15 Apart from the
ligands was initiated with the aim of building chainlike species.
However, in most cases the NO groups of the radical do not
coordinate even strongly electrophilic metal ions.18,19 Mean-
while, the particular azido anion has emerged as a versatile
ligand, generally leading to high-nuclearity systems with metals
such as Mn(II),5-12 Cu(II),13,14and Ni(II).15 It stands furthermore
as a good superexchange pathway, both antiferromagnetic in
µ1,3-N3 (end-to-end) coordination and ferromagnetic in µ1,1-N3
(end-on) coordination. In the latter case ferromagnetic coupling
has been interpreted by the spin polarization concept.20 The most
important examples of these systems are those exhibiting a
spontaneous magnetization between 16 and 40 K.5 The majority
† King Fahd University of Petroleum and Minerals.
‡ Universite´ de Rennes 1.
§ ICMCB.
(1) Kahn, O. Molecular Magnetism; VCH: New York, 1993.
(2) Inoue, K.; Hayamizu, T.; Iwamura, H.; Hashizume, D.; Ohashi, Y. J.
Am. Chem. Soc. 1996, 118, 1803.
(3) Caneschi, A.; Gatteschi, D.; Sessoli, R.; Rey, P. Acc. Chem. Res. 1989,
22, 392.
(4) Fegy, K.; Luneau, D.; Belorizky, E.; Novac, M.; Tholence, J. L.;
Paulsen, C.; Ohm, T.; Rey, P. Inorg. Chem. 1998, 37, 4524.
(5) Escuer, A.; Vicente, R.; Goher, M. A. S.; Mautner, F. A. Inorg. Chem.
1997, 36, 3440.
(11) Escuer, A.; Vicente, R.; Goher, M. A. S.; Mautner, F. A. Inorg. Chem.
1995, 34, 5707.
(12) Cortes, R.; Lezama, L.; Pizarro, J. L.; Arriortua, M. I.; Solans, X.;
Rojo, T. Angew. Chem., Int. Ed. Engl. 1994, 33, 2488.
(13) Sikorav, S.; Bkouche-Waksman. I.; Kahn. O. Inorg. Chem. 1984, 23,
490.
(14) Agnus, Y.; Louis, R.; Gisselbrecht, J. P.; Weiss, R. J. Am. Chem.
Soc. 1984, 106, 93.
(6) Escuer, A.; Vicente, R.; Goher, M. A. S.; Mautner, F. A. Inorg. Chem.
1998, 37, 782.
(7) Escuer, A.; Vicente, R.; Goher, M. A. S.; Mautner, F. A. J. Chem.
Soc., Dalton. Trans. 1997, 4431.
(8) Cortes, R.; Drillon, M.; Solans, X.; Lezama, L.; Rojo, T. Inorg. Chem.
1997, 36, 677.
(15) Ribas, J.; Monfort, M.; Ghosh, B. K.; Cortes, R.; Solans, X.; Font-
Bardia, M. Inorg. Chem. 1996, 35, 864 and references therein.
(16) Richardson, P. F.; Kreilick, R. W. J. Am. Chem. Soc. 1977, 99, 8183.
(17) Richardson, P. F.; Kreilick, R. W. J. Phys. Chem. 1978, 82, 1149.
(18) Caneschi, A.; Ferraro, F.; Gatteschi, D.; Rey, P.; Sessoli, R. Inorg.
Chem. 1990, 29, 4217.
(9) Escuer, A.; Vicente, R.; Goher, M. A. S.; Mautner, F. A. Inorg. Chem.
1996, 35, 6386.
(10) De Munno, G.; Julve, M.; Viau, G.; Lloret, F.; Faus, J.; Viterbo, D.
Angew. Chem., Int. Ed. Engl. 1996, 35, 1807.
(19) Luneau, D.; Risoan, G.; Rey, P.; Grand, A.; Caneschi, A.; Gatteschi,
D.; Laugier, J. Inorg. Chem. 1993, 32, 5616.
(20) Charlot, M. F.; Kahn, O.; Chaillet, M.; Larrieu, C. J. Am. Chem. Soc.
1986, 108, 2574.
10.1021/ic9905751 CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/19/1999

3968 Inorganic Chemistry, Vol. 38, No. 18, 1999
Fettouhi et al.
Table 1. Crystallographic Data for A, B and C
of the polynuclear azido systems reported so far are pyridine-
like or bipyridine adducts.5-12 Therefore, in order to couple the
two synthetic approaches, namely, realizing chainlike networks
built with the azido ligand and metallic centers coordinated by
spin- carrying ligands, we came up with the idea of using
nitronyl nitroxide substituted pyridines. As a first approach the
system Mn(II), N3, PNN, or PN was investigated. Unlike the
multicentercoordination schemes µ1,1 and µ1,3 adopted by the
azido moieties in the 1D and 2D pyridine adducts, only
mononuclear species were obtained.
Three new complexes have hence been isolated. We report
herein the synthesis, X-ray structures, and magnetic character-
izations of the three compounds A [Mn(PNN)2(PhCOO)2-
(H2O)2], B [Mn(PNN)4(N3)2], and C [Mn(PN)4(N3)2], where
PNN and PN stand for 2-(p-pyridyl)-4,4,5,5-tetramethylimida-
zoline-1-oxyl-3-oxide and 2-(p-pyridyl)-4,4,5,5-tetramethylimi-
dazoline-1-oxyl, respectively.
A
B
C
formula
fw
cryst syst
T (K)
MnN6C38H46O10 MnN18C48H64O8 MnN18C48H64O4
801.75
triclinic
293
1076.11
triclinic
293
1012.11
triclinic
293
0.710 73
1.258
λ (Å)
0.710 73
1.258
0.371
0.710 73
1.333
0.315
F
calc (g cm-1
)
µ (mm-1
a (Å)
b (Å)
)
0.306
6.859(1)
11.271(2)
13.978(6)
88.88(2)
89.38(2)
78.28(2)
1057.9(5)
1
7.177(4)
13.767(3)
13.928(4)
90.20(2)
102.94(4)
91.86(4)
1340.4(9)
1
7.004(2)
13.885(1)
14.036(2)
90.34(1)
101.42(2)
92.92(1)
1336.0(4)
1
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
R (Iobs g 2σ(I)) 0.0392(4118)
wR2 0.0898
0.0418(4554)
0.1041
0.0485(5291)
0.1225
a R ) Σ||Fo| - |Fc||/Σ|Fo| b wR2 ) {Σ[w(Fo2 - Fc2)2]/Σ[w (Fo2)2]}1/2
.
Experimental Section
Selected bond lengths and angles are given in Table 2. Complete bond
lengths and bond angles, anisotropic thermal parameters, and calculated
hydrogen coordinates are deposited as Supporting Information. Figures
1, 2, and 3 give the ORTEP atomic labeling schemes for compounds
A, B, and C, respectively.
Syntheses. 2-(p-Pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-
oxide and 2-(p-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl were
prepared using literature procedures.21
Compound A. To a solution of manganese acetate tetrahydrate (2
mmol) and benzoic acid (4 mmol) in 40 mL of absolute ethanol was
added a solid sample of PNN (4.3 mmol). The filtrate was kept in the
dark for 1 week. Black crystals were then obtained.
Results and Discussion
Anal. Obsd (calcd) for MnN6C38H46O10: Mn, 6.7 (6.9); N, 10.76
(10.49); C, 57.15 (56.93); H, 5.96 (5.74). IR (KBr): 3455, 1615, 1541,
Crystal Structures. The crystal structure of compound A,
[Mn(PNN)2(PhCOO)2(H2O)2], is displayed in Figure 4. The
manganese ion lies on an inversion center and adopts a distorted
octahedral coordination. Its coordination sphere is formed by
two nitrogen atoms of the pyridyl groups, two carboxylic oxygen
atoms, and two other oxygen atoms of the water molecules.
The Mn-N bond length of 2.44(2) Å is somewhat longer than
the corresponding one observed in compounds B and C
described below. In the pyridyl nitronyl nitroxide ligand, the
dihedral angle between the ON-C-NO fragment and the
pyridyl ring is 24.7(1)°. The shortest intermolecular contact is
observed in the c axis direction, between two nitronyl nitroxide
groups of adjacent centrosymmetric Mn(Ph2)(PNN)2(H2O)2 units
(O4‚‚‚O4 ) 3.77 Å). Intramolecular hydrogen bonding takes
place between one oxygen atom of the carboxylic group and a
hydrogen atom of the water molecule.
The molecular structures of compounds B (Mn(PNN)4(N3)2)
and C (Mn(PN)4(N3)2), with the labeling schemes, are shown
in Figures 2 and 3, respectively. These two compounds are
isostructural. Similarly, the manganese ion lies on an inversion
center and adopts a distorted octahedral coordination. It is bound
to four nitrogen atoms of the pyridyl groups in addition to two
azido ligands occupying the apical positions. The Mn-N (azido)
bond distances are 2.198(2) and 2.160(2) Å for B and C,
respectively, whereas the average Mn-N (pyridyl) bond distance
is 2.322(2) Å. These values are close to those observed for other
manganese complexes.5-12 The N2-N1-Mn1 angles are
129.7(2)° and 126.5(2)° for compounds B and C, respectively.
The dihedral angles between the NO-C-NO moieties and the
pyridyl rings for the two independent PNN ligands are 34.8(1)°
and 27.0(1)° for B and 31.7(1)° and 26.5(2)° for C, respectively,
whereas the dihedral angles between the two adjacent NO-
C-NO moieties of an asymmetric fragment are 74.3(2)° and
72.0(2)° for compounds B and C, respectively.
1395, 1330, 1225, 1145, 1083, 1029, 851, 729, 687 cm-1
.
Compound B. An aqueous solution of sodium azide (6 mmol) was
added dropwise to a solution of manganese(II) chloride tetrahydrate
(3 mmol) and PNN (12 mmol) in 50 mL of a (1/1) mixture of ethanol/
water. The filtrate was allowed to stand for 3 days in the dark at room
temperature. Black parallelepipedic crystals were obtained.
Anal. Obsd (calcd) for MnN18C48H64O8: Mn, 5.0 (5.1); N, 23.41
(23.44); C, 53.67 (53.58); H, 6.13 (5.95). IR (KBr): 2055, 1609, 1557,
1440, 1402, 1377, 1329, 1218, 1166, 1142, 1072, 1007, 831, 654 cm-1
.
Compound C. This compound was synthesized in a similar way as
B, starting from PN instead of PNN. Shiny red crystals were hence
obtained.
Anal. Obsd (calcd) for MnN18C48H64O4: Mn, 5.5 (5.4); N, 24.44
(24.92); C, 56.93 (56.96); H, 6.36 (6.32). IR (KBr): 2052, 1615, 1544,
1415, 1377, 1330, 1263, 1220, 1158, 1140, 1063, 1011, 842, 660 cm-1
.
Spectral and Magnetic Measurements. The IR spectra were
measured on Perkin-Elmer spectrophotometer. Magnetic measurements
were carried out using a Quantum Design SQUID SPMS-SS magne-
tometer working down to 2 K with magnetic fields up to 50 kOe. Data
were corrected for the contribution of the sample holder, and diamag-
netic contributions were estimated from Pascal’s constants.
X-ray Data Collection and Structure Determination. Crystals were
mounted on an Enraf-Nonius CAD4 diffractometer equipped with
graphite-monochromatized Mo KR radiation (λ ) 0.710 73 Å). The
unit cell parameters were obtained by least-squares fits of the
automatically centered 25 reflections.
Intensity data were corrected for Lorentz and polarization effects.
Data reduction and absorption correction using the ψ-scan method were
performed with MolEN programs22 and structure solution and refine-
ments conducted with SHELXS-86 and SHELXL-97, respectively.23,24
Crystallographic data are given in Table 1. Hydrogen atoms were found
by Fourier syntheses or included at calculated positions with isotropic
thermal parameters proportional to those of the connected carbon atoms.
(21) Ullman, E. F.; Call, L.; Osiecki, J. H. J. Org. Chem. 1970, 35, 3623.
(22) Crystal Structure Analysis (MolEN); Enraf-Nonius: Delft, The
Netherlands, 1990.
(23) Sheldrick, G. M. SHELXS-86, Program for the solution of Crystal
Structures; University of Go¨ttengen: Go¨ttingen, Germany, 1985.
(24) Sheldrick, G. M. SHELXL-93, Program for the refinement of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1993.
The crystal structures of B and C are displayed in Figures 5
and 6 as a projection on the bc plane.
For compound B, the shortest intermolecular contacts are
established between the centrosymmetric oxygen atoms of the

Manganese(II) Coordination Complexes
Inorganic Chemistry, Vol. 38, No. 18, 1999 3969
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for A, B, and C
A
B
C
Mn1-O2
Mn1-O1
Mn1-N1
O2-C1
2.1363(12)
2.2596(13)
2.4384(17)
1.258(2)
1.2635(19)
1.2761(17)
1.3205(19)
1.338(2)
1.416(2)
1.591(2)
1.566(2)
1.3533(19)
Mn1-N1
Mn1-N4
Mn1-N7
N1-N2
N2-N3
N7-C13
N7-C17
O1-N9
O2-N8
N9-C18
N9-C20
N8-C18
N8-C19
N4-C1
N4-C5
O3-N6
O4-N5
N6-C6
N6-C8
N5-C6
N5-C7
2.198(2)
2.3204(18)
2.3239(19)
1.130(3)
1.167(3)
1.341(3)
1.338(3)
1.267(2)
1.274(2)
1.342(3)
1.501(3)
1.344(3)
1.510(3)
1.340(3)
1.342(3)
1.265(2)
1.273(2)
1.343(3)
1.495(3)
1.348(3)
1.508(3)
Mn1-N1
Mn1-N7
Mn1-N4
N1-N2
N2-N3
N5-O1
N5-C6
N5-C7
N6-C6
N6-C8
N4-C1
N4-C5
N8-O2
N8-C18
N8-C19
N9-C18
N9-C20
N7-C13
N7-C17
2.1603(17)
2.3214(16)
2.3242(16)
1.182(2)
1.146(3)
1.258(3)
1.390(3)
1.495(3)
1.284(3)
1.495(3)
1.335(2)
1.342(2)
1.266(2)
1.388(3)
1.489(3)
1.282(3)
1.489(3)
1.332(2)
1.333(2)
O3-C1
O4-N2
O5-N3
N1-C12
N1-C8
N3-C15
N2-C14
N3-C13
O2-Mn1-O1
O2-Mn1-N1
O1-Mn1-N1
90.61(5)
87.58(5)
96.36(5)
N1-Mn1-N4
N1-Mn1-N7
N4-Mn1-N7
91.71(7)
88.79(7)
88.25(6)
N1-Mn1-N7
N1-Mn1-N4
N7-Mn1-N4
88.94(6)
89.07(6)
91.80(6)
Figure 1. ORTEP view of the molecular structure of A. Thermal
ellipsoids are drawn at the 50% probability level.
Figure 3. ORTEP view of the molecular structure of C. Thermal
ellipsoids are drawn at the 50% probability level.
Figure 2. ORTEP view of the molecular structure of B. Thermal
ellipsoids are drawn at the 50% probability level.
Figure 4. View of the structure of A in the bc plane.
NO groups: O1‚‚‚O1 (3.53 Å) and O4‚‚‚O4 (3.60 Å). Such a
contact scheme has a 2D (parallel bc plane) character ensured
by the centrosymmetric parallel ON-C-NO fragments. How-
ever, for compound C, only a short 1D contact scheme is
observed through centrosymmetric nitronyl groups (O1‚‚‚O1:
3.77 Å) in the c axis direction.
Magnetic Properties. The temperature dependences of the
magnetic susceptibility for compounds A-C were measured
in the temperature range 2-300 K, with an applied field of 1000
Oe. The plots of øMT versus T, where øM is the molar magnetic
susceptibility corrected for the core diamagnetism and T the

3970 Inorganic Chemistry, Vol. 38, No. 18, 1999
Fettouhi et al.
Figure 5. View of the structure of B in the bc plane.
Figure 8. Temperature dependence of øMT for complex B.
Figure 6. View of the structure of C in the bc plane.
Figure 9. Temperature dependence of øMT for complex C.
attributed to the intramolecular Mn-radical interaction as
observed in similar compounds.25,26The decrease of øMT below
15 K indicates the occurrence of intermolecular antiferromag-
netic interactions which may result from the rather close
proximity of the NO moieties of neighboring molecules
(O4‚‚‚O4′, 3.77 Å), as revealed by the X-ray structure analysis.
Therefore the temperature dependence of the magnetic suscep-
tibility was modeled with an expression of øM taking into
account the intramolecular interaction between the Mn(II) ion
and the two paramagnetic ligands, a theoretical expression
deduced from the spin Hamiltonian H ) -J(SMnSrad1
+
SMnSrad2). To take into account the decrease of øMT below 15
K, a weak intermolecular interaction based on the mean-field
approximation, zJ′, was introduced. Least-squares fitting of the
experimental data led to J ) 3.1 cm-1 and zJ′ ) - 0.3 cm-1
(gMn and grad were taken equal to 2.00).
The temperature dependence of the magnetic susceptibility
for Mn(PNN)4(N3)2, B, is shown in Figure 8. At high temper-
ature (150-300 K), øMT corresponds to the expected value, 5.9
Figure 7. Temperature dependence of øMT for complex A.
temperature, are shown in Figures 7, 8, and 9 for compounds
A, B, and C, respectively.
For Mn(PNN)2(PhCOO)2(H2O)2, A, the room temperature
øMT value is approximately equal to 5.12 cm3 K mol-1, the
expected value for the isolated SMn) 5/2 and two Srad ) 1/2 spins.
As the temperature is lowered, øMT increases to reach a
maximum of 5.6 cm3 K mol-1 at 15 K and then decreases
rapidly to about 3.4 cm3 K mol-1 at 2 K (Figure 7). The profile
of the curve is consistent with ferromagnetic interactions
(25) Chen, Z. N.; Qiu, J.; Gu, J. M.; Wu, M. F.; Tang, W. X. Inorg. Chim.
Acta 1995, 233, 131-5.
(26) Caneschi, A.; Ferraro, F.; Gatteschi D.; Rey, P.; Sessoli, R. Inorg.
Chem. 1990, 29, 4217.

Manganese(II) Coordination Complexes
Inorganic Chemistry, Vol. 38, No. 18, 1999 3971
cm3 K mol-1, for the uncorrelated spins of the Mn(II) and the
nitronyl nitroxides. As the temperature is lowered from 150 to
2 K, øMT first increases to reach a maximum of 6.6 cm3 K mol-1
at 18 K and then decreases rapidly to about 3.6 cm3 K mol-1 at
2 K. As for compound A, the profile of this curve indicates the
coexistence of ferromagnetic and weaker antiferromagnetic
interactions. This behavior is well reproduced by a spin
Hamiltonian taking into account both Mn-radical, J, and
radical-radical, J′, intramolecular magnetic interactions (eq 1),
the intermolecular interactions being considered in the mean-
field approximation as zJ′′. However, considering the distances
between the nitronyl nitroxide units within a molecule, no
significant radical-radical magnetic interaction is expected so
that J′ was fixed to zero. Least-squares fitting of the experi-
mental data led to J ) 4.7 cm-1, J′ ) 0 (fixed)27 and zJ′ )
-0.5 cm-1 (gMn and grad were taken equal to 2.00). These results
are in good agreement with the magnetic interactions found in
compound A and compounds described in the literature,25,26
confirming the ferromagnetic nature of the magnetic interaction
between the Mn(II) ion and the nitronyl nitroxide radical through
the p-pyridyl moiety.
forming thus a pair of radicals with a substantial antiferromag-
netic interaction. It can be noticed that the plateau reached by
the curve between 50 and 30 K corresponds approximately to
5
the expected øMT value for the uncorrelated spins SMn ) /2
1
and two Srad
)
/
spins. This confirms that the magnetic
2
moments of two radical units of the molecule have been
canceled by the intermolecular antiferromagnetic interactions.
Below 30 K the intramolecular ferromagnetic interactions
between the Mn(II) ion and its paramagnetic ligands start to
take place, hence increasing the global moment of the molecule.
The consequence is an increase of øMT value, which however
drops rapidly as soon as the ferromagnetic cluster corresponding
to the molecule orders antiferromagnetically with its neighboring
clusters via the radical-radical interaction, leading normally
to a nonmagnetic ground state. The high-temperature (300-50
K) decrease of øMT can be well reproduced by a pair model of
1
S ) /2 spins to yield an exchange parameter J ) -130 cm-1
.
Related rather strong through-space antiferromagnetic interac-
tions between aminoxyl radicals have been described recently.28
For compound B, the anomaly observed in the øMT plot at
around 130 K suggests the occurrence of a structural transition.
Concluding Remarks
H ) -JSMn(Srad1 + Srad2 + Srad3 + Srad4) -
Unlike the high nuclearity of the systems of formula
MnII(L)m(N3)n (L ) pyridine-like ligand) reported so far, only
mononuclear species were obtained when the ligand L was PNN
or PN. These results raise once again the ambiguity of the subtle
factors governing the coordination mode of the azido moiety
and the influence of the ligand L. From the magnetic point of
view, the p-pyridine moiety behaves as an intramolecular
ferromagnetic pathway between the Mn(II) and the nitronyl
centers as found in similar magnetic clusters while antiferro-
magnetic intercluster interactions dominate at low temperature.
Such magnetic interactions are related to the encountered
molecular packing schemes allowing significant overlap between
adjacent molecular magnetic orbitals.
J′(Srad1Srad2 + Srad2Srad3 + Srad3Srad4 + Srad4Srad1) (1)
Despite the structural similarity between B and C, their
magnetic behavior is quite different. The temperature depen-
dence of the magnetic susceptibility for compound C is shown
in Figure 9 in the form of the variation of øMT versus T. At
high temperature øMT of C is slightly below the expected value
of 5.9 cm3 K mol-1. As the temperature is lowered, øMT
decreases slowly to reach near 50 K a kind of plateau at ca. 5.2
cm3 K mol-1. Then, it increases again to reach a maximum of
5.5 cm3 K mol-1 at 7 K before dropping to 4.1 cm3 K mol-1 at
2 K. Compared to A and B, there is clearly a rather strong
antiferromagnetic interaction occurring in compound C. It is
likely to be the result of the interaction between two of the
nitronyl groups of C with one radical unit of two neighboring
complexes. This is supported by the X-ray analysis. In fact,
two of the four radicals of a molecule have been found to be
close to that of two neighboring molecules. The NO centers
are nearly parallel to each other with N‚‚‚O distances of 3.71
and 3.77 Å, an ideal position for the overlap of the p orbitals
Acknowledgment. Support from King Fahd University of
Petroleum and Minerals, Saudi Arabia, is gratefully acknowl-
edged.
Supporting Information Available: Three X-ray crystallographic
files, in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC9905751
(28) Akabane, R.; Tanaka, M.; Matsuo, K.; Koga, N.; Matsuda, K.;
Iwamura, H. J. Org. Chem. 1997, 62, 8854. Ishimaru, Y.; Kitano,
M.; Kumada, H.; Koga, N.; Iwamura, H. Inorg. Chem. 1998, 37, 2273.
(27) Least-squares fitting of the experimental data with all three contribu-
tions led to J ) 4.5 cm-1, J′ ) 2 cm-1, and zJ′ ) - 0.5 cm-1
.