Synthesis, magnetic properties, and electronic spectra of bis(β-diketonato)chromium(III) and nickel(II) complexes with a chelated imino nitroxide radical: X-ray structures of [Cr(acaMe)2(IM2py)]PF6 and [Ni(acac)2(IM2py)]

Article abstract of DOI:10.1021/ic011282m

Two new series of each of four Cr(III) and Ni(II) imino nitroxide complexes with various kinds of β-diketonates, [Cr(β-diketonato)₂(IM2py)]PF₆, and [Ni(β-diketonato)₂(IM2py)] (IM2py = 2-(2′-(pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxy)) have been synthesized, and their structures and magnetic and optical properties have been examined. The X-ray analysis demonstrated that a IM2py ligand coordinated to Cr(III) and Ni(II) acts as a five-membered bidentate chelate. The variable-temperature magnetic susceptibility measurements indicated the antiferromagnetic and ferromagnetic interaction of Cr(III) and Ni(II) with IM2py, respectively, giving a variety of the magnetic coupling constant J values with varying the β-diketonato ligands. The UV-vis shoulders around (19-20) x 10³ and (17-18) x 10³ cm^-1 for the Cr(III) and Ni(II) complexes, respectively, characteristic of the IM2py complexes were assigned to the metal-ligand charge-transfer transitions, Cr(t_2g)-SOMO(π*) and Ni(e_g)-SOMO(π*) MLCT in terms of the resonance Raman spectra and the variable-temperature absorption spectra. The absorption components centered around (13-14) x 10³ cm^-1 for the Cr(III) and Ni(II) complexes were due to the formally spin-forbidden d-d transition within the t_2g and e_g subshells, associated with the intensity enhancement. The spectroscopic behavior with varying the β-diketonato ligands is discussed in connection with the antiferromagnetic or ferromagnetic coupling constant J values on the basis of the exchange mechanism along with the coligand effect.

Full text of DOI:10.1021/ic011282m

Inorg. Chem. 2002, 41, 4363−4370
Synthesis, Magnetic Properties, and Electronic Spectra of
Bis(â-diketonato)chromium(III) and Nickel(II) Complexes with a Chelated
Imino Nitroxide Radical: X-ray Structures of [Cr(acaMe)2(IM2py)]PF6 and
[Ni(acac)2(IM2py)]
Yasunori Tsukahara, Takayuki Kamatani, Atsushi Iino, Takayoshi Suzuki, and Sumio Kaizaki*
Department of Chemistry, Graduate School of Science, Osaka UniVersity,
Toyonaka, Osaka, 560-0043, Japan
Received December 14, 2001
Two new series of each of four Cr(III) and Ni(II) imino nitroxide complexes with various kinds of â-diketonates,
[Cr(â-diketonato)2(IM2py)]PF , and [Ni(â-diketonato)2(IM2py)] (IM2py ) 2-(2′-(pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-
6
1H-imidazol-1-oxy)) have been synthesized, and their structures and magnetic and optical properties have been
examined. The X-ray analysis demonstrated that a IM2py ligand coordinated to Cr(III) and Ni(II) acts as a five-
membered bidentate chelate. The variable-temperature magnetic susceptibility measurements indicated the
antiferromagnetic and ferromagnetic interaction of Cr(III) and Ni(II) with IM2py, respectively, giving a variety of the
magnetic coupling constant J values with varying the â-diketonato ligands. The UV−vis shoulders around (19−20)
-1
× 103 and (17−18) × 103 cm for the Cr(III) and Ni(II) complexes, respectively, characteristic of the IM2py complexes
were assigned to the metal−ligand charge-transfer transitions, Cr(t2g)-SOMO(π*) and Ni(eg)-SOMO(π*) MLCT in
terms of the resonance Raman spectra and the variable-temperature absorption spectra. The absorption components
centered around (13−14) × 103 cm-1 for the Cr(III) and Ni(II) complexes were due to the formally spin-forbidden
d−d transition within the t2g and eg subshells, associated with the intensity enhancement. The spectroscopic behavior
with varying the â-diketonato ligands is discussed in connection with the antiferromagnetic or ferromagnetic coupling
constant J values on the basis of the exchange mechanism along with the coligand effect.
Introduction
e.g., mono-, tri-, tetra-,9 and one-dimensional ferrimagnetic
polynuclear10 complexes with Cu(II) and Mn(II) ions in
connection with magneto-optical molecular devices have
been studied where the spin-forbidden transition intensity
enhancements on cooling were examined in a quantitative
manner. Our recent studies have focused on the magnetic
A number of paramagnetic transition metal complexes with
nitronyl or imino nitroxide radicals such as several deriva-
tives of 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-3-
oxide-1-oxyl (NIT)1,2 or 4,4,5,5-tetramethyl-4,5-dihydro-1H-
imidazol-1-oxy (IM)3,4 have been investigated from the
viewpoint of molecular magnets. On the other hand, elec-
tronic spectra of multi-spin systems of paramagnetic transi-
tion metal complexes have been extensively investigated;5-12
(4) (a) Luneau, D.; Laugier, J.; Rey, P.; Ulrich, G.; Ziessel, R.; Legoll,
P.; Drillon, M. J. Chem. Soc., Chem. Commun. 1994, 741. (b) Romero,
F. M.; Luneau, D.; Ziessel, R. J. Chem. Soc., Chem. Commun. 1998,
551. (c) Luneau, D.; Romero, F. M.; Ziessel, R. Inorg. Chem. 1998,
37, 5078.
(5) Marvilliers, A.; Pei, Y.; Boquera, J. C.; Vostrikova, K. E.; Paulsen,
C.; Rivie`re, E.; Audie`re, J.-P.; Mallah, T. Chem. Commun. 1999, 1951.
(6) Vostrikova, K. E.; Luneau, D.; Wernsdorfer, W.; Rey, P.; Verdaguer,
M. J. Am. Chem. Soc. 2000, 122, 718.
* Author to whom correspondence should be addressed. E-mail:
kaizaki@chem.sci.osaka-u.ac.jp.
(1) (a) Caneschi, A.; Gatteschi, D.; Rey, P. Prog. Inorg. Chem. 1991, 39,
331 and references therein. (b) Kahn, O. Molecular Magnetism;
VCH: New York, 1993; (c) Coronado, E.; Dellhae`s, P.; Gatteschi,
D.; Miller, J. S., Eds. Molecular Magnetism: From Molecualr
Assemblies to the DeciVes; NATO ASI Ser. 321; Kluwer Academic
Publishers: Dordrecht, The Netherlands, 1996.
(7) Gu¨del, H. U. In Magnetic-Structural Correlations in Exchange Coupled
Systems; Willet, R. D., Gatteschi, D., Kahn, O., Eds.; Reidel:
Dordrecht, The Netherlands, 1985;, p 297.
(2) Luneau, D.; Risoan, G.; Rey, P.; Grand, A.; Caneschi, A.; Gatteschi,
D.; Laugier, J. Inorg. Chem. 1993, 32, 5616.
(3) Luneau, D.; Rey, P.; Laugier, J.; Belorizky, E.; Cogne, A. Inorg. Chem.
1992, 31, 3578.
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Bellitto, C.; Day, P. J. Chem. Soc., Dalton Trans. 1986, 847. (c)
Bellitto, C.; Day, P. J. Mater. Chem. 1992, 2, 265.
10.1021/ic011282m CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/24/2002
Inorganic Chemistry, Vol. 41, No. 17, 2002 4363

Tsukahara et al.
Chart 1
pentanedione), HacaPh (3-phenyl-2,4-pentanedione), Hhfac (1,1,1,-
5,5,5-hexafluoro-2,4-pentanedione), and Htfac (1,1,1-trifluoro-2,4-
pentanedione).
Materials: Starting and Reference Complexes. The starting
complexes [Cr(â-diketonato)2(H2O)2]BF4 16 and [Ni(â-diketonato)2-
(H2O)2]22 and the reference complexes [Cr(acac)2(en)]PF623 and [Ni-
(acac)2(tmen)]24 were synthesized according to literature methods.
Syntheses of IM2py Complexes: (1) [Cr(acaMe)2(IM2py)]-
PF6 (3a). To a solution of 1 mmol of [Cr(acaMe)2(H2O)2]BF4 in
50 mL of CH3CN was added 1 mmol of IM2py with stirring in an
ice-methanol bath at -15 °C. After 3 h of stirring, the solution
was evaporated under reduced pressure. The residue was dissolved
in 50 mL of EtOH, and to this solution was added 1 mmol of NaPF6.
After 6 h of stirring, the solution was evaporated under reduced
pressure around 15 °C. The crude product was obtained by adding
dichloromethane. The residue was dissolved in 10 mL of CHCl3.
This solution was loaded on a preparative HPLC (LC-908 (Japan
Analytical Industry Co. Ltd.)), and the column was eluted with
CHCl3. The first band was found to be the desired product of [Cr-
(acaMe)2(IM2py)]+. After the eluate was condensed, orange red
needlelike crystals were obtained by a vapor diffusion method in
dichloromethane-ether.
The other â-diketonato complexes [Cr(â-diketonato)2(IM2py)]-
PF6 (â-diketonato ) acac (1a), dbm(2a), and acaPh (4a)) were
obtained by a method similar to that for the acaMe complex.
(2) [Ni(acac)2(IM2py)] (1b). To a suspension of 1 mmol of [Ni-
(acac)2(H2O)2] in 20 mL of Et2O was added 1 mmol of IM2py
with stirring at room temperature. [Ni(acac)2(H2O)2] was not
dissolved in Et2O, but IM2py was dissolved in Et2O. This
suspension was stirred for 1 h. Then, the solid was dissolved and
the solution became clear. To the solution was added 30 mL of
n-heptane. This was evaporated under reduced pressure. When the
crude crystals began to deposit, evaporation was stopped. After the
crude crystals were dissolved again by warming, the solution was
allowed to stand in a refrigerator. The solid was collected on a
glass filter. Complex [Ni(acac)2(IM2py)] was obtained as deep
brown platelike crystals by a vapor diffusion method in dichloro-
methane-ether. The other â-diketonato complexes [Ni(â-diketonato)2-
(IM2py)] (â-diketonato ) bzac (2b), hfac (3b), and tfac(4b)) were
prepared by a method similar to that for the acac complex.
Crystallography. An orange-red crystal of [Cr(acaMe)2(IM2py)]-
PF6 with approximate dimensions of 0.35 × 0.22 × 0.06 mm3 was
glued on the top of a glass fiber with epoxy resin. The X-ray
intensities (2θmax ) 55) were collected on a Rigaku RAXIS-RAPID
Imaging Plate.
and spectroscopic properties of [M(â-diketonato)2(NIT2py)]n+
(M ) Ni(II)13-15 and Cr(III)13,16), as well as of [MIICl2-
(IM2py)2] complexes (M ) Zn, Ni, Mn, Co)17 and tetrahedral
complexes [MIICl2(NITmepy or IMmepy)] (M ) Zn, Co,
Ni),18 following the related diamagnetic Co(III) complexes19
with unidentate NITnpy and IMnpy (n ) 3 and 4) and the
chelated IM2py lanthanide(III) complexes.20 For the NIT2py
Ni(II) and Cr(III) â-diketonates complexes, the relations have
been examined between the antiferromagnetic exchange
coupling constants and the spin-forbidden d-d transition
intensities14.16 or the NMR contact shifts,15 demonstrating
the coligand effect. There remain open questions on the
coligand effect and quantitative complementarity between
optical and magnetic properties. To get some clues for these
problems, comparison of the magnetic and optical properties
between the NIT2py complexes and the IM2py complexes
is inevitable. However, there is no magnetic and optical study
on the IM2py Ni(II) and Cr(III) â-diketonate complexes so
far.
In this article, the synthesis of a series of Cr(III) and
Ni(II) IM2py complexes with various â-diketonates and their
magnetic and spectroscopic properties together with the
X-ray structural analysis are described in comparison with
those of the Cr(III) and Ni(II) NIT2py complexes.14,16
Experimental Section
Ligands. The radical ligand IM2py was prepared by the literature
method.21 The â-diketones were commercially available (â-diketone
) Hacac (2,4-pentanedione), Hdbm (1,3-diphenyl-1,3-propanedi-
one), Hbzac (1-phenyl-1,3-butanedione), HacaMe (3-methyl-2,4-
(10) (a) Mathonieres, C.; Kahn, O.; Daran, J. C.; Hilbig, H.; Kohler, F. H.
Inorg. Chem. 1993, 32, 4057. (b) Mathonieres, C.; Kahn, O. Inorg.
Chem. 1994, 33, 2103. (c) Cador, C.; Mathonieres, C.; Kahn, O. Inorg.
Chem., 1997, 36, 1923. (d) Cador, C.; Mathonieres, C.; Kahn, O.;
Costes, J. P.; Verelst, M.; Lecante, P. Inorg. Chem. 1999, 38, 2643.
(e) Cador, C.; Mathonieres, C.; Kahn, O. Inorg. Chem. 2000, 39, 3799.
(11) Benelli, C.; Dei, A.; Gatteschi, D.; Gu¨del, H. U.; Pardi, L. Inorg. Chem.
1989, 28, 3089.
(12) Sokolowski, A.; Bothe, E.; Bill, E.; Weyhermuller, T.; Wieghardt, K.
J. Chem. Soc., Chem. Commun. 1996, 1671.
(13) Yoshida, T.; Kanamori, K.; Takamizawa, S.; Mori, W.; Kaizaki, S.
Chem. Lett. 1997, 603.
(14) Yoshida, T.; Suzuki, T.; Kanamori, K.; Kaizaki, S. Inorg. Chem. 1999,
38, 1059.
(15) Yoshida, T.; Kaizaki, S. Inorg. Chem. 1999, 38, 1054.
(16) Tsukahara, Y.; Iino, A.; Yoshida, T.; Suzuki, T.; Kaizaki, S. J. Chem.
Soc., Dalton Trans. 2002, 181.
A deep brown crystal of [Ni(acac)2(IM2py)] was sealed in a glass
capillary with epoxy resin. The X-ray intensities (2θmax ) 55) were
collected on an automated Rigaku AFC-5R four-circle diffracto-
meter.
Absorption corrections were applied by a multiscan method.25
The structure was solved by the direct method with the SHELXS86
program26 and refined on F2 with all independent reflections with
the SHELXL97 program.26 All calculations were carried out with
a TeXsan27 software package.
(17) Yamamoto, Y.; Suzuki T.; Kaizaki, S. J. Chem. Soc., Dalton Trans.
2001, 1566.
(18) Yamamoto, Y.; Suzuki T.; Kaizaki, S. J. Chem. Soc., Dalton Trans.
2001, 2943.
(19) (a) Suzuki, T.; Ogita, M.; Kaizaki, S. Acta Crystallogr., Sect. C 2000,
56, 532. (b) Ogita, M.; Yamamoto, Y.; Suzuki, T.; Kaizaki, S. Eup.
J. Inrog. Chem. 2002, 886.
(20) (a) Tsukuda, T.; Suzuki, T.; Kaizaki, S., J. Chem. Soc., Dalton Trans.
2002, 1721. (b) Tsukuda, T.; Suzuki, T.; Kaizaki, S. Mol. Cryst. Liq.
Cryst., in press.
(22) Charles, R. G.; Pawlikowsky, M. A. J. Phys. Chem. 1958, 62, 440.
(23) Kaizaki, S.; Hidaka, J.; Shimura, Y. Inorg. Chem. 1973, 12, 135.
(24) (a) Fukuda, Y.; Sone, K. Bull. Chem. Soc. Jpn. 1970, 43, 2282. (b)
Fukuda, Y.; Sone, K. J. Inorg. Nucl. Chem. 1972, 34, 2315. (c) Fukuda,
Y.; Sone, K. J. Inorg. Nucl. Chem. 1975, 37, 455.
(25) Higashi, T. Abscor-Empirical Absorption Correction based on Fourier
Series Approximation;.Rigaku Corporation: Tokyo, Japan. 1995.
(26) Sheldrick, G. M. SHELXS-86, Program for Crystal Structure Deter-
mination; University of Go¨ttingen: Go¨ttingen, Germany, 1986.
(27) TEXSAN, Single-Crystal Structure Analysis Software, Ver. 1.7;
Molecular Structure Corperation: The Woodland, TX, 1995.
(21) Ullman, E. F.; Call, L.; Osiecki, J. H. J. Org. Chem. 1970, 35, 3623.
4364 Inorganic Chemistry, Vol. 41, No. 17, 2002

Bis(â-diketonato)chromium(III) and Nickel(II) Complexes
Table 1.
Table 3. Selected Bond Distances (Å), Bond Angles (deg), Torsion
Angles (deg), and Dihedral Angles (deg) of [Cr(acaMe)2(IM2py)]PF6
and [Ni(acac)2(IM2py)].
found (calcd) /%
compds
C
H
N
M
Cr
Ni
1a [Cr(acac)2(IM2py)]PF6
2a [Cr(dbm)2(IM2py)]PF6
3a [Cr(acaMe)2(IM2py)]PF6 45.1(44.9)
4a [Cr(acaPh)2(IM2py)]PF6
1b [Ni(acac)2(IM2py)]
2b [Ni(bzac)2(IM2py)]
3b [Ni(hfac)2(IM2py)]
4b [Ni(tfac)2(IM2py)]
43.7 (43.1) 5.14 (4.93) 6.67 (6.85)
58.0 (58.5) 4.44 (4.44) 4.86 (4.88)
5.34 (5.34) 6.75 (6.55)
51.6 (53.3) 4.82(5.00) 5.20 (5.49)
bond distances (Å)
M-O(21)
M-O(22)
M-O(41)
M-O(42)
M-N(1)
M-N(3)
O(1)-N(2)
N(1)-C(1)
N(2)-C(3)
N(3)-C(8)
N(3)-C(12)
C(1)-C(8)
1.931(3)
2.026(2)
2.016(2)
2.020(2)
2.020(2)
2.151(3)
2.103(3)
1.266(4)
1.280(4)
1.492(5)
1.351(4)
1.329(4)
1.466(4)
1.941(3)
1.927(3)
1.947(3)
2.040(3)
2.121(4)
1.265(5)
1.308(5)
1.493(6)
1.369(5)
1.335(6)
1.447(6)
55.4 (55.6) 6.26 (6.36) 8.91 (8.84)
64.1 (64.1) 5.88(5.72)
38.0 (38.2) 2.68(2.63)
6.89 (7.01)
5.88 (6.08)
7.32 (7.21)
45.3(45.3)
4.12(4.15)
Table 2. Crystallographic Data
[Cr(acaMe)2(IM2-py)]PF6
[Ni(acac)2(IM2-py)]
formula
fw
T, °C
cryst syst
space group
a, Å
b, Å
c, Å
â, deg
V, Å3
Z
C24H34F6N3CrPO5
641.515
-50
C22H30N3NiO5
475.20
23
monoclinic
C2/c
29.018(3)
10.095(3)
17.972(4)
111.45(1)
4900(2)
8
0.71072
1.288
0.827
0.045
0.135
bond angles (deg)
N(1)-M-O(21)
N(1)-M-O(22)
N(1)-M-O(41)
N(1)-M-O(42)
N(1)-M-N(3)
O(21)-M-O(22)
O(21)-M-O(41)
O(21)-M -O(42)
O(21)-M-N(3)
O(22)-M-O(41)
O(22)-M-O(42)
O(22)-M-N(3)
O(41)-M-O(42)
O(41)-M-N(3)
O(42)-M-N(3)
O(1)-N(2)-C(1)
C(1)-N(1)-Cr
C(2)-N(1)-Cr
M-N(3)-C(8)
170.85(14)
91.60(13)
96.93(14)
89.96(13)
78.70(14)
89.31(12)
92.16(13)
89.10(13)
92.21(13)
90.43(13)
178.40(13)
89.16(13)
89.74(13)
175.60(13)
90.80(13)
125.8(4)
96.11(10)
95.06(10)
169.24(10)
84.73(10)
77.51(10)
90.41(10)
88.78(9)
monoclinic
P21/c
10.7128(4)
15.7194(5)
17.1131(6)
94.473(2)
2873.05(17)
4
178.84(10)
88.80(9)
λ, Å
0.71069
1.483
0.531
0.074
0.2382
calcd, g cm-3
94.48(10)
90.31(10)
172.40(10)
90.26(10)
93.06(10)
90.61(10)
127.0(3)
111.4(2)
135.6(2)
118.0(3)
126.1(2)
F
µ(Mo KR), cm-1
R1a
wR2b
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑w(Fo - Fc )2/∑wFo ]1/2
.
113.3(3)
136.9(3)
114.8(3)
127.3(3)
Measurements. UV-vis absorption spectra were obtained on a
Perkin-Elmer Lambda 19 spectrophotometer. Low-temperature
UV-vis absorption spectra were measured at 200, 100, 50, and 10
K by the same spectrophotometer in a film made from cellulose
acetate in acetonitrile with an Oxford CF1204 cryostat.
M-N(3)-C(12)
torsion angles (deg)
2.0(6)
N(1)-C(1)-C(8)-N(3)
-8.0(4)
Magnetic susceptibility data were obtained at 2000 Oe between
2 and 300 K by using a SQUID susceptometer (MPMS-5S,
Quantum Design). Pascal’s constants were used to determine the
constituent atom diamagnetism. The inter- and intramolecular
magnetic interaction of the hfac complex 3b were estimated from
the remeasured magnetic susceptibility data in terms of the present
analysis. The J and J′ values were not so much different from those
of literature values.3
dihedral angles (deg)
plane 1a vs plane 2b
plane 1 vs plane 3c
plane 2 vs plane 3
3.036
3.199
2.440
9.633
13.27
15.57
a Plane 1 (N(1), C(1), N(2), O(1)). b Plane 2 (N(3), C(8), C(9), C(10),
C(11), C(12)). c Plane 3 (M, N(1), N(3)).
Resonance Raman spectra were measured in the solid state by a
Jasco NR1800 Raman spectrophotometer at room temperature,
using Ar laser lines (488.00, 514.50 nm), He-Ne laser line (632.80
nm), and dye laser lines (584.47, 606.27 nm) as excitation sources.
Results and Discussion
Preparations and Characterization. The reaction time
for the IM2py Cr(III) complexes is shorter than that for the
corresponding NIT2py complexes. Thus, the IM2py radical
is more easily coordinated to the starting Cr(III) complex
than the NIT2py radical. No crystallization solvent as inferred
from the elemental analyses in Table 1 suggests the chelation
of IM2py ligand on Cr(III) and Ni(II) ions as confirmed by
the X-ray analysis (vide infra).
Molecular Structure. The crystallographic data for 3a
([Cr(acaMe)2(IM2py)]PF6) and 1b ([Ni(acac)2(IM2py)]) are
listed in Table 2. Selected bond lengths, bond angles, torsion
angles, and dihedral angles are summarized in Table 3. As
shown in Figures 1 and 2, they are octahedral complexes
Figure 1. View of the molecular structure of [Cr(acaMe)2(IM2py)]PF6
(3a).
which are coordinated by a five-membered chelate through
the pyridyl nitrogen and IM nitrogen of the IM2py together
Inorganic Chemistry, Vol. 41, No. 17, 2002 4365

Tsukahara et al.
cation and IM2py radical (S ) 3/2 + 1/2) (øM‚T ) 2.25 emu
K mol-1) and decrease on lowering the temperature, sug-
gesting the antiferromagnetic interaction between the
Cr(III) and IM2py radical. The other â-diketonato complexes
give a similar pattern for the øM‚T vs T plots. The magnetic
coupling constants Jobs involving a Cr(III) ion and IM2py
were obtained by least-squares fits for the magnetic suscep-
3
tibilities øM to a two-spin system (H ) -2JS1‚S2; S1 ) /2,
1
S2 ) /2)28 including a Weiss constant θ. The coupling
constants Jobs with g ) ca. 2.0 and R ) 10-4-10-5 range
from -96.4 to -188 cm-1 together with θ ) -0.33 to -0.06
cm-1 as summarized in Table 4 and Table S1. These results
show that the intramolecular magnetic interaction in the
Cr(III) IM2py complexes is antiferromagnetic as predicted
for the corresponding NIT2py complexes16 and that the
intermolecular interaction is small in accordance with that
inferred from the long intermolecular distance (∼6 Å) in the
crystal packings of complex 3a.
Figure 2. View of the molecular structure of [Ni(acac)2(IM2py)] (1b).
As shown in Figure 4, the øM‚T products for [Ni(acac)2-
(IM2py)] at room temperature are larger than the calculated
value (øM‚T ) 1.375 emu K mol-1) for uncorrelated systems
containing a nickel(II) cation and IM2py and increase on
lowering the temperature, suggesting the ferromagnetic
interaction between the Ni(II) and IM2py radical. The
decrease of the øM‚T values at low temperature may result
in the intermolecular interaction. The other [Ni(â-diketonato)2-
(IM2py)] complexes give similar patterns for the øM‚T vs T
plots. The Jobs for the magnetic interaction between a nickel-
(II) ion and IM2py and the intermolecular interaction θ
values were estimated by fitting the magnetic susceptibilities
1
øM to a two-spin system (H ) -2JS1‚S2; S1 ) 1, S2 ) /2),
Figure 3. Temperature dependence of the magnetic susceptibilities for
[Cr(acac)2(IM2py)]PF6 (1a) (O) in the form of øMT vs T. The solid line
represents the best-fit calculated data.
with g ) 2.13-2.22 ranging from Jobs ) +0.400 to +95.3
cm-1 and from θ ) -0.25 cm-1 with R ) 10-5 for 1b to θ
) -9 cm-1 with R ) 70 × 10-4 for 4b.
with each of two chelated â-diketonato ligands. The O(1)-
N(2) lengths (1.265(5) Å for 3a and 1.266(4) Å for 1b) of
the IM moiety substantiate the existence of the imino
nitroxide radical. As shown in Table 3, the dihedral angles
around the coordinated IM2py moiety together with the
torsion angle of N(1)-C(1)-C(8)-N(3) for 3a are much
smaller than the corresponding values for [Cr(dpm)2(NIT2py)]-
PF6.16 This indicates that the IM2py chelate in 3a is more
planar than those in the Cr(III) NIT2py complexes. A similar
situation is encountered between the Ni(II) IM2py(1b) and
NIT2py complexes.14 As compared with the dihedral and
torsion angles around the IM2py moiety in the Cr(III)
complex 3a and the Ni(II) complex 1b, the IM2py chelate
in the Cr(III) complex is more planar than that of the Ni(II)
complex as found in the case of the corresponding pair of
NIT2py complexes.14.16 The molecular structures of the other
â-diketonato Cr(III) and Ni(II) complexes are expected to
be the same as those of 3a and 1b, respectively, in view of
the similarity in the UV-vis spectral pattern and SQUID
magnetic susceptibility as described below.
The intermolecular interaction (θ) in the acac complex 1b
is weak as suggested from the long intermolecular distance
(6.22 Å). For the tfac complex 4b with the short inter-
molecular N-O‚‚‚O-N distance (3.46 Å) demonstrated by
preliminary X-ray analysis, the re-estimated magnetic inter-
action including the J′ value instead of θ is -6.08 cm-1 with
R ) 9.4 × 10-4, in accordance with the case (3.97 Å)3 for
the hfac complex with J′ ) -2.80 cm-1.
The intramolecular magnetic interactions (J) thus obtained
between the IM2py complexes of Cr(III) and Ni(II) will be
discussed below.
Visible Absorption Spectra: Intraligand and Charge-
Transfer Transitions. The spectral characteristics of the
IM2py Cr(III) and Ni(II) complexes are different from those
of both the IM2py itself and the nonradical Cr(III) and
Ni(II) complexes as shown in Figures 5 and 6. The vibronic
structure around 20.0 × 103 cm-1 for both the Cr(III) and
Ni(II) complexes may originate from the intraligand transi-
tion, in view of the similarity in intensity and shift behavior
for variation of the â-diketonates as well as for the IM2py
Co(II), Ni(II), and Ln(III) complexes17,20 and IM3py and
IM4py Co(III) complexes.19b New absorption shoulders are
Magnetic Properties. The products of the molar magnetic
susceptibilities (øM) and temperature (T) (øM‚T) vs T plots
for [Cr(acac)2(IM2py)]PF6 are shown in Figure 3. The øM‚T
products at room temperature are smaller than the calculated
values for uncorrelated systems containing a chromium(III)
(28) Wojciechowski, W. Inorg. Chim. Acta 1967, 1, 324.
4366 Inorganic Chemistry, Vol. 41, No. 17, 2002

Bis(â-diketonato)chromium(III) and Nickel(II) Complexes
Table 4. Magnetic and Optical Data of the IM2py Cr(III) and Ni(II) Complexes
obsd (g)a
ꢀSF
ꢀCT
923.1
1044
1367
1160
ECT
∆ECT
(ꢀSF/ꢀCT)(∆ECT)2/(ECT
)
b
c
d
e
f
compd
J
1a
2a
3a
4a
[Cr(acac)2(IM2py)] PF6
[Cr(dbm)2(IM2py)] PF6
[Cr(acaMe)2(IM2py)] PF6
[Cr(acaPh)2(IM2py)] PF6
-188 (2.01)
-96.4 (2.05)
-102 (2.03)
-119 (2.01)
145.9
159.7
191.1
184.0
19.12
19.16
19.57
19.57
4.96
5.13
5.80
5.60
0.203
0.210
0.241
0.255
1b
2b
3b
4b
[Ni(acac)2(IM2py)]
[Nir(bzac)2(IM2py)]
[Ni(hfac)2(IM2py)]
[Ni(tfac)2(IM2py)]
41.2 (2.12)
0.397 (2.13)
95.3 (2.15)
71.2 (2.13)
11.6
8.91
4.86
4.92
107.0
17.04
17.06
17.47
17.30
4.32
4.34
4.64
4.46
0.119
0.123
0.133
0.129
79.9
44.9
43.9
a Obserbed J values in cm-1
.
b Molar absorption coefficient for the spin-forbidden in mol-1dm3cm-1
.
c Molar absorption coefficient for the MLCT transition
d
f
in mol-1dm3cm-1
.
MLCT transition energy in 103 cm-1
.
e The energy difference between the spin-forbidden and MLCT transition/103cm-1
.
JAF in 103
cm-1
.
Figure 4. Temperature dependence of the magnetic susceptibilities for
[Ni(acac)2(IM2py)] (1b) (O) in the form of øMT vs T. The solid line
represents the best-fit calculated data.
Figure 6. Absorption spectra of [Ni(acac)2(IM2py)] (1b) (-), [Ni(acac)2-
(tmen)] (- -), and IM2py (- - -) in CH2Cl2 at room temperature.
significant intensity enhancements were observed, especially
for the vibrational bands at 1545 and 1480 cm-1, which
can be assigned to the N-O stretching vibrations of the
O-NdC moiety in the IM2py ligand. The remaining
enhanced peaks at 340 and 630 cm-1 for 1a and at 255 cm-1
for 1b may be due to Cr-N,N and Ni-N,N stretching,
respectively. Thus, the observed resonance enhancements of
the Raman bands lead to the assignment of the shoulders
around (19-20) × 103 cm-1 for 1a and (17.0-18.0) × 103
cm-1 for 1b to the charge-transfer transition.
The absorption intensity enhancement on lowering the
temperature for the Cr(III) complex 1a (Figure 7) substanti-
ates the rigorous assignment to the triplet-triplet metal t2g-
to-ligand SOMO π* (MLCT) transition(Figure 8).
The MLCT intensities of the Ni(II) IM2py complexes are
much weaker than those of the Ni(II) NIT2py complexes.14
This is due to the depopulation of the doublet ground state
for the ferromagnetically coupled IM2py Ni(II) complex as
supported by the intensity decrease on lowering the temper-
ature as shown in Figure 9. This suggests that the MLCT is
not quartet (t2g-π*) but doublet (eg-π*), similarly to the
Ni(II) NIT2py complexes14 (Figure 10).
Near-Infrared Absorption Spectra: Spin-Forbidden
d-d Transitions. For the radical complexes [Cr(â-diketo-
nato)2(IM2py)]+, the absorption peaks around 14 × 103 cm-1
were intensified by several hundred times as compared with
Figure 5. Absorption spectra of [Cr(acac)2(IM2py)]PF6 (1a) (-), [Cr-
(acac)2(en)]PF6 (- -), and IM2py (- - -) in CH3CN at room temperature.
Inset: Enlarged absorption curve with the logarithmic ordinate of [Cr(acac)2-
(en)]PF6 (- -) in CH3CN at room temperature.
observed around (19.0-21.0) × 103 cm-1 for the Cr(III)
complexes and (17.0-18.0) × 103 cm-1 for the Ni(II)
complexes. These shoulders may be assigned to neither the
intraligand nor the d-d transitions, but to the charge-transfer
transitions as supported by the resonance Raman spectra
given below.
In the Raman resonance spectra in the region (19.0-21.0)
× 103 cm-1 for 1a and (17.0-18.0) × 103 cm-1 for 1b,
Inorganic Chemistry, Vol. 41, No. 17, 2002 4367

Tsukahara et al.
Figure 7. Absorption spectra of [Cr(acac)2(IM2-py)]PF6 (1a) in an acetate
cellulose film at 300 (-), 200 (- - -), 100 (- - -), and 10 K (- - -).
Figure 10. Energy levels of the spin-allowed and spin-forbidden d-d
transition: (f) absortion spectra; (f) spin-forbidden absorption; and (T)
configurational interactions between the ground and CT states with the same
spin multiplicity.
d-d transitions are ascribed to the exchange coupling
between Cr(III) and the IM2py.
The exchange coupling of the ground 4A2 and the excited
2Γ (Γ ) E and/or T1) states with the IM2py radical gives
the triplet 3LQ (4A2), the quintet 5LQ (4A2) and the triplet 3LD
(2Γ), the singlet 1LD(2Γ) states as shown in Figure 8.
Therefore, the quartet-doublet spin-forbidden d-d transi-
tions of [Cr(â-diketonato)2(IM2py)]+ become formally spin-
forbidden or substantially spin-allowed between the exchange
coupled triplets as a result of the breakdown of ∆S ) 0
restriction. This assignment is supported by the absorption
band intensity enhancement on lowering the temperature
(Figure 7), in accordance with the antiferromagnetic interac-
tion observed by the variable-temperature magnetic suscep-
tibility measurements.
Figure 8. Energy levels of the spin-allowed and spin-forbidden d-d
transition: (f) absortion spectra; (f) spin-forbidden absorption; and (T)
configurational interactions between the ground and CT states with the same
spin multiplicity.
For [Ni(â-diketonato)2(IM2py)], the absorption intensities
originating from the spin-forbidden 3A2f1E d-d transitions
around 13 × 103 cm-1 were enhanced by several tens of
times as compared with those of the nonradical complex [Ni-
(acac)2(tmen)] as shown in Figure 6. The exchange couplings
3
1
in the ground A2 state and the excited E state with the
IM2py lead to the LT(3A2), the LT(3A2), and the LS(1Ε)
states as shown in Figure 10. This results in the breakdown
of the ∆S ) 0 restriction by which the triplet-singlet
transitions become spin-allowed between the exchange-
coupled doublets. This assignment is substantiated by the
absorption band intensity diminution on lowering the tem-
perature (Figure 8), resulting from the Boltzmann depopula-
tion of the doublet level in the ground state, in agreement
with the ferromagnetic interaction observed by the magnetic
susceptibility measurements (vide supra). As compared with
the NIT2py Ni(II) complexes, the intensities of the IM2py
Ni(II) complexes were much smaller. The large difference
4
2
2
Figure 9. Absorption spectra of [Ni(acac)2(IM2py)] (1b) in an acetate
cellulose film at 300 (-), 250 (- - -), 200 (- - -), and 150 K (- - -).
those of the nonradical complex [Cr(acac)2(en)]+ in the
corresponding region as shown in Figure 5. Such enormous
4
intensity enhancements in the spin-forbidden A2f2E,2T1
4368 Inorganic Chemistry, Vol. 41, No. 17, 2002

Bis(â-diketonato)chromium(III) and Nickel(II) Complexes
(ꢀSF(IM2py) ) 4.86-11.6 dm3 cm-1 mol-1; ꢀSF(NIT) )
111-465 dm3 cm-1 mol-1) between the two series reflects
the difference in magnetic interaction.
considering the antiferromagentic interaction through the
charge-transfer integral hCT,30 the exchange coupling constant
JAF is expressed as in eq 3.
Changes of the absorption band intensity on lowering the
temperature for the Cr(III) and Ni(II) complexes do not
reproduce the magnetic coupling constants (J) estimated from
the magnetic susceptibility measurements as is similarly
found for the corresponding NIT2py complexes.14,16
The difference in the magnetic and optical properties
among the IM2py Cr(III) and Ni(II) complexes with various
kinds of â-diketonates is examined in terms of the exchange
mechanism for the intensity enhancement in the formally
spin-forbidden d-d transition region of the IM2py complexes
as discussed below.
The integrated intensity ISF in eq 3 may be approximated
by the product of the molar absorption coefficient (ꢀ) and
the half-bandwidth (∆1/2) as previously proposed.14,16 The
ratios ꢀSF/ꢀCT31 are almost constant, and the MLCT transition
energy ECT31 and the energy difference (∆ECT) between the
MLCT and the spin-forbidden transition energy do not
change with different kinds of â-diketonates. Accordingly,
as shown in Table 4, (ꢀSF/ꢀCT)(∆ECT)2/ECT ) or JAF is almost
constant for variations of â-diketonates, in contrast to the
large difference of the Jobsd values from ca. -96.4 to -188
cm-1 (Cr) or from ca. +0.400 to +95.3 cm-1 (Ni) (Table
4). That is, the antiferromagnetic interaction is not affected
by the coligands in the two series. Since the Jobsd values are
the sum of the antiferromagnetic and ferromagnetic contribu-
tion, i.e., Jobsd ) JAF + JF, the variation of the Jobsd values
seems to reflect that of the JF values. In other words, in both
series the coligand effect may be operative in the ferromag-
netic interaction, but not in the antiferromagnetic one.
Therefore, the magnetic interactions of the IM2py bis(â-
diketonato) Cr(III) and Ni(II) complexes are mainly varied
with the change of the ferromagnetic coupling. This behavior
is the same as that of the NIT2py Cr(III) complexes,16 but
different from that of the NIT2py Ni(II) complexes.14
The hypothetical variation of the JF values depending on
coligands may be accounted for theoretically by considering
the lowest quintet LMCT for Cr(III) and the quartet MLCT
for Ni(II). The highest or lowest spin multiplet level of the
ground state is stabilized due to the configurational inter-
action, leading to the ferromagnetic or antiferromagnetic or
varying the JF or JAF values, respectively, as in Figures 8
and 10.32 That is, the ferromagnetic interaction concerns the
5LMCT(HOMOπ-eg) for Cr(III) and 4MLCT(t2g-π*) for
Ni(II), whereas the antiferromagnetic interaction concerns
Correlation between the Absorption Intensity and the
Magnetic Coupling Constant through the Coligand Effect.
According to the exchange mechanism,30 the spin-forbidden
3
2
2
3LQ(4A2) f LD(2Γ) and LT(3A2) f LS(1Ε) transitions of
the IM2py Cr(III) and Ni(II) complexes, respectively, are
made allowable by the mixing of the MLCT excited states
through the perturbation, i.e., the electron-transfer integral
between the Cr(t2g)-IM2py(SOMO π*) and the Ni(eg)-
IM2py(SOMOπ*) as examined in our previous paper.14,16
The spin-forbidden transitions attain the transition intensity
or the integrated intensity ISF by acquiring a small part (b )
hCT/∆ECT) of that of the spin-allowed MLCT. hCT is the
electron-transfer integral: Ψ0(3LQ(4A2))|hCr|Φ(3MLCT) for
Cr(III) and Ψ0(2LT(3A2))|hNi|Φ(2MLCT) for Ni(II). ∆ECT
is the difference (ECT - ESF) between the transition energies
where ECT and ESF refer to the transition energies from the
3
3
triplet ground state to the MLCT (ECT ) and the LD(2Γ)-
(ESF) for Cr(III) or those from the doublet ground state to
2
2
the MLCT (ECT ) and the LS(1Ε) (ESF) for Ni(II). Then,
the following equation is given.
ISF ) b2ICT ) (hCT/∆ECT )2ICT
(1)
3
2
the MLCT(t2g-π*) for Cr(III) and the MLCT(eg-π*) for
Ni(II).33 Consequently, the variation in Jobs values with the
coligand effect can be interpreted by considering the JF and
JAF, namely, both the magnetic orbital orthogonality and
overlap. The difference in degree of the electron-transfer
integral or overlap integral between the IM2py and NIT2py
with the metal d orbitals is compared on the basis of the
chelate ring planarity (IM2py being more planar than
The configurational interaction with the triplet (Cr) or doublet
(Ni) excited MLCT state results in stabilizing the triplet (Cr)
or the doublet (Ni) ground-state level by hCT2/ECT (Figures
8 and 10 and eq 2).
JAF ) hCT2/ECT
(2)
≈ (ISF/ICT)(∆ECT)2/ECT (ꢀSF /ꢀCT )(∆ECT)2/ECT (3)
(32) (a) Kahn, O. Molecular Magnetism; VCH Publisher Inc.: New York,
1993; Chapter 8. (b) Tuczek, F.; Solomon, E. I. Inorg. Chem. 1993,
32, 2850. (c) Tuczek, F.; Solomon, E. I. Coord. Chem. ReV. 2001,
219-221, 1075-1112.
Thus, in terms of the ratio of the spin-forbidden transition
intensities to the MLCT values31 from eqs 1 and 2 by
(33) Ψ0(3LD(4A2)) and Φ0(3MLCT) for Cr(III) or Ψ0(2LT(3A2)) and
+
+
(29) Ullman, E. F.; Osieccki, J. H.; Rey, P.; Sessoli, R. J. Am. Chem. Soc.
1972, 94, 7049.
(30) Ferguson, J.; Guggenheim, H. J.; Tanabe, Y. J. Phys. Soc. Jpn. 1966,
21, 692.
(31) The positions of the MLCT shoulders were estimated by the second
derivatives of the absorption envelope, and the intensities were
approximated by the values of the ordinate (molar absorption coef-
ficients) corresponding to those of the estimated abscissa (wavenum-
ber). The difference in the estimated positions and intensities of the
MLCT (Table 4) among the â-diketonato complexes may be intrinsic,
but does not result from the influence of the shorter wavelength intense
intraligand n-π* transition of the IM2py, since the latter is not so
much varied with the â-diketonate coligands.
Φ0(2MLCT) for Ni(II) are approximated to represent |t2g t2g t2g+π*-
|
and |t2g t2g+π*-π*+| or |(t2g)6eg+eg+π*-| and |(t2g)6eg+π*+π*-|, re-
+
Cr
spectively. That is, htπ*
)
Ψ0(3LQ(4A2))|h|Φ0(3MLCT)
)
Ni
|t2g t2g+π*-|h|t2g t2g+π*-π*+| ) |t2g |h|π*+| for Cr(III) or heπ*
+
+
+
) Ψ0(2LT(3A2))|h|Φ0(2MLCT) ) |(t2g)6eg+eg+π*-|h|(t2g)6eg+π*+π*-
|
+
) |eg |h|π*+| for Ni(II). In a similar manner, the quintet π-eg LMCT
(|π-t2g t2g t2g+eg+π*+| from |π-π+t2g t2g t2g+π*+|) and the quartet
+
+
+
+
t2g-π* MLCT (|(t2g)4t2g+eg+eg+π*+π*-| from |(t2g)4t2g t2g-eg+eg+π*+|)
+
Cr
+
Ni
give the electron transfer integral heπ
)
|eg |h|π+| and htπ*
)
-
|t2g |h|π*-| , respectively. The stabilization of the quintet and quartet
level in the ground state or the variation of JF values could result
from the difference in the configurational interaction between the
ground and excited states.
Inorganic Chemistry, Vol. 41, No. 17, 2002 4369

Tsukahara et al.
NIT2py) as revealed by the X-ray analysis (vide supra). JF
results from the difference in magnetic orbitals of Cr(III)
and Ni(II), t2g(dπ) and eg(dσ), which are orthogonal to each
other.
Cr
+
Cr
and |JAF| depend on heπ
)
|eg |h|π+| and htπ*
)
+
-
|t2g |h|π*+| , respectively, for Cr(III) and htπ*Ni ) |t2g |h|π*-|
+
and heπ*Ni ) |eg |h|π*+| ,33 respectively , for Ni(II), assuming
The coligand effect on the Jobs or JF values could arise
from the difference in electronic properties of the â-diketo-
nates. The Jobsd values of the IM2py Cr(III) and Ni(II)
complexes tend to increase on decreasing the acid dissocia-
tion exponent pKa of the â-diketones except for the dbm
Cr(III) complex. Thus, the JF values increase on decreasing
the pKa or increasing the Lewis acidity of the metal sites in
the M(â-diketonate)2 moiety, making the interaction between
IM2py and metal ions stronger. This may result in an increase
of the orbital overlap between the eg(dσ) and the π+ for
Cr(III) or the t2g(dπ) and the SOMO(π*) for Ni(II), leading
to an increase of the ferromagnetic interaction JF. It is noted
that this situation is also found to be the same as that for the
NIT2py Cr(III) complexes,16 but in contrast to the case for
the corresponding NIT2py Ni(II) complexes where the JAF
values were correlated with the Hammett constant.14
that the respective factors 1/ECT in eq 2 for JAF and in the
analogous equation for JF are not so much different on going
from the NIT2py Cr(III) or the NIT2py Ni(II) complexes to
the corresponding IM2py Cr(III) or Ni(II) complexes.
For Cr(III), the overlap between the eg(dσ) and π orbitals
+
(heπCr ) |eg |h|π+| ) or between the t2g(dπ) orbitals and the
+
π* (htπ*Cr ) |t2g |h|π*+| ) leads to the relationship heπCr(NIT)
> heπCr(IM) or htπ*Cr(NIT) < htπ*Cr(IM), which gives the
following inequalities (eqs 4 and 5).
JF(NIT) > JF(IM)
(4)
(5)
-|JAF(NIT)| > -|JAF(IM)|
For Ni(II) with the eg(dσ) magnetic orbital, the situation
is completely reverse in terms of the relationship heπ*Ni(NIT)
< heπ*Ni(IM) and htπ*Ni(NIT) > htπ*Ni(IM);34
Conclusions
JF(NIT) < JF(IM)
(6)
(7)
The differences in magnetic behavior among the Cr(III)
and Ni(II) complexes with IM2py and NIT2py are elucidated
by considering the overlaps between the magnetic orbital
(t2g or eg) and the SOMO π*, LUMO π*, or HOMO π in
the nitroxide radicals.
For the Cr(III) and Ni(II) IM2py complexes, a large
variation of Jobsd is found to originate from the ferromagnetic
interaction (JF), but not from the antiferromagnetic interaction
(JAF) as found for the corresponding Ni(II) complexes14 on
the basis of the exchange mechanism. This coligand effect
on JF arises from the electronic effect of the â-diketonates
as revealed from the correlation with the Lewis acidity (pKa)
of the â-diketones themselves. Quantitative elucidation must
await further research underway in our laboratory.
-|JAF(NIT)| < -|JAF(IM) |
The addition of these inequalities ((4) + (5) and (6) + (7))
leads to the following inequalities.
For Cr(III):
-|JAF(NIT)| + JF(NIT) > -|JAF(IM)| + JF(IM)
For Ni(II):
-|JAF(NIT)| + JF(NIT) > |JAF(IM)| + JF(IM)
These are in accordance with the following findings.
Acknowledgment. We gratefully acknowledge support
of this research by a Grant-in-Aid for Scientific Research
(A) (No.10304056) from the Ministry of Education, Culture,
Science and Technology.
For the Cr(III) complexes:
J
obs(NIT) > Jobs(IM)
For the Ni(II) complexes:
Supporting Information Available: Variable-temperature mag-
netic susceptibilities with each fitting for the two-spin system, UV-
vis absorption spectra, resonance Raman spectra for the complexes,
and the relation between the Jobs values and pKa, and the magnetic
data (Figures 1S-17S and Table 1S), as well as X-ray crystal-
lographic files in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
J
obs(NIT)< Jobs(IM)
This accounts for the difference in magnetic behavior among
the Cr(III) and Ni(II) complexes with IM2py and NIT2py
by considering both the antiferromagnetic and ferromagnetic
contributions to the Jobs through the exchange mechanism.
It is seen that the reverse behavior in the inequal relations
IC011282M
4370 Inorganic Chemistry, Vol. 41, No. 17, 2002