Synthesis, magnetic properties, and electronic spectra of mixed ligand bis(β-diketonato)chromium(III) complexes with a chelated nitronyl nitroxide radical: X-ray structure of [Cr(dpm)2(NIT2py)]PF6

Article abstract of DOI:10.1039/b104092h

A new series of nine nitronyl nitroxide Cr(III) complexes with various β-diketonates, [Cr(β-diketonato)₂(NIT2py)]PF₆, have been synthesized where NIT2py is 2-(2′-(pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-3-oxide- 1-oxyl

Full text of DOI:10.1039/b104092h

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Synthesis, magnetic properties, and electronic spectra of
mixed ligand bis(ꢀ-diketonato)chromium(III) complexes
with a chelated nitronyl nitroxide radical:
X-ray structure of [Cr(dpm)2(NIT2py)]PF6 †
Yasunori Tsukahara, Atsushi Iino, Takafumi Yoshida, Takayoshi Suzuki and Sumio Kaizaki*
Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka,
560-0043, Japan. E-mail: kaizaki@chem.sci.osaka-u.ac.jp
Received 9th May 2001, Accepted 6th November 2001
First published as an Advance Article on the web 17th December 2001
A new series of nine nitronyl nitroxide Cr() complexes with various β-diketonates, [Cr(β-diketonato)2(NIT2py)]PF6,
have been synthesized where NIT2py is 2-(2Ј-(pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-3-oxide-1-oxyl,
and their structures, magnetic and optical properties have been examined. X-Ray analysis of [Cr(dpm)2(NIT2py)]PF6
(monoclinic, space group P21/a, a = 13.960(4), b = 31.19(1), c = 13.940(4), Z = 4) demonstrated that NIT2py
coordinated to Cr() as a bidentate six-membered chelate. Variable-temperature magnetic susceptibility
measurements indicated the antiferromagnetic interaction between Cr() and NIT2py with a variety of the magnetic
coupling constant J values for these complexes. From the variable temperature or solvent dependent UV–vis spectra
and MCD, and/or the resonance Raman spectra of the bis(β-diketonato)(NIT2py) Cr() complexes, the absorption
components centered around 13.3 × 103 cmϪ1 were assigned to the formally spin-forbidden d–d transition within the
t
2g subshell associated with the intensity enhancement and the newly appeared vibronic bands around (16.0–18.0) ×
103 cmϪ1 were due to the metal–ligand charge transfer [t2g–SOMO(π*) MLCT] transitions. The change of their
spectroscopic characteristics with varying the β-diketonato ligands is discussed in connection with the
antiferromagnetic coupling constant J values on the basis of the exchange mechanism along with the
coligand effect. The luminescence spectra which show a large Stokes shift suggest the antiferromagnetic
interaction in the lowest excited states originates from the 2E or 2T1 level of the Cr() moiety.
the spectroscopic methods with no X-ray structural evidence
Introduction
for the NIT2py chelation. Further investigation extending to a
series of this moderately coupled type of β-diketonato Cr()
complexes with t2g magnetic orbitals could be invaluable in
order to explore expected regulations of the magnetic and
optical properties with variation of β-diketonates in com-
parison with the corresponding Ni() complexes with eg
magnetic orbitals. This may be impossible for the strongly
coupled types of semiquinone and phenoxy radical Cr()
complexes, because in this case there has been demonstrated to
be large temperature independent intensity enhancement in the
spin-forbidden transitions due to strong antiferromagnetic
interactions in the ground state.5
In this article, the synthesis of a series of NIT2py Cr()
complexes with various types of β-diketonates and a magnetic
and spectroscopic study together with X-ray structural analysis
is presented in order to reveal the relation between the optical
and magnetic properties in comparison with those of the Ni()
complexes.
There have been a number of spectroscopic investigations1–6 on
multi-spin exchange coupled systems consisting of paramag-
netic metal ions, while metal complexes containing nitroxide
radicals such as several derivatives of 4,4,5,5-tetramethyl-4,5-
dihydro-1H-imidazolyl-3-oxide-1-oxyl (NIT) have been studied
from a viewpoint of molecular magnets.7–10 The first spectro-
scopic study on paramagnetic metal complexes with the
NIT2py radical ligand was our recent report of two series of
mixed ligand NIT2py nickel() complexes,11 [Ni(β-diketonato)2-
(NIT2py)] and [Ni(β-diketonato)(tmen)(NIT2py)]ϩ (tmen =
N,N,N Ј,NЈ-tetramethylethylenediamine) with various types of
β-diketonate ligands, followed by a preliminary communication
for a limited number of bis(β-diketonato) (NIT2py) complexes
of Ni() and Cr().12 The NIT2py Ni() complexes were found
to show a coligand effect (or a substituent effect) of the β-
diketonates on the formally spin-forbidden d–d transition
intensities or NMR contact shifts in relation to the magnetic
exchange coupling constants. Very recently, we reported the
magnetic and spectroscopic properties of cis (Cl)–trans (py)-
[MCl2(IM2py)2] {M = Mn(), Co(), Ni() or Zn(); IM2-
py = 2-(2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxy}13 and
tetrahedral four-coordinate [MCl2(NITmepy)] [NITmepy =
4,4,5,5-tetramethyl-2-(6-methyl-2-pyridyl)imidazolin-1-oxide 3-
Experimental
Ligand
The radical ligand NIT2py was prepared by the literature
method.15 The β-diketonates were commercially available [β-
diketone = Hacac (2,4-pentanedione), Hdbm (1,3-diphenyl-1,3-
propanedione), Hbzac (1-phenyl-1,3-butanedione), HacaMe
(3-methyl-2,4-pentanedione), HacaEt (3-ethyl-2,4-pentane-
dione), HacanBu (3-n-butyl-2,4-pentanedione), HacaPh
(3-phenyl-2,4-pentanedione), Hdpm (2,2,6,6-tetramethyl-3,5-
heptanedione), HMehp (6-methyl-2,4-heptanedione].
oxyl; M᎐Co(), Ni(), Zn()].14 On the other hand, only one
᎐
of the corresponding Cr() complexes, [Cr(acac)2(NIT2py)]-
ClO4,12 has been prepared and preliminarily characterized by
† Electronic supplementary information (ESI) available: variable tem-
perature magnetic susceptibilities with each fitting for the two spin sys-
tems and UV-vis spectra for complexes 2–9. See http://www.rsc.org/
suppdata/dt/b1/b104092h/
DOI: 10.1039/b104092h
J. Chem. Soc., Dalton Trans., 2002, 181–187
This journal is © The Royal Society of Chemistry 2002
181

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Syntheses of starting and reference complexes
intensities (2θmax = 55) were measured at 23 ЊC with graphite-
monochromate Mo-Kα radiation (λ = 0.71069 Å) on an auto-
mated Rigaku AFC-7R four-circle diffractometer using the
ω–2θ scan technique. There was no serious decomposition. The
structure was solved by direct methods using the SHELXS86
program17 and refined on F 2 with all independent reflections
using the SHELXS97 program.17 All calculations were carried
out using a TeXsan18 software package.
Starting complexes for bis(ꢀ-diketonato)di(aqua)chromium(III)
complexes. β-diketone (2 mmol) was added to a solution of
1 mmol of Cr(NO3)3ؒ9H2O in 50 mL of EtOH with stirring at
room temperature. After the mixture was dissolved completely
in EtOH, 2 mmol of NaOEt was added to this solution. After
3 days stirring, the reaction solution was evaporated under
reduced pressure. The crude product was obtained by the
addition of dichloromethane and the precipitate was filtered
off. The solid was dissolved in 5 mL of EtOH. This solution was
loaded on a cation exchange column [SP-TOYOPEARL 550
(Naϩ) resin]. The adsorbed band was eluted with 0.1 mol dmϪ3
NaBF4 ethanol solution. Only one band was eluted, which was
supposed to be a unipositive ion, leaving highly charged cation
complexes on the top of the column. This eluate was evapor-
ated to dryness. After the residue was desalted by extraction
with dichloromethane, a powder was obtained. Such powders
were characterized as [Cr(β-diketonato)2(H2O)2]BF4 by elem-
ental analysis and were used as the starting complexes without
further characterization.
CCDC reference number 164248.
See http://www.rsc.org/suppdata/dt/b1/b104092h/ for crystal-
lographic data in CIF or other electronic format.
Measurements
UV–vis spectra were obtained by a Perkin Elmer Lamda 19
spectrophotometer. Low-temperature UV–vis spectra were
measured at 200,100,50 and 10 K using the same spectro-
photometer with an Oxford Cryostat DN1704 (static) in a film
made from cellulose acetate with acetonitrile. Magnetic sus-
ceptibility data were measured 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 con-
stituent atom diamagnetism values. Resonance Raman spectra
were measured in the solid state using a Jasco NR1800 spectro-
photometer at room temperature using Ar laser lines (488.00
and 514.50 nm), the He–Ne laser line (632.80 nm), and dye laser
lines (584.47 and 606.27 nm) as excitation sources. Lumin-
escence measurement were carried out in the solid state using a
Jasco NR1800 spectrophotometer at 5, 50, 120 and 150 K.
Reference complex. [Cr(acac)2(en)]PF6 was synthesized
according to the literature method.16
Syntheses of NIT2py complexes
(1) [Cr(acac)2(NIT2py)]PF6 1. [Cr(acac)2(NIT2py)]PF6 was
synthesized by a modification of the previously reported
method.11a Yield 75%. Found: C, 42.0; H, 4.97; N, 6.58.
C22H30CrF6N3O6P requires C, 42.0; H, 4.80; N, 6.68%.
Results and discussion
(2) [Cr(dbm)2(NIT2py)]PF6 2. NIT2py (1 mmol) was added
to a solution of 1 mmol of [Cr(dbm)2(H2O)2]BF4 in 50 mL of
CH3CN with stirring in an ice–methanol bath at Ϫ15 ЊC. After
12 h stirring, the solution was evaporated under reduced pres-
sure. The residue was dissolved in 50 mL of EtOH, and 1 mmol
of NaPF6 was added to this solution. After 1 day stirring, the
solution was evaporated under reduced pressure. The crude
product was obtained by adding dichloromethane. The residue
was dissolved in 10 mL of CHCl3. This solution was loaded on
a HPLC column (LC-908, Japan Analytical Industry Co. Ltd.)
and eluted with CHCl3. The first band eluted with CHCl3 con-
tained the desired cation [Cr(dbm)2(NIT2py)]ϩ. After the eluate
was condensed, crystallization was carried out by vapor dif-
fusion from dichloromethane–diethyl ether. This complex was
recrystallized as violet–green needle-like crystals. Yield 80%.
Found: C, 57.2; H, 4.28; N, 4.82. C42H38CrF6N3O6P requires C,
57.5; H, 4.36; N, 4.79%.
The other β-diketonato complexes [Cr(β-diketonato)2(NIT2-
py)]PF6 [β-diketonato = bzac (3), acaMe (4), acaEt (5), acanBu
(6), acaPh (7), dpm (8), Mehp (9)] were obtained by a similar
method to that for the dbm complex. Most were obtained as
powdery solids except the crystalline acac, dbm and dpm com-
plexes. 3: 60%. Found: C, 50.2; H, 4.44; N, 5.38.C32H34CrF6-
N3O6P requires C, 51.0; H, 4.55; N, 5.58%. 4: 65%. Found: C,
43.8; H, 5.16; N, 6.33. C24H34CrF6N3O6P requires C, 43.8; H,
5.21; N, 6.39%. 5: 60%. Found: C, 45.0; H, 5.61; N, 6.27.
C26H38CrF6N3O6P requires C, 45.6; H, 5.59; N, 6.13%. 6: 60%.
Found: C, 48.2; H, 6.14; N, 5.64. C30H48CrF6N3O6P requires
C, 48.5; H, 6.50; N, 5.64%. 7: 60%, Found: C, 51.7; H, 4.94;
N, 5.35. C34H38CrF6N3O6P requires C, 52.2; H, 4.90; N, 5.38%.
8: 75%. Found: C, 51.0; H, 6.78; N, 5.47. C34H54CrF6N3O6P
requires C, 51.2; H, 6.82; N, 5.27%. 9: 65%. Found: C, 46.9;
H, 5.91; N, 5.81. C28H42CrF6N3O6P requires C, 47.1; H, 5.93;
N, 5.89%.
Preparation and characterization
The starting complexes [Cr(β-diketonato)2(H2O)2]BF4 for the
synthesis of a series of NIT2py complexes could not be pre-
pared by the preparative method in aqueous solution used for
the corresponding bis(acac) diaqua complexes.19 The present
method in ethanol solution is generally adopted in the prepar-
ation of the β-diketonato diaqua complexes. The diaqua com-
plexes were obtained as an equilibrium mixture of cis and trans
geometrical isomers, but no separation was attempted. The syn-
thetic method of the new [Cr(β-diketonato)2(NIT2py)]PF6
complexes was different from that of [Cr(acac)2(NIT2py)]PF6.12
When [Cr(β-diketonato)2(H2O)2]BF4 and NIT2py were mixed
in acetonitrile, [Cr(β-diketonato)2(NIT2py)]PF6 complexes
were found to form in solution at room temperature. However,
during the formation reaction at room temperature, NIT2py
was changed to IM2py. The most remarkable case was seen for
the hexafluoroacetylacetonato (hfac) complex. It was con-
firmed from UV–vis spectra in acetonitrile that NIT2py in
[Cr(hfac)2(NIT2py)]ϩ is deoxygenated to IM2py in [Cr(h-
fac)2(IM2py)]ϩ in 1 day at room temperature. The deoxygen-
ation reaction rates were found to depend on the nature of the
β-diketonate. Deceleration of the reaction rate by lowering the
temperature resulted in the successful isolation of the pure
complexes [Cr(β-diketonato)2(NIT2py)]PF6 except for the hfac
complex. Satisfactory elemental analyses for these complexes
confirmed the chemical formula with no crystallization solvent,
suggesting the chelation of NIT2py.
Molecular structure
Crystallographic data for [Cr(dpm)2(NIT2py)]PF6 are listed in
Table 1. Selected bond lengths, bond angles, torsion angles, and
dihedral angles are summarized in Table 2 and the molecular
structure is shown in Fig. 1. This is an octahedral complex char-
acterized by chelation of NIT2py through the pyridyl nitrogen
and nitronyl nitroxide oxygen leading to a six-membered ring,
together with two chelated dpm ligands. The other β-diketonato
complexes are expected to have the same structure as the dpm
Crystallography
A violet–green square columnar crystal of [Cr(dpm)2(NIT2-
py)]PF6 was sealed in a glass capillary with epoxy resin. X-Ray
182
J. Chem. Soc., Dalton Trans., 2002, 181–187

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Table 1 Crystallographic data for [Cr(dpm)2(NIT2py)]PF6 8
Formula
Formula weight
T /ЊC
C54H34CrF6N3O6P
797.786
23
Crystal system
Space group
Monoclinic
P21/a
a/Å
b/Å
c/Å
13.940(4)
31.19(1)
13.960(4)
137.03(2)
4137(2)
4
1.281
0.385
0.070
0.296
β/Њ
V/Å3
Z
Dc/g cmϪ3
µ(Mo-Kα)/cmϪ1
R1a
wR2b
a R1 = Σ||Fo| Ϫ |Fc||/Σ|Fo|. b wR2 = [Σw(|Fo| Ϫ |Fc|)2/Σw|Fo|2]1/2, wϪ1
=
σc2(Fo) ϩ (0.00022|Fo|)2.
Table 2 Selected bond distances (Å), bond angles (Њ), torsion angles (Њ)
and dihedral angles (Њ) for [Cr(dpm)2(NIT2py)]PF6
Fig. 1 View of the molecular structure of [Cr(dpm)2(NIT2py)]PF6 8.
Bond distance
Cr–O(1)
Cr–O(21)
Cr–O(22)
Cr–O(41)
Cr–O(42)
Cr–N(3)
tBu groups have been omitted for the sake of clarity.
1.962(5)
1.929(5)
1.947(5)
1.932(5)
1.936(5)
2.091(6)
1.293(7)
O(2)–N(2)
N(1)–C(1)
N(2)–C(3)
N(3)–C(8)
N(3)–C(12)
C(1)–C(8)
1.250(8)
1.331(9)
1.524(10)
1.342(8)
1.335(9)
1.452(9)
absolute configuration (Fig. 1) with the three chelates around a
Cr() ion adopting a λ(lel) conformation of NIT2py and vice
versa, in terms of the IUPAC definition, as found in the Ni()
complexes.11a There seems to be no intermolecular interactions
in view of the crystal packing around the complex.
O(1)–N(1)
Bond angle
Magnetic properties
O(1)–Cr–O(21)
O(1)–Cr–O(22)
O(1)–Cr–O(41)
O(1)–Cr–O(42)
O(1)–Cr–N(3)
O(21)–Cr–O(22)
O(21)–Cr–O(41)
O(21)–Cr–O(42)
O(21)–Cr–N(3)
O(22)–Cr–O(41)
178.6(2)
89.1(2)
88.0(2)
92.4(2)
88.1(2)
89.7(2)
91.2(2)
88.8(2)
92.8(2)
89.3(2)
O(22)–Cr–O(42)
O(22)–Cr–N(3)
O(41)–Cr–O(42)
O(41)–Cr–N(3)
O(42)–Cr–N(3)
Cr–O(1)–N(1)
O(1)–N(1)–C(1)
O(2)–N(2)–C(1)
Cr–N(3)–C(8)
Cr–N(3)–C(12)
178.4(2)
92.9(2)
90.3(2)
175.5(2)
87.7(2)
122.5(4)
126.6(5)
128.5(7)
126.1(5)
116.6(5)
The product of the molar magnetic susceptibility (χM) and tem-
perature (T ) χMT vs. T plot for [Cr(acac)2(NIT2py)]PF6 is
shown in Fig. 2. The χMT values are smaller at room temper-
Torsion angle
N(1)–C(1)–C(8)–N(3) Ϫ17.5(11)
Dihedral angle
Plane 1a–plane 2b
18.59
31.02
Plane 1–plane 4d 146.89
Plane 2–plane 3 20.94
Plane 1–plane 3c
a Plane 1: O(1), N(1), C(1), N(2), O(2). b Plane 2: N(3), C(8), C(9),
C(10), C(11), C(12). c Plane 3: Cr, O(1), N(3). d Plane 4: Cr, O(1), N(1).
complex in view of the similarity of their UV–vis spectral and
SQUID magnetic susceptibility as described below.
The two N–O lengths of the nitronyl nitoxide moiety are
similar to those of other NIT2py complexes.9–11a This fact sug-
gests the existence of the nitronyl nitroxide radical in the com-
plex. The coordinated O(1)–N(1) length [1.293(7) Å] for the
NIT2py is longer than the uncoordinated O(2)–N(2) length
[1.250(8) Å] as observed for other metal NIT2py complexes [e.g.,
NIT2py Ni() complex11a). The Cr–O bond lengths for the dpm
ligands are in the range 1.93–1.95 Å, which are shorter than
those (1.94–1.98 Å) of the corresponding [Cr2(dpm)4(µ-MeO)2]
complexes.20 As shown in Table 2, the bite angle 88.1(2)Њ
Fig. 2 Temperature dependence of the magnetic susceptibilities for
[Cr(acac)2(NIT2py)]PF6 1 (᭺) and [Cr(acaMe)2(NIT2py)]PF6 4 (∆) in
the form of χMT vs. T .
ature than the calculated value for an uncorrelated system
containing a chromium() cation and NIT2py radical (S᎐3/2 ϩ
᎐
1/2) (χMT = 2.25 emu K molϪ1) and decrease with lowering the
temperature, suggesting an antiferromagnetic interaction4a
between Cr() and the NIT2py radical. χMT vs. T plots for
other [Cr(β-diketonato)2(NIT2py)]PF6 complexes show a sim-
ilar pattern. The magnetic coupling constants Jobs involving a
chromium() ion and NIT2py were estimated by fitting the
magnetic susceptibilities to a two-spin system (S1 = 3/2, S2 =
1/2).21 The Jobs values thus obtained with R values equal to
10Ϫ3–10Ϫ4 are summarized together with the g values in Table 3.
The Jobs values range from Ϫ9.05 to Ϫ98.6 cmϪ1; [Cr(acaX)2-
(NIT2py)]PF6 type complexes with 3-substituted acac ligands
for O(1)–Cr–N(3) is larger compared with those of [Ni(β-
11a
diketonato)(tmen)(NIT2py)]PF6
and [Ni(hfac)2(NIT2py)].9
The dihedral angles around the NIT2py moiety together with
the torsion angle of N(1)–C(1)–C(8)–N(3) for the NIT2py are
11a
smaller than those of [Ni(β-diketonato)(tmen)(NIT2py)]PF6
and [Ni(hfac)2(NIT2py)].9 This indicates that the NIT2py
chelate in the Cr() complex is more flattened than those of the
Ni() complexes. Judging from the sign of the torsion angle of
N(1)–C(1)–C(8)–N(3) for the NIT2py, this complex has a ∆
J. Chem. Soc., Dalton Trans., 2002, 181–187
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Table 3 Magnetic and optical data for the NIT2py Cr() complexes
b
c
d
e
f
Compound
Jobs (g)a
εSF
εCT
ECT
ECT
(εSF/εCT)(ECT)2/(ECT
)
1 [Cr(acac)2(NIT2py)] PF6
2 [Cr(dbm)2(NIT2py)] PF6
3 [Cr(bzac)2(NIT2py)] PF6
4 [Cr(acaMe)2(NIT2py)] PF6
5 [Cr(acaEt)2(NIT2py)] PF6
6 [Cr(acanBu)2(NIT2py)] PF6
7 [Cr(acaPh)2(NIT2py)] PF6
8 [Cr(dpm)2(NIT2py)] PF6
9 [Cr(Mehp)2(NIT2py)] PF6
Ϫ61.6 (2.00)
Ϫ70.0 (2.00)
Ϫ30.2 (2.01)
Ϫ9.05 (2.06)
Ϫ36.5 (2.02)
Ϫ12.3 (2.01)
Ϫ19.4 (2.02)
Ϫ98.6 (2.00)
Ϫ49.0 (2.00)
327
325
285
230
265
279
234
346
337
641
648
589
504
536
567
479
631
660
17.3
17.3
17.3
17.1
17.2
17.1
17.2
17.3
17.3
4.00
4.05
4.00
4.01
4.07
4.01
4.03
3.97
4.02
0.472
0.475
0.447
0.429
0.477
0.462
0.460
0.500
0.478
a Observed J values/cmϪ1
MLCT transition/molϪ1 dm3 cmϪ1
cmϪ1. f JAF/103 cmϪ1
. .
b Molar absorption coefficient for the spin-forbidden transition/molϪ1 dm3 cmϪ1 c Molar absorption coefficient for the
.
d MLCT transition energy/103cmϪ1
.
e The energy difference between the spin-forbidden and MLCT transition/103
.
tend to give small Jobs values in the range from Ϫ9.05 to Ϫ36.5
cmϪ1. These Jobsd values of [Cr(β-diketonato)2(NIT2py)]PF6
exhibit no relationship with the Hammett constants unlike the
corresponding Ni() complexes,11a but are fairly well related to
the acid dissociation constants Ka of the β-diketones; Jobsd
values increasing with increasing Ka.
assigned to a π–π* transition or charge transfer originating
from the acetylacetonato ligand. Similarly to the Ni() com-
plexes,11a the additive character of the non-radical bis(β-
diketonato) Cr() complexes and NIT2py is also seen for the
electronic transitions in the ultraviolet region of the NIT2py
Cr() complexes, and they are not necessarily affected by
NIT2py coordination.
UV absorption spectra
Visible absorption spectra: charge-transfer transitions
UV spectra of [Cr(acac)2(NIT2py)]PF6 are compared with
those of NIT2py and the nonradical ethylenediamine (en)
complex [Cr(acac)2(en)]PF6 as shown in Fig. 3. The absorption
In the visible region where the n–π* intraligand band of
4
NIT2py or the first spin-allowed d–d A2
4T2 band for the
non-radical Cr() complexes are observed, the characteristics
of the absorption spectra of [Cr(β-diketonato)2(NIT2py)]ϩ at
16.0–17.0 × 103 cmϪ1 are different from those of NIT2py and
the non-radical Cr() complex as shown in Fig. 3. The band
intensities are much larger than those of the latter compounds
and the band envelope has vibronic structure; two components
were observed with a spacing of about 2000 cmϪ1 as peaks or
shoulders. The molar absorption coefficients (ε) of each com-
ponent in this region was found to be influenced by the nature
of the β-diketonato coligands as seen in Fig. 4. From these spec-
Fig.
3
Absorption spectra of [Cr(acac)2(NIT2py)]PF6 1 (——),
[Cr(acac)2(en)]PF6 (– – –) and NIT2py (– ؒ – ؒ –) in CH3CN at room
temperature. Inset: enlarged absorption curve with a logarithmic
ordinate for [Cr(acac)2(en)]PF6 (– – –) in CH3CN at room temperature.
bands of NIT2py at 17.8 × 103 cmϪ1 and 27.4 × 103 cmϪ1 are
due to intraligand n–π* and π–π* transitions, respectively.22 The
absorption bands around 27 × 103 cmϪ1 of [Cr(β-diket-
onato)2(NIT2py)]ϩ correspond to the π–π* transitions of
NIT2py in view of the similarity in position and intensity with
the β-diketonato (acac, Mehp, dpm) complexes. However, the
absorption intensities of [Cr(dbm or bzac and acaX)2-
(NIT2py)]ϩ are larger than those for the other β-diketonato
complexes, owing to the overlap with the phenyl π–π* transition
in the dbm or bzac ligands with the π–π* transition or charge
transfer transition originating from the other acaX ligands.
Since the higher frequency absorption band of [Cr(acac)2-
(NIT2py)]ϩ at 30.0 × 103 cmϪ1 is analogous in position and
intensity to that of [Cr(acac)2(en)]ϩ at 30.0 × 103 cmϪ1, this is
Fig. 4 Absorption spectra of complex 1 (——), 2 (– ؒ – ؒ –), 4 (– – –)
and 8 (ؒ ؒ ؒ ؒ ؒ ) in CH3CN at room temperature.
tral observations, the visible absorption bands are assigned not
to intraligand nor d–d transitions, but rather to charge transfer
(CT) transitions as verified by resonance Raman spectra (vide
infra).
The resonance Raman spectra of [Cr(acac)2(NIT2py)]PF6 in
the solid state are shown for a variaty of excitation wavelengths
184
J. Chem. Soc., Dalton Trans., 2002, 181–187

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Fig. 6 Excitation profiles (bottom) with visible spectra (top) of
[Cr(acac)2(NIT2py)]PF6 1; 1530cmϪ1 (ᮀ); 1468cmϪ1 (᭛); 614cmϪ1 (᭺);
258cmϪ1 (᭝).
Fig.
5
Resonance Raman spectra of [Cr(acac)2(NIT2py)]PF6 1
recorded using 488.00, 514.50, 584.47, 606.27 and 632.80 nm laser lines.
in Fig. 5. In Raman spectra in resonance with the components
in the region (16.0–18.0) × 103 cmϪ1, significant intensity
enhancement was observed, especially for the bands around
1530 and 1468 cmϪ1 which can be assigned to the stretching
vibrations of the O–N᎐C moiety of the NIT2py ligand. The
᎐
remaining enhanced peak at 614 cmϪ1 may be due to the Cr–
O,N stretching. Thus, as seen from the excitation profile in Fig.
6, the (16–18) × 103 cmϪ1 components are assignable to charge
transfer transitions. The intensity enhancement of this band on
lowering the temperature (Fig. 7) substantiates an assignment
(Fig. 8) to the triplet–triplet metal t2g
ligand SOMO π*
(ML(CT) transition. This is also supported by the blue shift in
more polar solvents (from CH2Cl2 and CH3CN to CH3OH).
Fig. 7 Absorption spectra of complex 1 in an acetate cellulose film at
30 K (——), 200 K (– – –), 100 K (ؒ ؒ ؒ ؒ ؒ ؒ), 50 K (— ؒ — ؒ —) and 10 K
(------).
Near-infrared absorption spectra: spin-forbidden d–d transitions
For the radical complexes [Cr(β-diketonato)2(NIT2py)]ϩ, the
absorption peaks around 13.3 × 103 cmϪ1 were intensified by
several hundreds times compared with those of the non-radical
complex [Cr(acac)2(en)]ϩ in the corresponding region as shown
in Fig. 3. As shown in Fig. 2 of our previous communication,12
the MCD sign in this region is similar to that of [Cr(acac)2-
(en)]ϩ, though their intensities were different from each other
along with small peak shifts. The CD peaks of the en complex
were observed in the same region as the near infrared band of
varied with the Jobs values in contrast to the case of the Ni (II)
complexes.11a
The exchange coupled ground quartet state consists of the
triplet 3L0 (4A2) and quintet 5L0 (4A2) states, whereas the excited
doublet state 2Γ (Γ = E and/or T1) generates the triplet 3LD (2Γ)
and singlet 1LD (2Γ) states as shown in Fig. 8. Therefore,
the quartet–doublet spin-forbidden d–d transitions of [Cr(β-
diketonato)2(NIT2py)]ϩ become formally spin-forbidden or
substantially spin-allowed between the exchange coupled trip-
lets as a result of the breakdown of ∆S = 0 restriction. This
assignment is supported by the absorption band intensity
enhancement with lowering the temperature (Fig. 7), which
result from the Boltzmann population of the triplet level in the
ground state, in accordance with the antiferromagnetic inter-
action observed by the magnetic susceptibility measurements.
The difference between the magnetic and spectroscopic proper-
ties for the NIT2py Cr() complexes with various β-diketonates
the NIT complexes.23 These facts suggest that they originate
4
from spin forbidden A
2E,2T d–d transitions. Such enor-
mous intensity enhancements in the spin-forbidden transition
are ascribed to exchange coupling between the Cr3ϩ and the
NIT2py radical. However, the extent of the enhancement is
much smaller as compared with the strong coupling case for the
semiquinone complexes.5 Such less enhanced intensities are
inferred from the moderate magnetic couplings, but are little
J. Chem. Soc., Dalton Trans., 2002, 181–187
185

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becomes invisible, as shown in Fig. 9. This variable temperature
behavior suggests intensity variation of two peaks on cool-
ing but no blue shift. A similar intensification of the higher
frequency peak around 11 800 cmϪ1 on cooling is seen for the
other [Cr(β-diketonato)2(NIT2py)]ϩ complexes. Apart from the
recent claim that the NIR luminescence in Ln() complexes
with two chelating NITBzImH radicals arises from ligand-
centered transitions,24 the suggestion that the present anom-
alous temperature variable luminescence originates from the
ligand field d–d transitions in view of the peak position is the
only other reported instance.
The origin of the intensification of the spin-forbidden transitions
Though the formally spin-forbidden 2L0 (3A2)
2Ls (1E) tran-
sitions of the NIT2py Ni() complexes with various types
of β-diketonates show a large range of absorption intensities
[120–480 molϪ1 dm3 cmϪ1 for the molar absorption coefficients
εmax(SF)],11a the corresponding Cr() complexes show only a
small range of 3L0 (4A2)
3LD (2Γ) transition intensities
[εmax(SF) = 234–346 molϪ1 dm3 cmϪ1] as shown in Table 3. The
range of the MLCT band intensity [εmax = (CT) = 479.0–
630.9 molϪ1 dm3 cmϪ1) for the Cr() complexes is also much
smaller than that of εmax (CT) for the Ni() complexes. In
addition εmax(SF) is found to increase with increasing εmax (CT)
as shown in Table 3. These facts strongly suggest a connection
among the εmax (CT), εmax(SF) and Jobs values in multi-spin
coupled systems; in other words, the formally spin-forbidden
Fig. 8 Energy levels of the spin-allowed and spin-forbidden d–d
transitions. ( ) absorption spectra; (ؒؒؒؒؒؒ) spin-forbidden absorption
and emission; (
) configurational interactions between the ground
and CT states with the same spin multiplicity.
are examined in terms of the exchange mechanism for the
intensity enhancement in the formally spin-forbidden d–d tran-
sition region as discussed below.
3L0 (4A2)
3LD (2Γ) transition can attain the integrated
Luminescence spectra
intensity ISF by borrowing the t2g–π* SOMO charge transfer
integrated intensity ICT through an exchange mechanism.25
According to the exchange mechanism, the spin-forbidden
The remeasurement of the luminescence spectra of [Cr-
(acac)2(NIT2py)]PF6 1 demonstrated similar behavior except
for the peak position to the previously reported value.12 The
observed peak at 11 350 cmϪ1 is located at a lower frequency
than the lowest frequency absorption peak corresponding to
3L0 (4A2)
3LD (2Γ) transition of Cr() radical complexes is
made allowed by mixing of the MLCT excited states through
perturbation, i.e., via modification of the electron transfer
integral between the Cr (t2g) and NIT2py (SOMO π*). The real
triplet (doublet) wave function is given by eqn (1).
the spin-forbidden [3L0 (4A2)
3LD (2Γ)] triplet–triplet tran-
sition around 13300 cmϪ1 as shown in Fig. 9. This shows a large
Ψ(3ꢀ(2Γ)) = Ψ0(3ꢀ(2Γ)) ϩ bΦ(3ꢀ(MLCT))
(1)
This contains an admixture of the triplet MLCT wave func-
tion, where Ψ0(3ꢀ2(Γ)) and Φ(3ꢀMLCT)) are approximated to
ϩ
ϩ
ϩ
represent |t2g t2g t2gϪπ*ϩ| and |t2g t2gϩπ*Ϫπ*ϩ|.
Spin-forbidden transitions attain a transition intensity or
integrated intensity ISF, by acquiring a small part,
b = <|t2g t2g t2gϪπ*ϩ|h|t2g t2gϩπ*Ϫπ*ϩ|>/∆ECT
= <|t2gϪ|h|π*Ϫ|>/∆ECT = h(CT)/∆ECT
ϩ
ϩ
ϩ
of that of the spin-allowed MLCT where ∆ECT is the differ-
ence (ECT Ϫ ESF) between the transition energies, ECT and ESF,
3
3
from the triplet ground state to the MLCT and the LD (2Γ),
respectively, as given by eqn. (2).
ISF = b2ICT = (h(CT)/∆ECT)2ICT
(2)
Fig. 9 Luminescence (solid lines) in the solid at 150, 120 and 50 K and
absorption spectra (dotted line) in CH3CN at room temperature of
[Cr(acac)2(NIT2py)]PF6 1. Inset: enlarged spectra at 120 and 150 K.
According to the experimental correlation between εmax(SF)
and εmax(CT) (vide supra), the term h(CT)/∆ECT in eqn. (2) is
constant with variation of the coligands.
Stokes shift involving the LD (2Γ)
3L0 (4A2) emission, for
1
which the corresponding absorption bands should be too weak
to observe owing to the definitely spin-forbidden triplet–singlet
transitions (Fig. 8). Thus, antiferromagnetic coupling is sug-
gested between the lowest excited doublet state of the Cr()
moiety and the ground doublet state of the NIT2py radical.
On cooling to 120 K, another luminescence peak appears at
a higher frequency of ca. 11 800 cmϪ1 other than the main
11350 cmϪ1 peak at 150 K. Further cooling to 50 K leads to the
11 800 cmϪ1 peak becoming more intense by about ten times
than that at 120 K, and the lower frequency 11 350 cmϪ1 peak
Spin-forbidden band intensity and magnetic interaction
The configurational interaction with the triplet MLCT excited
state results in stabilizing the triplet ground state level by
h(CT)2/ECT (Fig. 8). Thus, the exchange coupling constant JAF
[eqn. (4)] is expressed in terms of the spin-forbidden and
MLCT transition intensities ratios from eqns. (2) and (3) with
JAF = h(CT)2/ECT
(3)
186
J. Chem. Soc., Dalton Trans., 2002, 181–187

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neglect of the exchange integral K or by considering the anti-
ferromagnetic interaction through the charge (electron) transfer
integral h(CT).26
is to pursue quantitative complementarity between optical and
magnetic properties; elucidating the disagreement between the
magnetic interaction derived from the magnetic susceptibility
measurements and that estimated from the variable temper-
ature absorption band intensity changes.
JAF = (ISF/ICT)(∆ECT)2∝{εmax(SF)/εmax(CT)}(∆ECT)2/ECT (4)
As in eqn. (4),27 the integrated intensity ISF may be approx-
Acknowledgements
imated by the product of the molar absorption coefficient(εmax
)
We gratefully acknowledge support of this research by a Grant-
in-Aid for Scientific Research (A)(2) (No.10304056) from the
Ministry of Education, Science and Culture.
and the half-bandwidth (∆1/2). Actually, since ∆1/2 of the MLCT
components are similar to those of the formally spin-forbidden
bands, the ratios of εSF/εCT are taken for simplicity instead of
ISF/ICT in the following discussion. The ratios of εSF/εCT are
almost constant as seen above, and the MLCT transition energy
ECT and the energy difference between the MLCT and the spin-
forbidden transition energy ∆ECT do not change with different
β-diketonates. This indicates that the value of
References
1 H. U. Güdel, in Magnetic–Structural Correlations in Exchange
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Dordrecht, The Netherlands, 1985, p. 297.
2 P. U. McCarthy and H. U. Güdel, Coord. Chem. Rev., 1988, 88, 69.
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JAF∝{εmax(SF)/εmax(CT)}(∆ECT)2/ECT
is almost constant for different β-diketonates, though the Jobs
values vary considerably from ca. Ϫ9 to Ϫ99 cmϪ1 (Table 3).
That is, the antiferromagnetic interaction is not affected by the
coligands [eqn. (5)].
5 C. Benelli, A. Dei, D. Gatteschi, H. U. Güdel and L. Pardi, Inorg.
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Jobs = JAF ϩ JF
(5)
6 A. Sokolowski, E. Bothe, E. Bill, T. Weyhermuller and K.
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7 A. Caneschi, D. Gatteschi and P. Rey, Prog. Inorg. Chem., 1991, 39,
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Since the Jobs values consist of the antiferromagnetic and
ferromagnetic contribution (JAF ϩ JF) as in eqn. (5), the
variation of the Jobs values is associated with that of the JF
values. In other words, the coligand effect is operative in the
ferromagnetic interaction, but not in the antiferromagnetic
one. Therefore, the magnetic interactions of the bis(β-
diketonato)Cr() complexes mainly vary with the change of
the ferromagnetic coupling of the second term (JF) of eqn. (5),
in contrast to the case of NIT2py Ni() complexes where vari-
ation of the Jobs values is governed through the antifer-
romagnetic coupling of the first term (JAF). The reverse role in
the coligand effect on the Jobs values may be ascribed to the
difference in the respective magnetic orbitals [t2g(dπ) for Cr()
and eg(dσ) for Ni()]. This point will be discussed in more detail
in connection with the magneto-optical properties for two series
of Ni() and Cr() IM2-py complexes.28
8 I. Dasna, S. Golhen, L. Ouahab, O. Pena, J. Guillevic and
M. Fettouhi, J. Chem. Soc., Dalton Trans., 2000, 129.
9 D. Luneau, F. M. Romero, P. Rey, A. Grand, A. Caneschi,
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Chem. Lett., 1997, 603.
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Conclusions
The present spectroscopic studies along with the magnetic
properties can elucidate in some detail the spectral behavior in
2
2
terms of position and intensity of the ligand field E and T1
states of the Cr() complexes which are coupled with the
NIT2py radical ligand. The Jobs values are found to vary with
the JF values, but we are not yet in a position to obtain a cor-
relation between them through the coligand effect. Although
our initial attempts to obtain any correlation between the mag-
netic and the spectroscopic properties through the coligand
effect are as yet unsuccessful, the hypothesis of variable JF
values with varying coligands may be rationalized by con-
sidering the lowest LMCT which generates only a quintet but
not a triplet according to the formulation for the ferromagnetic
interaction in superexchange for dinuclear metal complexes.29
25 J. Ferguson, H. J. Guggenheim and Y. Tanabe, J. Phys. Soc. Jpn.,
1966, 21, 692.
26 O. Kahn, Molecular Magnetism, VCH publisher Inc., 1993, ch. 8.
27 Eqn. (3) in our previous paper11a was oversimplified on the
assumption of ESF Ӷ ECT as presented in refs. 5 and 25. In the case
of the low energy MLCT (ESF < ECT), this should be expressed as the
present formulation given in eqn. (4). However, this change has no
effect on the subsequent discussion in either case of Ni() or Cr()
complexes.
28 Y. Tsukahara, T. Kamatani, A. Iino, T. Suzuki and S. Kaizaki, to be
submitted.
29 (a) J. Glerup, D. J. Hodgson and E. Pedersen, Acta, Chem. Scand.,
Ser. A, 1983, 37, 161; (b) H. Weihe and U. Güdel, Inorg. Chem.,
1997, 36, 3632; (c) H. Weihe, U. Güdel and H. Toftlund, Inorg.
Chem., 2000, 39, 1351.
The quintet LMCT is a π
eg transition to the quintet
excited state |t2g t2g t2gϩegϩπϪπ*ϩ| from the quintet ground
ϩ
ϩ
ϩ
ϩ
state |t2g t2g t2gϩπϪπϩπ*ϩ|. The stabilization of the quintet
level in the ground state or the variation of JF values could
result from a difference in the configurational interaction
(<|egϩ|h|πϩ|>) between the quintet ground and excited states.
Our future goal will be to explore more detailed or quantitative
correlations between JF and the electronic properties of the
coligands as inferred from the relation between Jobs or JF and
the acid dissociation constants of the β-diketones. Another aim
J. Chem. Soc., Dalton Trans., 2002, 181–187
187