Synthesis and characterization of mononuclear and binuclear chromium(III) complexes of α-benzoin oxime

Article abstract of DOI:10.1081/SIM-120003792

Irradiation of a THF solution containing Cr(CO)₆ and α-benzoin oxime (H₂BNO) under various atmospheric conditions, of either argon or vacuum, gave two dichromium(III) complexes of the formulae [Cr(μ-OH)(BNIM)(HBNO)]₂ (1) and [Cr(BNIM)(HBNO)(THF)]₂O (2) where BNIM is the α-benzoin imine radical anion formed as a result of homolytic cleavage of the oxime N-OH bond. When a related reaction of α-benzoin oxime was carried out with Cr(NO)₃ in open atmosphere, Cr(HBNO)₃ (3) was obtained. Variable-temperature magnetic susceptibility measurements show a magnetic moment of 8.12 B.M. for the μ-dihydroxo complex (1) at room temperature. This high magnetic moment value strongly confirms the formulation of a dimer consisting of two benzoin imine radicals (S = 1/2 coordinated to bridged dichromium(III) (S = 3/2) ions. In contrast, the μ-oxo complex (2) shows a magnetic moment of 3.10 B.M. which is even smaller than that observed for the monomeric complex Cr(HBNO)₃ (3) at room temperature. Moreover, IR spectral analysis was very informative in assigning the vibrations due to Cr₂(OH)₂⁴⁺ and Cr₂O⁴⁺ moieties for the complexes (1) and (2), respectively. Thermal gravimetric analysis (TGA) of complex (2) has shown decompositions due to coordinated THF molecules, followed by gradual decomposition of bezoin imine ligands. Interestingly, a low-energy transition in the Visnear-IR region assigned as BNIM → Cr(III) LMCT transition was observed for the [Cr(BNIM)(HBNO)(THF)]₂O complex.

Full text of DOI:10.1081/SIM-120003792

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SYNTHESIS AND CHARACTERIZATION OF MONONUCLEAR
AND BINUCLEAR CHROMIUM(III) COMPLEXES OF α-
BENZOIN OXIME
A. S. Attia a b , S. F. El-Mashtouly a & M. F. El-Shahat a
a Chemistry Department , UAE University , P. O. Box 17551, Al-Ain, UAE
b Chemistry Department , Ain Shams University , Cairo, Egypt
Published online: 15 Feb 2007.
To cite this article: A. S. Attia , S. F. El-Mashtouly & M. F. El-Shahat (2002) SYNTHESIS AND CHARACTERIZATION OF
MONONUCLEAR AND BINUCLEAR CHROMIUM(III) COMPLEXES OF α-BENZOIN OXIME, Synthesis and Reactivity in Inorganic and
Metal-Organic Chemistry, 32:3, 509-527, DOI: 10.1081/SIM-120003792
To link to this article: http://dx.doi.org/10.1081/SIM-120003792
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SYNTH. REACT. INORG. MET.-ORG. CHEM., 32(3), 509–527 (2002)
SYNTHESIS AND CHARACTERIZATION
OF MONONUCLEAR AND BINUCLEAR
CHROMIUM(III) COMPLEXES
OF a-BENZOIN OXIME
A. S. Attia,* S. F. El-Mashtouly, and M. F. El-Shahat
Chemistry Department, Faculty of Science,
UAE University, Al-Ain, P. O. Box 17551, UAE
ABSTRACT
Irradiation of a THF solution containing Cr(CO)6 and
a-benzoin oxime (H2BNO) under various atmospheric con-
ditions, of either argon or vacuum, gave two dichromium(III)
complexes of the formulae [Cr(m-OH)(BNIM)(HBNO)]2 (1)
and [Cr(BNIM)(HBNO)(THF)]2O (2) where BNIM is the
a-benzoin imine radical anion formed as a result of homolytic
cleavage of the oxime N-OH bond. When a related reaction of
a-benzoin oxime was carried out with Cr(NO)3 in open
atmosphere, Cr(HBNO)3 (3) was obtained. Variable-tem-
perature magnetic susceptibility measurements show a mag-
netic moment of 8.12 B.M. for the m-dihydroxo complex (1) at
room temperature. This high magnetic moment value strongly
confirms the formulation of a dimer consisting of two benzoin
imine radicals (S ¼ 1=2 coordinated to bridged dichromiu-
*Corresponding author. Present temporary address: Chemistry Department, Faculty
of Science, Ain Shams University, Cairo, Egypt. E-mail: attia.attia@uaeu.ac.ae
509
Copyright # 2002 by Marcel Dekker, Inc.
www.dekker.com

510
ATTIA, EL-MASHTOULY, AND EL-SHAHAT
m(III) (S ¼ 3=2) ions. In contrast, the m-oxo complex (2)
shows a magnetic moment of 3.10 B.M. which is even smaller
than that observed for the monomeric complex Cr(HBNO)3
(3) at room temperature. Moreover, IR spectral analysis was
very informative in assigning the vibrations due to
4þ
Cr2(OH)2 and Cr2O4þ moieties for the complexes (1) and
(2), respectively. Thermal gravimetric analysis (TGA) of
complex (2) has shown decompositions due to coordinated
THF molecules, followed by gradual decomposition of bezoin
imine ligands. Interestingly, a low-energy transition in the Vis-
near-IR region assigned as BNIM ? Cr(III) LMCT transition
was observed for the [Cr(BNIM)(HBNO)(THF)]2O complex.
INTRODUCTION
Transition metal complexes containing paramagnetic organic ligands,
such as semiquinones1,2, mono-oximes3,4 and diimines577 are of great
interest because of their redox and magnetic properties. The unique mag-
netic properties displayed by these complexes result from antiferromagnetic
or ferromagnetic metal-ligand and=or ligand-ligand exchange interactions,
demonstrating the radical characters of the paramagnetic ligand2. More-
over, dimeric complexes containing paramagnetic metal and ligand centers,
e.g., [Fe(m-OMe)(DBSQ)2]2, (DBSQ ¼ di-tert-butyl-1,2-benzosemiquinone)
show a complicated magnetic behavior that results from the coupling effects
of Fe-SQ, SQ-SQ and Fe-Fe exchange interactions8.
Recently, there has been a growing interest in the coordination chemistry
of oximes with transition metals9713, which stems from their utilization in a wide
range of applications. There are several examples in the chemical literature of the
uses of oximes as analytical reagents14716, and their complexes as models for
biological systems17,18. In addition, an a-benzoin oxime complex of copper(II) is
used in industry for the formation of non-silver photographic images19.
As most of the work on oxime complexes has been performed with
transition metal salts, in this paper we report the first reactions between
a-benzoin oxime (H2BNO) and Cr(CO)6. These reactions result in the
formation of two new dimeric chromium(III) complexes, [Cr(m-OH)
(BNIM)(HBNO)]2 (1) and [Cr(BNIM)(HBNO)(THF)]2O (2) have been
obtained as stable solids. They represent a new addition to the oxime
coordination chemistry of chromium, and to complexes consisting of
a radical ligand (S ¼ 1=2) coordinated to bridged Cr(III) (S ¼ 3=2) metal
ions, and offers an opportunity to study the magnetic behavior that may

CHROMIUM(III) COMPLEXES
511
result from the spin interactions between Cr-BNIM and Cr-Cr centers.
Furthermore, the complexes were designed to contain various bridging
ligands (m-oxo and m-dihydroxo), connecting the two Cr(III)(BNIM) units,
to control the magnitude of magnetic interactions between these units. We
also describe the synthesis and characterization of the monomeric tris-
complex, Cr(HBNO)3. This compound, that does not contain a benzoini-
mine radical, has offered useful information for the structure, electronic and
magnetic characterizations of the dimeric complexes.
EXPERIMENTAL
Materials
Chromium hexacarbonyl, Cr(CO)6, and chromium(III) nitrate,
Cr(NO3)3, and a-benzoin oxime were used as received from Aldrich. Sol-
vents were purified by distillation: tetrahydrofuran from sodium benzo-
phenone; petroleum ethers from sodium hydride; chloroform from P4O10;
ethanol from magnesium ethoxide.
Physical Measurements
Variable-temperature (207298K) magnetic susceptibility data were
obtained with an OxfordFaradaymagnetometer. Diamagnetic corrections were
estimated from Pascal’s constants to obtain the corrected molar paramagnetic
susceptibilities of the compounds (wM )
corr 20,21. The magnetic moments were
calculated by the equation meff ¼ 2.828 (wMcorr.T)1=2. Infrared spectra
(40007400 cmꢀ1) were recordedas KBr pellets on a Unicam Mattson 1000 FTIR
spectrometer. The electronic absorption spectra were recorded by using a model
1601 Shimadzu spectrometer. Samples of 476610ꢀ6 M concentrations in DMF
were measured against the solvent in the reference cell. Elemental analyses were
performed using a Perkin-Elmer 2400 CHN elemental analyzer. Mass spectra
were obtained on a Finnigan MAT SSQ 7000 mass spectrometer. Thermal
gravimetric analyses (TGA) were carried out under nitrogen with a heating rate
of 10 ꢁC=min. using a Perkin-Elmer thermal analyzer.
Complex Syntheses
[Cr(OH)(BNIM)(HBNO)]2 (1)
Cr(CO)6 (0.13 g, 0.59 mmol) and a-benzoin oxime (0.30 g, 1.32 mmol)
were dissolved in 50 mL of degassed tetrahydrofuran under argon. The

512
ATTIA, EL-MASHTOULY, AND EL-SHAHAT
solution was irrꢁadiated for 48 h with an UV-lamp and heated in a paraffin
oil bath at 70 C. The solution which was initially colorless changed to
yellow and finally to brown. During the reaction strong effervescence was
observed. The volume of the brown solution was reduced with a flow of
argon gas. Fine, dark brown crystals of the complex were obtained in 0.14 g
(48%) yield. Anal. Calcd for C56H48N4O8Cr2 (MW 1009.01): C, 66.66; H,
4.79; N, 5.55. Found: C, 66.31; H, 4.54; N, 5.58. Mass spec. M=z (pþ),
ꢁ
1012. M. p. decomp >160 C.
[Cr(BNIM)(HBNO)(THF)]2O (2)
a-Benzoin oxime (0.20 g, 0.88 mmol) and Cr(CO)6 (0.12 g, 0.54 mmol)
were mixed in 50 mL tetrahydrofuran. The mixture was irradiated with
an UV-lamp and heated in a paraffin oil bath at 70 ꢁC under reduced
pressure for 3 days. Evaporation of the solvent resulted in a yellow-brown
solid. Fine crystals of [Cr(BNIM)(HBNO)(THF)]2O were obtained by
crystallization from CHCl3. The crystals were washed several times with
petroleum ether (bp. 40760 ꢁC). The product was obtained in 0.17 g (67%)
yield. Anal. Calcd for C64H62N4O9Cr2 (MW 1135.2): C, 67.71; H, 5.50; N,
4.93. Found: C, 67.55; H, 5.39; N, 4.60. Mass spec. M=z (pþ), 1136. M. p.
decomp > 80 ꢁC.
Cr(HBNO)3 (3)
A mixture of Cr(NO)3ꢂ9H2O (0.50 g, 1.64 mmol) and a-benzoin oxime
(1.00 g, 4.4 mmol) was dissolved in 100 ml of THF-EtOH (3:1). The solution
was heated at 70 ꢁC for 2 h, then the volume of the resulting green solution
was reduced. The green solid obtained was washed several times with hot
petroleum ether (bp. 60780 ꢁC) and then recrystalized from ethanol. The
green crystals of Cr(HBNO)3 were obtained in 0.86 g (80%) yield. The
complex was left to dry in vacuo for 2 h. Anal. Calcd for C42H36N3O6Cr
(MW 730.8): C, 69.03; H, 4.96; N, 5.75. Found: C, 68.57; H, 5.14; N, 5.60.
þ
ꢁ
Mass spec. M=z (p ), 728. M. p. 187 C.
RESULTS AND DISCUSSION
Reactions of oximes with Ni(II) have been used to study the oxidative
addition to Ni(II) metal ions22. Recently, Hank et al.11 have shown that a
stoichiometric reaction of a-benzoin oxime with Ni(II) results in the

CHROMIUM(III) COMPLEXES
513
formation of a stable Ni(IV)(OH)2(BNIM)2 complex. In addition, the
coordinated OH is formed by the cleavage of the oxime N-OH.
In the present study under various atmospheric conditions two dif-
ferent complexes were prepared from the reaction of a-benzoin oxime and
Cr(CO)6. Irradiation of a solution containing chromium hexacarbonyl and
a-benzoin oxime (H2BNO) under an inert atmosphere of argon, yielded the
m-dihydroxochromium(III) dimer, [Cr(III)(OH)(BNIM)(HBNO)]2 (1).
While changing the reaction condition to reduced atmospheric pressure (i.e.,
under vacuum) resulted in the formation of the m-oxo dimer, [Cr(III)(B-
NIM)(HBNO)THF)]2O (2) (eq. 1). The tris-derivative, Cr(III)(HBNO)3,
was prepared to aid in elucidating the structures of the other complexes by
comparing their spectroscopic data. The tris-derivative was prepared by
reacting a-benzoin oxime with Cr(NO)3 in THF (eq. 2).
Infrared Spectra
Generally, oximes23,24 are characterized by three IR absorption bands
at 330073150 cmꢀ1 (OH str.), 169071620 cmꢀ1 (C¼N str.), and ꢃ 950 cmꢀ1
(N-O str.). In the present study the a-benzoin oxime ligand shows these
bands at 3326, 1670 and 990 cmꢀ1 due to OH, C¼N and N-O stretching
modes, respectively.
The infrared spectrum of the tris-complex, Cr(HBNO)3 (3), reveals a
strong-intensity band at 1549 cmꢀ1 due to the C¼N stretching vibration.
This band has shifted by 121 cmꢀ1 to lower frequency from the C¼N
stretching frequency of the parent ligand. This significant lowering in the
stretching frequency evidently indicates the involvement of the oxime C¼N
in coordination. Such behavior was previously observed for chromium
and copper complexes of oxime ligands4,25. In contrast, the bands

514
ATTIA, EL-MASHTOULY, AND EL-SHAHAT
corresponding to OH and N-O stretches, observed at 3360 and 1028 cmꢀ1
,
respectively, are slightly shifted to higher frequencies with respect to those
of the a-benzoin oxime ligand. Furthermore,ꢀthe absence of vibrations
characteristic of free or coordinated26 NO3 exclude them from the
structure of the complex. On the basis of the above IR spectral studies,
which are further supported by elemental analyses and mass spectroscopic
data, the structure given in Fig. 1 is suggested for the tris-chromium(III)
complex.
The IR spectral data for the two dimeric complexes (1) and (2), pre-
sented in Table I, are drastically different from that observed for the tris-
complex (3). It is interesting that both complexes, (1) and (2), show two
Figure 1. Suggested structure for the tris-complex, Cr(III)(HBNO)3.

CHROMIUM(III) COMPLEXES
515

516
ATTIA, EL-MASHTOULY, AND EL-SHAHAT
bands in the characteristic range for the C¼N stretching frequency. These
bands are observed for complex (1) at 1535 and 1447 cmꢀ1 and for complex
(2) at 1544 and 1445 cmꢀ1. The first C¼N band for both complexes is
comparable to that observed for the tris-complex (3) at 1549 cmꢀ1 while the
second one is absent in the spectrum of both the tris-complex (3) and the
free ligand and is assigned to the coordinated C¼N stretching mode.
Moreover, complex (1) displays a strong absorption band at 955 cmꢀ1 which
is unique among the spectra of the three chromium complexes. This band is
assigned to a vibration mode associated with the Cr2(OH)24þ structural unit
and suggests a dimeric complex in which the two chromium(III) metal ions
are bridged by two OH groups. Our interpretion of the dihydroxo dimeric
structure derives additional support from the fact that the infrared bands of
4þ
4þ
[Cr(pic)2OH]2 and [Fe(pic)OH]2 due to Cr2(OH)2 and Fe2(OH)2 are
observed at 970 and 950 cmꢀ1, respectively27. On the other hand, the IR
spectrum for complex (2) shows a strong absorption at 735 cmꢀ1 which may
be assigned to nasym of the Cr-O-Cr moiety and compares very well with that
reported for several oxo-bridged complexes28730 in the range 7307745 cmꢀ1
.
Vibrations characteristic of coordinated THF are observed for complex (2)
at 2957 and 2872 cmꢀ1 (C-H str.) and 1119 cmꢀ1 (vibration associated with
C-O-C moiety). Consistent with this assignment (which is further supported
by TGA), coordinated THF was reported31733 to show bands at 2950, 2860
and 1118 cmꢀ1
.
It must be recalled that the second band due to C¼N observed for
both complexes (1) and (2) is assigned to C¼N of an imine radical formed as
a result of homolytic cleavage of the oxime N-OH bond by photolysis
(Fig. 3). This interpretation is confirmed by magnetic studies of the com-
plexes (vide infra) and gains further support from the IR data reported for
chromium and molybdenum complexes containing oximate radicals3,4
.
These results, therefore, seem in conformity with the structures given in
Fig. 2.
Magnetism
Magnetic exchange between paramagnetic metal ions and radical
ligands of monomeric, dimeric and polymeric complexes has been a subject
of several of our studies8,33736. Variable-temperature (207298K) magnetic
susceptibility data were collected for complexes (1) and (2) as solids. Plots of
the effective magnetic moment per molecule (meff=molecule) vs. temperature
are given in Fig. 4. At room temperature the magnetic moment for complex
(1), [Cr(m-OH)(BNIM)(HBNO)]2, is 8.12 B.M. If the complex had two
Cr(III) metal ions as the only source for the measured magnetic moment,

CHROMIUM(III) COMPLEXES
517
Figure 2. Suggested structures of [Cr(III)(OH)(BNIM)(HBNO)]2 (1) and
[Cr(III)(BNIM)(HBNO)THF)]2O (2) complexes.

518
ATTIA, EL-MASHTOULY, AND EL-SHAHAT
Figure 3. Suggested mechanism for the formation of [Cr(OH)(BNIM)(HBNO)]2 (1)
and [Cr(BNIM)(HBNO)(THF)]2O (2) complexes.

CHROMIUM(III) COMPLEXES
519
Figure 4. Plots of effective magnetic moments per molecules for solid samples of
[Cr(OH)(BNIM)(HBNO)] (1) (j) and [Cr(BNIM)(HBNO)(THF)] O (2) ( ) com-
plexes as a function of temperature.
ꢄ
2
2
spin interactions between the two Cr(III) centers (Scr ¼ 3=2) would result in
S ¼ 0 as a ground-spin state and S ¼ 1, 2 and 3 excited-spin states37. It would
be expected for such dimer to have a maximum magnetic moment of 6.93 mB.
However, the high meff value observed for complex (1) may be explained if
one views this compound in terms of two chromium(III), SCr ¼ 3=2, metal
ions interacting with two benzoin imine (BNIM), S ¼ 1=2, radicals. An S ¼ 0
ground state and S ¼ 1, 2, 3 and 4 excited states would be expected. In case
of a negligible exchange interaction, there would be thermal population of
these excited states, and the spin-only value of meff=molecule would be 8.94
B.M., whereas, a strong exchange interaction would lead to a thermal
population in the ground state S ¼ 0. Now it is obvious that the magnetic
moment for complex (1) shows a very weak interaction behavior, which is
temperature dependent, with the magnetic moment per dimer dropping

520
ATTIA, EL-MASHTOULY, AND EL-SHAHAT
from a value of 8.12 B.M. at 298K to 3.09 B.M. at 20K. The decrease of the
effective magnetic moment with temperature was observed for the
[Fe(DBSQ)(DBCAT)(dmbpy)] complex38. This compound showed a weak
antiferromagnetic coupling between Fe(III) metal ion (S ¼ 5=2) and the
semiquinone ligand (S ¼ 1=2). In contrast, complex (2) shows strong intra-
molecular exchange interactions that include Cr(III)-BNIM as well as
Cr(III)-Cr(III) coupling through the bridging oxygen. A magnetic moment
per dimer of 3.10 B.M. at 297.5K was observed which slightly drops to 1.53
B.M. at 20.5K. It is evident that the discrepancy of the magnetic behavior of
the two dimers is attributed to the difference in bridging ligands connecting
the two chromium metals8,39,40. In addition, mono-oxo-bridged metal ion
dimers exhibit a shorter M-O-M bond distances and a greater degree of
linearity, therefore, they exhibit a greater degree of exchange39. Such mag-
netic behavior compares very well with that reported41,32,30 for
[Cr(phen)2OH]24þ, [Fe(m-OMe)(3,6-DBSQ)2]2, and [Fe(salen)2]2O. Unfor-
tunately, the EPR spectra for the two chromium dimers in the solid state at
room temperature are relatively uninformative. Broadened spectra were
obtained with no hyperfine coupling resolved. The isotropic g values (giso
)
for complexes (1) and (2) were found to be 1.969 and 1.954, respectively,
with a peak-to-peak width of ꢃ 230 G.
The effective magnetic moment per molecule (meff=molecule) recorded
at room temperature for the tris-complex, Cr(HBNO)3, is 3.64 B.M. This
value is quite close to the spin-only value of a d3 metal ion (S ¼ 3=2)42 and
fits best to the formulation of a complex containing chromium metal ion in
the 3þ oxidation state.
Thermal Analysis
Thermal analysis (TGA) carried out on a solid sample of complex (2)
at a heating rate of 10 ꢁC=min. over a temperature range from room tem-
perature to 800 ꢁC is characterized by four decomposition steps (Table II).
The first step commences at 73 ꢁC, showing a slow decomposition up to
150 ꢁC. In this stage the complex mainly loses one of the THF molecules
(Table IV). This is followed by an abrupt loss in weight up to 350 ꢁC with
the elimination of the second THF molecule, a BNIM ligand, and a phenyl
group of the other BNIM ligand. In the third stage, which covers a tem-
perature range of 3507533 ꢁC, decomposition of the remaining BNIM
ligand takes place, in addition to the elimination of a phenyl group from the
HBNO ligand. The fourth stage covers the temperature range of 5337750 ꢁC
with the elimination of a CO molecule.

CHROMIUM(III) COMPLEXES
521
Table II. Percentage Weight Losses and Corresponding Detached Species
Encountered Upon Heating of the [Cr(BNIM)(HBNO)(THF)]2O (2) Complex
%Weight
Loss
Temp.
(ꢁC)
Species
decomposed
Step
Fragment
Mol. Wt
Calc. Found
First
Second
150
350
C60H52N4O8Cr2 1061.05 THF 2H
6.35
37.18 37.56
6.24
C36H24N3O4Cr2
666.58 THF BNIM
2 OH, 2 H C6H5
458.35 2 C6H5O-C-C¼N 31.23 30.93
430.34 CO 6.10 5.75
Third
Fourth
533
750
C22H14N2O3Cr2
C21H14N2O2Cr2
Table III. The Electronic Absorption Data for a-Benzoin Oxime and Its
Chromium Complexes Recorded in DMF
Compound
l, nm (e, Mꢀ1 cmꢀ1
)
H2BNO
Cr(HBNO)3
324 (11090), 370a (6073)
313 (13200), 558 (2547)
[Cr(BNIM)(HBNO)(THF)]2O
[Cr(m-OH)(BNIM)(HBNO)]2
310 (18452), 573 (2656), 992 (542)
319 (12041), 568 (2493)
aShoulder.
Table IV. UV-Vis Spectroscopic Data of the Investigated Complexes
Complex
UV-Vis lmax (e L molꢀ1 cmꢀ1 a
)
(1) [Re(TTA)(H2O)4]Cl2
(2) [Re(TTA)2(H2O)2]Cl
(3) [ReO(TTA)(H2O)2]Cl2
(4) [ReO(TTA)2]Cl
(5) [In(TTA)(H2O)4]SO4
(6) [In(TTA)(HOTCA)2]
(7) [In(TTA)(HTZT)2]
(8) [ReO(TTA)(HOTCA)[Cl
212 (250)
212 (410)
212 (410)
213 (380)
264 (710)
205 (320)
202 (470)
203 (390)
264 (510)
265 (820)
260 (810)
267 (720)
336 (950)
225 (700)
263 (350)
263 (710)
336 (510)
335 (800)
336 (950)
33 (950)
340 (300)
337 (220)
330 (900)
ae are in the range of 1027103 for six-coordinate complexes of low symmetry (spin-
allowed, Laporte-forbidden).

522
ATTIA, EL-MASHTOULY, AND EL-SHAHAT
Electronic Spectra
Electronic absorption spectra recorded for the three chromium com-
plexes in DMF solution over the range of 19071100 nm are summarized in
Table III. The resemblance of the spectra of the three complexes in the
1907800 nm range reinforces the similarity in charge distribution for these
complexes. The UV-Vis region is characterized by two absorptions dis-
played by all three complexes. The intense absorption at the shorter wave
length is assigned to a p-p* transition4,43,44, while the absorption at the
Figure 5. Electronic absorption spectra of Cr(HBNO)3 (a), [Cr(BNIM)(HB-
NO)(THF)2O (b) and [Cr(m-OH)(BNIM)(HBNO)]2 (c) in DMF solution.

CHROMIUM(III) COMPLEXES
523
longer wave length could be due to a chromium(III) 4A2 ? 4T2 transition45,
and=or n-p* transition, which commonly displays a bathochromic shift
upon complexation4. The other higher-energy d-d transitions characteristic
for Cr(III) centers are obscured by the intense p-p* transition and this is
usually the case for complexes when the ligand associated with the metal
contains aromatic rings46. The dominant feature of the electronic absorption
spectrum for the [Cr(BNIM)(HBNO)(THF)]2O (2) complex, shown in Fig. 5,
is a broad, low-energy band at 992 nm (e ¼ 542 Mꢀ1 cmꢀ1). This band is
most likely assignable to charge transfer associated with the central chro-
mium metal ion and the chelate rings of the coordinated benzoin imines.
Surprisingly, this ligand-to-metal charge transfer transition is not observed
for the [Cr(OH)(BNIM)(HBNO)]2 (1) complex. This may be attributed to
the instability of the complex in DMF solution since it decomposes within a
few minutes. The low solubility or the decomposition in most of the solvents
limits solution studies of these complexes.
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1. Pierpont, C.G.; Buchanan, R.M. Transition Metal Complexes of
o-Benzoquinone, o-Semiquinone and Catecholate Ligands. Coord.
Chem. Rev. 1981, 38, 45.
2. Pierpont, C.G.; Lange, C.W. The Chemistry of Transition Metal
Complexes Containing Catechol and Semiquinone Ligands. Coord.
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46. Mathoniere, C.; Kahn, O.; Daran, J.C.; Hilbig, H.; Kohler, F.H.
Complementarity and Internal Consistency between Magnetic and
Optical Properties for the MnIICuII Heterodinuclear Compound
[Mn(Me6-[14]ane-N4)(Cu(oxpn)](CF3SO3)2 (Me6-[14]ane-N4 ¼ 5,7,7,12,14,-
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Received November 16, 2000
Accepted December 13, 2001
Referee I: C. G. Pierpoint
Referee II: C. A. L. Mahaffy