Synthesis and characterization of oxo-bridged, trinuclear, six-coordinate mixed-carboxylato complexes of chromium (III)

Article abstract of DOI:10.1016/j.molstruc.2005.03.050

Mixed-carboxylato β-diketonato complexes of chromium (III) (R=C ₁₃H₂₇, C₁₅H₃₁ or C ₁₇H₃₅ and L=CH₃OH) [Cr₃O(OOCR) ₃(dike)₃L₃]⁺ have been synthesized by enforced substitution reactions of [Cr₃O(OOCCH₃) ₇(H₂O)] first with straight chain fatty acids (myristic, palmitic or stearic acid) and then with β-diketones (Hβ-dike) like acetylacetone(Hacac) or benzoylacetone(Hbzac) in toluene under reflux. These are new type of oxo-bridged chromium(III) complexes in which one ligand is a fatty acid while the other one is a β-diketone. The complexes were characterized by elemental analyses, spectral (infrared, electronic, FAB mass and powder XRD) studies, molar conductance and magnetic susceptibility measurements. Bridging coordination modes for both carboxylate and β-diketonate anions were indicated by presence of ν_asym(Cr₃O) vibrations in the infrared spectra. Trinuclear nature of the complexes and their structural features have been discussed on the basis of physicochemical studies.

Full text of DOI:10.1016/j.molstruc.2005.03.050

Journal of Molecular Structure 750 (2005) 72–77
www.elsevier.com/locate/molstruc
Synthesis and characterization of oxo-bridged, trinuclear,
six-coordinate mixed-carboxylato complexes of chromium (III)
*
B.P. Baranwal , Talat Fatma
Department of Chemistry, Coordination Chemistry, DDU Gorakhpur University, Gorakhpur 273 009, India
Received 13 December 2004; revised 18 March 2005; accepted 21 March 2005
Available online 15 June 2005
Abstract
Mixed-carboxylato b-diketonato complexes of chromium (III) (RZC13H27, C15H31 or C17H35 and LZCH3OH) [Cr3O(OOCR)3(dike)3L3]C
have been synthesized by enforced substitution reactions of [Cr3O(OOCCH3)7(H2O)] first with straight chain fatty acids (myristic, palmitic
or stearic acid) and then with b-diketones (Hb-dike) like acetylacetone(Hacac) or benzoylacetone(Hbzac) in toluene under reflux. These are
new type of oxo-bridged chromium(III) complexes in which one ligand is a fatty acid while the other one is a b-diketone. The complexes
were characterized by elemental analyses, spectral (infrared, electronic, FAB mass and powder XRD) studies, molar conductance and
magnetic susceptibility measurements. Bridging coordination modes for both carboxylate and b-diketonate anions were indicated by
presence of nasym(Cr3O) vibrations in the infrared spectra. Trinuclear nature of the complexes and their structural features have been
discussed on the basis of physicochemical studies.
q 2005 Elsevier B.V. All rights reserved.
Keywords: Carboxylates; Chromium (III); Oxo-bridged; Mixed-ligand complexes; b-diketonates
1. Introduction
properties. Compound [Cr3O(OOCC2H5)6 (H2O)3](NO3) is
a functional biomimetic [9] for the low-molecular-weight
chromium-binding substance (LMWCr) which has been
identified as the biologically active form of chromium in
mammals, activating the kinase activity of insulin receptor
in the presence of insulin [10]. Both LMWCr and its
biomimetic complex have been proposed as possible
nutritional supplements and therapeutics for adult-onset
diabetes [11]. Recently few groups [12–15] made the break
through to large chromium (III) carboxylate clusters, the
number of species remains small. The synthesis of
chromium (III) carboxylate complexes could be useful as
models [10,16] of the Cr-containing protein LMWCr, which
is an anionic, carboxylate-rich polypeptide possessing a
molecular weight of ca.1500 and binding four chromic ions
[10]; charge balance and other considerations suggest that
LMWCr possesses an anion-bridged, multinuclear Cr-
carboxylate assembly.
Transition-metal carboxylate chemistry has played a key
role in the conceptual development of modern inorganic
chemistry. Extensive structural and physicochemical studies
of these compounds have been crucial for increasing our
understanding of bonding and electronic interactions
between proximate metal centers, topics with implications
ranging from industrial catalysis and industrial magnetism
to metalloprotein structure and function [1–4]. The current
interest in the trinuclear, oxo-centered metal carboxylate
assemblies of the general composition [M3O(OOCR)6L3]C
(where MZtrivalent 3-d metal, LZmonodentate ligand) is
due to these complexes have served as important models to
test theories of magnetic and electronic coupling between
metal ions [5,6], these are valuable precursors for the
synthesis of higher nuclearity clusters exhibiting interesting
magnetic properties [7,8]. In the particular case of
chromium(III), oxo-complexes have exciting biological
This paper is devoted to the new oxo-bridged, trinuclear
complexes [Cr3O(dike)3(OOCR)3] (OOCCH3)$3CH3OH
which are proving invaluable in our attempts to synthesize
polynuclear chromium(III) carboxylate clusters with inter-
esting properties. Herein, we report the one-pot synthesis
following substitutions of acetate ions from m3-oxo-(aqua)
*
Corresponding author. Tel.: C91 551 2338445.
E-mail address: drbpbaranwal@yahoo.com (B.P. Baranwal).
0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2005.03.050

B.P. Baranwal, T. Fatma / Journal of Molecular Structure 750 (2005) 72–77
73
heptakis(acetato)chromium(III) and structural characteriz-
ation by magnetic, spectroscopic and powder XRD
techniques.
2.3. Synthesis of [Cr3O(acac)3(OOCC15H31)3]-
(OOCCH3)$3CH3OH
A toluene solution of palmitic acid (5.39 g; 20.94 mmol)
was slowly added with constant stirring to the toluene
suspension of [Cr3O(OOCCH3)7(H2O)] (4.21 g; 6.98 mmol).
The reaction mixture was refluxed for 12 h with slow and
continuous azeotropic fractionation of liberated water and
acetic acid and with toluene. The product thus obtained was
solublein toluene. After removingthe excesssolvent invacuo,
a dark green solid [Cr3O(OOCCH3)4(OOCC15H31)3] (1) was
obtained. A similar procedure was adopted to synthesize other
fatty acid derivatives with formula [Cr3O(OOCCH3)4-
(OOCR)3] [19].
2. Experimental
2.1. Materials and analytical methods
Organic solvents (Qualigens) were dried and distilled
before use by standard methods. The carboxylic acids were
used after distillation under reduced pressure (mp of myristic
acid: 53 8C; palmitic acid: 63 8C and stearic acid: 70 8C). b-
diketones (Aldrich) were distilled. Chromium(III) acetate
hydroxide, [Cr3(OH)2(OOCCH3)7], Aldrich, was used as
received. Chromium was determined gravimetrically as lead
chromate [17]. The liberated acetic acid in the collected
azeotrope was determined by titration with standard sodium
hydroxide solution using phenolphthalein as indicator.
A toluene solution of 1 was precipitated by adding three
times excess volume of methanol. The supernatant liquid
was decanted carefully and the precipitate was dried in
vacuo to yield complex having formula [Cr3O(OOCCH3)4
(OOCC15H31)3 (CH3OH)3]. To a toluene solution of this
complex (3.15 g; 2.48 mmol) acetylacetone (0.76 g;
7.59 mmol) was added. The contents were refluxed for
12–15 h with slow and continuous fractionation of acetic
acid-toluene-methanol azeotrope (bp 103–107 8C). Excess
solvent was removed in vacuo to yield a green solid, soluble
in benzene and insoluble in methanol. The product was
reprecipitated from its benzene solution by adding
three times excess methanol. The precipitate was finally
dried in vacuo (1 mm, 40 8C, 3 h). The analytical
results (Table 1) indicated this complex was [Cr3O(acac)3-
(OOCC15H31)3](OOCCH3)$3CH3OH. A few more com-
plexes were synthesized following the same experimental
procedure and the analytical results are given in Table 1.
2.2. Physicochemical measurements
FAB mass data were obtained on a JEOL SX 102/DA-
6000 mass spectrometer using m-nitrobenzyl alcohol (NBA)
as a matrix at the CDRI, Lucknow, India. Powder X-ray
diffraction data were collected on a PW 1717 from Jammu
University, Jammu, India. Electrical conductance was
measured on a century CC-601 digital conductivity meter
with a dip type cell using 10K2–10K4 molar solutions in
nitrobenzene. The other physicochemical studies were
carried out in the similar manner as reported by us earlier
[18].
Table 1
Analytical results for the chromium (III) complexes
Reactantsa (g; mmol)
Productb (colour), (%yield)
Found (Calculated)
CH3COOHc(g)
Cr
C
H
[Cr3O(OOCCH3)3(OOCC13H27)3]-
(OOCCH3)$3CH3OH (2.87; 2.42)C
Hacac(0.73; 7.29)
[Cr3O(acac)3(OOCC13H27)3]-
(OOCCH3)$3CH3OH (Green)
(81)
0.40(0.44)
11.90(11.94)
57.08(57.01)
9.01(9.03)
[Cr3O(OOCCH3) (OOCC13H27)3]-
3
[Cr3O(bzac)3(OOCC13H27)3]-
(OOCCH3)$3CH3OH (Dark
green) (79)
0.38(0.44)
0.48(0.45)
0.40(0.36)
0.36(0.40)
0.36(0.38)
10.40(10.45)
11.24(11.22)
9.83(9.89)
61.95(61.96)
58.70(58.72)
62.20(63.22)
60.20(60.26)
64.26(64.35)
8.34(8.30)
9.32(9.35)
8.60(8.63)
9.62(9.64)
8.90(8.92)
(OOCCH3)$3CH3OH (2.91; 2.45)C
Hbzac(1.17; 7.21)
[Cr3O(OOCCH3)3(OOCC15H31)3]-
(OOCCH3)$3CH3OH (3.15; 2.48)C
Hacac(0.76; 7.59)
[Cr3O(acac)3(OOCC15H31)3]-
(OOCCH3)$3CH3OH (Bluish
green) (73)
[Cr3O(OOCCH3)3(OOCC15H31)3]-
(OOCCH3)$3CH3OH(2.55; 2.01)C
Hbzac(0.99; 6.10)
[Cr3O(bzac)3(OOCC15H31)3]-
(OOCCH3)$3CH3OH (Dark
green) (84)
[Cr3O(OOCCH3)3(OOCC17H35)3]-
(OOCCH3)$3CH3OH (3.01; 2.22)C
Hacac(0.65; 6.49)
[Cr3O(acac)3(OOCC17H35)3]-
(OOCCH3)$3CH3OH (Green)
(70)
10.54(10.58)
9.35(9.39)
[Cr3O(OOCCH3)3(OOCC17H35)3]-
(OOCCH3)$3CH3OH (2.89; 2.13)C
Hbzac(1.06; 6.54)
[Cr3O(bzac)3(OOCC17H35)3]-
(OOCCH3)$3CH3OH (Dark
green) (77)
a
Refluxing time 12–15.
b
Precipitated product in benzene–methanol (1:3).
In azeotrope.
c

74
B.P. Baranwal, T. Fatma / Journal of Molecular Structure 750 (2005) 72–77
3. Results and discussion
O–H group. The bands observed at w460 cmK1 may be due
to Cr–O vibrations [21].
Mixed carboxylato complexes of chromium(III) were
synthesized by the reaction of m3-oxo-(aqua)heptakis(ace-
tato)chromium(III) with carboxylic acids in toluene under
reflux.
3.2. Electronic spectra and magnetic moments
Electronic spectra of the complexes were recorded in the
range 30,000–10,000 cmK1 in benzene solution as well as
nujol mulls and were found quite similar to one another. The
spectra exhibited two spin-allowed bands (Fig. 1; Table 2)
in the regions 16,900G150 and 23,400G400 cmK1 which
could be assigned to the transitions from 4A2g(F) to 4T2g (F)
½Cr3OðOOCCH3Þ7ðH2Oފ C3RCOOH
/½Cr3OðOOCCH3Þ4ðOOCRÞ3Š CH2O[C3CH3COOH[
Where RZC13H27, C15H31 or C17H35.
Acetate ions of these products were further substituted
with b-diketonate anions in refluxing toluene.
4
(n1) and T1g (F) (n2), respectively. The spectra may be
interpreted on the basis of an octahedral environment for
chromium (III) in these complexes [23]. The position of the
third spin-allowed transition n3[4A2g (F)/4T1 g (P)] could
be calculated along with the ligand field splitting energy
(10Dq), interelectronic repulsion parameter (B) and
covalency factor (b) by the equations of Underhill and
Billing [24]:
½Cr3OðOOCCH3Þ4ðOOCRÞ3Š C3H bdike/½Cr3OðdikeÞ3
!ðOOCRÞ3ŠðOOCCH3Þ C3CH3COOH[
Where dikeZacac or bzac.
Reprecipitation of these products in benzene–methanol
mixture (1:4 ratio) yielded the complexes [Cr3O(dike)3
(OOCR)3](OOCCH3)$3CH3OH.
340Dq2 K18ðn2 Cn3ÞDq Cn2n3 Z 0
B Z ðn3 Cn2 K30DqÞ=15
(1)
(2)
(3)
The use of toluene as a solvent was a very suitable
choice in these substitutions because it pushes the
reactions in the forward direction after removal of acetic
acid in the form of azeotrope, which could be fractionated
out at lower temperature (below 110 8C, bp of toluene).
All the (carboxylato)(b-diketonato) complexes were sol-
uble in organic solvents like toluene, benzene or
nitrobenzene. Determination of molar conductances of
all the complexes in nitrobenzene at 10K2–10K4 molar
b Z Bcomplex=Bfree ion
:
The values of interelectronic repulsion parameter (B) are
found to be below the free ion value for chromium(III) ion
(933 cmK1) [23] which indicate a considerable covalent
nature of the metal-ligand bond in all of the complexes with
the covalency factor (b) varying in the range 0.71G0.06.
The magnetic moment values for the complexes were
measured at the room temperature which lie in the range
3.39G0.04 BM. (Table 2). The chromic ions of trinuclear
oxo-centred chromium(III) (SZC3/2) carboxylate assem-
blies are antiferromagnetically coupled, resulting in an
SZC1/2 ground state [5]. Consequently the room
temperature magnetic moment of these complexes
concentrations (in the range 32-41 ohmK1 cm2 molK1
)
indicated them to be 1:1 electrolytes [20]. The complexes
do not have sharp melting points and all of them were
decomposed at around 400 8C temperature.
3.1. Infrared spectra
In infrared spectra of all the complexes, no band was
observed in the region 3600–3300 cmK1. This indicated the
absence of O–H stretching vibrations of carboxylic acids and
b-diketones. The band at 1710 cmK1 (CO stretching) in these
ligands and at 935 cmK1 (OH deformation offree carboxylic
acids) were also found absent in spectra of the complexes in
which two strong bands were observed at w1610 cmK1
(nasym OCO) and at w1430 cmK1 (nsymOCO) stretching
vibrations. The coordination mode of the carboxylate and
b-diketonate ligands can be assigned on the basis of the
difference (DnZw180 cmK1) of these two frequencies [21],
which indicated the presence of bridging mode of coordi-
nation of both the ligands in the complexes. A medium band
observed at w680 cmK1 could be assigned to d(OCO) of
bridging carboxylate anion [22]. An absorption near
545 cmK1 could be assigned to asymmetric stretching of
the Cr3O core [21,22]. The lowering of n(O–H) frequencies
compared to be in free methanol indicated the coordination of
Fig.
1.
Room-temperature
electronic
spectrum
of
[Cr3O(acac)3(OOCC13H27)3](OOCCH3)$3CH3OH.

B.P. Baranwal, T. Fatma / Journal of Molecular Structure 750 (2005) 72–77
75
Table 2
Electronic spectral data and magnetic moment values for Cr(III) complexes
4
Complex
Observed transition (in cmK1) from A2g(F) to
B (in cmK1
)
b
meff (BM)
4T2g(F) (n1 10Dq)
4T1g(F) (n2)
4T1g(P) (n3)
[Cr3O(acac)3(OOCR0)3](OOCR)$3ROH
[Cr3O(bzac)3(OOCR0)3](OOCR)$3ROH
[Cr3O(acac)3(OOCR00)3](OOCR)$3ROH
[Cr3O(bzac)3(OOCR00)3](OOCR)$3ROH
[Cr3O(acac)3(OOCR00)3](OOCR)$3ROH
[Cr3O(bzac)3(OOCR00)3](OOCR)$3ROH
16,750
16,775
16,900
16,990
16,830
17,015
23,800
23,470
23,800
23,390
23,650
23,180
37,190
36,490
37,330
37,560
37,460
37,110
716
672
695
665
708
616
0.77
0.72
0.75
0.71
0.76
0.66
3.41
3.36
3.39
3.43
3.40
3.35
R, CH3; R0, C13H27; R00, C15H31 and R00, C17H35
.
Fig. 2. FAB mass spectrum of [Cr3O(acac)3(OOCC15H31)3](OOCCH)3$3CH3OH.
(TZ290–298 K) are generally in the range 3.1–3.5 BM.
The obtained values of magnetic moments fall within this
range. These are lower than the calculated mS.O. value for
Cr3C (d3 system, 3.87 BM.) which could be interpreted
due to the presence of antiferromagnetic interaction
providing p-pathways through O-atom. This is common
to the trinuclear carboxylates of chromium (III) contain-
ing a central m3-O [5]. For one product [Cr3O(acac)3
(OOCC17H35)3](OOCCH3)$3CH3OH, the magnetic sus-
ceptibility was determined in the temperature range
TZ100–300 K; the plot of T vs. 1/XM (cor.) was found
to be a straight line with qZ0 K in accordance with
Curie Law.
chromium assemblies. The spectrum indicated the presence
of a trinuclear unit (m/z; 1329), weaker peaks at 1297 and
1233 showed the fragmentation of one residual methanol
and one acetate ion (Table 3). The peaks at lower values of
m/z correspond to the loss of one carboxylate anion at each
interval. A peak observed at 471 in the spectrum showed the
presence of [Cr3O(acac)3]4C unit (calculated m/z; 469).
Other peaks at lower side revealed a distinctive set of ions
Table 3
FAB mass data of [Cr3O(acac)3(OOCC15H31)3] (OOCCH3)$3CH3OH
Peak position Expected fragmentation species
Calcd mol. wt
1327
1329
1297
[Cr3O(acac)3 (OOCC15H31)3]
(OOCCH3)$CH3OH
[Cr3O(acac)3 (OOCC15H31)3]
3.3. FAB mass, powder XRD and structural elucidation
1295
(OOCCH3)
1233
982
729
471
[Cr3O(acac)3 (OOCC15H31)3]C
[Cr3O(acac)3 (OOCC15H31)2]2C
[Cr3O(acac)3 (OOCC15H31)]3C
[Cr3O(acac)3]4C
1236
980
725
469
The FAB mass spectral pattern of the representative
complex [Cr3O(acac)3 (OOCC15H31)3](OOCCH3)$
3CH3OH is given in Fig. 2. It is interpreted on the basis
of an earlier report [25] on oxo-bridged multinuclear

76
B.P. Baranwal, T. Fatma / Journal of Molecular Structure 750 (2005) 72–77
Fig. 3. XRD spectrum of [Cr3O(acac)3(OOCC13H27)3](OOCCH3)$3CH3OH.
with masses corresponding to the cation and fragments of
the parent derived by loss of acetylacetonate ligand [26].
The X-ray powder diffraction spectrum for a representa-
tive sample [Cr3O(acac)3(OOCC13H27)3](OOCCH3)$
3CH3OH is given in Fig. 3 and the peaks have been indexed
by the crystal system, with lattice parameters (d, 2q, height
and area) indicated the pattern of the sample which matched
well with those of other symmetric trinuclear chromiu-
m(III)oxo carboxylate assembly, reported recently [6,9,15].
The peak widths were found to be higher than those
introduced by instrumental broadening of XRD data. On the
basis of the findings from the spectral, magnetic and
conductivity studies on the complexes, their plausible
structure is established in Fig. 4 in which chromium(III) is
situated in an octahedral environment. There is a carbo-
xylate ion and a b-diketonate ion in between two chromium
(III) ions as shown in Fig. 4. A similar structure of many
other chromium(III) oxobridged complexes has been
reported recently [6,9,15].
Acknowledgements
Author are grateful to the UGC, New Delhi, for financial
support. Dr I. B. Sharma, Jammu University and Dr
(Ms)Fouzia Zia, DDU Gorakhpur University are also
thanked for XRD and magnetic studies and CDRI, Lucknow
for FAB mass spectra.
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