Geometrical isomers of tris(β-diketonato)metal(III) complexes for M = Cr or Co: Synthesis, X-ray structures and DFT study

Article abstract of DOI:10.1016/j.ica.2016.03.019

Solid state crystal data of three tris(β-diketonato)metal(III) complexes (M = Cr or Co), representative of the three different molecular symmetries that these complexes can obtain, namely D₃, C₃ or C₁ symmetry, are present

Full text of DOI:10.1016/j.ica.2016.03.019

Accepted Manuscript
Geometrical isomers of Tris(β-diketonato)metal(III) complexes for M = Cr or
Co: synthesis, X-ray structures and DFT study
Renyuan Liu, Petrus H. van Rooyen, Jeanet Conradie
PII:
S0020-1693(16)30111-6
http://dx.doi.org/10.1016/j.ica.2016.03.019
ICA 16954
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
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8 March 2016
11 March 2016
Please cite this article as: R. Liu, P.H. van Rooyen, J. Conradie, Geometrical isomers of Tris(β-diketonato)metal(III)
complexes for M = Cr or Co: synthesis, X-ray structures and DFT study, Inorganica Chimica Acta (2016), doi:
http://dx.doi.org/10.1016/j.ica.2016.03.019
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Geometrical isomers of Tris(β-diketonato)metal(III) complexes for M = Cr or Co: synthesis,
X-ray structures and DFT study.
Renyuan Liu,a Petrus. H. van Rooyen,b Jeanet Conradiea,*
a Department of Chemistry, PO Box 339, University of the Free State, 9300 Bloemfontein, Republic
of South Africa.
b Department of Chemistry, University of Pretoria, Private Bag X20, Hatfield, 0028, South Africa.
*Contact author details:
Name: Jeanet Conradie
Tel: +27-51-4012194
Fax: +27-51-4446384
e-mail: conradj@ufs.ac.za
Keywords
Chromium(III); Cobalt(III); β-Diketone; Structure; DFT
Graphical abstract
1

Synopsis
Synthesis, crystal and electronic structures of selected symmetrical, fac and mer isomers of tris(β-
diketonato)metal(III) complexes.
Highlights
Crystal structures of fac and mer isomers of tris(β-diketonato)cobalt(III)
Crystal structure of tris(β-diketonato)chromium(III)
Molecular energy levels and orbitals of [Cr(acac)3] and [Co(acac)3]
Structures with D3, C3 and C1 molecular symmetry
Abstract
Solid state crystal data of three tris(β-diketonato)metal(III) complexes (M = Cr or Co),
representative of the three different molecular symmetries that these complexes can obtain, namely
D3, C3 or C1 symmetry, are presented and compared with related experimental structures. Density
functional theory calculations show that both fac and mer isomers of tris(β-diketonato)metal(III)
complexes containing unsymmetrical β-diketonato ligands can exist, in agreement with
experimental NMR data. The orbital ordering for both the S = 3/2 [Cr(acac)3] and S = 0 [Co(acac)3]
(Hacac = acetylacetone) complexes in order of increasing orbital energy is: dxy < dxz, dyz < dz2, dx2-
.
y2
1
Introduction
β-diketones, RCOCH2COR', have two carbonyl groups separated by the α-carbon atom;
whereas in many cases, the keto-enol tautomer possesses the highest stability. The most known β-
diketone is acetylacetone (Hacac), where the substituents R and R' on both carbonyl groups are
methyl groups. Due to the keto–enol tautomerism of β-diketones, the hydrogen atom of the enolic
from of β-diketone ligands can be replaced by a metal to form a metal-β-diketonate complex. An
important application of β-diketones is thus the solvent extraction of metals [1]. The well-known
class of metal β-diketonates, the tris(β-diketonato)metal(III) complexes, has various applications in
catalysis. For example, [Co(acac)3] has made a significant impact in the oxidation of alkanes [2],
alkenes [3] and alcohols [4], and is also used for the hydrogenation of unsaturated organic
molecules [5] and cross-coupling of organozinc compounds [6]. Examples of the catalytic use of
2

[Cr(acac)3] are cross-coupling of organozinc compounds [7] and the polymerization of propylene
and ethylene [8]. Factors influencing the catalytic activity of transition metal complexes include the
arrangement of the ligands round the metal (steric factors) as well as the electronic properties of the
ligands attached to the metal [9]. Tris(β-diketonato)metal(III) complexes have a six-coordinate
metal(III) ion and the coordination polyhedron can be described by an octahedron. For octahedral
tris(β-diketonato)metal(III) complexes containing β-diketonato ligands (RCOCHCOR')− with
substituents R ≠ R', the stereochemistry is adequately defined using the terms fac or mer. In this
contribution, we present the synthesis, X-ray and electronic structure of three examples of tris(β-
diketonato)metal(III) complexes, [Cr(dbm)3] (1), the fac isomer of [Co(tfba)3] (2) and the mer
isomer of [Co(ba)3] (3), see Scheme 1.
Scheme 1: The tris(β-diketonato)metal(III) complexes of this study: [Cr(dbm)3] (1), the fac isomer
of [Co(tfba)3] (2) and the mer isomer of [Co(ba)3] (3).
2
Experimental
2.1 Materials and methods
Reagents were obtained from Merck and Sigma-Aldrich. Solid reagents employed in
preparations were used directly, without further purification. Liquid reactants and solvents were
dried and distilled prior to use. Melting points (m.p.) were determined by an Olympus BX51
system microscope, assembled on top of a Linkam THMS600 stage, and connected to a Linkam
TMS94 temperature programmer. MALDI-TOF spectra were collected by a Bruker Microflex
LRF20 in the positive mode with the minimum laser power required to observe signals.
Complexes (1) – (3) were synthesized as shown in Scheme 2. [Cr(dbm)3] (1) was
synthesised according to Rahman’s method [10] as reported previously [11].
Tris(β-
diketonato)cobalt(III) complexes (2) and (3) were synthesised by the method of Bryant and
3

Fernelius with minor changes [12]. Cobalt(II) carbonate (1.25 g, 0.0105 mole) and 10 mmole β-
diketone dissolved in 10 mL of ethanol were preheated to 95°C while stirring. 15 ml 25 %
hydrogen peroxide was added drop-wise to the suspension over 30 minutes. The precipitate was
filtered and washed with excess water, and then dried in a desiccator at room temperature.
Complexes (1) – (3) were recrystallized from diethyl ether.
R'
(a)
R
O
Cr
O
R
O
O
O
O
.
O
OH
CH3CH2OH
+ CrCl3 6H2O
+ 3 NaCl + CH3COOH + 6H2O
3
RT, 2 h
R
R'
+ CH3COONa
R'
R'
R
(b)
R'
O
R
R
O
O
O
O
OH
CH3CH2OH
O
.
+ 2CoCO3 xH2O
Co
O
2
+ (4+2x) H2O + 2CO2
R'
6
90°C, 30 min
R
R'
+ H2O2
R'
R
Scheme 2: General synthetic method for the (a) tris(β-diketonato)chromium(III) and (b) the tris(β-
diketonato)cobalt(III) complexes.
Characterization data for Cr(C6H5COCHCOC6H5)3, (1) [Cr(dbm)3]
Yield 90 %. Colour: Yellow. Melting point 76°C; MS (m/z): Calc. 723.06 [M], Found 722.0 [M-
1]−. Elemental analysis, Calculated for CrC45H33O6: C, 74.9; H, 4.6 Found: C, 74.8; H, 4.6.
Characterization data for Co(C6H5COCHCOCF3)3, (2) [Co(tfba)3]
1
Yield 13 %. Colour: Dark green. M.P. 107.6 °C. H NMR: (300 MHz, δ/ppm, CDCl3): 6.74 (s, 3H,
3xH, fac), 6.73, 6.75, 6.78 (s, 3H, 3xH, mer), 7.40-7.65 (m, 9H, 3xPh, fac + mer), 7.90-8.05 (m, 6H,
3xPh, fac + mer), MS (m/z): Calc.704.38 [M], Found: [M-1]− 703.98.
Characterization data for Co(C6H5COCHCOCH3)3, (3) [Co(ba)3]
Yield 66 %. Colour: Dark green. M.P. 184.3 °C. 1H NMR: (300 MHz, δ/ppm, CDCl3): 2.36 (s, 9H,
3×CH3, fac), 2.38, 2.40, 2.42 (s, 9H, 3xCH3, mer), 6.24, 6.26, 6.27 (s, 3H, 3xH, fac + mer), 7.28-
7.46 (m, 9H, 3xPh, fac + mer), 7.85-7.95 (m, 6H, 3xPh, fac + mer). MS (m/z): Calc. 542.47,
Found: 542.11.
4

2.2 Crystal structure analysis
Data for the crystals, obtained from solutions in diethyl ether, were collected on a Bruker D8
Venture kappa geometry diffractometer, with duo Iµs sources, a Photon 100 CMOS detector and
APEX II [13] control software using Quazar multi-layer optics monochromated, Mo-Kα radiation
by means of a combination of φ and ω scans, at 150(2)K. Data reduction was performed using
SAINT+ [13] and the intensities were corrected for absorption using SADABS [13]. The structure
was solved by intrinsic phasing using SHELXTS and refined by full-matrix least squares, using
SHELXTL + [14] and SHELXL-2013+ [14]. In the structure refinement, all hydrogen atoms were
added in calculated positions and treated as riding on the atom to which they are attached. All non-
hydrogen atoms were refined with anisotropic displacement parameters; all isotropic displacement
parameters for hydrogen atoms were calculated as X × Ueq of the atom to which they are attached,
where X = 1.5 for the methyl hydrogens and 1.2 for all other hydrogens. Crystal data and structural
refinement parameters are given in the electronic supplementary information.
2.3 Density functional theory (DFT) calculations
Density functional theory (DFT) calculations were carried out, using the ADF (Amsterdam
Density Functional) 2013 programme [15], with a selection of GGA (Generalized Gradient
Approximation) functionals, namely PW91 (Perdew-Wang 1991) [16], BP86 (Becke-Perdew)
[17,18], and OLYP (Handy-Cohen and Lee-Yang-Parr) [19,20], as well as the hybrid functional
B3LYP (Becke 1993 and Lee-Yang-Parr) [21,22]. The TZP (Triple ζ polarized) basis set, with a
fine mesh for numerical integration and full geometry optimization, applying tight convergence
criteria, was used for minimum energy searches.
3
Results and Discussion
3.1 Synthesis
Scheme 2 gives the synthetic approach for complexes (1) – (3). To obtain the [CrIII(β-
diketonato)3] complex, chromium(III) chloride was dissolved in the minimum amount of water and
added drop-wise to a solution of the β-diketone in ethanol. A saturated sodium acetate solution was
added drop-wise to deprotonate the β-diketone, which completed the coordination with the CrIII
cation. To obtain the [CoIII(β-diketonato)3] complexes, cobalt(II) carbonate was oxidized to
cobalt(III) oxide by the hydrogen peroxide. In the presence of the β-diketonato ligand in the
suspension, the coordination to the freshly oxidized CoIII cation proceeded immediately.
5

[Co(β-diketonato)3] complexes have a low spin singlet state (S = 0) [23] while [Cr(β-
diketonato)3] complexes are paramagnetic with a quartet spin state (S = 3/2) [24]. The three
methine hydrogen atoms (and also the hydrogen atoms of the R and R' groups) of fac [Co(β-
diketonato)3] are NMR equivalent, since they are related by a threefold rotation axis. The 1H NMR
spectrum of fac [Co(tfba)3] and fac [Co(ba)3] thus have one singlet for the three methine hydrogen
atoms. However, the three methine hydrogen atoms (and the hydrogen atoms of the R and R'
groups) of mer [Co(β-diketonato)3] are not related through symmetry. For the three methine
hydrogen atoms, the three hydrogen atoms are NMR inequivalent and the 1H NMR spectrum of mer
[Co(tfba)3] and mer [Co(ba)3] each thus have three singlets for the three methine hydrogen atoms.
A mixture of fac and mer [Co(β-diketonato)3] will thus have four singlets for the methine hydrogen
atoms and similarly the hydrogen atoms of the R and R' groups. Both the fac and mer isomers were
observed on 1H NMR for [Co(tfba)3] (2) and [Co(ba)3] (3), with the mer isomer the main isomer.
3.2 Molecular structures
Perspective drawings [25] of the molecular structures of [Cr(dbm)3] (1), the fac isomer of
[Co(tfba)3] (2) and the mer isomer of [Co(ba)3] (3), showing the crystallographic numbering scheme
used, are presented in Figure 1, Figure 2 and Figure 3 respectively. The central metal atom in each
of the three tris(β-diketonato)metal(III) complexes (M = Cr or Co), displays the expected octahedral
geometry. The angles between the three planes that include the β-diketonato ligands in each of (1),
(2) and (3) (Table 1), vary significantly more in (1) and (2), when compared to the angles observed
for (3). This is most likely the result of the increased steric requirements of the substituents on the
β-diketonato ligands. This variation is also evidenced by the larger deviations of atoms from least
squares planes in each of the three rings (R1, R2 and R3, see Table 1) observed in (1) and (2). The
crystallographic packing observed in (1) and (2) shows significant π-π interactions which is absent
in the packing of (3) (Supporting Information Figures S1 – S8, Table S1). There are also stronger
intermolecular hydrogen bonds present in complexes (1) and (2) compared to those observed for
(3). In addition, one F-atom in each of the three CF3 groups of (2) is involved in this hydrogen
bonding.
6

Figure 1: A perspective drawing of the molecular structure of [Cr(dbm)3], (1), showing the atom
numbering scheme. Atomic displacement parameters (ADPs) are shown at the 50 % probability
level.
Figure 2: A perspective drawing of the molecular structure of the fac isomer of [Co(tfba)3], (2),
showing the atom numbering scheme. Atomic displacement parameters (ADPs) are shown at the 50
% probability level.
7

Figure 3: A perspective drawing of the molecular structure of the mer isomer of [Co(ba)3], (3),
showing the atom numbering scheme. Atomic displacement parameters (ADPs) are shown at the 50
% probability level.
Table 1: Selected geometric parameters for [Cr(dbm)3], the fac isomer of [Co(tfba)3] and the mer
isomer of [Co(ba)3].
[Cr(dbm)3] (1) [Co(tfba)3] (2) [Co(ba)3] (3)
M-O(1) / Å
M-O(2) / Å
M-O(3) / Å
M-O(4) / Å
M-O(5) / Å
M-O(6) / Å
M-O (ave) / Å
O(1)-C(1) / Å
O(2)-C(3) / Å
O(3)-C(4) / Å
O(4)-C(6) / Å
O(5)-C(7) / Å
O(6)-C(9) / Å
O-C (ave) / Å
1.945(2)
1.971(2)
1.939(2)
1.949(2)
1.959(2)
1.954(2)
1.953(2)
1.274(4)
1.279(4)
1.288(4)
1.283(4)
1.274(4)
1.275(4)
1.279(4)
1.881(3)
1.882(3)
1.875(3)
1.872(3)
1.869(3)
1.885(3)
1.877(3)
1.265(5)
1.270(5)
1.264(4)
1.275(5)
1.261(5)
1.272(5)
1.267(5)
174.28(13)
90.04(13)
1.875(2)
1.883(2)
1.879(2)
1.889(3)
1.874(2)
1.888(3)
1.881(2)
1.272(4)
1.270(4)
1.278(4)
1.273(4)
1.273(4)
1.264(5)
1.272(4)
175.5(11)
90.04(13)
O-M-O / ° (trans, ave) 177.71(10)
O-M-O / ° (cis, ave) 90.00(10)
8

Table 2: Crystallographic and refinement data for [Cr(dbm)3] (1), the fac [Co(tfba)3] (2) and mer
[Co(ba)3] (3).
[Cr(dbm)3] (1)
C45 H33 Cr O6
721.71
[Co(tfba)3] (2)
C30 H18 Co F9 O6
704.37
[Co(ba)3] (3)
C30 H27 Co O6
542.44
Empirical formula
Formula weight (g mol-1)
Crystal system
space group
Monoclinic
P 21/c
Monoclinic
P 21/c
Triclinic
P -1
a (Å)
b (Å)
c (Å)
α (°)
16.9785(6)
9.7442(3)
21.2271(7)
90
92.772(2)
90
3507.7(2)
4
1.367
0.378
1500
9.5067(7)
31.906(2)
9.9607(8)
90
108.122(2)
90
2871.4(4)
4
1.629
0.700
1416
9.7546(5)
9.8755(6)
14.2557(8)
83.883(2)
84.5540(10)
77.6970(10)
1330.41(13)
2
β (°)
γ (°)
Volume (Å3)
Z
Dcalc (Mg/m3)
1.354
0.687
564
Absorption coefficient (mm-1)
F(000)
Reflections collected
Data used / restraints / parameters
Goodness-of-fit on F2
Final R indices [I>2sigma(I)]
32119
7168 / 0 / 469
1.068
R1 = 0.0658, wR2 R1 = 0.0669, wR2
= 0.1356 = 0.1532
R1 = 0.1114, wR2 R1 = 0.0782, wR2
= 0.1534 = 0.1577
0.448 and -0.640 0.970 and -0.805
88047
5878 / 0 / 415
1.219
48015
4879 / 0 / 334
1.058
R1 = 0.0510,
wR2 = 0.1214
R1 = 0.0809,
wR2 = 0.1343
0.469 and -0.229
R indices (all data)
Largest diff. peak and hole (e.Å-3)
Selected average structural parameters of [Cr(dbm)3] are compared to the structural
parameters of previously published crystal structures of [Cr(acac)3] [26], [Cr(dpm)3] [27] (Hdpm =
dipivaloylmethane or 2,2,6,6-tetramethyl-3,5-heptanedione, also abbreviated as Hthd) and
[Cr(hfaa)3] [28] (hfaa = 1,1,1,5,5,5-hexafluoroacetylacetonato) in Table 3. Data for [Co(ba)3 and
[Co(tfba)3] as well as previously published crystal data of [Co(acac)3] [29], [Co(dbm)3] [30],
[Co(dpm)3] [31] and [Co(tfaa)3] [32], are also included in Table 3 (see the electronic supporting
information for more detail). The average Cr–O bond lengths and O-Cr-O bond angles are all
similar within 0.02 Å and 1.2° except for [Co(hfaa)3]. For the four [Cr(β-diketonato)3] complexes,
the average Cr–O bond lengths and O-Cr-O bond angles are all similar within 0.01 Å and 1.1°.
Since the difference between Cr–O bond lengths and O-Cr-O bond angles in the same crystal are of
a similar order (up to 0.03 Å and 1.2°), temperature has a slight influence on experimental structural
parameters [33,34] (published experimental data are obtained at different temperatures, see
electronic supporting information), and the differences in the average experimental data in Table 3
are close to the experimental error of the structural data (taken as three times the uncertainty in the
bond length/angle), no conclusions or trends can be drawn from the average experimental data in
Table 3 for a specific metal. Generally, the average experimental Cr–O bond lengths are slightly
9

larger and the O–Cr–O bond angles thus slightly smaller than the experimental Co–O bond lengths
and O–Co–O bond angles, as expected for Cr with a larger atomic radius than Co.
Table 3: Selected average geometric parameters for Co(β-diketonato)3 and Cr(β-diketonato)3
complexes.
Co(β-diketonato)3
Cr(β-diketonato)3
Reference
β-
Co-O O-Co-O / C-C-C / Cr-O / O-Co-O C-C-C / Co(β-
Cr(β-
diketonato)3 diketonato)3
diketonato
/ Å
°
°
Å
/ °
°
hfaa
dpm
dbm
acac
tfaa (mer)
tfba (fac)
ba (mer)
2.07
1.87
1.88
1.89
1.89
1.88
1.88
87.6
95.8
95.5
95.0
96.1
95.8
96.0
94.9
123.0
125.0
124.9
124.8
125.4
122.7
125.1
124.8
1.94
1.95
1.95
1.96
90.6
90.0
90.2
91.1
119.9
124.5
124.7
125.1
28
27
32
31
30
29
this study,
26
32
this study
this study
average all 1.90
1.95
90.8
124.3
3.3 Computational chemistry study
Tris(β-diketonato)metal(III) complexes have an octahedral structure. No crystallographically
imposed symmetry was observed in the crystal structures of complexes (1) – (3), however,
depending on the β-diketonato ligand, and the arrangement of the ligands round the metal, the
tris(β-diketonato)metal(III) complexes can either have a D3, C3 or C1 molecular symmetry: D3
symmetry applies to complexes with symmetrical β-diketone ligands, e.g. M(acac)3; C3 rotational
symmetry applies to complexes with unsymmetrical β-diketone ligands, arranged in such a way that
they have a 3-fold rotational axis (called fac isomers); and C1 symmetry applies to complexes with
unsymmetrical β-diketone ligands, arranged in such a way that they have no symmetry elements
(called mer isomers). Complexes (1) – (2) are examples of tris(β-diketonato)metal(III) complexes
displying molecular symmetry of D3 ([Cr(dbm)3] (1)) or C3 (fac [Co(tfba)3] (2)) that were not
observed in the solid state.
Neutral CrIII(β-diketonato)3 complexes are d3 complexes with a spin state of S = 3/2 (three
unpaired electrons) [24]. Input geometries in lower symmetries than D3 (C1, C2 and C3) all
converged to the same lowest energy geometry with D3 symmetry (all Cr–O bonds and O–Cr–O
angles converged to the same value as given in Table 4). The most important bond lengths and
angles in organometallic complexes are those involving the metal centre. The average experimental
Cr–O bonds and in [Cr(acac)3] obtained from 18 published X-ray structures are slightly
overestimated by 0.02 (PW91, BP86 and B3LYP) – 0.05 (OLYP) Å for the calculated bonds. This
10

is expected, since gas phase calculations as well as GGA density functionals tend to overestimate
bonds lengths [35]. Longer calculated bond lengths, compared to crystallographic measured bond
lengths, have also been found for [Fe(acac)3] [36] and other metal β-diketonato complexes [37,38].
The calculated O–Cr–O angle of [Cr(acac)3] is within 0.4° of the average experimental value,
except for the OLYP calculated value. Generally differences between experimental and calculated
bond lengths below the threshold of 0.02 Å, are considered meaningless [39]. Results discussed in
the previous section indeed showed that Cr–O bond lengths and O–Cr–O bond angles in the same
[Cr(acac)3] crystal differ up to 0.03 Å and 1.2° (also see electronic supporting information). Thus
the PW91, BP86 and B3LYP functionals all give a good agreement with experimental geometric
parameters.
The d6 CoIII(β-diketonato)3 complexes are low spin with S = 0 [23]. Input geometries in
lower symmetries that D3 (C1, C2 and C3) all converged to the same lowest energy geometry of D3
symmetry with Cr–O bonds and O–Cr–O angles as given in Table 4. The calculated DFT Co–O
bonds and O–Cr–O bond angles for [Co(acac)3] are again slightly longer than the average
experimental values obtained from 11 published X-ray structures, namely 0.02 (PW91, BP86 and
B3LYP) – 0.04 (OLYP) Å for the bonds and up to 2° for the angles (see electronic supporting
information). Thus, for [Cr(acac)3], the PW91, BP86 and B3LYP functionals all give structural
parameters in good agreement with experimental geometric parameters.
Table 4: DFT-calculated Cr – O bond lengths (Å) and O – Cr – O bond angles (deg) obtained by
the indicated functionals, for [Cr(acac)3] and [Co(acac)3] optimized with D3 symmetry constraint.
Experimental
B3LYP OLYP BP86 PW91
(average)
[Cr(acac)3]
d(Cr – O)
O – Cr – O
[Co(acac)3]
d(Co – O)
O – Co – O
1.983
91.5
2.008
89.7
1.980
91.4
1.978
91.5
1.96 [29]
91.1 [29]
1.911
96.7
1.925
96.1
1.908
97.0
1.906
97.1
1.89 [26]
95.0 [26]
The DFT calculated energies of the fac and mer isomers of the [Co(ba)3], [Co(tfba)3] and
[Co(tfaa)3] are given in Table 5. DFT calculated energy differences below 1 kcal mol-1 (4.184 kJ
mol-1) are within the accuracy of DFT [40]. The small energy differences obtained between the fac
and mer isomers thus indicate that both isomers should be experimentally possible, as was
1
experimentally observed on H NMR for [Co(tfba)3] (2) (in this study) and [Co(ba)3] (3) (in this
study) and [Co(tfaa)3] [41]. Experimentally, crystals of mer isomers were found for [Co(ba)3] and
11

[Co(tfaa)3] and crystals of fac isomers were found for [Co(tfba)3]. Packing forces in the crystal may
favour a specific isomer above another.
Table 5: DFT calculated electronic (E) and Gibbs free (G) energies for the indicated Co(β-
diketonato)3 complexes and functionals.
X-tal
structure
Relative E (kJ mol-1)
Relative G (kJ mol-1)
B3LYP OLYP BP86 PW91 B3LYP OLYP BP86 PW91
Co(ba)3
fac
mer
0.0
0.5
7.3
0.0
3.1
0.0
0.2
0.0
6.3
0.0
0.9
0.0
0.0
0.5
5.5
0.0
1.2
0.0
0.0
0.0
3.8
0.0
0.7
0.0
2.1
0.0
4.7
0.0
7.8
0.0
0.3
0.0
8.6
0.0
0.9
0.0
0.0
5.3
2.9
0.0
3.5
0.0
0.0
2.4
3.8
0.0
0.8
0.0
mer
fac
Co(tfba)3 fac
mer
Co(tfaa)3 fac
mer
mer
Figure 4 and Figure 5 show the molecular energy level diagram of [Cr(acac)3] and
[Co(acac)3] optimized under the D3 symmetry constraint, including plots of the d-based frontier
MOs. In an octahedral complex, the presence of ligands leads to the splitting of the five metal-d-
orbitals into two different levels, due to the breaking of orbital degeneracy. The bottom three
energy levels are collectively referred to as t2g and consist of dxy, dxz, and dyz. The two upper energy
levels are collectively referred to as eg and consist of dx2−y2 and dz2. Distortion from the octahedral
orbital energies (Oh) towards D3 ligand symmetry, splits the degenerate t2g-orbitals further into a
and e components. The orbital ordering for both [Cr(acac)3] and [Co(acac)3] complexes in order of
increasing orbital energy are: dxy < dxz, dyz < dz2, dx2-y2. The S = 3/2 [Cr(acac)3] and S = 0
[Co(acac)3] complexes may be described by the following electronic configurations:
1
1
1
0
2
2
2
0
dxy dxz dyz dz2 dx2-y20 and dxy dxz dyz dz2 dx2-y20. The same orbital ordering was obtained for the high
1
1
1
1
spin Fe(acac)3 complex with electronic configurations: dxy dxz dyz dz2 dx2-y21 [36].
12

Figure 4: Molecular orbital Kohn-Sham energy levels (eV) for [Cr(acac)3], with S = 3/2, optimized
under the D3 symmetry constraint. Plots of the d-based MOs are included. The black arrows
represent the three unpaired electrons in the top α HOMOs. The MOs of α d-orbitals are illustrated
on the left of the figure, and the β d-orbitals are illustrated on the right.
Figure 5: Molecular orbital Kohn-Sham energy levels (eV) for [Co(acac)3], with S = 0, optimized
under the D3 symmetry constraint. Plots of the d-based MOs are included. The black arrows
represent electrons in the top HOMOs.
4
Conclusions
DFT calculations showed that both fac and mer isomers of the [Co(ba)3], [Co(tfba)3] and [Co(tfaa)3]
can exist. Experimentally in CDCl3 solution, on 1H NMR, both isomers were observed with mer as
the main isomer. In the solid state, crystals of mer isomers for [Co(ba)3] and [Co(tfaa)3] and
13

crystals of fac isomer for [Co(tfba)3] were isolated. Packing forces in the crystal may favour a
specific isomer above the other.
Supporting Information
Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre with
numbers: 1440622-1440624. Copies can be obtained, free of charge, on application to CCDC, 12
Union
Road,
Cambridge
CB2
1EZ,
UK
[fax:
+44
(0)1223
336033
or
ww.ccdc.cam.ac.uk/products/csd/request/]. Selected crystallographic data and the optimized
coordinates of the DFT calculations are given in the Supporting Information.
Acknowledgements
This work has received support from the South African National Research Foundation and the
Central Research Fund of the University of the Free State, Bloemfontein, South Africa. The High
Performance Computing facility of the UFS is acknowledged for computer time.
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