Observation of novel oxygen?oxygen interaction in supramolecular assembly of cobalt(iii) Schiff base complexes: a combined experimental and computational study

Article abstract of DOI:10.1039/c5ra13960k

Two mononuclear cobalt(iii) Schiff base complexes with azide [Co(L)(N₃)(L′)] (1) and [Co(L)(N₃)(L″)] (2) {where HL = 1-((2-(diethylamino)ethylimino)methyl)naphthalene-2-ol, HL′ = 2-hydroxy-1-naphthaldehyde and HL″ = acetylacetone} have been synthesized and characterized by elemental analysis, IR and UV-Vis spectroscopy and single crystal X-ray diffraction studies. Both complexes show mononuclear structures with azide as terminal coligand. Structural features have been examined in detail that reveal the formation of interesting supramolecular networks generated through non-covalent forces including hydrogen bonding, C-H?H-C and C-H/π interactions. These interactions have been studied energetically by means of theoretical DFT calculations. We have also analyzed the unexpected O?O interactions observed in one complex between the oxygen atoms of the coordinated aldehyde groups using several computational tools, including Bader's "atoms-in-molecules" (AIM) and natural bond orbital (NBO) analyses.

Full text of DOI:10.1039/c5ra13960k

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Observation of novel oxygen···oxygen interaction in
supramolecular assembly of cobalt(III) Schiff base
complexes: A combined experimental and computational
study
Mithun Dasa, Biswa Nath Ghoshb, Antonio Bauzác, Kari Rissanenb, Antonio
Fronterac,* and Shouvik Chattopadhyaya,*
a Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata 700032, India.
b
Department of Chemistry, Nanoscience Center, University of Jyväskylä, P.O. Box 35, 40014
Jyväskylä, Finland.
cDepartament de Química, Universitat de les Illes Balears, Crta. de Valldemossa km 7.5, 07122
Palma de Mallorca (Baleares), Spain. E-mail: toni.frontera@uib.es
E-mail: shouvik.chem@gmail.com
Tel:+91-33-24572941
Abstract
Two mononuclear cobalt(III) Schiff base complexes with azide [Co(L)(N3)(L′)] (1) and
[Co(L)(N3)(L′′)] (2) {where HL = 1-((2-(diethylamino)ethylimino)methyl)naphthalene-2-ol, HL′
= 2-hydroxy-1-naphthaldehyde and HL′′ = acetylacetone} have been synthesized and
characterized by elemental analysis, IR and UV–Vis spectroscopy and single crystal X-ray
diffraction studies. Both complexes show mononuclear structures with azide as terminal
coligand. Structural features have been examined in detail that reveal the formation of interesting
supramolecular networks generated through non-covalent forces including hydrogen bonding, C–
1

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H···H–C and C–H/π interactions. These interactions have been studied energetically by means of
theoretical DFT calculations. We have also analyzed the unexpected O···O interactions observed
in one complex between the oxygen atoms of the coordinated aldehyde groups using several
computational tools, including Bader’s “atoms-in-molecules” (AIM) and natural bond orbital
(NBO) analyses.
Keywords: Schiff base; Cobalt(III); Azide; O···O interactions; Theoretical study.
Introduction
Synthesis and structural characterization of transition metal complexes with Schiff base
ligands have attracted much attention for their fascinating structures, interesting properties and a
variety of appealing applications.1 Encouragement for their syntheses and characterization is
provided by their potential abilities in biological modeling application, e.g. to mimic the active
sites of many enzymes.2 Focusing to cobalt(III) Schiff base complexes, there is extensive
attention in the domain of metallo-organic and coordination chemistry due to their synthetic
accessibility, fascinating structural features and diverse range of applications covering organic
synthesis, photochemistry,3 magnetic and electrochromism,4 and catalysis.5 They also have
important biological applications.6 Recently, dinuclear and mononuclear cobalt(III) Schiff base
complexes have also been used as precursor for the synthesis of Co3O4 nanoparticles via thermal
decomposition method.7 Synthesis of Co3O4 nanoparticles via thermal decomposition cobalt(III)
Schiff base complexes have also been reported in literature.7
On the other hand, the formation of supramolecular assemblies is an appealing research
topic now-a-days. The most commonly used approach is to employ hydrogen bonds for
engineering the structures of such complexes.8 The crystalline architecture of the complexes
could also be managed by several other well established non-covalent interactions, such as, π-
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stacking, cation–π, and C–H/π forces, which have also attracted considerable interest due to their
relative strength, directionality, and ability to act synergically, thereby providing an organizing
force for the association of molecules into distinct supramolecular assemblies.9 Moreover, other
less recognized forces, such as, σ/π-hole, lone-pair/π and anion/π interactions, have also been
utilized in sensors, probes, photonic devices, catalysis etc.10 In this context, it is not irrelevant to
state that intra-molecular non-bonded interaction between sulfur and sulfur (or oxygen or
nitrogen) atoms has also been observed in a large number of organosulfur complexes controlling
the conformation of small and large molecules.11 In these molecules, the non-bonded S···S,
S···O or S···N distances are significantly shorter than the sum of the corresponding van der
Waals radii (3.62, 3.32 or 3.35 Å) in the crystalline structure.12
In the present work, we have synthesized two new cobalt(III) azide complexes with a
tridentate N2O donor Schiff base and 2-hydroxynaphthyl-1-carboxaldehyde or acetylacetone.
The structures have been confirmed by single crystal X-ray diffraction analysis. Structural
features have been examined in detail that reveal the formation of interesting supramolecular
networks generated through non-covalent forces including hydrogen bonding, C–H···H–C and
C–H/π interactions.13 These interactions have been studied energetically by means of theoretical
DFT calculations.14 Most interesting observation is the existence of O···O interactions in the
supramolecular assembly of one complex. The O···O interactions between the oxygen atoms of
the coordinated aldehyde groups have also been analyzed using several computational tools,
including Bader’s “atoms-in-molecules” (AIM)15 and natural bond orbital (NBO)16 analyses.
Experimental Section
Materials and Measurements
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All chemicals were of reagent grade and purchased from Sigma-Aldrich. They were used
without further purification. Caution!!! Since the azide complexes of metal ions are potentially
explosive, only small amounts of the materials should be handled with care.
Elemental analysis was performed using a PerkinElmer 240C elemental analyzer. IR
spectra in KBr (4500–500 cm-1) were recorded using a PerkinElmer Spectrum Two FT-IR
spectrophotometer. 1H NMR spectrum was recorded on Bruker DRX-300 NMR Spectrometer at
300 MHz using DMSO-d6 as solvent. Electronic spectra in DMSO (800–300 nm) were recorded
in PerkinElmer LAMBDA 35 UV/Vis spectrophotometer. Fluorescence spectra were obtained on
SHIMADZU RF-5301PC Spectrofluorophotometer at room temperature. Lifetime measurements
were recorded using Hamamatsu MCP photomultiplier (R3809) and were analyzed by using
IBHDAS6 software. Intensity decay profiles were fitted to the sum of exponentials series
where Ai is a factor representing the fractional contribution to the time resolved decay of
the component with a lifetime of τi. The intensity averaged life times (τav) are determined using
the following equation:
Preparation of ligand HL (1-((2-(diethylamino)ethylimino)methyl)naphthalen-2-ol)
The tridentate Schiff base ligand, HL, was synthesized by refluxing N,N-diethyl-1,2-
diaminoethane (1 mmol, 0.14 mL) with 2-hydroxynaphthyl-1-carboxaldehyde, (HL′), (1 mmol,
172 mg) in methanol solution (20 mL) for ca. 30 minutes following literature method.17 The
ligand was not isolated. The methanol solution was used for the syntheses of the complexes.
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Preparation of complex [Co(L)(N3)(L′)] (1)
A methanol solution (15 mL) of 2-hydroxynaphthyl-1-carboxaldehyde, (HL′), (1 mmol,
172 mg) and cobalt(II) acetate tetrahydrate (1 mmol, 250 mg) was stirred for ca. 30 minutes. The
methanol solution (20 mL) of Schiff base ligand, HL, was then added followed by aqueous
solution (5 mL) of sodium azide in stirring condition. The resulting solution was stirred for
additional 1 hr and then kept aside in room temperature. Solid product started to separate on
standing overnight. Suitable single crystal of 1 for X-ray diffraction was obtained from the
acetonitrile solution.
Yield: 340 mg (63%). Anal. Calc. for C28H28CoN5O3 (541.48): C, 62.11; H, 5.21; N,
12.93%. Found: C, 62.3; H, 5.3; N, 12.7%. FT-IR (KBr, cm−1): 1618, 1602 (C=O), 1583 (C=N),
2023 (N3). UV-Vis, λmax (nm) [εmax(L mol-1 cm-1)](Acetonitrile): 219 (90235), 268 (62163) 319
1
(24645), 404 (6446), 422 (6076). H NMR (DMSO-d6) (ppm) δ: 9.77 (s, 1H, -CH=O), 8.83 (s,
1H, -CH=N), 8.71 (d, J = 4.18 Hz, 1H, Ar-H), 8.37 (d, J = 4.11 Hz, 1H, Ar-H), 7.60 (m, 3H, Ar-
H), 7.45 (m, 3H, Ar-H), 7.28 (dd, J = 7.34, 7.22 Hz, 1H, Ar-H), 7.15 (dd, J = 7.61, 7.53 Hz, 1H,
Ar-H), 6.87 (dd, J = 9.07 Hz, 9.09 Hz, 1H, Ar-H), 6.68 (d, J = 9.05 Hz, 1H, Ar-H), 4.38 (m, 2H,
-CH2CH3), 3.15 (m, 1H, -CH2CH2), 2.79 (m, 2H, -CH2CH3), 2.06 (m, 2H, -CH2CH2), 1.21 (m,
1H, -CH2CH2), 1.55 (t, 6.19 Hz, 3H, -CH2CH3), 0.76 (t, 6.14 Hz, 3H, -CH2CH3).
Preparation of complex [Co(L)(N3)(L′′)] (2)
It was prepared in a similar method to that of 1 except that acetylacetone, (HL′′), (1
mmol, 0.1 mL) was used instead of 2-hydroxynaphthyl-1-carboxaldehyde (HL′). Solid product
separated on standing overnight. Single crystal suitable for X-ray diffraction was obtained from
the acetonitrile solution.
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Yield: 290 mg (62%). Anal. Calc. for C22H28CoN5O3 (469.42): C, 59.41; H, 5.58; N,
13.86%. Found: C, 59.6; H, 5.4; N, 13.7%. FT-IR (KBr, cm−1): 1621 (C=O), 1577 (C=N), 2023
(N3). UV-Vis, λmax (nm) [εmax(L mol-1 cm-1)](Acetonitrile): 224 (58391), 266 (57462), 408
1
(5601), 424 (5455). H NMR (DMSO-d6) (ppm) δ: 8.72 (s, 1H, -CH=N), 8.14 (d, J = 4.26 Hz,
1H, Ar-H), 7.65 (d, J = 3.97 Hz, 1H, Ar-H), 7.55 (d, J = 4.55 Hz, 1H, Ar-H), 7.44 (dd, J = 7.14,
7.56 Hz, 1H, Ar-H), 7.16 (dd, J = 7.65, 7.46 Hz, 1H, Ar-H), 7.08 (d, J = 4.57 Hz, 1H, Ar-H),
5.62 (s, 1H, -COCH-), 4.29 (m, 2H, -CH2CH3), 3.15 (m, 1H, -CH2CH2), 2.91 (m, 1H, -
CH2CH2), 2.68 (m, 2H, -CH2CH3), 2.37 (m, 1H, -CH2CH2), 2.27 (s, 3H, -COCH3), 1.85 (m, 1H,
-CH2CH2), 1.63 (s, 3H, -COCH3), 1.06 (t, 6.77 Hz, 3H, -CH2CH3), 0.81 (t, 6.99 Hz, 3H, -
CH2CH3).
X-ray Crystallography
The structural analysis of 1 was performed using Bruker-Nonius Kappa CCD
diffractometer equipped with APEX II detector with graphite-monochromatized Mo-Kα (λ =
0.71073 Å) radiation at 123 K. Collect software was used for the data measurement.18 and
DENZO-SMN for the processing.19 Absorption correction was applied using the multi-scan
SADABS programme.20 The unit cell of 1 includes a disordered acetonitrile molecule, which
could not be modeled as discrete atomic sites. So the structure was subjected to the SQUEEZE
procedure from the PLATON suite to calculate the diffraction contribution of the solvent
molecules and there by produce a set of solvent-free diffraction intensities. A void volume of 129
Å3 contained a minimum of 16 electrons. Prior to SQUEEZE, all non-hydrogen atoms were
made anisotropic and all hydrogen atoms were inserted at their calculated positions. Details of
the SQUEEZE procedure are given in the CIF file. Data for 2 were collected at 123 K on an
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Agilent Super Nova Dual diffractometer with Atlas detector using mirror-monochromatized Mo-
Kα (λ = 0.71073 Å) radiation. CrysAlis Program was used for the data collection and
processing.21 The intensities were corrected for absorption using the Analytical face index
absorption correction method.22 The structures were solved by charge flipping method with
SUPERFLIP23 and refined by full-matrix least-squares methods using the WinGX software,24
which utilizes the SHELXL-97 module.25 All non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms were introduced in calculated positions with isotopic
thermal parameters using the ‘riding model’. Selected crystallographic data and experimental
details of complexes 1 and 2 are summarized in Table 1. CCDC 1047381 (for 1) and 1047382
(for 2).
Theoretical Methods
The geometries of the complexes included in this study were computed at the BP86-
D3/def2-TZVP level of theory using the crystallographic coordinates within the TURBOMOLE
7.0 program.26 This level of theory that includes the latest available dispersion correction (D3) is
adequate for studying non-covalent interactions dominated by dispersion effects like π-
stacking.27 In some complexes, we have also evaluated the noncovalent interactions using the RI-
MP2/def2-TZVP level of theory in order to validate the DFT method used herein. We have used
high spin configuration for the cobalt(III). The basis set superposition error for the calculation of
interaction energies has been corrected using the counterpoise method.28 The “atoms-in-
molecules” (AIM) analysis of the electron density has been performed at the same level of theory
using the AIMAll program.29
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We have also optimized the position of the hydrogen atoms of the crystal structures in
order to compare the theoretically calculated (BP86-D3/def2-TZVP) positions to those obtained
using isotopic thermal parameters (riding model). The positions of the hydrogen atoms are
similar and consequently we have used the crystallographic ones. The geometries of the
theoretically calculated complexes are included in the ESI.
Scheme 1: Syntheses of complexes 1 and 2.
Catalytic oxidation of o-aminophenol
In order to examine the phenoxazinone synthase-like activity of the complexes, 1.02 ×
10−5 M solutions of 1 and 2 in methanol were treated with a 10-3 M solution of o-aminophenol
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(OAPH) under aerobic conditions at 25 °C. The reaction was followed spectrophotometrically by
monitoring the increase in the absorbance as a function of time at ca. 433 nm, which is
characteristic of the phenoxazinone chromophore. To determine the dependence of rate on the
substrate concentration and various kinetic parameters, 1.02 × 10−5 M solutions of the complexes
were mixed with at least 10 equivalents of substrate to maintain pseudo-first order conditions.
Moreover, to check the rate dependency on catalyst concentration, a similar set of experiments
was performed at a fixed concentration of substrate with varying amounts of catalyst. The rate of
a reaction was derived from the initial rate method, and the average initial rate over three
independent measurements was recorded.
Results and discussion
Syntheses
Schiff base ligand, HL (1-((2-(diethylamino)ethylimino)methyl)naphthalene-2-ol) was
prepared by the 1:1 condensation of the 2-hydroxy-1-naphthaldehyde (HL′) with N,N-
diethylethylenediamine in methanol following the literature method.17 The methanol solution
was then made to react with cobalt(II) acetate tetrahydrate and sodium azide in presence of two
different coligands. In both cases octahedral cobalt(III) complexes 1 and 2 are formed when HL′
and HL′′ (acetylacetone) was used as coligands respectively.
Structure description of Complexes [Co(L)(N3)(L′)] (1) and [Co(L)(N3)(L′′)] (2)
Complex 1 crystallizes in tetragonal space group P43212, whereas, complex 2 crystallizes
in orthorhombic space group P212121. Both the complex feature mononuclear octahedral
cobalt(III) complex as shown in Figure 1. Selected bond lengths and bond angles of complexes
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are given in Tables 2 and 3 respectively. In both the cases, central cobalt(III), Co(1), is
coordinated meridionally by two nitrogen atoms, N(1) and N(2), and one oxygen atom, O(1), of
the deprotonated Schiff base ligand (L)- and two oxygen atoms, O(2) and O(3), of deprotonated
coligand [(L′)- for complex 1 and (L′′)- for complex 2]. Another nitrogen atom, N(3), of azide
coordinates to complete the distorted octahedral geometry of cobalt(III). The saturated five
membered rings [Co(1)–N(1)–C(23)–C(24)–N(2)] (for 1) and [Co(1)–N(1)–C(12)–C(13)–N(2)]
(for 2) show envelope conformation on C(24) and C(13) with puckering parameters q(2) =
0.453(3)Å, ϕ(2) = 113.1(3)° and q(2) = 0.445(3)Å, ϕ(2) = 115.5(3)° respectively.30
Supramolecular interactions and theoretical study
We have divided the theoretical study into two parts. In the first one we have analyzed
the interesting C–H/π and hydrophobic C–H···H–C interactions observed in the solid state of
complexes 1 and 2 that are crucial for understanding the final 3D architecture of the complexes.
In the second part we have analyzed the counter intuitive chalcogen-chalcogen O···O
interactions observed in complex 1 involving two symmetrically related aldehyde groups of the
ligand.
Both the aliphatic and aromatic parts of the tridentate ligand in complex 1 establish a
variety of C–H/π interactions in the solid state forming a supramolecular 1D chain (Figure 2A).
Interestingly, the hydrogen atoms of the diethylamino group interact to one face of the π-system
of the naphthalene moiety and concurrently the aromatic hydrogen atoms of the same ligand
interact to the opposite face facilitating the formation of the 1D chain. We have studied this
interaction using a dimeric model (Figure 2B) to keep the size of the system computationally
approachable. We have computed the interaction energy of this dimer which is ꢀE1 = –17.5
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kcal/mol, which is large because of the enhanced acidity of the interacting hydrogen atoms due
to the coordination of the ligand to the cobalt(III) metal center that strengthens the C–H/π
interactions. As a matter of fact, if the calculations are performed without the cobalt(III)
transition metal (protonating the ligand to keep the model neutral), the interaction energy is
reduced to –7.5 kcal/mol for the aliphatic and to –5.2 kcal/mol for the aromatic C–H/π
interaction. To corroborate this explanation, we have computed the atomic charges of the
interacting hydrogen atoms, which are 0.17 and 0.19 e for the aliphatic and aromatic hydrogen
atoms, respectively, when the ligand is coordinated to cobalt(III). The charges are reduced to
0.15 e (for both hydrogen atoms) when the ligand is not coordinated, thus supporting the acidity
enhancement of the interacting hydrogen atoms of the ligand upon complexation.
In addition to the C–H/π bonding network, non-covalent C–H···H–C interactions are also
observed in the solid state of complex 1 (see Figure 3) in the formation of a self-assembled dimer
where the azide coligand also participates establishing hydrogen bonding interactions. Recently a
combined computational and CSD study has demonstrated the importance of C–H···H–C
interactions weak interactions in the solid state.31 In fact the binding energy of the dimer of
dodecahedrane,31 that is stabilized only by C–H···H–C interactions, is approximately 3 kcal/mol.
To further analyze the importance on this interaction, we have computed the binding energies in
a series of theoretical dimers based on the X-ray structure (see Figure 3). In Figure 3B the dimer
retrieved from the X-ray geometry is shown where, in addition to the C–H···H–C interactions, a
hydrogen bonding network with the participation of the azide and aldehyde moieties is present.
The interaction energy of this dimer is very large and negative (ꢀE2 = –20.9 kcal/mol) due to the
hydrogen bonding interactions. We have also computed a theoretical model where the azide
coligands have been replaced by hydrides (Figure 3C) and consequently the hydrogen bonding
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interactions involving azide are not present. As a result the interaction energy is significantly
reduced to ꢀE3 = –12.7 kcal/mol that corresponds to the hydrogen bonding interactions
established between the symmetrically related aldehyde groups [coordinated to cobalt(III)] and
the C–H···H–C interactions. Moreover, we have computed another theoretical model where the
azide coligands and the ethyl substituents of the amino group have been replaced by hydrides
and hydrogen atoms, respectively (Figure 3D) and consequently only the hydrogen bonding
interaction involving coordinated aldehyde groups are evaluated. As a result the interaction
energy is further reduced to ꢀE4 = –6.9 kcal/mol that corresponds to both complementary
CO···HC hydrogen bonding interactions. The contribution of the C–H···H–C interaction can be
evaluated as ꢀEC–H···H–C = ꢀE4 – ꢀE3 = –5.8 kcal/mol. The large binding energy for this
interaction can be rationalized by means of the large number of individual and additive C–
H···H–C contacts. Similar results have been observed in long open chain alkanes, where the
number of side-on contacts between neighbouring molecules increases resulting in large dimer
dissociation energies, as high as 4.5 kcal/mol for n-hexane.31b
It can be observed in Figure 3 that the O···O distance between the oxygen atoms of the
coordinated aldehyde groups is shorter than the sum of van der Waals radii (∑RvdW = 3.04 Å).
We have investigated the physical nature of this counterintuitive “like-like” interaction between
the oxygen atoms (red dashed line in Figure 3). We have first computed the non-covalent
interaction (NCI) plot of the dimer of 1 using the crystallographic coordinates to analyze the
intermolecular non-covalent interactions. The NCI plot is a visualization index based on the
electron density and its derivatives, and enables identification and visualization of non-covalent
interactions efficiently. The isosurfaces correspond to both favorable and unfavorable
interactions, as differentiated by the sign of the second density Hessian eigenvalue and defined
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by the isosurface color. NCI analysis allows an assessment of host-guest complementarity and
the extent to which weak interactions stabilize a complex. The information provided by NCI
plots is essentially qualitative, i.e. which molecular regions interact. The color scheme is a red-
yellow-green-blue scale with red for ρ+ (repulsive) and blue for ρ− (attractive). Yellow and
cut
cut
green surfaces correspond to weak repulsive and weak attractive interactions, respectively.32 In
Figure 4A we show the representation of the NCI plot computed for the dimer. It can be
observed a region between the ethyl groups that confirm the existence of favorable C–H···H–C
interactions. In addition, there is long isosurface that corresponds to the hydrogen bonding
network in the region of the azide ligand. Finally there is also a green surface in the region where
the aldehyde groups interact. In order to further analyze the aldehyde-aldehyde interaction we
have optimized a model dimer using formaldehyde and the interaction energy is ꢀE5 = –1.6
kcal/mol that is modest (Figure 4B). Since this value is small, we have also computed the
interaction energy at a higher level of theory, i.e. CCSD(T)/def2-TZVP. As a result, the
interaction energy is similar (–1.3 kcal/mol), giving reliability to the DFT-D3 method. Moreover,
we have also used the symmetry adapted perturbation theory (SAPT)33a to provide quantitative
information on the magnitude of the electrostatic, induction and dispersion interactions. For the
formaldehyde dimer complex the DFT-SAPT method gives a total interaction energy of
Et(SAPT) = – 1.0 kcal/mol, that is in reasonable agreement with the CCSD(T) interaction energy.
The partitioning of the interaction energy into the individual contributions is as follows:
electrostatic term is Eele = – 2.4 kcal/mol, exchange term is Eexc = +3.1 kcal/mol, induction term
is Eind = –0.2 and dispersion term is Edisp = –1.5 kcal/mol. Therefore the aldehyde-aldehyde
interaction is dominated by the electrostatic and dispersion terms. Strikingly the AIM analysis
shows that the interaction is characterized by a bond critical point (red sphere) and a bond path
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that connects both oxygen atoms confirming the existence of chalcogen-chalcogen interaction.
The distribution of critical points does not show a bond critical point connecting the hydrogen to
the oxygen atoms. It should be mentioned that the optimized O···O distance (2.98 Å) is very
similar to the distance observed in the X-ray structure (2.94 Å). We have also analyzed how the
interaction energy is affected by the O···O distance (Table 4). The interaction energy is reduced
in more than 50% upon increasing 0.5 Å the O···O distance. At this point, in order to investigate
the O···O interaction from an orbital point of view we have performed Natural Bond Orbital
(NBO)33b calculations in the formaldehyde dimer focusing our attention on the second order
perturbation analysis that is very useful to study donor acceptor interactions. Interestingly, we
have found that the lone pair (lp orbital) of one oxygen atom interacts with the C–O and C–H
antibonding orbitals of the opposite formaldehyde anion and vice-versa with a concomitant
second order stabilization energy of E(2) = 0.18 kcal/mol for each interaction. In addition the
energetic differences between the lp and σ∗ orbitals are only 0.50 and 0.75 a.u. for the C–H and
C–O bonds, respectively Therefore the orbital stabilization energy that can be attributed to the
O···O is approximately 0.34 kcal/mol in the dimer. We have also computed the AIM analysis of
the model dimer of complex 1 shown in Figure 3C in order to investigate if the distribution of
critical points is similar to the one found in the formaldehyde dimer. The distribution is shown in
Figure 5A. It can be observed that the distribution in this model of complex 1 also exhibits two
bond critical points that characterize the hydrogen bonds in addition to the bond CP and bond
path that characterize the O···O interaction. This results agrees with the larger binding energy
obtained for the dimer of complex 1 compared to the formaldehyde dimer. The properties of the
charge density at the bond CPs are summarized in Table 5. The values of the Laplacian are
positive as is common in closed shell interactions. The Kinetic energy and the total energy
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density are higher at the bond CP that connects the oxygen atoms than that at the bond CP that
characterizes the hydrogen bonds. Finally, we also represent in Figure 5B the distribution of
critical points that characterizes the C–H···H–C interaction described above (see Figure 3A).
This interaction is characterized by the presence of three bond CPs and bond paths that inter-
connect three hydrogen atoms of the ethyl groups.
For complex 2 we have focused our attention to the C–H/π interactions and the influence
of the metal center of the strength of the interaction. The dimers retrieved from the X-ray
structure are shown in Figure 6. In one dimer the hydrogen atoms that participate in the
interaction belong to the aliphatic part of the tridentate Schiff base ligand (Figure 6A) and in the
other dimer the hydrogen atoms belong to the acetylacetonate (4-oxopent-2-en-2-olate) coligand
(Figure 6B). The interaction energies of the dimers are ꢀE6 = –11.1 kcal/mol for the Schiff base
interaction and ꢀE7 = –8.2 kcal/mol for the acetylacetonate coligand. We have used two
theoretical models to evaluate the influence of the coordination to the transition metal upon the
interaction energies. In both models we have eliminated the metal center and protonated the
oxygen atom of the ligand that is coordinated to the cobalt(III) ion in the real system (Figure 6C
and D). As a result both interaction energies weaken to ꢀE8 = –6.0 kcal/mol and ꢀE9 = –6.1
kcal/mol, confirming that the coordination of the ligand to the metal center increases the acidity
of the interacting hydrogen atoms and enhances the C–H/π interactions. For the latter models we
have also computed the interaction energy at a higher level of theory (RI-MP2) in order to
validate the DFT method used in this study. The interaction energies are in reasonable
agreement, giving reliability to the methodology used herein (Figure 6).
We have also confirmed the existence of the aforementioned C–H/π interactions using the
AIM analysis of critical points and bond paths (Figure 7). In the dimer where the interacting
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hydrogen atoms belong to the aliphatic part of the tridentate Schiff base ligand (Figure 7A) the
distribution of critical points show four bond CPs that connect several hydrogen atoms with the
carbon atoms of the aromatic rings. The interaction is further characterized by the presence of
several ring CPs that are generated as a consequence of the formation of several supramolecular
rings. In the other dimer the hydrogen atoms belong to the acetylacetonate coligand (Figure 7B)
and the interaction is characterized by the presence of three bond CPs that connect three
hydrogen atoms with three carbon atoms of the aromatic part of the ligand. The interaction is
further characterized by the presence of two ring CPs. The properties of the density at two
representative bond CPs are summarized in Table 5.
Phenoxazinone synthase like activity
The phenoxazinone synthase mimicking activity of the complex 2 was studied by
monitoring the oxidation of o-aminophenol (OAPH) spectrophotometrically in dioxygen
saturated methanol solution at room temperature. Before going into detailed kinetic
investigation, it is necessary to get an estimation of the ability of the complexes to oxidize o-
aminophenol, and for this purpose 1.0 × 10−5 M solutions of the complexes were mixed with a
10-3 M solution of OAPH, and the spectra were recorded for up to 2 h in dioxygen saturated
methanol at room temperature. The time dependent spectral profiles for a period of 2 h after the
addition of OAPH are shown in Figure 8 for 2. The spectral scans reveal the progressive increase
of peak intensity at ca. 433 nm, characteristic of the phenoxazinone chromophore, suggesting the
catalytic conversation of OAPH to 2-aminophenoxazine-3-one in aerobic conditions. A blank
experiment without catalyst under identical conditions does not show significant growth of the
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band at 433 nm. These spectral behaviors conclude that complex 2 show phenoxazinone
synthase-like activity under aerobic conditions.
Kinetic studies were performed to understand the extent of the catalytic efficiency. For
this purpose, 1.0 × 10−5 M solutions of the complex was treated with substrate, maintaining
pseudo first order conditions. For a particular complex substrate mixture, time scans at the
maximum band of 2-aminophenoxazine-3-one were carried out for a period of 30 min, and the
initial rate was determined by linear regression from the slope of the absorbance versus time, and
each experiment was performed thrice and average values were noted. As shown in Figure 9, the
initial rates of the reaction versus concentration of the substrate plots show rate saturation
kinetics. This observation indicates that an intermediate complex substrate adduct formed at a
preequilibrium stage and that the irreversible substrate oxidation is the rate determining step of
the catalytic cycle. This type of saturation rate dependency on the concentration of the substrate
can be treated with the Michaelis–Menten model, which upon linearization gives a double
reciprocal Lineweaver–Burk plot to analyze values of the parameters Vmax, KM, and Kcat. The
observed and simulated initial rates versus substrate concentration plot and the Lineweaver–Burk
plot for 2 are shown in Figure 9 and 10, respectively. Analysis of the experimental data yielded
Michaelis binding constant (KM) value of 4.39×10−3 and Vmax value of 8.09×10−5 M−1. The
turnover number (Kcat) value is obtained by dividing the Vmax by the concentration of the
complex used, and is found to be 8.32 s-1.
IR and NMR spectra
In the IR spectra of 1 and 2 distinct bands due to azomethine (C=N) groups appear at
1583 and 1577 cm-1 respectively whereas, bands for carbonyl (C=O) groups are noticed at 1618,
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and 1621 cm-1 respectively.34,35 Appearance of strong band at 2023 cm-1 in the IR spectra of both
complexes indicates the presence of azide.36
1H NMR spectra data for complexes 1 and 2 have been summarized in experimental
section and the spectra have been depicted in Figure S1 and 11 respectively. Imine protons (-
CH=N) of 1 and 2 appear as a singlet at 8.83 ppm and 8.72 ppm respectively whereas, aldehyde
protons (-CH=O) of 1 appears at 9.77 ppm. A sharp singlet at 5.62 ppm and two sharp singlet at
2.27 ppm and 1.63 ppm appears in the spectrum of 2 suggesting the presence of acetylacetonate
moiety (Figure 11).
Electronic spectra and photophysical study
In electronic spectra of both complexes, two absorption bands of similar fashion are
observed at 404 nm, 421 nm (for 1, see Figure S2) and 408 nm, 424 nm (for 2, see Figure S3) in
the visible region which are assigned as LMCT.34 In UV region few other absorption bands are
observed at 220 nm, 269 nm, 319 nm for 1 and 224, 266 for 2. These bands are attributed to π-π*
transition of azomethine and naphthalene chromophores. No d-d bands are observed at higher
wavelength in the electronic spectra of both complexes. The bands are not observed due to the
low concentration (10-4 mol L-1) of the complexes solution. These bands should be low in
intensity in the region of 500-600 nm.
Complex 1 shows emission at 344 nm in the UV region, whereas complex 2 exhibits
emission in the visible region at 431 nm upon irradiation with UV-light (300 nm) in acetonitrile
at room temperature (Figure 12). The exited state mean lifetimes of complexes 1 and 2 are 3.17
ns and 6.7 ns respectively. This emission band can be attributed to intra ligand fluorescent
1(π→π*) emission of the coordinated ligand.37
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Relative fluorescence quantum yields for two complexes were measured in acetonitrile
using quinine sulfate (in 0.5 (M) H2SO4, Φ = 0.54) as the quantum yield standard.38 The
fluorescence quantum yields of complexes 1 and 2 are 0.0046, 0.0041 respectively. If Φx and Φs
are the quantum yields of a given fluorophore species ‘x’ and the standard ‘s’, respectively, then
Φx = Φs(Fx/Fs)(Ax/As)
where Fx, Fs are the wavelength integrated emission intensities of the two samples, As, Ax are the
optical densities at their corresponding wavelengths of excitation.39
Conclusion
Herein, we report the synthesis, X-ray characterization and photoluminescence properties
of two new coordination cobalt(III) Schiff base complexes. Hydrogen bonding, C–H/π and O···O
interactions play an important role in the packing of the complexes in the solid state. The metal
coordination of the ligands enhances the C–H/π interaction as has been demonstrated by means
of DFT calculations. A special attention has been paid to the short O···O interactions between
the aldehyde groups that has been analyzed using a variety of computational tools. It has a
favorable orbital contribution that is rationalized by means of donor-acceptor orbital interactions
(lp → σ∗ electron donation).
Acknowledgments
S.C. acknowledges DST, India under FAST Track Scheme (Order No. SR/FT/CS-
118/2010, dated 15/02/2012). K.R. gratefully acknowledges the financial support from the
Academy of Finland (Project No’s. 122350, 140718, 256259, 265328 and 263256). A.B. and
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A.F. thank MINECO of Spain (project CONSOLIDER INGENIO 2010 CSD2010-00065,
FEDER funds) and the CTI (UIB) for free allocation of computer time.
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Table 1: Crystal data and refinement details in complexes 1 and 2
Complexes
Formula
1
C28H28CoN5O3
541.48
2
C22H28CoN5O3
Formula Weight
Crystal Size (mm)
Temperature (K)
Crystal system
Space group
469.42
0.14 x 0.21 x 0.32
170
0.12 x 0.18 x 0.24
123
Tetragonal
P43212
Orthorhombic
P212121
a(Å)
10.2252(2)
10.2252(2)
50.0545(10)
8
9.55072(15)
12.7917(2)
18.0663(3)
4
b(Å)
c(Å)
Z
dcalc (g cm-3)
1.375
1.413
ꢁ (mm-1)
5.459
0.811
F(000)
2256
984
Total Reflections
Unique Reflections
Observed data [I > 2σ (I)]
R(int)
9712
28107
4839
5055
4405
4429
0.051
0.056
R1, wR2 (all data)
R1, wR2 [I > 2 σ (I)]
Largest diff. in peak and hole (eÅ-3)
0.0442, 0.0940
0.0403, 0.0927
0.33, -0.21
0.0472, 0.0845
0.0364, 0.0782
0.36, -0.25
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Table 2: Selected bond lengths around cobalt(III) (Ǻ) in complexes 1 and 2.
Complex
1
2
Co(1)–O(1)
Co(1)–O(2)
Co(1)–O(3)
Co(1)–N(1)
Co(1)–N(2)
Co(1)–N(3)
1.894(2)
1.909(2)
1.909(2)
1.883(2)
2.068(2)
1.932(2)
1.874(2)
1.922(2)
1.915(2)
1.887(2)
2.066(2)
1.949(2)
Table 3: Selected bond angels around cobalt(III) (°) in complexes 1 and 2.
Complex
1
2
O(1)–Co(1)–O(2)
O(1)–Co(1)–O(3)
O(1)–Co(1)–N(1)
O(1)–Co(1)–N(2)
O(1)–Co(1)–N(3)
O(2)–Co(1)–O(3)
O(2)–Co(1)–N(1)
O(2)–Co(1)–N(2)
O(2)–Co(1)–N(3)
O(3)–Co(1)–N(1)
O(3)–Co(1)–N(2)
O(3)–Co(1)–N(3)
N(1)–Co(1)–N(2)
N(1)–Co(1)–N(3)
N(2)–Co(1)–N(3)
89.75(9)
87.33(8)
93.56(8)
176.66(8)
88.74(1)
92.32(8)
87.52(8)
93.58(8)
178.11(9)
179.10(9)
92.85(9)
86.49(9)
86.28(9)
93.69(9)
87.95(1)
87.60(7)
86.03(7)
93.49(8)
179.02(8)
90.68(8)
93.87(7)
88.61(8)
93.19(8)
178.28(8)
177.46(8)
94.51(8)
85. 85(8)
85.94(8)
91.66(9)
88.53(8)
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Table 4: Interaction energy (BSSE corrected) of the formaldehyde dimer at different distances
O···O distance (Å)
ꢀE (kcal/mol)
–1.6
2.98
3.06
3.21
3.46
–1.5
–1.0
–0.7
Table 5: Properties (a.u.) at the bond critical point for some model dimers of complexes 1 and 2.
See Figures 5 and 7 and for the labelling of critical points.
∇2ρ
Complex
CP1
ρ
H(r)
G(r)
V(r)
ε
0.0092
0.0090
0.0050
0.0051
0.0042
0.0057
0.0043
0.0387
0.0420
0.0206
0.0206
0.0155
0.0154
0.0128
0.0019
0.0021
0.0013
0.0013
0.0008
0.0006
0.0007
0.0078
0.0084
0.0039
0.0039
0.0031
0.0032
0.0025
-0.0059
-0.0064
-0.0026
-0.0027
-0.0023
-0.0026
-0.0019
0.4139
0.7379
0.1496
0.1395
0.0594
0.3015
0.6784
CP2
CP3
CP4
CP5
CP6
CP7
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Figure 1: Perspective views of complexes 1 and 2 with selective atom numbering scheme.
29