Synthesis, structural characterization and DFT analysis of an unusual tryptophan copper(II) complex bound via carboxylate monodentate coordination: Tetraaquabis(L-tryptophan) copper(II) picrate

Article abstract of DOI:10.1016/j.ica.2018.06.021

Herein, we report the synthesis, spectroscopy, crystal structure and theoretical studies of a new Cu(II) complex with formula [Cu(L-trp)₂(H₂O)₄](pic)₂·4H₂O (1), which exhibits an unusual tryptophan coordination to copper(II) via carboxylate monodentate binding. The complex has been prepared using the mixed ligand approach. Single crystal X-ray diffraction structural analysis of 1 revealed that, in the complex, the Cu(II) site shows a distorted octahedral geometry, and the tryptophan (trp) ligand is coordinated through the carboxylate group in a monodentate fashion. The picric acid (pic) co-ligand, which is present in the anionic form, resides outside the coordination sphere forming a charge-separated complex. Density Functional Theory (DFT; including time-dependent DFT) has been employed to calculate the equilibrium geometry of the complex as isolated species in dimethylsulfoxide (DMSO) solution, and also its electronic spectrum, which was used to understand details of the experimentally obtained ultraviolet-visible (UV-Vis) spectrum. The structural analysis was complemented by additional spectroscopic studies [infrared (IR) and electron paramagnetic resonance (EPR) spectroscopies], and the thermal stability of the complex evaluated by thermogravimetric analysis (TGA).

Full text of DOI:10.1016/j.ica.2018.06.021

Accepted Manuscript
Synthesis, structural characterization and DFT analysis of an unusual tryptophan
copper(II) complex bound via carboxylate monodentate coordination: tetraa-
quabis(L-tryptophan) copper(II) picrate
Ahmad Husain, Girijesh Kumar, Trisha Sood, Shiwani Walia, Licínia L.G.
Justino, Rui Fausto, Rakesh Kumar
PII:
S0020-1693(18)30542-5
https://doi.org/10.1016/j.ica.2018.06.021
ICA 18313
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
11 April 2018
11 June 2018
13 June 2018
Please cite this article as: A. Husain, G. Kumar, T. Sood, S. Walia, L.L.G. Justino, R. Fausto, R. Kumar, Synthesis,
structural characterization and DFT analysis of an unusual tryptophan copper(II) complex bound via carboxylate
monodentate coordination: tetraaquabis(L-tryptophan) copper(II) picrate, Inorganica Chimica Acta (2018), doi:
https://doi.org/10.1016/j.ica.2018.06.021
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Synthesis, structural characterization and DFT analysis of an unusual tryptophan
copper(II) complex bound via carboxylate monodentate coordination: tetraaquabis(L-
tryptophan) copper(II) picrate
Ahmad Husain,1,* Girijesh Kumar,2 Trisha Sood,1 Shiwani Walia,1 Licínia L. G. Justino,3,*
Rui Fausto,3 Rakesh Kumar1,*
1
Department of Chemistry, DAV University, Jalandhar-144011, Punjab, India. [E-mail:Ahmad
Husain: ahmad.husain@outlook.com, Rakesh Kumar: rakesh_nitj@yahoo.co.in]
2
Department of Chemistry & Centre for Advanced Studies in Chemistry, Panjab University
Chandigarh, Chandigarh, India.
3 CQC, Department of Chemistry, University of Coimbra, Rua Larga 3004-535 Coimbra Portugal.
[E-mail: Licínia L. G. Justino: liciniaj@ci.uc.pt]
ABSTRACT
Herein, we report the synthesis, spectroscopy, crystal structure and theoretical studies of a
new Cu(II) complex with formula [Cu(L-trp)2(H2O)4](pic)2·4H2O (1), which exhibits an
unusual tryptophan coordination to copper(II) via carboxylate monodentate binding. The
complex has been prepared using the mixed ligand approach. Single crystal X-ray diffraction
structural analysis of 1 revealed that, in the complex, the Cu(II) site shows a distorted
octahedral geometry, and the tryptophan (trp) ligand is coordinated through the carboxylate
group in a monodentate fashion. The picric acid (pic) co-ligand, which is present in the
anionic form, resides outside the coordination sphere forming a charge-separated complex.
Density Functional Theory (DFT; including time-dependent DFT) has been employed to
calculate the equilibrium geometry of the complex as isolated species in dimethylsulfoxide
(DMSO) solution, and also its electronic spectrum, which was used to understand details of
the experimentally obtained ultraviolet-visible (UV-Vis) spectrum. The structural analysis
was complemented by additional spectroscopic studies [infrared (IR) and electron
paramagnetic resonance (EPR) spectroscopies], and the thermal stability of the complex
evaluated by thermogravimetric analysis (TGA).
Keywords: Crystal structure, tryptophan, picric acid, TGA, UV-Visible, IR, EPR, DFT.
1

1. Introduction
Non-classical interactions, such as hydrogen bonds, π---π stacking of aromatic rings,
cation---π interactions, and charge-transfer, play a crucial role in the structure and function of
macromolecules [1-7]. Metal–(amino acid) complexes, in particular, have been extensively
used to mimic non-classical interactions present in biological macromolecules [8].
Copper has been reported to be ubiquitous in redox-active metalloproteins [9]. The Cu(II)
complexes of amino acids and their derivatives have attracted much attention as they are
associated with proteasome inhibition [10], and show both DNA cleavage activity and
cytotoxic effects toward various cell lines [11]. Tryptophan (trp), an α-amino acid having one
amino, one indolic and one carboxylic group, has been shown to be a versatile ligand in
coordination chemistry due to the electron donor capability of its amino acid and N-
heterocyclic ring moieties [12,13]. The indole group of tryptophan has great potential for
noncovalent interactions in mixed ligand complexes. There are several reports in the
literature describing the structure of coordination compounds where either the amino groups
or both amino and carboxylate groups of tryptophan act as coordination moieties [14,15].
However, according to the CSD 2017 database [16], there are no reports of complexes where
tryptophan coordinates through the carboxylate oxygens in a monodentate way.
Herein, we report a tryptophan-based copper(II) complex, with formula [Cu(L-
trp)2(H2O)4](pic)2·4H2O (1), where the tryptophan carboxylate group is coordinated to the
Cu(II) ion in a monodentate fashion, and the picrate anion acts as counter ion. Picric acid is
known to act as an acceptor to form various π stacking complexes, and also as an acidic
ligand through specific electrostatic or hydrogen bond interactions. We determined the
molecular structure of 1 in crystalline phase, by single crystal X-ray diffraction, and
interpreted it based also in calculations for the isolated Cu(II)-(L-trp)2(H2O)4 nucleus
performed using density functional theory (DFT). The structural analysis was complemented
2

by additional spectroscopic studies [infrared (IR), ultraviolet-visible (UV-Vis), electron
paramagnetic resonance spectroscopy (EPR)], and the thermal stability of the complex
probed by thermogravimetric analysis (TGA).
2. Experimental and Computational Methods
2.1. Materials and instrumentation
All chemicals (including solvents) used in this study were of high purity. L-Tryptophan
was supplied by Loba Chemie and used without purification. Picric acid was used after
recrystallization. The IR spectrum was recorded with a Perkin–Elmer FTIR 2000
spectrometer with 4 cm-1 resolution and 64 accumulations, for the compound in a KBr pellet.
The thermogravimetric analysis (TGA) measurements were performed on an SDT Q600
(V20.9 Build 20) instrument (Artisan Technology Group, Champaign, IL), under N2
atmosphere, with a heating rate of 10 °C min-1. UV-Visible absorbance spectra were recorded
on a Shimadzu UV-1800 spectrophotometer. Powder X-ray diffraction (PXRD)
measurements were performed on a Bruker AXS D8 Advance X-ray (Cu-Kα1 radiation, λ =
1.5406 Å) diffractometer in the 5–35° 2θ range using a 0.02° s-1 step size. The sample was
prepared by making a thin film from the finely powdered sample (~30 mg) over a glass slide.
EPR measurements were carried out in a BRUKER Xenon EMX-ER073 spectrometer.
Elemental analyses for C, H, and N were undertaken using a FLASH Ea1112 SERIES CHNS
analyser.
2.2. Single crystal X-ray diffraction analysis
Single crystal X-ray diffraction data collection was carried out on a Bruker SMART
APEX II CCD diffractometer, using graphite monochromated Mo-Kα (λ = 0.71073 Å)
radiation. The frames were collected at T = 298 K. Data reduction and unit cell refinement for
1 was performed using SAINT-Plus [17]. The SHELXL-2014/7 [18,19] software was used to
3

solve the structure of 1 by the direct method, the refinement procedure being done by full-
matrix least-squares, based on F2 values against all reflections. Hydrogen atoms were fixed at
the calculated positions with isotropic thermal parameters. The hydrogen atoms of the lattice
water molecules could not be located from the difference Fourier map; however their
contributions were included in the empirical formula. Details of the crystallographic data
collection are given in Table 1. CCDC number 1558367 contains the supplementary
crystallographic data for this paper. This data can be obtained free of charge via www.
ccdc.cam.ac.uk/data request/cif [or from the Cambridge Crystallographic Data Centre
(CCDC), 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44(0)1223-336033; e-mail:
deposit@ccdc.cam.ac.uk].
Table 1. Crystallographic data for complex [Cu(L-trp)2(H2O)4](pic)2·4H2O.
Compound
[Cu(L-trp)2(H2O)4](pic)2·4H2O
Empirical formula
Formula weight
Crystal system
Crystal size
C34H44CuN10O26
1072.32
monoclinic
0.32 × 0.20 × 0.12
C2
Space group
Unit cell dimensions
a = 34.88(2) α = 90
b = 7.542(5) ß = 94.877(8)
c = 9.030(6) γ = 90
2367(3), 2
1.488
0.560
(Å), ( ° )
Volume (Å3), Z
ρcalc (g cm-3)
µ (cm-1)
F(000)
1086
Range of data collection
Limiting frequency
Total reflections
2.264 - 25.800
–42 ≤ h ≤ 41, -9 ≤ k ≤9, –10 ≤ l ≤10
13114
Independent reflections
Completeness to theta = 25.00
Refinement method
Data / restraints / parameters
Goodness of fit on F2
Final R indices [I > 2 (I)]
R indices (all data)
Largest diff. peak & hole (e Å3)
4371 [Rint = 0.0877]
99.7 %
Full matrix least squares on F2
4371 / 88 / 323
0.948
R1 = 0.0807, wR2 = 0.2053
R1 = 0.1365, wR2 = 0.2319
0.88 and –0.46
4

2.3. Theoretical calculations
The geometry of the open shell d9 (doublet state) Cu(II) complex 1 was optimized at the
unrestricted DFT/CAM-B3LYP [20] level of theory, with the LANL2DZ (Los Alamos
National Laboratory 2 double-ζ) [21–23] valence and effective core potential (ECP) functions
for copper and the standard double-ζ plus polarization 6-31G(d,p) basis set for the remaining
atoms. The X-ray determined structure was used as input for geometry optimization, and a C2
symmetry constrain was imposed, in accordance with the solid state symmetry of the
complex. The bulk solvent effects of dimethyl sulfoxide (DMSO) were considered through
the polarizable continuum model (PCM) using the integral equation formalism variant
(IEFPCM) [24, 25]. Calculation of the vibrational frequencies was carried out for the
optimized geometry to certify that the stationary point found was a true minimum (i.e., with
no imaginary frequencies). Time-dependent DFT (TD-DFT) calculations were carried out for
the optimized geometry using the CAM-B3LYP functional to obtain the vertical excitation
energies for the lowest 30 excited states (up to 5.3 eV). The bulk solvent effects of DMSO
were considered by using the IEFPCM [24, 25] solvent model. To assess the occurrence of
spin-contamination in the U-DFT calculations, the value obtained for was compared
with the expected S(S + 1) for the spin state (doublet state). In all the calculations spin-
contamination was found to be smaller than 3 %, and therefore, negligible. DFT and TD-DFT
calculations for the picrate anion were also carried out using the same functional, basis set (6-
31G(d,p)) and solvent model. The DFT calculations were performed using Gaussian 09
program and Gauss View was used for visualization of the structure, simulated UV/Vis
spectra and molecular orbitals [26].
2.4. Synthesis of [Cu(L-trp)2(H2O)4](pic)2·4H2O (1)
Copper picrate was synthesised by a modified literature procedure [27]. Picric acid (2
mmol; 458 mg) was added to a 1 mmol basic copper carbonate (221 mg) solution in distilled
5

water. The solution was stirred for an hour and heated at 50 ºC, with evolution of CO2. The
obtained green coloured solution was filtered and left for slow evaporation. Green coloured
crystals of copper picrate were formed after 1 week. An aqueous solution of copper picrate
0.5 mmol (114.55 mg) was then added to a clear solution of 1 mmol of tryptophan (204.22
mg) in 6–8 mL of distilled water, giving rise to a solution of yellowish green colour. This
solution was stirred overnight at room temperature. A red coloured complex was afforded,
which was collected by filtration and dried in desiccator. Yield: 0.804 g (75%). Anal. Calcd.
for C34H44CuN10O26: C, 38.08; H, 4.14; N, 13.06. Found: C, 38.42; H, 3.12; N, 13.15. IR
spectrum (KBr disk, selected peaks; ): 3403, 3154, 1625, 1598, 1565, 1533, 1416, 1327,
1294, 1165, 1080, 917, 743, 705, 509 cm–1 (see Figures S1 and S2, Supporting Information).
3. Results and Discussion
3.1. Synthesis and crystal structure description of [Cu(L-trp)2(H2O)4](pic)2·4H2O (1).
The copper picrate was synthesised using copper carbonate and picric acid in water, as
described in Section 2.5. The reaction between copper picrate and tryptophan afforded a
highly crystalline product of formula of [Cu(L-trp)2(H2O)4](pic)2·4H2O (1) (see Scheme 1).
The complex 1 was first characterized by elemental analysis and infrared spectroscopy (see
above). Single crystal X-ray diffraction analysis showed that 1 crystallizes in a monoclinic
unit cell, space group C2, exhibiting an asymmetric unit that consists of half of Cu(II),
located at the inversion centre, one molecule of tryptophan, one picrate anion, two
coordinated aqua ligands and two lattice occluded water molecules (Scheme 1 and Figure 1).
The overall charge of the complex nucleus is balanced by the lattice picrate anions. Complex
1 contains a distinctive structural motif containing a unique Cu(II) ion in a distorted
octahedral coordination geometry, with the tryptophan ligands exhibiting the unusual
monodentate coordination via the carboxylate group.
6

Scheme 1. Synthesis of the complex [Cu(L-trp)2(H2O)4](pic)2·4H2O.
The ORTEP diagram of the complex is shown in Figure 1. Selected bond lengths and
bond angles are listed in Table 2. As shown in Figure 1, the octahedral geometry at the Cu(II)
center is satisfied with two carboxylate oxygen atoms belonging to two different tryptophan
molecules, in a monodentate coordination mode and in trans-fashion, and by four water
molecules. The two carboxylate groups of the coordinated tryptophan molecules are nearly
perpendicular to each other, as shown by the torsion angle (73.37±0.10º) between the planes
of the two carboxylate groups determined experimentally. The tryptophan molecule exhibits a
staggered conformation with respect to indole ring, and gauche relatively to the amino group
[28].
The coordinated aqua molecules are H-bonded to the phenolic oxygen atom and nitro
functionality of the picrate anion (see Figure 2 and Table S1 in Supporting Information). The
protonated amino group of the tryptophan ligands is also H-bonded, with all the three H
atoms being involved in the H-bond network and being connected to oxygen atoms belonging
to the carboxylate group of a neighboring tryptophan molecule, the phenolate moiety of the
picrate anion, or a water molecule. The carbonyl oxygen atom of the tryptophan ligand forms
7

a four membered ring with graph set (4), defined by two tryptophan neighboring molecules
+
and involving also the NH3 moieties of the same two molecules (see Figure 2). The pyrrolyl
hydrogen atom is also H-bonded to an oxygen atom of a nitro group of picrate anion,
connecting two adjacent molecules. The nitro groups of the picrate moiety are twisted out of
the benzene ring plane by 5–19°. Noteworthy, the 2D structure of 1 shows only 2.8% solvent
accessible voids (SAVs) per unit volume, as calculated using PLATON [29], demonstrating
the dense packing of the molecules in the crystal. Crystal packing shows the indole and
picrate rings aligned parallel to each other, indicating aromatic ring stacking interaction
between them, as displayed in Figure 3. Although these interactions, as measured by the
distances between the interacting rings (4.208-4.916 Å) are week, they certainly play a
relevant role in the stabilization of the three dimensional network of the crystal.
Fig. 1. ORTEP drawing of complex 1 depicting 30% thermal ellipsoid probability, with atom labeling
scheme. H-atoms have been omitted for clarity. Symmetry transformations used to generate
equivalent atoms are: (i) –x, y, –z.
Fig. 2. H-bond interactions (N–H---O and O–H---O) in the crystal of 1, viewed along the b-axis.
8

Fig. 3. Display of π…π stacking interactions between the indole and picrate rings, which align parallel
to each other.
Table 2. Selected geometric parameters obtained for the crystal of [Cu(L-trp)2(H2O)4](pic)2·4H2O (1)
by X-ray diffraction and calculated at the unrestricted CAM-B3LYP/LanL2DZ level for the [Cu(L-
trp)2(H2O)4]2+ complex in DMSO solvent.
Bond distances (Å)
Cu(1)-O(2)i
Experimental
1.911(6)
CAM-B3LYP (in DMSO)
Deviation a
1.943
1.943
2.185
2.185
2.158
2.158
0.032
0.032
0.088
0.088
0.060
0.060
Cu(1)-O(2)
1.911(6)
Cu(1)-O(1W)i
Cu(1)-O(1W)
Cu(1)-O(2W)
Cu(1)-O(2W)i
2.097(11)
2.097(11)
2.098(16)
2.098(16)
Bond Angles (º)
O(2)i-Cu(1)-O(2)
177.1(8)
86.1(4)
92.0(4)
92.0(4)
86.1(4)
99.4(7)
83.5(5)
98.7(5)
89.4(5)
169.8(6)
98.7(5)
83.5(5)
169.8(6)
89.4(5)
82.3(10)
170.6
82.4
90.9
90.9
82.4
89.1
91.3
95.6
92.4
177.4
95.6
91.3
177.4
92.4
86.0
-6.5
-3.7
-1.1
-1.1
-3.7
-10.3
7.8
-3.1
3.0
7.6
-3.1
7.8
7.6
3.0
3.7
O(2)i-Cu(1)-O(1W)i
O(2)-Cu(1)-O(1W)i
O(2)i-Cu(1)-O(1W)
O(2)-Cu(1)-O(1W)
O(1W)i-Cu(1)-O(1W)
O(2)i-Cu(1)-O(2W)
O(2)-Cu(1)-O(2W)
O(1W)i-Cu(1)-O(2W)
O(1W)-Cu(1)-O(2W)
O(2)i-Cu(1)-O(2W)i
O(2)-Cu(1)-O(2W)i
O(1W)i-Cu(1)-O(2W)i
O(1W)-Cu(1)-O(2W)i
O(2W)-Cu(1)-O(2W)i
a
Mean absolute deviation between the theoretical and experimental structures is 0.06 Å for bond
lengths and 4.9º for bond angles. See Figure 1 for atom numbering.
To observe the homogeneity and crystallinity of the bulk material, the powder X-ray
diffraction (PXRD) pattern of the sample was also recorded, which fits nicely the simulated
9

pattern generated from the obtained single-crystal X-ray data and, thus, confirmed the
homogeneity and purity of the crystalline phase (Figure 4).
2
Fig. 4. The powder X-ray pattern of 1: simulated from the single crystal X-ray structure using the
Mercury 3.9 software (lower, blue trace) and the as-synthesized sample (upper, black trace).
3.2. DFT studies
To gain insight into the geometry and electronic structure of complex 1, DFT
calculations were performed at the CAM-B3LYP/6-31G(d,p)+LanL2DZ level of theory using
the geometric parameters obtained from X-ray analysis as starting structure. The structural
parameters calculated for the isolated complex considering the bulk solvents effects of
DMSO are shown in Table 2, where they can be compared with the experimental X-ray
structure. The mean absolute deviation between the theoretical and experimental structures is
0.06 Å for bond lengths and 4.9° for bond angles. These differences are small and involve, to
a major extent, the coordination of the four inner sphere water molecules. Taking into
account that the picrate anions in the crystal are involved in H-bonding with the coordinated
water ligands, and also that these counter ions were not included in the geometry
optimization, these structural differences are expected. The interactions arising from the
packing in the crystal are also absent in the theoretical calculations and may also contribute to
the differences found. The geometry around the metal center in the calculated structure
(Figure 5a) is distorted octahedral, as found experimentally for the complex (Figure 5b).
10

Similarly to what was found for the solid state structure, intramolecular H-bonds between the
protonated amino groups and the oxygen atoms of the carbonyl groups of tryptophan are
observed in the calculated structure.
Fig. 5. (a) Unrestricted CAM-B3LYP optimized molecular structure of the [Cu(L-trp)2(H2O)4]2+
complex. (b) ORTEP drawing showing the distorted octahedral geometry of complex 1 as determined
experimentally. Symmetry transformations used to generate equivalent atoms are: (i) –x, y, –z.
3.3. Hirshfeld surface analysis
The Hirshfeld surface analysis [30] was performed, using Crystal Explorer 3.1 [31,32],
to access details of the intermolecular interactions in the crystal. The Hirshfeld surfaces of
complex 1 are illustrated in Figure 6. The surfaces have been mapped over dnorm and 2D
fingerprint plots (plot of di versus de), allowing identification of different types of
intermolecular interactions. In the figure, the dnorm Hirshfeld surfaces are displayed using a
red–white–blue colour scheme, where red highlights shorter contacts, white is used for
contacts around the rvdW separation, and blue is for longer contacts. As it can be seen from the
figure, the O···H interactions are dominant, covering ~33% of the total Hirshfeld surface,
with two distinct spikes in the 2D fingerprint plots (see Figure 6b,c), and that can be viewed
as dark red spots on the dnorm surface (Figure 6a). The C···H and O···O interactions have also
a significant contribution to the total Hirshfeld surfaces, as displayed in Figure S3
(Supporting Information).
11

Fig. 6. (a) Hirshfeld surfaces representation for 1 mapped with dnorm with all types of intermolecular
interactions considered. (b, c) Fingerprint plots of 1: (b) all contacts accounted for, and (c) resolved
into O…H interactions, showing the percentages of contacts contributing to the total Hirshfeld surface
area.
3.4. Electronic spectroscopy and results of TD-DFT calculations
The electronic absorption spectrum of [Cu(L-trp)2(H2O)4](pic)2·4H2O (1) was recorded
in DMSO (5.12 × 10-5 M) at room temperature (Figure 7a). The complex exhibits an intense
absorption band with a maximum at 370 nm. Interestingly, the tryptophan-picrate complex
shows a charge-transfer band at 380 nm, due to the stacking between the indole and the
picrate rings, as shown in Figures S4 and S5. For [Cu(L-trp)2(H2O)4](pic)2·4H2O this band is
blue shifted to 370 nm, indicating the transfer of some electron density to the metal orbitals,
thus suggesting ligand to metal charge-transfer that is coupled with the charge-transfer
between the indole and the picrate rings. A weak broad band is also observed at 747 nm,
ascribable to a Cu(II) d–d transition.
TD-DFT calculations were carried out to obtain further information on the nature of the
excited states and help interpret the UV-Vis absorption spectrum. The absorption spectrum in
12

DMSO solution was calculated for both the [Cu(L-trp)2(H2O)4]2+ complex and the picrate
anion by TD-DFT with the long-range corrected CAM-B3LYP functional, which has been
found to give improved results for charge-transfer excitations compared with the B3LYP
functional [20]. The simulated absorption spectra are shown in Figure 7b, in comparison with
the experimental spectrum for the complex with the picrate anions.
Fig. 7. (a) UV-Visible spectrum of [Cu(L-trp)2(H2O)4](pic)2·4H2O (1) in DMSO; (b) Simulated TD-
DFT/CAM-B3LYP absorption spectra for the [Cu(L-trp)2(H2O)4]2+ complex (red line) and the picrate
anion (black line) in DMSO. Note that the higher energy band seen in the theoretical spectrum of the
complex is outside the region studied experimentally.
The comparison of the absorption spectra calculated for the [Cu(L-trp)2(H2O)4]2+
complex and for the picrate anions separately (Figure 7b) with the experimental spectrum
obtained for the [Cu(L-trp)2(H2O)4](pic)2·4H2O system (Figure 6a), suggests that, in the
experimentally observed spectrum, there is a superposition of the absorptions from the two
species, and that the picrate anions have a very significant contribution to the total spectrum.
Studies
undertaken
in
similar
compounds,
specifically
Cu(glycine)picrate,
Cu(phenylalanine)picrate, and tryptophan-picrate confirm the dominance of the picrate bands
also in the UV-Vis spectra of these compounds (Figure S5). In fact, since the concentration of
picrate anions in solution is twice the concentration of [Cu(L-trp)2(H2O)4]2+, the picrate
13

contribution to the experimental UV-Vis spectrum of the studied compound shall be even
larger than it is suggested by Figure 7b. In here, one will first analyze the theoretical
spectrum calculated for the core structure of the complex (without taking into account the
picrate counterions). The corresponding absorption spectrum was calculated for energies up
to 5.3 eV and the vertical excitation energies, oscillator strengths and main contributions to
the excited states of [Cu(L-trp)2(H2O)4]2+ are summarized in Table 3.
Table 3. TD-DFT/CAM-B3LYP calculated vertical excitation energies (eV), wavelengths (/ nm),
oscillator strengths (f) and main contributions to the excited states (only the contributions ≥ 10 % are
given) for the [Cu(L-trp)2(H2O)4]2+ complex in DMSO.
Energy (eV) λ (nm)
f a
Main contributions (%)
H-28 → L (55%) + H-27 → L (15%) + H-34 → L (11%)
H → L (98%)
1.80
3.76
4.20
4.43
4.99
5.13
689
330
295
280
249
242
0.0006
0.018
0.225
0.030
0.018
0.158
H-10 → LH-11 → L
H-11 → LH-10 → LH-7 → L
H-14 → LH-12 → LH-8 → L
H → L+2 (12%) + H → L+1 (12%) + H-2 → L+2 (10%) +
H-3 → L+1(10%)
5.22
238
0.197
H-1 → L+1(12%) + H-1 → L+2 (12%)
a Results are listed for excited states with oscillator strength ≥ 0.018 and also for the d-d transition at
689 nm.
The TD-DFT calculations indicate that the absorption spectrum of the complex has three
dominant bands in the region studied theoretically, with maxima at 295 nm, 242 nm and 238
nm. The band at 295 nm corresponds to a β-spin HOMO-10 (highest occupied molecular
orbital-10) to LUMO (lowest unoccupied molecular orbital) transition (56%), together with a
β-spin HOMO-11 → LUMO transition (35%). The electronic density distribution in the
orbitals involved in these transitions (Figure 8) indicates that this excitation has a dominant
ligand to metal charge transfer character (LMCT). The two bands at 242 and 238 nm are
very close and are observed in the simulated spectrum (see Figure 7b) as a single band. These
bands involve contributions from HOMOβ, HOMO-1β, HOMO-2β to LUMO+2β and from
HOMOα, HOMO-1α and HOMO-3α to LUMO+1α, and are ligand based transitions with a
14

dominant π → π* character. The weak band predicted at 689 nm corresponds to the
experimental band observed at 747 nm and involves contributions from β-spin HOMO-28
(55%), β-spin HOMO-27 (15%) and β-spin HOMO-34 (11%) to β-spin LUMO. The orbital
LUMOβ is essentially localized on the metal and to a small extent on the water ligands and
on the carboxylic groups. This orbital involves a combination of metal d orbitals, with a large
contribution from the
orbital. The orbitals HOMO-28β, HOMO-27β and HOMO-34β
are delocalized over the whole molecule, and include significant charge on the metal d
orbitals. Therefore, the band at 689 nm can be considered to have d-d, together with LMCT,
characters. The remaining excitations listed on Table 3, with absorptions at 330, 280 and 248
nm, are weaker and involve transitions from HOMOβ, HOMO-7β, HOMO-8β, HOMO-10β,
HOMO-11β, HOMO-12β and HOMO-14β to LUMOβ. These are essentially LMCT
transitions.
If we consider now the absorption spectrum calculated for the picrate anion, this
spectrum reproduces the shape of the experimental UV-Vis spectrum obtained for the [Cu(L-
trp)2(H2O)4](pic)2·4H2O system in DMSO. The bands are, however, shifted to higher energies
by ca. 60 nm, which is within the expected error for TD-DFT/CAMB3LYP calculations.
Table 4 summarizes the results for the calculated vertical excitation energies, oscillator
strengths and main contributions to the excited states of the picrate anion in DMSO solution.
The calculated absorption spectrum of the picrate anion has a dominant band with a
maximum at 307 nm (experimental maximum at 370 nm), which corresponds to a HOMO →
LUMO+1(82%), together with a HOMO-1 → LUMO+2 (13%) excitation. A second strong
band has its maximum at 367 nm (observed experimentally as a shoulder around 420 nm) and
corresponds to a HOMO → LUMO (100%) transition. Additional weaker bands have
calculated absorption maxima at 222, 193 and 187 nm. Figure 9 shows the molecular orbitals
of picrate involved in the main contributions to the excitations at 307 and 367 nm and the
15

molecular orbitals involved in the weaker excitations at 222, 193 and 187 nm can be seen in
the supplementary material (Figure S6).
Fig. 8. Schematic representation of the main contributions to the TD-DFT/CAM-B3LYP calculated
excitations for the [Cu(L-trp)2(H2O)4]2+ complex.
Table 4. TD-DFT/CAM-B3LYP calculated vertical excitation energies (eV), wavelengths (/ nm),
oscillator strengths (f) and main contributions to the excited states (only the contributions > 15 % are
given) for the picrate anion in DMSO.
Energy (eV) λ (nm)
f a
Main contributions (%)
H → L (100%)
3.38
4.04
5.59
6.41
6.64
367
307
222
193
187
0.206
0.328
0.183
0.157
0.115
H → L+1(82%) + H-1 → L+2 (13%)
H-2 → L (60%) + H-5 → L (25%)
H-7 → L (79%)
H-8 → L+1 (72%)
a Results are listed for excited states with oscillator strength ≥ 0.100.
16

Fig. 9. CAM-B3LYP optimized molecular structure of the picrate anion and schematic representation
of the main contributions to the TD-DFT/CAM-B3LYP excitations at 307 and 367 nm.
3.5. Thermal analysis
The termal stability of any compoud is the prime feature to use it as functional material.
Thereby, thermogravimetric analysis (TGA) measurements were performed for complex 1
under nitrogen atmosphere (Figure S7). The first weight loss ~15.75% was observed in the
temperature range 25–120 °C which corresponds to loss of nine water molecules. However,
there are only eight water molecules present in the crystal lattice. This inconsistancy for one
water molecule may be related to the the existance of moisture content in the crystaline
sample used for the TGA measurements. The complex 1 is further decomposed in subsequent
steps in the range of 190–500 °C, which correspond to the progressive destruction of organic
framework.
3.6. EPR spectroscopy studies
The polycrystalline powder EPR spectrum of [Cu(L-trp)2(H2O)4](pic)2·4H2O (1) at room
temperature (Figure 10) is characteristic of a typical for axial-type Cu(II) complex with
(2.530) > (2.109) > 2.03, suggesting a
ground state consistent with a distorted
octahedral geometry [33]. The > 2.3 indicates a considerable covalent character [34,35],
and the broadening of the spectrum is probably due to spin relaxation. The calculated G
17

value, [( –2.0023)/( –2.0023)] > 4, indicates that the spin exchange interaction between
Cu(II) centers in the solid state is negligible [36].
Fig. 10. X-band EPR spectrum of [Cu(L-trp)2(H2O)4](pic)2·4H2O (1).
4. Conclusion
The copper(II)-tryptophan amino acid complex [Cu(L-trp)2(H2O)4](pic)2·4H2O (1) was
prepared and structurally characterized. The metal complex exhibits distorted octahedral
geometry (Jahn-Teller distortion), and an unusual tryptophan coordination to copper(II) via
carboxylate monodentate binding. The picric acid (pic) co-ligand, which is present in the
anionic form, resides outside the coordination sphere forming a charge-separated complex.
Comparison between the experimental and theoretical structural results indicates that the
structure optimized at the unrestricted DFT/CAM-B3LYP level of theory, with the
LANL2DZ (Los Alamos National Laboratory 2 double-ζ) valence and effective core potential
(ECP) functions for copper and the standard double-ζ plus polarization 6-31G(d,p) basis set
for the remaining atoms, reproduces well that observed in the crystalline phase, with the
major differences in the two sets of structural parameters occurring for the parameters
associated with the coordination of the four inner sphere water molecules. These differences
might be ascribed to the larger uncertainties in the experimental determination of the
18

parameters of these water molecules and also to the fact that picrate counterions, with which
the water ligands are involved in H-bond type interactions, were not included in the
calculations.
The electronic structure of the complex was also evaluated theoretically and the UV-Vis
spectrum predicted through TD-DFT calculations, the calculated spectrum matching well that
obtained experimentally. The calculations also reveal that the electronic spectrum of the
compound in solution has a dominant contribution from the picrate ions. In addition, the
theoretical results indicate that the LUMO orbital has a significant contribution from the
metal
orbital, in accordance with the EPR experimental results, and that the main
transitions contributing to the observed spectrum involve charge transfer from the ligand to
the metal.
The thermal stability of the complex evaluated by TGA. The first weight loss was
observed in the temperature range 25–120 °C, and corresponds to loss of water molecules.
Subsequent weight losses take place in the 190–500 °C range, and correspond to the
progressive destruction of organic framework.
Acknowledgements: Financial support from Project PTDC/QEQ-QFI/3284/2014 – POCI-01-0145-
FEDER-016617, funded by the Portuguese “Fundação para a Ciência e a Tecnologia” (FCT) and
FEDER/COMPETE 2020-EU is acknowledged. The Coimbra Chemistry Centre (CQC) is supported
by FCT, through the project UI0313/QUI/2013, also co-funded by FEDER/COMPETE 2020-EU.
L.L.G.J. acknowledges FCT for the Post-doctoral Grant #SFRH/BPD/97026/2013. L.L.G.J. and R.F.
thank Prof. José António Paixão (CFisUC – Department of Physics, University of Coimbra) for the
helpful discussions on the X-ray analyses. G. Kumar thanks to Department of Science and
Technology (DST), Government of India for the financial assistant in the form of INSPIRE Faculty
Award with Ref. No. [DST/INSPIRE /04/2015/001300]. The authors are grateful to Prof.
Parthasarathi Dastidar, Department of Organic Chemistry, Indian Association for the Cultivation of
Science Kolkata, India for providing the single crystal X-ray diffraction facility. A. Husain gratefully
acknowledged UGC networking resource centre (UGC-NRC) School of Chemistry, University of
Hyderabad for providing research facilities to carry out part of this research.
19

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23

Highlights
The reaction between Cu(pic)2·6H2O (pic = picrate anion) and tryptophan (trp) in aqueous solution
resulted in a mononuclear Cu(II) complex Cu(L-trp)2(H2O)4](pic)2·4H2O, which exhibits an unusual
tryptophan coordination to Cu(II) via carboxylate monodentate binding in trans-fashion.
24