Molecular structure and DFT investigations on new cobalt(II) chloride complex with superbase guanidine type ligand

Article abstract of DOI:10.1007/s12039-015-0976-x

The new [Co(btmgn)Cl ₂] complex and the 1,8-bis(tetramethylguanidino)naphthalene (btmgn) ligand were synthesized and characterized. The X-ray single crystal investigation showed distorted tetrahedral geometry around the Co(II) ion. The geometry of the btmgn and [Co(btmgn)Cl ₂] complex was optimized using the B3LYP/6-311G(d,p) method. The calculated geometric parameters at the optimized structure of the [Co(btmgn)Cl ₂] complex showed good agreement with our reported X-ray structure. The two tetramethylguanidino groups are in a cis-type position to the naphthalene ring plane both in the free and coordinated btmgn. The large red shift of the υ _C=N mode upon coordination indicates the strong ligand-metal interactions. The calculated natural charges using natural bond orbital (NBO) analysis at the two coordinated Cl-atoms are not equivalent. Also the two LP(4)Cl →LP?(3)Co intramolecular charge transfer interaction energies (E ⁽²⁾) are 29.00 and 39.17 kcal/mol, respectively. The two Co-Cl bonds are not equivalent where the longer Co-Cl bond has more electronegative chlorine atom than the shorter one. Molecular electrostatic potential (MEP) study of the btmgn ligand showed that the N4 and N7 atoms are the most reactive nucleophilic centers for the coordination with the Co ²⁺ ion. The [Co(btmgn)Cl ₂] complex has higher polarizability (α ₀), first hyperpolarizability (β ₀) and lower energy gap (ΔE) than the free ligand. The TD-DFT calculations predicted the transition bands at 337.2 nm (f =0.2299, H →L) and 342.6 nm (f =0.1465, H-2/H →L) for the btmgn and [Co(btmgn)Cl ₂], respectively.

Full text of DOI:10.1007/s12039-015-0976-x

J. Chem. Sci. ꢀc Indian Academy of Sciences.
DOI 10.1007/s12039-015-0976-x
Molecular structure and DFT investigations on new cobalt(II) chloride
complex with superbase guanidine type ligand
a,b,∗
b,∗
c
SAIED M SOLIMAN
, MORSY A M ABU-YOUSSEF , JÖRG ALBERING and
b,d
AYMAN EL-FAHAM
Department of Chemistry, College of Science & Arts, King Abdulaziz University,
P.O. Box 344 Rabigh 21911, Saudi Arabia
a
b
Department of Chemistry, Faculty of Science, Alexandria University, P.O. Box 426 Ibrahimia,
2
1321 Alexandria, Egypt
c
Graz University of Technology, Mandellstrasse 11 (III), A-8010 Graz, Austria
d
Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451,
Saudi Arabia
e-mail: saied1soliman@yahoo.com; morsy5@Alexu.edu.eg; joerg.albering@tugraz.at;
aymanel_faham@hotmail.com
MS received 1 June 2015; revised 24 July 2015; accepted 10 August 2015
Abstract. The new [Co(btmgn)Cl2] complex and the 1,8-bis(tetramethylguanidino)naphthalene (btmgn) lig-
and were synthesized and characterized. The X-ray single crystal investigation showed distorted tetrahedral
geometry around the Co(II) ion. The geometry of the btmgn and [Co(btmgn)Cl ] complex was optimized
2
using the B3LYP/6–311G(d,p) method. The calculated geometric parameters at the optimized structure of the
[
Co(btmgn)Cl ] complex showed good agreement with our reported X-ray structure. The two tetramethylguani-
2
dino groups are in a cis-type position to the naphthalene ring plane both in the free and coordinated btmgn. The
large red shift of the υ mode upon coordination indicates the strong ligand–metal interactions. The calcu-
C=N
lated natural charges using natural bond orbital (NBO) analysis at the two coordinated Cl-atoms are not equiva-
(2)
lent. Also the two LP(4)Cl→LP*(3)Co intramolecular charge transfer interaction energies (E ) are 29.00 and
3
9.17 kcal/mol, respectively. The two Co-Cl bonds are not equivalent where the longer Co-Cl bond has more
electronegative chlorine atom than the shorter one. Molecular electrostatic potential (MEP) study of the btmgn
ligand showed that the N4 and N7 atoms are the most reactive nucleophilic centers for the coordination with
the Co2 ion. The [Co(btmgn)Cl ] complex has higher polarizability (α ), first hyperpolarizability (β ) and
+
2
0
0
lower energy gap (ꢀE) than the free ligand. The TD-DFT calculations predicted the transition bands at 337.2
nm (f=0.2299, H→L) and 342.6 nm (f=0.1465, H-2/H→L) for the btmgn and [Co(btmgn)Cl ], respectively.
2
Keywords. Guanidine; superbases; Cobalt(II); DFT; NBO; MEP.
1
. Introduction
has attracted great attention because of their potential
applications in heterogeneous catalysis for the synthesis
6
In the past decades, a number of organic proton of fine chemicals. The peralkylated bifunctional nitro-
sponges and superbases were synthesized and applied gen ligands of Cu and Fe were designed and used in
1
7,8
for various purposes. The classical proton sponge 1,8- the field of biomimetic coordination chemistry. Also,
bis(dimethylamino)naphthalene was described already the Mo and Ru complexes of these ligands were pre-
2
in 1968 by Alder et al. More recently, Sundermeyer pared and their catalytic performance in the hydrogena-
et al. reported on the new superbasic proton sponge 1,8- tion process was demonstrated. Late-transition metals,
9
bis(tetramethylguanidino)naphthalene (btmgn) as an especially Ni, Pd, and Pt coordinated to diimine ligands
3
example for an aromatically bridged bisguanidine. Due were used to catalyze polymerization of ethylene and
10
to resonance stabilization of the conjugated acid, guani- other α-olefin molecules. Moreover, guanidine groups
dines are much stronger bases than amines. In coordina- (arginine, creatine) play an important role in biologi-
tion chemistry, the neutral guanidine ligands represent cal systems, in particular for proton transfer processes
4
,5
11,12
not only excellent σ-donor, but also π-donor ligands.
in proton channels.
Studies of the structural, elec-
In the last decade, the design of organic–inorganic hybrid tronic and spectroscopic properties of guanidine ligands
mesoporous materials with strong basic functionalities containing aromatic group as potentially strong electron
donors and their complexes with Co(II) and Ni(II) were
∗
13–15
For correspondence
still rare in literature.

Saied M Soliman et al.
In the present work the new CoCl complex of preparations needed longer time). Uniform, deep blue
2
the guanidine-type ligand, bis(tetramethylguanidino) crystals, crystals suitable for X-ray measurements were
naphthalene (btmgn) has been synthesized. The ligand collected and air-dried. There was a yield of 0.85 g,
and the new [Co(btmgn)Cl ] complex were charac- about 76% with respect to the metal. FTIR: 3059-
2
terized using different spectroscopic techniques. The
2
860 (C-H), 1558 (C=N), 1520 (C=C aromatic), 1278
(C-N) cm . UV-Vis. 237.0, 348.0, 611.1, 648.0 nm
Nujal mull).
X-ray structure of the [Co(btmgn)Cl ] complex is deter-
−1
2
mined. The electronic and spectroscopic aspects of the
(
ligands as well as the [Co(btmgn)Cl ] were discussed
2
with the aid of DFT quantum chemical calculations.
2.4 XRD measurement
2
. Experimental and Computational
Single crystal X-ray diffraction data were collected on
an Enraf-Nonius 590 Kappa CCD single crystal diffrac-
tometer with graphite monochromated MoKα (λ =
2.1 General
0
.71073 Å) radiation. Intensity data were collected at
The solvents used were of HPLC reagent grade. Melting
points were determined with a Mel-Temp apparatus and
are uncorrected. Magnetic resonance spectra ( H NMR
and C NMR spectra) were recorded on a JOEL 400
MHz spectrometer with chemical shift values reported cell refinement and data reduction were carried with
room temperature using ϕ and ω scans; the crystal
to detector distance was 40 mm. Data processing was
performed using the maXus program package,16 the
1
13
17
in δ units (ppm) relative to an internal standard. Follow- Denzo and Scalepak’s programs; the crystal struc-
up of the reactions and checks of the purity of the com- tures were solved by direct methods using the SHELXS
18
pounds was done by TLC on silica gel-protected alu- program which revealed the positions of all non-
minum sheets (Type 60 GF254, Merck) and the spots hydrogen atoms. The crystal structure was refined by
2
were detected by exposure to UV-lamp at λ=254 nm full matrix least square refinement based on F using the
1
8
for a few seconds.
SHELX program package. The temperature factors
of all non-hydrogen atoms were refined anisotropically.
The hydrogen atoms were calculated using a riding
model with C-H = 0.96 Å. Multiscan absorption cor-
rections were applied to all data sets using the program
2.2 Synthesis of 1,8-bis(tetramethylguanidino)
3
naphthalene (btmgn)
19
SORTAV. Crystallographic data and data collection
details are summarized in Table 1.
Supplementary data are available from the CCDC,
2 Union Road, Cambridge CB2 1EZ, UK on request,
Chloroformamidinium salt 1 (TCFH, 5.7 g, 20 mmol)
was added slowly to a solution of 1,8-diaminonaph-
thalene 2 (1.6 g, 10 mmol) and triethylamine (2.0 g, 20
mmol) in 10 mL DMF. The reaction was carried out at
1
◦
9
0 C in a water bath for 6 h. The mixture was left to quoting the deposition number CCDC-1037138 or
cool to room temperature, and then poured into 50 mL the DOI for this paper. These data can be obtained
of ice cold water. The solid product formed was filtered free of charge at www.ccdc.cam.ac.uk/conts/retrieving.
directly and then washed with water. The crude product html or fax: (internat.) +44-1223/336-033; E-mail:
btmgn was recrystallized from CH Cl /n-hexane.
deposit@ccdc.cam.ac.uk.
2
2
The product btmgn was obtained as a gray solid in
◦
7
6% yield, M.p. 214-216 C (dec). R : 0.59 (CH Cl /
f
2
2
−
1
MeOH 9:1). IR (KBr, cm ): 3050-2855 (C-H), 1640 2.5 Computational Details
1
(
C=N), 1570-1540 (C=C aromatic), 1281 (C-N). H-
All calculations for the [Co(btmgn)Cl ] complex as
NMR (DMSO-d ) ppm: δ 2.86 (s, 24 H, 8 CH ), 6.49 (d,
2
2
6
3
13
well as the free ligand (btmgn) were carried out using
H, aromatic), 7.32-7.42 (m, 4 H, aromatic). C-NMR
2
0
(
(CD ) CO) ppm: δ 40.79, 115.12, 119.50, 123.12, Gaussian 03W software. The quantum chemical cal-
3 2
1
27.47, 137.93, 143.69, 160.62.
culations have been performed using the B3LYP level
of theory supplemented with the 6–311G(d,p) basis set.
GaussView4.1 and Chemcraft have been used to
draw the structures of the optimized geometries and to
21
22
2.3 Synthesis of [Co(btmgn)Cl2]
To an aqueous solution, 10 mL, of cobalt(II) chloride draw the HOMO and LUMO pictures. The input file
1.0 g, 0.77 mmol) a 15 mL ethanolic solution of the was taken from the CIF obtained from our reported
(
organic ligand (btmgn), (3 g, ∼0.85 mmol) was added X-ray single crystal measurement. The geometries
dropwise with constant stirring over about 15 min. The were optimized by minimizing energies with respect
clear solution was allowed to stand for 2 weeks (some to all geometrical parameters without imposing any

Co(II)-guanidine complex; DFT and structure studies
Table 1. Crystal data and structure refinement of the [Co(btmgn)Cl2] complex.
Empirical formula
Formula weight
Temperature
C20H30Cl2CoN6
484.33
298(2) K
Wavelength
Crystal system
Space group
0.71073 Å
monoclinic
P2 /c
1
Unit cell dimensions
a = 11.385(2) Å
b = 11.318(2) Å
c = 18.339(4) Å
α = 90◦
◦
β = 95.59(3)
γ = 90◦
3
Volume
2351.8(8) Å
Z
4
3
Density (calculated)
Absorption coefficient
F(000)
1.368 Mg/m
0.975 mm
1012
−
1
3
Crystal size
Theta range for data collection
Index ranges
0.12 × 0.04 × 0.03 mm
◦
3.00 to 24.99
−13<=h <=13, −13 <=k<=12, −21<=l<=21
Reflections collected
Independent reflections
Completeness to theta = 24.99
Max. and min. transmission
Refinement method
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I> 2σ(I)]
R indices (all data)
Extinction coefficient
Largest diff. peak and hole
7011
4115 [R(int) = 0.0408]
◦
99.30%
0.9713 and 0.8920
2
Full-matrix least-squares on F
4115 / 0 / 271
1.009
R1 = 0.0397, wR2 = 0.0704
R1 = 0.0859, wR2 = 0.0829
0.0020(4)
−
3
0.251 and −0.267 e.Å
molecular symmetry constraints. The true energy min-
imum at the optimized geometry of the studied com-
pounds was confirmed as indicated by the absence
of any imaginary frequency modes. The electronic
spectra of the studied compounds were calculated by
the TD-DFT method. The natural bond orbital (NBO)
calculations were performed using NBO 3.1 program
as implemented in the Gaussian 03W package at the
DFT/B3LYP level. The second order Fock matrix was
carried out to evaluate the donor–acceptor interactions
in the NBO basis.24
23
Scheme 1. Synthesis of Ligand.
−1
showed an absorption peak at 1640 cm correspond-
ing to the C=N groups, and the expected bands for
aromatic structure. The corresponding band in the
−1
[
Co(btmgn)Cl ] is located at about 1558 cm . The
2
large red shift upon coordination indicates the strong
ligand–metal interaction.
3
. Results and Discussion
1H NMR spectrum of the btmgn showed a multiplet
corresponding to the four aromatic protons H-2, H-3,
3.1 Structure of the ligand
The ligand (btmgn) was prepared by reaction of TCFH H-6 and H-7 in the region 7.42-7.32 ppm. A doublet
and 1,8-diaminonaphthalene using DMF as a solvent (δ=6.49, J=7.1 Hz) corresponding to the two protons
and following the reported method for preparation H-4 and H-5 was observed. A singlet at δ 2.86 ppm cor-
of guanidine using TCFH25 (scheme 1). The NMR responding to the twenty four hydrogens of the eight
spectra are presented in figure S1 (Supplementary equivalent N-methyl groups was also observed.
13
Information).
C-NMR spectrum of btmgn showed the expected
The spectral analysis of the btmgn ligand confirmed signals. A signal at 160.2 ppm corresponding to the
the expected structure. The IR spectrum of the btmgn two equivalent C=N groups was observed. Six signals

Saied M Soliman et al.
corresponding to the ten aromatic carbons showed in agent have in common, that the two tetramethyl-
the region 143.69-115.12 ppm. A signal at δ 40.78 ppm guanidino groups are in a cis-type position to the
assigned to the carbons of the eight equivalent methyl plane formed by the naphthalene rings. This was also
groups was also observed.
found for the very similar compound bis(tetramethyl-
guanidino)benzenedichlorocobalt(II).
27
Figure 2 depicts the packing of monomeric com-
plex molecules in a view along the crystallographic
b axis. It is evident that the crystal structure contains
non-polar and polar regions. The non-polar regions are
formed by the naphthalene rings located perpendicu-
lar to the bc plane at the height x = 0. In the range
between x ∼ 1/4 and x ∼ 1/3 numerous polar interac-
tions between the complex molecules can be observed.
A number of hydrogen bridge bonds between some pro-
tons of methyl residuals in the tetramethylguanidino
groups and the chlorine atom with Cl-H distances well
below 3.00 Å can be found (see figure 3). Additional
Cl-H bridge occurs between the H15 hydrogen atom of
the naphthalene ring and the Cl1 atoms of the adjacent
molecular layers.
3
.2 X-ray Crystal Structure of [Co(btmgn)Cl2]
The complex [Co(btmgn)Cl ], crystallizes with mono-
clinic symmetry in the space group P2 /c. The asym-
2
1
metric unit of the cell contains one formula unit of
the monomeric complex, thus the unit cell comprises
four symmetrically equivalent complex molecules. The
cobalt atom is distorted tetrahedrally coordinated by
two chlorine atoms and the chelating guanidine lig-
and. Figure 1 shows the molecular structure of this
monomeric complex, the interatomic distances as well
as the bond angles are listed in table S1 (Supple-
mentary Information). The Cl-Co-Cl bond angle is
1
◦
12.94(4) and thus close to the ideal tetrahedral angle.
As expected, the two Co-N distances in this complex are
very similar: Co-N1: 1.994(2) Å and Co-N4: 1.996(2)
Å. The N-Co-N bite angle of the chelating guanidine
3
.3 Optimized molecular structure
◦
ligand is 90.18 . This value compares quite well with
The optimized geometric structures and atomic num-
bering of the studied complex are shown in figure 4.
The predicted geometric parameters (bond lengths and
bond angles) of the investigated structures are collected
in table S2 (Supplementary Information). Most of the
bond lengths are overestimated except some of the C–N
bonds which have smaller bond lengths than the exper-
imental values. Generally, the bond lengths and bond
angles are predicted very well. The maximum devi-
ations of the calculated bond length values from the
experimental data are 0.051 and 0.031 Å for the Co–Cl2
and Co–Cl3, respectively. The calculated Co–Cl2 and
Co–Cl3 bond distances are 2.316 Å (exp. 2.265 Å) and
◦
◦
the data of 89.69 and 90.12 found for the corre-
26
sponding complexes of NiCl and NiBr , respectively.
2
2
These two compounds crystallize in the same space
group and with similar unit cell dimensions and thus
can be regarded to be isotypic with the presently dis-
cussed [Co(btmgn)Cl ] complex. All compounds using
2
1,8-bis(tetramethylguanidino)naphthalene as chelating
2
.264 Å (exp. 2.233 Å). The percentage error in these
Figure 1. Structure and atom numbering scheme of the
Co(btmgn)Cl2] complex. The hydrogen atoms have been
omitted from the drawing for clarity. All atoms are drawn Figure 2. Unit cell content of [Co(btmgn)Cl2] complex.
with ellipsoids of 30% probability level. The structure is shown in a view along the monoclinic b-axis.
[

Co(II)-guanidine complex; DFT and structure studies
Figure 3. Hydrogen bonds in the structure of [Co(btmgn)Cl2] complex. The
Cl-H interactions shorter than 3.00 Å are shown as red bars (A: Cl1-H1B: 2.762
Å; B: Cl1-H2B: 2.841 Å; C: Cl1-H15: 2.731Å; D: Cl2-H2A: 2.930 Å; E:
Cl2-H4B: 2.858Å). The different Cl-H interactions are marked by the capital
lettersA-E in this figure. The structure is shown in a view along the monoclinic
b-axis.
bond distances does not exceed 2.3%. These deviations in the range of the values of the ranges should be 39.7–
◦
◦
are mainly due to the phase difference between the cal- 46.2 and 32.0–36.9 , respectively. We noted that the
culations and the experimental data. The former is free formation of the Co–N bonds decreases the deviation of
from the crystal field effects. In agreement with the the two –C(N(CH ) ) from the naphthalene ring plane.
3
2 2
experimental X-ray structure, the calculations predicted Also, the two tetramethylguanidino groups are in a cis-
that the two Co–Cl bond distances are not identical. type position to the plane formed by the naphthalene
In the present section, the structural variations in rings both in the btmgn and [Co(btmgn)Cl ].
2
the ligand geometric parameters due to coordination
2
+
between the Co -ion and the ligand through N4 and N7
atoms have been discussed. Most of the ligand geomet-
ric parameters undergo very little variations due to the
3.4 Natural atomic charges
formation of the Co–N bonds. The C10–N4 and C11– Distribution of positive and negative charges has vital
N7 bond distances are increased by 0.04 Å while the role in the application of quantum chemical calculations
C10–N10 and C11–N9 bond distances are decreased to molecular system because of atomic charges affect
by the same magnitude. The rest of the bond distances dipole moment, molecular polarizability, electronic
are changed very little. Moreover, the most significant structure, acidity–basicity behavior and other proper-
28
bond angle changes occur around the coordination sites ties of molecular system. In this regard, the natural
N4 and N7). The C10–N4–C40, N4–C10–N5, N4– atomic charges (NAC) were calculated using the DFT
(
C10–N6, C11–N7–C43, N7–C11–N8 and N7–C11–N9 B3LYP/6–311G(d,p). The calculated natural charges at
◦
bond angle changes are in the range of 4.36-4.74 . the different atomic sites of the [Co(btmgn)Cl ] com-
2
The C10–N4–C40–C44, C10–N4–C40–C41, C11–N7– plex as well as the free ligand (btmgn) are given in
C43–C46 and C11–N7–C43–C41 dihedral angles of the table 2. All the H–atoms are electropositive where the
free ligand and [Co(btmgn)Cl ] complex are calculated natural atomic charge values at these sites are in the
2

Saied M Soliman et al.
Figure 4. The optimized molecular structure of the free ligand btmgn (lower) and the complex [Co((btmgn))Cl2] (upper).
range 0.2209-0.2641. Moreover the aromatic protons of the free and coordinated btmgn ligand are almost the
are more electropositive than the methyl H–atoms. In same. Furthermore, the NAC values of the Co1, Cl2
contrast, the aromatic and the methyl carbon atoms are and Cl3 are calculated to be +1.4314, −0.7942 and
electronegative. The C10, C11, C40 and C43 atoms −0.7686, respectively. The two coordinated Cl-atoms
have positive natural atomic charges as these C-sites are not equivalent. We noted that the longer Co–Cl2
bonded to the strong electronegative N-atoms. The bond has more electronegative chlorine atom than the
charge densities at all the N-atoms of the free ligand shorter Co–Cl3.
(
btmgn) are negative (−0.4859 to −0.5374). The NAC
values at the N4 and N7 atomic sites are calculated
3
.5 Molecular electrostatic potential
to be −0.5373 and −0.5374 respectively indicating the
higher basicity of these N-sites compared to the others. Molecular electrostatic potential (MEP) is a very useful
It is noted that the NAC values at the N4 (−0.7436) and property for analyzing and predicting molecular reac-
N7 (−0.7435) of the [Co(btmgn)Cl ] complex are more tive behavior. It is very useful three dimensional dia-
2
negative than the free ligand. The coordination between grams used to visualize the charge distributions and
the Co(II) ion and the ligand (btmgn) via the N4 and charge related properties of molecules as well as in
N7 coordination centers increases their negative charge assessing the molecules reactivity towards positively
values by 0.21 unit. The NAC values at the other N-sites or negatively charged reactants. The MEP is typically

Co(II)-guanidine complex; DFT and structure studies
Table 2. The calculated natural atomic charges of the [Co(btmgn)Cl2] com-
plex and the free ligand (btmgn).
Atom a [Co(btmgn)Cl2]
Btmgn
Atom [Co(btmgn)Cl2]
btmgn
Co1
Cl2
Cl3
N4
N5
N6
N7
N8
N9
C10
C11
C12
H13
H14
H15
C16
H17
H18
H19
C20
H21
H22
H23
C24
H25
H26
H27
C28
H29
H30
1.4314
−0.7942
−0.7686
−0.7436
−0.4715
−0.4373
−0.7435
−0.4715
−0.4376
0.6775
H31
C32
H33
H34
H35
C36
H37
H38
H39
C40
C41
C42
C43
C44
H45
C46
H47
C48
H49
C50
H51
C52
H53
C54
H55
C56
H57
H58
H59
0.2292
−0.4833
0.2428
0.2600
0.2209
0.2165
−0.4763
0.2418
0.2103
0.2249
−0.5373
−0.5041
−0.4860
−0.5374
−0.5041
−0.4859
0.6395
−0.4996
−0.4734
0.2640
0.2540
0.2271
0.2540
0.1764
−0.0780
−0.0378
0.1763
0.2201
0.2156
0.1795
−0.0787
−0.0421
0.1795
0.6774
0.6396
−0.4833
−0.4763
0.2429
0.2418
0.2209
0.2600
0.2249
0.2103
−0.2779
−0.2937
0.2354
0.2320
−0.4726
−0.4695
−0.2778
0.2354
−0.2937
0.2320
0.2310
0.2251
0.2320
0.2418
0.2226
0.2379
−0.2359
−0.2445
0.2374
0.2310
−0.4996
−0.4734
−0.2213
0.2406
−0.2301
0.2347
0.2539
0.2156
0.2271
0.2641
0.2201
0.2540
−0.2358
−0.2445
0.2374
0.2310
−0.4725
−0.4695
−0.2213
0.2406
−0.2301
0.2347
0.2418
0.2380
0.2320
0.2311
0.2226
0.2251
−0.4813
−0.4780
0.2485
0.2480
−0.4813
−0.4780
0.2293
0.2294
0.2166
0.2195
0.2484
0.2480
0.2294
0.2195
a
Atom numbering refers to Figure 4
visualized through mapping its values onto the surface are not equivalent. These results are in good agreement
reflecting the molecules boundaries so it allows us to with the calculated NAC values.
visualize the size and shape of molecules. The MEP dia-
grams of the free ligand (btmgn) and its [Co(btmgn)Cl2]
complex are shown in figure 5. In this particular, the
3.6 Nonlinear optical properties
regions of high electron density appear in red (elec- The calculated electronic properties such as dipole
trophilic reactivity) while the blue regions are related to moment (μ), polarizability (α ), first hyperpolarizabil-
0
electron deficiency (nucleophilic reactivity). The result- ity (β ), HOMO and LUMO energies as well as their
0
ing overall MEP of the ligand (btmgn) leaves no doubt energy gap are given in table 3. These electronic
that the nitrogen atoms carrying methyl groups (N5, N6, properties have strong relations to the nonlinear opti-
N8 and N9) already are not attracted to a positive test cal activity of compound. The present calculations
charge such as Co2 while the opposite is true for the indicated that the [Co(btmgn)Cl ] has higher dipole
+
2
N4 and N7 atoms. From the present investigations, we moment and polarizability than the free ligand. Also,
could conclude that the N4 and N7 atoms are the most the calculated first hyperpolarizability (β ) values of
0
reactive nucleophilic centers for the coordination with the btmgn and [Co(btmgn)Cl ] are 457.4870 a.u and
2
2
+
the Co ion. Hence, the high basicity of the two guani- 837.3271 a.u, respectively. Moreover, the lower energy
dinic nitrogen centers (N4 and N7) is evident. More- gap (ꢀE) indicates the more facile electronic transition
over, the MEP picture of the [Co(btmgn)Cl ] complex which is a basic requirement for good nonlinear optical
2
indicated that the MEP values around the two Cl-sites materials. The [Co(btmgn)Cl ] has lower energy gap
2

Saied M Soliman et al.
Figure 5. Molecular electrostatic potentials mapped on the electron density
surface for free ligand (btmgn) and [Co(btmgn)Cl2] complex calculated by
DFT/B3LYP method. For the [Co(btmgn)Cl2], the H-atoms are omitted for
clarity.
Table 3. The dipole moments components μ (D), the average polarizability α0 (a.u) and the first hyperpolarizability
β0 (a.u) of the studied compounds.
Parameter
[Co(btmgn)Cl2]
btmgn
Parameter
[Co(btmgn)Cl2]
btmgn
μx
μy
μz
μtot
αxx
αxy
αyy
αxz
αyz
αzz
α0
EHOMO
ELUMO
−0.004
4.470
8.683
−0.006
−3.950
2.759
βxyy
βyyy
βxxz
βxyz
βyyz
βxzz
βyzz
βzzz
βxyy
βyyy
βx
0.8458
−232.7684
−1.1379
914.8886
240.9203
0.1853
−0.5237
−130.1947
0.8055
591.0694
12.5230
0.2940
236.6741
0.0335
−125.2531
61.6926
0.3152
9.766
4.818
358.823
−0.033
356.760
0.063
−12.255
232.653
316.080
−5.018
−1.186
3.832
327.880
−8.937
249.173
26.499
63.602
248.899
275.320
−4.380
−0.334
4.046
15.2282
−0.0390
−74.8864
320.3787
−0.3311
607.2338
576.5272
837.3271
βy
βz
β0
335.6216
310.8897
457.4870
ꢀE
(ꢀE) than the free ligand. These results indicate that HOMO and LUMO levels are mainly localized over
the [Co(btmgn)Cl ] has better nonlinear optical (NLO) the ligand π-system. There is significant orbital densi-
2
2
+
properties than the free organic ligand.
ties present on the Co and the coordinated Cl-atoms
in case of [Co(btmgn)Cl ]. The electronic transition in
2
case of the ligand is mainly π-π* excitation while in the
[
Co(btmgn)Cl ] the electronic transition is mixed π-π*
3.7 Electronic spectra
2
and d-d transitions.
The electronic transition from the ground to the first
The more accurate electronic transitions of the ligand
excited state is mainly described by one electron exci- (btmgn) and the complex [Co(btmgn)Cl
tation from the highest occupied molecular orbital investigated by using the time-dependent density func-
] have been
2
(
HOMO) to the lowest unoccupied molecular orbital tional theory. Electronic absorption spectra calculated
29
(
LUMO). The excitation energy involves the differ- by the TD-DFT are presented in figure 7. For the free
ence between them (ꢀE=ELUMO-EHOMO). The HOMO ligand, the TD-DFT calculations predicted moderate
and LUMO plots of the [Co(btmgn)Cl ] complex as intensity transition band at 337.2 nm (f=0.2299) which
2
well as the free ligand (btmgn) drawn by DFT–B3LYP is mainly due to HOMO→LUMO (83%) excitation. Of
method are shown in figure 6. In both molecules the the fifty singlet-singlet electronic excitations calculated

Co(II)-guanidine complex; DFT and structure studies
H
L
(
btmgn)
[Co(btmgn)Cl2]
Figure 6. The ground state isodensity surface plots for the frontier molecular orbitals
contributed in the electronic transition.
for the [Co(btmgn)Cl ] complex, a very broad transi- 3.8 Natural bond orbital analysis
2
tion band with low intensity (f=0.1465) has been cal-
The natural bond orbital (NBO) calculations were per-
culated at 342.6 nm (exp. 348.0 nm) due to the HOMO-
formed in order to understand various interactions
between the filled and vacant NBOs, which are mea-
sure of the intramolecular delocalization of electrons
2
→LUMO (53%) and HOMO→LUMO (28%) excita-
tions. The rest of the transitions bands have very low
intensity (table S3, Supplementary Information).

Saied M Soliman et al.
Figure 7. The calculated electronic spectra of the studied compounds using TD-DFT method.
(2)
Table 4. The second order perturbation energies E (kcal/mol) of the
most important charge transfer interactions (donor–acceptor) of the studied
compounds using B3LYP method.
(2)
E
kcal/mol
Donor NBO (i)a
Acceptor NBO (j)
btmgn
[Co(btmgn)Cl2]
BD(2)N4-C10
BD(2)N7-C11
BD(2)C40-C44
BD(2)C40-C44
BD(2)C41-C42
BD(2)C41-C42
BD(2)C41-C42
BD(2)C41-C42
BD(2)C43-C46
BD(2)C43-C46
BD(2)C48-C50
BD(2)C48-C50
BD(2)C52-C54
BD(2)C52-C54
LP(1)N4
LP(1)N4
LP(1)N5
LP(1)N6
LP(1)N7
LP(1)N7
LP(1)N8
LP(1)N9
LP(1)Cl2
BD*(2)C40-C44
BD*(2)C43-C46
BD*(2)C41-C42
BD*(2)C48-C50
BD*(2)C40- C44
BD*(2)C43- C46
BD*(2)C48- C50
BD*(2)C52- C54
BD*(2)C41- C42
BD*(2)C52-C54
BD*(2)C40-C44
BD*(2)C41-C42
BD*(2)C41-C42
BD*(2)C43-C46
BD*(1)N5-C10
BD*(2)C40-C44
BD*(2)N4-C10
BD*(2)N4-C10
BD*(1)N8-C11
BD*(2)C43-C46
BD*(2)N7-C11
BD*(2)N7-C11
LP*(3)Co1
7.66
7.67
5.38
5.36
13.20
20.90
19.87
19.87
15.08
15.08
13.20
20.90
15.56
17.83
17.83
15.56
18.45
13.70
28.00
36.18
18.45
13.70
28.01
36.24
9.35
9.35
8.80
8.80
5.57
3.17
20.60
29.06
5.57
3.17
20.61
29.03
5.14
LP(4)Cl2
LP(4)Cl2
LP*(3)Co1
LP*(5)Co1
29.00
7.73
LP(1)Cl3
LP(4)Cl3
LP(4)Cl3
LP*(3)Co1
LP*(3)Co1
LP*(5)Co1
6.38
39.17
3.97
LP(1)N4
LP(1)N4
LP*(3)Co1
LP*(4)Co1
14.10
8.85
LP(1)N4
LP(1)N7
LP(1)N7
LP*(6)Co1
LP*(3)Co1
LP*(4)Co1
6.29
14.09
9.09
LP(1)N7
LP*(6)Co1
6.18
(
2)
*
Bold for the significant E variations of the n→ π* at N4 and N7.
a
Atom numbering refers to figure 4.

Co(II)-guanidine complex; DFT and structure studies
(2)
Table 5. The spin density at the different atomic sites of
in the studied system. The stabilization energies (E )
deduced from the NBO calculations for the most
significant intramolecular charge transfer interactions
are reported in table 4. The stabilization energy E
values associated with delocalization of electron Cl3
the [Co(btmgn)Cl2] complex.
Co1
Cl2
2.78714
0.05262
0.06195
0.01957
0.00516
0.00591
0.01958
0.00515
0.00590
0.00354
0.00353
−0.00015
0.00001
0.00044
0.00021
−0.00034
0.00009
0.00026
0.00009
0.00063
0.00006
0.00084
0.00014
−0.00034
0.00009
0.00026
0.00009
−0.00009
0.00001
0.00049
H31
C32
H33
H34
H35
C36
H37
H38
H39
C40
C41
C42
C43
C44
H45
C46
H47
C48
H49
C50
H51
C52
H53
C54
H55
C56
H57
H58
H59
0.00006
−0.00015
0.00001
0.00021
0.00044
0.00063
0.00013
0.00084
0.00006
−0.00508
0.01015
−0.00611
−0.00509
0.01202
−0.00056
0.01203
−0.00056
0.00967
−0.00042
−0.00570
0.00028
0.00968
−0.00042
−0.00570
0.00028
−0.00008
0.00001
0.00006
0.00049
(2)
N4
N5
density between the occupied and unoccupied NBOs
correspond to stabilizing donor–acceptor interactions.
N6
These stabilization energies are estimated using the
N7
second–order perturbation theory. The delocalization of
N8
electrons due to the π–π*, n–π* and n–σ* interac- N9
C10
tions are responsible for conjugation of the respective
π–bonds leading to a maximum stabilization in the
system. It is noted that most of the π–π* ICT inter-
actions in the naphthalene ring do not occur in the
C11
C12
H13
H14
[
Co(btmgn)Cl ] complex compared to the free one H15
2
while the rest of the π–π* ICT interactions are desta- C16
H17
bilized. Moreover, the changes in the stabilization ener-
(2)
H18
H19
C20
H21
gies E of the n–π* and n–σ* ICT delocalizations
are in the range of 7.12-12.88 kcal/mol. The maxi-
(2)
mum changes in the E values of these interactions
occur for the LP(1)N4→BD*(1)N5-C10/BD*(2)C40- H22
H23
C24
H25
H26
H27
C28
C44 and LP(1)N7→BD*(2)N7-C11/BD*(2)N7-C11
ICT. It seems that the coordination between the Co(II)
and the ligand (btmgn) via the N4 and N7 atoms
strongly destabilize the n–π* interactions resulting
from LP(1)N4/N7 more than the others.
Interestingly, the NBO calculations predicted the H29
H30
ICT LP(4)Cl2→LP*(3)Co and LP(4)Cl3→LP*(3)Co
(2)
interactions having E of 29.00 and 39.17 kcal/mol,
a
Atom numbering refers to figure 4
respectively. On the other hand, the LP(1)N→LP*Co
(
1
6.29–18.85 kcal/mol) and LP(1)N4→ LP*Co (6.18–
4.09 kcal/mol) have been predicted. It is clear that the,
LP*(3)Co
LP*(5)Co
Figure 8. The second order perturbation theory computed donor–acceptor
NBO plot for the HS state in complex showing the most significant donor-
acceptor charge transfer Co-Ligands interactions.

Saied M Soliman et al.
presented [Co(btmgn)Cl ], the spin density of the
2
Co(II) ion is found to be 2.78714. All the atoms coor-
dinated to Co(II) are found to have positive spin. This
illustrates that the spin delocalization mechanism is
operational.
4
. Conclusions
The cobalt(II) chloride complex, [Co(btmgn)Cl ] of
2
1,8-bis(tetramethylguanidino)naphthalene (btmgn) lig-
and has been synthesized and characterized. The X-
ray crystal structure of the [Co(btmgn)Cl ] is reported.
2
The geometry of the complex as well as the free btmgn
ligand have been calculated using DFT B3LYP/6-
3
11G(d,p) method. The calculated geometric param-
eters showed that the structure of the free ligand is
almost the same as in [Co(btmgn)Cl ] where the two
Figure 9. The computed spin density plots for the HS state
of complexes; the blue and green for the positive and negative
spin densities, respectively.
2
tetramethylguanidino group are in a cis-type position
to the plane formed by the naphthalene ring. The
the two N→Co interactions are almost equivalent while
the Cl→Co ICT interactions are not. These results are
in good agreement with our reported structure inves-
tigations. The two Co-N bonds are nearly equivalent
while the Co-Cl2 is weaker (longer) than the Co-Cl3. To
explore the nature of the d-orbitals involved in the most
significant donor acceptor interactions, the NBO plots
of these interactions are shown in figure 8. The com-
[
Co(btmgn)Cl ] has distorted tetrahedral geometry with
2
two equivalent Co-N bonds while the two Co-Cl bonds
are not. The calculated natural atomic charges and
molecular electrostatic potential (MEP) map showed
that the two guanidinic N-atoms (N4 and N7) have
the highest basic character and the most favored nucle-
ophilic centers for the Co(II) ion attack. The electronic
spectra of the btmgn and [Co(btmgn)Cl ] have been cal-
2
position analysis of these NBOs showed that, the d
x2−y2
culated using the TD-DFT method. The latter showed
broad band at 342.6 nm due to the H-2/H→L excita-
tions. The NBO calculations were used to study the dif-
ferent intramolecular charge transfer interactions occur
in the studied systems. The FTIR spectra showed strong
as well as the d are the most important metal ion d-
z2
orbitals involved in the donor-acceptor charge transfer
interactions.
3.9 Spin density analysis
red shift for the υ
mode due to the coordination
C=N
between the Co(II) and the ligand via N4 and N7 atoms.
This large red shift upon coordination indicates a strong
ligand–metal interaction.
In order to investigate the mechanism by which the
unpaired d-electrons of the studied high spin Co(II)
complex can distribute some spin density at the other
atoms, the spin densities at the different atoms are cal-
culated and listed in table 5. Also, the spin density plots Supplementary Information
of the high-spin state of the studied Co(II) complex
1
13
The H and C NMR spectra of the ligand are given in
figure S1. The experimental bond distances and angles
obtained from the X-ray structure of the complex are
shown in table S1. The calculated bond distances and
angles are given in table S2. The TD-DFT results of
the studied compounds are given in table S3. Sup-
plementary Information is available at www.ias.ac.in/
chemsci.
are shown in figure 9. Generally, we have two possi-
ble mechanisms: (1) the molecular orbital that hosts an
unpaired electron (metal d orbital) mixes in the atomic
orbitals of the ligands (mostly from the donor atoms).
As a result, a distribution of positive spin density occurs
through out the molecule, hence the resulting distri-
bution of spin density is said to arise from a spin
delocalization mechanism; (2) the positive spin at
the paramagnetic center may induce some spin den-
sity of the opposite sign at the atoms bonded to
it which is called the spin polarization mechanism.
In this case, a concentration of negative spin den-
sity is favored around the ligand donor atoms. In the
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