Synthesis, structure and DFT study of novel Ga(III) complexes containing a tetradentate ligand

Article abstract of DOI:10.1016/j.molstruc.2019.127334

The synthesis, structure and density functional theory (DFT) calculations of two novel Ga(III) complexes, the first containing a tetradentate Schiff base ligand (L1) and the second containing a tetradentate carboxamide ligand (L2), are presented. The comp

Full text of DOI:10.1016/j.molstruc.2019.127334

Journal of Molecular Structure xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis, structure and DFT study of novel Ga(III) complexes
containing a tetradentate ligand
**
*
James C. Truscott, Dumisani V. Kama, Hendrik G. Visser , Jeanet Conradie
Department of Chemistry, PO Box 339, University of the Free State, 9300, Bloemfontein, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 August 2019
Received in revised form
The synthesis, structure and density functional theory (DFT) calculations of two novel Ga(III) complexes,
the first containing a tetradentate Schiff base ligand (L1) and the second containing a tetradentate
carboxamide ligand (L2), are presented. The complexes are isolated as octahedral complexes with two
30 October 2019
water molecules attached to the axial positions. Both experimental and DFT calculations show that when
Accepted 31 October 2019
Available online xxx
.
[
Ga(L1)(H
2
O)
2
] NO
3
is dissolved in dimethyl sulfoxide (DMSO), spontaneous substitution of axial water
.
groups by DMSO occur to form [Ga(L1)(DMSO)
2
] NO
3
.
©
2019 Elsevier B.V. All rights reserved.
Keywords:
Gallium
DFT
Schiff base ligand
Tetradentate ligand
Carboxamide ligand
1. Introduction
exposed to open air conditions, adding to the benefits working with
these compounds [12].
Tetradentate ligands have sparked wide interest for uses in co-
ordination, biological, synthetic, and supramolecular chemistry [1].
We have focused on two different tetradentate ligands, namely
Schiff base and carboxamide ligands. Carboxamide complexes are
widely studied for their relatively inexpensive synthesis and their
uses in catalytic chemistry [2e4]. Schiff base ligands are mainly
studied for their optical activity for the use in OLED’s (optically
active light emitting diodes) [5e7] and catalytic activity which is
usually increased by complexation. These studies result in a better
understanding of the properties of both ligands and metal, and can
lead to the synthesis of highly active compounds [8]. The influence
of metals on the biological activity of these compounds and their
chemical interest as multidentate ligands have prompted a
considerable increase in the study of their coordination behaviour,
as well as in the radiopharmaceutical industry which extends
mostly towards organic actions involving antifungal, antibacterial,
antitumor, antidiabetic, herbicidal, anti-proliferative, anticancer
and anti-inflammatory actions [9e11]. When complexed to a metal,
the Schiff bases are known to be stable for long periods, even when
Gallium is a member of Group III of the Periodic Table of Ele-
ments, and was selected in this study due to the large interest in the
current radiopharmaceutical applications, OLED’s and the catalytic
industries of this metal over the past two decades which directly
relates to the current studies of the carboxamide and Schiff base
ligands. The most notable fields being semiconductor materials,
lasers, cancer and malaria chemotherapy, and dental materials
[13,14].
In this contribution we present the synthesis, characterization,
selected crystal structures and a density functional theory (DFT)
study of two novel octahedrally coordinated gallium complexes.
Gallium complex 1 is coordinated to a tetradentate Schiff base
N2O2 ligand (L1) and gallium complex 2 is coordinated to a
dicarboxamide N4 ligand (L2), see Fig. 1.
2. Experimental
2.1. General
Chemicals were of analytical grade while all the solvents were
purified and dried prior to use. IR spectra were recorded with a
Hitachi 270-50 spectrophotometer while the NMR spectra were
obtained at 298 K on a Bruker Avance DPX 300 NMR spectrometer
(300.130 MHz) and a Bruker Avance II 600 NMR spectrometer
*
Corresponding author.
** Corresponding author.
E-mail addresses: visserhg@ufs.ac.za (H.G. Visser), conradj@ufs.ac.za
(
J. Conradie).
https://doi.org/10.1016/j.molstruc.2019.127334
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Please cite this article as: J.C. Truscott et al., Synthesis, structure and DFT study of novel Ga(III) complexes containing a tetradentate ligand,
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Fig. 1. Structure of the gallium complexes 1a (left) and 2 (right) of this study. The anion is NO
3
. Gallium complex 1b contains DMSO in the axial positions instead of H
2
O.
(
600.130 MHz). The chemical shifts were reported relative to
NMR (75 MHz, DMSO) d 163.85 (s), 151.08 (s), 148.31 (s), 140.00 (s),
1
dimethyl sulfoxide (DMSO, 0.00 ppm) for the H spectra as well as
the C spectra. The absorption spectra were collected by a Perki-
nElmer Lambda 950 UVeVIS spectrophotometer.
136.71 (s), 124.16 (s), 120.89 (s), 119.81 (s), 118.93 (s), 115.67 (s),
13
ꢀ1
ꢀ1
56.08 (s), 19.57 (s). UV/vis:
l
max ¼ 270 nm, ε ¼ 222 479 M cm
.
2 24 4
Elemental analysis (C, N, H); calculated for C24N H O : C 71.27; H
5.98; N 6.93%, found: C 71.32; H 5.55; N 6.99%.
2.2. Synthesis and crystallization
2.2.1.2. N,N’-(4-nitro-1,2-phenylene)dipicolinamide/4, N,N’-(4-nitro-
A tetradentate Schiff base N2O2 ligand (L1) and a dicarboxamide
1,2-phenylene)bis(pyridine-2-carboxamide) (ligand L2).
N4 ligand (L2) were synthesized and coordinated to Ga(III) to form
gallium complexes 1 and 2 respectively.
Pyridine-2-carboxylic
acid
(Picolinic
acid)
(1.2368 g,
10.01 mmol) was added to a suspension 4-nitro-phenylenediamine
2
2
.2.1. Ligands L1 and L2
(0.0.7567 g, 4.94 mmol) in dry pyridine 10 ml. The reaction was
stirred at 40 C for 40 min. Triphenylphosphite (10 mL) was added
ꢁ
0
.2.1.1. 6,6’-((1e,1 E)-((4,5-dimethyl-1,2-phenylene)bis(azanylyli-
dropwise over a period of 10 min. The reaction temperature was
dene))bis(methanylylidene))bis(2-methoxyphenol) (ligand L1).
ꢁ
then increased to 95 C and stirred under reflux for 24 h [15]. The
Ortho-vanillin (1.5252 g, 0.01 mol) was added to 4,5-dimethyl-
white precipitate that was formed was filtered off using suction
1
,2-phenylendiamine (0.5435 g, 0.005 mol) and placed in a ball mill
filtration. Yield (0.4825 g, 26.6%).1H NMR (300 MHz, DMSO)
(s, 2H), 8.68 (d, 3H), 8.16 (d, 6H), 7.71 (s, 2H.IR (cm ): n
d
11.06
(C]
max ¼ 265 nm,
ꢀ
1
at 25 Hz for 40 min. After completion the product obtained was
bright orange. The reaction mixture was left to dry, and needle-like
crystals formed upon drying. Yield (1.267 g, 62.7%). H NMR
O) ¼ 1699.61 and
n
(NeH) ¼ 3333.99 UV/vis:
l
1
ꢀ1
ꢀ1
ε ¼ 272 479 M cm . Elemental analysis (C, N, H); calculated for
(
300 MHz, DMSO)
d
13.16 (s, 2H, OH), 10.25 (d, J ¼ 8.8 Hz, 2H), 8.91
18 5 13 4
C N H O : C 59.50; H 3.61; N 19.28; %, found: C 59.56; H 3.59; N
(
6
s, 2H), 7.27 (s, 2H), 7.23 (d, J ¼ 7.1 Hz, 2H), 7.11 (d, J ¼ 7.9 Hz, 2H),
19.32%
13
.89 (t, J ¼ 7.9 Hz, 2H), 3.82 (d, J ¼ 11.8 Hz, 6H), 2.29 (s, 6H, CH3).
C
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3
2
2
.2.2. Ga-complexes
Ligand
2 (0.1433 g, 0.394 mmol), dissolved in chloroform
(
20 mL), was slowly added to a solution of gallium (III) nitrate hy-
2 2 3
.2.2.1. Gallium complex 1a - [Ga(L1)(H O) .
].NO
Ligand 1 (0.1582 g, 0.393 mmol) dissolved in chloroform (20 mL)
was slowly added to a solution of gallium (III) nitrate hydrate
0.1003 g, 0.393 mmol) in methanol (10 mL), followed by a solution
drate (0.1018 g, 0.391 mmol) in methanol (10 mL), followed by a
solution of sodium acetate (0.0405 g, 0.493 mmol) in methanol
(10 mL). The reaction was stirred under reflux at 70 C for 24 h. The
ꢁ
(
of sodium acetate (0.0400 g, 0.472 mmol) in methanol (10 mL). The
reaction was stirred under reflux at 70 C for 24 h. The filtrate
filtrate formed upon cooling and was then filtered off. Yield:
ꢁ
1
(0.1698 g, 92.2%). H NMR (300 MHz, DMSO)
d
9.51 (d, J ¼ 2.8 Hz,
formed upon cooling was then filtered off. Yield: (0.1657 g, 88.5%).
1H), 9.29 (d, J ¼ 6.1 Hz, 2H), 8.81 (d, J ¼ 9.0 Hz, 1H), 8.62e8.56 (m,
4H), 8.20e8.11 (m, 2H), 8.06 (dd, J ¼ 9.0, 2.8 Hz, 1H). UV/vis:
1
H NMR (300 MHz, DMSO)
d 9.29 (s, 2H), 7.95 (s, 2H), 7.18 (dt,
ꢀ
1
ꢀ1
J ¼ 14.6 Hz, 4H), 6.79 (t, J ¼ 7.6 Hz, 2H), 3.83 (d, J ¼ 18.3 6H), 2.37 (s,
H). Crystal structure was collected. UV/vis:
max ¼ 220 nm,
ε ¼ 254 090 M cm . Elemental analysis (C, N, H); calculated for
GaC24 C 50.55; H 4.60; N 7.37; %, found: C 50.53; H 4.55; N
l
max ¼ 270 nm, ε ¼ 318 471 M cm . Elemental analysis (C, N, H);
6
l
calculated for GaC18
6 15 9
N H O : C 40.86; H 2.86; N 15.88; %, found: C
ꢀ
1
ꢀ1
40.80; H 2.88; N 15.83%
3 26 9
N H O
7.40%.
2.3. Crystal structure analysis
2 2 3
.2.2.2. Gallium complex 1b- [Ga(L1)(OSMe ) .
].NO
2
Crystal data, data collection and structure refinement details for
Complex 1b was formed by dissolving complex 1a in DMSO and
collecting the crystal once formed. H NMR (300 MHz, DMSO)
two different crystal structure determinations (1a) and (1b), of
gallium complex 1, are summarized in Table 1. For (1a) and (1b), all
H atoms were positioned in geometrically idealized positions and
1
d
7
2
10.23 (s, 1H), 9.23 (s, 2H), 7.86 (s, 2H), 7.23 (dt, J ¼ 11.2, 5.6 Hz, 2H),
.15 (dt, J ¼ 27.5, 13.8 Hz, 2H), 6.80 (t, J ¼ 7.9 Hz, 2H), 3.83 (m, 14H),
constrained to ride on their parent atoms, with CeH ¼ 0.93 Å and
2
.33 (s, 6H), 1.91 (s, 1H). Elemental analysis (C, N, H); calculated for
U
U
iso(H) ¼ 1.2Ueq(C) for sp CH, and with CeH ¼ 0.96 Å and
iso(H) ¼ 1.5Ueq(C) for methyl groups. Disorders of the ethanol
GaC28
36 2 6 2
H N O S C 53.34; H 5.76; N 4.44; %, found: C 53.33; H 5.70;
N 4.40%.
solvent species were observed for both complexes and during
refinement, distance and similarity restraints were applied to the
chemically equivalent bond lengths and angles.
2 2 3
.2.2.3. Gallium complex 2 - [Ga(L2)(H O) .
].NO
2
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J.C. Truscott et al. / Journal of Molecular Structure xxx (xxxx) xxx
Table 1
Experimental details.
Crystal Data
1a- [Ga(L1)(H
70Ga
2
O)
2
].NO
3
þ EtOH
1b - [Ga(L1)(OSMe
2
)
2
].NO
3
þ OSMe
2
Chemical formula
C
54
H
2
N
6
O
21
C
30
H
40GaN
3 10 3
O S
M
r
1282.63
768.55
Crystal system, space group
Temperature (K)
Triclinic, P-1
100
Triclinic, P-1
100
a, b, c (Å)
12.081 (5), 13.090 (5), 19.471(5)
91.044 (5), 90.158(5), 112.947(5)
2834.8 (18)
2
10.194(3), 11.686(4), 15.796(5)
111.335(10), 92.429(12), 90.716(11)
1750.5 (10)
2
ꢁ
a
,
b
,
g
( )
3
V (Å )
Z
Radiation type
Mo K
1.04
a
Mo Ka
1.02
ꢀ
1
m
(mm
)
Crystal size (mm)
Data collection
0.47 ꢂ 0.27 ꢂ 0.14
0.45 ꢂ 0.15 ꢂ 0.1
Diffractometer
Absorption correction
No. of measured, independent and observed
Bruker APEX-II CCD
None
41 255, 12 745, 7332
Bruker APEX-II CCD
None
27 419, 8461
[I > 2s(I)] reflections
R
int
0.067
0.102
0.053
0.109
(
sin
q/l)
max (Åꢀ1)
Refinement
2
2
2
R [F > 2
s(F )], wR (F ), S
0.128, 1.454, 1.02
0.118, 0.274, 0.112
No. of reflections
No. of parameters
No. of restraints
H-atom treatment
12 745
741
3
8461
440
18
H atoms treated by a mixture of independent and
constrained refinement
3.79, ꢀ2.51
H atoms treated by a mixture of independent and
constrained refinement
2.14, ꢀ1.77
Drmin (e Å 3
ꢀ
)
Drmax
,
2
.4. Computational method
optimized using density functional theory calculations. Both DFT
PW91/TZP gas phase optimized isomers are non-planar. L1 Isomer
1, which exhibits the same orientation as the experimental struc-
ture, has the lowest energy, see Fig. 2 a. The energy of L1 isomer 2 is
too high to exist, since when applying the Boltzmann equation to
the energies of L1 isomer 1 and L1 isomer 2, a population of 99.99
and 0.01% for the two isomers respectively, are obtained. L1 Isomer
1 has the correct orientation for tetradentate coordination with a
metal. Upon coordination to gallium (III) an octahedral complex is
formed. Since Ga(III) prefers to be six coordinated, two water
molecules that are available in the reaction mixture, fill the axial
DFT calculations were carried out in the gas and solvent phase
with the PW91 (Perdew-Wang 1991) [16] GGA (Generalized
Gradient Approximation) functional and the all-electron Slater-
type TZP (Triple
z
polarized) basis set as implemented in the ADF
Amsterdam Density Functional) 2014 programme and updates
17], and with the B3LYP functional using the triple- basis set 6-
11G (d,p) as implemented in the Gaussian 09 package [18]. The
(
[
3
z
implicit solvent Polarizable Continuum Model (PCM) [19] that uses
the integral equation formalism variant (IEFPCM) [20] was used for
solvent calculations in Gaussian and the COSMO (Conductor like
Screening Model) model of solvation [21e23] as implemented [24],
was used for solvent calculations in ADF. The type of cavity used is
Esurf [25] and solvent used was DMSO with ε ¼ 46.7 or ethanol
positions to complete the octahedral coordination. The DFT PW91/
þ
TZP optimized geometry of gallium complex 1a, [Ga(L1)(H
2
O)
2
] , is
.
2 2 3
shown in Fig. 3. However, when [Ga(L1)(H O) ] NO is dissolved in
DMSO, the axial waters are replaced by DMSO to form gallium
.
(
EtOH) with ε ¼ 24.55. The input coordinates for the compounds
complex 1b, [Ga(L1)(OSMe
geometry of [Ga(L1)(OSMe
2
)
)
2
] NO
3
. The DFT PW91/TZP optimized
þ
were constructed using Chemcraft [26], as well as available crystal
data. The optimized coordinates of the DFT calculations are pro-
vided in the Supporting Information.
2
2
] , is also shown in Fig. 3.
The tetradentate N4 carboxamide ligand L2 was synthesized by
adapting a published method for the unsubstituted ligand N,N’-
(
1,2-phenylene)bis (pyridine-2-carboxamide), L2a, from literature
[15]. A published crystal structure of the unsubstituted ligand L2a
CSD reference code GUKWEO [30]) showed that the both pyridine-
3
. Results and discussion
(
3
.1. Synthesis and structure
2-carboxamide groups are orientated in the same direction (similar
as in Fig. 2 b, L2a isomer 1), stabilized by a hydrogen bond between
the carboxamide oxygen and an amine hydrogen. This orientation
is unfavourable for N4 e coordination to a metal. However, PW91/
TZP density functional theory calculations of the two possible ste-
reo isomers of L2a (Fig. 2 b) showed that these two isomers are near
iso-energetic, with a Boltzmann calculated population of 25.6 and
74.4% for L2a isomer 1 and L2a isomer 2, respectively. Thus both
orientations are possible. A similar result was obtained for the three
possible isomers of L2, see (Fig. 2 d). Experimentally both isomers
of the 4,5-dimethyl substituted ligand, 4,5-dimethyl-1,2-phenylene
L2b were isolated in the solid state (CSD reference codes AVURIS
and QEDFEM [4,31]). Consistent with experimental observation, the
PW91/TZP energies of L2b isomer 1 and L2b isomer 2 are also near
iso-energetic (Fig. 2 c), with a Boltzmann calculated population of
33.0 and 67.0%, respectively. Isomer 2 of L2, L2a and L2b has the
The tetradentate Schiff base ligand L1 is synthesized by the ball
milling assisted solvent and catalyst free method, where the re-
actants ortho-vanillin and 4,5-dimethyl-1,2-phenylendiamine were
shaken at a high speed in the absence of solvent inside a vessel
containing grinding balls. The high speed produces enough force to
facilitate the chemical reaction which makes an amorphous
mixture of the reagents [27]. The reason for the use of ball milling
over conventional methods of chemical synthesis, is to enable a
chemical reaction in the absence of solvents, resulting in a cleaner
synthetic procedure [28]. The solid-state structure of L2 is known
(
CSD reference code PUJGIL [29]). The L2 free ligand in the solid
state deviates severely from planarity due to the repulsion between
the hydroxy and methoxy groups and has the structure of L1 isomer
1
in Fig. 2 a. The two different stereo isomers possible for L1, are
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5
Fig. 2. DFT PW91/TZP gas phase optimized geometries of possible isomers of (a) L1, (b) L2a, (c) L2b and (d) L2. The relative electronic energies (eV) are given in brackets. Colour code
for atoms (online version): O (red), N (blue), C (dark grey), H (white).
lowest energy and the correct orientation for tetradentate coordi-
nation of the four nitrogens with a metal.
with four nitrogens in the equatorial plane and two water mole-
cules attached to the axial positions to complete the octahedral
coordination. The DFT PW91/TZP optimized geometry of the cation
Coordination of ligand L2 to gallium leads to a Ga(III) complex,
Please cite this article as: J.C. Truscott et al., Synthesis, structure and DFT study of novel Ga(III) complexes containing a tetradentate ligand,
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J.C. Truscott et al. / Journal of Molecular Structure xxx (xxxx) xxx
Fig. 3. DFT PW91/TZP gas phase optimized geometry that cations of gallium complexes 1a, 1b and 2. Colour code for atoms (online version): Ga (yellow), O (red), N (blue), C (dark
grey), H (white).
2 2 3
Fig. 4. Perspective drawings, showing the atom numbering scheme, of the molecular structure of the two molecules of gallium complex 1a, [Ga(L1)(H O) ]$NO . Hydrogen atoms,
solvents and counter ions have been omitted for clarity. Atomic displacement parameters (ADPs) are shown at the 50% probability level.
þ
of gallium complex 2, [Ga(L2)(H
2
O)
2
] , is shown in Fig. 3. The tet-
were grown in ethanol and DMSO as solvents, producing structures
(1a - Ga (L1) (H O) ] NO ) and (1b - [Ga(L1)(OSMe ) ]$NO )
2 2 2 3 2 2 3
.
radentate N4-ligand in the DFT optimized geometry is nearly flat
with only the two hydrogens of the pyridine groups that are
pointing towards each other, pointing up and below the N4-plane
to minimize interaction. The same tetradentate N4-ligand L2 in
the solid state structure of the related Co(III) complex tetraethy-
lammonium dibromo-(N,N’-(4-nitro-1,2-phenylene)dipyridine-2-
carboxamidato)-cobalt (III) (CSD reference code SIWYAA [32]) is
also nearly flat, while L2 in (Dimethylformamide-O)-nitrato-(4-
nitro-1,2-bis(2-pyridylcarboxamido)benzene)-iron (III) deviated
slightly from planarity (CSD reference code BERJUE [33]). The large
respectively. Repeated recrystallization of gallium complex 1a led
to the same quality crystals. However, although the quality of the
crystals obtained was found to be relatively poor, good agreement
between the bond lengths obtained for the different crystals with
the DFT calculated molecules was obtained, pointing towards the
reliability of the experimental structure. Perspective drawings [34],
showing the crystallographic numbering scheme used for the
molecular structure (1a) and (1b), are shown in Fig. 4 and Fig. 5
respectively. Table 2 lists selected geometrical parameters of the
two crystals in the asymmetric unit of (1a) and the crystal of (1b).
ꢁ
DFT calculated Npy-Ga-Npy angle of 112.1 is due to the strain of the
three 5-membered rings that formed upon coordination of L2 to
gallium. The related DFT calculated O-Ga-O angle in complex 1 is
much nearer to 90 , namely 92.7 , since L1 forms two 6-membered
and one 5-membered ring upon coordination to gallium.
2 2 3
The crystal (1a - [Ga(L1)(H O) ]$NO ) of gallium complex 1
crystallized in the triclinic, P-1 space group with two formula units
per asymmetric unit, forming a distorted octahedral geometry
(Fig. 4). The central metal atoms are bound to the tetradentate li-
gands via two nitrogen and two oxygen bonds. Two water mole-
cules are also attached to the central metal in each case to complete
the octahedral environments. Two nitrate counter ions and two
solvent molecules (ethanol) complete the asymmetric units in each
case. The GaeN and GaeO (Table 2) bond lengths of the two mol-
ecules differ with less than 0.02 Å and are considered to fall within
ꢁ
ꢁ
3.2. X-ray structure of gallium complex 1a and 1b
].NO
3
, was successfully syn-
thesized and characterised using UV/vis, elemental analysis and H
NMR. To further characterize this novel complex, single crystals
Gallium complex 1a, [Ga(L1)(H
2
O)
2
1
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7
geometry, due to the larger strain in the 5-membered N-Ga-N-C-C
ꢁ
ring, while the O-Ga-O angle in the two crystals was close to 90 .
The N-Ga-O angels in the two crystals, on the other hand, were ca.
ꢁ
ꢁ
9
3 , larger than the expected 90 since it forms part of bigger 6-
membered rings, coordinated to Ga.
3
The crystal (1b - [Ga(L1)(H O) ]$NO ) of gallium complex 1 also
2
2
crystallized in the triclinic, P-1 space group, but with one formula
unit per asymmetric unit, forming a distorted octahedral geometry
(Fig. 4). The central metal atoms are bound to the tetradentate li-
gands via two nitrogen and two oxygen bonds, while two DMSO
molecules are also attached to the central metal in each case to
complete the octahedral environment. One nitrate counter ion and
one solvent molecule (DMSO) complete the asymmetric unit. The
GaeN and GaeO (Table 2) bond lengths are within 0.03 Å similar to
the bonds of the two molecules of [Ga(L1)(H
Ga (1)-N (2) angle of 80.52 (17) is c. a. 1 smaller than found for the
2
O)
2 3
]$NO . The N (1)-
ꢁ
ꢁ
two [Ga(L1)(H
is c. a. 3 larger.
2
O)
2
]$NO
3
crystals, while the N (1)-Ga (1)-N (2) angle
ꢁ
The flexibility of L1 in complex 1 that forms two 6-membered
and one 5-membered ring upon coordination to gallium, can be
seen in the deviation of the 2-methoxyphenol rings (adjacent to the
6-membered rings) from planarity through the Ga-N-N-O-O plane
of L1 in the two different structures of gallium complex 1, see Fig. 6.
However, Ga and the two coordinating O’s and N’s are in the same
plane with all atoms (Ga, O and N), less than 0.03 Å off the Ga-N-N-
O-O plane.
Fig. 5. Perspective drawings, showing the atom numbering scheme, of the molecular
structure of gallium complex 1b, [Ga (L1)(OSMe ]$NO . Atomic displacement pa-
rameters (ADPs) are shown at the 50% probability level.
2
)
2
3
3.3. DFT
the normal range [35]. The deviation from perfect octahedral ge-
ometry of the complex are best illustrated by the N (1)-Ga (1)-N (2)
Selected bond lengths and angles of the optimized structure of
ꢁ
ꢁ
gallium complexes 1a and 1b, are given in Table 2. In addition to gas
phase optimization, complex 1a was optimized in ethanol, the
experimental crystallization solvent of 1a, and complex 1b was
and N (3)-Ga (2)-N (4) angles of 82.36 (21) and 81.78 (21) . The N-
ꢁ
Ga-N angle is smaller than the expected 90 for a perfect octahedral
Table 2
Selected experimental and PW91/TZP DFT gas phase calculated geometric parameters for gallium complex 1.
Bond
a - [Ga(L1)(H
O (4)eGa (1)
N (1)eGa (1)
N (2)eGa (1)
O (3)eGa (1)
Bond distance XRD (Å)
Bond distance DFT (Å)
1
2
O)
2
]$NO
3
molecule 1
1.881 (6)
2.026 (8)
2.012 (8)
1.901 (6)
molecule 2
1.898 (6)
2.024 (8)
2.002 (7)
1.883 (6)
DFT gas
1.898
2.018
2.010
1.895
DFT gas
1.898
2.018
2.010
1.895
DFT gas
1.925
2.028
2.037
1.926
DFT solvent (EtOH)
1.903
2.016
2.020
1.906
DFT solvent (EtOH)
1.903
2.016
2.020
1.906
DFT solvent (DMSO)
1.927
2.029
2.041
1.929
1a - [Ga(L1)(H
2
O)
2
]$NO
3
O (1)eGa (2)
N (4)eGa (2)
N (3)eGa (2)
O (2)eGa (2)
1b - [Ga(L1)(OSMe
2
)
2
]$NO
3
O (1)eGa (1)
N (1)eGa (1)
N (2)eGa (1)
O (2)eGa (1)
1.885 (4)
2.020 (4)
2.031 (4)
1.896 (4)
Angle
Bond angle XRD (o)
Bond angle DFT (o)
1
a - [Ga(L1)(H
2
O)
2
]$NO
3
3
molecule 1
89.8 (3)
94.2 (3)
82.4 (3)
93.6 (3)
molecule 2
91.0 (3)
93.9 (3)
81.8 (3)
93.4 (3)
DFT gas
92.5
94.0
81.9
92.2
DFT gas
92.5
94.0
81.9
92.2
DFT solvent (EtOH)
91.3
93.3
81.7
O (3)-Ga (1)-O (4)
O (4)-Ga (1)-N (1)
N (1)-Ga (1)-N (2)
O (3)-Ga (1)-N (2)
93.6
1a - [Ga(L1)(H
2
O)
2
]$NO
DFT solvent (EtOH)
91.3
93.3
81.7
O (1)-Ga (2)-O (2)
O (1)-Ga (2)-N (4)
N (3)-Ga (2)-N (4)
N (3)-Ga (2)-O (2)
93.6
1
b - [Ga(L1)(OSMe
2
)
2
]$NO
3
DFT gas
91.5
94.2
81.3
93.2
DFT solvent (DMSO)
O (1)-Ga (1)-O (2)
O (2)-Ga (1)-N (2)
N (1)-Ga (1)-N (2)
O (1)-Ga (1)-N (1)
93.72 (16)
91.80 (17)
80.55 (17)
93.92 (16)
91.4
93.0
81.3
94.3
Please cite this article as: J.C. Truscott et al., Synthesis, structure and DFT study of novel Ga(III) complexes containing a tetradentate ligand,
Journal of Molecular Structure, https://doi.org/10.1016/j.molstruc.2019.127334

8
J.C. Truscott et al. / Journal of Molecular Structure xxx (xxxx) xxx
Fig. 6. Deviation from planarity of the tetradentate ligand in the two complex structures of gallium complex 1: 1a - [Ga (L1)(H
2
O)
2
]$NO
3
left and 1b - [Ga(L1)(OSMe
2
)
2
]$NO
3
right,
o
as measured by the angle between the planes through the 2-methoxyphenol rings and the plane through Ga-N-N-O-O, giving the maximum deviation from planar as 6.104 (3) and
1
8.146 (5)o for 1a and 1b respectively.
optimized in DMSO, the experimental crystallization solvent of 1b.
When optimizing coordination compounds, the accuracy of the
metal-ligand bonds is of importance. The PW91/TZP gas phase and
recrystallized in DMSO, the energetics of the substitution reaction
have been determined using two different DFT functionals (both in
gas and solvent phase):
solvent phase (ethanol) DFT optimized structures of gallium com-
þ
þ
þ
2
) ]
plex 1a, [Ga(L1)(H
2
O)
2
]
reproduce the solid state experimental
[Ga(L1)(H
2
O)
2
]
þ 2(OS(CH
3
)
2
) -> [Ga(L1)(OS(CH
3
)
2
þ 2(H
2
O)
structure within 0.02 Å for both the GaeO and GaeN bond lengths.
Both the PW91/TZP gas phase and solvent phase (DMSO) DFT
optimized
Ga-ligand
bond
lengths
of
complex
1b,
þ
[Ga(L1)(OSMe
2
)
2
] , compare well with the experimental crystal X-
D
E ¼ ꢀ0.92 eV (PW91/TZP gas phase); ꢀ0.37 eV (PW91/TZP
solvent phase)
E ¼ ꢀ0.95 eV (B3LYP/6-311G
phase); ꢀ0.49 eV (B3LYP/6-311G (d,p) solvent phase)
G ¼ ꢀ0.85 eV (B3LYP/6-311G (d,p)
phase); ꢀ0.38 eV (B3LYP/6-311G (d,p) solvent phase)
ray structure values, namely within 0.04 Å for GaeO bond lengths
and 0.02 Å for GaeN bond lengths. The small differences between
the solid state structures and the DFT calculated optimized struc-
tures can also be ascribed to the lack of solid state environment,
cations and explicit solvent molecules in the DFT calculations,
compared to the solid state structures. Generally differences be-
tween experimentally measured metal-ligand bond lengths and
calculated bond lengths below a threshold of 0.02 Å, are considered
as meaningless [36]. It should be noted that the GaeO and GaeN
bond lengths in the experimental structures 1a and 1b, differ
D
(d,p)
gas
gas
D
With E ¼ electronic energy change of the reaction and
D
D
G ¼ free energy change of the reaction. The results with negative
reaction energies, clearly show a spontaneous reaction, i.e. the
þ
product [Ga(L1)(OS(CH
3
)
2
)
2
]
is more stable than the reactant
10
þ
with 0.02 and 0.01 Å respectively. The d Ga(III) complexes have a
[Ga(L1)(H
2
O)
2
] , in agreement with experimental observation.
2
2
2
2
2
d electron configuration of dxz
yz xy z2
d d d dx2-y2. The Ga-d-based
molecular orbitals of gallium complexes 1a and 2 are shown in
Fig. 7. The highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) of gallium complexes
4
. Conclusion
The synthesis and structure of two novel Ga(III) complexes, the
1
a and 2 are shown in Fig. 8. The HOMO and LUMO of both gallium
first containing a tetradentate Schiff base ligand (L1) and the sec-
ond containing a tetradentate carboxamide ligand (L2), is pre-
sented. Since Ga(III) prefers to be six coordinated, two water
molecules that are available in the reaction mixture, are attached to
the axial positions to complete the octahedral coordination.
complexes 1a and 2 are ligand based, while the gallium-metal
based orbitals are much lower in energy.
To understand why the axial water molecules have been
þ
substituted by DMSO when the [Ga (L1)(H
2
O)
2
] , complex 1a, was
þ
þ
Fig. 7. The KohneSham Ga-d-based molecular orbitals of complexes [Ga(L1)(H
2
O)
2
] , 1a, and [Ga(L2)(H
2
O)
2
] , 2, obtained from PW91/STO-TZP DFT calculation. The MO plots use a
3
contour of 60 e/nm . Colour code of atoms (online version): Ga (green), C (magenta), O (red), N (orange), H (blue).
Please cite this article as: J.C. Truscott et al., Synthesis, structure and DFT study of novel Ga(III) complexes containing a tetradentate ligand,
Journal of Molecular Structure, https://doi.org/10.1016/j.molstruc.2019.127334

J.C. Truscott et al. / Journal of Molecular Structure xxx (xxxx) xxx
9
þ
þ
Fig. 8. Selected KohneSham frontier molecular orbitals of complexes [Ga(L1)(H
2
O)
2
] , 1a, and [Ga(L2)(H
2
O)
2
] , 2, obtained from PW91/STO-TZP DFT calculation. The MO plots use a
3
contour of 60 e/nm . Colour code of atoms (online version): Ga (green), C (magenta), O (red), N (orange), H (blue).
However, when [Ga(L1)(H
taneous substitution of axial water groups by DMSO occur to form
2
O)
2
]$NO
3
is dissolved in DMSO, spon-
Sci. 4 (2015) 119e133, https://doi.org/10.1016/j.bjbas.2015.05.004.
9] K. Brodowska, E. Lodyga-Chruscinska, Schiff bases - interesting range of ap-
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[
[
Ga(L1) (DMSO)
2
].NO
3
. DFT calculations, consistent with experi-
https://doi.org/10.1002/chin.201511346.
þ
ꢀ
mental observation, predict the [Ga (L1)(H
2
O)
2
]
þ 2 (OS(CH
3
)
2
) ->
[10] E.N. Oiye, M.F.M. Ribeiro, J.M.T. Katayama, M.C. Tadini, M.A. Balbino,
þ
I.C. Eleot eꢀ rio, J. Magalh a~ es, A.S. Castro, R.S.M. Silva, J.W. da Cruz Júnior,
[
Ga(L1)(OS(CH
3
)
2
)
2
]
þ 2(H
2
O) to be a spontaneous reaction.
E.R. Dockal, M.F. de Oliveira, Electrochemical sensors containing Schiff bases
and their transition metal complexes to detect analytes of forensic, pharma-
ceutical and environmental interest. A review, Crit. Rev. Anal. Chem. 49 (2019)
Future work will focus on the application of these gallium com-
plexes in the development of OLEDs and radio pharmacy (gallium-
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6
8).
[
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This work has received support from the South African National
Research Foundation (Grant numbers 113327 and 96111) and the
Central Research Fund of the University of the Free State, Bloem-
fontein, South Africa. The High Performance Computing facility of
the UFS and the CHPC of South Africa is acknowledged for computer
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Journal of Molecular Structure, https://doi.org/10.1016/j.molstruc.2019.127334