NMR, X-Ray Crystal Structure Studies and Mechanism for Formation of a Novel Di-gallium Complex and 5-Methoxy-4,5,6-triphenyl-4,5-dihydro-1,2,4-triazene-3(2H)-thione

Article abstract of DOI:10.1007/s10870-018-0745-z

Gallium complexes of bis-thiosemicarbazones were prepared and characterized using NMR and X-ray crystallography. Formation of a gallium nitrate complex was proven by NMR spectroscopy. Surprisingly, this complex was found to convert on standing to a new hy

Full text of DOI:10.1007/s10870-018-0745-z

Journal of Chemical Crystallography
https://doi.org/10.1007/s10870-018-0745-z
ORIGINAL PAPER
NMR, X-Ray Crystal Structure Studies and Mechanism for Formation
of a Novel Di-gallium Complex and 5-Methoxy-4,5,6-triphenyl-4,5-
dihydro-1,2,4-triazene-3(2H)-thione
T. K. Venkatachalam1 · Paul V. Bernhardt2 · G. K. Pierens1 · D. C. Reutens1
Received: 19 July 2017 / Accepted: 24 October 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Gallium complexes of bis-thiosemicarbazones were prepared and characterized using NMR and X-ray crystallography. For-
mation of a gallium nitrate complex was proven by NMR spectroscopy. Surprisingly, this complex was found to convert on
standing to a new hydroxido-bridged di-gallium complex. The X-ray crystal structure of the digallium complex is described.
Similarly, a triazene compound was formed during the preparation of the gallium complex comprising a tetraphenyl unit in
the bis-thiosemicarbazone structure. We propose a mechanism for the formation of both the di-gallium species as well as
triazene compound in solution. The lattice parameters for the digallium complex is as follows: a: 17.6854(13) Å, b:16.6492(7)
Å, c: 21.4659(14) Å, β=112.383(8)°.
Graphical Abstract
Gallium complexes of bis-thiosemicarbazones were prepared and were found to convert to a new hydroxido-bridged di-gallium complex
Keywords Synthesis · NMR · X-ray crystallography · Triazene · Mechanism
Introduction
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10870-018-0745-z) contains
Bis-thiosemicarbazones are a class of compound possess-
ing N- and S-donor atoms that confer an ability to coordi-
nate to a wide range of metal ions. These complexes can
be prepared [1–3] with relative ease and often exhibit high
stability. Some complexes with a thiosemicarbazone or bis-
thiosemicarbazone framework also exhibit interesting bio-
logical characteristics [4–21] including potent anticancer
activity. Recently, much attention has been devoted to metal
supplementary material, which is available to authorized users.
* T. K. Venkatachalam
t.venkatachalam@uq.edu.au
1
2
Center for Advanced Imaging, The University
of Queensland, Brisbane 4072, Australia
School of Chemistry and Molecular Biosciences, The
University of Queensland, Brisbane 4072, Australia
Vol.:(0123456789)
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Journal of Chemical Crystallography
coordinated thiosemicarbazones as potential imaging agents,
with the introduction of radio metals [22]. A variety of radio
metals can be used to coordinate with thiosemicarbazones.
Pascu et al. prepared 68Ga complexes as potential imaging
agents [22–24]. They found that the 68Ga complexes of tet-
radentate bis-thiosemicarbazones formed a square-based
pyramidal chloridogallium(III) complex.
NMR parameters (nmr = giao) were calculated with a
single-point calculation, using the functional and basis set
combinations, mpw1pw91/6-311+G(2d,p), using the opti-
mized structures from the B3LYP/6-311+G(2d,p) calcula-
tion. This functional and basis set combination was selected
in light of previous calculations which gave good agreement
between experimentally measured and calculated chemical
shifts. The integrated equation formalism polarized con-
tinuum model (IEFPCM) for chloroform was used in the
NMR calculations. The computed NMR shielding tensors
were converted to chemical shifts by using empirical scal-
ing factors that are derived from linear regression analysis
of a test set of molecules. The slope and intercept values of
the scaling factors for proton chemical shifts were −1.0719
and 31.8731 respectively and for carbon chemical shift were
−1.042 and 186.3569 respectively.
We were interested in developing imaging agents incor-
porating radio-metals, in gallium complexes of bis-thiosem-
icarbazones. Here, we describe the mechanism of formation
of di-gallium complex and its X-ray crystal structure and the
instability of the gallium nitrate complex resulting in forma-
tion of this di-gallium complex.
Experimental
All chemicals were purchased from Sigma-Aldrich Chemi-
cal Company and used as received. Anhydrous solvents
were also obtained from Sigma-Aldrich in Sure/Seal™ bot-
tles and transferred to reaction vessels via cannula under
nitrogen atmosphere. Nuclear Magnetic Resonance (NMR)
spectra were recorded on a Bruker 700 MHz instrument and
chemical shifts are reported in parts per million (ppm) rela-
tive to residual DMSO signal at 2.5 ppm, which was used
as the internal standard. 13C NMR spectra were recorded
in d6-DMSO on the same instrument by using a proton
X‑Ray Crystal Structure Determination
X-ray quality single crystals were obtained by slow diffusion
of hexane into tetrahydrofuran solution. Crystallographic
data were acquired at 190 K on an Oxford Diffraction Gem-
ini CCD diffractometer employing graphite-monochromated
Cu Kα radiation (1.5418 Å) and operating within the range
2<2θ<125 Å. Temperature control was achieved with an
Oxford Cryosystems Desktop Cooler. Data reduction and
empirical absorption corrections (multiscan) were performed
with Oxford Diffraction CrysAlisPro software. The structure
was solved by direct methods with SHELXS and refined
by full-matrix least-squares analysis with SHELXL-97 [26]
within the WinGX graphical user interface [27]. All non-H
atoms were refined with anisotropic thermal parameters. The
molecular structure diagram was produced with ORTEP-3
[28]. The data in CIF format have been deposited at the
Cambridge Crystallographic Data Centre with CCDC depo-
sition number 1561094.
decoupling technique. The chemical shifts reported for 13
C
NMR spectra are referenced to residual DMSO at 39.5 ppm.
COSY, HSQC and HMBC spectra were acquired with a cry-
oprobe attachment.
DFT Calculations
Monte Carlo Conformational searching was performed using
Macromodel [25]. Torsional sampling using a Monte Carlo
Multiple Minimum (MCMM) search was performed with
1000 steps per rotatable bond. Each step was minimized with
the OPLS-2005 force field using the Truncated Newton Con-
jugate Gradient (TNCG) method with a maximum number
of iterations of 50,000 and an energy convergence thresh-
old of 0.02. All other parameters were left at their default
values. The lowest energy conformations (< 5 kcal/mol, 3
conformations) were optimized in Gaussian. All conformers
were optimized with Gaussian 09 using B3LYP/631G(d) in
vacuum and the vibrational frequencies were checked for a
true minimum, i.e. no negative frequencies. All true min-
ima were compared to remove identical conformations and
conformations comprising<1% of the Boltzmann popula-
tions. The single conformations were further optimized with
B3LYP/6-311+G (2d,p) and a Polarizable Continuum Model
(PCM) for chloroform and the vibrational frequencies where
checked again for a true minimum.
Bis‑thiosemicarbazones
Bis-thiosemicarbazones were prepared (compounds 1–3) by
condensing appropriately substituted diketones with 4-phe-
nyl-3-thiosemicarbazide in methanol medium containing a
catalytic amount of dilute hydrochloric acid. The precipi-
tated bis-thiosemicarbazone was washed with methanol,
dried under vacuum and crystallized using methanol.
General Procedure for Preparation of Complexes
of Gallium
Gallium complexes were made by reacting the bis-thiosem-
icarbazone with sodium methoxide in methanol and subse-
quently adding gallium nitrate. The mixture allowed to reflux
for 4–6 h to yield highly colored solids with varying colors
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Journal of Chemical Crystallography
from orange to red. The precipitated complex was filtered,
washed with methanol and hexane and dried in vacuum [29].
diffused into the mixture (5–7 days) and colorless crystals of
(S)-5-methoxy-4,5,6-triphenyl-4,5-dihydro-1,2,4-tirazene-
3(2H)-thione (7 mg) were obtained. The red solution was
slowly pipetted out from the crystals and the crystals were
washed with hexane and dried under vacuum before NMR
and X-ray analysis. Of note, the red solution when left stand-
ing at room temperature for several days became completely
colorless upon formation of the triazene.
Isolation of (S)‑5‑Methoxy‑4,5,6‑triphenyl‑4,5‑di‑
hydro‑1,2,4‑tirazene‑3(2H)‑thione (Compound 4)
The prepared gallium methoxide complex (25 mg) was
placed in a 20 mL scintillation vial. Using a dry syringe,
10 ml of dichloromethane was added to obtain an orange-red
solution. Without disturbing the solution, 10 mL of ethanol
was layered over the dichloromethane solution. The ethanol
Results and Discussion
The synthesis of gallium complexes of bis-thiosemicarba-
zone was accomplished following Scheme 1. In brief, the
appropriately substituted diketones were condensed with
thiosemicarbazide in acid medium to furnish the bis-thio-
semicarbazones. This report describes specifically the sin-
gle crystal X-ray structure of a novel di-gallium complex
of bisthiosemicarbazone. Other structurally related gallium
thiosemicarbazones did not produce X-ray quality crystals
under our experimental conditions and therefore are not
described in this report. Treatment of the bis-thiosemicar-
bazones with gallium nitrate in sodium methoxide in metha-
nol yielded the gallium complexes as orange/red solids. The
structures of compounds described in the present study are
shown in Fig. 1.
During the preparation of these complexes, it is possible
that the nitrate complex is formed initially and converted
into the methoxide by substitution of the labile nitrate group.
We have indeed isolated the methoxide complex in our ear-
lier studies and confirmed the structure of the compound by
single crystal X-ray crystallography [29]. In the initial exper-
iments, we studied the NMR characteristics of this gallium
Scheme 1 Synthetic scheme used for preparation of gallium com-
plexes
Fig. 1 Structures of compound described in the present study
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Journal of Chemical Crystallography
1
complex. The H NMR spectrum of the asymmetrically
substituted (methyl/ethyl) diphenyl bis-thiosemicarbazone
gallium complex (1) in d6-DMSO is shown. (supplemental
data).
The proton NMR spectrum showed two singlets at
10.3 ppm and 10.4 ppm corresponding to the NH protons
associated with the aminophenyl moiety in the molecule. A
doublet at 7.7 ppm corresponds to the ortho-protons of the
phenyl ring, followed by two triplets assigned to the meta-
and para-protons associated with the structure. A quartet
at approximately 2.8 ppm indicated the presence of a CH2
group; a singlet at 2.54 ppm corresponded to the isolated
methyl group and finally a triplet at 1.2 ppm indicated the
presence of the methyl group associated with the ethyl moi-
ety. Further confirmation of the structure of the complex
came from 13C, COSY, HSQC and HMBC spectral data
(supporting information). The methoxy group was absent
based on the NMR data. The absence of any extra 13C NMR
peaks rules out coordination of d6-DMSO so the likely
ligand in the axial position is nitrate.
Fig. 3 The asymmetric unit showing the dimeric complex, two THF
For single crystal X-ray crystallography, we dissolved
this compound in THF followed by diffusion with hexane.
After 10 days, dark orange colored crystals were obtained
along with some precipitate in the vial. The colored crys-
tals were carefully separated and subjected to X-ray crystal-
lography (Fig. 2, 3). Based on the NMR data, we expected
to observe the nitrate complex and were surprised to find
that this crystal had two gallium atoms connected through
hydrxido bridge as shown below. The crystal parameters are
shown in Table 1.
molecules, hexane and nitrate
donors to the other THF molecule (N1B–H1B⋯O6 1.99 Å,
163.6°) and to the nitrate anion which bridges adja-
cent complex cations (N1A–H1A⋯O2; 2.10 Å, 167.2°;
N1B–H1B⋯O6 1.99 Å, 163.6°; N6A–H6G⋯O4 1.97 Å,
170.1°; N6B–H6H⋯O2′ 2.28 Å, 129.8°; N6B–H6H⋯O3′
2.45 Å, 157.7°). The two Ga ions adopt close to square
pyramidal geometries according to the well-established τ5
parameter [30] where τ5=the difference between the two
largest coordinate angles divided by 60; τ5= 0 for square
pyramidal and τ5 = 1 for trigonal bipyramidal). For Ga1
τ5=0.035 and for Ga2 τ5=0.104);
There is extensive H-bonding in the structure. The
hydroxido ligand donates an H-bond to one of the THF
molecules O1–H1⋯O5′ 1.82 Å, 172.0° (see Fig. 4). The
NH groups of both tetradentate ligands also act as H-bond
Selected bond lengths and angles for this di-gallium
bis-thiosemicarbazone complex (compound 5) are shown
in Table 2. The longest coordinate bond lengths are to the
S-donors while the axial hydroxido ligand exhibits the
shortest bonds; the two Ga-O bonds being the same within
experimental uncertainty. Indeed, no differences are appar-
ent between the two coordination spheres which are square-
pyramidal as expected.
Based on our observations, we propose that formation of
this gallium complex takes place according to the mecha-
nism depicted in Scheme 2. The asymmetrical methyl ethyl
diphenyl bis-thiosemicarbazone gallium nitrate is formed
initially, according to the NMR evidence presented in this
report. We postulate that the nitrate complex was hydrolyzed
in the presence of THF (containing traces of water). Due to
the high charge of the metal, the aqua ligand spontaneously
ionizes then the hydroxido ligand thus formed reacts with
another gallium nitrato complex resulting in the formation
of the hydroxido-bridged digallium complex. This reaction
Fig. 2 X-ray crystal structure of asymmetrical methyl ethyl diphenyl
bis-thiosemicarbazone gallium nitrate complex (compound 5) after
diffusion of solvent for 10 days (the compound has slowly hydrolyzed
forming the dimeric compound)
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Journal of Chemical Crystallography
Table 1 Crystal data and structure refinement for 1705v5
Identification code
Empirical formula
Formula weight
Temperature
1705v5
C52 H71 Ga2 N13 O6 S4
1241.90
190(2) K
1.54184 Å
Wavelength
Crystal system
Space group
Unit cell dimensions
Monoclinic
P21/n
a=17.6854(13) Å; α=90°
b=16.6492(7) Å; β=112.383(8)°
c=21.4659(14) Å; γ=90°
5844.4(6) Å3
4
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
1.411 Mg/m3
2.947 mm–1
2592
0.2×0.1×0.04 mm3
3.46 to 62.47°
−20≤h≤20, −18≤k≤19,
−24≤l≤24
Fig. 4 Intermolecular interactions within the asymmetric unit of the
Ga–OH–Ga dimer. Generated with Mercury vers. 3.10 and rendered
with PovRay vers. 3.7
Reflections collected
35,379
9284 [R(int)=0.0783]
99.7%
Semi-empirical from equivalents
1 and 0.96902
Full-matrix least-squares on F2
9284/9/669
Independent reflections
Completeness to theta=62.47°
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I>2sigma(I)]
R indices (all data)
(compounds 2 & 3), complexes were obtained as deep red
colored solids. When this complex was subjected to sol-
vent vapor diffusion during the preparation of X-ray crys-
tals, colorless crystals were formed despite the solutions
initially being highly colored and subsequently colorless
on prolonged periods of time forming additional colorless
crystals of triazene. (compound 4) The crystals were sepa-
rated from the solution and their NMR characteristics are
illustrated (Supplemental data). Tables (Supplemental data)
show the chemical shift obtained for each proton and carbon
in the compound. It also shows DFT calculated chemical
shifts for each proton in the compound. The DFT calcula-
tions were done to further assess the proton chemical shifts
due to extreme over-lap of signals in the aromatic region. A
single methoxy group was attached to a chirotopic carbon.
In general, DFT calculated proton chemical shifts agreed
well with the experimentally observed chemical shifts. We
also examined the 13C NMR chemical shifts and the values
obtained along with DFT calculated chemical shifts are pro-
vided (Supplemental data).
1.040
R1=0.0617, wR2=0.1531
R1=0.0972, wR2=0.1793
0.982 and −0.682 e/Å3
Largest diff. peak and hole
(Compound # 5) (digallium compound)
presumably occurred on standing of the nitrate complex in
solution since NMR was performed in solvent immediately
after the preparation of the compound and showed that the
nitrate complex was intact. Therefore, we conclude that this
reaction must be slow as evidenced from the formation of a
fine white residue on the top of the crystals showing partial
decomposition of the original nitrate complex. We note that
the symmetrically ethyl substituted gallium nitrate complex
was isolated and its X-ray crystal structure was solved dem-
onstrating better stability. Thus, replacement of the ethyl
group with a methyl group was associated with a change in
the reactivity of the complex with moisture present in the
solvent.
The X-ray crystal structure of the triazene compound has
been previously described [31]. The triazene possesses three
aromatic rings and a methoxy group attached at the apex of
the molecule. We postulate that the mechanism for the for-
mation of the racemic mixture involves a planar transition
During the preparation of tetra phenyl substituted dithi-
osemicarbazone gallium chloride/methoxide complexes
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Journal of Chemical Crystallography
Table 2 Selected bond lengths (Å) and angles (°) for the Ga–OH–Ga
In Scheme 3, the gallium complexes are first formed as
evidenced by the NMR data. X-ray crystal data shows that
the compound slowly decomposes with time in the solvent.
Previously, we observed that gallium nitrate and methoxide
complexes of diphenyl substituted bis-thiosemicarbazone
were not stable over prolonged periods in solution [29]. We
propose that the partially decomposed bis-thiosemicarba-
zone cyclizes in the presence of methoxide and forms the
tirazene molecule through a quaternary nitrogen as depicted
in the scheme. This proposal is consistent with the forma-
tion of both “R” and “S” enantiomers in the final product.
The transition state product, a tetracyclic planar compound,
can be attacked by methanol from either side of the plane
resulting in racemization.
complex
N(2A)–N(3A)
N(2B)–N(3B)
1.363(3)
1.374(7)
N(3A)–Ga(1)
N(3B)–Ga(2)
2.057(4)
2.041(5)
N(4A)–N(5A)
N(4A)–Ga(1)
1.363(6)
2.042(4)
N(4B)–N(5B)
N(4B)–Ga(2)
1.367(6)
2.043(4)
O(1)–Ga(1)
O(1)–Ga(2)
1.904(3)
1.908(3)
S(1A)–Ga(1)
S(1B)–Ga(2)
S(2A)–Ga(1)
S(2B)–Ga(2)
2.2852(15)
2.3329(16)
2.2938(15)
2.2842(15)
98.25(16)
98.93(16)
75.86(17)
107.28(11)
148.68(13)
82.41(12)
103.80(11)
82.90(13)
150.81(14)
107.55(6)
100.11(18)
100.28(16)
98.25(16)
98.93(16)
75.86(17)
107.28(11)
148.68(13)
82.41(12)
103.80(11)
82.90(13)
150.81(14)
107.55(6)
100.11(18)
100.28(16)
Previous authors used dilute hydrochloric acid to obtain
the triazene compound and obtained the bis-thiosemicar-
bazone as a product when concentrated hydrochloric acid
was used [31]. We used excess base in our experiments and
therefore the triazene must have formed from the decom-
position of the complex in the medium. Bermejo et al. [7]
reported the formation of triazene compounds using cop-
per acetate; it is likely that a similar reaction mechanism to
that proposed above also applies in this setting. An alterna-
tive explanation is that during the synthesis of the gallium
complexes, the gallium salts influenced the formation of
the triazene product before the formation of the complex.
Although the triazene formed easily with phenyl substituted
bis-thiosemicarbazones, the methyl and ethyl substituted bis-
thiosemicarbazone did not produce triazenes demonstrating
the importance of multiple phenyl rings in the structure of
the molecule.
O(1)–Ga(1)–N(4A)
O(1)–Ga(1)–N(3A)
N(4A)–Ga(1)–N(3A)
O(1)–Ga(1)–S(1A)
N(4A)–Ga(1)–S(1A)
N(3A)–Ga(1)–S(1A)
O(1)–Ga(1)–S(2A)
N(4A)–Ga(1)–S(2A)
N(3A)–Ga(1)–S(2A)
S(1A)–Ga(1)–S(2A)
O(1)–Ga(2)–N(3B)
O(1)–Ga(2)–N(4B)
O(1)–Ga(1)–N(4A)
O(1)–Ga(1)–N(3A)
N(4A)–Ga(1)–N(3A)
O(1)–Ga(1)–S(1A)
N(4A)–Ga(1)–S(1A)
N(3A)–Ga(1)–S(1A)
O(1)–Ga(1)–S(2A)
N(4A)–Ga(1)–S(2A)
N(3A)–Ga(1)–S(2A)
S(1A)–Ga(1)–S(2A)
O(1)–Ga(2)–N(3B)
O(1)–Ga(2)–N(4B)
In summary, we have demonstrated that certain gallium
nitrate complexes of bis-thiosemicarbazones are not stable
over prolonged periods in solvent and that the substituents
play a major role in regard to stability. Additionally, we have
provided experimental evidence that the tetraphenyl substi-
tuted bis-thiosemicarbazone gallium complexes can lead to
structurally interesting triazene compounds. Further work is
in progress to establish the structural features promoting sta-
bility for complexes with potential biological applications.
state. This allows the nucleophile to approach from either
side of the plane of the molecule. We propose the follow-
ing mechanism (Scheme 3) for the formation of the triazene
from the gallium complex.
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Journal of Chemical Crystallography
Scheme 2 Scheme illustrat-
ing the proposed pathway for
formation of the di-gallium
complex (compound 5)
Scheme 3 Proposed mecha-
nism of formation of (R) and
(S)-5-methoxy-4,5,6-triphenyl-
4,5-dihydro-1,2,4-tirazene-
3(2H)-thione (compound 4)
showing the hydrogen bonding
characteristics in the presence
of base
Acknowledgements This work was partially supported by an ARC
Linkage Project Grant LP 130100703 (D.R.). We thank our colleagues
at the Centre for Advanced Imaging for their help during the course
of this investigation.
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