Synthesis and characterization of 5- and 7-donor Schiff base ligands and spectroscopic evidence for tautomerism: A crystal structure showing tautomeric forms within one ligand

Article abstract of DOI:10.1016/j.molstruc.2019.02.054

An improved method for the synthesis of fifteen Schiff base ligands, of which six are novel, are reported. Optimised yields were obtained and the ligands have been fully characterized via several analytical techniques. It was found that tautomeric forms o

Full text of DOI:10.1016/j.molstruc.2019.02.054

Accepted Manuscript
Synthesis and characterization of 5- and 7-donor Schiff base ligands and
spectroscopic evidence for tautomerism: A crystal structure showing tautomeric forms
within one ligand
Hezron F.O. Ogutu, Waheed Saban, Rehana Malgas-Enus, Robert C. Luckay
PII:
S0022-2860(19)30184-X
DOI:
https://doi.org/10.1016/j.molstruc.2019.02.054
MOLSTR 26211
Reference:
To appear in: Journal of Molecular Structure
Received Date: 7 September 2018
Revised Date: 25 October 2018
Accepted Date: 13 February 2019
Please cite this article as: H.F.O. Ogutu, W. Saban, R. Malgas-Enus, R.C. Luckay, Synthesis and
characterization of 5- and 7-donor Schiff base ligands and spectroscopic evidence for tautomerism: A
crystal structure showing tautomeric forms within one ligand, Journal of Molecular Structure (2019), doi:
https://doi.org/10.1016/j.molstruc.2019.02.054.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Synthesis and characterization of 5- and 7-donor Schiff base ligands and spectro-
scopic evidence for tautomerism: a crystal structure showing tautomeric forms
within one ligand
Hezron F. O. Ogutu, Waheed Saban, Rehana Malgas-Enus and Robert C. Luckay*
Department of Chemistry & Polymer Science, Stellenbosch University, Private Bag X1, Matieland, Stel-
lenbosch, 7602, South Africa. Tel: +27 21 8083333, email:rcluckay@sun.ac.za.
Abstract
An improved method for the synthesis of fifteen Schiff base ligands, of which six are novel,
are reported. Optimised yields were obtained and the ligands have been fully characterized
via several analytical techniques. It was found that tautomeric forms of these novel ligands
exist and spectroscopic evidence for this phenomenon is provided. A crystal structure illus-
trating both enol and keto tautomeric forms in a new ligand, complements spectroscopic ev-
idence obtained. To our knowledge, this is the first reported evidence of tautomeric forms
within the same ligand.
1. Introduction
The condensation reaction of substituted salicylaldehyde’s, or substituents of 2-hydroxy
naphthylaldehydes with primary amines usually results in the formation of multi-donor Schiff
base ligands 1-3. Symmetrical salen type ligands such as saldienH2, salnH3 or saltrin (see fig-
ure 1 for structures of these ligands) have found various applications 4,5 which include making
dyes or pigments and in making of polymers6. These ligands are malleable with the ability to
have multiple donor groups. One of the properties of these ligands is their abilityto complex
to a range of metal ions forming very stable metal complexes. The formed complexes have
been studied for catalysis, photochromism and biological studies7. Because these ligands are
multi-donor with various donor groups, they also have high chelation ability and therefore
can be used in hydrometallurgical solvent extraction studies 4,8,9
.
There has been considerable effort given to the different ways of synthesising salen type lig-
ands4,10-12. From literature El Sheriff et. al. 4 reviews two of the commonly used methods; the
Schiff base condensation and the template synthesis method. For saldienH2, salnH3 and sal-
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trin type ligands, the Schiff base condensation method is the preferred method 13-17. For lig-
ands with stereogenic centres the template synthesis method is the preferred method 4.
The template synthesis often involves the use of an appropriate template; for example, the
use of metal ion to form the desired products18. The formed complex can then be reduced to
the desired ligand as shown in Scheme 1. The Schiff base condensation method on the other
hand is often associated with relatively moderate to low yield and difficulties in purification4.
The difficulties in the synthesis of these ligands are often associated with the sensitivity of
the imine bond and the difficulty in separation of the formed products from the reactants.
The sensitivity of the formed imine bond in the presence of an acid or base is often accom-
panied with re-hydrolysis of the formed imine to the starting reactants 4.
SaldienH2 (class A) and salnH3 (class B)
for n =1 and 2
saltrin (class C) ligands
Figure 1 Representation of the N-donor starting amines and aldehydes (L1- L5). The typical
reaction involves 1 equivalent of amine with 2 equivalents of aldehyde. For class C ligands, 3
equivalents of aldehyde are required.
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HO
H
R
H
H
N
H
N
O
H
O
O
O
M
R
M
M
N
R
H
Scheme 1 The template synthesis of Schiff base complexes according to El-Sheriff et al4
OH
O
R1
R2
+H/OH-
-H2O
R1
R2
R1
NH2
R1
R2
+
NH
+H2O
N
R
R
N-substituted imine
primary
Amine
aldehyde or ketone
carbinolamine
Scheme 2 The condensation reaction of a primary amine with a carbonyl group incorporating the mechanistic
arrows according to Zoubi et al10.
There are extensive efforts that have been developed in the synthesis of these Schiff base
6
ligands . Most of these methods deal with the various modifications that have been under-
taken to increase yield and purity. In this paper we synthesize fifteen Schiff base ligands
which we have divided into three classes as shown in Figure 1. Various optimization tech-
niques are used to increase yield and purity. Six new ligands AL4, AL5, BL2, BL4, BL5 and CL5 are
synthesised and discussed while the other nine ligands have previously been synthesised and
are given in supplementary material, sections S1 and S2. The electronic effect of the various
substituents and different alkyl groups added between the central N-donor atoms are also
studied. The tautomeric behaviour of ligand BL3 is explicitly elucidated through spectroscopic
means. For the first time, a crystal structure of the new ligand AL4 is shown to display tauto-
meric behaviour within the same ligand which is also discussed.
2. Results and Discussion
2.1 Synthetic procedures
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Method 1: Synthesis of class A ligands
For ligand AL1 two equivalents of salicylaldehyde, was weighed and dissolved in ethanol un-
der reflux. One equivalent of diethylenetriamine was weighed and immediately dissolved in
ethanol. This was then added slowly to the reaction solution and then allowed to reflux for 24
hours to obtain a clear yellow solution. The ethanol solution was evaporated using rotary
evaporation. As the solvent was reduced, a yellow crystalline solid was obtained, filtered,
dried and characterized. The reaction equation is given in Scheme 3. The same method was
used for all other ligands under this class. All the molecular weights are given in the supple-
mentary materials provided.
Scheme 3 Synthesis of Ligand AL1 through condensation reaction with salicylaldehyde
For ligand AL1, AL3 and AL4 these were obtained as solids with yields of 84 %, 86 % and 89 %
respectively. The new ligand AL5 was obtained as a yellow viscous liquid at 89 % yield. The lig-
and AL3 was obtained in excessively higher yield than reported by Heindel et al19 who re-
ported a 45 % yield. The ligand AL2 was obtained as a red black solid that slowly turned com-
pletely black. This black mass was also reported by Parida et al20. The analysis of this black
mass indicated a mixture of imine and the various products of incomplete reaction and there-
fore this ligand was resynthesized using method 2 below.
Method 2:
The second method used chloroform in place of ethanol as used in method 1. Therefore, two
equivalents of L2 aldehyde (4-methoxy salicylaldehyde) was dissolved in 40 ml of chloroform
under reflux. One equivalent of diethylenetriamine was measured and dissolved in 10ml eth-
anol. The diamine was added slowly to the chloroform solution over a 1hr period. The volume
of the resultant yellow solution was reduced using a rotary evaporator and a yellow solid was
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obtained after almost all the solvent was removed. This was analysed as pure and in high
yield of 83 %.
Method 3: Synthesis of class B ligands
An attempt was made to synthesise ligand BL1 using the same Schiff base condensation
method 1 above, however the products obtained were in moderate yield (65 %) as had also
been reported by Heindel et al19 and upon analyses it showed the presence of some of the
starting materials. The ligand was therefore synthesised using an excess of the diamine. Since
the use of excess amine could potentially lead to formation of unsymmetrical ligand the dia-
mine was added over two hours with reflux to form the yellow solution. The product was al-
lowed to reflux for a further 22 hours, the ethanol was subsequently evaporated using the
rotary evaporator and the resulting product washed 3 times to get rid of amine and unsym-
metrical products. The final “wet” product was dried under vacuum for 3 days and analysed
as pure.
The same method was used for the other ligands under the same class ligand BL1, BL2, BL4
and BL5 were obtained with high purity at 86 %, 67 %, 68 % and 89 %. For ligand BL3 two
products were obtained BL3a and BL3b at 22 % and 70 % yield respectively. The melting point
for the yellow compound is 215 – 217 °C and the orange compound melted at 217 – 219 °C.
These two products are discussed in the tautomerism study section in 2.6.
Synthesis of class C ligands
These ligands were synthesised using both methods 1 and 2. The ligands CL1 and CL2 were
synthesised using method 1 to obtain crystalline solids that could be recrystallized in ethanol
to obtain highly pure products at 89 % yield each, this value is improved compared to the lit-
erature value by Mustapha et al16 who recorded 55 % for CL1 while Kim et al21 reported a
comparable 90 % yield. The ligands CL3, CL4 and CL5 were synthesised using method 2 and CL3
was obtained as a yellowish green powder while CL4 and CL5 were obtained as very viscous
yellow oils. These were obtained in 87 %, 85 % and 86 % yield respectively. The ligand CL3 had
previously been reported at 76% yield using THF solution by Kaur et al17, while CL2 was syn-
thesised by Bhattacharyya et al22, however, no yield was given. NMR spectra for CL1 is shown
in figures S15 and S16 in the supplementary material on pages 14 and 15.
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Method 4: Synthesis of tautomeric product BL3
Ligand BL3 was initially synthesised using method 2 above and the results obtained showed
two products, see Figure 2. The results of the product is further discussed in section 2.6. To
counter the above formation of the two products Method 4 was then used for the optimised
synthesis of BL3. An appropriate mass of 2-hydroxy naphthylaldehyde was weighed and com-
pletely dissolved in 40ml chloroform. Half an equivalent of bis(3-aminopropyl)amine was
weighed and dissolved in 10 ml ethanol. The resultant mixture was allowed to stir at room
temperature using a magnetic stirrer for 24 hours to obtain a yellow solution. The solution
was evaporated using a rotary evaporator at room temperature to remove the chloroform
and the resultant product washed 3 times with water so as to remove excess amine and un-
symmetrical products. The obtained “wet” yellow solid was vacuum dried for 3 days to obtain
dry yellow lumps, yield 86 %, mp 215-217 °C. The melting point was the same as previously,
but with an improved yield.
Figure 2 The yellow solid product obtained for BL3a (right) and orange product, BL3b (left)
2.2 Study of the electronic effect (IR spectroscopy)
From the IR spectra, the presence of the Schiff base product was confirmed by the signal of
the imine peak shown by the vibrational bands ((C=N)ʋ); at 1627 cm-1 for ligand AL1, 1631 cm-
1, for AL2, 1610 cm-1 for ligand AL3, 1633 cm-1 for ligand AL4 and 1632 cm-1 for ligand AL5.
Some IR spectra are shown for starting materials and for ligand AL1 in Figure S2 in the sup-
plementary material on page 8. The appearance of this peak together with the disappearance
of the aldehyde peaks confirmed the presence of the imine product. These results further-
more indicate a slight signal shift in the IR of the various ligands which is caused by the differ-
ent substituents on the phenyl ring. The naphthylaldehyde moiety shows a shift in the IR
spectrum. Upon change of the spacer group given by the increased number of methylene
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carbon groups from two to three for the class B ligands or the use of the tripodal ligands in
the class C ligands, the same trend is observed in the IR with the imine peaks being observed
at 1628 cm-1, 1634 cm-1, 1614 cm-1, 1635 cm-1, 1634 cm-1 for the ligands BL1, BL2, BL3, BL4, BL5
respectively. For CL1, CL2, CL3, CL4 and CL5 being observed at 1630 cm-1, 1633 cm-1, 1620 cm-1,
1631 cm-1 and 1631 cm-1 respectively. The shift of this imine peak for class A and class B lig-
ands compared to the shift due to the various substituents on the benzene ring indicated a
lower shift between class A and class B ligands than the shift due to the substituents on the
phenyl ring. This low shift can be attributed to the low effect of the added -CH2- group in the
spacer mainly due to the dominating nature of the donor group23. The general analysis of the
IR spectra indicates a higher shift due to the naphthyl group than the benzyl group and its
substituents. This can be attributed to both the high stability brought about by the aromatic
system of the naphthyl group which is electron rich. This shift was also observed by an analo-
gous ligand bearing a cyclic system next to the imine by Shamsipur et al 24. The imine signals
from IR for all ligands are captured in Table 1.
2.3 Discussion of the NMR results
1
The H NMR analyses were obtained for the different ligands showing in general two signals
of doublets at 2.99 ppm and 3.70 ppm for ligand AL1. NMR spectra are shown in figures S2, S3
and S4 on pages 9 and 10 in the supplementary material. These were assigned to the -CH2-
CH2- group. Each of these integrates to four equivalent protons. The proton signals indicated
by the peak at 2.99 ppm were assigned to the hydrogen protons on NH-CH2 while the down-
field ones at 3.70 ppm were assigned to the CN-CH2 protons next to the imine. The signals
between 6.83 ppm and 7.30 ppm were assigned to the four aromatic protons on the phenyl
group. The typical signal at around 8.34 ppm was assigned to the imine protons. The lack of
the carbonyl proton peak at 9-11 ppm for the starting aldehydes indicated the formation of
the Schiff base product. It is important to note that the NH proton on the linker is usually very
difficult to observe due to the high shielding effect of the N-donor atom. The addition of the
different substituents led to the slight shift of the imine proton and a slight shift in both the
aromatic protons and the spacer protons. For ligands AL2, AL4 and AL5 the added phenyl sub-
stituent resulted in an added signal in the downfield(aliphatic) region next to the spacer pro-
ton signals, while the aromatic proton signals are observed in the same region. The imine sig-
nal was therefore observed at 8.31 ppm, 8.79 ppm, 8.37 ppm and 8.35 ppm respectively for
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the AL2, AL3, AL4 and AL5 ligands. The lack of any extraneous peaks indicates good purity of the
product.
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Table 1 Summary of the IR and NMR spectra of the imine in different Schiff base products synthesized by the different linkers and aldehydes
Substituted Aldehyde
Schiff base product synthesized by Schiff base product synthesized by Schiff base product synthesized by
using diethylenetriamine linker
aminoethyl)ethane-1,2-diamine
using bis(3-aminopropyl)amine linker using
2-N,N-bis(2-
NCH
NCH
NCH
IR
1H NMR 13C{1H}NMR
IR
1H NMR 13C{1H}NMR
IR
1H
NMR
(in
13C{1H}NMR
(in CDCl3)
(in
CDCl3)
(in CDCl3)
(in
CDCl3)
(in CDCl3)
CDCl3)
cm-1
ppm
8.34
ppm
cm-1
ppm
8.33
ppm
cm-1
ppm
ppm
AL1 1627
AL2 1631
AL3 1610
166.15
BL1 1628
165.04
CL1 1630
7.82
166.27
O
OH
O
8.31
8.79
165.90
159.14
BL2 1634
BL3 1614
8.27
8.76
165.15
157.99
CL2 1633
CL3 1622
8.31
8.72
165.88
158.93
O
OH
O
OH
CH3
H3C
AL4 1633
8.37
8.35
166.58
165.67
BL4 1635
8.33
8.32
165.88
165.58
CL4 1631
8.35
8.35
166.34
166.46
O
H3C
OH
AL5 1632
BL5 1634
CL5 1631
O
OH
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For the proton decoupled 13C NMR, ligand AL1 is used as a representative ligand in the discus-
sion. The formation of a strong signal at 166.15 ppm in the spectrum is assigned to the imine
(HC=N) carbon as indicated. The four signals between 116 ppm to 162 ppm are assigned to
the four aromatic carbons. The alkyl group (aliphatic) signals between the imine and the
amine appear as two signals at 59.60 ppm and at 49.75 ppm. The lack of any signal above the
190-ppm range which is for the starting carbonyl group confirms the synthesis of the Schiff
base product AL1. The lack of any other signals in this case also indicates the formation of the
pure Schiff base product. For the other ligands in this class the imine signal was observed at
166 ppm, 165 ppm,158 ppm, 166 ppm and 165 ppm for AL1, AL2, AL3, AL4 and AL5 respective-
ly. The analysis of the 13C NMR spectra for the class B ligands showed that they were relative-
ly similar to the results of the previously synthesized class A ligands. The comparative results
of the shift of the imine in the IR spectra, important signals in the 1H NMR and 13C {1H} NMR
for all 15 ligands are listed in Table 1 giving a summary of important signals of these N-donor
ligands.
There is a limited inductive effect that is observed with the increase of the number of the
methylene groups between the donor groups. This is due to the higher inductive effect of the
donor group. This inductive effect is subsequently referred to as the hidden inductive effect
23
.
Finally, for the class C tripodal ligands all signals were observed to be very close to that of the
class A and class B ligands. Structural formulae of AL2 and BL2 as well as their appearance is
shown in figures S5 and S6 on page 10 in the supplementary material.
2.4 Mass spectrometry (MS)
The products obtained from the synthesis of these ligands were subsequently analysed
through the MS technique. The obtained fragmentation patterns for these ligands were rela-
tively similar for the class A and B ligands. Both classes of ligands showed that the species
have two main fragmentation species [M+H and [(M/2) + H] + species. For the class C ligands
with three salicylaldehyde groups these were observed to show 4 fragmented species as
shown in Figure 3.
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HO42
MS_Direct_170209_3 14 (0.080) Cm (13:26)
1: TOF MS ES+
8.58e6
419.3356
100
(a)
(c)
(b)
(d)
304.2516
419.8370
304.7530
837.6633
420.3387
426.3431
426.8447
607.4961
838.6665
311.2592
311.7611
608.4990
621.5115
650
279.8926
839.6700
852.6818
412.3275
400
427.3462
450
189.1679
200
983.8153
0
m/z
150
250
300
350
500
550
600
700
750
800
850
900
950
1000
Figure 3 Mass spectrum of ligand CL5 with x- axis showing the m/z value and the y-axis show-
ing the % abundance of the different fragments.
D1
Figure 4 Ligand CL5 with the area circled and labelled D1 as a fragment which is lost in the MS
analysis
The representative mass spectrum obtained for the tripodal ligand CL5 as illustrated in Figure
3 showed that the ligand has three ionisable fragments. These are proposed to contain dif-
ferent species. The species (c) is proposed to be the base peak at 100 % relative abundance
representing [M-C27H42N2O1.5] + or the [(M/2) +H] + species. Species (a) would represent the
ionisable mass fragment at 25 % relative abundance which confirms the mass of the Schiff
+
base product. The fragmented species (b) represents the ionisable [M-D1] species, where
the species D1 is shown in Figure 4. The species (d) represents the species [M-C36H39N3] + or
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[(M-D1)/2] at 60 % abundance. It is important to remember that all these species are only
present in the ionisable environment and are not products of the synthesis.
Analyses of the spectra obtained for the Class A and B ligands showed that, the general trend
observed in the addition of bulky alkyl substituents on the phenolic ring has the effect of in-
creasing the bulkiness of the Schiff base product and this leads to stabilization of the ionisa-
ble species. Therefore, the ligands AL5 and BL5 containing the bulky nonyl-substituents
showed one fragmented species with the species [(M/2) + H] representing the base peak. The
[M+H] species was observed at 50 % relative abundance. For AL4 and BL4 ligands both had the
species [M+H] as the base peak at 100 % relative abundance while the species [(M/2) + H]
species was observed to be at 25 % relative abundance. For class A and B ligands made from
aldehydes L1 to L3, these only showed the [M+H] peak at 100 % relative abundance. This in-
dicates the stability of these ligands under the MS conditions analysed. Further mass spectra
for AL5, BL3a and BL3b are shown in figures S18, S19 and S20 on pages 16 and 17 in the sup-
plementary material.
2.5 Elemental analysis
All the ligands synthesised were subsequently analysed and confirmed to contain the Schiff
base product in high purity through CHN microanalyses. It is also important to note that lig-
ands AL2, BL3, CL2 and CL4 could not be dried completely so they contained some reaction
solvents such as methanol and ethanol. This was also the case for the crystals obtained for
the class C ligands. The results from CHN analyses are therefore summarized in Table 2 be-
low.
Table 2 Comprehensive CHN analysis table for all the ligands
FORMULA
%CALCULATED
%OBTAINED
C
H
N
C
H
N
AL1 C18H21N3O2
AL2 C20H25N3O4(H2O)
AL3 C26H25N3O2
AL4 C26H37N3O2
AL5 C36H57N3O2
69.43 6.80
61.68 6.94
75.89 6.12
73.72 8.80
13.49
10.79
10.21
9.92
69.27 6.62
61.82 6.59
75.07 6.32
73.91 8.44
13.48
10.11
10.06
9.80
76.68 10.19 7.45
76.14 10.08 7.62
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BL1 C20H25N3O2
BL2 C22H29N304
70.77 7.42
66.14 7.32
74.20 6.65
74.46 9.15
12.38
10.52
8.65
70.29 7.26
66.17 7.03
74.07 6.14
74.23 9.11
12.38
10.13
9.07
BL3 C28H29N302(C2H5OH)
BL4 C28H41N3O2
BL5 C38H61N3O2
9.30
9.42
77.11 10.39 7.10
77.89 10.34 7.95
CL1 C27H30N4O2
70.72 6.59
64.63 7.12
73.68 6.18
73.18 8.99
12.22
9.42
8.81
8.33
70.86 6.61
64.79 6.57
73.77 6.38
73.29 8.51
11.70
9.85
9.23
8.33
CL2 C30H36N4O6(C2H5OH)
CL3 C39H36N4O3.2H20
CL4 C39H54N4O3(C2H5OH)
CL5 C54H84N4O3
77.46 10.11 6.69
77.43 10.09 6.07
2.6 Tautomeric study of the BL3 ligand
2.6.1 Spectroscopic Evidence
The results from the synthesis of BL3 ligand using method 2 showed the presence of two
products that were isolated in high purity at 72 % yield of BL3a product and 22 % BL3b product
(Figure 2). The analysis of the two products using IR spectra showed the presence of the
imine . Further IR analyses shows a broad peak in the region of 3340 cm-1 due to the –OH
group in the yellow product BL3a which is absent in the case of the orange product BL3b. Use
of the imine and carbonyl peaks is not definitive due to the overlap of these peaks in this re-
gion (see supplementary material figure S9, S10). To elucidate the structures the analysis of
1
the two products via H NMR spectroscopy indicated the formation of the desired imine
1
product. These H NMR spectra showed identical signals in CDCl3 (see figures S11, S12, S13
and S14 in the supplementary data). While this ligand as indicated above is known in the lit-
erature 25, Amirnasr et al25 synthesised and reported a 90% yield of the product, but did not
however report the method or the formation of these two products as shown in Figure 2.
To try to understand the reason for the difference of the colour, this sample set was then re-
analysed for 1H NMR in the same NMR tube by mixture of these two samples at a mass ratio
of 1:10 for the orange to yellow product respectively. This indicated a slight difference in the
spectra as shown by a shift of some of the proton signals as seen in Figure 5. This spectrum
indicated a second imine signal of integration ratio 1:10. From various studies of the salic-
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ylaldehyde based ligands these ligands are known to undergo tautomerism between the -OH
tautomer and the –C=O tautomer16, 26-30. This tautomeric effect occurs through the formation
of a H-bond between the phenolic hydrogen and the imine N-atom first, this later results in
complete transfer of the H-atom from the phenolic group to the imine nitrogen26. The result-
ant product is the –C=O tautomer 27. It was therefore hypothesized that the two compounds
are possible tautomers of each other. The tautomerism was attributed to a Proton transfer
which causes a change in the p-electron configuration and consequently in the molecular
conformation. The analysis of the two products using 13C {1H} NMR in CDCl3 indicated the
formation of the carbonyl group in the orange product as shown in Figure 6. The possible
mechanism of tautomer formation is given by Scheme 4 and the MS of the two tautomer’s
give in S20 and S21 in the supplementary data provided.
H15/17
Figure 5 The 1H NMR of ligand BL3a and BL3b mixed together in a 1:10 ratio of orange to yellow product in CDCl3
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Figure 6 The stacked 13C {1H} NMR spectra of Ligand BL3b (top) in d-DMSO and BL3a (bottom) in CDCl3
Scheme 4 Tautomerism of the Schiff base product BL3, (a) represents the hydroxyl tautomer
product (yellow) and (b) represents the –C=O tautomer product (orange).
The above results therefore confirmed the yellow product to be the OH tautomer and the
orange product to be the –C=O tautomeric product. The different tautomer’s and the odds of
15

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getting each of the tautomers is directed by heat or light28. Since most of the Schiff base con-
densation reactions are accompanied by refluxing, the predominance of the –C=O form (or-
ange product) is shown through the various crystal structures obtained for salicylaldehyde
Schiff base ligands which predominantly exist in the –C=O form (90% in CCD29 search 02-08-
2018). In our investigation we therefore show a crystal structure of the novel ligand AL4 dis-
cussed in the next section 2.6.2 with one end forming the OH form while the other end
shows the –C=O form. To try and control the formation of the orange product the ligand was
resynthesized without heat and light using a simple condensation method 4 above. Further
spectroscopic evidence is shown in section S5 on pages 11-14 in the supplementary material.
2.6.2 Crystal structure discussion
Single crystals of ligand AL4 were obtained by crystallizing the formed yellowish red ligand in
ethanol and using diethyl ether by the vapour diffusion method. The yellow crystal was ob-
tained in a monoclinic C2/c crystal system as shown in Table 3.
Table 3 Crystal structure data for AL4
Identification code
SW038_25_05_15_0m
C26H36N2O3
424.57
Empirical formula
Formula weight
Temperature (K)
100.03
Crystal system (Space group)
monoclinic (C2/c)
14.407(2)
13.744(2)
24.944(4)
90.00
a/Å
b/Å
c/Å
α/°
β/°
91.152(2)
90.00
γ/°
Volume/Å3
4938.2(13)
8
Z
Volume (ρcalcg/cm3 )
Radiation
1.142
MoKα (λ = 0.71073)
3.26 to 53.04
2Θ range for data collection/°
Index ranges
-18 ≤ h ≤ 17, -12 ≤ k ≤ 17, -30 ≤ l ≤ 31
Reflections collected
Goodness-of-fit on F2
Final R indexes [I>=2σ (I)]
14330
1.086
R1 = 0.0606, wR2 = 0.1679
Final R indexes [all data]
Largest diff. peak/hole / e Å-3
R1 = 0.0786, wR2 = 0.1827
0.80/-0.40
16

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The crystals show a disordered spacer group that was refined in two positions. The structure
obtained shows one of the salicylaldehyde ends of the symmetrical molecule forming the –
C=O tautomers while the other end forming the -OH tautomer. Both these opposite ends
show internal H-bonding between the appropriate atoms as shown in Figure 7. The structure
also confirms synthesis of the ligand, complementing all other analytical data.
Figure 7 Ortep diagram of the ligand AL4 showing the hydrogen bonding of the NH and the
phenyl O-atom, ellipsoids are drawn at 50% probability and picture quality generated by
Olex30
Analysis of the bond lengths and bond angles shows slight differences of the two ends of the
asymmetrical unit. The O1 C1 bond length was observed to be 1.280 Å this which is slightly
shorter than 1.352 Å which is for the O2 C18 bond length. The shorter O1 C1 indicates the
formation of the carbonyl group, while the longer O2 C18 indicates the single bonded phe-
nolic group. The C11 N1 bond length (1.303 Å) and the imine N3A C16A at 1.3020 (19) Å are
within experimental deviation of the same size. This shows that the electrons in the system
are delocalised between the benzyl system and the imine system. The one C-O bond length is
shorter than the standard single C-O bond length at 1.34 Å 31. The measured torsion angle
around the molecule however shows that the molecule is slightly strained around the OH
tautomer compared to the –C=O tautomer section. The selected bond lengths, bong angles
and torsion angles are provided in Table 4.
17

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Table 4 Selected bond lengths, bond angles and torsion angles
selected bond
lengths
selected bond angles
selected torsion angles
Bond Angle ( )
C1 C2 C11 N1 2.1(3)
bond
O1 C1
Length
(Å)
bonds
Angle ( )
̊
̊
1.280(2)
C11 N1
C12B
O1 C1 C2
127.4(7)
122.82(18)
115.5(3)
124.1(3)
N1 C11 1.303(3)
N2B C14B
C15B N3B
N1 C12A
165.0(5)
57.2(8)
N1
C12B
N1
1.460(2)
1.461(1)
C14A N2A
C13A
N3A C16A
C17
C13A N2A
C18 C17 C16B 13.2(9
N3B
C12A
N1 H1 O.9995
C2 C11 1.421(3)
O2 C18 C17 120.9(2)
O1 C1 C2 C11 5.6(3)
C16B C17 C18 17.5(6)
O2
N1 C11 C2
122.2(2)
C3 C4
1.375(3)
N3B C16B
C17
117.0(4)
C16A C17
C18 O2
9.4(4)
Figure 8 Ball and stick diagram showing the inter-molecular H-bonding in the packing of the
AL4 ligand (All H-bonding are indicated by broken lines)
The analysis of the packing system of the ligand shows that there are various other intermo-
lecular H-bonding interactions between the subsequent groups as shown by Figure 8. Select-
ed H-bonding and angles are given in Table 5.
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Table 5 Selected H-bonding with donor acceptor bond length and bond angles for ligand AL4
Bond
Donor -Acceptor length (Å
̊
Bond Angle ( )̊
N(1) --H(1) ..O(1)
2.02
2.29
1.76
1.98(3)
2.51
2.56
113
115
143
162(2)
136
134
N(1) --H(1) ..N(2A)
O(2) --H(2) ..N(3A)
N(2A) --H(2A) ..O(1)
C(12A)--H(12C) ..N(2A)
C(12A)--H(12D) ..O(2)
C(14A)--H(14A) ..O(2)C
2.59
167
3. Conclusion
Fifteen different multidentate Schiff base ligands were synthesized containing the N3O2 and
N4O3 donor groups of which six ligands are new. They were comprehensively analysed
through different analytical techniques. The ligands AL1, AL3, AL4, AL5, BL1, BL4 and BL5 were all
synthesized with the Schiff base condensation method 2, while ligands bearing the methoxy
substituent AL2 and BL2 were synthesized using a modified method 3. The ligand with naph-
thylaldehyde BL3 with the propyl spacer was also synthesized using method 3 and the prod-
ucts obtained were confirmed to undergo tautomerism. The synthesized ligands were further
studied for the electronic and steric effects due to the addition of the different substituents
on the phenyl ring and the effect of adding the alkyl group between the donor groups. The
results indicated that the addition of the substituent on the phenyl group had a much greater
effect on the shift of the imine on both the IR and NMR analysis. The effect of adding the al-
kyl groups between the donor atoms was on the other hand observed to have minimal shift
of the imine signals. The heptadentate tripodal ligands showed a much greater shift of the
imine due to the increased number of donor groups and the increased bulkiness of these lig-
ands. The crystal structure of the novel AL4 ligand was also discussed showing the formation
of an unsymmetrical molecule with one end showing the -OH tautomer and the other end
showing the –C=O tautomer.
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4. Experimental
4.1 Reagents
The following chemicals and reagents were used in the synthesis and characterization proce-
dure and include; 2-hydroxyl phenylaldehyde (L1), 5-methoxy-2-hydroxylphenylaldehyde (L2),
2-hydroxyl naphthylaldehyde (L3) which were all purchased from Sigma Aldrich. The 5-tert
butyl-2-hydroxyl phenylaldehyde (L4) and 5-nonyl-2-hydroxyl phenylaldehyde (L5) were syn-
thesized and obtained in high yield using a literature procedure by Aldred et al32 (See sup-
plementary information, section S1B). The two corresponding alcohols 4-tert butyl-phenol
and 4-nonyl-phenol together with paraformaldehyde were all purchased from Sigma Aldrich.
The magnesium turnings were purchased from Merck Schuchardt OHG. The toluene solvent,
sulfuric acid (H2SO4) and nitric acid (HNO3) were bought from Merck laboratory supplies. All
these reagents were used without further purification. The corresponding three diamines
bearing the added N-donor group; N-(2-aminoethyl)ethane-1,2-diamine, N-(3-
aminopropyl)propane-1,3-diamine and N,N-bis(2-aminoethyl)ethane-1,2-diamine were also
bought from Sigma Aldrich and used without further purification. The D-chloroform and D6
DMSO used for NMR studies were bought from Sigma Aldrich. The 99% pure ethanol solution
and 99% methanol were bought from B & M Scientific and were used after dry distillation.
The cold ethanol was obtained by placing the dried ethanol (in a sealed 250 ml volumetric
flask) in an ice bath for 24 hours. All aqueous solutions were prepared using double distilled
deionized water from an econoPure16 reverse osmosis system.
4.2 Instrumentation
The IR analysis was carried out using the Nicolet Avatar 330 FT-IR instrument with ATR acces-
sory with a ZnSe/Diamond crystal. The analysis was carried out in transmission mode with 32
scans at a range between 4000-400 cm-1. The 1H NMR and the 13C NMR were carried out us-
ing the Varian Unity Inova 300 Liquid State NMR Spectrometer, Varian Unity Inova 400 Liquid
State NMR Spectrometer and the higher magnitude NMR Varian Unity Inova 600 Liquid State
NMR Spectrometer which was used only when necessary to obtain better resolution of the
spectra. Approximately 20 mg of the synthesized samples were weighed into an NMR tube
and CDCl3 or D6 DMSO depending on the sample was then added and analysed. The mass
spectral analysis and CHN analyses were carried out on a Waters Synapt G2 High Resolution
20

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Mass Spectrometer, with a cone voltage of 15V. The analysis was done on a TOF (Time of
flight) positive and negative electrospray ionisation (ESI+) and (ESI-). The CHN analysis on the
other hand was carried out using a Perkin-Elmer CHNS elemental analyser model 240. Crys-
talline samples or powder samples were analysed for melting points on a Stuart Scientific
Melting Point apparatus SMP30. The crystal structure was carried out using a Bruker SMART-
APEX II DUO diffractometer instrument under liquid nitrogen stream at 100 K.
The synthetic procedure of all the ligands previously made is given in the supplementary ma-
terial (sections S2A, S2B and S2C), however, that of the new ligands and ligand BL3 is given
below:
4.3 Synthesis and characterization of class A ligands
4.3.1 Characterization of 2, 2’-{iminobis[ethane-2,1-diylnitrilo(E) methylylidene]}di-(4-tert
butylphenol) (Ligand AL4, using method 1)
A 4.33 g (0.0242 moles) of 4-tert butyl salicylaldehyde and 1.30 g (0.0126 moles) of diethy-
lenetriamine was used. Yellowish orange viscous oil that solidified slowly mp 64.9-66.2 °C,
yield 88 %, For C26H37N3O2: EA calculated (%) C (73.72%) H (8.80%) N (9.92%), EA Found (%); C
(73.91%), H (8.44) ,N(9.80); 1H NMR (300 MHz, CDCl3) δ H ppm; 8.37 (2 H, s, NCH), 7.39 – 6.79
(6 H, m, ArH), 3.76 – 3.66 (4 H, m, (NCH2CH2)), 3.01 (4 H, d, J 5.4 Hz, (NCH2CH2)), 1.30 (18 H,
s, C(CH3)3). 13C {1H} NMR. (300 MHz, CDCl3) δ ppm 166.58 (NCH), 159.97 (CArOH), 141.15
C
(CArH), 129.77 (CArH), 127.88 (CArH), 118.06 (CArCN), 116.41 (CArH), 59.73 (NCH2CH2), 49.97
(NCH2CH2), 31.87 (C(CH3)3), 31.54 (CH3)3. IR (ATR, neat, cm-1); 2865 (m, C-H); 1633 (s, C=N);
1589 (m, C=C); 1491 (m, C-H); 1266 cm-1 (s, CAr-O); ESI-MS (ES+); Expected m/z (%) 424.3, Ob-
tained m/z(%) 212.65 (25%), [(M/2)+H]+, 424.3 (100) [M+H]+.
4.3.2 Characterization of (2, 2’-{iminobis[ethane-2,1-diylnitrilo(E) methylylidene]}di-(4-
nonylphenol) (Ligand AL5, using method 1)
A 5.81 g (0.0234 moles) of 4-nonyl salicylaldehyde and 1.13g (0.0109 moles) of diethylenetri-
amine was used. Orange viscous oil was obtained, yield 87 %, For C36H57N3O2 EA calculated
1
(%); (76.68%) H (10.19%) N (7.45%), EA Found (%): C (76.14%) H (10.08) N (7.62); H NMR
(300 MHz, CDCl3) δ H ppm; 13.04 (2 H, s, OH), 8.35 (2 H, s, NCH), 7.39 – 7.01 (4 H, m, ArH),
6.88 (2 H, d, J 8.6 Hz, ArH), 3.87 – 3.50 (4 H, m, NCH2CH2), 3.00 (4 H, t, J 5.9 Hz, NCH2CH2),
1.61 – 0.33 (36 H, m, (CH2)8CH3). 13C {1H} NMR. (300 MHz, CDCl3) δ ppm; 166.67 (NCH),
C
21

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158.71 (CArOH), 138.16 (CArC9H19), 130.24 (CArH), 117.99 (CArCN), 116.42 (CArH), 59.90
(NCH2CH2), 50.10 (NCH2CH2), 41.56- 8.75 (C9H19). IR (ATR, neat, cm-1); 2955 (m, C-H), 2870
(m, C-H); 1632 (s, C=N); 1588 (m, C=C); 1490 (m, C-H); 1280cm-1 (m, CAr-O); ESI-MS (ES+); Ex-
pected m/z(%) 564.5 Obtained m/z(%) 282.7 (100) [(M/2)+H]2+, 564.5 (50) [M+H]+.
4.4 Synthesis and characterization of class B ligands
4.4.1 Characterization of 2,2’-{iminobis[propane-3,1-diylnitrilo(E)methyl idene]}di-2-hydroxy
naphthalene) (ligand BL3, using method 3
)
A 2.66 g (0.0150 moles) of 2-hydroxy naphthylene and 1.02 g (0.00800 moles) of bis(3-
aminopropyl)amine was used, Yellow dry lumps were obtained mp 215-217 °C yield 86%, For
Anal. Calc. For C28H29N302(C2H5OH) EA calculated (%) C (74.20) H (6.65%) N (8.65%), EA Found
(%): C (74.07) H (6.14) N (9.07); 1HNMR (400 MHz, CDCl3) δ H ppm; 8.76 (2 H, s, NCH), 7.83 (2
H, d, J 8.3 NCH), 7.64 (2 H, d, J 9.3 Hz, ArH), 7.57 (2 H, d, J 7.9 Hz, ArH), 7.38 (2 H, dd, J 6.8 Hz,
1.1 Hz, ArH), 7.20 (2 H, ddd, J 8.0 Hz, 7.1 Hz, 1.0 Hz, ArH), 6.89 (2 H, d, J 9.3 Hz, ArH), 3.67 (4
H, t, J 5.9 Hz, NCH2CH2CH2), 2.99 (4 H, t, J 5.8 Hz, NCH2CH2CH2), 1.23 (4 H, t, J 7.0 Hz,
(NCH2CH2CH2). 13C {1H} NMR. (75 MHz, CDCl3) δ ppm; 176.14(CArOH), 158.35(NCH) 137.30
C
(CArCN), 133.84 (CArH, 129.29 (CArH), 128.08 (CArH), 122.87 (CArH), 106.92 (CArH), 53.83
(NCH2CH2CH2), 49.50 (NCH2CH2CH2), 29.90 (NCH2CH2CH2), IR (ATR, neat, cm-1); 2955 (m, C-H),
2738 (m, C-H); 1614 (m, C=N); 1541 (s, C=C); 1491 (m, CH); 1273cm-1 (m, CAr-O); ESI-MS
(ES+); Expected m/z(%) 440.54, obtained m/z(%) 440.23 (100) [M+H]+.
4.4.2 Characterization of 2, 2’-{iminobis [propane-3,1-diylnitrilo(E)methylylidene]} di-(4-tert
butylphenol) (ligand BL4, using method 3)
A 5.59g (0.0313 moles) of 4-tert butyl-2 hydroxyl aldehyde and 2.10g (0.0160 moles) of bis(3-
aminopropyl)amine. Orange viscous oil was obtained, yield 84%, For Anal Calc. For
C28H41N3O2; EA calculated (%); C (74.46%) H (9.15%) N (9.30%), EA Found (%); C (74.23%) H
1
(9.11) N (9.42); H NMR (300 MHz, CDCl3) δ ppm; 8.33 (2 H, s, NCH), 7.33 – 7.27 (2 H, m,
H
ArH), 7.19 (2 H, d, J 2.2 Hz, ArH), 6.85 (2 H, d, J 8.6 Hz, ArH), 3.56 (4 H, t, J 33.5 Hz,
NCH2CH2CH2), 2.68 (4 H, d, J 6.8 Hz, NCH2CH2CH2), 1.88 – 1.82 (4 H, m, NCH2CH2CH2), 1.30 –
1.21 (18 H, s, C(CH3)3). 13C {1H} NMR. (75 MHz, CDCl3) δ C ppm; 165.88 (NCH); 158.71 (CArOH),
142.03 (CArC(CH3)3), 126.23 (CArH), 116.52 (CArH), 115.03 (CArH), 57.12 (NCH2CH2CH2), 48.22
(NCH2CH2CH2), 40.46 (NCH2CH2CH2), 31.49 (C(CH3)3). IR (ATR, neat, cm-1); 2960(m, C-H),
22

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28649 (m, C-H); 1635 (s, C=N); 1590 (m, C=C); 1493 (m, CH); 1281 cm-1 (m, CAr-O); ESI-MS
(ES+); Expected m/z(%) 452.3 Obtained m/z (%) 452.3 (100) [M+H]+.
4.4.3 Characterization of 2,2’-{iminobis[propane-3,1-diylnitrilo(E)methylylidene]} di-(4-
nonylphenol) (Ligand BL5, using method 3)
A 7.52 g (0.0303 moles) of 4-nonyl salicylaldehyde and 1.98 g (0.0151 moles) of bis(3-
aminopropyl)amine. Dark reddish-yellow viscous oil, yield 87%, For Anal. Calc. For C38H61N3O2:
EA calculated (%) C (77.11%) H (10.39%) N (7.10%), EA Found (%); C (77.89%) H (10.34) N
(7.95); 1HNMR (400 MHz, CDCl3) δ H ppm; 8.32 (2 H, s, NCH), 7.07 (2 H, d, J 7.2 Hz, ArH), 6.87
(2 H, d, J 8.3 Hz, ArH), 6.70 (2 H, d, J 8.6 Hz, ArH), 3.60 (4 H, s, NCH2CH2CH2), 2.72 (4 H, d,
NCH2CH2CH2), 1.85 (4 H, d, J 6.0 Hz, NCH2CH2CH2), 1.72 – 0.45 (38H, m, C9H19). 13C {1H} NMR.
(151 MHz, CDCl3) δ ppm; 165.58 (NCH), 158.53 (CArOH), 116.49 (CArCH2), 116.41 (CArH),
C
57.66 (NCH2CH2CH2), 47.78 (NCH2CH2CH2), 43.52 (NCH2CH2CH2), 37.41-8.77 (C9H19). IR (ATR,
neat, cm-1); 2956(w, C-H); 2871 (w, C-H); 1634 (s, C=N); 1589 (s, C=C); 1492 (m, C-H);
1283cm-1 (m, CAr-O); ESI-MS (ES+); Expected m/z(%) 592.5 Obtained :m/z (%) 592.8 (100)
[M+H]+.
4.5 Synthesis and characterization of class C ligands
Characterization of 5-nonyl salicylaldehyde and 2,2'-[ethane-1,2-diylbis(oxy)] diethanamine (ligand
CL5, using method 2)
A 6.37 g (0.0256 moles) of 5-nonyl salicylaldehyde and 1.26 g (0.00862 moles) of 2-N,N-bis(2-
aminoethyl)ethane-1,2-diamine. Product obtained as yellow viscous oil, yield 87.3 %, FM
C54H84N4O3, EA calculated (%) C (77.46) H (10.11) N (6.69) EA Found (%) C (77.43) H (10.09) N
(6.07), 1H NMR (600 MHz, CDCl3) δ H ppm; 8.28 (3 H, s, NCH), 7.15 (3 H, t, J 13.3, CArH), 7.10
(3 H, d, J 17.0, CArH), 6.88 (3 H, d, J 8.4, CArH), 3.67 (6 H, d, J 14.5Hz, NCH2CH2), 2.94 (6 H, s,
NCH2CH2), 1.75 – 0.52 (57 H, m, (C9H19)3): 13C {1H} NMR. (151 MHz, CDCl3) δ ppm; 166.46
C
(NCH), 158.77 (CArOH), 138.41 (CArCN), 130.35 (CArCH2), 118.12 (CArH), 116.49 (CArH), 116.42
(CArH), 58.64(NCH2CH2), 56.14 (NCH2CH2), 41.57- 14.23 (C9H19): IR (ATR, neat, cm-1); 2969(m,
C-H), 2864 (m, C-H), 1631 (m, C=N). 1587 (s, C=C), 1490 (m, CH), 1279 cm-1 (m, CAr-O): ESI-MS
(ES+); Expected m/z(%) 837.6, Obtained m/z (%) 304.3(65) (([M-C16H26O])/2)+, 419.3 (100)
[(M/2)+H]+, 607.5 (15) [M-C16H26O]+, 837.7) (15) [M+H]+.
23

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5. Supplementary Information
Supplementary data are available for this publication. Crystallographic data including atomic
coordinates, bond distances and angles have been deposited with the CCDC (Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336 033;
email: deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk) and may be obtained
free of charge by quoting the deposition number CCDC 1418740
.
6. Acknowledgements
We wish to thank Stellenbosch University and the National Research Foundation (NRF) for
financial support for this work.
7. Conflict of Interest
There is no conflict of interest for this work.
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Highlights
This publication displays the following highlights:
(i)
6 new Schiff base ligands have been synthesized and characterized
The yields on nine previously made Schiff bases have been optimized by various
methods
(ii)
(iii)
(iv)
A ligand which was previously made came out in 2 different colours and the
structure of these “2 ligands” was explored using various spectroscopic
techniques – this led us to the fact that tautomerism was at display
A crystal structure of a new ligand is obtained which displays keto and enol
tautomeric forms within the same ligand