Solid and solution structures of bulky tert-butyl substituted salicylaldimines

Article abstract of DOI:10.1016/j.molstruc.2011.02.033

A series of tert-butyl substituted salicylaldimines was synthesized and characterized by IR, ¹H NMR, ¹³C NMR, UV-vis, X-ray diffraction and elemental analyses to investigate the effect of two bulky tert-butyl groups on the keto-enol

Full text of DOI:10.1016/j.molstruc.2011.02.033

Journal of Molecular Structure 995 (2011) 9–19
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Solid and solution structures of bulky tert-butyl substituted salicylaldimines
⇑
Jahir Uddin Ahmad, Martin Nieger, Markku R. Sundberg, Markku Leskelä, Timo Repo
Department of Chemistry, Laboratory of Inorganic Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland
a r t i c l e i n f o
a b s t r a c t
A series of tert-butyl substituted salicylaldimines was synthesized and characterized by IR, 1H NMR, 13C
NMR, UV–vis, X-ray diffraction and elemental analyses to investigate the effect of two bulky tert-butyl
groups on the keto-enol tautomerization in solution and solid state. According to X-ray studies all sali-
cylaldimines have an enol tautomeric form in solid state and due to strong O–HꢀꢀꢀN hydrogen bonding
they form a nearly planar intramolecular six-membered ring. Based on NMR, IR and UV–vis studies,
the enol form is also present in solutions. Computational studies carried out by the density function the-
ory (DFT) method verifies that in each sterically hindered keto-enol pair the enol form is more stable,
although the differences in energy vary from 5.5 to 10.1 kJ/mol (in terms of total energy obtained from
Gaussian for the optimized moieties). For the tert-butyl substituted salicylaldimines, solvatochromism
is not observed in polar hydrogen bonding solvents. As N-alkyl and N-alkylphenyl substituted salicylaldi-
mines without tert-butyl groups have a solvatochromic band, the sterically bulky tert-butyl groups seem
to have a key role in stabilizing the intramolecular hydrogen bonding.
Article history:
Received 24 September 2010
Received in revised form 11 January 2011
Accepted 14 February 2011
Available online 1 March 2011
Keywords:
Schiff bases
Salicylaldimine
Hydrogen bond
Crystal structure
Tautomerization
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
equilibrium between enol and trans-keto forms, and thermochro-
mism arises from the shift in equilibrium between enol and cis-
Schiff base ligands are universal in inorganic chemistry due to
their straightforward preparation, high yield and easy purification.
Their strong coordination affinities for metal ions are of great inter-
est in the synthesis of transition metal complexes, which play an
important role in many fields of chemistry and biochemistry
[1,2]. For example, Schiff base complexes have been used as highly
efficient catalysts for cyclopropanation [3–6], enantioselective ring
opening of mesoaziridine [7], oxidation [8–11], polymerization
[12–16], epoxidation [17–19] and Hetero Diels–Alder reactions
[20]. In biochemistry, Schiff base copper complexes are known to
exhibit good antibacterial activity [21] and have considerable anti-
malarial activity [22]. It is worth noting that Schiff base ligands
alone, such as N-hydroxy-N1-aminoguanidines, have antitumor
and antiviral activities and can also act as inhibitors of leukemia
cell growth [23].
keto isomers [25,26]. In photochromic materials, salicylaldimines
are non-planar (i.e. they lack O–Hꢀ ꢀ ꢀN hydrogen bonding), while
in thermochromic materials they are planar [27]. As a result, mol-
ecules in photochromic materials are packed rather loosely in a
crystal and may undergo some conformational changes, while
thermochromic materials are packed tightly to form one-dimen-
sional columns [28].
The position and nature of substituents in the phenyl ring of sal-
icylaldimines influence the keto-enol tautomerization [29] and
they can also cause a dramatic change in the catalytic properties
of salicylaldimine complexes [30]. Salicylaldimines with tert-butyl
groups in the phenyl moiety at positions 3 and 5 are particularly
interesting as they enable peripherally-bound, stable phenoxyl
radical complexes to form [31]. For example, copper complexes
based on 3,5-di-tert-butyl salicylaldimines form highly efficient
catalytic systems for the aerobic oxidation of primary benzylic,
allylic and heterocyclic alcohols under mild reaction conditions
[30].
Salicylaldimines are particularly interesting as such because of
their self-isomerization or tautomerism properties, which are asso-
ciated with changes in the
p-electron density distribution within
the molecule and which subsequently cause changes in molecular
conformations [24]. In solid state, molecular conformations can be
influenced by crystal packing forces, giving different photochromic
and thermochromic properties to crystals of the same salicylaldi-
mine. The photochromic behavior arises from the shift in
In solid state, the keto tautomer (Oꢀ ꢀ ꢀH–N) is particularly
stabilized by functional groups which facilitate the formation of
intermolecular H-bonds [32]. On the other hand, the enol form (O–
Hꢀ ꢀ ꢀN) appears to be predominant in solution when bulky substitu-
ents on the salicyl moiety are present [33]. We report herein a
detailed study of 3,5-di-tert-butyl salicylaldimines that was done
to gain further insight into their structure in solution and in solid
state. The series consists of seven tert-butyl substituted salicylaldi-
mines, including some with electron releasing (Me) or withdrawing
⇑
Corresponding author. Tel.: +358 9191 50194; fax: +358 9191 50198.
E-mail address: timo.repo@helsinki.fi (T. Repo).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.02.033

10
J. Uddin Ahmad et al. / Journal of Molecular Structure 995 (2011) 9–19
Compound
R
R
R
NH2
N
O
1
EtOH, Reflux
2
CH3
NO2
OH
OH
3
4
5
6
7
Scheme 1.
(NO2) groups, the main focus being on the structures in solution and
solid state (Scheme 1).
2.3. Ligand synthesis
Schiff base ligands 1–7 were prepared with high yields (80–90%)
as yellow to orange crystalline solids by published procedures
[36,37] involving the condensation of 3,5-di-tert-butyl-salicylalde-
hyde with the appropriate amine in refluxing ethanol in the
presence of a catalytic amount of formic acid (2–3 drops) as shown
in Scheme 1 (for more details concerning the ligand synthesis, see
Supplementary materials).
2. Experimental
2.1. General techniques
All chemicals were obtained from commercial sources and were
used without further purification. Melting points were obtained
with an electrothermal melting point apparatus and are not cor-
rected.1H and 13C NMR spectra were collected on a Varian Mercury
300 MHz spectrometer. Chemical shifts for 1H NMR and 13C NMR
were referenced with respect to CHCl3 and TMS, respectively. IR
spectra were collected on a Perkin Elmer Spectrum GX spectrome-
ter and a Perkin Elmer Spectrum one spectrometer for solution and
solid samples, respectively. UV–vis spectra were recorded with a
Hewlett Packard 8453 spectrophotometer from EtOH, CHCl3, tolu-
ene and n-hexane solutions. Elemental analyses were made using
an EA 1110 CHNS–OCE instrument. EI–mass spectra were run with
a JOEL JMS–SX 102 mass spectrometer (ionization voltage 70 eV)
from solid samples.
2.4. Computational study
Salicylaldimines 1–7 were fully optimized at the B3LYP/6–311Gꢁ
level of theory. The experimental atomic coordinates obtained from
the single-crystal X-ray diffraction studies were used as inputs. The
computed IR spectra for the optimized ligands did not show any
imaginary frequencies. The results were obtained using the Gauss-
ian03 program suite [38]. NBO analysis was carried out by subrou-
tines of Gaussian03 [39].
3. Results and discussion
2.2. X-ray crystallography
3.1. IR and NMR spectra
Dilute methanol solutions of 1 and 4–6 were kept at room tem-
perature and by slow evaporation of solvent, single crystals suit-
able for X-ray studies were obtained. Crystals of 2 and 7 were
obtained from saturated toluene and n-heptane solutions, respec-
tively. Compound 2 crystallizes with two and 4 with three inde-
pendent molecules in the asymmetric unit.
Crystal data, data collection parameters and details of the
refinements are summarized in Table 1. All single-crystal X-ray dif-
fraction studies were carried out on a Bruker–Nonius Kappa-CCD
The IR spectra for 1–7 were measured from solid samples and
solutions and their corresponding characteristic data are listed in
Table 2. For comparison also calculated frequencies for enol and
keto forms are included. The calculations were made at B3LYP/6–
311G⁄ level of theory and the obtained frequencies are multiplied
by scaling factor (0.9632) to compensate the shortcomings due to
the electron correlation approximate treatment, anharmonicity ef-
fects, basic set deficiencies etc. [40]. For 1 the calculated IR spectra
including both enol and keto form together with experimental
spectra in solid state and in CHCl3 solution are given in Fig. 1.
Accordingly, the calculated and observed IR frequencies confirm
the presence of 1 in enol form rather than keto form both in solu-
tion and solid state.
diffractometer at 123(2) K and 173(2) K, using Mo K
a radiation
(k = 0.71073 Å). Direct methods (SHELXS-97) were used for struc-
ture solution and refinement was carried out using SHELXL-97
(full-matrix least-squares on F2) [34]. Hydrogen atoms were local-
ized by difference electron density determination and refined
using a riding model, and H(O) atoms were refined free. In 2 and
6 a tert-butyl group at carbon 4 is disordered. The absolute struc-
ture of 1 and 5 could not be determined reliably by refinement
of Flack’s x-parameter [35].
Crystallographic data (excluding structure factors) for the struc-
tures reported in this work have been deposited with the Cambridge
Crystallographic Data Centre as Supplementary Publication Nos.
CCDC 734930 (1), CCDC 734931 (2), CCDC 734932 (3), CCDC
734933 (4), CCDC 734934 (5), CCDC 734935 (6) and CCDC 734936
(7). Copies of the data can be obtained free of charge on application
to The Director, CCDC, 12 Union Road, Cambridge DB2 1EZ, UK (Fax:
int. code + (1223)336 033; e-mail: deposit@ccdc.cam.ac.uk).
In solid state IR spectra, a sharp and intense band is observed in
the range 2949–2966 cmꢂ1 due to
m(O–H). The presence of this
band at such a relatively low frequency is characteristic of these
compounds and is an indication of strong hydrogen bonding [41].
In the calculated IR spectra, the most intense stretching vibration
is assigned to O–H mode at 2997–3074 cmꢂ1. Very sharp and
strong bands in the region of 1433–1440 cmꢂ1 are due to stretch-
ing of the phenolic C–O band [40(a),42], suggesting that all com-
pounds in the solid state exist as an enol-imino (O–Hꢀ ꢀ ꢀN)
tautomer. The calculated value for C–O band is in the range
1428–1439 cmꢂ1. The characteristic
is observed in the region 1570–1619 cmꢂ1 for 1–3, which have an
N-phenyl substituent. The N-alkyl substituted 4–5 give the (C@N)
m(C@N) band in the solid state
m

J. Uddin Ahmad et al. / Journal of Molecular Structure 995 (2011) 9–19
11
Table 1
Crystal data and structure refinement parameters for 1–7.
1
2
3
4
5
6
7
Empirical formula
Formula weight
Temperature (K)
Radiation,
C
21H27NO
C22H29NO
323.46
123(2) K
C22H29N2O3
354.44
173(2)
C18H29NO
275.42
123(2)
C21H33NO
315.48
173(2)
C22H29NO
323.46
123(2)
C23H31NO
337.49
173(2)
309.44
123(2)
Mo K
a
, 0.71073 Å Mo K
a, 0.71073 Å Mo K
a, 0.71073 Å Mo K
a
, 0.71073 Å Mo K
a, 0.71073 Å Mo K
a
, 0.71073 Å Mo K
a, 0.71073 Å
wavelength
Crystal system
Space group
a (Å)
b(Å)
c(Å)
Orthorhombic
Pna21 (No. 33)
12.305(1)
8.917(1)
16.568(2)
90
90
90
1817.9(3)
4
1.131
Monoclinic
P21/c (No. 14)
19.782(2)
10.715(1)
20.713(2)
90
117.80(1)
90
3883.7(7)
8
Orthorhombic
Pccn (No. 56)
13.406(2)
24.064(4)
12.006(1)
90
90
90
3873.2(9)
8
1.216
Triclinic
P-1 (No. 2)
12.020(2)
15.566(2)
15.920(2)
72.11(1)
80.74(1)
75.29(1)
2730.6(7)
6
Orthorhombic
Pna21 (No. 33)
11.964(8)
9.947(4)
16.848(4)
90
Monoclinic
P21/c (No. 14)
17.074(5)
9.889(2)
11.483(2)
90
93.13(2)
90
1936.0(8)
4
Monoclinic
P21/c (No. 14)
10.418(2)
10.0118(18)
20.286(3)
90
101.035(15)
90
2076.8(7)
4
a
(°)
b (°)
90
90
c
(°)
V (Å3)
2005.0(16)
4
1.045
Z
Dcal (g cmꢂ3
Absorption
)
1.106
0.067
1.005
0.061
1.110
0.067
1.079
0.065
0.068
0.081
0.063
coefficient
(mmꢂ1
F(0 0 0)
)
672
1408
1520
912
696
704
736
Crystal size (mm)
2 Thetamax. (°)
0.50 ꢃ 0.45 ꢃ 0.40 0.35 ꢃ 0.15 ꢃ 0.05 0.55 ꢃ 0.45 ꢃ 0.15 0.45 ꢃ 0.30 ꢃ 0.20 0.50 ꢃ 0.12 ꢃ 0.08 0.50 ꢃ 0.35 ꢃ 0.15 0.50 ꢃ 0.40 ꢃ 0.20
55
55
50
50
50
50
50
Limiting indices
ꢂ15 ꢄ h ꢄ 15
ꢂ11 ꢄ k ꢄ 11
ꢂ20 ꢄ l ꢄ 21
22,817
ꢂ20 ꢄ h ꢄ 25
ꢂ13 ꢄ k ꢄ 13
ꢂ26 ꢄ l ꢄ 24
31,777
ꢂ15 ꢄ h ꢄ 9
ꢂ27 ꢄ k ꢄ 28
ꢂ14 ꢄ l ꢄ 11
10,132
ꢂ14 ꢄ h ꢄ 14
ꢂ18 ꢄ k ꢄ 18
ꢂ18 ꢄ l ꢄ 18
33,617
ꢂ14 ꢄ h ꢄ 14
ꢂ11 ꢄ k ꢄ 11
ꢂ19 ꢄ l ꢄ 20
20,744
ꢂ20 ꢄ h ꢄ 20
ꢂ11 ꢄ k ꢄ 9
ꢂ13 ꢄ l ꢄ 9
7607
ꢂ12 ꢄ h ꢄ 12
ꢂ11 ꢄ k ꢄ 10
ꢂ23 ꢄ l ꢄ 24
12,641
No. of data
collected
No. of unique data 4047
8881
3324
9374
3502
3277
3632
Rint.
0.0296
0.0606
436/98
0.0390
238/1
0.0468
550/9
0.1042
211/2
0.0311
211/80
0.0558
229/1
No. of parameters/ 211/2
restraints
Ra [for I > 2
r(I)]
0.0330
0.0795
1.07
0.0784
0.2096
1.03
0.0418
0.0976
1.06
0.0525
0.1247
1.06
0.0666
0.1757
1.05
0.0694
0.1748
1.05
0.0475
0.1161
1.05
b
wR2 (for all data)
Goodness of fit
Absolute structure ꢂ0.1(11)
ꢂ1(3)
parameter x
Largest diff. peak
and hole (e Aꢂ3
0.215/ꢂ0.177
0.793/ꢂ0.550
0.193/ꢂ0.176
0.455/ꢂ0.237
0.354/ꢂ0.247
0.817/ꢂ0.519
0.165/ꢂ0.231
)
a
R =
wR2 ¼ ½
R
||Fo| ꢂ |Fc||/
R|Fo|.
2
2
1=2
R
½wðF2o ꢂ F2c Þ ꢅ=
R
½wðF2oÞ ꢅꢅ
.
b
band at 1629 cmꢂ1 while the band appears at 1631–1635 cmꢂ1 for
the N-alkylphenyl substituted 6 and 7. The relatively low C@N
stretching frequencies in N-phenyl salicylaldimines (1–3) com-
pared to N-alkyl (4–5) or alkylphenyl salicylaldimines (6–7) are
presumably due to the conjugation between the azomethine
withdrawing phenyl group de-shields the azomethine proton and
shifts the resonance to relatively low field (see Table 2). The CH@N
resonance appears as a broad singlet for all compounds in CDCl3,
providing further evidence that in solution these compounds exist
mainly in the enol-imino tautomeric form. In all cases a singlet for
OH has a distinct down-field resonance in the range of 13.10–
14.03 ppm, characteristic for the acidic proton involved in a strong
intramolecular hydrogen bond [47]. This phenomenon is further
confirmed by 13C NMR spectra (see Table 2) where the phenolic
carbon C1 resonance exists from 158.3 to 158.8 ppm, whereas it
appears further down field in the case of keto-imino tautomers,
(C1)OH = 149–161 ppm and (C1)NH = 179–183 ppm [47(b),48].
(CH@N) and the
p system of the N-phenyl ring. A similar phenom-
enon is observed in solution [42] as well as in gas phase measure-
ments. The calculated values for
m(C@N) in the enol form are
between 1560 and 1636 cmꢂ1. It is noteworthy that in the calcu-
lated IR spectra for the keto form, the most intense fundamental
vibration frequency is assigned to C@N at 1276–1633 cmꢂ1. The
presence of phenolic m
C–O at 1433–1440 cmꢂ1 and the absence
of a band at 1728–1731 cmꢂ1 for C@O [37,43] in the CHCl3, toluene
and n-hexane spectra of the compounds is the evidence for the pre-
dominant species of enol form (Nꢀ ꢀ ꢀH–O) with intramolecular
hydrogen bonding in the solutions. In keto form, the calculated fre-
quency for C@O is assigned at 1525–1551 cmꢂ1 which is eventu-
ally absence in the experimental IR spectra of the compounds
(1–7). A narrow band is observed for the compounds in the range
3.2. UV–vis spectra
Electronic spectra of 1–7 were recorded in the 200–900 nm
range in polar and nonpolar solvents such as CHCl3, EtOH, toluene
and n-hexane (Fig. 2). The corresponding data for the salicylaldi-
mines in the different solvents are also listed in Table 3. Previous
work on Schiff base derivatives has suggested that enolic intramo-
lecular hydrogen bonding predominates in the ground state in
nonpolar environments [49–51]. The absorption band at 400–
500 nm can be attributed to the keto form whilst the enol form
has no appreciable absorbance in this region [52].
2854–2973 cmꢂ1 due to the
groups [44].
m(C–H) stretching mode of the t-Bu
The presence of the enol form with intramolecular hydrogen
bonding in solution can also be ascertained by 1H NMR spectros-
copy as well [36,45,46]. The azomethine proton (CH@N) resonance
is observed within the range of 8.63–8.66 ppm for N-phenyl sali-
cylaldimines (1–3), around 8.36 ppm for N-alkyl (4–5) and 8.27–
8.44 for N-alkylphenyl (4–7) salicylaldimines as the electron
The UV–vis spectra of 4–5 and 6–7, having N-alkyl and N-alkyl-
phenyl groups respectively, in polar and nonpolar solvents show an
intense absorption band in the range of 328–333 nm. In general,

12
J. Uddin Ahmad et al. / Journal of Molecular Structure 995 (2011) 9–19
Table 2
Characteristic IR (calculated and experimental), 1H and 13C NMR (Experimental) data for 1–7.
Ligand
t(C–H of t-Bu)
t
(O–H)
t(C@N)
t
(C@O)
t(O–Cphenolic
)
1H NMRc
OH
13C NMRc
C–OH
CH@N
CH@N
1enol
1keto
2994
2980
3017
–
1614
1629
–
1538
–
1429
–
8.64
13.69
164.0
158.5
1a
2945/2908/2913/2908
2963/2964/2962/2940
1615/1618/1605/1620
1437/1440/1448/1450
exp
2enol
2keto
2992
2979
3678
–
1627
1629
–
1439
–
8.63
8.66
8.36
8.37
8.44
8.27
8.64
13.80
13.10
13.97
14.03
13.72
13.81
13.69
163.1
166.6
163.3
163.5
166.9
166.3
164.0
158.4
158.8
158.4
158.5
158.3
158.4
158.5
1531
2a
2910/2910/2854/2892
2995
2983
2947/2940/2854/2972
2991
2979
2961/2908/2869/2973
2979
2978
2941/2858/2856/2857
2992
2979
2909/2910/2855/2933
2993
2975
2907/2855/2854/2946
2979
2979
2955/2960/2961/2941
3074
–
2966/2964/2962/2988
2997
–
2964/2966/2965/2988
3055
–
2956/2933/2932/2935
3023
–
2960/2963/2962/2982
3000
–
2949/2963/2962/2978
3041
–
1619/1620/1620/1621
1560
1276
1570/1577/1579/1581
1635
1628
1629/1631/1630/1633
1630
1628
1629/1630/1629/1631
1636
1633
1439/1440/1440/1438
1427
–
1433/1433/1437/1434
1433
–
1441/1441/1441/1436
1428
–
1441/1441/1437/1442
1434
–
1436/1441/1442/1435
1433
–
1440/1442/1437/1442
1430
exp
3enol
3keto
–
1551
3a
exp
4enol
4keto
–
1530
–
4a
exp
5enol
5keto
–
1527
5a
exp
6enol
6keto
–
1525
–
–
1532
–
6a
1631/1632/1632/1634
1636
1633
exp
7enol
7keto
7a
1635/1632/1632/1634
exp
Renol
Rketo
1642
1642
1648
–
1532
1731
Rb
exp
a
b
c
Experimental IR (cmꢂ1) in solid state, CHCl3, toluene and n-hexane.
Experimental IR (cmꢂ1) of N-methyl-3,5-di-tert-butylsalicylaldimine [43].
Experimental 1H and 13C NMR (ppm) in CDCl3.
Fig. 1. Calculated (a and b) and experimental (c and d) IR spectra of 1 (a) keto form, (b) enol form, (c) solid state, (d) in CHCl3.

J. Uddin Ahmad et al. / Journal of Molecular Structure 995 (2011) 9–19
13
Fig. 2. UV–vis absorption spectra for 1–7 in different solvents (a) EtOH, (b) CHCl3, (c) toluene, (d) n-hexane.
Table 3
The maximum absorption wavelength and absorbance for 1–7 in different solvents.
Ligands
Solvent
Wavelength kmax/[(nm) Abs(AU)]
1
EtOH
CHCl3
Toluene
n-C6H14
307(1.02)
308 (1.16)
309(0.49)
307(0.99)
352(0.84)
354(0.91)
354(0.41)
355(0.82)
2
3
4
5
6
7
EtOH
CHCl3
Toluene
n-C6H14
311(0.96)
313 (0.86)
314(0.64)
311(0.91)
326(0.95)
327(0.84)
328(0.64)
326(0.86)
352(0.86)
355(0.76)
357(0.59)
355(0.81)
EtOH
CHCl3
Toluene
n-C6H14
330(0.97)
330 (0.92)
328(0.63)
318(0.91)
377(0.45)
374(0.56)
485(0.004)
EtOH
CHCl3
Toluene
n-C6H14
332(0.92)
328 (0.63)
329(0.92)
329(0.92)
488(0.008)
492(0.007)
EtOH
CHCl3
Toluene
n-C6H14
332 (0.91)
328 (0.64)
330(0.48)
330(0.93)
412(0.04)
421(0.02)
EtOH
CHCl3
Toluene
n-C6H14
333(0.95)
330 (0.67)
332(0.60)
333(0.97)
488(0.008)
488(0.002)
EtOH
CHCl3
Toluene
n-C6H14
333(0.98)
330 (0.97)
331(0.90)
333(0.97)
488(0.002)
conjugation should cause bathochromic shifts in the absorption
maxima of chromophores and the observed bathochromic shifts
with 1–3 are attributed to the partial conjugation between CH@N
and the neighboring phenyl group. Absorption spectra of 1–3 exhi-
bit 2 or 3 intense bands between 307 and 377 nm. In addition, the
presence of electron releasing or withdrawing group at the p-posi-
tion of the phenyl ring affects the absorption maxima of chro-
mophores. The electronic spectrum of 1 with p-H, in both polar
and nonpolar solvents shows two bands at around 308 and
353 nm, while three intense bands at around 312, 326 and

14
J. Uddin Ahmad et al. / Journal of Molecular Structure 995 (2011) 9–19
355 nm are observed for 2, which has an electron releasing methyl
group at the p-position. Compound 3, with an electron withdraw-
ing p-nitro group, gives two bands at around 328 and 374 nm.
In a more detailed analysis of the spectra of 3, 4, 6 and 7 a very
weak absorption above 480 nm is detectable in nonpolar solvents.
In the case of 5, a similar, very weak absorption band that appears
at 412 nm in protic polar solvent is red shifted to 421 nm in CHCl3.
These absorptions may be due to the presence of traces of the keto
form. It was earlier considered that a bulky o-substituent on a
phenoxy moiety, simply for steric reasons, could compel the OH
proton to be close to the N atom and thus favor the keto tautomer
[53(a)]. To confirm the effect of a tert-butyl group in keto-enol tau-
tomerization, a new series of salicylaldimines without the tert-
butyl groups (1’–7’) were also synthesized and examined in various
solvents (see Supplementary materials). Of those, only salicylaldi-
mines with N-phenyl (10) and N-p-methylphenyl (20) imine substit-
uents show a weak absorption band at longer wavelengths than
480 nm in nonpolar solvents. In addition, N-p-nitrophenyl
substituted 30 exhibits a weak absorption band both in polar and
nonpolar solvents.
creases [59]. Surprisingly, solvatochromism does not appear in
the tert-butyl substituted 1–7 but a strong solvatochromic band
is observed at around 400 nm for 40–70, which have N-alkyl and
N-alkylphenyl groups, respectively (see Supplementary materials,
Fig. S1). This agrees with previous reports for 60 (N-(benzyl) salicyl-
aldimine) and 70 (N-(2-phenylethyl) salicylaldimine) [57(a),60]. It
seems that the steric bulkiness of the tert-butyl groups in 1–7 on
the proximity of O–Hꢀ ꢀ ꢀN hampers the formation of an intermolec-
ular hydrogen bond between the imine and a solvent molecule
[33,54–56].
3.3. Crystal structures of 1–7
Compounds 1–7 were crystallized and their solid state struc-
tures are shown below. They all have the enol-imino conformation
with an (E)-configured imine. As a consequence of a strong intra-
molecular hydrogen bond (O1–H1ꢀ ꢀ ꢀN8), the salicylaldimine moie-
ties form nearly planar six-membered rings [61]. The phenol ring is
only slightly rotated relative to the imine moiety (between 0.8°
and 9.8°), while the aromatic substituents in 1, 2 and 3 are twisted
by 24° (1), 31°/37° (2) and 39° (3) relative to the salicylaldimine
plane. Selected bond distances and angles are given as Supplemen-
tary material and hydrogen bond geometries are listed in Table 4.
Only for 1, 4 and 5 weak cooperative intermolecular C–Hꢀ ꢀ ꢀO(phe-
nol) hydrogen bonds are found, while in 3 the NO2-group is in-
volved in weak intermolecular hydrogen bonds (see Table 4)
[62]. The crystal structures of 1–7 including atom numbering
schemes are shown in Figs. 3–9.
To summarize, the presence of substantial absorption bands for
1–7 within the region of kmax = 307–377 nm and the IR and NMR
studies above reveal that the enol form is the predominant species
in solution. Very weak absorption at 400–500 nm indicates that
only minor amounts of keto tautomer are present. The influence,
if there is any, of tert-butyl groups on keto-enol tautomerization
within the series is minor and cannot be unambiguously detected
[53,43].
In general, the absorption spectra of salicylaldimines in polar
hydrogen bonding solvents such as MeOH and EtOH give a solvato-
chromic band around 400 nm. This arises from intermolecular
hydrogen bonding between guest and host molecule [54–56]. It
has also been reported that in aprotic polar solvent the solvato-
chromic band appears around 400–480 nm for salicylaldimines
that have a spacer between the imine N and the phenyl ring
[57,58]. The spacer eliminates the resonance of the N lone pair
Crystallographic analysis reveals that the bond distances in the
phenolic ring of salicylaldimines 1–7 follow the blinking sequence:
C1–C2 > C2–C3 < C3–C4 > C4–C5 < C5–C6 > C6–C1, which is a typi-
cal resonance form in enol tautomers. The differences between the
shortest and longest bond distances are about 0.03 Å. In the case of
the keto form, the distances vary between 0.06 and 0.08 Å
[47(a),48]. In general, the average C1–O1 (1.360(4) Å), C6–C7
(1.462(6) Å) and C7 = N8 (1.286(5) Å) bond distances are in good
accordance with literature values (Car–OH 1.362(15) Å, Car–Csp2
with the
p system of phenyl; thus the basicity of the N atom in-
Table 4
Hydrogen bond geometry (Å, °).
Ligands
D–Hꢀ ꢀ ꢀA
Intramolecular hydrogen bond
d
d(D–H)
d(Hꢀ ꢀ ꢀA)
d(Dꢀ ꢀ ꢀA)
\(DHA)
rH /
c DH
1
O1–H1ꢀ ꢀ ꢀN8
0.883(14)
0.848(18)
0.868(17)
0.877(15)
0.862(12)
0.864(13)
0.868(13)
0.864(19)
0.871(19)
0.858(16)
1.748(15)
1.81(2)
1.76(2)
1.831(16)
1.774(16)
1.774(16)
1.732(15)
1.83(3)
2.5656(15)
2.585(3)
2.563(3)
2.6467(19)
2.572(2)
2.579(2)
2.550(2)
2.620(5)
2.579(3)
2.604(2)
152.8(17)
152(3)
154(3)
153.8(19)
153.0(19)
154.0(19)
156.0(19)
151(5)
0.011/0.04
0.024/0.05
0.004/0.00
0.006/0.02
0.004/0.06
0.001/0.05
0.010/0.00
0.06/0.17
2a
O1–H1ꢀ ꢀ ꢀN8
3
O1–H1ꢀ ꢀ ꢀN8
O1–H1ꢀ ꢀ ꢀN8
4b
5
6
7
O1–H1ꢀ ꢀ ꢀN8
O1–H1ꢀ ꢀ ꢀN8
O1–H1ꢀ ꢀ ꢀN8
1.76(2)
1.804(18)
156(4)
154(2)
0.013/0.02
0.006/0.04
Weak intermolecular hydrogen bondse
Symmetry code
1
C11–H1ꢀ ꢀ ꢀO1
0.95
2.65
3.561(2)
161.3
0.5 + x, 1.5 ꢂ y, z
2a
3
None
C42–H42cꢀ ꢀ ꢀO16
C5–H5ꢀ ꢀ ꢀO17
C7–H7ꢀ ꢀ ꢀO1b
C7a–H7aꢀ ꢀ ꢀO1
C7b–H7bꢀ ꢀ ꢀO1a
C44–H44bꢀ ꢀ ꢀO1
None
0.98
0.95
0.95
0.95
0.95
0.98
2.67
2.59
2.54
2.60
2.56
2.63
3.609(3)
3.492(2)
3.242(3)
3.272(3)
3.272(3)
3.538(5)
160.2
157.8
130.7
128.2
132.3
153.5
1.5 ꢂ x, y, ꢂ0.5 + z
ꢂ0.5 + x, 1 ꢂ y, 1.5 ꢂ z
4b
1 ꢂ x, 1 ꢂ y, 1 ꢂ z
5
6
7
1.5 ꢂ x, ꢂ0.5 + y, 0.5 + z
None
a
b
c
Two independent molecules in the asymmetric unit.
Three independent molecules in the asymmetric unit.
Mean deviation from the plane O1–C1–C6–C7–N8.
Deviation of H1 from the plane O1–C1–C6–C7–N8.
Refs. [61,62].
d
e

J. Uddin Ahmad et al. / Journal of Molecular Structure 995 (2011) 9–19
15
Fig. 3. Molecular structure of 1 (displacement parameters are drawn at 50% of the probability level).
Fig. 4. Molecular structure of 2, one of the two independent molecules (displacement parameters are drawn at 50% of the probability level).
Fig. 5. Molecular structure of 3 (displacement parameters are drawn at 50% of the probability level).
(conjugated) 1.470(15) Å, Car–Csp2 (unconjugated) 1.488(12) Å,
sp2@N(2) 1.279(8) Å) [63,64]. The C@N bond distances of N-alkyl
(4–5), N-alkyl phenyl (6–7) and N-phenyl (1–3) salicylaldimines
A slight lengthening of the C@N bond in N-phenyl salicylaldimines
C
is explained by the electron withdrawing phenyl group. The
distances (N8–C9) between imine N atoms and C atoms of phenyl
or alkyl groups are 1.422(2) –1.477(3) Å, corresponding to the C–N
are 1.277–1.283 Å, 1.282–1.286 Å and 1.287–1.298 Å, respectively.

16
J. Uddin Ahmad et al. / Journal of Molecular Structure 995 (2011) 9–19
Fig. 6. Molecular structure of 4, one of the three independent molecules (displacement parameters are drawn at 50% of the probability level).
Fig. 7. Molecular structure of 5 (displacement parameters are drawn at 50% of the probability level).
single bond (1.469 Å) [63,64]. All other bond distances and angles
of the phenyl rings of parent amine and tert-butyl groups are with-
in expected values.
The existence of intramolecular hydrogen bonds between the
phenolic hydroxyl oxygen and azomethine nitrogen in solid state
has been studied for various Schiff base derivatives. According to
the literature [47(a),48,64–67], the Oꢀ ꢀ ꢀN distance is significantly
shorter (for example, 2.555 Å in N-(p-nitrophenyl) salicylaldimine,
ently values for single bonds, i.e. for the enol tautomers. Because
of the double bond character of the keto form, the bond distances
appear to be clearly shorter, varying from 1.268 to 1.312 Å [29,37].
The bond distances between the azomethine N and hydroxyl H
are also longer in the enol form than in the keto form. The Hꢀ ꢀ ꢀN
distances for 1–7 are in the range of 1.76–1.83 Å, being substan-
tially longer than those reported for the keto (N–H) form, 1.03–
1.11 Å [68]. It can be concluded that 1–7, irrespectively of any
electron releasing (Me) or withdrawing (NO2) group, have the
enol tautomeric form. This is accordance with their solution
structures.
2.604(3)
Å
in N-(2-methyl-5-chlorophenyl) salicylaldimine,
2.598(2) Å in N-(3-methoxyphenyl) salicylaldimine, 2.515(2) in
N-(2-hydroxy-5-chlorophenyl) salicylaldimine) than the sum of
their Van der Waals radii (3.07 Å). In 1–7, the Oꢀ ꢀ ꢀN distance ranges
from 2.550 to 2.647 Å due to the presence of strong intramolecular
hydrogen bonding.
3.4. Computational results
The most significant indicator of the type of tautomeric form is
the bond distances within the six-membered ring. The C–O bond
distances in the enol form are a bit longer than those observed in
the keto form due to the double bond character of the latter. The
distinctive C–O bond distances of N-phenyl (1–3), N-alkyl phenyl
(6–7) and N-alkyl (4–5) salicylaldimines are 1.352–1.359 Å,
1.358–1.362 Å and 1.362–1.365 Å, respectively. These are appar-
To study the relative stability of the two possible tautomers,
their respective geometries were fully optimized. No internal coor-
dinates have been frozen. The experimental atomic coordinates
from the X-ray studies were used as input for the enol form. Then
the optimized structure was modified to the corresponding keto
tautomer, and the structure was reoptimized (see Supplementary
materials). In each keto-enol pair the enol form is more stable,

J. Uddin Ahmad et al. / Journal of Molecular Structure 995 (2011) 9–19
17
As shown in the crystallographic study, all of the studied sali-
cylaldimines belong to the point group C1 in the solid state. How-
ever, as stated in Ref. [12], there also exists the possibility that the
molecules can display a higher symmetry. Accordingly, two sets of
salicylaldimines were optimized. In the first set the experimental
atomic coordinates were used as input, whereas for the second
set the molecules were optimized in the point group Cs. In the
latter set only the optimized molecules with an aliphatic substitu-
ent (4–5) represented the real energy minimum in their potential
energy surfaces. The optimized salicylaldimines represented the
lowest energy for each compound (see Supplementary materials).
Selected geometrical parameters are given in Tables 5 and 6.
Variation in the geometrical parameters for the optimized com-
pounds is small. Notable variation is seen only in the N8–C9 bond
distance, which is formally a single bond. The longest values for
the N8–C9 bonds are found in the cases where the carbon atom is ali-
phatic. The Wiberg bond indices show clearly, that there are no pure
single or double bonds connecting the atoms C1, C6, C7 and N8. In-
stead, the values are suggestive of electron delocalization. To further
characterize the bonding in the molecules, NBO analysis was also
carried out. Calculated natural charges are shown in Table 7.
There is a clear trend in the six-membered ring stabilized by
hydrogen bonding: starting from H1 to N8 the charges are alter-
nately positive and negative. Interestingly, the charge at C9 clearly
varies. When carbon C9 is aromatic, it always carries a positive
charge. When the carbon is aliphatic, the charge is negative.
In order to get an insight into the equilibrium between the keto
and enol forms, second order perturbation analysis of Fock matrix
in NBO basis was carried out. The results are shown in Table 8. In
the enol form there is a stabilizing charge transfer from the elec-
tron lone pair of nitrogen into the O–H antibonding molecular orbi-
Fig. 8. Molecular structure of 6, minor disordered component omitted for clarity
(displacement parameters are drawn at 50% of the probability level).
although the differences in energy vary from 5.5 to 10.1 kJ/mol (in
terms of total energy obtained from Gaussian for the optimized
moieties). It must be noted that in the solid state there are inter-
molecular hydrogen bonds that might stabilize one of the tautom-
ers more than the other.
Fig. 9. Molecular structure of 7 (displacement parameters are drawn at 50% of the probability level).
Table 5
Selected bond distances (Å) and bond angles (°) for the optimized compounds showing energy minima at the B3LYP/6–311Gꢁ level of theory.
Compound
Point group
C7@N8
N8ꢀꢀꢀH1
N8ꢀꢀꢀO1
O1–H1
N8–C9
O1–C1–C6
C1–C6–C7
C6–C7–N8
C7–N8–C9
1
2
3
4
5
6
7
C1
C1
C1
Cs
Cs
C1
C1
1.289
1.288
1.292
1.280
1.285
1.280
1.281
1.727
1.733
1.741
1.727
1.723
1.728
1.725
2.621
2.627
2.630
2.624
2.631
2.623
2.622
0.993
0.992
0.990
0.993
1.001
0.993
0.993
1.408
1.408
1.400
1.460
1.457
1.449
1.451
120.37
120.18
120.44
120.30
120.00
120.16
120.29
121.61
121.77
121.74
121.36
121.58
121.46
121.36
123.22
123.27
123.45
123.63
123.56
123.46
123.60
121.41
121.30
121.29
119.86
119.88
119.54
119.66

18
J. Uddin Ahmad et al. / Journal of Molecular Structure 995 (2011) 9–19
Table 6
phenol group and the imine nitrogen the structures of the ligands
are already preorganized and act as a bidentate chelate ligand
rather than a bridging ligand in complexation reactions.
Selected dihedral angles (°) for the optimized compounds showing energy minima at
the B3LYP/6–311Gꢁ level of theory.
Compound Point
group
O1–C1–C6– C1–C6–C7– C6–C7–N8– H–O1–C1–
C7
N8
C9
C6
Acknowledgments
1
2
3
4
5
6
7
C1
C1
C1
Cs
Cs
C1
C1
ꢂ0.09
0.27
0.46
ꢂ0.64
0.81
ꢂ177.45
177.35
0.21
0.15
ꢂ0.35
0.00
Financial support from the Graduate School of Inorganic
Material Chemistry and the International Student Grant of Univer-
sity of Helsinki is gratefully acknowledged. MRS acknowledges the
generous computing time given by CSC (CSC–IT Center for Science
Ltd., Finland).
ꢂ0.33
ꢂ176.82
0.00
0.00
180.00
0.00
0.00
180.00
0.00
ꢂ0.02
ꢂ0.74
ꢂ179.54
2.00
ꢂ0.33
ꢂ0.42
0.38
ꢂ179.85
Appendix A. Supplementary material
Table 7
Natural charges for the optimized compounds calculated at the B3LYP/6–311Gꢁ level
of theory.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2011.02.033.
Atoms
1
2
3
4
5
6
7
H1
O1
C1
C6
C7
N8
C9
0.498
ꢂ0.690 ꢂ0.692 ꢂ0.683 ꢂ0.696 ꢂ0.697 ꢂ0.697 ꢂ0.695
0.397 0.392 0.402 0.392 0.388 0.389 0.392
ꢂ0.177 ꢂ0.174 ꢂ0.181 ꢂ0.175 ꢂ0.172 ꢂ0.175 ꢂ0.176
0.167 0.165 0.178 0.155 0.156 0.164 0.156
ꢂ0.544 ꢂ0.540 ꢂ0.553 ꢂ0.547 ꢂ0.538 ꢂ0.538 ꢂ0.538
0.150 0.142
0.182 ꢂ0.001 ꢂ0.007 ꢂ0.183 ꢂ0.155
0.498
0.498
0.497
0.497
0.499
0.497
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Second order perturbation theory analysis of Fock matrix. Stabilization energies in
kcal/mol.a
Ligands
1
2
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4
5
6
7
Enol
Keto
r
n(N) ?
r
a(O–H) 25.54 24.82 23.64 n.d.
16.65 17.84 16.79 15.56 16.65 16.61 15.47
3.72 3.82 3.74 3.57 3.72 3.66 3.47
20.37 21.66 20.53 19.13 20.37 20.27 18.94
5.17 3.16 3.11 4.81 4.90 6.88
25.18 25.17 25.82
r
n(O) ?
r
a(N–H)
Difference
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a
In the keto form the minor component is shown italics and the sum value is
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4. Conclusions
A series of sterically hindered, 3,5-di-tert-butyl salicylaldimine
derivatives were synthesized and characterized by spectroscopic
methods. Solid state structures of the compounds were determined
by X-ray crystallography. In solid state, regardless of whether there
was an electron releasing (Me) or withdrawing (NO2) group, all
molecules possessed
a strong intramolecular hydrogen bond
(O1–Hꢀ ꢀ ꢀN8), forming a nearly planar six-membered ring. Accord-
ing to UV–vis, IR and NMR studies, this structure is also present in
solution. Computational results also revealed that in each
keto-enol pair, the enol form is more stable and the differences
in energy vary from 5.5 to 10.1 kJ/mol. It is noteworthy that the
tert-butyl groups in 1–7, due to their bulkiness and proximity to
the O–Hꢀ ꢀ ꢀN hydrogen bond, hamper the formation of an intermo-
lecular hydrogen bond between the imine and a solvent molecule.
Due to the strong intramolecular hydrogen bonding between the
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