Strengthening of the intramolecular O · · · H · · · N hydrogen bonds in Schiff bases as a result of steric repulsion

Article abstract of DOI:10.1016/S0022-2860(98)00660-7

Three ortho hydroxy Schiff bases 2-(N-methyl-α-iminoethyl)-phenol (I), 2-(N-benzyl-α-iminoethyl)-phenol (II) and 2-(N-benzyl-α-iminopropyl)- phenol (III) were synthesised in which the hydrogen atoms in C-C(H)=N group were replaced by an alkyl substituent.

Full text of DOI:10.1016/S0022-2860(98)00660-7

Journal of Molecular Structure 484 (1999) 75–89
… …
Strengthening of the intramolecular O H N hydrogen bonds in
Schiff bases as a result of steric repulsion
Aleksander Filarowskia,b, Tadeusz Głowiakaa, Aleksander Kolla,
*
aFaculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie Str., 50-383, Wrocław, Poland
bArchitectural-Building Academy of Tyumen, 3 Lunacharsky Str., 625003 Tyumen, Russia
Received 28 September 1998; accepted 25 October 1998
Abstract
Three ortho hydroxy Schiff bases 2-(N-methyl-a-iminoethyl)-phenol (I), 2-(N-benzyl-a-iminoethyl)-phenol (II) and 2-(N-
benzyl-a-iminopropyl)-phenol (III) were synthesised in which the hydrogen atoms in C–C(H)yN group were replaced by an
alkyl substituent. The crystal structures were determined and ab initio calculations at the B3LYP/6-31G** basis set were
…
…
performed. One of the shortest known O–H N hydrogen bonds were found with O N distance equal to 2.459(3), 2.497(3)
˚
and 2.494(3) A, respectively in I, II and III. Steric repulsion of substituted alkyl results in this unusual strengthening of the
hydrogen bonds. FT-IR spectra in solution and in the solid state in function of the temperature were determined. Strong
influence of the temperature on the character of the hydrogen bond was observed. ᭧ 1999 Elsevier Science B.V. All rights
reserved.
Keywords: Schiff bases; Crystal structure; Ab initio; Steric repulsion; Hydrogen bonds
1. Introduction
behaviour is strongly related to the double fluores-
cence observed in salicyl anilides [3].
The ortho-hydroxy Schiff bases form the intramo-
lecular hydrogen bonds, which reveal very interesting
properties like thermochromism [1], attributed to the
intramolecular proton transfer reaction. It suggests
that the proton transfer state is comparatively easy
accessible-the energy of keto and enol forms should
not be very different.
A large group of Schiff bases show photochromism
that is explained by photo-induced proton transfer.
Such behaviour suggests the possibility of using
these compounds as elements for constructing the
optical switches or optical memory devices [2]. This
It was shown that double fluorescence is not limited
only to aromatic–aromatic (condensation products of
salicyl aldehydes and anilines) systems, but is also
observed in aromatic–aliphatic ortho-hydroxy Schiff
bases. Grabowska et al. [4,5] have performed the
luminescence studies with the aim to establish the
structure of the basic molecular units responsible for
double fluorescence. They have found that 2-(N-
methyliminophenyl)-phenol is such a unit. More
specific question arises–what is the structure of the
state after the proton transfer? Does it contain more
zwitterionic or keto form? (Scheme 1).
In our previous studies on the structure of two
chloro derivatives the 2-(N-methyliminomethyl)-4-
Cl-phenol and 2-(N-methyliminomethyl)-2,4-di-Cl-
phenol the strong coupling of CyN double bond
* Corresponding author. Tel.: ϩ 48-71-204200; fax: ϩ 48-71-
222348.
E-mail address: akoll@wchuwr.chem.uni.wroc.pl (A. Koll)
0022-2860/99/$ - see front matter ᭧ 1999 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(98)00660-7

76
A. Filarowski et al. / Journal of Molecular Structure 484 (1999) 75–89
with the phenol ring was demonstrated [6]. In the
…
structure with O–H N intramolecular hydrogen
bond about 20% of keto resonance form was found,
…
while in the proton transfer (PT) O H–N form the
amount of keto resonance form was estimated to be
about 50%. Strong coupling between chelate and
phenol rings leads to the increase in the strength of
hydrogen bond and to lowering of the energy of PT
Scheme 1.
Table 1
Summary of data collection and processing parameters (w ˆ 1=‰s2ꢀF02† ϩ ꢀa·P†2 ϩ b·PŠ where P ˆ ‰f·Max: of ꢀ0 or F02† ϩ ꢀ1 Ϫ f† × Fc2Š
Compound
I
II
III
Formula
Color
C9H11NO
Yellow
C15H15NO
Yellow
C16H17NO
Yellow
Molecular weight
Space group
Temperature,ЊC
Cell constants
Crystal system
149.19
P1
225.28
Pccn
293(2) K
25 ref., 25.0 Ͻ 2u Ͻ 38.0
Orthorhombic
21.838(2)
15.745(3)
7.020(4)
239.31
P21/n
293(2) K
50 ref., 21.2 Ͻ 2u Ͻ 38.3
Monoclinic
10.187(3)
6.847(2)
293(2) K
50 ref., 20.6 Ͻ 2u Ͻ 33.5
Triclinic
7.548(2)
7.560(2)
16.379(3)
94.78(3)
100.95(3)
113.94(3)
825.2 (3)
4
˚
a (A)
˚
b (A)
˚
c (A)
18.850(4)
a (Њ)
b (Њ)
g (Њ)
93.50(3)
Ϫ3
˚
Cell volume (A
)
2414(2)
8
1.240
Kuma KM4/v-2u
CuKa
1312.3(4)
4
1.211
Formula units/unit cell
D
cal(Mg × mϪ3
)
1.201
Difractometer/Scan
Radiation (A), graph
˚
MoKa
MoKa
monochromator
Max. crystal dimensions (mm)
Scan width (deg)
0.35 × 0.40 × 0.60
Variable
0.20 × 0.25 × 0.25
Variable
0.45 × 0.45 × 0.55
Variable
No. of stand. Ref. and int.
Reflections measured
2u range (deg)
Range of h, k, l
Reflections observed [Fo Ͼ
4s(Fo)]
3 (100 ref.)
2667
5.1–50.1
Ϫ 8/8, Ϫ 8/8, 0/19
1490
3 (100 ref.)
2401
6.0–160.4
0/27, 0/20, Ϫ 8/0
1333
3(100 ref.)
2730
4.3–52.1
0/12, 0/8, Ϫ 23/23
1427
Corrections applied
Computer programs
Source of structure factors used
Structure solution
Lorentz and polarization effects
Shelxs 86 [10], Shelxl 93 [11]
Programs
Direct method
Treatment of hydrogen atoms
No of parameters varied
Weights (a, b, f)
Refined parameters: x, y, z, Uiso
160
0., 0., 1/3
0.957
0.0435
0.1210
212
169
a
0.0801, 0.0316, 1/3
1.034
0.0428
0.0503, 0.2022, 1/3
1.029
0.0334
GOF
P
P
R1 ˆ (͉Fo͉ Ϫ Fc͉)/ (͉Fo͉)
wR2 ˆ {[wFo2 Ϫ Fo2)2]/
0.1200
0.0900
P
[w(Fo2)2]}1/2
Function minimized
P
w(F2)2
0.154 and Ϫ 0.197
Largest feature final diff. Map
0.200 and Ϫ 0.193
0.158 and Ϫ 0.118
Ϫ3
˚
(eA
)

A. Filarowski et al. / Journal of Molecular Structure 484 (1999) 75–89
77
Table 2
state. The ab initio calculations at the level of B3LYP/
6-31G** predicted that the PT state is located
4.5 kcal/mol above the molecular O–H N (HB)
Final atomic coordinates and equivalent thermal parameters with
esd’s in parentheses for 2-(N-methyl-a-iminoethyl)-phenol (I)
…
state in 2-(N-methyliminomethyl)-4-Cl-phenol and
3.1 kcal/mol in 2-(N-methyliminomethyl)-2,4-di-Cl-
phenol. However, the ab initio calculations were
performed for the gas phase. In condensed phases
these differences could be remarkably lower. Suppo-
sition about the thermal activation of the intra mole-
cular proton transfer appears to be realistic.
In this work we have performed studies of a new
class of ortho hydroxy Schiff bases. Replacement of
the H atom in the C–C(H)yN group by methyl or
ethyl substituent introduces perturbations to intramo-
lecular hydrogen bonds and modifies the potential for
the intramolecular proton transfer.
One can expect some strengthening of the hydrogen
bond by these substituents owing to inductive influ-
ence on the basicity centre and the opposite effect
owing to lowering the acidity of phenol. An average
electronic effect will be diminished owing to mutual
compensation.
x/a
y/b
0.2959(2)
z/c
Ueq/Uiso
O11
O21
N11
N21
C11
C12
C13
C14
C15
C16
C17
C18
C19
C21
C22
C23
C24
C25
C26
C27
C28
C29
0.7665(2)
0.1787(3)
0.6776(3)
0.3052(3)
0.5833(3)
0.4430(3)
0.2539(3)
0.2047(4)
0.3455(5)
0.5299(4)
0.4983(4)
0.3485(4)
0.7560(5)
0.1571(3)
0.2056(3)
0.1779(3)
0.1072(4)
0.0623(3)
0.0868(3)
0.2821(3)
0.3294(4)
0.3800(4)
0.61779(11)
0.88210(11)
0.46336(11)
1.03659(11)
0.61790(12)
0.54181(12)
0.5478(2)
0.0891(5)
0.0884(5)
0.0767(5)
0.0769(5)
0.0641(5)
0.0591(5)
0.0805(6)
0.0951(8)
0.0924(8)
0.0814(7)
0.0660(5)
0.1025(9)
0.1083(9)
0.0647(5)
0.0603(5)
0.0818(7)
0.0949(8)
0.0918(8)
0.0810(6)
0.0661(5)
0.1034(9)
0.1097(9)
0.1488(2)
0.2684(2)
0.2146(3)
0.2750(3)
0.2473(2)
0.2265(3)
0.2323(3)
0.2598(3)
0.2800(3)
0.2427(2)
0.2091(4)
0.2691(4)
Ϫ0.0345(3)
Ϫ0.0993(3)
Ϫ0.2941(3)
Ϫ0.4196(3)
Ϫ0.3513(4)
Ϫ0.1642(4)
0.0379(3)
Ϫ0.0323(4)
0.3671(4)
0.6245(2)
0.6973(2)
0.69387(14)
0.46066(12)
0.37992(15)
0.3887(2)
0.88217(12)
0.95825(12)
0.9520(2)
0.8753(2)
0.8025(2)
0.80616(14)
1.03939(12)
1.12012(15)
1.1111(2)
The steric interaction of alkyl substituent, which by
pushing the centre of basicity in direction of OH
…
group decreases the O N distance. Similar effect
was shown earlier in the case of 2-(N,N-dimethylami-
nomethyl)-3,4,6-tri-CH3-phenol [7]. Steric shortening
of the hydrogen bridge led to the increase of the spec-
troscopic consequences of the hydrogen bond forma-
tion [7]. Opposite effect can also be anticipated if the
alkyl groups make the chelate rings less planar as a
result of steric repulsion. (Scheme 2).
Table 3
Final atomic coordinates and equivalent thermal parameters with
esd’s in parentheses for 2-(N-benzyl-a-iminoethyl)-phenol (II)
x/a
y/b
z/c
Ueq/Uiso
Three Schiff bases with methyl and ethyl
substituents in C–CyN group were synthesized:
2-(N-methyl-a-iminoethyl)-phenol (I), 2-(N-benzyl-
a-iminoethyl)-phenol (II) and 2-(N-benzyl-a-imino-
propyl)-phenol (III). The molecular structures were
determined by X-ray crystallography and ab initio-
B3LYP/6-31G** calculations. Performed studies
indicate that the steric effects lead to strong strength-
ening of intramolecular hydrogen bonds. In all the
systems studied here the intramolecular hydrogen
N
O
0.47671(7)
0.49437(6)
0.43364(8)
0.39373(8)
0.33097(9)
0.30814(10) 0.0989(2)
0.34773(10) 0.03096(14)
0.40961(10) 0.04378(12)
0.29171(9)
0.13502(9)
0.12571(11)
0.19641(11)
0.17954(14)
0.0521(3)
0.0503(4)
0.0577(4)
0.0471(4)
0.0464(5)
0.0633(6)
0.0726(7)
0.0670(6)
0.0574(5)
0.0474(5)
0.0605(6)
0.0551(5)
0.0503(5)
0.0606(6)
0.0705(7)
0.0719(7)
0.0690(7)
0.0604(6)
0.0555(2)
0.0585(3)
0.0571(3)
0.0591(4)
0.0630(4)
0.0671(4)
0.0629(3)
0.0554(3)
0.0565(4)
0.0507(4)
0.0490(3)
C1
C2
C3
C4
C5
C6
C7
C8
C9
0.41815(8)
0.37509(10) 0.35762(12)
0.50569(9)
0.28348(11)
0.37527(12)
0.36744(11)
…
O–H N bonds belong to the shortest reported in
literature so far.
C10 0.57425(9)
C11 0.60864(10) 0.41167(13) Ϫ0.0840(3)
C12 0.67175(11) 0.40851(15) Ϫ0.0817(4)
C13 0.70148(11) 0.35958(15)
C14 0.66774(11) 0.31427(15)
C15 0.60504(10) 0.31838(13)
0.0517(4)
0.1820(4)
0.1816(3)
2. Experimental
Synthesis of a Schiff base from stoichiometric

78
A. Filarowski et al. / Journal of Molecular Structure 484 (1999) 75–89
Fig. 1. Molecular structure and atoms labelling system of 2-(N-methyl-a-iminoethyl)- phenol(I).
mixtures of particular salicylaldehyde and methyla-
mine in methanol was performed with the standard
procedure [8]. After recrystallization of the solid
products from methanol they were studied by X-ray
diffraction. Summary of the data collected is given in
Table 1. Anisotropic thermal parameters for non-H
atoms were applied [9]. Final atomic coordinates are
given in Tables 2–4.
3. Results
Molecular structures and atoms labelling system
are demonstrated in Figs. 1–3. In I two non-equiva-
lent molecules in basic crystal unit were found. Table
5 presents selected bond distances and valence angles.
Selected torsional angles are presented in Table 6.
Vibrational spectra were measured by FT-IR
Nicolet-205 spectrophotometer, at the resolution
of 2 cmϪ1, in CCl4 solutions, in cells with KBr
windows. Low temperature measurement was
performed on the specimens melted between KBr
windows.
4. Discussion
4.1. Molecular structure
There are two non-equivalent molecules in the
Table 4
Final atomic coordinates and equivalent thermal parameters with
esd’s in parentheses for 2-(N-benzyl-a-iminopropyl)-phenol (III)
x/a
y/b
z/c
Ueq/Uiso
O
N
0.58989(13)
0.0760(2)
0.71937(7)
0.64831(7)
0.74375(8)
0.72280(7)
0.75048(9)
0.79611(10) 0.0654(5)
0.81654(11) 0.0690(5)
0.79074(10) 0.0617(5)
0.67220(8)
0.64948(9)
0.59118(10) 0.0641(5)
0.59696(10) 0.0592(5)
0.0587(4)
0.0471(3)
0.0460(4)
0.0405(4)
0.0532(4)
0.71804(13) Ϫ0.1552(2)
0.5269(2) Ϫ0.0786(2)
0.55769(15) Ϫ0.2709(2)
0.4855(2)
0.3881(2)
0.3602(2)
0.4289(2)
0.65950(15) Ϫ0.3062(2)
0.6848(2)
0.5840(2)
C1
C2
C3
C4
C5
C6
C7
C8
C9
Ϫ0.4230(2)
Ϫ0.3907(3)
Ϫ0.2026(3)
Ϫ0.0488(3)
0.0414(4)
0.0493(4)
Ϫ0.5124(2)
Ϫ0.5786(3)
Ϫ0.1633(3)
C10 0.8199(2)
C11 0.79025(15) Ϫ0.0220(2)
0.53692(8)
0.0463(4)
C12 0.8660(2)
C13 0.8367(2)
C14 0.7318(2)
C15 0.6564(2)
C16 0.6857(2)
0.1411(3)
0.2679(3)
0.2305(4)
0.0715(4)
Ϫ0.0534(3)
0.52833(11) 0.0621(5)
0.47153(13) 0.0771(6)
0.42474(11) 0.0751(6)
0.43324(10) 0.0741(6)
Fig. 2. Molecular structure and atoms labelling system of 2-(N-
benzyl-a-iminoethyl)- phenol(II).
0.48868(9)
0.0627(5)

A. Filarowski et al. / Journal of Molecular Structure 484 (1999) 75–89
79
Fig. 3. Molecular structure and atoms labelling system of 2-(N-benzyl-a-iminopropyl)- phenol(III).
crystal unit cell of I. Differences between related para-
meters are in average, less than the experimental s
values and average parameter values for these two
molecules are used for discussion. One such exclusion
is the geometry of hydrogen bonds, where two struc-
tures will be distinguished.
(Table 5). The third, C15–C16 was of the same length
in I and in V. For atom labelling see Fig. 1.
On the basis of the structural parameters of the
phenol ring one should neither predict a strong elec-
tronic coupling in the molecule, nor strong strength-
ening of hydrogen bond, owing to resonance effects
[12,13].
Analysis of the bond lengths in I shows, that intra-
molecular hydrogen bonds exist in the molecular
(HB) form. Lesser amount of the ortho quinoic reso-
nance structure was found in the structure of phenyl
ring of I than in 2-(N-methyliminomethyl)-4-Cl-
phenol (V), which presents the experimentally
known structure of 2-(N-methyliminomethyl)-phenols
[6]. Of all the bonds shown in keto resonance structure
as double bonds (see Scheme 1), shortening of the
bonds is an indication of participation of o-quinoic
structure, two are longer than in V; C17–C12
In other (II, III) compounds the phenyl ring situa-
tion seem to be similar. The bonds which are double in
o-quinoic resonance structure are shortened (3-4, 5-6)
in comparison to those which are formally single ones
(1-2, 6-1, 5-4, 2-3). Especially elongated are 1-2
bonds, characteristic for collective effect of both
substituents, and extended by the formation of the
chelate ring. Like in I all those effects are less
pronounced than in V, and in comparison to the calcu-
lated structure of ortho hydroxy Schiff base without
substituents (IV) (Table 5). Thus coupling is not very
effective as it leads to mild differentiation of bond
lengths. As it was shown earlier [6] such a coupling
increases the content of keto form and leads to bond
differentiation in opposite to general resonance
˚
(1.47 A in I besides of 1.448 (in V) and C13–C14
˚
˚
(1.378 A besides of 1.355 A in V). Calculations for
the ionic form (about 50% of keto form was found
previously in 2-(N-methyliminomethyl)-2,4-Cl-
phenol [6]) show these bonds to be much shorter
Scheme 2.

80
A. Filarowski et al. / Journal of Molecular Structure 484 (1999) 75–89
Table 5
˚
Experimental and calculated (B3LYP/6-31G**) bond lengths (A) and angles (Њ) with esd’s in parentheses for 2-(N-methyl-a-iminoethyl)-
phenol (I) , 2-(N-benzyl-a-iminoethyl)-phenol (II) and 2-(N-benzyl-a-iminopropyl)-phenol (III)
b
˚
Bond lengths /A
Valence angles/Њ
X-ray
HB
Calculated
HB
IVa
HB
X-ray
HB
Calculated
HB
Bonds
PT
Angles
PT
O21–C21
O11–C11
N21–C27
N11–C17
N21–C29
N11–C19
C21–C26
C11–C16
C21–C22
C11–C12
C22–C23
C12–C13
C22–C27
C12–C17
C23–C24
C13–C14
C24–C25
C14–C15
C25–C26
C15–C16
C27–C28
C17–C18
1.328(2)
1.327(2)
1.280(3)
1.278(3)
1.454(3)
1.456(3)
1.384(3)
1.381(3)
1.414(3)
1.412(3)
1.393(3)
1.395(3)
1.473(3)
1.468(3)
1.378(4)
1.378(4)
1.376(4)
1.375(4)
1.345(3)
1.350(3)
1.500(3)
1.495(3)
Ϫ
Ϫ
Ϫ
C27–N21–C29
C17–N11–C19
O21–C21–C22
O11–C11–C12
C26–C21–C22
C16–C11–C12
C23–C22–C21
C13–C12–C11
C23–C22–C27
C13–C12–C17
C21–C22–C27
C11–C12–C17
C24–C23–C22
C14–C13–C12
C25–C24–C23
C15–C14–C13
C26–C25–C24
C16–C15–C14
C25–C26–C21
C15–C16–C11
N21–C27–C22
N11–C17–C12
N21–C27–C28
N11–C17–C18
123.5(2)
123.5(2)
121.3(2)
121.0(2)
119.4(2)
119.8(2)
117.2(2)
117.2(2)
123.0(2)
122.4(2)
119.9(2)
120.4(2)
122.0(2)
121.8(2)
119.3(2)
119.4(2)
120.4(2)
120.5(2)
121.8(2)
121.4(2)
117.0(2)
116.7(2)
123.3(2)
122.9(2)
Ϫ
Ϫ
1.3358
Ϫ
1.2759
Ϫ
1.3403
Ϫ
123.49
Ϫ
127.91
Ϫ
1.2948
Ϫ
1.3263
Ϫ
1.2847
Ϫ
122.52
Ϫ
122.71
Ϫ
1.4489
Ϫ
1.4481
Ϫ
1.4488
Ϫ
119.89
Ϫ
116.89
Ϫ
1.4066
Ϫ
1.4389
Ϫ
1.4038
Ϫ
117.34
Ϫ
118.77
Ϫ
1.4315
Ϫ
1.4673
Ϫ
1.4224
Ϫ
122.53
Ϫ
122.29
Ϫ
1.4114
Ϫ
1.4263
Ϫ
1.4039
Ϫ
120.14
Ϫ
118.94
Ϫ
1.4790
Ϫ
1.4300
Ϫ
1.4548
Ϫ
122.31
Ϫ
121.99
Ϫ
1.3869
Ϫ
1.3726
Ϫ
1.3871
Ϫ
119.89
Ϫ
119.45
Ϫ
1.4013
Ϫ
1.4202
Ϫ
1.4029
Ϫ
120.29
Ϫ
121.17
Ϫ
1.3852
Ϫ
1.3705
Ϫ
1.3890
Ϫ
120.77
Ϫ
121.73
Ϫ
1.5202
1.5094
Ϫ
117.41
Ϫ
117.89
Ϫ
121.98
118.74
2-(N-benzyl-a-iminoethyl)-phenol (II)
N–C7
N–C9
O–C1
1.286(2)
1.460(2)
1.335(2)
1.393(3)
1.414(2)
1.396(2)
1.471(2)
1.365(3)
1.375(3)
1.367(3)
1.499(2)
1.502(3)
1.384(3)
1.386(3)
1.379(3)
1.376(3)
1.374(3)
1.371(3)
1.294
1.458
1.338
1.406
1.428
1.411
1.476
1.387
1.402
1.386
1.516
1.518
1.400
1.399
1.395
1.395
1.396
1.395
1.329
1.454
1.276
1.439
1.468
1.427
1.427
1.372
1.421
1.371
1.507
1.524
1.401
1.399
1.395
1.396
1.395
1.396
C7–N–C9
O–C1–C6
O–C1–C2
121.5(2)
118.5(2)
121.7(2)
119.8(2)
117.1(2)
122.2(2)
120.7(2)
122.4(2)
119.6(2)
120.4(2)
120.7(2)
117.1(2)
123.1(2)
119.9(2)
111.0(2)
118.1(2)
121.7(2)
120.3(2)
121.0(2)
120.0(2)
119.4(2)
120.7(2)
120.9(2)
122.9
117.9
122.4
119.7
117.9
121.4
120.7
120.0
119.3
120.5
120.7
117.8
123.9
118.3
112.1
118.8
121.3
119.9
120.8
120.0
119.6
120.3
120.5
126.6
120.6
122.8
116.8
118.9
122.0
119.0
121.9
119.4
121.3
121.7
118.0
118.2
123.8
114.0
119.1
120.8
120.0
120.3
120.3
119.7
120.0
120.6
C1–C6
C1–C2
C2–C3
C2–C7
C3–C4
C4–C5
C5–C6
C7–C8
C9–C10
C10–C15
C10–C11
C11–C12
C12–C13
C13–C14
C14–C15
C6–C1–C2
C3–C2–C1
C3–C2–C7
C1–C2–C7
C4–C3–C2
C3–C4–C5
C6–C5–C4
C5–C6–C1
N–C7–C2
N–C7–C8
C2–C7–C8
N–C9–C10
C15–C10–C11
C15–C10–C9
C11–C10–C9
C12–C11–C10
C13–C12–C11
C14–C13–C12
C15–C14–C13
C14–C15–C10

A. Filarowski et al. / Journal of Molecular Structure 484 (1999) 75–89
81
Table 5 (continued)
˚
Bond lengths /A
Valence angles/Њ
X-ray
HB
Calculated
HB
IVa
HB
X-ray
HB
Calculated
HB
Bonds
PT
Angles
PT
2-(N-benzyl-a-iminopropyl)-phenol (III)
O–C1
N–C7
N–C10
C1–C6
C1–C2
C2–C3
C2–C7
C3–C4
C4–C5
C5–C6
C7–C8
C8–C9
C10–C11
C11–C12
C11–C16
C13–C14
C13–C12
C14–C15
C15–C16
1.334(2)
1.288(2)
1.463(2)
1.390(2)
1.415(2)
1.395(2)
1.471(2)
1.371(3)
1.379(3)
1.370(3)
1.503(2)
1.526(2)
1.506(2)
1.372(2)
1.374(2)
1.368(3)
1.396(3)
1.348(3)
1.369(3)
1.338
1.296
1.452
1.406
1.430
1.411
1.479
1.387
1.402
1.386
1.521
1.541
1.526
1.400
1.401
1.395
1.396
1.396
1.395
1.276
1.329
1455
C7–N–C10
O–C1–C6
O–C1–C2
124.3(2)
118.8(2)
121.5(1)
119.7(2)
117.4(2)
121.9(1)
120.7(1)
122.2(2)
119.7(2)
120.0(2)
121.0(2)
117.0(1)
124.3(1)
118.6(1)
111.4(1)
110.8(1)
117.9(2)
121.5(2)
120.6(2)
120.0(2)
120.1(2)
120.2(2)
119.7(2)
122.1(2)
124.7
117.8
122.3
119.9
117.5
122.0
120.5
122.2
119.4
120.4
120.7
117.1
123.9
119.1
112.3
112.6
118.8
121.8
119.3
120.4
120.4
119.5
120.1
120.8
127.3
120.6
122.7
116.7
119.1
121.7
119.3
121.8
119.4
121.3
121.7
117.8
119.4
122.8
112.5
114.0
119.1
120.0
120.9
120.0
120.9
119.7
120.3
120.3
1.439
1.466
1.426
1.427
1.373
1.421
1.371
1.513
1.541
1.524
1.399
1.400
1.395
1.396
1.396
1.395
C6–C1–C2
C3–C2–C1
C3–C2–C7
C1–C2–C7
C4–C3–C2
C3–C4–C5
C6–C5–C4
C5–C6–C1
N–C7–C2
N–C7–C8
C2–C7–C8
C7–C8–C9
N–C10–C11
C12–C11–C16
C12–C11–C10
C16–C11–C10
C14–C13–C12
C11–C12–C13
C15–C14–C13
C14–C15–C16
C15–C16–C11
b Calculated at the same basis set for 2-(N-methylimino)-phenol (IV).
a Parameters for atoms directly involved into hydrogen bond formation are given in Table 7.
effects, which for example gives complete equaliza-
tion of all bonds in benzene ring.
Despite this, an analysis of the geometry of the
chelate ring shows existence of very strong intramo-
lecular hydrogen bond, with predominance of mole-
cular form (Table 7).
Chelate rings are planar (Table 6) and coplanar with
the phenol ring; torsion angles characterizing devia-
tions from the last plane are less than 2Њ. The C8 atom
is also practically located in the phenol ring plane
(torsion angle Ͻ 3Њ).
shortening of non-bonded distances with respect to
appropriate sum of the van der Waals radii. Following
van der Waals radii were applied, after Bondi [14]:
ꢀ
˚
˚
RC ˆ 1:70 A, RN ˆ 1.55 A, RH ˆ 1.20 A, RO
ˆ
˚
1.52 A. Such an analysis cannot be performed for
…
atoms 1 3 forming the 1-2-3 angle, called geminal
…
interactions. The angle between 1 3 axis and 1-2,
2-3 bonds are less than 90Њ and because of non-spheric
atom–atom potentials a special van der Waals radii
should be established for such directions. However,
this type of procedure would be very arbitrary. Among
non-geminal interactions the C18–C13 and C18–C19
˚
distances equal to 2.975 and 2.895 A, respectively
4.2. Analysis of steric effects
˚
shortened by 0.425 and 0.505 A with respect to an
˚
appropriate sum of the van der Waals radii (3.4 A)
Detailed analysis of the structure of I compounds
shows that the intramolecular steric repulsions are the
main factor, which is responsible for shortening of the
hydrogen bridge.
were found.
Other distances between C18 atom (and hydrogen
atoms attached to it) and C13 and C19 atoms (and
hydrogen atoms attached to it) are shortened. It
These repulsions can be characterized by

82
A. Filarowski et al. / Journal of Molecular Structure 484 (1999) 75–89
Fig. 4. Molecular conformers of 2-(N-methyl-a-iminoethyl)-phenol (I) obtained in ab initio B3LYP/6-31G** calculations.
generates a strong strain within the molecules, which
is partially released by change of some valence
angles. The situation is illustrated in Scheme 3,
where valence angles in I, II and III are compared
with related angles in compounds (IV–VI) with C–
C(H)yN moiety.
Compound IV is a liquid and its calculated struc-
ture (not experimental) (ab initio B3LYP/6-31 G**
level) is shown.
The structural data for VI are taken from [15].
Compound I can be compared with IV and V while
II and III with VI as all the last compounds contain
the N-benzyl group, while the I, IV and V contain the
N-methyl group.
geminal) interactions of C-substituted methyl (or
ethyl) groups with C3 and C9 atoms as well as H
atoms attached to C3 and C9 atoms (labelling as in
Fig. 2). Such calculations were also performed for V
and VI compounds, which can be assumed as refer-
ence structures. Parameters of the potential from
MM3 force field were applied [16].
Calculated values of strain energy are 3.982 kJ/mol
in I, 4.147 kJ/mol in II and 4.821 kJ/mol in III, which
can be compared with the results of analogous calcu-
lations for V and VI compounds; 1.737 and 1.483 kJ/
mol, respectively. The results clearly distinguish
between compounds I–III type, and IV–VI type.
(On deciding appears the introducing of a substituent
at C(sp2) atom in C–C(H)yN group.) Neither did the
replacement of N-methyl by N-benzyl group nor the
replacement of methyl substituent by the ethyl
(Scheme 3) give any radical increase of steric repul-
sion.
One can observe that within the group of
compounds containing N-methyl substituent as well
as in the group with N-benzyl substituent the angles
C(H3)– C–C are lager than related H–C–C angles in
IV–VI compounds. The steric repulsion also leads to
a strong increase of C(H3)– CyN and CyN–C angles
The above mentioned effects cause a decrease in
…
H
…
in I–III compounds. Decrease of
N–CH3
the O N distance and “external” strengthening of
angle and especially pronounced decrease of NyC–C
angle.
the hydrogen bonds where some adaptation of lone
electron pair direction also takes place (see Scheme 4).
In order to estimate this effect one can calculate
values of angles (q, c) which makes the lone pair
direction (assumed to lay on bisector of CyN–C
angle) with the N O line, which is an “ideal”
direction of lone electron pair in linear complexes.
Calculated values of c are 24.8Њ, 25.9Њ and 24.9Њ
Despite this strong deformation of valence angles,
pronounced shortening of distances between CH3
group and neighbour C and H atoms were observed
in experimental structures of I, II and III compounds.
Total steric interaction between substituents can be
characterized by the energy of non-bonded (but not
…

A. Filarowski et al. / Journal of Molecular Structure 484 (1999) 75–89
83
Scheme 3.
for I, II and III, respectively while for the
compounds with less steric repulsions w values
are 32.3Њ, 32.2Њ and 31.5Њ for IV, V and VI,
respectively. Such differences in direction of
lone electron pair of N-atoms interacting with
the hydrogen bond can be very important for the
properties of hydrogen bond as it was shown by
Scheiner [17].
4.3. Hydrogen bonding
Detailed description of the hydrogen bond studied
Scheme 4.

84
A. Filarowski et al. / Journal of Molecular Structure 484 (1999) 75–89
Table 6
Selected torsion angles (Њ) for 2-(N-methyl-a-iminoethyl)-phenol (I), 2-(N-benzyl-a-iminoethyl)-phenol (II) and 2-(N-benzyl-a-iminopropyl)-
phenol (III) with esd’s in parentheses
I
II
III
O11–C11–C12–C13
C16–C11–C12–C13
O11–C11–C12–C17
C16–C11–C12–C17
C11–C12–C13–C14
C12–C13–C14–C15
C13–C14–C15–C16
C14–C15–C16–C11
C19–N11–C17–C12
C19–N11–C17–C18
C13–C12–C17–N11
C11–C12–C17–N11
C13–C12–C17–C18
O21–C21–C22–C23
C21–C22–C27–C28
180.0(2)
0.2(3)
Ϫ0.5(3)
179.7(2)
Ϫ0.1(3)
0.2(3)
Ϫ0.3(3)
0.4(3)
Ϫ179.6(2)
0.5(3)
178.3(2)
Ϫ1.2(2)
Ϫ1.8(3)
Ϫ180.0(2)
178.8(2)
O–C1–C2–C3
C6–C1–C2–C3
O–C1–C2–C7
C6–C1–C2–C7
C1–C2–C3–C4
C7–C2–C3–C4
C2–C3–C4–C5
C3–C4–C5–C6
C4–C5–C6–C1
O–C1–C6–C5
179.5(2)
Ϫ0.4(3)
Ϫ1.0(3)
179.1(2)
0.2(4)
Ϫ179.2(2)
0.7(4)
Ϫ1.5(4)
1.4(4)
179.7(2)
Ϫ179.9(2)
0.5(4)
Ϫ179.5(2)
1.1(3)
Ϫ179.2(2)
179.4(2)
Ϫ52.4(3)
129.4(2)
Ϫ1.5(3)
176.8(2)
1.3(4)
Ϫ0.1(4)
Ϫ0.9(4)
0.7(4)
O–C1–C2–C3
C6–C1–C2–C3
O–C1–C2–C7
Ϫ179.30(14)
0.7(2)
Ϫ1.0(2)
178.99(14)
0.1(2)
Ϫ178.2(2)
Ϫ0.8(3)
0.6(3)
Ϫ179.01(13)
Ϫ1.6(2)
178.16(15)
Ϫ0.1(2)
0.6(2)
Ϫ177.66(13)
Ϫ96.5(2)
80.9(2)
130.6(2)
111.2(2)
Ϫ69.0(2)
Ϫ0.7(3)
179.2(2)
0.6(3)
C6–C1–C2–C7
C1–C2–C3–C4
C7–C2–C3–C4
C2–C3–C4–C5
C3–C4–C5–C6
C10–N–C7–C2
C10–N–C7–C8
C3–C2–C7–N
C9–N–C7–C2
C9–N–C7–C8
C3–C2–C7–N
C1–C2–C7–N
C1–C2–C7–N
C3–C2–C7–C8
C1–C2–C7–C8
N–C7–C8–C9
C1–C2–C7–C8
C7–N–C9–C10
N–C9–C10 C15
N–C9–C10 C11
C15–C10–C11–C12
C9–C10–C11–C12
C10–C11–C12–C13
C11–C12–C13–C14
C12–C13–C14–C15
C13–C14–C15–C10
C9–C10–C15–C14
C2–C7–C8–C9
C7–N–C10–C11
N–C10–C11–C12
N–C10–C11–C16
C16–C11–C12–C13
C10–C11–C12–C13
C14–C13–C12–C11
C12–C13–C14–C15
C12–C11–C16–C15
C10–C11–C16–C15
0.0(3)
0.3(3)
Ϫ179.6(2)
Ϫ177.8(2)
˚
in I–III compound in comparison to two structures
with lower steric repulsion (V and VI) is given in
Table 7.
localization was found to be dO–H ˆ 1.15(4) A,
…
˚
dH
ˆ 1.38(2) A while the length of the hydrogen
N
˚
bond is almost the same as 2.460(3) A. The 2.46–
˚
In one of the two non-equivalent molecules in the
unit cell of I, very short hydrogen bond was found
2.47 A distance seems to be the critical one (the
…
shortest) in O–H N type of hydrogen bridges.
…
…
ꢀ
(dN
H
ˆ 2:459ꢀ3† A), shortest of all the known O–
Position of proton in II and III is more on the side
of oxygen and lengths of hydrogen bond are higher
than in I, which results from the decrease of basicity
while one replaces the N-methyl group with N-benzyl.
Nevertheless the hydrogen bonds in II and III with
O
N hydrogen bonds in the literature.
The proton is localized near the central position
…
˚
…
˚
dO
ˆ 1.20 A, dH
ˆ 1.32(3) A. The OHN
H
N
angle is 153Њ larger than in V (142.5Њ). The COH
angle strongly diminished from 111Њ in V to 102Њ in
compound I. This hydrogen bond can be compared
only with intramolecular O–H N hydrogen bond
in imazaryl [18], where O H-1.16(2) A, H N-
…
˚
N
O distances less than 2.5 A, belong to the shortest
…
known hydrogen bonds of O–H
N type. In
…
compounds V and VI the hydrogen bonds are
evidently longer, OH bonds are shorter than 1 A and
…
˚
…
˚
˚
…
O
˚
…
˚
1.32(2) A and
N-2.465(2) A distances were
H
N distances are larger than 1.7 A, while in I–III
˚
found. It was almost linear (a(OHN) ˆ 170(1)Њ)
they are less than 1.5 A. Hydrogen bond becomes
more linear; the a(OHN) angle is 10Њ larger in I–
III than in V, VI. The strengthening of the hydrogen
bond also gives a very characteristic [19] decrease of
COH angle-from 110.5(3)Њ in V, VI to 103(1)Њ in I–
III.
hydrogen bond forming seven membered ring,
1
where H NMR shift was found [18] to be 19 ppm,
independent of concentration, solvent and tempera-
ture.
In the second molecule more asymmetric proton

A. Filarowski et al. / Journal of Molecular Structure 484 (1999) 75–89
85
Table 7
Hydrogen bond parameters and selected bond lengths (A) and angles (Њ), with esd’s in parentheses, in pseudoaromatic chelate rings
˚
Form of HB
Measured X-ray
OH
ON
HN
a (OHN)
b (COH)
g (C7NH)
d (C9NH)
…
I
O
H–N
1.20(4)
1.15(4)
1.10(3)
1.07(3)
0.977
2.459(3)
2.460(3)
2.497(3)
2.494(3)
2.559
1.32(3)
1.38(2)
1.47(3)
1.48(3)
1.715
153(1)
153(1)
152(3)
155(1)
142.5
102(1)
103(2)
103.1(12)
102(1)
110.8
106(1)
106(1)
104.8(9)
104(1)
101.5
130(1)
131(1)
133.7(9)
132(1)
139.9
…
…
…
…
II
III
V
O–H
O–H
O–H
O–H
N
N
N
N
VI
0.990
2.587
1.730
142.6
110.4
102.4
138.7
a
Calculated ab initio B3LYP 6-31G**
DE
…
I
O
H–N
…
1.503
1.014
1.521
1.009
1.520
1.015
1.636
1.000
1.647
1.000
1.631
0.999
2.484
2.526
2.490
2.548
2.492
2.525
2.551
2.617
2.664
2.377
2.546
2.619
1.074
1.595
1.067
1.627
1.068
1.591
1.053
1.713
1.051
1.712
1.053
1.718
148.7
150.2
147.8
149.4
148.2
150.6
142.1
148.4
141.7
148.17
142.2
148.0
101.5
105.3
101.5
105.7
101.4
105.0
103.7
107.0
103.6
107.0
103.6
107.2
110.3
104.4
110.5
103.8
110.4
104.4
110.7
99.6
111.0
99.9
110.7
99.7
121.8
132.1
122.2
133.3
121.9
130.8
124.1
141.1
123.8
140.7
123.9
141.0
2.77
0.00
2.76
0.00
2.29
0.00
5.00
0.00
4.57
0.00
4.87
0.00
O–H
O H–N
N
…
II
…
O–H
N
…
III
IV
V
O H–N
…
O–H
N
…
O
H–N
…
O–H
N
…
O
H–N
…
O–H
N
…
O
VI
H–N
…
O–H
N
a DE (kcal/mol)-relative energy of states for particular compounds. Energy of molecular forms was assumed as 0.
No real strengthening of the hydrogen bonding is
earlier one can distinguish two classes of compounds.
observed in III in comparison to II resulting from the
CH3 group replacement by C2H5 , which should be the
case if electronic and not steric effects decide about
strengthening of hydrogen bond in I–III.
The first (I–III compounds) with large steric effects
and short O N distance reveal about two times lower
…
difference in energy between molecular and PT state
than in the second class (IV–VI compounds). Short-
ening of the hydrogen bridges strongly modifies the
potential for the proton transfer making the PT form
easily accessible. It can give pronounced conse-
quences in thermochromic properties of these systems
as well as its luminescence. The studies are in
progress [20].
Stronger hydrogen bonds were observed in the
experimental structures of I–III compounds than in
the calculated structures. Crystal packing forces prob-
ably increase the intramolecular steric repulsion.
Strengthening of hydrogen bond in I–III
compound give remarkable spectroscopic effects.
Observed effects can be explained from the point of
view of the steric interaction and the change of basi-
city upon replacement of N-methyl substituent by
N-benzyl one; pKa values of base part of these intra-
molecular complexes decrease more than 1 unit of
pKa (pKa of methyl amine is 10.7 while benzylamine
9.4 [21]).
Table 7 also contains the results of ab initio
B3LYP-calculations for both molecular and zwitter
ionic forms of intramolecular hydrogen bonding.
An interesting feature appears after calculations–
two conformers resulting from the N–CH3 group rota-
tion were found (Fig. 4). One conformation is as in the
experimental structure (Fig. 4a). The second one is
characteristic of direct contact between four H
atoms of both CH3 groups (Fig. 4b). It appears that
the b conformers are more stable, for example in I, by
0.435 kcal/mol in molecular form and 0.626 kcal/mol
in zwitterionic one. It can be probably explained by
very small shortening of the hydrogen bond while
steric repulsion between methyl groups increases.
Crystal packing conditions in the solid state are
however in favour of conformers of a type.
In the last column of Table 7 are presented the
differences in calculated energy of molecular
(lower) and zwitterionic (PT) states. As discussed

86
A. Filarowski et al. / Journal of Molecular Structure 484 (1999) 75–89
Fig. 5. FT-IR spectra of 2-(N-methyl-a-iminoethyl)-phenol (I) and 2-(N-benzyl-a-iminoethyl)-phenol (II) in CCl4 solutions.
The differences in IR spectra in the n(OH) absorp-
repulsion in compounds with N-methyl substituent.
In Fig. 7 are compared spectra of VI, II and III. All
of these compounds contain the N-benzyl group.
As in Fig. 6 the effects of the increase of steric repul-
sion when going from compounds with NyC(H)– C
group through compounds with NyC(CH3)– C group
tion range between I and II ( Fig. 5) demonstrate the
last effect. No free n(OH) bands are observed. Intra-
molecular hydrogen bonds are stable in CCl4 solutions
and the spectra do not change upon dilution.
Fig. 6 shows the effect of increase of steric
Fig. 6. FT-IR spectra of 2-(N-methyl-a-iminoethyl)-phenol (I), 2-(N-methylimino)-phenol (IV) and 2-(N-methyl-a-iminopropyl)-phenol (VII)
in CCl4 solutions.

A. Filarowski et al. / Journal of Molecular Structure 484 (1999) 75–89
87
Fig. 7. FT-IR spectra of 2-(N-benzyl-a-iminoethyl)-phenol (II), 2-(N-benzyl-a-iminopropyl)-phenol (III) and 2-(N-benzylimino)-phenol (VI)
in CCl4 solutions.
to compounds with NyC(C2H5)–C group are
demonstrated.
bonds with large dynamics of proton movement
… …
within the O
H
N bridge.
A strong increase of Dn(OH) is observed between
VI and II compounds, while an extension of steric
repulsion by replacement of methyl group by the
ethyl group is rather small. The same conclusions
follow from comparison of compounds with N–CH3
group (Fig. 6). Increase of steric repulsion is mostly
pronounced when NyC(H)– C group is replaced by
NyC(CH3)– C group.
Such strong temperature modifications of the
potential for the proton transfer are interesting for
further more extensive structural and spectroscopic
studies.
5. Conclusions
It agrees with the structural effects, where a
strong shortening of hydrogen bridge is observed
upon introduction of methyl substituent and while
passing from the compound with methyl to the
compound with ethyl substituent do not modify the
structure seriously.
A group of three ortho hydroxy Schiff bases with
methyl and ethyl substituents in the NyC(H)– C unit
were synthesised and its molecular structure by crys-
tallographic method and ab initio calculations was
determined. Very strong strengthening of the intramo-
…
lecular O–H N hydrogen bridge was achieved
Very special properties of I can be mentioned. In
this compound with the strongest O–H N hydrogen
owing to steric shortening of the hydrogen bridge.
…
One of the compounds (I) contains the shortest O–
…
…
bridge, very labile potential for the proton transfer
was detected. Decrease of the temperature causes a
strong modification of IR spectra in n(OH) absorption
range (Fig. 8).
Gradual decrease of the absorption above
2500 cmϪ1 and strong increase of the absorption
below 1700 cmϪ1 is observed. Below 100 K spectrum
it can be classified as a spectrum with continuous
absorption, characteristic for very short hydrogen
H
N hydrogen bridge O N distance equal to
˚
2.46 A known so far in the literature.
Two others also belong to the very short hydrogen
…
˚
bridges with O N distance less than 2.5 A. Detailed
analysis of intramolecular steric interactions was
performed and structure modifications resulting from
these interactions were specified. Replacement of N–
CH3 group by N-benzyl moiety does not change
pronouncedly the steric effects, but decreases the

88
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A. Filarowski et al. / Journal of Molecular Structure 484 (1999) 75–89
89
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Very large liability of the potential for the proton
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compounds was stated in comparison to related
compounds without alkyl substituents in the central
part of a molecule. Steric modification of the potential
for the proton movement discussed in this paper, can
be a source of the compounds with desirable physico-
chemical properties, including potential practical
applications resulting from thermochromic and photo-
chromic properties of Schiff bases.
¨
Inorganic Chemistry, University of Gottingen, German.
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Acknowledgements
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402.
Authors acknowledge the financial support of KBN
grant No 3TO9A04210. We would also like to
´
acknowledge Mr. Jerzy Janski for the help in
performing quantum mechanical calculations.
[18] P. Camilleri, C.A. Marby, B. Odell, H.S. Rzepa, R.N. Shep-
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