The self-assembly switching of the group 13 tetrahedral Schiff base complexes by changing the character of coordination centre

Article abstract of DOI:10.1039/b406288d

The reaction of Et₃B with one equivalent of N-substituted salicylideneimine (HsaldR′) yields the monomeric O,N-chelate complexes Et₂B(saldR′), where R′ = Me (1) or Ph (2). The crystal structure of the resulting complexes has been determined by X-ray crystallography. The molecular structure of these complexes consists of monomeric four-coordinate chelates and their primary arrangement in the crystal structure is determined by the C-H...O_aryloxide hydrogen bonds. An extended crystal structure analysis reveals that the adjacent monomeric moieties of 1 are interconnected by C-H_imino...O hydrogen bridges resulting in a 1-D motif infinite H-bonded chain, whereas the crystalline complex 2 comprises dimeric molecules linked through a pair of intermolecular C-H_arom...O interactions. The supramolecular arrangement of both compounds is discussed with relation to the structure of analogous aluminium and gallium complexes, and the role played by the coordination centre on the molecular assembly is analyzed. It is shown that the steric congestion at the hydrogen bond donor and acceptor sites, as a result of changes in the N-alkyl substituents or the coordination centre environment, affect the strength of the intermolecular C-H...O hydrogen bonds and may lead to the self-assembly switching of the bidentate Schiff base complexes.

Full text of DOI:10.1039/b406288d

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P A P E R
The self-assembly switching of the group 13 tetrahedral Schiff base
complexes by changing the character of coordination centre
a
a
a
a
´
Janusz Lewinski,* Janusz Zachara, Patryk Stolarzewicz, Maciej Dranka,
Edyta Kołodziejczyk,b Iwona Justyniakb and Janusz Lipkowskib
a Department of Chemistry, Warsaw University of Technology, Noakowskiego 3
00-664 Warsaw, Poland. E-mail: lewin@ch.pw.edu.pl
b Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52
01-224 Warsaw, Poland
Received (in Durham, UK) 26th April 2004, Accepted 28th July 2004
First published as an Advance Article on the web 18th October 2004
The reaction of Et3B with one equivalent of N-substituted salicylideneimine (HsaldR0) yields the monomeric
O,N-chelate complexes Et2B(saldR0), where R0 ¼ Me (1) or Ph (2). The crystal structure of the resulting
complexes has been determined by X-ray crystallography. The molecular structure of these complexes
consists of monomeric four-coordinate chelates and their primary arrangement in the crystal structure is
determined by the C–Hꢀ ꢀ ꢀOaryloxide hydrogen bonds. An extended crystal structure analysis reveals that the
adjacent monomeric moieties of 1 are interconnected by C–Himinoꢀ ꢀ ꢀO hydrogen bridges resulting in a 1-D
motif infinite H-bonded chain, whereas the crystalline complex 2 comprises dimeric molecules linked through
a pair of intermolecular C–Haromꢀ ꢀ ꢀO interactions. The supramolecular arrangement of both compounds is
discussed with relation to the structure of analogous aluminium and gallium complexes, and the role played
by the coordination centre on the molecular assembly is analyzed. It is shown that the steric congestion at
the hydrogen bond donor and acceptor sites, as a result of changes in the N-alkyl substituents or the
coordination centre environment, affect the strength of the intermolecular C–Hꢀ ꢀ ꢀO hydrogen bonds and
may lead to the self-assembly switching of the bidentate Schiff base complexes.
lates3 as model complexes. These studies have revealed that
Introduction
even minor differences in the subunit structure can have a
profound effect on the molecular and crystal structure. An
integral part of this work has been to study the structure of
group 13 metal alkyl chelate complexes with the salicylidenei-
minate anion as the O,N-bidentate ligand (M ¼ Al, Ga and
In).4 The latter complexes were found to be a good basic model
system for exploiting the role of hydrogen bonding on the
supramolecular structure of Schiff base metal complexes. For
instance, the crystal structure analysis of the aluminium and
gallium derivatives of N-methyl- and N-phenyl-salicylidenei-
mine (HsaldPh and HsaldMe, respectively) provided interest-
ing data concerning the effect of N-alkyl substituents on the
molecular assembly of the tetrahedral chelate complexes. Here
we report the structure investigations of the diethylboron
derivatives of N-phenyl- and N-methyl-salicylideneimine,
Et2B(saldR0). In an effort to elucidate factors controlling the
supramolecular structure of bidentate Schiff base complexes,
we discuss the geometrical parameters of both compounds with
relation to the structure of the related aluminium and gallium
complexes.
The great potential for producing novel materials with inter-
esting useful functions (with desirable properties) has led to the
rational design and synthesis of metal–organic polymeric net-
works becoming an intensely studied subject. The rational
construction of metal–organic supramolecular frameworks
involves a cooperation between the metal first-coordination
sphere and the second-sphere non-covalent active sites. Schiff
bases have been for decades among the most fundamental
chelating systems in coordination chemistry, and exploitation
of this group of compounds in material science is currently a
rapidly developing field.1 Not surprisingly, recent studies have
highlighted the potential importance of hydrogen bonding in
tuning the properties of materials based on Schiff base com-
plexes.1 However, the vast majority of investigations focus on
the metal first-coordination sphere of Schiff base complexes,
while the ligand frameworks have various donor and acceptor
sites capable of participating in an intermolecular hydrogen
bond to an adjacent metal-bound ligand, and the fundamental
structural consequences exhibited by the second-sphere hydro-
gen bonding interactions on the reactivity and properties of
these type complexes is not a well-developed field. Undoubt-
edly, the identification of the hydrogen bond preferred mode
for Schiff base metal complexes is of fundamental importance
in the design of solid-state supramolecules involving this type
of ligands and there is considerable current interest in elucidat-
ing the role of hydrogen bonding interactions using small-
molecule models.
Results and discussion
Synthesis and structure of diethylboron derivatives of
N-substituted salicylaldimine
The reaction of Et3B with one equivalent of N-substituted
salicylideneimine (HsaldR0) allows for the isolation of mono-
meric complexes Et2B(saldR0) almost quantitatively, where
R0 ¼ Me (1) or Ph (2). The resulting compounds were isolated
as yellow crystalline solids following evaporation to dryness of
the reaction mixture and subsequent recrystallization from
hexane/CH2Cl2 solution at low temperature. Both compounds
For the last few years we have been engaged in an investiga-
tion of relationships between the intra- and intermolecular
forces resulting from the donor–acceptor and hydrogen bond-
ing interactions using the group 13 alkoxides2 and carboxy-
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
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have been characterized in a solution by NMR and IR spectro-
scopy (see Experimental section) and in the solid state their
structures have been determined by X-ray crystallography. The
1H NMR spectra of Et2B(saldR0) shows no complexity and
characteristic multiplets of the B–Et protons, the single reso-
nance of N CH proton, and multiplets of aromatic protons
Q
are observed. The 11B NMR spectra consist of a single
resonance at 7.6 and 8.9 ppm for 1 and 2, respectively,
corresponding to a four-coordinate boron centre.5 The IR
spectra of 1 and 2 revealed the presence of a strong nCQ
N
band of the imine group at 1656 and 1628 cmꢁ1, respectively,
which are surprisingly shifted slightly towards higher frequen-
cies with respect to the free ligands values (1640 and
1620 cmꢁ1). Thus, the spectroscopic data are consistent with
the monomeric chelate structure of studied complexes.
Fig. 2 An ORTEP diagram of hydrogen bonded dimer of Et2B
(saldPh) (2) in the solid state showing a 30% probability of thermal
ellipsoids. The atoms labelled with a prime (0) are at symmetry
equivalent position (1/2 ꢁ x, 1/2 ꢁ y, 1 ꢁ z).
The solid structures of 1 and 2 comprise, like the correspond-
ing aluminium and gallium derivatives, monomeric species
with the extended hydrogen bond network. Compound 1
crystallises in the orthorhombic space group Pbca, while 2
shapes monoclinic crystals in C2/c space group. In an effort to
elucidate factors controlling the supramolecular structure of
bidentate Schiff base complexes, first we shall focus on the
geometrical parameters of 1 and 2 and the related aluminium
and gallium complexes of type Me2M(saldR0). The molecular
structures of 1 and 2 are shown in Figs. 1 and 2, respectively,
while selected interatomic distances and angles are collected in
Table 1, together with data obtained previously for the alumi-
nium and gallium analogues.4 In both structures, the boron
centre has a distorted tetrahedral geometry with angles ranging
from 93.19(14) to 125.3(3)1 and from 104.29(16) to 116.19(19)1
in 1 and 2, respectively. The most acute angle in each case is
associated with the bite of the chelating ligand. The six-
membered chelate-ring system O(1)C(1)C(2)C(7)N(1)B(1) in 1
is essentially planar (puckering amplitude Q ¼ 0.137(3) A6). In
contrast, the analogue heterocycle in 2 is highly deviated from
planarity (puckering amplitude Q ¼ 0.349(2) A) and presents
an envelope conformation with B(1) atom displaced 0.486(3) A
out of the almost flat ligand plane. Similar deformations of the
chelate ring were observed in the aluminium and gallium
derivatives of N-phenylsalicylideneimine where the relevant
displacement of Al and Ga atoms from the ligand plane is
equal 0.250(3) and 0.312(5) A, respectively. Interestingly, in all
cases this deformation is realised by the bending on the
aryloxide oxygen atom [the torsion angles M(1)–O(1)–C(1)–
C(2) range from ꢁ9.3(4) (1) to ꢁ29.8(3)1 (2)], while the metal–
imine linkage remains essentially planar (the torsion angles
nic interactions within the chelate ring which is a significant
contributor to the molecular structure of 1 and 2. The observed
B–O and B–N bond lengths are significantly shorter than the
M–O and M–N distances in the aluminium and the gallium
analogues (Table 1). Nevertheless, these differences are sub-
stantially consistent with the increasing covalent radii of boron
(0.83 A), aluminium (1.18 A) and gallium (1.26 A). One
additional interesting structural feature of the boron complexes
is the consistently widened O–B–N angle by comparison to the
O–M–N angles for the aluminium (3) and gallium (5) com-
Table 1 Selected bond lengths (A), bond angles (deg) and torsion
angles (deg) for Et2B(saldMe) (1), Et2B(saldPh) (2), Me2Al(saldPh) (3),
and Me2Ga(saldPh) (5)
Et2B(saldMe) (1)
B(1)–O(1)
Et2B(saldPh) (2)
1.515(4) B(1)–O(1)
1.610(4) B(1)–N(1)
1.607(4) B(1)–C(8)
1.505(3)
1.642(3)
B(1)–N(1)
B(1)–C(8)
1.617(3)
B(1)–C(10)
1.617(4) B(1)–C(10)
1.336(3) O(1)–C(1)
1.291(3) N(1)–C(7)
1.405(4) C(1)–C(2)
1.428(4) C(2)–C(7)
1.592(3)
O(1)–C(1)
1.332(3)
N(1)–C(7)
1.296(2)
C(1)–C(2)
1.400(3)
C(2)–C(7)
1.429(3)
C(8)–B(1)–C(10)
O(1)–B(1)–N(1)
C(1)–O(1)–B(1)
C(7)–N(1)–B(1)
O(1)–C(1)–C(2)
C(1)–C(2)–C(7)
N(1)–C(7)–C(2)
B(1)–O(1)–C(1)–C(2)
B(1)–N(1)–C(7)–C(2)
114.3(2)
C(10)–B(1)–C(8)
O(1)–B(1)–N(1)
C(1)–O(1)–B(1)
C(7)–N(1)–B(1)
O(1)–C(1)–C(2)
C(1)–C(2)–C(7)
C(2)–C(7)–N(1)
B(1)–O(1)–C(1)–C(2)
B(1)–N(1)–C(7)–C(2)
116.06(19)
104.28(16)
121.71(16)
120.19(16)
120.31(18)
118.52(19)
122.62(19)
ꢁ29.8(3)
4.8(4)
107.1(2)
124.9(2)
123.1(2)
121.4(3)
119.1(3)
122.5(2)
ꢁ9.3(4)
4.5(4)
M–N C–C are less than 51). The B(1)–O(1) distance in
Q
compound 1 [1.515(4) A] is slightly longer, whereas the B(1)–
N(1) distance [1.610(4) A] is shorter, than the corresponding
distances in 2 (1.505(3) and 1.642(3) A, respectively). Undoubt-
edly, the identity of the N-alkyl substituents affect the electro-
C(17)–C(12)–N(1)–C(7) 55.9(3)
Me2Al(saldPh) (3)4
Me2Ga(saldPh) (5)4
Al(1)–O(1)
1.772(2) Ga(1)–O(1)
1.889(3)
2.023(3)
1.942(5)
1.940(5)
1.324(5)
1.288(5)
1.407(6)
1.432(6)
Al(1)–N(1)
1.963(2) Ga(1)–N(1)
1.943(3) Ga(1)–C(8)
1.946(3) Ga(1)–C(9)
1.337(3) O(1)–C(1)
1.296(3) N(1)–C(7)
1.402(3) C(1)–C(2)
1.434(3) C(2)–C(7)
119.09(14) C(8)–Ga(1)–C(9)
Al(1)–C(8)
Al(1)–C(9)
O(1)–C(1)
N(1)–C(7)
C(1)–C(2)
C(2)–C(7)
C(8)–Al(1)–C(9)
O(1)–Al(1)–N(1)
C(1)–O(1)–Al(1)
C(7)–N(1)–Al(1)
O(1)–C(1)–C(2)
C(1)–C(2)–C(7)
C(2)–C(7)–N(1)
125.2(3)
95.14(9)
O(1)–Ga(1)–N(1)
93.21(14)
127.1(3)
122.1(3)
123.7(4)
123.7(4)
127.3(4)
129.71(17) C(1)–O(1)–Ga(1)
121.68(18) C(7)–N(1)–Ga(1)
121.6(2)
123.2(2)
126.5(3)
O(1)–C(1)–C(2)
C(1)–C(2)–C(7)
C(2)–C(7)–N(1)
Al(1)–O(1)–C(1)–C(2) ꢁ13.4(4)
Al(1)–N(1)–C(7)–C(2) 0.7(4)
C(15)–C(10)–N(1)–C(7) 46.0(4)
Ga(1)–O(1)–C(1)–C(2) ꢁ15.5(7)
Ga(1)–N(1)–C(7)–C(2) 1.9(7)
C(15)–C(10)–N(1)–C(7) 45.1(6)
Fig. 1 An ORTEP diagram of molecular structure of Et2B(saldMe)
(1) in the solid state showing a 50% probability of thermal ellipsoids.
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Table 2 Selected intermolecular contacts (A, deg) in 1 and 2
1
C–Hꢀ ꢀ ꢀO
C–H
0.95
0.98
C–H
0.98
Hꢀ ꢀ ꢀO
2.57
2.50
Cꢀ ꢀ ꢀO
C–Hꢀ ꢀ ꢀO
152
160
C(7)–H(7)ꢀ ꢀ ꢀO(10)a
C(12)–H(12A)ꢀ ꢀ ꢀO(10)a
C–Hꢀ ꢀ ꢀp
3.441(3)
3.438(3)
Cꢀ ꢀ ꢀCg
4.031(3)
Hꢀ ꢀ ꢀCgf
C–Hꢀ ꢀ ꢀCg
151
C(12)–H(12C)ꢀ ꢀ ꢀp(Ph00)b
3.14
2
C–Hꢀ ꢀ ꢀO
C–H
0.93
C–H
0.93
0.93
Hꢀ ꢀ ꢀO
Cꢀ ꢀ ꢀO
C–Hꢀ ꢀ ꢀO
171
C(6)–H(6)ꢀ ꢀ ꢀO(10)c
C–Hꢀ ꢀ ꢀp
2.54
3.462(3)
Cꢀ ꢀ ꢀCg
3.581(3)
3.768(4)
Hꢀ ꢀ ꢀCgf
2.82
3.10
C–Hꢀ ꢀ ꢀCg
140
130
C(4)–H(4)ꢀ ꢀ ꢀp(N-Ph00)d
C(16)–H(16)ꢀ ꢀ ꢀp(Ph000)e
Symmetry codes:a (ꢁ1/2 þ x, y, 1/2 ꢁ z); (–x, –1/2 þ y, 1/2 ꢁ z);
b
c
d
e
(1/2 ꢁ x, 1/2 ꢁ y, 1 – z); (1/2 – x, ꢁ1/2 ꢁ y, 112–z); (21 ꢁ x, 121 ꢁ y,
f
1 ꢁ z) Cg-phenyl ring centroid.
plexes. This is a consequence of the relatively short B–O and B–
N distances, and the rigidity of the ligand core. Other bond
lengths and angles within the salicylideneiminate ligand of 1
and 2 are typical of such moieties. Furthermore, in regard to
the N-phenyl complexes, i.e. compound 2 and the isostructural
Al and Ga complexes, it should be noted the significant
Fig. 3 A view of the 2D hydrogen bonding network in the crystal
structure of Et2B(saldMe) (1). The dashed and the dotted lines
represent C–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀp(Ph) hydrogen bonds, respectively.
Hydrogen atoms are omitted excluding those involved in the H-bond
formation.
twisting of the N-phenyl aromatic ring relative to the M–N
Q
C plane. Of the three compounds discussed, in 2 the N-phenyl
group is twisted towards the chelate ring in the largest degree.
The C(17)–C(12)–N(1)–C(7) torsion angle is equal 55.9(3)1,
accordingly, it is about 101 larger than in the Al and Ga
derivatives.
in the latter interaction. The distances and the angles between
the C–H groups and appropriate geometrical centroids of
the phenyl rings are included in Table 2. Interestingly, the
A detailed inspection of the crystal structures of 1 and 2
revealed that the weak intermolecular hydrogen bond systems
play a substantial role in the molecular assembly of crystalline
complexes. An analysis of intermolecular contacts showed that
adjacent monomeric moieties of 1 are held together in chains
by double C–Hꢀ ꢀ ꢀO hydrogen bridges. Both the imine hydro-
gen atom H(7) and the hydrogen atom H(12a) of the methyl
group are involved in hydrogen bond interactions with the
aryloxide oxygen O(10) of an adjacent molecule (Table 2).
The resulting infinite H-bonded chain is propagated by the
a-glide plane as depicted in Fig. 3. The length of the glide
vector is equal to 6.1205(5) A. Parallel chains are further
interconnected by weak C–Hꢀ ꢀ ꢀp(Ph) interactions (dotted
lines in Fig. 3) forming grid-like 2-D network on the (001)
crystallographic plane. It should be noted that a very similar
primary chain structure was recently revealed for the gallium
derivative of N-methylsalicylideneimine, Me2Ga(saldMe)
(4).4,7 For the latter complex molecules are linked together
by the C–Himinoꢀ ꢀ ꢀOaryloxide hydrogen bonds and are related
by the c-glide plane [glide vector equals 6.0343(5) A]. The
secondary structure for this compound is achieved also in a
similar way. Chains are assembled by weak C–Hꢀ ꢀ ꢀp(Ph)
interactions into the 2D double layers lying on the (100)
crystallographic plane. Adjacent layers are aligned parallel to
each other without interpenetration.
The supramolecular structure of compound 2 is presented in
Fig. 4. Surprisingly, it consists of a hydrogen-bonded dimer
formed by a pair of adjacent molecules as basic motif. Thus,
the observed intermolecular hydrogen bond motif for 2 is
entirely different from that found for 1 and the related alumi-
nium and gallium derivatives of N-phenylsalicylideneimine,
3 and 5. The association of the molecular complexes in 2 is
attained through a pair of intermolecular C–Haromꢀ ꢀ ꢀO (2.54 A)
interactions between the aryloxide oxygen atom and the hydro-
gen of the salicylideneimine aromatic ring (Table 2), thus
leading to the H-bonded dimer located on the crystallographic
centre of symmetry. In addition, the dimeric species are further
connected by C–Haromꢀ ꢀ ꢀp(Ph) interactions to form infinite
layers, parallel to the bc plane. Both phenyl rings are engaged
Fig. 4 A view of the 2D hydrogen bonding network in the crystal
structure of Et2B(saldPh) (2). The dashed and the dotted lines represent
C–Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀp(Ph) hydrogen bonds, respectively. Hydrogen
atoms are omitted excluding those involved in the H-bond formation.
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N
C–H
hydrogen in 2 is essentially not involved in the
ring and greatly affect the Lewis basicity of the aryloxide
oxygen and may affect the Lewis acidity of potential hydrogen
bond donor sites. The planar M–O–C linkage favours the
conjugation of the 2pp nonbonding orbital of the aryloxide
oxygen with the p-conjugated ligand bond system and
strengthens the stabilization of the oxygen lone pairs. Con-
versely, the pyramidalisation on the aryloxide oxygen atom
strengthens the Lewis basicity of this centre as well as it
changes the directionality of the oxygen lone pairs. However,
the observed conformational differences between the boron
complex and the aluminium and gallium analogues seem to be
too small to account for the observed self-assembly switching.
Furthermore, the relative orientation of the N-phenyl aromatic
Q
imino
hydrogen bond formation (the nearest contact of this proton to
the aromatic carbon atom is 3.01 A).
A Cambridge Structural Database8 search revealed that up
to date only few simple boron Schiff base derivatives of the
form R2B(O,N) have been structurally characterized, all bear-
ing exclusively the diphenylboron moieties.9–12 Our analysis of
the available structure data points that the presence of phenyl
groups bonded to the boron centre effectively prevents the
association of molecular complexes through intermolecular
C–Hꢀ ꢀ ꢀO interactions. Instead of the intermolecular hydrogen
bonds, both boron bonded phenyl rings, adopting appropriate
orientations, are engaged in the intramolecular C–Hphenꢀ ꢀ ꢀO or
(and) C–Hphenꢀ ꢀ ꢀN interactions. The observed Hphenꢀ ꢀ ꢀO
and Hphenꢀ ꢀ ꢀN contacts are equal ca. 2.5 and 2.6 A, respec-
tively.10–12 However, the crystallization of diphenylboron
Schiff base derivative in the presence of the strong H donor,
e.g. methanol, leads to the formation of a H-bonded solvate.9
ring with respect to the M–N C plane also should affect
Q
both the p-conjugation of the N-phenyl aromatic ring with the
C
N
Q
N imine group p system and the steric congestion at the
Q
C–H
hydrogen, but the observed out-of-plane con-
imino
formation of the N-phenyl ring are not significantly distinct for
the complexes in question. So, there must be another effect that
influences the hydrogen bond motifs. As was mentioned above,
the radius of boron is significantly smaller than that of
aluminium and gallium which results in a concomitant de-
crease in the B–O and B–N bond length. One consequence of
this difference is the increase of steric congestion at the aryl-
oxide oxygen in 2. The formation of the C–Himinoꢀ ꢀ ꢀOaryloxide
hydrogen bond chains requires a relatively close approach of
Toward factors affecting supramolecular structure of
bidentate Schiff base metal complexes
The presented above results demonstrate that the molecular
structure of R2M(saldR0) (where M ¼ B, Al or Ga) complexes
consists of isomorphous monomeric four-coordinate chelate
moiety independently of the character of the coordination
centre. However, unexpectedly, the crystal structures of these
complexes differ significantly. Although in all cases the supra-
molecular structure is determined by intermolecular hydrogen
bonds, the resulting hydrogen bond motifs are distinct. For the
boron derivative of N-phenylsalicylideneimine, it is the hydro-
the –(Ph)N C–H
hydrogen to the metal bonded arylox-
Q
imino
ide oxygen which in the case of boron complex causes a more
significant repulsion between the phenyl ring and the sterically
congested Et2BO tetrahedron face. Thus, in our opinion the
different preference of the C–H donor sites for the intermole-
cular hydrogen bonding is dominantly due to the different
steric congestion of the Et2BON core in 2 as compared to
the Me2MON core in the related heavier group 13 derivatives
of N-phenylsalicylideneimine, and it could affect the strength
of the C–Hꢀ ꢀ ꢀO hydrogen bond in the solid state. In turn, a
considerably greater congestion in the structure based on the
C–Himinoꢀ ꢀ ꢀOaryloxide hydrogen bond promotes the formation
of the double hydrogen bond dimer, where the cooperative
C–Haromꢀ ꢀ ꢀO interactions sufficiently compete with the chain
array of C–Hꢀ ꢀ ꢀO bonds involving the stronger hydrogen bond
donor site. It should also be emphasised that additional
hydrogen bond formations are observed in these complexes
which undoubtedly support the basic hydrogen bond motifs.
For instance, the hydrogen-bonded dimers of 2 are further
connected by C–Haromꢀ ꢀ ꢀp(Ph) interactions which lead to the
formation of infinite layers.
gen-bonded dimer formed by
a pair of intermolecular
C–Haromꢀ ꢀ ꢀO bridges between the aryloxide oxygen atom and
the ortho-positioned hydrogen of the salicylideneimine aro-
matic ring (Scheme 1a). In contrast, for the related aluminium
and gallium derivatives of N-phenylsalicylideneimine as well as
the boron and gallium derivatives of N-methylsalicylidenei-
mine, the monomeric moieties are linked by the C–Himi-
hydrogen bonds leading to the formation of
noꢀ ꢀ ꢀOaryloxide
infinite chain structures (Scheme 1b). Thus, a common feature
of the observed hydrogen bond motifs is the aryloxide oxygen
acting as the hydrogen acceptor and the difference concerns a
competition between the C–Himino and the C–Harom hydrogen
donor sites (from the CSD search it appears that the C–
Himinoꢀ ꢀ ꢀO could be expected as a dominant interaction13). In
this regard arises the question: why is there a change in the
hydrogen bond donor sites in this homologous series of N-
phenylsalicylideneimine derivatives passing from boron to the
heavier group 13 derivatives?
Further support for the assumption that steric congestions at
the hydrogen donor and acceptor sites strongly affect the
supramolecular structure of tetrahedral complexes stabilized
by monoanionic bidentate Schiff base ligands is provided by
the comparison of the crystal structure of the aluminium and
gallium derivatives of N-methyl- and N-phenyl-salicylidenei-
mine, Me2M(saldPh) and Me2M(saldMe) complexes. The mo-
lecular structure of these complexes consists of isostructural
monomeric four-coordinate chelates and their primary ar-
rangement in the crystal structure is determined by the C–
Himinoꢀ ꢀ ꢀOaryloxide hydrogen bonds. However, the replacement
of the N-methyl to N-phenyl group in the bidentate salicylide-
neiminate ligand leads to significant changes in the molecular
assembly. The gallium complex 4 with the N-methyl ligand
forms the C–Himinoꢀ ꢀ ꢀOaryloxide hydrogen bonded molecular
chains (vide supra). On the other hand, for the N-phenyl
complexes 3 and 5, the monomeric moieties related by 41 (43)
screw axis are linked by the C–Himinoꢀ ꢀ ꢀOaryloxide hydrogen
bonds which lead to the formation of infinite helices as
depicted in Fig. 5.4 The parallel alternating left- and right-
handed helical chains are further joined by C–Hꢀ ꢀ ꢀp(Ph)
interactions to form a tetragonal net. Except for the slightly
different ionic radii of aluminium and gallium, the two
In order to ascertain whether the observed hydrogen bond
motifs are determined by the molecular shape, molecular size
or other factors, it is instructive to consider the silent features
of the molecular structures of R2M(saldR0) complexes. Thus,
the relative conformation of the six-membered MOCCCN
chelate-ring system appears to be flexible, however one may
easily recognise that the extent of the chelate ring deformation
is closely related to the geometry of the M–O–C linkage. It is
apparent that the change of the M–O–C unit geometry is
important in determining the conjugation of the aryloxide
oxygen lone pair with the extended p-system of the chelate
Scheme 1
N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 3 2 0 – 1 3 2 5
1323

View Article Online
Fig. 5 (a) A view of the helical chain structure of Me2Al(saldPh) (3). (b) A view of the molecular helix along c axis. The dashed lines represent
C–Hꢀ ꢀ ꢀO hydrogen bonds. Hydrogen atoms are omitted excluding those involved in H-bond formation.
complexes are isostructural which indicates that in this case the
metal centre does not exert a significant influence on the
molecular assembly. So, there appears that small differences
in the molecular shape of Me2M(saldPh) and Me2M(saldMe)
complexes alter the mode of the molecular assembly in the
solid state. To compare, the N-methyl complex possesses a
Table 3 X-ray data collection and structure refinement parameters for
compounds 1 and 2w
Et2B(saldMe) (1)
C12H18BNO
203.08
Et2B(saldPh) (2)
C17H20BNO
265.15
Formula
M
Temperature/ K
Crystal system
Space group
Radiation
a/A
100(2)
Orthorhombic
Pbca (no. 61)
293(2)
Monoclinic
C2/c (no. 15)
relatively uncongested environment at the N C–H hydrogen,
Q
while in the N-phenyl complex the considerably greater steric
congestion imposed by the imine ligand may be responsible for
the observed switch in the molecular assembly. This change in
Mo-Ka (l ¼ 0.7107 A)
12.2407(9)
13.557(2)
14.531(2)
90
22.3787(8)
10.7423(6)
15.2876(9)
123.090(2)
3079.1(3)
8
b/A
c/A
the steric congestion at the N C–H hydrogen appears to
Q
greatly affect the spatial arrangement of the component mole-
cules and to change the hydrogen bonding pattern.
b/1
U/A3
2411.4(5)
8
1.119
In conclusion, while the group 13 compounds R2M(saldR0)
complexes exist in the solid state as isostructural monomeric
four-coordinate chelate species, small differences in the mole-
cular shape and molecular size significantly affect their crystal
structure. Two basic hydrogen bond motifs are observed for
this group of complexes: the hydrogen-bonded dimer formed
by a pair of intermolecular C–Haromꢀ ꢀ ꢀO interactions between
the aryloxide oxygen atom and the ortho-positioned hydrogen
of the salicylideneimine aromatic ring, and the array of the
C–Himinoꢀ ꢀ ꢀOaryloxide hydrogen bonds leading to the formation
of infinite chains. Thus, a common feature of the hydrogen
bond motifs is the aryloxide oxygen acting as the hydrogen
acceptor and the difference concerns a competition between the
C–Himino and the C–Harom hydrogen donor sites. The analysis
of the structural parameters of R2M(saldR0) complexes indi-
cates that the steric congestion at the hydrogen bond donor
and acceptor sites, as a result of changes in the N-alkyl
substituents or the coordination centre environment, affect
the strength of the internal C–Hꢀ ꢀ ꢀO hydrogen bond and
may lead to self-assembly switching of the bidentate Schiff
base metal complexes.
Z
Dc/g cmꢁ3
m/ mmꢁ1
F(000)
1.144
0.069
1136
0.069
880
Crystal size/ mm
2y Range/1
Independent refls (Rint
0.18 ꢂ 0.22 ꢂ 0.24
5.3 to 45.0
1567 (0.060)
1567, 0, 140
1.077
0.15 ꢂ 0.20 ꢂ 0.30
5.4 to 50.0
2702 (0.051)
2702, 0, 184
1.033
)
Data, restraints, param.
Goodness of fit on F2
R1, wR2 [I 4 2s(I)]a
wR2 (all data)
0.0552, 0.1009
0.112
0.0481, 0.1070
0.134
Weights a, bb
0.0366, 0.885
0.0059(9)
þ0.154/ꢁ0.164
2
0.0557, 0
0.0077(8)
þ0.125/ꢁ0.102
b
Extinction coeff. wc
Max, min Dr/e Aꢁ3
a
R1 ¼ S||Fo| – |Fc||/S|Fo|, wR2 ¼ [Sw(Fo ꢁ Fc2)2/Sw(Fo4)]1/2
;
w ¼
1/[s 2(Fo2) þ (a ꢀ P)2 þ b ꢀ P], where P ¼ (Fo þ 2 Fc2)/3. Fc* ¼
2
c
kFc[1 þ 0.001 ꢀ w ꢀ Fc2l3/sin(2y)]ꢁ1/4, where k ¼ overall scale factor.
1056(m), 1068(m), 1124(m), 1152(s), 1232(m), 1316(s), 1336(m),
1392(m), 1420(m), 1436(m), 1460(s), 1480(s), 1540(m), 1560(m),
1608(s), 1656(vs), 2824(m), 2868(m), 2908(m), 2948(m).
Et2B(saldPh) (2). A solution of N-phenylsalicylideneimine
(0.43 g, 2.18 mmol) in toluene (6 mL) was cooled to –78 1C,
and Et3B (0.32 cm3, 2.18 mmol) was added dropwise. After the
addition was completed, the reaction mixture was warmed to
room temperature and stirred for 1.5 h, then the volatiles were
removed in vacuo. Complex 1a was obtained as yellow crystals
after a recrystallization from hexane/CH2Cl2 solution at 5 1C.
Anal. Calcd for C17H20BNO: C, 77.00; H, 7.60; N, 5.28.
Experimental
All operations were carried out under dry nitrogen using
standard Schlenk techniques. Solvents and reagents were pur-
ified and dried by standard techniques. NMR spectra were
recorded on a Varian Mercury (400 MHz) spectrometer in
CDCl3 solutions. The IR spectra were recorded on a Specord
M80 spectrophotometer in CH2Cl2 solutions.
1
Et2B(saldMe) (1). A solution of N-methylsalicylideneimine
(0.52 g, 3.85 mmol) in toluene (5 mL) was cooled to ꢁ78 1C,
and Et3B (0.56 cm3, 3.85 mmol) was added dropwise. After the
addition was completed, the reaction mixture was warmed to
room temperature and stirred for 24 h, then the volatiles were
removed in vacuo. The resulting complex was obtained as
yellow oil. Yellow crystals were obtained after crystallization
from pentane solution at ꢁ15 1C. Anal. Calcd for C12H18BNO:
C, 70.97; H, 8.93; N, 6.90. Found: C, 70.74; H, 9.21; N, 6.22.
1H NMR (CDCl6): d 0.48 (q, 4H, B–CH2), 0.84 (t, 6H, CH3–
CH2), ), 3.21 (s, 3H, N–CH3), 6.6 – 7.4 (m, 4H, ArH), 7.85
Found: C, 76.84; H, 7.76; N, 5.06. H NMR (CDCl3): d 0.51
(q, 4H, B–CH2), 0.76 (t, 6H, CH3–CH2), 6.7–7.5 (m, 9H, ArH),
8.00 (s, 1H, CH N). 11B NMR (CDCl3): d 8.9. IR (cmꢁ1):
Q
456(m), 556(m), 692(m), 724(m), 784(m), 808(m), 896(m),
920(m), 944(m), 1012(m), 1028(m), 1056(m), 1068(m), 1128(m),
1152(m), 1200(s), 1240(m), 1320(m), 1340(m), 1376(m), 1420(m),
1436(m), 1460(m), 1476(m), 1492(m), 1508(m), 1552(s), 1596(m),
1628(vs), 2868(m), 2908(m), 2952(m).
w CCDC reference numbers [CCDC NUMBER(S)]. See http://
www.rsc.org/suppdata/nj/b4/b406288d/ for crystallographic data in
.cif or other electronic format.
(s, 1H, CH N). B NMR (CDCl ): d 7.6. IR (cmꢁ1): 456(m),
1
Q
524(w), 596(w), 704(w), 812(m), 928(s), 968(m), 1012(m),
3
1324
N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 3 2 0 – 1 3 2 5

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2
´
(a) J. Lewinski, J. Zachara and K. B. Starowieyski, J. Chem. Soc.,
X-ray crystallography
Dalton Trans., 1997, 4217; (b) J. Lewin
K. B. Starowieyski, J. Lipkowski, I. Justyniak and E. Kozodziejc-
zyk, Eur. J. Inorg. Chem., 2001, 1123; (c) J. Lewinski, J. Zachara
and I. Justyniak, Organometallics, 1997, 16, 4597; (d) J. Lewinski,
J. Zachara and I. Justyniak, Chem. Commun., 2002, 1586; (e)
J. Lewinski, J. Zachara, P. Horeglad, D. Glinka, J. Lipkowski and
I. Justyniak, Inorg. Chem., 2001, 40, 6086.
(a) J. Lewinski, J. Zachara and I. Justyniak, Inorg. Chem., 1998,
37, 2575; (b) J. Lewinski, I. Justyniak, J. Lipkowski and J.
Zachara, Inorg. Chem., Commun., 2000, 3, 700; (c) C. S. Branch,
J. Lewinski, I. Justyniak, S. G. Bott, J. Lipkowski and A. R.
Barron, J. Chem. Soc., Dalton Trans., 2001, 1253.
J. Lewinski, J. Zachara, K. B. Starowieyski, Z. Ochal, I. Justyniak,
T. Kopec
22, 3773.
J. Lewin
ski, J. Zachara, T. Kopec,
´ ´
Single crystals of 1 and 2 suitable for X-ray diffraction studies
were placed in a thin walled capillary tubes (Lindemann glass)
in an inert atmosphere, plugged with grease and flame sealed.
X-ray diffraction data were collected on a Nonius Kappa-CCD
diffractometer. Crystal data, data collection and refinement
parameters are given in Table 3. The data were corrected for
Lorentz-polarization effects. The crystal structures were solved
by direct methods using the SHELXS-86 program.14 Full-matrix
least-squares refinement method against F2 values was carried
out by using the SHELXL-97.15 All non-hydrogen atoms were
refined with anisotropic displacement parameters. The hydro-
gen atoms were introduced at geometrically idealized coordi-
nates with a fixed isotropic displacement parameter equal to
1.2 or 1.5 (methyl groups) times the value of the equivalent
isotropic displacement parameter of the parent carbon. An
extinction correction was applied for both data sets during the
final stages of refinement. The final Fourier-difference maps
have no significant chemical meaning. Computations for the
crystal structure discussions were carried out with the PLATON
program.16 ORTEP drawings were made using ORTEP3 for
Windows.17
´
´
´
3
´
´
´
4
5
´
´
, P. Stolarzewicz and M. Dranka, Organometallics, 2003,
ski, in Encyclopedia of Spectroscopy and Spectrometry,
´
eds. J. C. Lindon, G. E. Tranter and J. L. Holmes, Academic
Press, New York, Vol. 1, 1999, pp. 691–703.
6
7
D. Cremer and J. A. Pople, J. Am. Chem. Soc., 1975, 97, 1354.
(a) V. I. Bregadze, N. G. Furmanova, L. M. Golubinskaya, O. Y.
Kompan, Yu. T. Struchkov, V. A. Bren, Zh. V. Bren, A. E.
Lyubarskaya, V. I. Minkin and L. M. Sitkina, J. Organomet.
Chem., 1980, 192, 1; (b) The refinement of the structure of 4 was
only performed for non-hydrogen atoms. In order to analyse
intermolecular interactions H-atoms were introduced in the
calculated positions (PLATON16).
8
9
The CSD 3D Graphics Search System v5.25 (November 2003):
F. H. Allen, Acta Crystallogr., Sect. B, 2002, 58, 380.
BONBAH, R. Allmann, E. Hohaus and S. Olejnik, Z. Natur-
forsch., B: Chem. Sci., 1982, 37, 1450.
Acknowledgements
We thank the State Committee for Scientific Research (Grant
No. 4 T09A 165 23) for support.
10 LEKQUN, H. Hopfl and N. Farfan, Can. J. Chem., 1998, 76,
1853.
11 MONALB, O. E. Kompan, N. G. Furmanova, Yu. T. Struchkov,
L. M. Sitkina, V. A. Bren and V. I. Minkin, Zh. Strukt. Khim.,
1980, 21, 90.
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N e w J . C h e m . , 2 0 0 4 , 2 8 , 1 3 2 0 – 1 3 2 5
1325