Simple generation of neutral bimetallic aluminium and zinc alkyls Schiff bases bridged by a central resorcinol moiety

Article abstract of DOI:10.2478/s11532-010-0107-y

Aluminium and zinc complexes bearing the N,O-chelating Schiff base ligand 4,6-bis-1-(2-(dimethylamino)ethylimino)ethyl)benzene-1,3-diol, (C₆H₂(OH)₂(NCH₂CH₂NMe₂)₂) (1a), have been

Full text of DOI:10.2478/s11532-010-0107-y

Cent. Eur. J. Chem. • 8(6) • 2010 • 1305-1310
DOI: 10.2478/s11532-010-0107-y
Central European Journal of Chemistry
Simple generation of neutral bimetallic
aluminium and zinc alkyls Schiff bases
bridged by a central resorcinol moiety
Research Article
Elham S. Aazam1* and Martyn P. Coles2
1Department of Chemistry, University of King Abdulaziz,
P.O. Box 6171, Jeddah 21442, Saudi Arabia
2Department of Chemistry and Biochemistry, School of Life Sciences,
University of Sussex, Brighton BN1 9QJ, UK
Received 18 July 2010; Accepted 30 August 2010
Abstract:Aluminium and zinc complexes bearing the N,O-chelating Schiff base ligand 4,6-bis-1-(2-(dimethylamino)ethylimino)ethyl)benzene-
1,3-diol, (C6H2(OH)2(NCH2CH2NMe2)2) (1a), have been synthesized. Bimetallic aluminium and zinc alkyl complexes (2a - 4a) were
prepared by treatment of the hexadentate 1a with the appropriate amount of AlMe3, ZnMe2 and ZnEt2, respectively. 2a has been
characterized crystallographically, it lies on a crystallographic two-fold rotation axis and each aluminium centre adopts a five coordinate
geometry. Complex 2a was tested as a catalyst in the ring-opening polymerisation of ε-caprolactone. We describe here the synthesis
of two neutral ligands (1a and (C6H2(OH)2(C6H5NH2) (C=O(CH3)) (1b)) and demonstrate their application in the synthesis of molecular
aluminium and zinc derivatives.
Keywords: Aluminium methyl • Zinc alkyl • Schiff bases • DAR • Binuclear complex
© Versita Sp. z o.o.
generation of symmetrical Schiff bases which are either
di- or tetra-basic with two symmetrical sets of either O2N
1. Introduction
or N2O tridentate chelating sites [23-27]. Nevertheless,
Schiff base ligands are easily synthesized and form
dinuclear Al(III) and Zn(II) alkyl complexes with 1,3-
complexes with almost all metal ions, many of their
dihydroxybenzene bridging motifs are lacking.
complexes show high catalytic activity [1]. Monometallic
Here we have synthesized the new double and
Al complexes containing a series of phenoxy-imine
single-Schiff-base ligands 1a and 1b, respectively, as
ligands or NN Schiff base ligands have been reported
[2-11]. The monometallic pendant-arm single Schiff
shown in Scheme 1, starting from DAR by Schiff-base
condensation with two equivalents of the appropriate
base complexes of aluminium provide active centers
amine. The resulting ligand (1a) provides hexadentate
for ethylene polymerization due to the liability of the
pendant donor arm, thus allowing a pathway for
ethylene to approach the aluminium centre [6]. So far,
considerable attention has been paid to the synthesis,
[N2O] binding pockets which are bridged by a central
resorcinol moiety, while (1b) provides both tridentate
N2O and bidentate OO chelating sites.
structural determinations, and catalytic activity of metal
2. Experimental Procedure
complexes based on aluminium and zinc [12-15].
Among the reported catalysts, bimetallic complexes are
relatively few [16-19], and those reported for binuclear
zinc alkyl, aryl and aryloxide involved Cl, N and O
bridging ligands [20-22].
2.1. Chemicals and physical measurement
All manipulations were carried out in an atmosphere
of dry nitrogen using standard Schlenk techniques or
in an inert-atmosphere glovebox. Solvents were dried
from the appropriate drying agent, distilled, degassed
The
bifunctional
carbonyl
compound
4,6-
diacetylresorcinol (DAR) serves as precursor for the
* E-mail: wayfield8@yahoo.com.
1305

Simple generation of neutral bimetallic aluminium and
zinc alkyls Schiff bases bridged by a central resorcinol moiety
and stored over 4 Å sieves. Chemicals were purchased solution in hexanes) at 25ºC. The color of the solution
from Sigma-Aldrich Company Ltd. and diethyl zinc changed from dark yellow to light brown and an off-
was procured from Acros organics. The 1H and 13C white powder precipitated immediately. The exothermic
NMR spectra were recorded on a Varian 400 MHz reaction was left to stir for 2 h. Filtration and drying
spectrometer or a Varian 500 MHz spectrometer. The under vacuum afforded 3a as an off white powder, pure
1H and 13C NMR spectroscopy chemical shifts are given by 1H NMR spectroscopy. (Yield: 0.24 g, 81.7% ). Anal.
relative to residual solvent peaks.
Calc. for 3a C20H34N4O2Zn2: C, 48.69 ; H, 6.95 ; N, 11.36.
Found: C, 48.83; H, 7.04; N, 11.21. 1H NMR (399 MHz,
DCM) δ: -0.99 (6H, s, ZnCH3), 2.31(6H, s, C-CH3),
2.48(12H, s, N(CH3)2, 2.75(4H, t, 3J=6.0, CH2CH2),
3.60(4H, t, 3J=5.9, CH2CH2), 5.99(1H, s, Ar-H), 7.49(1H,
s, Ar-H). 13C NMR (399 MHz, DCM) δ: -18.23(ZnCH3),
2.2. Synthesis of the compounds
2.2.1. Synthesis of Schiff base ligand, 1a
The Schiff base ligand was prepared by the addition of
2-dimethylaminoethylamine (5.14 mmol, 0.56 mL) to a
colorless solution of (1,3-diacetyl-4,6-dihydroxybenzene
((resodiacetophenone, (RDP)) in the molar ratio 2:1
(0.5 g, 2.57 mmol) in ethanol (50 mL). The solution was
refluxed for 4 h, at which time the color had changed to
yellow. The volatile components were removed by rotary
evaporation to give a yellow oil. Washing with pentane
(3×10) mL resulted in the precipitation of a yellow
powder. The precipitate was collected by filtration,
washed with pentane then diethylether and finally dried
in vacuo overnight (yield: 0.55 g, 64.2%). Anal.Calc. for
C18H30N4O2: C, 64.64 ; H, 9.04 ; N, 16.75. Found: C,
64.01 ; H,8.81; N15.19, EI MS: m/z 334 (38%). 1H NMR
(500 MHz, DMSO) δ: 2.18(12H, s, N(CH3)2), 2.38(6H, s,
C-CH3), 3.31(4H, s, br, CH2CH2), 3.58(4H, s, CH2CH2),
5.73(1H, s, Ar-H), 7.79(1H, s, Ar-H), 17.14(2H, s, Ar-
OH). 13C NMR (500 MHz, DMSO) δ: 13.98(CCH3),
44.67(CH2CH2),45.21(N(CH3)2),58.47(CH2CH2),105.35
(C-H), 109.83 (C-CCH3), 132.70 (C-H)),172.53(C=N and
C-OH).
19.33(CCH3),
45.46(N(CH3)2),
47.10(CH2CH2),
58.74(CH2CH2), 112.12, 116.58, 125.9, 128.72, 129.53
and 136.46 (Ar-C), 173.19 (C=N).
2.2.4 Synthesis of 4a
To a stirred solution of 1a (0.2 g, 0.60 mmol) in toluene
(30 mL) was added drop-wise ZnEt2 (1.2 mL of 1 M
solution in hexane) at 25ºC. A white powder precipitated
immediately. The exothermic reaction was left to stir for
1 h. Filtration and drying under vacuum afforded 4a as
1
a white powder pure by H NMR spectroscopy. (Yield:
0.26 g, 85% ). Anal.Calc. C22H38N4O2Zn2: C, 50.68; H,
1
7.35; N, 10.75. Found: C, 50.57; H, 7.48; N, 10.70. H
NMR (500 MHz, DCM) δ: -1.91(4H, q, J=7.9, ZnCH2CH3),
-0.82(6H, t, ZnCH2CH3), 2.31(6H, s, C-CH3), 2.48(12H, s,
N(CH3)2, 2.75(4H, t, 3J=6.0, CH2CH2), 3.60(4H, t, 3J=5.9,
CH2CH2), 5.99(1H, s, Ar-H), 7.49(1H, s, Ar-H). 13C NMR
(500 MHz, DCM) δ: -2.86(ZnCH2CH3), 13.67(ZnCH2CH3)
19.53(CCH3),
45.78(N(CH3)2),
47.31(CH2CH2),
59.26(CH2CH2), 112,47, 116.95, 126.01, 128.93, 129.74
and 135.37 (Ar-C), 173.24 (C=N).
2.2.2. Synthesis of 2a
To a stirred solution of 1a (0.2g, 0.60 mmol) in toluene
(30 mL) was added drop wise AlMe3 (0.6 mL of a 2 M
solution in hexanes) at 25ºC. The color of the solution
changed from dark yellow to light yellow and an off
white powder precipitated immediately. The exothermic
reaction was left to stir for 1 h. Filtration and washing
with toluene (5 mL) afforded 2a as an off white powder,
pure by 1H NMR spectroscopy. The obtained solid
was dissolved in THF and crystallized at -20ºC to give
colorless crystals of 2a. (Yield: 0.25 g, 93.3%).Anal.Calc.
for C22H40Al2N4O2: C, 59.17 ; H, 9.03 ; N, 12.55. Found:
C, 59.50; H, 9.15; N, 12.31. EI MS: m/z 446 (32%).
1H NMR (500 MHz, DMSO) δ: -1.09 (12H, s, AlCH3),
2.13(12H, s, N(CH3)2, 2.36(6H, s, C-CH3), 2.72(4H, t,
3J=6.6, CH2CH2), 3.63(4H, t, 3J=6.5, CH2CH2), 5.62(1H,
s, Ar-H), 7.93(1H, s, Ar-H).
2.2.5 Synthesis of Schiff base 1b
The Schiff base ligand was prepared by the addition of
о-phenylenediamine (5.15 mmol, 0.56 g) to a colorless
solution of RDP in the molar ratio 2:1 (0.5 g, 2.57 mmol)
in ethanol (50 mL). The solution was refluxed for 5 h, at
which time the color changed to yellow. The precipitate
was collected by filtration and recrystallised from hot
toluene and finally dried in vacuo overnight (yield based
on RDP: 0.63 g, 86%). Anal.Calc. for C16H16N2O3: C,
67.59; H, 5.67; N, 9.85. Found: C, 67.67; H, 5.58; N,
159.86, EI MS: M+. m/z 284 (20%), M+.-Me m/z 269
(100%).1HNMR(500MHz,DMSO)δ:2.25(3H,s,C-CH3),
2.49(3H, s, C-CH3), 4.89(4H, s, NH2), 6.12(1H, s, Ar-
H(DAR)), 6.47-6.71(1H, s, Ar-H(о-phenylenediamine),
8.21(1H, s, Ar-H(DAR)), 12.69 (1H, s, Ar-OH), 16.11(1H,
s, Ar-OH). 13C NMR (500 MHz, DMSO) δ: 16.96, 27.17
(CCH3), 112.80-141.04 (Ar-C), 165.88, 172.00 and
174.01(C=N and C-OH), 203.08 (C=O).
2.2.3 Synthesis of 3a
To a stirred solution of 1a (0.2 g, 0.60 mmol) in toluene
(30 mL) was added drop wise ZnMe2 (0.6 mL of a 2 M
1306

E. S. Aazam, M. P. Coles
2.2.6 Synthesis of 3b
2.4. Crystal structure determination
To a stirred solution of 1b (0.1 g, 0.35 mmol) in toluene
(30 mL) was added drop-wise ZnMe2 (0.35 mL of a
2 M solution in hexanes) at 25ºC. The color of solution
changed from dark yellow to light brown and an off
white powder precipitated immediately. The exothermic
reaction was left to stir for 2 h. Filtration and drying
under vacuum afforded 3b as an off-white powder, pure
Single crystals of 2a were grown by crystallization
at -20ºC from THF. The data for the complex were
collected at 173(2) K. Data collection: COLLECT [31];
cell refinement: HKL DENZO and SCALEPACK [32];
data reduction: HKL SCALEPACK and DENZO [32];
program(s) used to solve structure: SHELXS97 [33];
program(s) used to refine structure: SHELXL97 [33];
molecular graphics: ORTEP-3 for Windows) [34];
software used to prepare material for publication:
WinGX [35]. CCDC 784404 contains the supplementary
crystallographic data for 2a. These data can be obtained
free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
1
by H NMR spectroscopy. (Yield: 0.12 g, 75.0%). Anal.
Calc. for 3b C18H20N2O3Zn2: C, 48.79; H, 4.55; N, 6.32.
1
Found: C, 48.84; H, 4.57; N, 6.37. H NMR (399 MHz,
DCM) δ: -1.27 (3H, s, ZnCH3), -1 (3H, s, ZnCH3), 2.19
(3H, s, C-CH3), 2.28 (3H, s, C-CH3), 4.54 (2H, s, NH2),
5.65 (1H, s, Ar-H(DAR)), 6.60-6.23 (4H, m, Ar-H(о-
phenylenediamine), 8.16 (1H, s, Ar-H(DAR)).
2.3. Typical polymerization procedure [22]
A solution of aluminium complex 2a in toluene was
injected to a solution of ε-caprolactone (ε-CL) in toluene
kept at 60ºC. The concentration of ε-CL was 1 M. The
molar ratio of [ε-CL] / [2a] = 300:1. 1 mLof polymerization
aliquots were withdrawn at appropriate time intervals
under the protection of nitrogen and quenched with
methanol. After removal of the volatiles, the residue was
subjected to 1H NMR analysis. Monomer conversion was
determined by observing the integration of monomer vs.
3. Results and Discussion
The
Schiff
base
ligand
4,6-bis-1-(2-
(dimethylamino)ethylimino)ethyl)benzene-1,3-diol
(C6H2(OH)2(NCH2CH2NMe2)2), 1a, was accessed
in moderate yield (> 64%) via standard imine
condensation reaction between 4,6-diacetyl resorcinol
with two equivalents of 2-dimethylaminoethylamine in
a ratio 1:2 to give the symmetrical bis-azomethine 1a,
1
obtained as yellow powder. The H NMR spectrum of
the ligand exhibited the disappearance of the phenolic
DAR peak at δ 12.50 ppm and the appearance of a new
phenolic peak down field at δ 17.14 ppm, the formation
1
polymer methylene resonance in the H NMR (CDCl3,
500 MHz) spectrum.
NMe2
N
N
Me2N
Zn
Zn
O
O
3a
1a
(iii)
(iv)
(ii)
NMe2
N
NMe2
N
N
N
N
Me2N
N
Me2N
Zn
N
N
Al
Zn
Al
O
O
O
O
HO
OH
OH
4a
2a
(i)
O
O
HO
(v)
O
N
NH2
HO
OH
1b
(vi) X
(vii)
X
N
N
O
O
NH2
NH2
Zn
Al
Zn
Al
O
O
O
O
2b
3b
Scheme 1.
Synthesis of 1a and 1b and their metal complexes. (i) 2 NH2CH2CH2NMe2, reflux 4 h, EtOH; (ii) 2 AlMe3, toluene; (iii) 2 ZnMe2,
toluene; (iv) 2 Zn Et2, Toluene; (v) 2 o-phenylenediamine, reflux 5 h, EtOH; (vi) 2 AlMe3, toluene; (vii) 2 ZnMe2, toluene
1307

Simple generation of neutral bimetallic aluminium and
zinc alkyls Schiff bases bridged by a central resorcinol moiety
Table 1. Crystal data, bond lengths [Å] and angles [°] for 2a
Crystal data
Empirical formula
C22 H40 Al2 N4 O2
F(000)
Crystal size
Theta range for data collection
Index ranges
968
3
Formula weight
Temperature
Wavelength
446.54
173(2) K
0.71073 Å
0.42 x 0.20 x 0.20 mm
3.50 to 27.09°.
-35<=h<=35, -9<=k<=9,
-15<=l<=15
Crystal system
Space group
Unit cell dimensions
a = 27.7413(7) Å
b = 7.5898(2) Å
c = 12.0513(3) Å
Volume
Monoclinic
Reflections collected
17968
C2/c (No.15)
Independent reflections
Reflections with I>2sigma(I)
Completeness to theta = 27.09°
Refinement method
2700 [R(int) = 0.053]
2480
98.2%
α= 90°.
2
β= 101.621(2)°.
γ = 90°.
Full-matrix least-squares on F
2700 / 0 / 140
Data / restraints / parameters
3
2
2485.40(11) Å
Goodness-of-fit on F
1.066
Z
4
Final R indices [I>2sigma(I)]
R1 = 0.040, wR2 = 0.109
-3
Density (calculated)
Absorption coefficient
1.19 Mg m
R indices (all data)
R1 = 0.044, wR2 = 0.113
-1
-3
0.14 mm
Largest diff. peak and hole
0.26 and -0.34 e.Å
Bond distances/Å
Bond angles/°
Al-O
1.8376(10)
O-Al-C(12)
100.06(6)
95.83(6)
117.44(7)
88.63(5)
111.38(6)
129.21(7)
162.29(5)
93.04(6)
88.55(6)
Al-C(12)
Al-C(11)
Al-N(1)
Al-N(2)
O-C(2)
1.9780(15)
1.9959(16)
2.0182(12)
2.3138(13)
1.3029(15)
1.3082(16)
1.4851(16)
O-Al-C(11)
C(12)-Al-C(11)
O-Al-N(1)
C(12)-Al-N(1)
C(11)-Al-N(1)
O-Al-N(2)
C(12)-Al-N(2)
C(11)-Al-N(2)
N(1)-C(5)
N(1)-C(7)
of azomethine methyl protons at δ 2.38 ppm instead Å] being substantially longer than that of the imino one
of the acetyl methyl protons at δ 2.60 ppm and the N1-Al [2.0182(12) Å], The azomethine C=N bond retains
appearance of two non-equivalent ethyl protons of the its double bond character [N1-C5, 1.30852(16) Å].
dimethylaminoethylamine moiety indicate the formation These bonds, however, are comparable to the related
of 1a, this was further supported by 13C NMR spectrum, single Schiff base chelate [(3,5-But2-2-(O)C6H2CHNNL)
elemental analysis and EI-MS.
AlMe2] [L = CH2CH2NMe2] [6]. The geometry of the five-
To demonstrate that the 1a compound described coordinate Al(III) centre by quantative method (τ = 0.55)
above can be used in the synthesis of coordination cannot be reasonably defined as either square pyramidal
complexes, a brief investigation into the application (sqp) or trigonal bipyramidal (tbp). For example the
of 1a as a ligand precursor in aluminium and zinc O-Al-N2 (axial) angles, expected to be 180° for tbp is
chemistry was conducted. The reaction between the rather narrow, 162.29(5)°. Similar deviations are found
neutral 1a with alkyl-aluminium and -zinc compounds in the structure of previously reported five-coordinate,
in toluene proceeded via protonolysis of the phenolic chelated aluminium complexes [28].
oxygen and an alkane elimination leads to the
The Al–C12 , Al-C11 and Al-O bond distances
formation of the complexes in good yields to afford the [1.9780(15) ,1.9959(16) and 1.8376(10) Å], are longer
bis(aluminium methyl), bis(zinc methyl and ethyl) Al(L) than those recently reported for the four coordinate
Me2 (2a), Zn(L)Me2 (3a) and Zn(L)Et2 (4a) respectively, AlMe2 complex [Me2Al[O-2,4-But2-6-{(2,6-Pri2C6H3)
[L = C6H2(O)2(NCH2CH2NMe2)2] (Scheme 1).
N=CH}C6H2] [Al–C: 1.955(2) and 1.949(3) Å; Al-O:
The mass spectrum and elemental analysis of 2a 1.7688(17) Å] and may indicate weaker coordination,
were consistent with two ‘AlMe2’ units attached to the however, the Al-O bond length is within the range
Schiff base ligand. The 1H NMR spectrum of 2a shows a suggested for σ-bond coordination [5]. The multinuclear
sharp singlet at -1.09 ppm due to the Al-Me protons as NMR spectra are consistent with the solid-state structure
predicted for a symmetrically substituted Al-centre.
being maintained in solution.
Crystals of 2a, suitable for X-ray structure
The binuclear symmetrical zinc alkyl complexes 3a
determination, were grown from THF at -20ºC. The and 4a were formulated on the basis of H, 13C NMR
molecular structure is shown in Fig. 1, Table 1. spectroscopyandelementalanalysis.Asingleresonance
Selected bond lengths and angles are given in Table 1. for the methyl and ethyl protons were observed in the 1H
Deprotonated 1a is seen to act as a hexadentate ligand, NMR spectra at δ -0.99 ppm for ZnCH3 in 3a and δ -1.91
binding to each aluminium atom via the phenolic oxygen and -0.82 ppm for ZnCH2CH3 in 4a, which was also
and the imino and amino nitrogen atoms. Monomeric supported by a single resonance signal in the 13C NMR
molecules of 2a lie on a crystallographic two-fold rotation spectra. Further evidence for a symmetrical solution-
axis. The amino nitrogen N2-Al bond length [2.3138(13) state geometry is implied by the single azomethine
1
1308

E. S. Aazam, M. P. Coles
analysis. 3b represents a rare example of a mixed
four and three-coordinate monomeric zinc(II) methyl
complex [29]; however, attempts to grow single crystals
suitable for X-ray analysis were unsuccessful. The
isolated complexes were stable at ambient temperature,
air and moisture-sensitive powders, poorly soluble in
aliphatic hydrocarbons (pentane, hexane) and aromatic
hydrocarbons (benzene, toluene), and very soluble in
dichloromethane, DMSO and THF.
2a is moderately reactive in the polymerization of
ε-caprolactone (ε-CL). The polymerization took place
rapidly (Al/ε-CL ) 1/300, (temp, 60 °C), and the great
increase in viscosity of the solution was observed after
Figure 1. Molecular structure of [Al2(C6H2(O)2(NCH2CH2NMe2)2)
(Me2)2], 2a
1
1 h with 100% conversion after 4 h based on H NMR
methyl resonances of the compounds at δ 2.31 ppm
for both 3a and 4a in the H NMR spectra. According
spectroscopy.
1
to these data, the Zn(II) ion in both complexes are
four-coordinate and may adopt a distorted tetrahedral
geometry.
4. Conclusion
In conclusion, we have found a simple and relatively
inexpensive method for the generation of alkyl-
aluminium and -zinc neutral species, based on readily
available 4,6-diacetylresorcinol core. The resulting Al
methyl species appeared to be efficient initiators in the
polymerization of ε-caprolactone. It has been known
that the activity toward ring-opening polymerization of
L-lactide is Mg>Zn>Al [30], so it might be predicted that
3a and 4a will be more efficient initiators.
The structure of 1b was confirmed by NMR
spectroscopy, which confirmed the isolation of an
unsymmetrical single Schiff base as the first major
insoluble product isolated from ethanol. Further
confirmation of ligand structure was supported by
microanalysis and EI-MS. Attempts to isolate Al
complex 2b were unsuccessful, however the Zn
methyl was isolated in good yield. The reaction of
dimethyl zinc with 1b in a molar ratio of 2:1 leads to
the expected methyl zinc complex 3b, which can be
stabilized without donor ligands. For 3b, two methyl
resonances were observed at -1.27 and -1.00 ppm and
Acknowledgements
a down-field shift in the NH2 resonance from δ 4.89 to The author acknowledges King Abdulaziz University for
4.54 ppm indicating its coordination to one of the Zn its support and Dr Robin Fulton for providing laboratory
atoms having a distorted tetrahedral geometry, while facilities.
the other Zn atom may adopt a trigonal geometry. The
structure of 3b was further supported by elemental
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