Simple generation of cationic aluminum alkyls and alkoxides based on the pendant arm tridentate Schiff base

Article abstract of DOI:10.1021/ic049337i

The prepared in situ methyl(chloro)aluminum complex (2) from Me ₂-AlCl and the pendant arm tridentate Schiff base (H-SChNMe ₂) was used to generate the methylaluminum cationic species [(SChNMe₂)AlMe]⁺ in further reaction with 1 equiv of AlCl₃ or NaBPh₄ as the chloride abstracting reagents. The exposure of the resulting methylaluminum cationic species to an excess of dry dioxygen at 0 °C afforded the alkoxyaluminum cationic species, [(SChNMe ₂)AlOMe]⁺ or [(SChNMe₂)AlOPh]⁺. The alkoxylaluminum cations proved to be a very efficient catalyst in the polymerization of ε-caprolactone.

Full text of DOI:10.1021/ic049337i

Inorg. Chem. 2004, 43, 5789−5791
Simple Generation of Cationic Aluminum Alkyls and Alkoxides Based
on the Pendant Arm Tridentate Schiff Base
,†
Janusz Lewin´ski,* Paweł Horeglad,† Maciej Dranka,† and Iwona Justyniak‡
Department of Chemistry, Warsaw UniVersity of Technology, Noakowskiego 3,
00-664 Warsaw, Poland, and Institute of Physical Chemistry, Polish Academy of Science,
Kasprzaka 44/52, 01-224 Warsaw, Poland
Received May 21, 2004
The prepared in situ methyl(chloro)aluminum complex (2) from Me -
degree in the polymerization of ethylene,1a,b,f-h,2b,3,4a,5 methyl
2
methacrylate,1a,g,i or heterocyclic monomers.1g,i,2b Neverthe-
less, knowledge about the reactivity and potential application
of these species is very limited, and undoubtedly, the
chemistry of well-defined cationic aluminum alkyls is just
emerging.6 On the other hand, aluminum alkoxides have
attracted much attention on account of their rich structural
and bonding features and potential applications. However,
cationic aluminum species with a terminal alkoxide or
aryloxide group are lacking, and only very recently, Jordan
reported the synthesis and molecular structure of the first
example of cationic dinuclear species with the bridging
alkoxide group, {(iPr2-ATI)Al(µ-OiPr)}22+ (ATI ) N,N-diiso-
propylamino-troponiminate ligand).1d Herein we describe a
simple method for the synthesis of alkylaluminum and alk-
oxyaluminum cationic species employing inexpensive re-
agents as well as demonstrate that the resulting alkoxyl-
aluminum cations are very efficient catalyst in the polymeriza-
tion of ꢀ-caprolactone.
AlCl and the pendant arm tridentate Schiff base (H−SchNMe2)
was used to generate the methylaluminum cationic species
[(SchNMe2)AlMe]+ in further reaction with 1 equiv of AlCl3 or
NaBPh4 as the chloride abstracting reagents. The exposure of the
resulting methylaluminum cationic species to an excess of dry
dioxygen at 0 °C afforded the alkoxyaluminum cationic species,
[(SchNMe2)AlOMe]+ or [(SchNMe2)AlOPh]+. The alkoxylaluminum
cations proved to be a very efficient catalyst in the polymerization
of ꢀ-caprolactone.
Cationic aluminum complexes have been very intensively
investigated over the past few years with the main aim of
developing catalysts for the polymerization of olefins.1-3
A
range of neutral dialkylaluminum chelate complexes contain-
ing N,N-1 and O,N-bidentate2 as well as N,N,N-3 and O,N,N-
tridentate4 spectator ligands have been used to generate three-
or four-coordinate alkylaluminum cationic complexes upon
further reaction with [CPh3][BPh4], [HNMe2Ph], [B(C6F5)4],
or B(C6F5)3 as the alkyl abstracting reagents. These various
cationic aluminum alkyls were found to be active to a certain
Treatment of Me2AlCl with 1 equiv of pendant arm
tridentate Schiff base 1 (H-SchNMe2) at low temperature
afforded the putative methyl(chloro)aluminum complex 2
(Scheme 1). The prepared in situ 2 was used to generate the
methylaluminum cationic species in further reaction with 1
equiv of AlCl3 or NaBPh4 as the chloride abstracting
reagent.7,8 The reaction with AlCl3 proceeded smoothly at
* To whom correspondence should be addressed. E-mail: lewin@
ch.pw.edu.pl.
† Warsaw University of Technology.
‡ Polish Academy of Science.
(1) (a) Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 1997, 119, 8125.
(b) Ihara, E.; Young, Jr., V. G.; Jordan, R. F. J. Am. Chem. Soc. 1998,
120, 8277. (c) Cosle´dan, F.; Hitchcock, P. B.; Lappert, M. F. Chem.
Commun. 1999, 705. (d) Korolev, A. V.; Guzei, I. A.; Jordan, R. F.
J. Am. Chem. Soc. 1999, 121, 11605. (e) Radzewich, C. E.; Guzei, I.
A.; Jordan, R. F. J. Am. Chem. Soc. 1999, 121, 8673. (f) Dagorne, S.;
Guzei, I. A.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 2000,
122, 274. (g) Korolev, A. V.; Ihara, E.; Guzei, I. A.; Young, Jr., V.
G.; Jordan, R. F. J. Am. Chem. Soc. 2001, 123, 8291. (h) Pappalardo,
D.; Tedesco, C.; Pellecchia, C. Eur. J. Inorg. Chem. 2002, 621. (i)
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(2) (a) Cameron, P. A.; Gibson, V. C.; Redshaw, C.; Segal, J. A.; White,
A. J. P.; Williams, D. J. J. Chem. Soc., Dalton Trans. 2001, 1472. (b)
Dagorne, S.; Lavanant, L.; Welter, R.; Chassenieux, C.; Haquette, P.;
Jaouen, G. Organometallics 2003, 18, 3732.
(3) Bruce, M.; Gibson, V. C.; Redshaw, C.; Solan, G. A.; White, A. J.
P.; Williams, D. J. Chem. Commun. 1998, 2523.
(4) (a) Cameron, P. A.; Gibson, V. C.; Redshaw, C.; Segal, J. A.; Bruce,
M. D.; White, A. J. P.; Williams, D. J. Chem. Commun. 1999, 1883.
(b) Robson, D. A.; Rees, L. H.; Mountford, P.; Schro¨der, M. Chem.
Commun. 2000, 1269. (c) Robson, D. A.; Bylikin, S. Y.; Cantuel, M.;
Male, N. A. H.; Rees, L. H.; Mountford, P.; Schro¨der, M. J. Chem.
Soc., Dalton Trans. 2001, 157. (d) Cameron, P. A.; Gibson, V. C.;
Redshaw, C.; Segal, J. A.; White, A. J. P.; Williams, D. J. J. Chem.
Soc., Dalton Trans. 2002, 415.
(5) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283.
(6) For the recent review concerning inorganic cationic aluminum
complexes see: Atwood, D. A. Coord. Chem. ReV. 1998, 176, 407.
(7) It is worth noting that, for example, attempts of No¨th and co-workers
to prepare the low-coordinate aluminum cations by reaction of the
aluminum halide adducts with NaBPh4 were unsuccessful and resulted
in phenylation products: Krossing, I.; No¨th, H.; Schwenk-Kircher,
H. Eur. J. Inorg. Chem. 1998, 927.
(8) The chloride was abstracted from higher-coordinate salen-type (N2O2)-
AlCl complexes to form higher coordinate cationic aluminum com-
plexes; see for example ref 6.
10.1021/ic049337i CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/17/2004
Inorganic Chemistry, Vol. 43, No. 19, 2004 5789

COMMUNICATION
Scheme 1
regarded as one of the most promising methods for the
synthesis of metal alkoxides. Thus, we examined the interac-
tion of the methylaluminum cations with dioxygen. The
exposure of the methylene chloride solution of 4 and 5 to
an excess of dry O2 at 0 °C for several hours affords a
mixture of alkoxyaluminum cationic species, [(SchNMe2)-
AlOMe]+ (6) and [(SchNMe2)AlOPh]+ (7) (Scheme 2). The
1H NMR spectrum of the postreaction mixture reveals the
presence of a methyl group bound to oxygen and the lack
of the Al-Me group, which implies complete oxygenation
of the Al-C bond. The 27Al NMR spectrum contains a single
resonance at 35 ppm, which indicates the presence of a five-
coordinate aluminum center.9 The latter result suggests that
the [(SchNMe2)AlOR]+ species might adopt a weakly
associated dimeric structure with bridging alkoxide groups.
The 11B NMR spectrum of the postreaction mixture shows
ambient temperature, while that with NaBPh4 required
prolonged reaction time. The resulting ionic compounds
[(SchNMe2)AlMe][AlCl4] (3) and [(SchNMe2)AlMe][BPh4]
(4) were characterized by multinuclear NMR spectroscopy.
The 1H NMR spectrum of 3 shows a sharp singlet at -0.82
ppm due to the Al-Me protons. The ionic character of salt
3 is evidenced by the 27Al NMR spectrum, which shows two
signals at 102 ppm (w1/2 ) 60 Hz) and 97 ppm (w1/2 ) 100
Hz). The lower field sharp signal is characteristic for the
AlCl4- anion,9 which nicely confirms the abstraction of the
Cl- anion from 2. The second resonace was assigned to the
[(SchNMe2)AlMe]+ cation; however, the observed chemical
shift is not univocally indicative for the metal center
coordination number, and does not allow us to distinguish
between the monomeric or higher coordinate cationic species.
We note that the tendency of methylaluminum cations
supported by the O,N-bidentate Schiff base to form the
dimeric species was recently nicely demonstrated by Dagorne
-
-
two resonances characteristic for BPh4 and BMePh3
anions, which confirms the presence of an equilibrium
between 4 and 5 in solution (Scheme 2). We were not able
to obtain crystals of 6 or 7 suitable for X-ray crystallography
using aromatic solvents. However, the corresponding crystal-
line material was obtained from a THF solution, and the
X-ray analysis revealed a new salt, [(SchNMe2)Al(OPh)-
(THF)2][BPh4] (8) (Figure 1).11
The six-coordinate aluminum center in the [(SchNMe2)-
Al(OPh)(THF)2]+ cation is stabilized by the tridentate Schiff
base, occupying equatorial positions along with the phenoxy
group, and two THF molecules in axial positions. The Al-O
distances associated with THF are long at 2.024(2) and
2.062(2) Å. The terminal aryloxide Al-O(1) distance of
1.760(2) Å is short and is only marginally longer than the
corresponding distances previously reported for neutral,
monomeric, four-coordinate, sterically crowded aluminum
aryloxides.12 The aryloxide Al-O(2) distance (1.813(2) Å)
associated with the Schiff base ligand is slightly longer and
is controlled by the π-interaction of the aryloxide oxygen
lone pair with the salicylideneimine extended π-system which
substantially weakens the Lewis basicity of the oxygen atom
and simultaneously strengthens the basicity of the chelating
et al.2b In the case of the reaction involving NaBPh4, the 11
B
NMR spectrum of the postreaction mixture revealed the
presence of two sharp signals at -6.6 ppm (w1/2 ) 20 Hz)
and at -11.7 ppm (w1/2 ) 37 Hz) corresponding to the BPh4-
and BMePh3 anions, respectively.9 However, the corre-
-
sponding 1H NMR spectrum is relatively complex in contrast
to that of 3, and several signals in the region characteristic
for AlsMe protons, two broad signals of the BsMe protons,
and two well resolved singlets of the CHdN proton could
be distinguished. The above observations imply the presence
of two cationic species, [(SchNMe2)AlMe]+ and [(SchNMe2)-
AlPh]+ (Scheme 2), which was further confirmed by the
Scheme 2
(10) Lewin´ski, J.; Zachara, J.; Grabska, E. J. Am. Chem. Soc. 1996, 118,
6794. Lewin´ski, J.; Zachara, J.; Kopec´, T.; Madura, I.; Prowotorow,
I. Inorg. Chem. Commun. 1999, 131. Lewin´ski, J.; Zachara, J.; Gos´,
P.; Grabska, E.; Kopec´, T.; Madura, I.; Marciniak, W.; Prowotorow,
I. Chem. Eur. J. 2000, 6, 3215. Lewin´ski, J.; Ochal, Z.; Bojarski, E.;
Tratkiewicz, E.; Justyniak, I.; Lipkowski, J. Angew. Chem., Int. Ed.
2003, 42, 4643. Lewin´ski, J.; Marciniak, W.; Justyniak, I.; Lipkowski,
J. J. Am. Chem. Soc. 2003, 125, 12698.
(11) Crystal data for [(SchNMe2)Al(OPh)(THF)2][BPh4] (8), C49H56-
Al1B1N2O4 C4H8O: triclinic, space group P1h (No. 2), a ) 12.2830-
(2) Å, b ) 12.6830(2) Å, c ) 15.1880(3) Å, R ) 87.6130(10)°, â )
68.9840(10)°, γ ) 88.8800(10)°, U ) 2206.73(7) Å3, Z ) 2, F(000)
) 466, Dc ) 0.655 g m3, R1 ) 0.0445, wR2 ) 0.0853 for 3695
reflections with Io > 2σ(Io). The structure was solved by direct methods
using the SHELXS97 program (Sheldrick, G. M. SHELXS97, Program
for Crystal Structure Analysis; University of Go¨ttingen: Go¨ttingen,
Germany, 1997) and was refined by full-matrix least-squares on F2
using the program SHELXL97 (Sheldrick, G. M. SHELXL97, Program
for Crystal Structure Analysis; University of Go¨ttingen: Go¨ttingen,
Germany, 1997). H-atoms were included in idealized positions and
oxygenation and polymerization reactions (vide infra).
Furthermore, a complex character of the 1H NMR spectrum
may also suggest the presence of higher aggregates in
solution (vide supra). Unfortunately, difficulties in isolating
the alkylaluminum ionic species precluded their structural
characterization by X-ray crystallography.
There has been our continued interest in the controlled
oxygenation of main group metal alkyls,10 which may be
refined isotropically. The GOF on F2 was equal to 1.080. A weighting
scheme w ) [σ2(Fo2 + (0.0418P)2 + 3.1964P]-1, where P ) (Fo
+
2
2Fc2)/3, was used in the final stage of refinment. The residual electron
(9) Lewin´ski, J. In Encyclopedia of Spectroscopy and Spectrometry;
Lindon, J. C., Tranter, G. E., Holmes, J. L., Eds.; Academic Press:
New York, 1999; Vol. 1, pp 691-703.
density ) 0.25/-0.26 e Å-3
.
(12) Healy, M. D.; Power, M. B.; Barron, A. R. Coord. Chem. ReV. 1994,
130, 63.
5790 Inorganic Chemistry, Vol. 43, No. 19, 2004

COMMUNICATION
Figure 2. MALDI TOF spectrum of polycaprolactone obtained with the
mixture of 6 and 7 as initiators.
Figure 1. Molecular structure of the [(Sch-NMe2)Al(OPh)(THF)2]+ cation
as 30% elipsoids; BPh4 anion is omitted for clearity.
-
AlOMe][AlCl4] (9) (the product resulting from the oxygen-
ation of 3) was used as catalyst for polymerization of ꢀ-CL,
the MALDI TOF spectrum of the resulting P-ꢀ-CL revealed
one set of signals with monomodal weight distribution
(Mw/Mn, 1.24) and the end-group of 32 Da corresponding to
a hydroxy and a methyl ester group (Figure 1S, Supporting
Information). These data also demonstrate that only the
terminal alkoxide or aryloxide group is involved in the
polymerization process and the aryloxide associated with the
Schiff base ligand acts as a spectator functionality, which is
consistent with the already mentioned weakening of the
Lewis basicity of this aryloxide oxygen. The above observa-
tions additionally confirm the presence of an equilibrium
between 4 and 5 (Scheme 2), as well as strongly indicate
that both the Al-Me and Al-Ph bonds undergo oxidation
to the Al-OMe and Al-OPh units, which are able to initiate
the polymerization of ꢀ-CL.
In conclusion, we have found a simple and relatively
inexpensive method for the generation of alkylaluminum and
alkoxyaluminum cationic species, based on readily available
organoaluminum Shiff base complexes and effective anion
abstractors such as AlCl3 or NaBPh4. The resulting alkoxide
cationic species appeared to be very efficient initiators in
the polymerization of ꢀ-caprolactone.
CdN group.13 The latter fact is well expressed by a marked
difference between the two Al-N bonds, with that to the
imine nitrogen being significantly shorter (1.985(2) Å) than
that to the amine nitrogen (2.122(2) Å). The multinuclear
NMR spectra are consistent with the solid-state structure
being maintained in solution (Supporting Information).
According to our knowledge, compound 8 comprises the first
structurally authenticated mononuclear cationic aluminum
species with a terminal alkoxide group. While we have not
succeeded in the structure characterization of “base-free” 6
and 7, the molecular structure of 8 is not less interesting
than the former compounds as it mimics a potential
intermediate in the ring-opening polymerization of hetero-
cyclic monomers.
Indeed, compounds 6 and 7, resulting from the oxygen-
ation of a mixtrure of 4 and 5, appeared to be highly reactive
in the polymerization of ꢀ-caprolactone (ꢀ-CL). The polym-
erization took place rapidly (Al/ꢀ-CL ) 1/50, temp ) 40
°C), and the great increase in viscosity of the solution was
observed after 15 min with 95% conversion after 1 h. For
example, the MALDI TOF spectrum of P-ꢀ-CL obtained with
generated in situ cationic alkoxides 6 and 7 as initiators
shows two sets of signals, each one representing the
monomodal weight distribution with 114 Da mass difference
(Mw/Mn, 1.30) (Figure 2). The masses of residual groups were
found to be 32 and 94 Da, which correspond to a hydroxy
group and methyl ester and phenyl ester end-groups,
respectively. Interestingly, when the putative salt [(SchNMe2)-
Acknowledgment. This work was supported by Warsaw
University of Technology and the State Committee for
Scientific Research (3 TO9A 066 19).
Supporting Information Available: Text giving synthetic
procedures and characterization of 2-9, MALDI-TOF spectrum
of polycaprolactone obtained with 9 as catalyst, and X-ray
crystallographic files for 8 in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
(13) For an extended discussion on the role of the π-interaction within the
salicylideneimine extended π-system on the structure of the group 13
organometallic complexes, see: Lewin´ski, J.; Zachara, J.; Starowieyski,
K. B.; Ochal, Z.; Justyniak, I.; Kopec´, T.; Stolarzewicz, P.; Dranka,
M. Organometallics 2003, 22, 3773.
IC049337I
Inorganic Chemistry, Vol. 43, No. 19, 2004 5791