Synthesis of Al complexes containing phenoxy-imine ligands and their use as the catalyst precursors for efficient living ring-opening polymerisation of ε-caprolactone

Article abstract of DOI:10.1039/b801561a

Al complexes containing phenoxy-imine ligands of type, Me ₂Al[O-2-R¹-6-(R²NiCH)C₆H ₃] [R¹ = Me, R² = 2,6-ⁱPr ₂C₆H₃ (1a), ^tBu (1b); R¹ = ^tBu, R² = 2,6-ⁱPr₂C ₆H₃ (2a), ^tBu (2b), cyclohexyl (2c), adamantyl (2d), C₆H₅ (2e), 2,6-Me₂C₆H ₃ (2f), C₆F₅ (2g)] have been prepared in high yields from AlMe₃ by treating with 1.0 equiv. of 2-R ¹-6-(R²NiCH)C₆H₃OH in n-hexane. Structures for 1a, 1b, 2a-e and 2g were determined by X-ray crystallography, and these complexes have a distorted tetrahedral geometry around Al; both the Al-O and the Al-N bond distances were influenced by substituents in both the aryloxo and the imino groups. Me₂Al[μ₂-O-2-(R ²NiCH)C₆H₄](AlMe₃) [R² = 2,6-ⁱPr₂C₆H₃ (3a), ^tBu (3b)] were prepared exclusively by reaction of AlMe₃ with 2-(R ²NiCH)C₆H₄OH, and these complexes form a distorted tetrahedral geometry around each Al centre with additional AlMe ₃ coordinating to the oxygen in the phenoxy-imine ligand. Complexes 1a, 1b and 2a-g were tested as catalyst precursors for ring-opening polymerisation (ROP) of ε-caprolactone (CL) in the presence of ⁿBuOH (1.0 equiv. to Al), and their catalytic activities were strongly influenced by the imino substituent (R²). The efficient ROP has been achieved using the C₆F₅ analogue (2g), with the ROP taking place in a living manner.

Full text of DOI:10.1039/b801561a

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis of Al complexes containing phenoxy-imine ligands and their use as
the catalyst precursors for efficient living ring-opening polymerisation of
e-caprolactone†
Jingyu Liu,a Naruhito Iwasaa,b and Kotohiro Nomura*a
Received 28th January 2008, Accepted 6th May 2008
First published as an Advance Article on the web 20th June 2008
DOI: 10.1039/b801561a
1
2
1
=
Al complexes containing phenoxy-imine ligands of type, Me2Al[O-2-R -6-(R N CH)C6H3] [R = Me,
R2 = 2,6-iPr2C6H3 (1a), tBu (1b); R1 = Bu, R2 = 2,6-iPr2C6H3 (2a), tBu (2b), cyclohexyl (2c), adamantyl
t
(2d), C6H5 (2e), 2,6-Me2C6H3 (2f), C6F5 (2g)] have been prepared in high yields from AlMe3 by treating
1
2
=
with 1.0 equiv. of 2-R -6-(R N CH)C6H3OH in n-hexane. Structures for 1a, 1b, 2a–e and 2g were
determined by X-ray crystallography, and these complexes have a distorted tetrahedral geometry
around Al; both the Al–O and the Al–N bond distances were influenced by substituents in both the
2
2
i
t
=
aryloxo and the imino groups. Me2Al[l2-O-2-(R N CH)C6H4](AlMe3) [R = 2,6- Pr2C6H3 (3a), Bu
2
=
(3b)] were prepared exclusively by reaction of AlMe3 with 2-(R N CH)C6H4OH, and these complexes
form a distorted tetrahedral geometry around each Al centre with additional AlMe3 coordinating to the
oxygen in the phenoxy-imine ligand. Complexes 1a, 1b and 2a–g were tested as catalyst precursors for
ring-opening polymerisation (ROP) of e-caprolactone (CL) in the presence of nBuOH (1.0 equiv. to Al),
and their catalytic activities were strongly influenced by the imino substituent (R2). The efficient ROP
has been achieved using the C6F5 analogue (2g), with the ROP taking place in a living manner.
1
2
1
2
=
2,4-R -6-{(2,4,6-R C6H2)N CH}C6H2OH [R , R = alkyl, aryl,
Introduction
2
3
halogen], in the presence of PhCH2OH showed moderate catalytic
activity for ROP of e-caprolactone (CL) in a living manner.6j
This is not only because no detailed mechanistic studies for the
catalytically active species including isolation of the Al complexes
have been reported,9 but also because it was reported that an
electronic effect of the imino substituent (aromatic or aliphatic
etc.) was not important; the fact seemed to be related to a
description that the electrophilicity of the metal centre is much
less important for the ROP of lactones.6g,i In contrast, it has been
known that the catalytic activity, molecular weight for the resultant
polymers, and the polymerisation behaviour for ethylene (and/or
propylene) polymerisation using a series of zirconium complexes
containing bis(phenoxy)imine ligands were highly affected by
substituents on both the phenoxy and the imino groups.10,11
We recently communicated that the rapid ROP of e-
caprolactone (CL) could be achieved using Me2Al[O-2-tBu-6-
Organoaluminium compounds are known to be important
reagents widely employed in organic synthesis,1 olefin polymeri-
sation catalysis,2 and as initiators for ring-opening polymerisation
(ROP) of cyclic esters, such as lactones and lactides.3 It has
been observed that the introduction of extremely bulky aryloxide
ligands to aluminium generates monomeric Lewis acid catalysts
that have been employed in a variety of organic transformations.4
Moreover, Al alkoxides have been known to polymerise cyclic
esters3 such as lactides,5 lactones,5j,5n,6 and certain aliphatic
polyesters possessing promising characteristics as biodegradable
and bioassimilable materials, not only due to their practical
biodegradability in light of recent concerns with the environ-
ment, but also due to their biocompatibility for medical and
pharmaceutical applications.7 Therefore, considerable attention
has been paid to the synthesis and structural determinations of
monomeric/dimeric Al complexes.8
n
12
=
{(C6F5)N CH}C6H3] in the presence of BuOH (1.0 equiv.).
Among examples concerning Al-initiated ROP of lactides5 and
lactones,5j,5n,6 we had an interest in the fact that in situ catalyst
systems consisting of AlEt3 and 2 equiv. of salicylaldimines,
t
2
=
The imino substituent in Me2Al[O-2- Bu-6-(R N CH)C6H3] was
found to play a crucial role for the efficient ROP in a controlled
manner; the catalytic activity turnover number (TON) in the ROP
increased in the order: R = C6F5 >> 2,6-iPr2C6H3 >> Bu >
t
aGraduate School of Materials Science, Nara Institute of Science and
Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0101, Japan.
E-mail: nomurak@ms.naist.jp
adamantyl (negligible activity). In this paper, we investigated the
synthesis and structural analysis of a series of four coordinate
bResearch Laboratories, Daiso Co., Ltd., 9 Otakasu, Amagasaki, Hyogo
660-0842, Japan
1
2
1
t
2
=
Me2Al[O-2-R -6-(R N CH)C6H3], [R = Me (1), Bu (2); R =
2,6-iPr2C6H3 (a), Bu (b), cyclohexyl (c), adamantyl (d), C6H5
t
† Electronic supplementary information (ESI) available: Crystal structure
1
2
1
(e), 2,6-Me2C6H3 (f), C6F5 (g)]. We also investigated the reaction
=
determinations, reports for Me2Al[O-2-R -6-(R N CH)C6H3] [R = Me
(1), Bu (2), H (3); R2 = 2,6-iPr2C6H3 (a),tBu (b), cyclohexyl (c), Ph (e),
t
2
=
of AlMe3 with 2-(R N CH)C6H4OH for comparison. We also
2
=
C6F5 (g)] (1a, 1b, 2c, 2e, 2g), and Me2Al[l2-O-2-(R N CH)C6H4](AlMe3)
explored their possibilities as the initiators for ROP of CL in the
presence of nBuOH (1.0 equiv.), including ligand effect toward the
catalytic activity (TON) (Scheme 1).
[R2 = 2,6-iPr2C6H3 (3a), Bu (3b)]. CCDC reference numbers 666959–
t
666966. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b801561a
3978 | Dalton Trans., 2008, 3978–3988
This journal is
The Royal Society of Chemistry 2008
©

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Scheme 1
Results and discussion
1. Synthesis and structural analysis of various aluminium
complexes containing phenoxy-imine ligand of type,
1
2
=
Me2Al[O-2-R -6-(R N CH)C6H3]
A series of aluminium complexes containing phenoxy-imine
1
2
1
=
ligands of the type, Me2Al[O-2-R -6-(R N CH)C6H3] [R
=
Me (1), Bu (2); R2 = 2,6-iPr2C6H3 (a), Bu (b), cyclohexyl (c),
t
t
adamantyl (d), C6H5 (e), 2,6-Me2C6H3 (f), C6F5 (g)], were prepared
by reaction of AlMe3 with 1.0 equiv. of the corresponding 2-R1-
2
=
6-(R N CH)C6H3OH in n-hexane, as shown in Scheme 2. These
reactions took place along with the evolution of methane, and
the analytically pure samples were collected from the chilled-
concentrated n-hexane solution containing Al complexes cooled
◦
in the freezer (−20 C). The resultant complexes were identified
by 1H and 13C NMR spectra and elemental analysis. The 1H
NMR spectra of these complexes showed no complexity; a sharp
2
2
=
Fig. 1 ORTEP drawings for Me2Al[O-2-Me-6-(R N CH)C6H3] [R
=
2,6-iPr2C6H3 (1a, top), tBu (1b, bottom)].13 Thermal ellipsoids are drawn
single resonance assigned to the Al–Me protons at (−0.16 to
at the 50% probability level and H atoms are omitted for clarity.
2
=
−0.30 ppm), a resonance ascribed to the R N CH proton at
(7.22–7.78 ppm, s), as well as resonances corresponding to the
ligand were observed. The resultant complexes were also identified
by elemental analyses. Crystals suitable for the crystallographic
analysis were grown from the chilled-concentrated n-hexane
solution containing 1a, 1b, 2a–e and 2g, and their structures
determined by X-ray crystallography as the monomeric four-
coordinate species as described below.13
t
i
=
to those in Me2Al[O-2,4- Bu2-6-{(2,6- Pr2C6H3)N CH}C6H2] re-
ported previously [115.1(2), 114.0(2), 110.4(2) and 93.6(1)◦,
respectively].8g The Al–C bond distances [1.955(2) and 1.949(3) A]
˚
8g
˚
are also close to the reported values [1.959(5) and 1.948(5) A].
˚
The Al–N bond distance in 1a [1.9821(18) A] is somewhat close to
t
8g
˚
or longer than those in the above 2,4- Bu2 analogue [1.972(3) A],
6e
˚
[ArNC(Me)CHC(Me)O]AlMe2 [1.955(7) A], but the distances
are significantly longer than those in EtAl[1,2-{N(PMes2)}2-
6l
˚
C6H4] [1.863(1) A, Mes = 2,4,6-Me3C6H2], HC[SiMe2N(4-
8p
˚
MeC6H4)]3Al(THF) [1.8300(9)–1.8458(9) A]. The results clearly
suggest that the N atom does not form a r-bond with Al.
˚
The Al–O bond distance in 1a [1.7688(17) A] is somewhat
shorter than those in the above reported 2,4-tBu2 analogue
8g
6e
˚
˚
[1.773(3) A], [ArNC(Me)CHC(Me)O]AlMe2 [1.795(5) A]. Al-
though the bond distance is longer than that for Al–OiPr
i
6k
˚
in {CH2NMeCH2C(CF3)2O}2Al(O Pr) [1.699(3) A], the above
results suggest that the oxygen atom strongly coordinates to Al.
The structure for 1b around the Al atom is similar to that in 1a,
with 1b forming a distorted four-coordinate tetrahedral geometry
around Al. The O(1)–Al–C(1) [109.16(6)◦] and O(1)–Al–C(2)
[110.79(7)◦] bond angles are smaller that those in 1a [111.89(11)◦
and 111.20(10)◦, respectively], whereas the C(1)–Al–C(2)
[116.91(8)◦] and O(1)–Al–N(1) [95.66(5)◦] bond angles are larger
than those in 1a [116.61(15)◦ and 93.68(7)◦, respectively]. The Al–
Scheme 2
Structures for 1a and 1b are shown in Fig. 1 and the selected
bond distances and angles for 1a, 1b, 2a and 2b12 are sum-
marised in Table 1.13 The crystal structure of 1a (Fig. 1, top)
shows that 1a forms a distorted tetrahedral geometry around
the Al metal centre, as seen in the bond angles for C(1)–Al–
C(2) [116.61(15)◦], O(1)–Al–C(1) [111.89(11)◦], and O(1)–Al–C(2)
[111.20(10)◦], although the O(1)–Al–N(1) bond angle is somewhat
small [93.68(7)◦]; these bond angles are somewhat analogous
˚
˚
O distance in 1b [1.7756(11) A] is longer than in 1a [1.7668(17) A]
and the Al–N distance [1.9797(13) A] is shorter than in 1a
˚
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©

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◦
1
2
1
2
i
t
1
t
2
˚
=
Table 1 Selected bond distances (A) and angles ( ) for Me2Al[O-2-R -6-(R N CH)C6H3] [R = Me, R = 2,6- Pr2C6H3 (1a), Bu (1b); R = Bu, R =
2,6-iPr2C6H3 (2a), tBu (2b)]13
R1; R2
Me; 2,6-iPr2C6H3 (1a)
Me; tBu (1b)
tBu; 2,6-iPr2C6H3 (2a)a
tBu; tBu (2b)a
˚
Bond distances/A
Al(1)–O(1)
Al(1)–N(1)
Al(1)–C(1)
Al(1)–C(2)
N(1)–C(9)
N(1)–C(11)
1.7688(17)
1.9821(18)
1.955(2)
1.949(3)
1.295(2)
1.455(2)
1.7756(11)
1.9797(13)
1.9607(19)
1.9640(19)
1.2935(18)
1.5142(18), N(1)–C(10)
1.7592(19)
1.975(2)
1.949(3)
1.943(2)
1.296(3)
1.762(2)
1.972(2)
1.958(3) Al(1)–C(4)
1.958(3) Al(1)–C(4*)
1.289(4) N(1)–C(3)
1.526(4), N(1)–C(5)
1.454(3), N(1)–C(14)
Bond angles/◦
O(1)–Al(1)–N(1)
C(1)–Al(1)–C(2)
Al(1)–O(1)–C(3)
Al(1)–N(1)–C(9)
Al(1)–N(1)–C(11)
O(1)–Al(1)–C(1)
O(1)–Al(1)–C(2)
N(1)–Al(1)–C(1)
N(1)–Al(1)–C(2)
93.68(7)
116.61(15)
130.81(12)
122.24(13)
120.69(14)
111.89(11)
111.20(10)
110.71(10)
110.44(9)
95.66(5)
116.91(8)
131.32(9)
120.53(9)
122.50(9), Al(1)–N(1)–C(10)
109.16(6)
110.79(7)
112.53(6)
109.76(6)
93.70(9)
115.96(14)
133.84(15)
122.24(17)
118.14(16), Al(1)–N(1)–C(14)
110.41(13)
112.40(10)
111.64(11)
110.61(11)
95.52(11)
118.99(14), C(4)–Al–C(4*)
133.9(2), Al(1)–O(1)–C(1)
120.1(2), Al(1)–N(1)–C(3)
122.4(2), Al(1)–N(1)–C(5)
108.45(10), O(1)–Al(1)–C(4)
108.45(10), O(1)–Al(1)–C(4*)
111.38(10), N(1)–Al(1)–C(4)
111.38(10), N(1)–Al(1)–C(4*)
a Cited from our preliminary communication.12
˚
[1.9821(18) A]. The observed differences may be due to an influ-
ence from the substituent in the imino group (2,6-iPr2C6H3 vs. tBu).
The structures of 2a and 2b are analogous to those of 1a and 1b;
these complexes also form a four-coordinate, distorted tetrahedral
geometry around the Al metal centre.12,13 The Al–O bond distances
˚
[1.7592(19) and 1.762(2) A], and the Al–N distances [1.975(2) and
˚
1.972(2) A] in 2a and 2b are somewhat shorter than those in 1a
˚
˚
and 1b [1.7688(17), 1.7756(11) A and 1.9821(18), 1.9797(13) A,
respectively], although no significant differences are seen in the
Al–C bond distances.
Note that the reactions of AlMe3 with 1.0 equiv. of 2-
2
2
i
t
=
(R N CH)C6H4OH [R = 2,6- Pr2C6H3 (a) and Bu (b)] in n-
hexane afforded a mixture of the reaction product and the free
ligand. The ratio was assumed to be 1 : 1 based on the 1H NMR
spectra, with resonances corresponding to protons for the Al–
1
Me bond (in the H NMR) that were observed as a mixture of
AlMe3 (s, ca. −0.2 ppm, 9H) and the product (s, −0.4 ppm,
6H) identified by the integration ratios. Resonances ascribed to
2
=
protons in 2-(R N CH)C6H4OH were no longer observed when
the reaction was performed with 2.0 equiv. of AlMe3 in n-hexane.
The reaction products were therefore identified as Me2Al[O-2-
2
1
(R N CH)C6H4](AlMe3) (3) based on H, 13C NMR spectra and
=
elemental analyses (Scheme 3).
Scheme 3
Colourless block microcrystals of 3a suitable for X-ray crys-
tallographic analysis were grown from the chilled-concentrated
n-hexane solution; the structures for 3a and 3b were determined
(Fig. 2),14 and the selected bond distances and angles are sum-
marised in Table 2.
2
=
Fig. 2 ORTEP drawings for Me2Al[l2-O-2-(R N CH)C6H4](AlMe3)
[R2 = 2,6-iPr2C6H3 (3a, top), Bu (3b, bottom].14 Thermal ellipsoids are
t
drawn at the 50% probability level and H atoms are omitted for clarity.
3980 | Dalton Trans., 2008, 3978–3988
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©

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◦
14
˚
Table 2 Selected bond distances (A) and angles ( ) for complexes 3a and 3b
i
=
Me5Al2[O-6-{(2,6- Pr2C6H3)N CH}C6H4] (3a)
˚
Bond distances/A
Al(1)–O(1)
Al(1)–C(1)
N(1)–C(9)
1.8522(16)
1.941(2)
1.288(2)
Al(1)–N(1)
Al(1)–C(2)
N(1)–C(13)
O(1)–C(3)
1.9849(17)
1.940(3)
1.462(2)
1.386(2)
Al(2)–O(1)
1.9761(19)
Bond angles/◦
O(1)–Al(1)–N(1)
Al(1)–O(1)–C(3)
Al(1)–N(1)–C(13)
O(1)–Al(1)–C(2)
N(1)–Al(1)–C(2)
O(1)–Al(2)–C(10)
C(10)–Al(2)–C(11)
91.80(6)
113.49(15)
123.54(11)
107.66(9)
107.79(10)
100.81(12)
115.04(11)
C(1)–Al(1)–C(2)
Al(1)–N(1)–C(9)
O(1)–Al(1)–C(1)
N(1)–Al(1)–C(1)
Al(1)–O(1)–Al(2)
O(1)–Al(2)–C(11)
C(10)–Al(2)–C(12)
118.71(12)
118.88(13)
115.38(11)
112.07(10)
124.87(7)
108.22(10)
115.97(14)
t
=
Me5Al2[O-6-( BuN CH)C6H4] (3b)
˚
Bond distances/A
Al(1)–O(1)
Al(1)–C(1)
N(1)–C(9)
1.8477(16)
1.943(3)
1.289(2)
Al(1)–N(1)
Al(1)–C(2)
N(1)–C(10)
O(1)–C(3)
1.988(2)
1.939(3)
1.515(3)
1.389(3)
Al(2)–O(1)
1.9564(19)
Bond angles/◦
O(1)–Al(1)–N(1)
Al(1)–O(1)–C(3)
Al(1)–N(1)–C(10)
O(1)–Al(1)–C(2)
N(1)–Al(1)–C(2)
O(1)–Al(2)–C(14)
C(14)–Al(2)–C(16)
92.50(8)
112.21(15)
122.87(14)
110.48(11)
117.70(12)
103.71(14)
116.4(2)
C(1)–Al(1)–C(2)
Al(1)–N(1)–C(9)
O(1)–Al(1)–C(1)
N(1)–Al(1)–C(1)
Al(1)–O(1)–Al(2)
O(1)–Al(2)–C(15)
C(15)–Al(2)–C(16)
118.45(12)
116.99(19)
109.79(10)
104.78(12)
125.88(9)
106.66(12)
115.41(19)
The structure of 3a forms a distorted tetrahedral geometry
around each Al metal centre, although the O(1)–Al(1)–C(1) and
O(1)–Al(1)–C(2) bond angles [115.38(11), 107.66(9)◦] are larger
than the O(1)–Al(2)–C(10) and O(1)–Al(2)–C(11) bond angles
The structures of 2c, 2e and 2g, determined by X-ray crystal-
lography are shown in Fig. 3, with selected bond distances and
angles summarised in Table 4.13 These complexes have a distorted
tetrahedral geometry around Al, and no remarkable differences
were observed for the bond angles and distances among these
complexes, 2a–e, 2g. The Al–O bond distances in a series of
◦
˚
[100.81(12), 108.22(10) ]. The Al(1)–O(1) bond [1.8522(16) A] in
˚
3a is longer than those in 1–2a [1.7688(17) and 1.7592(19) A,
t
2
˚
=
Table 1], but the Al(2)–O(1) bond [1.9761(19) A] is much longer
Me2Al[O-2- Bu-6-(R N CH)C6H3] (2a–e, 2g) increased in the
order: 2e [R = Ph, 1.7792(11) A] > 2c [cyclohexyl, 1.7746(12) A],
2
˚
˚
than the Al(1)–O(1) bond. These results clearly indicate that the
Al(1)–O(1) bond forms a r-bond and Al(2)–O(1) forms a p-bond.
˚ ˚
2g [C6F5, 1.7737(16) A] > 2d [adamantyl, 1.7708(11) A] > 2b
[ Bu, 1.762(2) A], 2a [2,6- Pr2C6H3, 1.7592(19) A]. The Al–N
bond distances increased in the order: 2e [1.9896(13) A] > 2d
t
i
˚
˚
The structure for 3b is analogous to that of 3a, with AlMe3 co-
t
˚
=
ordinating to oxygen in the Me2Al[O-2-( BuN CH)C6H4] moiety.
These results clearly indicate that the ortho-substituents in the
phenoxy group plays an important key role to generate/form
the mononuclear Al structure. The observed differences in the
structures for 1a and 3a would provide better explanations for the
observed difference in the catalytic activity for the ring-opening
polymerisation (ROP) of e-caprolactone using a mixed catalyst
system consisting of AlMe3 and iminophenols (1.0 equiv.) in the
[1.9859(10)] > 2g [1.9780(19)], 2a [1.975(2)], 2b [1.972(2)] > 2c
[1.9664(14)]. Although we could not find an exact appropri-
ate explanation for the observed difference, we could at least
say that the bond distances and angles in Me2Al[O-2-R1-6-
2
=
(R N CH)C6H3] are influenced by the substituents in both the
imino and aryloxy groups. As demonstrated above, the presence
of two substituents in the 2,6-position of the aryloxo ligands are
important in forming monomeric four-coordinate (tetrahedral)
structures.
n
presence of BuOH (1.0 equiv.), as shown in Table 3. Moreover,
the activities of the catalyst systems consisting of AlMe3 and
t
t
=
=
2-( BuN CH)C6H4OH and 2-Me-6-( BuN CH)C6H3OH were
negligible under these conditions (runs 2 and 4); this therefore
suggests that the imino substituent strongly affects the activity.
As described below (in Table 5), the imino substituent signifi-
cant affected the activity in the ROP. In contrast, importantly,
the ortho-substituent in the phenoxide appears to be required
The differences in the resonances of the imino protons between
the Al complex and the free ligand would also be dependent upon
the imino substituent employed [e.g. Dd (ppm, C6D6) = 0.81 (R2 =
C6F5, 2g) > 0.76 (Ph, 2e) > 0.65 (tBu, 2b) > 0.46 (2,6-iPr2C6H3, 2a),
0.45 (2,6-Me2C6H3, 2f) = 0.45 (cyclohexyl, 2c), > 0.27 (adamantyl,
2d)]. The differences may be somewhat related to the above bond
distances and angles, with the imino nitrogen in certain complexes
being partially dissociated,15,16 which would affect the catalytic
activity as described below.
for the generation of the catalyst precursors, Me2Al[O-2-R1-6-
i
=
{(2,6- Pr2C6H3)N CH}C6H3], by eliminating the possibility of
coordination with AlMe3.
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1
2
1
2
=
Table 3 Ring-opening polymerisation of e-caprolactone using catalyst systems consisting of AlMe3 and 2-R -6-(R N CH)C6H3OH [R = H, Me; R =
2,6-iPr2C6H3, tBu]a
c
c
Run
R1; R2
Time/min
Yield/mg (%)
TONb
Mn × 10−4
Mw/Mn
1
2
3
4
Me; 2,6-iPr2C6H3 (1a)
Me; tBu (1b)
60
30
60
30
163 (29)
Trace
42 (8)
Trace
73
—
20
—
2.4
—
1.1
—
1.3
—
1.1
—
H; 2,6-iPr2C6H3 (3a)
H; tBu (3b)
Conditions: AlMe3 20 lmol (in n-hexane, 19 lL), 2-R -6-(R N CH)C6H3OH 20 lmol (in toluene, 30 lL), BuOH 20 lmol, CL 5.0 mmol, 60 ◦C.
b TON = (molar amount of CL reacted)/(molar amount of Al). c GPC data in THF vs.polystyrene standards.
a
1
2
n
=
t
2
2
Fig. 3 ORTEP drawings for Me2Al[O-2- Bu-6-(R N CH)C6H3] [R = cyclohexyl (2c, top left), Ph (2e, top right), C6F5 (2g, bottom)].13 Thermal
=
ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity.
◦
t
2
2
13
˚
=
Table 4 Selected bond distances (A) and angles ( ) for Me2Al[O-2- Bu-6-(R N CH)C6H3] [R = cyclohexyl (2c), adamantyl (2d), Ph (2e), C6F5 (2g)]
R2
Cyclohexyl (2c)
Adamantyl (2d)a
Ph (2e)
C6F5 (2g)
Al(1)–O(1)
Al(1)–N(1)
Al(1)–C(1)
Al(1)–C(2)
N(1)–C(5)
N(1)–C(14)
O(1)–Al(1)–N(1)
C(1)–Al(1)–C(2)
Al(1)–O(1)–C(3)
Al(1)–N(1)–C(5)
Al(1)–N(1)–C(14)
O(1)–Al(1)–C(1)
O(1)–Al(1)–C(2)
N(1)–Al(1)–C(1)
N(1)–Al(1)–C(2)
1.7746(12)
1.9664(14)
1.954(2)
1.956(2)
1.291(2)
1.491(2)
94.75(6)
117.05(11)
127.22(11)
118.85(11)
124.88(11)
110.73(8)
111.54(8)
107.66(9)
112.85(8)
1.7708(11)
1.9859(10)
1.9578(15)
1.7792(11)
1.7737(16)
1.9780(19)
1.951(2)
1.9896(13)
1.9482(18)
1.9607(18)
1.3024(18)
1.430(2)
1.9634(15)
1.941(2)
1.2971(15), N(1)–C(9)
1.4967(17), N(1)–C(10)
94.68(4)
119.32(7)
124.04(9)
116.98(9), Al(1)–N(1)–C(9)
122.22(7), Al(1)–N(1)–C(10)
107.36(6)
109.92(5)
117.15(5)
1.300(2), N(1)–C(3)
1.436(2), N(1)–C(10)
93.17(7)
93.44(5)
119.36(8)
130.64(10)
119.74(11)
122.94(8)
111.93(7)
110.51(8)
108.90(6)
109.59(6)
118.12(12)
133.89(14), Al(1)–O(1)–C(9)
122.97(14), Al(1)–N(1)–C(3)
120.55(14), Al(1)–N(1)–C(10)
113.73(10)
113.71(10)
105.34(9)
105.57(5)
109.26(10)
a Cited from our preliminary communication.12
3982 | Dalton Trans., 2008, 3978–3988
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t
2. Ring-opening polymerisation (ROP) of e-caprolactone using
(2c, 70, run 13) > Bu (2b, 20, run 10) > adamantyl (2d, trace,
run 12).18 It is thus clear that both electronic and steric factors
in the imino substituent (R2) affected the catalytic activity. This
fact would be an interesting contrast to that of the in situ catalyst
systems consisting of AlEt3 and 2.0 equiv. of salicylaldimines, 2,4-
1
2
=
Me2Al[O-2-R -6-(R N CH)C6H3] (1, 2)
Ring-opening polymerisation (ROP) of e-caprolactone (CL) using
1
2
=
Me2Al[O-2-R -6-(R N CH)C6H3] (1a, 1b and 2a–g) was con-
n
1
2
1
2
=
ducted upon the addition of BuOH (1.0 equiv. to Al), and the
results are summarised in Table 5. The addition of BuOH was
R -6-{(2,4,6-R C6H2)–N CH}C6H2OH [R , R = alkyl, aryl,
2
3
n
halogen], in the presence of PhCH2OH,6j because it was described
that an electronic effect of the imino substituent (aromatic or
aliphatic etc.) was not important towards the activity.
essential and the polymerisation did not take place in the absence
of nBuOH,12 suggesting that the polymerisation was initiated with
Al-alkoxide via a coordination insertion mechanism as previously
proposed and demonstrated.3,17
The ROP with the C6F5 analogue (2g) reached completion after
30 min under the same conditions (runs 17–18, Al 20 lmol at
60 ◦C); the ROP of 2g also reached high conversion of CL (86%
conversion after 30 min, run 23) even when the amount of Al
complex was reduced (Al 10 lmol). The Mn values could be
controlled by varying the CL : Al molar ratios (runs 17–18, 21–22
and 24), and a linear relationship between the Mn and the TON
values was observed.12 The ROP initiated by the 2g–PhCH2OH
system also reached completion under the same conditions (runs
19–20), and the Mn values [4.24 × 104 (run 19) and 4.42 × 104 (run
The catalytic activities [TON values, TON = CL consumed,
reacted (mmol)/Al (mmol)] of the 2,6-iPr2C6H3 analogues (1–2a)
t
were higher than the Bu analogues (1–2b), suggesting that the
imino substituents affected the observed catalytic activity. The
activity of 1b was somewhat higher than 2b, probably due to the
t
increased steric bulk of 2b containing two Bu groups in both
the aryloxo and the imino groups. The activity of 2a was slightly
higher than 1a. It seems that the observed activity was affected by
the imino substituent rather than the phenoxy substituent based
on these results (runs 5, 6, 9 and 10). Therefore, as described below,
the ROP with a series of tBu analogues with various substituents in
the imino group (2a–g) was conducted under the same conditions.
The activity (TON after 60 min at 60 ◦C) in the ROP of CL
1
20)] estimated by the H NMR spectrum (based on methylene
protons at the polymer chain end derived from PhCH2OH) was
very close to the exact values [4.07 × 104 (run 19) and 4.23 ×
104 (run 20)] calculated and corrected based on the Mn value
by GPC measurement versus polystyrene standards.19 The result
t
2
n
=
with a series of Me2Al[O-2- Bu-6-(R N CH)C6H3] increased in
the order: R2 = C6F5 [2g, TON >248 (430) after 30 min, run 17
(run 23)] >> 2,6-Me2C6H3 (2f, 185 after 30 min, run 8) > 2,6-
iPr2C6H3 (2a, 209, run 6) > C6H5 (2e, 113, run 16) > cyclohexyl
thus assumes that the ROP was initiated with Al–OR (R = Bu,
PhCH2) fragment as proposed.2a,c,17 The Mn value of the 2g–
PhCH2OH system was higher than that of the 2g–nBuOH system,
the difference can be explained as the difference in the initiation
1
2
1
t
Table 5 Ring-opening polymerisation (ROP) of e-caprolactone (CL) by [Me2Al[O-2-R -6-(R N CH)C6H3] (1a, 2a–g) [R = Me (1), Bu (2); R2
=
=
2,6-iPr2C6H3 (a), tBu (b), cyclohexyl (c), adamantyl (d), C6H5 (e), 2,6-Me2C6H3 (f), C6F5 (g)]: ROP under bulk conditionsa
T/◦C
Time/min
Yield/mg (%)
TONc
Mn × 10−4
Mw/Mn
d
d
Run
Complex (R2)
Al/lmol
CL : Alb
5
1a (2,6-iPr2C6H3)
2a (2,6-iPr2C6H3)
2a (2,6-iPr2C6H3)
2f (2,6-Me2C6H3)
1b (tBu)
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
15
12
10
10
20
20
20
20
20
20
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
250
333
417
500
500
250
250
250
250
250
250
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
50
50
50
50
50
50
60
60
30
30
60
60
90
120
60
60
120
60
30
30
30
30
30
30
30
45
60
60
30
30
30
30
427 (76)
472 (84)
192 (34)
413 (74)
106 (20)
46 (8)
106 (21)
227 (41)
156 (28)
Trace
190
209
85
185
50
20
27
100
70
—
4.50
4.35
2.87
4.84
1.85
1.64
2.04
2.88
2.07
—
1.61
1.61
1.24
1.64
1.26
1.10
1.24
1.39
1.19
—
1.06
1.38
1.64
1.53
1.61
1.66
1.69
1.64
1.64
1.86
1.27
1.74
1.42
1.39
1.40
1.41
6e
7e
8
9
10e
11e
12e
13
2b (tBu)
2b (tBu)
2b (tBu)
2c (Cyclohexyl)
2d (Adamantyl)
2d (Adamantyl)
2e (C6H5)
14e
15e
16
43 (8)
19
0.97f
2.83
5.60
5.57
7.26
7.56
6.22
8.21
9.34
10.05
2.58
5.39
3.13
3.12
5.27
5.00
258 (45)
551 (99)
548 (98)
551 (99)
550 (98)
549 (99)
515 (92)
481 (86)
538 (95)
198 (35)
449 (80)
193 (34)
191 (34)
480 (86)
456 (81)
113
248
245
248
245
330
383
430
475
88
200
85
85
215
203
17e
18g
19e,h
20g,h
21e
22e
23e
24e
25e
26
2g (C6F5)
2g (C6F5)
2g (C6F5)
2g (C6F5)
2g (C6F5)
2g (C6F5)
2g (C6F5)
2g (C6F5)
2a (2,6-iPr2C6H3)
2f (2,6-Me2C6H3)
2f (2,6-Me2C6H3)
2f (2,6-Me2C6H3)
2g (C6F5)
27
28g
29e
30g
2g (C6F5)
a Conditions: Al 10–20 lmol (in toluene, 50 lL), nBuOH 1.0 equiv. to Al, CL 5.0 mmol, 60 ◦C. b Initial molar ratios of CL : Al. c TON = (molar amount
of CL reacted)/(molar amount of Al). d GPC data in THF vs. polystyrene standards. e Cited from our preliminary communication.12 f High molecular
weight (Mn = 2.3 × 104) shoulder was also observed. g Independent experimental runs for reproducibility. h PhCH2OH was used in place of nBuOH [Mn =
4.24 × 104 (run 19), 4.42 × 104 (run 20) estimated by 1H NMR spectra].12
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efficiency. In fact, the efficiency for the ROP of 2g in the presence
of PhCH2OH estimated from the Mn value by GPC19e,h (polymer
yields and the Mn value corrected by that measured by GPC) was
68% (run 19), whereas the efficiency of the 2g–nBuOH system (run
17) was 88%.
The ROPs with effective catalysts, 2a, 2f and 2g, were also
◦
conducted at 50 C, and the observed activities were lower than
those performed at 60 ◦C (runs 25–30). A similar trend, the effect
of substituents in the imino group toward the activity (2g > 2f >
2a), was observed and the polymerisation by 2g reached a high
conversion (86%, run 29) even after 30 min.
Fig. 4 Time course plots of log[CL]/[CL]0. [CL] = concentration of CL
in mmol mL−1. The conditions are described in Table 6 (polymerisation at
50 ◦C, runs 40–43).
t
The ROPs of CL by 2b (R2 = Bu), 2c (R2 = cyclohexyl), 2g
(R2 = C6F5) were conducted under rather dilute conditions (initial
CL concentration 1.0 mmol mL−1 toluene) in the presence of
nBuOH (1.0 equiv.), and the results are summarised in Table 6.
The ROP with 2g proceeded at a significant rate even under the
diluted conditions (370 turnovers after 20 min, run 38), affording
poly(CL) with a narrow molecular weight distribution (Mw/Mn =
1.19), and the Mn value increased after 30 min with a narrow
distribution (run 39). In contrast, the activities of 2b (runs 32–
34) were significantly lower than those performed under the bulk
conditions as described in Table 5 (runs 10–12): the monomer
conversion reached 86% after 48 h (run 34), and the Mn value
increased upon increasing the monomer conversion (runs 33, 34).
The monomer conversion reached 80% after 12 h in the ROP
by 2c, however the molecular weight distribution became broad
(Mw/Mn = 3.56, run 37) without any notable increase in the
Mn values (runs 36, 37). This fact would suggest a possibility
of (probably intramolecular) transesterification accompanied in
the catalytic polymerisation as the side reaction.20
Al molar ratios (runs 17–18, 21–22 and 24).12 Taking these facts
into account, it is therefore clear that the ROP using 2g took place
in a living manner with an exclusive initiation efficiency under
certain optimised conditions.
Concluding remarks
Al complexes containing phenoxy-imine ligands of type, Me2Al[O-
1
2
1
2
i
t
=
2-R -6-(R N CH)C6H3] [R = Me, R = 2,6- Pr2C6H3 (1a), Bu
(1b); R1 = Bu, R2 = 2,6-iPr2C6H3 (2a), Bu (2b), cyclohexyl
(2c), adamantyl (2d), C6H5 (2e), 2,6-Me2C6H3 (2f), C6F5 (2g)]
have been prepared in high yields from AlMe3 by treating
with 1.0 equiv. of 2-R -6-(R N CH)C6H3OH in n-hexane, and
their structures (1a, 1b and 2a–e, 2g) were determined by X-
ray crystallography as mononuclear complexes with tetrahedral
t
t
1
2
=
geometry around Al. In contrast, the similar reaction of AlMe3
2
=
with 2-(R N CH)C6H4OH (0.5 and/or 1.0 equiv. to Al) afforded
The ROP (at 50 ◦C) proceeded at a significant rate at the
beginning and the rate gradually decreased; as shown in Fig. 4,
the rates were dependent upon the CL concentration with the
first order, suggesting that the polymerisation proceeded without
deactivation. The Mn value increased linearly upon increasing
TON values (polymer yields) consistent with low Mw/Mn values
(Mw/Mn = 1.08–1.13). The number of polymer chains estimated
from the Mn value by GPC analysis19 (10.1–10.5 lmol) was close
to the Al initiator employed under these conditions (Table 6, runs
40–43), and the Mn values could be controlled by varying the CL :
=
2
2
i
Me2Al[l2-O-2-(R N CH)C6H4](AlMe3) [R = 2,6- Pr2C6H3 (3a),
tBu (3b)] exclusively, and their structures form a distorted
tetrahedral geometry around each Al and the additional AlMe3
coordinates to oxygen in the phenoxy-imine ligand.
Complexes 1a, 1b and 2a–g were employed as catalyst precursors
for the ring-opening polymerisation (ROP) of e-caprolactone (CL)
in the presence of nBuOH ◦(1.0 equiv. to Al). The catalytic activity
(TON after 60 min at 60 C) in the ROP of CL with a series of
t
2
2
=
Me2Al[O-2- Bu-6-(R N CH)C6H3] increased in the order: R =
C6F5 (2g, TON >248 after 30 min) >> 2,6-Me2C6H3 (2f, 185 after
t
2
2
t
a
=
Table 6 Ring-opening polymerisation of e-caprolactone (CL) by Me2Al[O-2- Bu-6-(R N CH)C6H3] [R = Bu (2b), cyclohexyl (2c), C6F5 (2g)]
T/◦C
Time
Yield/mg (%)
TONb
Mn × 10−4
Mw/Mn
Nd/lmol
c
c
Run
Complex/lmol
31
32
33
34
35
36
37
38
39
40
41
42
43
2b (20)
2b (20)
2b (20)
2b (20)
2c (20)
2c (20)
2c (20)
2g (10)
2g (10)
2g (10)
2g (10)
2g (10)
2g (10)
60
60
60
60
60
60
60
60
60
50
50
50
50
6 h
12 h
24 h
48 h
3 h
Trace
—
50
120
215
—
—
—
111 (20)
270 (48)
479 (86)
Trace
138 (25)
447 (80)
413 (74)
481 (86)
208 (37)
299 (53)
426 (75)
481 (86)
1.20
1.56
3.33
—
1.27
1.73
7.68
8.67
3.67
5.33
7.33
8.15
2.61
1.67
1.72
—
1.86
3.56
1.19
1.17
1.08
1.10
1.13
1.13
9 h
63
12 h
20 min
30 min
30 min
1 h
1.5 h
2 h
200
370
430
185
265
375
430
9.6
9.9
10.1
10.0
10.4
10.5
a Conditions: Al 10 or 20 lmol (in toluene, 100 lL), nBuOH 1.0 equiv. to Al (in toluene, 9 lL), CL 5.0 mmol, toluene 4.37 mL (initial CL concentration
1.0 mmol mL−1), 50 or 60 ◦C. b TON = (molar amount of CL reacted)/(molar amount of Al). c GPC data in THF vs. polystyrene standards. d Estimated
number of polymer chain (lmol) = polymer yield (mg)/{0.56 × Mn(GPC)}.19e,h
3984 | Dalton Trans., 2008, 3978–3988
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30 min) > 2,6-iPr2C6H3 (2a, 209) > C6H5 (2e, 113) > cyclohexyl
(2c, 70) > Bu (2a, 20) > adamantyl (2d, trace), indicating that
138.6, 142.5, 142.6, 164.0, 173.7. Colourless block microcrystals
suitable for crystallographic analysis were grown from the
chilled-concentrated n-hexane solution containing 1a and the
structures were determined by X-ray crystallography. For further
details, see the ESI.†
t
the imino substituents in 2 strongly affect the catalytic activity.
The C6F5 analogue (2g) showed remarkable activity and the Mn
values for the resultant poly(CL) increased upon increasing the
TON values (polymer yields) consistent with low Mw/Mn values
(Mw/Mn = ca. 1.1) under optimised conditions (Table 6); the
number of polymer chain estimated was also close to the Al
initiator employed. These results strongly suggest that the ROP
took place in a living manner with high initiation efficiency under
certain optimised conditions.
We are currently exploring in more detail the ROP with Al
complexes of this type, including mechanistic details (initiation
and propagation mechanism) and the effect of ligands toward the
activity, and we are also exploring possibilities of these complexes
as catalysts for the ROP of other cyclic esters; these results will be
reported in the near future.
t
=
Synthesis of Me2Al[O-2-Me-6-( BuN CH)C6H3] (1b). Syn-
thesis of 1b was carried out according to the same procedure as that
of 1a, except 2-Me-6-( BuN CH)C6H3OH (0.41 g, 2.15 mmol)
t
=
1
was used. Yield 0.39 g (73.4%). H NMR (C6D6): d −0.22 (s,
t
6H, AlMe2), 1.01 (s, 9H, Bu), 2.27 (s, 3H, Me), 6.55 (t, J =
7.72 Hz, 1H, aromatic), 6.68 (d, J = 7.72 Hz, 1H, aromatic), 7.11
(d, J = 6.6 Hz, 1H, aromatic), 7.74 (s, 1H, CH N). 13C NMR
=
(C6D6): d −6.1, 16.3, 29.6, 59.5, 116.9, 118.5, 130.3, 133.0, 137.4,
163.0, 168.2. Anal. calcd for C14H22AlNO: C 67.99; H 8.97; N 5.66.
Found: C 67.99; H 9.40; N 5.62%.
t
i
=
Synthesis of Me2Al[O-2- Bu-6-{(2,6- Pr2C6H3)N CH}C6H3]
(2a). An n-hexane solution containing 2-tBu-6-{(2,6-
i
Experimental
=
Pr2C6H3)N CH}C6H3OH (1.0 g, 3.0 mL of n-hexane, 3.0 mmol)
was added slowly into a stirred n-hexane solution containing
AlMe3 (220 mg, 15 mL of n-hexane, 3.0 mmol) at −20 ◦C in
a drybox. The solution was allowed to warm slowly to room
temperature, and was stirred for a further 3 h. The mixture was
then concentrated in vacuo, and the chilled solution (−20 ◦C)
General procedures
All experiments were carried out in a nitrogen atmosphere in a
Vacuum Atmospheres drybox or using standard Schlenk tech-
niques. Anhydrous grade n-hexane (Kanto Kagaku Co., Ltd.) was
transferred into a bottle containing molecular sieves (a mixture
of 3A 1/16, 4A 1/8, and 13X 1/16) in the drybox under a N2
stream, and were passed through a short alumina column under a
N2 stream before use. All chemicals used were of reagent grade and
were purified by standard purification procedures. Reagent grade
AlMe3 in n-hexane (Kanto Kagaku Co. Ltd.) was stored in the
drybox and was used as received. Various salicylaldimines (imino-
phenols) containing different substituent on both the aryloxo and
1
afforded colourless microcrystals of 2a (926 mg, 77% yield). H
NMR (400 MHz, C6D6): d −0.30 (s, 6H), 0.80 (d, 6H), 1.18 (d,
6H), 1.52 (s, 9H), 3.13 (septet, 2H), 6.59 (t, 1H), 6.72 (dd, 1H),
7.01–7.03 (m, 2H), 7.08–7.11 (m, 1H), 7.40 (dd, 1H), 7.81 (s, 1H).
13C NMR (400 MHz, C6D6): d −9.06, 22.6, 25.9, 28.4, 29.5, 35.4,
117.7, 119.4, 124.5, 128.5, 133.7, 135.3, 142.0, 142.5, 142.7, 164.9,
174.1. Anal. calcd for C25H36NOAl: C 76.30; H 9.22; N 3.56%.
Found: C 76.33; H 9.46; N 3.37%.
1
2
1
t
=
the imino groups, 2-R -6-(R N CH)C6H3OH [R = Me (1), Bu
(2); R2 = 2,6-iPr2C6H3 (a), tBu (b), cyclohexyl (c), adamantyl (d),
Ph (e), 2,6-Me2C6H3 (f), C6F5 (g)] were prepared according to
literature procedures.10 Elemental analyses were performed using a
PE2400II Series (Perkin-Elmer Co.). All 1H, and 13C NMR spectra
were recorded on a JEOL JNM-LA400 spectrometer (399.65 MHz
t
t
=
Synthesis of Me2Al[O-2- Bu-6-( BuN CH)C6H3] (2b). Syn-
thesis for 2b was performed according to the same procedure as
t
t
=
that of 2a, except that 2- Bu-6-( BuN CH)C6H3OH (1.0 g, 3.0 mL
of n-hexane, 4.3 mmol) was added slowly into a stirred n-hexane
solution containing AlMe3 (310 mg, 15 mL of n-hexane, 4.3 mmol)
◦
at −20 C. The off-white microcrystals were then collected from
for H, 100.40 MHz for 13C). All spectra were obtained in the
solvent indicated at 25 C unless otherwise noted, and chemical
1
the chilled solution (942 mg, 76% yield). 1H NMR (C6D6): d −0.23
(s, 6H), 0.98 (s, 9H), 1.55 (s, 9H), 6.61 (t, 1H), 6.69 (dd, 1H), 7.39
(dd, 1H), 7.72 (s, 1H). 13C NMR (C6D6): d −6.41, 29.5, 29.6, 35.2,
59.3, 116.9, 119.8, 133.6, 133.8, 141.1, 163.6, 168.7. Anal. calcd
for C17H28NOAl: C 70.56; H 9.75; N 4.84%. Found: C 70.53; H
10.08; N 4.73%.
◦
shifts are given in ppm and are referenced to SiMe4 (d 0.00,
1H, 13C).
i
=
Synthesis
C6H3] (1a). Into
of
Me2Al[O-2-Me-6-{(2,6- Pr2C6H3)N CH}-
a
stirred solution of 2-Me-6-{(2,6-
i
=
Pr2C6H3)N CH}C6H3OH (0.63 g, 2.15 mmol) in n-hexane
(3.0 mL), AlMe3 (1.1 M n-hexane solution (1.62 g, 2.37 mmol Al))
was added drop-wise over a 10 min period at −20 ◦C. The solution
was allowed to warm to room temperature and was stirred for
3 h. The reaction mixture was placed in a rotary evaporator, and
the solution was concentrated in vacuo. The chilled-concentrated
n-hexane solution was placed in the freezer (−20 ◦C) and afforded
t
=
Synthesis of Me2Al[O-2- Bu-6-{(cyclohexyl)N CH}C6H3] (2c).
Synthesis of 2c was carried out according to the same procedure
t
=
as that of 1a, except that 2- Bu-6-{(cyclohexyl)N CH}C6H3OH
(0.56 g, 2.15 mmol) was used in place of 2-Me-6-{(2,6-
Pr2C6H3)N CH}C6H3OH. Yield 0.61 g (90.1%). 1H NMR
i
=
(C6D6): d −0.22 (s, 6H, AlMe2), 0.90 (m, 3H, cyclohexyl), 0.90–1.51
1
t
complex 1a (0.63 g, 83.5% yield) as colourless microcrystals. H
(m, 7H, cyclohexyl), 1.56 (s, 9H, Bu), 2.65 (m, 1H, cyclohexyl),
NMR (C6D6): d −0.30 (s, 6H, AlMe2), 0.84 (d, J = 6.6 Hz, 6H,
(CH3)2CH–), 1.19 (d, J = 6.96 Hz, 6H, (CH3)2CH–), 2.28 (s,
3H, Me), 3.14 (m, 2H, (CH3)2CH–), 6.52 (t, J = 7.72 Hz, 1H,
aromatic), 6.68 (d, J = 7.72 Hz, 1H, aromatic), 7.02–7.11 (m,
6.66 (t, J = 7.68 Hz, 1H, aromatic), 6.76 (d, J = 7.72 Hz, 1H,
aromatic), 7.42 (d, J = 7.32 Hz, 1H, aromatic), 7.40 (s, 1H,
CH N). 13C NMR (C6D6): d −7.5, 25.1, 25.4, 29.5, 33.7, 35.3,
=
68.5, 116.9, 119.8, 133.2, 133.8, 141.3, 164.0, 170.0. Anal. calcd
for C19H30AlNO: C 72.35; H 9.59; N 4.44. Found: C 72.28; H 9.88;
N 4.38%.
4H, aromatic), 7.80 (s, 1H, CH N). 13C NMR (C6D6): d −9.0,
=
16.4, 22.7, 25.8, 28.5, 117.8, 118.3, 124.5, 128.5, 131.3, 132.8,
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t
=
Synthesis of Me2Al[O-2- Bu-6-{(adamantyl)N CH}C6H3] (2d).
Synthesis of 2d was performed according to the same procedure
t
=
as that of 2a, except that 2- Bu-6-{(adamantyl)N CH}C6H3OH
(0.67 g, 2.15 mmol) was added slowly into a stirred n-hexane so-
lution containing AlMe3 (171 mg, 15 mL of n-hexane, 2.37 mmol)
at −20 ◦C. The chilled solution (−20 ◦C) afforded yellow
1
microcrystals (0.67 g, 85% yield). H NMR (C6D6): d −0.16 (s,
6H), 1.36 (dd, 6H), 1.57 (s, 9H), 1.66 (s or d, 6H), 1.79 (s, 3H),
6.67 (t, 1H), 6.82 (d, 1H), 7.42 (d, 1H), 7.79 (s, 1H). 13C NMR
(C6D6): d −6.1, 29.5, 29.7, 35.3, 35.8, 42.4, 60.3, 116.9, 120.0,
133.6, 133.8, 141.3, 163.8, 167.5. Anal. calcd for C23H34AlNO: C
75.17; H 9.33; N 3.81%. Found: C 75.10; H 8.95; N 3.84%.
t
=
Synthesis of Me2Al[O-2- Bu-6-(PhN CH)C6H3] (2e). Synthe-
sis of 2e was performed according to the same procedure as that of
t
=
1a, except that 2- Bu-6-(PhN CH)C6H3OH (0.54 g, 2.15 mmol)
was used. Yield: 0.57 g (85.5%). 1H NMR (C6D6): d −0.21 (s, 6H,
AlMe2), 1.55 (s, 9H, tBu), 6.59–6.67 (m, 2H, aromatic), 6.90–6.98
(m, 5H, aromatic), 7.41 (dd, J = 7.32, 1.84 Hz, 1H, aromatic),
7.47 (s, 1H, CH N). 13C NMR (C6D6): d −8.6, 29.7, 35.5, 117.6,
=
120.6, 122.6, 130.0, 134.3, 135.2, 141.9, 147.2, 165.0, 170.6. Anal.
calcd for C19H24AlNO: C 73.76; H 7.82; N 4.53. Found: C 73.68;
H 8.06; N 4.51%.
t
=
Synthesis of Me2Al[O-2- Bu-6-{(2,6-Me2C6H3)N CH}C6H3]
(2f). Synthesis of 2f was performed according to the same
t
=
procedure as that of 1a, except that 2- Bu-6-{(2,6-Me2C6H3)N
CH}C6H3OH (0.60 g, 2.15 mmol) was used. Yield 0.68 g (93.9%).
1H NMR (C6D6): d −0.31 (s, 6H, AlMe2), 1.55 (s, 9H, tBu), 1.97
(s, 6H, Me), 6.61 (t, J = 7.72 Hz, 1H, aromatic), 6.65 (dd, J =
7.68, 1.84 Hz, 1H, aromatic), 6.81–6.91 (m, 3H, aromatic), 6.89
=
(t, J = 8.4 Hz, 1H, aromatic), 7.22 (s, 1H, CH N), 7.41 (dd, J =
7.36, 1.84 Hz, 1H, aromatic). 13C NMR (C6D6): d −8.6, 18.4, 29.5,
35.3, 117.3, 119.8, 127.4, 129.0, 131.7, 133.8, 135.0, 141.8, 145.1,
164.8, 174.6. Anal. calcd for C21H28AlNO: C 74.75; H 8.36; N 4.15.
Found: C 74.60; H 8.47; N 4.02%.
t
=
Synthesis of Me2Al[O-2- Bu-6-{(C6F5)N CH}C6H3] (2g).
Synthesis of 2g was carried out according to the same procedure as
t
=
that of 1a, except that 2- Bu-6-{(C6F5)N CH}C6H3OH (0.74 g,
1
2.15 mmol) was used. Yield 0.70 g (81.6%). H NMR (C6D6): d
t
−0.29 (s, 6H, AlMe2), 1.47 (s, 9H, Bu), 6.54 (m, 1H, aromatic),
=
6.65 (m, 1H, aromatic), 7.30 (s, 1H, CH N), 7.40 (m, 1H,
aromatic). 13C NMR (C6D6): d −10.0, 29.3, 35.3, 118.0, 119.3,
121.5, 134.7, 136.8, 137.2, 139.4, 140.6, 141.7, 142.4, 143.0, 166.4,
176.9. Anal. calcd for C19H19AlF5NO: C 57.15; H 4.80; N 3.51.
Found: C 57.14; H 4.66; N 3.45%.
i
=
Synthesis of Me2Al[l2-O-2-{(2,6- Pr2C6H3)N CH}C6H4]-
i
=
(AlMe3) (3a). Into a stirred solution of 2-{(2,6- Pr2C6H3)N
CH}C6H4OH (0.60 g, 2.15 mmol) in n-hexane (3.0 mL), AlMe3
(1.1 M n-hexane, 3.24 g, 4.7◦4 mmol Al) was added drop-wise
over a 10 min period at −20 C. The solution was then allowed
to warm to room temperature and was stirred overnight. The
reaction mixture was then placed in a rotary evaporator to
concentrate the reaction mixture under reduced p◦ressure. The
chilled solution that was placed in the freezer (−20 C) afforded
1
complex 3a (0.81 g, 92.1% yield) as colourless microcrystals. H
NMR (C6D6): d −0.45 (s, 6H, AlMe2), −0.21 (s, 9H, AlMe3),
0.90 (d, J = 6.96 Hz, 6H, (CH3)2CH–), 1.19 (d, J = 6.96 Hz, 6H,
(CH3)2CH–), 2.99 (m, 2H, (CH3)2CH–), 6.56 (t, J = 7.32 Hz, 1H,
3986 | Dalton Trans., 2008, 3978–3988
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aromatic), 6.61 (dd, J = 7.68, 2.20 Hz, 1H, aromatic), 6.98–7.16
(m, 4H, aromatic), 7.57 (d, J = 7.68 Hz, 1H, aromatic), 7.61 (s,
References
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Wiley–VCH, Weinheim, Germany, 2000, p. 283.
1H, CH N). 13C NMR (C6D6): d −10.2, −5.2, 23.1, 25.1, 28.7,
=
121.9, 122.9, 124.6, 124.8, 129.0, 133.4, 137.0, 141.8, 157.1, 172.7.
Anal. calcd for C24H37Al2NO: C 70.39; H 9.11; N 3.42. Found: C
70.32; H 9.21; N 3.30%.
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N. Spassky, A. Le Borgne and M. Wisniewski, Macromolecules, 1996,
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Llobet and W. J. Simonsick, Jr., Macromolecules, 2001, 34, 885; (m) N.
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124, 1316; (o) D. Chakraborty and E. Y.-X. Chen, Organometallics,
2003, 22, 769.
6 Examples for ROMP of lactones etc.: (a) B.-T. Ko and C.-C. Lin,
Macromolecules, 1999, 32, 8296; (b) I. Taden, H.-C. Kang, W. Massa,
T. P. Spaniol and J. Okuda, Eur. J. Inorg. Chem., 2000, 441; (c) D.
Chakraborty and E. Y.-X. Chen, Organometallics, 2002, 21, 1438;
(d) M.-L. Hsueh, B.-H. Huang and C.-C. Lin, Macromolecules, 2002,
35, 5763; (e) R.-C. Yu, C.-H. Hung, J.-H. Huang, H.-Y. Lee and J.-
T. Chen, Inorg. Chem., 2002, 41, 6450; (f) G. Zheng and H. D. H.
Sto¨ver, Macromolecules, 2003, 36, 7439; (g) L. M. Alcazar-Roman,
B. J. O’Keefe, M. A. Hillmyer and W. B. Tolman, Dalton Trans., 2003,
3082; (h) C.-T. Chen, C.-A. Huang and B.-H. Huang, Macromolecules,
2004, 37, 7968; (i) J. Lewinsky, P. Horeglad, E. Tratkiewicz, W. Grzenda,
J. Lipkowski and E. Kolodziejczyk, Macromol. Rapid Commun., 2004,
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molecules, 2005, 38, 5363; (k) A. Amgoune, L. Lavanant, C. M. Thomas,
Y. Chi, R. Welter, S. Dagorne and J.-F. Carpentier, Organometallics,
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t
=
Synthesis of Me2Al[l2-O-2-( BuN CH)C6H4](AlMe3) (3b).
Synthesis of 3b was carried out according to the same procedure
t
=
as that of 3a, except 2-( BuN CH)C6H4OH (0.38 g, 2.15 mmol)
was used. Yield 0.68 g (93.9%). 1H NMR (C6D6): d −0.37 (s, 6H,
AlMe2), −0.26 (s, 9H, AlMe3), 0.90 (s, 9H, tBu), 6.55–6.63 (m, 2H,
aromatic), 6.95–7.00 (m, 1H, aromatic), 7.22 (d, J = 8.08 Hz, 1H,
aromatic), 7.44 (s, 1H, CH N). 13C NMR (C6D6): d −7.7, −5.0,
=
28.7, 61.5, 121.8, 122.2, 124.5, 133.2, 135.7, 156.5, 165.5. Anal.
calcd for C16H29Al2NO: C 62.93; H 9.57; N 4.59. Found: C 62.63;
H 9.86; N 4.55%.
Ring-opening polymerisation (ROP) of e-caprolactone (CL).
Typical polymerisation procedures (Table 5, run 5) are as follows.
Into a sealed Schlenk tube containing a toluene solution of 1a
(0.020 mmol, 0.050 mL of toluene), nBuOH (1.8 lL, 0.020 mmol)
was added in a drybox at room temperature. The solution was
stirred for 10 minutes, and then e-caprolactone (5.0 mmol) was
added to the solution. The re◦action mixture was then placed into
an oil bath pre-heated at 60 C, and the solution was stirred for
the prescribed time (60 min). The polymerisation mixture was
then quenched by adding methanol (1.0 mL), and the resultant
solution was then poured into methanol (400 mL). The ring-
opened polymer was then collected as the methanol insoluble white
precipitates; the resultant polymer was then collected on filter
paper and was dried in vacuo. 1H NMR (CDCl3): d 1.33 (m, 2H),
1.51–1.63 (m, 4H), 2.25 (t, 2H), 4.00 (t, 2H). 13C NMR (CDCl3):
=
d 24.5, 25.5, 28.3, 34.1, 64.1, 173.5 (C O). The ROP of CL
using a mixed catalyst system consisting of AlMe3 and Me2Al[O-
2-R -6-(R N CH)C6H3OH was performed using an analogous
procedure for those with 1 and 2, except that AlMe3 and the
imino-phenol was pre-mixed for 10 min before adding nBuOH.
1
2
=
Crystallographic analysis
All measurements were made on a Rigaku RAXIS-RAPID
Imaging Plate diffractometer with graphite monochromated Mo-
Ka radiation. The selected crystal collection parameters are listed
below (Table 7), and the detailed results are described in the
attached reports, see ESI.†13,14 All structures were solved by direct
methods and expanded using Fourier techniques,21 and the non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were included but not refined. All calculations were performed
using the crystal structure crystallographic software package.22
7 (a) M. K. Cox, in Biodegradable Polymers and Plastics, ed. M. Vert,
J. Feijen, A. Albertsson, G. Scott and E. Chiellini, The Royal Society
of Chemistry, Cambridge, UK, 1992, p. 95; (b) Y. Doi, Y. Kumagai,
N. Tanahashi, K. Mukai, in Biodegradable Polymers and Plastics,
ed. M. Vert, J. Feijen, A. Albertsson, G. Scott and E. Chiellini, The
Royal Society of Chemistry, Cambridge, UK, 1992, p. 139; (c) M.
Okada, Prog. Polym. Sci., 2002, 27, 87.
8 (a) D. A. Atwood, J. A. Jegier and D. Rutherford, Inorg. Chem., 1996,
35, 63; (b) B. Qian, D. L. Ward and M. R. Smith, III, Organometallics,
1998, 17, 3070; (c) C. E. Radzewich, M. P. Coles and R. F. Jordan,
Acknowledgements
The authors express their thanks to Mr Shohei Katao (NAIST)
for structure analyses, and to Prof. Michiya Fujiki (NAIST) for
helpful comments. J. L. expresses her thanks to JSPS (Japan
Society for the Promotion of Science) for a postdoctoral fellowship
(P05397). N.I. and K.N. express their thanks to Daiso Co., Ltd.
for support.
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Dalton Trans., 2008, 3978–3988 | 3987
©

View Article Online
J. Am. Chem. Soc., 1998, 120, 9384; (d) P. A. Cameron, V. C. Gibson,
C. Redshaw, J. A. Segal, M. D. Bruce, A. J. P. White and D. J. Williams,
Chem. Commun., 1999, 1883; (e) M.-A. Mun˜oz-Herna´ndez, T. S. Keiser,
B. Patrick and D. A. Atwood, Organometallics, 2000, 19, 4416; (f) S. Liu,
M.-A. Munoz-Hernandez and D. A. Atwood, J. Organomet. Chem.,
2000, 596, 109; (g) P. A. Cameron, V. C. Gibson, C. Redshaw, J. A. Segal
and G. A. Solan, J. Chem. Soc., Dalton Trans., 2001, 1472; (h) M. S.
Hill, A. R. Hutchison, T. S. Keiser, S. Parkin, M. A. Van Aelstyn and
D. A. Atwood, J. Organomet. Chem., 2001, 628, 71; (i) C. Redshaw and
M. R. J. Elsegood, Chem. Commun., 2001, 2016; (j) P. A. Cameron,
V. C. Gibson, C. Redshaw, J. A. Segal, A. J. P. White and D. J. Williams,
J. Chem. Soc., Dalton Trans., 2002, 415; (k) D. Chakraborty and E. Y.-
X. Chen, Macromolecules, 2002, 35, 13; (l) J. Lewin´ski, J. Zachara,
K. B. Starowieyski, Z. Ochal, I. Justyniak, T. Kopec, P. Stolarzewicz
and M. Dranka, Organometallics, 2003, 22, 3773; (m) J. Lewinski, P.
Horeglad, M. Dranka and I. Justyniak, Inorg. Chem., 2004, 43, 5789;
(n) S. Dagorne, L. Lavanant, R. Welter, C. Chassenieux, P. Haquette
and G. Jaouen, Organometallics, 2003, 22, 3732; (o) W. Braune and J.
Okuda, Angew. Chem., Int. Ed., 2003, 42, 64; (p) H. Zhu and E. Y.-X.
16 From these results, as described in the text, we assumed that the imino
nitrogen in certain complexes especially in the adamantyl analogue
(2d) may be dissociated partially, and the assumption by 2d may also
˚
be supported by a rather long Al–N bond distance [1.9859(10) A],
˚
and although they give a similar Al–N bond distance [1.9896(13) A],
a significant difference in the chemical shift (Dd 0.76 ppm) was seen
in the phenyl analogue (2e). A reviewer commented that the downfield
shifts of the resonances of the imino-protons should be influenced by
electronic effects more than by bulkiness of the related substituents.
17 W. Dittrich and R. C. Shultz, Angew. Makromol. Chem., 1971, 15,
109.
18 As described in text, the Al complexes containing alkyl substituents in
the imino group showed low activities (runs 10, 13–14), and that the
number of polymer chains (N, catalyst efficiencies calculated based on
the molar ratio of N : Al) in the resultant poly(CL)s were lower than
those in the Al complexes containing aryl substituents in the imino
groups under the same conditions (60 ◦C after 60 min, runs 6, 8, 16–
18). The observed difference may be due to the catalyst efficiency. We
also speculated that the imino group in the adamantyl analogue (2d)
may be weakly coordinated to Al estimated by the difference in the
chemical shifts of the imino nitrogen between 2d and the free ligand,
presumably leading to the extremely low catalytic activity.
Chen, Inorg. Chem., 2007, 46, 1481.
t
=
9 Synthesis of Me2Al[O-2,4- Bu2-6-(RN CH)C6H2] (R
=
2,6-
iPrC6H3, tBu), structural analysis for Me2Al[O-2,4-tBu2-6-{(2,6-
i
=
Pr2C6H3)N CH}C6H2], and reactions with B(C6F5)3 were reported in
19 Reports concerning comparison between exact number average molec-
ular weights and Mn values by GPC vs. polystyrene standards: (a) A.
Duda, Z. Florjanczyk, A. Hofman, S. Slomkowski and S. Penczek,
Macromolecules, 1990, 23, 1640; (b) S. J. McLain and N. E. Drysdale,
Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.), 1992, 33, 174;
(c) H. Pasch and K. Rhode, J. Chromatogr., A, 1995, 699, 24; (d) H. R.
Kricheldorf and S. Eggerstedt, Macromol. Chem. Phys., 1998, 199,
283; (e) A. Duda, A. Kowalski and S. Penczek, Macromolecules, 1998,
31, 2114; (f) A. Kowalski, J. Libiszowski, A. Duda and S. Penczek,
Macromolecules, 2000, 33, 1964; (g) H.-L. Chen, B.-T. Ko, B.-H.
Huang and C.-C. Lin, Organometallics, 2001, 20, 5076; (h) M. Save,
M. Schappacher and A. Soum, Macromol. Chem. Phys., 2002, 203,
889; (i) H. R. Kricheldorf and S. Rost, Polymer, 2005, 46, 3248. In this
paper, the exact Mn values for ring-opened poly(CL)s were corrected
ref. 8g. NMR study for reactions of AlEt3 with 1.0 or 2.0 equiv. of 2,4-
t
=
Me2-6-{(2,4,6- Bu3C6H2)N CH}C6H2OH in toluene-d8 was reported
in ref 6j (and in the ESI†), however no attempts for isolation of Al
complexes, including structural determinations were made.
10 Syntheses of various salicylaldimines and Ti, Zr and V complexes
containing bis(phenoxy-imine) ligands: T. Fujita, Y. Tohi and M.
Mitani, Eur. Pat., 0 874 005 A1, 1998.
11 (a) S. Matsui, M. Mitani, J. Saito, Y. Tohi, H. Makio, N. Matsukawa, Y.
Takagi, K. Tsuru, M. Nitabaru, T. Nakano, H. Tanaka, N. Kashiwa and
T. Fujita, J. Am. Chem. Soc., 2001, 123, 6847; (b) M. Mitani, J. Mohri,
Y. Yoshida, J. Saito, S. Ishii, K. Tsuru, S. Matsui, R. Furuyama, T.
Nakano, H. Tanaka, S. Kojoh, T. Matsugi, N. Kashiwa and T. Fujita,
J. Am. Chem. Soc., 2002, 124, 3327; (c) M. Mitani, R. Furuyama,
J. Mohri, J. Saito, S. Ishii, H. Terao, T. Nakano, H. Tanaka and T.
Fujita, J. Am. Chem. Soc., 2003, 125, 4293; (d) H. Terao, S. Ishii, J.
Saito, S. Matsuura, M. Mitani, N. Nagai, H. Tanaka and T. Fujita,
Macromolecules, 2006, 39, 8584.
12 N. Iwasa, J. Liu and K. Nomura, Catal. Commun., 2008, 9, 1148.
13 Crystal structure reports for 1a, 1b, 2c–e and 2g are shown in the
ESI,†and the crystallographic data (including CIF files) for 1a, 1b, 2a–
e and 2g were deposited with the Cambridge Crystallographic Data
Centre as CCDC 666959–666964, respectively. The structure reports
for 2a, 2b and 2d are shown in the ref. 12, and the data (including CIF
files) were deposited in the Cambridge Crystallographic Data Centre
as CCDC 645924–645926, respectively.
14 Crystal structure reports for 3a and 3b are shown in the ESI,†and
the data (including CIF files) for 3a and 3b were deposited in
the Cambridge Crystallographic Data Centre as CCDC 666965 and
666966, respectively.
15 Related reports concerning synthesis of half-titanocenes containing
phenoxy-imine ligands where the imino nitrogens were not coordinated
to Ti: H. Zhang, S. Katao, K. Nomura and J. Huang, Organometallics,
2007, 26, 5967.
from the Mn values by GPC vs. polystyrene standards according to the
19e,h
following equation: Mn(PCL) = 0.56 × Mn(GPC vs. polystyrene standards)
.
.
20 Selected reports concerning transesterification in the ROP of CL and
DL-lactide: (a) A. Duda and S. Penczek, Macromolecules, 1995, 28,
5981; (b) G. Montaudo, M. S. Montaudo, C. Puglisi, F. Samperi, N.
Spassky, A. Le Borgne and M. Wisniewski, Macromolecules, 1996, 29,
6461; (c) N. Spassky, V. Simic, M. S. Montaudo and L. G. Hubert-
Pfalzgraf, Macromol. Chem. Phys., 2000, 201, 2432; (d) D. Jhurry,
A. Bhaw-Luximon and N. Spassky, Macromol. Symp., 2001, 175,
67; (e) J. Cayuela, V. Bounor-Legare´, P. Cassagnau and A. Michel,
Macromolecules, 2006, 39, 1338.
21 P. T. Beurskens, G. Beurskens, R. de Gelder, S. Garcia-Granda, R. O.
Gould, R. Israel and Jan M. M. Smits, The DIRDIF-94 program system,
Crystallography Laboratory, University of Nijmegen, The Netherlands,
1998.
22 (a) Crystal Structure 3.6.0: Crystal Structure Analysis Package, Rigaku
and Rigaku/MSC, 9009 New Trails Dr., The Woodlands, 77381 TX,
USA, 2000–2004; (b) D. J. Watkin, C. K. Prout, J. R. Carruthers and
P. W. Betteridge, CRYSTALS Issue 10, Crystallography Laboratory-
Oxford, UK 1996.
3988 | Dalton Trans., 2008, 3978–3988
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