Synthesis and styrene polymerisation catalysis of η5- and η1-pyrrolyl-ligated cationic rare earth metal aminobenzyl complexes

Article abstract of DOI:10.1039/b719182k

The cationic rare earth metal aminobenzyl complexes bearing mono(pyrrolyl) ligands are synthesised and structurally characterised, and the coordination mode of the pyrrolyl ligands is found to show significant influence on the polymerisation of styrene. T

Full text of DOI:10.1039/b719182k

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Synthesis and styrene polymerisation catalysis of g5- and
g1-pyrrolyl-ligated cationic rare earth metal aminobenzyl complexesw
Masayoshi Nishiura, Tomohiro Mashiko and Zhaomin Hou*
Received (in Cambridge, UK) 13th December 2007, Accepted 4th February 2008
First published as an Advance Article on the web 25th February 2008
DOI: 10.1039/b719182k
The cationic rare earth metal aminobenzyl complexes bearing
mono(pyrrolyl) ligands are synthesised and structurally charac-
terised, and the coordination mode of the pyrrolyl ligands is
found to show significant influence on the polymerisation of
styrene.
structure of 1 is shown in Fig. 1(a). The pyrrolyl ligand is
bonded to the metal centre in an Z5-fashion as an azacyclo-
pentadienyl ligand and the two aminobenzyl ligands are all
bonded to the metal centre in a chelating fashion via both the
benzyl CH2 carbon atom and the amino unit. The average
bond length of the Sc–pyrrolyl ring atoms (2.531(2) A) is
almost the same as that of the Sc–Cp ring carbons (2.539(2) A)
found in (Me3SiC5Me4)Sc(CH2C6H4NMe2-o)2.2a In com-
plexes 1–3, the bond distances around the metal centres
decrease in the order of 3 (La) 4 2 (Y) 4 1 (Sc) as the
decrease in their metal size (e.g., Ln–pyrrolyl bond (av.) 1:
2.531(2), 2: 2.678(2), 3: 2.829(8) A).
The cationic rare earth (group 3 and lanthanide metals) alkyl
complexes bearing mono(cyclopentadienyl) ligands have at-
tracted growing interest due to their high potential in poly-
merisation catalysis.1,2 For the stabilisation of such half-
sandwich alkyl complexes, cyclopentadienyl derivatives with
bulky substituents or with side arms that have amino or ether
functional groups have often been employed.3 In contrast,
studies on the influence of heteroatom-containing cyclopenta-
dienyl ligands on the olefin polymerisation activity of the rare
earth complexes remained scarce.4
The similar reaction of Sc(CH2C6H4NMe2-o)3 with 1 equiv
of tetramethylpyrrole C4Me4NH gave (Z1-C4Me4N)-
Sc(CH2C6H4NMe2-o)2 (4) in 64% isolated yield. An X-ray
crystallographic study revealed that the pyrrolyl ligand in 4 is
in an Z1-N-s coordination fashion and the scandium atom lies
in the pyrrolyl plane with an average deviation of only 0.017 A
(Fig. 1(b)). The Sc(1)–N(3) (pyrrolyl) bond distance in 4
(2.119(3) A) is much shorter than that in 1 (2.478(2) A),
consistent with the difference in pyrrolyl coordination modes
between 4 and 1. The Z1-bonding fashion of the tetramethyl-
Pyrrolyl is isoelectronic with cyclopentadienyl and can show
more flexible metal-coordination modes (ranging from N-Z1
(s) to Z5 (p)) than the cyclopentadienyl analogues. Although a
number of pyrrolyl-ligated divalent5 and trivalent6 rare earth
complexes and group 4 metal complexes7,8 have been reported,
the influence of the coordination mode of the pyrrolyl ligands
on the polymerisation activity of a metal complex catalyst has
hardly been examined, and a structurally characterised catio-
nic pyrrolyl metal complex has not been reported previously.
We report herein the synthesis, structural characterisation,
and styrene polymerisation catalysis of the cationic rare earth
(o-dimethylaminobenzyl) complexes bearing Z5- and Z1-pyr-
rolyl ligands. We found that the coordination mode (Z1 or Z5)
of the pyrrolyl ligands can show dramatic influence on the
polymerisation activity.
pyrrolyl ligand in
4 is in sharp contrast with what
was observed previously in other tetramethylpyrrolyl-co-
ordinated metal complexes such as (Z5-NC4Me4)Ti(SPh)3,8a
(Z5-NC4Me4)TiCl3,8a
(Z5-NC4Me4)TaMe2Cl2,8b
and
Ru(Z5-NC4Me4)2,8c in which the pyrrolyl ligands all adopted
an Z5-bonding mode. As far as we are aware, complex 4
represents the first example of a structurally characterized
Z1-bonding tetramethylpyrrolyl complex.
The acid–base reaction of the rare earth tris(o-dimethyla-
minobenzyl) complexes Ln(CH2C6H4NMe2-o)3 with 1 equiv
of 2,5-t-Bu2C4H2NH gave in high yields the corresponding
mono(pyrrolyl)-ligated bis(aminobenzyl) complexes (Z5-2,5-t-
Bu2C4H2N)Ln(CH2C6H4NMe2-o)2 (Ln ¼ Sc (1), Y (2), La (3))
from the smallest scandium to the largest lanthanum (Scheme
1).z X-Ray analyses revealed that all of these complexes adopt
a similar overall structure. As a typical example, the X-ray
Organometallic Chemistry Laboratory, RIKEN (The Institute of
Physical and Chemical Research), Hirosawa 2-1, Wako, Saitama
351-0198, Japan. E-mail: houz@riken.jp; Fax: +81 48 462 4665;
Tel: +81 48 467 9393
w Electronic supplementary information (ESI) available: Experimental
details, and scanned NMR spectra of syndiotactic polystyrene. Crys-
tallographic data for CCDC 659963–659966 and 668429. See DOI:
10.1039/b719182k
Scheme 1 Synthesis of rare earth metal bis(o-dimethylaminobenzyl)
complexes bearing mono Z5- and Z1-pyrrolyl ligands.
ꢀc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 2019–2021 | 2019

View Article Online
Fig. 2 ORTEP drawing of the cationic part of 6 with 30% prob-
ability of thermal ellipsoids. Selected bond lengths (A): Sc(1)–C(1)
2.228(2), Sc(1)–C(2) 2.654(2), Sc(1)–C(3) 2.751(2), Sc(1)–O(1) 2.242(2),
Sc(1)–O(2) 2.216(2), Sc(1)–N(1) 2.368(2), Sc(1)–N(2) 2.382(2), Sc(1)–
C(10) 2.482(2), Sc(1)–C(11) 2.541(2), Sc(1)–C(12) 2.523(2), Sc(1)–
C(22) 2.453(2) Sc(1)–C(1)–C(2) 89.3(2).
Fig. 1 ORTEP drawings of 1 (a) and 4 (b) with 30% probability of
thermal ellipsoids. Selected bond lengths (A) and angles (1): 1: Sc(1)–C(1)
2.290(2), Sc(1)–C(10) 2.285(2), Sc(1)–N(1) 2.489(2), Sc(1)–N(2) 2.471(2),
Sc(1)–N(3) 2.478(2), Sc(1)–C(19) 2.527(2), Sc(1)–C(20) 2.561(2),
Sc(1)–C(21) 2.556(2), Sc(1)–C(22) 2.532(2), Sc(1)–C(1)–C(2) 101.4(2),
Sc(1)–C(10)–C(11) 107.7(2). 4: Sc(1)–C(1) 2.223(4), Sc(1)–C(10)
2.221(4), Sc(1)–N(1) 2.309(3), Sc(1)–N(2) 2.306(3), Sc(1)–N(3) 2.119(3)
Sc(1)–C(1)–C(2) 103.2(2), Sc(1)–C(10)–C(11) 105.0(2).
Sc–C1–phenyl(C2) angle in 6 (89.3(2)1) is significantly smaller
than those in 1 (101.4(2) and 107.7(2)1). To our knowledge,
complex 6 represents the first example of a structurally char-
acterised cationic pyrrolyl complex for any metal.
Addition of 1 equiv of [Ph3C][B(C6F5)4] to 1 in toluene at
25 1C gave the corresponding cationic mono(aminobenzyl)
complex [(2,5-t-Bu2C4H2N)Sc(CH2C6H4NMe2-o)][B(C6F5)4]
(5) in ca. 90% yield (Scheme 2). Recrystallisation of 5 from
The reaction of the Z1-tetramethylpyrrolyl-ligated complex
4
with 1 equiv of [Ph3C][B(C6F5)4] in THF afforded
a
mixed hexane–toluene–DME solution afforded single
[(C4Me4N)Sc(CH2C6H4NMe2-o)(thf)3][B(C6F5)4] (7), which
possessed three thf ligands as shown by 1H NMR analysis.
Attempts to obtain single crystals of 7 were, however, not
successful.
crystals of [(2,5-t-Bu2C4H2N)Sc(CH2C6H4NMe2-o)(dme)]-
[B(C6F5)4] (6). An X-ray diffraction study established that 6
is a separated ion pair that possesses a chelating dme ligand at
the Sc centre (Fig. 2). The pyrrolyl ligand is bonded to the Sc
atom in an Z5-fashion as observed in its neutral precursor 1.
Because of the greater electron deficiency of the cationic metal
centre in 6, the bond distances of the Sc–pyrrolyl ring bonds in
6 (2.476(2) A) are significantly shorter than that in 1 (2.531(2)
A), and so are the Sc–benzyl (6: 2.228(2) A, 1: av. 2.288(2) A)
and Sc–amino (6: 2.368(2) A, 1: av. 2.480(2) A) bond dis-
tances. Moreover, interactions between the Sc centre and some
phenyl atoms of the benzyl group are also observed in 6
(Sc–C2: 2.654(2) A, Sc–C3: 2.751(2) A), and therefore, the
In the presence of 1 equiv of [Ph3C][B(C6F5)4], the scandium
bis(aminobenzyl) complex 1 showed high activity for the
polymerisation of styrene, yielding syndiotactic polystyrene
with high molecular weight and moderate molecular distribu-
tion (Table 1, run 1).w The yttrium complex 2 could also
produce syndiotactic polystyrene but with much lower activ-
ity, whereas the lanthanum analogue 3 showed no activity
under the same conditions. These results, including the trend
in metal-effect, are comparable with those observed in the case
of the C5Me4SiMe3-based complexes (C5Me4SiMe3)-
Table 1 Syndiospecific polymerisation of styrene by complexes
1–6,8a
Run Cat. Ln t/min Yield (%) Activityb sPS Mn (104)c Mw/Mn
c
1
2
3
4
5
6
7
a
1
2
3
Sc o1 100
43100 100 14.7
17 100 1.7
1.99
2.09
—
Y 30
La 30
13
0
—
—
—
5d Sc o1 100
43100 100 20.6
1.83
6d Sc
4
8
30
30
25
0
0
95
—
—
—
—
—
—
Sc
Sc
118 100 6.8
2.46
Conditions: 25 mmol Ln, 25 mmol [Ph3C][B(C6F5)4]; Ln : monomer ¼
b
1 : 500 (mol : mol), V ¼ 7.2 mL (toluene), unless otherwise noted. kg
c
of polymer per mol-Sc h. Determined by GPC in o-dichlorobenzene at
d
Scheme 2 Synthesis of a cationic scandium aminobenzyl complex
bearing an Z5-pyrrolyl ligand.
145 1C against polystyrene standard. Borate compound was not used.
ꢀc
This journal is The Royal Society of Chemistry 2008
2020 | Chem. Commun., 2008, 2019–2021

View Article Online
Ln(CH2SiMe3)2(thf).2e,9 The isolated cationic benzyl species 5
alone also showed high activity for the syndiospecific poly-
merisation of styrene, while the neutral complex 1 was inactive
under the same conditions. The cationic complex 6 showed no
activity, apparently due to the strong coordination of the dme
ligand to the metal centre. The Z1-C4Me4N-ligated Sc complex
4 did not show any activity for the polymerisation of styrene
either in the presence or absence of [Ph3C][B(C6F5)4]. In
comparison, the analogous Z5-tetramethylcyclopentadienyl
scandium complex (C5Me4H)Sc(CH2C6H4NMe2-o)2 (8)2a
afforded syndiotactic polystyrene in the presence of 1 equiv
of [Ph3C][B(C6F5)4] (Table 1, run 7). These results demon-
strate that an Z5-p-bonding ligand system is superior to an Z1-
bonding analogue for such types of polymerisation catalysts.
In summary, we have demonstrated that the pyrrolyl-ligated
rare earth bis(o-dimethylaminobenzyl) complexes ranging
from the largest La to the smallest Sc can be prepared by
the acid–base reaction between the tris(o-dimethylaminoben-
zyl) complexes Ln(CH2C6H4NMe2-o)3 and a pyrrole ligand
such as 2,5-t-Bu2C4H2NH (or C4Me4NH). The reaction of the
Sc complex (Z5-2,5-t-Bu2C4H2N)Sc(CH2C6H4NMe2-o)2 (1)
with [Ph3C][B(C6F5)4] has afforded the first structurally char-
acterized cationic pyrrolyl–metal complex, [(Z5-2,5-t-
Bu2C4H2N)Sc(CH2C6H4NMe2-o)(dme)][B(C6F5)4] (6). The
coordination mode (Z1 or Z5) of the pyrrolyl ligands has been
found to show a dramatic influence on the polymerisation of
styrene.
659965. [(2,5-t-Bu2C4H2N)Sc(CH2C6H4NMe2-o)][B(C6F5)4] (5). A to-
luene solution (10 mL) of 1 (0.246 g, 0.50 mmol) was added to
[(Ph3C)][B(C6F5)4] (0.461 g, 0.50 mmol) in toluene (15 mL) at room
temperature. After stirring for 5 min, the solvent was removed under
reduced pressure. The residue was washed by hexane. Drying
under vacuum gave a dark brown powder of 5 (0.471 g, ca. 90%)
with a small amount of an unidentified impurity. [(2,5-t-Bu2C4H2N)-
Sc(CH2C6H4NMe2-o)(dme)][B(C6F5)4] (6). DME (0.50 mL) was added
to a toluene solution (2 mL) of 5 (0.103 g, 0.10 mmol) at room
temperature. After stirring for 5 min, the solvent was removed under
reduced pressure. The residue was washed by hexane. Drying under
vacuum gave 6 (0.101 g, 88%) as an orange powder. Single crystals
suitable for X-ray analysis were obtained from a hexane–toluene–
DME solution at room temperature. CCDC 659966. [(2,3,4,5-
Me4C4N)Sc(CH2C6H4NMe2-o)(thf)3][B(C6F5)4] (7). To
a benzene
solution (2 mL) of 4 (0.044 g, 0.10 mmol) was added a benzene
solution (2 mL) of [(Ph3C)][B(C6F5)4] (0.092 g, 0.10 mmol) at room
temperature. After stirring for 5 min, 0.5 mL of THF was added and
the solvent was removed under reduced pressure. The residue was
washed by hexane. Drying under vacuum gave 7 (0.091 g, 76%) as a
dark brown powder with a small amount of an unidentified impurity.
1 Selected reviews on cationic rare earth polymerization catalysts: (a)
Z. Hou, Y. Luo and X. Li, J. Organomet. Chem., 2006, 691,
3114–3121; (b) P. M. Zeimentz, S. Arndt, B. R. Elvidge and J.
Okuda, Chem. Rev., 2006, 106, 2404–2433; (c) S. Arndt and J.
Okuda, Adv. Synth. Catal., 2005, 347, 339–354; (d) J. Gromada, J.
F. Carpentier and A. Mortreux, Coord. Chem. Rev., 2004, 248,
397–410; (e) W. E. Piers and D. J. H. Emslie, Coord. Chem. Rev.,
2002, 233–234, 131–135; (f) Z. Hou and Y. Wakatsuki, Coord.
Chem. Rev., 2002, 231, 1–22.
2 (a) X. Li, M. Nishiura, K. Mori, T. Mashiko and Z. Hou, Chem.
Commun., 2007, 4137–4139; (b) X. Li, J. Baldamus and Z. Hou,
Angew. Chem., Int. Ed., 2005, 44, 962–965; (c) X. Li and Z. Hou,
Macromolecules, 2005, 38, 6767–6769; (d) L. Zhang, Y. Luo and Z.
Hou, J. Am. Chem. Soc., 2005, 127, 14562; (e) Y. Luo, J. Baldamus
and Z. Hou, J. Am. Chem. Soc., 2004, 126, 13910–13911.
3 Selected reviews on mono(cyclopentadienyl) rare earth complexes:
(a) Z. Hou, Bull. Chem. Soc. Jpn., 2003, 76, 2253–2266; (b) S.
Arndt and J. Okuda, Chem. Rev., 2002, 102, 1953–1976.
4 (a) F. Jaroschik, T. Shima, X. Li, K. Mori, L. Ricard, X.-F. Le
Goff, F. Nief and Z. Hou, Organometallics, 2007, 26, 5654–5660;
(b) Y. Yang, B. Liu, K. Lv, W. Gao, D. Cui, X. Chen and X. Jing,
Organometallics, 2007, 26, 4575–4584; (c) E. L. Roux, F. Nief, F.
This work was partly supported by a Grant-in-Aid for
Scientific Research on Priority Areas (No. 18065020, ‘‘Chem-
istry of Concerto Catalysis’’) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan, and a
Grant-in-Aid for Scientific Research (A) (No. 18205010) and
for Young Scientists (B) (No 19750053) from the Japan
Society for the Promotion of Science.
Jaroschik, K. W. Tornroos and R. Anwander, Dalton Trans., 2007,
4866–4870.
¨
Notes and references
5 (a) T. Dube
Vasapollo, Angew. Chem., Int. Ed., 1999, 38, 3657–3659; (b) M.
Ganesan, C. D. Berube, S. Gambarotta and G. P. A. Yap,
´
, S. Conoci, S. Gambarotta, G. P. A. Yap and G.
z (2,5-t-Bu2C4H2N)Sc(CH2C6H4NMe2-o)2 (1). A THF solution (5
mL) of 2,5-t-Bu2C4H2NH (0.540 g, 3.0 mmol) was added to a THF
solution (20 mL) of Sc(CH2C6H4NMe2-o)3 (1.343 g, 3.0 mmol) at
room temperature. After stirring for 12 h at 70 1C, the solvent was
removed under reduced pressure. The residue was washed by diethyl
ether. Recrystallisation by toluene–hexane gave 1 as yellow crystals
(1.329 g, 90%). CCDC 659963. (2,5-t-Bu2C4H2N)Y(CH2C6H4N-
Me2-o)2 (2). To a THF solution (6 mL) of Y(CH2C6H4NMe2-o)3
(0.492 g, 1.0 mmol) was added a THF solution (4 mL) of 2,5-t-
Bu2C4H2NH (0.179 g, 1.0 mmol) at room temperature. After stirring
overnight at 50 1C, the solvent was removed under reduced pressure.
The residue was washed by diethyl ether to give 2 as a light-yellow
powder (0.385 g, 72%). Single crystals suitable for X-ray analysis were
obtained by recrystallisation from hexane solution. CCDC 668429.
(2,5-t-Bu2C4H2N)La(CH2C6H4NMe2-o)2 (3). To a THF solution (10
mL) of La(CH2C6H4NMe2-o)3 (0.542 g, 1.0 mmol) was added a THF
solution (5 mL) of 2,5-t-Bu2C4H2NH (0.179 g, 1.0 mmol) at room
temperature. After stirring for 1 h at room temperature, the solvent
was removed under reduced pressure. The residue was washed by
hexane and dissolved in 30 mL of toluene. Slow evaporation of solvent
gave 3 as orange cubic crystals (0.338 g, 69%). CCDC 659964. (2,3,4,5-
Me4C4N)Sc(CH2C6H4NMe2-o)2 (4). To a THF solution (20 mL) of
Sc(CH2C6H4NMe2-o)3 (1.343 g, 3.0 mmol) was added a THF solution
(5 mL) of 2,3,4,5-Me4C4NH (0.478 g, 3.3 mmol) at room temperature.
After stirring for 12 h at 70 1C, the solvent was removed under reduced
pressure. The residue was washed by hexane to give 4 as a yellow
powder (0.833 g, 64%). Single crystals suitable for X-ray analysis were
obtained from a concentrated hexane solution at ꢁ30 1C. CCDC
´
´
Organometallics, 2002, 21, 1707–1713.
6 (a) Y. Yang, S. Li, D. Cui, X. Chen and X. Jing, Organometallics,
2007, 26, 671–678; (b) C. Pi, Z. Zhang, R. Liu, L. Weng, Z. Chen
and X. Zhou, Organometallics, 2006, 25, 5165–5172; (c) C. Cui, A.
Shafir, C. Reeder and J. Arnold, Organometallics, 2003, 22,
3357–3359; (d) S. Arndt, A. Trifonov, T. P. Spaniol, J. Okuda,
M. Kitamura and T. Takahashi, J. Organomet. Chem., 2002, 647,
158–166; (e) Y. Matsuo, K. Mashima and K. Tani, Organometal-
lics, 2001, 20, 3510–3518; (f) H. Schumann, E. C. E. Rosenthal, J.
Winterfeld and G. Kociok-Kohn, J. Organomet. Chem., 1995, 495,
¨
C12–C14; (g) H. Schumann, P. R. Lee and A. Dietrich, Chem. Ber.,
1990, 123, 1331–1334.
7 A review: Y. Yoshida, S. Matsui and T. Fujita, J. Organomet.
Chem., 2005, 690, 4382–4397.
8 (a) A. R. Dias, A. M. Galvao, A. C. Galvao and M. S. Salema, J.
Chem. Soc., Dalton Trans., 1997, 1055–1061; (b) K. G. Parker, B.
Noll, C. G. Pierpont and M. R. DuBois, Inorg. Chem., 1996, 35,
3228–3234; (c) C. C. McComas, J. W. Ziller and D. L. Van
Vranken, Organometallics, 2000, 19, 2853–2857; (d) A. R. Dias,
A. P. Ferreira and L. F. Veiros, Organometallics, 2003, 22,
5114–5125.
9 As mentioned previously,2a the Ln–CH2C6H4NMe2-o unit is
as active as Ln–CH2SiMe3 for the initiation of olefin polymeriza-
tion. After olefin insertion, the benzyl (or alkyl) group will get
away from the metal center and show little influence on the
propagation.
ꢀc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 2019–2021 | 2021