Preparation and structure of iminopyrrolyl and amidopyrrolyl complexes of group 2 metals

Article abstract of DOI:10.1021/om201210y

Reactions of N-aryliminopyrrolyl ligand 1a, 2-(2,6-ⁱPr ₂C₆H₃N=CH)-C₄H₃NH (Imp^Dipp-H), with dibenzylcalcium gave two types of pyrrolylcalcium complexes, bis(iminopyrrolyl)calcium

Full text of DOI:10.1021/om201210y

Article
pubs.acs.org/Organometallics
Preparation and Structure of Iminopyrrolyl and Amidopyrrolyl
Complexes of Group 2 Metals
Tarun K. Panda,†,‡ Keishi Yamamoto,† Koji Yamamoto,† Hiroshi Kaneko,† Yi Yang,† Hayato Tsurugi,†
and Kazushi Mashima*,†
†Department of Chemistry, Graduate School of Engineering Science, Osaka University, and CREST, JST, Toyonaka, Osaka 560-8531,
Japan
‡Department of Chemistry, Indian Institute of Technology Hyderabad, Ordnance Factory Estate, Yeddumailaram 502205,
Andhra Pradesh, India
S
* Supporting Information
ABSTRACT: Reactions of N-aryliminopyrrolyl ligand 1a, 2-
(2,6-iPr2C6H3NCH)-C4H3NH (ImpDipp-H), with dibenzyl-
calcium gave two types of pyrrolylcalcium complexes,
bis(iminopyrrolyl)calcium (2a) and (amidopyrrolyl)calcium
(3a), via alkane elimination and ligand alkylation reaction,
respectively. Preparation of a mono(iminopyrrolyl) complex, (iminopyrrolyl)Ca[N(SiMe3)2](THF)2 (4a), was accomplished by
the addition of 1 equiv of 1a to Ca[N(SiMe3)2]2(THF)2. A series of group 2 metal bis(iminopyrrolyl) complexes,
[(ImpDipp)2M(THF)3] (M = Sr (5a), Ba, (6a)) and [(ImpMe)2Ca(THF)2] (2b) (2-(4-MeC6H4NCCH3)-C4H3NH (ImpMe-H)),
was selectively prepared via amine elimination reactions, and their molecular structures were clarified by X-ray diffraction
studies.
supported by bidentate iminopyrrolyl ligands. Ligand alkylation
and deprotonation reactions using Ca(CH2Ph)2(THF)0.5 as the
precursor led to the formation of (amidopyrrolyl)- and
bis(iminopyrrolyl)calcium complexes. A mono(iminopyrrolyl)
complex was prepared from Ca[N(SiMe3)2]2(THF)2 and 1
equiv of the N-aryliminopyrrolyl ligand. We also prepared
bis(iminopyrrolyl) complexes of strontium and barium by the
amine elimination reaction of M[N(SiMe3)2]2(THF)3 (M = Sr, Ba)
with 2 equiv of the N-aryliminopyrrolyl ligand. Effects of ionic
radii of the alkaline-earth metals on the coordination number of
bis(iminopyrolyl)metal complexes were clarified by X-ray
diffraction studies.
INTRODUCTION
■
Alkaline-earth metal compounds have attracted recent interest
because of their catalytic applications to ring-opening polymer-
ization of cyclic esters,1,2 polymerization of styrene and dienes,3
and hydroamination and hydrophosphination reactions of
alkenes and alkynes.4 Determining the structure and reactivity
of alkaline-earth metal species is an important step toward the
design and development of efficient catalysts; however, full
realization of the catalytic potential of these elements still
requires substantial advances in understanding their basic
coordination and organometallic chemistry. To stabilize these
extremely oxophilic and electropositive metals, a wide variety of
nitrogen-based ancillary ligands, such as tris(pyrazolyl)-
borates,2e,g,5 aminotrop(on)iminates,6 β-diketiminates,2f,7 and
bis(imino)pyrroles,8 have been introduced to prepare well-
defined alkaline-earth metal complexes. We and others are
interested in synthetic studies of bidentate and tridentate
iminopyrrolyl ligands for early transition metals,9 rare-earth
metals,8,10 and main group elements.11 Iminopyrrolyl ligands
were introduced to the metal center through not only
deprotonation reaction of the acidic pyrrolyl proton but also
alkylation reaction of the imine moiety by using alkylmetal
precursors.9d,i,10g Roesky et al. recently reported the prepara-
tion of heavier alkaline-earth metal complexes of tridentate
2,5-bis(imino)pyrrolyl ligands, as efficient catalysts for intra-
molecular hydroamination reactions,8 and iminopyrrolyl
complexes of magnesium through deprotonation reaction.11
Few studies have explored the coordination behavior and
chemical reactions of iminopyrrolyl complexes of alkaline-earth
metals other than magnesium. Herein, we report the synthesis
and characterization of heavier alkaline-earth metal complexes
RESULTS AND DISCUSSION
■
We have continuously studied the complexation reaction of
imine-based ligands to alkylmetal precursors;12 for example,
when tetrabenzylzirconium was treated with iminopyrrolyl
ligands, an alkyl migration reaction was observed.9i We thus
used Ca(CH2Ph)2(THF)0.5 as an entry for preparing heavier
alkaline-earth metal complexes with iminopyrrolyl ligands.
Treatment of Ca(CH2Ph)2(THF)0.5 with 1 equiv of bulky
iminopyrrolyl ligand 1a resulted in the formation of two types
of calcium complexes, mononuclear bis(iminopyrrolyl)calcium
complex 2a and dinuclear (amidopyrrolyl)calcium complex 3a.
1
By monitoring the complexation reaction in the H NMR
spectrum, we observed the resonances corresponding to the
complexes 2a and 3a in a 1:1.6 ratio; a toluene elimination
reaction smoothly proceeded to afford 2a, while a benzyl
Received: December 5, 2011
Published: March 14, 2012
© 2012 American Chemical Society
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migration to the imine moiety of the ligand gave 3a (eq 1).
Aiming at preparing (amidopyrrolyl)calcium complex 3a in a
selective manner, we conducted a slow addition of ligand 1a to
the solution of Ca(CH2Ph)2(THF)0.5 at low temperature;
however, both complexes 2a and 3a were formed in all runs
examined. Finally, complex 3a was isolated by washing with
pentane and THF followed by crystallization and was fully
1
characterized by H and 13C NMR spectroscopy. Noteworthy
was that three correlating resonances in THF-d8, determined by
1
the H,1H-COSY spectrum, were observed at δ 2.31, 2.81, and
4.13, assignable to the PhCH2CH moiety in the benzylated
ligand backbone. Three broad signals for the dissymmetric
pyrrolyl moiety appeared at δ 6.5, 5.5, and 4.8, and the signals
were observed in a higher magnetic field than those of 2a due
to the η5-coordination of the pyrrolyl ring to the calcium atom.
Similar intramolecular alkylation of iminopyrrolyl ligands by
alkylmetal precursors of group 3 and 4 metals was noted
previously.9i,10g The connectivity and nuclearity of 3a as shown
in eq 1 are supported by an X-ray diffraction study, the quality
of which is unfortunately too low for publication. Some
examples of the μ-η1:η5-coordination mode of the pyrrolyl ring
bridging two metal centers have been reported to date.10g,13,14
In contrast to the alkane elimination reaction, amine
elimination reaction of Ca[N(SiMe3)2]2(THF)2 with 1 equiv
of 1a resulted in the formation of mono(iminopyrrolyl)calcium
complex 4a, and a ligand amination reaction was not observed
Figure 1. Molecular structures of complexes (a) 2a and (b) 4a with
50% thermal ellipsoids. All hydrogen atoms are omitted for clarity.
Table 1. Bond Distances (Å) and Angles (deg) of
(Iminopyrrolyl)calcium Complexes 2a and 4a
2a
Ca−N1
Ca−N3
Ca−O1
N1−Ca−N2
N2−Ca−N4
2.422(2)
2.393(2)
2.389(2)
72.38(7)
177.91(7)
Ca−N2
Ca−N4
Ca−O2
N1−Ca−N3
O1−Ca−O2
2.526(2)
2.534(2)
2.411(2)
100.18(8)
89.29(7)
1
in this case (Scheme 1). In the H NMR spectrum of 4a, a
singlet resonance due to the bis(trimethylsilyl)amido group was
observed at δ 0.23. On the other hand, addition of 2 equiv of 1a
to Ca[N(SiMe3)2]2(THF)2 resulted in the selective formation
4a
1
of bis(iminopyrrolyl)calcium complex 2a. In the H NMR
Ca−N1
Ca−N3
2.388(2)
Ca−N2
Ca−O1
2.467(2)
spectrum of 2a, one singlet resonance assignable to the imine
moiety was observed at δ 7.90, indicating a symmetric structure
in solution. The molecular structures of 2a and 4a were clarified
by X-ray diffraction studies (both complexes depicted in Figure 1),
and bond distances and angles are listed in Table 1. The
calcium center of 2a adopts a six-coordinated distorted
octahedral geometry surrounded by two chelating iminopyr-
rolyl ligands and two cis-arranged THF molecules. The bond
distances of Ca−Npyr (Ca−N1, 2.422(2) Å; Ca−N3, 2.393(2) Å)
are shorter than those of Ca−Nimine (Ca−N2, 2.526(2) Å; Ca−N4,
2.534(2) Å). The bond lengths of Ca−O (Ca−O1, 2.389(2) Å;
Ca−O2, 2.411(2) Å) are in the range of normal Ca−O
bonds.15 The pyrrolyl nitrogen atoms are oriented mutually cis
with an N1−Ca−N3 angle of 100.18(8)°, and the imine
nitrogen atoms occupy the trans positions (N2−Ca−N4,
177.91(7)°). The calcium complex 4a has a five-coordinated
2.312(2)
2.3890(18)
Ca−O2
2.4040(18)
72.93(7)
N1−Ca−N2
N2−Ca−N3
Ca−N3−Si1
N1−Ca−N3
O1−Ca−O2
Ca−N3−Si2
147.59(8)
170.88(7)
111.31(11)
139.48(7)
117.05(11)
distorted trigonal-bipyramidal geometry with one iminopyrrolyl
ligand, one bis(trimethylsilyl)amido ligand, and two THF
located at the axial position. The bond distances of Ca−Namido
(2.312(2) Å) are shorter than that of Ca−Npyr (2.388(2) Å)
due to the effective donation of the nitrogen lone pair to the
Lewis acidic metal center.
With the selective formation of calcium complexes 2a and 4a
in hand, we carried out the preparation of bis(iminopyrrolyl)
complexes of strontium (5a) and barium (6a) via an amine
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(Sr−N2, 2.6691(19) Å; Ba−N2, 2.823(4) Å). The elongation
of the metal−nitrogen covalent bond is probably due to steric
crowding of the equatorial plane.7a,c,8,16
Scheme 1. Preparation of Mono- and Bis(iminopyrrolyl)
calcium Complexes
In contrast to the heavier group 2 metal complexes,
magnesium complex 7a has a five-coordinated distorted
square-pyramidal geometry surrounded by two iminopyrrolyl
ligands at the basal plane of the square pyramid and one THF
located at the axial position. The magnesium atom is located
0.526 Å above the basal plane containing the N1, N2, N3, and
N4 atoms. The low coordination number of 7a is ascribed to
the smaller ionic radius of the magnesium atom and the
bulkiness of the 2,6-diisopropylphenyl group, which occupies
the opposite side of the THF molecule.
When the less bulky iminopyrrolyl ligand 1b was introduced
to the smaller group 2 metals, i.e., calcium and magnesium, by
the amine or alkane elimination procedures (eq 3), we obtained
elimination method as outlined in eq 2. Treatment of
M[N(SiMe3)2]2(THF)3 (M = Sr, Ba) with 2 equiv of
iminopyrrolyl ligand 1a in toluene at 90 °C gave bis(iminopyrrolyl)
complexes 5a and 6a in 89% and 92% yield, respectively. The
1H NMR spectra of both complexes revealed one set of
resonances corresponding to the iminopyrrolyl ligands, but no
signal due to the bis(trimethylsilyl)amido groups attached to
the metal center was observed, suggesting the complete
elimination of 2 equiv of HN(SiMe3)2. Notably, the corre-
sponding mono(iminopyrrolyl) complex, (ImpDipp)Ba[N-
(SiMe3)2], was not detected in the 1H NMR spectrum,
probably due to the increased basicity of the bis(trimethylsilyl)-
amido group bound to the barium atom after introducing one
iminopyrrolyl ligand and facial approach of a second
iminopyrrolyl ligand to the larger central metal compared
with the calcium atom. Both complexes 5a and 6a were char-
acterized spectroscopically as well as by combustion analysis.
Furthermore, we prepared magnesium complex 7a, (ImpDipp)2Mg-
(THF), via an alkane elimination reaction of Mg(CH2Ph)2 with
2 equiv of 1a (eq 2), according to the synthetic route of
bis(iminopyrrolyl)magnesium complexes reported by Roesky
et al.11 In contrast to the reaction in eq 1, we did not detect any
ligand-alkylated product from the reaction mixture.
The molecular structures of bis(iminopyrrolyl) complexes
5a−7a were determined by single-crystal X-ray diffraction
studies and are depicted in Figure 2. Selected bond distances
and angles of 5a−7a are summarized in Table 2. The metal
centers of strontium complex 5a and barium complex 6a are
coordinated by two chelating iminopyrrolyl ligands and three
THF molecules, adopting a seven-coordinated distorted
pentagonal bipyramidal geometry. Two nitrogen atoms of the
imine group are located at the axial site (N2−Sr−N2*,
178.20(9)°; N2−Ba−N2*, 178.27(19)°), while two pyrrolyl
nitrogen atoms and three THF molecules occupy the equatorial
sites. The bond distances of M−Npyr (Sr−N1, 2.651(2) Å;
Ba−N1, 2.821(5) Å) are almost the same as those of M−Nimine
bis(iminopyrrolyl) complexes 2b and 7b, both of which
contained 2 equiv of THF, as confirmed by their H NMR
spectra.
1
The molecular structures of 2b and 7b were also clarified by
X-ray diffraction studies to reveal that both complexes are
isostructural. Figure 3 shows the structure of 2b, while the
structure of 7b is shown in the Supporting Information.
Selected geometries of 2b and 7b are listed in Table 3. The
metal center has a six-coordinated octahedral geometry with
two pyrrolyl nitrogen atoms, imine nitrogen atoms, and two
oxygen atoms trans to each other. In contrast to 2a, the two
chelating iminopyrrolyl moieties of 2b are coplanar due to the
less bulky aromatic substituent on the imine nitrogen atom.
The different bulkiness of the iminopyrrolyl ligand in 1a and 1b
affected the coordination number of the magnesium complexes
7a and 7b; the second THF coordinates to the metal center at
the position trans to the other THF molecule.
With the same series of group 2 metal bis(iminopyrrolyl)
complexes in hand, we preliminarily studied their catalytic
performance for ring-opening polymerization of ε-caprolactone.17
Bis(iminopyrrolyl)calcium complexes 2a or 2b showed low
catalytic activity (less than 20% yield), whereas quantitative
conversions of the monomers were observed for amidocalcium
complexes 3a and 4a. In contrast to the polymerization reaction
by amidocalcium complexes 3a and 4a, the reaction mixture of
bis(iminopyrrolyl)strontium and -barium complex 5a or 6a and
ε-caprolactone (100 equiv) became highly viscous in 3 min
with the consumption of all the monomers. GPC measure-
ments of the poly(ε-caprolactone)s revealed the formation of
very high molecular weight polymers (Mn = 1.0 × 105 for 5a;
Mn = 1.8 × 105 for 6a), probably due to the low initiation
efficiency of the heavier alkaline-earth metal catalysts.
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Figure 2. Molecular structures of bis(iminopyrrolyl) complexes of group 2 metals with 50% thermal ellipsoids. All hydrogen atoms are omitted for
clarity. (a) (ImpDipp)2Sr(THF)3 (5a), (b) (ImpDipp)2Ba(THF)3 (6a), and (c) (ImpDipp)2Mg(THF) (7a).
Table 2. Bond Distances and Angles of Group 2 Metal
Bis(iminopyrrolyl) Complexes 5a−7a
5a
6a
7a
M−N1
M−N2
M−O1
M−O2
N1−M−N2
N1−M−N1*
N2−M−N2*
O1−M−O2
2.651(2)
2.821(5)
2.823(4)
2.746(5)
2.773(4)
62.44(12)
152.3(2)
178.27(19)
143.25(9)
2.122(2)
2.142(2)
2.0265(19)
2.6691(19)
2.647(2)
2.624(3)
66.38(6)
79.46(8)
a
151.62(10)
178.20(9)
143.87(4)
162.57(9)
b
140.04(9)
a
b
N1−Mg−N3. N2−Mg−N4.
SUMMARY
■
We selectively prepared mono- and bis(iminopyrrolyl)calcium
complexes 2a and 4a by the reaction of Ca[N-
(SiMe3)2]2(THF)2 with 1 or 2 equiv of iminopyrrolyl ligand
1a. When Ca(CH2Ph)2(THF)0.5 was used as a starting material,
alkane elimination and ligand alkylation reactions of imino-
pyrrolyl ligand 1a gave 2a and 3a. Characterization of 3a, formed
via imine alkylation reaction of 1a, revealed the dimeric structure
through μ-η1:η5-pyrrolyl coordination of the ligand. Bis-
(iminopyrrolyl) complexes of strontium and barium were
prepared by the amine elimination reaction using M[N(SiMe3)2]2-
(THF)3 (M = Sr, Ba). The heavier group 2 metal complexes of
strontium (5a) and barium (6a) had a seven-coordinated
Figure 3. Molecular structure of (ImpMe)2Ca(THF)2 (2b) with 50%
thermal ellipsoids. All hydrogen atoms are omitted for clarity.
pentagonal-bipyramidal geometry, and the metal−Npyr bonds
were relatively longer than the other amidometal complexes of
barium and strontium. Bis(iminopyrrolyl)complexes of stron-
tium (5a) and barium (6a) were highly active for the ring-
opening polymerization of ε-caprolactone, affording high
molecular weight poly(ε-caprolactone)s compared to the
polymers produced by calcium and magnesium complexes.
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(m, 3H, Ar), 6.91−6.97 (m, 1H, pyr), 6.65−6.61 (m, 1H, pyr), 3.56−
Table 3. Bond Distances (Å) and Angles (deg) of 2b and 7b
3
3.66 (m, 8H, THF), 3.14 (sept, J = 6.8 Hz, 2H, CH(CH3)2) 1.25−
1.31 (m, 8H, THF), 1.20 (d, 3J = 6.8 Hz, 6H, CH(CH3)2), 1.11 (d, 3J =
6.8 Hz, 6H, CH(CH3)2), 0.23 (s, 18 H, N(SiMe3)2); 13C{1H} NMR (100
MHz, 30 °C, C6D6) δ 162.2 (CN), 149.2 (Ar), 140.9 (Ar), 137.3 (pyr),
136.6 (pyr), 125.3 (Ar), 123.6 (Ar), 122.7 (pyr), 112.3 (pyr), 69.7
(THF), 28.0 (CH(CH3)2), 25.7 (CH(CH3)2), 25.3 (THF), 23.4
(CH(CH3)2), 5.23 (N(SiMe3)2). Anal. Calcd for C31H55CaN3O2Si2: C
62.26, H 9.27, N, 7.03. Found: C 62.22, H 9.36, N 7.16.
2b
Ca−N1
Ca−O1
N1−Ca−O1
2.398(4)
2.349(4)
88.38(14)
Ca−N2
N1−Ca−N2
N2−Ca−O1
2.448(3)
110.01(14)
91.59(13)
7b
Mg−N1
Mg−O1
N1−Mg−O1
2.1241(11)
2.1483(13)
89.70(4)
Mg−N2
N1−Mg−N2
N2−Mg−O1
2.1950(13)
102.44(5)
89.69(5)
Synthesis of [(ImpDipp)2Ba(THF)3] (6a). [Ba{N(SiMe3)2}2(THF)3]
(200 mg, 0.30 mmol) and [ImpDipp-H] (170 mg, 0.67 mmol) were
dissolved in toluene (5 mL). The reaction mixture was stirred for 12 h
at 90 °C. The reaction mixture was then cooled to room temperature,
and all volatiles were removed under reduced pressure. The remaining
solid was washed with pentane (5 mL) and then dried in vacuo to give
white solids, which were recrystallized from THF/pentane (v/v = 1/2)
at −35 °C as block-shaped colorless crystals, 6a (261 mg, 92%): mp
145−151 °C (dec). Evacuation of the colorless crystals for a long time
led to the formation of white powders, probably due to the loss of
packing and coordinating THF molecules to form (ImpDipp)2Ba-
EXPERIMENTAL SECTION
■
General Procedures. All manipulations involving air- and
moisture-sensitive organometallic compounds were carried out under
argon using the standard Schlenk technique or an argon-filled
glovebox. Tetrahydrofuran, pentane, and toluene were dried and
deoxygenated by distillation over sodium benzophenone ketyl under
argon and then distilled over CaH2 prior to storing in the glovebox.
Benzene-d6 was dried over Na/K alloy and stored in the glovebox.
1H NMR (300 MHz, 400 MHz) and 13C NMR (75 MHz, 100 MHz)
spectra were measured on a Varian-Unity-INOVA-300 and a Bruker
AVANCEIII-400 spectrometer. Elemental analyses were performed on
a Perkin-Elmer 2400 microanalyzer at the Faculty of Engineering Science,
Osaka University. Iminopyrrolyl ligands,9e [M{N(SiMe3)2}2(THF)n]
(M = Sr, n = 3; Ba, n = 3; Ca, n = 2),18 [Ca(CH2Ph)2(THF)0.5],15a and
[Mg(CH2Ph)2]19 were prepared according to the literature pro-
cedures. Gel permeation chromatographic analysis was carried out at
40 °C by using a Shimadzu LC-10A liquid chromatograph system and
a RID 10A refractive index detector, equipped with a Shodex KF-806 L
column, which was calibrated versus commercially available poly-
styrene standards (Showa Denko).
1
(THF)2. H NMR (400 MHz, 30 °C, THF-d8) δ 7.75 (s, 2H, N
CH), 7.08 (d, 3J = 7.8 Hz, 4H, m-Ar), 7.00 (br, 2H, pyr), 6.98 (t, 3J =
3
4
7.8 Hz, 2H, p-Ar), 6.54 (dd, J = 3.3 Hz, J = 1.2 Hz, 2H, pyr), 6.03
(dd, 3J = 3.3 Hz and 3J = 1.5 Hz, 2H, pyr), 3.59−3.66 (br, 8H, THF),
3
3.18 (sept, J = 6.9 Hz, 4H, CH(CH3)2), 1.74−1.79 (m, 8H, THF),
1.12 (br, 24H, CH(CH3)2); 13C{1H} NMR (100 MHz, 30 °C, THF-
d8) δ 161.7 (CN), 152.1 (ipso-Ar), 141.0 (o-Ar), 138.4 (pyr), 137.3
(pyr), 124.2 (p-Ar), 123.6 (m-Ar), 122.4 (pyr), 110.5 (pyr), 68.2
(THF), 28.8 (CH(CH3)2), 26.4 (THF), 26.1 (CH(CH3)2), 23.2
(CH(CH3)2). Anal. Calcd for C46H66BaN4O3: C 64.21, H 7.73, N 6.51.
Found: 63.82, H 7.56, N 6.31.
Other heavier alkaline earth bis(iminopyrrolyl) complexes were
prepared in similar manner to 6a.
[(ImpDipp)2Ca(THF)2] (2a). Colorless crystals were obtained in 90%
Synthesis of (Amidopyrrolyl)calcium Complex 3a. [Ca-
(CH2Ph)2(THF)0.5] (300 mg, 1.16 mmol) and [ImpDipp-H]
(294 mg, 1.16 mmol) were placed in a Schlenk flask, and THF (12 mL)
was added. The reaction mixture was stirred for 3 h at room temperature.
All volatiles were removed under reduced pressure. The remaining
solid was washed with THF/pentane (v/v = 1/2) and then dried in
vacuo to give white solids of 3a (45% yield, 273 mg, 0.259 mmol). 3a was
recrystallized from THF/pentane (v/v = 1/1) at −35 °C as block-shaped
1
yield: mp 132−138 °C (dec); H NMR (300 MHz, 35 °C, C6D6) δ
7.90 (br, 2H, NCH), 7.24 (br, 2H, p-Ar), 7.10 (br, 6H, m-Ar and
pyr), 6.96−7.00 (m, 2H, pyr), 6.60 (br, 2H, pyr), 3.41 (br, 8H, THF),
3.22 (sept, 3J = 6.6 Hz, 4H, CH(CH3)2), 1.21 (br, 8H, THF), 1.16 (d,
3J = 6.6 Hz, 12H, CH(CH3)2), 1.08 (d, 3J = 6.6 Hz, 12H, CH(CH3)2);
13C{1H} NMR (75 MHz, 35 °C, C6D6) δ 162.5 (CN), 150.0 (ipso-
Ar), 141.1 (Ar), 137.6 (pyr), 137.4 (p-Ar), 124.9 (Ar), 123.6 (pyr),
121.9 (pyr), 112.0 (pyr), 68.8 (THF), 28.3 (CH(CH3)2), 25.9
(CH(CH3)2), 25.4 (THF), 23.2 (CH(CH3)2). Anal. Calcd for
C42H58CaN4O2: C 73.00, H 8.46, N 8.10. Found: C 72.77, H 8.23,
N 7.99.
1
colorless crystals: mp 133−134 °C (dec); H NMR (400 MHz, −60 °C,
THF-d8) δ 6.75−6.95 (m, 10H, m-iPr2C6H3, m- and p-Ph), 6.40−6.57
(m, 8H, p-iPr2C6H3, o-Ph, and pyr), 5.5 (br, 2H, pyr), 4.8 (br, 2H,
pyr), 4.56 (m, 2H, CH(CH3)2), 4.09−4.17 (m, 2H,
N-CHCH2Ph), 3.67 (m, 2H, CH(CH3)2), 3.55−3.65 (m, 16H,
THF), 2.76−2.86 (m, 2H, N-CHCH2Ph), 2.25−2.36 (m, 2H,
[(ImpMe)2Ca(THF)2] (2b). White crystals were obtained in 95%
1
yield: mp 173−177 °C (dec); H NMR (300 MHz, 35 °C, C6D6) δ
3
N-CHCH2Ph), 1.72−1.84 (m, 16H, THF), 1.26 (d, J = 6.0 Hz, 6H,
7.17−7.21 (m, 2H, pyr), 6.94−7.06 (m, 6H, Ar and pyr), 6.55−6.63
(m, 4H, Ar), 6.48−6.53 (m, 2H, pyr), 3.43−3.53 (m, 8H, THF),
2.15−2.17 (br s, 6H, CH3), 2.03−2.06 (br s, 6H, CH3), 1.25−1.35
(m, 8H, THF); 13C{1H} NMR (75 MHz, 35 °C, C6D6) δ 167.4 (CN),
148.2 (ipso-Ar), 139.2 (pyr), 136.8 (Ar), 133.6 (Ar), 129.8 (Ar), 123.7
(pyr), 116.4 (pyr), 111.7 (pyr), 68.4 (THF), 25.5 (THF), 21.0 (CH3),
16.7 (CH3). Anal. Calcd for C34H42CaN4O2: C 70.55, H 7.31, N 9.68.
Found: C 70.21, H 7.11, N 9.39.
3
3
CH(CH3)2), 1.13 (d, J = 6.4 Hz, 6H, CH(CH3)2), 1.08 (d, J = 6.4
Hz, 6H, CH(CH3)2), 0.88 (d, J = 6.0 Hz, 6H, CH(CH3)2); 13C{1H}
3
NMR (100 MHz, −60 °C, THF-d8) δ 159.8 (ipso-iPr2C6H3), 146.5
(o-iPr2C6H3), 145.5 (pyr), 144.9 (o-iPr2C6H3), 144.8 (ipso-Ph), 130.8
(o-Ph), 127.0 (m-iPr2C6H3), 124.0 (m-Ph), 122.9 (p-Ph), 122.5 (pyr),
117.1 (p-iPr2C6H3), 104.8 (pyr), 104.4 (pyr), 72.7 (N-CH), 68.2
(THF), 50.3 (CH2Ph), 27.6 (CH(CH3)2), 26.7 (CH(CH3)2), 26.5
(CH(CH3)2), 26.39 (THF), 26.35 (CH(CH3)2), 26.0 (CH(CH3)2).
Resonances for both carbon atoms of m-iPr2C6H3 appeared at δ 127.0.
A signal of one methyl carbon was overlapped with the resonance of
THF. Anal. Calcd for C64H88Ca2N4O4: C 72.68, H 8.39, N 5.30.
Found: C 72.25, H 8.54, N 5.32.
[(ImpDipp)2Sr(THF)3] (5a). Colorless crystals were obtained in 89%
1
yield: mp 142−146 °C (dec); H NMR (300 MHz, 35 °C, C6D6) δ
7.91 (br, 2H, NCH), 7.27 (br, 2H, p-Ar), 7.10 (br, 6H, m-Ar and pyr),
6.90 (br, 2H, pyr), 6.46 (br, 2H, pyr), 3.47 (br, 12H, THF), 3.11
(br, 4H, CH(CH3)2), 1.30 (br, 12H, THF), 1.08 (br, 24H, CH3);
13C{1H} NMR (75 MHz, 35 °C, C6D6) δ 162.1 (CN), 149.8 (ipso-
Ar), 140.7 (Ar), 138.0 (pyr), 137.6 (p-Ar), 124.9 (Ar), 123.6 (pyr),
122.3 (pyr), 112.2 (pyr), 68.3 (THF), 28.6 (CH(CH3)2), 25.6 (THF),
23.3 (CH(CH3)2). Anal. Calcd for C46H66N4O3Sr: C 68.15, H 8.20, N
6.91. Found: C 67.87, H 7.95, N 6.62.
Synthesis of [(ImpDipp)Ca(N(SiMe3)2)(THF)2] (4a). Toluene (20 mL)
was added to
a Schlenk flask containing solid [Ca{N-
(SiMe3)2}2(THF)2] (750 mg, 1.48 mmol) and [ImpDipp-H] (375 mg,
1.48 mmol) at room temperature, and then the reaction mixture
was stirred for 12 h. All volatiles were removed under reduced
pressure. The remaining solid was washed with pentane (5 mL) and
then dried in vacuo to give white solids of 4a (597 mg, 67%). 4a was
recrystallized from pentane and several drops of THF at −35 °C as
block-shaped colorless crystals: mp 111−115 °C; 1H NMR (400 MHz,
30 °C, C6D6) δ 7.88 (s, 1H, NCH), 7.47 (s, 1H, pyr), 7.11−7.16
Synthesis of [(ImpMe)2Mg(THF)2] (7b). Mg(CH2Ph)2 (103 mg,
0.50 mmol) and [ImpMe-H] (198 mg, 1.0 mmol) were dissolved in
toluene (5 mL). The reaction mixture was stirred for 12 h at 90 °C.
After cooling the reaction mixture to room temperature, all volatiles
were removed under reduced pressure. The remaining solid was
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dx.doi.org/10.1021/om201210y | Organometallics 2012, 31, 2268−2274

Organometallics
Article
washed with pentane (5 mL) and dried in vacuo to give white solids,
which were crystallized from THF/pentane (v/v = 1/2) at −35 °C.
Block-shaped colorless crystals of 7b were obtained (250 mg, 88%):
Pal for his contribution to X-ray analyses of the alkaline-earth
metal complexes. This work was supported by the Core
Research for Evolutional Science and Technology (CREST)
program of the Japan Science and Technology Agency (JST),
Japan.
1
mp 147−153 °C (dec); H NMR (400 MHz, 30 °C, C6D6) δ 7.01−
7.08 (m, 6H, Ar and pyr), 6.73−6.78 (m, 4H, Ar), 6.46−6.52 (m, 4H, pyr),
3.47−3.39 (m, 8H, THF), 2.18 (s, 6H, CH3), 2.02 (s, 6H, CH3),
1.23−1.30 (m, 8H, THF); 13C{1H} NMR (75 MHz, 35 °C, C6D6) δ
167.5 (CN), 148.2 (ipso-Ar), 139.2 (pyr), 136.7 (p-Ar), 133.5
(m-Ar), 129.7 (o-Ar), 123.7 (pyr), 116.3 (pyr), 111.7 (pyr), 68.3 (THF),
25.5 (THF), 20.9 (CH3), 16.4 (CH3). Anal. Calcd for C34H42MgN4O2: C
72.53, H 7.52, N 9.95. Found: C 72.27, H 7.41, N 9.72.
REFERENCES
■
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Similar to the synthesis of 7b, colorless crystals of 7a were obtained
1
in 93% yield: mp 197−201 °C (dec); H NMR (300 MHz, 35 °C,
4
C6D6) δ 7.77 (d, J = 1.1 Hz, 2H, NCH), 7.10−7.30 (m, 6H, Ar),
6.80 (dd, 3J = 3.5 Hz, 4J = 1.0 Hz, 2H, pyr), 6.32 (dd, 3J = 3.5 Hz, 4J =
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N 8.92.
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2
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2
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2
4
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was used to draw the molecules.
ASSOCIATED CONTENT
■
S
* Supporting Information
Molecular structure of 7b, polymerization of ε-caprolactone
catalyzed by bis(iminopyrrolyl) complexes of group 2 metals,
and the 1H NMR spectrum of the representative poly(ε-
caprolactone), and CIF files. These materials are available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mashima@chem.es.osaka-u.ac.jp. Tel/Fax: +81-6-
6850-6245.
(8) Jenter, J.; Koppe, R.; Roesky, P. W. Organometallics 2011, 30,
̈
Notes
1404.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We are grateful for the JSPS Fellowship to T.K.P. H.K. and K.Y.
thank the Global COE (Center of Excellence) Program “Global
Education and Research Center for Bio-Environmental
Chemistry” of Osaka University. K.Y. expresses his special
thanks for the financial support provided by the JSPS Research
Fellowships for Young Scientists. We acknowledge Dr. Kuntal
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