Transition Metal Complexes Containing C2-Symmetric Bis(imidazolin-2-imine) Ligands Derived from a 1-Alkyl-3-arylimidazolin-2-ylidene

Article abstract of DOI:10.1002/zaac.201500569

The new N-heterocyclic carbene 1-(2, 6-diisopropylphenyl)-3, 4,5-trimethylimidazolin-2-ylidene (IPrMe) (3) was prepared in three steps from methyl isothiocyanate and 2, 6-diisopropylaniline, affording N-methyl-N′-2, 6-diisopropylthiourea (1). Reaction of 1 with 3-hydroxybutanone gave the imidazolin-2-thione (IPrMe)S (2), which was reduced with elemental potassium to the carbene IPrMe (3). Treatment of 3 with trimethylsilyl azide (Me₃SiN₃) in boiling toluene, followed by desilylation in methanol, gave the imidazolin-2-imine (IPrMe)NH (4). The ethylene-bridged diimine ligand N,N′-bis[1-(2, 6-diisopropylphenyl)-3, 4,5-trimethylimidazolin-2-ylidene]-1, 2-ethanediamine, (IPrMe)NCH₂CH₂N(IPrMe) (5) was prepared by reaction of 4 with 1, 2-ethyleneditosylate and subsequent deprotonation with KOtBu. The reaction of 5 with metal dichlorides furnished the tetrahedral complexes [(5)MCl₂] [M = Fe (6), M = Ni (7), M = Zn (8)] and the square-planar complex [(5)PdCl₂] (9), as revealed by X-ray diffraction analyses. The X-ray crystal structures of 2, 3, and 4 were also established. Complex 8 and related zinc(II) bis(imidazolin-2-imine) complexes were tested in the polymerization of d, l-lactide.

Full text of DOI:10.1002/zaac.201500569

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DOI: 10.1002/zaac.201500569
Transition Metal Complexes Containing C2-Symmetric Bis(imidazolin-2-
imine) Ligands Derived from a 1-Alkyl-3-arylimidazolin-2-ylidene
Thomas Glöge,[a] Frouke Aal,[a] Sabina-Alexandra Filimon,[b][‡] Peter G. Jones,[a]
Janna Michaelis de Vasconcellos,[b] Sonja Herres-Pawlis,[c] and Matthias Tamm[a]
Dedicated to Professor F. Ekkehardt Hahn (“Ekki”) on the Occasion of His 60th Birthday
Keywords: N-heterocyclic carbenes; N-donor ligands; Guanidines; Transition metals; Lactide polymerization
Abstract. The new N-heterocyclic carbene 1-(2,6-diisopropylphenyl)- ylphenyl)-3,4,5-trimethylimidazolin-2-ylidene]-1,2-ethanediamine,
3,4,5-trimethylimidazolin-2-ylidene (IPrMe) (3) was prepared in three
steps from methyl isothiocyanate and 2,6-diisopropylaniline, affording
N-methyl-NЈ-2,6-diisopropylthiourea (1). Reaction of 1 with 3-hy-
(IPrMe)NCH2CH2N(IPrMe) (5) was prepared by reaction of 4 with
1,2-ethyleneditosylate and subsequent deprotonation with KOtBu. The
reaction of 5 with metal dichlorides furnished the tetrahedral com-
droxybutanone gave the imidazolin-2-thione (IPrMe)S (2), which was plexes [(5)MCl2] [M = Fe (6), M = Ni (7), M = Zn (8)] and the square-
reduced with elemental potassium to the carbene IPrMe (3). Treatment planar complex [(5)PdCl2] (9), as revealed by X-ray diffraction analy-
of 3 with trimethylsilyl azide (Me3SiN3) in boiling toluene, followed
by desilylation in methanol, gave the imidazolin-2-imine (IPrMe)NH
(4). The ethylene-bridged diimine ligand N,NЈ-bis[1-(2,6-diisoprop-
ses. The X-ray crystal structures of 2, 3, and 4 were also established.
Complex 8 and related zinc(II) bis(imidazolin-2-imine) complexes
were tested in the polymerization of d,l-lactide.
the ligands BLR,RЈ (Scheme 1), with the contribution of the
dipolar mesomeric form increasing significantly upon coordi-
nation to transition metal complex fragments. As a conse-
Introduction
Bis(imidazolin-2-imine) ligands such as BLMe and BLiPr,[1]
and related guanidine-type ligands,[2] have found widespread
use in organometallic and coordination chemistry,[3] in particu-
lar as ancillary ligands in homogeneous catalysis.[4] Because
the imidazole moiety can efficiently stabilize a positive
charge,[5] these diimine species are highly basic and can serve
as strong N-donor ligands towards transition metals. The re-
sulting strong polarization of the exocyclic C=N bond can be
described by the two limiting resonance structures A and B for
* Prof. Dr. M. Tamm
Fax: +49-251-391-5309
E-Mail: m.tamm@tu-bs.de
# née Börner
[a] Institut für Anorganische und Analytische Chemie
Technische Universität Braunschweig
Hagenring 30
38106 Braunschweig, Germany
[b] Institut für Anorganische Chemie
Department Chemie
Universität Paderborn
Warburger Str. 100
33098 Paderborn, Germany
[c] Lehrstuhl für Bioanorganische Chemie
Institut für Anorganische Chemie
RWTH Aachen University
Landoltweg 1
52074 Aachen, Germany
Scheme 1. Mesomeric structures (A, B) for bis(imidazolin-2-imine)
[‡] née Börner.
ligands BLR,RЈ (BLMe: R = RЈ = Me; BLiPr: R = RЈ = iPr; BLDipp,Me
:
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201500569 or from the au-
thor.
R = 2,6-diisopropylphenyl, RЈ = Me); schematic presentation of C2-
symmetric bis(imidazolin-2-imine) metal complexes (box).
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quence of this charge separation, the imidazole rings in the
resulting metal complexes usually adopt a perpendicular orien-
tation relative to the N–M–N plane, precluding any significant
π-interaction with the metal-bound nitrogen atoms.
Accordingly, bis(imidazolin-2-imine) ligands with a sym-
metric N,NЈ-substitution pattern (R = RЈ), such as BLMe and
BLiPr, usually form Cs- or C2v-symmetric chelate complexes
with four magnetically equivalent nitrogen substituents.[1] The
introduction of unsymmetric imidazolin-2-ylidene moieties
with nitrogen substituents of distinctly different size, however,
might afford BLR,R’ ligands (R = BIG ϶ RЈ = small) that are
able to form chiral C2-symmetric complexes by adopting a
conformationally stable anti-orientation of the R and RЈ pairs
of substituents as illustrated in Scheme 1 (box). In principle,
this would allow the synthesis of chiral complexes for applica-
tions in asymmetric catalysis such as transfer hydrogen-
ation.[4c,4f,4g] A similar concept was developed for related α-
diimine ligands and their respective nickel(II) and palladi-
um(II) complexes, which serve as catalysts for the production
of highly branched polyethylene.[6]
To test this hypothesis, we report herein the preparation of
the bis(imidazolin-2-imine) ligand BLDipp,Me (5), which con-
tains large 2,6-diisopropylphenyl (Dipp) and small methyl
(Me) substituents, and its reaction with the metal dihalides
FeCl2, NiCl2, ZnCl2, and PdCl2. This affords complexes of the
type [(BLDipp,Me)MCl2], which were structurally characterized
to elucidate whether chiral C2-symmetric complexes are
formed.
Scheme 2. Preparation of bis(imidazolin-2-imine) complexes.
Results and Discussion
Ligand Synthesis
Following the established synthetic route for imidazolin-2-
imines,[5] an unsymmetrical N-heterocyclic carbene (NHC)[7]
with one large and one small nitrogen substituent was required.
We chose to synthesize the unknown 1-(2,6-diisopropyl-
phenyl)-3,4,5-trimethylimidazolin-2-ylidene (IPrMe) (3).
Metal complexes of a related NHC, 1-(2,6-diisopropylphenyl)-
3-methylimidazolin-2-ylidene, with hydrogen atoms in the 4,5-
positions have been reported and were generally prepared from
the corresponding imidazolium salts without isolation of the
free carbene.[8]
The preparation of the required carbene 3 commences with
the reaction between methyl isothiocyanate and 2,6-diisopropyl-
aniline, which afforded thiourea 1 according to a modified
literature procedure.[9] Treatment of 1 with 3-hydroxybutanone
in 1-hexanol gave the imidazolin-2-thione 2 in satisfactory
yield (65%, Scheme 2).[10] The molecular structure of 2 was
established by X-ray diffraction analysis, after suitable single
Figure 1. ORTEP diagram of 2 with thermal displacement parameters
drawn at 50% probability. Selected bond lengths /Å and angles /°:
C1–S 1.6801(12), C1–N1 1.369(15), C1–N2 1.3582(15); N1–C1–N2
105.15(10).
Reduction of thione 2 proceeded by treatment with potas-
crystals had been obtained by diffusion of n-hexane into a satu- sium metal in refluxing THF, furnishing the free Arduengo-
rated THF solution (Figure 1). As expected, the phenyl plane type carbene 3 in satisfactory yield (79%) as a pale yellow
of the Dipp substituent adopts a perpendicular orientation rela- solid after sublimation (Scheme 2).[10] The carbene carbon
tive to the imidazole ring with an interplanar angle of 88.9°, atom gives rise to a characteristic low-field 13C NMR signal
so that the molecule possesses approximate mirror symmetry at δ = 215.3 ppm. Single crystals suitable for X-ray diffraction
(r.m.s. deviation 0.14 Å). The C–S bond length of analysis were obtained by cooling a saturated n-hexane solu-
1.6801(12) Å falls in the range that has been established for tion of 3 to –35 °C. The resulting molecular structure repre-
similar compounds.[11]
sents one of only few available structures of asymmetrically
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substituted N-heterocyclic carbenes.[12] It consists of two inde- C1–N1 bond length of 1.295(2) Å is observed that exceeds the
pendent but similar molecules differing in the orientation of value of about 1.28 Å expected for a typical C(sp2)–N(sp2)
one isopropyl group; one molecule is shown in Figure 2. The bond length.[15] Likewise, surprisingly long internal C–N dis-
C1–N1 and C1–N2 bond lengths of 1.3690(13) and tances of 1.369(2) Å (C1–N2) and 1.387(2) Å (C1–N3) are ob-
1.3617(13) Å and the N1–C1–N2 angle of 101.66(8)° are in served together with a C2–N1–C3 angle of 104.54(12)°, which
good agreement with the structural parameters found for other lies between the values reported for imidazolium ions and
carbenes of the imidazolin-2-ylidene-type.[13]
imidazolin-2-ylidenes, respectively.[13] These structural param-
eters confirm that, from a structural point of view, electron
delocalization within the heterocycle does not play an impor-
tant role for such imine systems.[5] Investigation of the crystal
packing of 4 reveals an intermolecular C–H···N contact of
2.64 Å, which falls below the van der Waals cut-off criterion
of 2.75 Å[16] and involves the imine nitrogen atom N1 and one
of the Dipp methyl groups, affording parallel zigzag chains of
the molecules 4 in the solid state.
The preparation of the diimine BLDipp,Me (5) requires the
linkage of two imine molecules 4 via an ethylene bridge,
which can be accomplished by the reaction of 1,2-ethylenedi-
tosylate with two equivalents of 4 followed by deprotonation
with KOtBu (Scheme 2). Single crystals suitable for X-ray dif-
fraction analysis were obtained by cooling a saturated n-hex-
ane solution to –25 °C. The asymmetric unit of 5 contains two
Figure 2. ORTEP diagram of one of the two crystallographically inde- independent molecules with very similar structural parameters,
pendent molecules 3 with thermal displacement parameters drawn at
50% probability. Selected bond lengths /Å and angles /°: C1–N1
ecule is further discussed (Figure 4). It exhibits exocyclic
1.3690(13), C1–N2 1.3617(13); N1–C1–N2 101.66(8).
and therefore, only the structure of the first independent mol-
C–N bonds of 1.2770(19) Å (C3–N1) and 1.2788(18) Å (C21–
N4), which are considerably shorter than the corresponding
The carbene 3 can be treated with trimethylsilyl azide to
undergo a Staudinger type[14] reaction, and subsequent dissol-
ution and stirring of the intermediate 2-(trimethylsilylimino)-
imidazoline in methanol yields the imidazolin-2-imine 4 as a
white solid (Scheme 2).[5] An X-ray diffraction analysis of 4
was performed on single crystals grown by diffusion of n-hex-
ane into a saturated THF solution, and the molecular structure
is shown in Figure 3. Compounds 2 and 4 are isotypic; accord-
ingly, the latter also displays approximate mirror symmetry
(r.m.s.d. 0.17 Å). The structural parameters are in excellent
agreement with those established for related 1,3-dialkyl- and
1,3-diarylimidazolin-2-imines.[5] Accordingly, a relatively long
bond in the free imine 4 [1.2947(17) Å], whereas the internal
C–N bonds are elongated. These structural features indicate
that charge separation and electron delocalization is even less
pronounced in 5 and that its solid-state structure is well pre-
sented by the resonance structure A depicted in Scheme 1 (vide
supra). As a measure of these effects, the parameter ρ = 2a/(b
+ c) has been introduced for guanidine systems,[17] with a, b,
and c representing the exo- (a) and endocyclic (b, c) C–N dis-
Figure 4. ORTEP diagram of one of the two crystallographically inde-
pendent molecules of BLDipp,Me (5) with thermal displacement param-
eters drawn at 50% probability. Selected bond lengths /Å and angles
/° of the first independent molecule: C1–N1 1.4579(18), C3–N1
1.2770(19), C3–N2 1.4021(18), C3–N3 1.3855(18), C2–N4
Figure 3. ORTEP diagram of imine 4 with thermal displacement pa-
rameters drawn at 50% probability. Selected bond lengths /Å and 1.4541(19), C21–N4 1.2788(18), C21–N5 1.4027(17), C21–N6
angles /°: C1–N1 1.295(2), C1–N2 1.369(2), C1–N3 1.387(2); N2–
C1–N3 104.54(13).
1.3849(19); N2–C3–N3 103.75(13), N5–C21–N6 104.15(12); ρ(C3) =
0.916, ρ(C21) = 0.917 (see footnote in Table 1 for a definition).
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tances within the CN3 moieties. For 5, ρ values of 0.916 and
0.917 well below unity are calculated for the two imidazolin-
2-imine sites and will be used for illustration of structural
changes upon metal coordination by comparison with the ρ
values in BLDipp,Me transition metal complexes (vide infra).
Transition Metal Complexes
The complexes [(BLDipp,Me)MCl2] [M = Fe (6), M = Ni (7),
M = Zn (8)] were prepared by the reaction of the diimine li-
gand 5 with [FeCl2(THF)1.5], [NiCl2(DME)] (DME = 1,2-di-
methoxyethane) or ZnCl2, respectively (Scheme 2). For all
three complexes, the composition was confirmed by elemental
analysis and mass spectrometry. Since the complexes 6 and 7
are paramagnetic, only the diamagnetic zinc complex 8 was
additionally characterized by NMR spectroscopy. It should be
Figure 6. ORTEP diagram of [(BLDipp,Me)NiCl2] (7) in 7·1.5THF with
thermal displacement parameters drawn at 50% probability.
1
noted that the H and 13C NMR spectra do not provide evi-
dence for the formation of a conformationally stable C2-sym-
metric complex in solution, and only one set of resonances is
observed for the isopropyl groups, namely one septet and two
1
doublet H NMR resonances for the tertiary CH and for the
diastereotopic CH3 groups, respectively. Single crystals for X-
ray diffraction analysis were grown by diffusion of n-hexane
into saturated solutions of 6 and 7 or by diffusion of diethyl
ether into a solution of 8 in CH2Cl2. All complexes incorporate
solvent upon crystallization, and the molecular structures of
the [(BLDipp,Me)MCl2] complexes in the solvates 6·THF,
7·1.5THF and 8·CH2Cl2 are shown in Figure 5, Figure 6, and
Figure 7. The nickel complex contains two independent mol-
ecules in the asymmetric unit; despite some differences in ring
orientations, these are generally similar, and therefore only the
structural parameters of the first independent molecule are fur-
ther discussed. Pertinent structural data of complexes 6, 7, and
8 are assembled in Table 1.
Figure 7. ORTEP diagram of [(BLDipp,Me)ZnCl2] (8) in 8·CH2Cl2 with
thermal displacement parameters drawn at 50% probability.
entation of the planes containing the MCl2 and MN2 coordina-
tion spheres deviates significantly from the perpendicular
alignment that would be expected for a regular tetrahedron;
complexes 6–8 display dihedral angles of 79.0°, 73.3°, and
78.1°, respectively, with the nickel complex showing the
largest deviation. Another method for describing the distortion
from ideal arrangements in four-coordinate was proposed by
Houser,[18] who introduced the geometry index τ4 = [360° –
(α + β)]/141°, where α and β represent the largest angles in the
four-coordinate species. The values of τ4 will range from 1.0
for a perfect tetrahedron to zero for a perfect square-planar
arrangement. Intermediate structures, including trigonal-py-
ramidal and seesaw, will afford values 0 Ͻ τ4 Ͻ 1.0. In our
series 6–8, the N–M–Cl angles are the largest (Table 1), and
the calculated τ4 values of 0.86 (6), 0.77 (7), and 0.82 (8) are
similar to those established for [(BLiPr)MCl2], where a greater
distortion was also observed for the corresponding nickel com-
plex in comparison with its iron and zinc congeners.[3e]
Figure 5. ORTEP diagram of [(BLDipp,Me)FeCl2] (6) in 6·THF with
thermal displacement parameters drawn at 50% probability.
In all complexes, the metal atoms are four-coordinate and
The metal-nitrogen and metal-chlorine distances are also
display distorted tetrahedral arrangements with the diimine li- similar to those established for the corresponding
gand forming a five-membered chelate ring with acute N1–M– [(BLiPr)MCl2] complexes (M = Fe, Ni, Zn)[3e,4d] and for related
N2 bite angles of about 86°. Similarly to the structures of re- bis(guanidine) iron, nickel, and zinc complexes.[19] As ex-
lated complexes of the type [(BLiPr)MCl2],[3e] the relative ori- pected, the ρ values increase upon metal coordination from
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Table 1. Selected bond lengths /Å and angles /° in metal complexes 6–9.
6·THF
7·1½THFe)
8·CH2Cl2
9·CH3CN
M–N1
M–N2
M–Cl1
M–Cl2
C3–N1
C3–N3
C3–N4
C21–N2
C21–N5
C21–N6
N1–M–N2
Cl1–M–Cl2
N1–M–Cl1
N1–M–Cl2
N2–M–Cl1
N2–M–Cl2
2.0660(17)
2.0816(18)
2.2911(7)
2.2934(6)
1.317(3)
1.369(3)
1.400(3)
1.325(3)
1.368(3)
1.381(3)
85.74(5)
117.38(2)
105.03(5)
118.54(6)
120.42(6)
106.21(5)
79.0
1.9885(11)
2.0006(11)
2.2477(4)
2.2624(4)
1.3202(17)
1.3653(17)
1.3704(17)
1.3220(17)
1.3696(17)
1.3714(17)
85.65(5)
111.63(2)
116.36(4)
105.01(3)
100.95(3)
135.39(3)
73.3
2.0286(9)
2.0418(9)
2.2402(3)
2.2586(3)
1.3170(14)
1.3595(14)
1.3796(13)
1.3183(14)
1.3684(13)
1.3758(13)
86.51(4)
110.26(1)
109.02(3)
118.97(3)
125.51(3)
105.64(3)
78.1
2.0455(11)
2.0307(11)
2.3231(3)
2.3090(3)
1.3243(16)
1.3573(16)
1.3673(16)
1.3245(16)
1.3544(16)
1.3714(15)
80.03(4)
90.40(1)
95.70(3)
173.04(3)
173.71(3)
93.61(3)
5.5
a)
MCl2/MN2
M–N–C–N
22.6, 21.5
0.86
41.4
45.8, 40.5
0.77
63.2
35.1, 46.5
0.82
63.0
–59.2, 44.9
0.09
3.3
b)
τ4
Imidazolec)
ρd)
0.951, 0.964
0.965, 0.965
0.962, 0.961
0.972, 0.972
a) Angle between MCl2 and MN2 planes. b) τ4 = [360°– (α + β)]/141, where α and β represent the largest angles in four-coordinate species. c)
Angle between imidazole planes. d) ρ = 2a/(b + c), with a, b and c representing the exo- (a) and endocyclic (b, c) distances within the CN3
guanidine moiety. e) Values for one of the two crystallographically independent molecules.
0.916 and 0.917 in the free BLDipp,Me ligand to ca. 0.95–0.97 square-planar coordination sphere around the palladium atom
(Table 1), indicating a substantial charge delocalization, albeit (τ4 = 0.09). The Pd–N and Pd–Cl bond lengths are similar to
to a smaller extent than found for half-sandwich cyclopen- those established for other bis(imidazolin-2-imine) palladi-
tadienyl and arene ruthenium complexes, where significantly um(II) chloride complexes,[20] e.g. [(BLiPr)PdCl2] (Table 1).[3f]
stronger metal–nitrogen interaction is observed.[3a,4c,4g] In- However, the complex displays Cs rather than the expected
deed, the ligands adopt the anticipated C2-symmetric confor- C2 symmetry (r.m.s.d. 0.01 Å), with the methyl and Dipp N-
mation shown in Scheme 1 (box); however, the M–N–C–N tor- substituents each pointing to the same side of the square-planar
sion angles are comparatively small and range from 21.5° in 6 PdN2Cl2 moiety. The acetonitrile molecule is well-ordered, and
to 46.5° in 8, thus deviating substantially from a perpendicular its hydrogen atoms (all clearly located) form C–H···Cl contacts
orientation of the imidazole rings relative to the MN2 planes. to the PdCl2 group.
The methyl substituents point towards the MCl2 moiety, and
inspection of the structures reveals that only a small barrier
can be expected for the interconversion between the enantio-
meric C2-symmetric conformations. This is corroborated by
the NMR spectra of the zinc complex 8, which reveal averaged
C2v symmetry in solution on the NMR time scale (vide supra).
Noteworthy, the dichloromethane solvent molecule of 8 is
well-ordered and its hydrogen atoms each make one C–H···Cl
contact to a chlorine atom of the ZnCl2 group.
Since the complexes 6–8 display distorted tetrahedral arran-
gements, the preparation of the square-planar complex of the
BLDipp,Me ligand was attempted, which might afford a higher
barrier because of a coplanar orientation of the MN2 and
MCl2 planes. Accordingly, BLDipp,Me (5) was treated with
Figure 8. ORTEP diagram of [(BLDipp,Me)PdCl2] (9) in 9·CH3CN with
thermal displacement parameters drawn at 50% probability.
[(COD)PdCl2] (COD = 1,4-cyclooctadiene) in THF, which af-
forded the complex [(BLDipp,Me)PdCl2] (9) in almost quantita-
1
tive yield as a red crystalline solid. The H NMR spectrum
exhibits broad signals for the isopropyl CH3 and CH and also
Since the observed line broadening in the 1H NMR spectrum
for the NCH3 hydrogen atoms, indicating that rotation around of 9 gave evidence for a fluxional process, we performed a
the exocyclic N–Cimidazole bonds is slow at room temperature temperature-dependent NMR study (see the Supporting Infor-
on the NMR time-scale. Single crystals for X-ray diffraction mation). Below –50 °C, separate signals indicate the presence
analysis were isolated from acetonitrile solution at –34 °C; the of two isomers in a 1:2.3 ratio, each giving rise to three sing-
resulting molecular structure (Figure 8) confirms the expected lets for the methyl CH3 groups together with two septets and
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four doublets for the isopropyl CH3 and CH groups, respec- stronger electron donating capability of the guanidine in com-
tively. We tentatively assign these resonances to Cs- and C2- parison with the pyridine ligand. However, the symmetry of
1
symmetric conformers, suggesting that interconversion be- the H NMR spectrum in [D6]acetone indicates fast exchange
tween chiral C2-symmetric enantiomers can be indeed slowed between the coordinated and uncoordinated imidazolin-2-im-
down at low temperature, but that their energy is (too) close ine arms in solution at room temperature, potentially by a five-
to that of the achiral Cs-symmetric conformer.
coordinate transition state.
For comparison and for the catalytic study described below,
Crystallization of 10 from dichloromethane/n-pentane solu-
the zinc(II) chloride complex 10 containing the pincer ligand tion gave single crystals of 10·HCl·CH2Cl2, in which the unco-
2,6-bis[(1,3-di-tert-butylimidazolin-2-imino)methyl]pyridine ordinated imine moiety has reacted with traces of hydrogen
(TLtBu [21] was prepared in high yield by reaction of TLtBu chloride from the chlorinated solvent (Figure 10). The zinc co-
)
with anhydrous ZnCl2 in THF (Scheme 3). Elemental analysis ordination sphere is similar to that in 10, whereas protonation
confirmed the general formula [(TLtBu)ZnCl2]. Single-crystals at nitrogen atom N3 affords a considerably longer exocyclic
of 10, suitable for X-ray diffraction analysis, were obtained C19–N3 bond, shorter endocyclic C19–N6 and C19–N7 bonds
from THF/n-hexane solution. The molecular structure in Fig- and consequently an increase of the ρ value from 0.92 in 10
ure 9 reveals a bidentate κ2 coordination mode of the ligand to 1.02 in 10·HCl (Table 2). The chloride counterion is in-
through the pyridine nitrogen atom N2 and the imidazolin-2- volved in a short contact N3–H03···Cl3, whose dimensions
imine nitrogen atom N1, with one imdiazolin-2-imine moiety Cl3–H03 2.32 Å and N3–H03–Cl3 167° indicate a strong
remaining uncoordinated. This resembles the situation found hydrogen bond.[22]
for related cobalt(II) and iron(II) complexes of the type
[(TLtBu-κN1:κN2)MCl2] (M = Co, Fe).[21c] The zinc atom re-
sides in a distorted tetrahedral environment (τ4 = 0.87), with
the Zn–N1imine bond [1.982(3) Å] being shorter than the Zn–
N2pyridine bond (2.088(3) Å], which can be explained by the
Figure 10. ORTEP diagram of 10·HCl in 10·HCl·CH2Cl2 with thermal
displacement parameters drawn at 50% probability.
Table 2. Selected bond lengths /Å and angles /° in 10·THF and
10·HCl·CH2Cl2.
10·THF
10·HCl·CH2Cl2
Zn–N1
Zn–N2
Zn–Cl1
Zn–Cl2
C1–N1
C1–N4
C1–N5
C19–N3
C19–N6
C19–N7
N1–Zn–N2
Cl1–Zn–Cl2
N1–Zn–Cl1
N1–Zn–Cl2
N2–Zn–Cl1
N2–Zn–Cl2
1.982(3)
2.088(3)
2.2391(10)
2.2543(9)
1.341(4)
1.365(4)
1.382(4)
1.293(4)
1.407(4)
1.400(4)
83.23(11)
113.18(4)
113.33(9)
119.98(9)
118.37(8)
105.42(8)
81.7
1.966(2)
2.091(2)
2.2320(8)
2.2363(8)
1.343(3)
1.372(3)
1.376(4)
1.379(3)
1.360(4)
1.356(3)
82.67(9)
110.27(3)
116.20(7)
121.43(7)
114.17(6)
108.95(7)
85.7
Scheme 3. Preparation of [(TLtBu-κN1:κN2)ZnCl2] (10).
a)
MCl2/MN2
Zn–N1–C1–N5
82.6
0.86
0.97, 0.92
84.8
0.87
0.98, 1.02
b)
τ4
ρc)
a) Angle between MCl2 and MN2 planes. b) τ4 = [360°– (α + β)]/141,
where α and β represent the largest angles in four-coordinate species.
c) ρ = 2a/(b + c), with a, b and c representing the exo- (a) and endo-
cyclic (b, c) distances within the CN3 guanidine moiety.
Figure 9. ORTEP diagram of [(TLtBu)ZnCl2] (10) in 10·THF with ther-
mal displacement parameters drawn at 50% probability.
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Lactide Polymerization
the TLtBu ligand does not represent a suitable ancillary ligand
for the development of ZnII lactide polymerization catalysts.
Determination of similar Pr values of ca. 0.5 for the complexes
[(BLDipp,Me)ZnCl2] (8) [(BLMe)ZnCl2] (11)[4d] reveals that the
stereochemistry of the resulting polymer is not influenced by
the presence of the potentially C2-symmetric ligand in 8.[25]
Similar results were reported for aluminum complexes with
bidentate N-donor ligands, which induced only slight enhance-
ment for isotactic enchainment despite C2 symmetry in the
crystal structures.[26]
It has been demonstrated that guanidine zinc complexes
serve as efficient catalysts for the ring-opening polymerization
of d,l-lactide,[4d,17b,19b,23] and in comparison with conven-
tional alkoxide or alkylzinc complexes,[24] these systems offer
the advantage of high stability towards air and moisture. In
addition, the (potential) C2 symmetry of the complex
[(BLDipp,Me)ZnCl2] (8) might have an impact on the tacticity
of the resulting polylactide (PLA) material. Therefore, the per-
formance of this complex in the bulk polymerization of d,l-
lactide was studied and compared to the performance of the
related complexes [(TLtBu)ZnCl2] (10), [(BLMe)ZnCl2] (11),[4d]
and [(BLiPr)ZnCl2] (12)[3e] (Figure 11). Accordingly, the initia-
tor and d,l-lactide (ratio 1:500, 0.2 mol-%) were heated at
150 °C. After reaction times of 24 h or 48 h, the melt was dis-
solved in dichloromethane, and then the PLA was precipitated
in cold ethanol, isolated, and dried in a vacuum at 50 °C. In
order to rate the catalytic activity of the complexes, the poly-
mer yield was determined and the molecular weights and the
polydispersity of the isolated PLA material was established by
gel permeation chromatography (Table 3). For two runs with
catalysts 8 and 11, the tacticity and the probability of heterot-
actic enchainment (Pr) was analyzed by homonuclear decou-
Conclusions
The bis(imidazolin-2-imine) ligand (IPrMe)NCH2CH2N-
(IPrMe) (BLDipp,Me) (5) was prepared via the new N-hetero-
cyclic carbene IPrMe with an asymmetric N,NЈ-substitution
pattern. BLDipp,Me forms transition metal complexes
[(BLDipp,Me)MCl2] (M = Fe, Ni, Co, Pd). The tetrahedral iron,
nickel, and zinc complexes exhibit, as expected, C2-symmetric
conformation in the solid state. In contrast, a Cs-symmetric
orientation is found for the square-planar palladium complex,
with the Dipp and Me groups each pointing to the same side
of the PdN2Cl2 coordination sphere. NMR studies reveal fast
interconversion between the C2- and Cs-symmetric conformers
in solution, and the use of these ligands in asymmetric homo-
geneous catalysis therefore seems to be less promising, as con-
firmed by use of the zinc complex [(BLDipp,Me)MCl2] as an
initiator for the polymerization of d,l-lactide. Nevertheless,
coordination to other complex fragments with larger ancillary
ligands and the potential to form stronger metal–nitrogen
bonds might afford C2-symmetric complexes of higher confor-
mational stability. This could be expected for half-
1
pled H NMR spectroscopy.[25]
sandwich
cyclopentadienyl
and
arene
ruthenium
complexes of the type [(η5-Cp)Ru(BLDipp,Me)]+ and [(η5-
arene)Ru(BLDipp,Me)]2+.[3a,4c,4g] In addition, the use of chiral
counterions in these systems would also facilitate chiral resolu-
tion, which is essential for the use of such complexes in asym-
metric hydrogen transfer catalysis.[4f]
Figure 11. Bis(imidazolin-2-imine) zinc(II) chloride complexes as ini-
tiators for lactide polymerization.
Table 3. Polymerization of d,l-lactide with 8, 9, 11, and 12.a)
Experimental Section
Initiator
Time /h Yield /% Mw /g·mol–1
PDb)
Prc)
General: All operations were performed in an atmosphere of dry argon
by using Schlenk and vacuum techniques. All solvents were purified
by standard methods and distilled prior to use. 1H and 13C NMR spec-
tra were recorded at room temperature (ca. 298 K) with Bruker DPX
200 and Bruker DPX 400 devices; chemical shifts are reported in ppm
relative to TMS. The spin coupling patterns are indicated as s (singlet),
d (doublet), m (multiplet), sept (septet) and br (broad, for unresolved
signals); o (ortho), m (meta), p (para), and i (ipso) indicate the position
of carbon atoms aromatic substituents. Elemental analyses (C, H, N)
were performed with a Vario EL III CHNS elemental analyzer. Mass
spectrometry was performed with a Finnigan MAT 90 device. Methyl
8
8
10
11d)
11d)
12
12
24
48
48
24
48
24
48
88
90
16
85
89
79
82
48,800
38,000
20,300
50,700
54,800
36,300
36,600
1.9
1.9
1.6
2.0
1.9
2.0
1.8
0.52
–
–
0.53
–
–
–
a) Reaction conditions: initiator (0.2 mol%), 150 °C. b) PD = Mw/Mn
where Mn is the number average molar mass. c) From analysis of the
1H homonuclear decoupled NMR spectrum using the equation Pr =
[sis]2/2. d) Values taken from Ref. [4d].
isothiocyanate,
2,6-diisopropylaniline,
3-hydroxybutanone,
All complexes containing bidentate bis(imidazolin-2-imine)
ligands gave high PLA yields (ca. 80%–90%) with compara-
tively narrow polydispersities (around 2.0) and molecular
weights ranging between 36,000 and 50,000 g·mol–1. In con-
[Ni(DME)Cl2], and ZnCl2 were used as received from Aldrich; azido-
trimethylsilane was distilled prior to use. [FeCl2(THF)1.5],[27]
[Pd(COD)Cl2],[28]
ethylene
glycolditosylate,[29]
TLtBu [21a]
,
[(BLMe)ZnCl2] (11),[4d] and [(BLiPr)ZnCl2] (12)[3e] were prepared ac-
trast, complex 10 afforded PLA in poor yield, indicating that cording to literature procedures.
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Gel Permeation Chromatography: The molecular weight and mole-
cular weight distribution of the isolated polylactide samples were de-
termined by gel permeation chromatography (GPC) in THF as mobile complete addition, the reaction mixture was heated under reflux for
phase at a flow rate of 1 mL·min–1. A combination of PSS SDV col-
24 h. The mixture was filtered and the solvent evaporated. Sublimation
umns with porosities of 105 Å and 103 Å was used together with a at 150 °C and 0.04 mbar afforded the product as a pale yellow solid
dissolved in toluene (40 mL) and azidotrimethylsilane (1.28 mL,
1.11 g, 9.63 mmol) was added dropwise at room temperature. After
1
HPLC pump (L6200, Merck Hitachi) and a refractive index detector
(Smartline RI Detector 2300, Knauer) detector. Universal calibration
was applied to evaluate the chromatographic results. Kuhn-Mark-Hou-
(1.81 g, 79%). H NMR (200 MHz, C6D6): δ = 7.19 (m, 2 H, p-CH),
7.14 (d, 1 H, m-CH), 2.94 (sept, 2 H, CHMe), 2.82 (s, 3 H, NCH3),
1.59 (s, 3 H, CCH3), 1.49 (s, 3 H, CCH3), 1.31 (d, 6 H, CHCH3), 1.08
(d, 6 H, CHCH3), 0.22 (s, 9 H, SiCH3) ppm. 13C NMR (50.26 MHz,
C6D6): δ = 148.5 (o-C), 143.9 (NCN), 133.7 (i-C), 129.3 (p-C), 123.9
wink (KMH) parameters for the polystyrene standards (KPS
=
0.011 ml/g, αPS = 0.725) were taken from the literature [J. E. Mark,
Polymer Data Handbook, Oxford University Press, New York, USA, (m-C), 114.4 (CMe), 113.9 (CMe), 28.7 (NCH3), 28.4 (CHCH3), 24.5
1999]. Previous GPC measurements utilizing online viscosimetry de-
(CHCH3), 23.8 (CHCH3), 9.5 (CCH3), 9.1 (CCH3), 4.4 (SiCH3). MS
(EI, 70 eV): m/z 357 (47) [M+], 342 (100) [M+ – CH3], 285 (31) [M+ –
TMS], 270 (71) [M+ – TMS – CH3]. C21H35N3Si (357.6): calcd. C
70.53, H 9.86, N 11.75%; found C 69.87, H 9.60, N, 11.59%.
tection revealed the KMH parameters for polylactide (KPLA
=
0.053 ml/g, αPLA = 0.610).[19b]
N-Methyl-NЈ-(2,6-diisopropylphenyl)thiourea (1): Methyl isothiocy-
anate (1.0 g, 13.7 mmol) was dissolved in acetonitrile (15 mL). 2,6-
Diisopropylaniline was added dropwise to the solution under ambient
conditions, and the mixture was stirred for 4 d. The solvent was evapo-
rated, and the residue was recrystallized from toluene. The solid was
isolated and washed with n-hexane (20 mL). The white product was
isolated after drying in vacuo (1.20 g, 35%). 1H NMR (200 MHz,
CDCl3): δ = 7.46 (s br, 1 H, NHDipp), 7.43–7.16 (m, 3 H, m-,p-CH),
5.32 (d br, 1 H, NHMe), 3.14 (sept, 2 H, CHMe), 3.08 (d, 3 H, NCH3),
1.18 (2 x d, 12 H, CHCH3) ppm. 13C NMR (50.26 MHz, CDCl3): δ =
182.5 (NCS), 147.8 (o-C), 132.3 (i-C) 129.9 (p-C), 124.6 (m-C), 31.9
(NCH3), 28.5 (CHMe), 24.4 (CHCH3), 23.3 (CHCH3) ppm. MS (EI,
70 eV): m/z = 250 (M+), 207 (M+-C3H7). C14H22N2S (250.4): calcd. C
67.15, H 8.86, N 11.19%; found C 66.91, H 8.35, N 11.07%.
1-(2,6-Diisopropylphenyl)-3,4,5-trimethylimidazolin-2-imine (4): 1-
(2,6-Diisopropylphenyl)-3,4,5-trimethyl-2-(trimethylsilyl)imino-
imidazoline (see previous protocol, 1.81 g, 5.07 mmol) was dissolved
in methanol (20 mL) and stirred for 24 h at ambient temperature. All
volatiles were removed under vacuum. The residue was recrystallized
from THF and isolated by filtration. After drying in vacuo, the product
was obtained as a white solid (1.45 g, 79%). 1H NMR (200 MHz,
C6D6): δ = 7.22 (m, 1 H, p-CH), 7.10 (m, 2 H, m-CH), 4.39 (s br, 1
H, NH), 3.44 (s, 3 H, NCH3), 2.90 (sept, 2 H, CHMe), 1.52 (s, 3 H,
CCH3), 1.45 (s, 3 H, CCH3), 1.19 (d, 6 H, CHCH3), 1.05 (d, 6 H,
CHCH3) ppm. 13C NMR (50.26 MHz, C6D6): δ = 155.7 (NCN), 150.0
(o-C), 131.4 (i-C), 130.5 (p-C), 125.1 (m-C) 115.8 (CMe), 114.3
(CMe), 29.2 (CHCH3), 28.9 (NCH3), 25.2 (CHCH3), 24.1 (CHCH3),
9.5 (CCH3), 9.1 (CCH3) ppm. MS (EI, 70 eV): m/z = 285 (39) [M+],
N-Methyl-NЈ-(2,6-diisopropylphenyl)imidazolin-2-thione (2):
1
(5.19 g, 20.73 mmol) and 3-hydroxybutanone (1.83 g, 20.73 mmol) 270 (100) [M+ – CH3]. C18H27N3 (285.4): calcd. C 75.74, H 9.53, N
were dissolved in 1-hexanol (150 mL) and heated under reflux for 3
d. The solvent was evaporated and the residue was washed with small
amounts of THF and then n-hexane. The remaining solid was dried in
vacuo to afford the product as a white solid (4.06 g, 65%). H NMR
(300 MHz, C6D6): δ = 7.26 (m, 1 H, p-CH), 7.14 (m, 2 H, m-CH),
14.72%; found C 75.49, H 9.50, N 14.04%.
1,2-Bis(1-(2,6-diisopropylphenyl)-3,4,5-trimethylimidazolin-2-
imino)ethylene (5, BLDipp,Me): Ethylene glycolditosylate (0.94 g,
2.54 mmol) and the imine 4 (1.45 g, 5.08 mmol) were dissolved in
1
3.19 (s, 3 H, NCH3), 2.74 (sept, 2 H, CHMe), 1.44 (m, 12 H, CHCH3), toluene (70 mL) and stirred for 72 h at 115 °C. Afterwards, the solvent
1.08 (s, 3 H, CCH3), 1.06 (s, 3 H, CCH3) ppm. 13C NMR (75.4 MHz, was evaporated, and the residue was washed with n-hexane
C6D6): δ = 165.3 (NCS), 147.1 (o-C), 132.7 (i-C), 129.7 (p-C), 124.1
(2ϫ20 mL) and diethyl ether (20 mL). The remaining solid (2.09 g,
(m-C), 120.4 (CMe), 120.2 (CMe), 31.3 (NCH3), 28.8 (CHMe), 24.4 2.22 mmol) was suspended in THF (60 mL) and KOtBu (597.9 mg,
(CHCH3), 23.5 (CHCH3), 9.1 (CCH3), 8.7 (CCH3) ppm. C18H26N2S
(302.5): calcd. C 71.48, H 8.66, N 9.26%; found C 71.23, H 8.74, N
9.31%.
5.33 mmol) was added. The mixture was stirred overnight at ambient
temperature, filtered and washed with THF (2ϫ10 mL). The solvent
was removed and the remaining solid was dried in vacuo to yield the
product as a pale yellow solid (1.31 g, 98%). 1H NMR (300 MHz,
C6D6): δ = 7.11–7.06 (m, 2 H, p-CH), 6.95–6.93 (m, 4 H, m-CH), 3.09
(s, 4 H, CH2), 3.04 (sept, 4 H, CHMe), 2.85 (s, 6 H, NCH3), 1.46 (s,
6 H, CCH3), 1.34 (s, 6 H, CCH3), 1.18 (d, 12 H, CHCH3), 0.99 (d, 12
H, CHCH3) ppm. 13C NMR (75.48 MHz, C6D6): δ = 148.2 (o-C),
145.0 (NCN), 134.8 (i-C), 128.7 (p-C), 123.3 (m-C), 114.8 (CMe),
113.4 (CMe), 50.9 (CH2), 28.3 (NCH3), 28.2 (CHCH3), 24.0 (CHCH3),
23.6 (CHCH3), 8.9 (CCH3), 8.7 (CCH3) ppm. MS (EI, 70 eV): m/z =
596 (M+), 298 (M+/2). C38H56N6 (596.9): calcd. C 76.46, H 9.46, N
14.08%; found C 75.62, H 9.25, N 13.65%.
1-(2,6-Diisopropylphenyl)-3,4,5-trimethylimidazolin-2-ylidene (3):
Potassium (2.39 g, 61.2 mmol) was added to a solution of thione 2
(8.42 g, 27.8 mmol) in THF (150 mL) at room temperature, and the
reaction mixture was heated under reflux overnight. The resulting sus-
pension was filtered through Celite and washed with THF (3ϫ20 mL).
The solvent was evaporated and the residue was sublimed at 120 °C
and 0.02 mbar to give a white crystalline solid (6.93 g, 92%). 1H
NMR (200 MHz, C6D6): δ = 7.30 (m, 2 H, p-CH), 7.19 (d, 1 H, m-
CH), 3.45 (s, 3 H, NCH3), 2.83 (2 H, sept, CHMe), 1.68 (m, 3 H,
CCH3), 1.62 (m, 3 H, CCH3), 1.27 (d, 6 H, CHCH3), 1.11 (d, 6 H,
CHCH3) ppm. 13C NMR (50.26 MHz, C6D6): δ = 215.3 (NCN), 146.9
(o-C), 137.6 (i-C), 128.8 (p-C), 123.7 (m-C), 124.2 (CMe), 122.5
(CMe), 35.7 (NCH3), 28.5 (CHMe), 25.1 (CHCH3), 23.1 (CHCH3),
9.5 (CCH3), 9.0 (CCH3) ppm. MS (EI, 70 eV): m/z 357 (47) [M+], 342
(100) [M+ – CH3], 285 (31) [M+ – TMS], 270 (71) [M+ – TMS-
CH3]. C18H26N2 (270.4): calcd. C 79.95, H 9.69, N 10.36%; found C
79.99, H 9.57, N 10.05%.
[(BLDipp,Me)FeCl2] (6):
A
THF solution (10 mL) of BLDipp,Me
(142.1 mg, 0.24 mmol) was added dropwise at ambient temperature to
a solution of [FeCl2(THF)1.5] (50.0 mg, 0.23 mmol) in THF (15 mL)
and stirred overnight. The volume of the solution was reduced to 5 mL,
and the product was fully precipitated by addition of n-hexane
(30 mL). Filtration, washing with n-hexane (2ϫ5 mL), and drying in
vacuo afforded the product as a white solid (107.6 mg, 63%). MS (EI,
1-(2,6-Diisopropylphenyl)-3,4,5-trimethyl-2-(trimethylsilyl)imino- 70 eV): m/z = 722 (M+), 298 (BLDipp,Me/2). C38H56N6Cl2Fe (723.7):
imidazoline (IPrNSiMe3): The carbene 3 (1.74 g, 6.42 mmol) was calcd. C 63.07, H 7.80, N 11.61%; found C 62.74, H 7.53, N 11.34%.
Z. Anorg. Allg. Chem. 2015, 2204–2214
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[(BLDipp,Me)NiCl2] (7): BLDipp,Me (100.0 mg, 0.17 mmol) in THF (CCH3), 9.1 (s, CCH3) ppm. C38H56Cl2N6Pd (774.2): calcd. C 58.95,
(5 mL) was added to a suspension of [(DME)NiCl2] (36.0 mg,
0.16 mmol) in THF (10 mL) at ambient temperature. The mixture was
stirred overnight, and the volume was reduced to 5 mL. The product
was precipitated with n-hexane (30 mL), filtered off, washed with n-
hexane (2ϫ5 mL) and dried in vacuo. The product was isolated as a
red solid (95.3 mg, 80%). MS (EI, 70 eV): m/z = 726 (M+), 298
(BLDipp,Me/2). C38H56N6Cl2Ni (726.5): calcd. C 62.82, H 7.77, N
11.57%; found C 60.87, H 7.54, N 10.83%.
H 7.29, N 10.85%; found C 58.92, H 7.17, N 11.84%.
[(TLtBu)ZnCl2] (10): A solution of TLtBu (200 mg, 0.405 mmol) in
THF (10 mL) was added dropwise to a solution of ZnCl2 (55.22 mg,
0.98 mmol) in THF (10 mL) at room temperature. During the addition
of the ligand, the formation of a pale yellow precipitate was observed.
The suspension was stirred for 12 h followed by reduction of THF
volume to 10 mL. The product was fully precipitated by addition of
n-hexane (20 mL). Filtration, washing with n-hexane (2ϫ10 mL) and
drying in vacuo afforded the product as a pale yellow solid (254 mg,
[(BLDipp,Me)ZnCl2] (8): A solution of BLDipp,Me (89.5 mg, 0.15 mmol)
in acetonitrile (5 mL) was added to a suspension of ZnCl2 (20.0 mg,
0.15 mmol) in acetonitrile (10 mL) at ambient temperature and stirred
for 72 h. The volume was reduced to 5 mL and the product precipitated
by addition of diethyl ether (30 mL). Filtration, washing with diethyl
ether (2ϫ10 mL) and drying in vacuo afforded the colorless product
1
99%). H NMR (200 MHz, CDCl3): δ = 7.83 (t, 1 H, p-Py), 7.59 (d,
2 H, m-Py), 6.59 (s, 4 H, NCH), 4.98 (s, 4 H, CH2), 1.66 (s, 36 H,
CH3) ppm. 13C NMR (50 MHz, CDCl3): δ = 161.3 (o-C), 150.5
(NCN), 138.9 (p-C), 119.8 (m-C), 111.1 (NCH), 67.9 (CH2), 57.5
(NCMe), 30.3 (CCH3) ppm. C29H47N7Cl2Zn (630.0): calcd. C 55.28,
H 7.52, N 15.56%; found C 55.17, H 7.71, N 13.84%.
1
(107.4 mg, 97%). H NMR (400 MHz, CDCl3): δ = 7.32 (t, 2 H, p-
CH), 7.09 (d, 4 H, m-CH), 3.61 (s, 6 H, NCH3), 2.73 (sept, 4 H,
CHMe), 2.39 (s, 4 H, CH2), 2.04 (s, 6 H, CCH3), 1.56 (s, 6 H, CCH3),
1.08 (2 x d, 24 H, CHCH3) ppm. 13C NMR (50.26 MHz, CDCl3): δ =
149.7 (NCN), 147.5 (o-C), 132.5 (i-C), 129.7 (p-C), 123.7 (m-C),
117.6 (CMe), 116.4 (CMe), 47.4 (CH2), 31.6 (NCH3), 28.0 (CHCH3),
24.3 (CHCH3), 23.6 (CHCH3), 9.5 (CCH3), 8.8 (CCH3) ppm. MS (EI,
70 eV): m/z = 733 (M+), 298 (BLDipp,Me/2). C38H56N6Cl2Zn (733.2):
calcd. C 62.25, H 7.70, N 11.46%; found C 62.22, H 7.78, N 10.49%.
General Procedure for D,L-Lactide Polymerization with Zinc Com-
plexes: d,l-Lactide (3.603 g, 25 mmol) and the initiator (I/M ratio
1/500) were weighed into a 50 mL flask, which was closed with a
glass stopper. The d,l-lactide was used as purchased from Purac with-
out further purification steps. The reaction vessel was heated at 150 °C.
After the reaction time of 24 or 48 h the polymer melt was allowed to
cool to room temperature and then dissolved in 25 mL of dichloro-
methane. The PLA was precipitated in 350 mL of ice-cooled ethanol
und dried under vacuum at 50 °C.
[(BLDipp,Me)PdCl2] (9): A solution of BLDipp,Me (210 mg, 0.35 mmol)
in THF (15 mL) was added to a suspension of [(COD)PdCl2] (100 mg,
0.35 mmol) in THF (2 mL). A red solution formed instantaneously.
X-ray Diffraction Analyses: Data are summarized in Table 4. Crystals
After stirring overnight, the product was precipitated by addition of were mounted in inert oil on glass fibres. Data were recorded at 100 K
hexane, isolated by filtration, washed with hexane (3ϫ5 mL), and
with an Oxford Diffraction diffractometers using monochromated Mo-
dried in vacuo. The product was isolated as a red solid (260 mg, 96%).
Kα or mirror-focused Cu-Kα radiation. Absorption corrections were
1H NMR (300 MHz, CD2Cl2): δ = 7.41 (m, 2 H, p-CH), 7.23 (m, 4 performed on the basis of multi-scans. Structures were refined aniso-
H, m-CH), 3.77 (br. m, 6 H, NCH3), 3.50–2.00 (m, 8 H, CHMe and
CH2), 2.05 (m, 6 H, CCH3), 1.62 (s, 6 H, CCH3), 1.07 (br. m, 12 H
CHCH3) ppm. 13C NMR (75 MHz, CD2Cl2): δ = 152.9 (NCN), 148.6 Exceptions and special features: The two independent molecules of
(o-C), 131.6 (i-C), 130.3 (p-C), 124.7 (m-C), 119.3 (CMe), 118.6 (s, compound 3 are related to a good approximation by the translation c/
CMe), 32.7 (NCH3), 28.2 (CHMe), 24.7 (CHCH3), 24.4 (CHCH3), 9.5 2, but the different orientations of several groups (notably the isopropyl
tropically on F2 using the program SHELXL-97.[30] Hydrogen atoms
were included using rigid idealized methyl groups or a riding model.
Table 4. Crystallographic data.
2
3
4
5
6·THF
7·1.5THF
8·CH2Cl2
9·CH3CN
10·2.5 THF
10·HCl·CH2Cl2
Empirical for-
mula
a /Å
b /Å
c /Å
C18H26N2S
C18H26N2
C18H27N3
C38H56N6
C42H64Cl2FeN6O C44H68Cl2N6NiO1.5 C39H58Cl4N6Zn C40H59Cl2N7Pd C39H67Cl2N7ZnO2.5 C30H50Cl5N7Zn
12.3432(8)
9.2267(6)
15.3639(8)
90
98.086(6)
90
22.3995(6)
6.6797(4)
23.0171(8)
90
103.730(4)
90
12.5623(10) 13.8023(12) 18.1232(4)
8.8476(6) 15.0429(14) 14.4546(4)
15.1410(14) 18.4481(18) 17.5963(6)
90
97.140(8)
90
10.9767(6)
18.7461(12)
23.3544(16)
69.943(6)
80.878(6)
85.304(6)
4455.2(5)
4
9.4516(6)
15.5494(4)
14.6054(4)
90
91.985(4)
90
10.6162(2)
14.7074(5)
15.2574(6)
116.578(4)
94.798(3)
101.245(3)
2049.55(11)
2
9.9010(10)
13.2950(12)
32.069(3)
90
91.282(10)
90
9.1131(10)
14.1319(14)
14.6013(14)
89.791(8)
84.212(8)
82.792(8)
1856.0(3)
2
α /°
β /°
88.386(8)
70.584(8)
76.085(8)
3500.9(6)
4
90
112.349(4)
90
4263.3(2)
4
γ /°
V /Å3
Z
1732.35(18) 3345.5(2)
4
1669.8(2)
4
2145.22(16)
2
4220.3(7)
4
8
Formula weight 302.47
270.41
P21/n
100(2)
0.71073
1.074
0.063
86259
285.43
P21/c
100(2)
0.71073
1.135
0.068
25757
596.89
P1
100(2)
0.71073
1.132
795.74
Cc
834.65
P1
100(2)
0.71073
1.244
818.08
P21
100(2)
0.71073
1.267
815.24
P1
100(2)
0.71073
1.321
810.27
P21/c
100(2)
1.54184
1.275
751.39
P1
100(2)
1.54184
1.345
¯
¯
¯
¯
Space group
T /K
P21/c
100(2)
0.71073
1.160
100(2)
0.71073
1.240
0.518
42068
λ /Å
D
calcd. /g·cm–3
μ /mm–1
0.183
0.067
68675
0.596
128780
0.855
56112
0.620
142675
2.302
57156
4.465
30927
Reflection col- 43466
lected
Independent re- 4296, Rint
=
9360, Rint
0.0444
1.082
=
2950, Rint
0.0822
0.813
=
14259, Rint
0.0721
0.835
=
10571, Rint
0.0443
0.930
=
23857, Rint
0.0420
0.912
=
10667, Rint
0.0209
1.022
=
12207, Rint
0.0409
1.051
=
7994, Rint
0.1162
0.893
=
7661, Rint
0.0662
1.045
=
flections
0.0365
Goodness of fit 1.049
on F2
R(Fo),
0.0371
0.1026
0.0468
0.1255
0.0368
0.0688
0.0417
0.0678
0.0318
0.0643
0.0317
0.0785
0.0174
0.0259
0.0594
0.0518
0.1190
0.0431
0.1070
[I Ͼ 2σ(I)]
2
Rw(Fo
)
0.0448
0.398/–0.302
Δρ /e·Å–3
0.395/–0.267 0.446/–0.338 0.139/–0.202 0.184/–0.177 0.323/–0.359
0.381/–0.248
0.482/–0.536
0.505/–0.458
0.662/–0.482
Z. Anorg. Allg. Chem. 2015, 2204–2214
2212
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Journal of Inorganic and General Chemistry
www.zaac.wiley-vch.de
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
T. Glöge, D. Petrovic, C. G. Hrib, C. Daniliuc, E. Herdtweck,
P. G. Jones, M. Tamm, Z. Anorg. Allg. Chem. 2010, 636, 2303–
group at C13–15) show that this translation symmetry is not exact.
The NH hydrogen atoms of compounds 4 and 10·HCl·CH2Cl2 were
refined freely. Compound 6 crystallizes in the non-centrosymmetric
space group Cc, but the symmetry is close to C2/c (the complex dis-
plays twofold symmetry within an r.m.s.d. of 0.09 Å). Nevertheless,
the poor refinement in the higher symmetry space group (some high
U values, high wR2 of 0.30) convinces us that Cc is correct. The struc-
ture was refined as an enantiomeric twin with relative component vol-
umes 0.51, 0.49(1). Compound 7 crystallizes with 3THF in the asym-
metric unit; two were refined satisfactorily, but the third had to be
removed mathematically using the routine SQUEEZE (SQUEEZE
forms part of the PLATON program package: A. L. Spek, University
of Utrecht, Netherlands). Similarly, compound 10 crystallized with
2.5THF in the asymmetric unit, of which 1.5 were removed with
SQUEEZE. The dichloromethane solvent molecule of 10·HCl·CH2Cl2
is disordered, but was refined satisfactorily. Compound 8 crystallizes
only by chance in the Sohncke space group P21; the Flack parameter
refined to –0.003(3). The carbon atoms of the chelate ring in com-
pound 9 are disordered over two positions, although one position pre-
dominates; suitable restraints were employed to improve refinement
stability, but dimensions of disordered groups should always be inter-
preted with caution.
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Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
numbers CCDC-1409537 (2), CCDC-1409538 (3), CCDC-1409539
(5), CCDC-1409540 (4), CCDC-1409541 (6·THF), CCDC-1409542
(7·1.5THF),
CCDC-1409543
(8·CH2Cl2),
CCDC-1409544
(10·2.5THF), CCDC-1409545 (10·HCl·CH2Cl2), CCDC-1409546
(9·CH3CN), CCDC-1409547 (5·2HCl·3CH2Cl2), and CCDC-1409548
([Rh(COD)(5)]BF4·3THF
(Fax:
+44-1223-336-033;
E-Mail:
deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
Supporting Information (see footnote on the first page of this article):
Further X-ray structure determinations of 5·2HCl·3CH2Cl2) and
[Rh(COD)(5)]BF4·3THF; selected NMR spectra.
Acknowledgements
We thank Dr. Cristian G. Hrib, Dr. Constantin G. Daniliuc, and Mr.
Dirk Bockfeld (M.Sc.) for X-ray crystallographic support. S. H.-P.
thanks Prof. Klaus Huber for GPC measurements.
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Received: July 2, 2015
Published Online: September 3, 2015
Z. Anorg. Allg. Chem. 2015, 2204–2214
2214
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim