Binuclear magnesium, calcium and zinc complexes based on bis(salicylaldimine) ligands with rigid bridges

Article abstract of DOI:10.1002/ejic.200800308

Four dinucleating ligands based on sterically protected salicylaldimine units bridged by a variety of rigid groups have been prepared: 1,4-bis[(3,5-di-tert-butylsalicylidene)amino]-benzene (PARA-H₂), 1,3-bis[(3,5-di-tert-butylsalicylidene)-amino]benzene (META-H₂), 2,6-bis[(3,5-di-tert-butylsalicylidene) amino]pyridine (PYR-H₂) and 1,4-bis[(3,5-di-tert-butylsalicylidene) amino]-2,3,5,6-tetramethylbenzene (Me₄PARA-H₂). Reactions with [(Me₃Si) ₂N]₂Mg·(thf)₂ gave three new heteroleptic dinuclear magnesium complexes: PARA-[MgN(SiMe₃) ₂·thf]₂, META-[MgN(SiMe₃) ₂·thf]₂ and PYR-[MgN(SiMe₃) ₂·thf]₂. The complex PARA-[MgN(SiMe ₃)₂·thf]₂ was characterized by X-ray diffraction. Reactions with [(Me₃Si)₂N] ₂Ca·(thf)₂ gave mixtures of homoleptic and heteroleptic species. The homoleptic and heteroleptic calcium complexes [PARA-Ca·(thf)₂]₂ and PARA-[CaN(SiMe ₃)₂· (thf)₂]₂ could be isolated and were structurally characterized. These complexes are not stable in solution. Attempted syntheses of Me₄PARA-[CaN(SiMe₃) ₂·(thf)₂]₂ gave insoluble coordination polymers of Me₄PARA-Ca. Reactions with [(Me₃Si) ₂N]₂Zn gave precipitates of homoleptic dinuclear zinc complexes of which PYR-Zn could be structurally characterized as a cyclic tetramer. The dinuclear magnesium complexes PARA-[MgN(SiMe₃) ₂·thf]₂, META-[MgN(SiMe₃) ₂·thf]₂ and PYR-[MgN(SiMe₃) ₂·thf]₂ are neither active in the polymerization of epoxides nor in the copolymerization of epoxide/CO₂ (cylohexene or propylene oxide) but initiate the polymerization of rac-lactide. Yields of isolated polylactide strongly depend on the bridging unit and the solvent. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800308

FULL PAPER
DOI: 10.1002/ejic.200800308
Binuclear Magnesium, Calcium and Zinc Complexes Based on
Bis(salicylaldimine) Ligands with Rigid Bridges
Sven Range,[a] Dirk F.-J. Piesik,[a] and Sjoerd Harder*[a]
Keywords: Alkaline earth metals / Magnesium / Calcium / Zinc / Polylactide
Four dinucleating ligands based on sterically protected sali-
cylaldimine units bridged by a variety of rigid groups have
been prepared: 1,4-bis[(3,5-di-tert-butylsalicylidene)amino]-
benzene (PARA-H2), 1,3-bis[(3,5-di-tert-butylsalicylidene)-
(thf)2]2 could be isolated and were structurally characterized.
These complexes are not stable in solution. Attempted syn-
theses of Me4PARA-[CaN(SiMe3)2·(thf)2]2 gave insoluble co-
ordination polymers of Me4PARA-Ca. Reactions with
amino]benzene (META-H2), 2,6-bis[(3,5-di-tert-butylsalicyli- [(Me3Si)2N]2Zn gave precipitates of homoleptic dinuclear
dene)amino]pyridine (PYR-H2) and 1,4-bis[(3,5-di-tert-butyl-
salicylidene)amino]-2,3,5,6-tetramethylbenzene (Me4PARA-
H2). Reactions with [(Me3Si)2N]2Mg·(thf)2 gave three new
heteroleptic dinuclear magnesium complexes: PARA-
[MgN(SiMe3)2·thf]2, META-[MgN(SiMe3)2·thf]2 and PYR-
[MgN(SiMe3)2·thf]2. The complex PARA-[MgN(SiMe3)2·thf]2
was characterized by X-ray diffraction. Reactions with
[(Me3Si)2N]2Ca·(thf)2 gave mixtures of homoleptic and heter-
oleptic species. The homoleptic and heteroleptic calcium
complexes [PARA-Ca·(thf)2]2 and PARA-[CaN(SiMe3)2·
zinc complexes of which PYR-Zn could be structurally char-
acterized as a cyclic tetramer. The dinuclear magnesium
complexes PARA-[MgN(SiMe3)2·thf]2, META-[MgN(SiMe3)2·
thf]2 and PYR-[MgN(SiMe3)2·thf]2 are neither active in the
polymerization of epoxides nor in the copolymerization of ep-
oxide/CO2 (cylohexene or propylene oxide) but initiate the
polymerization of rac-lactide. Yields of isolated polylactide
strongly depend on the bridging unit and the solvent.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
rigid groups (Scheme 1), (b) explore the feasibility to syn-
thesize well-defined heteroleptic dinuclear alkaline-earth
Introduction
The use of well-defined alkaline-earth metal complexes metal complexes that are stable against ligand exchange
in catalysis is rapidly growing.[1] This especially holds for (Scheme 1), (c) investigate the use of these dinucleating li-
applications in ring opening polymerization of polar mono- gands in the syntheses of closely related zinc complexes and
mers in which case the biocompatibility of the catalyst is of (d) test heteroleptic dinuclear metal complexes in the cata-
major importance.[2]
lytic polymerization of polar monomers.
As increasing research efforts on multinuclear metal cata-
lysts show that cooperative effects of neighbouring metal
sites considerably influence activities and selectivities in
catalytic transformations,[3] we became interested in binu-
clear alkaline-earth metal catalysts. The bimetallic nature of
such a complex could not only drastically change its cata-
lytic behaviour but also might allow conversions that are
not possible with monometallic systems. For instance, reac-
tions like the epoxide homopolymerization (or epoxide/CO2
copolymerization) are highly dependent on the close prox-
imity of two or more metals.[4] Metalloenzymes efficiently
exploit such cooperative effects of metals at close range.[5]
In our current research we define the following goals: (a)
syntheses of a range of dinucleating ligands based on steri-
cally protected salicylaldimine units bridged by a variety of
[a] Anorganische Chemie, Universität Duisburg-Essen,
Universitätsstraße 5-7, 45117 Essen, Germany
E-mail: sjoerd.harder@uni-due.de
Scheme 1.
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Binuclear Magnesium, Calcium and Zinc Complexes
the permethylated p-phenylene bridge (Me4PARA-H2) fix
the metal···metal distance in the range of circa 5.5–8.5 Å.[8]
The ligand with the m-phenylene bridge (META-H2, for
which a dinuclear Zn complex is known)[9] is designed for
bimetallic complexes in which the metal···metal distances
can vary over a much larger range (circa 2.5–8 Å).[8] The
ligand with the 2,5-pyridyl bridge (PYR-H2) has been in-
corporated in these studies for potential coordination of the
bridge’s nitrogen atom to the metal(s).
Results and Discussion
Ligand Choice and Syntheses
As ligands based on the N,O-chelating salicylaldimine
unit are among the most easily accessible and tunable sys-
tems,[6] we concentrated on the syntheses of bis(salicylaldi-
mine) complexes. Whereas flexible bridges allow fourfold
chelation of the O,N,N,O-ligand, rigid bridges enable ex-
clusive isolation of bimetallic complexes. Bulky tBu substit-
uents in the o-position are essential in order to prevent
bridging of the phenolate oxygen atom and formation of
coordination polymers. Four bis(salicylaldimine) ligands
with rigid bridges were obtained in good yields (63–88%)
by condensation of 3,5-di-tert-butylsalicylaldehyde with
Bis-magnesium Complexes
Bimetallic heteroleptic magnesium amide complexes have
several diamines (Scheme 2). These ligands are abbreviated been prepared by twofold deprotonation of the bridged
according to the nature of the bridge. The ligand with the bis(salicylaldimine)ligandswithtwoequivalentsof[(Me3Si)2-
rigid p-phenylene bridge (PARA-H2, which has been used N]2Mg·(thf)2 in benzene at room temperature. The com-
in the syntheses of dinuclear Al and Ga complexes)[7] and plexes PARA-[MgN(SiMe3)2·(thf)]2, META-[MgN(SiMe3)2·
Scheme 2.
Scheme 3.
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S. Range, D. F.-J. Piesik, S. Harder
FULL PAPER
Figure 1. Crystal structures (hydrogen atoms omitted for clarity) of (a) PARA-[MgN(SiMe3)2·(thf)]2, (b) [PARA-Ca·(thf)2]2, (c) PARA-
[CaN(SiMe3)2·(thf)2]2 and (d) [PYR-Zn]4 (tBu substituents omitted for clarity; view along the crystallographic twofold rotation axis).
Selected geometric parameters for all structures have been summarized in Table 1.
(thf)]2 and PYR-[MgN(SiMe3)2·(thf)]2 could be obtained Bis-Calcium Complexes
crystalline pure in good yields (63–67%, Scheme 3).
The crystal structure of PARA-[MgN(SiMe3)2·(thf)]2
Deprotonation of the bridged salicylaldimine ligands
(Figure 1a), described in detail below, shows two crystallo- with two equivalents of [(Me3Si)2N]2Ca·(thf)2 in benzene
graphic identical (salicylaldimine)MgN(SiMe3)2·(thf) units is fast, however, in all cases mixtures of homoleptic and
connected by the p-phenylene bridge. The metal centers are heteroleptic complexes are observed (Scheme 3). This dem-
situated at opposite sides of the bridge plane and show es- onstrates the difficulties encountered in switching from
sentially no interaction [the Mg···Mg distance is magnesium to the considerably larger and softer calcium.
8.186(2) Å]. Crystals of META-[MgN(SiMe3)2·(thf)]2 and Only in the case of the PARA ligand could we isolate a
PYR-[MgN(SiMe3)2·(thf)]2 were too small for X-ray struc- pure product: homoleptic [PARA-Ca·(thf)2]2 was obtained
ture determination. Their compositions, however, indicate in the form of small yellow cube-like crystals. The crystal
that the metal coordination geometries in these complexes structure (Figure 1, b), which is discussed in detail below,
are similar to that in PARA-[MgN(SiMe3)2·(thf)]2.
shows a centrosymmetric dimeric complex. As the 1H
In benzene or thf solutions all complexes are stable NMR spectrum of [PARA-Ca·(thf)2]2 dissolved in C6D6
towards ligand exchange and the Schlenk equilibrium is, displays numerous signals for the PARA-ligand, it should
also at higher temperatures (60 °C), fully at the side of the be concluded that in solution either several species exist or
heteroleptic species. The 1H NMR spectra of PARA- that a complicated aggregate with inequivalent ligand envi-
[MgN(SiMe3)2·(thf)]2, META-[MgN(SiMe3)2·(thf)]2 and ronments is formed (e.g. by dissociation of thf and reforma-
PYR-[MgN(SiMe3)2·(thf)]2 consist of one set of signals for tion to higher aggregates).
the bridging group and both salicylaldimine units as well
Interestingly, when PARA-H2 is deprotonated by two
as one singlet for the amide ligands. This implies essentially equivalents of [(Me3Si)2N]2Ca·(thf)2 in benzene, the initially
free rotation of the salicylaldimine units in respect to the formed small yellow cube-like crystals of [PARA-Ca·
bridge at room temperature.
(thf)2]2 slowly disappear and several large yellow-orange
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Binuclear Magnesium, Calcium and Zinc Complexes
crystals with the shape of hexagons start to form (in one ated phenylene bridge (like in Me4PARA) could avoid for-
occasion the smaller crystals converted to just one new mation of the homoleptic dimer and might be the key to a
large hexagon). Single-crystal X-ray diffraction of these stable heteroleptic bis-calcium complex Me4PARA-[CaN-
larger crystals revealed conversion of [PARA-Ca·(thf)2]2 (SiMe3)2·(thf)2]2. Reaction of Me4PARA-H2 with two
into the heteroleptic complex PARA-[CaN(SiMe3)2·(thf)2]2. equivalents of [(Me3Si)2N]2Ca·(thf)2 gave a precipitate that
The crystal structure, described below in detail, shows a C2- is completely insoluble in benzene as well as in thf. 1H
1
symmetric monomer (Figure 1, c). As can be seen by H NMR analysis of the solid quenched in CD3OD revealed a
NMR analysis on crystalline pure samples of PARA-[CaN- homoleptic product that only contains the Me4PARA-li-
(SiMe3)2·(thf)2]2 dissolved in C6D6, in solution PARA- gand. We propose a polymeric structure in which Ca2+ ions
[CaN(SiMe3)2·(thf)2]2 converts back to [(Me3Si)2N]2Ca· bridge between different ligands (Scheme 3).
(thf)2 and several homoleptic PARA-Ca·(thf)x species over
a time period of several hours. In solution, this Schlenk
equilibrium can be completely shifted back again to the het-
eroleptic species, PARA-[CaN(SiMe3)2·(thf)2]2, by addition
of circa two equivalents of [(Me3Si)2N]2Ca·(thf)2. The equi-
libria between solid state and solution structures are repre-
sented in Scheme 4.
Scheme 4.
Figure 2. Partial structure of [PARA-Ca·(thf)2]2. (a) The view along
the bridge planes shows the attractive interaction between the
phenylene bridges. (b) The view perpendicular on the bridge plane
shows the off-set stacking of the π-π interaction.
Although the compounds [PARA-Ca·(thf)2]2 and PARA-
[CaN(SiMe3)2·(thf)2]2 are not stable in solution, solid-state
structures could be obtained in both cases. Especially note-
worthy is complete conversion of the small crystals (cube-
like) of the homoleptic complex [PARA-Ca·(thf)2]2 into
very large crystals (hexagons) of the heteroleptic complex
PARA-[CaN(SiMe3)2·(thf)2]2. Although homoleptic species
Bis-Zinc Complexes
are favoured in solution, the heteroleptic complex is fav-
Reaction of the bis(salicylaldimide) ligand PARA-H2
oured in the solid state. This is likely due to the size of with two equivalents of Et2Zn in benzene at room tempera-
the crystals. According to the Gibbs–Thomson rule small ture resulted in the expected evolution of ethane but gave a
crystals dissolve more readily than larger ones whereas complex mixture of species from which no distinct product
large crystals grow faster; i.e. under reversible crystalli- could be isolated. Reactions of the ligands META-H2 or
zation/dissolution conditions larger crystals grow at cost of PYR-H2 with two equivalents of Et2Zn gave exclusive pre-
smaller ones. According to this principle, which is called cipitation of the homoleptic complexes META-Zn (which
Ostwald ripening,[10] the larger crystals of the heteroleptic has been described before)[9] and PYR-Zn. Even with a
complex PARA-[CaN(SiMe3)2·(thf)2]2 are favoured. Al- large excess of Et2Zn (Ͼ 10 equiv.) only homoleptic com-
though Ostwald ripening has been successfully applied in plexes could be isolated. Also addition of Lewis bases like
enantiomer trapping,[11] this is the first example of con- thf did not enable isolation of heteroleptic species. Likewise,
verting a crystalline homoleptic calcium complex into a reactions of the ligands with [(Me3Si)2N]2Zn gave exclu-
crystalline heteroleptic complex that is unstable in solution. sively homoleptic complexes. For one of the homoleptic
Detailed analysis (vide infra) of the crystal structure of products we have been able to isolate single crystals suitable
[PARA-Ca·(thf)2]2 shows that the phenylene bridges attract for X-ray diffraction. The crystal structure of (PYR)Zn
each other (Figure 2). This favourable π-π stacking interac- (Figure 1, d), which is described in detail below, shows a
tion might be the driving force for formation of homoleptic tetrameric aggregrate: [(PYR)Zn]4. Selforganization to this
complexes in solution. We reasoned that use of a permethyl- supramolecular quadrangle can be rationalized by the fact
Eur. J. Inorg. Chem. 2008, 3442–3451
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S. Range, D. F.-J. Piesik, S. Harder
FULL PAPER
that two mutual perpendicular N,O-chelating salicylald- 124.5(3)°. No obvious agostic Mg···H interactions are ob-
imine units coordinate at a common Zn center in tetrahe- served. This non-planar arrangement of salicylaldimine
dral fashion, thus creating 90° angles between these ligands. units and the imine substituents is commonly observed in
Such selforganization to intriguing metallosupramolecular metal complexes with chelating salicylaldimine ligands and
structures is quite common for salicylaldimine zinc com- is related to the bland yellowish colour of these compounds:
plexes.[12]
planar arrangements give rise to delocalization and stronger
colours (usually orange-red like in the protonated li-
gands).[13]
Crystal Structures
[PARA-Ca·(thf)2]2 crystallizes as a crystallographic cen-
trosymmetric dimer [Figure 1 (b), Table 1]. Each Ca2+ ion
binds to two N,O-chelating salicylaldimine units and two
THF ligands. The distance between the two Ca2+ ions mea-
sures 8.380(2) Å. The salicylaldimine ligands at a common
metal center are nearly coplanar [dihedral angle N1–Ca–
O1/N2Ј–Ca–O2Ј 7.3(2)°]. The N,O-bite angle of the salicyl-
aldimine unit (average: 74.6°) is considerably smaller than
that in the Mg complex PARA-[MgN(SiMe3)2·(thf)]2.
Therefore, the coordination geometry for Ca2+ is slightly
distorted from octahedral. The oxygen atoms of the THF
ligands bend somewhat inwards [O3–Ca–O4 164.7(2)°] on
account of the bulky tBu substituents in the periphery. The
salicylaldimine units make angles of 57.3(2)° and 63.2(2)°
with the phenylene bridge thus partially shutting off delo-
calization in the PARA ligand system. As these dihedral
angles are larger than those in PARA-[MgN(SiMe3)2·
(thf)]2, repulsion between the N=CH hydrogen atom and
the bridge is relaxed. The phenylene bridge itself is con-
siderably deviated from planarity as can be seen from a view
parallel to the bridge (Figure 2, a). The carbon atoms C17,
C18, C20 and C21 are arranged in a plane (maximum devi-
ation from a least-squares plane is 0.006 Å) but carbon
atoms C16 and C19 are situated 0.691(6) Å and 0.694(6) Å,
respectively, out of this plane. This attraction between the
bridges can be explained by a strong π-π stacking interac-
tion.[14] The ring–ring distance of 3.412(8) Å is indeed sim-
ilar to that of the layer distance in graphite (3.40 Å). The
view perpendicular on the bridge planes shows that the
rings are slipped in respect to each other [Figure 2 (b); ring
slippage 2.117 Å]. This off-set stacking interaction allows
for an optimal mutual attraction between Cδ––Hδ+ dipoles
of one phenylene unit and the π-system of the other bridge.
PARA-[CaN(SiMe3)2·(thf)2]2 crystallizes as a monomer
with approximate C2-symmetry [Figure 1 (c), Table 1]. The
Ca–N(SiMe3)2 units are situated on the same side of the
phenylene bridge with a Ca···Ca distance of 8.251(2) Å.
Each Ca2+ ion is bound to one N, O-chelating salicylald-
imine unit, one (Me3Si)2N– ion and two THF ligands. The
calcium coordination geometries can be described as dis-
torted trigonal bipyramidal. For Ca1 axial positions are en-
visioned for N1 and O4 and for Ca2 the atoms N2 and O6
occupy the axial positions. For each Ca2+ ion one rather
large N–Ca–O angle is observed in the equatorial plane
[N3–Ca1–O3 142.4(2)° and N4–Ca2–O5 139.8(2)°]. This is
due to an agostic interaction between a Me3Si substituent
and Ca2+ as is evident from Ca···C distances considerably
shorter than the sum of the van der Waals radii of Ca and
C (3.49 Å): the Ca1···C42 and Ca2···C45 distances are
3.153(8) Å and 3.191(8) Å, respectively. These agostic
PARA-[MgN(SiMe3)2·(thf)]2 crystallizes as a crystallo-
graphically C2-symmetric complex [Figure 1 (a), Table 1].
Each Mg2+ ion binds to a N,O-chelating salicylaldimine
unit, a (Me3Si)2N– ion and a THF ligand. The coordination
geometry of Mg2+ can be described as a distorted tetrahe-
dron. The Mg–O/Mg–N bond lengths and the N,O-bite an-
gle of the salicylaldimine unit [90.4(1)°] are comparable to
those in another salicylaldimine magnesium complex.[13]
The dihedral angle between the salicylaldimine plane and
the phenylene bridge is 35.2(1)°. A coplanar arrangement
of the salicylaldimine unit and the phenylene bridge is un-
favourable on account of repulsion between the N=CH hy-
drogen atom and a hydrogen atom in the phenylene bridge.
This repulsive interaction is not only underscored by short
H···H distances but also by the acute C17–C16–N1 angle
of 117.4(3)° and the obtuse C18–C16–N1 angle of
Table 1. Selected bond lengths [Å] and angles [°] for the crystal
structures of PARA-[MgN(SiMe3)2·(thf)]2, [PARA-Ca·(thf)2]2,
PARA-[CaN(SiMe3)2·(thf)2]2 and [PYR-Zn]4.
PARA-[MgN(SiMe3)2·(thf)]2
Mg–O1
Mg–N1
Mg–O2
Mg–N2
1.914(3)
2.097(3)
2.032(3)
1.980(3)
N1–Mg–O1
N1–Mg–N2
O2–Mg–N2
90.4(1)
122.1(2)
117.3(1)
N1–Mg–O2
O1–Mg–O2
O1–Mg–N2
98.2(1)
98.7(1)
123.9(1)
[PARA-Ca·(thf)2]2
Ca–N1
Ca–O1
Ca–O3
Ca–O4
2.520(5)
2.215(4)
2.362(4)
2.382(4)
N1–Ca–O1
N2Ј–Ca–O2Ј 74.5(2)
O3–Ca–O4
O1–Ca–O4
74.7(2)
O1–Ca–O2Ј
N1–Ca–N2Ј
O1–Ca–O3
96.8(2)
114.4(2)
97.1(1)
164.7(2)
92.2(2)
PARA-[CaN(SiMe3)2·(thf)2]2
Ca1–O1 2.179(4)
Ca1–N1 2.459(6)
Ca1–O3 2.413(5)
Ca1–N3 2.328(5)
Ca1–O4 2.401(5)
Ca2–O2 2.157(4)
Ca2–N2 2.507(6)
Ca2–N4 2.292(5)
Ca2–O5 2.398(5)
Ca2–O6 2.383(6)
N1–Ca1–O1
74.8(2)
85.1(2)
161.3(2)
101.2(2)
96.2(2)
75.2(2)
81.0(2)
156.7(2)
105.2(2)
97.9(2)
O1–Ca1–O4
O1–Ca1–N3
O3–Ca1–O4
O3–Ca1–N3
O4–Ca1–N3
O2–Ca2–O6
O2–Ca2–N4
O5–Ca2–O6
O5–Ca2–N4
O6–Ca2–N4
90.5(2)
121.3(2)
85.1(2)
142.4(2)
96.3(2)
89.6(2)
122.2(2)
83.7(2)
139.8(2)
97.8(2)
N1–Ca1–O3
N1–Ca1–O4
N1–Ca1–N3
O1–Ca1–O3
N2–Ca2–O2
N2–Ca2–O5
N2–Ca2–O6
N2–Ca2–N4
O2–Ca2–O5
[PYR-Zn]4
Zn1–O1 1.904(2)
Zn1–N1 1.999(3)
Zn1–O3 1.895(2)
Zn1–N3 1.990(3)
Zn2–O2 1.913(2)
Zn2–N2 1.991(3)
Zn2–O4Ј 1.919(4)
Zn2–N4Ј 2.026(3)
O1–Zn1–N1
O1–Zn1–O3
O1–Zn1–N3
96.9(1)
122.5(1)
116.5(1)
N1–Zn1–O3
N1–Zn1–N3
O3–Zn1–N3
114.6(1)
111.8(1)
95.5(1)
O2–Zn2–N2
O2–Zn2–O4Ј 120.8(1)
O2–Zn2–N4Ј 115.6(1)
96.6(1)
N2–Zn2–O4Ј 116.3(1)
N2–Zn2–N4Ј 114.0(1)
O4Ј–Zn2–N4Ј 95.0(1)
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Binuclear Magnesium, Calcium and Zinc Complexes
Me···Ca interactions also cause significant tilting of the rac-lactide (rac-LA). Although magnesium complexes are
amide ligand with respect to the Ca–N axes: one of the Ca– generally among the most active catalysts for lactide poly-
N–Si angles is widened [Ca1–N3–Si1 121.9(3)° and Ca2– merization,[2c,2d] the bimetallic magnesium catalysts are
N4–Si4 121.8(3)°] whereas the other is squeezed [Ca1–N3– only moderately active. Whereas the β-diketiminate catalyst
Si2 109.8(3)° and Ca2–N4–Si3 112.3(3)°].
1 reaches full conversion of monomer after one minute,[2c]
[PYR-Zn]4 crystallizes as a crystallographically C2-sym- PARA-[MgN(SiMe3)2·(thf)]2 gave under similar conditions
1
metric cyclic tetrameric aggregate [Figure 1 (d), Table 1]. 70% conversion after 60 min (as determined by H NMR).
Both crystallographically unique Zn metals (Zn1 and Zn2) It is, however, comparable in activity with a recent mononu-
are chelated by two nearly perpendicularly oriented salicyl- clear salicylaldimine magnesium catalyst 3, for which under
aldimine units [dihedral angles: N1–O1–Zn1/N3–O3–Zn1 similar conditions 59% conversion was observed after one
89.2(1)° and N2–O2–Zn2/N4Ј–O4Ј–Zn2 89.0(1)°]. In com- hour.[2i] The results of a series of polymerization experi-
parison to the other structures described in this paper, the ments with three different catalysts in three different sol-
bis(salicylaldimine) ligands in [PYR-Zn]4 display a more vents (thf, CH2Cl2 and toluene) and work-up after five
pronounced planarity. The dihedral angles between salicyl- hours have been summarized in Table 2.
aldimine and pyridyl planes are 8.0(1)°, 14.4(1)°, 31.9(1)°
Table 2. Polymerization of rac-LA using the heteroleptic magne-
and 37.6(1)°. Whereas the conformations of the PARA and
META ligands are partially determined by repulsion be-
sium complexes PARA-[MgN(SiMe3)2·thf]2, META-[MgN-
(SiMe3)2·thf]2 and PYR-[MgN(SiMe3)2·thf]2; ratio [LA]:[Mg] =
50:1 (expected Mn = 7.2ϫ103 Da); polymerization time = 5 h.
tween the N=CH hydrogen atom and the bridge, the PYR
ligand in [PYR-Zn]4 avoids such repulsive interaction by
the cisoid arrangement of N=CH hydrogen and pyridyl ni-
trogen atoms. Instead, attractive non-classical C–H···N hy-
drogen bridges are observed: the H···N distances range
from 2.15–2.46 Å and are well below the sum of their van
der Waals radii (2.75 Å; C–H···N angles range from 101–
108°). The Zn–O and Zn–N bond lengths are within the
range observed for other salicylaldimine zinc complexes.[15]
Catalyst
T [°C] Solvent Yield (%) Mnϫ103 PDI
PARA-[MgN(SiMe3)2·thf]2
META-[MgN(SiMe3)2·thf]2
PYR-[MgN(SiMe3)2·thf]2
PARA-[MgN(SiMe3)2·thf]2
META-[MgN(SiMe3)2·thf]2
PYR-[MgN(SiMe3)2·thf]2
20
20
20
20
20
20
thf
thf
thf
CH2Cl2
CH2Cl2
CH2Cl2
79
56
35
43
22
81
36
30
72
18.3
18.7
17.5
19.9
17.6
16.6
14.6
13.4
17.1
1.97
1.76
1.62
1.79
1.67
1.44
1.42
1.45
1.66
PARA-[MgN(SiMe3)2·thf]2 80[a] toluene
META-[MgN(SiMe3)2·thf]2 80[a] toluene
PYR-[MgN(SiMe3)2·thf]2
80[a] toluene
[a] Due to the poor solubility of rac-LA in toluene the temperature
has been raised to 80 °C.
Polymerization Studies
At room temperature the heteroleptic bimetallic com-
plexes PARA-[MgN(SiMe3)2·(thf)]2, META-[MgN(SiMe3)2·
(thf)]2 and PYR-[MgN(SiMe3)2·(thf)]2 do not react with cy-
clohexene oxide or propylene oxide. Increasing the reaction
temperature to 60 °C gave protonation of the amide
(Me3Si)2N–. Although no products could be isolated from
this reaction, we presume that the amide base deprotonated
the epoxide to form an enolate. Similar enolate formation
has been unequivocally proven in reaction of 1 with cyclo-
hexene oxide.[2d] Like the monometallic complex 1, also the
bimetallic magnesium catalysts are not active in the homo-
polymerization of cyclohexene and propylene oxide. As re-
cently a bimetallic magnesium catalyst (2) has been intro-
duced for the epoxide/CO2 copolymerization,[16] our bime-
tallic magnesium amides were also tested in the copolymer-
ization of neat cyclohexene oxide with CO2 (10 bar). Under
various conditions no activity for copolymerization could
be observed.
Isolated yields strongly depend on the solvent used and
the nature of the bridging unit (Table 2). The p-phenylene-
bridged catalysts generally give higher polylactide yields
than the m-phenylene-bridged ones. Whereas the pyridyl-
bridged catalyst is not very active in thf, it is superior to
the phenylene-bridged systems in less polar solvents like
CH2Cl2 and toluene. This indicates that in an apolar me-
dium the intramolecular coordination of the pyridyl nitro-
gen at the metal center might play a role in the polymeriza-
tion mechanism.
The molecular weights of the polymers obtained are in
all cases considerably higher than the molecular weight ex-
pected based on the Mg/lactide ratio of 1:50 (7200 Da).
This indicates either that propagation is faster than initia-
tion or that partial hydrolysis of the initiating functionality
has occurred. As it has been observed before that the
(Me3Si)2N ligand is slow in initiating lactide polymeriza-
tion,[2c,2d] slow initiation is the most likely explanation.
Whereas the salicylaldimine magnesium catalyst 3 gave po-
lylactides with very narrow molecular weight distributions
(PDI: 1.05–1.10), the bimetallic magnesium catalysts gave
less controlled polymerization (PDI: 1.42–1.97). We tenta-
tively attribute the broad molecular weight distributions to
slow initiation as well as transesterification processes (back-
biting).
All heteroleptic bimetallic complexes PARA-[MgN-
Although the tacticities of the obtained polymers vary
(SiMe3)2·(thf)]2, META-[MgN(SiMe3)2·(thf)]2 and PYR- slightly with the nature of the bridge and the solvent used,
[MgN(SiMe3)2·(thf)]2 are active in the polymerization of all polymers should be regarded as mainly atactic. This con-
Eur. J. Inorg. Chem. 2008, 3442–3451
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3447

S. Range, D. F.-J. Piesik, S. Harder
FULL PAPER
META-H2: Orange solid (yield: 3.55 g, 82%); m.p. 187 °C. 1H
NMR (300 MHz, C6D6, 25 °C): δ = 14.1 (s, 2 H, OH), 8.13 (s, 2
trasts with the mononuclear magnesium catalyst 1 which in
a polar medium (thf) gives heterotactic polylactide.[2d]
4
4
H, CHN), 7.67 [d, J(H,H) = 2.3 Hz, 2 H, Haryl], 7.09 [d, J(H,H)
4
= 2.4 Hz, 2 H, Haryl], 7.04 [t, J(H,H) = 8.1 Hz, 1 H, Haryl], 6.82–
6.77 (m, 3 H, Haryl) 1.71 (s, 18 H, tBu), 1.36 (s, 18 H, tBu) ppm.
13C NMR (300 MHz, C6D6, 25 °C): δ = 164.6, 159.2, 149.9, 140.7,
137.5, 130.1, 127.5, 119.7, 118.9, 114.2, 35.5, 34.3, 31.7, 29.7 ppm.
C36H48N2O2 (540.80): calcd. C 79.96, H 8.95; found C 80.27, H
8.98.
PYR-H2: Orange solid (yield: 2.73 g, 63%); m.p. 237 °C. 1H NMR
(300 MHz, C6D6, 25 °C): δ = 14.5 (s, 2 H, OH), 9.43 (s, 2 H, CHN),
Conclusions
Heteroleptic bimetallic magnesium amide complexes
with a variety of bridged bis(salicylaldimine) ligands have
been obtained. These complexes are also stable in solution
(C6D6 as well [D8]THF) even under forced reflux condi-
tions. Switching to the heavier congener Ca led to dynamic
mixtures of homoleptic and heteroleptic complexes. How-
ever, homoleptic as well as heteroleptic calcium complexes
could be obtained in the solid state. Interestingly, crystals
of the homoleptic complex convert to crystals of the hetero-
leptic complex on account of Ostwald ripening. Stable het-
eroleptic bimetallic zinc amide complexes cannot be ob-
tained with the bis(salicylaldimine) ligands used in this
study. Instead, spontaneous self-organization to large
supramolecular multimetallic zinc aggregates is observed.
Although the heteroleptic bimetallic magnesium amides
were not effective in the homopolymerization of epoxides
or in the copolymerization of CO2 and epoxides, polylactide
could be obtained in a variety of solvents. The isolated yield
of polymer is strongly dependent on the nature of the bridg-
ing unit and the solvent used. Whereas the phenylene-
bridged catalysts are more effective under polar conditions,
the pyridyl-bridged catalyst shows higher activity in apolar
solvents. This indicates that the intramolecular coordina-
tion of the pyridyl nitrogen at the metal center might play
a role in the polymerization mechanism.
4
4
7.64 [d, J(H,H) = 2.3 Hz, 2 H, Haryl], 7.12 [d, J(H,H) = 2.3 Hz,
3
3
2 H, Haryl], 7.04 [t, J(H,H) = 7.6 Hz, 1 H, Haryl], 6.72 [d, J(H,H)
= 7.7 Hz, 2 H, HAryl], 1.69 (s, 18 H, tBu), 1.21 (s, 18 H, tBu) ppm.
13C NMR (300 MHz, C6D6, 25 °C): δ = 166.2, 160.1, 140.9, 140.3,
137.5, 129.1, 128.6, 119.0, 118.7, 35.5, 34.2, 31.5, 29.7 ppm.
C35H47N3O2 (541.78): calcd. C 77.59, H 8.74; found C 77.72, H
8.51.
Me4PARA-H2: Pale yellow solid (yield: 3.34 g, 70%); m.p. 298 °C.
1H NMR (300 MHz, C6D6, 25 °C): δ = 14.1 (s, 2 H, OH), 7.88 (s,
2 H, CHN), 7.67 [d, 4J(H,H) = 2.0 Hz, 2 H, Haryl], 7.10 [d, 4J(H,H)
= 2.1 Hz, 2 H, Haryl], 1.96 (s, 12 H, CH3), 1.69 (s, 18 H, tBu), 1.34
(s, 18 H, tBu) ppm. 13C NMR (300 MHz, C6D6, 25 °C): δ = 168.8,
159.2, 146.4, 137.7, 128.1, 127.1, 125.2, 118.7, 35.5, 34.3, 31.6, 29.8,
15.3 ppm. C40H56N2O2 (596.90): calcd. C 80.49, H 9.46; found C
80.61, H 9.32.
PARA-[MgN(SiMe3)2·thf]2: To a stirred solution of [(Me3Si)2N]2-
Mg·(thf)2 (1.96 g, 4.01 mmol) in benzene (5.0 mL) was added
PARA-H2 (1.00 g, 1.85 mmol). The yellow solution was stirred for
16 h at room temperature. The solution was concentrated to slightly
less than half of its original volume. After slowly cooling to 5 °C
yellow crystals of PARA-[MgN(SiMe3)2·thf]2 precipitated: 1.28 g
(66%); m.p. 231 °C. 1H NMR (300 MHz, C6D6, 25 °C): δ = 8.21
4
(s, 2 H, NCH), 7.77 [d, J(H,H) = 2.6 Hz, 2 H, Haryl], 7.48 (s, 4 H,
4
Haryl), 7.09 [d, J(H,H) = 2.6 Hz, 2 H, Haryl], 3.50 (m, THF), 1.74
(s, 18 H, tBu), 1.35 (s, 18 H, tBu), 1.09 (m, THF), 0.40 (s, 36 H,
Me3Si) ppm. 13C NMR (300 MHz, C6D6, 25 °C): δ = 171.7, 169.3,
149.5, 141.7, 135.9, 131.6, 130.8, 123.7, 120.2, 69.3, 35.8, 34.1, 31.6,
30.0, 24.9, 6.13 ppm. C56H98Mg2N4O4Si4 (1052.38): calcd. C 63.91,
H 9.39; found C 63.74, H 9.21.
Experimental Section
General Remarks: All manipulations were performed under a dry
and oxygen-free atmosphere (argon or nitrogen) using Schlenk line
and glove box techniques and freshly dried solvents. Following
complexes have been prepared according to literature: 3,5-di-tert-
META-[MgN(SiMe3)2·thf]2: To a stirred solution of [(Me3Si)2N]2-
Mg·(thf)2 (1.90 g, 3.89 mmol) in benzene (5.0 mL) was added
META-H2 (1.00 g, 1.85 mmol). The yellow solution was stirred for
16 h at room temperature. All solvents were removed under reduced
pressure and the yellow solid was dissolved in hexane. After cooling
slowly cooling the hexane solution –28 °C small yellow crystals pre-
cipitated. The crystalline product was isolated, washed three times
with cold hexane and dried at 40 °C under high vacuum: 1.22 g
(63%); m.p. 231 °C. 1H NMR (300 MHz, C6D6, 25 °C): δ = 8.36
butylsalicylaldehyde,[17] [(Me3Si)2N]2Mg·(thf)2,[18] [(Me3Si)2N]2Ca·
[18]
(thf)2
and (Me3SiN)2Zn.[19]
General Synthesis of Bridged Bis(salicylaldimine) Ligands (see
Scheme 1): A stirred solution of 3,5-di-tert-butylsalicylaldehyde
(3.75 g, 16.0 mmol) and 8.0 mmol of the selected diamine in meth-
anol (150 mL) was heated at reflux temperature for 2 h. The reac-
tion mixture was cooled to room temperature and stirred overnight.
The coloured precipitate was filtered and washed with methanol
(5ϫ80 mL). The resulting solid was dissolved in thf (120 mL) and
dried with molecular sieves for 6 h at reflux temperature. After sep-
aration of the molecular sieves the solvent was removed under re-
duced pressure to give a coloured solid.
4
(s, 2 H, NCH), 7.77 [d, J(H,H) = 2.3 Hz, 2 H, Haryl], 7.43 (s, 1 H,
4
Haryl), 7.33 (s, 3 H, Haryl), 7.20 [d, J(H,H) = 2.3 Hz, 2 H, Haryl],
3.54 (m, THF), 1.73 (s, 18 H, tBu), 1.36 (s, 18 H, tBu), 1.08 (m,
THF), 0.39 (s, 36 H, Me3Si) ppm. 13C NMR (300 MHz, C6D6,
25 °C): δ = 172.4, 169.4, 152.9, 141.7, 136.0, 131.8, 131.0, 121.3,
120.1, 114.3, 69.6, 35.8, 34.1, 31.6, 29.9, 24.8, 6.54 ppm.
C56H98Mg2N4O4Si4 (1052.38): calcd. C 63.91, H 9.39; found C
63.66, H 9.28.
PARA-H2: Orange solid (yield: 3.81 g, 88%); m.p. 307 °C. 1H
NMR (300 MHz, C6D6, 25 °C): δ = 14.2 (s, 2 H, OH), 8.13 (s, 2
4
4
H, CHN), 7.67 [d, J(H,H) = 2.3 Hz, 2 H, Haryl], 7.08 [d, J(H,H)
= 2.3 Hz, 2 H, Haryl], 6.89 (s, 4 H, Haryl), 1.71 (18 H, tBu), 1.37 (s, PYR-[MgN(SiMe3)2·thf]2: To a stirred solution of [(Me3Si)2N]2-
18 H, tBu) ppm. 13C NMR (300 MHz, C6D6, 25 °C): δ = 163.5,
159.1, 147.1, 140.7, 137.5, 128.2, 127.4, 122.4, 119.0, 35.5, 34.3,
31.7, 29.8 ppm. C36H48N2O2 (540.80): calcd. C 79.96, H 8.95;
found C 80.15, H 8.81.
Mg·(thf)2 (1.96 g, 4.01 mmol) in benzene (5.0 mL) was added PYR-
H2 (1.00 g, 1.85 mmol). The orange solution was stirred for 16 h at
room temperature. All solvents were removed under reduced pres-
sure and the orange solid was dissolved in hexane. Slowly cooling
3448
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 3442–3451

Binuclear Magnesium, Calcium and Zinc Complexes
the hexane solution to –80 °C gave a precipitate of fine orange nee-
dle-like crystals which were isolated and washed three times with
cold hexane. The product was dried at 40 °C under high vacuum:
(s, 2 H, NCH), 7.72 (d, 4J(H,H) = 2.5 Hz, 2 H, Haryl), 7.62 (s, 4 H,
Haryl), 7.23 [d, 4J(H,H) = 2.5 Hz, 2 H, Haryl], 3.64 (m, 16 H, THF),
1.75 (s, 18 H, tBu), 1.30 (s, 18 H, tBu), 0.34 (s, 36 H, Me3Si) ppm.
13C NMR (300 MHz, C6D6, 25 °C): δ = 170.4, 168.7, 151.9, 140.2,
133.9, 131.4, 130.0, 128.3, 123.5, 122.1, 69.5, 35.8, 34.0, 31.8, 30.0,
1
1.31 g (67%); m.p. 138 °C. H NMR (300 MHz, C6D6, 25 °C): δ =
4
9.18 (s, 2 H, NCH), 7.78 [d, J(H,H) = 2.6 Hz, 2 H, Haryl], 7.52–
7.46 (m, 3 H, Haryl), 7.23 [d, 4J(H,H) = 2.6 Hz, 2 H, Haryl], 3.58 25.1, 5.92 ppm. C64H114Ca2N4O6Si4 (1228.15): calcd. C 62.59, H
(m, THF), 1.72 (s, 18 H, tBu), 1.36 (s, 18 H, tBu), 1.04 (m, THF), 9.36; found C 62.18, H 9.16.
0.41 (s, 36 H, Me3Si) ppm. 13C NMR (300 MHz, C6D6, 25 °C): δ
[PYR-Zn]4: To a stirred solution of PYR-H2 (1.00 g, 1.85 mmol) in
benzene (8.0 mL) was added Et2Zn (1.85 mL of a 1  solution in
hexane, 1.85 mmol) which resulted in immediate gas evolution and
precipitation of an orange solid. After stirring for 4 h at room tem-
perature the precipitate was isolated, washed three times with hex-
ane and dried under high vacuum: 0.77 g (69%). Crystals for the
X-ray structure determination were obtained by recrystallization
from warm benzene; m.p. 343 °C. 1H NMR (300 MHz, C6D6,
25 °C): δ = 9.17 (s, 4 H, NCH), 7.80 [d, 4J(H,H) = 2.1 Hz, 4 H,
Haryl], 7.06 [d, 4J(H,H) = 2.1 Hz, 4 H, Haryl], 6.90 [d, 3J(H,H) =
= 171.0, 159.7, 142.2, 140.1, 136.2, 132.7, 131.5, 127.9, 119.7, 115.8,
69.2, 35.8, 34.1, 31.5, 29.0, 25.0, 6.18 ppm. C55H97Mg2N5O4Si4
(1053.37): calcd. C 62.71, H 9.28; found C 62.55, H 9.12.
[PARA-Ca·(thf)2]2: To a stirred solution of [(Me3Si)2N]2Ca·(thf)2
(1.96 g, 3.88 mmol) in benzene (12.0 mL) was added PARA-H2
(1.00 g, 1.85 mmol). The yellow solution was stirred for 5 h at room
temperature. Slow cooling to +5 °C gave overnight small yellow
cube-like crystals which were isolated, washed twice with cold ben-
zene and dried under high vacuum: 1.98 g (74%); m.p. 238 °C
(dec.). The crystals dissolve poorly in C6D6 and [D8]THF and in
3
8.1 Hz, 4 H, Haryl], 6.13 [t, J(H,H) = 8.1 Hz, 4 H, Haryl], 1.67 (s,
1
both cases numerous sets of signals are observed in the H NMR
36 H, tBu), 1.41 (s, 36 H, tBu) ppm. 13C NMR (300 MHz, C6D6,
25 °C): δ = 171.8, 170.1, 156.4, 142.9, 136.8, 132.7, 132.1, 117.8,
116.1, 35.9, 34.0, 31.2, 29.7 ppm. C140H180N12O8Zn4 (2420.58): C
69.47, H 7.50; found: C 69.62, H 7.72.
spectra. It is likely that in solution either several species exist or
that a complicated aggregate with inequivalent ligand environments
is formed. C88H124Ca2N4O8 (1446.15): calcd. C 73.09, H 8.64;
found C 72.73, H 8.67.
PARA-[CaN(SiMe3)2·(thf)2]2: To a stirred solution of [(Me3Si)2N]2-
Ca·(thf)2 (1.02 g, 2.02 mmol) in benzene (8.0 mL) was added
PARA-H2 (0.50 g, 0.93 mmol). The yellow solution was stirred for
5 h at room temperature. Slow cooling to +5 °C gave overnight
small yellow cube-like crystals (0.1ϫ0.1ϫ0.1 mm) which after
standing for 3 d at +5 °C completely dissolved again and new
rather large yellow/orange crystals (4ϫ4ϫ0.5 mm) in the form of
a hexagon precipitated. The new crystals were isolated, washed
twice with cold benzene and dried under high vacuum: 0.50 g
(44%); m.p. 172 °C. 1H NMR (300 MHz, C6D6, 25 °C): δ = 8.37
General Procedure for Lactide Polymerization: A Schlenk flask was
charged in a glovebox with a solution of rac-lactide (250 mg,
1.73 mmol) in 2.5 mL of the specified solvent. A solution of the
initiator (0.017 mmol) in the same solvent (500 mg) was added
rapidly. The mixture was immediately stirred with a magnetic stir
bar at the desired temperature for five hours. The reaction was
quenched with methanol (10 mL) containing a few drops of con-
centrated aqueous HCl, and the polymer was precipitated with ex-
cess methanol. The polymer was then dried under vacuum to con-
stant weight. Molecular weight distributions have been measuered
Table 3. Crystal data for PARA-[MgN(SiMe3)2·(thf)]2, [PARA-Ca·(thf)2]2, PARA-[CaN(SiMe3)2·(thf)2]2 and [PYR-Zn]4.
Compound
PARA-[MgN(SiMe3)2·(thf)]2 [PARA-Ca·(thf)2]2
PARA-[CaN(SiMe3)2·(thf)2]2 [PYR-Zn]4
Formula
C56H98Mg2N4O4Si4·(C6H6)2
C88H124Ca2N4O8·(C6H6)4
C64H114Ca2N4O6Si4·(C6H6)
C140H180N12O8Zn4·(C6H6)12
MW
1208.58
0.2ϫ0.2ϫ0.1
monoclinic
C2/c
1758.50
0.1ϫ0.1ϫ0.1
monoclinic
P21/c
1306.23
0.5ϫ0.4ϫ0.3
orthorhombic
Pca21
3357.83
0.5ϫ0.2ϫ0.2
monoclinic
C2/c
Size [mm3]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α
43.349(5)
10.3258(11)
16.6335(18)
90
13.7721(11)
18.4509(14)
20.3988(15)
90
18.5829(10)
17.3548(9)
26.4108(15)
90
38.021(5)
17.104(2)
35.059(4)
90
β
γ
110.401(7)
90
7300.3(14)
4
1.100
0.144
95.727(5)
90
5157.6(7)
2
1.132
0.167
90
90
8517.6(8)
4
1.019
106.109(7)
90
21904(5)
4
1.018
0.484
V [Å3]
Z
ρ [gcm–3]
µ(Mo-Kα) [mm–1]
0.195
T [°C]
–70
–70
–70
–100
θ (max.)
24.0
24.2
24.3
26.1
Total reflections, unique
Rint
34661, 5679
0.128
13628, 7155
0.109
146368, 13654
0.139
93728, 21702
0.069
Obsd. reflections [IϾ2σ(I)]
3350
2597
9159
14400
Parameters
382
580
813
1087
R1
wR2
GOF
0.0626
0.1842
1.01
–0.38/0.28
–
0.0592
0.1966
0.82
–0.28/0.35
–
0.0828
0.2274
1.04
–0.31/0.50
0.13(6)
0.0654
0.1740
1.06
–0.64/0.37
–
max./min. residual e– [eÅ–3]
Flack parameter
Eur. J. Inorg. Chem. 2008, 3442–3451
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3449

S. Range, D. F.-J. Piesik, S. Harder
FULL PAPER
by GPC measurements of polymer dissolved in chloroform. 1H
NMR measurements have been performed in CDCl3.
Darensbourg, W. Choi, C. P. Richers, Macromolecules 2007, 40,
3521–3523; l) H.-Y. Chen, H.-Y. Tang, C.-C. Lin, Polymer
2007, 48, 2257–2262.
Crystal Structure Determinations: All data were collected on a Sie-
mens SMART CCD diffractometer (Table 3). The structures have
been solved by direct methods (SHELXS-97)[20] and were refined
with SHELXL-97.[21] The geometry calculations and graphics have
been performed with PLATON.[22] All crystals contain one or more
benzene molecules in the lattice. In the crystals of PARA-
[MgN(SiMe3)2·(thf)]2, and [PARA-Ca·(thf)2]2 the enclosed benzene
molecules were ordered and could be refined. The crystals of
PARA-[CaN(SiMe3)2·(thf)2]2 and [PYR-Zn]4 contained some or-
dered benzene molecules that have been refined and some disor-
dered benzene molecules which were treated with the SQUEEZE
procedure incorporated in the program PLATON.[22] Crystals of
[PYR-Zn]4 are extremely sensitive and decompose fast in paraffin
oil. This is likely due to large amounts of incorporated solvent (7
relatively ordered and 2 disordered benzene molecules are found in
the asymmetric unit).
[3]
a) M. Sawamura, M. Sudoh, Y. Ito, J. Am. Chem. Soc. 1996,
118, 3309–3310; b) R. C. Mathews, D. K. Howell, W.-J. Peng,
S. G. Train, W. D. Treleaven, G. G. Stanley, Angew. Chem.
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Acknowledgments
We acknowledge the Deutsche Forschungsgemeinschaft (DFG) for
support of this project within the AM2-net framework. Prof. Dr.
R. Boese and D. Bläser (University Duisburg-Essen) are thanked
for collection of X-ray diffraction data.
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Received: March 26, 2008
Published Online: July 7, 2008
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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