Zinc and enolato-magnesium complexes based on bi-, tri- and tetradentate aminophenolate ligands

Article abstract of DOI:10.1039/b812754a

The coordination chemistry of a series of potentially tetra-, tri- and bi-dentate aminophenol pro-ligands ([L¹]H-[L⁵]H) with magnesium and zinc derivatives has been studied. Reactions of the pro-ligands [L¹]H, [L²]H and [L³]H with one equiv. of ZnEt₂ or Zn(N(SiMe₃)₂)₂ in toluene at room temperature afford cleanly, via ethane or amine elimination, the ethyl- and amido-zinc complexes 1-4. Complexes [L⁴]ZnEt (5) and [L ⁴]Zn(N(SiMe₃)₂)₂ (6) derived from the tridentate pro-ligand [L⁴]H were prepared in 84% and 76% yields, following similar alkane and amine elimination protocols, respectively. The 1:1 reactions of bidentate pro-ligand [L⁵]H with ZnEt₂ or Zn(N(SiMe₃)₂)₂ in toluene using different reaction protocols systematically yielded mixtures of the bis(ligand) complex [L⁵]₂Zn (7) and the corresponding ethyl-{[L ⁵]ZnEt}_n (8; n = 1 or 2) or amido- {[L⁵] Zn(N(SiMe₃)₂)}_n complexes (9; n = 1 or 2). The synthesis of magnesium-enolate complexes 10 and 11 was carried out in a one-pot, two-step procedure by first reacting pro-ligands [L¹]H and [L ⁴]H with one equiv. of Mg(n,sBu)₂ to generate the corresponding {[Lⁿ]Mg(n,sBu)}_n species, which were further reacted with one equiv. of 2,4,6-Me₃C₆H₂COMe. All complexes have been characterized by multinuclear NMR, elemental analysis, and by single-crystal X-ray diffraction studies for five-coordinate magnesium complex 10, and four-coordinate Zn complexes 3-7. Preliminary studies indicate that magnesium species 10 and 11 as well as zinc complexes 1 and 5 are not active initiators for the polymerization of methyl methacrylate, even in combination with one equiv. of Li[OC(=CH₂)(2,4,6-Me₃C ₆H₂)], while zinc complexes 2, 4 and 6 are effective initiators for the ring-opening polymerization of ε-caprolactone and rac-lactide at 20°C. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008.

Full text of DOI:10.1039/b812754a

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Zinc and enolato-magnesium complexes based on bi-, tri- and
tetradentate aminophenolate ligandsw
a
a
a
a
´
Zhanjiang Zheng, Gang Zhao, Remy Fablet, Miloud Bouyahyi,
Christophe M. Thomas,z*a Thierry Roisnel,b Osvaldo Casagrande Jrc
and Jean-Franc¸
ois Carpentier*a
Received (in Montpellier, France) 24th July 2008, Accepted 29th August 2008
First published as an Advance Article on the web 14th October 2008
DOI: 10.1039/b812754a
The coordination chemistry of a series of potentially tetra-, tri- and bi-dentate aminophenol pro-
ligands ([L1]H–[L5]H) with magnesium and zinc derivatives has been studied. Reactions of the
pro-ligands [L1]H, [L2]H and [L3]H with one equiv. of ZnEt2 or Zn(N(SiMe3)2)2 in toluene at
room temperature afford cleanly, via ethane or amine elimination, the ethyl- and amido-zinc
complexes 1–4. Complexes [L4]ZnEt (5) and [L4]Zn(N(SiMe3)2)2 (6) derived from the tridentate
pro-ligand [L4]H were prepared in 84% and 76% yields, following similar alkane and amine
elimination protocols, respectively. The 1 : 1 reactions of bidentate pro-ligand [L5]H with ZnEt2
or Zn(N(SiMe3)2)2 in toluene using different reaction protocols systematically yielded mixtures of
the bis(ligand) complex [L5]2Zn (7) and the corresponding ethyl-{[L5]ZnEt}n (8; n = 1 or 2) or
amido- {[L5]Zn(N(SiMe3)2)}n complexes (9; n = 1 or 2). The synthesis of magnesium-enolate
complexes 10 and 11 was carried out in a one-pot, two-step procedure by first reacting pro-
ligands [L1]H and [L4]H with one equiv. of Mg(n,sBu)2 to generate the corresponding
{[Ln]Mg(n,sBu)}n species, which were further reacted with one equiv. of 2,4,6-Me3C6H2COMe. All
complexes have been characterized by multinuclear NMR, elemental analysis, and by single-
crystal X-ray diffraction studies for five-coordinate magnesium complex 10, and four-coordinate
Zn complexes 3–7. Preliminary studies indicate that magnesium species 10 and 11 as well as zinc
complexes 1 and 5 are not active initiators for the polymerization of methyl methacrylate, even in
combination with one equiv. of Li[OC(QCH2)(2,4,6-Me3C6H2)], while zinc complexes 2, 4 and 6
are effective initiators for the ring-opening polymerization of e-caprolactone and rac-lactide
at 20 1C.
[L2NO]ꢀ ligands were focused on the coordination chemistry of
Introduction
Cu(II),1 Zn(II),2,3 Cu(I),4,5 and Ni(II).6 Triaminophenolate ligands
have been shown to adopt bidentate (Al(III)), tridentate (Mg(II),
Ca(II), Zn(II)) and tetradentate (K(I), Cr(III), Fe(II), Co(II)) binding
modes.7 Phenolate ligands functionalized with triazacyclononanes
have been reported as well for the coordination of a variety of
early transition metals (Sc(III), Y(III), Ti(IV), V(III), Cr(III)) and
related group 13 metals (Al(III), In(III), Ga(III), Tl(III)).8 Very
recently Lappert and coll. introduced piperazinylphenolate
ligands onto Zn(II) centers and showed they behave as bidentate
chelates.9 Other recent applications involve the coordination of
tripodal [L2NO]ꢀ ligands onto group 3 and 4 metals, essentially
for olefin and polar monomers polymerization purposes.10–12
Mononuclear13a and dinuclear13b zinc complexes supported by
bulky tridendate diamino-phenolate ligands have been prepared
and the former mononuclear complexes have been shown to
polymerize lactide with good control and at a rate faster than
any other Zn-containing system reported previously. Also,
bidentate aminophenolate ligands have been used for preparing
low-coordinate complexes of group 13 metals (Al(III), Ga(III))14
and magnesium complexes,15 which are useful for ring-opening
polymerization catalysis.
The coordination chemistry of aminophenolate ligands has been
intensively studied in recent years. Of particular interest are ligands
of general type A, of which relevant examples disclosed in the
literature are shown in Chart 1. According to the number and
nature of extra donor arms, such monoanionic compounds can
accommodate a broad range of coordination geometries as
bidentate, tridentate or tetradentate ligands for various oxophilic
metals and some of the resulting complexes can be used as efficient
(pre)catalysts in a variety of processes. Early reports on tripodal
a
´
Catalyse et Organometalliques, Sciences Chimiques de Rennes,
UMR 6226 CNRS-Universite de Rennes 1, 35042 Rennes Cedex,
´
France. E-mail: christophe.thomas@univ-rennes1.fr.
E-mail: jean-francois.carpentier@univ-rennes1.fr
b
c
´
Centre de Diffractometrie X, Sciences Chimiques de Rennes, UMR
6226 CNRS-Universite de Rennes 1, 35042 Rennes Cedex, France
´
´
´
Laboratorio de Catalise Molecular, Instituto de Quimica-
Universidade Federal do Rio Grande do Sul, Av. Bento Gonc¸alves
9500, Porto Alegre, RS, 90501-970, Brazil
w Crystallographic data for complexes 3–7 and 10. CCDC reference
numbers 672301–672306. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b812754a
´
z Current address: Ecole Nationale Superieure de Chimie de Paris,
11 rue Pierre et Marie Curie, 75231, Paris Cedex 05, France.
E-mail: christophe-thomas@enscp.fr
This report describes the synthesis and structural charac-
terization of a series of new amido- and alkyl-zinc and
ꢁc
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Chart 1 Examples of aminophenolate-type ligands useful in coordination chemistry and catalysis.
enolato-magnesium complexes with such aminophenolate
ligands. The denticity of the latter has been varied from
4- to 2-coordinate ligands in order to evaluate the stabilization
of the metal center. The presence of non-bound donor arms,
which could readily exchange in solution with bound arms,
may be an important factor in this respect, with possible
important implications for catalytic applications. Preliminary
investigations of the reactivity of some of the prepared com-
plexes toward polar monomers are also reported.
temperature afford cleanly, via ethane or amine elimination,
the ethyl- and amido-zinc complexes 1–4 (Scheme 2). These
compounds were isolated in good yields (65–87%) as colorless
or pale yellow solids, which are readily soluble in hydrocarbon
(benzene, toluene, pentane, hexanes) and chlorinated
(CH2Cl2) solvents. In our hands, [L1]ZnEt (1), which was
previously reported as unstable over hours,2a could be stored
for weeks in the glovebox at ꢀ30 1C without signifi-
cant decomposition. Compound 1 and the three other
new complexes [L2]Zn(N(SiMe3)2) (2), [L3]ZnEt (3) and
[L3]Zn(N(SiMe3)2) (4) were authenticated by elemental
analyses and 1H and 13C NMR spectroscopy. The 1H and
13C NMR spectra of complexes 1, 2 and 3 in C6D6 at room
temperature show each one set of resonances for equivalent
(on the NMR timescale) 2-pyridylmethyl and (3,5-dimethyl-
1-pyrazolyl)ethyl groups, respectively. Variable-temperature
NMR spectroscopy of complex 3 in toluene-d8 revealed
decoalescence of the resonances upon lowering the tempera-
ture, eventually resulting in the observation of individual
resonances for four methyl-pyrazolyl and two NCH2CH2-
pyrazolyl groups at ꢀ80 1C. As further supported by the solid
state structure of 3 (vide infra), these observations are con-
sistent with only one of the pyrazolyl groups coordinated onto
the metal center and fast exchange of the coordinated and free
pyrazolyl groups on the NMR timescale at relatively high
temperature. The same situation, that is a coordinated pyridyl
group and a free pyridyl group, can be envisioned for com-
plexes 1 and 2. Alternatively, we cannot rule out that both
pyridyl groups are coordinated to the zinc center in 1 and 2,
either in a symmetric or dissymmetric fashion; the latter
situation was observed in zinc complexes supported by
aminophenolate ligands bearing 2-methoxyethyl arms, in
Results and discussion
Synthesis of aminophenol pro-ligands
The aminophenol pro-ligands used in this study were prepared in
a straightforward manner, by reductive amination, following a
one-pot, two-step procedure similar to that reported for known
bis(pyridylmethyl) derivative [L1]H (Scheme 1).16 The new com-
pounds [L2]H, [L3]H, [L4]H and [L5]H were prepared in 74%,
40%, 38% and 46% overall yields (not optimized), starting from
bis(picolyl)amine,
N,N-bis[2-(3,5-dimethyl-1-pyrazole)ethyl]-
amine, N-benzyl-N-(2-methyl-pyridine)amine, and commercially
available N,N-bis(benzyl)amine, respectively. The ligand precur-
sors were obtained as colorless or yellowish powders, and their
identity was confirmed by NMR and high-resolution mass
spectrometry. Such ligand architecture allows steric and electro-
nic variations on both the phenol and the pyridine or pyrazole
donor fragments.
Synthesis and structure of zinc complexes
Reactions of the pro-ligands [L1]H, [L2]H and [L3]H with
one equiv. of ZnEt2 or Zn(N(SiMe3)2)2 in toluene at room
Scheme 1 Synthesis of aminophenol pro-ligands [Ln]H.
ꢁc
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these conditions (toluene, ꢀ78 1C to 20 1C) to form 3, which
could be isolated in pure form, though in modest yield (see
Experimental section). Actually, the released ethanol slowly
hydrolyzes the Zn–O(phenolate) bond of 3, leading to free
[L3]H and Zn products that could not be identified. This
competitive, undesired reaction was independently confirmed
by the reactions of 3 with ethanol, benzyl alcohol or 2-propanol
(1–5 equiv, toluene, 20 1C). Similarly, the reaction of 4 with
2-propanol appeared to be plagued by the release of free [L2]H
and, despite the fact that alcoholysis of the amido residue
takes place as evidenced by the formation of HN(SiMe3)2
(identified by NMR), the desired zinc 2-propoxide complex
could not be obtained in a pure form.
The new complexes [L4]ZnEt (5) and [L4]Zn(N(SiMe3)2) (6)
derived from the tridentate pro-ligand [L4]H were prepared in
84% and 76% yields, following similar alkane and amine
elimination protocols, respectively (Scheme 3). Those com-
plexes are stable solids under inert atmosphere at room
temperature, which feature the same solubility as above-
Scheme 2 Preparation of zinc aminophenolate complexes 1–4.
which one methoxy arm is tightly coordinated and the other
one is weakly coordinated onto the metal center.3 On the other
hand, the 1H and 13C NMR spectra for complex 4 in C6D6 at
room temperature show four methyl-pyrazolyl and two
NCH2CH2-pyrazolyl groups. This also indicates that, in this
complex, one of the pyrazolyl groups is coordinated to the
metal center while the second one is free (or more weakly
coordinated), but that these two groups do not exchange
rapidly on the NMR timescale under these conditions.
Apparently, the different bulkiness of the ethyl and bis-
(trimethylsilylamido) groups may account for this different
fluxional behavior.
1
mentioned for 1–4. They were both characterized by H and
13C NMR solution spectroscopy and X-ray diffraction studies
of single crystals grown from pentane solutions at 20 1C. Both
the 1H and 13C{1H} NMR spectra of 5 and 6 in C6D6 or
CD2Cl2 at room temperature contained a single set of sharp
resonances, which could be assigned in details based on 2D
1
1H–1H COSY, and H–13C HMQC and HMBC experiments
(see Experimental section), and which are consistent with the
existence of a single species on the NMR timescale under these
conditions. Complementary investigations in the case of 5
indicated that those NMR features were essentially unchanged
in toluene-d8 and CD2Cl2 down to ꢀ80 1C; in particular, no
significant broadening of the signals was noticed at such low
temperature. The above observations are consistent with the
formation of a single diastereomer of 5 and 6, because the
relative configurations at the chiral bridging N atom and Zn
center in these complexes are related.
The crystal structures of complexes 3 and 4 were determined
by X-ray diffraction studies. Details of crystal and refinement
data are reported in Table 1. Both complexes are monomeric
in the solid state and feature a four-coordinated zinc center in
a distorted tetrahedral geometry (Fig. 1 and 2). Consistent
with the NMR observations in solution (vide supra), only one
of the two pyrazolyl groups per molecule is coordinated,
together with the bridging N and O atoms of the ligand,
forming two six-membered metallacycles. Thus, though po-
tentially tetradentate, the tripodal aminophenolate is actually
tridentate.7 The coordination sphere is completed by the alkyl
or amido ligand. The Zn–O and Zn–N bond distances fall in
normal ranges for similar complexes.2 Significant differences
between those two structures include a shorter Zn(1)–N(21)
bond distance in 4 (2.121(2) A) than in 3 (2.206(2) A), likely
reflecting the strong s-donor effect of the amido group. The
most acute angle among the two six-membered metallacycles is
associated with the bite of the N,N chelate ring, although the
difference is much more sensitive for 2 (86.86(7) vs. 95.01(6)1)
than for 3 (93.50(7) vs. 95.71(6)1). Thus, rather unexpectedly,
the bulkier N(SiMe3)2 group in 4 (as compared to ethyl in 3)
apparently does not induce a more acute bite angle of the N,O
chelate ring, but a more opened bite angle of the N,N chelate
ring. Both six-membered metallacycles in the two complexes
adopt boat conformations, as observed in related tetrahedral
trisaminophenolate zinc complexes.2,7
The solid-state structures of 5 and 6 have been determined
by X-ray diffraction studies. The unit cells of 5 and 6 contain
respectively three and two independent molecules, but both
these sets of molecules have very similar overall geometry,
bond distances and angles. The molecular structures of 5 and 6
are monomeric and feature a four-coordinated zinc center in a
distorted tetrahedral geometry (Fig. 3 and 4). Details of crystal
and refinement data are reported in Table 1. First, it is worth
noting that the same relative configurations at the N and Zn
atoms are observed in 5 and 6. The Zn–O and Zn–N bond
distances fall in normal ranges for similar complexes,2,7 and
compare well in particular with those found in 3 and 4 (and the
bond angles as well). The angles at the metal involving the
aminophenolate ligand are in the range 79.1(2)–118.9(3)1.
The most acute is in both complexes logically associated with
the bite of the five-membered N,N chelate ring, the extra
methylene in the six-membered N,O-chelate ring allowing this
latter chelate to adopt a less strained coordination geometry.
As in complexes 3 and 4, the six-membered metallacycles in 5
and 6 adopt boat conformations. The five-membered ring in 5
has a twisted conformation with the N(21) atom lying 0.56 A
out of the N(11)–Zn(1)–C2 plane, while in 6 the atoms of the
five-membered ring are coplanar to within ca. 0.14 A.
Attempts to prepare an ethoxide-zinc complex [L3]ZnOEt
(potentially useful for polymerization catalysis) from [L3]H
and ZnEt(OEt) did not afford the desired compound
(Scheme 2). 1H NMR monitoring of this reaction showed that
only ethanol, and no ethane, elimination proceeded under
ꢁc
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Table 1 Summary of crystal and refinement data for complexes 3, 4, 5, 6, 7 and 10
3
4
5
6
7
10
Empirical
formula
Formula
weight
C31H49N5OZn
C35H62N6OSi2Zn
C30H40N2OZn
C34H53N3OSi2Zn
C58H72N2O2Zn
C38H47MgN3O2
602.10
573.12
704.46
510.01
641.34
894.55
Crystal
system
Space group C2/c
Monoclinic
Triclinic
Triclinic
Orthorhombic
Triclinic
Monoclinic
ꢀ
P1
ꢀ
P1
ꢀ
P1
Pca21
15.2406(8)
10.5921(6)
48.038(3)
90
90
90
7754.8(8)
8
P21/c
a/A
b/A
c/A
a/1
31.6107(11)
11.7495(4)
14.5448(5)
14.7216(5)
62.3027(17)
88.2672(17)
67.8521(17)
2030.70(13)
2
13.9651(2)
17.5917(2)
21.1494(3)
76.8990(10)
79.3830(10)
67.2880(10)
4641.00(11)
6
10.646(2)
14.508(3)
16.750(4)
85.050(10)
75.882(10)
80.339(10)
2470.6(9)
2
10.4793(6)
22.4925(13)
15.5746(9)
90
108.702(4)
90
3477.2(3)
4
1.15
0.087
1296
11.2967(3)
17.4711(6)
90
96.688(2)
90
b/1
g/1
V/A3
Z
6196.4(3)
8
Dc/Mg mꢀ3 1.229
1.152
0.696
760
1.095
0.815
1632
1.099
0.722
2752
1.203
0.540
960
m/mmꢀ1
F(000)
0.823
2464
Crystal
size/mm
y range/1
Limiting
indices
0.15 ꢂ 0.17 ꢂ 0.25 0.08 ꢂ 0.14 ꢂ 0.32 0.03 ꢂ 0.20 ꢂ 0.30 0.06 ꢂ 0.35 ꢂ 0.40 0.04 ꢂ 0.15 ꢂ 0.35 0.03 ꢂ 0.20 ꢂ 0.22
2.55 to 27.59 3.03 to 27.48 2.26 to 26.65 2.96 to 27.47 2.52 to 26.44 2.74 to 27.69
ꢀ40 r h r 38, ꢀ14 ꢀ17 r h r 17, ꢀ22 ꢀ17 r h r 17, ꢀ22 ꢀ11 r h r 13, ꢀ19 ꢀ13 r h r 13, ꢀ18 ꢀ13 r h r 11, ꢀ29
r k r 10, ꢀ22 r l r k r 21, ꢀ26 r l r k r 21, ꢀ26 r l r k r 19, ꢀ62 r l r k r 18, ꢀ21 r l r k r 29, ꢀ14 r l
r 22
r 26
r 26
r 61
r 21
r 20
Refl.
collected
Refl. unique 7145 (5024)
[I 4 2s(I)]
Data/
28 903
19 863
62 706
67 188
33 514
35 729
9258 (7219)
9258/0/406
18 987 (10056)
18 987/0/919
17 364 (13933)
17 364/1/745
11 241 (8736)
11 241/0/640
8082 (4998)
8082/0/403
7145/0/343
restraints/
param.
Goodness- 1.035
1.008
1.064
1.179
1.012
1.012
of-fit on F2
R1 [I 4
2s(I)] (all
data)
0.0414 (0.0686)
0.0410 (0.0583)
0.0913 (0.1534)
0.0843 (0.1008)
0.0391 (0.0582)
0.0542 (0.1053)
wR2 [I 4
2s(I)] (all
data)
0.1072 (0.1196)
0.0970 (0.1059)
0.2698 (0.2910)
0.2289 (0.2364)
0.0831 (0.0914)
0.1196 (0.1402)
Largest
diff./e Aꢀ3
0.415 and ꢀ0.502
0.445 and ꢀ0.311
2.243 and ꢀ1.35
0.869 and ꢀ1.213
0.300 and ꢀ0.415
0.385 and ꢀ0.345
The 1 : 1 reactions of bidentate pro-ligand [L5]H with ZnEt2
or Zn(N(SiMe3)2)2 in toluene using a variety of protocols
(reaction temperature, introduction order) systematically
yielded mixtures of the bis(ligand) complex [L5]2Zn (7) and
the corresponding ethyl- {[L5]ZnEt}n (8; n = 1 or 2) or amido-
{[L5]Zn(N(SiMe3)2)}n complexes (9; n = 1 or 2) (Scheme 4).
These mixtures never contained more than 80% of the desired
complexes and could not be efficiently separated by recrystal-
lization; only small amounts of 7, which proved to be suitable
for X-ray diffraction studies (vide infra), were isolated from
some batches. Formation of 8 appeared to be somewhat
favored by using excess of ZnEt2 (5 equiv.), but all attempts
to purify the desired complex from these mixtures also failed.
Therefore, we could not probe whether homoleptic bis(ligand)
complex 7 is a redistribution/disproportionation product of
thermodynamically unstable heteroleptic complexes 7 or 8 (as
observed by Lappert et al.),9 or rather results from a kineti-
cally fast consecutive reaction of 8 or 9 with [L5]H.
being centrosymmetric, with quite differentiated O–Zn–N
angles within the two metallacycles (94.39(6) and 99.11(5)1).
Also, one six-membered ring has a twisted envelop conforma-
tion with the N(11) atom lying ca. 0.79 A out of the
Zn(1)–O(51)–C(73)–C(72)–C(71) plane, while the second
six-membered ring has a boat conformation with the O(1)
and C(31) atoms lying ca. +0.38 and +0.65 A out of the
C(23)–C(22)–Zn(1)–N(1) plane. The Zn–O and Zn–N
distances in 7 are slightly smaller than those observed in
complexes 3–6; this is likely due to the lower steric hindrance
induced by the formation of a single metallacycle per ligand
unit (vs. two in 3–6). Overall, the bond distances and bond
angles in 7 compare very well those observed in a related
bis(aminophenolate)zinc complex reported by Lappert and
colleagues.9
Synthesis and structure of magnesium complexes
Expectedly, the solid-state structure of 7 features a mono-
meric molecule with the Zn center in a distorted tetrahedral
geometry coordinated by the four heteroatoms of the two
aminophenolate ligands (Fig. 5). The complex is far from
The synthesis of magnesium-enolate complexes was carried
out in a one-pot, two-step procedure (Scheme 5). Tri-
and diaminophenol pro-ligands [L1]H and [L4]H17 were first
reacted with one equiv. of Mg(n,sBu)2 to generate the
ꢁc
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Fig. 3 Solid-state structure of [L4]ZnEt (5) (ellipsoids drawn at the
50% probability, H atoms omitted for clarity). Selected bond distances
(A) and angles (1) (data given for the independent molecule depicted
above, with [values] for the two other independent molecules):
Zn(1)–O(31), 1.959(4) [1.935(5), 1.976(4)]; Zn(1)–C(1), 1.987(7)
[1.969(7)]; Zn(1)–N(11), 2.146(5) [2.122(6), 2.136(6)]; Zn(1)–N(21),
2.179(6) [2.170(5), 2.187(5)]; O(31)–Zn(1)–C(1), 121.8(2) [119.7(9),
120.1(3)]; O(31)–Zn(1)–N(11), 106.0(2) [101.2(2), 104.4(2)];
O(31)–Zn(1)–N(21), 94.4(2) [94.6(2), 95.3(2)]; C(1)–Zn(1)–N(21),
134.1(3) [134.7(3), 135.0(3)]; C(1)–Zn(1)–N(11), 112.2(2) [114.8(3),
Fig. 1 Solid-state structure of [L3]ZnEt (3) (ellipsoids drawn at the
60% probability, H atoms omitted for clarity). Selected bond distances
(A) and angles (1): Zn(1)–O(31), 1.909(1); Zn(1)–N(11), 2.100(2);
Zn(1)–N(21), 2.206(2); Zn(1)–C(1), 1.978(2); O(31)–Zn(1)–C(1),
128.33(9); O(31)–Zn(1)–N(11), 100.93(7); O(31)–Zn(1)–N(21),
95.01(6); C(1)–Zn(1)–N(21), 117.09(8); C(1)–Zn(1)–N(11), 119.05(9);
N(11)–Zn(1)–N(21), 86.86(7).
118.4(3)];
N(11)–Zn(1)–N(21),
79.1(2)
[78.1(2),
79.0(2)];
C(22)–N(21)–Zn(1), 121.8(4) [119.7(4), 125.9(5)].
Fig. 2 Solid-state structure of [L3]Zn(N(SiMe3)2) (4) (ellipsoids
drawn at the 60% probability, H atoms omitted for clarity). Selected
bond distances (A) and angles (1): Zn(1)–O(31), 1.932(2); Zn(1)–N(11),
Fig. 4 Solid-state structure of [L4]Zn(N(SiMe3)2) (6) (ellipsoids
drawn at the 50% probability, H atoms omitted for clarity). Selected
bond distances (A) and angles (1) (data given for the independent
molecule depicted above, with [values] for the other independent
molecule): Zn(1)–O(131), 1.936(5) [1.933(5)]; Zn(1)–N(101), 1.923(7)
[1.916(6)]; Zn(1)–N(111), 2.160(7) [2.175(6)]; Zn(1)–N(121), 2.091(6)
2.058(2);
Zn(1)–N(21),
2.121(2);
Zn(1)–N(1),
1.931(2);
108.62(7);
116.65(7);
O(31)–Zn(1)–N(1),
O(31)–Zn(1)–N(21),
121.17(7);
95.71(6);
O(31)–Zn(1)–N(11),
N(1)–Zn(1)–N(21),
N(1)–Zn(1)–N(11), 116.17(7); N(11)–Zn(1)–N(21), 93.50(7).
[2.087(6)];
O(131)–Zn(1)–N(101),
119.1(3)
[118.7(2)];
O(131)–Zn(1)–N(121), 98.1(2) [98.0(2)]; O(131)–Zn(1)–N(111),
96.3(2) [96.3(2)]; N(101)–Zn(1)–N(121), 120.6(3) [121.9(2)];
N(101)–Zn(1)–N(111), 130.9(3) [130.6(2)]; N(121)–Zn(1)–N(111),
82.8(3) [82.2(2)]; C(112)–N(111)–Zn(1), 120.1(6) [121.5(4)].
corresponding {[Ln]Mg(n,sBu)}n species, which were not iso-
lated but further reacted with one equiv. of 2,4,6-Me3C6H2-
COMe. This sterically demanding ketone was selected for its
Scheme 3 Preparation of zinc aminophenolate complexes 5–6.
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well-known excellent crystallization properties that its metal
enolates can have.18 Actually, recrystallization of the crude
products from toluene allowed isolating the desired enolate
complexes [Ln]Mg{OC(QCH2)Mes} (Mes = 2,4,6-Me3C6H2)
10 and 11 as white crystals in 63% and 21% yield, respectively.
Both complexes were characterized in solution by 1H and 13
C
NMR spectroscopy and, in the case of 10, by a single-crystal
X-ray diffraction study.
The solid-state structure of 10 features a five-coordinated
Mg center in a distorted trigonal-bipyramidal geometry, with
coordination of the phenolato oxygen, the two pyridine nitro-
gens, the tertiary bridging nitrogen, and the enolate oxygen
(Fig. 6). Complex 10 represents a rare example of a monomeric
magnesium complex with a terminal O–bound enolate ligand,
in addition to Mg(enolato)(dpp-bianH)(THF)2 com-
plexes (enolato = OC(QCHPh)CH2Ph, OC(QCPh2)CH3, or
Scheme 4 Formation of zinc aminophenolate complexes 7–9.
camphor; dpp-bian
=
1,2-bis{(2,6-diisopropylphenyl)-
imino}acenaphthene) recently reported by Schumann and
co-workers.19 Other examples magnesium enolate complexes
are concerned with dimeric or oligomeric structures with
m-bridging O(enolate) ligands,20 as usually observed for
oxophilic, hard metals.21 The Mg–O(enolato) distance of
1.888(2) A in 10 compares well with those observed in
Mg(enolato)(dpp-bianH)(THF)2
complexes
(1.902(3),
1.921(2) and 1.890(2) A, respectively), and lies in between
those of 1.845(3) and 2.012(3) A observed for the two termi-
nally bound enolate groups in the tetrameric complex
[Mg4{OC(QCH2)Mes}8{OQC(Mes)Me}2(C6H5Me)2]5.20e
Fig. 5 Solid-state structure of complex [L5]2Zn (7) (ellipsoids drawn at the
50% probability, H atoms omitted for clarity). Selected bond distances (A)
and angles (1): Zn(1)–O(1), 1.897(2); Zn(1)–O(51), 1.911(2); Zn(1)–N(1),
2.113(2); Zn(1)–N(11), 2.083(2); O(1)–Zn(1)–O(51), 119.62(6); O(1)–Zn(1)–
N(11), 101.61(6); O(51)–Zn(1)–N(11), 94.39(6); O(1)–Zn(1)–N(1), 99.11(5);
O(51)–Zn(1)–N(1), 106.95(6); N(11)–Zn(1)–N(1), 137.39(6).
Fig.
6
Solid-state structure of complex [L1]Mg{OC(QCH2)(2,4,6-
Me3C6H2)} (10) (ellipsoids drawn at the 50% probability, H atoms
omitted for clarity). Selected bond distances (A) and angles (1):
Mg(1)–O(41), 1.888(2); Mg(1)–O(21), 1.903(2); Mg(1)–N(1), 2.143(2);
Mg(1)–N(11), 2.178(2); Mg(1)–N(17), 2.273(2); O(41)–C(41), 1.319(2);
C(41)–C(42), 1.333(3); O(41)–Mg–(1)–O(21), 107.61(7); O(41)–Mg(1)–
N(1), 92.22(7); O(41)–Mg(1)–N(11), 102.00(7); O(41)–Mg(1)–N(17),
163.93(7); O(21)–Mg(1)–N(1), 120.64(7); O(21)–Mg(1)–N(11), 115.82(7);
O(21)–Mg(1)–N(17), 87.79(6); N(1)–Mg(1)–N(11), 113.40(7); N(1)–
Mg(1)–N(17), 75.56(6); N(11)–Mg(1)–N(17), 74.24(7); O(41)–C(41)–
C(42), 126.0(2).
Scheme 5 Preparation of magnesium aminophenolate enolate
complexes 10 and 11.
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They are only slightly smaller than those of 1.908(2)
and 1.921(2) A observed for C,O-bridging enolate groups in
the 1H–1H COSY spectrum in CD2Cl2] at d 5.00 and 3.35 ppm
(J = 12.7 Hz) and d 3.45 and 3.10 ppm (J = 13.5 Hz),
respectively. Unfortunately, attempts to grow suitable crystals
of 11 for X-ray diffraction to assess the exact nuclearity of this
compound in the solid state were unsuccessful. The formula-
tion of complex 11 (as in Scheme 3) appears to be only possible
on the basis of elemental analysis and by analogy with that of
complex 10.
the dimeric [(BDI)Mg(m,m-OC(QCH2)Mes)]2 (BDI
=
HC(C(Me)QN-2,6-iPr2C6H3)2).20f The bond distances and
angles associated to the aminophenolate ligand are similar to
those observed in a related [N3O]Mg(iPr) complex.7 The two
five-membered rings in 10 have an envelop conformation
with the N(17) atom lying ca. 0.59
A out of the
Mg(1)–N(11)–C(15)–C(16) plane, and ca. 0.63 A out of the
Mg(1)–N(1)–C(5)–C(6) plane.
Preliminary studies on the reactivity towards polar monomers
The 1H and 13C NMR spectra of 10 contain each one set of
well resolved resonances which were fully assigned on the basis
of 2D 1H–1H COSY, and 1H–13C HMQC and HMBC experi-
ments (see Experimental section). Key data include only one
signal in both the 1H and 13C NMR spectra in C6D6 for
magnetically equivalent CHH-pyridine groups. This is indica-
tive that, in benzene solution, both pyridine groups are
coordinated to the magnesium center, and that the structure
found in the solid state is maintained in solution. Other im-
Zinc and magnesium complexes modified by ancillary ligands
have been used with great success for the controlled poly-
merization of polar monomers such as (meth)acrylates, lactide
(LA), and lactones.22–26 We are therefore interested in inves-
tigating the catalytic activity of the new mononuclear zinc and
magnesium complexes that we have prepared and we report
here some preliminary observations.
Reactivity towards methyl methacrylate. Zinc and magne-
sium enolate complexes have recently been shown to feature
high performances for the polymerization of methyl metha-
crylate (MMA), affording PMMAs with a high degree of
control in terms of molecular weight, molecular weight dis-
tribution, and tacticity.21,25 The reactivity of the prepared com-
plexes towards MMA was therefore investigated, either as
single initiators or in combination with Li[OC(QCH2)-
(2,4,6-Me3C6H2)]. Both zinc complexes 1 and 5 showed
no activity, even when 1 equiv. of the bulky Li-enolate
was added to produce in situ the enolate-zincate species.
More surprisingly, isolated Mg-enolate complexes 10 and 11
also showed no activity towards MMA polymerization.
Indeed, even though metal enolates are known to adopt a
variety of structural motifs, which are dependent on the choice
of metal, solvent, and steric bulk of the ligands,27 enolates
of electropositive metals such as 10 and 11 prefer O-bound
structures,28 and are usually reactive in MMA polymeriza-
tions.29 More, Gibson reported recently very high activities for
MMA polymerization with related imidinate-Mg-enolate
[(BDI)Mg(OC(QCH2)(2,4,6-Me3C6H2))]2. One might explain
this inactivity by the high coordination number of the metal
center in 10 and 11, which would prevent coordination of the
MMA monomer, compared to Gibson’s initiator which is
3-coordinated.25
1
portant data for 10 include two finely-coupled H NMR reso-
nances (J o 0.3 Hz) at d 5.10 and 4.47 ppm for the OCQCHH
hydrogens of the enolate group. These resonances are more
deshielded than those reported for the mixed Mg–Na–enolate
complexes of the type [Na2Mgx{m-OC(QCH2)Mes}y-
(TMEDA)2] (d 4.69–5.01 and 4.02–4.20 ppm in C6D6),20e
[(BDI)Mg{m,m-OC(QCH2)Mes}]2 (d 4.10 and 3.72 ppm in
C6D6),20f and even more than for (BDI)Mg{OC(QCH2)Mes}-
(THF) (d 3.58 and 2.98 ppm in THF-d8).20f Except in the latter
case where change of the NMR solvent (THF for benzene) is
likely to have a major impact on chemical shifts, we assume that
these NMR trends reflect the different coordination modes of
the enolate groups in those complexes. The latter trends are
more obvious in 13C NMR spectroscopy. In fact, the key 13C
NMR chemical shifts for 10 (d (O(CQCH2)) 166.86; d (QCH2)
81.80 ppm) compare well with those in (BDI)Mg{OC(QCH2)-
Mes}(THF) (d (O(CQCH2)) 165.83; d (QCH2) 84.97 ppm),20f
which has also a terminal O–bounded enolate group, while these
for bridging enolate groups in [Na2Mgx{m-OC(QCH2)Mes}y-
(TMEDA)2] are more shielded (d (O(CQCH2)) 161.23, 163.12
and 165.17; d (QCH2) 89.74, 87.02 and 83.91 ppm).20e
The 1D and 2D NMR spectra of 11 in either C6D6
(300 MHz) or CD2Cl2 (500 MHz) at 20 1C were much more
complicated, and could not be fully interpreted. Though most
of the resonances were relatively sharp, severe overlapping
occurred and the chemical shifts of the many diastereotopic
hydrogens appear to be significantly affected by anisotropic
shielding of the aryl groups, hampering a straightforward
interpretation. The number and similar intensity of 1H
NMR resonances for the tBu and Ar–Me groups, and of the
many AB systems [also identified in the 1H–1H COSY
spectrum] for the CHH groups as well, suggest the existence
of two sets of resonances in a ca. 1 : 1 ratio. This might be
indicative of the presence of two [mononuclear] isomers in
comparable proportions or, more likely, of a dinuclear species
with non-equivalent ligands. Consistent with these hypotheses
is the observation in the 13C NMR spectrum of two resonances
of similar intensity for the enolate methylenic group at d 93.34
Reactivity towards e-caprolactone and rac-lactide. The ability
of the zinc complexes to catalyze the ROP of e-caprolactone
(e-CL) and rac-lactide (rac-LA) was also preliminarily exam-
ined (Scheme 6). Some first results are given in Table 2. Zinc
complexes 4 and 6 proved significantly active and productive
initiators for the ROP of e-CL, allowing conversions of
200–3700 equiv. of monomer at room temperature (Entries
1–4). The degree of control over the polymerization (as judged
by the correspondence of experimental and theoretical mole-
cular weights, and molecular weight distributions) was rela-
tively good for 6 but appeared to be quite poor for 4,
presumably due to transesterification reactions and/or side
reactions with impurities that act as transfer agents.30
Complexes 2, 4 and 6 are also active for the polymerization
of rac-lactide at room temperature, affording atactic31 PLAs
1
and 81.31 ppm. The latter resonances correlate in the H–13C
HMQC spectrum with two sets of doublets [authenticated in
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Experimental
General procedures
All experiments involving metal complexes were carried out
under purified argon using standard Schlenk techniques or in a
glove box (o1 ppm O2, 5 ppm H2O). Hydrocarbon solvents,
diethyl ether and tetrahydrofuran were distilled from Na/
benzophenone, toluene and pentane were distilled from Na/K
alloy under nitrogen and degassed by freeze-thaw-vacuum
cycles prior to use. Methanol was distilled from Mg chips.
Chlorinated solvents were distilled from calcium hydride.
Deuterated solvents were purchased from Eurisotop and
purified before use. 3,5-Di(tert-butyl)-2-hydroxybenzaldehyde,
N,N-bis(picolyl)amine, N-benzyl-N-(2-methylpyridine), bis(2-
chloroethyl)amine hydrochloride, NaBH3CN, ZnEt2 (15 wt%
solution in hexane) and Mg(n,s-Bu)2 (1.0 M solution in
heptane) were purchased from Acros or Aldrich Co. and
used as received. Zn[N(SiMe3)2]2 was prepared following the
reported procedure.34 ZnEt(OEt)2 was prepared by ethanolysis
of ZnEt2 in toluene. e-Caprolactone was dried over calcium
hydride and then distilled at reduced pressure (2 mmHg, 85 1C)
before use.
Scheme 6 Ring-opening polymerization of e-caprolactone (e-CL)
and rac-lactide (rac-LA).
(Entries 5–7). Significant differences in terms of activity and of
degree of control were observed, suggesting a major influence
of the substituents on the phenolate ligands and the nature of
the N-donor fragments, as well.
These preliminary results indicate that some of the above
systems feature high polymerization activity, good molecular
weight control and give relatively narrow molecular weight
distributions; this is notably the case of complex 6 for the ROP
of lactide. Overall, such performances compare well with those
reported for some related, efficient aminophenolate-zinc sys-
tems.13 A somehow disappointing feature of these amino-
phenolate-zinc systems remains the absence of stereocontrol
achieved in the ROP of racemic lactide, whatever the sub-
stituents on the phenolate ligands and the nature of the
N-donor fragments, while other Zn(II) systems, such as those
supported by BDI-type ligands (BDI = CH(CMeNC6H3-2,
6-iPr2)2), lead to highly heterotactic PLAs.23 These observa-
tions further confirm the crucial role of ancillary ligands in
chain-end stereocontrolled ROP processes.
NMR spectra were recorded on Bruker AC-200, AC-300
and AM-500 spectrometers (in Teflon-valved NMR tubes for
complexes) at 20 1C otherwise stated. 1H and 13C NMR
chemical shifts were determined using residual solvent reso-
nances and are reported vs. SiMe4. Assignment of signals was
made from 2D 1H–1H COSY and 1H–13C HMQC and HMBC
NMR experiments. Coupling constants are given in Hertz.
Elemental analyses (C, H, N) were performed using a Flash
EA1112 CHNS Thermo Electron apparatus and are the
average of two independent determinations. High resolution
mass spectra of complexes were obtained by ESI in the positive
mode (6 keV) using a Micromass ZabSpec TOF spectrometer.
Size exclusion chromatography (SEC) of PCL and PLA poly-
mers was performed in THF at 20 1C using a Polymer
Laboratories PL-GPC 50 plus apparatus (PLgel 5 mm
MIXED-C 300 ꢂ 7.5 mm, 1.0 ml minꢀ1, RI and Dual angle
LS detector (PL-LS 45/90)). The number average molecular
masses (Mn) and polydispersity index (Mw/Mn) of the poly-
mers were calculated with reference to a universal calibration
vs. polystyrene standards. Mn values were corrected by a
Mark–Houwink factor of 0.56 and 0.58 for PCL32 and
PLA,33 respectively, due to the use of PS standards.
Conclusions
In conclusion, we have reported a new series of magnesium
enolato and zinc alkyl or amido complexes supported by
readily available polydentate monoanionic aminophenolate
ligands. The ligands designed confer interesting activity and
productivity to the zinc derivatives, which readily initiate the
ROP of e-CL and rac-lactide. Thorough studies are currently
being pursued to evaluate and rationalize the influence of the
N-donor fragments and of the substituents on the phenolate
ligands on the activity and degree of control in these poly-
merizations, and will be reported in due course.
Table 2 Preliminary results obtained in the ring-opening polymerization of e-caprolactone and rac-lactide promoted by complexes 2, 4, and 6a
Mn,calcb/g molꢀ1
Mn,expc/g molꢀ1
Mw/Mn
c
Entry
Monomer M
[I]
[M]/[I]
t/min
1
2
3
4
5
6
7
e-CL
e-CL
e-CL
e-CL
rac-LA
rac-LA
rac-LA
4
4
6
6
2
4
6
200
3700
500
1000
100
2
210
1
2
15
22 800
422 000
57 000
114 000
14 400
14 400
14 400
7600
14 700
65 500
179 000
15 500
6100
1.8
1.6
1.6
1.6
3.6
1.9
1.7
100
100
1440
180
13 800
a
All reactions performed at 20 1C in toluene at [monomer] = 1–2 M (i.e., slurry for rac-lactide), until completion as determined by the integration
of 1H NMR methyl resonances of rac-LA and PLA or methylene resonances of e-CL and PCL. Mn of PCL or PLA calculated from the monomer
b
c
conversion: Mn,calc = ([e-CL or rac-LA]/[Zn]) ꢂ conversion ꢂ M(monomer). Mn and Mw/Mn of polymer determined by SEC-RI in THF at RT
using polystyrene standards; Mn values were corrected by a Mark–Houwink factor of 0.56 and 0.58 for PCL32 and PLA,33 respectively, due to the
use of PS standards.
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Syntheses
N(CH2CH2-pyraz)2), 3.61 (t, 3J = 6.0, 4H, N(CH2CH2-
pyraz)2), 5.69 (s, 2H, pyraz-H).
[L1]H.16. To a 100 mL Schlenk flask containing 3,5-di-tert-
butyl-2-hydroxybenzaldehyde (2.34 g, 10.0 mmol) in anhy-
drous MeOH (60 mL), bis(picolyl)amine (1.99 g, 10.0 mmol)
was added slowly at room temperature, followed by several
drops of aqueous acetic acid as catalyst. After stirring at room
temperature for about 1 h, NaBH3CN (0.63 g, 10.0 mmol) was
slowly added in by portions, and the reaction mixture
was stirred at room temperature for 3 days. The mixture was
acidified by adding 4 M aqueous HCl (ca. 10 mL) and then
evaporated to dryness under vacuum. The residue was dis-
solved in saturated aqueous Na2CO3 (ca. 50 mL) and
extracted with CHCl3 (3 ꢂ 50 mL). The combined extracts
were evaporated to dryness to give a brown oil, which was
further purified by flash chromatography (CHCl3–methanol
= 3 : 1) to give [L1]H as a pale yellow powder (2.71 g, 65%).
1H NMR (CDCl3, 500 MHz): d 1.29 (s, 9H, tBu), 1.48 (s, 9H,
tBu), 3.83 (s, 2H, CH2-phenyl), 3.90 (s, 4H, CH2-pyridyl), 6.91
(b, 1H, phenyl-H), 7.18 (t, J = 6.5, 2H, pyridyl-4H), 7.23
(s, 1H, phenyl-H), 7.41 (d, J = 7.5, 2H, pyridyl-2H), 7.66
(t, J = 7.5, 2H, pyridyl-3H), 8.59 (d, J = 4.9, 2H, pyridyl-5H),
10.6 (br s, 1H, OH).
2,4-Di(tert-butyl)-6-{bis[(3,5-dimethyl-1-pyrazole)ethylamino]-
aminomethyl}phenol ([L3]H). To
a solution of 3,5-di-
(tert-butyl)-2-hydroxylbenzaldehyde (1.96 g, 8.4 mmol) in
methanol (20 mL) were added bis[2-(3,5-dimethyl-1-pyrazole)-
ethyl]amine (2.20 g, 8.4 mmol) and a small amount of acetic
acid. Sodium cyanotrihydroborate (0.53 g, 8.4 mmol) was
added by portions to the resulting solution with stirring. After
the solution was stirred for three days at room temperature, it
was acidified by adding 4 M aqueous HCl (ca. 10 mL) and
then evaporated under reduced pressure. The residue was
dissolved in saturated aqueous Na2CO3 (50 mL) and extracted
with CHCl3 (3 ꢂ 50 mL). The combined extracts were dried
over anhydrous MgSO4 and evaporated under vacuum to give
a brown oil which was purified on a silica-gel column with
chloroform–methanol to give [L3]H as a colorless powder
(1.60 g, 40%). 1H NMR (300 MHz, CDCl3) d 9.65 (s, 1H,
PhOH), 7.23 (d, J = 2.4, 1H, phenol-CH), 6.82 (d, J = 2.4,
1H, phenol-CH), 5.76 (s, 2H, pyr-CH), 4.03 (t, 4H, J = 6.6,
NCH2CH2NCH2CH2N), 3.81 (s, 2H, NCH2PhOH), 2.98
(t, J = 6.6, 4H, NCH2CH2NCH2CH2N), 2.19 (s, 6H, CH3-
pyraz), 2.07 (s, 6H, CH3-pyraz), 1.39 (s, 9H, phenol-tBu), 1.29
(s, 9H, phenol-tBu). 13C NMR (75 MHz, CDCl3): d 10.81
(CH3-pyraz), 13.45 (CH3-pyraz), 29.59 (C(CH3)3), 37.67
[L2]H. Aminophenol [L2]H was prepared following the
procedure described above for [L1]H, starting from 5-chloro-
3-methyl-2-hydroxy-benzaldehyde (0.50 g, 2.9 mmol), bis-
(picolyl)amine (0.64 g, 3.2 mmol), and NaBH4 (0.50 g,
13.2 mmol), to give after workup [L2]H as a white powder
(C(CH3)3),
46.12
(NCH2CH2NCH2CH2N),
53.94
(NCH2CH2NCH2CH2N), 59.51 (ZnCH2-phenol), 105.16
(CH-pyraz), 123.41 (C-pyraz), 123.83 (C-pyraz). ESI-HRMS:
calcd for [M + H]+: 480.37024; found: 480.3711; calc. for
[M + Na]+: 502.35218; found: 502.3519.
1
(0.76 g, 74%). H NMR (CDCl3, 300 MHz): d 2.29 (s, 3H,
Me), 3.76 (s, 2H, CH2-pyridyl), 3.89 (s, 4H, CH2), 6.92 (s, 1H,
phenol-H), 7.05 (s, 1H, phenol-H), 7.20 (t, J = 7, 2H, pyridyl-
4H), 7.36 (d, J = 7.7, 2H, pyridyl-2H), 7.67 (t, J = 7.5, 2H,
pyridyl-3H), 8.62 (d, J = 4.0, 2H, pyridyl-5H), 11.17 (s, 1H,
OH). 13C{1H} NMR (CDCl3, 75 MHz): d 16.18 (CH3), 56.61
(CH2), 59.10 (2CH2), 122.32 (pyridyl-C4), 123.27 (pyridyl-C2),
127.20 (phenol-C), 129.79 (phenol-C), 136.83 (pyridyl-C3),
149.06 (pyridyl-C5), 154.40 (phenol-C1), 158.04 (pyridyl-C1).
Anal. Calc. for C20H20ClN3O (353.85): C, 67.89; H, 5.70; N,
11.88. Found: C, 68.0; H, 5.9; N, 11.9.
N-Benzyl-N-(2-methylpyridine). This compound was pre-
pared using a modified procedure from that reported in the
literature.35 To a 100 mL Schlenk flask containing 2-amino-
methyl(pyridine) (7.55 g, 70 mmol) in anhydrous MeOH
(50 mL), benzaldehyde (7.45 g, 70 mmol) was added slowly
in at room temperature. Then, several drops of aqueous acetic
acid were added as catalyst. After stirring at room temperature
for about 1 h, the solution was cooled to 0 1C and then NaBH4
(3.97 g, 105 mmol) was slowly added in by portions. After
stirring for 30 min, the solution was poured into a flask and
acidified with 4 M HCl till pH = ca. 1. KOH pellets were then
added to the solution until pH = 14 was reached, and the
solution was extracted with CH2Cl2 (ca. 50 mL). The organic
phases were combined, dried over Na2SO4 and the solvents
were removed under vacuum to give yellowish oil. The oil
was distilled under vacuum (180 1C/0.1 mmHg) to afford
N-benzyl-N-(2-methylpyridine) as a colorless oil (11.1 g,
N,N-Bis[2-(3,5-dimethyl-1-pyrazole)ethyl]amine.
To
a
100 mL Schlenk containing a suspension of NaH (1.38 g,
57.4 mmol) in DMF (50 mL) was added dropwise a solution of
3,5-dimethylpyrazole (3.68 g, 38.3 mmol) in DMF (10 mL).
The reaction mixture was stirred for 2 h at room temperature.
Then a solution of bis(2-chloroethyl)amine hydrochloride
(3.41 g, 19.1 mmol) in DMF (10 mL) was added dropwise to
the mixture. The reaction mixture was further stirred for 2 h at
70 1C, yielding a cream white dispersion which was cooled and
filtered. The filtrate was evaporated to dryness in vacuo, and
hot water (80 1C, 100 mL) was added. Cooling to room
temperature gave white needle crystals. Finally, a colorless
oil-like product (2.20 g, 45%) was obtained by further remov-
1
80%). H NMR (CDCl3, 300 MHz): d 8.56 (d, J = 4.2, 1H,
pyridyl-2H), 7.63 (t, J = 6.3, 1H, pyridyl-3H), 7.17–7.32
(m, 7H, pyridyl-H and phenyl-H), 3.92 (s, 2H, CH2-pyridyl),
3.84 (s, 2H, CH2-phenyl), 2.14 (br s, 1H, NH).
1
ing the residual water by azeotropic distillation overnight. H
[L4]H. Aminophenol [L3]H was prepared following the
procedure described above for [L1]H, starting from 3,5-
di-tert-butyl-2-hydroxybenzaldehyde (2.34 g, 10.0 mmol),
N-benzyl-N-(2-methylpyridine) (1.98 g, 10.0 mmol), and
NaBH3CN (0.63 g, 10.0 mmol), to give after workup [L4]H
as a white powder (1.58 g, 38%). 1H NMR (CDCl3, 300 MHz):
NMR (200 MHz, CDCl3): d 2.16 (s, 6H, pyraz-CH3), 2.19
(s, 6H, pyraz-CH3), 2.95 (t, 3J = 6.0, 4H, N(CH2CH2-
pyraz)2), 4.01 (t, 3J = 6.0, 4H, N(CH2CH2-pyraz)2), 5.74
1
(s, 2H, pyraz-H). H NMR (200 MHz, C6D6): d 1.81 (s, 6H,
pyraz-CH3), 2.29 (s, 6H, pyraz-CH3), 2.70 (t, J = 6.0, 4H,
3
ꢁc
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d 1.29 (s, 9H, tBu), 1.47 (s, 9H, tBu), 3.71 (s, 2H, CH2-pyridyl),
3.80 (s, 4H, CH2), 6.88 (d, J = 2.4, 1H, Ph-2,6-H), 7.10–7.40
(m, 7H, pyridyl-H and phenyl-H), 7.68 (t, J = 17.1, 1H,
pyridyl-5H), 8.58 (d, J = 5.0, 1H, pyridyl-6H). 13C{1H} NMR
(C6D6, 75 MHz): d 29.60 (C(CH3)3), 31.66 (C(CH3)3), 34.13
(C(CH3)3), 34.91 (C(CH3)3), 57.90 (CH2), 59.36 (CH2), 122.30
(pyridyl-C5), 122.94 (Ph-C), 124.13 (pyridyl-C3), 128.49 (Ph-
C), 129.93 (Ph-C), 135.48 (phenol-C6), 136.62 (pyridyl-C4),
140.63 (phenol-C4), 149.13 (pyridyl-C6), 157.92 (pyridyl-C2).
[L3]ZnEt (3). Synthesis from ZnEt2: Complex 3 was pre-
pared in 87% yield following a protocol similar to that
described above for 1. Synthesis from EtZnOEt: To a solution
of zinc ethylethoxide (0.072 g, 0.52 mmol) in toluene (3 mL)
was slowly added [L2]H (0.248 g, 0.52 mmol) in toluene (5 mL)
at ꢀ78 1C. The mixture was stirred for 24 h at room tempera-
ture. The volatiles were removed in vacuo leaving a viscous oil,
which was further recrystallized from pentane at ꢀ30 1C to
give 3 as a pale yellow crystalline solid (0.050 g, 17%). 1H
NMR (500 MHz, C6D6): d 0.76 (q, 2H, J = 8.0, ZnCH2CH3),
1.59 (s, 6H, CH3-pyraz), 1.62 (s, 9H, tBu), 1.89 (t, 3H, J = 8.0,
ZnCH2CH3), 1.95 (s, 9H, tBu), 2.34 (s, 6H, CH3-pyraz), 2.79
(m, 4H, NCH2CH2NCH2CH2N), 3.53 (t, 4H, J = 5.5,
NCH2CH2NCH2CH2N), 3.62 (s, NCH2-phenol), 5.55 (s, 2H,
pyraz-CH), 7.05 (d, 1H, J = 2.6, phenol-CH), 7.76 (d, 1H,
J = 2.6, phenol-CH). 13C{1H} NMR (125 MHz, C6D6): d ꢀ0.47
(ZnCH2CH3), 10.12 (CH3-pyraz), 13.16 (CH3-pyraz), 13.75
(ZnCH2CH3), 29.86 (C(CH3)3), 32.22 (C(CH3)3), 33.96
(C(CH3)3), 35.57 (C(CH3)3), 44.50 (NCH2CH2NCH2CH2N),
55.44 (NCH2CH2NCH2CH2N), 63.69 (phenol-CH2), 105.42
(pyraz-CH), 124.34 (phenol-CH), 125.67 (phenol-CH). Anal.
Calc. for C31H49N5OZn (573.14): C, 64.96; N, 12.22; H, 8.62.
Found: C, 65.1; N, 11.9; H, 8.8.
ESI-HRMS: calc. for C28H37N2O [M + H]ꢃ+, 417.2906;
+
found, 417.2893; calc. for C28H36N2ONa [M + Na]ꢃ
439.2725; found, 439.2725.
,
[L5]H. Aminophenol [L4]H was prepared following the
procedure described above for [L1]H, starting from 3,5-di-
tert-butyl-2-hydroxybenzaldehyde (2.34 g, 10.0 mmol), N,N-
dibenzylamine (1.97 g, 10.0 mmol), and NaBH3CN (0.63 g,
10.0 mmol), to give after workup [L3]H as a white powder
(1.89 g, 46%). 1H NMR (C6D6, 300 MHz): d 1.37 (s, 9H, tBu),
1.69 (s, 9H, tBu), 3.31 (s, 4H, CH2), 3.46 (s, 2H, CH2), 6.9–7.5
(m, 12H, phenyl-H). 13C{1H} NMR (C6D6, 75 MHz): d 29.58
(C(CH3)3), 31.69 (C(CH3)3), 34.13 (C(CH3)3), 34.91
(C(CH3)3), 57.61 (CH2), 58.07 (CH2), 121.23, 122.88,
123.84, 127.51, 129.69, 128.51, 129.69, 137.04, 140.50, 153.85
+
(C arom). ESI-HRMS: calc. for C29H38NO [M + H]ꢃ
416.2953; found, 416.2960.
,
[L3]ZnN(SiMe3)2 (4). This compound was prepared in
a similar manner to [L2]ZnN(SiMe3)2 (2) starting from
[L3]H (0.400 g, 0.83 mmol) and Zn[N(SiMe3)2]2 (0.320 g,
0.83 mmol) in toluene (5 mL). Compound 4 was obtained as
a colorless powder (0.380 g, 65%). 1H NMR (300 MHz,
C6D6): d 0.51 (s, 18H, N(SiMe3)2), 1.14 (s, 3H, CH3-
pyraz), 1.59 (s, 9H, tBu), 1.85 (s, 9H, tBu), 2.08 (s, 3H,
CH3-pyraz), 2.28 (m, 1H, NCH2CH2NCH2CHHN), 2.38
(s, 3H, CH3-pyra), 2.57 (s, 3H, CH3-pyra), 2.92
(m, 1H, NCH2CH2NCH2CHHN), 3.11 (d, 1H, J = 1.9,
[L1]ZnEt (1). Complex 1 was prepared according to the
reported method.2 To a 50 mL Schlenk flask were added [L1]H
(0.200 g, 0.48 mmol), toluene (10 mL), and ZnEt2 (0.390 g of a
15 wt% solution in hexane, 0.48 mmol). The reaction mixture
was stirred at room temperature overnight. After evaporation
of volatiles under vacuum, the residue was washed with
pentane at ꢀ78 1C to give 1 as a white powder (0.205 g,
1
84%). H NMR (C6D6, 200 MHz): d 0.76 (q, J = 8.2, 2H,
ZnCHH-phenol),
3.24
(m,
4H,
ZnCHH-phenol,
ZnCH2CH3), 1.37 (s, 9H, tBu), 1.71 (t, J = 8.2, 3H,
ZnCH2CH3), 1.85 (s, 9H, tBu), 3.45 (s, 2H, CH2-phenolate),
3.74 (d, J = 15.0, 2H, CH2-pyridyl), 4.04 (d, J = 15.0, 2H,
CH2-pyridyl), 6.40-7.40 (m, 8H, arom), 8.19 (d, J = 4.6, 2H,
pyridyl-5H).
NCH2CHHNCHHCHHN), 3.93 (m, 1H, NCH2CH2N-
CHHCH2N), 4.18 (m, 1H, NCHHCH2NCH2CH2N), 4.47
(m, 1H, NCHHCH2NCH2CH2N), 4.63 (d, 1H, J = 1.9,
ZnCHH-phenol), 5.33 (s, 1H, pyraz-CH), 5.83 (s, 1H, pyraz-
CH), 7.03 (d, 1H, J = 2.4, phenol-CH), 7.66 (d, J = 2.4, 1H,
[L2]ZnN(SiMe3)2 (2). To a solution of [L2]H (0.400 g,
0.83 mmol) in toluene (10 mL) was added a solution of
Zn[N(SiMe3)2]2 (0.320 g, 0.83 mmol) in toluene (5 mL). The
mixture was stirred for 24 h at room temperature and volatiles
were removed in vacuo leaving a powder, which was washed by
a minimum amount of cold pentane to give 2 as a colorless
phenol-CH). 13C{1H} NMR (75 MHz, C6D6):
d 5.87
(N(SiMe3)2), 9.92 (CH3-pyraz), 10.36 (CH3-pyraz), 13.75
(CH3-pyraz), 13.81 (CH3-pyraz), 29.96 (C(CH3)3), 32.17
(C(CH3)3), 33.93 (C(CH3)3), 35.42 (C(CH3)3), 42.32
(NCH2CH2NCH2CH2N), 42.61 (NCH2CH2NCH2CH2N),
53.05 (NCH2CH2NCH2CH2N), 56.44 (NCH2CH2NCH2-
CH2N), 61.84 (ZnCH2-phenol), 105.55 (pyraz-CH), 105.94
(pyraz-CH), 124.66 (phenol-CH), 125.35 (phenol-CH). Anal.
Calc. for C35H62N6OSi2Zn (704.46): C, 59.67; N, 11.93; H,
8.87. Found: C, 59.4; N, 11.8; H, 9.0.
1
powder (0.310 g, 65%). H NMR (C6D6, 500 MHz): d 0.60
(s, 18H, N(SiMe3)2), 2.33 (s, 3H, Me), 3.04 (s, 2H, CH2), 3.26
(d, J = 15, 2H, CH2-pyridyl), 3.38 (d, J = 15, 2H, CH2-
pyridyl), 6.33 (d, J = 7.5, 2H, pyridyl-3H), 6.66 (t, J = 6.5,
2H, pyridyl-5H ), 6.82 (s, 1H, phenol-H), 7.02 (s, 1H, phenol-
H), 6.91 (t, J = 7.5, 2H, pyridyl-4H), 9.13 (d, J = 4.0, 2H,
[L3]ZnEt (5). Complex 5 was prepared following the pro-
cedure described above for 3, starting from [L3]H (0.200 g,
0.48 mmol), toluene (10 mL), and ZnEt2 (0.390 g of a 15 wt%
solution in hexane, 0.48 mmol). Recrystallization of the final
solid from pentane at ꢀ30 1C gave 5 as colorless crystals
pyridyl-2H). 13C{1H} NMR (C6D6, 75 MHz):
d 6.09
(N(SiMe3)2), 17.65 (CH3), 60.04 (CH2), 115.55 (phenol-C),
121.40 (phenol-C), 122.39 (pyridyl-C4), 123.07 (pyridyl-C2),
127.14 (phenol-C), 130.15 (phenol-C), 138.06 (pyridyl-C3),
148.90 (pyridyl-C5), 153.77 (pyridyl-C1), 165.38 (phenol-C1).
Anal. Calc. for C26H37ClN4OSi2Zn (578.60): C, 53.97; H, 6.45;
N, 9.68. Found: C, 54.1; H, 6.5; N, 9.8.
1
(0.210 g, 84%). H NMR (C6D6, 300 MHz): d 0.76 (m, 2H,
ZnCH2CH3), 1.34 (s, 9H, tBu), 1.71 (m, 3H, ZnCH2CH3),
1.88 (s, 9H, tBu), 3.02–3.70 (m, 4H, CH2-pyridyl and
ꢁc
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2288 | New J. Chem., 2008, 32, 2279–2291

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CH2-phenol), 3.80 (s, 2H, CH2-phenyl), 5.90–7.20 (m, 8H,
arom), 7.40 (m, 1H, pyridyl-3H), 7.82 (d, J = 2.3, 1H, pyridyl-
(d, J = 12.6, 2H, CHH), 4.58 (br d, J = 13.5, 2H, CHH), 4.90
(br d, J = 13.5, 2H, CHH), 7.10–7.40 (m, 20H, H arom), 7.77
(d, J = 2.5, 2H, H phenolato). 8 (selected key resonances): d
0.61 (q, J = 8.1, 2H, ZnCH2CH3), 1.31 (t, J = 8.1, 3H,
ZnCH2CH3), 1.53 (s, 9H, tBu), 1.97 (s, 9H, tBu), 3.45 (d, J =
13, 2H, CHH), 3.65 (m, 4H, 2 CHH). 13C{1H} NMR (C6D6,
125 MHz): 7: d 29.95 (C(CH3)3), 32.03 (C(CH3)3), 34.04
(C(CH3)3), 35.53 (C(CH3)3), 55.63 (NCH2-phenolate)), 61.07
(NCH2Ph), 119.58, 124.06, 128.93, 130.18, 132.07, 135.09,
135.31, 138.59, 163.41 (C arom). 8 (selected key resonances)
d 0.55 (ZnCH2CH3), 12.23 (ZnCH2CH3), 29.88 (C(CH3)3),
32.01 (C(CH3)3), 33.89 (C(CH3)3), 35.63 (C(CH3)3), 57.73
(CH2), 60.09 (CH2).
1
6H). H NMR (CD2Cl2, 500 MHz): d 0.34 (m, J = 8.1, 2H,
ZnCH2CH3), 1.20 (s, 9H, tBu), 1.30 (t, J = 8.1, 3H,
ZnCH2CH3), 1.39 (s, 9H, tBu), 3.40 (d, J = 11.6, 1H,
CHH), 3.63 (d, J = 15.5, 1H, CHH), 3.90 (d, J = 11.6, 1H,
CHH), 4.19 (d, J = 2.5, 2H, CH2Ph), 4.22 (d, J = 15.5, 1H,
CHH), 6.67 (d, J = 2.6, 1H, H arom), 6.97 (d, J = 2.6, 1H, H
arom), 7.01 (d, J = 7.8, 1H, H arom), 7.21 (dt, J = 1.6 and
6.3, 1H, pyridyl-5H), 7.45–7.50 (m, 5H, H arom Ph), 7.64 (dt,
J = 1.6 and 6.3, 1H, pyridyl-4H), 8.35 (d, J = 4.6, 1H,
pyridyl-6H). 13C{1H} NMR (C6D6, 75 MHz): d ꢀ1.72
(ZnCH2CH3), 13.86 (ZnCH2CH3), 30.09 (C(CH3)3), 32.26
(C(CH3)3), 33.83 (C(CH3)3), 35.58 (C(CH3)3), 58.28 (CH2),
60.29 (CH2), 71.76 (CH2), 121.71, 122.67, 123.48, 125.63,
126.73, 128.11, 131.35, 138.20, 147.69, 150.48 (C arom).
13C{1H} NMR (CD2Cl2, 125 MHz): d ꢀ2.68 (ZnCH2CH3),
13.07 (ZnCH2CH3), 29.37 (C(CH3)3), 31.60 (C(CH3)3), 33.53
(C(CH3)3), 34.89 (C(CH3)3), 58.64 (CH2), 59.55 (CH2), 61.13
(CH2Ph), 121.75, 122.22, 123.11 (pyridyl-C5), 123.55, 125.55,
128.39, 128.48, 131.39, 133.20, 134.18, 136.90, 138.78 (pyridyl-
C4), 147.81, 156.05, 164.26 (pyridyl-C6). Anal. Calc. for
C30H40N2OZn (510.01): C, 70.65; N, 5.49; H, 7.90. Found:
C, 70.5; N, 5.6; H, 8.0.
1 : 1 Reaction of [L5]H with Zn(N(SiMe3)2). Generation of
[L5]2Zn (7) and {[L5]Zn(N(SiMe3)2)}n (9). This reaction was
carried out as described above, starting from [L4]H (0.200 g,
0.48 mmol), Zn(N(SiMe3)2)2 (0.185 g, 0.48 mmol) in toluene
(15 mL). Workup afforded a colorless powder (0.100 g), which
proved to be by 1H NMR a ca. 80 : 20 mixture of 7 and 9. 1H
NMR (200 MHz, C6D6) for {[L4]Zn(N(SiMe3)2)}n (9): d 0.31
(s, 18H, ZnN(SiMe3)2), 1.45 (s, 9H, tBu), 1.78 (s, 9H, tBu),
3.53 (s, 2H, NCH2-phenol), 3.62 (s, 2H, NCH2Ph), 3.81 (s, 2H,
NCH2Ph), 6.85 (m, 2H, H-Ph), 7.07 (m, 4H, H-Ph), 7.10
(m, 4H, H-Ph), 7.61 (2 overl. s, 2 ꢂ 1H, H-phenolate).
[L4]ZnN(SiMe3)2 (6). This compound was prepared in a
similar manner to [L2]ZnN(SiMe3)2 (2) starting from [L4]H
(0.310 g, 0.74 mmol) and Zn[N(SiMe3)2]2 (0.278 g, 0.72 mmol)
in toluene (15 mL). Compound 5 was obtained as a colorless
[L1]Mg{OC(QCH2)(2,4,6-Me3C6H2)} (10). To a 50 mL
Schlenk flask containing [L1]H (0.200 g, 0.48 mmol) in toluene
(10 mL), Mg(n,sBu)2 (0.48 mL of a 1.0 M solution in heptane,
0.48 mmol) was added slowly in at ꢀ78 1C. The solution was
slowly warmed to room temperature for 30 min; then, under
stirring, 2,4,6-Me3C6H2COMe (0.077 g, 0.48 mmol) was added
in via syringe under argon. A white precipitate formed after
10 min. The solution was concentrated under vacuum to ca.
5 mL, and the white residue was recrystallized from toluene
(ca. 30 mL) at ꢀ30 1C to give 10 as white crystals (0.190 g,
1
powder (0.350 g, 76%). H NMR (500 MHz, C6D6): d 0.55
(s, 18 H, N(SiMe3)2), 1.18 (s, 9H, tBu), 1.59 (s, 9H, tBu), 2.97
(d, 1H, J = 11.4, NCHHPh), 3.07 (d, 1H, J = 15.5, NCHH-
phenol), 3.83 (d, 1H, J = 15.5, NCHH-phenol), 4.08 (d, 1H,
J = 11.4, NCHHPh), 4.13 (d, 1H, J = 13.7, NCHH-pyridyl),
4.26 (d, 1H, J = 13.7, NCHH-pyridyl), 5.95 (d, 1H, J = 7.8,
pyridyl-CH), 6.22 (t, 1H, J = 6.3, pyridyl-CH), 6.33 (d, 1H,
J = 2.5, pyridyl-CH), 6.61 (dt, 1H, 2J = 7.7, 3J = 1.6, pyridyl-
CH), 7.00 (d, 1H, J = 2.6, phenol-CH), 7.89 (d, 1H, J = 4.7,
1
63%). H NMR (C6D6, 500 MHz): d 1.63 (s, 9H, tBu), 2.06
(s, 9H, tBu), 2.45 (s, 3H, p-Me), 2.84 (d, J = 15.5, 2H, CH2-
phenolate), 3.11 (s, 6H, 2 ꢂ o-Me), 3.45 (m, 4H, CH2-pyridyl),
4.47 (s, 1H, OCQCHH), 5.10 (s, 1H, OCQCHH), 6.23
(d, J = 7.7, 2H, pyridyl-3H), 6.56 (t, J = 12.5, 2H, pyridyl-
5H), 6.83 (t, J = 15.3, 2H, pyridyl-4H), 7.12 (s, 1H, phenolate-
5H), 7.17 (s, 2H, phenyl-3,5-H), 7.74 (s, 1H, phenolate-3H),
9.30 (d, 2H, J = 5.0, 2H, pyridyl-6H). 13C{1H} NMR (C6D6,
125 MHz): d 20.80 (o-Me), 21.12 (p-Me), 30.07 (C(CH3)3),
32.34 (C(CH3)3), 33.83 (C(CH3)3), 35.74 (C(CH3)3), 57.52
(CH2-phenolate), 60.79 (CH2-pyridyl), 81.80 (CQCH2),
122.00 (pyridyl-C3), 123.20 (pyridyl-C4), 124.32 (phenolate-
C3), 124.95 (phenolate-C5), 128.00 (phenyl-3,5-C), 133.21
(phenolate-C4), 134.03 (phenyl-C4), 135.21 (phenyl-2,6-C),
138.52 (pyridyl-C5), 138.89 (phenolate-C6), 144.89 (phenyl-
C1), 151.13 (pyridyl-C6), 156.83 (pyridyl-C2), 164.57 (pheno-
late-C1), 166.86 (COMg). Anal. Calc. for C38H47MgN3O2
(602.10): C, 75.80; N, 6.98; H, 7.87. Found: C, 75.7; N, 7.1;
H, 8.1.
pyridyl-CH). 13C{1H} NMR (125 MHz, C6D6):
d 6.02
(N(SiMe3)2), 29.65 (C(CH3)3), 31.83 (C(CH3)3), 33.39
(C(CH3)3), 35.04 (C(CH3)3), 58.69 (NCH2-phenolate), 61.33
(NCH2Ph), 63.48 (NCH2-pyridyl), 121.11 (pyridyl-CH),
122.60 (pyridyl-CH), 123.49 (phenolate-CH), 124.75 (pyridyl-
CH), 127.88, 131.34, 132.65, 134.22, 137.90 (pyridyl-CH),
147.47
(pyridyl-CH),
154.54
(phenolate-C),
164.31
(pyridyl-C6). Anal. Calc. for C34H53N3OSi2Zn (641.37): C,
63.67; N, 6.55; H, 8.33. Found: C, 63.4; N, 6.5; H, 8.4.
1 : 1 Reaction of [L5]H with ZnEt2. Generation of [L5]2Zn (7)
and {[L4]ZnEt}n (8). To a 50 mL Schlenk flask containing
[L4]H (0.199 g, 0.48 mmol) in toluene (10 mL), ZnEt2 (0.390 g
of a 15 wt% solution in hexane, 0.48 mmol) was slowly added
in at ꢀ78 1C. The solution was warmed to room temperature
and stirred overnight. After evaporation of volatiles under
vacuum, the residue was recrystallized from pentane at ꢀ30 1C
1
to give white crystals (0.190 g), which proved by H NMR to
[L4]Mg{OC(QCH2)(2,4,6-Me3C6H2)} (11). Complex 11
was prepared following the procedure described above for
10, starting from [L4]H (0.199 g, 0.48 mmol), (n,sBu)2Mg
(0.48 mL of a 1.0 M solution in heptane, 0.48 mmol), and
1
be a mixture of [L4]2Zn (7, 23%) and [L4]ZnEt (8, 77%). H
NMR (C6D6, 500 MHz) 7: d 1.61 (s, 18H, tBu), 1.87 (s, 18H,
tBu), 3.22 (br d, 2H, CHH), 3.83 (d, J = 14.5, 4H, CHH), 4.19
ꢁc
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2,4,6-Me3C6H2COMe (0.077g, 0.48 mmol), to give 11 as a
white powder (0.062 g, 21%). 1H NMR (CD2Cl2, 500 MHz),
selected resonances: d 1.1–1.8 (several singlets, tBu), 2.1–2.5
(several singlets, o-Me and p-Me), 3.10 (d, J = 12.7, 1H,
OCQCHH), 3.35 (d, J = 12.7, 1H, OCQCHH), 3.45 (d, J =
13.5, 1H, OCQCHH), 3.50 (d, J = 13.6, 1H, CHH), 3.90 (d,
J = 15.5, 1H, CHH), 4.30 (br d, J = ca. 12, 1H, CHH), 4.43 (br
d, J = ca. 11, 1H, CHH), 4.70 (d, J = 13.6, 1H, CHH), 5.00
(d, J = 12.7, 1H, OCQCHH), 5.15 (d, J = 15.5, 1H, CHH),
6.2-8.0 (m, H arom), 8.20 (d, 1H, J = 4.9), 8.35 (br d, 1H),
9.38 (d, 2H, J = 4.4, 1H, pyridyl-6H). 13C{1H} NMR
(CD2Cl2, 125 MHz), selected resonances: d 18.80 (p-Me),
20.78 (o-Me), 28.17 (C(CH3)3), 29.40 (C(CH3)3), 31.71
(C(CH3)3), 33.50 (C(CH3)3), 34.40 (C(CH3)3), 35.74
(C(CH3)3), 50.05, 55.72, 58.24, 62.64 (CH2-phenolate and
CH2-pyridyl), 81.31 (CQCH2), 93.34 (CQCH2), 122.31,
124.97, 127.83, 128.37, 131.94, 135.43, 137.17, 138.50, 140.00,
148.45, 159.08, 164.36 (C arom). Anal. Calc. for
C39H50MgN2O2 (603.13): C, 77.66; N, 4.64; H, 8.36. Found:
C, 77.2; N, 4.9; H, 8.2.
General procedure for lactide polymerization
In the glovebox, a Schlenk flask was charged with a solution of
the initiator in toluene (0.2 mL) or THF (0.2 mL). To this
solution was added rapidly a slurry of the monomer (rac-LA)
in the appropriate ratio in toluene (3.0 mL). The mixture was
immediately stirred with a magnetic stir bar at room tempera-
ture. Aliquots were periodically removed with a pipette for
1
monitoring by H NMR. After the desired time, the reaction
was quenched with acidic methanol (0.5 mL of a 1.2 M HCl
solution), and the polymer was precipitated with excess metha-
nol (100 mL). The polymer was then dried under vacuum to
constant weight.
Acknowledgements
We thank Arkema Co. for a postdoctoral fellowship grant to
ZZ, the Centre National de la Recherche Scientifique, and
CAPES-COFECUB (grant 556/07) for financial support of
this work. J. F. C. gratefully thanks the Institut Universitaire
de France for a Junior IUF fellowship (2005–2009).
Solid-state structure determination of complexes 3–7 and 10
References
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2290 | New J. Chem., 2008, 32, 2279–2291

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