Chemistry of BDI*M(2+) complexes (M = Mg, Zn) and their role in lactide polymerization where BDI* is the anion derived from methylenebis(C-tBu, N-2,6-diisopropylphenyl)imine BDI*H

Article abstract of DOI:10.1039/b910669c

The bulky diimine CH₂{C(^tBu)N-2,6-Prⁱ ₂C₆H₃}₂, BDI*H, is shown to exist in two isomers trans-trans, 1a and cis-trans, 1b that interconvert slowly on the NMR time-scale. Treatment of the diimine with ⁿBuLi in hexane proceeds slowly to give the lithium β-diketoiminate LiBDI*, 2, which upon hydrolysis yields the eneimine tautomer of BDI*H which has been characterized by X-ray studies in three rotamers, 3a, 3b and 3c. Dialkylmetal compounds (M = Mg, R = Bu; M = Zn, R = Et) react with either 1 or 3 in hydrocarbon and ether solvents to give BDI*Mg(ⁿBu)THF, 4, and BDI*ZnEt, 5, which have been structurally characterized. M(N(SiMe ₃)₂)₂ compounds where M = Mg, Zn or Ca failed to react with either 1 or 3 in hydrocarbon solvents. The reactions between ZnCl₂ and 2 yield BDI*ZnCl, 6, and a further reaction with LiNMe₂ yields BDI*ZnNMe₂, 7. The compounds 5 and 6 are shown to contain trigonal planar, M(2+) ions. Compounds 4, and 7 initiate ring-opening polymerization, ROP, of rac-lactide to give atactic polylactide, PLA. The rate of ROP depends on the metal M = Mg > Zn and is slower than that observed for the related β-diketoiminate complexes where the bulky- ^tBu group is replaced by Me.

Full text of DOI:10.1039/b910669c

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PAPER
www.rsc.org/dalton | Dalton Transactions
Chemistry of BDI*M(2+) complexes (M = Mg, Zn) and their role in lactide
polymerization where BDI* is the anion derived from methylenebis(C-tBu,
N-2,6-diisopropylphenyl)imine BDI*H†
Carla N. Ayala, Malcolm H. Chisholm,* Judith C. Gallucci and Clemens Krempner
Received 3rd June 2009, Accepted 4th August 2009
First published as an Advance Article on the web 2nd September 2009
DOI: 10.1039/b910669c
The bulky diimine CH2{C(tBu)N-2,6-Pri2C6H3}2, BDI*H, is shown to exist in two isomers trans-trans,
1a and cis-trans, 1b that interconvert slowly on the NMR time-scale. Treatment of the diimine with
nBuLi in hexane proceeds slowly to give the lithium b-diketoiminate LiBDI*, 2, which upon hydrolysis
yields the eneimine tautomer of BDI*H which has been characterized by X-ray studies in three
rotamers, 3a, 3b and 3c. Dialkylmetal compounds (M = Mg, R = Bu; M = Zn, R = Et) react with
either 1 or 3 in hydrocarbon and ether solvents to give BDI*Mg(nBu)THF, 4, and BDI*ZnEt, 5, which
have been structurally characterized. M(N(SiMe3)2)2 compounds where M = Mg, Zn or Ca failed to
react with either 1 or 3 in hydrocarbon solvents. The reactions between ZnCl2 and 2 yield BDI*ZnCl, 6,
and a further reaction with LiNMe2 yields BDI*ZnNMe2, 7. The compounds 5 and 6 are shown to
contain trigonal planar, M(2+) ions. Compounds 4, and 7 initiate ring-opening polymerization, ROP,
of rac-lactide to give atactic polylactide, PLA. The rate of ROP depends on the metal M = Mg > Zn
and is slower than that observed for the related b-diketoiminate complexes where the bulky-tBu group is
replaced by Me.
BDIZnOiPr.14 A related magnesium initiator was found to be more
rapid in ROP of rac-LA but showed less stereo-selectivity unless
Introduction
the solvent was THF.15,16 Since THF is a good donor ligand to
hard metals and evidence of its coordinating ability was seen in
the molecular structure of BDIMg(OtBu)THF, we reasoned that
an increase in steric pressure at the metal center was enhancing the
influence of end-group control in discriminating diastereoselective
transition state energies, the DDG‡s. Given that the Coates,
BDIZnOR system already showed ~90% diastereoselectivity in
yielding heterotactic PLA from rac-LA,14 we reasoned that the
more bulky BDI* where the back-bone methyl groups are replaced
by tert-butyl groups could well lead to a significant further
enhancement in selectivity. The influence of the bulky BDI* in
controlling the coordination geometry in relation to the related
BDI ligand has been well documented by Holland17,18 in the
chemistry of iron(2+) and by Mindiola19-21 in the stabilization
of low coordinate carbine and imido group 4 metal complexes.
Bailey22,23 has also shown the influence of this bulky ligand
on the coordination chemistry of allyl and methyl magnesium
compounds. As we show here this simple idea did not prove to
be correct and only atactic PLA was obtained from reactions
employing rac-LA with BDI*M(2+) initiators (M = Mg, Zn). The
chemistry of these complexes and, indeed, the free ligand BDI*H
has, however, proved interesting in being very different from its
less sterically demanding sister ligand.
The ring opening polymerization of lactides (L, rac and meso)
by single-site metal alkoxide catalysts has attracted considerable
attention in recent years as biodegradable and biocompatible
polymers from renewable resources are gaining a market place
alongside the now traditional polymers derived from petroleum.1,2
Just as the physical properties of a poly(a-olefin) are determined
by molecular weight, polydispersity and stereosequence so too
are the properties of polylactides, PLAs, and currently isotactic,
syndiotactic, heterotactic, stereoblock and stereoplex polymers are
known.3 Polymerization of meso-LA having two different methine
stereocenters can yield either syndiotactic PLA with sss tetrads
or heterotactic PLA with sis/isi tetrads. Polymerization of rac-
LA can give heterotactic or stereoblock polymers. In both of the
above examples the ROP must be stereoselective otherwise atactic
PLA will be formed, but just how stereoselectivity is achieved
is not well understood. The interplay between enantiomorphic
site control where the metal has attendant chiral ligands and
end-group control due to the chiral methine centers near to the
metal, MOC*HMeC(O)OC*HMe of the growing polymer chain,
is clearly complex and can be influenced by the nature of the
solvent.4 This is well evident from studies employing chiral and
achiral salen and salan aluminum complexes.5-13 Some of the
most impressive stereoselective polymerizations have employed
the achiral initiators bearing the b-diketoiminate ligand BDI =
CH{C(Me)N-2,6-Pri2C6H3}2 as in the work of Coates employing
Results and discussion
Department of Chemistry, The Ohio State University, Columbus, OH 43210,
USA
Synthesis and characterization of BDI*H
† Electronic supplementary information (ESI) available: Ortep plots of
3a–3c. CCDC reference numbers 735238–735243. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b910669c
In contrast to BDI-H which can be simply made from the reac-
tion between 2,5-pentanedione and 2,6-diisopropylphenylamine,
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Scheme 1 Synthesis of the diimine BDI*H 1.
Scheme 2 Trans-trans 1a versus cis-trans 1b form of diimine BDI*H 1.
BDI*-H has to be synthesized in a stepwise manner as shown
in Scheme 1. This leads to the bisimine form of the free
ligand, though 1H NMR data reported in the literature indi-
cates that this is sometimes contaminated with the eneimine
tautomer.
The reaction between the bis-imine BDI*H and nBuLi in
hexane is very sluggish and requires heating to yield the lithium
b-diketoiminate, BDI*Li, 2, as a pale yellow microcrystalline
material. Careful hydrolysis of 2 in hexanes yields the eneimine
form of the BDI*H ligand exclusively. This form of the ligand
is also obtained by hydrolysis of the BDI* metal complexes of
zinc and magnesium, and due to adventitious water, crystals
of BDI*H have been obtained and examined by single crystal
X-ray crystallography leading to the structural characterization
of the eneimine as three polymorphs 3a, 3b and 3c. The latter, 3c,
also contains 1.5 independent molecules in the asymmetric unit,
one of which has a crystallographic two-fold rotation axis. These
molecular structures differ only slightly and a summary of the
structural parameters for the core of the molecule depicted in
Fig. 2 are given in Table 1. It can be seen that the C(1)–N(1)
distance for the non symmetric molecules is notably longer
1
The H NMR spectra for the bis-imine BDI*H are shown in
Fig. 1 at various temperatures. The spectrum at room temperature
(298 K) is particularly ambiguous but clearly indicative of a
dynamic process which even upon heating to 338 K is not rapid
on the NMR time-scale. Upon cooling to 223 K the situation
becomes clearer as the major species present in solution has one
i
resonance and two Pr methyl doublets due to diastereotopism.
This and other signals given in the Experimental can reasonably
be assigned to the trans-trans-bisimine isomer 1a. The cis-trans-
t
isomer 1b has two different Bu groups one of which, the cis
one, lies in the face of the phenyl ring and is thus shifted upfield
by the aromatic magnetic anisotropy. The interconversion of the
isomers 1a and 1b are shown in Scheme 2 and has a calculated
activation energy on the order of 14( 1) kcal mol-1, though a
detailed line shape analysis has not been attempted. The molecular
structures of 1a and 1b were previously reported by Bailey et al.24
who found two unique molecules in the asymmetric unit, namely
1a and 1b.
˚
˚
1.35 A (ave) than the C(3)–N(2) distance 1.31(1) A. Similarly the
˚
C(1)–C(2) distances 1.38 A (ave) are notably shorter than the C(2)–
˚
C(3) distances 1.43 A (ave.). These parameters are consistent with
the eneimine description of the molecule having an asymmetric
N–H ◊ ◊ ◊ N bond as depicted in Fig. 2. In the molecule that is
found for 3c that has a crystallographic C2 axis of symmetry this
distinction is lost and the structural parameters are intermediate
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Fig. 1 VT 1H-NMR of the diimine BDI*H 1 in toluene-D8.
as would be found for a molecule with a symmetric N ◊ ◊ ◊ H ◊ ◊ ◊ N
bond. Full crystallographic details for the three structural deter-
minations of the eneimine 3a, 3b and 3c are given in the ESI.†
The 1H NMR spectrum of 3 is remarkably more simple
than that of 1. The spectrum of 3 is consistent with a
molecule having time averaged C2v symmetry and a symmetrical
N ◊ ◊ ◊ H ◊ ◊ ◊ N bond. Aside from the aromatic proton signals there is
an isopropyl methine septet and two isopropyl methyl doublets for
the diastereotopic methyl groups. The tBu groups and the methine
proton of the NCCHCN backbone give rise to singlets in the
integral ratio of 18:1.
The bisimine 1, and the eneimine 3 do not interconvert
in hydrocarbon solutions though in the presence of acid 3
and 1 form an equilibrium involving the protonated form of
Fig. 2 Ene-imine tautomer with atom numbering scheme.
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◦
˚
Table 1 Selected bond lengths [A] and angles [ ] of 3a–c
3a 3b
3c (non-symm.)
3c (C2-symm.)
C1–N1
C3–N2
C1–C2
C2–C3
N1–C1–C2
N2–C3–C2
C1–C2–C3
C12–N1–C1
C24–N2–C3
N1–C1–C2–C3
N2–C3–C2–C1
C2–C1–N1–C12
C2–C3–N2–C24
C8–C1–N1–C12
C4–C3–N2–C24
C13–C12–N1–C1
C25–C24–N2–C3
1.358(2)
1.306(2)
1.376(2)
1.439(2)
119.3(1)
118.5(1)
129.1(1)
130.5(1)
129.1(1)
2.0(2)
1.333(2)
1.319(2)
1.397(2)
1.413(2)
118.9(2)
118.9(2)
129.4(2)
130.8(2)
128.2(1)
1.3(3)
1.354(2)
1.305(2)
1.382(3)
1.433(3)
118.6(2)
117.7(2)
128.3(2)
130.8(2)
132.5(2)
3.2(3)
C36–N3
—
C36–C37
—
N3–C36–C37
—
C36–C37–C36
C42–N3–C36
1.323(2)
—
1.404(2)
—
118.4(2)
—
128.2(2)
131.3(2)
—
5.6(1)
—
154.8(2)
—
32.1(3)
—
123.3(2)
—
—
N3–C36–C37–C36
—
C42–N3–C36–C37
—
C42–N3–C36–C38
—
C43–C42–N3–C36
—
1.8(2)
0.5(3)
16.7(3)
159.4(2)
171.6(1)
26.4(2)
9.9(3)
65.6(2)
101.7(2)
168.0(2)
172.1(2)
16.5(3)
11.5(3)
77.3(2)
104.2(2)
152.4(2)
162.6(2)
26.7(4)/45.0(5)a
21.4(3)
66.9(3)
76.9(3)
a C8 is disordered.
Scheme 3 Diimine–enamine-interconversion of BDI*H.
+
+
1: BDH*-H2 (1) ꢀ BDI*-H2 (3). These interconversions are sum-
marized in Scheme 3 following the formation of 1 as outlined in
Scheme 1.
in the presence of THF which presumably facilitates proton
transfer and also activates reagents such as LiBu.
BDI*MgBun(THF), 4
Reactions involving BDI*H and organometal complexes
n
From the reaction between a hexane solution of MgBu2 and
Reactions involving alkyllithium, alkylzinc and alkylmagnesium
reagents and BDI*H (either as 1 or 3) are extremely sluggish
in hydrocarbon solvents and under similar conditions BDI*H
and M(N(SiMe3)2)2, where M = Mg, Ca or Zn, fail to react.
From this we conclude that the kinetics of proton transfer from 1
(C–H) and 3 (N–H) is severely limited by steric factors. Certainly
the reactivity is remarkably different from the methyl analogue
BDI-H. Reactions involving BDI*-H proceed more conveniently
BDI*H in THF under reflux the compound BDI*MgBun(THF),
4 was obtained as yellow crystals. The molecular structure of
this complex is shown in Fig. 3 and selected bond distances and
angles are given in Table 2. All reactions involving 4 and alcohols
i
(ButOH, PrOH) failed to yield the desired magnesium alkoxide
BDI*MgOR(THF) and BunH. In addition to the formation
of BunH, the BDI*H ligand was liberated in the eneimine
form, 3.
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Table 2 Selected bond lengths [A] and angles [ ] of 4, 5 and 6
BDI*MgBu(THF) (4)
BDI*ZnEt (5)
Molecule A
Molecule B
BDI*ZnCl (6)
Mg–N1
Mg–N2
C1–N1
C3–N2
C1–C2
C2–C3
N1–Mg–N2
N1–C1–C2
N2–C3–C2
C1–C2–C3
C12–N1–C1
C24–N2–C3
Mg–C40
2.067(2)
2.087(2)
1.348(2)
1.322(2)
1.401(3)
1.429(3)
93.2(1)
121.3(2)
121.1(2)
133.2(2)
122.3(2)
126.2(2)
2.122(2)
Zn1–N1
Zn1–N2
C1–N1
C3–N2
C1–C2
C2–C3
N1–Zn1–N2
N1–C1–C2
N2–C3–C2
C1–C2–C3
C12–N1–C1
C24–N2–C3
Zn1–C36
1.961(2)
1.968(2)
1.339(3)
1.335(3)
1.401(3)
1.413(3)
97.9(1)
121.2(2)
121.2(2)
133.7(2)
126.2(2)
125.0(2)
1.964(3)
1.962(2)
1.957(2)
1.335(3)
1.335(3)
1.407(3)
1.408(3)
97.8(1)
120.9(2)
121.2(2)
133.6(2)
128.0(2)
127.0(2)
1.958(3)
Zn–N1
—
C1–N1
—
1.910(1)
—
1.334(2)
—
1.411(2)
—
103.8(1)
121.3(1)
—
C1–C2
—
N1–Zn–N1a
N1–C1–C2
—
C1–C2–C1a
C12–N1–C1
—
134.8(2)
128.5(1)
—
Zn–Cl
2.130(1)
a Symmetry transformation used to generate this atom: -x, y, -z + 1/2.
Fig. 3 Structure of 4 in the solid state. The thermal ellipsoids correspond to 30% probability. Hydrogen atoms are omitted for clarity. Only one site is
shown for the disordered atoms in the THF and n-butyl ligands for clarity.
BDI*ZnEt, 5
and angles are given in Table 2. The trigonal planar coordination
of Zn(2+) is not surprising in view of the related structures of
BDIZnX compounds, where X = OBut and N(SiMe3)2.16 However,
the steric pressure exerted by the But ligands is evidenced by the
decreased Zn–N–C(Aryl) angles. Compound 5 fails to react with
alcohols and does not react with lactide even at 100 ◦C.
The reaction between ZnEt2 and BDI*H, 1, in refluxing hexane
(24 h) yielded BDI*ZnEt, 5 as off white crystals upon crystal-
lization. The molecular structure of 5 determined from a single
crystal X-ray study is shown in Fig. 4 and selected bond distances
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Fig. 4 Structure of 5 in the solid state. The ellipsoids correspond to 30% probability. Hydrogen atoms are omitted for clarity. Only one of the molecules
in the asymmetric unit is drawn.
BDI*ZnX, where X = Cl (6) and NMe2 (7)
BDI*MX compounds, the decrease in reaction rate and loss of
stereoselectivity indicates that this is not important in the ROP
process. Quite possibly for this bulky chelate one of the arms may
dissociate during the process of ring-opening of a lactide molecule.
We noted before that for Tp’ZnOR initiators, where Tp’ represents
a 3 substituted trispyrazolylhydroborate, the rate of ROP increased
when the bulkiness of the Tp’ reached a certain point, which led
us to suspect a k3 ꢀ k2 equilibrium for the Tp’ ligand.27 This work
further underscores the notion that little can be inferred about
reactivity from an examination of ground state structures.
The reaction between ZnCl2 and LiBDI* in THF proceeds to give
BDI*ZnCl, 6, as a white crystalline compound which in turn reacts
with LiNMe2 to yield BDI*ZnNMe2, 7. The molecular structure
of the trigonal planar Zn ion in 6 is shown in Fig. 5 and selected
structural parameters are summarized in Table 2.
One measure of the steric pressure exerted by the But groups
in the BDI* complexes compared to the Me groups on the back-
bone of the BDI ligand is seen in the Zn–N–C (aryl) angles of
3-coordinate complexes. For BDIZnX compounds where X =
OBut or NPri2, the Zn–N–C angles span a range from 117 to
119◦ whereas for the BDI* compounds 5 and 6 the angles span
the range from 108 to 112◦.
Experimental
The manipulation of air-sensitive compounds involved standard
Schlenk line and dry box techniques. THF, benzene, toluene and
hexanes were distilled under nitrogen from alkali metals and stored
Reactions with rac-LA
˚
over activated molecular sieves (4 A). Acetonitrile and CH2Cl2
In order to investigate the stereoselectivity of the ROP of rac-LA,
NMR tube reactions were carried out wherein 100 equivalents of
rac-LA was allowed to react with each of compounds 4 and 7
in benzene. PLA was formed in both cases though the reaction
involving 7 was notably slower than related reactions involving
BDIZn initiators. The 1H decoupled methine 1H signal of the PLA
was examined in order to interrogate the stereosequences in the
polymer.3,25,26 In both cases atactic PLA was detected indicating
little if any stereoselectivity in the ROP reaction involving these
BDI* containing initiators.
were distilled under nitrogen from CaH2 and stored over activated
˚
molecular sieves (4 A). rac-Lactide was purified by sublimation
under vacuum three times. Benzene-D6, was dried over activated
˚
molecular sieves (4 A) and stored in a glove box. BDI*H (1)
and BDI*Li(THF) (2) were synthesized according to literature
procedures.28 LiNMe2, ZnEt2 (1M in hexanes) and MgBun (1M
2
1
in hexanes) were purchased from Aldrich. H-, 13C- and NMR
spectra were obtained from either Bruker DPX-400 or DPX-250
spectrometers using benzene-D6 as solvent and elemental analyses
were performed by the Atlantic Microlab, Inc.
Concluding remarks
BDI*H (3)
The chemistry associated with the two b-diketoiminate ligands,
Water (10 mL) was added to a solution of 2 (0.2 g, 0.35 mmol)
in hexanes (10 mL). The organic phase was dried with MgSO4,
filtered and all volatiles were evaporated to quantitatively leave
BDI and BDI* are notably different. While there is no doubt
t
that the presence of the bulky Bu ligands push the aryl groups
1
to decrease the size of the pocket for the M–X group in
3 as a yellow crystalline solid. H NMR (C6D6, 400 MHz): 11.7
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Fig. 5 Structure of 6 in the solid state. The ellipsoids correspond to 30% probability. Hydrogen atoms are omitted for clarity. The molecule contains a
crystallographic two-fold rotation axis. Symmetry related atoms are designated by the superscript i, i.e., C1 and C1i are related by the symmetry operation
-x, y, -z + 1/2.
1
(br, NH, 1 H), 7.05 (s, CH-arom., 6 H), 5.51 (s, CH, 1 H), 3.37
(sept, J = 6.8 Hz, CH(CH3)2, 4 H), 1.29, 1.11 (2d, J = 6.8 Hz,
CH(CH3)2, 2 ¥ 12 H), 1.16 (s, C(CH3)3, 18 H) ppm. 13C NMR
(C6D6, 100.6 MHz): 169.2 (C N), 142.5, 141.2 (Ar-quart), 124.6,
122.9 (CH-arom.), 94.4 (CH), 41.1 [C(CH3)3], 31.1 [C(CH3)3], 28.4
[CH(CH3)2], 25.0, 22.3 [CH(CH3)2] ppm.
crystals. H NMR (C6D6, 400 MHz): 7.02–7.11 (m, CH-arom.,
6 H), 5.38 (s, CH, 1 H), 3.64 (m, THF, 4 H), 3.32 (sept, CH(CH3)2,
4 H), 1.32, 1.31 (2d, CH(CH3)2, 2 ¥ 12 H), 1.19 (s, C(CH3)3, 18 H),
1.37 (m, THF, 4 H), 1.11 (m, MgCH2(CH2)2CH3, 4 H), 0.87 (t,
=
MgCH2(CH2)2CH3, 3 H), -0.54 (t, MgCH2(CH2)2CH3, 2 H). 13
C
=
NMR (C6D6, 100.6 MHz): 176.1 (C N), 146.1, 141.7 (Ar-quart),
124.6, 123.5 (CH-arom.), 95.5 (CH), 69.1 (THF), 43.8 [C(CH3)3],
32.9 [C(CH3)3], 31.7, 31.5 [MgCH2(CH2)2CH3], 27.9 [CH(CH3)2],
26.0, 23.8 [CH(CH3)2], 25.0 (THF), 14.1 [MgCH2(CH2)2CH3],
6.4 [MgCH2(CH2)3CH3] ppm. Anal. Calc. for C43H70ON2Mg
(655.334): C, 78.81; H, 10.77. Found: C, 78.56; H, 10.69.
BDI*H (1):
1H NMR (C6D6, 400 MHz, 338 K): 7.11–7.13, 7.00–7.02 (2 m, CH-
arom., 6 H), 3.39 (s, CH2, 2 H), 2.75 (br, CH(CH3)2, 4 H), 1.26 (d,
J = 7.2 Hz, CH(CH3)2, 12 H), 1.14 (br, CH(CH3)2, 12 H), 1.13
(br, C(CH3)3, 18 H) ppm. 13C NMR (C6D6, 100.6 MHz, 343 K):
=
172.4 (C N), 145.0, 135.8 (Ar-quart), 123.5, 122.3 (CH-arom.),
BDI*ZnEt (5)
42.0 [C(CH3)3], 33.6 (CH2), 28.9 [C(CH3)3], 28.5 [CH(CH3)2], 23.8,
21.4 [CH(CH3)2] ppm; 1H NMR (toluene-D8, 400 MHz, 232 K):
6.99–7.16 (m, CH-arom., 6 H), 3.38 (s, CH2, 2 H), 3.36 (s, CH2,
2 H), 3.13, 2.30 (2 sept, J = 6.4 Hz, CH(CH3)2, 2 ¥ 2 H), 2.85, 2.71
(2 sept, J = 6.4 Hz, CH(CH3)2, 2 ¥ 2 H), 1.52, 0.52 (2 s, C(CH3)3,
2 ¥ 9 H), 1.34 (d, J = 6.4 Hz, CH(CH3)2, 12 H), 1.21, 0.93 (2d,
J = 6.4 Hz, CH(CH3)2, 2 ¥ 6 H), 1.13 (br, C(CH3)3, 18 H) ppm.
A mixture of 20 mL (20 mmol) ZnEt2 (1.0 M, hexanes) and 1
(2 g, 3.98 mmol) was refluxed for 24 h. After evaporation of the
solvent, the resulting residue was dried under vacuum (0.01 mbar,
80 ◦C) for ca. 2 h to leave a white solid. Recrystallization from
1
hexanes afforded 1.6 g (68%) of the title compound. H NMR
(C6D6, 400 MHz): 7.01–7.10 (m, CH-arom., 6 H), 5.50 (s, CH,
1 H), 3.30 (sept, J = 6.8 Hz, CH(CH3)2, 4 H), 1.30, 1.29 (2d,
J = 6.8 Hz, CH(CH3)2, 2 ¥ 12 H), 1.21 (s, C(CH3)3, 18 H), 0.67
BDI*MgBun(THF) (4)
(t, J = 8.0 Hz, CH2CH3, 3 H), -0.01 (q, J = 8.0 Hz, CH2CH3,
13
=
A mixture of THF (1 mL), 1 (1 g, 1.99 mmol) and 8 mL (8 mmol) of
Mg(Bun)2 (1 M, hexanes) was refluxed for 5 h. After evaporation
of the solvent the residue was dried under vacuum (0.01 mbar,
80 ◦C) for ca. 2 h to leave a yellow solid. Crystallization from
hexanes afforded 0.99 g (76%) of the title compound as yellow
2 H). C NMR (C6D6, 100.6 MHz): 174.3 (C N), 146.7, 140.5
(Ar-quart), 124.9, 123.1 (CH-arom.), 94.6 (CH), 43.5 [C(CH3)3],
32.7 [C(CH3)3], 28.2 [CH(CH3)2], 24.4, 22.9 [CH(CH3)2], 10.9
(ZnCH2CH3), 0.3 (ZnCH2CH3) ppm. Anal. Calc. for C37H58N2Zn
(596.280): C, 74.53; H, 9.80. Found: C, 74.27; H, 9.63.
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Table 3 Summary of crystallographic details for 3a, 3b, 3c, 4, 5 and 6
3a
3b
3c
4
5
6
∑
Formula
C35H54N2
502.80
C35H54N2 0.1(H2O)
504.69
C35H54N2
502.80
C43H70MgN2O
655.32
C37H58N2Zn
596.22
C35H53ClN2Zn
602.61
Formula Wt.
Crystal system
monoclinic
16.6935(1)
12.0280(1)
17.8619(2)
117.121(1)
3192.13(5)
150(2)
monoclinic
15.5478(2)
9.7599(1)
22.1039(3)
106.180(1)
3221.30(7)
150(2)
monoclinic
14.9388(1)
13.0230(1)
50.4676(5)
90.194(1)
9818.3(1)
230(2)
monoclinic
12.1942(1)
26.3031(2)
13.3982(1)
107.503(1)
4098.44(6)
150(2)
monoclinic
16.7410(2)
17.8475(2)
24.4862(3)
108.584(1)
6934.6(1)
150(2)
monoclinic
16.8648(1)
9.2141(1)
22.8397(2)
106.180(1)
3408.57(5)
150(2)
˚
a, A
˚
b, A
˚
c, A
b, ◦
U, A
3
˚
Temp., K
Space group
Z
P21/c
P21/a
C2/c
P21/n
P21/a
C2/c
4
4
12
4
8
4
Crystal size, mm
m(MoKa), mm-1
2q range, ◦
0.19 ¥ 0.31 ¥ 0.31 0.19 ¥ 0.38 ¥ 0.42
0.060
4.24 to 50.02
52317
0.19 ¥ 0.27 ¥ 0.27
0.058
4.14 to 50.08
39200
0.19 ¥ 0.35 ¥ 0.38
0.076
4.26 to 50.06
54911
0.12 ¥ 0.27 ¥ 0.35
0.734
4.18 to 50.08
81289
0.12 ¥ 0.31 ¥ 0.35
0.823
5.02 to 54.96
32613
0.059
4.6 to 50.02
41657
Reflections measured
Unique ref.’s (Rint
R1 (I >2s(I))
wR2 (all data)
)
5637 (0.048)
0.044
0.120
0.51, -0.29
1.041
5671 (0.036)
0.049
0.130
0.33, -0.28
1.043
8612 (0.051)
0.054
0.148
0.20, -0.18
1.035
7242 (0.045)
0.052
0.150
0.23, -0.23
1.044
12236 (0.049)
0.044
0.118
0.44, -0.54
1.036
3906 (0.037)
0.033
0.082
0.55, -0.26
1.058
3
˚
Max., min. Dr, e/A
Goodness of fit on F2
BDI*ZnCl (6)
collected by filtration and dried. The stereochemistry of the PLA
was determined by examination of the CH 1H resonance with
irradiation of the methyl proton signal in CDCl3 solution.25,26
A solution of 2 (2.18 g, 3.76 mmol) and ZnCl2 (0.565 g, 4.15 mmol)
in THF (20 mL) was refluxed for 24 h. After removal of all volatiles
under vacuum (0.01 mbar, 80 ◦C), the remaining residue was
extracted several times with hot benzene. Recrystallization from
hexanes gave 1.2 g (55%) ofthe title compoundas colorless crystals.
1H NMR (C6D6, 400 MHz): 7.01–7.10 (m, CH-arom., 6 H), 5.58 (s,
CH, 1 H), 3.23 (sept, J = 6.8 Hz, CH(CH3)2, 4 H), 1.39, 1.25 (2d,
Crystallographic studies
X-ray diffraction data were measured on a Nonius KappaCCD
diffractometer. Crystallographic details are summarized in
Table 3. All work was done at low temperature using an Oxford
Cryosystems Cryostream Cooler. The structures were solved by
the direct methods procedure in SHELXS-97.29 Full-matrix least-
squares refinements based on F2 were performed in SHELXL-
97,29 as incorporated in the WinGX package.30 The methyl group
hydrogen atoms are included in the model at calculated positions
using a riding model with U(H) = 1.5*Ueq (bonded carbon atom).
For some of these groups, the torsion angle, which defines the
orientation of the methyl group about the C–C, bond was refined.
The other hydrogen atoms are included in the model at calculated
positions using a riding model with U(H) = 1.2*Ueq (bonded
carbon atom).
The structures of three polymorphs of BDI*H have been
determined: 3a, 3b, and 3c. The structure of 3a is an imine-enamine
tautomer. The hydrogen atoms bonded to N(1) and to C(2) were
refined isotropically. One of the isopropyl groups was modeled as
being disordered over two orientations: C31A, C32A and C31B,
C32B with atom C(30) common to both orientations.
The structure of 3b contains several disorder problems. One of
the isopropyl groups is disordered over two orientations. These
two orientations are modeled with atoms C(31A) and C(32A)
for one and atoms C(31B) and C(32B) for the other, with atom
C(30) common to both orientations. The disorder can be described
as two orientations of the isopropyl group related by rotation
about the C29–C30 bond. DFIX and SADI restraints were used
in modeling this disorder.
A difference electron density map indicated that both nitrogen
atoms in 3b are bonded to hydrogen atoms. Since both hydrogen
atoms cannot be simultaneously present, they were included in the
model with occupancy factors of 0.4 and 0.6 (based on their peak
J = 6.8 Hz, CH(CH3)2, 2 ¥ 12 H), 1.15 (s, C(CH3)3, 18 H) ppm.
13
=
C NMR (C6D6, 100.6 MHz): 177.9 (C N), 144.3, 141.4 (Ar-
quart), 126.4, 123.6 (CH-arom.), 95.1 (CH), 44.0 [C(CH3)3], 32.8
[C(CH3)3], 28.6 [CH(CH3)2], 25.1, 23.0 [CH(CH3)2] ppm. Anal.
Calc. for C35H53ClN2Zn (602.672): C, 69.75; H, 8.86. Found: C,
69.54; H, 8.79.
BDI*ZnNMe2 (7)
A suspension of 6 (0.7 g, 1.16 mmol), LiNMe2 (0.07 g, 1.37 mmol)
and benzene was stirred at room temperature for one day and at
70 ◦C for ca. 2 h. After separating the suspension by centrifugation
and filtration, the solvent of the filtrate was evaporated and the
remaining residue was dried under vacuum (0.01 mbar, 70 ◦C) to
1
leave 0.6 g (85%) of 7 as a yellowish powder. H NMR (C6D6,
400 MHz): 7.00–7.06 (m, CH-arom., 6 H), 5.43 (s, CH, 1 H),
3.33 (sept, J = 6.8 Hz, CH(CH3)2, 4 H), 1.97 (s, N(CH3)2,
6 H), 1.41, 1.28 (2d, J = 6.8 Hz, CH(CH3)2, 2 ¥ 12 H), 1.17
13
=
(s, C(CH3)3, 18 H) ppm. C NMR (C6D6, MHz): 175.7 (C N),
146.6, 141.7 (Ar-quart), 125.8, 123.7 (CH-arom.), 94.0 (CH), 45.9
[N(CH3)2], 43.9 [C(CH3)3], 33.0 [C(CH3)3], 28.6 [CH(CH3)2], 24.4,
23.0 [CH(CH3)2] ppm. Anal. Calc. for C37H59N3Zn (611.294): C,
72.70; H, 9.73. Found: C, 72.48; H, 9.65.
Lactide polymerization
The polymerization of rac-LA was carried out in benzene with
typically 100 equiv. of LA to 1 equiv. of 4 or 7 at room temperature.
The PLA was precipitated by the addition of MeOH and was
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heights in the difference map), and they were refined isotropically.
So this structure is considered to be a 60:40 mixture of two imine-
enamine tautomers. The bond lengths in the N–C–C–C–N portion
of the molecule support this idea.
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There is also a region of electron density consisting of a single
peak located within the cavity of this molecule. This was modeled
as a partially occupied oxygen atom with an occupancy factor of
0.105(4), which would presumably be part of a water molecule.
This water molecule would be present only when the C(31B),
C(32B) orientation of the disordered isopropyl group is also
present, as it is too close to the other orientation of this group.
Structure 3c contains 1.5 molecules in the asymmetric unit; one
molecule is in a general position and the other molecule contains a
crystallographic two-fold rotation axis. The two tBu groups on the
molecule in the general position are disordered and were modeled
t
with two orientations each. For the Bu group involving atom
C(4), the two orientations are composed of the set of atoms C(5),
C(6), C(7) and C(5A), C(6A), C(7A). These two orientations are
related to each other by rotation about the C(3)–C(4) bond. For
t
the other disordered Bu group, the whole group is disordered
such that C(8), C(9), C(10), and C(11) represent one orientation
and C(8A), C(9A), C(10A), and C(11A) represent a second
orientation. Various restraints (SADI, SIMU, DELU) were used
in refining these disordered groups along with EADP constraints.
A difference electron density map indicated that both nitrogen
atoms, N1 and N2, in the molecule on a general position are
bonded to hydrogen atoms. Since both hydrogen atoms cannot
be simultaneously present, they were included in the model with
occupancy factors of 0.7 and 0.3, and were refined isotropically.
For the molecule on the two-fold rotation axis, the hydrogen atom
bonded to N3 was assigned an occupancy factor of 0.5, since it is
disordered because of the two-fold rotation axis. So both of these
molecules are a mixture of imine-enamine tautomers: the molecule
on the general position is a 70:30 mixture, while the one containing
the two-fold axis is a 50:50 mixture. The hydrogen atoms bonded
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two positions for atoms C(41), C(42), and C(43). SADI restraints
were used in the refinement of these disordered regions.
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Acknowledgements
We thank the Department of Energy, Office of Basic Energy
Sciences, Chemistry Division for support.
30 WinGX, Version 1.64.05: L. J. Farrugia, J. Appl. Crystallogr., 1999, 32,
837–838.
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 9237–9245 | 9245
©