Aluminum and zinc complexes based on an amino-bis(pyrazolyl) ligand: Synthesis, structures, and use in MMA and lactide polymerization

Article abstract of DOI:10.1021/ic061749z

The coordination chemistry of bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine (1, LH) with aluminum- and zinc-alkyls has been studied. Reaction of 1 with AIR₃ affords the adducts [LH]·AIR₃ (R = Me, 2; Et, 3), which undergo alkane elimina

Full text of DOI:10.1021/ic061749z

Inorg. Chem. 2007, 46, 328−340
Aluminum and Zinc Complexes Based on an Amino-Bis(pyrazolyl)
Ligand: Synthesis, Structures, and Use in MMA and Lactide
Polymerization
Bing Lian,† Christophe M. Thomas,† Osvaldo L. Casagrande Jr.,§ Christian W. Lehmann,|
Thierry Roisnel,‡ and Jean-Francois Carpentier*,†
¸
Catalyse et Organome´talliques, UMR 6226, UniVersity of Rennes 1, 35042 Rennes Cedex, France,
Centre de Diffractome´trie X, Sciences Chimiques de Rennes, UniVersite´ de Rennes 1, 35042
Rennes, France, Laborato´rio de Cata´lise Molecular, Instituto de Quimica-UniVersidade Federal
do Rio Grande do Sul, AV. Bento Gonc¸alVes, 9500, Porto Alegre, RS, 90501-970, Brazil, and
Max-Planck-Institut fu¨r Kohlenforschung, Chemical Crystallography, Postfach 101353, 45466
Mu¨lheim an der Ruhr, Germany
Received September 14, 2006
The coordination chemistry of bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine (1, LH) with aluminum- and zinc-alkyls
has been studied. Reaction of 1 with AlR3 affords the adducts [LH] AlR3 (R Me, 2; Et, 3), which undergo alkane
elimination upon heating to yield the amido complexes [L]AlR2 (R Me, 4; Et, 5). Reaction of LiO(iPrO)C CMe2
with 2 proceeds via N H deprotonation to give Li[L]AlMe3 (6), while the former enolate adds to 4 to generate
[Me2C C(OiPr)OLi] [L]AlMe2 (7). Similarly, the 1:1 reaction of ZnEt2 with 1 gives [LH] ZnEt2 (9), which is transformed
into [L]ZnEt (10) upon heating. When an excess of ZnEt2 was used in the latter reaction, the bimetallic complex
[L]ZnEt ZnEt2 (11) was isolated beside 10. Performing the same reaction in the presence of O2 traces yielded
selectively the dinuclear ethyl-ethoxide complex [L]Zn2Et2( -OEt) (12), which was alternatively prepared from the
reaction of 10 and ZnEt(OEt). Zinc chloride complexes [LH] ZnRCl (R Et, 13; p-CH3C6H4CH2, 14) and [L]ZnCl
‚
)
)
d
−
d
‚
‚
‚
µ
‚
)
(15) were prepared in high yields following similar strategies. Ethyl abstraction from 10 with B(C6F5)3 yields
[L]Zn+EtB(C6F5)3- (16). All complexes have been characterized by multinuclear nuclear magnetic resonance (NMR),
elemental analysis, and single-crystal X-ray diffraction studies for four-coordinate Al complexes 2, 4, and 6 and Zn
complexes 9
monomer conversions are improved in the presence of neutral complexes 2 or 4, respectively; however, these
methyl methacrylate (MMA) polymerizations are uncontrolled. Polymerization of rac-lactide takes place at 20 C in
−12 and 14. Aluminate species 6 and 7 initiate the polymerization of methyl methacrylate, and the
°
the presence of zinc ethoxide complex 12 to yield atactic polymers with controlled molecular masses and relatively
narrow polydispersities.
Introduction
have the potential to function as very active catalysts because
of their less coordinatively saturated nature, as compared to
more usual tetradentate (e.g., SALEN) ligated complexes.
Aluminum-based initiators are also of great utility for the
The chemistry of alkyl-aluminum derivatives bearing
nitrogen-based bidentate or tridentate ligands has recently
attracted much attention because of the rich structural
chemistry of this class of complexes1 and their applicability
as unique catalysts in fine chemical synthesis2 and even more
in olefin polymerization.3 Such neutral or cationic species
(1) (a) Trepanier, S. J.; Wang, S. Angew. Chem., Int. Ed. Engl. 1994, 33,
1265-1266. (b) Trepanier, S. J.; Wang, S. Organometallics 1994, 13,
2213-2217. (c) Trepanier, S. J.; Wang, S. J. Chem. Soc., Dalton Trans.
1995, 2425-2429. (d) Trepanier, S. J.; Wang, S. Can. J. Chem. 1996,
74, 2032-2040. (e) Lewinski, J.; Zachara, J.; Mank, B.; Pasynkiewicz,
S. J. Organomet. Chem. 1993, 454, 5-7.
(2) (a) Ooi, T.; Maruoka, K. Lewis Acids in Organic Synthesis; Yamamoto,
H., Ed.; Wiley-VCH: Weinheim, Germany, 2000; Vol. 1, pp 191-
281. (b) Wulff, W. D. Lewis Acids in Organic Synthesis; Yamamoto,
H., Ed.; Wiley-VCH: Weinheim, Germany, 2000; Vol. 1, pp 283-
354.
* To whom correspondence should be addressed. Fax: (+33)(0)223-
236-939. E-mail: jean-francois.carpentier@univ-rennes1.fr.
† Universite´ de Rennes 1.
‡ Centre de Diffractome´trie X, Rennes.
§ Universidade Federal do Rio Grande do Sul.
| Max-Planck-Institut fu¨r Kohlenforschung.
328 Inorganic Chemistry, Vol. 46, No. 1, 2007
10.1021/ic061749z CCC: $37.00
© 2007 American Chemical Society
Published on Web 11/23/2006

Complexes Based on an Amino-Bis(pyrazolyl) Ligand
Scheme 1
controlled polymerization of polar monomers such as (meth)-
acrylates, propylene oxide, and lactones, and alkyl-aluminum
complexes bearing tridentate nitrogen ligands designed for
this purpose have been recently disclosed.4 A few zinc
complexes of tridentate nitrogen donor ligands have also been
prepared, mostly to study their coordination chemistry,5 for
example, as models of metalloenzymes.6 A large number of
those tridentate N,N,N-ligated Al and Zn complexes include
neutral pyridine-2,6-diimine or monoanionic pyrroyl-2,5-
diimine ligands. This type of framework is nowadays
ubiquitous to all coordination/catalysis chemists since the
discovery of highly active iron and cobalt ethylene polym-
erization catalysts derived from such bulky tridentate pyri-
dine-diimine ligands.7
Recently, we have reported the synthesis of amino-bis-
(pyrazolyl) ligands as new readily available tridentate N,N,N-
ligands for the nickel-catalyzed selective dimerization of
ethylene to 1-butene.8 We found also that this class of ligands
is very valuable for the chromium-catalyzed oligomerization
of ethylene.9 As an extension of our continuous interest in
this field, we have investigated the reactions of trialkylalu-
minum and dialkylzinc with bis[2-(3,5-dimethyl-1-pyrazolyl-
)ethyl]amine (1, LH), which can act as a neutral or
monoanionic ligand. We selected this tridentate ligand
because of its potential ability to stabilize both neutral and
cationic metal centers, having in mind that tridentate amine
or amidoamine initiators have shown remarkable perfor-
mance in polymerization of polar monomers, though only a
few of them have been investigated thus far.4 We report in
this paper the syntheses, structures, and reactivity of a series
of mononuclear and dinuclear complexes as well as applica-
tions of some of these derivatives in the addition and ring-
opening polymerization of methyl methacrylate and lactide.
Results and Discussion
Synthesis and Structure of Neutral Aluminum Com-
plexes Supported by Tridentate Ligand Bis[2-(3,5-dim-
ethyl-1-pyrazolyl)ethyl]amine (1). The reaction of ligand
1 (LH) with 1 equiv of AlMe3 or AlEt3 at room temperature
in benzene or toluene affords the corresponding adduct
complexes [LH]‚AlR3 (R ) Me, 2; Et, 3), which were
isolated as blocky colorless crystals in high yield (>80%)
(Scheme 1). Both complexes 2 and 3 were characterized by
elemental analysis and nuclear magnetic resonance (NMR)
1
spectroscopy. The room-temperature H NMR spectra in
(3) (a) Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 1997, 119, 8125-
8126. (b) Coles, M. P.; Swemson, D. C.; Jordan, R. F.; Young, V. G.,
Jr. Organometallics 1997, 16, 5183-5194. (c) Ihara, E.; Young, V.
G., Jr.; Jordan, R. F. J. Am. Chem. Soc. 1998, 120, 8277-8278. (d)
Radzewich, C. E.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 1998,
120, 9384-9385. (e) Aeilts, S. L.; Coles, M. P.; Swenson, D. C.;
Jordan, R. F.; Young, V. G., Jr. Organometallics 1998, 17, 3265-
3270. (f) Coles, M. P.; Swenson, D. C.; Jordan, R. F.; Young, V. G.,
Jr. Organometallics 1998, 17, 4042-4048. (g) Bruce, M.; Gibson, V.
C.; Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J. Chem.
Commun. 1998, 2523-2524. (h) Radzewich, C. E.; Guzei, I. A.;
Jordan, R. F. J. Am. Chem. Soc. 1999, 121, 8673-8674. (i) Korolev,
A. V.; Guzei, I. A.; Jordan, R. F. J. Am. Chem. Soc. 1999, 121, 11605-
11606. (j) Dagorne, S.; Guzei, I. A.; Coles, M. P.; Jordan, R. F. J.
Am. Chem. Soc. 2000, 122, 274-289. (k) Korolev, A. V.; Ihara, E.;
Guzei, I. A.; Young, V. G., Jr.; Jordan, R. F. J. Am. Chem. Soc. 2001,
123, 8291-8309. (l) Matsuo, Y.; Tsurugi, H.; Yamagata, T.; Tani,
K.; Mashima, K. Bull. Chem. Soc. Jpn. 2003, 76, 1965-1968.
(4) (a) Emig, N.; Nguyen, H.; Krautscheid, H.; Re´au, R.; Cazaux, J.-B.;
Bertrand, G. Organometallics 1998, 17, 3599-3608. (b) Jegier, J. A.;
Atwood, D. A. Inorg. Chem. 1997, 36, 2034-2039. (c) Saunders,
Baugh, L.; Sissano, J. A. J. Polym. Sci., Part A: Polym. Chem. 2002,
40, 1633-1651.
various solvents (benzene-d6, toluene-d8, CD2Cl2, THF-d8)
and the 13C NMR spectra in C6D6 of 2 and 3 show all a
single set of resonances for a symmetrically coordinated
ligand and three magnetically equivalent Al-R (R ) Me,
Et) groups. Coordination of 1 via the central NH atom in 2
was confirmed by single-crystal X-ray diffraction, the result
of which is shown in Figure 1, together with a selection of
bond lengths and angles. The solid-state structure of 2 shows
the Al center in a four-coordinated geometry with the two
pyrazolyl groups pointing in opposite directions from the
Al center.
Methane and ethane elimination from complexes 2 and 3,
respectively, takes place slowly at room temperature in
aromatic hydrocarbons. Heating 2 and 3 at 80 °C for 24 h
gave the corresponding amido-dialkyl complexes [L]AlR2
(R ) Me, 4; Et, 5) in quantitative NMR yield (Scheme 1).
Complex 4 was isolated as colorless crystals and was fully
characterized by elemental analysis, NMR spectroscopy, and
X-ray diffraction analysis; the parent diethyl complex 5 was
generated on an NMR scale. The solid-state structure of 4
(Figure 2, Table 1) shows that only one pyrazolyl moiety is
coordinated to the Al center, which illustrates again the
strongly favored formation of four-coordinated Al species.10
(5) Luo, B.; Kucera, B. E.; Gladfelter, W. L. Polyhedron 2006, 25, 279-
285 and references cited therein.
(6) See for example: Gosiewska, S.; Cornelissen, J. J. L. M.; Lutz, M.;
Spek, A. L.; van Koten, G.; Klein Gebbink, R. J. Inorg. Chem. 2006,
45, 4214-4227. and references cited therein.
(7) (a) Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Maddox, P.
J.; Mc Tavish, S. J.; Solan, G. A.; White, A. J. P.; Williams, D. J.;
Chem. Commun. 1998, 849-850. (b) Small, B. L.; Brookhart, M.;
Bennett, A. M. A. J. Am. Chem. Soc. 1998, 120, 4049-4050.
(8) Ajellal, N.; Kuhn, M. C. A.; Boff, A. D. G.; Hoerner, M.; Thomas,
C. M.; Carpentier, J.-F.; Casagrande, O. L., Jr. Organometallics 2006,
25, 1213-1216.
(10) (a) Healy, M. D.; Ziller, J. W.; Barron, A. R. J. Am. Chem. Soc. 1990,
112, 2949-2954. (b) Trepanier, S. J.; Wang, S. Organometallics 1996,
15, 760-765.
(9) Junges, F.; Kuhn, M. C. A; Ajellal, N.; Thomas, C. M.; Carpentier,
J.-F.; Casagrande, O. L., Jr. Manuscript in preparation.
Inorganic Chemistry, Vol. 46, No. 1, 2007 329

Lian et al.
Figure 1. Solid-state structure of 2 (ellipsoids drawn at the 50%
probability; hydrogen atoms omitted for clarity). Selected bond lengths (Å)
and angles (deg): Al(1)sC(1), 1.9865(12); Al(1)sC(2), 1.9797(13); Al-
(1)sC(3), 1.9800(15); Al(1)sN(1), 2.0370(11); C(1)sAl(1)sC(2), 114.82-
(6); C(1)sAl(1)sC(3), 113.47(6); C(2)sAl(1)sC(3), 115.53(5); N(1)s
Al(1)sC(1), 105.02(5); N(1)sAl(1)sC(2), 102.45(5); N(1)sAl(1)sC(3),
103.52(5).
Figure 4. Solid-state structure of 6 (ellipsoids drawn at the 50%
probability; hydrogen atoms omitted for clarity). Selected bond lengths (Å)
and angles (deg): Al(1)sC(1), 2.001(2); Al(1)sC(2), 1.987(2); Al(1)s
C(3), 2.013(2); Al(1)sN(21), 1.9151(19); Li(1)sC(3), 2.395(4); Li(1)s
N(21), 2.050(4); Li(1)sN(22), 1.957(4); Li(1)sN(11), 1.985(4); C(1)s
Al(1)sC(2), 114.61(10); C(1)sAl(1)sC(3), 109.94(10); C(3)sAl(1)s
N(21), 104.98(8); C(2)sAl(1)sC(3), 106.80(9); Al(1)sC(3)sLi(1), 76.29(10);
Al(1)sN(21)sLi(1), 87.31(12); C(3)sLi(1)sN(21), 88.69(14).
N(pyrazolyl) bond length formed in 4 (AlsN(1), 2.0025-
(10) Å). No difference is observed in the AlsC bond lengths
in both complexes (AlsC, 1.9732(13) and 1.9698(13) in 4
vs 1.9797(13) and 1.9800(15) in 2). The variable-temperature
1H NMR spectrum of 4 in toluene-d8 solution is also in
agreement with a four-coordinated aluminum species (Figure
3): at low temperature (0 °C), four [broadened] singlets are
observed for the four inequivalent Me pyrazolyl groups, and
the two AlsMe groups appear as two singlets. At room
temperature, both the free and coordinated pyrazolyl groups
exchange fast on the NMR time scale, and the Me pyrazolyl
and AlMe2 groups appear as two sharp singlets and one
singlet, respectively.
Figure 2. Solid-state structure of 4 (ellipsoids drawn at the 50%
probability; hydrogen atoms omitted for clarity). Selected bond lengths (Å)
and angles (deg): Al(1)sC(16), 1.9732(13); Al(1)sC(17), 1.9698(13); Al-
(1)sN(1), 2.0025(10); Al(1)sN(8), 1.8267(10); C(16)sAl(1)sC(17),
117.32(6); C(16)sAl(1)sN(1), 102.06(5); C(16)sAl(1)sN(8), 115.06(5);
C(17)sAl(1)sN(1), 111.44(5); C(17)sEnDashAl(1)sN(8), 112.89(5);
N(1)sAl(1)sN(8), 95.02(4).
The preparation of Al-enolate complexes, potentially active
species for methyl methacrylate (MMA) polymerization,11
was attempted following different routes. In the reaction of
AlMe3-adduct 2 with 1 equiv of Li-ester enolate LiO-
(iPrO)CdCMe2, deprotonation of the NsH moiety was
found to proceed faster than enolate addition, to give
selectively the bimetallic aluminum-lithium amido complex
Li[L]AlMe3 (6), with concomitant release of 1 equiv of
isopropyl isobutyrate (identified by 1H and 13C NMR)
(Scheme 2). This pathway was not unexpected considering
the enhanced acidity of NsH because of its coordination
onto Al (as compared to that of free-ligand 1). An X-ray
diffraction analysis of a single crystal of 6 confirmed its
molecular monomeric structure and showed that both Al and
Li are in a tetracoordinate environment (Figure 4, Table 1).
The Li atom is actually coordinated by the three N atoms of
the ligand, thus generating two six-membered metallacycles,
via an agostic interaction with one of the AlsCH3 groups.
The short contact between lithium and the methyl carbon
Figure 3. Selected region of the variable-temperature 1H NMR spectra
(300 MHz, Tol-d8) of 4. Descriptors a and s refer, respectively, to the CH3
pyrazolyl groups of 4 and the residual solvent resonances (Tol-d8).
The AlsN(amido) bond length involving the sp2-hybridized
nitrogen-bridge atom in complex 4 (sum of angles around
N(8) ) 357.81°; AlsN(8), 1.8267(10) Å) is logically much
shorter than the corresponding AlsN(amino) bond length
in adduct complex 2 (sum of angles around N(1) ) 336.47°;
AlsN(1), 2.037(11) Å). The latter is similar to the new Als
(11) Rodriguez-Delgado, A.; Chen, E. Y.-X. J. Am. Chem. Soc. 2005, 127,
961-974.
330 Inorganic Chemistry, Vol. 46, No. 1, 2007

Complexes Based on an Amino-Bis(pyrazolyl) Ligand
Table 1. Crystal Data and Structure Refinement for Aluminum Complexes 2, 4, and 6
2
4
6
empirical formula
fw/g‚mol-1
temperature/K
wavelength/Å
crystal system
space group
a [Å]
C17H32AlN5
333.46
100(2)
0.71073
triclinic
C
16 H28AlN5
C17H31AlLiN5
339.39
373(2)
0.71069
triclinic
P1h
7.822(5)
11.327(5)
12.250(5)
83.801(5)
78.221(5)
72.077(5)
1009.7(9)
2
317.41
100
0.71073
monoclinic
P21/c (no. 14)
13.0200(3)
9.5441(2)
14.5474(3)
90
94.8040(10)
90
1801.37(7)
4
P1h
8.8262(4)
b [Å]
c [Å]
10.9199(5)
11.7634(5)
112.305(2)
93.821(2)
106.374(2)
986.95(8)
R/deg
â/deg
γ/deg
volume/Å3
Z
2
density (cacld) Mg‚m-3
absorp coeff/mm-1
F(000)
1.122
0.110
364
0.6 × 0.4 × 0.3
2.66-32.00
-13 e h e 13
-16 e k e 16
-17 e l e17
22 398
1.170
0.117
688
1.116
0.108
368
crystal size/mm3
θ range for data collection/deg
index ranges
0.44 × 0.07 × 0.07
0.7 × 0.4 × 0.2
2.94-31.02
2.48-27.94
-18 e h e 18
-13 e k e13
-9 e h e 10
-14 e k e14
-16 e l e16
16 887
-21 e l e 21
48 457
reflns collected
indep reflns
6760 [Rint ) 0.0302]
semiempirical from equivalents
full-matrix least squares on F2
6760/0/212
1.035
R1 ) 0.0444
wR2 ) 0.1339
R1 ) 0.0500
wR2 ) 0.1409
0.542 and -0.482
5746 [Rint ) 0.0543]
semiempirical from equivalents
full-matrix least squares on F2
5746/0/205
4677 [Rint ) 0.0546]
absorp correction
refinement method
data /restraints/params
goodness-of-fit on F2
final R indices[I > 2σ(I)]
none
full-matrix least squares on F2
4677/0/217
1.060
R1 ) 0.0526
wR2 ) 0.1548
R1 ) 0.0726
wR2) 0.1654
0.435 and -0.404
1.039
R1 ) 0.0461
wR2 ) 0.1073
R indices (all data)
R1 ) 0.0587
wR2 ) 0.1140
largest diff. peak and hole/e‚Å-3
0.378 and -0.321
Scheme 2
Scheme 3
atom (LisC(3), 2.395(4) Å) falls in the upper range for such
Li...Me...Al interactions (2.185-2.36 Å)12 and generates
another four-membered bismetallacycle which is nearly
planar (torsion angle Al(1)sC(3)sLi(1)sN(21) ) 14.4°).
The AlsN bond length involving the tetragonal nitrogen
bridge atom in 6 (AlsN(21), 1.9151(19) Å) is intermediary
to those found in 2 and 4 (vide supra).
-0.31, δ 13C -8.51 in 4). These observations suggest that
aluminate species did not form and only coordination
between the Li and N atoms of the ligand takes place in 7.
The formation of aluminate-enolate species is likely ham-
pered by the favorable four-coordination of the Al center in
4.
The addition of 1 equiv of Li-ester enolate to dimethyl-
amido complex 4 was monitored by 1H NMR and was found
to generate a unique species, [Me2CdC(OiPr)OLi]‚[L]AlMe2
(7, Scheme 3). In the 1H and 13C NMR spectra of 7 in C6D6,
only some of the resonances for the N-CH2CH2 group (δ 1H
3.63 and 3.55, δ 13C 54.43 and 49.69 ppm) are significantly
shifted as compared to those of 4 (δ 1H 3.62 and 3.20, δ 13C
51.03 and 49.74 ppm); in particular, the resonances for AlMe2
An Al-enolate species was obtained by the addition of 1
equiv of 2,4,6-trimethylacetophenone to either complex 2
or 4 (Scheme 4). Methane elimination was not observed in
the reaction of 4 with the bulky enolizable ketone, but
reprotonation of the N(amido) atom takes place to form [LH]-
AlMe2(OC(2,4,6-Me3Ph)dCH2) (8) quantitatively within 2
h at room temperature. The same product was obtained from
the methane elimination reaction between the trimethylalu-
minum adduct 2 and 2,4,6-trimethylacetophenone; the kinet-
ics of this reaction are much slower in comparison with the
route from 4 (35% conversion after 15 h at room tempera-
ture), and a higher temperature is required to go to comple-
tion.
1
1
are not affected (δ H -0.34, δ 13C -8.90 in 7 vs δ H
(12) (a) Armstrong, D. R.; Clegg, W.; Davies, R. P.; Liddle, S. T.; Linton,
D. J.; Raithby, P. R.; Snaith, R.; Wheatley, A. E. H. Angew. Chem.,
Int. Ed. 1999, 38, 3367-3370. (b) Boss, S. R.; Cole, J. M.; Haigh,
R.; Snaith, R.; Wheatley, A. E. H.; McIntyre, G. J.; Raithby, P. R.
Organometallics 2004, 23, 4527-4530. (c) Uhl, W.; Layh, M.; Massa,
W. Chem. Ber. 1991, 124, 1511-1516. (d) Fryzuk, M. D.; Giesbrecht,
G. R.; Rettig, S. J. Organometallics 1997, 16, 725-736. (e) Armstrong,
D. R.; Davies, R. P.; Linton, D. J.; Snaith, R.; Schooler, P.; Wheatley,
A. E. H. Dalton Trans. 2001, 2838-2843.
Synthesis of Neutral and Cationic Zinc Complexes
Supported by Tridentate Ligand Bis[2-(3,5-dimethyl-1-
pyrazolyl)ethyl]amine (1). The reaction of ligand 1 with 1
Inorganic Chemistry, Vol. 46, No. 1, 2007 331

Lian et al.
Scheme 4
Scheme 7
Scheme 8
Scheme 5
Scheme 9
Scheme 6
(Scheme 8). Heating 13 at 60 °C (to undergo alkane
elimination) yielded a mixture of products, as evidenced by
complicated NMR spectra, which could not be fully char-
acterized. Similar treatment of benzylic complex 14 gave
the chloride complex [L]ZnCl (15) in 86% NMR purity, but
some problems were encountered in the final purification.
equiv of ZnEt2 in toluene at room temperature gives the
adduct complex [LH]‚ZnEt2 (9) in high yield (90%) (Scheme
5). Heating complex 9 at 60 °C yielded quantitatively the
pure anticipated complex [L]ZnEt (10), together with release
of 1 equiv of ethane (Scheme 5). Initially, we carried out
this sequential reaction directly at high temperature, using
an excess (3 equiv) of ZnEt2 versus 1, to drive it to
completion. Under these conditions, the amido-ethyl-zinc
complex 10 was formed in ca. 80% selectivity beside ca.
20% of bimetallic complex [L]ZnEt‚ZnEt2 (11), which arises
from coordination of an extra ZnEt2 unit to the amide-N atom
of 10 (Scheme 6). Obviously, exact stoichiometry must be
respected in this reaction to ensure the selective formation
of desired complex 10. Also, when the reaction between
ligand 1 and 3 equiv of ZnEt2 was carried out at 120 °C in
the presence of traces of O2, the mixed dinuclear ethyl-
ethoxide complex [L]Zn2Et2(µ-OEt) (12) was recovered in
high yield as the only product (Scheme 7). We assume that
this reaction proceeds primarily by the formation of 11,
followed by addition of dioxygen in one of the ZnsC bonds
and final structural rearrangement.13 Alternatively, we found
that complex 12 can also be readily synthesized by the
reaction of 10 with 1 equiv of ZnEt(OEt) (Scheme 7).
Finally, the cationic complex [L]Zn+ EtB(C6F5)3- (16) was
cleanly generated in CD2Cl2 by abstraction of the ethyl group
of 10 with 1 equiv of B(C6F5)3 (Scheme 9). This product,
1
which was fully characterized in solution by H, 13C, 11B,
and 19F NMR spectroscopy, is stable at least 2 days at room
temperature. However, attempts to grow single crystals were
unsuccessful.
Structures of Zinc Complexes. Complexes 9-12 and 14
have been characterized by single-crystal X-ray diffraction
studies. Crystallographic details are reported in Table 2, and
the molecular structures as well as selected bond lengths and
angles are given in Figures 5-9. With the exception of
complex 10 which is dimeric, all the other zinc complexes
feature a monomeric structure in the solid state.
The zinc atom in the diethylzinc-adduct 9 presents a
distorted tetrahedral geometry with an acute N(pyrazolyl)s
ZnsN(bridge) angle (N(1)sZn(1)sN(3), 86.23(12)°) and
(13) Similar dioxygen addition (adventitious) reactions have been re-
ported: (a) Bailey, P. J.; Dick, C. M. E.; Fabre, S.; Parsons S. Dalton
Trans. 2000, 1655-1661. (b) Dove, A. P.; Gibson, V. C.; Hormnirun,
P.; Marshall, E. L.; Segal, J. A.; White, A. J. P.; Williams, D. J. Dalton
Trans. 2003, 3088-3097. (c) Conway, B.; Hevia, E.; Kennedy, A.
R.; Mulvey, R. E.; Weatherstone, S. Dalton Trans. 2005, 1532-1544.
(d) Lewinski, J.; Zachara, J.; Gos, P.; Grabska, E.; Kopec, T.; Madura,
I.; Marciniak, W.; Prowotorow, I. Chem. Eur. J. 2000, 6, 3215-3227.
(e) Lewinski, J; Sliwinski, W.; Dranka, M.; Justyniak, I.; Lipkowski,
J. Angew. Chem., Int. Ed. 2006, 45, 4826-4829. and references cited
therein.
The reaction of ligand 1 with 1 equiv of Zn(R)Cl at room
temperature cleanly afforded the corresponding zinc adduct
[LH]‚ZnRCl (R ) Et, 13; p-CH3C6H4CH2, 14) in high yields
332 Inorganic Chemistry, Vol. 46, No. 1, 2007

Complexes Based on an Amino-Bis(pyrazolyl) Ligand
Inorganic Chemistry, Vol. 46, No. 1, 2007 333

Lian et al.
Figure 5. Solid-state structure of 9 (ellipsoids drawn at the 50%
probability; hydrogen atoms omitted for clarity). Selected bond lengths (Å)
and angles (deg): Zn(1)sC(1), 2.006(4); Zn(1)sC(3), 2.017(4); Zn(1)s
N(1), 2.214(3); Zn(1)sN(3), 2.263(3); C(1)sZn(1)sC(3), 134.78(16);
C(1)sZn(1)sN(1), 106.35(15); C(1)sZn(1)sN(3), 111.90(14); N(1)sZn-
(1)sN(3), 86.23(12).
Figure 6. Solid-state structure of 10 (ellipsoids drawn at the 50%
probability; hydrogen atoms omitted for clarity). Selected bond lengths (Å)
and angles (deg): Zn(2)sC(1), 2.0124(12); Zn(2)sN(5), 2.0825(10); Zn-
(2)sN(5*), 2.0877(10); Zn(2)sN(22*), 2.1197(10); C(1)sZn(2)-N(5),
122.48(5); C(1)sZn(2)sN(22*), 109.65(5); C(1)sZn(2)sN(5*), 124.63-
(5); N(22*)sZn(2)sN(5*), 95.69(4); N(5)sZn(2)sN(5*), 90.27(4); Zn-
(2)sN(5)sZn(2*), 89.73(4).
an opened CsZnsC angle (C(1)sZn(1)sC(3), 134.78(16)°)
(Figure 5). These values fall in the ranges of those observed
for related adducts of ZnEt2 with a four-membered chelating
diamine which feature acute NsZnsN angles but narrower
EtsZnsEt bond angles14 and those of adducts with two
monoamine ligands which have more opened NsZnsN
angles.15 The ZnsC and ZnsN bond lengths are comparable
to those observed in the aforementioned adducts.14,15
The solid-state structure of dimeric ethyl-amido complex
10 features also two four-coordinated Zn centers, with
µ-bridging via the N atom of the ligand bridge (Figure 6).
The overall structure of 10, centrosymmetric around the
square Zn2N2 plane, and its geometric features (bond lengths
and angles) are reminiscent to other dimeric alkyl-(amino-
µ-amido) Zn complexes of general formula [R1ZnNR2(CH2)n-
NHR2]2 (R1 ) H, Me, Et; R2 ) Me, Bn; n ) 2, 3),5,16 also
prepared from the reaction of ZnR21 with primary diamines.
The molecular structure of dinuclear complex 11 reveals
a four-coordinated Zn center in a distorted tetrahedral
environment, together with a ZnEt2 unit singly coordinated
to the amido-N atom in a planar trigonal geometry (Figure
7). In the latter respect, 11 is a rare example of a structurally
characterized trigonal adduct of a dialkylzinc onto a Lewis
base.17 The geometry around the ZnEt2 fragment in 11 (Zn-
(2)sN(8), 2.100(3); Zn(2)sC(18), 1.988(5); Zn(2)s
C(20),1.995(5) Å; C(18)sZn(2)sC(20), 137.0(2)°) com-
Figure 7. Solid-state structure of 11 (ellipsoids drawn at the 50%
probability; hydrogen atoms omitted for clarity). Selected bond lengths (Å)
and angles (deg): Zn(1)sC(16), 1.994(5); Zn(1)sN(15), 2.105(4); Zn(1)s
N(1), 2.091(4); Zn(1)sN(8), 2.045(3); Zn(2)sN(8), 2.100(3); Zn(2)sC(18),
1.988(5); Zn(2)sC(20), 1.995(5); C(18)sZn(2)sC(20), 137.0(2); C(16)s
Zn(1)sN(1), 121.65(19); C(16)sZn(1)sN(15), 107.7(2); N(1)sZn(1)s
N(15), 102.68(15); Zn(1)sN(8)sZn(2), 93.73(13).
(14) Andrews, P. C.; Raston, C. L.; Skelton, B. W.; White, A. H.
Organometallics 1998, 17, 779-782; ZnEt2[Me2NCH2CH2NMe2],
N-Zn-N, 80.68°, C-Zn-C, 118.08°.
pares remarkably to that of Arduengo’s nucleophilic carbene-
ZnEt2 adduct (carbene: 1,3-di(1-adamantyl)imidazol-2-
ylidene; ZnsC(carbene), 2.096; ZnsC(Et), 1.994 and 2.009
Å; C(Et)sZnsC(Et), 132.16°).17a The ZnsN(amido) bond
lengths in 10 (2.0825-2.0877 Å) and 11 (2.100(3) Å) are
similar, indicating that the mononuclear unit in dimeric 10
has an equivalent effect as the ZnEt2 fragment in 11.
The two metal centers in the dinuclear mixed ethyl-
ethoxide complex 12 are doubly bridged by the central
N(amido) atom of the ligand and the O ethoxide atom, as
observed in many zinc alkoxides (Figure 8). An approximate
(noncrystallographic) symmetry axis goes through these two
bridging atoms, provided one does not take into account the
(15) (a) Weng, S. Main Group Met. Chem. 1999, 22, 447-451; ZnEt2-
[substituted piperidine], N-Zn-N, 102.13°, C-Zn-C, 135.61°. (b)
Wissing, E.; Havenith, R. W. A.; Boersma, J.; Smeets, W. J. J.; Spek,
A. L.; van Koten, G. J. Org. Chem. 1993, 58, 4228-4236; ZnEt2-
[substituted pyridine], N-Zn-N, 89.41°, C-Zn-C, 145.75°.
(16) (a) Bell, N. A.; Moseley, P. T.; Shearer, H. M. M.; Spencer, C. B. J.
Chem. Soc., Chem. Commun. 1980, 359-360. (b) Bell, N. A.; Kassyk,
A. L. Inorg. Chim. Acta 1996, 250, 345-349. (c) Malik, M. A.;
O’Brien, P.; Motevalli, M.; Jones, A. C. Inorg. Chem. 1997, 36, 5076-
5081. (d) Bette, V.; Mortreux, A.; Lehmann, C. W.; Carpentier, J.-F.
Chem. Commun. 2003, 332-333.
(17) (a) Arduengo, A. J., III; Dias, H. V. R.; Davidson, F.; Harlow, R. L.
J. Organomet. Chem. 1993, 462, 13-18. (b) Boss, S. R.; Haigh, R.;
Linton, D. J.; Schooler, P.; Shields, G. P.; Wheatley, A. E. H. Dalton
Trans. 2003, 1001-1008.
334 Inorganic Chemistry, Vol. 46, No. 1, 2007

Complexes Based on an Amino-Bis(pyrazolyl) Ligand
Scheme 10
Table 3. MMA Polymerization Initiated by Aluminum Complexesa
Li-enolate
entry complex (equiv) yield (%) rrb (%) Mnc (g.mol-1
)
Mw/Mn
c
1
2
3
4
5
6
2 or 4
traces
45
20
40
25
traces
8
15
2
2
4
4
8
8
8
0.5
1.0
0.5
1.0
58
24
29
31
17 300
8300
15 500
9400
1.41
1.26
1.36
1.32
7d
8d,e
58
66
50 900
33 600
3.97
2.88
Figure 8. Solid-state structure of 12 (ellipsoids drawn at the 50%
probability; hydrogen atoms omitted for clarity). Selected bond lengths (Å)
and angles (deg): Zn(1)sC(11), 1.9954(19); Zn(1)sO(32), 2.0214(17); Zn-
(1)sN(10), 2.0732(17); Zn(1)sN(7), 2.1010(16); Zn(2)sN(10), 2.0595-
(18); Zn(2)sN(26), 2.1144(16); Zn(2)sO(32), 2.0087(15); Zn(2)sC(30),
1.9993(19); C(11)sZn(1)sO(32), 122.08(7); C(11)sZn(1)sN(7), 117.09-
(8); O(32)sZn(1)sN(10), 86.08(6); Zn(1)sO(32)sZn(2), 94.22(6); Zn-
(1)sN(10)sZn(2), 91.20(6); N(10)sZn(2)sO(32), 86.77(6).
a Reaction time ) 24 h (unoptimized), T ) 20 °C, solvent ) toluene
(2.0 mL), [MMA]/[Al] ) 200; cat. ) 0.05 mmol, Li-enolate )
LiO(iPrO)CdCMe2. b Determined by 1H NMR. c Mn and Mw/Mn values
determined by GPC in THF vs polystyrene standards. d B(C6F5)3 (1.0 equiv
vs Al) was added. e (2,6-di-tBu-4-Me-C6H2O)2AlMe (1.0 equiv vs Al) was
added.
with a larger N(pyrazolyl)sZnsN(bridge) angle (N(11)s
Zn(1)sN(21), 92.55(5)°) and a more acute CsZnsCl angle
(C(1)sZn(1)sCl(2), 125.46(6)°). Both the ZnsN(pyrazolyl)
(2.0876(14) Å) and ZnsN(bridge) (2.1290(13) Å) bond
lengths are reduced as compared to those in 9 (2.214(3) and
2.263(3) Å, respectively), reflecting the electron-withdrawing
effect of the chlorine ligand.
MMA Polymerization Promoted by Aluminum Com-
plexes. Methyl methacrylate (MMA) polymerization was
investigated with the prepared aluminum complexes (Scheme
10). Representative results are given in Table 3. Neutral
complexes 2, 4, and 8 are inactive toward MMA in toluene
solution at 20 °C (entries 1 and 6). However, addition of
the Li-ester enolate derived from isopropyl isobutyrate to 2
or 4, to generate in situ the corresponding anionic aluminum
species 6 and 7 (Schemes 2 and 3), leads to active systems.
When 1 equiv of Li-ester enolate versus Al was used, poor
conversions (20-25%) were observed from either 2 or 4
(entries 3 and 5). Higher conversions (40-45%) were
observed upon adding only 0.5 equiv of Li-ester enolate
versus Al (entries 2 and 4). These results evidence that the
combination of an anionic Al species (i.e., 6 or 7) as the
initiator with a neutral Al species (2 or 4, respectively; left
over because of the default of Li-ester enolate) generates a
more effective system. This beneficial effect is assumed to
arise from monomer activation by the Lewis acidic neutral
species, as highlighted recently by Rodriguez-Delgado and
Chen.11 The systems generated under these conditions are
not living but offer a certain degree of control as the PMMAs
had relatively narrow polydispersities and experimental
(uncorrected) number-average molecular weights slightly
higher than the calculated values. Interestingly, only the
system derived from 2 led to a significant syndiotacticity.
Poor results were observed with the enolate-aluminum
complex 8 (entries 6-8). Addition of 1 equiv of the methyl
abstractor B(C6F5)3 to generate in situ cationic Al-enolate
species (entry 7), further combined with 1 equiv of the bulky
aluminum Lewis acid (BHT)2AlMe ((2,6-di-tBu-4-Me-
Figure 9. Solid-state structure of 14 (ellipsoids drawn at the 50%
probability; hydrogen atoms omitted for clarity). Selected bond lengths (Å)
and angles (deg): Zn(1)sC(1), 2.0182(17); Zn(1)sCl(2), 2.2789(6); Zn-
(1)sN(11), 2.0876(14); Zn(1)sN(21), 2.1290(13); Cl(2)sZn(1)sN(11),
106.71(4); Cl(2)sZn(1)sN(21), 105.41(4); N(11)sZn(1)sN(21), 92.55-
(5); C(1)sZn(1)sCl(2), 125.46(6); C(2)sC(1)sZn(1), 107.35(11).
ethyl residue of the ethoxide ligand. The ZnsO and Zns
N(amido) bond lengths in 12 (ZnsO, 2.0214(17)-2.0087-
(15); ZnsN(10), 2.0595(18)-2.0732(17) Å) compare well
with those observed in a mixed ethyl-oxide Zn complex of
trimethylethylenediamine (ZnsO, 1.973-2.028; ZnsN
2.001-2.092 Å)16b and a tetranuclear ethyl-ethoxide Zn
complex of tridentate pyridine ligands (ZnsO, 2.131-2.187;
ZnsN, 2.144 Å).18 The ZnOZnN metallacycle in 12 is
slightly distorted from an ideal plane (torsion angle: 9.8°),
as it is also in the two latter related compounds (torsion
angles: 2.1 and 7.3°, respectively16b,18).
The solid-state structure of the benzyl-zinc chloride adduct
14 (Figure 9) is comparable to that of diethylzinc adduct 9.
The metal center is in a less distorted tetrahedral geometry
(18) Olmstead, M. M.; Grigsby, W. J.; Chacon, D. R.; Hascall, T.; Power,
P. P. Inorg. Chim. Acta 1996, 251, 273-284.
Inorganic Chemistry, Vol. 46, No. 1, 2007 335

Lian et al.
Scheme 11
efficiency, as compared to 12 and even 10, arguably call for
fundamental mechanistic differences.20 The homo-decoupled
1H NMR spectra of the methine regions of PLAs, both
produced in THF and toluene by 12, show an atactic
microstructure.
C6H2O)2AlMe, also-called MAD)11 (entry 8), improved
MMA conversions. However, the initiation efficiency re-
mained low (ca. 5-10%), and the molecular weight distribu-
tions of the PMMAs, though monodisperse, were broad.
rac-Lactide Polymerization Initiated by Zinc Com-
plexes. Zinc complexes 10, 12, and 16 were assessed in the
polymerization of rac-lactide (Scheme 11).19 Representative
results are summarized in Table 4. All three complexes
proved significantly active, allowing conversions of 100-
400 equiv of rac-lactide at room temperature. For instance,
polymerization initiated by 12 in dichloromethane at 20 °C
proceeded with turnover frequencies of ca. 60 h-1 (Figure
10). In terms of degree of control of the polymerizations,
the ethoxide dinuclear complex 12 proved a much better
initiator than the ethyl-amido complex 10 and its cationic
derivative 16. As a matter of fact, the PLAs (polylactides)
produced with 12 had a relatively narrow molecular weight
distribution (Mw/Mn ) 1.23-1.40), and the experimental
(uncorrected) number-average molecular weights are in close
agreement with the calculated values. Also, the Mn values
of the PLAs increased linearly with the [LA]/[12] ratio
(Figure 11). These results are consistent with a controlled-
living polymerization model. On the other hand, the PLAs
produced by 10 and 16 had broader polydispersities (Mw/
Mn ) 1.71-1.97) and either lower or larger Mn values
(entries 9 and 17). Two possibilities can account for the
relatively broad polydispersities observed with complex 10:
(1) because of the use as initiator of a Zn-alkyl(ethyl) group,
which is known to be less nucleophilic than alkoxides (i.e.,
ethoxide in 12), initiation is delayed versus propagation19
and (2) backbiting reaction/transesterification takes place as
side reactions, resulting in the formation of macrocycles with
a wide range of molecular weight distribution. The lower
Mn values (as compared to those expected for a well-
controlled polymerization) observed with 10 are also diag-
nostic of transfer reactions. These results highlight the
importance of differences in the coordination sphere between
the mononuclear complex 10 and the dinuclear complex 12
on their polymerization performance. On the other hand, the
significantly poorer control featured by cationic complex 16
in lactide polymerization, in particular its low initiation
Experimental Section
General Procedures. All experiments were carried out under
purified argon using standard Schlenk techniques or in a glovebox
(<1 ppm O2, 5 ppm H2O). Hydrocarbon solvents, diethyl ether,
and tetrahydrofuran were distilled from Na/benzophenone, and
toluene and pentane were distilled from Na/K alloy under nitrogen
and were degassed by freeze-thaw-vacuum cycles prior to use.
Chlorinated solvents were distilled from calcium hydride. Deuter-
ated solvents were purchased from Eurisotop and were purified
before use. Methyl methacrylate (MMA, Acros) was distilled twice
under argon over CaH2. rac-Lactide (Aldrich) was sub-
limed twice before use. 3,5-Dimethylpyrazole, bis(2-chloroethyl)-
amine hydrochloride, 2,4,6-trimethylacetophenone, NaH, AlMe3,
AlEt3, ZnEt2, and ZnCH2C6H4-p-CH3(Cl) (all Aldrich) and ZnCl2
(Acros) were used as received. B(C6F5)3 (Boulder Scientific Co.)
was sublimed twice before use.
NMR spectra were recorded on Bruker AC-200, AC-300, and
AM-500 spectrometers in Teflon-valved NMR tubes at 23 °C unless
otherwise stated. 1H and 13C NMR chemical shifts were determined
using residual solvent resonances and are reported versus SiMe4.
Assignment of signals was made from H-13C HMQC and H-
13C HMBC 2D NMR experiments. Coupling constants are given
in hertz. Elemental analyses (C, H, N) were performed by the
Microanalytical Laboratory at the Institute of Chemistry of Rennes
and are the average of two independent determinations. Molecular
weights of PMMA (or PLA) were determined by gel permeation
chromatography (GPC) at room temperature on a Waters apparatus
equipped with five PL gel columns (Polymer Laboratories Ltd),
an autosampler Waters WISP 717, and a differential refractometer
Shimadzu RID 6A. THF was used as eluent at a flow rate of 1.0
mL.min-1. Polystyrene standards were used for molecular weight
universal calibration. The microstructure of polymers was deter-
1
1
1
mined by H NMR in CDCl3.
Bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine (1). A modified
method21 was used to synthesize ligand 1. 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 °C, yielding a cream white dispersion which
was cooled and filtered. The filtrate was evaporated to dryness in
vacuo, and then hot water (80 °C, 10 mL) was added. Cooling gave
1
white needle crystals of 1 (3.24 g, 65%). H NMR (CDCl3): δ
(19) For lactide polymerization initiated by discrete Zn-alkoxide complexes,
see: (a) Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G.
W. J. Am. Chem. Soc. 1999, 121, 11583-11584. (b) Chamberlain, B.
M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates,
G. W. J. Am. Chem. Soc. 2001, 123, 3229-3238. (c) Chisholm, M.
H.; Gallucci, J. C.; Zhen, H.; Huffman, J. C. Inorg. Chem. 2001, 40,
5051-5054. (d) Chisholm, M. H.; Huffman, J. C.; Phomphrai, K.
Dalton Trans. 2001, 222-224. (e) Williams, C. K.; Breyfogle, L. E.;
Choi, S. K.; Nam, W.; Young, V. G., Jr.; Hillmyer, M. A.; Tolman,
W. B. J. Am. Chem. Soc. 2003, 125, 11350-11359. (f) Jensen, T. R.;
Schaller, C. P.; Hillmyer, M. A.; Tolman, W. B. J. Organomet. Chem.
2005, 690, 5881-5891. (g) Chisholm, M. H.; Gallucci, J. C.;
Phomphrai, K. Inorg. Chem. 2005, 44, 8004-8010. (h) Chen, H.-Y.;
Tang, H.-Y.; Lin, C.-C. Macromolecules 2006, 39; 3745-3752.
5.74 (s, 2H, pyrazole-H), 4.01 (t, 3J ) 6.0, 4H, NCH2CH2-pyrazole),
3
2.95 (t, J ) 6.0, 4H, NCH2CH2-pyrazole), 2.19 (s, 6H, pyrazole-
CH3), 2.16 (s, 6H, pyrazole-CH3). 1H NMR (C6D6): δ 5.69 (s, 2H,
pyrazole-H), 3.61 (t, 3J ) 6.0, 4H, NCH2CH2-pyrazole), 2.70 (t, 3J
(20) Contrary to polymerizations of rac-lactide carried out with 10 and
12, which likely proceed via a coordination-insertion mechanism, those
with 16 are assumed to occur via a cationic mechanism, though this
was not established in the present case.
(21) Martens, C. F.; Schenning, A. P. H. J.; Feiters, M. C.; Berens, H. W.;
van der Linden, J. G. M.; Admiraal, G.; Beurskens, P. T.; Kooijman,
H.; Spek, A. L.; Nolte, R. J. M. Inorg. Chem. 1995, 34, 4735-4744.
336 Inorganic Chemistry, Vol. 46, No. 1, 2007

Complexes Based on an Amino-Bis(pyrazolyl) Ligand
Table 4. rac-Lactide Polymerization Initiated by Complexes 10, 12, and 16a
Mn,calcb (g.mol-1
)
Mn,expc (g.mol-1
)
Mw/Mn
c
entry
complex
[LA]/[complex]
solvent
conv. (%)
9
10
11
12
13
14
15
16
17
10
12
12
12
12
12
12
12
16
100
100
100
200
100
200
300
400
100
THF
THF
93
95
98
13 400
13 700
14 100
26 800
14 400
28 900
43 300
57 700
14 100
8500
14 400
12 500
22 700
14 700
22 500
29 500
38 500
111 000
1.71
1.23
1.32
1.30
1.34
1.29
1.38
1.40
1.97
toluene
toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
93
>99
>99
>99
>99
98
a Reaction time ) 30 h (unoptimized), T ) 20 °C, solvent ) 2.5 mL, complex ) 0.01 mmol. b Calculated Mn,calc values considering one polymer chain
per initiator. c Experimental Mn and Mw/Mn values determined by GPC in THF vs PS standards.
pyrazole-H), 3.94 (t, 3J ) 6.1, 4H, NCH2CH2-pyrazole), 2.90 (t, 3J
) 6.1, 4H, NCH2CH2-pyrazole), 2.16 (s, 6H, pyrazole-CH3), 2.06
(s, 6H, pyrazole-CH3), -0.97 (s, 9H, Al(CH3)3). 13C{1H} NMR
(C6D6): δ 147.62 (pyrazole-CCH3), 138.71 (pyrazole-CCH3),
105.00 (pyrazole-CH), 47.84 (NCH2CH2-pyrazole), 44.52 (NCH2CH2-
pyrazole), 13.39 (pyrazole-CH3), 10.29 (pyrazole-CH3), -8.33 (Al-
(CH3)3). Anal. calcd. for C17H32AlN5 (333.45): C, 61.23; H, 9.67;
N, 21.00; found: C, 60.72; H, 9.34; N, 21.50.
Bis[2-(3,5-dimethyl-1-pyrazolyl)ethylamine]AlEt3 (3). The same
procedure was used as that described for 2, starting from AlEt3
(1.45 mL of a 1.9 M solution, 2.75 mmol) and bis[2-(3,5-dimethyl-
1-pyrazolyl)ethyl]amine (0.72 g, 2.75 mmol), yielding block
Figure 10. Kinetics of rac-lactide conversion with ethoxide zinc complex
12 (for polymerization conditions, see Table 4, entry 13).
1
colorless crystals of 3 (0.85 g, 82%). H NMR (C6D6): δ 5.54 (s,
2H, pyrazole-H), 5.11 (s br, 1H, NH), 3.56 (m, 4H, NCH2CH2-
pyrazole), 2.72 (m, 4H, NCH2CH2-pyrazole), 2.12 (s, 6H, pyrazole-
CH3), 1.71 (s, 6H, pyrazole-CH3), 1.45 (t, 3J ) 8.1, 9H, AlCH2CH3),
0.21 (q, 3J ) 8.1, 6H, AlCH2CH3). 13C{1H} NMR (C6D6): δ 147.62
(pyrazole-CCH3), 138.71 (pyrazole-CCH3), 105.30 (pyrazole-CH),
47.48 (NCH2CH2-pyrazole), 43.98 (NCH2CH2-pyrazole), 13.45
(pyrazole-CH3), 10.22 (pyrazole-CH3), 10.09 (AlCH2CH3), -0.29
(AlCH2CH3). Anal. calcd. for C20H38AlN5 (375.53): C, 63.97; H,
10.20; N, 18.65; found: C, 62.90; H, 10.15; N, 18.92.
[Bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amide]AlMe2 (4). In the
glovebox, a Teflon-valved NMR tube was charged with 2 (50.0
mg, 0.15 mmol), and C6D6 (ca. 0.5 mL) or toluene-d8 (ca. 0.5 mL)
was vacuum transferred in. The tube was sealed and kept at 80 °C
for 24 h. NMR spectroscopy was recorded, which revealed
Figure 11. Variation of the number-average molecular weights of PLAs
as a function of monomer-to-initiator ratio (for polymerization conditions,
see Table 4, entries 13-16).
1
quantitative formation of 4, together with release of CH4 (δ H
1
(C6D6) 0.15 ppm, δ H (toluene-d8) 0.22 ppm). The solution was
dried in vacuo, and the white solid residue was recrystallized from
pentane at -34 °C to give colorless crystals of 4 (42 mg, 88%). A
suitable crystal was selected for X-ray diffraction analysis. 1H NMR
(C6D6): δ 5.48 (s, 2H, pyrazole-H), 3.62 (t, 3J ) 6.1, 4H,
) 6.0, 4H, NCH2CH2-pyrazole), 2.29 (s, 6H, pyrazole-CH3), 1.81
(s, 6H, pyrazole-CH3).
Bis[2-(3,5-dimethyl-1-pyrazolyl)ethylamine]AlMe3 (2). To a
50-mL Schlenk containing AlMe3 (0.38 mL of a 2.0 M solution,
0.76 mmol) in pentane (2 mL) was added dropwise bis[2-(3,5-
dimethyl-1-pyrazolyl)ethyl]amine (0.20 g, 0.76 mmol) in toluene
(5 mL). The mixture was stirred for 6 h at room temperature. The
solvent was removed under vacuum, leaving a colorless oil that
was recrystallized with pentane to give block colorless crystals of
2 (0.22 g, 87%). A suitable crystal was selected for X-ray diffraction
3
NCH2CH2-pyrazole), 3.20 (t, J ) 6.1, 4H, NCH2CH2-pyrazole),
2.21 (s, 6H, pyrazole-CH3), 1.68 (s, 6H, pyrazole-CH3), -0.31 (s,
6H, Al(CH3)2). 1H NMR (Tol-d8): δ 5.52 (s, 2H, pyrazole-H), 3.65
(t, 3J ) 5.5, 4H, NCH2CH2-pyrazole), 3.19 (t, 3J ) 5.5, 4H, NCH2-
CH2-pyrazole), 2.23 (s, 6H, pyrazole-CH3), 1.77 (s, 6H, pyrazole-
1
CH3), -0.38 (s, 6H, Al(CH3)2). H NMR (toluene-d8, 0 °C): δ
5.50 (s, 1H, pyrazole-H), 5.49 (s, 1H, pyrazole-H), 3.63 (s, 2H,
NCH2CH2-pyrazole), 3.62 (s, 2H, NCH2CH2-pyrazole), 3.20 (s, 2H,
NCH2CH2-pyrazole), 3.19 (s, 2H, NCH2CH2-pyrazole), 2.25 (s, 3H,
pyrazole-CH3), 2.24 (s, 3H, pyrazole-CH3), 1.75 (s, 3H, pyrazole-
CH3), 1.74 (s, 3H, pyrazole-CH3), -0.33 (s, 3H, Al(CH3)2), -0.34
(s, 3H, Al(CH3)2). 13C{1H} NMR (toluene-d8): δ 147.03 (pyrazole-
CCH3), 139.79 (pyrazole-CCH3), 104.91 (pyrazole-CH), 51.03
(NCH2CH2-pyrazole), 49.74 (NCH2CH2-pyrazole), 12.98 (pyrazole-
CH3), 10.22 (pyrazole-CH3), -8.51 (Al(CH3)2). Anal. calcd. for
C16H28AlN5 (317.41): C, 60.54; H, 8.89; N, 22.06; found: C, 60.92;
H, 8.78; N, 22.35.
1
analysis. H NMR (C6D6): δ 5.53 (s, 2H, pyrazole-H), 3.56 (m,
4H, NCH2CH2-pyrazole), 2.80 (m, 4H, NCH2CH2-pyrazole), 2.12
(s, 6H, pyrazole-CH3), 1.71 (s, 6H, pyrazole-CH3), -0.39 (s, 9H,
Al(CH3)3). 1H NMR (Tol-d8): δ 5.54 (s, 2H, pyrazole-H), 4.57 (br
s, 1H, NH), 3.60 (m, 4H, NCH2CH2-pyrazole), 2.85 (m, 4H, NCH2-
CH2-pyrazole), 2.13 (s, 6H, pyrazole-CH3), 1.80 (s, 6H, pyrazole-
CH3), -0.46 (s, 9H, Al(CH3)3). 1H NMR (CD2Cl2): δ 5.80 (s, 2H,
pyrazole-H), 4.10 (m, 4H, NCH2CH2-pyrazole), 3.17 (m, 4H, NCH2-
CH2-pyrazole), 2.24 (s, 6H, pyrazole-CH3), 2.16 (s, 6H, pyrazole-
1
CH3), -0.90 (s, 9H, Al(CH3)3). H NMR (THF-d8): 5.65 (s, 2H,
Inorganic Chemistry, Vol. 46, No. 1, 2007 337

Lian et al.
[Bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amide]AlEt2 (5). The
same procedure was used as for 4, starting from 3 (50.0 mg, 0.130
mmol), giving complex 5 over 95% NMR purity. 1H NMR
(C6D6): δ 5.50 (s, 2H, pyrazole-H), 3.67 (t, 3J ) 6.1, 4H,
(s, 6H, pyrazole-CH3), 2.00 (s, 3H, p-CH3-Ph), 1.80 (s, 6H,
pyrazole-CH3), -0.52 (s, 6H, Al(CH3)2). 13C{1H} NMR (C6D6):
δ 153.22 (OCdCH2), 147.54 (pyrazole-CCH3), 138.73 (Ph), 138.38
(pyrazole-CCH3), 136.82 (Ph), 132.75 (Ph), 128.33 (Ph), 104.50
(pyrazole-CH), 95.54 (CdCH2), 49.14 (NCH2CH2-pyrazole), 47.90
(NCH2CH2-pyrazole), 20.78 (p-CH3-Ph), 20.05 (o-CH3-Ph), 13.60
(pyrazole-CH3), 10.57 (pyrazole-CH3), -9.89 (Al(CH3)2).
Reaction of 2 with 2,4,6-Trimethylacetophenone: Generation
of 8. Complex 2 (12.0 mg, 0.036 mmol) and 2,4,6-Me3C6H2COCH3
(5.8 mg, 0.036 mmol) were charged in a Teflon-valved NMR tube,
and C6D6 (ca. 0.5 mL) was vacuum transferred in. 1H NMR
spectroscopy was recorded, which revealed the formation of 8
(>95%) within 6 h at 60 °C. The NMR data of complex 8 were
the same as those reported above.
3
NCH2CH2-pyrazole), 3.24 (t, J ) 6.1, 4H, NCH2CH2-pyrazole),
2.34 (s, 6H, pyrazole-CH3), 1.69 (s, 6H, pyrazole-CH3), 1.38 (t, 3J
3
) 8.1, 6H, AlCH2CH3), 0.33 (q, J ) 8.1, 4H, AlCH2CH3). 13C-
{1H} NMR (C6D6): δ 147.55 (pyrazole-CCH3), 140.26 (pyrazole-
CCH3), 105.41 (pyrazole-CH), 51.63 (NCH2CH2-pyrazole), 50.23
(NCH2CH2-pyrazole), 13.14 (pyrazole-CH3), 10.68 (pyrazole-CH3),
10.16 (AlCH2CH3), 1.37 (AlCH2CH3). Anal. calcd. for C18H32AlN5
(345.46): C, 62.58; H, 9.34; N, 20.27; found: C, 61.85; H, 9.67;
N, 20.78.
Reaction of 2 with LiO(iPrO)CdCMe2: Generation of 6. In
the glovebox, complex 2 (21.6 mg, 0.065 mmol) and LiO(iPrO)Cd
CMe2 (8.8 mg, 0.065 mmol) were charged in a Teflon-valved NMR
[Bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine]ZnEt2 (9). To a
solution of ligand 1 (0.306 g, 1.17 mmol) in toluene (5 mL) was
slowly added ZnEt2 (1.01 g, 15 wt % solution in hexane, 1.23 mmol)
at room temperature. The reaction mixture was stirred for 2 h. After
evaporation of volatiles under vacuum, the residue was recrystal-
lized from a ca. 1:10 toluene/pentane solution at -34 °C to give
colorless crystals of 9 (0.406 g, 90%). A suitable crystal was
1
tube, and C6D6 (ca. 0.5 mL) was vacuum transferred in. H NMR
spectroscopy was recorded, which revealed the quantitative forma-
tion of 6, together with release of 1 equiv of (CH3)2CHCO2iPr.
The solution was dried in vacuo, and the white solid residue was
recrystallized from pentane at -34 °C to give colorless crystals of
6 (17 mg, 75%), of which some were suitable for X-ray diffraction
1
selected for X-ray diffraction analysis. H NMR (C6D6): δ 5.61
1
3
analysis. H NMR (C6D6): δ 5.53 (s, 2H, pyrazole-H), 3.72 (m,
(s, 2H, pyrazole-H), 3.52 (t, J ) 5.5, 4H, NCH2CH2-pyrazole),
2.68 (q, 3J ) 5.5, 4H, NCH2CH2-pyrazole), 2.29 (s, 6H, pyrazole-
CH3), 1.70 (s, 6H, pyrazole-CH3), 1.55 (t, 3J ) 8.1, 6H, Zn-
(CH2CH3)2), 0.42 (q, 3J ) 8.1, 4H, Zn(CH2CH3)2). 13C{1H} NMR
(C6D6): δ 147.64 (pyrazole-CCH3), 138.78 (pyrazole-CCH3),
104.89 (pyrazole-CH), 49.11 (NCH2CH2-pyrazole), 46.61 (NCH2CH2-
pyrazole), 13.99 (Zn(CH2CH3)2), 13.47 (pyrazole-CH3), 10.45
(pyrazole-CH3), 2.83 (Zn(CH2CH3)2). Anal. calcd. for C18H33N5-
Zn (384.88): C, 56.17; H, 8.64; N, 18.20; found: C, 55.83; H,
8.32; N, 18.65.
[Bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amide]ZnEt (10). In the
glovebox, complex 9 (0.105 g, 0.273 mmol) was charged in a
Teflon-valved NMR tube, and C6D6 (ca. 0.5 mL) was vacuum
transferred in. The tube was sealed and kept at 60 °C for 2 days.
1H NMR spectroscopy was recorded, which revealed the quantita-
tive formation of 10, together with release of CH3CH3 (δ 1H (C6D6)
0.79 ppm). Cooling the solution gave large amounts of colorless
crystals of 10 (92 mg, 94%), of which some were suitable for X-ray
diffraction analysis. 1H NMR (C6D6): δ 5.58 (s, 2H, pyrazole-H),
2H, NCH2CH2-pyrazole, syn), 3.58 (m, 2H, NCH2CH2-pyrazole,
syn), 3.49 (m, 2H, NCH2CH2-pyrazole, anti), 2.53 (m, 2H, NCH2-
CH2-pyrazole, anti), 2.09 (s, 6H, pyrazole-CH3), 1.59 (s, 6H,
pyrazole-CH3), -0.39 (s, 9H, Al(CH3)3). 13C{1H} NMR (C6D6):
δ 147.56 (pyrazole-CCH3), 139.72 (pyrazole-CCH3), 104.63 (pyra-
zole-CH), 54.43 (NCH2CH2-pyrazole), 49.06 (NCH2CH2-pyrazole),
13.06 (pyrazole-CH3), 10.43 (pyrazole-CH3), -8.10 (Al(CH3)3).
NMR data for (CH3)2CHCO2iPr: 1H NMR (C6D6): δ 5.01 (sept,
3J ) 6.9, 1H, OCH(CH3)2), 2.36 (sept, 3J ) 6.3, 1H, (CH3)2CHCO),
1.05 (d, 3J ) 6.9, 6H, OCH(CH3)2), 1.02 (d, 3J ) 6.3, 6H, (CH3)2-
CHCO). 13C{1H} NMR (C6D6): δ 176.51 (COOiPr), 67.02 (OCH-
(CH3)2), 34.43 ((CH3)2CHCO), 21.49 (OCH(CH3)2), 18.87 ((CH3)2-
CHCO). Anal. calcd. for C17H31AlLiN5 (339.38): C, 60.16; H, 9.21;
N, 20.64; found: C, 60.32; H, 9.18; N, 20.75.
Reaction of 4 with LiO(iPrO)CdCMe2: Generation of 7.
Complex 4 (30.0 mg, 0.090 mmol) and LiO(iPrO)CdCMe2 (12.2
mg, 0.090 mmol) were charged in a Teflon-valved NMR tube, and
1
C6D6 (ca. 0.5 mL) was vacuum transferred in. H NMR spectros-
3
3
4.05 (t, J ) 6.0, 4H, NCH2CH2-pyrazole), 3.30 (t, J ) 6.0, 4H,
NCH2CH2-pyrazole), 2.25 (s, 6H, pyrazole-CH3), 1.75 (s, 6H,
copy was recorded, which revealed the quantitative formation of
1
3
7. H NMR (C6D6): δ 5.54 (s, 2H, pyrazole-H), 4.54 (sept, J )
6.2, 1H, OCH(CH3)2), 3.63 (m, 2H, NCH2CH2-pyrazole, syn), 3.55
(m, 2H, NCH2CH2-pyrazole, syn), 3.33 (m, 2H, NCH2CH2-pyrazole,
anti), 2.48 (m, 2H, NCH2CH2-pyrazole, anti), 2.18 (s, 6H, pyrazole-
CH3), 1.94 (s, 3H, (CH3)2CdC), 1.84 (s, 3H, (CH3)2CdC), 1.59
(s, 6H, pyrazole-CH3), 1.27 (d, 3J ) 6.2, 6H, OCH(CH3)2), -0.34
(s, 6H, Al(CH3)2). 13C{1H} NMR (C6D6): δ 150.59 ((CH3)2Cd
C), 147.60 (pyrazole-CCH3), 139.06 (pyrazole-CCH3), 104.85
(pyrazole-CH), 84.78 ((CH3)2CdC), 68.01 ((CH3)2CHO), 54.43
(NCH2CH2-pyrazole), 49.69 (NCH2CH2-pyrazole), 22.25 ((CH3)2-
CHO), 18.02 ((CH3)2CdC), 13.11 (pyrazole-CH3), 10.45 (pyrazole-
CH3), -8.90 (Al(CH3)2).
3
3
pyrazole-CH3), 1.54 (t, J ) 8.0, 3H, ZnCH2CH3), 0.55 (q, J )
8.0, 4H, ZnCH2CH3). 13C{1H} NMR (C6D6): δ 147.47 (pyrazole-
CCH3), 139.03 (pyrazole-CCH3), 104.87 (pyrazole-CH), 54.47
(NCH2CH2-pyrazole),47.91(NCH2CH2-pyrazole),14.35(ZnCH2CH3),
13.46 (pyrazole-CH3), 10.56 (pyrazole-CH3), 1.10 (ZnCH2CH3).
Anal. calcd. for C16H27N5Zn (354.81): C, 54.16; H, 7.67; N, 19.74;
found: C, 54.05; H, 7.48; N, 19.86.
[Bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amide]ZnEt.ZnEt2 (11).
To a solution of 1 (128 mg, 0.49 mmol) in toluene (5 mL) was
slowly added ZnEt2 (1.33 mL of a 1.1 M solution in toluene, 1.46
mmol) at room temperature. The reaction mixture was stirred for
16 h at 120 °C under strictly anaerobic conditions. Volatiles were
removed under vacuum, and the solid residue was analyzed by 1H
NMR in C6D6, revealing the formation of 10 (ca. 80%) with other
multiple, complex resonances assigned to the diethylzinc adduct
(11; ca. 20%). The residue was recrystallized from toluene at -34
°C to give a few colorless crystals of 11, which proved suitable
for X-ray diffraction analysis.
Reaction of 4 with 2,4,6-Trimethylacetophenone: Generation
of 8. Complex 4 (11.5 mg, 0.036 mmol) and 2,4,6-Me3C6H2COCH3
(5.8 mg, 0.036 mmol) were charged in a Teflon-valved NMR tube,
and C6D6 (ca. 0.5 mL) was vacuum transferred in. 1H NMR
spectroscopy was recorded, which revealed the quantitative forma-
tion of 8 within 2 h. 1H NMR (C6D6): δ 6.68 (s, 2H, Ph), 5.67 (s,
2
2
2H, pyrazole-H), 4.79 (d, J ) 2.4, 1H, CdCHH), 4.03 (d, J )
2.4, 1H, CdCHH), 3.61 (t, 3J ) 6.0, 4H, NCH2CH2-pyrazole), 2.71
(q, 3J ) 6.0, 4H, NCH2CH2-pyrazole), 2.32 (s, 6H, o-CH3-Ph), 2.26
[Bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amide]Zn2Et2(µ-
OEt) (12). (a) Synthesis of 12 from ZnEt2 and 1 under Aerobic
338 Inorganic Chemistry, Vol. 46, No. 1, 2007

Complexes Based on an Amino-Bis(pyrazolyl) Ligand
Conditions. The reaction was carried out as described above,
starting from commercial (unpurified) ZnEt2 (1.33 mL of a 1.1 M
solution in toluene, 1.46 mmol), ligand 1 (128 mg, 0.49 mmol),
and carrying out the reaction without special care (i.e., traces of
O2 were present). Recrystallization of the final solid residue from
toluene at -34 °C gave colorless crystals of 12 (88 mg, 36%),
(NCH2CH2-pyrazole), 20.67 (CH3Ph), 13.48 (pyrazole-CH3), 10.15
(pyrazole-CH3). Anal. calcd. for C22H32ClN5Zn (467.37): C, 56.54;
H, 6.90; N, 14.98; found: C, 56.08; H, 7.12; N, 15.37.
[Bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amide]ZnCl (15). A
Teflon-valved NMR tube was charged with 14 (50.0 mg, 0.110
mmol) and ca. 0.5 mL THF-d8 was vacuum transferred in. The
tube was sealed and kept at 80 °C for 2 days, resulting in the
formation of 15 as a white precipitate. 1H NMR (CD2Cl2): δ 5.89
(s, 2H, pyrazole-H), 4.31 (m, 2H, NCH2CH2-pyrazole), 3.24 (m,
2H, NCH2CH2-pyrazole), 2.30 (s, 6H, pyrazole-CH3), 2.19 (s, 6H,
pyrazole-CH3).
1
some of which proved suitable for X-ray diffraction analysis. H
3
NMR (C6D6): δ 5.46 (s, 2H, pyrazole-H), 4.27 (q, J ) 6.9, 2H,
OCH2CH3), 4.11 (m, 4H, NCH2CH2-pyrazole), 3.05 (m, 4H, NCH2-
3
CH2-pyrazole), 2.35 (s, 6H, pyrazole-CH3), 1.71 (t, J ) 8.1, 6H,
3
ZnCH2CH3), 1.52 (s, 6H, pyrazole-CH3), 1.47 (t, J ) 6.9, 3H,
3
1
OCH2CH3), 0.67 (q, J ) 8.1, 4H, ZnCH2CH3). H NMR (CD2-
Cl2): δ 5.94 (s, 2H, pyrazole-H), 4.34 (m, 4H, NCH2CH2-pyrazole),
3.76 (q, 3J ) 6.8, 2H, OCH2CH3), 3.18 (m, 4H, NCH2CH2-
pyrazole), 2.35 (s, 6H, pyrazole-CH3), 2.29 (s, 6H, pyrazole-CH3),
1.13 (t, 3J ) 8.1, 6H, ZnCH2CH3), 1.09 (t, 3J ) 6.9, 3H, OCH2CH3),
0.01 (q, 3J ) 8.1, 4H, ZnCH2CH3). 13C{1H} NMR (C6D6): δ 148.64
(pyrazole-CCH3), 139.86 (pyrazole-CCH3), 105.24 (pyrazole-CH),
61.89 (OCH2CH3), 57.04 (NCH2CH2-pyrazole), 48.56 (NCH2CH2-
pyrazole), 22.35 (OCH2CH3), 13.94 (ZnCH2CH3), 12.97 (pyrazole-
CH3), 10.37 (pyrazole-CH3), 0.48 (ZnCH2CH3). Anal. calcd. for
C20H37N5OZn2 (494.32): C, 48.59; H, 7.54; N, 14.17; found: C,
48.36; H, 7.67; N, 14.35.
Generation of [{Bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine}-
Zn]+[EtB(C6F5)3]- (16). Complex 10 (14.9 mg, 0.042 mmol) and
B(C6F5)3 (21.3 mg, 0.042 mmol) were charged in a Teflon-valved
NMR tube, and CD2Cl2 (ca. 0.5 mL) was vacuum transferred in.
1H NMR spectroscopy was recorded, which revealed the quantita-
tive formation of 16 within 10 min 1H NMR (CD2Cl2): δ 6.16 (s,
2H, pyrazole-H), 4.26 (m, 2H, NCH2CH2-pyrazole, syn), 3.95 (m,
2H, NCH2CH2-pyrazole, anti), 3.50 (m, 2H, NCH2CH2-pyrazole,
syn), 3.27 (m, 2H, NCH2CH2-pyrazole, anti), 2.31 (s, 6H, pyrazole-
CH3), 2.07 (s, 6H, pyrazole-CH3). 13C{1H} NMR (CD2Cl2): δ
150.64 (pyrazole-CCH3), 146.11 (pyrazole-CCH3), 107.31 (pyra-
zole-CH), 53.87 (NCH2CH2-pyrazole), 47.11 (NCH2CH2-pyrazole),
12.55 (pyrazole-CH3), 11.04 (pyrazole-CH3). NMR data for
EtB(C6F5)3 anion: 1H NMR (CD2Cl2): δ 1.16 (q br, J ) 6.0,
2H, BCH2CH3), 0.56 (t, 3J ) 6.0, 3H, BCH2CH3). 19F NMR (CD2-
Cl2): δ -133.0 (d,3JF-F ) 22, 6F, o-F), -165.5 (t,3JF-F ) 22, 3F,
p-F), -168.2 (t, 3J ) 22, 6F, m-F). 11B NMR (CD2Cl2): δ -12.69
(s, BCH2CH3). 13C{1H} NMR (CD2Cl2): δ 148.2 (dm, JC-F )233,
o-C6F5B), 137.4 (dm, JC-F )233, p-C6F5B), 136.2 (dm, JC-F )233,
m-C6F5B), 127.7 (Cipso), 11.64 (CH3CH2B).
Solid-State Structure Determination of Complexes 2, 4, 6,
9-12, and 15. A suitable single crystal of 2, 4, 6, 9-12, and 15
was mounted onto a glass fiber using the “oil-drop” method.
Diffraction data were collected at 100 K using a NONIUS Kappa
CCD diffractometer with graphite monochromatized Mo KR
radiation (λ ) 0.71073 Å). A combination of ω- and æ-scans was
carried out to obtain at least a unique data set. Crystal structures
were solved by means of the Patterson method, and the remaining
atoms were located from difference Fourier synthesis, followed by
full-matrix least-squares refinement on the basis of F2 (programs
Shelxs-97 and Shelxl-97).22 Many hydrogen atoms could be found
from the Fourier difference. Carbon-bound hydrogen atoms were
placed at calculated positions and were forced to ride on the attached
carbon atom. The hydrogen atom contributions were calculated but
not refined. All non-hydrogen atoms were refined with anisotropic
displacement parameters. The locations of the largest peaks in the
final difference Fourier map calculation as well as the magnitude
of the residual electron densities were of no chemical significance.
Crystal data and details of data collection and structure refinement
are given in Table 1 and Table 2. Crystallographic data for 2, 4, 6,
9-12, and 15 are also available as cif files (see Supporting
Information).
(b) NMR-Scale Generation of 12 from ZnEt(OEt) and 10.
Complex 12 was also quantitatively generated in a Teflon-valved
NMR tube by the reaction of ZnEt(OEt) (5.9 mg, 0.042 mmol)
with complex 10 (15.0 mg, 0.042 mmol) in C6D6 (ca. 0.5 mL) at
room temperature.
-
3
[Bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine]ZnEt(Cl) (13).
To a solution of ZnEt(Cl), in situ generated from ZnCl2 (117 mg,
0.86 mmol) and ZnEt2 (106 mg, 0.86 mmol), in Et2O (5 mL), was
added 1 (225 mg, 0.86 mmol) in toluene (5 mL). The reaction
mixture was stirred for 2 h at room temperature. After evaporation
of volatiles under vacuum, the residue was recrystallized from
pentane at -34 °C to give colorless crystals of 13 (0.290 g, 85%).
1H NMR (THF-d8): δ 5.87 (s, 2H, pyrazole-H), 4.28 (m, 4H,
NCH2CH2-pyrazole), 3.24 (m, 4H, NCH2CH2-pyrazole), 2.28 (s,
3
6H, pyrazole-CH3), 2.22 (s, 6H, pyrazole-CH3), 1.11 (t, J ) 8.1,
3H, ZnCH2CH3), 0.06 (q, 3J ) 8.1, 2H, ZnCH2CH3). 13C{1H} NMR
(THF-d8): δ 148.59 (pyrazole-CCH3), 141.44 (pyrazole-CCH3),
105.03 (pyrazole-CH), 49.42 (NCH2CH2-pyrazole), 45.62 (NCH2CH2-
pyrazole), 12.50 (pyrazole-CH3),12.05 (ZnCH2CH3), 10.40 (pyra-
zole-CH3), -0.85 (ZnCH2CH3). Anal. calcd. for C16H28ClN5Zn
(391.27): C, 49.11; H, 7.21; N, 17.90; found: C, 49.63; H, 7.45;
N, 17.98.
[Bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine]Zn(CH2C6H4-p-
CH3)(Cl) (14). The same procedure was used as that described for
13, starting from 1 (180 mg, 0.69 mmol) and ClZnCH2(p-CH3C6H4)
(1.38 mL of a 0.5 M solution, 0.69 mmol), to give 14 as colorless
crystals (232 mg, 72%). A suitable crystal was selected for X-ray
1
3
diffraction analysis. H NMR (C6D6): δ 6.83 (d, J ) 7.5, 2H,
3
Ph-H), 6.68 (d, J ) 7.5, 2H, Ph-H), 5.02 (s, 2H, pyrazole-H),
3.70 (m, 2H, NCH2CH2-pyrazole), 2.94 (m, 2H, NCH2CH2-
pyrazole), 2.72 (m, 2H, NCH2CH2-pyrazole), 2.58 (m, 2H, NCH2-
CH2-pyrazole), 2.38 (s, 6H, pyrazole-CH3), 2.19 (s, 3H, CH3Ph),
Typical Procedure for MMA or rac-Lactide Polymerization.
To a 10-mL flask equipped with a magnetic stirrer, containing the
catalyst (0.05 mmol) in toluene (or THF or CH2Cl2) solvent, was
introduced Li-enolate, MAD, or B(C6F5)3 (for MMA polymeriza-
tion) in toluene (or THF or CH2Cl2) solvent. Then, the proper
1
2.17 (s, 2H, p-CH3PhCH2), 1.60 (s, 6H, pyrazole-CH3). H NMR
(THF-d8): δ 6.61 (d, 3J ) 7.5, 2H, Ph-H), 6.44 (d, 3J ) 7.5, 2H,
Ph-H), 5.84 (s, 2H, pyrazole-H), 3.85 (m, 4H, NCH2CH2-pyrazole),
3
3.04 (q, J ) 6, 4H, NCH2CH2-pyrazole), 2.24 (s, 6H, pyrazole-
(22) (a) Sheldrick, G. M. SHELXS-97, Program for the Determination of
Crystal Structures; University of Goettingen: Goettingen, Germany,
1997. (b) Sheldrick, G. M. SHELXL-97, Program for the Refinement
of Crystal Structures; University of Goettingen: Goettingen, Germany,
1997.
CH3), 2.19 (s, 6H, pyrazole-CH3), 2.09 (s, 3H, CH3Ph), 1.58 (s,
2H, p-CH3PhCH2). 13C{1H} NMR (C6D6): δ 149.76 (Ph), 148.09
(pyrazole-CCH3), 137.79 (pyrazole-CCH3), 128.57 (Ph), 125.25
(Ph), 105.41 (pyrazole-CH), 48.71 (NCH2CH2-pyrazole), 45.25
Inorganic Chemistry, Vol. 46, No. 1, 2007 339

Lian et al.
amount of monomer (MMA or rac-Lactide) was rapidly added.
The polymerization was carried out for a time period. The reaction
was then quenched by addition of acidified methanol (3% HCl,
200 mL). The precipitated polymer was filtered and dried overnight
under vacuum at 60 °C.
Scientifique for financial support of this work. We gratefully
thank Dr. C. Navarro, Dr. M. Glotin, and Dr. G. Meunier
(Arkema Co.) for valuable discussions.
Supporting Information Available: Crystallographic data for
2, 4, 6, 9-12, and 15 as CIF files. This material is available free
of charge via the Internet at http://pubs.acs.org
Acknowledgment. We thank Total and Arkema Co.
(grant to B. L.) and the Centre National de la Recherche
IC061749Z
340 Inorganic Chemistry, Vol. 46, No. 1, 2007