Between enamide and azaallyl Structures: Novel flexible N-chelate ligands in the lanthanide chemistry

Article abstract of DOI:10.1021/om049655p

Novel types of hexa-1,5-diene-1,6-diamide neodymium complexes were prepared from the corresponding neodymium bromide NdBr₃(THF)_3.5 with the appropriate dilithium hexa-1,5-diene-1,6-diamide reagent. The molecular structures of the new

Full text of DOI:10.1021/om049655p

Organometallics 2005, 24, 797-800
797
Between Enamide and Azaallyl Structures: Novel
Flexible N-Chelate Ligands in the Lanthanide Chemistry
Volker Lorenz,†,‡ Helmar Go¨rls,§ Sven K.-H. Thiele,| and Joachim Scholz*,†
Department of Chemistry, Institute of Sciences, University of Koblenz-Landau,
Universita¨tsstrasse 1, D-56070 Koblenz, Germany, Institute of Inorganic and Analytical
Chemistry, Friedrich-Schiller-University, August-Bebel-Strasse 2, D-07743 Jena, Germany,
and Buna Sow Leuna Olefinverbund GmbH, Site Schkopau, Building F 17, CR & D Catalysis
Laboratory, Synthetic Rubber, D-06258 Schkopau, Germany
Received May 14, 2004
Scheme 1. Course of the Reaction of the
1-Aza-1,3-diene 1 with Lithium
Summary: Novel types of hexa-1,5-diene-1,6-diamide
neodymium complexes were prepared from the correspond-
ing neodymium bromide NdBr3(THF)3.5 with the appro-
priate dilithium hexa-1,5-diene-1,6-diamide reagent. The
molecular structures of the new complexes revealed that
the chelating hexa-1,5-diene-1,6-diamide can be bound
either as an enamide or as an azaallyl-type ligand.
Several lanthanide compounds are known to catalyze
the polymerization of olefins.1 These are mainly metal-
locene complexes, as so far the development of the lan-
thanide chemistry has been associated with the use of
stabilizing Cp and Cp* ligands.2 Inspired by work on the
very high polymerization activity of titanium and other
transition-metal complexes with dianionic bis(amide)
ligands,3 recently a growing interest has also been di-
rected toward lanthanide complexes with polydentate
amide ligands.4 In comparison with the well-established
metallocene-based catalysts, those based on amide com-
plexes have advantages, because their complex centers
are often coordinatively unsaturated and therefore are
exposed to lower steric strain and have a higher Lewis
acidity.5 Moreover, the steric and electronic situation
at the metal center can be selectively influenced by
variation of the ligand substituents, even though the
coordination geometry is mostly predetermined by the
limited flexibility of the polydentate diamide ligands.
Recently we reported on the reduction of the 1-aza-
1,3-diene 1 to the 1-azabut-2-ene-1,4-diyl dianion 3,
wheresinitially unexpected but nearly quantitativelys
the dilithium hexa-1,5-diene-1,6-diamide 2 was gener-
ated as an intermediate during the course of the
reaction (Scheme 1).6
However, it is the 2-fold enamide structure rather
than the exceptional preparative approach that makes
2 such an interesting novel ligand system for the
chemistry of lanthanides: different from alkyl-bridged
diamide ligands, 2 also offers, in addition to the amide
functions, π-electron systems for the ligand metal bond-
ing. Thus, depending on the steric and electronic situ-
ation at the complex center, the hexa-1,5-diene-1,6-
diamide 2 with both ligand termini can be bound either
as an enamide ligand (a, η1- or σ-azaallyl) or as an
azaallyl ligand (b, η3- or π-azaallyl).7
* To whom correspondence should be addressed. E-mail: scholz@
uni-koblenz.de.
† University of Koblenz-Landau.
‡ Current address: Otto-von-Guericke-University, Universita¨tsplatz
2, D-39108 Magdeburg, Germany.
§ Friedrich-Schiller-University.
| Buna Sow Leuna Olefinverbund GmbH.
(1) Selected examples for the utilization of organolanthanide com-
pounds as catalysts for the olefin polymerization: (a) Watson, P. L.;
Parshall, G. W. Acc. Chem. Res. 1985, 18, 51-56. (b) Jeske, G.; Schock,
L. E.; Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc.
1985, 107, 8103-8110. (c) Anwander, R. In Applied Homogeneous Cat-
alysis with Organometallic Compounds; Cornils, B., Hermann, W. A.,
Eds.; VCH: Weinheim, Germany, 1996; pp 866-892. (d) Edelmann, F.
T. Top. Curr. Chem. 1996, 179, 247-276. (e) Anwander, R. In Applied
Homogeneous Catalysis with Organometallic Compounds; Cornils, B.,
Herrmann, W. A., Eds.; VCH: Weinheim, Germany, 2002; pp 974-1014.
(2) (a) Schumann, H.; Meese-Marktscheffel, J. A.; Esser, L. Chem.
Rev. 1995, 95, 865-986. (b) Edelmann, F. T. Angew. Chem., Int. Ed.
Engl. 1995, 34, 2466-2488. (c) Edelmann, F. T.; Freckmann, D. M.
M.; Schumann, H. Chem. Rev. 2002, 102, 1851-1896.
(3) (a) Scollard, J. D.; McConville, D. H. J. Am. Chem. Soc. 1996,
118, 10008-10009. (b) Tinkler, S.; Deeth, R. J.; Duncalf, D. J.;
McCamley, A. J. Chem. Soc., Chem. Commun. 1996, 2623-2624. See
also the following reviews: (c) Britovsek, G. J. P.; Gibson, V. C.; Wass,
D. F. Angew. Chem., Int. Ed. 1999, 38, 428-447. (d) Gibson, V. C.;
Spitzmesser, S. K. Chem. Rev. 2003, 103, 283-315.
(4) (a) Shah, S. A. A.; Dorn, H.; Roesky, H. W.; Lubini, P.; Schmidt,
H.-G. Inorg. Chem. 1997, 36, 1102-1106. (b) Gountchev, T. I.; Tilley, T.
D. Organometallics 1999, 18, 2896-2905. (c) Spangenberg, A.; Oberthu¨r,
M.; Noss, H.; Tillack, A.; Arndt, P.; Kempe, R. Angew. Chem., Int. Ed.
1998, 37, 2079-2082. (d) Cloke, F. G. N.; Elvidge, B. R.; Hitchcock, P.
B.; Lamarche, V. M. E. Dalton 2002, 2413-2414. (e) Roesky, P. W.
Organometallics 2002, 21, 4756-4761. (f) Estler, F.; Herdtweck, E.;
Anwander, R.; Eickerling, G.; Scherer, W. Organometallics 2003, 22,
1212-1222.
In the following we will demonstrate that in novel
neodymium complexes this flexible bonding behavior is
actually realized. Here we report for the first time on
the syntheses and structures of these complexes and
also on their suitability as catalysts for the polymeri-
zation of buta-1,3-diene.
10.1021/om049655p CCC: $30.25 © 2005 American Chemical Society
Publication on Web 01/27/2005

798 Organometallics, Vol. 24, No. 5, 2005
Communications
Scheme 2. Synthesis of the
Hexa-1,5-diene-1,6-diamide Neodymium Complexes
4-6
Figure 1. Molecular structure and numbering scheme of
4. The hydrogen atoms have been omitted for clarity. Due
to the poor crystal quality, a listing of bond lengths and
angles was abandoned.
Figure 2. Molecular structure of 5. The hydrogen atoms
have been omitted for clarity. Selected bond lengths (Å)
and angles (deg): Nd-Br1 ) 2.9186(5), Nd-Br2 ) 3.0021(5),
Nd-N1 ) 2.352(4), Nd-N2 ) 2.553(4), Li-Br1 ) 2.597(9),
Li-N2 ) 2.043(9), Nd-C1 ) 2.698(4), Nd-C2 ) 2.864(4),
Nd-C5 ) 2.938(5), Nd-C6 ) 2.704(5), N1-C1 ) 1.374(6),
N2-C6 ) 1.383(5), C1-C2 ) 1.360(6), C2-C3 ) 1.526(6),
C3-C4 ) 1.541(6), C4-C5 ) 1.546(5), C5-C6 ) 1.360(6);
Br1-Nd-Br2 ) 79.47(1), Br1-Nd-Br2A ) 145.29(1),
Br2-Nd-Br2A ) 73.18(1), Nd-Br1-NdA ) 106.82(1),
N1-Nd-N2 ) 157.2(1), N1-C1-C2 ) 124.0(4), N2-C6-
C5 ) 125.7(4), C2-C3-C4-C5 ) 52.8(4).
Treatment of NdBr3(THF)3.5 with 1 equiv of the dilith-
ium hexa-1,5-diene-1,6-diamide compound 2 in THF as
solvent led to the formation of the very air- and mois-
ture-sensitive (hexa-1,5-diene-1,6-diamide)neodymium
complex 4 in 65% yield (Scheme 2). Corresponding to
its ionic constitution, 4 is very soluble in THF and DME
but only moderately soluble in diethyl ether. Nearly color-
less crystals of a monoclinic as well as of a triclinic modi-
fication were obtained by recrystallization from THF.8
Unfortunately, in both cases the crystals were not of
high quality, but the main features of their structures,
which are very similar, could be clearly identified.
As expected, the hexa-1,5-diene-1,6-diamide (Figure
1) is coordinated as an ethylene-bridged bis(π-azaallyl)
chelate ligand using the two amide functions N1 and
N2 as well as the adjacent π-systems C1dC2 and C5dC6,
respectively, for bonding to the neodymium. The charge
centers of the π-azaallyl units and the two bromide
ligands define the tetrahedral coordination geometry
around the neodymium, which meets the common struc-
tural pattern known from most of the lanthanide metal-
locene complexes.2 Unfortunately, the 1H NMR spectra
of 4 and the other neodymium complexes reported here
are uninformative, due to their very broad low-intensity
resonances combined with a strong paramagnetic shift.
Surprisingly, as a result of our efforts to improve the
crystal quality of the neodymium complex 4 by repeated
recrystallization from DME, the solubility of the com-
plex increased significantly. After prolonged standing at
-10 °C colorless crystals precipitated again. The X-ray
structure analysis now reveals the dimeric neodymium
complex 5 bridged by two bromide ligands (Figure 2).8
Probably the dimerization of 4 can be ascribed to the
high Lewis acidity of the neodymium additionally
promoted by its low coordination number. However, the
loss of DME in the course of the crystallization of 5
results in a situation different from that in 4, since the
Li+ ions are no longer completely surrounded by solvent
molecules. Instead, the Li+ ions are closely bound to the
terminal bromide ligands Br2 and Br2A, respectively
(Li-Br ) 2.597(9) Å), as well as to the amide functions
N2 and N2A, respectively (Li-N2 ) 2.043(9) Å).
These contacts lead to elongated Nd-N2 and NdA-
N2A bonds, respectively (Nd-N2 ) 2.553(4) Å), in
contrast to Nd-N1 (2.352(4) Å), but the η3 coordination
of the two π-azaallyl units of the hexa-1,5-diene-1,6-
diamide ligand is maintained. In any event, the differ-
ences between the Nd-C bonds (Nd-C1 ) 2.698(4) Å
and the Nd-C2 ) 2.864(4) Å or Nd-C6 ) 2.704(5) Å
and Nd-C5 ) 2.938(5) Å, respectively) are striking.
They indicate that the η3-azaallyl coordination in 5
(5) Gibson, V. C.; Kimberley, B. S.; White, A. J. P.; Williams, D. J.;
Howard, P. Chem. Commun. 1998, 313-314.
(6) Lorenz, V.; Go¨rls, H.; Scholz, J. Angew. Chem., Int. Ed. 2003,
42, 2253-2257.
(7) Caro, C. C.; Lappert, M. F.; Merle, P. G. Coord. Chem. Rev. 2001,
219-221, 605-663.

Communications
Organometallics, Vol. 24, No. 5, 2005 799
differs markedly from that of a symmetrical π-coordina-
tion as is found, for example, in the (η3-allyl)neodymium
complexes (η3-C3H5)3Nd(DME)9a and [(η3-C3H5)2Nd(µ-
Cl)(THF)]2.9b On the other hand, the bond distances and
angles largely agree with those of the, to our knowledge,
only structurally characterized (η3-azaallyl)lanthanide
complexes, [Me3SiCHC(tBu)N(SiMe3)]2SmI(THF) and
[Me3SiCHC(tBu)N(SiMe3)]2Yb,10 whose π-azaallyl ligands,
however, are not connected by an alkyl bridge.
When the mole ratio of the two reactants 2 and NdBr3-
(THF)3.5 was increased to 2:1, then the nearly colorless,
air- and moisture sensitive neodymium complex 6 was
obtained in 75% yield (Scheme 2). Complex 6 can be
formally considered as a derivative of the ionic com-
pound 4, in which the two bromine ligands are substitu-
ted by a second hexa-1,5-diene-1,6-diamide ligand. There-
fore, it is not much of a surprise that, at least in the
crystalline state, the [(hexa-1,5-diene-1,6-diamide)2Nd]-
Figure 3. Molecular structure of the anion of 6. The hy-
drogen atoms have been omitted for clarity. Selected bond
lengths (Å) and angles (deg): Nd-N1A ) 2.393(3), Nd-
N2A ) 2.364(3), Nd-N1B ) 2.380(3), Nd-N2B ) 2.397(3),
Nd-C1A ) 2.842(4), Nd-C2A ) 3.170(4), Nd-C5B )
3.101(4), Nd-C6 ) 2.834(4), N1A-C1A ) 1.370(5), N2A-
C6A ) 1.374(5), C1A-C2A ) 1.358(6), C2A-C3A ) 1.519(6),
C3A-C4A ) 1.558(6), C4A-C5A ) 1.523(5), C5A-C6A )
1.337(5), N1B-C1B ) 1.382(4), N2B-C6B ) 1.368(5),
C1B-C2B ) 1.335(5), C2B-C3B ) 1.520(5), C3B-C4B )
1.560(5), C4B-C5B ) 1.525(5), C5B-C6B ) 1.356(5);
N1A-Nd-N2A ) 124.4(1), N1B-Nd-N2B ) 125.8(1),
N1A-C1A-C2A ) 124.3(4), N2A-C6A-C5A ) 129.1(4),
N1B-C1B-C2B ) 128.5(3), N2B-C6B-C5B ) 124.1(4).
(8) (a)Crystaldatafor4.ModificationI: [C12H30O6Li]+[C26H34Br2N2Nd]-,
Mr ) 955.91, colorless prism, size 0.10 × 0.08 × 0.06 mm3, monoclinic,
space group P21, a ) 12.1509(3) Å, b ) 28.9797(8) Å, c ) 12.8885(3) Å,
â ) 96.685(2)°, V ) 4507.6(2) Å3, T ) -90 °C, Z ) 4, Fcalcd ) 1.409 g
cm-3, µ(Mo KR) ) 29.65 cm-1, ψ-scan, minimum transmission 0.7559,
maximum transmission 0.8422, F(000) ) 1948, 17 486 reflections in h
(-14 to +15), k (-37 to +24), l (-16 to +12), measured in the range
1.59° e θ e 27.11°, completeness θmax ) 92.7%, 12 240 independent
reflections, Rint ) 0.063, 10 476 reflections with Fo > 4σ(Fo), 896
parameters, 1 restraint, R1obsd ) 0.083, wR2obsd ) 0.201, R1all ) 0.097,
wR2all ) 0.219, GOF ) 1.085, Flack parameter 0.07(2), largest
difference peak/hole 7.089/-1.453 e Å-3. Modification II: triclinic, space
group P1h, a ) 12.1499(7) Å, b ) 12.8905(7) Å, c ) 29.0650(10) Å, R )
86.015(3)°, â ) 88.674(3)°, γ ) 83.328(3)°, V ) 4509.8(4) Å3, T ) -90
°C, Z ) 4, Fcalcd ) 1.787 g cm-3, µ(Mo KR) ) 65 cm-1, ψ-scan, minimum
transmission 0.5626, maximum transmission 0.6244, F(000) ) 2388,
18 964 reflections in h (-12 to +15), k (-16 to +16), l (-27 to +37),
measured in the range 1.59° e θ e 27.45°, completeness θmax ) 98.6%.
The X-ray data of both of the two modifications of compound 4 are of
minor quality. Therefore, only their crystallographic data and the
conformation of 4 are published. Bond lengths and angles are not listed.
anion and the [Li(DME)3]+ cation existsmuch like in the
case of 4sas a solvent-separated ion pair (Figure 3).8
Due to the limited space in the coordination sphere
of the metal center, the two hexa-1,5-diene-1,6-diamide
ligands are forced into the more space-saving enamide
function (η1- and σ-azaallyl, respectively), each at the
expense of one η3-azaallyl function. As a consequence
thereof, the configuration of the double bonds C5AdC6A
and C1BdC2B changes from E to Z and thus prevents
an increase of the ring tension. The result of the crystal
structure analysis furthermore indicates that the Nd-C
distances of the other two η3-azaallyl units are slightly
longer than those of 5 (Nd-C1A ) 2.842(4) Å, Nd-C2A
) 3.170(4) Å, Nd-C6 ) 2.834(4) Å, Nd-C5B ) 3.101(4)
Å). However, the bonding mode of the hexa-1,5-diene-
1,6-diamide ligands in 6 is still best described as η3-
azaally η1-enamide bonding.
Ziegler-Natta catalysts containing neodymium have
been used in the industrial production of poly-cis-1,4-but-
adiene for some time now.11 The above-mentioned (η3-
allyl)neodymium complexes (η3-C3H5)3Nd(dioxane)9a and
[(η3-C3H5)2Nd(µ-Cl)(THF)]2 9b as well as (η3-C3H5)NdCl2-
(THF)2 9b have actually been shown to be extremely ac-
tive and highly selective complex catalysts for the 1,4-cis
polymerization of buta-1,3-diene. To obtain preliminary
information about the catalytic activity of the novel aza-
allyl neodymium complexes 4 and 6, polymerization of
buta-1,3-diene was conducted with them using MMAO
as cocatalyst (molar ratio of about 0.1:30) and cyclohex-
ane as the solvent at 80 °C. For comparison we also stud-
ied the hexa-1,5-diene-1,6-diamide lanthanum complex
7, whose structure was established by 1H NMR and re-
sembles that of 5 (see Experimental Section). The re-
sults of the polymerization experiments are listed in
Table 1. As expected, the coordinatively less saturated
Crystal data for 5:
C60H88Br4Li2N4Nd2O4‚2C4H10O, Mr ) 1699.58,
colorless prism, size 0.10 × 0.09 × 0.06 mm3, triclinic, space group
P1h, a ) 12.3196(5) Å, b ) 13.2774(5) Å, c ) 13.5081(6) Å, R ) 94.649-
(3)°, â ) 112.393(2)°, γ ) 105.774(2)°, V ) 1923.22(14) Å3, T ) -90
°C, Z ) 1, Fcalcd ) 1.467 g cm-3, µ(Mo KR) ) 34.59 cm-1, semiempirical,
minimum transmission 0.623, maximum transmission 0.834, F(000)
) 858, 22 302 reflections in h (-15 to +15), k (-16 to +17), l (-17 to
+13), measured in the range 1.89° e θ e 27.44°, completeness θmax
99.6%, 8743 independent reflections, Rint ) 0.084, 6529 reflections with
Fo > 4σ(Fo), 371 parameters, 0 restraints, R1obsd ) 0.046, wR2obsd
0.097, R1all ) 0.075, wR2all ) 0.110, GOF ) 1.024, largest difference
)
)
peak/hole 0.716/-1.414 e Å-3. Crystal data for 6: [C12H30LiO6]+[C52H68
-
N4Nd]-, Mr ) 1170.64, colorless prism, size 0.10 × 0.10 × 0.09 mm3,
monoclinic, space group P21/c, a ) 10.8383(3) Å, b ) 37.8916(9) Å, c )
15.8874(4) Å, â ) 94.797(1)°, V ) 6501.8(3) Å3, T ) -90 °C, Z ) 4,
Fcalcd ) 1.196 g cm-3, µ(Mo KR) ) 8.47 cm-1, F(000) ) 2484, 40 749
reflections in h (-14 to +13), k (-49 to +35), l (-20 to +20), measured
in the range 1.96° e θ e 27.44°, completeness θmax ) 97.6%, 14 472
independent reflections, Rint ) 0.055, 9932 reflections with Fo > 4σ(Fo),
717 parameters, 0 restraints, R1obsd ) 0.049, wR2obsd ) 0.108, R1all
)
0.088, wR2all ) 0.125, GOF ) 1.007, largest difference peak and hole:
0.875/-0.635 e Å-3. (b) The detailed crystallographic data of this
publication are available as Supplementary Publications CCDC-215583
(5) and CCDC-215584 (6) from the Cambridge Crystallographic Data
Centre. The data can be ordered without charge via www.cam.ac.uk/
conts/retrieving.html (or by postal mail in Great Britain: Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ,
U.K.; fax (+44) 1223-336-033; e-mail deposit@ccdc.cam.ac.uk).
(9) (a) Taube, R.; Windisch, H.; Maiwald, S.; Hemling, H.; Schu-
mann, H. J. Organomet. Chem. 1996, 513, 49-61. (b) Maiwald, S.;
Taube, R.; Hemling, H.; Schumann, H. J. Organomet. Chem. 1998,
552, 195-204. (c) Maiwald, S.; Sommer, C.; Mu¨ller, G.; Taube, R.
Macromol. Chem. Phys. 2000, 202, 1446-1456. See also reports about
allylsamarium complexes which are efficient catalysts for the polym-
erization of methyl methacrylate: (d) Woodman, T. J.; Schormann, M.;
Bochmann, M. Organometallics 2003, 22, 2938-2943. (e) Woodman,
T. J.; Schormann, M.; Hughes, D. L.; Bochmann, M. Organometallics
2003, 22, 3028-3033.
(11) (a) Witte, J. Angew. Makromol. Chem. 1981, 94, 119-124. (b)
Wilson, D. J. Polym. Int. 1996, 39, 235-242. (c) Taube, R.; Sylvester,
G. In Applied Homogeneous Catalysis with Organometallic Compounds;
Cornils, B., Herrmann, W. A., Eds.; VCH: Weinheim, Germany, 1996;
pp 280-318. (d) Thiele, S. K.-H.; Wilson, D. R. J. Macromol. Sci., Part
C-Polym. Rev. 2003, C43, 581-628.
(10) (a) Hitchcock, P. B.; Lappert, M. F.; Tian, S. J. Organomet.
Chem. 1997, 549, 1-12. (b) Lappert, M. F.; Liu, D.-S. J. Organomet.
Chem. 1995, 500, 203-217.

800 Organometallics, Vol. 24, No. 5, 2005
Communications
Table 1. Selected Results of the Buta-1,3-diene
Polymerization with the
5. A 2.50 g portion (2.62 mmol) of 4 was dissolved in
50 mL of DME with mild heating. Subsequently, the sol-
ution was filtered and stored at -20 °C for 24 h. Color-
less crystals of the dimeric neodymium complex 5 precip-
itated. This procedure was repeated several times. Mp:
158-160 °C dec. Anal. Calcd for C68H108Br4N4O6Li2-
Nd2: C, 48.05; H, 6.40; N, 3.30. Found: C, 48.09; H,
6.42; N, 3.38.
6. A 3.14 g portion (4.94 mmol) of NdBr3(THF)3.5 was
added to a solution of 5.30 g (9.88 mmol) of 2 in 200 mL
of DME at -20 °C. The reaction mixture was warmed to
room temperature and stirred for 48 h. Then, the solvent
was distilled off under vacuum and the remaining solid
was dissolved in 200 mL of Et2O. Initially, at -20 °C,
crystalline LiBr(DME)2 precipitated from the solution
and was separated by filtration. The volume of the fil-
trate was reduced to 100 mL. After the mixture stood
for another 48 h at -20 °C, 4.33 g of colorless crystals
of 6 (3.70 mmol, 75% referenced to the starting
NdBr3(THF)3.5) was obtained. Mp: 124 °C. Anal. Calcd
for C64H98N4O6LiNd: C, 65.66; H, 8.44; N, 4.79.
Found: C, 65.53; H, 8.50; N, 4.83.
Hexa-1,5-diene-1,6-diamide Complexes 4 and 6 and
also with the Lanthanum Complex 7a
microstructure of
complex
activityb
yieldc
the polybutadiened
4
6
7
116.4
23.3
82.1 (2 h 15 min)
5.1 (1 h 39 min)
84.5 (1 h 34 min)
93.5/5.5/1.0
not defined
93.7/4.7/1.7
529.1
a The polymerization experiments were performed at Dow BSL
Olefinverbund GmbH. b Activity in g of polymer (mmol of complex)-1
h-1 c Yield of polybutadiene in percent. d Content of isomers of
.
1,4-cis-/1,4-trans-/1,2-polybutadiene in percent.
bromide-containing neodymium complex 4 displays a
markedly higher activity than the bis(hexa-1,5-diene-
1,6-diamide)neodymium complex 6. In any event, the
high activity and selectivity of the lanthanum complex 7
is consistent with that of comparable Ziegler-Natta sys-
tems, as for example Nd(versatat)3/MAO (1:264) with a
turnover frequency of approximately 12 000 mol of buta-
1,3-diene (mol of Nd)-1 h-1 and a cis selectivity of 91%.11b
The results at hand show that with the easily availa-
ble hexa-1,5-diene-1,6-diamide a ligand system with a
truly unexpected versatility is available to the chemistry
of lanthanides: a combination of the amide and the al-
kene functions within the hexa-1,5-diene-1,6-diamide re-
sults in the formation of azaallyl units. Thus, the bonding
mode may vary in the range between a bis-enamide
structure on the one hand and a bis-azaallyl structure on
the other, depending on the electronic and steric situa-
tionatthecomplexcenter.Theconsequentstructuralvar-
iety of size and geometry of the chelate ring and the cat-
alytic activity of the novel neodymium complexes offer
enough reasons for more extended investigations in this
field.
7. To a solution of 4.20 g (7.83 mmol) of 2 in 100 mL
of DME was added 4.94 g (7.83 mmol) of LaBr3(THF)3.5
at -20 °C. The reaction mixture was warmed to room
temperature and stirred for 24 h. The mixture was then
evaporated to dryness, and the product was dissolved
in diethyl ether (200 mL). Crystalline LiBr(DME)2,
which precipitated first, was separated by filtration.
After the mixture stood for 48 h at -20 °C, a yield of
4.10 g (2.66 mmol, 68% referenced to the starting LaBr3-
(THF)3.5) of colorless needlelike crystals of the dimeric
lanthanum complex 7 was obtained. Mp: 143-145 °C
1
dec. H NMR (THF-d8, 300 MHz, 20 °C): δ 7.53 (br d,
Experimental Section. All experiments were per-
formed under an atmosphere of dry argon using stan-
dard Schlenk techniques. Solvents were distilled from
sodium/benzophenone ketyl (THF, diethyl ether) or
LiAlH4 (pentane) under argon and stored over activated
8H, o-Ph), 6.92-6.68 (m, 12H, m-Ph, p-Ph), 6.22 (s, 4H,
CHdCMe), 3.98 (br s, 4 H, CHPh), 3.53 (s, 8H, OCH2,
3
DME), 3.35 (s, 12H, OCH3, DME), 2.91 (sept, J(H,H)
) 6.4 Hz, 4H, NCHMe2), 1.60 (s, 12H, CHdCMe), 1.06
3
(br d, J(H,H) ) 6.4 Hz, 24H, NCHMe2). Anal. Calcd
4 Å molecular sieves. The lanthanide bromide complexes
for C60H88Br4Li2N4La2O4: C, 46.77; H, 5.76; N, 3.64.
Found: C, 46.59; H, 5.79; N, 3.71.
12
LaBr3(THF)3.5 and NdBr3(THF)3.5
as well as the
dilithium hexa-1,5-diene-1,6-diamide 26 were prepared
according to literature procedures. The H NMR spec-
Butadiene Polymerization. The polymerization ex-
periments were carried out in a double-walled steel re-
actor at +80 °C in cyclohexane. The polymerization was
started by addition of the lanthanide complex to the cy-
clohexane solution of buta-1,3-diene and the activator
MMAO (300-600 equiv per lanthanide complex). For the
termination of the polymerization process, the polymer
solution was transferred into methanol which contained
Irganox 1520 as polymer stabilizer. The microstructure
of the polybutadiene was analyzed spectroscopically.
1
trum of 7 was recorded in THF-d8 on a Varian 300 BB
spectrometer (1H NMR at 300.075 MHz). Elemental
analyses were carried out by the analysis laboratory at
the Martin-Luther-University of Halle-Wittenberg.
4. To a solution of 6.78 g (12.64 mmol) of 2 in 200 mL
of DME was added 8.04 g (12.64 mmol) of NdBr3(THF)3.5
at -20 °C. The reaction mixture was warmed to room
temperature and stirred for 24 h. Then, the mixture was
evaporated to dryness, and the product was dissolved in
diethyl ether (200 mL). After the mixture stood for 12 h
at -20 °C crystalline LiBr(DME)2 precipitated at first
and was separated by filtration. The filtrate was concen-
trated to 100 mL and stored for another 48 h at -20 °C.
A yield of 7.68 g (8.22 mmol, 65% referenced to the start-
ing NdBr3(THF)3.5) of colorless crystals of 4 was obtain-
ed. Mp: 132-135 °C dec. Anal. Calcd for C38H64Br2N2O6-
LiNd: C, 47.74; H, 6.75; N, 2.93. Found: C, 47.70; H,
6.81; N, 2.99.
Acknowledgment. This work was supported by the
Deutsche Forschungsgemeinschaft. We thank Prof. D.
Steinborn for providing laboratory facilities at the
Institute of Inorganic Chemistry at the University of
Halle-Wittenberg, Halle-Wittenberg, Germany, and Dow
BSL Olefinverbund GmbH, Schkopau, Germany, for
helpful collaboration.
Supporting Information Available: Listings of atomic
coordinates, thermal parameters, and bond distances and
angles for complexes 4-6; crystallographic data are also
available as CIF files. This material is available free of charge
via the Internet at http://pups.acs.org.
(12) LaBr3(THF)3.5 and NdBr3(THF)3.5 were prepared by reaction of
chippings of the lanthanide metals with CH2Br2 in THF. See also:
Taube, R.; Maiwald, S.; Sieler, J. J. Organomet. Chem. 2001, 621, 327-
336.
OM049655P