Mononuclear and dendritic nickel(II) complexes containing N,N′-iminopyridine chelating ligands: generation effects on the catalytic oligomerization and polymerization of ethylene

Article abstract of DOI:10.1021/om0509084

A series of carbosilane dendritic compounds Gn-ONNMe_m, containing one (n = 0), four (n = 1), eight (n = 2), or 16 (n = 3) terminal pyridylimine ligands, substituted with m methyl groups (m = 0, 2, 3), and nickel complexes Gn-ONNMe_mNiBr₂, comprising monometallic to metallodendritic structures, have been synthesized. The nickel complexes, in combination with methylaluminoxane (MAO), are active catalysts for the concurrent transformation of ethylene into mixtures of toluene-insoluble polyethylene and oily oligomers. Oligomers consist of mixtures of olefins that follow a Schulz - Flory distribution, and polymers are found to be highly branched low molecular weight polyethylene. The variation of the pyridylimine ligand framework by methyl substituents has a decisive influence on the activities of the nickel compounds. Also, the size (i.e., generation n) of the dendritic precursor acutely affects the catalyst performance and the microstructure of the insertion products. Thus, higher generation catalysts show superior oligomerization activities and produce less branched polyethylene polymers with higher molecular weights.

Full text of DOI:10.1021/om0509084

3876
Organometallics 2006, 25, 3876-3887
Mononuclear and Dendritic Nickel(II) Complexes Containing
N,N′-Iminopyridine Chelating Ligands: Generation Effects on the
Catalytic Oligomerization and Polymerization of Ethylene†
Jose´ M. Benito, Ernesto de Jesu´s,* F. Javier de la Mata, Juan C. Flores,*
Rafael Go´mez, and Pilar Go´mez-Sal
Departamento de Qu´ımica Inorga´nica, UniVersidad de Alcala´, Campus UniVersitario,
28871 Alcala´ de Henares, Madrid, Spain
ReceiVed October 19, 2005
A series of carbosilane dendritic compounds Gn-ONNMem, containing one (n ) 0), four (n ) 1), eight
(n ) 2), or 16 (n ) 3) terminal pyridylimine ligands, substituted with m methyl groups (m ) 0, 2, 3),
and nickel complexes Gn-ONNMemNiBr2, comprising monometallic to metallodendritic structures, have
been synthesized. The nickel complexes, in combination with methylaluminoxane (MAO), are active
catalysts for the concurrent transformation of ethylene into mixtures of toluene-insoluble polyethylene
and oily oligomers. Oligomers consist of mixtures of olefins that follow a Schulz-Flory distribution,
and polymers are found to be highly branched low molecular weight polyethylene. The variation of the
pyridylimine ligand framework by methyl substituents has a decisive influence on the activities of the
nickel compounds. Also, the size (i.e., generation n) of the dendritic precursor acutely affects the catalyst
performance and the microstructure of the insertion products. Thus, higher generation catalysts show
superior oligomerization activities and produce less branched polyethylene polymers with higher molecular
weights.
ethylene derivatives ranging from light oligomers to high
molecular weight polymers,2,7-11 with linear to hyperbranched
microstructures as a result of the so-called “chain-walking
mechanism”.12 Considerable effort has been made to expand
the scope of chelating ligands on nickel and palladium com-
plexes useful in ethylene chemistry,1 including studies with
P/O,13 N/O,14 P/P,15 and N/P16 bidentate systems. Asymmetrical
N/N pyridylimine-type compounds have also been synthesized
and used as catalysts in ethylene oligomerization and poly-
merization reactions.17-21 In addition, this class of ligands has
been successfully applied in solid-phase organometallic synthesis
Introduction
The interest in the field of well-defined late transition metal
catalysts for olefin polymerization has been renewed during the
last decade.1 Following the disclosures by Brookhart2 and by
Gibson,3 most of these catalysts have been based on nickel and
palladium R-diimine-based complexes, or iron or cobalt com-
pounds bearing bis(imino)pyridyl ligands. Earlier work by Keim
showed that Ni(II) complexes containing bidentate P/O chelating
ligands are effective homogeneous catalysts for the oligomer-
ization of ethylene (SHOP process),4 affording R-olefins in the
industrially desirable C4-C20 range.5 The oligomers are pro-
duced due to a highly competitive chain transfer reaction relative
to chain propagation.6 The chain termination rate is retarded
by using Brookhart-type Ni(II) catalysts with sterically demand-
ing R-diimine ligands, which block the axial sites of the active
center, impeding the chain transfer process and thus enhancing
its growth.2,6 Consequently, depending on the substitution of
the ligand and the reaction conditions, these catalysts produce
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† Dedicated to Prof. Dr. Victor Riera on the occasion of his 70th birthday.
* Corresponding authors. Tel: +34 91 885 4603 (E.J.); +34 91 885
4607 (J.C.F.). Fax: +34 91 885 4683. E-mail: ernesto.dejesus@uah.es
(E.J.); juanc.flores@uah.es (J.C.F.).
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10.1021/om0509084 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 07/07/2006

Effects of Dendritic Ni-Catalysts in Polymerization
Organometallics, Vol. 25, No. 16, 2006 3877
and radical polymerization, by immobilizing molybdenum
carbonyl22 and copper halide complexes,23 respectively.
Research on metallodendrimers has also been a prominent
area during the last few years, and an increasing number of
reports have been published on the incorporation of transition
metals at the core, branches, or periphery of dendrimers.24
Encouraged by the fact that dendrimers can be used as well-
defined supports for active centers in a homogeneous phase,
their catalytic applications are being widely studied,25 including
those in polymerization,18b,26-29 and oligomerization processes.30
We have recently initiated a program focused on the
chemistry of carbosilane dendrimers containing early transition
metal complexes located at the dendritic periphery, or at the
focal point,31 and usually bonded through O-32 or N-donor33
Figure 1. Numbering scheme for pyridylimine fragment.
anchoring ligands. As part of this research and in an attempt to
expand the range of useful synthetic routes available to attach
metal complexes to dendrimers, we decided to evaluate the
possible application of pyridylimine ligands (Figure 1) in the
field of carbosilane metallodendritic chemistry and analyze
relationships between the dendritic nature of nickel derivatives
and their polymerization behavior. Since dendritic moieties bring
about distinctive catalytic environments, and ethylene poly-
merization with late transition metal catalysts is sensitive to the
active site surroundings, we decided to extend and explore the
capacity of metallodendrimer chemistry in this type of process.
This paper describes the synthesis and characterization of
different pyridylimine-ended carbosilane dendrimers and their
subsequent complexation with nickel(II), as well as monome-
tallic nickel model complexes. In addition, the results using the
nickel compounds as catalyst precursors for ethylene poly-
merization are also presented. The influence of the dendrimer
generation and the substitution of the N/N anchoring ligand on
the catalytic potential are analyzed. A preliminary account of
part of this work has been published previously.33d
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Results and Discussion
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Synthesis of Ligands and Mononuclear Nickel Com-
pounds. The pyridylimine ligands used in this work are the
N-donor bidentates 1a-c (Scheme 1), with an unprotected
phenolic hydroxy group effective for the functionalization of
dendrimers, and the trimethylsilyl-protected derivatives 2a-c,
useful as models of the dendritic carbosilanes described further
on. They were synthesized adapting a procedure described in
the literature for 1a and 2a, which consists of the condensation
of pyridine-2-carbaldehyde or 6-methylpyridine-2-carbaldehyde
with the corresponding p-aminophenol compound, accompanied
in the case of 2 by the phenolysis of chlorotrimethylsilane.22a
The sequence of reactions can be inverted as demonstrated in
the preparation of 2a (see Experimental Section and Scheme
1). Compounds 1 and 2 were isolated in high yields as yellow
solids and oils, respectively. All compounds gave satisfactory
elemental analyses, and their mass spectra (EI/MS) exhibit a
peak assignable to the molecular ion. The relevant feature of
their 1H and 13C NMR spectra is the appearance of two singlets
for the Me13 and Me14 groups (numbering in Figure 1; δ )
(23) Haddleton, D. M.; Kukulj, D.; Radigue, A. P. Chem. Commun. 1999,
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2004, 689, 4016. (i) Rossell, O.; Seco, M.; Angurell, I. C. R. Chim. 2003,
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Kreiter, R.; Kleij, A. W.; Gebbink, R. J. M. K.; van Koten, G. Top. Curr.
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3878 Organometallics, Vol. 25, No. 16, 2006
Benito et al.
Scheme 1
2.1-2.3 and 15.6-17.5 for proton and carbon, respectively)
and an additional singlet for the pyridinic Me15 group (δ ) 2.6
and 24.4 for proton and carbon, respectively) on going from
derivatives a to c. Another singlet is observed downfield for
the imine group (CH7, δ = 8.5 and 157 for proton and carbon,
respectively). This group is also characterized by a strong IR
ligands is favored in polar solvents,13d and the evolution of
mono(P/O) to bis(P/O) nickel compounds by addition of MeOH
has been reported recently.28a The NMR spectra of complexes
3 and 4 are not informative because of the interfering effect
caused by their paramagnetism. The mass spectrometric data
(EI/MS) reveal the molecule ion in the case of complex 3c and
the fragments after the loss of one or two bromide atoms for
3a and 3c, together with peaks derived from the bidentate ligand
present in each complex. The fragmentation pattern with halide
loss has been shown to occur for similar nickel compounds.9,10,19a
In all cases the peak isotope distributions match calculated
patterns. The positive ion electrospray mass spectra (ESI+/MS)
or atmospheric pressure chemical ionization (APCI/MS) of 3a-
c, carried out in acetonitrile solutions, exhibit the fragments after
the loss of a bromine atom and coordination of an acetonitrile
molecule, whereas in the case of the bis(pyridylimine) com-
pound 4 the coordination of the solvent is not detected and the
[M - Br]+ fragment is observed. The formation of the [M -
Br + solvent]+ ion from related nickel complexes has been
described to occur with concomitant appearance in the spectrum
of peaks due to dimetallic species formed by halide bridges.34
Compounds 3, including 3bOH, also show this behavior. For
instance, the APCI/MS spectrum of 3c dissolved in CH3CN
shows the [M - Br + CH3CN]+ peak (m/z 492), together with
that corresponding to the dinuclear cation [M2 - Br]+ (m/z 982),
and the bis(chelate)metal ion [(2c)2NiBr]+ (m/z 763) probably
derived from the bis(chelate) dibromonickel complex produced
by the addition of the polar solvent.
ν
CdN band at ca. 1627 cm-1 present in all the derivatives, which
appears together with other two intense absorptions in the range
1590-1565 cm-1 assigned to the pyridine ring and, in some
cases, with a weaker band at ca. 1600 cm-1 due to the aromatic
C-C vibrations.
The nickel complexes 3a-c were synthesized by the dis-
placement of the labile ligand in [NiBr2(DME)] by the corre-
sponding chelate ligand 2 in dichloromethane (Scheme 1). These
compounds are soluble in the reaction solvent and in other polar
solvents such as acetone, THF, or acetonitrile, and insoluble in
aromatic or aliphatic hydrocarbons. Noteworthy is that their
solubility increases with the number of methyl groups on the
pyridylimine ligand. The reference complex 3bOH, accordingly
prepared from ligand 1b (Scheme 1), is found to be slightly
soluble in polar solvents. Compound 3bOH is isolated as a yellow
solid and 3a as an orange solid, whereas 3b and 3c are red
solids. In the solid state, they can be stored in air without
decomposition for unlimited time. However, attempts to crystal-
lize 3a from ethanol/pentane as a solvent mixture resulted in
the protolysis of the Si-O bonds and the redistribution of the
ligand, leading to the formation of red crystals of the hexaco-
ordinated complex 4. This compound was synthesized at a
preparative scale by an alternative procedure based on the
reaction of 2 equiv of the phenol 1a as the entering ligand and
[NiBr2(DME)] in ethanol (Scheme 1). It is known that the
formation of bis(chelate)nickel complexes bearing bidentate P/O
(34) (a) Sun, W.-H.; Zhang, W.; Gao, T.; Tang, X.; Chen, L.; Li, Y.;
Jin, X. J. Organomet. Chem. 2004, 689, 917. (b) Wilson, S. R.; Wu, Y.
Organometallics 1993, 12, 1478.

Effects of Dendritic Ni-Catalysts in Polymerization
Organometallics, Vol. 25, No. 16, 2006 3879
Figure 2. Partial IR spectra of the model ligand 2a, the dendritic ligands 5b, 6b, and 7b (top), and their corresponding mono- (3a) and
polymetallic (8b-10b) nickel complexes (bottom).
Figure 3. ORTEP diagram of the adduct 3c‚H2O.
The IR spectra of compounds 3 and 4 show a single strong
absorption at ca. 1589-1596 cm-1, together with two much
weaker bands, instead of the group of three intense absorptions
observed in the range 1565-1627 cm-1 for the imine and
pyridine groups in the free ligands (Figure 2). The disappearance
Figure 4. ORTEP drawing of enantiomer ∆ of compound 4.
or shift of the CdN vibration in the IR spectra of related
complexes is explained as a result of the bonding of the
pyridylimine ligand to the metal center, reducing the electron
density in the CdN bond and, consequently, leading to a lower
νCdN value.18c
Table 1. Selected Bond Distances (Å) and Angles (deg) for
3c‚H2O
Bond Distances
N(2)-Ni(1) 2.056(6)
N(1)-Ni(1) 2.028(7)
Ni(1)-O(2) 2.013(5)
Br(2)-Ni(1) 2.5009(16) C(7)-N(2)
N(2)-C(6) 1.285(10) C(12)-C(15) 1.508(10)
C(6)-C(5) 1.449(11) C(1)-C(13) 1.510(10)
1.425(10)
Molecular Structures of Compounds 3c and 4. The
molecular structure of mono(chelate) 3c and bis(chelate) 4 nickel
compounds has been determined by single-crystal X-ray dif-
fraction studies. Figures 3 and 4 show their ORTEP representa-
tions, while Tables 1 and 2 summarize relevant structural data.
Complex 3c crystallized from a solution in wet CH2Cl2.
Therefore, one H2O molecule is coordinated to the metal center
in order to compensate the electron deficit of the complex.
Increased coordination numbers (i.e., 5 or 6) are commonly
found in the solid state for nickel(II) compounds of this type,
achieved by bonding to donor ligands (e.g., H2O, MeCN) or
halide bridging, leading to dimeric structures.17b,c,34,35 The
molecular structure of 3c consists of discrete molecules, with a
five-coordinated nickel center. The coordination polyhedra can
be regarded as a slightly deformed trigonal bipyramid in which
the equatorial plane includes the nitrogen atoms of the chelating
ligand and the oxygen atom of the coordinated H2O. The
Br(1)-Ni(1) 2.5384(17) N(1)-C(5) 1.373(8)
Bond Angles
N(1)-Ni(1)-N(2)
N(2)-Ni(1)-O(2)
O(2)-Ni(1)-N(1)
Br(1)-Ni(1)-N(2)
Br(1)-Ni(1)-N(1)
Br(1)-Ni(1)-Br(2)
82.2(3)
160.5(3)
117.3(3)
95.53(19)
90.5(2)
Br(1)-Ni(1)-O(2)
Br(2)-Ni(1)-O(2)
C(7)-N(2)-C(6)
Si(1)-O(1)-C(10)
N(1)-C(1)-C(13)
85.5(2)
87.5(2)
120.6(6)
127.4(6)
116.7(8)
171.14(5)
bromine atoms occupy the axial coordination sites and are
somewhat tilted from the perpendicular position to the equatorial
plane, toward the H2O molecule, due to the steric repulsion
between the large bromine substituents and the pyridylimine
ligand. The positioning of the coordinated H2O is asymmetrical
with the respect to the chelate ligand, being closer to the pyridine
donor atom (bond angle O(2)-Ni(1)-N(1) ) 117.3°) and
almost trans to the imine group (O(2)-Ni(1)-N(2) ) 160.5-

3880 Organometallics, Vol. 25, No. 16, 2006
Benito et al.
Scheme 2
Table 2. Selected Bond Distances (Å) and Angles (deg) for
Complex 4
and compared to the structure of 3c, the aryl groups are rotated
with respect to the N-Ni-py plane (torsion angle C(1)-N(1)-
C(7)-C(8) ) 41.7°) and are substituted by hydroxy instead of
siloxy groups.
Bond Distances
Ni(1)-Br(1)
2.5930(3) Ni(1)-N(1) 2.1387(14) N(1)-C(1) 1.288(2)
Ni(1)-Br(1a)a 2.5930(3) O(1)-C(10) 1.363(2)
C(1)-C(2) 1.459(2)
C(2)-N(2) 1.352(2)
Synthesis and Characterization of Dendrimers. The py-
ridylimine ligands described above have been used as linkers
between carbosilane dendrimers and metal complexes. Thus,
the methodology applied for the preparation of ligands 2 and
metal complexes 3 has been extended to synthesize G1-
ONNMem (5), G2-ONNMem (6), G3-ONNMem (7), and their
polymetallic nickel derivatives (8-10, Scheme 2, Figure 5).
Dendritic ligands G1-ONNMem (5a, m ) 0; 5b, m ) 2; 5c,
m ) 3), G2-ONNMem (6a, m ) 0; 6b, m ) 2; 6c, m ) 3), and
G3-ONNMem (7b, m ) 2; 7c, m ) 3) were obtained by
phenolysis of the terminal silyl-chloride groups in carbosilanes
G1-Cl, G2-Cl, and G3-Cl,38 with pyridylimines 1a-c in the
presence of NEt3 in THF. After workup, they were isolated in
good yield as yellow (5a-c) or brownish (6a-c and 7b,c)
viscous oils. Their solubility increases with the number of
methyl groups on the pyridylimine ligand and diminishes with
the increase of dendrimer generation. MALDI-TOF mass spectra
carried out for 5b,c and 6b,c, using dithranol as a matrix, show
the protonated parent peak [M + H]+. The complete substitution
of the chloride atom at the end of the branches in the starting
dendrimers Gn-Cl by pyridylimine groups in 5-7 was confirmed
Ni(1)-N(2)
2.0833(14) N(1)-C(7) 1.428(2)
Bond Angles
N(1)-Ni(1)-N(2)
N(1)-Ni(1)-N(2a)a
N(1a)-Ni(1)-N(2)a
N(1)-Ni(1)-N(1a)a
Br(1)-Ni(1)-N(2)
78.62(5)
98.24(5)
98.24(5)
88.06(7)
94.81(4)
N(2)-Ni(1)-Br(1a)a
88.26(4)
175.70(8)
173.32(4)
91.70(4)
89.30(1)
N(2)-Ni(1)-N(2a)a
N(1)-Ni(1)-Br(1)
N(1)-Ni(1)-Br(1a)a
Br(1)-Ni(1)-Br(1a)a
a Symmetry transformations used to generate equivalent atoms a: -x,
y, -z+1/2.
(3)°). Worth mentioning is that the aryl ring lies nearly on the
plane formed by the metal, the imine nitrogen, and the pyridine
ring (torsion angle C(8)-C(7)-N(2)-C(6) ) 9.5°), whereas
in related complexes with dimeric structure, as two bridging
bromine atoms connect the metal centers, the phenyl ring adopts
a position close to perpendicular to the pyridine ring.17c The
distances N(2)-C(6) and C(6)-C(5) are indicative of localiza-
tion of the imine double bond.
Complex 4 crystallized by slow diffusion of a pentane layer
into a concentrated solution in ethanol. The asymmetric part of
the unit cell consists of half of the molecule and a crystallization
H2O molecule, and the whole molecule is formed by two halves
related by a binary axis. The nickel atom is a six-coordinated
center with a distorted octahedral geometry, with two bromine
atoms and two N/N chelating ligands. Although the coordination
number 6 is frequent in nickel chemistry, a small number of
bis(pyridylimine) octahedral compounds are known so far.36 The
positioning of the ligands around the metal center in compound
4 follows the maximum hardness principle (MHP), and there-
fore, the hardest ligand (imine) is in trans position to the softest
one (bromo).37 Figure 4 depicts enantiomer ∆, although the two
possible enantiomers are present in the unit cell. In this case
(35) (a) Jie, S.; Zhang, D.; Zhang, T.; Sun, W.-H.; Chen, J.; Ren, Q.;
Liu, D.; Zheng, G.; Chen, W. J. Organomet. Chem. 2005, 690, 1739. (b)
Tang, X.; Sun, W.-H.; Gao, T.; Hou, J.; Chen, J.; Chen, W. J. Organomet.
Chem. 2005, 690, 1570. (c) Britovsek, G. J. P.; Baugh, S. P. D.; Hoarau,
O.; Gibson, V. C.; Wass, D. F.; White, A. J. P.; Williams, D. J. Inorg.
Chim. Acta 2003, 345, 279.
(36) (a) Garoufis, A.; Kasselouri, S.; Raptopoulou, C. P.; Terzis, A.
Polyhedron 1999, 18, 585. (b) Riggle, K.; Lynde-Kernell, T.; Schlemper,
E. O. J. Coord. Chem. 1992, 25, 117.
(37) (a) Pearson, R. G. J. Chem. Edu. 1999, 76, 267. (b) Arnaiz, A.;
Garcia-Herbosa, G.; Cuevas, J. V.; Lavastre, O.; Hillairet, C.; Toupet, L.
Collect. Czech. Chem. Commun. 2002, 67, 1200.

Effects of Dendritic Ni-Catalysts in Polymerization
Organometallics, Vol. 25, No. 16, 2006 3881
the range 1565-1627 cm-1 for the corresponding free dendritic
ligand (representative examples in Figure 3).
Ethylene Catalyses Using Pyridylimine Nickel Com-
pounds. Nickel monometallic (3, 4) and dendritic compounds
(8-10) were activated using MAO and screened for ethylene
catalysis under the same mild conditions and concentration of
nickel centers. The nickel compounds combined with the
cocatalyst became readily soluble in toluene, and we found the
starting material [NiBr2(DME)] to be totally inactive under the
same reaction conditions. The catalytic results are summarized
in Table 3.
As shown in the table, roughly all catalysts gave a toluene-
soluble fraction of oligomers together with solid polyethylene
(PE), with low to moderate activities according to Gibson’s
classification.39 The oligomeric oily products are mainly com-
posed of even olefins up to C34, with number average molecular
weight in the range Mn ) 220-370, and they are characterized
by Schulz-Flory chain length distributions with R values from
0.45 to 0.70 (R ) kp/(kp + kct) ) mol of Cn+2/mol of Cn; kp )
rate of propagation, kct ) rate of chain transfer).40 Small amounts
of even alkanes (<5 wt %) are also detected in the GC analysis,
which is consistent with the occurrence of chain transfer to
“free” trimethylaluminum present in the MAO solution used
as cocatalyst.41,42 Among the olefinic products obtained, the
selectivity in R-olefins is between 45 and 65 mol %, and the
remainder are almost exclusively 2-alkenes (cis- and trans-CH3-
CHdCHR). This observation, together with the relatively low
branching observed by 1H NMR (3-5 branches per 100 C, for
C4 to C34 oligomers),16b,c indicates that chain walking of the
oligomerization-active species proceeds, in most cases, as much
to the â-carbon under these experimental conditions.
A much higher degree of branching is found in the solid
polymers. Some of them are as densely branched as PE polymers
prepared from palladium-diimine catalysts under comparable
conditions, and considered hyperbranched.43 It is noteworthy
that the later class of catalysts produces from linear to dendritic
PE by tuning the ethylene pressure or the substitution on the
ancillary ligand.44 Tm values of all samples (116-125 °C, Table
3) are in agreement with their level of chain branching and are
lower than those typically found for HDPE (g135 °C). GPC
analysis of the solid materials shows that the weight average
molecular weights are low (Mw ) 1600-177 000) with wide
distributions (PDI ) 3-20). The quantitative 13C NMR analyses
of the polymers formed (a representative spectrum is shown in
Figure 6) show branching densities in the range of 60-170 total
branches per 1000 main-chain carbons (Table 3), and according
to Galland et al.,45 an abundant amount of branches are relatively
long (g6 C). In addition, all peaks in the carbon-13 spectra
Figure 5. Metallodendrimer 10b.
by 1H and 13C{1H} NMR. 29Si{1H} NMR spectra show a singlet
for every generation silicon nuclei with chemical shifts almost
insensitive to the dendrimer generation and to the pyridylimine
group substitution: δ 18-19 for the SiMe2 groups and 0-1
for the central Si atom and the inner SiMe groups. The IR
spectra of all dendritic ligands 5-7 show the characteristic group
of three intense absorptions mentioned above for the imine group
and the pyridine ring (for examples see Figure 3).
Dendrimers 5-7 are useful supports to coordinate 4, 8, or
16 metal centers through the asymmetrical N/N bidentate
terminal groups (Scheme 2, Figure 5). Thus, reactions with
[NiBr2(DME)] in CH2Cl2 led to the formation of metalloden-
drimers 8-10, which were isolated as mustard-yellow or orange
paramagnetic air-stable solids that can be stored in conventional
vials exposed to air without decomposition. They are only
slightly soluble in solvents such as CH2Cl2 or THF, and again,
their solubility increases with the number of methyl subtituents
on the pyridylimine group and decreases with the generation
number of the dendrimer. The solubility is a relevant fact since
low solubilities severely limit the preparation of high-generation
dendrimers.
The paramagnetism of nickel dendrimers 8-10 makes
ineffective their characterization by NMR studies. They were
characterized by elemental analysis and IR. Mass spectrometry
was only informative in the case of G1-ONNMe3NiBr2 (8c),
which was soluble enough in CH2Cl2 to allow the formation of
a solid sample in a dithranol matrix by evaporation. The
MALDI-TOF mass spectrum from that sample exhibits a peak
at m/z ) 2180.3 corresponding to the [M - Br]+ fragment, the
cation after the loss of only one of the eight bromine atoms
present. The IR spectra of all metallodendrimers 8-10 are also
consistent with coordination of the pyridylimine groups, showing
a strong absorption at ca. 1597 cm-1, together two weaker bands,
instead of the group of three intense absorptions observed in
(39) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int.
Ed. 1999, 38, 428.
(40) (a) Schulz, G. V. Z. Phys. Chem., Abt. B 1935, 30, 379. (b) Flory,
P. J. J. Am. Chem. Soc. 1940, 62, 1561.
(41) (a) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem.
Soc. 1998, 120, 4049. (b) Chen, Y.; Chen, R.; Qian, C.; Dong, X.; Sun, J.
Organometallics 2003, 22, 4312. (c) Britovsek, G. J. P.; Cohen, S. A.;
Gibson, V. C.; van Meurs, M. J. Am. Chem. Soc. 2004, 126, 10701.
(42) Even PMAO-IP (polymethylaluminoxane, improved process), com-
mercialized by Akzo-Nobel, is known to contain some residual TEA. For
instance see: Busico, V.; Cipullo, R.; Cutillo, F.; Friederichs, N.; Ronca,
S.; Wang, B. J. Am. Chem. Soc. 2003, 125, 12402.
(43) Cotts, M. P.; Guan, Z.; McCord, E.; McLain, S. Macromolecules
2000, 19, 6945.
(44) (a) Popeney, C.; Guan, Z. Organometallics 2005, 24, 1145. (b) Chen,
G.; Guan, Z. J. Am. Chem. Soc. 2004, 126, 2662. (c) Guan, Z. Chem. Eur.
J. 2002, 8, 3087. (d) Guan, Z.; Cotts, P. M.; McCord, E. F.; McLain, S. J.
Science 1999, 283, 2059.
(45) Galland, G. B.; de Souza, R. F.; Mauler, R. S.; Nunes, F. F.
Macromolecules 1999, 32, 1620.
(38) (a) van der Made, A. W.; van Leeuwen, P. W. N. M. J. Chem.
Soc., Chem. Commun. 1992, 1400. (b) Alonso, B.; Cuadrado, I.; Mora´n,
M.; Losada, J. J. Chem. Soc., Chem. Commun. 1994, 2575. (c) Seyferth,
D.; Kugita, T.; Rheinglod, A. L.; Yap, G. P. A. Organometallics 1995, 14,
5362.

3882 Organometallics, Vol. 25, No. 16, 2006
Benito et al.
Table 3. Ethylene Oligomerization/Polymerization Catalyzed by Monometallic and Dendritic Pyridyliminine Nickel Precursorsa
oligomers
polymer
Cn branches/1000 Ci
(°C)h n ) 1/2-5/g6 total Mw
oligomer
R-olefin
branches/ polymer
polymer
Tm
10-3
f
j
entry
precatalyst
TOFb,c (h-1
)
Rd,e (mol %)e Mn
100 Cf,g yield (g) TOFb (h-1
)
PDIj
1
2
3
4
5
6
7
8
9
3a
240
0.63
0.70
66
48
219.6
265.8
3
3
0.30
8.90
0.08
1.40
0.05
0.07
0.20
10.02
3.45
3.24
0.02
32
944
8
149
5
7
21
1063
366
344
2
120.3
117.1
121.4
122.0
121.4
119.6
121.3
120.5
123.2
124.7
119.0
12/24/80
12/36/97
66/41/64
10/18/48
n.d.l
11/21/56
15/10/34
17/22/62
13/11/38
12/10/36
4/35/19
116
145
171
76
2.75 2.74
1.64 5.58
6.01 4.19
5.88 6.61
3b
3c
372
tracesk
264
3bOH
4
0.65
45
341.1
4
tracesk
293
n.d.l 11.32 11.35
8a (G1-ONNNiBr2)
9a (G2-ONNNiBr2)
8b (G1-ONNMe2NiBr2)
9b (G2-ONNMe2NiBr2)
0.67
0.61
0.60
0.51
0.45
60
62
55
46
45
286.1
371.9
284.5
342.5
327.6
3
5
3
3
3
88
59
101
62
58
58
27.89 20.12
177.04 2.84
2.16 2.77
10.62 11.25
17.07 14.06
n.d.l n.d.l
306
594
729
10 10b (G3-ONNMe2NiBr2)
11 8c (G1-ONNMe3NiBr2)
957
tracesk
aPolymerization conditions: 50 mL of toluene; n(Ni) ) 14 µmol; Al/Ni ) 1000; tp ) 24 h; Tp ) 20 °C; PC2H4 ) 2 bar; activities ( 7%. b Turnover
frequency: mol of C2H4 converted/(molNi × h). c Corrected to consider the lower olefin lost during workup. d Schulz-Flory parameter, R ) rate of propagation/
(rate of propagation + rate of chain transfer).40 e Determined by GC. f Determined by 1H NMR.16b,c g Total number of methyl per 100 C. h Determined by
DSC (second heating run). i Determined by 13C NMR.45 j Determined by GPC. k Less than 3 mg of very impure oligomers. l Not determined.
Figure 7. Variation with time of Tp and yield of PE and oligomers
produced by 3b.
Figure 6. 13C NMR spectrum of PE made using catalyst 8b.
The production of oligomers and polymers by eventual
formation of bis(pyridylimine)nickel species or splitting of the
Si-O bond by the addition of the cocatalysts is excluded
because of the observed performances of compounds 3bOH and
4 (entries 4 and 5, Table 3). The latter is scarcely active, and
the former also gives the two types of ethylene products,
although behaving differently than 3b and related dendrimers.
Therefore, protection at the phenolic oxygen, even with a very
simple carbosilane group (SiMe3), seems to be positive in terms
of polymerization activity (compare the performance of 3bOH
vs 3b), and the presence of a second chelate ligand deactivates
the metal center probably by saturation of the coordination
sphere (compare 4 vs 3bOH).
were assigned except two low-intensity resonances (δ 17.7 and
12.7), which are believed to be due to a branch-on-branch
structure.43 These findings imply that polymerization-active
species walk along the chain much further than the oligomer-
ization ones.
Although oligomers and polymers might be formed by the
same or by two different active centers, the latter is more
feasible. It has been recently shown that C1 symmetry precursors
lead simultaneously to PE or oligomeric products depending
on the catalytic face where alkyl propagation takes place.46 This
could be the case for compound 3b and related metalloden-
drimers 8b-10b, with a hindered rotation of the aryl group
about the C-N bond. However the parent compound 3a is a Cs
symmetric precursor and behaves similarly, producing R-olefins
and polymer.
The effect on the catalytic behavior of the methyl substituent
in the 6-position of the ligand pyridine ring in compounds c
(3c and 8c) concurs with earlier findings.17c,19a Thus, the
activities in oligomerization or polymerization for both com-
pounds are almost negligible as a result of steric obstruction
exerted by the methyl group pointing to the equatorial coordina-
tion sites. On the other hand, it is well established that ligand
substitution on the ortho aryl positions produces a steric
protection of the axial coordination sites, which has a profound
effect on the catalytic activity of this type of precursors and on
the properties of the catalytic products.1-3 Furthermore, it has
been described that catalysts without enough steric bulk in those
ortho positions of the pyridylimine ligand are more easily
deactivated through the interaction with the alkylaluminum
Moreover, these active species seem to have different
lifetimes. Figure 7 illustrates the variation of yields and
temperature over a 24 h period using 3b; similar profiles are
observed for the other mono- and polymetallic precursors. The
data reveal that almost 90% of the oligomers are alredy formed
30 min after the reaction start, coinciding with the exotherm
maximum, whereas the polymerization-active species show long
lifetimes under these conditions, with low loss of activity over
the period studied.
(46) Bianchini, C.; Giambastiani, G.; Guerrero, I. R.; Meli, A.; Passaglia,
E.; Gragnoli, T. Organometallics 2004, 23, 6087.

Effects of Dendritic Ni-Catalysts in Polymerization
Organometallics, Vol. 25, No. 16, 2006 3883
These results might be interpreted as a consequence of the
combination of the “steric pressure” on the growing chains and
“microenvironment protection” of higher generation dendrimers
on the polymerization species. Their bulkiness enhances chain
transfer processes and favors the performance of the oligomer-
ization species, as reflected by a decrease in the R parameter
and an increase in activity, respectively. A larger size also affects
the polymerization-active species, preventing the catalyst chain-
walking process from going too deep into the coiled polymer
chain (see Figure 9), leading to less branched polymers. At the
same time, the polymers are probably produced by more
protected active centers in larger dendrimers, with lower activity,
but higher Mw (and PDI). For instance, the latter is reasonable
if back-folding of terminal active species into the dendrimer
matrix becomes significant at higher generation.
-1
Figure 8. Turnover frequencies (mol of C2H4 converted × molNi
× h-1) producing oligomers and polymers by Gn-ONNMe2NiBr2
Conclusions
under the conditions described in Table 3.
cocatalyst.47 The data summarized in Table 3 are in agreement
with these remarks and show a trend in activity: compounds b
> a > c. Compounds 3a, 8a, and 9a show oligomerization
turnover frequencies slightly inferior and polymerization activi-
ties much lower than those corresponding to 3b, 8b, and 9b. In
general, the selectivity for R-olefins is higher for the nonsub-
stituted catalysts a, whereas substituted compounds b give rise
to more branched polymers, with lower molecular weights.
Several trends arise when analyzing the data in Table 3 with
regard to the metallic nuclearity or generation of the precursors.
The activity leading to oligomers slightly increases (see entries
1, 6, and 7 for precatalysts a and entries 2, 8, 9, and 10 for b),
whereas the activity producing polymer diminishes for higher
generations (see corresponding entries for the a, b, and c series
in Table 3). For instance, Figure 8 depicts turnover frequencies
in the production of polymer and oligomer with precursors b
(3b, 8b, 9b, and 10b) under the same reaction conditions. It is
shown that monometallic compound 3b is superior, yielding
PE rather than oligomer, while the contrary is observed for
polynuclear dendrimer 10b, with 16 Ni centers. The Schulz-
Flory parameter (R) for the most active compounds b also
decreases with increasing nuclearity. Conversely, higher mo-
lecular weights (and PDIs) are generally observed for polymers
produced by higher generations. The branching degree of the
polymers is also generation-dependent and decreases progres-
sively. For instance, it drops from 145 for the monometallic
complex 3b to 58 branches per 1000 C for G3 dendrimer 10b;
this reduction in the number of branches is larger for the longest
branches than for methyl ones.
In summary, the carbosilane dendrimers containing peripheral
pyridylimine groups reported herein represent a new set of
versatile dendritic ligands that have been used for the preparation
of new nickel metallodendrimers. Upon activation with MAO
under mild conditions, the nickel derivatives are active for
ethylene oligomerization and polymerization, and their catalytic
properties are definitely sensitive to the ligand structure and
the dendrimer generation. Although the precise nature of the
active species for each type of insertion product remains elusive
to date, significant ligand and dendrimer effects have emerged.
Steric protection of axial coordination sites by methyl substit-
uents on the aryl ring increases the activities for both oligomers
and polyethylene. However, the presence of a methyl group on
the 6-position of the pyridine moiety, hindering the equatorial
coordination sites, causes the reverse. The size of the dendrimer
regulates the production of ethylene insertion products (oligomer
vs polymer), the oligomer chain-length distribution (R), and the
branching density, molecular weight, and polydispersity of the
polymers. These findings may be useful for controlling the
structure/properties of ethylene insertion products.
When the pioneering work on dendrimer-bound metal cata-
lysts was published in 1994, it was remarked that such systems
had the adequate shape and size to be removed from the solution
of products using nanofiltration methods, at the same time
retaining the characteristics found for the monomeric model.48
If the metal centers are directly available for the substrate in
the periphery-type of dendrimeric catalysts, as in mononuclear
homogeneous catalysts, then comparable activities and selectivi-
ties are expected for both systems. However, dendritic effects
Figure 9. Representation of a branched polyethylene chain growing at the dendrimer periphery.

3884 Organometallics, Vol. 25, No. 16, 2006
Benito et al.
operating at 135 °C with 1,2,4-trichlorobenzene (TCB) solvent and
polystyrene calibration standards.
Preparation of N-(4-Hydroxyphenyl)-2-pyridylmethanimines
(1). Compound 1a was synthesized as described in the literature,22a
whereas 1b and 1c were synthesized by adapting this procedure as
already reported for 1b.33d
on activity, both positive and negative, or selectivity have been
observed and ascribed to the proximity of active sites and steric
crowding on the surface of the dendrimers.49 Whereas catalyst
recycling plays a central role with palladium and other rare
catalysts, oligomerization/polymerization of olefins with den-
drimers can benefit from improvements in the stability of active
species or in the control of the microstructure, molecular weight,
or stereochemistry of the polymers. Recently, the activity of a
(P,O)nickel complex has been improved by embedding such a
catalyst in a dendritic framework, which restricts the formation
of inactive bis(P,O)nickel complexes.30a In this paper we have
shown an example in which the size of the dendrimer regulates
the characteristics of ethylene insertion products. It is noteworthy
that this dendritic effect has been obtained with peripheral
metallodendrimers when core or focal-point dendrimers are a
priori more suited systems for these purposes.
1c. The preparation was similar to that of 1b but starting from
6-methylpyridine-2-carbaldehyde (0.88 g, 7.3 mmol) and 4-amino-
2,5-dimethylphenol (1.00 g, 7.29 mmol). Compound 1c was found
to be soluble in toluene, CH2Cl2, and THF and insoluble in hexanes.
The yellow solid could be purified by recrystallization from toluene
or by sublimation (160 °C, 10-3 mmHg). Yield: 1.50 g (86%).
Anal. Calc for C15H16N2O (240.30): C, 74.97; H, 6.71; N, 11.66.
Found: C, 74.87; H, 6.72; N, 11.69. 1H NMR (CDCl3): δ 2.21 (s,
3H, Me14), 2.33 (s, 3H, Me13), 2.62 (s, 3H, Me15), 5.16 (br s, 1H,
OH), 6.64 (s, 1H, H3), 6.90 (s, 1H, H6), 7.19 (d, 1H, JH,H ) 7.5
Hz, H11), 7.66 (pt, 1H, H10), 8.03 (d, 1H, JH,H ) 7.7 Hz, H9), 8.48
(s, 1H, H7). 13C{1H} NMR (CDCl3): δ 15.6 and 17.5 (Me13,14),
24.4 (Me15), 116.8 (C3), 118.4 (C6), 119.8 (C9), 121.8 (C5), 124.3
(C11), 132.4 (C2), 136.8 (C10), 142.5 (C1), 152.7 (C4), 154.6 (C8),
157.6 (C7) 158.0 (C12). IR (KBr): ν 1620 cm-1 (s, CdN), 1607
(w, CdC), 1594 and 1573 cm-1 (s and vs, py-ring). MS (70 eV,
EI): m/z 240 [M]+.
Preparation of N-(4-Trimethylsiloxyphenyl)-2-pyridylmetha-
nimines (2). Compounds 2a-c were prepared in high yields (80-
90%), starting from the corresponding pyridylimine 1, as described
previously for 2a22a and 2b.33d The former was alternatively
synthesized as follows.
2a. 4-Trimethylsiloxyaniline (0.80 g, 4.4 mmol),33a pyridine-2-
carbaldehyde (0.48 g, 4.5 mmol), MgSO4 (5 g), and ethyl acetate
(60 mL) were heated under reflux for 2 h and then stirred overnight
at room temperature. The solvent was removed under reduced
pressure, and the residue extracted with diethyl ether (2 × 30 mL).
Removal of the ether afforded spectroscopically pure 2a as an
orange-yellow oil, soluble in all common organic solvents. Yield:
0.95 g (80%).
Experimental Section
Reagents and General Techniques. All operations were per-
formed under an argon atmosphere using Schlenk or drybox
techniques. Unless otherwise stated, reagents were obtained from
commercial sources and used as received. [NiBr2(DME)],50 and
carbosilane dendrimers Gn-Cl (n ) 1, 2, 3)38 were prepared
according to literature procedures. Solvents were previously dried
prior to use and distilled under argon as described elsewhere.51
NMR spectra were recorded on Varian Unity 500+, Varian Unity
VR-300, or Varian Unity 200 NMR spectrometers. Chemical shifts
(δ) are reported in ppm referenced to SiMe4 for 1H, 13C, and 29Si,
and assignments for the iminopyridyl ligands are according to the
numbering scheme depicted in Figure 1. IR spectra were recorded
on a Perkin-Elmer FT-IR Spectrum-2000 spectrophotometer.
Elemental analyses and mass spectra were performed by the
Microanalytical Laboratories of the University of Alcala´ (MLUAH)
on a Heraeus CHN-O-Rapid mycroanalyzer and on a Hewlett-
Packard 5988A Quadruplo (EI) or a Thermo Quest Finningan
Automass Multi (ESI) mass spectrometer. MALDI-TOF MS were
performed by the Analytical Services of the Universidad Auto´noma
de Madrid in a Bruker Reflex II spectrometer, using samples in a
1,8,9-trihydroxyanthracene (dithranol) matrix. Polymethylalumi-
noxane improved process (PMAO-IP) in toluene (13 wt % Al) was
purchased from Akzo Nobel. Oligomer products were analyzed on
a Chrompack CP 9001 gas chromatograph using a CP-Sil 5CB
capillary column (10 m, 0.25 mm i.d., 0.12 µm df) under the
following conditions: injector and detector temperature, 250 °C;
oven temperature program, 100 °C/5 min, 5 °C/min ramp, 250 °C/
15 min. DSC melting endotherms of polymers were performed on
a Perkin-Elmer DSC 6 instrument. Polyethylene molecular weight
determinations were made by MLUAH on a Waters GPCV 2000
2c: orange-yellow oil. Yield: 91%. Anal. Calc for C18H24N2-
OSi (312.49): C, 69.19; H, 7.74; N, 8.96. Found: C, 68.93; H,
1
7.66; N, 8.87. H NMR (CDCl3): δ 0.26 (s, 9H, SiMe3), 2.14 (s,
3H, Me14), 2.34 (s, 3H, Me13), 2.60 (s, 3H, Me15), 6.63 (s, 1H,
H3), 6.92 (s, 1H, H6), 7.17 (d, 1H, JH,H) 7.7, H11), 7.65 (pt, 1H,
H10), 8.03 (d, 1H, JH,H ) 7.7 Hz, H9), 8.49 (s, 1H, H7). 13C{1H}
NMR (CDCl3): δ 0.6 (SiMe3), 16.3 and 17.6 (Me13,14), 24.4 (Me15),-
118.3 (C3), 119.7 (C6), 120.7 (C9), 124.2 (C11), 126.8 (C5), 131.6
(C2), 136.6 (C10), 142.9 (C1), 152.4 (C4), 154.6 (C8), 157.7 (C7),
158.0 (C12). 29Si{1H} NMR (CDCl3): δ 18.9 (SiMe3). IR (Nujol/
CsI): ν 1627 cm-1 (s, CdN), 1589 and 1572 cm-1 (s, py-ring).
MS (70 eV, EI): m/z 313 [M]+.
Preparation of Nickel Complexes (3, 4). The synthesis of the
nickel compounds 3a-c is exemplified by the preparation of 3a.
The syntheses of compounds 3b and 3bOH have been reported in a
preliminary communication.33d
(47) Britovsek, G. J. P.; Mastroianni, S.; Solan, G. A.; Baugh, S. P. D.;
Redshaw, C.; Gibson, V. C.; White, A. J. P.; Williams, D. J.; Elsegood, M.
R. J. Chem. Eur. J. 2000, 6, 2221.
NiBr2(Me3SiO-4-Ph-NdCH-2-Py) (3a). Ligand 2a (1.18 g, 4.36
mmol), [NiBr2(DME)] (1.34 g, 4.34 mmol), and dichloromethane
(50 mL) were placed in a Schlenk tube and stirred at room
temperature for 48 h. During the course of the reaction the dark
mixture slowly changed to orange. The solvent was removed in
vacuo and the residue washed with hexane (3 × 20 mL) and cold
CH2Cl2 (2 × 10 mL) to give 3a as an orange paramagnetic solid.
Yield: 1.80 g (85%). Anal. Calc for C15H18N2OSiBr2Ni (488.91):
C, 36.85; H, 3.71; N, 5.73; O, 3.27. Found: C, 37.14; H, 3.92; N,
5.79; O, 3.13. IR (Nujol/CsI): ν 1624 cm-1 (w), 1596 cm-1 (vs,
CdN), 1559 cm-1 (w). MS (70 eV, EI): m/z 409 [M - Br]+, 270
[2a]+; (ESI+ in CH3CN) m/z 450 [M - Br + CH3CN]+.
NiBr2[Me3SiO-4-(2,5-Me2Ph)-NdCH-2-(6-MePy)] (3c): red,
paramagnetic solid. Yield: 68%. Anal. Calc for C18H24N2OSiBr2-
Ni (530.99): C, 40.72; H, 4.56; N, 5.28. Found: C, 40.70; H, 4.17;
(48) Knapen, J. W. J.; van der Made, A. W.; de Wilde, J. C.; van
Leeuwen, P. W. N. M.; Wijkens, P.; Grove, D. M.; van Koten, G. Nature
1994, 372, 659.
(49) (a) Mizugaki, T.; Ooe, M.; Ebitani, K.; Kaneda, K. J. Mol. Catal.
A: Chem. 1999, 145, 329. (b) Kleij, A. W.; Gossage, R. A.; Jastrzebski, J.
T. B. H.; Boersma, J.; van Koten, G. Angew. Chem., Int. Ed. 2000, 39,
176. (c) Breinbauer, R.; Jacobsen, E. N. Angew. Chem., Int. Ed. 2000, 39,
3604. (d) Kleij, A. W.; Gossage, R. A.; Gebbink, R. J. M. K.; Brinkmann,
N.; Reijerse, E. J.; Kragl, U.; Lutz, M.; Spek, A. L.; van Koten, G. J. Am.
Chem. Soc. 2000, 122, 12112. (e) Ropartz, L.; Morris, R. E.; Foster, D. F.;
Cole-Hamilton, D. J. Chem. Commun. 2001, 361. (f) Dahan, A.; Portnoy,
M. Chem. Commun. 2002, 2700. (g) Drake, M. D.; F. V.; Bright, M. R.
Detty, J. Am. Chem. Soc. 2003, 125, 12558. (h) Delort, E.; Darbre, T.;
Reymond, J.-L. J. Am. Chem. Soc. 2004, 126, 15642.
(50) Ward, L. G. L. In Inorganic Syntheses; Cotton, F. A., Ed.; McGraw-
Hill: New York, 1972; Vol. 13, p 154.
(51) Perrin, D. P.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: Oxford, 1988.

Effects of Dendritic Ni-Catalysts in Polymerization
Organometallics, Vol. 25, No. 16, 2006 3885
Table 4. Crystal Data and Structure Refinement for Compounds 3c and 4
3c‚H2O 4‚(H2O)2
C18H26Br2N2NiO2Si
empirical formula
fw
C24H24Br2N4NiO4
650.98
549.03
color
red
red
temperature
wavelength
cryst syst, space group
unit cell dimens
200(2) K
100(2) K
0.71073 Å
0.71073 Å
monoclinic, P21n
a ) 14.797(7) Å
orthorombic, Pbcn
a ) 15.3360(3) Å
b ) 9.4940(8) Å
c ) 16.9100(8) Å
2462.1(2) Å3
b ) 7.446(4) Å; â ) 107.87(3)°
c ) 21.455(11) Å
2250(2) Å3
volume
Z, calcd density
absorp coeff
4, 1.621 g/cm3
4, 1.756 g/cm3
44.8 cm-1
40.73 cm-1
F(000)
1104
1304
cryst size
θ ranges
limiting indices
no. of reflns collected/unique
no. of reflns obsd
absorp corr
0.35 × 0.27 × 0.2 mm
0.4 × 0.35 × 0.3 mm
5.00 to 27.53°
2.41 to 27.50°
-15 e h e 19, -9 e k e 8, -27 e l e 25
11527/4876 [R(int) ) 0.11781]
2767
-19 e h e 19, -12 e k e 12, -18 e l e 21
48364/2829 [R(int) ) 0.0381]
2502 [I > 2σ(I)]
none
multiscan
max. and min. transmn
refinement method
no. of data/restraints/params
goodness of fit on F2
final Ra indices [I > 2σ(I)]
R indices (all data)
largest diff peak and hole
1.476, 079
full-matrix least-squares on F2
4876/0/263
full-matrix least-squares on F2
2829/0/167
0.933
R1 ) 0.021, wR2 ) 0.0506
R1 ) 0.0259, wR2 ) 0.053
0.477 and -0.638 e/Å3
1.014
R1 ) 0.0727, wR2 ) 0.1341
R1 ) 0.1476, wR2 ) 0.1616
1.017 and -1.205 e/Å3
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) {[∑w(Fo - Fc )]/[∑w(Fo )2]}1/2
.
N, 5.14. IR (KBr): ν 1627 cm-1 (w), 1595 cm-1 (vs, CdN), 1565
cm-1 (w). MS (70 eV, EI): m/z 531 [M]+, 451 [M - Br]+, 371
[M - 2Br]+, 312 [2c]+; (ESI+ and APCI in CH3CN) m/z 492 [M
- Br + CH3CN]+, 313 [2c + H]+.
in both structures. Hydrogen atoms were included from geometrical
calculations and refined using a riding model. All the calculations
were made using the WINGX system.53
Preparation of G1-Pyridylimine Dendritic Ligands (5). Ter-
minal-functionalized carbosilanes 5a-c were prepared as illustrated
for 5a. Characterization data for 5b have already been reported in
a previous account.33d
NiBr2(HO-4-Ph-NdCH-2-Py)2 (4). Ethanol (30 mL) was added
to a mixture of ligand 1a (0.77 g, 3.9 mmol) and [NiBr2(DME)]
(0.60 g, 1.94 mmol). The dark red solution was stirred overnight
at room temperature and then added dropwise over pentane (150
mL) and vigorously stirred. The precipitated solid was separated
by filtration and dried under vacuum to give 4 analytically pure,
as a red, paramagnetic solid. Yield: 1.12 g (94%). Crystalline 4
was also obtained in attempts to recrystallize 3a from ethanol/
pentane as a solvent mixture. Anal. Calc for C24H20N4O2Br2Ni
(614.95): C, 46.88; H, 3.28; N, 9.11. Found: C, 47.11; H, 3.42;
N, 8.99. IR (Nujol/CsI): ν 1623 cm-1 (m), 1592 cm-1 (vs, CdN),
1563 cm-1 (m). MS (70 eV, EI): m/z 197 [1a - H]+; (ESI+ in
CH3CN) m/z 535 [M - Br]+, 198 [1a]+.
X-ray Crystallographic Studies. Suitable monocrystals of 3c
were obtained from a dichloromethane solution exposed to air, and
by slow diffusion of a pentane layer into a ethanol solution in the
case of complex 4. A summary of crystal data, data collection, and
refinement parameters for the structural analysis is given in Table
4. Red crystals of the compounds 3c or 4 were glued to a glass
fiber using an inert polyfluorinated oil and mounted in an N2 stream
in a Bruker-Nonius Kappa-CCD diffractometer with area detector,
and data were collected using graphite-monochromated Mo KR
radiation (λ ) 0.71073 Å). Data for compound 3c were collected
at low temperature (200 K), with an exposure time of 34 s per
frame (four sets; 263 frames; omega scans 1.7° scan-width). Data
for compound 4 were collected at low temperature (100 K), with
an exposure time of 30 s per frame (six sets; 939 frames; phi and
omega scans 1.0° scan-width). Raw data were corrected for Lorenz
and polarization effects.
G1-ONN (5a). A solution of chlorosilane G1-Cl (2.45 g, 4.3
mmol) in THF (20 mL) was added via cannula to a stirred mixture
of 1a (3.45 g, 17.4 mmol) and Et3N (2.5 mL, 18 mmol) in THF
(40 mL) at 0 °C. The reaction was allowed to proceed at room
temperature overnight, precipitating Et3N‚HCl. The white solid was
separated by filtration, the solvent of the filtrate was removed at
reduced pressure, and the yellow residue was extracted with diethyl
ether or toluene (2 × 30 mL). The yellow viscous oil obtained
after solvent evaporation was characterized as 5a, which was found
to be soluble in all common organic solvents. Yield: 5.20 g (99%).
Anal. Calc for C68H84N8O4Si5 (1217.89): C, 67.06; H, 6.95; N,
1
9.20. Found: C, 66.40; H, 7.09; N, 9.36. H NMR (CDCl3): δ
0.22 (s, 6H, SiMe2), 0.58 (m, 2H, SiCH2), 0.80 (m, 2H, CH2SiMe2),
1.41 (m, 2H, CH2CH2CH2), 6.84 (AA′ part of an AA′BB′ spin
system, 2H, H3,5), 7.22 (BB′ part of an AA′BB′ spin system, 2H,
H2,6), 7.31 (m, 1H, H11), 7.75 (m, 1H, H10), 8.14 (d, 1H, JH,H ) 8.0
Hz, H9), 8.58 (s, 1H, H7), 8.66 (d, 1H, JH,H ) 4.8 Hz, H12). 13C-
{1H} NMR (CDCl3): δ -1.2 (SiMe2), 17.0 (CH2), 17.7 (CH2),
21.4 (CH2), 120.5 (C3,5), 121.6 (C9), 122.6 (C2,6), 124.8 (C11), 136.6
(C10), 144.2 (C1), 149.5 (C12), 154.7 (C4), 154.8 (C8), 158.2 (C7).
29Si{1H} NMR (CDCl3): δ 19.5 (SiMe2), 0.7 (central Si). IR (Nujol/
CsI): ν 1626 cm-1 (s, CdN), 1596 (w, CdC), 1582 and 1568 cm-1
(vs and s, py-ring).
G1-ONNMe3 (5c): yellow, viscous oil. Yield: 97%. Anal. Calc
for C80H108N8O4Si5 (1386.22): C, 69.32; H, 7.85; N, 8.08. Found:
1
C, 69.32; H, 8.25; N, 8.04. H NMR (CDCl3): δ 0.21 (s, 6H,
Structures were solved by direct methods, completed by subse-
quent difference Fourier techniques, and refined by full-matrix least
squares on F2 (SHELXL-97).52 Anisotropic thermal parameters were
used in the last cycles of refinement for the non-hydrogen atoms
SiMe2), 0.59 (m, 2H, SiCH2), 0.80 (m, 2H, CH2SiMe2), 1.42 (m,
2H, CH2CH2CH2), 2.12 (s, 3H, Me14), 2.32 (s, 3H, Me13), 2.59 (s,
3H, Me15), 6.60 (s, 1H, H3), 6.91 (s, 1H, H6), 7.16 (d, 1H, JH,H
)
7.5, H11), 7.63 (pt, 1H, H10), 8.01 (d, 1H, JH,H ) 7.5 Hz, H9), 8.48
(s, 1H, H7). 13C{1H} NMR (CDCl3): δ -0.9 (SiMe2), 16.4 and
(52) Sheldrick, G. M. SHELXL-97. Program for Crystal Structure
Refinement (Release 97-2); University of Go¨ttingen: Go¨ttingen, Germany,
1998.
(53) WinGX System. Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.

3886 Organometallics, Vol. 25, No. 16, 2006
Benito et al.
17.7 (Me13,14), 17.1 (CH2), 17.9 (CH2), 21.9 (CH2), 24.4 (Me15),
118.3 (C3), 119.6 (C6), 120.5 (C9), 124.1 (C11), 126.7 (C5), 131.7
(C2), 136.6 (C10), 142.8 (C1), 152.5 (C4), 154.7 (C8), 157.6 (C7),
158.0 (C12). 29Si{1H} NMR (CDCl3): δ 18.2 (SiMe2), 0.7 (central
Si). IR (Nujol/CsI): ν 1626 cm-1 (s, CdN), 1589 and 1573 cm-1
(vs and s, py-ring). MS (ESI+ in CH3CN): m/z 1087 [M - 1c -
SiMe2]+. MS (MALDI-TOF in dithranol): m/z 1386.7 [M + H]+.
Preparation of G2-Pyridylimine Dendritic Ligands (6). Car-
bosilane dendrimers 6a-c were prepared as illustrated for 6a,
starting from the corresponding hydroxyphenylpyridylimines 1.
Characterization data for 6b have already been reported in a
previous communication.33d Dendrimers 6b,c were purified by silica
gel chromatography using CH2Cl2 (6b) or ethyl acetate (6c) as
eluents.
2.31 (s, 12H, Me13), 2.58 (s, 12H, Me15), 6.59 (s, 4H, H3), 6.89 (s,
4H, H6), 7.13 (d, 4H, JH,H ) 7.0, H11), 7.60 (pt, 4H, H10), 7.99 (d,
4H, JH,H ) 7.2 Hz, H9), 8.46 (s, 4H, H7). 13C{1H} NMR (CDCl3):
δ -4.7 (SiMe, only one peak can be distinguished), -0.7 (SiMe2),
16.6 and 17.8 (Me13,14), 18.0, 18.7, 19.2, and 21.9 (CH2), 24.5
(Me15), 118.1 (C3), 119.5 (C6), 120.4 (C9), 124.0 (C11), 126.6 (C5),
131.5 (C2), 136.4 (C10), 142.6 (C1), 152.3 (C4), 154.4 (C8), 157.3
(C7), 157.6 (C12). 29Si{1H} NMR (CDCl3): δ 17.9 (SiMe2), 0.7
(outermost SiMe), 0.5 (SiMe), 0.0 (central Si). IR (Nujol/CsI): ν
1626 cm-1 (s, CdN), 1589 and 1572 cm-1 (s, py-ring).
Preparation of G1-Pyridylimine-NiBr2 Dendritic Complexes
(8). Compounds 8a-c were synthesized as described above for 3
starting from the corresponding G1-pyridylimine dendritic ligands
(5). The preparation of 8a is reported as an example, whereas
characterization data for 8b have been reported previously.33d
G1-ONNNiBr2 (8a). [NiBr2(DME)] (697 mg, 2.26 mmol) was
added to a solution of compound G1-ONN (5a) (688 mg, 0.56
mmol) in dichloromethane (25 mL), and the mixture was stirred at
room temperature for 3 days. A yellow solid precipitated out of
the dark suspension, which was separated by filtration and washed
with CH2Cl2 (3 × 20 mL) and hexane (3 × 20 mL) to give 8a as
a yellow paramagnetic solid scarcely soluble in any common
solvents. Yield: 804 mg (69%). Anal. Calc for C68H84N8O4Si5Br8-
Ni4: C, 39.04; H, 4.05; N, 5.36. Found: C, 39.10; H, 4.20; N,
5.12. IR (Nujol/CsI): ν 1622 cm-1 (w), 1597 cm-1 (vs, CdN),
1560 cm-1(w).
G1-ONNMe3NiBr2 (8c): orange, paramagnetic solid soluble in
CH2Cl2. Yield: 60%. Anal. Calc for C80H108N8O4Si5Br8Ni4
(2260.22): C, 42.51; H, 4.82; N, 4.96. Found: C, 42.96; H, 5.11;
N, 4.88. IR (Nujol/CsI): ν 1625 cm-1 (w), 1595 cm-1 (vs, CdN),
1561 cm-1(w). MS (MALDI-TOF in dithranol): m/z 2180.3 [M -
Br]+.
Preparation of G2- and G3-Pyridylimine-NiBr2 Dendritic
Complexes (9 and 10b). Compounds 9a,b and 10b were synthe-
sized similarly to that described above for 3 starting from the
corresponding Gn-pyridylimine dendritic ligands (6 or 7b). The
preparation of 9a is reported as an example, and data for compounds
9b and 10b have been reported previously.33d
G2-ONNNiBr2 (9a). [NiBr2(DME)] (152 mg, 0.49 mmol)
suspended in dichloromethane (50 mL) was added to a solution of
compound G2-ONN (6a) (170 mg, 0.06 mmol) in the same solvent
(25 mL), and the mixture was stirred at room temperature for 3
days. A yellow solid precipitated out of the dark mixture, which
was separated by filtration and washed with CH2Cl2 (3 × 15 mL)
and hexane (3 × 20 mL) to give 9a as a yellow, paramagnetic
solid scarcely soluble in any common solvents. Yield: 220 mg
(81%). Anal. Calc for C152H204N16O8Si13Ni8Br16 (4496.52): C,
40.60; H, 4.57; N, 4.98. Found: C, 40.18; H, 5.30; N, 5.04. IR
(KBr): ν 1625 cm-1 (m), 1599 cm-1 (vs, CdN), 1560 cm-1 (w).
G2-ONN (6a). A solution of chlorosilane dendrimer G2-Cl (2.11
g, 1.45 mmol) in THF (15 mL) was added to a mixture of 1a (2.31
g, 11.6 mmol) and Et3N (2.0 mL, 14 mmol) in THF (30 mL) at 0
°C. The white suspension was stirred overnight at room temperature.
Then, the solvent was removed at reduced pressure, and the residue
extracted with toluene (3 × 30 mL). Removal of the toluene
afforded 6a as a brownish, sticky oil. Yield: 3.00 g (75%). Anal.
Calc for C152H204N16O8Si13 (2748.51): C, 66.42; H, 7.48; N, 8.15.
1
Found: C, 66.14; H, 7.41; N, 8.34. H NMR (CDCl3): δ -0.08
(s, 3H, SiMe), 0.22 (s, 12H, SiMe2), 0.57 (m, 8H, SiCH2), 0.80
(m, 4H, CH2SiMe2), 1.29 (m, 2H, CH2CH2CH2), 1.41 (m, 4H,
CH2CH2CH2), 6.83 (AA′ part of an AA′BB′ spin system, 4H, H3,5),
7.21 (BB′ part of an AA′BB′ spin system, 4H, H2,6), 7.30 (m, 2H,
H11), 7.75 (m, 2H, H10), 8.14 (d, 2H, JH,H ) 7.6 Hz, H9), 8.58 (s,
2H, H7), 8.66 (d, 2H, JH,H ) 4.0 Hz, H12). 13C{1H} NMR (CDCl3):
δ -5.0 (SiMe), -1.1 (SiMe2), 17.7, 18.4, 18.6, 19.1, 21.4, and
21.5 (CH2), 120.5 (C3,5), 121.5 (C9), 122.5 (C2,6), 124.7 (C11), 136.4
(C10), 144.1 (C1), 149.4 (C12), 154.5 (C4), 154.6 (C8), 158.1 (C7).
IR (Nujol/CsI): ν 1627 cm-1 (s, CdN), 1596 (w, CdC), 1582 and
1568 cm-1 (vs and s, py-ring).
G2-ONNMe3 (6c): brownish, viscous oil. Yield: 57%. Anal.
Calc for C176H252N16O8Si13 (3085.15): C, 68.52; H, 8.23; N, 7.26.
1
Found: C, 68.42; H, 8.70; N, 7.14. H NMR (CDCl3): δ -0.08
(s, 3H, SiMe), 0.20 (s, 12H, SiMe2), 0.57 (m, 8H, SiCH2), 0.80
(m, 4H, CH2SiMe2), 1.29 (m, 2H, CH2CH2CH2), 1.42 (m, 4H,
CH2CH2CH2), 2.12 (s, 6H, Me14), 2.32 (s, 6H, Me13), 2.58 (s, 6H,
Me15), 6.60 (s, 2H, H3), 6.90 (s, 2H, H6), 7.15 (d, 2H, JH,H ) 7.7,
H11), 7.62 (pt, 2H, H10), 8.00 (d, 2H, JH,H ) 7.2 Hz, H9), 8.47 (s,
2H, H7). 13C{1H} NMR (CDCl3): δ -5.0 (SiMe), -0.9 (SiMe2),
16.3 and 17.6 (Me13,14), 17.8, 18.5, 18.6, 19.2, and 21.8 (CH2),
24.3 (Me15), 118.4 (C3), 119.7 (C6), 120.7 (C9), 124.2 (C11), 126.8
(C5), 131.7 (C2), 136.6 (C10), 143.0 (C1), 152.7 (C4), 154.9 (C8),
157.8 (C7), 158.1 (C12). 29Si{1H} NMR (CDCl3): δ 18.0 (SiMe2),
0.8 (SiMe), 0.1 (central Si). IR (Nujol/CsI): ν 1626 cm-1 (s, Cd
N), 1589 and 1572 cm-1 (vs and s, py-ring). MS (MALDI-TOF in
dithranol): m/z 3085.8 [M + H]+.
General Procedure for Ethylene Catalytic Reactions. Ethylene
was prepurified by passage through an ALLTECH gas purification
system (AT-Indicating Oxy Cartridge). The corresponding catalyst
precursor was weighed (5-10 mg) into a 250 mL screw-capped
glass pressure reactor equipped with a mechanical stirrer. The
reactor was capped and sealed with a septum, purged by repeated
argon/vacuum operations, charged with purified toluene (50 mL),
and flushed with ethylene gas fed at constant pressure (2 bar).
Polymerization was initiated, at least, 10 min later by the addition
of a toluene solution of PMAO-IP (Al/Ni ) 1000) with vigorous
stirring. After a desired time interval the ethylene pressure was
released, the polymerization reaction was quenched with acidified
methanol (2% HCl), and the mixture was stirred overnight. The
polymer was filtered, washed with methanol, and dried in an oven
to constant weight. The reaction filtrate was washed with water
and treated with MgSO4, and the volatiles were removed in a rotary
evaporator. To minimize experimental error, the polymerization data
represent the average of a number of runs disregarding the data
Preparation of G3-Pyridylimine Dendritic Ligands (7). Car-
bosilane dendrimers 7b,c were prepared as illustrated for 7c. The
data for 7b have been reported previously.33d 7b was purified by
silica gel chromatography using CH2Cl2 as eluent.
G3-ONNMe3 (7c). A solution of chlorosilane dendrimer G3-Cl
(0.87 g, 0.27 mmol) in THF (15 mL) was added to a mixture of 1c
(1.05 g, 4.39 mmol) and Et3N (1.0 mL, 7.2 mmol) in THF (40
mL) at 0 °C. The white suspension was stirred overnight at room
temperature, the solvent was then removed at reduced pressure,
and the residue was extracted with toluene (3 × 30 mL) and further
purified by silica gel chromatography in ethyl acetate. Dendrimer
7c was isolated as a brownish oil, soluble in all common organic
solvents. Yield: 1.68 g (96%). Anal. Calc for C368H540N32O16Si29
(6483.02): C, 68.18; H, 8.40; N, 6.91. Found: C, 68.35; H, 8.90;
N, 6.39. 1H NMR (CDCl3): δ -0.09 (s, 9H, SiMe), 0.20 (s, 24H,
SiMe2), 0.55 (m, 20H, SiCH2), 0.79 (m, 8H, CH2SiMe2), 1.28 (m,
6H, CH2CH2CH2), 1.41 (m, 8H, CH2CH2CH2), 2.11 (s, 12H, Me14),

Effects of Dendritic Ni-Catalysts in Polymerization
Organometallics, Vol. 25, No. 16, 2006 3887
from those with yields deviating more than 7%. The mixtures of
oligomers were analyzed by GC in pentane as a solvent. The
Schulz-Flory R parameter of each sample (R ) mol of Cn+2/mol
of Cn) was calculated as described below from the GC data and
the equation log(mol % of Cn) ) 2n log R + A, where n is the
chain length in carbon number and A is a constant (see for instance
Supporting Information in ref 41c). First, the data of lighter olefins
(below C16) were omitted because they are partially or completely
lost by evaporation in the workup procedure, and only the heavier
oligomers that gave well-resolved peaks of significant area in the
GC of the considered sample were utilized for calculations
(minimum from C16 up to C24 and usually up to C30, between 5
and 8 data points). Subsequently, the product distribution of the
selected oligomers, expressed as log(mol % of oligomer), was
plotted versus the oligomer chain length n. As expected for a
Schulz-Flory distribution, least-squares linear fits of reasonably
good quality (correlation coefficients in the range 0.960-0.999)
were obtained for all the samples. Finally, the R parameter
determined from the linear fitting was used to estimate the quantity
of lower olefins lost in the workup.8 The microstructure of these
materials was also determined by 1H NMR (oligomers, rt, CDCl3)
or
13C{1H} NMR (polymers, 100 °C, 20% v/v benzene-d6/TCB).
Acknowledgment. We acknowledge the financial support
from the DGI-Ministerio de Educacio´n y Ciencia (project
CTQ2005-00795/BQU) and Comunidad de Madrid (project GR/
MAT/0733/2004). In addition, we thank Dr. Carmen Mart´ınez
and Dr. Carlos Mart´ın, REPSOL YPF, for their valuable
assistance with 13C NMR polymer analyses.
Supporting Information Available: The CIF files for com-
pounds 3c‚H2O and 4 are available free of charge at http://
pubs.acs.org.
OM0509084