Monometallic nickel(II) complexes containing N,N′-iminopyridine chelating ligands with dendritic substituents: The influence of dendrimer topology on the catalytic oligomerization and polymerization of ethylene Metallodendrimers Special Issue

Article abstract of DOI:10.1016/j.ica.2013.07.025

Iminopyridine ligands of general formula Gn-PBE-ONN (Gn-PBE = Fréchet-type dendritic wedges of generation G0 (1), G1 (2), G2 (3), and G3 (4); -ONN = -O-4-(2,5-Me₂C₆H₂)-NCH-2-py) have been prepared starting from H-ONN. Reaction of these bidentate ligands with [NiBr₂(DME)] results in the nickel(II) complexes [NiBr ₂(Gn-PBE-ONN)] (5-8). The methylaluminoxane (MAO)-activated monometallic complexes have been studied in the transformation of ethylene into mixtures of polyethylene and oily oligomers, and their catalytic performance scrutinized as a function of dendritic size. A comparison with the behavior observed for related polymetallic dendrimers has enabled the dendritic features that affect various catalytic parameters to be determined. Thus, it has been possible to discern whether the dendritic effect detected in the catalysis should be ascribed to the metallic nuclearity rather than the bulkiness of the metallodendrimer.

Full text of DOI:10.1016/j.ica.2013.07.025

Inorganica Chimica Acta 409 (2014) 156–162
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Inorganica Chimica Acta
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Monometallic nickel(II) complexes containing N,N0-iminopyridine
chelating ligands with dendritic substituents: The influence of
dendrimer topology on the catalytic oligomerization and polymerization
of ethylene
⇑
⇑
Francisco Martínez-Olid, Ernesto de Jesús , Juan C. Flores
Departamento de Química Orgánica y Química Inorgánica, Universidad de Alcalá, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 31 July 2013
Iminopyridine ligands of general formula Gn-PBE-ONN (Gn-PBE = Fréchet-type dendritic wedges of
generation G0 (1), G1 (2), G2 (3), and G3 (4); –ONN = -O-4-(2,5-Me2C6H2)-N@CH-2-py) have been pre-
pared starting from H-ONN. Reaction of these bidentate ligands with [NiBr2(DME)] results in the nickel(II)
complexes [NiBr2(Gn-PBE-ONN)] (5–8). The methylaluminoxane (MAO)-activated monometallic com-
plexes have been studied in the transformation of ethylene into mixtures of polyethylene and oily olig-
omers, and their catalytic performance scrutinized as a function of dendritic size. A comparison with the
behavior observed for related polymetallic dendrimers has enabled the dendritic features that affect var-
ious catalytic parameters to be determined. Thus, it has been possible to discern whether the dendritic
effect detected in the catalysis should be ascribed to the metallic nuclearity rather than the bulkiness
of the metallodendrimer.
Metallodendrimers Special Issue
Keywords:
N ligands
Dendrimers
Metallodendrimers
Nickel
Dendritic effects
Polymerization
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
of olefin polymerization catalyzed by well-defined late-transition
metal complexes has provided a rationale for new catalyst design
Amongst other attractive applications, dendritic architectures
[1] have emerged as promising, well-defined supports for catalyt-
ically active metal sites that operate under homogeneous
conditions [2], a field in which organocatalysis [3] and dendri-
mer-encapsulated nanoparticles have also received a great deal
of attention [4]. The location of metal complexes, which for cata-
lytic purposes is generally at the periphery or at the core or focal
point of the dendritic framework, greatly affects the properties of
metallodendrimers [5]. For instance, whilst high local concentra-
tions of metal sites present at the surface of peripheral metalloden-
drimers are expected to cause crowding or cooperative outcomes,
site isolation phenomena can be envisioned for core-functionalized
structures, where the metal nanoenvironment is gradually estab-
lished with increasing dendrimer generation. For either metallo-
dendrimer topology, dendritic effects—distinct features imparted
by the dendrimer that would not otherwise be achievable—can
come into play and influence numerous properties and applica-
tions [6], including catalytic performance [6b].
and reaction control aimed at the synthesis of ethylene derivatives
ranging from light oligomers to high molecular weight polymers,
with linear to hyperbranched—and dendritic [8]—microstructures,
or polyethylenes incorporating polar co-monomers [9]. Among the
prevalent pre-catalyst types explored, studies carried out with aryl
N,N0-iminopyridyl complexes of late-transition metal halides have
shown that variation of the ligand substitution greatly influences
the catalytic performance in terms of activity and selectivity (i.e.,
oligomers vs. polymers, linear vs. branched) [10]. Nickel(II) cata-
lysts of this kind are the most relevant to the present contribution
[11].
Although metallodendrimers have also been applied in poly-
merization processes, apart from a limited number of interesting
studies concerning the ROMP [12] or vinyl polymerization [13] of
norbornene, most such studies have concentrated on the polymer-
ization of ethylene. A few of these have involved group 4 metallo-
cene-type Ziegler–Natta catalysts embedded at the periphery [14]
or core [15] of carbosilane (CS) structures, which generally afford
lower catalytic activities and linear polyethylenes with higher
polydispersities compared with those of related non-dendritic
complexes. Systems based on dendrimers functionalized with
late-transition metals have offered greater diversity of outcomes.
Thus, Mapolie et al. have found that a first-generation DAB-PPI
Following the seminal work published by Brookhart’s group in
1995 [7], the renewed widespread research interest in the field
⇑
Corresponding authors. Tel.: +34 91 8854607; fax: +34 91 8854683.
E-mail address: juanc.flores@uah.es (J.C. Flores).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.07.025

F. Martínez-Olid et al. / Inorganica Chimica Acta 409 (2014) 156–162
157
dendrimer with alkyl(imido)pyridylpalladium complexes bound to
its periphery is more active than its non-dendritic counterparts for
the polymerization of ethylene, whereas the activity and selectivity
of related salicylaldimine Ni(II) dendrimers change significantly
with the generation (e.g., short oligomers for G1, and polymers
for G2) [16]. Li and coworkers have reported peripheral
bis(imino)pyridyliron(II) CS dendrimers of up to the second gener-
ation (G2), which are able to display much higher activity towards
polyethylene formation and produce polymers of much higher
molecular weight than the corresponding monometallic reference
complex [17]. In contrast, studies published by the Moss group
solvent decreasing progressively as the generation number (n)
increases.
The 1H and 13C{1H} NMR spectra of compounds 1–4 show two
singlets for the Me13 and Me14 groups (numbering in Scheme 1;
d (1H/13C) 2.2–2.4/16–18), together with another singlet at lower
field for the imine group (CH7, d (1H/13C) 8.5/157). The protons of
the dendritic moieties give sets of resonances in the three regions
characteristic for this type of poly(benzyl ether) dendrons [22a].
The distinctive IR band at 1625–1610 cmꢀ1 for the imine double
bond, and that for the pyridine ring at 1565 cmꢀ1, are only ob-
served in the spectra of 1 and 2, whereas for the compounds with
larger dendritic wedges (3 and 4) they are hidden by the very
strong absorption due to the aromatic C–C vibrations (ca. 1595
and 1450 cmꢀ1). Similarly, the relative intensity of the asymmetric
(ca. 1300 cmꢀ1) and symmetric C–O–C stretching vibrations (ca.
1160 and 1050 cmꢀ1) increases significantly on going from 1 to
4. The positive-ion electrospray mass spectra (ESI+-TOF/MS), car-
ried out in 5 mM ammonium formate solution in CH2Cl2/MeOH,
exhibit peaks that can be assigned to the protonated molecular
ions [M+H]+ with the expected isotopic distributions.
on ethylene oligomerization using bis(imino)pyridyliron or a-dii-
minepalladium complexes symmetrically substituted with poly(-
benzyl ether)—also known as Fréchet-type—or CS dendrons
concluded that the type and size of the dendrons have little effect
on catalytic performance and suggested that this result opens up
the possibility to separate the active species from the product
stream by means of filtration techniques [18]. Reek, van Leeuwen,
and coworkers have described a site-isolation effect in the oligo-
merization of ethylene promoted by a SHOP-process type complex
substituted with CS dendrons, in which the dendritic P,O-ligand
suppresses deactivation of the nickel center by avoiding the coor-
dination of a second chelating ligand, thus resulting in much more
active catalyst than in the non-dendronized case [19]. Our group
has previously described series of CS dendrimers bearing
aryl(imido)pyridylnickel(II) complexes at their surface, and found
a strong dependence of the ethylene oligomerization/polymeriza-
tion outcomes on the dendritic generation, which affects the com-
position of the final insertion product (oligomer/polymer ratios)
and other parameters (i.e., activities, polydispersities, and molecu-
lar weights). These effects were ascribed to steric interactions of
the growing polymer chain and the nanosized catalyst, as well as
to the cooperative effects of several metal centers in close proxim-
ity [20]. We reached similar conclusions when using the palla-
dium(II) counterparts of these species for the syndiotactic and
alternating copolymerization of CO and 4-tert-butylstyrene [21].
Since metallodendritic architectures result in distinctive metal
environments, and in light of the fact that the polymerization of
ethylene with late-transition metal complexes is influenced by
the active site surroundings, we set out to extend our previous
studies by exploring the reactivity of monometallic, core-function-
alized dendrimers in this type of process. Our studies on the orga-
nization of Fréchet-type dendritic arms around metal centers
located at the focal point of the dendritic structures, in both solu-
tion and the solid state, revealed that the accessibility of the metal
center varies with the size of the dendritic ligand [22]. As such, we
reasoned that the dendrimer arrangements might modulate the
catalytic properties. Herein we report the synthesis and character-
ization of nickel(II) complexes containing an aryl-N,N0-iminopyri-
dine ligand substituted with Fréchet-type dendrons of up to the
third generation. The influence of dendrimer generation on the cat-
alytic behavior of these pre-catalysts is also analyzed, and the re-
sults compared to those found for the related peripheral
polymetallic dendrimers [20].
Chelates 1–4 reacted with one equivalent of [NiBr2(DME)]
(DME = dimethoxyethane-j2O,O0) in dichloromethane to form
complexes [NiBr2(Gn-PBE-ONN)] 5–8 by displacement of the labile
DME ligand (Scheme 1 and Fig. 1). The initial yellow-orange sus-
pensions of the nickel precursor turned red-brown over the course
of the reaction. Complexes 5–8 were isolated in good yields (>70%)
as orange paramagnetic solids that are stable to air as solids and in
solution and are insoluble in alkanes but soluble in acetone and in
chlorinated and aromatic solvents.
The paramagnetism of nickel complexes 5–8 precludes their
characterization by NMR spectroscopy. However, their identities
were established by IR spectroscopy, mass spectrometry, magnetic
susceptibility measurements and elemental analysis. Coordination
of the iminopyridine ligands to the metal center in 5 and 6 is sup-
ported by the collapse of the three intense C@N and C@C bands ob-
served in the range 1626–1565 cmꢀ1 in the free ligand into a single
absorption, at around 1590 cmꢀ1, accompanied by two much
weaker bands. The shift or disappearance of the C@Nimine vibration
is attributed to a reduction in the electron density of the double
bond upon coordination [20b]. The intense absorption of the den-
dritic moieties at 1595 cmꢀ1 overlaps the bands of interest in the
case of the higher generation complexes 7 (G2) and 8 (G3). The
molecular peak was only observed in the ESI+-TOF mass spectrum
of the G3 complex 8. The protonated free ligand, or fragments
thereof arising due to the loss of one or two bromides and the addi-
tion of ionizing agents, solvent or a second ligand, were the more
characteristic peaks observed in the mass spectra of 5–7 (see Sec-
tion 4). Magnetic susceptibilities were measured in the solid state
and in solution (Table 1) and corrected to take into account the dia-
magnetic contribution, which varies considerably because of the
large differences in molecular magnitude of the complexes under
study (from 535 to 2021 g/mol). The resulting solid-phase para-
magnetic moments lie in the range 2.70–3.06
close to the spin-only moment expected for two unpaired electrons
(2.82 B) and to reported values for other tetrahedral iminopyridyl
Ni(II) complexes [23]. Similar moments were obtained in acetone
(2.84–3.22 B), thus indicating that the tetrahedral metal environ-
lB and are therefore
l
2. Results and discussion
l
2.1. Synthesis of ligands and nickel complexes
ment is maintained in solution.
Bidentate ligands 1–4 were prepared by treating the hydroxyl-
iminopyridine HONN with the corresponding benzyl bromide
Gn-PBE-Br (Scheme 1), in warm acetone, with K2CO3 as the base,
and in the presence of 18-crown-6. After appropriate work up, they
were isolated in good yields (>74%) as yellow solids that are solu-
ble in acetone and chlorinated solvents, insoluble in alkanes, and
scarcely soluble in diethyl ether, with their solubility in this
2.2. Ethylene oligomerization/polymerization
In the presence of ethylene, and upon activation with MAO,
nickel complexes 5–8 promote the formation of mixtures of a tol-
uene-soluble fraction of oligomers together with solid polyethyl-
ene under mild conditions. Oligomer formation is the
consequence of a competitive rate of chain transfer (kct) relative

158
F. Martínez-Olid et al. / Inorganica Chimica Acta 409 (2014) 156–162
Scheme 1.
to chain propagation (kp) [24] in a process that is sensitive to the
ability of the ligand to sterically block the axial sites of the active
center, thus delaying chain transfer and enhancing chain growth
[7,24]. The simultaneous formation of oligomers and polymers
has been proposed to be caused by hindered rotation of the aryl-
N bond, which results in two different propagating faces at the me-
tal [25].
Table 2 lists the catalytic results and data corresponding to the
polyolefinic products. The oligomeric oily fractions are composed
of even-numbered olefins up to C34 that follow Schulz–Flory
chain-length distributions with
a values ranging from 0.70 to
0.81 ( = kp/(kp + kct) = mol of Cn+2/mol of Cn) [26], with a selectivity
a
of approx. 50 mol% for terminal alkenes. The number average
molecular weight (Mn) of these oligomers is in the range 240–
265, and their 1H NMR spectra indicate a relatively low branching
(4 branches per 100 C) [27]. The solid polymers have low average
molecular weights, relatively wide polydispersities (Mw = 1000–
2500 and PDI = 2.5–4.0), a low branching (essentially methyl
groups in a ratio of 12–16 branches per 1000 main chain carbons)
[28], and Tm values (115–118 °C) lower than the regular melting
temperatures for HDPE (P135 °C).
Fig. 1. Metallodendrimer 8.
The size of the PBE dendron has a limited influence on the olig-
omerization/polymerization process. Thus, nickel compounds 5–8
exhibit comparable activities in terms of oligomer and polymer
Table 1
production. In studies with Fe bis(imino)pyridyl and Ni and Pd a-
Magnetic susceptibilities and moments for nickel complexes [NiBr2(Gn-PBE-ONN)]
(5–8).
diimine monometallic complexes, Moss and coworkers also ob-
served that the size (and type) of the dendritic substituent at the
para position of the ArN group had a minor effect on the catalytic
activity [18]. Amongst the parameters shown in Table 2, only the
Complex 106 vD Solid phasea
Solution phaseb
c
(cm3/
mol)
106 vM 106
leff
lB
106 vM 106
leff
lB
d
f
d
f
e
e
(cm3/
mol)
vM
(
)
(cm3/
mol)
vM
(
)
0
0
Schultz-Flory constant
a changes regularly with the generation
(cm3/
mol)
(cm3/
mol)
number, although the differences between consecutive values are
scarcely significant except for the larger dendrons. Other parame-
ters, as the polymer Mw, show an irregular and small variation
when the generation number increases. A comparison of the cata-
lytic data of the G0–G3 complexes 5–8 with that of the related
complexes [NiBr2(RONN)] where R = H (I) and SiMe3 (II) (Table 3)
suggests that the catalytic outcome is more sensitive to the elec-
tronic nature than to the size of the O-substituent at the aryloxy
ring of the ligand.
5
6
7
8
–274
–414
–694
–1254
3287
2638
3133
2675
3561
3052
3827
3929
2.91 3111
2.70 3040
3.02 3648
3.06 2830
3385
3454
4342
4084
2.84
2.87
3.22
3.12
a
b
c
d
e
f
At r.t.
At 20 °C, in acetone-d6, [Ni] = 1.12 ꢁ 10ꢀ2 M.
Diamagnetic susceptibility calculated using Pascal’s constants.
Measured molar magnetic susceptibility.
Corrected magnetic susceptibility.
These results are in contrast with the strong influence of the
dendrimer generation in ethylene catalysis that we observed in
lB = Bohr Magneton.

F. Martínez-Olid et al. / Inorganica Chimica Acta 409 (2014) 156–162
159
Table 2
Ethylene oligomerization/polymerization catalyzed by [NiBr2(Gn-PBE-ONN)] (5–8)/MAO.a
Precatalyst
Oligomers
Polymer
Yield (g)
f
h
act.b,c
ad,e
a
-Olefin (mol%)e
Mn
act.b,c
Tm (°C)g
Mw
PDIh
5 (G0)
6 (G1)
7 (G2)
8 (G3)
3.70
2.02
3.27
3.13
0.81
0.79
0.76
0.70
50
50
52
50
266.7
240.1
238.3
242.4
1.58
1.68
1.66
2.63
2.35
2.50
2.47
3.91
116.4
115.4
117.1
118.2
1800
960
2320
2530
2.69
2.53
2.88
3.97
a
b
c
Conditions: 50 mL toluene; n(Ni) = 14
Activity: g/(mmolNi ꢁ h ꢁ bar).
lmol; Al/Ni = 1000; tp = 24 h; Tp = 20 °C; Pethylene = 2 bar; activities 7%.
Corrected to consider the lower olefin lost during work up.
d
e
f
Schulz-Flory parameter,
Determined by GC.
a = kp/(kp + kct) [26].
Determined by 1H NMR spectroscopy [27].
Determined by DSC (2nd heating run).
Determined by GPC.
g
h
Table 3
Ethylene oligomerization/polymerization catalyzed by [NiBr2(HONN)] (I) and carbosilane metallodendrimers Gn-CS-(ONNNiBr2)m (II–V)/MAO.a,b
.
Precatalyst
Oligomers
Activity
Polymer
Activity
a
Mn
Mw
PDI
6.61
3.58
2.77
11.25
14.06
I
3.70
5.21
8.32
10.21
13.40
0.65
0.70
0.60
0.51
0.45
341.1
265.8
284.5
342.5
327.6
2.09
13.21
14.88
5.12
5880
1640
2160
10620
17070
II (G0)
III (G1)
IV (G2)
V (G3)
4.82
a
Conditions and considerations as in Table 2.
Data from Ref. [20].
b
our previous studies with CS dendrimers bearing one (G0), four
(G1), eight (G2), or sixteen (G3) aryl(imido)pyridylnickel(II) com-
plexes at their periphery (II–V, Table 3) [20]. These polymetallic
dendrimers also afforded mixtures of oligomers and polymers with
a low degree of branching, similar to that observed for 5–8 (it
should be noted that we erroneously described the presence of
large branches in the polymer fraction due to a misassignment of
some of the 13C resonances in the previous report [20]).1 The com-
parison of data in Tables 2 and 3 reveals the different evolution of
some parameters on going from G0 to G3 for the two series of cata-
lysts, PBE-based monometallic (5–8) and CS-based polymetallic den-
drimers (II–V). Thus, a higher generation for II–V results in more
oligomers and fewer polymers (from 5.2 to 13.4 and from 13.2 to
4.8 g mmolNiꢀ1 hꢀ1 barꢀ1, respectively) whereas the productivities
for 5–8 are fairly insensitive to the dendrimer generation (2.0–3.5
and 2.5–4.0 g mmolNiꢀ1 hꢀ1 barꢀ1 for oligomers and polymers,
respectively; see Fig. 2). In the case of the Schulz–Flory parameter,
a steady and sharp decrease is observed for the polymetallic dendri-
mers II–V (from 0.70 to 0.45) whereas 5–8 show a more moderate
decrease. The polymer molecular weight (also the polydispersity,
Fig. 3) increases in one order of magnitude for II–V (from 1600 to
17000) in contrast with the modest increase observed for 5–8 (from
1800 to 2500).
Fig. 2. Activities by monometallic (5–8) and peripheral (II–V) metallodendrimers of
generation G0 to G3.
controlled parameters that include size, shape, surface chemistry,
flexibility/rigidity, architecture or elemental composition [6c].
We initially interpreted the dendritic effects observed with the
polymetallic CS dendrimers II–V as being partly due to the steric
interaction between the dendritic structure and the growing
polymer chains [20]. From the above comparison between the
Tomalia pointed out that dendritic effects are related to one or
more and perhaps the concurrent interaction of several structure
1
We thank one of the referees for comments leading us to correct our previous
branching analysis for the polymers.

160
F. Martínez-Olid et al. / Inorganica Chimica Acta 409 (2014) 156–162
commercial sources and used as received. [HO-4-(2,5-Me2Ph)-
molecular weight
polydispersity
N@CH-2-py] (HONN) [20a], [NiBr2(DME)] [29], and the dendritic
benzyl bromides Gn-PBE-Br [30] were prepared according to liter-
ature procedures. Solvents were dried prior to use and distilled un-
der argon as described elsewhere [31]. NMR spectra were recorded
using Varian Unity 500+, VR-300 or 200 NMR spectrometers.
Chemical shifts (d) are reported in ppm relative to SiMe4, and were
measured relative to the 13C and residual 1H resonances of the deu-
terated solvents. Assignments for the iminopyridyl fragments are
given according to the numbering of the positions depicted in
Scheme 1. Coupling constants (J) are given in Hz. The following
abbreviations/notations are used: Ph refers to an aromatic ring of
the terminal benzyl groups, Ar to the internal rings of benzyl
ethers, and ipso refers to the first ring position on going from the
bidentate iminopyridyl ligand. IR spectra were recorded using a
Perkin–Elmer FT-IR Spectrum-2000 spectrophotometer. The mag-
netic susceptibilities of the nickel compounds were measured in
the solid state at room temperature in an Evans balance (Magway
MSB MK1, Sherwood Scientific Ltd.) instrument calibrated using
Hg[Co(NCS)4] as standard, whereas solution (1.12 ꢁ 10ꢀ2 M) mea-
surements were determined by the method proposed by Evans
[32], using Wilmad coaxial NMR tubes in a Varian VR-300 NMR
spectrometer, in acetone-d6, at 20 °C, and SiMe4 as a reference.
For both methods, corrections for underlying diamagnetism were
applied to the data using tabulated Pascal’s constants [33]. Elemen-
tal analyses were performed by the Microanalytical Laboratories of
the University of Alcalá using a LECO CHNS-932 microanalyzer.
ESI-TOF mass spectra were recorded by the Research Services at
the Universidad Autónoma de Madrid (SIDI) using Applied Biosys-
tems spectrometers. Poly(methyl aluminoxane) improved process
(PMAO-IP) in toluene (13 wt% Al) was purchased from Akzo Nobel.
Oligomer products were analyzed using a Chrompack CP 9001 gas
chromatograph equipped with a CP-Sil 5CB capillary column
18
15
12
9
6
3
G0
0
G1
G2
Gn-CS-(ONNNiBr2)m (I–IV)
Generation
G3
[NiBr2(Gn-PBE-ONN)] (5–8)
Fig. 3. Molecular weight and polydispersities of the polymeric fraction obtained by
monometallic (5–8) and peripheral (II–V) metallodendrimers.
monometallic and polymetallic G0–G3 series of catalysts, we can
now suggest that the dendritic effects observed with the latter in
the polymerization of ethylene are more likely connected with
the topology and, more specifically, the metallic nuclearity than
with the size (bulkiness) of the dendrimer. Although other features
of the CS structures (flexibility, for instance) might influence in the
observation of dendritic effects with the polymetallic systems, it is
noteworthy that Moss and coworkers observed only a minor im-
pact of the type of dendrimer (CS or PBE) on the catalytic activity
of related precursors [18a]. Nevertheless, a steric-based mecha-
nism can yet be considered to explain the observed dendritic ef-
fects in II–V but considering the interaction between the high
local concentration of growing polymer chains in the limited sur-
face of high-generation dendrimers. We also noted previously that
another possible contribution to the changes observed with the
generation could arise from poor local heat dispersion at the den-
dritic surface where the active centers are concentrated [21b].
(10 m, 0.25 mm i.d., 0.12 lm d.f.) under the following conditions:
injector and detector temperature: 250 °C; oven temperature pro-
gram: 100 °C/5 min, 5 °C/min ramp, 250 °C/15 min. DSC melting
endotherms of polymers were recorded using a Perkin-Elmer DSC
6 instrument. Polyethylene molecular weight determinations were
performed by the Polymer Technology Laboratory (LTEP) at the
Universidad Rey Juan Carlos using a Waters GPCV2000 instrument
operating at 145 °C with 1,2,4-trichlorobenzene (TCB) solvent
(Number and type of columns: 2 PL Gel 10 micras Mixed B + 1 PL
Gel 10 micras 10E6) and polystyrene calibration standards.
3. Conclusions
Aryliminopyridyl ligands substituted with poly(benzyl ether)
dendritic wedges of up to the third generation have been obtained
by alkylation of HO-4-(2,5-Me2Ph)-N@CH-2-py at the phenolic
oxygen. Displacement of the labile ligand in [NiBr2(DME)] by the
bidentate N,N0-ligands leads to straightforward formation of the
corresponding NiBr2 chelates. Upon activation with MAO, the nick-
el derivatives produce ethylene oligomerization and polymeriza-
tion products under mild conditions. The catalytic properties are
much less sensitive to dendrimer generation for these monometal-
lic precursors than those found previously for polymetallic dendri-
mers with different amounts of the same nickel complex at their
periphery. The oligomer/polymer distributions or the polymer
molecular weights and polydispersities are likely regulated by
the metallic nuclearity of the dendrimer, whereas the Schulz-Flory
distribution might also be affected in some extent by the dendri-
mer bulkiness.
4.2. Preparation of dendrons Gn-PBE-ONN (1–4)
Benzyl bromide (n = 0) or the corresponding Fréchet wedge Gn-
PBE-Br (n = 1–3), 18-crown-6, K2CO3, and HONN were stirred in
acetone (10–20 mL) at 70 °C for 48 h. The mixture was then filtered
through Celite, the solvent removed in vacuo, and the residue trea-
ted as described below to give 1–4 as yellow solids.
4.2.1. G0-PBE-ONN (1)
Benzyl bromide (0.18 mL, 1.5 mmol), 18-crown-6 (80 mg,
0.30 mmol), K2CO3 (315 mg, 2.28 mmol) and HONN (339.4 mg,
1.5 mmol). The oily crude reaction mixture was washed with
diethyl ether and hexane. Yield: 376 mg (79.0%). Anal. Calc. for
C21H20ON2 (316.40): C, 79.72; H, 6.37; N, 8.85. Found: C, 79.80;
H, 6.36; N, 8,86%. 1H NMR (CDCl3): d 2.26 (s, 3 H, Me14), 2.31 (s,
3 H, Me13), 5.08 (s, 2 H, CH2O), 6.78 (s, 1 H, H3), 6.98 (s, 1 H, H6),
7.3–7.5 (m, 6 H, Ph and H11), 7.80 (t, JH–H = 7.6 Hz, 1 H, H10), 8.24
(d, JH–H = 7.9 Hz, 1 H, H9), 8.56 (s, 1H, H7), 8.67 (d, JH–H = 4.3 Hz, 1
H, H12). 13C{1H} NMR (CDCl3): d 16.2 and 17.9 (Me13,14), 70.2
(CH2O), 113.5 (C3), 119.8 (C6), 121.4 (C9), 124.6 (C11), 125.7 (C5),
127.8 (p-Ph), 127.2 and 128.4 (o- and m-Ph), 131.9 (C2), 136.8
(C10), 137.4 (ipso-Ph), 142.2 (C1), 149.5 (C12), 155.3 (C4), 155.7
4. Experimental
4.1. Reagents and general techniques
All operations were performed under argon using Schlenk
techniques. Unless otherwise stated, reagents were obtained from

F. Martínez-Olid et al. / Inorganica Chimica Acta 409 (2014) 156–162
161
(C8), 157.5 (C7). IR (KBr pellet):
m
1626 (s, C@N), 1602 and 1454 (s,
H10), 8.20 (d, JH–H = 7.9 Hz, 1 H, H9), 8.51 (s, 1 H, H7), 8.66
(d, JH–H = 4.3 Hz, 1 H, H12). 13C{1H} NMR (CDCl3): d 16.2 and 17.9
(Me13,14), 70.0 (G0-CH2O, G1-CH2O and G2-CH2O, overlapping),
70.1 (PhCH2O), 101.5 (G0-p-Ar), 101.6 (G1-p-Ar and G2-p-Ar, over-
lapping), 106.0 (G0-o-Ar), 106.4 (G1-o-Ar and G2-o-Ar overlap-
ping), 113.5 (C3), 119.8 (C6), 121.3 (C9), 124.6 (C11), 125.1 (C5),
128.0 (p-Ph), 127.5 and 128.6 (o- and m-Ph), 131.9 (C2), 136.5
(C10), 136.7 (ipso-Ph), 139.1 (G2-ipso-Ar), 139.2 (G1-ipso-Ar),
139.9 (G0-ipso-Ar) 142.3 (C1), 149.5 (C12), 155.3 (C4), 155.8 (C8),
157.5 (C7), 160.0 (G0-m-Ar) 160.1 (G1-m-Ar), 160.2 (G2-m-Ar). IR
C@C), 1563 (s, py-ring) 1303 (m, C–O–Cas), 1199 and 1023 cmꢀ1
(vs, C–O–Csym). MS (ESI+-TOF in CH2Cl2/MeOH, NH4HCOO 5 mM):
m/z 317.16 [M + H]+.
4.2.2. G1-PBE-ONN (2)
G1-PBE-Br (500 mg, 1.30 mmol), 18-crown-6
(70 mg,
0.262 mmol), K2CO3 (271.3 mg, 1.965 mmol) and HONN (296 mg,
1.31 mmol). The crude reaction mixture was washed with diethyl
ether and hexane. A second crop of compound 2 was obtained by
combining the filtrates and cooling in
a
freezer overnight
(KBr pellet): m 1595 (vs, C@N and C@C overlapping), 1449 (s,
(ꢀ20 °C). Combined yield: 580 mg (84.0%). Anal. Calc. for
C@C), 1297 (s, C–O–Cas), 1155 and 1047 cmꢀ1 (vs, C–O–Csym). MS
(ESI+-TOF in CH2Cl2/MeOH,NH4HCOO 5 mM): m/z 1801.76 [M+H]+.
C
35H32O3N2 (528.64): C, 79.52; H, 6.10; N, 5.30. Found: C, 79.67;
H, 6.01; N, 5.32%. 1H NMR (CDCl3): d 2.23 (s, 3 H, Me14), 2.38
(s, 3 H, Me13), 5.00 (s, 2 H, ArCH2O), 5.04 (s, 4 H, PhCH2O), 6.56
4.3. Preparation of [NiBr2(Gn-PBE-ONN)] (5–8)
4
4
(t, JH–H = 2.1 Hz, 1 H, p-Ar), 6.69 (d, JH–H = 2.1 Hz, 2 H, o-Ar),
6.73 (s, 1 H, H3), 6.96 (s, 1 H, H6), 7.3–7.5 (m, 11 H, Ph and H11 over-
lapping), 7.77 (t, JH–H = 7.9 Hz, 1 H, H10), 8.22 (d, JH–H = 7.9 Hz, 1 H,
H9), 8.52 (s, 1 H, H7), 8.67 (d, 1 H, JH–H = 4.3 Hz, H12). 13C{1H} NMR
(CDCl3): d 16.1 and 17.9 (Me13,14), 70.1 (ArCH2O), 70.4 (PhCH2O),
101.5 (p-Ar), 106.1 (o-Ar), 113.6 (C3), 119.8 (C6), 121.4 (C9), 124.6
(C11), 125.2 (C5), 128.0 (p-Ph), 127.5 and 128.6 (o- and m-Ph),
131.9 (C2), 136.5 (C10), 136.9 (ipso-Ph), 139.9 (ipso-Ar), 142.3 (C1),
149.5 (C12), 155.4 (C4), 155.9 (C8), 157.5 (C7), 160.1 (m-Ar). IR
A solution of the corresponding ligand Gn-PBE-ONN (1–4) in
CH2Cl2 (15 mL) was slowly added to a stirred suspension of
[NiBr2(DME)] in the same solvent (15 mL) at room temperature.
Stirring was continued for 24 h. The initial yellow-orange suspen-
sion turned red-brown over the course of the reaction. The mixture
was filtered through Celite, the solvent removed in vacuo to dry-
ness, and the residue washed with several portions of hexane
and dried in vacuo to give 5–8 as orange paramagnetic solids.
(KBr pellet):
m 1611 (s, C@N), 1593 and 1452 (vs, C@C), 1565 (m,
py-ring), 1306 (s, C–O–Cas), 1166 cmꢀ1 (vs, C–O–Csym). MS
4.3.1. [NiBr2(G0-PBE-ONN)] (5)
(ESI+-TOF in CH2Cl2/MeOH,NH4HCOO 5 mM): m/z 529.25 [M+H]+.
G0-PBE-ONN (1, 48 mg, 0.15 mmol) and [NiBr2(DME)] (46 mg,
0.15 mmol). Yield: 61 mg (76.0%). Anal. Calc. for C21H20ON2NiBr2
(534.90): C, 47.15; H, 3.77; N, 5.24. Found: C, 46.80; H, 4.18; N,
4.2.3. G2-PBE-ONN (3)
G2-PBE-Br (500 mg, 0.62 mmol), 18-crown-6 (33 mg,
0.124 mmol), K2CO3 (128 mg, 0.93 mmol) and HONN (140 mg,
0.62 mmol). The crude reaction mixture was washed with diethyl
ether, and the resulting oily solid treated with hexane. Yield:
453 mg (76.7%). Anal. Calc. for C63H56O7N2 (953.13): C, 79.39; H,
5.92; N, 2.94. Found: C, 78.92; H, 5.84; N, 2.88%. 1H NMR (CDCl3):
d 2.23 (s, 3H, Me14), 2.38 (s, 3H, Me13), 4.97 (s, 4 H, G1–ArCH2O),
5.00 (s, 2 H, G0–ArCH2O), 5.01 (s, 8 H, PhCH2O), 6.53 (t, 1 H,
4.77%. IR (KBr pellet): m 1600 (m, C@N), 1597 (s, C@C), 1306 (m,
C–O–Cas), 1201 and 1021 cmꢀ1 (s, C–O–Csym). MS (ESI+-TOF in CH2-
Cl2/MeOH, NH4HCOO 5 mM): m/z 969.46 [(M –Br)2 + NH4HCOO]+,
735.24 [(1)2Ni + HCOO]+, 313.16 [1+H]+.
4.3.2. [NiBr2(G1-PBE-ONN)] (6)
G1-PBE-ONN (2, 80 mg, 0.15 mmol) and [NiBr2(DME)] (46 mg,
0.15 mmol). Yield: 80 mg (71.0%). Anal. Calc. for C35H32O3N2NiBr2
(747.14): C, 56.26; H, 4.32; N, 3.75. Found: C, 56.45; H, 4.84; N,
4
4JH–H = 1.9 Hz, G0-p-Ar), 6.56 (t, JH–H = 1.9 Hz, 2 H, G1-p-Ar), 6.67
4
(d, JH–H = 1.9 Hz, 6 H, G0-o-Ar and G1-o-Ar), 6.74 (s, 1 H, H3),
3.91%. IR (KBr pellet): m 1595 (vs, C@N and C@C overlapping),
6.95 (s, 1 H, H6), 7.3–7.4 (m, 21 H, Ph and H11 overlapping), 7.77
(t, JH–H = 7.9 Hz, 1 H, H10), 8.21 (d, JH–H = 7.9 Hz, 1 H, H9), 8.52 (s,
1 H, H7), 8.67 (d, JH–H = 4.3 Hz, 1 H, H12). 13C{1H} NMR (CDCl3): d
16.1 and 17.9 (Me13,14), 70.0 (G0-CH2O and G1-CH2O overlapping),
70.1 (PhCH2O), 101.4 (G0-p-Ar), 101.6 (G1-p-Ar), 106.1 (G0-o-Ar),
106.3 (G1-o-Ar), 113.5 (C3), 119.8 (C6), 121.3 (C9), 124.6 (C11),
125.2 (C5), 128.0 (p-Ph), 127.5 and 128.6 (o- and m-Ph), 131.9
(C2), 136.5 (C10), 136.8 (ipso-Ph), 139.3 (G1-ipso-Ar), 139.9
(G0-ipso-Ar), 142.3 (C1), 149.5 (C12), 155.3 (C4), 155.8 (C8), 157.5
1305 (m, C–O–Cas), 1151 and 1058 cmꢀ1 (vs, C–O–Csym). MS
(ESI+-TOF in CH2Cl2/MeOH, NH4HCOO 5 mM): m/z 1159.41 [(2)2-
Ni + HCOO]+, 697.16 [M–Br+MeOH]+, 529.24 [2+H]+.
4.3.3. [NiBr(G2-PBE-ONN)] (7)
G2-PBE-ONN (3, 95 mg, 0.10 mmol) and [NiBr2(DME)] (31 mg,
0.10 mmol). Yield: 84 mg (71.6%). Anal. Calc. for C63H56O7N2NiBr2
(1171.63): C, 64.58; H, 4.82; N, 2.39. Found: C, 64.14; H, 4.94; N,
2.21%. IR (KBr pellet):
m 1595 (vs, C@N and C@C overlapping),
(C7), 160.0 (G0-m-Ar), 160.2 (G1-m-Ar). IR (KBr pellet):
m
1595
1297 (m, C–O–Cas), 1156 and 1054 cmꢀ1 (vs, C–O–Csym). MS
(ESI+-TOF in CH2Cl2/MeOH, NH4HCOO 5 mM): m/z 2009.76 [(3)2-
Ni + HCOO]+, 1055.34 [M–2 Br+HCOO]+, 953.42 [3+H]+.
(vs, C@N and C@C overlapping), 1447 (s, C@C), 1296 (s, C–O–Cas),
1162 and 1057 cmꢀ1 (vs C–O–Csym). MS (ESI+-TOF in CH2Cl2/
MeOH,NH4HCOO 5 mM): m/z 953.42 [M+H]+.
4.3.4. [NiBr(G3-PBE-ONN)] (8)
4.2.4. G3-PBE-ONN (4)
G3-PBE-ONN (4, 100 mg, 0.055 mmol) and [NiBr2(DME)]
(17 mg, 0.055 mmol). Yield: 86 mg (77.4%). Anal. Calc. for C119H104-
O15N2NiBr2 (2020.64): C, 70.73; H, 5.19; N, 1.39. Found: C, 70.15;
G3-PBE-Br (500 mg, 0.30 mmol), 18-crown-6 (16 mg,
0.06 mmol), K2CO3 (63 mg, 0.45 mmol) and HONN (68 mg,
0.30 mmol). The crude reaction mixture was washed with diethyl
ether, and the resulting oily solid treated with hexane. Yield:
400 g (74.0%). Anal. Calc. for C119H104O15N2 (1802.10): C, 79.31;
H, 5.82; N, 1.55. Found: C, 78.67; H, 5.82; N, 1.38%. 1H NMR
(CDCl3): d 2.23 (s, 3 H, Me14), 2.37 (s, 3 H, Me13), 4.94 (s, 8 H,
G2-ArCH2O), 4.96 (s, 4 H, G1-ArCH2O), 4.97 (s, 2 H, G0-ArCH2O),
4.99 (s, 16 H, PhCH2O), 6.5–6.6 (3 ꢁ t, 7 H, G0-p-Ar, G1-p-Ar and
G2-p-Ar, overlapping), 6.6–6.7 (3 ꢁ d, 14 H, G0-o-Ar, G1-o-Ar and
G2-o-Ar, overlapping), 6.72 (s, 1 H, H3), 6.94 (s, 1 H, H6), 7.2–7.4
(m, 41 H, Ph and H11 overlapping), 7.76 (t, JH–H = 7.9 Hz, 1 H,
H, 4.47; N, 1.17%. IR (KBr pellet): m 1595 (vs, C@N and C@C overlap-
ping), 1296 (m, C–O–Cas), 1155 and 1053 cmꢀ1 (vs, C–O–Csym). MS
(ESI+-TOF in CH2Cl2/MeOH, NH4HCOO 5 mM): m/z 2016.5 [M]+,
1801.75 [4+H]+.
4.4. General procedure for ethylene catalytic reactions
The corresponding catalyst precursor was weighed (7.5–
28.3 mg) into a 250-mL screw-capped glass pressure reactor
equipped with a mechanical stirrer. The reactor was capped and

162
F. Martínez-Olid et al. / Inorganica Chimica Acta 409 (2014) 156–162
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This work was supported by the Spanish Ministerio de Eco-
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grateful to the Ministerio de Educación for an FPU Doctoral
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