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A 'Dendritic Effect' in Homogeneous Catalysis with Carbosilane-Supported Arylnickel (II) Catalysts: Observation of Active-Site Proximity Effects in Atom-Transfer Radical Addition

01 Dec 2000-Journal of the American Chemical Society (American Chemical Society)-Vol. 122, Iss: 49, pp 12112-12124

AbstractTransmetalation of polylithiated, carbosilane (CS) dendrimers functionalized with the potentially terdentate ligand [C6H2(CH2NMe2)2-2,6-R-4]- ( = NCN) with NiCl2(PEt3)2 produced a series of nickel-containing dendrimers [G0]-Ni4 (4), [G1]-Ni12 (5), and [G2]-Ni36 (7) in moderate to good yields. The metallodendrimers 4, 5, and 7 are catalytically active in the atom-transfer radical addition (ATRA) reaction (Kharasch addition reaction), viz. the 1:1 addition of CCl4 to methyl methacrylate (MMA). The catalytic data were compared to those obtained for the respective mononuclear compound [NiCl(C6H2{CH2NMe2}2-2,6-SiMe3-4)] (2). This comparison indicates a fast deactivation for the dendrimer catalysts beyond generation [G0]. The deactivation of [G1]-Ni12 (5) and [G2]-Ni36 (7) is caused by irreversible formation of catalytically inactive Ni(III) sites on the periphery of these dendrimers. This hypothesis is supported by results of model studies as well as ESR spectroscopic investigations. Interestingly, the use of two alternative nickelated [G1] dendrimers [G1]*-Ni12 (11) and [G1]-Ni8 (15), respectively, in which the distance between the Ni sites is increased, leads to significantly improved catalytic efficiencies which approximate those of the parent derivative 2 and [G0]-Ni4 (4). Preliminary membrane catalysis experiments with [G0]-Ni4 (4) and [G1]-Ni12 (5) show that 5 can be efficiently retained in a membrane reactor system. The X-ray crystal structure of the Ni(III) complex [NiCl2(C6H2{CH2NMe2}2-2,6-SiMe3-4)] (16), obtained from the reaction of 2 with CCl4, is also reported.

Summary (1 min read)

Introduction

  • Current research in their group concentrates on the use of dendrimers1-4 and hyperbranched polymers5 to create new materials that bridge the gap between homogeneous and heterogeneous catalysis.
  • The authors detail a novel synthetic methodology for the production of metallodendrimer catalysts via a simple procedure involving attachment of the “pincer” ligand to the dendrimer periphery via a Si-C bond.

Results and Discussion

  • The same conditions were used as reported in their previous studies on this type of catalysis.6,8.
  • Molecular modeling performed on5 indicated that the Ni sites in this dendrimer are in much closer proximity when compared to4 and the nickelated [G1] dendrimer described earlier .

Conclusions

  • In summary, the authors have detailed a new and simple synthetic methodology for the production of metal-centered carbosilane dendrimers by using polylithiated precursors.
  • These new metallodendrimers were successfully employed as homogeneous catalysts in the ATRA (i.e., Kharasch) reaction.
  • The use of ultrafiltration membrane technology as a tool for the separation of macromolecular catalysts from the product stream was demonstrated.
  • The size properties of [G1]-Ni12 (5) (∼25-30 Å diameter) allowed the catalysis to be operated under continuous conditions with retention of the catalyst in the membrane reactor and with no significant loss of catalytic material.
  • The formation of a purple precipitate, which contains Ni(III) sites, was a limiting factor.

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A “Dendritic Effect” in Homogeneous Catalysis with
Carbosilane-Supported Arylnickel(II) Catalysts: Observation of
Active-Site Proximity Effects in Atom-Transfer Radical Addition
Arjan W. Kleij,
Robert A. Gossage,
†,
Robertus J. M. Klein Gebbink,
Nils Brinkmann,
Ed J. Reijerse,
|
Udo Kragl,
Martin Lutz,
§
Anthony L. Spek,
§,#
and Gerard van Koten*
,†
Contribution from the Debye Institute, Department of Metal-Mediated Synthesis, Utrecht UniVersity,
Padualaan 8, 3584 CH, Utrecht, The Netherlands, Forschungszentrum Ju¨lich GmbH,
Institut fu¨r Biotechnologie, D-52425 Ju¨lich, Germany, BijVoet Center for Biomolecular Research,
Department of Crystal and Structural Chemistry, Utrecht UniVersity, Padualaan 8, 3584 CH Utrecht,
The Netherlands, Department of Molecular Spectroscopy, UniVersity of Nijmegen, ToernooiVeld,
6525 ED, Nijmegen, The Netherlands
ReceiVed July 19, 2000
Abstract: Transmetalation of polylithiated, carbosilane (CS) dendrimers functionalized with the potentially
terdentate ligand [C
6
H
2
(CH
2
NMe
2
)
2
-2,6-R-4]
-
( ) NCN) with NiCl
2
(PEt
3
)
2
produced a series of nickel-
containing dendrimers [G0]-Ni
4
(4), [G1]-Ni
12
(5), and [G2]-Ni
36
(7) in moderate to good yields. The
metallodendrimers 4, 5, and 7 are catalytically active in the atom-transfer radical addition (ATRA) reaction
(Kharasch addition reaction), viz. the 1:1 addition of CCl
4
to methyl methacrylate (MMA). The catalytic data
were compared to those obtained for the respective mononuclear compound [NiCl(C
6
H
2
{CH
2
NMe
2
}
2
-2,6-
SiMe
3
-4)] (2). This comparison indicates a fast deactivation for the dendrimer catalysts beyond generation
[G0]. The deactivation of [G1]-Ni
12
(5) and [G2]-Ni
36
(7) is caused by irreversible formation of catalytically
inactive Ni(III) sites on the periphery of these dendrimers. This hypothesis is supported by results of model
studies as well as ESR spectroscopic investigations. Interestingly, the use of two alternative nickelated [G1]
dendrimers [G1]
*
-Ni
12
(11) and [G1]-Ni
8
(15), respectively, in which the distance between the Ni sites is
increased, leads to significantly improved catalytic efficiencies which approximate those of the parent derivative
2 and [G0]-Ni
4
(4). Preliminary membrane catalysis experiments with [G0]-Ni
4
(4) and [G1]-Ni
12
(5) show
that 5 can be efficiently retained in a membrane reactor system. The X-ray crystal structure of the Ni(III)
complex [NiCl
2
(C
6
H
2
{CH
2
NMe
2
}
2
-2,6-SiMe
3
-4)] (16), obtained from the reaction of 2 with CCl
4
, is also
reported.
Introduction
Current research in our group concentrates on the use of
(carbosilane) dendrimers
1-4
and hyperbranched polymers
5
to
create new materials that bridge the gap between homog-
eneous and heterogeneous catalysis. In this regard, we pioneered
the first “metallodendrimer catalyst”, in which catalytically
active metal centers were connected to an inert carbosilane
framework via an appropriate (carbamate) linker group (see
Figure 1).
6
These compounds were designed to take advantage
of the activity, selectivity, and ease of characterization and
modification of homogeneous catalysts in combination with the
size properties that are necessary for effective recovery (i.e.,
nanofiltration) of the catalytic species (cf., heterogeneous
systems).
3b,7
* To whom correspondence should be adressed. Telephone: +3130
2533120. Fax: +3130 2523615. E-mail: g.vankoten@chem.uu.nl.
Department of Metal-Mediated Synthesis, Utrecht University.
Institut fu¨r Biotechnologie.
§
Department of Crystal and Structural Chemistry, Utrecht University.
|
University of Nijmegen.
Current address: Department of Chemistry, Acadia University, Wolfville,
Novia Scotia, Canada BOP1XO.
#
Address correspondence pertaining to crystallographic studies to this
author. E-mail: a.l.spek@chem.uu.nl.
(1) For reviews on this subject, see: (a) Newkome, G.; Moorefield, C.
N.; Vo¨gtle, F. Dendritic Molecules - Concepts, Syntheses, PerspectiVes;
VCH: Weinheim, 1996. (b) Newkome, G., Ed. AdVances in Dendritic
Macromolecules; JAI Press: Greenwich, CT, 1995. For more recent reviews
in dendrimer chemistry, see: (c) Bosman, A. W.; Janssen, H. M.; Meijer,
E. W. Chem. ReV. 1999, 99, 1665. (d) Newkome, G.; He, E.; Moorefield
Chem. ReV. 1999, 99, 1689. (e) Fischer, M.; Vo¨gtle, F. Angew. Chem., Int.
Ed. 1999, 38, 885 (f) Frey, H.; Lach, C.; Lorenz, K. AdV. Mater. 1998, 10,
279. (g) Gorman, C. ibid. 1998, 10, 295. (h) Gudat, D. Angew. Chem., Int.
Ed. Engl. 1997, 36, 1951. (i) Fre´chet, J. M. J.; Hawker, C. J.; Gitsov, I.;
Leon, J. W. J. Macromol. Sci. Pure Appl. Chem. 1996, A33, 1399. Dendritic
compounds have been made primarily from carbon-based materials, although
the “branching points” are frequently heteroatoms such as nitrogen
2
,
phosphorus,
3
and silicon.
4
(2) (a) Tomalia, D. A.; Taylor, A. M.; Goddard, W. A., III. Angew. Chem.
1990, 102, 119-156, Angew. Chem., Int. Ed. Engl. 1990, 29, 138-175.
(b) Tomalia, D. A. Sci. Am. 1995, 272,62-70.
(3) (a) Majoral, J. P.; Caminade, A. M. Chem. ReV. 1999, 99, 845-880
and references therein. (b) de Groot, D.; Eggeling, E. B.; de Wilde, J. C.;
Kooijman, H.; Haaren, R. J.; van der Made, A. W.; Spek, A. L.; Vogt, D.;
Reek, J. N. H.; Kamer, P. C. J.; van Leeuwen, P. W. N. M. Chem. Commun.
1999, 1623. (c) Petrucci-Samija, Guillemette, V.; Dasgupta, M.; Kakkar,
A. K. J. Am. Chem. Soc. 1999, 121, 1968. (d) Alper, H.; Bourque, S. C.;
Manzer, L. E.; Arya, P. J. Am. Chem. Soc. 2000, 122, 956.
(4) (a) van der Made, A. W.; van Leeuwen, P. W. N. M. J. Chem. Soc.,
Chem. Commun. 1992, 1400-1401. (b) Zhou, L. L.; Roovers, J. Macro-
molecules 1993, 26, 963-968. (c) Roovers, J.; Zhou, L. L.; Toporowski,
P. M.; van der Zwan, M.; Iatrou, H.; Hadjichristidis, N. ibid. 1993, 26,
4324-4329. (d) van der Made, A. W.; van Leeuwen, P. W. N. M.; de Wilde,
J. C.; Brandes, R. A. C. AdV. Mater. 1993, 5, 466-468.
(5) Schlenk, C.; Kleij, A. W.; Frey, H.; van Koten, G. Angew. Chem.,
Int. Ed. 2000, 39, 3445-3447.
(6) 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,
659-663.
12112 J. Am. Chem. Soc. 2000, 122, 12112-12124
10.1021/ja0026612 CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/22/2000

The original dendrimer catalysts gave comparable activity
per metal atom when compared with the monomeric analogues
(Figure 2, X ) Br).
8
Specifically, these Ni-based complexes
are active catalysts for the atom-transfer radical (i.e., Kharasch)
addition of polyhalogenated alkanes to olefins
9
and are char-
acterized by the incorporation of a bis(ortho) chelating monoan-
ionic “pincer” ligand. This fragment contains a formal aryl
carbanion in combination with two trans-positioned tertiary
amine donor groups and is derived from the simple para-
substituted arenes 1-R-3,5-bis[(dimethylamino)methyl]benzene
(Figure 2).
8
Ligands of this class are currently an area of active
study due to the unique catalytic properties of a variety of metal
“pincer” complexes.
10
In this report, we detail a novel synthetic methodology for
the production of metallodendrimer catalysts via a simple
procedure involving attachment of the “pincer” ligand to the
dendrimer periphery via a Si-C bond. The resulting pincer-
containing, dendrimer ligands were selectively lithiated and
transmetalated,
11
affording catalytic units which are bound to
the dendritic framework via a direct Si-C bond. This latter
feature makes the metallodendrimers chemically even more
robust than our earlier systems (Figure 1). However, removal
of the carbamate-based linker will dramatically affect the nature
of the sterical crowding at the periphery of the CS-dendritic
species and, in particular, the mutual distance between the Ni
catalytic sites. We have further explored the catalytic activity
of these new metallodendrimers in the atom-transfer radical
addition reaction and compared the turnover numbers and
electrochemical properties of these materials with the respective
mononuclear analogue [NiCl(C
6
H
2
{CH
2
NMe
2
}
2
-2,6-R-4)] (Fig-
ure 2, R ) SiMe
3
).
In an earlier preliminary communication, we have reported
that the higher-generation metallodendritic catalysts have dra-
matically lower catalytic activities and that the Ni sites no longer
act as independent catalytic units.
12
However, it was also shown
that an increase in spatial separation between the nickel sites
brought about by the use of sterically less congested CS-
dendritic supports, led to retention of catalytic activity. The
underlying mechanistic reasons for the deactivation of the
higher-generation nickel-containing dendrimers have been stud-
ied in more detail and are described herein. Moreover, recent
results demonstrate that modern ultrafiltration membrane reac-
tors can be used to effectively recover the dendrimer catalysts
and hence can be employed for continuous-operation applica-
tions in homogeneous catalysis.
13
Results and Discussion
Preparation of Nickelated Carbosilane Dendrimers. Re-
cently, we reported the synthesis of carbosilane dendrimers
functionalized with terminal bidentate (C,N) or tridentate
(N,C,N) ligand fragments.
11
These systems could be selectively
and quantitatively converted into their corresponding polylithi-
ated analogues. We anticipated that these lithiated carbosilanes
would be useful starting materials for the synthesis of metal-
lodendrimers containing various (N)CN-ligated transition metal
fragments. Therefore, we extended these earlier studies and
prepared a series of novel nickel-containing carbosilane den-
drimers with the NCN-NiCl fragments attached to the dendritic
backbone via a direct, stable Si-C bond. We prepared the model
compounds [NiCl(C
6
H
2
{CH
2
NMe
2
}
2
-2,6-SiMe
3
-4)] (2) and
Me
2
Si[C
6
H
2
{CH
2
NMe
2
}
2
-3,5-(NiCl)-4]
2
(3) to serve as refer-
ence compounds in catalysis and for electrochemical investiga-
tions using cyclic voltammetry.
The mono-nickelated derivative [NiCl(C
6
H
2
{CH
2
NMe
2
}
2
-2,6-
SiMe
3
-4)] (2) was obtained by a two-step reaction sequence.
Lithiation of 1-trimethylsilyl-3,5-bis[(dimethylamino)methyl]-
benzene
14
with t-BuLi in hexane at room temperature (RT) for
18 h, followed by treatment of the lithiated intermediate with
[NiCl
2
(PEt
3
)
2
]
15
affords 2 as an orange solid (61% yield). The
bis-nickelated derivative Me
2
Si[C
6
H
2
{CH
2
NMe
2
}
2
-3,5-(NiCl)-
4]
2
(3) was produced in a 66% yield by a similar approach using
the silane derivative Me
2
Si[C
6
H
3
{CH
2
NMe
2
}
2
-3,5]
2
(1)as
starting material. This latter compound was obtained as a yellow
oil and isolated in quantitative yield by treatment of Me
2
SiCl
2
with an excess of the lithium reagent [Li(C
6
H
3
{CH
2
NMe
2
}
2
-
3,5)]
11a
( ) Li-NCN, Scheme 1). The dendrimer analogues of
(7) (a) Hovestad, N. J.; Eggeling, E. B.; Heidbu¨chel, H. J.; Jastrzebski,
J. T. B. H.; Kragl, U.; Keim, W.; Vogt, D.; van Koten, G. Angew. Chem.,
Int. Ed. 1999, 111, 1763. (b) Brinkmann, N.; Giebel, D.; Lohmer, G.; Reetz,
M. T.; Kragl, U. J. Catal. 1999, 183, 163-168. (c) Giffels, G.; Beliczey,
J.; Felder, M.; Kragl, U. Tetrahedron: Assymmetry 1998, 9, 691-696.
(8) (a) van de Kuil, L. A.; Luitjes, H.; Grove, D. M.; Zwikker, J. W.;
van de Linden, J. G. M.; Roelofsen, A. M.; Jenneskens, L. W.; Drenth,
W.; van Koten, G. Organometallics 1994, 13, 468-477. (b) Grove, D. M.;
Verschuuren, A. H. M.; van Koten, G. J. Mol. Catal. 1988, 45, 169. (c)
Grove, D. M.; Verschuuren, A. H. M.; van Koten, G.; van Beek, J. A. M.
J. Organomet. Chem. 1989, 372,C1-C6.
(9) (a) Gossage, R. A.; van de Kuil, L. A.; van Koten, G. Acc. Chem.
Res. 1998, 31, 423. (b) van de Kuil, L. A.; Grove, D. M.; Gossage, R. A.;
Zwikker, J. W.; Jenneskens, L. W.; Drenth, W.; van Koten, G. ibid. 1997,
16, 4985-4994. (c) Kharasch, M. S.; Jensen, E. V.; Urry, W. H. Science
1945, 102, 128.
(10) For NCN-type ligands, see: (a) van Koten, G. Pure Appl. Chem.
1989, 61, 1681-1694. (b) Rietveld, M. H. P.; Grove, D. M.; van Koten,
G. New J. Chem. 1997, 21, 751-771. (c) Stark, M. A.; Jones, G.; Richards,
C. J. Organometallics 2000, 19, 1282. For PCP-type ligands, see: (d) Dani,
P.; Karlen, T.; Gossage, R. A.; Gladiali, S.; van Koten, G. Angew. Chem.,
Int. Ed. 2000, 39, 743. (e) Gupta, M.; Hagen, C.; Kaska, W.; Cramer, R.
E.; Jensen, C. M. J. Am. Chem. Soc. 1997, 119, 840. (f) Lee, D. W.; Kaska,
W. C.; Jensen, C. M. Organometallics 1998, 17, 1. (g) Jensen, C. M. Chem.
Commun. 1999, 2443. (h) Liu, F.; Goldman, A. S. Chem. Commun. 1999,
655. (i) Longmire, J. M.; Zhang, X. Organometallics 1998, 17, 4374. (j)
Gorla, F.; Togni, A.; Venanzi, L. M.; Albinati, A.; Lianza, F. Organome-
tallics 1994, 13, 1607. For SCS-type ligands, see: (k) Loeb, S. J.; Shimizu,
G. K. H. J. Chem. Soc., Chem. Commun. 1993, 1395. (l) Moulton, C. J.;
Shaw, B. L. J. Chem. Soc., Dalton Trans. 1975, 1020. (m) Bergbreiter, D.
E.; Osburn, P. L.; Liu, Y.-S. J. Am. Chem. Soc. 1999, 121, 9531. For a
review on C-C bond activation by “pincer” type complexes, see: (n)
Rybtchinski, B.; Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 870.
(11) (a) Kleij, A. W.; Klein, H.; Jastrzebski, J. T. B. H.; Smeets, W. J.
J.; Spek, A. L.; van Koten, G. Organometallics 1999, 18, 268. (b) Kleij, A.
W.; Klein, H.; Jastrzebski, J. T. B. H.; Spek, A. L.; van Koten, G.
Organometallics 1999, 18, 277.
(12) For a preliminary communication of this work, see: Kleij, A. W.;
Gossage, R. A.; Jastrzebski, J. T. B. H.; Boersma, J.; van Koten, G. Angew.
Chem., Int. Ed. 2000, 39, 179.
(13) (a) Kragl, U. and Dreisbach, C. Angew. Chem., Int. Ed. Engl. 1996,
35, 642. (b) Kragl, U.; Dreisbach, C.; Wandrey, C. Membrane reactors in
Homogeneous Catalysis. Applied Homogeneous Catalysis with Organome-
tallic Compounds; Cornils, B., Herrmann, W. A., Eds.; VCH: Weinheim,
1996; pp 833-843.
(14) Steenwinkel, P.; James, S. L.; Grove, D. M.; Veldman, N.; Spek,
A. L.; van Koten, G. Chem. Eur. J. 1996, 2, 1440.
(15) This transmetallating reagent is easily prepared from the reaction
between NiCl
2
6H
2
O and PEt
3
in EtOH. See: Jensen, K. A. Z. Anorg. Allg.
Chem. 1936, Band 229, 273.
Figure 1. Schematic representation of a metallodendritic catalyst.
6
Figure 2. Schematic structure of para-functionalized nickel complexes
derived from the NCN ligand.
Carbosilane-Supported Arylnickel(II) Catalysts J. Am. Chem. Soc., Vol. 122, No. 49, 2000 12113

2 and 3 were synthesized in moderate to good yields (32-86%)
by the reaction of their polylithiated precursors with an
appropriate amount of [NiCl
2
(PEt
3
)
2
] at RT as described below.
The synthesis of the metallodendrimers [G0]-Ni
4
(4), [G1]-
Ni
12
(5), and [G2]-Ni
36
(7) (Schemes 2 and 3) began with the
preparation of the polylithiated precursors of the dendritic ligand
precursors [G0]-SiMe
2
-NCN,
11a
[G1]-SiMe
2
-NCN
11a
and [G2]-
SiMe
2
-NCN (6). The second generation NCN-functionalized
carbosilane dendrimer 6 could be prepared in a moderate yield
(48%) as an extremely viscous oil by treatment of the poly-
chlorosilane [G2]-SiMe
2
Cl
4a
with an excess of Li-NCN as
described for the [G0]- and [G1]-generation derivatives (vide
supra).
11a
The polylithiated reagents were obtained as amor-
phous, insoluble red solids by addition of excess of t-BuLi to
these ligand systems and were subsequently purified by remov-
ing the excess t-BuLi by washing with dry hexanes. After this
step, the transmetallating reagent (i.e., [NiCl
2
(PEt
3
)
2
]) was added
as a diethyl ether solution which afforded the multimetallic
dendrimers 4, 5, and 7 after appropriate workup.
To no surprise, the use of these extremely moisture-sensitive
lithium reagents typically resulted in partial hydrolysis during
workup. As a result, we found that nickelation of the ligand
sites beyond 80-90% could not be achieved.
17
Spectroscopic
evidence for this is found in the
1
H and
13
C{
1
H} NMR spectra
of [G0]-Ni
4
(4), [G1]-Ni
12
(5), and [G2]-Ni
36
(7), in which
signals that were attributed to nonmetalated NCN fragments,
were clearly visible. A single resonance pattern is found for
both metalated as well as non-nickelated NCN sites. The
1
H
NMR spectroscopic signal integration of all NCN sites cor-
responded with the signal integration for the SiMe
2
groups of
the CS-dendritic support. The microanalyses of 4, 5, and 7 were
in agreement with the calculated percentage of metalation as
obtained by
1
H NMR spectroscopy (i.e., signal integration). It
should be noted that dendrimer [G2]-Ni
36
(7)isless soluble
than [G0]-Ni
4
(4) and [G1]-Ni
12
(5), indicating that beyond the
first generation the solubility behavior is increasingly controlled
by the peripheral organometallic groups (i.e., size). The
decreased solubility features found for 7 could explain its low
isolated yield.
In addition, we also prepared two less “condensed” [G1]
nickelated carbosilane dendrimers. These alternative [G1] den-
drimers were constructed to be able to vary the proximity
between the surface-bonded nickel sites and their accessibility
for substrate molecules (vide infra).
Two different approaches were used to increase the separation
between the external organonickel(II) chloride groups. The first
approach involves the introduction of an inert carbosilane tether
(i.e., -CH
2
CH
2
CH
2
Si(Me)
2
-) on the [G1] periphery. The syn-
thetic pathway leading to the extended nickel-containing den-
drimer 11 (abbreviated as [G1]*-Ni
12
) is outlined in Scheme 4.
The synthesis of [G1]*-Ni
12
(11) begins with an allylation
reaction of the known chlorosilane [G1]-SiMe
2
Cl
4a
with an
excess of allylMgBr to yield the dodeca-olefin [G1]-SiMe
2
-
CH
2
CHdCH
2
(8) as a viscous oil (88% yield). Compound 8
was quantitatively converted into [G1]-C
3
H
6
-SiMe
2
Cl (9)by
hydrosilylation in the presence of a large excess of HSiMe
2
Cl
using [(NBu
4
)
2
PtCl
6
] as catalyst. Treatment of 9 with an excess
of Li-NCN
11a
afforded the extended dendrimer ligand [G1]-
C
3
H
6
-SiMe
2
-NCN (10) as a yellow, vicous oil in 87% yield.
Finally, the nickelated derivative [G1]*-Ni
12
(11) was prepared
(50% yield) by a similar lithiation/transmetalation procedure
as described earlier for 4, 5, and 7. The moderate yield of 11 is
probably a result of the increased solubility behavior of this
compound. This rendered the purification and workup of 11
less efficient. The dendritic materials 8-11 were identified by
1
H,
13
C{
1
H}, and
29
Si{
1
H} NMR spectroscopy, as well as by
MALDI-TOF mass spectrometry and combustion analyses.
A second approach to increase the separation between the
catalytic sites is depicted in Scheme 5 and was aimed at the
introduction of less [G1] periphery-bonded nickel(II) complexes.
This leads to the [G1] nickelated dendrimer 15 (abbreviated as
[G1]-Ni
8
), which was prepared in three consecutive steps. The
carbosilane dendrimer species [G1]-(CH
2
CHdCH
2
)
8
(12) was
prepared using a procedure described in the literature
4
and fully
characterized
17
before its use in subsequent syntheses.
The first step in the synthesis of [G1]-Ni
8
(15) was the
hydrosilylation of 12 in neat HSiMe
2
Cl under platinum catalysis
as described for 9 (vide supra) which gave dendrimer [G1]-
(SiMe
2
Cl)
8
(13: 95% yield). The latter was immediately
converted into the air stable dendrimer [G1]-(NCN)
8
(14)by
treatment with an excess of Li-NCN as described for 10. The
nickelated dendrimer 15 was obtained as an orange solid in 79%
yield by using a similar lithiation/transmetalation sequence as
was used for 11. The structures of 12-15 were confirmed by
NMR spectroscopy (
1
H,
13
C{
1
H}, and
29
Si{
1
H}), mass spec-
trometry (MALDI-TOF), and elemental analyses.
As for the other nickelated dendrimers, an incomplete met-
alation was observed for 11 and 15. The percentage of nickel
incorporation was estimated by
1
H NMR spectroscopy and ele-
mental analyses and indicated a nickel loading for both den-
drimers (11: 79%; 15: 89%) falling in the same range as was
established for 4, 5, and 7 (viz. 80-90%). These results clearly
demonstrated the limitations but also the reproducibility of the
lithiation/transmetalation sequence for introducing metal sites
into this type of NCN-containing dendrimer.
Characterization of (Nickelated) Dendrimers: NMR Spec-
troscopy. As for the related model compounds 2 and 3,
18
NMR
(16) We have found that similar transmetalation reactions with Pt
II
salts
gave comparable results.
12b
(17) To our knowledge, no analytical data was available for this particular
dendrimer.
Scheme 1: Synthesis of Model Compounds 2 and 3
a
a
Reagents and conditions: (i) 2 equiv t-BuLi, Et
2
O, -78 °C; (ii)
0.5 equiv Me
2
SiCl
2
,Et
2
O, -78 °C f RT; (iii) excess Me
3
SiCl, Et
2
O,
78 °C f RT; (iv) t-BuLi, pentane, RT; NiCl
2
(PEt
3
)
2
,Et
2
O.
12114 J. Am. Chem. Soc., Vol. 122, No. 49, 2000 Kleij et al.

(
1
H and
13
C{
1
H}) spectroscopic characterization of 4-15
18
(C
6
D
6
;
RT) revealed single resonances patterns for the CH
2
N, N(CH
3
)
2
,
and Si(CH
3
)
2
groups. This result suggests free rotation for both
the dendritic branches around the central Si (core) atom and
the ligands located on the dendrimer periphery. Of particular
note is that in the
13
C{
1
H} NMR spectrum of 5, the inner and
periphery CH
2
fragments of the carbosilane skeleton gave rise
to five separate resonances, while a theoretical sixth resonance
is most probably coincident with one of the signals observed
for the other methylenic groupings. In the 75 MHz
13
C{
1
H}
NMR spectrum obtained for the [G2] dendrimer ligand 6;
however, the signals corresponding for the methylenic carbon
atoms are overlapped at this field, and as a result, only three
broad peaks were observed. This suggests that the intramolecular
rotational freedom in these dendrimer molecules decreases with
increasing generation number.
29
Si{
1
H} NMR spectroscopy
proved to be a very helpful technique for a further structural
assignment of 6. This NMR spectrum showed four (expected)
resonances at 1.48, 1.14, 0.75, and -3.76 ppm, respectively.
Strangely, one additional resonance (δ )-3.07 ppm) is
observed, and this could indicate the presence of structural
defects present in 6. This latter resonance is ascribed to a silicon
center which is bonded to an aryl group. The observation of
this extra resonance could be caused by the introduction of, for
example, Markovnikov addition of SiMe
2
Cl groups, followed
by arylation of the resulting β-SiCl groups. We estimated this
defect to be present in only small amounts (4%) by means of
signal integration.
The NMR spectra of the extended dendrimer ligand precursors
8-10 is likewise helpful for structural assignment. Of particular
note is the
13
C{
1
H} NMR spectrum of 8, which displayed six
separated resonances for the methylenic groups of the carbosi-
lane support of which three were significantly smaller in
intensity. The
29
Si{
1
H} NMR spectrum of the same compound
showed three distinctive resonances (δ ) 1.10, 1.04, and 0.89
ppm, respectively). The hydrosilylation of 8 to give [G1]-C
3
H
6
-
SiMe
2
Cl (9) could be easily monitored, as the resonances of
the olefinic groups in the
1
H and
13
C{
1
H} spectra of 8
disappeared with time. This was accompanied by the appearance
of additional resonances assigned to new CH
2
and Si(CH
3
)
2
groups. The
29
Si{
1
H} NMR spectrum of 9 clearly showed the
new signal (δ ) 30.76 ppm) attributed to the introduced SiMe
2
-
Cl fragment. The NMR spectra recorded for [G1]-C
3
H
6
-
SiMe
2
-NCN (10) and [G1]*-Ni
12
(11) showed an increased
overlap between the different dendrimer segments and hence
the presence of broader resonance patterns. Similar NMR
spectroscopic observations were encountered for the dendrimer
ligand precursors 12-14 and [G1]-Ni
8
(15).
Mass Spectrometric Analysis. Although all nickelated
dendrimers are sensitive to oxidation, one can readily perform
mass spectrometric analyses for the model derivatives 2, 3, and
4. For this purpose, fast atom bombardment (FAB) mass
spectrometry proved to be an effective method for analyzing
these relatively small molecules. This procedure afforded spectra
containing many ion clusters with well-resolved isotopic pat-
terns. The FAB-MS spectra of 2-4 displayed characteristic
(18) The NMR spectra of the (nickelated dendrimer) species tend to suffer
from line-broadening which is most likely caused by the presence of traces
of paramagnetic Ni
III
. To overcome this problem, NMR samples containing
these compounds can be treated with CO(g) to reduce traces of Ni(III) prior
to spectroscopic measurement which reduces the line-broadening. See:
Grove, D. M.; van Koten, G.; Mul, W. P.; van der Zeijden, A. A. H.;
Terheijden, J. Organometallics 1986, 5, 322-326.
Scheme 2: Synthesis of [G0] and [G1] Dendrimers 4 and 5
a
a
Reagents and conditions: (i) t-BuLi, pentane, RT; NiCl
2
(PEt
3
)
2
,Et
2
O.
Carbosilane-Supported Arylnickel(II) Catalysts J. Am. Chem. Soc., Vol. 122, No. 49, 2000 12115

isotope distributions for their molecular ions (m/z ) 356.2,
626.1, and 1566.2, respectively) while the most intensive peaks
in these spectra were attributed to the [M - Cl]
+
fragment ions
(m/z ) 321.2, 591.1, and 1529.1, respectively). Despite the
complexity of these spectra, the presence of molecular ions with
correct isotopic patterns is consistent with the proposed stoi-
chiometry of 2-4. The FAB-MS spectrum of 4 shows clear
isotopic patterns at m/z ) 1472.3, 1437.3, 1378.4, and 1343.4
which are attributed to incomplete nickelated molecular and
fragment ions [G0]-Ni
3
Cl
3
, [G0]-Ni
3
Cl
2
, [G0]-Ni
2
Cl
2
, and [G0]-
Ni
2
Cl
1
, respectively. This can, obviously, be a consequence of
incomplete metalation (vide supra) during the synthesis of 4.
We applied MALDI-TOF-MS as an additional analytical tool
for the relatively larger dendrimer molecules 5-15. As could
be expected, the mass spectrometric characterization of the larger
dendrimers was hampered by the appearance of very broad
peaks in the spectra of the nickelated dendrimers 5, 7, and 11.
This feature was partially ascribed to an incomplete nickel
incorporation that gives a distribution of dendrimer species, of
which the expected isotopic distributions of the molecular and
fragment ions most likely strongly overlap. The broadness of
these peaks may also be influenced by the oxygen-sensitivity
of these nickelated dendrimers.
We were able to obtain a reasonable mass spectrum for [G2]-
SiMe
2
-NCN (6)(m/z ) 11453, calcd 11646). The difference
(193) can be explained by an acid-mediated cleavage of one
NCN-H ligand precursor (calculated mass 192) ligand from
the [G2] periphery and the resultant formation of a silicon cation.
Also for [G1]-Ni
8
(15) a reasonable MALDI-TOF-MS analysis
could be performed. The mass spectrum obtained for 15 clearly
showed peaks at m/z ) 3446.72, 3409.43, and 3349.42 which
were attributed to the (incompletely nickelated) molecular and
fragment ions [G1]-Ni
8
Cl
8
, [G1]-Ni
8
Cl
7
, and [G1]-Ni
7
Cl
7
.
The mass spectra collected for the precursor compounds and
ligand systems [G1]-SiMe
2
-CH
2
CHdCH
2
(8), [G1]-C
3
H
6
-
SiMe
2
-NCN (10), [G1]-(CH
2
CHdCH
2
)
8
(12), and [G1]-
(NCN)
8
(14) showed in all cases a clear peak for the molecular
ion consistent with the proposed stoichiometry. It should be
noted that for 8 and 12 a silver salt was added to increase the
peak resolution, which resulted in the observation of (M + Ag)
+
molecular ion clusters. In the case of 10 and 14, an acidic matrix
was used to provoke the in situ formation of (M + H)
+
molecular ions.
Molecular Modeling of [G0]-Ni
4
(4) and [G1]-Ni
12
(5).
Molecular mechanics calculations (MM2)
19
were performed on
the [G0] and [G1] nickel-containing species to give some
impression about their gas-phase three-dimensional structure.
The size and shape of 5 is shown in Figure 4, which represents
one of the calculated conformations. For 4, a molecular radius
of 20 Å was calculated, while for 5 this radius was extended
to 30 Å. It should be noted that these idealized structures may
be exaggerated and only give information about the volume of
a fully “stretched” (i.e., spherical) dendrimer. We found a
profound difference between the calculated three-dimensional
structures of 4 and 5. The “front” view of 5 showed this
molecule to be a generalized spherical molecule like in 4. The
“side” view (90° rotation) of 5, however, showed a much more
densely packed system. This phenomenon has also been reported
by other authors.
20
Nevertheless, this information is useful in
cases where a certain molecular size is prerequisite for the
effective use of ultrafiltration techniques or membrane technol-
ogy for catalyst recovery (vide infra).
7,13
While in 4 the Ni sites are well-separated (average Ni-Ni
distance 19 Å), in 5 these sites are situated in a much smaller
volume and are in close proximity to each other (closest Ni-
Ni distance about 8-11 Å). It is important to note that the
nonrandom distribution of the nickel complexes in the dendrimer
system 5 could have an effect on the accessibility of the Ni(II)
sites in catalytic applications. In our earlier dendrimer species
(see Figure 1),
6
the nickel sites were connected to the dendrimer
periphery via (long) carbamate linkers, and it was anticipated
that the nickel sites are well-separated as in [G0]-Ni
4
(4).
Although we were not able to perform molecular modeling for
[G2]-Ni
36
(7),
21
one could imagine that in this metallodendrimer
the catalytic sites are even more closely packed when compared
to [G1]-Ni
12
(5).
(19) CAChe Molecular Mechanics at the MM2 level, Oxford molecular
group.
(20) Huck, W. T. S. Thesis Noncovalent Synthesis of Nanosize Metal-
lodendrimers. Technical University of Twente, Twente, 1997.
(21) We were unable to perform Molecular Modeling for the [G2]
derivative 7 with our software (CAChe, MM2) because of the large number
of atoms present in the molecule.
Scheme 3: Synthesis of [G2] Dendrimers 6 and 7
a
a
Reagents and conditions: (i) excess Li-NCN, Et
2
O, -78 °C f
RT; (ii) t-BuLi, pentane, RT; NiCl
2
(PEt
3
)
2
,Et
2
O.
12116 J. Am. Chem. Soc., Vol. 122, No. 49, 2000 Kleij et al.

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