Titanocene dichloride complexes bonded to carbosilane dendrimers via a spacer of variable length - Molecular dynamics calculations and catalysis of allylic coupling reactions Metallodendrimers Special Issue

Article abstract of DOI:10.1016/j.ica.2013.07.020

Carbosilane metallodendrimers of the first generation Si{CH ₂CH₂CH₂SiMe₂CH₂CH ₂SiMe₂(CH₂)_nC₅H ₄TiCl₂C₅H₅

Full text of DOI:10.1016/j.ica.2013.07.020

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Accepted Manuscript
Titanocene dichloride complexes bonded to carbosilane dendrimers via a spacer
of variable length – molecular dynamics calculations and catalysis of allylic
coupling reactions
Tomá š Straš ák, Florian Jaroschik, Marek Malý, Jan Čermák, Jan Sýkora, Radek
Fajgar, Jindřich Karban, Dominique Harakat
PII:
S0020-1693(13)00391-5
http://dx.doi.org/10.1016/j.ica.2013.07.020
ICA 15563
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Please cite this article as: T. Straš ák, F. Jaroschik, M. Malý, J. Čermák, J. Sýkora, R. Fajgar, J. Karban, D. Harakat,
Titanocene dichloride complexes bonded to carbosilane dendrimers via a spacer of variable length – molecular
dynamics calculations and catalysis of allylic coupling reactions, Inorganica Chimica Acta (2013), doi: http://
dx.doi.org/10.1016/j.ica.2013.07.020
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Titanocene dichloride complexes bonded to carbosilane dendrimers via a spacer of variable
length – molecular dynamics calculations and catalysis of allylic coupling reactions
Tomáš Strašáka, Florian Jaroschikb, Marek Malýc, Jan Čermáka,c*, Jan Sýkoraa, Radek Fajgara,
Jindřich Karbana, and Dominique Harakatb
aInstitute of Chemical Process Fundamentals, v.v.i., Academy of Sciences of the Czech
Republic, Rozvojová 135, 165 02 Prague 6, Czech Republic
bInstitut de Chimie Moléculaire de Reims, UMR, CNRS 7312, Case postale 44
UFR des Sciences Exactes et Naturelles, BP 1039, 51687 Reims Cedex 2, France
cFaculty of Science, J. E. Purkinje University Ústí nad Labem, České mládeže 8, 400 96 Ústí
nad Labem, Czech Republic
Abstract
Carbosilane metallodendrimers of the first generation
Si{CH2CH2CH2SiMe2CH2CH2SiMe2(CH2)nC5H4TiCl2C5H5}4 and
Si{CH2CH2CH2SiMe[CH2CH2SiMe2(CH2)nC5H4TiCl2C5H5]2}4 (n = 1, 5) and of the second
generation Si{CH2CH2CH2SiMe[CH2CH2CH2SiMe2CH2CH2SiMe2(CH2)nC5H4TiCl2C5H5]2}4
(n = 1, 5) and
Si{CH2CH2CH2SiMe[CH2CH2CH2SiMe(CH2CH2SiMe2(CH2)nC5H4TiCl2C5H5)2]2}4 (n = 5)
were prepared, purified using GPC-NMR and GPC, and characterized by multinuclear NMR,
IR and ESI-TOF mass spectrometry.
Computer atomistic models of the second generation metallodendrimers with the length of
carbon spacer n = 1, 3, and 5 carbon atoms were created, parametrized, and consequently
simulated in THF solvent using molecular dynamics. The equilibrated dendrimer model
structures were analyzed to provide inter alia density distribution of selected molecular
components. It was shown that i) average distance between the closest pairs of Ti atoms
(around 11 Å), ii) permanent accessibility of Ti atoms from many sites even if they were in
close contact with another Ti atom, and iii) high dynamics of titanocene groups were
favourable conditions to fully use all Ti atoms for the catalytic reaction in case of the longest
five carbon spacer.
Dendrimers of the second generation with 8 and 16 end groups and the five carbon spacer
were used in the catalytic system with manganese metal to catalyze allylic coupling reactions
of cinnamyl and geranyl bromides with low catalyst loadings.
Keywords
Metallodendrimers; Titanocene dichloride; Molecular dynamics; Allylic homocoupling
1. Introduction
Dendrimers represent a recently much studied class of nanomaterials with broad applications
in medicine, in the construction of sensors, in the synthesis of soft materials with unique
properties e.g for electronics and photonics, and last but not least in catalysis of chemical
reactions. Although the first dendrimers were purely organic, efforts to incorporate other than
typically organic elements into dendritic structures appeared soon.

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Among hundreds of currently known dendritic polymer families, carbosilane dendrimers [1-
5], first reported at the beginning of 1990’s, quickly attracted attention, also due to their
properties well suited to catalytic applications. Carbosilane metallodendrimers were first used
in catalysis in 1991 [6] and great impetus for further research was provided after a paper by
van Koten et al appeared in Nature [7]. Catalysis by carbosilane metallodendrimers [8] and
catalysis by metallodendrimers in general [9-19] were reviewed.
Metallocenes are known to play a very important role in catalysis, the focus being centered to
polymerization reactions. However, metallocenes catalyze many other important reactions
[20,21].
Recently, we synthesized carbosilane metallodendrimers bearing titanocene dichloride end
groups up to the second generation with 16 peripheral metal units [22]. Dendrimer interiors
were built from propylidene units whereas the outermost layer had favorably ethylidene units.
The capping reaction was hydrosilylation of double-bond-terminated dendrimers with a Si-H
bond containing complex (so called normal hydrosilylation). A three carbon spacer between
the Si-H bond and the Cp ring of the titanocene dichloride was used.
In this work, a straightforward extension of this methodology leading to the synthesis of
dendrimers with different lengths of spacers is described. The structures of the obtained
dendrimers were modeled using molecular dynamics and the largest dendrimers were applied
in catalysis of C-C coupling reactions. So far, when metallodendrimers and dendrons with
titanocene or zirconocene units were synthesized, their typical use in catalysis was in
polymerization reactions [23-25].
2. Experimental Section
2.1 General methods and procedures
All syntheses were carried out under argon atmosphere using Schlenk or dry box techniques.
The organic solvents used were dried and purified according to standard procedures and
stored under argon.
Literature procedures were followed in the preparation of vinyl terminated carbosilane
dendrimers [22, 26-29] and starting complex 3 [30]. Starting vinyl terminated dendrimers are
labelled Nv where N is the structure number of the corresponding metallodendrimer (Schemes
1 and 2). The Nv structure is derived from N structure by imaginary removal of all atoms of
the branches from periphery up to (and including) the outermost silicon atoms and their
corresponding -hydrogens. Synthesis of metallodendrimers 11 and 14 has been reported
previously [22], metallodendrimer 13 was not obtained pure enough and its structure is
included because it was modeled. Karstedt’s catalyst (obtained as a 2-3 % solution in
xylenes), and cyclopentadienyl sodium (2M solution in THF) were purchased from Aldrich.
Complex [( 5-C5H5)TiCl3] was obtained from Acros Organics and used as received.
1H, 13C{1H}, and 29Si{1H} NMR spectra were recorded on a Varian Mercury/Inova 300 or 500
MHz spectrometers. Chemical shifts (δ/ppm) are given relative to the solvent signal (CDCl3:
δH 7.26, δC 76.99) or internal standard hexamethyldisilane (29Si). Deuterochloroform, CDCl3
(LC-NMR grade) was purchased from Euriso-top.
HPLC-NMR: A commercial HPLC system (Varian ProStar 230) with 250×4.6 mm GPC
column (Polymer Laboratories, PLgel, 5 μm particles, 300 Ǻ pore size), was employed in the
GPC-NMR experiment described. Approximately 20mg of the reaction mixture were
dissolved in 1.0 mL of CDCl3 and 20 µL were injected into HPLC. The separation was run in
pure CDCl3 (flow-rate 0.5 mL/min) and was monitored at 254 nm and by on-flow 1H NMR
detection. 1H NMR observations were conducted on Varian INOVA 500 MHz spectrometer
equipped with HCN triple resonance (60 µL active volume) microflow probe. The operating

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spectrometer frequency was 499.872 MHz, standard LC-NMR software vnmrC 6.1 was used
throughout. The spectrometer was equipped with a X,Y,Z-Performa gradient module, had four
r.f. channels, two of them fitted with waveform generators. All the separations and NMR
detection were conducted at ambient temperature (22 oC).
The 1H-NMR data were collected in on-flow mode employing WET multiple frequency
solvent suppression [31]. The signal of residual CHCl3 (δ = 7.26 ppm) and H2O (δ = 1.57
ppm) were suppressed using one scout scan prior to data collection. The data acquisition
during 1 s acquisition time covering the spectral width of 10 kHz followed after 90° RF pulse
(3.4 μs), four transients were accumulated in each spectrum. No relaxation delay was
employed. Data were zero-filled to 262K and the initial 3 data points were obtained by
backward projection (32 LP coefficients) and the residual signals of water and chloroform
were eliminated by digital solvent subtraction before Fourier transform; spline baseline
correction was applied to the spectra. No other data processing was employed.
Preparative GPC: Chromatograph Watrex with UV detection (310 nm), PLgel column
(Polymer Laboratories, 250×4,6 mm, particle size 5 μm, pore size 300 Ǻ), flow rate 5 ml/min
of CHCl3.
Electrospray ionization mass spectrometry (ESI-MS) : All experiments (MS and HRMS) were
obtained on a hybrid tandem quadrupole/time-of-flight (Q-TOF) instrument, equipped with a
pneumatically assisted electrospray (Z-spray) ion source (Micromass, Manchester, UK)
operated in positive and negative mode. The electrospray potential was set to 3 kV, and the
extraction cone voltage was usually varied between 30-80V, the samples were dissolved in
THF, acetonitrile or dichloromethane and introduced continuously at a flow rate of 5 l/min. In
order to increase the Na concentration a 1.0 mM sodium acetate solution in acetonitrile was
used for the dendritic compounds.
2.2 Preparations of starting compounds and metallodendrimers
(Dimethylsilyl)pentylcyclopentadiene, (2)
A solution of cyclopentadienylsodium in THF (50.1 mL, 0.11 mol) was added dropwise to a
solution of 3-iodopentyldimethylsilane (23.3 g, 91.1 mmol) in dry THF (20 cm3). After the
addition, the reaction mixture was stirred for 12 h and the solvent was evaporated under
vacuum. The residue was extracted repeatedly with dry pentane (3 × 15 mL) and filtered. The
filtrate was evaporated under vacuum, and the crude product was purified by flash
chromatography with CH3CN as the eluent, affording the required product as a mixture of two
isomers. Yield: 9.4 g (48.1 mmol, 53%) of a colorless liquid.
EI-MS: m/z (%): 194 (80) [M+], 179 (7), 151 (13), 113 (25), 93 (31), 80 (100), 59 (44).
1H NMR: 0.06 (d, 2JHH = 3.5 Hz, 6H, SiCH3); 0.60 (m, 2H, SiCH2); 1.37 (m, 4H,
SiCH2CH2CH2); 1.54 (m, 2H, SiCH2CH2CH2CH2); 2.38 (m, 2H, SiCH2CH2CH2CH2CH2);
2.88; 2.95 (m, 2H, CH2-Cp); 3.52 (2×heptet, 2JHH = 3.5 Hz, 1H, SiH); 6.00; 6.15; 6.25; 6.43
(4×m, 3H, Hcp). 13C NMR:
(SiCH3); 14.0; 14.1; 24.2; 28.6; 29.4; 29.8; 30.6; 32.9; 33.0
(CH2CH2CH2CH2CH2); 41.2; 43.2 (CH2-Cp); 125.7; 126.1; 130.3; 132.4; 133.5; 134.8
(CHcp); 147.4; 150.2 (Ccp). 29Si NMR: -13.10.
[TiCl2{η5-C5H4(CH2CH2CH2CH2CH2SiMe2H)}(η5-C5H5)], (4)
(Dimethylsilyl)pentylcyclopentadiene (1.78 g, 9.1 mmol) was added dropwise to a vigorously
stirred solution of n-BuLi (5.6 mL, 9.1 mmol) in 150 mL of hexane at −40°C. After the
addition the solution was slowly warmed to room temperature. The reaction mixture was
further stirred for 3 h at room temperature; the solvent was then removed at reduced pressure
and the residue washed with cold pentane. The substituted lithium cyclopentadienide in THF