Olefin hydrosilylation catalysts based on allyl Bis(phenolato) complexes of the early lanthanides

Article abstract of DOI:10.1002/asia.201000598

Non-metallocene allyl complexes of early lanthanides (lanthanum, cerium, neodymium, and samarium), that contain a sulfur-linked bis(phenolate) ligand, catalyze the regioselective hydrosilylation of styrene. Copyright

Full text of DOI:10.1002/asia.201000598

DOI: 10.1002/asia.201000598
Olefin Hydrosilylation Catalysts Based on Allyl Bis(phenolato) Complexes of
the Early Lanthanides
Elise Abinet, Thomas P. Spaniol, and Jun Okuda*[a]
Dedicated to Professor Eiichi Nakamura on the occasion of his 60th birthday
Olefin hydrosilylation is commonly catalyzed by platinum
compounds, such as the Speier[1a] or Karsted catalysts.[1b] Ho-
mogeneous late-transition-metal (Co, Rh, Pd, Pt) catalysts
have been extensively optimized with respect to activity, re-
gioselectivity, and stereoselectivity.[1c] Lanthanide complexes
are also known to efficiently catalyze the hydrosilylation of
olefins.[2] Their catalytic activity increases with the size of
the central metal, as the crucial steps involve s-bond meta-
thesis. Therefore, inert ancillary ligands other than the ubiq-
uitous metallocene frameworks are not numerous in the lit-
erature.[2,3]
unstable and difficult to manipulate,[7c] we took advantage
of the thermal stability of the tris(allyl) [Ln 3A(diox)]
(h3-C3H5)
G
E
CHTUNGTRENNUNG
(Ln=Y, La, Ce, Pr, Nd, Sm; diox=1,4-dioxane)[8–10] com-
plexes for the synthesis of precatalysts. Herein, we report
that non-metallocene allyl complexes of early rare-earth
metals containing a 1,w-dithiaalkenediyl-bridged bis(pheno-
late) ligand show high activity and regioselectivity as preca-
talysts for the hydrosilylation of styrene.
A series of allyl bis(phenolato) complexes [Ln(L)
C3H5)(thf)1À2] (L=tbbp, Y 1, La 2; L=etbbp, La 3, Ce 4,
Nd 5, Sm 6) were synthesized in tetrahydrofuran by reacting
the tris(allyl) complex [Ln 3A
(h3-C3H5) (diox)][10] with the bis-
(phenol) through propene elimination (Scheme 1). The li-
ACHTUNGTRENNUNG
(h3-
AHCTUNGTRENNUNG
Some time ago, sulfur-linked bis(phenolato) ligand scaf-
folds that contain two hard anionic oxygen donors along
with a soft thioether moiety were introduced, and these scaf-
folds can be broadly modified in terms of their steric
bulk.[4a–c] These scaffolds have allowed the development of
efficient Group 4 metal catalysts for the stereoselective
polymerization of styrene,[4d–f] and are also effective for sup-
porting larger lanthanide central atoms, such as monomeric
A
R
CHTUNGTRENNUNG
AHCTUNGTRENNUNG
gands containing both one and two sulfur donor atoms in
the backbone were found to be suitable as supporting li-
gands for the monoACTHNUGRTENUNG(allyl)-lanthanide fragment.
[Sm
methylphenolate)) and silylamido complexes of scandium,
yttrium, and lutetium [Ln(L){N(SiMe3)2}] (L=(OSSO)-type
1,w-dithiaalkenediyl-bridged bis(phenolate) ligand).[6] The
alkyl complexes of lutetium [Lu(L)(CH2SiMe3)(thf)n] (L=
ACHTUNGTRENNUNG(tbmp)ACHTUNGTRENNUNG(OAr)ACHTUNGTRENNUNG
(thf)][5] (tbmp=6,6’-thiobis(2-tert-butyl-4-
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
tbmp, mtbmp: 6,6’-methylenebis(sulfanediyl)bis(2-tert-butyl-
4-methylphenolate), etbmp (6,6’-ethane-1,2-diylbis(sulfane-
diyl)bis(2,4-di-tert-butylphenolate)), n=1–2) and the corre-
sponding hydride complexes showed high activity and regio-
selectivity in the hydrosilylation of 1-hexene.[7] As non-met-
allocene hydride complexes of the lanthanides are extremely
Scheme 1. [Ln
(h3-C3H5)
(thf)1À2] (Ln=La 3, Ce 4, Nd 5, Sm 6).
(h3-C3H5)
ACHTUGNETNR(NUNG tbbp)CAHTUGTNRENNGU ACHNUTEGRTGNN(UN thf)3À4] (Ln=Y 1, La 2) and [LnACHTUNGTRENNUNG(etbbp)-
N
ACHTUNGTRENNUNG
The medium-sized yttrium as well as the largest lantha-
nide, lanthanum, were straightforwardly coordinated with
the (OSO)-type bis(phenol) tbbp ligand. Recrystallization of
yttrium complex 1 from a tetrahydrofuran/n-pentane mix-
ture gave crystals suitable for X-Ray diffraction, which re-
vealed that the geometry around the yttrium center could
be best described as pentagonal bipyramidal with three tet-
rahydrofuran molecules in the plane (Figure 1).
[a] Dipl.-Chem. E. Abinet, Dr. T. P. Spaniol, Prof. Dr. J. Okuda
Institut fꢀr Anorganische Chemie
AHCTUNGTRENNUNG
RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
Fax : (+49)241-80-92644
E-mail: jun.okuda@ac.rwth-aachen.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000598.
Chem. Asian J. 2011, 6, 389 – 391
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
389

COMMUNICATION
sulfur atom of the (OSSO)-type ligand occupying the axial
positions. Cerium complex 4, which crystallized with an iso-
typic arrangement of the atoms as the lanthanum complex
3, showed slightly shorter bonds (ca. 1%) to the metal
center.
The monoACHTUGNETNRU(NNG allyl) complexes [LnCAHUTNGTREN(NUG etbbp)ACHTUNGTNERGUN ACHTUNGTRENNUNG(thf)1À2]
(h3-C3H5)
(Ln=La 3, Ce 4, Nd 5, Sm 6) were tested in the hydrosilyla-
tion reactions of different olefins (Table 1). Hydrosilylation
Table 1. Hydrosilylation of olefins catalyzed by early rare-earth metal
bis(phenolato) complexes.[a]
Complex RSiH3
AHCTUNGTRENGN(UN metal)
Olefin
t [h]
T [8C] Conv. [%][b] 1,2:2,1[c]
3 (La)
4 (Ce)
5 (Nd)
6 (Sm)
3 (La)
Ph
Ph
Ph
Ph
Ph
1-hexene
1-hexene
1-hexene
1-hexene
1.5-hexa-
diene
1.5-hexa-
diene
1.5-hexa-
diene
1.5-hexa-
diene
1.5-hexa-
diene
styrene
styrene
styrene
styrene
trans-stil-
bene
18
14
9
16
48
25
50
50
50
25
>99
>99
>99
>99
>99
94:6
93:7
93:7
95:5
95:5
Figure 1. ORTEP diagram of [{YACHTUNGTREN(NGUN tbbp)ACHTUNRTGENNUNG CATHNUGTREN(NUNG thf)3}·thf] 1. Thermal el-
(h3-C3H5)
lipsoids drawn at the 30% probability level. Hydrogen atoms as well as
the non-coordinated THF solvent molecules are omitted for clarity. thf=
coordinated tetrahydrofuran.[15] IUPAC recommended differentiation be-
tween coordinated molecules (lower case) and non-coordinated solvent
molecules (upper case).
4 (Ce)
4 (Ce)
5 (Nd)
5 (Nd)
Ph
nBu
Ph
14
20
10
17
50
50
50
50
>99
90
91:9
38:62
93:7
>99
92
The larger lanthanide metals lanthanum, cerium, neody-
mium, and samarium all accommodated the (OSSO)-type
bis(phenolate) ligand. The tetrahydrofuran ligands were
stable to substitution at room temperature. Variable-temper-
ature 1H NMR spectroscopy of 3 in [D8]tetrahydrofuran
showed the typical fluxional behavior for the allyl protons;
distinct syn and anti protons could only be observed at
À308C along with the multiplet for the methine proton (see
the Supporting Information). Recrystallization of the lantha-
num and cerium complexes 3 and 4 from a tetrahydrofuran/
n-pentane solution led to monomeric bis(tetrahydrofuran)
nBu
67:33
3 (La)
4 (Ce)
5 (Nd)
6 (Sm)
4 (Ce)
Ph
Ph
Ph
Ph
Ph
<30 min
<20 min
25
25
25
50
50
>99
>99
>99
>99
51
3:97
3:97
5:95
6:94
–
3
7
14
[a] Reaction conditions: 40:1 substrate/precatalyst molar ratio, a silane/
olefin ratio of 1.03:1 in C6D6 with a drop of [D8]THF. [b] Calculated by
1H NMR spectroscopy. [c] Ratio of the 1,2:2,1-regioisomers was deter-
mined by integration of the corresponding resonances in the 1H NMR
spectra.
adducts [LnACHTUNGTRENNUNG(etbbp)ACHTUNGTRENNUNG ACHTGUNTREN(NUNG thf)2] (Ln=La 3’, Ce 4’). Both
(h3-C3H5)
compounds adopted a distorted-pentagonal-bipyramidal ge-
ometry in the solid state (Figure 2). One sulfur atom and
the two oxygen atoms from the (OSSO)-type tetradentate
ligand, and two tetrahydrofuran molecules were found in
the equatorial plane, with the allyl group and the second
of 1-hexene with phenylsilane using complexes 3–6 led to
the formation of 93–95% of the anti-Markovnikov or pri-
mary product. The regioselectivity did not differ significant-
ly from metal to metal, but the conversion time was differ-
ent, with the neodymium complex 5 being the fastest. 1,5-
Hexadiene was hydrosilylated with phenylsilane and n-bu-
tylsilane. With phenylsilane, the regioselectivity for the for-
mation of the linear disilylated product was found to be be-
tween 91 (4) and 95% (3). Full conversion was achieved at
508C within 10–14 hours. With n-butylsilane, only 67%
linear product was formed for the neodymium complex 5
and 38% for the cerium complex 4. The reaction time was
also slower than that with phenylsilane.
The hydrosilylation of styrene with phenylsilane were re-
markably efficient (Scheme 2). The regioselectivity was
highest with the lanthanum 3 and cerium 4 compounds and
full conversion was achieved in less than 30 minutes at room
temperature, giving 97% Markovnikov or secondary prod-
uct. Neodymium complex 5 gave full conversion within
3 hours and samarium 6 compound required 7 hours and
higher temperatures for the reaction to proceed to comple-
Figure 2. ORTEP diagram of [CeACHTUNGTREN(NGUN etbbp)ACHTUNRTGENNUNG CAHTNUGTREN(NUGN thf)2] (4). Thermal el-
(h3-C3H5)
lipsoids drawn at the 30% probability level. Hydrogen atoms are omitted
for clarity.[15]
390
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Chem. Asian J. 2011, 6, 389 – 391

Olefin Hydrosilylation Catalysts Based on Allyl Bis(phenolato) Complexes of the Early Lanthanides
[1] a) J. L. Speier, J. A. Webster, G. H. Barnes, J. Am. Chem. Soc. 1957,
79, 974–979; b) J. L. Speier, D. E. Hook (Dow coming corp.), US
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Scheme 2. Hydrosilylation of styrene with phenylsilane.
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tion. The selectivity of the samarium compound 6 was only
slightly lower. The hydrosilylation of the less-reactive sub-
strate trans-stilbene with phenylsilane catalyzed by the
cerium compound 4 gave 51% conversion after heating at
508C for 14 hours.
[4] The ligand tbmp was originally introduced for Group 4 metal poly-
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In conclusion, we have shown that the larger early-lantha-
nide allyl complexes can serve as versatile catalyst precur-
sors for the hydrosilylation of olefins. When optimized, both
activity and regioselectivity could approach values for late-
metal catalysts. To the best of our knowledge,[7c,11–13] lantha-
num and cerium compounds 3 and 4 are among the most-
active, regioselective homogeneous rare-earth metal cata-
lysts for the hydrosilylation of styrene. At least in the case
of readily available cerium this catalyst system would offer
some advantage over precious metal catalysts.[14] Investiga-
tions into the asymmetric hydrosilylation of styrene using
catalysts with the chiral, enantiopure variant of the (OSSO)-
type ligand are currently underway.[4f]
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Experimental Section
3: A solution of etbbpH2 (151 mg, 0.3 mmol) in THF (5 mL) was added
dropwise to a solution of [LaACHTNUGRTNEUNG
(h3-C3H5)3(1,4-dioxane)] (105 mg, 0.3 mmol)
in THF (2 mL). The resulting mixture was stirred for 2 h, then concen-
trated to 3 mL under vacuum, and stored at À308C for crystallization for
a few days. Pale-yellow crystals of 3 (186 mg, 82%) were obtained after
decantation of the supernatant and drying under vacuum. 1H NMR
([D8]THF): d=1.23 (s, 18H, p-tBu), 1.41 (s, 18H, o-tBu), 1.78 (m, 6H, b-
thf), 2.49 (br. s, 2H, anti-CH2CHCH2), 2.77 (s, 4H, SCH2CH2S), 3.18 (br.
s, 2H, syn-CH2CHCH2), 3.62 (m, 6H, a-thf), 6.10 (quint, 3JHH =12.1 Hz,
1H, CH2CHCH2), 7.13 (d, 4JHH =2.5 Hz, 2H, C3-H), 7.15 ppm (d, JHH
=
4
2.5 Hz, 2H, C5-H). 13C NMR ([D8]THF): d=26.37 (b-thf), 30.02 (o-tBu,
[9] L. F. Sꢂnchez-Barba, D. L. Hughes, S. M. Humphrey, M. Bochmann,
Organometallics 2005, 24, 3792–3799.
C
C
N
ACHTUTNGRENNU(G CH3)3), 34.58 (p-tBu, CACHTUNGTRENN(UGN CH3)3), 35.96 (o-tBu,
[10] D. Robert, E. Abinet, T. P. Spaniol, J. Okuda, Chem. Eur. J. 2009,
44, 11937–11947.
[11] Y. Horino, T. Livinghouse, Organometallics 2004, 23, 12–13.
[12] M. Rastꢃtter, A. Zulys, P. Roesky, Chem. Eur. J. 2007, 13, 3606–
3616.
(C2), 125.53 (C5), 129.21 (C3), 137.27 (C4 and C6), 147.18 (CH2CHCH2),
169.20 ppm (C1). Anal. Calcd. for C37H57LaO3S2 (M: 752.90 gmolÀ1):
C 59.03, H 7.63; Found: C 58.58, H 7.63. Diffraction quality crystals of
the bisACHTUNGTRENNUNG(THF)-adduct 3’ were obtained from an n-pentane/THF mixture
at À408C.[15]
[13] P-F. Fu, L. Brard, Y. Li, T. J. Marks, J. Am. Chem. Soc. 1995, 117,
7157–7168.
[14] T. Hayashi, K. Yamasaki in Comprehensive Organometallic Chemis-
try III, Vol. 10, Elsevier, Amsterdam, 2007, Chap. 10.18, p. 815–838.
[15] CCDC 736509 (1), 736510 (3), and 736511 (4) contain the supple-
mentary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre at www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
This work was supported by the Fonds der Chemischen Industrie, the
Deutsche Forschungsgemeinschaft, and the Cluster of Excellence “Tailor-
Made Fuels from Biomass”.
Received: August 23, 2010
Published online: November 12, 2010
Keywords: allyl ligands · hydrosilylation · lanthanides ·
olefins · styrene
Chem. Asian J. 2011, 6, 389 – 391
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
391