O-benzylated-calix[4]arenes as ancillary ligands in organolanthanide chemistry

Article abstract of DOI:10.1039/b205200h

The synthesis and molecular structure of the O-benzylated-calix[4] arene supported lanthanide complexes was reported. The ligands showed distorted trigonal bipyramidal geometry at the center with ethereal oxygen atoms at the apical positions. The ancillar

Full text of DOI:10.1039/b205200h

View Article Online / Journal Homepage / Table of Contents for this issue
O-Benzylated-calix[4]arenes as ancillary ligands in organo-
lanthanide chemistry†
Frank Estler, Eberhardt Herdtweck and Reiner Anwander*
Anorganisch-chemisches Institut der Technischen Universität München, Lichtenbergstraße 4,
D-85747 Garching, Germany. E-mail: reiner.anwander@ch.tum.de
Received 28th May 2002, Accepted 11th July 2002
First published as an Advance Article on the web 19th July 2002
25,27-O-Benzylated-calix[4]arenes H2L react with Ln-
[N(SiHMe2)2]3(thf )x (Ln ؍ Sc (x ؍ 1), Lu (x ؍ 2)) via an
extended silylamide route to yield monomeric, heteroleptic
calix[4]arene complexes with a N(SiHMe2)2 ligand bonded
to the metal centre which is suitable for further ligand
exchange reactions.
Compounds 3a and 3b can be crystallized from toluene
in high yields (80–90%). Spectroscopic data and elemental
analyses are consistent with the presence of one bis(dimethyl-
silyl)amido ligand at the metal centre. Displacement of all of
the THF donor ligands occured which is probably due to the
steric demand of the benzyloxo groups and the better chelating
donor capabilities of its O(ether) atoms. The resulting steric
and/or electronic saturation of the metal centres is also indi-
cated by the absence of any Ln ؒ ؒ ؒ H–Si ß-agostic inter-
actions.12 Non-agostic bis(dimethylsilyl)amide ligands are
clearly indicated by the ν(Si–H) stretching vibration at ν = 2083
(3a) and ν = 2052 (3b) cmϪ1 in the IR spectrum and a SiH
proton displaying a septet at δ = 5.79 (3a) and δ = 5.71 (3b),
considerably downshifted compared to its synthetic precursors
(2a, δ = 5.03; 2b, δ = 4.95). The signals of the benzylic methylene
groups also appear significantly downshifted at δ = 5.64 (3a)
and δ = 5.44 (3b) (1a, δ = 4.71). Coordination of the metal
renders the ring methylene groups diastereotopic as revealed by
two doublets at δ = 4.30/2.93 (3a) and δ = 4.19/2.93 (3b). This
pattern and the signals of the tert-butyl groups at δ = 1.41 (3a)
and δ = 0.71 (3b) are in agreement with a cone or a 1,3-alternate
conformation. The 13C NMR chemical shifts of the ring methyl-
ene groups of δ 33.8 (3a) and 34.1 (3b) propose a cone conform-
ation in solution (cf., δ(CH2)cone = 32.7 for 4-tert-butylcalix-
[4]arene).13 In the solid state, the cone conformation of the
scandium derivative 3a could be unequivocally established by
an X-ray structure analysis (Fig. 1).¶
Linked alkoxide (aryloxide) ligands have attracted considerable
attention in early transition metal chemistry as ancillary ligands
of chiral precatalysts for olefin polymerization (“postmetallo-
cenes”) and organic synthesis.1,2 Calix[4]arene derived anions
comprise a set of oxygen donor atoms preorganized in a quasi-
planar fashion making them interesting aryloxide ligands for
rare earth metals.3 The overall charge and steric demand of
these ligands can be effectively tuned by partial functionaliz-
ation of the O4 set (at the lower rim), e.g., via alkylation or
silylation.4 Moreover, derivatization at the upper rim affects the
acidity of the phenol groups.5 Complexes of rare earth metals
with deprotonated calixarene ligands featuring various ring
sizes were previously obtained by salt metathesis reactions
in the presence of base molecules.6–9 Herein, we describe the
synthesis and characterization of 4-tert-butylcalix[4]arene and
calix[4]arene supported lanthanide complexes utilising our
extended silylamide route.10 ‡
The lower rim-functionalized “divalent” precursors 25,27-
dibenzyloxy-4-tert-butylcalix[4]arene 1a and 25,27-dibenzyl-
oxycalix[4]arene 1b were synthesized from the corresponding
calix[4]arenes using literature methods.11 1a was reacted with
one equivalent of the amide complexes Ln[N(SiHMe2)2]3(thf )x
(2a, Ln = Sc, x = 1; 2b, Ln = Lu, x = 2) at ambient temperature
to yield heteroleptic [LnL{N(SiHMe2)2}] (3) as shown in
Scheme 1. Remarkably, derivatives 3 of the larger rare earth
metal centers Y and La are not formed under the prevailing
reaction conditions, probably due to an intrinsic steric misfit
of the functionalized calix[4]arene bowl and the size of the
lanthanide–silylamide moiety.§
Fig. 1 Molecular structure of 3a. Selected bond lengths (Å) and
angles (Њ): Sc–N 2.060(2), Sc–O1 1.912(2), Sc–O2 2.278(2), Sc–O3
1.912(2), Sc–O4 2.248(2); O1–Sc–N 113.9(1), O2–Sc–N 100.0(1),
O1–Sc–O3 126.2(1), O2–Sc–O4 162.65(8).
The coordination geometry at the metal centre is best
described as distorted trigonal bipyramidal. Two aryloxide
oxygen atoms (O1, O3) and the nitrogen atom form the equa-
torial plane, while the ethereal oxygen atoms (O2, O4) occupy
the apical positions (O2–Sc–O4, 162.65(8)Њ). The Sc–O1/O3
bond lengths average 1.912 Å and as expected are considerably
longer than those in the formally three-coordinate complex
Scheme 1 Synthesis of discrete metal–organic rare earth calix[4]arene
complexes according to the extended silylamide route.
† Electronic supplementary information (ESI) available: synthesis
and characterization data. See http://www.rsc.org/suppdata/dt/b2/
b205200h/
3088
J. Chem. Soc., Dalton Trans., 2002, 3088–3089
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b205200h

View Article Online
Sc(OC6H2tBu2-2,6-Me-4)3 (av. 1.869 Å)14 and are comparable
to those found in other formally five-coordinate scandium
aryloxide complexes, e.g., [Sc2(4-tert-butyloxacalix[3]-arene)2-
(DMSO)2]ؒ2DMSO (1.970 Å).9b The Sc–O2/O4 bond lengths
average 2.263 Å and lie in the range for other metallated
calix[4]arene complexes featuring a formally five-coordinate
exo isomer, e.g., Al[4-tert-butylcalix[4]arene(OMe)2]H (2.125
Å)15 or Fe[4-tert-butylcalix[4]arene(OMe)2]CPh2 (2.225 Å).16
The Sc–N bond length of 2.060(2) Å compares to the av. 2.069
Å found in Sc[N(SiHMe2)2]3(thf ).17
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft and Degussa
AG for financial support. Generous support from Prof.
Wolfgang A. Herrmann is gratefully acknowledged. Frank
Estler thanks the Fonds der Chemischen Industrie for the
award of a fellowship.
Notes and references
The reaction of 1b with Lu[N(SiHMe2)2]3(thf )2 was carried
out in toluene at ambient temperature according to Scheme 1
because of the poor solubility of 1b in hexane. After crystalliz-
ation from toluene, [LuL{N(SiHMe2)2}] (4) was obtained in
high yield (90%). Its spectroscopic details are almost identical
to those of 3b. The ν(Si–H) stretching vibration of the lutetium
bonded bis(dimethylsilyl)amido ligand appears at ν = 2053 cmϪ1
‡ For convenience, the term “lanthanide” is used synonymous with the
rare earth elements Sc, Y, La and Ln = Ce–Lu.
§ We propose that also for reasons of steric discrimination, a more
complicated reaction occurred when Ln[N(SiMe3)2]3 was used as a syn-
thetic precursor.
¶ Crystallographic data for 3a: C83H104NO4ScSi2, M = 1280.81, mono-
clinic, space group Cc, a = 17.5600(4), b = 24.5441(7), c = 17.2735(4) Å,
β = 92.577(2)Њ, V = 7437.3(3) Å3, Z = 4, ρcalc = 1.144 g cmϪ3, F(000) =
2760, µ(Mo-Kα) = 0.180 mmϪ1, λ = 0.71073 Å, T = 123 K. The 20851
reflections measured on a Nonius Kappa CCD area detector yielded
12306 unique data (Θmax = 25.34Њ, Rint = 0.037), R1 = 0.0532, wR2 =
0.1276.
1
and the H NMR spectrum shows a septet at δ = 5.74. The
signals for the ring methylene groups and for the benzylic
methylene groups are at δ = 4.19/2.93 and at δ = 5.49, respect-
ively. Also for complex 4, the 13C NMR chemical shift of the
ring methylene groups of δ = 32.8 suggests a cone conformation
in solution (cf., δ(CH2) = 31.6 for calix[4]arene).13 The molec-
ular structure of 4 could be unequivocally proven by X-ray
crystallography (Fig. 2).¶
¯
For 4: C53H56NO4LuSi2, M = 1002.14, triclinic, space group P1, a =
11.3794(1), b = 14.0015(2), c = 16.9067(3) Å, α = 69.3972(6), β =
75.8968(6), γ = 69.8248(8)Њ, V = 2343.69(6) Å3, Z = 2, ρcalc = 1.420 g
cmϪ3, F(000) = 1024, µ(Mo-Kα) = 2.203 mmϪ1, λ = 0.71073 Å, T = 173
K. The 21461 reflections measured on a Nonius Kappa CCD area
detector yielded 8139 unique data (Θmax = 25.38Њ, Rint = 0.042), R1 =
0.0329, wR2 = 0.0748. CCDC reference numbers 182440 (3a) and
182441 (4). See http://www.rsc.org/suppdata/dt/b2/b205200h/ for crys-
tallographic data in CIF or other electronic format.
1 G. J. P. Britovsek, V. C. Gibson and D. F. Wass, Angew. Chem., Int.
Ed., 1999, 38, 428.
2 M. Shibasaki and H. Gröger, Top. Organomet. Chem., 1999, 2,
199.
3 J. M. Harrowfield, M. I. Odgen, W. R. Richmond and A. H. White,
J. Chem. Soc., Dalton Trans., 1991, 2153.
4 (a) B. Castellano, E. Solari, C. Floriani, N. Re, A. Chiesi-Villa and
C. Rizzoli, Organometallics, 1998, 17, 2328; (b) R. Anwander,
J. Eppinger, I. Nagl, W. Scherer, M. Tafipolsky and P. Sirsch, Inorg.
Chem., 2000, 39, 4713.
5 K. Araki, K. Iwamoto, S. Shinkai and T. Matsuda, Bull. Chem. Soc.
Jpn., 1990, 63, 3480.
6 (a) I. L. Fedushkin, M. Weydert, A. A. Fagin, S. E. Nefedov,
I. L. Eremenko, M. N. Bochkarev and H. Schumann, Z. Natur-
forsch., Teil B, 1999, 54, 466; (b) L. M. Engelhardt, B. M. Furphy,
J. M. Harrowfield, D. L. Kepert, A. H. White and F. R. Wilner,
Aust. J. Chem., 1988, 41, 1465.
7 (a) P. D. Beer, M. G. Drew, A. Grieve, M. Kan, P. B. Leeson,
G. Nicholson, M. I. Odgen and G. Williams, Chem. Commun., 1996,
117; (b) P. D. Beer, M. G. Drew, A. Grieve, M. Kan, P. B. Leeson,
G. Nicholson, M. I. Odgen and G. Williams, Inorg. Chem., 1996, 35,
2202.
8 M. B. Furphy, J. M. Harrowfield, M. I. Odgen, B. W. Skelton,
H. A. White and F. R. Wilner, J. Chem. Soc., Dalton Trans., 1989,
2217.
9 (a) Z. Asfari, J. M. Harrowfield, M. I. Odgen, J. Vicens and
H. A. White, Angew. Chem., Int. Ed. Engl., 1991, 30, 854; (b)
C. E. Daitch, P. D. Hampton and E. N. Duesler, Inorg. Chem., 1995,
34, 5641.
10 R. Anwander, Top. Organomet. Chem., 1999, 2, 1.
11 J.-D. van Loon, A. Arduini, L. Coppi, W. Verboom, A. Pochini,
R. Ungaro, S. Harkema and D. N. Reinhoudt, J. Org. Chem., 1990,
55, 5639.
12 J. Eppinger, M. Spiegler, W. Hieringer, W. A. Herrmann and
R. Anwander, J. Am. Chem. Soc., 2000, 122, 3080.
13 C. Jaime, J. de Mendoza, P. Prados, P. M. Nieto and C. Sánchez,
J. Org. Chem., 1991, 56, 3372.
14 P. B. Hitchcock, M. F. Lappert and A. Singh, J. Chem. Soc., Chem.
Commun., 1983, 1499.
15 M. G. Gardiner, G. A. Koutsantonis, S. M. Lawrence, P. J. Nichols
and C. L. Raston, Chem. Commun., 1996, 2035.
Fig. 2 Molecular structure of 4. Selected bond lengths (Å) and angles
(Њ): Lu–N 2.186(4), Lu–O1 2.011(3), Lu–O2 2.351(3), Lu–O3 2.011(3),
Lu–O4 2.356(3); O1–Lu–N 121.1(1), O2–Lu–N 99.4(1), O1–Lu–O3
121.8(1), O2–Lu–O4 156.86(9).
The lutetium centre in 4 shows the same coordination geom-
etry as scandium in 3a. The Lu–O (aryloxide) and Lu–O (ether)
bond lengths average 2.011 and 2.353 Å, respectively, and are
comparable to those found in five-coordinate aryloxide com-
plex Lu(OC6H3iPr2-2,6)3(thf )2 (av. 2.044 and 2.296 Å)18 and are
in agreement with other five-coordinate lanthanide calix-
[4]arene complexes, e.g., 3a and [Y(4-tert-butylcalix[4]arene-
(OSiHMe2)(thf )]2 (2.065 and 2.260 Å) taking into account the
different ionic radii.4b The Lu–N bond length, 2.186(4) Å,
equals those found in Lu[N(SiHMe2)2]3(thf )2 (2.184(3), 2.238(3)
and 2.253(3) Å).17 The different Ln–O bond lengths and the
different substitution pattern at the upper rim of complexes 3a
and 4 implicate distinct conformational orientations of the
phenyl rings. The two opposite O-alkylated phenyl rings form
angles of 69.4Њ (3a) and 76.9Њ (4) with the least square plane
formed by the four carbon atoms of the methylene groups. The
phenolic rings are tilted against this C4 plane by 45.5Њ (3a) and
48.6Њ (4), respectively. The overall pinched cone conformational
arrangement is also revealed by the O ؒ ؒ ؒ O distances of the
ether moieties (O2 ؒ ؒ ؒ O4, 4.475 Å (3a) and 4.611 Å (4)) and
those of the more flattened phenol units (O1 ؒ ؒ ؒ O3, 3.411 Å
(3a) and 3.514 Å (4)).
16 M. Giusti, E. Solari, L. Giannini, C. Floriani, A. Chiesi-Villa and
C. Rizzoli, Organometallics, 1997, 16, 5610.
17 R. Anwander, O. Runte, J. Eppinger, G. Gerstberger, E. Herdtweck
and M. Spiegler, J. Chem. Soc., Dalton Trans., 1998, 847.
18 D. M. Barnhart, D. L. Clark, J. C. Gordon, J. C. Huffman,
R. L. Vincent, J. G. Watkin and B. D. Zwick, Inorg. Chem., 1994, 33,
3487.
Preliminary investigations show that activation of complexes
3 and 4 for catalytic applications can be achieved by silylamide
ligand displacement involving Brønsted (HOMe) and Lewis
acidic reagents (AlR3).
J. Chem. Soc., Dalton Trans., 2002, 3088–3089
3089