Calixarenes as ligands for transition-metal catalysts: A bis(calix[4]arene-11,23-dicarboxylato) dirhodium complex

Article abstract of DOI:10.1039/b110478k

A novel dirhodium tetracarboxylate complex is described in which two calix[4]arene macrocycles, bridged at the upper rim by a Rh-Rh unit, serve as ligands and whose solid-state structure shows an unusual coordination of a toluene molecule in the axial pos

Full text of DOI:10.1039/b110478k

Calixarenes as ligands for transition-metal catalysts: a
bis(calix[4]arene-11,23-dicarboxylato) dirhodium complex
Jürgen Seitz and Gerhard Maas*
Division of Organic Chemistry I, University of Ulm, Albert-Einstein-Allee 11, D-89081 Ulm, Germany.
E-mail: gerhard.maas@chemie.uni-ulm.de
Received (in Cambridge, UK) 16th November 2001, Accepted 4th January 2002
First published as an Advance Article on the web 31st January 2002
A novel dirhodium tetracarboxylate complex is described in
which two calix[4]arene macrocycles, bridged at the upper
rim by a Rh–Rh unit, serve as ligands and whose solid-state
structure shows an unusual coordination of a toluene
molecule in the axial position at each rhodium atom.
methylene carbons of the macrocycle and the aromatic ring
planes are 146.2, 77.1, 145.5 and 78.5°, and the interplanar
angle between the benzene rings bridged by the dirhodium
moiety is 24.5°.
Dinuclear quadruply bridged rhodium carboxylate com-
plexes exist typically in the form Rh
2
4 2
[m-OOCR) L where L is
Calixarenes are under active investigation as three-dimensional
a donor molecule that occupies the available axial position at
1
,2
11
scaffolds for functional molecular systems.
The use of
each octahedrally coordinated metal atom. In the case of 2, a
calixarenes as ligands for transiton metals is also increasing,1
with an emphasis on complex formation at the lower rim where
hydroxy groups and their derivatives offer excellent multiple
coordination sites. Examples for metal coordination at the upper
,3
toluene molecule is found at the axial coordination site, and this
represents a rare example of arene coordination at a dirhodium
tetracarboxylate complex. The Rh–Rh axis is almost perpendic-
ular to the toluene ring plane (angle Rh1–Cp-tolyl–Ci-tolyl 97.3°)
and the shortest distances are: Rh1–Cpara 2.580, Rh1–Hpara 2.64
(H in calculated position), Rh1–Cmeta1 2.968, Rh1–Cmeta2 3.027
Å; the values for the second independent molecule are similar.
2
rim are still rare: A calix[4]arene with para-PPh substitution at
opposite rings was found to form trans-chelate complexes with
4
several transition metals (Ag, Pd, Pt, Ru), and related water-
soluble calixarene–phosphanes were investigated as ligands for
the Rh-catalysed two-phase hydroformylation of 1-alkenes.5
Upper-rim mono-functionalised calixarene–diphenylphosphane
rhodium complexes have also been prepared.6
This geometry is different from the one-dimensional coordina-
11f
tion copolymer [Rh
2
(OOCCF
3
)
4
·C
6
Me
6
]
8
,
which features a
2
h -coordination of rhodium at the axial hexamethylbenzene
ligand with Rh–C distances of 2.770–2.786 Å. The metal–arene
coordination in the present case resembles the calculated
With a view to the importance of dimeric rhodium(II) acetate
1
2
[
Rh
2
(OAc)
4
] and related carboxylate and amidate complexes as
3
structures of benzene with Lewis acids MX (M = B, Al )
7
catalysts for carbene transfer reactions with diazo compounds,
we set out to bridge the upper rim of calix[4]arenes by an Rh–
Rh unit and to evaluate the catalytic activity of the resulting
where the metal is also positioned over one aromatic ring
carbon. However, the geometry of our complex could also be
interpreted as a C–H coordination of rhodium, an interesting
aspect in the research on aromatic C–H activation by electro-
2 4
complexes. Rh (OAc) reacts with the calix[4]arene-11,23-di-
8
13
carboxylic acid 1 in boiling toluene under complete exchange
of ligands to form the bis(5,17-dibromocalix[4]arene-11,23-di-
carboxylato) dirhodium complex 2 (Scheme 1).†
philic transition-metal complexes.
Fig. 2 shows that the complexed toluene molecules also fit
quite well in the clefts which are formed by the vicinal p-
bromophenyl rings of the two calixarene units. Although this
arrangement is unsymmetrical, it resembles the perpendicular
structures of the benzene dimer14 and might contribute to the
From a toluene solution of 2, dark-green solvent-containing
crystals separate within a few days; when exposed to ambient
atmosphere, they rapidly lose toluene and form a green powder.
The XRD analysis at 280 °C reveals the composition of 2·4
toluene.‡ Two solvent molecules are part of the complex, while
the remaining two occupy positions between the complex
entities. As expected, a Rh–Rh unit bridges a cone-shaped
calixarene molecule through coordination of two bidentate
carboxylate groups, and it connects two calixarenes in a head-
to-head fashion to form an expanded capsula open at two
opposite faces9 (Fig. 1). The complexation of the two
carboxylate groups at the upper rim generates a highly distorted
pinched-cone conformation of each calixarene macrocycle with
two flattened and two nearly parallel opposing aromatic rings.10
The angles between the least-squares plane defined by the four
2
Fig. 1 (Left) ORTEP drawing of complex 2(toluene) . Only one of the two
independent molecules is shown. The ellipsoids of thermal vibration
represent a 20% probability. Selected bond distances (values in square
brackets refer to the second molecule): Rh1–Rh1A (at –x, 1 2 y, 1 2 z)
2
.399(2) [2.395(2)], Rh–O 2.009(6)–2.043(6) [2.026(6)–2.040(6)], Rh–
p-tolyl 2.580(10) [2.605(10)] Å. Bond angles: O1–Rh1–O3 83.8(2), O1–
Rh1-O4 95.7(2), O1–Rh1–Rh1A 88.8(2), O4–Rh1–Rh1A 88.3(2)°. (Right)
Space filling model of the complex 2(toluene)
C
Scheme 1 Synthesis of the bis(calix[4]arene) dirhodium complex 2.
2
.
3
38
CHEM. COMMUN., 2002, 338–339
This journal is © The Royal Society of Chemistry 2002

dark-green prism, 0.3130.4230.54 mm; Stoe IPDS diffractometer. The
structure was solved using SIR 92 and refined by the full-matrix least-
2
squares method on F using SHELX 97. Hydrogen atoms (except for the
methyl protons of one toluene molecule) were included at calculated
positions and treated as riding on their bond neighbours. Refinement of
1
146 variables (8 restraints) gave R = 0.0641 and R
w
= 0.1522 for 7340
= 0.1662 for 12010
reflections with I > 2s(I) and R = 0.1038, R
w
independent reflections. CCDC reference number 170079. See http://
www.rsc.org/ suppdata/cc/b1/b110478k/ for crystallographic data in CIF or
other electronic format.
Fig. 2 Reactions catalyzed by 2.
1
2
V. Böhmer, Angew. Chem., Int. Ed. Engl., 1995, 34, 713.
C. D. Gutsche, Calixarenes Revisited, The Royal Society of Chemistry,
Cambridge, 1998.
energetic stabilisation of this host–guest arrangement.15 For the
two independent complex molecules in the unit cell, the shortest
distances between H atoms of toluene and C atoms of the
calixarene are in the following ranges: Hmeta–Cipso(Br) 3.01–3.21
Å, Hmeta–Cortho(Br) 3.30–3.39 Å, Hortho–Cortho(Br) 3.25-3.32 Å.
A preliminary evaluation of the catalytic activity of 2 was
carried out for two important carbene transfer reactions, alkene
cyclopropanation and intramolecular C–H insertion (Scheme
3 C. Wieser, C. B. Dieleman and D. Matt, Coord. Chem. Rev., 1997, 165,
93; S. Steyer, C. Wieser, D. Armspach, D. Matt and J. Harrowfield, in
Calixarenes 2001, ed. Z. Asfari, V. Böhmer, J. Harrowfield and J.
Vicens, Kluwer, Dordrecht, 2001, p. 513.
4
C. Wieser-Jeunesse, D. Matt and A. De Cian, Angew. Chem., Int. Ed.,
998, 37, 2861; I. A. Bagatin, D. Matt, H. Thönnessen and P. G. Jones,
1
Inorg. Chem., 1999, 38, 1585.
S. Shimizu, S. Shirakawa, Y. Sasaki and C. Hirai, Angew. Chem., Int.
Ed., 2000, 39, 1256.
5
2
). The formation of cyclopropanes 4 from alkenes 3 and methyl
diazoacetate (MDA) was readily catalysed by 1 mol% of 2 at 20
C in CH Cl with the following results+styrene, 98% yield,
E+Z = 72+28; cyclohexene, 63%, exo+endo 72+28; 2-methyl-
-butene, 52%, anti:syn = 45+55. A comparison with the
Rh (OAc) -catalysed cyclopropanation of the same alkenes
6 M. Vézina, J. Gagnon, K. Villeneuve, M. Drouin and P. D. Harvey,
Chem. Commun., 2000, 1073.
°
2
2
7 M. P. Doyle, M. A. McKervey and T. Ye, Modern Catalytic Methods for
Organic Synthesis with Diazo Compounds — From Cyclopropanes to
Ylides, John Wiley & Sons, New York, 1998; A. Padwa and K. E.
Krumpe, Tetrahedron, 1992, 48, 5385; J. Adams and D. M. Spero,
Tetrahedron, 1991, 47, 1765.
2
2
4
1
6
with MDA (93%, 62+38; 90%, 79+21; 97%; 60+40) indicates
that steric factors operate in the case of catalyst 2: the yields
decrease for the more highly substituted olefins, and the
diastereoselectivity is reversed in the case of the trisubstituted
alkene. While a full control over the diastereoselectivity in this
8
9
M. Larsen and M. Joergensen, J. Org. Chem., 1996, 61, 6651.
Capsule-shaped coordination motifs have recently been reported for
calix[4]arene-p-sulfonates which are connected by aquo-bridged Na
ions: H. R. Webb, M. J. Hardie and C. L. Raston, Chem. Eur. J., 2001,
7, 3616.
7
type of cyclopropanation reactions is rarely achieved, the steric
conditions at the catalytic site are more influential in the case of
intramolecular reactions since the latter have more compact
transition states. Therefore, we were pleased to find that
complex 2 catalyses the intramolecular C–H insertion of a-
diazo-b-ketoesters 5a,b with a high preference for carbene
insertion into the aromatic C–H bond (R = Me: 6:7 = 90+10,
10 Other calix[4]arenes with a short diametrical bridge at the upper rim: (a)
M. Pitarch, V. McKee, M. Nieuwenhuyzen and M. A. McKervey, J.
Org. Chem., 1998, 63, 946; (b) O. Struck, J. P. M. van Duynhoven, W.
Verboom, S. Harkema and D. N. Reinhoudt, Chem. Commun., 1996,
1
517; (c) P. Lhoták and S. Shinkai, Tetrahedron Lett., 1996, 37, 645; (d)
A. Arduini, S. Fanni, A. Pochini, A. R. Sicuri and R. Ungaro,
Tetrahedron, 1995, 29, 7951; (e) H. Goldmann, W. Vogt, E. Paulus and
V. Böhmer, J. Am. Chem. Soc., 1988, 110, 6811.
7
7% total yield; R = Ph+97+3, 46%). This site selectivity
contrasts with the same reaction catalysed by Rh (OAc)
55+45 ratio for R = Me) but it is close to the selectivity
as catalyst18 for which the steric
obtained with Rh (OOCCPh
2
4
11 Selected examples: (a) T. R. Felthouse, Prog. Inorg. Chem., 1982, 29,
73; (b) E. B. Boyar and S. D. Robinson, Coord. Chem. Rev., 1983, 50,
1
7
(
2
3
)
4
109; (c) L = I : F. A. Cotton, E. V. Dikarev and M. A. Petrukhina,
2
Angew. Chem., Int. Ed., 2000, 39, 2362; (d) L = alkene: F. A. Cotton,
L. R. Falvello, M. Gerards and G. Snatzke, J. Am. Chem. Soc., 1990,
effect of the bulky triphenylacetate ligands was demonstrated.
In conclusion, we have presented a novel type of upper-rim
functionalization of calixarenes with transition-metal units. The
coordination motif in 2 that links two calixarene molecules in a
head-to-head fashion should also be applicable to other metals.
We have shown that the rhodium center in 2 retains its catalytic
properties typical for dirhodium carboxylate complexes and that
the enhanced steric shielding of the catalytic site may be useful
for the control of stereo- and regio-selectivity in catalytic
processes.
1
12, 8979; (e) L = alkyne: F. A. Cotton, E. V. Dikarev, M. A.
Petrukhina and S.-E. Stiriba, Organometallics, 2000, 19, 1401; (f) L =
h -arene: F. A. Cotton, E. V. Dikarev and S.-E. Stiriba, Organome-
tallics, 1999, 118, 2724.
2
12 P. Tarakeshwar, J. Yong Lee and K. S. Kim, J. Phys. Chem. A., 1998,
102, 2253; P. Tarakeshwar, S. Joo Lee and K. S. Kim, J. Phys. Chem.
B, 1999, 103, 184; P. Tarakeshwar and K. S. Kim, J. Phys. Chem. A,
3
1999, 103, 9116. The benzene–AlX complexes are predicted to have a
shorter C–Al contact than the C–B complexes and to have binding
energies dominated by electrostatic interactions. Our toluene–Rh
This research was supported by the Fonds der Chemischen
Industrie.
3
complex is comparable with the AlX complexes.
1
3 Two general mechanisms are discussed for this reaction type, concerted
metal insertion into the aromatic C–H bond and formation of a metal
arenium complex: A. Vigalok, O. Uzan, L. J. W. Shimmon, Y. Ben-
David, J. M. L. Martin and D. Milstein, J. Am. Chem. Soc., 1998, 120,
12539; A. E. Shilov and G. B. Shul’pin, Chem. Rev., 1997, 87, 2879; W.
D. Jones and F. J. Feher, Acc. Chem. Res., 1989, 22, 91.
14 P. Hobza and Z. Havlas, Chem. Rev., 2000, 100, 4253.
15 High level ab initio calculations suggest that the energetic stabilization
of the T-shaped benzene dimer results mainly from dispersion
interactions: P. Hobza, H. L. Selzle and E. W. Schlag, J. Phys. Chem.,
1996, 100, 18790.
16 M. P. Doyle, R. L. Dorow, W. E. Buhro, J. H. Griffin, W. H. Tamblyn
and M. L. Trudell, Organometallics, 1984, 3, 44.
17 D. F. Taber and R. E. Ruckle Jr., J. Am. Chem. Soc., 1986, 108,
7686.
18 S. Hashimoto, N. Watanabe and S. Ikegami, J. Chem. Soc. Chem.
Commun., 1992, 1508.
Notes and references
†
Physical and spectroscopic data for 2: green fine powder from MeOH
o
1
after drying at 50 C/0.001 mbar; yield: 1.46 g (88%); decomp. 4290 °C. H
NMR (500.1 MHz, CDCl ): d 0.83 (t, 6H), 1.06 (t, 6H), 1.76–1.84 (m, 8H),
.06 (d, 4H), 3.63 (t, 4H), 3.90 (t, 4H), 4.30 (d, 4H), 6.75 (s, 4H), 7.29 (s,
3
3
4
1
13
H). C NMR (CDCl
3
): d 9.7, 10.7, 22.8, 23.4, 30.6, 76.5, 77.1, 114.5,
25.3, 129.6, 131.8, 132.0, 138.6, 156.7, 158.6, 185.0. MS (MALDI-TOF,
+
+
dhb matrix): 1879.5 [M ], 1900.5 [(M + Na) ]. C84
calc.: C, 53.69; H, 4.72; found: C, 53.79; H 5.12%.
Crystal data for 2(C Me) : C84 16Br Rh ·4C
triclinic, space group P1 (no. 2); a = 12.801(2), b = 16.916(3), c =
5.080(5) Å, a = 72.15(2), b = 87.38(2), g = 85.48(2)°, V = 5151.9(17)
4 2
H88Br O16Rh (1879.0):
‡
6
H
5
4
88
H O
4
2
7 8
H : M = 2247.5,
¯
2
3
Å , Z = 2 (two independent molecules, each with a crystallographic centre
of symmetry), D
= 1.449 g cm2 , m(Mo-Ka) = 1.936 mm , T = 193 K;
3
21
c
CHEM. COMMUN., 2002, 338–339
339