Substituent-dependent formation of organotransition-metal bimetallic calix[4]arene inclusion complexes

Article abstract of DOI:10.1021/om049568x

Reaction of 1,3-di-O-substituted calixarene ligands with Et₂Zn gives new organotransition-metal bimetallic calix[4]arene inclusion complexes when the substituent is larger than methyl. The resulting complexes show strong interactions between th

Full text of DOI:10.1021/om049568x

4
540
Organometallics 2004, 23, 4540-4543
Su bstitu en t-Dep en d en t F or m a tion of
Or ga n otr a n sition -Meta l Bim eta llic Ca lix[4]a r en e
In clu sion Com p lexes
Ella Bukhaltsev, Israel Goldberg, and Arkadi Vigalok*
School of Chemistry, The Raymond and Beverly Sackler Faculty of Exact Sciences,
Tel Aviv University, Tel Aviv 69978, Israel
Received J une 14, 2004
Summary: Reaction of 1,3-di-O-substituted calixarene
ligands with Et2Zn gives new organotransition-metal
bimetallic calix[4]arene inclusion complexes when the
substituent is larger than methyl. The resulting com-
plexes show strong interactions between the incorporated
organometallic fragment and the hydrophobic cavity of
the calixarene group.
In 1996 Raston and co-workers reported an unusual
bimetallic zinc complex formed upon treatment of the
calixarene 1,3-dimethyl ether with 2 equiv of diethyl-
zinc.8 The solid-state structure of this complex revealed
the distorted flattened-cone conformation of the calix-
arene moiety with two phenolate units lying nearly in
the same plane (A, Scheme 1). Each of the Zn atoms is
bound to three oxygen atoms and the ethyl group, and
the organometallic fragments are equivalent both in
solution and in the solid state. As bimetallic zinc
The unusual three-dimensional properties of calix-
arene scaffolds have attracted much attention, particu-
larly with regard to molecular recognition and inclusion
9
1
complexes are of great importance in catalysis, we were
chemistry. Although the latter is normally associated
interested in the possibility of preparing bimetallic zinc
calixarene inclusion complexes. We hypothesized that
small methyl substituents can be responsible for ring
flipping in A (vide infra) and decided to evaluate the
reactivity of calixarenes with substituents other than
methyl. Interestingly, we discovered that room-temper-
ature stirring of the calixarene 1,3-dibenzyl ether 1a
with 2 equiv of diethylzinc in toluene resulted in the
clean formation of the new complex 2a (Scheme 1),
which had two distinctly nonequivalent metal-bound
with organic host-guest interactions, a few examples
of metal incorporation into the calixarene cavity have
also been reported.2,3 There were several recent publica-
tions on bimetallic complexation to the fully or partially
deprotonated lower (phenolic) rim, with one of the metal
centers penetrating the hydrophobic cavity of the calix-
arene fragment.4 While most of this research was
performed on alkali-metal and alkaline-earth-metal
ions, examples of such inclusion complexes with transi-
tion metals remain scarce.5,6 Incorporation of an orga-
1
10
ethyl groups in the H NMR spectrum. These groups
give rise to two sets of signals, a quartet (CH2) and
triplet (CH3), with the ∆δ value between the appropriate
signals in each set being about 2 ppm (-1.54 ppm vs
nometallic fragment into the calixarene cavity appears
particularly attractive, due to the expected change in
the reactivity pattern of the guest. Ishii and co-workers
recently communicated the stepwise preparation of a
heterobimetallic complex with an organometallic guest
fragment coordinated to the lower rim by penetrating
0
.97 ppm for CH2; -0.11 ppm vs 1.79 ppm for CH3). The
7
(5) For recent reviews on transition-metal calixarene chemistry
the hydrophobic calixarene cavity. Herein we describe
see: (a) Steyer, S.; J eunesse, C.; Armspach, D.; Matt, D.; Harrowfield,
J . In Calixarenes 2001; Asfari, Z., et al., Eds.; Kluwer Academic:
Dordrecht, The Netherlands, 2001; p 513. (b) Sliwa, W. Croat. Chem.
Acta 2002, 75, 131. (c) Wieser, C.; Dieleman, C. B.; Matt, D. Coord.
Chem. Rev. 1997, 165, 93. (d) Harvey, P. D. Coord. Chem. Rev. 2002,
the substituent-dependent formation of bimetallic calix-
arene inclusion complexes with spontaneous discrimi-
nation between the organometallic fragments.
2
33-234, 289.
*
To whom correspondence should be addressed. Tel: 972-3-640
617. Fax: 972-3-640 6205. E-mail: avigal@post.tau.ac.il.
1) (a) Bohmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713. (b)
Casnati, A.; Sansone, F.; Ungaro, R. Acc. Chem. Res. 2003, 36, 246.
c) Ludwig, R. Fresenius J . Anal. Chem. 2000, 367, 103. (d) Calixarenes
Revisited; Gutsche, C. D., Ed.; Springer: New York, 1998.
2) For a general review see: Ikeda, A.; Shinkai, S. Chem. Rev. 1997,
7, 1713.
3) For metal ion complexation inside a calixarene cavity see: (a)
(6) For examples of alkali-metal or alkaline-earth-metal guest ions
8
in polynuclear transition-metal calixarene complexes see: (a) Petrella,
A. J .; Roberts, N. K.; Raston, C. L.; Craig, D. C.; Thornton-Pett, M.;
Lamb, R. N. Eur. J . Inorg. Chem. 2003, 4153. (b) Caselli, A.; Solari,
E.; Scopelliti, R.; Floriani, C.; Re, N.; Rizzoli, C.; Chiesi-Villa, A. J .
Am. Chem. Soc. 2000, 122, 3652. (c) Zanotti-Gerosa, A.; Solari, E.;
Giannini, L.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Chem. Commun.
1997, 183. (d) Gibson, V. C.; Redshaw, C.; Clegg, W.; Elsegood, M. R.
J . Chem. Commun. 1997, 1605. For inclusion chemistry of π-coordi-
nated transition-metal calixarene complexes see: (e) Staffilani, M.;
Hancock, K. S. B.; Steed, J . W.; Holman, K. T.; Atwood, J . L.; J uneja,
R. K.; Burkhalter, R. S. J . Am. Chem. Soc. 1997, 119, 6324 and
references therein.
(7) Iwasa, K.; Kochi, T.; Ishii, Y. Angew. Chem., Int. Ed. 2003, 42,
3658.
(8) (a) Gardiner, M. G.; Lawrence, S. M.; Raston, C. L.; Skelton, B.
W.; White, A. H. Chem. Commun. 1996, 2491. (b) Upon reaction of
2
Et Zn with the unsubstituted ligand, an exo-bimetallic dicalixarene
complex was observed: Atwood, J . L.; J unk, P. C.; Lawrence, S. M.;
Raston, C. L. Supramol. Chem. 1996, 7, 15.
(
(
(
9
(
Inokuchi, F.; Miyahara, Y.; Inazu, T.; Shinkai, S. Angew. Chem., Int.
Ed. Engl. 1995, 34, 1364. (b) Ikeda, A.; Shinkai, S. J . Am. Chem. Soc.
994, 116, 3102. For ligand guest molecules coordinated to a metal
1
center see: (c) Wieser-J eunesse, C.; Matt, D.; De Cian, A. Angew.
Chem., Int. Ed. 1998, 37, 2861. (d) Lejeune, M.; J eunesse, C.; Matt,
D.; Kyrisakas, N.; Welter, R.; Kintzinger, J .-P. Dalton 2002, 1642. (e)
Wieser, C.; Matt, D.; Toupet, L.; Bourgeois, H.; Kintzinger, J .-P. J .
Chem. Soc., Dalton Trans. 1996, 4041. (f) Vigalok, A.; Swager, T. M.
Adv. Mater. 2002, 14, 368. (g) Corazza, F.; Floriani, C.; Chiesi-Villa,
A.; Rizzoli, C. Inorg. Chem. 1991, 30, 4465. (h) Gardiner, M. G.;
Koutsantonis, G. A.; Lawrence, S. M.; Nichols, P. J .; Raston, C. L.
Chem. Commun. 1996, 2035.
(9) (a) Kitamura, M.; Suga, S.; Kawai, K.; Noyori, R. J . Am. Chem.
Soc. 1986, 108, 6071. (b) Pu, L.; Yu, H.-B. Chem. Rev. 2001, 101, 757
and references therein.
(
4) (a) Dubberley, S. R.; Blake, A. J .; Mountford, P. Dalton 2003,
418 and references therein. (b) Guillemot, G.; Solari, E.; Rizzoli, C.;
Floriani, C. Chem. Eur. J . 2002, 8, 2072.
2
(10) See the Supporting Information for full NMR data.
1
0.1021/om049568x CCC: $27.50 © 2004 American Chemical Society
Publication on Web 09/01/2004

Communications
Organometallics, Vol. 23, No. 20, 2004 4541
Sch em e 1. Rea ction of 1,3-Disu bstitu ted Ca lixa r en e Liga n d s w ith Et Zn
2
coordinate arrangement, while the other is fully im-
mersed in the calixarene cavity in the formal three-
coordinate environment. The incorporation of the
ethylzinc fragment into the calixarene cavity is related
to that in the pentanuclear dicalixarene zinc system,
where three exocyclic zinc atoms fuse together two
8
a
ethylzinc calixarene moieties. The bond distances
inside the Zn(1)-O(3)-Zn(2)-O(4) rectangle lie between
1
.9661(19) and 1.9994(19) Å, comparing well with the
appropriate bond distances in the pentanuclear dicalix-
arene complex as well as those in the recently reported
bimetallic ethylzinc procatalysts for heterocycle ring-
1
2
opening polymerization. Also, the Zn(1)-Zn(2) dis-
tance of 3.065 Å is similar to those in the bimetallic
polymerization procatalysts (3.016 and 3.084 Å). The
ethyl groups at the zinc atoms lie in the same plane and
are parallel. There is a rather large discrepancy between
the bond lengths of the two oxygen atoms of the
coordinated benzyl ether to Zn(2) (2.367 and 2.576 Å),
the longer distance corresponding to the diverging
substituent. The nonbonding distances between the
hydrogen atoms of the encapsulated ethyl group and the
hydrogen atoms of the calixarene tert-butyl group are
almost within the sum of two hydrogen atom van der
Waals radii (2.499 and 2.506 Å for the CH3 and CH2
groups, respectively). A similar close distance is ob-
served for the CH2 group hydrogen atom of the second
ethyl group and one of the CH2 group hydrogen atoms
of the benzyl ether (2.402 Å).
F igu r e 1. ORTEP view of a molecule of 2a with thermal
ellipsoids shown at 50% probability.
chemical shifts for the same signals in the symmetrical
A (-0.21 ppm for CH2 and 0.92 ppm for CH3) fall
between the two sets in 2a . Two sets of signals were
also observed in the 13C NMR spectrum of 2a : -1.88
The close proximity of the ethyl hydrogen atoms to
the hydrogens of the calixarene scaffold in 2a was
and -6.22 ppm for the CH2 carbon atoms (cf. -2.14 ppm
in A). In light of this difference, it was crucial to
determine the structure of 2a by a single-crystal X-ray
analysis.
1
further evaluated by H NOESY solution studies. The
2
D spectrum in C6D6 (Figure 2) clearly shows strong
interactions between the entrapped ethyl group protons
and one set of the tert-butyl groups, as well as an
interaction between the CH3 of this ethyl group and the
meta protons of the calixarene aromatic ring. Similarly,
the CH2 fragment of the second ethyl group interacts
with the methylene hydrogens of the benzyl ether group.
Therefore, the whole ethylzinc fragment remains fully
immersed in the calixarene cavity, both in solution and
in the solid state. The intercalation of the ethyl group
inside the hydrophobic calixarene cavity is strongly
manifested by the aromatic shielding of the ethyl
hydrogen atoms as well as through-space interactions.
As the structure of A does not indicate any steric
barriers toward having a bulkier benzyl group in the
similar environment, we were puzzled as to the cause
of such different behaviors of two closely related calix-
Colorless crystals of 2a were obtained by slow crystal-
lization from pentane at -30 °C.11 The X-ray structure
of 2a (Figure 1) shows coordination of two ethylzinc
groups to a single calixarene unit in the pinched-cone
conformation. Interestingly, the two ethylzinc fragments
demonstrate different binding modes, with one of the
zinc atoms capping the calixarene cone in the five-
(
11) X-ray structure data for 2a : C62
H
76
O
4
Zn
0.2 × 0.15 mm , monoclinic, space group P2 /c, a ) 12.3180(3) Å, b
42.2240(15) Å, c ) 11.1870(4) Å, â ) 112.691(2)°, V ) 5368.2(3) Å ,
2 r
, M ) 1015.97, 0.35
3
×
1
3
)
-3
Z ) 4, Fcalcd ) 1.257 g cm , 2θmax ) 25.0°, Nonius KappaCCD
diffractometer, Mo KR radiation (λ ) 0.710 73 Å), graphite monochro-
mator, T ) 110(2) K, 19 500 collected reflections, 9162 unique
reflections (Rint ) 0.036). The structure was determined by direct
2
methods (SIR-97) and refined anisotropically by least squares on F
data (SHELXL-97, 627 parameters with no restraints). The structure
of 2a at ca. 110 K is fully ordered and characterized by a relatively
high precision, with R1 ) 0.041 and wR2 ) 0.097 for 6966 data with
I > 2σ(I) and R1 ) 0.062 and wR2 ) 0.109 for all unique data. The file
CCDC-226862 (Cambridge Crystallographic Data Base) contains full
crystallographic data for this compound.
(12) (a) Williams, C. K.; Breyfogle, L. E.; Choi, S. K.; Nam, W.;
Young, V. G.; Hillmyer, M. A.; Tolman, W. B. J . Am. Chem. Soc. 2003,
125, 11350. (b) Nakano, K.; Nozaki, K.; Hiyama, T. J . Am. Chem. Soc.
2003, 125, 5501.

4
542 Organometallics, Vol. 23, No. 20, 2004
Communications
Sch em e 2. P r op osed Mech a n ism of Rea ction betw een Dieth ylzin c a n d th e Ca lixa r en e Liga n d
Sch em e 3. Syn th esis of th e P h en ylzin c In clu sion Com p lexes
internal group. In all cases, however, the bimetallic Zn
complexes, similar to 2a , were isolated, indicating that
the inclusion of the organometallic fragment into the
calixarene cavity is independent of the electronic or
steric (n-propyl and larger) nature of the substituent.
Reaction of 1a with diethylzinc in the coordinating THF
did not affect the formation of 2a , indicating that it is
the thermodynamic product. Since 1a -d and their
dimethyl analog exist in the cone conformation as the
most stable form, it is conceivable that the different
structures obtained from the dimethyl ether and the rest
of the ligands are the result of conformational differ-
ences at the stage prior to the formation of the bimetallic
complexes. As the propyl group is the smallest one
known to prevent the phenolic ring flipping in the
calixarene system to form a partial cone or 1,3-alternate
1
3
conformers, this process must be responsible for the
formation of different products in the cases of the
dimethyl and dipropyl calixarene ligands. It is notewor-
thy that the reaction of the 1,3-diethyl calixarene ether
1
e with 2 equiv of diethylzinc resulted in a mixture of
products, in which 2e could be identified as the major
product (Scheme 1). Scheme 2 illustrates the proposed
mechanism for the formation of A and 2a -d . In a
stepwise approach, the monoethylzinc calixarene inter-
mediate is initially formed. When the substituent is
small, this intermediate undergoes ring flipping. Reac-
tion with the second diethylzinc molecule results in
arresting of the calixarene fragment in the 1,3-alternate
conformation to form A. When no ring flipping is
F igu r e 2. NOESY spectrum of complex 2a in benzene-
d
6
. Through-space interactions of the endo-cavity Zn-
CH CH fragment with the calixarene are shown by
arrows.
2
3
arene ligands. To that end, we prepared a series of
disubstituted calixarene ligands 1b-d and studied their
reactivity toward diethylzinc (Scheme 1). Replacing the
ether ligand in 1a with the ester in 1d led to dramatic
changes in both the 1H and C NMR spectra of the
external ethyl group, while only slightly influencing the
13
(13) Ghidini, E.; Ugozzoli, F.; Ungaro, R.; Harkema, S.; El-Fadl, A.
A.; Reinhoudt, D. N. J . Am. Chem. Soc. 1990, 112, 6979.

Communications
Organometallics, Vol. 23, No. 20, 2004 4543
possible, due to the steric bulk of the substituents, the
second diethylzinc molecule must approach the remain-
ing phenolic group by penetrating the hydrophobic
calixarene cavity.
Further evidence to support this stepwise mechanism
came from the reaction of the calixarene ligands with
diphenylzinc. Treatment of 1d with 2 equiv of Ph2Zn in
toluene resulted in the bimetallic inclusion complex 3
In summary, the unusual self-assembly of organ-
otransition-metal bimetallic calixarene inclusion com-
plexes has been described. This transformation is
strongly dependent on the steric properties of the
substituent at the calixarene lower rim, with larger ones
favoring the discrimination between the equivalent
organometallic fragments. These findings allow the
preparation of stable organometallic calixarene inclu-
sion complex to become a predictable process. This
predictability factor remains strikingly missing in calix-
arene-metal chemistry. The reactivity studies of the
new complexes are currently underway.
(
Scheme 3). Due to aromatic shielding, two protons of
1
the guest phenylzinc fragment appear in the H NMR
spectrum as a doublet at 5.46 ppm. Importantly, when
a 1:1 mixture of Et2Zn and Ph2Zn was reacted with 1
equiv of 1d , the formation of 2d and 3 was observed
along with complex 4. Complex 4 has the external
ethylzinc fragment with the phenylzinc fragment bound
through the hydrophobic cavity. This coordination mode
Ack n ow led gm en t. This work was supported by a
US-Israel Binational Science Foundation Start Up grant
and by a Yigal Alon New Faculty Award from the Israel
Council for Higher Education to A.V. A.V. is the
incumbent of the Raymond and Beverly Sackler Career
Development Chair. We thank Dr. Hadassah Shinar for
performing the NOESY experiments.
1
is manifested in the overlapped H NMR signals of the
external ethylzinc protons as well as the overlapped
proton signals of the aromatic groups at 8.31 ppm in
2
d and 4. The overlapping signal integration corre-
sponds well with the doublet at 5.46 ppm of the internal
phenylzinc. No formation of another isomer of 4, which
would have the ethylzinc guest fragment and the
external phenylzinc group, was observed. The 1 C NMR
spectrum of the reaction mixture also showed exclu-
sively the signals of the external ethyl group. Therefore,
it is likely that, in the stepwise process, the more
reactive Et2Zn occupies the easily accessible external
position, leaving the remaining Et2Zn and Ph2Zn mol-
ecules to compete for the calixarene cavity to form 2d
and 4, respectively.
3
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details for compounds 2-4 and a CIF file giving X-ray
data for 2a . The material is available free of charge via the
Internet at http://pubs.acs.org. The CIF file is also available
from the Cambridge Crystallographic Data Base as the file
number CCDC-226862.
OM049568X