A Calix[4]arene Carceplex with Four Rh24+ Fasteners

Article abstract of DOI:10.1021/ja0396899

It is shown that a spheroidal carceplex can be assembled by linking two bowl-shaped calix[4]-arenes via four dimetal units, (DAniF)₂Rh ₂ (DAniF = N, N′-di-p-anisylformamidinate), with a molecule (diethyl ether) or a cation (tetraethylammonium ion) trapped inside. The tetraethylammonium carceplex, 1b, has been characterized by X-ray crystallography, ¹H NMR, IR, and mass spectrometry. The tetraethylammonium ion fits snugly in the interior of the spheroidal carceplex. A two-fold axis of the tetrahedral cation coincides with the idealized four-fold axis of the cage, and the cation is disordered over two rotational orientations. Only one axial position on each dirhodium unit is occupied by a ligand, CH₃CN or H₂O. The carceplex is very stable, and the axial ligands can be exchanged in single crystals without disrupting the crystallinity of the samples. In this way, a red crystal of the complex with all axial positions occupied by acetonitrile can be transformed to a green crystal of the complex with two axial positions having acetonitrile and the other two having water by simply putting the crystal in contact with distilled water. The calix[4]arene used to make the carciplex structure is 25,26,27,28-tetra-n-propoxycalix[4]arene-5,11,17,23-tetracarboxylic acid. By employing 25,26,27,28-tetrapropoxy-5,17-dibromo-calix[4]arene-11, 23-dicarboxylic acid, two 1:1 dimetal: calixarene compounds have also been made and characterized: 2, cis-Rh₂(DAniF)₂(calix)(CH ₃OH), and 3, cis-Mo₂(DAniF)₂(calix). The Rh-Rh distances in lb are in the range of 2.410(2)-2.428(2) A, that in 2 is 2.4383(4) A, and the Mo-Mo distance in 3 is 2.0931(4) A.

Full text of DOI:10.1021/ja0396899

Published on Web 01/17/2004
4+
A Calix[4]arene Carceplex with Four Rh2 Fasteners
F. Albert Cotton,*,† Peng Lei,† Chun Lin,† Carlos A. Murillo,*,† Xiaoping Wang,†
Shu-Yan Yu,†,‡ and Zhong-Xing Zhang‡
Contribution from the Laboratory for Molecular Structure and Bonding, Department of
Chemistry, P.O. Box 30012, Texas A&M UniVersity, College Station, Texas 77842-3012, and
State Key Laboratory of Polymer Physics and Chemistry and Joint Laboratory of Polymer
Science and Materials, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 10080, P. R. China
Received November 19, 2003; E-mail: cotton@tamu.edu; murillo@tamu.edu; syu@iccas.ac.cn
Abstract: It is shown that a spheroidal carceplex can be assembled by linking two bowl-shaped calix[4]-
arenes via four dimetal units, (DAniF)2Rh2 (DAniF ) N,N′-di-p-anisylformamidinate), with a molecule (diethyl
ether) or a cation (tetraethylammonium ion) trapped inside. The tetraethylammonium carceplex, 1b, has
been characterized by X-ray crystallography, 1H NMR, IR, and mass spectrometry. The tetraethylammonium
ion fits snugly in the interior of the spheroidal carceplex. A two-fold axis of the tetrahedral cation coincides
with the idealized four-fold axis of the cage, and the cation is disordered over two rotational orientations.
Only one axial position on each dirhodium unit is occupied by a ligand, CH3CN or H2O. The carceplex is
very stable, and the axial ligands can be exchanged in single crystals without disrupting the crystallinity of
the samples. In this way, a red crystal of the complex with all axial positions occupied by acetonitrile can
be transformed to a green crystal of the complex with two axial positions having acetonitrile and the other
two having water by simply putting the crystal in contact with distilled water. The calix[4]arene used to
make the carciplex structure is 25,26,27,28-tetra-n-propoxycalix[4]arene-5,11,17,23-tetracarboxylic acid.
By employing 25,26,27,28-tetrapropoxy-5,17-dibromo-calix[4]arene-11,23-dicarboxylic acid, two 1:1 dimetal:
calixarene compounds have also been made and characterized: 2, cis-Rh2(DAniF)2(calix)(CH3OH), and
3, cis-Mo2(DAniF)2(calix). The Rh-Rh distances in 1b are in the range of 2.410(2)-2.428(2) Å, that in 2
is 2.4383(4) Å, and the Mo-Mo distance in 3 is 2.0931(4) Å.
Introduction
carboxylates) to give various kinds of supramolecular assemblies
such as loops, squares, triangles, and more complex polyhedra.4
The use of Rh2 units is especially appealing because many
dirhodium compounds have important and varied catalytic
activity.5
There is currently great interest in the design and preparation
of supramolecular materials. Many may be constructed with
single metal atom vertices (e.g., square planar PdII and PtII or
tetrahedral ZnII),1 or as in our laboratory by using metal-metal
bonded (M2) dimetal atom vertices (especially those with M2
) Mo24+,2 Rh24+,2 and Ru25+ 3). Useful precursors for preparing
4+
Another possible application that we have envisioned for our
polygonal assemblies is their use as hosts for guest molecules
of appropriate size. Arrangements capable of encapsulating
molecules have been the target of many studies.6 Of special
interest are the so-called carceplexes, that is, complexes with
permanently imprisoned guests.7 In this context, an attractive
n+
the latter have cis-M2(formamidinate)2 moieties with two
cisoid formamidinate anions which remain fixed along with
some labile ligands such as acetonitrile molecules or acetate.
These react with appropriate polyfunctional linkers (e.g., di-
† Texas A&M University.
(4) Cotton, F. A.; Lin, C.; Murillo, C. A. Acc. Chem. Res. 2001, 34, 759.
(5) See, for example: (a) Doyle, M. P.; Ren, T. Prog. Inorg. Chem. 2001, 49,
113. (b) Estevan, F.; Herbst, K.; Lahuerta, P.; Barberis, M.; Pe´rez-Prieto,
J. Organometallics 2001, 20, 950. (c) Doyle, M. P.; Phillips, I. M.; Hu,
W. J. Am. Chem. Soc. 2001, 123, 5366. (d) Doyle, M. P.; McKervey, M.
A.; Ye, T. Modern Catalytic Methods for Organic Synthesis with Diazo
Compounds - From Cyclopropanes to Ylides; John Wiley & Sons: New
York, 1998.
(6) (a) Seidel, S. R.; Stang, P. J. Acc. Chem. Res. 2002, 35, 972. (b) Fujita, M.
Chem. Soc. ReV. 1998, 27, 417. (c) Yu, S.-Y.; Kusukawa, T.; Biradha, K.;
Fujita, M. J. Am. Chem. Soc. 2000, 122, 2665. (d) Kusukawa, T.; Fujita,
M. J. Am. Chem. Soc. 1999, 121, 1397. (e) Terpin, A. J.; Ziekler, M.;
Johnson, D. W.; Raymond, K. N. Angew. Chem., Int. Ed. 2001, 40, 157.
(f) Fujita, M.; Nagao, S.; Ogura, K. J. Am. Chem. Soc. 1995, 117, 1649.
(g) O’Leary, B. M.; Szabo, T.; Svenstrup, N.; Schalley, C. A.; Lu¨tzen, A.;
Scha¨fer, M.; Rebek, J., Jr. J. Am. Chem. Soc. 2001, 123, 11519. (h)
Shivanyuk, A.; Rebek, J., Jr. J. Am. Chem. Soc. 2002, 124, 12074. (i) Reek,
J. N. H.; Schenning, A. P. H. J.; Bosman, A. W.; Meijer, E. W.; Crossley,
M. J. Chem. Commun. 1998, 11.
‡ Chinese Academy of Sciences.
(1) (a) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100, 853 and
references therein. (b) Swiegers, G. F.; Malefetse, T. J. Chem. ReV. 2000,
100, 3483. (c) Espinet, P.; Soulantica, K.; Charmant, J. P. H.; Orpen, A.
G. Chem. Commun. 2000, 915. (d) Navarro, J. A. R.; Lippert, B. Coord.
Chem. ReV. 1999, 185/186, 653. (e) Whang, D.; Kim, K. J. Am. Chem.
Soc. 1997, 119, 451. (f) Mann, S.; Huttner, G.; Zsolnai, L.; Heinze, K.
Angew. Chem., Int. Ed. Engl. 1996, 35, 2808. (g) Scherer, M.; Caulder, D.
L.; Johnson, D. W.; Raymond, K. N. Angew. Chem., Int. Ed. 1999, 38,
1588. (h) Lai, S.-W.; Chan, M. C.-W.; Peng, S.-M.; Che, C.-M. Angew.
Chem., Int. Ed. 1999, 38, 669. (i) Jones, C. J. Chem. Soc. ReV. 1998, 27,
289. (j) Klausmeyer, K. K.; Wilson, S. R.; Rauchfuss, T. B. J. Am. Chem.
Soc. 1999, 121, 2705. (k) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O.-
M. Nature 1999, 402, 276.
(2) Cotton, F. A.; Lin, C.; Murillo, C. A. Proc. Nat. Acad. Sci. U.S.A. 2002,
99, 4810.
(3) Angaridis, P.; Berry, J. F.; Cotton, F. A.; Murillo, C. A.; Wang, X. J. Am.
Chem. Soc. 2003, 125, 10327.
9
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J. AM. CHEM. SOC. 2004, 126, 1518-1525
10.1021/ja0396899 CCC: $27.50 © 2004 American Chemical Society

A Calix[4]arene Carceplex with Four Rh24+ Fasteners
A R T I C L E S
family of compounds are the calix[n]arenes because these well-
known toroidal or chalice-like molecules are capable of accom-
modating various organic molecules within the bowl.8 The thrust
of the work we report here is to design systems in which two
such bowls are united to form spheroidal cages. A few such
molecules have already been made with the aid of hydrogen
bonding interactions,9 or with organic covalent linkers,10 but
we report here for the first time the use of dimetal moieties to
link two calixarene bowls to form a spheroidal carceplex.11
Several years ago, a study of the reaction of a corner piece
precursor [cis-Rh2(DAniF)2(CH3CNeq)4(CH3CNax)2]2+, DAniF
) N,N′-di-p-anisylformamidinate, and the anions of a calix[4]-
arenetetracarboxylic acid, I, produced a cage compound with
two calix[4]arene ligands united by covalent bonds to four [Rh2]
Experimental Section
Materials, Methods, and General Procedures. Unless otherwise
stated, all manipulations were carried out under nitrogen using standard
Schlenk techniques. Solvents were dried by conventional methods and
were freshly distilled under nitrogen before use. The starting materials,
cis-[Rh2(DAniF)2(CH3CNeq)4(CH3CNax)2](BF4)2,12 cis-[Mo2(DAniF)2(CH3-
CN)4](BF4)2,13 calix[4]arene(CO2H)4 ) 25,26,27,28-tetra-n-propoxycalix-
[4]arene-5,11,17,23-tetracarboxylic acid14 and 25,26,27,28-tetrapropoxy-
5,17-dibromo-calix[4]arene-11,23-dicarboxylic acid14a were prepared
following published methods. Et4NOH (35% aqueous solution) and
2+
units (where [Rh2] represents the cis-Rh2(DAniF)2 unit)
through the eight carboxylate groups of the ligands. This
1
compound was characterized by H NMR spectroscopy and a
crystal structure, and it was found that an ether molecule was
inside a cage, 1a. Further work showed that this synthesis was
difficult to reproduce, but a highly reproducible preparation of
a more stable product has now been found. This compound
Bun NOH (1 M methanol solution) were purchased from Aldrich. All
+
-
4
encapsulates a NEt4 cation and has a BF4 counteranion
external to the cage, so that the formula can be compactly
represented as {NEt4⊂[cis-Rh2(DAniF)2L]4[calix[4]arene-
(CO2)4]2}BF4, 1b, where L ) acetonitrile or H2O. The core is
very stable and remains intact even under the conditions
necessary for mass spectrometric measurements. This compound
has been characterized structurally and in other ways including
1H NMR and mass spectrometry. For comparison, simple
dinuclear compounds with one cis-M2(DAniF)2 unit, M ) Rh
and Mo, and a calix[4]arene]dicarboxylate ligand, II, have also
been made. These are cis-Rh2(DAniF)2(Br2calix[4]arene-
(CO2)2)(CH3OHax), 2, and cis-Mo2(DAniF)2(Br2calix[4]arene-
(CO2)2), 3.
other commercially available chemicals were used as received.
Abbreviations Used. DAniF ) N,N′-di-p-anisylformamidinate; [Rh2]
) cis-(DAniF)2Rh22+; calix[4]arene(CO2H)4 ) 25,26,27,28-tetra-n-
propoxycalix[4]arene-5,11,17,23-tetracarboxylic acid; Br2calix[4]arene-
(CO2H)2 ) 25,26,27,28-tetrapropoxy-5,17-dibromocalix[4]arene-11,23-
dicarboxylic acid; [Rh24+] ) the unit [cis-Rh2(DAniF)2L]2+, where L
is a neutral axial ligand such as water or acetonitrile; {NEt4⊂cage}-
BF4 ) [cis-Rh2(DAniF)2L]4[calix[4]arene(CO2)4]2‚NEt4BF4 (also re-
ferred to as 1b or 1c); TBAH ) Bun NPF6.
4
1
Physical Measurements. The H NMR spectra were recorded on
either a Varian Inova-500 or a Bruker Avance DMX500 spectrometer
for 1b or a Varian XL-300 instrument for 1a, 2, and 3. The infrared
spectrum was collected on KBr pellets on a Bruker Tensor 27
spectrophotometer. The MALDI-TOF mass spectrum of 1b was
collected on a Bruker Biflex instrument. Mass spectral simulation was
carried out using the software provided free of cost by the University
of Sheffield (http://www.shef.ac.uk/chemistry/chemputer/isotopes.html).
Cyclic voltammetry was performed in dichloromethane using a CH
Instruments model CH1620A electrochemical analyzer equipped with
Pt working and auxiliary electrodes and 0.1 M TBAH as the supporting
electrolyte. Potentials were referenced to an Ag/AgCl electrode. The
values of E1/2 were taken as (Epa + Epc)/2, where Epa and Epc are the
anodic and cathodic peak potentials. Elemental analyses were performed
by the Microanalytical Laboratory at the Institute of Chemistry, Chinese
Academy of Sciences for 1b, or Canadian Microanalytical Service,
Delta, British Columbia, Canada for 2 and 3.
Preparation of (NEt4)4(calix[4]arene(CO2)4). To a solution of calix-
[4]arene(CO2H)4 (77 mg, 0.10 mmol) in methanol (5 mL) was carefully
added a solution of NEt4OH (20% aqueous solution:methanol, 1:5) until
a pH of 6.5 to 7.0 was attained and some solid had formed. After
filtration, the solid was washed with methanol which dissolved most
of it. The colorless filtrate and the methanol washing were combined,
and the solvent was removed at room temperature under vacuum. The
solid was then heated under vacuum at 35 °C for 72 h. A white powder
(128 mg) was obtained in essentially quantitative yield.
(7) (a) Cram, D. J.; Cram, J. M. Container Molecules and Their Guests; Royal
Society of Chemistry: Cambridge, 1994. (b) Jasat, A.; Sherman, J. C. Chem.
ReV. 1999, 99, 931. (c) Chapman, R. G.; Sherman, J. C. Tetrahedron 1997,
53, 15911. (d) Naumann, C.; Sherman, J. C. In Calixarenes 2001; Asfari,
Z., Bo¨hmer, V., Harrowfield, J., Vicens, J., Eds.; Kluwer Academic
Publishers: Dordrecht, 2001. (e) Hardie, M. J.; Raston, C. L. J. Chem.
Soc., Dalton Trans. 2000, 2483. (f) Rebek, J., Jr. Chem. Commun. 2000,
637. (g) Warmuth, R.; Yoon, J. Acc. Chem. Res. 2001, 34, 95. (h)
Mungaroo, R.; Sherman, J. C. Chem. Commun. 2002, 1672.
(8) (a) Gutsche, C. D. Calixarenes ReVisited; Royal Society of Chemistry:
London, 2000. (b) Calixarenes in Action; Mandolini, L., Ungaro, R., Eds.;
Imperial College Press: London, 2000. (c) Calixarenes 2001; Asfari, Z.,
Bo¨hmer, V., Harrowfield, J., Vicens, J., Eds.; Kluwer Academic Publish-
ers: Dordrecht, 2001. (d) Calixarenes: A Versatile Class of Macrocyclic
Compounds; Vicens, J., Bo¨hmer, V., Eds.; Kluwer Academic Publishers:
Dordrecht, 1991. (e) Fan, M.; Zhang, H.; Lattman, M. Chem. Commun.
1998, 99. (f) Yu, S.-Y.; Huang, H.; Liu, H.-B.; Chen, Z.-N.; Zhang, R.;
Fujita, M. Angew. Chem., Int. Ed. 2003, 42, 686. (g) Macrocycle
Synthesis: A Practical Approach; Parker, D., Ed.; Oxford University
Press: Oxford, 1996.
(9) (a) Kobayashi, K.; Shirasaka, T.; Yamaguchi, K.; Sakamoto, S.; Horn, E.;
Furukawa, N. Chem. Commun. 2000, 41. (b) Shimizu, K. D.; Rebek, J., Jr.
Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 12403. (c) Scheerder, J.; van
Duynhoven, J. P. M.; Engbersen, J. F. J.; Reinhoudt, D. N. Angew. Chem.,
Int. Ed. Engl. 1996, 35, 1090. (d) Mogck, O.; Bohrner, V.; Vogt, W.
Tetrahedron 1996, 52, 8489. (e) Gibb, C. L. D.; Stevens, E. D.; Gibb, B.
C. J. Am. Chem. Soc. 2001, 123, 5849.
(10) (a) MacGillivray, L. R.; Atwood, J. L. Angew. Chem., Int. Ed. 1999, 38,
1018. (b) Gibb, C. L. D.; Stevens, E. D.; Gibb, B. C. Chem. Commun.
2000, 363.
(11) The use of single-metal atom units to create calix[n]arene or resorc[n]-
arene cages has been reported, but the number of structurally characterized
molecules is very limited. See, for example: (a) Fox, O. D.; Dalley, N.
K.; Harrison, R. G. J. Am. Chem. Soc. 1998, 120, 7111. (b) Fox, O. D.;
Dalley, N. K.; Harrison, R. G. Inorg. Chem. 1999, 38, 5860. (c) Fochi, F.;
Jacopozzi, P.; Wegelius, E.; Rissanen, K.; Cozzini, P.; Marastoni, E.;
Fisicaro, E.; Manini, P.; Fokkens, R.; Dalcanale, E. J. Am. Chem. Soc.
2001, 123, 7539. (d) Fox, O. D.; Drew, M. G. B.; Beer, P. D. Angew.
Chem., Int. Ed. 2000, 39, 136.
(12) Catalan, K. V.; Mindiola, D. J.; Ward, D. L.; Dunbar, K. R. Inorg. Chem.
1997, 36, 2458.
(13) Chisholm, M. H.; Cotton, F. A.; Daniels, L. M.; Folting, K.; Huffman, J.;
Iyer, S.; Lin, C.; MacIntosh, A. M.; Murillo, C. A. J. Chem. Soc., Dalton
Trans. 1999, 1387.
(14) (a) Larsen, M.; Jrrgensen, M. J. Org. Chem. 1996, 61, 6661. (b) Sansone,
F.; Barboso, S.; Casnati, A.; Fabbi, M.; Pochini, A.; Ugozzoli, F.; Ungaro,
R. Eur. J. Org. Chem. 1998, 897. (c) Gutsche, C. D.; Pagoria, P. F. J. Org.
Chem. 1985, 50, 5795.
9
J. AM. CHEM. SOC. VOL. 126, NO. 5, 2004 1519

A R T I C L E S
Cotton et al.
Synthesis of {NEt4⊂[cis-Rh2(DAniF)2L]4[calix[4]arene(CO2)4]2}-
BF4, 1b. To [cis-Rh2(DAniF)2(CH3CNeq)4(CH3CNax)2](BF4)2 (114 mg,
0.100 mmol) in 2:1 CH3CN-CH2Cl2 (3 mL) was added dropwise 5
mL of an acetonitrile solution of (NEt4)4(calix[4]arene(CO2)4) (64 mg,
0.050 mmol), which had been heated to aid the dissolution of the
calixarene salt. The reaction mixture was stirred at ca. 37-40 °C for
7 days. After filtration, a dark red solution was collected and layered
with diethyl ether. After 2 days, a small amount of unreacted [cis-
Rh2(DAniF)2(CH3CNeq)4(CH3CNax)2](BF4)2 precipitated as red crystals.
After filtration in air, the volume of the filtrate was reduced to 3 mL
under vacuum and the resulting solution was chromatographed using
a silica gel column and CH2Cl2/CH3CN (2:1 v/v) as eluent. Three bands
were observed: a major green band, and then a tan band, that was
followed by a broad, very slow moving, blue band. As observed by
mass spectrometry, the tan band contained a very small amount of [cis-
Rh2(DAniF)2(CH3CNeq)4(CH3CNax)2](BF4)2. The green band was col-
lected, and the solvent was evaporated under reduced pressure. The
product was dried under vacuum. Yield: 16.5 mg (15%). Dark-red,
single crystals suitable for X-ray crystallographic analysis were obtained
after 2 weeks from a solution of the product in CH2Cl2/CH3CN (5:1
v/v), which had been layered with hexane at room temperature. This
gave the product with L ) CH3CN. Green crystals used for the structural
determination were obtained by rinsing single crystals of the acetonitrile
analogue with distilled water (L ) a 50% mixture of H2O and
acetonitrile). MALDI-TOF for ([NEt4⊂cage without axial solvents]+):
4523.7. Anal. Calcd for C220H238N19O42Rh8BF4, {(NEt4⊂cage)BF4 with
2 acetonitrile and 2 water molecules in axial positions)}: C, 55.86; H,
5.07; N, 5.63. Found: C, 55.46; H, 5.08; N, 5.70. IR (KBr, cm-1):
3442 (br, s), 2960 (br, m), 1614 (s), 1503 (s), 1464 (w), 1400 (s), 1334
(w), 1292 (w), 1244 (s), 1220 (s), 1176 (m), 1122 (m), 1083 (m), 1035
(m), 958 (w), 901 (vw), 831 (w), 807 (w), 776 (w), 725 (vw), 598
(vw), 531 (w). 1H NMR δ (ppm, in CD2Cl2): 7.766 (s, 16H, aromatic/
calixarene), 7.255 (s, 8H, NCHN), 6.961 (d, 32H, aromatic/anisyl),
6.707 (d, 32H, aromatic/anisyl), 4.508 (d, 8H, Hax of ArCH2Ar/
calixarene), 3.832 (t, 16H, CH2CH2CH3), 3.751 (s, 48H, OCH3/anisyl),
3.353 (d, 8H, Heq of ArCH2Ar/calixarene), 1.941 (s, 12H, CH3CN),
1.919 (m, 16H, CH2CH2 CH3), 0.925 (t, 24H, CH2CH2 CH3), 0.791
(broad, 8H, CH2CH3), -1.560 (broad, 12H, CH2CH3).
8H), 1.14 (t, 6H), 0.86 (t, 6H). CV: one reversible wave at 0.15 V in
the scan range from -0.20 to +0.60 V. Anal. Calcd for C72H74Br2N4O12-
Mo2: C, 56.19; H, 4.85; N, 3.64. Found: C, 55.97; H, 5.12; N, 3.44.
X-ray Structure Determinations. For the Rh
8-cage compounds,
three crystal structures were determined, 1a, 1b, and 1c.15 For each, a
single crystal suitable for X-ray diffraction analysis was mounted on
the tip of a quartz fiber with a small amount of silicone grease and
attached to a goniometer head. Data were collected on a Bruker SMART
1000 CCD system equipped with a low-temperature controller cooled
by liquid nitrogen. In each case, 60 frames were collected first to
determine the orientation matrix. The cell parameters were calculated
using an autoindexing routine, and a hemisphere of data was collected.
Data reduction and integration were performed with the software
package SAINT,16 while absorption corrections were applied by using
the program SADABS.17 Positions of the Rh atoms and most of the
non-hydrogen atoms were found by using the direct methods program
in the Bruker SHELXTL software package.18 Subsequent cycles of least-
squares refinement followed by difference Fourier syntheses revealed
the positions of the remaining non-hydrogen atoms. Some of the anisyl
groups in the DAniF ligands in 1a19 and 1b were found disordered,
and they were refined isotropically. The crystal of 1b contains two
independent cage molecules with an encapsulated Et4N+ ion, designated
1b′, 1b′′. Each resides on a crystallographic inversion center, and the
+
internal volume is occupied by a disordered NEt4 cation. Although
1b′, 1b′′ are not crystallographically equivalent as the result of disorder
in the crystal structure, they are chemically equivalent, having one axial
ligand in each of the dirhodium units. These are acetonitrile molecules
in two of the Rh24+ units and water in the other two.20 The BF4- anions
are disordered in two positions in the unit cell, and they were refined
with some constraints in bond distances. Hydrogen atoms were placed
in calculated positions in the final structure refinement. Data collection
for 2 and 3 was similar to that for 1b; refinement proceeded
straightforwardly. Crystal data and structural refinement information
are given in Table 1. Selected bond distances and angles are listed in
Tables 2-5.
Results and Discussion
Reaction of the corner piece precursor [cis-Rh2(DAniF)2(CH3-
Synthesis of cis-Rh2(DAniF)2(Br2calix[4]arene(CO2)2)(CH3OHax),
2. To a stirred solution of red [cis-Rh2(DAniF)2(CH3CNeq)4(CH3CNax)2]-
(BF4)2 (57 mg, 0.050 mmol) in 10 mL of CH3CN was added
CNeq)4(CH3CNax)2](BF4)2 in which the cation has a singly
bonded Rh2 unit, two formamidinate groups in a cisoid
conformation, and six labile acetonitrile molecules in equatorial
and axial positions with salts of calix[4]arene(CO2H)4 allows
the isolation of a cage complex having four [Rh24+] units capped
4+
(Bun N)2(Br2calix[4]arene(CO2)2) (50.0 mg, 0.050 mmol) in 15 mL of
4
CH3CN. The reaction mixture was stirred at ambient temperature (22-
25 °C) for ca. 24 h. A reddish precipitate gradually formed. This was
collected by filtration and then washed several times with CH3CN. The
crude product was extracted with CH2Cl2 (2 × 5 mL). Methanol was
carefully layered on the top of the solution to afford a dark crystalline
by two calix[4]arene ligands. Depending on the reaction
4+
conditions (vide infra), the axial ligand in each of the [Rh2
]
units can be water or acetonitrile, and the cage can encapsulate
+
1
an ether molecule or a NEt4 cation.
material after several days. Yield: 69 mg, 86%. H NMR δ (ppm, in
Structural Considerations. The cage molecule was charac-
terized in independent crystallographic studies. In 1a (Figure
CD2Cl2): 7.42 (s, 4H), 7.36 (br, 2H), 7.02 (d, 8H), 6.81 (s, 4H), 6.77
(d, 8H), 4.43 (d, 4H), 4.01 (t, 4H), 3.81 (s, 12H), 3.71 (t, 4H), 3.20 (d,
4H), 2.02-1.80 (m, 8H), 1.13 (t, 6H), 0.86 (t, 6H). CV: two reversible
waves at 0.21 and 1.07 V in the scan range from -0.10 to +1.30 V.
Anal. Calcd for C73H78Br2N4O13Rh2: C, 55.32; H, 4.96; N, 3.53.
Found: C, 55.01; H, 5.15; N, 3.38.
(15) Crystals of 1c were obtained by allowing the single crystals of the
acetonitrile analogue to stand in air over a period of three weeks. These
crystals are isomorphous with those of 1b having two independent cage
molecules but differing slightly in composition. In 1c, a partially occupied
CH3CONH2 molecule is found on the axial position of a Rh2-subunit. The
CH3CONH2 molecule is formed by reaction of acetonitrile with water, in
a process that is known to be catalyzed by transition metal species. (See,
for example: Eglin, J. E. Comments Inorg. Chem. 2002, 23, 23 and
references therein.) Other crystallographic data for 1c and 1b are essentially
the same, and they are provided in the Supporting Information.
(16) SAINT. Data Reduction Software. Version 6.36A; Bruker Advanced X-ray
Solutions, Inc.: Madison, WI, 2001.
(17) SADABS, Version 2.05; Area Detector Absorption and Other Corrections;
Bruker Advanced X-ray Solutions, Inc.: Madison, WI, 2001.
(18) Sheldrick, G. M. SHELXTL. Version 6.12; Bruker Advanced X-ray
Solutions, Inc.: Madison, WI, 2002.
(19) Crystals of 1a were obtained from a reaction similar to that described for
the synthesis of 1b, except that the reaction was done at 22 °C over a
period of 2 days, and without chromatographic purification.
(20) These crystals were prepared, as reported above, by taking single crystals
of the acetonitrile analogue and rinsing them with distilled water.
Synthesis of cis-Mo2(DAniF)2(Br2calix[4]arene(CO2)2), 3. To a
stirred orange solution of [cis-Mo2(DAniF)2(CH3CN)4](BF4)2 (104 mg,
0.100 mmol) in 15 mL of CH3CN was added (Bun4N)2(Br2calix[4]arene-
(CO2)2) (97.8 mg, 0.100 mmol) in 15 mL of CH3CN. An immediate
reaction took place with formation of a bright yellow precipitate, which
was collected by filtration, washed several times with CH3CN, and dried
under vacuum. The crude product was extracted with CH2Cl2 (3 × 5
mL). A mixture of isomeric hexane was then carefully layered on top
of the solution to afford a yellow crystalline material after several days.
1
The yield was essentially quantitative. H NMR δ (ppm, in CD2Cl2):
8.46 (s, 2H), 7.34 (s, 4H), 7.03 (s, 4H), 6.68 (dd, 16H), 4.47 (d, 4H),
4.04 (t, 4H), 3.77 (t, 4H), 3.72 (s, 12H), 3.22 (d, 4H), 1.99-1.82 (m,
9
1520 J. AM. CHEM. SOC. VOL. 126, NO. 5, 2004

A Calix[4]arene Carceplex with Four Rh24+ Fasteners
A R T I C L E S
Table 1. Crystal Data and Structure Refinement
1a‚10CH CN
1b‚3.5H O
2
3
C72H74Br2Mo2N4O12
1539.06
P1h
3
2
empirical formula
fw
C
240H260N30O41Rh8
5044.06
P1h
C
220H245B1F4N19O45.5Rh8
C73H78Br2N4O13Rh2
1585.03
4793.44
P1h
space group
a, Å
b, Å
c, Å
P1h
17.577(2)
19.640(3)
19.871(3)
69.092(2)
69.336(3)
85.822(3)
5985(1)
1.400
20.257(9)
24.161(10)
26.445(11)
75.223(7)
67.977(7)
66.997(7)
10 955(8)
1.453
12.0208(6)
13.1643(6)
23.138(1)
77.851(1)
77.749(1)
80.174(1)
3467.4(3)
1.518
12.1936(8)
14.496(1)
20.993(1)
106.990(1)
95.559(1)
105.863(1)
3350.1(4)
1.526
R, deg
â, deg
γ, deg
V, Å3
d, g/cm3
Z
T, K
1
2
1
1
213(2)
173(2)
213(2)
213(2)
R1,a wR2b [I > 2σ(I)]
R1,a wR2b (all data)
quality of fitc
0.0879, 0.2010
0.1683, 0.2363
1.127
0.0824, 0.1834
0.1630, 0.2340
1.182
0.0469, 0.1219
0.0672, 0.1367
1.028
0.0436, 0.1017
0.0728, 0.1178
1.018
2
2
2
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo - Fc )2]/∑w(Fo )2]1/2, w ) 1/[σ2(Fo ) + (aP)2 + bP], where P ) [max(0 or Fo ) + 2(Fc )]/3. c Quality
of fit ) [∑[w(Fo - Fc )2]/(Nobs - Nparams)], based on all data.
2
2
Table 2. Selected Bond Lengths [Å] and Angles [deg] for 1a
Rh(1)-Rh(2)
Rh(3)-Rh(4)
Rh(1)-N(1)
Rh(1)-N(3)
Rh(1)-N(9)
Rh(2)-N(2)
Rh(2)-N(4)
Rh(3)-N(6)
Rh(3)-N(7)
Rh(4)-N(5)
2.429(1)
2.422(2)
2.04(1)
2.04(1)
2.16(1)
1.99(1)
2.03(1)
2.02(1)
2.01(1)
2.05(1)
Rh(4)-N(8)
Rh(4)-N(10)
Rh(1)-O(9)
Rh(1)-O(11)
Rh(2)-O(10)
Rh(2)-O(12)
Rh(3)-O(14)
Rh(3)-O(15)
Rh(4)-O(13)
Rh(4)-O(16)
2.04(1)
2.16(1)
2.075(9)
2.090(9)
2.043(9)
2.071(9)
2.073(8)
2.048(9)
2.088(9)
2.086(8)
N(1)-Rh(1)-N(3)
N(1)-Rh(1)-O(9)
N(3)-Rh(1)-O(9)
N(1)-Rh(1)-O(11)
N(3)-Rh(1)-O(11)
O(9)-Rh(1)-O(11)
N(1)-Rh(1)-N(9)
N(3)-Rh(1)-N(9)
O(9)-Rh(1)-N(9)
N(5)-Rh(4)-O(16)
N(8)-Rh(4)-O(13)
N(5)-Rh(4)-O(13)
O(16)-Rh(4)-O(13)
N(8)-Rh(4)-N(10)
N(5)-Rh(4)-N(10)
91.7(4)
O(11)-Rh(1)-N(9)
N(2)-Rh(2)-N(4)
N(2)-Rh(2)-O(10)
N(4)-Rh(2)-O(10)
N(2)-Rh(2)-O(12)
N(4)-Rh(2)-O(12)
O(10)-Rh(2)-O(12)
N(7)-Rh(3)-N(6)
N(7)-Rh(3)-O(15)
N(6)-Rh(3)-O(15)
N(7)-Rh(3)-O(14)
N(6)-Rh(3)-O(14)
O(15)-Rh(3)-O(14)
N(8)-Rh(4)-N(5)
N(8)-Rh(4)-O(16)
87.0(4)
175.4(4)
87.3(4)
87.9(4)
175.0(4)
92.7(3)
93.4(5)
98.1(4)
91.2(4)
87.3(4)
88.5(4)
175.2(4)
90.8(3)
92.8(5)
93.3(5)
92.9(4)
176.1(4)
88.2(4)
86.4(4)
176.1(4)
92.2(3)
88.2(4)
176.5(5)
89.8(4)
90.4(4)
175.9(5)
91.5(3)
93.1(4)
175.2(4)
Figure 1. A drawing of the cage 1a with the encapsulated diethyl ether
molecule removed for clarity. Each of the four dirhodium units has one
acetonitrile molecule occupying one axial site. An inversion center relates
the two halves of the neutral molecule. Displacement ellipsoids are drawn
at the 30% probability level.
simple compound with only one [Rh2] unit and a calix[4]-
arenedicarboxylate derivative was made. It can be seen in Figure
2 that this has essentially the same environment as each of the
dirhodium units in the cage with two formamidinate, two
carboxylate groups, and only one axial methanol ligand, and
has a Rh-Rh distance of 2.4383(4) Å which is comparable to
those in the cage. Other Rh-N and Rh-O distances (see Table
3) are also similar. However, in this compound the two
carboxylate groups are provided by the same ligand. This
arrangement resembles somewhat that found in cis-Rh2(acetate)2-
(m-C6H4(OCMe2CO2)2)22 and cis-Mo2(DAniF)2(m-C6H4(OCMe2-
CO2)2).23The structure of 2 is also very similar to that of the
molybdenum analogue 3 (see Supporting Information) except
that the latter does not have axial ligands, in accord with the
established lesser tendency toward axial ligation of quadruply
bonded, dimolybdenum paddlewheel compounds relative to
those of the Rh analogues.24
1), the compound crystallizes in the triclinic space group P1h.
The center of the cage resides on a crystallographic inversion
center, and the inside of the spheroidal cage is occupied by a
disordered Et2O molecule. The cage possesses four dirhodium
units but only two independent Rh-Rh distances of 2.429(1)
and 2.422(2) Å (Table 2). Each of the Rh2 moieties is
equatorially bound to two formamidinate ligands arranged in a
cisoid conformation and two carboxylate groups which are trans
to the formamidinate ligands. One of the four carboxylate groups
from each calixarene ligand forms bonds to each dirhodium unit.
Thus, these dirhodium units serve as fasteners that hold together
the two bowls of the calixarenes. Because of this arrangement
with two formamidinates and two carboxylates, each dimetal
unit has a paddlewheel structure. Additionally, each of the
dimetal units has one axial acetonitrile ligand. This is somewhat
unusual for dirhodium compounds which typically crystallize
with two axial interactions. However, a few examples are known
with only one axial interaction.21 To understand whether this
arrangement is due to the presence of the large molecule or if
it is due to the electronic environment created by the ligands, a
(21) Cotton, F. A.; Hillard, E. A.; Liu, C. Y.; Murillo, C. A.; Wang, W.; Wang,
X. Inorg. Chim. Acta 2002, 337, 233.
(22) Bonar-Law, R. P.; McGrath, T. D.; Singh, N.; Bickley, J. F.; Steiner, A.
Chem. Commun. 1999, 2457.
(23) Berry, J. F.; Cotton, F. A.; Ibragimov, S. A.; Murillo, C. A.; Wang, X. J.
Chem. Soc., Dalton Trans. 2003, 4297.
9
J. AM. CHEM. SOC. VOL. 126, NO. 5, 2004 1521

A R T I C L E S
Cotton et al.
Table 4. Selected Bond Lengths [Å] and Angles [deg] for 1b′ and
1b′′
Rh(1)-Rh(2)
Rh(3)-Rh(4)
Rh(5)-Rh(6)
Rh(7)-Rh(8)
Rh(7B)-Rh(8B)
Rh(1)-N(11)
Rh(1)-N(12)
Rh(2)-N(21)
Rh(2)-N(22)
Rh(3)-N(3)
Rh(3)-N(31)
Rh(3)-N(32)
Rh(4)-N(41)
Rh(4)-N(42)
Rh(5)-N(5)
Rh(5)-N(51)
Rh(5)-N(52)
Rh(6)-N(61)
2.417(2)
2.410(2)
2.428(2)
2.413(4)
2.412(4)
2.04(1)
2.024(9)
2.020(9)
2.030(9)
2.16(1)
2.03(1)
2.03(1)
2.05(1)
2.02(1)
2.20(1)
2.02(1)
2.03(1)
2.01(1)
Rh(6)-N(62)
Rh(7)-N(71)
Rh(7)-N(72)
Rh(8)-N(81)
Rh(8)-N(82)
Rh(1)-O(10)
Rh(1)-O(12)
Rh(2)-O(22)
Rh(3)-O(32)
Rh(4)-O(42)
Rh(5)-O(52)
Rh(6)-O(62)
Rh(7)-O(71)
Rh(7)-O(72)
Rh(7B)-O(8B)
Rh(8)-O(80)
Rh(8)-O(81)
Rh(8)-O(82)
2.08(1)
2.07(1)
2.02(1)
2.009(9)
2.005(9)
2.316(8)
2.053(7)
2.086(7)
2.094(8)
2.087(8)
2.054(8)
2.072(8)
2.06(1)
2.050(9)
2.20(1)
2.34(2)
2.103(7)
2.105(7)
N(12)-Rh(1)-N(11)
N(12)-Rh(1)-O(12)
N(11)-Rh(1)-O(12)
N(12)-Rh(1)-O(10)
N(11)-Rh(1)-O(10)
O(12)-Rh(1)-O(10)
N(12)-Rh(1)-Rh(2)
N(21)-Rh(2)-N(22)
N(21)-Rh(2)-O(22)
N(52)-Rh(5)-O(52)
N(52)-Rh(5)-Rh(6)
87.3(4)
93.0(3)
175.9(4)
96.5(3)
102.7(4)
81.4(3)
88.9(3)
91.0(4)
175.0(3)
87.9(4)
87.6(3)
N(22)-Rh(2)-O(22)
N(21)-Rh(2)-Rh(1)
N(22)-Rh(2)-Rh(1)
O(22)-Rh(2)-Rh(1)
N(31)-Rh(3)-N(32)
N(31)-Rh(3)-O(32)
N(32)-Rh(3)-O(32)
N(31)-Rh(3)-N(3)
N(32)-Rh(3)-N(3)
O(32)-Rh(3)-N(3)
N(11)-Rh(1)-Rh(2)
86.9(3)
87.0(3)
87.4(3)
88.3(2)
92.0(5)
173.6(4)
88.2(4)
93.8(4)
97.0(4)
92.5(3)
89.6(3)
Figure 2. A plot of 2 with displacement ellipsoids drawn at the 40%
probability level. The drawing shows the similarity of the dirhodium unit
with those of the cages 1a and 1b. Note, however, that in the cage
compounds each of the carboxylate groups is provided by a different
calixarene ligand.
Table 3. Selected Bond Lengths [Å] and Angles [deg] for 2
Rh(1)-Rh(2)
Rh(1)-N(1)
Rh(1)-N(3)
Rh(2)-N(2)
Rh(2)-N(4)
2.4383(4)
2.026(3)
2.038(3)
2.026(3)
2.012(3)
Rh(1)-O(1S)
Rh(1)-O(5)
Rh(1)-O(7)
Rh(2)-O(6)
Rh(2)-O(8)
2.301(2)
2.048(2)
2.088(2)
2.088(2)
2.063(3)
The cores of these molecular cages or capsules resemble those
of compounds with cavitand tetracarboxylic acids in which the
two units are joined by four 2-aminopyrimidine molecules.9a
However, in the latter arrangement, the “molecular glue” is
provided by hydrogen bonds, not covalent interactions such as
those between the dirhodium units and carboxylate groups in
the cores of molecules 1.
Reactivity and Synthetic Considerations. The assemblage
reaction for cage compound 1b is carried out in CH3CN and is
represented by eq 1.
N(1)-Rh(1)-N(3)
N(1)-Rh(1)-O(5)
N(3)-Rh(1)-O(5)
N(1)-Rh(1)-O(7)
N(3)-Rh(1)-O(7)
O(5)-Rh(1)-O(7)
N(1)-Rh(1)-O(1S)
N(3)-Rh(1)-O(1S)
O(5)-Rh(1)-O(1S)
O(7)-Rh(1)-O(1S)
N(1)-Rh(1)-Rh(2)
N(3)-Rh(1)-Rh(2)
91.4(1)
175.9(1)
91.2(1)
92.8(1)
173.1(1)
84.3(1)
N(2)-Rh(2)-O(6)
O(8)-Rh(2)-O(6)
N(4)-Rh(2)-Rh(1)
N(2)-Rh(2)-Rh(1)
O(8)-Rh(2)-Rh(1)
O(6)-Rh(2)-Rh(1)
O(5)-Rh(1)-Rh(2)
O(7)-Rh(1)-Rh(2)
O(1S)-Rh(1)-Rh(2)
N(4)-Rh(2)-N(2)
N(4)-Rh(2)-O(8)
N(2)-Rh(2)-O(8)
173.6(1)
84.0(1)
89.28(9)
88.23(8)
88.46(6)
86.70(7)
88.26(6)
86.47(6)
168.81(7)
91.9(1)
96.3(1)
101.7(1)
86.24(9)
83.29(9)
88.70(8)
88.12(8)
4[cis-Rh2(DAniF)2(CH3CNeq)4(CH3CNax)2](BF4)2 +
2(NEt4)4(calix[4]arene(CO2)4) f
175.5(1)
91.9(1)
{NEt4⊂[cis-Rh2(DAniF)2L]4[calix[4]arene(CO2)4]2}BF4 +
7NEt4BF4 (1)
The second cage, 1b, also forms crystals in the triclinic space
group P1h, but there are two independent cage molecules, 1b′
and 1b′′. These are chemically equivalent, and their cores are
structurally similar to that of 1a. Each has an inversion center,
The reaction conditions are very important for the success
of the reaction and thus worthy of discussion. The reaction
temperature must be kept above room temperature at 35-40
°C. Furthermore, a long reaction time appears to be necessary.
Under these conditions, the reaction is highly reproducible.
When the reaction is done under an inert atmosphere, the
compound is isolated as red crystals with L ) CH3CN. When
water molecules replace two of the four axial CH3CN molecules,
the color of the crystals changes from red to green. This is
consistent with the well-established fact that the colors of
paddlewheel compounds having the dirhodium unit are highly
dependent on the nature of the axial ligands.25 We found by
structural studies and mass spectroscopy (vide infra) that the
+
but the internal volume is occupied now by a disordered NEt4
cation; a BF4- counteranion is located outside the cage. Again,
however, only one axial position is occupied on each of the
crystallographically independent dirhodium units. Four of the
eight axial sites are occupied by two acetonitrile and two water
molecules. The Rh-Rh distances are in the narrow range of
2.410(2)-2.428(2) Å (Table 4). The structure of the core of
one of the molecules and a schematic representation of the
reaction are shown in Figure 3. The molecule has been drawn
with all axial ligands and anisyl groups removed, and with a
space filling diagram of the encapsulated tetraethylammonium
ion. It appears that this is trapped and thus unable to escape
from inside the cage (vide infra). Therefore, the species can be
represented by the formula {NEt4⊂cage}BF4.
(25) (a) Cotton, F. A.; Hillard, E. A.; Murillo, C. A. J. Am. Chem. Soc. 2002,
124, 5658. (b) Johnson, S. A.; Hunt, H. R.; Neumann, H. M. Inorg. Chem.
1963, 2, 960. (c) Trexler, J. W., Jr.; Schreiner, A. F.; Cotton, F. A. Inorg.
Chem. 1988, 27, 3265. (d) Miskowski, V. M.; Schaefer, W. P.; Sadeghi,
B.; Santarsiero, B. D.; Gray, H. B. Inorg. Chem. 1984, 23, 1154. (e)
Lichtenberger, D. L.; Pollard, J. R.; Lynn, M. A.; Cotton, F. A.; Feng, X.
J. Am. Chem. Soc. 2000, 122, 3182.
(24) Cotton, F. A.; Walton, R. A. Multiple Bonds between Metal Atoms, 2nd
ed.; Oxford University Press: Oxford, 1993.
9
1522 J. AM. CHEM. SOC. VOL. 126, NO. 5, 2004

A Calix[4]arene Carceplex with Four Rh24+ Fasteners
A R T I C L E S
Figure 3. A schematic drawing showing the formation of the carceplex 1b. The drawing on the right has a space filling diagram of one orientation of the
encapsulated tetraethylammonium ion in one of the two crystallographically independent molecules. The anisyl groups of all formamidinate ligands and the
axial ligands (two water and two acetonitrile molecules which occupy one axial position of each of the four Rh24+ units) have been removed for clarity. For
a balanced equation for the process, see eq 1.
core of the cage remained unchanged upon substitution of the
axial ligands. Furthermore, when single crystals of the corre-
sponding acetonitrile cage with the encapsulated NEt4 cation
chromatographic support. Nevertheless, the green band produced
a very pure sample of the cage compound as shown below.
+
Attempts to prepare the cage species using (NBun ) (calix-
4 4
[4]arene(CO2)4) instead of the NEt4+ analogue were unsuccess-
ful, which is a good indication that the stability of the cage is
adversely affected by the presence of the much larger cation
which does not fit into the cavity. We also tried the crystal-
lization process in the presence of adamantane, but again no
inclusion of this hydrocarbon was observed. Thus, encapsulation
of the NEt4+ cation seems to be favored and appears to be quite
selective.
are exposed to moist air over a period of many days, or rinsed
with distilled water, the diffraction pattern remains essentially
unchanged even though the color of the crystals changes. This
indicates that the cage is quite sturdy and persists. Although
exposure of the aqua complex to an atmosphere of acetonitrile
did not reverse the process, water can be removed under vacuum
and then replaced by placing the solid in contact with aceto-
nitrile. We found that the cage in its green form having some
axially coordinated water molecules is more stable and easier
to handle than the corresponding compound with acetonitrile
molecules in all axial positions.
The complex with encapsulated diethyl ether, 1a, is signifi-
cantly less stable, and the crystals appear to lose crystallinity
very rapidly upon exposure to air. This crystalline form was
obtained in very low yield from a reaction carried out at 22 °C
over a 2 day period.26
Also important for the isolation of a pure crystalline product
with an encapsulated NEt4+ cation is the purification by column
chromatography over silica gel, although this has some unwel-
come consequences too. The progress of the reaction mixture
was monitored by thin-layer chromatography which showed that
the cage compound was the major product with a small amount
of unreacted starting material also present. However, the use
of column chromatography produced a third band (blue in color)
that trailed the green band of the product and a tan band
containing the Rh2 starting material. This blue band is due to
decomposition of the product upon prolonged contact with the
The synthesis of the simple molecule 2 was accomplished in
very high yield in acetonitrile at 22 °C upon reaction of the
corner piece and the calix[4]arenedicarboxylate, as shown in
eq 2. The crystalline compound with one axial ligand, Lax
)
CH3OH, can be isolated upon recrystallization from a methanol
solution:
[cis-Rh2(DAniF)2(CH3CNeq)4(CH3CNax)2](BF4)2 +
(NBun ) Br2calix[4]arene(CO2)2 f
4 2
cis-Rh2(DAniF)2[Br2calix[4]arene(CO2)2]Lax
+
2NBun BF4 (2)
4
The red crystalline compound 2 thus obtained is spectroscopi-
cally pure as shown by the 1H NMR spectrum, and there is no
need to purify it further by column chromatography. While it
is possible that an analogue of this type might occur during the
assemblage reaction of 1, no such compounds were isolated from
the preparations of 1. The higher temperatures and longer
periods of time used for the preparation of the cage compounds
might be attributed to the need to rearrange such intermediates
into the cage products. A rearrangement would be necessary
because in the dinuclear species the two carboxylate groups are
(26) The reproducibility of this reaction is not very good. Because of this, the
molecule with an encapsulated Et2O was characterized only by X-ray
crystallography and 1H NMR spectroscopy, and no further efforts to make
it again have been made. Higher crystallization temperatures give exclu-
sively, and reproducibly, the complex with an encapsulated NEt4+ cation,
for which extensive characterization was done.
9
J. AM. CHEM. SOC. VOL. 126, NO. 5, 2004 1523

A R T I C L E S
Cotton et al.
Table 5. Selected Bond Lengths [Å] and Angles [deg] for 3
provided by only one calixarene ligand, while in the cage each
carboxylate group is attached to a different dirhodium unit.
The molybdenum analogue 3 was prepared similarly in very
high yield and excellent purity according to eq 3:
Mo(1)-Mo(2)
Mo(1)-N(1)
Mo(1)-N(3)
Mo(2)-N(2)
Mo(2)-N(4)
2.0931(4)
2.132(3)
2.116(3)
2.104(3)
2.126(3)
Mo(1)-O(5)
Mo(1)-O(7)
Mo(2)-O(6)
Mo(2)-O(8)
2.124(2)
2.134(2)
2.155(2)
2.133(2)
[cis-Mo2(DAniF)2(CH3CNeq)4](BF4)2 +
Mo(2)-Mo(1)-N(3)
Mo(2)-Mo(1)-O(5)
N(3)-Mo(1)-O(5)
Mo(2)-Mo(1)-N(1)
N(3)-Mo(1)-N(1)
O(5)-Mo(1)-N(1)
Mo(2)-Mo(1)-O(7)
N(3)-Mo(1)-O(7)
N(2)-Mo(2)-O(6)
93.04(8) O(5)-Mo(1)-O(7)
92.27(6) N(1)-Mo(1)-O(7)
80.04(9)
91.1(1)
92.83(8)
92.39(8)
95.2(1)
92.01(7)
89.7(1)
173.2(1)
91.14(6)
(NBun ) Br2calix[4]arene(CO2)2 f
4 2
92.5(1)
Mo(1)-Mo(2)-N(2)
92.63(8) Mo(1)-Mo(2)-N(4)
cis-Mo2(DAniF)2[Br2calix[4]arene(CO2)2] +
96.0(1)
170.0(1)
91.52(6) N(2)-Mo(2)-O(8)
171.4(1)
168.7(1)
N(2)-Mo(2)-N(4)
Mo(1)-Mo(2)-O(8)
2NBun BF4 (3)
4
N(4)-Mo(2)-O(8)
Mo(1)-Mo(2)-O(6)
Like other compounds described here, it is diamagnetic because
of the closed shell structure of the metal-metal bonded M2
4+
species.
Accompanying the trapped NEt4+ cation in {NEt4⊂cage} is
Other Spectroscopic Observations. The high purity of all
crystalline products was assessed by elemental analyses and 1H
NMR spectroscopy. The latter, as well as mass spectrometric
studies, provide evidence that the solid-state structures remain
unchanged in solution and in the gas phase.
-
a BF4 counterion which is found outside the cage. Spectro-
scopic evidence for this is provided by the appearance of
bands in the infrared spectrum at 1083 and 531 cm-1 which
are similar to those reported by Nakamoto30 for the BF4- anion
11
and those in cis-[Rh2(DAniF)2(CH3CNax)4(CH3CNeq)2](BF4)2
1
The H NMR spectra of 2 and 3 show that the molecule is
and [Mo2(DAniF)2(CH3CN)4](BF4)2,12 and absent in (NEt4)2-
(calix[4]arene(CO2)4). Elemental analysis also supports this
formulation.
highly symmetrical having only one signal for the methine
proton of the formamidinate group at 7.36 ppm for 2 and at the
lower field of 8.46 ppm for 3. This shift is consistent with the
Further evidence for the stability of the {NEt4⊂cage}BF4
species is provided by mass spectroscopic studies. The MALDI-
TOF spectrum shows a series of peaks distributed about the
most intense signal at 4523.7. The signals are consistent with
the isotopic distribution for [NEt4⊂cage]+ devoid of axial
ligands which are known to be easily lost. There is another signal
at 14 mass units lower having significantly lower intensity which
can be assigned to the same ion but with a methoxy group from
an anisyl moiety replaced by a hydrogen atom. Because of this,
the mass spectra of the red form (axial acetonitrile) and the green
form (axial water or a mixture of acetonitrile/water) are
indistinguishable. Very little else appears in the spectrum, and
it is very notable that there are no signals in the region
4+
higher magnetic anisotropy of the quadruple bond in the Mo2
species.27 All other signals are consistent also with a highly
symmetrical species such as that found in the solid state.
The proton NMR spectrum of 2 is very helpful in assigning
that of the cage 1b as there are only small shifts (in ppm
referenced to CH2Cl2) because of the similarity of the chemical
4+
environment of the singly bonded Rh2 units in these com-
pounds. The singlet at 7.766 ppm corresponds to the 16 aromatic
protons in the calix[4]arene ligand, while those on the 8
methylene groups appear as expected as a pair of doublets at
4.508 and 3.353 ppm. By analogy to those of the corresponding
acid, the axial doublet is at lower field than the equatorial one.14a
More importantly, there are signals at very high field that
+
corresponding to the mass of the cationic cage without the NEt4
+
correspond to the encapsulated NEt4 cation (broad signals at
fragment. This is evidence that the entire crystalline sample
contains only species with the encapsulated cation and that these
cations remain inside the cage even in the gas phase. This is
one of only a few examples in which gas-phase interactions of
calixarene cavitands with molecular guests have been observed
by mass spectrometry.31
0.791 and -1.560 ppm). These have been assigned by analogy
to those in other carceplexes and related species containing
trapped organic entities which show strong shielding and signals
at higher fields relative to those of the same molecules in
ordinary solutions. Prior examples are provided by encapsulated
+
NEt4 cations28 and a variety of species.29 The broadening of
Comparison with Other Calix[4]arene Compounds Hav-
ing Dimetal Units. There is a small number of dinuclear
compounds having one metal-metal bonded unit and calix[4]-
arene ligands.32 It should be noted that there are only two
the tetraethylammonium bands in the NMR spectrum is
consistent with some motion of the cation within the cage which
might be due to an interconversion between the two disordered
positions which exist in the solid state. Other spectral assign-
ments are shown in the Experimental Section.
complexes with the quadruply bonded Mo2 unit.32,33 Like 3,
4+
these have two cisoid ligands, either acetate or formamidinate
groups, and a capping calixarene, as shown in III. Unlike 2
and 3, however, the calixarene is bonded to the dimetal units
through the oxygen atoms of alkoxo or phenolic components
of the calixarene. Interestingly, the Mo-Mo distances differ
significantly, with those in 3 being 2.0931(4) Å (Table 5) while
(27) See, for example: Cotton, F. A.; Daniels, L. M.; Lei, P.; Murillo, C. A.;
Wang, X. Inorg. Chem. 2001, 40, 2778 and references therein.
(28) (a) Caulder, D. L.; Bru¨ckner, C.; Powers, R. E.; Ko¨nig, S.; Parac, T. N.;
Leary, J. A.; Raymond, K. N. J. Am. Chem. Soc. 2001, 123, 8923. (b)
Parac, T. N.; Caulder, D. L.; Raymond, K. N. J. Am. Chem. Soc. 1998,
120, 8003. (c) Vysotsky, M. O.; Pop, A.; Broda, F.; Thondorf, I.; Bo¨hmer,
V. Chem.-Eur. J. 2001, 7, 4403.
(29) (a) Aoki, S.; Shiro, M.; Kimura, E. Chem.-Eur. J. 2002, 8, 929. (b) Chopra,
N.; Sherman, J. C. Angew. Chem., Int. Ed. 1999, 38, 1955. (c) Chapman,
R. G.; Sherman, J. C. J. Org. Chem. 2000, 65, 513. (d) Fox, O. D.; Leung,
J. F.-Y.; Hunter, J. M.; Dalley, N. K.; Harrison, R. G. Inorg. Chem. 2000,
39, 783. (e) Atwood, J. L.; Szumna, A. J. Am. Chem. Soc. 2002, 124, 10646.
(f) Tucci, F. C.; Rudkevich, D. M.; Rebek, J., Jr. J. Am. Chem. Soc. 1999,
121, 4928. (g) Ikeda, A.; Yoshimura, M.; Udzo, H.; Fukuhara, C.; Shinkai,
S. J. Am. Chem. Soc. 1999, 121, 4296. (h) Park, S. J.; Hong, J.-I. Chem.
Commun. 2001, 1554. (i) Zhong, Z.; Ikeda, A.; Ayabe, M.; Shinkai, S.;
Sakamoto, S.; Yamaguchi, K. J. Org. Chem. 2001, 66, 1002.
(30) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination
Compounds. Part A: Theory and Applications in Inorganic Chemistry, 5th
ed.; John Wiley & Sons: New York, 1997.
(31) (a) Vincenti, M.; Irico, A. Int. J. Mass Spectrom. 2002, 214, 23. (b) Schalley,
C. A.; Castellano, R. K.; Brody, M. S.; Rudkevich, D. M.; Siuzdak, G.;
Rebek, J., Jr. J. Am. Chem. Soc. 1999, 121, 4568.
(32) For a review of calixarenes with dimetal units up to the year 2001, see:
Cotton, F. A.; Daniels, L. M.; Lin, C.; Murillo, C. A. Inorg. Chim. Acta
2003, 347, 1.
9
1524 J. AM. CHEM. SOC. VOL. 126, NO. 5, 2004

A Calix[4]arene Carceplex with Four Rh24+ Fasteners
A R T I C L E S
the others are 2.122(3)32 and 2.1263(6) Å.33 Other compounds
having two calixarene ligands and M2 units, M ) Mo, W, Nb,
commonly have the structural motif shown in IV, and one has
the structure shown in V.34 Recently, the compound Rh2(Br2-
calix[4]arene(CO2)2)2‚2toluene was made by reacting Rh2(O2-
CCH3)4L2 and Br2calix[4]arene(CO2H)2 in refluxing toluene and
has been shown to catalyze the intramolecular C-H insertion
of R-diazo-â-ketoesters.35 Notably, full substitution of the four
labile acetate ligands was accomplished here. This contrasts with
the reaction leading to the formation of 2 where the less labile
Concluding Remarks
A highly reproducible synthesis of a carceplex consisting of
two tetracarboxylato-calixarene bowls joined covalently by four
singly bonded dirhodium units has been established. The cage
molecules encapsulate tetraethylammonium ions selectively over
tetrabutylammonium ions to give a very stable species which
remains intact in solution, and even under the conditions
necessary for mass spectroscopy. Simpler compounds with only
one dirhodium or one dimolybdenum unit are also described
here. These represent the first known compounds of their class
in which there is only one cis-[M2(formamidinate)2n+] unit
attached to carboxylate groups of a calixarene ligand. Efforts
to improve the yield of 1b and study its catalytic activity are in
progress.
4+
formamidinate ligands remain attached to the Rh2 unit.
Acknowledgment. We thank Dr. L. M. Daniels for early
crystallographic work on 1a, and the National Science Founda-
tion and the National Science Foundation of China (No.
90206013) for financial support. We also thank Johnson Matthey
for a loan of rhodium and Zhejiang University for allowing us
to use their Avance DMX500 NMR spectrometer.
Supporting Information Available: X-ray crystallographic
data for 1a, 1b, 1c, 2, and 3 (CIF). Fully labeled plots of the
cores of the molecules in 1a and 1b, to facilitate the interpreta-
tion of the tables of bond distances and angles, and a plot of 3
(PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
(33) (a) Acho, J. A.; Lippard, S. J. Inorg. Chim. Acta 1995, 229, 5. (b) Acho,
J. A.; Ren, T.; Yun, J. W.; Lippard, S. J. Inorg. Chem. 1995, 34, 5226.
(34) See, for example: (a) Chisholm, M. H.; Folting, K.; Streib, W. E.; Wu,
D.-D. Inorg. Chem. 1999, 38, 5219. (b) Caselli, A.; Solari, A. R.; Scopelliti,
L. R.; Floriani, C.; Re, N.; Rizzoli, A. C.; Chiesi-Villa, A. J. Am. Chem.
Soc. 2000, 122, 3652.
(35) Seitz, J.; Maas, G. Chem. Commun. 2002, 338.
JA0396899
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