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Binding of Boron and Alkali Metal Cations by a Pseudocryptand.

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[12] K. B. Wiberg. K . E. Laidig. 1 Am. Chrm. Sor. 1987, 109, 5935.
[13] S. Yamada. Angrw. Chem. 1993, 10.5, 1128: Angrw. Chem. In/. Ed. Eng/. 1993.
32, 10x3.
[14] Linewidths a1 halfheights are as follows: 1 a. 270 Hz; 1 b. 260 Hz: 1 c, 540 Hz:
Id, 780 H r ; l e . 490 H r : 2a. X7 Hz; Zb, 180 Hz; 2c, 360 Hz.
[15] R. F. W Bader. T. T. Nguyen-Dang, Adr. Quunmn C/7em. 1981, 14. 63.
[16] R. F W. Badcr. K. E. Cheeseman, K. E. Laidig, K. B. Wiberg, C. M. Breneman. .I A m Chem. Soc. 1990. 112. 6530: C. M . Breneman. K. 8. Wiberg, J.
C o m p i i . Choii. 1990. I / , 361; C. L. Perrin, J. Am. Chem. Soc. 1991. 113. 2865;
A. Greenherg. Y.-Y. Chiu, J. L. Johnson. J. F. Liebman. Strurt. Chem. 1991,2,
117: K. E Laidig, R. F. W. Bader, J. Am. Chem. Suc. 1991. 113, 6312; K. B.
Wiberg. C . M. Breneman. ihid 1992. 114. 831: A. Greenherg, C. A. Venanzi.
ihd.1993. 115. 6951. K. B. Wiberg, P. R. Rablen, ibid. 1993. 115,9234; R. D.
Bach. I. Mintcheva, 1 Org. Chmm. 1993, SR, 6135: F. J. Luque. M. Orozco, ibirl.
1993. SK. 6391.
[17] This work. Full details of the crystal structure will he published elsewhere.
polyethyleneglycol chains.["] Compounds bearing the [2.2]
macrocycle and two bipyridine moieties are known and their
ability to bind both transition and alkaline metal cations has
been described." 31 Triaza and tetraaza macrocycles bearing
three and four pendant catecholate units, re~pectively,['~]
as well
as macrobicyclic tris-catecholate ligands that serve as siderophore analogues have also been reported."
Binding of Boron and Alkali Metal Cations
by a Pseudocryptand**
Ernest Graf,* M i r Wais Hosseini,* R o m a i n R u p p e r t ,
Nathalie Kyritsakas, Andrk De Cian, Jean Fischer,
Christine Estournes, and Francis Taulelle
Since the discovery of cryptands, in particular the [2.2.2] macrobicyclic ligand 3, some 25 years ago by Dietrich, Lehn, and
Sauvage"] and of the crown ethers by Pedersen"] an exciting
new field of research named supramolecular chemistry has
opened to inorganic, organic, and physical chemists.[3- 51
Whereas at the outset much attention was focused on the selective binding of spherical monoatomic alkali metal and alkalineearth metal cations, very rapidly considerable achievements
were reported with regard to the recognition of molecular
cations.['] Many aspects of supramolecular chemistry remain to
be developed. for example, new materials based on self-assembly may be targeted by using weak interactions."]
The early design of cryptands involved the construction of
cage molecules capable of binding cationic substrates within
their cavities leading to the formation of cryptates. Since the
[2.2.2]cryptand 3 is neutral, for the sake of charge neutrality the
cationic cryptate complex must be accompanied by a counter
anion. In order to overcome the problems associated with the
presence of the anion, we designed a cryptand bearing a negative
charge within its framework. Inspired by the naturally occurring
antibiotics such as boromycin['I and aplasm~mycin,[~]
both bear a borate ester moiety, we synthesized the pseudocryptand
This compound combines the [2.2] macrocyclic core" ' ] of the [2.2.2]cryptand with two bidentate dianionic catecholate units. A similar concept was previously reported
for podands and coronands bearing two catechol units linked by
[*] Dr. E. Graf. Prof. Dr. M. W. Hosseini, Dr. R. Ruppert
Laboratoire de Chimie de Coordination Organique (URA au CNRS 422)
Universite Louis Pasteur. Institut Le Be1
4, rue Blaise Pascal, F-67000 Strasbourg (France)
Telefax: Int. code f88416266
N. Kyritsakas. Dr. A. De Cian, Prof. Dr. J. Fischer
Lahoratoire de Christdllochimie et Chimie Structurale (URA au CNRS 424)
Universite Louis Pasteur. lnstitut Le Bel, Strasbourg (France)
Dr. C . Estournis, Dr. E Taulelle
Laboratoire de RMN et Chimie du Solide (UMR an CNRS 50)
Universite Louis Pasteur. Institut Le Bel. Strasbourg (France)
This work was supported by the CNRS and by the lnstitut Universitaire de
France. The lii-st publication ahout [2.2.2]cryptands by B. Dietrich, J.-M. Lehn,
and I - P . Sawage appeared in 1969 (ref. [I]); this work is dedicated to them.
lni. Ed. E q l . 1995, 34. N o . 10
[M'. 21
Compound 1 may also be regarded as a binucleating ligand
capable of binding two cations. Indeed, the binding of a main
group element such as boron by 1 would lead to the negatively
charged pseudocryptand 2, a complex of the Boeseken type['61
found in boromycin['] and in a p l a s m ~ m y c i n . [In
~ ] turn, this
complex should bind an alkali metal cation, affording the neutral cryptate [M' . 21. Although the design and synthesis of binuclear complexes have been extensively studied over the past
twenty years," '1 only recently has considerable effort been invested in the synthesis of heterobinuclear complexes bearing
both a hard alkali or alkaline-earth metal cation and a soft
transition metal cation."']
Treatment of 1 [ ' O 1 in EtOH with one equivalent of B(OH),
and one equivalent of M O H in H,O at room temperature led
exclusively to the formation of the [M' . 21 complex. A single
recrystallization of the raw material afforded the pure [M' 21
complex (M = Li, Na, K, Rb, Cs, NH,).
The binding ability of 2 towards alkali metal cations was
studied in solution by N M R spectroscopy in CDCl, and in
CD,OD. In CDCl,, the 'H N M R spectrum of the parent compound 1 in the region 6 = 2-4.5 showed two triplets at 6 = 2.82
and 3.69 for CH,N and NCH,CH,O, respectively, and two
singlets at 6 = 3.61 and 3.81 for OCH,CH,O and the benzylic
CH,N protons, respectively (Fig. 1 a). The coordination of
boron in 1, caused dramatic changes in the 'H N M R spectrum.
In particular, the benzylic CH,N protons in 2 give rise to an AB
quartet. Furthermore, the chemical shift values in the ' H N M R
spectrum appear to be strongly dependent on the nature of the
cation complexed in the cavity. This was especially true for the
benzylic CH, protons for which the observed chemical shift
difference (aA- 6,) varied from about 375 Hz for Na' to about
554 Hz for Kf(Fig. 1 ) . For thecomplex [K' . 21 all ' H a n d I3C
signals were assigned based on a 'H-13C correlation experiment. For the other complexes the same assignment was assumed to be correct. Since the positions of the 'H signals for the
[M 21 complexes appeared to be cation-dependent, the selectivity and stability were studied by 'H N M R spectroscopy. The
stability of [M' '21 complexes (M = Na'. K'. Cs') was investigated by competition experiments with [2.2.2]cryptand 3 in
CD,OD. Addition of less than one equivalent of 3 to a solution
of [Na' . 2 ] caused partial decomplexation of the Na' ion leading to a mixture of [Na' . 21 and [Na' . 31 complexes. Complete
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Fig. 1. Selected regions (6 =1.8-4.5) in the ' H NMR spectra (CDCI,): a) free
ligandl,b)[Nat ' 2 ] , c ) [ K + . 2 ] , a n d d ) [ C s t .2].ThequartetinFig. I d a t d = 3.72
is attributed to residual EtOH. The quartet of AB benzylic protons are indicated
with an *.
removal of N a + from 2 was attained by addition of more than
one equivalent of 3, indicating that the [Na' . 21 complex was
less stable than the [Na' . 31 complex. On the other hand, addition of up to 100 equivalents of 3 to the [K+ . 21 complex did not
lead to the removal of the K' ion from 2, indicating that the
[K' .2] complex was at least two orders of magnitude more
stable than the [K' .3] complex. In other words, since the binding constant K , for the [K' .3] complex in methanol was initially estimated to be greater than 10' L mol - ,[I9] and subsequently determined to be 10'0.49in methanol in the presence of 0.5 M
NEt,C104,[201for the [K' . 21 complex this K, value should be
greater than 1012.5Lmol-'. However, owing to the high stability of the [K' 21 complex we have so far been unable to determine its binding constant accurately using conventional methods. The selectivity of 2 towards Na', K', and Cs' ions was
likewise investigated by competition experiments in CD,OD.
Whereas addition of less than one equivalent of KI to the
[Na' . 21 complex led to a mixture of both [Na ' . 21 and [K' 21
complexes, upon addition of one equivalent of KI, only the
characteristic 'H NMR pattern of the [K+ . 21 complex was
observed. These findings indicated a stronger binding of K +
than Na' by 2. Further confirmation of this was obtained from
the following experiment. Addition of up to lo3 equivalents of
NaI to the [K+ . 21 complexes led to no changes in the 'H NMR
spectrum, indicating that the selectivity factor between Na' and
K + lay in the favor of the latter by at least a factor of lo3. For
comparison, a selectivity factor of about 300: 1 for K'/Na+ was
reported in methanol for the cryptand [2.2.2] (3).['01 Analogous
competition experiments between Cs' and K ' revealed that 2
binds K + more strongly than Cs+ with a selectivity factor
greater than lo2.
The binding ability of 2 towards alkali metal cations was also
studied by 23NaNMR spectroscopy (Fig. 2). In CD,OD, the
[Na' . 21 complex gave a major signal at 6 = - 15.7 with a line
width of 113 Hz; and a minor signal at 6 = - 4.1 is attributed
to some sodium contamination ( 5 M NaCl in D,O was used as
external reference) (Fig. 2a). For NaI under the same conditions values of 6 = -3.91 and line width = I 6 1 Hz were observed. For the [Na' '31 cryptate the 23Na NMR signal occurs
at 6 = - 11.4 and the line width Av is 29 Hz in CD,OD/D,O:
95/5.[211Since 2 is considerably less symmetrical than 3, the
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Fig. 2.
[Na' 21
[Na' . 21
N M R spectra (105 MHz, CD,OD,
25°C). a) [Nat . 21, b)
+ 1 equiv Nal, c) [Na' . 21 + 1 equiv KI, d) "a+ . 2) + 1 equiv CsBr, e)
+ 1 equiv 3. 5 M NaCl in D,O was used as external reference.
increase in the line width values from 29 Hz for the "a+ . 31
complex to 113 Hz for the [Na' .2] complex may be expected
because of the nuclear quadrupolar moment of 'jNa ( I = 3/2).
Upon addition of one equivalent of NaI (Fig. 2 b) to the solution containing the [Na' . 21 complex, two signals were observed at 6 = -15.4 and -3.7 with line widths of 238 and
192 Hz, respectively; this indicates a slow exchange process be= 55"C,
tween the free and the complexed Na' at 25°C
k , = 2.7 x lo3 s-', AG* = 15.5 kcalmol-I). The addition of
one equivalent of KI to this solution restored the signal at
6 = - 3.9 with a line width of 18.9 Hz corresponding to free
Na' ions (Fig. 2c). This thus confirmed the results of the
'H NMR investigations namely that the K + ion was more
strongly complexed by 2 than the N a + ion. Furthermore, addition of one equivalent of CsBr to a solution of the [Na' . 21
complex gave two signals at 6 = -15.5 and -4.0, which are
assigned to the free and complexed sodium, indicating that the
affinity of 2 for Na' and Cs' was in the same range (Fig. 2d).
Finally, addition of the [2.2.2]cryptand 3 to a solution of the
[Na' . 21 complex led to a single signal at 6 = - 11.8 with a line
width of 21.3 Hz. This signal corresponds to the [Na' . 31 complex, confirming the results of the 'H NMR studies according to
which 3 showed a higher affinity for the Na' ion than 2 (Fig. 2e).
Although X-ray data were collected for the [M' . 21 complexes with M = K', Cs', and NH;, only the crystal structure of
[K' . 21 is reported here.r221
The X-ray crystal structure analysis
of the racemic potassium complex (Fig. 3) showed that K + is
Fig. 3. X-ray structure of the [K' . 21 complex. The enantiomer shown is arbitrary.
Side view (left) and view along the boron-potassium axis (right). Selected bond
lengths [A] and angles ["I (average values given): K - 0 2.82, K-N 3.15, B - 0 1.48;
0 - B - 0 105.0 and 109.9. N-K-N 179.
U570-0833195/1U1U-/1163 10.00+ .2S/U
Angen. Chem. I n [ . Ed. Engl. 1995, 34, No. 10
located within the cavity formed by the negatively charged
cryptand 2. The coordination geometry around the boron atom
is indeed tetrahedral with two sets of 0 - B - 0 angles of about 105
and 109.9", and an average B - 0 distance of about 1.48 A. As
expected, because of the tetrahedral coordination around the
boron atom, R and S isomers are present in the unit cell. The
lone pairs of the two nitrogen atoms are oriented towards the
interior of the cavity (in& conformation) and the K' ion is
located at position that is almost equidistant from the two
bridgehead nitrogen atoms (N-Kdistance ca. 3.15 8,); the NK-Nangle is 179". Among the eight oxygen atoms present in 2,
only six of them, namely the four oxygen atoms of the ether
junctions and two borate oxygen atoms, are localized within a
bonding distance of K + (average distance of ca. 2.82 A). The
coordination polyhedron of the Kfion in the [Kt . 21 complex,
in which the K + ion is surrounded by six oxygen and two nitrogen atoms at distances of 2.82-3.15 A, is irregular. The average
K - 0 distance of about 2.82 8, is considerably shorter than the
average K-N distance of about 3.15 8,. In contrast the K +ion
in [K' . 31 is located almost in the center of the cage with average K-0 and K-N distances of about 2.78 8, and 2.87 A, respecti ~ e I y .3l1 ~
In conclusion, we have demonstrated that the pseudocryptand
1 participates in a cascade-type complexation process. Indeed,
binding of the boron atom by both catecholate moieties in 1
leads to the borate ester 2, thus creating a preorganized cavity
bearing one negative charge. Compound 2 acts as a cryptand
and binds an alkali metal cation, leading to a neutral [M+ . 21
complex. To our knowledge, both the estimated binding constant of about 10'2.smo1L-l for the potassium complex in
methanol and the selectivity factor of the ligand 2 for K + over
Na' and Cs' cations of more than lo3 and lo2,respectively, are
the highest values obtained to date for artificial K+ receptors.
Received: December 27. 1994 [Z 7580 IE]
German version: Angel?. Chem 1995. 107, 1204
Keywords: alkali metal compounds . boron compounds .
cryptands macrocycles . molecular recognition
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C. J. Pedersen, J. Am. C%em.Sor. 1967. 89, 2495, 7017.
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27. 1009.
J.-M. Lehn. Angrw. Clietn. 1988, /00, 91 ; Angeu. Chem. Int. Ed. Engl. 1988.27.
F. Vogtle. Slrpromo/ecu/ur Chemi.trry, Wiley. Chichester, 1991, G. Gokel,
Crowti Ethcr.r rind Cr,vpruncl.\, Royal Society of Chemistry, Cambridge, 1991;
D. J. Cram. J. M. Cram. Contuiner Molecules und Their Guests, Royal Society
of Chemistry, Cambridge, 1994; B. Dietrich. P. Viout, J:M. Lehn, Mucroc:,,clic
C h ~ w i . \ / r , tVCH.
Weinheim, 1993.
J:M. Lehn, A n g w . Chem. 1990,102,1347; Angew. Chem. I n [ . Ed. Engl. 1990.
29, 1304:G. M. Whitesides. J. P. Mathias,T. Seto, Soenre 1991,254,1312; J. S.
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G. Vogel. W. Voser. H. Zihner. Hclv. Chirn. Acru 1967.50, 1533; J. D. Dunitz.
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ihid. 1971. 54. 1709. W. Marsh. J. D. Dunitz, D. N. J. White, ibid. 1974, 57. 10.
T. Okazaki. T. Kitahara. Y. Okanii. J Anlibiut. 1975, 28. 176; T. J. Stout, J.
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B. Dietrich. J.-M. Lehn, J.-P. Sauvage. ibid. 1973. 29, 1647.
Y. Kobuhe. Y. Sumida. M. Hayashi, H . Ogoshi, Angrw. Chem. 1991.103,1513;
A n g r i i ~C ' h ~ ~ r h
n .i . Ed. Engl. 1991. 30, 1496.
T. Nabeshima. T. Inaba. T. Sagae, N . Furukawa, Terruhedron Lerr. 1990, 31,
Arigeii.. C ' h m i . Inr. Ed. Engl.
1995, 34. No. 10
[14] F. L. Weitl, K. N. Raymond, J. Am. Chem. Soc. 1980, lU2, 2289; K. N. Raymond, G. Miiller, B. F. Matzanke, Top. Curr. Chem. 1984. 123. 49.
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Engl. 198423,714; P. Stutte, W. Kiggen, F. Vogtle. Tetrahedron 1987,43,2065;
T. J. McMurry, S. J. Rodgers, K. N. Raymond. J Am. C/irm. Soc. 1987, 109,
3451 ; T. J. McMurry. M. W. Hosseini, T. M. Garrett. F. E. Hahn, 2. E. Reyes,
K. N. Raymond, hid. 1987. 109. 7196, T. M. Garrett. T. J. McMurry. M. W.
Hosseini. Z. E. Reyes. F. E. Hahn, K. N. Raymond, ibid 1991, 113, 2965.
1161 J. Boeseken, Ad,. Curbohydr. Chem. 1949.4. 189.
1171 J.-M. Lehn in Frontiers o/Chernistr.v, I U P A C (Ed.: K. J. Laidler). Pergamon.
Oxford, 1982. S. 265; R. M. Izatt, K. Pawlak, J. S . Bradshaw. Chem. R r r . 1991,
91. 1721; P. Zanello, S . Tamburini, P. A. Vigato. G . A Mazzocchin, Coord.
Chem. Rra. 1987. 77. 165; D. C. Smith, H. B. Gray, Chwii. Rev. 1990. 90. 169.
1994, 94.
[lS] F. C. J. M. Van Veggel, W. Verboom, D. N. Reinhoudt. C h ~ mRev.
[19] J.-M. Lehn, J.-P. Sauvage. J Am. Chem. Soc. 1975, 97. 6700.
[20] H. J. Buschmann, Inorg. Chim. Acru 1986, 125. 31.
1211 J:P. Kintzinger, J.-M. Lehn, J. Am. Chern. Soc. 1974. 96, 3313.
. CH,Cl,. M , = 637.4,
[22] Crystal data for [ K t 21 complex: C,,H,,BN,O,K
monoclinic, space group P2,/r, u =11.813(3), h =15.576(4), c =17.205(5)&
[j = 106.12(2)", Y = 3041.2 A',
= 1.392 gcm-',
Z = 4. p(CnK,) =
36.177 cm-' (graphite monochromator). A total of 3646 reflections were collected using a Philips PW1100/16 automatic diffractometer. Quantitative data
were obtained at - 100 "C using a self-made gas flow device. The resulting data
set was analyzed with the Enraf-Nonius SDPWAX package. The structure was
solved with MULTAN and refined to R ( F ) = 0.032 ( R w , ( F ) = 0.050) using
2410 reflections with I > 30(1). Further details of the crystal structure investigation are available on request from the Director of the Cambridge Crystallographic Data Centre. 12 Union Road, GB-Cambndge CB2 1EZ (UK). on
quoting the full journal citation.
[23] D. Moras. B. Metz, R. Weiss, Actu CrystuNogr. Sect. B 1973, 29, 383.
Synthesis and Characterizationof the First Mixed
Alkali Metal Enolate Containing Amine Ligands:
A Novel "Open-Stack" Structure and Its
Implications for Aldol Addition**
Kenneth W. Henderson, Paul G. Williard,* and
Peter R. Bernstein
During the past two decades there has been an increasing
awareness of the role that aggregation plays in the reactions of
lithium enolates.['] This focus has been created by interest in the
aldol reaction and in particular in the possibility of forming
chiral materials from simple ketones [Eq. (a)] .[*I The synthetic
1. R ~ C H O
+ R2NH
2 . H2O
utility of this carbon -carbon bond-forming reaction has proved
itself in numerous natural product syntheses, leading to an evolution in its methodology. Generally, lithium amides such as
lithium diisopropylamide (LDA) or lithium hexamethyldisi['I
Prof. P. G. Williard. Dr. K . W. Henderson
Department of Chemistry, Brown University
Providence, RI 02912 (USA)
Telefax: Int. code. +(401) 863-2594
Dr. P. R. Bernstein
Zeneca Pharmaceuticals
[**I This work was funded by Zeneca Pharmaceuticals strategic research fund,
grant no. 235. P. G. W. wishes to thank the NIH for grant no. GM-35982.
VCH I'erlugsge.s.sL.//.\~/iuft
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metali, alkali, pseudocryptand, cation, binding, boron
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