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Coelenterands A New Class of Metal-Encapsulating Ligands.

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Experinwntd Procedure
5a.b; 7. To a solution of 1.0mmol sodium aryltellurolate or -selenolaie (freshly
prepared from diarylditellurium and -diselenium. respectively. and sodium borohydride in 20 mL of ethanol) was added triphenyltelluronium chloride (0.39 g,
1 .O mmol) in methanol (20 mL) at -60 C. Immediately a microcrystalline product
precipitated After filtration at -60 -C and washing with ethanol it was dried in
VHCUO. Crystals suitable for X-ray diffraction were obtained by diffusion of the
dissolved starting materials at - 30 C. 5a (triphenyltelluronium4-methylphenyltellurolate): Yield 0.51 g (89%). Elemental analysis of C25H12Te2:calcd C 51.98, H
3.84: found C 57 75. H 4 00. 5b (triphenyltelluronium4-methoxyphenyltellurolate):
Yield 0.47 g (XO0/o) Elemental analysis of C,,H,,OTe,: cakd C 50.58. H 3.74;
found C 50.48. H 3 75 7 (triphenyltelluronium 4-methylphenyl selenolate): Yield
0.35 g (67%) Elemental analysis of C,,H,,SeTe: calcd C 56.76. H 4.19. found C
56.03, H 3.96.
Received: March 21, 1996
Revised version: June 14, 1996 [Z89551E]
German version: Angew. Chmt. 1996. 108. 2822 -2824
Keywords: cations
pounds
. chalcogen
compounds
- tellurium com-
[ l ] F. Seel. A h . Iirorg. Chmr. Rudirichein. 1974. 16, 297.
[2] P. Bottchei-. U Kretschmann, Z.Anorg. AlIg. Cheni. 1985, 523. 145.
. Ed. Engl. 1988.
[3] P. Bdtcher. Ari,qeii~Chern. 1988. IOU. 781 : Angea-. C h ~ n iIn/.
27.759
[4] M. G. Kanatxdis. Angrit Clieni. 1995. 107. 2281 -2283: Angew. Cheni. Int. Ed.
Eilgi. 1995. 34. 2109 -2111.
[ 5 ] A. F Wells. Srrwrurui Inorgnnic Chenrictry. 5th ed., Clarendon Press. Oxford.
Coelenterands: A New Class of
Metal-Encapsulating Ligands
Chris M. Hartshorn and Peter J. Steel*
Heterocyclic tripodal ligands have long been used in coordination and organometallic chemistry,[’] most notably the anionic tripyrazolylborates (scorpionates) ,I231 and their neutral carbon analogues, the tripyrazolylmethane~.[~]
A separate area of
chemistry that has received much attention is the study of
x-arene complexes of transition metals. Benzene forms stable
organometallic complexes with many metals, for example,
[Ru(q6-C6H6)J2 ,Is1 but half-sandwich complexes of ruthenium
(e.g., [Ru(q6-C6H6)LJ2+; L = monodentate ligand) are less
well studied.15. 6] In this paper we bring together for the first time
these two important areas of chemistry and introduce a new
class of ligands, the ~oelenterands.[’~
The structures of these
molecules is such that a coordinated metal is simultaneously
involved in $-bonding to the benzene ring and tripodal coordination to suitably placed heterocycles.
We are currently engaged in the synthesis and study of an
extensive range of compounds (represented by the generalized
structure 1) that comprise a benzene (or other arene) ring to
+
1984.
161 G. Becker. 0 . Mundt in l ~ ’ i r l i o n ~ ~ e n / i Werhs(,/u.irkungeri
oii~~~/~~
in der Chemie
I ~ W / U / / ~ . Y C ~ OEI ’l m w r c , . (Ed. B. Krebs), VCH, Weinheim, 1992, pp. 199-217.
[7] H. 1. Breunig. S . Qiilcc in Dnkonwntionelle Wechsehr-irkungm in dw Chemie
B. Krebs), VCH. Weinheim. 1992, pp. 218--230.
nrcIu//lsl~/t~V
~ / ( ‘ J i i l W / c(Ed.
~,
[8J A. 1. Ashe. A d r . Orgiinorncr. C l i ~ n i1990.
.
30. 77.
W~~t./i,selr~irk~~ngeii
in der Clirmre mrtulli[9] W.-W, du Mont in C~rrh.ori~~mrior~elle
. d 7 l v Elcvnmic,. (Ed.: B. Krebs), VCH. Weinheim, 1992. pp. 231 -244.
[lo] S. Sato. N. Kondo. N. Furukawa, OrgnntiniernNic~ 1994, 13. 3393-3395; S.
Sato, N. Kondo. N. Furukawa. ihid 1995, 14. 5393-5398.
[ l l ] M. Wieher. E. Schmidt, Z.Anorg. Ailg. Chrm. 1988. 556. 189-193.
[12] M. Wieber. S. Rohse. Z.Anorg. Allg. Cltem. 1991. 592. 202-206.
[13] Structure determinations: 5a: C,,,H,,Te,. M , = 1155.25. triclmic. space group
Pi. (I = 12.390(4). h =13.955(4). c = 14.221(4) A. z = 93.28(2), p = 99.96(2),
;.=11425(?) : 1 ’ = 7 1 X 5 . 5 ( 1 1 ) ~ ’ ; % = 2 , p c r i c d = 1 . 7 5 6 M g m ~ ’ : i =
0.71073 pm. 7 = I73 K . the crystal (0.60 x 0.41 x 0.06 mm) was mounted in
inert oil. The intensities were measured to 20 50 using Mo,, radiation on a
Sirinens R 3 diffractometer. Of 7755 reflections. 7383 were unique (R,,, =
0.01XI) and uscd for all calculations (programm SHELXL-93). After absorption correction (psi-scans). the structure was solved by direct methods and
relined anisotropically on F2.The final wR2 war 0.0959 with conventional
R ( F ) 0 0359. 489 parameters. and 468 restraints, max. Ap 1262
5b:
C,,,H,,O,Te,.
.M, = 1187.25. triclinic, space group Pi: u = 12 344(5).
h = 1 3 . 5 0 3 ( 6 ) . ( =15267(6)A.1=93.51(4)./j= 98.99(4).;=115.30(4) : V =
2248.2(16),&. % = 2 ; pLJ,cd
=1.754Mgm-’. .; =0.71073pm, T = 1 7 3 K ;
0.80 x 0.54 x 0.03 mm, 213 range 6-55 . ahsorption correction (SHELXA).
XX1X data. 8x17 unique, structure solved and refined a s above, ir-RZ = 0.1803.
Rl = 0 0542, 507 parameters. and 474 restraints; max. A p 1496enm-’. 7:
C,,,H,,Sc,Te,.
M ,= 3057.97; triclinic. space group Pi: (I =12.108(3).
h=13.96X(3). ( =14.169(3)8\. ~ = 9 3 . 7 5 1 ( 1 0 ) ,p=100.64(1),;. =114.04(1) :
b‘= 3 1 2 4 2 ( 8 ) A ” . Z = 2;p,,,,, =1.654Mgm-3.j.=0.71073pm, T = 1 4 3 K :
0.50 x 0.50 x 0.35 mm. 20 range 6-50 , absorption correction (psi-scans), 7969
data. 7589 unique. STOE Stadi-4 diffractometer; structure solved by using
non-hydrogen ;itom coordinates of 5a. refinement as above, wR2 = 0.0763.
R1 = 0.0267.4X9 parameters and 468 restraints: max. A p 948 enm-’. Because
of similar cell dimensions and atomic positions these crystal structures can be
treated as isostructural. Crystallographic data (excluding structure factors) for
the structures reported in this paper have been deposited with the Cambridge
Crystnllographic Data Centre as supplementary publication no. CCDC-17911 3 . (‘opies o f the data can be obtained free of charge on application to The
Director. CCDC. 17 Union Road. Cambridge CB2 1EZ. UK (fax: int.
code + (1723).336-033, e-mail: teched!o chemcrys.cam.ac.uk)
[14] (12-Te-5)means that 12 valence electrons surround the central atom Te. which
has 5 substitucnts C W Perkins. J. C. Martin. A. J. Arduengo 111. W. Lau, A.
J . 1980. 102. 7753.
Alegriii. J K. Kochi. J. Ani. C ~ P I ?Sot..
[15] I. Haiduc. R. B. King. M. G. Newton, Chein. Rey. 1994, 94. 301-326.
[16] C. .I;iniak. unpublished results.
Anpew. C h m . Inr. Ed. Engl. 1996. 35. No. 22
7‘:
1
4
3
which are appended multiple heterocyclic rings connected
through a variety of spacer groups. These polyheteroaryl-substituted arenes exhibit a variety of modes of coordination to different transition metals.[*] We recently reported[’’ the syntheses of
a number of poly(pyrazo1-I -ylmethyl)benzenes including the
1,3,5-tri(pyrazol-l -ylmethyl)benzenes (2a,b), and speculated
that such compounds might act as tripodal chelating ligands, as
in 3, or even encapsulate a metal with additional coordination
to the benzene ring, as in 4. We now report the successful realization of this second mode of coordination.
Reaction of 2b, readily available in two steps from
mesitylene,[’] with [Ru(dmso),C1,] in ethanol/water at reflux
[*I
Dr. P. J. Steel, C. M. Hartshorn
Department of Chemistry
University of Canterbury, Christchurch (New Zealand)
Fax. Int. code +(3)3642110
e-mail: p.steel(u chem.canterbury.ac.nz
VCH Verlug.~geseNschuftmbH, 0-69451 Weinheim, 1996
o57o-oR33/96/3522-265S3 1 5 . U O i .25:0
2655
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gave a product that was isolated in 42% yield as the hexafluorophosphate salt (Scheme 1). X-ray quality crystals of 5 proved
very elusive; of the several counterions tried, only the tetrachlorozincate anion furnished single crystals.[101
5
Scheme 1. Preparation of 5.
Figure 1 shows a perspective view of the structure of cation
5.[".
This clearly shows that the ligand encapsulates the
ruthenium atom, with joint chelation by the three pyrazole nitrogens and n-coordination by the benzene ring, in this 18-electron species. The cation has approximate C,, symmetry; the
coordination about the ruthenium atom is distorted octahedral.
The benzene ring is measurably distorted towards a chair conformation: the pyrazolylmethyl-substituted carbons are displaced towards the ruthenium atom (mean deviation from the
plane 0.029 A). Whereas the three methyl substituents lie in the
plane of the benzene ring (maximum deviation from the plane
0.069 A), the three attached methylene carbons are severely displaced from this plane (mean deviation 0.470 A), an indication
of significant pyramidalization of the aromatic carbons. By necessity, the the structure has an eclipsed, rather than staggered,
piano-stool-type arrangement.
ruthenium atom from the arene ring; in 5 the ruthenium lies
1.579 8, from the centroid of the benzene ring, whereas in the
previously reported structures the corresponding distances are
significantly longer, lying in the range 1.67-1.70 8,. This indicates a strong metal-arene interaction in 5, which is undoubtedly reinforced by multiple chelate effects. In the published structures[6. 1 3 - 1 7 ] th e N-Ru-N angles are generally acute, whereas
in the case of 5 the internal attachments to the benzene rings
result in these angles being obtuse (92.6-96.7").
According to the similarity of the spectroscopic and electrochemical [18] properties of 5 to those of Ru(@-arene) complexes
with additional nitrogen donors, this structure is maintained in
solution.[6. 14, '', 'I In particular, the large upfield shifts (about
30 ppm) of the arene carbons in the I3C NMR spectrum clearly
establish their coordination to the ruthenium atom.
Having demonstrated the ability of these ligands to encapsulate a ruthenium atom, we are currently studying their complexation of other metal ions. We are also synthesizing other coelenterands with different heterocyclic rings and different spacer
groups. For example, we have prepared the new, less symmetrical ligand 6, with a 2-pyridoxy group, from 2,4,6-trimethylpheno1 by a reaction sequence involving heteroarylation,[*] double
bromomethylation,[201 and phase-transfer-catalyzed alkylationE2'Iwith pyrazole (Scheme 2). This new coelenterand also
reacts with [Ru(dmso),Cl,J to form a complex analogous to that
of 5.
0
0
U
Fig. 1. Perspective view and atom labeling of the X-ray crystal structure of the
cation 5. Selected bond lengths [A] and angles I"]:Rul -N12 2.107(8). Rul -N32
2.091(8), Rul -N52 2.094(8), Rul -C1 2.103(9), Rul -C2 2.186(9), Rul -C3
2.096(9), Rul-C4 2.156(9), Rul-C5 2.083(9), Rul-C6 2.149(9); N12-Rul-N32
96.7(3), N12-Rul-NS2 94.5(3), N32-Rul-NS2 92.6(3).
A search of the Cambridge Crystallographic Data Base for
[Ru($-arene)] structures with three additional nitrogens coordinated to the ruthenium disclosed five related structures, with
B(Pz),,['~] (p~H),pz,"~' (PhNH-),,"sl (MeCN),,[161 and
pyCH,N(Me)CH,CH,py'61 (where pz = pyrazole and py =
pyridine) as additional ligands. The structures of two pcymeneruthenium complexes of a tripyrazolylborate and a
tripyrazolylmethane have also recently been reported." 71 In
none of these literature structures is any of the nitrogen atoms
linked to the benzene ring. The most significant difference between these structures and that of 5 is in the distance of the
2656
0 VCH
Verlagsgesellschaft mbH, 0-69451 Weinherm, 1996
Scheme 2. Preparation and complexation of 6 . a) 2-Bromopyridine/K2C0,/A;
b) CH,O/HBr; c) pyrazole/NaOH/NBu,OH; d) [Ru(dmso),CI,]; e) NH,PF,
In conclusion, we here describe the formation of novel
organometallic compounds derived from a new class of ligands,
the coelenterands, which encapsulate metal ions with simultaneous $-arene coordination and tripodal chelation from within
the same ligand.
Experimental Procedure
5 : An equimolar mixture of 2b and [Ru(dmso),C1,] was heated at reflux in ethanol/
water (3/1) for 12 h. The ethanol was removed under reduced pressure to give the
chloride salt of5. High-resolution MS: calcd for [C,,H,,-N,3SC1'02Ru]t 497.0795,
found 497.0801. This was dissolved in water and filtered. Ammonium hexafluorophosphate was added to the filtrate to give a pale yellow/green precipitate of 5 as
the PF6 salt in 42% overall yield. M.p. ~ 2 5 0 ' C(decomp). UV/Vis (CH,CN):
0570-0833/96/3522-26S6S I5.00+ ,2510
Angen. Chem. Int. Ed Engl. 1996. 35. No. 22
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tor. CCDC. 12 Union Road. Cambridge CB2 1% UK (fax: int. code
.; [nm] (c [ # - ' c m - ' ] ) = 222 (11200). 344 (625). 'HNMR (300MHz, CD,CN):
+(1223) 336-033; e-mail: techedia chemcrys.cam.ac.uk).
6 = 2.19 ( s . YH). 5.48 (s, 6H), 6.70 (t. 3H), 7.69 (d, 3H), 8.12 (d, 3Hj. 13C NMR
(75 MHz. CD,CN): d =13.0 (CH,), 51.6 (CH,). 100.6 (C2,4.6), 109.2 (C1,3,5),
[I21 The structure i s held together by a complex network of hydrogen bonds be109.6 (C4). 135.1 (CS), 140.7 (C3). Alternatively, zinc chloride (2 equiv) and
tween the tetrachlorozincate anion and three water solvate molecules, one of
hydrochloric acid were added to the above filtrate, which, over a period of weeks,
which is disordered over two sites.
furnished yellow crystals of the ZnCI:- salt of 5, which were suitable for single
[13] R. J. Restivo, G. Ferguson, D. J. O'Sullivan. F. J. Lalor, Inor,?. Chem. 1975, 14,
3046 - 3052.
crystal X-ray structure analysis. C,H,N anal. calcd. for C,,H,,N,RuZnCI,-2H,O:
C 35.79. H 4.00. N 11.92; found: C 35.60, H 3.91. N 11.83
I141 D. Carmona, J. Ferrer, L. A. Oro, M. C. Apreda, C. Foces-Foces. F. H. Cano,
J. Elguero, M. L. Jimeno, J Chem. SOC.Dulton Trans. 1990. 1463-1476.
6: a ) 2.4,6-TrimethylphenoI (2 equiv), 2-bromopyridine (1 equiv) and potassium
1151 G. C. Martin, G. J. Palenik, J. M. Boncella, lnorg. Chem. 1990,29,2027-2030.
carbonate (1 equiv) were heated, with stirring, at 220-230'C for 8 hours. The
[16] W. Luginbuhl, P. Zbinden, P. A. Pittet. T. Armbruster. H.-B. Biirgi, A. E.
resulting mixture was extracted repeatedly with diethyl ether, and the extracts comMerbach. A. Ludi, Innrg. Chem. 1991, 30,2350-2355.
bined and washed with aqueous sodium hydroxide Removal of the solvent under
[17] S. Bhambri, D. A Tocher, Pol.yhedron 1996, 15, 2763-70.
reduced pressure yielded 2,4,6-trimethy1(2-pyridoxy)benzenein 62% yield. b) A
[181 In acetonitrile 5 showed only a series of irreversible reductions starting
mixture of 2.4.6-trimethyl(2-pyridoxy)benzene (I equiv), paraformaldehyde
at -0.91 V (vs. SCE) and no observable oxidation process within the solvent
(4 equiv), and 40% hydrobromic acid was heated at reflux in acetic acid for 14 days;
limits. Ru(+benzenej complexes with tridentate nitrogen Iigands behave simadditional hydrobromic acid was added to the reaction mixture every second day.
ilarly [6]. We thank Dr. A. J. Downard for these measuremcnts.
The resulting solution was poured into water, and the solid filtered and recrystal[19] M. Stebler-Rothlisberger. A. Ludi, PoLvhedron 1986. 5. 1217-1221.
lized from petroleum ether to give 3,5-bis(bromomethyl)-2,4.6-trimethy1(2-pyri[20] A. W. van der Made, R. H. van der Made. J Org. Chem. 1993,58. 1262- 1263.
doxy)benzene in 29 % yield. M.p. = 160-161 'C. C.H,N anal. calcd. for
I211 A. J. Downard, G. E. Honey, P. J. Steel. lnorg. Chem 1991. 30. 3733-3737,
C,,H,,Br,NO. C4X.15, H 4.29, N 3.51; found: C48.57, H 4.28. N3.44. 'HNMR
and references therein.
(300 MHz. CDCI,): 6 = 2.18 ( s , 6H), 2.47 (s. 3H), 4.59 (s, 4H). 6.90 (d. 1 H). 6.95
(1, 1 H), 7.69 (t, 1 H). 8 11 (d, 1 H). I3C NMR (75 MHz, CDCI,): 6 =12.79, 14.92,
29.71. 109.96, 117.84. 131 73, 133.52. 134.26. 139 56, 147.75, 148.53, 162.95. c) A
mixture of 3,5-bis(bromomethyl)-2,4.6-trimethyl(2-pyridoxy)benzene(1 equiv)
pyrazole (2 equiv) and tetrabutylammonium hydroxide was heated at reflux in benzeneiaqueous sodium hydroxide for 18 hours. The benzene layer was then separated, dried (sodium sulhte), and concentrated under reduced pressure to give a crude
product. which was rccrysta!lized from petroleum ether to give the pyridoxybenzene
6 as a colorless solid in 58% yield. M.p. =127"C. C,H.N anal. calcd. for
C,,H,,N,O: C 70.76. H 6.21. N 18.75; found: C 70.51. H 6.05, N 18.66 ' H N M R
(300 MHz. CDCI,): d = 2 15 (s, 6 H ) , 2.27 (s. 3H). 5.43 (s, 4H). 6.20 (t, 2H). 6.94
(d,1H),6.95(t,lH).7.09(d,2H),7.53(d,2H).7.70(t.lH),8.10(d,1H).'3C
NMR (75 MHz, CDCI,): 6 =13.24, 15.58, 50.42, 105.33, 110.07, 117.86. 127.85,
Christophe Canevet, Jacqueline Libman, and
131.36, 132.52. 135.55, 139.22. 139.53, 147.67. 148.88, 162.88.
Molecular Redox-Switches by Ligand
Exchange**
Abraham Shanzer*
Received: April 19, 1996 [29053IE]
German version: Angew. Chem. 1996. 108. 2818-2820
Keywords: arenes . chelate ligands
- ruthenium compounds
[l] A. Shaver in Comprdiensive Coordinurion Chemistry. Vol. 2 (Eds.: G. Wilkinson, R. D. Gillard, J A McCleverty). Pergamon, Oxford, 1987, p. 245.
121 S. Trofimenko, Prog. lnorg. Clrem. 1986, 34. 115-210; Chem. Rev. 1993, 93,
943-980: G Parkin. A h . Inorg. Chem. 1995.42.291 -393; N. Kitajima, W. B.
Tolman. Prog. Inorg Chem. 1995, 43. 419-531.
[3] S. Trofimenko. .I. An?. Chem. Sor. 1967,#9, 3170-3177.6288-6294.
[4] P. K. Byers. A. J. Canty, R. T. Honeyman, Adv. Orgunomef. Chem. 1992, 34,
1 -65; T. Astley. J. M. Gulbis. M. A. Hitchman. E. R. T. Tiekink, J Chem. Sor.
Dalton 7run.s. 1995. 509-515: D. L. Reger, J. E. Collins, R. Layland. R. D.
Adams. lnorg C h m . 1996, 35. 1372- 1376, and references therein.
[5] M. A. Bennett, M. 1. Bruce. T. W. Matheson in Comprehensive Orgunometatlir
Chemi.>-/rr.&I. 4 (Eds.: G. Wilkinson. F. G. A. Stone, E. W. Abel). Pergamon,
Oxford, 1982. p. 691: M. A. Bennett in Comprehensive OrganomefuNit Chemrslry 11, Vol. 7 (Eds.: E. W. Abel. F. G. A. Stone, G. Wilkinson), Pergamon,
Oxford. 199.5. p. 549.
[6] 2. Shirin, R. Mukherjee, J. F. Richardson, R. M Buchanan, J Chem. SOC.
Dulton 7ians. 1994, 465 -469. and references therein.
[7] We propose the name coelenterand for this type of ligand and coelenterate for
its metal complex. after the animal phylum Ccelenterata. from the Greek for
"hollow stomach" We thank Dr. J. A. Gerrard for suggesting this name.
[Sl C. M. Hartshorn. P. J. Steel, Inorg. Chem., in press.
[Y] C. M . Hartshorn. P. J. Steel, Aust. J. Chem. 1995. 48, 1587-1599.
[lo] We thank Prof. D. A. House for this useful suggestion.
I l l ] Crystal structure analysis of C2,H,,N,Ru-ZnCI;3H20:
M , =722.75, triclinic. space group Pi, u =10.330(2), h =11.557(3), c =12.835(3),&,
a =I14 86(2). a = 102.61(2). ;' = 93.59(2)", V =1336.0(5) A', F(000) =728,
p,,,,,(Z = 2) = 1.797 gcm-'. p = 1.90 mm-', approximate crystal dimensions
0.40 x 0.29 x 0.06 mm. 2U,,, = 5 0 , radiation Mo,, (1. = 0.71073 A), w scans,
T = - 85 C. 4303 measured data, 4078 unique data. Lp corrections, no absorption correction, Patterson/Fourier (SHELXS). full-matrix least-squares
refinement on F 2 with all data (SHELXL93). 338 parameters, H atoms in
calculated positions, conformational orientation of methyl hydrogens deduced
from circular Fourier syntheses, residual electron density < 1.6 e k ' .
GOF = 0.92. wR(al1 data) = 0.156. conventional R value 12571 data with
I > 2o(l)] = 0.061. Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-179-115.
Copies of the data can be obtained free ofcharge on application to the DirecAnpi%
Clreiit IU. Ed. Gtgl. 1996. 35. No. 22
The increasing interest in molecule-based technologies[" is
stimulating extensive research on the synthesis of task-oriented
molecules such as molecular wires,123'I diodes,14. 51 light converters,I6I and switches." - 141 Recently, several molecular switches
have been introduced that respond to external triggers such as
photons, protons, and electrons. Examples include photochromic materials that undergo reversible, light-induced cyclizations,[' 'I rotaxanes in which macrocyclic structures shuttle
between two stations on a rodlike molecule,['2] catenates in
which redox processes of a guest ion promote swinging motions
of two rings relative to each other," and triple-stranded helices
that undergo electron-induced translocation of an iron atom
between neighboring
Here we introduce a novel redox-switch that interconverts between two distinct states by
ligand exchange. At the heart of this switch is a molecule that
possesses two sets of ion-binding groups: one set of "hard" and
one set of "soft" ligating groups. The two sets are anchored o n
a calix[ll]arene ring["I in a n alternating fashion, such that they
can form either a "hard" o r a "soft" ion-binding cavity; one
cavity is formed at the exclusion of the other. Calix[4]arene was
selected as the anchor since it directs both sets of ion-binding
groups to the same face of the ring and allows either set to
converge by means of a rocking motion. When loaded with Fe"',
the "hard" binding groups, hydroxamates, converge to embrace
the "hard" metal ion, while the soft groups diverge. Upon reduction, the ligand rearranges to engulf Fe" with its "soft"
[*] Prof. A. Shanzer, C. Canevet, Dr. J. Libman
Weizmann Institute of Science
Department of Organic Chemistry
Rehovot 76100 (Israel)
Fax: Int. code f8934-4142
e-mail: coshanzr@weizmann.weimann.ac 11
[**I This work was financially supported by the Israel Academy of Sciences and
Humanities and by the Minerva Foundation. We thank Dr. S . J. Harris, Dublin
University, Ireland. for sharing his knowledge of calix[/l]arene chemistry. The
French- Israeli I'Arc-en-ciel Program is acknowledged.
VCH Verlagsgesellsrhafi mbH, 0-69451 Weinheim. 1996
0570-083319613522-2657$ i5.00+ .25/0
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