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quenched with a solution o f citric acid. The mixture was diluted with DCM
(100 mL) and washed with sodium bicarbonate (sat.), water, and brine (25 mL
each). The organic layer was dried (N a2S 0 4), evaporated, and the residue was
dissolved in D C M /M eO H (98/2) and filtered through a plug o f silica gel. Evapora­
tion gave the crude precursor o f 2 (773 mg). To this crude product (746 mg) in dry
T HF (40 mL) was added methanesulfonic acid (3U0 |.iL). After stirring the mixture
at RT for 72 h under nitrogen, pyridine (400 pL), DIPEA (1.5 m L), and acetic
anhydride were added at 0"C. After 2 h the solution was diluted with DCM
(100 mL), washed with sodium bisulfate (lM ). water, sodium bicarbonate (sat.),
water, and brine (2 5 mL each). The organic layer was dried (N a2S 0 4), filtered,
evaporated, and chromatographed (silica gel, DCM /M eOH (98/2)) to afford 2
(360 mg, 40 % based on X).
3: To 2 (60 mg, 0.051 mmol) in DCM (1 mL), TFA (0.5 mL) was added dropwise
and stirred For 1 h at 0 °C .T h e reaction mixture was diluted with DCM (50 mL) and
washed three times with sodium bicarbonate (sat.) then water and brine (10 mL
each). The organic layer was dried (N a2S 0 4), evaporated, and redissolved in DCM
(0.5 m L ). Boc-(2S,3/?,47?,6.£')-3-liydoxy-4-methyl-2-(methylamino)-6-octenoic acid
(Boc-MeBmt)[21] (18.7 mg, 0.062 m m ol), bromotripyrrolidinophosphonium hexafluorophosphate (PyBroP) [22] (27 mg, 0.060 mmol) and DIPEA (27 pL) were
added at 0 °C under nitrogen. After 2 h at 0 ‘C the reaction was allowed to warm to
RT, diluted with DCM (50 mL), washed with sodium bisulfate (1 m ) , water, sodium
bicarbonate (sat.), water, and brine (20 mL each). The organic layer was dried
(N a2S 0 4), filtered, evaporated and chromatographed (flash silica gel, EtOAc/acetone (9/1)) to give 3 (56 mg, 81 %).
4: To 3 (2 7 mg, 0.022 mmol) in D C M (1 mL), TFA (0 .5 mL) was added dropwise
and stirred for 1 h at 0 °C, diluted with DCM (50 mL) and washed three limes with
with sodium bicarbonate (sat.), water and brine (10 mL each). The organic layer
was dried (N a2S 0 4), filtered and evaporated. To the residue in DCM (30 jiL),
Fm oc-N -M elle (12 mg, 0.032 inm ol), PyBroP (14 mg, 0.032 mmol) and DIPEA
(15 |iL) were added and the mixture was stirred for 6 h at 0°C and 30 min at RT.
Fm oc-N-M elle (5 mg, 0.013 m m ol), PyBroP (5 mg, 0.011 mmol) were added again
at 0 °C and allowed to warm to RT overnight. The mixture was diluted with DCM
(50 mL), washed with sodium bisulfate (1 m), water, sodium bicarbonate (sat.),
water, brine (20 mL each). The organic layer was dried (N a2S 0 4), filtered, evapo­
rated, and chromatographed (silica gel, EtOAc/acetone 9:1) to afford 4 (19 mg,
60% ).
[16] R. E. Ireland, W. J. Thompson, Tetrahedron L ett. 1979, 4705.
[17] L, A. Carpino, F. A. El, J. Org. Chem. 1994, 59, 695,
[18] R. E, H&ndschutnacher, M . W. Harding, J. Rice, R. X Drugge, D. W. Speicher,
Science 1984, 226, 544.
[19] H. Husi, M . Zurini, Anal, Biochem. 1994, 222, 251.
[20] L, D . Zydowsky, F. A . Etzkorn, H. Y. Chang, S. B. Ferguson, L. A. Stolz, S. I.
H o, C .T. Walsh, Protein Sci. 1992, 1 , 1092.
[21] W. D. Lubeil, T. F. Jamison, H. Rapoport, J. Org. Chem. 1990, 55, 3511.
[22] J. Coste, E. Frerot, P. Jouin, X Org. Chem. 1994, 59, 2437.
[23] A, Nicholls, K. A. Sharp, B. Honig, Prot. Struct. Funct. Genet. 1991,1L 283.
Strong Binding of Paraquat and Polymeric
Paraquat Derivatives by Basket-Shaped Hosts**
Albertus P. H. J. Schenning, Bas de Bruin,
Alan E. R owan,* H uub Kooijm an, Anthony L. Spek,
and Roeland J. M . N o lte
C lip -sh ap ed h o st m o lecu les o f typ e 1 can bind uncharged
arom atic g u est m o le c u le s, fo r exam p le resorcin ol, b y n - n stack­
in g an d h y d ro g en b o n d in g in te ra c tio n s.[1] B asket-shaped
derivates o f 1 c o n ta in in g crow n ether m oieties (com p ou n d s of
ty p e 2) are, in a d d itio n , ab le to b in d alk ali m etal io n s and proto n a ted a m in e s.121 W e rep ort here o n th e bin ding affinities o f
these h o s t m o le c u le s to w a rd s ch arged arom atic com poun ds,
su ch as p a ra q u a t 3 a n d th e p o ly m eric p araq u at derivatives 4 and
5: To 4 (4.6 mg, 0.0028 mm ol) in THF/water (10:1, 200 pL), D B U (3 pL) and
lithium bromide (2 mg) were added. After stirring overnight at RT, D B U (4 pL) and
lithium bromide (3 mg) were added again. After 5 h, the reaction was quenched with
acetic acid (20 pL) and purified by reverse-phase HPLC (Beckman ODS ultrasphere
5 n 10 mm x 25 cm, 0.1 % TFA /M eCN 70/30 ->10/90 in 30 min, 70°C, 3 runs) to
yield the peptide precursor o f 5 (2.1 mg, 60% ), A solution o f this peptide precursor
(1.2 mg, 970 nmol), AOP[17] ( 4 mg, 0.009 mmol) and 2,6-lutidine (4 pL) in DCM
(1.2 mL) was stirred for 48 h at RT. The reaction mixture was quenched with acetic
acid (30 pL), the DCM evaporated, and the residue dissolved in acetonitrile and
purified by reverse-phase HPLC (Beckman ODS ultrasphere 5 p 10 mm x 25 cm,
0.1 % TFA/M eCN 70/30 -»10/90 in 30 min. 70 °C) to afford the pure cyclic peptide
5 (0.65 mg, 55% ),
Received: June 9, 1995 [Z8080IE]
German version: Angew, Chem. 1995, 107, 2313-2317
3
K eyw ords: cy clo p h ilin - c y clo sp o rin - im nrunop hilin s • p r o te in
dim erization
p f
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
D , J. Austin, G. R. Crabtree, S. L. Schreiber, Chem,
B iol 1994, J%131.
M. A. Lemmon, J. Schlessinger, Trench Biochem. Sei, 1994, 19, 459.
A . C. Chan, D . M. Desai, A. Weiss, Awm. Rev. Immunol, 1994, 12, 555.
W, H, Landschulz, P, E Johnson, S. L. McKnight, Science 1988, 240, 1759.
D . M. Spencer, T. J. Wandless, S, L. Schreiber, G. R. Crabtree, Science 1993,
262, 1019.
M. N . Pruschy, D . M. Spencer, T. M. Kapoor, H. Miyaki, G. R. Crabtree, S.
S. L. Schreiber, Chem. & Biol. 1994, 1, .163,
H. Ke, D. Mayrose, P. J, Beishaw, D . G. Alberg, S. L. Schreiber, Z. Y. Chang,
F. A. Etzkorn, S. Ho, C T. Walsh, Structure (London) 1994, 2, 33.
G. Pflugl, J. Kailen, X Schirmer, J. N . Jansonius, M, G. Zurini, M. D . Walkinshaw, Nature 1993, 361, 91.
V. F. Quesniaux, M. H, Schreier, R. M . Wenger, P. C, Hiestand, M, W Hard­
ing, M. H. V. Van Regcnmortel, Ear. J. Immunol. 1.987,17, 1359.
T. Clackson, J. A. Wells, Science 1995, 267, 383.
J. S. Richardson, D . C. Richardson in Prediction o f protein structure and the
principles o f protein conformation, Vol. X III; (Ed. G. D . Fasman), Plenum,
New York, 1989,
G. Sarkar, S. S. Sommer, BioTechmques 1990, Ä, 404,
P. X Beishaw, S. L. Schreiber, Chem. & Biol., submitted,
R. Oliyai, V. J. Stella, Pharm, Res, 1992, 9, 617.
A . Ruegger, M . Kuhn, H. Lichti, H. R. Loosli, R. Huguenin, C. Quiquerez,
W A. Von, Hetv. Chim. Acta. 1976, 59, 1075.
2132
©
VCH Verhgsgesellschafl mbH, D-69451 Weinheim, 1995
■~ 0
6"
-
p f
0
h
c
sh
, 2 - 0 C ( 0 ) - ( C H 2 ) 6- ( 0 ) C 0 - ( C H 2)2 —
n
5
5. T here is currently a g r ea t d eal o f in terest in paraqu at-bind in g,
w h ic h h as resu lted in th e d e sig n a n d co n stru ctio n o f new m o lec ­
ular stru ctu res, as e x em p lified by the e leg a n t w ork by Stoddart
et al, o n c a te n a n d s a n d r o ta x a n e s.[31 W e describe here th at com [*] Dr. A. E. Rowan, Dipl.-Chem. A. P. H. J. Schenning, Dipl.-Chem. B. de Bruin,
Prof. Dr, R. J. M. N olte
Department o f Organic Chemistry, N SR Center
University of Nijmegen
Toernooiveld, NL-6525 ED Nijmegen (The Netherlands)
Telefax: Int. code + (8 0 ) 553450
Dr, H, Kooijman, Dr. A . L. Spek
Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry
Utrecht University (The Netherlands)
[**] This work was supported by the Netherlands Foundation for Chemical Re­
search (SON) with financial aid from the Netherlands Organization for Scien­
tific Research (N W O ).
0570-083319513419-2132 $ J 0 .0 0 + .2 5 /0
Ange ir. Chem. Int. Ed. Engl. 1995, 34, No. 19
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p o u n d 2 is a n exceptionally g oo d host for paraquat. T his opens
the possibility o f clipping host m olecules o f type 2 to polym eric
chains c o n ta in in g p a r a q u a t units. T h e X - r a y s tr u c tu r e and
propert ies o f the c o m p le x between 2 and p a r a q u a t are also p r e ­
sented.
C om pounds 1 a n d 2 were synthesized as described p re v io u s­
ly.1'1,21 F o r the synthesis o f 2 b , 4,4'-diniethylbenzil was used as
the sta rtin g m aterial. C o m p o u n d s 3 a n d 4 w ere p re p a re d by
using literature p ro c ed u re s.1'1,41 C o m p o u n d 5 was p re p a re d by
the c o n d e n s a tio n o f /V1./V '-bis(2-hydroxyethyl)-4,4'-bipyridiniurn hexafluorophosphate in acetonitrile w i t h o n e eq u iv a len t o f
adipoyl c hlo ride with trie thyla m ine as the b a se ,151 a n d purified
by precipitation from ethyl acetate.
A d d itio n o f 3-CI2 to a solution o f h ost 2 b in for exam ple
m eth anol/ch loroform led to an im m e d ia te c o lo r c h an g e fr om
colorless to yellow-orange, indicative o f the f o r m a t i o n o f a
c h a rg e -tra n sfe r c o m p le x .1-'1 i H N M R s p e c tro s c o p ic studies re­
vealed th a t in this c om ple x the p a r a q u a t gue st is located in the
c left of the cavity.161 F a s t a to m b o m b a r d m e n t m ass spectrom e­
try (FA B M S ) sh o w ed a peak at m jz 1094 (m a trix : nitrobenzyl
alc o h o l), which c o r re s p o n d s to the mass o f the h o s t - g u e s t c o m ­
plex. Crystals o f the com plex between 2 b a n d 3-(P F 6) 2 suitable
for X -ray analysis were grown from a m ixture o f 2 b and tetrab u ty la r n m o n iu m h e x aflu o r o p h o sp h a te dissolve d in c h lo ro f o r m ,
w h ich was layered with 3 - C l2 dissolved in m e t h a n o l. T he X - ra y
stru c tu r e o f com plex is sho w n in Figure 1 ( t o p ) . 171 It reveals a
is n o t tilted. T h e b in ding c o n s t a n t o f th e b is ( p a ra p h e n y le n e )[ 3 4 ]cro w n -t0 m acrocycle w ith 3 -(P F 6) 2 in a c e to n e is 730 m “ 1,
T h e b in d in g p ro p e rties o f 1 a n d 2 were e v a l u a t e d b y r e c o r d in g
the intensity o f the c h a r g e - tr a n s f e r a b s o r p t i o n b a n d a t a p p r o x i ­
m ate ly 425 ntn at differe n t h o s t - g u e s t ra tio s. T h e results for
differe nt so lv e n ts a r e presented in T a b le :t. T h e basket-shap ed
host m olecules 2 bind p a r a q u a t a p p r o x i m a t e l y 2 5 - 7 5 tim es
st r o n g e r th a n the b is (p a ra p h e n y le n e )-[ 3 4 ] c ro w n - 1 0 m a c r o ­
cycle.131 T h is result c an be exp lain ed from the fact that: c o m ­
p o u n d s 2 are m o r e p r e o r g a n i z e d for b in d in g than S t o d d a r t ’s
c o m p o u n d .[8J O n ly a very sm a ll K a o f 80 m-1 w a s fo u n d fo r the
b in d in g o f 3 in the c avity o f clip m o le c u le 1. T h i s clearly d e m o n ­
s tra te s the i m p o r t a n c e o f the c r o w n e th e r m o ie tie s in 2 fo r the
c o m p l e x a t io n o f p a r a q u a t .
T a b l e I . Bi n d in g c o n s t a n t s [a] o f th e c o m p l e x e s b e tw e e n h o s t s I a n d 2 a n d g u e s ts 3,
4, a n d 5.
Host
Guest
/ U m - ']
i
3-CI2
2a
3 - tP F ,. ) ,
5 7 0 0 0 [c]
2 it
3-< PF„ ),
3 -Cl,
2 0 0 0 0 [ci]
21)
2a
2a
2a
80 [ b I
2 2 0 0 0 [b]
4 a If]
t 80 0 [c, e]
‘I b f g l
5 [li|
1 9 0 0 0 |c, e|
4 500 [c, e|
fa] A s s o c i a t i o n c o n s t a n t s w e r e c a l c u l a t e d us in g t h e B e n e s i - H i l d e b r a n d e q u a t i o n
[13]. G o o d c o r r e l a t i o n s ( R > 0. 99 5 ) we re o b t a i n e d f o r all t i t r a t i o n c u rv e s a s s u m i n g
a 1:1 h o s t " g u e s t c o m p l e ju i ti o n . E s t i m a t e d e r r o r in A.',, is 1 0 % . F o r th e A', m e a s u r e d
in a c e t o n e , t h e e r r o r is .50% d u e to t h e p o o r s o lu b i li t y o f t h e h o s t m o le c u le in this
s o lv e n t , [b] M e t h u r i o l - e h l o m f o r m ( 1 : 1 , v/V), [cj A c e t o n it r i l e . [d] A c e t o n e . [e| Pe r
p o l y m e r r e p e a t uni t, [f] in tr in s i c vi sc o s i ty o f p o l y m e r [f/J = 0 . 0 0 6 d L g
1 ( a c c t o n i-
(rile, 25 " C ) . [g| Inirin.sic visc os ity o f p o l y m e r [r/| = 0.2 93 d L g “ 1 (a c e t o n it ri l e ,
2 5 ° C ) , [li| A v e r a g e d e g re e o f p o l y m e r i z a t i o n
F i g . Î. T o p : Cr y s ta l .structure o f the c o m p le x b e tw e e n 2 b a n d 3-{P F6) 2. L e ft t o p
v i e w , right side view [14]. B o t t o m : C o m p u t e r - g e n e r a t e d m o d e l o f t h e c o m p l e x
b e t w e e n 2 b a n d 5.
p e r fe c t c o m p le m e n ta r ity between host a n d guest. T h e p a r a q u a t
g u e s t sits sy m m etrically within the walls o f the b a s k e t; the
m e t h y l su b s titu e n ts are partially e n c a p su la te d by the c ro w n
e t h e r rings. T h e crystal stru c ture is different f ro m th a t o f the
c o m p l e x o f p a r a q u a t with “ bis(p a raph e nyle ne )-[3 4]c row n-1 0”
( t h e sam e m acrocycle as c o m p o u n d 2 a, b u t w it h o u t the
d i p h e n y l g l u c o l u r il unit), as published by S t o d d a r t et a l.1J| In the
c a s e o f the latter the bipy rid inium guest is flat, w h ereas in the
c r y s t a l structure described herein the two bipyridinium units are
t w i s t e d by an angle o f 22.5(3)°. This is a c o n s e q u e n c e o f being
le s s sterically constrained by the a ro m a tic side w alls o f the b a s ­
k e t . In S t o d d a r t ’s c o m p o u n d the b ip y rid in iu m is tilted a t an
a n g l e o f a b o u t 28° with respect to the 0 - 0 axis o f the p a r a p h en y len e unit; in ou r case the bipy rid inium gu est in the basket
Angew. Chem. Int. Ed. Engl. 1995, 34, No. 19
©
4 ( N M R , e n d g r o u p a n a ly s i s) .
T h e ele ctroc hem ica l b e h a v i o r o f the c o m p l e x be tw ee n 3(P F 6) 2 a n d c o m p o u n d 2 a w a s stu d ie d in ace tonitrile . In this
so lv e n t the guest s h o w e d tw o reversible o n e - e le c tr o n tra n s fe rs:
£ l/2(2 + /1 + ) = - 0.423 V, £ ’1/2( 1 + / 0 ) = - 0.840 V (vs. S C E ,
fo r b o t h t r a n s it i o n s A E = 60 m V ). U p o n the a d d it i o n o f o n e
e q u iv a l e n t o f 2 a the first r e d o x t r a n s f e r shifted 100 m V to m o r e
negative p o t e n t i a l , w h e re a s th e s e c o n d e le c tr o n tr a n s fe r p o t e n ­
tial r e m a in e d u n a f fe c te d . T h ese d a t a ind ic a te t h a t 2 a binds a n d
stabilizes the d o u b ly charged p a r a q u a t species, w hich results in
a m o r e ne g ative re d o x p o t e n t i a l for the first re d o x transfer. It is
well k n o w n t h a t in this type o f h o s t - g u e s t sy s te m the gue st
r e d u c e d by o n e e le ctro n d isso c iate s f o rm the h o st, a n d this e x ­
p lains the u n a lte re d s e c o n d re d o x p oten tial.[J|
P o ly m e r ic p a r a q u a t d e r iv a tiv e s ha v e p re v io u s ly been investi­
g a te d as r e d o x -a c tiv e films and m o r e recently as optical d a t a
s t o ra g e m a te ria ls a n d sh o w a wide v a rie ty o f ele ctroc hro rnic
a n d f h e r m o c h r o m i c b eh avior.i4,9l l0J T h e polym eric p a r a q u a t
d e riv a tive w ith losy late c o u n t e r i o n s has been s h o w n to possess
liq uid c ry stallin e properties.111,121 W e felt that h o s t - g u e s t c o m p le x a tio n m ig h t be a n i n te r e s tin g way to m o d if y a n d c o n tr o l the
physical p r o p e r t i e s o f this in te re s tin g class o f poly m ers. T h e
results o f bin d in g studies carried o u t in a c e to n itr ile , sh o w e d that
2 a can be c lip p e d to p o ly m e r ic p a r a q u a t derivatives 4 a, 4 b , and
5 w ith a s s o c i a t i o n c o n s t a n t s o f 1800 m " 1, 4500 m - 1 , a n d
1 9 00 0 M~
respectively (p e r p o l y m e r r e p e a t u n i t, see T a b le 1,
Fig. 1). T h e b in d in g c o n s t a n t s o f 4 a a n d 4 b a r e low er th an tho se
o b s e rv e d for p a r a q u a t itself. M o l e c u l a r m o d e l i n g studies have
revealed t h a t in the case o f 4 a a n d 4 b , c o m p lex a tio n o f a b a sk e t
to a p a r a q u a t unit in the p o l y m e r is sterically h in dere d by b a s ­
kets c o m p l e x e d to a d ja c e n t p a r a q u a t units. T h e overall low er
b in d in g c o n s t a n t s reflect this ste ric h i n d r a n c e , w h i c h is less for
VCH Verlagsgesellschaft mhf-I, D-69451 WeMwtm, 1995
0570- 0833¡ 95¡ 34 l 9- 2 l 33 $ 10.00+ .2510
2133
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the p olym er w ith a lon ger sp acer (4 b ) than fo r the on e w ith the
sm aller sp acer ( 4 a ). V isc o sity m easu rem en ts sh ow ed th a t the
sp ecific v isc o sity o f the p o ly m e rs so lu b ilized in a ceton itrile in­
creases u p o n the a d d ition o f 0.03 eq u ivalen ts o f the h ost m o le ­
cule 2 a p er rep eat unit: fo r exam p le, 4 a : tjsp — 0.006, 4 a + 2 a
'iSp = 0 .0 0 8 ; 4 b : ))sp = 0 .0 6 8 , 4 b + 2 a )?sp = 0.070. T h ese results
indicate th a t the average m o lecu la r m ass o f the polym ers
increases o n co m p lex a tio n w ith 2 a . T he red ox p o te n tia ls o f
the p o ly m ers w ere also in flu en ced b y the h o sts. P olym er 4 a
disp layed th e fo llo w in g redox tran sition s in aceton itrile:
E U2( 2 + / i + ) = - 0.387 V, E m ( 1 + / 0 ) = - 0.8 4 0 V (vs. SC E,
for b oth tran sition s AE p = 60 m V ) . U p o n th e ad d ition o f one
eq u ivalen t o f 2 a per rep eat un it the first red ox transfer sh ifted
20 m V to m ore negative p o te n tia l, w hereas the second redox
transfer rem ained u n ch an ged . A d d itio n o f three eq u ivalen ts o f
2 a resulted in a 35 m V sh ift in th e n eg a tiv e direction . T h ese shift
values are sm aller than the 100 m V sh ift m easured for paraquat
(3) which m a y be the result o f the low er bin ding affinity o f 2 a
for the p olym eric p araq u at d erivatives. T h ese prelim inary re­
sults in d icate that the electroch em ical b eh avior o f polym eric
paraqu at d erivatives can ea sily be tu n ed by the ad d ition o f bas­
ket m o lec u les. T his m ay b e o f interest for futu re a p p lica tio n o f
these p o ly m ers as op tical d ata sto ra g e system s or as m olecular
sw itches, etc.
In c o n c lu sio n , w e have sh o w n th a t h o s t m olecu les o f the type
2 strongly bind paraqu at 3 an d th at it is p o ssib le to clip these
m olecu les to p olym eric p a ra q u a t derivatives. Further studies
are underw ay.
Received: March 14, 1995 [Z 7792 IE]
German version: Angew. Chem. 1995, 107, 2288-2289
Keywords: c ro w n ethers • p a ra q u a t • su p ram olecu lar chem istry
[1] R. P. Sijbesma, A. P. M, Kentgens, E. T. G, Lutz, J. H. van der Maas, R. J. M.
Nolte, J Am. Chem. Soc. 1993, 115, 8999.
[2] J. W. H. Smeets, R. P. Sijbesma, L. van Dalen, A. L. Spek, W. J. J. Smeels,
R. J. M. Nolte, J. Org. Chem. 1989, 54, 3710.
[3] a) B. L. Allwood, N. Spencer, H. Shahriari-Zavareh, J. F. Stoddart, D. J.
Williams, J. Chem. Soc. Chem. Commun. 1987, 1064; b) P. L. Auelii, P. R.
Asluon, R. Ballnrdini, V. Balzani, M. Delgado, M. T. Gandolfi, T. T. Goodnow, A. E. Kaifer, D . Philp, M. Pietraszkiewiez, L. Prodi, M. V. Reddingkm,
A. M. Z. Slawin, N . Spencer, J. F. Stoddart, C. Vicent, D . J. Williams, J. Am.
Chem. Soc. 1992, 114, 193.
[4] T. Ohsaka, M. Nakanishi, O. Hatozaki, N . Oyama, Elccirochim. Acm 1990,35,
63.
[5] P. R. Ashton, D . Philip, M. V. Reddington, A. M. Z. Slavvin, N. Spencer, J. F.
Stoddart, D. J. Williams, J. Chem. Soc. Chem. Commun. 1991,1680.
[6] The following 'H N M R signals o f 2a are shifted by complexation with 3(PF„)2 ( 1:1 ) : (200 MHz, CDjCN/CDCl., (7/3, v /v), 25 °C) : A<5 = — 0.81 (s, 4 H,
Ar-H), + 0 .07 (AB quartet, 4 H , C H 2-N ), - 0 .8 7 (m, 8H . C H ,0-A r); likewise
for 3-(PF6)2 by complexation with 2a: AcS = - 1 .0 8 (d, 4H , Ar-H), +0.12 (d,
4H , Ar-H), + 0 .2 2 (s, 6H , CH.,).
[7] Crystal data for [(2b) • (3) • 2PFr>• 2CHC1J: CMH ,4N 60 ,,P 2FI2CI6,
M, = 1622,0, monoclinic, space group C2/e, ¡1 = 32.968(5), l> = 13.6135(14),
c = 17.875(2) Â , (i =118.375(11)°,
K - 7058.6(17) Â 3, Z = 4,
«
1.5263(4)g e m M o Ka radiation (graphite monochromator), fi = 3.9 cm “ \
7352 intensity data collected at 150 K, 6778 unique, Lp correction, no absorp­
tion correction. The structure was solved by direct methods (SHELXS-86) and
refined on F 2 (SHELXL-93), no observance criterion applied during refine­
ment. Final \vR2 = 0,226. R i = 0,099 (for 3 1 6 2 1 > 2<r(/)), S -1 .0 0 . Residual
electron density was in the range -0 .3 4 ,0 .5 0 (near PF„). Further details of the
crystal structure investigation may be obtained from the Director o f the Cam­
bridge Cristallographie Data Centre, 12 Union Road, GB-Cambridge
CB2 1EZ (U K ), on quoting the full journal citation.
[8] E. P. Kyba, R. C. Helgeson, K, Madan, G, W, Gokel, T. L, Tarnowski, S. S.
Moore, D. J. Cram, J. Am. Cham. Soc. 1977, 99, 2564.
[9] T. Nagunuira, Y. Isoda, J, Chem. Soc. Chem. Commun. 1991, 72.
[10] L. P. Yu, E. T. Samulski in Oriented Fluids and Liquid Crystals, Vol. 4 (Eds.;
A. C. Griffin, J. K Johnson), Plenum, New York, 1984, p. 697.
[11] P. K, Bhowmik, W, Xu, H, Han, /. Polym. Set, Part A: Palym. Chem. 1994,32,
3205.
[12] P. K. Bhowmik, H. Han, J Polvm. Sci. Pari A : Pofym. Chem. 1995, 33,1745.
[13] J. A. Hildebrand, H. A. Benesi,' J. Am, Chem. Soc. 1949, 7 t , 2703.
2134
©
VCH Verlagsgesellschaft mbH, D-694S1 Weinhehn, 1995
Formation of a “meso-Helicate”
by Self-Assembly of Three Bis(catecholate)
Ligands and Two Titanium(iv) Ions
Markus Albrecht* and Sirpa Kotila
D ou b le- or triple-helical oligonuclear coord in ation com ­
p ou n d s th at are form ed sp ontan eou sly by self-assem bly o f two
or three oligod en tate ligan ds and several m etal ion s are called
heiicates. D u e to their helicity, such m olecules are chiral.m On
the other hand, m<?io-helicates are analogou s m etal complexes
w ith tw o differently con figu rated helical units and are thus achiral. In a to p o lo g ica l sense, th e w&vo-helicates are helices that
inverse their helical tw ist.121
A n octahedral com p lex form ed by one m etal center and three
bidentate ligands is the m ost sim ple helical unit. It can have two
configurations (A or A ) . m Triple bridging betw een the ligands
o f tw o such octahedrons results in the form ation o f three differ­
ent coord in ation c o m p ou n d s: the enantiom eric A, A- and A,Ahelicates, and the diasterom eric ¿1,/1-form — the m ost simple
triple-stranded mes o- helicate (Fig. 1).
The
coord in ation
chem istry
of
linear
oligod en tate
nitrogen
d on or ligands w ith soft
m etal ions has recently
becom e the focus o f a
great deal o f attention.
h elicate
m eso-helicate
To our kn ow led ge, in ­
(A,A-form )
(A,A-form)
vestigations tow ards the
m etal-directed self-as­
b>
sem bly o f heiicates have
resulted only in the
characterization o f d o u ­
ble- 11,41 and triple-heli­
cal [5!
and
d o u b le­
stranded
non-helical
m etal com plexes, w h ich
do n ot possess units
i
i = bidentate ligand
w ith helical chirality.161
0 / ^
= A/A-configurated
In this con text, the
octahedral metal center
triple-stranded
mesoFig. 1. Schematic representation of a) a heli­
helicate is a m issing
cate (here: A,/1-form) and the corresponding
structural m otif.
mejo-helicate (4,/(-form) and b) a binuclear
C,-bridged helicate and wejo-helicate (only
This stim ulated our
one o f the ligand strands is shown).
interest in self-assem bly
processes based on o x y ­
gen don or ligands in com b in ation with hard m etal ion s.17,81 The
use o f early transition m etals sh ould provide access to new
supram olecular aggregates, w h ose properties should differ from
those o f the “ tradition al” com poun ds.
C atechol ligands seem to be ideal bidentate chelating units for
this pu rp ose.181 In itially w e chose the (CI-I2) 3 group as a spacer
[*] Dr. M. Albrecht
Institut für Organische Chemie der Universität
Richard-Willstiitter-Allee, D-76131 Karlsruhe (Germany)
Telefax: Int. code + (721)698529
Dr. S. Kotila
Organisch-Chemisches Institut der Universität Münster (Germany)
[**] This work was supported by the Fonds der Chemischen Industrie (Liebig
Stipendium), Ihe Deutsche Forschungsgemeinschaft, and the Academy of
Finnland. Dr. H. Röttele is thanked for the NMR measurements, Mrs. B.
Wibbeling’s and Dr. R. Fröhlich’s help with the X-ray structure analysis is
gratefully acknowledged.
0570-0833/95/3419-2134 $ 10.00+.25/D
Angew. Chem. Ini. Ed. Engl. 1995, 34, No. 19