[l] Recent reviews: a) L. E. Orgel, Nulure 1992.358,203-209; b) A. Eschen- moser. E. Loewenthal. Chrm. Soc. Rev. 1992, 1-16; c) S. Hoffmann, Angrw. Chem. 1992, 104, 1032- 1035: Angen. Chem. lnt. Ed. Engl. 1992, 31,1013- 1016; d) M. Famulok, J. S. Nowick, J. Rebek, Acra Chem. Scund. 1992. 46, 315- 324; e) G. von Kiedrowski, J. Helbing. B. Wlotzka, S. Jor- dan. M. Matzen, T. Achilles, D. Sievers, A. Terfort, B. C. Kahrs, Nuchr. Chcm. Tech. Lub. 1992. 40. 578-588.

[2] Sequence notations principally concern oligodeoxynucleotide derivatives ; the prefix d for deoxy is omitted here. Abbreviations: Me = 5'-0-methyl, M T M = S-0-methylthiomethyl. q 3 = 5'-azido-5'-deoxy. H2N = 5'-amino-5'- deoxy. Ho = 5'-hydroxy, p = 3-phosphate. p = 3'-(2-chlorophenyl)- phosphate, pPT1: = 3'-(2-phenylthioethyI)phosphate, pn = 3'-5'-phospho- amidate (the sequences, when not otherwise specified, are always in the 5' + 3' direction); HPLC = high-performance liquid chromatography, RPC' = reversed-phase column.

(31 G . von Kiedrowski, Angrn.. Chem. 1986, 98, 932-934; Angew. Chem. 1121. Ed. Engl. 1986. 25, 932- 935.

[4] a ) For the theory of the square root law and parabolic growth see: G. von Kiedrowski. Bioorg. Chern. Front. 1993, 3, 113- 146. b) Interestlngly, parabolic growth is also described in connection with the propagation of computer viruses: J. 0. Kephart. S. R. White Proc 1991 IEEE Computer S11~. Symp. Rex Securucj undPrivucy, IEEE Computer Society, Washing- ton, 1991, 343-359.

[S] G. von Kiedrowski, B. Wlotzka, J. Helbing, M. Matzen, S. Jordan, Angeic.. Clr~wi. 1991. 103, 456-459. 1066; Angew. Chem. In/ . Ed. Engl. 1991, 30. 423-426. 892.

[61 Further self-replicating systems with self-complementary templates from t u o starting materials: a ) W. S. Zielinski, L. E. Orgel, Nururr 1987, 327, 346- 347: b) G. von Kiedrowski, B. Wlotzka. J. Helbing. A n g e n . Chem. 1989. lo/, 1259-1261; Angew. Chem. Int. Ed. Engl. 1989,2X. 1235-1237; c) T. T]ivikua, P. Ballester. J. Rebek, Jr., J. Am. Chew Soc. 1990, 112. 1749 - 1250; d) J. S. Nowick, Q. Feng. T. Tjivikua. P. Ballester. J. Rebek, Jr . ibid. 1991. 113,8831 -8839; e) I - I . Hong, Q. Feng. V. Rotello, J. Rebek. S&nw 1992. ,755. 848-850; f) Q , Feng, T. K. Park, J. Rebek, [hid. 1992, 2.36. 1 1 79 ~ 1180; g) A. Terfort, G. von Kiedrowski. Angnr. Ciiem. 1992, 1114. 626 -628: Angeic.. Chrm. In!. Ed. Engl. 1992, 31, 654-656. Related examples: h) W. S. Zielinski, L. E. Orgel, J. Mol. Ewl . 1989. 29. 281 -283, i ) K . E. Ng, L. E. Orgel, ibid. 1989, 29. 101 - 107; j) J. T. Goodwin, D. G. Lynn.J Am. Chem. Soc. 1992. 114.9197-9198; k) F. Persico, J. D. Wuest, J. 0rg . Chem. 1993, 5X. 95.

[7] Other systems: Self-replicating micelles: a) P. A. Bachmann. P. Walde, P L. Luisi. J. Lang, J. Am. Chrm. Soc. 1990. 112. 8200-8201; b)P . A. Bachmann. P. L. Luisi, J. Lang. Nature 1992, 357, 57-59. En route to self-replicating ribozyines: c) J. A. Doudna, J. W. Szostak, hid . 1989, 339, 519; d) J. A. Doudna. J. Couture, J. W. Szostak, Science 1991, 251,1605- 3610. e) For self-replicating algorithms. see articles in Arr[/icrd LiJe. The Procrc~ding.\ N/ un Interdisciplinury Workshop on the Synthesis und Simulu- 1ioii o/ Lri'mg Systems Held September, 1987 in Los Atumos, New Mexico (Ed.: C. G. Langton). Addison-Wesley, Redwood City, USA, 1989; f ) J. A. Reggia, S. L. Armentrout, H.-H. Chou, Y Peng, Science 1993,259. 1282- 1287.

[8] a ) J. Helbing. Dissertation, Gottingen, 1990; b) T. Achilles. G. von Kiedrowski. unpublished; c) synthesis method employed: V. A. Efimov, 0. G. Chakhmakhcheva, Y. A. Ovchinnikov, Nuclic Acids Res. 1985, 13, 3651 3670.

[9] The following standard conditions were chosen for all described reactions: T = 20 'C; 0.2 M EDC; 0.1 M MeIm/pH 7 2; reactant concentrations: 2 mM.

[lo] Nucleosil ' 3 8 . 4 x 250 mm; A: 0.1 M sodium hydrogen carbonate, B: ace- tonitrile/water 30:70 (v/v); flow rate 1 mLmin- ' ; gradient (B): 18%- 2 8 % i n 5 m i n , 2 8 % - 4 2 % i n 2 m i n , 4 2 % - 5 0 % i n 4 m i n , 5 0 % - 7 5 % i n 2.5 min, and 0.5 min 75%; detection at 1. = 254 nm.

11 11 As trace products in the conversion of (A + B + C), all possible oligomer- ization products of the type B,, AB,,, B,C. and AB,C are assumed (n > 11, because each amino reactant (B,,, B,C) can in principle be condensed with each 3'-terminal phosphate reactant (AB,. B,) to give the corresponding 3'S-phosphoamidate. However, with the exception of ABC, higher con- densation products could not be reliably identified in the HPLC profile.

[12] The elution behavior of the nucleotide reactants on C1X is determined mainly by the terminal groups. Compounds with polar termini, for ex- ample B and BB. elute immedialely and hence cannot be separated.

[13] AC'. A B and A B were prepared by solid-phase synthesis (phosphoamid- itc method). The formation ofphosphodiesters between 3-phosphates and the terminal hydroxy groups of these templates is negligible under the reaction conditions; the templates can be regarded as inert.

[14] B. Wlotzka, Dissertation. Gottingen, 1992. [15] a) S. M . Freier, A. Sinclair, T. Neilson, D. H. Turner, J. Mol. Biol. 1985,

1x5. 645 -648, b) K. H. Breslauer. R. Frank. H. Bliicker, L. A. Marky. Proc. Null. Acud. Sci. USA 1986. 83, 3746- 3749.

[16] E Smthmiry. 1. Gladkih, J. Thror. Bid. 1989. 138. 55-58.

Metallomesogens with a Cholesteric Mesophase** By Maria J. Baena, Julio Buey, Pablo Espinet,* Heinz-S. Kitzerow, and Gerd Heppke

Metallomesogens, liquid crystals containing coordinated metal atoms, have seen a rapid development in the last few years."] In a very short period metallomesogens displaying almost every mesophase previously found in organic meso- gens--calamitic (nematic and smectic phases), discotic, poly- meric, and amphiphilic phases-have been reported. Rather surprisingly, chiral metallomesogens are scarce; only one material displaying a chiral smectic C mesophase (and hence ferroelectric) had been reportedf21 until very recently, when two paramagnetic chiral smectic C metallomesogens were described.[31 There is no representative of the chiral nematic (cholesteric, Ch) mesophase. We report here the first metal- lomesogens displaying cholesteric behavior, which have been obtained within the now-large family of metallomesogens derived from ortho-palladated imines and azines.I2. 41

p-C,H,,0~~C,H,~CH=N-C,H,-OC,H,3-~ 1

Imine 1 reacts with Pd(OAcf, in acetic acid (50"C, overnight) to give complex 2a (X = OAc) which shows no mesogenic behavior (crystalline/217"C/isotropic) (Scheme I).['] Treatment of 2a in CH,Cl, with HCI (Pd/

CC.H

*

y 3

* CHCl

CH3 CH,

Scheme 1. Za: X = OAc; Zb: X = CI.

HCI = 1 /I) affords 2 b (X = CI) (crystalline/l76 "C/ SJ247 "C/isotropic) which, on treatment with ( R ) - K0,CCHClMe in CH,Cl,/acetone (1 /I) , yields complex 3 (crystalline/l52 "C/S,/l86 "C/isotropic/203 "C/Sa/224 "C/ isotropic). All the complexes gave satisfactory C,H,N-analy- ses. The mesogenic behavior of the planar complex 2 b and the nonmesogenic behavior of the folded compound 2a are to be expected from our previous result^,[^.^' but the meso-

['I Prof. Dr. P. Espinet, Dr. M. J. Baena, J. Buey Departamento de Quimica Inorginica Facultad de Ciencias Universidad de Valladolid. E-47 005 Valladolid (Spain) Telefax: Int. code + (83)423-013 Dr. H.-S. Kitzerow. Prof. Dr. G. Heppke. Technische Universitit Berlin -ER 11- Strasse des 17. Juni 135, D-10623 Berlin (FRG)

[**I This work was supported by the Comision Interministerial de Ciencia y Tecnologia (Project MAT90-0325), the Consejeria de Cultura de la Junta de Castilla y Leon. Iberdrola, (stipend for M. J. B.), and the Deutsche Forschungsgemeinschaft (SFB 335).

Angew Chcm I n / . Ed. Engl. 1993. 32, N o . 8 VCH VeriugsgeselBrhufi mhH. 0-69451 Weinheim, 1993 O57O-O833j93jOSO~-l20l 3 IO.OO+ .25:0 1201

genic behavior of the folded complex 3 is unexpected and remarkable. We had expected that the 2-chloropropionato ligand, which is bulkier than the acetato ligand, would dis- turb the molecular packing and hinder mesogenic behavior more; however, the dipole moment introduced with the C-CI bond apparently increases the strength of the intermolecular interactions enough to compensate for the unfavorable steric effect, and 3 is indeed mesomorphic. The compound is also remarkable for its double-melting

Treatment of 3 with mercaptans HSC,H,,+, (Pd/HSR = 2/1) produces the mixed-bridge compounds 4 (4a: n = 6; 4b: n = l o ; 4c: n =IS). The complexes gave satisfactory C,H,N-analyses; molecular weight determinations in CHCI, and 'H NMR spectra are also consistent with a bridged di- nuclear compound. We discuss only the data obtained for 4a; the results for 4b and 4c are similar. The easily observ- able signals for H, (6 =7.83, s) and H, (6 =7.49 d, J = 2.34 Hz) indicate a unique cis arrangement of the two imine moieties in the complex.['] In contrast, the chloro- and carboxylato-bridged complexes 2 and 3, respectively, have trans structures. To the best of our knowledge bridged dimers with ortho-palladated ligands either have trans struc- tures (and hence have no net dipole moment) or are mixtures of trans and cis isomers, in which the latter is typically the very minor component. The carboxylato/thiolato bridges of the compounds described here seem to force a cis arrange- ment. This is particularly interesting in view of possible ap- plications of these compounds as frequency doublers in non- linear optics, where a net dipole moment is required.[*]

All three compounds 4 a 4 c show a cholesteric mesophase in a certain range of temperatures (4a: crystalline/l40 "C/ cholesteric/l57 "C/isotropic; 4b: crystalline/l09.5 "C/S,/ 115.2 "C/cholesteric/l37.6 "C/isotropic; 4c: crystalline/92 "C/ S,/128.3 "C/cholesteric/l31.7 "C/isotropic). In order to characterize their chirality in mesophase condition~,[~] we measured the pitch of the cholesteric phase induced by 4a and 4c in mixtures of the respective chiral metallomesogen and RO-TN 404 (a commercial, wide-range nematic solvent from Hoffmann-La Roche, containing cyano-substituted biphenyls and pyrimidine derivatives), by applying the Grandjean-Can0 method. [ ''1

The cholesteric mixtures show a temperature dependence of the pitchp which is characterized by a divergence o fp and an inversion of the helical screw sense with changing temper- ature (Fig. 1). At low temperatures we found a right-handed cholesteric helix (p > 0), at high temperatures a left-handed

200 t I t l

t # I 100 1 # I

P [PI I

' I I I

-200 I I 1 , I . I

SO 30 50 70 90 110

T ["C] - Fig. 1. Temperature dependence of the cholesteric pitch p for a mixture of 21.2 wt% of 4 a and 78.8 wt% of the commmercial nematic solvent RO- TN 404.

Ilpc

one (p < 0). Similar inversions of the helical pitch have been previously reported in organic systems.[' ' - 14] The induced chirality can be described by the helical twisting power k = p- ' c- ', which is independent on the concentration c of the chiral compound for most substances. From the temper- ature dependence of this quantity (Fig. 2) the inversion tem- peratures 7; = 89.8 "C for compound 4a and 7: = 100.4 "C (extrapolated) for compound 4c are obtained.

-1.5 ' ' ' I ' 1 ' I ' I ' 0 20 40 60 80 100 120

T ["C] - Fig. 2. Temperature dependence of the helical twisting power for 4 a and 4c ( p = cholesteric helical pitch, c = concentration of the chiral compound in wt Yo).

The relationship between the unusual temperature depen- dence of the pitch of the cholesteric helix and the chemical structure of the new mesogens calls to mind earlier investiga- tions on mesogenic esters of (S)-l,2-propanediol and (R,R)- 2,3-butanediol which also show a helix inversion,'''] Like compounds 4 a 4 c , the structure of these esters is character- ized by two large mesogenic groups that are connected to each other. It was proposed that the inversion effect is due to the presence of two conformers, which induce cholesteric structures with opposing helical screw senses, and which are in temperature-dependent thermodynamic equilibrium. The observations presented here seem to confirm that molecules containing two mesogenic wings tend to favor helix inver- sion. However, it should be noted that twist inversion has also been observed in compounds exhibiting a different type of structure.['3. l41

Received: November 27, 1992, Revised version: February 11,1993 [Z 5707 IE] German version: Angew. Chem. 1993, 105, 1238

[l] Reviews: A. M. Giroud-Godquin, P. M. Maitlis, Angew. Chem. 1991,103. 370; Angew. Chem. Int. Ed. Engl. 1991.30, 375; P. Espinet. M. A. Esteru- elas, L. A. Oro, J. L. Serrano, E. Sola, Coord. Chem. Rev. 1992, 117, 215; Inorganic Materials (Eds.: D. Bruce, D. OHare), Wiley, New York, 1992.

121 P. Espinet, J. Etxebarria, M. Marcos, J. Perez, A. Remon, J. L. Serrano, Angew. Chem. 1989,101,1076; Angew. Chem. Int. Ed. Engl. 1989,28,1065.

I31 M. Marcos, J. L. Serrano, T. Sierra, M. J. Gimenez, Angew. Chem. 1992, 104, 1523; Angew. Chem. Inl. Ed. Engl. 1992, 31, 1471.

[4] J. Barbera, P. Espinet. E. Lalinde, M. Marcos, J. L. Serrano, Liq. Cryst. 1987, 2, 833; P. Espinet, E. Lalinde, M. Marcos, J. Perez, J. L. Serrano, Organomerallics 1990, 9, 555; P. Espinet, J. Perez, M. Marcos, M. B. Ros, J. L. Serrano, J. Barbera, A. M. Levelut, &id. 1990,9,2028; M. B. Ros, N. Ruiz, J. L. Serrano, P. Espinet, Liq. Crysr. 1991, 9, 77; M. J. Baena, P. Espinet, M. B. Ros, J. L. Serrano, Angew. Chem. 1991, 103. 716; Angew. Chem. Inr. Ed. Engl. 1991, 30, 711.

[S] The phases observed were characterized with a polarizing microscope; the abbreviation S, that appears later in the text refers to a smectic A phase characterized by an untilted, liquid-crystalline layer structure.

1202 0 VCH Verlagsgesellschafi mhH. 0-69451 Weinhem, 1993 0570-0833i93j0808-1202 S 10.00+.25jO Angew. Chem Int. Ed. Engl. 1993, 32. No. 8

[6] Other examples ofcompounds with double-melting behavior: K. Ohta. M. Yokohama, S . Kusabayashi, H. Mikawa, J Chem. Sor. Chem. Commun. 1980,392; K. Ohta. H. Muroki, K. 1. Hatdda, 1. Yamamoto, K. Matsuza- ki. Mol. Cryst. Liy. Cryst. 1985, 130, 249; K. Ohta, H. Muroki. K. I. Hatada. A. Takagi. H. Ema, I . Yamamoto, K. Matsuzaki, ibid. 1986, 140, 163; K. Ohta. H. Ema. H. Muroki, I. Yamamoto. K. Matsuzaki, &id. 1987. 147, 61.

[7] The nonequivalence of the two halves of the dimer caused by the chiral center becomes apparent only at - 20 "C in the 'H NMR spectrum: at this temperature the t w o nonequivalent Haatoms give rise to two very close but distinct singlets. Modified complexes of type 4 have been measured. and frequency dou- bling was observed: J. Zyss, I. Ledoux, J. Martin, J. Buey, P. Espinet, unpublished. [a];" of the complexes ( c =1, CH,CI,): 4a, -8.8; 4b, -8.0; 4c, -7.7. R. Cano. Bull. Soc. Fr. Mineral. Cristallogr. 1967, 90. 333. Cholesteric mixtures containing about 20 wt% of the chiral additive were placed he- tween a flat glass slide and a spherical lens. Both surfaces were rubbed to provide a uniform planar anchoring. The disclination rings in the sample, which correspond to steps ofp/2. were observed using a polarizing micro- scope. The pitch p was calculated from the diameters of these rings. H. Finkelmann, H. Stegemeyer, Z. Naturforsch. A 1973, 28, 799. G. Heppke. D. Lotzsch. F. Oestreicher, 2. Naturforsch. A 1987, 42, 279. L. Komitov, S. T. Lagerwall, B. Stebler. G . Anderson, K . Flatiscler, Fer- roelimrics 1991. 114, 167. A. J. Slaney, I. Nishiyama, P. Styring, J. W. Goodhy, J. Mater. Chem. 1992, 2. 805.

Improved Properties of Ferroelectric Liquid Crystals from Palladium fl-Diketone Complexes** By Maria Jesus Baena, Pablo Espinet,* Maria Blanca Ros, Jose Luis Serrano," and Amaya Ezcurra

Ferroelectric liquid crystals (FLCs)"] are one of the main interests in research on liquid crystals. Although many or- ganic mesogens are known for FLCs, there are few examples of analogous metal complexes['. 3l and most of them have high transition temperatures that affect their thermal stabil- ity. The first FLC metallomesogen reported,['] a dimeric ortho-palladated azine complex (a mixture of isomers), dis- played some undesirable drawbacks : high transition temper- atures (K 102°C SmC* 119°C SmA 149°C I) leading to thermal instability in the mesophases and a very long switch- ing time (nearly one second). We then discovered that alter- ing the highly symmetrical molecular shape of these dimers results in lower melting Whereas dimeric com- pounds [{(C-N)PdCl),] (C-N = ortho-palladated imine with long alkyl chains) form SmA and SmC phases at high tem- p e r a t u r e ~ , ~ ~ ~ monomeric complexes [ (C-N)Pd(acac)] form N

[*] Dr. P. Espinet, M. J. Baena Quimica Inorganica, Facultad de Ciencias Universidad de Valladolid E-47005 Valladolid (Spain) Telefax: Int. code + (83)423-013 Dr. J. L. Serrano, Dr. M. 8 . Ros Quimica Organica, Facultad de Ciencias-I.C.M.A. Universidad de Zaragoza-C.S.I.C. E-50009 Zaragoza (Spain) Telefax: In t . code + (76)567920 Dr. A. Ezcurra Fisica Aplicada 11, Facultad de Ciencias Universidad del Pais Vasco, Aptdo. 644 E-48080 Bilbdo (Spain)

["I This work was financed by the Comision lnterministerial de Ciencia y Tecnologia (Projects MAT90-0325, MAT90-0813, and MAT91-0962C02- 01) and the Consejeria de Cultura de la Junta de Castilla y Leon. M. J. B. thanks Iberduero for a fellowship. We thank Dr. J. Barhera for the X-ray measurements and M. M. Zurbano for supplying the 8-di- ketone. ~ Abbreviations: C = crystalline, SmC' = chiral smectic C phase, SmA = smectic A phase, I = isotropic, N = nematic, FLC = ferroelectric liquid crystal.

and SmA phases at noticeably lower temperatures. Thus the monomeric complexes may be taken repeatedly to the isotropic state without signs of decomposition.[41

Taking these two observations into account, we aimed to synthesize ferroelectric metallomesogens with low transition temperatures, which would allow potential applications as both liquid crystals and in nonlinear optics.16' Our first ap- proach was the introduction of chirality in the alkyl chains of the above-mentioned acac complexes,[41 but this did not lead to the desired SmC* phases. However, analogues 3a (n = 10) and 3 b ( n = 14) with modified 8-diketonate ligands formed a ferroelectric SmC* liquid crystalline phase at lower temperatures, showed thermal stability under working con- ditions, and displayed switching times a thousand times shorter than that of the first ferroelectric metallomesogen described.[']

The synthesis of compounds 3 from 1 (Scheme 1) proceed- ed via the dinuclear complexes 2. The reaction of 2a, bC5] with thallium(1) P-diketonate['I in CH,Cl, provided the mononuclear complexes 3a and 3b, respectively. Both com- plexes gave correct elemental analyses, and 'H NMR and IR spectra in full agreement with the structure proposed.

d, -

l a (n : 10) l b ( n : 14)

Pa (n : 10) 2b (n : 14)

3a (n : 10) 3b (n : 14)

Scheme 1. a) Pd,(AcO),/AcOH; b) HCIIMeOH; c) TI [p-H,,C,,OC,H,CO),. CH]/CH,CI,.

The thermotropic behavior of the 8-diketonate derivatives was studied by optical microscopy and differential scanning calorimetry (DSC) at 10 Kmin-' (Table 1). Smectic A and chiral smectic C phases were assigned on the basis of their optical textures and confirmed by X-ray diffraction studies. The chiral smectic C phase showed a characteristic focal- conic texture, and no dechiralization lines were observed. The ferroelectric properties of these two 8-diketonate com- plexes-spontaneous polarizations (P,), switching times (T~), viscosities (y,), and tilt angles (+are summarized in Table 1 .['I Figure 1 shows the dependence of P, and 0 of the ferroelectric palladium complexes on temperature.

These metallomesogens have moderate P, values with maxima at 29 and 22 nCcm-' for 3a and 3 b respectively, and maximum optical tilt angles very close to the optimum tilt angle in FLCs of 22.5". It is noteworthy that the tilt angles could not be calculated from the X-ray diffraction measurements, as no significant differences between the lay- er thickness at the SmA and SmC* phases were detected.['']

The switching times of the new ferroelectric palladium complexes 3a, b are on the order of milliseconds. Although this is noticeably slower than the switching times of typical organic FLCs (microseconds), these new complexes switch three orders of magnitude faster than the first palladium

Angen.. Chem. Int. Ed. Engl. 1993, 32, No. 8 0 VCH Verlagsgesellschafi mbH. 0-69451 Weinheim, 1993 0570-0833/93/0808-1203 $ 10.00+ ,2510 1203