Stereochemical Effects on Monolayer Formation of a Chiral

The complex was resolved into pure optical isomers by being eluted with methanol on a column packed with an ion-exchange adduct of Δ-[Ru(phen)3]2+ (p...
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Langmuir 1997, 13, 1689-1694

1689

Stereochemical Effects on Monolayer Formation of a Chiral Amphiphilic Ruthenium(III) Complex Akihiko Yamagishi,* Noriyasu Sasa, and Masahiro Taniguchi Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo 060, Japan

Akira Endo, Maki Sakamoto, and Kunio Shimizu Department of Chemistry, Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyoda-ku, Tokyo 102, Japan Received June 21, 1996X A chiral amphiphilic ruthenium(III) complex, [Ru(acac)2(C12-acac)] (acac ) 2,4-pentanedionato, C12acac ) 3-dodecyl-2,4-pentanedionato), has been synthesized. The complex was resolved into pure optical isomers by being eluted with methanol on a column packed with an ion-exchange adduct of ∆-[Ru(phen)3]2+ (phen ) 1,10-phenanthroline) and a synthetic hectorite clay. The surface pressure-molecular area (π-A) curves of the pure enantiomer and the racemic mixture were obtained in the temperature range 10-30 °C. The π-A curve of the pure enantiomer was little affected by the change of temperature, while that of the racemic mixture was displaced toward the larger molecular area with the increase of temperature. The pure enantiomer was concluded to form a more packed monolayer than the racemic mixture. When the monolayer of either the ∆- or Λ-enantiomer was formed on a subphase containing ∆-[Ru(phen)3]Cl2, the molecular area of the monolayer was expanded due to the binding of ∆-[Ru(phen)3]2+ in a stereoselective way. The AFM images of the chiral and racemic monolayers deposited on mica at 20 mN m-1 showed that the chiral molecules formed an oblique lattice (a ) 0.59 nm, b ) 0.55 nm, and φ ) 63 °C), while the racemic mixture exhibited no two-dimensional surface order.

1. Introduction A two-dimensional molecular assembly on a solid surface such as Langmuir-Blodgett films and self-assembled monolayers has been applied to develop a functional material on a molecular scale.1 Metal complexes in this field occupy a unique position because of their characteristic electronic and magnetic properties.2 Polarity, hydrogen bonding, and van der Waals interactions have been used as a main tool for controlling molecular orientation and ordering. One of the promising routes to an organized assembly is to make use of molecular chirality. The stereochemical effects on molecular interactions in a packed state have a possibility for achieving higher molecular ordering in the arrangements of molecules. For these purposes, it is of fundamental importance to investigate the effects of molecular chirality when chiral molecules form a two-dimensional lattice. Although the chirality effects on organic monolayers have been reported previously,3-5 little is known about the monolayer formations of chiral metal complexes. This is contrasted with the enormous amount of evidence that has been accumulated for the stereoselective packings of metal complexes in a crystalline state.6 * To whom correspondence should be addressed. Fax: 81-011746-5232. Telephone: 81-011-706-2769. X Abstract published in Advance ACS Abstracts, February 15, 1997. (1) (a) Roberts, G. G. Adv. Phys. 1985, 34, 475-512 and references therein. (b) Richardson, T.; Roberts, G. G. Thin Solid Films 1988, 160, 231-239. (2) Richardson, T.; Roberts, G. G.; Polyska, M. E. C.; Davies, S. G. Thin Solid Films 1989, 179, 405-411. (3) Eckhart, C. J.; Peachey, N. M.; Swanson, D. R.; Takacs, J. M.; Khan, M. A.; Gong, X.; Kim, J. H.; Wang, J.; Upheus, R. A. Nature 1993, 362, 614-616. (4) Arnett, E. M.; Harvey, N. G.; Rose, P. L. Acc. Chem. Res. 1989, 22, 131-138 and references therein. (5) Tachibana, T.; Yoshizumi, T.; Hori, K. Bull. Chem. Soc. Jpn. 1979, 52, 34-41.

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We have been studying the chirality effects on the monolayers of metal complexes at an air-water interface. The mixed monolayer of tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) and stearic acid exhibited different layered structures, depending on the pure enantiomer and racemic mixture of the metal complexes.7 The difference was rationalized in terms of the orientation of a stearate anion intervening the cationic metal complexes. As an extension of these results, the present work has been focused on the monolayer formation of neutral metal complexes. No external ion was incorporated in the monolayer. Interactions were solely determined by the stereoselective intermolecular interactions. For this purpose, we have prepared a purely enantiomeric amphiphilic Ru(III) complex, [Ru(acac)2(C12-acac)] (acac ) 2,4-pentanedionato, C12-acac ) 3-dodecyl-2,4-pentanedionato). On a molecular model, the diameter of the freerotating head group and the length of the molecule along the alkyl chain are estimated to be 0.9 nm and 2.1 nm, respectively (Figure 1). The compound forms a reversible monolayer at an air-water interface. The effects of homoand heterochirality on monolayer formation have been investigated. The monolayer of the enantiomeric complex showed higher packing efficiency and stability than that of the racemic mixture. These stereochemical effects were confirmed by the atomic force microscopy (AFM) observations of monolayer films deposited on mica. 2. Experimental Section 2.1. Preparation and Resolution of [Ru(acac)2(C12acac)]. 3-Dodecyl-2,4-pentanedione (C12-acacH) was prepared by refluxing 1-iodododecane (50 g) with acetylacetone (17 g) and K2CO3 (23.5 g) in acetone for about 60 h. After the solvent was evaporated, the solid product was washed with water. [Ru(acac)2(6) Mason, S. F. Molecular Optical Activities and the Chiral Discriminations; Cambridge University Press, Cambridge, 1982; pp 169175. (7) Yamagishi, A.; Goto, Y.; Taniguchi, M. J. Phys. Chem. 1996, 100, 1327-1832.

© 1997 American Chemical Society

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Figure 2. Circular dichroism spectra of the fractions at the first (curve 1) and second (curve 2) peaks in the choromatographic resolution on a column packed with an ion-exchange adduct of ∆-[Ru(phen)3]2+ and a synthetic hectorite clay. Solvent was CHCl3.

Figure 1. Structure of [Ru(acac)2(C12-acac)]; molecular skeleton (upper) and model structures as seen in parallel with (middle) and perpendicular to (lowest) the alkyl chain. (C12-acac)] was synthesized by reacting [Ru(acac)2(CH3CN)2] (0.1 g), C12-acacH (10 g), and KHCO3 (0.5 g) in ethanol for about 3 h.8 The product was purified by being eluted on a silica gel column with 1:1 (v/v) dichloromethane/benzene. Analy. Found: C, 57.48; H, 8.07. Calcd for RuC27H45O6: C, 57.21; H, 8.02. 1H-NMR (CDCl3, room temperature) δ ) -28.59 (s, 2H, H(d)), -13.37 (s, 6H, CH3(c), -6.30 (s, 6H, CH3(a)), -2.27 (s, 6H, CH3(b)), 0.87 (s, 3H, H(e)); 1.27 (s, 20H, CH2(f)); 1.87 (s, 2H, CH2(g)) (the positions of protons are indicated in Figure 1.) The molar extinction coefficients in the electronic absorption spectrum in a chloroform solution were obtained to be  ) 17 000 (285 nm), 8410 (355 nm), and 2070 (510 nm). For optical resolution, a methanol solution of the compound was mounted on a column (25 mm × 4 mm (i.d.)) packed with an ion-exchange adduct of ∆-[Ru(phen)3]2+ and synthetic hectorite (Laponite XLG) and eluted with methanol at 40 °C.9 The elution was monitored by the absorbance at 520 nm. The fraction of a pure optical isomer was obtained, as will be described in the results section. 2.2. Instruments. A surface pressure versus area per molecule (π-A) curve was obtained with a Langmuir trough and a film balance controller (FSD-100, USI Co., Japan). Compression of a surface was started 30 min after a chloroform solution of the complex was spread over pure water. The rate of compression was 10 cm2 min-1. A Langmuir-Blodgett (LB) film was prepared by transferring a monolayer to a hydrophobic glass plate at the dipping rate of 5 mm min-1. The hydrophobic glass plate was prepared by being soaked in a 2% (v/v) toluene solution of diphenyldimethoxysilane for 6 h. An X-ray diffraction pattern was recorded with an X-ray diffractometer (Rigaku Co., Japan) at the wavelength of Cu KR (0.1540 nm) under the conditions of (8) (a) Endo, A.; Kajitani, M.; Mukaida, M.; Shimizu, K.; Sato, G. P. Inorganic Chim. Acta 1988, 150, 25-34. (b) Endo, A.; Hoshino, Y.; Hirakata, K.; Takeuchi, Y.; Shimizu, K.; Furushima, Y.; Ikeuchi, H.; Sato, P. G. Bull. Chem. Soc. Jpn. 1989, 62, 709-716. (9) (a) Yamagishi, A. J. Am. Chem. Soc. 1985, 107, 732-734. (b) Yamagishi, A. J. Coord. Chem. 1987, 16, 131-211.

40 kV and 30 mA. A circular dichroism (CD) spectrum was recorded with a spectropolarimeter (J-500, JASCO Co., Japan). A cyclic voltammogram was measured on an acetonitrile solution containing 0.1 M tetraethylammonium perchlorate, using a Ag/ AgCl electrode as a reference electrode. The AFM measurements were performed with an atomic force microscope, Nanoscope III AFM (Digital Instruments, California). A 10 µm × 10 µm scan head and a silicon nitride tip on a cantilever with a spring constant of 0.12 N m-1 were used. The images were taken in the force mode. A monolayer was transferred onto mica by vertical dipping at 5 mm min-1 in the upward direction. Transfer ratios were close to unity. The same atomic images were obtained for at least two independent samples in each case. In order to eliminate the thermal drift, the measurements were started about 5 h after the sample was mounted. Two scan directions were chosen to be 0 and 45° with respect to the horizontal direction.

3. Results and Discussion 3.1. Resolution and Properties of [Ru(acac)2(C12acac)]. The racemic mixture of [Ru(acac)2(C12-acac)] was eluted on a column packed with an ion-exchange adduct of ∆-[Ru(phen)3]2+ and a synthetic hectorite clay at 40 °C. Curves 1 and 2 in Figure 2 are the circular dichroism spectra of the fractions at the first and second peaks, respectively. When the spectra were compared with those of the enantiomers of tris(2,4-pentanedionato)ruthenium(III) ([Ru(acac)3]), the negative and positive peaks at 350 and 275 nm in curve (1) corresponded to the negative and positive peaks at 350 and 275 nm for the Λ-enantiomer of [Ru(acac)3], respectively.10 Thus from the observed elution order, the ∆-isomer of the complex showed higher affinity toward ∆-[Ru(phen)3]2+ adsorbed on a clay. This selectivity (called here “enantiomeric selectivity”) was the same as in the resolution of [Ru(acac)3] on the same column.9b The compound was insoluble in water and soluble in organic solvents such as CH3OH, CHCl3, and benzene. When a cyclic voltammogram was measured on an acetonitrile solution of the racemic complex, two reversible redox processes were observed; the first oxidation and reduction peaks were observed at -0.85 and -0.79 V (vs Ag/AgCl electrode), respectively, and the second oxidation and reduction peaks at 0.90 and 0.96 V (vs Ag/AgCl electrode), respectively. These processes were ascribed to the redox reactions of Ru(II)/Ru(III) and Ru(III)/Ru(IV), respectively. The values were compared with the redox peaks of [Ru(acac)3] (-0.78/-0.70 and 0.98/1.06 V).11 The (10) Kobayashi, H.; Matsuzawa, H.; Kaizu, Y.; Ichida, A. Inorg. Chem. 1987, 26, 4318-4323. (11) Measured in our laboratory under the same conditions.

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Figure 4. Change of the surface area as a function of time when the monolayers of (A) ∆- and (B) racemic [Ru(acac)2(C12acac)] were compressed up to 20 mN m-1 and kept at that surface pressure.

Figure 3. Surface pressure vs molecular area curves of (A) ∆and (B) racemic [Ru(acac)2(C12-acac)] at various temperatures. Subphase was pure water.

shift of the redox peaks of [Ru(acac)2(C12-acac)] toward the lower potential was ascribed to the formation of the aggregate of [Ru(acac)2(C12-acac)] in an acetonitrile solution. The other possible reason for this was the electron-donating effect of the dodecyl group in C12acac-.8b,12 3.2. Monolayer Behaviors. The surface pressurearea isotherms were measured in the temperature range 10-35 °C. When the compression experiments were repeated after 30 min, the identical curves were obtained at all temperatures investigated, confirming the reversible formation of a surface film. The results for the ∆-isomer and the racemic mixture are shown in Figure 3 (A and B, respectively). As for the enantiomeric complex, the slope in the π-A curves changed little when the temperature was varied from 9.6 to 32.8 °C. In this temperature range, the curve showed a lift-off area at about 0.80 nm2 molecule-1 where the surface pressure rose from zero. This area was compared with the geometrical area of the freerotating head group of the complex (0.64 nm2). If the molecule tilts its alkyl group from the vertical direction, it will occupy a larger molecular area. Thus it was concluded that the surface pressure appeared when the molecules came into contact with each other. On further compression, the surface pressure increased gradually, indicating that the molecular packing in the film became denser with the concomitant increase of the repulsive interaction. The slope of the surface pressure vs area curve was low (less than 20 mN m-1 nm2), and no region existed in which the surface pressure rose almost verti(12) Patterson, G. S.; Holm, R. R. Inorg. Chem. 1972, 11, 22852288.

Figure 5. Surface pressure vs molecular area curves when enantiomeric [Ru(acac)2(C12-acac)] was spread on the subphase of pure water or an aqueous solution containing ∆-[Ru(phen)3]Cl2: (a) ∆-[Ru(acac)2(C12-acac)]/water; (b) Λ-[Ru(acac)2(C12acac)]/water; (c) ∆-[Ru(acac)2(C12-acac)]/∆-[Ru(phen)3]Cl2 (1.4 × 10-4 M); (d) Λ-[Ru(acac)2(C12-acac)]/∆-[Ru(phen)3]Cl2 (1.4 × 10-4 M); (e) ∆-[Ru(acac)2(C12-acac)]/∆-[Ru(phen)3]Cl2 (2.5 × 10-4 M); (f) Λ-[Ru(acac)2(C12-acac)]/∆-[Ru(phen)3]Cl2 (2.5 × 10-4 M).

cally, as seen in the case of the monolayer of stearic acid.13 On further compression, the surface pressure attained a saturated value at about 30 mN m-1, where the molecular area was 0.4 nm2 molecule.-1 The monolayer was considered to collapse at molecular areas less than this value. The π-A curve of the racemic mixture shifted toward larger molecular area with the increase of temperature. It indicated that the monolayer of the racemic mixture became expanded at higher temperature. The surface pressure at the plateau region decreased with the increase of temperature in the same way as for the enantiomer. Comparing the π-A curves at the same temperature, we found the enantiomers were more closely packed in a monolayer than the racemic mixture at temperatures higher than 15 °C. At 30 °C and 10 mN m-1, for example, the enantiomer formed a more compact monolayer than the racemic mixture by about 14%. The observed effects of homo- and heterochirality are to be compared with the molecular packings of the same compound in a solid state. At present, however, we have no knowledge on the densities or crystal structures for either racemic or enantiomeric [Ru(acac)2(C12-acac)]. Instead the results are compared with the reported facts of tris(2,4-pentanedionato)metal(III).10,14 The density of the racemic crystal of [Cr(acac)3] (1.362 g cm-3), for (13) Defay, R.; Prigogine, I.; Sanfeld, A. J. Colloid Interface Sci. 1977, 58, 498-510. (14) Kuroda, R.; Mason, S. F. J. Chem. Soc., Dalton Trans. 1979, 273-278.

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Figure 6. Unenhanced (A) and enhanced (B) 8.0 nm × 8.0 nm AFM images of the monolayer of ∆-[Ru(acac)2(C12-acac)] transferred onto mica at 10 °C and 20 mN m-1. The inset in part B is the two-dimensional Fourier transform pattern.

example, is 6.0% higher than that of the enantiomeric one (1.285 g cm-3).6 The closer packing in the racemic crystal is achieved by the stacking of 2,4-pentanedionato ligands for a racemic pair where the complexes are facing with their threefold symmetry axes parallel.14 According to the π-A curves of [Ru(acac)2(C12-acac)], the enantiomer formed a denser monolayer than the racemic mixture. Thus the molecular packings in the monolayer state of the present complex were quite different from those of [M(acac)3] in a crystalline state. One reason might be that the presence of the dodecyl group in [Ru(acac)2(C12acac)] hinders the packing of the racemic pair with their threefold symmetry axes in parallel. Instead the results suggested that the enantiomers were packed with their twofold symmetry axes parallel. Under such orientations, the enantiomeric pair was expected to attain higher molecular packing than the racemic pair. 3.3. Preparation of LB Films. The stability of the monolayer was studied by monitoring the time course of the surface area under the constant surface pressure 20 mN m-1 at 20 °C. The results are shown in Figure 4. The area of the monolayer of ∆-[Ru(acac)2(C12-acac)] showed little change with time (curve A), while that of the racemic mixture decreased considerably (curve B). In other words, the monolayer of the enantiomeric complex was more stable than that of the racemic mixture under constant surface pressure. The monolayer of ∆-[Ru(acac)2(C12-acac)] was transferred onto a hydrophobic glass plate at 20 mN m-1 with the transfer ratios 0.00-0.05 and 0.90-0.95 for the downward and upward depositions, respectively. Thus they formed an LB film of Z-type. From the X-ray diffraction analyses for the 30-layer film of the enantiomeric complexes, the interlayer distances were determined to be 2.35 nm. This was nearly equal to the length of the molecule along the alkyl chain (2.1 nm). Thus the molecules oriented the alkyl chain vertically with respect to the surface of the plate. 3.4. Interaction of a Monolayer with [Ru(phen)3]2+ Ions in a Subphase. The possibility of chiral discrimination by a chiral monolayer was examined by spreading the chloroform solutions of the chiral complexes on an

aqueous subphase containing another kind of soluble chiral metal complex. For such a chiral metal complex, [Ru(phen)3]Cl2 was chosen because the complex was reported to interact with tris(2,4-pentanedionato)metal(III) in a homogeneous solution due to its highly hydrophobic nature.15 The same complex was used as a chiral modifier on a clay surface in our chromatographic resolution (see the Experimental Section). Curves a and b in Figure 5 were the π-A curves for the ∆- and Λ-isomers of [Ru(acac)2(C12-acac)] on a subphase of pure water. Curves c and d were the π-A curves for the ∆- and Λ-isomers of [Ru(acac)2(C12-acac)] on a subphase containing 1.4 × 10-4 M ∆-[Ru(phen)3]Cl2, respectively. In both of the curves, the lift-off areas increased from 0.80 to 1.1 nm2 molecule-1 when the subphase of pure water was replaced with an aqueous ∆-[Ru(phen)3]Cl2 solution. Such a result implied that a chiral cation in a subphase, ∆-[Ru(phen)3]2+, was associated with [Ru(acac)2(C12-acac)] to expand the monolayer. On compressing the area, the curve of ∆-[Ru(acac)2(C12-acac)] approached the one on a pure water subphase, while the curve of Λ-[Ru(acac)2(C12-acac)] retained a wider molecular area by about 0.15 nm2 molecule-1 even at the stage of collapse. The results indicated that ∆-[Ru(phen)3]2+ was dissociated from ∆-[Ru(acac)2(C12-acac)] on compression but the same metal complex remained bound with the monolayer of Λ-[Ru(acac)2(C12-acac)] until the monolayer collapsed. Thus the binding of ∆-[Ru(phen)3]2+ occurred stereoselectively, showing higher affinity toward the Λ-isomer of [Ru(acac)2(C12-acac)] (called here “racemic selectivity”). It should be noted that this tendency was opposite to the “enantiomeric selectivity”, as observed in the chromatographic resolution. These facts indicated that the binding modes were different between an air-water interface and a clay surface. The observed oppposite tendency might be related to the different orientational restrictions of adsorbed molecules at these two interfaces. Curves e and f in Figure 5 are the π-A curves for the ∆- and Λ-isomers of [Ru(acac)2(C12-acac)] on a subphase containing 2.5 × 10-4 M ∆-[Ru(phen)3]Cl2, respectively. (15) Iwamoto, E.; Yamamoto, M.; Yamamoto, Y. Inorg. Nucl. Chem. Lett. 1977, 13, 399-402.

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Figure 7. Molecular packing model when four ∆-[Ru(acac)2(C12-acac)] molecules are placed at the lattice points in a unit lattice as deduced from Fiugre 6B.

Figure 8. Unenhanced (A) and enhanced (B) 8.0 nm × 8.0 nm AFM images of the monolayer of racemic [Ru(acac)2(C12-acac)] transferred onto mica at 10 °C and 20 mN m-1. The inset in part B is the two-dimensional Fourier transform pattern. The possible stacking structure of a racemic pair is drawn in part B.

Under these conditions, the curves showed nearly the same lift-off area, ca. 1.8 nm2 molecule-1, indicating that ∆-[Ru(phen)3]2+ remained bound to both isomers of [Ru(acac)2(C12-acac)] at this higher concentration. 3.5. AFM Observation of a Monolayer Transferred on Mica. Parts A and B of Figure 6 are the 8.0 nm × 8.0 nm unenhanced and enhanced AFM images of ∆-[Ru(acac)2(C12-acac)] deposited at 20 mN m-1 and 10 °C, respectively. The inset in Figure 8B is the 2D-FT pattern of the enhanced image. The four bright spots in the 2D-FT pattern formed a skewed rectangle and

corresponded to the oblique lattice with a ) 0.59 nm, b ) 0.55 nm, and φ ) 63°. The area per molecule was 0.29 nm2. This value was smaller than the molecular area for the monolayer state at an air-water interface (0.41 nm2 molecule-1), indicating the occurrence of film contraction during the deposition. The lattice had no axis of mirror symmetry and was chiral. Thus the chirality of molecules became a dominant factor in determining molecular packing. Figure 7 is the molecular packing model when four ∆-[Ru(acac)2(C12-acac)] moleucles were placed at the lattice points in a unit lattice, as deduced by the AFM

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measurements. In this model, the molecules are so close to each other that they have a part of their ligands overlapped with those of the neighboring molecules. At this level of molecular packing, it is a natural consequence to expect that they interact stereoselectively to result in the formation of a chiral lattice. Parts A and B of Figure 8 are the unenhanced and enhanced 8.0 nm × 8.0 nm AFM images of the racemic monolayer deposited under the same conditions as in Figure 6. The film possessed no two-dimensional surface order, but it consisted of the zigzag alleys. The distance between the alleys was estimated to be 0.62 nm. Each alley consisted of a bow-shaped unit of about 2 nm in length. We noted that the length was close to the molecular length of [Ru(acac)2(C12-acac)] along the alkyl chain. Thus it was suspected that the molecules oriented their alkyl chains horizontally in the racemic film. The possible molecular stacking is shown in Figure 8B, where two antipodal molecules make a pair with their head groups stacked. As far as we know, the chirality effects on the monolayer formation of metal complexes as observed here are the

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first observations which promise the use of chirality for achieving two-dimensional order in arranging metal complexes. 4. Conclusion A novel amphiphilic ruthenium(III) complex, [Ru(acac)2(C12-acac)], was synthesized. The compound was completely resolved by being eluted with methanol on a column packed with an ion-exchanged adduct of ∆-[Ru(phen)3]2+ and a synthetic hectorite clay. The compound formed a reversible monolayer at an air-water interface. The monolayer of the enantiomer was more compact and stable than that of the racemic mixture. The monolayer was transferred onto a hydrophobic glass plate to form an LB film of Z-type. When a subphase contained another kind of chiral metal complex, ∆-[Ru(phen)3]Cl2, the monolayer was expanded due to the binding of ∆-[Ru(phen)3]2+ in a stereoselective way. The AFM images of the film deposited on mica confirmed the stereochemical effects on the molecular packing in the monolayers. LA9606153