Tetradecylferrocene: Ordered Molecular Array of an Organometallic

Katrin Wedeking, Zhongcheng Mu, Gerald Kehr, Roland Fröhlich, Gerhard Erker*, Lifeng Chi*, and Harald Fuchs. Organisch-Chemisches Institut der Univer...
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Langmuir 2006, 22, 3161-3165

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Tetradecylferrocene: Ordered Molecular Array of an Organometallic Amphiphile in the Crystal and in a Two-dimensional Assembled Structure on a Surface Katrin Wedeking,† Zhongcheng Mu,‡ Gerald Kehr,† Roland Fro¨hlich,†,§ Gerhard Erker,*,† Lifeng Chi,*,‡ and Harald Fuchs‡ Organisch-Chemisches Institut der UniVersita¨t Mu¨nster, Corrensstr. 40, 48149 Mu¨nster, Germany, and Physikalisches Institut der UniVersita¨t Mu¨nster, Wilhelm-Klemm-Str. 10, 48149 Mu¨nster, Germany ReceiVed NoVember 22, 2005 Tetradecylferrocene (4, Fc-(CH2)13CH3) was synthesized via lithiation of ferrocene by treatment with tert-butyl lithium, followed by alkylation with 1-bromotetradecane. Complex 4 forms a physisorbed ordered molecular monolayer on the surface of highly oriented pyrolytic graphite (HOPG) that was analyzed by scanning tunneling microscopy (STM). It features a lamellar structure with single rows of ferrocenyl moieties separating connecting areas formed by the long alkyl chains which are arranged parallel to each other. The ordering principle of 4 on the surface can be related to the arrangement of Fc-(CH2)13CH3 molecules in the three-dimensional crystal lattice.

Introduction The formation of ordered two-dimensional structures by selfassembly of suited molecules on surfaces is of high current interest.1 Much is known about the formation of such highly ordered physisorbed monolayers of organic molecules.2,3 Much less is known about the two-dimensional supported assembly of related organometallic systems,4,5 although the defined arrangement of metal-containing systems might eventually bear some potential for future application in nano science. We had recently shown that oligomethylene-bridged diferrocenes Fc-(CH2)nFc (1) (e.g., with n ) 14, 18, or 22) form two-dimensional lamellar structures on highly oriented pyrolytic graphite (HOPG) from saturated phenyloctane solutions.6 These consist of parallel double * To whom correspondence should be addressed. E-mail: [email protected]; [email protected]; [email protected]. † Organisch-Chemisches Institut der Universita ¨ t Mu¨nster. ‡ Physikalisches Institut der Universita ¨ t Mu¨nster. § Contact this author for information regarding X-ray crystal structure determinations. (1) De Feyter, S.; Gesquie´re, A.; Abdel-Mottaleb, M. M.; Grim, P. C. M.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mu¨llen, K. Acc. Chem. Res. 2000, 33, 520-531. (2) (a) McGonigal, G. C.; Bernhardt, R. H.; Thomson D. J. Appl. Phys. Lett. 1990, 57, 28-30. (b) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424-427. (c) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600-1615. (3) (a) Grim, P. C. M.; De Feyter, S.; Gesquie´re, A.; Vanoppen, P.; Rucker, M.; Valiyaveettil, S.; Moessner, G.; Mu¨llen, K.; De Schryver, F. C. Angew. Chem. 1997, 109, 2713-2715; Angew. Chem., Int. Ed. Engl. 1997, 36, 2601-2603. (b) Okawa, Y.; Aono, M. Nature 2001, 409, 683-684. (c) Okawa, Y.; Aono, M. J. Chem. Phys. 2001, 115, 2317-2322. (4) (a) Lu, X.; Hipps, K. W. J. Phys. Chem. B 1997, 101, 5391-5396. (b) Scudiero, L.; Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 1189911905. (c) Wong, K. T.; Lehn, J. M.; Peng, S. M.; Lee, G. H. Chem. Commun. 2000, 22, 2259-2260. (d) Qiu, X.; Wang, C.; Yin, S.; Zeng, Q.; Xu, B.; Bai, C. J. Phys. Chem. B 2000, 104, 3570-3574. (e) Scudiero, L.; Barlow, D. E.; Mazur, U.; Hipps, K. W. J. Am. Chem. Soc. 2001, 123, 4073-4080. (f) Abdel-Mottaleb, M. M. S.; Schuurmans, N.; De Feyter, S.; Van Esch, J.; Feringa, B. L.; De Schryver, F. C. Chem. Commun. 2002, 1894-1895. (g) Ziener, U.; Lehn, J. M.; Mourran, A.; Moller, M. Chem. Eur. J. 2002, 8, 951-957. (h) Yin, S.; Wang, C.; Xu, B.; Bai, C. J. Phys. Chem. B 2002, 106, 9044-9047. (5) (a) Hoeppener, S.; Chi, L. F.; Wonnemann, J.; Erker, G.; Fuchs H. Surf. Sci. 2001, 487, 9-14. (b) Hoeppener, S.; Wonnemann, J.; Chi, L.; Erker, G.; Fuchs, H. ChemPhysChem. 2003, 4, 490-494. (c) Lenhert, S.; Zhang, L.; Mueller, J.; Wiesmann, H. P.; Erker, G.; Fuchs, H.; Chi, L. AdV. Mater. 2004, 7, 619-624. (d) Zhang, L.; Gaponik, N.; Mu¨ller, J.; Plate, U.; Weller, H.; Erker, G.; Fuchs, H.; Rogach, A. L.; Chi, L. Small 2005, 1, 524-527. (6) Wedeking, K.; Mu, Z.; Kehr, G.; Cano Sierra, J.; Mu¨ck Lichtenfeld, C.; Grimme, S.; Erker, G.; Chi, L.; Wang, W.; Zhong, D.; Fuchs, H. Chem. Eur. J. 2006, 12, 1618-1628.

Figure 1. View of the molecular structure of 4. Selected bond lengths (Å) and angles (°): Fe-C1 2.057(2) Fe-C2 2.046(2), FeC3 2.041(2), Fe-C4 2.044(3) Fe-C5 2.046(3), Fe-C(C21-C25) 2.032(2) to 2.050(3), C1-C6 1.504(3), C-C (C6-C7 to C17C18) 1.516(4) to 1.527(3), C18-C19 1.529(4); C-C-C (C1-C6C7 to C17-C18-C19) 113.0(2) to 115.2(2).

rows of closely neighboring ferrocenyl headgroups separated by maximally extending parallel oligomethylene spacers. The overall pattern of the two-dimensional Fc-(CH2)n-Fc (1) surface structures in many cases was reminiscent of the molecular arrangements of 1 in specific planes in the three-dimensional crystal lattice of these systems as determined by single-crystal X-ray diffraction studies.6 It was an interesting question whether a physisorbed two-dimensional structure of a long chain monoferrocenylalkane, a much more open system, would behave analogously when compared to the corresponding threedimensional solid state structure or if the less confined molecular structure would lead to a substantially different molecular arrangement on the HOPG surface. We have therefore synthesized a typical example, tetradecylferrocene (4), and compared its molecular arrangements in the crystal and on a surface.

Results and Discussion The synthesis of the target molecule Fc-(CH2)13CH3 (4) was straightforward. Monolithiation of ferrocene (2) was effected by treatment with tert-butyllithium in THF at 0 °C.7 The resulting Li-ferrocene reagent (3) was then reacted with 1-bromotetradecane (12 h, -78 °C to room temperature) to yield the product 4. It was isolated in 33% yield after chromatographic separation from residual ferrocene. The 1H NMR spectrum of 4 (in d6benzene) shows a Cp-singlet at δ 4.02 (s, 5H) [13C: δ 68.8]. The substituted η5-C5H4 ring system gives rise to resonances at δ (7) Herberhold, M.; Ayazi, A.; Milius W.; Wrackmeyer, B. J. Organomet. Chem. 2002, 656, 71-80.

10.1021/la0531594 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/02/2006

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Figure 2. Two projections of the crystal packing of Fc-(CH2)13CH3 (4) molecules in the ac plane showing the typical double rows of ferrocenyl units. Scheme 1

4.00 (m, 2H) and 3.97 (m, 2H) [1H] and δ 89.5 (C1), 68.4 (C2, C5), and 67.5 (C3, C4) [13C], respectively. The elongated chain gives rise to separate 1H NMR signals at δ 2.29 (m, 2H, 6-H) and δ 1.53 (m, 2H, 7-H) plus a broad unresolved multiplet of signals centered around δ 1.31 for the remaining (-CH2-)11 fragment plus a cleanly separated CH3 signal at δ 0.91 (m, 3H). Single crystals of the ferrocene derivative 4 were obtained from a concentrated n-heptane solution at +6 °C that was suited for an X-ray crystal structure analysis.8 The molecular structure of complex 4 (see Figure 1) is conventional. The ferrocene unit features an eclipsed metallocene conformation.9 The side chain is found in a maximally extended form with all of the saturated hydrocarbyl subunits showing an ideal antiperiplanar conformation. The overall length of the molecule of 4 is ca. 18.9 Å, determined by the distance of the terminal methyl carbon (C19) to the centroid of the η5-C5H4 ring. The mean chain is oriented almost normal to the ferrocene main vector, but not ideally: the best vector of the (-CH2-)13 unit forms an angle of ca. 85° with the Cpcentroid-Fe-Cpcentroid vector. Consequently, the C6-C7 bond is not completely coplanar with the ferrocene C5H4 plane (θ C2-C1-C6-C7 152.8 (2)°). We had shown previously6 that the packing pattern of the Fc-(CH2)14-Fc (1a) molecules resembled the surface structure of the mono-molecular layer of physisorbed 1a molecules on highly oriented pyrolytic graphite (HOPG) under a saturated 1a solution in phenyloctane as analyzed by scanning tunneling microscopy (STM). On HOPG 1a formed lamellar structures (as (8) Data set was collected with a Nonius KappaCCD diffractometer, equipped with a rotating anode generator. Programs used: data collection COLLECT (Nonius B. V., 1998), data reduction Denzo-SMN (Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307-326), absorption correction SORTAV (Blessing, R. H. Acta Crystallogr. 1995, A51, 33-37. Blessing, R. H. J. Appl. Crystallogr. 1997, 30, 421-426), structure solution SHELXS-97 (Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467-473), structure refinement SHELXL-97 (Sheldrick, G. M. Universita¨t Go¨ttingen, 1997), graphics XP (BrukerAXS, 2000), CCDC-Mercury V 1.3. (9) Dunitz, J. D.; Orgel, L. E.; Rich, A. Acta Crystallogr. 1956, 9, 373-375.

schematically depicted in Figure 5, left) where the extended (all anti-periplanar) (-CH2-)14 chains that are connecting the terminal ferrocenyl units were all arranged parallel to each other. The single resulting lamellar structures were separated by double rows of ferrocenyl end groups. A similar arrangement of 1a molecules was found in a major plane of the three-dimensional crystal structure of Fc-(CH2)14-Fc (1a).6 Because of this close resemblance of the surface structure of 1a physisorbed on HOPG and the crystal packing of 1a, it was tempting to investigate whether the much simpler system Fc(CH2)13CH3 (4) showed a similar relationship. Therefore, we will first look at the crystal packing of 4. Figure 2 shows two projections from the lattice of 4 in the ac plane. The Fe atoms of the individual molecules reside in the ac plane. The C14 chains are oriented in pairs parallel to each other slightly above (ca. 1.7 Å) and below this plane. We again notice the structural motif of the double rows of ferrocenyl end groups that seem to be characteristic of this general type of organized structures of long chain alkyl-substituted ferrocene molecules inside the three-dimensional crystal lattice. Adjacent ferrocenyl subunits in the ac plane have a short Fe‚‚‚ Fe separation of 5.39 Å and they are oriented close to perpendicular; the angle between the Cpcentroid-Fe-Cpcentroid vectors and two Fe neighbors from each Fc-(CH2)13CH3 ribbon is 76°. The Fe‚‚‚Fe distance between two closest ferrocenyl neighbors inside a ribbon amounts to 9.82 Å. The coplanar C14 alkyl chains form an angle of ca. 70° with the mean orientation of the ferrocenyl double rows in the ac plane (see Figure 2). There are two additional planes inside the three-dimensional structure of 4 that contain the iron atoms. Figure 3 shows two views of the ab plane. Here the parallel C14 hydrocarbon chains are C2-symmetrically located above and below the plane with a shortest lateral C-C separation of 3.89 Å (max. distance: 4.22 Å). The ferrocenyl units within each individual ribbon are oriented parallel with their Cpcentroid-Fe-Cpcentroid vectors. Their Fe‚‚‚

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Figure 3. Two views of the ribbon structure of 4 in the crystallographic ab plane.

Figure 4. Two views of the Fe atom containing plane (0.11x + 0.76y + 0.64z ) 12.14) in the crystal lattice of 4.

Fe separation amounts to 7.92 Å. The closest ferrocenyl neighbors of the adjacent ribbons are tilted by ca. 82.9°. The average ribbon width in this projection amounts to ca. 27.66 Å (as compared to 27.27 Å for the ribbon structures in the ac plane, see above). There is an additional cut through the crystal outside its major axes that reveals a coplanar arrangement of the Fe atoms (see Figure 4). The overall arrangement of the resulting ribbons is similar to the patterns observed in the ac and ab planes (see above). Again, double rows of the ferrocenyl groups are found at the edges of oriented C14 chains (min. lateral Fe‚‚‚Fe separation: 5.39 Å). The orientation of the chains is slightly more complicated than in the two former examples, building up a recurring step-type pattern of four individual chains (see Figure 4). Fc-(CH2)13CH3 (4) forms a physisorbed monolayer on HOPG from a saturated solution in phenyloctane. Its structural features were analyzed by STM. It shows a highly ordered lamellar structure (see Figure 5, right). Narrow rows of single bright spots are separated by broader bands that appear much darker in the image. The parallel rows of the bright spots are equidistant. Their average separation amounts to ca. 2.2 nm. It is likely that the bright spots originate from individual ferrocenyl end groups of the ordered Fc-(CH2)13CH3 chains. The observed lattice

constant of the physisorbed molecular pattern is in the order of the molecular dimension of 4. We assume that the observed pattern can be attributed to a packing scheme as it is depicted in Figure 5 (lower right illustration). The observed monolayer pattern of 4 on the HOPG surface is different from that of its relative 1a. For Fc-(CH2)13CH3, single rows of ferrocenyl moieties are observed (Figure 5, right), whereas Fc-(CH2)14-Fc (1a) features a surface pattern that exhibits characteristic double rows of ferrocenyl units (Figure 5, left),6 similar as was observed in the crystal. It seems that the surface structure of 1a can be analyzed by closely comparing it with the packing of the Fc-(CH2)14-Fc molecules in the threedimensional crystal, whereas 4 seems to form markedly dissimilar structures in the crystal and on the HOPG surface. A closer inspection, however, reveals that the unexpected arrangement of the Fc-(CH2)13CH3 molecules in the physisorbed monolayer on HOPG can probably be systematically derived from the threedimensional crystal packing by taking some characteristic features of the latter into account. A close inspection of the ribbon structures obtained by the specific cuts through the crystal as depicted in Figures 2 and 3 reveals that each of these molecular projections contains large unoccupied sections (ac, ab plane) or a noncoplanar arrangement of the chains (Figure 4). In the ac plane, each of these “holes”

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Figure 5. Comparison of the STM images of Fc-(CH2)14-Fc (1a, left: 20.0 nm × 20.0 nm, U ) +0.8V, I ) 80.0 pA) and Fc-(CH2)13CH3 (4) (right: 15.5 nm × 15.5 nm, U ) -1.233V, I ) 650.0 pA) with schematic representations of the different corresponding molecular orientations in the physisorbed monolayers.

Figure 6. Schematic representation of the hypothetical interconversion of the observed molecular packing pattern of 4 in the ab crystal plane to the ordered physisorbed monolayer structure observed on HOPG.

has an area of ca. 9.8 × 5.4 Å (Fe‚‚‚Fe distances), and in the ab plane, each of these empty spots measures an area of ca. 7.9 × 5.4 Å. In the three-dimensional lattice, this does not result in any appreciable destabilization since the analogous molecular layers above and below are shifted such that their ferrocenyl units occupy the space on the top and on the bottom of these areas and thus lead to a closed stable three-dimensional structure. In a monolayer, this stabilizing effect is absent, and consequently, the “two-dimensional lattice” must undergo a defined distortion to close these large destabilizing gaps. As can be seen, this can easily be achieved by shifting of the adjacent ribbon e.g. in the ab plane by one lattice constant in a ca. 45° angle (see schematically depicted in Figure 6) to achieve a stable twodimensional close packing similar as it is probably observed in the STM image of the physisorbed molecular monolayer of 4 on HOPG (see Figure 5, right).

Conclusion Although the surface structures of 1a and 4 at first sight seem to be rather different, the characteristics of the physisorbed patterns of both systems on HOPG can probably be derived from their packing diagrams in the three-dimensional crystal packing, if certain differences of the crystal structures are taken into account. This might mean that the typical structural features of physisorbed molecules such as these ferrocenylalkanes can potentially be

better understood by analyzing their possible connections with conventional three-dimensional structural features. Whether this is just so in special cases, such as these organometallic amphiphiles, or will hold for other systems as well might be worth investigating more closely. Experimental Section General. All reactions with air and moisture sensitive compounds were carried out under dry argon in Schlenk-type glassware or in a glovebox. Solvents were dried and distilled prior to use. Commercially available reagents were used as received. For additional general conditions, including a list of instruments used for a physical characterization of the compounds, see ref 6. Most NMR assignments were secured by a variety of 2D NMR experiments.10 STM Analysis. STM investigations were performed using a commercial Nanoscope III scanning tunneling microscopy (Digital Instrument Co., Santa Barbara, CA) with mechanically cut Pt/Ir (90/10) tips at ambient temperature. For the measurement at the solution-substrate interface, a saturated solution of ferrocenyltetradecane was applied onto a freshly cleaved surface of HOPG (Grade ZYB, mosaicity 0.8° ( 0.2°). Measurement conditions are given in the corresponding figure captions. Different tips and samples were used to check for reproducibility and to ensure there are no image artifacts caused by the tips or sample. Flattening of the images (10) Berger, S.; Braun, S. 200 and More NMR Experiment: A Practical Course, 3rd ed.; Wiley-VCH: Weinheim, Germany, 2004 and references therein.

Tetradecylferrocene was carried out to compensate for tilting of the substrate and scan line artifacts, and a low-pass filtered transform was employed to remove scanning noise in the STM images. Tetradecylferrocene (4).11 The monolithiation of ferrocene (2) was carried out under the conditions described by Herberhold7 using ferrocene (4.70 g, 25.1 mmol) and tert-butyllithium (1.5 M in pentane; 17 mL, 25.5 mmol) in tetrahydrofuran (25 mL) under argon. The generated lithio-ferrocene (3) was used in situ. Tetrahydrofuran (60 mL) was added and the solution cooled to -78 °C. 1-Bromtetradecane (4.00 g, 14.4 mmol) was added, and the reaction mixture was stirred for 16 h, while slowly warming to room temperature. The solvent was removed under reduced pressure. The residue was dissolved in pentane, the solvent volume was reduced, and the remaining ferrocene (2) was allowed to crystallize out of the mixture. Compound 4 remained in solution. Afterwards the crude product was purified by column chromatography on silica gel (pentane). The second fraction was collected. (Rf (2) ) 0.27; Rf (3a) ) 0.35) After evaporation of the solvent compound 4 was obtained as a red solid (1.80 g, 33%). Single crystals for X-ray crystal structure analysis were obtained from a n-heptane solution at +6 °C. 1H NMR (400.1 MHz, 300 K, d6-benzene): δ ) 4.02 (s, 5H, Cp), 4.00 (pt, 2H, 2,5-H), 3.97 (pt, 2H, 3,4-H), 2.29 (m, 2H, 6-H), 1.53 (m, 2H, 7-H), 1.31 (m, 22H, (11) For a different and more complicate synthesis of 4, see e.g.: Wang, K.; Gokel, G. W. J. Phys. Org. Chem. 1997, 10, 323-334. See also: (a) Okada, Y.; Oguri, K.; Sakamoto, K.; Miyako, Y.; Hayashi, T. Magn. Reson. Chem. 2002, 40, 795-796. (b) Maharaj, F.; McDonagh, A.; Scudder, M.; Craig, D.; Dance, I. Cryst. Eng. Comm. 2003, 5, 305-309.

Langmuir, Vol. 22, No. 7, 2006 3165 8,9,10,11,12,13,14,15,16,17,18 -H), 0.91 (m, 3H, 19-H); 13C{1H} NMR (100.6 MHz, 300 K, d6-benzene): δ ) 89.5 (C1), 68.8 (Cp), 68.4 (C2,5), 67.5 (C3,4), 32.3 (C17), 31.7 (C7), 30.15, 30.15, 30.15, 30.15, 30.12, 30.10, 30.02, 30.02, 30.01, 29.79 (C6, C8, C9, C10, C11, C12, C13, C14, C15, C16), 23.1 (C18), 14.3 (C19); elemental analysis calcd. (%) for C24H38Fe: C 75.38, H 10.02; found C 75.39, H 10.00 MS (MALDI-TOF (DCTB)): m/z (%): 382.1 (100) M+. X-ray crystal structure analysis of 4: formula C24H38Fe, M ) 382.39, yellow crystal 0.35 × 0.30 × 0.03 mm, a ) 55.313(1), b ) 7.919(1), c ) 9.818(1) Å, β ) 99.63(1) °, V ) 4239.9(7) Å3, Fcalc ) 1.198 g cm-3, µ) 7.15 cm-1, empirical absorption correction (0.788 e T e 0.979), Z ) 8, monoclinic, space group C2/c (No. 15), λ) 0.71073 Å, T ) 198 K, ω and φ scans, 13820 reflections collected ((h, (k, (l), [(sin θ)/λ] ) 0.67 Å-1, 5232 independent (Rint ) 0.067) and 3487 observed reflections [I g 2 σ(I)], 227 refined parameters, R ) 0.059, wR2 ) 0.112, max. residual electron density 0.42 (-0.47) e Å-3, hydrogen atoms calculated and refined as riding atoms.

Acknowledgment. Financial support from the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged. Supporting Information Available: Crystallographic data (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. LA0531594