Langmuir 2002, 18, 5055-5057
5055
Selective Borohydride Reduction Using Functionalized Atomic Force Microscopy Tips Landy K. Blasdel,†,‡ Sarbajit Banerjee,§ and Stanislaus S. Wong*,†,§ Materials and Chemical Sciences Department, Brookhaven National Laboratory, Building 480, Upton, New York 11973, and Department of Chemistry, SUNY at Stony Brook, Stony Brook, New York 11794 Received April 27, 2002 A powder of a selective reducing agent, sodium triacetoxyborohydride (Na(OAc)3BH4), has been attached to an atomic force microscopy tip and used to selectively reduce a monolayer of imines (formed from the reaction of cystamine molecules with cyclopentanone) to their corresponding secondary amines within a well-defined region. Confirmation of the completion of the reaction came through the use of surface mid-IR results as well as with the use of the chloranil test in which the secondary amines were reacted with acetaldehyde to form an enamine that was then conjugated with chloranil to form a localized colored complex, observable by optical means, in a spatially distinct area, traced out by the scanning tip.
The notion of carrying out controllable, in situ reactions that occur on a small though discernible scale is not totally unprecedented.1 Linear chain polymerization on the nanometer scale has been induced using a scanning tunneling microscope (STM) tip.2 The electrochemical transformation3 of a surface-bound redox species has been monitored using atomic force microscopy (AFM)-based adhesion measurements. More recently,4 the kinetics of alkaline hydrolysis of ester groups in self-assembled monolayers was followed by a combination of AFM and FT-IR spectroscopy. In a recent study,1 a platinum-coated AFM tip was used to catalytically convert terminal azide groups to amines with high spatial resolution in the presence of hydrogenated 2-propanol. The reaction was effectively visualized by subsequent covalent modification of the reacted area with aldehyde-modified latex beads or with 3-(2-furoyl)quinoline-2-carboxaldehyde. Catalytic hydrogenation with platinum is economical and effective, but it can also give a mixture of products and low yields depending on the molar ratio and the structure of the reactants.5 Moreover, hydrogenation carried out in this fashion, using either Pt or Pd metal, has limited applicability to compounds containing carbon-carbon multiple bonds as well as in the presence of reducible functional groups such as nitro and cyano groups. A more general and useful methodology utilizes hydride reducing agents,6 which are stable in strong acids, are soluble in hydroxylic solvents, and remain active at different pH values. Herein, we report the use of an AFM tip, coated with a hydride reducing agent, to selectively reduce, in situ, a spatially defined region of a * To whom correspondence should be addressed. Phone: 631632-1703; 631-344-3178. E-mail:
[email protected];
[email protected]. † Brookhaven National Laboratory. ‡ Current address: Department of Chemistry, University of California at Berkeley, Berkeley, CA 94720. § SUNY at Stony Brook. (1) Mu¨ller, W. T.; Klein, D. L.; Lee, T.; Clarke, J.; McEuen, P. L.; Schultz, P. G. Science 1995, 268, 272. (2) Okawa, Y.; Aono, M. Nature 2001, 409, 683. (3) Green, J.-B. D.; McDermott, M. T.; Porter, M. D. J. Phys. Chem. 1996, 100, 13342. (4) Scho¨nherr, H.; Chechik, V.; Stirling, C. J. M.; Vancso, G. J. J. Am. Chem. Soc. 2000, 122, 3679. (5) Skita, A.; Keil, F. Chem. Ber. 1928, 61B, 1452. (6) Borch, R. F.; Bernstein, M. D.; Durst, H. D. J. Am. Chem. Soc. 1971, 93, 2897.
monolayer of imines. This process mimics an important, more general solution reaction on a much smaller, localized scale without the use of electric current or potentially hazardous reaction conditions. More specifically, we investigate the reaction of cystamine, a primary amine, with cyclopentanone to form an imine that upon reaction with the reducing agent, sodium triacetoxyborohydride,7 is converted into a secondary amine. This borohydride agent is nontoxic, exhibiting high selectivity as a reducing agent. It reduces aldehydes selectively over ketones.8,9 Moreover, it is mild, because the steric and electron-withdrawing effects of the three acetoxy groups stabilize the boron-hydrogen bond.10 The presence of the generated secondary amine, the product in question, was directly visualized by means of the chloranil test. To confirm the feasibility of the surface reaction and, hence, the accuracy of our scanning probe results, the successive reaction steps described in this Letter were systematically reproduced on 1 mm × 1 mm gold wafers; the molecules formed and adsorbed on these wafers were analyzed using surface mid-IR spectroscopy. The reaction scheme coupled with the chloranil test is shown in Figure 1. Gold-coated Si wafers were initially soaked overnight in a 3.74 mM solution of cystamine (Aldrich), which formed a stable, crystalline monolayer on the surface. Immediately prior to each experiment, a wafer was removed from the cystamine solution, washed with water, and ultimately soaked in pure cyclopentanone (Aldrich) in the presence of an excess of 3 Å molecular sieves (Fisher). The molecular sieves absorb the expected water byproduct, driving the reaction to completion.11 Once the imine formed uniformly on the surface, the wafer was rinsed with MeOH and dried, immediately prior to imaging in an atomic force microscope (MultiMode Nanoscope IIIa scanning probe microscope, Digital Instruments, Santa Barbara, CA). The probes utilized were initially prepared by coating conventional silicon pyramids (force modulation etched (7) Abdel-Magid, A. F.; Carson, K. G.; Harris, B. D.; Maryanoff, C.; Shah, R. D. J. Org. Chem. 1996, 61, 3849. (8) Gribble, G. W.; Ferguson, D. C. J. Chem. Soc., Chem. Commun. 1975, 1975, 535. (9) Nutaitis, C. F.; Gribble, G. W. Tetrahedron Lett. 1983, 24, 4287. (10) Gribble, G. W.; Nutaitis, C. F. Org. Prep. Proced. Int. 1985, 17, 317. (11) Layer, R. W. Chem. Rev. 1963, 63, 489.
10.1021/la0258695 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/21/2002
5056
Langmuir, Vol. 18, No. 13, 2002
Figure 1. A 3.74 mM solution of neat cystamine (I) is chemisorbed onto a gold-coated Si wafer substrate for 12 h, forming an amine-terminated species, II, on the surface. Reaction with cyclopentanone (III) generates an intermediate imine (IV). The sample is then scanned in methanol with an atomic force microscopy tip functionalized with sodium triacetoxyborohydride (V). Areas of the sample where the tip has scanned have been transformed from the imine (IV) to the secondary amine (VI) via the localized reduction reaction initiated by the roaming probe. The presence of colorless VI is confirmed through the chloranil test which involves reacting the structure with equal amounts of solutions of acetaldehyde (VII) and tetrachloro-p-benzoquinone (VIII). Upon heating the gold-coated wafer, onto which these materials have been adsorbed, for 15 min, the surface complex turns a bright blue, due to the presence of (IX).
silicon probes, k ) 1-5 N/m) with Cr (50 Å) followed by gold (500 Å) through thermal evaporation. By use of an inverted optical microscope (Nikon Epiphot 200) with an in-house micromanipulation setup, micrometer-sized chunks of the reducing compound Na(OAc)3BH4 (Aldrich), a white powder, were then attached to the tip using a thin film of UV-curable glue (Norland Products); this particular inert glue was chosen after a number of trials with other adhesive materials so as to ensure that the reducing agent would not detach from the tip during scanning. Use of an excess amount of the borohydride powder ensured that the intermediate glue layer could not physically come into contact with the surface, thereby avoiding potential contamination. As a control, an untreated Au-coated tip was utilized under identical conditions; no epoxy was used though. Optical images of the tip before and after AFM imaging revealed little change in the tip morphology and structure. Scans were run in nondestructive Tapping Mode in fluid, with resonant frequencies in the range of 25-30 kHz. Pure anhydrous MeOH (Mallinckrodt) was used as the imaging solvent to allow for protonation of the reduced amine following hydride transfer; the stability of the borohydride reagent in this solvent has been documented12 and, moreover, no further dehydrating agent, acid catalysis, or heating is needed under these conditions. All efforts were made to keep the reaction cell and its components free of any water, as water would not only degrade the Na(OAc)3BH4 powder on the tip but also react with the imines present on the surface, thereby reversing the reaction back to the starting reagents, cyclopentanone and primary amine. Variations in scan parameters were attempted; in general, relatively slow scan speeds (
