Self-Assembled Diisocyanide Monolayer Films on Gold and Palladium

Jeremy S. Hamilton, and J. Campbell Scott. IBM Research Division, Almaden Research Center, San Jose, California 95120. Received November 4, 2004...
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Self-Assembled Diisocyanide Monolayer Films on Gold and Palladium Sally A. Swanson,* Richard McClain, Katherine S. Lovejoy, Neda B. Alamdari, Jeremy S. Hamilton, and J. Campbell Scott IBM Research Division, Almaden Research Center, San Jose, California 95120 Received November 4, 2004. In Final Form: February 23, 2005 Self-assembled monolayers (SAMs) of the aromatic diisocyanides, 1,4-phenylenediisocyanide, 2,3,5,6tetramethyl-1,4-phenylenediisocyanide, 4,4′-biphenyldiisocyanide, 3,3′,5,5′-tetramethyl-4,4′-biphenyldiisocyanide, and 4,4′′-p-terphenyldiisocyanide, were prepared on gold and palladium surfaces. The SAMs were characterized by ellipsometry, polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS), and grazing-angle attenuated total reflectance infrared spectroscopy (GATR). Based on the position of the metal-coordinated isocyanide stretching band, the SAMs on gold were found to bind in the terminal (η1) geometry, while the SAMs on palladium prefer a different geometry which is possibly a triply bridging (µ3-η1) geometry. A side-reaction of the unbound isocyanide in the SAM was identified as oxidation to an isocyanate group.

Introduction The formation of self-assembled monolayers (SAMs) onto metals is well-known with the most widely studied systems being alkanethiols on gold.1 Another interesting system is the organoisocyanides on metals. With a vertical orientation of isocyanides on many metal surfaces as well as the delocalized dπ-pπ orbital system allowing electron density to shift between the metal and the molecule, a small contact barrier is expected.2 Isocyanides have been suggested as possible “alligator clips” for molecular wires,3 and the electronic transport between metal contacts (gold or palladium) and 1,4-phenylenediisocyanide (PDI)4-6 and longer diisocyanide phenylene oligomers has been explored.7 There have been previous studies exploring the adsorption of organic isocyanides on metal surfaces. The absorption of methyl isocyanide and its bonding geometry has been studied on nickel,8-10 rhodium,11,12 palladium,13 silver,14 and platinum.15-17 Alkyl isocyanides can be * Corresponding author. Phone: (408) 927-1629. Fax: (408) 9273310. E-mail: [email protected]. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Chen, J.; Wang, W.; Klemic J.; Reed, M. A.; Axelrod, B. W.; Kaschak, D. M.; Rawlett, A. M.; Price, D. W.; Dirk, S. M.; Tour, J. M.; Grubisha, D. S.; Bennett, D. W. Ann. N.Y. Acad. Sci. 2002, 960, 69. (3) Seminario, J. M.; Zacarias, A. G.; Tour, J. M. J. Am. Chem. Soc. 1999, 121, 411. (4) Chen, J.; Calvet, L. C.; Reed, M. A.; Carr, D. W.; Grubisha, D. S.; Bennett, D. W. Chem. Phys. Lett. 1999, 313, 741. (5) Lee, J.-O.; Lientschnig, G.; Wiertz, F.; Struijk, M.; Janssen, R. A. J.; Egberink, R.; Reinhoudt, D. N.; Hadley, P.; Dekker, C. Nano Lett. 2003, 3, 113. (6) Dupraz, C. J.-F.; Beierlein, U.; Kotthaus, J. P. ChemPhysChem 2003, 4, 1247. (7) Hong, S.; Reifenberger, R.; Tian, W.; Datta, S.; Henderson, J.; Kubiak, C. P. Superlattices Microstruct. 2000, 28, 289. (8) Hemminger, J. C.; Muetterties, E. L.; Somorjai, G. A. J. Am. Chem. Soc. 1979, 101, 62. (9) Friend, C. M.; Stein, J.; Muetterties, E. L. J. Am. Chem. Soc. 1981, 103, 767. (10) Friend, C. M.; Muetterties, E. L.; Gland, J. L. J. Phys. Chem. 1981, 85, 3256. (11) Cavanagh, R. R.; Yates, J. T., Jr. J. Chem. Phys. 1981, 75, 1551. (12) Semancik, S.; Haller, G. L.; Yates, J. T., Jr. J. Chem. Phys. 1983, 78, 6970. (13) Murphy, K.; Azad, S.; Bennett, D. W.; Tysoe, W. T. Surf. Sci. 2000, 467, 1. (14) Ceyer, S. T.; Yates, J. T., Jr. J. Phys. Chem. 1985, 89, 3842. (15) Avery, N. R.; Matheson, T. W. Surf. Sci. 1984, 143, 110.

selectively adsorbed over disulfides on surface pretreated platinum substrates.18 Mono-, bi-, and tridentate alkyl isocyanides adsorb on the surface of gold powder with all isocyanide groups bound to the metal.19,20 At higher concentrations, 1,6-diisocyanohexane adsorbs on gold and platinum substrates with only one isocyanide, leaving a free isocyanide available to polymerize on exposure to Ni(II) solution.21 This type of SAM has been used to subsequently attach platinum or palladium nanoparticles to gold surfaces using 1,8-diisocyanooctane tethers.22 Aromatic isocyanides were adsorbed on chromium surfaces, but these films were unstable in air or solvents.23 Alkyl and aryl isocyanides have been found to bond endon to single gold atoms on powdered gold.24,25 SAMs of double-ended aryl diisocyanides on silver and gold were found to orient vertically with one isocyanide group bound to the surface and the other remote.26,27 The remote group could be used for subsequent attachment of metal complexes,28 clusters, or nanoparticles.29 Aryl isocyanide SAMs were found to adsorb more strongly with synergistic σ/π interactions on palladium than with the σ-bonds formed on gold.30 Alternating diisocyanide multilayers using Co(II) bridges have been assembled on silica tethers, although a polymerization side-reaction also occurs.31,32 More (16) Avery, N. R.; Matheson, T. W.; Sexton, B. A. Appl. Surf. Sci. 1985, 22/23, 384. (17) Szilagyi, T. Appl. Surf. Sci. 1988, 35, 19. (18) Hickman, J. J.; Laibinis, P. E.; Auerbach, D. I.; Zou, C.; Gardner, T. J.; Whitesides, G. M.; Wrighton, M. S. Langmuir 1992, 8, 357. (19) Ontko, A. C.; Angelici, R. J. Langmuir 1998, 14, 1684. (20) Ontko, A. C.; Angelici, R. J. Langmuir 1998, 14, 3071. (21) Lin, S.; McCarley, R. L. Langmuir 1999, 15, 151. (22) Horswell, S. L.; O’Neil, I. A.; Schiffrin, D. J. J. Phys. Chem. B 2001, 105, 941. (23) Clot, O.; Wolf, M. O. Langmuir 1999, 15, 8549. (24) Robertson, M. J.; Angelici, R. J. Langmuir 1994, 10, 1488. (25) Shih, K.-C.; Angelici, R. J. Langmuir 1995, 11, 2539. (26) Han, H. S.; Han, S. W.; Joo, S. W.; Kim, K. Langmuir 1999, 15, 6868. (27) Henderson, J. I.; Feng, S.; Bein, T.; Kubiak, C. P. Langmuir 2000, 16, 6183. (28) Huc, V.; Bourgoin, J.-P.; Bureau, C.; Valin, F.; Zalczer, G.; Palacin, S. J. Phys. Chem. B 1999, 103, 10489. (29) Henderson, J. I.; Feng, S.; Ferrence, G. M.; Bein, T.; Kubiak, C. P. Inorg. Chim. Acta 1996, 242, 115. (30) Murphy, K. L.; Tysoe, W. T.; Bennett, D. W. Langmuir 2004, 20, 1732.

10.1021/la047284b CCC: $30.25 © 2005 American Chemical Society Published on Web 04/21/2005

Self-Assembled Diisocyanide Monolayer Films Chart 1. The Aromatic Diisocyanide Molecules Studied: 1,4-Phenylenediisocyanide (PDI), 2,3,5,6-Tetramethyl-1,4-phenylenediisocyanide (TMPDI), 4,4′-Biphenyldiisocyanide (BPDI), 3,3′,5,5′-Tetramethyl-4,4′-biphenyldiisocyanide (BXyDI), 4,4′′-p-Terphenyldiisocyanide (TPDI)

recently, the adsorption of aryl mono- and diisocyanides on silver and gold nanoparticles has been investigated.33-38 In this work, the SAM adsorption of a series of aromatic diisocyanides on gold and palladium substrates was studied using ellipsometry and infrared spectroscopy. The molecules used in this study, shown in Chart 1, are a series of phenylene oligomers that have either methyl groups or hydrogens at the positions ortho to the isocyanides. The infrared frequencies of the isocyanide groups will be used to indicate the binding modes and geometries of the SAMs. The inductive effect of the methyl groups on the infrared frequency of the isocyanide groups will also be used to help decipher the SAM chemistry. Experimental Section PDI, 2,3,5,6-tetramethyl-p-phenylenediamine and 3,3′,5,5′tetramethylbenzidine were purchased from Aldrich. Hydrazobenzene was purchased from Fluka. 4,4′′-Diamino-p-terphenyl was obtained from Lancaster Synthesis Ltd. 1H NMR spectra were obtained on an Avance 400 spectrometer and referenced to the deuterated solvent. IR spectra were taken on a Thermo Nicolet Nexus 670 FT-IR spectrometer equipped with a liquid nitrogen cooled MCT-A detector. A Thermo Nicolet Smart OMNI-Sampler HATR (horizontal attenuated total reflectance) accessory was used for neat samples using 32 scans in the range of 4000-675-1 at a resolution of 4 cm-1. Polarizationmodulation infrared reflection-absorption spectroscopy (PMIRRAS)39 was performed on this same spectrometer equipped with a PEM module including a Hinds Instruments PEM-90 photoelastic modulator system. The difference reflectance measurements at 83° angle of incidence were collected for total of 1024 scans in the range of 4000-750 cm-1 at a resolution of 4 cm-1. The ratio taken between the spectra and those of previously scanned metal substrates was used to obtain the desired signal from the thin films. To record the grazing angle ATR (GATR) spectra, a Harrick Scientific Corp. accessory with a Ge hemi(31) Ansell, M. A.; Zeppenfeld, A. C.; Yoshimoto, K.; Cogan, E. B.; Page, C. J. Chem. Mater. 1996, 8, 591. (32) Ansell, M. A.; Cogan, E. B.; Page, C. J. Langmuir 2000, 16, 1172. (33) Bae, S. J.; Lee, C.-r.; Choi, I. S.; Hwang, C.-S.; Gong, M-s.; Kim, K.; Joo, S.-W. J. Phys. Chem. B 2002, 106, 7076. (34) Joo, S.-W.; Kim, W.-J.; Yoon, W. S.; Choi, I. S. J. Raman Spectrosc. 2003, 34, 271. (35) Kim, H. S.; Lee, S. J.; Kim, N. H.; Yoon, J. K.; Park, H. K.; Kim, K. Langmuir 2003, 19, 6701. (36) Lee, C.-r.; Kim, S. I.; Yoon, C-j.; Gong, M-s.; Choi, B. K.; Kim, K.; Joo, S.-W. J. Colloid Interface Sci. 2004, 271, 41. (37) Joo, S.-W.; Kim, Y.-S. Colloids Surf., A 2004, 234, 117. (38) Joo, S.-W.; Kim, W.-J.; Yun, W. S.; Hwang, S.; Choi, I. S. Appl. Spectrosc. 2004, 58, 218. (39) Buffeteau, T.; Desbet, B.; Turlet, J. M. Appl. Spectrosc. 1991, 45, 380.

Langmuir, Vol. 21, No. 11, 2005 5035 spherical ATR crystal at a 65° fixed incident angle was used to collect 32 scans in the range of 4000-675 cm-1 at a resolution of 4 cm-1. Baseline correction was performed for the spectral areas between 2400 and 1450 cm-1 using the Thermo Nicolet Omnic software. Ellipsometry. Ellipsometry measurements were obtained on a Gaertner L116S Stokes ellipsometer using a HeNe 6328 Å laser. The refractive index (ns) and extinction coefficient (ks) for the metal substrates were calculated using a least-squares fitting routine from three measurements of Ψ and ∆ each at six angles between 45° and 70°. The thickness of the absorbed organic layer was found by a similar fitting routine with the known ns and ks and using estimated values of n for the SAM molecules calculated from group contributions to the molar refraction40 using known n of model compounds. TMPDI. 2,3,5,6-Tetramethyl-p-phenylenediamine (1.64 g, 10 mmol) was dissolved in 200 mL of methylene chloride (CH2Cl2). Next, 200 mL of 45% potassium hydroxide solution was added followed by benzyltriethylammonium chloride (11.4 mg, 0.25 mol %). To the biphasic mixture was added chloroform (2.65 g, 22.2 mmol), and the reaction mixture was refluxed overnight. After cooling to room temperature, the mixture was transferred to a separatory flask, diluted with 400 mL of H2O, and extracted twice with H2O and once with saturated NaCl. The organic layer was dried over magnesium sulfate, filtered, and the solvent was removed by evaporation yielding 1.91 g of brown solid. The product was purified by column chromatography on silica gel using 1:3 ethyl acetate/hexane as eluent yielding 0.75 g of tan solid (41% yield) and further purified by vacuum sublimation. IR spectrum, (HATR): ν(NtC) 2110 cm-1. 1H NMR (CDCl3, 400 MHz) δ [ppm]: 2.39 (s, CH3). 13C NMR (CDCl3, 400 MHz) δ [ppm]: 16.58, 132.11, 141.89, 169.45. BXyDI. 3,3′,5,5′-Tetramethylbenzidine (1.00 g, 4.16 mmol) was dissolved in 200 mL (CH2Cl2). Next, 200 mL of 45% potassium hydroxide solution was added followed by benzyltriethylammonium chloride (4.9 mg, 0.25 mol %). To the biphasic mixture was added chloroform (1.1 g, 9.21 mmol), and the reaction mixture was refluxed overnight. After cooling to room temperature, the mixture was transferred to a separatory flask, diluted with 400 mL of H2O, and extracted twice with H2O and once with saturated sodium chloride. The organic layer was dried over magnesium sulfate, filtered, and the solvent was removed by evaporation yielding 1.23 g of orange solid. The product was further purified by crystallization from CH2Cl2 (92% yield) and vacuum sublimation. IR spectrum, (HATR): ν(NtC) 2121 cm-1. 1H NMR (CDCl3, 400 MHz) δ [ppm]: 2.46 (s, 12H, CH3), 7.25 (s, 4H, ArH). 13C NMR (CDCl3, 400 MHz) δ [ppm]: 19.31, 126.72, 135.68, 140.35, 169.02. TPDI. TPDI was prepared according to a published procedure.29 The product was further purified by sublimation. IR spectrum, (HATR): ν(NtC) 2130 cm-1. 1H NMR (CDCl3, 400 MHz) δ [ppm]: 7.48 (d, J ) 8.5 Hz, 4H, ArH), 7.65 (d, J ) 8.5 Hz, 4H, ArH), 7.67 (s, 4H, ArH). 13C NMR (CDCl3, 400 MHz) δ [ppm]: 127.14, 127.95, 128.17, 136.44, 137.86, 165.68. BPDI. Hydrazobenzene (18.42 g, 100 mmol) and 88% formic acid (100 mL) were stirred at room temperature under nitrogen overnight. The N,N′-biphenyl-4,4′-diyl-bisformamide was filtered free of a brown liquid, washed with water, and dried under vacuum to a tan solid which was purified by triturating with chloroform (200 mL), filtered, and dried under vacuum to yield 10.53 g of off-white solid (41% yield). The bisformamide (2.54 g, 10 mmol) was dissolved in CH2Cl2 (400 mL) with triethylamine (TEA) (90 mL) and benzyltriethylammonium chloride (0.45 g, 2 mmol) under nitrogen and cooled to 0 °C. A solution of triphosgene (5.93 g, 20 mmol) in 50 mL of CH2Cl2 was added dropwise over 2 h, and the reaction mixture was allowed to warm to room temperature overnight. The mixture was then transferred to a separatory flask and washed with DI H2O (3×). The aqueous washes were extracted with CH2Cl2, and the combined organics were dried over MgSO4, filtered, and evaporated under vacuum to 2.22 g of brown solid, which was redissolved in ethyl acetate and filtered through silica gel yielding 1.73 g of tan solid (85% yield from the bisformamide). This was further purified by (40) Van Krevelen, D. W. Properties of Polymers; Elsevier: New York, 1997.

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Scheme 1. Synthesis of Aromatic Diisocyanides from Aromatic Diamines via a Modified Hoffmann Carbylamine Reaction

Swanson et al. Table 1. IR Stretching Frequencies for Aromatic Diisocyanides, Dicarbonitriles, and Diisocyanates

aromatic structure p-phenyl 2,3,5,6-tetramethyl-1,4-phenyl 4,4′-biphenyl 3,3′,5,5′-tetramethyl-4,4-biphenyl 4,4′′-p-terphenyl

isocyanide ν(NtC) (cm-1)a

nitrile ν(CtN) (cm-1)

isocyanate ν(NdCdO) (cm-1)

2129 2110

2234b 2222d

2264c 2285e

2127 2121

2228b

2266c 2274e

2130

2265e

a HATR of sublimed samples. b SDBSWeb: http://www.aist.go.jp/ RIODB/SDBS/ (National Institute of Advanced Industrial Science and Technology), 2004. c Valli, V. L. K.; Alper, H. J. Org. Chem. 1995, 60, 257. d Suzuki, M.; Ikeno, T.; Osoda, K.; Narasaka, K.; Suenobu, T.; Fukuzumi, S.; Ishida, A. Bull. Chem. Soc. Jpn. 1997, 70, 2269. e Hogarth, G.; Humphrey, D. G.; Kaltsoyannis, N.; Kim, W.-S.; Lee, M. Y.; Norman, T.; Redmond, S. P. J. Chem. Soc., Dalton Trans. 1999, 2705.

Scheme 2. Synthesis of BPDI from Hydrazobenzene via Benzidine Rearrangement and Dehydration of the Bisformamide

vacuum sublimation. IR spectrum (HATR): ν(NtC) 2127 cm-1. 1H NMR (CDCl , 400 MHz) δ [ppm]: 7.46 (dd, J 3 A,B ) 8.7 Hz, JA,A′ ) 2.0 Hz, 4H, ArH), 7.56 (dd, JB,A ) 8.7 Hz, JB,B′ ) 2.0 Hz, 4H, ArH). 13C NMR (CDCl3, 400 MHz) δ [ppm]: 127.27, 128.34, 140.68, 165.66. Preparation of Substrates and Self-Assembled Monolayers (SAMs). Metal surfaces were prepared on silicon (100) wafers by coating with a 50 Å Cr adhesion layer followed by 1000 Å gold or palladium. Self-assembled monolayer SAMs for all molecules were formed by soaking overnight in approximately 15 mM dimethyl sulfoxide (DMSO) solutions of the diisocyanide. The substrates were then thoroughly rinsed twice with DMSO followed by ethyl acetate and blown dry in a nitrogen stream.

Results and Discussion Synthesis of Diisocyanides. The diisocyanides, TMPDI, BXyDI, and TPDI, were prepared directly from the corresponding diamines as shown in Scheme 1. BPDI was prepared as shown in Scheme 2 starting from hydrazobenzene via the benzidene rearrangement in formic acid to the bisformamide and from thence to the diisocyanide. IR Spectroscopy. IR spectroscopy is a useful tool for the analysis of isocyanide bonding because of the strong NtC stretching vibration (ν(NtC)). The bulk (HATR) IR spectra of the diisocyanides show only one ν(NtC) band for each molecule, and Table 1 shows the ν(NtC) peak position for each molecule. The addition of electrondonating methyl groups, which increases the π-acidity of the isocyanide,41 at both ortho positions, as in BXyDI, changes the frequency of this stretch by -6 cm-1 (ν(Nt C)BXyDI - ν(NtC)BPDI). When all positions are methyl (41) Wagner, N. L.; Laib, F. E.; Bennett, D. W. J. Am. Chem. Soc. 2000, 122, 10856.

Figure 1. (a) HATR spectra of PDI solid. (b) PM-IRRAS spectrum of PDI self-assembled on gold. (c) GATR spectrum of PDI self-assembled on gold. (d) PM-IRRAS spectrum of PDI self-assembled on palladium. (e) GATR spectrum of PDI selfassembled on palladium.

substituted, as in TMPDI, the frequency is decreased by 19 cm-1 (ν(NtC)TMPDI - ν(NtC)PDI). Examination of the literature, as summarized in Table 1, reveals a similar decrease of 12 cm-1 is upon methyl substitution in the ν(CtN) of aromatic nitriles. Aromatic isocyanates, however, show an increase in frequency for ν(NdCdO) stretching upon methyl substitution (8 cm-1 for ortho only and 21 cm-1 for both ortho and meta). Upon SAM formation on either gold or palladium, the PM-IRRAS and GATR spectra of the diisocyanides split into two or three ν(NtC) stretching bands as shown in Figures 1-5 and as summarized in Table 2. The PMIRRAS and GATR spectra are very similar, which is to be expected for monolayer films, because both techniques are sensitive to modes that have a transition dipole perpendicular to the metal surface. We began this study using the PM-IRRAS technique but found that the GATR gave almost identical information in less time with better signal-to-noise. Both spectra are shown for most experiments. In all cases, there is one band shifted slightly (by 2-9 cm-1) relative to the bulk compound. This has been assigned to the unbound isocyanide NtC stretch.24,29 This

Self-Assembled Diisocyanide Monolayer Films

Figure 2. (a) HATR spectra of TMPDI solid. (b) PM-IRRAS spectrum of TMPDI self-assembled on gold. (c) GATR spectrum of TMPDI self-assembled on gold. (d) PM-IRRAS spectrum of TMPDI self-assembled on palladium. (e) GATR spectrum of TMPDI self-assembled on palladium.

Figure 3. (a) HATR spectra of BPDI solid. (b) PM-IRRAS spectrum of BPDI self-assembled on gold. (c) GATR spectrum of BPDI self-assembled on gold. (d) PM-IRRAS spectrum of BPDI self-assembled on palladium. (e) GATR spectrum of BPDI self-assembled on palladium.

slight shift has been suggested to be the result of the conjugation through the molecule.26,35 The frequency of this unbound isocyanide band appears to be unaffected by the identity of the substrate metal but is still modified by the presence or absence of adjacent methyl groups (omethyl groups lower the frequency by 7-9 cm-1). The peak shape is sharp and very similar to that of the bulk material. The second band is assigned to the isocyanide, which is coordinated to the metal surface. The position of this second band is highly dependent on the metal composition and the bonding geometry. Isocyanides could bind to metal surfaces by any of the geometries shown in Figure 6.27,30 On gold, the position of ν(NtC) shifts 43-63 cm-1 higher to 2170-2190 cm-1, which has been identified as consistent with an end-on or terminal (η1) coordination of the isocyanide to the metal.24,27 These peaks are broader than those of the unbound isocyanides, which has been at-

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Figure 4. (a) HATR spectra of BXyDI solid. (b) PM-IRRAS spectrum of BXyDI self-assembled on gold. (c) GATR spectrum of BXyDI self-assembled on gold. (d) PM-IRRAS spectrum of BXyDI self-assembled on palladium. (e) GATR spectrum of BXyDI self-assembled on palladium.

Figure 5. (a) HATR spectra of TPDI solid. (b) PM-IRRAS spectrum of TPDI self-assembled on gold. (c) GATR spectrum of TPDI self-assembled on gold. (d) PM-IRRAS spectrum of TPDI self-assembled on palladium. (e) GATR spectrum of TPDI self-assembled on palladium. Table 2. IR Assignments for ν(NtC) of Diisocyanide SAMS on Au and Pd ν(NtC) (cm-1) compound

bulk (HATR)

unbound

bound to Au

bound to Pd

unknown

PDI TMPDI BPDI BXyDI TPDI

2129 2110 2127 2121 2130

2121 2112 2121 2114 2121

2172 2170 2190 2171 2185

1980 (br) 1985 (br) 1987 (br) 1983 (br) 2000 (br)

2272 2279 2270 2279 2274

tributed to the inhomogeneity of the surface adsorption sites.24 The effect of adjacent methyl groups on peak position is still evident. On palladium, however, the ν(NtC) positions shift 127149 cm-1 lower to 1980-2000 cm-1 and are very broad. In metal complexes with isocyanide ligands, the shift of

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Figure 6. Possible bonding geometries for isocyanide on a metal (M) surface.

Figure 7. PM-IRRAS spectra of BPDI self-assembled on gold heated at 60 °C in air for (a) 15 min, (b) 2 h; and heated in nitrogen for (c) 15 min, (d) 2 h.

the ν(NtC) peak upon metal coordination is attributed to the σ-donor and π-acceptor abilities of the metal-ligand bond.42 The monopalladium/2,6-dimethylphenyl isocyanide complex with terminal (η1) isocyanide structures have ν(NtC) bands between 2139 and 2192 cm-1,43,44 and the perpendicular (η1) adsorption of methyl isocyanide at high surface coverage on Pd(111) surface also has ν(NtC) bands that are very close to those of the gas-phase compound,13 so this geometry is unlikely. Likewise, the geometry of µ2-bridging isocyanides coordinated to two zerovalent metals or to Pd(111) at low surface coverage, which has ν(NtC) in the range of 1880-1580 cm-1,13,45 can also be eliminated. A bent µ1-η2 geometry, such as is found in tungsten-isocyanide complexes that have dominant π back-donation from the metal,41 has been assigned to the ν(NtC) band position at 1960 cm-1 for PDI SAMs on palladium.30 However, another interpretation can be obtained by comparison with isocyanide complexes with open-faced trinuclear metal clusters, in which the metal triangles are locked in place in a planar arrangement. These clusters have been found to be useful mimics of the chemical properties of (111) fcc and (0001) hcp metal surfaces.46 Ligands can bind to these open-faced clusters in the same geometries as described above for metal surfaces.47 In these open-faced palladium clusters, TMPDI has been found to bind in the µ3-η1 geometry with ν(NtC) (42) Ryu, H.; Knecht, S.; Subramanian, L. R.; Hanack, M. Synth. Met. 1995, 72, 289. (43) Rashidi, H.; Vittal, J. J.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans. 1994, 1283. (44) Tanase, T.; Ohizumi, T.; Kobayashi, K.; Yamamoto, Y. Organometallics 1996, 15, 3404. (45) Yamamoto, Y. Coord. Chem. Rev. 1980, 32, 193. (46) Puddephatt, R. J.; Manojlovic-Muir, L.; Muir, K. W. Polyhedron 1990, 9, 2767. (47) Bradford, A. M.; Kristof, E.; Rashidi, M.; Yang, D.-S.; Payne, N. D.; Puddephatt, R. J. Inorg. Chem. 1994, 33, 2355.

bands in the range of 1929-1988 cm-1, 48 which corresponds well to our observations. Thus, it is also possible that the aromatic isocyanides bind to palladium in the triple-bridging µ3-η1 geometry. The third broad band appears between 2270 and 2279 cm-1 and is unaffected by the metal composition. This third peak has been described previously for SAMs of PDI on gold, and an aggregated (antiparallel dimer) arrangement of PDI on the surface was hypothesized as the source of this peak.28 Another explanation for these higher frequency peaks could be the effect of a higher oxidation state for the coordinating metal atom. Yet, the ν(NtC) values of aromatic isocyanide ligands on Au(I) are found between 2220 and 2240 cm-1 25,49 and 2211 and 2220 for o-methyl substituted aromatic isocyanides.50 The increase (ca. 100 cm-1) in the ν(NtC) is attributed to the σ donation of the antibonding carbon lone pair to gold upon complexation,51 but this increase is still too low. Similarly, aromatic isocyanides coordinated to Pt(II) and Pd(II) ions display the ν(NtC) absorption in the range of 2200-2230 cm-1 with a ∆ν of ca. 80-100 cm-1 as expected for isocyanide coordinated to metal ions.52 Again, this is not in the range of the observed peaks. Notice also that the o-methyl substituent effect is inversed (o-methyl groups shift the position to higher frequencies) for this band, so it is unlikely that this band is due to an isocyanide stretch. However, we observe that the appearance and size of this peak can be amplified by illumination53 or heating in air. The photooxidation of isocyanides to isocyanates in (48) Rashidi, M.; Kristof, E.; Vittal, J. J.; Puddephatt, R. J. Inorg. Chem. 1994, 33, 1497. (49) Benouazzane, M.; Coco, S.; Espinet, P.; Martin-Alvarez, J. M.; Berbera´, J. J. Mater. Chem. 2002, 12, 691. (50) Irwin, M. J.; Jia, G.; Payne, N. C.; Puddephatt, R. J. Organometallics 1996, 15, 51. (51) Sarapu, A. C.; Fenske, R. F. Inorg. Chem. 1975, 14, 247. (52) Facchin, G.; Michelin, R. A.; Mozzon, M.; Pugliese, S.; Sgarbossa, P.; Tassan, A. Inorg. Chem. Commun. 2002, 5, 915.

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Table 3. Ellipsometric Film Thicknesses for Diisocyanide SAMs on Au and Pd

compound

n (calculated)

calculated thickness (Å)a

T on Au (Å)

T on Pd (Å)

PDI TMPDI BPDI BXyDI TPDI

1.55 1.54 1.60 1.59 1.63

10.3 10.2 14.7 14.6 19.0

12.7 17.4 13.9 23.9 35.1

10.4 12.5 12.0 39.9 19.3

a Theoretical thicknesses were calculated using CambridgeSoft Chem 3D software (http://www.cambridgesoft.com/).

Ellipsometry. The ellipsometry results are summarized in Table 3 and agree well with theoretical lengths calculated for PDI and BPDI on both surfaces. The small discrepancies can be attributed to the accuracy of the measurement technique for metal surfaces that are not perfectly flat, and to our choice of fitting procedure, which fixes n and varies only the thickness. In some cases, however, the measurement gives values much thicker than expected (TMPDI and TPDI on gold, and BXyDI on both substrates), so bilayers or aggregates may have formed. Conclusions

acetonitrile solution has been described,54 and evidence for thermal oxidation can be observed in Figure 7, which shows the IR spectra for BPDI SAMs on gold after heating at 60 °C in air or under nitrogen. Therefore, it is suggested that some of the unbound isocyanide groups have oxidized to isocyanate. This would explain the increase in frequency upon o-methyl substitution that is observed in aromatic isocyanates (see Table 1). The slight shifts (4-9 cm-1) of these peaks from those of the diisocyanate molecules are on the same scale and in the same direction (positive for TMPDI and negative for the rest) as the shifts of the unbound isocyanides upon metal coordination. The broadness of the peaks also is consistent with the isocyanate bands. (53) The illumination of the SAMs during ellipsometry measurements was sufficient to observe this peak. This explains the difference between the GATR and PM-IRRAS spectra for the PDI SAM on Pd shown in Figure 1. (54) Boyer, J. H.; Ramakrishnan, V. T.; Srinivasan, K. G.; Spak, A. J. Chem. Lett. 1981, 43.

IR and ellipsometry have been used to study SAM formation of diisocyanides on gold and palladium. The ellipsometric measurements indicate the formation of a densely packed organic layer at the metal surface, and the IR data show that the molecules have a high degree of alignment perpendicular to the interface. The IR data suggest that the isocyanide SAMs bind on gold with terminal η1 geometry, but the geometry on palladium is different and is possibly a triply bridging µ3-η1 geometry. An additional peak at high frequency has been identified as an isocyanate group resulting from either photo- or thermal oxidation. Acknowledgment. This work was partially supported by a grant from the National Science Foundation (CHE-9625628). LA047284B