Equilibrium and Saturation Coverage Studies of Alkyl and Aryl

Erik R. Klobukowski , Robert J. Angelici , and L. Keith Woo ... Mihaela Lazar, Bolin Zhu, and Robert J. Angelici. The Journal .... Song Lin and Robin ...
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Langmuir 1995,11, 2539-2546

2539

Equilibrium and Saturation Coverage Studies of Alkyl and Aryl Isocyanides on Powdered Gold Kuo-Chen Shih and Robert J. Angelici" Department of Chemistry and Ames Laboratory, Iowa State University, Ames, Iowa 50011 Received August 1, 1994@ Isocyanides (CsN-R) adsorbed from 1,2-dichloroethane(DCE)solution on powdered gold were studied by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Concentration measurements showed that adsorption equilibrium constants decreasein the order: CNPh > CNCHzC(0)OEt> 4-CNCeH4NO2 > CNBu". This is a differentorder than that (CNBut CNBu" > CNCHzC(0)OEt> CNPh > 4-CNC&NOz) observed for the binding of isocyanidesin the complex AuCl(C=N-R); this latter order indicates that the isocyanides with the highest a-donor ability bind the most strongly. At saturation coverage, the number of moles of adsorbed CEN-R per gram ofAu decreases as the size of the R group increases: CNBu" > cN(c-C~H11)> CNBut > 2,4,6-CNC6H2(But)3. For the least bulky isocyanide (CNBu"), the ratio of CNBu" molecules to surface Au atoms is approximately U3.9.From calculated cross-sectional areas of the R groups in the isocyanides, it is estimated that two-thirds of the surface area is covered at saturation coverage for all of the isocyanides. Gold powder that is surface-oxidizedby Clz adsorbs CNBunbut AuCl(CNBu") desorbs into the DCE solution.

-

Introduction An understanding of the interactions of organic molecules with surfaces of metallic solids is essential in many areas of chemistry including heterogeneous cata1ysis.l Most metals, however, form surface oxides when exposed to 02.2,3 Therefore studies on metals are carried out under conditions of ultrahigh vacuum or in a reducing atmosphere. On the other hand, gold metal is known to be resistant to oxidation; thus, it can be handled easily under atmospheric conditions. Molecular 0 2 adsorbs only a t low temperature (ca. 150 KI3 on clean gold surfaces, and no surface oxide form^.^,^ Water4b,4c,6 and ethylene6,Ido not bind to gold surfaces a t or near room temperature. However, extensive studies show that alkanethiols (RSH) and disulfides (RSSR) in solution chemisorb strongly on gold films to give RS groups on the surface.8 Triphenyl compounds of the group 15 elements (i.e., NPh3, PPh3,

*

Abstract published in Advance A C S Abstracts, May 1, 1995. (1) (a) Campbell, I. M. Catalysis at Surfaces; Chapman and Hall: New York, 1988. (b) Bond, G. C. Heterogeneous Catalysis, 2nd ed.; Oxford Science Publication: Oxford, UK, 1987. ( c ) Muetterties, E. L. Angew. Chem., Int. Ed. Engl. 1978, 17, 545-620. (2) Andreoni, W.; Varma, C. M. Phys. Rev. B 1981,23, 437. (3) Kamath, P. V.; Yashonath, S.; Srinivasan, A.; Basu, P. K.; Rao, C. N. R. J . Indian Chem. SOC.1982, 59, 153. (4) (a)Trapnell, B. M. W. Proc. Roy. SOC.London, Ser. A. 1953,A218, 566-577. (b) Chesters, M. A,; Somorjai, G. A. Surf. Sci. 1976,52,2128. ( c ) Schrader, M. E. J . Colloid Interface Sci. 1977,59,456-460. (d) Schrader, M. E. Surf. Sci. 1978, 78, L227-232. (e) Canning, N. D. S.; Outka, D.; Madix, R. J. Surf. Sci. 1984, 141, 240. (0Pireaux, J. J.; Chtaib, M.; Delrue, J. P.; Thiry, P. A.; Liehr, M.; Caudano, R. Surf. Sci. 1984,141,211. (g) Sault, C. R.; Madix, R. J.;Campbell, C. T. Surf. Sci. 1986,169, 347. ( 5 ) (a) Brundle, C. R.; Roberts, M. W. Surf. Sci. 1973,38, 234. (b) Kay, B. D.; Lykke, K. R.; Creigh, J. R.; Ward, S.J. J . Chem. Phys. 1989, 91, 5120. (c) Wells, R. L.; Fort, T., J. R. Surf. Sci. 1972, 32, 554. (6) Franken, P. E. C.; Ponec, V. Surf. Sci. 1976, 53, 341. (7) Outka, D. A.; Madix, R. J. J . A m . Chem. SOC.1987, 109, 1708. (8) (a) Ulman, A. A n Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly;Academic Press Inc: New York, 1991. (b) Bain, C. D.; Whitesides, G. M. J . Am. Chem. SOC.1988,110, 3665. (c) Bain, C. D.; Evall, J.; Whitesides, G. M. J . Am. Chem. SOC. 1989, 111, 7155. (d) Amdt, T.; Schupp, H.; Schrepp, W. Thin Solid Films 1989,178,319. (e)Bryant, M. A,; Pemberton, J. E. J . Am. Chem. SOC.1991,113,8284, (0Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990,6,682-691. (g) Laibinis, P. E.; Whitesides, G. M. J . Am. Chem. SOC.1992,114, 1990. (h) Alves, C. A.; Porter, M. D. Langmuir 1993, 9,3507. (i)Alves, C. A.; Smith, E. L.; Porter, M. D. J . A m . Chem. Chem. 1992,114, 1222. (i) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. SOC.1993,115,9389.

0743-7463/95/2411-2539$09.00/0

AsPh3, SbPh3, and BiPh3),9 P [ ( C H ~ ) I & H ~and ] ~ , long~~ chain alkanenitriles and dinitriles'O in solution also adsorb on gold. Gaseous CO adsorbs to give CO groups that are bonded terminally to one Au atomell Previously, a brief report described the adsorption of the isocyanide CN(CHZ)ZZCH~ on a gold film.8c And we reported in an earlier paper12 DRIFTS studies of isocyanides (CNPh, 1,4-(cN)zC&, CNBut) adsorbed from methanol solution on Au powder. These studies showed that the isocyanides are bonded terminally to one Au atom through the carbon atom, and there was no evidence for bridging isocyanides. The DRIFT studies also indicated that the amounts of the isocyanides adsorbed increase as the concentrations of the isocyanides in solution increase; at higher concentrations, the Au surfaces become saturated with the isocyanides. In the present paper, we describe equilibrium studies of the adsorption of isocyanides (CrN-R) on powdered Au as a function of the electronic properties of the R group. Also reported are investigations of the steric effect of the R group on the amount of isocyanide that can be adsorbed.

Experimental Section The isocyanides CNPh,134-CNCW02,14J5and 2,4,6-CNC,&(But)316J7were prepared by literature methods. CNBut and CNBu" were purchased from Aldrich. CN(c-&H11) was purchased from Strem. AuCl(CNR)(CNR = CNPh, 4-CNeHJV02, CNBut,CNBun,or CNCH2C(O)OEt)were prepared by literature (9) Steiner, U.B.; Neuenschwander, P.; Caseri, W. R.; Suter, U.W. Langmuir 1992,8,90-94. (10)Steiner, U.B.; Caseri, W. R.; Suter, U.W. Langmuir 1992, 8, 2771-2777. ( l l ) ( a ) Kavtaradze, N. N.; Sokolova, N. P. Russ. J . Phys. Chem. 1962,36, 1529. (b) Guerra, C. R.; Schulman, J. H. Surf. Sci. 1967, 7, 229. (c) Yates, D. J. C. J . Colloid Interface Sci. 1969, 29, 194. (d) Bradshaw, A. M.; Pritchard, J. Pro. Roy. SOC.London, A 1970,316,169. (e) Kottke, M. L.; Greenler, R. G.; Tompkins, H. G. Surf. Sci. 1972,32, 231. (0 Schwank, J.;Parravano, G.; Gruber, H. L. J . Catal. 1980,61, 19-28. (g)Dumas,P.;Tobin,R.G.;Richards,P.L.Surf.Sci.1986,171, 579. (12) Robertson, M. J.; Angelici, R. J . Langmuir 1994, 10, 1488. (13) Weber, W. P.; Gokel, G . W.; Ugi, I. K. Angew. Chem., Int. Ed. Engl. 1972, 11, 530. (14) Obrecht, R.; Henmann, R.; Ugi, I. Synthesis 1986,400. (15) Barton, D. H. R.; Ozbalik, N.;Vacher, B. Tetrahedron 1988,44, 3501

(16) Burgers, J.;Hoefnagel,M.A.;Verkade,P.E.;Visser, H.; Wepster, B. M. Recl. Trav. Chim. Pays-Bas. 1968, 77, 491. (17) Yamamoto, Y.; Aoki, K.; Yamazaki, H. Inorg. Chem. 1979,18, 1681.

0 1995 American Chemical Society

2540 Langmuir, Vol. 11, No. 7, 1995 methods fromAuCl(SMe2).18Jg 1,2-Dichloroethane(DCE, 99+%) was purchased from Aldrich. The Au powder was prepared a s described previously.12,20 Routine BET surface area measurements were performed on a Micromeritics AccuSorb 2100E instrument. With Kr as the adsorbate gas a t 77 K, the surface area of the Au powder used in these studies was determined to be 0.27 mVg. The surface area ofthe standard ( A l 2 0 9 ) measured a t the same time was 0.46 m2/g; the accepted value for this standard is 0.45 f 0.03 m2/g. FTIR and DRIFT spectroscopy. All infrared spectra were recorded on a Nicolet 710 spectrophotometer equipped with a TGS detector in the main compartment and a MCT detector in the auxiliary experiment module (AEM). The AEM housed a Harrick diffuse reflectance accessory. All the solution IR spectra were recorded using a NaCl cell (0.50 mm) in the main compartment with the instrument set for 4 cm-l resolution and 128 scans. All DRIFT spectra were recorded with the samples in the Harrick microsampling cup with 600 scans and 4 cm-1 resolution. The background used for the DRIFT spectra was clean gold powder. Spectra were routinely baseline corrected and purge corrected. The abscissa of the spectra were recorded and are reported in absorbance units even though the radiation collected is actually reflectance. Kubelka-Munk transformations of the spectra were not possible due to the low intensities of all bands; the bands were eliminated by the transformation. The pseudo-absorbance units used do appear t o obey Beer's law, a t least approximately (vide infra). However, measurements of the amounts of the isocyanides on Au using DRIFTS are not necessarily expected to be quantitative.21

General Procedures for the Adsorption of Isocyanides on Au Powder. Stock solutions of isocyanides in DCE were prepared using 25.00 mL volumetric flasks. Typically,a solution of the desired concentration of isocyanide in DCE (0.50 mL) was mixed with the Au powder (300 mg) in a test tube, followed by shaking for 60 s on a Genie vortex mixer and then centrifuging for 10 min a t room temperature. The concentration of the CNR in the separated liquid was analyzed by the intensity of its v(CN) band. Calibration plots of the v(CN) absorbances versus their solution concentrations (standard error is 0.01 mM) are linear. They all follow Beer's law in the range studied (0.10-10 mM). The gold powder was dried under oil pump vacuum (typically 10 min), and exactly 50 mg of it was then introduced through a microfunnel into a microsampling cup for DRIFT analysis.

Reaction of CNBu"and Au Powder Pretreated with Clz. Au powder (300 mg) was placed in a Schlenk flask. After degassing the flask with Nz, 30 p L (1.2 x 10+ mol) (room temperature and pressure) of Clz was syringed into the flask. After shaking for 10 min, the flask was pumped under vacuum for 1min and filled with Nz. A solution of CNBu" (10mM) in DCE (0.50 mL) was syringed into the flask for adsorption studies as described above.

Reaction of AuCl(SMe2)with Au Powder Covered with CNR. Au powder (300 mg) was mixed with a solution of CNBun (5.0 mM) in DCE (0.50 mL) in a test tube. After shaking for 1 min and standing for another 10 min, the Au solid was separated from the liquid by centrifugation and dried under vacuum for 10 min. The Au solid was then mixed for 10 min with a solution ofAuCl(SMe2)(2.0 mM) in DCE (0.50 mL). After centrifugation, the DRIFT spectrum of the solid and infrared spectrum of the solution were obtained.

Measurements of Equilibrium Constants for the Exchange of CNR in AuCl(CNR). A solution of AuCl(CNBut) (2.0 mM) and CNR (R = Bun, CHZC(O)OEt, Ph, and 4-NO&&) (2.0 mM) in DCE (3.0 mL) was stirred a t room temperature (23 "C) and monitored by FTIR. The reaction reached equilibrium within 10 min after which no change occurred in the IR spectrum of the solution. Similar experiments were carried out with 2.0 mMAuC1(CNBut)and 1.0 mM CNR. Concentrations oftheAuC1(CNR) and CNR species in solution were determined by the intensities of their v(CN) absorptions and their extinction coefficientsin units of cm-1 M-l: for AuCl(CNR),where R = But (7801, Bun (5401, CHzC(0)OEt (3801, Ph (890) and 4-NOzCsH4 (18)McCleverty, J. A,; de Mota, M. M. M. J . Chem. Soc., Dalton Trans. 1973, 2571. (19)Phillips, F. C. J.A m . Chem. Soc. 1901,23, 257. (20) Block, B. P. Inorg. Synth. 1953,4, 14. (21) Mackenzie, M. W. Advances in Applied Fourier Transform Infrared Spectroscopy; John Wiley & Sons: New York, 1988.

Shih and Angelici

1 1

i

0.007

0.006

0.0

0.6

1.0

1.6

2.0

8.0

2.6

8.6

Concenttationof CNR (mM) in DCE bebm adsorption

Figure 1. Intensities of v(CN)bands in DRIFT spectra of CNPh

and 4-CNCeHfiO2 adsorbed on Au powder (300 mg) with increasing concentrations of CNR in DCE solution.

Table 1. v(CN) Frequencies for CNR,cm-l

CNPh 4-CNCsH4NOz CNBut CNBu" CNCHzC(0)OEt

in DCE solution 2129 2127 2138 2151 2163

adsorbed on Aua 2190 2187 2207 2223 2232

AuCl(CNR)b 2225 2220 2238 2254 2261

At saturation coverage. In DCE solution. (1400); for CNR, where R = But (2501, Bun (200), CHZC(0)OEt (280), Ph (320), and 4-NOzCsH4 (600).

Results In general, adsorptions of the isocyanides on powdered gold were performed by shaking the Au in 172-dichloroethane (DCE) solutions (0.50 mL) of the isocyanides CNR (R = Ph, 4-NOzCsH4, Bu", But, C - C ~ H I2,4,6-CgH2(But)3, ~, and CHzC(0)OEt)a t room temperature. After separating

the powder from the solution, the DRIFT spectrum of each Au sample revealed a v(CN) band in the region between 2100 and 2300 cm-l. The position of this band remains the same within f 2 cm-1 over the concentration range shown in Figure 1. The band was 70-40 cm-l higher than that of the free CNR (Table 1). As discussed in the previous paper,l2 the position of these bands indicates that the isocyanides are end-on bonded through the carbon atom to one Au atom on the surface. In some of the DRIFT spectra there is a weak v(CN)band in the region near that of the free CNR, which indicates the presence of more than a monolayer of the isocyanide. The random occurrence and very weak intensity of this band suggests that the Au powder was not completely separated from the CNR/DCE solution in these experiments. Also shown in Table 1are v(CN)values for the complexes AuCl(CNR)in which the isocyanide is end-on bonded through the carbon, as established for the structure of (NC)Au(CNCH3).22 The 4CN) values for the isocyanides in the compounds are approximately 100 cm-l higher than those of t h e free isocyanides. The solutions separated from the Au(s)/(CNR)powder were analyzed for the amounts of CNR by measuring the intensities of the d C N ) bands and converting them to CNR concentrations using Beer's law. Subtracting these concentrations from the original concentrations of the CNR (22) Steinar, E. Acta Chem. Scand. 1976, A 30, 527.

Langmuir, Vol. 11, No. 7, 1995 2541

Studies of Isocyanides on Powdered Gold Table 2. Adsorption of Varying Amounts of CNPh (a) and 4-CNC&NOz (b) from 0.5 mL of DCE on Au Powder (300 mg) [CNR] (mM) mol of CNR [CNRI (mM) intensity of adsorbed v(CN) of CNR after before

adsorption

from DRIFTS

adsorption

(a) CNPh 0.0104 2.20 0.0102 1.22 0.0100 0.50 0.0098 0.27 0.0087 0.15 0.0072 0.07 (b) 4-CNCsH4N02 0.0120 2.28 0.0119 1.27 0.0111 0.85 0.0105 0.43 0.0100 0.34 0.0091 0.09

3.00 2.02 1.27 0.98 0.79 0.57 2.92 1.90 1.47 0.97 0.85 0.51

on 1g of Au

1.33 x 1.33 x 1.28 x 1.18 x 1.07 x 0.87 x

h t 0'5

1.07 x 1.05 x 1.03 x 0.90 x 10-6 0.85 x 0.70 x

solutions permits the calculation of the amount of CNR adsorbed on the Au surface. Test of the bversibility of the Adsorption (eq 1). In order to determine if a Au surface containing one preadsorbed isocyanide will react with another isocyanide in solution to give a n equilibrium distribution of the two isocyanides, we performed the experiments outlined in eqs 2 and 3. Two samples of Au powder (300 mg) were Au(s)/(CNBu') 2.6 x 1O-4mmol

+

CNBP(DCE)

1-

4.7 x 1O-4mmol

in 0.50 mL

preadsorbed

A

Au(s)/(CNBP) 4.7 x 10-4mmol

preadsorbed

+

CNBut(DCE) 2.6 x 10-4mmol in 0.50 mL

(2)

Au(s)/(CNBU~)(CNBU') 3.4 x 10-4mmol CNBu" (3) 1 .O x 10-4mmol CNBd

reacted separately with solutions (0.50 mL) of CNBut (5.0 mM) and CNBu" (5.0 mM), respectively. The solution IR mmol) of CNBut spectra showed that 0.52 mM (2.6 x and 0.93 mM (4.7 x mmol) of CNBu" were adsorbed. After removing the CNBut solution by centrifugation, the Au(s)/(CNBut)sample was treated with 0.50 mL of a 0.93 mM CNBu" solution in DCE (eq 2). The Au(s)/(CNBun) separated from the CNBu" solution was treated similarly with 0.50 mL of a 0.53 mM CNBut solution in DCE (eq 3). Infrared spectra of the solutions from the reactions in eqs 2 and 3 reveal that the same amounts of CNBut (0.32 mM) and CNBu" (0.27 mM) are present in both solutions, indicating that both reactions are in equilibrium. The reactions were complete within 10min since the intensities of the two v(CN) bands were unchanged even after 60 min. The DRIFT spectra of these two Au samples separated from the solutions also show the same 4CN) band a t 2222 cm-l. These results strongly suggest that an equilibrium exists between the free isocyanide in DCE and the adsorbed isocyanide on the Au surface. Although the isocyanides readily displace one another from the surface, washing of the surface by solvent does not remove all of the isocyanide. When Au powder covered with CNBu" was washed with DCE or CHsOH, it was found, on the basis of DRIFT spectra of the powder, that not all of the CNBu" was removed after several washings. After three washings with 0.5 mL of DCE, approximately one-third of the CNBu" remained on the surface. However, CNBu" at saturation coverage on Au is completelyremoved with one washing with a DCE solution of AuCl(SMe2);in the DCE wash solution AuC1(CNBun)is identified by its v(CN) band. Adsorption of Varying Amounts of CNR. Gold powder (300 mg) was shaken with 0.50 mL of DCE

I 1 0.0

2.6

0.0

0.6

1.0 1.6 2.0 [CNR](mM) InDCE after adsorption

Figure 2. Amounts of CNPh and 4-CNCsHfi02 adsorbed on mol/g) with increasing concentrationsof CNR in DCE solution after adsorption.

Au powder (

solutions of CNR (R = Ph and 4-N0&&4) ranging in concentration from 0.50 to 3.00 mM. DRIFT intensities of the v(CN) bands of the isocyanides adsorbed on the Au surface increased as the solution concentrations increased (Table 2). From the concentration of the isocyanide present in solution after reaction as determined by IR spectroscopy, the amount of the isocyanide adsorbed on 1 g of the Au powder was obtained. Figure 1shows DRIFT intensities of the v(CN) bands of CNR (R = Ph and 4-NOzCsHd) adsorbed on the Au with increasing concentrations of the isocyanide solutions used. For each isocyanide, the v(CN) band intensity reaches a plateau which indicates that the surface of the Au powder is saturated with isocyanide a t this point. This result is confirmed by FTIR studies of the solutions in these reactions which show (Figure 2) that the amount of the isocyanide adsorbed on Au also reaches a plateau. This occurs when the concentration of the 4-CNCsH4N02 in solution is larger than ca. 1.5 mM; for CNPh, it occurs above ca. 1.0 mM. The fact that a less concentrated CNPh solution is required to cover the surface of the Au implies (vide infra) that CNPh binds more strongly than 4-CNC6H4N02 to the Au. Langmuir Treatment of Adsorption Data. In order to determine quantitatively the relative binding abilities of CNPh and 4-CHC6H4N02, the Langmuir equation (eq 4) was sed.^^,^^ In this equation, K1 is the equilibrium

Kl = S/[Cl(n,,

- S)

(4)

constant for the adsorption of the isocyanide in solution on the Au powder (eq l ) , S is the number of moles of the isocyanide on 1 g of Au, [C] is the equilibrium molar concentration of the isocyanide in solution, and nl, is the total number of moles of adsorption sites per gram of Au. The term (nl, - S ) gives the number of free sites a t equilibrium. Rearrangement of eq 4 produces the familiar form of the Langmuir equation (eq 5).26For a homogeneous

[CYS = 1/(K1n1J + [CI/n1,

(5)

adsorbent with only one type of adsorption site, a plot of [C]/S vs [C] gives a straight line over the entire concentration range. For a heterogeneous adsorbent with more than one kind of adsorption site, curvature is found in the - - - .-.~ (23)Young, D. M.; Crowell, A. D. Physical Adsorption of Gases; Butterworths: Washington, DC, 1962. (24) Lim, Y. Y.; Drago, R. S.;Babich, M. W.; Wong, N.; Doan, P. E. J.Am. Chem. SOC.1987,109, 169. (25)Adamsoq A. W. Physical Chemistry of Surfaces, 3rd ed.; Interscience: New York, 1976.

2542 Langmuir, Vol. 11, No. 7, 1995

Shih and Angelici Scheme 1

Au(s) t CNPh t 150 mg 11.8 x lO-4mmol in 0.5 mL

(AIa

9.6 x mmol in 0.5 mL

CNCH2CO2Et 3.4 x lO-4mmol in 0.5 mL

3.1 x

DCE = Au(s)/(CNPh)(CNCHzCO2Et)

2.2 x 0.3 x

mmol

in 0.5 mL

mmol of CNPh mmol of CNCH2CO2Et

aBefore adsorption. bAfter adsorption. DCE

Au(s) t C N C G H ~ N O ~ t CNCHzCO2Et 150 mg 12.0 x mmol 3.6 x mmol in 0.5 mL in 0.5 mL

(A)a

10.4 x 10-4"01 in 0.5 mL

(B)b

2.7 x 10-4"01 in 0.5 mL

2.0

1.6

1.0

0.5

0.0 0.0

0.5

1.0

1.I

2.0

2.5

rc1(mM) Figure3. Langmuirplots (eq 5 )of [CYS vs [Cl for the adsorption of CNPh and 4-CNCsHdN02 on Au powder.

plot with regions approximating straight lines with different slopes.26 Values of [Cl and S were calculated from the data in Table 2 based on the intensities of v(CN) in solutions in which 300 mg of Au was stirred with 0.50 mL solutions (0.50-3.0 mM) of the isocyanide in DCE. The isocyanide adsorption isotherms are plotted in Figure 3. The linear plots of [CYS vs [Cl demonstrate that type I behavior is followed according to the classification scheme of Brunauer.25 Based on eq 5, nl, and K1 can be obtained from the intercepts and slopes of these plots. As shown in Table 3, the equilibrium constant (K1)for CNPh adsorbed on Au is 2.3 times larger than that of 4-CNCsH4N02; nl, for 4-CNCsH4N02 appears to be slightly smaller than that for CNPh. Similar Langmuir isotherm studies for alkyl isocyanides were unsuccessful due to the low extinction coefficients of their v(CN) bands in the solutions.27 The relative binding abilities of CNPh and 4-CNCsH4NO2 on Au were also measured indirectly in competition reactions (eqs 6 and 7, Scheme 1) between CNPh and 4-CNC&N02 with CNCH2C(O)OEton Au powder in the following two experiments: (A) A mixture of CNPh (2.36 mM) and (ethoxycarbony1)methyl isocyanide (0.67 mM) in DCE (0.50 mL) was reacted with Au powder (150 mg). After shaking, IR studies of these solutions indicated that there was 0.43 mM (2.2 x loT4mmol) CNPh and 0.06 mM (0.3 x mmol) (ethoxycarbony1)methyl isocyanide adsorbed on the surface of Au from the solution. (B)For (26) Sincar,S. J. Chem. SOC.,Faraday Trans. I 1984,80,1101.

= Au(s)/(CNC~H~NO~)(CNCH~CO~E t) 1.6 x 0.9 x

(7)

mmol of CNCeH4N02 mmol of CNCHzC02Et

the similar reaction between the mixture of 4-CNCsH4NO2 (2.39 mM) and (ethoxycarbony1)methyl isocyanide (0.71 mM) with Au powder (150 mg), 0.32 mM (1.6 x mmol) 4-CNCsHfi02, and 0.18 mM (0.9 x mmol) (ethoxycarbony1)methyl isocyanide adsorbed to the surface. Comparing the results of experiments A and B, it is evident that more CNPh (0.43 mM, 2.2 x mmol) adsorbed onto the Au powder than 4-CNCsH4N02 (0.32 mM, 1.6 x mmol) from the mixtures of CNPh and 4-CNCsH4N02 with the same concentration of the ester isocyanide. Supporting this conclusion is the observation that a smaller amount of ester isocyanide (0.06 mM, 0.3 x mmol) was adsorbed from the solution containing CNPh than from that containing4-CNCsH4N02(0.18mM, 0.9 x mmol). Additional evidence comes from DRIFT spectra of the two Au samples: In experiment B, two bands were observed a t 2180.2 and 2228.7 cm-l, corresponding to 4-CNCsH4N02 and the ester isocyanide, respectively. However, in experiment A, only one band a t 2186.5 cm-l was observed; this band is due primarily to the v(CN) of CNPh, the small amount of the ester isocyanide shifting it to lower wavenumber. All of these results support the conclusion that CNPh adsorbs more strongly than 4-CNCsH4NO2. Competition Reactions of Other Isocyanides. In order to compare the relative binding abilities of a broader range of isocyanides, studies of Au powder (300 mg) with mixtures of CNR (R = Ph, 4-CsH4N02, CH2C(O)OEt)and CNBu" in DCE were performed. The total concentration of the CNR and CNBu" isocyanide mixtures was about 5 mM, but the ratio of the isocyanides varied from 6:l to 1:2. DRIFT spectra (Figure 4) of the Au powder obtained from reactions with mixtures of CNBu" and CNPh show two v(CN)bands of adsorbed CNBu"(2221cm-l) and CNPh (2190 cm-'). The changes in relative intensities of these two bands are consistent with changes in the relative amounts of these two isocyanides in DCE, measured by solution IR spectroscopy. Similar results (Figure 5 ) are observed for the Au powder obtained from reactions with mixtures of CNBu" and 4-CNCsH4N02. For the reactions of Au with mixtures of CNBu" and CNCH2C(O)OEt,an unresolved band is observed in the v(CN) region of the DRIFT spectra. However, the v(C=O) band of CNCH2C(0)OEt a t 1750 cm-I (Figure 6) increases in intensity as the amount of the ester isocyanide on the Au surface increases. It should be noted that the v(C=O) value of (27) Malatesta, L.; Bonati, F. Isocyanide Complexes of Metals; John Wiley & Sons Ltd.: 1969.

Studies of Isocyanides on Powdered Gold

Langmuir, Vol. 11, No. 7, 1995 2543

Table 3. Langmuir Results (eq 5) Obtained from Figure 3

intercept

CNR

CNPh 4-CNCsHdNOz a

slope 7.37 (f 0.03) 105 9.02(i0.11) 105

2.7 (f0.4) x lo4 7.1 (f 1.3)x lo4

For eq 1. Units of M-l.

10-3 K , ~ J 27 f 4 12 f 2

nlsC 1.36 (f 0.05)x 1.11 (f 0.12)x 10-6

Moles of adsorption sites per gram of Au.

n

-\

1 : 1.1

A

1 : 1.3

2 A70

WAVENUMBER 9

9

Figure 4. DRIFT spectra of various compositions of CNBun

and CNPh adsorbed on Au powder.

n

Figure 6. DRIFT spectra of various compositions of CNBu*

and CNCH&(O)OEt adsorbed on Au powder.

In a n attempt to quantify the relative binding abilities of two isocyanides to Au powder, the Langmuir approach was applied. If two isocyanides, a and b, in solution are in equilibrium (eq 8) with the adsorbed isocyanides, the 1 :0.23

1 :0.64

1 :1.3

0:l WWENUHBER

Figure 6. DRIFT spectra of various compositions of CNBu"

and 4-CNCsH4N02 adsorbed on Au powder. the adsorbed ester isocyanide is the same as that (1750 cm-l) of the free ester isocyanide in DCE solution, thus indicating that the ester group is not coordinated to the Au surface.

ratio (Kab,eq 9) of the equilibrium constants for isocyanides can be derived from eq 4. This approach assumes that isocyanides a and b have the same nl, values. That this is nearly true is indicated by the following measured nls moYg, Table 31, CNC6H4N0z values: CNPh (1.36 x (1.11x Table 31, CNBu" (1.6 x Table 51, and CNCHzCOzEt (1.5 x In eq 9, K, and Kb are the equilibrium constants for the reactions (eq 1)ofisocyanides a and b with the surface of the Au, and Kab is the ratio of these equilibrium constants. Saand S b are the amounts (mmol) of the isocyanides adsorbed on the Au surface. C , and c b indicate the concentrations (mM) of isocyanides a and b in solution a t equilibrium. From measurements of the intensities of the v(CN1 bands of the isocyanides in the solutions, C , and c b were determined; from these values and knowing the original concentrations of a and b in solution, Saand s b were calculated. These results together with Kab values (eq 9) for competitive reactions of4-CNC6HaOz, CNPh, and CNCHzC(0)OEtwith CNI3u" (isocyanide b) are given in Table 4. The results show that

Shih and Angelici

2544 Langmuir, Vol. 11, No. 7, 1995 Table 4. Competitive Adsorption (eq 9) of 4-CNCd&NOz (a), CNPh (b), and CNCH&(O)OEt (c) with CNBunon Au PowdeP experiment number 2 3

1

CNO;

0.51 3.57 7.5 x 10-5 3.3 x 10-4 0.19 1.6

CBu" sNO;

sBu"d DNO; K,J

0.33 C B ~ " * 3.64 sphh 1.7 x 10-4 sBu*d 2.3 x 10-4 0.42 Dph' 7.9 K,hf cphg

0.72

Cestej

C B ~ " 3.31

sesterk2.0 x

10-4 2.6 x 10-4

sBund Desterl

0.43

(a) 4-CNCsH4N02 1.30 2.88 2.65 2.38 1.6 x 10-4 1.9 x 2.4 x 10-4 2.2 x 0.39 0.47 1.3 0.70 (b) CNPh 0.84 0.68 4.00 2.19 2.4 10-4 2.6 1.9 x 10-4 1.6 x 0.55 0.62 5.2 5.9 (c) CNCHzC(0)OEt 1.12 1.42 1.80 3.15 2.6 x 10-4 3.2 x 2.3 x 10-4 1.8 x 0.53 0.64 3.2 2.3

4 2.78 1.10 2.2 x 10-4 1.7 x 10-4 0.57 0.51

10-4 10-4

2.17 2.15 3.3 10-4 1.0x 10-4 0.76 3.2

10-4

10-4

2.85 1.73 3.7 x 10-4 1.2 x 10-4 0.76 1.9

10-4

10-4

3.5 K,J a CNO is~[4-CNCsH4N021(mM) in DCE. C B ~is" [CNBu"] (mM) in DCE. S N O is the ~ mmoles of 4-CNCsH4N02 adsorbed on 300 mg of Au. S B ~is"the mmoles of CNBu" adsorbed on 300 mg of Au. e DNO* = SNO~/(SNO~ + SB~"). fKabis calculated from eq 9. g c p h is

[CNPh] (mM) in DCE. s p h is the mmoles of CNPh adsorbed on Ceskris [CNCHzC(O)OEtI 300 mg of Au. Dph = Sph/(Sph + is the mmoles of CNCHzC(0)OEt adsorbed (mM) in DCE. Seater on 300 mg of Au. Dester = SesteASester + SB~"). J

8.0

M4 4.0

6.0

2.0

it

CNC~C(0)OEVCNBu"

\

CNC,H,NOdCNBu"

0

I

It

0

0

0 0

0

e 0

i

.

0.0

0.4

0.2

0.0

D ,,

0.8

0.8

1.0

Mole fraction of CNR adsorbed op1 Au

Figure 7. Plots of Kab vs mole fraction of CNR (&NE) for the competitive adsorption of CNPh, 4-CNC&NOz, and CNCHzC(0)OEt with CNBu" on Au powder (see Table 4 and text). Kab increases

a s the mole fraction of isocyanide a on the surface (D, = sa/@, S b ) ) decreases (Figure 7). For 4-CNCsH4N02, Kab increases from 0.51 to 1.6 as D N O ~ decreases from 0.57 to 0.19. For CNPh, Kabincreases from 3.2 to 7.9 as Dph decreases from 0.76 to 0.42; and for CNCH2C(O)OEt,&b increases from 1.9 to 3.5 as Dester decreases from 0.76 to 0.43. Reaction between Au(s)/CNR and AuC1(SMe2).28 When Au powder (300 mg) completely covered with CNBu" was reacted with 0.50 mL of a 2 mM AuCl(SMe2)solution in DCE, the IR spectrum of the solution phase showed that AuCNCNBu")was present. The DRIFT spectrum of the solid showed that there was no CNBu" remaining on the Au surface. Nor is AuCUCNBu") observed on the surface of the Au. When this Au powder was subsequently

+

(28)Nifontova, G. A.; Lavrentiev, I. P. Transition Met. Chem. 1993, 18, 27-30.

Table 5. Saturation Coverage of Isocyanides on Au Powder as a Function of Isocyanide Size (Cone Angle/ Cross-sectional Area) CNBU" cone angle (deg) 58 cross-sec ional 19 area ( i 2 ) adsorbedCNRa 1.6 x 10-6 max. adsorbed 2.4 x 10-6 CNRb adsorbed 0.67 maximumc

CN(c-csH11) CNBut 64 76 28 35

CNC6Hz(But)3 85 52

1.6 x

0.87 x 1.3 x

0.60 x 0.87 x

0.69

0.67

0.69

1.1 x

a Moles of CNR adsorbed on 1.0 g of Au determined experimentally. Maximummoles ofCNRadsorbed on 1.0 x ofAu, calculated from the cross-sectional area of each CNR. Ratio of moles of CNR adsorbed (experimenta1)almaximum moles of CNR adsorbed (cal-

treated with a 5.0 mMDCE solution (0.50mL) of CNCH2C(O)OEt,the IR spectrum of the liquid separated from the Au shows a v(CN) band a t 2261 cm-l, characteristic of AuCl(CNCH2C(O)OEt). This result indicates that [AuClI fragments remain on the Au surface where they react with CNCH2C(O)OEtto form AuCKCNCH2C(O)OEt). Adsorption of CNJ3un on Au Powder Pretreated with Clz. Gold powder (300 mg) was treated with Cl2 (30 pL, 1.2 x mol) followed by reaction with 0.50 mL of a DCE solution of CNBu" (10 mM). Infrared spectra of the separated solution showed that the CNBu" concentration had decreased by 1.96 mM. A band a t 2253 cm-' in the solution spectrum also indicated the presence of AuCl(CNBu"). The DRIFT spectrum of the Au powder showed a v(CN) band a t 2236 cm-l, which is higher than that (2222 cm-', Table 1) of CNBu" on Au powder. Apparently, the surface ofAu powder was oxidized by Cl2, which increases the v(CN)of adsorbed CNBu" by 14 cm-l. Subsequent washings of the Au sample with DCE solutions of CNBu" showed the presence of AuCI(CNBu") in the wash solutions; in DRIFT spectra of the Au powder the v(CN) band of the adsorbed CNBu" shifted from 2236 to 2222 cm-l, the value of CNBu" on pure Au, after several washings. Adsorption of a Mixture of CNPh and PPhs on Au powder. When a mixture of CNPh (3.00 mM) and PPh3 (3.00 mM) in DCE (0.50 mL) was reacted with 150 mg of Au powder, 0.31 mM (1.6 x mmol) of the CNPh was adsorbed on the Au powder. Since 0.43 mM (2.2 x mmol) of CNPh completely covered the surface when PPh3 was not present, it appears that PPh3 adsorbed onto the surface of Au powder, reducing the amount of CNPh that was adsorbed. This conclusion is also supported by the v(CN) value of the adsorbed CNPh (2182 cm-l), which is 8 cm-l lower than that of CNPh (2190 cm-') adsorbed on Au powder in the absence of PPhs. Amounts of CNRon Au at Saturation Coverage as a Function of the Bulkiness of R. Solutions (0.50 mL) of 3.00 and 5.00 mM of CNBu", CN(c-C6H11),CNBut, and 2,4,6-CNC&z(But)3, which were more than sufficient to saturate the surface (Figures 1and 2), were reacted with Au powder (300 mg). The amounts (mmol) of CNR on the surface were determined from IR spectra of the solutions. Table 5 shows these amounts per 1g of Au powder for the different isocyanides. The bulkiness of each isocyanide was estimated from its cone angle.29 A plot (Figure 8) of the amount of the isocyanide adsorbed per 1g ofAu versus the cone angle of the isocyanide shows that the amount of CNR adsorbed a t saturation coverage decreases as the cone angle of the isocyanide increases. (29) de Lange, P. P. M. D.; Fruhauf, H.-W.;Kraakman, M. J. A.; van Wijnkoop, M.; Kranenburg, M.; Groot, A. H. J. P.; Vrieze, K.; Fraanje, J.;Wang, Y.; Numan, M. Organometallics 1993, 12, 417-427. (30) Soriaga, M. P.; Hubbard, A. T. J . Am. Chem. SOC.1982, 104, 2735.

Studies of Isocyanides on Powdered Gold

Langmuir, Vol. 11, No. 7, 1995 2545

Cross-sectional area (A*) A

$ 1.8

10

j

20

30

40

50

60

70

80

90

t

t

t

1.4 0

'

3

4 4

0.6

f

0.4

l

,

,

10

20

30

,

,

,

tj

40 50 60 Cone angle (9

70

80

90

o

,

/

2,4,6-CNCP,(BuSs a

Figure 8. Saturation coverage dependence on the cone angle (solid circles)/cross-sectionalarea (open circles) of the isocyanides CNBu", CN(c-CGH11),CNBut, and 2,4,6-CNCsHz(But)3on Au powder. Another, more appropriate, measure of the size of each isocyanide ligand is the rectangular cross sectional area as defined by Hubbard and S ~ r i a g a .Following ~~ their approach, we used the CSC Chem 3D (Cambridge Scientific Computing, Inc.) computer program to establish space-filling models for each of the four isocyanides; the atoms were scaled to 100% of the van der Waals' radii. A projection of the CNR ligand bonded in a n on-top site onto the Au surface was obtained. From a hard copy of this projection, a rectangular cross sectional area (A21 was calculated for each of the isocyanides. The areas (Table 5) increase with the R group in the CGN-R ligand in the same order that the cone angles increase. Both the cone angles and cross-sectional areas are plotted versus the numbers of moles of the isocyanides adsorbed on 1.0 g of Au a t saturation coverage in Figure 8. Equilibrium Constant Studies for the Exchange of CNR in AuCl(CNR). In order to determine the relative binding tendencies of different isocyanides in a simple complex, we studied the exchange reaction in eq 10. AuCl(CNBut)

+ CNR

--L

AuCUCNR)

+ CNBut

(10)

R = Bun, CH,C(O)OEt, Ph, a n d 4-CNC6H4N0, Substitution reactions of AuC1(CNBut)by the free CNR in DCE produce the corresponding AuCl(CNR) and free CNBut. The amounts of AuCl(CNR1 and CNR were determined by the intensities of their dCN) bands. The concentrations of the species did not change after 10 min, indicating a fast equilibrium reaction. The calculated (eq 11)

& = [CNBut][AuC1(CNR)I/[CNRl[AuC1(CNBut)l (11) equilbrium constants (Kz)a t 23 "C for the reactions in eq 10 decrease in the following order:

-

CNBut (1.0) CNBu" (0.91) > CNCH,C(O)OEt (0.11) > CNPh (0.067) > 4-CNC,&NO2 (0.010) (12) This trend suggests that the binding ability of the isocyanide decreases as its a-donor ability decreases.

Discussion Isocyanide Bonding on Powdered Au. In organometallic complexes,isocyanides are good u-donors but they may also be n-acceptors depending on the electron density

of the meta1.27p31In AuCl(CNR)complexes,the a-donating ability of the isocyanide dominates. This is reflected in the much higher v(CN) values of the isocyanides in these complexes than in the corresponding free isocyanides (Table lhS2 In the electron-rich complex Fe(CNR)S, v(CN)is lower than in the free CNR, indicating substantial n-backbonding from the metal to the CNR ligands.31 In the DRIFT spectra of isocyanides adsorbed on Au powder (Table l), the v(CN) values are higher than those of the free isocyanides but not as high as those in AuCl(CNR). Thus, the v(CN) values for isocyanides on Au powder do not exclude the possibility of n-backbonding from the Au. We sought to determine the importance of n-bonding by measuring equilibrium binding constants for isocyanides with electron-donating and -withdrawing R groups. Results (Table 3, Figure 3) of Langmuir isotherm studies on Au powder show that CNPh binds more strongly than its less electron donating analog 4-CNC6H4N02, as reflected by the 2.3 times larger equilibrium constant (K1) (12). Z Experiments for CNPh (27) than for C N C ~ H ~ N O involving the competitive adsorption of CNPh, 4-CNC6HdNO2, and CNCHzC(0)OEt against CNBu" on Au powder also show that CNPh binds more strongly than 4-CNCsH4NO2. In addition, these experiments demonstrate that the relative binding affinities (Kab in Table 4, Figure 7) of these isocyanides decrease in the order

CNPh > CNCHZC(0)OEt > 4-CNC,&NO,

>

CNBu" (13) This order is quite different from that observed for the equilibrium constants for CNR binding in AuCl(CNR) complexes, which decrease in the order shown in eq 12. This latter trend appears to be dominated by the cr-donating ability of the i ~ o c y a n i d esince ~ ~ , ~the ~ most strongly coordinated ligands are those with the most electrondonating R groups. Therefore, the trend (eq 13) for isocyanide binding on Au is not determined by the a-donating ability of the isocyanide. On the other hand, it does not follow a trend which suggests that n-backbonding is important, since CNPh binds more strongly than 4-CNCsH4NOz. At this point, it is not clear what determines the adsorption trend (eq 13). It is possible that interactions between the R groups of the adsorbed isocyanides contribute to their binding abilities.8j Perhaps the stronger adsorption of CNPh as compared with CNBu" is the result of aromatic stacking of the phenyl groups of CNPh on the surface. Effects of Surface Composition on Isocyanide Adsorption. Reaction of Cl2 with Au(ll1) produces we reacted Au surface AuCl and A u C ~ In ~ our . ~ studies, ~ powder with Clz to form unidentified surface species. Adsorption of CNBu" from DCE solution on this surface gives a n isocyanide dCN) band which is 14 cm-l higher than that of Au itself. Thus, oxidation of the Au surface increases the v(CN). This is the same trend that is observed in metal complexes where metals in high positive oxidation states give isocyanide complexes with the highest v(CN) values.36 When CNBu" reacts with Clztreated Au, it also produces AuC1(CNBun)in solution. In fact, by repeatedly washing the surface with CNBu" in DCE, it appears that all of the C1- can be removed as (31)Yamamoto, Y. Coord. Chem. Rev. 1980,32,193-233. (32)Sarapu, A. C.; Fenske, R. F. Inorg. Chem. 1975,14,247. (33)Jones, P. J.; Williams, A. F. J . Chem. SOC.,Dalton Trans. 1977, 1430. (34)Bonati, F.; Minghetti, G. Gam. Chim. Ital. 1973,103,373. (35)(a)Kishi, K.; Ikeda, S. J.Phys. Chem. 1974,78,107.(b)Spencer, N,D.; Lambert, R. M. Su$. Sci. 1981,107,237. (c) Sesselmann, W.; Marinero, R. M.; Chuang, T. J. Surf. Sci. 1986,178,787. (36)Plummer, D.T.;Angelici, R. J. Znorg. Chem. 1983,22,4063.

2546 Langmuir, Vol. 11, No. 7, 1995

Shih and Angelici

AuCKCNBu"), leaving CNBu" on the surface with a v(CN) value that is the same as that on pure Au. While the v(CN) of CNBu" on Clz-treated Au is higher than that of CNBu" on Au, coadsorption of PPh3 and CNPh on Au powder decreases the v(CN) value, as seen in the DRIFT spectrum of the Au powder. The 8 cm-l lower v(CN) value for CNPh on the mixed CNPWPh3 surface than on the surface covered only by CNPh suggests that PPh3 is a stronger electron donor than CNPh. The PPh3 presumably reduces CNPh a-bonding to the surface and increases n-backbonding, which reduces 4CN) for the adsorbed CNPh. As the adsorption of PPh3 changes the coordination of Au to a n isocyanide, so does the adsorption of one isocyanide on the surface of Au change the ability of the Auto interact with another isocyanide. Figure 7 and Table 4 show the dependence of the ratio of equilibrium constants (Kab,eq 9) on the relative amounts of CNBu" and another isocyanide on the surface of Au powder. For example, for the aromatic isocyanide 4-CNC&N02, the relative equilibrium constant ( & , ) increases as the mole fraction of 4-CNCsH4N02 (DNoJon the Au surface decreases. This result can be interpreted by noting that CNBu" is a stronger a-donor than 4-CNC6H4N02;when the amount of CNBu" is high, more electron density is available for n-backbonding to the adsorbed 4-CNCsH4N02, which strengthens the bonding of 4-CNCeH4N02 . -to the Au. Like 4-CNcsH4N02,the relative equilibrium constant (Kab) for CNPh increases as the CNPh mole fraction (&h) on the surface decreases. For the a h h a t i c isocyanide CNCHzC(O)OEt, the relative equilibrium consfjant Kab also i n creases as Dester decreases. These trends in Kabwith the CNWCNBu" ratio may all be explained by noting that CNBu" is the stronger a-donor and the greater the amount of it on the surface, the greater the n-backbonding to the other isocyanide. However, it should be noted that interactions between the R groups may also contribute to the observed trends. Nevertheless, these studies illustrate the importance of surface composition on the binding abilities of CNR on Au. Saturation Coverage of Gold Powder by Isocyanides of Different Sizes. The surface area of the Au powder is 0.27 m2/g, measured using the BET method. Assuming the structure of the Au surface is (1111, the area of a unit cell face will be 7.204 x m2.37 Since this face has one Au atom, the number of Au atoms on a (111)surface will be 2.3 x mol Au atom/m2. On the basis of the measured surface area, the number of surface Au atoms for 1g of Au is 6.2 x moUg Au. Since the surface structure of powdered gold is not known, this value is only a n estimate. If it is assumed that the surface is (110) or (1001, the calculated surface Au atoms per gram ofAu wouId be 3.8 x and 5.4 x respectively. For the purpose of the following discussion we will use the (111)value of 6.2 x moVg. At saturation coverage, we observe that 1.6 x mol of CNBu" is adsorbed on 1.0 g of Au. On the basis of the estimate in the previous paragraph, this means that there are 3.9 (6.2 x 10-W.6 x surface Au atoms per adsorbed CNBun. Porter and c o - w o r k e r ~found ~ ~ 8.40 f 0.7 x 10-lo mol/cm2for full surface coverage by a thiolate

(CH3(CH2),SH, n = 3-18) monolayer onAu(ll1)film. The J3 x d3 R30" thiolate overlayer on the Au film was also established by scanning tunneling m i ~ r o s c o p y .This ~~ overlayer structure corresponds to one adsorbed nalkanethiolate for every 3.0 surface Au atoms. For a slightly more bulky substrate, CF3(CF2)7(CH2)2SH,the number of moles for full surface coverage was smaller (5.7 f 0.7 x 10-lo moVcm2),and the overlayer pattern changed to 2 x 2, i.e., one thiolate for every four Au surface atoms.8h Coverage by an even bulkier thiolate group on a Au(ll1) film gave only one adsorbate for every 15 Au surface atoms.39 Thus, for self-assembled thiolate monolayers, the bulkiness of the adsorbate influences the number of moles of thiolate on Au films. We sought to examine the effect of the bulkiness of the isocyanide R group on the number of moles of CNR that could be adsorbed. The cone anglesz9and cross-sectional areas of each CNR are chosen as the measures of their bulkiness; the larger the cone angle or cross-sectional area, the more bulky the isocyanide. As is evident in Table 5 and Figure 8, the number of moles of CNR adsorbing a t saturation coverage on 1 g of Au decreases from 1.6 x moVg for the smallest isocyanide (CNBu") to 0.60 x moVg for the largest isocyanide investigated (2,4,6CNC&z(But)3. Assuming a (111)Au surface as discussed above, this means the ratio of adsorbed CNR to surface Au atoms decreases in the following order:

(37) (a) Wyckoff, R. W. G. Crystal Structures, 2nd ed.; Interscience: New York, 1963;Vol1. (b)Kittel, C. Introduction to Solid State Physics, 5th ed.; Wiley: New York, 1976. (38) Widrig, c.A.; Alves, c.A.;Porter, M. D. J.Am. Chem.sot. 1991, 113,2805. (39) Kwan, W. S. V.; Atanasoska, L.; Miller, L. L. Langmuir 1991,

Acknowledgment. We thank Professor Marc D. Porter for helpful discussions. This work was supported by the Office of Basic Energy Sciences, Chemical Sciences ~ i v i ~ofthe i ~U.S. ~ , Department OfEnergyunder Contract No. W-7405-Eng*-82. We appreciate partial support Of this project by a NATO Collaborative Research Grant.

7, 1419. (40)Heyraud, J. C.; Metois, J. J. Acta Metallurgica 1980,28, 1789. (41) Iijima, S.; Ichihashi, T. Jpn. J . Appl. Phys. 1985,24, L125.

C N B (1/3,g) ~ ~ > CN(~-C,H,,) ( p j . 7 ) > C N B d (1/7.2) < 2,4,6-CNC,H2(But), (1/10) From the least bulky (CNBu")to the most bulky isocyanide (2,4,6-CNCsH2(But)3),the number of required surface Au atoms per isocyanide increases from 3.9 to 10. From the cross-sectionalareas of each of the isocyanides, the maximum number of moles of an isocyanide that will fit on the surface (0.27 m2)of 1.0 gofAu can be calculated. These values along with the experimentally determined moles of isocyanide actually adsorbed on 1.0 g of Au are given in Table 5. For each isocyanide the calculated maximum moles of isocyanide is larger than the actual moles of adsorbed isocyanide, and the ratio of the experimental moles of adsorbed isocyanide to the maximum mmoles is 0.67-0.69 (Table 5) for the four isocyanides. This means that two-thirds of the Au surface is covered by isocyanide regardless of the size (cross-sectional area) ofthe isocyanide. This result is not consistent with a description of the Au particles in which the active Au sites are separated from each other by inactive sites; in this case, more of the surface would be covered by bulky than nonbulky ligands, which is not observed. However, it is consistent with the view that there are regions of active sites on the surface and regions of inactive sites. The active regions are capable of adsorbing either nonbulky or bulky isocyanides until these regions are covered. The active regions occupy two-thirds of the entire Au surface. The nature of the active regions on these Au powders is not clear. However, studies of Au crystallites supported on graphite40 and silicon4l show ( l l l ) , (loo), and (110) facets. Perhaps one or two of these facets are the active regions on the Au powders. Further studies are required to support these tentative proposals.

LA9406012