Equilibrium and Saturation Coverage Studies of Alkyl and Aryl Isocyanides on Powdered Gold

Isocyanides (C=-N-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 decrease in the order: CNPh > CNCH{sub 2}C(O)OEt > 4-CNC{sub 6}H{sub 4}NO{sub 2} > CNBu{sup n}. This is a different order than that (CNBu{sup t} nearly equals CNBu{sup n} > CNCH{sub 2}C(O)OEt > CNPh > 4-CNC{sub 6}H{sub 4}NO{sub 2}) observed for the binding of isocyanides in the complex AuCl(C=N-R) ; this latter order indicates that the isocyanides with the highest {sigma}-donor ability bind the most strongly. At saturation coverage, the number of moles of adsorbed C=N-R per gram of Au decreases as the size of the R group increases: CNBu{sup n} > CN(c-C{sub 6}H{sub 11}) > CNBu{sup t} > 2,4,6-CNC{sub 6}H{sub 2}(Bu{sup t}){sup 3}. For the least bulky isocyanide (CNBu{sup n}), the ratio of CNBu{sup n} molecules to surface Au atoms is approximately 1/3.9. 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-oxidized by Cl{sub 2} adsorbs CNBu{sup n} but AuCl(CNBu{sup n}) desorbs into the DCE solution. 41 refs., 8 figs., 5 tabs.