Studies of the adsorption of bi- and tridentate isocyanides on gold powder

Diffuse reflectance Fourier transform spectroscopic (DRIFTS) studies of diisocyanides [C=N-(CH2)(x)N=C, where x = 2, 4, 6, 8, and 12; m- and p-xylyl(NC)(2), xylyl = -CH2-C6H4-CH2-] and triisocyanides [1,1,1-tris(isocyanomethyl)ethane (Tripod(NC)(3)) and tris[2-isocyanoethyl]amine (Tren(NC)(3))] adsorbed on gold (Au) powder show that all of their -NC groups are coordinated to the surface. The nu(NC) values (cm(-1)) for the adsorbed ligands are similar to 2220 cm(-1), which indicates that each of the -NC groups is bound through the carbon to a single Au atom. The saturation coverages (n(1s)) for the diisocyanides decrease as the linking -(CH2)(x)-group lengthens from x = 2 to x = 12. At saturation coverage, the number of moles of -N=C groups coordinated for C-12(NC)(2) is similar to that for the monoisocyanide n-C18H37NC, whereas twice as many -NC groups are adsorbed for C-2(NC)(2) than n-C18H37NC. Qualitative kinetic measurements show that all of the monoisocyanide n-C18H37NC adsorbed on Au powder is displaced by C-4(NC)(2) within 90 min. However, only 39% of the diisocyanide m-xylyl((NC)-C-13)(2) is displaced by C-4(NC)(2), even after 120 h, demonstrating that only 34-39% of the diisocyanide m-xylyl((NC)-C-13)(2) is exchangeable and the remaining 61-66% of the diisocyanide is kinetically inert to exchange. The existence of two adsorption regimes, low coverage (less than 61-66%) and high coverage (greater than 61-66%), on the Au powder is supported by a variety of evidence. Reaction quotients (Q(ab)), which probably include both kinetic and thermodynamic factors, for the adsorption of diisocyanides on Au increase significantly as the -(CH2)(x)-link between the -NC groups becomes shorter. The C-2(NC)(2) ligand has the highest Q(ab) value. These studies also show that the relative binding affinities of the isocyanides increase as the number of -NC groups in the ligand increases (RNC < R(NC)(2) < R(NC)(3)).