Studies of the Adsorption of Bi- and Tridentate Isocyanides on Gold Powder

Diffuse reflectance Fourier transform spectroscopic (DRIFTS) studies of diisocyanides [C⋮N−(CH 2 ) x −N⋮C, where x = 2, 4, 6, 8, and 12; m- and p- xylyl(NC) 2, xylyl = −CH 2 −C 6 H 4 −CH 2 −] 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 ν(NC) values (cm - 1 ) for the adsorbed ligands are ∼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 −(CH 2 ) 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- C 18 H 37 NC, whereas twice as many −NC groups are adsorbed for C 2 (NC) 2 than n- C 18 H 37 NC. Qualitative kinetic measurements show that all of the monoisocyanide n- C 18 H 37 NC adsorbed on Au powder is displaced by C 4 (NC) 2 within 90 min. However, only 39% of the diisocyanide m- xylyl(N 13 C) 2 is displaced by C 4 (NC) 2, even after 120 h, demonstrating that only 34−39% of the diisocyanide m- xylyl(N 13 C) 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 −(CH 2 ) 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 ).