Chemisorption trimethylphosphine

Lunsford et al. (202) used trimethylphosphine as a probe molecule in their 31P MAS NMR study of the acidity of zeolite H-Y. When a sample is activated at 400°C, the spectrum is dominated by the resonance due to (CH3)3PH+ complexes formed by chemisorption of the probe molecule on Bronsted acid sites. At least two types of such complexes were detected an immobilized complex coordinated to hydroxyl protons and a highly mobile one, which is desorbed at 300°C. (see Fig. 45)... 

The stepped nickel 9(lll)x(lll) and stepped-kinked Ni 7(lll)x(310) surfaces displayed a benzene coordination chemistry that was quantitatively and qualitatively identical with that of the Ni(lll) surface except that not all the benzene was displaced by trimethylphosphine indicating that either a small percentage ( 10%) of the benzene on these surfaces either was present in different environments or was dissociatively (9) chemisorbed see later discussion of stereochemistry. Benzene chemisorption behavior on Ni(110) was similar to that on Ni(lll) except that the thermal desorption maximum was lower, vl00°C, and that trimethylphosphine did not quantitatively displace benzene from the Ni(110)-C H surface. In these experiments, no H-D exchange was observed with CgHs + C D mixtures. 

Toluene surface coordination chemistry was quite different from that of benzene. Toluene chemisorption on all the clean surfaces was thermally irreversible. In addition, toluene was not displaced from these surfaces by trimethylphosphine nor by any other potentially strong field ligand examined to date, e.g., carbon monoxide or methyl isocyanide. In the thermal decomposition of toluene on these surfaces (attempted thermal desorption experiments), there were two thermal desorption maxima for H2 (or D2 from perdeuterotoluene) with the exception of the Ni(110) surface. This is illustrated in Figure 6 for Ni(lll)-C7Dg. 

Ethylene and Acetylene. On nickel (111), both ethylene and acetylene are irreversibly chemisorbed neither can be thermally desorbed. We also find that trimethylphosphine cannot displace ethylene or acetylene from these surfaces. There have been suggestions that ethylene and acetylene are not present on the surface as molecules but as molecular fragments. Many ultra-high vacuum studies of ethylene and of acetylene chemisorption on nickel crystal planes have been reported. Most of these studies seem to implicate states in which C-H bond cleavage reactions have accompanied the basic chemisorption process (19). 

Acetonitrile and Methyl Isocyanide (8). Acetonitrile forms an ordered chemisorption state on the fully flat nickel surfaces, a p(2x2) and a c(2x2) on Ni(lll) and Ni(100), respectively. Acetonitrile thermal desorption from these two surfaces was nearly quantitative (a small amount of acetonitrile decomposed at the temperatures characteristic of the reversible thermal desorption from these surfaces). Importantly from an interpretive context, acetonitrile was quantitatively displaced from these two flat low Miller index planes by trimethylphosphine (8). However, the displacement was not quantitative (only 90-95% complete) from the stepped and stepped-kinked surfaces. For the super-stepped (110) surface, chemisorption was nearly irreversible and no acetonitrile could be displaced from this surface by trimethylphosphine. 

Trimethylphosphine. Trimethylphosphine is very strongly chemisorbed on all the nickel surfaces. On the Ni(lll) surface, thermal decomposition occurs readily and CHi, and H2 are desorbed as decomposition products with desorption maxima at 90 and 98°C respectively. Chemisorption of this phosphine initially must involve a donor-acceptor interaction centered at the phosphorus atom. Models show that the methyl hydrogen atoms can then closely approach the surface metal atoms. Cleavage of C-H bonds probably occurs at or near 25°C, and P-C-Ni bonds are then irreversibly formed. This surface chemistry qualitatively mirrors that of trimethylphosphine in the coordinately unsaturated complex, Fe[P(CH3)3K, which is primarily HFe[r)2-CH2P(CH3)i][P(CH3)3]3 in the solution state (21). 

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