Tracking the chemistry of unsaturated C3H3 groups adsorbed on a silver surface:: Propargyl-allenyl-acetylide triple bond migration, self-hydrogenation, and carbon-carbon bond formation

A diverse array of unsaturated C-1 (methylene and methylidyne) and C-2 (vinyl, vinylidene, ethylidene, and ethylidyne) bound to metal center(s) and surfaces has received much attention. In sharp contrast to the effort devoted to C-1 and C-2 ligands, complexes or surfaces bearing C-3 fragments have been less explored, especially the M-C3H3 Systems, which include propargyl (M-CH2C CH), allenyl (M-CH=C=CH2), and acetylide (M-C CCH3) forms. To understand the bonding and reactivity of these C3 species appended to an extended metal structure, proprargyl bromide (Br-CH2C CH) was utilized as a precursor to generate C3H3 fragments on a Ag(111) surface under ultrahigh vacuum conditions. The molecular transformation process was explored by a combination of temperature-programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and X-ray photoemission spectroscopy (XPS) techniques. In addition, density functional theory (DFT) calculations were conducted to obtain the optimized geometries and energies for the various surface intermediates. The computed IR spectra facilitated the vibrational mode assignments. TPD spectra show that C3H3(ad) self -hydrogenates to C3H4 around 300 and 475 K, respectively. In addition to hydrogenation, a C-C coupling product C6H6 (2,4-hexadiyne) is also unveiled as part of the desorption feature at 475 K. Identification of the possible C3H4 isomers (propyne and/or allene) was equivocal, but it was circumvented by using an (x,cc-dimethyl-substituted propargylic species -(CH3)(2)C-alpha-C CH, which results in hydrogenatioin products, alkynic (CH3)(2)CH-C CH and allenic (CH3)(2)C=C=CH2, distinguishable by the mass spectrometry. The substitution experiments clarify that in the normal case the convoluted TPD feature around 300 K, in fact, consists of both allene at 260 K and propyne at 310 K, while the last hydrogenation product at 475 K is solely propyne. The RAIR spectroscopy demonstrates that at 200 K C3H3(ad) on Ag(111) readily adopts the allenyl formalism involving concerted C-Br bond scission and [1,3]-sigmatropic migration (i.e., Br-(CH2C)-C-star CH -> (CH2)-C-star=C=CH-Ag), in which the sigma bond moves to a new metal location across the pi-periphery. Single hydrogen incorporation to the a-carbon of the surface allenyl rationalizes the allene formation at 260 K. When the surface is heated to the range of 250-300 K, both RAIR and XP spectra reveal drastic changes, indicative of a new species whose spectral characteristics could be duplicated by separate measurements from 1-propyn-1-yl iodide (CH3-C C-I) being a direct source for the surface methylacetylide (CH3-C C-Ag). It is thus suggested that allenyl is further reorganized to render acetylide presumably via [1,3]-hydrogen shift (i.e., (CH2)-C-star=C=CH-Ag -> (CH3)-C-star-C C-Ag). The presence of this third Ag-C3H3 isomeric form demonstrates an unprecedented propargyl-allenyl-acetylide multiple rearrangements on a metal surface. Migration of the triple bond from the remote terminal position into the chain, through the stage of allenic structure, is driven by hermodynamic stabilities, supported by the DFT total energy calculations. Consequently, the evolutions of propyne at 310 and 475 K, as well as 2,4-hexadiyne (bismethylacetylide), can all be reasoned out.