486
Langmuir 1991, 7, 486-492
Metal Oxide Surface Modification with Iron Pentacarbonyl: Ligation to Surface-Tethered 2-(Trimethoxysilyl)ethyl-2-pyridine M. Mahon, K. W. Wulser, and M. A. Langell* Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304 Received May 11, 1990. In Final Form: June 24, 1990 Pyridine-tethered iron tetracarbonyl has been attached to the surface of a thin titanium oxide film by first silylating the oxide surface with 2-(trimethoxysilyl)ethyl-2-pyridineand subsequently treating this film with Fe(C0)5. The pyridine-tethered iron carbonyl that results is stable under ultrahigh vacuum to 570 K and is present in approximately monolayer amounts. Characterization of the silyl-modified surfaces has been carried out by X-ray photoelectron spectroscopy,Auger electron spectroscopy,and temperature programmed desorption techniques.
Introduction Organosilanes are commonly used to attach organic and organometallic functionalities to solid ~urfacesl-~ in order to impart desirable catalytic, adhesive, optical, and/or electron transfer characteristics to them and to prevent undesirable surface corrosion. The silylating agents, chloro- or alkoxysilanes appropriately functionalized for the desired modifying properties, covalently attach to oxides through condensation with hydroxyl groups which form readily at the surfaces of these substrates. The material to be silylated need not be of bulk oxide composition, as even the relatively thin oxide layers that form on air-exposed gold foil will often support this form of surface m~dification.~ If the silylating agent contains a suitable ligand functionality, transition-metal complexes may be covalently attached to the silyl-modified surface by exchange of one of the metal complex ligands for that of the ligand tethered to the metal oxide surface. In the case of volatile or highly reactive complexes, this may provide a mechanism whereby standard ultrahigh vacuum (UHV)-based methods of analyses can be used to investigate the chemical nature and reactivity of the metal complex. Silyl-tethered ligands with demonstrated utility include cyclopentadienes (meta l l ~ c e n e s )pyrrole^,^ , ~ ~ ~ and ~ y r i d i n e .A~large majority of this surface-modification work has been directed at elucidating electrode surface behavior a t liquid-solid interfaces. However, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) have been used successfully to characterize modified electrode samp l e ~indicating ,~ that silylated surfaces are vacuum compatible and amenable to analysis by electron spectroscopies. Metal carbonyls, in general,5-13and iron pentacarbonyl,
more specificallyB-11,have been investigated under UHV for their adsorption properties on various metal and semiconductor surfaces primarily in application to chemical vapor deposition techniques because of their high volatility and ease of decomposition through thermal, photon, and electron assisted loss of CO. The metal carbonyls decompose directly upon room temperature adsorption5-’3 but physisorb largely intact to form multilayers below a minimum decomposition temperature (
