Direct Organometallic Synthesis:  The Metal-Etching Reactions of Isobutyl Iodide on Al(111)¹

Abstract

We report a study of the thermal decomposition and reactions of isobutyl iodide on Al(111). Using temperature-programmed reaction and Auger electron spectroscopies, it was found that more than one product-forming pathway involving the alkyl moiety exists on this surface. A first-order, β-hydride elimination reaction converts surface-bound isobutyl groups derived from the dissociation of the C−I bond to gas phase isobutene and dihydrogen at temperatures above ∼420 K. Competing with this unimolecular process is a collection of complex associative reactions which effect the etching of the aluminum surface via the formation of volatile organometallic species. This includes formation and subsequent desorption of diisobutylaluminum iodide (desorption peak maximum at ∼490 K), diisobutylaluminum hydride (∼515 K), methylaluminum dihydride (∼725 K), and AlI x , x = 1−3 (∼620 K). The kinetics of the processes yielding the various aluminum hydrides are coupled to that of the β-hydride elimination pathway (which serves as the hydrogen atom source) and are strongly coverage dependent. The formation of MeAlH2 reveals the occurrence of a kinetically competitive β-methyl elimination reaction of the surface alkyl groups.

The Crystal Structures Of Dimeric Di(tert-butyl)Aluminium And -Gallium Iodides

“…Among these organometallic aluminium halide compounds the most reactive are alkyl-aluminium iodides, such as Me n AlI 3−n (n = 1, 2) [3] or Et n AlI 3−n [4]. In contrast to the established butyl-substituted organoaluminium systems i Bu n AlI 3−n and n Bu n AlI 3−n [5], to the best of our knowledge, t Bu 2 AlI has not been reported so far.…”

Collision-induced activation of the β-hydride elimination reaction of isobutyl iodide dissociatively chemisorbed on Al(111)

“…The depletion of the surface-bound isobutyl groups also results ͑to a lesser degree͒ from complex, associative metal etching reactions which yield both hydrido-and haloaluminum and isobutylaluminum complexes ͓AlI, diisobutylaluminum hydride ͑DIBAH͒, disobutylaluminum iodide ͑DIBAI͔͒. 41 A common fragment useful for the mass spectroscopic detection of these desorption products ͑excluding AlI͒ is the ion C 3 H 5 ϩ (m/eϭ41). The desorption yield detected at this mass can be used to estimate the cross sections of the collisioninduced reactions described in this report.…”

“…The leading edge intensity is weighted by the associative desorption of diisobutyl iodide ͑DIBAI, T max at ϳ490 K͒. 41 The latter portion of the peak is more heavily weighted by the desorption of isobutene ͑which is believed to be the predominant thermal reaction product͒ and diisobutyl aluminum hydride ͑DIBAH, T max at ϳ515 K͒. The product partitioning is sensitive to coverage, but weakly varies at coverages within ϳ20% of saturation.…”

“…The thermally activated barrier for the ␤-hydride elimination reaction is estimated by TPRS to be in the range of ϳ1 eV. 37,41 On varying the incident beam energy for Ar from 1 to 1.4 eV, only negligible changes were seen in the TPRS data for m/eϭ41 ͑data not shown͒. This suggests an insignificant cross section for CID by the Ar atoms in this energy range.…”

“…Specific details regarding sample preparation and TPRS acquisition are provided elsewhere. 41 The kinetic energy distribution of the beam was determined by a standard time of flight method. 39 The time of flight was measured between a rotating chopper in the second stage, at a distance of ϳ15 cm from the nozzle, and the line-of-sight mass spectrometer in the scattering chamber.…”