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The role of kinetics vs. thermodynamics in surface-assisted Ullmann coupling on gold and silver surfaces

DOI: 10.1021/jacs.8b11473 DOI Help

Authors: Massimo Fritton (Technical University of Munich; Deutsches Museum) , David A. Duncan (Technical University of Munich; Diamond Light Source) , Peter S. Deimel (Technical University of Munich) , Atena Rastgoo-lahrood (Technical University of Munich; Deutsches Museum) , Francesco Allegretti (Technical University of Munich) , Johannes V. Barth (Technical University of Munich) , Wolfgang M. Heckl (Technical University of Munich; Deutsches Museum) , Jonas Björk (Linköping University) , Markus Lackinger (Technical University of Munich; Deutsches Museum)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Journal Of The American Chemical Society

State: Published (Approved)
Published: February 2019

Abstract: Surface-assisted Ullmann coupling is the workhorse of on-surface synthesis. Despite its obvious relevance, many fundamental and mechanistic aspects remain elusive. In order to shed light on individual reaction steps and their progression with temperature, Temperature-Programmed X-ray Photoelectron Spectroscopy (TP-XPS) experiments are performed for a prototypical model system. The activation of the coupling by initial dehalogenation is tracked by monitoring Br 3d core levels, whereas the C 1s signature is used to follow the emergence of metastable organometallic intermediates and their conversion to the final covalent products upon heating in real time. The employed 1,3,5 tris(4 bromophenyl)benzene precursor is comparatively studied on Ag(111) vs. Au(111), whereby intermolecular bonds and network topologies are additionally characterized by Scanning Tunneling Microscopy (STM). Besides the well-comprehended differences in activation temperatures for debromination, the thermal progression shows marked differences between the two surfaces. Debromination proceeds rapidly on Ag(111), but is relatively gradual on Au(111). While on Ag(111) debromination is well explained by first-order reaction kinetics, thermodynamics prevail on Au(111), underpinned by a close agreement between experimentally deduced and Density Functional Theory (DFT) calculated reaction enthalpies. Thermodynamically controlled debromination on Au(111) over a large temperature range implies an unexpectedly long life-time of surface-stabilized radicals prior to covalent coupling, as corroborated by TP-XPS of C 1s core levels. These insights are anticipated to play an important role regarding our ability to rationally synthesize atomically precise low-dimensional covalent nanostructures on surfaces.

Subject Areas: Chemistry

Facility: BESSY