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J Am Chem Soc. 2016 May 4;138(17):5585-93. doi: 10.1021/jacs.5b13461. Epub 2016 Apr 20.

Control of Reactivity and Regioselectivity for On-Surface Dehydrogenative Aryl-Aryl Bond Formation.

Author information

1
Institute of Experimental and Applied Physics, University of Regensburg , 93053 Regensburg, Germany.
2
Department of Chemistry and Biochemistry, University of Bern , 3012 Bern, Switzerland.
3
Institute of Physics of Czech Academy of Sciences , 16200 Prague, Czech Republic.
4
Charles University in Prague , Faculty of Mathematics and Physics, Department of Surface and Plasma Science, 18000 Prague, Czech Republic.

Abstract

Regioselectivity is of fundamental importance in chemical synthesis. Although many concepts for site-selective reactions are well established for solution chemistry, it is not a priori clear whether they can easily be transferred to reactions taking place on a metal surface. A metal will fix the chemical potential of the electrons and perturb the electronic states of the reactants because of hybridization. Additionally, techniques to characterize chemical reactions in solution are generally not applicable to on-surface reactions. Only recent developments in resolving chemical structures by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) paved the way for identifying individual reaction products on surfaces. Here we exploit a combined STM/AFM technique to demonstrate the on-surface formation of complex molecular architectures built up from a heteroaromatic precursor, the tetracyclic pyrazino[2,3-f][4,7]phenanthroline (pap) molecule. Selective intermolecular aryl-aryl coupling via dehydrogenative C-H activation occurs on Au(111) upon thermal annealing under ultrahigh vacuum (UHV) conditions. A full atomistic description of the different reaction products based on an unambiguous discrimination between pyrazine and pyridine moieties is presented. Our work not only elucidates that ortho-hydrogen atoms of the pyrazine rings are preferentially activated over their pyridine equivalents, but also sheds new light onto the participation of substrate atoms in metal-organic coordination bonding during covalent C-C bond formation.

PMID:
27059121
DOI:
10.1021/jacs.5b13461

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