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Spontaneous Grafting of Diazonium Salts: Chemical Mechanism on Metallic Surfaces
journal contribution
posted on 2012-08-14, 00:00 authored by Alice Mesnage, Xavier Lefèvre, Pascale Jégou, Guy Deniau, Serge PalacinThe spontaneous reaction of diazonium salts on various
substrates
has been widely employed since it consists of a simple immersion of
the substrate in the diazonium salt solution. As electrochemical processes
involving the same diazonium salts, the spontaneous grafting is assumed
to give covalently poly(phenylene)-like bonded films. Resistance to
solvents and to ultrasonication is commonly accepted as indirect proof
of the existence of a covalent bond. However, the most relevant attempts
to demonstrate a metal–C interface bond have been obtained
by an XPS investigation of spontaneously grafted films on copper.
Similarly, our experiments give evidence of such a bond in spontaneously
grafted films on nickel substrates in acetonitrile. In the case of
gold substrates, the formation of a spontaneous film was unexpected
but reported in the literature in parallel to our observations. Even
if no interfacial bond was observed, formation of the films was explained
by grafting of aryl cations or radicals on the surface arising from
dediazoniation, the film growing later by azo coupling, radical addition,
or cationic addition on the grafted phenyl layer. Nevertheless, none
of these mechanisms fits our experimental results showing the presence
of an Au–N bond. In this work, we present a fine spectroscopic
analysis of the coatings obtained on gold and nickel substrates that
allow us to propose a chemical structure of such films, in particular,
their interface with the substrates. After testing the most probable
mechanisms, we have concluded in favor of the involvement of two complementary
mechanisms which are the direct reaction of diazonium salts with the
gold surface that accounts for the observed Au–N interfacial
bonds as well as the formation of aryl cations able to graft on the
substrate through Au–C linkages.