From Molecular Building Blocks to Condensed Carbon Nitride Networks: Structure and Reactivity

The scope of this thesis was defined by three major issues, which
have been arising from the requirement to further extend and to
deepen research on carbon nitride chemistry from a materials
chemistry point of view. Access to highly condensed CNx species
and, ultimately, binary carbon nitride C3N4, was primarily sought
by using suitable carbon nitride precursor species, based on the
following leitmotifs: 1. Gaining a deeper understanding of the
reactivity of precursors and the mechanisms governing solid-state
reactions, the latter being the key to the directed synthesis of
novel precursor systems as well as to extended carbon nitride
solids with tailored properties. 2. Developing novel CNx precursors
based on the evaluation of reactivity principles and solid-phase
reaction trajectories thus established. 3. Providing an
experimental basis for the predominantly speculative discussion
centered on the structure of graphitic carbon nitride-type systems,
with the major focus being on the nature of the structural
building-blocks of polymeric CNxHy solids. The interplay between
structural requirements of suitable CNx precursors and their
thermal reactivity was demonstrated by a combined 2H solid-state
NMR and neutron diffraction study of ammonium dicyanamide, as well
as by a comprehensive spectroscopic study of the thermal
decomposition of ammonium cyanoureate. Various novel non-metal
dicyanamides and tricyanomelaminates were synthesized, structurally
characterized and screened for potential thermally induced
solid-state reactivity. Their suitability as CNx precursors for the
synthesis of graphitic carbon nitride g-C3N4 was evaluated, leading
to the observation that the formation of melamine C3N3(NH2)3 is
favored in all systems at elevated temperatures. Therefore,
particular emphasis was placed on the study of the thermal behavior
of the prototypic precursor melamine, whose pyrolysis, including
the identities of the intermediates, has been a highly
controversial issue during the past decades. The present work
provides the structures of the two “missing links” in melamine
condensation, melam [(H2N)2C3N3]2NH and melon [C6N7NH(NH2)]n. In
addition, the identities of two further intermediates were
resolved, which could be identified as co-crystallisates made up
from melamine and melem in the distinct ratios 2:1 and 1:2,
respectively. Ultimately, the identity of a CNxHy polymer obtained
by pyrolysis of melamine at T = 893 -913 K was resolved by a
concerted approach based on electron diffraction, solid-state NMR
spectroscopy, and theoretical investigations. It was demonstrated
that the material commonly associated with a hydrogen-contaminated
graphitic carbon nitride material is in fact melon, a 1D polymer
composed of NH-bridged heptazine rings first described by Liebig in
1834. Melon represents a so far unique example of a structurally
characterized, 1D polymeric carbon nitride material, and at the
same time sheds new light on the present discussion regarding the
identity and structure of graphitic carbon nitride.