Towards novel C–N materials

Two modifications of the novel guanidinium dicyanamide have been obtained by means of ion-exchange reaction in aqueous or methanolic solution. The hygroscopic compounds were characterized by solution state NMR, mass spectrometry and vibrational spectroscopy. The crystal structures of the polymorphs were elucidated by means of single-crystal X-ray diffraction at 200 K {β-[C(NH2)3][N(CN)2]: Pna21, Z=8, a=1373.1(3), b=495.5(1), c=1802.9(4) pm, U=1226.7(4)×106 pm3; α-[C(NH2)3][N(CN)2]: P21/c, Z=8, a=1924.9(4), b=496.0(1), c=1372.4(3) pm, β=110.46(3)°, U=1227.5(4)×106 pm3} and were found to be largely equivalent in terms of the overall assembly of the molecular ions. Thermodynamic and kinetic aspects of the temperature behaviour of the polymorphs, which is characterized by a succession of thermal events in the temperature region between 240 and 440 K, were assessed by means of temperature-dependent X-ray powder diffraction and thermal analysis. Due to the chemical composition of the novel dicyanamide (C3N6H6), which is formally identical with that of melamine C3N3(NH2)3, and its thermal reactivity, which is represented by the facile conversion into melamine around 400 K, guanidinium dicyanamide may be ideally suited as a molecular precursor for chemical approaches toward highly condensed graphitic carbon nitride materials

From Triazines to Heptazines

The first non-metal tricyanomelaminates have been synthesized via metathesis reactions and characterized by means of single-crystal X-ray diffraction and vibrational and solid-state NMR spectroscopy. The crystal structures of [NH4]2[C6N9H] (1) (P21/c, a = 1060.8(2) pm, b = 1146.2(2) pm, c = 913.32(18) pm, β = 112.36(3)°, V = 1027.0(4) × 106 pm3), [C(NH2)3]3[C6N9]·2 H2O (2) (P212121, a = 762.12(15) pm, b = 1333.6(3) pm, c = 1856.6(4) pm, V = 1887.0(7) × 106 pm3) and [C3N6H7]2[C6N9H]·2.5 H2O (3) (P1̄, a = 1029.5(2) pm, b = 1120.3(2) pm, c = 1120.7(2) pm, α = 104.22(3)°, β = 112.74(3)°, γ = 104.62(3)°, V = 1064.8(4) × 106 pm3) are composed of singly protonated (1 and 3) or nonprotonated (2) tricyanomelaminate ions, which, together with the respective counterions, form two-dimensional, layered structures (1 and 3) or a quasi three-dimensional network (2). Particular emphasis has been placed on the elucidation of the thermal reactivity of the three molecular salts by means of thermal analysis and vibrational and NMR spectroscopy, as well as temperature-dependent X-ray powder diffraction. The title compounds were found to be promising candidates as molecular CNx precursors for the synthesis of graphitic carbon nitride materials. Upon being heated, ammonium and guanidinium tricyanomelaminate uniformly pass the crystalline, heptazine (C6N7)-based intermediate melem (C6N7(NH2)3), which decomposes and forms a semi-amorphous CNxHy material with a pronounced layered structure. Identical pyrolysis products are obtained for the melaminium salt, a classical triazine (C3N3)-based CNx precursor, after passing an intermediate, possibly cross-linked phase at low temperatures. Preliminary solid-state NMR investigations of the final products best conform to heptazine-based structure models for g-C3N4 that have commonly been rather disregarded in favor of triazine-based ones

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

Reorientational Dynamics and Solid-Phase Transformation of Ammonium Dicyanamide into Dicyandiamide

The reorientational dynamics of ammonium dicyanamide ND4[N(C≡N)2] and the kinetics as well as the mechanism of the solid-state isomerization reaction from ammonium dicyanamide into dicyandiamide (N≡C-N=C(NH2)2) was studied by means of 2H and 14N solid-state NMR spectroscopy in a temperature range between 38 and 390 K. Whereas in previous investigations the mechanism of the solid-state transformation was investigated by means of vibrational and magic angle spinning solid-state NMR spectroscopy as well as neutron diffraction, we here present a comprehensive 2H study of the ammonium ion dynamics prior to and during the course of the reaction, thereby highlighting possible cross correlations between dynamics and reactivity involving the ammonium ion. The ND4+ group was found to undergo thermally activated random jumps in a tetrahedral potential, which is increasingly distorted with increasing temperature, giving rise to an asymmetrically compressed or elongated tetrahedron with deviations from the tetrahedral angle of up to 6°. The correlation time follows an Arrhenius law with an activation energy of Ea = 25.8(2) kJ mol-1 and an attempt frequency of τ0-1 = 440(80) THz. The spin−lattice relaxation times were fitted according to a simple Bloembergen−Purcell−Pound type model with a T1 minimum of 4 ms at 230 K. Temperature-dependent librational amplitudes were extracted by line-shape simulations between 38 and 390 K and contrasted with those obtained by neutron diffraction, their values ranging between 5 and 28°. The onset and progress of the solid-phase transformation were followed in situ at temperatures above 372 K and could be classified as a strongly temperature-dependent, heterogeneous two-step reaction proceeding with rapid evolution of ammonia and comparatively slow subsequent reintegration into the solid. On the microscopic level, this correlates with a rapid proton transferpossibly triggered by a coupling between the ammonium ion dynamics and phonon modes on the terahertz time scaleand an essentially decoupled nucleophilic attack of ammonia at the nitrile carbon, giving rise to significantly differing time constants for the two processes

Optoelectronics Meets Optoionics: Light Storing Carbon Nitrides and Beyond

Known for decades, Liebig's carbon nitrides have evolved into a burgeoning class of macromolecular semiconductors over the past 10+ years, front and center of many efforts revolving around the discovery of resource‐efficient and high‐performance photocatalysts for solar fuel generation. The recent discovery of a new class of “ionic” 2D carbon nitrides—poly(heptazine imide) (PHI)—has given new momentum to this field, driven both by unconventional properties and the prospect of new applications at the intersection between solar energy conversion and electrochemical energy storage. In this essay, key concepts of the emerging field of optoionics are delineated and the “light storing” ability of PHI‐type carbon nitrides is rationalized by an intricate interplay between their optoelectronic and optoionic properties. Based on these insights, key characteristics and general principles for the de novo design of optoionic materials across the periodic table are derived, opening up new research avenues such as “dark photocatalysis”, direct solar batteries, light‐driven autonomous systems, and photomemristive devices

Synthetic routes toward MOF nanomorphologies.

As metal–organic frameworks (MOFs) are coming of age, their structural diversity, exceptional porosity and inherent functionality need to be transferred into useful applications. Fashioning MOFs into various shapes and at the same time controlling their size constitute an essential step toward MOF-based devices. Moreover, downsizing MOFs to the nanoscale triggers a whole new set of properties distinguishing nanoMOFs from their bulk counterparts. Therefore, dimensionality-controlled miniaturization of MOFs enables the customised use of nanoMOFs for specific applications where suitable size and shape are key prerequisites. In this feature article we survey the burgeoning field of nanoscale MOF synthesis, ranging from classical protocols such as microemulsion synthesis all the way to microfluidic-based techniques and template-directed epitaxial growth schemes. Along these lines, we will fathom the feasibility of rationally designing specific MOF nanomorphologies—zero-, one- and two-dimensional nanostructures—and we will explore more complex “second-generation” nanostructures typically evolving from a high level of interfacial control. As a recurring theme, we will review recent advances made toward the understanding of nucleation and growth processes at the nanoscale, as such insights are expected to further push the borders of nanoMOF science

Solving the COF trilemma: towards crystalline,stable and functional covalentorganic frameworks

Covalent organic frameworks (COFs) have entered the stage as a new generation of porous polymers which stand out by virtue of their crystallinity, diverse framework topologies and accessible pore systems. An important – but still underdeveloped – feature of COFs is their potentially superior stability in comparison to other porous materials. Achieving COFs which are simultaneously crystalline, stable, and functional is still challenging as reversible bond formation is one of the prime prerequisites for the crystallization of COFs. However, as the COF field matures new strategies have surfaced that bypass this crystallinity – stability dichotomy. Three major approaches for obtaining both stable and crystalline COFs have taken form in recent years: Tweaking the reaction conditions for reversible linkages, separating the order inducing step and the stability inducing step, and controlling the structural degrees of freedom during assembly and in the final COF. This review discusses rational approaches to stability and crystallinity engineering in COFs, which are apt at overcoming current challenges in COF design and open up new avenues to new real-world applications of COFs

Tackling the stacking disorder of melon—structure elucidation in a semicrystalline material

In this work we tackle the stacking disorder of melon, a layered carbon imide amide polymer with the ideal composition (C6N7(NH)(NH2)). Although its existence has been postulated since 1834 the structure of individual melon layers could only recently be solved via electron diffraction and high-resolution 15N solid-state NMR spectroscopy. With only weak van der Waals interactions between neighboring layers its long range stacking order is poorly defined preventing an efficient use of diffraction techniques. We, therefore, rely on a combination of solid-sate NMR experiments and force field calculations. The key information is obtained based on heteronuclear (1H–13C) and homonuclear (1H–1H) second moments M2 acquired from 1H–13C cross polarization experiments. To allow for an interpretation of the polarization transfer rates the resonances in the 13C MAS spectra have to be assigned and the hydrogen atoms have to be located. The assignment was performed using a two-dimensional 15N–13C iDCP experiment. For the determination of the position of the hydrogen atoms NH and HH distances were measured via 1H–15N Lee–Goldburg CP and 1H–1H double-quantum build-up curves, respectively. Furthermore, the homogeneity of the material under examination was investigated exploiting 15N spin-diffusion. Based on force field methods 256 structure models with varying lateral arrangements between neighboring layers were created. For each model the M2 were calculated allowing them to be ranked by comparing calculated and measured M2 as well as via their force field energies. This allows the creation of markedly structured hypersurfaces with two distinctly favored shift vectors for the displacement of neighboring layers

Structure elucidation of polyheptazine imide by electron diffraction — a templated 2D carbon nitride networkw

Structure elucidation of a condensed carbon IV) nitride with a stoichiometry close to C3N4 by electron diffraction reveals a two-dimensional planar heptazine-based network containing isolated melamine molecules in the trigonal voids