Molecular structure cyanamide

Summary Precursor-derived quaternary Si-B-C-N ceramics frequently possess an enhanced thermal stability compared to SiC, SisN4 or Si-C-N ceramics. The stability of the materials towards crystallization and/or decomposition is directly coimected to the molecular structure and the elemental composition of the polymeric precursors. This paper highlights recent investigations on the synthesis of boron-modified polysilazanes and polysilylcarbodiimides. Hydroboration of polyvinylsilazanes and dehydrocoupling reactions of boron-modified silanes with ammonia or amines as well as cyanamide are described. It is shown that simple organosilicon chemistry provides a means to efficiently optimize ceramic yields and tune elemental composition as well as thermal properties of the polymer-derived ceramics. 

Problem 20.25 Give the electronic structure of the cyanamide anion, (NCN) Discuss its molecular shape, bond lengths, and location of charge. 

Within CaNCN, the NCN anion exists in the so-called carbodiimide form, that is, a linear N=C=N with two N=C double bonds. Thus, calcium "cyanamide" should rather be called a carbodiimide, but the name had been given to CaNCN many years before the crystal structure was clarified. We reiterate that molecular carbodiimide (H-N=C=N-H) is highly unstable. The same (false) naming applies to most other metal "cyanamides" of which most are, in fact, carbodiimides because they contain the N=C=N anion. 

As a fundamental class of compounds in the fields of synthetic solid-state (and also molecular) chemistry, cyanamides and carbodiimides have gained increasing attention within the past decade. Because of their 2-fold anionic charge, both cyanamide and carbodiimide structural units allow the realization of nitrogen-based pseudo-oxide chemistry since NCN is able to replace in a wide variety of novel materials. A number of alkali metal, alkaline-earth metal, main-group metal,divalent transition-metal, trivalent rare-earth metal,and also triva-lent transition-metal cyanamides/carbodiimides were obtained following different synthetic routes. The only carbodiimide containing divalent lanthanide ions was reported by DiSalvo et al., who found that EuNCN is isostructural to the already known a-SrNCN. ... 

Structural adhesive FM 34 marketed by American Cyanamid is based on a similar chemistry using the reactants shown in Fig. 16. The bis(orf/jo-acid ester) 33 is prepared in situ by the reaction of BTDA with ethyl alcohol in polar aprotic solvents. After 4,4 -methylenebisbenzeneamine 34 has been added in stoichiometric amount, the solution is heated to 130-150°C. The reaction mechanism leading to high molecular weight polyimides when ODA is opposed to the diethyl esters of different tetracarboxylic acids has been published [31]. 

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