Polymeric boron-nitrogen compounds

In hydrogen cyanide, H —C=N, the values for the carbon-nitrogen bond are XE = 5.5 and AE = 0.5. The triple bond is thus stable, but weaker than the nitrogen-nitrogen triple bond. Correspondingly, hydrogen cyanide is also known in polymeric form. In the boron-nitrogen compounds, falls to 5.0,... 

Generally, during hydroboration polymerization of dicyano compounds, the formation of the borazine structures that have a six-membered boron-nitrogen ring (scheme 19a) and dihydroborated end groups (scheme 19b) as a structural defect is unavoidable. The borazine cross-linked structures often cause the gelation, and dehydroboration causes a decrease in molecular weight. 

The chemistry of boron-phosphorus compouuds has been reviewed. Numerous boron-phosphorus derivatives have been reported, but relatively few boron-arsenic or boron-antimony compounds have been described. Boron-phosphorus compounds are similar in many ways to boron nitrogen derivatives, but the teudeucy to share boudiug electrons in covalent tetrahedral compounds is much more evident with phosphorus thau with uitrogeu. lu fact, most boron phosphorus chemistry iuvolves tetrahedral borou. They are typically either phosphiue-boraue complexes, such as R3P BR j, or phosphinoboranes (R2PBR2) , cyclic or polymeric derivatives of the hypothetical H3P BH3. The chemistry of these compounds and that of boron phosphate and thiophosphate is described below. Boron phosphides are discussed in Section 2.6. 

One attractive molecular species upon which polymeric boron nitride precursors could be based is the ring-compound borazine, BaNsHe The compound is easily made and has both the correct boron/nitrogen ratio and a hexagonal structure. Indeed, Paine and Paciorek have each demonstrated the use of linked polyborazine gels or polymers as precursors for boron nitride. 

In hydroboration polymerization with dicyano compounds, dialkyl-boranes can also be used as borane monomers (Scheme 6). In this case, the substituents at boron atoms in poly(cyclodiborazane)s should be two alkyl groups, while alkyl and hydrogen groups in the case of monoalkylborane monomer used. When polymerization between di(/2-butyl)borane and adiponitrile was carried out in bulk under nitrogen, the... 

Cyclic species of equal numbers of alternating boron and nitrogen atoms may be considered as condensation products of monomeric B—N units and recent work by Paetzold 8-11> gives some credence to Wiberg s view that boronimides, XB=NR, are possible intermediates in the formation of cyclic B—N compounds. Boronimides may polymerize or may be trapped by 1,3-dipolar agents 8 11>. In similar fashion, the formation of the cyclotetrazenoborane ring system 1... 

Like boron, most other metalloids are potential Lewis acids. Therefore, it is reasonable that simple cyanides Ge(CN)4, As(CN)3, Sb(CN)3 and Bi(CN), 2Cl i are insoluble in organic solvents and presumably polymeric (Bither, Knoth, Lindsey and Sharkey, 1958 also, Menzer, 1958). The unusually low volatility of phosphorus tricyanide and arsenic tricyanide prompted Emerson and Britton (1964, 1963) to determine the structures of these compounds. Phosphorus tricyanide is composed of pyramidal molecules which appear to associate through N---P interactions. The nitrogen-phosphorus distances, 2.85 to 2.97 A, are shorter than the esti-... 

The compounds [CymB(Pz)3]Tl (136) and [FcB(Pz)3]Tl show polymeric structures, with bridging B(Pz)3 fragments in the solid state. This is a result of unfavorable steric interaction between the substituent on boron and the hydrogen atoms on the pyrazolyl ring 5-position. The structure of [Cym B(Pz)3]Tl is somewhat related, but it adopts a macrocyclic tetrameric structure rather than a linear polymeric structure. Ligands with secondary donors on the backbone may form additional bonds to the thallium atom. For example, in [HB(3-(2-pyridyl)Pz)3]Tl, weak TF"N interactions between pyridyl nitrogens and the T1 atom have been observed. HB(3-(2-MeOC6Fl4)Pz)3]Tl features close intramolecular TF 0 interactions. ... 

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