Borazine and Its Derivatives: Synthesis, Mechanochemical Synthesis Reactions and Outlooks for Technological Applications

Chemistry for Sustainable Development 17 (2009) 225234 225 Borazine and Its Derivatives: Synthesis, Mechanochemical Synthesis Reactions and Outlooks for Technological Applications V. V. VOLKOV, K. G. MYAKISHEV and E. A. ILINCHIK Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, Pr. Akademika Lavrentyeva 3, Novosibirsk 630090 (Russia) E-mail: [email protected] (Received September 22, 2008; revised November 20, 2008) Abstract Review of publications on chemistry, properties and outlooks of the use of boron and nitrogen containing compound borazine (borazol), or cyclo-Í3Â3N3H3 and some its derivatives, as well as the starting compounds for synthesis, since 1926 (the date of borazine discovery) till now is presented. It is shown that the information described in classical works is in many aspects urgent also at present. With the use of borazine, promising methods were developed for obtaining insulating layers and films based on boron nitride h-BN and carbonitroborides through thermal or plasmachemical decomposition of vapours, as well as coatings composed of condensed heterocyclic borazine derivatives. The data obtained by the authors of the review on solid- phase mechanochemical methods of the synthesis of borazine and its derivatives N-trimethylborazine and N-triethylborazine are presented. Advantages and possibilities of technical implementation of these methods in comparison with other known methods are substantiated. Key words: borazine, borazine derivatives, mechanochemical synthesis, physicochemical properties, application INTRODUCTION Numerous derivatives of B3N3H6 with a com- mon name Borazines are known, first of all The subject considered in the present work the products of hydrogen substitution in the is borazine or cyclo-Í3Â3N3H3 (B3N3H6), which [B3N3] heterocycle by the atoms of elements is a six-membered heterocyclic boron and ni- or organic radicals (R); condensed heterocycles trogen containing compound with B and N at- are also known [4]. oms alternating in the cycle. The trivial name The foundations of the chemistry of boranes of this compound is also used: borazol (as a were formed by the classical works of A. Stock structural analogue of benzene) and the sys- in 1912, later on generalized in monograph [5]. tem name according to IUPAC nomenclature The development of modern chemistry of bo- s-triazatriborine. Homoatomic cycles formed by ranes and their derivatives started in late 1940es. boron atoms similarly to organic ones, for ex- Boron compounds are used in various areas of ample Ñ6Í6, Ñ6Í12, Ñ5Í5 etc., are not typical materials science, in catalyst, inorganic and for the chemistry of boron. Chemistry of dif- organic synthesis, biochemistry, medicine, ag- ferent-type heteroatomic cycles with boron at- riculture etc. Some boranes and their deriva- oms has developed into an extensive area of tives are manufactured on the industrial scale, modern inorganic and hetero-organic chemis- so their application for technical and experi- try [13]. In addition to boron atoms, hetero- mental purposes is possible. cycles can contain C, O, N, P, Si, S atoms in Classification of chemical binding in borane different combinations. Examples of the struc- structure was considered in review [6]. The first ture of these molecules are presented in Fig. 1. type of boranes and their derivatives includes 226 V. V. VOLKOV et al. properties [14]. The next two decades 19301950 are marked with extensive efforts in borazine chemistry. The primary information on the synthesis of borazine, its physicochemical properties and derivatives was collected and generalized in monograph [5] and review [15]. The similarity of the physical properties of B3N3H6 with the properties and structure of C6H6 was stressed, so borazine was called inorganic benzene. The initial ideas about the structure of B3N3H6 molecule were based on its physical and chemical properties [4, 5]. Later on, the structure of borazine was determined by means of electron diffraction, XSA and vibrational spectroscopy [16, 17]. The overall results of the initial stage of development of borazine chemistry (19261960) were generalized in review [4] where the information about more than 190 its derivatives was presented, demonstrating the onrush of borazine chemistry. It is noteworthy that a list of 194 publications was cited in that review. This information is of interest also under Fig. 1. Schematic sketch of the molecules of heterocyclic modern conditions. The authors of the review boron compounds (a) and two-dimensional product describe the use of borazine as an additive to of borazine condensation (b). fuel, as a component of polymer materials, and the vapour of this compound is used in counters the simplest representatives, such as borane for neutron detection in atomics. Later works ⋅ report the application of borazine for layer BH3, its adducts with Lewis bases ÂÍ3 L, tet- deposition and for obtaining fibres composed rahydroborate anion BH , diborane(6) B H . 4 2 6 of BN and BCN, formation of condensed The compounds of this class serve as the start- structures (see Fig. 1), polymers, ceramics and ing components for the synthesis of complicat- nanosystems [3, 1820]. ed borane compounds including borazines. The The first classical method of B N H syn- second type includes cluster hydrides of bo- 3 3 6 thesis is based on the reaction of B H with ron: boranes B H , B H , decaborane (14) and 2 6 3 9 10 14 NH under heating the gas mixture in a tight its derivatives, for example B H L , polyhe- 3 10 12 2 apparatus [5, 14, 15]. As a result, borazine was 2 dral anions having the composition BHnn, for obtained and its properties were determined, example BH2 , BH2 , ortho-, meta-, para- but it is difficult to use this method because of 10 10 12 12 the high reactivity and toxicity of B H . Tech- carboranes(12) B C H and their derivatives 2 6 10 2 12 nologically, it may be implemented in the case π BCH9212, -complexes of transition metals (M) if modern apparatus arrangement of the pro- III π−η5 with the composition [M ( -B9C2H11)2] , cess is realized on the basis of stationary pro- II metal boranes Sn B9C2H11, cluster polymers etc. duction of diborane. Borazine synthesis meth- [2, 613]. The third type of borane derivatives ods using boron halides BX3, alkyl amines R3N incorporates heterocycles (see Fig. 1) including etc. are known [14]. Reactions are carried out in Â3N3H6 and its analogues. the inert atmosphere in organic solvents (dig- The date of the start of development of lyme, triglyme, tetraglyme, ethers, THF, chlo- borazine chemistry is considered to be 1926 robenzene etc.), their preparation and regen- when A. Stock and E. Pohland published a work eration are complicated procedures; in addition, describing its synthesis and investigation of its it is possible to introduce admixtures with them. BORAZINE AND ITS DERIVATIVES 227 Efficient methods may be solid-phase reactions Borazine and isoelectronic benzene C6H6 are activated by the mechanical action on the mix- close to each other in molecular structure: both tures of starting reactants simple and com- have D3h symmetry, bond lengths in B3N3H6 mercially available substances, in particular molecule are, Å: ÂN 1.44, NH 1.02, NaBH4, NH4Cl and their analogues [2124]. BH 1.20, in C6H6 ÑÑ 1.397, ÑÍ 1.084 Å. Reactions involving mechanical activation (MA), The melting point of borazine is 58.0 °C, as a rule, are carried out without solvents, boiling point 53.0 °C [5].In H-substituted which simplifies complete synthesis cycles and borazines, the length of BN bonds varies improves the purity of the products. This is a within the range 1.4131.42 Å, while the length substantial advantage of these reactions. The of bonds in benzene in the structures of layers modern state of mechanochemistry was con- of hexagonal boron nitride h-BN is 1.446 Å [4, sidered in review [25]. In the present work we 7, 26]. Aromaticity, that is, delocalization of discuss the principles of synthesis and applica- electrons over the atoms of the cycle, is tion of borazines with focus on MA reactions. characteristic of borazines to a lesser extent than of benzene [6, 7]. Nevertheless, elements of π-bonding are realized also in the cycles of BORAZINE AND ITS DERIVATIVES. borazine [18, 20]. Identical stoichiometry, PRINCIPLES OF SYNTHESIS AND APPLICATION similarity of size and analogy of the elements The first works on chemistry and synthesis of structure of borazine molecules and h-BN can become a favourable circumstance for the of borazine according to the reaction of B2H6 design of materials based on boron nitride and with NH3 stated the complicated character of the formation of this heterocycle from the its derivatives (Fig. 2). Borazine molecule is a simplest starting components and specificity of structural blank for h-BN. Thermodynamic probability of the formation of h-BN is high BN chemical bond in B3N3H6 molecule [5, 14, 15, 26]. The electron configuration of boron because the standard heat of its formation 2 2 1 ∆=o atom (Z = 5) is 1s 2s 2p . In the formation of 251.14Hf298 kJ/mol. It is known that compounds, the valence electrons form ceramic parts and films made of BN possess 2 1 2 energetically more stable configuration 1s 2s 2p valuable dielectric characteristics [26]. Borazine in which boron possesses four valence atomic is thermally stable in the gas phase, but it orbitals (AO) 2s, 2px, 2py, 2pz and only three valence electrons [6, 7, 27]. The electrons of boron atom are localized in the strong field of the nucleus, thus the 2pz AO is vacant. This feature allows the light-weight element boron with the simplest electronic structure of the atom to form very complicated compounds with many types of chemical bonding and structures [6]. Boron atom is able to form σ-bonds with the participation of AO having the sp2 configuration and at the same time π-bonds in which the vacant 2ðz AO is involved, for example in molecules BX3 (where X is a halogen) etc.

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