Chemistry for Sustainable Development 17 (2009) 225–234 225

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 [1–3]. 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 1930–1950 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 (1926–1960) 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, 18–20]. 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, 6–13]. 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: –N 1.44, N–H 1.02, NaBH4, NH4Cl and their analogues [21–24]. B–H 1.20, in C6H6 Ñ–Ñ 1.397, Ñ–Í 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 B–N bonds varies improves the purity of the products. This is a within the range 1.413–1.42 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 B–N 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. The vacant AO provides the electro acceptor ability of boron atom in molecules

BX3, BR3 (R = H, alkyl, aryl etc.). Its interaction with unshared electron pairs of donor molecules leads to the formation of molecular complexes, ⋅ ⋅ ⋅ for example BH3 NH3, BH3 R3N, BX3 NH3, ⋅ BF3 Me2O. These compounds are known as Fig. 2. Details of the structure of borazine molecule B3N3H6 (a), initial or intermediate products of borazine hexagonal boron nitride h-BN (b) and layer structure in synthesis processes [4, 5, 14, 15]. h-BN (ñ). 228 V. V. VOLKOV et al.

→ slowly polymerizes in the liquid phase when stored Al(CH3)3 + 2B2H6 Al(BH4)3 + 2B(CH3)3 (5) for a long time at room temperature [5, 29]. Tetrahydroborates LiBH4 and Be(NH4)2 were The classical principle of the synthesis of synthesized similarly. During the development

B3N3H6 from NH3, B2H6 or other boranes [5, of the chemistry of metal tetrahydroborates, 14, 15] is important because the details of its the methods of the synthesis of borohydrides mechanism are essential for synthesis methods of alkaline metals (M´ÂÍ4): LiBH4, NaBH4 and that are under development at present. Am- KBH4 were developed (1940–1950). Reviews and monia interacts with boranes to form nonvola- monographs were published on this subject [1, ⋅ ⋅ tile products such as Â2Í6 2NH3, B4H10 4NH3, 30, 35–38]. For example, the authors of [30] ⋅ B5H9 4NH3. Their heating in a tightly closed ap- considered the principles and apparatus ar- paratus leads to complicated overall reactions like rangement for the production of Ì´ÂÍ4. ⋅ 3(Â2Í6 2NH3) = 2(cyclo-B3N3H6) + 12H2 (1) Reactions of the synthesis of M(ÂÍ4)n can When this process is carried out at a tempera- be related to the following classes: ture of 180–220 C for 0.3–3 h, the yield of – ° A. Formation of BH4 anion [30, 35, 37]: borazine is 23–41 % [15]. Side products are also 2LiH + B H → 2LiBH ; in ether, formed; they contain B, H, N, so the yields 2 6 4 LiBH4 is solvated (6) decrease. The final stage of the formation of → 4M´H + B(OCH3)3 M´BH4 + 3MOCH3; heterocycle takes place at increased tempera- M´ = Li, Na (7) ture similarly to the case of the formation of → Na2B4O7 + 7SiO2 + 16Na + 8H2 4NaBH4 organic cyclic structures like C6H6 and others + 7Na SiO (8) from non-cyclic hydrocarbons CH , C H [28]. 2 3 4 2 2 Reaction (8) is carried out at 450–500 °C and There are many reactions involving the syn- the pressure of H2 equal to 3 atm, the yield is thesis of borazine derivatives like (XBNR)3 90 %. Liquid ammonia is used to isolate NaBH4 where X stands for halogens, R = H or or- from the reaction mixture [30]. ganic radicals, more frequently Me, Et, Bu, B. Reaction of BH– anion exchange [23, 35, Ph [2, 4, 18, 29]. The authors of [4, 29] de- 4 scribed the syntheses of some of these com- 37, 39, 40]: → pounds including the following stages: NaBH4 + LiCl LiBH4 + NaCl; THF, → ⋅ dimethylformamide (9) BX3 + NR3 X3B NR3 ⋅ → NaBH + KOH → KBH + NaOH; aqueous X3B NR3 X2BNR2 + RX (2) 4 4 → medium, KOH in excess (10) 3(X2BNR2) cyclo-(XBNR)3 + 3RX → The second and the third stages usually pro- 4NaBH4(s) + ZrCl4(s) Zr(BH4)4(liquid, vapour) ceed in reaction mixture under heating. + 4NaCl(s) (11) Examples may also be the syntheses of bo- Covalent bonding type is realized in M(ÂÍ4)n structures with the participation of bridging razine derivatives from BCl3 and NH4Cl in boil- H atoms according to the scheme ̖͖ÂÍ ing chlorobenzene C6H5Cl (boiling point 132 °C) (Ì = Be, Al, Zr, Hf, Th, Ti(III), U(IV)) with and from BCl3 and aniline Ph-NH2 [2, 29, 30]: → the formation of volatile non-ionic compounds 3BCl3 + 3NH4Cl cyclo-(ClBNH)3 + 9HCl (3) → M(BH ) [36, 39, 40]. Compounds Ì ÂÍ (M = 3BCl3 + 6PhNH2 cyclo-(ClBNPh)3 4 n ´ 4 ´ ⋅ Na, K, Rb, Cs) belong to the ionic type. + 3PhNH2 HCl (4) At present, LiBH , NaBH , KBH are man- Borazine derivative cyclo-(ClBNPh)3 was ob- 4 4 4 tained as long ago as in 1889 according to reac- ufactured on an industrial scale and are com- tion (4), but this discovery was not noted by mercially available [41]. For the synthesis of contemporaries [2]. Reactions like (3) and (4) are borane compounds under MA conditions it is interesting because they proceed without par- reasonable to use more stable, relatively cheap δ δ- – ticipation of hydride atoms Í . and handy NaBH4 as a carrier of H . A nodal event in the development of mod- Reaction sequences with Ì´ÂÍ4 are impor- ern chemistry of boranes was the discovery of tant for the synthesis of borazines [2, 24, 42]: metal tetrahydroborates by Schlesinger and ′′+25 oC→+ M BH44 NH Cl [NH 44 BH ](s) M Cl (12)

Brown [31–34] in the interaction between B2H6 o NH BH106 →+C H B(NH ) BH (s) 2H (13) and organometallic compounds MRn: 44 2 324 2 BORAZINE AND ITS DERIVATIVES 229

120–140 oC SOLID-PHASE REACTIONS OF BORAZINE SYNTHESIS 2324→ HB(NH)BH cyclo-B3N3H6 (vapour) ++2 (BHNH)n H (14) Let us pass to the consideration of solid- phase reactions of borazine synthesis. The first re- Tetrahydroborates Ì´ÂÍ4 are carriers of ac- δ tive hydride hydrogen Í -. This feature found action of this type was performed by H. Schelisnger application in selective substitution of elements [55], and then reproduced and improved by V. I. Mi- bound to boron or nitrogen atoms of borazine kheeva and V. Yu. Markina [56]: → for H atoms [4]: 3LiBH4(s) + 3NH4Cl(s) cyclo-H3B3N3H3(vapour) → + 3LiCl(s) + 9H (17) 2R3N3B3Cl3 + 6M´BH4 2R3N3B3H3 2 The reaction is carried out in a flask under stir- + 3B2H6 + 6M´Cl → ring the mixture of reagents at 250–300 C. Due 3BCl3 + 3NH3 cyclo-2,4,6-Cl3B3N3H3 (15) ° + 6HCl to the formation of side products, the yields → (18–42 %) are poorly reproducible. The use of 2Cl3B3N3H3 + 6NaBH4 2B3N3H6 + 3B2H6 + 6NaCl; diglyme, yield 46 %. LiBH4 has some disadvantages: a) lithium is a Reaction (15) with varying R and X is typical rare alkaline metal, its compounds are expen- for the synthesis of borazine derivatives. sive, recovery is necessary; b) LiBH4 is a sub- ⋅ Alkylaminoboranes R3–nHnN BH3 serve as stance inflammable when in contact with mois- the starting and sometimes intermediate ture, so special precautions are to be taken in compounds in the synthesis of borazine the case if large amounts of reagents are in- derivatives. An example of these processes is the volved. For these reasons, laboratory process sequence of reactions (2). These compounds were (17) with LiBH4 did not win technological ap- described for the first time by Wiberg in 1948 plication. It is reasonable to perform syntheses [43], discussed in reviews [44, 45] and using the commercially available compound monographs [1, 29, 30, 46]. Many methods of NaBH4. However, NaBH4 is less active in reac- ⋅ the synthesis of R3N BH3 are based on reactions tions than LiBH4. In the version with NaBH4, performed in organic solvents. At present, some MA of the reagent mixture in mills is required. ⋅ R3N BH3, their solutions and derivatives are Information about mechanochemical syntheses manufactured on a commercial scale [41]. of borazine from NaBH4 and NH4Cl was pre- Taking into account the applied significance sented in [21–24, 42, 57–60]. Synthesis proceeds ⋅ of complexes L BH3, efficient mechanochem- similarly to reaction (17): ical methods of their synthesis were developed, → 3NaBH4(s) + 3NH4Cl(s) B3N3H6(vapour) possessing the above-indicated advantages [47, ∆ o ⋅ + 3NaCl(s) + 9H2, – H298 = 233.63 kJ/mol (18) 48]. The features of the synthesis of R3N BH3 were studied during MA according to the reac- The first laboratory set-up for borazine syn- tions of the type thesis and operation mode for MA according to ⋅ → ⋅ reaction (18) were described in detail in [58]. R3N HCl(s) + M´BH4(s) R3N BH3(liquid, The reagents used in the syntheses were NaBH vapour) + M´Cl(s) + H2 (16) 4 where R = Me, Et; Ì´ = Li, Na, K; apparatus of 96.5 % purity and NH4Cl of Kh. Ch. reagent is a rotary vacuum ball or vibratory mill, t = grade (chemically pure) at the molar ratio τ ⋅ NaBH /NH Cl = 0.31. Under optimized condi- 70–100 °C, MA duration = 3–6 h, R3N BH3 4 4 yield varies within the range 74–87 % depend- tions, the yield of borazine reaches 38 % with τ respect to NaBH . The characteristics of thus ing on R3N, M´, t, [48]. By means of MA 4 ⋅ obtained borazine are: T = (–58±1) C; IR spec- according to reaction (16) complexes L BH3 m ° trum, ν, cm–1: 3450, 2535, 1465, 1070, 920, were synthesized, with L = Me2HN, Et2HN, 700, 520 and 402 – correspond to the published (C4H9)HN, (C6H5CH2)3N, C5H5N, tribenzylamine, N2H4 with the yield of 34–98 % [22–24, 47– data [61, 62]. The yield is close to that achieved 54]. These complexes may be the initial com- in the classical works. A disadvantage of the pounds for the synthesis of borazines. Volatile set-up with oscillating reactor [58] is the possi- ⋅ compounds L BH3 containing simple radicals bility to disturb impermeability. To eliminate it, Me and Et are suitable for depositing carboni- borazine synthesis mode with preliminary MA troboride films and coatings like BxCyNz on sub- of the mixture of NaBH4 with NH4Cl at room strates from the gas phase [22–24]. temperature was developed [24, 60]. The mix- 230 V. V. VOLKOV et al.

3– ture was subjected to MA in a tightly closed and PO4 form undesirable gases in contact with rotating ball mill without connecting it at the BH– : Í S and ÐÍ [9, 37]. MA stage to the gas withdrawal system. The 4 2 3 set-up in which the synthesis principle with preliminary MA of the mixture was described MECHANOCHEMICAL METHODS OF THE SYNTHESIS in [60]. It was used for technological syntheses OF N-TRIMETHYLBORAZINE AND N-TRIETHYLBORAZINE of borazine at the Novosibirsk Capacitor Plant (MEI USSR). A typical synthesis mode involved The necessity to develop mechanochemical charging 250 g of NaBH4 and 500 g of NH4Cl methods of the synthesis of non-ionic N-tri- into the reactor (molar ratio NH4Cl/NaBH4 = methylborazine (TMB) or cyclo-(ÑÍ3NBH)3, and 1.5), MA duration was 1.5 h. Then reactor tem- N-triethylborazine (TEB), or cyclo-(Ñ2Í5NBH)3 perature was gradually increased to 270 °C. The was dictated by the possibility to deposit car- resulting mixture of vapour and gases was bonitride layers and films with the composition passed through cooled condenser traps. The yield ÂxCyNz (ideally BCN) on substrates using ther- of the raw product containing 0.2 % B2H6 and mal or plasmachemical decomposition of their 0.1 % NH3 is 34–51 g (22–30 %) [60]. After re- vapour [71–73]. The primary information on ob- peated recondensation at –78 °C, no foreign taining boron carbonitride (BCN), a valuable elec- admixtures were detected. Replacement of tric insulating material, by means of high-tem-

NaBH4 by LiBH4 or KBH4 has no substantial perature metal ceramics was published in [74]. The effect on the yield. technology of obtaining metal ceramics is unsuit- For the synthesis of larger (up to 150 g) able for manufacturing the films for microelec- amounts of borazine according to reaction (18), tronic devices, so attention is paid to the use of a vacuum vibratory set-up with the vertical TMB vapour. The stoichiometric composition of arrangement of units was developed [63, 64]. TMB holds up the atomic composition (BCN) and Due to the complicated maintenance, it was possibly the elements of its structure, similarly replaced by a simpler set-up [60]. to the resemblance of compositions and struc- The possibility of the mechanochemical syn- tures of borazine with h-BN marked above (see thesis of borazine at lower temperatures Fig. 3). As far as TEB is concerned, it should be (~80 °C) was demonstrated in [65]: noted that its stoichiometry does not meet the 4LiBH4(s) + (NH4)2ZnCl4(s) = 2/3B3N3H6(vapour) composition (BCN) in carbon content. The effect + B2H6 + 4LiCl + Zn + 7H2 (19) of this feature on the properties of resulting films Thermal or plasmachemical decomposition of bo- needs more detailed investigation. razine vapour was used to obtain insulating films The problem of the synthesis of alkyl de- composed of BN [24, 66–69] according to reaction ′ rivatives of borazines RNBR3333 was consid- →600–800 o C ered in [2, 4, 29]. TMB was obtained for the BNH(vapour)336 3BN(s) + (20) first time according to the sequence of trans- 3H2 (g) formations of Â2Í6 and methylamine Í3C–NH2 Increased attention to the use of borazine under heating in the absence of the air [75, 76]: to obtain inorganic polymers and films is stressed → ⋅ B2H6(g) + 2ÑÍ3NH2(vapour) 2ÑÍ3NH2 BH3(s) in [70]; described is the method of borazine syn- o 3CH NH⋅ BH100 C→+ (CH NHBH ) 3H thesis in solution according to reaction 32 3 3 232 3(NH ) SO + 6NaBH → 2B N H + 3Na SO 200 oC→ (22) 4 2 4 4 3 3 6 2 4 (CH323 NHBH ) cyclo-(CH3NBH)3 +3H2 + 18H (21) 2 Remarkable features of the process are the for- –+ At the molar ratio of BH44 /NH = 0.65 in tet- mation of monomethylaminoborane τ ⋅ raglyme, temperature 120–140 °C and = 3 h ÑÍ3NH2 BH3 at the first stage and the forma- the yield of borazine was 10–20 g (58–60 % tion of TMB cycle at 200 °C at the last one with respect to BH– ). It is possible to use acid (a usual condition for the formation of hetero- 4 cycle [B N ]). Provided that Â Í is available, salts (NH )HSO and (NH ) HPO in reaction 3 3 2 6 4 4 4 2 4 the synthesis according to reactions (22) can be 2– (21), but the results are worse because SO4 used for technological purposes. BORAZINE AND ITS DERIVATIVES 231

Almost simultaneously, a method of the The effect of the molar ratio n = M´BH4/ ⋅ τ synthesis of TMB through the interaction of EtNH2 HCl, process duration , nature of LiBH4 with methylamine hydrochloride M´BH4 and MA conditions was studied. Stage- ⋅ ÑÍ3NH2 HCl in diethyl ether in which LiBH4 by-stage pyrolysis of the products of MA re- is soluble was proposed [77, 78], while NaBH4 action at 90 °C and then at 240 °C leads to the and KÂÍ4 are insoluble in ether. The first stage formation of TEB. The maximal yields of TEB of the process is were 79, 60 and 74 % for LiBH4, NaBH4 and o KBH , respectively, and for n = 1.3–1.7. The LiBH+ CH NH ⋅ HCl 25 C →⋅ CH NH BH 4 432 323 product was characterized on the basis of melt- + 3 LiCl+H2 ing point –48 °C, density of the liquid 0.836 g/cm After evaporation of ether, the reaction mass 20 = at 20 °C, refractive index nD 1.4365 and pres- was heated to 250 C. Thus methylaminoborane ° sure of saturated vapour 3.0 mm Hg at 24 °C, was transformed into volatile TMB. The which correspond to literature data for TEB [78]. synthesis of TMB using NaBH4 in boiling The IR spectra of synthesized TMB and TEB triglyme was also reported [78]. It is interesting are presented in Fig. 3. These spectra confirm that this work was published as early as in 1960 the nature of the substances and are suitable in Chemistry and Industry (the USA). In the for analytical purposes. Investigation of the sequences of reactions (22) and (23) TMB synthesis of TMB and TEB provides evidence molecules are formed during pyrolysis of of the efficiency of MA method and its prom- ⋅ CH3NH2 BH3 which is formed at the first stage ising character for obtaining the considered sub- of synthesis and relates to alkylaminoboranes stituted borazines. ⋅ R3–nHn BH3 with n = 2. It was demonstrated For the synthesis of h-BN, interesting re- ⋅ above that R3N BH3 is easily obtained by means actions may be thermolysis of hexahydrobora- of MA from crystal borohydrides of alkaline zine (called also hexahydroborazole or cyclot- ⋅ metals and R3N HCl without using Â2Í6 and riborazene), cyclo-Í6Â3N3H6, that is, hydro- organic solvents. The conditions of the synthesis ⋅ of CH3NH2 BH3 by means of MA from ⋅ MeNH2 HCl and Ì´ÂÍ4 for Ì´ = Na, Li, K: ⋅ CH3NH2 HCl(s) + M´BH4(s) → ⋅ CH3NH2 BH3(s) + M´Cl(s) + H2(g) (24) were studied in [80]. The reaction was carried out in a vibratory ball mill [23, 24]. The next stage ⋅ → 3CH3NH2 BH3 (CH3NBH)3 + 6H2 (25) was performed under stepwise temperature rise up to 100 and 250 °C with gas removal. The synthesized TMB was identified on the basis of melting point (–7 °C), pressure of sat- urated vapour at 25 °C (11.7 mm Hg), density 3 of the liquid phase at 20 °C (0.850 g/cm ), re- 20 = fractive index ( nD 1.440 ) and IR spectrum cor- responding to the characteristics reported in [75,

77, 80–83]. The yields of TMB with LiBH4, NaBH4, KBH4 are 60, 34 and 36 %, respectively. Similarly to the synthesis of TMB accord- Fig. 3. IR spectra of TMB (1), TEB (2) and a scheme of ing to reactions (24) and (25), the synthesis of H3B3N3R3 molecules. Spectrophotometer: UR-20; samples: films pressed between discs made of KBr. Band positions, TEB under MA of the mixture of ethylamine ν, cm–1: TMB – ν(CH) 2948 m, 2818 w; ν(BH) 2491 s; ⋅ ν δ ρ δ ρ hydrochloride EtNH2 HCl with Ì´ÂÍ4 for Ì´ = (BN) 1470 v.s; (CH) 1410 v.s; (CH), (BH) 1070 m; (ÂÍ), Li, Na, K was investigated for the sequence ρ(CN) 878 s; TEB – ν(CH) 2970 s, 2935 m, 2893 m, 2875 ν ν δ ⋅ → ⋅ sh.; (BH) 2492 s; (BN) 1443 v.s; (CH3) 1480 s, 1379 s; EtNH2 HCl + M´BH4 EtNH2 BH3 δ(CH ) 1443 v.s; ρ(CC) 1118 w, 1093 m, 886 s; δ(BH) 1079 w; → → 2 (EtNHBH2)n cyclo-(EtNBH)3 (26) γ(BH) 982 w, 886 s. 232 V. V. VOLKOV et al. genated derivative of borazine [84–87]. 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