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Neutral Lewis base adducts of silicon tetraazide.

The field of binary main-group element azides has enjoyed a renaissance in the last decade, leading to many fascinating compounds. Binary azides of Group 14 elements are a class of rare, highly endothermic compounds. Their isolation and handling poses considerable challenges to experimentalists due to the combination of high energy content, excessive sensitivity and thermal lability. Therefore, it is not surprising that to date only the primary explosive a-Pb(N3)2 [4] and the ions [C(N3)3] + [5] and [E(N3)6] 2 (E = Si–Pb) 6, 7] have been structurally characterized. Recently, the extremely hazardous compound C(N3)4 was isolated in tiny amounts and transformed into various organic products. Si(N3)4 has been reported to be a violently explosive substance, which could not be obtained in pure form. Experimental evidence for the presence of pure Ge(N3)4 is lacking, [10] and Sn(N3)4 and Pb(N3)4 are presently not known. Nitrogen-rich silicon compounds are of special interest due to their potential as a viable replacement for lead azide to avoid its deleterious environmental impact and as precursors for new materials. Herein we present the large-scale synthesis and full characterization of conveniently accessible, thermally stable, and highly energetic Lewis base adducts of Si(N3)4, and the safe synthesis and handling of solutions of pure Si(N3)4. Addition of SiCl4 to a suspension of 7.3 equiv of NaN3 in acetonitrile at room temperature afforded selectively the disodium salt of hexaazidosilicate (1; Scheme 1). Evidence for the formation of 1 was provided by its selective chemical functionalization (see below) and the solution IR spectra, which displayed one strong nasym(N3) absorption band at 2109 cm 1 and one weak nsym(N3) absorption band at 1317 cm 1 after completion of the reaction. Both bands appear at the same positions as those reported for (PPN)2[Si(N3)6] (4 ; PPN + = N(PPh3)2 ). Compound 1 forms colorless solutions in acetonitrile that are sensitive to hydrolysis but can be stored for several weeks under exclusion of air at 28 8C and used as stock for the syntheses of derivatives of Si(N3)4. Treatment of 1 with a slight excess of the Lewis bases 2,2’-bipyridine (bpy) and 1,10-phenanthroline (phen) afforded, after precipitation of NaN3, exclusively the Lewis base adducts [Si(N3)4(bpy)] (2) and [Si(N3)4(phen)] (3), respectively (Scheme 1). After work-up and recrystallization from acetonitrile, compound 2 and the MeCN hemisolvate of 3 were isolated as colorless, analytically pure needles in 57– 60% yields (from SiCl4). No explosions occurred during the repeated preparations of 2 and 3·0.5 MeCN, which can be scaled-up to several grams of the desired compound. Both compounds are not sensitive to friction and are moderately soluble in CH2Cl2, THF, and MeCN. Although solutions of 2 and 3·0.5 MeCN are rapidly hydrolyzed, releasing HN3 and the Lewis bases (bpy or phen), the crystalline compounds can be stored and handled safely at ambient temperature under dry air. Under vacuum, compound 2 melts at 212 8C, whereas 3 decomposes upon melting at 215 8C. The remarkable thermal stability of 2 and 3·0.5 MeCN is surprising in view of their reactive nitrogen contents of 44–48% and the extreme sensitiveness of Si(N3)4. The thermochemical properties of 2 and 3·0.5 MeCN were studied in more detail by differential scanning calorimetry (DSC) and compared with those of the analogous germanium compounds [Ge(N3)4(bpy)] (2a) and [Ge(N3)4(phen)]·0.5MeCN (3 a·0.5 MeCN). [13] Representative thermograms of 2 and 3·0.5 MeCN are depicted in Figure 1. The thermogram of 2 reveals that melting at the extrapolated onset temperature Ton ex = 211 8C (endothermic peak temperature Tp endo = 212 8C, DHm = + 110 Jg ) is followed by a distinct decomposition process, which begins at Ton ex = 265 8C (Tp exo = 294 8C), and liberates a large heat of decomposition (DHd = 2.4 kJg ). The germanium analogue 2a shows a similar behavior. In comparison, compound 3·0.5MeCN Scheme 1. Syntheses and reactions of Si(N3)4 and its Lewis base adducts. For an alternative synthesis of compound 4, see Ref. [6].