GROUP 15 TRIAZIDES: A COMPREHENSIVE THEORETICAL STUDY AND THE PREPARATION OF BISMUTH TRIAZIDE Thomas M. Klapötke* and Axel Schulz Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK Abstract: The geometries and frequencies of all group 15 triazides have been theoretically predicted using quantum mechanical ab initio methods at the Hartree-Fock level of theory and density functional theory (B-LYP). For As, Sb and Bi quasi-relativistic effective core potentials were applied. The calculated IR frequencies (HF level) are in reasonable agreement with those which have been observed experimentally (E = P, As, Sb, Bi). The geometries and energies of two different structural isomers are compared. For nitrogen and phosphorus triazide the cis structure was calculated to be slightly lower in energy, whereas the cis structure becomes clearly less favorable compared to the trans structure when X is As, Sb, and Bi. The electronic structure of all species has been investigated using the natural bond orbital population analysis (NBO). Detailed information is given about the nature of the X-N bond and the hybridization and the atomic population of the central atom X in both possible azid structures. The synthesis for Bi(N3)3 is reported as well as an attempted synthesis for N(N3)3. Introduction In contrast to the chemistry of halogen azides, which has been extensively explored in the last years,la,b studies on binary Group 15 azide compounds are still very limited. However, in a recent study + the structures and stabilities of the azidamines N(N3)3, HN(N3)2, the N(N3)2" anion, and the N(N3)4 cation have been theoretically predicted.10 We have recently been studying the reactions of various nitrogen, phosphorus and arsenic halides with silver azide and activated sodium azide.1,2 Phosphorus 3 triazide, P(N3)3, was first reported by Schmidt in 1968. In 1995 we described the synthesis of the first + 4 binaiy arsenic azide species As(N3)3 and [As(N3)4] . Quite recently, we also reported on the reaction of 5 Sbl3 with freshly prepared silver azide. Reaction of Sbl3 and AgN3 in acetonitrile results in the formation of the binary antimony triazide Sb(N3)3 [eq. (1)]. Pure Sb(N3)3 was separated by extraction of the crude product with acetonitrile at room temperature and isolated in high yield as a white explosive l4 solid. Subsequently, Sb(N3)3 was identified from its N NMR and IR spectrum. CH3CN, rt Sbl3 + 3 AgN3 > 3 Agl + Sb(N3)3 (1) Naturally, the preparation of Sb(N3)3 led to the attempted preparation of bismuth triazide. In this contribution we want to report on preparation of Bi(N3)3 as well as on an attempted preparation of N(N3)3. We also present a comprehensive theoretical study of all group 15 triazides X(N3)3 (X = Ν, P, As, Sb, Bi). Dedicated to Professor Hartmut Köpf on the occasion of his 60th birthday. 325 Vol. 20, No. 5, 1997 Group 15 Triazides: A Comprehensive Theoretical Study and the Preparation of Bismuth Triazide Materials and Methods CAUTION: All group 15 triazides, silver azide and nitrogen trichloride are explosive. Materials. Silver azide, AgN3, was always freshly prepared prior to use according to the previously 6 published procedures and checked by IR spectroscopy. Nitrogen trichloride, NC13, was also always freshly prepared in H20/CC14 solution by direct chlorination of NH,C1 and isolated as a pure compound 7 by fractional condensation. Bil3 (Aldrich) was used as supplied. All solvents [CH3CN (Fisons), CH2C12 (Fisons), CFC13 (Merck)] were dried over P4O10 and distilled prior to use. All manipulations were routinely performed under an inert gas atmosphere (N2, dry box). Spectroscopy. Infrared spectra were recorded at 20°C as Nujol mulls between KBr plates on a Philips PU9800 FTIR spectrometer. 3 Preparation ofBi(N3)3. AgN3 (0.176 g, 1.17 mmol) was suspended in 10 cm CH3CN and treated with dark green-black Bil3 (0.213 g, 0.34 mmol) at 20°C and the slurry stirred in the dark. After 12 h the color of the insoluble material had changed from initially green-black (Bil3) to yellow (mixture of Agl and Bi(N3)3). All attempts to separate the insoluble products (Agl and Bi(N3)3) by extraction with either CH2C12 or CH3CN were unsuccessful. An attempt to sublime Bi(N3)3 resulted in an explosion of the reaction mixture. The only product identified after the explosion was metallic bismuth (and possibly nitrogen) but no bismuth nitride. We also tried to prepare Bi(N3)3 from Bil3 and activated sodium azide, which is easier to handle and can be stored for some time. However, no reaction was observed in acetonitrile. This is probably due to the low solubility in CH3CN of both NaN3 and Nal while Agl is "completely" insoluble. Attempted preparation of N(N3)3. Experiment 1: Addition of AgN3 (0.60 g, 4.00 mmol) to a solution of NC13 (0.16 g, 1.33 mmol) in CFC13 at -78°C resulted in an immediate reaction and the formation of nitrogen gas which was identified by gas discharge. After the reaction was completed at -78°C we immediately recorded a MN NMR spectrum at this temperature. However, no nitrogen containing compound (beside dissolved N2 gas: δ —70.0 ppm rel. to CH3N02) could be detected in the MN NMR spectrum. Experiment 2: A reaction vessel was loaded with AgN3 (1.00 g, 6.67 mmol, excess) which was suspended in CFC13. The AgN3 was then converted into a fine powder by a combination of magnetic stirring and ultra sound application. In the next step all CFC13 was pumped off under dynamic vacuum at -78°C and pure NC13 (0.16 g, 4.00 mmol) was condensed onto the silver azide. The condensation of the NC13 onto the AgN3 caused an immediate and violent explosion which destroyed both the glass reaction vessel and the steel Dewar flask. Computations. The structures and the vibrational frequencies of all the E(N3)3 azide molecules (E = Ν, P, As, Sb, Bi) were computed ab initio at the HF level of theory. In addition, the geometries of all species were also computed using the DFT theory. Becke's functional where the non-local correlation is provided by the LYP expression (Lee, Yang, Parr correlation functional) was used which is implemented in the program package Gaussian 94.8 For a concise definition of the B-LYP functional see ref. 9. For Ν and Ρ a 6-31+G* basis set was used. For the heavy atoms As, Sb and Bi quasi-relativistic pseudopotentials (As: ECP28MWB, Sb: ECP46MWB, Bi: ECP78MWB)10 and a (5s5p)/[3s3p]-DZ basis set extended with a single set of d functions [As: dexp = 0.293, Sb: deXp = 0.211, Bi: dexP = 0.185] were used.11 Natural bond orbital analyses (NBO) were carried out to account for non-Lewis contributions to the most appropriate valence structure.12 In the quantum mechanical computation (NBO analysis, subjecting 326 Τ.Μ. Klapotke and Α. Schulz Main Group Metal Chemistry the HF density matrix as represented in the localized NBOs to a second-order perturbative analysis) the energy was computed according to equation (2) with hF being the Fock operator. (2) <<phV> f -2- ΨΦ* (2) Εφ* Εφ Results and Discussion Preparative Aspects All attempted preparations of N(N3)3 failed and led instead either to rapid decomposition under formation of nitrogen gas or to an explosion of the reaction mixture. We therefore state that N(N3)3 is not a stable molecule at or above -78°C but decomposes according to equation (3) to give elemental nitrogen. T2>-78°C N(N3)3 > 5 N2 (3) Bismuth triazide, Bi(N3)3, was prepared according to equation (4) and undoubtedly identified by its IR spectrum compared with the theoretically predicted vibrational data (see below). However, we have so far been unable to separate the bismuth azide from the reaction by-product silver iodide due to very similar poor solubility of both compounds in most common solvents and explosive decomposition of Bi(N3)3 upon attempted sublimation. 20° C, acetonitrile Bil3 + 3 AgN3 > Bi(N3)3 + 3 Agl (4) Computational Aspects Recently, the geometries and vibrational spectra of all halogen azides XN3 (X = F, CI, Br, I) and HN3 were computed employing ab initio methods at the Hartree-Fock level of theory (HF, MP2) and density functional theory calculations at the self-consistent-field level with the nonlocal exchange functional of Becke (Β) and the nonlocal correlation functional of Lee-Yang-Parr (LYP).1,13 The general agreement between the computed geometries at correlated levels (MP2, B-LYP) and the observed (micro wave, electron diffraction) structures is very good. Therefore we decided to to investigate the series of group 15 triazides X(N3)3, X = Ν, P, As, Sb, Bi, using the density functional theory (B-LYP) and quasi - relativistic pseudopotentials for the havier elements (As, Sb, Bi). In order to save cpu time we carried out the frequency analyses only at the HF level in which case scaling is required.13 There are no experimental geometries available which may be used to compare our results for both structures types A and Β (Fig. 1). Experimental frequencies, however, can be compared with at HF level calculated frequencies for P, As, Sb, and Bi (references see Table 4). The scaled frequencies are in reasonable agreement with those experimentally observed (cf. Table 3 and 4). As the first step in our investigation, we calculated the azides X(N3)3 in Cι symmetry at HF level.