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) < 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. The only structures found which represent minimum structures are the X(N3)3 structures A and Β of C3 symmetry as shown in Fig. 1 (X trans to N3). The difference between structure A and Β is the Vol. 20, No. 5, 1997 Group 15 Triazides: A Comprehensive Theoretical Study and the Preparation of Bismuth Triazide position of the N3 group relative to the lone pair (or a dummy atom Y) localized at the central atom X. Structure A resembles a trans like structure (Y trans to N2, A2 > 90.0°) and Β resembles a as like structure (Y cis to N2, A2 < 90.0°). These structures were further optimized using the DFT method (B- LYP) as described above. The calculated energies and geometries are listed in Table 1 and 2.

N3b

Fig. 1 Schematic representation of the structure A and Β of X(N3)3 (X = Ν, P, As, Sb, Bi; Y and Ζ = dummy atoms; dihedral angle Y-X-Nl-Z = 0.0°).

The calculated energies predict that for N(N3)3 and P(N3)3 structure Β is lower in energy whereas for

X(N3)3 (X = As, Sb, Bi) structure A is more stable. The energy difference between both structure is less 1 than 2 kcal mol" at B-LYP level (for P, As, Sb). The geometry optimization of structure A for N(N3)3 at B-LYP level failed because the optimization always led to structure B. Fig. 2a shows the potential of

N(N3)3 at the HF level and Fig. 2b at the B-LYP level. At HF level the potential shows a very flat minimum for A which is separated from Β by an energy barrier of only 1.3 kcal mol1 (A2 = 108.4 ° in the transition state structure). It follows that A is not a minimum on the potential energy surface of

N(N3)3 at B-LYP level. Our computed structural parameters of isomer Β of N(N3)3 are in excellent lc agreement with an earlier study by Christe, Dixon et al. For Bi(N3)3 neither at HF nor at B-LYP level a minimum structure of type Β was found. It seems that the larger the atom X the more unstable becomes structure Β since the repulsion between the lone pair (Y) localized on X and the N1-N2 bonding electrons increases (Fig. 1).

Furthermore, for N(N3)3 and P(N3)3 structure Β is favored since in structure A the repulsion between the p-lone pair localized at Nla and Nlb/lc is larger than for X(N3)3 (X = As, Sb, Bi) resulting in an

328 Τ.Μ. Klapotke and Α. Schulz Main Group Metal Chemistry

unusually large Nla-N-Nlb angle of 113.4° for N(N3)3. With increasing X-Nl bond length this repulsion decreases. In all cases, the Nla-X-Nlb angles are slighty larger in structure A compared to structure Β due to this repulsion.

Table 1 Calculated energies for group 15 azides a

-Em( A) B) Δ/?™(Α-Β) -£B"LYP( A) -£®"LYP(B) M®-LYP(A-B)

N(N3)3 544.10514 544.12812 + 14.4 b -547.17432 b

P(N3)3 830.54915 830.55220 +1.91 833.90590 833.90859 +1.69

As(N3)3 495.86113 495.85693 -2.63 498.75679 498.75481 -1.24

Sb(N3)3 495.09823 495.09076 -4.69 497.99476 497.99220 -1.61

Bi(N3)3 495.05959 b b 497.96468 b b

Due to the stereochemical^ active lone pair on the Gp. 15 central atom structure A resembles a trans-like structure (Y trans to N2) and Β resembles a cü-like structure (Y cis to N2); Y is a dummy atom, cf. Fig. 1.

For N(N3)3 only at HF level a minimum structure for A was found. For Bi(N3)3 neither at HF nor at B-LYP level a minimum structure for Β was found.

As expected for covalently bound azides, X(N3)3 (X = Ν, P, As, Sb, Bi) displays a bent configuration with an N-N-N bond angle between 172 and 177°. The N-N bond in N(N3)3 is rather long 4 with 1.496 Ä, comparable to 1.49 A in F2N-NF2.' The value of the N-Nl distance corresponds to a bond 15 order less than one (typical values: N-N single bond, 1.447 Ä in N2H4, N=N double bond, 1.252 A in 16 N2H2). This slighly longer single X-N bond can be predicted for all X(N3)3 (X = Ν, P, As, Sb, Bi). The predicted N1-N2, and N2-N3 bond lengths are very similar to experimental 17 and theoretical 13 bond 1S distances found for the halogen azides, XN3 (X = H, F, CI, Br, I, At ) and show the same trend: c?(Nl- N2) decreases the larger X is whereas t/(N2-N3) increases.

The theoretically predicted harmonic vibrational frequencies and IR intensities of all X(N3)3 are presented in Table 3. The deviation from the experimentally obtained frequencies (Table 4) may be compensated by using scaling factors. The calculated frequencies at HF level are usually too high (ca. 10%). The scaling required may also be different for various vibrational modes which are present in the 13019 molecule. According to Table 3, ω ι and co2 are almost purely assymmetric stretching modes of the

N1N2N3 moiety. Due to the C3 symmetry the assymmetric stretching mode is splitted (as are all normal modes) into two vibrations (A, all in phase) and (E, 180° shifted). The next highest lying frequencies, co3 and ω4, represent the symmetric stretching N1N2N3 modes. The assignments of ω5 and co6 are not as straightforward as with ω1/2 and co3/4. These motions can be approximately described as a mixture of X- N1 stretching along with N2-N3 bending. As expected, the vibrational frequency for the X-N stretching mode (approximate assignment) decreases from Ν to Bi (Table 3 and 4). The remaining normal modes are either in-plane deformations (XNNN plane) or out-of-plane deformation vibrations.

329 Vol. 20, No. 5, 1997 Group 15 Triazides: A Comprehensive Theoretical Study and the Preparation of Bismuth Triazide

25-

20- ο Ε Ss s 15-

10 / E> ω c. / tu 05 /

0-

—I 1 r~ I —I— —I— —I 20 40 60 80 100 120 140 160 180 angle N2-N1-Z in deg.

Fig. 2a Energy potentials for N(N3)3 at HF level.

Fig. 2b Energy potentials for N(N3)3 at B-LYP level.

330 T.M. Klapotke and Α. Schulz Main Group Metal Chemistry

Table 2 Calculated structures of group 15 azidesa (bond lengths in Ä, angles in °)

A(HF) A (B-LYP) Β (HF) Β (B-I

N(N3)3

1.417 - 1.421 1.496

rf(N2-N3) 1.098 - 1.096 1.155

<(Nla-N-Nlb) 113.4 - 106.7 103.8

<(N-N1-N2) 114.4 - 107.5 108.5

<(N1-N2-N3) 172.6 - 173.9 170.9 a <(Y -N-Nl-N2) -141.5 - -27.5 -30.2

<(N-N1-N2-N3) 148.8 - -174.0 -178.2

P(N3)3

d(P-Nl) 1.736 1.792 1.722 1.781

AS(N3)3

rf(As-Nl) 1.864 1.928 1.851 1.917 rf(Nl-N2) 1.230 1.249 1.232 1.252 J(N2-N3) 1.098 1.158 1.098 1.159 <(Nla-As-Nlb) 99.8 100.8 95.5 94.3 <(As-Nl-N2) 119.5 120.7 116.3 117.4 <(N1-N2-N3) 176.5 174.8 175.6 173.5 <(Y-As-Nl-N2)a -135.4 -137.0 -47.7 -43.8 <(As-Nl-N2-N3) 167.6 167.3 179.4 179.6

Sb(N3)3

rf(Sb-Nl) 2.054 2.115 2.045 2.105 i/(Nl-N2) 1.225 1.247 1.223 1.247 J(N2-N3) 1.100 1.160 1.101 1.161 <(Nla-Sb-Nlb) 97.2 98.7 93.4 91.5 <(Sb-Nl-N2) 121.7 122.3 121.4 120.4 <(N1-N2-N3) 176.8 174.8 176.1 173.8 <(Y -Sb-Nl-N2)a -138.3 -138.4 -60.6 -35.8 <(Sb-Nl-N2-N3) 169.4 170.0 179.1 178.4

331 Vol. 20, No. 5. 1997 Group 15 Triazides: A Comprehensive Theoretical Study and the Preparation of Bismuth Triazide

Table 2 (continued)

Bi(N3)3

d{Bi-Nl) 2.165 2.215 rf(Nl-N2) 1.221 1.245 rf(N2-N3) 1.102 1.162 <(Nla-Bi-Nlb) 97.4 99.7 <(Bi-Nl-N2) 122.1 122.1 <(N1-N2-N3) 177.0 174.7 <(Y-Bi-Nl-N2)a -141.1 -139.8 <(Bi-Nl-N2-N3) 171.7 172.1

Due to the stereochemically active lone pair on the Gp. 15 central atom structure A resembles a trans-liks structure (Y trans to N2) and Β resembles a c/s-like structure (Y cis to N2); Y is a dummy atom, cf. Fig. 1.

Table 3 Calculated characteristic frequencies (in cm"1) and IR intensities (in parentheses) (HF/6-31+G*)a for structure A and Β of group 15 azides

Ν(Ν3)3 Ρ(Ν3)3 AS(N3)3 Sb(N3)3 Bi(N3)3

structure A

v^iNa), A 2461 (621) 2558 (945) 2542 (1051) 2531 (1344) 2506 (1544)

ω2) ν„(Ν3), Ε 2426 (477) 2520 (690) 2505 (714) 2491 (721) 2463 (737)

ω3, Vs(N3), A 1206 (122) 1378 (488) 1372 (515) 1397 (612) 1408 (638)

ω4, V,(N3), Ε 1260 (208) 1338 (498) 1334 (448) 1360 (420) 1373 (400)

ω5, V(X-N3), Ε 1108 (59) 778 (128) 484 (103) 449 (94) 422 (88)

Ω β, V(X-N3), A 965 (35) 761 (7) 528 (28) 486 (21) 457 (13)

ω7, δ(ΧΝΝΝ), Ε 765 (6) 632 (50) 749 (44) 726 (27) 716(18)

ω8, δ(ΧΝΝΝ), Α 781 (17) 632 (33) 750 (6) 727 (7) 716 (9)

ω9, δ(ΧΝΝΝ), Ε 578 (3) 585 (70) 645 (12) 665 (14) 674 (16) ω™, δ(ΧΝΝΝ), Α 579 (8) 663 (53) 648 (16) 663 (14) 675 (11) ω„, δ(ΧΝΝΝ), Α 467 (4) 445 (30) 356 (46) 286 (54) 239 (42)

ω12, δ(ΧΝΝΝ), Ε 442 (1) 330 (2) 276 (7) 228 (10) 199 (12) zpe / kcal mol"1 29.1 27.6 26.7 26.2 25.8

332 TM. Klapolke and Α. Schulz Main Group Metal Chemistry

Table 3 (continued)

structure Β

ω,, vas(N3), A 2485 (444) 2547(111) 2522 (43) 2511 (12)

ω2, vas(N3). Ε 2455 (771) 2523 (1455) 2598 (1718) 2485 (2114)

Ω3, Vs(N3), A 1272(125) 1361 (18) 1358 (3) 1405 (19)

ω4, Vs(N3), Ε 1221 (331) 1312 (898) 1320 (911) 1376 (983)

co5, V(X-N3), Ε 1129 (40) 812 (122) 514 (121) 464 (110)

co6, V(X-N3), A 1098 (5) 840(102) 528 (15) 484 (29)

ω7) δ(ΧΝΝΝ), Ε 793 (12) 665 (17) 765 (32) 725 (18)

ω8, δ(ΧΝΝΝ), A 635 (0) 660 (15) 771 (45) 727 (16)

co9, δ(ΧΝΝΝ), Ε 626 (13) 621 (106) 658 (10) 673 (11)

ω10, δ(ΧΝΝΝ), Α 592 (0) 589 (3) 664 (22) 674 (1) ω„, δ(ΧΝΝΝ), Α 539 (15) 387 (2) 305 (6) 235 (14)

ω 12, δ(ΧΝΝΝ), Ε 466 (11) 330 (10) 279 (9) 227 (7) zpe / kcal mol"1 29.7 27.9 26.7 26.2

Table 4 Experimentally observed IR frequencies (in cm"1)

3c 4a 5 a Ρ(Ν3)3 As(N3)3 Sb(N3)3 Bi(N3)3

Vas(N3), A 2186 (Raman) 2130 m 2161 m 2137 w

Vas(N3), Ε 2175 vs 2082 vs 2095 s 2093 s

VS(N3), A 1290 (Raman) 1238 s 1260 s 1262 w

- V,(N3), Ε 1260 s 1205 m 1202 w

V(X-N3), Ε 775 m 442 m 420 m -

V(X-N3), A - 400 m -

δ(ΧΝΝΝ), Ε - 683 w 670 w

δ(ΧΝΝΝ), Α 604 m - 668 w 650 w δ(ΧΝΝΝ), Ε 566 s 561 w 566 w (br) 550 w (br)

δ(ΧΝΝΝ), Α - 566 w (br) - δ(ΧΝΝΝ). Ε 445 s

a This work

In the following section we want to investigate the electronic structure of the group 15 azides quantitatively using the NBO population scheme.12 Of particular interest are the natural atomic composition of the X-Nl bond orbital and the intramolecular donor acceptor interactions. Table 5 summerizes the natural atomic hybrids of which the natural bond orbital X-Nl (X = Ν, P, As, Sb and Bi) is composed, the polarization coefficents (c) and spxd|J composition. For all molecules the polarization

333 Vol. 20, No. 5, 1997 Group 15 Triazides: A Comprehensive Theoretical Study and the Preparation of Bismuth Triazide

coefficients c(X) is smaller than c(Nl) corresponding to the higher electronegativity of N1 in the azid group. Both kind of structures (A and B) show the same trend. From nitrogen to bismuth c(X) gets smaller corresponding to an increasing polarization of the X-Nl bond whereas c(Nl) gets larger (cf.

Table 7). In case of the N(N3)3 molecule, the N-Nl bond represents an almost covalent bond [c(N) ~ c(Nl] and the hybrid composition corresponds roughly to the qualitative concept of interacting sp3 hybrids which agrees with the Nla-N-Nlb angle of 104° for structure Β (Table 2). This situation changes significantly going down the row from 76% p-character for nitrogen (sp3 26) to 95% for bismuth (sp17 64) explaining the decreasing Nla-x-Nlb angle from Ν to Bi (Table 2 ). The other side of this effect is that the sp-character of the lone pair localized at X changes significantly from X = Ν to X = Bi. In 2 32 N(N3)3 it resembles an sp hybrid (30 % s-character, 70 % p-character) whereas the p-character

decreases constantly to Bi(N3)3 where the lone pair occupies almost an 6s orbital which contains some p- character (84% s, 16 % p). Acoording to the most likely Lewis structure (Fig. 3), there are two lone pairs localized at Nl. One lone pair corresponds to a p-orbital the other one represents an spx hybrid ( λ=0.84 to 0.57 from Ν to Bi). All X-N bonds are rather long (Tab. 3). The value of the X-N distance corresponds to a bond order less than one (Tab. 6) indicating the increasing polarization of the X-N bond. The N-Nl bond represents more or less a non-polar two electron bond whereas the ionic character of the bond increases from Ν to

Bi. Apart from N(N3)3 the σ-bond X-Nl in X(N3)3 systems is found to be significantly ionic. The use of d-functions is very small and d-functions act primarily as polarization functions.

Table 5 Natural atomic hybrids of which the natural bond orbital X-Nl (X = Ν, P, As, Sb and Bi) is composed; the polarization coefficents (c) and the spxdH composition a

X c(X) λ(Χ) μ(Χ) c(Nl) λ(Ν1) μ(Ν1)

structure A

Ν 0.7155 2.81 0.01 0.6986 3.13 0.01 Ρ 0.4803 6.21 0.21 0.7578 3.01 0.01 As 0.4553 8.89 0.13 0.8904 4.86 0.01 Sb 0.4030 10.59 0.05 0.9152 7.92 0.02 Bi 0.3969 17.64 0.04 0.9179 13.01 0.02

structure Β

Ν 0.7203 3.26 0.02 0.6937 3.49 0.02 Ρ 0.4957 6.20 0.17 0.8685 3.26 0.01 As 0.4721 8.72 0.10 0.8816 5.20 0.02 Sb 0.4194 10.27 0.050 0.9078 8.49 0.02

Bi ------

a σ(Χ-Ν1) = c(X)h(X) + c(Nl)h(Nl) = c(X) sp'd^X) + c(Nl) spV(Nl); a pair of valence hybrids [h(X): hybrid localized on atom X; h(Nl) hybrid localized on atom Nl)] in the NHO basis give rise to a 2 2 bond σ(Χ-Ν1) in the NBO basis. (100[cx] and 100[cNi] gives the percentage of the NBO on each hybrid). SCF density used.

334 Τ.Μ. Klapotke and Α. Schulz Main Group Metal Chemistry

Fig. 3 Lewis structure.

The bond order indices for the N1-N2 bond indicate an intermediate bonding situation between a single and a double bond increasing from Ν to Bi (1.06 - 1.20). The N2-N3 bond is nearly a triple bond.

It is interesting to mention that in case of the N(N3)3 molecule all three N-N bonds are different: N-Nl corresponds to a bond order less than unity, N1-N2 lies between a single and a double bond whereas N2- N3 represents almost a triple bond (bond order between 2 and 3).

Table 6 Atom-Atom linear NLMO/NPA bond order BO(X-N 1) and energies due to hyperconjugation (E2 in kcal mol"1)

N(N3)3 P(N3)3 AS(N3)3 Sb(N3)3 Bi(N3)3

structure A

fiO(X-Nl) 0.97 0.50 0.44 0.36 0.34 £2[σ(Χ-Ν1)-> 7.8 27.1 39.4 56.8 70.6 π*(Ν2-Ν3)] £2[p-LP(Nla)-> 12.3 16.0 14.7 13.7 11.6 a*(X-Nlb)] £2[p-LP(Nla)-> 1.72 0.0 0.0 0.0 0.0 a*(X-Nlc)]

structure Β

ßO(X-Nl) 0.95 0.52 0.46 0.37 £2[σ(Χ-Ν1)-> 9.4 26.4 39.0 56.8 π*(Ν2-Ν3)] Ε2[ p-LP(Nla)-> 2.3 0.5 0.6 0.7 a*(X-Nlb)] £2[p-LP(Nla)-> 9.4 13.3 12.6 11.9 o*(X-Nlc)]

335 Vol. 20, No. 5, 1997 Group 15 Triazides: A Comprehensive Theoretical Study and the Preparation of Bismuth Triazide

Moreover, as indicated by the NBO analysis, there is a significant interaction of all three σ(Χ-Ν1) orbitals with the unoccupied, antibonding π*(Ν2-Ν3) orbital which lies in the X-N1-N2-N3 plane. This σ(Χ-Ν1) π*(Ν2-Ν3) donor-acceptor interaction (hyperconjugation) accounts for the rather long X-N bond and a decreasing bond order, respectively (Fig. 4). This hyperconjugation inreases from Ν to Bi because of the inceasing p-character of the X-N sigma bond which leads to a better overlap. Due to

C(N,)b, >c(N,)SB > CCNOA, > c(N,)p > C(N,)N the σ(Χ-Ν) orbital (Table 6) the electron density of the

binding electron pair is more localized at the N1 in the Bi species (c(N,)Bl = 0.92) than in the Ν species

(C(N1)n = 0.69) resulting in a better electron donation (stronger hyperconjugation). Furthermore, all three p-orbital like lone pairs e.g. p-LP(Nla) interact with the antibonding A*(X-Nlb) which actually weakens the σ-bond system but strengthens the π-bond system of the X-Nl bond. This interaction shows the derealization of π-electron denistiy over the entire molecule (Fig. 5). This non covalent effect and the interaction of the σ(Χ-Ν1) orbitals with the π*(Ν2-Ν3) orbitals (as described above) is larger in structure Α und counts for the larger X-N bond distance in structure A as well. The lone pair localized at atom X plays no role in the donor acceptor interactions. Table 7 succinctly descibes the molecular charge distribution in terms of NPA charges. The X-Nl (X = P, As, Sb and Bi) bond is essentially a single bond with a polarity of Χδ+-Ν1δ\ Going down in group 15 the X-Nl bond (X = Ν, P, As, Sb and Bi) gets more and more polarized as mentioned above. Similar to the situation of the halogen azides were only the very unstable fluorine azide has a polarity of F^-Nl5"; the nitrogen triazide, similarly unstabile, shows the same polarity. Comparison of structure A and Β indicates that in all cases X is more positive in structure A than in structure B.

Ρ

Fig. 4

Fig. 5 p-LP(Nla) -» a*(X-Nlb) donor-acceptor interaction for X = P. 336 Τ.Μ. Klapotke and A. Schulz Main Group Metal Chemistry

Table 7 NPA charges of group 15 azides a

Χ Ν1 Ν2 Ν3

N(N3)3 A -0.02 -0.31 +0.27 +0.05 Β -0.09 -0.31 +0.29 +0.05

P(N3)3 A + 1.51 -0.83 +0.32 +0.01 Β + 1.42 -0.80 +0.32 +0.01

AS(N3)3 A +1.65 -0.86 +0.33 -0.02 Β + 1.58 -0.83 +0.32 -0.03

Sb(N3)3 A +1.90 -0.91 +0.33 -0.Ό 5 Β + 1.85 -0.88 +0.32 -0.06

Bi(N3)3 A + 1.95 -0.89 +0.32 -0.08

Β - - - -

SCF density used

Acknowledgments. Financial support by the University of Glasgow is gratefully acknowledged.

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Received: December 20, 1996 - Accepted: January 21, 1997 - Accepted in revised camera-ready format: February 12, 1997

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