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The intermediate [Cd(4-PTZ)2(H2O)4] [4-PTZ is 5-(4-pyridyl N-oxide)­tetrazolate, C6H4N5O], (I), in the synthesis of 4-HPTZ, (II), from the cyclo­addition reaction of 4-cyano­pyridine N-oxide with NaN3 in water using CdCl2 as catalyst, was structurally characterized. The unique Cd atom lies on a twofold axis and the coordination geometry of the Cd atom is that of a slightly distorted octahedron, involving four water mol­ecules and two tetrazolate ligands. The catalytic role of the Cd2+ ion in the tetrazole generation reaction stems from the formation of (I). In acidic solution, (I) can be disassociated and the free ligand, (II), can be released.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270104005864/de1236sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270104005864/de1236Isup2.hkl
Contains datablock I

CCDC reference: 237921

Comment top

Tetrazoles form an increasingly popular functional group (Butler, 1996), which has applications in coordination chemistry as a ligand (Franke & Groeneveld, 1980; Erbe & Beck, 1983; Kreutzer et al., 1983), in medicinal chemistry as a metabolically stable surrogate for a carboxylic acid group (Singh et al., 1980) and in various materials sciences, including specialty explosives (Ostrovskii et al., 1999). It has long been known that simple heating of azide salts with a nitrile in solution produces the corresponding tetrazole (Dimroth et al., 1910). Demko & Sharpless (2001) have recently modified this transformation in an environmentally friendly, relatively simple, route. In this general reaction, the tetrazole is prepared by the addition of azide to nitriles in water, with the catalysis of Lewis acids, such as zinc salts. However, the intermediate was not characterized, and thus the catalytic mechanism of the Lewis acid is unknown. Although Xiong and co-workers (Xue et al., 2002; Xiong et al., 2002; Wang et al., 2003) have recently tried to isolate and characterize these intermediates, the exact intermediate is still unknown, because potentially bridging substituted nitriles were used in their experiments and a series of complicated coordination tetrazole polymers were formed. A structural investigation of this intermediate would be very interesting because it may provide important clues to the role of the Lewis acid in the tetrazole synthesis reaction, which in turn may allow synthetic chemists to optimize this synthetic approach further. For the feasibility of generation of? high-quality crystalline material for X-ray structural analysis, water-soluble nitrile (4-cyanopyridine N-oxide) is chosen in this paper. In addition, the selected nitrile has a strong electron-withdrawing group, which can accelerate the cycloaddition reaction to form tetrazole (Demko & Sharpless, 2001). We report here the X-ray structure of the intermediate [Cd(4-PTZ)2(H2O)4], (I), in the formation of 4-H-PTZ, (II), from the reaction of 4-cyanopyridine N-oxide with NaN3, with CdCl2 as catalyst.

A crystallographic twofold axis passes through the Cd atom, relating the two equal parts of the molecule. The coordination geometry of the Cd atom is a slightly distorted octahedron, formed from the coordination of four water molecules and two tetrazolate ligands. The O atoms from the four water molecules form a square-planar arrangement around the Cd center, and the tetrazolate ligands coordinate to the Cd atom via atom N1. The dipolar cycloaddition of azides with nitriles can lead to two isomers, i.e. a 4-R or a 5-R coordinated ligand may be formed (Fig. 2). As observed in the indium tetrazolate complex (Guilard et al., 1987), only the 4-R tetrazolate is formed here because of its sterically favored configuration. The coordinated tetrazole ring is a planar pentagon, with bond lengths [1.317 (2)–1.339 (2) Å] intermeditate between those of single and double bonds. The dihedral angle between the tetrazolate plane and the C1-bonded 4-pyridyl ring is only 18.5°, favoring an orbital conjugation between the two cycles.

With the elucidation of the structure of the intermediate, we can state that the catalytic mechanism exhibited by Lewis acids in the synthesis of tetrazole from the reaction of nitrile with azide stems from the formation of a metal–tetrazolate complex. With the disassociation of the complex in acidic media, the tetrazole can be released.

Experimental top

4-Cyanopyridine N-oxide (0.060 g, 0.5 mmol) was dissolved in distlled water (30 ml), and sodium azide (0.033 g, 0.5 mmol) and cadmium chloride (0.050 g, 0.3 mmol) were dissolved in water (10 ml). The two solutions were mixed and allowed to stand at room temperature for a few days. The cycloaddition reaction formed the tetrazole ligand in situ, and white block-like crystals of (I) (0.103 g, 81%) were obtained after washing thoroughly with distilled water and methanol. Analysis calculated for C12H16CdN10O6: C 28.23, H 3.16, N 27.46%; found: C 28.01, H 3.45, N 27.19%. The free tetrazole (II) was obtained by dissolving (I) in 2 N HCl solution followed by extration with ethyl acetate and evaporation of the organic layer. 1H NMR (DMSO-d6): δ 8.17 (d, 2H, J = 6.7 Hz), 7.98 (d, 2H, J = 6.7 Hz). Analysis calculated for C6H5N5O: C 44.16, H 3.09, N 42.94%; found: C 44.38, H 3.42, N 42.62%.

Refinement top

Water H atoms were located in difference maps and their parameters were refined isotropically [O—H = 0.76 (3)–0.80 (3) Å]. All other H atoms were positioned geometrically and treated as riding, with C—H distances of 0.94 Å and Uiso(H) values of 1.2Ueq(C).??

Computing details top

Data collection: KappaCCD Software (Nonius, 1998); cell refinement: HKL SCALEPACK (Otwinowski & Minor, 1997); data reduction: HKL DENZO (Otwinowski & Minor, 1997) and maXus (Mackay et al., 1998); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Sheldrick, 1998); software used to prepare material for publication: SHELXL97.

Figures top
[Figure 1]
Figure 1. A view of (I), showing the atom0labeling scheme and 50% probability displacement ellipsoids.

Figure 2. Two isomers of the metal tetrazolate complexes.
Tetraaquabis[5-(4-pyridyl N-oxide)tetrazolato-κN2]cadmium top
Crystal data top
[Cd(C6H4N5O)2(H2O)4]F(000) = 1016
Mr = 508.75Dx = 1.923 Mg m3
Monoclinic, C2/cMo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2ycCell parameters from 12672 reflections
a = 21.871 (4) Åθ = 3.0–30.0°
b = 7.0825 (14) ŵ = 1.30 mm1
c = 11.417 (2) ÅT = 223 K
β = 96.52 (3)°Polyhedron, white
V = 1757.2 (6) Å30.30 × 0.15 × 0.10 mm
Z = 4
Data collection top
Siemens P4
diffractometer
2338 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.028
Graphite monochromatorθmax = 30.0°, θmin = 3.0°
ω scansh = 3029
Absorption correction: empirical (using intensity measurements)
(Blessing, 1995, 1997)
k = 99
Tmin = 0.793, Tmax = 0.880l = 1515
12672 measured reflections4200 standard reflections every 250 reflections
2519 independent reflections intensity decay: none
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.026Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.066H atoms treated by a mixture of independent and constrained refinement
S = 1.05 w = 1/[σ2(Fo2) + (0.0431P)2 + 0.2051P]
where P = (Fo2 + 2Fc2)/3
2519 reflections(Δ/σ)max < 0.001
148 parametersΔρmax = 0.76 e Å3
0 restraintsΔρmin = 0.40 e Å3
Crystal data top
[Cd(C6H4N5O)2(H2O)4]V = 1757.2 (6) Å3
Mr = 508.75Z = 4
Monoclinic, C2/cMo Kα radiation
a = 21.871 (4) ŵ = 1.30 mm1
b = 7.0825 (14) ÅT = 223 K
c = 11.417 (2) Å0.30 × 0.15 × 0.10 mm
β = 96.52 (3)°
Data collection top
Siemens P4
diffractometer
2338 reflections with I > 2σ(I)
Absorption correction: empirical (using intensity measurements)
(Blessing, 1995, 1997)
Rint = 0.028
Tmin = 0.793, Tmax = 0.8804200 standard reflections every 250 reflections
12672 measured reflections intensity decay: none
2519 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0260 restraints
wR(F2) = 0.066H atoms treated by a mixture of independent and constrained refinement
S = 1.05Δρmax = 0.76 e Å3
2519 reflectionsΔρmin = 0.40 e Å3
148 parameters
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Cd0.50000.16671 (2)0.25000.02557 (7)
O10.52955 (7)0.4199 (2)0.36845 (13)0.0350 (3)
O20.52750 (7)0.0719 (2)0.38149 (13)0.0353 (3)
O30.06385 (6)0.16621 (17)0.09405 (13)0.0334 (3)
N10.40315 (7)0.1573 (2)0.30861 (15)0.0305 (3)
N20.38677 (7)0.1301 (3)0.41481 (15)0.0329 (3)
N30.32556 (7)0.1246 (3)0.40767 (14)0.0331 (3)
N40.35365 (7)0.1697 (2)0.22916 (15)0.0310 (3)
N50.12221 (7)0.16044 (18)0.14220 (14)0.0260 (3)
C10.30687 (8)0.1488 (2)0.29291 (16)0.0244 (3)
C20.24267 (8)0.1510 (2)0.24149 (16)0.0241 (3)
C30.22749 (8)0.1218 (3)0.12179 (16)0.0312 (4)
H3A0.25860.09660.07360.037*
C40.16717 (9)0.1295 (3)0.07309 (17)0.0326 (4)
H4A0.15720.11310.00860.039*
C50.13517 (8)0.1867 (3)0.25878 (17)0.0300 (4)
H5A0.10310.20760.30560.036*
C60.19510 (8)0.1831 (3)0.30993 (16)0.0286 (4)
H6A0.20400.20240.39150.034*
H2A0.5521 (13)0.073 (4)0.439 (2)0.044 (7)*
H2B0.5011 (16)0.136 (4)0.387 (3)0.049 (9)*
H1A0.5405 (14)0.391 (5)0.431 (3)0.046 (8)*
H1B0.5015 (13)0.486 (4)0.373 (2)0.043 (7)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Cd0.01654 (10)0.03543 (11)0.02459 (10)0.0000.00166 (6)0.000
O10.0310 (7)0.0426 (8)0.0298 (7)0.0054 (6)0.0030 (6)0.0055 (6)
O20.0251 (7)0.0462 (8)0.0329 (7)0.0035 (6)0.0044 (6)0.0096 (6)
O30.0160 (6)0.0441 (8)0.0379 (7)0.0006 (5)0.0068 (5)0.0019 (5)
N10.0168 (7)0.0434 (9)0.0307 (8)0.0007 (6)0.0003 (6)0.0024 (6)
N20.0190 (7)0.0507 (9)0.0285 (8)0.0013 (6)0.0004 (6)0.0032 (7)
N30.0193 (7)0.0527 (9)0.0268 (8)0.0018 (6)0.0006 (6)0.0044 (7)
N40.0180 (7)0.0466 (9)0.0279 (8)0.0003 (6)0.0009 (6)0.0024 (6)
N50.0162 (6)0.0302 (7)0.0305 (7)0.0007 (5)0.0019 (5)0.0002 (5)
C10.0175 (7)0.0292 (8)0.0262 (8)0.0003 (6)0.0008 (6)0.0002 (6)
C20.0177 (7)0.0280 (8)0.0261 (8)0.0003 (5)0.0002 (6)0.0017 (5)
C30.0214 (8)0.0464 (10)0.0258 (8)0.0019 (7)0.0033 (6)0.0031 (7)
C40.0247 (9)0.0485 (11)0.0238 (8)0.0011 (7)0.0003 (7)0.0030 (7)
C50.0187 (8)0.0432 (10)0.0281 (9)0.0003 (6)0.0033 (7)0.0026 (7)
C60.0196 (8)0.0417 (10)0.0244 (8)0.0010 (6)0.0016 (6)0.0023 (6)
Geometric parameters (Å, º) top
Cd—N1i2.2938 (17)N3—C11.339 (2)
Cd—N12.2938 (17)N4—C11.330 (2)
Cd—O12.2945 (15)N5—C51.342 (2)
Cd—O1i2.2945 (15)N5—C41.347 (2)
Cd—O2i2.2948 (15)C1—C21.459 (2)
Cd—O22.2948 (15)C2—C31.385 (3)
O1—H1A0.76 (3)C2—C61.389 (2)
O1—H1B0.78 (3)C3—C41.373 (3)
O2—H2A0.80 (3)C3—H3A0.9400
O2—H2B0.74 (3)C4—H4A0.9400
O3—N51.3317 (19)C5—C61.373 (3)
N1—N21.317 (2)C5—H5A0.9400
N1—N41.334 (2)C6—H6A0.9400
N2—N31.332 (2)
N1i—Cd—N1176.66 (8)N1—N2—N3108.93 (15)
N1i—Cd—O189.26 (6)N2—N3—C1104.42 (15)
N1—Cd—O193.35 (6)C1—N4—N1103.65 (15)
N1i—Cd—O1i93.35 (6)O3—N5—C5119.33 (16)
N1—Cd—O1i89.26 (6)O3—N5—C4119.49 (16)
O1—Cd—O1i77.17 (9)C5—N5—C4121.17 (16)
N1i—Cd—O2i88.27 (6)N4—C1—N3112.44 (16)
N1—Cd—O2i89.28 (6)N4—C1—C2122.94 (16)
O1—Cd—O2i175.23 (5)N3—C1—C2124.62 (16)
O1i—Cd—O2i98.90 (6)C3—C2—C6117.95 (16)
N1i—Cd—O289.28 (6)C3—C2—C1120.49 (16)
N1—Cd—O288.27 (6)C6—C2—C1121.56 (16)
O1—Cd—O298.90 (6)C4—C3—C2120.35 (17)
O1i—Cd—O2175.23 (5)C4—C3—H3A119.8
O2i—Cd—O285.15 (9)C2—C3—H3A119.8
Cd—O1—H1A113 (2)N5—C4—C3120.06 (17)
Cd—O1—H1B110 (2)N5—C4—H4A120.0
H1A—O1—H1B106 (3)C3—C4—H4A120.0
Cd—O2—H2A130 (2)N5—C5—C6120.21 (17)
Cd—O2—H2B111 (2)N5—C5—H5A119.9
H2A—O2—H2B112 (3)C6—C5—H5A119.9
N2—N1—N4110.55 (15)C5—C6—C2120.23 (17)
N2—N1—Cd129.03 (12)C5—C6—H6A119.9
N4—N1—Cd120.31 (13)C2—C6—H6A119.9
Symmetry code: (i) x+1, y, z+1/2.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O2—H2A···N2ii0.80 (3)2.05 (3)2.847 (2)169.46 (4)
O2—H2B···O3iii0.74 (3)2.02 (3)2.764 (2)173.52 (4)
O1—H1A···O3iv0.76 (3)1.92 (3)2.670 (2)175.27 (4)
O1—H1B···O3v0.78 (3)1.98 (3)2.756 (2)171.70 (4)
Symmetry codes: (ii) x+1, y, z+1; (iii) x+1/2, y1/2, z+1/2; (iv) x+1/2, y+1/2, z+1/2; (v) x+1/2, y+1/2, z+1/2.

Experimental details

Crystal data
Chemical formula[Cd(C6H4N5O)2(H2O)4]
Mr508.75
Crystal system, space groupMonoclinic, C2/c
Temperature (K)223
a, b, c (Å)21.871 (4), 7.0825 (14), 11.417 (2)
β (°) 96.52 (3)
V3)1757.2 (6)
Z4
Radiation typeMo Kα
µ (mm1)1.30
Crystal size (mm)0.30 × 0.15 × 0.10
Data collection
DiffractometerSiemens P4
diffractometer
Absorption correctionEmpirical (using intensity measurements)
(Blessing, 1995, 1997)
Tmin, Tmax0.793, 0.880
No. of measured, independent and
observed [I > 2σ(I)] reflections
12672, 2519, 2338
Rint0.028
(sin θ/λ)max1)0.704
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.026, 0.066, 1.05
No. of reflections2519
No. of parameters148
H-atom treatmentH atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å3)0.76, 0.40

Computer programs: KappaCCD Software (Nonius, 1998), HKL SCALEPACK (Otwinowski & Minor, 1997), HKL DENZO (Otwinowski & Minor, 1997) and maXus (Mackay et al., 1998), SHELXS97 (Sheldrick, 1990), SHELXL97 (Sheldrick, 1997), SHELXTL (Sheldrick, 1998), SHELXL97.

Selected geometric parameters (Å, º) top
Cd—N12.2938 (17)N4—C11.330 (2)
Cd—O12.2945 (15)N5—C51.342 (2)
Cd—O22.2948 (15)N5—C41.347 (2)
O3—N51.3317 (19)C1—C21.459 (2)
N1—N21.317 (2)C2—C31.385 (3)
N1—N41.334 (2)C2—C61.389 (2)
N2—N31.332 (2)C3—C41.373 (3)
N3—C11.339 (2)C5—C61.373 (3)
O1—Cd—O1i77.17 (9)O1—Cd—O298.90 (6)
O1—Cd—O2i175.23 (5)
Symmetry code: (i) x+1, y, z+1/2.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O2—H2A···N2ii0.80 (3)2.05 (3)2.847 (2)169.46 (4)
O2—H2B···O3iii0.74 (3)2.02 (3)2.764 (2)173.52 (4)
O1—H1A···O3iv0.76 (3)1.92 (3)2.670 (2)175.27 (4)
O1—H1B···O3v0.78 (3)1.98 (3)2.756 (2)171.70 (4)
Symmetry codes: (ii) x+1, y, z+1; (iii) x+1/2, y1/2, z+1/2; (iv) x+1/2, y+1/2, z+1/2; (v) x+1/2, y+1/2, z+1/2.
 

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