DESCRIPCIÓN DE LA AVESTRUZ COMO ESPECIE PROMISORIA

The azido-N-cyanoformamidine (cyanoguanyl azide) (32) which is formed as an intermediate can be isolated as long as no base is present. In general, the tetrazole ring may exist in equilibrium with the acyclic imidoyl azide form, and depending on the substituent at the imidoyl fragment present, both isomers can be observed. Electron withdrawing groups such as CN or NO2 favor the open azide chain isomer, imidoyl azide, whereas electron donating groups

(e.g. alkyl) or bases are required effecting the cyclization to the isomeric tetrazole.3,40,41

2.2 Results and Discussion

2.2.1 Synthesis and properties of CIT salts

The two step synthesis of 5-cyaniminotetrazoline salts (CIT) starts from amino-1H- tetrazolium monohydrate (5-AT), which after neutralization with sodium hydroxide, was reacted with cyanogen bromide to give azido-N-cyanoformamidine (32). 32 represents an easily accessible source for an NCN-N3 unit. Treating a methanolic or ethanolic solution of 2 with

bases (e.g. MOH, M = Li, Na, K, Cs; amines), immediate deprotonation and cyclization of 32

can be observed resulting in the formation of the stable 5-cyaniminotetrazolinediide salts (Scheme 3.2). Raman and IR spectroscopy are particularly suitable to identify CIT salts very fast with the help of the CN stretching mode at ca. 2150 and the missing N3 stretching at 2190

cm–1 (for approximate assignment of all normal modes on the basis of DFT calculation, see Appendix A Table A-1 – A-3);

Pure alkali and ammonium CIT salts are stable at ambient temperature, and are neither heat nor shock sensitive. To determine the thermal properties of CIT salts, combined IR and DSC experiments were carried out. DSC experiments of the cesium CIT salt (33) revealed three interesting features: (i) an endothermic reversible change of the modification at ca. 253° (∆H = 34.8 J/g for α→β modification, Figure 3.3) which was supported by powder diffraction studies (Figure 3.4), (ii) melting at 335 °C (onset), and (iii) the decomposition of Cs2CIT after melting

in cesium azide and caesium dicyanamide. It is assumed that the presence of an over the entire dianion delocalised π-system probably accounts for the remarkable kinetic stability of CIT salts (see below).

Figure 3.3. DSC experiments showing the decomposition of β-Cs2C2N6 (left) and the non-reversibility after melting

of β-Cs2C2N6 (right).

2.2.2 DSC of Cs2CIT (34)

The results of the different scanning calorimetry (DSC) experiments show that there are two endotherms (Figure 3.3, left), at 252 and 334 °C. The first endotherm is related to the transition of α-Cs2C2N6 to the metastable β-Cs2C2N6 modification, which is in accordance with

the transformation that can be observed from the in situ data shown in Figure 3.4. The enthalpy of the strongly endothermic transformation was measured to be 34.8 J/g. The first transition was found to be reversible, which can be seen from the temperature-resolved in situ powder diffraction patterns in Figure 3.4. The second endotherm corresponds to the melting of β- Cs2C2N6. This process is not reversible (Figure 3.3, right), which can be explained due to

(right) does not exhibit distinctive features which can be related to 34, and therefore is indicative of an irreversible conversion of 34 to CsN3 and CsN(CN)2. The obtained melt starts

to decompose in an exothermic reaction under formation of nitrogen above 395 °C.

2.2.3 In Situ X-ray Powder Diffraction of Cs2CIT (34)

High-temperature in situ X-ray diffractometry was performed on a STOE Stadi P power diffractometer (Mo Kα1 radiation, λ = 70.093 pm), with an integrated furnace and unsealed

glass capillaries as sample containers from 180-260 °C and 260–180 °C in one run. The data collection was done with a 2θ range of 4.0 – 22.0° and a single scan collection of 8 min under isothermic conditions. The sample was heated to the starting temperature by applying a heating rate of 1 °C min-1.

Figure 3.4. Temperature-dependent X-ray diffraction measurement from 180 °C (top) to 260 °C and 260 °C to 204 (bottom) °C between 4.0 and 22.0 in 2θ. To ensure a sufficient small time window for the detection off structural changes and to simultaneously maintain an acceptable data quality, the data acquisition time for a single scan was fixed to 8 min. A reversible phase transition between the α and β modification is observed. No overlap between of the different phases is observed.

The sequences of X-ray powder patterns collected from 160Æ260Æ204 °C, respectively, are shown in Figure 3.4. A prominent feature of the variable-temperature studies is the absence of a unique onset temperature for the observed phase transition, and there is no transitory regime observed, where no reflections are detected, indicate the non occurrence of an amorphous phase or intermediate glas transition, indicating also the reversibility of the first transition of α-Cs2C2N6 Æ β-Cs2C2N6 Æα-Cs2C2N6.

2.2.4 X-ray Crystal Structure of (iPrNH3)2CIT•MeOH (35) and Cs2CIT (34).

Figures 3.5 and 3.6 show the molecular units with the atom labeling scheme of 35 and 34, respectively. Selected bond lengths and angles are presented in Table 3.1. 35 crystallizes in the triclinic space group P-1 with two formula units in the cell, and 5 in the monoclinic space group P21/n with 4 units in the cell.

Figure 3.5. Formula unit and labeling scheme for 35 (ORTEP Plot, thermal ellipsoid represents 50% probability).

As shown in Figure 3.4, there are only small cation-anion interactions (hydrogen bridges) in the solid state structure of 35 (The bond distances and angles in the isopropylammonium and MeOH are as observed for many other salts with isopropylamine or MeOH as co-solvent and

therefore we abstain a discussion),42 in contrast to the very complex bonding situation between anions and cations in 34.

Table 3.1. Comparison of selected interatomic distance (Å) and bond angles (°) of the C2N6 frame in 34, 35, 36 and 37.a 34 36 35 37 CITb CIT-SO3b Bond Length C1–N1 1.337(2) 1.344(3) 1.338(4) 1.359(6) 1.358 1.352 N1–N2 1.348(2) 1.357(3) 1.362(4) 1.379(5) 1.355 1.359 N2–N3 1.301(2) 1.298(3) 1.309(4) 1.274(6) 1.321 1.294 N3–N4 1.348(2) 1.358(3) 1.357(4) 1.362(6) 1.358 1.366 C1–N4 1.331(2) 1.335(3) 1.337(4) 1.330(7) 1.357 1.379 C1–N5 1.368(2) 1.364(3) 1.388(4) 1.342(7) 1.387 1.344 C2–N5 1.301(2) 1.299(3) 1.302(4) 1.318(7) 1.293 1.306 C2–N6 1.159(2) 1.157(3) 1.174(4) 1.147(7) 1.197 1.186 Bond Angle N1–C1–N4 111.2(1) 109.8(2) 112.6(3) 106.9(5) 110.9 106.7 C1–N1–N2 104.8(1) 106.3(2) 104.0(2) 108.6(4) 104.9 106.4 N1–N2–N3 109.5(1) 108.2(2) 109.5(2) 105.8(4) 109.7 112.0 N2–N3–N4 109.6(1) 110.3(2) 109.8(2) 111.9(4) 109.7 106.5 N3–N4–C1 104.9(1) 105.4(2) 104.1(2) 106.8(4) 104.8 108.5 C1–N5–C2 116.1(1) 119.0(2) 116.8(3) 117.4(5) 123.6 121.0 N5–C2–N6 175.9(2) 173.3(2) 174.0(3) 173.2(7) 171.6 172.2

a _{for numbering see Figure 3.7;} b _{theoretical gas phase data: B3LYP/aug-cc-pvTZ}

Figure 3.7. Molecular model and numbering scheme of the C2N6 frame in 5-cyanimino-

tetrazolinediide.

The structure of 34 consists of an infinite three-dimensional network of repeating Cs2CIT

units. Each anion is bonded to twelve cesium cations while each Cs+ _{has contacts to nine N}δ–

atoms in six different CIT anions (Figure 3.8) with Cs-N bond distances of 3.131(2) to 3.586(3) Å. All Coulombic interactions (listed in Table 3.2) are either larger than the sum of the ionic radii of Cs+ (1.92 Å, KZ = 9) and N3- (1.32 Å)43 or in the range, indicating that the structure is predominantly ionic. The sum of the calculated valency units for Cs1+ and Cs2+ accounts to

1.026 and 1.051, respectively, consistent with a single positive charge on both Cs+ and a doubly negatively charged CIT ion (Table 3.2).

Figure 3.6. Formula unit and labeling scheme for 34 (ORTEP Plot, thermal ellipsoid represents 50% probability).

Table 3.2.Observed Cs-N contacts and calculated valency units in 34

contacta,b distance (Å) valency unitsc contacta,b distance (Å) valency unitsc

Cs1–N1ii 3.403(3) 0.0969 Cs2–N1i 3.481(3) 0.0836 Cs1–N2iii _{3.290(3) 0.1213 Cs2–N2}iv _{3.302(3) 0.1184} Cs1–N2v 3.505(3) 0.0799 Cs2–N3ii 3.324(3) 0.1136 Cs1–N3vi 3.320(3) 0.1143 Cs2–N4 3.178(3) 0.1525 Cs1–N3v 3.233(3) 0.1362 Cs2–N4vi 3.586(3) 0.0687 Cs1–N4vi 3.131(2) 0.1679 Cs2–N5 3.483(3) 0.0832 Cs1–N5 3.330(3) 0.1120 Cs2–N5vi 3.265(3) 0.1276 Cs1–N5ii 3.571(3) 0.0706 Cs2–N6i 3.154(3) 0.1606 Cs1–N6vii 3.266(3) 0.1273 Cs2–N6vii 3.208(3) 0.1433 sum of valency units 1.0264 1.0515

a _{Locations of contacts are shown in Figure 4.} b _{Symmetry codes: (i) 1-x, 1-y, -z; (ii) 0.5-x, 0.5+y, 0.5-z; (iii) 1-x, 1-y, -z; (iv) -}

0.5+x, 0.5-y, -0.5+z; (v) x, 1+y, z; (vi) -x, 1-y, -z; (vii) -0.5+x, 1.5-y, -0.5+z. c _{Valency units were calculated using Brown’s}

expression s = (R/R0)-N where R is the observed contact distance; R0 is the distance corresponding to a valency unit of 1, and N is

a fitted constant. Values of R0 for contacts to N and O differ by an average of 0.10 to 0.15 Å for other nuclei; R0 for Cs–O is

reported to be 2.24 Å, as no Cs–N contacts are given, R0 was set to 2.39 Å. The value of N for Cs–O contacts is 6.6, and this

Figure 3.8.View of cesium coordination environment in Cs2CIT

2.2.5 Structure and bonding of the CIT dianion.

In agreement with DFT calculations, the CIT anion (33) is almost planar in the solid state. The tetrazole ring in 34 and 35 is planar with the attached exocyclic nitrogen atom of the NCN moiety lying within the tetrazole plane (e.g. with a maximal deviation of 0.0034 Å in 35). The cyanoimino moiety is slightly outside of the tetrazole plane (<N4–C1–C2–N6; 35: 9.4(4)°, 34: 5.4 (5)°). Similar to the situation found in covalent bound azides,44 a trans bent geometry is found for the NCN group (<NCN, 35: 175.9(2)°, 34: 174.0(3)°). Actually, the CIT dianion is isoelectronic to the related azidotetrazolate monoanion (replacement of the middle N atom of the azido group by C–), hence the hitherto unknown structure of azidotetrazolate can be expected to have similar features like those of CIT. As displayed in Table 3.1, the distances and angles of CIT in different salts are not significantly different. Thus it can be assumed that the geometry of the C2N6 dianion is little affected by coordination to a metal (X-ray data of 36,

below) or substitution at a ring nitrogen (X-ray data of 37, below). The calculated gas phase geometry of CIT at B3LYP/aug-cc-pvTZ level of theory is similar to that found in the solid state and is given in Table 3.1.

As expected, for all CIT salts the N2–N3 bonds are significantly shorter than the N1–N2 and C1–N1 bonds (Figure 3.7, e.g. 34: 1.309(4), 1.362(4), and 1.338(4) Å), which is

length is significantly smaller than all other C–N or N–N bond lengths, indicating a stronger π- interaction along the cyano group of the NCN unit.The bond length found for the cyano group lies in a normal range (e.g. 35: 1.159(2) Å; 34: 1.174(4) for C2–N6; Table 2).45 These relatively short N–N and C–N bond lengths, together with the planarity, indicate the presence of delocalization of π-bonds over the whole anionic species. MO and NBO calculations displayed the existence of a 10π-electron, 8-center bond unit (Figures 3.9 and 3.10).46 _{In Figure 3.9, A}

and B are the energetically preferred Lewis representations of NNCM according to NBO analysis which is in agreement with the calculated Wiberg bond indices47 (WBI) and the calculated partial charges (Figure 3.9). Investigation of the intramolecular donor-acceptor interactions utilizing the NBO partitioning scheme, clearly indicates a highly delocalized 10π- system according to resonance between Lewis representations A – I.

Figure 3.9. Nine possible Lewis representations according to NBO analysis along with WBI’s (blue color) and NPA charges (red color).

The calculated WBIs indicate a stronger π-bond along the NCN moiety and the N3-N4

unit. The largest negative NPA net charge (NPA, natural population analysis) is found on the two N atoms of NCN unit. Summing the net charges of the NCN and the tetrazole moiety reveals for both units a negative charge close to -1 (NCN: -1.04; N4C: -0.96e).

LUMO, 6 A"

5 A''

4 A" 3 A3''

2 A" 1 A"

Figure 3.10. π-Type MOs (B3LYP/aug-cc-pvTZ) of CIT displaying the 10π-8c-bond

2.2.6 Crystal structure of [Pd(C2N6)(NH3)3]·H2O (36)

Complexes of CN-species with transition metals are well known and often exhibit interesting magnetic properties.48 Due to the potential of being polydentate, several transition metal salts of CIT (M(cit)(NH3)2; M = Cu, Ni, Pd) were synthesized of which only the

palladium complex will be introduced. These complexes are easily obtained from the reaction of the according transition metal(II)chlorides and 2 in ammonia solution.

The pale yellow palladium complex crystallizes in the monoclinic space group P21/n with

planar arrangement composed of three NH3 and one CIT ligand (coordination via N1 of the

tetrazole ring), with N–Pd–N angles ranging from 89.58(9) to 90.22(9)° (mean deviation from the square plane is 0.0707 Å). All Pd-N distances are in the range of 2.008(2) to 2.046 (2) Å (Table 3.3) and lie well within the range of distances recorded previously.49 The CIT ligand is not part of the square-plane as the tetrazole ring is twisted by 31.1° (N7–N9–N1–C1). The molecular parameters are comparable with those found in 35 and 34 (Table 3.1). It is interesting to note that the cyanoimino group is not involved in any bonding to the Pd metal.

Figure 3.11. (I) The dimer [Pd(C2N6)(NH3)3]•H2O (36), showing the atom-numbering scheme

and displacement ellipsoid at 50% probability level; (II) Part of the crystal structure of 36, showing the π-π-stacking interaction which links the dimers to chains along [010] axis. Atoms marked with i _{are at the symmetry position –x, –y, –z.}

Table 3.3. Selected structural parameters (Å, °) of 36 Pd–N1 2.008(2) Pd–N8 2.043(2) Pd–N7 2.046(2) Pd–N9 2.043(2) N1–Pd–N7 90.22(9) N1–Pd–N8 176.90(8) N7–Pd–N8 89.82(9) N7–Pd–N9 179.67(9) N8–Pd–N9 90.37(9) Pd–N1–C1 132.5(2) N9–Pd–N1 89.57(9) Pd–N1–N2 120.9(2)

In the solid state, the complex is composed of dimers where the two complexes are related via an inversion centre. The long Pd–Pdi distance of 4.2743(2) Å in the dimer excludes the possibility of the existence of a metal-metal interaction. A closer inspection of the molecular structure as shown in Figure 3.11 reveals two agostic interactions (N7–H7C····Pd1i; d(Pd···N7)i = 3.536(3) Å; symmetry code: (i) -x, 1-y, -z) between the two Pd complex fragments forming the dimer. Apparently, these two interactions, together with an intermolecular hydrogen bond (N8–H8C····N2i_{), can be regarded as the driving force for the dimerization.}

Table 3.4. Hydrogen bond geometry (Å, °) of 36

D–H····A D–H H····A D····A D–H····A O1–H1A···N6i 0.948(2) 2.072(3) 2.994(3) 164.0(2) O1–H1B···N4 0.947(3) 2.037(4) 2.966(4) 166.3(2) N7–H7A···O1ii _{0.89(2) 2.11(1) 2.989(3) 172.4(9)} N7–H7B···N4iii 0.891(1) 2.154(6) 3.031(4) 157.4(5) N8–H8B···N5iv 0.891(1) 2.16(1) 3.031(4) 167.4(9) N8–H8C···N2v 0.890(9) 2.187(1) 3.066(3) 169.1(7) N9–H9B···N6vi 0.890(8) 2.302(6) 3.151(4) 159.4(5) Symmetry codes: (i) -0.5+x, -0.5-y, -0.5+z; (ii) -0.5+x, 0.5-y, 0.5+z;

(iii) -x, -y, -z; (iv) 0.5-x, 0.5+y, 0.5-z ; (v) –x, 1-y, -z ; (vi) x, 1+y, z.

Moreover, the dimers are linked by π-π-stacking interactions to chains along the [010] axis. The parallel tetrazole rings of the CIT ligands at (x, y, z) and (–x, –y, –z) have an interplanar stacking spacing of 3.307(2) Å. Additionally, several hydrogen bonds, including the water molecule, are found linking the dimers to a three dimensional network (Table 3.4).

2.2.7 The reaction of Cs2CIT with SO2.

A frozen mixture of Cs2C2N6 (35) and SO2 reacts on thawing and formation of a bright

yellow solution which turns into a deep red solution within a few minutes. From this solution a gummy residue separates within 20 minutes from which a pale yellow solution was separated after one day. From this pale yellow solution, dicesium 5-cyaniminotetrazoline-1- sulfonatediide·SO2, Cs2CITSO3·SO2 (37) crystallized as pale yellow plates in an all over yield

of 40% according eq [1]. Interestingly, the pale yellow crystals of 37 contain one SO2 molecule

(solvent) per formula unit and are surprisingly stable at room temperature even without SO2

atmosphere. The loss of SO2 starts only slowly from the surface of the crystals. It can be

assumed that in the first reaction step SO2 is attached to the N1 atom of the tetrazole ring

followed by oxidation to SO3 by SO2 yielding the total reaction

2 Cs2C2N6 + 3 SO2 ⎯SO⎯ →⎯2 2 Cs2C2N6SO3 + 1/8 S8 [1]

This reaction represents a typical disproportion reaction (3 SO2 → 2 SO3 + 1/8 S8). The

reaction of Cs2CIT/SO2 system is related to the known reaction of CsF with SO2 resulting in

CsSO3F and S8.50 Two more things are worth mentioning: (i) in contrast to the CsF/SO2

reaction, no ultrasonic activation is needed for the spontaneous reaction of Cs2CIT with SO2,50

and (ii) the related cesium cyano-tetrazolate was prepared in SO2 without any reaction with the

solvent SO2.30

Unfortunately, the direct synthesis of 37 from CsN3 and CsN(CN)2 in SO2 according

Scheme 3.3 did not yield the sulfonate 37.