Tetrazole anion

Despite the weak basicity of isoxazoles, complexes of the parent methyl and phenyl derivatives with numerous metal ions such as copper, zinc, cobalt, etc. have been described (79AHC(25) 147). Many transition metal cations form complexes with Imidazoles the coordination number is four to six (70AHC(12)103). The chemistry of pyrazole complexes has been especially well studied and coordination compounds are known with thlazoles and 1,2,4-triazoles. Tetrazole anions also form good ligands for heavy metals (77AHC(21)323). 

Azidopyrazoles exist as such (459), but their anions (460) are in equilibrium with tetrazole anions (461) which can be trapped as (462). 

Tetrazolate, 5-(3-chlorobenzyldimethylammonium)-structures, 5, 796 Tetrazolate, 5-cyclopropyl-rearrangement, 5, 810 Tetrazolate, 5-(p-substituted)phenyl-alkylation, 5, 818 Tetrazolates anions... 

Upfield shifts, negative, downfield shifts positive. 5-Substituted tetrazolate anion shifts in DjO relative to external NH NOj have also been listed >(86MRC984>. Methyl sulfate salt in CDnOD. Some confusion on N-3 and N-4 shifts in early spectra has been corrected <92MRC558>. 

The tetrazolate anion is isoelectronic with the aromatic cyclopentadienyl ring and is a potential source of -coordination that can be enhanced by... 

Reactions between 5-cyanotetrazole and transition metals, when performed in boiling acetone, lead to hydrolysis of the cyano group and formation of 5-carbamyl tetrazolate complexes (68). Complexes containing 1- or 5-substituted tetrazolate anions can also be obtained by 1,3-dipolar cycloaddition of organic isonitriles (RNC) (15) or nitriles (RCN) (61), respectively, to coordinated azide ligands [Eqs. (3) and (4)]. 

Generally monodentate N-1-, N-2-, or C-5-coordinated tetrazolate anions appear to adopt planar, regular pentagonal structures with... 

Infrared spectra have been recorded for the parent tetrazole (ttaH), its sodium salt, and a range of transition metal derivatives (94). In subsequent work the infrared spectra of sodium tetrazolate and of the copper complex Cu(tta)2 H20 were assigned and a vibrational analysis was reported for the tetrazolate anion (77). Infrared data have been... 

Far infrared data have been reported and v(M-N) frequencies assigned for a variety of cobalt, nickel, copper, zinc, cadmium, and mercury tetrazolate complexes (153). An inverse relationship between the basicity of the tetrazolate anions and the values of v(M-N) has been interpreted as evidence of significant M - L n bonding (153). 

Besides pyrazoles and imidazoles, we have extended these studies to other azoles, for instance to C-halogeno 1/7-1, , 4-triazoles 60 (X = Cl, Br) [73] (for DFT studies of other C-substituted 1,2,4-triazoles, see [74]) and to 5-substituted tetrazoles 61 (X = H, CH3, t-Bu, C6H5, Cl, CF3, N02) [75], The 1,2,4-triazole series 60 was examined regarding tautomerism in solution and in the solid state (the most stable is 60a and the least stable 60c). The paper on 5-substituted tetrazoles 61 concerns, besides tautomerism, their ionization (proton loss) to tetrazolate anion 62. 

We have already discussed the acidity of NH-tetrazoles 61 to form tetrazolate anions 62 [75], In the same paper, we also studied the protonation. The substituent X strongly affects the tautomeric balance between 1,3-H,H+- (82) and 1,4-H,locations (84). In the case of electron-withdrawing substituents, the most preferred form of the conjugated acid is the 1,3-H,H+- form (82) structures 83 and 85 are much less stable. The acidity measures in solution and in the gas phase satisfactorily correlate with each other. In all cases, these relationships do not hold for 5-phenyltetrazole (56, X = Ph). This could be explained by the difference in solvation of this compound compared to other 5-X tetrazoles as well as by some peculiarity of its electronic structure, for example, the strong conjugation between the phenyl substituent and the tetrazole ring. 

Table 3 Selective X-ray diffraction data of cycle geometry of neutral tetrazoles, tetrazolate anions, and tetrazolium cations... 

C NMR spectra of tetrazolate anions are similar to the spectra of 2-substituted tetrazoles. For instance, the chemical shift of the signal of tetrabutylammonium tetrazolate in CDCI3 is 149.8 ppm <2001JOC3291 >, and for the sodium 5-nitrotetrazolate in acetone- it is 167.5 ppm <2006MI631>. 

Thus it is possible to conclude that in the 13C NMR spectra of tetrazolate anions, 2//-tctrazolcs, and 1,3-tetrazolium ions the signal of the endocyclic carbon atom is considerably shifted downfield compared to the corresponding signal in the spectra of 1-substituted neutral tetrazoles and 1,4-disubstituted tetrazolium ions, and also of the fuzed tetrazolo-azines. It is just in the tetrazolate anions and tetrazole derivatives with a substituent at position 2 that a lesser variation in the bond distances in the ring and stronger aromaticity are observed (cf. Sections 6.07.3.1 and 6.07.4.1). 

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