The Syntheses and Structure of the Vanadium(IV) and Vanadium(V) Binary Azides V(N₃)₄, [V(N₃)₆]²⁻, and [V(N₃)₆]⁻

Abstract

During the last decade, there has been much interest in inorganic polyazide chemistry. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Because of the energetic nature of the azido group, polyazides are highly endothermic compounds, the energy content of which increases with an increasing number of azido ligands. It is, therefore, not surprising that the synthesis of molecules with a high number of azido groups is very challenging owing to their explosive nature and shock sensitivity.A significant number of pentavalent binary azido compounds of the heavier Group 5 elements have been prepared and characterized, namely Nb(N 3 ) 5 , [17] and also the vibrational and electronic spectra of the [V(N 3 ) 6 ] 3À ion [18] have been reported. For the higher oxidation states of vanadium, only ternary or quatenary azides, such as VOCl 2 N 3 , [19] [VO(N 3 ) 4 ] 2À , [18,20,21] and [V(N 3 ) 3 (N 3 S 2 )] 2À , [22] have been reported, and no binary vanadium(V) compounds are known except for VF 5 , VF 6 À , and V 2 O 5 . By analogy with our previous syntheses of binary Group 5 azides, [14] As expected for a covalently bonded polyazide, [23] solid V(N 3 ) 4 is very shock-sensitive. It can explode violently upon the slightest provocation, for example when touched with a metal spatula or by a rapid change in temperature (such as freezing with liquid nitrogen or a fast warm-up). All attempts to obtain single crystals of V(N 3 ) 4 by recrystallization were unsuccessful. The identity of the vanadium tetraazide was established by the observed weight, a low-temperature Raman spectrum, and by quantitative conversion with either Ph 4 P2À salts. These salts were characterized by their material balances, crystal structures, and vibrational spectra of the crystals and the bulk material, which were identical. The recording of the Raman spectrum of V(N 3 ) 4 was very challenging owing to the black color of the sample, its amorphism, and its extreme shock sensitivity. In spite of these difficulties, we succeeded to record several reproducible Raman spectra of amorphous samples and one spectrum of a crystalline sample before it exploded. In the Supporting Information, Table S16, the vibrational frequencies and intensities observed for the amorphous samples are compared with those calculated for the free molecular species at the MP2/MCP-TZP level of theory. The vibrational frequencies calculated for V(N 3 ) 4 at the B3LYP/MCP-TZP level are given in the Supporting Information, Table S1. There is a very good agreement between the calculated and observed frequencies (Supporting Information, Table S16), suggesting that amorphous V(N 3 ) 4 is only weakly associated and permits assignments to the individual modes. In contrast, the Raman spectrum obtained for the crystalline sample (Supporting Information, Table S16, footnote [a]) deviated significantly from that of the amorphous sample. It showed three intense bands in the region of the V-N stretching modes, indicating association.The MP2/MCP-TZP calculations resulted in a minimum energy stru...

Homoleptic Poly(nitrato) Complexes of Group 14 Stable at Ambient Conditions

“…This method is akin to the synthesis of the related hexaazido complexes. [32][33][34][35][36][37][38][39][40][41][42][43][44][45][46] The indirect approach (ii) involves the in situ formation of tetranitrates, which are then "trapped" by Lewis acid-base reactions with nitrate anions.…”

The Vanadium(V) Oxoazides [VO(N3)3], [(bipy)VO(N3)3], and [VO(N3)5]2−

“…The average V À N azido distance of 2.007(2) f or the equatorial ligands and V À N azido distance of 2.159(2) f or the axial ligand are considerably longer than the average V À N azido distance found for [(bipy)VO(N 3 ) 3 ]( 1.970 (2) ). This elongation of the VÀNb onds can be attributed to the doubly negative charge of the dianion, resulting in more ionic azido ligands.Asaresult of the increased ionicity of the N 3 groups, their average internal N À Nd istance (1.199 (3) ) is more similar to the terminal N À Nd istance (1.145 (3) 5 ] 2À ,t he two levels of theory found different minimum-energy structures,w hich differ in the orientation of the various azido ligands.T his is in good agreement with previous results that there are only subtle energy differences of less than 5kcal mol À1 between various orientations of the azido ligands in metal polyazides.A s ar esult, polyazido compounds may adopt several structures with only very little energy difference among them. [10] Tw od ifferent minimum-energy structures (structures A and Bi nF igure 3) with C 1 symmetry were found for the bipyridine adduct [(bipy)VO(N 3 ) 3 ].…”

“…As aresult, the synthesis of molecules with ahigh number of azido groups is very challenging because of their explosive nature and shock sensitivity.Ithas also been well established that polyazido compounds of metals in higher oxidation states are generally more sensitive and less stable than the ones of the same metal in alower oxidation state. Theb inary vanadium(IV) azides [V(N 3 ) 4 ] [5] and [V(N 3 ) 6 ] 2À , [5] theb inary vanadium(V) azide [V(N 3 ) 6 ] À , [5] and the species [VOCl 2 N 3 ], [6] [VO(N 3 ) 4 ] 2À , [7] and [V(N 3 ) 3 -(N 3 S 2 )] 2À [8] have been reported for the higher oxidation states but no vanadium(V) polyazide has been structurally characterized. [3] Another approach for the stabilization of high-oxidation-state azides is the introduction of oxygen atoms,a st he oxidation potential of am etal oxohalide is generally lower than that of the corresponding oxygen-free metal halide with the metal in the same oxidation state.W hile as ignificant number of azido compounds of the heavier Group 5elements Nb and Ta have been prepared and characterized, [3,4] only al imited number of vanadium azides are known.…”

“…[5] Thec olor can be attributed to the fact that many vanadium compounds are deeply colored and that the presence of multiple chromophores,such as azido groups,can further enhance the color intensity. [5] Thec olor can be attributed to the fact that many vanadium compounds are deeply colored and that the presence of multiple chromophores,such as azido groups,can further enhance the color intensity.…”

The Vanadium(V) Oxoazides [VO(N3)3], [(bipy)VO(N3)3], and [VO(N3)5]2−

“…When . [5] Thec olor can be attributed to the fact that many vanadium compounds are deeply colored and that the presence of multiple chromophores,such as azido groups,can further enhance the color intensity. In both molecules,the coordination geometry around the central vanadium atom is derived from adistorted octahedron with the three azido ligands as well as one bipyridine nitrogen atom occupying the equatorial positions.T he oxygen atom and the second bipyridine nitrogen atom occupy the axial positions.I nb oth molecules,t he central vanadium atom is located 0.27(2) a bove the plane containing the four equatorial nitrogen atoms.This distortion from the octahedral arrangement is in good agreement with valence-shell electron repulsion theory (VSEPR) and can be attributed to increased repulsion from the sterically more demanding oxygen ligand.…”