1,3-Dipolar cycloadditions to coordinated azide in cobalt chelate complexes of the type LCo(chelate)N3

Azide Chemistry – An Inorganic Perspective, Part II[‡][3+2]‐Cycloaddition Reactions of Metal Azides and Related Systems

“…that of co- [220,350] L 5 = 1,11-diamino-3,6,9-triazaundecane (tetren) [354] ) with the free azide ion, revealed a two-step process via a facile cycloaddition to give the N 1 -bonded tetrazolate followed by a slow N 1 Ǟ N 2 -linkage isomerisation (Scheme 12). The latter is in contradiction with electronic considerations which favour N 1 -coordination, [152,184,219] but obviously driven by the steric congestion existing between the tetrazole ring substituent in the 5-position and the four cis-ammines (cis-Nligands) of the pentaamminecobalt(III) (or [Co(tetren)]) unit. The rate of the linkage isomerization turned out to be largely dependent on the hydrogen ion concentration, i.e., the more weakly bound protonated neutral ligand complexes rearrange much faster.…”

“…[151,171] Conversely, the strictly regiospecific N 2 -coordination of the ambidentate tetrazolate anion to the (octahedral) cobaloxime fragment Bu n 3 PCo(DH) 2 , irrespective of the presence of electron-withdrawing or -releasing substituents in the 5-position and in spite of a more nucleophilic N 1 (MINDO/3 calculations), is sterically induced (see also 1.5.3). [152,184,219] 2 , an assumption which has later been abandoned in favour of additional trans isomers. Their ready formation upon dissolution of the solid with clear cis-geometry [142] throws light on the configurational lability of the square-planar palladium(II) system; that also some, albeit slow, tetrazole exchange occurs, is proven, e.g., by the presence of both N 1 and N 2 bonded tetrazolato ligands in various solutions of cis-[Pd{N 4 CMe} 2 (PMe 2 Ph) 2 ] whose X-ray crystal structure showed only N 1 -bonded tetrazolate.…”

“…[184,317] What mainly affects the tautomerism or the isomer ratio, however, is the electronic and spatial structure of the substituent at the C atom. This applies all the more, when an N-substituent is introduced into the tetrazole ring in the place of a hydrogen or metal atom capable of migration and dissociation.…”

“…So it would appear that in both cases the initial products form directly, viz. by attack of the γ-nitrogen of the azide ligand on the nitrile carbon to give the N 2 -www.zaac.wiley-vch.de [184] or, respectively, by attack of the free N 3 -ion on the co-ordinated nitrile resulting in the expected N 1 -form; it was not possible, however, to distinguish between a concerted 1,3 dipolar cycloaddition I or a mechanism involving the formation of an imidoyl azide intermediate II which then rapidly cyclizes to afford the final product. [356] The latter course is adopted by the reaction of SiMe 3 (N 3 ) with Cl 3 CCN, for example (Table 2).…”

“…4-6 and Table 1). [183,184] While the latter only reacted with highly electron-deficient nitriles to give (presumably) N 2 -bound tetrazolato complexes some of which had lost their PPh 3 base -the related diazido complex AsPh 4 [Co(N 3 ) 2 (DH) 2 ] did not react at all, and complex Co(N 3 )(DH) 2 PPh 3 reacted much more slowly -both azidoiron porphyrins, Fe(N 3 )(OEP) and Fe(N 3 )(TMP), underwent cycloaddition with a series of electron-rich nitriles ( Table 1). The linkage modes of the axial tetrazolato ligands of two representative complexes, Fe{N 4 CMe}(OEP) and Fe{N 4 CPh}(OEP), have been determined by X-ray and found to occur through N 1 and N 2 , respectively.…”