2,6-Diacetylpyridine, transition metal

Transition Metal Complexes with Bis(Hydrazone) Ligands of 2, 6-Diacetylpyridine. Hepta-Coordination of 3d Metals... 

TRANSITION METAL COMPLEXES WITH BIS(HYDRAZONE) LIGANDS OF 2,6-DIACETYLPYRIDINE. HEPTA-COORDINATION OF 3d METALS... 

An extensive family of tetraaza (17) and pentaaza (18) macrocycles with a range of ring sizes are obtained by reactions of 2,6-diacetylpyridine with tri- or tetr-amines in the presence of suitable metal ions, which include as well as the usual M11 transition metal ions a variety of non-transition metal ions.19 Large rings, such as (19), can be formed by 2 2 condensations in the presence of suitable larger metal ions,20 Reactions of a, to diamines which have other internal heteroatoms with 2,6-diacetylpyridine (or 2,5-diformylfuran) produce a variety or related mixed heteroatom macrocycles. 

In many cases it is possible to utilise the hole size effects for the synthesis of specific types of macrocycle. Thus, a tetradentate macrocycle (6.33) is expected to be obtained from a template condensation of 2,6-diacetylpyridine with 1,5,9-triazanonane in the presence of small, first-row transition metal dications. The hole size of 6.33 closely matches the size of these metal ions. This is indeed what happens when Ni2+ (r = 0.8 A) is used as a template for the condensation and the nickel(n) complex of 6.33 is obtained in good yield (Fig. 6-32). However, when Ag+ (r = 1.0 A) is used as a template, the metal ion... 

Figure 6-32. The condensation of 2,6-diacetylpyridine with the diamine in the presence of nickel(n) or other first-row transition metal dications leads to complexes of the tetraazamacrocycle 6.33.

So far, we have concentrated upon reactions resulting from discrepancies between the size of the metal ion and the size of the macrocyclic cavity. However, it is not only the size of the metal ion that may result in a mismatch what happens if the favoured conformation of the ligand does have an arrangement of donor atoms that matches with the preferred co-ordination geometry of the metal ion This is exactly the situation that we observe with metal complexes of some pentadentate macrocycles. We have previously observed the formation of tetraaza macrocycles from the template condensation of 2,6-diacetylpyridine with diamines in the presence of a transition metal ions. We also noted that if the size of the metal ion were incorrect, it was possible to get [2+2] or other products. Now let us look at this topic in a little more detail. 

Retrosynthetically, preparation of N-azolyl 2,6-bis(imino)pyridyl metal complexes as precatalysts for polymerization involves (i) synthesis of N-amino azoles, (ii) condensation of 2,6-diacetylpyridine with two equivalents of N-amino azole, and (iii) complexation of die appropriate transition metal halide with the terdentate ligands. The latter two reactions present no real challenge and are more or less analogous to fhe published preparation of N-aryl 2,6-bis(imino)pyridyl complexes [13], and fhe more demanding synthesis of N-amino-azoles was covered in a preceding section. 

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