Naphthyridines as Ligands

The availability of 1,8-naphthyridine and its alkyl derivatives has spawned a veritable explosion of studies aimed at examining the behavior of this ring system as a ligand. Much of this work has been done by Hendricker and co-workers, initially caused by the availability of 1,8-naphthyridine synthesized by Kress and Paudler. 

Metal ion complexes with 1,8-naphthyridine could be conceived as possessing any or all of the following structures (a, b, or c)  

The monodentate behavior exemplified by a could also involve an equilibrium between the structure shown and the corresponding species where M is bonded to N2. 

The X-ray crystallographic structure of the Fe(II) complex showed that the 1,8-naphthyridine was part of an eight-coordinate Fe(II) complex in which one of the 1,8-naphthyridines is more tightly bonded than the other three.90 In solution the complex dissociates into a tris-l,8-naphthyridine-Fe(II) species. 

The Mossbauer 57Fe spectrum of the Fe(ll) complex has shown that it has the largest quadrupole splitting (4.49 mm/sec) thus far encountered for any Fe(II) species.92-93 

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Cr(CO)f,] reacts with 1,8-naphthyridine, 2-methyl-1,8-naphthyridine, and fran.s-decahydro-l,8-naphthyridine (L) to form [Cr(CO)5L], [Cr(CO)4L2], and Cr[(CO)4L] complexes, the latter involving L as a bidentate ligand. With 2,7-dimethyl-l,8-naphthyridine and 2,9-dimethyl-l,10-phenanthroline (L ), [Cr(CO)4L ] complexes were obtained. 

Although not a purine, the ligand properties of 7-azaindole (81) strongly resemble those of deprotonated purines. Metal binding of this anionic ligand may occur to one or two metal ions, just as with 1,8-naphthyridine (Scheme 8).158... 

The 1,8-naphthyridine derivative (60) reacts with LiPPh2 giving the product (61) (55% yield), which has been used as a ligand for rhodium <92JCS(D)2367>. 

Naphthyridine [21 ] acts as a stable monodentate ligand, and the contact shift data reported for Co(ii) and Ni(ii) complexes of this ligand (122) have been compared with those for other heterocyclic nitrogen ligands. The complexes are octahedral but in solution the following equilibrium exists ... 

Finally, the structural characteristics of the 1,8-naphthyridines favour their behavior as bidentate and dinucleating ligands, thus leading to short metal-metal separations in late transition-metal complexes,24"26 and to the stabilization of mixed-valence complexes.27,28 The dinuclear ruthenium complex [ Ru(napy)(H20)2 (/i-Cl)(/i-0H)](C104)2 is a stable and active catalyst for the oxidation of alcohols and the epoxidation of alkenes, while its mononuclear precursor is much less active.29... 

In recent years, 1,8-naphthyridine derivatives have attracted growing interest as promising ligand systems, which coordinate metal ions of different groups of the periodic table, such as Cu (1984IC3633, 1992JOC2215, 1998ICA129), Au... 

Naphthyridine (41), used as a chelating ligand, has been studied to investigate the molecular geometry when the molecule is not in a chelating environment. The molecule is non-planar, with the two rings twisted in... 

The ligand, 1,8-naphthyridine, forms complexes with what appear to be very high coordination numbers. With perchlorate as the anion the complexes R(napy)s(C104)3 (R = La-Pr) and R(napy)s(C104)3 (R = Nd-Eu) are isolated from ethyl acetate solutions (Foster et al., 1972). The infrared evidence indicates that in all cases the naphthyridine is functioning as a bidentate ligand and that the perchlorate is present in the ionic form. This would require that the larger lanthanide ions be twelve-coordinate and that the heavier lanthanides be... 

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