Pyridines 1+1 coordination chemistry

E. Ochial, Aromatic Amine Oxides. Elsevier, Amsterdam, 1967. Pyridines, coordination chemistry ... 

The coordination chemistry of ancillary amidinate ligands with a pyridine functionality has been described. Magnesium, aluminum, zirconium, and lanthanum complexes have been prepared in which the amidinate anions act as tridentate, six-electron-donor ligands Amidinate ligands containing quinolyl substituents were constructed in the coordination sphere of lanthanide... 

N-heterocyclic compounds containing six-membered rings (pyridine and analogues) behave as excellent -acceptors and in turn they provide a rather soft site for metal ion coordination. The 7r-excessive five-membered pyrazole is a poorer 7r-acceptor and a better 7r-donor and it acts as relatively hard donor site. Inclusion of six- and five-membered N-heterocycles like pyridine and pyrazole in one ligand system leads to very attractive coordination chemistry with variations of the electronic properties.555 The insertion of a spacer (e.g., methylene groups) between two heterocycles, which breaks any electronic communication, makes the coordination properties even more manifold. 

The chemistry of coordination compounds comprises an area of chemistry that spans the entire spectrum from theoretical work on bonding to the synthesis of organometallic compounds. The essential feature of coordination compounds is that they involve coordinate bonds between Lewis acids and bases. Metal atoms or ions function as the Lewis acids, and the range of Lewis bases (electron pair donors) can include almost any species that has one or more unshared pairs of electrons. Electron pair donors include neutral molecules such as H20, NH3, CO, phosphines, pyridine, N2, 02, H2, and ethyl-enediamine, (H2NCH2CH2NH2). Most anions, such as OH-, Cl-, C2042-, and 11, contain unshared pairs of electrons that can be donated to Lewis acids to form coordinate bonds. The scope of coordination chemistry is indeed very broad and interdisciplinary. 

The organic chemistry of the bipyridines has not previously been reviewed, although short sections devoted to aspects of bipyridines are included in treatises on pyridine chemistry. A booklet on some aspects of bipyridines has appeared in Russian. This review provides a thorough coverage of the chemistry of the bipyridines to the end of 1981 and includes work reported in most chemical journals to mid-1982. The early chemistry of the bipyridines, which has received some discussion in the reviews of pyridine chemistry, is mentioned briefly to provide continuity. The review excludes the extensive metal and nonmetal coordination chemistry of the bipyridines... 

The importance of the carboxylate donors is underlined by a study of the lanthanide coordination chemistry of the similar terdentate ligand 2,6 -bis( 1 -pyrazol-3 -yl)pyridine, L24 (63). The complex structure of [Tb(L24)3][PF6]3, shown in Fig. 11, appears to be fairly robust in methanolic solution, with Horrocks analysis (q = 0.6) suggesting the 9-coordinate structure is retained the small quenching effect of outer sphere coordination explains the q-value. However, in aqueous solution, the lability of the ligands dramatically changes the luminescence. Whilst the emission decays are not exactly single exponential, approximate lifetimes in H20 and DoO suggest a solvation value of 4-5. 

An example of the different points of view and different tastes in the matter of acid-base definitions was provided to one of the authors in graduate school while attending lectures on acid-base chemistry from two professors. One Felt that the solvent system was very useful, but that the Lewis concept went too far because it included coordination chemistry. The second used Lewis concepts in all of his work, but felt uncomfortable with the Usanovich definition because it included redox chemistry To the latter s credit, however, he realized that the separation was an artificial one, and he suggested the pyridine oxide example given above. 

Undoubtedly, pyridine, C5H5N (2), is the best-known heterocyclic nitrogen ligand and its coordination chemistry has been studied in great detail, as have its simple derivatives bearing a non-coordinating substituent. For the physical properties, the reader is referred to the heterocyclic literature.1 3,5,9 The basic properties of pyridine have been mentioned above. Alkyl-substituted derivatives are slightly more basic [pA (base) values of about 5-7]. 

Our third contribution is by Alexander Sadimenko, of the University of Fort Hare, (Republic of South Africa) and continues the series of organo-metallic complexes of heterocycles. The present contribution covers a broad class of chelating ligands constituted by phosphinopyridines and related compounds. These interesting ligands possess both hard (pyridine nitrogen) and soft (phosphorus) coordination sites, which provides them with special properties in coordination chemistry. 

Argent, S. P., Adams, H., Riis-Johannessen, T., etal, Coordination chemistry of tetradentate N-donor ligands containing two pyrazolyl-pyridine units separated by a 1,8-naphthyl spacer Dodecanuclear and tetranuclear coordination cages and cyclic helicates. Inorg. Chem. 2006, 45, 3905-3919. 

Complexes of aluminum iodide with pyridine (1 1 and 1 3) have been described as being formed by direct reaction of the two compounds.1 However, the limited availability of high-purity A1I3 has hampered the development of the coordination chemistry of this compound. The synthesis described here makes use of an A1I3 ethereal solution, prepared in situ, as the starting material, and it is based on the reaction ... 

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