Optical resolution of octahedral metal

Mechanism of Optical Resolution of Octahedral Metal Complexes... 

Although the resolution into their enantiomeric forms of octahedral metal complexes bearing bidentate ligands was accomplished at an early stage,19 the application of such materials for the preparation even of dinuclear complexes was greatly delayed.20 Only recently has the preparation of a particular stereoisomer of a diruthenium complex been accomplished by the resolution of the bis-o-phenanthroline complex (9). The subsequent use of the optically active material in a stereoselective reaction with 2,5-bis(2-pyridyl)pyrazine (10) yielded the dinuclear com-... 

The demonstration of the optical activity of octahedral complexes was important in confirming Alfred Werner s intuitive ideas about coordination chemistry. Early work involved the resolution of complexes characterized by optical rotations. Modem instmments for optical rotatory dispersion were developed first, but circular dichroism (CD) spectra proved to be more useful. CD has been a powerful tool for detailed studies of the stereochemistry of octahedral complexes. Contributions to rotational strength of chelate ring conformational, configurational, and vicinal contributions are additive. Chiral metal complexes are now used in enantioselective synthesis of chiral pharmaceuticals. 

Scheme 4 shows some optically active resolving agents used in the resolution of organometallic transition metal compounds of tetrahedral, octahedral, and square pyramidal geometry. Schemes 2 and 3 demonstrate the application of the menthoxide ion. The aminophosphine shown will be used in an example discussed later on. The pyridine imine chelate ligand has been the chiral auxiliary for the resolution of octahedral compounds [9,10], not described in detail here, and for the resolution of square pyramidal compounds to be discussed next [11]. 

The structure of A1(acac)3 contains an octahedral A106 core, with At—O distance of 189.2 pm. Differences of the monoclinic a form and orthohombic y form (a racemate) lie chiefly in the orientations of molecules within the crystal bond distances are almost identical.181 Resolution of the optical isomers of Al(acac)3 has twice been achieved by column chromatography at low temperature.182183 The hexafluoro compound, Al[(OCCF3)2CH]3, m.p. 74 °C, is more volatile than Al(acac)3. Its structure, determined by vapour phase ED,184 reveals a slightly distorted A106 unit in which the A1—O distance of 189.3 pm is the same as that in Al(acac)3. Comparative studies have been made of the Raman spectra of M(acac)3 (M= Al, Ga or In)185 and of other j8-diketonates of these metals.186... 

Although the existence of circular dichroism and anomalous optical rotatory dispersion for the visible d—d transitions of transition metal complexes was discovered by Cotton (7), the first resolution of an octahedral complex was achieved by Werner (2), for the chloroamminebis-(ethylenediamine)cobalt(III) ion (I, X = C1, Y = NHs). In the course of a few years he resolved (3) the trisethylenediaminecobalt(III) ion (II), a number of bis- and tris-chelated octahedral complexes of cobalt, chro-... 

Thus, chromatography on Sephadex ion-exchangers is very effectively applied to multivalent complex cations. Jensen and Woldbye have reviewed the optical activity of coordination compounds resolutions of racemic octahedral transition metal complexes through both diastereoisomer and chromatographic techniques are summarized. 

It is fitting to begin this brief historical overview of chirality in organometallic and coordination chemistry with the name of Alfred Werner (1866-1919) who, as far back as 1893, applied van t Hoff and Le Bel s stereochemical ideas of the tetrahedral nature of the carbon atom to the structure of hexacoordinated metal complexes. He established their octahedral structure and predicted that some could exist in an enantiomeric form with the power of optical rotation. This prediction was followed in 1911 by the resolution of the two enantiomers of the complexes [Co (en)2(NH3)X]X2 (X = Cl, Br) (2.1)-X2 (en = ethylene diamine) (Figure 2.1). This overall work won him the Nobel Prize for Chemistry in 1913, following which he then went on to resolve the inorganic complex Co (OH)6[Co (NH3)4]3 Br6 (2.2)-Br6 (Figure 2.2). ... 

This paper traces the development of optical activity in inorganic compounds to the point where Alfred Werner was able to use optical resolution to such striking effect in proving the octahedral stereochemistry of several metal ions by resolving chelated compounds, in particular the fully inorganic cation now called "Werner s hexol". This achievement came to fruition (1) in the years just prior to World War I. 

Alfred Werner conjectured as early as 1899 that octahedrally coordinated metal complexes should occur in nonidentical minor image isomers. For such objects. Lord Kelvin, in 1893, had coined the adjective chiral , a term never used by Werner. It can be proved by examination of the original sample of [Co(NC)2)2(en) Br, prepared by Edith Humphrey, a Ph.D. student of Wemer s, that crystals of optically pure samples were obtained in Werner s laboratory as early as 1899 or 1900. However, Werner did not publish die first successful resolution of an octahedral metal complex until 1911. Presently, interest in chirality in coordination compounds is booming, mainly because of the importance of coordination compounds in enantioselective homogeneous catalysis. Other interesting sq )plications are enantioselective interactions of chiral coordination species with biomolecules, and the stereoselective synthesis of multicenter systems. 

Ligand (69) coordinates to nickel such that the four donors and the metal ion form a planar array whereas (68) coordinates around one face of an octahedral arrangement. Each complex type exhibits a characteristic kinetic inertness which no doubt arises from the operation of the macro-cyclic effect. Indeed, because of the inertness of the cation [Ni(tri)(H20)3]2+, its resolution into optical isomers has been possible... 

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