Metal complexes from aryl and hetarylazocompounds

Significance of the phase transfer catalysis for the synthesis of less accessible aromatic and heterocyclic compounds is emphasized. A series of novel azo compounds containing pyrazole and quinoline fragments has been prepared and employed in the synthesis of 3d-metal coordination compounds with desired types of the coordination site


Introduction
The formulation of the basic principles of phase transfer catalysis advanced by Mieczyslaw Makosza [1][2][3] dates back to the middle of sixties, and the method continues to be one of the powerful tools of the synthetic organic chemistry.In recent years phase transfer catalysis has assumed a new importance in organometallic [4][5][6] and coordination metal chemistry 7,8 , in particular as a useful approach to the preparation of ligands  . Whil being employed mostly in the reactions of nucleophiles, the method may be applied also to the transformations occurring with the participance of electrophilic reactants in two-and triphase catalytic systems.5,7 An amply studied example of such type transformation is azo coupling reactions affording diverse less accessible aromatic and heterocyclic azo compounds in high (80-85%) yields 5,30 .Many of these compounds serve as useful ligands in the synthesis of metal coordination compounds.This paper presents a concise review of the structural and preparative aspects of metal coordination compounds with azo-group containing aromatic and heterocyclic ligands.
Not only aromatic, but also heterocyclic diazonium salts were used in the synthesis of azo ligands of metal coordination compounds.Thus, a series of azo compounds 4 was obtained based on the diazonium salts prepared from 2-amino-1-alkylbenzimidazoles (4). 36he azo compounds containing a mercapto group in the ring are rather unstable and are readily oxidized in the air.Therefore o-thiocyanateazobenzenes 5 37,38 were used as their precursors in the synthesis (5) of the metal chelates with MN 2 S 2 coordination sites.
ISSN 1424-6376 Page 34 Stereochemistry of the metal chelates.Annulation of a pyrazole fragment to the metal chelate ring results in the tetrahedral deformation of the coordination site in the Ni(II), Zn(II) complexes 11 (X=NAr, O). 41,42,71 According to the X-ray diffraction data the complex 19 possesses pentacoordinated trigonal bipyramidal structure.The same structural effect was observed also for the azomethine analogues of 11. 8,16,17,49 On the contrary, the sulfur complexes 11 (M=Ni, n=2, X=S) have a planar configuration of the coordination site. 54,71 he metal ions at higher oxidation numbers, e.g.Co(III) 42 and Ru(III) 47 can form octahedral complexes with bidentate azo ligands.The octahedral configuration of a metal center has been proven by X-ray determinations for the complexes 20 43 and 21 72 with tridentate ligands, as well as [11 .MeOH] (M = Ni, R 1 = Ph, R 2 = Me, Ar = p-tert-Bu-С 6 H 4 , n = 2). 42N Intrachelate isomerism.This type of bond-linkage isomerism 19,20 is characteristic of the metal chelate complexes containing several competitive donor centers in a coordination site.In the azo ligands 1, 4, 6, 8, 9 such a role may be played by each of the two nitrogens of an ambidental azo group. 8,20,67 Te possibility of the formation of the five-membered chelate ring due to donation of an electron pair by the nitrogen atom adjacent to the arene ring was considered as an explanation of the unexpected planarity of the coordination site of the Co(II) complex 22. 31,73 N H However, X-ray studies of the analogous Ni(II) complex as well as of its oxygen analogue witness the six-membered trans-planar structures 23. 67The first direct evidence of the inclusion of a metal center into the two five-membered rings formed by azo groups was obtained using Xray determinations of the complexes 24a [74][75][76][77] and 24b 38,39 .By varying the structure of the azo ligands (X and R in 25), metal chelates 25a 78 and 25b 79 with both five-and six-membered ring within a molecule have been obtained and structurally characterized.Examples of the similar type intrachelate isomers became known for the complexes 26 52 and 27 53 with heterocyclic azo ligands.

Conclusions
The data above considered point to the possibilities of the significant variation of the structure and, therefore, properties of metal chelate complexes caused by diverse structural modifications of the azo-containing ligands.Less studied in this respect is the problem of the influence of strong electron-withdrawing and electron-releasing substituents in the aryl rings of the aromatic and heterocyclic azo compounds, which is essential for defining basicity and ligating abilities of the nitrogen centers in the azo groups.It is expected that the investigation into this problem may be promoted by the further extension of the series of the relevant azo ligands and gaining deeper insight into the mechanism and preparative application of the azo coupling reactions under the phase transfer catalysis conditions.

Experimental Section
General Procedures. 1 H NMR spectra were registered on "Varian UNITY-300" spectrometer.Magnetic measurements were carried out on SQUID magnetometer "Quantum Design MPSM-5 S" at 299 K.

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, Ni, Pd, Zn A monochelate Cu(II) complex 13 was obtained from by the redox reaction.