Cobalt complexes multidentate

To confirm whether multidentate calixarenes could limit NP growth under solvothermal conditions, we prepared hexadecanuclear cobalt complexes 1 and 2 from Co2(CO)g, a common precursor for Co NP synthesis, and octa-O-propargyl derivatives of Cl and Cll resorcinarene (3 and 4 respectively) [14, 15]. Coje-calixarenes 1 and 2 were stable under ambient conditions, but decomposed rapidly above 90 °C. An x-ray crystal stmcture of 1 shows all eight pendant Co2(CO)e units to be on the same face of the calixarene platform [14], and MALDl-MS analysis of 2 indicates facile decarbonylation into zerovalent Coie clusters [15], implying formation of a metastable capped-cluster intermediate (Fig. 34.3). 

Sargeson and his coworkers have developed an area of cobalt(III) coordination chemistry which has enabled the synthesis of complicated multidentate ligands directly around the metal. The basis for all of this chemistry is the high stability of cobalt(III) ammine complexes towards dissociation. Consequently, a coordinated ammonia molecule can be deprotonated with base to produce a coordinated amine anion (or amide anion) which functions as a powerful nucleophile. Such a species can attack carbonyl groups, either in intramolecular or intermolecular processes. Similar reactions can be performed by coordinated primary or secondary amines after deprotonation. The resulting imines coordinated to cobalt(III) show unusually high stability towards hydrolysis, but are reactive towards carbon nucleophiles. While the cobalt(III) ion produces some iminium character, it occupies the normal site of protonation and is attached to the nitrogen atom by a kinetically inert bond, and thus resists hydrolysis. 

Some multidentate ligands as models of CA (A) tris-(3,5-dimethyl-l-pyrazolylmethyl) amine (cobalt[HJ complex) (B) tris (4,5-diisopropylimidozal-lyOphosphine" (zinc Il complex) (C) bis(histamino) -cyclodextrin. 

Cobalt(—I) complexes arsenic ligands, 769 bipyridyl, 691 cyanides, 646 phenanthroline, 691 phosphines bidentate, 728 monodentate, 718 multidentate, 738 tridentate, 738 phosphinites, 747 phosphites, 747 phosphonites, 747 terpyridyl, 691 Cobalt(O) Complexes arsenic ligands, 769 bipyridyl, 691 carbon disulfide, 646 cyanides, 646 phenanthroline, 691 phosphines, 718 bidentate, 728 multidentate, 738 tridentate, 738... 

Linear and cyclic polyamine complexes of mthenium(III) have been little studied relative to those of first-row transition elements such as cobalt(III) and nickel(II), presumably due to a lack of general routes of syntheses. A convenient and reproducible method for the syntheses of trans-[RuLCl2], where L is any multidentate aliphatic ligand with a four-amine donor combination, has been developed. Procedures for the preparation of trans-[RuLCl2] [L = (en)2,J 2,3,2,-tet,f and cyclamj] are described here. The literature method is closely followed except that the much safer hexafluorophosphate(l — ) anion is employed in place of perchlorate. 

Syntheses reported for the pentaammine(trifluoromethanesulfonato-O) complexes can be readily adapted for other amine or multidentate amine analogs. Syntheses of coIbalt(III) complexes with 1,2-ethanediamine or A -ethyl-l,2-eth-anediamine ligands have been reported earlier in this series. To exemplify the procedures further, trifluoromethanesulfonato-O complexes of cobalt(III), chro-mium(III), and rhodium(III) with unidentate methylamine ligands based on the readily prepared [M(NH3)5Cl]Cl2 precursors are reported here. The following sections report syntheses of 1,2-ethanediamine complexes of Rh(III) and Irflll) and of Ru(II) and Os(II) diimines with trifluoromethanesulfonato ligands. Such syntheses indicate the diversity of the synthesis technique, and the complexes described are excellent precursors for other compounds. 

Table 93 Structures Multidentate Thioether-Cobalt(Ill) Complexes...

Five-co-ordinate Complexes.—review on five-co-ordinate cobalt(u) complexes has appeared. All of the five-co-ordinate complexes described here contain multidentate ligands in four trigonal-bipyramidal geometry is found, whereas in two the metal co-ordination is square pyramidal. 

Reactions of zinc(n) and cobalt(n) complexes of ida, Hida, nta , Eten, and dien with cydta - and Hcydta have been studied. The steric effect of the cyclohexane ring in cydta on the rate of substitution is discussed. Other reports of the replacement of one multidentate ligand by another have appeared for the systems Ni(Hida) + dtpa, Ni(edta-OH) -I- dtpa, and Co(gedta) + (edta-OH) [Hida = A-(2-hydroxyethyl)iminodiacetate, (edta-OH) = lV-(2-hydroxyethyI)ethylenedi-amine-AW A -triacetate, gedta = 2,2 -ethylenedioxybis(ethyIamine)-AAW JV -tetra-acetate). Similar studies of aminopolycarboxylate replacement have appeared for zinc(n), lead(n), and cadmium(n). ... 

Multidentate Leaving Groups.—The hydrolysis of [Co(ox)a] - and of [Co(ox)2(OH2)2], which ultimately produces cobalt(n) and carbon dioxide, involves the formation of an intermediate containing a unidentate oxalate ligand previous to the rate-determining step. Free radical intermediates are thought unlikely in the decomposition of these oxalato-complexes, but malonate ion-radicals are thought to be intermediates both in the thermal and photochemical hydrolysis of the [Co(mal)3] anion. Kinetics are reported for a third example of these aquation-redox processes, [Co(acac)2] in acidic solution. ... 

Sharrad CA, LUthi SR, Gahan LR (2003) Embracing ligands. A synthetic strategy towards new nitrogen-thioether multidentate ligands and characterization of the cobalt(lll) complexes. Dalton Trans 3693-3703... 

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