Tris-chelate complexes

Figure B The absolute configuration of the optical isomers of a metal tris-chelates complex such as [Co(en)3]. (a) A configuration and (b) A configuration.

Dynamics of intramolecular metal-centred rearrangement reactions of tris-chelate complexes. L. H. Pignolet, Top. Curr. Chem., 1975,56,93-137 (85). 

Encapsulation of [Co(bpy)3]2+ within zeolite frameworks has also been shown to have a remarkable influence on the electronic spin state of the complex.240 Distortions imparted on the tris-chelate complex by the confines of the zeolite supercage are found to be responsible for stabilizing the unusual low-spin electronic ground state.241,242 The [Co(bpy)3]3+/2+ couple has been measured for the encapsulated complex and it has been found that the complexes remain within the zeolite and do not exchange with the bulk solution.243 Electrochemistry of [Co(bpy)3]3+/2+ immobilized within a sol-gel has also been studied.244... 

The [Co(phen)3]3+ complex is photoactive and a powerful oxidant in its excited state. The ion has no H-bonding groups and hence is considerably more hydrophobic1279 than hexaamine relatives. These properties have proven particularly useful. Aryl and alkyl substituted [Co(phen)3]3+ complexes have received a great deal of attention due to their ability to intercalate within the helical structure of DNA through a combination of electrostatic and hydrophobic forces. The chirality of the tris-chelate complex is crucial in determining the degree of association between the complex and... 

Chelating aldehydes such as 2-pyridine carbaldehyde and 2-dimethylamino benzaldehyde improve the stability of the aldehyde complexes via N,0 chelation. NMR studies show that the complexes are present in solution without an excess of aldehyde and can be formed in the presence of donor ligands. The X-ray structures showed longer and weaker Zn—O bonds when more than one chelating ligand was present. IR demonstrates the variation in C=0 bond strengths and how the environment of the zinc ion will influence potential catalytic activity via reaction rates or pathways. Tetrahedral chelate complexes, and octahedral bis- and tris-chelate complexes, were isolated.843... 

Pignolet, L. H. Dynamics of Intramolecular Metal-Centered Rearrangement Reactions of Tris-Chelate Complexes. 56, 91-137 (1975). 

A large number of complexes have been prepared in which metalla-/3-diketonate anions coordinate to either metal ions or B(X)(Y) moieties. The chemistry of such metal complexes has been reviewed previously (7). To summarize briefly, tris-chelate complexes (3) of Al(III) or Ga(III)... 

The PFg" salts of [Ru(bpy)2(110)] and [Ru(110)3] and analogous complexes containing 4,4 -bis(substituted) ferrocenyl ligands (110 ), have been synthesized and characterized the tris(chelate) complexes are either poorly soluble or insoluble. Electropolymerization of [Ru(110 )3][PF6]2 produces an electrochromic film. The complex [Ru(bpy)2(lll)] undergoes electropolymerization on Pt and glassy carbon electrodes, although the related complex [Ru(bpy)2(112)] does not. Electrochemical and spectroscopic properties of the films indicate that they form by both head-to-tail and tail-to-tail monomer coupling. ... 

Complexes of simple amino adds with chromium(lll) were first prepared by Ley.1148 The isomers possible for tris chelated complexes of this type are illustrated below (248-251). The consequences of such isomerism were first seriously considered by Gillard.1149 Red complexes of the formulae [Cr(gly)3] and [Cr(L-ala)3] were prepared by neutralizing refluxed solutions of hexaaquachromium(Ill) and the amino add in ratios between 1 5 and 1 10. These complexes were shown to be isomorphous with 0-[Co(gly)3] and D- -[Co(L-ala)3] respectively. The crystal structure of red j8-[Cr(gly)3] has also been reported.1150... 

Figure 11. Sokolov s strategy for constructing a trefoil knot on an octahedral tris-chelate complex [39].

It was mentioned above that tris(chelate) complexes of the type (Co(en) ],+ lack an improper axis of rotation. As a result, such complexes can exist in either of two enantiomeric forms (or a racemic mixtire of the two). Figure 12.20 illustrates the complex ions (Co(en)j]3+ and (Crfoxy3-. each of which ts chiral with Di symmetry. 

In general, substituents in 2,9-positions of phen and 6,6 -positions of bipy prevent the formation of tris-chelate complexes due to the steric hindrance of the ligands and consequently the mono- and bis-chelate complexes become preferred. The two complexes [NiX2(N—N)] (102 X = C1, Br) formed by 2,9-dimethyl-l,10-phenanthroline have been found to be square pyramidal dinuclear with both bridging and terminal halides,848 while the iodo derivative [NiI2(N—N)] (103) is monomeric and pseudotetrahedral.847... 

Values of the normalized bite b and the twist angle 8 for these complexes are given in Table 14. In sharp contrast to other tris(chelate) complexes, it is very clear that 8 is not a simple function of b. In all cases 8 is lower than predicted, and in many cases the trigonal prism with 8 = 0° is observed. These dithiolate ligands are clearly different from other bidentate ligands, even other sulfur donors such as MeSCH=CHS, dithioacetylacetonate, dithiooxalate, dithiophosphates, dithiocarbamates and xanthates. 

A variety of racemizations are known to be induced by heating or applying pressure to solids. O Brien has reviewed these reactions.12 Most studies to date have focused on tris-chelate complexes. 

For a given M and z, the ease of oxidation of [MfS R ]2 decreases in the order R = H, alkyl > Ph > CF3 > CN and R = Ph > CF3 > CN for the analogous tris-chelated complexes. The series parallels the election-donating tendency of the. substituent R. For the dianion, another stability trend was noted the oxidative stability increases across the first row transition metals Fe Co < Ni < Cu. 

An interesting variation is to exchange an optically pure tris chelate complex on to montmorillonite, e.g. A-[Ru(phen)j]2+. If this clay is used to modify the surface of an Sn02 electrode, it is found that the electrochemical oxidation of [Co(phen)3]2+ at 30 °C produced A-[Co(phen)3]2+ with 7% optical purity.78... 

M.T. Carter, M. Rodriguez and A.L. Bard, Voltammetric studies of the interaction of metal chelates with DNA. 2. Tris-chelated complexes of cobalt(III) and iron(II) with 1,10-phenanthrofine and 2,2 -bipyridine, J. Am. Chem. Soc., Ill (1989) 8901-8911. 

Dynamics of Intramolecular Metal-Centered Rearrangement Reactions of Tris-Chelate Complexes... 

Several reviews have recently appeared which cover the topics of inter-1 6 and intramolecular1, s-n rearrangement reactions of metal complexes. In order to minimize duplication this review will be limited to intramolecular metal-centered rearrangementjeactions of six-coordinate tris-chelate complexes. Both kinetically slow (rates < 10 2 sec"1) and fast (rates % 10" 2 sec"1 )b complexes will be considered mainly from a mechanistic point of view. In addition various structural and electronic parameters of tris-chelate complexes will be scrutinized in order to assess their affect on the rate and mechanism of rearrangement. 

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