Ethylenediamine-metal complexes

A very promising short-cut to reliable absolute configurations of certain complexes has been indicated by COREY and BAICAR (8). The basis for this is the fact originally found by THEILACKER(9) that the five-membered rings formed in ethylenediamine-metal complexes are not planar but twisted somewhat like cyclopentane. As pointed out (8) this means that in a tris(en) metal complex the C-C bond in each of the en molecules may occupy one of two positions, namely nearly parallel ("lei") to or obliquely slanted ("ob") with respect to the trigonal axis (there is, of course, strictly speaking a trigonal axis only if all of the three en molecules have the same conformation). 

In transition metal complexes, two main coordination modes have been described for urea ligands W-monohapto, or N,0-chelates. In the case of molecules containing more than one urea function, the molecules act as Ar,AT-chelates, so that one of the urea functions always behaves as monohapto-ligand. For example, complexes of Co(III) with N-(2-pyridylmethyl)urea and ethylenediamine have been characterised by X-ray crystallographic analysis (Scheme 7). The urea group is coordinated through only one of its N... 

Ni — N bond breaks and one NH2 group of an ethylenediamine ligand dissociates from the metal complex. 

Metals may also be linked through an oxygen or nitrogen atom to form a stable metal complex without a carbon-metal bond. These include metal complexes of ethylenediamine tetraacetate (EDTA), diethylenetriamine pentaacetate (DTPA), or ethylenediamine tetramethylphosphonate (EDTMP). Metalloid compounds include antimonyl gluconate and bismuth salicylate. 

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

Fig. 7.1 A simple representation of the two conformations of the five membered puckered ring in metal complexes of ethylenediamine and derivatives. Rings are viewed along the plane containing the two nitrogens and the metal. The C-C bond is nearer the observer than the two nitrogens. It is skewed down to the right for a 5 conformation and to the left for a X conformation.

Eichhom and his co-workers have thoroughly studied the kinetics of the formation and hydrolysis of polydentate Schiff bases in the presence of various cations (9, 10, 25). The reactions are complicated by a factor not found in the absence of metal ions, i.e, the formation of metal chelate complexes stabilizes the Schiff bases thermodynamically but this factor is determined by, and varies with, the central metal ion involved. In the case of bis(2-thiophenyl)-ethylenediamine, both copper (II) and nickel(II) catalyze the hydrolytic decomposition via complex formation. The nickel (I I) is the more effective catalyst from the viewpoint of the actual rate constants. However, it requires an activation energy cf 12.5 kcal., while the corresponding reaction in the copper(II) case requires only 11.3 kcal. The values for the entropies of activation were found to be —30.0 e.u. for the nickel(II) system and — 34.7 e.u. for the copper(II) system. Studies of the rate of formation of the Schiff bases and their metal complexes (25) showed that prior coordination of one of the reactants slowed down the rate of formation of the Schiff base when the other reactant was added. Although copper (more than nickel) favored the production of the Schiff bases from the viewpoint of the thermodynamics of the overall reaction, the formation reactions were slower with copper than with nickel. The rate of hydrolysis of Schiff bases with or/Zw-aminophenols is so fast that the corresponding metal complexes cannot be isolated from solutions containing water (4). 

We prepared a series of pendant-type polymer-metal complexes having a uniform structure by the substitution reaction between a polymer ligand and a Co(III) or Cr(III) chelate, the chelate being inert in ligand-substitution reactions1,2 A poly-mer-Co(III) complex, e.g. ci s-[Co(en)2(PVP)Cl]Cl2 (en=ethylenediamine, PVP= poly(4-vinylpyridine)) i 7, was prepared as follows11 ... 

The peculiar metal ion specificity of the ATP cleavage reaction may perhaps be explained by reference to some studies on the metal complexes of Schiff bases, which have provided clues to many aspects of biological metal catalysis. It was shown that metal ions will split the carbon-nitrogen double bond in thiophenalde-hyde-ethylenediamine (18, 21) as a consequence of the electronic-drift-to-metal... 

A Ni(salen) complex [salen = bis(salicylidene)ethylenediamine] encapsulated in zeolite is highly efficient in the hydrogenation of simple alkenes (cyclohexene, cyclooctene, 1-hexene).451 This method, which is the encapsulation of metal complexes into the cavities of zeolites, offers the unique possibility of shape-selective... 

The chelate effect is the ability of multidentate ligands to form more stable metal complexes than those formed by similar monodentate ligands.4 For example, the reaction of Cd(H,0)g+ with two molecules of ethylenediamine is more favorable than its reaction with four molecules of methylamine ... 

Transition metal complexes encapsulated in the channel of zeolites have received a lot of attention, due to their high catalytic activity, selectivity and stability in field of oxidation reactions. Generally, transition metal complex have only been immobilized in the classical large porous zeolites, such as X, Y[l-4], But the restricted sizes of the pores and cavities of the zeolites not only limit the maximum size of the complex which can be accommodated, but also impose resistance on the diffusion of substrates and products. Mesoporous molecular sieves, due to their high surface area and ordered pore structure, offer the potentiality as a good host for immobilizing transition complexes[5-7]. The previous reports are mainly about molecular sieves encapsulated mononuclear metal complex, whereas the reports about immobilization of heteronuclear metal complex in the host material are few. Here, we try to prepare MCM-41 loaded with binuclear Co(II)-La(III) complex with bis-salicylaldehyde ethylenediamine schiff base. 

The pH of the medium always has a strong effect on metal binding. Competition with protons means that metal complexes tend to be of weak stability at low pH. Anions of carboxylic acids are completely protonated below a pH of 4 and a metal can combine only by displacing a proton. However, at pH 7 or higher, there is no competition from protons. On the other hand, in the case of ethylenediamine, whose pKa values are 10.2 and 7.5 (Table 6-9), protons are very strong competitors at pH 7, even with a strongly com- c... 

A series of complexes of the type ML(SCN) (C104)2-, (M - Zn, Cd, or Hg x = 1 or 2) has been prepared where L is en and its tetramethyl derivative, diethylenetriamine and its pentamethyl derivative, triethylenetetramine and its hexamethyl derivative, and bis(ethylenediamine). The complexes are either monomeric with four-, five- or six-coordinate metal, or polymeric containing bridging thiocyanate the perchlorate is always ionic. The thiocyanate is generally bonded through nitrogen to zinc and cadmium (and through sulfur to mercury).181... 

Other methods of synthesis are widely reviewed by Dayagi and Degani.2 However, it must be observed that few Schiff bases commonly used as ligands have been prepared and characterized in the uncomplexed state, since the corresponding metal complexes have been directly obtained by other procedures. For example, many metal complexes containing salenH2 may be obtained directly by reaction between metal ions, salicylaldehydes and ethylenediamine.3... 

In contrast to using metal complex developers in solution, metal salt developer precursors such as CuNCS, C0CO3, CoC204 or MnC204 can be incorporated directly into silver halide emulsion layers. After exposure, the emulsion is developed by immersion in an ethylenediamine or other amine solution, generating a complex ion developer in situ. 2... 

Ethylenediamine tetraacetic acid (EDTA) was introduced originally as a water-softener and as a textile dyeing assistant because of its ability to form very stable, water soluble complexes with many metal ions, including calcium and magnesium. The equilibria involved in chelation of metal ions by EDTA and related ligands have been exhaustively studied, notably by G. Schwarzenbach and his colleagues, and provide the basis for complexometric methods of chemical analysis. EDTA and its metal complexes have also become probably the most familiar examples of agents used in chelation therapy. 

Several types of Werner complexes have been investigated over the last few years by TDDFT methods. They include metal oxide, metal halide, metal oxyhalide compounds, and transition metal complexes with bidentate ligands such as ethylenediamine and acetylacetonato. 

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