Complexes of magnesium

To our knowledge there is no example in the literature of a stable alkene (or alkyne) complex with magnesium as central atom, n Complexes have, however, been postulated as intermediates in the rearrangement reactions of some alkenyl Grignard reagents (86). 

Mono-n-cyclopentadienylmagnesium compounds have so far only been isolated in the form of adducts with oxygen or nitrogen bases. The bis-(tetrahydrofuran) complexes of cyclopentadienyl(phenyl)- and -(methyl)-magnesium have been synthesized by Whitesides and co-workers (96) according to Eq. (4). 

These compounds are monomeric in solution and are thought to possess the n structures as indicated in XXIVa (Fig. 6). Reaction of cyclopentadiene with dimethylmagnesium in diethyl ether leads to an ether adduct of cyclo-pentadienyl(methyl)magnesium (97) [Eq. (5)]. 

From IR data it appears that in the solid state the magnesium atoms are associated via methyl bridge bonds and that the cyclopentadienyl rings are approximately pentahapto bonded as indicated in structure XXIVb. When dissolved in benzene, the methyl bridge bonds are retained. 

MeMgBr + C5H3(SiMe3)3 + TMEDA ------- [C5H2(SiMe3)3]MgBrTMEDA + MeH 

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Mg(THF), when the stoichiometry was 1 2. Monomeric and dimeric amidinate complexes of magnesium have been studied in detail with respect to potential applications of these compounds in the chemical vapor deposition of magnesium-doped Group 13 compound semiconductor films. The reactions and products are summarized in Scheme 16. ... 

Recently, very effective asymmetric conjugate addition of 1,3-dicarbonyl compounds to nitroalkenes has been reported, as shown in Scheme 4.10. The reaction of ethyl acetoacetate with nitrostyrene is carried out in the presence of 5 mol% of the preformed complex of magnesium triflate and chiral bis(oxazoline) ligands and a small amount ofW-methylmorpholine (NMM) to give the adduct with selectivity of 91%. The selectivity depends on ligands. The effect of ligands is presented in Scheme 4.10.63... 

Complexes are also formed by main-group metal ions, and many of them are of great importance. Chlorophyll, for instance, which gathers energy from the Sun and without which life on Earth would be very primitive, is a complex of magnesium. However, the /-metals form the richest variety of complexes, and we concentrate on them. 

Coordinated a-amino amides can be formed by the nucleophilic addition of amines to coordinated a-amino esters (see Chapter 7.4). This reaction forms the basis of attempts to use suitable metal coordination to promote peptide synthesis. Again, studies have been carried out using coordination of several metals and an interesting early example is amide formation on an amino acid imine complex of magnesium (equation 75).355 However, cobalt(III) complexes, because of their high kinetic stability, have received most serious investigation. These studies have been closely associated with those previously described for the hydrolysis of esters, amides and peptides. Whereas hydrolysis is observed when reactions are carried out in water, reactions in dimethyl-formamide or dimethyl sulfoxide result in peptide bond formation. These comparative results are illustrated in Scheme 91.356-358 The key intermediate (126) has also been reacted with dipeptide... 

Ate complexes of magnesium are less commonly encountered than those of, for example, aluminum, and they have not yet found many applications in synthesis.+ However, there are indications that they may be useful reagents when electron transfer reaction pathways are desired. 

Organomagnesium compounds sometimes behave as single-electron donors. Such behaviour is most likely to be observed in highly polar, cation-solvating media and/or when steric hindrance inhibits alternative pathways. Ate complexes of magnesium can also act as single electron donors (see p. 97). It should be noted that the presence of traces of transition metals may lead to electron transfer processes [2]. 

Fluorimetric methods [34] Fluorimetric sequential injection analysis of magnesium in commercial drinking waters is based on the complexation of magnesium (II) with 8-hydroxyquinoline-5-sulfonic acid in the presence of ethylene glycol-bis(6-aminoethyl ether)-N,N,N, N -tetra-acetic acid as a masking agent and cetyltrimethylammonium chloride as the fluorescence enhancer. 

The initial complexation of magnesium species with ketone could be demonstrated by measuring the rate constants of reactions of methylmagnesium bromide with mixtures of pinacolone and magnesium bromide the reaetion followed second-order law ... 

Magnesium aluminum silicate is a polymeric complex of magnesium, aluminum, silicon, oxygen, and water. The average chemical analysis is conventionally expressed as oxides ... 

Curve A in Figure 17-6 is a plot of data for the titration in Example 17-4. Curve B is the titration curve for a solution of magnesium ion under identical conditions. The formation constant for the EDTA complex of magnesium is smaller than that of the calcium complex, which results in a smaller change in the p-function in the equivalence-point region. 

Other kinetic studies of complexation using relaxation techniques can be found in Bridger et al. (1983) [hydroxo and chloro complexes of Fe(III)], Bridger et al. (1982) [complexation of Fe(III) by picolinic and dipicolinic acids], Strahm et al. (1979) (complexation of aqueous iron chloride), and Patel and Taylor (1973) (complexation of magnesium pyrophosphates). 

Coordination compounds play important roles in nature. Chlorophyll, which is involved in photosynthesis in plants, is a coordination complex of magnesium. Hemoglobin, the oxygen transporter in the human body, is a coordination complex of iron. Vitamin B12, necessary for the prevention and cure of pernicious anemia, is a coordination complex of cobalt. In all three compounds, the metal ion is in an approximately octahedral environment, its coordination number is 6, and bonded to it are the four nitrogen atoms of a planar porphyrin-hke ring. The basic planar ring structure is closely related to that of the extremely stable blue pigment, Cu(II)phthalo-cyanine. 

Cayley and Hague [70a] have employed the temperature-jump relaxation method to measure the rate of formation and dissociation of the 1 1 complex of magnesium(II) and 5-nitrosalicyclic acid (NSA) and also of the ternary complexes formed between this ligand and Mg(II) nitrilotriacetate (NTA), adenosine triphosphate (ATP) and polytriphosphate (TP) complexes, the last in an effort to gather information about the effect of bound ligands on the rate of substitution of a metal ion by a second type of group. It is expected that the presence of a bound ligand should... 

With the correspondingly more reactive methylphosphonic dichloride a mixture was obtained which contained traces of phosphinic acids. More important was the observation that methylphosphonic difluoride condensed smoothly with alkyl Grignard reagents to provide unsynametrical phosphinyl halides which precipitated as insoluble complexes of magnesium salts. Decomposition of the complexes with base gave the corresponding phosphinic acids, the yields of which increased as R increased... 

In the study of the effect of beryllium on fibroblast and myoblast cultures, anomalous mitoses were observed and the cell division was stopped in the metaphase, which was greatly extended. Practically no telophase and anaphase were observed. Beryllium was demonstrated histochemically in the liver and particularly in chromosomes of dividing cells. In the case of fibroblasts, there was a remarkable inhibition of the synthesis of nude-oproteins. Damage to the mechanism of the RNA synthesis is assumed to occur by action of beryllium. Beryllium is bound to DNA in the same way as magnesium, however, its affinity is many times higher. The complex of magnesium with DNA can be easily hydrolysed, whereas the complex with beryllium is stable. It was thus demonstrated that beryllium modifies physico-chemical characteristics of DNA. 

J. H. Dahl, D. Espersen, and A. Jensen, Diffential Kinetic Analysis and Flow Injection Analysis. Part 1. The trans-l,2-Diaminocyclohexane-Tet-raacetate Complexes of Magnesium and Strontium. AnaL Chim. Acta, 105 (1979) 327. 

The first example of chiral tetranuclear supramolecular assembly 5.27 was reported by Saalfrank and coworkers in 1988. Thus the complex [(NH3)4n(Mg (22)5] (5.27) was obtained by serendipity upon treatment of the doubly bidentate ligand 22, which is formed in situ from malonic ester and oxalyl chloride, with MeMgl in aqueous ammonium chloride solution (Scheme 5.13). Later an improved one-pot reaction was used to prepare such tetrahedral complexes of magnesium, manganese, cobalt and nickel by treating the bis(chelate) ligand 22, obtained in situ, with MeLi/MCl2 followed... 

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