Metal pyridine compounds

Comparative studies [1127] of the kinetics of decomposition of similar salts containing related pyridine ligands have been used to investigate the strength of M—N bonds in coordination compounds. Non-isothermal DSC measurements were used to determine values of E for the reactions 

Activation energies were in the sequence py 4-mepy 3-mepy which is the order of M—N bond strengths. 

Isothermal a—time curves for the decomposition at 363—464 K of the pseudo octahedral compounds NiL2(NCS)2 (L = py, 3-picoline or quinoline) to NiL(NCS)2 and volatilized L, obeyed the contracting volume equation [eqn. (7), n = 3]. E values decreased in the sequence L = py 3-picoline quinoline and this order was ascribed to the effect of increasing ligand volumes [1128]. 

Coordination compounds containing bidentate ligands are often thermally more stable than those comprised of related monodentate ligands, e.g. ethylenediamine (en) complexes dissociate at a higher temperature than those of ammonia or pyridine. Compounds containing a ring structure, such as coordinated salicylaldehyde (sal) and acetyl-acetonate (acac), are particularly stable, and may often be sublimed 

The corresponding chromium compounds [Cr(en)3]X3 evolve ethylenediamine [1131] and the values of E determined using non-isothermal measurements were 105 and 182 kJ mole 1 for X = Cl and SCN , respectively. Hughes [1132] reported a value of E = 175 kJ mole 1 for X = Cl and showed that the decomposition rate is sensitive to sample disposition. Amine evolution from both the (en) and propenediamine (pn) compounds was catalyzed by NH4C1 [1132,1133] or NH CN [1133,1285], addition of small amounts of these substances resulting in a substantial reduction of E. The influence of NH4C1 is ascribed [1132] to the dissociation products, since HC1 promoted the reaction but NH r and NH4I showed no such effect. 

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That this reaction does not take place in the metal-ammines was proved in the following way.2 A tertiary amine was chosen, namely, pyridine, in which no labile hydrogen is possible, and therefore no forked chain could be formed if the compound were united with metal salt. It was found that when pyridine and the metallic salt are allowed to interact, metal-pyridine compounds are obtained analogous in every way to the metal-ammines. It seemed probable, therefore, that the metal-ammines do not contain the (NH4) group. 

PPha, pyridine) organic groups (olefines, aromatic derivatives) and also form other derivatives, e.g. halides, hydrides, sulphides, metal cluster compounds Compounds containing clusters of metal atoms linked together by covalent (or co-ordinate) bands, metaldehyde, (C2H40) ( = 4 or 6). A solid crystalline substance, sublimes without melting at I12 1I5" C stable when pure it is readily formed when elhanal is left in the presence of a catalyst at low temperatures, but has unpredictable stability and will revert to the monomer, ft is used for slug control and as a fuel. 

Another compound, the antimicrobial action of which is associated with chelation, is 2-pyridinethiol-A/-oxide [3811-73-2] (Omadine). Activity has been shown to depend on coordinating property. The iron chelate is active, but not the free pyridine compound (200). In the form of its zinc chelate it is found in shampoos to control seborrheic dermatitis (201). Other appHcations of this useful chemical include preservation of adhesives, plastics, latex paints, polyurethane foam, and metal working fluids (202). 

Attempts to synthesize transition metal alkyl compounds have been continuous since 1952 when Herman and Nelson (1) reported the preparation of the compound C H6>Ti(OPri)3 in which the phenyl group was sigma bonded to the metal. This led to the synthesis by Piper and Wilkinson (2) of (jr-Cpd)2 Ti (CH3)2 in 1956 and a large number of compounds of titanium with a wide variety of ligands such as ir-Cpd, CO, pyridine, halogen, etc., all of which were inactive for polymerization. An important development was the synthesis of methyl titanium halides by Beerman and Bestian (3) and Ti(CH3)4 by Berthold and Groh (4). These compounds show weak activity for ethylene polymerization but are unstable at temperatures above — 70°C. At these temperatures polymerizations are difficult and irreproduceable and consequently the polymerization behavior of these compounds has been studied very little. In 1963 Wilke (5) described a new class of transition metal alkyl compounds—x-allyl complexes,... 

It is more difficult to study equilibria between transition metal allyl compounds and bases, olefins, etc. In the case of Zr (allyl) 4 and pyridine, a valency change occurs as shown by Eq. (8), and the process is irreversible. The polymerization is considered to be preceded by displacement of one allyl group by the monomer (12) as shown in Eq. (1). In the methyl methacrylate/Cr(allyl)3 system it was not possible to detect any interaction between the olefin and catalyst with infrared radiation, even with equimolar concentrations because of the strong absorption by the allyl groups not involved in the displacement processes. Due to the latter, evidence for equilibrium between monomer and catalyst is less likely to be found with these compounds than with the transition metal benzyl compounds. 

Finally, compound (iv) is condensed with either trimethyl(6-methyl-3-pyridyl)tin or the boronate ester by means of Pd(PPh3)4 to afford etoricoxib. The metallated pyridine (vii) is obtained by esterification of 3-hydroxy-2-methylpyridine with triflic anhydride to give the corresponding triflate, which is treated with a tin reagent to yield the target tin intermediate. The boron lithium salt (viii) is prepared by treatment of 5-bromo-2-methylpyridine with butyllithium followed by addition of triisopropyl borate. 

Although the large majority of pyridine compounds use the free electron pair to bind to metal ions in a monodentate fashion, bridging coordination using the nitrogen atom is also possible, although stabilization in a so-called crevice seems to be required, as shown in the compound [MoO(Cg H9 PS2)2OS]2py.5 ... 

Similarly to the analogous reaction with furo[2,3-Z>]pyridines, metallation of compound (20) with n-butyllithium in TMEDA followed by the addition of DMF afforded the 2-formyl derivative in 66% yield. Likewise, metallation of the isomer (22) under the same conditions afforded the 2-formyl derivative in comparable yield <84JHC785>. Introduction of a carbonyl group at C-3 of compound (20) was accomplished by halogen-metal exchange between 3-bromothieno[2,3-6]pyridine and n-butyllithium. Table 45 lists some of the thieno[2,3-Z>]pyridine derivatives formed by this method <74JHC355>. 

Also included in this section are procedures for the synthesis of the chromium analog and a pyridine substituted vanadium(II) saccharinate. All of these compounds are potentially useful in the synthesis of other metal(II) compounds. They are prepared in good yields and with high purity. 

The pK values of 2-, 3-, and 4-methylpyridine have been determined to be ca. 34, 37, and 32, respectively (Sect. 2, Ref. [1]). Di- and trimethylpyridines are expected to be somewhat less acidic. Since the pK value of ammonia lies within this range, interaction between the heterocyclic compounds and the alkali amides will give rise to certain equilibrium concentrations of the metallated pyridine derivatives. The extent of ionization is expected to decrease in the following order 4-CH3 > 2-CH3 > 3-CH3. As in the metallations of isoprene and a-methylstyrene... 

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