New Ring Systems of Elements of Main Groups Iv and V

Holmes, Robert R.; Day, Roberta O.; Chandrasekhar, Vadapalli; Shafeizad, S.; Harland, John J.; Rau, Denise N.; Holmes, Joan M.

doi:10.1080/03086648608072800

Cited by 8 publications

(1 citation statement)

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“…For example, the mimmolecular hydrophobic association of two alkyl chains (of oligoethylene glycol diethers and diesters) in aqueous solution has been described. 4 This observation corresponds to the intramolecular hydrophobic interaction of the two isopropyl groups in the Cu(L-leucinate)2 complex.22 Recently, also the dimerization of the 1:2 Cu2+-decanoate complex (CuA2) according to 2CuA2 ^Cu2A4 was quantified: it was observed50 that the dimerization constant increased with the dioxane content of the aqueous solution, i.e. from 0.3 to 0.4 and 0.5 mole fraction of dioxane.…”

Section: Discussionmentioning

confidence: 99%

Ternary complexes in solution. 50. Dependence of intramolecular hydrophobic ligand-ligand interactions on ligand structure, geometry of the coordination sphere of the metal ion, and solvent composition. Opposing solvent effects

Liang¹,

Tribolet²,

Sigel³

1988

Inorg. Chem.

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Stability constants of mixed-ligand M(phen)(z'-A1CA)+ complexes [M = Cu2+, Zn2+; phen = 1,10-phenanthroline; z'-AICA' = 2-methylpropionate (2-MPr"), 3-methylbutyrate, 4-methylvalerate, 5-methylhexanoate, 6-methylheptanoate (6-MHp")] have been determined by potentiometric pH titration in aqueous solution and in 50% (v/v) ethanol-water or dioxane-water and compared with the stabilities of the corresponding formate or acetate complexes. The ternary complexes containing isoalkanecarboxylates (z'-AICA") are significantly more stable due to intramolecular hydrophobic interactions between the alkyl residue of the z'-AICA" ligands and the phen molecule. These intramolecular hydrophobic interactions have been confirmed by *H NMR shift measurements in water and 50% (v/v) ethanol-water for the whole series of the Zn(phen)(z'-A1CA)+ complexes. The formation degree of the intramolecular hydrophobic adducts in the Cu2+ and Zn2+ complexes (closed species) was calculated, and the position of the intramolecular equilibrium between the opened and closed forms was determined: the closed forms occur between a trace percent and about 50%, depending on the geometry of the coordination sphere of the bridging metal ion (Cu2+, tetragonal; Zn2+, tetrahedral or octahedral), on the number of methylene units between the isopropyl residue and the coordinating carboxylate group (i.e., on the size of the alkyl moiety), and on the solvent composition. The influence of the solvent composition has been further studied by measuring the complex stabilities for M(phen)(2-MPr)+ and M(phen)(6-Mhp)+ (M = Cu2+, Zn2+) in water and in 30-90% (v/v) ethanol-water. There are indications, and these are discussed, that the structure of the closed species is solventdependent: in water, a "simple" (though not rigid) intramolecular hydrophobic ligand-ligand adduct is formed, while in the mixed solvents, in addition, probably a series of structurally somewhat different closed species (orientation of the ligand moieties, degrees of solvation, intercalated organic solvent molecules, etc.) may occur. As there is at present no way to identify with certainty such different structures, the whole observed stability increase between M(phen)(z'-A1CA)+ and M(phen)(Ac)+ or M(phen)(HCOO)+ was simply attributed to a (single) so-called "closed" species. The formation degree of the closed species is influenced by the solvent: addition of some ethanol (or dioxane) to. an aqueous solution favors their formation, contrary to the experience with simple unbridged hydrophobic adducts, which are destabilized. Such a destabilization of the closed ternary species occurs only at high concentrations of the organic solvent (usually more than 70%). It should be noted that the overall stability of the interaction between M2+ and z'-AICA" is governed by the polarity of the solvent, while the position of the intramolecular equilibrium is influenced by the hydrophobic solution properties of the solvent molecules. The relevance of the present results with regard to biological systems is indicated.

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Section: Discussionmentioning

confidence: 99%