Colin D. Hubbard

A review of proton transfer reactions between various carbon-acids and amine bases in aprotic solvents

Abstract The subject of proton transfer between carbon acids and nitrogen bases in aprotic solvents is reviewed. Equilibrium and rate constants that characterize such reactions are most often determined utilizing UV–visible spectrophotometry. At ambient temperature reaction rates are sufficiently rapid that fast reaction methods, for example, the stopped-flow and temperature-jump techniques are required in many cases. Variation of the properties of the donor and acceptor reaction pairs enables electronic and steric effects upon thermodynamic and kinetic parameters of proton transfer to be assessed. Determination of the kinetic isotope effect (KIE), i.e. k (protium)/ k (deuterium) led to the conclusion that, under certain circumstances and when the KIE is greater than seven, the proton undergoes reaction with a significant degree of quantum mechanical tunneling, consistent with a theoretical prediction advanced several decades earlier. In fact this aspect may be one of the most significant outgrowths of these studies. Many reactions have been characterized (by tunneling) but rarely are the reacting systems experimentally amenable to obtaining all the experimental criteria that support tunneling. Controversy that has arisen regarding treatment of experimental data and resulting conclusions from them is visited in this review. The structural nature of the product state of reaction is formulated based on spectroscopic evidence, in favorable cases, and probable structures of the transition state can be inferred.

Solubilities of salts and kinetics of reaction between hydroxide ions and iron(II)–di-imine complexes in water–methanol mixtures. Derivation of single-ion transfer chemical potentials and their application to analysis of solvent effects on kinetic parameters

Kinetic data are reported for the reaction at 298 K and ambient pressure between two iron(II)–di-imine complex cations and hydroxide ions in water–methanol mixtures. Solubility data are reported for a range of inorganic salts containing simple and complex ions. Methods for calculating transfer chemical potentials of single ions are examined and, depending on the extrathermodynamic assumption, shown to predict different trends in the properties of ions in these aqueous mixtures. Further, calculated initial- and transition-state solvation effects on the kinetics are different: in some cases dramatically so. The solvation characteristics are compared for various ions in methanol–water mixtures as calculated using the tetraphenylphosphonium tetraphenylborate (TPTB) assumption, which sets the transfer chemical potential of tetraphenylphosphonium ions equal to that of tetraphenyl-boronate ions. Arguments are advanced for adopting single-ion transfer chemical potentials based on this assumption. Relationships are examined between the transfer parameters for H+, H3O+, ROH+2 and H9O+4 ions in binary aqueous mixtures, ROH + H2O.

LIGAND SUBSTITUTION REACTIONS

Publisher Summary This chapter discusses recent progress in the area of kinetics and mechanisms of ligand substitution. The chapter describes classical complexes and a selection of bioinorganic reactions reflecting the growing importance of this bridging area, a few references of relevance to inorganic pharmacology, and organometallic systems. A rapidly developing area where mechanistic information is currently minimal but could usefully be greatly increased is that of self-assembly and supramolecular chemistry. The chapter focuses on catenanes, rotaxanes, helicates, and knots, to provide a qualitative view of the development of mechanistic investigation and understanding in this field. Substitution at copper(I) is an important feature of supramolecular chemistry, especially in relation to catenane formation. Mechanistic developments should be imminent in relation to the ligand-replacement reactions that are an integral part of the entry of copper into cells and of its subsequent central role in metalloprotein mediation of such processes as iron uptake and energy generation through electron transfer.

Mechanistic studies of reactions of coordination compounds. Some recent highlights

Detailed kinetics studies of reactions of coordination compounds in solution have been invaluable in determining the relevant reaction mechanisms. A selection of reports that highlights very recent progress in mechanistic investigations of coordination chemistry is presented. The methods and techniques employed in pursuit of these mechanistic goals are referred to mostly through the latest authoritative reports. Reactions chosen for inclusion are classified according to the comprehensive table of contents. In summary, solvent exchange, ligand substitution, redox reactions, some reactions of nitric oxide and a few other reactions, whose mechanisms have been scrutinised, are covered.

VOLUMES OF ACTIVATION FOR DISSOCIATION OF THE CATIONS OF [TRIS-2,2'-BIPYRIDYL]IRON(II), [TRIS-1,10-PHENANTHROLINE]IRON(II) AND OF OTHER DIIMINE IRON(II) COMPLEXES IN AQUEOUS-SOLUTION

Abstract The kinetics of dissociation of several iron(II) diimine complexes by aqueous hydroxide ions have been studied spectrophotometrically at atmospheric pressure and elevated pressures, at 298.2 K. Values of ΔV* are between + 10 and + 16 cm3 mol−1, and are interpreted as arising from a significant loss of electrostricted water from the hydroxide ion as it reacts with the iron(II) complex associatively. Dissociation of the [Fe(5NO2phen)3]2+ and [Fe(5Brphen)3]2+ complexes in the presence of aqueous EDTA is reported. For both, ΔV* is + 22 cm3 mol−1. These values can be understood by considering the reaction as a process comparable to aquation in acidic medium, a dissociative process. Activation enthalpy data support this idea.

Solvation and reactivity of the low-spin trisdiimine iron(II) complex of the schiff base ligand derived from 2-benzoylpyridine and 3,4-dimethylaniline, [Fe(Me2bsb)3]2+

Abstract The reactivity and solvation of the iron(II) tris complex made from the bidentate ligand obtained by condensing phenyl 2-pyridyl ketone and 3,4-dimethylaniline have been examined in aqueous solution and in aqueous solvent mixtures. Base hydrolysis kinetics measurements upon the complex ion, [Fe(Me 2 bsb) 3 ] 2+ , show an increasing rate of dissociation with increasing co-solvent concentration, due largely to the destabilization of the hydroxide ion. The predicted preference the lipophilic complex would exhibit toward solvents less polar than water is an expectation that is realized when transfer (from water to aqueous solvent mixtures) chemical potential trends are analysed. The latter parameters were obtained from solubility measurements. When corresponding transfer chemical potentials for the transition state are also considered, the factors controlling reactivity patterns are shown; this is described by an initial state—transition state analysis. Base hydrolysis kinetics carried out at high pressures (up to 1 kbar) yield the volumes of activation, Δ V *, which provide a complementary view of solvation. Some measurements with a similar complex, [Fe(Me 2 tsb) 2 ] 2+ , are reported for comparison.

Transfer chemical potentials for ions, solubilities of salts and kinetics of reactions involving inorganic complex ions at ambient pressure and 298.2 K in binary aqueous mixtures containing ethanol and propan-2-ol

Solubilities of several complex salts at ambient temperature and pressure are reported for aqueous solutions and for solutions in binary aqueous organic mixtures. The organic cosolvents are ethanol and propan-2-ol. These and earlier published solubilities are analysed using the TATB assumption which sets the transfer chemical potentials of Ph4As+ ions equal to those of Ph4B– ions. The calculations yield transfer parameters for various ions including complex metal ions. Derived transfer parameters for ions are compared for a range of solvent systems comprising binary aqueous mixtures, including those systems where the organic cosolvents are methanol, ethanol, propan-2-ol and acetone. These transfer parameters are used in an analysis of kinetic data describing chemical reaction between hydroxide ions and iron(II) complex cations, [Fe(phen)3]2+ in aqueous solutions. Definitions of standard state chemical potentials for solutes and solvents are considered with reference to descriptions of composition of these systems using the mole fraction scale.

Use of high pressure techniques for measuring reaction rates in liquid solutions

Abstract This review describes the instrumental methods employed for measuring reaction rates in liquid solutions at elevated pressures. The current status as well as the developments leading to it are presented. The value of such measurements in elucidating reaction mechanisms is a principal focus. A wide range of examples from inorganic chemistry, organic chemistry, bioinorganic chemistry, biochemistry and organometallic chemistry which illustrates the instruments employed, general methods, and mechanistic insight, is provided.

Transfer chemical potentials for iron(II)-diimine complexes from water into aqueous methanol

SummarySolubilities of perchlorate and thiocyanate salts of several iron(II)-diimine complexes of Schiff base ligands in methanolwater mixtures are reported. From these results and published values for transfer chemical potentials of the perchlorate and thiocyanate anions, transfer chemical potentials for the complex cations are calculated. Trends with solvent composition suggest that preferential solvation varies from negligible to very strongly by methanol, depending on the size and hydrophilic-hydrophobic character of the complex.

The interpretation and mechanistic significance of activation volumes for organometallic reactions

Publisher Summary This chapter describes the value and significance of volumes of activation in a wide range of organometallic chemistry reactions. The methods, techniques, apparatus, and instruments relating to determination of volumes of activation for organic, inorganic, and bioinorganic reactions, in most cases, can be applied in organometallic chemistry. For many inorganic and bioinorganic reactions the volume of activation has been obtained from measurements at 25°C. The application of the hydrostatic pressure variable in a multitude of reaction kinetics studies leading to the volume of activation has been clearly demonstrated to be of inestimable value in mechanistic studies of organometallic chemistry reactions. Its relative simplicity of definition and precision of typical experimental values render the volume of activation to be of far superior mechanistic value than the entropy of activation. In many cases the volume of activation adds the extra dimension to mechanistic elucidation. The breadth of techniques and variety of reaction-monitoring methods described in this chapter has added broader scope, interest, and depth to this account.