Robert I. Haines

Solvatochromic behaviour of intramolecular charge-transfer spectra of inorganic diimine complexes

SummaryThe dependence of charge-transfer maxima (vmax) on solvent has been investigated for a variety of diimine (LL) complexes M(CO)4(LL) and Fe(LL)2(CN)2. These and previously published results for compounds of this general type are collated and summarised in the form of solvent sensitivities, which are the slopes of plots vmaxversus solvent ET values. The dependence of such solvent sensitivities on the nature of the metal, of the diimine ligand, and of other ligands present, is discussed.

Structure, solvatochromism, and solvation of trans-[CoIII(cyclam)(NCS)2](NCS) and the structure of [CoII(Me4cyclam) (NCS)]2[Co(NCS)4]MeOH

The structures of trans-[CoIII(cyclam)(NCS)2](NCS) and of [CoII(Me4cyclam)(NCS)]2[Co(NCS)4]·MeOH have been established by X-ray diffraction methods. The solvatochromic behavior of the trans-[Co(cyclam)(NCS)2]+ cation in several binary aqueous solvent mixtures is reported. Transfer chemical potentials for this complex from H2O into MeOH-H2O mixtures have been established from solubility measurements on its thiocyanate salt. The solvatochromic behavior of this cation is discussed in the context of other solvatochromic inorganic complexes; its transfer chemical potentials are discussed in relation to those of other cobalt(III) complexes.

Kinetics and mechanisms of formation, and of reactions, of intermediates in the iron(II)–1,10-phenanthroline–cyanide and related systems

Rate laws and rate constants are reported for the two stages of the reactions of [Fe(L–L)2(CN)2](L–L = 1,10-phenanthroline, 5-chloro-1,10-phenanthroline, or 2,2′-bipyridyl) with cyanide ion in aqueous solution. The preparation of [Fe(5NO2-phen·CN)2(5NO2-phen)], an intermediate in the reaction of [Fe(5NO2-phen)3]2+ with cyanide to give [Fe(5NO2-phen)2(CN)2], is described, and the kinetics and mechanisms of some of its reactions, especially with hydroxide and with cyanide, are reported and discussed.

Kinetics of reactions of tris(2,2′-bipyridyl)iron(II) with cyanide and with hydroxide ion in binary aqueous solvent mixtures

Rates of reaction of [Fe(bipy)3]2+ with cyanide ion in binary aqueous solvent mixtures containing ethylene glycol glycerol, acetonitrile, dimethyl sulphoxide (dmso), or tris (dimethylamino)phosph ine oxide (tdpo), and with hydroxide ion in binary aqueous solvent mixtures containing t-butyl alcohol, dmso, or tdpo, are reported. Reactivity trends in the various solvent mixtures are discussed and, in the case of the reaction with hydroxide in aqueous dmso, compared with reactivity trends for similar inorganic and organic reactions. Where the existence of the required thermodynamic data on Gibbs free energies of transfer of single ions permits, it is shown that the effect of solvation, and thence chemical-potential, changes for cyanide and hydroxide ions is dominant in determining reactivies in their reactions with [Fe(bipy)3]2+.

Solvent effects on discrimination in the dissociative substitution of pentacyano(ligand)ferrate(II) comptexes in alcohol–water mixtures

Solvent effects on the discriminatory properties of the transient [Fe(CN)5]3– intermediate in the dissociative (D) substitution at the complex ions [Fe(CN)5L]3–(L = 3- or 4-cyanopyridine) are described. The solvents employed are methanol—, ethanol—, and t-butyl alcohol–water mixtures; in each case the incoming group is cyanide ion.

Kinetic and equilibrium properties of pentacyano(3,5-dimethylpyridine)-iron(II) and related anions in mixed aqueous solvents

The kinetic pattern for reaction of [Fe(CN)5(3,5-Me2py)]3– and [Fe(CN)5,(3-CNpy)]3– with a range of incomlng groups, in water and in 40% glycol and 40% t-butyl alcohol, is consistent with the operation of a D mechanism under all the conditions studied. Rate constants for the reaction of [Fe(CN)5,(3.5-Me2py)]3– with cyanide ion have been determined in binary aqueous mixtures containing up to 40% by volume of methanol, ethanol, t-butyl alcohol, glycerol, or tetrahydrofuran. Correlation of these rate constants with Grunwald–Winstein Y values gives a value of m of –0.1; this small value reflects the low sensitivity of the rate to solvent composition. Free energies of activation determined from the observed rate constants have been plotted against excess Gibbs free energies of mixing (GE) for the respective solvent mixtures. The resulting pattern indicates how far the thermodynamic characteristics of mixing of the solvent are reflected in the kinetics of this substitution reaction. The relation between the relative stabilities of [Fe(CN)5(3,5-Me2py)]3– and its pyridine analogue, and solvent mixture GE values, has also beenexamined. Thesolvatochromic behaviourofthecharge-transfer band of [Fe(CN)5,(N-methylpyrarinium)]2– has been investigated, and contrasted with that of organic analogues.

Initial state and transition state solvation in the peroxodisulphate oxidation of dicyanobis(2,2′-bipyridyl)iron(II)

Solvent effects on the kinetics of peroxodisulphate oxidation of dicyanobis(2,2′-bipyridyl)iron(II), [Fe(bipy)2(CN)2], in aqueous methanol have been analysed into initial state and transition state components; the latter dominate the observed reactivity trend.

Volumes of activation for dissociative (D) mechanisms: the substitution of pentacyanoferrate(II) complexes by various nucleophiles

The dissociative step of a dissociative, or D, substitution mechanism exhibits a volume of activation, ΔV1‡, which is independent of the nature of the incoming nucleophile. For the reaction of [Fe(CN)5(3,5Me2-py)]3–(3,5Me2-py = 3,5-dimethylpyridine) with [CN]–, ΔV1‡= 20.5 ± 0.8 cm3 mol–1; with pyrazine, ΔV1‡= 21.2 ± 1.0 cm3 mol–1; and with imidazole, ΔV1‡= 20.3 ± 1.0 cm3 mol–1. For the reaction of [Fe(CN)5(3CN-py)]3–(3CN-py = 3-cyano-pyridine) with [CN]–, ΔV1‡= 20.6 ± 0.5 cm3 mol–1. These volumes of activation are shown to correspond to stretching of the Fe–N bond in the activated complex from 65 to 88% of the distance corresponding to complete dissociation of the leaving group from the [Fe(CN)5]3– moiety.

Initial state and transition state contributions to solvent effects in the reaction of 2,2′-bipyridyl with nickel(II) in aqueous methanol

Kinetic and thermodynamic results show that the variation with solvent composition of the activation enthalpy for reaction of nickel(II) with 2,2′-bipyridyl in aqueous methanol represents the small difference between large destabilisations both of the nickel(II) and of the transition state (intermediate) as the proportion of methanol increases.

Kinetics of reactions of Schiff-base complexes of iron(II). Part 6. The preparation and kinetics of reactions of complexes of multidentate ligands

Preparations of iron(II) complexes of bi-, tri-, quadri-, quinque-, and sexi-dentate Schiff bases are reported. The Schiff-base ligands are derived from pyridine-2-carbaldehyde, phenyl 2-pyridyl ketone, pyridine-2,6-dicarb-aldehyde. or 2,6-diacetylpyridine, and appropriate amines. All these complexes aquate in acid solution, and react with cyanide to give ternary iron(II)–cyanide–Schiff-base complexes. The kinetics of aquation of several of these Schiff-base complexes, and the kinetics of cyanide attack at all the complexes, are described, and reactivities are compared with those for analogous reactions of other iron(II) di-imine complexes. Solvent effects on the re-activity with cyanide of one of the complexes of the sexidentate Schiff bases in a range of binary aqueous mixtures are discussed.