Octahedral cobalt(III) complexes in dipolar aprotic solvents.  XII.  Ion association studies in N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide by conductance measurements

Millen, W. A.; Watts, D. W.

doi:10.1021/ja01002a009

Cited by 25 publications

(6 citation statements)

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“…Final confirmation of these ideas was obtained when chlorohaemin solutions in DMA were shown to have only very small conductivities above that of the pure solvent. With the chloride molar conductance value in DMA of 44-4 Q-1 Cm.2 mole-' (Millen & Watts, 1967) our results indicate that chlorohaemin is less than 2% dissociated in DMA. The spectrum obtained on addition of aqueous alkali to chlorohaemin in DMA is shown in Fig.…”

Section: Resultsmentioning

confidence: 64%

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Solution structures of ferrihaem in some dipolar aprotic solvents and their binary aqueous mixtures

Brown

Lantzke

1969

Biochemical Journal

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1. Conductivity and u.v. and visible spectroscopic techniques were used to investigate the solution structure of the prosthetic group of the ferric haemoproteins (ferrihaem) in dimethyl sulphoxide, NN-dimethylacetamide, NN-dimethylformamide and sulpholane, and certain of their aqueous mixtures. 2. In neutral or acid dimethyl sulphoxide, chlorohaemin is monomeric and completely dissociated into Cl(-)ion and a ferrihaem species with dimethyl sulphoxide molecules in the fifth and sixth co-ordination positions on iron. 3. In neutral NN-dimethylacetamide and NN-dimethylformamide chlorohaemin is monomeric but is largely undissociated, giving different spectra from that of chlorohaemin in dimethyl sulphoxide. On acidification, dissociation occurs and the dimethyl sulphoxide type of spectrum results. 4. Studies in a fourth solvent, sulpholane, indicate that solvent co-ordinating power (ligand strength) rather than bulk dielectric constant is responsible for dissociation of chlorohaemin. 5. In neutral dimethyl sulphoxide-water mixtures chlorohaemin remains monomeric and completely dissociated, and spectra are independent of mixture composition, except at high water concentrations, when precipitation occurs. In alkaline dimethyl sulphoxide-water mixtures, where the complete solvent mixture range is accessible, ferrihaem is polymeric (probably dimeric) and spectra are dependent on solvent composition. A quantitative analysis indicates that the spectral changes are due to replacement by water of one molecule of co-ordinated dimethyl sulphoxide per ferrihaem aggregate, and do not involve a two-molecule replacement as has been suggested for the alkaline pyridine-water system.

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Section: Resultsmentioning

confidence: 64%

“…In addition, the proportion of the conductance attributable to the large ferrihaem species would be small. The mobility of Clion in DMSO has been estimated by Millen & Watts (1967) as 25-2+0-2 i-1cm.2 mole-1 at 250.…”

Section: Resultsmentioning

confidence: 99%

Solution structures of ferrihaem in some dipolar aprotic solvents and their binary aqueous mixtures

Brown

Lantzke

1969

Biochemical Journal

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“…combined into a single scale for transfer from DMA to water, as in (12). The values are shown in Table I and in Figure 1.…”

Section: Resultsmentioning

confidence: 99%

“…The trans isomer thus associates with anions and interacts with polar solvents less strongly than the cis isomer. 12 The role of hydrogen bonding in accentuating ion association in these cations is comparable to the role of the nitrogen proton +4.0" + 3.3"…”

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confidence: 95%

Isomerization, solvolysis, ion association, and solvation of cis- and trans-dichlorobis(ethylenediamine)cobalt(III) cations in protic and in dipolar aprotic solvents

Fitzgerald¹,

Parker²,

Watts³

1968

J. Am. Chem. Soc.

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“…19) which clearly show that two nitrogen protons, those trans to the cis chloro-ligands and along the octahedral edge trans to the edge occupied by these chloro-ligands, are more deshielded in the ion pair and as a result their absorption is shifted to lower fields. It is thus clear that at least for these cis-complexes hydrogen bonding between the ligands in the co-ordination sphere and ions in the solvation sphere is an extension of the metal centred structure and produces marked kinetic and thermodynamic consequences.…”

Section: The Substitution Reactions Of Transition Metal Complexesmentioning

confidence: 99%

Solvents and their role in determining inorganic mechanisms

Watts¹

1979

Pure and Applied Chemistry

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-Two aspects of the mechanisms of the reactions of metal complexes in nonaqueous solutions are discussed in this paper. Firstly, the contribution made by studies in non-aqueous solvents, and in particular dipolar aprotic solvents, to the understanding of the mechanisms of substitution reactions at transition metal centres is considered. Secondly, the mechanisms of electron transfer reactions between transition metal centres are discussed with particular reference to recent work on the role of the solvent. THE SUBSTITUTION REACTIONS OF TRANSITION METAL COMPLEXESIt is helpful to an understanding of these substitution processes to discuss why it is that dipolar aprotic solvents have had a smaller impact on the study of transition metal substitution kinetics than on similar studies in carbon chemistry. The value of the dipolar aprotic solvent to the understanding of the chemistry of carbon has derived from the importance of the bimolecular processes of substitution. In the bimolecular mechanisms the rate of nucleophilic attack by an anionic nucleophile shows a first order dependence on the concentration of the nucleophile, and the rate in any solvent is directly proportional to the activity of the nucleophilein the solvent. Thus anions, which have activities most characteristically dependent on transfer from protic to dipolar aprotic solvents, have rates of nucleophilic attack which can vary dramatically with the nature of the solvent.The dependence of such rates on solvent characteristics is now well understood and was most elegantly expressed by my Australian colleague, Prof.A.J.Parker in his important review in 1969 (Ref.1). In simple terms, Parker showed that small anions, hard in the Pearson sense (Ref.2), had greatly incrsd rates of bimolecular nucleophilic attack in dipolar aprotic solvents compared with protic solvents mainly because of the hydrogen bonding contribution to solvation of these anions in the latter solvents. Provided the changes in the solvation of the attacked substrate and the transition state are not important in comparison with these changes in anion solvation, differences in rate of as much as i6 fold can be found for bimolecular nucleophilic attack by say chloride ion in dimethylacetamide (DMA) compared with the rate in methanol, being faster in dimethylacetamide. Large polarizable anionic nucleophiles, soft in the Pearson sense (Ref.2), do not show this tendency, indeed their reactions are often slower in dipolar aprotic solvents because these ions are better solvated in these media.Unimolecular processes in carbon chemistry can also show marked solvent dependence particularly in those cases where considerable charge separation occurs in the transition state and thus the departing anion can manifest a good deal of its separate ion character. In such cases, if the departing ion is for instance the chloride ion, the reaction is significantly faster in a protic solvent compared with an aprotic solvent because of the magnitude of the hydrogen-bonding contribution to the solvation of...

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Octahedral cobalt(III) complexes in dipolar aprotic solvents. XII. Ion association studies in N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide by conductance measurements

Cited by 25 publications

References 2 publications

Solution structures of ferrihaem in some dipolar aprotic solvents and their binary aqueous mixtures

Solution structures of ferrihaem in some dipolar aprotic solvents and their binary aqueous mixtures

Isomerization, solvolysis, ion association, and solvation of cis- and trans-dichlorobis(ethylenediamine)cobalt(III) cations in protic and in dipolar aprotic solvents

Solvents and their role in determining inorganic mechanisms

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